Functional oxynitride films for sustainable development

Resume : Oxynitride perovskites with the general formula AB(O,N)3 possess smaller band gaps than the corresponding pure oxides. This property gives rise to a number of potential applications like pigments, photocatalysts or novel semiconductors. In addition, many perovskite oxynitrides show good thermal and chemical stabilities. In this contribution, we report on hydrothermal synthesis techniques yielding oxide precursors, which after ammonolysis lead to nano-sized oxynitrides of very good crystallinity and large specific surface areas. Their structures and optical properties were examined and the photocatalytic activities in the solution-based decomposition of organic dyes were investigated. The use of co-catalysts was found to strongly improve the catalytic properities. For some oxynitrides, high and nearly temperature-independent dielectric permittivities have been reported but many investigations suffer from the low sintering activity of the samples. Here, we report on facile approaches to obtain dense oxynitride ceramics and even thin single-crystalline samples, whose dielectric properties were investigated depending on frequency and temperature.

Resume : For many years, perovskite materials have been studied regarding the wide variety of their properties achieved by substitutions on A and B cations-sites in the basic ABO3 structure. Moreover, by replacing nitrogen for oxygen, oxynitride perovskite compounds were also synthesized. They present original properties compared to their parent oxides, such as an absorption in the visible region resulting in colored materials with potential applications in the fields of visible light-driven photocatalysis and pigments. Since recently, attention has been focused on their dielectric properties; high permittivity values ranging from some tens to several thousands have been reported on ceramics and thin films. The dielectric and ferroelectric behavior of perovskite oxynitrides is currently the subject of many studies; a relaxor-type ferroelectric behavior has been proposed, which would originate from an O/N order in the perovskite structure. The purpose of the present contribution is to present an overview of the dielectric and ferroelectric properties of oxynitride perovskite compounds. It will be illustrated by our own results on oxynitride perovskite films such as LaTiO2N, SrTaO2N and (Sr1-xLax)(Ta1-xTix)O2N solid solution. This will include dielectric and ferroelectric measurements in macro and nano scales, in low and high frequencies and versus temperature.

Resume : The last years perovskite-type oxynitrides AB(O,N)3 and oxyfluoronitrides AB(O,N,F)3 have attracted great interest due to their applicability in various fields, e.g. photoanodes for solar water splitting (SWS).[1] For SWS application a band gap energy of 2.1 eV is beneficial. Fortunately the perovskite structure offers a great tolerance towards substitution on the A-, B- and O-site resulting in a tunable band gap and band edge positions. A partial anionic substitution of O2– by N3– leading to AB(O,N)3 (A = Mg, Ca, Sr, Y, La; B = Ti, Zr) causes a reduction of the optical band gap (oBG) up to 2 eV. By using cationic co-substitution for charge compensation an opening of the oBG might result due to an enhanced tilting of the octahedral network.[2] For double anionic substitution the presence of N and F narrowed the oBG and additionally lowered the anionic vacancy concentrations in comparison to only cationic substituted AB(O,N)3. References: [1] S. Yoon, A. E. Maegli, L. Karvonen, A. Shkabko, S. Populoh, K. Gałązka, L. Sagarna, M. H. Aguirre, P. Jakes, R. A. Eichel, A. Weidenkaff, Z. Anorg. Allg. Chem. 2014, 640, 797. [2] M. Widenmeyer, C. Peng, A. Baki, R. Niewa, A. Weidenkaff, Solid Sate Sci. 2015, in press.

