Transient absorption study on photogenerated carrier dynamics in visible light responsive photocatalysts GaN:ZnO

Abstract

Transient absorption of visible light responsive powder photocatalysts, solid solution of GaN and ZnO (denoted as GaN:ZnO), and other related materials were measured by using femtosecond diffuse reflectance spectroscopy in order to evaluate the nature of photogenerated electrons and holes through spectral information in visible and near-infrared region as well as kinetics from 100 fs to 500 ps. The GaN:ZnO is known to be one of successful photocatalysts which are able to split water into oxygen and hydrogen molecules under visible-light irradiation. Since photoexcitation generates electrons and holes in the conduction and valence bands, respectively, it is important to understand their trapping and recombination processes in details. Generally efficient and quick trapping and slow recombination of them are required to increase the chance of charge transfer of them to protons and water molecules. We have elucidated that the charge trapping was within time resolution (< 1 ps) and recombination time was about 100 ps for 26% of carriers and much longer for 74%, clearly indicating that most of photogenerated carriers have long lifetime. Other photocatalysts with lower photocatalytic activity showed shorter lifetimes. These results indicated that the long carrier lifetime in GaN:ZnO is one of the reasons for the efficient reactivity.