Influence of pH and Internal Electric Field on the Photochemical Reactivity of Oxide Catalysts

Water splitting promoted by particulate photocatalysts is a promising way to solve energy challenges and reduce greenhouse-gas emissions. Photocatalysts are less expensive than other solar energy harvesting and conversion devices, yet the efficiency is not yet commercially competitive. The ultimate goal of this work is to improve the efficiency of particulate photocatalysts so that they can provide commercially competitive energy solutions. One of the main factors limiting the efficiency of photocatalysts is the recombination of photogenerated

charge carriers and the back reaction of intermediates. The electric field has been shown to be an effective approach for separating and transporting the photogenerated electrons and holes. As a result, the two half reactions are also spatially separated. The electric field can be from inside the

photocatalyst material, or from the photocatalyst-solution interface. In this work, we will focus on the ferroelectric polarization, a form of the internal electric field, and the pH of the solution, the effect of which is analogous to applying an external electric field. An external electric field can be applied to the particulate photocatalyst through changing the

pH of the solution. In the first study, the relative photochemical reactivity of BaTiO₃ was investigated as a function of the surrounding aqueous solution pH. The observations are consistent with the idea that increasing the pH increases the net negative charge adsorption on the surface, increasing the upward band bending and promoting the oxidation half reaction, which is the rate-limiting factor for the overall reaction. As a result, the rate of Ag+ reduction

increases with pH. This method can be combined with internal field engineering to further improve the photochemical reactivity. The spontaneous polarization in ferroelectric materials is a powerful tool to introduce an

internal electric field. Understanding the relationship between the magnitude of ferroelectric polarization and photochemical reactivity can provide a guide to improving the photochemical reactivity through ferroelectric domain engineering. In the second study, the influence of the

magnitude of the polarization on photochemical reactivity is investigated. We found that the photochemical reactivity can only be improved by a factor of ~ 3 when the polarization

magnitude increases by a factor of ~10. Our result has the potential to be applied to materials with internal fields arising from other mechanisms. To investigate the effect on photochemical reactivity when the internal electric field and the external electric field are tuned at the same time, we used a high-throughput method for the measurement of the hydrogen evolution rate in the third and the fourth study. In the third study, the rates of hydrogen production from six different BaTiO₃/TiO₂ core/shell photocatalysts were

measured as a function of pH (from 1 to 12) using a parallelized and automated photochemical reactor. The pH dependence of the performance of the photocatalysts was similar: the rate of hydrogen production increased with pH, except for a local minimum between pH 3 and 6.

Compared to the other catalysts at intermediate pH, the core/shell catalyst annealed at 600 °C had the highest reactivity. In the fourth study, the effect of internal and external fields on the photochemical hydrogen production of Al-doped SrTiO₃/TiO₂ core/shell heterostructured

catalysts were investigated. The core/shell sample whose core was treated in a SrCl₂ flux, doped with 2% Al₂O₃, and annealed at 1000°C showed the highest hydrogen generation rate. The optimal pH was 6 for the core and 9 or 12 for the core/shell. The results demonstrate that the

parallelized and automated photochemical reactor is an effective way to compare the performance of hydrogen producing catalysts with different structures and in different operating conditions.