Improved Solar-Driven Photocatalytic Activity of Hybrid Graphene Quantum Dots/ZnO Nanowires: A Direct Z-Scheme Mechanism

Herein, an electrochemical technique as a cost-effective and one-step approach was utilized to fabricate graphene quantum dots (GQDs). Different amounts of GQDs (0, 0.2, 0.4, 0.8, and 1.2 wt %) were decorated uniformly on the surface of anodized ZnO nanowires (NWs) forming GQD/ZnO NWs. Transmission electron microscopy and atomic force microscopy confirmed formation of GQDs on the ZnO NWs, 12-22 nm in width and 1-3 graphene layers thick. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were employed to verify the functional groups on the surface of GQDs, and the results indicated that GQDs readily anchored on the surface of ZnO NWs. The GQD/ZnO NWs exhibited a considerable improvement on the photocatalytic degradation of methylene blue under solar irradiation, due to efficient light absorption. In addition, the results indicated that the optimized GQD (0.4 wt %)/ZnO NWs showed the highest photoactivity with about 3-fold enhancement as compared to pure ZnO NWs. Finally, a mechanism of charge carrier generation, transport, and separation was proposed using different scavengers to probe the potential reaction pathway following a direct Z-scheme approach.