Photocatalytic Performance Of G-C₃N₄-based Composite Photocatalyst In The Microfluidic System

Photocatalytic degradation technology has been proven to be a promising technology for the degradation of organic compounds.As one of the most widely used photocatalysts,titanium dioxide（TiO₂）has excellent chemical and thermal stability,low toxicity,low price,and high quantum yield.However,the wide band gap of TiO₂makes it only be excited to ultraviolet light,which restricts its photocatalytic efficiency.Therefore,it is necessary to develop a photocatalyst that could be excited to visible light.Graphite-like carbon nitride（g-C₃N₄）is commonly used in photocatalytic reactions that absorb visible light.It can be prepared by thermal condensation of low-cost and nitrogen-rich precursors.However,the small specific surface area,the narrow absorption range of visible light,and the rapid recombination of photogenerated electron-hole pairs limited the photocatalytic efficiency of the original g-C₃N₄.Combining with zinc oxide（ZnO）to construct heterojunction composite photocatalyst can significantly improve the above problems of g-C₃N₄.Photocatalytic reactors for water treatment are generally divided into reactors with suspended photocatalyst particles and reactors with the photocatalyst fixed on a continuous inert carrier.These reactors have some problems such as low energy efficiency,slow reaction kinetics,and in some cases of photocatalyst pollution.For this reason,this work combines optics and microfluidics to study the photocatalytic water treatment in optofluidic microreactors.The specific content is as follows:（1）A simple pyrolysis method was used to synthesize g-C₃N₄ photocatalysts（labeled as m-CN and t-CN,respectively）from melamine and thiourea.Then a new type of optofluidic microreactor based on the g-C₃N₄ film was designed.The photocatalytic degradation experiment of MB solution under the visible light range proved its feasibility.The effects of flow rate and light intensity were studied,and the results showed that increasing the residence time and light intensity could improve the degradation efficiency.Also,the reactor’s performance loaded with t-CN was better than that of the reactor loaded with m-CN,which indicated that the t-CN sample possessed better photocatalytic performance,coinciding with the specific surface area,UV-vis diffuse reflectance spectra（UV-vis DRS）and photoluminescence（PL）spectra analysis.（2）Thiourea and zinc acetate were used as raw materials to synthesize ZnO/g-C₃N₄ composite materials with different ZnO content by the impregnation method and then fixed in the optofluidic microreactor.The photocatalytic performance of the samples was evaluated by conducting photocatalytic degradation of methylene blue（MB）,neutral red（NR）,malachite green（MG）,rhodamine B（Rh B）under visible light.The structural morphology and optical properties of the samples were analyzed through a series of characterization techniques.The degradation experiments show that 6 wt.%ZnO/g-C₃N₄ composite had the best photocatalytic performance,and the degradation efficiencies of MB,NR,MG,and Rh B are 85.8%,96.4%,97.5%and 98.9%,respectively.Correspondingly,this composite had a weaker PL emission intensity,reflecting the decrease in the recombination rate of photogenerated electron-hole pairs.