State of the art and perspectives on materials and applications of photocatalysis over TiO2

TLDR: A short review of recent trends, accomplishments, and problems pertaining to photocatalysis over TiO2 and related semiconductors can be found in this paper, where the authors focus on the recent developments in photocatalytic water or air treatment as well as new applications.

Abstract: We present a short review of some recent trends, accomplishments, and problems pertaining to photocatalysis over TiO2 and related semiconductors. We have focused in particular on developments in TiO2-based materials, and on applications including continuing progress in longstanding uses such as photocatalytic water or air treatment as well as more recent ones such as self-cleaning surfaces and, finally, new applications. This article will not consider the topics of degradation mechanisms or reactor design, each of which would merit a lengthy review itself. Furthermore, the references cited are not intended to be comprehensive (for that we refer the reader to an extensive bibliography [1]), but to provide some highlights and examples regarding recent research and basic knowledge. Photon absorption by TiO2 results in the promotion of an electron from the valence band to the conduction band of the titania, leaving behind a ‘‘hole’’ (i.e, electron vacancy) in the valence band, provided the photon has an energy at least equal to that of the bandgap of the photocatalyst. The bandgaps for anatase and rutile being 3.2 and 3.0 eV, corresponding to wavelengths = 385 and 410 nm, respectively, ultraviolet light is needed for photoexcitation. The electron–hole pairs can either recombine or participate in chemical reactions with surface/adsorbed species (Scheme 1). Oxidation of water/hydroxide ion by the valence-band hole (hVB) can produce the hydroxyl radical, OH. The conduction-band electron (e CB) can react with molecular oxygen to form the superoxide radical-anion, O2 , which can be involved in further reactions (Scheme 1). In addition, hVB and e CB can react directly with adsorbed pollutants (Scheme 1). The radicals thus formed, dioxygen and water can participate in further reactions, resulting ultimately in mineralization of the organic pollutants.