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

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).