Construction And Photocatalytic Performance Of Bismuth Oxybromide Hybrids

Nowadays,water resource pollution is becoming more serious,which is closely linked to people's daily work and life.As known,there are various pollution sources to water body.Among them,dye wastewater is difficult to deal with,because its chroma is high and the biodegradability is low.So the great concern of environmental pollution improvement is to develop an efficient,stable,and energy conservation method to treat dye wastewater.Photocatalysis technology is a newly developed method for wastewater treatment,originating from the advanced oxidation technology.It has a good treatment effect for some special contaminants,with features of complete removal,no secondary pollution,moderate reaction conditions,and low cost.The main principle of photocatalysis technology is described as: after absorption of light with appropriate energy,electrons in semiconductors are motivated,transferred from the valence band to the conduction band.Correspondingly,holes and electrons are obtained on the conduction band and valence band,respectively.The resultant electrons and holes own strong redox ability.So far,the most researched semiconductor is titania,owning to reported good merits.However,its applications in photocatalysis are greatly restrained,as the result of large energy gap,low light absorption and utilization ability,and difficult recovery.Therefore,it is quite desired to modify titania or develop other materials to fulfill the requirement of photocatalytic system with high efficiency,stability and strong visible light response.The bismuth-based semiconductors gained much attention,due to the appropriate energy gap and special stratified structure.The most representative one is bismuth oxybromide?BiOBr?in a tetragonal phase.BiOBr owns unique electron structure and excellent photocatalytic ability,having potential applications towards catalytic degradation of toxic organic contaminants.However,its photocatalytic performance is greatly restrained by the high recombination rate of electron-holes,which can be solved by coupling BiOBr with other ingredients with proper band structures.In this study,a series of BiOBr-graphitic carbon nitride?g-C₃N₄?hybrids were constructed,characterized,and then subjected to photocatalytic evaluation.A possible catalytic mechanisom was proposed basing upon experimental results.In the first section,mesoporous g-C₃N₄ was fabricated through the direct pyrolysis of nitric acide solution treated melamine.The presence of narrow slit-shaped pores on surface was demonstrated by SEM and TEM images.The partial destruction of structurewas confirmed by X-ray diffraction patterns and FT-IR spectra,which may possibly result into the increase of specific surface area.These samples were able to efficiently remove Rhodamine B?RhB?in solutions,with an apparent kinetic rate constant 6.2 times that of g-C₃N₄.Active species trapping measurements and ESR measurements indicated that ·O₂^- was main active species.The second section was relevant to the synthesis and photocatalytic performance of another g-C₃N₄ materials.The direct pyrolysis of melamine produced bulk g-C₃N₄,which was pyrolyzed again to generate exfoliated C₃N₄ nanosheets,denoted as CNs.A similar structure of CNs to that of bulk g-C₃N₄ was confirmed by X-ray diffraction and FT-IR spectra.The sepecifc surface area is obviously enhanced.In addition,atomic force microscope revealed 4-atom layered structure in CNs.CNs showed much higher photocatalytic efficiency upon Rh B degradadation than bulk g-C₃N₄.According to the experiment,CNs was choosen as an ingredient to fabricate hybrids in the following step.Various BiOBr-CNs hybrids were prepared through a chemical deposition precipitation method.TEM elemental mapping images showed that five elements were evenly dispersed in the testing region,revealing the coexistence of two ingredients in a uniform manner.The specific surface area of BiOBr hybrids was markedly increased.With the increase of CNs,the specific surface area was improved.X-ray diffraction,FT-IR spectra,SEM and TEM images indicated that BiOBr and CNs were not destroyed and formed heterojunction structure between both interfaces.During the process of degrading RhB and 2,4-dichlorophenol?2,4-DCP?,samples 0.5CNs-BiOBr and CNs-BiOBr exhibited the highest photocatalytic activity.In the process of degradation of Rh B,the photocatalytic reaction rate of 0.5CNs-BiOBr is 2.7 and 6.8 times of pure BiOBr and CNs.In the process of degradation of 2,4-DCP,the photocatalytic reaction rate of CNs-BiOBr is 7.5 and 2.5 times of pure BiOBr and CNs.Active specises trapping experiment demonstrated that h+ and ·O₂^- played leading roles in the process of photocatalytic degradation of RhB.·OH and ·O₂^- played leading roles in the process of photocatalytic degradation of 2,4-DCP.