Chapter I. Introduction I.1 Hydrogen production by water splitting

 Their counterpart Si-rich silicides or monosilicides are not ferromagnetic. For Nanowires

General introduction

Transition metal silicides are a family of intermetallic compounds delivering various properties including heterogeneous catalysis and magnetism. Recently, dihydrogen production by water splitting received tremendous attention because of the energy transition. Some transition metal silicides have been demonstrated electrocatalytically active for electrolysis of alkaline water.

On the other hand, transition metal monosilicides have shown a surface related defect-induced ferromagnetism. Further tuning of the properties of these materials can be achieved by nanostructuration. However, the synthesis approaches towards nanostructures of transition metal silicides remain few, because the formation of their relatively high energy bonds requires high temperature, which cannot be afforded by traditional colloidal synthesis.

Hence, this thesis is dedicated to the design of transition silicide nanoparticles, as well as to the study of their electrocatalytic properties of water splitting in alkaline medium and to their magnetic properties. This thesis includes five chapters:

In the first chapter, we present a brief state-of-the-art concerning the understanding of reaction pathways for electrocatalytic electrolysis of alkaline water, we then highlight state-of-theart electrocatalysts. Then, the properties of transition metal silicides are presented. Traditional synthesis methods and recently developed approaches towards these phases are reviewed. Finally, two molten salts synthesis strategies using nanoparticles as precursors are proposed.

The second chapter is the proof-of-concept of our first synthesis strategy by insertion of metal species into silicon nanoparticles in molten salts. The obtained nanoparticles of iron silicide, nickel silicide and iron-doped nickel silicides are assessed as electrocatalysts for water oxidation.

Post mortem studies are conducted to elucidate the origin of the outstanding activity and stability of iron-doped nickel silicides.

The third chapter presents cobalt monosilicide nanoparticles synthesized through the same method as above. Core-shell and homogeneous CoSi nanoparticles can be respectively achieved upon different reaction temperatures. Then, these nanoparticles are measured in OER electrocatalysis and post mortem analysis are carried out. Besides, these CoSi nanoparticles show peculiar interface-related ferromagnetic properties, discussed based on magnetic measurements.

The fourth chapter focuses on an important silicon precursor, sodium silicide Na4Si4, for our second synthesis strategy. A reproducible and straightforward approach to high purity Na4Si4 is detailed. The impact of the silicon source, the argon flow rate and the reaction time on synthesis outcome are discussed. The selective thermal decomposition of Na4Si4 towards type I and type II sodium silicon clathrates is then studied to assess the efficiency as precursor of Na4Si4 prepared in this work.

The fifth chapter presents our second synthesis strategy. P-doped Ni2Si and original ternary nickel silicophosphide nanoparticles are obtained by reacting P-doped nickel nanoparticles and Na4Si4 in molten salts. The electrocatalytic properties in overall alkaline water splitting of the ternary silicophosphide nanoparticles are discussed in respect to those of the binary nickel silicide.

Preliminary post mortem results further reveal the different behaviors of ternary and binary phases in electrocatalysis.

enhanced. Nevertheless, failures of structure mechanical integrity arise due to the high operation temperature. 6 Table I-1 Characteristics of typical water electrolysis cell (published in 2017). 6 As for membrane electrode assemblies, polymeric membranes not only replace the corrosive alkaline liquid electrolyte of conventional alkaline electrolysis to improve the system safety, but also fulfill the role of gas separators to enable H2/O2 separation. Thanks to this compact structure, a varying operation pressure is feasible. Moreover, the system response time is largely reduced.

The improved response rate can better adapt to the transient renewable energy production. 4,12 Amongst the two types of membrane electrode assemblies, proton exchange membranes is more developed than anion exchange membranes, because H + conductor membranes are relatively easier to achieve. However, the acidic regime provided by Nafion proton exchange membranes (equivalent pH ~2) limits the range of suitable electrode catalysts to noble metals. 13 This is the main limitation of proton exchange membranes. In anion exchange membranes, water electrolysis occurs in alkaline medium, so that non-noble catalysts could be used. Thus, anion exchange membranes combining the advantages of AWE and proton exchange membranes shows great potential as upgraded technology in water electrolysis. 13 Hence, researches on lowcost electrocatalysts for alkaline water electrolysis are widely conducted. [14][15][16]

I.2 Electrocatalysis of water oxidation in alkaline medium

The two half-cell reactions (1) and ( 2) of water electrolysis -hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode, are hampered by kinetic factors, thus high overpotentials must be imposed to enable water splitting. In order to make water electrolysis a cost-competitive hydrogen production technology, electrocatalysts are required to lower the consumption of electricity. 17 Among the two electrode reactions, OER involving a four-electron transfer exhibits a higher overall activation energy than HER transferring only two electrons. Therefore, electrocatalysts for OER are of great importance for efficient water electrolysis.

Cathode (HER)

2H 2 O + 2e -→ 2OH -+ H 2 ↑ (1)
Anode (OER)

4OH -→ 4e -+ 2H 2 O + O 2 ↑ (2)
Understanding the alkaline OER mechanism is an essential task attracting extensive attention for designing advanced catalysts. Different methods have been developed to elucidate the rate determining step of OER. Song et al. employed the operando X-ray absorption spectroscopy to monitor the evolution of chemical species. 18 Hao et al. designed a series of experiments using reactive probes to scavenge different oxygen intermediates appearing during the OER to study their individual effects. 19 Besides, density functional theory (DFT) calculations of Gibbs free energies can give an insight in the thermodynamics of intermediate reaction steps. 20 Nonmetal materials are rarely active in OER electrocatalysis. 21 *OOH. 22 As AEM seems to apply to more catalysts, it will be discussed in the following.

In the four-step adsorption evolution mechanisms, the electrocatalytic efficiency is intrinsically determined by the binding energies of these species on the catalyst surface, which should be neither too strong nor too weak to favor the adsorption/desorption process. For example, if *O binds strongly onto the catalyst, the formation of M-OOH would be limited.

Contrarily, when the M-O bond is too weak, the transformation from M-OH to M-O is unfavorable. 23 To achieve an appropriate binding strength, other elements (metals and/or non-metals) can be introduced for modulation. [24][25][26] For instance, many bimetallic OER electrocatalysts achieve very low overpotential, thanks to the synergistic effect of two metals. [27][28][29][30] As regard to the exact mechanism, people have not reached a consensus yet. In the case of NiFe-based catalysts, the oxygen evolving site has been considered as either Ni or Fe in the literature. [31][32][33][34] One view considered that the outstanding activity of NiFe-based materials originated from the Fe assisted catalysis on Ni sites. 31,32 Li et al. concluded that Fe 3+ is a strong Lewis acid, which promotes the oxidation of its neighbor Ni 3+ , and that the resulting high valence Ni 4+ is responsible for the high catalytic activity. 31 On the other hand, other studies suggested that Fe is the active site. 33,34 lattice oxygen mechanisms (LOM). 22 M denotes the surface active metal sites.

Noble metals are used as electrocatalysts for OER thanks to their high stability in corrosive environment, even in acidic medium. 4 However, the expensive raw materials of these catalysts limits enlargement of the production scale. Considering the high demand of cost-efficient water electrolysis, many researches have been done on earth-abundant electrocatalysts for OER. 17,35 In 2014, Trotochaud et al. revealed for the first time the essential role of the incorporated-Fe in Ni (oxy)hydroxide for efficient OER electrocatalysis. 28 Many studies have been conducted on NiFebased materials afterward. The latter are now among the most active electrocatalysts for OER in alkaline media, delivering high electrocatalytic activities with only earth-abundant elements. 14,36 While many studies have reported the OER electrocatalysis efficiency of NiFe layered (oxy)hydroxides, 37 their layered structure exhibits moderate electrical conductivity because of intercalated water or anions, 37,38 together with a low surface density of active sites and a tendency towards aggregation, thus leading to fast degradation. 14,37,38 Therefore, designing composite electrodes with an electrically conductive support is required. 16,39,40 Within the most-used support materials for oxidic NiFe-based electrocatalysts, there are mainly carbon-based materials, such as graphene, reduced graphene oxide (rGO) and carbon nanotubes. 14,36 However, according to the Pourbaix diagram of carbon, 41 the carbon support is thermodynamically unstable under the anodic conditions of water electrolysis in alkaline medium. The possible failure of carbon supports could be detrimental to the stability of the electrode. Joo et al. have investigated the deactivation of IrO2 catalysts supported by carbon black. On one hand, even if the carbon corrosion is slow in the typical OER conditions, dissolution of a small amount of carbon could cause the electrical isolation of IrO2, thus resulting in a physical deactivation. On the other hand, the oxidates of carbon (CO and/or CO2) could poison the IrO2 surface to chemically deactivate the catalysts. 42 Therefore, other solutions to the conductivity problem have been proposed.

Figure I-2

In situ electrochemical formation of core-shell NiFe disulfide -oxyhydroxide for alkaline OER electrocatalysis 43 Alloying of Ni and Fe with p-block elements can improve their oxidation resistance and keep the electrical conductivity of the metals. Non-oxidic NiFe-p-block element compounds such as borides 44,45 , phosphides 46 , nitrides 47 , sulfides 43,48 , and selenides 49 were investigated as precatalysts for OER through in situ complete or partial oxidation during OER. Among them, those undergoing only partial oxidation have emerged as potential solutions to the electrical conductivity limitation. Indeed, under OER conditions a thin surface layer of crystalline 43,50 or amorphous 44 metal (oxy)hydroxide/oxide was formed. These species are highly active for the OER electrocatalysis. 43,44 Meanwhile, the in situ generated layer enable charge percolation through the particle cores, which resist oxidation and maintain a conductive nature (Figure I-2). 43,44 These in situ formed core-shell composite electrocatalysts exhibit excellent activity and stability on OER electrocatalysis. 43,44 Although NiFe silicides have never been studied as OER electrocatalysts, one can anticipate that if their oxidation during OER is only superficial, they will deliver high electrocatalytic activity, thanks to their metal-like conductivity (~10 μΩ cm for nickel silicides), 51,52 combined with a very low-cost and low environmental impact due to the natural abundance of the constitutive elements in the Earth's crust. Reaching this goal requires the development of NiFe silicides exhibiting a high surface-to-volume ratio for enhanced electrocatalytic properties.

I.3 Transition metal silicide nanomaterials

Transition metal silicides are a class of intermetallic compounds, which combine at least one transition metal element and silicon. The difference of electronegativities of constituents in such intermetallics is greater than in alloys. According to the Hume-Rothery rule, the formation of long-range ordered compound is favorable in this case. 53 Metal silicide intermetallics exhibit a metallocovalent character because of the hybridization of Si p-and metal d-orbitals, 54 which results in a reinforced stability versus oxidation and leads to a higher hardness at room temperature than their metal components. Most transition metal silicides exhibit a low metal-like resistivity. 55,56 Research on transition metal silicides started at the beginning of the last century. Bulk metal silicides were produced by reacting the constituent elements at high temperature, but this method usually led to a large concentration of impurities. 57 Aside from powder metallurgy, diffusion couple methods, such as decomposition of silane gas on a metal substrate, 58 co-sputtering of metal and silicon, and the reaction between deposited metal and Si substrate, 59 were also widely used to obtain silicide thin films. Metal silicides exhibit good thermal stability and corrosion resistance because of the formation of a SiO2 protective layer. 60 Owing to these characteristics, transition metal silicides were used as heating elements for electrical furnaces and as protective coatings for jet engines made of Mo or W. 57,60 In the late 1970s, along with the development of Si technology, several transition metal silicide thin films have been investigated in Si integrated circuits as electrical contact materials for microelectronics and as interconnect materials in complementary metal oxide semiconductors (CMOS), thanks to their good resistance against oxidation, thermal stability and high electrical conductivity. 60 Murarka has listed the resistivities of some transition metal silicide thin films prepared by different methods (Table I-2). 57 Most of these silicide thin films exhibit a low resistivity of ca. 10 -5 Ω cm, which can be considered conductive, while the resistivities of certain silicides like FeSi2 and CrSi2 (ca. 10 -4 Ω cm to 1 Ω cm) behave closer to semiconductors with narrow bandgap.

Thermoelectric materials can convert temperature gradient to electrical voltage, which can be used for sustainable energy production. Optimal thermoelectric materials exhibit high Seebeck coefficient S, high electrical conductivity σ, and low thermal conductivity κ, i.e., a large figure of merit: ɀT= S 2 σT/κ. In this domain, lots of progresses in materials design have been made in the last decades. [67][68][69] However, efficient thermoelectric materials usually contain low abundance elements, such as Te, Bi and Ag, etc. 69 In contrast, transition metal silicides achieving high Seebeck coefficient and consisting of metals and silicon with high crustal abundance, are attractive because of the cost of raw materials. 67 The most promising silicides FeSi2, CrSi2, Mg2Si and MnSix exhibit a decent figure of merit compared to some benchmarks such as lead telluride. 67 On the other hand, nanostructuring has been demonstrated an effective pathway to enhance the thermoelectric properties of materials by selective decrease of κ, 67 which encourages the development of synthesis methods towards metal silicide nanoparticles.

Table I-2

Reported values of silicide thin film resistivities. 57 The tabulated values of each column are from Robins, 61 Padnos, 62 Neshpor & Samsonov, 63 Keiffer & Benesovsky, [START_REF] Wang | Boosting Bifunctional Oxygen Electrolysis for N-Doped Carbon via Bimetal Addition[END_REF] Wehrmann, 65 and Murarka, 66 respectively.

The magnetic properties of transition metal silicides have also been investigated. 61,[70][71][72][73][74] While instance, FeSi is paramagnetic. NiSi and CoSi are diamagnetic. 72,[START_REF] Petrova | Thermodynamic, and Electronic Properties of MnSi, FeSi, and CoSi[END_REF][START_REF] Stishov | Self-Doping Effects in Cobalt Silicide CoSi: Electrical, Magnetic, Elastic, and Thermodynamic Properties[END_REF] However, many nanowires of binary disilicides or monosilicides exhibit magnetic order. [START_REF] Lin | Growth of Single-Crystalline Nickel Silicide Nanowires with Excellent Physical Properties[END_REF][START_REF] Seo | Magnetic Properties of Single-Crystalline CoSi Nanowires[END_REF][START_REF] Liang | Magnetic Iron Silicide Nanowires on Si(110)[END_REF] This peculiar magnetism of nano-objects was attributed to unpaired spins produced by crystal defects, undercoordinated metal atoms and dangling bonds at the interface and/or grain boundaries of silicides, 71 which could be tuned by the size and morphology of materials. Therefore, one can expect that the synthesis of nanoscaled transition metal silicides is of great interest towards higher magnetic moment silicides.