Resume : Oxynitride perovskite materials with ABO3-xNx formula present interesting properties in various applications (pigment, photocatalysis, …), due to bandgap tuning by controlling nitrogen amount. However, the mains oxynitride perovskite synthesis are reported for B cations with stable high valency state (in order to keep electroneutrality), such as LaTiO2N, LaTiOxNx, LaVO3-xNx, CaTaO2N, etc. The nitrogen doping of perovskites with rare earth elements and transition metal cations without stable high valency state (such as iron) is almost not reported yet, but should lead to similar results. We have studied the case of one-step LaFeO3-xNx thin film synthesis, by magnetron sputtering using O2 and N2 as reactive gases and from two metallic targets. A large effort has been carried out to optimize deposition conditions (dual sputtering in presence of two reactive gas), with synthesis of multilayers with different deposition atmospheres analyzed by SIMS to link O and N contents into the film in relation to the gas composition. Then, different LaFeO3-xNx thin films have been deposited with following Ar/O2/N2 flow rate ratios: 21/7/7 and 21/1/6, and at different temperatures (up to 800°C). Optical measurements (UV-visible and FTIR spectrophotometry) have been performed and revealed a decrease of the bandgap and new IR vibration modes. Nitrogen quantification has been investigated using different methods (WDS, EDS, ERDA), and seems to be limited to few at% but leads to modification of physical and optical properties.

Resume : The bond strength of a metal–ceramic system is affected by the adhesion between its ceramic and metallic parts, being determined by their chemical, mechanical and thermal compatibility. For this purpose, Cr silico-oxynitrides in monolayer structure were deposited by cathodic arc technique on CoCr dental alloys, as a function of substrate bias voltage. Moreover, functionally graded layers consisting of Cr/CrSi/CrSiN/CrSiON were also obtained, and the influence of Cr and CrSi layers added at the metal interface was investigated. Since the layers are intended to be used in oral environment, their electrochemical behaviour in artificial saliva was analysed by means of polarization measurements. Additionally, odd random phase multisine electrochemical impedance spectroscopy was employed to assess the long term performance of the obtained layers. The characterization of the new metal-layer-ceramic systems was based on the requirements specified in ISO 9693:2012 standard, and correlations with elemental, phase composition and adhesion results were made. It was shown that better protective performance and higher bond strength were exhibited by the monolayers deposited at higher substrate bias voltage, ascribed to the beneficial effect of the more intense ion bombardment during film deposition. The addition of the adhesion layers to the CrSiN/CrSiON structure was shown to be beneficial to the electrochemical behaviour and metal-ceramic bond strength.

Resume : Photoelectrochemical (PEC) splitting of water appears to be the most promising, economically viable and sustainable way for the production of hydrogen. Although, there are a series of new engineering strategies which have emerged towards development of highly efficient PEC water splitting for hydrogen generation. Yet, finding an optimal material for PEC cell is a very difficult task due to three main requirements in a single material system: 1) chemical stability 2) visible light absorption and 3) band edges matching to redox levels of water. This talk will summarize the results obtained for photoelectrochemical water splitting using metal oxide semiconductor based heterostructured photoanodes such as ZnO/TiO2 , Fe2O3/TiO2 and BiVO4/Fe2O3. These heterostructures were modified by various engineering strategies such as band structure engineering, material disordering, nano/micro engineering, surface/interface engineering to improve the performances of metal oxide-based materials, especially favoring the photoelectrochemical activity under simulated sunlight. Recently, a new engineering strategy, i.e., band re-alignment at the heterostructure was achieved by gas–phase modification technique, which strongly promotes interfacial interaction at the junction and leads to an effective interfacial charge separation and charge transport. The modified heterostructures exhibit significant enhancement of visible light absorption and improve the photoelectrochemical response.

Resume : Photocatalytic water splitting into H2 and O2 is now attracting much interests from conversion of solar to chemical energy. There are several catalysts, mainly, Ta based oxide, are reported as active catalyst. However, active catalyst is generally wide band gap semiconductor resulting in using only UV light. In contrast, it is reported that GaN doped with ZnO is active for water splitting by using visible light. However, details of effects of composition on photocatalytic water splitting are not reported up to now. In this study, effects of N and O content on water splitting activity of GaN:ZnO catalyst were studied. Since ZnO is evaporated during nitridization of Ga2O3 -ZnO mixture with NH3, control of composition is rather difficult. In this study, effects of nitridization temperature and period on the composition were studied systematically and it was found that the optimized nitridization condition with NH3 is 1123K for 8.5h. H2 and O2 formation rate increases with increasing N content and at 74%, the highest H2 and O2 formation rate was obtained. On the other hand, the optimized content of Zn exists around 15%. On the optimized composition, complete water splitting into H2 and O2 was achieved. Combination of GaN:ZnO with organic dye is also studied for increasing photocatalytic activity to water splitting.

