GENERAL INTRODUCTION

The manuscript of Habilitation à Diriger des Recherches (HDR) offers the opportunity to highlight the research that I have conducted after my thesis, including the postdoctoral contributions. Throughout the manuscript, I will expose major findings in which my contribution has been essential. In particular, I will put an emphasis on the projects that I've designed in an independent manner, the innovative approaches I've employed and the ideas that I've implemented to move the projects forwards. While the topics of the research concern experimental investigations, essentially on model systems, the potential applications will be presented. Eventually, the prospect for more innovative research topics for years to come will be discussed.

The general framework of my research activities concerns the properties of heterogeneous interfaces and materials at the nanoscale. In this regard, the atomic-level understanding of fundamental properties of metallic nanoparticles (NPs) and individual molecules supported on metaloxide and semiconducting surfaces, represents the main focus of this research. One of the original approaches followed herein was the investigation of these nanostructures under reactive conditions in real-time while the chemical reactions take place. While this approach represented a significant challenge, it provided however the opportunity for better understanding of phenomena that govern their properties and their reactivity.

My research projects can be inscribed within two main topics. The first one concerns the design and in-situ investigation of the properties of supported metallic nanoparticles and oxide nanostructures with view to potential applications in heterogeneous catalysis. The second one deals with strategies for the nano-patterning of pristine Si(001) surface and real-time investigations of the adsorption mechanisms of organic molecules on these surfaces. Despite the differences between the two topics, they nevertheless have in common the methodology of real-time and in-situ investigation of chemical (adsorption, compositions, bonding, oxidation states, gas phase) and physical (morphology, shape, electronic, arrangement) phenomena at the atomic scale.

Regarding the first topic, which is dedicated to the rational design of efficient and strong nanostructured catalysts, its perspectives are for applications in the environment remediation and energy generation systems. My approach within the framework of transition-metal heterogeneous catalysis tackles major limitations inherent to these nanocatalysts such as sintering, deactivation and active sites poisoning on the one hand. It also seeks suitable pathways to better investigate major chemical reactions in in situ conditions such as the CO oxidation on the other hand. The original approach followed here consists in monitoring these phenomena (chemical reactions, surface segregation…) in in situ conditions in the presence of the reactive environment. In particular, I design supported bimetallic nanoparticles with controlled size, shape and surface composition with drastic reduction of the precious metal loads. The use of more abundant but efficient materials and the possibility to tune the structure as a function of the desired chemical reaction and durability expectations represent an exquisite goal. Probing their properties at the atomic level will provide access to valuable insights into the understanding of central phenomena that govern the activity of catalysts such as the structure/nature of active sites and catalyst-reactant interface. Ultimately, we will take advantage of these phenomena to provide long-life catalysts with enhanced activity.

The second topic focuses on Si-nanopatterning and on the study of the adsorption mechanism of water and organic molecules on pristine and modified silicon (001) surfaces. It explores new possibilities to use the atomically-rich electronic nanostructures in these inorganic-organic interfaces for the development of nanoscale devices. The originality of my approach lies in the investigations of adsorption phenomena and electronic properties at the atomic scale in real time while the reactions take place. This approach provides access to the electronic structure of silicon atoms, the surface properties and the morphology of the adsorbed organic molecules. In addition, the design of innovative hybrid inorganic semiconductor and organic molecules, which requires a perfect control of the functionalization of the surfaces, is facilitated by the use of local probe microscopies as they give access to the properties at the atomic scale. Thus, the adsorption modes, the mobility of molecules and the localized electronic charges were determined and demonstrated.

Finally, it is of paramount interest to use suitable tools that enable the access at the atomic scale to these properties and under these specific conditions. Indeed, regarding this aspect, we use stateof-the-art techniques, (i) real-time Scanning Tunneling Microscopy/Spectroscopy (STM-STS) and (ii) Near-Ambient Pressure X-ray Photoemission Spectroscopy (NAP-XPS) both operating in environmental conditions (under chemical reaction conditions). These two techniques are complementary which provides powerful investigation capabilities. In particular, STM investigations are particularly essential to achieve the structural understanding at the atomic and molecular levels of our supported nanostructures. XPS on the other hand provides information on the chemical surface composition, gas phase in the vicinity of the surface and oxidation states of species during the reaction under reactive environment. This approach which correlates changes in the surface morphology with the underlying chemical states provides the opportunity to better understanding the catalysts performances of these systems and their related issues. In addition, the real-time monitoring of surfaces while dosing with molecules allows the identification of preferential adsorption sites and intermediates species.

FORWORD

My research interests as stated above concern two main topics. However, the first one will be discussed hereafter in two sections, A and B, while the second one will be discussed in a single chapter, C. In addition, a final part will be dedicated to projects and perspectives.

TOPIC 1: Nanocatalysis

Section A: Supported Size-selected metallic and bimetallic nanoparticles on TiO2(110): design, composition effects and thermal stability Section B: In situ studies of nanocatalysts in near ambient pressure conditions

TOPIC 2: Silicon surfaces and interfaces

Section C: Molecular adsorption on pristine and water-saturated Si(001)

PROJECT AND PERSPECTIVES:

ZnO-based inverse oxide/metal nanocatalysts and organic/inorganic hybrid solar cells

A.1. Introduction

The investigation of supported metallic nanoparticles (NPs) has attracted widespread interest, particularly in the quest for new materials that exhibit efficient catalytic properties and enhanced stability against deactivation. [START_REF] Hugon | Supported gold catalysts for selective hydrogenation of 1,3-butadiene in the presence of an excess of alkenes[END_REF][START_REF] Bond | Catalysis by Gold[END_REF] Thus, the understanding of both the electronic and chemical properties that govern the reactivity of NPs represents the key motivation towards this goal. The size of the NPs, the inter-particles distance, the binding of reactants to specific sites and their activation, the nature of the active phase, the chemical composition, the alloying, and the shape of the active material are among the key parameters that play a paramount interest in this regards. [START_REF] Somorjai | Molecular Factors of Catalytic Selectivity[END_REF][START_REF] Schauermann | Model Approach in Heterogeneous Catalysis: Kinetics and Thermodynamics of Surface Reactions[END_REF][START_REF] Freund | Cluster, facets, and edges: Site-dependent selective chemistry on model catalysts[END_REF][START_REF] Mirdamadi-Esfahani | Bimetallic Au-Pt nanoparticles synthesized by radiolysis: Application in electro-catalysis[END_REF][START_REF] Shan | Nanoscale Alloying in Electrocatalysts[END_REF][START_REF] Naitabdi | Local investigation of the electronic properties of size-selected Au nanoparticles by scanning tunneling spectroscopy[END_REF][START_REF] Delannoy | Surface Segregation of Pd from TiO2-Supported AuPd Nanoalloys under CO Oxidation Conditions Observed Insitu by ETEM and DRIFTS[END_REF] Indeed, one of the main effect that I have investigated concerned the thermal stability of Pt and bimetallic AuFe NPs supported on the TiO2(110) surface. [START_REF] Naitabdi | Enhanced thermal stability and nanoparticle-mediated surface patterning: Pt/TiO2(110)[END_REF][START_REF] Naitabdi | Thermal stability and segregation processes in self-assembled size-selected AuxFe1-x nanoparticles deposited on TiO2(110): composition effects[END_REF][START_REF] Naitabdi | Formation, thermal stability, and surface composition of size-selected AuFe nanoparticles[END_REF] The performances of a supported catalyst rely on its stability and reproducibility during the activation reactions. A major process leading to the deactivation of a metal catalyst is the thermal diffusion and sintering under reactive gases which quit often result in larger size clusters well above the critical dimension required for the reactivity. [START_REF] Morgan | Metal Redispersion Strategies for Recycling of Supported Metal Catalysts: A Perspective[END_REF] The sintering which is a thermodynamic-driven process, results from the large surfaceto-volume ratio inherent to nanoparticles. [START_REF] Plessow | Sintering of Pt Nanoparticles via Volatile PtO2: Simulation and Comparison with Experiments[END_REF] This ultimately affects strongly the physical and the chemical properties of supported metal clusters. The size has been proven to play a crucial role in the catalytic activity and the electronic properties of metallic NPs. [START_REF] Naitabdi | Local investigation of the electronic properties of size-selected Au nanoparticles by scanning tunneling spectroscopy[END_REF][START_REF] Cao | Size-and shape-dependent catalytic performances of oxidation and reduction reactions on nanocatalysts[END_REF][START_REF] Valden | Onset of catalytic activity of gold clusters on titania with the appearance of nonmetallic properties[END_REF][START_REF] Miller | The effect of gold particle size on Au-Au bond length and reactivity toward oxygen in supported catalysts[END_REF] Pioneering groups in France and elsewhere have indeed understood these aspects and have studied these aspects on model catalysts and developed techniques to monitor them. [START_REF] Valden | Onset of catalytic activity of gold clusters on titania with the appearance of nonmetallic properties[END_REF][START_REF] Henry | Surface studies of supported model catalysts[END_REF][START_REF] Henry | Catalytic activity of supported nanometer-sized metal clusters[END_REF][START_REF] Freund | Model Studies in Heterogeneous Catalysis[END_REF][START_REF] Cabie | Direct Observation of the Reversible Changes of the Morphology of Pt Nanoparticles under Gas Environment[END_REF][START_REF] De Clercq | Core-shell Pd-Pt nanocubes for the CO oxidation[END_REF] Indeed, from the electronic structure point of view, the band structure of metal NPs could behave as a molecule instead of a metallic state, in stark contrast with their bulk counterparts. In particular, large electronic density are usually located closer to the Fermi level while the d band becomes narrower. This molecular-like electronic state was explained to play a decisive role in the peculiar high catalytic activity of small NPs. [START_REF] Cao | Size-and shape-dependent catalytic performances of oxidation and reduction reactions on nanocatalysts[END_REF] The alloy formation and segregation processes as a function of the composition were other phenomena that I have investigated in details. Alloying Pt or Au with another metal has been found

to alter fundamentally their electronic and chemical properties and hence their catalytic activity. [START_REF] Singh | Synergistic Catalysis over Bimetallic Alloy Nanoparticles[END_REF][START_REF] Louis | Chemical Preparation of Supported Bimetallic Catalysts. Gold-Based Bimetallic, a Case Study[END_REF] Several mechanisms are believed to be responsible for such enhanced catalytic performance, including the presence of synergetic effects and improved resistance against poisoning and subsequent deactivation. In addition, the catalytic activity can be tuned by changing the elemental composition of the NPs. [START_REF] Lai | Recent Advances in the Synthesis and Electrocatalytic Applications of Platinum-Based Bimetallic Alloy Nanostructures[END_REF] Indeed, Au-Fe system was found to be highly active for catalyzing lowtemperature CO oxidation. [START_REF] Li | Origin of the high activity of Au/FeOx for low-temperature CO oxidation: Direct evidence for a redox mechanism[END_REF] Au which is widely used as a catalyst owing to its high activity & selectivity in a large number of important chemical reactions, 2,27-29 is gradually minimized by the incorporation of Fe and yet its performances are preserved if not improved. [START_REF] Naitabdi | Formation, thermal stability, and surface composition of size-selected AuFe nanoparticles[END_REF][START_REF] Kang | Engineering Catalytic Contacts and Thermal Stability: Gold/Iron Oxide Binary Nanocrystal Superlattices for CO Oxidation[END_REF][START_REF] Qi | Stability improvement of Au/Fe-La-Al2O3 catalyst via incorporating with a FexOy layer in CO oxidation process[END_REF] Indeed, the Au-FeO system is highly active for catalyzing low-temperature CO oxidation. [START_REF] Li | Origin of the high activity of Au/FeOx for low-temperature CO oxidation: Direct evidence for a redox mechanism[END_REF] However, theses intermetallics systems generate interfaces with considerable complexity. Since the interface represents the most critically active area, it focuses considerable investigation efforts. Especially, the FeO-Au nanostructures which contain interface-confined coordinatively unsaturated ferrous atoms (CUF)

show promises for low-temperature CO oxidation and represent a good candidate to reduce the use of noble metals. [START_REF] Kang | Engineering Catalytic Contacts and Thermal Stability: Gold/Iron Oxide Binary Nanocrystal Superlattices for CO Oxidation[END_REF] The CUF [START_REF] Kang | Engineering Catalytic Contacts and Thermal Stability: Gold/Iron Oxide Binary Nanocrystal Superlattices for CO Oxidation[END_REF][START_REF] Guo | Ferrous Centers Confined on Core-Shell Nanostructures for Low-Temperature CO Oxidation[END_REF][START_REF] Fu | Interface-Confined Ferrous Centers for Catalytic Oxidation[END_REF] promote the activation of O2, essential to the CO oxidation reaction, while they prevent the metal from being poisoned by CO. [START_REF] Fu | Interface-Confined Ferrous Centers for Catalytic Oxidation[END_REF] In addition, the surface segregation process is another phenomena that inevitably occurs in bimetallic NPs owing to their high structure complexity and flexibility under operating conditions. [START_REF] Naitabdi | Thermal stability and segregation processes in self-assembled size-selected AuxFe1-x nanoparticles deposited on TiO2(110): composition effects[END_REF][START_REF] Freund | Model Studies in Heterogeneous Catalysis[END_REF][START_REF] Liao | Surface Segregation in Bimetallic Nanoparticles: A Critical Issue in Electrocatalyst Engineering[END_REF] Therefore, the atomistic-level understanding of underlying surface processes remains one of the primary goals for catalysis research and long-term stability. Thus, I have implemented synthetic methods that enable fine control of size, shape, morphology and composition of metal nanoparticles at an atomic level. Moreover, composition and chemical states changes in bimetallic nanoparticles in response to the annealing in UHV have been investigated.

