Unraveling the Surface Reactivity of Pristine and Ti-Doped Hematite with Water.

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.

Theoretical investigation of the platinum substrate influence on BaTiO3 thin film polarisation.

Density functional theory calculations are performed to study the out-of-plane polarisation in BaTiO3 (BTO) thin films epitaxially grown on platinum. Prior to any polarisation calculation, the stability of the Pt(001)/BaTiO3(001) structure is thoroughly discussed. In particular, the nature of the Pt/BTO and BTO/vacuum interfaces is characterised. The growth of BTO is shown to start with a TiO2 layer while the nature of the surface termination does not broadly modify the stability. Therefore both upper terminations are considered when describing the ferroelectric behaviour in Pt/BTO interfaces. The geometric and electronic effects of the substrate on the polarisation are investigated. To isolate the electronic influence of platinum, the out-of-plane polarisation in Pt/BTO systems is compared to the one in isolated BTO slabs constrained to the same lattice mismatch induced by the epitaxial growth on platinum. The ferroelectric phase is favoured as soon as the thickness is larger than 23 Å, both for isolated and deposited BTO, for the smallest width. The Pt substrate will modify the size of polarisation domains, while an upper BaO layer through the use of asymmetric [TiO2/BaO] systems will induce an increase of the polarisation. One could take advantage of this experimentally.

The nature of the Pt(111)/α-Fe2O3(0001) interfaces revealed by DFT calculations.

Density functional theory calculations are performed to give a thorough description of structural, energetic, and electronic properties of Pt(111)/α-Fe2O3(0001) systems by spin-polarized calculations, accounting for the on-site Coulomb interaction. Toward the better understanding of Pt(111)/α-Fe2O3(0001) interfaces, two terminations of α-Fe2O3(0001) surface, namely, the single Fe- and the O3-termination, are considered and coupled with the four possible (top, hcp, fcc, and bridge) sites on Pt(111). The effect of the strain on clean hematite surfaces due to the lattice mismatch between the substrate and the overlayer is included in the analysis. Among the possible adsorption configurations, bridge sites are unstable, while the most favorable configurations are the ones at hollow sites. The stability of the interfaces is not only influenced by the termination of the overlayer but also influenced by the degree of its structural relaxation and the relative position of the first layer of O atoms in hematite with respect to Pt. To elucidate the different nature of the two terminations of the overlayer on Pt, projected density of states and 3D charge density difference plots are also discussed.