Visible-Light-Active Titania Photocatalysts: The Case of N-DopedTiO2s—Properties and Some Fundamental Issues

Abstract

This article briefly reviews some factors that have impacted heterogeneous photocatalysis with next generationTiO2photocatalysts, along with some issues of current debate in the fundamental understanding of the science that underpins the field. Preparative methods and some characteristics features of N-dopedTiO2are presented and described briefly. At variance are experimental results and interpretations of X-ray photoelectron spectra (XPS) with regard to assignments of N 1s binding energies in N-dopedTiO2systems. Relative to pristine nominally cleanTiO2with absorption edges at 3.2 eV (anatase) and 3.0 eV (rutile), N-dopedTiO2s display red-shifted absorption edges into the visible spectral region. Several workers have surmised that the(intrinsic) band gapofTiO2is narrowed by coupling dopant energy states with valence band (VB) states, an inference based on DFT computations. With similar DFT computations, others concluded that red-shifted absorption edges originate from the presence of localized intragap dopant states above the upper level of the VB band. Recent analyses of absorption spectral features in the visible region for a large number of dopedTiO2specimens, however, have suggested a common origin owing to the strong similarities of the absorption features, and this regardless of the preparative methods and the nature of the dopants. The next generation of (doped)TiO2photocatalysts should enhance overall process photoefficiencies (in some cases), since dopedTiO2s absorb a greater quantity of solar radiation. The fundamental science that underpins heterogeneous photocatalysis with the next generation of photocatalysts is a rich playing field ripe for further exploration.