Manipulating the ferroelectric polarization state of BaTiO 3 thin films

International audienceControlling the ferroelectric polarization at macroscopic or microscopic levels is crucial in the framework of the development of ferroelectric materials used in yet challenging photo-electrochemical (PEC) cells and spintronic applications. We report here on polarization methods allowing to electrically polarize prototypical samples of BaTiO 3 (001) films. Epitaxial single crystalline layers were grown up to a thickness of 25 nm by atomic oxygen assisted molecular beam epitaxy on 1 at.% Nb doped SrTiO 3 (001) single crystals. The samples were both microscopically and macroscopically polarized using Piezoresponse Force Microscopy and electrochemical poling in an electrolyte respectively. In addition we demonstrate the possibility to retrieve a quasi-native mixed ferroelectric polarization state after annealing. These polarization methods may be applied to many other ferroelectric thin films

Local electronic structure and photoelectrochemical activity of partial chemically etched Ti-doped hematite

International audienceThe direct conversion of solar light into chemical energy or fuel through photoelectrochemical water splitting is promising as a clean hydrogen production solution. Ti-doped hematite (Ti:α-Fe 2 O 3) is a potential key photoanode material, which despite its optimal band gap, excellent chemical stability, abundance, non-toxicity and low cost, still has to be improved. Here we give evidence of a drastic improvement of the water splitting performances of Ti-doped hematite photoanodes upon a HCl wet-etching. In addition to the topography investigation by atomic force microscopy, a detailed determination of the local electronic structure has been carried out in order to understand the phenomenon and to provide new insights in the understanding of solar water splitting. Using synchrotron radiation based spectromicroscopy (X-PEEM), we investigated the X-ray absorption spectral features at the L 3 Fe edge of the as grown surface and of the wet-etched surface on the very same sample thanks to patterning. We show that HCl wet etching leads to substantial surface modifications of the oxide layer including increased roughness and chemical reduction (presence of Fe 2+) without changing the band gap. We demonstrate that these changes are profitable and correlated to the drastic changes of the photocatalytic activity

Mussel as a Tool to Define Continental Watershed Quality

Bivalves appear as relevant sentinel species in aquatic ecotoxicology and water quality assessment. This is particularly true in marine ecosystems. In fact, several biomonitoring frameworks in the world used mollusks since several decades on the base of contaminant accumulation (Mussel Watch, ROCCH) and/or biological responses called biomarker (OSPAR) measurements. In freshwater systems, zebra and quagga mussels could represent alternative sentinels, which could be seen as the counterparts of mussel marine species. This chapter presents original studies and projects underlying the interest of these freshwater mussels for water quality monitoring based on contaminant accumulation and biomarker development measurements. These sentinel species could be used as a tool for chemical/biological monitoring of biota under the European water framework directive and for the development of effect-based monitoring tools

Films minces épitaxiés à base d’hématite comme photo-anodes pour la photo-électrolyse de l’eau

Using hydrogen as an energy carrier for solar energy storage and/or fuel alternative to oil is very appealing, especially as it can be cleanly produced by solar water splitting. In this process, electron-hole pairs, generated in illuminated semiconductors dipped in an aqueous solution, realize the water oxidoreduction reactions (oxygen production at the photoanode and hydrogen production at the photocathode). Transition metal oxides, in particular hematite (α-Fe2O3) which features a quasi ideal band-gap for this application, are the most promising photoanodes materials.Hematite thin films were deposited on single crystals by oxygen plasma assisted molecular beam epitaxy. These model samples along with the use of high-end techniques, in particular using synchrotron radiation, make possible the identification of the relevant parameters affecting the photoelectrochemical properties. I firstly focused on the impact of the crystallographic structure, the stoichiometry and the surface morphology. Then the effects of doping with titanium were investigated, demonstrating the existence of an optimal doping level and an increase of the charges diffusion length inducing a high photocurrent gain. In addition, I studied the electronic structure and the surface recombinations dynamics of TiO2 - Ti-doped hematite heterojunctions, revealing a diffuse interface. Lastly, the internal electric field created by a ferroelectric thin film of BaTiO3/Nb:SrTiO3 was considered in order to enhance the performances of photoanodes. A first step toward the comprehension of the link between ferroelectric polarization and photocurrent was achieved through the evidence of an internal electric field favourable for the separation of charges.Utiliser l'hydrogène en tant que vecteur énergétique pour stocker l'énergie solaire et/ou remplacer le pétrole comme carburant est très attrayant, d'autant qu'il peut être produit de façon propre par photo-électrolyse de l'eau. Dans ce procédé, des paires électron-trou, générées par éclairement dans des semi-conducteurs immergés dans une solution aqueuse, réalisent les réactions d’oxydo-réduction de l’eau (production d'oxygène à la photo-anode et production d'hydrogène à la photo-cathode). Les oxydes de métaux de transition, en particulier l'hématite (α-Fe2O3) qui présente un gap quasi-idéal pour cette application, sont les matériaux de photo-anode les plus prometteurs.Des films minces d'hématite ont été déposés sur des monocristaux par épitaxie par jets moléculaires assistée par plasma d’oxygène. Ces échantillons modèles ainsi que l’utilisation de techniques de pointe, notamment utilisant le rayonnement synchrotron, rendent possible l’identification des paramètres pertinents influençant les propriétés de photo-électrolyse. Je me suis d'abord intéressé à l'impact de la structure cristallographique, de la stœchiométrie et de la morphologie de surface. Ensuite, les effets d'un dopage avec du titane ont été analysés, montrant l'existence d'un taux de dopage optimal et l'augmentation de la longueur de diffusion des porteurs de charges induisant un fort gain en photo-courant. J'ai également étudié la structure électronique et la dynamique des recombinaisons en surface d'hétérojonctions TiO2 - hématite dopée Ti, révélant une interface diffuse. Enfin, le champ électrique interne créé par un film mince ferroélectrique de BaTiO3/Nb:SrTiO3 a été considéré pour améliorer les propriétés des photo-anodes. Un premier pas vers la compréhension du lien entre polarisation ferroélectrique et photo-courant a été fait, mettant en évidence un champ électrique interne favorable pour séparer les charges