Furthermore, in the last century, the superconductivity of some binary and ternary metal silicides was revealed. [START_REF] Matthias | Superconducting Properties of Cobalt Disilicide[END_REF][START_REF] King | Topological Aspects of the Chemical Bonding in Superconducting Transition-Metal Borides, Silicides, and Alloys[END_REF] Among those whose critical temperature for superconducting transition is above 2 K, one can cite V3Si (17 K), WSi0.7 (2.84 K), ThSi2 (3.16 K), Sc5Ir4Si10 (8.4 K), LaRu3Si2 (7.3 K), and ThIrSi (6.5 K), etc. [START_REF] King | Topological Aspects of the Chemical Bonding in Superconducting Transition-Metal Borides, Silicides, and Alloys[END_REF][START_REF] Hardy | Superconducting Silicides and Germanides[END_REF] Moreover, reinforcement materials in metal-matrix nanocomposites are another potential application of nanoscaled metal silicides. For example, Ti5Si3 has showed an excellent wetting behavior with Al in the preparation of Al-Ti5Si3 nanocomposite. [START_REF] Estruga | Facile and Scalable Synthesis of Ti5Si3 Nanoparticles in Molten Salts for Metal-Matrix Nanocomposites[END_REF] Recently, transition metal silicides have been reported as efficient catalysts for diverse reactions. Ryabchuk et al. reported a reversible hydrogenation/dehydrogenation of Nheterocycles promoted by nickel silicide@SiO2, which could be auspicious for hydrogen storage. [START_REF] Ryabchuk | Heterogeneous Nickel-Catalysed Reversible, Acceptorless Dehydrogenation of N-Heterocycles for Hydrogen Storage[END_REF] Hausmann et al. found that FeSi supported on Ni foam is capable to achieve selective oxygenation of 5-hydroxymethylfurfural. [START_REF] Hausmann | Evolving Highly Active Oxidic Iron(III) Phase from Corrosion of Intermetallic Iron Silicide to Master Efficient Electrocatalytic Water Oxidation and Selective Oxygenation of 5-Hydroxymethylfurfural[END_REF] Besides, transition metal silicides also deliver enhanced catalytic activity on hydrodesulfurization, [START_REF] Chen | Structure Investigation and Dibenzothiophene Hydrodesulfurization Properties of Fe-Substituted Ni-Si Intermetallics[END_REF][START_REF] Chen | Synthesis and Characterization of Ferromagnetic Nickel-Cobalt Silicide Catalysts with Good Sulfur Tolerance in Hydrodesulfurization of Dibenzothiophene[END_REF] CO methanation, [START_REF] Chen | Silicon-Nickel Intermetallic Compounds Supported on Silica as a Highly Efficient Catalyst for CO Methanation[END_REF] the Rochow process, [START_REF] Frank | Catalytic Formation of Silanes on Copper-Silicon Alloys[END_REF] hydrodechlorination, [START_REF] Acker | Reactivity of Intermetallic Compounds: A Solid State Approach to Direct Reactions of Silicon[END_REF] hydrodeoxygenation, [START_REF] Yang | One-Step Modification of Active Sites and Support in Ni/Al2O3 Catalyst for Hydrodeoxygenation of Lignin-Derived Diphenyl Ether[END_REF] oxidation of H2 and CO, [START_REF] Il'chenko | Kinetics and Mechanism of Oxidation of Hydrogen on Metal-like Nitrides, Borides, and Silicides[END_REF][START_REF] Vlasenko | Catalytic Activity of Transition Metal Silicides in Hydrogen Oxidation[END_REF][START_REF] Polisski | In Situ Synthesis and Catalytic Activity in CO Oxidation of Metal Nanoparticles Supported on Porous Nanocrystalline Silicon[END_REF] and formation of carbon nanotube, [START_REF] Hang | Synthesis of Uniform Double-Walled Carbon Nanotubes Using Iron Disilicide as Catalyst[END_REF] etc. 55 Aside from heterogeneous catalytic reactions, transition metal silicide nanoparticles have been also studied as emerging photoelectrocatalysts and electrocatalysts for both electrode reactions of water splitting. 55 TiO2/TiSi2 heterostructures were demonstrated as high-efficiency photoelectrocatalyst for water splitting.

I.4 Today's challenges towards transition metal silicide nanomaterials

As mentioned above, technologies such as powder metallurgy or arc-melting were widely adopted for metal silicides synthesis in the early years. These methods can achieve a large range of thermodynamically stable metal silicides by varying the reaction equilibrium temperature and the metal-to-silicon ratio according to phase diagrams. On one hand, they allow producing homogeneous products in a highly reproducible way. The intrinsic properties of silicides have been evaluated by using these bulk single-phase silicides. On the other hand, these methods also restrict the range of accessible products. Metastable phases cannot be isolated at the high temperature involved, and the resulting products are usually large grains. With the attractiveness of various properties tuned by sample size, morphology and composition, many other synthesis methods have been developed. 55 Chen and Liang reviewed the reported methods towards transition metal silicides. 55 These methods use either dual-source precursors (Table I-3) or single-source precursors (Table I-4).

Self-propagating high temperature synthesis relies the combustion between metal and silicon upon ignition, which is enabled by the exothermicity of this chemical reaction. The reaction occurs only in a few second, which consumes few energy, compared to traditional powder metallurgy. [103][104][105][106][107][108][109][110][111][112][113][114][115] By using additives such as KNO3 to trigger outgassing during the reaction, porous material can be produced. 116 However, poor control on product purity is the main drawback of this method.

High energy ball milling can enable the solid-state reaction of elemental metal and silicon. 55 This method produces materials of fine grains with a low cost and simple process. [117][118][119][120][121] To further downscale the resulting product to the nanoscale, ZrSi2 nanoparticles were prepared by adding CaCl2 as diluent. 121 Grain refinement can be achieved by extending the milling time to dozens of hours. 118,119 Cooling intervals for the equipment could be required, because the mill's motor could easily wear due to the heating of components. 121 This results in a modest efficiency of ball milling synthesis.

Solid-state metathesis reactions use highly reactive alkali or alkaline earth silicides to reduce transition metal halides or oxides and to form transition metal silicides. In many cases, the reaction is complete within few seconds without external heating or by a simple initiation, e.g. gas flam. [122][123][124][125] During the process, the byproducts alkali and alkaline earth halides were generated as molten fluxes that slightly dilute the reaction medium, which can be easily removed after reaction.

Such a dilution can moderate the reaction for a better control of grain size. However, the temperatures during processing can generally reach the melting point of the produced silicides because of the large exothermic energy of reactions, 55 so that this method only yields bulk materials.

Table I-3

Synthesis methods using dual-source precursor towards metal silicides reviewed by Chen and Liang. 55 Nanoparticles (including supported nanoparticles) are underscored in the table.

Dual-source precursor methods

General precursors

General morphologies

Achieved phases and references

Arc melting

Dual-source precursor methods

General precursors

General morphologies

Achieved phases and references

Silicon sources with metal substrates

SiH4 or organic Si (SiCl4, (CH3)2SiCl2, etc.) + metal substrate or metal oxide nanoparticles Nanowires, thin films, nanoparticles Ni2Si, 157,158 Ni3Si2, 157 NiSi, [157][158][159] Ni-Si@SiO2, 92 FeSin@SiO2, 160,161 Ni1-xCoxSi2, [START_REF] Chen | Synthesis and Characterization of Ferromagnetic Nickel-Cobalt Silicide Catalysts with Good Sulfur Tolerance in Hydrodesulfurization of Dibenzothiophene[END_REF] Ni1-xFexSi2 Microwave-assisted synthesis is a well-known method providing large heating rate, which can favor the nucleation over the growth of grains, and thus decrease the particle size. Mg2Si

nanoparticles have been achieved by microwave irradiation of elemental Mg and Si. 134 Besides, Pd2Si nanowires have been grown by microwave plasma enhanced chemical vapor deposition, 138 which shows the flexibility of microwaves for coupling with other synthesis methods. However, for solid-state reactions, the use of microwaves is not optimal for reactants that are poor microwave absorbers. In this case, susceptors such as SiC and carbon must be used. Based on this feature, Zhang et al. prepared carbon nanotubes (CNTs)-supported CoSix nanoparticles. 139,140 Different from the previous solid-state reactions, chemical vapor deposition methods involve gas-solid or gas-gas reactions, and produce typically nanowires, which are compatible with the microelectronic processing. [START_REF] Seo | Magnetic Properties of Single-Crystalline CoSi Nanowires[END_REF] The gas-gas reaction by simultaneous deposition of metaland Si-containing vapors is yet challenging because of the difficulty of independent control over the pressures of two vapors. [141][142][143][144] As for the two other sub-methods (Table I-3), the reaction between the metal halide vapor and the Si substrate has been extensively studied for producing various transition metal silicide (M = Cr, Fe, V, Co ,Ti, Mn, Ni, W, Ta, etc.) nanowires. [START_REF] Seo | Magnetic Properties of Single-Crystalline CoSi Nanowires[END_REF][146][147][148][149][150][151][152][153][154][155][156] The substrates are generally held at ca. 900 °C. 55 By controlling the vapor pressure of the halides, the stoichiometry of the resulting product can be tuned. 55 When using Si-containing vapors, volatile SiH4, SiCl4, and some organic Si species are usually involved. In this case, silicification can not only occur on metallic substrates to form nanowires, 157-159 but also occurs in metal oxide particles to yield supported or unsupported transition metal silicide nanoparticles. [START_REF] Chen | Structure Investigation and Dibenzothiophene Hydrodesulfurization Properties of Fe-Substituted Ni-Si Intermetallics[END_REF][START_REF] Chen | Synthesis and Characterization of Ferromagnetic Nickel-Cobalt Silicide Catalysts with Good Sulfur Tolerance in Hydrodesulfurization of Dibenzothiophene[END_REF][START_REF] Chen | Silicon-Nickel Intermetallic Compounds Supported on Silica as a Highly Efficient Catalyst for CO Methanation[END_REF]160,161 Despite the processing simplicity delivered by solid-state reactions and the large range of achieved phases, liquid phase syntheses under mild conditions are more adaptive for nanomaterials design, especially because the reactions can occur in a diluted medium.

NixSi-C8H17 colloids of 2 nm have been obtained in tetrahydrofuran (THF) at 55-65 °C . 163 However, better crystallinity requires higher reaction temperature. With the purpose of extending the range of transition metal silicide nanoparticles, enlarged medium temperature range is needed compared to usual solvents. Molten salt synthesis is a likely choice, where reactions take place in an inorganic solvent. A typical molten salt synthesis allows operating at temperatures ranging from 100-1000 °C. 166,167 Molten salts provide a diluted reaction medium in which reagents are diluted or dissolved, combined with high temperatures, which can trigger extensive nucleation, increase the reactant mobility and enhance the reaction rate. Unlike organic solvents, the nanoparticles produced in molten salts are organic ligand-free at the benefit of catalytic active site exposure. Previously, molten NaF-assisted synthesis was conducted to produce MoSi2 nanoparticles at 1000-1200 °C . 164 Ti5Si3 nanoparticles have been prepared in the LiCl-KCl eutectic at 700 °C. [START_REF] Estruga | Facile and Scalable Synthesis of Ti5Si3 Nanoparticles in Molten Salts for Metal-Matrix Nanocomposites[END_REF] Apart from the works presented in and NiSi nanocrystals respectively at 300 and 500 °C . 102 In these cases, high temperature phases were achieved as nanoparticles, which makes molten salt synthesis a promising nanoparticle designing pathway to reach a large versatility of transition metal silicide phases. 166,168 However, there are still some limitations of molten salt synthesis, such as the poor control over morphology of nanoparticles because the organic surface stabilizing agents cannot be used, as well as the necessary purification step for removing inorganic salts. These points will be addressed in this thesis.

Apart from dual-source precursor methods, single-source precursor synthesis using a precursor containing both metal and silicon has been also studied (Table I-4). With volatile molecules containing metal-silicon bonds e.g. M(SiH3)x(CO)y (M = Co, Fe, Mn, Re) and M(SiCl3)x(CO)y (M = Co, Fe, and Mn), metal organic chemical vapor deposition has been conducted. Metal silicide thin films, nanowires and supported nanoparticles have been successfully prepared. [169][170][171][172][173][174][175][176] Since the metal-to-silicon ratio is fixed by the precursors, this method allows easier and precise control over the stoichiometry. But this feature also restricts the accessible silicide phases because of the availability of suitable volatile precursors. 55 Another approach by pyrolysis of metal-containing polysilanes, which are usually in the solid state, enable tuning the metal-silicon ratio through the preparation of precursors. This method only yields supported nanoparticles or nanoparticles embedded in a matrix. 175,[177][178][179] Table I-4 Synthesis methods using single-source precursor towards metal silicides reviewed by Chen and Liang. 55 Nanoparticles (including supported nanoparticles) are underscored in the table.

Singlesource precursor methods

General precursors General morphologies

Achieved phases and references

Metal organic chemical vapor deposition M(SiH3)x(CO)y (M = Co, Fe, Mn, Re), M(SiCl3)x(CO)y (M = Co, Fe, Mn) Thin films, nanowires, supported nanoparticles CoSi@SiO2, 169,170 M5Si3 + MSi1.25 (M = Mn, Re), 171 FeSi2, 171 FeSi, 172 Fe1- Aiming at metal silicide freestanding nanoparticles, several essential factors for synthesis have been considered, including reaction medium, precursors of silicon and metal, etc. As for reaction media, molten salt synthesis has been adopted in this work, where liquid-mediated reactions can be triggered at higher temperature than in organic solvents. These liquid media enable reactions at lower temperature than in the solid state by enhancing the mobility of reactants. They also provide diluted media to produce small particles because of the favorable nucleation over growth. Moreover, the insulating organic surface ligands of nanoparticles, which could be detrimental for the electrocatalysis, can be avoided by this synthesis method.

One of the challenges for nanomaterials design in molten salts is the poor control over product morphology. In some cases, it could be overcome by topochemical methods, i.e., using nanoparticles as precursors, so that they could also act as nanotemplates. 168 Furthermore, nanoparticle reagents also present higher reactivity. 184 Hence, silicon nanoparticles and metal nanoparticles are chosen as precursors (Figure I-3). As for insertion species, metal halides are used for the reaction with silicon nanoparticles in this work, because the redox reaction between metal halide and elemental silicon is known feasible. [144][145][146][147][148][149][150][151] For the other pathway using metal nanoparticles, the choice of silicon sources is more delicate, this precursor should be ( 1

Chapter II. Synthesis and electrocatalytic properties of Fe-doped Ni 2 Si nanoparticles II.1 Introduction

NiFe oxide/(oxy)hydroxides are among the most active species for OER electrocatalysis in alkaline media, delivering high electrocatalytic activities with only earth-abundant elements. 1,2 However, their moderate electrical conductivity (Table II-1), low surface density of active sites and aggregation tendency result in fast degradation of the electrocatalytic properties. 1,3,4 Therefore, designing composite electrodes with an electrically conductive support is usually required. [5][6][7] Most-used carbon-based supports suffer from corrosion in OER conditions, which is detrimental for performance stability. 8 Non-oxidic compounds of Ni, Fe and p-block elements such as borides, 9,10 phosphides, 11 nitrides, 12 sulfides, 13,14 and selenides 15 are possible pre-catalysts forming NiFe layered (oxy)hydroxides (LDH) by in situ oxidation. Among these materials, some undergo only partial oxidation and are potential solutions to the electrical conductivity limitation. By forming a thin surface layer of crystalline 14,16 or amorphous 9 metal (oxy)hydroxide/oxide under OER conditions, these materials enable charge percolation through the particles core that maintains a conductive nature. 9,14 However, the stability of such heterostructures is debatable.

The existence and origin of resistance against in-depth corrosion remains to be solved. Nickel iron silicides have never been studied as OER electrocatalysts. One can anticipate that if their oxidation during OER were only superficial, they would deliver high electrocatalytic activity, thanks to their metal-like conductivity (~10 -5 Ω cm for nickel silicides), 22,23 combined with a very low cost and low environmental impact due to the natural abundance of the constitutive elements in the Earth's crust. Designing nanostructured architectures where NiFe layered (oxy)hydroxides would be bound to a conductive silicide core, in turn protected versus corrosion, would enhance the surface area of the catalyst material, the electrical conductivity of the whole material, and its stability. These features could be achieved by forming core-shell nanostructures. Therefore, high performance electrocatalysts could be reached by developing

NiFe silicide nanocrystals able to form a layered (oxy)hydroxide shell at their surface, with an interface stable versus corrosion.

Besides thin films and nanowires, very few occurrences of nanostructured metal silicides have been reported. [24][25][26][27] Among them, McEnaney et al. obtained Cu, Pd and Ni silicide nanoparticles through a colloidal synthesis pathway. 28 Kumar et al. established recently a molten salt synthesis of nickel silicide nanoparticles from nickel salts and sodium silicide. Despite a low control of morphological features, they revealed significant OER electrocatalytic activity of materials derived from nickel silicides. 29 Nanostructured NiFe silicides are even rarer 30 and have not been studied for OER electrocatalysis, although they have been sparsely assessed for other types of catalysis, like hydrodesulfurization where Si-rich NiFe silicides were derived from silicification of nickel iron oxides. 30 In the present chapter, we design metal-rich nickel iron silicide nanocrystals by exploiting the high reactivity of silicon nanoparticles as precursors towards nanostructured non-oxides. 31 Using nano-objects as reagents enables size and morphological control in the synthesis of multielement nano-objects when the initial objects are reacted in low temperature organic solvents. [31][32][33][34][35] These procedures rely mostly on metal, oxide, chalcogenide and halide nanoparticles.

In this work, we use molten salts as high temperature solvents to trigger the reactivity of covalent silicon nanoparticles and convert them into size-controlled NiFe silicides. The low vapor pressure of molten salts 36,37 offers the opportunity to run liquid-phase reactions at relatively high temperature and under dynamic vacuum. This feature enables shifting the equilibrium of the reaction between metal salts and silicon nanoparticles and performing for the first time this reaction while eliminating byproducts during the synthesis. The as-obtained Fe-doped Ni2Si nanocrystals exhibit high and stable performances in alkaline OER electrocatalysis. We then specifically address the origin of the stability of the electrochemical behavior by scrutinizing the structural and morphological changes occurring during electrocatalysis.