Resume : Zinc nitride (Zn3N2) is an attractive material for microelectronics due to its band gap (1.23 eV), high mobility (~100 cm2/V*s), carrier density (1018-1020 cm-3), and low resistivity (10-2-10-3 Ohm*cm) [1]. Indeed, the development of thin film transistors and based on Zn3N2 has been recently reported [2,3]. Despite this progress, zinc nitride is still poorly studied because it is a metastable material, reacting with air in normal ambient conditions and experiencing a progressive oxidation that leads to a zinc oxide (ZnO) phase [4]. Therefore, the understanding and control of the exact mechanisms inducing this transformation is essential to build new devices based on this material. In this work we have analysed the oxidation of Zn3N2 layers grown by RF magnetron sputtering of a pure Zn target (99.995 %) with reactive N2 plasma (99.999 %) at room temperature. Different ex-situ oxidation processes has been tested, including air, thermal treatments under O2 gas and plasma (ranging from 300 C up to 900 C), and water. The passivation of the surface has been also studied, obtaining successful results with thin capping layers of ZnO. The phase transformation in both as-grown and passivated systems was monitored through scanning electron and atomic force microscopy (SEM, AFM), spectroscopic ellipsometry (SE), and elastic recoil detection analysis with time-of-flight telescope (ERDA-TOF) using 30 MeV I5+ ions. As a result of these data, we have developed electrical sensors on transparent solid (glass) and flexible (PEDOT) substrates, evaluating the response to air/water contact. These devices have been characterized with a probe station, measuring the resistivity change as a function of different parameters. Our results demonstrate that Zn3N2-based sensors can be potentially applied as chemical sensors of different gases or liquids containing O. [1] C. Garc�a N��ez, J.L. Pau, M.J. Hern�ndez, M. Cervera, E. Ru�z, J. Piqueras, Thin Solid Films 522, 208 (2012). [2] C. Garc�a N��ez, J.L. Pau, E. Ru�z, J. Piqueras, Appl. Phys. Lett. 101, 253501 (2012). [3] S. Sinha, D. Choudhury, G. Rajaramanb, S.K. Sarkar, RSC Adv. 5, 22712 (2015) [4] C. Garc�a N��ez, J.L. Pau, M.J. Hern�ndez, M. Cervera, and J. Piqueras, Appl. Phys. Lett. 99, 232112 (2011).

Resume : Silicon Oxynitride films are environmental friendly and show remarkable properties for applications like solar energy conversion by frequency conversion or light emitting devices. Indeed such thin films are ideal matrices for a rare earth ions doping thanks to the presence of nitrogen which will increase the doping limit in oxide films. The aim of this study is to elaborate luminescent Ce3+ doped SiOxNy films by using radio frequency sputtering technique with N reactive flow. By adjusting the deposition parameters, different oxynitride compositions have been produced with optical indices going from 1.5 up to 3.8 (at 1.95 eV). RBS measurements indicate the chemical compositions evolution which explains the index variation. Simulations of the films compositions evolution depending on the index deduced from a two (Si3N4/SiO2, Si/Si3N4 and Si/SiO2) and three phases mixing (SiO2/Si/Si3N4) will be linked to the experimental results. Such an index variation affects the structural environment and more precisely the 5d band of Ce3+ offering thus interesting optical properties. Photoluminescence measurements show a wide and bright luminescence centered at 480 nm under a 325 nm wavelength excitation. A carefully investigation of the emission as a function of the excitation wavelength and for various N incorporation is lead to show a blue shift up to 350 nm emission wavelength.

