Facet-Dependent Electrical, Photocatalytic, and Optical Properties of Semiconductor Crystals and Their Implications for Applications

Recent studies on the electrical conductivity and photocatalytic activity properties of semiconductor nanocrystals such as Cu2O, Ag2O, TiO2, PbS, and Ag3PO4 exposing well-defined surfaces have revealed strong facet effects. For example, the electrical conductivity of Cu2O crystals can vary from highly conductive to nonconductive, and they can be highly photocatalytically active or inactive depending on the exposed faces. The crystal surfaces can even tune their light absorption wavelengths. Our understanding is that the emergence of these unusual phenomena can be explained in terms of the presence of an ultrathin surface layer having different band structures and degrees of band bending for different surfaces, which affects charge transport and photons into and out of the crystals. This review uses primarily results from our research on this frontier area of semiconductor properties to illustrate the existence of semiconductor facet effects. A simple adjustment to normal semiconductor band diagram allows good understanding of the observed phenomena. Recognizing that facet-dependent behaviors are intrinsic semiconductor properties, we should pay attention to their influence in the explanation of the measured photocatalytic properties, and consider ways to enhance photocatalytic efficiency or design electrical components utilizing the facet effects. There should be many opportunities to advance applications of semiconductor nanocrystals and nanostructures with continued research on the facet-dependent properties of various semiconductor materials.