A.2. Synthesis of supported size-selected nanoparticles by inverse micelles encapsulation method

The method is based on a chemical route in which self-assembled and size-controlled NPs are formed following 5 steps as schematized in Fig. A1. The initial step consists in the formation of inverse micelles of a diblock copolymer polystyrene-block-poly(2-vinylpyridine) [PS(x)-P2VP(y), where x and y are the molar weights of the PS and P2VP monomers, respectively. The dissolution of the PS-P2VP in the toluene assuming a solution ratio polymer-to-toluene equals to 5. Since the toluene is a non-polar solvent, the apolar (polar) PS (P2VP) tale (core) will (not) dissolve in the solution forming inverse micelles. In this process the core acts as a host for the metal salt precursor where the pyridine group assumes the main chemical reaction. After adding of metal precursor ions will be bounded as counterions in the polar core of the micelles (by protonating the pyridine rings). The complete encapsulation of the metal salt compounds inside the micelles was achieved when a metal salt to PS-P2VP ratio concentration of 0.6 was used. Following this synthesis method, for a given particle size [determined by the molar weight of the polymer core (P2VP)], the alloy composition can also be easily tuned by changing the relative concentration of the two metal salts when bimetallic NPs are desired.

The inter-particle distances are also controlled by tuning the length of the PS tales (from the molar weight of the PS). Indeed, the self-assembly of NPs on the surface is obtained by dip-coating (step 4 in Fig. A1). Thanks to this procedure, single monolayer of well-ordered NPs into a hexagonal arrangement is routinely obtained on surfaces. Finally, the removal of the polymeric group was performed in the UHV using a soft O2 plasma treatment. It is noteworthy that this treatment keeps the NPs shape and initial hexagonal arrangement unchanged as can be seen in the STM image shown in The complete removal of the micelles using our O2 plasma treatment is a suited approach as it provides access directly to the metallic-oxide nanoparticles supported on a desired surface in a regular distribution. [START_REF] Roldan-Cuenya | Thermally stable nanoparticles on supports[END_REF] The total elimination of the micelles was monitored through the C 1s core-level spectra using XPS characterization. Indeed, the C 1s of the polymeric group which arises at the binding energy (BE) of 285 eV is suppressed after plasma treatment (see Fig.

A.3.Thermal stability of Pt size-selected nanoparticles on TiO2(110)

It is therefore possible to monitor the properties of NPs and their evolution on the surface as a function of the annealing treatment. Indeed, in order to investigate the thermal stability of the NPs prepared by this method, I have conducted a step-wise annealing from the room temperature (RT) up to 1060°C. While Pt NPs prepared by electron beam evaporation method have been shown to exhibit high mobility at rather low temperature i.e. 170°C, [START_REF] El-Azab | Binding and diffusion of Pt nanoclusters on anatase TiO2(001)-(1 x 4) surface[END_REF] the NPs prepared by this method exhibits strikingly a remarkable stability up to 1040°C. [START_REF] Naitabdi | Enhanced thermal stability and nanoparticle-mediated surface patterning: Pt/TiO2(110)[END_REF] Indeed, the initial hexagonal arrangement was preserved up to 1060°C indicating that no noticeable NPs diffusion has occurred (see Fig. A3). The average inter-particle distance of 30  2 nm and the initial apparent height of 3.4  0.2 nm remained nearly constant up to 1060°C. Meanwhile however, a strong TiO2(110) surface rearrangement has occurred. In particular, first a strong smoothing of the terraces and the decrease of the number of steps were seen to increase with the temperature. Second, striking features corresponding to the formation of TiO2 nanostripes running preferentially along the [001] direction of the TiO2(110) on the Pt-decorated surface are formed upon high temperature annealing (Fig. 3A). Interestingly, the width of these nanostripes seems to be related to the size of the nanoparticles, and their spatial distribution to the interparticle distance.

The widening of the terraces, while the NPs remain stable, was due to mobility of TiOx clusters or Ti-O molecules on the surface. This effect results from a singular strong Pt-TiO2(110) interactions well above the renowned strong metal-support interaction (SMSI) phenomena for which the Pt/TiO2 is a prominent system. [START_REF] Dulub | Imaging cluster surfaces with atomic resolution: The strong metal-support interaction state of Pt supported on TiO2(110)[END_REF] The particular enhanced stability of Pt NPs prepared by inverse micelles method can be attributed to the formation of a titanium carbide (TiC) interface between the TiO2 and Pt NPs. The carbon originates from the residual PS chains beneath the NPs. Indeed, the TiC is known for its outstanding thermal stability. 38

A.4. Alloy formation and segregation processes in size-selected AuFe NPs

When it comes to bimetallic NPs, it is imperative to investigate the structural and chemical evolutions as a function of the composition and the temperature. Phenomena such as phase separation, alloy, and atoms segregation are common occurrences in these systems when bimetallic NPs are annealed in UHV or exposed to reactive gases. The use of XPS measurements are well suited to track these effects. Indeed, I have conducted a step-wise annealing from 300°C up to 1000°C while monitoring the structure and chemical states of Au and Fe in bimetallic AuxFe1-x NPs supported on TiO2(110) as a function of the composition (x = 1, 0.8, 0.5, 0.2, 0). [START_REF] Naitabdi | Thermal stability and segregation processes in self-assembled size-selected AuxFe1-x nanoparticles deposited on TiO2(110): composition effects[END_REF][START_REF] Naitabdi | Formation, thermal stability, and surface composition of size-selected AuFe nanoparticles[END_REF] In addition, the thermal stability depends strongly on the composition in bimetallic NPs due to thermodynamic effects and cohesion energy optimization as it is clearly the case in our AuFe NPs.

Thorough XPS investigations were conducted on AuFe NPs in order to reach a comprehensive picture on the chemical phases and structure evolution as summarized in Fig A4 . The size of these AuFe NPs is in average 4.6  1.4 nm in height. For comparison, the phase contents obtained for the pure Fe NPs are also monitored and striking differences with bimetallic AuFe NPs were seen. Thus, the effect of incorporating gold in the AuFe NPs, such as alloy formation (see blue arrows in Fig. A4), was revealed as a function of the temperature and composition. Therefore, while Au0.8Fe0.2, Au0.5Fe0.5 and Au0.2Fe0.8 NPs share similar trends below 500°C, they exhibit clear differences above that temperature. In the temperature range RT-500°C, Au and Fe coexist in separated phases and no significant alloy was seen as expected since gold and iron are immiscible materials at low temperatures. In this range, the Fe content is dominated by the Fe-oxide compounds and the presence of Au does not provide any effect on its oxidation state. Indeed, Fe shows similar evolution invariably in pure Fe NPs or in bimetallic AuFe NPs. It has been reported that the decomposition of Fe2O3 on the surface of pure 7 nm Fe nanoparticles [oxide-shell/bcc-Fe core] occurred at about 500 °C. 39 The formation of Au-Fe alloy was observed above 500°C where for example a majority alloy phase was obtained for the composition Au0.5Fe0.5 in the range of 600°C-800°C while it is shifted to higher temperature, 800°C-900°C, for Au0.2Fe0.8 and Au0.8Fe0.2. Another important phenomena arises above 800°C namely gold segregation to the surface of AuFe NPs. The segregation of gold can be driven by the tendency of the Au-Fe NPs to lower their surface free energy by altering the composition of their surface. Since Au has a much lower surface energy (γAu = 1.61 J/m 2 ) than Fe (γFe = 3.28 J/m 2 ), [START_REF] Vitos | The surface energy of metals[END_REF][START_REF] Skriver | Surface-Energy and Work Function of Elemental Metals[END_REF] the segregation of Au atoms to the NP surface upon annealing in UHV must be thermodynamically favored. Therefore, this effect was found to play a role in the thermal stability of the AuFe NPs depending on the composition. Indeed, Au0.8Fe0.2 NPs exhibit a particular high stability up to 1000°C (Fig. A5). The mechanism of this stability can be described as follow. First, the higher melting temperature of Fe (Tm = 1535 °C) as compared to Au (Tm = 1064 °C) could induce an upward shift of the Au melting temperature once an alloy is formed. Second, the diffusion of Au atoms to the surface of the Au-Fe NPs, owing to its lower surface energy and higher atomic volume (VAu = 12.5 Å 3 ) compared with those of Fe, VFe = 8.0 Å 3 ), may reduce the mechanical stress on the atoms at the NP's core (presumably Fe-rich upon high temperature annealing), enhancing the NPs stability. In the particular, the composition of Au0.8Fe0.2, seems to favor the formation of core(Fe) shell (Au) NPs in which the stress energy was optimized. Similar effect has been observed by Mottet et al. for bimetallic AgCu small nanoclusters where the substitution of some Ag atoms by Cu ones induced a decrease in the stress at the core of these clusters which stabilized them against thermal disordering. [START_REF] Mottet | Single impurity effect on the melting of nanoclusters[END_REF] The trends observed in these AuFe NPs reveal a clear difference with the behavior expected from the Au-Fe bulk phase diagrams. According to this phase diagram for a bulk alloy with an average composition of 50 at. % Fe (and 50 at. % Au), [START_REF] Massalski | Binary Alloy Phase Diagrams[END_REF] a phase segregation would occur at 300 °C, resulting in nearly pure bcc Fe and in a Au-rich fcc Au-Fe alloy containing roughly 4 at. % Fe. However, the phase separation line on the Au-rich side is not well-known. If the temperature is increased to 700 °C, such an alloy would segregate into a bcc Fe-Au alloy containing ∼1 at. % Au, and in a fcc Au-Fe alloy with 40 at. % Fe. For 50 at. % Fe, a temperature of at least 800 °C is required in order to form a solid solution (alloy) of fcc Fe0.5Au0.5. This occurs for example at 900 °C. A comparison with my results obtained for these NPs with nominally 50 at. % Fe, shows that the phase diagram is shifted to lower temperatures as compared to bulk Au-Fe alloys. For instance, our results indicate the formation of a an Fe-rich core (bcc-Fe) and a Au-rich surface above 800°Cpromoting the stability of NPs, particularly in the case of Au0.8Fe0.2, (iv) an Fe-Ti-O alloy after gold desorption above 900°C.

While this study conducted in UHV has provided valuable insights and understanding on the properties of bimetallic NPs, their investigation under reactive gases at near ambient pressures is crucial and has to be accomplished. Thus, the main subject of the following section (B) will be to address this goal through one typical catalytic reaction.

B.1. Introduction

The investigation of nanocatalysts under their realistic conditions of pressure and temperature represents a real strategy toward a deeper understanding of their chemical state and reactivity. 21,44- 47 Therefore, I have designed mono-(Pt, Zn) and bimetallic (ZnPt) NPs on TiO2(110) and implemented methods to monitor their catalytic activity using state-of-the-art environmental techniques. [START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF] Indeed,

In-situ studies of nanocatalysts are among the current trends being actively explored in heterogeneous catalysis. [START_REF] Zorn | Oxidation on Technological Pd-Al2O3 Catalysts: Oxidation State and Activity[END_REF][START_REF] Zhang | In-Situ Studies of Nanocatalysis[END_REF][START_REF] Tao | Action of bimetallic nanocatalysts under reaction conditions and during catalysis: evolution of chemistry from high vacuum conditions to reaction conditions[END_REF] While Pt is one of the most prominent transition metal catalysts owing to its relevance in a wide range of industrial applications, in particular in the composition of environmentally friendly energy generation solutions such as hydrogen fuel cells, [START_REF] Gandhi | Automotive exhaust catalysis[END_REF][START_REF] Carrette | Fuel Cells -Fundamentals and Applications[END_REF] it however suffers from several drawbacks including rapid deactivation due to CO poisoning, material dissolution and high cost. [START_REF] Vesborg | Addressing the terawatt challenge: scalability in the supply of chemical elements for renewable energy[END_REF] In this regards, I have designed PtZn NPs in order to deal with these issues.

