Aude Roland

Arthur Dupuy

Denis Machon

Frédérique Cunin

Nicolas Louvain

Bernard Fraisse

Abderaouf Boucherif

Laure Monconduit

In-depth study of annealed porous silicon: Understand the morphological properties effect on negative LiB electrode performance

Silicon (Si) used as negative electrode in a Li-ion battery (LIB) is highly attractive for its high energy density, safe cycling, and nontoxicity. However its alloying mechanism with Li induces material pulverization, which leads to a rapid capacity fade. In this work, annealing post treatment was used in order to tune the morphological properties of porous silicon. Playing on annealing temperature, the morphological modification induces electrochemical behavior changes in LIB. The porosification is an interesting way to accommodate the volume expansion occurring during the alloying process.

Increase of the annealing temperature leads to porous Si pores and walls reorganization, which has a positive impact on battery performance likely due to a higher wettability of the Si electrode with electrolyte. 700°C appeared to be the optimized annealing temperature.

I. INTRODUCTION

Silicon (Si) is one of the most attractive candidate material for next-generation Li-ion battery (LiB) anodes. Indeed, Si anode works at low potential (370 mV vs Li +/ Li) [1] leading to a high energy density and safe cycling conditions. It could theoretically lead to ten time higher gravimetric capacity (3579 mAh g -1 ) [START_REF] Xu | Nanosilicon anodes for high performance rechargeable batteries[END_REF] than currently commercialized graphite anodes. Furthermore, being the second most abundant element on earth it is inexpensive and it is nontoxic. However, despite its remarkable advantages, silicon shows some limiting intrinsic properties. The Li/Si alloying reaction induces a large volume expansion (310 %) causing material pulverization which leads to active material electrical isolation. [START_REF] Daniel | Handbook of Battery Materials: Second Edition[END_REF] This volume change is the first cause responsible for rapid capacity fade of silicon based negative electrodes. The use of nanostructured silicon is one of the key of success for Si-based negative electrode in Li-ion battery application. Indeed, reducing the silicon dimensionality limits the structural stress during the lithiation/delithiation mechanism responsible for its pulverization. Huang J. et al. have demonstrated that no cracks occur with a silicon particle size limit below 150 nm [1]. Moreover downsizing active materials influences Li ion diffusion kinetics and the stability of SEI layer [2, [START_REF] Roy | Nanostructured anode materials for lithium ion batteries[END_REF]. That is the reason why many efforts are presently still provided by proposing new rapid and cost-effective methods to produce nanostructured silicon materials. Many different types of structures have been produced and tested in Li-ion batteries applications: from 0D (Si nanospheres/nanoparticles), 1D (nanowires or nanotubes), 2D (Si thin films) to 3D nanostructured materials as porous silicon [START_REF] Zuo | Silicon based lithium-ion battery anodes: A chronicle perspective review[END_REF]. It is also well established that Si-C composites are efficient to stabilize the electrode electrochemical behavior. In fact limiting the Si content in the composite allows decreasing the volume expansion drawbacks [START_REF] Beattie | Si Electrodes for Li-Ion Batteries-A New Way to Look at an Old Problem[END_REF]. On the other hand controlling the interactions binder-Si [START_REF] Bridel | In Situ Observation and Long-Term Reactivity of Si/C/CMC Composites Electrodes for Li-Ion Batteries[END_REF], [START_REF] Ding | Improvement of cyclability of Si as anode for Li-ion batteries[END_REF], [START_REF] Mazouzi | Critical roles of binders and formulation at multiscales of silicon-based composite electrodes[END_REF] or improving the electron transfer by adding conducting coating at Si surface have shown to be interesting ways. [START_REF] Fu | Surface modifications of electrode materials for lithium ion batteries[END_REF] [START_REF] Terranova | Si/C hybrid nanostructures for Li-ion anodes: An overview[END_REF] The choice of the electrolyte plays also a key role in capacity retention especially since most of them are electrochemically unstable in the working potential window and lead to a solid electrolyte interface (SEI) continually generated on the Si electrode surface. In order to form stable SEI layer, chemicals additives like vinylene carbonate (VC) and fluoro-ethylene carbonate (FEC) can be added in the electrolyte. [START_REF] Du | Strategies to succeed in improving the lithium-ion storage properties of silicon nanomaterials[END_REF] Regarding cycling conditions, it is recognized that limiting the low potential window to prevent the Li15Si4 crystalline phase formation may improve the cyclability. Despite these numerous limitations, good electrochemical performance can be observed in literature for porous Si structures thanks to the free spaces in the material able to buffer the volume expansion. Highly porous Si sponge structure (80% of porosity, pore size around 50 nm, wall thickness of 10 nm) with high surface area of ~495 m 2 .g -1 , showed interesting performance with a capacity of 750 mAh g -1 and more than 80% of its capacity could be retained after over 1000 cycles [START_REF] Fang | Sponge-like reduced graphene oxide/silicon/carbon nanotube composites for lithium ion batteries[END_REF]. Even higher capacity has been reached (2500 mAh g -1 at a current rate of 0.5 C for 100 cycles) with 3D porous silicon particles [START_REF] Kim | Three-dimensional porous silicon particles for use in high-performance lithium secondary batteries[END_REF], presenting larger interconnected pores (200 nm) and wall thickness ( 40 nm). Similarly, Ge et al. reported porous doped Si nanowires synthesized from P-doped Si wafer [START_REF] Ge | Porous doped silicon nanowires for lithium ion battery anode with long cycle life[END_REF]. Such nanowires exhibit smaller pore diameter and wall thickness (8 nm) and lead to extremely stable cycling behavior over 2000 cycles. These studies verified that managing the porosity can improve the capacity retention by providing a free volume for the Si expansion and maintaining the structural integrity of the Si anodes. To the best of our knowledge, no systematic study showing the link between silicon morphological properties and electrode performance has been reported. For example, for mesoporous silicon sponge particles [START_REF] Li | Mesoporous silicon sponge as an anti-pulverization structure for high-performance lithium-ion battery anodes[END_REF], the optimal overall porosity was proposed around 70% to limit volume expansion under 75%. On the other hand Si with walls of 10 nm and pores of 10 nm have shown a capacity of 570 mAh g -1 and 87% of capacity retention over 1000 cycles. At last porous Si microparticles [START_REF] Ikonen | Electrochemically anodized porous silicon : Towards simple and affordable anode material for Li-ion batteries[END_REF] with an average pore size ranged between 5-16 nm and a porosity around 50% have shown a better behavior for 10-20 µm sized particles. Needless to say, a comparative analysis of Si based electrodes is tricky due to the huge variety of possible cycling conditions which have a considerable impact on performance.

