Imaging photogenerated charge carriers on surfaces and interfaces of photocatalysts with surface photovoltage microscopy

Understanding photogenerated charge separation on the nano- to micrometer scale is the key to optimizing the photocatalytic solar energy conversion efficiency. In the past few years, spatially resolved surface photovoltage (SPV) techniques have opened up new opportunities to directly image localized charge separation at surfaces or interfaces of photocatalysts and thus provided deep insights into the understanding of photocatalysis. In this review, we reviewed the SPV techniques, in particular Kelvin probe force microscopy (KPFM) based spatially resolved SPV techniques and their applications in charge separation imaging. The SPV principle was explained with regard to charge separation across a space charge region (SCR) in a depletion layer at a semiconductor surface and to diffusion. The center of charge approach, relaxation of SPV signals and measurement of SPV signals including SPV transients with fixed capacitors were described. Then, we highlighted the fundamental principle and development of the spatially resolved SPV technique and its application in photocatalysis. Important progress made by the spatially resolved SPV technique in this group is given, focusing on understanding the nature of charge separation and providing insights into the rational design of highly efficient photocatalytic systems. Finally, we discuss the prospects of further developments of the spatially resolved SPV technique that would help in understanding photocatalysis for solar energy conversion with high temporal resolution and operated under in operando conditions.