II.2 Experimental methods

Reagents. Lithium iodide (99%, Alfa Aesar), potassium iodide (99%, Sigma-Aldrich), silicon nanoparticles (99%, Nanomakers © , France, Figure II-1), bulk Si powder (99 %, Sigma-Aldrich), nickel (II) chloride (99%, Alfa Aesar) and iron (II) chloride (99.5%, Alfa Aesar) were stored and manipulated as received in an Ar-filled glovebox (H2O < 0.5 ppm, O2 < 0.5 ppm).

Methanol (VWR Normapur grade) was used for washing. The mixture was loaded in a quartz tube (Ø28×H345 mm) which was then connected to a N2flowed Schlenk line. An intermediate liquid nitrogen trap between the quartz tube and the Schlenk line was set to condensate the volatile byproduct SiI4. A vertical furnace from Eraly® was pre-heated to the reaction temperature. Afterward, the quartz tube was evacuated and placed into the furnace. The reaction medium was maintained at the targeted temperature for 6 hours under dynamic vacuum (10 -3 mbar). Then, the hot quartz tube was taken out and cooled down to room temperature. The as-obtained mixture was washed in methanol by seven cycles of centrifugation/redispersion and was later dried in a Schlenk tube under vacuum. The resulting powder was transferred and stored in an Ar-filled glovebox. A typical synthesis yields about 100 mg of powders, which corresponds to a yield of about 90 %. Electrolyte purification. 38 ca. 1 g of as-prepared Ni(OH)2 solid was put into 200 mL asprepared 0.1M KOH. The suspension was sonicated for 3 min and then rested for at least 6 h. The purified 0.1M KOH supernatant was separated by centrifugation, stored under nitrogen and later used as electrolyte. The electrolyte was bubbled by O2 for at least 20 min before measurement and during the measurement. The rotating rate of the RDE was set at 1600 rpm for all cyclic voltammetry (CV) measurements. CV and LSV measurements were performed at scan rate of 20 and 10 mV s -1 , respectively, from 0 to 1.0 V vs. Ag/AgCl in saturated KCl (equivalent to 1.0-2.0 V vs. RHE). For the chronopotentiometry (CP), the current density was set at 10 mA cm -2 to monitor the potential evolution. Since a non-rotating GC sheet was used as working electrode substrate, a Teflon-covered stir bar stirred the electrolyte at 350 rpm during the measurements to decrease the limitation on activity due to ion depletion. The electrochemical impedance spectroscopy (EIS) was recorded in a frequency range of 50 000 to 1 Hz with a voltage amplitude of 10 mV.

For all potentials reported in this work, the ohmic drop iR was compensated, where the resistance is approximately equal to the total impedance measured at a frequency of 50 kHz. The potentials vs. RHE are obtained from conversion of measured potentials based on the equation: ERHE = Emeasured + 0.197 + 0.0591×pH, where pH = 13.

II.3 Results and Discussion

II.3.1 Synthesis and characterization

The reactivity between metal chlorides and elemental silicon is appealing to produce metal silicides because of the low cost of these reagents and their abundancy. These reactions have been well explored by chemical vapor deposition (CVD) to yield metal silicide nanowires 24,25,27,39- 42 usually at ca. 900 °C. 25,42 The same reaction has been performed in the solid-state and in molten salts for CrSi2 at 800 °C. 43,44 This temperature is poorly compatible with the recovery of high surface area nano-objects. To decrease further reaction temperatures, we have used a molten salt medium to ensure liquid-phase reaction, and 40 nm silicon nanoparticles (diamond-like crystal structure, noted d-Si) as reagents with enhanced reactivity compared to bulk silicon.

Schematically (Figure II-3a), in molten LiI-KI (eutectic molar ratio 0.63:0.37) with a melting point at 286 °C, the metal (II) cations were reduced by Si(0) nanoparticles and further reacted with the Si core by forming metal silicides. In the meantime, Si(0) was oxidized to Si(IV), and combined with iodides from the molten salts to form gaseous SiI4 (boiling point at 290 °C under atmosphere pressure). 45 The byproduct was collected and analyzed by XRD, and then confirmed to be SiI4 (Figure II-4). The chemical surface states of the NiFe silicides were analyzed by X-ray photoelectron spectroscopy (XPS) (Figure II-8). In the region of Ni 2p3/2, the peaks at 856.5 eV and 853.1 eV can be respectively ascribed to the surface oxidic Ni species and Ni-Si. 29,[47][48][49] The latter only slightly shifts to higher binding energy compared to that of Ni, because the electronegativities of Ni (ꭓNi = 1.91) and Si (ꭓSi = 1.90) are close. 50 Similarly, the Fe 2p3/2 spectrum shows two oxidation states of Fe. The peak at 711.2 eV corresponds to surface Fe(III), 51 and the peak at 707.0 eV, which is slightly higher than that of metallic Fe, fits to Fe-Si (ꭓFe = 1.83). 50,52,53 In the Si 2p region, the peak at 103.3 eV can be assigned to silicate or SiO2, and the peak at 99.5 eV is characteristic of metal silicides. 54,55 Accordingly, the O 1s spectrum shows a peak at high binding energy 532.6 eV that can be attributed to oxidized silicon, 47 and a peak at 530.8 eV that is ascribed to oxidized metal. 47,51

II.3.2 Electrocatalytic properties

We then assessed the electrochemical properties of NiFe silicides. In order to avoid the fortuitous influence of in situ Fe deposition on the catalyst activity as well as on the durability in alkaline aqueous electrolytes, 56,57 we adopted a previously reported protocol to remove the Fe impurities in KOH solution by co-precipitation of Ni and Fe. 56 In this work, all electrochemical measurements were performed in iron-free 0.1 M KOH (see Experimental methods for details).

The electrocatalytic activity was investigated on rotating disk electrodes made of hybrid nanocrystals/Nafion films deposited on glassy carbon substrates. Although the electrocatalytic activity would be enhanced by using carbon or Ni foam substrates, 58 this configuration enables direct comparison of activities normalized versus the catalyst mass loading and electrode surface.

With the as-prepared electrode, cyclic voltammograms (CVs) were recorded at 20 mV s -1 . For all silicide samples, a reversible anodic wave appearing at lower potential than the water oxidation front is attributed to an oxidation of nickel species. 56 which would lead to a significant decrease in the Fe content for this sample containing originally the smallest amount of iron. 5 Indeed, Fe-leaching has been identified as the main origin of deactivation for NiFe-based OER electrocatalysts. 56,57 On the opposite, the negative increment for NiFeSi-12 shows self-activation in the first 5 h. The NiFeSi-12 properties after activation demonstrate a great durability in 0.1 M KOH at 10 mA cm -2 for more than 85 h. The comparison of NiFe silicides with the reported NiFe-based catalysts further confirms this outstanding stability (Table II-5). Moreover, NiFeSi-12 exhibits higher activity and stability than previously reported for nickel silicide nanomaterials synthesized in molten salts. 29 Electrochemical impedance spectroscopy (EIS) was then conducted to assess charge transport in the electrodes and at the electrode-electrolyte interfaces for silicide catalyst at different stages of activation process (Figure II-10d). An equivalent circuit with three main components of solution resistance (Rs), charge transfer resistance (Rct) and mass transfer related to reaction intermediates adsorbed at the electrode surface (Rads) was used to fit the experimental curves. 18,59 As shown in The overall composition of post-OER catalyst samples was also determined by SEM-EDX (Table II-6). The Ni:Fe ratio does not change, while a loss of Si can be detected in the first 65 h.

Nonetheless, the overall composition stabilizes after 65 h, with a (Ni+Fe)/Si ratio of 2.6, in agreement with a maintained (Ni1-xFex)2Si core and an outer Si-free shell of nickel iron (oxy)hydroxide. This is consistent with partial dissolution of SiO2 or silicates 75

II.4 Conclusion

We synthesized FeSi, Ni2Si and NiFe silicide nanocrystals through a redox reaction between silicon nanoparticles and metal chlorides by designing a strategy to fasten and bring this reaction to completion hundreds of degrees below the usual temperature of the solid-state reaction between bulk reagents: the high reactivity of silicon nanoparticles was combined with two specific features of molten salts, namely their ability to operate as liquid media at 300-400 °C and their low vapor pressure that enables running reactions under dynamic vacuum to shift equilibria. This synthesis strategy delivers a pathway to a wide range of nanomaterials.

The resulting NiFe silicide nanoparticles delivered high activity for OER electrocatalysis, with an activity stable for at least 85h of continuous electrolysis at 10 mA cm -2 . These high performances arise from the NiFe silicide nanocrystals acting as pre-catalysts, forming upon silicon leaching an electrocatalytically active crystalline Fe-doped NiOOH shell. The resulting nanostructures are however highly stable because of a dense oxidic intermediate layer containing metal and silicon that avoids contact of the silicide core with the electrolyte. Therefore, the metallic core can play its role of enhancing charge transfer within the electrode. This in situ evolution of nickel iron silicides towards core-shell-shell nanostructures with a passivating intermediate layer has not been observed yet for other metal-p block element compounds, including nickel silicides formed in molten salts. Therefore, this original behavior shows the ability to design hierarchically structured electrocatalysts built from Earth abundant elements.

Chapter III. Ferromagnetic and electrocatalytically active CoSi nanoparticles III.1 Introduction

The nanoscale largely impacts the magnetic and catalytic properties of transition metal silicides. These materials exhibit a large diversity of magnetic properties. [1][2][3][4][5][6] Generally, binary transition metal silicides only show magnetic ordering in the metal-rich phases (e.g. Co3Si, Fe3Si). 2,7 Their counterpart Si-rich silicides or monosilicides do not show magnetic order. For example, FeSi is paramagnetic. NiSi and CoSi are diamagnetic. 3,8,9 However, unusual magnetic order has been demonstrated in many nanowires of binary disilicides or monosilicides. [10][11][12] This ferromagnetism has been attributed to the unpaired spins produced by crystal defects, undercoordinated metal atoms and dangling bonds at the interface and/or grain boundaries.

These features can be tuned by the size and morphology of the nano-objects. 2 Nanoscaled metal silicides have also raised interest in hydrogenation catalysis 13 and electrocatalysis of water splitting [14][15][16] . Although cobalt silicide nanoparticles have not been investigated as catalysts for the hydrogen evolution and oxygen evolution reactions (HER and OER), other cobalt-p-block element compounds [17][18][19][20][21][22][23] , like carbides, nitrides, phosphides, sulfides and borides have demonstrated outstanding OER electrocatalytic activity. They undergo an electrode reconstruction by dissolution of p-block-element under the oxidizing conditions of OER, which results in a partial or complete conversion to electrocatalytically active cobalt (oxy)hydroxides. Because the nature of the p-block element could influence the local structure, geometry and thickness of the generated (oxy)hydroxides due to differences in radii, electronegativity and bonding nature with Co, 23 It is necessary to evaluate the OER electrocatalytic properties of nanoscaled cobalt silicides.

CoSi nanowires have previously been grown, 11 while CoSi nanoparticles supported on graphene or other solid matrices achieved by chemical vapor deposition have been investigated

as efficient catalysts for hydrogenation, [24][25][26] oxidation of methanol/ethanol, 27,28 but their magnetic property have not been reported. [24][25][26][27][28] As the interaction between nanoparticles and the support could change the exposed facets and especially the electronic state of surface atom of nanoparticles, 29 one can foresee that designing freestanding cobalt silicide nanoparticles could deeply impact electrocatalytic activity and magnetism.

In

III.2 Experimental methods

Reagents. Lithium iodide (99%, Alfa Aesar), potassium iodide (99%, Sigma-Aldrich), silicon nanoparticles (99%, Nanomakers © , France) and cobalt(II) chloride (99.7%, Alfa Aesar)

were stored and manipulated as received in an Ar-filled glovebox (H2O < 0.5 ppm, O2 < 0.5 ppm).

Methanol (VWR Normapur grade) was used for washing. Eraly® was pre-heated to the reaction temperature 300 and 400 °C for core-shell and homogenous nanoparticles, respectively. Then, the quartz tube was put in the furnace, followed by 6 hours of thermal treatment under dynamic vacuum (10 -3 mbar). Later, the hot quartz tube was taken out and cooled down to room temperature under vacuum. The as-prepared mixture a new phase in the mixture. Scale factor ratio were adapted between 0 and 1 (0.1 steps) to obtain the best R reliability factor.

Synthesis

Crystalline cobalt was also tested as additional phase (in addition to CoSi, amorphous SiO2 and crystalline Si) with PDF analysis. The refinement got worse with this phase addition (R=15.9 %), meaning that the presence of crystalline Co is highly improbable.

Amorphous Co was also tested in addition to the CoSi / amorphous SiO2 / crystalline Si mixture, and it did not improve the refinement (R=15.

Magnetic measurements Vibrating sample magnetometry (VSM) on quantum design

PPMS was used to determine the curves of magnetization versus applied field at 5 and 300 K for the samples. Zero-field-cooled and field-cooled magnetization versus temperature curves were acquired on a Quantum Design MPMS-XL superconducting quantum interference device (SQUID) magnetometer at magnetic field of 20 Oe from 5 to 300 K. All measurements were done on powder samples.

III.3 Results and discussion

The reactivity between silicon and cobalt chloride has been previously employed in the chemical vapor deposition (CVD) of CoSi nanowires at 850 °C. 11 Herein, we employ the synthesis approach developed in the previous section for nickel iron silicides. Silicon nanoparticles are reacted with metal chlorides in LiI-KI molten salts under vacuum. The reaction temperature is largely decreased compared to the CVD process to be compatible with nanoparticle design.

III.3.1 Characterization of CoSi nanoparticles

The powder X-ray diffraction (XRD) reflections ( The surface states of core-shell CoSi were analyzed by X-ray photoelectron spectroscopy (XPS). In the region of Co 2p3/2 (Figure III-6a), the peaks at 778.0 and 781.7 eV can be respectively ascribed to Co-Si and oxidized cobalt formed by air exposure. [32][33][34] As for the Si region, the peak at 103.4 eV corresponds to silicate and SiO2. The component at 98.6 eV can be assigned to metal silicides. [34][35][36] The peak at 100.2 eV is due to the photoelectron emission from the Co 3s state. 37 Unlike bulk CoSi, CoSi nanoparticles obtained in this work react to a magnet. In order to eliminate the possible influence of amorphous elemental Co, the core-shell CoSi sample was analyzed by pair distribution functions (PDFs). PDF analysis based on characterisation of the G(r) curve describing the inter-atomic distances from X-ray total diffusion analysis is a powerful technique to assess the local structure when X-ray diffraction fails to probe amorphous materials, defects and small clusters. Indeed, it has been recently used to identify amorphous iron phosphide catalysts, 38 or to attest the presence of embedded molecular catalysts 39 or Ni clusters 40 as well as their post-catalytic structural integrity. It is used here to assess the presence or absence of additional phases that could be unseen from X-ray diffraction and yet responsible for the observed magnetic solid behaviour.

The best local structure description is obtained when the calculated G(r)calc from the refined structure is the closest to the experimental G(r)exp, where the reliability R factor is at its lowest.

Refinement of the crystalline CoSi led to a fairly acceptable (reliability factor R=20.9 %) but yet the law of approach to saturation. 41,42 The linear fitting of M versus 1/H gives Ms values of 7.9 and 10.6 emu.g -1 at 300 K and 5K, respectively, for core-shell CoSi particles, corresponding to 0.17 and 0.22 μB/Co. The saturated magnetizations for homogeneous CoSi particles are 2.6 emu.g -1 (0.048 μB/Co) and 3.6 emu.g -1 (0.067 μB/Co) at 300 and 5 K, respectively. These values are much lower than that of metallic Co (1.72 μB/Co at 4 K), 11 in agreement with the absence of Co clusters deduced by PDF analysis. Bulk CoSi is reported as intrinsically diamagnetic (Table III-1). 9 CoSi thin films are non-magnetic. 7 The ferromagnetism of single crystal CoSi nanowires has been ascribed to the Co atoms in some defective positions, e.g. interfaces, grain boundaries and crystal defects. 2,11 Herein, as the polycrystalline core-shell CoSi nanoparticles demonstrate rich boundaries and interfaces, this effect is therefore more pronounced than for the reported CoSi single crystal nanowires (0.03 μB/Co at 300 K) 11 and the homogeneous CoSi nanoparticles prepared in this work (0.05 μB/Co at 300 K). to the oxidation of cobalt and/or silicon. 32 The latter could be followed by dissolution of silicon. 23 Moreover, this process is not observed again in the second cycle, and is then irreversible. As the current density of oxygen evolution remained almost unchanged afterward, it can be deduced that the quantity of active sites for OER was retained after the evolution of the material during the first CV. The overpotential at current density of 10 mA cm -2 in 0. corresponds to CoOOH and/or Co3O4. [48][49][50][51] In the O 1s region, the peak at the lowest binding energy 529.5 eV is attributed to cobalt oxide. The component at 530.5 eV can be assigned to typical non-stoichometric oxygen in spinel (Co3O4) or cobalt hydroxide. 33,48 These results are further confirmed by the decrease of the signal-to-noise ratio of the Si 2p signal, which is ascribed to Si-leaching. The residual Si shows a peak at 102.9 eV. It could be attributed to Si oxoanions 52 implanted in the interlayer regions of cobalt oxide/(oxyhydroxide), which may assist the formation of the layered structure. 23 Hence, according to STEM-EDX and XPS results, cobalt oxide/(oxy)hydroxide is identified as the active electrocatalytic species after OER, which is consistent with the full conversion of the initial catalyst observed from TEM (Figure III-9b).

