Visible-Light-Driven Water Oxidation with Nanoscale Co3O4: New Optimization Strategies

Water oxidation is the bottleneck of artificial photosynthesis on the way to clean and sustainable solar fuels production. Although spinel-type Co3O4 keeps attracting considerable research interest as a robust and low-cost water oxidation catalyst (WOC), full control over its manifold performance parameters remains a preparative and analytical challenge. The present study screens a wide spectrum of influential factors (such as surface area and adsorption processes, morphology, crystallinity, and cobalt oxidation states) with respect to straightforward WOC optimization strategies. Tuning of crystallinity and cobalt valence states emerges as a major guideline for spinel catalyst synthesis, thus adding a key complementary factor to the main design paradigm of surface area maximization. In contrast to the prevailing amorphous characteristics of electrocatalytic WOCs, crystallinity was found to be crucial for photochemical WOC development. The interplay of synthetic history, crystallinity, and cobalt valence states in hydrothermally synthesized Co3O4 WOCs was investigated with a variety of analytical techniques, including electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and various electron microscopy and spectroscopic techniques. Complementary top down and bottom up optimization strategies introduce new Co3O4 WOC design parameters for facile catalyst production.