Semiconductor crystals generally possess facet-dependent electrical conductivity, photocatalytic activity, and optical properties. From results of DFT calculations, all these phenomena can be understood to arise from the presence of a thin surface layer with slight bond length variations and bond direction deviations to yield dissimilar band structures for different crystal surfaces. Molecular functionalization can tune the surface band structure and thus enhance photocatalytic activity. Efforts have begun to use XRD and HR-TEM methods to probe the existence of the surface layer.