Photocatalytic H₂ evolution over TiO₂ nanoparticles. The synergistic effect of anatase and rutile

Photocatalytic H₂ evolution over aqueous TiO₂ suspension, with methanol as holes scavenger, is systematically studied as a function of anatase and rutile phase compositions. The highly crystalline, flame-synthesized TiO₂ nanoparticles (22-36 m² 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 H₂ evolution in suspension form; that is, without potential bias. Here, synergistic effects in terms of H₂ 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 TiO₂ conduction band, that is, current-doubling, for heterogeneous (instead of homogeneous) H₂ evolution. © 2010 American Chemical Society.