By essence the catalysts are flexible and often undergo structural and surface composition changes under chemical reaction conditions, [START_REF] Cabie | Direct Observation of the Reversible Changes of the Morphology of Pt Nanoparticles under Gas Environment[END_REF][START_REF] De Clercq | Core-shell Pd-Pt nanocubes for the CO oxidation[END_REF] so that their properties before and after the reaction may differ noticeably. [START_REF] Freund | Model Studies in Heterogeneous Catalysis[END_REF][START_REF] Tao | Reaction-Driven Restructuring of Rh-Pd and Pt-Pd Core-Shell Nanoparticles[END_REF][START_REF] Zhu | Shape Evolution of Metal Nanoparticles in Water Vapor Environment[END_REF] Thus, determining the active phase, chemical composition and morphology of the catalysts is better achieved when in situ investigation techniques are used. [START_REF] Cabie | Direct Observation of the Reversible Changes of the Morphology of Pt Nanoparticles under Gas Environment[END_REF][START_REF] De Clercq | Core-shell Pd-Pt nanocubes for the CO oxidation[END_REF] In particular, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which fits perfectly within these requirements, provides direct access to the surface elemental composition, gas phase in the vicinity of the surface (product of the chemical reaction), oxidation states of the catalysts and their evolution during the reaction under gas pressure up to 25 mbar. [START_REF] Salmeron | Ambient pressure photoelectron spectroscopy: A new tool for surface science and nanotechnology[END_REF][START_REF] Bluhm | Methanol oxidation on a copper catalyst investigated using in situ X-ray photoelectron spectroscopy[END_REF] It also allows the monitoring of a chemical reaction as it takes place, for example the CO oxidation can be observed through the CO2 gas phase uptake near the surface and the chemical nature of the adsorbates. [START_REF] Blomberg | In Situ X-Ray Photoelectron Spectroscopy of Model Catalysts: At the Edge of the Gap[END_REF] In addition, synchrotron-based NAP-XPS provides the possibility to obtain a depth profile of atomic composition near the surface of the nanoparticles during the chemical reaction owing to the tunable x-ray photon energy. This is particularly suitable in the case of bimetallic NPs when they are investigated in situ under reactive gases since surface segregation phenomena or oxide thin films can form on these systems such as in the present study. Although a remarkable and relatively significant number of studies have been dedicated to the in situ investigation of the oxidation and CO oxidation on Pt single crystal surfaces under near ambient conditions XPS [START_REF] Miller | Oxidation of Pt(111) under Near-Ambient Conditions[END_REF][START_REF] Butcher | In Situ Oxidation Study of Pt(110) and Its Interaction with CO[END_REF][START_REF] Zhu | Formation of Nanometer-Sized Surface Platinum Oxide Clusters on a Stepped Pt(557) Single Crystal Surface Induced by Oxygen: A High-Pressure STM and Ambient-Pressure XPS Study[END_REF][START_REF] Zhu | Structure and Chemical State of the Pt(557) Surface during Hydrogen Oxidation Reaction Studied by in Situ Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy[END_REF][START_REF] Saveleva | Operando Near Ambient Pressure XPS (NAP-XPS) Study of the Pt Electrochemical Oxidation in H2O and H2O/O2 Ambients[END_REF] , the investigation of similar reactions on Ptbased NPs is now attracting growing attention.

The investigation of CO oxidation, which is also considered as a paradigmatic reaction that provides valuable insights on more complicated catalytic reactions, remains still challenging under reaction conditions due to the changing of the structure and chemical composition on bimetallic nanocatalysts. A common occurrence under high pressure exposure to reactive gases, such as O2, are dynamic changes of the surface chemical composition and structure. [START_REF] Cabie | Direct Observation of the Reversible Changes of the Morphology of Pt Nanoparticles under Gas Environment[END_REF][START_REF] Zhu | Formation of Nanometer-Sized Surface Platinum Oxide Clusters on a Stepped Pt(557) Single Crystal Surface Induced by Oxygen: A High-Pressure STM and Ambient-Pressure XPS Study[END_REF][START_REF] Tao | Evolution of Structure and Chemistry of Bimetallic Nanoparticle Catalysts under Reaction Conditions[END_REF] Therefore, real time monitoring of the CO oxidation reaction in in situ conditions is greatly needed, particularly in bimetallic Pt-based nanocatalysts in order to identify the nature of the active sites, the gas-surface interaction mechanism and the role of each constituents. Because of a wealth of physical and chemical phenomena occurring, CO oxidation studies address also the key practical issue of CO pollution control, in automotive exhaust gas [START_REF] Bowker | Automotive catalysis studied by surface science[END_REF][START_REF] Herreros | Enhancing the low temperature oxidation performance over a Pt and a Pt-Pd diesel oxidation catalyst[END_REF] and in the CO-abatement in H2 produced via steam reforming of natural gas (still the main source of hydrogen). [START_REF] Lee | Understanding the Bifunctional Effect for Removal of CO Poisoning: Blend of a Platinum Nanocatalyst and Hydrous Ruthenium Oxide as a Model System[END_REF][START_REF] Liu | Pt Alloy Electrocatalysts for Proton Exchange Membrane Fuel Cells: A Review[END_REF] In hydrogen fuel cells CO contaminant leads inevitably to the poisoning of Pt catalyst at the anodes. [START_REF] Liu | Pt Alloy Electrocatalysts for Proton Exchange Membrane Fuel Cells: A Review[END_REF] Herein, the catalysts play a prominent role, therefore new research strategies are emerging in order to address these issues and produce catalysts with enhanced activity and long-term stability. [START_REF] Fonseca | Preferential CO oxidation over nanosized gold catalysts supported on ceria and amorphous ceriaalumina[END_REF][START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF][START_REF] Gatalo | Positive Effect of Surface Doping with Au on the Stability of Pt-Based Electrocatalysts[END_REF][START_REF] Shao | Nanostructured Electrocatalysts for PEM Fuel Cells and Redox Flow Batteries: A Selected Review[END_REF][START_REF] Shao | Recent Advances in Electrocatalysts for Oxygen Reduction Reaction[END_REF][START_REF] Jung | Pt-based nanoarchitecture and catalyst design for fuel cell applications[END_REF][START_REF] Jaouen | Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells[END_REF][START_REF] Ehteshami | A review of electrocatalysts with enhanced CO tolerance and stability for polymer electrolyte membarane fuel cells[END_REF] Within these current trends, efforts are deployed towards the design of strongly stable Pt-based catalysts. Thus, Zn, a has recently emerged as one of the most prominent candidate to form enhanced Pt-based catalysts owing in part to its greater "affinity" with oxygen compared with that of the noble metal Pt, and the stability of its ZnO oxide under oxidizing or reducing conditions. [START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF][START_REF] Martynova | CO oxidation over ZnO films on Pt(111) at near-atmospheric pressures[END_REF][START_REF] Behrens | The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts[END_REF][START_REF] Lei | Combining Electronic and Geometric Effects of ZnO-Promoted Pt Nanocatalysts for Aqueous Phase Reforming of 1-Propanol[END_REF][START_REF] Kang | Shape Control, and Methanol Electro-oxidation Properties of Pt-Zn Alloy and Pt3Zn Intermetallic Nanocrystals[END_REF] Thus, the present section considers the oxidation of CO over pure platinum, pure zinc and zincplatinum nanoparticles deposited in the metallic state on rutile TiO2 (110). The surface chemistry is examined using NAP-XPS as the main investigation tool. This comparative approach aims at identifying possible synergetic effects due to the presence of the two metals in the NPs. While the catalytic effect of Pt single crystals in CO oxidation is well documented in surface science, including NAP-XPS, [START_REF] Wu | Structure and Dynamics of Reactant Coadsorption on Single Crystal Model Catalysts by HP-STM and AP-XPS: A Mini Review[END_REF] the in-situ XPS examination of Pt NPs are not that common (see e.g. [START_REF] Tao | Reaction-Driven Restructuring of Rh-Pd and Pt-Pd Core-Shell Nanoparticles[END_REF] ). For its part, the CO oxidation catalytic activity of ZnO, either in the bulk state or in the form nanoparticles, has been little studied by surface techniques until now, [START_REF] Martynova | CO oxidation over ZnO films on Pt(111) at near-atmospheric pressures[END_REF] and a NAP-XPS study is completely new.

B.2. Characterization of the morphology of Pt, Zn and ZnPt NPs on TiO2(110)

The growth method used to prepare small NPs is based on electron beam physical vapor deposition from research-grade materials in UHV. In particular, in order to design bimetallic NPs, I have followed a consecutive approach which took advantage of the SMSI effect. Here, the idea is that Pt clusters serve as seeds for the nucleation of Zn, and so that the atomic diffusion of the Zn is hindered.

This effect enhances the formation of bimetallic NPs and reduces the presence of monometallic ones.

Indeed, to form the ZnPt bimetallic NPs with an average stoichiometry close to 1:1, we first deposit Pt, and then Zn in exactly the same conditions. Since Pt interacts strongly with the TiO2(110) surface, Pt NPs with an average diameter of 2.3  0.7 nm are distributed homogenously on the support with a high surface density of 7.8 × 10 12 NPs /cm 2 (see Fig. B1(a)). Assuming a cap shape for the NPs, an estimation of the surface density of Pt atoms is ~4 × 10 14 /cm 2 . I have also investigated pure Zn NPs deposited on TiO2(110) surface and found that they are also distributed homogeneously on the support, however with lower surface density (3.0 × 10 12 NPs/cm 2 ) compared with that Pt (Fig. B1(b)).

Similarly, assuming a cap shape for the Zn NPs an estimate of the Zn atom surface density is ~3 × 10 14 /cm 2 , a value very close to the estimated amount of platinum. In addition, while the free TiO2 area fraction left uncovered after Pt deposition is about 65 %, the TiO2 free area left after Zn deposition is ~84% of the surface. Thus, when bimetallic ZnPt are sought it is necessary to perform in a consecutive manner the deposition in which Pt is deposited first then Zn.

For the growth of the bimetallic ZnPt NPs, where the same quantity of both metals has been deposited as discussed above, the resulting overall density of NPs is 10.8 × 10 12 NPs /cm 2 (Fig. 1B(c)).

With respect to the pure Pt NP sample, an increase in NP density by 38% is relatively moderate, much less than a mere doubling expected if pure Zn NPs stay apart from pure Pt NPs. Therefore the STM images show that Zn atoms are attracted by the pre-existing Pt NPs. Naturally some pure Zn NPs may also appear on the surface. The average diameter of the NPs measured after Zn deposition is 2.6  1.1 nm. The distribution width is broader than that of the pure Pt NPs. The apparent average height also increases to 0.7 ± 0.17 nm. Although heights do not only depend on the physical shape but also on the electronic density, this may be a further indication that Zn is added to the Pt mounds.

B.3. Monitoring of the In situ CO oxidation reaction

The real-time monitoring of the CO oxidation reaction (2CO + O2 → 2CO2) was made possible using the NAP-XPS measurements. In order to determine whether the combination of the two metals in the same ZnPt NP is effective, a detailed study of the pure materials, Pt and Zn NPs is required. The pure NPs must have a surface density and average sizes comparable to the bimetallic NPs. This was controlled in this work using (STM) as discussed above. While the catalytic effect of Pt single crystals in CO oxidation is well documented in surface science, including NAP-XPS, [START_REF] Wu | Structure and Dynamics of Reactant Coadsorption on Single Crystal Model Catalysts by HP-STM and AP-XPS: A Mini Review[END_REF] the in-situ XPS examination of Pt NPs are scarce (see e.g. [START_REF] Tao | Reaction-Driven Restructuring of Rh-Pd and Pt-Pd Core-Shell Nanoparticles[END_REF] ). Moreover, the CO oxidation catalytic activity of ZnO, either in the bulk state or in the form of nanoparticles, has been little studied by surface techniques until now, [START_REF] Martynova | CO oxidation over ZnO films on Pt(111) at near-atmospheric pressures[END_REF] and a NAP-XPS study is completely new.

