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

The present chain of five papers considers the concept of defect engineering in processing TiO ₂-based photosensitive semiconductors for solar-to-chemical energy conversion. The papers report the effect of chromium on the key performance-related properties of polycrystalline TiO ₂ (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 TiO ₂, including the band gap and mid-gap energy levels. It is shown that chromium incorporation into the TiO ₂ lattice results in a decrease of the band gap from 3.04 eV for pure TiO ₂ to 1.4 and 1.3 eV, for Cr-doped TiO ₂ (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 TiO ₂ processed in the gas phase of controlled oxygen activity. [Figure not available: see fulltext.].