Resume : Ta3N5 nanostructures are widely explored as anodes for photoelectrochemical (PEC) water splitting. Although the material shows excellent semiconductive properties for this purpose, the key challenge is its severe photocorrosion when used in typical aqueous environments. In the present work we introduce a NiFe layered double hydroxide (LDH) co-catalyst that dramatically reduces photocorrosion effects. To fabricate the Ta3N5 electrode, we use through-template-anodization of Ta and obtain oxide nanorod arrays that then are converted to Ta3N5 by high temperature nitridation. After modification with our co-catalyst system, we obtained solar photocurrents of 6.3 mA cm-2 at 1.23 VRHE in 1 M KOH, and an electrode that maintains about 80% of the initial activity for extended irradiation times.

Resume : Zinc nitride (Zn3N2) is a metastable material that reacts with air in normal ambient conditions, suffering a progressive oxidation that leads to a zinc oxide (ZnO) layer [1]. This characteristic can limit the performance of electronic devices based on this material, such as thin film transistors [2]. However, the details of this physical/chemical process are still relatively unknown and difficult to control. In this work we study O diffusion processes in Zn3N2 layers grown by RF magnetron sputtering of a pure Zn target (99.995 %) with reactive N2 plasma (99.999 %) at room temperature. Different ex-situ oxidation processes has been analysed including aging in air and deionized water, and several thermal treatments at 300 ºC, 450 ºC, 600 ºC under O2 gas (using a horizontal tube furnace) and O2 plasma (using electron cyclotron resonance). The passivation of the surface through ZnO capping layers was also explored for three different thicknesses. The morphology of the films was studied by scanning electron microscopy (SEM). Optical characterization by spectroscopic ellipsometry (SE) was used to track the O in-diffusion with time and the thermal treatments. These modelling was confirmed by elastic recoil detection analysis with time-of-flight (ERDA-TOF). This compositional analysis was carried out with 30 MeV I5+ ion beam produced with the 5 MV Cockcroft–Walton accelerator of the Centre of Micro-Analysis of Materials. Overall, the results show that O diffusion proceeds faster in water (about two orders of magnitude) than in air. Thermal treatments evidence that, in addition to the oxidation, there is partial intermixing with the substrate that affects to the final stoichiometry. Interestingly, the passivation with a thin ZnO cap prevents in a major way the oxidation of the films, what can significantly influence the design of future Zn3N2-based devices. [1] C. García Núñez, J.L. Pau, M.J. Hernández, M. Cervera, E. Ruíz, J. Piqueras, Thin Solid Films 522, 208 (2012). [2] C. García Núñez, J.L. Pau, E. Ruíz, J. Piqueras, Appl. Phys. Lett. 101, 253501 (2012).

Resume : For photovoltaic applications, the antireflection layer has a very important role; it can significantly re-duce the optical losses at the cell surface. A single layer of silicon nitride is generally used as anti-reflection layer. Unfortunately, this layer acts as a quarter-wave plate depends on the wavelength which gives a zero reflectivity for a single wavelength. For this, an application of a multilayer combining several materials on the front of the cells is a proposed way to reduce optical loss and improve conversion efficiency. In this study, we explored the possibility to exploit the wide range of refractive indices of different materials to develop antireflective multi-layer and minimize optical losses. we realized 3 antireflective structures, single antireflective layers, 4 and 6 lay-ers antireflective, silicon nitride and silicon oxynitride were deposited by plasma enhanced chemical vapor deposition (PECVD, 13.56 MHz). A theoretical study validated by reflectivity measurements and ellipsometry was used to optimize the various multilayers. The different results found, predict the best reflectivity for a multilayer with 6 layers R = 1.98% compared to a 4 layer (3.26%) and a single layer (6.1%) over a textured surface.

