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

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 degrees 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 degrees C annealed samples and at 0.13-0.20% after annealing at 400-500 degrees 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 degrees 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.