Priyanka Ray

Nadia El Kolli

Anais Lehoux

Abdel Karim

El Omar

Mohamed El

Mehdi Zekri

me a lot of help and made my life meaningful in Paris-Sud

LIST OF FIGURES

 4Photochemical processes in a semiconductor followed by deexcitation events. Where A is electron acceptor and D is electron donor. From ref. [39] ............ 18

General Introduction

Photocatalysis with semiconductors has recently emerged as a promising method for environmental decontamination in water and air treatment. Titanium dioxide (TiO 2 ) has proven to be the most promising semiconductor photocatalyst not only because of its excellent photoactivity, but also because of its non-toxicity, chemical stability and low cost. However, the fast charge-carrier recombinations and the wide band-gap of TiO 2 limit its application under solar light. Hence, many efforts have been devoted to solve these problems, such as modification with metal nanoparticles or other semiconductors, doping with transition metals or nitrogen and sulfur… In this thesis work, we modified TiO 2 with gold and/or copper to enhance its photocatalytic activit y.

Semiconductors with intrinsic band-gap suitable for visible light photocatalysis havealso attracted considerable attention in the past decades. Radiolytic synthesis of copper oxides and copper sulfides with special morphologies and their photocatalytic activity were also investigated in this work.

This manuscript contains four chapters:

The first chapter lays the foundation for this thesis, it will outline several themes.

We will make a brief review of various conventional advanced oxidation processes used for wastewater treatment. Basic principles of the photocatalysis processes are described. We will review significant effects on photocatalytic activity of semiconductors. We also will summarize ways to improve the photocatalytic activity.

The main deposition methods for semiconductor surface modification will be introduced. The principles of radiolytic synthesis of metal nanoparticles will be presented. We also will describe the research front of non-TiO 2 visible light photocatalysis, and particularly radiolytic synthesis of copper oxides and copper sulfides.

In the second chapter, we describe the experimental conditions, including materials, synthetic protocols for the modification of TiO 2 and preparation of copper compounds, the characterization techniques and the equipments used for the photocatalytic reactions of different model pollutants. In chapter 4, the radiolytic synthesis of copper oxides and copper sulfides of different morphologies and their photocatalytic activity are presented.

Finally, we present the conclusion of this work with some perspectives.

CHAPTER I FOUNDATION IN

PHOTOCATALYSIS

Chapter 1 Foundation in Photocatalysis

Water Pollution

In recent years, global environmental pollution has become a serious problem that needs to be solved urgently. The on-going industrialization has been bringing about serious pollution to air and water. Water is essential to our existence. The pollution from both the atmosphere and soil will eventually enter water by deposition and dissolution. Thus, water remediation has emerged as a high national and international priority over the past decades.

Hazardous wastes caused a big threat to water. Both advanced industrialized nations and developing countries encounter tremendous environmental problems related to the remediation of hazardous wastes. The hazardous wastes exist in military installations and civilian effluents. For military installations, problems with hazardous wastes are related to the disposal of hazardous chemicals in lagoons, underground storage tanks, and dump sites.

Consequently, the surrounding soil or underlying groundwater aquifers were contaminated with various kinds of hazardous wastes. In the civilian sector, the elimination of toxic and hazardous chemical wastes such as halogenated hydrocarbons from waste effluents and previously contaminated sites has become a major concern. [1] Commonly, general classes of compounds of concern include: volatile organics, chlorinated aromatics, dibenzofuran, dioxins, chlorophenols, chlorinated aliphatic and olefinic compounds, nitrogenous compounds, hydrocarbons, carboxylic acids, alcohols, halocarbons, heteroatom compounds… [2] Besides, aqueous soluble dyes and other aromatic molecules, such as rhodamine B, methyl orange, and phenol etc. are widely used in textile and other industries. These compounds are considerably stable in the environment under natural light.

Conventional methods for elimination of these pollutants are aeration, extraction, high-temperature incineration, filtration, distillation, reverse osmosis, ion exchange, absorption, biological treatment, ozonation, and etc. However, most of the organic compounds are resistant to conventional chemical and biological treatments. The so-called advanced oxidation processes (AOPs) have been reported to be effective for the near ambient CHAPTER I degradation of soluble organic contaminants from waters and soils, because they can contribute to an almost total degradation. [3][4] Of these, advanced oxidation processes (AOPs) will probably constitute the best option in the near future. [5] 1.2 Advanced Oxidation Processes (AOPs)

As defined broadly, AOPs are mainly considered to be the aqueous phase oxidation processes which are based primarily on the oxidizing hydroxyl radical in the mechanism resulting in the destruction of the model pollutants or xenobiotic or contaminant compounds. [5][6] Highly reactive hydroxyl radicals (HO • ) [E 0 (HO • /H 2 O = + 2.8 V NHE )] are traditionally known to be the active species responsible for the decomposition of organic pollutants. [7][8] Owing to their high standard reduction potential of 2.8 V vs NHE (Normal standard hydrogen electrode) in acidic media, [9] these radicals are able to oxidize almost all organic compounds to carbon dioxide (CO 2 ) and water. [10] AOPs show considerable similarities because of the participation of hydroxyl radicals in most of the mechanisms that are operative during the reaction. Hydroxyl radicals are extremely unstable because of their high reactivity. Considering that the hydroxyl radicals are unstable and reactive, they must be produced continuously in-situ through chemical or photochemical reactions.

Generally, AOPs use photolysis (UV), ozone (O 3 ), hydrogen peroxide (H 2 O 2 ), ozone in combination with UV (O 3 /UV), hydrogen peroxide and UV light (H 2 O 2 /UV), ozone plus hydrogen peroxide (O 3 /H 2 O 2 ), Fenton's reagent (see page 11) and photocatalysis (which is based on interaction of light with semiconductors). [5,[11][12] Compared to the AOPs based on O 3 / U V, H 2 O 2 / U V, or on photo-Fenton processes (techniques which have been practically employed to carry out the mineralization of organic compounds), semiconductor-based photocatalysis has also shown a promising potential in organic pollutant degradation in recent years. The various AOPs are described in detail in the following paragraph.

Photolysis (UV)

This process is based on excitation of organic molecules by UV irradiation. [5,13] Direct photolysis involves the interaction of light with molecules-in addition to water-to bring about their dissociation into fragments. It is reported that photolysis can be more effective when UV irradiation is combined with ozone, hydrogen peroxide, homogeneous, heterogeneous catalysis or photocatalysis. [11,14]

O 3

In the O 3 AOP, two possible ways of oxidizing action may be taken into account: the direct way owing to the direct reaction between O 3 and the dissolved molecules, [15] and the radical way (indirect way) because of the reaction triggered by the radicals generated in the ozone decomposition (hydroxyl radicals). [5] Compared to other oxidizing reagents, O 3 is more efficient in organic pollutant degradation. In addition, it is not harmful for most of the organisms. To our knowledge, ozonation has been widely used for drinking water disinfection. [16] However, the limitation of its application in water treatment is due to its high energy consumption. [11]

O 3 /UV

Photolytic ozonation (O 3 /UV process) is an effective means for the oxidation and destruction of organic pollutants in aqueous solution. [17][18][19] It has a crucial potential for wastewater treatment. In principle, aqueous systems saturated with O 3 are irradiated with UV light at 253.7 nm. In this case, the extinction coefficient of O 3 at 253.7 nm is 3300 M -1 cm -1 , which is much higher than that of hydrogen peroxide. [11] O 3 /UV process generates HO• radicals by the light-induced homolysis O 3 and the subsequent by the interaction of O ( 1 D) with water. [20][21] The global reaction is shown in equation 1.1:

310nm 32 2 O H O 2HO O • +  → + hν< (1.1)
O 3 /UV is more effective for the decomposition of some organic compounds than either UV or O 3 alone. [11] In this O 3 /UV process, it is worth noting a high synergetic effect between O 3 and UV photolysis.

H O Fe

Fe HO OH

+ +•- +→++ (1.6)
The mechanism for the ferric ion in decomposition of H 2 O 2 in acidic solution to release hydroxyl radicals has been intensively discussed in former studies. [25] 1.

Photocatalysis

Photocatalysis has proven to be a promising technology for degrading aromatic compounds. [26][27][28][29] Compared to the other conventional AOPs discussed above, photocatalysis may be more effective and more economicas the involved semiconductor can continuously catalyze the decomposition of organic pollutants under irradiation. The energy is transformed directly from solar to chemical without consumption of oxidizing reagents.

Electron-hole pairs are produced in the semiconductors under irradiation (in most cases UV light). These charged carriers can recombine or migrate to the surface (when they escape from

Photocatalysis for decomposition of organic dyes

Studies on photocatalysis by TiO 2 have been reported since the start of 20 th century. In 1938, Doodeve and Kitchener reported that photodegradation of dyes by TiO 2 occurred both in vacuum and under oxygen. But at that time, this phenomenon was identified as a mechanism of photo-bleaching. [31] It was also found that TiO 2 itself did not change through the photo-bleaching reaction. However the terminology of "photocatalyst" was not used for TiO 2 but it was called a photosensitizer.

After the report of photocatalytic water splitting by Fujishima, [32] there were some studies on other applications of the photocatalytic activity of TiO 2 in decomposition of organic pollutants such as organic dyes . Frank and Bard reported for the first time the decomposition of cyanide in the presence of aqueous TiO 2 suspension. [33] A mechanism was given for this photodecomposition process based on the known procedure on TiO 2 electrodes.

The electrons in the conduction band reduce oxygen while the holes in the valence band cause the oxidation of CN -:

VB 2 CN 2OH 2 CNO H O h - -+ - + +→ + (1.8)
And the further oxidation of CNO -is possible. Although the quantum efficiencies here were lower than those found for a single crystal TiO 2 electrode coupled to a platinum cathode in a photoelectrochemical cell, their are still respectable.

In a photodecomposition system which includes the photocatalyst and the organic molecules, photo-induced molecular transformations or decompositions occur at the surface of the catalyst. Depending on where the initial excitation takes place, photocatalysis can be generally divided into two classes of processes:

1) Catalyzed photoreaction. The initial photoexcitation occurs in an adsorbate molecule which then interacts with the ground state catalyst substrate.

2) Sensitized photoreaction. The initial photoexcitation takes places in the catalyst substrate and the photo-excited catalyst then transfer an electron or energy into a ground state molecule, the process is called a sensitized photoreaction.

The initial photoexcitation of the system is followed by subsequent electron transfer and (or) energy transfer. In consequence, chemical reactions in the photocatalysis processes are attributed to the de-excitation processes by electron transfer or energy transfer. To show more details, the two half reactions at the anode and the cathode were featured and H + . The excited electron moved to the platinum particle because the Fermi energy level of platinum is lower than that of conduction band of TiO 2 . Finally, hydrogen was generated from the H + reduction by the excited electrons on the surface of the platinum electrode.

Rising of Photocatalysis

It must be figured out that there are two indispensable requirements for the thermodynamically favorable water-splitting reaction; the first one is that the conduction band should have higher energy than the hydrogen-evolution potential and the second one is that the valence band should be lower than the oxygen-evolution potential.

By UV irradiation of TiO 2 , it is then possible to decompose water without employing any external voltage, according to the following equations: When semiconductor photocatalysts are used in organic pollutant degradation, the redox potential of the generated holes should be sufficiently positive to produce hydroxyl radicals.

Furthermore, the redox potential of the electrons must be negative to be able to reduce adsorbed O 2 to superoxide (O 2-. ). [36][37] The hydroxyl radical can be generated by the According to the discussion above, these two half reactions in photocatalysis could be used to solve not only the problem of energy shortage, but also to treat the serious environmental pollution. versus the NHE are also indicated with dashed red lines. From the scheme, we can see that for the water-splitting reaction to take place without external potential application, the band-gap should straddle these redox potentials, that means that the CB should have higher energy (more negative potential) than the hydrogen evolution potential while the VB should be lower than the oxygen evolution potential.

Although the band-gap energy is a significant property for photocatalysis. the ability of a photocatalyst to absorb visible and/or UV light is the crucial factor, but other properties have also to be considered such as: the photocatalyst should be also i) chemically and biologically inert and photostabe, ii) nontoxic, and iii) inexpensive. The semiconductors TiO 2 , ZnO, CeO 2 , SrTiO 3 , WO 3 , Fe 2 O 3 , Bi 2 S 3 , GaN, CdS and ZnS can all act as photoactive materials for redox/charge-transfer processes owing to their electronic structures which are characterized by a filled valence band and an empty conduction band. Among these semiconductors, TiO 2 is the most widely used photocatalytic material because it fulfills all of the above requirements as well as exhibiting adequate conversion values. [36,38] TiO 2 has many advantages such as it has high photon harvesting performance in UV light region; it widely exists on the earth and exhibits an extremely chemical stability; it is biologically inert and non-toxic; moreover, it is cheap and easy to synthesize. In this thesis, one part of the work is surface modification of TiO 2 to enhance its photocatalytic activity under UV and visible illumination.

1.5 Basic Principles in the Photocatalysis Process Where A is electron acceptor and D is electron donor. From ref. [39] 1.

Characteristic Times

The general mechanism for the heterogeneous photocatalysis on TiO 2 on the basis of laser flash photolysis measurements has been proposed by Hoffman et al. [1] A succession of photochemical processes takes place in the volume and on the surface of the solid. Figure 1.5

shows the scheme of characteristic times in the photocatalysis processes, where the photoreactions are initiated from the charge-carrier generation. The absorption of a photon occurs very rapidly, on the order of 10 -15 s. [39] Then, the charge-carrier trapping occurs, including electrons trapping and holes trapping.

The >TiOH in the equations represents the primary hydrated surface functionality of TiO 2 . [1,40] {>Ti 4+ OH • } is the surface-trapped VB hole (hydroxyl radical), and {>Ti 3+ OH} is the surface-trapped CB electron. The eq. 1.15 represents a dynamic equilibrium indicating a reversible trapping of CB electrons in a shallow trap below the CB edge. So, there is a finite probability that e -could transfer back into the CB at room temperature.

From this characteristic times scheme, we could clearly conclude that the two routes to enhance the photocatalytic activity are: i) increasing of charge-carrier yields, ii) trapping the charge-carrier to postpone their recombination. The efficiency of the photocatalytic process is measured by a quantum yield which is defined as the number of events occurring per absorbed photon. In the case of a photodecomposition reaction, the quantum yield (Φ) is defined as the ratio of the amount of molecules decomposed over that of photons absorbed. It is very difficult to measure the actual absorbed light in heterogeneous systems because of the scattering of light by the semiconductor surfaces. However, it is generally assumed that all the light is absorbed and the efficiency is quoted as an apparent quantum yield. Sometimes the QE is measured as the yield of a particular product in spite of several products produced in the photocatalytic reactions.

The QE for an ideal system follows the simple relationship below:

CT CT R = Φ + k kk (eq. 1.21)
In this equation 1.21, k CT represents the rate of charge transfer, and k R refers to the rate of the electron-hole recombination.

Recombination Effects

Apparently, a semiconductor with a lower band-gap is able to absorb more photons in the visible region, which would lead to a better photocatalytic activity under sunlight. In fact, there are many semiconductors with low band-gap energy in the visible region, such as GaAs (E g is around 1.4 eV) shown in Figure 1.3. However these semiconductors are not efficient in photocatalysis. One of the main reasons for their poor photocatalytic performance is the high recombination rate of their generated electron-hole pairs. The recombination effect could scavenge the electron-hole pairs and thus lower the possibility of electron-hole transfers from the semiconductor to the adsorbed molecules. This effect dominates in many of the semiconductors with small band-gap energies over electron-hole transfer making these semiconductors inefficient.

Obviously, electron-hole recombination is detrimental to the efficiency of a semiconductor photocatalyst. Therefore, modifications of semiconductor surface such as deposition of metal, doping, or combinations with other semiconductors can be beneficial in decreasing the electron-hole recombination rate and thereby increasing the quantum efficiency of the photocatalysts.

1.6 Ways to enhance the photocatalytic activity 1.6.1 Quantum size effects

The size of the semiconductor particles can have an effect on the band-edge positions;

these are known as quantum size effects. Quantum size effects occur for semiconductor particles on the order of 10-100 Å in size. The anomalies arise when the size of the semiconductor particles become comparable to the de Broglie wavelength of the charge-carriers in the semiconductor. These effects could increase the band-gap of the semiconductors as the nanoparticle size gets smaller and smaller. The reason for the increased band-gap with decreasing size can be ascribed to the confinement of electron and hole in a potential well of small geometrical dimensions. The confinement produces a quantization of discrete electronic states and thus increases the effective band gap of the semiconductor. [39,[42][43] These changes on the band-edge positions of the valence band and the conduction band influence the photocatalytic properties of the semiconductor, providing an interesting yet challenging aspect to the design of novel photocatalysts.

Metal Semiconductor Modification

Semiconductors can be modified by deposition of metal particles on their surface. There are different mechanisms proposed to explain the enhancement caused by modification with plasmonic metal NPs under visible illumination. Plasmonic metal nanostructures are characterized by their strong interaction with resonant photons through an excitation of surface plasmon resonance (SPR). SPR can be described as the resonant photo-induced collective oscillation of surface electrons, established when the frequency of photons match the intrinsic frequency of surface electrons oscillating against the restoring force of positive nuclei. [63] The first view, most commonly, is that incident photons are absorbed by metal NPs injecting electrons into the conduction band of TiO 2 . [64][65][66][67][68][69] The metallic plasmonic NPs (mainly Au, Ag, Cu which have a plasmon resonance in the visible range) essentially act like a dye sensitizer, absorbing resonant photons and transferring the excited electron to the nearby semiconductor particle (Figure 1.8). Besides, the electron transfer implies that plasmon activation of metal particles on semiconductor particles by visible light generates positive charges on the metal particles and conduction-band electrons on the semiconductor particles. The oxidation and reduction reactions occur simultaneously as with conventional photocatalysts activated by UV-light. [39,70] Because of the excellent mobility of charge-carriers and high absorption cross-sections, which is up to 10 5 larger than the cross-section of typical dye sensitizer molecules, plasmonic nanostructure of noble metals represent very promising sensitizers. [71] Another advantage for the noble metal NPs is that they are very stable under light compared to dye sensitizers. Besides the charge injection mechanism, near-field electromagnetic mechanism was also proposed to interpret SPR-induced enhancements in the photocatalytic activity of semiconductors on systems where the semiconductor and the plasmonic metal were separated from each other by thin, non-conductive spacers preventing any direct charge exchange between the two building blocks. [76][77][78] This mechanism is based on the interaction of the semiconductor with the strong SPR-induced electric fields localized nearby at the metallic nanostructure. The electric fields close to plasmonic nanostructures (a few nanometers) are orders of magnitude higher than the field of incident photons. Photo-excited plasmonic nanostructures essentially play a role of concentrators that can amplify the local light intensity.

As the rate of electron-hole formation in a semiconductor is proportional to the local intensity of the electric field (more specifically |E| 2 ), [79][80] the electron-hole generation in some regions of the semiconductor increases by a few orders of magnitude (even grow out of nothing when the light energy is not sufficient to excite the semiconductor). Most recently, TiO 2 -graphene composite was found to be a promising photocatalyst under visible illumination. Some publications are related to this novel titania composite. [89][90][91][92][93] Such composite possesses a great adsorptivity of dyes, extended photo-responding range (by the formation of a chemical Ti-O-C bond), and enhanced charge separation and transportation properties simultaneously. The enhancement in photocatalytic activity was ascribed to the giant two-dimensional planar structure of graphene, which served as a better platform for adsorption of dyes and charge transportation and separation. This provides a new possibility in addressing various environmental issues and energy crisis.

1.6.4 Doping B [108], P [109], F [110], and Cl [111], where the shift of the absorption threshold of TiO 2 into the visible range and the photocatalytic activity under visible illumination were reported.

The mechanism of photoactivity under visible light for the non-metal doped semiconductors is still in debate. [112][113] One school proposed that corresponding increases in photoactivity of TiO 2 were owing to a narrowing of the intrinsic band-gap, while others contended that the band gap of TiO 2 was not narrowed but rather that the ensuing oxygen vacancies formed upon doping. However, doping with non-metal elements normally requires harsh preparation conditions and the degree of doping is difficult to determine with chemical analyses.

Surface Sensitization

Surface sensitization of a wide band-gap semiconductor can be achieved by chemisorbing or physisorbing photosensitive dyes. The photo-sensitization process can also expand the wavelength range of excitation for the semiconductor. Electrons are excited from the sensitizer and then transferred to the semiconductor. [39,114] Grätzel showed that TiO 2 could be photo-sensitized with a dye in the presence of a redox couple to produce electrical power; this has since become known as the Grätzel solar cell. Meanwhile, the electron in the CB of the semiconductor can go back to the organic dye (electronic recombination, Figure 1.12b, pathway 3) or also undergo de-excitation to the semiconductor VB (Figure 1.12b, pathway 4). This fact is drawback to the photocatalysis process and must be avoided to improve the photocatalytic activity. This can be achieved by modifying the surface of semiconductor, which can lead to significant improvements in adsorbing kinetics of dyes or preventing electronic recombination by forming a high energy barrier on the return route of electron to the dye. [START_REF] Kay | [END_REF] 1

.7 Deposition methods

The parameters such as particle size, dispersion and loading play an important role on the photocatalytic activity of the modified semiconductor photocatalysts. All these parameters are strongly dependent on the preparation procedure. In general, there are three deposition methods: colloid deposition, photo-deposition, and deposition precipitation method. The details, advantages and disadvantages of these procedures are demonstrated in the following paragraphs.

Colloid Deposition

Colloid deposition method is the simplest method, consisting in preparation of metal colloidal solution followed by adsorption of the metal NPs on the substrate surface. For this deposition procedure, there are different methods to prepare metal colloidal solution in water or other solvents. For instance, the most popular methods are reduction of AuCl 4 -with citrate in aqueous phase (the Turkevich method) [125] or the two-phase protocol consisting of reduction of AuCl 4 -by NaBH 4 in water and extraction into toluene of the Au NPs by means of a phase transfer agent (the Brust method). [126] Gold NPs of 5 nm with narrow size distribution can be obtained. Besides, an alternative is reduction of Au III ions by tetrakis (hydroxymethyl) phosphonium chloride (THPC), giving a hydrosol of gold NPs with average diameter 1~2 nm. [127] THPC acts simultaneously as reducing and stabilizing agent.

Moreover, a characteristic method induced by radiolytic reduction was used to synthesize metal colloidal suspensions in the presence of surfactants or polymer stabilizing agents. [128][129] Some strong reducing species such as aqueous electrons, hydrogen radicals generated in the radiolysis process are able to reduce metal ions to lower valences and finally to metal atoms. The radiolytic synthesis will be discussed in detail in the following section.

These prepared metal colloids can be consequently used as precursors for adsorption on a solid surface. However, it has been often observed that in the adsorption stage the prepared metal colloidal particles undergo coalescence and agglomeration forming metal islands of particle size above 50 nm. This would result in inactivation of the photocatalyst due to the weak interaction with the substrate or the small surface/volume ratio of metal NPs.

[69]

Photo-Deposition

Besides deposition of prepared metal colloids, photo-deposition has been widely used to prepare titania supported metal NPs. The method of photo-deposition is valid when the support is a semiconductor such as titania. Electrons and holes are able to be generated when the semiconductor is irradiated with light of sufficient energy. The photo-generated electrons are so reductive that they can reduce metal ions into metal NPs that will be simultaneously deposited on the semiconductor surface. Normally, hole scavengers such as an alcohol have to be added to remove the oxidative radicals. This scheme based on generation of Au NPs on the in the case of photo-deposition of gold on TiO 2 . [130] In contrast, Hidalgo et al. reported photo-deposition using irradiation at high light intensity appeared as an ineffective method to obtain Au-TiO 2 catalysts for photocatalytic applications because their conditions lead to very large and heterogeneously distributed gold deposits. [131] This investigation has an identical conclusion to earlier results that deposited gold NPs in smaller size and homogeneous distribution result in an improved photocatalytic activity under UV-vis irradiation.

Deposition-Precipitation (DP)

The most widely used method for preparing Au-TiO 2 catalysts is deposition-precipitation method. [131][132] For many catalytic reactions, supported gold catalysts are more active when metal gold particles are smaller than 5 nm. [START_REF] Bond | Catalysis by Gold[END_REF] The gold catalysts method of deposition precipitation with NaOH is developed by Haruta et al. [START_REF] Haruta | [END_REF][135][136] The DP NaOH method was selected for the production of reference gold catalysts (World Gold Council, www.gold.org). In the DP method, the metal precursors are mixed with the supports forming an aqueous suspension, subsequently precipitated as a hydroxide by increasing the pH value.

The key point of the DP method is the prevention of precipitation away from the surface of the supports. The DP method is very simple and convenient and leads to homogeneous samples with reproducible properties and very small metal nanoparticle size (less than 5 nm). the main species in solution is AuCl 3 (OH) -at room temperature and remains the same up to 150 °C. [138][139] When the DP time increases and therefore the pH value reaches PZC TiO2 or much higher (7 or 8 instead of 2), the gold speciation in solution is different, probably AuCl 2 (OH) -at 80 °C. The mechanism of adsorption of gold hydroxy chlorides is not an electrostatic interaction but the formation of a surface complex by reaction with surface OH on TiO 2 . [140] As in eq. 1.22, the gold complexes can form Ti-O-AuCl 2 :

2 2 22 TiOH+AuCl (OH) TiOAuCl H O OH - - ↔ ++ (1.22)
This adsorption mechanism can interpret the DP NaOH results: (i) the amount of deposited gold reaches a maximum at pH 6, when the surface charge is neutral; (ii) Au is bound to oxygen atoms of the TiO 2 surface (Au-O-Ti) (no gold hydroxide are formed). [135] CHAPTER I

After the deposition takes place, the samples are reduced to obtain gold NPs on the TiO 2 surface. Various routes are used for the reduction, such as thermal treatment under hydrogen gas and radiolytic reduction in aqueous suspension.

There are some disadvantages of DP with NaOH, such as the maximum metal loading is low (≤3 wt.%) when pH is in the range 7-10, deposition yield is less than 100% and inapplicability of some supports with a point of zero charge below 5 [137,140]. For this

CO(NH ) 3H O 2NH CO 2OH

+-

+ → ++ (1.23)
This gradual addition of hydroxyl ions avoids local increase in pH and precipitating of metal hydroxide in solution. Compared to DP NaOH procedure, DP urea method can result in a complete deposition of Au on the TiO 2 substrate. The depositon-precipitation mechanism could first involve an initial electrostatic interaction between anionic gold species AuCl 4 -or AuCl 3 (OH) -and the positively charged TiO 2 surface at acidic pH, then the growth of the particles of gold precipitate on these sites, which act as nuclei. When the pH increases in the DP urea procedure, the surface charge intensity of the gold precipitate clusters is modified, leading to a fragmentation, and then to a decrease in the gold particle size with increasing deposition time. [137] We can conclude that the DP urea method, as well as the DP NaOH, is a suitable method for the preparation of gold catalysts on various oxide substrates with point of zero charge in the range of 6-8 (TiO 2 , Al 2 O 3 , CeO 2 ) but not for silica (PZC ~2).