Therefore, the CO oxidation reaction on pure Pt, pure Zn and ZnPt NPs samples was monitored in situ through the evolution of C 1s (Fig. B2) and O 1s (Fig. B3) core-level spectra. The experiments were conducted in an oxygen rich environment where the total pressure of CO:O2 mixture (partial pressure ratio of 1:4) was 1 mbar. Indeed, in the C 1s and O 1s binding energy (BE) window several carbon and oxygen peaks arising from both the gas phase in the vicinity of the surface and adsorbed molecules were evidenced. The C 1s and O 1s binding energies (measured from the substrate Fermi level) of the reactive gases CO and O2, (denoted CO(g) and O2(g), respectively), and of the reaction product CO2 (denoted CO2(g)) are found at significantly higher values than the components of the surface species, e.g. adsorbed CO (denoted CO(ad)) or carbonates (denoted Carb*). In the C 1s (O 1s) window the peaks above 290 (534) eV are related to the gas phase. This synoptic view of the gas and solid phases enables an immediate determination of the activation temperature of CO oxidation and makes NAP-XPS particularly suited for the monitoring of this reaction. [START_REF] Salmeron | Ambient pressure photoelectron spectroscopy: A new tool for surface science and nanotechnology[END_REF][START_REF] Bluhm | Methanol oxidation on a copper catalyst investigated using in situ X-ray photoelectron spectroscopy[END_REF] The onset of CO oxidation reaction is marked by the appearance of the characteristic CO2(g) spectrum in the C 1s and O 1s spectral windows. For the pure Pt NP and ZnPt NPs samples the product CO2(g) is detected at ~380 K, together with the reactants CO(g) and O2(g). The pure Zn NPs sample also demonstrate their capacity to activate the oxidation of CO, but the onset of CO2(g) production is seen at a higher temperature of 396 K. The activation of CO oxidation coincides with a significant decrease in the intensities of CO(g). For heating steps at higher temperatures CO(g) is no longer detected (this is better seen in the C 1s window, Fig. B2). The temperature at which the CO(g) signal disappears

indicates that oxidative conditions are reached (396 K for the pure Pt and ZnPt NPs samples, and 450 K for the pure Zn NPs sample one). This means that the CO(g) partial pressure in the gas volume probed by the beam above the surface becomes so low that the CO(g) spectrum becomes non-measurable.

(The experimental geometry makes that the irradiated gas volume has a height of ~0.12 mm above the surface plane, however as the analyzer is focused on the surface, not all gas "layers" contribute equally.) All CO molecules arriving "in the vicinity of the surface" are quickly transformed into CO2, and the gas layers close to the active surface are depleted from CO molecules. Mass transport limitation was also observed by Blomberg and coworkers [START_REF] Blomberg | In Situ X-Ray Photoelectron Spectroscopy of Model Catalysts: At the Edge of the Gap[END_REF] in their NAP-XPS study of CO oxidation on Pd(100). Thus, At this high temperature, the CO(g) signal (see Figs. B2 andB3) has disappeared (we have reached the mass transport limitation regime).

B.3.1. Adsorbed CO and carbonates formation on nanoparticles under 1 mbar of CO:O2

The monitoring of the evolution as a function of the temperature of adsorbed CO (CO(ad)) and carbonates (carb*) provides valuable indications on the structure and reactivity of NPs. In particular, the presence of CO(ad) is an indication on the elemental composition on the surface of the NPs.

Additionally, the BE value of C 1s associated with CO(ad) points to the nature of the adsorption sites on the NPs, such as bridge or atop sites. These information are relevant to better understand the reactivity of nanocatalysts.

Indeed, I observed the presence of CO(ad) mainly on Pt and ZnPt NPs up to 380 K while it is lacking on Zn NPs (Figs. B2-3). Adsorbed CO is associated with the presence of triplet peaks at BEs of 285.9 eV, 286.7 eV and 287.3 eV that grows considerably in intensity once the gas mixture is introduced at room temperature. CO is very likely adsorbed on the Pt NPs. The calculated chemisorption energies of CO (atop sites) are in the 1.51-1.54 eV range for the Pt(111) surface. [START_REF] Bleakley | Density functional theory study of the interaction between CO and on a Pt surface: CO/Pt(111)[END_REF] The CO molecule occupies sequentially the atop and the bridge sites. [START_REF] Steininger | On the Adsorption of Co on Pt(111)[END_REF][START_REF] Feibelman | The CO/Pt(111) puzzle[END_REF] The C 1s binding energies of CO in atop and bridge sites are 286.7 and 286.0 eV, respectively. [START_REF] Liao | FTIR study of adsorption and photoassisted oxygen isotopic exchange of carbon monoxide, carbon dioxide, carbonate, and formate on TiO2[END_REF] Assuming that (111 Besides CO(ad) species, the C 1s and O 1s BE areas show the surprising presence of carbonate (Carb*) species on the NPs sample once the gas mixture is introduced at room temperature. The main component is found for Pt NPs at a BE of ~289 eV. [START_REF] Koitaya | Real-Time Observation of Reaction Processes of CO2 on Cu(997) by Ambient-Pressure X-ray Photoelectron Spectroscopy[END_REF][START_REF] Deng | Surface chemistry of Cu in the presence of CO2 and H2O[END_REF] When temperature increases, the carbonate signal diminishes in the case of pure Pt NPs. It almost vanishes at 380 K, although a sizeable CO(ad) signal remains. The two phenomena are certainly uncorrelated. The formation of carbonates should follow a formal reaction of the type Pt( 4+ )O2+CO→PtCO3. In marked contrast with the pure Pt NPs, the carbonate species persist on Zn and PtZn NPs. In the case of Zn NPs, the carbonates appear as broad C 1s peak (in the 288.5-289.0 eV range) (Fig. B2b). In fact it is made of two components separated by ~1 eV. While the lower binding energy component is the most intense at low temperatures, the high binding one becomes preponderant at 450 K. This suggests the existence of two different bonding configurations for CO2 on oxidized zinc, with different chemisorption energies. In his review of zinc oxide surfaces Wöll gives a recension of the binding energies of CO and CO2 adsorbed on O-ZnO and ZnO(101 ̅ 0) and leading to "CO3" surface compounds. [START_REF] Woll | The chemistry and physics of zinc oxide surfaces[END_REF] There are indications in the literature that CO can be oxidized on the ZnO surfaces. Four decades ago, Esser and coworkers [START_REF] Esser | Comparative-Study on the Reactivity of Polycrystalline and Single-Crystal Zno Surfaces -Catalytic-Oxidation of Co[END_REF] observed the thermal desorption of CO and CO2 after CO exposure to stoichiometric ZnO powders at 300 K. They noted that the coadsorption of O2 increased the production of CO2. More recently, infra-red spectroscopy [START_REF] Hussain | Oxidation of CO by O-2 over ZnO Studied by FTIR Spectroscopy[END_REF] showed the presence of carbonates irreversibly adsorbed at 300 K on the surface of finely divided oxidized ZnO powders, the subsequent addition of O2 enhancing the formation of carbonates.

)
The fact that the carbonate signal in the C 1s (Figs. B2b-c) and O 1s (Figs. B3b-c) spectra does not vanish when CO oxidation conditions are met means that CO2 is strongly bonded to the oxidized zinc compound. The in-situ NAP-XPS study shows that CO is oxidized to CO2 already at room temperature on the oxidized zinc surface exposed to the CO:O2, mixture and readily bonded as a carbonate. The evolution of CO2(g) is only possible when the temperature is raised. We can propose that a steady-state regime is reached, with CO2 molecules continuously formed on the zinc oxide and desorbed when a sufficient temperature is reached. Finally, our oxidized Zn NPs behaves in a way very similar to the prismatic ZnO(101 ̅ 0) surface (the major orientation found in ZnO powders). Esser et al. [START_REF] Esser | Comparative-Study on the Reactivity of Polycrystalline and Single-Crystal Zno Surfaces -Catalytic-Oxidation of Co[END_REF] found that the stationary regime of CO oxidation starts at 400 K and is maximum at ~530 K in a CO:O2 (1:1) mixture, under a pressure of 1.610 -5 mbar. The reaction mechanism is possibly of the Mars-van Krevelen type, [START_REF] Mars | Oxidations carried out by means of vanadium oxide catalysts[END_REF] in which a surface oxygen atom oxidizes the CO molecule, leaving an oxygen vacancy at the surface, that is then replenished by reaction with an O2 molecule from the gas phase.

B.3.2. Chemical changes at the ZnPt NPs surface in the presence of 1 mbar of CO:O2 gas mixture

The in situ studies under a reactive gas allow the monitoring of the surface chemistry composition and its evolution, which is particularly of paramount interest in bimetallic NPs. Indeed, the measurements of core-level XPS spectra of Pt 4f and Zn 3p provides indications on the surface composition of ZnPt NPs. In stark contrast to what happened for the pure Zn NPs sample, we found that the Zn 3p in ZnPT NPs does not exhibit any binding energy shift once the CO:O2 mixture is introduced at room temperature up to 380 K which is the onset of CO oxidation reaction (Fig. B4a).

This suggests that metallic Zn is protected from any contact with the oxygen molecules. Given the oxophilicity of Zn and the absence of CO bonding to this metal, most of the zinc atoms must have penetrated in the pre-existing Pt NPs, whose surface is platinum-rich, despite the fact that (i) Pt deposition preceded Zn deposition, and (ii) the surface energy of Zn(0001) (0.993 J.m -2 ) is less than that of Pt(111) (2.489 J.m -2 ). [START_REF] Vitos | The surface energy of metals[END_REF][START_REF] Chen | Surface structure and stability of PdZn and PtZn alloys: Density-functional slab model studies[END_REF] Therefore a chemically driven phenomenon drags the platinum atoms to the surface. We can attribute it to the formation of Pt-CO bonds. Indeed, in the C 1s energy window (Fig. B2c), the CO(ad) signal "titrates" the platinum atoms in contact with the gas mixture. The fact that the CO(ad) signal is absent in the same conditions for the pure Zn NPs sample is a further confirmation that it is related to CO(ad) on platinum. Indeed the fate of CO(ad) is identical on Pt and ZnPt NPs surfaces. CO(ad) is still observed at the onset of the oxidation reaction (380 K, identical to the pure Pt NPs case), and disappears at 396 K, when the CO(g) signal vanishes, i.e. in oxidative conditions. The Zn 3p binding energy, until then constant, increases suddenly, by 0.5-0.6 eV (the same shift as that seen for pure Zn NPs), indicating that Zn is readily oxidized to ZnO, when CO, previously bonded to Pt Further indications on the changes on the surface of NPs was gained when considering one of the most interesting property of the synchrotron radiation, that is the tenability of the photon energy.

In particular, the photon energy dependence of the platinum atomic weight (Pt/(Pt+Zn)) displayed in Fig. B4b illustrates clearly how the ZnPt NPs initially surface rich become Zn rich above 380 K. Indeed,

The Pt/(Pt+Zn) atomic weight systematically increases when the photon energy increases (i.e. when the inelastic mean free path increases) from room temperature to 380 K, i.e. when CO molecules are adsorbed on the surface. This means that Pt segregates at the surface of NPs in this temperature range (this is coherent with the absence of Zn oxidation, as discussed before). Once oxidative conditions are reached when the temperature is increased, we observe the opposite: the atomic weight of Pt is the greatest when the inelastic mean free path is the largest. This is fully coherent with the image of a ZnO shell (on which carbonates are bound) and a metallic Pt core. The segregation of Zn (as an oxide) can be simply explained by thermochemical effects: In oxidative conditions, thermodynamic data suggest that the oxidation of Zn should be preferred over that of Pt, as the enthalpy of formation of ZnO (-350 kJ/mol) [START_REF] Cox | CODATA key values for thermodynamics[END_REF] is much more negative than the corresponding value of PtO2 (-80 kJ/mol). [START_REF] Nagano | Standard enthalpy of formation of platinum hydrous oxide[END_REF] Therefore we observe chemically-driven phase separations, to be distinguished from a segregation due to surface energy considerations.

The role of Zn is more severe in pure oxidative (O2 atmosphere) conditions as it provides significant protection against Pt oxidation (Fig. 5b). [START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF] The presence of Zn in ZnPts NPs shows that it keeps Pt in a majority metallic state. This is again as discussed above related to thermodynamic effects where the ZnO formation is more favorable than PtO or PtO2 occurrences. As discussed in details in one of my previous studies, [START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF] the formation of PtO was found to be the driving force for the encapsulation by the TiOX support. Therefore, the presence of Zn represents a significant added value regarding the stability of ZnPt NPs.

