Electrochemical Water Oxidation Catalysed by CoO-Co₂O₃-Co(OH)₂ Multiphase-Nanoparticles Prepared by Femtosecond Laser Ablation in Water

Nanoparticle formation via laser ablation in liquid is known to produce functional materials. However, there have been few applications of this technique to the synthesis of electrochemical catalysts for energy conversion. Herein, we report the detailed effects of femtosecond laser ablation in water on the structure and activity of a catalyst intended to promote the electrochemical oxygen evolution reaction (OER) in association with water oxidation. The femtosecond laser ablation of submicron-sized Co–CoO particles induced a drastic size reduction (from approximately 500 to 5 nm) to give highly dispersed CoO nanoparticles. X-ray absorption near edge structure (XANES) and X-ray diffraction (XRD) data demonstrated that these particles also contained Co₃O₄ and CoO(OH) but not metallic Co. These 5 nm-CoO nanoparticles showed higher mass-based-activity and lower over-potential values than those of submicron-sized Co–CoO during the OER in a nearly neutral solution. XANES data suggest that Co containing Co₂O₃ and Co(OH)₂ formed during the OER functioned as the actual OER catalyst.