Chelation-mediated in-situ formation of ultrathin cobalt (oxy)hydroxides on hematite photoanode towards enhanced photoelectrochemical water oxidation

Abstract In this work, a facile chelation-mediated route was developed to fabricate ultrathin cobalt (oxy)hydroxides (CoOOH) nanosheets on hematite photoanode (Fe2O3). The route contains two steps of the adsorption of [Co-EDTA]2− species on Fe2O3 nanorod array followed by the hydrolysis in alkaline solution. The resulting CoOOH/Fe2O3 exhibits a remarkably improved photocurrent density of 2.10 mA cm−2 at 1.23 V vs. RHE, which is ca. 2.8 times that of bare Fe2O3. In addition, a negative shift of onset potential ca. 200 mV is achieved. The structural characterizations reveal the chelate EDTA plays important roles that enhance the adsorption of Co species and the formation of contact between CoOOH and Fe2O3. (Photo)electrochemical analysis suggests, besides providing active sites for water oxidation, CoOOH at large extent promotes the charge separation and the charge transfer via passivating surface states and suppressing charge recombination. It also found CoOOH possesses some oxygen vacancies, which could act as trapping centers for photogenerated holes and facilitate the charge separation. Intensity modulated photocurrent spectroscopy (IMPS) shows that, under low applied potential the water oxidation mainly occurs on CoOOH, while under high applied potential the water oxidation could occur on both CoOOH and Fe2O3. The findings not only provide an efficient strategy for designing ultrathin (oxy)hydroxides on semiconductors for PEC applications but also put forward a new insight on the role of CoOOH during water oxidation.