Combined Experimental and Theoretical Insights into the Synergistic Effect of Cerium Doping and Oxygen Vacancies in BaZrO_3−δ Hollow Nanospheres for Efficient Photocatalytic Hydrogen Production

The long-standing debate over the influence of oxygen vacancies and various dopants has been the center point in perovskite-based compounds for their photocatalytic applications. Hydrothermally synthesized cerium-doped BaZrO₃ (BZO) hollow nanospheres have been systematically studied by experimental and theoretical calculations to understand the effect of cerium doping and oxygen vacancies on the photocatalytic properties. Compounds synthesized by a template-free route were composed of hollow nanospheres generated by Ostwald ripening of spherical nanospheres, which were formed by agglomeration of nanoparticles. The high alkaline condition and high temperature during the hydrothermal condition may lead to the formation of local disorders and oxygen vacancies in the compounds, confirmed by ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) analysis, and density functional theoretical (DFT) calculations. Combination of oxygen vacancies and progressive doping of Ce in BaZr_1–xCe_xO₃ (x = 0.00–0.04), creates additional energy levels stipulated by vacancy defects and Ce mixed valence states within the band gap of BZO, thereby reducing its band gap. The photocatalytic efficacy of the compounds has been examined by photodriven H₂ generation concomitant with oxidation of a sacrificial donor. In this study, BaZr_0.97Ce_0.03O₃ shows the highest efficiency (823 μmol h^–1 g^–1) with an apparent quantum yield (AQY) of 6% in photocatalytic H₂ production among all five synthesized samples. The data obtained from the UV–vis DRS, XPS, ESR analysis, and DFT calculations, the synergistic effect of decreasing the band gap due to Ce doping and the presence of Ce­(III)/Ce­(IV) pairs along with oxygen vacancies and lattice distortions could be the reasons behind the enhanced photocatalytic efficacy of BaZr_1–xCe_xO₃ (x = 0.00–0.04) under UV–vis light.