ISSP - The institute for Solid State Physics

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Lippmaa Group
Associate Professor
LIPPMAA,
Mikk

Research Associate
TAKAHASHI,
Ryota

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Photocatalytic water splitting for hydrogen production presents an interesting challenge for oxide semiconductor development. The purpose is to design an oxide material that is chemically stable in water, has a bandgap of about 2 eV, and has high photocarrier mobility at room temperature. The first two requirements can be met by using noble metal doped SrTiO3, but the best photocatalysts, Ir:SrTiO3 and Rh:SrTiO3 have very low photocarrier mobilities. We study the possibility of avoiding the mobility problem by placing self-organized nanoscale metal electrodes inside the oxide semiconductor. Spontaneous noble segregation in a perovskite forms arrays of nanoscale metal pillars (Fig. 1), which can form Schottky-type depletion layers in the surrounding semiconductor (Fig. 2). The required photocarrier diffusion length thus becomes shorter and we can extract photogenerated charge very efficiently from an intrinsically low-mobility semiconductor.

Fig. 1. STEM images of spontaneously formed noble metal nanopillars in SrTiO3. Pillar formation has been observed for Ir, Pt, Pd, and Rh. A suitable depletion layer forms around Ir nanopillars in an Ir:SrTiO3 host matrix.
Fig. 2. Schematic illustration of a nanoscale metal nanopillar in a thin film (a). A Schottky junctions forms between the metal and the oxide (b), effectively separating photogenerated holes and electrons before recombination. The metal nanopillar provides an efficient charge transport path to the photocatalyst surface for the oxygen evolution reaction.

Research Subjects

  1. Growth of thin oxide films and heterostructures by pulsed laser deposition
  2. Development of oxide photoelectrode materials for photocatalytic water splitting
  3. Polar oxides and multiferroic coupling
  4. Synthesis of nanostructures and nanocomposite thin films