Identifying rate-limiting steps in photocatalysis: a temperature-and light intensity-dependent diagnostic of charge supply vs. charge transfer

Abstract

Photocatalysis fundamentally relies on two interconnected processes: charge supply, which includes carrier generation, separation, and migration, and charge transfer, which involves interfacial redox reactions driving chemical transformations. Differentiating these two processes is critical, as addressing the wrong bottleneck can lead to ineffective optimization strategies. Here, we introduce a simple yet powerful diagnostic based on the distinct temperature sensitivities of these two processes. While charge transfer follows Arrhenius-type kinetics and accelerates significantly with increasing temperature, charge supply is comparatively temperature-insensitive. By systematically varying both temperature and light intensity, we pinpoint the Onset Intensity for Temperature Dependence (OITD), the threshold where surface reactions begin to bottleneck overall photocatalysis. Applying this method to ZnO and TiO₂, it is revealed that ZnO has sufficient carrier generation but sluggish surface reactions, whereas TiO₂ suffers from insufficient carrier supply at lower irradiance. Experiments with TiO₂ powders of varying crystallinity and morphology reveal that smaller particles, which ensure better surface accessibility within the carrier's mean free path, contribute more to performance improvements than enhanced crystallinity. By clarifying which step actually limits performance, this approach provides a straightforward roadmap for targeted catalyst optimization and a deeper understanding of key processes in photocatalysis.