Graphene quantum dots enhance UV photoresponsivity and surface-related sensing speed of zinc oxide nanorod thin films

Thin films of almost vertically-grown zinc oxide nanorods (ZnO NR) were prepared via a solvothermal method, and graphene quantum dots (GQDs) were synthesized by pyrolyzing citric acid. Various amounts of GQDs were loaded onto ZnO NR thin films by applying a dip-coating process for different times. Field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray diffractometry (XRD) were employed to characterize the structures. UV-visible spectra of the thin films showed no significant change in the bandgap (similar to 3.3 eV) after incorporating GQDs, which is appropriate for visible-blind UV sensing applications. Photoluminescence (PL) spectra showed a significant decrease in the PL intensity of the ZnO NR thin film after incorporating GQDs, suggesting that the generated electrons can be better separated from their hole counterparts and hence contribute to the generated photocurrent. Finally, UV sensing analysis was performed, indicating that GQDs can enhance both the UV-induced photocurrent and the sensing speed of ZnO NR thin films; however, the improvement in the surface-related UV sensing was higher than in the bulk-related one, which could be attributed to the numerous active sites and edge effects of GQDs. The enhanced photocurrent was discussed based on the band diagram. (C) 2017 Elsevier Ltd. All rights reserved.