Resume : An important aspect that has to be considered for load bearing implants is the action of the biological environment which can initiate corrosive processes followed by release of ions and corrosion products. The combined effect of mechanical and chemical stimuli can decrease the implant service life, with impact on patients’ health. This hazard can be overcome by coating the metallic components of load bearing prosthesis with wear and corrosion resistant thin films. Different multilayers based on chromium oxinitrides were obtained by cathodic arc method on CoCr alloy substrate. The architecture of the multilayer coatings was based on different thickness layers with different properties. Due to its ceramic characteristics, CrSiON was considered as the main layer to provide corrosion and friction resistance, and CrSiN and CrSi layers were added to ensure the gradual transition from the metallic substrate to the CrSiON outer layer. Moreover, in order to improve the adhesion of the coating to the substrate, a Cr layer was used. Uncoated and coated CoCr samples were characterized in terms of hardness, corrosion resistance and tribological performance (after 1500 m sliding distance under a constant normal load of 3N). The elemental composition and surface morphology of the wear tracks at the end of the sliding test were also determined. Of the investigated coatings, Cr/CrSi/CrSiN/CrSiON exhibited the best tribological performance, both in friction coefficient and wear rate.

Resume : Many studies are now devoted to the research of photocatalysts presenting activity under visible irradiation. Using Titanium Tetraisopropoxide (TTIP) as a precursor, titania nanoparticles were synthesized by the laser pyrolysis technique. By adding NH3 in the reactive medium, a reducing effect was obtained and Ti(O,N) nanoparticles were synthesized in a cubic structure1. In the same manner, more complex nanoparticles Pd-Ti(O,N) were synthesized by dissolving a Pd precursor in the TTIP. However, due to the high carbon content in the precursors, the nanoparticles contain C removed by thermal treatment under air. Up to 300°C annealing temperature, the powders save the Ti(O,N) organization and exhibit absorption strongly extended in the visible range by comparison to TiO2. Using a monochromatic diode with an emission at 485 nm, the photoactivity under illumination of these nanoparticles was tested for the decoloration of methylene blue (MB) and decomposition of formic acid. In these conditions, the decoloration of MB is observed in both cases. By contrast, the decomposition of formic acid is much more efficient with the sample containing Pd illustrating the positive effect of the metallic part for the charge transfer. 1 - P. Simon et al, Chem mater, 22(12), 3704-3711, (2010)

Resume : Oxynitrides are promising photocatalytic materials for visible light water splitting due to the reduction in bandgap by the substitution of O with N. Many metal oxynitrides have been investigated as photoanodes for water splitting, such as TaON,[1, 2] Ta3N5,[2] BaTaO2N,[3] LaTiO2N,[4] LaTaON2,[5] etc. We are searching for novel oxynitrides with suitable bandgaps for efficient visible light water splitting. In this work, we focused on the rarely studied Y-Ta-O-N system. Pyrochlore, fluorite and perovskite structures of Y-Ta-O-N system have been synthesized, and their PEC (photoelectrochemical) performances are investigated. In order to improve the photocurrent, we studied the effects of various heterojunctions and cocatalysts on Y-Ta-O-N system. [1] R. Abe, M. Higashi, K. Domen, J. Am. Chem. Soc. 2010, 132, 11828. [2] M. Higashi, K. Domen, R. Abe, Energy Environ. Sci. 2011, 4, 4138. [3] M. Higashi, K. Domen, R. Abe, J. Am. Chem. Soc. 2013, 135, 10238. [4] T. Minegishi, N. Nishimura, J. Kubota, K. Domen, Chemical Science 2013, 4, 1120. [5] L. Zhang, Y. Song, J. Feng, T. Fang, Y. Zhong, Z. Li, Z. Zou, Int. J. Hydrogen. Energ 2014, 39, 7697.

