Study Of Surface Plasmon Enhanced AgX(X=Cl,Br,I) & Its Composite Materials' Preparation,Characterization And Photocatalytic Property

Now the world faces the challenges of energy shortage, environment pollution and global warming. It's necessary to solve all these problems for our country to realize the sustainable development, raise people's living standard and guarantee national security. Photocatalyst can be used to split water, photodegrade organic pollution, photo-reduce CO2 and so on. Photocatalyst can convert the solar energy (low density) to hydrogen energy (high density), which is suitable for storage and transportation. Using hydrogen doesn't produce any pollution. Photocatalyst can use the sun light to degrade the organic pollution without producing the second pollution. Photocatalyst can also photo-reduce CO2 to organic alkane, minishing the content of CO2 in atmosphere, and the alkane can be used as fuel. Therefore, photocatalyst has prospect of application in solving the problem of energy shortage, environment pollution and global warming. However, the traditional photocatalyst TiO2 requires ultraviolet (UV) light (λ<400 nm), which provides sufficient energy for the electron excitation across its band gap (i.e.,3.2eV for anatase TiO2). Only about 4% of the solar spectrum can be utilized by pure TiO2. Thus, it is highly desirable to develop photocatalysts that can yield high reactivity under visible light. The visible light photocatalyst has become the research hotspot. In the latest 30 years, to extend the absorption band-edge of TiO2 from UV to visible light region, a number of different approaches have been developed, including doping and combining TiO2 with other semiconductors, heterojunction photocatalysts, dye-sensitized photocatalysts and other newly composite semiconductor photocatalysts.Different from other's research, we have put forward plasmonic photocatalyst based on the AgX materials. The plasmonic photocatalyst combines the property of noble metal nanoparticles' surface plasmon resonance (SPR), metal-semiconductor contact and semiconductor photocatalyst. The SPR can be modulated by tuning the size, shape and surrounding of the noble metal nanoparticles. The SPR can enhance photocatalyst's visible light absorption, leading to high photocatalytic efficiency. Due to the low Fermi level, noble metal can reduce the contact energy between the semiconductor and H2O, CO2 and other organic molecule. The noble metal also works as electron trap, making the separation between electrons and holes more efficient. Therefore more photo-generated electrons and holes take part in the photocatalytic reaction, leading to high photocatalytic efficency. The plasmonic photocatalyst extends the absorption band-edge of photocatalyst, makes the photo-generated electrons and holes separate more efficiently, explores a new way to design and fabricate highly efficient photocatalysts.In this thesis, we mainly introduced the new plasmonic photocatalysts, composite photocatalyst and composite photocatalyst based on the SPR. The plasmonic photocatalysts mainly include Ag＠AgCl, Ag＠AgBr, Ag＠Ag(Br,I), Ag＠Ag(Cl,Br) and Ag＠Ag(Cl,I). The AgX is fabricated by the ion exchange process between Ag2MoO4 and HX. The Ag＠AgX is fabricated by the light-induced chemical reduction process. The plasmonic photocatalysts have strong absorption in visible light region, and show high photocatalytic efficiency. The composite photocatalyst is H2WO4·H2O/AgCl. By theoretical calculation, we find that the valence band and conduction band of H2WO4·H2O are lower than that of AgCl. For such a system, photons may be absorbed in both AgCl and H2WO4·H2O semiconductors forming electrons and holes. However, an electron at the conduction band bottom of AgCl would migrate to that of the H2WO4·H2O, hence being prevented from combining with an Ag+ ion, whereas a hole at the valence band top of AgCl would remain there. In contrast, a hole at the valence band top of H2WO4·H2O would migrate to that of AgCl, but an electron at the conduction band bottom of the SBG semiconductor would remain there. The composite photocatalyst H2WO4·H2O/AgCl makes the separation between electrons and holes more efficient, showing high photocatalytic efficiency. The composite photocatalyst based on SPR is Ag/AgBr/WO3·H2O, which combines the advantages of plasmonic photocatalyst and composite photocatalyst.In Chapter one, we briefly introduced the background and the latest progress of related works. Firstly the mechanism of semiconductor photocatalyst was introduced based on semiconductor's energy band theory. The properties, crystal structures and applications of AgX were briefly introduced. The latest progress in visible light photocatalysts was reviewed in detail. Secondly, the noble metal's SPR, influencing factors and application were introduced. The conception and the latest progress of plasmonic photocatalyst were introduced. Finally, the significance