Green Emission to Probe Photoinduced Charging Events in ZnO−Au Nanoparticles. Charge Distribution and Fermi-Level Equilibration

Abstract

Photoinduced electron accumulation in ZnO nanoparticles results in the bleaching of the exciton band as well as quenching of green emission. In the absence of an electron scavenger, photogenerated electrons are stored near the conduction band edge and promote charge recombination via a nonradiative process. By exposing the UV-irradiated ZnO suspension to an electron acceptor (O 2 or thionine dye) the stored electrons are discharged and the original excitonic band and the visible emission are restored. Titration of electrons stored in ZnO nanoparticles with an electron acceptor, thionine dye, shows a linear relationship between stored electrons and the emission quenching. When gold nanoparticles are added to pre-UV-irradiated ZnO colloids, only partial recovery of the emission is seen. Pt nanoparticles on the other hand caused almost complete recovery of the quenched emission as the electrons are discharged into the solution. The charge distribution between UV-irradiated ZnO and gold nanoparticles results in equilibration of the Fermi level. Furthermore, the transfer of electrons to the metal nanocore followed by equilibration continues until the Fermi level reaches close to the conduction band edge of ZnO. Basic understanding of the interaction between the semiconductor and metal layers leading to Fermi-level equilibration is important for evaluating the role of noble metals in photocatalytic reactions.