Radiolytic Synthesis

Various physical and chemical routes have been used to prepare metal clusters or nanoparticles. In an aqueous phase, the method of preparation of metal NPs involves in general reduction of metal precursor salt by a suitable reducing agent (such as sodium borohydride, ascorbic acid, formaldehyde and hydrazine). Besides, the reduction of metal precursor salt may also be carried out using UV-irradiation, microwave radiation and ionizing radiations like γ, X-rays or electron (or ion) beams. Among these methods, radiolytic synthesis in solution exhibits the advantage of simple physicochemical conditions (such as room temperature, atmospheric pressure, absence of contaminants) and leads to homogeneous reduction and nucleation in solutions and heterogeneous media. [START_REF] Wishart | Recent Trends in Radiation Chemistry[END_REF] When aqueous solutions are irradiated by high energy radiation (γ-rays, electron beams, X-rays) excitation and ionization of water takes place as following: [START_REF] Belloni | [END_REF]

ray 2 aq 3 2 2 2 H O e , H O , H ,OH , H , H O γ - - +• •  → (1.24)
During the irradiation of deoxygenated water, many active intermediates are produced, including strong reductive hydrated electrons (e aq -) and hydrogen atoms (H

) = 0.26, G(H • ) = 0.06 μmol J - 1 .
[ [145][146] These radicals can reduce dissolved metal ions to the zero valence state. In the case of a free or complexed monovalent metal ion M + , the reduction proceeds through:

0 aq Me M +- +→ (1.25) 0 M H MH + • + + → + (1.26)
For the multi-valent metal ions, they are firstly reduced to lower valences and finally to zero-valent state according to a multi-step reaction mechanism involving intermediate valencies (by disproportionation for example). These redox reactions in the case of Au III are demonstrated in detail in chapter 3.3.

In the meantime, strong oxidative hydroxyl radicals

• OH [ E 0 ( • OH /H 2 O) = + 1.9 V NHE at pH 7, G( • OH) = 0.27μmol J -1
] are also generated during the irradiation of deoxygenated water. [145] In this case, hydroxyl scavengers are added to the aqueous solutions to remove oxidative species, and thus obtain a reductive environment (eq. 1.27). Primary and secondary alcohols (such as 2-propanol) or formate are chosen as scavengers, which also react with hydrogen atoms to form reducing radicals (eq. 1.28). [START_REF] Belloni | [END_REF]147]:

9 -1 -1 3 3 3 3 22 CH CHOHCH OH CH C OHCH H O = 1.9 10 L mol s k •• +→ + × (1.27) 7 -1 -1 3 3 3 3 23 CH CHOHCH H CH C OHCH H = 7.4 10 L mol s k •• +→ + × (1.28) 22 HCOO HO (or H ) COO H O (or H ) - • • •- + →+ (1.29)
Due to their redox potentials [E 0 ((CH 3 ) 2 CO/(CH 3 ) 2 C • OH) = -1.8 V NHE at pH 7, and

E 0 (CO 2 /COO •-) = -1.9 V NHE ],

M (CH ) C OH M (CH ) CO H

+• + + →+ + (1.30) 0 2 M COO M CO + •- + →+ (1.31)
Owing to the quasi-uniform energy distribution in the irradiated medium, the metal ions are homogeneously reduced leading to a homogeneous nucleation throughout the solution.

Considering the binding energy between two metal atoms is stronger than the atom-solvent or atom-ligand bond energy, the atoms dimerize when they encounter. Then, by a cascade of coalescence processes, M 2 progressively coalesce to growing clusters (Figure 1.15):

00 2 MM M +→ (1.32) MM M +→ mp n
(1.33) The atoms or clusters also associate with unreduced ions and these association processes are fast. Finally, according to a multi-step coalescence process, metal clusters are formed with a remarkable monodispersity:

0 2 MM M + + + → (1.34) 22 2 4 MM M ++ + +→ (1.35) MM M ++ +→ m m+1 (1.36) MM M +→ x+ y+ z+ m+x p+y n+z (1.37) aq MeM +-+→ n+1 n+1 (1.38)
where m, n and p represent the cluster nuclearities (number of reduced atoms), and x, y and z refer to the number of associated ions. [START_REF] Belloni | [END_REF][145]148]

Determination of Radiolytic Yield of Metal Species Reduction

In the experiments, we have to calculate the dose (D) required to totally reduce metallic salts with a certain concentration C 0 . Thus, for a solution containing low metal salt concentrations, the maximum reduction yield for an aqueous solution containing oxidant radical scavengers like 2-propanol is:

aq 1 red e H OH G max G G G 0.6 mol J m -• • - = ++ ≈ ( ) (1.39)
Therefore, in the case of a metallic salt solution with a concentration (C 0 ) and oxidation state (Z), the minimum dose (D min ) for the complete reduction can be presented as follow:

1 0 min 1 red Z C (mol L ) D G (max) (mol L ) - - ⋅ = (1.40)
The absorbance of plasmonic metals increases with the dose of irradiation. According to Beer-Lambert law: A = εlc, the concentration (C) of ions reduced by a dose of irradiation (D) can be deduced from the absorbance (A).

The plateau (A max ) towards which the curve bends at high dose is due to the total reduction of the metal ions to atoms (Figure 1.16). Supposing that the final concentration of the metal atoms is equal to the initial concentration (C 0 ) of the metal ions, the molar extinction coefficient can be deduced from the A max :

max 0 A C l ε = ⋅ (1.41)
where l is the optical path length.

Assuming the molar extinction coefficient (ε in L mol -1 cm -1 ) of the metal at a certain wavelength is a constant whatever the aggregation state and the size (what is in fact not really exact), we can deduce the radiolytic reduction yield (linear part of the curve A = f(D),

Figure 1.16): Z () G () A lD ε D = × ⋅D 1.42
The curve, C = f(D) can be deduced from the absorbance spectra (if ε does not change with the size of the aggregates) recorded at different doses. Radiolysis has been used successfully to synthesize various noble (such as silver, gold and platinum) and non-noble (nickel, copper and iron) metal nanoparticles in aqueous solution (and in other solvents such as alcohols) and in heterogeneous media (on supports, in polymeric membranes, in mesophases…). [147][148][149][150] The nanosized metal clusters or particles are readily oxidized due to their relatively low redox potential compared to that of the bulk counterparts. For this reason, the deoxygenation (by purging with an inert gas) of the solutions prior to irradiation must be performed to prevent the possible oxidation. Moreover, since water radiolysis leads to the formation of protons in addition to that of hydrated electrons, radio-induced acidification of the solution may lead to non-noble metal cluster corrosion. Therefore, preparing a slight basic solution of a non-noble metal salt is able to avoid the back oxidation by protons.

Effect of dose rate

The dose rate effect on the mean size of the radiolytic synthesized nanoparticles has been widely studied. [148,[151][152][153][154] Increasing the dose rate (the deposited energy into the irradiated medium, expressed in Grays (1 Gy = 1 J kg -1 ) leads to more reduced metal ions in the same time and then to more seeds. As a consequence, the metal particles become smaller and less dispersed in size at a high dose rate. In contrast, in the steady irradiation regime (at lower dose rates), the association of M + ions with reduced atoms and the subsequent coalescence processes are faster than the generation rate of the reducing species. Therefore, the reduction of M + ions occurs probably in-situ on the charged clusters M n+1 + already formed.

As a result, because of the continuous growth of the seed (development) larger particles are obtained by radiolysis at low dose rate.

For the bimetallic (or multi-metallic) systems, the dose rate can have an effect on the structure of the multi-component metal particles. Different bimetallic systems were studied, such as Ag-Au, Ag-Pd, Au-Pd, Au-Pt nanoparticles. Alloyed or core-shell nanoparticles can be obtained depending on the dose rate. For example in case of the Au/Ag system, different structures can be obtained depending on the dose rate and on the metal precursors. [155] When an aqueous solution containing Au III (AuCl 4 -

) and Ag + is γ-irradiated at a low dose rate of 3.8 kGy h -1 , the evolution of the optical absorption spectra with increasing γirradiation doses exhibits a maximum which is shifting from 520 nm to 430 nm with increasing the initial ratio

[Ag + ]/[Au III ]
. These two maxima correspond to the plasmon resonance absorption bands of gold and silver respectively. Meanwhile, the colloidal solution turns from pink to yellow. This evidences that gold NPs are obtained at the beginning with a lower dose, whereas Au core -Ag shell NPs are formed at higher dose. The structure of the nanoparticles has been confirmed by EDS. In the case of low dose rate, the reduction process is controlled by the redox potential of each metal complex. The reduction of gold ions is thus more efficient than that of silver. Silver ions are initially reduced however the silver atoms will transfer electrons to gold ions as long as gold ions are present in solution. Thus, the silver reduction occurs only when gold ions are completely reduced. In particular, the silver ions are adsorbed at the surface of gold clusters, leading to Au core -Ag shell nanoparticles.

In the case of irradiation of the same gold and silver precursor solution at a higher dose rate of 35 kGy h -1 , the absorption spectra display a constant maximum at 480 nm, which is an intermediate value between the plasmon bands of gold and silver. Furthermore, the absorption peak does not shift with the dose while its intensity progressively increases showing that the composition and structure of the NPs is identical from the beginning to the end of irradiation.

EDS analysis of the nanoparticles at different irradiation times has confirmed that they were formed by silver and gold, and that the composition is the same along the diameter. At such a higher dose rate, the radio-induced reduction of metal ions is faster than the inter-metallic electron transfer leading to alloyed Au-Ag nanoparticles.

The structure of multi-metallic NPs can also be affected by the constitution of mixed metal salts. For example, alloyed Cu 3 Pd, CuPd, NiPt, CuAu and AgPt clusters or particles could be obtained even at low dose rate (γ-irradiation with moderate dose rate, less than 30 kGy h -1 ). [156] By contrast, alloying may require a higher dose rate (35 kGy h -1 ) for Ag-Au and Ag-Pd bimetallic systems. [155] 1.8.3 Effect of stabilizing agent

In order to limit the size of the nanoparticles and to avoid cluster coalescence into large particles, stabilizing agents are added to the solutions prior to irradiation. Polymers

[poly(vinyl)alcohol PVA, poly acrylic acid PAA, polyacrylate PA, poly(acrylamide) PAM, poly(vinyl)pyrolydone PVA etc.] or surfactants (sodium dodecylsulphate SDS, cetyl-trimethylammonium bromide CTAB etc.) were often used. Functional groups with high affinity for the metal ensure the anchoring of the molecule at the cluster surface while the polymeric chain protects the cluster from coalescence. Moreover, the stabilizing agents should not chemically reduce the ions adsorbed on the clusters so as to prevent their growth. In these conditions, small metal clusters or particles were successfully stabilized. [157][158][159] Radiolysis has been proven to exhibit significant advantages compared to chemical or electrochemical methods:

The radiation is distributed homogeneously at all points in the solvent and thus induces an initial dispersion of the reducing agents.

Radiolytic syntheses take place at room temperature and atmospheric pressure.

This process does not require the addition of a chemical reducing agent, as it is the solvent which provides the reductants.

e sol -is a very strong reducing species. Therefore, metals such as Fe, Ni or cobalt which are difficult to reduce by chemical methods can be reduced easily by radiolysis.

The great penetration power of the ionising radiation allows reductions in solution, in heterogeneous media and in porous matices. The reduction kinetics can be monitored by controlling the dose rate. Therefore the size or shape of nanoparticles, and the composition and structure of bimetallic nanoparticles can be easily controlled by changing the dose rate. prepared spindle-like nanocrystalline CuS by γ-irradiation in an ethanol system. They did not report the photocatalytic properties of the synthesized CuS. [169] Therefore, more research is needed in this area.

Summary

In this chapter, a brief introduction is given on potential applications of photocatalysis in water treatment, the fundamental photocatalytic mechanism and routes of photoactivity improvement. Various deposition methods and the principle of radiolytic synthesis of metal nanoparticles are also discussed. In addition, non-TiO 2 visible light photocatalysis (in particular with copper oxides and sulfides) is described and the potential application of the semiconductor in photocatalysis under visible light is discussed.

Aim of this research and its significance

The aim of this thesis work is to improve the photocatalytic efficiency of TiO

Chapter 2 Experimental

This chapter firstly presents the materials used in the syntheses.Then, the protocols for syntheses are introduced. We used radiolysis and chemical reduction (reduction with THPC)

to synthesize metal nanoparticles (NPs) (Au, Au and Au-Cu) in solution. Deposition precipitation method with urea was also employed to deposit metal ions on TiO 2 , followed by radiolytic reduction. In chapter 3, we will compare the various preparation methods and then we will present the catalytic activity of modified TiO 2 . Radiolytic synthesis of copper sulfides and copper oxides is also decribed. Characterization techniques associated with this research are presented in the third section of this chapter, and the principles of the equipment used are illustrated in the appendix. Finally, the photocatalytic tests of different model pollutants (rhodamine B, phenol and methyl orange) are described, and the photocatalytic reactors or apparatus are shown in detail in appendix.

Materials

In the experiments of colloid deposition method by radiolytic or chemical route, all the reagents were used as received without further purification unless otherwise indicated. After γ-irradiation, colloids with a purple or violet color were obtained. They were deposited in a second step on TiO 2 . In the preparation of Au-Cu colloids with another stabilizer, such as PVA, the concentration of each stabilizer was kept the same (0.1 M).

After the Au-Cu bimetallic colloids were prepared, the deposition on TiO 2 was performed. The adequate volume of the prepared colloid was put in contact with TiO 2 . The required mass of TiO 2 depends on the metal NPs loading. Here we assumed that the Au ions were reduced completely into Au atoms. In the case of Au-Cu-P25 1%, (the percentages indicated in the thesis manuscript are the mass ratio: metal NPs/TiO 2 ) the weight of P25 was 100 times higher than that of Au NPs in the solution. After that, P25 was added to the Au-Cu colloids. The purple color of the mixed slurry varied to a certain extent depending on the total mass of Au-Cu bimetallic NPs. The mixtures were stirred (magnetic stirring) for 24 hour in the dark to ensure complete deposition. And then, the slurry was centrifuged to remove the supernatant at a speed of 4000 rpm. The supernatant was analized by UV-visible spectroscopy to check the efficiency of the deposition. After that, the purple precipitate was dried in an oven at 60 °C.

Colloid-Deposition Method by Radiolysis

The preparation of Au-Cu bimetallic NPs modified P25 was done in two steps (see However, one significant difference is that the total nominal metal content was kept at 0.5% in weight for all the modified P25 samples. In the case of monometallic Au supported on P25, the nominal metal content of Au is 0.5 % in weight. While for the preparation of bimetallic Au-Cu modified P25 with an Au/Cu molar ratio of 1:1, the nominal Au and Cu percentages are 0.38 % and 0.12 %, respectively. The preparation procedure is described below. 1.90 mL of HAuCl 4 (10 -2 M) and 1.90 mL of CuCl 2 (10 -2 M) were added into a flask of 25 mL, followed by adding 5 mL of PAA (0.5 M), 192 μL of 2-propanol and completed with water.

The solution was kept in the dark to prevent possible photochemical reduction of Au III . After that, the solution was purged with N 2 to remove O 2 prior to irradiation. Radiolysis of the solution was carried out using a 60 Co panoramic gamma source with a radiation dose rate of 1.4 kGy h -1 . [2] The exposure time was 5 hours (dose = 7 kGy). The second step was deposition of the synthesized metal NPs by a colloid-deposition route. Deposition was attempted by stirring 1.00 g of P25 into the Au-Cu colloid followed by adding 5 mL of PVA (0.5 M). After 24 hours, the surface modified P25 was separated by centrifugation and washed repeatedly with water to remove the excess of polymer. The resulting Au-Cu/P25

composite was dried at 60 °C for 24 hours.

Except when specifically mentioned, the Au and Au-Cu samples were calcinated under air, and then reduced again under H 2 at 500 °C (∼100 mg of catalyst under 100 mL.min -1 , first, under air with a heating rate of 5 °C.min -1 then a 15 min plateau. After, a second 15 min plateau under H 2 at 500 °C). After reduction, the samples were cooled down to room temperature under N 2 flow, and transferred in air before characterization.

Deposition-Precipitation with Urea

A typical synthesis of bimetallic Au-Cu NPs modified P25 at a total metal content of 0.5 wt. % is described as below. 0.478 mL of HAuCl 4 solution (10 -2 M) was added to a jar of 25 mL of distilled water. After that, 250 mg of P25 and 37.5 mg of urea were added in the jar to form a white suspension. Then, the jar was heated up to 90 °C in a water bath with stirring.

The temperature of water bath was kept at 90 °C for 2 hours. At this point, the well dispersed suspension turned into a well formed precipitate. After that 0.478 mL of CuCl 2 (10 -2 M) was added into the jar. The temperature decreased naturally to room temperature with keeping magnetic stirring. 0.192 mL of 2-propanol was added to scavenge the oxidative OH • radicals generated during γ-irradiation. The jar was sealed with a rubber cap, followed by purging with nitrogen for 10 minutes. Consequently, the jar was put in a 60 Co gamma ray source located in USTC, Hefei (See Appendix I). The dose rate was around 170 Gy/min, and the irradiation time was 180 min. A self-made magnetic agitator was used for stirring during the irradiation.

After irradiation, the fine white suspension had turned to violet, indicating the formation of procedure was repeated three times). The samples were then dried at 50 °C for 48 hours. For the bimetallic Au-Cu catalysts, the unique difference with the protocols of synthesis of monometallic Au catalysts was that gold and copper were simultaneously introduced. The total metal mass percentage was kept at 0.5 wt. % for the monometallic and bimetallic photocatalysts. The compositions of the synthesized photocatalysts are listed in Table 3.8.

After drying, all the samples were stored in the dark. Except when specifically mentioned, the Au and Au-Cu modified samples were thermally treated according to the procedure which was described in Section 2.2.2.

Preparation of Copper Oxides

In a typical procedure for synthesizing CuO nanowires, 0.270 g of CuCl AuCu31PAA@P25(1)

AuCu11PAA@P25(1)

AuCu13PAA@P25(1)

AuPVA@P25(1)

AuCu13PP@P25(1)

Samples Preparations Details Descriptions

AuCu13PAA@P25(0.5)

AuCu11PAA@P25(0.5)

AuCu31PAA@P25(0.5)

Au/P25

Characterization Techniques

X-ray diffraction (XRD). The compositions of copper oxides and sulfides were determined by powder XRD, which was performed on a Rigaku TTR-III X-ray diffractometer with Cu Kα radiation (λ=1.541 78 Å).

Scanning electronic microscopy (SEM). The morphology of the as-synthesized products was observed using field-emission scanning electron microscopy (SEM; FEI Sirion 200).

Transmission electron microscopy (TEM). The surface morphology of modified TiO 2 was studied by Transmission Electron Microscopy (TEM, JOEL JEM 100CX II) operating at 100 kV in collaboration with Patricia Beaunier (LRS, UPMC, Paris 6). The irradiated suspensions were first sonicated for a few minutes then a few drops of the suspension were deposited on copper coated carbon grids for TEM observations. Particles size measurements were performed manually using Nano Measurement software on digitized graphs. We measured at least the size of 80 metal nanoparticles. The average size was calculated using the formula followed:

ii i d nd n = ∑∑
, where n i is the number of particles of diameter d i . [2] High-Resolution TEM and EDX were performed using a JEOL 2010 microscope, equipped with a LaB 6 filament and operating at 200 kV. The images were collected with a 4008 X 2672 pixels CCD camera (Gatan Orius SC1000).

Scanning Transmission Electron Microscopy (STEM). High Angle Annular Dark

Field-Scanning Transmission Electronic Microscopy (JOEL JEM-ARM200F: 200 kV FEGSTEM/TEM equipped with a CEOS Cs corrector on the illumination system) was also employed to characterize the morphology of modified TiO 2 ; STEM-Electron dispersive X-ray spectroscopy (EDS) was used to analyze the compositions of the deposited Au-Cu NPs. The probe current used for acquiring the High-angle annular dark field (HAADF)-and the bright field (BF)-STEM images was 9C (23.2 pA); the condenser lens aperture size was 40 μm.

HAADF-STEM images were acquired with a camera length of 8 cm/6 cm and the collection angle of 68-280 mrad/90-270 mrad. The BF-STEM images were obtained using a 3 mm/1mm aperture and a collection angle of 17 mrad/5.6 mrad (camera length in this case was 8 cm). The HAADF as well as the BF images were acquired using a digiscan camera. EDS measurements for line scan profiles as well chemical maps for the various elements were obtained with a solid state detector and software for two dimensional mapping from EDAX.

These experiments were done in collaboration with Miguel José Yacaman (University of Texas at Austin, USA)

UV-vis spectroscopy. The UV-vis absorption spectra of the solutions were recorded using a HP Agilent diode array 8453 UV-Visible spectrophotometer and also using a Shimadzu UV-1800 UV-Visible spectrophotometer.

Diffuse reflectance spectrophotometry (DRS).

The diffuse reflectance spectra of the modified TiO 2 samples were obtained using a Cary 5E spectrophotometer equipped with Cary 4/5 diffuse reflection sphere. The baseline was recorded using a poly(tetrafluoroethylene) reference. Besides, a Shimadzu DUV-3700 spectrophotometer was also used to record the UV-vis diffuse reflectance spectra of the samples in the region of 250 to 800 nm.

Thermal gravimetric analysis (TGA). DSC-TGA was performed on a SDT Q600 instrument. Samples were heated in air (the flow rate is 100.0 mL min -1 ) at 500 °C (with a heating rate of 3 °C min -1 ).

Nitrogen adsorption-desorption. N 2 adsorption isotherms were performed by BET measurements using a Tristar II 3020M surface area analyser. Brunauer-Emmett-Teller (BET)

surface area was determined by a multipoint BET method in the relative pressure (P/P 0 ) rang of 0.07-0.2. The pore diameter distribution and the BET surface area were calculated using the Barrett-Joiner-Halenda (BJH) and BET method based on the adsorption isotherm. after UV laser excitation were determined by microwave absorption experiments using the TRMC method. The principle of TRMC and the experimental set-up have been widely described in a previous paper. [6] It is also described in detail in Appendix V.

X-ray photoelectron spectroscopy (XPS). XPS analysis was performed on

Photocatalytic Reactions

In order to investigate the application of the synthesized photocatalysts for water treatment, we chosed three model pollutants: rhodamine B (RhB) , phenol and methyl orange (MO). All of these compounds are widely used in chemical industries and are not sensitive to the irradiation with natural light. Therefore, it is reasonable to carry out photocatalytic reactions of these model pollutants under UV and visible light. The photocatalytic experiments are described in the following sections.

Photodegradation of Rhodamine B

For the RhB photocatalytic degradation under both of UV-visible and visible light, the photocatalytic reaction system for these tests is described in detail in Appendix III. The optical path (in quartz) included water filter and glass filter (GG) to cut off infrared and ultraviolet illumination. An optical filter (GG450) was used to cut off wavelength shorter than 450 nm. The optical path of the cuvette is 10 mm and the volume is about 4 mL. The photocatalyst powders were dispersed in aqueous RhB solution with the initial concentration of 10 -4 M, keeping the same numerical ratio between the mass of the photocatalysts and the volume of RhB solution as 1.0 g/L. Then, the suspension was put in the ultrasound basin for 30 seconds to disperse the powders into tiny particles. After that, the cuvette was kept stirring on a magnetic stirrer in dark for 15 minutes to reach the equilibrium between adsorption and desorption of RhB on the surface of the photocatalysts. After that, the suspension was irradiated for 10 min (under UV-visible light) or 100 min (under visible light > 450 nm) under a fixed flow of oxygen and magnetic stirring. An aliquot of 500 μL suspension was taken from the cuvette at fixed time intervals. The aliquots were transferred into glass jars for further centrifugation to remove the photocatalyst nanoparticles. After that, the degraded RhB solutions were analized with the UV-Vis spectrophotometer in a 2 mm quartz cell. The kinetics of the photodegradation was achieved by monitoring the main absorption peak at 554 nm.

Photodegradation of Phenol

Phenol photodegradation under UV-visible light. Appendix II shows in detail the photocatalytic reaction system for these tests. The initial phenol concentration is 2×10 -4 M.

The photodegradation reaction processes are nearly the same according to section 2.3.4.1. The difference is that high performance liquid chromatography (HPLC) was used for the determination of the phenol concentration .

Phenol photodegradation under visible light. In the case of visible illumination, another photocatalytic reaction system was used (see Appendix III). A 300 mL three-mouth flask was used for the container. 300 mg of photocatalyst was dispersed in 300 mL of phenol solution (2×10 -4 M, 18.8 ppm). Then, the suspension was ultrasonicated for 30 seconds to disperse the powder into tiny particles. The suspension in the container was bubbled with oxygen at a flow rate of 20 cm 3 per min. A visible lamp (MASTER TL Mini Super 80 8W/840 from Philips, the emission spectrum is shown in Appendix III) with a power of 8 W was used as the light source. A peristaltic pump is used to drive the suspension cycle in this system. The speed of the pump was set at 200 rpm. A 4 mL aliquot was taken at certain time interval. Millipore filters were used to remove the suspended powders and to obtain clear supernatants.

The supernatants were analyzed using high performance liquid chromatography.

Analyses were carried out by using a Varian Prostar 230 ternary gradient pump combined with a Prostar 330 photodiode array detector (D2 lamp), by a method developed in our laboratory (LCP). For elution, an isocratic mobile phase consisting in 75 % of H 2 O and 25 % ACN, at a 1 mL min -1 flow rate, was used. Identification of degraded phenol or intermediates was done by the means of UV absorbance at 270 nm. The column was Adsorbosphere C18 reverse phase (5mm, l: 150 mm, ID: 4.6 mm, Alltech) combined with All-Guard cartridge system TM (7.5 × 4.6 mm, Alltech). For data acquisition, Star software was used.

Photodegradation of Methyl Orange

The photocatalytic experiments were carried out in a XPA-7 photochemical reactor (Nanjing Xujiang Machine-electronic Plant, China). The detailed description of the apparatus can be found in Appendix IV.

For the photocatalytic reactions under UV illumination, a 300 W mercury lamp was used as the light source. Optical filters were employed to cut off the unwanted light. Only the illumination longer than 270 nm and shorter than 400 nm can pass through the filter. Typically, 20 mg of synthesized photocatalyst was dispersed in 20 mL of methyl organe (MO) with an initial concentration of 20 mg L -1 . The suspension was loaded in a silica tube, followed by ultrasonication for 30 seconds to disperse the powder. After that, the solution was kept stirring on a magnetic stirrer in the dark for 30 minutes in order to reach the equilibrium between adsorption and desorption of MO on the surface of photocatalysts. After that, the slurry was subjected to UV irradiation under magnetic stirring and oxygen bubbling (flow rate of 200 mL min -1 ). A 2 mL syringe equipped with a long needle was used to take aliquots from the reactor at fixed time intervals. The suspensions were centrifuged at 4000 rpm for 20 minutes to obtain clear supernatants. Then, the supernatants were analyzed by spectrophotometry.

In the case of visible photocatalytic degradation, a 500 W Xenon lamp was used as the light source, equipped with another optical filters cutting wavelengths shorter than 450 nm.