Porous Si can be prepared following two classes of methods including top-down or bottom up. The top-down approach provides the pore formation from a bulk material while the bottom-up approach generally uses a porous template that will be filled and then dissolved. Generally, the top-down methods leads to material dissolution by hydrofluoric acid (HF) attack. The morphological properties of the porous material, as the pore shape and size, the walls shape and thickness or the overall porosity, can be tuned by controlling the applied current density and HF concentration [START_REF] Ge | Review of porous silicon preparation and its application for lithium-ion battery anodes[END_REF]. The electrochemical performance of the silicon electrode varies significantly with the morphology, and the latter is highly dependent on the nature (p or n) and concentration of the dopant within the Si wafer [19], [START_REF] Domi | Effect of Phosphorus-Doping on Electrochemical Performance of Silicon Negative Electrodes in Lithium-Ion Batteries[END_REF]. It is also possible to start the synthesis from porous silica and modify it by magnesiothermic reaction [START_REF] Jia | A novel approach to synthesize micrometer-sized porous silicon as a high performance anode for lithium-ion batteries[END_REF][START_REF] Shivaraju | High-rate and long-cycle life performance of nano-porous nano-silicon derived from mesoporous MCM-41 as an anode for lithium-ion battery[END_REF], aluminothermic reaction [START_REF] Mishra | High performance porous Si@C anodes synthesized by low temperature aluminothermic reaction[END_REF] or from an alloy as Si-Al dissolving specifically Al [START_REF] Xing | A novel durable double-conductive core-shell structure applying to the synthesis of silicon anode for lithium ion batteries[END_REF], and also from silicon particles by electroless etching [START_REF] Liang | Silicon-based materials as high capacity anodes for next generation lithium ion batteries[END_REF][START_REF] Shen | In Situ and Ex Situ TEM Study of Lithiation Behaviours of Porous Silicon Nanostructures[END_REF]. Those technics allow complex architecture but a lower control of the porous silicon morphology determined by the mother material, the global material porosity is more often discussed than the pores shape and dimensions. [START_REF] Ge | Review of porous silicon preparation and its application for lithium-ion battery anodes[END_REF] In this work, our approach consists in using electrochemical etching of silicon wafers as the easiest and most reproducible method to obtain highly porous silicon films and to tune their morphological properties by applying a rapid thermal annealing post treatment. This process allows a wide variety of pore morphologies and textures without modifying the dopant level. The main goal is to study the morphological properties impact on silicon-based anode performance in Li-ion batteries.

II. EXPERIMENTAL

A. Porous silicon preparation a. Electrochemical etching process Commercially available, boron doped p-type crystalline Si wafers (0.001 -0.002 Ω cm) were porosified in a single bath electrochemical etching Teflon cell. The Si wafer ((100) oriented) was placed between the Cu working electrode and Pt counter-electrode. The porous Si film was obtained by applying a pulsed current in a HF (49 %): ethanol (1:1, v:v) electrolyte. Anodic and cathodic current density were respectively 150 mA cm -2 and 0 mA cm -2 during a same pulse time of 1 sec. The total etching time was 100 min, which led to a 200 µm-thick porous silicon. Then, the separation of the porous Si layer from its mother substrate was performed by electropolishing at a current density of 300 mA cm -2 during 3 sec in an HF (49 %): ethanol (1:3, v:v) electrolyte. Finally, the porous film was rinsed with ethanol several times and dried under a N2 flow. The layer porosity was experimentally evaluated around 60 % by gravimetry (SI.1.).

b. Annealing post treatment

Rapid Thermal Annealing device, JetFirst from J.I.P.ELEC, was used to perform the annealing post-treatment. Samples were placed under a nitrogen environment and exposed to three different annealing temperature 500, 700 and 900°C respectively with a same duration of 30 sec.

c. Characterization i. SEM

The porosity evolution with annealing temperature was observed by scanning electron microscopy (SEM) using Hitachi S-4800. All dimension (pore diameters, wall thicknesses) measurements were realized using imageJ software.

ii. Nitrogen sorption analysis

The surface area was evaluated according to the Bruner-Emmett-Teller (BET) nitrogen gas sorption method with ASAP2000 system. Prior to each measurement, a degassing step of 12 h at 50°C under a pressure of 0.01 torr was performed.

iii. XRD -Rocking curve scan

The phase characterization was performed by powder X-ray diffraction (XRD) on a Philips X'Pert 2theta/omega diffractometer equipped with an X'Celerator detector and Cu Kα1 radiation (λ = 1.510562 Å). The results are displayed in SI.2. Rocking curve measurements have been carried out using a PANalytical X'Pert MRD diffractometer in high-resolution configuration. This configuration includes one mirror and one primary beam monochromator to remove the Kα2 and Kβ, so the radiation source is pure Cu-Kα1 (λ = 1.5405929 Å). As the wafer is (100) orientated, the measurements were carried out around the Si(400) reflection, using the substrate peak centered on 34.5645° ω as reference. The two axes ω and ϕ were systematically optimized before each measurement to ensure the reproducibility of the results. The spatial homogeneity of the sample has been verified by collecting five rocking-curves for different positions of the sample (sample center, centers of four quarter of the sample).

Rocking-curves ω scan were measured for different ϕ values, from 0° to 180°, to investigate the parallelism between the bulk and the porous frameworks. Cartography and rotation measurements are presented in SI.3.

iv. Surface wettability analysis

The contact angle analysis of the electrolyte solution droplets (LiPF6 1M in EC/PC/3DMC + 1 % VC + 5 % FEC) at the porous Si material surfaces was performed with an automated goniometer. 5 μL droplets were deposited and two images of their profiles were taken using a CCD camera (at 10 s time intervals) and ImageJ software for the analysis.

B. Preparation of electrodes and coin cell fabrication

Silicon porous films were grinded in agate mortar and used as active material, Super P (H30253 Carbon black Alpha Aesar) and Vapor growth fibers (VGCF-H, BET = 15 m 2 /g, Showa Denko) were used as conducting agent in a 50:50 weight ratio and sodium carboxymethyl cellulose (MW = 250 000; DS = 0.9, Sigma Aldrich) was used as binder. The electrode components were thoroughly mixed in a pH = 3 buffered aqueous solution (700 µL) with a weight ratio of 18:70:12 (Si:VGCG/SP:CMC) respectively to prepare a slurry. The slurry was spread on copper foil with a controlled thickness of 150 µm using a 3540 bird film applicator from Elcometer, and subsequently air-dried at room temperature for 24 h. The electrodes were then punched into 12.7 mm-diameter disks and dried at 80 °C under dynamic vacuum overnight. The dry electrodes were not calendered. The total material loading was approximately 1.2 mg cm -2 (0.25 mg cm -2 for Si), corresponding to a density of 0.4 g cm -3 approximately.

Coin cells (2032) were assembled in an Argon filled glovebox and used for the evaluation of electrochemical cycling performance. The coin cell was composed of the porous Si-based working electrode and a lithium foil at the counter electrode, the two electrodes were separated by a glass-fiber paper (GF/D, Whatman) soaked with the electrolyte. The electrolyte used was LiPF6 1 M in EC/PC/3DMC (Solvionic), 5%v of FEC (98% purity, Alfa Aesar) and 1%v VC (80 ppm BHT as stabilizer, 99% purity) were added.

C. Electrochemical measurements

The electrochemical galvanostatic measurements were performed between 1.5 and 0.01 V vs Li + /Li at a current density of C/20 (178 mA g -1 ) for the first cycle, referred to as the formation cycle, and then at C/5 (716 mA g -1 ) for the further cycles. The weight of carbons (Super P, carbon fibers) and the silicon particles were considered for the gravimetric capacity calculated per gram of silicon and carbon. The expected specific capacity is approximately 775 mA h g -1 (Si+C) corresponding to a theoretical amount of inserted Li + close to 0.40 in the electrode. This capacity is calculated by considering the molar composition of the composite from its mass composition (C0,9Si0,1), and the values of the experimental capacity of the carbon composite (185 mA h g -1 ) and theoretical capacity of silicon (3580 mA h g -1 ) considering the formation of the alloy Li15Si4 as an end-product of the lithiation. The expected capacity calculation with complete explanation is shown in SI. [START_REF] Roy | Nanostructured anode materials for lithium ion batteries[END_REF]. In all case, it is important to note that capacity value is not perfectly reproducible, these variations come from the masse evaluation depending on the film homogeneity. The following tests were carried out using a Neware BTS4000 battery cycler at 25 °C. All the electrochemical measurements were repeated a minimum of three times. Variable current density tests have also been carried out from C/20 to 20C to investigate the rate capability. In order to complete the textural organization understanding, nitrogen sorption analysis was performed. The results are presented in Fig. 2. PSi(REF) and PSi(500 °C) pre and post annealed materials respectively, exhibit type IV isotherms typical from mesoporous structures. In the case of PSi(REF) the hysteresis loop does not show parallel and vertical adsorption and desorption branches probably indicating a large pore size distribution and interconnected pores. After the first annealing at 500 °C (Fig. 2b), an important reduction of the surface area is observed (from 272 to 145 m 2 g - 1 ), consistently with the increase of the pore diameters. Moreover, the two adsorption/desorption branches are vertically aligned indicating a better pore size homogeneity. The isotherms obtained for PSi(700°C) and PSi(900°C) are consistent with a macroporous structure and exhibit lower surface areas with a decreasing contribution of the microporosity confirming the small pores coalescence and so the possibility to use annealing post treatment as an efficient way to tune the porosity.

