Prof Allain Billard

Prof Juan Creus

Tabassom, Morganne Guillaume

Adria Fanny

Soraya

Cesar

Sara

Dorina

Maxime Raquel Antalya

João Clement

Lorenzo

Abdou, Mirasbek, Eugene, Getnet, Damir, Klaasjan, Monica, Charlotte, Cécile, Chiara, José, Guislain Pierre

Keywords: atmospheric pressure MOCVD dye-sensitized solar cells Electrochemical Impedance Spectroscopy FCC, Face-centered cubic FTIR, Fourier transform infrared spectroscopy HSS, High speed steel Open Circuit plasma assisted chemical vapor deposition Reactive oxygen species SEM, scanning electron microscopy Tributyltin oxide TEM, transmission electron microscopy TM, Transition metals UV, ultraviolet VB, valence band XRD, X-ray diffraction XPS, X-ray photoelectron spectroscopy TiO2, Titanium dioxide

with whom I have shared amazing time. You truly made my lab life enjoyable and unforgettable. Thanks for every good moment shared together.

I express my deepest gratitude to my parents, Marcia and Carlos, and my brother, Pedro, for encouraging me in all of my pursuits and inspiring me to follow my dreams. Even though they miss me, they always encourage me to see the world. I feel very lucky to having you as my parents, always supporting me and

First of all, I would like to express my profound gratitude to my three supervisors Prof. Frédéric Sanchette, Prof. Akram Alhussein and Dr. Carmen Jimènez for their patience, motivation and immense knowledge. Their guidance was unquestionably fundamental to overcome all the difficulties during all the time of research and the writing of the thesis. Thank you for giving me a lot of opportunities to visit different laboratories and to attend conferences over the world. Those were extremely useful for extend my vision to a wider research community. Both of you always showed an extraordinary positive attitude, and it was really a great pleasure to work under your supervision.

I would like to particularly thank to Dr. Carmen Jimènez for opening me the door of LMGP and for the hard questions which incented me to widen my research and life from various perspectives.

Besides my direct three supervisors, I would like to specially thank Dr. Frédéric Schuster for the support during these last 3 years, I thank you for the words of encouragement and for his implication in my future professional career. I would like to thanks University of Technology of Troyes (UTT) and doctoral school for this wonderful opportunity to do a PhD. In UTT, I would like to thank Marie Lecomte for all the administrative help and to be always available when I needed. I also want to thank Isabelle Leclercq and Pascale Denis for their administrative help and advices.

I would like to thank prof. Elisabeth Blanquet for all the help and support during my first year of PhD and for giving me the possibility to work in SIMaP lab.

General Introduction

Once a metal is submerged into seawater, interactions between metal, living organisms and marine environment result in biofouling and in corrosion of metal components, almost at the same time. The development of micro and macro-organisms is the origin of the biofouling, which causes a significant industrial problem. To develop an antibiofouling coating system, one should take into account the environment, the engineered coating and the substrate.

At the same time that biological changes give rise to biofilm accumulation, a sequence of inorganic changes occurs on the metal surface instantly after submerging it in an aggressive aqueous medium. This consists in the process of metal dissolution and development of corrosion product. While corrosion and corrosion product accumulation develop from the metal surface towards the solution, biofilm development is the result of accumulation processes directed from the bulk towards the metal surface. In this thesis, we propose a novel multi-functional bilayer coating composed of Al-Zr and TiO2 as an antibiofouling system on steel substrates. Aluminum-transition metal alloys have demonstrated to be potential candidates for protection of steel parts against galvanic corrosion. The transition metals are added to compensate the weak mechanical characteristics of pure aluminum. An optimized aluminum alloy with a homogeneous microstructure is deposited before the antifouling layer.

The material chosen as antibiofouling is based on titanium dioxide (TiO2), a promising candidate thanks to its photocatalytic activity and the hydrophilic / hydrophobic behavior in order to slow down the biofouling process in the marine structures. PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition) processes are suitable industrial technologies for the deposition of functional and protective thin films. Indeed, In this chapter I the state of the art for the chosen materials and their functionalities are presented. Firstly, the use of aluminium-transition metals as sacrificial anodes and the corrosion of aluminium alloys in aqueous solutions are discussed. The choice of Al-Zr coating for anti-corrosion applications is exposed. In a second part, the discussions are based on mechanisms and applications of photocatalysis by titanium dioxide semiconductors. Major aspects of biofouling and corrosion in marine structures are also presented.

I.1. Corrosion of aluminium and aluminium alloys in aqueous environment

Corrosion of Al alloys is normally presented as a local nature, since there is a separation of anodic and cathodic reactions.

The basic anodic reaction is metal dissolution: Al → Al 3+ + 3e -I. 1

Whereas the cathodic reactions are oxygen reduction: O2+2H2O + 4e-→ 4OH -I. 2 Or hydrogen reduction in acidified solution being the result of Al ion hydrolysis:

2H + + 2e → H2 I. 3
So the corrosion of Al can be described as the interaction between local anodes and cathodes and the alloy matrix that leads to all forms of corrosion in Al alloys [1]. It includes pitting corrosion, selective dissolution, trenching and intergranular attack.

Al and its alloys are known to be resistant against corrosion in moderately aggressive aqueous environments. The thermodynamic strength of Al alloys in corrosive environment is due to the protective oxide layer, which acts as a physical barrier and is capable to repair itself in oxidizing environments if damaged [1], although the passive layer breakdown mechanism by chloride ions is still in debate [2] as a result of the complexity of the process [3].

In general, localized attack initiates the adsorption of destructive anions and creation of soluble transitional compounds with the cations at the oxide surface. The passivity aspect that rules the corrosion behavior of Al is summarized by Pourbaix-type analysis. A plot of potential (VSCE) vs. pH in accordance with the electrochemical reaction of the species involved, known as Pourbaix diagram [4], is presented in Figure I-1. Pourbaix, 1974 [4]). The lines (a) and (b) are the water stability and its decomposed product.

Al is slightly passive in the pH range of ~ 4 to 9 as a result of the existence of an Al2O3 film. In environments that diverge from the neutral range, the permanence of Al2O3 film can be disrupted.

Consequently, the film becomes soluble, making it easier the rapid dissolution of the alloy. In the acidic range, Al is oxidized by creating Al 3+ , while AlO2 -happens in alkaline range.

Corrosion prediction can be interpreted as a straightforward process when Pourbaix diagram is taken in account. On the other hand, in actual engineering applications, there are numerous variables that weren't taken into account by Pourbaix. These comprise (i) the existence of alloying elements in the vast majority of engineering metals (ii) the presence of substances, like chloride, in the electrolyte, (iii) the working temperature of the alloy, (iv) the mode of corrosion, and (v) the reaction rate. Gimenez et al. [5] have taken these factors into account and they have created a revised version of a potential (VSCE) vs. pH diagram for 5000 series aluminium alloys in 0. Gimenez, Rameau et al., 1981[5]).

The conditions where localized attack is strongly possible in the theoretical passive region is shown in Figure I-2. Gimenez et al. [5] reported that localized attack is possible over the whole range of pH according to the specific potential.

Cathodic polarisation provides alloy degradation in two manners. Primarily, the accumulation of hydroxyl ions at the Al surface induces chemical dissolution of the Al. In the second place, Al is a strong hydride former, and hydrogen from the cathodic reaction at such negative potentials will associate with Al to create hydrides [6].

I.2. Electrochemical basis of corrosion

Corrosion generally takes place at an equilibrium level determined among the different electrochemical reactions. The anodic reaction is one of the reactions, in which a metal is oxidized, releasing electrons inside the metal. The other type is known as the cathodic reaction, in which solution species (usually O2 or H + ) are reduced, extracting electrons from the metal. When both

Chapter I: State of the art reactions are in equilibrium, the flow of electrons from respective reaction is balanced, and there is no electrical current. Both reactions can occur on one metal or on two different metals that are electrically linked.

Figure I-3 exposes this process. The vertical axis represents the electrical potential and the horizontal axis is the logarithm of absolute current. The straight lines correspond to the theoretical current for the anodic and cathodic reactions. The sharp point in the curve corresponds to the point in which the current reverses polarity as the reaction alters from anodic to cathodic, or vice versa [7].

The open-circuit potential Eoc is defined as an equilibrium potential assumed by the metal when there is no electrical connections to the metal. In the majority of electrochemical corrosion tests, the first step is the measurement of Eoc. The value of both anodic and cathodic current at Eoc is named the corrosion current, Icorr. The Icorr is used to determine the corrosion rate of the metal.

Classic Tafel analysis is carried out by extrapolating the linear portions of a logarithmic current against potential plot back to their intersection, as represented in Figure I-3. The value of either the anodic or the cathodic current at the intersection is Icorr. The measured data are fit to Eq. I.4 by setting the values of Ecorr, Icorr, βa, and βc [7].

𝐼 = 𝐼 𝐶𝑜𝑟𝑟 (𝑒 ) I. 4 Where I is the measured current from the cell in A (amperes).

Icorr is the corrosion current in A (amperes).

E is the electrode potential.

Ecorr is the corrosion potential in V (Volts).

βa is the anodic β Tafel constant in V/ decade.

Βc is the cathodic β Tafel constant in V/ decade. potential is situated at point A. At this potential the sum of the anodic and cathodic reaction rates on the electrode surface is equal to zero. When the potential increases, it moves towards the region B, which is the active region. In that zone, metal oxidation is the principal reaction taking place.

Chapter I: State of the art

The passivation potential is represented by the point C, and as long as the applied potential rises over this value the current density decreases with increasing potential (region D) until a low, passive current density is achieved (Passive region -Region E) [8]. When the potential reaches an adequate positive value (Point F) the applied current quickly increases (region G). This increase can be a result of different phenomena, depending on the alloy and environment. For some systems such as aluminium alloys in salt water, this sudden increase in current may be pitting, which means a localized breakdown of passivity, whereas for others it may be transpassive dissolution. Transpassivation occurs when a passivated metal starts rapid dissolution (increase in corrosion rate) if the metal's electrode potential becomes too positive.

I.3. Al-transition metal coatings used as sacrificial anodes

Sacrificial Anodes are highly active metals used to protect a less active material surface from corroding. Sacrificial Anodes are made from a metal alloy with a more negative electrochemical potential than the metal which should be protected: the anode mentioned "sacrificial" will be consumed instead of the protected metal.

Since the sacrificial anode works by introducing another metal surface with a more negative and much more anodic surface, the current will move from the recently introduced anode and the protected metal becomes cathodic forming a galvanic cell. The oxidation reactions are transferred from the metal surface to the galvanic anode and will be sacrificed in favor of the protected metal structure.

Chapter I: State of the art

Carbon steels are susceptible to corrosion in most environments and will, unless protected, oxidize spontaneously in state of high thermodynamic stability. The integrity of steel structures in marine environments can be ensured by Cathodic Protection (CP) [9]. The cathodic protection links the base metal at risk (steel) to a sacrificial metal that corrodes instead of the base metal. The technique of supplying cathodic protection to steel preserves the metal by providing a highly active metal that can work as an anode and supply free electrons.

In order to ensure a general protection of carbon steel immersed in seawater, a protection potential Ep equal to or less than -850 mV/SCE is needed [10]. Normally, pitting corrosion, stresscorrosion cracking, and intergranular corrosion may be annihilated by maintaining the electrode potential below a given critical value, known as the protection potential [11]. Aluminium and its alloys are extensively used in many industrial applications, mainly in automotive and aerospace

industries. Its open circuit potential is more negative than that of steel implying its use as sacrificial materials for protection of steel substrates. It has been demonstrated that nanostructured aluminium alloys show high mechanical properties and corrosion resistance in contrast to their bulk counterparts due to the very great number of crystallizations germs, stable intermetallic compounds and increasing of the germination velocity due to a rapid solidification process [12,13]. The usefulness of pure Al as an anode material in seawater is reduced significantly due to the formation of a protective oxide film, which limits both its current and potential output. In order to improve the efficiency of Al anodes they are typically alloyed with other elements to encourage depassivation (breakdown of the oxide film) and/or shift the operating potential of the metal to a more electronegative direction. The incorporation of alloying elements has been reported as a possible method to improve the corrosion resistance of Al coatings, essentially for limiting the sensitivity to localized corrosion in saline environments [14,15].

The transition metals (TM) have a low solubility in Al, so, when a PVD method is used, it leads to deposition of extended supersaturated solid solutions of TM in Al, which are associated with enhanced mechanical properties. Additionally, TM are characterized by a low diffusivity in Al, leading to a better thermal stability of these metastable alloys. Nevertheless, high TM content in Al alloy conducts also to the detriment of sacrificial properties [16,17]. The ideal solution in terms of steels protection is a compromise between mechanical and corrosion properties.

Over the last decades, a lot of researches focused on the nanocrystalline Al-based coatings reinforced by addition of transition metals (TM) e.g. Al-Cr [18][19][20][21][22][23][24] , Al-Mo [25] , Al-Ti [19][20][21]26] and Al-Mn [15,27]. Regarding Al-Cr alloys, the best compromise has been achieved for amorphous coatings with approximately 20 at.% Cr [22]. Addition of Mo or Mg in Al led to an improvement of mechanical properties and the pitting resistance of Al-Mo and Al-Mn coatings [16,25] . As previously explained, alloying elements are added to pure Al for various purposes, mainly for the enhancement of mechanical properties. Alloying elements induce the heterogeneity of microstructure, which is the main cause of localized corrosion starting in the form of pitting.

I
Zr, Cr, Sc, Ti, W and Sr are normally added individually in small amounts in order to refine grain size, reduce crystallisation and decrease the effect of intermetallic compounds [28].

Intermetallic compounds can be produced at high temperatures in the Al melt with adding Zr and Ti, presenting dispersed particles of Al3Zr and Al3Ti inside the solidified matrix. These particles with fine size (i.e << 1 µm) show a minimal impact on corrosion [29]. Al3X particles where X can be Zr, Ti, Sc, W, Ti and Sr, take the form of fine insoluble dispersoids which are efficient in grain inoculation and refinement.

I.3.2. Kinetic stability of aluminium and its alloys

Kinetics is defined as the reaction during corrosion. Metals tend to stabilize to a value of electrochemical potential, characteristic of the material and its composition for a given electrolyte, when subjected to an aqueous environment. This potential can be described as the potential at which anodic and cathodic reactions over the metal surface are the same, and the value of this potential is strongly influenced by elements that may change the relative rates of anodic or cathodic reaction effectiveness over the metal surface.

Generally, a potentiodynamic polarization test is applied in order to characterize the corrosion performance of an alloy. This technique provides essential kinetic information, for example, current density over a range of potentials, corrosion potential, pitting potential, passive current density, etc. Alloy behavior and dominant reactions can be compared in a quantitative way.

The anodic part of the polarization curve provides information related to the anodic/ dissolution reaction, whereas the cathodic part symbolizes the reduction reaction, known as oxygen reduction, as explained before. value of electrode potential which is known as the pitting potential (Epit) [30]. [30].

Addition of more noble alloying elements normally boosts the corrosion potential to more noble values [31]. This ennoblement is not necessarily correlated to the corrosion rate. Moreover, the major threat for Al alloys is the localized attack, in particular pitting. Therefore, more noble value of pitting potential does not necessarily indicate a better corrosion resistance [32]. Normally, the electrochemical reactions of Al-alloys are greatly influenced by the microstructure and the chemistry. In order to understand better the overall kinetic stability of a metal over the potential-pH space, the staircase potentio-electrochemical impedance spectroscopy (SPEIS) method can be employed to determine kinetic stability diagrams, as demonstrated for pure Al [30].

Chapter I: State of the art

I.4. Choice of Al-Zr coating for corrosion applications

Zirconium is a transition metal known as a great grain refiner in traditional aluminium alloys [33]. It boosts mechanical properties [33] and improves considerably the corrosion resistance in deaerated borate-boric acid solution containing Cl - [34]. Contrary to the traditional methods where the Zr solid solubility is very low (maximum solubility is 0.083 at.% in the peritectic reaction [35]), the non-equilibrium processing methods like rapid solidification [36], mechanical alloying [37] and sputtering technique [34,38,39] offer the possibility to achieve an extended supersaturated single solid solution with high Zr contents.

Chaudhury et al. [36,40] have shown that rapid solidification and vapour deposition results in an enlargement of the solid solubility of zirconium up to 3 at.%. Ho et al. [38] have produced, by alternate sputtering deposition, a face-centered cubic phase supersaturated solution of 7.3 at. % Zr in Al. Yoshioka et al. [34] obtained a supersaturated phase with Zr content up to 11 at. % by sputter deposition.

Creus et al. presented the open circuit potential classification in saline solution of metals and

Al based alloys deposited by PVD as represented in Figure I-6 [14]. The corrosion potentials of Al-Zr coating are more negative than the potential threshold needed to obtain cathodic protection of steels in saline solution. It was reported that the addition of elements such as V, Mn, Cr, Ti, Mo, Si or Zr induces an ennoblement of the corrosion potential compared with pure Al when the alloying element content is increased. The reaction of hydrogen evolution (REH) corresponds to a new cathodic reaction which takes place on the alloy surface for high negative potentials. In fact, very negative corrosion potentials contribute significantly to this cathodic reaction that could degenerate the subjacent substrate. Consequently, it is essential to limit the mass content of these alloying elements. Crossland [41] on the anodization of different binary Al-X alloys showed an improvement of the resistance of passive film in saline medium. This improvement was mainly associated to the Chapter I: State of the art incorporation of more stable oxide in the passive film that is initially mainly composed of aluminium oxide or oxo-hydroxide, and which will permit to modify the net oxide charge in contact with the corrosive medium and will affect the stage of adsorption of the chloride ions during the passive film growth [14].

Figure I-7. Polarization curves after 1 h of immersion in saline solution of binary Electron-beam physical vapor deposition and magnetron sputtering Al-X alloys deposited on glass substrates with

concentration close to 20 wt.% ( Reproduced from J. Creus et al.,2012 [14]).

Some researchers found that Zr has a tendency to be enriched in the passive film, because of its low electro-negativity and high oxygen affinity compared to Al [42]. Zr addition enhances the protective efficiency of the passive film most likely due to the corrosion products formed on Al alloys [START_REF] Yoshioka | Anodic polarization behaviour of sputter-deposited AlZr alloys in a neutral chloride-containing buffer solution[END_REF]. The coupling between Al-Zr and steel was studied by Sanchette et al. [START_REF] Reffas | Intrinsic structural, mechanical and corrosion properties of sputtered Al-Zr thin films[END_REF] in 5 wt.% NaCl solution using potentiodynamic polarisation, linear polarisation resistance and electrochemical impedance spectroscopy (EIS). The results have shown an enhancement of corrosion resistance and a decrease of corrosion kinetics with Zr content. In addition, they reported that the corrosion resistance of Al-Zr coatings is influenced by structure and porosity of coatings [START_REF] Reffas | Intrinsic structural, mechanical and corrosion properties of sputtered Al-Zr thin films[END_REF].

Liu et al. [START_REF] Liu | An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part II.: EIS interpretation of corrosion behaviour[END_REF] reported that permeable defects like pores or pinholes are prejudicial to the corrosion performance as they afford direct paths to permit the corrosive electrolyte to access to the steel substrate.

I.5. The heterogeneous photocatalysis: principles and generalities

Photocatalysis is employed as an environmentally friendly, versatile and low-cost treatment alternative for water and air purification [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF], destruction of microorganisms such as bacteria [START_REF] Gumy | Solar photocatalysis for detoxification and disinfection of water: Different types of suspended and fixed TiO2 catalysts study[END_REF] and viruses [START_REF] Maness | Bactericidal Activity of Photocatalytic TiO2 Reaction : toward an Understanding of Its Killing Mechanism Bactericidal Activity of Photocatalytic TiO2 Reaction : toward an Understanding of Its Killing Mechanism[END_REF], nitrogen fixation [START_REF] Petriconi | Photocatalytic fixation of nitrogen by aqueous suspensions of silver iodide[END_REF], the inactivation of cancer cells [START_REF] Kabachkov | Research of photocatalytic degradation of HeLa cells at the TiO2 interface by ATR-FTIR and fluorescence microscopy[END_REF], the clean-up of oil spill [START_REF] Hsu | Photocatalytic degradation of spill oils on TiO2 nanotube thin films[END_REF] and the water splitting to generate molecular hydrogen as high-energy and ecologically clean fuel [START_REF] Al-Rasheed | Water Treatment By Heterogeneous Photocatalysis an Overview, 4th SWCC[END_REF]. Palmisano and Sclafani [START_REF] Mart | Physico-chemical properties of WO3/TiO2 systems employed for 4-nitrophenol photodegradation in aqueous medium[END_REF] described the heterogeneous photocatalysis as a "catalytic process during which one or more reactions steps occur by means of electron-hole pairs photogenerated on the surface of semiconductor materials illuminated by light of suitable energy".

When a photocatalyst is irradiated, it creates excited states that can provide resulting processes such as reduction-oxidation reactions and substrate transformations. The fundamental mechanism of heterogeneous photocatalysis, studied by many researchers [START_REF] Hoffmann | Environmental Applications of Semiconductor Photocatalysis[END_REF][START_REF] Litter | Heterogeneous photocatalysis: Transition metal ions in photocatalytic systems[END_REF], is presented in Figure I-8. When a semiconductor is irradiated with light of energy equal or higher than its band gap energy (Eg), electrons (e -) from the valence band jump to the conduction band, at the same time leaving positive holes (h + ) in the valence band [START_REF] Schneider | Understanding TiO2 Photocatalysis: Mechanisms and Materials[END_REF]. The photogenerated electron-hole pairs are able to induce oxidation-reduction reactions with electron donor and electron acceptor adsorbed on the catalyst surface. The redox process competes with a potential electrons-holes recombination that takes place during a few nanoseconds. Consequently of electron-hole recombination phenomena, an input energy dissipation is formed. The thermodynamic prerequisite required for the occurrence of this process is a more negative potential of the electron acceptor, and a more positive potential for the holes of the valence band against the oxidation potential of the donor.

An ideal photocatalyst must present the following characteristics [START_REF] Al-Rasheed | Water Treatment By Heterogeneous Photocatalysis an Overview, 4th SWCC[END_REF]:

 High photoactivity.
 Biological and chemical inactivity.

 Stability against corrosion.

 Mechanical robustness.

 Suitability towards visible or near UV light.

 Low cost.

 Lack of toxicity.

Chapter I: State of the art

A wide range of inorganic semiconductors is used for photocatalytic applications. Band gap energies of the widely used semiconductors are presented in Table I. 1.

Table I. 1. Band gap energies of semiconductors employed in photocatalytic processes [START_REF] Loddo | Membranes for photocatalysis in water and wastewater treatment[END_REF].

Some of the photocatalysts do not guarantee long-term activity within the photocatalytic process. Binary metal sulfides like CdS, CdSe or PbS, with narrow band gap providing the advantage of absorbing the visible light, illustrate an example of unstable semiconductor for catalysis in aqueous media when they experience photo-anodic corrosion. To prevent this drawback, numerous works suggest the addition of sulfide and sulfite to the contacting solution [START_REF] Beydoun | Role of Nanoparticles in Photocatalysis[END_REF]. These materials are worldwide known as toxic compounds.

Hematite (α -Fe2O3), presenting a band gap of about 2.2 eV, could be a potential photocatalyst in the visible range, but demonstrates much lower activity than TiO2 or ZnO, certainly because of corrosion or the creation of short-lived metal-to-ligand or ligand-to-metal charge-transfer states as demonstrated by Trillas et al [START_REF] Trillas | Photo-oxidation of phenoxyacetic acid by TiO2illuminated catalyst[END_REF]. Both ZnO and TiO2 anatase have band gap energy of about 3.2

eV, but zinc oxide is known to be more unstable in illuminated aqueous solution, with hydroxide species being created on the particle surface allowing the catalyst deactivation [START_REF] Navaladian | Thermal decomposition as route for silver nanoparticles[END_REF]. Wu et al [START_REF] Wu | Enhanced TiO2 photocatalysis by Cu in hydrogen production from aqueous methanol solution[END_REF] also detected a higher TiO2 photocatalytic activity in comparison to that of SnO2.

Heterogeneous photocatalysis based on oxide semiconductors is a promising technique for the inhibition of microbial proliferation and to reduce the growth of microorganisms. Titanium dioxide is capable to catalyze the degradation of organic substances and it becomes highly hydrophilic under UV illumination, which turns TiO2 to be an interesting antifouling material [START_REF] Kallio | Antifouling properties of TiO2: Photocatalytic decomposition and adhesion of fatty and rosin acids, sterols and lipophilic wood extractives, Colloids Surfaces A Physicochem[END_REF].

I.6. TiO2 structures, electronic properties and photocatalysis

Titanium dioxide (TiO2) presents high photocatalytic activity and chemical stability, nontoxicity and low cost, and consequently it has become the most widely used photocatalyst. TiO2 has been widely used as white pigment in paints, plastic, paper, cosmetics and foodstuffs.

TiO2 has three crystalline forms in nature: anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic) as represented in Figure I-9. The majority of these phases appear exclusively at high pressure. The unit cells of the TiO2 crystal structures are presented in Table I. 2 [START_REF] Esch | Surface structures and thermodynamics of low-index of rutile, brookite and anatase -A comparative DFT study[END_REF].

Rutile is stable and chemically inert. Rutile is able to be excited by both UV and visible light [START_REF] Hashimoto | TiO2 Photocatalysis: A Historical Overview and Future Prospects[END_REF]. Anatase can be converted into rutile at high temperatures between 400 and 700 °C, and is able to be excited by UV light. Both phases show a tetragonal, ditetragonal and dipyramidal crystal structure; although they diverge from each by the different space group lattices. Brookite goes through a non-reversible phase transformation into rutile when heated at high temperatures between 700 °C and 900 °C. Additionally, TiO2 can exist in less common crystal forms, for example, baddeleyite, columbite, ramsdellite, hollandite and monoclinic structures [START_REF] Kavan | Optically Transparent Cathode for Dye-Sensitized Solar Cells Based on Graphene Nanoplatelets[END_REF]. Table I. 2. Crystal structure parameters of anatase and rutile TiO2 [START_REF] Mathews | Titania Nano-architectures for Energy[END_REF].

Rutile form presents a more stable thermodynamic behavior and a higher refractive index than anatase, which makes it more suitable for optical applications. In contrast, anatase is the most adequate form for the photocatalytic process, solar cells and gas sensing, as a result of greater charge carrier mobility and its catalytic properties [START_REF] Banerjee | The design, fabrication, and photocatalytic utility of nanostructured semiconductors: Focus on TiO2-based nanostructures[END_REF]. The crystal structure parameters of the TiO2 anatase and rutile are given in Table I. 2 [START_REF] Mathews | Titania Nano-architectures for Energy[END_REF]. The anatase-rutile phase transformation is affected by several causes, for example, particle size, stress, texture and also pressure. It has been found

that the temperature of anatase-rutile transformation increases when the particle size is increased.

Li et al. [START_REF] Li | Size dependence of thermal stability of TiO2 nanoparticles[END_REF] showed that a decrease in the nanoscale particle size, e.g. from 23 to 12 nm, resulted in a higher anatase stability.

TiO2 presents a band gap energy of approximately 3.2 eV, therefore, its photoactivation demands radiation with light of wavelength less than or equal to approximately 380 nm, having a maximum activation at approximately 340 nm [START_REF] Mathews | Titania Nano-architectures for Energy[END_REF]. The valence band of wide-gap rutile and anatase is formed by O2p states, whereas the conduction band is made of Ti 3d states [START_REF] Hervieu | The Surface Science of Metal Oxides[END_REF]. The electrical properties are dependent on the crystallographic direction. Titania crystals are considered a wide-gap semiconductor, which present high resistivity (~1015 Ωcm) [START_REF] Ardakani | Electrical and optical properties of in situ "hydrogen-reduced" titanium dioxide thin films deposited by pulsed excimer laser ablation[END_REF] and present various sorts of ionic imperfections: titanium interstitials, titanium vacancies, bulk oxygen vacancies and reduced crystal surfaces that are considered to create shallow electron donor levels cooperating with the electric conductivity of TiO2 [START_REF] Paxton | Electronic structure of reduced titanium dioxide[END_REF].

