Construction Of High Efficient Visible-light Active Photocatalysts And Their Photocatalytic Enhancement Mechanism

In the past few decades,semiconductor photocatalysis has attracted extraordinary attention as one of the most advanced,green chemical,and promising technologies to solve the problem of environmental deterioration and energy crisis.At present,the research hotspot of photocatalytic technology is still focused on exploring and developing visible light photocatalysts with wide spectrum response,high-efficient charge carrier separation and good surface chemical reaction efficiency.Among all photocatalysis materials,Ag3PO4 and Bi2WO6 have gained extensive research interest in light energy transformation and pollutant degradation due to their good photocatalytic performance,unique chemical property and suitable energy band structure,which display great research value and application potential.In this paper,Ag3PO4 and Bi2WO6 are served as primary catalysts,morphology control and heterojunction construction and some other strategies are used to improve the photocatalytic efficiency and stability of Ag3PO4 and Bi2WO6 based visible-light photocatalysts.The core content of the thesis is to explore the reason of superior performance of the synthesized photocatalytic system in light absorption,separation and transformation of photogenerated charge carriers,and surface chemical reaction process,and finally reveal the synergistic enhancement mechanism of multiple modification strategies.Besides,photocatalytic degradation of water pollution(including Rh B,phenol,Cr(VI)heavy metal ion and tetracycline hydrochloride simultaneously),photocatalytic selective oxidation for organic synthesis and photocatalytic removal of NOx pollutants under visible light irradiation have been studied in order to expand the photocatalytic application ranges.The main contents of this dissertation are summarized as follow:(1)Porous Ag3PO4 nanotubes(PNTs)photocatalyst was synthesized via a surface anion exchange reaction from the pre-grown Ag2CO3 nanorods(NRs)template.The diameter and length of Ag3PO4 PNTs is about 350 nm and 2.1 μm,respectively.Nanoholes with diameters in range of 40~200 nm are existed on the side wall of the tubular structure.Through the preparing condition comparison and structural characterization,the rationally release rate of H+ and PO43-is proved to be the key factor for the formation of Ag3PO4 PNTs.Compared with irregular Ag3PO4(Ag3PO4 IRs),Ag3PO4 PNTs have a bigger specific surface area,good porous structure,better light absorption and lower chargerecombination rate,which are responsible for the superior photocatalytic performance of Ag3PO4 PNTs in the degradation of Rh B and phenol.The photocatalytic activity of Ag3PO4 PNTs is about 3.02 times that of Ag3PO4 IRs.Furthermore,Ag3PO4 PNTs maintain an efficient and stable photocatalytic activity even after five cycles.(2)Mesoporous nanoplate multi-directional assembled Bi2WO6 architecture(MN-Bi2WO6)was successfully prepared under solvothermal synthesis and calcination process and applied for the photocatalytic removal of NOx pollutants at low concentrations.MN-Bi2WO6 exhibits a excellent conversion efficiency in the photocatalytic oxidation of gaseous NO,which can reach 90% conversion after 6 min under visible light irradiation and almost completely degrade NO in only 2 min under UV-vis irradiation.Meanwhile,this hierarchical mesoporous Bi2WO6 also has excellent photocatalytic durability and reusability.Furthermore,after the comparative study of morphology,specific surface area,pore environment,light absorption,and photogenerated electron-hole pairs separation of different samples,the excellent photocatalytic performance of MN-Bi2WO6 could be attributed to its special hierarchical mesoporous structure with an appropriate pore size and interconnected porous network,which impart good gas permeability and fast mass transfer during the NO oxidation.(3)Ag3PO4@Mo S2 nano-2D heterojunctions were fabricated by a stirring-ultrasonic exfoliation method and an organic phase in situ growth strategy.The exfoliated Mo S2 has a quantum dots/few-layered nanosheets structure and a wide direct bandgap of 1.93 e V.The obtained Ag3PO4@Mo S2 nanocomposites displays significantly enhanced activity for photodegradation of Rh B and photocatalytic selective oxidation of benzyl alcohols to benzaldehyde compared with pure Ag3PO4,and the activity of Ag3PO4@Mo S2-6 in two applications is about 2.85 and 2.79 times that of Ag3PO4.The reactive-species-trapping experiments and the energy band structure analysis demonstrate that the electrons of Mo S2 with higher reducibility and the holes of Ag3PO4 with higher oxidability are the real participants in photocatalytic reactions.The improved performance of the Ag3PO4@Mo S2 can be attributed to the enhanced light adsorption ability,more active sites,efficient interfacial charge transfer and separation through the Z-scheme migration mechanism and intimate contact core@shell interface.In addition,the introduction of Mo S2 can protect the nano-Ag3PO4 from photocorrosion,maintaining the activity and stability of the composite.(4)On the basis of above Bi2WO6 morphology control about mesoporous hierarchical structure,an all-solid-state narrow bandgap Co3O4/Ag/Bi2WO6 Z-Scheme heterojunctionwith good contact and interaction between components was constructed through a multi-step synthesis.And it was first time applied to the removal of heavy metal ion Cr(VI)and tetracycline hydrochloride(TCH)simultaneously.Compared to single Bi2WO6,Ag/Bi2WO6 and Co3O4/Bi2WO6 heterojunctions,Co3O4/Ag/Bi2WO6 exhibited best photocatalytic activity,the photocatalytic efficency of Co3O4/Ag/Bi2WO6 in degradation of Cr6+and TCH under visible light irradiation is about 7.89 and 5.65 times higher than that of pure Bi2WO6,respectively.Meanwhile,the photocatalytic efficency of removal of Cr6+-TCH coexistence system was found to be 6.86(Cr6+)and 2.58(TCH)times higher than that in its single system due to better utilization of electrons and holes.Besides,Co3O4/Ag/Bi2WO6 heterojunction is proved to show a Z-Scheme charge separation and transformation mechanism,and the enhanced photocatalytic performance of this Z-scheme photocatalytic system is concluded to its wide spectral response,high efficient photogenerated charge separation and utilization,and strong redox capacity.(5)Ultrathin Bi2WO6 nanosheet and solid few-layer Mo S2 nanosheet were fabricated by solvothermal method and heat treating-liquid exfoliation method,respectively.Surface plasmons coupled two-dimensional chemical Au/Bi2WO6-Mo S2 heterojunction was first obtained through microwave-assisted reduction method and selective chemical-assembly process,and the photocatalytic performance of the composites were evaluated by degradation of Cr6+ and TCH simultaneously.Au nanoparticles are attached to the side of Bi2WO6 nanosheet,Bi2WO6 and Mo S2 are combined together with S-O bonds existing at the 2D/2D interfaces.Moreover,this particular system is designed to provide a directional flowage for charge separation,transportation and consumption process.The hot electrons of Au nanoparticles from localized surface plasmon resonance(LSPR)can be injected to the conduction band of Bi2WO6,the photogenerated electrons of Bi2WO6 and hot electrons can further transferred to Mo S2 nanosheet via the interfacial S-O bonds,and are finally consumed by Cr6+reduction,then the holes left in Bi2WO6 can catalyze TCH oxidation.The two-dimension layer junction was also proved to be the structure foundation for the enhanced photocatalytic activity and corresponding enhancement mechanism.