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A coating strategy to achieve effective local charge separation for photocatalytic coevolution

Significance Particulate photocatalysis is a promising approach to solar fuels production at scale. Herein, we present a general design by using conformal coatings and attaching nanoscale cocatalysts to achieve local charge separation and, at the same time, to stabilize photocatalysts that are easily photocorroded otherwise. With spatial charge separation, the nanometer-spaced reductive and oxidative surface sites can coevolve to produce H2 and O2, or to produce H2 and oxidize redox mediators, or to produce O2 and reduce redox mediators. This work investigates the charge separation strategy for the semiconductor/coating/cocatalyst structure both by tuning barrier height energetics and by building numerical models. Semiconductors of narrow bandgaps and high quantum efficiency have not been broadly utilized for photocatalytic coevolution of H2 and O2 via water splitting. One prominent issue is to develop effective protection strategies, which not only mitigate photocorrosion in an aqueous environment but also facilitate charge separation. Achieving local charge separation is especially challenging when these reductive and oxidative sites are placed only nanometers apart compared to two macroscopically separated electrodes in a photoelectrochemical cell. Additionally, the driving force of charge separation, namely the energetic difference in the barrier heights across the two type of sites, is small. Herein, we used conformal coatings attached by nanoscale cocatalysts to transform two classes of tunable bandgap semiconductors, i.e., CdS and GaInP2, into stable and efficient photocatalysts. We used hydrogen evolution and redox-mediator oxidation for model study, and further constructed a two-compartment solar fuel generator that separated stoichiometric H2 and O2 products. Distinct from the single charge-transfer direction reported for conventional protective coatings, the coating herein allows for concurrent injection of photoexcited electrons and holes through the coating. The energetic difference between reductive and oxidative catalytic sites was regulated by selectivity and local kinetics. Accordingly, the charge separation behavior was validated using numerical simulations. Following this design principle, the CdS/TiO2/Rh@CrOx photocatalysts evolved H2 while oxidizing reversible polysulfide redox mediators at a maximum rate of 90.6 μmol⋅h−1⋅cm−2 by stacking three panels. Powered by a solar cell, the redox-mediated solar water-splitting reactor regenerated the polysulfide repeatedly and achieved solar-to-hydrogen efficiency of 1.7%.