Construction And Photocatalytic Performance Of Ag@AgCl And Ag₃PO₄ Base Heterojunction Photocatalysts

Photocatalytic technology is one of the most promising ways to solve current energy and environmental problems.The key of photocatalytic technology is the development of efficient photocatalysts.Construction of heterojunction photocatalyst is a kind of classical method for improving the separation efficiency of photoinduced electron and hole.However,the transfer and separation of photoinduced electron and hole always couple with the decrease of redox ability of heterojunction photocatalyst.Therefore,the biggest challenge to futher imporve the performance of heterojunction photocatalyst is how to promote the separation efficiency and redox ability of photoinduced electron and hole simultaneously.In this dissertation,Ag@AgCl and Ag₃PO₄ were choosed as substrate to constructe heterojunction photocatalysts.The separation efficiency and redox ability of photoinduced electron and hole were enhanced by designing the components matching of heterojunction photocatalysts.The key of our strategy is enhancement of the photoinduced carrier transfer from solar energy captur unit to the surface catalytic unit.The main contents are focused on the following four aspects:?1?In order to construct a heterojunction photocatalyst with good performance,ZIF-8 was used as a photoinduced electron transfer and utilization center to improve the photocatalytic performance of Ag@AgCl.A novel Ag@AgCl/ZIF-8?50%?heterojunction photocatalyst was designately prepared by anchoring Ag@AgCl nanoparticles on the surface of ZIF-8.The photocatalytic performance of Ag@AgCl/ZIF-8?50%?was conducted by removing of Rhodamine B?RhB?from aqueous solution under visible light irradiation?? > 420 nm?.The experimental results showed that the as-synthesized Ag@AgCl/ZIF-8?50%?exhibited higher photocatalytic activity than Ag@AgCl and ZIF-8.After four recycling runs,the Ag@AgCl/ZIF-8?50%?maintained high photocatalytic activity and structure stability.ZIF-8 in heterojunction photocatalyst not only has the adsorption ability for RhB by its large specific area,but also has the ability for the formation of O2·-by an appropriate position of conduction band?CB?,which improves the transfer and utilization efficiency of photoinduced electrons.?2?Based on the previous work,an Ag@AgCl/Ag nanofilm/ZIF-8 heterojunction photocatalyst was successfully constructed by two-step method including ion-exchange and photo-reduction.The photocatalytic activity was evaluated by photocatalytic degradation of methylene blue?MB?under visible light irradiation.The first-order kinetic constant of MB degradation over Ag@AgCl/Ag nanofilm/ZIF-8(0.2460 min^-1)was nearly 2 times of Ag@AgCl/ZIF-8(0.1322 min^-1).Moreover,the enhanced stability was observed after four cycles.The electrochemical impedance spectra,photoelectric conversion spectra and ESR DMPO-O2·-species spectra revealed that Ag@AgCl/Ag nanofilm/ZIF-8 had a lower resistance,higher photocurrent density and more generation of superoxide radical than those of Ag@AgCl/ZIF-8,meaning that the extended interface between Ag nanofilm and ZIF-8 could promote the quick electrons transfer and electron-hole pairs separation.The formed Ag nanofilm was harnessed as a mediator to transfer photoinduced electrons from the plasmonic Ag@AgCl nanostructure to ZIF-8 for enhancing the transfer and utilization of photoinduced electrons.?3?TiO_2-x was used as a photoinduced electron transfer and utilization center to improve the photocatalytic performance of Ag₃PO₄.A novel TiO_2-x/Ag₃PO₄ heterojunction photocatalyst with controllable oxygen vacancies?Vo.?amount was prepared by a two-step method including co-precipitation and ion exchange.The first-order kinetic constant of RhB degradation over TiO_2-x400/Ag₃PO₄(0.48 min^-1)was 4.4 times higher than that of Ag₃PO₄(0.11 min^-1).Compared with Ag₃PO₄,TiO_2-x400/Ag₃PO₄ photocatalytic system showed obviously higher stability after four cycles.The photoelectric conversion spectra,Ag 3d XPS narrow scan spectra,ESR DMPO-O2·-species spectra revealed that the TiO_2-x/Ag₃PO₄ had a higher photocurrent density,lower photocorrosion and more generation of superoxide radical than those of Ag₃PO₄.The Vo.on TiO_2-x was found as molecular oxygen activation center trapping electrons and O2 to form ·O2-,avoided the electrons combined with h+ and/or Ag+.?4?Ag₃PO₄/g-C₃N₄ Z-scheme heterojunction photocatalyst was synthesized successfully via an ion-exchange method using g-C₃N₄ as photoinduced electron transfer and utilization center.The photocatalytic activity was evaluated by means of the phenol degradation under visible light?? > 420 nm?and the photocatalytic mechanism was discussed based on the active species during photocatalytic process as well as band structure.The experimental studies have shown that the as-prepared Ag₃PO₄/g-C₃N₄ exhibited higher photocatalytic activity than pure Ag₃PO₄ and g-C₃N₄.The high photocatalytic performance of Ag₃PO₄/g-C₃N₄ heterojunction was attributed to its enhanced redox ability.Meanwhile,the Ag₃PO₄/g-C₃N₄ heterojunction was regenerated by H2O2,which could fully recover the photoactivity of the photocatalyst.