Chapter 3 Modification of Titania with Au and Cu Bimetallic

Nanoparticles and Their Applications in Water Treatment

In this chapter, surface modification of TiO

Introduction

TiO 2 has been extensively studied for decades since it was used in photocatalytic splitting of water initiated by Fujishima. Afterwards, TiO 2 has attracted much attention because of its photocatalytic properties, its excellent resistance to photocorrosion, and in addition it is also non-toxic and environment friendly. These properties make it the most suitable material for photocatalysis. TiO 2 exists in Nature in three crystallographic phases:

anatase, rutile and brookite. Among them, anatase and rutile are most commonly used in photocatalysis, their band-gap energy are 3.1 and 3.0 eV respectively. Electron-hole pairs can be generated in the core of TiO 2 irradiated with light with wavelength shorter than 387 nm.

The great shortcoming of TiO 2 in photocatalysis is the requirement of high energy UV light to excite the electrons from the valence band to the conduction band. In the past decades, metallic nanoparticles have been widely studied due to their excellent properties compared to those of the corresponding bulk ones. Because of the confinement of the charge carriers in such a small nano-scale, there exists a shift of the plasmon resonance absorption, non-linear optical effects, non-metallic conductivity, and nano-catalytic effects. Therefore, metal nanoparticles have potential applications in the field of optics, catalysis, photocatalysis, chemical sensing, and biomedicine, etc. In addition, bimetallic and multi-metallic nanoparticles could often exhibit enhanced catalytic, electronic magnetic properties, which differ from their monometallic counterparts. Particularly, bimetallic nanoparticles show better catalytic performances in terms of activity, selectivity, and stability, compared to the separate components.

To the best of our knowledge, a lot of methods are employed for preparing metal nanoparticles, such as hydrothermal, solvothermal, chemical vapor deposition (CVD), lithography and so forth. [40][41][42] In addition to those common methods, radiolysis synthesis of metal nanoparticles is a characteristic approach. Fujita was the first scientist who introduced the radiolytic technique into aqueous synthesis of inorganic and metal nanoparticles. In 1962, he prepared Au sol from Chloroauric Acid (HAuCl 4 ) solution via γ-Irradiation. [43] By this method, a variety of metallic nanostructures with specific properties have been synthesized, including platinum nanowires, gold nanorods and porous Pt-Pd nanoballs. [44][45][46] The advantage of radiolysis synthesis is that it induces a homogeneous reduction and nucleation, therefore generates uniform nanoparticles. Radiolysis is also a powerful method to synthesize bimetallic nanoparticles of controlled structure (alloys or core-shell).

In particular, bimetallic nanoparticles based on gold have attracted increased attention because of their optical properties and their application in catalysis and electrocatalysis. [47][48][49] Only a few studies report on synthesis, structure and properties of Au-Cu nanoparticles. [50][51][52][53] Recently, it has been shown that Au-Cu bimetallic nanoparticles exhibit higher catalytic activities than monometallic gold catalysts, for both CO oxidation and PROX reactions. From Figure 3.1a it can be seen that there is no absorption in the visible region before irradiation. The absorption peaks in the visible increase with irradiation time. At the beginning stage of the γ-irradiation (until 60 min), the absorption in the visible does not increase (Figure With increasing dose, an absorption peak emerged at ~530 nm which corresponds to the plasmon band of gold. The intensity of the absorption peak increased with irradiation dose and reaches a plateau after 180 min irradiation (dose = 4.2 kGy ). For doses higher than 4.2 kGy, the spectra are the same. The radiolytic reduction rate of Au III is equal to 0.6 μmol J -1 .

The polymer PAA plays a significant role in the stabilization of Au colloids, preventing the further coalescence of small Au nanoparticles. Moreover, absorption spectra of Au solution (Figure 3.1b) show that the Au colloids are stable for several days (more than 6 days).

Gamma radiolysis of

Radiolytic synthesis of Au-Cu bimetallic nanoparticles

Bimetallic nanoparticles often display better catalytic and electrocatalytic properties than the corresponding monometallic counterparts. [69][70][71][72]. Here, we chose gold and copper as the two components of bimetallic nanoparticles. Au/Cu metallic colloids of different molar ratios were prepared by radiolysis of aqueous solutions containing Cu II nanoparticles is 3.6 nm. The size of AuCu13PAA varies from 1.9 nm to 6.6 nm (Figure

3.3a,b).

The nanoparticle size of AuCu11PAA is relatively larger, varying from 3 nm to 13 nm.

The size of the AuCu31PAA NPs is comparatively much larger than that of AuCu13PAA and AuCu11PAA NPs. A few large nanoparticles more than 20 nm can be seen in the TEM images (Figure 3.3d). Additionally, few triangular crystals can also be observed. Such a triangle structure is usually observed in the preparation of monometallic Au NPs in the presence of a polymer stabilizer. The triangular structure is one of the typical Au crystal morphologies and it is easily formed. Therefore, it can be obviously concluded that the Au-Cu bimetallic NPs are getting smaller and more homogeneous along with the increase of the Cu proportion in the bimetallic NPs. The probable mechanism we proposed is that the presence of Cu ions handicaps the deposition of Au ions on the surface of Au 0 clusters. Hence, the comproportionation reaction between Au 0 and Au III may be inhibited. Consequently, the nuclei number of Au increased. As a result, the growth of Au crystal was limited and the crystal size decreased. When PVA was used in the radiolytic synthesis of Au-Cu NPs, the nanoparticle sizes presented two distributions in size with a first maximum at around 3.6 nm (Figure 3.5a) and a second maximum located at around 13 nm: indeed NPs with sizes ranging from 9.7 to 17.5 nm were also observed (Figure 3.5c,d).

Considering the inhomogeneous dispersion on the Au-Cu NP size, the polymer PVA is not as fine a stabilizer as PAA for the preparation of Au-Cu NPs by radiolysis.

3.6d).

By contrast, the absorbance at 540 nm for the parallel synthesis with PAA is nearly negligible (Figure 3.2a,c,e). Moreover, when the two stabilizers, PVA and PAA, were employed simultaneously, the absorbance is located in between those obtained for the individual ones. A probable mechanism is proposed: the nucleation is the dominant process in the presence of PAA while the crystal growth of Au is more predominant with the PVA stabilizer.

Deposition of Au-Cu NPs on the support P25

All of the modified photocatalysts are pink or pink-violet. Details of the prepared photocatalysts are summarized in Table 3.2. The strong absorption bands at around 300 nm is attributed to the intrinsic absorption of TiO 2 , resulting from excitation of O 2p electrons to the Ti 3d level. [75] The absorbance for the modified P25 in the visible region is always higher than that for the pristine P25. Note that wide absorption with a maximum at 550 nm is obtained with Au-Cu modified P25. This absorption results in the pink -violet color of the modified TiO 2 samples. The band-gap value E g for pure P25 can be calculated from the extrapolated intercept on axis of wavelength.

According to Einstein's equation E = hν and the transform E = 1240/λ, the E g obtained from experiment is 3.1 eV, which is consistent with the characteristic band-gap value of TiO 2 . It is worth noting the DRS spectra for the modified P25 show a slight shift in the band-gap transition to longer wavelengths. Approximately, the shift toward lower energy (longer wavelength) increases with the copper nominal loading. For example, the band-gap of AuCu31PAA@P25(0.5) is estimated to be equal to 2.98 eV (absorption edge of 416 nm) and AuCu13PAA@P25(0.5) is 2.90 eV (427.5 nm). This effect was observed in the case of Ag- 2.0 AuCu13PAA@P25(0.5) AuCu11PAA@P25(0.5) AuCu31PAA@P25(0.5) P25 AuCu11PAA@P25(1)

Abs

Wavelength (nm)

band with a maximum at around ~520 nm and ~570 nm in aqueous solution, respectively.

This plasmon band is sensitive to the environment and can be shifted depending on the substrate or on the stabilizer. The plasmon bands in case of and Au-Cu/TiO 2 are red-shifted because of the coupling between the metal nanoparticles and TiO 2 substrate having a high reflective index (the absorption coefficient and refractive index for anatase phase at a wavelength of 380 nm are 90 cm -1 and 2.19, respectively). [78-79]

Photocatalytic activity under UV illumination

The photocatalytic activities of the modified P25 were evaluated by photodegrading RhB.

RhB aqueous solution is considerably stable subjected to UV and visible illumination. As is shown in Figure 3.10a, it is a typical plot for RhB photodegradation. The RhB display two absorbance maxima at 554 nm and at 258 nm. The main absorption peak decreased remarkably with UV irradiation time. The decrease of the photoactivity can be due to several reasons as follows. Firstly, the Au-Cu NPs cover the surface of P25 so that less photons are incident onto P25 surface, consequently inducing less electron-hole pairs.

Another explanation could be also proposed that the metal NPs act in this case as recombination center for the charge carriers.

It should also be pointed out that the modified P25 with a lower metal loading of 0.5% is slightly higher in photocatalytic activity, although both of them are worse than pristine P25. The photoactivity of the modified samples was also investigated for the degradation of phenol under UV illumination. As it can be seen in Figure 3.11, pure P25 is highly photoactive in phenol degradation under UV light. After 330 minutes of irradiation, 97% of phenol was degraded. The catalyst AuCu13PAA@P25(0.5) exhibits a slightly higher photocatalytic activity than AuCu11PAA@P25(0.5). It can be concluded that the photoactivity increases with the copper proportion in the bimetallic NPs.

Figure 3.11 Time runs of phenol relative concentration in photodegradation in the presence of pristine P25, AuCu13PAA@P25(0.5), and AuCu11PAA@P25(0.5) under UV illumination.

Photocatalytic activity under visible illumination

The L mol -1 cm -1 and rhodamine (0 ethyl) is 8.4 (see Table 3.3). Hence, from the Beer-Lambert law: A = εcl, it can be concluded the absorbance decreased at the beginning and increased

P25

AuCu13PAA@P25(0.5) AuCu11PAA@P25(0.5) afterwards. Certainly, this case occurred just when the photo-bleaching could be ignored compared to N-deethylation. For the second observation, the reason is that after the N-deethylation completed, the aromatic structures in rhodamine cannot be destroyed with the modified P25 by visible photocatalysis. Thus the main absorption peaks at 495 nm are due to a stable intermediate of rhodamine degradation generated by the effect of N-deethylation. It is complicated to evaluate the photodegradation kinetic processes due to the various intermediates existing in the reaction system. Nevertheless, we could just consider the absorbances at 554 nm to assess the N-deethylation capability of a series of modified P25 photocatalysts. Figure 3.13 shows the time runs of RhB photodegradation under visible illumination. As it can be seen that RhB in solution is quite stable under visible light irradiation. All of the modified P25 photocatalysts showed better activities than the bare P25.

The most efficient one is AuCu13PAA@P25(1). Generally, it can be concluded that the Au-Cu-P25 photocatalysts with a higher Cu proportion leads to a higher photocatalytic activity in N-de-ethylation.

In order to evaluate the rate of RhB N-de-ethylation process with various photocatalyst, wavelength (at absorption peaks) change as a function of irradiation time was plotted in reported that the pH values can affect the rate of the RhB N-de-ethylation process. The higher the pH, the slower the N-de-ethylation. In the solution of pH 10, no N-de-ethylation can be observed in the P25 suspension with RhB. [80] In our experiments, considering that the Au precursor chloroauric acid is acidic, a higher Au content means a low pH value of the solution, which decreases the amount of OH -on the TiO 2 surface. In consequence, a higher Au content in the modified P25 is related to a lower pH value of P25 suspension with RhB. In brief, a higher Au proportion results in a higher rate of N-de-ethylation of RhB. Figure 3.13 Time runs of RhB relative concentration in photodegradation with no photocatalyst (solid square ■), P25 (solid circle •), AuPAA@P25(1) (solid triangle ▲), AuCu31PAA@P25(1) (empty square □), AuCu11PAA@P25(1) (empty circle ○), AuCu13PAA@P25(1) (empty triangle △). The initial concentration of RhB is 10 -4 M; illumination λ > 450 nm; optical path: 2 mm. We also studied the photocatalytic activity of the synthesized catalysts with polymers PAA and PVA. As shown in Figure 3.15, in the degradation of RhB under visible irradiation, the same trend as in the case of the presence of PAA only was observed: the absorbance maxima shifted to shorter wavelength. Furthermore, the wavelength change is larger for the sample AuCu11PP@P25(1) than for AuCu13PP@P25(1). This is consistent with the conclusion that a higher Au proportion results in a higher rate of N-de-ethylation of RhB.

Influence of wash-treatment on the photocatalytic activity

In order to remove the stabilizers PAA and PVA from the surface of supported metal nanoparticles on P25, wash-treatment with hot water, according to Lopez-Sanchez's method, with some modifications, was attempted.

Decomposition of the stabilizing polymers followed by TGA

The colors of the thermal-treated samples changed to grey-or dark-purple. The color changed more obviously with the increase of Cu proportion in the Au-Cu bimetallic NPs. For AuCu31PAA@P25(0.5), the color slightly changed to light purple, while the color turned to grey for the AuCu13PAA@P25(1).

DRS-TGA was used to investigate the polymer decomposition as a function of temperature in order to optimize the calcination temperature. The DSC-TGA curves of modified photocatalysts are illustrated in Figure 3.17. When the temperature was below 250 °C, the slow decrease of weight was attributed to the removal of the adsorbed surface water molecules, accounting for 2% (Figure 3.17a,b). The major weight loss of 3.5% and 7% in the temperature range from 220 °C to 350 °C was assigned to the decomposition of PAA in AuCu31PAA@P25(0.5) and AuCu11PAA@P25(1), respectively. In the case of AuCu13PAA@P25(1), another sharp weight loss at 200 °C appeared, probably attributed to the abundant residual chloride ions owing to higher CuCl 2 ratio. The photocatalytic activity of the thermal-treated samples was investigated for RhB degradation. Evolutions of UV-vis spectra of RhB in the photocatalytic reaction with the samples of AuCu31PAA@P25(0.5), AuCu11PAA@P25(1), and AuCu13PAA@P25(1) after thermal treatment are shown in Figure 3.18. It is worth noting that the absorbance maxima have no obvious shift in the presence of the three thermal treated samples. The sample AuCu31PAA@P25(0.5) has the highest activity. After 100 minutes of illumination under visible light, 55% of RhB was degraded. However, the degradation rate is still lower than that for the pristine P25 reference (75%). In the case of AuCu13PAA@P25(1), nearly no degradation of RhB was observed, indicating a negative effect on the photocatalytic activity of Au-Cu modified TiO 2 induced by the thermal treatment.

From the analysis above, we knew that calcination under air is harmful to the photoactivity of modified TiO 2 . For this reason, reducing under Hydrogen at 500 °C was performed after the calcination in the later experiments. compared to the pristine P25 under UV illumination. Among the synthesized catalysts, the optimal one is that with a higher copper proportion (Au/Cu 1:3) or suitable metal loading of 0.5%. Under visible illumination, the modified photocatalysts in the presence of PAA have great capability in N-de-ethylation of RhB. The increase of Cu ratio in Au-Cu bimetallic NPs led to a higher activity of RhB degradation. Besides, the higher Au proportion results in a higher rate of N-de-ethylation of RhB, due to the lower pH value of the suspension. b τ 1/2 : is the time to obtain intensity I max /2 of the TRMC signal, for pure P25 τ 1/2 is 88.9 ns.

Conclusions of this section

Modification of

TEM Characterization

The morphologies of the surface modified P25 were observed using TEM technique (Figure 3.19). The TEM observations show small nanoparticles distributed on P25. The mean size of Au/P25 was 5.8 nm. When Au was associated to Cu, the average size decreased. For the sample AuCu11/P25 and AuCu13/P25, the mean size decreased to 4.4 nm and 3.8 nm respectively. Besides the decrease of particle size, the distributions also became more homogeneous when Cu was associated to Au. In the case of Au monometallic modified P25, nanoparticles up to 13 nm were found. However, there was no particles larger than 6 nm as it can be seen from the TEM images of the sample AuCu13/P25. This fact confirms the result that we found in the radiolytic preparation of Au-Cu colloids; the size decreases and distribution is more narrow with the increase of Cu proportion in bimetallic Au-Cu NPs.

UV-vis Spectra of the Supernatants

The synthesized PAA stabilized Au-or Au-Cu colloids were characterized using UV-vis spectrophotometry. The results are shown in

DRS Analysis

The optical properties were investigated using diffuse reflectance spectroscopy. Cu nanoparticles are known to exhibit a plasmon band with a maximum at 520 nm and 570 nm, respectively. Hence, it can be assumed that the shift toward longer wavelength is caused by the increase of Cu.

Abs

Wavelength (nm)

wavelengths were observed. However, a significant difference is that the absorption maxima for all the modified samples are at 535 nm. Note that the absorbance in the range from 550 nm to 800 nm for the sample AuCu31/P25 is higher than that for Au/P25, indicating the existence of Cu in a form of atoms or clusters. It can be ascertained that the Au-Cu nanoparticles are not alloys at this step, because the increase of the amount of Cu did not result in a shift of the plasmon band maximum of Au-Cu. The Au-Cu bimetallic structure might be in a form of Au-Cu core-shell. After the modified samples were calcinated at 500 °C, the Au-Cu bimetallic nanoparticles restructured and formed an alloyed structure. In the case of alloyed Au-Cu bimetallic structure, the proportion of Cu had a great influence on the wavelength of plasmon resonance maximum (Figure 3.22). loop as a result of capillary condensation of N 2 inside the pores is ascribed to the existence of mesopores. [87][88] The BET surface area was obtained as 41.5 m 2 g -1 . Compared to the pristine P25 (51.4 m 2 g -1 ), a considerable decrease in the BET surface area was found, attributed to the calcination at 500 °C. [85,88] The pore size distribution for the modified P25 was obtained by the Barrett-Joyner-Halenda (BJH) method [89] and is presented in

Photocatalytic Tests

The photocatalytic activities of the prepared samples were investigated for RhB and Phenol degradations under UV and visible light.

Photocatalytic Activity under UV Irradiation

For the RhB degradation under UV illumination, Evonik P25 is highly photoactive, which is used as the reference. After 10 minutes illumination in the presence of P25, 99% of In the case of TiO 2 , the TRMC signal can be attributed to the electrons because their mobility is much larger than that of the positive holes. [40,91] The decrease of the TRMC signal is then probably due to efficient electron scavenging by the nanoparticles deposited on TiO 2 . It implies a decrease of the charge-carrier recombination that is beneficial to the photoactivity. Such acceleration has also been reported for the Ag modified TiO However, it is different from our previous observations made with Pt-and Pd-modified TiO 2 , [77,92] where a slowdown of the overall decay was observed. Indeed, contrary to metals such as Pt and Pd which provide an ohmic contact, metals such as Ag, Au and Cu exhibit capacitive properties. [93] In the modified TiO 2 , electrons migrate from the semiconductor to Cu metal owing to the higher work function that Cu displays. region there is no direct light absorption of pure TiO 2 but the metal NPs deposited on modified titania (Au and Au-Cu) are absorbing light through their plasmon resonance. Since no TRMC signal is observed at this excitation wavelength, one can conclude that no electron transfer from the NPs to the conduction band of TiO 2 . This is consistent with the photocatalytic results under visible light. As it is discussed above, the photoactivity obviously decreased under visible illumination owing to no excess electrons excited from metal NPs to P25, furthermore, the excited electrons from photosensitive dyes were probably trapped by metal NPs.

TRMC Signals

Conclusion of this section

Modification of TiO 2 with Au-Cu bimetallic nanoparticles by radiolysis, deposition-precipitation route with Urea

The resulting modified monometallic catalyst was 0.5 wt% Au on P25, referred as Au/P25DPU, where DPU refers to deposition-precipitation method with urea.

Correspondingly, AuCu11/P25DPU represents the sample P25 modified with Au-Cu NPs with a molar ratio of 1:1. The total nominal metal content was kept at 0.5 wt%. The compositions of the modified P25 by DPU are listed in details in Table 3.5.

Deposition of Au-Cu ions on P25

In order to evaluate the deposition efficiency of the Au-Cu ions with the DPU method, the supernatants of the mixture were monitored using UV-vis absorption spectrophotometer.

Typically, the spectrum of AuCu1:1/P25DPU supernatant before irradiation (Figure 3.30a)

shows no absorption peaks in both of UV and visible region except for the peak at 200 nm, which is attributed to urea in the solution. This illustrates that the [AuCl 4 ] -and Cu 2+ are not present in the supernatant, namely they completely deposited on the surface of TiO 2 . The spectrum of AuCu1:1/P25DPU after irradiation was almost the same as that before irradiation except for the small absorption peak at 262 nm. We assumed that this small peak was due to the instrinsic absorption of TiO 2 in the UV region. This was confirmed by UV-Vis absorption spectra of the supernatant along with filtration cycling. The absorption peak at 262 nm obvioulsy decreased after the first filtration by a millipore filter whose pore size is about 0.45 μm. For the second filtration, the same filter was repeatly used. The decrease of the absorption peak at 262 nm was considerably smaller than the first time. This is reasonable because after filtration the number of larger TiO 2 NPs decreases. Nevertheless, the smaller TiO 2 NPs could not be filtrated (by the filter we used). The peak at 262 nm deceases in intensity with the increase of filtration times until reaching a plateau (see Figure 3.34b-f). In order to confirm that the absorption peak at 262 nm is due to the intrinsic absorption of TiO 2 , a filter with an average pore of 0. The first chemical step that occurrs during DPU preparation of Au on TiO 2 is the formation of a surface complex. The complex AuCl 3 (OH) -sticks to the TiO 2 surface by the interaction with surface OH groups to form Ti-O-AuCl 2 (instead of an electrostatic interaction). In the case of preparation of Au modified P25 by irradiating a mixture of P25 and Au III , the Au nanoparticleswere not well deposited on TiO 2 . In the case of DPU, the adsorption of gold hydroxy chlorides on TiO 2 increased as the pH increased; increasing the pH value means more OH on the surface. The pH (~6.5) was slightly higher than PZC TiO2 after the end of the deposition. Thus, the surface of TiO 2 is negatively charged. Furthermore, the AuCl(OH) 3 -, AuCl 2 (OH) 2 -and AuCl 3 (OH) -are also negative. At this moment, Cu 2+ was introduced into the suspension, it can adsorb on the TiO 2 surface or can interact electrostatically with the Au complexes.

Characterization of the photocatalysts 3.4.2.1 TEM Characterization

The morphologies of the surface modified catalysts were characterized by TEM. As is shown in Figure 3. 35(a,c,e), the darkest dots are the metal NPs (mono-Au or bi-Au-Cu NPs) distributed on the TiO 2 surfaces. One noteworthy thing is that the size of metal NPs decreased slightly with the increase of Cu proportion in the bimetallic NPs (for monometallic Au NPs on P25, the mean size is around 8.4 nm, for AuCu1:3/P25DPU, the mean size is around 6.4 nm).

It has been reported that the size of Au-Cu alloyed NPs decreased with increasing the amount of Cu in the bimetallic NPs. In the case of monometallic Cu NPs on TiO 2 , no particles could be observed on the TiO 2 support, probably because of the small contrast between copper and titianium (the images are not shown). Furthermore, in the case of the sample AuCu1:3/P25DPU, the spacing value of the Au-Cu bimetallic nanoparticle on P25 is smaller (0.218 nm, Figure 3.35f), indicating a higher proportion of Cu in the bimetallic Au-Cu NPs. The HRTEM results probably evidenced the alloyed bimetallic nanostructure for the supported Au-Cu NPs on P25. However, it should also be pointed out that the Vegard's law is not necessarily followed by nanoalloys.

EDS Analysis

The composition of the prepared Au-Cu modified P25 was further characterized by STEM-Electron dispersive X-ray spectroscopy (EDS). EDS profile scans were carried out

DRS Analysis

Diffuse reflectance spectroscopy (DRS) is a useful method to characterize the electronic status of optical materials. Figure 3.37a shows the UV-Vis spectra of pristine and modified P25. P25 has a significant absorbance of UV light (λ is shorter than 400 nm), which is attributed to the large band gap (3.1 eV) of P25. Apparently, all of the modified P25 have notable absorbance in the visible region with respect to pure P25. The absorption maxima of Au/P25DPU, AuCu1:1/P25DPU and AuCu1:3/P25DPU appeared at 552 nm, 565 nm and 563 nm, respectively. The absorption bands with maxima at various wavelengths result in the diverse violet color of the modified photocatalysts. For the sample Cu/P25DPU, the absorbance maximum was found at 750 nm. It is well known that Au and Cu nanoparticles exhibit respectively a plasmon band with a maximum at ~520 nm and ~570 nm in aqueous solution. The absorption shift of the modified P25 photocatalysts is because the plasmon band is sensitive to the environment and depended on the stabilizer or on the substrate. The plasmon bands in case of Au/TiO 2 and Au-Cu/TiO 2 are red-shifted because of the coupling between the metal nanoparticles and TiO 2 support having a high reflective index,. However in case of Cu/TiO 2 , the metal nanoclusters are very small and the wide absorption band in the visible and near infra-red region observed in this case could be due to inter-band transitions in the Cu clusters deposited on different phases and sites of TiO 2 and with strong interaction with the support. It should also be pointed out there is a slight shift in band-gap transition to longer wavelengths from the DRS spectra of the modified photocatalysts. A similar effect was also previously observed with Pt-and Ag-modified TiO 2 . This effect can be ascribed to a stronger stabilization of the conduction band of TiO 2 by the conduction band of the Au-Cu NPs compared to the stabilization of the valence band. The band-gap of the as-prepared samples could be calculated by the equation:

() ( ) n g h kh E αn n = - (3.9)
where α is the absorption coefficient, k is the parameter related to the effective mass associated with the valence and conduction bands, n equals to 1/2 for the indirect transition semiconductor, hν is the discrete photon energy of incident light and E g is the band-gap energy. [19,95] Plot of (αhν) 1/2 versus the energy of absorbed light hν based on the DRS response is shown in Figure 3.37b, giving the E g value according to the extrapolated intercept.

The absorption band edges of the modified P25 reduced from 3.1 eV of the pristine P25 to around 2.95 eV. Approximately, modified P25 with a higher Cu proportion in the bimetallic NPs leads to a lower band edge (AuCu1:3/P25DPU is estimated to 2.93 eV, and Au/P25DPU is 2.96 eV). The extended absorption to visible light region means an enhanced light-harvesting efficiency and results in a possible capability in solar-driven photocatalysis.

XPS Analysis

In order to analyze the chemical composition of the prepared samples and to identify the chemical status of Cu element in the samples, X-ray photoelectron spectroscopy (XPS) measurements were attempted. The XPS survey spectrum of AuCu1:3/P25DPU is shown in peaks are located at 931.7 and 951.5 eV, respectively, which agree with those reported previously. Furthermore, the Auger CuLMM spectrum shows the peak position of kinetic energy is found at around 920 eV, illustrating that the main chemical state of Cu is in the zero valence.

Photocatalytic Tests

The photocatalytic activity of the modified P25 was investigated for the degradation of methyl orange (MO).

