Mechanism And Performance Of Transition Metal Doping Modified Photocatalysts

The continuous development of textile industry,pharmaceutical industry,pesticide industry and other industries lead to a large number of organic pollutants in water.This phenomenon is bound to harm the health of human and other organisms.In addition,the problem of global energy shortage makes it necessary for mankind to search for renewable energy sources.Therefore,it is urgent to find a green and efficient water purification technology.Semiconductor photocatalysis technology,which has emerged since the 1970s,is driven by the solar energy.This technology produces a series of oxidation-reduction reactions with photo-excited carriers to destroy organic pollution molecules.Since it fits in with the idea of sustainable development,this technology plays an important role in controlling environmental pollution and alleviating energy shortage,and is considered as a water treatment method with great potential.According to literature reports,although traditional semiconductor photocatalysts,such as Ti O₂and Zn O,have the superiority of non-toxicity and good stability,their practical application value is still not satisfying due to their large band gap and low utilization rate of sunlight.Therefore,it is particularly important to find new semiconductor photocatalysts with more comprehensive advantages.The Ag-based semiconductor photocatalyst has become another hot research project because of excellent light response ability.Ag₃PO₄,as a representative Ag-based semiconductor with good absorption capacity of visible light,is considered as a potential photocatalyst due to its excellent photocatalytic performance.Apart from that,emerging Bi-based semiconductors have been studied extensively for their ideal chemical stability,low toxicity and excellent photocatalytic activity.Among them,bismuth silicate（Bi₂Si O₅）belonging to the Aurivillious family has a special two-dimensional layered structure,which can help extend the life of photogenic carriers.Moreover,the BET specific area of Bi₂Si O₅ is relatively large.It can provide more active sites for the chemical reaction of decomposing organic matters.These characteristics make Bi₂Si O₅ become a popular topic in the research direction of photocatalysis.Unfortunately,when either Ag-based or Bi-based semiconductor light is used as the photocatalyst alone,some problems inevitably arise,such as limited solar capture ability and serious photo-generated charge recombination,which greatly reduce the ability of catalyst to degrade contaminants.Among many modification methods,transition metal doping is one of the commonly used methods to restrain the recombination of carriers and enhance the light absorption capacity.In this work,pure Ag₃PO₄ nanospheres and Co-bulk-doped Ag₃PO₄ were synthesized smoothly by a simple light-avoidance deposition method at room temperature.The successful incorporation of cobalt ions was proved by XPS spectra and ICP-MS test.The results of degradation experiments showed that the modified sample 2%CAP had the best performance in all the samples prepared.The degradation rate of 2%CAP for methyl orange and phenol was 4.4 times and 3.3 times that of Ag₃PO₄,respectively.Furthermore,the mineralization ability of the 2%CAP was significantly improved,while the free radical capture experiment showed that the photocatalytic reaction depended on the participation of holes and superoxide radicals.The reasons for the improved performance of the photocatalyst were analyzed and put forward,including the enlarged light absorption range of the catalyst and the inhibited carrier separation and migration.Next,considering that the photocatalytic reaction mainly occurs on the surface of the photocatalysts,the performance improvement effect of doping only the surface part of the material was explored.Concretely,a novel Fe-surface-doped Bi₂Si O₅ composite photocatalyst was synthesized by oil bath method.The degradation rate constants of the best performance sample FB-2 for ciprofloxacin and Rhodamine B were 3.8 times and5.0 times that of the unmodified sample,respectively.Beyond that,the research results on the practical value of photocatalyst show that FB-2 process the good mineralization capacity and the outstanding stability.In addition,the influence factors on the degradation performance were also analyzed,including the initial concentration and p H of solution,the dosage of catalyst,and the coexistence of inorganic anions.The free radical capture experiments and ESR measurements made clear that the synergistic action of superoxide radical and hole in photocatalysis promoted the redox reaction.Comprehensively analyzing the reasons for the improved performance,it can be attributed to the fact that the separation and migration of charge were promoted,the utilization range of sunlight of the catalyst was expended and the ability of electrons to reduce the dissolved oxygen was enhanced.