Effect of Sn doping in ZnO on the photocatalytic activity of ZnO-Graphene nanocomposite with improved activity

Development of novel photocatalysts is necessary in environmental cleaning of industrial effluents by photocatalysis. The photocatalyst Sn ion doped (0 to 10 wt %) ZnO-graphene was synthesized by hydrothermal method. ZnO was formed with hexagonal wurtzite crystal structure. Band gap energy of the composite has decreased from 3.12 to 2.84 eV, whereas the photoluminescence lifetime has increased from 15.05 to 21.60 ns with Sn ion doping. Photocatalytic activity of them was tested on anionic dyes (methyl orange, MO and methyl red) and cationic dyes (methylene blue and rhodamine-B) under UV light. The MO was additionally tested under direct sunlight irradiation. Maximum degradation of MO was obtained to be 89.3 and 99.1% under UV and sunlight irradiation respectively, that are 9 and 8% higher than the same shown by undoped ZnO-graphene composite. The best photocatalytic activity was achieved with 1 wt% of doping in ZnO and this enhancement was attributed to optimum doping content, decreased band gap (2.87 eV), sustaining lifetime (15.72 ns), reduced recombination of charge carriers and increased generation of center dot OH radicals during photocatalysis. The photocatalyst could effectively photodegrade anionic dyes in acidic medium and cationic dyes in basic medium as dictated by the type of charges on their surface induced by the pH of the media. This photocatalyst was found efficient in simultaneously degrading the cationic and anionic dyes of mixed solution. The Sn ion doped ZnO with graphene nanocomposite is the first of its kind reported for photocatalytic applications.