I.6.1. Mechanisms of TiO2 photocatalytic activity

Photocatalyst properties of TiO2 are strongly based on its hydrophilic character and ability to catalyze the degradation of organic substances. Both of them are induced by UV illumination. The UV radiation provided by a strong daylight presents enough intensity to maintain the strong hydrophilicity and photocatalytic degradation. Photocatalytic decomposition is the major cleaning mechanism in applications such as water and air purification [START_REF] Blake | Bibliography of Work on the Heterogeneous Photocatalytic Removal of Hazardous Compounds from Water and Air[END_REF]. The photocatalysis performance depends on the quantity of radicals created from the reactions of H2O and O2 on defect sites of UV illuminated TiO2 surfaces. Radicals (such as O2 -, •OOH, •OH), other active species (H2O2), electrons or electron holes of TiO2 can react with organic matters. Moreover, dissolved oxygen plays a considerable function in the degradation reactions [START_REF] Schwitzgebel | Role of the Oxygen Molecule and of the Photogenerated Electron in TiO2-Photocatalyzed Air Oxidation Reactions[END_REF]. The end products are predominantly CO2 and H2O. Depending on the decomposed compound, numerous intermediate products can be formed during the degradation [START_REF] Krumdieck | Chapter I: State of the art Polson[END_REF]. TiO2 being a semiconducting oxide that, under irradiation, behaves as a powerful oxidizing material, reducing the activation energy for the decomposition of organic/inorganic molecules. The necessary energy to promote electrons is influenced by the semiconductor band gap that represents the lowest energy required to transform the material into an electrically conductive material. TiO2 anatase presents a band gap of about 3.2 eV and rutile phase presents a band gap of approximately 3.0 eV, which corresponds to photons with a wavelength of 388 nm and 413 nm, respectively [START_REF] Lasa | Photocatalytic Reaction Engineering[END_REF].

The adsorption of organic compounds or water on TiO2 surface is advantaged by the superhydrophilicity of the TiO2 surface that leads to a contact angle less than 5° under UV light irradiation. The superhydrophilicity originates from modifications in the chemical configuration of the semiconductor surface [START_REF] Wang | Light-induced amphiphilic surfaces[END_REF]. As presented above, the major part of the photogenerated holes produces hydroxyl radicals by reacting with the adsorbed species, whereas the others are grabbed by lattice oxygen vacancies and, reacting with the titania molecules, induce a weakening of the lattice titanium-oxygen ions bonds. The inhibition of these bonds allows water to produce additional hydroxyl groups on the TiO2 surface under UV light irradiation [START_REF] Figoli | Photocatalytic activity and synthesis procedures of TiO2 nanoparticles for potential applications in membranes[END_REF].

The effect of light modifications on the titania photo-induced performance and its structure changes has received considerable attention from researchers, whose objective is to improve the semiconductor performance. It is possible to understand better water and air pollution control, selfcleaning glasses and antifogging by controlling the hydrophilicity phenomenon.

I.6.2. Self-cleaning properties of TiO2

A photocatalytic material can be considered self-cleaning if the rate of destruction/removal of the surface contaminants is higher than the rate of deposition of the contaminants. If a too thick layer of contaminants is accumulated on the surface, it will eventually impede all the essential UV light and the photocatalytic reactions will finish.

In several situations, photocatalysis alone is not sufficient to provide the surface self-cleaning in practice. TiO2 surfaces have another property, in addition to photocatalysis, which leads to the self-cleaning ability: the photoinduced superhydrophilicity. This effect was first reported in 1997 by Wang et al. [START_REF] Wang | Light-induced amphiphilic surfaces[END_REF]. Many studies have shown that under UV irradiation the surface of TiO2 becomes highly hydrophilic. The contact angle between a water droplet and the surface approaches zero and instead of forming droplets water forms a thin film on the surface which remains clear and transparent.

Different explanations for the photoinduced superhydrophilic effect have been given in the literature. The first proposed model assumes the ejection of oxygen atoms from the TiO2 lattice leading to the production of surface oxygen vacancies [START_REF] Wang | Light-induced amphiphilic surfaces[END_REF]. These defects are known to induce water splitting which would then lead to increase the number of surface hydroxyl groups and improve the surface hydrophilicity [START_REF] Thompson | Surface Science Studies of the Photoactivation of TiO2 New Photochemical Processes[END_REF]. The second model suggests that an important reconstruction in the surface hydroxyl groups happens under UV irradiation [START_REF] Sakai | Quantitative Evaluation of the Photoinduced Hydrophilic Conversion Properties of TiO2 Thin Film Surfaces by the Reciprocal of Contact Angle[END_REF]. A more hydrophilic surface is produced when an increased quantity of metastable weakly bound hydroxyl groups are presented after UV irradiation. The third explication, and the most simple, proposes that superhydrophilicity is an intrinsic property of clean TiO2 surfaces and that decrease in water contact angles under UV irradiation is just created by the photocatalytic oxidation of surface hydrocarbon impurities [START_REF] Thompson | Surface Science Studies of the Photoactivation of TiO2 New Photochemical Processes[END_REF].

Although the ongoing discussion about the current mechanisms, the revelation of the superhydrophilic property resulted in a big interest towards TiO2 surfaces and many applications began emerging on the market. Photocatalytic TiO2 thin films can be applied as self-cleaning surfaces to combat fouling problems in many types of environments.

The most famous examples are self-cleaning windows fabricated by Pilkington, PPG and Saint Gobain. Photocatalytic reactions on the surface of TiO2 decompose the accumulated organic dirt, and consequently clean the surface with the presence of UV light. Superhydrophilicity of the irradiated TiO2 facilitates the removal of any dirt on the surface by washing with water.

Additionally to the windows, photocatalytic self-cleaning surfaces impede fouling in many other applications like bathroom tiles, lamp covers in highway tunnels, building walls, etc [START_REF] Fujishima | TiO2 Photocatalysis: Fundamentals and Applications[END_REF].

I.6.3. Structural design and properties of TiO2 photocatalysts

TiO2 nanostructures in various shapes are used in multiple applications because of their particular size-related properties. The energy band structure becomes discrete for nanometer-scale TiO2, and its surface, photochemical, and photophysical properties are rather different from those of the bulk ones as a result of the quantum size effect. For this reason, a lot of works have focused on targeted synthesis of nanocrystalline TiO2 with a large specific surface area [START_REF] Ding | TiO2 hollow spheres with large amount of exposed (001) facets for fast reversible lithium storage[END_REF][START_REF] Kim | Synthesis and photocatalytic activity of mesoporous TiO2 with the surface area, crystallite size, and pore size[END_REF].

Many aspects have been considered with the maximized surface area. The diffusion of reactants in and out of the surface region must be quick. Since light is necessary for the photocatalytic reactions the piece geometry must allow the light to reach the surface area as more as possible. Thus, photocatalysts having high surface areas have been made by introducing materials with nanoscale geometries. Thin film deposition process can be employed to develop nanostructured photocatalysts but because of large surface areas and complex three-dimensional structures involved, highly conformal film growth is necessary.

The construction of TiO2 nano-or micro-structures with interesting morphologies and properties has recently attracted considerable attention [START_REF] Chen | Synthesis of Titanium Dioxide (TiO2) Nanomaterials[END_REF] and many TiO2 nanostructural materials, such as spheres [START_REF] Wang | Large Scale Synthesis and Gas-Sensing Properties of Anatase TiO2 Three-Dimensional Hierarchical Nanostructures[END_REF], nanorods [START_REF] Cozzoli | Low-Temperature Synthesis of Soluble and Processable Organic-Capped Anatase TiO2 Nanorods[END_REF], fibers [START_REF] An | Synthesis and Characterization of Thermally Stable Nanotubular TiO2 and Its Photocatalytic Activity[END_REF], tubes [START_REF] Nakata | Fabrication and Photocatalytic Properties of TiO2 Nanotube Arrays Modified with Phosphate[END_REF], sheets [START_REF] Aoyama | Mesocrystal nanosheet of rutile TiO2 and its reaction selectivity as a photocatalyst[END_REF], and interconnected architectures [START_REF] Arabatzis | Synthesis of Porous Nanocrystalline TiO2 Foam[END_REF], have been fabricated. Nanostructured TiO2 materials are widely used not only in photocatalysis, but also in dye-sensitized solar cells (DSSCs) [START_REF] Hou | Highly crystallized mesoporous TiO2 films and their applications in dye sensitized solar cells[END_REF], lithium-ion batteries [START_REF] Ding | TiO2 hollow spheres with large amount of exposed (001) facets for fast reversible lithium storage[END_REF], and electrochromic displays [START_REF] Periyat | Rapid microwave synthesis of mesoporous TiO2 for electrochromic displays[END_REF].

In the different TiO2 applications, its antimicrobial action has attracted a lot of interest in the last few decades. A lot of works have been published on the efficiency of TiO2 thin films as antimicrobial systems, for bacteria, viruses, fungi and algae disruption. The TiO2 unification also allows the reduction of fouling and at the same time highly improves water permeability and selfcleaning ability.

It is widely known that various elements influence significantly the photocatalytic performance of TiO2, such as the crystallite size, specific surface area, pore volume, pore structure, crystalline

Chapter I: State of the art phase, and the exposed surface facets. Consequently, the enhancement of this performance by setting these elements remains the aim of photocatalysis research. Structural dimensionality is also an element that can influence the photocatalytic performance, and presents an important effect on the properties of TiO2 materials as represented in Figure I-10. For instance, a sphere with zero dimensionality presents a high specific surface area, leading to a higher rate of photocatalytic decomposition of organic pollutants [START_REF] Liu | Mesoporous TiO2 Core-Shell Spheres Composed of Nanocrystals with Exposed High-Energy Facets: Facile Synthesis and Formation Mechanism[END_REF]. One-dimensional fibers or tubes have advantages with respect to less recombination as a result of the short distance for charge carrier diffusion [START_REF] Lu | A General Preparation Strategy for Hybrid TiO2 Hierarchical Spheres and Their Enhanced Solar Energy Utilization Efficiency[END_REF], lightscattering properties [START_REF] Yao | Electrospinning and Stabilization of Fully Hydrolyzed Poly(Vinyl Alcohol) Fibers[END_REF], and fabrication of self-standing nonwoven mats [START_REF] Lee | Characterization of nano-structured Chapter I: State of the art poly(ε-caprolactone) nonwoven mats via electrospinning[END_REF].

Two-dimensional nanosheets present high adhesion and smooth surfaces [START_REF] Katsumata | Preparation and Characterization of Self-Cleaning Glass for Vehicle with Niobia Nanosheets[END_REF], as long as threedimensional monoliths can have high carrier mobility as a consequence of their interconnecting structure and be used in eco-friendly decontamination. TiO2 materials with the appropriate dimensionalities allow us to take full advantage of the extraordinary properties provided by TiO2 materials.

I.6.4. Role of crystal size and surface area of TiO2 on its photoactivity

It has been reported that thin films having a large surface area with high porosity and good transparency to the visible light can be efficient photocatalyst materials [START_REF] Negishi | Preparation of a transparent thin-film photocatalyst for elimination of VOC[END_REF].

Increasing the light absorption is well-known in small nanocrystals where the surface to volume ratio is very high. The interface improvement of the absorption becomes significant when the particle size is about 20 nm [START_REF] Banerjee | The design, fabrication, and photocatalytic utility of nanostructured semiconductors: Focus on TiO2-based nanostructures[END_REF]. An extra advantage acquired in nanoparticles in the few nanometer size regimes is that the large surface-to-volume ratio allows the timely usage of photogenerated carriers in interfacial process [START_REF] Enright | Spectroscopic determination of electron and hole effective masses in a nanocrystalline semiconductor film[END_REF].

As photocatalysis is a surface phenomenon, surface area is crucial in determining the quantity of reacting sites, and charge carriers have to be employed correctly to enhance their ability to launch surface reactions. At the same time, TiO2 crystallinity must be high to avoid the recombination of charge carriers (e -and h + ). The energy conversion efficiency on TiO2 is influenced by many factors, for example, surface area, e --h + recombination rate, and solar energy spectrum [START_REF] Banerjee | The design, fabrication, and photocatalytic utility of nanostructured semiconductors: Focus on TiO2-based nanostructures[END_REF]. The surface area and crystallinity of the TiO2 powder must to be improved in order to have an efficient photocatalysis.

Gerischer [START_REF] Gerischer | Photocatalysis in aqueous solution with small TiO2 particles and the dependence of the quantum yield on particle size and light intensity[END_REF] created a model based on the mechanistic stages in photocatalysis on TiO2 that concludes an important increase in quantum yield (defined as the ratio of electron-hole pairs involved in redox reactions at the surface of the titanium dioxide particles to total electron-hole pairs produced) when particle size decreases from 1000 to 10 nm. As long as the performance of light absorption reduces with decreasing the particle size, less electron-hole pairs are generated.

The increase in quantum yield predicated by Gerischer's model depends on a large portion of electron-hole pairs participating in redox reactions at the surface when few electron-hole pairs are created inside the particle.

Grela and Colussi [START_REF] Grela | Kinetics of Stochastic Charge Transfer and Recombination Events in Semiconductor Colloids. Relevance to Photocatalysis Efficiency[END_REF] created a computer stochastic model that indicates an increase in quantum yield when the particle size increases from 3 to 21 nm. The increasing of quantum yield was also accompanied with a slow rate of electron-hole recombination at the surface [START_REF] Wang | Photocatalytic decomposition of halogenated organics over nanocrystalline titania[END_REF].

Grela and Colussi model shows that slower electron-hole recombination when particle size increased to 21 nm is a consequence of two hypotheses made in the model: (i) the average initial distance between electron-hole pairs is proportional to the radius of the TiO2 particle (R), and (ii) the average number of jumps before a free hole at the surface recombined with a trapped electron is proportional to R 2 [START_REF] Grela | Kinetics of Stochastic Charge Transfer and Recombination Events in Semiconductor Colloids. Relevance to Photocatalysis Efficiency[END_REF].

I.6.5. Synthesis of TiO2 nanomaterials

Nanostructure of TiO2 can be synthesized through several methods. The most used methods are sol-gel, hydrothermal method, direct oxidation, microwave method, chemical or physical vapor deposition and also atomic layer deposition.

Generally, liquid phase deposition methods take place at low temperature and create amorphous TiO2; a separate annealing step is essential in order to crystallize the film. TiO2 thin films made by the sol-gel technique usually require annealing at around 400-500 °C in order to be crystallized [START_REF] Langlet | Photocatalytic activity and photo-induced superhydrophilicity of sol-gel derived TiO2 films[END_REF]. Polycrystalline TiO2 can be produced directly more easily by vapor phase methods, as crystalline anatase TiO2 at much lower temperatures, close to 200 °C. Besides, gas phase methods normally ensure high quality thin films with lower impurity and higher film densities.

In the present work, TiO2 films were synthetized by aerosol assisted chemical vapor deposition method (AACVD).

I.6.6. Synthesis of TiO2 films by Chemical Vapor Deposition and its photocatalysis properties

Chemical vapor deposition (CVD) is a widely employed material-processing technology, based on a deposition method where chemical precursors are transferred in a vapor state to decompose on a heated substrate to create a film as illustrated in Figure I-11. The films ca be epitaxial, polycrystalline or amorphous as a function of materials characteristics and reactor conditions. This technology is able to deposit films with a conformal and complex coverage. The CVD can be performed in various environments with improved equipment and different products in order to deposit a large diversity of materials. CVD has become the main method of film deposition for the semiconductor industry as a result of its high capacity, high purity and low cost operation.

Moreover, CVD is commonly used for optoelectronics, optical and wear resistant coatings. In general, CVD may be conducted in hot-wall or cold-wall reactors usually bellow 10 -6 Pa and up to atmospheric pressure, with or without carrier gases, and at temperatures normally varying from 200 to 1600°C. The advanced CVD methods include the use of plasma, ions, photons, laser or combustion reactions in order to boost deposition rate and/or decrease deposition temperature.

The CVD derivatives involve metal-organic CVD, organo-metallic CVD and inorganic CVD [START_REF] Jones | Malandrino, Chemical Vapour Deposition. Precursors, Processes and Applications[END_REF]. CVD has been employed in various studies to develop photocatalytic TiO2 thin films [START_REF] Justicia | Designed Self-Doped Titanium Oxide Thin Films for Efficient Visible-Light Photocatalysis[END_REF] [106]. Generally, CVD reaction is employed where a titanium alkoxide vapor is controllable over the substrate at a defined temperature where the titanium precursor molecules decompose leading to TiO2 growth [107] [108]. During the chemical reaction, O2 reacts with the titanium precursor [START_REF] Byun | Photocatalytic TiO2 deposition by chemical vapor deposition[END_REF]. Plasma enhanced CVD (PECVD) has been applied to deposit photocatalytic TiO2 thin films using Ti(iOPr)4 and O2 plasma [START_REF] Guillard | Physical properties and photocatalytic efficiencies of TiO2films prepared by PECVD and sol-gel methods[END_REF]. A halide precursor, TiCl4, in conjunction with ethyl acetate as the oxygen source can also be employed in the CVD of photocatalytic TiO2 thin films [106] , [START_REF] O'neill | Atmospheric pressure chemical vapour deposition of titanium dioxide coatings on glass[END_REF]. The films were photocatalytically active in the degradation of stearic acid by UV light and also become superhydrophilic after irradiation.

It was reported that when the CVD deposition temperature is increased for TiO2 thin films with about 1-2 µm of thickness, the layer presents a (112) preferential orientation and it corresponds to an optimum photocatalytic Benzene decomposition rate. As the deposition temperature increases, columnar structure is formed in vertical direction from the interface between the film and substrate that leads to a higher degree of the (112)-preferred orientation [START_REF] Byun | Photocatalytic TiO2 deposition by chemical vapor deposition[END_REF]. This agrees well with Weinberger and Garber [START_REF] Byun | Photocatalytic TiO2 deposition by chemical vapor deposition[END_REF] who said that the columnar structure of TiO2 films grown with

Chapter I: State of the art preferred orientation revealed more surface area for photocatalytic reaction (columns with preferred orientation are separated by small voids).

Krumdieck et al [START_REF] Krumdieck | Chapter I: State of the art Polson[END_REF] reported that the photocatalytic activity of the coatings is greatly dependent on deposition temperature. The films deposited by CVD at 375°C had very low photocatalytic activity and the films deposited at 400°C and 425°C were slightly more active but the film deposited at 450°C presented the highest photocatalytic activity [START_REF] Krumdieck | Chapter I: State of the art Polson[END_REF]. The high temperature of rod sample presenting very small diameter expands through the film thickness, and leads to high growth rate in the preferred growth direction. Consequently, the surface-to-volume ratio of the samples increases, which according to Manna et al. [START_REF] Manna | Epitaxial Growth and Photochemical Annealing of Graded CdS/ZnS Shells on Colloidal CdSe Nanorods[END_REF] ensures higher interfacial charge carrier transfer rate and increases the delocalization of carriers in the samples, hence reduces the electronhole recombination probability. This is consistent with the increasing of methylene blue dye reduction rate for the films deposited at 450 °C [START_REF] Krumdieck | Chapter I: State of the art Polson[END_REF].

Maury et al. [START_REF] Duminica | Growth of TiO2 thin films by AP-MOCVD on stainless steel substrates for photocatalytic applications[END_REF] reported that decomposition rate of Orange G by TiO2 films increases strongly with TiO2 thickness until approximately 300 nm for coatings deposited by AP-MOCVD on stainless steel. It was reported that when the film is dense and too thick, most of the electron-hole pairs are generated too deeply in the bulk of TiO2 and they never reach the surface. On the other hand, if the film is too thin, only a small part of the incident light is absorbed by the TiO2 film and the photocatalysis is low. Maury et al showed that the critical thickness necessary to obtain superhydrophilic films seems to be the same one found for the maximum photocatalytic activity [START_REF] Duminica | Growth of TiO2 thin films by AP-MOCVD on stainless steel substrates for photocatalytic applications[END_REF].

They have also reported that TiO2 film deposited at 400°C containing only anatase phase with (1 0 1) preferred orientation presents a higher decomposition rate of Orange G solution compared to films deposited at higher temperatures containing rutile phase.

Some researchers have compared the photoconductivity between anatase and rutile phases, and they have shown that charge carrier, electrons and positive holes lifetime are higher for anatase.

The rate of recombination is significantly higher for rutile. This recombination slows the photodegradation of the pollutants because it limits the formation of oxidizing species necessary for the mineralization of organic matter on the surface [START_REF] Schindler | Charge-carrier dynamics in titania powders[END_REF]116].

Neatu et al reported [START_REF] Neaţu | Solar light photocatalytic CO2 reduction: General considerations and selected bench-mark photocatalysts[END_REF] several important features, which make photocatalysts more efficient, as represented in Table I. 3.

I.7. Biofouling and corrosion in marine structures

Biofouling can be clearly defined as the growth of very small organisms (microorganisms) and macroorganisms such as bacteria, fungi, and algae on surfaces. If these organisms provoke an accumulation, it will induce the formation of destructive biofilms that cause problems in a wide range of industrial processes [START_REF] Al-Ahmad | Biofuoling in RO membrane systems Part 1: Fundamentals and control[END_REF]. Different methods are used such as membrane filtration systems, cooling towers in the energy sector, and ship hulls in the marine environment. It menaces food safety, food security, health, safe drinking water, and electricity production.

A lot of research has been made in order to investigate the mechanisms of bacterial attachment to surfaces and biofilm formation leading to biofouling [119].

Biofouling consists in two main aspects: microfouling and macrofouling. Microfouling means the formation of biofilm and adhesion to the surface, and macrofouling is described as the adhesion of organisms like barnacles, diatoms and sea weed to create a fouling community. The growing bacteria and the chemicals produce microfouling, also mentioned as 'slime' that expands on objects immersed in water. After few days, macrofouling expands as unicellular eukaryotes, such as protozoa and diatoms that colonize the surface. Multicellular eukaryotes start colonizing the surface during many weeks and include colonization of meroplankton larvae and algal spores.

Marine environment is a hard milieu that causes corrosion and biofouling for marine structures.

Biofouling creates large operational losses for the shipping industry. A great extent of fouling on the ship's hull notably increases the requested drag force, contributing to the drop in the whole

Chapter I: State of the art hydrodynamic performance of the vessel and increasing the fuel consumption. For these reasons, ships' owners are concerned with using high-performance coatings that inhibit corrosion and fouling development on the ships' hull. Rigorous environmental legislations and the needs to reduce fuel consumption have led to the development of new antifouling technologies. It has been demonstrated that the increase of roughness made by a heavily fouled ship hull may lead to get a penalty of up to 86 % at cruising speed and rather light fouling by diatom slimes can produce 10 -16 % of penalty [START_REF] Schultz | Effects of coating roughness and biofouling on ship resistance and powering[END_REF].

Marine coating producers are in general careful in adopting new technologies. Nevertheless, more and more strict environmental legislation, accompanied by customer inclination for more eco-friendly solutions, is pushing innovation in the market.

Diverse methods have been employed to minimize and control the accumulation of biofilms on surfaces. The employ of chemical products has been dominating for a long time, although their use is under serious inquiry as a result of injurious effects they have on the environment. Tributyltin oxide (TBT) for example was used as an additive to coatings for more than 40 years and it was very efficient. The use of this chemical agent has been prohibited in almost all countries because of its detrimental effects on the environment causing the toxicity of organisms and marine fauna leading consequently to carcinogenicity in human beings [START_REF] Champ | Economic and environmental impacts on ports and harbors from the convention to ban harmful marine anti-fouling systems[END_REF]. Physical methods such as manual cleaning are not appropriate for all surfaces and these limit their applicability.

Research over the last two decades was mostly concentrated on surface modification in order to develop self-cleaning surfaces.

Fouling can be divided in four main forms such as scaling, silting, biofouling and organic fouling [START_REF] Herzberg | Biofouling of reverse osmosis membranes: Role of biofilmenhanced osmotic pressure[END_REF][START_REF] Kruithof | Integrated multiobjective membrane systems for surface water treatment: pretreatment of reverse osmosis by conventional treatment and ultrafiltration[END_REF], as shown in Figure I-12. The various types of fouling usually happen at the same time and may interact with each other [START_REF] Persson | Characterisation of the behaviour of particles in biofilters for pre-treatment of drinking water[END_REF]. For all these types, controlling of biofouling is complicated, which is due to the antimicrobial resistance.

I.7.1. Biofouling process

Wet environments provide best conditions for the increase of microorganisms. In the natural mediums, the majority of bacterial mass exists in biofilms attached onto surfaces. This can be explained by the fact that nutrients are adsorbed on surfaces. As a biofilm, bacteria are less likely to be swept off to aggressive circumstance and are less susceptible to antibiotics [START_REF] Norde | Surface-tethered polymers to influence protein adsorption and microbial adhesion[END_REF]. A lot of environmental parameters, for example, temperature, pH, iron availability and oxygen tension can stimulate the development of microorganisms from planktonic cells (individual microorganisms) to biofilm mode [START_REF] Stanley | Environmental signals and regulatory pathways that influence Chapter I: State of the art biofilm formation[END_REF].

The consecutive steps of biofilm development seem to be preserved among a large extent of bacteria [START_REF] Toole | Biofilm formation as microbial development[END_REF]. As reported by Flemming and Schaule [129], the process of biofouling can be separated in various stages. The processes from first surface settlement to a full biofilm maturation are illustrated in Figure I-13 [START_REF] Tiranuntakul | Evaluation of fouling in a pilot scale membrane bioreactor[END_REF]. All industrial systems except operating under sterile conditions, are vulnerable to biofilm formation [START_REF] Bishop | The role of biofilms in water reclamation and reuse[END_REF]. This generates hygienic and functional issues to several kinds of devices and equipment, resulting in important financial losses. Metals and their alloys are quite susceptible to microbial colonization and this induces biofouling and finally material failure.

I.7.2. Current research in antibiofouling fields

Nowadays, the dominant tendency to resolve the biofouling is to do modifications in the surface characteristics. Developing of self-cleaning surfaces presenting antimicrobial function is the main objective to remove the attached microbial compounds. When the biofilms colonize a surface, they change the surface properties and establish adhesion properties. Consequently, the modification of surfaces to avoid the first steps of biofilm attachment is indispensable for biofouling controlling.

Many researchers develop advanced materials with hydrophilic surfaces as a good method to control fouling [START_REF] Yokwana | Development of functionalized doped carbon nanotube/polysulfone nanofiltration membranes for fouling control[END_REF]. It is generally considered that hydrophilic surfaces present lower fouling potential compared to their hydrophobic counterparts [START_REF] Zhang | Effects of hydrophilicity/hydrophobicity of membrane on membrane fouling in a submerged membrane bioreactor[END_REF]. It is supported by the fact that a hydrophobic material may generate other nonpolar or hydrophobic compounds to be adsorbed onto its surface generating fouling [START_REF] Lim | A Short Review of Laboratory and Field Testing of Environmentally Benign Antifouling Coatings[END_REF].

Chapter I: State of the art

The principal challenge in developing an active fouling-resistant coating is that the variety of fouling organisms is enormous, as represented in Figure I-14, and the type of mechanisms applied is diversified. The settlement steps of fouling organisms vary in size from few micrometers to hundreds of micrometers or millimeters [START_REF] Callow | Trends in the development of environmentally friendly foulingresistant marine coatings[END_REF]. The surface size and topography are important in order to avoid the growth of fouling organisms. When an organism settles its needs to colonize a surface and grows into adult, its growth in a violent marine environment is influenced by the capacity of the macromolecular adhesive polymers secreted by the settled organisms. This is related to the interfacial molecular interactions that are in turn depend on the properties of the surface or coating at the molecular or nanoscale level [START_REF] Callow | Trends in the development of environmentally friendly foulingresistant marine coatings[END_REF].

The vast majority of commercial fouling resistant coatings are made with poly (dimethylsiloxane) elastomers (PDMSe). These polymers are hydrophobic, non-polar and present low surface energy (~22 mN m -1 ) and for this reason they are presumed to have low adhesion of polar molecules because of limited opportunities for H-binding and polar interactions [START_REF] Finnie | Paint and Coatings Technology for the Control of Marine Fouling[END_REF].