III.4 Conclusion

Based on the synthesis method developed in the previous chapter, freestanding CoSi nanoparticles are obtained as core-shell Si-CoSi nanoparticles and homogeneous nanoparticles by reacting Si nanoparticles with a cobalt salt at 300 or 400 °C respectively. Unlike diamagnetic bulk CoSi, the cobalt silicide nanoparticles are ferromagnetic below 300 K. This magnetic ordering arises from Co atoms at the interfaces, grain boundary or crystal defects. Hence, the polycrystalline core-shell CoSi nanoparticles obtained in this work exhibit higher magnetic moment (0.17 μB/Co at 300 K) than the reported CoSi single crystal nanowires (0.03 μB/Co at 300 K) 11 and the homogeneous CoSi nanoparticles prepared in this work (0.05 μB/Co at 300 K).

On the other hand, the core-shell CoSi nanoparticles deliver high OER electrocatalytic activity surpassing IrO2 through in situ Si-assisted conversion to a layered Co oxide/(oxy)hydroxide. The modified electrode demonstrates stability for at least 15 h.

Chapter IV. Synthesis of Na 4 Si 4 IV.1 Zintl phase Na4Si4

This chapter is dedicated to the synthesis of an important precursor for our synthesis approaches. Na4Si4 is a Zintl phase that contains tetrahedral polyanions [Si4] 4-, which are Unfortunately, Na4Si4 is usually accompanied by elemental silicon and sodium impurities, 2,3,5-10 which are detrimental to the precise control of the reactions where sodium silicide is used afterward. For instance, Si impurities are relatively inert and remain in the final product. Na impurities can also act as additional reducing agent, 11 although they might be separated from Na4Si4 by vacuum heating at 230~300 °C . 5,6,8 Hence, a synthesis procedure for high purity Na4Si4 (ca. 90 mol. %) is yet to be developed.

The reaction between elemental sodium and silicon is the most known approach to obtain Na4Si4 (Table IV-1). 5,8 To avoid sodium evaporation, the reagents must be sealed by arc-welding in tantalum or niobium tubes, further introduced into silica ampules or steel pressure vessels and heated at 650~750 °C for several days to one week. The procedure requires specific equipment and metal containers not readily available in most laboratories. It also requires special care to avoid the presence of sodium at the welded closure, which is detrimental to sealing and reproducibility. 7 Overall, the resulting material encompasses Na4Si4 and excess elemental silicon and/or sodium. For instance, a stoichiometric mixture of silicon and sodium yields a mixture of Si and Na4Si4 with a purity of ca. 44 mol. % (Table IV-1). 3 Another approach involves the reaction of sodium with silica gel to yield ca. 15 mol. % Na4Si4 mixed with Na2SiO3 (Table IV-1). 12,13 A third method of Na4Si4 synthesis was reported first by Ma et al., 7 who triggered the reaction between previously ball-milled sodium hydride and silicon powders at 420 °C under argon flow over several days. The procedure did not require any specific container nor sealing and then appeared simple, but a long ball-milling pretreatment of 0.5-1 h was required. 6,14 More important, the outcome strongly depended on experimental parameters, including the preliminary ball-milling time and especially the argon flow rate, since reaction intermediates (sodium) and byproducts (dihydrogen) are volatile or gaseous at the reaction temperature. Therefore, despite the attractiveness of this procedure, reaching high purity (ca. 90 mol. %) sodium silicide remains an arduous task: a large excess of sodium hydride is required, while elemental silicon and sodium are commonly encountered as impurities. Herein we report a procedure to enhance the reactivity of silicon with sodium hydride by using silicon nanoparticles as reagents and by carefully controlling experimental parameters (Figure IV-2). Thanks to this approach, we demonstrate the ability with regular lab equipment to reproducibly produce sodium silicide with a purity of ca. 98 mol.% and at the scale of several hundreds of milligrams, both features strongly exceeding those previously reported. 7 We then deliver a simple scalable protocol that will enable wide-spread use of this material. To exemplify the use of this material, we show that it is efficient in the selective synthesis of silicon clathrates.

IV.2 Experimental methods

Reagents: Commercial NaH (95 %, Sigma-Aldrich) and bulk Si powder (99 %, Sigma-Aldrich) were used as received. Commercially available silicon nanoparticles (99 %) were obtained from Nanomakers © (Figure IV-3), France. All the reagents and products were stored and manipulated in an argon-filled glovebox free of H2O (< 0.5 ppm) and O2 (< 0.5 ppm) and with standard Schlenk procedures. Caution: the Na4Si4 powders produced by the procedure described herein are air sensitive and burn spontaneously when exposed to water. All manipulations described were performed by handling these powders under inert atmosphere, except when stated otherwise. The as-synthesized sample was made of a dark pellet with another white pellet on the top. The latter corresponds to Na2O and NaOH coming from the reaction between NaH and the native silicon oxide layer on the surface of Si nanoparticles (Figure IV-3). The upper white pellet was scratched away. The synthesis of Na4Si4 under the optimal conditions described here was repeated 20 times to ensure reproducibility.

Na4Si4 synthesis from bulk Si powder:

The same procedure and reagents proportion described above were followed, but bulk Si powder was used instead of Si nanoparticles. The thermal treatment was performed at 395 °C for 10 hours.

Si clathrates synthesis:

The Na4Si4 powder (4 mmol) synthesized as described above was loaded in a pyrolytic h-BN crucible (Ø25×H60 mm) previously dried at 400 °C under vacuum (10 -3 mbar) for 10 hours to remove adsorbed moisture. The crucible was placed in a bottomclosed quartz tube. The tube was heated in a vertical furnace. Na8Si46 (type I) and NaxSi136 (type II) were obtained by heat treatment at 470 °C and 440 °C, respectively, under dynamic vacuum (10 -3 mbar) for 90 min. These temperatures were chosen according to the literature. 2 For Na8Si46 (type I), the h-BN crucible was covered with an h-BN cap prior heating, to maintain a higher Na vapor pressure. For NaxSi136 (type II), the crucible was maintained opened. During the reaction, a thin deposit of Na appeared on the upside cold part of the quartz tube. After dwelling and cooling down, the quartz tube with the h-BN crucible and its containment was transferred into an argon-filled glovebox without exposure to air.

IV.3 Results and discussion

IV.3.1 Optimal synthesis of Na4Si4

Reaction (1) between NaH and Si nanoparticles (1.1:1 mol. ratio) was performed for 24 hours at 395 °C with an argon flow of 55 mL min -1 :

4NaH (s) + 4Si (s) ∆ → Na 4 Si 4 (s) + 2H 2 (g) (1)
ca. -110 and -40 ppm 16,17 nor any contribution of d-Si expected at ca. -80 ppm 18 Overall, EDX, XRD and NMR support the formation of crystalline Na4Si4. EDX yields a purity of ca. 98 mol. %, and amorphous Si is the main impurity. The synthesis yield is about 80 %, which corresponds to ca. 700 mg of powders, i.e., 10 times increase compared to previous approaches using the reaction between sodium hydride and silicon (Table IV-1). This result is achieved with only a slight excess of sodium hydride reagent (10 mol. %) compared to previous reports (60-90 mol. % excess) (Table IV-1), 6,14 which enables bypassing purification steps (Table IV-1). The preliminary ball-milling time is also shortened from over 30 min to only 2 min and the reaction time is further reduced to 24 h. A comparison with the most commonly used reaction between elemental sodium and silicon (Table IV-1) shows that the reaction time and the temperature are strongly reduced with the approach we describe herein, while purification steps and complex set-ups encompassing metal containers are avoided.

IV.3.2 Impact of the Si source

To assess the role of the silicon source in the synthesis, we have followed the same protocol as above but by using bulk Si powder as precursor. The reaction was performed over 10 h under unreacted Na and Si. Therefore, silicon nanoparticles play a primary role in the completion of the reaction at low temperature and short time. Their higher reactivity ensures a reaction in closeto-stoichiometry reagents ratio compared to bulk Si that required a large excess of NaH (NaH:Si = 1.9:1 mol.) to yield Na4Si4 as major product. 7,21 Note also that while bulk Si required ball-milling over 1 hour, 6,7 using Si nanoparticles enables shortening the milling step down to 2 min. Likewise, the reaction was performed over 10 to 24 h compared to more than two days as reported for bulk silicon. 7 These tetrahedral frameworks can adopt a wide range of elemental substitutions to tune thermal, electrical and thermoelectric properties among others. 23,24 To assess the usefulness of the assynthesized sodium silicide as precursor for inorganic synthesis, we have studied its thermal decomposition as a well-known pathway to type I Na8Si46 and type II NaxSi136 (0 ≤ x ≤ 24) silicon clathrates. Solid red square, blue circle, yellow and green triangles represent data reported by Beekman et al., 25 Stefanoski et al., 26 Beekman et al., 27 and Renny et al., 24 respectively, while the empty red square corresponds to this work. The broken line corresponds to data linear fit.

The electronic and thermal properties of NaxSi136 (0 ≤ x ≤ 24) type II clathrates depend strongly on the occupancy of structural cages by Na. 24 Therefore, we have estimated the x value according to the dependency of the a cell parameter as a function of x (Figure IV-12). We retrieve an overall composition of Na11.86(2)Si136 corresponding to a partial occupancy of Na sites. 2,28

IV.5 Conclusion

Herein we have shown that using silicon nanoparticles as chemical reagents provides a convenient way to solve usual limitations of the synthesis of silicon compounds with poor thermal stability and from poorly reactive elemental silicon: the high reactivity of silicon nanoparticles enables synthesis at high yield, close to the theoretical reagent stoichiometry, and at lowered temperature below the decomposition threshold of the product, thus significantly enhancing its purity. It also enhances the robustness of the synthesis versus experimental conditions, especially by eliminating the dependence of the reaction versus gas flow. We demonstrated that the as-obtained Na4Si4 powder is a convenient precursor towards the selective synthesis of type I and type II silicon clathrates, thus supporting the relevance of the material for further use towards silicon-based materials. In the next chapter, the use of Na4Si4 as reagent for synthesis of intermetallic binary nickel silicides and ternary nickel silicophosphides will be discussed.

Chapter V. Synthesis and electrocatalytic

properties of a ternary nickel silicophosphide

V.1 Introduction

Intermetallic compounds combining transition metals and p-block elements are receiving tremendous attention because of their diverse properties on semiconductivity, magnetism, thermoelectricity and catalysis. 1,2 The catalytic properties of binary borides, phosphides and silicides can be strongly improved by synergetic effects in ternary bimetallic systems. 3 On the opposite, the combination of two nonmetals with a metal and its impact on catalysis is understudied. In the case of Si and P, despite many compositions and crystal structures of ternary metal silicophosphides (M-Si-P), the properties of most them remain unreported. 4,5 Very recently, some attractive properties of Si-and P-rich silicophosphides have been revealed (Table V-1). For example, silicophosphides of Cu, Co, Zn and Fe have been investigated

as Li-ion battery anode materials. The silicophosphide anodes exhibit highly improved cycling stability and rate capability compared to Si-and P-based anodes. [6][7][8][9][10][11][12] Furthermore, ternary M-Si-P are considered more likely to form noncentrosymmetric phases than binary metal silicides M-Si or phosphides M-P. This statement has led to outstanding nonlinear optical properties. 13,14 On the other hand, research on metal-rich silicophosphides has been seldom conducted to date. Amorphous ternary Ni-rich borophosphides have demonstrated catalytic activities in hydrogenation superior to their binary counterpart borides and phosphides. [15][16][17] The enhanced activities were attributed to the variation of the metal electron density induced by metalloid elements. 17 Similar effects may arise in silicophosphides, as well as other behaviors due to special crystal structures. Therefore, studying catalytic properties of silicophosphides is highly relevant.

This also implies developing nanoscaled silicophosphides to benefit from their high surface area.

However, transition metal silicophosphide nanoparticles have not been reported, while many properties (e.g., thermoelectrics, magnetism and catalysis) could be impacted, if not improved through nanostructuring of crystalline phases. 2,18,19 Accessing nanoscaled ternary transition metal silicophosphide materials requires design of new synthesis methods. Inorganic molten salts have been demonstrated as appropriate reaction media for achieving nanomaterials with high crystallization temperature. [20][21][22][23][24] Therefore, they could be adapted for the synthesis of nanoparticles of ternary silicophosphides enabling the combination of two p-block elements into crystal structures bearing a covalent character. The Ni-Si-P ternary system presents a variety of thermodynamic phases. 5 By reacting elemental powders, bulk Ni2Si0.75P0.25 has been obtained at 800 °C. 28 In this work, we demonstrate the synthesis of ternary Ni2Si0.75P0.25 nanoparticles by reacting P-doped Ni nanoparticles and Zintl phase Na4Si4 in a molten LiI-KI eutectic mixture at 500 °C. Herein, nanoparticles act as reactive reagents and nanotemplate for the ternary phase. 29,30 Ni2Si nanoparticles are also obtained by tuning down the P content in the Ni nanoparticle precursor. A preliminary investigation of their electrocatalytic properties on water splitting is conducted. The obtained ternary Ni2Si0.75P0.25 nanoparticles show higher electrocatalytic activities in alkaline oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) than binary Ni2Si nanoparticles synthesized in this work.

Post mortem studies unveil the evolution of morphology, composition and chemical states of Ni2Si0.75P0.25 nanoparticles under cathodic and anodic conditions of water electrolysis.

V.2 Experimental methods

Reagents. Lithium iodide (99%, Alfa Aesar), potassium iodide (99%, Sigma-Aldrich), silicon nanoparticles (99%, Nanomakers © , France), sodium hydride (95 %, Sigma-Aldrich), Nickel (II) acetylacetonate (95%, Ni(acac)2, Strem Chemicals) and trioctylphosphine (TOP, 97%, P(C8H16)3, Strem Chemicals) are stored and manipulated as received in an Ar-filled glovebox (H2O < 0.5 ppm, O2 < 0.5 ppm). Oleylamine (technical grade 70%, Sigma-Aldrich) is stored and used in the air. Acetone (VWR Normapur grade), tetrahydrofuran (THF, VWR Normapur grade), methanol (VWR Normapur grade) was used for washing.

Synthesis of Ni nanoparticles.

The synthesis is based on a reported protocol. 31 20.8 g (25.6 mL, 78 mmol) oleylamine and a Teflon-coated stir bar were put in a 50 mL three-necked round-bottomed flask connected to a reflux condenser and Schlenk line. Then, oleylamine was stirred and degassed at room temperature under vacuum during 30 min to remove dissolved air.

Afterward, 2 g (7.8 mmol) Ni(acac)2 and 2.1 mL (4.7 mmol) TOP were added. With a heating mantle, the mixture was degassed at 100 °C for 3 min to remove the side product water. Then, the contents were rapidly heated to 220 °C and maintained for 2 h under flowed nitrogen. The suspension was cooled to room temperature and transferred to a centrifuge tube for wash. The nanoparticles were isolated by centrifugation at 21000 rpm for 3 min. After removing the supernatant, 30 mL acetone and 10 mL THF were added. Then, this process was repeated nine times. The washed nanoparticles were dried in a Schlenk tube under vacuum and then stored in an Ar-filled glovebox for further manipulation.

