Nickel Oxide Nanoclusters for Electrocatalytic Water Oxidation

The growth and/or deposition of uniformly dispersed nanoclusters is a significant challenge for the field of electrocatalysis science. Here we employ ALD technique to obtain ultrafine nickel oxide nanoclusters uniformly dispersed on flat electrodes. Atomic layer deposition (ALD) is a versatile technique to the precise growth of a variety of materials. Specifically, the key point in ALD process is that precursor molecules evaporated at an appropriate temperature react only with chemically reactive surface sites (e.g. hydroxyl groups on surfaces), and these reactions are self-limiting, i.e. controllable at the atomic level. Thus, we utilize a self-limiting nature of ALD for nanocluster formation. Meso-tetra(4-carboxyphenyl)porphyrin (TCPP) monolayer is adsorbed parallel to the electrode surface, and subsequently metallated with Mn (or Fe)-OH (denoted by MnTCPP or FeTCPP) as reactive sites for nickel oxide growth. The nickel oxide nanoclusters, then, are grown on the -OH functional groups of MnTCPPs (or FeTCPPs) via ALD. The growth of nickel oxide nanoclusters is demonstrated in detail by transmission electron microscopy images and grazing incidence small angle X-ray scattering analysis, as well as by in-situ monitoring mass gain during deposition. Additionally, to evaluate the electrocatalytic water oxidation activity of nanoclusters, TCPP molecules are removed from nanocluster films through post-ozone-treatments at a relatively low temperature. Based on electrochemical analysis, 1-2 nm sized-nanocluster films show a significantly higher water oxidation activity (i.e. higher water oxidation currents for unit number of nickel atoms deposited on films) compared with that of a very thin film. Consequently, this shows that the uniformly dispersed nanoclusters should be very useful for many applications including electrocatalysis, catalysis, and supercapacitors.