Observation of individual semiconducting nanoparticle collisions by stochastic photoelectrochemical currents.

We describe a method to detect individual semiconducting nanoparticles (NPs) using the photoelectrochemical (PEC) current measured at an ultramicroelectrode (UME). We use photooxidation of MeOH by TiO2 NPs as a model system of photocatalysis in solution. NPs suspended in MeOH under constant illumination produce valence-band holes that oxidize MeOH. The electrons are collected at the UME, and the current-versus-time data show discrete current changes that are assigned to particle-by-particle interactions of the NPs with the UME. The stepwise changes in the photocurrent denote irreversible attachment of NPs to Pt UMEs (<30 μm diameter). We found that accumulation of electrons in the conduction band by the NPs is not enough to explain the stochastic PEC currents. We propose that the observed anodic steps have a PEC nature and are due to photooxidation of MeOH by the NPs at the electrode surface.