Multiscale probing of the influence of the defect-induced variation of oxygen vacancies on the photocatalytic activity of doped ZnO nanoparticles

Here, we demonstrate that the density of oxygen vacancies generated by the doped transition metal (TM) ions at the surface of ZnO nanoparticles (TM-doped ZnO NPs; TM = Cr or Co) determines the photocatalytic degradation (PCD) activity for organic pollutants; to this end, we employ multimodal microscopy and spectroscopy that encompass material probing at multiple length scales, ranging from the atomic to mesoscale. Following the doping treatment of pristine ZnO NPs as a post-synthetic process, Cr ion doping leads to a significant enhancement of the PCD activity of the ZnO NPs, whereas Co ion doping results in a negative effect. Atomic-scale observations and site-specific spectroscopy confirm that these two TM ions substitute for Zn, in different valence states (Cr3+ and Co2+, respectively), without inducing a phase change of the host ZnO matrix (wurtzite structure). Cr-doped ZnO NPs have the highest density of oxygen vacancies originating from charge mismatch, which contributes to a notably enhanced PCD effect on the tested pollutants, whereas the other two samples (pristine and Co-doped ones) have similar amounts of oxygen vacancies. Our multimodal and multiscale spectroscopic probing provides a clear insight into the significant role of charge mismatch, induced by the substitution of the host cation by the doped cation, in controlling the oxygen vacancies. This can serve as a practical guideline for atomically precise control of oxygen vacancies toward the development of high-performance metal oxide photocatalysts.