Despite the fact that PDMSe show the requested association of low surface energy (to reduce the work of adhesion) and low modulus, these types of coatings present some disadvantages. They are complex to bond to a substrate without an adequate tie coat as they present low surface energy.

At the same time, they are less durable, more easily broken than other sorts of coatings and they are not capable to decompose slimes governed by diatoms that bind strongly to hydrophobic surfaces [START_REF] Molino | Development of the initial diatom microfouling layer on antifouling and fouling-release surfaces in temperate and tropical Australia[END_REF].

The antifouling technology is considered potent when employed in high-activity for fastmoving ships (> 15 knot) and is less appropriate for ships that spend long time in a port or that use lower speeds to keep fuel efficiency. For these interests, there are extensive studies to develop antifouling technologies [START_REF] Finnie | Paint and Coatings Technology for the Control of Marine Fouling[END_REF].

Field immersion test in natural seawater is required to examine the antifouling property as revealed in laboratory circumstances because there are several environmental conditions that cannot be reproduced in the laboratory [START_REF] Henrikson | A new antifouling assay method: results from field experiments using extracts of four marine organisms[END_REF]. The majority of static field test, not directly affected by high current velocities, and conducted in coastal or port waters is touched by turbulence generated by wind and boat wakes that can influence larval growth [START_REF] Koehl | Mini review: Hydrodynamics of larval settlement into fouling communities[END_REF].

Short-term field tests are commonly employed in field exposures having a duration of not more than one month. This test may be enough to expose the coating to a great number of fouling organisms in the field and investigate the physical integrity of the coating over short period under real conditions. Short-term field testing is mostly employed to collect initial fouling settlements.

The best coating performer can be elected after screening quickly the large-scale arrays [START_REF] Lim | A Short Review of Laboratory and Field Testing of Environmentally Benign Antifouling Coatings[END_REF].

Following the first evaluation of activity, the next step would implicate a scale-up and implementation of inspiring coatings onto huge panels composed of thick multi-layered coatings that agree with the ASTM requirements [140]. Promising coatings can be put in normal 10 by 12 inches panels using ASTM [140] rules for coating field characterization. Smaller panels 4 by 8 inches can be employed if dimensions and cost are limited. The effective time of the field immersion tests can differ from one test to another due to the variation in composition and density of fouling communities according to seasonality. Whatever the experiment, it is crucial to compare the results with a controlled coating.

Priyankha et al. [START_REF] Sathe | Bioinspired nanocoatings for biofouling prevention by photocatalytic redox reactions[END_REF] reported that ZnO nanorods (nanocoating) synthesized by sol-gel process prevents biofouling by producing reactive oxygen species under visible light irradiation. Other works have also reported that photocatalytic materials like TiO2 and ZnO are able to degrade an extensive variety of contaminants [START_REF] Sathe | Self-decontaminating photocatalytic zinc oxide nanorod coatings for prevention of marine microfouling: a mesocosm study[END_REF].

It is reported that under irradiation, ZnO generates reactive oxygen species (ROS) which cause microbial cell membrane damage and increase oxidative stress ultimately leading to cell death as represented in Figure I-15 [START_REF] Sathe | Bioinspired nanocoatings for biofouling prevention by photocatalytic redox reactions[END_REF].

I.8. An additional advantage of TiO2 on steel substrates: photocathodic protection

TiO2 films coated on metallic substrates like steel can perform a source of electrons under UV light and can provide a cathodic protection [START_REF] Bu | A review on photoelectrochemical cathodic protection semiconductor thin films for metals[END_REF].

Yuan et al. [START_REF] Yuan | Characterization of Sol-Gel-Derived TiO2 Coatings and Their Photoeffects on Copper Substrates[END_REF] reported that the TiO2 coatings can be applied to cathodic protection of metal upon illumination. It implies that a semiconductor materials as TiO2 act as a photoanode, which is known as the photocathodic protection. During this process, the photoelectrons created in the conduction band can be moved to the metal to be protected under UV illumination. Consequently, the potential of the metal can be shifted in the negative direction [START_REF] Park | A novel photoelectrochemical method of metal corrosion prevention using a TiO2 solar panel[END_REF]. If the potential is more negative than the potential of the metal to be oxidized, the metal can be protected against corrosion.

Ohko et al. [START_REF] Ohko | Photoelectrochemical anticorrosion and selfcleaning effects of a TiO2 coating for type 304 stainless steel[END_REF] showed that TiO2 film coated stainless steel produces photoelectrons which are transferred directly to the metal substrate to provide a cathodic protection of the stainless steel substrate under UV irradiation as represented in Bamoulid et al. [START_REF] Bamoulid | An efficient protection of stainless steel against corrosion: Combination of a conversion layer and titanium dioxide deposit[END_REF] studied the performance of TiO2 films for improving the anticorrosion properties of a metallic substrate. They combined the use of a conversion coating and a sol-gel TiO2 top-coat to protect a ferritic stainless steel against corrosion. The TiO2-coated samples resisted the salt spray test within 500 h whereas the bare samples and the samples coated with a single conversion coating presented clear signs of corrosion.

Gluszek et al. [START_REF] Głuszek | Structural and electrochemical examinations of PACVD TiO2 films in Ringer solution[END_REF] employed plasma assisted chemical vapor deposition (PACVD) to grow TiO2 coatings on the surface of surgical grade 316L samples. The corrosion resistance of the coated samples was evaluated through polarization curves and EIS (Electrochemical Impedance Spectroscopy) measurements in Ringer's solution. The presence of the TiO2 layer eliminated pitting corrosion, expanding the passivity region of the 316L samples up to 3 V. The bare alloy presented a breakdown potential at approximately 0.2 V. The corrosion potential was shifted to nobler values and the corrosion current density decreased two orders of magnitude.

Krishna et al. [START_REF] Krishna | Thermally oxidised rutile-TiO2 coating on stainless steel for tribological properties and corrosion resistance enhancement[END_REF] prepared TiO2 coatings on stainless steel, and they found that the corrosion resistance of stainless steel was improved with TiO2 coatings.

Shen et al. [START_REF] Shen | Corrosion protection of 316 L stainless steel by a TiO2 nanoparticle coating prepared by sol-gel method[END_REF] reported the corrosion protection of 316 L stainless steel by uniform TiO2 nanoparticulate coatings prepared by sol-gel method. They found that the TiO2 coatings exhibited very good corrosion resistance by acting as a protective barrier on the steel surface.

Antunes et al. [START_REF] Antunes | Effect of the Deposition Temperature on the Corrosion Stability of TiO2 Films Prepared by Metal Organic Chemical Vapor Deposition[END_REF] investigated the effect of the deposition temperature on the corrosion performance of the TiO2 films deposited by MOCVD on Silicon wafers. The TiO2 films were stable during the polarization tests, independently of the deposition temperature. The current density decreased sharply at the beginning of the test, indicating the passive nature of the TiO2 films.

I.9. Objective of PhD thesis

The rising demands for eco-friendly materials presenting antibiofouling and anticorrosion properties for marine applications present a challenge to develop advanced coatings. In this context, the main objective of this thesis is to employ high quality DC magnetron sputtering and chemical vapor deposition techniques to deposit Al-Zr/TiO2 bilayer coatings combining anticorrosion and antibiofouling properties. Al-Zr is known for protection of metallic structures against corrosion and TiO2 films improve their photocatalytic activity under UV illumination. As well, biofouling field tests in natural seawater, at a depth of 50 cm (Atlantic Ocean, Kernevel Harbour, France)

were carried out in order to evaluate the films performance in the real environment. Given the promising characteristics of Al-Zr/TiO2 films, this work will focus on the elaboration and characterizations of TiO2 and Al-Zr bilayer films. In PVD processes, the coating is deposited by condensation from a flux of neutral or ionized atoms of metals. The different PVD processes can be distinguished via the type of the transfer of metallic species and the plasma conditions employed during the deposition process [1].

Magnetron sputtering is a PVD technology commonly used because of its high deposition rate and good film performance (reproducibility, homogeneity, etc.). This technique is compatible with the synthesis of films on large surfaces. Sputtering method uses argon ions for bombarding a cathodically connected target made of the coating material. Sputtered target species are condensed on the cold substrate surface leading to layer growth.

In order to develop new Al-Zr coatings as sacrificial anodes we first studied the effect of the zirconium content on the properties of Al-Zr alloys deposited by DC magnetron sputtering.

The first part of this chapter will be devoted to the description of the production process as well as the deposition conditions for the coating synthesis. The microstructural and physical properties will be discussed in the second part.

II.1. Direct Current Magnetron Sputtering

II.1.1. Principle of magnetron sputtering

Nowadays, sputtering technique is used at industrial scale for many applications. An abnormal glow discharge is produced by applying a voltage in the order of 1 to 3 kV between the cathode (the target) and the anode (the walls of the grounded reactor) in a rarefied atmosphere of argon at a pressure reduced generally between 0.1 and 10 Pa. As a result, partially ionized plasma composed of electrons, ions and neutral species is formed between the two electrodes. Positive ions are accelerated towards the negatively polarized target under the effect of a local electric field and these ions bombard the target's surface. Several mechanisms can be produced on the surface of the target:

-Ejection of an atom from the target by momentum transfer.

-Implantation of the incident ion.

-Reflection of the incident ion neutralized by charge transfer.

-Emission of secondary electrons which ensure the self-maintenance of the discharge.

The ejected atom acquires enough energy to cross the plasma and finally condenses on the first surface encountered. The growth of a thin layer takes place in many stages:

-Adsorption of atoms on the substrate surface.

-Interaction between deposited species and / or those of the substrate forming a stable bond.

-Nucleation.

-Volume growth.

-Diffusion of atoms in volume in the layer and / or in the substrate.

Close to the surface, the incident atom is subjected to a potential energy formed by the atoms of the surface. The interactions involved are Van der Waals interactions. This atom diffuses on the substrate surface during a time that depends on the interactions between the adatoms, the substrate, the temperature and the deposition rate. During its diffusion, it chemically binds with the substrate on a favorable site (Chemisorption). This type of site is called germination site. When the adatom arrives on one of the sites already occupied, it binds to the germ in the course of formation.

Nucleation of germs is preferably carried out at the level of the defects present on the substrate surface.

The growth of films can be divided into different stages of germination and growth. These processes are determined by thermodynamic parameters as well as kinetic parameters. The actual growth mode is determined mostly by the conditions such as deposition rate, temperature and pressure.

A thermodynamic criterion developed by Bauer [2] for the growth mode in equilibrium conditions is given by the following equation: If Δσ ≤ 0, the atoms are bound more strongly to the substrate than the ones to the others and the films tend to extend over the substrate to minimize total energy. In this condition, a full recovery of the substrate by the films is favorable and the growth mode of Frank van der Merwe (layer-bylayer type) is observed.

If Δσ ≥ 0, the atoms are bonded to each other more strongly than they are bound to substrate. In this case, the Volmer-Weber growth mode (of the island type) is obtained. There is a third possibility: Δσ can change from a negative value to a positive one depending on the thickness of the film. This comes from the strain energy, proportional to the thickness of the film, which can bring additional energy to Δσ. In in this case, the growth mode of Stranski-Krastanov (mixed type) is obtained. In most practical cases, the growth of the films takes place in conditions out of thermodynamic equilibrium, and is strongly influenced by kinetic processes. Therefore, the final macroscopic state of the film is not necessarily the most stable thermodynamically but the most favorable from the kinetics point of view. Morphology of thin layers strongly depends on the deposition conditions and is the subject of a so-called structure zone model (the SZMs). The first model of structure zone for metallic thin films was proposed by Thornton in 1977 [3]. In this model, the most important deposition parameters are the working pressure and the homologous temperature (the ratio of the substrate temperature and the melting temperature of the deposited material was used: Ts / Tm).

The SZM's concept was extended to a more generalized situation by Anders in 2010 [3] as Zone 1 (T / Tf < 0.25): this zone is composed of grains in the form of porous fibers separated by gaps. This porous morphology is due to the shading effect, which occurs during growth, and at a very limited surface diffusion.

presented
Zone T (transition): the film growth takes place in the form of assembled fibers in a dense morphology, and the shading effects are minimized compared to zone 1. Consequently, the surface appearance is relatively smooth.

Zone 2 (0.25 < T / Tf < 0.5): the width of the columns is fixed on the thickness of the layer and the columns are organized in a dense manner separated by well-defined grain boundaries. Shading effects are very little present in this area where surface diffusion is important.

Zone 3 (T / Tf > 0.5): The volume diffusion is predominant in this zone. Consequently, the recrystallization produces grains that are no longer columnar but equiaxed.

II.1.2. Description of DC magnetron sputtering reactor

The Dephis 4 reactor (Figure II-3) allows the placement of four targets with a diameter of 200 mm. The targets are placed on the bottom of the chamber. A carbon paper spacer ensures electrical and thermal conduction and provides sufficient mechanical clamping of the target to the cathode.

The rotating substrate holder consists in a stainless steel tray with 600 mm in diameter. The distance between the substrate holder and the targets is adjustable and could be varied between 6 and 45 cm. In our study, we kept this distance constant at 10 cm.

A base vacuum of about 10 -4 Pa is reached through a pumping system consisting of a primary pump and a turbomolecular pump.

The gases are introduced and controlled using brand mass flow meters Tylan. In order to allow stripping and bias of the substrate, the sample holder is connected to a RF power supply. This reactor, equipped with 4 targets, allows deposition of alloys with controlled chemical composition.

The working pressure is controlled by either argon flow or a throttle valve.

II.1.3. Substrates

Al-Zr alloy films were deposited on three types of substrates to be used for performing multi types of characterizations. Glass slides with dimensions 76 × 26 × 1 mm 3 were used for characterizations by XRD and electrochemical measurements. High speed steel disc substrates (Φ 30 mm x 1 mm thick) were polished and used for nanoindentation and electrochemical characterizations. Silicon wafers were also used to perform observations using a scanning electron microscope (SEM).

The substrates were cleaned with ethanol and acetone in an ultrasonic bath and then dried in hot air before ion cleaning and film deposition.

II.2. Al-Zr films deposited by DC magnetron sputtering

Al-Zr alloy films were deposited from pure metallic Al and Zr targets in a pure argon atmosphere at floating temperature (< 80°C). The total mass flow of argon and working pressure were kept constant at 50 sccm and Par = 0.33 Pa, respectively. The deposition system is shown in The substrates were positioned on a rotating substrate-holder (10 rounds per minute) ensuring a good homogeneity in thickness and composition of coatings.

Bias voltage of the substrate holder allowed an ion etching with applying a power of 500W at an argon pressure of 0.3 Pa for 30 minutes before each deposition stage in order to improve the film adhesion.

Evolution of Zr content was ensured by fixing the aluminium discharge current at 2.0 A and by changing that of the Zr one between 0 and 1.3 A. The deposition conditions of Al-Zr alloys are shown in Table II. 1.

Film thickness was measured using fused quartz substrates by a tactile method using an Altysurf 500 profilometer produced by ALTIMET. This device is equipped with an inductive sensor, allowing a precision of the order of 10 nm.

The chemical composition of Al-Zr alloy films deposited on silicon were evaluated by means of EDS (Figure II-4). Thickness also increased with the zirconium content, reaching 4.3 m for a zirconium content greater than 27 at. %. The incorporation of zirconium in aluminium coating increases the deposition rate, which is obviously associated with the increase of the total power dissipated on both targets. In general, most of coatings presents stresses, which depend on the deposition parameters as well as on the differences in properties between the coating and the substrate. The appearance of compressive and tensile stresses is generally observed in films deposited by PVD [5].

Samples I Al (A) I Zr (A) Al (V) Zr (V) Power Al (W)

From a qualitative perspective, thin film total stress can be written as follow:

σtot = σext + σth + σint II. 2
where σext means the external stress due to the external loading, σth the thermally-induced stress and σint the internal stress [6]. In the case of our coatings σext is equal to zero, because no external load was applied and σth is negligible since our substrates were not heated during deposition.

There are several methods that can be exploited for the determination of internal residual stresses. As part of our study we chose Stoney's formula [6]. The following hypotheses were considered in order to apply this method: homogenous substrate and coating, isotropic and linear elastic behavior, substrate much thick compared to the film thickness, infinitesimal deformations and rotations, constant stress and curvature radius on the whole surface of the sample [6].

The Stoney's formula can be expressed according to the following formula:

𝜎 = 𝐸 𝑆 6 ( 1-𝑣 𝑠 ) ℎ𝑠 2 ℎ 𝑓 ( 1 𝑅 - 1 𝑅 0 ) II. 3
Where, Iron wires were used a substrate to determine the residual stresses. Before deposition, a heat treatment at 550°C was carried out on these samples, in a Mo device, in order to start from a zero stress state.

𝐸S

The curvature radius was measured by an optical profilometer Altysurf 500 before and after deposition considering the whole surface of the sample. A tensile stress is achieved when the curvature of the sample is concave and a compressive stress is considered when this curvature radius is convex. The positive sign is used respecting the international convention for tensile stress.

In the opposite case, a compressive film stress is obtained corresponding to a negative value. Generally, the residual compressive stresses are measured when a growing coating is bombarded by ions having energies between ten and hundreds of eV.

It was noticed that residual stresses take place as a result of a decrease in spacing between growing crystallites [7]. It will be described in more detail in the morphology discussion which follows in section II.3.

II.3. Structural characterization and morphology of Al-Zr thin films

The structure and lattice parameters of the Al-Zr films were studied by standard X-ray The intermetallic compounds like Al3Zr, stable in Al-Zr alloys made by conventional techniques are not observed in these Al-Zr films, which can be explained by the non-equilibrium character of magnetron sputtering technology [13].

Yang et al. [14] have developed a thermodynamic model concerning the structure of Al-Zr alloys deposited by co-sputtering. They showed that Al-Zr alloys having a zirconium content between 23.5 and 67.5 at. % present an amorphous phase thermodynamically more stable compared to the crystalline phases. Co-sputter-deposited Al-Zr alloys containing 7 at. % Zr presents a polycrystalline fcc structure and for films containing a Zr content higher than 22 at. %, an amorphous structure is detected.

The structure of Al-Zr (4 at. % Zr) as-deposited film after annealing at 500 °C was studied by X-ray diffraction in order to check its thermal stability at temperature conditions used for depositing TiO2 top coating by aerosol CVD. A low working pressure favors the growth of compact films. In our case the working pressure was about 0.3 Pa, which is considered low for magnetron sputtering technique. In this study, the coatings become more compact as the transition metal content, Zr in our case, increases.

Microstructure, morphology and stress evolutions depend on the same mechanisms. The atomic penning, which is the bombarding effect on the growing film surface by an energetic particle-flux of working gas and sputtered species, favors compactness of films, compressive stress and grain size refinement [15].

Flux and energy of neutralized and reflected working gas atoms are proportional to Mt/ Mg ratio where Mt is the atomic mass of the target material and Mg that of the gas, Argon in our case. The transition metal (Zr) is heavier than Al and consequently, increasing the Zr target power is equivalent to increase the Mt / Mg ratio, which increases the in situ bombardment of the growing film [16]. When Zr discharge current was increased with the objective of adding more Zr, the Mt / Mg ratio raised up and the bombardment of the growing film was enhanced.

II.4. Nanohardness measurements of Al-Zr coatings

The nanohardness of Al-Zr as-deposited films were measured by a TI 980 TriboIndenter using dynamic nanoindentation technique to provide a continuous measurement of elastic-plastic properties related to indentation depth. The nanoidentation was performed at an ambient temperature of 20°C with an applied load of 10 mN. Twenty indentations were performed per measurement.

The main constituents in a nanoindentation test are the test material, the sensors used to apply and measure the mechanical load and indenter displacement, and the indenter tip. The indenter tip is made of diamond having a sharp and symmetric shape such as the three-sided Berkovich pyramid. During the nanoindentation test, force and displacement are registered during the time that the indenter tip is penetrated into the test material's surface with a pre-set load. The response of interest is the load-displacement curve [17].

The Hardness of the films is defined as its resistance to plastic deformation and were deduced using the equation II.4.

𝐻 = 𝑃

𝑚𝑎𝑥 𝐴 II. 4
Where H is hardness, Pmax is the peak load, and A is the projected area at peak load.

Evolution of nanohardness with the Zr content is presented in

II.5. Electrochemical tests

Samples were degreased in ethanol, rinsed in Mili-QTM water and dried before electrochemical tests. The active surface was delimited using an insulating varnish and the area was 2.25 cm2. The electrochemical measurements were performed in an aerated and stirred chlorine solution, 5 wt.% NaCl, using an EG&EG 273 A potentiostat driven by M352 software. The solution temperature was controlled at 25°C using a Julabo F32 cryothermostat and the pH was fixed at 7 by adding a dilute NaOH solution. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a large platinum grid.

The polarisation curves were registered using a potential range of ± 150 mV around the open circuit potential (OCP) with a scan rate of 0.2 mV/s after 1h of immersion in the saline solution.

The corrosion potential Ecorr and the corrosion current density icorr were predicted using Tafel extrapolation [20]. The evolution of OCP was recorded during 48h.

II.5.1. Electrochemical behaviors of Al-Zr alloys deposited on glass

Al-Zr films deposited on glass were prepared in order to analyze their intrinsic electrochemical behavior.

The electrochemical performances fully depend on zirconium content. The polarisation curves present a stable passive region followed by an abrupt rise in current at the pitting potential, which is correlated with the breakdown of the passive film and the beginning of pitting corrosion. It is clear that the passive region is well defined and the corrosion current density depends on the Zr content.

The ennoblement of the alloys is related to the addition of Zr, which is characterized by an increase of the corrosion potential and also by the decrease of the corrosion current density. When

Zr content was increased, an extension of the passive region was observed, which is correlated with an improved pitting resistance. Table II The decrease of the current density seems to be a result of improvement of the quality of passive film formed on Al-Zr alloys. The incorporation of Zr in the passive film reduces films reactivity and, consequently, decreases the corrosion rate.

Yoshioka and al. have shown that Zr has a tendency to be enriched in the passive film as a consequence of its lower electronegativity and higher oxygen affinity compared to Al [21]. Zr also increases the protective efficiency of the passive film since corrosion products are formed on Al alloys [22]. It was identified by XPS analysis, the existence of Zr 4+ species integrated in the Al passive film when they analyzed the passive film formed on Al-Zr alloys in deaerated borate-boric solid solution having chloride ions.

The grain size refinement when Zr content increases also contributes to the corrosion resistance improvement. Song and al. have shown the influence of zirconium grain refinement on the corrosion performance of magnesium-rare earth alloy in saline solution. They have noticed a better corrosion resistance of alloy as a result of the increase of the barrier effect of the grain boundary phase [23].

In the present work, Al-Zr film containing 4 atomic percent of Zr shows lower open circuit potential and corrosion current density compared to others Al-Zr alloys. An ennoblement of corrosion potential is also noticed for this film.

II.5.2. Corrosion performance of Al-Zr films deposited on high speed steel (HSS) substrates

The corrosion performance of Al-Zr films deposited on HSS was studied by potentiodynamic polarisation. A significant reduction of cathodic and anodic current takes place when Zr content increases and, consequently, the corrosion current. For Al-Zr alloys having the lowest content of Zr, the corrosion potentials are more negative in comparison with that of high speed steel, which reveals a sacrificial protection of Al-Zr coatings. For Al-Zr alloys having Zr content higher than 10 at. %, the corrosion potential reaches that of high speed steel, and consequently the sacrificial protection is not preserved. The coating becomes progressively nobler than the speed steel substrate when Zr content increases. As a result, the coating depends on the presence of imperfections that locally favors electrolyte penetration on the steel substrate, representing a risk of confinement of corrosion phenomena situated on these defects [24].

The coating ennoblement is followed by decrease of corrosion current densities. The pitting potential of Al-Zr alloys increases with Zr content. Tsuda and al. [25] obtained similar results when they analyzed the corrosion resistance of the electrodeposited Al-Zr coatings on Cu substrates in deaerated 0.1 mol/L NaCl solutions. They found an increase of pitting potential for coatings having Zr content between 0 and 8 at. %. A stabilization of pitting potential was noticed for Zr content up to 17 at. %.

In order to study the stability of sacrificial behavior in function of time, the OCP potential of Al-Zr alloys was verified for long immersion time. The OCP potential grows fast for all Al-Zr alloys and remains constant for six hours. The abrupt increase of OCP values can be attributed to the fast growth of a passive film on coatings surface.

OCP decreases lightly with immersion time suggesting the dissolution of the passive film.

Numerous OCP fluctuations were noticed for pure aluminium coating and Al-Zr coating having 3 atomic percent of Zr. It is due to the fact that these coatings are more porous and the increase of zirconium content leads to a passivation of coatings by the formation of a dense and more stable passive film.

Pores and defects are generally the pitting initiation sites on coatings because when the immersion increases, the pitting corrosion is situated at these pores and propagates by dissolving the steel. The detrimental defects like pores or pinholes are prejudicial to the corrosion performance since they offer paths to allow the corrosive electrolyte to entry to the steel substrate [26].

II.6. Conclusions

Al-Zr films with Zr contents between 0 and 27 at. % were deposited on both glass slides and high speed steel (HSS) substrates by DC magnetron sputtering. Films were deposited from pure metallic Al and Zr targets in a pure argon atmosphere at floating temperature (< 80°C). Evolution of Zr content was ensured by fixing the aluminium discharge current at 2.0 A and by changing that of the Zr one between 0 and 1.3 A.

Al-Zr films having Zr content below 10 at. % present well-defined diffraction peaks, which correspond to fcc structure, indicating the growth of a supersaturated solid solution of Zr in Al. For Zr contents above 10 at. %, the diffraction peaks becomes broader. The peak broadening is the result of a grain refinement occurred when Zr content increases. Above 10 at.% Zr, Al-Zr films are probably a mixture of the solid solution of Zr in fcc-Al and an amorphous phase.

SEM shows that at low Zr content, the coatings grow with a columnar morphology, while for Al-Zr films with higher Zr contents the films are more compact and the grain refinement is observed.

A low working pressure favors the growth of compact films. In our case the working pressure was about 0.3 Pa, which is considered low for magnetron sputtering technique. Microstructure, morphology and stress evolutions depend on the same mechanisms.

Evolution of nanohardness with the Zr content is due to a combined effect of the diminution of the average grain size (Hall-Petch effect), and also to the solid solution hardening effect.

The corrosion performance of Al-Zr films deposited on glass and HSS substrates was studied by potentiodynamic polarisation. The ennoblement of the alloys is related to the addition of Zr. For Al-Zr alloys having Zr content higher than 10 at. % deposited on HSS, the corrosion potential reaches that of high speed steel, and consequently the sacrificial protection is not preserved. The coating becomes progressively nobler than the speed steel substrate when Zr content increases.

Besides, Al-Zr film deposited on glass containing a zirconium content of 4 at. % shows an optimal corrosion resistance after 1 hour of immersion in saline water, as it presents lower current density and a higher corrosion potential. As Al-Zr with 4 at.% Zr led to the best compromise between intrinsic corrosion resistance, sacrificial character/ steels, and mechanical reinforcement, it is chosen as the first layer in order to provide anti-corrosion properties. The chemical vapor deposition technique (Chemical Vapor Deposition or CVD) allows the deposition of a material on a substrate through precursors gases that chemically react under the influence of temperature. The deposition conditions are strongly dependent on the precursor nature and the reactor setup, for instance, the pressure can vary from atmospheric pressure to high vacuum (10 -7 Pa). In particular, CVD systems working at atmospheric pressure (without vacuum system) are very attractive for industrial applications.

As previously mentioned, CVD is a deposition method based on a chemical reaction that allows to grow films if the reaction takes place in heterogeneous phase vapor-solid at the interface between the heated substrate and the reactive gases. This method can be used to grow different types of materials: metals, nitrides, sulphides, carbides and oxides [1].

In this work we have used an aerosol-assisted metalorganic chemical vapor deposition (AAMOCVD) to synthetize TiO2 films as photocatalysts top layer. The first part of this chapter will be dedicated to the study of the influence of the deposition parameters on the growth process of TiO2 films on silicon. The microstructural, physical properties and photocatalytic activity will be discussed in the second part. The objective is to define the deposition parameters to grow TiO2 films in anatase phase, corresponding to the optimal photocatalyst conditions.

