Preparation And Photocatalytic Properties Of ZnO-based Nanoheterojunctions

With the rapid development of industry,water pollution is becoming more and more serious.The traditional methods of wastewater treatment are limited and easy to cause secondary pollution,the green photocatalytic technology with low consumption,thus,the high efficiency and non-toxic environmental protection provides more optimized solutions for environmental problems.Zinc oxide?ZnO?has attracted extensive attention of researchers due to its non-toxicity,low production cost and good biocompatibility.However,ZnO is a wide band gap semiconductor with low absorption of solar energy and insufficient chemical stability in water.Moreover,the aggregation of nanomaterials reduces their specific surface area and surface energy.These weaknesses affect the application prospects of ZnO as the photocatalyst.In view of the above drawbacks,the doping modification of zinc oxide was carried out in this thesis.The morphology of zinc oxide,carbon content in C@ZnO,surface state of C@ZnO and Zn_0.91Mn_0.09O,Au content in C@ZnO@Au and other factors affecting the catalytic performance of photocatalysts were systematically studied in terms of specific surface area,surface bulk defects and quantum efficiency.The main results are as follows:?1?Carbon sphere templates were prepared by hydrothermal method,and mesoporous zinc oxide hollow spheres and nanorods were synthesized.The morphology and specific surface area of zinc oxide nanostructures can be changed by controlling the concentration of zinc acetate.The increase of zinc acetate concentration will increase the number of nanorods in ZnO.When the concentration of zinc acetate was 0.171 mol/L,the specific surface area of the prepared ZnO mesoporous hollow spheres was larger.The degradation efficiency of rhodamine B?RhB?reached 99.43%in 50 minutes,and the degradation rate constant was0.0978 min^-1.Compared with nanorods,mesoporous ZnO hollow spheres have higher specific surface area and void concentration in the range of 2-50 nm,so their photocatalytic activity is superior.?2?The mesoporous C@ZnO hollow spheres were synthesized by a carbon sphere template combined with hydrothermal method.By changing the calcination time,the carbon content in the C@ZnO hollow sphere can be adjusted.The calcination time is extended from 1to 12 hours,and the mass fraction of carbon in the product is reduced from 5.9%to 0.1%.The results show that as the carbon content decreases,the photocatalytic degradation rate increases first and then decreases.The proper carbon content?3%by mass?has the best photocatalytic activity.The degradation efficiency is 91.72%and the degradation rate constant is 0.133 min^-1at 30 minutes of photocatalysis.The change of photocatalytic performance with carbon content is mainly due to the competition of two factors:the improvement of the separation efficiency of photogenerated carriers and the decrease of the contact area of ZnO and RhB.?3?The mesoporous C@ZnO and Zn_0.91Mn_0.09O hollow spheres prepared by the carbon sphere template method were subjected to acid etching and post-etching annealing.For C@ZnO hollow spheres,the photocatalytic degradation efficiencies of the samples after pickling and pickling annealing reached 97.16%and 96.06%,respectively,the degradation rate constants were 0.1174 min^-1 and 0.1064 min^-1,respectively.For Zn_0.91Mn_0.09O hollow spheres,the efficiency of the untreated sample was 96.95%after degradation for 60 minutes,and the degradation rate constant was 0.056 min^-1.The degradation efficiencies of the samples after pickling and pickling annealing were 50 and 40 minutes respectively.The degradation rate constants were 0.073 min^-1 and 0.099 min^-1,respectively,reaching 97.71%and 98.25%.The results show that after hydrochloric acid modification,the oxygen deficiency increases,and the acid center can also serve as a reaction site.After low temperature annealing,the lattice and crystal mass structure of the solid solution are improved,and the bulk phase defects are repaired,thereby suppressing photo-generated charge recombination,prolonging the lifetime of photogenerated carriers,and improving photocatalytic activity.?4?Mesoporous C@ZnO@Au hollow spheres were prepared by photoreduction method.The content of Au in heterojunctions could be controlled by changing the concentration of chloroauric acid solution.The results show that the photocatalytic activity of the photocatalyst is lower than that of C@ZnO when the molar ratio is 1:10000.With the increase of Au content,the degradation efficiency increases sharply at first,then decreases slowly.The specific surface area of the sample with the molar ratio of Au to C@ZnO of 1:1000 is 120.5 cm³/g.The photocatalytic activity of the sample is the best.The degradation efficiency of RhB reaches 99.87%in 25 minutes and the degradation rate constant is 0.183 min^-1.The performance of the sample is much better than that of commercial titanium dioxide P25(0.139min^-1).Due to the formation of Schottky barrier between Au and C@ZnO,it is beneficial to the separation of photogenerated carriers and the improvement of photocatalytic activity.?5?Under ultraviolet light,the degradation rates of C@ZnO@Au hollow spheres were0.0383 min^-1,0.0270 min^-1,0.0225 min^-1 and 0.0185 min^-1,respectively.The results show that the longer the corrosion time,the worse the performance,but the corrosion time has little effect on the performance.The degradation rate reached 59.15%after photoetching for 30minutes at 55 minutes.The photocatalytic activity of the nano-materials decreases significantly due to the decrease of oxygen ions adsorbed on the surface and grain boundaries,and the increase of the recombination rate of photogenerated electron-hole pairs.After annealing,the oxygen vacancies in the photoelectron capture center increased,so the degradation efficiency of RhB reached 98.209%in 30 minutes and the degradation rate constant increased to 0.1833 min^-1.?6?Mesoporous C@ZnO@Au hollow spheres with different Au contents have excellent Pb²⁺adsorption capacity and photocatalytic activity for degrading RhB after adsorption of Pb²⁺.The removal efficiency of Pb²⁺by C@ZnO@Au is higher than that of C@ZnO,and the content of Au has little effect on the ability of samples to adsorb Pb²⁺.After adsorption of Pb²⁺,C@ZnO@Au,with the Au content increases,the photocatalytic performance of the sample increases first and then decreases.The sample with a molar ratio of Au to C@ZnO of 5/10000has the fastest degradation efficiency,and its degradation rate is 0.161 min^-1.