Chemically derived defects in zinc oxide nanocrystals and their enhanced photo-electrocatalytic activities

This paper reports the influence of surface defects on the photocatalytic degradation of methylene blue (MB) for zinc oxide (ZnO) nanocrystals (NCs) synthesized in different organic solvents. A simple chemical approach has been adopted for the promotion of oxygen vacancies in pristine ZnO using solvents namely dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and dimethylsulfoxide (DMSO). This alters the growth of NCs through the promotion of oxygen vacancies depending on the fact that the solvent with minimum viscosity supports faster nucleation and growth exhibiting maximum surface defects. DMF with minimum viscosity results in largest particle size and superior photocatalytic activity. Further, X-ray diffraction, UV-visible reflectance spectroscopy and transmission electron microscopy confirm that the DMF supports the faster growth of NCs as compared to NMP and DMSO. Electron paramagnetic resonance, Raman, X-ray photoelectron, and photoluminescence spectroscopies confirm different states of oxygen vacancies in the NCs and their dependence on the nature of solvents. The photocatalytic activities of these NCs were investigated against the degradation of MB as a model dye. The results show that the oxygen defects at the surface of NCs are more responsible for higher photocatalytic activity than the specific surface area of NCs. The electrochemical investigations of MB degradation suggest that these defects upon interaction with MB influence the storage capacity and charge-discharge profiles of NCs. During degradation, MB passivates these defects, which has been explained in terms of increased charge-discharge time and storage capacity.