Engineering of cobalt oxide-integrated nitric acid-functionalized Zr-Fe₂O₃ nanocoral photoanodes for photoelectrochemical water splitting

A strategy involving CoO_x catalyst adsorption and acid treatment for modifying the surface properties of Zr-doped hematite nanocoral (NC) photoanodes has been proposed. Acid-treated Zr-Fe₂O₃ NCs exhibited a reduced electrode/electrolyte interface resistance and improved surface charge-injection efficiency. X-ray photoelectron spectroscopy data confirmed an increased number of hydroxy groups and the formation of Fe²⁺ sites on the surface of the acid-treated Zr-Fe₂O₃ NC, suggesting surface amendment. Furthermore, after surface amendment with a 1 mM Co(OAc)₂ solution, the CoO_x encapsulated acid-treated Zr-Fe₂O₃ NC photoanode exhibited a photocurrent density of 1.83 mA/cm² at 1.23 V vs. RHE, following a 200-mV cathodic shift in the photocurrent onset compared with the bare Zr-Fe₂O₃ NC. The experimental results indicated the efficacy of acid treatment and CoO_x adsorption for reducing the surface state and enhancing the photoelectrochemical water-oxidation performance. The acid-treatment step in the CoO_x-encapsulated acid-treated Zr-Fe₂O₃ NC provided a CoO_x catalyst adsorption bond and permitted the kinetics of the charge-transfer process. This synergistic strategy of acid treatment and CoO_x adsorption for enhancing the PEC performance provides a reference for accelerating the charge separation for other photoelectrodes.