Spinel-type Co3O4 finds applications in a wide range of technological fields, including gas sensing and clean energy conversion, where nanostructured Co3O4 may provide a cost-efficient alternative to Pt- and Ir-based catalysts for electrocatalytic water-splitting. As a particular challenge, however, the oxygen-based redox process involves a complex four-electron transfer resulting in poor reaction kinetics. Hence, efficient and robust catalysts promoting the oxygen evolution reaction (OER) are in great demand.
We here explore a synthetic approach based on the decomposition of thermally unstable solution-grown oleate precursors, where a similar methodology was previously described by Ehlert et al. for the preparation of ZnO nanoparticles. The reaction conditions are optimized by systematically varying reactant concentrations and solvent mixtures as well as the decomposition temperature. Furthermore, we investigate the stabilizing and structure-directing effects of various other fatty acid ligands, such as stearate, myristate, and caprate. The reaction products are routinely characterized by UV/Vis spectroscopy, transmission electron microscopy, and small-angle x-ray scattering. Most importantly, we investigate the processes of nanoparticle formation under different reaction regimes by in-situ small-angle x-ray scattering in a temperature-controlled reaction cell.
Our synthetic strategy leads to ligand-stabilized CoOx nanoparticles, and thus offers the opportunity for a dedicated ligand exchange . This enables an ordered assembly of the nanoparticles into superlattices via selective loading into specific microdomains of phase-separated blockcopolymer systems, where post-synthetic conversion into Co3O4 is possible. For the direct synthesis of Co3O4 , cobaltous hydroxide precursors are employed which offer superior possibilities regarding size- and morphology control.
In order to study the effects of particle morphology and size on the functional properties, the nanoparticles are investigated as electrocatalysts for the oxygen evolution reaction (OER). For this purpose, the materials are isolated as powders and immobilized on a rotating disc electrode for analysis in cyclic voltammetry. The resulting overpotentials towards the OER associated with these very well-defined, but randomly oriented nanoparticles will be discussed in comparison to reference materials as well as micron-scale mesocrystalline particles comprising a substructure based on co-oriented Co3O4 nanoparticles interspersed by pores.
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