B.3.3. A comparative perspective on the pure Pt, Zn and bimetallic ZnPt NPs

As a summary of this study, the scheme given in figure B6 shows an overview picture of the complete evolution of the three samples as a function of the temperature. The pure Pt NPs sample behaves in a way similar to the crystalline surfaces of bulk platinum (or bulk palladium) [START_REF] Blomberg | In Situ X-Ray Photoelectron Spectroscopy of Model Catalysts: At the Edge of the Gap[END_REF] exposed to a CO:O2 mixture. Considering an increasing reaction temperature, Pt NPs adsorb CO, and the reaction starts at a critical temperature when CO is replaced by atomic oxygen, i.e. when platinum oxidizes.

With respect to the former case, the pure Zn NPs sample exhibits a very contrasted behavior, due to the fact that CO does not bond to metallic Zn. In fact, Zn is readily oxidized in the CO:O2 gas mixture to ZnO at room temperature. The oxide provides O species to oxidize the CO molecule (in a

Mars van Krevelen scheme), and the resulting CO2 species remains bound as a carbonate. Our observation of an appreciable catalytic activity for the pure Zn NPs at 450 K, points to the importance of considering what crystallographic faces are exposed to the gas mixture when the activity of ZnO is discussed. The observation of carbonates is only explainable if non-polar faces like ZnO(101 ̅ 0) and (112 ̅ 0) are in contact with the gas mixture, because of the large calculated chemisorption energies of CO2 (0.83 and 0.79 eV, 92 respectively) on the latter ones. On the polar faces CO2 has a very low calculated chemisorption energy (<0.1 eV). [START_REF] Tang | Adsorption of CO2 at ZnO: A Surface Structure Effect from DFT+U Calculations[END_REF] The bimetallic ZnPt NPs present a more complex behavior. Although Pt is deposited first, the alloyed NPs are Pt-terminated at room temperature and CO can poison the catalyst by bonding to platinum. At a critical temperature, Zn moves to the external surface where it forms an oxide, while

Pt goes to the core where it remains metallic. Then in these oxidative conditions, CO2 is produced. As in the case of pure Zn NPs, carbonates are observed and they likely play a role in CO oxidation. Yet the role of platinum that remains metallic (in contrast with the pure Pt NPs sample that is oxygencovered) remains to be fully elucidated. We note that the onset of CO oxidation occurs at 380 K, a temperature lower than for the pure Zn NPs case (396 K). Therefore the presence of platinum is clearly beneficial. The metallic platinum core can be a provider of electrons in a "Cabrera-Mott picture" facilitating the dissociation of O2 molecules. As an alternative explanation, we may also consider the fact that Pt doping decreases the oxygen vacancy formation energy at prismatic ZnO surfaces. [START_REF] Pala | Modification of the oxidative power of ZnO(10(1)over-bar0) surface by substituting some surface Zn atoms with other metals[END_REF] Thus more atomic oxygen could be provided to oxidize CO in a Mars van Krevelen scheme.

By virtue of these examples, we demonstrate how model studies on complex nanostructured materials may provide an atomistic view of processes at the gas-solid interface related to heterogeneous catalysis.

SECTION C: Molecular adsorption on pristine and water-saturated Si(001)

This section is dedicated to the study of adsorption mechanism of organic and water molecules on Si(001) surface and its electronic properties. This project is the subject study of the head of the team "Surface fonctionnalisées et environnementales" to which I belong, Prof. François Rochet. This project was initiated in my postdoctoral term at the UPMC and continued afterwards as a faculty in the same institution. My involvement consists on the STM studies which represent a significant part of this project.

Therefore, I was in charge of these STM studies. My original approach here is the investigation at the atomic scale of the adsorption and electronic phenomena in real time using a scanning while dosing method.

C.1. Introduction

Si(001) is one of the most studied semiconductor surface owing to its technological relevance in the current microelectronic devices and in the promising and complex molecular electronics applications when it is modified by adsorbed molecules. [START_REF] Wolkow | Controlled molecular adsorption on silicon: Laying a foundation for molecular devices[END_REF][START_REF] Filler | The surface as molecular reagent: organic chemistry at the semiconductor interface[END_REF] In this regard, efforts are deployed to understand the local electronic properties of the surface at the atomic scale, the chemical reactions and adsorption mechanism taking place at the interface. The ultimate goal of silicon-based technologies is to develop atomic scale devices in which the charge and/or spin of individual Si atoms can be manipulated. It is a prerequisite, for example, in the development of a Si-based solid-state quantum computers or in the functionalization of Si surfaces using organic molecules for molecular electronics. [START_REF] Kane | A silicon-based nuclear spin quantum computer[END_REF][START_REF] Piva | Field regulation of single-molecule conductivity by a charged surface atom[END_REF] Indeed, we have explored various configurations of the Si(001) surface including:

pristine, water-saturated, and adsorbed organic molecules such as triethylamine and benzaldehyde.

98-100

C.2. Triethylamine adsorption on pristine Si(001) surface

The reactivity of the Si(001) surface which is dictated by its topology and the charge borne by the surface atom plays a key role in the adsorption of organic molecules. In particular, the zwitterionic nature of the silicon dimer suggests that surface reactions can be controlled by charges. In this respect, the reaction of amines molecules such as triethylamine (TEA) (a tertiary amine), which bear a nitrogen lone-pair electron, allows the investigation of the adsorption mechanisms in a Lewis acid-base reaction and the role of electronic charges. Thus, the nitrogen of TEA which is a Lewis base (electron rich) can indeed engage its lone-pair in the empty dangling bond of the Si dimer atom (an electron-poor, acidic site) to form a dative bonding.

In the following, I will first discuss the electronic properties of a clean Si(001) surface.

C.2.1. Morphology and electronic properties of a pristine Si(001) surface

The study of the basic properties of silicon clean surface has led to establish that one of the (001) ideal surface reconstruction was the symmetric Si(001)-(21) at above a transition temperature. The evolution from the original (11) towards the (21) structure, through the formation of surface Si-Si dimers, occurs so that the total surface energy is lowered. 101 Thus the clean Si(001) consists of large terraces with apparent symmetric Si-Si dimer rows running along the [001] direction (see Fig. C1). Further decrease of the surface energy is obtained when the dimers are buckled, 102-105 one atom moving upward, denoted 'Si-up' (Siu), while the other one moves downward, denoted 'Si-down' (Sid). 102 The electronic structure of this buckled surface is dominated by a negative charge transfer from the "down" Sid dimer atom to the "up" Siu dimer atom; thereby the Sid atom becomes electron-deficient (electrophilic), whilst the Siu becomes electron-rich (nucleophilic). Although the surfaces shown in Fig. 1C are almost perfect, they exhibit a typical defect, the socalled C-defect (see the feature indicated by the circle in Fig. C1b). It is associated with the dissociation of a water molecule which occurs spontaneously at RT almost without an energy barrier leading to the formation of Si-H and Si-OH fragments (Fig. C1c). The asymmetric contrast of the C-defect originates from the uneven electronic distribution between its reacted and unreacted Si atoms. Upon dissociation of H2O on two adjacent dimers, a transfer of charge takes place from the Si-H and Si-OH sites; whereby two dangling bonds are eliminated, towards their opposite unreacted Si atoms (couple of two bright protrusions in Fig. C1b).

C.2.2. Adsorption mechanism of triethylamine on Si(001)

The adsorption of TEA from the gas phase on this surface is molecular (non-dissociative) at RT.

It takes place though the nitrogen atom which engages its lone-pair electron in the empty dangling bond of the Sid dimer atom (an electron-poor, acidic site) to form a dative bonding (a Lewis acid-base reaction). [START_REF] Naitabdi | Triethylamine on Si(001)-(2 x 1) at 300 K: Molecular Adsorption and Site Configurations Leading to Dissociation[END_REF] The presence of datively-bonded TEA molecule on the surface appears as a bright protrusion in the filled-state STM images (Fig. C2). Besides, the unreacted silicon dimers appear now buckled. The static buckling of bare dimers, along the dimer row surrounding the adsorbed molecules, is characteristic of dative bonding. Interestingly, the surface exhibits completely a c(4×2) pattern for a TEA coverage occupying only ~0.6% of the total area. Therefore the adsorption of a few TEA molecules on a dynamically buckled area of the Si(001)-(21) surface generates a static c(42) structure at RT. This effect represents a templating process at RT since buckled silicon dimer is a zwitterion, with a negatively silicon atom (Siu) and a positively charged silicon atom (Sid).

Additionally, unlike "on-dimer'' di- bonding molecular of π-bonded molecules, or the dissociative bonding of water and ammonia, for which two fragments cap the two dangling bonds, tertiary amines dative bonding leave one dimer silicon atom uncapped (free dangling bond). Indeed, this surface can promote chemical reactions following a Lewis acid-base scheme, where an electron donor can engage in dative bonding with the Siu atom. Further exposure to TEA leads to the formation of locally well-ordered domains of molecularly adsorbed TEA molecules which can be described as a c(42) cell (Fig. C2b). The adsorbed TEA molecules exhibit a clear ternary symmetry (three grey spots are observed), expected for the datively bonded adduct, for which the CC bond axes point toward vacuum. We note that one triangle summit is systematically oriented towards the free atom of the reacted silicon dimer. The stability of datively-adsorbed TEA molecules was monitored as a function of time. Indeed, 2 hours after the deposition, I have noticed the appearance of new features with apparent morphology different from that of the initial datively-bonded TEA as can be seen in Fig C2c . They are dissociated pairs of two TEA molecules siting on the same row on two adjacent dimers (Fig. C2cc'). The dissociation occurs through the N-C bond cleavage and leads to the formation of Si-N(CH2CH3)2

and Si-CH2CH3 moieties on a symmetric dimer. This induced mode is strikingly different from the dative mode as the two dangling bonds are occupied by molecular fragments, which suggests a weak electronic coupling of the reacted dimer with the adjacent bare dimers. Consequently, the dynamic buckling resumes again as can be seen in the STM image in Fig. C2c. As dissociated molecules are always paired, the dative-to-dissociated state conversion may be not unimolecular, in other terms, dissociation of one molecule is facilitated by the presence of a second molecule sitting on an adjacent dimer (maybe via a concerted path). This effect implies that a TEA molecule is wandering across the surface until it encounters a partner TEA molecule in order to dissociate. Therefore, this suggests that datively-adsorbed TEA molecules are mobile on the surface. Indeed, the mobility phenomena is clearly observed and evidenced hereafter.

C.2.3. Mobility and conductance properties of triethylamine on Si(001)

The scanning-while-dosing approach was particularly convenient in order to monitor the mobility of TEA molecules on the surface for a given time span (see Fig. C3). Indeed, for example two molecules indicated by two arrows (1 and 2) in Figure C3 moved from their initial locations, toward new adsorption sites. The hopping of TEA molecules observed here is independent of the scan direction and thus it is not mechanically-induced by the tip. This mobility can be tracked geometrically. Hence, the displacement of the molecule (1) can be quantified as Jumps are observed both along the dimer rows and perpendicular to it, within a characteristic time-span of a few minutes. Surface mobility is a further evidence for a molecular, dative bonding adsorption mode. Surface diffusion barriers are substantially lower than the calculated desorption energy barriers Ea of the datively bonded molecule (as dative bonding is barrierless), Ea is equal to -Eads and thus ranges from 1.1 eV to 2.0 eV, according to coverage. [START_REF] Naitabdi | Triethylamine on Si(001)-(2 x 1) at 300 K: Molecular Adsorption and Site Configurations Leading to Dissociation[END_REF] In fact, the residence time is 10 -13 e (Ea/kT) corresponding to 10 6 s for Ea = 1.1 eV and 510 21 s for Ea = 2.0 eV. One of the key issue that molecular conductance measurements could help address concerns the nature of the molecule-substrate bonding, and the degree of hybridization between the adduct and the substrate. In this regard, using scanning tunneling spectroscopy (STS), I have performed normalized differential conductance measurements, which correspond, with a good approximation, to the local density of states (LDOS) (noted ) of TEA adducts on Si(001)-(21) near the Fermi level, in three various adsorption modes: (i) dative single molecule, (ii) dative molecule embedded within a well-ordered c(42) molecular domain and (iii) dissociated molecule (Fig. C4). 100 In effect, The LDOS of the molecules datively bonded to silicon depends strongly on coverage as illustrated by the presence of peaks at different energies. The peak denoted at -0.72 eV is associated to the Siu atom band (the so called π band). In the empty state, the peaks at + 0.25 eV (arrow) and at +0.84 eV are attributed to the Sid band (the so-called π* band), and to the backbonds (BB), respectively. The LDOS of the clean surface exhibits a surface band gap of 0.5 eV.

