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International audience; We report on a new class of metamaterials, which consists of meta-foams, optimized for high performance in enhanced solar steam generation. The design of these meta-foams involves precise dimensional control of a periodic two-dimensional micro-pore array. Proof-of-concept of meta-foams efficiency is demonstrated using two fabrication technologies. The first one involves microscale plasma etching onto a nanostructured black silicon substrate. As an alternative low-cost technique, we also use 3D printing of a graphene-containing polymer material. The experimental validation shows that the best evaporation rate reaches 1.34 kg/(h·m2) under 1 sun illumination, in open-air conditions under normal temperature and relative humidity conditions of 20 °C and 58%, an unmatched value so far under similar conditions, while the theoretical limit is estimated at 1.5 kg/(h·m2). This corresponding to a 89% conversion efficiency. Experimental data concur well with the predicted performance obtained by numerical simulations. The proposed model simultaneously accounts for both heat and mass transfer, including photothermal conversion and water phase change, to guide the optimization towards the maximization of the evaporation rate, an important figure of merit in many applications, including solar heat harvesting and solar water purification.

Photoelectrocatalysis takes advantage of electrocatalytic and photocatalytic processes [...]

International audience; To reduce the energy cost and environmental impact of biological nitrogen removal in wastewater treatment plants, it would be advantageous to treat urea contained in urine at the source. In this perspective, FTO/Ti-Fe2O3 (nanorods) photoelectrodes decorated with Ni as catalyst are developed and tested for urea photoelectrocatalytic oxidation under solar illumination. Gains up to 0.50 V in oxidation onset potential vs. metallic Ni are obtained thanks to a Ni photoelectrodeposition method. In situ transmission measurements (based on NiOOH light absorption) during electrochemical cycling allowed to evaluate the state of active Ni sites and confirmed that urea oxidation mechanism is of EC type. Photoelectrolyses give faradaic efficiencies of 10–18% and 9-35% for N2 and O2 formation, respectively. A significant and unexpected NO2− production (∼65%) is detected indicating another or incomplete reaction pathway. The photoelectrocatalytic removal of nitrogen from urea solutions is demonstrated but requires catalysts with higher selectivity towards N2.

To shift towards the greener city, photocatalytic urban infrastructures have emerged as a promising solution for pollution remediation. To reach this goal, the large bandgap semiconductors, such as nontoxic Zinc Oxide (ZnO), already proved their excellent photocatalytic performances. However, integrating and developing cost-effective and greener photocatalytic surfaces with an easily scaled-up synthesis method and without energy and chemical product overconsumption is still challenging. Therefore, this work proposes to develop a depolluting Zinc (Zn) roof covered by ZnO nanostructures (NSs) using a one-step seedless hydrothermal growth method in 2 h. The feasibility of this synthesis was firstly studied on small areas of Zn (1.25 cm2) before being scaled up to medium-sized areas (25 cm2). The efficiency of this functionalization route for ZnO NSs grown without seed layer was attributed to the presence of Zn2+ sites and the native oxide film on the Zn surface. Their photocatalytic efficiency was demonstrated by removing in less than 3 h the Methylene Blue (MB) and Acid Red 14 (AR14) in both DI water and rainwater under UV-light. Promising results were also recorded under solar light. Therefore, the photocatalytic Zn roof functionalized by ZnO NSs is a promising route for rainwater purification by photocatalysis.

Nanoporous materials with 3-D interconnected channels and large surface areas have been known to be technologically useful for a wide spectrum of applications, particularly in the fields of energy storage and conversion, supported catalysis and separation technology. For the first time, we herein report upon an original approach to elaborate nanoporous polymers, especially thermosets, with bicontinuous morphology. The versatility of this strategy stems from the possibility of using nanoporous gold (np-Au) as a template to replicate any polymeric material. Depending on the solubility behavior of the polymer, the infiltration solution used in the step of pore filling is different, i.e. a preformed polystyrene (PS) solution or pure liquid dicyanate monomers followed by in-situ polymerization in the case of thermosetting polycyanurate (PCN). After selective and quantitative dissolution of the np-Au template in a KI/I2 solution, the resulting np-PS or np-PCN, respectively, takes the inverse morphology of np-Au, as evidenced by SEM and Fourier transform analysis. Both np-PS and np-PCN frameworks present well-defined nanostructures with a homogeneous size distribution over the entire volume of the processed samples. This new pathway is very promising not only because of its versatility in terms of polymer diversity but also for the possibility of precisely tuning the nanostructure size of the resulting nanoporous polymers.

