Abstract
Anatase TiO2 (a-TiO2) exhibits a strong X-ray absorption linear dichroism with the X-ray incidence angle in the pre-edge, the XANES and the EXAFS at the titanium K-edge. In the pre-edge region the behaviour of the A1-A3 and B peaks, originating from the 1s-3d transitions, is due to the strong $p$-orbital polarization and strong $p-d$ orbital mixing. An unambiguous assignment of the pre-edge peak transitions is made in the monoelectronic approximation with the support of ab initio finite difference method calculations and spherical tensor analysis in quantitative agreement with the experiment. It is found that A1 is mostly an on-site 3d-4p hybridized transition, while peaks A3 and B are non-local transitions, with A3 being mostly dipolar and influence by the 3d-4p intersite hybridization, while B is due to interactions at longer range. Finally, peak A2 which was previously assigned to a transition involving pentacoordinated titanium atoms exhibits a quadrupolar angular evolution with incidence angle. These results pave the way to the use of the pre-edge peaks at the K-edge of a-TiO2 to characterize the electronic structure of related materials and in the field of ultrafast XAS where the linear dichroism can be used to compare the photophysics along different axes.
Highlights
Titanium dioxide (TiO2) is one of the most studied large-gap semiconductors due to its present and potential applications in photovoltaics [1] and photocatalysis [2]
A complementary approach using experimental titanium atom (Ti) K edge linear dichroism (LD) measurements of Anatase TiO2 (a-TiO2) single crystals, abinitio finite difference method (FDM) calculations and spherical tensor analysis provides an unambiguous assignment of the pre-edge features
We show that A1 is mainly caused by a dipolar transition to on-site hybridized 4px,y − 3dxz, 3dyz final states, which give a strong dipolar LD to the transition with a weak quadrupolar component from (3dxz, 3dyz, 3dx2−y2 ) states
Summary
Titanium dioxide (TiO2) is one of the most studied large-gap semiconductors due to its present and potential applications in photovoltaics [1] and photocatalysis [2]. K-edge x-ray absorption spectroscopy (XAS) is an element-specific technique that is used to extract the local geometry around an atom absorbing the x radiation, as well as about its electronic structure [5]. The absorption edge (typically > 50 eV), the extended x-ray absorption fine structure (EXAFS) contains information about bond distances. (ii) The edge region and slightly above it (< 50 eV) represents the x-ray absorption near-edge structure (XANES), which contains information about bond distances and bond angles around the absorbing atom, as well as about its oxidation state. In contrast to EXAFS, XANES features require more complex theoretical developments due to the multiple scattering events and their interplay with bound-bound atomic transitions. (iii) The pre-edge region consists of bound-bound transitions of the absorbing atom. The Ti K-edge absorption spectrum of anatase TiO2 (a-TiO2) exhibits four pre-edge features labeled A1, A2, A3, and B, while rutile TiO2 only
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