III. RESULTS AND DISCUSSION

Figure 3: a) Rocking curve of porous silicon membrane PSi(REF) with different ϕ orientation b) Rocking curves of the annealed materials PSi(500°C), PSi(700°C), PSi(900°C) compared to PSi( REF).

No broadening nor shift of the Si peaks can be observed using regular Bragg-Brentano powder XRD (SI.2). Wafer crystalline purity and orientation are conserved by electrochemical etching and annealing post treatment. Higher accuracy is needed to observe the crystalline modifications (SI.2). The crystalline properties of the porous silicon (PS) film obtained by ω scan rocking curve X-ray diffraction are presented in Fig. 3. For the PSi(REF) sample presented in Fig. 3a), three peaks can be observed.

The peak at Ω = 34.57° corresponds to the expected position of the lattice planes parallel (400) to the Si(100) bulk Si surface reflection. The two other peaks observed at lower ω angle are due to the porous layer which suffers from lattice distortion and lower crystalline quality. As already demonstrated by Bellet et al., the porous lattice parameter mismatchs the wafer's one as a function of the layer porosity. [START_REF] Ogata | Structural change in p-type porous silicon by thermal annealing[END_REF]. These peaks appear at a smaller angle than the bulk silicon and thus corresponds to a higher lattice parameter. This shift toward lower angle is due to the chemical adsorption of hydrogen on the porous surface which constrains the lattice parameter [START_REF] Sugiyama | Microstructure and lattice distortion of anodized porous silicon layers[END_REF]. The two peaks coming from porous structures observed in the PSi(REF) sample, are probably due to the formation of a heterogeneous porous membrane. Indeed, in the case of thick membrane elaboration, the fluoride ions diffusion can be limited modifying the etching rate and creating different porosities [START_REF] Lomov | X-Ray Diffraction Study of Thin Porous Silicon Layers[END_REF]. Moreover, the peak observed at the lowest ω position shows a fluctuating intensity (SI.3 -Fig. 2.a) while observing different positions of the surface sample. This also plead for a non-homogenous layer. After 500°C annealing, only two peaks left, it seems that the peak previously observed at lowest ω vanished and the one coming from the porous structure decreased significantly due to the material reorganization. The peak from the porous layer appears larger due to the surface change during annealing. Indeed, the reorganization starts by hydrogen desorption around 260°C and from 350°C the lattice spacing starts to decrease [START_REF] Ogata | Structural change in p-type porous silicon by thermal annealing[END_REF]. Hydrogen desorption creates dangling bonds increasing Si-Si bonds formation. Moreover, Ogata et al. explained that amorphous-crystalline transition takes place when the temperature is higher than 700 °C justifying the diffuse pattern observed for the 700°C annealed. With higher temperature (900 °C), the porous layer reorganizes entirely leading to the smallest in plane lattice mismatch. Fourier transform infrared (ATR-FTIR) was used to characterize the sample chemical surface evolution with annealing (Figure 4). The PSi(REF) exhibits a silicon hydride surface with the characteristic vibration bands for Si-H bond at 2100 cm -1 and 622 cm -1 . In addition, low surface oxidation was observed with the presence of a relatively small band at 1070 cm -1 corresponding to the Si-O vibration mode. [START_REF] Canham | Properties of Porous Silicon[END_REF] The 500 °C annealing post treatment did not induce significant change in the chemical surface properties of the porous Si film. In contrast, annealing at higher temperatures (700 °C and 900 °C) induced a larger surface oxidation of the porous Si film with a more intense Si-O vibration band observed at 1070 cm -1 , and with the appearance of a broad band centered at 3300 cm -1 assigned to the -OH bond vibration. These two observations are in good accordance with the previous rocking curve measurements.

Literature indicates that the surface of freshly etched silicon without or with low temperature annealing treatment exhibits hydrogen on the surface and that higher temperature annealing leads to hydrogen desorption and so a higher reactivity to atmospheric oxygen inducing surface oxidation. [START_REF] Sugiyama | Microstructure and lattice distortion of anodized porous silicon layers[END_REF] These different characterizations have shown that the annealing post treatment proved its efficiency to adjust the porous silicon morphological properties as pore shape, pore size and wall thickness (crystallite size). Indeed, annealing allows material reorganization, both the pores and the walls grow reducing the surface area and retaining a highly crystalline and porous structure. This surface driven reorganization also plays on the later surface oxidation. At 500°C annealing temperature, only a slight morphological change is observed which already double the pore diameter and thickness. Above 700°C, the morphology is impressively changed with increasing characteristic dimensions. It is interesting to study the impact of these structural/morphological properties on the electrochemical behaviour in lithium battery. The silicon-based electrodes were electrochemically tested by galvanostatic assays in order to compare their performance versus Li + /Li. The first cycle profiles measured at a current of 178 mA g -1 (Si+C), the second discharge measured at 716 mA g -1 (Si+C) between 0.01 and 1.5 V. The corresponding derivative curves, as well as the capacity retention and the coulombic efficiency over 100 cycles are represented on Fig. 5.

In first discharge the potential of PSi(REF) decreases slowly down to 0.01 V, which represents the consumption of 0.3 Li. This characteristic potential evolution can be ascribed to the electrolyte reduction to form the solid electrolyte interphase (SEI) which leads to a very low coulombic efficiency for the first cycle of 46 %. The derivative curve of the 1 st discharge shows a small pic at 0.1 V corresponding to the lithium silicon amorphous alloy formation. [START_REF] Liu | In situ TEM experiments of electrochemical lithiation and of individual nanostructures[END_REF] In further cycling around 0.25 Li are reversibly exchanged resulting in a capacity of 450 mAh g -1 (Si+C). Considering 3,75 Li/Si as the higher theoretical lithiation (Li15Si4) with a capacity of 3580 mA g -1 , the experimental capacity of carbon additives around 185 mAh g -1 (Si+C) (SI.4) and the atomic ratio between silicon and carbon in the electrode, a capacity of 775 mAh g -1 (Si+C) is expected (insertion of 0,40 Li in C0,9Si0,1). The low lithiation of PSi(REF) electrode could be due to a limited electrolyte accessibility to the silicon. [START_REF] Shin | Porous silicon negative electrodes for rechargeable lithium batteries[END_REF] The galvanostatic curves obtained for the 500 °C annealed porous Si-based electrode is very close to that of PSi(REF). In all the three cases of annealed samples the potential progressively decreases in first discharge from 1.3 V to 0.1 V where a voltage plateau takes place. However for higher annealing temperatures 700 °C and 900 °C, an increasing reactivity vs Li is observed, through a longer voltage plateau at 0.1 V.

