The water splitting cycle for hydrogen production at photo-induced oxygen vacancies using solar energy: experiments and DFT calculation on pure and metal-doped CeO2

Abstract

Metal oxides can produce photo-induced oxygen vacancies under ultraviolet irradiation, where the oxygen vacancies can help produce hydrogen during the process of thermocatalytic decomposition of water. Therefore, a water splitting cycle on metal oxides, with consecutive photochemical and thermochemical reaction stages, can be established. In this study, cerium oxide (CeO₂) was proved to have the ability of generating photo-induced oxygen vacancies after irradiation, and thereafter the water splitting reaction was performed at the photo-induced oxygen vacancies. The formation and consumption of photo-induced oxygen vacancies on CeO₂ during the cycling process were detected by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). A hydrogen yield of 9.45 μmol g⁻¹ h⁻¹ for pure CeO₂ was achieved. To improve the reactions in the photochemical stage, various transition and lanthanide metal ion doped CeO₂ samples were prepared by the sol–gel method. Cu doped CeO₂ showed the best hydrogen yield of 18.36 μmol g⁻¹ h⁻¹, which is 2 times that of pure CeO₂. The testing results from XPS, Raman, photoluminescence (PL), and EPR indicated that metal ion doping improved light absorption performance and thus effectively promoted the generation of surface oxygen vacancies. Further DFT calculations highlighted that metal ion doping significantly reduced the formation energy of surface oxygen vacancies, and the improved H₂ yield from various metal doped CeO₂ showed an obviously negative correlation with the formation energy of surface oxygen vacancies. This indicated that oxygen vacancies had a dominant role in affecting the efficiency of hydrogen production. However, the metal doping inhibited the thermal reaction to some extent. Accordingly, the calculated energy barrier in the thermochemical stage appeared to be another factor affecting the H₂ yield.