Design of visible-light photocatalysts by coupling of narrow bandgap semiconductors and TiO2: effect of their relative energy band positions on the photocatalytic efficiency

Abstract

According to relative energy band positions between TiO₂ and visible-light-absorbing semiconductors, three different types of heterojunction were designed, and their visible-light photocatalytic efficiencies were analyzed. In Type-A heterojunction, the conduction band (CB) level of sensitizer is positioned at a more negative side than that of TiO₂, whereas in Type-B system its valence band (VB) level is more positive than that of TiO₂ and in Type-C system the sensitizer energy level is located between the CB and VB of TiO₂. In evolving CO₂ from gaseous 2-propanol (IP) under visible-light irradiation, the Type-B systems such as FeTiO₃/TiO₂, Ag₃PO₄/TiO₂, W₁₈O₄₉/TiO₂, and Sb-doped SnO₂ (ATO)/TiO₂ demonstrated noticeably higher photocatalytic efficiency than the Type-A such as CdS/TiO₂ and CdSe/TiO₂, while the Type-C such as NiTiO₃/TiO₂, CoTiO₃/TiO₂, and Fe₂O₃/TiO₂ did not show any appreciable improvement. Remarkably high visible-light photocatalytic activity of Type-B heterojunction structures could be explained by inter-semiconductor hole-transfer mechanism between the VB of the sensitizer and that of TiO₂. The evidence for the hole-transport between sensitizer and TiO₂ was also obtained by monitoring the hole-scavenging reactions with iodide (I⁻) and 1,4-terephthalic acid (TA).