Electrochemical features of electrodes modified with Multiple Nano Contacts (MNCs) from colloidal noble metal particles

Abstract

The key problem for the direct conversion of solar energy in photoelectrochemical cells is catalysis. The deposition of electrocatalytic metals onto the surface of the semiconductor electrode can provide the necessary catalysis for chemical reactions. In photoelectrochemical devices these reactions are driven by the minority charge carriers produced upon illumination. To avoid a simultaneous increase of the majority carrier transfer—the loss current which decreases the efficiency of the photon energy conversion device—a new way of metallizing semiconductor surfaces has been developed. By depositing colloidal metal particles onto semiconductor surfaces, well defined Multiple Nano Contacts (MNCs) are prepared. The surface modification by MNCs changes the catalytic properties of the semiconductor surface dramatically with negligible disturbance of the semiconductor surface energetics. This has been verified here by performing electrochemical experiments with gold MNCs of different sizes (6–100 nm) deposited on p-GaAs electrodes. Calculations and experiments using bulk gold electrodes prove that a very small surface coverage of metal particles is sufficient to prevent charge carrier diffusion problems in the electrolyte. Investigations performed with platinized gold particles elucidate the great influence of the spherical diffusion for the transfer of ions across the diffusion layer. The different electrochemical properties of semiconductor electrodes modified with deposits from metal ion solutions compared to MNC modified electrodes are verified with time resolved potentiostatic measurements. These current/time measurements reveal that a gold coverage of less than half a monolayer deteriorates the advantageous properties of the semiconductor/electrolyte interface for energy conversion.