Photocatalytic H₂ Evolution over TiO₂ Nanoparticles. The Synergistic Effect of Anatase and Rutile

Abstract

Photocatalytic H 2 evolution over aqueous TiO 2 suspension, with methanol as holes scavenger, is systematically studied as a function of anatase and rutile phase compositions. The highly crystalline, flame-synthesized TiO 2 nanoparticles (22-36 m 2 g -1 ) were designed to contain 4-95 mol % anatase, with the remaining being rutile. Although the amount of photocurrent generated under applied potential bias increases with increasing anatase content, a different trend was observed during photocatalytic H 2 evolution in suspension form; that is, without potential bias. Here, synergistic effects in terms of H 2 evolution were observed for a wide range of anatase contents, from 13 to 79 mol %. At the optimal 39 mol % anatase, the photocatalytic activity was enhanced by more than a factor of 2 with respect to the anatase-and rutile-rich phases. The synergistic effect in these mixed anatase-rutile phases was thought to originate from the efficient charge separation across phase junctions. No synergistic effect was observed for the physically mixed anatase and rutile particles due to insufficient physical contact. Here, we also identify the formation of highly reducing hydroxymethyl radicals during the simultaneous oxidation of methanol, which efficiently inject additional electrons into the TiO 2 conduction band, that is, current-doubling, for heterogeneous (instead of homogeneous) H 2 evolution.

Brookite versus anatase TiO2 photocatalysts: phase transformations and photocatalytic activities

“…This will lead to a better charge carrier separation and thus to an increase of the photocatalytic activity. An analogous synergistic effect is also commonly observed between anatase and rutile phases [32][33][34][35] and is moreover experimentally supported by photodeposition experiments of Ag. 36 However, in the case of brookite-rich nanoparticles, this synergistic effect has not been observed before, as pure brookite showed higher photocatalytic activity than the brookite-rich nanoparticles (see Fig.…”

Hydrogen Production by Photoreforming of Renewable Substrates

“…However, there is not univocal interpretation on the activity of the different crystal phases. Anatase is often considered the most active single phase photocatalyst, though its coupling with rutile may lead to the most interesting results [44]. Indeed, the photogenerated electrons in the anatase crystals may transfer to the conduction band of the rutile crystal and contribute to the reduction of H + .…”

Nanomaterials for renewable energy production and storage

“…176 This synergistic effect of anatase and rutile has also been suggested in other studies. 180 Recently, Paracchino et al reported a highly active photocathode for solar H 2 production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide, and activated for hydrogen evolution with electrodeposited Pt nanoparticles. 181 A schematic representation of the electrode structure and an SEM image of the Cu 2 O grains covered with a 5 Â (4 nm ZnO/0.17 nm Al 2 O 3 )/11 nm TiO 2 atomic layer deposition protective layer and Pt nanoparticles are presented in Fig.…”