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International audience; Developing micro- and nanomaterials for environmental pollution remediation is currently a pertinent topic. Among the plethora of strategies, designing supported nanocatalysts for the degradation of pollutants has achieved prominence. In this context, we are addressing one of the UN Sustainable Development Goals by valorizing agrowaste as a source of biochar, which serves as a support for bimetallic nanocatalysts. Herein, olive pit powder particles were impregnated with copper and nickel nitrates and pyrolyzed at 400 °C. The resulting material consists of bimetallic CuNi-decorated biochar. CuNi nanocatalysts were found to be as small as 10 nm and very well dispersed over biochar with zero valent copper and nickel and the formation of copper–nickel solid solutions. The biochar@CuNi (B@CuNi) exhibited typical soft ferromagnet hysteresis loops with zero remanence and zero coercivity. The biochar@CuNi was found to be an efficient catalyst of the reduction in methyl orange (MO) dye, taken as a model pollutant. In sum, the one-pot method devised in this work provides unique CuNi-decorated biochar and broadens the horizons of the emerging topic of biochar-supported nanocatalysts.

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International audience; Nanoporous gold and platinum electrodes are used to pattern n-type silicon by contact etching at the macroscopic scale. This type of electrode has the advantage of forming nanocontacts between silicon, the metal and the electrolyte as in classical metal assisted chemical etching while ensuring electrolyte transport to and from the interface through the electrode. Nanoporous gold electrodes with two types of nanostructures, fine and coarse (average ligament widths of ∼30 and 100 nm, respectively) have been elaborated and tested. Patterns consisting in networks of square-based pyramids (10 ×10 µm 2 base × 7 µm height) and U-shaped lines (2, 5, and 10 µm width × 10 µm height × 4 µm interspacing) are imprinted by both electrochemical and chemical (HF-H 2 O 2) contact etching. A complete pattern transfer of pyramids is achieved with coarse nanoporous gold in both contact etching modes, at a rate of ∼0.35 µm min −1. Under the same etching conditions, U-shaped line were only partially imprinted. The surface state after imprinting presents various defects such as craters, pores or porous silicon. Small walls are sometimes obtained due to imprinting of the details of the coarse gold nanostructure. We establish that np-Au electrodes can be turned into "np-Pt" electrodes by simply sputtering a thin platinum layer (5 nm) on the etching (catalytic) side of the electrode. Imprinting with np Au/Pt slightly improves the pattern transfer resolution. 2D numerical simulations of the valence band modulation at the Au/Si/electrolyte interfaces are carried out to explain the localized aspect of contact etching of n-type silicon with gold and platinum and the different surface state obtained after patterning. They show that n-type silicon in contact with gold or platinum is in inversion regime, with holes under the metal (within 3 nm). Etching under moderate anodic polarization corresponds to a quasi 2D hole transfer over a few nanometers in the inversion layer between adjacent metal and electrolyte contacts and is therefore very localized around metal contacts.

Ni100-xRhx/C composites consisting in bimetallic nanoparticles (10 wt.%) dispersed in high surface area graphite were synthesized and their electrocatalytic performances for urea oxidation tested in alkaline media by cyclic voltammetry using a cavity microelectrode. The composites are synthesized by impregnation of a graphite powder with an aqueous solution of nickel and rhodium salts, followed by drying and reduction under H2/Ar at 500 °C. TEM observations reveal that the Rh fraction influences the average nanoparticle size which varies from 15 nm for pure Ni to 2 nm for pure Rh. XRD analysis indicates that solid solutions are obtained for all compositions. Ni100-xRhx with up to 30 at.% of rhodium have a higher electrocatalytic activity than pure Ni and their voltammograms exhibit two urea oxidation peaks rather than one, the additional peak being at a potential 50 mV less positive than that of pure nickel. When the rhodium content increases above 30 at.%, the current of urea oxidation decreases and becomes almost zero for pure rhodium. These variations in current as the fraction of rhodium increases are only related to the decrease in nanoparticle size and in nickel content (within a surface layer of ∼1 nm). Hence, there is no synergistic effect between nickel and rhodium for the oxidation of urea in alkaline media.