The associated derivate curves confirms it through the intensity rise of the peak at 0.1 V in first discharge but also by the presence of the two other broad peaks at 0.25 and 0.1 V in further discharges corresponding to the electrochemically-driven solid state amorphisation accompanying the Li-Si alloys formation [START_REF] Liu | In situ TEM experiments of electrochemical lithiation and of individual nanostructures[END_REF]. In second cycle reversible oxidative peaks appear at 0.3 and 0.45 V. These derivative peaks are in good agreement with those described in the literature for the lithiation/delithiation of Si [START_REF] Liu | In situ TEM experiments of electrochemical lithiation and of individual nanostructures[END_REF]. The cyclability over 100 cycles with a current density of 715 mA g -1 (Si+C) was measured for the four samples and the performance are reported in table 1. Close performance are reported for PSi(REF) and PSi(500°C) electrodes with low reversible capacity of 475 and 427 mAh g -1 in first cycle but a good stability with a capacity retention above 70 % after 100 cycles with a coulombic efficiency around 99.5 %. As discussed above the first cycle coulombic efficiency is improved for high temperature annealing Si electrodes from 46 % for PSi(REF) and 40 for PSi(500°C) to around 59 % for PSi(700°C) and to 68 % for PSi(900°C). PSi(900 °C) shows the highest first coulombic efficiency which is correlated with the lowest surface area accessible for the electrolyte reduction. Lowering the surface area limits SEI formation and consequently the first irreversible capacity. However, only 73% of the capacity is retained after 100 cycles. This is probably due to the volume buffer effect loss. Indeed, at such a high annealing temperature, the PSi(900°C) material exhibits the largest constituting pore walls inducing internal stress rise during lithiation.

In comparision, PSi(700°C) still shows higher initial coulombic efficiency than PSi(REF)

and PSi(500°C) thanks to its lower surface area. PSi(700°C) also exhibits a higher capacity which could be linked with a more efficient contact between the electrolyte and the active material, in fact increasing the electrolyte access with larger pore, probably enhances lithium diffusion and thus Si reactivity. Finally, a better cyclability is also observed for PSi(700°C) compared with PSi(900°C) attributed to its limited crystallites size composition. In order to confirm the hypothesis of the better electrolyte penetration in the annealed material PSi(700°C) and PSi(900°C) and explain the variations in electrochemical performance, contact angle experiments with the electrolyte were performed. The contact angle images are available in SI.5 and Table 2 summarizes the results. For the PSi(REF) and the PSi(500°C) the electrolyte drop forms a contact angle around 30°

and 20° respectively and it is not possible to observe the drop absorption before its evaporation. In these two examples the electrolyte shows limited penetration in the porous material, this behavior can be associated either to poor chemical interaction between electrolyte and silicon hydride surface or to too small pores limiting the electrolyte diffusion. From 700°C, the drop disappears quickly indicating enhanced electrolyte penetration in the porous structure which can still be related with the oxidized surface or pore size. These results (cf. Table2) show a positive impact of the annealing post treatment on the electrode wettability by the electrolyte leading to obvious improvement of the electrochemical performance. wafer follows the (100) direction and the resulting porous material exhibits (111) facets at the bottom of the pores and (110) along the walls. [START_REF] Gabouze | Macroporous Silicon[END_REF] According to Kuzma-Filipek [START_REF] Kuzma-Filipek | Sintering of Porous Silicon[END_REF] the annealing of a porous material causes the reorganization of the surface Si atoms to minimize the energy and this would thus favor the formation of (111) and also (100) facets but in a smaller amount, which possess the lowest energies.

To use as a negative electrode active material in LiBs, it is interesting to note that the Li diffusion rate varies with respect to the crystalline orientation. A theoretical study suggests, for example, that lithium diffuses at least two orders of magnitude quicker along the Si crystal direction (100) than along the (111) [START_REF] Zhang | First-principles approaches to simulate lithiation in silicon electrodes[END_REF], which should be responsible for the anisotropic volume expansion of silicon. The insertion of lithium would be energetically favored according to (110) plans compared with (100) or (111).

These calculations perfectly match the experimental observations. [START_REF] Mcdowell | 25th Anniversary Article: Understanding the Lithiation of Silicon and Other Alloying Anodes for Lithium-Ion Batteries[END_REF] In this study and according to the literature, the annealing post treatment should increase the number of facets (111) and facets (100) and these facets do not facilitate the lithiation for kinetic reasons. Therefore, the observed improvement in cycling performance cannot benefit from it and should be entirely due to the optimization of pore size and surface composition. shown that a simple 700°C annealing treatment leads to i) a highly porous structure able to limit the pulverization which is a consequence of the volume changes during the alloying mechanism, ii) large pores with limited surface area, beneficial for the electrolyte penetration and which limit the SEI formation, and to iii) a limited crystallite size to avoid structural fragility with lithium insertion.

Figure 1 :

 1 Figure 1: SEM image of a)PSi(REF) ,b) PSi(500°C), c) PSi(700°C), d) PSi(900°C) from top of the film ; Cross section of e)PSi(REF) ,f) PSi(500°C), g) PSi(700°C), h) PSi(900°C) ; Electrode view of I) PSi(REF) ,j) PSi(500°C), k) PSi(700°C), l) PSi(900°C)

Figure 2 :

 2 Figure 2: Nitrogen sorption isotherms for a) PSi(REF), b) PSi(500°C), c) PSi(700°C), d) PSi(900°C)

Figure 4 :

 4 Figure 4: ATR-FTIR spectra of porous REF film and Porous Si film annealed at 500°C, 700°C, and 900°C

Figure 5 :

 5 Figure 5: a)d)g)j) Galvanostatic profils of the first and a half cycles at 0,178 A g -1 (Si+C) for the first cycle and 0,716 A g -1 (Si+C) b)e)h)k) Derivatives c)f)i)l) Capacity retention over 100 cycles. Formulation 18%SiP/70%(SuperP/VGCF)/12%CMC -0,01-1.5 V

Figure 6 :

 6 Figure 6 : Variable Current-rate measurements -0.01-1.5V Vs Li + /Li

Table 1 :

 1 Capacity evolution over 100 cycles

		Capa. D1 EC (%)	Capa. D2 Capa.	EC (%)	Capa.	EC (%)	100/
	Active material		D1		D10	D10	D100	D100	D2
	PSi(REF)	1029,7	46,1	525.4	458,5	97,3	369,4	99,3	70%
	PSi(500°C)	1054,3	40,6	460,6	404,5	98,1	362,8	99,4	78%
	PSi(700°C)	1302,0	58,8	796,9	740,2	98,2	616,3	99,4	77%

Table 2 :

 2 Electrolyte wettability test sum-up As presented on the Fig.6the material annealed at 700°C and 900°C exhibits correct resistance of the capacity to current rise, a capacity of 600 mAh g -1 (Si+C) is maintained up to C. This good behavior observed at high rate for the annealed material confirms the benefit from larger pore on the total interface resistance and Li diffusion. The poor cycling at high rate for the PSi(REF) and PSi(500°C) seems to verify this hypothesis, the smallest pores may be blocked by the large amount of SEI in the pores. Higher rate shows a rapid capacity fading. The porous Si obtained here by etching and annealing at 700 and 900 °C has shown morphological and chemical modification, which appears favorable for the lithiation/delithiation in the electrode.

		Left angle	Right angle	Observations
	PSi(REF)	32,2	30,7	The drops does not penetrate and
				had time to evaporate
	PSi(500°C)	20,6	18,4	The drops does not penetrate and
				had time to evaporate
	PSi(700°C)	close to 0	close to 0	The drop disappears in 8s
	PSi(900°C)	0	0	The drop disappears in 5s

Finally, variable current-rate (C-rate) tests were carried out on porous Si-based electrode.