A representative plot for the photodegradation of MO under UV-light with the sample AuCu1:3/P25DPU is shown in a N/A: Cu nanoparticles are unable to be observed from the TEM images b AuCu1:3/P25DPU-s: DPU-s is deposition precipitation method with urea, but Au and Cu ions were added simultaneously before water bath.

c AuCu1:3/P25DP: DP means deposition precipitation method, without urea or other reagents.

The Effect of Urea on the Deposition and Photoactivity

We have also carried out experiments to evaluate the effect of urea in the deposition precipitation processes. These experiments were divided into two parts. In the first group, deposition precipitation was done with urea. However, Au and Cu ions were introduced to the solution simultaneously. After that, the suspension was heated with a water bath at 90 °C.

Therefore, these samples are named ending with DPU-s. In another group, the deposition precipitation has been performed in the absence of urea. Photocatalysts prepared by this method were named ending with DP. Details of the prepared photocatalysts are summarized in Table 3.7.

The supernatant was measured using UV-vis spectrophotometer to check the probable residual metal ions. The photocatalytic activities of the prepared catalysts were tested for RhB degradation under UV and visible light. For the samples prepared by DPU-s method, the photoactivities under UV illumination decreased overall compared to that of P25 (Figure 3.46a). Especially, the sample Cu/P25DPU-s exhibits an obvious decrease in the photocatalytic activity with respect to Au/P25DPU-s and AuCu1:3/P25DPU-s. In the case of visible illumination, all of the modified photocatalysts show great decrease in the photocatalytic activity (Figure 3.46b).

The sample AuCu/P25DPU-s and AuCu13DPU-s even present no degradation of RhB. In this case, the modification with Cu incorporated Au NPs act probably as recombination centers for the charge carriers decreasing the photocatalytic activity.

In the case of deposition-precipitation without urea, the sample AuCu1:3/P25DP exhibits a slight higher photoactivity while Au/P25DP shows slower degradation under UV illumination (Figure 3 It can be concluded from the results discussed above that the modification with Au-Cu NPs by two-step deposition-precipitation in the presence of urea can result in small and homogeneous metallic NPs supported P25, and consequently leads to an enhancement in the photocatalytic activity.

Conclusion of this section

Small Au-Cu NPs (~7 nm) were synthesized by radiolysis in-situ on the P25 support after deposition precipitation in the presence of urea. Au and Cu ions were introduced at different stages during the deposition precipitation process to ensure an efficient deposition. As mentioned before, it has been already reported that the size of the bimetallic Au-Cu particles decreases as the amount of copper in the NPs increased. [39,48] In case of Cu on TiO 2 , we did not observe any particle on the support (Figure 3.48d), probably because of their small size and the small contrast between Cu and titania. We have also carried out EDAX elemental mapping for the respective elements, Typical values are given in Table 3.9. An interesting feature concerns the Cu/P25_TiO 2 NPs for which we detected easily both Cu and TiO 2 signals demonstrating the presence of Cu NPs on TiO 2 particles, which was very difficult to observe by electron microscopy. So the global characterization performed by XPS agrees with the local ones, that means that bimetallic NPs supported on P25 TiO 2 can be considered as homogeneously distributed.

DRS Analysis

The optical properties of the modified TiO 2 have been studied in details by Diffuse Reflectance Spectroscopy (DRS). Figure 3.56a shows the spectra of pure and modified TiO 2 .

It should also be pointed out that the DRS of the modified samples show a slight shift in the band-gap transition to longer wavelengths for all kinds of surface-modified photocatalysts.

This effect was previously observed with Pt-and Ag-modified TiO 2 . As mentioned before, this effect can be attributed to a stronger stabilization of the conduction band of TiO 2 by the conduction band of the Au-Cu NPs compared with the stabilization of the valence band. The absorbance in the visible region is always higher for the modified than for pure P25. Note that wide absorptions with maxima at 550 and 750 nm are obtained respectively with Au and Cu-modified P25 (Figure 3.56a). Au-Cu-modified P25, absorb in the visible region with maxima respectively at 580 and 600 nm for AuCu11/P25-T and AuCu13P25-T. These absorptions result in violet or grey-violet colors of the modified TiO 2 samples. Au and Cu nanoparticles are known to exhibit a plasmon band with a maximum respectively at around 520 nm and 570 nm in water. This plasmon band is sensitive to the environment and can be shifted depending on the stabilizer or on the substrate. Because of the coupling between the metal nanoparticles and TiO 2 support, the plasmon bands in case of Au/TiO 2 and Au-Cu/TiO 2 are red-shifted. However in case of Cu/TiO 2 , the metal nanoclusters are very small and the wide absorption band in the visible and near infra-red region observed in this case could be due to inter-band transitions in the Cu clusters deposited on different phases and sites of TiO 2 and with strong interaction with the support.

By contrast, we give the DRS results of the modified P25 before thermal treatment (Figure 3.56b). The similar shifts in the band-gap transition toward longer wavelengths were observed. However, a significant difference is the absorption maxima for all the modified samples at 548 nm. Note that the cross at 610 nm and the absorbance in the range from 610 nm to 800 nm for the sample AuCu13/P25-T is higher than that for Au/P25-T, indicating the existence of Cu probably in the form of small clusters. XPS results (not shown here) are very close to those obtained after thermal treatment for all the samples. They also attest in this case that Cu is in the zero valence.

Photocatalytic Tests

The photocatalytic activities of the prepared samples were investigated for the degradation of RhB and Phenol under UV and visible light. be approximately regarded as first-order kinetics. [98-99] Rate constant (k) was used to evaluate the photocatalytic activities. According to the equations 3. We conclude that modification of P25 with Au-Cu alloyed NPs leads to higher photocatalytic activity under UV light. The mechanism will be studied by TRMC. NPs by this THPC method gave much better results for the nanoparticle size and distribution and photocatalytic activity than by the colloid-deposition method. Besides, this method is more facile than deposition precipitation method with urea.

Conclusions

Different methods were developed to modify the surface of TiO 2 with Au and/or Cu nanoparticles for photocatalytic applications. Some interesting results were obtained: i) Au-Cu bimetallic NPs with small size (~4 nm) and homogeneous distribution were synthesized by radiolysis in the presence of stabilizing agent PAA. Polymer PVA can facilitate the deposition of metal NPs on the TiO 2 surface in the colloid deposition method.

ii) When synthesizing Au-Cu bimetallic NPs, the addition of Cu ions can lead to a smaller particle size and homogeneous size distribution compared to monometallic Au NPs.

The trends exist for both colloidal deposition (by radiolysis and by THPC method) and deposition precipitation with urea.

iii) Deposition precipitation with urea followed by radiolytic reduction leads to Nanosized semiconductors such as CuO and Cu 2 O have been intensively investigated for their particular optical and electronic properties. As one of the most popular p-type semiconductor with a narrow band gap (1.2eV), CuO is employed as anodes for lithium ion batteries, [20][21][22] catalysts, [23][24][25] To study the effect of NaOH on the product compositions, an experiment with an intermediate concentration (between sample 1 and sample 2) was conducted. For this sample, the molar ratio of OH -to Cu 2+ was 2.10 ( that in the [111] direction. The ratio of growth rate of the [100] direction to that of [111] is defined as R. Kuo

Effect of SDS on the morphology of copper oxides

The morphology of the synthesized products was observed using SEM technique. The SEM image in Figure 4.4a shows that sample 1 is composed of wires with a diameter of ~20 nm. These CuO nanowires interlace together, as it can be seen from the image. The length of the nanowires, which could not be precisely measured, is larger than 5 μm.

The SDS plays a role in the formation of CuO nanowires. To understand this effect, parallel experiments without SDS were carried out. In this case only uniform nanoleaves are formed (Figure 4.5a). This proves that SDS affects the growth of CuO crystals, probably by poisoning specific facets. One dimension nanostructures such as nanotubes, nanorods and nanowires have currently drawn much attention owing to their unique electric, photonic and thermal properties and their unique applications in fabrication of nanoscale devices. [33,38,42] Commonly, templates are used to restrict the growth of certain crystal orientation to form 1-D nanostructures. In other cases, linking agents are employed to prepare such nanostructures.

Conclusion of this section

Therefore, a facile and efficient route for synthesis of 1-D nanostructures is still a challenge.

Copper sulfide is a typical p-type semiconductor, which can form a variety of stoichiometric compounds including covellite (CuS), anilite (Cu 7 S 4 ), digenite (Cu 9 S 5 ), djurleite (Cu 1.95 ) and chalcocite (Cu 2 S). [43] They were widely applied in the field of optical recording, bio-medical, photothermal conversion, superionic materials, optical filters and solar cell devices. [44][45] In addition, CuS was also found to be a potential photocatalyst with activity under visible illumination. [46][47][48][49] The structure and size of CuS have a great influence on its photocatalytic activity. On the other hand, radiolysis is a powerful method to synthesize metal and semiconductors nanoparticles of controlled size and shape. [56] Here, we synthesized, for the first time, the homogeneous CuS nanotubes via γ-irradiation route under ambient temperature and pressure. No template was employed in the preparation of CuS nanotubes. The photocatalytic activity was examined for decomposition of RhB, a model pollutant, under visible illumination.

Results and Discussion

Characterizations

The obtained XRD pattern of the synthesized structure is shown in Figure 4.10. The peaks correspond to the ones of the hexagonal covellite CuS (standard JCPDF no. 06-464; a=3.792Å, b=3.792Å, c=16.344Å; α=90°, β=90°, γ=120°). No other characteristic peaks were observed, implying that the product is a pure-phase covellite CuS. In addition, it can be seen from the XRD pattern that the width of peaks are broadened, giving the information that the grain sizes of the synthesized CuS products are very small. The possible reaction processes for the formation of CuS nanotubes could be described The morphology of the as-prepared CuS products was observed by transmission electronic microscopy (TEM) and High-Resolution TEM (HRTEM).

Photocatalytic Tests

The photocatalytic activity of these nanotubes under visible light was studied for RhB degradation. First, we studied the adsorption properties of these nanostructures. When RhB is put in dark in contact with the CuS nanostructures, the absorbance at its characteristic absorption peak at 554 nm decreased with time until a factor of 0.

Conclusions and Perspectives

Conclusions

In this thesis work, two main aspects of photocatalysts were studied, i.e. the enhancement of photocatalytic activity of TiO 2 by modification with Au-Cu bimetallic NPs and the preparation of non-TiO 2 visible light photocatalysts.

Colloid deposition and deposition precipitation methods were used to prepare Au-Cu modified TiO 2 photocatalysts.

For the colloid deposition method by radiolysis, colloidal solutions of Au-Cu bimetallic NPs with small sizes (around 4 nm in diameter) were prepared in the presence of the stabilizing agent PAA. Addition of PVA facilitates the deposition of Au-Cu NPs on TiO 2 surface.

For the colloid deposition method by chemical method, very small Au-Cu bimetallic NPs homogeneous in size (around 3 nm in diameter) were synthesized and deposited on P25. We have found that very small amount of metal (0.5 % wt.) can activate TiO 2 for photocatalytic applications. We employed Cu, which is an abundant and a cheap metal to modify TiO 2 , and the modification of the photocatalyst (with Cu or with Au-Cu NPs) resulted in an enhancement of the photocatalytic activity under UV light, thus, the cost of photocatalyst preparation is relatively low. TRMC results showed that the NPs act as efficient electron scavengers increasing the quantum yield of light conversion.

For the deposition precipitation method with urea followed by radiolytic reduction, Au-Cu bimetallic NPs with a size of about 7 nm were synthesized in-situ on the P25 substrate.

This method ensures an efficient deposition performance. The modified Au-Cu bimetallic NPs are alloys without thermal treatment. This is convenient with respect to the colloid deposition method. P25 modified with Au/Cu1:3 bimetallic NPs shows a quite higher photocatalytic activity in MO degradation than the pristine P25, and the photocatalytic efficiency under UV light increased by more than 2 times.

The nanoparticle size decreases for the Au-Cu bimetallic NPs compared with those of the monometallic Au ones in the same preparation conditions. This trend exists in both cases of the colloid deposition method (by THPC or radiolysis) and the deposition precipitation method. Besides, the size distribution becomes more homogeneous when Cu is associated to Au NPs.

Modification of P25 with Au-Cu bimetallic NPs leads to a higher UV photocatalytic activity than that with monometallic Au or Cu ones all through the different deposition methods.

For all the deposition methods we used, modification of P25 with Au/Cu1:

Perspectives

It is necessary to optimize the ratio of Au to Cu in the deposited Au-Cu NPs to obtain the highest photocatalytic activity. The cost should also be taken into account to balance the photocatalytic efficiency and economy.

Deposition precipitation method with urea is a promising method for depositing various metals such as Cu, Ag, Pt, Au on TiO 2 . It also has potential applications in the preparation of In this thesis work, we modified P25, which is a mixture of anatase and rutile. P25 is the best TiO 2 photocatalyst under UV light. However, it is known that it is very difficult to improve its activity under UV light. Here, we succeeded in this task by modifying its surface with Cu and Au-Cu nanoparticles. To better understand the effect of the metal nanoparticles on TiO 2 photocatalytic activity, the same study will be conducted on pure anatase TiO 2 , in this case the system will be less complex to study.

It is also important to try to enhance the photoactivity of TiO 2 under visible light. For In the bright field mode, the incident electron beam can be deflected by collisions across the sample. The probability of a collision increases with the atomic number Z of the constituents of the sample and the sample thickness. The deflected electrons are not observed which caused by the most elastic collisions, so the areas are darker while areas with lower Z atoms and/or reduced thickness appear bright. This method is widely used to observe the metal structures for the metal atoms with a high atomic number, and provide excellent contrast against the bright background.

In the diffraction mode, the focal length of the intermediate lens, which allows passing a distance diffraction picture mode, is placed in the image focal plane. This mode allows us to observe the electron diffraction as a diffraction pattern. The shape of the diffraction pattern can initially determine whether the observed area is amorphous, polycrystalline or crystalline.

A crystal structure defines spots while a polycrystalline structure present diffraction rings. The analysis of the diffraction patterns to determine these crystal structures through the interatomic distances observed. In the case of a bimetallic alloy, the distance between two diametrically opposed spots is intermediate between the distances of each metal characteristics in proportion to the ratio between the two metals. This technique can provide Luminescent screen/CCD information on the organization of atoms but also on the composition, provided that the atoms have interatomic distances sufficiently distinct. In the diffraction mode, it is also possible to make a SAED ("Selected Area Electron Diffraction"). A diaphragm area of selection is then introduced into the assembly at the image plane of the objective lens, for defining the area participating in the diffraction. In a modern X-ray diffraction instrument X-rays of fixed wavelength (usually CuKα=1.54 Å) are directed onto a crystal and the reflections are measured as a function of the incident angle. In powder X-ray diffraction there is no single crystal since the powder is composed of many small crystals with random orientations and thus the orientation is not an issue. These crystals "reflect" the incident X-ray beam at the same angle of incidence thus making the angle between the incident and diffracted beams equal to 2θ. [6] In this thesis work, XRD measurements were carried out on a Rigaku TTR-III X-ray diffractometer with Cu Kα radiation (λ=1.541 78 Å) in Hefei National Laboratory for Physical Sciences at the Microscale at USTC, Hefei. In our experiments, the X-ray photons generated from the K line of aluminum (1486.6 eV). The electrons ejected by these photons come regardless of the valence shell or layer of heart and are recorded as a function of their energy by the detector. The results are presented as a spectrum consisting of peaks according to their energy. The integration of the peaks can be traced back to the composition, the area under the peak is proportional to the amount of the element in question to the surface of the sample (up to 5 nm, sometimes more due to the porosity of the material). As the peak shape and binding energies are sensitive to oxidation phenomena and chemical state of the transmitter atom, XPS also provides information on the chemical bonding of the peak deconvolution. [4] These experiments were performed in collaboration with Arnaud Etcheberry (ILV, Université Versailles Saint Quentin en Yvelines). The X-ray photoelectron spectroscopy (XPS)

X-ray Photon electron Spectroscopy

analysis was performed on a Thermo Electron ESCALAB 220i-XL. The XPS analyzer was a Thermo Electron ESCALAB 220i-XL. Either a non-monochromatic or a monochromatic X-ray Al Kα line was used for excitation.

Besides, in the section of deposition precipitation method with urea, XPS analysis was performed in USTC at a Perkin-Elmer RBD upgraded PHI-5000C ESCA system.

Isotherm Absorption

Adsorption is the measure of deposition of absorbates on absorbents at a set temperature --Isothermal Type I: The main feature is the presence of a horizontal reflecting the saturation of the adsorbent. The adsorbent for this type of isotherm is an adsorbent with micropores that fill quickly.

--Isotherm type II: the amount of adsorbed gas is gradual and continuous function of the relative pressure. This type of isotherm is characteristic of the adsorption of several molecular layers and appears in non-porous or macroporous adsorbents.

--Type IV isotherm: This isotherm has the same shape at low pressures the isotherm type II. For higher pressures, it is characterized by the appearance of a level of saturation (sometimes just a small inflection point) corresponding to a mesoporous material. Inside the mesopores there is a capillary condensation. Desorbing the nitrogen condensed in the capillary is not reversible and generates a hysteresis of desorption with respect to the adsorption.

--Isothermal type III and V: They differ respectively from isothermal II and IV by the shape of the curve adsorption isotherm at low pressures, because the interactions between adsorbent and adsorbable are low, as in the case of the water vapor adsorption on a hydrophobic surface.

3. 1 3 . 4 . 1 Chapter 4

 13414 Introduction ........................................................................................................ 3.2 Colloid-deposition of Au-Cu metallic NPs on P25 by radiolysis in the presence of stabilizers ...................................................................................... 3.2.1Radiolytic synthesis of Au monometallic nanoparticles .......................... 3.2.2 Radiolytic synthesis of Au-Cu bimetallic nanoparticles ......................... 3.2.3 Deposition of Au-Cu NPs on the support P25 ........................................ 3.2.4 Photocatalytic activity under UV illumination ........................................ 3.2.5 Photocatalytic activity under visible illumination ................................... 3.2.6 Influence of wash-treatment on the photocatalytic activity .................... 3.2.7 Decomposition of the stabilizing polymers followed by TGA ............... 3.2.8 Conclusions of this section ...................................................................... 3.3 Modification of TiO 2 with Au-Cu bimetallic NPs by radiolytic colloid-deposition route .................................................................................... 3.3.1 Characterization of the Photocatalysts .................................................... 3.3.2 Photocatalytic Tests ............................................................................... 3.3.3 TRMC Signals ....................................................................................... 3.3.4 Conclusion of this section ..................................................................... 3.4 Modification of TiO 2 with Au-Cu bimetallic nanoparticles by radiolysis, deposition-precipitation route with Urea ........................................................ VII CONTENTS Deposition of Au-Cu ions on P25 ......................................................... 3.4.2 Characterization of the photocatalysts .................................................. 3.4.3 Photocatalytic Tests ............................................................................... 3.4.4 The Effect of Urea on the Deposition and Photoactivity ...................... 3.4.5 Conclusion of this section ..................................................................... 3.5 Colloid-deposition of Au-Cu metallic NPs on P25 by chemical method with THPC .............................................................................................................. 3.5.1 Characterization of Photocatalysts ........................................................ 3.5.2 Photocatalytic Tests ............................................................................... 3.5.3 TRMC Signals ....................................................................................... 3.5.4 Conclusion of this section ..................................................................... 3.6 Conclusions ...................................................................................................... References .............................................................................................................. Radiolytic synthesis of Cu sulfides and oxides nanostructures: application in photocatalysis under visible illumination ...................................................................... 4.1 Synthesis of CuO and Cu 2 O nanostructures of controlled morphology: application in photocatalysis under visible light ............................................ 4.1.1 Introduction ........................................................................................... 4.1.2 Results and discussion ........................................................................... 4.1.3 Conclusion of this section ..................................................................... 4.2 Radiolytic synthesis of CuS nanotubes with photocatalytic activity under visible light ..................................................................................................... 4.2.1 Introduction ........................................................................................... 4.2.2 Results and Discussion .......................................................................... 4.2.3 Conclusion of this section ..................................................................... References ..............................................................................................................

Figure 1 . 1 13 Figure 1 . 2 15 Figure 1 . 3 16 Figure 1 .

 1113121513161 Figure 1.1 Schematic diagram of photoelectrochemical cell. (1) n-type TiO 2 electrode; (2) platinum black counter electrode; (3) ionically conducting separator; (4) gas buret; (5) load resistance; and (6) voltmeter. From ref. [32] .................................................. 13 Figure 1.2 Scheme of water splitting process in a particle-based semiconductor catalyst linked with a platinum particle. ...................................................................... 15 Figure 1.3 CB and VB positions for various semiconductors in aqueous electrolytes at pH = 1. ......................................................................................................................... 16 Figure 1.4 Photochemical processes in a semiconductor followed by deexcitation events. Where A is electron acceptor and D is electron donor. From ref. [39] ............ 18

Figure 1 . 5 19 Figure 1 . 6 21 Figure 1 . 7 23 Figure 1 . 8 24 Figure 1 . 9 25 Figure 1 . 10

 15191621172318241925110 Figure 1.5 Scheme of characteristic times in the photocatalysis process. .................. 19 Figure 1.6 Quantum size effect on semiconductor band-gap of (a) CdS, and (b) CdSe and metal oxides. [39, 41] ............................................................................................ 21 Figure 1.7 Schematic illustrations of (a) photo-excited metal-modified semiconductor particle, and (b) RhB degradation under UV irradiation on Pt cluster modified TiO 2 . [39, 50] ......................................................................................................................... 23 Figure 1.8 Mechanism of SPR-induced charge transfer. (i) electrons near the metal Fermi level, Ef are excited to surface plasmon states; (ii) the electrons transfer to a nearby semiconductor particle; (iii) this activates electron-driven processes such as the hydrogen-evolution half-reaction.[63] ................................................................... 24 Figure 1.9 Proposed mechanism for Aerobi oxidation of Alcohol on the Au/P25 catalyst under visible light illumination.[70] (P25 is a commercial TiO 2 that is a mix of anatase and rutile) .................................................................................................... 25 Figure 1.10 Schematic diagram representing inter-particle electron transfer process from the conduction band (CB) of photoexcited CdS to the CB of TiO 2 .[88]............. 26

Figure 1 .

 1 Figure 1.11 (a) Optical absorption spectra of TiO 2-x N x and pristine TiO 2 films. (b) N 1s XPS spectra of TiO 2-x N x and TiO 2 films. (c) CO 2 evolution as a function of irradiation time during the photodegradation of acetaldehyde gas in the presence of TiO 2-x N x (solid circles) and pristine TiO 2 (open squares) under UV illumination and visible irradiation. [97-98] ........................................................................................... 28

Figure 1 . 29 Figure 1 . 13 31 Figure 1 . 14 33 XIVFigure 1 . 15 37 Figure 1 . 16

 129113311143311537116 Figure 1.12 (a) Various electron excitations in TiO 2 photocatalysis,[123] and (b) electronic transitions in the dye sensitizing photodegradation process.[120] .............. 29 Figure 1.13 Mechanism of the photo-deposition method. .......................................... 31 Figure 1.14 The scheme of deposition-precipitation with NaOH method. ................. 33

Figure 2 . 1 56 Figure 2 . 2 57 Figure 2 . 3

 2156225723 Figure 2.1 Procedure for the colloid deposition method by radiolysis. ...................... 56 Figure 2.2 Procedure for the deposition-precipitation method with urea. .................. 57 Figure 2.3 Procedure for the colloid deposition method by THPC method. ............... 58

Figure 3 . 1 ( 74 Figure 3 . 2 77 Figure 3 . 3 78 Figure 3 . 4 79 Figure 3 . 5 80 Figure 3 . 6 81 Figure 3 . 7 83 Figure 3 . 8 83 Figure 3 . 9 85 Figure 3 . 10 87 Figure 3 . 11 88 Figure 3 . 89 Figure 3 . 13 91 Figure 3 . 14 91 Figure 3 . 15

 31743277337834793580368137833883398531087311883893139131491315 Figure 3.1 (a) Evolution of UV-Visible absorption spectrum with irradiation time of a HAuCl 4 solution containing PAA and 2-propanol, [HAuCl 4 ] = 10 -3 M, (b) evolution of spectrum with standing time in irradiated HAuCl 4 solution for 180 min. [PAA] = 0.1 M, [2-propanol] = 0.1 M, optical path: 2 mm. The solutions were degassed with N 2 prior to irradiation, dose rate: 1.4 × 10 3 Gy/h. ............................................................. 74 Figure 3.2 Evolution of UV-Visible absorption spectra with irradiation time of irradiated HAuCl 4 /CuCl 2 solutions containing PAA (0.1 M) and 2-propanol (0.1 M) (a) AuCu13PAA, [HAuCl 4 ] = 5×10 -4 M, [CuCl 2 ] = 1.5×10 -3 M, (c) AuCu11PAA, [HAuCl 4 ] = 10 -3 M, [CuCl 2 ] = 10 -3 M, (e) AuCu31PAA, [HAuCl 4 ] = 1.5×10 -3 , [CuCl 2 ] = 5×10 -4 M; and evolution of UV-visible spectra with standing time of corresponding solutions (b) AuCu13PAA (irradiated 210 min), (d) AuCu11PAA (irradiated 180 min). Optical path: 2 mm. The solutions were degassed with N 2 prior to irradiation, dose rate: 1.4 × 10 3 Gy/h. ..................................................................................................... 77 Figure 3.3 TEM images of Au-Cu nanoparticles with different compositions (a) AuCu13PAA, (c) AuCu11PAA, (d) AuCu31PAA, prepared by radiolysis in the presence of PAA. Inset: the corresponding SAED patterns. (b) Size distribution of the sample AuCu13PAA. ................................................................................................... 78 Figure 3.4 TEM images of the sample AuCu31PP (a, b),AuCu11PP (c, d), and AuCu13PP (e, f), prepared by radiolysis in the presence of PAA and P VA . ............... 79 Figure 3.5 TEM images of the sample AuCu13PVA prepared by radiolysis in the presence of PVA, at a lower (a) and a higher (c) magnification; size distributions of the sample AuCu13PVA at (b) the larger size region and (d) smaller size region. ..... 80 Figure 3.6 Evolution of UV-Vis absorption spectra of the sample (a) AuCu31PP, [HAuCl 4 ] = 1.5×10 -3 M, [CuCl 2 ] = 5×10 -4 M, (b) AuCu11PP, [HAuCl 4 ] = 10 -3 M, [CuCl 2 ] = 10 -3 M, (c) AuCu13PP, [HAuCl 4 ] = 5×10 -4 M, [CuCl 2 ] = 1.5×10 -3 M, and (d) AuCu13PVA, [HAuCl 4 ] = 5×10 -4 M, [CuCl 2 ] = 1.5×10 -3 M. [PAA] = 0.1 M and [PVA] = 0.1 M, [2-propanol] = 0.1 M. Dose rate = 1.4 kGy/h. Optical path: 2mm. .............. 81 Figure 3.7 TEM images of the sample AuCu13@P25PAA(1) at a lower (a-c) and

Figure 3 . 93 Figure 3 . 94 Figure 3 . 18 95 Figure 3 . 98 Figure 3 . 99 Figure 3 . 100 Figure 3 . 101 Figure 3 .