of topic selection, train of though and outline of the thesis were summarized.In Chapter two, we studied the preparation, characterization and photocatalytic property of plasmonic photocatalyst Ag@AgCl.The plasmonic photocatalyst Ag@AgCl was fabricated by the ion-exchange process and light-induced chemical reduction reaction. The plasmonic photocatalyst Ag@AgCl was characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and UV/Vis spectroscopy (UV-DRS). The photocatalytic property was evaluated by measuring the decomposition of methylic orange (MO). The mechanism and photo-stability were discussed based on SPR and metal-semiconductor contact.In Chapter three, we studied the preparation, characterization and photocatalytic property of plasmonic photocatalyst Ag＠AgCl with various structures.The precursor Ag2MoO4 was synthysized by microwave-hydrothermal method. By tuning the pH of starting solution, Ag2MoO4 with various structures were got. The plasmonic photocatalyst Ag＠AgCl with various structures were fabricated by the ion-exchange process and light-induced chemical reduction reaction. The plasmonic photocatalysts were characterized by XRD, SEM, X-ray photoelectron spectroscopy (XPS) and UV-DRS. The photocatalytic property was evaluated by measuring the decomposition of MO. The Ag＠AgCl with hollow sphere structure shows higher photocatalytic efficiency. By morphology control, high efficient photocatalysts can be got.In Chapter four, we studied the preparation, characterization and photocatalytic property of plasmonic photocatalyst Ag@AgBr.The plasmonic photocatalyst Ag@AgBr was fabricated by the ion-exchange process and light-induced chemical reduction reaction. The plasmonic photocatalyst was characterized by XRD, SEM and UV-DRS. The photocatalytic property was evaluated by measuring the decomposition of MO. Ag@AgBr shows higher efficiency in photo-degradation of MO than that of Ag@AgCl, the mechanism was discussed.In Chapter five, we studied the preparation, characterization and photocatalytic property of plasmonic photocatalyst Ag@Ag(X1,X2) (X1,X2=Cl,Br,I).The plasmonic photocatalyst Ag@Ag(Br,I) was fabricated by the ion-exchange process and light-induced chemical reduction reaction. The plasmonic photocatalyst was characterized by XRD, SEM, XPS and UV-DRS. From the results of the calculation, we can predict that the reducing ability of Ag(Br,I) is stronger than that of AgBr. The reducing ability of the plasmonic photocatalysts has been checked. The plasmonic photocatalyst Ag@Ag(Br,I) shows high efficiency in photo-reduction of Cr(VI) than that of Ag@AgBr. The introduction of iodine element enhanced the reducing ability of the plasmonic photocatalyst. The plasmonic photocatalysts Ag@Ag(Cl,Br) and Ag@Ag(Cl,I) were also fabricated by the same method, and the properties were characterized by XRD, SEM, UV-DRS and photo-reduction of Cr(VI).In Chapter six, we studied the preparation, characterization and photocatalytic property of composite photocatalyst H2WO4·H2O/AgCl.The band position of H2WO4 was determined by theoretical calculation. The newly high efficient photocatalyst H2WO4·H2O/AgCl was designed based on the band theory and band positions of semiconductors. The composite photocatalyst H2WO4-H2O/AgCl was fabricated by the ion-exchange process. The photocatalyst was characterized by XRD, SEM and UV-DRS. The photocatalytic property was evaluated by measuring the decomposition of MO. H2WO4·H2O/AgCl shows high efficiency. The composite photocatalyst enhanced the separation between electrons and holes, leading to high photocatalytic efficiency.In Chapter seven, we studied the preparation, characterization and photocatalytic property of composite photocatalyst Ag/AgBr/WO3·H2O.The band position of WO3·H2O was determined by theoretical calculation. The newly high efficient photocatalyst Ag/AgBr/WO3·H2O was designed based on SPR, band theory and band positions of semiconductors. The photocatalyst Ag/AgBr/WO3·H2O was fabricated by the ion-exchange process and light-induced chemical reduction reaction. The photocatalyst was characterized by XRD, SEM, XPS and UV-DRS. The photocatalytic property was evaluated by measuring the decomposition of MO and bacterial destruction. Ag/AgBr/WO3·H2O shows high efficiency in bacterial destruction. The introduction of WO3·H2O enhanced the bacterial destruction ability of the photocatalyst.In Chapter eight, the summary and prospect were given. The problems remained to be solved were discussed. At last, a plan for the future work was made and the futurity was expectation.In summary, the plamsonic photocatalyst solves the visible light absorption and separation of electons-holes, exploring a new way to fabricate high efficient photocatalyst. The plasmonic photocatalyst and composite photocatalyst are of great theoretical guidance for the practical application in solving the problem of energy shortage, environment pollution and global warming in the future.