Quantifying the prospect of a visible-light-absorbing oxysulfide photocatalyst by probing transient absorption and photoluminescence†

Photocatalytic water splitting is an emerging renewable technology for producing green hydrogen fuel from sunlight and water on a large scale. Identifying charge-carrier transport properties is critical for establishing a design pathway for exciting visible-light-absorbing oxysulfide-based photocatalysts. Herein, the dynamics of distinct charge carriers in the Gd₂Ti₂O₅S₂ (GTOS) photocatalyst is revealed by transient optical spectroscopies (transient diffuse reflectance (TDR) and transient photoluminescence (TPL) spectroscopies) and theoretical modeling. We demonstrate that TDR and TPL signals can probe the evolution of photoexcited mobile electrons and holes separately for GTOS. The decay of optical signals primarily originates from bimolecular recombination of mobile electrons with detrapped holes from shallow trap states close to the valence band. Using different estimated parameters, the effects of the size reduction and charge carrier extraction rate k_e (surface to electrolyte) on the internal quantum efficiency (IQE) are determined. Our results indicate that the IQE can be tremendously improved by simultaneously reducing particle size and increasing k_e. After particle size reduction, we show that the high apparent quantum yield (∼30%) GTOS was achieved by improving k_e (from surface treatment and optimizing the cocatalyst loading method) as compared to Y₂Ti₂O₅S₂ (0.7%). Our work presents a comprehensive methodology that identifies the critical photophysical properties of visible-light-absorbing photocatalysts for efficient and scalable particulate photocatalyst-based solar water splitting systems.