 39339431895398399310031013 Figure 3.16 (a) Evolution of the UV-Vis spectra of RhB photodegraded in the presence of the sample AuCu13PAA@P25(1) after washing with water, (b) time

Figure 3 . 104 Figure 3 . 25 105 Figure 3 . 26 106 Figure 3 . 27 107 Figure 3 . 108 Figure 3 . 109 Figure 3 . 112 Figure 3 . 113 Figure 3 . 113 Figure 3 . 114 Figure 3 . 114 Figure 3 . 116 Figure 3 .

 3104325105326106327107310831093112311331133114311431163 Figure 3.24 (a) N 2 adsorption-desorption isotherms for the sample AuCu13/P25. Inset: BET surface area plot, (b) the corresponding pore size distribution. ........................ 104 Figure 3.25 The time runs of relative RhB concentration in photodegradation in the presence of pristine and modified P25 under (a) UV, and (b) visible illumination. .. 105 Figure 3.26 Histogram of rate constants of the first-order kinetic of RhB or phenol photodegradation in the presence of pristine and modified P25 under UV light. ...... 106 Figure 3.27 The time runs of relative RhB concentration in photodegradation in the presence of pristine and modified P25 under (a) UV, and (b) visible illumination without thermal treatment. ......................................................................................... 107 Figure 3.28 TRMC signals of pristine and the modified P25 Au/P25, AuCu11/P25 and AuCu13/P25 after thermal treatment. ................................................................. 108 Figure 3.29 TRMC signals of pristine P25 and the modified P25 Au/P25, AuCu31/P25, AuCu11/P25 and AuCu13/P25, without thermal treatment. ............... 109 Figure 3.30 UV-Vis absorption spectra of (a) AuCu1:1/P25DPU supernatant before irradiation, (b) AuCu1:1/P25DPU supernatant after irradiation, (c) 1.25×10 -4 M of Au

Figure 3 . 120 Figure 3 . 121 Figure 3 . 39

 31203121339 Figure 3.37 (a) Diffuse reflectance UV-Vis spectra of the modified photocatalysts and P25, and (b) the plot of transformed Kubelka-Munk function versus the energy of incident light, inset is the partial enlarged view. ........................................................ 120 Figure 3.38 (a) XPS survey spectrum and (b) Cu2p region of the XPS spectra of sample AuCu1:3/P25DPU. The inset in b is Auger CuLMM spectrum of the sample. .................................................................................................................................... 121 Figure 3.39 Cu2p region of the XPS spectra of sample AuCu1:3/P25DPU, after photocatalytic reaction under UV illumination. The inset is Auger CuLMM spectrum of the sample. ............................................................................................................. 122

Figure 3 . 123 Figure 3 .

 31233 Figure 3.40 (a) N 2 adsorption-desorption isotherms for the sample AuCu1:3/P25DPU. Inset: BET surface area plot of, (b) the corresponding pore size distribution. .......... 123 Figure 3.41 UV-vis absorption spectra of MO in photodegradation under UV illumination with the sample AuCu1:3/P25DPU. ...................................................... 124

Figure 3 . 126 Figure 3 . 43 127 Figure 3 . 129 Figure 3 . 130 Figure 3 . 46 131 Figure 3 . 47 131 Figure 3 . 134 Figure 3 . 49 135 Figure 3 . 50 135 Figure 3 . 137 Figure 3 . 52

 31263431273129313034613134713131343491353501353137352 Figure 3.42 (a) The time runs of relative MO concentration, and (b) first-order

Figure 3 . 139 Figure 3 . 54 140 Figure 3 . 55 141 Figure 3 . 143 Figure 3 . 144 Figure 3 . 58 145 Figure 3 . 146 Figure 3 . 148 Figure 3 . 61 149 Figure 3 .

 313935414035514131433144358145314631483611493 Figure 3.53 (a) HAADF-STEM image of AuCu11/P25-T and EDS elemental mapping analysis at the NPs; (b) EDS analysis of the selected area in a. ................. 139 Figure 3.54 Au 4f region of the XPS spectra of the sample AuCu1:1/P25 (blue) and Au/P25 (red)............................................................................................................... 140 Figure 3.55 Cu 2p region of the XPS spectra of the sample AuCu1:1/P25 (blue) and

Figure 4 . 1 162 Figure 4 . 2 164 Figure 4 . 3 164 Figure 4 . 4 166 Figure 4 . 5

 4116242164431644416645 Figure 4.1 XRD patterns of (a) the synthesized sample 1, CuO nanowires; (b) the synthesized sample 2, Cu 2 O octahedra. ..................................................................... 162Figure 4.2 XRD pattern of sample 5. ........................................................................ 164 Figure 4.3 TEM images of the sample 5 at a lower magnification (a), and a higher magnification (b). ...................................................................................................... 164Figure 4.4 SEM images of sample 1-4 (a, b, c and d); TEM images of sample 2 (e), and sample 4 (f); HRTEM image of sample 2 (g) and its corresponding SAED pattern (h). .............................................................................................................................. 166Figure 4.5 TEM images of CuO (a) and Cu 2 O (b) prepared without SDS, the experimental conditions are corresponding to sample 1 and sample 2, respectively. 168

Figure 4 . 6

 46 Figure 4.6 UV-vis absorption spectra showing the photodegradation of RhB in with truncated octahedral Cu 2 O under visible illumination. Optical path: 2 mm. ............. 169

Figure 4 . 7 (XXFigure 4 . 8 Figure 4 . 9 Figure 4 .Figure 4 .Figure 4 .

 474849444 Figure 4.7 (a) The time runs of MO concentration and (b) pseudo first-order kinetic fittings for MO photodegradation in the presence of (i) no catalyst, (ii) CuO nanowires, (iii) Cu 2 O octahedra and (iv) Cu 2 O truncated-octahedra under visible illumination. ............................................................................................................... 171

Figure 4 .

 4 Figure 4.13 (a, b) TEM images and (c. d) SEM images of the synthesized CuS hollow spheres; the molar ratio of Na 2 S 2 O 3 to CuCl 2 is 3:1. ..................................... 178

Figure 4 .Figure 4 .Figure A. 1 Figure A. 2 Figure A. 3 Figure A. 4 Figure A. 5 Figure A. 6 Figure A. 7 Figure A. 8 Figure A. 9 Figure A. 11

 4412345678911 Figure 4.14 (a) UV-vis absorption spectra of RhB photodegration by CuS nanotubes under visible illumination, (b) time runs of relative RhB concentration in photodegradation with CuS (balck squares) and witout catalyst (red circles). Optical path: 2 mm. ................................................................................................................ 179Figure 4.15 UV-Vis absorption spectra of RhB in the adsorption experiment with prepared CuS NTs in the dark. ................................................................................... 179

Figure A. 12

 12 Figure A.12 Diagram of components of a transmission electron microscope.[4] .....

Figure A. 13

 13 Figure A.13 (a) TEM mass contrast imaging mechanism. (b) TEM diffraction contrast .......................................................................................................................

Figure A. 14

 14 Figure A.14 Schematic of the STEM.[5] ..................................................................

Figure A. 15 A

 15 Figure A.15 A graphic representation of Bragg's Lawfrom a crystal surface.[6] .....

Figure A. 16

 16 Figure A.16 The principle of photoelectron emission. ..............................................

Figure A. 17

 17 Figure A.17 The classification of the adsorption isotherm. ......................................

Chapter 3

 3 presents the results obtained for modification of TiO 2 with Au, Cu monometallic nanoparticles or Au-Cu bimetallic nanoparticles by various deposition methods, including colloid deposition by radiolysis, colloid deposition by chemical method, deposition-precipitation method with urea. The photocatalytic properties of the modified TiO 2 were studied for degradation of rhodamine B, phenol and methyl orange under UV and visible light. Time resolved microwave conductivity (TRMC) technique was used to follow the kinetics of excited electrons induced after laser excitation of TiO 2 . The results of the photocatalytic activities of TiO 2 under UV excitation related to lifetime of the photogenerated charge-carriers are presented.

FujishimaFigure 1 . 1 Figure 1 . 1

 1111 Figure 1.1 Schematic diagram of photoelectrochemical cell. (1) n-type TiO2 electrode; (2) platinum black counter electrode; (3) ionically conducting separator; (4) gas buret;(5) load resistance; and (6) voltmeter. From ref.[32]

Figure 1 . 2

 12 Figure 1.2 Scheme of water splitting process in a particle-based semiconductor catalyst linked with a platinum particle.

Figure 1 . 3

 13 Figure 1.3 CB and VB positions for various semiconductors in aqueous electrolytes at pH = 1.

1. 5 . 1 Figure 1 . 4

 5114 photocatalysis in a semiconductor.

Figure 1 . 5

 15 Figure 1.5 Scheme of characteristic times in the photocatalysis process.

Figure 1 . 6

 16 illustrates the increases in band gap for CdS and CdSe when the particle size decreases. For CdS, the band-gap dramatically increases from its characteristic value for the bulk CdS of ~2.6 eV to 3.0 eV for the particles in the order of 26 Å in size. In the case of CdSe, the band-gap increases by 0.32 eV as the particle size decreased from 4.2 nm to 2.8 nm. The more positive valence band implies an increased oxidizing capability of the holes. As well, the more negative conduction band means an enhancement on reducing capability of the generated electrons.

Figure 1 . 6

 16 Figure 1.6 Quantum size effect on semiconductor band-gap of (a) CdS, and (b) CdSe andmetal oxides.[39, 41]

Figure 1 . 7

 17 Figure 1.7 Schematic illustrations of (a) photo-excited metal-modified semiconductor particle, and (b) RhB degradation under UV irradiation on Pt cluster modified TiO2. [39, 50]

Tsukamoto

  et al. put forward a new idea on the charge transfer mechanism.[70] It is that the joint active site consisting of Au/anatase/rutile phases facilitates smooth e -transfer from the photoactivated Au NPs to TiO 2 (Figure1.9). In a system of anatase/rutile modified with Au NPs, a) plasmon activation of gold NPs transfers e -to the tightly bound rutile; b) e -then move to well-conjugated anatase; and c) O 2 is reduced there by e -. Usually, e -transfer from rutile to anatase is thermodynamically unfavorable because the conduction band potential (E CB ) of rutile (-0.05 V vs NHE) is less negative than the E CB of anatase (-0.25 V).[72] However, the e -injection from Au NPs to rutile accumulates e -in its conduction band and contributes to the negative shift of E CB (Figure1.9b).[73-75]As a result, the consecutive e - transfer from Au particles to rutile and then to anatase leads to a photoactivity under visible irradiation.

Figure 1 . 8

 18 Figure 1.8 Mechanism of SPR-induced charge transfer. (i) electrons near the metal Fermi level, Ef are excited to surface plasmon states; (ii) the electrons transfer to a nearby semiconductor particle; (iii) this activates electron-driven processes such as the hydrogen-evolution half-reaction.[63]

Figure 1 . 9 Figure 1 .

 191 Figure 1.9 Proposed mechanism for Aerobi oxidation of Alcohol on the Au/P25 catalyst under visible light illumination.[70] (P25 is a commercial TiO2 that is a mix of anatase and rutile)

  2 to transfer photoexcited carriers to reactive sites at the photocatalysts surface within their lifetime. Condition ii and iii require the use of anionic species for the doping rather than cationic metals, which often offer quite localized d states deeply in the band gap of TiO 2 and contribute to recombination centers of charge-carriers. The doped TiO 2 was yellowish owing to the noticeable absorption in the CHAPTER I region of visible light (< 500 nm) (Figure 1.11a). The doped photocatalysts showed activity under visible illumination for degradation of acetaldehyde and decoloration of methylene blue solution (Figure 1.11c). Nitrogen atoms were believed to substitute for lattice oxygen atoms, evidenced by the peak at 396 eV in the N 1s XPS spectra (Figure 1.11b).[97]

Figure 1 .

 1 Figure 1.11 (a) Optical absorption spectra of TiO2-xNx and pristine TiO2 films. (b) N 1s XPS spectra of TiO2-xNx and TiO2 films. (c) CO2 evolution as a function of irradiation time during the photodegradation of acetaldehyde gas in the presence of TiO2-xNx (solid circles) and pristine TiO2 (open squares) under UV illumination and visible irradiation. [97-98]

  photosensitive dyes are used as sensitizers including erythrosine B, thionine, and analogs of Ru(bpy) 3 2+ .[118-119] Figure 1.12a presents the mechanism of the photosensitized degradation on a TiO 2 particle, compared to other kinds of photoexcitation processes. If the oxidative energy level of the excited state of the dye molecule with respect to the conduction band edge of the semiconductor is favorable (more negative, Figure 1.12b), the excited dye molecule can then transfer electrons to the conduction band of the semiconductor. The electrons can be captured by adsorbed species on the surface, such as hydroxyl anions or O 2 groups. Owing to the high surface area of nanosized semiconductor systems and the adequate spectrum characteristic of common dyes, the joined systems might absorb a high proportion of the incident solar energy flow (even high as 46%), despite that obtained efficiencies, in solar simulation under AM 1.5 conditions, are still low ( around 7-11%).[120][121][122]

Figure 1 .

 1 Figure1.12 (a) Various electron excitations in TiO2 photocatalysis,[START_REF] Li | Preparation and Characterization of Porous Visible Light Photocatalysts[END_REF] and (b) electronic transitions in the dye sensitizing photodegradation process.[120]

TiO 2 surface by UV irradiation is illustrated in Figure 1 Figure 1 .

 11 Figure 1.13 Mechanism of the photo-deposition method.

Figure 1 .Figure 1 .

 11 Figure 1.14 illustrates the steps of the deposition-precipitation method.

Figure 1 .

 1 Figure 1.15 Scheme of metal ion reduction in solution by ionizing radiation in the presence of alcohol for scavenging oxidizing species. The isolated atoms formed M 0 coalesce into clusters. They associate excess ions. They are stabilized by ligands, polymers or supports. The redox potential E 0 (M n+ /M n ) increases with the nuclearity. The smallest oligomers may undergo corrosion.[144]

Figure 1 .

 1 Figure 1.16 Evaluation of the advancement of radiolytic reduction as a function of the dose absorbed by the sample.

  Typically, for the preparation of Au-Cu1:1 (1:1 corresponds to the atomic ratio Au/Cu) bimetallic colloid, 2.5 mL of HAuCl 4 solution (10 -2 M) and 2.5 mL CuCl 2 solution (10 -2 M) were added into a 25 mL flask. 5 mL of PAA (0.5 M) and 191 μL of 2-propanol were added consequently to get a concentration of 0.1 M after filling up to constant volume. Then, the mixed solution was transferred to jars and sealed with rubber caps and followed by purging with N 2 for 15 minutes. The mixed solutions were covered with aluminium foils because Au III is sensitive to light. After that, the degassed jars were placed in a γ-irradiation source at a dose rate of 1.4 kGy/h. The γ-irradiation source is a panoramic Cobalt ( 60 Co) gamma facility (LCP, University of Paris-Sud, to see Appendix I). The initial activity of the source in 1995 was 7000 Curies during installation. The maximum dose rate in October 2011 was about 1.4 kGy•h -1 .[1]

Figure 2 . 1 )

 21 Figure 2.1). The first step is the synthesis mono-Au or bi-metallic Au-Cu NPs by radiolysis in the presence of PAA. The procedure is similar to what was described in Section 2.2.1.

Figure 2 . 1

 21 Figure 2.1 Procedure for the colloid deposition method by radiolysis.

Figure 2 . 3

 23 Figure 2.3 Procedure for the colloid deposition method by THPC method.

  photoelectrons were detected perpendicularly to the support. A constant analyzer energy mode was used with pass energy of 20 eV. Element atomic percentage determinations were performed using peak area corrected by sensitivity factors using the Thermo Avantage data

  photoactivity under UV illumination. The bimetallic NPs modified P25 show better photoactivity than monometallic Au NPs under UV light. However, the modification with Au-Cu NPs resulted in a decrease of the photocatalytic activity under visible illumination.

  Many endeavorshave been attempted to resolve this challenge, such as doping with N, C, S, P or halogen elements,[1][2][3][4][5][6][7] surface modification with semiconductor and plasmonic metal nanoparticles, and TiO 2 composite with Graphene Oxide or CNTs.[8][9][10][11][12][13][14][15][16][17] All of the above approaches are intended to narrow the band-gap, or to introduce transition energy level between the valence band and conduction band.For the UV light driven photocatalysis, it is important to restrain the recombination of excited electrons and holes. Therefore the photoactivity of TiO 2 systems (especially anatase) was typically enhanced by modification with noble metals such as Pt, Pd, Ir, Au and Ag on the surface.[18][19][20][21][22][23][24][25][26][27][28][29][30] Furthermore, anatase-oxide mixtures using ZrO 2 , SnO, WO 3 , Al 2 O 3 , Y 2 O 3 , or other systems aiming at influencing the electron-hole separation was successfully attempted.[31][32][33][34][35][36][37][38] Only a few studies report modification of titania with bimetallic nanoparticles. Recently, improved photoactivity under visible light irradiation has been reported for modified titania with bimetallic silver-gold NPs prepared by microemulsion method.[39]

  [54][55] Bimetallic Au-Cu NPs on TiO 2 were found more active and selective towards propene epoxidation by nitrous oxide than monometallic Au or Cu samples supported on TiO 2 .[48] Carbon electrodes modified by bimetallic Au-Cu NPs were found promising for electrocatalytic oxidation of glucose in alkaline solution.[56] Brodie-Linder et al. reported Cu II grafted silica based mesoporous materials (SBA-15) by the impregnation method. The presence of the [Cu(NH 3 ) 3 (H 2 O) 3 ] 2+ complex in the reaction solution is the key to control the Cu II loadings from 6-20%. The presence of Cu II on the surface of SBA-15 materials reduces and inhibits the production of H 2 by electron irradiation.[57][58] Many methods are developed to deposit metal nanoparticles on oxide surfaces such as colloid-deposition, precipitation-deposition, and photo-deposition. In our experiments, colloid deposition and deposition-precipitation were used to prepare modified photocatalysts. For the colloid deposition method, the primary step is to synthesize small NPs homogeneous in size and well dispersed in solution. Moreover, stabilizers have an influence on the deposition of metal NPs on the substrates. Hence, choosing appropriate stabilizing agents is important. In our experiments, Poly Acrylic Acid (PAA) was used for preparation of small and homogeneous metallic NPs. The subsequent addition of Polyvinyl Alcohol (PVA) facilitated the deposition of metallic NPs on the support. Deposition-precipitation with urea has been developed on the basis of a deposition precipitation method with NaOH initiated by Haruta.[59] In the DP method, the metal precursors are mixed with the supports forming an aqueous suspension, subsequently precipitated as a hydroxide by increasing the pH value. The key point of the DP is the prevention of precipitation away from the surface of the supports.[60-61] Zanella and Hugon et al. synthesized Au, Pd or some other noble metal NPs supported on TiO 2 and Al 2 O 3 with DPU method.[48, 62-63] However, these works just focused on the deposition of noble metals and monometallic systems. Furthermore, calcination and H 2 reduction are necessary to obtain metal particles on the support. In this thesis, we, for the first time, attempted to prepare noble Au and non-noble Cu bimetallic NPs supported on TiO 2 by a modified DPU method. Au and Cu were deposited at different stages followed by gamma reduction to obtain Au-Cu bimetallic NPs modified titania. The prepared photocatalysts were studied for degradation of model pollutants, such as phenol, rhodamine B (RhB) and methyl orange (MO) under UV and visible illumination.TRMC measurements were carried out to reveal the correlation of electron-hole recombination with photocatalytic activity after UV excitation.

3. 2

 2 Colloid-deposition of Au-Cu metallic NPs on P25 by radiolysis in the presence of stabilizers 3.2.1Radiolytic synthesis of Au monometallic nanoparticles

Figure 3 . 1 (

 31 Figure 3.1 (a) show the evolution with dose of the spectra of a solution containing AuCl 4 -in the presence 2-propanol (as electron scavenger) and of PAA (as stabilizer).

Figure 3 .

 3 Figure 3.1 (a) Evolution of UV-Visible absorption spectrum with irradiation time of a HAuCl4 solution containing PAA and 2-propanol, [HAuCl4] = 10 -3 M, (b) evolution of spectrum with standing time in irradiated HAuCl4 solution (180 min). [PAA] = 0.1 M, [2-propanol] = 0.1 M, optical path: 2 mm. The solutions were degassed with N2 prior to irradiation, dose rate: 1.4 × 10 3 Gy/h.

  CuSO 4 solutions containing polyacrylic acid and polyvinyl alcohol induces the formation of copper nanoclusters absorbing at 570 nm, and which dissolve readily when brought into contact with oxygen. Oligomers of lower nuclearity stabilized by the polyacrylate ions are shown to absorb at 292, 350 and 455 nm.[68]

  and Au III in the presence of polyacrylic acid (PAA) and/or polyvinyl alcohol (PVA). 2-propanol was added in the solution to scavenge the OH• oxidative radicals produced by radiolysis. The concentrations of 2-propanol and of each polymer were kept at 0.1 M. The details of the synthesized Au-Cu colloids are listed in Table 3.1.

3 . 2 .

 32 colloidal solution of Au-Cu bimetallic NPs with a molar ratio of 1:3 prepared by radiolysis in the presence of stabilizer PAA. b AuCu11PP: PP indicates the mixture of polymer PAA and PVA. The UV-Vis absorption spectra of the prepared Au-Cu colloids with different Au/Cu ratios are shown in Figure The absorption peak at ~530 nm increases with gamma irradiation time. These absorption is due to the surface plasmon resonance (SPR) of gold and copper particles. Au and Cu are known to exhibit a plasmon band with maxima respectively at 520 and 570 in aqueous solution. It is known that surface plasmon resonance is due to the collective oscillation modes of the free conduction band electrons induced by the localized electromagnetic field. The intensity of plasmon band is proportional to the concentration of metal atoms.[73-74] As a result, the maximum intensities of absorption peaks with various Au-Cu ratios (in case of alloys) are relatively proportional to the overall concentration of Au and Cu. It should be pointed out the absorption peak is located at the same wavelength and does not shift with irradiation time (see Figure 3.2a,c,e). It can be assumed that the compositions of Au-Cu bimetallic nanoparticles were the same from the beginning to the end of irradiation, namely the synthesized Au-Cu bimetallic nanoparticles have probably an alloyed structure. The as-prepared Au-Cu colloids are stable even after more than 6 days (Figure 3.2b,d).

Figure 3 . 2

 32 Figure 3.2 Evolution of UV-Visible absorption spectra with irradiation time of irradiated HAuCl4/CuCl2 solutions containing PAA (0.1 M) and 2-propanol (0.1 M) (a) AuCu13PAA, [HAuCl4] = 5×10 -4 M, [CuCl2] = 1.5×10 -3 M, (c) AuCu11PAA, [HAuCl4] = 10 -3 M, [CuCl2] = 10 -3 M, (e) AuCu31PAA, [HAuCl4] = 1.5×10 -3 , [CuCl2] = 5×10 -4 M; and evolution of UV-visible spectra with standing time of corresponding solutions (b) AuCu13PAA (irradiated 210 min), (d) AuCu11PAA (irradiated 180 min). Optical path: 2 mm. The solutions were degassed with N2 prior to irradiation, dose rate: 1.4 × 10 3 Gy/h.

Figure 3 . 3

 33 Figure 3.3 TEM images of Au-Cu nanoparticles with different compositions (a) AuCu13PAA, (c) AuCu11PAA, (d) AuCu31PAA, prepared by radiolysis in the presence of PAA. Inset: the corresponding SAED patterns. (b) Size distribution of the sample AuCu13PAA.

Figure 3 . 4

 34 Figure 3.4 TEM images of the sample AuCu31PP (a, b), AuCu11PP (c, d), and AuCu13PP (e, f), prepared by radiolysis in the presence of PAA and PVA.

Figure 3 . 5

 35 Figure 3.5 TEM images of the sample AuCu13PVA prepared by radiolysis in the presence of PVA, at a lower (a) and a higher (c) magnification; size distributions of the sample AuCu13PVA at (b) the larger size region and (d) smaller size region.

Figure 3 . 6 3 . 6 .

 3636 Figure 3.6 Evolution of UV-Vis absorption spectra of the sample (a) AuCu31PP, [HAuCl4] = 1.5×10 -3 M, [CuCl2] = 5×10 -4 M, (b) AuCu11PP, [HAuCl4] = 10 -3 M, [CuCl2] = 10 -3 M, (c) AuCu13PP, [HAuCl4] = 5×10 -4 M, [CuCl2] = 1.5×10 -3 M, and (d) AuCu13PVA, [HAuCl4] = 5×10 -4 M, [CuCl2] = 1.5×10 -3 M. [PAA] = 0.1 M and [PVA] = 0.1 M, [2-propanol] = 0.1 M. Dose rate = 1.4 kGy/h. Optical path: 2mm.

a

  AuCu31PAA@P25(1): The modified P25 with AuCu31PAA bimetallic NPs, (1) indicates the mass percentage of Au to P25 is 1%.

Figure 3 . 7

 37 Figure 3.7 TEM images of the sample AuCu13@P25PAA(1) at a lower (a-c) and higher magnification (d), inset: the fourrier transform of the image, the mass percentage of Au to P25 is 1%.

Figure 3 . 8

 38 Figure 3.8 TEM images of AuCu13@P25PP(1).

Figure 3 . 9

 39 Figure 3.9 Diffuse reflectance UV-Vis spectra of the modified photocatalysts and pure P25.

and

  Pt-modified TiO 2 .[76][77] Such an effect can be attributed to a stronger stabilization of the conduction band of TiO 2 by the conduction band of the Au-Cu NPs compared to the stabilization of the valence band. Au and Cu nanoparticles are known to exhibit a plasmon

3 COOHScheme 3 - 1

 331 Scheme 3-1 Molecular formula of rhodamine B.

Figure 3 .

 3 Figure 3.10 Evolution of UV-Vis absorption spectra of RhB in photocatalytic reaction with (a) AuCu13PAA@P25 (1), and (b) AuCu13PAA@P25(0.5) under UV illumination, optical path 2mm, (c) time runs of RhB relative concentration in photodegradation in the presence of pristine and modified P25 under UV illumination.

3 . 12 .

 312 photodegradation of RhB with Au-Cu/P25 photocatalysts under visible illumination are shown in Figure As it was studied widely, the photodegradation of RhB follows different processes: photo-bleaching and N-deethylation.[80-83] Considering the molecular structure of RhB is N,N,N′,N′-tetraethylrhodamine (Scheme 3-1), it can be deethylated into various intermediates by N-deethylation. As is shown in Table 3.3, the intermediates have different absorption peaks, decreasing to shorter wavelength with the N-deethylation processing. It can be obviously seen from Figure 3.12 that the absorption peaks shift a lot and the shift is accompanied by a decrease of the absorbance. It is observed that: (1) the absorbance maxima decreased at the beginning of illumination and increased at longer illumination time; (2) the absorption peaks did not shift at longer irradiation time. Here, we give the probable interpretations. For the first question, the intermediates generated in the N-deethylation process have different extinction coefficient. Apparently, ε increases with the decrease of ethyl in the molecule (not including RhB). For instance, ε of TER (3 ethyl) is 5.5

Figure 3 .

