Th. Pauporté

Heteroepitaxial electrodeposition of zinc oxide films on gallium nitride

Epitaxial zinc oxide films have been prepared on gallium nitride (0002) substrates by cathodic electrodeposition in an aqueous solution containing a zinc salt and dissolved oxygen at 85 °C. The films have the hexagonal structure with the c axis parallel to that of GaN and the [100] direction in ZnO parallel to the [100] direction in GaN in the (0002) basal plane. The structural quality is attested by the values of the full width at half maximum in θ/2θ x-ray diffraction (XRD) diagrams [0.07° for the (0002) peak] and in five circles XRD diagrams [0.74° for the ZnO (1011) planes compared to 0.47° for the GaN (1011) planes]. The morphology of the layers has been studied by scanning electron microscopy. Before coalescence, arrays of epitaxial single crystalline hexagonal columns are observed with a low dispersion in size, indicating instantaneous tridimensional nucleation. Preliminary results on luminescence properties of the films before and after annealing are presented.

Hydrogen Peroxide Oxygen Precursor for Zinc Oxide Electrodeposition I. Deposition in Perchlorate Medium

Cathodic electrodeposition of zinc oxide is demonstrated using dissolved hydrogen peroxide as the oxygen precursor. Unlike molecular oxygen, H 2 O 2 is a very soluble compound in water and can consequently be used at high concentrations allowing high deposition rates. Unlike nitrate ions, the reduction reaction does not give rise to by-products, which may progressively contaminate the deposition bath. ZnO films have been deposited at 70°C and - 1.4 V vs. a saturated sulfate mercurous electrode onto tin oxide over a large range of peroxide concentration with perchlorate as the supporting electrolyte. The films are well covered and crystallized with the wurtzite structure and textured with the c axis perpendicular to the electrode surface. A transition between dense films and porous films is observed at the higher H 2 O 2 concentration investigated. At 40 mM H 2 O 2 , the porosity of the films deposited in the presence of 80 mM zinc perchlorate is estimated at 33% with deposition rate of ca. 16 μm h -1 .

Hydrogen peroxide oxygen precursor for zinc oxide electrodeposition II—Mechanistic aspects

Zinc oxide thin films have been deposited cathodically at 70 °C from a chloride aqueous solution containing dissolved hydrogen peroxide and a zinc(II) chloride. The electrodeposition process has been studied by cyclic voltammetry (CV), chronoamperometry, and quartz crystal microbalance techniques. A parametric study has been carried out by varying the solution composition and the substrate. It is shown that the film growth is under kinetic control since, the catalytic activity of ZnO surfaces for the reduction of hydrogen peroxide species is low under these conditions. The nature of the substrates (tin oxide, gold) and their treatments prior to the deposition experiments are shown to have a marked influence on the electrochemical behavior of the system. Besides, we show that it is possible to normalize the film growth behavior by taking the mean current density as the key parameter. For values below the limiting diffusion current of zinc ions, the faradaic efficiency is close to 100%, whereas, above this value the faradaic efficiency decreases markedly. The results have been modeled with the help of a simple competition mechanism in which we assume that the hydroxide ions produced in excess at the surface diffuse towards the solution and react partly with zinc ions diffusing towards the surface. That reaction in solution reduces the availability of zinc ions for the heterogeneous deposition reaction at the surface. We have proposed an empirical law to describe this phenomenon.

Cathodic electrodeposition of mixed oxide thin films

Abstract Cathodic electrodeposition is a rather unexplored route for the synthesis of mixed oxides. Two different mechanisms are generally reported which lead to oxide thin film formation: (i) the direct reduction of the oxidation state of the metallic element, (ii) an interfacial pH increase and local supersaturation followed by oxide precipitation. We emphasize the major requirements for mixed thin film formation via electrosynthesis and present results obtained with two different systems of special interest. The first, ZnO/Eu(OH) x , is based on the second mechanism. The second, TiO 2 /WO 3 , is a mixed mechanism process. We describe the conditions under which europium oxide can be deposited cathodically. This oxide is obtained by co-deposition with zinc oxide. After a heat treatment at 400 °C, X-ray diffraction shows that the films are mainly made of monoclinic Eu 2 O 3 . The successful WO 3 –TiO 2 mixed film electrodeposition in a wide compositional range is also reported.

