Development of Ni doped CuWO4 thin films for photoelectrochemical water oxidation

Abstract

CuWO4 is a promising material in solar water oxidation owing to a suitable band gap for visible-light absorption as well as high stability. With a view to improving its activities, we tried to introduce impurities into the host lattice and recognized Ni as the best dopant to others. Polycrystalline Ni doped CuWO4 with various ratios were synthesized to examine the photocatalytic conversion capability. Spray pyrolysis was employed to fabricate the thin films onto FTO glasses at a fixed condition of temperature, flow rate, pressure of carrier gas and distance between the spray gun and substrates. Material characteristics were identified by X-ray diffraction (XRD) and X-ray spectroscopy (XPS). Those spectra reveal that nickel atoms were successfully inserted into copper vacancies of the host lattice without any segregation of the dopant phase. UV-vis absorbance measurements demonstrate negligible influence of Ni on CuWO4 since all spectra have similar curves, suggesting a preliminary band gap of 2.3 eV. Photoelectrochemistry of pristine and Ni doped CuWO4 with various target ratios was performed under solar simulation with neutral condition afforded by a 0.1M potassium phosphate (KPi). 1% Ni doped samples presented the highest photocurrent densities in either linear sweep voltammetry (LSV) or bulk electrolysis, which are twice performances of bare CuWO4. Furthermore, its steady-state photocurrent increased gradually by 15.89% after 1 hour illuminated while performances of other samples stayed the same. Oxygen and hydrogen evolutions were employed to examine photocatalytic water reaction of Ni doped CuWO4. Faradaic efficiencies are ca. 76.8% and 82.7% for oxygen and hydrogen generation rates, respectively.To elucidate the reasons of such upgraded performances, we borrowed two diagnostic investigations. Incident photon-to-current efficiency (IPCE) and electrochemical impedance spectroscopy (EIS) measurements conducted under the same condition revealed pertinent trends with photoactivities, perfectly showing origin of enhancements. In case of 1% Ni doped CuWO4, doubled IPCE represented improved solar conversion. Meanwhile, its resistance accomplished from EIS was reduced two times. Due to decrease in charge-transfer resistance at the interface of the semiconductor and electrolyte, recombination possibility was eliminated across the bulk, hence increased solar conversion and photocatalytic performances.