 3 Figure3.12 UV-vis absorption spectra of RhB degraded under visible light in the presence of (a) AuPAA@P25(1), (b) AuCu31PAA@P25(1), (c) AuCu11PAA@P25(1), (d) AuCu13PAA@P25(1). Optical path: 2 mm; illumination, λ > 450 nm; photocatalyst suspended 1 g/L.

Figure 3 . 14 .

 314 Figure 3.14. Without photocatalyst, there is no shift in the position of the absorption wavelength maximum of the RhB with irradiation time (Figure 3.14B). The modified Au-Cu/P25 results in larger wavelength changes compared to the pristine P25. The ultimate absorption peaks of photodegraded RhB with the modified photocatalysts were located at 498 nm, indicating the intermediates of rhodamine degradation accumulated. Among the as-prepared photocatalysts, more Au proportion leads to faster N-de-ethylation. Qu et al.

Figure 3 .

 3 Figure 3.14 Wavelength shifts of the absorbance maxima in photocatalytic degradation of RhB with no photocatalyst (B, solid square ■), P25 (C, solid circle •), AuP25PAA@P25(1) (D, solid triangle ▲), AuCu31PAA@P25(1) (E, empty square □), AuCu11PAA@P25(1) (F, empty circle ○), AuCu13PAA@P25(1) (G, empty triangle △); illumination, λ > 450 nm.

Figure 3 .

 3 Figure 3.15 Evolution of UV-Vis spectra of RhB photodegraded in the presence of the sample (a) AuCu11PP@P25(1), and (b) AuCu13PP@P25(1) under visible light. Illumination > 450 nm, optical path: 2 mm.

[ 84 ] 3 . 16 .

 84316 Au-Cu/TiO 2 samples were treated with water under stirring at 80 °C for 150 minutes. The supernatant was slightly pink. The precipitate is dried in an oven at 60 °C and then finely ground. The color of the photocatalyst obtained after wash-treatment turned to a slightly lighter colour than before. The results of photocatalytic degradation of RhB under visible light in the presence of AuCu13PAA@P25(1) after treatment are illustrated in Figure The absorbance maxima decreased and the wavelength of absorbance maxima shifted toward shorter wavelengths with illumination. Compared to AuCu13PAA@P25(1) without treatment, a decrease of the photocatalytic activity was observed as well as a shift in the absorbance maximum wavelength with irradiation time(Figure 3.16b). It can be concluded that washing caused partial desorption of the Au-Cu bimetallic NPs leading to a decrease in the photocatalytic activity.

Figure 3 .

 3 Figure 3.16 (a) Evolution of the UV-Vis spectra of RhB photodegraded in the presence of the sample AuCu13PAA@P25(1) after washing with water, (b) time courses of RhB relative concentration and wavelength changes of absorbance maxima in photocatalytic degradation of RhB in the presence of the sample AuCu13PAA@P25(1) (square), and after wash-treatment with water (circle).

Figure 3 .

 3 Figure 3.17 DRS-TGA of the sample (a) AuCu31PAA@P25(0.5), (b) AuCu11PAA@P25(1), and (c) AuCu13PAA@P25(1).

Figure 3 .

 3 Figure 3.18 Evolution of UV-Vis spectra of RhB photodegraded in the presence of the sample (a) AuCu31P25PAA(0.5) after calcination, (b) AuCu11P25PAA(1) after calcination, and (c) AuCu13P25PAA(1) after calcination. Illumination > 450 nm. Optical path: 2 mm.

  Au-Cu bimetallic NPs were synthesized by radiolysis and deposited on the surface of P25 via a colloid-deposition route. The polymers PAA and PVA were used, separately or together, as stabilizers. The obtained Au-Cu NPs in the presence of polymer PAA are smaller and more homogeneous in size compared with the NPs stabilized by PVA. The mean nanoparticle size decreased obviously when Au is associated with copper (3.6 nm for AuCu13PAA NPs). The presence of PVA improved the deposition efficiency of Au-Cu bimetallic NPs on P25. The synthesized photocatalysts exhibit reduced photoactivities

  TiO 2 with Au-Cu bimetallic NPs by radiolytic colloid-deposition route 3.3.1 Characterization of the Photocatalysts Details of the synthesized photocatalysts are summarized in Table 3.4. The resulting modified monometallic catalyst was 0.5 wt% Au on P25, referred as Au/P25. The bimetallic Au-Cu modified catalysts were 0.45 wt% Au and 0.05 wt% Cu on P25 (Au/Cu molar ratio = 3:1), referred as AuCu31/P25. In the same manner, the supported bimetallic photocatalysts of 0.38 wt% Au and 0.12 wt% Cu on P25, 0.25 wt% Au and 0.25 wt% Cu on P25 were named as AuCu11/P25 and AuCu13/P25, respectively.

Figure 3 .

 3 Figure 3.19 TEM images of metallic nanoparticle modified P25 and the corresponding size distributions of the supported metallic NPs (a, b) Au/P25, (c, d) AuCu11/P25, and (e, f) AuCu13/P25.

Figure 3 . 20 (

 320 trace a-c). The absorption peaks at ~525 nm reveal the formation of Au-or Au-Cu nanoparticles. In addition, the broadening of the absorption bands for AuCu31PAA and AuCu13PAA toward longer wavelength region was observed, indicating the formation of bimetallic Au-Cu nanoparticles. The bimetallic nature of the nanoparticles cannot fully be determined by UV-vis spectra alone. Though it cannot be definitively ascertained whether the nanoparticles are alloys or core-shells, it strongly suggests that the Au-Cu nanoparticles are bimetallic in nature.After deposition of the PAA/PVA stabilized Au-Cu colloids on P25, the supernatants were monitored by UV-vis spectra (Figure3.20 trace d-f). There was no absorption in visible region, demonstrating that all the NPs were deposited on the support. The absorbance in the UV region was attributed to the stabilizers, PAA or PVA which are absorbing strongly in the UV region (Figure3.20 g, h).

Figure 3 .

 3 Figure 3.20 UV-vis spectra of the colloid (a) AuPAA, (b) AuCu13PAA, (c) AuCu31PAA, the supernatant of (d) Au/P25, (e) AuCu11/P25, (f) AuCu31/P25 and the solution of (g) PVA (0.1 M), and (h) PAA and PVA (0.1 M of each).

3 . 21 .

 321 Fringes with uniform thickness at the edges of modified P25 particles without thermal treatment and hydrogen reduction can be seen in Figure Although the Au-Cu modified P25 samples were washed several times with water to remove the stabilizing polymers, the metal nanoparticles or substrate P25 were still covered by the polymer molecules. The polymers on the surface are unfavourable to the photocatalytic activity because of the decrease of contact between photocatalyst surface and model dyes. It is necessary to remove these polymers. Hence calcination, following by H 2 reduction, was attempted later.

Figure 3 .

 3 Figure 3.21 TEM images of (a) Au/P25, (b) AuCu11/P25, and (c) AuCu13/P25 at a high magnification, all the samples are without thermal treatment and H2 reduction.

Figure 3 .

 3 22 shows the spectra of pristine and modified P25. It is worth noting that the modified samples exhibit a light shift in the band-gap transition to larger wavelengths. For pristine P25, the absorption edge is 398 nm, and for the modified P25 it increases to 420 nm. This effect was previously observed in the case of Ag-and Pt-modified TiO 2 . This effect can attributed to a stronger stabilization of the conduction band of TiO 2 by the conduction band of the Au-or Au-Cu particles compared to the stabilization of the valence band. The absorbance in the visible region is always higher for the modified TiO 2 than pure P25. The absorbance maxima of the modified photocatalysts Au/P25, AuCu31/P25 and AuCu13/P25 are at 561 nm, 573nm and 580 nm, respectively. These maxima explain the pink-violet color of the modified TiO 2 samples. The sample Au/P25 shows a deep purple color while AuCu13/P25 is pink-violet. The absorption peak of Au/P25 shows a shift toward longer wavelength compared to that of the stabilized Au colloid (at 525 nm), indeed the plasmon band is sensitive to the environment and the substrate. The plasmon bands in case of Au/TiO 2 and Au-Cu/TiO 2 are blue-shifted because of the coupling between the metal nanoparticles and TiO 2 support having a high reflective index. It should also be pointed out that the absorption peak exhibits a shift towards longer wavelength with the increase of Cu proportion in the bimetallic nanoparticles. Au and

Figure 3 .

 3 Figure 3.22 DRS spectra of pure P25 and the modified samples with thermal treatment.

Figure 3 .

 3 Figure 3.23 DRS spectra of pristine and modified P25 without thermal treatment.

Figure 3 .

 3 24b. The sample AuCu13/P25 shows bimodal pore size distributions with a maximum pore diameter of 2.5 nm and 29 nm respectively, indicating the presence of intra-aggregated and inter-aggregated pores (represented by the hysteresis loop in the higher P/P 0 range). It was reported that Degussa P25 titania powders prepared by flame hydrolysis of TiCl 4 exhibited a monomodal pore-size distribution with a maximum pore diameter at 2.3 nm. The significant difference in pore-size distribution for the modified sample is probably due to the non-uniform drying stresses when the modified P25 slurry was drying.[85, 90] It is noteworthy that the pore volume of intra-aggregated pore (at 3.1 nm, Figure3.24b) is remarkably smaller compared to that of photocatalysts prepared by THPC or DPU method (Figure3.57b and Figure3.40b). The possible cause is that the high-concentrated stabilizing polymer PAA and PVA induced a uniform particle packing on the account of electrostatic repulsion.[85]

Figure 3 .

 3 Figure 3.24 (a) N2 adsorption-desorption isotherms for the sample AuCu13/P25. Inset: BET surface area plot, (b) the corresponding pore size distribution.

  photocatalytic activity compared to pure P25 (k = 6.72 × 10 -3 s -1 ). AuCu13/P25 exhibited the highest activity among the modified P25 catalysts, even higher than pristine P25.

Figure 3 .

 3 Figure 3.25 The time runs of relative RhB concentration in photodegradation in the presence of pristine and modified P25 under (a) UV, and (b) visible illumination.

3. 3 . 2 . 1 3 . 25 .

 321325 photo-sensitivity of RhB, namely the injection of excited electrons from RhB molecules to the conduction band of TiO 2 , consequently resulting in oxidation of RhB molecules. The results of visible light driven degradation of RhB are shown in Figure 3.25. All of the modified P25 resulted in slower degradation of RhB compared to bare P25. Furthermore, the rate for RhB photodegradation decreased with the rising copper proportion. AuCu13/P25 (v = 0.40 mmol L - 3 min -1 ) has the lowest photocatalytic activity, decreasing by nearly half compared with pure P25 (0.70 mmol L -3 min -1 ).

Figure 3 .

 3 Figure 3.26 Histogram of rate constants of the first-order kinetic of RhB or phenol photodegradation in the presence of pristine and modified P25 under UV light.

Figure 3 .

 3 Figure 3.27 The time runs of relative RhB concentration in photodegradation in the presence of pristine and modified P25 under (a) UV, and (b) visible illumination without thermal treatment.

  Time resolved microwave conductivity (TRMC) experiments were conducted to study the electronic properties of the modified P25. The TRMC measurements realized under UV laser irradiation show TRMC signals presented in Figure 3.29. The TRMC signal has two principal parameters: the maximum value (I max ) and the decay I(t). Halftime life τ 1/2 is the time to obtain intensity I max /2 of the signal. The surface modification with Au-or Au-Cu NPs show a strong influence on the charge-carrier decay of P25. The overall decays of the signal for all the modified P25 are accelerated (Figure 3.28). τ 1/2 of pure P25, Au/P25, AuCu11/P25, and AuCu13/P25 are 88.9 ns, 22.7 ns, 55.4, and 36.3 ns, respectively (Table3.4). This influence on the decay can be related to the activity in the case of RhB or Phenol degradation under UV light.

Figure 3 .

 3 Figure 3.28 TRMC signals of pristine and the modified P25 Au/P25, AuCu11/P25 and AuCu13/P25 after thermal treatment.

  photoactivity. Such acceleration has also been reported for the Ag modified TiO 2 .[76]

Figure 3 .

 3 Figure 3.29 TRMC signals of pristine P25 and the modified P25 Au/P25, AuCu31/P25, AuCu11/P25 and AuCu13/P25, without thermal treatment.

  Au-Cu bimetallic NPs were synthesized by radiolysis in the presence of the stabilizing polymer PAA, and deposited on P25 in the presence PAA and PVA. The affixing of Cu to Au NPs resulted in a smaller nanoparticle size; especially the smallest NPs are formed at a ratio of Au/Cu 1:3 (mean size is 3.8 nm). The Au-Cu NPs modified P25 were characterized using TEM, DRS, and TRMC. The Au-Cu bimetallic NPs have an alloyed structure. The TRMC signal showed that the Au-Cu alloyed NPs supported on P25 are efficient in scavenging electrons, contributing to the enhancement of the photoactivity under UV illumination. The bimetallic NPs modified P25 exhibit better photoactivity than TiO 2 modified with monometallic Au NPs under UV light. The modification with Au-Cu NPs resulted in the decrease of the photocatalytic activity under visible illumination.

Figure 3 .

 3 Figure 3.30 UV-Vis absorption spectra of (a) AuCu1:1/P25DPU supernatant before irradiation, (b) AuCu1:1/P25DPU supernatant after irradiation, (c) 1.25×10 -4 M of Au solution, (d) 1.25×10 -4 M of Cu solution, and (e) 2.5×10 -2 M of urea solution after water bath at 90 °C for 2 hours. Optical path: 10 mm.

Figure 3 .

 3 Figure 3.31 UV-Vis absorption spectra of (a) supernatant of Au/P25DPU after irradiation, (b) 2.5×10 -4 M of Au solution, and (c) 2.5×10 -2 M of urea solution after water bath at 90 °C for 2 hours. Optical path: 10 mm.

Figure 3 .Figure 3 .

 33 Figure 3.32 UV-Vis absorption spectra of (a) the supernatant of Cu/P25DPU before irradiation, (b) supernatant of Cu/P25DPU after irradiation, (c) 2.5×10 -4 M Cu solution, and (d) 2.5×10 -2 M of urea solution after water bath at 90 °C for 2 hours. Optical path: 10 mm.

3 . 34 .

 334 22 μm was used to remove the TiO 2 colloidal particles. The UV-Vis absorption spectra are shown in Figure After the first filtration, the absorption peak at 262 nm decreased significantly. A series of filtration induced a decrease in the absorption at 262 nm. However, the peak remains with a low intensity, probably because the very small TiO 2 NPs cannot be removed by the filter (see Figure 3.34c,d,e,f).

Figure 3 .

 3 Figure 3.34 UV-Vis spectra of Au/P25DPU supernatants before and after several filtration cyclings. Optical path: 10 mm.

Figure 3 .

 3 Figure 3.35 TEM and HR-TEM images of (a,b) Au/P25DPU, (c,d) AuCu1:1/P25DPU, (e,f) AuCu1:3/P25DPU. Dose rate: 170 Gy/min, dose in total: 20 kGy.

3 . 36 .Figure 3 . 36 ,

 336336 across the different Au-Cu NPs of the sample AuCu1:1/P25DPU. A representative nanoparticle is shown in Figure The Au-Lα1 and Cu-Kα1 line scan signals show the intensity disperse apparently symmetrically along with the scanning regions of the NPs, implying that the Au and Cu atoms are approximately homogeneously dispersed. Both of the Au-Lα1 and Cu-Kα1 peaks in the center of the Au nanoparticle demonstrate that the Au and Cu atoms are apparently homogeneously dispersed. Therefore, the EDS profile scans evidenced the alloyed structure of bimetallic Au-Cu NPs. Considering the low diffraction capability of Cu, the Cu signal is much lower than that of Au. Indeed, the atomic ratio of Au to Cu was around 1:2 from the EDS spectrum, deviated from the nominal ratio of 1:1. Furthermore, EDS elemental mapping was also attempted to detect respective elements (bottom). AuLα1 and CuKα1 maps clearly identified the alloyed structure of Au-Cu bimetallic NPs. HR-TEM images and EDS analyses show that the supported bimetallic Au-Cu NPs on P25 are nanoalloys.

Figure 3 .

 3 Figure 3.36 Top. HAADF-STEM image of AuCu1:1/P25DPU (left) and the corresponding Energy Dispersive X-ray Spectroscopy line scan across an Au/Cu1:1 nanoparticle (right. The The profile was taken along the purple line, the blue line corresponds to CuK and the red one to AuL signal). Bottom. Mapping EDS analysis performed at a metallic nanoparticle of AuCu1:1/P25DPU.

Figure 3 .

 3 Figure 3.37 (a) Diffuse reflectance UV-Vis spectra of the modified photocatalysts and P25, and (b) the plot of transformed Kubelka-Munk function versus the energy of incident light, inset is the partial enlarged view.

Figure 3 .

 3 Figure 3.38a. The peaks of Cu2p, Au4f, Ti2p, O1s, and C1s can be clearly detected. The weak peaks of C are contamination peaks arising mainly from CO 2 adsorbed on the surface of the sample .[96] The high resolution XPS spectrum of Cu in the region 2p for the sample

Figure 3 .Figure 3 .Figure 3 . 40 .

 33340 Figure 3.38 (a) XPS survey spectrum and (b) Cu2p region of the XPS spectra of sample AuCu1:3/P25DPU. The inset in b is Auger CuLMM spectrum of the sample.

Figure 3 .

 3 Figure 3.40 (a) N2 adsorption-desorption isotherms for the sample AuCu1:3/P25DPU. Inset: BET surface area plot of, (b) the corresponding pore size distribution.

Figure 3 . 41 .

 341 The absorption of the main peak at 465 nm is proportional to the concentration of MO. The slight absorbance decrease at 0 minute was due to the adsorption of MO on P25 surface. The modified P25 is highly photoactive in degradation of MO under UV light. 40% of MO was decomposed after 5 minutes, and nearly 98% of MO was degraded after 20 minutes (complete disappearance of absorption maxima at 465 nm and 272 nm).

Figure 3 .Table 3 . 6

 336 Figure 3.41 UV-vis absorption spectra of MO in photodegradation under UV illumination with the sample AuCu1:3/P25DPU.

Figure 3 .

 3 Figure 3.42 (a) The time runs of relative MO concentration, and (b) first-order kinetics in photodegradation under UV illumination in the presence of P25 (filled square), Au/P25DPU (filled circle), AuCu1:1/P25DPU (filled triangle), AuCu1:3/P25DPU (open square), Cu/P25DPU (open circle), and no photocatalyst (open triangle).

Figure 3 .

 3 Figure 3.43 Rate constants of the first-order kinetic of MO photodegradation under UV illumination with, (a) P25, (b) Au/P25DPU, (c) AuCu1:1/P25DPU, (d) AuCu1:3/P25DPU, and (e) Cu/P25DPU.

Figure 3 .

 3 44a shows the absorption spectra of the supernatant of Au/P25DP. The absorbance maximum at 220 nm can be attributed to Au III . 1.25×10 -4 M of HAuCl 4 solution was used as the reference. The remained Au concentration was calculated as around 7.35×10 -5 M. Compared to the initial Au concentration (5×10 -4 M) in the suspension, at least one seventh of Au ions was used in the deposition procedure.

Figure 3 .

 3 Figure 3.44 UV-vis absorption spectra of (a) the supernatant of Au/P25DP, and (b) 1.25×10 -4 M HAuCl4 solution. Optical path: 10 mm.

Figure 3 .

 3 Figure 3.45 TEM images of the photocatalysts (a) Au/P25DPU-s, (b) AuCu1:3/P25DPU-s, (c) Au/P25DP at dose rate of 20 Gy/min; and (d) Au/P25DP at dose rate of 170 Gy/min.

  .47a). For the degradation of RhB under visible light, AuCu1:3/P25DP exhibits nearly the same degradation kinetics as bare P25, and Au/P25DP shows a decreased activity for RhB photodegradation (Figure 3.47b).

Figure 3 .

 3 Figure 3.46 The time runs of RhB concentration in photodegradation under (A) UV, and (B) visible light with (a) P25, (b) Au/P25DPU-s, (c) AuCu1:3/P25DPU-s, (d) Cu/P25DPU-s, and (e) no photocatalyst.

Figure 3 .

 3 Figure 3.47 The time runs of RhB concentration in photodegradation under (A) UV, and (B) visible light with (a) P25, (b) Au/P25DP, (c) AuCu1:3/P25DP, and (d) no photocatalyst.

  photoactivity was obtained with modified P25 with Au/Cu 1:3 NPs, increasing by two times compared with pristine P25. The supported Au-Cu NPs are stable in the photocatalytic reaction as confirmed by TEM and XPS techniques. The modification of P25 with Cu nanoparticles also induces larger enhancement in the photocatalytic activity compared with its modification with Au NPs. This implies a more economical way for improving the photocatalytic activity of TiO 2 . The two-step deposition precipitation with urea is efficient for depositing Au and Cu ions on P25, and it is applicable to deposition of other metals on TiO 2 or on other substrates.

3. 5 8 .

 58 Colloid-deposition of Au-Cu metallic NPs on P25 by chemical method with THPC3.5.1 Characterization of PhotocatalystsThe resulting modified monometallic catalyst was 0.5 wt% Au on P25, referred as Au/P25-T, where T refers to the agent THPC in this chemical method. Correspondingly, AuCu11/P25-T represents the sample P25 modified with Au-Cu NPs with a molar ratio of 1:1.The total nominal metal content was kept at 0.5 wt%. The compositions of the modified P25 are listed in details in Table3.The colours of the modified P25 are varied depending on the metal supported. The sample Au/P25-T is typically violet because of the plasmon band of Au NPs. The colour of AuCu13/P25-T altered to grey-violet. While Cu/P25-T exhibited light-green colour, owing to the Cu deposition on P25.

  b before: the prepared samples were before thermal treatment; c after: the prepared samples were thermal treated. d AuCu13/P25-T: modified P25 with Au-Cu NPs with a molar ratio 1:3, prepared by chemical method with THPC.3.5.1.1 TEM CharacterizationTEM observations show small nanoparticles homogenously dispersed on the surface of P25 (Figure3.48). Calcination (used to remove the THPC residue) and hydrogen treatment (used for further reduction) induces an increase in NPs size. For the bimetallic Au-Cu NPs modified samples, the respective increases in average size are 2.3 nm and 1.9 nm for AuCu11/P25-T and AuCu13/P25-T. However, the monometallic Au modified P25 increased by 3.8 nm for the mean size (Figure3.49 and Figure3.50). The mean particle size had slightly (after calcinations and H 2 treatment) decreased from 6.9 nm (for pure gold NPs) to 4.8 nm (for AuCu13 NPs) when gold is associated to Cu. In the latter case, smaller NPs and more homogeneous in size were obtained (see Figure3.48c).

Figure 3 .

 3 Figure 3.48 TEM images of the modified P25 with Au and Cu NPs. (a) Au/P25-T, (b) AuCu11/P25-T, (c) AuCu13/P25-T, and (d) Cu/P25-T.

Figure 3 .

 3 Figure 3.49 Size distributions of Au and Au-Cu NPs deposited on P25 prepared by chemical method with THPC, (a) Au/P25-T, (b) AuCu11/P25-T, and (c) AuCu13/P25-T.

Figure 3 .Figure 3 .

 33 Figure 3.50 Size distributions of the modified P25 by chemical method with THPC, (a) Au/P25-T, (b) AuCu11/P25-T, and (c) AuCu13/P25-T. The samples were without thermal treatment.

Figure 3 .

 3 Figure 3.51c,d shows the atomic composition in a Au-Cu nanoparticle. The contrast is roughly proportional to Z 2 where Z is the atomic number. Therefore due to the difference of the atomic number of Au and Cu the contrast is easy to see. Every spot in the image of Figure 3.51c corresponds to an atomic column. The bright spots correspond therefore to column richer in Au and the lighter spots to columns richer in Cu. We found that the nominal composition of the sample (Au/Cu = 1:1) is roughly followed in the image.

Figure 3 . 3 . 52 .

 3352 Figure 3.51 HAADF-STEM and BF-STEM images of (a, b) Au/P25-T, (c, d) AuCu11/P25-T, and (e) AuCu13/P25-T.

Figure 3 .

 3 Figure 3.52 Energy dispersive X-ray spectroscopy line scans across external nanowires of nanoparticles (profiles were taken along the green or violet lines) and corresponding HAADF-STEM images for the samples of (a1, a2) Au/P25-T, (b1, b2) AuCu11/P25-T, and (c1, c2) AuCu13/P25-T.

Figure 3 .

 3 53a shows chemical maps for Au-Cu bimetallic NPs of the sample AuCu11/P25-T. Au (K, M) and Cu (K, L) maps clearly revealed the presence of the bimetallic Au-Cu alloys (Composite image of Au-Cu). The information obtained from high resolution TEM images, HAADF-STEM and EDS profile analyses of the individual NPs confirm the formation of bimetallic alloy Au-Cu NPs.

Figure 3 .

 3 Figure 3.53 (a) HAADF-STEM image of AuCu11/P25-T and EDS elemental mapping analysis at the NPs; (b) EDS analysis of the selected area in a.

3. 5 . 1 . 4

 514 XPS Analysis XPS analysis which is performed under 400 µm spot size probes a large number of particles layered onto the holder. It is a statistical measurement that provides the global response of a large number of nanoparticles. It is a complementary information of the other ones issued from local approaches. Obviously XPS demonstrates homogeneity of the preparations because signals for different layering of NPs on substrates are very reproducible in global chemical composition as in energy distribution whatever the core level investigated. Different preparations of Au/Cu-P25 TiO 2 NPs have been investigated. For each preparation XPS observed expected responses: Au, Ti, O core levels for Au/P25 NPs; Au, Cu, Ti, O core levels for Au/Cu/P25 NPs; Cu, Ti, O core levels for Cu/P25 NPs. Using atomic % determination for each element, we observed specific ratio ((% Au or % Au+ % Cu or % Cu) /(% Ti + %O)), which characterized the "metallic NPs /P25_TiO 2 " networks and reflected the relative dispersion for both component after calcinations under air then reduction under H 2 at 500 °C. Again the different ratios are rather reproducible indicating the homogeneity of the dispersions.

Figure 3 .

 3 Figure 3.54 Au4f region of the XPS spectra of the sample AuCu1:1/P25 (blue) and Au/P25 (red).

Figure 3 .

 3 Figure 3.55 Cu2p region of the XPS spectra of the sample AuCu1:1/P25 (blue) and Cu/P25 (red).

Figure 3 .

 3 Figure 3.56 DRS spectra of pure and modified P25 with Au and Cu nanoparticles (a) thermal treated, (b) without thermal treatment, prepared by the chemical method with THPC.

3. 5 . 1 . 6 Figure 3 . 57 .