Composites consisting of rhodium, copper, and copper-rhodium nanoparticles (2 nm in average diameter) dispersed in a high-surface area graphite powder (~ 10 wt.% of metal) have been synthesized by a wet chemical method. After characterization by ICP-OES and TEM, they have been tested for the electrochemical reduction of nitrates in alkaline media (10−1 mol L−1 KOH) using a cavity microelectrode. It is found that in the 0.02–0.5 V/RHE potential range, bimetallic composites exhibit a much higher electrocatalytic activity than single-metal composites. The peak current describes a volcano plot as a function of the composition, with a maximum for CuRh, which is 7.5 times higher than that obtained with pure rhodium (under identical metal wt.%). This synergistic effect can be rationalized directly from the electrochemical response of pure metals. It is then tentatively attributed to the fact that the first (rate determining) reduction step, corresponding to the formation of nitrites, takes place efficiently in copper-rich areas while the subsequent steps of nitrite reduction in ammonia (via hydroxylamine formation) occur in rhodium-rich areas. For the same mass of rhodium, the electrocatalytic conversion of nitrates to ammonia is 12 times more effective with CuRh than with pure rhodium. With the additional gain in active surface area due to the nanoparticle morphology compared to bulk or thin film forms, these results represent a step-forward in cost reduction of rhodium-based electrocatalysts for the conversion of nitrates to ammonia.

There is a growing interest in sonochemistry for either the controlled design of nanostructured materials or for the synthesis of polymers and polymer composites. It is fast and highly efficient method that provides materials with exceptional and enhanced structural and chemical properties. Herein, we take advantage of the versatile sonochemical process in order to design core/double layered shell nanocomposite denoted by Fe3O4@NH2-mesoporous silica@ PPy/Pd. This magnetic, multicomponent material was designed in a three-step sono-process: (i) synthesis of magnetic core, (ii) cure of mesoporous silica, and (iii) sonochemical deposition of PPy/Pd. This last step was achieved within 1 h, a much shorter duration compared to conventional routes which usually take several hours to few days. The final nanocomposite can be recovered with a simple magnetic stick. X-ray diffraction patterns highlighted the presence of zerovalent palladium on the surface of the magnetic nanocomposite. The catalytic activity of the solid support was investigated by the study of the p-nitrophenol (p-NP) reduction and the Methyl Orange (MO) degradation in aqueous media. Results showed a very high catalytic efficiency, a high conversion yield of p-NP into 4-aminophenol (more than 94%) and an almost entire degradation of MO (99%) with a fast kinetics fitting to the first order model. This work demonstrates conclusively the benefits of sonochemistry in the design of metal nanoparticle-decorated inorganic/polymer hybrid system with outstanding performances.

Rh-based nanoparticles supported on a porous carbon host were prepared with tunable average sizes ranging from 1.3 to 3.0 nm. Depending on the vacuum or hydrogen environment during thermal treatment, either Rh metal or hydride is formed at nanoscale, respectively. In contrast to bulk Rh that can form a hydride phase under 4 GPa pressure, the metallic Rh nanoparticles (∼2.3 nm) absorb hydrogen and form a hydride phase at pressure below 0.1 MPa, as evidenced by the presence of a plateau pressure in the pressure-composition isotherm curves at room temperature. Larger metal nanoparticles (∼3.0 nm) form only a solid solution with hydrogen under similar conditions. This suggests a nanoscale effect that drastically changes the Rh-H thermodynamics. The nanosized Rh hydride phase is stable at room temperature and only desorbs hydrogen above 175 °C. Within the present hydride particle size range (1.3-2.3 nm), the hydrogen desorption is size-dependent, as proven by different thermal analysis techniques.

International audience; — New Si processes based on Metal Assisted Chemical Etching (MACE) are explored for solar cells texturization. Pt and Au are considered as catalysts for MACE of p and n-type Si substrates. 2D band bending modeling at the nanoscale shows that Pt nanoparticles (NPs) make ohmic contacts and induce delocalized etching. Accordingly, cone-shaped macropores, very efficient in reducing the reflectivity (

The hydrogen sorption properties of palladium particles synthesized with different sizes (2, 6 and 18 nm) dispersed on high surface area graphite powders are studied by both solid/gas and electrochemical methods. We demonstrate that we can directly evaluate by cyclic voltammetry the influence of the particle size on Pd hydride phase formation despite the ohmic losses and capacitive currents associated with powder electrodes. The size-dependent Pd hydride compositions calculated from electrochemical signals and pressure-composition-isotherms under ambient conditions are in good agreement and show the same trend: as the Pd particle size is reduced, the hydrogen content increases in the α phase and decreases in the β phase. Changes in electrochemical signals during cycling are observed when the potential domain covers the Pd oxide region, corresponding to an increase of the mean size of the smallest particles from 2 to ∼12 nm (as confirmed by ex situ X-ray diffraction) whereas no modification occurs if it is restricted to the hydrogen region. This highlights that voltammograms can be used to readily detect in situ size modifications of Pd nanomaterials.

