Toward sustainable energy: photocatalysis of Cr-doped TiO2: 1. electronic structure

Abstract

The present chain of five papers considers the concept of defect engineering in processing TiO2-based photosensitive semiconductors for solar-to-chemical energy conversion. The papers report the effect of chromium on the key performance-related properties of polycrystalline TiO2 (rutile), including (i) electronic structure, (ii) chromium-related photocatalytic properties, (iii) oxygen-related photocatalytic properties, (iv) electrochemical coupling and (v) surface versus bulk properties. The present work reports the effect of chromium on defect disorder and the related electronic structure of TiO2, including the band gap and mid-gap energy levels. It is shown that chromium incorporation into the TiO2 lattice results in a decrease of the band gap from 3.04 eV for pure TiO2 to 1.4 and 1.3 eV, for Cr-doped TiO2 (1.365 at% Cr) after annealing at 1373 K in the gas phase of controlled oxygen activity, 21 kPa and 10−10 Pa, respectively. The optical properties determined using the ultraviolet-vis spectroscopy (in the reflectance mode) indicate that chromium incorporation results in the formation of mid-band energy levels. In this work, we show that chromium at and above the concentrations levels of 0.04 and 0.376 at% results in the formation of acceptor-type energy levels at 0.57 and 1.16 eV (above the valence band), respectively, which are related to tri-valent chromium in titanium sites and titanium vacancies, respectively. Application of well-defined protocol leads to the determination of data that are well reproducible. The new insight involves the determination of the band gap of TiO2 processed in the gas phase of controlled oxygen activity.