 516357 BET AnalysisNitrogen sorption isotherms were generated to investigate the porous structure and the Brunauer-Emmett-Teller (BET) surface areas of the modified photocatalysts. The data are shown in The sample AuCu13/P25-T exhibits a typical-IV isotherm pattern and it shows steep hysteresis loop at high relative pressure (Figure3.57a).[85][86][87][88] The hysteresis loop as a result of capillary condensation of N 2 inside the pores is ascribed to the existence of mesopores.[87][88] The BET surface area was obtained as 50.09 m 2 g -1 . Compared to the reference P25 (51.4 m 2 g -1 ), a very slight decrease in the BET surface area was found. The pore size distribution for the modified P25 was obtained by the Barrett-Joyner-Halenda (BJH) method[89] and is presented in Figure3.57b. The sample AuCu13/P25-T shows bimodal pore size distributions with a maximum pore diameter of 2.32 nm and 43.4 nm respectively, indicating the presence of intra-aggregated pore and inter-aggregated pore (represented by the hysteresis loop in the higher P/P 0 range). Degussa P25 titania powders prepared by flame hydrolysis of TiCl 4 exhibited a monomodal pore-size distribution with a maximum pore diameter at 2.3 nm. The significant difference in the pore-size distribution for the modified sample and the emergence of peak at 43.4 nm are probably due to the non-uniform drying stresses when the modified P25 slurry was dried.[85, 90] TiO 2 particles aggregate to generate larger pores.

Figure 3 .

 3 Figure 3.57 (a) N2 adsorption-desorption isotherms for the sample AuCu13/P25-T. Inset: BET surface area plot, (b) the corresponding pore size distribution.

3. 5 . 2 . 1

 521 Photocatalytic activity under UV illumination Evonik P25 is highly photoactive in phenol degradation under UV-Vis light. A representative plot on kinetics degradation of phenol in the presence of pristine P25 under UV and characterisation of degradation products by HPLC are shown in Figure 3.58. After 2 minutes of illumination, 50% of phenol was degraded. Meanwhile, intermediates, benzoquinone and hydroquinone were produced and accumulated. Their concentrations reached maxima at 3 minutes. After 6 minutes, phenol and the intermediates were completely degraded.

Figure 3 .

 3 Figure 3.58 The time runs of phenol, benzoquinone, and hydroquinone concentrations in photodegradation with pristine P25 under UV illumination. The initial concentration of phenol is 2×10 -4 M (equal to 18.82 ppm).

Figure 3 .

 3 Figure 3.59a shows the time runs of relative phenol concentration photodegraded under UV light in the presence of pure and modified P25. Compared to pure P25, all of the modified catalysts exhibit faster decreases for phenol concentration. These photocatalytic reactions can

7 and 3 . 8 ,

 38 the rate constants of the first-order kinetic of phenol photodegradation for the bare-and modified TiO 2 are shown in Table 3.10 and plotted in Figure 3.59b. Surprisingly, modification with Cu leads to better enhancement in the photocatalytic properties compared to modification with Au. Modification of P25 by Au-Cu NPs induces the highest enhancement in the photocatalytic properties. The best photocatalyst is the Au-Cu/P25 with Au/Cu 1:3. The photocatalytic efficiency for the sample AuCu13/P25-T increased by 40% than that for pure P25 based on the rate constants.

Figure 3 .

 3 Figure 3.59 (a) The time runs of phenol relative concentration in photodegradation under UV illumination with pure P25, and modified P25, (b) the rate constants of the corresponding photodegradation.

Figure 3 .

 3 Figure 3.60 (a) The time runs of RhB relative concentration in photodegradation under UV illumination with no catalyst, pure P25, and modified P25, (b) the rate constants of the corresponding photodegradation.

3. 5 . 2 . 2 Figure 3 . 61 .

 522361 Photocatalytic tests have been conducted under visible light. The results are shown in Figure 3.61. In that case, no improvement of the photocatalytic activity of modified titania was observed and the photocatalytic activity even decreased.

Figure 3 .

 3 Figure 3.61 The time runs of phenol relative concentration in photodegradation under visible illumination with no catalyst, pure P25, and modified P25 without thermal treatment.

3. 5 . 3 Figure 3 . 62 .Figure 3 .

 533623 Figure 3.62. The surface modification with Au, Cu and Au-Cu NPs show a strong influence on the charge-carrier decay of P25. This influence on the decay can be related to the activity in the case of phenol degradation with UV-light.

3. 5 . 4

 54 Conclusion of this sectionSmall Au, Cu and bimetallic Au-Cu nanoparticles homogeneous in size were synthesized on P25 TiO 2 by the THPC method. HRTEM, HAADF-STEM, EDS and XPS have demonstrated the alloyed structure of the Au-Cu NPs. Titania surface modification with Au, Cu and bimetallic Au-Cu NPs enables the increase of the photocatalytic activity under UV-visible irradiation. We have found that very small amount of metal (0.5% wt.) can activate titania for photocatalytic applications, thus the costs of photocatalyst preparation are relatively low. TRMC measurements show that under UV irradiation Au, Cu and Au-Cu NPs act as electron scavengers hindering charge recombination in modified P25. The modification of titania by Cu nanoclusters induces a larger enhancement in the photocatalytic activity compared with its modification with Au NPs. The highest photocatalytic activity is obtained with P25 modified with Au-Cu 1:3. TRMC signals show that bimetallic nanoparticles are better electron scavenger than Cu and Au NPs. These bimetallic NPs can also have applications in catalysis and electrocatalysis. Modification of P25 with Au-, Cu-or Au-Cu

Chapter 4

 4 modification of TiO 2 with Au-Cu bimetallic NPs with a high deposition yield and direct formation of Au-Cu alloyed NPs without thermal treatment. iv) Modification of P25 with Au or/and Cu metal NPs by THPC method exhibited much better results than for the nanoparticle size and distribution and photocatalytic activity than by the radiolytic colloid-deposition method. The THPC method is an efficient and facile method to modify TiO 2 with gold, copper or other metals. v) The monometallic NPs (Au and Cu) and alloyed Au-Cu bimetallic NPs are efficient for electron scavengers. Modification of TiO 2 with alloyed Au-Cu bimetallic NPs prompts electron-hole separation, consequently contributing to the enhancement of the photocatalytic activity under UV illumination. Radiolytic synthesis of Cu sulfides and oxides nanostructures: application in photocatalysis under visible illumination Although TiO 2 has been widely investigated because of its excellent photocatalytic activity, his wide band-gap restricts its application under visible illumination. As demonstrated in the former chapter, modification with noble metals or doping with non-metal atoms are costly and not easy methods. Therefore, non-titanium oxides or compounds are developed as potential photocatalysts. In this thesis, synthesis of copper oxides and copper sulfides with special morphologies and their photocatalytic activity were investigated. The study is divided into two sections. In the first section, the controllable synthesis of copper oxides including CuO and Cu 2 O was carried out, and their photocatalytic performance under visible light was investigated. Copper oxide (CuO) nanowires and cuprous oxide (Cu 2 O) nanocrystals have been synthesized through γ-irradiation of aqueous solutions containing CuCl 2 , NaOH, sodium dodecyl sulfate (SDS) and isopropyl alcohol under ambient conditions. The product composition can be changed by modulating the amount of the NaOH base. The morphology of the products can be accurately controlled by changing the concentrations of SDS and isopropyl alcohol. The effects of the surfactant and isopropyl alcohol on the product morphology were discussed. A possible formation mechanism was also proposed. The photocatalytic property of the synthesized copper oxides was evaluated for the degradation of a model pollutant (methyl orange, MO) under visible illumination. Cu 2 O nanocrystals with specific morphology have an excellent photocatalytic activity for dye degradation under visible illumination. Especially, truncated octahedral Cu 2 O exhibits the highest photoactivity. In the second section, uniform CuS nanotubes (NTs) with a diameter of 200 nm were synthesized without template by gamma irradiation of aqueous solutions of CuCl 2 and NaS 2 O 3 . The length of the CuS NTs are from hundreds nanometers up to ~2 micrometers. The external diameter and wall thickness are respectively ~220 nm and ~40 nm. The morphology of CuS NTs depends on the ratio of NaS 2 O 3 to CuCl 2 . The photocatalytic activity of the as-prepared CuS NTs was investigated for decomposition of rhodamine B (RhB) in aqueous solution. The synthesized CuS NTs exhibit a photocatalytic activity under visible light.4.1 Synthesis of CuO and Cu 2 O nanostructures of controlled morphology: application in photocatalysis under visible light 4.1.1 IntroductionMaterials in nano-scale, because of their small-size and the quantum confinement effect, exhibit interesting physical and chemical properties, which differ from the corresponding bulk.[1][2][3] Different ways such as hydrothermal or solvothermal methods are used for the synthesis of nanomaterials.[4][5] γ-Irradiation is a distinctive approach which was first discovered for preparing nanomaterial byFujita et al. in 1962.[6] In recent years, γ-Irradiation route has been well developed for the synthesis of semiconductors and inorganic nanomaterials.[7][8][9][10][11][12][13][14][15][16][17][18][19]

Figure 4 .

 4 Figure 4.1a shows the XRD pattern of the synthesized CuO (sample 1). The XRD

Figure 4 . 1

 41 Figure 4.1 XRD patterns of (a) the synthesized sample 1, CuO nanowires; (b) the synthesized sample 2, Cu2O octahedra.

Figure 4 . 2

 42 presents the XRD pattern of the synthesized sample 5. It can be clearly seen that the product exhibits a two-phase composition: narrow peaks can be ascribed to Cu 2 O, while broad peaks attributed to CuO.

Figure 4 . 2

 42 Figure 4.2 XRD pattern of sample 5.

Figure 4 . 3

 43 Figure 4.3 TEM images of the sample 5 at a lower magnification (a), and a higher magnification (b).

4. 1 . 2 . 2 4 . 4

 12244 Effect of Isopropyl AlcoholTo evaluate the effect of isopropanol on the morphology of Cu 2 O crystals, blank experiments with other amounts of isopropanol were carried out. Panel b, c and d of Figure show the SEM images of the Cu 2 O products obtained with different quantity of isopropanol. In the presence of 5 mL isopropanol, the products are composed of uniform octahedral Cu 2 O (Figure 4.4b), with sizes from 900 nm to 1100 nm. When the amount of isopropanol increased to 12 mL, the Cu 2 O structures show the morphology of the truncated octahedra (Figure 4.4c). While this amount increased to 20 mL, nanocubes of Cu 2 O are obtained with sizes in the same range as for sample 2 (Figure 4.4d). The results demonstrated above indicate that the amount of isopropanol has an important effect on the morphology of the obtained Cu 2 Ostructures. The growth of Cu 2 O crystals may be explained as this: at the early stage of irradiation, small Cu 2 O seeds are formed. These small seeds then aggregate into larger particles. Isopropanol is employed as the • OH oxidative radicals scavenger.[36] So more isopropanol is added, faster the small Cu 2 O particles grow under γ-irradiation. Thereby, the amount of isopropanol might favor the growth rate along the [100] direction compared to

Figure 4 . 4

 44 Figure 4.4 SEM images of sample 1-4 (a, b, c and d); TEM images of sample 2 (e), and sample 4 (f); HRTEM image of sample 2 (g) and its corresponding SAED pattern (h).

Figure 4 . 5

 45 Figure 4.5 TEM images of CuO (a) and Cu2O (b) prepared without SDS, the experimental conditions are corresponding to sample 1 and sample 2, respectively.

Figure 4 .

 4 4g gives the high-resolution TEM (HRTEM) image of the obtained Cu 2 O crystals. The image clearly shows the lattice fringes with interplanar spacing of 0.282 nm, which is close to the distance between the (110) lattice planes of the cubic crystal structure of Cu 2 O (0.302nm, standard in JCPDS card 05-667). The corresponding selected-area electron diffraction (SAED) pattern (Figure 4.4h) also verifies that the as-prepared products are signal of cubic Cu 2 O crystals, in agreement with the XRD pattern results.

Figure 4 . 6

 46 Figure 4.6 UV-vis absorption spectra showing the photodegradation of RhB in with truncated octahedral Cu2O under visible illumination. Optical path: 2 mm.

Figure 4 . 7 (Figure 4 .

 474 Figure 4.7 presents the photodegradation of MO under visible light in the presence of CuO nanowires, octahedral and truncated octahedral Cu 2 O structures. In the blank experiment

Figure 4 . 4 . 8 ,

 448 Figure 4.7 (a) The time runs of MO concentration and (b) pseudo first-order kinetic fittings for MO photodegradation in the presence of (i) no catalyst, (ii) CuO nanowires, (iii) Cu2O octahedra and (iv) Cu2O truncated-octahedra under visible illumination.

[50- 51 ]

 51 Huang et al. synthesized uniform CuS nanotubes by a sacrificial templating method, which exhibit photocatalytic activity under UV illumination.[52-54] Ding et al. prepared CuS nanoflowers by a polyol route and they showed their photocatalytic activity under visible illumination.[55]

Figure 4 .

 4 Figure 4.10 XRD pattern of the as-prepared CuS NTs, the bottom is the JCPDF card no. 06-464.

  radicals) are generated by water radiolysis (Eq. 4.9). Cu 2+ and S 2-were released from the reduction of [Cu(S 2 O 3 ) 2 ] 2-by e aq -(Eq. 4.10). They combined in-situ with each other to form CuS molecules. Consequently, the CuS complexes aggregated to form nano-clusters and finally CuS nanotubes (Eq. 4.11).

Figure 4 .

 4 Figure 4.11 TEM images of as-prepared CuS nanotubes at low magnification (a), high magnification (b); HRTEM of CuS nanoparticles (c); EDS of as-prepared CuS NTs (d).

Figure 4 .

 4 11a shows that the CuS products have a one-dimensional structure, with external diameters of around 220 nm, and lengths are from several hundred nanometers up to about 2 micrometers. In addition, the difference in brightness between the edges and central parts gives the evidence that the one-dimension nanostructures of CuS exhibit a hollow structure. The internal diameter is about 140 nm. In Figure4.11b, the structure of tubes is clearer. Moreover, the CuS nanotubes are composed of 1D-assembled small crystalline grains.The X-ray energy dispersive spectrum (EDS) revealed that the products are composed of Cu and S (Figure4.11d). Besides the peaks of Cu and S, a weak signal corresponding to oxygen (probably due to the surface adsorption of air or to the oxidation by air), and the peak of carbon (due to the TEM grid) are obtained. The atomic ratio of Cu to S (determined from the area under the peaks of S Kα and Cu Kα) was found to be 48: 52, which is in well accordance with the stoichiometry of CuS.

Figure 4 .

 4 Figure 4.11c shows an HR-TEM image of the nanotubes. The measured distances between fringes were 3.01 Å and 3.04 Å, and correspond to (102) planes of covellite CuS (JCPDF no. 06-464). HR-TEM images show also that the synthesized CuS nanotubes are polycrystalline in nature.

Figure 4 .

 4 Figure 4.12 FE-SEM images of as-prepared CuS nanotubes at a low magnification (a), and high magnification (b).

Figure 4 .

 4 Figure 4.12 shows the SEM images of the synthesized CuS nanotubes which are quite monodisperse in size with uniform diameter of ~220 nm and a length of a few micrometers.

Figure 4 .

 4 Figure 4.13 (a, b) TEM images and (c. d) SEM images of the synthesized CuS hollow spheres; the molar ratio of Na2S2O3 to CuCl2 is 3:1.

55 and reached a plateau after 20 min (Figure 4 . 15 ).

 415 This is due to the adsorption of RhB on the surface of CuS. This important adsorption can be ascribed to the nanotubes structure displaying a tremendous surface area, allowing sufficient contact between RhB and CuS. Photocatalytic tests were conducted after reaching this adsorption equilibrium. The results of the photocatalytic degradation of RhB under visible light are shown in Figure 4.14. The concentration of RhB decreased gradually with irradiation time. The absorption peak at 255 nm decreased and finally disappeared, indicating the cleavage of aromatic structures in the RhB molecules.[53] RhB is quite stable under visible illumination (Figure 4.14b). However in the presence of the CuS photocatalyst, the degradation of RhB reached up to 70% after 100 minutes illumination. This result indicates that these porous CuS nanotubes are very photoactive under visible light. Further investigations will be attempted to study the stability of the photocatalyst under illumination and to optimize the photocatalytic properties of the synthesized CuS NTs.

Figure 4 .

 4 Figure 4.14 (a) UV-vis absorption spectra of RhB photodegration by CuS nanotubes under visible illumination, (b) time runs of relative RhB concentration in photodegradation with CuS (balck squares) and witout catalyst (red circles). Optical path: 2 mm.

Figure 4 .

 4 Figure 4.15 UV-Vis absorption spectra of RhB in the adsorption experiment with prepared CuS NTs in the dark.

3

 3 Conclusion of this sectionIn summary, we have synthesized CuS nanotubes through a non-template route by γ-irradiation of an aqueous solution containing CuCl 2 and Na 2 S 2 O 3 . The lengths of the CuS nanotubes are from several hundreds of nanometers to 2 micrometers. The external diameter and wall thickness of the synthesized CuS nanotubes are respectively ~220 nm and ~40 nm.The initial molar ratio of the precursors Na 2 S 2 O 3 to CuCl 2 has a significant influence on the final structure of CuS. The synthesized CuS nanotubes have a great ability in adsorbing RhB and they exhibit a high photocatalytic activity under visible light. Therefore, these nanostructures are promising for water treatment.

  3 bimetallic NPs resulted in an enhancement of the photocatalytic activity under UV light. TRMC measurements show that alloyed Au-Cu bimetallic NPs are very efficient in electron trapping. They are even more efficient than monometallic Au and Cu in electron scavenging. These results explain the better photocatalytic activity obtained with Au-Cu modified TiO 2 compared to Au/TiO 2 and Cu/TiO 2 . CuO nanowires and Cu 2 O crystals with shape evolution from cubic to octahedral were prepared by γ-irradiation method. The product composition for CuO or Cu 2 O can be easily changed by modulating the molar ratio of the reactants NaOH to CuCl 2 . The morphology of Cu 2 O crystals can be accurately controlled by choosing the amount of isopropanol. Cu 2 O truncated octahedral structures show excellent photocatalytic activity in MO degradation under visible light. Uniform CuS nanotubes were synthesized by radiolysis through a non-template route. The external diameter and wall thickness of the synthesized CuS nanotubes are ~220 nm and ~40 nm, respectively. The initial molar ratio of the precursors Na 2 S 2 O 3 to CuCl 2 has a significant influence on the final structure of CuS. The synthesized CuS nanotubes show a high capability to adsorb RhB and exhibit a very good photocatalytic activity under visible illumination.

Al 2 O 3 ,

 23 photoactivity and the size of Au-Cu NPs caused by different preparation methods. Besides, the location and the interaction manner of Au-Cu metallic NPs with the support can also be a possible reason for the different photoactivity of modified P25 by various preparation methods both under UV and visible light. Experimental or theoretical studies need to be conducted to understand the effect of the size of the metal NPs and their interaction with TiO 2 on the photocatalytic activity of the system.

Figure A. 10

 10 Figure A.10 UV-visible transmission spectra of (a) visible optical filter, (b) UV optical filter.

Figure A. 11

 11 Figure A.11 Scheme of TRMC experimental set-up.[2]

Figure A. 12

 12 Figure A.12 Diagram of components of a transmission electron microscope.[4]

Figure A. 13

 13 Figure A.13 (a) TEM mass contrast imaging mechanism. (b) TEM diffraction contrast

Figure A. 14

 14 Figure A.14 Schematic of the STEM.[5]

  Deposits for the XPS analyses are performed on a sheet of indium, previously prepared by a succession of baths of sulfuric acid, hydrochloric acid and washed several times with deionized water to remove the oxide layer formed on its surface. The sample is first suspended in a solution and then dried under a stream of N 2 . The photoelectrons are detected perpendicular to the support.The XPS is a technique based on the photoelectric effect. The absorption of a photon by an atom, a molecule or surface, has the effect of ejecting an electron, if the incident photon energy is higher than the binding energy of the electron. The principle of the photoelectron emission is shown in Figure A.16.

Figure A. 16

 16 Figure A.16 The principle of photoelectron emission.

Figure A. 17

 17 Figure A.17 The classification of the adsorption isotherm.

  This AOP is based on the O 3 /H 2 O 2 process. The efficiency of this process is tremendously improved by association with UV irradiation. It is considered to be the most effective process for extremely polluted effluents.[24] However, the process is not an

	economic method.
	1.2.7 Fenton's reagent
	In this process, hydroxyl radicals are generated by means of the reaction between H 2 O 2
	and iron (II). The overall reaction for the hydroxyl radical generation in acidic solution is
	shown in equation 1.6:
		23
	22
	+	•	→	++	•	(1.4)
	1.2.5 O 3 /H 2 O 2
	In this process, hydroxyl radicals are generated by a radical-chain mechanism by
	interaction between O 3 and the H 2 O 2 .[5, 22-23] Equation 1.5 shows the global reaction:
	3 2O H O 22 +→+ 2HO 3O •	2	(1.5)
	1.2.6 O 3 /H 2 O 2 /UV

The generated hydroxyl radical can react with organic pollutants by hydrogen abstraction, electrophilic addition and electron transfer mechanisms. However, the disadvantage of this AOP is the absorption coefficient of H 2 O 2 and quantum efficiency of HO• is very low at wavelength longer than 250 nm.

electron trapping by deposited metal NPs is illustrated in Figure 1.7. This

	The
	metal that modifies TiO 2 does not form part of the crystalline framework, but exists in a
	separate phase in interfacial contact with TiO 2 particles. In order to obtain stable and durable
	materials, the metal used in this approach should be chemically inert. Based on this, noble
	metals are most suitable for this purpose.
	In the past decades, various metals such as Au, Ag, Pt, Pd, Rh, Ni, Co, Ir, and Cu were
	used to modify the surface of TiO 2 or other substrates.[44-54] The modification with metal
	nanoparticles (NPs) can result in an enhancement in photocatalytic activity under UV
	illumination. The approved interpretation is that the metal NPs act as electron traps, which
	can decrease the electron-hole pair recombination rate, consequently prompt the holes to
	diffuse to the semiconductor surface where oxidation of organic species can occur. The
	scheme of the mechanism was confirmed by time resolved microwave conductivity (TRMC) measurements,
	where a reduction in the photoconductance was observed for the semiconductor excited with
	UV laser.[50] Takai et al. used methylene blue (MB) as an indicator to prove the electrons
	were stored in the TiO 2 supported Ag NPs.[55] Moreover, modification with bimetallic NPs
	can result in better catalytic activity compared to the monometallic ones, ie. Ag-Au[35, 56-57],
	Au-Cu,[58-61] and Pt-Au [48] modified titania. We have shown in this work that the
	bimetallic Au-Cu alloyed nanostructures were more efficient in electron scavenging than the
	monometallic Au and Cu ones.[62]

  reason, Geus et al. improved the method of DP with urea (CO(NH 2 ) 2 ), acting as the precipitating base.[141][START_REF] Van Dillen | Proceedings of the 6th International Congress on Catalysis[END_REF] Urea can gradually and homogeneously release hydroxyl ions throughout the whole solution, as the following equation:[137]

	22	2	4	2

  the radicals formed by reactions(1.28) and(1.29) are almost as powerful reducing agents as H• atoms (-2.3 V NHE ). Therefore, these secondary radicals are able to directly reduce metal ions into lower valencies for the multi-valent metal ions and even into a zero valence state for the monovalent metal cations. The reactions leading to the monovalent cations are the following:

	0
	32	32

  reported that CuS/ZnS composite was prepared by surface modification of ZnS porous nanosheets with CuS. The synthesized CuS/ZnS porous nanosheets reach a high H 2 -production rate under visible light illumination. The high visible light photocatalytic H 2 -production activity is owing to the interfacial charge transfer (IFCT) from the valence band of ZnS to CuS. It is also pointed out that the reduction of partial CuS to Cu 2 S enhances H 2 -production activity. This shows a possibility for replacing noble metal catalysts by economical CuS in the photocatalysis and also exhibits a facile method for enhancing photocatalytic activity by photo-induced IFCT.[167] To our best knowledge, very little work on the radiolytic synthesis of copper oxides and copper sulfides have been reported.Liu et al. synthesized Cu 2 O crystals with various shapes,

	1.9 Non-TiO 2 Visible Light Photocatalysis
	Bismuth tungstate (Bi 2 WO 6 ) is one of promising semiconductor materials in terms of
	visible photocatalysis. Recently, Bi 2 WO 6 with various nanostructures such as nanoparticles,
	nanorods, nanoplates, nanooctahedra, spherelike superstructures, flowerlike nanostructures,
	nanocages and hierarchical hollow spheres have been prepared by a variety of methods to
	improve its photoactivity.[160]

Copper compounds, such as copper oxide and copper sulfide have also been intensively studied in a form of single-phase material or composite photocatalyst. Owing to their low band-gap energy compared to TiO 2 , they can be excited by visible illumination. The morphology of copper oxide or copper sulfide significantly affects the photocatalytic properties of the catalysts. Li et al. reported that a spindly CuO exhibited a high photocatalytic activity in the degradation of dye pollutants, including rhodamine B, methyl orange and methylene blue in the presence of H 2 O 2 under halogen tungsten lamp irradiation.[161] Kuo et al. reported octahedral CuO exhibited higher photocatalytic activity under visible illumination than cubic ones, suggesting the (111) faces of Cu 2 O nanostructures are more photoactive than the (100) faces.[162] Similar results were also reported by Xu et al. where octahedral Cu 2 O particles were found to be more photoactive than cubic Cu 2 O.[163] Hara et al. reported that Cu 2 O had stable photocatalytic activity in splitting of water under visible light irradiation.[164] Ding et al. prepared CuS nanoflowers by a polyol route and they showed their photocatalytic activity under visible illumination.[165] CuO/ZnO nanocomposites were prepared by Saravanan et al. and were used for degradation of real textile waste water under visible light illumination.[166] Since the coupling of CuO/ZnO reduces the band gap of ZnO, extending the wavelength range to visible light region. The presence of CuO also improves the charge-carrier separation. The maximum photocatalytic efficiency is observed for 5% CuO loaded on ZnO. Zhang et al. have such as eight-pod cubes, six-armed starlike and octahedra etc. by γ radiolysis, however the photocatalytic properties of these Cu 2 O productions were not addressed.[168] Qiao et al.

  rate is significant for their photocatalytic activity. The size and distribution of the deposited metal NPs on the substrate TiO 2 are crucial for the electron trapping. Colloid deposition and deposition precipitation methods are employed to modify TiO 2 with Au-Cu bimetallic NPs.Noble metal Au associated with non-noble metal Cu can be a good option for achieving better photocatalytic activities at low cost.Additionally, the use of solar energy for photocatalysis in the hydrogen generation or environmental purification is the future goal in scientific and industrial fields. Visible photoactive Cu 2 O and CuS with special structures synthesized by radiolysis open up possibilities for the development of solar-induced photocatalysts. These nanostructures may also have applications in other fields such as catalysis, water splitting, and solar cells.

2 by various modification methods and to synthesize non-TiO 2 based visible photocatalysts (copper oxide and sulfide nanostructures). For the TiO 2 based photocatalysts, charge-carrier recombination CHAPTER I

  14 H 14 N 3 SO 3 Na), were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water (Milli Q with 18.6 MΩ•cm) was used for all the synthesis and precipitation methods and for the photocatalytic experiments.