International audience

International audience; Cavity MicroElectrodes (CMEs) offer a valuable platform to evaluate the electrocatalytic performance of micro-and nano-particulate materials. The technical factors and physicochemical processes affecting the electrochemical response at CMEs are to be recognized, specifically, the accessibility of redox species to the electrocatalyst surface. With this aim, the voltammetric response of CMEs is investigated through a joint experimental and theoretical approach including a comparative study of cyclic and square wave voltammetry (SWV). Experiments reveal a capacitive distortion of the response that increases with the powder surface area, but with a faradaic response analogous to that of recessed or inlaid microdisks, i.e. with electrochemical reactions occurring essentially on the first layer of the powder load. We show that SWV is well suited to discriminate Faradaic processes at CMEs and we present accurate mathematical expressions to describe it. These results provide guidelines for the design and analysis of CME voltammetric measurements.

A study of the CO2 electrochemical reduction reaction (CO2RR) at Pd100−xCux solid solution type nanoalloys is presented. Cyclic voltammetry and chronoamperometry are used in combination with a cavity microelectrode (CME) for the first time. Low-distorted voltammetric signals obtained with the CME evidence that for alloys with low or moderate Cu content (up to 50 at%) the typical CO2RR inhibition peak, related to CO blocking Pd active sites, becomes a current plateau. Interestingly, the plateau height matches the theoretical value corresponding to the steady state current of a microdisc. This fact indicates a better tolerance of Pd100−xCux towards CO poisoning and also the attainment of a pseudo-steady state where mass transport should be controlled by diffusion. The synergy between Cu and Pd is thus established for the first time at the level of the voltammograms. The chronoamperometric responses exhibit three well defined regions corresponding to (1) double layer charging and diffusion/reaction establishment, (2) beginning of CO poisoning by formation of a CO adlayer and (3) final collapse of the catalytic activity at a critical time, tc, which depends directly on the bimetallic composition. A plot of tc vs. Pd content exhibits a clear volcano shape, from which ~ Pd72Cu28 is identified as a close to optimal composition in terms of CO tolerance and Pd35Cu65 as the composition from which the advantage of alloying Pd with Cu is lost. An in-depth analysis of the I/V response and its evolution with composition and time is performed in the CO2RR and CO stripping regions. As a result, the possibility of developing a method to further increase the catalyst lifetime based on periodical releasing of “CO-like” intermediates by CO stripping is proposed.

Photocatalysis is proven to be the most efficient and environmentally friendly method for the degradation of organic pollutants in water purification. To meet the requirement of large-scale water treatment, there are two important points: One is the lifetime and chemical stability of the photocatalyst material, especially in the complex and harsh aqueous conditions. The other is the ease of synthesis of such photocatalysts with specific nano-morphology. In this work, two common photocatalyst materials, zinc oxide (ZnO) and titanium dioxide (TiO2), are selected to form more sustainable photocatalysts with high chemical stability. This involves the combination of both TiO2 and ZnO in a two-step simple synthesis method. It appears advantageous to exploit the conformal deposition of atomic layer deposition (ALD) to achieve nanometer-thick TiO2 coating on ZnO nanowires (NWs) with a high aspect ratio, which are firmly anchored to a substrate and exhibit a large specific surface area. The high chemical stability of the ALD TiO2 coating has been investigated in detail and proven to be effective under both strong acid and strong alkaline aqueous solutions. In addition, photocatalysis experiments with organic dyes show that via this simple two-step synthesis method, the produced ZnO/TiO2 tandem photocatalysts does indeed exhibit improved chemical stability in a harsh environment, while allowing efficient photodegradation.

