Zinc oxide-carbon nitride and zinc oxide-reduced graphene oxide composite for photodegradation of phenol and hydrogen production from water

Abstract

Design of highly efficient photocatalysts that is workable for various photocatalytic processes such as organic pollutant degradation and hydrogen production from water is crucial. Zinc oxide (ZnO) is the suitable candidate for such photocatalysis, owing to its superior activity under UV light. For phenol degradation, ZnO prepared by precipitation method showed ca. 10% degradation. This activity was twice higher compared with the ZnO prepared by calcination method due to higher degree of crystallinity, larger surface area (15 m2g-1) and higher interactions with phenol (Ksv = 0.0051 ppm-1). Unfortunately, poor responses to visible light and high rate of electron hole pair recombination have limited the use of ZnO as a photocatalyst. In order to overcome the drawbacks of the ZnO, carbon nitride-zinc oxide (CN-ZnO) and reduced graphene oxide-zinc oxide (rGO-ZnO) composites were prepared by impregnation and photoreduction methods, respectively. The effects of zinc to carbon mole ratio (Zn/C) for the CN-ZnO composites and GO loading amounts for the rGO-ZnO composites towards physical and chemical properties were studied. Both series of CN-ZnO and rGO-ZnO composites showed improved absorption in the visible light region, as proven by diffuse reflectance ultraviolet-visible (DR UV-visible) spectra. Fluorescence and electrochemical impedance spectroscopies (EIS) confirmed that the increased loading of CN or GO on ZnO led to the suppression of electron hole recombination in the ZnO. The transmission electron microscopy (TEM) images revealed that intimate contact was formed between ZnO to CN and ZnO to rGO. The prepared CN-ZnO and rGO-ZnO samples were studied for photodegradation of phenol and photocatalytic hydrogen production from water under visible light and UV irradiation, respectively. After 5 hours reaction under visible light, the best photocatalyst for the CN-ZnO series was the CN-ZnO(1) that showed ca. 43% phenol degradation, while ZnO only achieved ca. 15% degradation. The improved photocatalytic efficiency of the CN-ZnO was due to the role of the CN to suppress electron-hole recombination and extend the absorption of ZnO to the visible light region. For the rGO-ZnO samples, after 6 hours of irradiation under UV light, the best photocatalyst was rGO(3)-ZnO with 31% phenol degradation, which was 3 times higher than ZnO with ca. 9% degradation. The optimum light intensity to produce rGO(3)-ZnO with low defects (ID/IG = 0.94) was 0.4 mW cm-2, while the irradiation time was 24 hours. The rGO(3)-ZnO sample was also the best photocatalyst for hydrogen production from water. The presence of Pt (0.25 wt%) increased the hydrogen production of the rGO(3)-ZnO from 20.2 to 99.3 μmol after 5 hours reaction under UV light in the presence of methanol as a sacrificial agent. Hydrogen production was dependent on the oxidation potential of the sacrificial agent, in the following order: methanol > hydroquinone > catechol > phenol.