Metalloporphyrin-Decorated Titanium Dioxide Nanosheets for Efficient Photocatalytic Carbon Dioxide Reduction.

In photocatalysis, the most efficient way to separate photogenerated electron-hole pairs has been extensively studied. However, the methods to increase the quantities of free electrons are neglected. Herein, we used a self-assembly method to fabricate MTCPP/TiO composite materials with a series of metalloporphyrins (MTCPPs, M = Fe, Co, Zn) as sensitizers to modify TiO nanosheets. First, abundant carboxyl and hydroxyl on porphyrin were adsorbed by metal ions. Then, the remaining carboxyl and hydroxyl on porphyrin were anchored on the surface of TiO nanosheets. Finally, MTCPP/TiO was obtained by a layer-by-layer self-assembly process. MTCPP broadens the light response of TiO from ultraviolet light to visible light and enhances the CO adsorption ability. Moreover, metal ions coordinating with porphyrin regulate the electron density of the porphyrin ring and provide a stronger π feedback bond, which promote charge separation. Consequently, by optimizing the type of metal ion, the yield of ZnTCPP/TiO composites reached 109.33 μmol/(g h) of CO and 9.94 μmol/(g h) of CH, which was more than 50 times that of pure TiO. This study proposes a possible visible-light-induced CO reduction mechanism of metal-ion-based photocatalysis, which provides great insights into optimizing the designation of efficient photocatalysis.