| Nowadays,there remains to be a great challenge to governance environment,alleviate energy crisis effectively for realizing sustainable development.Photocatalytic materials and technologies which can make use of the inexhaustible solar energy in nature to degrade pollutant and store energy under mild conditions provide a new way to solve the energy and environmental problems.However,the traditional UV-responsive photocatalytic materials represented by TiO2 are limited by the low utilization of solar energy(<5%).Therefore the visible-light responsive photocatalytic materials have gradually become a research hotspot.In this paper,a typical visible light responsive semiconductor such as graphite carbon nitride(g-C3N4)was studied.In order to solve the problems of low specific surface area and easy recombination of photo-generated carriers in bulk g-C3N4 prepared by the tradiltional solid phase method,uniform g-C3N4 nanoparticles were prepared by using template-free concentrated-acid stripping method.Based on such g-C3N4 nanoparticles,novel metal-free composite photocatalytic materials were constructed,therefore achieving visible-light response and effective separation of photo-generated electron-hole pairs.Moreover,the nanonization of g-C3N4 also increased the redox ability of material.Hence,the photocatalytic activity of composite materials was increased due to the synergistic effect of above factors.The main research contents and conclusions are as follows:(1)Based on the protonation process of g-C3N4,g-C3N4 nanoparticles were prepared by the reflux of concentrated acid,and the influences of preparation conditions on the structure and properties of g-C3N4 nanoparticles were also investigated.The results show that concentrated acid is the key to the preparation of g-C3N4 nanoparticles,which can make bulk g-C3N4 thinning and further etch it into nanoparticles.The temperature affects the acid etching rate to g-C3N4,while the stirring condition affects the uniformity of acid etching.The nanonization of bulk g-C3N4 can effectively shorten the migration path of photo-generated carrier from the body to the surface,thus inhibiting the recombination of photo-generated electrons and holes.In addition,the nanonization of g-C3N4 leads to the blue shift of the absorption edge,resulting in the increase of the optical band gap of 9-C3N4 from 2.75eV to 3.40eV.This means that g-C3N4 nanoparticles have lost visible-light photocatalytic activity,only ultraviolet light can inducethe photocatalytic performance of g-C3N4.However,the increase of optical band gap also enhances the redox ability of photo-generated electrons and holes of g-C3N4to a certain extent.(2)Graphene oxide(GO)was served as the carrier,g-C3N4 nanoparticles(CN-NP)were loaded on the GO surface through electrostatic assembly,thus obtaining a composite photocatalytic material(CNNP/GO).Then the relationship between the structure and the properties of the composites was studied.The results show that g-C3N4 nanoparticles can.be loaded on GO surface uniformly and firmly by forming C-O-C covalent bond with GO,which is favorable for the transfer of photo-generated carriers between different materials,and further improves the separation efficiency of photo-generated electrons and holes.More importantly,the introduction of GO decreases the bandgap of g-C3N4 nanoparticles,therefore enhancing the absorption and utilization of visible light,So CNNP/GO shows much higher visible-light photocatalytic activity than CNNP.With the increase of the content of GO,the photocatalytic performance of CNNP/GO increases first and then decreases.The highest value of the first order reaction rate constant k for decomposition of methylene blue can reach 0.343 h"1,which is 8.6 times higher than that of the CNNP.The free radical trapping experiment indicates that ·O2-and ·OH-generated from photo-generated electrons are main reactive species in the photocatalysis reaction.(3)A micro nano-composite photocatalytic material was prepared by loading g-C3N4 nanoparticles on bulk g-C3N4 through electrostatic self-assembly.The structure-property relationship of the composite photocatalytic materials was studied.With the increase of the content of g-C3N4 nanoparticles,the g-C3N4 nanoparticles on the surface of the composite gradually increase until completely covered.Under visible-light irradiation,g-C3N4 nanoparticles act as electron acceptors for bulk g-C3N4,therefore reducing the recombination probability of photo-generated carriers,and thus the quantum efficiency of composites is higher than that of single bulk g-C3N4.Moreover,the composites have enhanced optical current and exhibit excellent visible-light photocatalytic performance.The highest value of the first order reaction rate constant k for decomposition of methyl orange can reach 1.02 h-1,which is 3 times higher than that of the CNNP.Based on the results of the free radical trapping experiment,it is believed that photo-generated electrons play a major role in the photocatalysis reaction,and can generate O2-and ·OH-,which are main reactive species in the reaction process. |
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