Deposition of TiO2 films on high speed steel (HSS) substrate is presented at the end of the chapter for comparison with TiO2 films deposited on silicon.

TiO2 films were deposited by AAMOCVD technique working at atmospheric pressure in order to develop optimal films presenting a good photocatalytic activity before depositing it on Al-Zr alloy to produce bilayer films, which will be discussed in greater detail in chapter IV.

III.1. Deposition of TiO2 films by aerosol assisted metalorganic chemical vapor deposition (AAMOCVD)

III.1.1.

Principle and reactions of CVD

Metal Organic Chemical Vapor Deposition: One variant of CVD is using Metal organic precursors for the CVD reaction, then the process is named Metal Organic Chemical Vapor Deposition (MOCVD). These precursors are composed of one or more metal atoms connected to organic functional groups by either directly metal-carbon (called organometallics) or through a nitrogen-oxygen coordination.

Implementation: Generally, a CVD reactor is divided in 2 main zones: a pre-treatment precursor zone to generate the precursor vapors (evaporation of liquid precursors, sublimation of solid precursors) and a reaction zone at the substrate. A carrier gas transports the precursor vapors to the reaction zone, while a reactive gas is usually delivered directly in the reaction zone. This area can be heated independently of the evaporation zone to activate the chemical reactions or to achieve a determined crystallographic phase.

Method used at LMGP: Aerosol-assisted CVD (AAMOCVD), also known as aerosol pyrolysis or pyrosol process, is based on the transport of precursor in liquid phase by generating an aerosol from the source solution. These small droplets are transported by a carrier gas to the reaction zone. The reaction zone is heated provoking in a very small region the evaporation of solvent and precursors, and the CVD reaction on the substrate.

As in all CVD process, the process depends on the deposition temperature and the nature of the chemical species. The specific parameters that influence the kinetics of the thin layer growth are the temperature gradient around the substrate and the speed and size of the aerosol droplets. -In the regime A, at low temperatures, the droplet remains liquid when arriving on the substrate.

Solvent evaporates and leaves a dry precursor that can decompose in powder.

-In the B mode, at medium temperature, solvent evaporates before the droplet reaches the area.

Precursor arrives on the surface and decomposes there.

-In the regime C, at high temperatures, solvent and precursor vaporize during the course of the droplet to the surface of the substrate. Precursor vapor reacts through a heterogeneous chemical reaction at substrate surface. This process is the CVD itself.

-In the regime D, at higher temperature, the solvent and the precursor vaporize before reaching the surface of the substrate. The chemical reaction takes place in gas or homogeneous phase and results in a non-adherent powdery deposit. We will seek to place ourselves in the regime C to deposit the TiO2 layers. This will allow us to obtain adherent layers on the substrate.

Aerosol production:

The aerosol can be generated by three techniques [2]:

-Electrostatic spraying; this technique involves charging the solution a narrow duct raised to high potential facing a pierced plate connected to the ground. The effect of electrostatic forces leads to the formation of droplet droplets from the solution. This technique makes it possible to obtain droplets of size controlled.

-Pneumatic spraying; this technique involves passing the solution under pressure through a nozzle. The Venturi effect makes it possible to create and transport the aerosol.

-The ultrasonic spraying (used in this work). The surface tension of the solution is symbolized by σs, its density by ρs and the excitation frequency is f. Solution of butanol and ethanol such as used in these works (σs = 24.20 mN•m -1 and ρs = 810kg • m-3 ) for butanol and (σs = 21.82 mN•m -1 and ρs = 790kg • m-3 ) for ethanol with a

Chapter III: Deposition and characterization of TiO2 thin films working frequency of 800 kHz lead to an average diameter of droplets of 3.6 μm if it is assumed that the precursors used have no surfactant effects.

𝜎 𝑆 = 𝑋 4 𝜇𝑠 𝜆 𝑐 𝑓 𝑝 𝑠 III. 2
Spraying occurs from a certain vibration amplitude of the surface of the piezoelectric transducer.

This amplitude αs can be estimated by empirical formula III-2 [5], where X is a coefficient between 3 and 6, and μs is the viscosity of the solution. For a solution of butanol this amplitude is between Solvent must have several specifications to be used in AAMOCVD, it must:

-Dissolve easily the precursors, (in our case, we work with a concentration of 0.03mol•l -1 ).

-Evaporate at a temperature below sublimation or evaporation temperature of the precursors.

-Have a low surface tension to be sprayed by ultrasound.

In this work, we used Butanol or Ethanol. To prepare a precursor solution with a concentration of 0.03 mol•l -1 , we add 4.71 g of precursor in 600 ml of alcohol solvent (butanol-01 or ethanol).

The mixture is stirred until complete dissolution using a magnetic bar with speed 300 cycle/min.

The precursor solution becomes clear yellow right after 30 minutes stirring: then the solution is ready for deposition. Once completed, the precursor solution is stable for several months. The substrates used for depositing TiO2 films were (100) Si, corning glass substrates (reference C1737-S111) and polished high speed steel substrates. The substrates were cleaned before deposition through ultrasonication for 10 minutes

III.1.3. Principle and description of AAMOCVD reactor

Aerosol Assisted CVD has the ability to use less volatile metal organic precursors during CVD deposition [2]. Previous works on the TiO2 deposition confirms the ability to obtain crystalline films [6][7][8]. In particular, Biswas et al [9] studied the growth of TiO2 microflowers structures for specific deposition conditions.

We used for this work a homemade AAMOCVD reactor, presented in Figure III-4. On left, a scheme of the reactor and, on right, a picture of the actual system.

The precursor solution is placed in a vessel. The ultrasounds is generated by a piezoelectric membrane, with a certain power and frequency. As a result, a white mist containing tiny droplets of the precursor solution is created at the surface of the solution. The frequency generator has been provided by the company RBI and allows to provide frequencies between 640 kHz and 870 kHz to the piezoelectric transducer. Two knobs regulate the frequency and power on an arbitrary scale between 0 and 10 units. The optimal frequency is settled by observing the surface of the solution while changing the potentiometer. When we approach the resonance frequency, the surface is covered by concentric waves of maximum intensity in the center of the pot. The optimal frequency is achieved at the maximum deformation of the surface. In the standard parameters of these works the potentiometer is set at 2.0 which coincides to a frequency of 800 kHz. The mist is then carried by a gas flow used as the carrier gas (CG#1) towards the reaction chamber. In our case, the carrier gas is an oxidizing gas flow consisting in 3 L min -1 of air (12% of O2). Before entering in the reaction zone, a process gas consisting in 3 L min -1 (CG#2) of air is added to the precursor flow.

This second flow is introduced from the middle part for sake of security, and accelerates the precursor onto the top heating plate, where the reaction takes place. A funnel at the top the substrate allows to spread the precursor aerosol in a bigger surface. The heating plate of 20 x 20 cm 2 allows to heat up to 550°C.

The decomposed precursor wastes and evaporated solvent are ventilated from the bottom, going through a liquid nitrogen cooled solvent trap. A ventilation system exhaust is placed at the bottom right below the hot plate. In our working conditions, an extraction pressure of 3 mm.H2O is used for all the depositions.

III.2. Effect of deposition temperature on the TiO2 films deposited on silicon

The goal of this study was to evaluate the effect of the deposition temperature on TiO2 films grown on silicon substrate as a first step to evaluate the best conditions for the deposition on metallic substrates. We will present here the results obtained by different characterization techniques.

The deposition temperature was varied between 350 °C and 550 °C, the other deposition parameters are represented in Table III. 1. Biswas et al. [9] previously proved that AAMOCVD employing Titanium(IV) oxide bisacetylacetonate as the precursor allows the deposition of TiO2 films crystallized in the anatase phase and containing hierarchical microflowers on the surface by adjusting the deposition conditions.

As this microstructure can be interesting for our application because it presents a very large active surface, optimal deposition parameters are needed to tune the morphology and density of the microflowers. The deposition temperature was determined to be the main parameter providing the development of TiO2 microflowers, maintaining all the other parameters constant, such as a solution feeding rate of 3 ml min -1 , a precursor solution concentration of 0.03 mol L -1 and a deposition time of 40 min. Microflowers characteristics revealed to be definitely sensitive to the temperature. SEM images of the deposited TiO2 films (Figure demonstrate that, at lower temperatures, such as 400°C, no flower is created. At 500 °C, some flowers can be observed, but much smaller and in smaller quantity than those developed at 550°C. At 550 °C, TiO2 microflowers of about 7-8 μm in diameter are clearly detected. Finally, at temperatures higher than 570 °C (not shown) bigger nanoparticles agglomerates are formed instead of flowers.

Biswas et at. [9] also demonstrated that a feeding rate higher than 3 ml min -1 is crucial to the development of microflowers. The creation of TiO2 nanopetals is associated to the metastable conditions that take place at the surface when the aerosol evaporates developing supersaturation circumstances, which perform as a leading force for TiO2 crystallization. During the evaporation of the solvent, the precursor is under supersaturation conditions in the droplet. The excess of the precursor can result in a spontaneous agglomeration process or a reaction in the homogeneous phase by reacting with oxygen and consequently crystallizing in a very thin structure. The petal thickness seems to be independent of the deposition parameters and it is not preferentially oriented; it have changed only when a second deposition on the microflowers is made [10].

Following the evaporation of the solvent, the precursor has a tendency to precipitate in the created petals in place of keeping moving in the gas phase. When a long deposition time is carried out, the precursor provides a growth of microflowers in the density and also the height. However, supersaturation happens far away from the surface, rendering impossible the creation of new nucleation sites.

The influence of deposition temperature on deposition rate of TiO2 films deposited by AACVD is shown in Figure III-6. Since our films are not homogenous in thickness, an average thickness is calculated using cross section SEM images for all the films deposited on silicon presenting a percentage error of +-5 %.

An evolution is observed as the deposition rate increases from 5.5 nm/min for films deposited at 350 °C to about 8 nm/min for films deposited at temperatures higher than 450 °C. It should be noticed that surface reaction phenomena are thermally-activated processes that have an visible dependence on temperature only at low values (the lower the T values, the lower the rate of the process), but at higher temperatures there is enough energy for any surface reaction to be achieved, and the dependence on T decreases.

The deposition rate is often an important processing variable in CVD processing. Not only can the rate affect the film growth; along with the deposition time, it is often used to control the total amount of material deposited.

III.2.2. Fourier transform infrared spectroscopy (FTIR)

Fourier transform infrared spectroscopy (FTIR) is a technique that allows the identification of chemical bounds in the films as a consequence of the absorption of infrared light. In this context, an infrared beam consisting of various wavelengths goes over the film, interacting with the chemical species. In the case of one specific chemical bond presents the same energy of the infrared light, it will be absorbed. The light beam is divided in two distinct beams, using a Michelson Interferometer that will goes through the sample and produce an interferogram (interference pattern) in the detector. The Fourier transform of the generated pattern will produce an infrared spectrum, employed for the identification of chemical bonds.

In the analysis of TiO2 films, this technique is interesting for the detection of Ti-O and Ti-O-Ti bonds. In case of incomplete decomposition of the precursor, residual organic substances can indeed be present in the film, which can lead to meagre physical properties. Fourier-transform infrared (FTIR) measurements were conducted in a Bruker Vertex 70V spectrometer, equipped with a CsI beamsplitter and working under vacuum. Spectra were recorded with a resolution of 4 cm -1 by accumulating 64 scans in transmission mode using silicon substrates, transparent to infrared light.

The FTIR spectrum was used to identify various functional groups present in the TiO2 thin films deposited at a substrate temperatures range from 400 °C to 550 °C as represented in Figure III-7.

The scanning range was 200-4000 cm -1 , but no absorption peak was noticed in the range of 2000-4000 cm -1 . Only small peaks in the region of 1200 to 1800 cm -1 were visible, mainly at 1425, 1600, 1718 cm -1 and with a lower intensity at 1250 cm -1 . These peaks correspond to characteristic absorption bands of acetylacetonate [11,12]. Also C-O-C stretch is detected at 1150 cm -1 .The bands for the Ti-O and Ti-O-Ti bonds are present in the 800-400 cm -1 region. These infrared bands are detected at 440 cm -1 and 265 cm -1 with a small shoulder at 660 cm -1 . These absorptions are related to the stretching vibration of Ti-O-Ti in anatase [9].

III.2.3. X-Ray diffraction (XRD)

The crystallinity and phase identification of the samples were studied by X-ray diffraction (XRD) in θ-2θ scanning mode using a Bragg-Brentano configuration between 10° and 80° at 0.011° intervals with an acquisition time of 2 s using a Bruker D8 Advance diffractometer with monochromatic CuKα1 radiation (λ = 0.15406 nm).

III.2.4. Raman Spectroscopy analysis

Regarding the chemical composition of the films, Raman spectroscopy is an effective tool to cover vibrational, rotational, and diverse low-frequency modes in a material. This type of spectroscopy technique is based on the Raman scattering (inelastic) taking place inside the samples over illumination with a monochromatic light source. Once a high intensity monochromatic light, in this matter a laser, strikes a material, the majority of photons are scattered with the same energy as the incident light. This elastically scattering technique is named Rayleigh scattering.

Nevertheless, a certain number of the scattered photons, around 1 in 10 millions, present a shift of energy compared to the original energy. In the case of a lower or higher energy, this inelastic technique is named Raman scattering. In this technique, it is normal to employ wavenumber, cm -1 , in place of wavelength, as long as we always present the shift from the original light source.

Concerning TiO2 thin films, this technique was important for the detection of the anatase phase.

Raman spectra were collected using a Jobin-Yvon/Horiba LabRam multichannel spectrometer equipped with a liquid N2 cooled CCD detector. Experiments were carried out in the micro-Raman mode at room temperature in a backscattering geometry. The 514.5 nm line of an Ar + laser (2.6 mW) was focused giving a spot size of about 1 μm. The recorded spectra were calibrated using Si spectra at room temperature. The integration times were adjusted in order to obtain a high signalto-noise ratio.

Many works have been reported in the literature about the Raman signature of the different TiO2 phases [13]. Moreover, this technique is more sensitive to anatase, rutile and amorphous phases of TiO2 films than XRD [14]. In addition, this technique is used to perform local measurements, allowing to probe the different microstructure phases of the sample surface. Raman shift (cm -1 ) Intensity (a.u)

E g B 1g A 1g /B 1g E g a b c

III.2.5. X-ray Photoelectron Spectroscopy (XPS)

XPS is a non-destructive surface analysis method that characterize the binding energy between atoms. The analyzed depth is about 10 nm.

A monochromatic beam of X-Ray bombards the sample, causing the ionization of the atoms.

By absorbing an X-ray photon of energy hv, a core electron is ejected: it is called a photoelectron.

To return to its initial stable state, the electronic levels are rearranged: a higher energy electron replace the vacant core electron. To do so, the atom gets rid of an excess of energy via the emission of an X-ray photon (X-ray fluorescence) or of high energy level electron (Auger electron).

The XPS spectra acquired represent the number of photoelectrons collected per unit of time depending on their binding energy. These energies are a characteristic of an element and are computed with the equation III. 3.

E binding = hv -E C III. 3
where hv is the energy of the photon, 𝐸 𝑏𝑖𝑛𝑑𝑖𝑛𝑔 is the binding energy of the electron and Ec the kinetic energy measured by the detector. Integrating the electrons at different binding energy generates a spectrum where the peaks correspond to characteristic orbitals of each element. In the context of this study, XPS was fundamental to detect alterations in titanium oxidation state, in order to identify the presence of Ti +4 in the films.

The analysis was conducted in Vacuum Generator XR3E2 device at SIMAP -Grenoble INP using Al Kα radiation (energy 1486.6.6 eV) as an X-ray source. Before acquisition, the samples are placed during 12 hours in an introduction chamber at the pressure of 10 -8 mbar in order to obtain a state desorption equilibrium, then introduced into the analysis chamber whose pressure is 10 -10 mbar.

XPS spectra were made between 100 and 1100 eV with a resolution of 0.5 eV allowing to determine all the elements present. The peak at binding energy 530.3 eV imply that the oxygen atoms exist as crystal lattice oxygen in Ti-O-Ti. Secondly, the peak at 531.9 eV is attributed to hydroxyl groups chemisorbed on the surface [16].

The Ti2p peaks are constituted of a doublet that arises from spin orbit-splitting. The doublet Ti2p3/2 presents a binding energy of 459.2 eV and Ti2p1/2 presents a binding energy of 464.9 eV.

The literature shows that these peaks are consistent with Ti 4+ in TiO2 lattice, since Ti2p3/2 shows a binding energy of 458.6 eV and Ti2p1/2 shows a binding energy of 464.4 eV [17]. In our case, the contributions Ti2p3/2 and Ti2p1/2 with a splitting energy of 5.7 eV confirms a normal state of Ti 4+ ion in the sample [18]. XPS proves that Ti 3+ is not presented in the film.

III.2.6. UV-VIS-IR transmittance

The optical characterization of the TiO2 layers consisting in measuring the total transmittance thanks to an integrated sphere of a diameter of 150 mm. The measurements from Ultraviolet to near-infrared were carried out in a Lambda 950 spectrophotometer from Perkin Elmer in the range of 250 nm to 2500 nm, displaying a wavelength step of 5 nm. The spectra were employed to calculate the band gap by tracing the Tauc plot using the equation III. 4 [19].

The transmittance of the films was measured on samples deposited on corning glass, and the bandgap of TiO2 films was determined by extrapolating the slope of the curve to the photon energy axis.

(αhv

) n = A(hv -E g ) III. 4
where α is the absorption coefficient, hv is the photon energy (eV), A is the proportionality constant, Eg (band gap energy) while n stands for the type of transition, which may be direct or indirect. In this case, the transition was indirect and as such n = 2.

The total optical transmittance and optical band gap calculated using Tauc plot of (αhv) 2 The band gap energies of TiO2 films are higher than the bulk value (3.2 eV) [20] of anatase.

Moses et al. [21] obtained a band gap of 3.65 eV for TiO2 films deposited at 450 °C by spray pyrolysis using titanyl (bis) acetylacetonate as precursor and it was explained that the band gap is larger than the bulk TiO2 (3.2 eV) due to the presence of nano-sized grains.

It is noticed that the optical band gap of TiO2 films varies from 3.78 eV to 3.55 eV when the deposition temperature increases from 400 °C to 550 °C .This can be explained by an increase of the crystallinity with deposition temperatures, which is supported by the XRD patterns of TiO2 films deposited at different temperatures. The optical absorption of samples demonstrates that absorption is located mainly in UV region as a consequence of its wide band gap. The strong UV absorption of TiO2 sample is due to electronic transition of an electron from valence band to the conduction band.

Fan Zhang et al [22] demonstrated that electron life time τ and band gap Eg decrease at elevated temperature. It is due to the thermal expansion of the lattice and renormalization of the band structure by electron-phonon interaction [23].

III.3. Photocatalytic characterization

Photocatalysis is based on photo-induced oxidation-reduction mechanisms leading to the decomposition (e.g. mineralization) of organic molecules adsorbed on the surface of a semiconductor. The kinetics of photocatalytic decomposition of molecules in aqueous solution generally follow a Langmuir-Hinshelwood mechanism, modified to account for the reactions taking place at the interface between the solid photocatalyst and the liquid, assimilating the liquid to a gas under strong pressure. Under these conditions, the rate of degradation of the reactant (r) is written:

𝑟 = - 𝑑𝐶 𝑑𝑡 = 𝑘 𝑐 . 𝐾 𝑎𝑑𝑠 . 𝐶 1 + 𝐾 𝑎𝑑𝑠 . 𝐶 III. 5
where kc is the kinetic constant of the reaction, Kads is the adsorption constant of the reactant, and It is important to fix two geometrical parameters to ensure the comparison of results from one sample to other. One parameter is the exposed area of the sample has to be the same for all the experiments. In our work the area was of 7.06 cm 2 for films deposited on round high speed steel substrate (d=3 cm) and 9 cm 2 for films deposited on silicon substrate. The second parameter is the distance of the layer to the bottom of the beaker. This distance fixes the volume of the solution in contact with the layer, and the distance to the UV lamps. We use a polymeric holder imposing a distance of 8 mm from the sample to the beaker bottom, which is placed at 20 mm of the UV lamps.

We insert the sample in the holder and then the sample and its support are placed at the bottom of the beaker. The face covered by the layer TiO2 (active face) is directed towards the bottom of the beaker as represented in Figure III-13. The whole is placed in an oven regulated in humidity (RH = 40 %) and temperature (T = 20 °C). In order to homogenize the solution and maintain constant adsorption/desorption conditions during UV exposure, the solution is stirred constantly during irradiation via a magnetic bar and a magnetic stirrer set at 500 rpm.

For our measurements of photocatalytic activity, 50 ml of an aqueous solution of OG (concentration 5.10 -5 M) are poured into a beaker. The beaker is then positioned on a support allowing it to be held an above three UV lamps (Philips PLS 11W). These lamps emit a continuous spectrum in the UV-A range (negligible UV-B and UV-C emission), with a maximum emission at 365 nm, and provide a light intensity at the sample level of approximately 5 mW/cm 2 .

Before irradiation, the system is kept in the dark for one hour in order to obtain a balance of adsorption of the OG on the surface of the photocatalyst. However, no significant variation in the concentration of OG in solution was observed during this preliminary stabilization step, regardless of the nature of the samples analyzed. It can be inferred that, under our conditions, not only the amount of GO adsorbed on the surface of the thin film photocatalyst is extremely weak but also that OG is chemically stable in the absence of UV and photocatalyst. After this step, the whole is irradiated for three hours via the device described above. A small amount of solution (approximately 3 ml) is taken every thirty minutes in order to measure the evolution of the concentration of OG by UV-Vis spectrometry and then reinjected into the solution. where α is the molar absorption coefficient at the measured wavelength and l the light path in the solution (1 cm in our case). In this study, the measurement of A is performed at 480 nm, which corresponds to the maximum absorption of the Orange G dye (and confers its orange color to this compound). Under our conditions, the parameters l and α are constant, which means that A is directly proportional to C. It is important to notice that microflowers structure were mainly present in samples deposited ant 550 °C, creating a higher roughness and a higher surface area for chemical exchange or reaction. For films without microflowers deposited at 400°C the k constant is one order of magnitude lower than films containing microflowers deposited at the same range of temperature.

It can be also explained by the fact that the band gap shift in the absorption band edge for TiO2 film deposited at 550 °C is attributed to the higher rate of electron-hole pairs on the photocatalyst surface, leading to higher photocatalytic activity [25].

The photocatalytic activity of Al-Zr/TiO2 bilayer films deposited at different temperatures with different deposition feeding rates (mL/min) will be presented and further analysed in chapter IV.

III.4. Reproducibility of TiO2 films on high speed steel substrates

Since TiO2 films is used as a top layer on Al-Zr coated High Speed Steel (see chapter IV) it is important to evaluate the reproducibility of TiO2 films presenting hierarchical microflowers on the surface when deposited on high speed steel substrates. Indeed, the presence of microflowers on the surface is important to improve the photocatalytic activity.

Deposition was carried out on different substrates like high speed steel, FTO and ITO-coated substrates. The same microstructure is obtained in all the cases as represented in Figure III-17.

XRD measurements were also performed in order to confirm the same crystalline structure for films deposited on high speed steel. The XRD patterns show that the obtained films are crystalline and anatase single phased. For films deposited on high speed steel at 540 °C and 550 °C, we observed a much higher intensity of the diffraction peaks compared to film deposited at 500 °C. The effect of temperature and presence of microflowers on the surface with regard to crystallite size is discussed in chapter IV.

III.5. Conclusions

AA-MOCVD using titanyl (bis) acetylacetonate as precursor and working at atmospheric pressure allows the direct synthesis of TiO2 microflowers dispersed at the top of a thin film of TiO2.

Optimum growth conditions are acquired by the combining the deposition temperature, precursor concentration and precursor feeding rate. The formation of the microstructure occurs at a specific thermal window that depends on the precursor concentration; more precisely when working with a 0.03 mol L -1 concentration solution and an aerosol feeding rate of 3 ml min -1 , flower-like structures are generated at 550 °C. At this temperature, the maximum flower density is about 40 %, in terms of area fraction coverage. The development of microflowers is definitely sensitive to the temperature. At lower temperatures, such as 400 °C, no flower is created. At 500 °C, some flowers can be observed and at 550 °C, TiO2 microflowers of about 7-8 μm diameter are clearly detected.

XRD and Raman Spectroscopy confirm the flowers and the thin film to be in the anatase phase.

TiO2 samples deposited at 550 °C shows better photocatalytic with a higher k (0.00153 min -1 ) and 30 % of mineralization of Orange G dye after 240 min under UV light when compared to others films deposited at lower deposition temperatures (500 and 400 °C). This chapter confirms the fundamental role of microflowers with an average size of 7 µm and its density on the surface layer on the photocatalysis process.

The optical band gap of TiO2 films decreases from 3.78 eV to 3.55 eV when the deposition temperature increases from 400 °C to 550 °C. It is related to the band gap shift in the absorption band edge for TiO2 film deposited at 550 °C, which is attributed to a higher rate of electron-hole pairs on the surface of the film, leading to higher photocatalytic activity. The corrosion of metals in natural seawater is a consequence of interactions between the metal, living organisms and seawater composition [1]. When a metal is submerged into seawater, corrosion and biofouling take place simultaneously [2]. This is usually pursued by the development of biofilm within some days after submersion [3] and after a continuous exposure, macrobiota like invertebrate larvae may combine with the biofilm and settle on the surface [4]. In order to solve this problem we focus on two different functional properties: photocatalysis, which is related to the surface properties to avoid initial step of attachment, and corrosion, to protect the steel used in the mechanical system.

This chapter presents the physico-chemical and functional properties of a multi-functional bilayer coating combining Al-Zr (4 at.% Zr) alloy deposited by DC Magnetron Sputtering on high speed steel substrates and TiO2 deposited on the top by AAMOCVD. Al-Zr (4 at.% Zr) film was used to provide sacrificial corrosion resistance of steels as presented in chapter II and, TiO2 was added as a top layer to induce photocatalytic activity and hydrophilic behavior which can generate antifouling properties in order to slow down the biofouling process.

The first part of this chapter will be dedicated to the microstructural and physical properties of the bilayer films. The functional properties such as the electrochemical behavior and the photocatalytic properties of bilayer films will be treated in the second part.

IV.1. Deposition conditions

Polished round High Speed Steel (HSS) substrates with 30 mm in diameter and 1 mm thick were systematically covered with a previously optimised composition of Al-Zr coating having 4 atomic percent of zirconium and a thickness of 3 microns, as explained in chapter I. Then, TiO2

films were deposited at different temperatures in order to achieve the optimal deposition conditions allowing the formation of flower-like microstructures as described in chapter III. As presented in chapter III, the titanium precursor (titanyl (bis) acetylacetonate) was dissolved in ethanol with a concentration of 0.03 mol L -1 and the solution consumption was around 3mL min -1 in order to develop microflowers in the TiO2 layer. A solution consumption between 3 and 4mL min -1 allows the formation of hierarchical microflowers. However, less than 3mL min -1

Chapter IV: Characterizations of Al-Zr/TiO2 bilayer thin films microflowers are not developed. The oxidizing gas flow rate was obtained by using 6 L min -1 of air (12% of O2) and the deposition temperature was varied between 500 °C and 550 °C for a deposition time of 40 minutes. The substrates used were Al-Zr coated HSS substrates.

IV.2. Morphology and roughness of HSS/Al-Zr/TiO2films

The morphology of as-deposited Al-Zr films observed by a SEM is shown in Surface roughness profiles of samples were obtained using a Form Talysurf 50 (Taylor Hobson Co Ltd) surface Profilometer and measured 25 mm crossing the centre along the surface of the sample. For these measurements, the pre-race length used was 0.30 mm, the spacing (y) was 0.1 mm, the measuring speed was 1 mm/ s and the number of points was 5000. As described in ASME B46.1, Ra is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Ra is the average of a set of individual measurements of surface peaks and valleys [5].

