Study On Controllable Synthesis Of Carbon-based And MoO_3-x Photocatalysts And Their Photocatalytic Performance

Environmental pollution and energy crisis have gradually become the serious problems for human society survival and develop since 21^th century.Photocatalysis,which is a green,environment protection and energy saving chemical method,has been well recognized as one of effective strategies to solve above problems.However,the traditional semiconductor photocatalysts still have some drawbacks such as low light utilization and photocatalytic reaction efficiencies,etc.,which directly affect the practical application of photocatalysis technique in our daily life.In respect of above issues,this study mainly focused on developing new series of photocatalyst,modifying the classic photocatalyst,tuning apparent shape and surface unsaturated atom coordination of materials towards MoO_3-x and carbon contained photocatalysts,expecting to improve the photocatalytic performances of the as-prepared semiconductors for their application in the fields of environmental purification and energy.The main contents of this thesis are as follows:（1）The nonmetal organic polymer（SCN）_n photocatalysts with three polymerization degrees were successfully prepared by a facile solid state grinding method by controlling the contents of oxidizing agent.The results of XRD,SEM,XPS,TEM,GPC and UV-vis DRS characterizations demonstrated that these three（SCN）_n have similar composition and shape,moreover,the bandgaps decreased with increasing polymerization degree.Mott schottky analysis and DFT calculation revealed that with increasing polymerization degree,HOMO and LUMO positions of（SCN）_n would move up and down,respectively,and the corresponding oxidation and reduction potentials decrease.This thesis used RhB,TC and Cr（VI）solution as pollutant targets to study the inner relationship and mechanism between polymerization degree and photocatalytic properties towards（SCN）_n.The results showed that sample SCN-1 with the lowest polymerization degree presented the best visible light induced photocatalytic oxidation and reduction properties as compared to other samples.This superior should be owing to two reasons.Firstly,sample SCN-1obtains the most negative conduction band position and then produce much more·O₂^–which were the main active species for the photocatalytic activities of（SCN）_n;Secondly,as compared to other two samples,sample SCN-1 have better migration and separation efficiency for photogenerated charge carriers.（2）Ultrathin N deficiency g-C₃N₄ was successfully prepared by heat condensation reaction by using the above as-synthesized（SCN）_n as precursors.The effect of heat condensation reaction temperature on the composition,shape feature,light absorption and band structure of samples were investigated by various characterizations.The results of XRD,FTIR,TG,XPS,XPS,NMR unveiled that during heat condensation process over（SCN）_n,the S was gradually released by the form of bubbles（e.g.H₂S,C₂S,etc.）,and when the heat temperature was increased to550^oC,there was no S existed in sample CN-550 but with N deficiency to some extent.It is obvious from TEM and AFM results that sample CN-550 has two dimensional ultrathin structure,which contains 3-4 layers of monolayer g-C₃N₄ and the specific surface area is about 67.25 m²·g^-1.This phenomenon disclosed that the ultrathin N deficiency g-C3N4 could be fabricated by a facile heat condensation method.The photocatalytic water splitting for hydrogen production and corresponding mechanism of samples prepared by different heat temperatures was studied under visible light irradiation.The results demonstrated that sample CN-550presented the optimal photocatalytic hydrogen production efficiency and the apparent quantum efficiency was about 4.7%.The enhancement of photocatalytic hydrogen production over sample CN-550 was mainly due to the peculiar two dimensional ultrathin N deficiency structure,which made the bandgap decrease and the visible light absorption improve and the migration and separation of photogenerated electron and hole fasten.Besides,deactivation reasons of samples CN-280 and CN-450 for photocatalytic hydrogen production were also discussed,which was probably contributed to that the existed S-S bonds in C₃N₄ significantly affected the adsorption of water and desorption of H₂.（3）Three different morphologies of AgSCN have been successfully prepared by ultrasonic assisted chemical precipitation method by using various silver sources.The physical composition,shape and light absorption,etc.of various AgSCN samples were studied by the means of XRD,SEM,XPS and UV-vis DRS,etc.Moreover,the DFT calculation was employed to study the electronic band structure of samples.Besides,the metronidazole and Cr（VI）solution were used as pollutant targets to investigate the photocatalytic oxidation and reduction properties of AgSCN synthesized by various silver sources.The photocatalytic results displayed that sample S1 prepared by using silver nitrate as source presented the best photocatalytic performances among three samples,which should be mainly owing to the efficiency charge separation in sample S1 as compared to those of other two samples.The trapping experiments showed that during metronidazole degradation process,·O₂^–and h⁺are the main active species,while active species·OH made little contribution,which was produced by two electrons oxidation towards photogenerated electron.（4）Reductive MoO_3-x with three various morphologies were successfully prepared by simple solvothermal method through tuning reaction conditions.The characterization results demonstrated that samples MoO_3-x-F、MoO_3-x-S、MoO_3-x-R had 6.96%、8.84%and 3.95%of Mo（V）related to total Mo ions,respectively.Owing to the existence of these reduced Mo（V）,the LSPR effect was produced.This effect made the light absorption in the range of visible and near infrared lights enhanced and also presented absorption tail with various intensities:MoO_3-x-S>MoO_3-x-F>MoO_3-x-R.The BET results exhibited that sample MoO_3-x-R obtained the largest specific surface area followed by MoO3-x-F and MoO3-x-S.Under the irradiation of visible light,the photocatalytic hydrolysis of NH₃BH₃ to produce hydrogen and corresponding reaction mechanism were studies for above three samples.The photocatalytic results showed that sample MoO_3-x-F presented the best NH₃BH₃hydrolysis properties among three samples,which should be mainly due to the synergetic effect of large specific surface area and LSPR effect induce by reduced Mo（V）.The free radical test revealed that the main active species in this photocatalytic reaction was h⁺.When the visible light irradiated the sample contained NH₃BH₃ solution,the B-N bonds in NH₃BH₃ would break quickly,which significantly improve the photocatalytic performance.Based on the above discussion,the work of this thesis would provide theoretical support and research reference for the development of new organic polymer photocatalyst,modification of classic semiconductors and the corresponding mechanism study.