Characterizing the Structure and Defect Concentration of ZnO Nanoparticles in a Colloidal Solution

The structure and defect concentration of colloidal solutions of ZnO nanoparticles, synthesized by a sol gel procedure (SG-NP), as well as commercially available ZnO nanoparticles (SA-NP) are investigated by UV-vis absorption spectroscopy, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). The XRD patterns, in agreement with the TEM images, reveal that the SG-NPs are more ellipsoidal than the SA-NPs. XRD and XAS reveal the presence of both zinc and oxygen vacancies. The concentration of the latter is independent of the NP size. This is not the case for the zinc vacancies, whose concentration increases sharply in the SG-NPs compared to the SA-NPs, and an similar to 40% oxygen excess in comparison to the expected stoichiometric ratio is found. Importantly, an extended X-ray absorption fine structure (EXAFS) analysis shows that this large concentration of zinc vacancies does not lead to distortions of the local lattice structure. Finally, the Zn K-edge X-ray absorption near edge structure (XANES) spectra show distinct changes in the rising edge and above edge regions, which supports the presence of zinc vacancies. In all cases, two weak pre-edge features are also observed and assigned to a small concentration of oxygen vacancies.