Interestingly, despite the presence of a TEA molecule, the LDOS Clean and dat-is, acquired on clean Si(001) and on a single datively-bonded TEA, respectively are very similar. This is an indication that the HOMO and LUMO orbitals are positioned on silicon atoms. In particular, the HOMO and nearly degenerate levels (HOMO+1) comprises the Siu atom on the reacted dimer and the Siu atom of the nonreacted adjacent dimer. 100 Thus, the covalent dative bonding of a TEA molecule conserves the strong electronic coupling between the reacted dimer levels and those of the adjacent bare dimers. The LDOS for a TEA datively-bonded molecule imbedded in the c(4  2) reconstructed molecular domain does not disrupt either the coupling of the adjacent unreacted Si dimers but exhibits an increase in the gap (now ~0.85 eV). This difference originates from the fact that bare dimers in the molecular c(42) are significantly less buckled than in the clean surface (5.5° versus 18.5 °) and their Si-Si bond is shorter (2.32 Å versus 2.36 Å). [START_REF] Naitabdi | Triethylamine on Si(001)-(2 x 1) at 300 K: Molecular Adsorption and Site Configurations Leading to Dissociation[END_REF]100 This is indicative of a stronger p bond. The π-π* splitting should increase, pushing the occupied and non-occupied bands apart. The coupling is however suppressed for dissociated TEA molecule as shown in the profile of the LDOS diss in Fig C3 . In this case all states related to the Si dangling bonds are suppressed as there is no contribution to the HOMO (LUMO) on the reacted dimer. C2). For the sake of clarity, the curves are shifted vertically: the horizontal line below each curve corresponds to the normalized differential conductance level equals zero. All data were acquired with the same tunneling conditions: a current set-point of 100 pA and bias of -2 V.

C.3. Benzaldehyde adsorption on water-saturated Si(001) surface

The adsorption of benzaldehyde on water-saturated Si(001), noted in the following as (H, OH)-Si(001)-(21), exploits the peculiar properties of isolated silicon dangling bonds (IDB) left unreacted on this surface. In particular, the IDB are rich localized electronic atomic sites which can be used as templates for selective molecular reactions.

C.3.1. Formation of isolated dangling bond and properties of a water-saturated Si(001)

The (H, OH)-Si(001)-(21) surface was prepared by exposing the clean surface at room temperature to 4.5 L of water [1 langmuir (L) = 10 -6 Torr  s] (under a nominal pressure of 0.75  10 -8 Torr). This exposure was sufficient to saturate the surface. The dissociation of water lead to the formation of Si-H and Si-OH moieties on the surface which are clearly visible on the STM images (see Fig. C5). Indeed, our atomically resolved images show the special arrangement of H and OH terminations, and can be identified based on their apparent heights. The medium bright elongated features are due to Si-OH which tend to pair on the same dimer (likely stabilized by hydrogen bonding) (Fig. C5).

Despite the surface saturation, unreacted Si atoms remain on the surface and they correspond to the silicon isolated dangling bond. The IDB appear as bright features in the STM images with an areal density of 0.020  0.005 defects/Si atoms (Fig. C5). These IDB are bandgap states resulting from the remaining valence electron left in an unsatisfied dangling bond state. From the energetic point of view, the IDB can be in three different states, (i) negative state IDB -(bearing two localized electrons), (ii) neutral state IDB 0 (bearing a single localized electron), (iii) positive state IDB + (bearing a localized hole). Thus, the IDB are attractive as reaction sites for the linkage of molecules such as benzaldehyde which is discussed in the following paragraph.

C.3.2. Benzaldehyde adsorption on (H, OH)-Si(001): radical adduct and concerted hydrosilylation

A scanning-while-dosing approach is used to monitor in real time the adsorption sites of benzaldehyde on (H, OH)-Si(001)-(21). The exposure to benzaldehyde at a nominal pressure of P = 3.75  10 -8 Torr (total dose = 31.5 L) shows a gradual change in the surface structure. Indeed the bright protrusions (IDB) go extinct with the dosing as a result of the reaction with benzaldehyde.

Thus, the real-time STM images show that the preferential reaction sites are the IDB. The IDB changes from bright protrusion to a circular dark halo after the reaction with benzaldehyde. In addition, The center of the halo is not exactly confounded with the location of the IDB. Actually, it extended towards the trench between the dimer rows. This asymmetry is due to bulky phenyl of the benzaldehyde ring that bent toward the trench. From complementary XPS measurements, we have shown that the IDB remain electrically active after the reaction. The formation of closed-shell adduct via the formation of Si-O bond, for example, should produce bonding and anti-bonding states out of the gap, and hence the bands should go flat. [START_REF] Pierucci | Benzaldehyde on Water-Saturated Si(001): Reaction with Isolated Silicon Dangling Bonds versus Concerted Hydrosilylation[END_REF] This is not the case, the observed band bending (-0.30 eV) on the reacted surface is equal to that of the pristine (H, OH)-Si(001). [START_REF] Pierucci | Benzaldehyde on Water-Saturated Si(001): Reaction with Isolated Silicon Dangling Bonds versus Concerted Hydrosilylation[END_REF] At high coverage (total dose of 58. electrons are captured by O2 molecules at the oxide surface to form superoxo O 2 -species that then dissociate into labile O species available to oxidize CO. The ideas are basically those put forward by Mott 121 and Cabrera and Mott. 122,123 However, to be effective, inverse catalysts need that the oxide surface be sufficiently "flexible" to accommodate the formation of the charged species. Thus, the control of the morphology, the thickness and the structure are among the parameters that will be addressed in my project. Indeed, in this project the system that will be explored concerns ZnO/Pt(111) owing to its relevance as a catalyst for CO oxidation. In addition, within the quest to reduce the cost of catalysts by minimalizing Pt loads and the need to stabilize Pt against oxidation, poisoning and subsequent decomposition, the incorporation of Zn is greatly promising. [START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF][START_REF] Martynova | CO oxidation over ZnO films on Pt(111) at near-atmospheric pressures[END_REF]124 The formation of Pt surface oxide (PtO) was found to be the factor that fosters the disappearance of Pt due to support encapsulation. [START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF] Indeed, in oxidative conditions, thermodynamic data suggest that the oxidation of Zn should be preferred over that of Pt, as the enthalpy of formation of ZnO (-350 kJ/mol) [START_REF] Cox | CODATA key values for thermodynamics[END_REF] is much more negative than the corresponding value of PtO2 (-80 kJ/mol). [START_REF] Nagano | Standard enthalpy of formation of platinum hydrous oxide[END_REF] The advantage provided by the addition of "sacrificial" Zn (oxidizing to ZnO) is the prevention of PtO formation and hence a promotion of the Pt metallic state stability at the surface. [START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF] The metal-oxide interfaces promote the activation of O2, essential to the CO oxidation reaction, while they prevent the metal from being poisoned by CO. Although these systems are probably wellunderstood from the structural point of view, there is no substantial evidence of the structural, oxidation & chemical states of the active elements. In-situ real-time NAP-XPS experiments will allow us to simultaneously monitor the oxidation states of the catalysts and chemical reactions as a function of the treatment (temperature, gas exposure). Therefore, following this approach, this project intends to establish the structure and chemical states reactivity relationship in ZnO/Pt(111) system.

Moreover, the size and thickness dependent electronic properties and hence catalytic performances are key factors that will be explored. The CO oxidation reaction will be used as a prototype using in situ NAP-XPS investigations. Thus, I will investigate how the CO oxidation can be controlled by tuning the following effects: (i) metal-oxide (ZnO) thickness and morphology, (ii) charge transfer from the metal subsurface atoms (Pt), (iii) surface and interface composition (alloy, segregation). The ZnO-Pt will act as an inverse catalyst where the ZnO represent the active surface and the metallic Pt serves as the reservoir for electrons necessary to the activation of O2.

In order to bridge the gap between the model systems and the real-world catalysts, I will implement the following strategy which will rely on two parallel design methods. The first consists resulting charge built up on the surface will activate O2 essential to the CO oxidation reaction. The charge transfer effect will be measured by XPS as it will lead to an upward shift in the binding energy of Zn 2p spectra and the d-band states of Pt. The presence of ZnO will prevent the Pt from being poisoned by CO molecules. Hence, in this project, I will explore in details possible correlation between the structure, electronic and catalytic properties of oxide-metal interfaces via the paradigmatic CO oxidation reaction under realistic conditions.

P.2.2. Synthesis of size-selected bimetallic ZnOPt nanoparticles: chemical routes

Since , the size, shape and surface crystallinity (high and low index facets) play a prominent role in the catalytic performances, [START_REF] Freund | Model Studies in Heterogeneous Catalysis[END_REF] tt is therefore of paramount interest to implement efficient synthesis Here, the oleylamine will act as the reducing agent, surfactant and solvent. The size (2.0 nm -5 nm)

and composition of the NPs will be tuned. While the ex situ synthesis of bimetallic NP through Oleylamine reduction of M1(acac)2 and M2(acac)2 has been proven efficient, the exact role of oleylamine remains still unclear. Since these NCs are intended for catalytic applications, the understanding of the ligands effects and their evolution as a function of the temperature and reactive gases are crucial. Therefore, we will perform the synthesis step by step in our laboratory ultra-high vacuum (UHV) chamber, where the reactants will be introduced through a pulse-controlled liquid injection process. The UHV system is also equipped with an advanced X-ray Photoelectron Spectroscopy (XPS) instrument that can be operated not only in UHV but also at intermediate pressures up to 510 -4 mbar. The availability of this option allows the in situ characterization under reactive gases. This method provides twofold advantages. First, the environment will be controlled and subsequent air exposure after the synthesis and before the characterization will be suppressed. Thus, the monitoring of the oxidation states by XPS after each step in operando conditions will be performed providing the opportunity to understand the initial state of the reactants and their final status after the chemical reaction. Second, the treatment will be performed on well-identified NCs from the oxidation state point of view. Here, the modification induced by air exposure, such as oxidation, will not be an issue allowing a better understanding of the surfactant and the effect of treatments.

Indeed, the presence of the ligands or surfactants on the surface of these NCs may be a limitation to their chemical reactivity, particularly in the case of CO oxidation. Therefore, prior to the catalytic reaction, the NCs will be first exposed in situ to a soft O2 plasma treatment (PO2 = 510 -7 mbar) in order to remove the ligands. Our UHV system is also equipped with a state of the art Plasma source so that this treatment will be performed in situ. We have conducted successfully this plasma treatment on size-selected Pt NPs prepared by inverse-micelle encapsulation method without any alteration of the structure and shape of NPs. [START_REF] Naitabdi | Enhanced thermal stability and nanoparticle-mediated surface patterning: Pt/TiO2(110)[END_REF][START_REF] Roldan-Cuenya | Thermally stable nanoparticles on supports[END_REF] Additionally, the exposure to O2 plasma will result in

GENERAL CONCLUSIONS

At the initial stage of my studies in the area of heterogeneous catalysis, I have first dedicated my efforts to the synthesis of supported mono-and bimetallic nanoparticles, in particular Pt and AuFe, using the inverse micelles of diblock copolymers. This method has been proven very efficient in providing size-selected NPs with tunable interparticle distance. In addition, thanks to the formation of a strong TiC interface between NPs and the support, the NPs exhibited excellent thermal stability which represents a decisive asset for their application as stable nanocatalysts. I have also shown thanks to STM and XPS investigations in UHV that the alloy formation and the stability of bimetallic AuFe NPs depended strongly on the composition, with the Au0.8Fe0.2 being the most thermally stable due to stress energy optimization resulting from a core (Fe)-shell (Au) formation.

Afterwards, I have conducted in situ studies on model metallic NPs such as Pt, Zn, and PtZn, supported on TiO2(110) using state of the art NAP-XPS which have provided atomistic view of processes at the gas-solid interface related to heterogeneous catalysis. In particular, in situ studies under realistic conditions of pressure and temperature using NAP-XPS have been proven decisive as they provided synoptic information on the gas phase, adsorbed species and oxidation states of catalysts. The investigation of CO oxidation, which is considered a paradigmatic reaction, has shown that pure Pt NPs sample behaved in a way similar to the crystalline surfaces of bulk platinum and adsorbed CO up to CO2 production regime. Initially formed metallic PtZn NPs transformed into ZnOPt, with the oxide on the shell, and exhibited comparable activity towards CO oxidation. These studies have highlighted the emerging property of ZnO as a very promising catalyst, as it is active for CO oxidation. The role of Zn is more severe in pure oxidative (O2 atmosphere) conditions as it provides significant protection against Pt oxidation. This is related to thermodynamic effects where the ZnO formation is more favorable than PtO or PtO2 occurrences.

