Laser-Induced Iron Doping of Spinel Thin Film Electrocatalyst: Impact of Pulsed Laser Processing on Lateral Cobalt Oxide Doping and Oxygen Evolution Reaction Activity

Laser-based synthesis of heterogeneous catalysts enables the scalable production of highly pure, defect-rich nanoparticles through kinetic control.1 Recently, pulsed laser defect engineering in liquids (PUDEL) was developed to tailor cation occupancy in spinel nanoparticles with single-pulse precision, using a liquid jet to deliver particles into a repetition-rate-matched laser beam.2 While PUDEL has been successfully applied to nanoparticle suspensions, its transfer to thin film catalysts remained unexplored. Here, we apply PUDEL to cobalt oxide thin films using iron-containing liquids to investigate pulse-dependent Fe doping. Iron cations are kinetically incorporated into the spinel surface from the liquid phase at millimolar concentration, pulse by pulse. The doping depth, characterized by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS), scales with the number of laser pulses and directly influences electrocatalytic oxygen evolution activity. We identify an optimum at three laser pulses, where Fe incorporation significantly enhances catalytic performance, consistent with earlier findings in nanoparticle powders. This demonstrates that PUDEL enables precise surface doping of Co3O4 thin films, offering an alternative to conventional bulk doping strategies. Our results highlight the potential of surface-selective, laser-assisted methods to rationally design active sites in spinel-type electrocatalysts.