Vanadium Doping of Anisotropic Cobalt Oxide Platelets: Effect of Increasing Laser Energy on Depth Penetration and Resulting Catalytic Activity

Cobalt oxide (Co3O4) is a promising alternative due its electro- and thermocatalytic activity in oxidation reactions. To identify the active sites within Co3O4 and gain deeper insights into its catalytic properties, model systems are required. In this study, hexagonal Co3O4 platelets with predominantly <111> orientation were investigated. Vanadium ions were integrated into the Co3O4 crystal structure via Pulsed Laser Defect Engineering in Liquid (PUDEL), a modification known to influence catalytic activity. A combination of established nanostructure characterization techniques (SEM, XRD, XPS STEM-EDX, ICP-MS, EELS, HAADF, SAEM, and EPR spectroscopy) confirmed and verified a successful incorporation of the vanadium cations into the Co3O4 lattice with pulse-by-pulse control over the amount and depth of doped vanadium. The platelet-like morphology, particle size, and phase purity of the Co3O4 spinel nanoparticles were maintained in all cases. XPS and EDX analysis both revealed a linear increase of Vanadium content with higher number of applied laser pulses and an average vanadium oxidation state of +4 and +5. Electron energy loss spectroscopy (EELS) and electron paramagnetic resonance (EPR) further confirm that the doped Co3O4 is partially reduced by the inserted vanadium and could potentially facilitate the formation of HS Co2+ at Oh-site. The vanadium-doped Co3O4 platelets were shown to enhance the oxidative conversion of cinnamyl alcohol (CA) and to lower the electrocatalytic overpotential of the oxygen evolution reaction (OER) that is reduced by ~40 mV to ~50 mV with increasing number of applied laser pulses during laser processing. Density functional theory (DFT) approximation reveal that a high *OH coverage on the Co3O4 surface leads to an increase in the OER activity. The DFT study also verified that vanadium incorporation further activates the Co3O4 surface for OER, in line with the experimental observations. The investigation of vanadium-doped Co3O4 spinel’s opens new avenues for further doping experiments with other oxides as well as thin-films instead of nanoparticles, to systematically evaluate the influence of individually oriented facets on activity.