In situ characterization of semiconductor photocatalysts

Semiconductor photocatalysts hold great potential for sustainable energy and environmental applications, but optimizing their performance requires understanding of their behavior under working conditions. In situ characterization techniques, enabling observations under light illumination and in the presence of reactants, are essential for capturing dynamic processes during photocatalysis. This review highlights the application of in situ methods, including X-ray photoelectron spectroscopy and X-ray absorption spectroscopy for monitoring oxidation states; Raman and infrared spectroscopy for analyzing chemical bonds and groups; electron spin resonance spectroscopy and total internal reflection fluorescence microscopy for studying unpaired charge carriers and radicals; photoluminescence, infrared spectroscopy and transient absorption spectroscopy for probing charge excitation and relaxation. These techniques have been employed to examine structural and compositional transformations, such as photocorrosion, metal species reduction and oxidation, and lattice oxygen loss; detect surface intermediates; investigate charge dynamics, such as charge separation and trapping; and identify active sites, including vacancies, metal-based sites, molecular sites, and active components in heterojunctions. The review concludes by discussing challenges and opportunities, specifically, the replication of ambient reaction conditions, mitigation of interference from light sources during spectroscopic measurements, exploration of morphological changes in semiconductors during photocatalysis, and understanding of the effects of photocatalysis on band bending at interfaces.