Photoreaction of Au/TiO2 for hydrogen production from renewables: a review on the synergistic effect between anatase and rutile phases of TiO2

The review focus is on hydrogen production from renewables using photocatalysis. In particular we focus on the role of synergism on the reaction rate. Among the most studied examples for this phenomenon are catalysts based on TiO2. TiO2 exists in two common phases: anatase and rutile, with the latter more thermodynamically stable. For hydrogen production the photocatalyst is often composed of nano-size precious metals deposited on TiO2 (such as Pt, Pd, or Au). It has been observed by many researchers over a decade that M/TiO2 rutile is far less active than M/TiO2 anatase. Yet, the presence of the two phases together results in considerable enhancement of the reaction rate when compared to M/TiO2 anatase alone. The main reason for this is the increase of the charge carriers' lifetime allowing for electron transfer to hydrogen ions and hole transfer to oxygen ions (and/or the sacrificial agent used). In this work we review the few proposed models, so far, explaining the way by which this charge transfer occurs across both phases.