Because of the complex composition of the apparent activation energy, the rate-determining step in a photocatalytic reaction like hydrogen evolution is still being explored even after sluggish oxygen evolution is replaced with efficient hole extraction. This issue severely limits the implementation of certain strategies like the synergistic thermal effect. Here, by developing a combined monitor method based on open-circuit potential decay, we demonstrate that semiconductor–cocatalyst interfacial electron transfer occurring on a decisecond to second time scale dominates photocatalytic hydrogen evolution. This time scale is approximately 6–12 orders of magnitude larger than the widely reported values of picoseconds to microseconds and is comparable to that predicted by Durrant et al. To improve photocatalytic hydrogen evolution, we manage to create more intermediate sites by electronically doping the semiconductor surface. This measure promotes semiconductor–cocatalyst interfacial electron transfer by charge recombination and makes the synergistic thermal effect very evident.

The major objective of the emerging photo-thermo-catalysis is using waste heat to boost the photocatalytic reaction, especially that powered by sunlight. Because of the complex composition of light-intensity-dependent apparent activation energies, the issue that principally hinders the synergistic thermal effect to photocatalysis has hardly been accurately explored. In this work, by virtue of mutual match of theoretical simulation and experimental behaviors, we demonstrate that photocatalytic reaction rates exhibit a sensitively positive correlation with temperature under weak illumination, in which charge recombination predominates the rate-determining step of semiconductor–cocatalyst interfacial electron transfer. Under high-intensity irradiation, however, the aggravation of charge leakage inherently accompanied by thermionic emission severely weakens the synergistic thermal effect or even slows down the reaction by raising the temperature. Inspired by these, we manage to maximize the photocatalytic solar utilization by spherical incidence of sunlight with the assistance of low-grade heat.

Because of very high potential barrier for thermionic emission and trap‐assisted charge recombination, photocatalytic reaction rate that determined by semiconductor‐cocatalyst interfacial electron transfer severely deviates from linearity to the photocatalyst dosage or to the light intensity. This makes it challenging to maximize utilization of practical irradiation by referring the parameters evaluated from method used in conventional catalysis. We here develop a model and predict that photocatalytic reaction rate positively correlates to photocatalyst concentration under weak illumination while the correlation becomes negative under intense irradiation. The theoretical simulation that matches the experimental values can be used to guide maximizing photocatalytic photon utilization under various intensity of irradiation. The strong correlation can rationalize photocatalytic evaluation instead of obtaining a numerically high value by excessively lowering the denominators. To realize efficient utilization of real‐time changing sunlight, we propose a reactor configuration that can optimize the amount of photocatalyst participating into the reaction.

Developing cocatalyst-free photocatalysts is highly desired because it could avoid the very slow interfacial electron transfer that makes photocatalytic photon utilization a dilemma. However, even in the optimal case, photocatalysts without the use of cocatalysts deliver comparable performance only for conventional construction. We demonstrate here that electronic doping not only provides catalytically active sites in cocatalyst-free photocatalysts but also plays certain additional roles. These electronic states can efficiently channel the trapped electrons to the semiconductor surface without suffering from time-consuming detrapping and can facilitate the extraction of photogenerated holes. These features endow our demonstrated tungsten-doped CdS with evident superiority in photocatalytic performance over conventional counterparts loaded with platinum cocatalysts.