Figure IV-4 presents the surface roughness profiles for the HSS/Al-Zr film and for the HSS/Al-Zr/ TiO2 bilayer films deposited at 500 °C and 550 °C. Ra values were measured using contact mode, where the measuring instrument was in direct contact with the surface.

Al-Zr monolayer film presents a surface with a Ra value of 87 nm. The roughness increases when TiO2 layer is added and also when the deposition temperature increases. The TiO2 film deposited at 500°C, reproduces the Al-Zr baseline, with sparse peaks corresponding to the presence of microflowers. For TiO2 deposited at 550 °C, the density of microflowers is so high that causes a high roughness with an average Ra= 524 nm. The highest peaks correspond to the height of the biggest microflowers dispersed at the top of the layer (about 7 µm in size). 101) and ( 200) diffraction peaks at 25° and 48° respectively. The XRD pattern of TiO2 thin film is also dominated by anatase phase with a preferential orientation of ( 101), without presence of rutile phase, even for the highest deposition temperature of 550 °C. This is favourable because rutile has a lower charge-transfer rate and higher recombination rate of photogenerated electron-hole pairs than anatase [6].

IV.3.2. Raman Spectroscopy analysis

Figure IV-7 shows the Raman spectra of HSS/Al-Zr/TiO2 bilayer films deposited at different temperatures ranging between 500 and 550°C. The Raman spectra of the films containing microflowers on the surface show characteristic modes of the TiO2 anatase phase [7]: two Eg modes at 141 cm -1 and 635 cm -1 , one A1g mode at 513 cm -1 and one B1g mode at 397.5 cm -1 . The sharp peak at 144 cm -1 clearly identifies the anatase phase of TiO2. Smaller peaks at 398 and 639 cm -1 can also be assigned to the anatase phase. No band corresponding to the rutile phase of TiO2 was observed. Moreover, the absence of a broad band background in the Raman spectra allows to discard the growth of amorphous TiO2. where C0 is the concentration of Orange G after the adsorptiondesorption equilibrium, Kapp is the apparent first-order rate constant (h -1 ).

The constant k was used to compare the photocatalytic activities of the different samples. k describes an apparent kinetic constant of the system as explained before in chapter III.

Al-Zr coating was tested for verification and, as expected, this layer did not present any The decomposition rate (DR) was calculated by multiplying the initial concentration of Orange G (C0 = 5x10 -5 M) by the constant k. These values are also shown in Table IV-3 for the Al-Zr/TiO2 coatings deposited at 500 °C and 550 °C.

Samples with microflowers

Bilayer films DR (10 -10 mol L -1 min -1 ) Crystallite Size (nm)

Al-Zr/TiO2 Tdep=500°C 510 18

Al-Zr/TiO2 Tdep=550°C 780 20

Samples without microflowers

Al-Zr/TiO2 Tdep=500°C 332 44

Al-Zr/TiO2 Tdep=550°C 297 F. Maury et al. [8] reported a maximum DR of Orange G solution (10 ppm) of about 280 X 10 - 10 mol L -1 min -1 for TiO2 films deposited at 400 °C by AAMOCVD containing pure anatase phase.

In our work, the increase in decomposition rate from 510 X 10 -10 mol L -1 min -1 for the TiO2/Al-Zr film deposited at 500 °C to 780 X 10 -10 mol L -1 min -1 for the sample deposited at 550 °C was observed only for films with microflowers. For samples without flowers, the DR was similar or even lower for the film deposited at the highest temperature 550 °C (297-332 X 10 -10 mol L -1 min - 1 ).

The presence of microflowers and small crystallite size contribute to a large specific surface, which enhances the photocatalytic activity and improves the degradation rate of organic pollutants by multiplying the active sites for absorption in TiO2 [9,10]. By increasing the surface area, adsorption of the reactants upon the photocatalyst and absorption of the light will increase. This is the double interest of the microflowers activity [11].

IV.5. Wettability of HSS/Al-Zr/TiO2 bilayer films

The film wettability was evaluated by measuring the contact angle and surface energy with water droplets. These measurements were performed with a goniometer KRÜSS G10 equipped with an image analysis system consisting of a CCD video camera, a card of acquisition connected Hydrophilicity is usually associated with photocatalytic activity [12]. This property can be enhanced, and even restored, by UV light. The self-cleaning property is directly related to this material ferature [12]. For a sample in contact with water, its high photoactivity means that the photogenerated holes attract hydroxyl groups from water and produce hydroxyl radicals (•OH). The higher the number of hydroxyl molecules on the surface, the higher its hydrophilicity is. This is presented as a small contact angle between the water droplet and the TiO2 surface. Figure Water contact angles as a function of TiO2 deposition temperature for HSS/Al-Zr/TiO2 bilayer films before UV illumination, after UV illumination and after six months without UV illumination.

IV.6. TEM Analysis of HSS/Al-Zr/TiO2 bilayer films

The equipment used for TEM imaging was a JEM -ARM 200F Cold FEG TEM/STEM operating at 200 kV and equipped with a spherical aberration (Cs) probe and image correctors (point resolution 0.12 nm in TEM mode and 0.078 nm in STEM mode). Precipitation of this metastable phase has already been observed in conventional alloys after annealing at temperatures below 500 °C [14][15][16] while annealing at higher temperatures can lead to the precipitation of the stable phase [17,18]. These precipitates have either a spherical or rod shape. Image obtained by high resolution STEM-HAADF (Figure shows the coherent interfaces, with adaptation of the parameter between the supersaturated solid solution matrix of Zr in Al, and the Al3Zr precipitates with higher zirconium content. There is a very small difference in the parameter between the f.c.c. aluminum matrix and the f.c.c. L12-type Al3Zr compound; and since the mismatch is slight, the L12-type compounds form coherent precipitates [19].

Scully et al. reported that these particles is an important grain refiner in commercial structural Al base alloys and presents a minimal impact on corrosion properties [20].

IV.7. Electrochemical tests

The electrochemical measurements were carried out in an aerated and stirred chlorine solution The OCP of the Al-Zr coating is quite constant during the immersion at around -0.62 V/Ag-AgCl, which seems to be closed to the pitting potential as mentioned in [23] for similar Al based coatings in saline solution. The OCP of HSS/Al-Zr/TiO2 bilayer coating is also quite constant during the immersion at around -0.69 V/Ag-AgCl, which is slightly more negative compared with the Al-Zr coating. The shift of the OCP towards more negative values was also reported by Dias et al. on TiO2 thin films deposited by atomic layer deposition on Al-Mn alloys [24].

The OCP measurements prove that Al-Zr and HSS/Al-Zr/TiO2 bilayer coatings keep the sacrificial character. It seems that the presence of the TiO2 coatings acts differently when is deposited on steel or aluminium based materials. This could be due to the intrinsic properties of the titanium oxide film [11]. The corrosion potential of the bare steel after one hour of immersion in stirred saline solution was around -0.52 V vs Ag/AgCl. A diffusional plateau is observed in the cathodic domain corresponding to the dioxygen reduction and a uniform corrosion is observed in the anodic domain.

The TiO2 film induces a shift of the corrosion potential towards more positive values as observed by Ramaprakash et al. [22]. The cathodic branch is not affected by the presence of the titanium oxide film, whereas the steel dissolution mechanism is modified. The anodic slope is decreased suggesting that the oxide film affect the charge transfer reactions during the steel dissolution.

Al-Zr coating presents a corrosion potential at around -0.59V vs Ag/AgCl which is slightly more negative compared to the HSS substrate. The anodic polarisation curve presents a sudden rise in current close to the corrosion potential corresponding to the initiation of pitting corrosion as presented by Juan Creus et al. [23]. We can notice that the initiation of the pitting corrosion occurs at a potential value that is close to the corrosion potential.

The polarization curve for the bilayer coated sample (HSS/Al-Zr/TiO2) is quite different from the other configurations. The influence of the TiO2 film on the corrosion behavior is different from HSS and Al-Zr material, probably due to the fact that steel suffers from a uniform corrosion whereas pitting corrosion is the main degradation observed on Al-Zr coating. The oxide film affects the cathodic domain with an increase of the cathodic reaction slope associated to the dioxygen reduction suggesting that the charge transfer reaction is slow down. The TiO2 film permits to reduce the contribution of the cathodic reactions leading to the decrease of the corrosion current density, down to 4.1 X 10 -7 A/cm 2 . As observed for the Al-Zr coating, the pitting corrosion occurs at potential values very close to the corrosion potential, but in the bilayer coating, we can suppose that the pit initiation is mainly localised at the oxide film defects. An important decrease of the anodic reaction kinetics is observed and probably due to the increase of the surface impedance associated to the TiO2 layer. So the pit propagation seems to be reduced in presence of the oxide film.

The optical observations of the corroded surfaces of the coated samples after the potentiodynamic polarization tests are presented in Figure IV-24. It shows that the mean pit diameter decreases when TiO2 is added as a second layer. It suggests that the TiO2 coating impedes the growth of pits. It seems that the TiO2 layer is able to increase the pitting resistance because of the low cathodic reaction kinetics.

IV.8. Conclusions

The potential functionality of HSS/Al-Zr/TiO2 bilayer coating combining anticorrosion and photocatalysis properties was tested using High speed steel (HSS) substrates. The Al-Zr thin films containing a saturated solid solution with 4 at. % Zr were deposited on HSS substrates by DC magnetron sputtering. Then, TiO2 coatings were deposited by Aerosol Assisted CVD in optimised conditions to grow a microstructure characterised by the presence of TiO2 microflowers physically attached to the TiO2 thin film. Optimal growth conditions were obtained by the combination of deposition temperature, precursor concentration and precursor feeding rate. XRD and Raman spectroscopy confirm that TiO2 microflowers and thin film reveal anatase phase. The presence of TiO2 microflowers having a size of about 7 µm and a crystallite size of 20 nm plays a crucial role in photocalysis of Orange G compound. This behaviour was enhanced on TiO2 samples deposited at 550 °C, showing a higher decomposition rate of Orange G (780 X 10 -10 mol L -1 min -1 ) under UV light when compared to other films deposited at lower deposition temperatures (500 and 540 °C).

This study confirms the fundamental role of roughness and superhydrophilicity on the TEM analysis revealed that after the AA-MOCVD process step at 550 °C, Al-Zr (4 at.% Zr) presented precipitates, which are richer in zirconium than the matrix. EDS analyzes have shown that the zirconium content of these precipitates is in the range 15.0 to 17.5 at. %. The structure formed after deposition at 550°C is clearly biphased ; an FCC solid solution of zirconium in aluminum and an ordered metastable Al3Zr compound.

Finally, HSS/Al-Zr/TiO2 bilayer coatings were found to be an interesting alternative to preserve sacrificial character for the protection of steel structures in saline environment, showing a corrosion potential of -0.61 V vs Ag/AgCl. Al-Zr/TiO2 bilayer coatings deposited on steel substrates offer good protection through the preferential oxidation of the bilayer presenting a lower corrosion current density of 4.01 X 10 -7 A/cm 2 . TiO2 as a second layer reduces the electrochemical activity of the films through the formation of a dense passive film, and also increases the pitting resistance when the cathodic reaction kinetics is lower.

Zhang et al. [3] reported that the substitution of Ti 4+ by Cu 2+ /Cu 1+ in the TiO2 lattice induces the creation of oxygen vacancies and additional impurity band that results in a significant reduction of both band gap and recombination rate of photo electron-hole pair. Thus, photocatalytic performance and the ability of visible light absorption of TiO2 have been improved.

Indeed, most single phase materials do not meet all the requirements to be an efficient visiblelight-active photocatalysts, comprising high surface area for adsorption of targeted species, high conductivity, long-charge lifetimes, direct pathways to carry photogenerated charge, and good chemical stability in water. Mixed phase or doped materials, tough, present higher charge separation at the interfaces leading to higher photocatalytic activities and have begun to attract greater attention as visible light-active photocatalysts [4] [5].

For environmental recovery and solar-energy conversion applications, TiO2 and CuxO mixtures reveal great potential, in part as a result of their low toxicity but also in lieu of their suitable energy band positions [6].

In this chapter the influence of incorporation of high contents of copper into TiO2 thin films on structural, optical, photocatalytic activity and antibiofouling properties of TiO2 thin films is proposed as new Cu-Ti-O films. Cu-Ti-O films with different Cu contents were deposited by aerosol-assisted metalorganic chemical vapor deposition using the same parameters used for depositing pure TiO2 films as presented in chapter III.

The properties of the compounds obtained were evaluated by measurements of X-ray diffraction (XRD), Raman spectroscopy, X-ray Photoelectron Spectroscopy, UV-vis spectrometry, Transmission and Scanning Electron Microscopy, and Photocatalytic degradation of Orange G.

Marine biofouling field test were carried out in Lorient's Harbor in France in order to evaluate the antifouling performance of the coatings.

V.1. AAMOCVD parameters and preparation of starting precursor solutions V.1.1. Preparation of starting precursor solutions

The preliminary step in the deposition process is the preparation of the solution. The precursor chosen for copper deposition was Cu(II) acetylacetonate (Cu(AcAc)2) (98%, STREM Chemicals).

This precursor has been already used in LMGP for Cu2O films deposition by AACVD [7]. Starting solutions with a concentration of 0.01 M and 0.03 M were prepared by adding 1.3 g and 3.9 g of precursor respectively, in 500 ml of absolute ethanol followed by a constant stirring. Since Cu(AcAc)2 has low solubility in alcohol solvents, ethylendiamine was added to increase the solubility, at a double concentration in relation with the total molar concentration of precursors [8].

In our case, at least 0.01 M ethylene diamine (99%, Sigma Aldrich) was added as dispersant (2.5 ml of ethylene diamine for 500 ml of 0.01 M solution). The solution became dark blue right after addition the dispersant, as represented in

V.1.2. Deposition conditions

The substrates used for depositing copper-titanium oxide films were (100)-oriented silicon single crystal and alkaline earth boroaluminosilicate glass (reference Corning C1737-S111). In some experiments, Al-Zr on HSS (d= 30 mm) were also used as substrates. Substrates were cut with a diamond pen into squares with size of about 1.5 x 5 cm 2 and 5 x 5 cm 2 and washed in an ethanol solution with ultrasounds for 10 minutes, rinsed with deionized water and dried with N2 flow.

The substrate deposition temperature was fixed at 550 °C with a deposition time of 40 minutes.

In this work, an extraction pressure of 3mm of H2O was used in order to homogenize the air in the deposition chamber. As explained in chapter III, air as carrier gas #2 was introduced with flow rate of 3 L/min and, consequently, when temperature reaches the set point, air as carrier gas #1 was introduced with flow rate 3 L/ min. Once precursor flow achieves the heating place the timer can be set.

The analysis of the microstructure and chemical composition of Cu-Ti-O films deposited by aerosol-assisted CVD using the two starting solutions were performed by scanning electron microscope SEM in a FEI Quanta 250 MEB FEG ESEM tool. Energy dispersive spectroscopy (EDS) analysis was conducted in order to investigate the chemical composition of the sample at the microscopic scale.

Table V. 1 shows the at.% Cu in film measured by EDS for films deposited on silicon, corning glass and Al-Zr coated high speed steel using the two different solutions.

One mean value of Cu content is given in Table V. 1 for each substrate for samples deposited using solution 1 and solution 2.

For Cu-Ti-O films deposited using the lowest Cu concentration (solution 1), the Cu content in film is 16 at.% Cu for films deposited on Si and corning glass substrates. This solution leads to reproducible films from run to run.

For Cu-Ti-O films deposited using the highest Cu concentration (solution 2), the film composition are less reproducible. Three runs were performed from the same solution and films present a Cu content that varies between 50 and 66 at.% Cu for films deposited on silicon, between 50 and 75 at.% Cu for films deposited on corning glass and between 25 and 50 at.% Cu for Al-Zr coated high speed steel (HSS). Since our films are not homogenous in thickness, an average thickness is calculated using cross section SEM images for all the films deposited on silicon presenting a percentage error of +-10 %.

Films containing 55 at.% Cu shows the maximum thickness of 500 nm for a deposition time of 40 minutes. Secondly, pure TiO2 films presented the smallest thickness presenting an average thickness of about 300 nm. This can be explained because the Ti solution has a molar concentration of 0.03 M, lower than mixed solutions.

It is important to emphasise that the molar concentration is not the same in the two mixed solutions since solution 1 has a total of 0.04 M and solution 2 has a total of 0.06 M. The total molar concentration does not seem to have influence on the deposition rate of films.

V.2. Morphological characterization

The No significant influence of the substrate was found on the morphology of these Cu-Ti-O films. In the group of microscopy analysis, another technique used in this work was atomic force microscopy to measure the roughness of the samples. The analysis was performed in Veeco D3100

AFM with 5μm x 5μm images. Cu-Ti-O films deposited on corning glass were also characterized by AFM, which allows to quantify the roughness through the root-mean-square (RMS) of height deviation, visible in Table V. 2. This quantitative technique agrees with the SEM observation indicating higher roughness with the increase of Cu content. The images of the AFM are

represented in Figure V-5.

V.3.1. X-Ray Diffraction (XRD) analysis

The XRD patterns of TiO2 film and Cu-Ti-O films with various at.% Cu were acquired using X-ray diffraction (XRD) in θ-2θ scanning mode using a Bragg-Brentano configuration between 10° and 80° at 0.011° intervals with an acquisition time of 2 s using a Bruker D8 Advance

V.3.2. Raman Spectroscopy analysis

Raman spectroscopy was used to clarify the early stage of phase transformation in these sets of samples. This technique is complementary to XRD analysis, but allows for a more local analysis. Finally, CuO phase are barely detected for Cu-Ti-O films with at.% Cu greater than 50, independently of the substrate. 298 cm -1 (Ag), 346 cm -1 (Bg) and 632 cm-1 (Bg) modes are presented for this films [15,16].

Raman

Films deposited with a mixed solution of Ti and Cu are mainly constituted of TiO2 and Cu2O.

However, as observed in Figure V-8 a) and Figure V-8 b), the intensity of the anatase peaks decreased and broadened for films containing more than 50 % of Cu, which indicates an increment in structural disorder [9].

The incorporation of Cu distorts the lattice structure of TiO2 since the ionic radius of Cu + (0.77 Å) is slightly larger than that of Ti +4 (0.68 Å). Furthermore, because of the charge difference between Cu + and Ti +4 , the addition of Cu + into the TiO2 matrix creates the oxygen vacancies to protect the charge neutrality in the lattice [14] and the higher concentration of Cu + compose more oxygen vacancies in the TiO2 lattice. Since these oxygen vacancies are formed, the lattice distorts and the all Raman peaks shift and broaden, with increasing Cu content.

An extra peak is observed at 755 cm -1 for Cu-Ti-O films with a Cu content greater than 16 at.% Cu for films deposited on silicon substrate and greater than 50 at.% Cu for films deposited on Al-Zr coated high speed steel substrate and corning glass substrates.

To evaluate if this band appears only for thick films, we characterized by Raman spectroscopy a big sample of CuTiO58C film. As can be seen in Figure V-9, the sample presents an inhomogeneous thickness. Nevertheless, the composition was confirmed at each point by EDS.

The spectra obtained at the defined points are represented in the figure, and all of them are similar.

The presence of TiO2, Cu2O and CuO bands are confirmed in all measured regions together with the absorption band at 755 cm -1 . We can infer that this band is not thickness dependent.

One hypothesis is that this extra peak existed with high Cu concentration can be related to A2u

IR active mode of anatase TiO2 [17,18]. In our case, the A2u mode (755 cm -1 ) of anatase is more prominent when the Cu content increases. Thus frequency band corresponds to the longitudinal optical (LO) phonons. There are six Raman active modes (A1g +2B1g +3Eg) and three IR active modes (A2u + 2Eu) for anatase TiO2. The Eg Raman modes originate from oxygen vibrations.

According to M. Grujić-Brojčin et al. [18], the additional modes can be understood in terms of IRforbidden modes [19], since the oscillator frequencies are very similar to the two strongest Eg modes in the Raman spectrum of anatase TiO2. In our case, the addition of Cu + could remove the neutrality of charges in the Eg vibrational modes, causing a slight lattice distortion and appearance of IR-forbidden modes. the anatase phase at 262 cm -1 , 435 cm -1 [20]. When increasing Cu content, the intensity of these bands decreases and a new band at 618 cm -1 appears, which corresponds to the stretching band of Cu2O. For the highest Cu contents a band at 500 cm -1 corresponds to the phonon spectrum of CuO [21].

FTIR results confirm XRD interpretation that Cu-Ti-O films are composed of a mixing of TiO2 and Cu2O, and CuO for higher Cu contents. However, the band detected by Raman spectroscopy at 755 cm -1 and attributed to a FTIR forbidden band could not be confirmed in these spectra.

V.6. X-ray Photoelectron Spectroscopy (XPS)

XPS is chemical sensible method widely used to study the composition and the oxidation state The Ti2p1/2 and Ti2p3/2 spectra were analysed using two components at binding energy of 464.7

eV and 459 eV, respectively, with a separation of 5.7 eV between the two peaks. These energy values were close to those reported in the literature for the TiO2 films at 458.9 and 464.6 eV [26].

When adding Cu to the films the area of the Ti2p components decrease and the FWHM values slightly increases, but not shift in energy is detected. We can so deduce that the oxidation state of Ti doesn't change with Cu incorporation and the Ti is in the Ti 4+ state [27,28].

The O1s spectrum (second column in The 250 nm scale bar is valid for all images.

V.8. Contact angle measurements of TiO2 and copper-doped films

Wettability measurements were performed for films deposited on corning glass substrate using a goniometer KRÜSS G10 equipped with an image analysis system consisting of a CCD video camera, a card of acquisition connected to a PC computer and DROP SHAPE image processing software ANALYSIS (DSA), as explained in chapter IV. As presented in chapter IV, pure TiO2 as deposited films shows a contact angle of 15°, and after 2 hours under UV irradiation these films become superhydrophilic with a contact angle of about 5°. This films remains superhydrophilic after 6 months without UV irradiation. Chapter V: New Cu-Ti-O films for photocatalysis and antibiofouling applications It has been already reported that copper oxides, such as Cu2O and CuO exhibit hydrophobicity [30]. The hydrophobic behavior can be so attributed to the presence of Cu2O in Cu-Ti-O films, in accordance with the XPS and XRD results.

V.9. Photocatalysis properties

The photocatalytic activity of each samples was performed using the same protocol explained in chapter III, measuring the decomposition of Orange G dye under UV visible light.

Pure TiO2 films was used as our internal reference. We remind that the high photocatalytic activity of these films was explained by the formation of microflowers with nanopetals that increase the efficient surface and decrease the crystal size. CuTiO50S film presents a maximum k value of 0.00168 min -1 compared to pure TiO2 film and CuTiO55S film that present a value of k equal to 0.00144 min -1 . Nevertheless, the difference is very small so we can consider that both films shows photocatalytic response comparable to our

Evaluation of antifouling activity following standard test

The antifouling activity of pure TiO2 films and Cu-Ti-O films deposited on corning glass substrates was tested by exposing the coatings in natural seawater and observing the degree of fouling by visual inspection after several months of immersion (ASTM D 3623) [34]. Fay et al. [35] reported that the Kernevel Harbour seems to be extremely favorable to the settlement of biofouling.

All of the species, which are considered as representative of biofouling can be observed. In addition, the degree of fouling, determined by visual inspection, distinguishes microfouling, barnacles, tubeworms, etc. Almeida et al. [36] also reported that the adhesion of the main kinds of macroorganism species can be evaluated by visual inspection.

Confocal laser scanning microscopy (CLSM) and SEM were used as complementary analytical tools in order to study microfouling behavior. The purpose is to link the observations of the marine microfouling with the macrofouling activity of samples. Scanning electron microscopy (SEM) has become an usual method to visualize and identify organisms directly attached to coated surfaces [37]. Nevertheless, SEM can be a laborious method that demands complicated sample preparation protocols, comprising the chemical fixation and dehydration of the sample accompanied by freeze drying with the purpose of conserving the 3D-structure of micro-foulers.

CLSM is a computerized microscope that produces three-dimensional images of microorganisms. This technique, combined with the use of fluorescent probes has proven to be useful for the analyses of the three-dimensional distribution of the total and active cells in solid structures such as biofilm [38].

Three different films were tested: pure TiO2 films, CuTiO16C and CuTiO50C films. Samples with two differ rent sizes were prepared: Square samples of 1.5 x 1.5 cm 2 for microfouling tests and 5 x 5 cm 2 for macrofouling. To take into account the variability of the experience, three samples were prepared for each composition on the large substrate and 12 samples for each compositon on small substrates. These samples were pasted on polycarbonate panels, which were stuck onto a

Chapter V: New Cu-Ti-O films for photocatalysis and antibiofouling applications Samples for SEM observations were prepared using a protocol adapted from Coy et al. [39].

Initially, samples were immersed in 3% glutaraldehyde solution during all the night and then dehydrated by several washings: phosphate buffer (10 min, 3 times); ethanol 70% (10 min, 3 times); ethanol 90% (10 min, 3 times) and absolute ethanol (10 min, 3 times). Therefore, the samples were dried by the carbon dioxide critical point method. Finally, a gold deposit is done in order to make samples conductive [35]. Images were obtained with a magnification of 500×, under an acceleration voltage of 7 keV, from secondary electrons. A representative area of the coating was chosen to be analyzed by microscopic methods.

As a representative example of the overall nature and degree of fouling found on films, SEM of both films after 1-5 weeks of immersion is shown in Figure V-18. All coatings were subjected to the same immersion conditions. Consequently, variations in microfouling communities indicates differences in the coating performances.

1. After 10 days of immersion just a few diatoms were observed for all the films.

2. After 25 days a diverse community of microorganisms, which included bacteria and diatoms, is observed in all the films, especially in CuTiO50C films. This composition presented a greater amount of microorganisms compared to the other coatings.

3. A few diatoms were observed after 38 days of immersion for pure TiO2 films and CuTiO16C films, which is in contrast with films containing higher Cu content. CuTiO50C films were almost totally covered by diatoms and bacteria. It is important to notice that the presence of microflowers on the surface is observed even after 38 days of immersion for pure TiO2 films deposited at 550°C. It confirms that the TiO2 microflowers are resistant to seawater and are also mechanically stable, which is quite important since microflowers are responsible for ensuring a better photocatalytic activity, as showed in chapters III and IV. After 4 weeks of immersion in seawater, the microfouling present on coatings with different compositions was clearly different.

After 10 days of immersion bacteria are presented in all the films. Indeed pure TiO2 films and CuTiO16C films presented a small amount of bacteria on the surface compared to CuTiO50C films.

After 17 days of immersion all the coatings developed a biofilm composed of bacteria, diatoms and micro-algae. Additionally, the presence of micro-algae on the surface are better noticed for CuTiO50C films.

In our case, CuTiO16C showed a clearly better resistance to biofilm and micro-algae development since SEM and CLSM observations presented a smaller growth potential.

V.10.3. Evaluation of macrofouling adhesion by visual inspection and image analysis

The percentage of surface covered by different fouling types was visually assessed at periodical time during 13 weeks after the static immersion. Some particular numbers have to be given:

 After 8 weeks pure TiO2 films presented an average value of 46% against 52% and 60%

for CuTiO16C and CuTiO50C films, respectively.

 After 10 weeks, it is observed a self-cleaning effect in one of CuTiO16C films which show clearly a decrease of biofouling from 54 % to 24 % of surface area covered.

 After 10 weeks CuTiO16C films showed better performance compared to the other compositions with an average value of 50 %. One of pure TiO2 coatings presented a good performance after 10 weeks showing 29 % of surface area covered by biofouling.

 After 10 weeks CuTiO50C presented a higher average percentage of surface covered with an estimated value of 80 % due to the growth of macro and micro-algae on the surface.

Figure V-22 shows that after static immersion for 13 weeks all the coatings tested presented a percentage of surface covered by fouling higher than 70 %, which is dominated by macro-algae.

Actually, juvenile barnacles and a calcareous tubeworms were not settled on coatings.

Comparison of macro and microfouling

In this experiment, the observation of biofilms on antifouling coatings revealed a good correlation between microscopic observation (microfouling observation) and visual inspection following ASTM standard testing (macrofouling observation).

