| Energy shortage and environmental pollution are the most serious problems for human beings, searching for environmental friendly and sustainable energy has become a hot topic in this field. Photocatalytic technique can utilize the clean and renewable solar energy, and thus exhibits its potential broad application in the fields of energy conservation and environment protection. Especially, photocatalytic technique can degrade organic pollutants under milder reactions conditions, and has the advantages of low cost and no secondary pollution. Therefore, it has become a promising technology for the treatment of environmental pollution.Graphite-like carbon nitride (g-C3N4) has been widely studied as a novel visible-light-active photocatalyst, owing to its suitable band-edge position, well-matched band-structure, high chemical stability and low prices. However, the photocatalytic activity of the bulk g-C3N4 prepared by thermal condensation of nitrogen-rich precursors is usually restricted by the small surface area and low quantum efficiency from sintering process. To solve these problems, we combined ball milling technique and protonation method under hydrothermal conditions to fabricate nanoporous g-C3N4 with high specific surface area. On this basis, visible-light active g-C3N4/BiOBr composite photocatalyst was systhesised by loading BiOBr grains on the nanoporous g-C3N4 matrix, leading to the formation of a heterojunction structure. The article focuses on the preparation, structure and properties of the photocatalyst, and the main contents and conclusions are as follows:(1) g-C3N4 was pretreated by using high energy ball milling technique, and the effects of ball milling processing parameters on the particle size distribution, surface morphology and photocatalytic activities of as-prepared g-C3N4 samples were analysed. The results show that the optimum processing parameters are as following: the ball-to-powder weight ratio is 30:1; the solid-to-liquid weight ratio is 1:1; the ball milling time is 4 h and the rotating speed is 200 rpm. Under these conditions, g-C3N4 achieves the narrowest particle size distribution. The specific surface area of g-C3N4 is improved after ball milling, and the adsorption rate of the ball-milled g-C3N4 for CR and RhB dyes is 2.24 and 1.03 times as high as that of the original one respectively, while the decompositon rate for CR and RhB dyes over the ball-milled g-C3N4 is 2.9 and 4.5 times as high as that over the original one respectively.(2) The ball-milled g-C3N4 was protonated under hydrothermal conditions with H2SO4 as a solvent to fabricate nanoporous g-C3N4, and the effect of protonation conditions on the morphology and structure of nanoporous g-C3N4 was investigated. Moreover, the spectroscopy characteristics, photoelectric properties and photocatalytic activities of the as-prepared samples were also studied. The results show that g-C3N4 exhibits the best nanoporous structure when the concentration of dilute sulphuric acid is 0.4 mol/L. The specific surface area of the obtained g-C3N4 is 30.9 m2/g, which is 8.9 as high as that of the original one. Owing to the porous structure, the decreased particle size and surface defects of nanoporous g-C3N4, the separation of photo-generated electrons and holes and the lifetime of charge carriers are improved, leading to the increase of quantum efficiency. Therefore, the adsorption ability and photocatalytic activity of g-C3N4 over CR are significantly improved. Furthermore, the nanoporous g-C3N4 also displays a selective feature in photocatalysis due to its positive surface charges determined by the protonation treatment.(3) g-C3N4/BiOBr composite photocatalyst was prepared by loading BiOBr grains on the nanoporous g-C3N4 matrix via hydrothermal method, and the morphology, structure, optical and photocatalytic properties, reusability and photocatalytic mechanism of the as-prepared samples were investigated. The results show that BiOBr grains grow uniformly on the surface of porous g-C3N4 matrix. The heterojunction structure is formed between porous g-C3N4 and BiOBr due to the matched band-structure, which can efficiently inhibit the recombination of the photo-generated electrons and holes, leading to the improvement of the quantum efficiency and photocatalytic activity. The porosity of g-C3N4 can enhance its adsorption ability and contact area for dyes and thus accelerates photodegradation rate. Moreover, the result also reveals that ·O2-1 and h+ are the main reactive species in the photocatalysis process. The photodegradation rate of MO(CR) over g-C3N4/BiOBr composite photocatalyst is 2.4(1.9) and 6.6(7.2) times as high as that over pure porous g-C3N4 and BiOBr samples, respectively. In addition, the photocatalytic activity of g-C3N4/BiOBr is stable and keeps at a high level after 4 cycles photodegradation of CR under visible light irradiation. |
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