Synthesis of Na4Si4

. This synthesis is an improvement of a reported protocol. 32 470 mg (20 mmol) of NaH and 500 mg (20 mmol) of Si nanoparticles were ball-milled for 2 min at 20 Hz.

Then, the fine mixture was loaded in a pyrolytic h-BN crucible (cylinder Ø25×H60 mm) covered with a cap of the same material, then placed in a bottom closed quartz tube. The tube was heated in a vertical furnace at 395 °C for 48 h under Ar flow. After cooling, the system was transferred to an Ar-filled glovebox. The as-synthesized sample was a dark pellet with another white pellet on the top. The latter corresponding to Na2O and NaOH was scratched away. The obtained Caution: The Na4Si4 powder is air sensitive and burn spontaneously when exposed to water. All manipulations described were performed by handling these powders under inert atmosphere, except when stated otherwise. Measurements of OER properties. Electrochemical measurements were performed with a typical three-electrode configuration which includes the working electrode, a Pt coil as counter electrode and an Ag/AgCl in saturated KCl electrode as reference electrode. 0.1 M KOH was used as electrolyte (pH = 13). In order to remove the Fe impurity in the electrolyte, ca. 1 g of asprepared Ni(OH)2 solid was put into 200 mL as-prepared 0.1M KOH. The suspension was sonicated for 3 min and then rested for at least 6 h. The purified 0.1M KOH supernatant was separated by centrifugation, stored under nitrogen and later used as electrolyte. Before measurements, the electrolyte was bubbled by O2 for 20 min. The rotating disk electrode rotated at 1600 rpm. Cyclic voltammograms were recorded in the potential range of 0 to 1.05 V vs. Ag/AgCl reference electrode at a scan rate of 20 mV s -1 . For the chronopotentiometry measurement, a carbon sheet was used for working electrode substrate, a Teflon-coated stir bar was put in the electrolyte and stirred at 400 rpm to enhance the ions diffusion. For the potentials reported in this work, the ohmic drop iR was corrected by considering the total impedance measured at a frequency of 50 kHz as the solution resistance. The measured potentials were converted by the equation: ERHE = Emeasured + 0.197 + 0.0591× pH, where pH=13.

Synthesis

Measurements of HER properties. The same configuration as for OER studies was used except that a carbon rod was used as counter electrode. 0.1 M KOH was used as electrolyte (pH = 13). Before measurements, the electrolyte was bubbled by Ar for 20 min. The rotating disk electrode rotated at 1600 rpm. The potential was cycled in the range of -0.5 to -2.0 V vs. Ag/AgCl reference electrode at a scan rate of 20 mV s -1 . The parameters for OER CP measurement were followed for HER CP measurement.

V.3 Results and discussion

V. 24 For the synthesis of nickel silicophosphide nanoparticles, the LiI-KI eutectic mixture (melting point = 286 °C) was chosen as the inorganic solvent, because of its large stability temperature range, low vapor pressure, inertness to Na4Si4. 24 and Si are detected as minor impurities. Ni2Si0.75P0.25 exhibits the same crystal structure as Ni2P.

One can then expect a solid solution range of Ni2SixP1-x compounds with Ni2P structure (Figure V-4). 28 According to EDX analysis (Table V- amorphous silicon oxide at the surface of nanoparticles. The excess P suggests the composition of ternary silicophosphide is not in stoichiometry of Ni2Si0.75P0.25, which is consistent with the literature that the crystal structure of Ni2Si0.75P0.25 can adapt a range of Si/P ratio. 5 The Ni/Si/P ratios are 2/1.3/0.1 for the Ni2Si product. The excess Si could be amorphous impurities of silicon or silicon oxide layer on the particle surface. On the other hand, the P/Ni ratio in P-doped Ni precursors is maintained in the ternary phase, as well as in Ni2Si (Table V-2). Ni oxide of the oxide layer evolved during handling. 24,[33][34][35] The binding energy of Ni silicophosphide (853.3 eV) is close to the reported values for Ni2P and Ni2Si (ca. 853.0 -853.5 eV), 24,34,36,37 showing a positive shift (ca. 0.5 eV) compared to elemental Ni (ca. 852.6 eV). 36,38 It indicates that Ni in the silicophosphide is also positively charged. The Si 2p spectrum shows two components. The low binding energy one at 99.6 eV corresponds to low oxidation state Si of silicide, while the one at 102.8 eV can be ascribed to silicate or SiO2. 38,39 As shown in P 2p spectrum, the initial P-doped Ni nanoparticles show a peak at 129.4 eV (2p3/2), which can be a

Ni-rich phosphide formed at the Ni surface, in agreement with previous results. 40 The peak at 130.9 eV corresponds to trioctylphosphine adsorbed on the Ni surface. 40 The peak at high binding energy 133.0 eV could be partly oxidized P species on the surface. 41 As for Ni2Si0.75P0.25, the phosphide contribution is observed at 129.9 eV. The adsorbed phosphine peak is not observed anymore for Ni2Si0.75P0.25. The peak at 133.5 eV is assigned to nickel phosphate. 42 (1840 eV), which can be assigned to non-oxidic silicide. This value is close to the reported binary silicide. 43 And the peak at higher energy is similar to SiO2 (1847 eV) references, corresponding to silicon oxide on the surface of nanoparticles. The shoulder at ca. 1844 eV can be assigned to the Si 3+ oxidation state of the central Si atom in Si-SiO3 tetrahedron. 44 Similarly, through the comparison with red phosphorus and Na3PO4, two main peaks observed on the P K-edge of The shoulder peaks at ca. 2147 eV and 2156 eV could be assigned to oxidized phosphorus with different coordination or neighbors compared to PO4 3-. 45 The feature at 2147 eV shows a more important contribution in P-doped Ni, because this energy could also be attributed to phosphine, 46 which was previously detected by XPS.

V.3.2 Electrocatalysis of water splitting

The electrocatalytic properties on HER and OER of ternary Ni2Si0.75P0.25 and binary Ni2Si obtained from the same synthesis method were investigated in 0.1 M KOH. Because the electrocatalytic properties of Ni-based catalyst can be strongly influenced by Fe impurity in electrolyte, 47,48 a reported Fe-elimination protocol by co-precipitation of Ni and Fe in the KOH electrolyte was adopted to assess the intrinsic properties of the ternary phase. 47 The electrochemical measurements were performed with electrodes made of hybrid nanocrystals/Nafion films deposited on glassy carbon sheet (see experimental section for more details). nanoparticles in Fe-free 1 M KOH demonstrates an overpotential at 410 mV for 10 mA cm -2 , nanoparticles exhibit an overpotential at 10 mA cm -2 (294 mV). Even if it is much lower than that of binary Ni2Si nanoparticles (445 mV), ternary silicophosphide in alkaline HER electrocatalysis is not as efficient as binary Ni2P, which can usually attain an overpotential below 200 mV at 10 mA cm -2 . 55,56 Indeed, phosphorus has been considered playing important roles as proton-acceptor to initiate HER and weakening the bond strength of metal and adsorbed hydrogen, 56 which is highly favorable for alkaline HER. 57 The EIS measurement ( Based on this synthesis method, freestanding polycrystalline CoSi nanoparticles core also achieved as core-shell Si-CoSi nanoparticles at 300 °C and as homogeneous nanoparticles at 400 °C. Different than bulk CoSi, the obtained CoSi nanoparticles in this work are ferromagnetic below 300 K. This magnetic order is attributed to the unpaired spins produced by undercoordinated Co atoms at interfaces, grain boundaries or crystal defects. 1 The core-shell CoSi nanoparticles exhibit small crystal domains, thus demonstrating higher magnetic moment (0.17 μB/Co at 300 K) than reported single crystal CoSi nanowires (0.03 μB/Co at 300 K) 2 and the homogeneous CoSi nanoparticles (0.05 μB/Co at 300 K) obtained in this work. The electrocatalytic properties of core-shell CoSi nanoparticles in OER were also investigated. Upon a complete conversion towards layered Si-doped Co oxide/(oxy)hydroxide, core-shell CoSi nanoparticles delivered high OER catalytic activity surpassing IrO2. This activity of modified electrodes was stable for at least 15 h. Aside from the above synthesis method, a second molten salt synthesis method using Ni nanoparticles and Na4Si4 as precursors was developed. Sodium silicide Na4Si4 is a reductive and reactive source of silicon highly relevant to design non-oxidic silicon materials, including clathrates, various silicon allotropes, and metal silicides. Despite the importance of this compound, its production in high amounts and high purity is yet a bottleneck with reported methods.

In this work, we demonstrated that readily available Si nanoparticles react with sodium hydride with a stoichiometry close to the theoretical one and at a temperature of 395 °C for shorter duration than previously reported. This enhanced reactivity of Si nanoparticles makes the procedure robust and less dependent on experimental parameters, such as gas flow. As a result, we delivered a procedure to achieve Na4Si4 with purity of ca. 98 mol.% at the gram scale. We showed that this compound is an efficient precursor to deliver selectively type I and type II

  Generally, OER elementary steps are considered occurring on high-valence metal sites. Two OER mechanisms have been proposed for different types of catalysts (Figure I-1), i.e. lattice oxygen mechanisms (LOM), for which the O-O bonding involves reversible formation of oxygen vacancy, and adsorption evolution mechanisms (AEM) involving three main adsorbed intermediates *OH, *O, and

Figure I- 1

 1 Figure I-1 Alkaline OER electrocatalysis: (a) adsorption evolution mechanisms (AEM) and (b)

  ) dispersible/soluble in molten salts, (2) reactive in the temperature range of molten salts, (3) without strong Si-O bonds, and (4) non-volatile at the reaction temperature. Few silicon sources respond all the requests. The Zintl phase sodium silicide Na4Si4 is used in this work as silicon species for silicification of metal nanoparticles. Both approaches in Figure I-3 will be discussed in the following section.

Figure I- 3

 3 Figure I-3 Reaction schema (a) insertion of metal species into Si nanoparticles, (b) insertion of

Figure II- 1

 1 Figure II-1 (a) Powder XRD pattern of Si nanoparticles used as reagents. Red drop lines indicate a diamond-like structure Si reference. (b) TEM images of Si nanoparticles used as reagents.

Figure II- 2

 2 Figure II-2 The three-electrode configuration used in the custom-made PMMA electrochemical cell.

Figure II- 3

 3 Figure II-3 (a) Schematic illustration of the reaction between Si nanoparticles and metal salts in molten salts and under vacuum. (b) HRTEM image with corresponding Fast Fourier Transform (FFT, inset) and (c) STEM-EDX chemical mapping of as-prepared NiFeSi-12 sample. The FFT is indexed along the o-Ni2Si structure. (d) XRD patterns of the as-prepared samples of Ni, Fe and NiFe silicides. The blue, green and red drop lines denote the references of o-Ni2Si, h-Ni2Si and FeSi, respectively. The broad bumps between 15-25 ° come from the beam scattering of the protective PMMA plastic dome used for confining the Ar atmosphere around the sample. The o-Ni2Si (bottom) and FeSi (top) crystal structures are also displayed on the right.

Figure II- 4

 4 Figure II-4 Powder XRD pattern of the evaporated and recondensed byproduct of metal silicide synthesis, which can be indexed along the SiI4 reference (indicated by red lines). The broad bump between 15 and 25 ° comes from the beam scattering of the protective PMMA plastic dome used for confining the Ar atmosphere around the sample.

Figure II- 5

 5 Figure II-5 Powder XRD pattern: (a) the final product from the reaction between Si nanoparticles and NiCl2 under 1 atm. argon at 395 °C for 6 h in LiI-KI. Nickel silicides were the major phase, however unreacted Si can be observed, (b) the final product from the reaction between bulk Si powder and NiCl2 under vacuum at 395 °C for 6 h in LiI-KI. Large amount of Si remains unreacted. Nickel silicides were formed in a minor fraction, (c) the final product from the solid-solid reaction between Si nanoparticles and NiCl2 under vacuum at 395 °C for 6 h. Unreacted Si and NiCl2 were found. NiCl2 appears hydrated because the XRD diagram was acquired in the air.

Figure II- 6

 6 Figure II-6 (a) Size distribution of NiFeSi-12. TEM images of (b) NiFeSi-50, (c) NiFeSi-12, (d) NiFeSi-3, (e) FeSi and (f) Ni2Si.

Figure II- 7

 7 Figure II-7 STEM-EDX mapping of the as-prepared NiFeSi-12 sample: superposition of oxygen and other elements. Si, Fe, Ni, O distributions are shown in green, yellow, red and purple, respectively.

Figure II- 8

 8 Figure II-8 XPS of as-prepared NiFeSi-12 sample corresponding to Fe-doped Ni2Si: (a) Ni 2p region, (b) Fe 2p region, (c) Si 2p region, (d) O 1s region.

  During activation (Figure II-9), the water oxidation current increases together with the Ni oxidation/reduction wave area, which indicates that more and more Ni are involved in this reversible redox reaction, so that the amount of highvalence Ni on the surface active for OER electrocatalysis increases. When the CVs reached a steady state, linear sweep voltammetry (LSV) was recorded at 10 mV s -1 to probe the activated catalysts.

Figure II- 9

 9 Figure II-9 CVs showing the electrochemical activation of NiFeSi-12 by continuous potential cycling from 0.96 to 2.01 V vs. RHE at 20 mV s -1 in O2-saturated 0.1 M KOH.

Figure II- 10

 10 Figure II-10 OER electrocatalytic properties of metal silicide nanocrystals. (a) LSV curves of the metal silicides nanoparticles and of reference commercial IrO2 nanoparticles in an aqueous O2saturated 0.1 M KOH electrolyte. (b) Tafel slopes and (c) Chronopotentiometry measurements at 10 mA cm -2 for NiFe silicides and IrO2. Insets show photographs of a NiFeSi-12 working electrode with glassy carbon substrate before and after 85h (the conductive copper foil and insulating cover were removed) of OER. Nyquist plots: (d) recorded under 1.60 V vs. RHE and corresponding fits for NiFeSi-12 at different stages of the electrocatalytic process. The equivalent circuit is shown in the inset. (e) recorded under 1.60 V vs. RHE of metal silicide nanocrystals and reference commercial IrO2 nanoparticles in O2-saturated 0.1 M KOH electrolyte. (f) EIS curves of NiFeSi-50 recorded at different potential from 50 000 to 1 Hz.

  EIS was later performed on activated silicides and IrO2. As shown in the Nyquist plot (Figure II-10e), the diameter of the semicircle in the high frequency region independent of the applied potential (Figure II-10f) represents Rct. NiFeSi-50 exhibits the smallest Rct (FigureII-10e).Rct increases with the Fe content of NiFe silicide. This observation is consistent with the higher electrical conductivity of nickel silicides than of iron silicides.23,60 The second semicircle in the low frequency region decreasing with the applied potential corresponds to Rads (FigureII-10f). FromFigure II-10e, the Rads values are very close for the three NiFe silicides, which indicates similar surface states for these materials during electrocatalysis. The low values Rct+Rads (Figure II-10e) highlight the low overall resistances of NiFe silicide catalysts.

II. 3 . 3

 33 a) NGF: N-doped graphene framework, b) measured in 0.1M NaOH, c) MM′-N/C: bimetaldecorated, pyridinic N-dominated large-size carbon tubes, d) MM′-NC: MM′ alloy and nitrogencodoped porous carbon, e) NS: layered double hydroxide (LDH) nanosheets (NSs, ~ 1.3 nm), f) N-GO: nitrogen-doped graphene oxide, g) oGSHs: oxidized graphene/single-walled CNT hybrids. Post mortem studies To elucidate the mechanisms at the origin of the high durability of the electrocatalytic performances of the NiFeSi-12 sample, the corresponding electrodes were analyzed after operation at 10 mA cm -2 for different durations. SEM images (Figure II-11a-c) show a roughening of the electrode surface. The XRD diagram of the post-OER electrode subjected 10 h to OER at 10 mA cm -2 (Figure II-11d) suggests that the main crystalline phase remains Ni2-xFexSi, although the low signal-to-background ratio due to low amounts of material does not enable identifying minor phases. Then, the elemental distribution was analyzed by STEM-EDX. The chemical maps (Figure II-11e and Figure II-12a-c) show that the outer oxidic layer of the particles is poor in Si and rich in Ni and Fe, which could be explained by the formation of a NiFe (oxy)hydroxide/oxide at the surface of the particles during OER. This is consistent with the selfactivation step observed Figure II-9, i.e., the catalyst activation corresponds to the building of aNiFe (oxy)hydroxide/oxide layer. This observation is also in agreement with the previously reported formation of an electrocatalytically active amorphous or crystalline NiFe (oxy)hydroxide/oxide layer at the surface of NiFe borides, phosphides, nitrides and sulfides.9,14

Figure II- 11

 11 Figure II-11 SEM images of a working electrode made of NiFeSi-12 nanoparticles deposited on a GC substrate (a) before electrochemical study, (b) post 10 h OER, and (c) post 85 h OER. (d) XRD patterns of a NiFeSi-12/glassy carbon electrode as-prepared and post mortem after 10 h of OER at 10 mA cm -2 . The pattern of a blank electrode without catalyst is shown as reference. (e) comparison of oxide layer content of pristine sample (top) and post 85 h OER sample (bottom).

