Revealing the Size Effect of Platinum Cocatalyst for Photocatalytic Hydrogen Evolution on TiO2 Support: A DFT Study

In heterogeneous catalysis metal particle morphology and size can influence markedly the activity, both in heat-driven and in electro-photocatalytic conditions. Because of the intimate coupling between the photoelectron transfer and the surface catalytic reaction, it has been challenging to determine the optimal metal particle size in photocatalytic reactions. Here, we utilize ab initio molecular dynamics and hybrid density functional theory calculations to reveal the size-dependent activity of photocatalytic hydrogen evolution reaction (HER) over Pt/TiO2 photocatalysts. By supporting four different Pt particles Pt5, Pt8, Pt13, and Pt19 on anatase TiO2(101), we determine the optimal geometries, the electronic structures, the photoelectron-transfer efficiency, and the surface hydrogen coupling reaction activity of these composite materials. We demonstrate that very small Pt clusters with less than two atomic layers are efficient photoelectron collectors from TiO2 bulk due to their low highest occupied molecular orbital level with respect to the oxide conduction band; by contrast, larger particles with more than two Pt layers are the active site for hydrogen coupling to catalyze HER. Our results suggest that quasi two-Pt-layer particles, corresponding to ～1 nm size, are the best catalyst for photocatalytic HER. Compared to heat-driven and electrocatalysis, we conclude that photocatalysis prefers even smaller metal particles to enhance photoelectron transfer, which should be a general guideline for designing optimal photocatalysts in the future