A novel strategy to achieve 3D pattern transfer into silicon in a single step without using lithography is presented. Etching is performed electrochemically in HF media by contacting silicon with a positively biased, patterned, metal electrode. Dissolution is localized at the Si/metal contacts and patterning is obtained as the electrode digs into the substrate. Previous attempts at imprinting Si using bulk metal electrodes have been limited by electrolyte blockage. Here, the problem is solved by using, for the first time, a nanoporous metal electrode that allows the electrolyte to access the entire Si/metal interface, irrespective of the electrode dimensions. As a proof of concept, imprinting of well-defined arrays of inverted pyramids has been performed with sub-micrometer spatial resolution over 1 mm2 using a nanoporous gold electrode of the complementary shape. Under a polarization of +0.3 V/SME in 5 M HF, the etch rate is ~0.5 μm min−1. The pyramidal pattern is imprinted independently of the Si crystallographic orientation. This maskless imprinting technique opens new opportunities in the fabrication of Si microstructures. Keywords: Silicon, Nanoporous gold, Imprinting, Microstructure, MACE

Composites made of ultra-small Rh hydride nanoparticles (NP) with controlled average sizes of 1.3, 1.9 and 2.3 nm dispersed in high surface area graphite powders were synthesized. The hydrogen electrosorption properties of the Rh nanohydride that is stable under ambient conditions were characterized by cyclic voltammetry under various scan rates in the hydrogen potential domain with the help of a cavity microelectrode. During the first cycles, an evolution of the voltammograms, characterized by an isopotential point, is observed that corresponds to a surface conversion of RhH x into Rh, the core of the NP remaining in the hydride phase. After stabilization, the voltammograms exhibit the classical hydrogen electrosoprtion peaks of Rh, but a shift to positive potentials indicates that H is more weakly bounded to the surface as the NP size decreases. The onset of the HER follows the same trend. Interestingly, it is observed that the quantity of electrosorbed hydrogen strongly increases when downsizing the NPs, with the H/Rh ratio reaching 0.47 ± 0.11 for NPs with an average size of 1.3 nm. This enhancement cannot be explained just by the increase in surface area by NP downsizing. It may arise from the creation of new multi-fold adsorption surface and sub-surface sites due to the presence of many corner and edge atoms in ultra-small NPs with strong surface curvature.

International audience; An in-depth study of metal assisted chemical etching (MACE) of p-type c-Si in HF/H2O2 aqueous solutions using Pt nanoparticles as catalysts is presented. Combination of cyclic voltammetry, open circuit measurements, chronoamperometry, impedance spectroscopy, and 2D band bendingmodeling of the metal/semiconductor/electrolyte interfaces at the nanoscale and under different etching conditions allows gaining physical insights into this system. Additionally, in an attempt to mimic the etching conditions, the modeling has been performed with a positively biased nanoparticle buried in the Si substrate. Following these findings, the application of an external polarization during etching is introduced as a novel efficient approach for achieving straightforward control of the pore morphology by acting upon the band bending at the Si/electrolyte junction. In this way, nanostructures ranging from straight mesopores to cone-shaped macropores are obtained as the Si sample is biased from negative to positive potentials. Remarkably, macroscopic cone-shaped pores in the 1−5 μm size range with a high aspect ratio (L/W ∼ 1.6) are obtained by this method. This morphology leads to a reduction of the surface reflectance below 5% over the entire VIS-NIR domain, which outperforms macrostructures made by state of the art texturization techniques for Si solar cells.

The hydrogen sorption properties of nanosized palladium particles dispersed in high surface area graphite powders are studied by both solid/gas and electrochemical methods. Using either classical powder electrodes or a cavity microelectrode we show that cyclic voltammetry peaks coming from the hydrogen sorption/evolution, that are usually strongly overlapped in powder electrodes, can be resolved at low scan rates. With the cavity microelectrode, we could measure voltammograms over a 3-decade range of potential scan rates and thus describe the sorption processes in this hybrid material.

