The charge distribution of the defect states at the reduced TiO(2)(110) surface is studied via a new method, the resonant photoelectron diffraction. The diffraction pattern from the defect state, excited at the Ti-2p-3d resonance, is analyzed in the forward scattering approach and on the basis of multiple scattering calculations. The defect charge is found to be shared by several surface and subsurface Ti sites with the dominant contribution on a specific subsurface site in agreement with density functional theory calculations.

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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

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

The growth, crystal and electronic structures, and photo-electrochemical properties of undoped and Ti doped hematite epitaxial films were studied. We evidence that Ti4+ substitutes Fe3+ in the hematite lattice inducing a slight modification of the oxygen octahedron. Ti doping is shown to induce a shift of the valence band toward higher binding energy due to a movement of the Fermi level toward the conduction band. The resulting modification of electrical conductivity appears as a possible origin of the improvement of photo-electrochemical properties in the doped sample.

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a b s t r a c tThe properties of CoFe 2 O 4 /BaTiO 3 artificial multiferroic multilayers strongly depend on the crystalline structure, the stoichiometry and the cation distribution between octahedral (Oh) and tetrahedral (Td) sites (inversion factor). In the present study, we have investigated epitaxial CoFe 2 O 4 layers grown on BaTiO 3 , with different Co/Fe ratios. We determined the cation distribution in our samples by X-ray magnetic circular dichroism (XMCD), a well accepted method to do so, and by X-ray photoelectron spectroscopy (XPS), using a fitting method based on physical considerations. We observed that our XPS approach converged on results consistent with XMCD measurements made on the same samples. Thus, within a careful decomposition based on individual chemical environments it is shown that XPS is fully able to determine the actual inversion factor.

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