Finally, in a second topic of my research program, I have shown that the adsorption of triethylamine on Si(001) tooke place via a covalent dative bonding which maintained the Si-Si coupling unchanged. In addition, TEA molecules were shown to pattern the Si(001) with a transition from (21) to a c(42) structures at room temperature. Moreover, silicon isolated dangling bonds, which acted as localized electronic states where found to be the preferential sites for the molecular adsorption of benzaldehyde molecules and the formation of stable radical adducts.

  SECTION A: Supported Size-selected metallic and bimetallic nanoparticles on TiO2(110): design, composition effects and thermal stability This work was initiated during the first part of my postdoctoral experience which took place at the University of Central Florida. However, the topic of supported nanoparticles on TiO2(110) was developed and emphasized afterwards in the research projects pursued in my faculty position at the Université Pierre et Maris Curie.

Fig

  Fig. 2A(a).

Figure A1 :

 A1 Figure A1: Schematic representation of the principle and steps on the synthesis of metallic NPs by the inverse micelle encapsulation in diblock copolymers method.

  A2(b)).

Figure A2 :

 A2 Figure A2: STM image of supported Pt NPs on TiO2(110) after O2 plasma treatment and annealing at 1000°C. The sample was prepared by the inverse micelle method. (b) XPS core-level spectra of C 1s of this sample before and after O2 plasma treatment.

Figure A3 :

 A3 Figure A3: Upper panel: 3D STM images (100 nm 100 nm)of supported Pt NPS on TiO2(110) after O2 plasma treatment and annealing at high temperature as indicated. Lower panel: line scans measured across the STM images (at larger scale). The staircases numbered 1 to 5 are a schematic illustration to show the number of surface steps after each temperature annealing. The scale bar is 10 nm.

Figure A4 :

 A4 Figure A4: Evolution of the relative content of the two different Fe phases (component 1 and component 2) obtained from the fits of Fe-2p XPS spectra (not shown here) acquired after sample annealing in UHV from RT to 1000 °C. The data ( and ) correspond to the following compositions: (a) Au0.8Fe0.2, (b) Au0.5Fe0.5, and (c) Au0.2Fe0.8 NPs. For comparison, the data ( and ) correspond to the phase content of the pure Fe NPs. The component labeled as 1 ( and ) corresponds to the low BE feature (Fe-2p3/2 at 706.9-707.7 eV) assigned to metallic Fe, while component 2 ( and ) corresponds to the high BE feature (Fe-2p3/2 at 709.8-712.2 eV and satellites at ∼720 eV) attributed to Fe-oxides + FeTiO3+δ compounds or Au-Fe alloy contribution depending on the temperature. The vertical arrows is shown to

Figure A5 .

 A5 Figure A5. Schematic illustration of phase evolutions on AuFe NPs as a function of the composition and the temperature. Legend: the grey balls are Fe, the brawn balls are Au and violet balls are Ti. In summary, as schematized in Fig A5, from XPS measurements a better understanding of key phenomena occurring on bimetallic AuFe NPs was reached, particularly the onset of Au-Fe alloy formation and segregation phenomena in nanoparticles with similar average size but distinct composition. In general, with increasing annealing temperature, four types of structures were inferred in a nanoparticle: (i) separated Fe and Au grains bellow 500°C, (ii) a heterogeneous mixture of fcc-Au-Fe alloy (majority phase) plus bcc Fe phase (minority phase) in the range 700°C-800°C, (iii)

Figure

  Figure B1: three-dimension empty-states STM images of pure Pt (a), pure Zn (c), and ZnPt (c) nanoparticles supported on rutile TiO2(110). The images were taken under UHV at room temperature. Scanning parameters: Vbias = 1.8 eV, It = 150 pA.

Figure B2 :

 B2 Figure B2: in situ core-level spectra of the C 1s recorded at a photon energy h = 400 eV: pure Pt nanoparticles (a), pure Zn nanoparticles (b) and bimetallic ZnPt nanoparticles (c). The spectrum labeled UHV was recorded on the samples as prepared at RT before exposure to gases. Afterwards, all the spectra were recorded in isobaric conditions under a nominal pressure of 1 mbar and with a CO:O2 ratio of 1:4 (oxygen rich mixture). The component noted CO(g) is associated with the CO gas phase. COad corresponds to adsorbed CO. CO2(g) is the CO2 gas phase. Carb* are carbonates species. C(gr) is the graphitic carbon. The room temperature (RT) is 293 K. The cyan structure at the highest binding energy (left) is due to the K 2p3/2 component of a potassium impurity commonly seen in TiO2(110) bulk. The latter can be used as a yardstick to compare signal intensities.

  Pt facets dominate, then the introduction of the gas mixture leads to the preferential occupation of atop positions by CO, with minority occupation of the bridge sites. Other sites must be envisaged owing to the nanometer size of the Pt NPs. CO could adsorb on steps where under-coordinated Pt atoms are present. Tentatively we attribute the component at 287.6 eV to CO bonded to the under-coordinated Pt atoms.

Figure B3 :

 B3 Figure B3: in situ core-level spectra of the O 1s recorded at a photon energy h = 635 eV: pure Pt NPs (a), pure Zn NPs (b) and bimetallic ZnPt NPs (c) supported on TiO2(110). The spectrum labeled UHV was recorded on these samples prepared at RT before exposure to gases. Afterwards, all the spectra were recorded in isobar conditions at nominal pressure of 1 mbar and with the CO:O2 ratio = 1:4 (oxygen rich). The components noted O2(g) and CO(g) are the O2 and CO gas phases, respectively. The CO2(g) is the produced CO2 gas phase. Carb* are carbonates species. O(TiO2) is the oxygen of the TiO2(110) surface. The component denoted with (*) can be attributed to dissociated moieties from residual water molecules on TiO2(110).

  has left the NPs surface in the CO oxidation reaction regime (above 380 K).Additional information on the chemical bonding and segregation was obtained from the Pt 4f spectral weight of the metallic component of Pt (Ptmet) in Zn Pt NPs(Fig B4b and Fig B5a). The Ptmet spectral weight decreases due to the adsorption of CO in the temperature window between RT and 380 K. From 396 K, when the CO(ad) signal disappears and Zn is oxidized to ZnO, Ptmet starts to increase again, until it reaches 100% (fully metallic) at 450 K. This can be explained by Zn segregating at the surface of the NPs and forming an oxidized compound. This strongly suggests that in oxidative conditions a ZnO shell (with carbonates) is formed over a metallic Pt core. Therefore, for the highest temperatures, zinc plays the role we expected it to have in ZnPt NPs, namely it forms a sacrificial oxide.

Figure B4 :

 B4 Figure B4: Pt 4f and Zn 3p binding energy window of ZnPt NPs measured at h = 400 eV in a CO:O2 mixture (1:4) and under a total pressure of 1 mbar at two temperatures 360 K and 410 K, before and within the CO2 production regime, respectively. The XPS spectra were normalized to the Zn integral intensity to illustrate the strong decrease of the Pt contribution above 380 K. The onset of the CO2 production regime was detected at 380 K. The schematics illustrate the segregation of Zn to the surface of the NPs during the CO2 production regime. (b) Pt atomic weight (Pt/(Pt+Zn)) ratio as a function of the temperature under 1 mbar of a CO:O2 mixture (1:4). The calculation is made from the XPS Pt 4f and Zn 3p integrated intensities, taking into account the cross-section corrections. The dotted arrow at 380 K indicates the temperatures at which the onset of the CO oxidation reaction is detected.

Figure B5 :

 B5 Figure B5: (a) metallic Pt (Ptmet) spectral weight in pure Pt NPs and ZnPt NPs versus temperature obtained from the Pt 4f XPS spectra as measured in UHV and under 1 mbar of a gas mixture CO:O2 at a ratio 1:4. (b) metallic Pt (Ptmet) spectral weight in pure Pt NPs and ZnPt NPs versus temperature obtained from the Pt 4f XPS spectra as measured in UHV and under 1 mbar of pure O2. PtOx and Pt-CO are contributions from Pt surface oxides and Pt bound to CO.

Figure B6 :

 B6 Figure B6: Synoptic comparison of chemical changes at the surface of pure Pt NPs, pure Zn NPs and ZnPt NPs samples as a function of temperature in a CO:O2 (1:4) mixture under a pressure of 1 mbar. CO(g) and O2(g) are the CO and O2 gas phases, respectively. CO(ad) are adsorbed CO molecules. Carb* are carbonates species.

Figure C1 :

 C1 Figure C1: Filled-state STM images (bias = -2V, It = 100 pA), of a pristine n-doped Si(001)-(21) (highly n-doped  = 0.003   cm). (a) large scale image, (b) high resolution STM with atomic resolution, and (c) the schematic representation of a part of the Si(001) surface shown in (b). The solid circle in (b) indicates a C-defect. The rectangle indicates a (21) unit cell.Below a transition temperature of ~150 K, the static buckling leading to a c(42) reconstruction was observed in STM images.106,107 Bright protrusions arise from the Siu (Sid) dimer atom in the occupied (unoccupied) state STM images. Above the transition temperature, the dimer buckling becomes dynamic due to the fast thermally induced oscillations between up and down positions. In

Figure

  Figure C2: filled-state STM images (V = -2 V, I = 100 pA) of (a) individual TEA molecule datively bonded (protrusion), of (b) a compact molecular domain (reconstructed c(4×2)) of datively-bonded TEA molecules, of (c) two dissociated TEA molecules (at low molecular surface density) on two adjacent dimers (note that the prominent features associated to Si-N(CH2-CH3)2 are on the opposite sides of the row). The cross hair indicates the position where the STS curves are measured (results are shown in Figure C4). (a'), (b') and (c') images are explanatory schemes of the above STM images. Open circles are, from large to small: "up" Si dimer atom (Siu), "down" Si dimer atom (Sid), 2 nd layer Si atom and 3 rd layer Si atom. Only Siu atoms are seen in the STM images. Nominal dosing pressure P = 510 -10 mbar in (a) and total exposure to TEA = 0.18 L. (b) obtained after increasing the dosing pressure to 510 -9 mbar with a total exposure to TEA = 2.3 L.

  The cell dimensions measured by STM are indeed ~15.6 Å along [-110] and ~7.9 Å along [110]. In the c(42) cell, one dimer in two is occupied, corresponding to a coverage of one molecule per two dimer (i.e. a molecular coverage of 0.25 ML).

2 ,

 2 [from Fig C3a to Fig. C3b]. The latter has eventually operated an additional translation of → b [from Fig. C3b to Fig. C3c], corresponding to a total displacement of transient position to its ultimate adsorption site. This effect is a clear indication of the mobility of TEA molecules into specific adsorption sites on the Si(001)-c(4×2) surface.

Figure

  Figure C3: (a, b, c) Real-time consecutive 3D STM images recorded on the same area of a Si(001) at RT while dosing triethylamine. Nominal dosing pressure P = 510 -10 mbar. Total exposure to TEA = 0.18 L. (a', b', c') are schematic representations of the STM images (a, b, c). TEA is represented in the ball and stick model where the blue is N, black is C and grey H atoms. Open circles are from large to small: Si updimer surface atom (Siu), Si down-dimer surface atom (Sid), Si 2 nd layer atom, and Si 3 rd layer atom. 𝑎 ⃗ and 𝑏 ⃗⃗ are vectors of the (11) unit cell.

Figure C4 :

 C4 Figure C4: Normalized differential conductance spectra obtained after numerical derivation of I(V) curves acquired by STS as a function of the adsorption geometry on four different locations (from bottom to top): bare clean dimer (clean), isolated datively bonded TEA molecule (dat-is), molecule in a compact c(4×2) reconstructed molecular domain (dat-dom), pair of dissociated molecules (diss). The zero energy represents the Fermi level, negative energy represents filled states (valence band), and positive energy represents empty states (conduction band). The 3D STM images in the inserts indicate examples of locations where the STS curves were acquired (see also FigureC2). For the sake of clarity, the curves are shifted vertically: the horizontal line below each curve corresponds to the normalized differential conductance level equals zero. All data were acquired with the same tunneling conditions: a current set-point of 100 pA and bias of -2 V.