A simple and effective method for producing Ag nanoparticles at room temperature and on the time scale of several minutes on the surface of hydrogenated Si nitride (SiN x :H) films is presented. Depending on the stoichiometry of SiN x :H layers, Ag nanostructures with different sizes and densities can be produced. The structural properties of the deposited Ag nanoparticles are described in details with respect to the stoichiometries of SiN x :H films.

Metal-assisted etching of silicon in HF/H2O2/H2O solutions with Ag nanoparticles as catalyst agents was investigated. SEM observations and etch rate measurements were carried out as a function of the etching solution composition. Depending on the relative amount of HF and H2O2, different regimes of dissolution take place and a strong similarity with the etching in HF–HNO3 solution is evidenced, for the first time. Formation of meso- and macroporous Si, etched craters and polished Si are observed as the HF/H2O2 ratio decreases. The dissolution mechanisms are discussed on the basis of a localized hole injection from the Ag nanoparticles into Si and in terms of the well known chemistry of Si dissolution in HF-based chemical and electrochemical systems. At high HF/H2O2 ratio, there is no formation of oxide at the surface. Hole injection and Si dissolution occur at the level of the Ag nanoparticle only, resulting in the formation of meso and macropores depending on the Ag nanoparticle size. At low HF/H2O2 ratio, the Si surface is oxidized, the injected holes are homogeneously distributed and thus polishing occurs. There is an intermediate range of composition in which injected holes diffuse away from the Ag nanoparticles and cone-shaped macropores, several tens of nm in diameter are formed.

Taking advantage of the electrochemical formation of macropores in aqueous HF under illumination, we have developed an efficient photoelectrochemical (PEC) texturization on n-type multicrystalline Si (mc-Si) wafers which leads to a decrease in the reflectivity of solar cells. An optimized mc-Si texturized surface exhibits an 8.5% effective reflectivity (λ=400-1000 nm, AM1.5 standard solar illumination).

MX2 nanoparticles, with M = Mo, W and X = S or Se, were synthesized via a chemical reaction between molybdenum carbonyl (or tungsten carbonyl) and chalcogen dissolved in para-xylene solution. X-ray diffraction and high-resolution transmission electron microscopy analysis showed that the MoS2, WS2, MoSe2 and WSe2 powders were constituted of rounded amorphous nanoparticles. Upon annealing at 550 °C the nanoparticles crystallized, losing their round shape. The four materials synthesized under similar conditions showed that the annealed selenide nanoparticles (MoSe2 and WSe2) are well crystallized, with aligned stacks of 5–10 layers (002 van der Waals (vdW) planes), while the annealed sulfide nanoparticles (MoS2 and WS2) exhibit curved and tangled 002 planes, randomly distributed in the particle.

Conditions of macropore formation on multicrystalline medium doped silicon were deduced from a systematic study of the macroporous morphology of (100) and (111) oriented monocrystalline silicon in function of the porous etching parameters and silicon doping. A new promising method to texturize the surface of multicrystalline silicon wafers has been developed which leads to a macroporous surface with a very low effective reflectivity ∼9%. Macroporous multicrystalline silicon solar cells exhibit high efficiency larger than 13.5%.

A porous silicon (PS) layer formed electrochemically in the outer part of the n emitter of p-n Si junctions can be used as an efficient antireflection coating (ARC). A two-step procedure is presented which can determine the electrochemical parameters leading to the formation of an optimized single-layer PS ARC. Single-layer PS ARCs achieving: 7% effective reflectance between 400 and 1000 nm are obtained on shallow p‐n junction solar cells. To reduce the reflectance further, the design of double-layer ARCs based on PS is investigated. PS layers with different porosities can be realized in a single experiment by modulating the current density during the electrochemical process. It is shown theoretically and experimentally that such PS structures can lead to an effective reflectance below 3%. © 2000 Elsevier Science S.A. All rights reserved.

The antireflection properties of electrochemically formed porous silicon (PS) layers in the 0.3 μm thick n+ emitter of Si p–n+ junctions, have been optimized for application to commercial silicon photovoltaic cells. The porosity and thickness of the PS layers are easily adjusted by controlling the electrochemical formation conditions (current density and anodization time). The appropriate PS formation conditions were determined by carrying out a two steps experiment. A first set of samples allowed to determine the optimal porosity and a second one to adjust the thickness of the PS layers, by evaluating the interference features of the reflectance produced by the layers. A PS layer with optimal antireflection coating (ARC) characteristics was obtained in 30% HF in only 3.5 s. The effective reflectance is reduced to 7.3% between 400 and 1150 nm which leads to a gain of up to 33% in the theoretical short circuit current of a p–n+ shallow junction compared to a reference junction without a PS layer. The effective reflectance with optimized PS layers is significantly less than that obtained with a classical TiO2 ARC on a NaOH pretextured multicrystalline surface (11%).