One-step electrodeposition of ZnO/eosin Y hybrid films from a hydrogen peroxide oxygen precursor

The deposition of eosin Y/zinc oxide hybrid films is performed by cathodic electrodeposition from a hydrogen peroxide oxygen precursor in chloride medium. Typically the deposition bath contains 5 mM ZnCl2 ,1 0 mM H 2O2 and less than 10 mM eosin Y. A rotating disk substrate is used with a rotation speed of 500 rpm. The deposition potentials range between /1.25 and /1.50 V versus the mercurous sulfate electrode (MSE). The resulting films are well crystallised and highly textured with the c axis perpendicular to the substrate. They exhibit an electrocatalytic activity for hydrogen peroxide reduction during the deposition. The role of reduced eosin bound to ZnO in the electrocatalysis is discussed. We have measured a maximum of dye loading at 0.21 M and we show that the dye is concentrated in the film compared to the dye/zinc ion molecular ratio in solution. The dye loading decreases slightly with deposition overvoltage. We discuss the loading mechanism in the light of the dye diffusion behaviour in solution and of the relationship between dye loading and film growth rate. The chemical diffusion coefficient of eosin Y under the present conditions is measured an equal to 1.9/10 5 cm 2 s 1 . We show that the complexation between reduced eosin Y and free zinc ion in solution and the coprecipitation reaction of this complex are likely to be the key steps of the loading process. # 2002 Elsevier Science B.V. All rights reserved.

Copper Diffusion in Copper Sulfide: a Systematic Study

Copper sulfide Cu2−xS is a model mineral for chalcogenides because of the existence of a non-stoichiometric compounds series in the range Cu2S - CuS in which properties change with x. For this reason, we have studied the influence of the mineral composition on the diffusion in this solid.Electrochemical Impedance Spectroscopy (EIS) applied to Cu2-xS/cupric sulfate electrolyte was the main investigation technique. It enabled us to work at the equilibrium potential at which the composition is fixed and known. Changing the composition by electrochemically removing (or adding) a known amount of Cu, we were able to determine the chemical diffusion coefficient of copper in the composition range (from x=0 to 0.066). In this work, we present the results obtained in the chalcocite and djurleite phases. These results were compared to other values reported in the literature. From this systematic study we discuss various diffusion mechanisms. Our observations support that in chalcocite and djurleite Cu diffuses via a vacancy mechanism.

Structural Changes in Electrochromic WO 3 Thin Films Induced by the First Electrochemical Cycles

We have investigated the effect of the very first electrochemical cycles on the local structure and bonding for two different amorphous transparent electrochromic WO 3 thin films, namely, a sputtered film and an electrodeposited film. The cycles are performed at a constant potential scan rate between 0.65 and -0.55 V vs. NHE in a 0.3 M LiClO 4 /propylene carbonate solution. We show that this treatment has a weak influence on the short range order in sputtered films. On the contrary, dramatic changes are found in electrodeposited ones. Both Raman spectroscopy and extended X-ray absorption fine structure (EXAFS) data show that the initial electrodeposited oxide films are highly disordered with a highly asymmetric structure. The Fourier transforms of the EXAFS spectra of these films show four different W-O distances in the first oxygen shell with values ranging between 1.5 and 2.4 A (after phase correction). After the electrochemical treatment, the radial distribution of the oxygen atoms is changed. The structure becomes similar to that classically reported for amorphous WO, films and can be fitted assuming two different main distances. Raman spectroscopy and EXAFS reveal a larger W-O distance distribution in the two different amorphous films compared to a monoclinic WO 3 reference sample. They show a higher dispersion of the W-O distances in cycled electrodeposited films, compared to the cycled sputtered ones.

Temperature effects on copper diffusion in natural chalcocite

Abstract Two samples of chalcocite with compositions Cu1.9984S and Cu1.9962 were studied. They were first characterised by electrochemical, electrical and X-ray diffraction measurements. For the monoclinic γ-phase, the variations with temperature and composition of the chemical diffusion coefficient of copper in these mixed conductors were investigated using two different methods: measurements of the time dependence of the resistance change of the samples after application of a small dc voltage at an ion blocking microelectrode (between room temperature and ≈100°C) and electrochemical impedance spectroscopy (EIS) between room temperature and 58°C. The chalcocite hexagonal β-phase (>100°C) was studied using a microelectrode, showing a small decrease of D Cu at the phase change.

Single Nanowire Nanosensors: A Case Study of the Effects of Metal Doping on ZnO

Individual nanowire/nanorod sensors of semiconducting oxides are of great interest for science and engineering. In current work, the effects of metal doping on ZnO nanowire properties are presented. It is found that metaldoping greatly improved the performance of ZnO nanowire nanosensors. Experimental results and possible mechanisms of the improved performance are discussed.

Effects of annealing temperature and thickness of nanoparticle ZnO aggregate layers on dye-sensitized solar cell performances

ZnO sphere made of aggregated nanoparticles with a mean diameter of 19 nm have been prepared by the forced thermohydrolysis in polyol medium technique which is a versatile synthesis method for the preparation of metal oxide particles with controlled properties. The sphere were polydisperse and sub-micrometric in size. Porous layers have been prepared using these building blocks. They showed a large specific surface area and were highly light scattering in the visible wavelength region. We have investigated the performances of D149-dyesensitized solar cells (DSSCs) based on these layers. The annealing temperature as well as the layer thickness has been optimized. Finally the best cells reached an overall conversion efficiency of about 4.7% for layers with a thickness ranging between 27 and 35 μm and annealed at 400°C.