Temporary and permanent changes to the defect equilibrium due to ultraviolet exposure: surface and bulk effects on ZnO nanostructures

Abstract

We report on the influence of prolonged exposure of above band gap illumination (UV) on the surface electronic structure (core and valence band) and bulk defect equilibrium of ZnO nanorods. We investigated two samples (ZnO1 and ZnO2) of mutually contrasting surface electronic structure as well as photoluminescence responses. The changes due to the above gap exposure were juxtaposed of as-prepared, UV-treated and healed samples. As prepared samples consist of CC, COC and COOH groups at the surface. The intrinsic surface/defect mediated photocatalytic activity under UV-illumination regenerated the lattice oxygen, oxidized the excess zinc and increased the COC fraction. The surface of ZnO2 was catalytically more active than that of ZnO1 due to zinc interstitials (Znis) and extended Znis (ex-Znis). Also, we identify chemisorbed oxygeneous species, interstitial hydrogen (Hi +), multidentate complexes and disassociated O2 molecule at oxygen vacancies (VO). As a result of the catalytic activity severe changes occurred to the valance band (VB) edge and deeper-VB structure. After the course of healing, the VB edge for ZnO1 recovered to its pristine condition, unlike ZnO2. Additionally, we note increased fraction of O2s component for both the samples which, after healing did not recover to their as prepared condition. In the context of defect equilibrium the UV-treatment reduced the density of charged oxygen vacancy (VO δ) and the thickness of the depletion layer which we attribute to the desorption of some chemisorbed gases and reconstruction of lattice oxygen. For ZnO1, ex-Znis are induced after UV-treatment, which subdued to an extent in the course of healing. In sharp contrast, for ZnO2 the UV-treatment subdued the ex-Zni related emission, which slightly recovered after healing in addition to a further loss of VO δ related emission. The slow recovery and reorganization of intrinsic defects are attributed to the diffusivity of Hi + and the associated lattice distortion, and Znis in the neighborhood of VOs. Furthermore, the non-Coulombic attractive interaction between neutral VOs and Znis mediate the migration of defects and subsequent stabilization on slower timescales. The changes due to UV illumination on the electronic structure and defect equilibrium enhance the applicability and understanding of ZnO nanostructures in optoelectronic applications.