Preparation And Photocatalytic Activity Of Graphite Carbon Nitride Based Composite Semiconductor Photocatalyst

With the rapid development of human society,energy shortage and environmental pollution problem has become a global problem urgently to be solved, In view of this situation, the semiconductor photocatalytic technology has attracted more and more attention to the researchers:Semiconductor photocatalysis is designed to play through the physical and chemical characteristics of the semiconductor material to convert solar energy into chemical energy, to achieve further use of chemical degradation of organic pollutants, heavy metals reduction, CO2 reduction, hydrolysis, organic synthesis and sterilization and disinfection process, Semiconductor photocatalytic technology absorbs the fruits of advanced materials research, integrating the research hotspot in the field of energy and environment, has the advantages of energy saving and environmental friendly, is a frontier research field and has broad application prospect.Graphitecarbon nitride（g-C₃N₄） is a new narrow band gap polymer semiconductor material, with cheap price of raw materials and simple preparation process, good chemical stability, excellent performance characteristics of optical absorption, became the hot spot in the field of photocatalysis lately.However, due to the narrow range of the visible light response, the light-generated electron/hole recombination rate and other reasons, g-C₃N₄ photocatalytic activity induced by visible light still needs to be improved. Thus, g-C₃N₄ as a base material to be appropriately modified form in line with a semiconductor photocatalyst is to improve the g-C₃N₄ effective way to respond to visible light photocatalytic activity.Based on the above background, the present paper using microwave-assisted synthesis prepared Schottky junction composite structure Ag/g-C₃N₄ and narrow bandgap coupling heterostructure Ag/Ag₂S/g-C₃N₄, prepared by In-Situ deposition of narrow bandgap coupled heterostructure MoS₂/g-C₃N₄ and wide band gap coupling heterostructures ZnS/g-C₃N₄, the physical and chemical properties of the composite structure were systematically characterized using rhodamine B（RhB） as a probemolecule evaluation of composite structures responsive to visible light catalytic activity.Details are as follows:1.With thiourea as the precursor, thermal polycondensation method of preparing the original g-C₃N₄, Ag/g-C₃N₄ and Ag/Ag₂S/g-C₃N₄ compound semiconductor photocatalysts were synthesized by a rapid microwave-assistedpolyol process in ethylene glycol. Its crystal structure, chemical composition and morphology were characterized by Transmission electron microscopy（TEM） and mapping analysis, X-ray diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）. The optical properties of the samples were characterized by UV-vis DRS and PL. Characterization results showed that the elemental Ag, Ag2 S had compounded to the g-C₃N₄ successfully. Compared with the original g-C₃N₄, the visible light response range is enhanced by the optical properties, In terms of photocatalysis, as in the Ag/g-C₃N₄ samples, when the conditions of microwave synthesis about 800 W, 120 ℃, 20 min and the Ag and g-C₃N₄ quality ratio is 2 wt%, the sample has the highest photocatalytic activity, the degradation rate reached to 98.1%. The degradation rate was still at 96.9% after four cycle tests. It is indicated that the Ag/g-C₃N₄ composite photocatalyst has a certain stability. For the synthesis of Ag/Ag₂S/g-C₃N₄ composite semiconductor photocatalyst, the characterization results showed its excellent crystal structure and high visible light absorption. Under the same conditions, Ag2 S accounts for g-C₃N₄ of the nominal weight ratio at 10 wt% had the best degradation efficiency, the degradation rate reached to 98%after 120 min reaction, four cycles later, the degradation rate was about 80%.2. The heterojunction type compound semiconductor photocatalyst MoS₂/g-C₃N₄、ZnS/g-C₃N₄ was synthesised by In-Situ synthesis methods under the tube furnace with the protectioning of N₂ by using thiourea as g-C₃N₄ precursor and sulfur source,respectively by using（NH4）2MoO4 as a source of Mo or using Zn（NO3）2 as a source of Zn. The same characterization method was adopted for the catalyst, characterization results showed that the formation of a heterojunction, an internal electric field is formed between the semiconductor interface when electron transferred which can restrain the recombination of the electron/hole pair through the heterojunction interaction. In the field of photocatalysis, when the amount of（NH4）2Mo O4 is 3 mg, MoS₂/g-C₃N₄ has the highest photocatalytic activity, the degradation rate reached 97.8% after 60 min reaction.Four cycles later, the degradation rate was about 93.8%. When the inputs of Zn（NO3）2is 1 g, ZnS/g-C₃N₄ has the highest photocatalytic activity, the RhB could be completely degraded after120 min, The degradation rate was still at 87.2% after four cycle tests.3. Photocatalytic reaction activity of species about those four kinds composite photocatalyst systems were captured for the experiment. The results show that the photoinduced hole and O2?- is the main active species of the process about visible light induced degradation of RhB. As about Ag/g-C₃N₄ catalysts, O2?- is the main active species, and photogenerated holes as the main active species of Ag/Ag₂S/g-C₃N₄,MoS₂/g-C₃N₄ and ZnS/g-C₃N₄ series catalysts. On this basis, we discussed the mechanism of the composite photocatalyst.In this paper, the two methods of synthesis have the characteristics of simple process and controllable reaction, which can be used as reference for the preparation of g-C₃N₄ based composite photocatalyst. Comprehensive comparison, in the preparation of the four series of photocatalyst, MoS₂/g-C₃N₄ showed the best photocatalytic activity and stability, has better application prospect. In the further work can continue to consider the optimization load of semiconductor and g-C₃N₄ energy band matching in order to improve the photocatalytic activity, and expand the application in the field of other photocatalytic.