Photocatalytic degradation of methylene blue in water using zinc oxide modified by sugar-derived carbon

Water pollution has recently emerged as one of the most concerns in the world. Particularly, water resources that are contaminated by organic pollutants which consist of pharmaceuticals and personal care products (PPCPs), persistent organic pollutants (POPs), and organic dyes. Photocatalysis, one of the advanced oxidation processes, is being noticed as one of the most efficient technologies to mitigate organic pollutants in water and is eco-friendly. Zinc oxide is one of the most used photocatalysts because of its extinguish properties, which are easy to synthesize, non-toxic, and low-cost. However, poor activity under visible light and the recombination of photogenerated electron and hole pairs are the limitations of zinc oxide. To fix these problems, the modification of zinc oxide with carbon quantum dots (CQDs) was carried out, because CQDs with a size less than 10 nm have up-conversion photoluminescence (UPCL) properties. They can act as spectral converters and electron reservoirs to suppress recombination which can enhance photocatalysis efficiency. In this work, sugar was used as a low-cost carbon source to modify zinc oxide. The modified zinc oxide was used as photocatalysts for the degradation of methylene blue with an initial concentration of 5 mg/L under UV-free LED (19 watts) irradiation. The weight ratios between zinc oxide and CQD solution were varied to determine the optimum ratio that gave the maximum efficiency. In this work, the optimum ratio of 1:1 was obtained with a degradation efficiency of 68.1%.