Anisotropy of Single-Crystal Semiconductors in Photo(electro)Catalysis

Anisotropy in single-crystal semiconductors has emerged as a key design principle for understanding and advancing photo(electro)catalytic systems. The exposure of well-defined facets in single-crystal semiconductors introduces anisotropic variations in atomic coordination, electronic structure, and surface energetics, giving rise to directional charge transport and facet-specific reactivity. Such intrinsic differences coordinate the entire photocatalytic process, from charge excitation and separation to interfacial reaction kinetics. In this regard, effective utilization of anisotropy requires clarifying its impact on electronic structure, charge transport, and interfacial reactivity, along with its sensitivity to microenvironmental changes under realistic operando conditions. In this review article, we systematically examine the role of crystallographic anisotropy in light harvesting, charge carrier dynamics, and surface reactivity. We summarize recent advances in anisotropic material design, trace the evolution of the concept, and provide mechanistic insights based on experimental studies, theoretical models, and advanced characterization techniques. We further discuss current challenges and propose strategies to guide the rational application of anisotropy in catalyst design, aiming to expand its scope across a broader range of photo(electro)catalytic systems.