Figure II- 12

 12 Figure II-12 STEM-HAADF and STEM-EDX chemical mapping of NiFeSi-12 catalyst: (a) post 10 h OER, (b) post 65 h OER, (c) post 85 h OER. HRTEM images and corresponding FFTs (insets) of NiFeSi-12 catalyst (d) post 10 h OER, (e) post 65 h OER, (f) post 85 h OER. TEM images of NiFeSi-12 catalyst (g) post 10 h OER, (h) post 65 h OER, (i) post 85 h OER at 10 mA cm -2 .

Figure II- 13

 13 Figure II-13 XPS of a NiFeSi-12 working electrode before OER, post 10h OER and post 85h OER in (a) Ni 2p and (b) Si 2p regions.

Figure II- 14

 14 Figure II-14 STEM-EDX line profile mapping of two NiFeSi-12 particles post 85 h OER at 10 mA cm -2 .

  of core-shell and homogeneous cobalt monosilicide nanoparticles. 63.2 mg Si nanoparticles (2.3 mmol), 194.8 mg CoCl2 (1.5 mmol), 2.9 g LiI (21.7 mmol) and 2.1 g KI (12.7 mmol) (molar ratio LiI:KI 0.63:0.37) were ball-milled during 2 min at 20 Hz (Retsch MM400 ball mill airtight vial of 50 mL, filled with one steel ball of 62.3 g with a diameter of 23 mm) to get a well-mixed fine powder. The mixture was loaded in a quartz tube (Ø28×H345 mm) which was then connected to a Schlenk line and was evacuated under vacuum. An intermediate liquid nitrogen trap between the quartz tube and the Schlenk line was set to condensate the byproduct SiI4 to avoid the pollution of the Schlenk line. A vertical furnace from

  8 %). A CoSi and amorphous Co mixture was tested as well and compared with CoSi alone. This led to a slightly worse reliability factor (21.0 %) than CoSi alone. The degradation of CoSi-alone refinement resulting from adding amorphous Co and the lack of improvement of the refined CoSi / amorphous SiO2 / crystalline Si mixture allow us to eliminate amorphous Co as a component of the material.

Figure

  Figure III-2). On contrary, the core exhibits a poor crystallinity and few areas can be attributed to d-Si (Figure III-2). This disordered state could be due to the lattice distortion induced by interstitial Co atoms in an Si matrix.

Figure III- 1

 1 Figure III-1 (a) Powder XRD pattern of the as-prepared core-shell CoSi. The red drop lines denote the CoSi reference. The asterisks mark indicative peaks of d-Si references. (b) HRTEM image with corresponding Fast Fourier Transform (FFT) in the inset. The FFT is indexed along the CoSi structure. (c) STEM-HAADF image and (d) STEM-EDX linear scan along the line indicated in the inset.

Figure III- 2

 2 Figure III-2 HRTEM images of core-shell CoSi nanoparticles and the FFTs of corresponding areas. The core and the shell are indexed along d-Si and CoSi structures, respectively. The lighter contrast outer layer shows no crystalline feature.

Figure III- 3

 3 Figure III-3 (a) TEM image and the three positions used for point STEM-EDX analysis 1, 2, and 3. (b) Schematic analyzed zones of each position.

Figure III- 4

 4 Figure III-4 Schema of synthesis towards core-shell CoSi and homogeneous CoSi nanoparticles.

Figure III- 5

 5 Figure III-5 Comparison of (a) powder XRD diagrams of CoSi samples obtained at 400 and 300 °C and TEM images of corresponding (b) homogeneous CoSi and (c) core-shell CoSi nanoparticles. Homogeneous CoSi nanoparticles: (d) HRTEM image and FFT of corresponding area. The FFT can be fully indexed along the CoSi structure. (e) STEM-HAADF image (inset) and elemental STEM-EDX linear profile along the line indicated in the inset.

Figure III- 6

 6 Figure III-6 XPS of as-prepared core-shell CoSi nanoparticles: (a) Co 2p and (b) Si 2p regions.

  improvable solution. The addition of crystalline Si and amorphous silica has thus led to the best refinement (R=15.8 %, Figure III-7a). More importantly, amorphous Co undetected by X-ray diffraction and crystalline Co could be excluded since their addition did not improve the reliability factor (FigureIII-7b-c). Therefore, the reaction of the core-shell sample to magnet is attributed to the CoSi phase (See experimental methods for details of refinement process).

Figure III- 7

 7 Figure III-7 PDF experimental G(r) curve (blue dotted line) of core-shell CoSi nanoparticles superimposed with the calculated G(r) curve (red line) of different models, and residual profile (green).

44

  The zero-field cooled (ZFC) and field cooled (FC) magnetization curves of core-shell CoSi nanoparticles at 20 Oe are shown in Figure III-8b. The ZFC curve shows a maximum at ca. 275 K. This result is in agreement with the recorded hysteresis loops at different temperatures. In a polycrystalline CoSi sample, the spins of each crystal domain are spontaneously oriented along their easy axis at room temperature. These macroscopic disordered spin orientations are frozen at low temperature (5 K) before the ZFC measurement, so that a low magnetization is recorded at the beginning of ZFC measurement. As the temperature increases, the spins are defrosted to be aligned under the applied field, so the magnetization increases. The rise of temperature also leads to thermal disturbance and decrease of coercivity. As a result, after reaching the maximum magnetization, the FC and ZFC curves converge above 300 K, where the sample becomes paramagnetic.

Figure III- 8

 8 Figure III-8 Magnetization curves of (a) core-shell CoSi nanoparticles and (b) homogeneous CoSi nanoparticles: as a function of magnetic field at 300 and 5 K. (b) Magnetization of coreshell CoSi nanoparticles as a function of temperature at applied field of 20 Oe, recorded after cooling in a 20 Oe field (FC) and after cooling in zero field (ZFC), respectively.

1 M

 1 KOH of core-shell CoSi nanoparticles is ca. 410 mV, versus 440 mV for commercial IrO2 nanoparticles, showing that core-shell CoSi nanoparticles exhibit a higher activity than IrO2. The activities normalized versus the amount of Co are further calculated and compared to Co-based catalysts previously reported (Table III-2). The normalized activity of core-shell CoSi nanoparticles is superior to the listed Co-based OER electrocatalysts.

Figure III- 9

 9 Figure III-9 The OER electrocatalytic properties of CoSi nanoparticles. (a) CVs of core-shell and homogeneous CoSi nanoparticles and of reference commercial IrO2 nanoparticles in an aqueous O2-saturated 0.1 M KOH electrolyte. (b) CP measurement at 10 mA cm -2 for core-shell CoSi nanoparticles. (c) TEM images and (d) STEM-EDX chemical mapping of the core-shell CoSi nanoparticles after 15 h CP measurement.

47 To

 47 elucidate the evolution of the surface chemical states and of the material, the post OER sample was analyzed by XPS. As shown in Figure III-10, after 15h of OER operation, the peaks of the Co 2p3/2 spectrum are shifted to higher binding energy (Figure III-6). The deconvolved peaks at 780.1 eV and 781.2 eV and the satellite features indicate the presence of Co 3+ , which

Figure III- 10

 10 Figure III-10 XPS of the core-shell CoSi sample after 15 h of OER at 10 mA cm -2 : (a) Co 2p region, (b) O 1s region, (c) Si 2p region. In (b), the Nafion peaks are identified according to the literature.33

Figure III- 11

 11 Figure III-11 XANES spectra at the Si K-edge recorded on the core-shell CoSi sample, the corresponding as-prepared electrode, post-15h-OER electrode, and the Si wafer and fumed silica references.

  isoelectronic and then isostructural to the P4 molecule (FigureIV-1). They are compensated by Na + cations as a result of charge transfer from electropositive sodium to silicon. The [Si4] 4- clusters are highly reactive, which has been discussed to provide a potential portable hydrogen source upon reaction with water.1 Additionally, the low oxidation state of silicon and the fact that Si is only involved in intra-cluster bonds, which provides high flexibility for designing siliconbased materials under mild conditions. For instance, depending on the conditions of decomposition, Na4Si4 yields readily silicon clathrates 2,3 or silicon nanoparticles.4

Figure IV- 1

 1 Figure IV-1 Schematic crystal structure of Na4Si4.

Figure IV- 2

 2 Figure IV-2 Scheme of the synthesis route towards high purity Na4Si4.

Figure IV- 3

 3 Figure IV-3 (a) Powder XRD pattern (bottom) of Si nanoparticles used as reagents. The XRD pattern is indexed along the silicon diamond structure (red bars). (b) TEM images of as-received Si nanoparticles used as reagents.

  is observed. It thus confirms the high purity of Na4Si4. The 23 Na spectrum (Figure IV-5) shows two quadrupolar resonance patterns at 56.3 ppm (CQ = 1.32 MHz, Q = 0.47) and 49.8 ppm (CQ =2.34 MHz, Q = 0.27) corresponding to two Na crystallographic sites in Na4Si4. In addition, no peak at ca. 50 ppm corresponding to Na2O could be detected,19 suggesting no contamination from the upper white pellet made mainly of Na2O and of a minor portion of NaOH (FigureIV-6). The thin peak at 18.5 ppm is assigned to crystalline NaOH accounting for less than 5 % of the total amount of sodium.20 Because crystalline NaOH could not be detected by XRD, its observation by NMR is ascribed to the degradation of Na4Si4 during preparation of NMR analyses.

Figure IV- 5

 5 Figure IV-5 29 Si and (b) 23 Na solid-state NMR spectra of the Na4Si4 powder obtained after 24 h at 395 °C under 55 mL min -1 Ar flow. Asterisks indicate spinning sidebands. The measured spectra are shown by the blue curves, and the fitting curve is shown in red.

Figure IV- 6

 6 Figure IV-6 Powder XRD pattern of the white pellet on the top of the as-prepared product of Na4Si4 synthesis.

15 mL min - 1

 1 Ar flow rate. The initial NaH:Si ratio was kept at 1.1:1 mol. The resulting product was pale grey aggregates. According to XRD (Figure IV-7), the main crystalline phases are

Figure IV- 7

 7 Figure IV-7 Powder XRD patterns of the Na4Si4 samples synthesized respectively with Si nanoparticles (black bottom curve) and bulk Si powder (blue top curve) at 395 °C for 10 hours with an Ar flow rate 15 mL min -1 . Red lines indicate Na4Si4 reference. Si, Na, and NaOH impurities are marked on the most informative peaks.

Figure IV- 9 IV. 4

 94 Figure IV-9 Powder XRD patterns of Na4Si4 samples synthesized with different Ar flow rates at 395 °C with a reaction time of 10 hours. Red lines account for Na4Si4 reference. Si, Na, and NaOH impurities are marked on the most informative peaks, where NaOH is ascribed to incidental oxidation of sample. (b) An enlarged view of the XRD pattern in the range of 26-32 °.

Figure IV- 10

 10 Figure IV-10 Schematic structural model of clathrate compounds: (a) type I Na8Si46 and (b) type II NaxSi136.2

Figure IV- 11

 11 Figure IV-11 (a, b) Le Bail analysis of the powder XRD patterns of products obtained by Na4Si4 thermal decomposition at (a) 470 and (b) 440 °C. The observed pattern, calculated pattern and difference curve are shown in red, black line and blue, respectively. Blue, green and red tick marks correspond to Na8Si46 (type I), NaxSi136 (type II) and Si, respectively. HRTEM images and corresponding Fast Fourier Transforms (FFTs) of powders obtained at (c) 470 °C and (d) 440 °C. FFTs are indexed along the (c) type I and (d) type II silicon clathrate structures.

Figure IV- 12 a

 12 Figure IV-12 a lattice parameter evolution with the Na content of NaxSi136 (0 ≤ x ≤ 24) clathrate.

  of ternary nickel silicophosphide nanoparticles. 88 mg (1.5 mmol) Ni nanoparticles, 45 mg (0.21 mmol) Na4Si4, 2.9 g LiI (21.7 mmol) and 2.1 g KI (12.7 mmol) were ball-milled during 2 min at 20 Hz to get a homogeneous mixture, which was later transferred in a glassy carbon crucible (Ø10×H200 mm). The crucible was placed in a bottom closed quartz tube and then connected to Schlenk line under Ar flow. A vertical furnace from Eraly® was preheated to the reaction temperature of 500 °C. Then, the quartz tube was put in the furnace. After holding the temperature for 4 h under Ar flow, the hot quartz tube was taken out and cooled down. The as-prepared mixture was washed in methanol by seven cycles of centrifugation/redispersion and was later dried in a Schlenk tube under vacuum overnight. The dried powder was transferred and stored in an Ar-filled glovebox.Working electrode preparation. Two types of working electrode substrates were used for different measurements. A glassy carbon (GC) rotating disk electrode (RDE) with a diameter of 5 mm was used for cyclic voltammetry (CV) and linear sweep voltammetry (LSV). A GC sheet was used for chronopotentiometry (CP) measurements. The catalyst film was prepared by drop casting. 3.5 mg of catalyst was sonicated in 480 μL absolute ethanol for 60 min, followed by adding 20 μL of Nafion solution (5% in alcohols and water, Sigma-Aldrich) for another 15 min of ultrasonication. The well-dispersed ink was then dropped onto the as-polished RDE or the GC sheet substrate to obtain a catalyst loading of 350 μgcatalyst.cm -2 electrode, and then the coating was dried under air for 30 min.

3 . 1

 31 Synthesis and characterizationThe P-doped Ni nanoparticle precursors of ca. 20 nm (Figure V-1a-f) has been obtained by a reported colloidal synthesis involving the reaction between Ni(acac)2, oleylamine (OA) and trioctylphosphine (TOP)31 . Here, TOP is used as surface stabilizing agent and phosphorus source (equivalent TOP per Ni = 0.6). The P/Ni atomic ratio of P-doped Ni nanoparticles determined by scanning electron microscope -analysis energy-dispersive X-ray spectroscopy (SEM-EDX) is 0.17 (TableV-2). The Si source Na4Si4 containing [Si4] 4-species was synthesized through the reaction of commercial Si nanoparticles and NaH (Figure V-1g).

  P-doped Ni nanoparticles and Na4Si4 were reacted in molten LiI-KI at 500 °C for 4 h under Ar flow. After reaction, the salts were removed from the resulting mixture by washing in methanol (see experimental section for more details of the synthesis). Following the same synthesis protocol by changing the TOP amount used in Ni the precursor synthesis (equivalent TOP per Ni = 0.2) results in orthorhombic Ni2Si as main phase (Figure V-2).

Figure V- 1

 1 Figure V-1 Ni nanoparticles used as reagents for Ni2Si0.75P0.25 synthesis: (a) powder XRD pattern, red drop lines indicating Ni reference, (b) TEM image and (c) size distribution. Ni nanoparticles used as reagents for Ni2Si synthesis: (d) powder XRD pattern, red drop lines indicating Ni reference, (e) TEM image and (f) size distribution. (g) Powder XRD pattern of as-prepared Na4Si4 used as reagent. Red drop lines indicate Na4Si4 reference.

Figure V- 2

 2 Figure V-2 Ni2Si nanoparticles: (a) powder XRD pattern of as-prepared Ni2Si. Black, blue, and red lines indicate o-Ni2Si, h-Ni2Si and Ni2Si0.75P0.25 references. (b) TEM image and (c) HRTEM and FFT of the corresponding area. The FFT can be fully indexed along the o-Ni2Si structure.

