Hélène Magnan

Maghemite (‐) is a promising non‐precious‐metal‐containing photocatalyst for water oxidation (OER). Despite being less studied than Hematite, it offers similar corrosion resistance and a favorable band structure, along with higher conductivity and the advantage of an adaptable spinel structure. Its catalytic performance can be optimized by tuning the concentration of a donor dopant. In this work, we synthesized via molecular beam epitaxy ultra‐thin (6 nm) Ni‐doped ‐ (111) films with different Ni concentrations. We present a comprehensive study that sheds light on the structural reconfigurations induced by tuning the Ni concentration and the resulting effects on the OER onset and photoconversion efficiency in alkaline media. Samples' electronic reconfiguration under operando conditions and the role of Ni dopant on the catalytic mechanism are studied with X‐ray Absorption Spectroscopy. Light‐driven surface chemistry modifications during OER are probed with a novel X‐ray fluorescence pump‐and‐probe approach: Fixed energy X‐ray absorption PhotoVoltammetry.

In this work, we determine the electronic structure and charge carrier dynamics of α-Fe₂O₃(0001) ultrathin film deposited on Pt(111) as a function of water pressure by combined near ambient pressure-time-resolved photoelectron spectroscopy (NAP-TR-PES) measurements and DFT calculations. Under ultrahigh vacuum (UHV) α-Fe₂O₃ exhibits the expected typical n-type semiconductor behavior with a surface photovoltage (SPV) shift of 31 meV. Surprisingly, when exposed to water a completely different comportment appears. At a partial pressure of water of PH₂O = 0.02 hPa, a much smaller SPV (7 meV) appears that is shifted in the opposite direction, which is thus characteristic of a p-type semiconductor. Finally, at PH₂O > 0.1 hPa, SPV is canceled, which is consistent with a layer of metallic nature. DFT calculations explain these experimental findings well as originating from a modification of the surface structure and electronic properties of the layer consecutive to water adsorption. It is shown that the Fermi level position in the band gap of hematite depends on the quantity of adsorbed water, giving rise to the ambipolar behavior of the surface, which is able to transport both negative and positive charges when exposed to the appropriate water pressure.

Electrochemical cell for operando XAS measurements down to the Ti K edge. Sample is probed from the electrolyte side. As a case study, we investigated Ni-doped maghemite epitaxially grown on Pt as a catalyst for the oxygen evolution reaction.

Epitaxial perovskite self-oxidized oxynitride N : BaTiO₃ thin films, deposited on 1% Nb-doped SrTiO₃(001) single crystals, were obtained by atomic nitrogen assisted molecular beam epitaxy without supplying additional oxygen gas.

Water adsorption and dissociation on undoped and Ti-doped hematite thin films were investigated using near-ambient pressure photoemission and DFT calculations. A fine understanding of doping effects is of prime importance in the framework of photoanode efficiency in aqueous conditions. By comparison to pure Fe₂O₃ surface, the Ti(2%)-Fe₂O₃ surface shows a lower hydroxylation level. We demonstrate that titanium induces wide structural modifications of the surface, preventing it from reaching full hydroxylation.

We studied the growth of epitaxial ferroelectric layers BaTiO3/Pt(001) in the framework of solar water splitting. The stoichiometry, i.e. the Ti/Ba ratio, appears as a crucial parameter not only for the crystallographic structure but also for the photocurrent generation when the sample is used as photoanode. We established an electrochemical poling method using a modulated potential, without altering the sample to control the electric polarization. Importantly, we show that the photoanode efficiency is improved when the polarization of the film is oriented towards the substrate. We demonstrate that this improvement is only due to the electrical field created by screening charges, which allows the separation of photo-generated charges.