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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.

International audience

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.

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.

A conventional flatbed scanner equipped with an additional diffusor is used for rapid measurements of an important figure of merit for optical surfaces, the effective reflectivity of devices such as solar cells. The application of the technique to multicrystalline silicon wafers with light-trapping structures obtained by electrochemical etching is shown. The use of this method for rapid quality control in production environments as well as in the lab is envisaged: even with a non-optimized diffusor, a spatial resolution of ∼0.1×0.1 mm2 can be achieved, with an accuracy of the reflectivity measurements of ∼1% and data-acquisition times around 10 s per wafer.

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.

Des cellules solaires extremely thin absorber (eta) ZnO/CdSe/CuSCN a base de nanofils de ZnO ont ete realisees avec succes en utilisant des techniques faciles d'acces de depot en solution et par voie electrochimique. Une couche constituee de nanofils de ZnO monocristallin de plusieurs microns de hauteur et de 100-200 nm de diametre a ete deposee sur un substrat de verre conducteur (SnO 2 :F) recouvert d'une couche de ZnO formee par pyrolyse de spray agissant comme couche electronique bloquante. Une couche mince de 30-40 nm de CdSe jouant le role d'un absorbeur a ete electrodeposee de maniere conforme le long des nanofils de ZnO. Les espaces entre les nanofils composite de ZnO/CdSe ont ete remplis par du CuSCN semiconducteur de type p ; l'ensemble formant une jonction p-i-n. La couche de nanofils composite de ZnO/CdSe presente un important effet de confinement optique de la lumiere, avec une absorption effective de ∼ 89% et une reflectivite effective de ∼ 8% dans la region 400-800 nm du spectre solaire (AMI.5). L'influence du recuit sur la taille des grains de CdSe et sur le rendement de conversion de l'energie de la cellule eta-solar cell a ete analysee. Le rendement de conversion observe des cellules apres recuit de la couche de CdSe est, pour la meilleure d'entre elles, de 2,3%, demontrant ainsi que la nanoheterostructure ZnO/CdSe/CuSCN constitue une option interessante pour developper un nouveau type de cellules photovoltaiques.

The ZnO/CdTe/CuSCN heterostructure was analyzed as a candidate to act as an inorganic eta -solar cell. A ZnO film consisting of single crystal nanocolumns was electrodeposited on a transparent conducting substrate which acts as n-type material. As absorber material we used CdTe, which was deposited on the ZnO columnar film by Metal Organic Chemical Vapor Deposition. In order to complete the eta -solar cell we deposited a CuSCN layer by chemical solution deposition. A conformal and uniform CdTe coverage of the ZnO columns was achieved, producing a very efficient light trapping effect. The effective absorption (∼87%) and effective reflectance (∼10%) of the complete heterostructure in the 400–800 nm (AM1.5) solar spectrum range are very favorable for the use of this heterostructure in photovoltaic devices. Preliminary current–voltage characterization made on this ZnO/CdTe/CuSCN heterostructure shows rectifying behavior and a photovoltaic effect.

First results of CdTe deposition on ZnO films consisting of free standing single crystalline columns of several micrometers height and ∼100– 200 nm diameter are presented. The ZnO films have been obtained by electrochemical deposition on conductive glass. Morphology, structural and optical properties of the nanostructured columnar CdTe/ZnO composite films have been studied. The cadmium telluride ( 40 nm thick) deposited by vapor-phase epitaxy under dynamical vacuum is lining the ZnO columns as a continuous smooth thin film with conformal coverage. The polycrystalline CdTe layer is a mixture of cubic and hexagonal phases and is semiconductor with a direct band gap optical transition of 1.56 eV . The nanocomposite CdTe/ZnO film shows strong light trapping effect and has considerable potential for use in photoelectric thin film devices.

Thin films molybdenum dichalcogenide, MoS 2 , were prepared by cathodic electrochemical deposition from aqueous and non-aqueous solutions of tetrathiomolybdate ions, at different temperatures. The films were X-ray amorphous as deposited. They consist of an amorphous matrix in which quantum sized nanocrystallites or clusters were embedded. Upon annealing at high temperatures, the films obtained from aqueous solutions become crystalline and highly texturized having their van der Waals planes oriented parallel to the substrate, whereas, those obtained from ethylene glycol solutions kept on the amorphous matrix, with slightly larger sizes MoS 2 nanoparticles embedded, than before annealing. Difference in the mechanism of the electrodeposition in aqueous and ethylene glycol solutions is discussed.

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.

Visible luminescence observed from the nanoporous layer of the two (100)-orinted low doped and highly doped PEC-etched n-type Si is explained as being due to the existence of single crystal silicon quantum wires within their structure. The nanometer-size tangled Si structure is contained and attached to a regular geometric Si macroarray. TEM studies also reveal subtle variations in morphology between the two layers studied, which could explain the blueshift in the spectrum of the low-doped specimen — thinner and more rigid irregular wires — as compared to the highly doped specimen.