Gavouze et al. indicate that the electrochemical etching of a p-type (100) oriented

Acknowledgment :

The SEM measurements were performed at IEM (UMR5635) by Didier Cot, the nitrogen sorption by Jérémy Rodriguez at ICGM (UMR5253). This work has benefited from State aid managed by the National Agency for Research under the program "Investments for future" bearing the reference ANR-10-LABX-05-01. [1] X.H. Liu, L. Zhong, S. Huang, S.X. Mao, T. Zhu, J.Y. Huang, Size-dependent fracture of silicon nanoparticles during lithiation, ACS Nano. 6 (2012) 1522-1531. doi:10.1021/nn204476h.


Joanna Wisniewska

Hazar Guesmi

Maria Ziolek

Frederik Tielens
email: frederik.tielens@vub.be

Stability of nanostructured silver-platinum alloys

Keywords: silver, platinum, stability, DFT, XRD, UV-Vis

HAL is

Introduction

In materials science, the range of properties of metallic systems can be much extended by taking mixtures of elements to generate intermetallic compounds and alloys. Nanoalloys have become of particular interest mainly because they may display properties which are distinct from those of the pure clusters. Their chemical and physical properties may be tuned by varying the composition and atomic ordering as well as the size of the clusters [START_REF] Ferrando | Nanoalloys : From Theory to Applications of Alloy Clusters and Nanoparticles[END_REF][START_REF] Zafeiratos | Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment[END_REF]. In many cases, specific properties are enhanced upon alloying due to synergistic effect, which has led to widespread applications of alloys in optoelectronics [START_REF] Gao | Bimetallic Ag-Pt hollow nanoparticles: Synthesis and tunable surface plasmon resonance[END_REF][START_REF] Aldirmaz | A shape memory alloy based on photodiode for optoelectronic applications[END_REF], medicine and catalysis [START_REF] Zafeiratos | Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment[END_REF][START_REF] Zhang | Glucose-directed synthesis of Pt-Cu alloy nanowires networks and their electro-catalytic performance for ethylene glycol oxidation[END_REF][START_REF] Gondal | Facile synthesis, characterization and photocatalytic performance of Au-Ag alloy nanoparticles dispersed on graphitic carbon nitride under visible light irradiations[END_REF][START_REF] Wang | Minute quantities of hexagonal nanoplates PtFe alloy with facile operating conditions enhanced electrocatalytic activity and durability for oxygen reduction reaction[END_REF][START_REF] Du | Pt-based alloy nanoparticles embedded electrospun porous carbon nanofibers as electrocatalysts for Methanol oxidation reaction[END_REF]. It is also worth noting that the properties of nanoalloys may be different from those of the ideal bulk because of the surface segregation and nanoscale effect [START_REF] Ferrando | Nanoalloys : From Theory to Applications of Alloy Clusters and Nanoparticles[END_REF][START_REF] Deng | Morphology, dimension, and composition dependence of thermodynamically preferred atomic arrangements in Ag-Pt nanoalloys[END_REF][START_REF] Peng | Designer platinum nanoparticles: Control of shape, composition in alloy, nanostructure and electrocatalytic property[END_REF]. The efficient development of bimetallic systems requires collaboration between theoretical modeling, experimental preparation and characterization techniques. It is still a challenge to obtain the atomic-scale structural and compositional information about nanoalloys at the experimental level. Furthermore, it is wellknown that the surface structure and atomic arrangement of experimentally prepared nanoalloys are influenced by many factors, e.g. the particle size, morphology and composition. This has become less problematic thanks to application of powerful theoretical methods [START_REF] Deng | Morphology, dimension, and composition dependence of thermodynamically preferred atomic arrangements in Ag-Pt nanoalloys[END_REF][START_REF] Dhifallah | First-principles study of Au -Cu alloy surface changes induced by gas adsorption of CO, NO or O2[END_REF][START_REF] Guesmi | Theoretical insights on the effect of reactive gas on the chemical ordering of gold-based alloys[END_REF].

From among all bimetallic systems, platinum-based nanoalloys (Pt-Cu [START_REF] Zhang | Glucose-directed synthesis of Pt-Cu alloy nanowires networks and their electro-catalytic performance for ethylene glycol oxidation[END_REF][START_REF] Lei | The structural and electronic properties of Pt-Cu alloy clusters: Embedding atom method combined with density functional theory study[END_REF][START_REF] Zhou | Pt-Cu Core-Shell and Alloy Nanoparticles for Heterogeneous NO[END_REF][START_REF] Guedes-Sobrinho | Structure , Electronic , and Magnetic Properties of Binary Pt n TM 55 -n ( TM = Fe , Co , Ni , Cu , Zn ) Nanoclusters : A Density Functional Theory Investigation[END_REF], Pt-Co [START_REF] Zafeiratos | Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment[END_REF][START_REF] Guedes-Sobrinho | Structure , Electronic , and Magnetic Properties of Binary Pt n TM 55 -n ( TM = Fe , Co , Ni , Cu , Zn ) Nanoclusters : A Density Functional Theory Investigation[END_REF][START_REF] Piotrowski | Platinum-based nanoalloys Pt nTM 55-n (TM = Co, Rh, Au): A density functional theory investigation[END_REF], Pt-Fe [START_REF] Wang | Minute quantities of hexagonal nanoplates PtFe alloy with facile operating conditions enhanced electrocatalytic activity and durability for oxygen reduction reaction[END_REF][START_REF] Guedes-Sobrinho | Structure , Electronic , and Magnetic Properties of Binary Pt n TM 55 -n ( TM = Fe , Co , Ni , Cu , Zn ) Nanoclusters : A Density Functional Theory Investigation[END_REF][START_REF] Zhou | Embedding platinumbased nanoparticles within ordered mesoporous carbon using supercritical carbon dioxide technique as a highly ef fi cient oxygen reduction electrocatalyst[END_REF], Pt-Ni [START_REF] Guedes-Sobrinho | Structure , Electronic , and Magnetic Properties of Binary Pt n TM 55 -n ( TM = Fe , Co , Ni , Cu , Zn ) Nanoclusters : A Density Functional Theory Investigation[END_REF], Pt-Pd [START_REF] Sosa | Oxygen adsorption on Pt-Pd nanoclusters by DFT and ab initio atomistic thermodynamics[END_REF][START_REF] Li | Theoretical insights into the relationship between structures and properties in tri-metallic Pd13-nNin@Pt42(n = 0-13) nanoparticles[END_REF], Pt-Rh [START_REF] Piotrowski | Platinum-based nanoalloys Pt nTM 55-n (TM = Co, Rh, Au): A density functional theory investigation[END_REF], Pt-Au [START_REF] Piotrowski | Platinum-based nanoalloys Pt nTM 55-n (TM = Co, Rh, Au): A density functional theory investigation[END_REF][START_REF] Song | DFT studies of Pt/Au bimetallic clusters and their interactions with the CO molecule[END_REF], Pt-Ru [START_REF] Semiz | Nanoporous Pt and Ru catalysts by chemical dealloying of Pt-Al and Ru-Al alloys for ultrafast hydrogen generation[END_REF]) have been widely studied to reduce the amount of expensive Pt in technological applications.

