Charge transfer across a ZnO/electrolyte interface induced by sub-bandgap illumination: Role of the surface states

The role of interfacial bandgap states in sub-bandgap photoinduced electron transfer across a ZnO/electrolyte junction has been analyzed using time-resolved photocurrent measurements in the millisecond regime. The crystallographic structure and morphology of ZnO samples were characterized using X-ray diffraction and scanning electron microscopy measurements. A kinetic model for charge-carrier transport at the ZnO/electrolyte interface based on the intermediacy of the surface states was developed, and the rate equations were analytically solved. A theoretical simulation of the intensity-dependent photocurrent transients was also conducted. Based on an analysis of the experimental data and theoretical predictions, the density of the surface states was determined to be 3.1 x 10(13) cm(-2) and the capture cross section was 1.5 x 10(-16) cm(2). The obtained experimental results are consistent with the developed kinetic model based on a surface-state mediated charge-transfer mechanism. (C) 2008 The Electrochemical Society.