Infrared and Near-Infrared Spectrometry of Anatase and Rutile Particles Bandgap Excited in Liquid
Zhebin Fu, Hiroshi Ōnishi
Abstract
Chemical conversion of materials is completed in milliseconds or seconds by assembling atoms over semiconductor photocatalysts. Bandgap-excited electrons and holes reactive on this time scale are key to efficient atom assembly to yield the desired products. In this study, attenuated total reflection of infrared and near-infrared light was applied to characterize and quantify the electronic absorption of TiO2 photocatalysts excited in liquid. Nanoparticles of rutile or anatase were placed on a diamond prism, covered with liquid, and irradiated by steady UV light through the prism. Electrons excited in rutile particles (JRC-TIO-6) formed small polarons characterized by a symmetric absorption band spread over 10000–700 cm–1 with a maximum at 6000 cm–1. Electrons in anatase particles (JRC-TIO-7) created large polarons and produced an asymmetric absorption band that gradually strengthened at wavenumbers below 5000 cm–1 and sharply weakened at 1000 cm–1. The absorption spectrum of large electron polarons in TIO-7 was compared with the absorption reported in a Sr-doped NaTaO3 photocatalyst, and it was suggested that excited electrons were accommodated as large polarons in NaTaO3 photocatalysts efficient for artificial photosynthesis. UV-light power dependence of the absorption bands was observed in N2-exposed decane liquid to deduce electron–hole recombination kinetics. With light power density P > 200 W m–2 (TIO-6) and 2000 W m–2 (TIO-7), the polaron absorptions were enhanced with absorbance being proportional to P1/2. The observed 1/2-order power law suggested recombination of multiple electrons and holes randomly moving in each particle. Upon excitation with smaller P, the power-law order increased to unity. The unity-order power law was interpreted with recombination of an electron and a hole that were excited by the same photon. In addition, an average lifetime of 1 ms was estimated with electron polarons in TIO-6 when weakly excited at P = 20 W m–2 to simulate solar-light irradiation.