Photoelectrochemical energy conversion represents a promising energy technology that converts solar energy to useful chemical fuels and electricity. The key component in a photoelectrochemical cell is a semiconductor material that upon illumination generates charge carriers (electrons and holes). Evaluation of the photoelectrocatalytic properties of these semiconductor materials are usually performed at an ensemble level. Here we use a single-molecule fluorescence microscopy technique to interrogate the photoelectrocatalytic process, at the single-particle and sub-particle level, of a semiconductor material with facet-dependent electronic structure. Using redox-selective probe molecules coupled with electrochemical modulation, we visualize the spatial distributions of the activities of both electrons and holes under different electrical potentials, providing for the first time quantitative information on the facet and potential dependences of charge carrier activities. Furthermore, using focused-laser beam illumination at a sub-facet level combined with theoretical simulation of charge carrier behaviors, we demonstrate intra-facet variation of important physical properties (photocurrent, incident-photon-to-current efficiency, flat band potential, quasi Fermi level) that are considered crucial performance metrics for photoelectrochemical energy devices. Our findings reveal the importance of edges in three-dimensional semiconductor crystals with facet-dependent electronic structure, opening up new opportunities in the rational design of energy materials with complex morphological and electronic properties.