Gradually Fe-Doped Co3O4 Nanoparticles in 2-Propanol and Water Oxidation Catalysis with Single Laser Pulse Resolution

Abstract

Controlling the surface composition of colloidal nanoparticles is still a challenging yet mandatory prerequisite in catalytic studies to investigate composition-activity trends, active sites, and reaction mechanisms without superposition of particle size or morphology effects. Laser post-processing of colloidal nanoparticles has been employed previously to create defects in oxide nanoparticles, while the possibility of laser-based cation doping of colloidal nanoparticles without affecting their size remains mostly unaccounted for. Consequently, at the example of doping iron into colloidal Co3O4 spinel nanoparticles, we developed a pulse-by-pulse laser cation doping method to provide a catalyst series with a gradually modified surface composition but maintained extrinsic properties such as phase, size, and surface area for catalytic studies. Laser pulse number-resolved doping series were prepared at a laser intensity chosen to selectively heat the Co3O4-NPs to roughly 1000 K and enable cation diffusion of surface-adsorbed Fe3+ into the Co3O4 lattice. The combination of bulk-sensitive X-ray fluorescence and surface-sensitive X-ray photoelectron spectroscopy was used to confirm the surface enrichment of the Fe-dopant. X-ray diffraction, magnetometry, and Mössbauer spectroscopy revealed an increasing interaction between Fe and the antiferromagnetic Co3O4 with arising number of applied laser pulses, in line with a herein proposed laser-induced surface doping of the colloidal Co3O4 nanoparticles with Fe. Using Fick’s second law, the thermal diffusion-related doping depth was estimated to be roughly 2 nm after 4 laser pulses. At the example of gas-phase 2-propanol oxidation and liquid-phase oxygen evolution reaction, the activity of the laser-doped catalysts is in good agreement with previous activity observations on binary iron-cobalt oxides. The catalytic activity was found to linearly increase with the calculated doping depth in both reactions, while only catalysts processed with at least one laser pulse were catalytically stable, highlighting the presented method in providing comparable, active, and stable gradual catalyst doping series for future catalytic studies.