Material Hunting of Advanced Metal Oxide Films for Electro- and Photoelectrocatalysis Using a Mixed Metal-Imidazole Casting (MiMIC) Method

Electrochemical and photoelectrochemical conversion of renewable energy sources into useful chemicals and fuels is of paramount importance for future sustainable technologies. Renewable energy conversion requires catalysts for multielectron redox reactions such as water oxidation and reduction (toward water splitting systems). Developing efficient catalysts for multielectron redox reactions is a great challenge in current science and technology. Metal oxides have been extensively researched to be applied to a large variety of photonic and electronic devices due to the wide range of electronic properties of conducting, semiconducting, and insulating and diverse catalytic properties at their surface depending on the exposing facet, as well as physical and chemical robustness under ambient conditions. We aspire to the development of an easy technique available for large-scale production of metal oxide films based on simple casting and calcination to adopt a strategy for controlling the formation and growth of metal oxide films by ligands to metal centers in precursors. We have developed an easy preparation technique of mono- and multimetallic oxide films, termed the “mixed metal-imidazole casting (MiMIC) method”, by which metal oxide films are generated tightly on various electrode substrates by casting precursor solutions or suspensions containing component metal salts in a mixed solvent of methanol/imidazole derivative as a ligand, followed by calcination. The general versatility of the MiMIC method encourages us to hunt new metal oxide films as efficient catalysts for the multielectron redox reactions, because the rigid adherability of films formed on a current collector electrode is necessary for essential evaluation of the catalytic performance of the metal oxide films. In this Account, we expound synthesis and characterization of a variety of mono- and multimetallic oxide films using the MiMIC method and its application to electro- and photoelectrocatalysis for water splitting and oxygen reduction, which are important key reactions in future sustainable technology. The adherability of these films onto the electrode surface is prominent although their morphology, crystallinity, and nanostructures depend on the metal oxide materials, which is one of the important factors to induce high performance of the metal oxide films for electro- and photoelectrocatalysis. Imidazole derivatives were found to act as a source of nitrogen for the N-doping to a metal oxide lattice, and a structure-directing agent for the anisotropic crystallization, as well as a binder among constituting nanoparticles to lead to the rigid adherability of films on the substrate. These findings surely expand material development to a great extent, by not only changing the metal compositions but also being based on band engineering due to doping of representative elements and crystal facet control of metal oxide films.