Single Crystalline Hematite Films for Solar Water Splitting: Ti-Doping and Thickness Effects

International audienceUndoped and Ti-doped (2 at. %) epitaxial hematite thin films, in the thickness range 5–50 nm, were grown by atomic oxygen assisted molecular beam epitaxy (AO-MBE) on Pt(111) substrates in the framework of hydrogen harvesting from sunlight-induced water splitting. Such single crystalline samples are suitable model systems to study thickness and doping effects on the photoelectrochemical properties; we demonstrate that they also allow disentangling intrinsic transport properties from mingled overall properties due to the usually unknown contributions from morphology or crystalline structure defects. From their photoelectrochemical characteristics (I(V) curves, incident photon to current efficiency measurements, and electrochemical impedance spectroscopy), we evidence the existence of an optimum layer thickness, which is higher for Ti-doped samples (30 nm) as compared to undoped ones (20 nm). Our results suggest that this effect is due to an increase of the carrier concentration combined with higher carriers’ diffusion lengths in the doped samples stressing intrinsic modifications of the hematite layer upon titanium doping that cannot be accounted for by simple structural or electronic structure changes

Substrate strain induced effects in La0.7Sr0.3MnO3 thin films

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Oxygen Vacancies Engineering of Iron Oxides Films for Solar Water Splitting

International audienceHematite (α-Fe 2 O 3) can be considered as one of the top candidates to act as photoanode in the framework of clean hydrogen production through solar water splitting. The O:Fe ratio, that in this material plays a crucial role in the definition of its photoelectrochemical properties, has been investigated in detail. For this purpose, we examined thermal magnetite oxidation and hematite reduction as two possible routes to produce semiconducting iron oxides layers with controlled stoichiometry. We report on properties of single crystalline nanometric films elaborated by atomic oxygen plasma assisted molecular beam epitaxy as model systems to disentangle structural phase transition effects from pure stoichiometry ones. We provide new insights into the mechanisms related to hematite properties modifications and their correlation with photocurrent changes upon the presence of oxygen vacancies and phase mixing with magnetite, with respect to the vacancies concentration regimes. We show on one hand that crystallographic structure mixing appears as strongly detrimental for photoanodes synthesis whatever the oxygen vacancies concentration. On the other hand, oxygen vacancies in the optimal concentration range, while preserving the α-Fe 2 O 3 corundum phase, is highly favorable for solar water splitting, inducing a substantial reduction of 0.2 V for the onset potential and an overall photocurrent increase of 50% with respect to stoichiometric hematite. The present study demonstrates more generally the possibility of using oxygen vacancies as a degree of freedom for the optimization of hematite photoanodes

Resonant PhotoEmission Spectroscopy Investigation of Fe2O3 – TiO2 Heterojunctions for Solar Water Splitting

International audienceThe implementation of semiconductor heterojunctions photo-anodes appears as a very promising way to improve the performances of devices for solar water splitting (sun light assisted hydrogen production from water). However, assembling different materials together results in the existence of interfaces which usually do not have the same electronic structure than the simply stacked individual layers. The electronic structure of the valence band being a key parameter for water splitting, it is necessary to investigate it for each layer and for the interfaces. A very powerful technique to tackle these issues is Resonant PhotoEmissionSpectroscopy (RPES). In this work, we present RPES results of the valence band performed on epitaxial TiO2/Ti-doped Fe2O3 heterojunctions. The provided insights concerning the interface and the electronic structure are correlated to the water splitting performances

Cellular and molecular complementary immune stress markers for the model species Dreissena polymorpha

International audienceThis study aimed to combine cellular and molecular analyses for better detail the effects of various stresses on a sentinel species of freshwater invertebrate. For this purpose, the hemocytes of the zebra mussel, Dreissena polymorpha, were exposed to different stresses at two different intensities, high or low: chemical (cadmium and ionomycin), physical (ultraviolet B), or biological ones (Cryptosporidium parvum and Toxoplasma gondii). After exposure, flow cytometry and droplet digital PCR analyses were performed on the same pools of hemocytes. Several responses related to necrosis, apoptosis, phagocytosis, production of nitric oxide and expression level of several genes related to the antioxidant, detoxification and immune systems were evaluated. Results showed that hemocyte integrity was compromised by both chemical and physical stress, and cellular markers of phagocytosis reacted to ionomycin and protozoa. While cadmium induced oxidative stress and necrosis, ionomycin tends to modulate the immune response of hemocytes. Although both biological stresses led to a similar immune response, C. parvum oocysts induced more effects than T. gondii, notably through the expression of effector caspases gene and an increase in hemocyte necrosis. This suggests different management of the two protozoa by the cell. This work provides new knowledge of biomarkers in the zebra mussel, at both cellular and molecular levels, and contributes to elucidate the mechanisms of action of different kinds of stress in this species