Charge kinetics is highly critical in determining the quantum efficiency of solar-to-chemical conversion in photocatalysis, and this includes, but is not limited to, the separation of photoexcited electron-hole pairs, utilization of plasmonic hot carriers and delivery of photo-induced charges to reaction sites, as well as activation of reactants by energized charges. In this review, we highlight the recent progress on probing and steering charge kinetics toward designing highly efficient photocatalysts and elucidate the fundamentals behind the combinative use of controlled synthesis, characterization techniques (with a focus on spectroscopic characterizations) and theoretical simulations in photocatalysis studies. We first introduce the principles of various processes associated with charge kinetics that account for or may affect photocatalysis, from which a set of parameters that are critical to photocatalyst design can be summarized. We then outline the design rules for photocatalyst structures and their corresponding synthetic approaches. The implementation of characterization techniques and theoretical simulations in different steps of photocatalysis, together with the associated fundamentals and working mechanisms, are also presented. Finally, we discuss the challenges and opportunities for photocatalysis research at this unique intersection as well as the potential impact on other research fields.

It is highly desirable to convert CO2 to valuable fuels or chemicals by means of solar energy, which requires CO2 enrichment around photocatalysts from the atmosphere. Here we demonstrate that a porphyrin-involved metal-organic framework (MOF), PCN-222, can selectively capture and further photoreduce CO2 with high efficiency under visible-light irradiation. Mechanistic information gleaned from ultrafast transient absorption spectroscopy (combined with time-resolved photoluminescence spectroscopy) has elucidated the relationship between the photocatalytic activity and the electron-hole separation efficiency. The presence of a deep electron trap state in PCN-222 effectively inhibits the detrimental, radiative electron-hole recombination. As a direct result, PCN-222 significantly enhances photocatalytic conversion of CO2 into formate anion compared to the corresponding porphyrin ligand itself. This work provides important insights into the design of MOF-based materials for CO2 capture and photoreduction.

The effect of defects on electron-hole separation is not always clear and is sometimes contradictory. Herein, we initially built clear models of two-dimensional atomic layers with tunable defect concentrations, and hence directly disclose the defect type and distribution at atomic level. As a prototype, defective one-unit-cell ZnInS atomic layers are successfully synthesized for the first time. Aberration-corrected scanning transmission electron microscopy directly manifests their distinct zinc vacancy concentrations, confirmed by positron annihilation spectrometry and electron spin resonance analysis. Density-functional calculations reveal that the presence of zinc vacancies ensures higher charge density and efficient carrier transport, verified by ultrafast photogenerated electron transfer time of ∼15 ps from the conduction band of ZnInS to the trap states. Ultrafast transient absorption spectroscopy manifests the higher zinc vacancy concentration that allows for ∼1.7-fold increase in average recovery lifetime, confirmed by surface photovoltage spectroscopy and PL spectroscopy analysis, which ensures promoted carrier separation rates. As a result, the one-unit-cell ZnInS layers with rich zinc vacancies exhibit a carbon monoxide formation rate of 33.2 μmol g h, roughly 3.6 times higher than that of the one-unit-cell ZnInS layers with poor zinc vacancies, while the former's photocatalytic activity shows negligible loss after 24 h photocatalysis. This present work uncovers the role of defects in affecting electron-hole separation at atomic level, opening new opportunities for achieving highly efficient solar CO reduction performances.