This glossary of terms covers phenomena considered under the very wide terms photocatalysis and radiation catalysis. A clear distinction is made between phenomena related to either photochemistry and photocatalysis or radiation chemistry and radiation catalysis. The term “radiation” is used here as embracing electromagnetic radiation of all wavelengths, but in general excluding fast-moving particles. Consistent definitions are given of terms in the areas mentioned above, as well as definitions of the most important parameters used for the quantitative description of the phenomena. Terms related to the up-scaling of photocatalytic processes for industrial applications have been included. This Glossary should be used together with the Glossary of terms used in photochemistry, 3^rd edition, IUPAC Recommendations 2006: (doi:10.1351/pac200779030293) as well as with the IUPAC Compendium of Chemical Terminology, 2^nd ed. (the “Gold Book”, 2006– doi:10.1351/goldbook) because many terms used in photocatalysis are defined in these documents.

This article briefly reviews some factors that have impacted heterogeneous photocatalysis with next generation TiO₂ photocatalysts, 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‐doped TiO₂ are 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‐doped TiO₂ systems. Relative to pristine nominally clean TiO₂ with absorption edges at 3.2 eV (anatase) and 3.0 eV (rutile), N‐doped TiO₂s display red‐shifted absorption edges into the visible spectral region. Several workers have surmised that the (intrinsic) band gap of TiO₂ is 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 doped TiO₂ specimens, 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) TiO₂ photocatalysts should enhance overall process photoefficiencies (in some cases), since doped TiO₂s 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.