Chapter V: New Cu-Ti-O films for photocatalysis and antibiofouling applications

In particular, CuTiO16C films showed a better resistance to the development of biofilms and micro-algae and also a lower percentage of the area covered by macroorganisms after 13 weeks of immersion. Pure TiO2 films are also coherent in the observation and the macrofouling results indicate a possible influence of photocatalytic activity. Nevertheless, CuTiO50C films present the worst resistance to microfouling and macrofouling activities.

These results revealed that TiO2 films and CuTiO50C films that presented a good performance in photocatalysis under UV radiation do not show the same resistance to the development of biofouling in seawater after 13 weeks. On the other hand, TiO2 films show better behavior up to 8 weeks of immersion, which can be explained by a higher photocatalytic activity in these films since the sample placed at the top of the panel showed better performance. The CuTiO16C films with a lower photocatalytic activity seems to enhance self-cleaning properties after 10 weeks of immersion. These results are encouraging but they are only a preliminary study that needs further investigation. Nevertheless, they allow to propose a not toxic approach to the marine biofouling problem.

V.11. Conclusions

New Cu-Ti-O films with high contents of copper were deposited by AAMOCVD at 550 °C with a deposition time of 40 minutes in order to develop an efficient visible-light-active photocatalyst material, presenting a high surface area for adsorption of targeted species, high conductivity, longcharge lifetimes, direct pathways to carry photogenerated charge, and good chemical stability in water.

Cu(AcAc)2 solutions with a concentration of 0.01 M and 0.03 M were prepared and then mixed with a TiO(AcAc)2 solutions dissolved in ethanol at a concentration of 0.03 M. For Cu-Ti-O films deposited using Cu(AcAc)2 as a start solution at a concentration of 0.03M, the Cu content in film varies between 50 and 66 at.% Cu for films deposited on silicon, between 50 and 75 at.% Cu for films deposited on corning glass and between 25 and 50 at.% Cu for Al-Zr coated high speed steel (HSS). Cu-Ti-O films deposited using Cu(Acac)2 as a start solution at a concentration of 0.01M, the Cu content in film is 16 at.% Cu for films deposited on Si and corning glass substrates.

XRD and Raman spectroscopy revealed that films with at.% Cu greater than 50 present Cu2O, TiO2 anatase and CuO phases on its composition. The UV-Visible study showed that the optical band gap energy decreases with increasing Cu content. CuTiO75C film presented a lower band gap of 2.88 eV.

Cu-Ti-O films remain hydrophobic after 2 hours under UV irradiation, contrary to pure TiO2 films which presents a superhydrophilic behavior.

CuTiO50S and CuTiO55S films present a good performance on photocatalytic activity compared to CuTiO16S and CuTiO66S films. CuTiO50S film presents a maximum k value of 0.00168 min -1 compared to pure TiO2 film and CuTiO55S film that present a value of k equal to 0.00144 min -1 . Nevertheless, the difference is very small so we can consider that both films shows a photocatalytic response comparable to our best TiO2 film.

Marine biofouling field tests reveals that pure TiO2 films show better behavior up to 8 weeks of immersion, which can be explained by a higher photocatalytic activity in these films since the sample placed at the top of the panel showed better performance. CuTiO16C films showed a lower percentage of the area covered by macroorganisms after 13 weeks of immersion. This composition also presented a better resistance to the development of biofilms and micro-algae compared to other Chapter V: New Cu-Ti-O films for photocatalysis and antibiofouling applications CuO phase also appear. Besides, band gap decreased for Cu-Ti-O films starting from a Eg TiO2 value of 3.55 eV, to the lowest value near to 2.88 eV with the higher Cu content (75 at.% Cu).

Cu-Ti-O films showed hydrophobic behavior even after 2 hours under UV irradiation, contrary to superhydropilic TiO2 films. Hydrophobic surfaces are known to show good 'fouling release'

properties, whereas the hydrophilic surfaces show resistance to protein adsorption and cell adhesion.

Photocatalysis activity was evaluated using the decomposition of Orange G dye under UV visible light. The photocatalytic activity of Cu-Ti-O film with 50 at.% of Cu is remarkable high and presents a maximum k value of 0.00168 min -1 compared to 0.00144 min -1 for pure TiO2 film and Cu-Ti-O film with 55 at.% Cu. Cu-Ti-O films a very promising materials for the photocatalytic degradation of organic compounds under visible light. Indeed, photocatalytic oxidation processes using TiO2 based catalysts appear to be a very useful technique for the antifouling properties, leading to the mineralization of the pollutants or to their conversion to more biodegradable compounds.

Marine biofouling field tests were carried out in Lorient's Harbor in order to evaluate the antifouling properties of pure TiO2, Cu-Ti-O (16 at.% Cu) and Cu-Ti-O (50 at.% Cu) films deposited on corning glass. Pure TiO2 films showed better behavior until 8 weeks of immersion, which can be explained by a higher photocatalytic activity while the sample placed at the top of the panel showed better performance. SEM images showed that TiO2 microflowers were present even after 38 days of immersion, which confirms that microflowers are resistant to seawater and mechanically stable. This fact is quite important since TiO2 microflowers are essential for ensuring a better photocatalytic activity.

Biofouling tests revealed that after 13 weeks of immersion in seawater, Cu-Ti-O (16 at.% Cu) films presented a better resistance to the development of biofilms, microalgae and macroalgae on the surface confirmed by SEM and CLSM analyzes, and also the visual observations. New biofouling tests are in progress in order to validate the reproducibility of the anti-fouling behavior of these coatings. Pure TiO2 films with different densities of micro-flowers on the surface are being test in order to check the real role of flowers on anti-fouling properties since this films have shown a good performance on photocatalysis. Moreover, the chemical stability of Cu-Ti-O films and TiO2 films in sea water needs to be evaluated in order to know if these films degrade when they are immersed.

Introduction

Une fois qu'un métal est immergé dans l'eau de mer, les interactions entre le métal, les organismes vivants et l'environnement marin provoquent un bio-encrassement et une corrosion des composants métalliques, presque simultanément. Le développement de micro et macroorganismes à l'origine de la bio-salissure est un problème industriel important qui touche au secteur des transports maritimes, à la purification de l'eau, à l'industrie automobile, aux produits pharmaceutiques, à la microélectronique et aux industries alimentaires. Afin de développer un système antisalissure, il faut prendre en compte l'environnement, le revêtement et le substrat, comme montré sur la Figure 1, proposée par R.Holland et al [1]. Le développement de la salissure a lieu sur presque toutes les surfaces. Ce processus commence par la fixation de molécules organiques. Il se poursuivit ensuite par l'adhérence de bactéries, diatomées et de microalgues afin de créer un biofilm qui est suivi de l'adhésion de macroorganismes, tels que les balanes, les bivalves et les macro-algues, comme indiqué sur la Figure 2. détriment des propriétés sacrificielles [16,17]. La solution idéale en matière de protection des aciers doit être un compromis entre propriétés mécaniques et propriétés anticorrosion.

Le zirconium est un métal de transition connu comme raffineur de gros grains dans les alliages d'aluminium traditionnels [18]. Il renforce les propriétés mécaniques [18]

III Dépôt et caractérisation de couches minces de TiO2

La première partie de ce chapitre est consacrée à l'étude de l'influence des paramètres de dépôt chimique en phase vapeur sur le processus de croissance des films de TiO2 sur silicium. La Ces résultats montrent qu'une forte teneur en cuivre est préjudiciable à l'activité photocatalytique.

Cela peut être expliqué par le taux de recombinaison électron-trou qu'augmente une fois l'augmentation de la concentration en Cu [25,26]. Des échantillons de 1,5 x 1,5 cm et de 5 x 5 cm ont été collés sur des panneaux de polycarbonate.

Les panneaux sont d'abord collés sur un support, puis immergés dans la mer, à une profondeur de 50 cm (océan Atlantique, port de Kernevel, France), comme représenté à la Figure 12. Ecole Doctorale "Sciences pour l'Ingénieur"

Thèse réalisée en partenariat entre :

5 I. 1 . 6 I. 2 . 8 I. 3 . 11 I. 4 . 15 I. 5 . 18 I. 6 . 21 I. 7 . 31 I. 8 .

 516283114155186217318 Chapter I: State of the art Chapter I: State of the art ............................................................................................................. Corrosion of aluminium and aluminium alloys in aqueous environment ...................... Electrochemical basis of corrosion ................................................................................ Al-transition metal coatings used as sacrificial anodes ............................................... Choice of Al-Zr coating for corrosion applications ..................................................... The heterogeneous photocatalysis: principles and generalities ................................... TiO2 structures, electronic properties and photocatalysis ............................................ Biofouling and corrosion in marine structures ............................................................. An additional advantage of TiO2 on steel substrates: photocathodic protection ......... 37 I.9. Objective of PhD thesis ............................................................................................... 39 References ..................................................................................................................................... 40

Figure I- 1 .

 1 Figure I-1. Potential-pH diagram for pure Al at 25˚C in aqueous solution (Reproduced fromPourbaix, 1974 [4]). The lines (a) and (b) are the water stability and its decomposed product.

5 M

 5 sodium chloride as represented in Figure I-2.

Figure I- 2 .

 2 Figure I-2. Mode of corrosion based on experimental data for AA5086 in saline solution (ReproducedfromGimenez, Rameau et al., 1981[5]).

Figure I- 3 .I. 2 . 1 .

 321 Figure I-3. Corrosion process presenting anodic and cathodic components of current.

Figure I- 4 .

 4 Figure I-4. Theoretical anodic polarization scan.[8]

. 3 . 1 .

 31 Influence of transition metal elements including Zr, Cr, Sc, Ti, W and Sr

Figure I- 5

 5 Figure I-5 presents the potentiodynamic polarization curve for pure Al in aerated aqueous solution containing 0.1 M of NaCl. Aluminium displays a passive region in which current remains unchanged by the modification in applied potential. It is noticed as a current density plateau, as shown in Figure I-5. Nevertheless, the current density suddenly increased after achieving a specific

Figure I- 5 .

 5 Figure I-5. Potentiodynamic curve of pure Al in aerated aqueous solution 0.1 M NaCl [30].

Figure I- 6 .

 6 Figure I-6. Open circuit potential classification in saline solution of binary Electron-beam physical vapor deposition of metals and Al based alloys deposited by PVD( Reproduced from J. Creus et al., 2012 [14]). The polarization curves for several binary Al-X alloys are represented in Figure I-7. The shapes of the polarization curves are quite similar in all cases. The presence of a large passivity domain is observed, followed by a brutal increase of anodic current representing the initiation and propagation of pits. The incorporation of these TMs improves the corrosion resistance of material by reinforcing the stability of passive film. Many works in the literature, in particular those of

  .[START_REF] Reffas | Intrinsic structural, mechanical and corrosion properties of sputtered Al-Zr thin films[END_REF] have shown that for Zr content up to 12 at.% in Al, the microstructural characterisation reveals the formation of a fcc supersatured extended solid solution of Zr in Al with a significant decrease of the mean grain size when increasing Zr content. The substitution of Al byChapter I: State of the artZr leads to increase the lattice parameter. This increase is related to a gradual refinement of the grain size. For higher Zr contents, the coatings are expected to be a mixture of fcc solid solution of Zr in Al and an amorphous phase. The electrochemical measurements in 5 wt. % NaCl solution performed on Al-Zr films deposited on glass, show an increase of the intrinsic corrosion resistance for Zr alloys and are correlated with microstructures of as-deposited films. The refinement of grain size increases the corrosion resistance and reduces the electrochemical activity of alloys through the development of a dense passive film containing Zr. When Zr is added, the compactness of alloys increases the Cl -blocking effect and consequently inhibits the pit propagation. The improved pitting corrosion resistance is ascribed to the more compact structure and also to the Zr enrichment in the passive film.

Figure I- 8 .

 8 Figure I-8. Simplified reaction scheme of photocatalysis for TiO2 anatase.

Figure I- 9 .

 9 Figure I-9. Crystal structures of TiO2 rutile, brookite and anatase , from left to right (Reproduced fromEsch et al., 2014 [63]).

Figure I- 10 .

 10 Figure I-10. Illustration of structural dimensionality of materials with expected properties (Reproduced from K. Nakata and A.Fujishima, 2012 [97]).

Figure I- 11 :

 11 Figure I-11: Schema of CVD system.

Figure I- 12 .

 12 Figure I-12. Different types of fouling [125].

Figure I- 13 .

 13 Figure I-13. Sequential steps in biofilm formation (Reproduced from M.Tiranuntakul, 2011 [130]).

Figure I- 14 .

 14 Figure I-14. Variety and size scale of typical fouling organisms (Reproduced from J.A Callow and M.E Callow, 2011 [135]).

Figure I- 15 .

 15 Figure I-15. Mechanism of antifouling action (Reproduced from S. Priyanka et al., 2017[START_REF] Sathe | Bioinspired nanocoatings for biofouling prevention by photocatalytic redox reactions[END_REF]).

Figure I- 16 .

 16

Figure I- 16 :

 16 Figure I-16: Mechanism of the photoelectrochemical anticorrosion effect of TiO2 for metals (Reproduced from Y. Bu and J.P Ao, 2017[143]).

Chapter II: 55 II. 1 . 56 II. 2 . 62 II. 3 . 66 II. 4 . 71 II. 5 .

 551562623664715 Deposition and Characterization of Al-Zr thin films Chapter II: Deposition and Characterization of Al-Zr thin films ........................................... Direct Current Magnetron Sputtering .......................................................................... Al-Zr films deposited by DC magnetron sputtering .................................................... Structural characterization and morphology of Al-Zr thin films ................................. Nanohardness measurements of Al-Zr coatings .......................................................... Electrochemical tests.................................................................................................... 72 II.6. Conclusions .................................................................................................................. 78 References ..................................................................................................................................... 79 Physical Vapor Deposition (PVD) technologies are widely used for the synthesis of thin layers.

  Δσ = σf + σi -σs II. 1 With: σs = surface energy of the substrate, σi = free enthalpy of the film-substrate interface, σf = free enthalpy of the film surface It is possible to distinguish three growth modes from this criterion (Figure II-1).

Figure II- 1 .

 1 Figure II-1. Diagram of the three growth modes of thin films: Morphology of a film by the mechanisms of (a) Frank van der Merwe, (b) Stranski-Krastanov and (c) Volmer-Weber.

  in Figure In this model, Anders proposed three axes: the generalized homologous temperature (T *, the homologous temperature as well as the temperature shift caused by the potential energy of particles arriving on the surface), the normalized flow of kinetic energy of the particles arriving on the surface (E *, the initial energy from the plasma as well as the variation related to the acceleration in the reactor), and the thickness of the film (t *).For magnetron sputtering, T * and E * are mainly influenced by substrate temperature and working pressure. During the deposition, the substrate temperature influences the mobility of the surface adatoms. The increase of the substrate temperature leads to increase the mobility of species adsorbed on the substrate surface and increasing their diffusion in volume leads to densify the deposited layer. The deposition pressure impacts the energy and angles between the normal of the Chapter II: Deposition and characterization of Al-Zr thin films substrate and the trajectory of the incident particles. Decreasing the deposition pressure has the same effect as increasing the deposition temperature. Anders' model, presented in Figure II-2, shows four different zones.

Figure II- 2 .

 2 Figure II-2. Anders's Structure zone diagram.[4]

Figure II- 3 .

 3 Figure II-3.

Figure

  Figure II-5 shows the variation of film thickness and Zr content as a function of current applied to the Zr target. It is possible to accurately control the zirconium content and confirms that the ratio between the atomic percentages of Zr and Al increases almost linearly with the intensity applied to the zirconium target.

Figure II- 4 .Figure II- 5 .

 45 Figure II-4. EDS analysis of Al-Zr films deposited on silicon substrate.

  : Young's Modulus of the substrate = 200 Gpa 𝜈S : Poisson's ratio of the substrate = 0.3 hs : thickness of the substrate = 220 µm hf : thicknesses of the film. R: Curvature radius of the substrate after deposition. 𝑅0: Curvature radius of the substrate before deposition.

Figure II- 6

 6 Figure II-6 shows the internal residual stresses calculated by Stoney's formula as a function of zirconium content in Al-Zr alloys. The films containing 4 and 7 at. % Zr present low tensile stress.By increasing the zirconium content above 10 at. %, the internal residual stresses are compressive reaching a value of -260 MPa for Al-Zr film containing 27 at. % of zirconium.

Figure II- 6 .

 6 Figure II-6. Variation of residual stress as a function of zirconium content in Al-Zr alloy films.

  diffraction technique using a Bruker AXS D8-Advance diffractometer with Cu Kα (λCu = 0.15418 nm) radiation, working at 40 KV and 40 mA. The measurements were carried out with 2θ varying 90°. The acquisition time was 1 s with steps of 0.03° in θ-2θ configuration (Figure II-7).Films morphology was observed by scanning electron microscopy (SEM) using a FEI Quanta 250 field-emission scanning electron microscope (FESEM) for high-resolution images. X-ray diffraction patterns of Al-Zr films deposited on glass by magnetron sputtering are shown in Figure II-6. For Zr contents below 10 at. %, the diffractograms show the well-defined diffraction peaks, which correspond to fcc structure, indicating the growth of a supersaturated solid solution of Zr in Al. When Zr contents are higher than 10 at. % diffraction peaks are broader. The peaks broadening is the result of grain refinement occurred when Zr content increased. Simultaneously, a shift of the diffraction peaks to lower values of 2θ angles was observed, which is due to a progressive increase of lattice parameter. The estimation of average lattice parameter of these Al-Zr alloys from the θ angles corresponding to the (111) planes confirms that the lattice parameter increased from a= 4.050 Å for pure Al to a= 4.060 Å for Al-Zr alloy having 27 at. % Zr. As far as Zr atomic radius (0.218 nm) is greater than that of Al (0.182 nm), the lattice increase is associated to the substitution of aluminium atom by zirconium atom in the fcc lattice. Above 10 at.% Zr, according to previous results on Al-Transition Metal (TM) coatings, the Al-Zr films are probably a mixture of the solid solution of Zr in fcc-Al and an amorphous phase. This evolution is consistent with binary Al-TM (Transition Metal) alloys deposited by magnetron sputtering [8-12].

Figure II- 8

 8 Figure shows the XRD patterns for the high speed steel substrate and Al-Zr (4 at. % Zr) film deposited by DC magnetron sputtering before and after annealing in air at 500°C. The well-defined diffraction peaks seem to show that there is no effect of annealing of Al-Zr (4 at. % Zr) in air at 500 °C; in both cases, the (111) diffraction peak of aluminum at 38° is observed, suggesting the presence of the -Al solid solution. Nevertheless, TEM analysis of the Al-Zr layer after TiO2 deposition shows precipitation of the metastable Al3Zr phase (see Chapter IV/ Section IV.6).

Figure II- 7 .

 7 Figure II-7. XRD patterns of Al-Zr coatings deposited on glass.

Figure II- 9 .

 9 Figure II-9. Morphology evolution of Al-Zr alloys deposited on silicon substrates.

Figure II- 10 .

 10 Figure II-10. Cross-section SEM images of Al-Zr films deposited on silicon wafers.

  Figure II-11. Evolution of nanohardness of Al-Zr films as a function of Zr content.

  Figure II-12 shows the polarisation curves after 1 h of immersion in saline solution.

Figure II- 12 .

 12 Figure II-12. Polarisation curves of Al-Zr films deposited on glass after 1h in saline solution (5wt. % NaCl). Al-Zr alloys Zr (at. %)

  Figure II-13 shows the polarisation curves of Al-Zr alloys after 1h of immersion in the 5 wt. % NaCl solution. It is shown that the Al-Zr coatings corrosion is under cathodic control and the general aspect of polarisation curves indicates a passive corrosion. This aspect decreases as Zr content increases.

  Figure II-14 presents the evolution of OCP potential versus time in 5 wt. % NaCl during 48 h of immersion.

Figure II- 13 .

 13 Figure II-13. Polarisation curves of Al-Zr films deposited on high speed steel after 1h in saline solution (5wt% NaCl).

Figure II- 14 .

 14 Figure II-14. Evolution of Open Circuit Potential (OCP) of Al-Zr films deposited on high speed steel in saline solution (5wt/% NaCl).

83 III. 1 .

 831 Chapter III: Deposition and characterization of TiO 2 thin films Chapter III: Deposition and characterization of TiO2 thin films ............................................ Deposition of TiO2 films by aerosol assisted metalorganic chemical vapor deposition (AAMOCVD) ........................................................................................................ 84 III.2. Effect of deposition temperature on the TiO2 films deposited on silicon ............ 89 III.3. Photocatalytic characterization ........................................................................... 101 III.4. Reproducibility of TiO2 films on high speed steel substrates ............................. 106 III.5. Conclusions ........................................................................................................ 107 References ................................................................................................................................... 109

Figure III- 1

 1 Figure III-1 shows the behavior of aerosol according to the temperature:

Figure III- 1 .

 1 Figure III-1. Behavior of the aerosol as a function of temperature.

  Ultrasonic spray pyrolysis involves generating ultrasound in the precursor solution. It will produce capillary waves at the free surface of the solution. The effect of electrostatic forces leads to the formation of droplets as shown in Figure III-2. This technique allows to create droplets of controlled size and relatively mono-disperse.

Figure III- 2 .

 2 Figure III-2. Solution atomization[3]. The average diameter of the droplets (dg) is proportional at the wavelength λc of the capillary wave. The experimental work of E.G Lierke and G. Griesshammer [4] resulted in formula: 𝑑 𝑔 = 0.34 𝜆 𝑐 = 0.34 ( 8𝜋 𝜎 𝑆 𝜌 𝑆 𝑓 2 ) 1/3

4 .

 4 55 μm and 9.11 μm in taking the extremal values of the coefficient X. III.1.2. TiO2 precursor and substrates AAMOCVD precursor have to be soluble in the solvent, and stable if other precursors are present in the solution. It must be volatile and should decompose at growth temperatures while being stable at room temperature. The titanium precursor used in these work is titanyl (bis) acetylacetonate (TiOAcAc)2, also known as Bis(acetylacetonato)titanium(IV) oxide, Bis(2,4-pentanedionato)titanium(IV) oxide, Bis(acetylacetonato)titanium(IV) oxide, or Bis(2,4-pentanedionato)titanium(IV) oxide. Its chemical formula is TiO (CH3COCHCOCH3)2, (see Figure III-3), purchased from Strem Chemicals and used without treatment additional. It is a white powder with a melting point at 200°C. It dissolved easily in alcohols.

Figure III- 3 .

 3 Figure III-3. Titanyl(bis)acetylacetonate.

Figure III- 4 .

 4 Figure III-4. Details of AACVD configuration.

  Analysis of the microstructure of the samples and the evaluation of their thickness were acquired by means of a scanning electron microscope (SEM). The SEM was performed in a FEI Quanta 250 MEB FEG ESEM tool.

Figure III- 5

 5 Figure III-5 presents SEM images of TiO2 films deposited at a range of temperature from 400°C to 550° that mostly comprise of two parts: one is a thin layer immediately over the substrate and the other is composed of the microflowers that are dispersed at the top of the layer. The flowers having an average diameter of about 7 µm are characterized by a homocentric association of TiO2 nanopetals forming a corolla as if growing from the receptacle of a flower (Figure III-5b). Figure III-5a) presents de cross-section of TiO2 film containing microflowers. It is noticed that microflowers rise from the layer, at first as single nanopetals to develop subsequently into a bundle of interconnected petals. The principal characteristics of these microstructure is that petal have a nanometric thickness (~ 5 nm) and are formed by nanocrystal of anatase phase [9]. ImageJ was used to quantify the density of the flowers by calculating the area fraction coverage of the substrate covered by the flowers (in percentage) from the images. The area fraction coverage by microflowers is about 40 % for TiO2 thin films samples obtained deposited at 550°C.

Figure III- 5 .

 5 Figure III-5. SEM images of microstructured TiO2 thin films deposited by AA-MOCVD at 400°C, 500°C and 550°C on silicon substrate a) cross section showing the interfaces between the substrate, TiO2 film and microflowers and b) zoomed in image of petals to show their nanometric thickness.

Figure III- 6 .

 6 Figure III-6. Evolution of deposition rate over deposition temperature for TiO2 films deposited by AACVD.

Figure III- 7 .

 7 Figure III-7. FTIR spectra of thin film samples on the Si substrate.

Figure III- 8 96 Figure

 896 Figure III-8 shows the diffraction patterns obtained for as-deposited films deposited at a range from 400 °C to 550 °C on silicon substrate with a deposition time of 40 min. The XRD patterns show that the obtained films are crystalline and consisted of pure anatase phase. The diffraction peaks were observed at 25.5°, 37.5°, 48° and 55.3° 2θ degrees, corresponding to the anatase (101), (004), (200) and (211) planes (JCPDS-21-1272). No evidence for rutile phase was found by XRD, even for TiO2 films deposited at 550 °C.

Figure III- 9

 9 Figure shows the Raman spectra of TiO2 films deposited on silicon at different temperatures ranging between 400 °C and 550 °C. The Raman spectra of the films show characteristic modes of the TiO2 anatase phase[15]: two Eg modes at 141 cm -1 and 635 cm -1 , one A1g mode at 513 cm -1 and one B1g mode at 397.5 cm -1 . The sharp peak at 144 cm -1 clearly identifies the anatase phase of TiO2. Smaller peaks at 398 and 639 cm -1 can also be assigned to the anatase phase. No band corresponding to the rutile phase of TiO2 was observed. Moreover, the absence of a broad band background in the Raman spectra allows to discard the growth of amorphous TiO2.

Figure III- 9 .

 9 Figure III-9. Raman spectra for TiO2 films deposited on silicon by aerosol CVD at a) 550 °C, b) 500 °C and c) 400 °C.

Figure III- 10

 10 Figure III-10 shows high resolution XPS spectrum of a TiO2 film deposited at 550°C on silicon substrate after rising with ethanol and distilled water.

Figure III- 10 .

 10 Figure III-10. XPS for a) Ti2p configuration, b) O1s configuration and c) C1s configuration for TiO2 film deposited at 540°C on silicon substrate.

  photon energy of TiO2 deposited on corning glass at different temperatures are shown in Figure III-11 a) and Figure III-11 b).

Figure III- 11

 11 Figure III-11 a) shows that TiO2 films presents a high transmission ( > 60 -80 %) in the visible range, films are highly transparent in the visible. The oscillation observed are the result of interferences with the interfaces airfilm and filmsubstrate.

Figure III- 11

 11 Figure III-11. a) Total transmittance of TiO2 films deposited at different temperatures. b) Optical band gap calculated using Tauc plot of TiO2 films deposited at different temperatures.

C

  Figure Studies published by other authors showed that the OG is not only decolorized by photocatalytic reactions but fully mineralized[24]. The mineralization is a set of chemical reactions including the dissociation of the central azo group and its elimination in the form of dediazote gas, as well as the multi-stage reduction of aromatic rings in carboxylic species, then CO2.

Figure III- 12 .

 12 Figure III-12. The molecular structure of orange G.

Figure III- 13 .

 13 Figure III-13. Diagram of the device used for the measurement of photocatalytic activity.

Calibration measurements under irradiation

  carried out in the presence of the substrate alone have shown that the concentration of Orange G in the solution increases slightly during the time due to slow evaporation of the water as a dilution medium (reconcentration mechanism). A reconcentration correction has therefore been done in order to determine the changes in Orange G concentration actually induced by the photocatalytic decomposition (Figure III-15).

Figure III- 14 .

 14 Figure III-14. UV-visible absorbance spectra of decomposed Orange G-dye solution over TiO2 film deposited at 550 °C on silicon under UV light illumination.

Figure III- 15 .