International audience; 4-benzenethiol-functionalized High Surface Area Graphite powder was prepared and decorated with bimetallic Cu 100-x Rh x nanoparticles (NPs) to serve as electrocatalysts for the reduction of nitrates. In the first step, the HSAG powder was grafted with in situ generated diazonium compounds from 4-aminothiophenol (ATP), in acidic medium using NaNO 2 for the diazotization process. The surface composition was tuned using different initial quantities of ATP. The surface XPS-determined S/C atomic ratio was found to increase stepwise with the initial amine quantity. In a second step, the grafted and untreated HSAG powders were decorated with Cu 100-x Rh x NPs by a wet chemical method and the elemental composition of the end composites assessed by EDS-SEM and ICP, whereas TEM and EDS-TEM served to characterize the NPs morphology and their composition at the nanometer scale. In all cases, the NP size was invariably found to be ~1.7 nm but with a size distribution becoming narrower under increasing grafting rate and the global composition enriched in copper. Voltammetry was performed with a cavity microelectrode to evaluate the electrocatalytic performances of the composites for nitrate reduction. Increasing diazonium grafting led to a progressive reduction of the peak current intensity and a shift of the peak potentials towards cathodic values. Maximum intensity was obtained for 0.005 µmol of diazonium salt per mg of HSAG, 2 with a gain of 40 % in comparison to the best untreated sample. This improvement and a change of the voltammogram characteristics after grafting seem to result from modifications of the local composition at the level of NPs that differs from the global composition. This work conclusively shows that diazonium surface modification is important not only to attach electrocatalytic NPs to carbon supports but also to provide narrower size distribution of the electrocatalysts together with finely tuned catalytic properties.

International audience

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International audience

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.

International audience

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.

International audience

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.

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 Fe

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 (

National audience

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.

International audience; Metal Assisted Chemical Etching (MACE) of Si has attracted the attention of academy and industry during the last decades as an efficient low-cost wet etching method to produce Si nanostructures with high aspect ratios (HAR). Several noble metals are known to be effective catalysts for MACE: Ag nanoparticles (NPs), for example, provide an extremely localized etching with the formation of mesopores or Si nanowire arrays; contrarily, MACE with Pt NPs is rather delocalized, resulting in the formation of large pores whose potential application in Si surface structuration has received less attention. In this work, MACE with Pt NPs under an applied external bias is presented as a novel approach to synthesize HAR Si nanostructures of controlled size and shape, with clear application as built-in blocks for photovoltaic devices, the reflectivity being < 3 % vs. ~10 % for the state of the art texturization technique (inverted pyramids). A combination of voltammetry, impedance spectroscopy and band bending modelling allowed complete physicochemical characterization of this MACE process. This simple method allows straightforward control of the pore morphology, such that nanostructures ranging from straight mesopores to cone-shaped macropores are readily obtained as Si is biased from negative to positive potentials. Because such morphologies are difficult to obtain even with techniques like cryogenic plasma, etching MACE with Pt may have a strong potential for Si surface structuration.

International audience; We present a study on metal assisted chemical etching of p-type c-Si with Pt nanoparticles, performed under controlled polarization of the Si sample in a 3-electrodes electrochemical setup. OCP measurements combined with impedance spectroscopy and cyclic voltammetry allow to give physical insights in the MACE process. In addition, the application of an external polarization during etching results in a straightforward control of the pore morphology, ranging from straight mesopores to cone-shaped macropores as the Si sample is positively biased. The latter morphology leads to a reduction of the surface reflectivity below 5% which compares favorably with state of the art texturization techniques for Si solar cells.

Although hydrogenated amorphous silicon is already widely examined regarding its structural and electronic properties, the chemical etching behavior of this material is only roughly understood. We present a detailed study of the etching properties of intrinsic hydrogenated amorphous silicon, (i)a-Si:H, layers on crystalline silicon, c-Si, within the framework of metal assisted chemical etching (MACE) using silver nanoparticles (Ag NPs). The etching processes are examined by in situ photoluminescence (PL) and in situ surface photovoltage (SPV) measurements, as these techniques allow a monitoring of the hole injection that takes place during MACE. By in situ PL measurements and SEM images, we could interpret the different stages of the MACE process of (i)a-Si:H layers and determine etch rates of (i)a-Si:H, that are found to be influenced by the size of the Ag NPs. In situ PL and in situ SPV measurements both enable researchers to determine when the Ag NPs reach the (i)a-Si:H/c-Si interface. Furthermore, a preferential MACE of (i)a-Si:H versus c-Si is revealed for the first time. This effect could be explained by an interplay of the different thermodynamic and structural properties of the two materials as well as by hole injection during MACE resulting in a field effect passivation. The presented results allow an application of the examined MACE processes for Si nanostructuring applications.