	In the preparations and photocatalytic reactions of copper oxides and copper sulfides,
	copper chloride dihydrate (CuCl 2 •2H 2 O), sodium hydroxide (NaOH), sodium thiosulfate
	(Na 2 S 2 O 3 •5H 2 O), sodium dodecyl sulfate (SDS), methyl orange (C 14 H 14 N 3 SO 3 Na), and
	isopropyl alcohol (2-propanol, C3H8O) were purchased from Sinopharm Chemical Reagent
	Co., Ltd.

Titanium dioxide (TiO 2 , Evonik P25, , 50 m²g -1 , 80% of Anatase, 20% of Rutile) was purchased from Evonik, Germany. Chloroauric Acid (HAuCl 4 •3H 2 O) (used as gold precursor) and Copper Chloride (CuCl 2 ) (used as copper precursor) were respectively purchased from Acros Chemicals and Rectapur. Rhodamine B (C 28 H 31 ClN 2 O 3 ) and phenol (C 6 H 5 OH) were purchased from Fluka. Poly acrylic acid (PAA, 60% in water), polyvinyl alcohol ( P VA , analytical pure), tetrakis (hydroxymethyl) phosphonium chloride (THPC, 80% in water) were purchased from Aldrich-Sigma. High purity (impurities less than 1 ppm) nitrogen gas from Air Liquid was used to degas the samples prior to gamma irradiation. The suspensions were bubbled with oxygen from Air Liquid during the photocatalytic reactions. Deionized water (Milli Q with 18.6 MΩ•cm) was used all through our experiments.

In the experiments of deposition precipitation with urea, chloroauric acid

(

HAuCl 4 •3H 2 O), copper chloride dihydrate (CuCl 2 •2H 2 O), and methyl orange (C 2.2 Syntheses of metal nanoparticles and copper sulfides and oxides 2.2.1 Preparation of Au-Cu Colloids by Radiolysis and Deposition on P25

  The ratios of CuCl 2 to NaOH for the preparation of Cu 2 O cubes and octahedra were slightly different. The amount of NaOH decreased to 0.3 M, while the concentrations of the other reactants were the same. In addition, isopropyl alcohol was added as a scavenger of oxidative radicals produced during irradiation. Then, the mixed solution was slowly stirred for 15 min to form a uniform blue suspension. Finally, the blue mixed solution was poured into a wide-mouth bottle and sealed. Then, the bottle was submitted to irradiation with a 2.22 × 10 15 Bq 60 Co γ-ray source at a dose rate of 30 Gy min -1 Na 2 S 2 O 3 •5H 2 O) was added into the solution with gentle stirring until all the reagents dissolved. After that, the color of the solution turned to green. In addition, 8 mL of 2-propanol was added as a scavenger of hydroxyl radicals produced during irradiation.Then, the mixed solution was poured into wide-mouth bottles and sealed. The bottle was exposed to a 2.22 × 10 15 Bq

	2.2.6 Preparation of Copper Sulfide
	In a typical procedure to prepare CuS nanotubes, 1.7 mmol of cupric chloride
	(CuCl 2 •2H 2 O) was dissolved in 25 mL water and a green-blue solution then formed. 3.5 mmol
	of sodium thiosulfate (

2 •2H 2 O was dissolved in 30 mL of distilled water. 10 mL of aqueous solution 0.375 M NaOH and 1.000 g of SDS were added in sequence.

for 72 h (total dose = 130 kGy) (Appendix I). At the end of irradiation, the precipitates were collected. The products were washed with a distilled water and ethanol solution several times and then dried in an oven at 50 °C. Table

4

.1 shows the detailed synthesis conditions of the copper oxide nanostructures.

60

Co γ-ray source (Appendix I). The dose rate was 16 Gy/min and the total irradiation dose was 64 kGy. The obtained black precipitate was washed several times with water. The precipitate was then dried in the air at 60 °C for 48 hours.

Table 2 .

 2 1 The overall descriptions of the preapared samples in this thesis.

	Samples	Preparations	Details	Descriptions
	AuCu13PAA
	AuCu11PAA			These are the Au-Cu colloidal
	AuCu31PAA			solutions prepared by radiolysis
	AuCu13PVA	Section 2.2.1	see Table 3.1	in the presence of various polymer stabilizers. The related
	AuCu13PP			results are shown in section
	AuCu11PP			3.2.2.
	AuCu31PP
	AuPAA@P25(1)
				These are the modified P25 with
				Au or Au-Cu colloidal NPs of
				Table 3.1. The samples are
		Section 2.2.1	see Table 3.2	without thermal treatment unless
				otherwise indicated. The related
				results are shown in section
				3.2.3.

Table 3 .7

 3

				These are the modified P25 with
				Au or Au-Cu NPs. PVA was
	AuCu31/P25 AuCu11/P25	Section 2.2.2	Table 3.4	added to facilitate the deposition. All the samples are with thermal
				treatment. The results are shown
	AuCu13/P25			in section 3.3.1.
	Au/P25DPU
				These are the modified P25 by
	AuCu1:1/P25DPU AuCu1:3/P25DPU	Section 2.2.3	Table 3.5	deposition precipitation method with urea. The results are shown
				in section 3.4.2.
	Cu/P25DPU
	Au/P25DUP-s			These are the control samples
				compared to those by DPU
	AuCu1:3/P25DPU-s	The preparation		method. DPU-s refers to Au and
	Cu/P25DPU-s	procedure is		Cu ions are introduced
				simultaneously before the water
	Au/P25DP	similar to that		bath. DP means deposition
	AuCu1:3/P25DP	in section 2.2.3.		precipitation without urea. The
				results are shown in section
	Cu/P25DP			3.4.4.
	Au/P25-T
	AuCu11/P25-T			These are the modified P25 by
		Section 2.2.4	Table 3.8	THPC method. The results are
	AuCu13/P25-T			shown in section 3.5.
	Cu/P25-T
	1
	2			These are copper oxides
	3	Section 2.2.5	Table 4-1	prepared by radiolysis. The results are shown in section
	4			4.1.2.
	5

  3.1a, 60 min). This indicates there are nearly no Au atoms generated during the first 60 minutes. The reduction of Au III into Au 0 is a multi-step process.[64][65][66] Pulse radiolysis studies have shown that Au II does not directly disproportionate but dimerizes into a long-lived dimer, (Au II ) 2 , which then dissociates into Au III and Au I . It could be concluded from the studies of the dose-dependent yields in the steady state radiolysis that a comproportionation reaction of nascent gold atoms Au 0 with excess Au III ions takes place to yield Au II and Au I . This reaction will take place until the reduction of all Au III complexes into Au I After, the coalescence of Au 0 atoms into gold clusters might occur. The reduction reactions of Au III to Au 0 are shown in the following equations:[64, 67]

				III Au Cl +e 4 --aq	→	II Au Cl	4	2	-	(3.1)
	III Au Cl	4	-	II (CH ) C OH (or H ) Au Cl + (CH3) CO + H (or H ) 2 + + 32 4 2 •• -+→	(3.2)
				II Au + Au	II	II (Au ) →→ 2	III Au + Au	I	(3.3)
				I Au + e	aq	-→	Au	0	(3.4)
	I Au + (CH ) C OH (or H ) Au + (CH ) CO + H (or H ) 0 ++ 32 32 •• →	(3.5)
				III Au + Au	0	→	II Au + Au	I	(3.6)

Table 3 .

 3 1 The Au-Cu Colloid samples with various Au/Cu ratios prepared by radiolysis in the presence of different stabilizers.

	Samples label	Molar Ratio of Au/Cu	[HAuCl 4 ] (mol/L)	[CuCl 2 ] (mol/L)

Table 3 .

 3 2 The List of the synthesized Au and Au-Cu NPs supported on P25.

			Content of metal
	Photocatalysts	Molar ratio of Au/Cu	precursor (wt %)	Stabilizer
			Au	Cu
	AuPAA@P25(1)	-	1	-	Poly Acrylic Acid
	a AuCu31PAA@P25(1)	3:1	1	0.11	Poly Acrylic Acid
	AuCu11PAA@P25(1)	1:1	1	0.32	Poly Acrylic Acid
	AuCu13PAA@P25(1)	1:3	1	0.97	Poly Acrylic Acid
	AuPVA@P25(1)	-	1	-	Polyvinyl Alcohol
	AuCu13PP@P25(1)	1:3	1	0.97	PAA and PVA
	AuCu13PAA@P25(0.5)	1:3	0.5	0.48	Poly Acrylic Acid
	AuCu11PAA@P25(0.5)	1:1	0.5	0.16	Poly Acrylic Acid
	AuCu31PAA@P25(0.5)	3:1	0.5	0.05	Poly Acrylic Acid

Table 3 .

 3 3 Spectroscopic Data for the Dyes in the Photocatalytic Reaction of RhB.[81]

		Absorbance maximum, nm	α ε max , × 10 -4
	Dyes	In water	In MeOH	L•mol -1 •cm -1
	Rhodamine B	555	547	11.5
	N,N,N′-triethylrhodamine (TER)	539	539	5.5
	N,N′-Diethylrhodamine (DER)	522	525	7.2
	N-Ethylrhodamine (MER)	510	516	6.1
	Rhodamine	498	503	8.4

Table 3 .

 3 4 List of modified P25 prepared by radiolysis followed by deposition onto the P25 substrate.

			Content of metal
	Photocatalyst	Molar ratio of Au/Cu	precursor/P25 (wt%)	a TEM size (nm)	b τ 1/2 (ns)
			Au	Cu
	Au/P25	1:0	0.5	0	5.8	22.7
	AuCu31/P25	3:1	0.45	0.05	-	-
	AuCu11/P25	1:1	0.38	0.12	4.4	55.4
	AuCu13/P25	1:3	0.25	0.25	3.8	36.3

a TEM size: this was measured manually with a program (Nano measurement);

Table 3 .

 3 5 The List of the Prepared Au, Cu, and Au-Cu Modified P25 by Deposition Precipitation with Urea.

	Photocatalyst	Molar Ratio of Au/Cu	Content of metal precursor/P25 (wt%) Au Cu	Color	mean size (nm) (from TEM pictures)
	Au/P25DPU	1:0	0.5	0	Violet	8.4
	AuCu1:1/P25DPU	1:1	0.38	0.12	Light-violet	8.2
	AuCu1:3/P25DPU	1:3	0.25	0.25	Grey-violet	6.4
	Cu/P25DPU	0:1	0	0.5	Light-green	a N/A

a N/A: Cu nanoparticles are unable to be observed from the TEM images

Table 3 .

 3 6 Rate constants of the pseudo first-order fit of MO photodegradation under UV illumination.

	Photocatalyst	k (rate constant) (10 -3 s -1 )	R (correlation coefficient)
	P25	1.2±0.1	0.998
	Au/P25DPU	0.78±0.04	0.992
	AuCu1:1/P25DPU	1.5±0.1	0.986
	AuCu1:3/P25DPU	3.2±0.4	0.972
	Cu/P25DPU	1.7±0.1	0.996

Table 3 .

 3 7 Summary of Au, Cu and Au-Cu modified P25 prepared by deposition precipitation and DPU-s methods.

	Photocatalyst	Molar Ratio of Au/Cu	Content of metal precursor/P25 (wt%) Au Cu	Color	TEM size (nm)
	Au/P25DUP-s	1:0	1	0	violet	14.0
	b AuCu1:3/P25DPU-s	1:3	0.5	0.5	violet	22.9
	Cu/P25DPU-s	0:1	0	1	Light-green	a N/A
	Au/P25DP	1:0	1	0	Violet	16.7
	c AuCu1:3/P25DP	1:3	0.5	0.5	violet	--
	Cu/P25DP	0:1	0	1	Light-green	N/A

Table 3 .

 3 8 Summary of the modified photocatalysts prepared by chemical method with THPC.

	Photocatalyst	Molar Ratio of Au/Cu	Content of metal precursor/P25 (wt%)	Color	a TEM size (nm)
			Au	Cu		b before	c after
	Au/P25-T	1:0	0.5	0	violet	3.1	6.9
	AuCu11/P25-T	1:1	0.38	0.12	Light-violet	3.9	5.2
	d AuCu13/P25-T	1:3	0.25	0.25	grey-violet	2.9	4.8
	Cu/P25-T	0:1	0	0.5	Light-green	N/A	N/A
	a TEM size: this was measured manually with a program (Nano measurement);

Table 3

 3

	.9 Specific atomic ratio deduced from the XPS spectra for the samples Cu/P25,
	Au/P25, AuCu1:3/P25, AuCu1:1/P25
	Sample	(%Au + %Cu)/(%Ti +%O)	%Au/% Cu
	Au/P25	0.0014	a N/A
	AuCu1:1/P25	0.003	0.7
	AuCu1:3/P25	0.0062	0.14
	Cu/P25	0.0093	N/A

a N/A: this value is not applicable.

Table 3 .

 3 11 The rate constants of the pseudo first-order fitting in RhB photodegradation under UV illumination.

	Samples	k (rate constant) (10 -3 s -1 )	R (correlation coefficient)
	P25	7.2±1.0	0.972
	Au/P25-T	7.6±0.4	0.994
	AuCu11/P25-T	7.9±0.4	0.996
	AuCu13/P25-T	11.8±1.1	0.988
	Cu/P25-T	9.6±0.6	0.996

  Previously, our group has reported radiolytic synthesis of cuprous oxides with various morphologies such as eight-pod cubes, six-armed starlike, octahedral, and spindle like structures.[8, 34] In this work, we further designed an environmentally convenient way to prepare CuO nanowires, octahedral and cubic Cu 2 O with smaller and more uniform distribution in size. Interestingly, through delicately modulating the ratio of copper ion and hydroxyl ion, we could control the synthesis of CuO nanowires and Cu 2 O of octahedral or cubic structures. Moreover, it has been found that the amount of isopropanol has a significant impact on the morphology of Cu 2 O crystals. The photocatalytic activity of the synthesized samples was investigated for degradation of methyl orange (MO) under visible illumination.

	4.1.2 Results and discussion
	4.1.2.1 Characterizations
	and gas sensor.[26-28] Cu 2 O, which is an important p-type
	semiconductor with a direct band gap (2.17 eV), has been demonstrated to be used as a
	photocatalyst, and for solar energy conversion,[29-30] photochemical decomposition of water
	into H 2 and O 2 under visible light irradiation.[31-32] The different morphologies of Cu 2 O
	nanocrystals can exhibit special properties. For example, octahedral Cu 2 O nanocrystals with
	entirely {111} surfaces show a better performance in the photocatalytic degradation of dye
	molecules than those of other shapes. [33]

Table 4 . 1

 41 Synthesis Conditions and Morphology of the Obtained Products.

		a NaOH		isopropyl
	sample	(mol/L)	b α	alcohol (mL) composition	morphology
	1	9.3×10 -2	2.35	5	CuO	nanowires
	2	7.5×10 -2	1.90	5	Cu 2 O	octahedra
	3	7.5×10 -2	1.90	12	Cu 2 O	truncated octahedra
	4	7.5×10 -2	1.90	20	Cu 2 O	nanocubes
	5	8.3×10 -2	2.10	5	CuO and Cu 2 O	nanowires and octahedra

a NaOH: the concentrations are proportional to the total volumn of solution, 40 mL. b α = molar ratio of OH -to Cu 2+ .

  The equations involved in the reaction processes are shown as follows: CH3) 2C • OH radicals (-1.1 V). When a small quantity of NaOH is added in the solution, the dissolved Cu 2+ can be mainly reduced into Cu + by e aq -and CH 3 C 3 (OH)CH 3 radicals (eq 4.3), while OH -quickly combines with Cu + to form CuOH, which finally decomposes into Cu 2 O (eqs 4.4 and 4.5) under γ-irradiation. Pure Cu 2 O is obtained because of the stoichiometric ratio (the molar ratio of OH -to Cu

			Cu OH +-+→	CuOH	(4.4)
		2CuOH	ray   → γ -	2 H O Cu O 2 +	↓	(4.5)
			Cu	2	+		+	2OH	-	→	Cu(OH)	2	(4.6)
			Cu(OH)	2	ray   → γ -	2 H O CuO +	↓	(4.7)
	Radiolysis of aqueous solution induces many active intermediates, such as strong
	reductive hydrated electrons (standard redox potential equal to -2.87 V), H radicals and
	oxidative radicals • OH (eq 4.1).[8, 34] Isopropyl alcohol is introduced to scavenge • OH,[35]
	leading to reducing (
	2 H O	ray   → γ -	+ e , H , HO , H O , H , H O aq 3 2 2 2 -• •	(4.1)
	3 CH CH(OH) CH + HO 3	•	→	3 CH C (OH)CH •	3	+	2 H O	(4.2)
	Cu	2	aq e or (CH C (OH) CH ) Cu 3 3 +-• + →	+	(4.3)

2+

; Table

4-

1) close to 2 (equals 1.90). When the concentration of NaOH was increased to a certain extent (α ~ 2.35; Table 1), Cu(OH) 2 precipitate formed. Then, Cu(OH) 2 decomposes into CuO under γ-irradiation (eqs 4.6 and 4.7), forming nanowires. In order to study the role of γ-irradiation in the formation of CuO nanowires, blank experiments without γ-irradiation were carried out, while keeping other conditions the same as above. In this case, no CuO nanowires were found. So, γ-irradiation is important for the formation of CuO nanowires. Scheme 4-1 shows the formation processes of CuO and Cu 2 O.

Scheme 4-1 Formation Processes of CuO and Cu2O

Table 4 -

 4

1, sample 5).

  and Wang reported that the morphology of Cu 2 O was dependent on the value of R.[33,[37][38] When R is ~ 0.58, perfect cubic Cu 2 O crystals are formed. When R is ~1.73, octahedral crystals are synthesized. When R is ~1.15, the morphology of Cu 2 O is truncated octahedra, a morphology between cubes and octahedra. In Kuo's study, the amount of the reducing agent NH 2 OH•HCl in a CuCl 2 -NH 2 OH•HCl-SDS-NaOH system influenced the value of R.[33] In these radiolytic syntheses, it is believed that the formation processes are in agreement with the reported case. The amount of 2-propanol has an influence on the reducing radicals, consequently affects the value of R, resulting in the morphology evolution from cubic to octahedral for Cu 2 O cystals.

  In summary, CuO nanowires and Cu 2 O nanocrystals with shape evolution from cubic to octahedral structures have been successfully prepared via a simple γ-irradiation route at room temperature and under ambient pressure. As far as we know, this is the first report of radiolytic synthesis of CuO nanowires and Cu 2 O nanocubes. It has been found that the products composition could be changed by modulating the molar ratio of OH -and Cu 2+ . For Cu 2 O crystals, the morphology can be accurately controlled by choosing the amount of isopropanol. A formation mechanism is proposed. Related studies surely enriched the applications of radiolysis in synthesis of nanomaterials.

	4.2 Radiolytic synthesis of CuS nanotubes with photocatalytic
	activity under visible light
	4.2.1 Introduction
	Truncated octahedra Cu 2 O exhibit a better performance in the photocatalytic degradation
	of MO molecules. Further studies are necessary to investigate the relation between the Cu 2 O
	morphology and its photoactivity, and to understand the related mechanisms. This Cu 2 O
	nanostructures with specific morphologies may find applications in photocatalysis and other
	solar conversion technologies.

  When mixed Cu 2+ and NaS 2 O 3 are mixed, a green-blue complex [Cu(S 2 O 3 ) 2 ] 2-(Eq. 4.8) forms. The reducing species (aqueous electrons and H .

	as follows:[11-12, 57]
		Cu	2	+	+	2 2[S O ] 2 3	-		→	2 [Cu(S O ) ] 2 3 2	-	(4.8)
		2 H O	ray   → γ -	aq e ,H ,HO ,H O ,H ,H O 3 2 2 2 -• • +	(4.9)
	2e	aq	-	+	2 [Cu(S O ) ] 2 3 2	-	→	Cu	22 2S +-++	2SO	3	2	-	(4.10)
		Cu	22 S +-+	→	CuS	↓→	(CuS)n	(4.11)

α In dilute (10 -6 ~ 10 -5 M) methanolic solution at 20 °C.

ACKNOWLEDGMENTS

First of all, I would like to thank my supervisor, Prof. Jiafu Chen for his supports and guidance during my Ph. D study at University of Science and Technology of China. His

This above method was named as DPU because it was deposition-precipitation method with urea. In order to compare the deposition efficiency, another deposition method, where the Au and Cu ions were introduced simultaneously, was carried out. Hence, such samples prepared by this method were named ending with DPU-s.

Modification of P25 by THPC Method

The monometallic gold catalysts (0.5 wt.% of Au) were prepared according to Duff and Baiker method. [3][4][5] This preparation was based on the preparation of a gold sol, which was then deposited onto a support. In a beaker, 1.5 mL of NaOH solution (0.2 M, freshly prepared) was added to 45 mL of deionized water under vigorous stirring. After 5 minutes, 1 mL of tetrakis (hydroxymethy1) phosphonium chloride (THPC, 0.05 M) was added to the mixture followed by 2.5 mL of HAuCl 4 solution (10 -2 M). THPC was used as reducing and stabilizing agent. The color change from transparent to brown was immediately observed indicating the reduction of Au III into Au 0 and the formation of Au NPs. 1 g of P25 support was then added to the mixed solution, which was maintained under vigorous stirring at room temperature during two hours. The solid was separated by centrifugation, washed with deionised water (the example, we can synthesize larger (25 or 50 nm) Au-Cu bimetallic NPs well deposited on the TiO 2 support to study the size effect on the photoactivity. In case of P25, we can also attempt to deposit the Au-Cu NPs on the joint sites between anatase and rutile, promoting the electron transfer from the photoactivated metallic NPs to rutile and consequently to anatase. In this case, P25 can be photoactivated by excited electrons from metallic NPs under visible irradiation.

Further investigation should be carried out to reveal the mechanism on the excellent visible photocatalytic efficiency of truncated octahedral Cu 2 O. The durability of these nanostructure will be studied. In addition, it is desirable to improve the hydrophilicity of Cu 2 O microcrystals. On one hand, it can lead to higher photoactivity; on the other hand, it promises practical applications in industrial or commercial areas.

Further studies are also necessary to be conducted to investigate the radiolytic synthesis of copper sulfides and the photocatalytic properties of CuS with different morphologies, such as nanospheres, hollow spheres and nanotubes.

APPENDICES Appendix I Gamma Ray Sources

In the preparations of copper oxides and copper sulfides and the doposition precipitation method with urea, the γ-irradiation source was a panoramic cobalt where Δn i (t) is the number of excess charge-carriers i at time t and μ i their mobility. The sensitivity factor A is independent of time but depends on different factors such as the microwave frequency or the dielectric constant.

Appendix II Photocatalytic Reaction System A

Appendix III Photocatalytic Reaction System B

Considering that the trapped species have a small mobility that can be neglected, Δn i is reduced to mobile electrons in the conduction band and holes in the valence band. In the specific case of TiO 2 , the TRMC signal can be attributed to electrons because their mobility is much larger than that of the holes. [3] The TRMC signal can be characterized by two parameters: the maximum value (Imax) and the decay I(t). Imax is determined by the electron mobility and by fast decay processes with an appreciable activity during the excitation (i.e. the first 10 ns). These two contributions cannot be separated easily for a system as TiO 2 powder. In general, a high value of I max can be regarded as reflecting good electronic properties due to good crystalline structure of the sample. Because the signal decay is not entirely exponential, the general decay shape is characterized by several halftime lives, τ 1/2 is the time to obtain intensity I max /2 of the signal, 2τ 1/2 to obtain I max /4.

Appendix VI Principle of Characterization Techniques (High Resolution) Transmission Electron Microscopy

TEM is a technique in which a stream of accelerated electrons interacting with a sample deposited on a grid to form an image. The scheme of a TEM apparatus is shown in Figure A.12.

the electron beam, a crystalline specimen may cause the direction of propagation of the electron beam to be diffracted away from the optical axis of the microscope. Contrast can be generated between those regions that deflect the beam and those regions that do not (Figure

A.13b).

A diffraction contrast image is a map of those regions in the sample that are participating in diffraction processes. The most basic description of the condition for diffraction is Bragg's Law:

In this expression, λ is the wavelength, d hkl is the distance between crystal planes (hkl) and θ is the Bragg angle. If this relationship between the wavelength, the interplanar spacing and the orientation of the electron beam with respect to the crystal planes happens to be satisfied, strong Bragg diffraction is expected. Diffraction contrast forms the basis for at least two different imaging modes: bright and dark field. Bright Field, where the TEM image is formed with those regions of the sample that are not being diffracted away from the optical axis; and Dark Field, where the TEM image is a map of the those regions of the specimen that are diffracting the electron beam away from the optic axis of the microscope such that they pass through an aperture inserted in the back focal plane of the objective lens, called the objective aperture. [5] Two TEM setups were used in this thesis work. The first one is a JEOL 100 CXII operating at 100 kV in Laboratoire de Réactivité de Surface de l'Université Paris VI in collaboration with Patricia Beaunier. It was used in bright field and diffraction mode. The diffraction condition from these crystal planes is given by Bragg's law (equation A.3)

Scaning Transmission Electronic Microscopy (STEM)

As illustrated in

The property from which this law is derived is illustrated in Figure A.15. Here the path difference between the two incident X-rays must be equal to integer multiples of the radiation over a range of pressures to form a graph called an adsorption isotherm. N 2 adsorption is by far the most practiced method due to its ability to accurately determine the properties of a wide range of adsorbents. [6] The relationship between the amount of a substance adsorbed by an absorbent material and the pressure in equilibrium at a constant temperature is called adsorption isotherm. In contrast, a desorption isotherm is for the release of adsorbed gas on the surface of solids when the relative pressure decreases.

The pace of adsorption isotherm reflects the texture of the material studied. This is an experimental data from which quantitative data, such as specific surface area (or specific surface area) and the average pore size can be extracted.

Several types of isotherms were differentiated. The easiest is the Langmuir isotherm and it is used to describe the formation of a molecular monolayer on the surface of the solid progressive occupation of all possible sites. The generalization of this theory to multilayer Langmuir is the BET theory. The classification of isotherms into five classes was performed by Brunauer, Deming and Teller then supplemented by a sixth isothermal steps.

All absorbents are generally classified into three basic types, microporous, mesoporous, and nonporous according to their pore width/diameter. These adsorbents have five very distinct isotherms (see Figure A.17, I~V), which are used to determine their properties.

--The isotherm type VI: This isotherm, also called multi-step isothermal is relatively rare and is observed for homogeneously energetic materials on which adsorbed layers are formed one after the other. [4] Isotherm adsorption measurements were carried out using a Tristar II 3020M surface area analyser by a multipoint BET method in the relative pressure (P/P 0 ) range of 0.07-0.2. This setup is also in Hefei National Laboratory for Physical Sciences at the Microscale at USTC, Hefei.

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GENERAL ABSTRACT

Photocatalysis is recently extensively studied because it implies a variety of potential industrial applications ranging from the hydrogen generation of water splitting to the treatment of waste water. Among all the semiconductors, TiO 2 has attracted the most attention.