Title: Investigating Possible Ferroelectric Properties of Rhodium-doped Strontium Titanate Using Surface Photovoltage Spectroscopy
Authors: Nathan Soland, Ruirui Han, Chengcan Xiao, and Frank E. Osterloh
Abstract: Photocatalytic water splitting is currently a field of intense research, with promising candidates for hydrogen production constantly being developed and improved. One such material is rhodium-doped strontium titanate, SrTiO3:Rh, a doped perovskite semiconductor with an effective bandgap of 2.4eV. SrTiO3:Rh nanocrystals were synthesized with a hydrothermal method and the charge transfer properties were studied using surface photovoltage spectroscopy (SPS), which measures the change in surface potential, known as the contact potential difference, upon illumination. On conductive gold substrates, photovoltages of 4V were repeatedly achieved which exceeded the theoretical limit of 3.5V, as determined on the basis of the band gap of the material and the standard reduction potentials of the electron donor and acceptor. The unusually high photovoltages have been explained by assuming that SrTiO3:Rh is a ferroelectric material, and a mechanism for polarization is proposed. The ferroelectric hypothesis was investigated using electric polarization experiments in the dark and under bandgap illumination coupled with transient SPS. Stability tests in vacuum confirm remnant polarization after 15 hours. Methanol is found to quench the ferroelectric polarization, suggesting the effect is connected to Rh4+ states in the material.