Hot Electron Generation in Aqueous Solution at Oxide-Covered Tantalum Electrodes. Reduction of Methylpyridinium and Electrogenerated Chemiluminescence of Ru(bpy)3

We describe the injection of hot electrons into an aqueous solution by use of a Ta/Ta2O5 electrode. Evidence for this process was obtained by studying the chemically irreversible reduction of N-methylpyridinium ion (NMP+, E° ) -1.37 V vs SHE). The reduction of NMP+ gives rise to a radical which undergoes rapid dimerization to produce a water-insoluble species, 1,1-tetrahydrobipyridine (TBP), that deposits on the electrode surface. The deposit was analyzed by AFM, XPS, IR-reflectance, and TOF/SIMS to provide evidence of TBP formation. We also investigated the electrochemical reduction of Ru(bpy)3 to confirm the generation of hot electrons on a Ta/Ta2O5 electrode in water. The electrochemical and spectroelectrochemical results present a well-defined reduction wave at high overpotential on Ta/Ta2O5 which is not observed on Pt or Au electrodes. Reduction of Ru(bpy)3 at a Ta/Ta2O5 electrode produces emission (electrogenerated chemiluminescence) and provides additional evidence of hot electron injection.