Overall Utilization of Photoexcited Charges for Simultaneous Photocatalytic Redox Reactions

Abstract: The photoconversion of CO2 to carbon-containing fuels, splitting water into H2, selective organic synthesis, reduction of N2 to NH3, and hazardous organic contaminant degradation represent feasible schemes for solving environmental and energy issues. In 1972, TiO2 was applied for decomposing water into H2 and O2 via photocatalysis. Owing to its the low visible-light utilization, fast charge recombination, and high energy barrier for water oxidation, overall photocatalytic water-splitting efficiency is extremely low. Because H2 is more economically valuable than O2, sacrificial agent-assisted photocatalytic H2 evolution has been extensively investigated. Because the sacrificial agent can quickly consume photoexcited holes and effectively reduce the water oxidation energy barrier, photocatalytic H2 evolution efficiency can be increased by 3–4 orders of magnitude compared to photocatalytic water splitting. However, the overuse of sacrificial agents contributes to wasted photoexcited holes and expensive processes, while presenting potential environmental issues. Recently, overall charge utilization and improved redox efficiency have been achieved by coupling photocatalytic reduction with oxidation reactions. Moreover, overall charge utilization can boost charge separation and increase photocatalyst durability. However, the photocatalytic mechanism of the overall redox reactions remains unclear, owing to the complex reaction processes and design difficulties. Herein, the basic principles of photocatalysis are discussed from the perspective of light harvesting, photoexcited charge separation, thermodynamics, and redox reaction kinetics. Photocatalytic redox reactions, including overall water photodecomposition, photocatalytic H2 evolution coupled with organic oxidation, photocatalytic CO2 reduction coupled with organic oxidation, photocatalytic H2O2 production coupled with organic oxidation, photocatalytic N2 reduction coupled with N2 oxidation, and photocatalytic organic reduction coupled with organic oxidation, can be systematically classified according to the coupling of photocatalytic oxidation reactions with photocatalytic reduction reactions. Subsequently, the design of photocatalytic redox reactions is considered in terms of the modulation of photocatalyst materials, reaction conditions, and diversity of reactants and products. In addition, the vital role of density functional theory (DFT) calculations for unveiling photoexcited charge transfer, rate-determining steps, and redox reaction barriers are discussed in the context of the work function, electron density difference, Bader charge, and variation in the intermediate adsorption free energy profiles. The activity and mechanism of various photocatalytic redox reactions were elaborately analyzed through in situ characterizations and DFT calculations using representative cases. Finally, the overall photocatalytic redox reactions were summarized with a focus on the construction of an S-scheme heterojunction photocatalyst, reasonable loading of co-catalysts, photocatalyst morphology regulation, novel photocatalyst development, reasonable selection of the oxidation half-reaction and reduction half-reaction for coupling, and combined in situ characterization and DFT calculations. This work provides a reference for promising design strategies and insight into the mechanism of overall photocatalytic redox reactions.