2 )

 2 , the atomic ratios Ni/Si/P of nickel silicophosphide are 2/1.1/0.3. Excess Si is present as crystalline Si and Ni2Si (Figure V-3a) or

Figure V- 3

 3 Figure V-3 (a) XRD patterns of the as-prepared Ni2Si0.75P0.25 nanoparticles. The red drop lines denote the references of Ni2Si0.75P0.25. The asterisks and circles mark respectively the indicative peaks of Si and o-Ni2Si references. The inset shows the crystal structure of Ni2Si0.75P0.25. (b) TEM image of the as-prepared Ni2Si0.75P0.25 nanoparticles. (c) HRTEM image and Fast Fourier Transform (FFT, inset) of the corresponding area. (d) STEM-HAADF image and EDX chemical mapping of Ni2Si0.75P0.25 nanoparticles.

Figure V- 4

 4 Figure V-4 Comparison of XRD patterns of Ni2P and Ni2Si0.75P0.25.

Figure V- 5

 5 Figure V-5 XPS of Ni2Si0.75P0.25 nanoparticles: (a) Ni 2p3/2 and (b) Si 2p region. (c) The comparison of P 2p regions for Ni2Si0.75P0.25 nanoparticles and P-doped Ni nanoparticles used as precursor.

Figure V- 6

 6 Figure V-6 XPS of o-Ni2Si nanoparticles in (a) Ni2p3/2, (b) Si 2p, (c) P 2p region.

Ni2Si0.75P0. 25

 25 and P-doped Ni (Figure V-7b) at 2142 and 2150 eV are attributed to elemental P (2142 eV) and phosphates (2150 eV).

Figure V- 7

 7 Figure V-7 XANES on (a) Si K-edge (b) P K-edge of Ni2Si0.75P0.25 and P-doped Ni nanoparticles.

Figure V-8a-c show

  Figure V-8a-c show electrocatalytic properties in OER of Ni2Si0.75P0.25 and Ni2Si with a comparison of their 1 st and 25 th cyclic voltammograms (CVs). The reversible anodic wave at ca. 1.5 V of CV curves (Figure V-8a) correspond to oxidation of nickel species.47 For Ni2Si, an activation process can be observed, i.e., the water oxidation current increases along with the Ni oxidation/reduction wave area upon cycling. This correlated increase indicates that the highvalence Ni involved in the reversible oxidation is the active surface site of the catalyst. Its increase upon cycling indicates a progressive surface evolution of the material which results in an enhanced electrocatalytic activity.

Figure V- 8

 8 Figure V-8 Electrochemical measurements of Ni2Si0.75P0.25 and Ni2Si nanoparticles in an aqueous Fe-free O2-saturated 0.1 M KOH electrolyte. OER electrocatalytic properties: (a) CV curves (b) Nyquist plot recorded at 1.80 V vs. RHE. (c) CP measurements at 10 mA cm -2 for Ni2Si0.75P0.25 nanoparticles. HER electrocatalytic properties: (d) CV curves (e) Nyquist plot recorded under -0.60 V vs. RHE. (f) CP measurements at -10 mA cm -2 for Ni2Si0.75P0.25 nanoparticles. Inset shows the EIS of the electrode before (blue dots) and after (red dots) CP measurement.

Figure V- 8e )

 8e Figure V-9a-b). The HRTEM images (Figure V-9c-d) show the amorphous nature of these oxide layers, while the crystalline core can be fully indexed to Ni2Si0.75P0.25. This partial oxidation reveals the good corrosion resistance of Ni2Si0.75P0.25 nanoparticles under anodic and cathodic conditions of water electrolysis. It is further confirmed by elemental mapping on post OER/HER samples (Figure V-9e-f), which demonstrates the homogeneous distribution of Ni, Si and P in Ni2Si0.75P0.25 nanoparticles after electrocatalysis. Compositional analysis carried out by SEM-EDX (Table V-4) shows that Si was partially leached from the nanoparticles during electrocatalysis. On the opposite, P was maintained in the material. Note that after initial leaching of Si, the Ni/(Si+P) ratio approaches the expected value for the sole silicophosphide.

Figure V- 9 Table V- 4

 94 Figure V-9 Ni2Si0.75P0.25 nanoparticles: TEM images (a) after 5 h OER and (b) after 15 h HER, HRTEM images and FFTs of the corresponding areas (c) after 5 h OER and (d) after 15 h HER, STEM-HAADF image and STEM-EDX chemical mapping after 15 h HER (e) after 5 h OER and (f) after 15 h HER.

Figure V- 10

 10 Figure V-10 XPS of as-prepared electrode, post 5h OER electrode and post 15h HER electrode in Ni 2p3/2, Si 2p and P 2p regions.

Figure V- 11 V. 4 Conclusion

 114 Figure V-11 XANES at (a) Si K-edge and (b) P K-edge of post electrocatalysis on as-prepared electrode (t = 0), electrode after 5 h OER and electrode after 15 h HER.

  [START_REF] Chen | Structure Investigation and Dibenzothiophene Hydrodesulfurization Properties of Fe-Substituted Ni-Si Intermetallics[END_REF]

  Solution synthesis of transition metal silicide nanoparticles is thus based on relatively high boiling point organic solvents. Fe3Si

	nanoparticles have been produced by using o-1,2-dichlorobenzene as solvent at 220-250 °C . 162
	McEnaney et al. reported a colloidal synthesis achieving Ni2Si, Pd2Si and Cu3Si nanoparticles at
	375 °C in a mixture of trioctylamine and squalane. 101 Even if they were mentioned as high
	temperature solution synthesis, it can be noticed that the reaction temperature in liquid medium
	is few hundred degrees lower than solid-state or gas-solid reactions. Conventional organic
	solvents can only be operated below 400 °C, 166 which is still limiting the exploration of metal
	silicide nanomaterials. Indeed, ordering the Si-Si bonds might require higher energy, so that most
	metal silicides obtained in solution synthesis are only metal-rich phases.

Table I-3, Kumar et al. recently explored a reaction between metal salts and Na4Si4 in LiI-KI molten medium to achieve Ni2Si

.5 Summary and strategy of this work

	IHydrogen produced by water electrolysis is an auspicious solution to the transient
	production of renewable energy. In order to lower the electricity consummation of electrolysis,
	electrocatalysts are required for kinetically slow electrode reactions of water electrolysis, especially
	for OER. High valence NiFe-based materials are the benchmarks of precious metal-free OER
	electrocatalysts to date. Because NiFe oxide/hydroxide/LDH electrocatalysts usually suffer from
	low electrical conductivity and self-agglomeration, hybrid electrocatalysts with an additional
	conductive material such as graphene, reduced graphene oxide, or carbon nanotubes are required.
	xCoxSi, Nevertheless, considering the potential instability of carbon-based supports under OER 172,173
	MnSi1.8, 174 FeSi@SiO2, 175 conditions, other electrocatalysts have been designed to overcome the conductivity problem.
	MnSi@SiO2 176 NiFe-p-block-element intermetallic compounds such as NiFe sulfides, borides, phosphides, etc.,
	ferrocene-polydimethylsilane, exhibit high electrical conductivity. Nanoparticles of these phases have been demonstrated highly
	Pyrolysis of metal-containing active on OER electrocatalysis, owing to their complete or partial oxidation during OER. Those poly(ferrocenylsilane), nickel-containing polycarbosilanes, polydimethylsilane-Ni(acac)2, Supported nanoparticles Fe3Si@SiO2, 175 Fe3Si@SiC/C, 177 Ni-Si@ceramics, 178 only partially oxidized at the surface form stable core-shell nanocomposite electrocatalysts, with
	polysilane an intermetallic conductive core and a highly electrocatalytically active oxidized shell. Due to the aminopyridinato palladium Pd2Si@SiCN 179
	complex rare occurrence of metal silicide nanoparticles, NiFe silicide nanoparticles only containing earth-
	Nanostructuration of transition metal silicides could bring enhancements on many physical abundant elements, have never been evaluated as OER electrocatalysts. Attracted by the diverse
	properties (e.g., thermoelectricity, magnetism, mechanical strength, etc.) and (electro)catalytic properties of metal silicides and their nanomaterials, the present work is dedicated to the design
	activity. However, despite the large versatility of silicide phases achieved as bulk materials or as of metal silicide nanoparticles.
	2D nanowires, the occurrences of transition metal silicide nanoparticles remain few. Moreover,
	many of them are supported nanoparticles. For instance, cobalt silicides have been only reported
	as supported nanoparticles. 139,169,170,180-182 Since the interaction between nanoparticles and the

Table II -

 II

	Catalysts	Rct (Ω)	Ref.
	NiFe LDH	1800	17
	NiFe oxide	50	18
	NiFe LDH/carbon network	353	19
	NiFe LDH	875	19
	NiFe LDH	70	20
	NiFe2O4/Ni(OH)2	500	21
	Catalysts	Rct (Ω)	Ref.
	NiFeSi-12	17	This work
	NiFeSi-50	5	This work
	Ni-Fe disulfide	10	14
	Ni-Fe hydroxide	17.5	14
	(Ni0.75Fe0.25)2P	5

1 Comparison of the conductivity of reported OER electrocatalysts according to their charge transfer resistances (Rct) obtained from electrochemical impedance spectroscopy (EIS). 16 NiFe borate/reduced graphene oxide 38 10 NiFe borate 81 10 NiFe LDH/oxidized graphene/carbon nanotube 100

Table II - 2

 II2 Reactant masses, amounts and temperatures for syntheses of Ni, Fe and NiFe silicides. Electrode preparation. A Pt wire and Ag/AgCl in saturated KCl electrode were respectively used as counter electrode and reference electrode for a typical three-electrode configuration. There are two types of substrates used for the working electrode in this work. A

	glassy carbon (GC) rotating disk electrode (RDE) with a diameter of 5 mm was used for cyclic
	voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy
	(EIS) measurements, and a GC sheet was used for chronopotentiometry (CP) measurements. All
	catalyst inks were prepared with the same protocol. 3.4 mg of catalyst was sonicated in 480 μL
	absolute ethanol for 15 min, followed by adding 20 μL of Nafion solution (5 % in alcohols and
	water, Sigma-Aldrich) for another 10 min of ultrasonication. A given volume of well-dispersed
	Samples	Reactant / mg (mmol) Si NiCl2	FeCl2	Reaction temperature (°C)
	Ni2Si	42.1 (1.5)	194.4 (1.5)	/	300
	NiFeSi-50	52.5 (1.9)	243.0 (1.9)	47.5 (0.4)	300
	NiFeSi-12	42.1 (1.5)	146.0 (1.1)	47.5 (0.4)	300
	NiFeSi-3	84.3 (3.0)	194.4 (1.5)	190.1 (1.5)	300
	FeSi	63.2 (2.3)	/	190.1 (1.5)	400
	Synthesis from bulk Si powder. 42.1 mg bulk Si powder (1.5 mmol), 194.4 mg NiCl2
	(1.5 mmol) and 5 g LiI-KI (0.63:0.37 mol.) were ball-milled during 2 min. The synthesis protocol
	described above was kept. The as-obtained mixture was washed in methanol and in deionized
	water, respectively by six cycles and two cycles of centrifugation/redispersion, and was later dried
	under vacuum.

NiFeSi-X where X is the Ni/Fe atomic ratio according to scanning electron microscopy (SEM)energy dispersive spectroscopy analysis (EDX) corresponding to the o-Ni2Si structure. Synthesis under Ar. 42.1 mg Si nanoparticles (1.5 mmol), 194.4 mg NiCl2 (1.5 mmol) and 5 g LiI-KI (0.63:0.37 mol.) were ball-milled during 2 min. The same synthesis procedure was followed with the same setup as the synthesis described above, except that the reaction was under Ar flow instead of vacuum. The as-prepared solid was washed in methanol and in deionized water, respectively by six cycles and two cycles of centrifugation/redispersion, and was later dried under vacuum.

Synthesis without molten salts. 42.1 mg Si nanoparticles (1.5 mmol) and 194.4 mg NiCl2 (1.5 mmol) were ball-milled during 2 min. Later, the same procedure was followed. After thermal treatment, the resulting powder was directly collected. ink was dropped onto the GC sheet or the as-polished RDE substrate to obtain a catalyst loading of 170 μgcatalyst.cm -2 electrode, and then the substrate was dried for at least 30 min under air. IrO2 nanoparticles were deposited according to the same protocol.

Table II - 3

 II3 Elements atomic ratio of as-prepared Ni, Fe and NiFe silicide samples determined by SEM-EDX.

		Ni2Si	NiFeSi-50	NiFeSi-12	NiFeSi-3	FeSi
	Ni	1.68	1.96	1.85	0.84	/
	Fe	/	0.04	0.15	0.31	0.58
	Si	1	1.17	1.74	1	1

Table II

 II

	Electrode	As-prepared	After 20 CVs	Post 85h-OER
	Rct (Ω)	6.17	5.34	5.23
	Rads (Ω)	14.9	6.87	0.11

-4, Rct decrease during activation, which could relate to the dissolution of the native Si-rich oxide layer. The decrease of Rads indicates a more efficient adsorption/desorption process on the catalyst surface after activation.

Table II-4 Comparison of Rct and Rads for electrodes (NiFeSi-12) at different stages of electrolysis.

Table II -

 II 5 OER electrocatalytic activity and stability of previously reported NiFe-based catalysts in 0.1 M KOH on GC substrate.

	Catalyst	Loading amount (mg cm -2 )	Overpotential (mV) @10mA cm -2	Stability (h)	Tafel slope (mV dec -1 )	Ref
	NiFeSi-50	0.17	337	85	63.8	This work
	NiFeSi-12	0.17	345	85	85.4	This work
	NiFeSi-3	0.17	393	85	75.3	This work
	nNiFe-LDH/NGF a)	0.25	337	3.3	45	61
	Amorphous Ni70Fe30(OH)x	0.14	292	2	30.4	62
	NiFe2O4/α-Ni(OH)2 b)	~0.2	340	2.7	41	21
	Ni3FeN	~0.13	355	3.3	70	12
	Fe 3+ -doped β-Ni(OH)2	0.25	260	10	32	63
	NiFe-N/C c)	0.4	320	2000 cycles	44.2	64
	NiFe-NC d)	0.2	380	2.8	115	65
	graphene/NiFe (oxy)hydroxides	0.25	372	2.2	76	66
	Ni II Fe III @NC	0.29	397	12	81	67
	NiFe-LDH NS e)	0.14	290	8	33.4	68
	Porous NiFe Oxide	0.14	328	12	42	18
	Ni2Co III Fe-LDH/N-GO f)	0.18	317	8.3	74.1	7
	Ni3Fe/N-C	0.13	370	3.3	77	69
	NiFe/C	0.5	330	21	57	70
	m-NiFe/CNx	0.51	360	5.6	59.1	71
	NiFe LDH/oGSH g)	0.25	350	2.8	54	17
	NiFe LDH	0.1	360	2	51	72
	NiFe@NCX	0.4	320	10	60.6

  in 0.1 M KOH in the first 65 h. Because the composition of the electrode material does not evolve after 65h and the conductive silicide core is maintained, the amorphous ~1.5 nm-thick intermediate layer also indicates that this shell is denser than the outer Fe-doped NiOOH shell, which implies further reactivity with water to form the outer oxyhydroxide shell. The intermediate layer is buried below this outer shell and is then not in contact with water. Overall, these reactivity considerations support the interpretation that this intermediate layer is a mixture of amorphous nickel oxide and silicates that may be doped with iron. Elemental composition determined by SEM-EDX of a pristine NiFeSi-12 sample and of corresponding post mortem electrode materials after 65 and 85 h of OER operation.

	observed by TEM (Figure II-12b-d) should play a role in passivating the silicide core and
	protecting it from oxidation and silicon leaching. XPS (Figure II-13) shows that this intermediate
	shell contains metal oxide, silicate and/or silicon oxide. This passivation layer generated in anodic
	and basic conditions can protect the silicide core from further dissolution. 76 TEM (Figure
	II-12b-d) Table II-6 Pristine	Post 65h-OER	Post 85h-OER
	Ni	1.85	1.85	1.85
	Fe	0.15	0.12	0.11
	Si	1.74	0.77	0.76

  this work, freestanding CoSi nanoparticles are achieved through the reactivity of cobalt chloride and Si nanoparticles in molten salts based on the synthesis approach developed in the previous chapter. The obtained CoSi nanoparticles show either homogeneous or core-shell

morphologies under different reaction temperatures (300-400 °C). The obtained CoSi nanoparticles show ferromagnetism below 300 K. Then, the OER electrocatalytic activity and stability of core-shell CoSi nanoparticles have been studied. Core-shell CoSi nanoparticles are a pre-catalyst for OER, yielding electrocatalytic activity surpassing IrO2 in the same conditions.