Although, numerous theoretical and experimental studies have been focused on the Ag-Pt nanosystem [START_REF] Deng | Morphology, dimension, and composition dependence of thermodynamically preferred atomic arrangements in Ag-Pt nanoalloys[END_REF][START_REF] Schaal | Theoretical and experimental studies of Ag-Pt interactions for supported Ag-Pt bimetallic catalysts[END_REF][START_REF] Feng | Preparation of Pt Ag alloy nanoisland/graphene hybrid composites and its high stability and catalytic activity in methanol electro-oxidation[END_REF][START_REF] Hart | Acta Materialia Revisiting the revised Ag-Pt phase diagram[END_REF][START_REF] Bin Yousaf | Single Phase PtAg Bimetallic Alloy Nanoparticles Highly Dispersed on Reduced Graphene Oxide for Electrocatalytic Application of Methanol Oxidation Reaction[END_REF][START_REF] He | Formation of AgPt alloy nanoislands via chemical etching with tunable optical and catalytic properties[END_REF][START_REF] Hwang | Property of Pt -Ag Alloy Nanoparticle Catalysts in Carbon Monoxide Oxidation[END_REF][START_REF] Peng | An electrochemical approach to PtAg alloy nanostructures rich in Pt at the surface[END_REF][START_REF] Li | The Influence of Pt Layers in Ag@Pt nL ( 111 ) ( n = 1 and 2 ) on Oxygen Reduction Reaction : A Theoretical Study[END_REF], still the controlled synthesis of AgPt alloy, the composition and structure dependence of its properties and stability need further exploration. Therefore, AgPt nanoalloy is an interesting system to investigate, especially considering the potential applications given the lower price of Ag and the limited resources of Pt. An additional inspiration for studying the stability of Ag-Pt bimetallic nanoparticles is motivated by the results obtained in our previous paper [START_REF] Wisniewska | Formation of Pt-Ag alloy on different silicas -surface properties and catalytic activity in oxidation of methanol[END_REF]. Indeed, it has been shown that bimetallic catalysts with Ag and Pt supported on silica (commercial and ordered mesoporous cellular foam -MCF) after calcination (the last step of preparation route) display Ag-Pt alloy-like particles. On the basis of these experimental results it has been postulated that the condition for alloy formation is the minimal Ag/Pt molar ratio equal to 2.5. On the majority of samples the alloy disappeared upon heating at 673 K for 2 hours in argon or hydrogen flow. Only bigger alloy particles remained after activation in inert gas. In the present study heating in N2 flow was applied, but at different temperatures (423 K, 473 K, 523 K and 573 K). For such a study amorphous silica or MCF were modified by silver and platinum species with Ag/Pt molar ratio in range 0.4 to 5.1. The goal of this work was to find the optimal composition of Ag-Pt bimetallic nanoalloy which will display the highest stability according to DFT study and to check experimentally Ag-Pt alloy resistance to decay upon temperature treatment.

Computational details

Methodology

The geometry optimization, the calculation of the total energy were performed using Perdew-Burke-Ernzerhof form of the spin-polarized generalized gradient approximation (GGA-PBE) [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF][32] functional, implemented in the Vienna Ab Initio Simulation Package (VASP) [START_REF] Kresse | Ab initio molecular dynamics for open-shell transition metals[END_REF] [START_REF] Kresse | Ab initio molecular-dynamics liquid-metal -amorphoussemiconductor simulation of the transition in germanium[END_REF]. The valence electrons are treated explicitly, and their interactions with the ionic cores are described by the projector augmented-wave method (PAW) [START_REF] Blochl | Projector augmented-wave method[END_REF], which allows the use of a low-energy cutoff equal to 400 eV for the plane-wave basis. To get the lowest energy of clusters each starting configuration was placed inside a large cubic box of 30 × 30 × 30 Å and ensured that image interaction between clusters of two nearest boxes is negligible. For the slab models, 3 × 3 atoms were considered in one layer and similar six layers were taken in the Z-direction.

For optimization of this slab, while atomic positions of top four layers were relaxed, atoms at 2 bottom layer were kept fixed at bulk values. The molecules are at large enough distance from each other, in order to avoid intermolecular interactions. The sampling in the Brillouin zone was performed employing k-points at a distance of each other of 30 Å -1 in the reciprocal space.

Description of models

The aim of the theoretical study was to investigate the stability of two kinds of Ag-Pt systems: a. 13-atoms cluster model: Ag13, Pt13, Ag13-nPtn (Fig. 1A) b. 54-atoms (6-layers) (111) surface slab model: 45Ag9Pt, Ag(111), Pt(111) (Fig. 1B) Cluster models differ in the content of silver and platinum atoms and their positions. Slab models display constant composition (45 atoms of Ag and 9 atoms of Pt) and differ just in the location of Pt atoms. Studied slab models (Fig. 1B) consist of 6 layers in which the top four layers were allowed to relax, whereas the two bottom layers were fixed at their crystallographic bulk positions. To analyze the stability of Ag13-nPtn clusters and 45Ag9Pt slab models, the excess energy per atom (ΔEexc) was calculated [START_REF] Dhifallah | First-principles study of Au -Cu alloy surface changes induced by gas adsorption of CO, NO or O2[END_REF]. The ΔEexc measure the stability of the studied systems with respect to the parent compounds. It is defined by the following equation:

∆𝐸𝐸 𝑒𝑒𝑒𝑒𝑒𝑒 = 𝐸𝐸 𝐴𝐴𝐴𝐴 13-𝑛𝑛 𝑃𝑃𝑃𝑃 𝑛𝑛 - 𝑛𝑛 13
𝐸𝐸 𝑃𝑃𝑃𝑃 13 -13-𝑛𝑛 13 𝐸𝐸 𝐴𝐴𝐴𝐴 13 13 for the cluster model

∆𝐸𝐸 𝑒𝑒𝑒𝑒𝑒𝑒 = 𝐸𝐸 45𝐴𝐴𝐴𝐴9𝑃𝑃𝑃𝑃 - 9 54 (1)
𝐸𝐸 𝑃𝑃𝑃𝑃(111) -45 54

𝐸𝐸 𝐴𝐴𝐴𝐴(111) 54 for the slab model,

with:

∆𝐸𝐸 𝑒𝑒𝑒𝑒𝑒𝑒 -the excess energy per atom, 𝐸𝐸 𝐴𝐴𝐴𝐴 13-𝑛𝑛 𝑃𝑃𝑃𝑃 𝑛𝑛 , 𝐸𝐸 45𝐴𝐴𝐴𝐴9𝑃𝑃𝑃𝑃 -total energy of the bimetallic model, 𝐸𝐸 𝑃𝑃𝑃𝑃 13 , 𝐸𝐸 𝑃𝑃𝑃𝑃(111) -total energy of Pt13 or bulk Pt(111) model, 𝐸𝐸 𝐴𝐴𝐴𝐴 13 , 𝐸𝐸 𝐴𝐴𝐴𝐴(111) -total energy of Ag13 or bulk Ag(111) model.

For pure metals (Ag13, Pt13, bulk Ag(111), bulk Pt(111)) ΔEexc is set to be 0.00 eV. A negative value of excess energy indicates a tendency to form nanoalloys (the mixing is energetically favorable), and the lower the value of excess energy, the higher the stability of the structure [START_REF] Guedes-Sobrinho | Structure , Electronic , and Magnetic Properties of Binary Pt n TM 55 -n ( TM = Fe , Co , Ni , Cu , Zn ) Nanoclusters : A Density Functional Theory Investigation[END_REF]. Additionally, segregation energy was calculated for selected clusters and slab models. The segregation energy (Eseg) of Pt atoms in silver slab was defined by equation [START_REF] Gao | Bimetallic Ag-Pt hollow nanoparticles: Synthesis and tunable surface plasmon resonance[END_REF].

