Highly efficient zinc oxide-carbon nitride composite photocatalysts for degradation of phenol under UV and visible light irradiation

In order to utilize solar light in an efficient way, a good photocatalyst shall absorb both UV and visible light. In this study, a series of composite photocatalyst consisting of zinc oxide (ZnO) and carbon nitride (CN) was successfully prepared through a physical mixing method. The ZnO is an ultraviolet (UV)-based photocatalyst, while the CN is known as a visible light-driven photocatalyst. The effect of zinc to carbon mol ratio (Zn/C) towards the properties and photocatalytic activities was investigated. X-ray diffraction (XRD) patterns revealed that the prepared ZnO-CN composite photocatalysts composed of wurtzite ZnO and graphitic CN. The presence of ZnO and CN made the composites have absorption at both UV and visible region, suggesting the potential application as photocatalysts under both UV and visible light. Fluorescence studies revealed that all ZnO-CN composites showed emission peaks at 445 and 460 nm when excited at 273 nm, but with lower intensity as compared to those of the CN. The lower emission intensity suggested the role of ZnO to reduce the charge recombination and improve the charge separation on the CN. The ZnO-CN composites were further evaluated for photocatalytic degradation of phenol. The amount of degraded phenol was determined by a gas chromatography, in which a flame ionization detector was used in this study (GC-FID). The composite photocatalyst with an optimum content of 1% Zn/C gave almost 1.15 times higher activity than the CN under visible light irradiation. On the other hand, the composite photocatalyst with an optimum content of 10% Zn/C showed 2.6 times higher activity than the CN under UV light. The improved photocatalytic efficiency on the ZnO-CN composite photocatalysts was caused by the synergic effect between ZnO and CN. The ZnO would boost the separation efficiency of photogenerated electrons on the CN, while the CN would enable ZnO to absorb visible light region as the ZnO-CN composites.