Figure C5 :

 C5 Figure C5: Filled-state 3D STM image of a water-saturated Si(001)-(21). Bias = -2V, 100 pA.

  7 L), and thanks to the use of dual-bias imaging, we have identified the opening of another reaction channel which does not involve the IDB. In this second reaction channel, benzaldehyde adsorbed on (H, OH)-Si(001)-(21) and suggest the breaking of the 𝐶 = O double bond.

Figure C6 :

 C6 Figure C6: Real-time (scanning-while-dosing) filled-state STM images of water-saturated Si(001)-(21) (lightly p doped ρ = 18 Ω  cm) exposed to benzaldehyde at nominal P = 3.75  10 -8 Torr: (a) pristine surface; (b) after 7 min dosing; and (c) after 14 min dosing. The white circles indicate isolated IDB, before (dashed circle) and after (solid circle) reaction with the molecule. Bias = -2 V, It = 100 pA.The adsorption mechanism of benzaldehyde on (H, OH) Si(001) is summarized by the schematics shown in Fig.C7. First, in the initial reaction channel on the quasi-neutral IDB, the nature of the product formed is tentatively identified as a radical molecule adduct (Fig.C7a). This is an openshell radical molecular species, corresponding to the attack of H-S-Si  or HO-Si-Si  (  denotes the unpaired electron) by the carbonyl and formation of a Si-O-CH  -C6H5 species (the CH bears formally an unpaired electron). It is noteworthy that in this adsorption scheme, no indication of the reformation of isolated dangling bond via the abstraction of a nearby H atom from a silicon

Figure C7 :

 C7 Figure C7: schematic illustrations of the two mechanisms of benzaldehyde interaction with (OH, H)-Si(001)-(21): (a) reaction on IDB and radical adduct formation process at low exposure, and (b) reaction onSi-H and concerted hydrosilylation process at high exposure.

  methods and characterization techniques in order to establish a comprehensive methodology in the rational design of the nanocatalysts that encloses these parameters.In this project, I propose an in situ approach in which both the nucleation and growth of nanocrystals (NCs), on the one hand, and their characterization in real time, on the other hand represents a suitable solution towards these goals.The design of our bimetallic NCs such as M-ZnO (where M is the metal: Pt or Cu) will rely on solution-phase synthesis methods.110 Specifically, NCs will be prepared in in situ conditions based on an organometallic route in which a simple bi-component systems (ClM(I)(PPh3)3 is solubilized in oleylamine. For this part, we have a LABEX Michem partnership with Prof. Christophe Petit and Dr.Caroline Salzemann of MONARIS laboratory (UPMC). These partners have a long standing experience in two-phase and single-phase (organometallic route through thermal decomposition) synthesis methods.110,[126][127][128] For example, the synthesis of M-Zn NP, will be performed by coreduction ofM(acac)2 and Zn(acac)2, where acac is the acetylacetonate, with oleylamine à 300°C (Fig P2).

Figure P2 :

 P2 Figure P2: (a) schematic representation of our synthesis and fabrication of the inverse oxide/metal M1(Cu, Pt)[core]M2-Ox(ZnO)[shell] nanocrystals in situ (UHV chamber). (b) TEM image of bimetallic capped oleylamine Pt(acac)2Zn(acac)2 recently synthesized in our lab (ex situ). See also Ref. 110 for ClCo(PPh3)3 for hcp NCs of Cobalt.

SECTION B: In situ studies of nanocatalysts in near ambient pressure conditions

This work was conducted in my group in the Université Pierre et Marie curie. I introduced and developed this project as a new topic in the Laboratoire de Chimie Physique Matière et Rayonnement following my arrival in this laboratory as a faculty member in September 2012. The original approach followed here is to investigate the chemical reactions in in situ conditions. This project highlights the wealth of information that can be obtained using the in situ studies approach, in particular by NAP-XPS, in the investigation of catalytic or chemical properties of nanoparticles.

PROJECT AND PERSPECTIVES:

The following project which will be deployed in my research group, deals with two strategies. (i) the inverse oxide/metal nanocatalysts relevant for environmental remediation and energy generation systems, and (ii) organic/inorganic material interfaces for solar cells applications. This project explores the advantages of the special structural and catalytic properties of these systems due to their distinctive metal-oxide interactions. The design and study of fundamental properties such as the composition, structure, thickness, and electronic states will be essential to better understanding the chemical reactions taking place in the inverse oxide/metal systems in the form of nanoparticles or thin film nanostructures. Besides, the understanding of the chemistry and energy levels alignment of organic/inorganic interfaces are the key factors towards enhanced efficiency and stability of these solar cells systems. One of the key strategies of this project will be to continue using the in situ approach via our state-of-the-art spectroscopies and microscopies tools.

P.1. Overview and Motivations

The design and understanding of key processes that govern oxidation reactions occurring in the transition metal-based heterogeneous nanocatalysts and the appropriate handling of their long overdue issues will provide significant contribution towards the advancement of low-emissions energy generators. [108][109][110] Metal oxide catalysts have attracted considerable attention owing to their use in many valuable industrial catalytic applications. In particular, the inverse oxide/metal catalysts, in which the oxide forms an ultrathin layer overcoating the metal, have been shown to exhibit remarkable catalytic performances in the CO oxidation and represent a promising stable catalytic system. [START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF][111][112][113] Recently, Zn has emerged as one of the most promising element in bimetallic catalysts owing in part to the stability of its ZnO oxide under reducing conditions [START_REF] Naitabdi | Oxidation of Small Supported Platinumbased Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen[END_REF][START_REF] Martynova | CO oxidation over ZnO films on Pt(111) at near-atmospheric pressures[END_REF]114 and its greater "affinity" with oxygen compared with that of Pt. Ellingham diagrams show that Zn is thermodynamically able to reduce PtO2 to Pt, forming ZnO. Therefore, Zn, an affordable metallic material, when combined with Pt will offer the opportunity to handle these limitations. Indeed, ZnO/ is active for CO oxidation [START_REF] Martynova | CO oxidation over ZnO films on Pt(111) at near-atmospheric pressures[END_REF] and promotes catalytic reactions relevant for fuel cells. 114 77 In this project,

I will first design supported ZnO nanostructures, nanoparticles and thin films, and will explore in details possible correlation between the structure, electronic and catalytic properties of oxidemetal interfaces via the paradigmatic CO oxidation reaction under realistic conditions.

Similarly, in a second axis, the organic solar cells (OSC) have shown great potential as efficient and affordable photovoltaic devices for solar energy conversion. 115 In particular, the use of ZnO in hybrid OSC has become increasingly the subject of great interest in order to improve their performances and their life-time. 116 Indeed, ZnO which is an n-type inorganic semiconductor with a direct gap of ~3.4 eV, 117 is one of the best choices in metal-oxide materials due to its features such as good optoelectronic properties, cost-effective, high stability, non-toxicity, and environment-friendly material. In particular, among current strategies to reach widespread practical applications of OSC, the design of perfect interfaces and the understanding of their chemistry and electronic transport properties are the principal goals of this project. In this regard, I will put an emphasis on the design of well-defined ZnO nanostructures (nanoparticles or thin films) and on the chemistry and energy levels alignment of inorganic/organic interfaces.

P.2. ZnO-based inverse oxide/metal nanocatalysts:

The study of chemical reactivity of inverse oxide/metal nanocatalysts offers great promises in the understanding of the role of oxide and oxide-metal interface in a catalytic reaction. The classical heterogeneous catalysts studies, which often consisted on metal NPs dispersed generally on an oxide, have on the one hand provided significant advances but limitations on the performances, such as premature deactivations and decomposition, remained persistent on the other hand. 118 In addition, the oxide support in that case has long been considered as inert in the catalytic process. However in recent years we have assisted to a paradigmatic shift, as synergetic effects of some metal/oxide couples have been observed. 112,119 The concept, termed inverse oxide/metal catalysts, was defined by Rodriguez and coworkers. 112,120 Indeed, the current project is within these trends as its goals are to design ZnO-based inverse oxide/metal nanostructures at the atomic level and explore fundamental properties of their interfaces (structure, morphology, composition, electronic states) which play a paramount interest on the catalytic performances.

The inverse oxide/metal nanocatalyst can be described as a "thin oxide layer/metal" system, in which the metal substrate acts as an electron reservoir. The original idea herein relies on a physical phenomenon which can be described as follow: electrons are transferred from the metal, through the oxide, to the surface, when the oxide surface remains at a "tunneling distance" from the metal. Then

on the design and characterization of model well-defined ZnO nanostructures on Pt(111) (Fig. P1). The second concerns the synthesis of their equivalent size-selected bimetallic ZnPt

nanoparticles and their subsequent oxidation into ZnOPt NPs using chemical routes (Fig. P2).

P.2.1. Model ZnO nanostructures on Pt(111)

The ZnO nanostructures, nano-islands or thin films, will be prepared by electron-beam deposition method in UHV on crystalline platinum surfaces, in particular Pt(111). Research grade Zn metal will be deposited by evaporation under an atmosphere of pure O2 at RT and subsequent oxidation at 600 K. Additional annealing at 520 K will be performed to achieve a well-ordered ZnO films. In this conditions, the ZnO films exhibit ordered triangular domains which are Zn-terminated polar (0001) surface (see Fig. P1.). The morphology and thickness of the films will be studied by STM and Auger. XPS measurements will be performed in order to study the electronic properties of these films (work function, bandgap …) as a function of the thickness (effect of boundaries) and the adoption of H2O, CO molecules. The stability of ZnO films, in particular the dewetting in reducing environment, will be investigated.

P.3. Organic/Inorganic material interfaces for solar cells applications

The knowledge gained from the design of well-defined ZnO thin films or nanoparticles will be implemented in the design of ZnO-based organic/inorganic solar cells. In this part of the project, I will focus on two applications: (i) ZnO/organic hybrid solar cells and (ii) ZnO as a cathode buffer layer in ZnO/organic hybrid solar cells. Besides, the fabrication of these systems, the analysis by two complementary techniques, STM and XPS-UPS, will provide a comprehensive characterization of the morphology, the chemistry, and the energy alignment of their interfaces.

P.3.1. ZnO/organic hybrid solar cells

The ultimate goal of this application is within the current efforts to replace the electron acceptor organic semiconductor of OSC by ZnO which offers excellent stability and high electron mobility. The ZnO films will be prepared in UHV as described above. Here, the fabrication of controlled and well-ordered ZnO thin film is essential to the quality of the interfaces. Indeed, the diffusion of excitons during their lifetime in the material toward their respective electrodes should not be hindered. 116 Here, ZnO will act as electron acceptor material in the active layer of ZnO/organic hybrid solar cell. Regarding the organic part which will act as the p-type material (electron-donor and hole transporting material), I will focus on thiophene-based conjugated oligomers owing to their several advantages. (i) they are well-defined molecular structures which will facilitate eventually repeatable device performances, (ii) their crystalline features allow long-range order suitable for charge carrier transport, and (iii) they can be deposited in UHV by evaporation useful for preparing high purity materials. Indeed, the self-assembly of these molecules will be investigated at the nanoscale by STM in UHV. The energy level alignment of the ZnO/tiophene oligomers interface, in particular the acceptor LUMO and donor HOMO, will be investigated by XPS-UPS.

P.3.2. ZnO as a cathode buffer layer in ZnO/organic hybrid solar cells

Besides acting as electron acceptor material in ZnO/organic hybrid solar cell, ZnO can be also used as efficient buffer layer between the active layer and the cathode. Indeed, after the dissociation of the excitons, generated upon solar illumination of the active layer, the resulting free carriers will diffuse as separated positive and negative charges towards the anode and cathode, respectively. The driving force for this process is the difference between the HOMO level of the donor and the LUMO of the acceptor. These energy levels will be measured by XPS-UPS and tuned as a function of the ZnO thickness and molecular adsorption. Therefore, the quality of the interface between the active layer and both electrodes is of paramount interest in order to reach an efficient harvesting of the carriers.

Ultimately, the performances of the hybrid OSC will be improved. The control of these interfaces at the atomic scale and the control of their energy level alignment will contribute to reach this goal.

Finally, further improvement of the electron mobility of the ZnO layer can be reached by doping this system using Al atoms. Indeed, Al:ZnO was demonstrated as a suitable transparent conductive oxide electrode in organic/inorganic solar cells. 116 It has been shown for example, that ZnO nanoparticles with tunable optical properties, via band gap alteration, were obtained by doping with Mg. 129 Here in my project, the engineering of well-defined interfaces which combine Al:ZnO and tiophene oligomers represents a suitable strategy towards the improvement of the OSC efficiencies.