Resume : Over the past few decades, many semiconductor materials have been developed for photocatalytic or photoelectrochemical (PEC) applications operating under UV light irradiation. The entire or partial substitution of oxygen by nitrogen in the oxide to form (oxy)nitride structures was found to narrow the band gap and thereby extend the photoactivity into the visible range. The present contribution reports the deposition and the characterization of perovskite oxynitride SrTaO2N thin films. The thin film can address the intrinsic properties of the material, and strictly elucidate which key parameters (crystalline quality, N/O level of substitution…) are most effective to improve efficiencies in PEC reactions. SrTaO2N films were deposited by RF magnetron sputtering using an oxide target under N2 reactive plasma. The optical properties were investigated by UV-Visible transmittance and several characteristics, such as absorption coefficient, band gap and refractive index in the visible range were measured. X-ray diffraction analysis provides information about the texture or the epitaxial growth nature of the films on MgO and Nb-doped SrTiO3 substrates. The dielectric characteristics of these materials were also measured. Photocurrent measurements versus an applied potential were performed in aqueous solution and SrTaO2N films were found to be promising material for the solar overall water splitting.

Resume : In recent years much attention has been paid to the study of ternary AlGaN alloys because of their potential for optoelectronic devices operating in ultraviolet (UV) spectral region. In this work, we report on a study of the optical proprieties of AlGaN layers grown on sapphire substrate using Si/N treatment method by atmospheric-pressure metal organic vapor phase epitaxy (AP-MOVPE). Atomic force microscopy (AFM), high-resolution x-ray diffraction (HRXRD), and photoreflectance (PR) spectroscopy are employed to characterize the studied layers. In particular, two groups of samples are investigated. In the first group, all samples are grown without GaN template and the thickness of AlGaN layer is varied. It is found that the structural and optical properties of these samples are improved with increasing layer thickness up to 0.6 µm. Furthermore, AFM images revealed a transition from 3D to 2D growth mode. After the optimization of the AlGaN layer thickness, a high temperature GaN template layer is inserted between the AlGaN layer and the sapphire substrate, in the second group of samples. The effect of the template thickness on the sample quality is studied. Our results show that the optimal thickness of the GaN template layer is about 1.3 µm. Keywords: AlGaN, GaN template, metal organic vapor phase epitaxy, photoreflectance.

Resume : InAsBi layers were elaborated on semi-insulating (100) GaAs substrates misoriented10° by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE) reactor. Spectral reflectance in the range of 200 to 1100 nm was employed to in situ monitor epitaxy. For determining the optical constants of InAsBi films, an optical model incorporating time-dependent surface roughness and time-dependent growth rate was used to simulate the in situ reflectance. A theoretical motivation for the introduction of these two parameters instead of a standard single rms roughness and growth rate is provided Several InAsBi samples grown at different growth temperatures were used to illustrate ways in which the parameters introduced can be evaluated. Reflectivity analysis was ex situ correlated by atomic force microscopy. Keywords: InAsBi, In situ spectral reflectance, optical model, refractive index Corresponding authors: ahmed.rebey@fsm.rnu.tn, hedi.fitouri@fsm.rnu.tn

Resume : High power impulse magnetron sputtering (HiPIMS) of pure Zn target in Ar/N2/O2 gas mixture was used to synthesize ZnOxNy thin films with the nitrogen content ranged from 0 to 6.2 at.% and the optical band-gap between 3.34 eV and 1.67 eV. The fine control of the nitrogen content in the deposited ZnOxNy thin films composition was possible by the reactive HiPIMS discharge’s stabilization in the metallic-to-compound target sputtering transition region. Various analytical techniques such as AFM, XPS, XRD, UV-Vis and Raman spectroscopy have been employed to characterize the properties of the deposited thin films. The photocatalytic activity, light excitation efficiency, photo sensibility and life time of photogenerated charge carriers in the ZnOxNy films were investigated by photo-electrochemical and photocurrent measurements during light irradiation on/off cycles. The highest photoresponse and photocatalytic activity of the ZnOxNy photocatalytic films has been found for a nitrogen doping concentration value of 3.4 at.%. This pointed out that the nitrogen doping process has an important role on the photocatalytic activity improvement for water molecule splitting by narrowing the optical band gap of zinc oxide (in order to extend the adsorption of catalyst to the visible light region) and by inhibition of recombination of photo-generated electron-hole pairs.