 15 Figure III-15. Kinetics of photocatalytic decomposition of OG on the surface of a thin layer of TiO2 after reconcentration correction (a). Kinetics in the presence (b) and in the absence (c) of the photocatalyst. Monitoring the photocatalytic decomposition of the OG over three hours makes it possible to deduce the photocatalytic activity according to the relationship. Given the low concentration of OG of the aqueous solutions used during this work (5.10 M -5 ), the term Kads. C is much lower than 1 and can be neglected in this equation, which becomes: 𝑟 = -𝑑𝐶 𝑑𝑡 = 𝑘 𝑐 . 𝐾 𝑎𝑑𝑠 . 𝐶 = 𝑘. 𝐶 III. 7Where k describes an apparent kinetic constant of the system. By integrating the equation III. 7, we obtain:

Figure III- 16 b

 16 Figure III-16.a) Relative photocatalytic activity of different TiO2 films deposited on silicon (about 250 nm thick) as deposited. b) Kinetics of photocatalytic decomposition of orange G measured for different TiO2 films. The evolution of the absorption band at 480 nm corresponding to Orange G allowed to quantify the concentration variation with UV exposure time. The concentration of orange G decreases when exposed to TiO2 films as represented in Figure III-16 a). Figure III-16 b) shows plots of Ln(C/C0) as a function of UV light irradiation time. The photocatalytic activity of three different TiO2 films deposited at 400 °C, 500 °C and 550 °C on silicon was analysed in order to evaluate the films performance. For films containing TiO2 microflowers, deposition temperature presents an influence on the degradation rate, with a maximum value in the case of the film deposited at 550 °C, which shows almost 30 % of mineralization after 240 min of UV light exposure. For the film

Figure III- 18

 18 Figure III-18 shows the diffraction patterns obtained for as-deposited films deposited at a range from 500 °C to 550 °C on high speed steel with a deposition time of 40 min.

Figure III- 17

 17 Figure III-17. a) SEM image of TiO2 microflowers on films deposited at 550 °C on a) FTO-coated glass b) ITO-coated glass and c) high speed steel substrate.

Figure III- 18 .

 18 Figure III-18. XRD of TiO2 films at deposited at high speed steel substrate at a) 500 °C, B) 540 °C and C) 550 °C.

Chapter IV: 113 IV. 1 . 114 IV. 2 . 115 IV. 3 . 118 IV. 4 . 121 IV. 5 . 126 IV. 7 .

 113111421153118412151267 Characterizations of Al-Zr/TiO 2 bilayer thin films Chapter IV: Characterizations of Al-Zr/TiO2 bilayer thin films ........................................... Deposition conditions ......................................................................................... Morphology and roughness of HSS/Al-Zr/TiO2films ........................................ Structural properties ........................................................................................... Photocatalytic properties of HSS/Al-Zr/TiO2 bilayer films ............................... Wettability of HSS/Al-Zr/TiO2 bilayer films ..................................................... 124 IV.6. TEM Analysis of HSS/Al-Zr/TiO2 bilayer films ................................................ Electrochemical tests .......................................................................................... 132 IV.8. Conclusions ........................................................................................................ 137 References ................................................................................................................................... 139

Figure IV- 1 presents

 1 HSS substrates coated with Al-Zr alloy and Al-Zr/TiO2 bilayer film.

Figure IV- 1 .

 1 Figure IV-1. Al-Zr (4 at. % Zr) alloy, on the left, and Al-Zr/TiO2 bilayer film on the right.

  Figure IV-2 (a-b)for low and high magnification conditions in the upper and lower pictures respectively, and in cross section represented in FigureIV-2 c). For low Zr content like in our case and in our deposition conditions, the coatings grow with a columnar morphology forming big grains of around 500 nm of diameter, as explained in chapter II. The Al-Zr thickness is 3 µm. When TiO2 was deposited by AACVD as a second layer on the Al-Zr coating, two kinds of features appeared. On one hand, a film consisting in small needle-like (dendritic microspheres) grains of 50 nm covered the Al-Zr grains in a very conformal way. This morphology can be observed in Figure IV-2 (e-h-k), for the three deposition temperatures used. We cannot see any difference or significant changes between these morphologies. The Al-Zr grains can be still identified even if TiO2 film thickness is around 400 nm. On the other hand, microflowers started to grow at deposition temperature higher than 500 °C, as explained before in chapter III. At 500 °C, represented in FigureIV-2 (d-f), some sparse flowers can be observed but much smaller than those shown in FigureIV-2 (g-i) and (j-l) for films deposited at 540 °C and 550 °C, respectively. For films deposited at 540 °C and 550 °C, the flowers density increases and a quite high density of microflowers is observed on the surface. Flowers having an average diameter of about 7 µm can be described by a homocentric association of TiO2 nanopetals forming a corolla as if growing from the receptacle of a flower. The main characteristics of these microstructures are that petals have a nanometric thickness (~ 5 nm) and are formed by nanocrystal of anatase phase as shown in the next section.

Figure IV- 2 .

 2 Figure IV-2. SEM images of samples: a-b-c) Al-Zr (4 at.% Zr) coating and TiO2 deposited at d-e-f) 500 °C, g-h-i) 540 °C and j-k-l) 550 °C. Images d, g and j were used to quantify the density of the flowers by calculating the area fraction covering the sample by the flowers (in percentage) as shown in Figure IV-3. The area fraction covered by microflowers was about 43 % for TiO2 thin films samples deposited at 550°C, 23 % for films deposited at 540°C and 8 % for TiO2 films deposited at 500°C.

Figure IV- 3 .

 3 Figure IV-3. SEM images processed by ImageJ for TiO2 films deposited at a) 500 °C, b) 540 °C and c) 550 °C.

Figure IV- 4 .

 4 Figure IV-4. Surface roughness profiles measured 25 mm along the surface.

  -Zr/TiO2 Tdep=550°C 53 XRD characterisation was also performed after deposition of TiO2 at 500, 540 and 550°C on HSS/Al-Zr substrates. The XRD patterns of these TiO2 coatings with and without microflowers are shown in Figure IV-5 and Figure IV-6, respectively. They reveal only anatase phase in all cases, mainly detected by the (

Figure IV- 5 .

 5 Figure IV-5. XRD patterns for Al-Zr/TiO2 films with microflowers deposited at a) 550°C, b) 540°C and c) 500°C on d) Al-Zr.

Figure IV- 6 .

 6 Figure IV-6. XRD Patterns for Al-Zr/TiO2 thin films without microflowers deposited at a) 500°C, b) 550°C on c) Al-Zr.

Figure IV- 7 .IV. 4 .

 74 Figure IV-7. Raman spectra for HSS/Al-Zr/TiO2 films deposited by aerosol CVD at a) 550°C, b) 540°C and c) 500°C.

  photocatalytic activity as indicated in Figure IV-8. On the contrary, the concentration of orange G decreased when exposed to TiO2 films. Two kinds of TiO2 films were deposited to highlight the influence of microflowers on phtotocatalytic properties. Using the same deposition temperature, the solution consumption is decreased to less than 3L.min -1 by decreasing the frequency of the

Figure IV- 8 .

 8 Figure IV-8. Linear regression of kinetics of orange G degradation by the HSS/Al-Zr/TiO2 thin films under UV light (371 nm) irradiation: On left, films with microflowers deposited at 1a) 550 °C, 1b) 540 °C, 1c) 500 °C; films without microflowers (on right) deposited at 2a) 500 °C and 2b) 550 °C; and (1d and 2c) Al-Zr films.

  to a PC computer and DROP SHAPE image processing software ANALYSIS (DSA), represented in Figure IV-9. This software first determines the baseline of the drop using its reflected mirror image on the surface of the sample, then measures the contact angles of the drop at the intersection with the baseline via the 'Circle fitting' method (Figure IV-10).A droplet of distilled water of 0.5 µl volume was placed on the film surface and the first contact angle was measured immediately. The contact angle was measured before and after the incidence of light in the UV-C region used for the evaluation of photocatalytic activities. Each value was an average of 3 measurements on different areas of the sample surface.

Figure IV- 9 .

 9 Figure IV-9. Schematic representation of the contact angle measuring device.

Figure IV- 10 .

 10 Figure IV-10. Determination of the contact angle using the DSA software.

  Al-Zr/TiO2 thin films were irradiated with UV light during 4 hours and the water contact angle was measured at different exposure times. The results are presented in Figure IV-11. For all samples, the contact angle decreased sharply with the time from about 60° to 0° after 4 hours indicating a photoinduced superhydrophilicity. After six months in the absence of UV irradiation, the samples remained superhydrophilic presenting a contact angle of about 5°, as shown in Figure IV-12.

Figure IV- 11 .

 11 Figure IV-11. Variation of water contact angles under UV illumination for HSS/Al-Zr/TiO2 bilayer films with TiO2 deposited at a) 500°C, b) 540°C and c) 550°C.

Figure IV- 13

 13 Figure IV-13 shows the microstructure of the TiO2 layer deposited by AA-MOCVD. The composition appears to be homogeneous (Elemental X-ray X-maps, Figure IV-14). The electron diffraction pattern confirms that TiO2 grows with the anatase structure.

  diffraction patterns of as-deposited Al-Zr coatings (4 at.% Zr) and annealed at a temperature of about 550 °C during TiO2 deposition (Figure II-8 in chapter II) do not show significant change. So, it seems that the structure and microstructure of the Al-Zr layer are stable up to this temperature.Before the AA-MOCVD process step, the Al-Zr alloy has a columnar microstructure (Figure. EDS analysis has shown that zirconium is homogeneously distributed throughout the layer.The columns are approximately 450 nm wide and the growth is not systematically from the interface to the surface of the film. This coating is clearly single-phased; it is an extended supersaturated solid solution of zirconium in aluminum. This solid solution grows without preferential orientation, which is not surprising for an aluminum coating deposited at low temperature without bias.After the AA-MOCVD process step at substrates temperature of about 550 °C, the Al-Zr coating has a different microstructure (Figure IV-14a, b). Indeed, dark precipitates appear clearly in the Bright field scanning transmission electron microscopy (STEM BF, Figure IV-14 a) Scanning-mode transmission electron microscopy coupled to a High-Angle Annular Dark-Field Detector (STEM-HAADF), due to the dependence of signal strength with Z, increases the contrast between phases with different atomic numbers. Figure IV-14a, b confirm that these precipitates are richer in zirconium than the matrix. These precipitates have essentially two morphologies; rod and spherical. The spherical ones crystallize mainly at the solid solution grains boundaries. EDS analyzes have shown that the zirconium content of these precipitates is in the range 15.0 to 17.5 at. %. The compound with the highest aluminum content in the phase diagram is Al3Zr. Its stable structure is of type DO23 (Figure IV-16).

Figure IV- 13 .

 13 Figure IV-13.TEM Bright field (a) and SAED (b, anatase TiO2) of TiO2 layer deposited on the Al-Zr coating.

Figure IV- 15 .

 15 Figure IV-15. TEM Bright field (a) and SAED (b, [110] zone axis) of Al-Zr layer deposited before AA-MOCVD process (before annealing).

Figure IV- 16 .

 16 Figure IV-16. Unit cell of D023 type for Al3Zr.The DO23 structure has a parameter a = 4.001 Å[13] to be compared with a = 4.032 Å for Al and a ratio of tetragonality c/a = 4.31. Because of this ratio c/a significantly higher than 4, such an Al3Zr coherent precipitate with the aluminum matrix will therefore be subjected to high elastic stresses. This makes it unlikely to precipitate this structure in the Al-Zr coating.The Al3Zr compound also has an ordered metastable structure based on FCC cell but which has the symmetries of a simple cubic; the L12 type structure (Figure. The parameter of this phase is very close to that of aluminum; a = 4.093 Å. The elastic effects related to the coherence

Figure IV- 17 .

 17 Figure IV-17. Unit cell of L12 type for Al3Zr.

Figure IV- 18

 18 Figure IV-18 shows the TEM bright field images and selected area electron diffraction (SAED)patterns corresponding to zone axis[START_REF] Guillard | Physical properties and photocatalytic efficiencies of TiO2films prepared by PECVD and sol-gel methods[END_REF]. The structure is clearly biphased; an FCC solid solution of zirconium in aluminum and an ordered Al3Zr compound of L12 type, which confirms the hypothesis of the precipitation of this ordered metastable phase.

Figure IV- 18 .

 18 Figure IV-18. TEM bright field (a, b zoomed) images, associated SAED pattern with the [110] zone axis (c) and darkfield (d) obtained from sport corresponding to Al3Zr precipitates.

Figure IV- 19

 19 Figure IV-19 clearly confirms that L12 type ordered Al3Zr precipitates are fine coherency with the Al(Zr) solid solution matrix in cube on cube relationship with [100]Al3Zr // [100]Al(Zr), [010] Al3Zr // [010] Al(Zr) and [001] Al3Zr // [001] Al(Zr).Precipitation of this metastable phase has already been observed in conventional alloys after

Figure IV- 19 .

 19 Figure IV-19. Model (red: Al3Zr and black: Al(Zr)) and SAED diffraction pattern of Al-Zr after AA-MOCVD process for TiO2 deposition (after annealing). [001]Al3Zr // [001]Al(Zr).

Figure IV- 20 .

 20 Figure IV-20. High resolution STEM-HAADF of Al-Zr coating after AA-MOCVD process for TiO2 deposition (after annealing).

( 5

 5 Figure IV-21. The measurements were made using an Autolab potentiostat driven by NOVA 2.0 software.A polarization cell was composed of an electrolyte solution, a reference electrode, a counter electrode and the metallic sample placed with a sample holder. The sample presented the working electrode and the other electrodes were connected to an electronic instrument named a potentiostat.The working reference and counting electrodes were located in the electrolyte solution, normally a solution that most closely represents the real application environment of the material being analysed. In the solution, an electrochemical potential (voltage) was produced between the various electrodes.In this work, the pH of the solution was adjusted to 6 by addition of 0.5 M HCl solution and each measurement was performed at room temperature. The potentials were referenced against an Ag/AgCl electrode and the counter electrode was a large platinum grid. The effective area of

Figure IV- 21 .

 21 Figure IV-21. Schema of polarization cell used for electrochemical measurements.

Figure IV- 22

 22 Figure IV-22 shows the evolution of the open circuit potential (OCP) versus time during 1h of immersion in 5 wt.% NaCl for different samples: HSS, HSS/Al-Zr, HSS/TiO2, and the HSS/Al-Zr/TiO2. The steel OCP was reported as a reference. The steel OCP rapidly decreased and then stabilized at around -0.55 V/Ag-AgCl. This evolution is typical for the progressive formation of iron based corrosion products on the steel surface[21]. The TiO2 coated steel presents a quite similar OCP evolution during the immersion suggesting that this evolution is directly related to the steel degradation through the open defects of the oxide film. However, the OCP values are nobler in presence of the titanium oxide layer. Ramaprakash et al.[22] also observed a slight ennoblement of the steel corrosion potential when is covered with TiO2 in 3.5% NaCl solution. This ennoblement

Figure IV- 22 .Figure

 22 Figure IV-22. OCP evolution of steel coated with Al-Zr (4 at. % Zr ), TiO2 and Al-Zr/TiO2 bilayer films vs immersion time in 5 wt.% NaCl.

Figure IV- 24 .

 24 Figure IV-24. Optical micrographs of pits formed on a) Al-Zr (4 at.% Zr), b) TiO2 and c) Al-Zr/TiO2 bilayer films deposited on HSS after 1h of immersion in 5 wt.% NaCl.

Chapter IV:

  photocatalysis process and encourage the application of TiO2 nano-coatings for the fabrication of components to avoid algal adhesion on underwater structures.

Figure V- 1 .

 1 After adding ethylendiamine, the solution was stirred with speed of 300 cycle/min until complete dissolution. The temperature of the solution is also increased to 50 ºC to promote the precursor dissolution. The stirring lasted 15 hours until a homogenously solution was obtained. The precursor molecule structure is shown below: The Cu(II)(AcAc)2 prepared solutions was then mixed with a Ti(IV)O(AcAc)2 solutions at a concentration of 0.03 M in ethanol, prepared as explained in chapter III. In this way, we obtained two different cation concentration starting solutions in ethanol: Solution I) a solution of 0.01M of Cu and 0.03 M of Ti, Solution II) 0.03 M of Cu and 0.03 M of Ti. The mixture was stirred until complete mixing using a magnetic bar with speed 300 cycle/min at room temperature. After 3 hours the solution was ready for deposition.

Figure V- 1

 1 Figure V-1 Effect of the addition of the dispersant in the Cu(acac)2 solution a) before and b) after addition.

  Figure V-2 a) presents the evolution of the amount of copper on the film with the percentage of copper in solution. Figure V-2 b) exhibits the evolution of film thickness when the concentration of copper increases. The evolution is clearly not linear.

Figure V- 2

 2 Figure V-2. a) Evolution of at.% Cu on the film with the % of Cu in solution for the three substrates. b)Evolution of film thickness measured on silicon with at.% Cu.

  microstructure morphology of Cu-Ti-O films deposited on silicon and glass observed by SEM is shown in Figure V-3. Figure V-3 a) presents pure TiO2 films containing the typical microflowers. Morphology is rather different for films containing copper in their composition and microflowers are no longer presented. When at.% Cu increases, the morphology becomes granular and rough as represent in Figure V-3 b, c, d, e and f. The same feature is noticed for Cu-Ti-O films deposited on silicon, corning glass and on Al-Zr coated high speed steel substrate (Figure V-4).

Figure V- 3 .

 3 Figure V-3. SEM images of microstructures corresponding to a) pure TiO2 film, b) Cu-Ti-O -16 at.% Cu film c) Cu-Ti-O -50 at.% Cu film d) Cu-Ti-O -55 / 58 at.% Cu film and e) Cu-Ti-O -66 / 75 at.% Cu film deposited by AA-MOCVD on silicon and corning glass substrates.

Figure V- 4 .

 4 Figure V-4. SEM images of a) Cu-Ti-O -25 at.% Cu film and b) Cu-Ti-O -50 at.% Cu film deposited on Al-Zr coated high speed steel substrate.

Figure V- 5 .

 5 Figure V-5. AFM images 5μmx5μm of the Cu-Ti-O filmsa) CuTiO16C, b) CuTiO58C and c) CuTiO75C.V.3. Structural properties

  diffractometer with monochromatic CuKα1 radiation (λ = 0.15406 nm). The results are represented in Figure V-6 a) for corning substrate and in Figure V-7 a) for silicon. To have an increased sensibility to secondary phases, GIXRD was performed on Cu-Ti-O films and the diffraction patterns are shown in Figure V-6 b) and Figure V-7 b). GIXRD patterns were acquired with a Bruker C40 D8 with incidence and exit angles maintained around 0.5°. All samples exhibits diffraction peaks for the TiO2 anatase phase. TiO2 diffraction peaks are in agreement with JCPDS Card No.21-1272[13]. The presence of Cu2O is confirmed in films presenting at.% Cu higher or equal to 50 % and diffraction peaks are in agreement with JCPDS card No. 05-0667. A small amount of CuO is detected for films deposited on both substrates for Cu content equal or higher to 50 %. This phase is particularly visible in the GIXRD diagrams, presenting CuO peaks corresponding to the crystal planes (-1 1 1) and (11 1) at 2θ = 35.55° and 38.75° (JCPDS card No.07-1375). No other secondary phase were found in the diffraction diagrams at any composition.

Figure V- 6 .

 6 Figure V-6. XRD diagrams of TiO2 and Cu-Ti-O films deposited on corning glass substrates at 550°C obtained. a) in a Bragg-Brentano configuration and b) in GI-XRD configuration.

Figure V- 7 .

 7 Figure V-7. XRD diagrams of TiO2 and Cu-Ti-O films deposited on silicon substrate at 550°C obtained. a) in a Bragg-Brentano configuration and b) in GI-XRD configuration.

  spectra of as-deposited Cu-Ti-O thin films are represented in Figure V-8 a), Figure V-8 b), and Figure V-8 c) for films deposited on corning glass, silicon and Al-Zr coated substrates, respectively. The prominent Raman peaks observed for all samples at 144, 400, 519 and 639 cm -1 are related to typical anatase phase of TiO2 as already presented in Chapter III. These peaks correspond to following Raman vibration modes: Eg modes at 143 and 639 cm -1 , B1g mode at 400 cm -1 and overlapped A1g and B1g modes at 519 cm -1 . The anatase phase of Cu-Ti -O films is retained well by the incorporation of Cu in the films. Besides, Raman spectroscopy confirmed the formation of the Cu2O phase based on the presence of standard associated modes, which were detected in all films with at.% Cu greater than 50 as represented in Figure V-8. Several vibrational active modes of Cu2O are identified: (T2u) ~ 88 cm -1 , (Eu) ~ 106 cm -1 , (T1u) ~ 148 cm -1 , (2Eu) ~ 215 cm -1 and (T1u) ~ 625 cm -1 [14].

Figure V- 8 .

 8 Figure V-8. Raman spectra for Cu-Ti-O films deposited by aerosol CVD at 550°C on a) Corning glass substrate, b) Silicon substrate and c) Al-Zr coated high speed steel.

Figure V- 9 .

 9 Figure V-9. Raman spectra for CuTiO50C film measured at different points on the film surface, described on the picture on the right.V.4. Fourier transform infrared spectroscopy (FTIR)

  of elements at the surfaces, probing the top 10 nm of a film. The surface of the as-deposited films deposited by AAMOCVD at 550°C on silicon substrate were investigated by XPS in order to determine the possible changes induced in Ti oxidation state by the Cu incorporation in the film, together with the determination of the Cu oxidation state.XPS (X-rayPhotoelectron Spectroscopy) analyses were carried out in Thermo Scientific™ K-Alpha™ XPS spectrometer. Sample surfaces were irradiated with Al Kα radiation at 15 kV for a current of 20 mA. To correct for any charging effect, all the binding energies were calibrated to the C-C component of the C1s core spectrum at 285.0 eV. Peak deconvolutions were performed using ThermoAvantage software using Gaussian-Lorentzian functions The energy windows used for the analyses of the elements were 448 -475 eV for Ti2p, 925-965 eV for Cu2p, 525 -545 for O1s, and 279 -298 eV for C1s. The XPS spectra for three compositions are presented Figure V-12. The samples are pure TiO2, CuTiO50S and CuTiO66S.

   shows a main peak at 530.3 eV, indicating Ti-O and Cu-O bonds , and a second component at 531.7 eV could be ascribed to hydroxyl groups on the surface[28]. The intensity of the main peak, and so the total intensity of the O1s band, decreases with Cu content. This can be explained by the decrease in the oxygen ration when replacing TiO2 by Cu2O.Concerning Cu2p spectra presented in the third columns of Figure V-12, any curve is presented for the pure TiO2 sample because, as expected, Cu signal was not detected. The main issue in the Cu2p spectra is the absence of satellites around 945 eV, which are the signature of the CuO presence. The main peak in the Cu2p3/2 band is located at 933 eV, attributed to reduced copper species, Cu 1+ or Cu 0[29]. In our case, taking into account the phase identification by other techniques, we can assume that this peak is related to Cu +1 . The intensity of the Cu2p peak increases when at.% Cu is increased from 50 to 67 % Cu, but the formation of Cu2p satellites is only distinguished for the highest Cu content. As the presence of CuO was detected by Raman and XRD; we can infer that the CuO formation takes place inside the film and not essentially at the surface.C1s bands are characteristics of surface contamination, mainly C=C at 285 eV, and small contribution around 289 eV is also detected, attributed to C=O or C-O ligands, but not specific trends were found with Cu content. This contribution can be associated to a less efficient decomposition of the organic precursors. As in previous results, no evidence of the formation of a ternary phase was detected in the Cu-Ti-O films.

Figure V- 12 .

 12 Figure V-12. XPS spectra of Ti2p and O1s in the left and center columns respectively, and C 1s and Cu 2p in the right column. Three compositions are presented: the first row corresponds to pure TiO2 (except last one), the second to CuTiO50S film and third row to CuTiO66S film. C1s spectra compares in the same graph the signal for the three samples.

Figure V- 13 .

 13 Figure V-13. HRTEM image of a Cu-Ti-O film with 55 at.% Cu: a) cross section view on the silicon interface region, b) ) detail of the film top part removed by scratching ; c) same diffraction pattern showing the presence of Cu2O structure and d) corresponding diffraction pattern identified as TiO2 anatase . STEM was used to analyse the composition of the CuTiO55S film in cross section. Moreover, EDS was performed punctually using spot analysis in STEM to identify the local distribution of Cu and Ti. Mappings for each element obtained from the CuTiO55S as deposited film are presented in Figure V-14. This analysis confirms the presence of Cu and Ti through all film without significant segregation in the grain boundaries.

Figure V- 14

 14 Figure V-14. a) Bright-field STEM image of Cu-Ti-O film with 55 at.% Cu deposited on silicon, b-e) Corresponding EDS-STEM elemental mapping of the b) Ti, c) Si, d) Cu, and e) O elements, respectively.The 250 nm scale bar is valid for all images.

Figure V- 15

 15 Figure V-15 shows the contact angle for Cu-Ti-O films as deposited on corning glass before and after 2 hours of UV irradiation. Cu-Ti-O films, behave in a completely different way; they are hydrophobic as deposited (angle contact of ~100 °) and remain hydrophobic after 2 hours under UV irradiation.

Figure V- 15 .

 15 Figure V-15. Variation of water contact angle for the Cu-Ti-O and TiO2 films deposited on corning glass at 550°C as deposited and after 2 hours of UV irradiation.

  The photocatalytic activity was tested on Cu-Ti-O films deposited on silicon substrates with different compositions. The degradation kinetics with UV irradiation are represented in Figure V-16 a). CuTiO50S and CuTiO55S films presented the best performance in photocatalytic decomposition of Orange G dye. Figure V-16 b) presents the evolution of the k constant for Orange G degradation calculated from the slope of the degradation rate as a function of copper content in films.

V. 10 .

 10 Marine biofouling field testImmersion in seawater enables the most authentic test to evaluate antibiofouling efficacy providing a propitious site to the development of different successive colonisers. In this work, biofouling tests were carried out in order to compare coatings with different compositions.V.10.1.

Figure V- 18 .

 18 Figure V-18. Observation of coatings colonisation by SEM after 38 days of immersion in the AtlanticOcean.

Figure V- 19

 19 Figure V-19 presents the film's surface covered by diatoms in red after 38 days of immersion. The number of diatoms was counted for each sample and the TiO2 pure film presents approximately 133 diatoms on surface. Besides, CuTiO16C and CuTiO50C films presents approximately 183 and 260 diatoms, respectively.

Figure V- 19 .

 19 Figure V-19. SEM images of pure TiO2 films and Cu-Ti-O films covered by diatoms in red.

  These results were confirmed by CLSM. For CLSM experiments, adherent cells on coatings were stained by inserting Syto 9 stains (Molecular Probes. They are noted after 10 min of incubation. Syto 9 (5 µM) was employed as a nuclear stain for live cells. It was excited at 488 nm, and the emission was gathered at 498-550 nm (red fluorescence). Microalgae were observed by their fluorescence at 633 nm, and the emission was collected at 638-720 nm.

Figure V- 20

 20 Figure V-20 presents micrographs of the different coatings taken at different immersion time.

Figure V- 20 .

 20 Figure V-20. Observation of coatings colonisation by CLSM after 10, 17 and 25 days of immersion in Atlantic Ocean. Bacteria were stained with Syto 61 red/Sytox green allowing live and dead cells to appear in red and green, respectively. Micro-algae were observed in red by autofluorescence.

  Figure V-21 summarizes the macroscopic observations of surfaces after 7, 8, 9, 10 and 13 weeks of immersion in natural seawater.

Figure V- 21 .

 21 Figure V-21. Images showing the evolution of biofouling of the sample surfaces on TiO2 and Cu-Ti-O films deposited on corning glass at different immersion time during a tota immersion of 13 weeks in KernevelHarbour.

Figure V- 22

 22 Figure V-22 presents the evolution of fouling development with immersion time by quantifying with ImageJ software the surface area of sample covered by biofouling. After 4 weeks pure TiO2 films shows the best performance, presenting the smallest percentage of surface area covered by biofouling, with an average value of 20 %. On the other hand,

Figure V- 22 .

 22 Figure V-22. Evolution of surface area of samples covered by biofouling with immersion time obtained from pictures shown in Figure V-21 for a) TiO2 films, b) CuTiO16C films and c) CuTiO50C films.

Figure 1 .

