Surface Photovoltage Spectroscopy Observes Sub-Band-Gap Defects in Hydrothermally Synthesized SrTiO₃ Nanocrystals

Abstract

SrTiO3 has been of interest as a photocatalyst for overall water splitting, but the energy conversion efficiency of this material is limited by recombination at surface and lattice defects. Here, we use surface photovoltage spectroscopy (SPS) to map defects in SrTiO3 nanocrystals made by hydrothermal synthesis, as a function of temperature during a subsequent thermal annealing step. Two types of defects, D1 and D2, can be identified on the basis of their photovoltage contributions at 2.0 eV in as-synthesized particles and at 2.0 and 2.7 eV in 300 °C annealed particles. Using Rh:SrTiO3 nanocrystals with a defined 2.9 eV photovoltage as an internal standard, defect concentrations in SrTiO3 are estimated at 0.47–1.10 atom % (based on Ti) in 25–300 °C annealed samples and at 0.13–0.20% after annealing at 400–500 °C in air. Based on electron paramagnetic resonance spectra at 77 K, the defects are assigned to Ti­(III) states associated with oxygen vacancies and to a Ni­(III) ion contamination from the NiFe stainless steel autoclave used in the synthesis of the nanocrystals. Furthermore, 400 and 500 °C annealed nanocrystal films show small positive photovoltage signals that are attributed to a 0.8 to 2.9 micrometer wide depletion layer with a +0.2 eV potential barrier. The ability of SPS to detect photoactive defects at low concentrations is relevant to the optimization of metal oxide particles for photocatalytic solar energy conversion.

Molecularly engineered photocatalyst sheet for scalable solar formate production from carbon dioxide and water

“…25 RuO 2 -BiVO 4 :Mo is expected to provide strong water oxidation power under visible light irradiation, 26 whereas SrTiO 3 :La,Rh was chosen as the light absorber for CO 2 RR owing to its relatively negative conduction band (-1.1 V versus NHE at pH 7). 27 The sheet was subsequently modified with the molecular CO 2 RR cocatalyst (CotpyP) by immersion from a methanolic solution. Fully assembled CotpyP-loaded SrTiO 3 :La,Rh|Au|RuO 2 -BiVO 4 :Mo sheets with relatively large active areas (ca.…”

Hydrothermal synthesis of single-crystal Cr-doped SrTiO3 for efficient visible-light responsive photocatalytic hydrogen evolution

“…Strontium titanate (SrTiO 3 ), a typical perovskite oxide semiconductor with excellent chemical and optical stability, is considered as promising photocatalyst [4]. However, the wide band-gap energy (3.2 eV) restricts its photocatalytic performance, which could be responsive to the ultraviolet light range [5]. Therefore, modifications to SrTiO 3 are widely attempted to narrow the band-gap and thus enlarge its light-harvesting region.…”

Photochemical Charge Separation and Dye Self-Oxidation Control Performance of Fluorescein, Rose Bengal, and Triphenylamine Dye-Sensitized Solar Cells

“…The photovoltage signal begins at approximately 2.0 eV, much before the 3.2 eV band gap of SrTiO 3 . A previous SPS study has attributed this subgap photovoltage to Ti 3+ defect states in SrTiO 3 resulting from oxygen vacancies . Similarly, for the bare TiO 2 films in Figure C,D, the major photovoltage signal begins near 3.0 eV, which corresponds to the tabulated band gap of rutile TiO 2 .…”

“…61 A previous SPS study has attributed this subgap photovoltage to Ti 3+ defect states in SrTiO 3 resulting from oxygen vacancies. 62 Similarly, for the bare TiO 2 films in Figure 5C,D, the major photovoltage signal begins near 3.0 eV, which corresponds to the tabulated band gap of rutile TiO 2 . 63 Contributions to the subgap photovoltage (at 2.0 eV) from Ti 3+ states are evident in some films, but they are not as pronounced as in SrTiO 3 .…”