Table III - 1

 III1 Comparison of the magnetic properties of different Co-based materials.

	Phase Morphology	Temperature (K)	Coercivity (Oe)	Saturation magnetization μB/Co emu/g	Ref.
	CoSi	Polycrystalline core-shell nanoparticles	300 5	15 945	0.17 0.22	7.7 10.6	This work This work
	CoSi	Polycrystalline nanoparticles (50 nm)	300 5	5 1000	0.05 0.07	2.6 3.6	This work This work
		Single crystal
	CoSi	nanowires	300	460	0.03	1	11
		(20-60 nm)
	CoSi	Thin film	-	non-magnetic		7
	CoSi	Bulk	<20	diamagnetic			9
	Co3B	Nanoparticles (10-20 nm)	300 5	1300 1400	-1.02	-91	33 33
	Co	Nanoparticles (1.5 nm)	5	-	1.94	185	43
		Nanoparticles
	Co	(1.5 nm) surface	5	-	0.58	55	43
		covered by CO
	Co	Thin film	4.2	1000	1.72	166

Table III -

 III 2 OER electrocatalytic normalized activity and stability of previously reported Cobased catalysts in 0.1 M KOH on GC substrate.

	Catalyst	Loading amount (mg cm -2 )	Overpotential (mV) @10mA cm -2	Overpotential (mV) @40mA cm -2 gCo -1	Stability (h)	Ref.
	Core-shell CoSi	0.17	410	360	15 h	This work
	Amorphous Co2B 0.21	380	370	60 h	22
	CoSe2	0.20	484	420	/	45
	CoB	0.21	370	390	51 h	46
	Co2B	0.20	405	360	1000 cycles

Table IV - 1

 IV1 Comparison of reported syntheses towards Na4Si4

	Na+Si Na in 10 mol.% excess	W crucible canister sealed in stainless steel	/		36h	650 °C	Na in excess	/	10
	Reactants Na+Si Stoichiometric Setup mixture Sealed Ta tube Sealed	Ball-milling time /	Reaction time 1h10min 800-1200 °C Reaction temperature Purity 44% (Si Purification in / excess) procedure	Ref. 3
	NaH+Si nanoparticles Na in 10 mol.% Na+silica gel BN crucible in a quartz flask Covered h-Erlenmeyer 2min /	24h	/	395 °C 400 °C	98% 15%	/	/	work This 13
	excess	tube
	NaH+Si Na in 90 mol.% excess	Covered alumina crucible in glass tube a silica	30min 48h		420 °C	Na XRD detected by	/	14
	NaH+Si Na in 60 mol.% excess	Covered alumina crucible	1h		48h		395 °C	/	Excess Na removed by vacuum at 250°C for 3h

Table IV - 2

 IV2 Lattice parameters obtained by Le Bail analysis of the XRD pattern of the clathrate powders obtained by Na4Si4 thermal decomposition at 470 and 440 °C.

		Space group	a470°C (Å)	a440°C (Å)
	Na8Si46	Pm-3n	10.1943(1)	10.2050(3)
	NaxSi136	Fd-3m		14.6581(1)
	Si	Fd3-m	5.4248(1)	5.4418(2)
	χ 2		4.78	4.17

Table V - 1

 V1 Reported Si-and P-rich transition metal silicophosphides and their properties.

	Properties	Phase	Precursors Synthesis method Morphology	Ref.
		Cu4SiP8 Cu, red P, Si Mechanochemistry	Micrograin	6
		CuSi2P3 Cu, red P, Si Mechanochemistry	Micro to nano grain 25
	Li-ion or Na-ion	CoSi3P3 Co, red P, Si Mechanochemistry	Micrograin	8
	battery anode	ZnSiP2 Zn, red P, Si Mechanochemistry	Micrograin	9
	materials	ZnSi2P3 Zn, red P, Si Mechanochemistry	Micrograin	10
		FeSi4P4 Fe, red P, Si Mechanochemistry	Micrograin	11
		FeSi4P4 Fe, red P, Si Mechanochemistry	Micrograin	26
		MnSiP2 Mn, red P, Si Solid-state reaction Bulk	14
	Nonlinear optical materials	RuSi4P4 RuSi4, red P Solid-state reaction Bulk	13
		IrSi3P3	IrSi3, red P	Solid-state reaction Bulk	13
	Thermoelectric	CuSi2P3 Cu, red P, Si Solid-state reaction -	27
	materials	CuSi4P3 Cu, red P, Si Solid-state reaction -	27

Table V - 2

 V2 Ni, Si and P ratio determined by EDX of Ni nanoparticle precursors and of their corresponding products.

		Ni	Si	P
	Ni nanoparticles (high P content)	2	/	0.3
	Ni2Si0.75P0.25	2	1.1	0.3
	Ni nanoparticles (low P content)	2	/	0.1
	Ni2Si	2	1.3	0.1

support could change the exposed facets and surface electronic states of nanoparticles, 183 the chemical and physical properties could be significantly changed for freestanding nanoparticles.

Amongst the synthesis approaches reported to date, freestanding nanoparticles of a small portion of transition metal silicide phases have been achieved through microwave-assisted synthesis, 134 chemical vapor deposition on metal oxide nanoparticles, 90,91 solution synthesis, 101,162,163 and molten salt synthesis. 87,164 Widening the range of transition metal silicide freestanding nanoparticles is not only an intriguing topic in fundamental research, but also an opportunity to develop promising materials with high cost-efficiency for the industry because of the crustal abundance of silicon.

The methods for their synthesis are thus the essential scientific bottleneck.

was washed in methanol by seven cycles of centrifugation/redispersion and was later dried in a Schlenk tube under vacuum during the night. The dried powder was transferred and stored in an Ar-filled glovebox.

Electrochemical measurements A typical three-electrode configuration was used, which includes a working electrode, a Pt coil as counter electrode and a Ag/AgCl saturated KCl electrode as reference electrode. There are two types of substrates used for the working electrode in this work. A glassy carbon (GC) rotating disk electrode (RDE) with a diameter of 5 mm was used for cyclic voltammetry (CV). Glassy carbon sheets were used for chronopotentiometry (CP) measurements. The working electrode was prepared by drop casting. 3.4 mg of catalyst was sonicated in 480 μL absolute ethanol for 15 min, followed by adding 20 μL of Nafion solution (5% in alcohols and water, Sigma-Aldrich) for another 10 min of ultrasonication. The welldispersed ink was then dropped onto the as-polished RDE or the GC sheet substrate to obtain a catalyst loading of 170 μgcatalyst.cm -2 electrode, and then the substrate was dried for at least 30 min under air.

A custom-made PMMA electrochemical cell was used for all measurements. The electrolyte was bubbled with O2 for at least 20 min before measurement. The rotating rate of the RDE was set at 1600 rpm for all cyclic voltammetry (CV) measurements. Cyclic voltammograms were recorded from 0 to 1.05 V vs. saturated KCl Ag/AgCl reference electrode at scan rate of 20 mV s - 1 , until overlapped cycles were obtained. For the chronopotentiometry (CP), the current density was set at 10 mA cm -2 to monitor the potential evolution.

The ohmic drop iR compensation was considered for the potentials reported, where the resistance is approximately equal to the total impedance measured at a frequency of 50 kHz. The potentials vs. RHE are obtained from conversion of measured potentials based on the equation: ERHE = Emeasured + 0.197 + 0.0591× pH.

PDF Refinements

were performed for the core-shell CoSi nanoparticles using PDFGui 30 in the range 1.3-20 Å. CoSi with 9 nm coherence length was used as a starting point. Cell parameters, scale factor, thermal parameters, atomic positions and peak broadening effect (sratio and rcut) parameters were allowed to vary, and lead to an acceptable reliability factor of 20.9 %.

Crystalline Si phase and amorphous SiO2 (coherence length 8 Å) were added in the structural model. The reliability factor R for (CoSi/amorphous SiO2/crystalline Si) mixture decreased down to 15.8 %, better than CoSi alone, CoSi/amorphous silica (R=20.1 %) or CoSi/crystalline Si (R=19.7 %). This is the decrease of the reliability factor that allows us to validate the addition of centered at -362 and -366 ppm corresponding respectively to the two crystallographic nonequivalent Si sites in the crystal structure. 5 This upfield shift compared to silicon is due to electron transfer from Na to Si. 8 Besides, neither broad signal of amorphous SiO2 and Si centered at 24 h. Note that the XRD pattern obtained at 10 hours is similar to the one previously reported for Na4Si4 produced from bulk silicon over 2 days. 7 ) resulted in excess Si, while residual Na was observed when using moderate argon flow rate (10 mL min -1 ). 7 This reaction sensitivity versus gas-flow-rate comes from two sides. Firstly, H2 gas forms during the reaction (1).

Secondly, reaction (1) proceeds by decomposition of NaH at ca. 317 °C22 into Na, which then which is 120 mV higher than in KOH with presence of Fe. 50 Note that the high overpotentials of the ternary silicophosphide and binary silicide compared to the reported OER catalysts might be related to the lack of Fe in the catalytic layer as for Ni2P. [50][51][52] We then questioned the role of the incorporation of both nonmetal elements on electrocatalysis.

Electrochemical impedance spectroscopy (EIS) was performed before the 1 st CV and after

General conclusion

The rare occurrence of transition metal silicide nanomaterials limits the investigation of their properties. This thesis is dedicated to the exploration of synthesis pathways towards transition metal silicide nanomaterials, and then to the study of their electrocatalytic for water splitting and magnetic properties.

In this thesis, two synthesis approaches in molten salts using nanoparticles as precursors have been developed towards nanoparticles of transition metal silicides (M-Si, M = Ni, Fe, NiFe, Co) and ternary nickel silicophosphides (M-Si-P). The electrocatalytic properties in alkaline OER

were measured for all materials. Ternary nickel silicophosphide nanoparticles were also assessed as HER electrocatalysts in alkaline media. Post mortem studies were conducted to investigate the evolution of catalysts on phase, composition and morphology. Apart from electrocatalysis, the magnetism of cobalt monosilicide nanoparticles was evaluated.

The first synthesis method used the redox reaction between Si nanoparticles and metal chlorides at 300-400 °C in a low vapor pressure molten salts LiI-KI, which allows to run reaction under dynamic vacuum to remove volatile byproducts. Ni2Si, FeSi and NiFe silicide nanoparticles of ca. 40 nm were achieved. The reaction temperature in our synthesis method is hundreds of degrees below the reaction temperature of traditional solid-state synthesis towards these phases.

The enhanced reactivity is achieved through three features of this synthesis method: (1) high temperature liquid media provided by inorganic solvents, (2) dynamic vacuum shift of reaction equilibria and (3) a reactive Si nanoparticle precursor. The latter also enables a better control over particle morphology by acting as a nanotemplate.

Among Post mortem studies revealed that the core of silicophosphide nanoparticles maintain its intermetallic nature after OER and HER electrocatalysis, while a thicker amorphous P-poor oxide layer was observed. Further mechanistic study on the role of Si and P in electrocatalysis will be conducted in the future.

Overall, the two synthesis pathways developed in this work provide a new insight of the design of transition metal silicide nanomaterials in molten salts. Preliminary results on ternary silicophosphide show the potential of these methods towards ternary or multinary compounds, which deliver improved properties in (electro)catalysis. Furthermore, the post mortem studies conducted in this thesis provide a basis of electrocatalytic mechanism of transition metal-p block element compounds for future research on multinary catalysts. Samples were transferred to ZrO2 rotors spun at a MAS rate of 10kHz for 29 Si and 20kHz for 23 Na. A single-pulse excitation was used with a recycle delay of 500s and 400 scans for 29 Si and 1s and 1960 scans for 23 Na. 29 Si and 23 Na chemical shifts were referenced to TMS and 0.1 M NaCl (aq) respectively. The spectra were treated with the DMFIT program. 31 Scanning electron microscopy (SEM) was carried out on a Hitachi SU-70 FESEM.

Appendix

Postmortem electrodes were observed without any surface coating, accelerating voltage settings as indicated in images.

Energy-dispersive X-ray spectroscopy (EDX) (Oxford Instruments -X-max) on the Scanning electron microscope (SEM) was performed on a SEM HITACHI S3400N at 20 kV to determine the sample composition. Cobalt was used for calibration of quantitative analyses.

Powder samples were flatly smeared on a carbon adhesive tape on sample holder. Carbon deposition was required for the Na4Si4 sample to enable the EDX analysis on insulating sample.

In order to avoid equipment damage and for safety concern, air-sensitive Na4Si4 samples were pre-neutralized by exposure to air during 4 days. X-ray absorption near edge structure (XANES) data were collected on the LUCIA in SOLEIL. The incident beam energy was monochromatized using a Si (111) double crystal monochromator. Data were collected in a primary vacuum chamber as fluorescence spectra with an outgoing angle of 5 ° using a Bruker silicon drift detector. The data were normalized by the intensity of incident energy and processed with the Athena software. Powder samples were measured as an ingot prepared by pressing mixtures of nanoparticles and boron nitride powders.

Transmission electron microscopy (TEM

Electrode samples were measured as prepared and as recovered for electrochemical measurement.

Song Yang -Thè se de doctorat -2021 Abstract:

Transition metal silicides are a family of intermetallic compounds, which have been widely studied as functional materials in integrated circuits, thermoelectricity, superconductivity, magnetism and heterogeneous catalysis. Nanostructuration offers the opportunity to extend the frontier of silicon-based materials science with novel phases and diverse properties. However, building transition metal silicides encompassing relatively high energy bonds usually requires high temperatures, which are not conducive for nanomaterial design and not compatible with the traditional colloidal synthesis methods. In this thesis, molten salts syntheses based on element insertion into nanoparticles are developed. Transition metal silicide nanoparticles (M-Si, M=Ni, Fe, NiFe, Co) and a ternary nickel silicophosphide are crystallized in high temperature inorganic solvents, where a diluted and carbon-free environment is provided. The obtained silicide nanoparticles are investigated in electrocatalysis of alkaline water oxidation and magnetism. NiFe silicides demonstrate outstanding activity and stability arising from an original in situ generated core-shell-shell structure, while defect-rich CoSi nanoparticles show an unusual surface related ferromagnetism. Moreover, the study of silicophosphide nanoparticles provides an insight in multinary material design in molten salts and the role of nonmetal elements in overall alkaline water splitting electrocatalysis.

Keywords: [transition metal silicides, nanoparticles, synthesis in molten salts, overall alkaline water splitting, magnetism]

[Conception de nanoparticules de siliciure de métal en sels fondus : propriétés électrocatalytiques et magnétiques]

Ré sumé :

Les siliciures de mé taux de transition sont une famille de composé s intermé talliques, qui ont é té largement é tudié s en tant que maté riaux fonctionnels dans les circuits inté gré s, la thermoé lectricité , la supraconductivité , le magné tisme et la catalyse hé té rogè ne. La nanostructuration offre la possibilité repousser les frontiè res de la science de ces maté riaux avec de nouvelles phases et des proprié té s diveres. Cependant, l'é nergie de liaison relativement é levé e des siliciures de mé taux de transition né cessite gé né ralement une tempé rature é levé e pour leur formation, ce qui n'est pas propice à la conception de nanomaté riaux et n'est pas compatible avec les mé thodes de synthè se colloï dales traditionnelles. Dans cette thè se, des synthè ses en sels fondus basé es sur l'insertion d'é lé ments dans des nanoparticules sont dé veloppé es. Des nanoparticules de siliciure de mé tal de transition (M-Si, M=Ni, Fe, NiFe, Co) et un silicophosphure de nickel ternaire sont cristallisé s dans des solvants inorganiques à haute tempé rature, où un environnement dilué et sans carbone est fourni. Les nanoparticules de siliciures obtenues sont é tudié es en é lectrocatalyse de l'oxydation de l'eau alcaline et du magné tisme. Les siliciures de NiFe dé montrent une activité et une stabilité exceptionnelles ré sultant d'une structure originale coeur-coquille-coquille gé né ré e in situ, tandis que les nanoparticules de CoSi riches en dé fauts pré sentent un ferromagné tisme inhabituel. De plus, l'é tude des nanoparticules de silicophosphure donne un aperç u de la conception de maté riaux multinaires dans les sels fondus et du rôle des é lé ments non mé talliques dans l'électrocatalyse de l'électrolyse de l'eau.

Mots clé s : [siliciures de mé tal de transition, nanoparticules, synthè se en sels fondues, global é lectrolyse de l'eau alcaline, magnétisme]