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

摘要

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.