The electrochemical formation of porous silicon (PS) layers in the n+ emitter of silicon p–n+ homojunctions for solar energy conversion has been investigated. During the electrochemical process under constant polarization, a variation of the current density occurs. This effect is explained by considering the doping impurity gradient in the emitter and by TEM characterization of the PS layer structure. Optical transmission measurements indicate that modifications of the refractive index and absorption coefficient of PS are mainly related to the porosity value. Reflectivity measurements, spectral response and I–V characteristics show that PS acts as an efficient antireflection coating layer. However, beyond a critical layer thickness, i.e. when PS reaches the p–n+ interface, the junction properties are degraded.

Pyramidal textures are commonly used to reduce reflections from silicon solar cells and improve light absorption by light trapping. They are generally modelled or characterised under normal incidence. In this work, a monolayer 3D ray tracing program taking into account the polarisation of light have been developed, validated and used to compute the directional-hemispherical reflectance versus the azimuth and incidence angles for both regular upright pyramids and inverted ones, with (1 1 1) facets. Results are given for a wavelength of 0.7 μ m . They show that this reflectance is not minimal at normal incidence but for an incidence angle near 20 ° and that upright pyramids can have a lower hemispherical reflectance than inverted ones for incidence angles in the middle range. The bihemispherical reflectance is 19.6% for regular upright pyramids and 20.7% for inverted ones. The effect of the pyramids aspect ratio on the hemispherical reflectance at normal incidence is also studied. This reflectance decreases with the aspect ratio of both textures. Above an aspect ratio of 0.51, inverted pyramids have a lower hemispherical reflectance. But their bihemispherical reflectance is lower only for aspect ratios below 0.23.

We report photoelectrochemical measurements of diffusion lengths in p-type multicrystalline silicon used for photovoltaic solar cells. We show that, under some conditions, this method provides a very practical means for rendering an account of the quality nature of this material through diffusion length variations. The results obtained during two years of use of this method in the industrial processing of photovoltaic cells are presented and discussed

We have prepared absorbing structures for photovoltaic cells with different nano-texturization, obtained by means of a femtosecond laser, without the use of corrosive gas (i.e. under vacuum). To take in account the 3D structured front surface, the emitter doping has been realized by using Plasma Immersion Ion Implantation (so-called PULSION). The results show a photocurrent increase up to 60 % in the laser texturized zones.

We report on a method based on photoelectrochemistry which allows the measurement of diffusion lengths of multicrystalline silicon before fabricating the photovoltaic device. The results obtained with this method after two years experience on a production line, by the photovoltaic manufacturer Photowatt Int. are presented. It concerns the variation of the diffusion length within the ingots and its correlation with the short-circuit current of the final cells, and the effect of industrial gettering. We also present diffusion length maps on a 10/spl times/10 cm/sup 2/ cell.

Optical and structural properties of thin porous silicon (PS) layers electrochemically formed in the outermost part of 0.3 μm thin emitters of commercial silicon shallow junctions were studied by spectroscopic ellipsometry (SE). The SE data were evaluated with a multiparameter fitting procedure based on the Bruggeman effective medium approximation by using a mixture of fine‐grain polycrystalline Si and voids. The fit results show that the thicknesses of PS films grown at a constant current density of in 30% hydrofluoric acid (HF) with etching times of 1.5 and 3.5 s are 54 and 105 nm, respectively. When the formation time is less than 2.5 s, the porosity is a constant 60% throughout the entire PS layer. A porosity gradient occurs across the layer for formation times greater than 2.5 s, resulting in a layer of up to 85% porosity near the surface after 3.5 s. This increasing porosity is due to chemical dissolution by the HF electrolyte, occurring as a simultaneous reaction to the electrochemical PS formation. All the PS layers grow at a rate of , which is independent of the PS formation time and is not affected by the phosphorus doping gradient in the emitter. The calculated number of elementary charges, z, needed to dissolve one Si atom from the bulk is 3.3. Comparison of the SE fit with total reflectance measurements confirms the validity of the SE modeling. PS layer thicknesses were measured independently of the SE experiments by combining secondary ion mass spectroscopy measurements and surface profiling. © 2000 The Electrochemical Society. All rights reserved.