ISSP - The institute for Solid State Physics

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Professor

LIPPMAA,
Mikk

Research Associate

TAKAHASHI,
Ryota

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The band gap and transport behavior of oxide semiconductors is generally determined by point defects and doping. In many doped oxide systems, the dominant site substitution mechanism can be assumed from the dopant valence and ionic radius, but explicitly determining the dopant site structure is generally impossible. X-ray fluorescence holography is a useful technique in this regard, as it produces directly the atomic positions of the nearest neighbors surrounding a dopant atom. The method is based on measuring the angular distribution of the dopant atom’s fluorescence x-ray intensity (Fig. 1) and solving for the positions of the nearest-neighbor scatterer atoms. We have analyzed the structure of Rh-doped SrTiO₃ photocatalysts and found that while the Rh⁴⁺ dopant substitutes at the Ti site without lattice distortion, a Rh³⁺ dopant forms clusters with oxygen vacancies or substitutes at the oxygen site (Fig. 2). Such clustering has a detrimental effect on photocarrier dynamics in Rh:SrTiO₃ photocatalysts and photoelectrodes used for the solar-powered water splitting reaction.

Fig. 1. Angular distribution of Rh fluorescence x-ray intensity for (a) Rh³⁺:SrTiO₃ and (b) Rh⁴⁺:SrTiO₃ thin films. The difference in the patterns indicates that the arrangement of the Rh nearest-neighbor atoms is different. The pattern in (b) corresponds to a perfect perovskite structure.

Fig. 2. Two main defect cluster types in a Rh³⁺:SrTiO₃ film grown at low oxygen pressure: (upper) Rh cluster with an oxygen vacancy, (lower) Rh substitution at the anion site.