Eseg. = [ EAgPt(Pt x-layer) -EAgPt(Pt 4th-layer)]/NPt ( 3
)
where EAgPt(Pt x-layer) represents the total energy of the AgPt alloy system (the slab of 54 atoms)

with Pt layer located in the upper x layers of Ag slab (x = 1, 2 or 3), and EAgPt(Pt 4th-layer) represents the total energy of the AgPt alloy system with the Pt located in the 4 th Ag layer slab, which corresponds to the presence of Pt atoms in the "bulk" silver. For cluster model this energy is calculated as the difference between cluster with the central Pt (Pt in the bulk) and corresponding structure with Pt segregated to the surface.

Experimental

Preparation of catalysts

Amorphous silica (Ultrasil) or mesoporous MCF were grafted with 3mercaptopropyltrimethoxysilane (MPTMS) (95%, Sigma-Aldrich) to functionalize its surface before modification with metals. To obtain bimetallic (Ag-Pt) catalysts the same procedure as in our previous work [START_REF] Wisniewska | The effect of niobium and tantalum on physicochemical and catalytic properties of silver and platinum catalysts based on MCF mesoporous cellular foams[END_REF] was used (S1-SD). The Ag/Pt molar ratios: 0.4, 1.6, 2.5, 3.6 and 5.1

Ag/Pt were applied. The total amount of metals did not exceed 3 wt. %. After metals loading, the samples were calcined in air at 773 K for 4 h. The actual approximate Ag/Pt molar ratio is indicated in the material symbol, e.g. 0.4AgPt/SiO2 means Ag/Pt molar ratio equal to 0.4.

Test of stability upon temperature treatment

The stability of AgPt alloy particles was tested by heating the prepared samples at different temperatures (423 K, 473 K, 523 K and 573 K) in N2 flow (40 mL/min) for 2 hours. Directly after cooling down the samples were analyzed with the use of UV-Vis spectroscopy and X-Ray Diffraction.

X-Ray Diffraction. XRD measurements were carried out on a Bruker AXS D8Advance diffractometer with Cu Ka radiation (k = 0.154 nm), with a step size of 0.05⁰ in the wide-angle range 6⁰ -60⁰.

UV-Vis spectroscopy. UV-Vis spectra were recorded using a Varian-Cary 300 Scan UV-Visible spectrophotometer. Powder samples, directly after heating and cooling down were placed in a cell equipped with a quartz window. The spectra were recorded in the range from 800 to 190 nm. Spectralon was used as the reference material.

Results and discussion

Theoretical results

The cluster model

Literature data [37][38] suggest that a stable series of cluster catalysts should have the topological magic number structure (e.g., Pt13, Pt55, …) with high symmetry. The Pt13 has a well-known icosahedral atomic coordination as part of the stable cluster series. Therefore, the Pt13 cluster as the initial structure was chosen to construct the alloy cluster. Different metals Ag/Pt ratios in the range 0.1 to 12.0 were applied.

Systematic study of 75 configurations of Ag13-nPtn alloy cluster was performed, (Fig.

S1-SD

). The most stable structures are presented in Fig. 2. Calculated total energies, excess energies and segregation energies per atom are collected in Table S1-SD. As presented in Fig. depends on Pt loading. For clusters with Ag/Pt molar ratio lower than 0.4 and higher than 2.0, the location of Pt on the surface is found to be highly unfavourable. Such alloy clusters display lower stability than clusters with Ag/Pt molar ratio in the range of 1.6 -0.4 with preferable surface location of platinum. This conclusion is in line with the experimental results, showed in the later section.

As presented in Fig. 5, the Ag-Pt distances (2.80Å, 2.77Å) in the bimetallic cluster are shorter in comparison with the Ag-Ag ones (2.92Å; 2.78Å) in the silver cluster and longer in comparison with Pt-Pt (2.75Å; 2.62Å) in the platinum cluster, located at the similar positions.

This indicates that the icosahedron structure is slightly distorted. Thus, different interactions between monometallic and bimetallic clusters with different reagents can be expected.

The slab model

The slab models considered are shown in Fig. 6. The (111) surface of Pt and Ag are thermodynamically the most stable so they were used as references S2-SD, the values of all calculated excess energies are close to 0 eV/atom, i.e within the precision of the calculation level. Thus, only general tendency of changes, not the actual values of excess energies can be considered. From the energy segregation of a layer of Pt in Ag structure and the effect of order (Table S2-SD) it can be seen the segregation tendency.

Surface location of Ag atoms is energetically favored (45Ag9Pt_3 rd and 45Ag9Pt_4 th models)

which is in agreement with the Monte Carlo studies performed on Ag-Pt system [START_REF] Deng | Morphology, dimension, and composition dependence of thermodynamically preferred atomic arrangements in Ag-Pt nanoalloys[END_REF]. The data presented indicate the limitation in construction of AgPt alloy structures, such as their size. The slab models which describe the surface of bigger bimetallic particles are much less stable than the cluster models describing particles with sizes less than 10 nm. These differences are caused by limited miscibility between Ag and Pt in bulk materials. Narrowing or complete disappearance of miscibility gaps for nanostructured platinum alloys is fundamentally interesting, and potentially very useful for controlling the properties that cannot be achieved through bulk alloys [START_REF] Peng | Designer platinum nanoparticles: Control of shape, composition in alloy, nanostructure and electrocatalytic property[END_REF][40] [START_REF] He | Design of AgM bimetallic alloy nanostructures (M = Au, Pd, Pt) with tunable morphology and peroxidase-like activity[END_REF]. When the particle size is reduced to nanoscale, often the change in phase behavior is observed and the phase diagram based on bulk bimetallic systems cannot be simply applied for nanoalloys [START_REF] Ferrando | Nanoalloys : From Theory to Applications of Alloy Clusters and Nanoparticles[END_REF][10] [START_REF] Peng | Ag-Pt alloy nanoparticles with the compositions in the miscibility gap[END_REF].

Experimental results

Fundamental characterization of catalysts calcined at 773 K in air has been presented in our previous paper [START_REF] Wisniewska | Formation of Pt-Ag alloy on different silicas -surface properties and catalytic activity in oxidation of methanol[END_REF]. After calcination, Ag and Pt are mainly in the form of Ag-Pt alloy nanoparticles. On the basis of TEM images particle size distribution was estimated and particles in the range of 4-6 nm were found to dominate. It means that the Ag13-nPtn cluster model is better equivalent of the actual AgPt alloy particles in prepared catalysts than semi-infinite surface presented by slab models. In Fig. 7 Ag/Pt molar ratios are shown corresponding to real values studied in the experimental part.

