Electrochemical detection and sizing of colloidal ZnO nanoparticles

We demonstrate the controlled reduction of colloidal ZnO nanoparticles (NPs) on a Hg ultramicroelectrode (UME) and its application to determine the redox potential for the deoxygenation of ZnO. The NPs are in colloidal suspension in acetonitrile (MeCN) and reach the electrode by migrational mass transport. At the electrode surface, the Zn(2+) in zinc oxide NPs is reduced to a Zn(Hg) amalgam. The redissolution (stripping) of Zn(2+) from the amalgam into MeCN is used to determine the amount of reduced ZnO. The stripping charge is proportional to the concentration of the colloidal ZnO NPs at potentials where the reduction is found to be limited by mass transport. The charge resulting from the reduction of ZnO is a function of the reduction potential and presents a sigmoidal curve that fits a quasireversible model and is used to determine the formal reduction potential, E(0)'.We observed that E(0)' is a strong function of NP size in the size regime of 11-80 nm in diameter. The ZnO particles studied are more stable than the Zn(Hg) amalgam, but smaller NPs are less stable compared to larger particles, making smaller NPs relatively easier to reduce. The reduction potential of ZnO NPs is a strong function of 1/r. This Kelvin effect is analogous to the electrochemical behavior of metal nanoparticles described by Plieth's model.