Direct probing of ultrafast heterointerfacial charge transfer in Cu2O/ZnO nanocrystalline composites

Abstract

Transition metal oxides ZnO and Cu₂O are widely used in solar driven photocatalytic water splitting for hydrogen production by their hetero-composites. Charge transfer on the interfacial heterojunctions is the most important responsible mechanism. However, direct photophysical indications for the formation of heterojunctions and for the charge-transfer dynamics on the Cu₂O/ZnO interfaces have not been reported. We investigate in this work transient absorption (TA) spectroscopic signatures for the hetero-interfacial charge-transfer process from Cu₂O to ZnO using femtosecond pump probe. Comparing the TA spectroscopic response of the Cu₂O/ZnO nanocomposites with that of pure ZnO nanorods and pure Cu₂O nanocubes, we were not only able to determine the electronic transition channels, but also resolve the charge-transfer route and its modulation on the transient excitonic absorption and stimulated emission performance. In particular, we assign the broadband stimulated emission with a much reduced lifetime as the fingerprint indication, where the lifetime is dominantly limited by the decay rate of the excitonic population on the conduction band of Cu₂O, although multiple mechanisms may have impact on the corresponding transition dynamics. These discoveries not only give deep insights into the photoelectronics on the Cu₂O/ZnO heterojunctions, but also laid basis for the design and applications of these semiconductors in photocatalysis.