XRD patterns (Fig. 8) of calcined samples with Ag/Pt ratio equal to 2.5, 3.6, 5.1 show reflections at 2θ ≈ 31.8⁰, 32.9⁰, 33.9⁰ from silver oxide planes [START_REF] Valverde-Aguilar | Silver Core -Silver Oxide Shell Nanoparticles Embedded on Mesostructured Silica Films[END_REF]; 2θ ≈ 39.8⁰, 46.2⁰ from metallic Pt 0 [43] [START_REF] Sobczak | Surface properties of platinum catalysts based on various nanoporous matrices[END_REF]; and 2θ ≈ 37.9⁰, 41.0⁰, 46.7⁰ from Ag-Pt alloy particles [START_REF] Wisniewska | Formation of Pt-Ag alloy on different silicas -surface properties and catalytic activity in oxidation of methanol[END_REF]. For the sample with the highest amount of Pt (Ag/Pt = 0.4) there are only reflections from metallic platinum, however it does not exclude the presence of very small alloy particles (below the limits of X-Ray Diffraction detection). The reflections from alloy are not observed neither in the XRD pattern of the sample with Ag/Pt = 1.6, although the metallic platinum and silver oxide are well distinguished. Types of active species in calcined samples were also estimated from UV-Vis spectra (Fig. 9). According to literature a surface plasmon resonance band of spherical metallic silver nanoparticles appears at 400 nm [START_REF] Suber | Preparation and the mechanisms of formation of silver particles of different morphologies in homogeneous solutions[END_REF], whereas the band at 267 nm can be assigned to metallic platinum [START_REF] Gharibshahi | Influence of dose on particle size and optical properties of colloidal platinum nanoparticles[END_REF] or cationic silver species [START_REF] More | Rational preparation of Ag and Au bimetallic catalysts for the hydrocarbon-SCR of NOx: Sequential deposition vs. coprecipitation method[END_REF]. Blue shift of UV-Vis band coming from metallic silver from 400 nm to lower wavelengths (300 -400 nm range) is caused by the formation of alloy Ag-Pt structure. The band at a lower than 400 nm wavelength originates from d-d electron transfer in alloy Ag-Pt particles [30][48]. Thus, the presence of AgPt alloy particles in all calcined samples is confirmed by UV-Vis spectra shown in Fig. 9.

XRD patterns of samples heated at 423 K under N2 flow permit differentiating between the stability of alloy particles in the studied samples. Reflections from alloy disappear for 5.1AgPt/SiO2, whereas in the other samples they remain (Fig. 8.). For the samples with higher amount of Pt (lower Ag/Pt molar ratio) analogous changes (disappearance of alloy reflections) but at higher temperatures: 473 K for 3.6AgPt/SiO2 and 523 K for 2.5AgPt/SiO2 are observed.

On the basis of XRD patterns it is not possible to conclude about stability of AgPt alloy in samples: 0.4AgPt/SiO2 and 1.6AgPt/MCF because the peaks from alloy were not visible in the parent, calcined materials. Therefore, the stability of alloy for these samples was studied by UV-Vis spectroscopy. In the UV-Vis spectra of both samples (Ag/Pt = 0.

3 .

 3 the formation of the alloy-like Ag13-nPtn nanoclusters is energetically favorable for almost all compositions as the excess energy is negative in most cases. The exceptions are clusters with Ag/Pt molar ratio in the low range (0.1 -0.3) with Ag atom located at the center. Taking into account the most stable clusters from each group with the same composition, the excess energy ΔEexc decreases (indicating the increasing stability of alloy) by replacing Ag atoms by Pt atoms in the cluster, and it reaches a minimum at n = 5 (Ag/Pt molar ratio = 1.6). Then, the excess energy per atom increases (indicating a decrease in stability of the nanocluster) and reaches 0.0 eV at n = 13 (Pt13). A similar tendency for Pt-containing bimetallic clusters has been reported in literature for PtnTM55-n (TM = Fe, Co, Ni, Cu, and Zn)[START_REF] Guedes-Sobrinho | Structure , Electronic , and Magnetic Properties of Binary Pt n TM 55 -n ( TM = Fe , Co , Ni , Cu , Zn ) Nanoclusters : A Density Functional Theory Investigation[END_REF] and PtnTM55-n (TM = Co, Rh, Au)[START_REF] Piotrowski | Platinum-based nanoalloys Pt nTM 55-n (TM = Co, Rh, Au): A density functional theory investigation[END_REF] systems. The most stable nanoclusters were characterized by the silver to platinum molar ratio of 0.4 -2.4 (4-9 Pt atoms in Ag13-nPtn clusters). It was also noticed that the central site is preferably occupied by Pt atom in the energetically favorable Ag13-nPtn nanoparticles, regardless of the Ag/Pt molar ratio (Fig.2.). It indicates that the Pt atom in the central site is responsible for strong stabilization of icosahedral structures. This is due to the strong cohesive energy of Pt (5.84 eV) compared to Ag (2.95 eV) as well as the lower surface energy of Ag compared to Pt. A similar bulk segregation phenomenon, with the Pt atoms at the cluster core has been observed for Pt-Pd nanoclusters[START_REF] Radillo-Díaz | Structural and electronic properties of PtPd and PtNi nanoalloys[END_REF]. In the Fig.4., the evolution of surface segregation energy of Pt in AgPt clusters (from the central position to the surface) with the increasing Ag/Pt molar ratio has been plotted. Interestingly, the segregation of Pt is found to

  . The goal was to determine the effect of platinum location on the stability of bimetallic AgPt in slab models. Six slab models were studied, with different Pt atom positions: (a) platinum on the top, (b) Pt in the 2 nd layer, (c) Pt in the 3 rd layer, (d) Pt in the 4 th layer (e) Pt atoms agglomerates (f) bimetallic surface. As seen in Table

Figure 1 .Figure 2 .

 12 FIGURES CAPTIONS

Figure 3 .

 3 Figure 3. Calculated ΔEexc (eV/atom) Ag13-nPtn as a function of the composition (number of Pt

Figure 4 .

 4 Figure 4. Relationship between Ag/Pt molar ratio and calculated segregation energy (eV/atom)

Figure 5 .

 5 Figure 5. The comparison of Ag-Pt, Ag-Ag and Pt-Pt bond lengths in monometallic and

Figure 6 .

 6 Figure 6. Studied 6-layers (111) surface slab models: 45Ag9Pt (Ag/Pt = 5.0), Ag (111), Pt(111);

Figure 7 .

 7 Figure 7. Relationship between temperature stability of AgPt alloy and its composition -

Figure 8 .

 8 Figure 8. XRD patterns of bimetallic Ag-Pt samples: calcined and after heating in N2 flow at

Figure 9 .

 9 Figure 9. UV-Vis spectra of bimetallic Ag-Pt samples: fresh and after heating in N2 flow at 423

Figure 1 .

 1 Figure 1. Studied models: 13-atoms Ag13-nPtn cluster (A) and 54-atoms 45Ag9Pt slab (B). All

Figure 2 .

 2 Figure 2. The most stable structures of Ag13-nPtn clusters (n=1-12); Ag -orange, Pt-green.

Figure 3 . 1 ∆E

 31 Figure 3. Calculated ΔEexc (eV/atom) Ag13-nPtn as a function of the composition (number of

Figure 4 .

 4 Figure 4. Relationship between Ag/Pt molar ratio and calculated segregation energy

Figure 5 .

 5 Figure 5. The comparison of Ag-Pt, Ag-Ag and Pt-Pt bond lengths in monometallic

Figure 6 . 6 -

 66 Figure 6. 6-layers (111) surface slab models: 45Ag9Pt (Ag/Pt = 5.0), Ag (111), Pt(111), Ag

Figure 7 .

 7 Figure 7. Relationship between temperature stability of AgPt alloy and its composition -

  decay , K -1 (*10 -2 )

ACKNOWLEDGEMENTS

J. Wisniewska would like to thank Dr. Jan Turek from Vrije Universiteit Brussel (Free University Brussels-VUB) for patiently answering all questions about VASP calculations, many useful advices and his selfless generosity not only in science field.

Computational resources and services were provided by the Shared ICT Services Centre funded by the Vrije Universiteit Brussel, the Flemish Supercomputer Center (VSC) and FWO.

National Science Centre in Poland (Grant No. 2015/19/N/ST5/00501) is acknowledged for the financial support of this work.

Supporting Information

S1 -Synthesis procedure Table S1. -The calculated total energies, excess energies per atom and segregation energies for Ag13-nPtn cluster models.