Highly efficient visible light driven photocatalytic activity of graphene and CNTs based Mg doped ZnO photocatalysts: A comparative study

Novel photocatalysts, Mg doped ZnO/CNTs (MZC) and Mg doped ZnO/graphene (MZG) composites synthesized via sol-gel method and characterized with a variety of tools such as XRD, SEM, TEM, BET surface area, EDX, UV-vis DRS, PL, photocurrent transient response, EIS and Mott-Schottky studies were used to study the photocatalytic methylene blue (MB) elimination under visible light illumination. When compared with pure ZnO nanoparticles, MZG catalyst showed considerably about 5.23 times higher photocatalytic activity in the elimination of MB, while about two times higher than MZC catalyst under 60 min visible light illumination. Similarly, the catalyst dose of 1 g/L resulted in great improvement of photocatalytic MB elimination over MZG catalyst under visible light illumination, reaching to MB elimination rate of 100% in just 30 min which is the highest among all ZnO based photocatalytic systems reported so far. Also, this elimination rate was six times higher than that of pure ZnO catalyst, while approximately 1.3 times greater than that of MZC photocatalyst. As initial solution pH treatment, the photocatalytic activity was gradually increased under visible light illumination up to optimal pH 10, identifying 4.5 and 1.4 times higher elimination rate for MZG catalyst than pure ZnO and MZC catalysts, respectively. Designing heterojunction, declining grain size extending optical absorption, improving surface area, inhibiting electron-hole recombination rate and quick transport of photo generated charge carriers to the surface of the photocatalyst due to its lower impedance were considered efficient strategies to enhance the photocatalytic performances. The MZG photocatalyst revealed efficient photocatalytic stability under visible light illumination retaining efficiency about 100% after five cycles of MB elimination. Similarly, XRD and PL reusability tests of MZG photocatalyst did not reveal any difference in its structural and emission responses. The efficient and inexpensive photocatalytic results and strong stability of MZG photocatalyst exemplarily highlights new pathways to design potential visible light driven photocatalysts for industrial applications.