Resume : The well-matching band gap of oxynitrides with the energy of visible light, which is due to the incorporation of nitrogen into the wide band-gap oxides, results in interesting optical and photocatalytic properties. These characteristics attracted many researchers to study oxynitrides as a potential approach to overcome the global energy demand through providing sustainable energy in the form of hydrogen fuel via solar water splitting. Among the different oxynitride materials reported in the literature, CaNbO2N and LaTiO2N powders have shown promising photoactivity towards water splitting. In this work, the effect of the crystallographic properties of the oxynitrides on the photoelectrochemical performance is studied using oxynitride thin films as model systems. The oxynitrides are fabricated as thin films using Pulsed Laser Deposition. Polycrystalline highly textured and epitaxial samples were grown as revealed by X-ray diffraction. The composition of the thin films is determined by Rutherford Backscattering Spectroscopy and Elastic Recoil Detection Analysis. Photoelectrochemical measurements are done to investigate the photoactivity of these oxynitride thin films towards water splitting. The role of cocatalysts, such as IrO2, Au, CoOX, or RuOX, in the hydrogen evolution is also investigated.

Resume :

Reactive high power impulse magnetron sputtering operated in multi-pulse mode (m-HiPIMS) of a pure Ti target in Ar/N₂/O₂ gas mixture (mass flow rates of 50, 2 and 0.1 sccm, respectively) has been used for the deposition of titanium oxinitride (TiO_xN_y) thin films with variable content of nitrogen (0.1 < y < 0.4). Increase of the nitrogen content of the deposited TiO_xN_y thin films determined a decrease of the optical bandgap energy and a corresponding increase of visible light adsorption. The structure and composition of deposited films have been investigated by AFM, XRD, Raman spectroscopy and XPS. The photocatalytic activity for water molecule splitting of the films deposited on copper substrate, which were used as the photo-anode in an electrochemical cell, is investigated by measurements of photocurrent versus biasing voltage during on/off cycles of visible light irradiation (sun light simulated by a xenon lamp). Results of this investigation will be presented during the symposium.

Resume : Hydrogen production by solar water splitting in photoelectrochemical cells (PEC) is an appealing technology for a future hydrogen economy. In this context, hematite (α-Fe2O3) has been considered as one of the most promising photoanode materials due to its proper band gap (~2.2 eV), higher stability, high abundance and low cost. However, the combination effect of low hole mobility, short hole diffusion length and high electron–hole recombination rate has prevented efficient solar energy conversion. In general, loading of oxygen evolution catalysts on the surface of hematite is one of the most popular methods to overcome the oxygen evolution reaction (OER) barrier. Herein, we decorated α-Fe2O3 surface by FeOOH catalyst to overcome such barriers. In this regard, we have fabricated α-Fe2O3/FeOOH photoanode by two step deposition route. The prepared α-Fe2O3/FeOOH photoanode exhibits enhanced photocurrent density and lead to decrease the over potential required to drive photoelectrochemical water oxidation compared to pristine α-Fe2O3. This can be ascribed to the high electrocatalytic activity of nanostructured FeOOH, which reduce the photogenerated electron-holes recombination therefore enhance the water oxidation kinetics. Furthermore, the Fe2O3/FeOOH films show visible light response and long-term stability for photoelectrochemical splitting of water.

