Enhanced Photocatalytic Activity Of Bismuth Oxyhalides Under Visible Light Irradiation

In recent years,the photo catalysis technology has been widely concerned in the developments of energy and environmental pollution control for its strong oxidation ability and good environmental affinity.The traditional photo catalysis is mainly based orn TiO2 semiconductor photocatalyst.Due to the wide band gap of TiO2,it can only absorb the ultraviolet light,whose energy is less than 5%of the total energy of the solar energy.So people begin to look for the new semiconductor photocatalytic materials with visible light activity.For the stable chemical properties and unique electronic structure,bismuth oxyhalides have attracted much attention in nitrogen fixation,photolysis of water for hydrogen production,environmental pollution control and etc.Among BiOX(X = Cl,Br,I),BiOCl has the strongest redox capacity,but it can't make use of visible light because of the large band gap.Although BiOI has the best visible light absorption,the oxidation ability of its photogenerated holes is not good enough,which limits its photocatalytic activity to some extent.Through this study,we hope to make BiOCl,BiOI have better photocatalytic activity under visible light.Firstly,as BiOCl could not make use of visible light,we synthesized BiOCl with oxygen vacancy and improved its visible light performance in the form of iron doping.By density functional theory calculation,we found that there was a new impurity level near the oxygen vacancy level after iron-doped.Nitrogen fixation activity test under visible light showed that iron doping could promote the photocatalytic activity of BiOCl effectively.Then,the results of EPR and N2-TPD suggested that oxygen vacancies could adsorb and activate the nitrogen,DRS,p.hotocurrent and fluorescence spectrum indicated iron doping could enhance the response of BiOCl in the visible region,promove the separation efficiency of photogenerated carriers and change the transition way of electrons in BiOCl under visible light.More electrons could be excited and transferred from both oxygen vacancy and BiOCl valence level to involve in the reduction of nitrogen.Thus,Fe-doped BiOCl with oxygen vacancies had great photocatalytic activity during nitrogen fixation under visible light.Then,as BiOI had good visible-light response but poor redox ability,we used photocatalytic degradation of pentachlorophenol sodium as a typical photocatalytic reaction,and persulfate was introduced to enhance the photocatalytic activity at the same time.The rate constants of the degradation of sodium pentachlorophenate by BiOI/PS system were 8 times higher than BiOI.Under visible light,photo-induced electrons and·O2-could implement the process of persulfate activation to produce the strong oxidizing·SO4-and-OH which enhanced the oxidative capacity of reaction system greatly and made up for the poor redox ability of photogenerated holes.Then,many intermediates during the oxidative degradation of sodium pentachlorophenate were detected by GC-MS,such as 2,3,5,6-tetrachloro-1,4-benzenediol and 3,4,5,6-tetrachloro-1,2-benzenediol.Therefore,we believed that strong oxidative ·SO4-and ·OH improved the degradation efficiency of sodium pentachlorophenate with BiOI under visible light by enhancing the oxidative dechlorination process.