 1 Figure1. Paramètres affectant un système de revêtement antisalissure[1].En même temps que les changements biologiques donnent lieu à l'accumulation de biofilms, une séquence de changements inorganiques se produit instantanément à la surface du métal après l'immersion dans un milieu aqueux agressif. Cette séquence induit un processus de dissolution du métal et la formation de produits réactionnels liés à la corrosion. Alors que la corrosion et l'accumulation de produits se développent de la surface métallique vers la solution, le développement de biofilms est un procédé inverse qui se développe de la solution vers la surface métallique.Dans cette thèse, nous proposons un nouveau revêtement multicouche multifonctionnel composé d'une première couche d'Al-Zr et une deuxième couche de TiO2. Il combine ainsi deux fonctionnalités : une couche antisalissure, le TiO2, déposé sur une couche anticorrosion Al-Zr, qui protège le substrat en acier.En effet, les alliages d'aluminium-métal de transition sont des candidats potentiels pour la protection contre la corrosion. Les métaux de transition sont ajoutés pour compenser les faibles caractéristiques mécaniques de l'aluminium pur. Un alliage d'aluminium optimisé avec une microstructure homogène a été utilisé dans ces travaux de thèse.

Figure 2 .

 2 Figure 2. Étapes dans la formation de biofilms. (Reproduit de M. Tiranuntakul,2011 [3]).

Figure 3 .

 3 Figure 3. Schéma réactionnel simplifié de la photocatalyse.Les nanostructures et les microstructures jouent un rôle important dans l'amélioration des performances des dispositifs, principalement en raison de l'augmentation de la surface spécifique[10], comme dans le cas des réactions photocatalytiques. Les structures de TiO2 qui ont une mésoporosité et une surface spécifique élevées sont attractives, car elles augmentent l'efficacité des phénomènes de photocatalyse qui se produisent en surface. La morphologie, la structure, la phase et la dimensionnalité ajustent les propriétés physiques et chimiques des nanostructures de TiO2.Tian[10] a démontré que le TiO2 en forme de fleur 3D présentait la plus grande absorption de la lumière UV lorsqu'il était intégré dans des cellules solaires à pigment photosensible comparé à des nanosphères et à des structures de nanorodes. L'activité photocatalytique (efficacité de dégradation de 97%) pour les fleurs était supérieure à celle des sphères ou des structures en forme de bâtonnets

Figure 4 .

 4 Figure 4.Classification du potentiel en circuit ouvert de métaux et des alliages à base d'Al déposes parPVD dans une solution saline (Reproduit de J.Creus et al., 2012 [24]).

Figure 5 .

 5 Figure 5. Schéma de la machine pulvérisation cathodique magnétron Delphis 4.

2 3 où

 23 photoactivité.

Figure 7 .

 7 Figure 7.a) Activité photocatalytique relative de différents films de TiO2 déposés sur du silicium (d'une épaisseur d'environ 250 nm). b) Cinétique de décomposition photocatalytique de l'orange G mesurée pour différents films de TiO2 sous radiation UV.

Figure 8 .

 8 Figure 8. Clichés MEB des films: a-b-c) Al-Zr (4 at.% Zr) et bicouche Al-Zr/TiO2 déposé à d-e-f) 500 °C, g-h-i) 540 °C and j-k-l) 550 °C.

Figure 10 .

 10 Figure 10. Spectres DRX des films de TiO2 et Cu-Ti-O déposés sur verre à 550 °C a) en mode Bragg-Brentano. b) En incidence rasante.

Figure 11 .

 11 Figure 11. a) Cinétique de décomposition photocatalytique de l'OG à la surface des films TiO2 et Cu-Ti-O avec différentes teneurs en Cu sous irradiation UV (371 nm). b) Evolution de la constant k avec la teneur en Cu. L'immersion dans l'eau de mer permet d'avoir le plus authentique test afin d'évaluer l'efficacité antisalissure, offrant un site propice au développement de différents micro et macro-organismes. Dans ce travail, des tests d'encrassement biologique ont été réalisés afin de comparer des revêtements avec différentes compositions. Des films de TiO2 purs et les films Cu-Ti-O ont été déposés sur un substrat de verre (Corning glass reference C1737-S111) puis immergés dans de l'eau de mer. Pendant les trois mois d'immersion, le degré de salissure par inspection visuelle a été estimée chaque semaine (ASTM D 3623) [27]. Trois films différents ont été testés : TiO2 pur, Cu-Ti-O 16 at.% Cu et Cu-Ti-O 50 at.% Cu.

Figure 12 .

 12 Figure 12. Films de TiO2 pur, Cu-Ti-O 16 at% Cu et Cu-Ti-O 50 at% Cu collés sur des panneaux de polycarbonate et immergé dans l'eau de mer naturelle (Océan Atlantique, Port de Kernevel, France). Trois échantillons pour chaque type de revêtement ont été immergés pour évaluer l'aptitude des films à empêcher la salissure par les macroorganismes. Dans un deuxième temps, douze échantillons pour chaque type pour analyser le potentiel pour empêcher la salissure par microorganismes. L'étude a débuté en avril 2019. Dans ce travail, le pourcentage estimé de la surface de l'échantillon couverte par l'encrassement biologique est obtenu après un traitement d'image à l'aide du logiciel ImageJ. Microscope confocal à balayage laser (MCBL) et le Microscope électronique à balayage (MEB) ont été utilisés comme outils analytiques complémentaires pour étudier le comportement des micro-organismes. L'objectif est de relier les observations de salissure par micro-organismes marins à l'activité de macroorganismes sur les échantillons.En ce qui concerne l'évaluation de la salissure par micro-organismes, après 10 jours d'immersion l'analyse MEB a permis de voir que seulement quelques diatomées ont été observées

Figure 13 .

 13 Figure 13. Clichés MEB de la colonisation des films après 38 jours d'immersion dans l'océan Atlantique. Ces résultats ont été confirmés par le Microscope confocal à balayage laser. Après 4 semaines d'immersion dans l'eau de mer, les micro-organismes présents sur des films de compositions différentes sont clairement différents.

Figure 14 .

 14 Figure 14. Evolution de la surface des échantillons recouverts de bio-encrassement avec le temps d'immersion pour a) TiO2 films, b) Cu-Ti-O 16 at.% Cu films et c) Cu-Ti-O 50 at.% Cu films.

  Yoshioka et al. have detected, by XPS analysis, the presence of Zr 4+ species incorporated in the aluminium based passive film when they studied the passive film formed from Al-Zr alloys in deaerated borate-boric solid solution containing chloride ions at pH=8.4. The passive film consists in a solid solution of Al 3+ and Zr 4+ ions containing a small amount of OH -

Table I .

 I 3. Desirable properties of a photocatalyst. Adapted from Neatu et al, 2014[START_REF] Neaţu | Solar light photocatalytic CO2 reduction: General considerations and selected bench-mark photocatalysts[END_REF].

How to accomplish the property Property Effect

	Small particle size	High surface area	High adsorption
	Crystalline material	Single site structure	Homogeneity
	Engineering band gap	Light absorption	Higher efficiency
	Preferential migration along	Efficient charge separation	Low recombination
	certain direction
	High crystallinity	High mobility of charge carries More efficient charge separation

Table II .

 II

1: Deposition conditions of Al-Zr films.

Table II . 2

 II2

	0	-1013	-1053	0.11	-671
	3	-930	-984	0.10	-599
	4	-747	-784	0.04	-595
	7	-945	-979	0.08	-532
	27	-890	-918	0.06	-350

. Electrochemical measurements of Al-Zr films deposited on glass substrates (deduced from polarization curves).

Table 1 :

 1 Precursor properties.

	Precursor	Molar mass (g/mol) Melting temperature (°C)	Purity	CASS Registry number
	TiO AcAC	262.08	184	> 95%	14024-64-7

Table III .

 III 1. AAMOCVD deposition parameters used in this work.

Table IV -

 IV 1: Effect of presence of microflowers on the crystallite size in TiO2 coatings deposited at 500°C and 550°C.

Table IV -

 IV 2. Constant k (kinetic constant of the system) for HSS/ Al-Zr/ TiO2 coatings deposited at different temperatures.

	Samples with microflowers	Constant k (min -1 )
	Al-Zr/TiO2 Tdep=500°C	0.00063
	Al-Zr/TiO2 Tdep=540°C	0.00102
	Al-Zr/TiO2 Tdep=550°C	0.00156
	Samples without	Constant k (min -1 )
	microflowers
	Al-Zr/TiO2 Tdep=500°C	0.00066
	Al-Zr/TiO2 Tdep=550°C	0.00059

Table IV -

 IV

3. Effect of presence of microflowers on the decomposition rate of orange G and crystallite size

for the TiO2 /Al-Zr coatings deposited at 500°C and 550°C.

Table IV

 IV

	Samples	Icorr (A/cm 2 )	Ecorr (V /( Ag/AgCl))
	HSS substrate	18	-0.52
	HSS/TiO2	9.25	-0.48
	HSS/Al-Zr ( 4 at.% Zr)	2.49	-0.59
	HSS/Al-Zr/TiO2	0.41	-0.61

.

4

. Icorr and Ecorr values for Al-Zr, TiO2, Al-Zr/TiO2 films and HSS substrate.

Table V .

 V 2. Roughness of Cu-Ti-O thin films.

	at.% Cu	RRMS (nm)
	16	23.77
	58	30.30
	75	36.90

  Cesméthodes permettent de contrôler les paramètres de dépôt afin d'obtenir les meilleures performances en fonction de l'application visée. Les techniques de dépôt par pulvérisation cathodique magnétron et CVD assistée par aérosol (AAMOCVD), également appelée pyrolyse d'aérosol, ont été sélectionnées pour le dépôt de films bicouches TiO2 /Al-Zr. Les films ont été caractérisés en termes de morphologie, structure cristallographique, texture, rugosité, mouillabilité de la surface, comportement électrochimique à la corrosion, l'activité photocatalytique et les propriétés antisalissures.La demande croissante de matériaux écologiques présentant des propriétés antisalissure et anticorrosion pour les applications marines fait face à un défi pour le développement de revêtements avancés. Dans ce contexte, l'objectif principal de cette thèse est d'utiliser des techniques de pulvérisation cathodiques magnétron à courant continu et dépôt chimique en phase vapeur afin de déposer un nouveau revêtement bicouche Al-Zr/TiO2 combinant les deux propriétés. Al-Zr est connu pour la protection des structures métalliques contre la corrosion et les films de TiO2 pour La salissure marine est définie comme la croissance et l'accumulation de micro et macroorganismes sur des surfaces mouillées dans un environnement salin. C'est un problème mondial qui affecte les industries aquatiques car cela peut réduire la vitesse des navires, augmenter la consommation de carburant et stimuler la corrosion, ce qui provoque la détérioration mécanique des structures statiques[2].

	I	Etat de l'art
	Ce chapitre présent un état de l'art sur les matériaux choisis et leurs fonctionnalités. Tout
	d'abord, l'utilisation des alliages d'aluminium-métal de transition, comme anodes sacrificielles, et
	Le dioxyde de titane (TiO2), candidat prometteur pour son activité photocatalytique et son la corrosion d'alliages d'aluminium dans des solutions aqueuses sont discutées en détail. Le choix
	comportement hydrophile/hydrophobe, ralentit le processus de bio-salissure dans les structures du revêtement Al-Zr pour les applications de corrosion est exposé. Dans une seconde partie, les
	marines. discussions portent sur les mécanismes et les applications de la photocatalyse par les semi-

Le dépôt physique en phase vapeur (PVD) et le dépôt chimique en phase vapeur (CVD) sont des méthodes appropriées pour le dépôt de couches minces fonctionnelles et protectrices. ses propriétés photocatalytiques sous radiation UV. Compte tenu des caractéristiques prometteuses des films Al-Zr/TiO2, ces travaux porteront sur l'élaboration et la caractérisation des films bicouches TiO2 et Al-Zr, afin de développer un revêtement multifonctionnel permettant d'éviter la corrosion et les biosalissures des structures utilisées en milieu marin. De plus, des essais de bioencrassement en eau de mer naturelle, à une profondeur de 50 cm (océan Atlantique, port de Kernevel, France) ont été effectués afin d'évaluer la performance des films de TiO2 dans l'environnement réel.

Le manuscrit de thèse est organisé en cinq chapitres. conducteurs en dioxyde de titane. Les principaux aspects de l'encrassement biologique et de la corrosion dans les structures marines est également présentée.

Développement de revêtements multifonctionnels combinant les propriétés anti- corrosion et antisalissure 202

  technique de pulvérisation cathodique[19,22,23] offrent la possibilité d'obtenir une solution monosolide sursaturée à haute teneur en Zr. Afin d'améliorer l'efficacité des anodes en aluminium, celles-ci sont généralement alliées à d'autres éléments pour favoriser le retrait de la couche de passivation (rupture du film d'oxyde) et /ou déplacer le potentiel de fonctionnement du métal dans une direction plus négative. L'ajout d'éléments comme Zr engendre une augmentation du potentiel de corrosion. De plus, l'augmentation de la teneur en Zr dans Al permet également de réduire la densité du courant de corrosion, ce qui est associé à une meilleure résistance à la corrosion. Creus et al. ont [24] présenté la classification du potentiel en circuit ouvert de métaux et d'alliages à base d'Al déposés par PVD dans une solution saline comme représenté à la Figure 4.Les potentiels de corrosion du revêtement Al -Zr sont plus négatifs que le seuil de potentiel nécessaire pour obtenir la protection cathodique des aciers en solution saline. Il a été signalé que l'ajout d'éléments tels que V, Mn, Cr, Ti, Mo, Si ou Zr induit un ennoblissement du potentiel de corrosion par rapport à celui de l'Al pur lorsque la teneur en éléments d'alliage est augmentée.

	et améliore
	considérablement la résistance à la corrosion dans une solution désaérée d'acide borate-borique
	contenant du Cl -[19]. Contrairement aux méthodes traditionnelles où la solubilité solide du Zr est

très faible (la solubilité maximale est de 0,083 at.% dans la réaction péritectique

[20]

), les méthodes de traitement hors équilibre telles que la solidification rapide

[21]

, l'alliage mécanique et les Résumé:

Figure II-3. View and scheme of the DC magnetron sputtering machine Dephis 4.

Figure 6. Schéma de la machine dépôt chimique en phase vapeur.L'optimisation des paramètres de dépôt permet le développement et la croissance de microfleurs de TiO2, de phase anatase, dispersées sur une couche mince de TiO2. Les conditions de croissance optimales ont été obtenues en ajustant la température de dépôt et le débit de précurseur (vitesse de consommation de la solution).Les paramètres optimaux de dépôt sont : le précurseur titanyl (bis) acétylacétonate avec une concentration de 0,03 M, une gamme de température comprise entre 500 °C à 550 °C, une vitesse de consommation de solution supérieure à 3 mL/min et une durée de dépôt de 40 minutes. Pour ces températures, des micro-fleurs de TiO2 d'environ 7 à 8 µm de diamètre sont formées. La diffraction des rayons X confirme la phase anatase des films de TiO2. Il est important de noter que la formation des micro-fleurs est très sensible à la température de dépôt. À des températures inférieures, telles que 400°C, aucune fleur n'est créée.

Acknowledgments i Acknowledgments During these three years of PhD, I have passed challenging and grateful moments which allowed me to develop both personally and professionally. Acknowledgments ii In LMGP, I thank Michele San Martin and Josiane Viboud for the administrative and technical help. I would like to acknowledge Odette Chaix, Hervé Roussel, Laetitia Rapenne and Etienne Pernot for support with the characterization techniques and consequent scientific consultations. Additionally, I would thank the scientific support and endless discussions with prof. Jean-Luc Deschanvres in the laboratory. Thanks for having always showed an extraordinary positive attitude and to give me lots of valuable suggestions during my thesis work. I would like to thank Dr. Michel Schlegel for the corrosions tests in CEA and for all the advices which helped me a lot during this PhD. I also want to thank prof. Juan Creus for all the great discussions and for all the scientific support when I needed help. I would like to thank prof. Claude Guet, Prof. Dong Zhili and Dr. Srikanth Narasimalu for the opportunity to work in Nanyang Technological University. It was a great personal and professional experience.

I also would like to thank Dr. Fabienne Fay for giving me the opportunity to collaborate with her making possible to perform important tests. Thanks for all the support and valuable advices. I would like to thank Prof. Albano Cavaleiro and Prof. Francis Maury for the honor they gave me

IV.3. Structural properties IV.3.1. X-Ray Diffraction (XRD) analysis

The crystallinity and phase identification of the samples were studied by X-ray diffraction (XRD) in θ-2θ scanning mode using a Bragg-Brentano configuration between 10° and 80° at 0.011° intervals with an acquisition time of 2 s using a Bruker D8 Advance diffractometer equipped with monochromatic CuKα1 radiation (λ = 0.15406 nm).

The crystallite size was determined using the Scherer's formula based on the FWHM (Full Width At half Maximum) of ( 101), ( 200) and (211) anatase peaks in XRD patterns (Eq. IV.1):

where DP is the crystallite size in nanometers, λCu = 0.15418 nm is the X-ray wavelength, θ is the diffraction angle of the ( 101), ( 200) and (211) peaks and β is defined as the peak width of the diffraction peak profile at half maximum height resulting from small crystallite size in radians and K is Scherrer's constant related to crystallite shape (in our study, K = 0.9). To determine the evolution of crystallite size with temperature on the dense TiO2 film without flowers, we performed XRD analysis on HSS/Al-Zr/TiO2 films deposited on polished HSS substrates. In this case, the grain size values obtained by Scherer's method given by formula IV.1 were 44 nm for film deposited at 500°C and 53 nm for films deposited at 550°C. However, if we calculate crystallite size from the samples containing flowers, these values are 18 nm and 20 nm for films deposited at 500 and 550°C respectively. As the mean value includes the crystallites size from the film that should be equivalent to previous values in TiO2 films, we can infer that crystallites size contained in the microflowers are rather smaller to 18 nm. The crystallite size values for TiO2/Al-Zr coatings deposited at 500°C and 550°C are resumed in

V.5. UV-VIS-IR transmittance

The optical properties of the Cu-Ti-O films deposited on corning glass at 550 °C were determined by UV-Visible-Near Infrared spectrometry. These measurements were carried out in a Lambda 950 spectrophotometer from Perkin Elmer in the range of 250 nm to 2500 nm working in transmission mode. Figure V-11 a) presents the total transmission spectra obtained for pure TiO2, CuTiO16C and CuTiO75C films using an integrating sphere.

It is seen that for pure TiO2 film as being colorless and transparent, the absorption starts at 390 nm in the UV-region. As the Cu atom is incorporated into the films, the color becomes darker, transmittance decreases and the absorption edge shifts to visible region.

The pure TiO2 thin film is transparent to visible light with an expected band gap energy of 3.2 eV [22] , thus enabling the absorption of only UV light with the wavelengths shorter than 390 nm.

When adding Cu in the film, a striking change is observed in the UV-vis absorption spectra of the Cu-Ti-O films. The absorption band of the Cu-Ti-O films shifted to longer wavelength region in visible light. It can be seen that the steep absorption edges shifted to the visible region obviously as the at.% Cu increases, which indicates a narrowing in the optical band gap. This behavior is correlated to the formation of Cu2O with an indirect gap energy of 2.17 eV [23]. The spectra were employed to calculate the band gap by tracing the Tauc plot using the equation:

where α is the absorption coefficient, hv is the photon energy (eV), A is the proportionality constant, Eg (band gap energy) while n stands for the type of transition, which may be direct or We obtained a gap energy of 3.55 eV for pure TiO2. The optical band gap energy shifted toward a visible regime with increasing Cu content, being of 3.4 eV for CuTiO16C and 2.88 eV for CuTiO75C. This last value is still higher than values reported for Cu2O films, probably to the effect of TiO2 mixing structure. Navas et al. [25] reported greater band gap reduction with increases in copper concentration due to the covalent character of the Cu-O interaction leading to new states at the valence band maximum.

V.7. TEM Analysis

In order to assess the location of Cu2O and TiO2 within the film structure (complete mixing or a two phase compound), CuTiO55S film was prepared in cross section by grating and by the tripod method for TEM observation. The equipment used for TEM imaging was a JEOL JEM 2010 microscope, operating at 200 kV (0.19 nm resolution), provided with an EDS system, INCA Energy TEM 100 X-Max 65T. Cross-section samples were prepared from films deposited on silicon.

The cross section of the sample can be seen in This result indicated there is an optimal value for the copper content and that the presence of a greater content of Cu is detrimental to the photocatalytic ability. It has been reported in the literature that loading with copper on TiO2 particles can lead to about twice higher photocatalytic activity than pure TiO2. Nevertheless most of the references concerned low loading values of Cu in TiO2 nanoparticles [31,32]. Nevertheless, they also found a decreases of photocatalytic activity with Cu content and they proposed two possible reasons for this decrease. Firstly, the active sites on the catalyst are covered with the excessive Cu. Secondly, the charge-recombination rate increases with the increase in the amount of Cu. Furthermore, these phenomena may be related to the formation of recombination center of CuO which accelerate the recombination of electron-hole pairs; because the conduction band of (CuO/Cu2O) is lower than that of TiO2. Hence, the excited electrons in TiO2 will be inclined to transfer to CuO instead of staying in TiO2 to recombine with holes [33]. This last explanation is maybe the most adapted to our case. In this work, the estimated percentage of the sample area covered by biofouling is obtained after using ImageJ image processing program.

V.10.2. Evaluation of microfouling adhesion by microscopy

Microfouling was evaluated after 38 days of immersion on small size coated substrates. Samples were taken out and the observations were conducted by two interdependent microscopic methods: SEM and CLSM.

Regarding the microfouling analysis, four samples for each type of films were observed by SEM on a JEOL 6460LV microscope and eight others by CLSM on a Zeiss LSM 710 microscope using the oil immersion 40× objective.

films. These preliminary studies propose a non-toxic approach to the marine biofouling problem and encourage a further investigation in order to check the reproducibility.

Chapter V: New Cu-Ti-O films for photocatalysis and antibiofouling applications

Conclusions and future works

The rising demands for eco-friendly materials presenting antibiofouling and anticorrosion properties for marine applications present a challenge to develop advanced coatings. Two different coatings, Al-Zr and TiO2, were successfully prepared in order to develop a potential bifunctional Al-Zr/TiO2 bilayer coating combining anticorrosion and photocatalytic properties using high-speed steel (HSS) as substrates.

First, the anticorrosion performance of Al-Zr films was studied. Al-Zr films with different Zr contents were deposited on HSS, glass and silicon substrates by direct current magnetron sputtering. Al-Zr films having Zr content below 10 at. % present well-defined diffraction peaks, which correspond to fcc structure, indicating a supersaturated solid solution of Zr in Al. Al-Zr (4 at.% Zr) was chosen to be used as an underlayer since it presents the best compromise between intrinsic corrosion resistance, sacrificial character/steels and mechanical reinforcement.

Secondly, TiO2 films proposed as photocatalysts were deposited by AACVD using TiO(AcAc)2

as precursor under optimized conditions to develop a microstructure characterized by the presence of TiO2 microflowers physically attached to TiO2 film surface. Formation of this microstructure occurs at a specific thermal window that depends on the precursor concentration; more precisely when working with a 0.03 mol L -1 concentration solution and an aerosol feeding rate of 3 ml min -1 .

The maximum flowers density was achieved for films deposited at 550 °C, presenting an area fraction coverage of about 40 %.

TiO2 with different structural and morphological characteristics were deposited on Al-Zr (4 at.% Zr) as a second layer in order to produce multifunctional Al-Zr/TiO2 bilayer films. X-ray diffraction and Raman spectroscopy show that the TiO2 micro-flowers as well as the TiO2 thin film crystallize

Conclusions and future works

in the anatase phase. This TiO2 phase presents higher charge mobility and consequently a better photocatalytic response.

Photocatalytic performance of bilayer films was evaluated by means of degradation of Orange

G dye under UV illumination. The presence of TiO2 microflowers having a size of about 7 µm and a crystallite size of 20 nm plays a crucial role in photocatalysis. The photoefficiency of TiO2 films, quantified in terms of kinetic rate constant increases with microflowers area fraction coverage. This behavior was clearly noticed for TiO2 samples deposited at 550 °C, showing a higher decomposition rate of Orange G (780 x 10 -10 mol L -1 min -1 ) under UV light as compared to other films obtained at lower deposition temperatures (500 and 540 °C).

TEM analysis revealed that after the AA-MOCVD process performed at 550 °C, Al-Zr (4 at.% Zr) presented precipitates, which are more rich in zirconium than the matrix. The structure formed after deposition at 550°C is clearly biphased; an FCC solid solution of zirconium in aluminum and an ordered metastable Al3Zr compound. Literature shows that these particles are an important grain refiner in commercial structural of Al-based alloys and present a minimal impact on corrosion properties.

Electrochemical tests were carried out in an aerated and stirred chlorine solutions (5 wt.% NaCl).

They revealed that Al-Zr/TiO2 bilayer coatings is an attractive alternative for preserving the sacrificial protection of steel structures in saline environments. These films offer good protection thanks to the preferential oxidation of the bilayer presenting a lower corrosion current density of 4.01 × 10 -7 A/cm 2 . TiO2 as a second layer reduces the electrochemical activity of the films through the formation of a dense passive film and also increases the pitting resistance when the cathodic reaction kinetic is lower.

In order to improve the properties of TiO2 wide band gap semiconductor, the incorporation of transition metals such as Cu was proposed as an efficient method. The influence of incorporation of copper into TiO2 thin films on structural, optical, photocatalytic activity and antibiofouling properties was studied. New Cu-Ti-O films with high copper contents were deposited by AAMOCVD at 550 °C in order to develop an efficient visible-light-active photocatalyst material.

TiO2 and CuxO mixtures reveal great potential due to their low toxicity and their suitable energy band positions.

Cu-Ti-O films were deposited on corning glass, silicon and Al-Zr coated substrates. Cu-Ti-O films are composed of Cu2O, TiO2 anatase and for films with a Cu content higher than 50 at.% the

Conclusions and future works

Additionally, more understanding of coatings surface properties in the immersed condition, rather than in dry is necessary, as immersion changes the dynamics of self-assembly and surface organization. When the coated samples are immersed in the natural marine environment, their properties might change further as a result of surface conditioning by adsorption of macromolecules and microorganisms, which will also change according to the season and geographic location. The anti-fouling behavior of novel Cu-Ti-O and TiO2 films for fouling-release needs to be tested more times in order to understand the adhesion mechanisms of micro and macroorganisms. Interdisciplinary studies combining chemistry, biology and advanced physical techniques for interfacial characterization will be crucial for advancing this research work.

The control of biofouling of surfaces in the marine environment is a challenge: the fouling organisms present the capability to attach and proliferate on the material surfaces in the highly turbulent marine environment. Regarding this aspect, dynamic test system is necessary to check the efficacy of coatings.

Moreover, the HSS/Al-Zr/TiO2 bilayer coatings combining both corrosion and antifouling properties need to be tested in real conditions.

Other antifouling approaches range from biocidal and self-cleaning coatings to various disinfecting techniques. For instance, medical devices use antimicrobial coatings to inhibit biofilm formation. Developing non-toxic foul-release films for medical, marine and industrial applications is the particular appeal of this thesis. Future research could focus on gathering the best properties from each coating and combining them to produce an effective antifouling surface resistant to a variety of foulers.

Finally, regarding the scalability, the development of low cost, non-vacuum CVD-based techniques like aerosol assisted CVD offers the potential to be scaled up for large area or mass production. This can widen the scope of engineering applications of CVD technique. Cu(II) acetylacetonate (Cu(AcAc)2) a été utilisé comme précurseur de cuivre et des solutions de départ avec une concentration de 0,01M et 0,03M ont été préparées en utilisant de l'éthanol comme solvant. Ethylène diamine à 0,01M a été ajouté en tant que dispersant. La solution préparée de Cu(AcAc)2 (0,01M ou 0,03M) est ensuite mélangée à une solution de Ti(AcAc)2 dissoute dans de l'éthanol à une concentration de 0,03M.

La spectroscopie de rayons X à dispersion d'énergie (EDS) a été utilisée afin de quantifier la quantité de Cu présent dans les films (Tableau 1). Les solutions contenant 33% de cuivre produisent

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