Hierarchical Growth of CoO Nanoflower Thin Films Influencing the Electrocatalytic Oxygen Evolution Reaction

The 3D architecture of Co(II) oxide (CoO) having oxygen defects has been recognized as a highly functional characteristic towards efficient electrocatalysis of water. Herein, different surface structures of CoO in the form of chemically deposited films were fabricated via AACVD technique, directly over the transparent fluorine-doped tin oxide (FTO) electrodes just by varying the deposition times. The as-prepared films were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). As the deposition time is varied, the surface structure of the CoO changes from nanoparticles that are formed just in 15 min to nanobuds at 30-min deposition, and finally to a homogeneously distributed dense population of nanoflowers in 45 min. The evolution of these structures was also accompanied by a preferential exposure of (111) facets and an increasing number of oxygen defects which resulted in an enhancement of electrocatalytic activity towards water oxidation. The CoO nanoflowers (CoO-NFs) with highest number of these active oxygen vacancies showed the best performance with an overpotential of 325 mV vs RHE for a current density of 10 mA/cm² while having a Taefl slope of 98 mV/dec, a mass activity of 35.2 A/g, and the electrochemically active surface area (ECSA) of 1069 μF. However, more importantly, the current density for CoO-NF jumped sharply to the values above 200 mA/cm² with potential less than 1.8 V vs RHE, thereby meeting the commercialization standards while still providing high stabilities of oxygen generation, current densities, and repeated cycling. Such a performance can be considered remarkable for a material fabricated via a rapid and facile synthetic route and is directly deposited on a low cost and relatively less conductive FTO substrate which can be attributed to the synergistic effect of the larger specific surface area of 3D structure and the high distribution of oxygen defects. [Figure not available: see fulltext.].