This dissertation presents studies on semiconductor photoelectrochemical systems, with 2 chapters detailing my major contributions and 3 appendices covering collaborative efforts with my minor contributions.Chapter 1 investigates how single cocatalyst particle depositions influence local and remote charge dynamics on particulate BiVO4 photoanodes. I led the experimental design, synthesis, single-molecule fluorescence imaging, and analysis. Results show that single cocatalyst particles enhance local catalytic activity, while remote suppression occurs for cocatalyst CoOx deposition on basal facets. Operando single-molecule imaging reveals synergistic effects for cocatalyst CoOx and anti-synergistic effects for cocatalyst Pt/PtOx, contributing to improved photocatalyst performance. This chapter comes from a completed first-authored manuscript. Chapter 2 explores overall water splitting on InGaN/GaN nanowires with integrated cocatalysts. I applied operando fluorescence microscopy to map electron- and hole-induced reactions by tracking resazurin reduction and amplex red oxidation. Imaging revealed reaction "hotspots" where electrons and holes converge at shared HER and OER cocatalyst sites, validating critical design principles for optimized nanowires. This chapter comes from a completed manuscript on which I am a co-first author. Appendix 1 examines long-range micropollutant adsorption on metal-promoted photocatalysts (Nature Catal. 2024, 7, 912–920). Appendix 2 investigates nanoscale cooperative adsorption for materials control (Nature Commun. 2021, 12, 4287). Appendix 3 uncovers energy conversion pathways in biohybrids using single-cell multimodal imaging (Nature Chem. 2023, 15, 1400–1407). This dissertation demonstrates the potential of operando single-molecule and single-particle microscopy for elucidating cocatalyst-induced mechanisms, advancing both photo(electro)chemical and biohybrid catalytic system design.