Time-Resolved Radio Frequency Conductivity (TRRFC) Studies of Charge-Carrier Dynamics in Aqueous Semiconductor Suspensions

Abstract

The time-resolved radio-frequency conductivity (TRRFC) method provides a useful tool for in situ measurements of charge carrier dynamics in aqueous suspensions of semiconductor particles. In this report, the effects of pH on surface states of ZnO and the effects of hole scavengers (2-propanol) are examined. The experimental results are interpreted in terms of surface mediated recombination processes, in which holes are trapped in a fast process by surficial sites on the ZnO. Recombination rates appear to be governed by the reaction rate of electrons with surface-trap sites. At higher pH (i.e., pH 12), electrostatic repulsion due to a negatively charged ZnO surface leads to slower surface recombination rates compared to lower pH (Le., pH 7) conditions. Addition of a hole scavenger significantly decreases the absolute charge-carrier concentration as detected by TRRFC. This decrease is attributed to the loss of trapped surface holes or surface-bound OH due to oxidation of the hole scavenger. When 2-propanol is used as a solvent, the holes react in a fast step with the solvent at the semiconductor interface within the time resolution of the experiment. The observed TRRFC signal is then due to electrons which are thought to be predominantly transferred to dissolved oxygen (O_2) leading to the formation of hydroperoxyl radicals (HO_2) and subsequently hydrogen peroxide (H_2O_2).