Photocatalysis with CdSe nanoparticles in confined media: Mapping charge transfer events in the subpicosecond to second timescales

Abstract

Photoinduced charge transfer events between 3 nm diameter CdSe semiconductor nanocrystals and an electron acceptor, MV 2+, have been probed in the subpicosecond-microseconds-seconds time scale by confining the reactants in an AOT/heptane reverse micelle. The probe molecule, methyl viologen (MV 2+) interacts with the excited CdSe nanoparticle and quenches its emission effectively. The ultrafast electron transfer to MV 2+, as monitored from the exciton bleaching recovery of CdSe and the formation of MV + radical, is completed with an average rate constant of 2.25×10 10 s -1. Under steady state irradiation (450 nm) the accumulation of MV + is seen with a net quantum yield of 0.1. Mediation of the electron transfer through TiO 2 nanoparticles is achieved by coupling them with the CdSe-MV 2+ system within the reverse micelle. This coupling of two semiconductor nanoparticles increases the quantum yield of MV 2+ reduction by a factor of 2. The dual roles of TiO 2 as an electron shuttle and a rectifier are elucidated by transient absorption spectroscopy and steady state photolysis. The presence of both TiO 2 and MV 2+ in the reverse micelle creates a synergistic effect to enhance the electron transfer rate constant by an order of magnitude. The time-resolved events that dictate the production and stabilization of electron transfer product provide an insight into the photocatalytic systems that are potentially important in solar hydrogen production and photocatalytic remediation. © 2009 American Chemical Society.