Enhanced Kinetics of Hole Transfer and Electrocatalysis during Photocatalytic Oxygen Evolution by Cocatalyst Tuning

Understanding photophysical and electrocatalytic processes during photocatalysis in a powder suspension system is crucial for developing efficient solar energy conversion systems. We report a substantial enhancement by a factor of 3 in photocatalytic efficiency for the oxygen evolution reaction (OER) by adding trace amounts (∼0.05 wt %) of noble metals (Rh and Ru) to a 2 wt % cobalt oxide modified Ta₃N₅ photocatalyst particulate. The optimized system exhibited high quantum efficiencies (QEs) of up to 28 and 8.4% at 500 and 600 nm in 0.1 M Na₂S₂O₈ at pH 14. By isolation of the electrochemical components to generate doped cobalt oxide electrodes, the electrocatalytic activity of cobalt oxide on doping with Ru or Rh was improved in comparison with cobalt oxide, as evidenced by the onset shift for electrochemical OER. Density functional theory (DFT) calculations show that the effect of a second metal addition is to perturb the electronic structure and redox properties in such a way that both hole transfer kinetics and electrocatalytic rates improve. Time-resolved terahertz spectroscopy (TRTS) measurement provides evidence of long-lived electron populations (>1 ns; with mobilities μ_e ≈ 0.1–3 cm² V^–1 s^–1), which are not perturbed by the addition of CoO_<i>x</i>-related phases. Furthermore, we find that Ta₃N₅ phases alone suffer ultrafast hole trapping (within 10 ps); the CoO_<i>x</i> and M/CoO_<i>x</i> decorations most likely induce a kinetic competition between hole transfer toward the CoO_<i>x</i>-related phases and trapping in the Ta₃N₅ phase, which is consistent with the improved OER rates. The present work not only provides a novel way to improve electrocatalytic and photocatalytic performance but also gives additional tools and insight into understand the characteristics of photocatalysts that can be used in a suspension system