Resume : The use of metal oxide semiconductor as a solar energy harvesting element in photoelectrochemical cell for water splitting is a topic of growing intrest for solar energy conversion to chemical fuel. Photoelectrochemical cell incorporates both solar energy collection and electrolysis of water in a single device. TiO2 which is a widely used wide band gap semiconductor due to its stability and efficient photocatalytic activity has certain limitation due to its wide bandgap resulting in poor absorption of solar radiation. Here, we demonstrate that thermal treatment on TiO2 nanocrystals under oxygen deficient environment (5% H2 in Ar) can significantly tailor the structural, optical and electrical properties for significantly improved photocatalytic and photoelectrochemical performance. The thermally treated TiO2 nanocrystals contained paramagnetic Ti3 centers and exhibited a higher visible light absorption cross-section as confirmed by electron paramagnetic resonance and diffuse reflectance spectra measurements. The thermally treated samples showed a noticeable improvement in photocatalytic activity under visible light (λ >380nm) which was demonstrated by degrading methylene blue dye and improved photoelectrochemical response in term of high photocurrent density.

Resume : < p>< div align="justify">Synthesis of efficient photocatalytic semiconductor materials uses strategies as increase of quantum efficiency for visible light, increase of life-time of charge carriers, increase of the availability of charge carriers for surface redox reactions and increase of effective surface area by control of surface topography at nanoscale. In a previous work we have shown that ZnON thin films deposited by High Impulse Power Magnetron Sputtering (HiPIMS) in Ar/N< sub>2< /sub>/O< sub>2< /sub> gas mixture show good photocatalytic activity in visible light. In the present work we study the effect of ZnON surface topography on the photocatalytic activity of these films. Control of the topography of HiPIMS deposited ZnON films was performed by deposition of relatively thin films (60 nm in thickness) on cooper substrates with controlled topography. The substrates were prepared by pulsed laser deposition (PLD) of cooper on metallic electrodes (10 x 10 mm< sup>2< /sup>). Substrates with cooper films with different roughness values were used to deposit ZnON thin films with roughness varying from few nanometers to few hundreds nanometers. The ZnON electrodes obtained by this technique were used as the photo-anode in an electrochemical cell irradiated with visible light (sun light simulated by a xenon lamp) for investigation of visible-light photocatalytic activity. Results of this investigation will be presented during the symposium.< /div>< /p>

Resume : ZnO, due to its bandgap, should be a good material for efficient optoelectronic devices, operating at blue and UV wavelengths. This application requires a stable material with p-type conductivity, and high carrier mobility. Nitrogen (N) known to form acceptor centres in ZnO films, due to its greater solubility and lower-energy acceptor level as compared with Phosporus or Arsenium. We investigated the characteristics of N doped ZnO thin films deposited on fused silica substrates by RF-magnetron sputtering at different temperatures using a ZnO target in reactive atmosphere (Ar+O2+N2) at a constant pressure (0.5 Pa); N doping was varied by modifying the nitrogen flow. The films exhibited a wurtzite structure with a preferred (002) orientation. The films' elemental and chemical composition were analysed (EDS, AES, FT-IR), and linked to their electrical characteristics (Hall measurements). The band gap values were obtained by optical measurements. Films' photoluminescence was also evidenced.

Resume : ZrSiON thin films were deposited on CoCr dental alloys by means of cathodic arc evaporation method for improvement of the metal-ceramic bond strength. The corrosion resistance is crucial for materials used in dental applications subjected to the action of the aggressive oral environment. For this purpose, a defect-free microstructure is required and therefore, the coatings were obtained at two different substrate bias voltages, Vb: -50 V and -200V), in order to select the optimum parameters for the preparation of coatings with superior properties. In this study, the long term protective performance of the ZrSiON thin films was investigated during 72h immersion in artificial saliva solution by odd random phase multisine electrochemical impedance spectroscopy (ORP- multisine EIS). This method allowed, in contrast to standard single sine EIS, the possibility of quality control of the experimental data and therefore the application of optimal measurement conditions, for obtaining a linear and stationary behaviour, and also a good signal-to-noise ratio. Based on this additional information, a statistical evaluation was made to assess the fitting procedure. The results showed that ZrSiON thin film deposited at Vb= -200 V exhibited the lowest porosity and the best protective properties. Moreover, this behaviour can be associated to the enhanced formation of a stable passive layer inhibiting the corrosion of the CoCr substrate.