Synthesis And Properties Of Highly Efficient Visible-Light-Driven Photocatalysts Silver Phosphate And Its Heterojunction

With the increasingly serious environmental pollution, human survival and development are threatened. It is more urgent to discovery new and efficient materials to control the environmental pollution. Photocatalytic degradation of pollutant using semiconductor material is an efficient and environmentally friendly, much to be concerned in recent years. TiO2has been extensively studied and applied in the field of photocatalysis because of its long-term stability, high activation and environment friendly. However, light response range of the TiO2photocatalyst is mainly in the ultraviolet region, and the use efficiency of solar energy is low (about4%). In addition, the carrier recombination rate is high and photocatalytic activity is low. So it is very necessary to develop new, efficient, visible light response photocatalytic materials.In the present work, we around a new photocatalyst Ag3PO4, prepared several photocatalysts such as Ag/Ag3PO4plasma, the Ag3PO4nano-particles, AgCl (Br)/Ag3PO4heterojunction and AgCl/AgVO3heterojunction using different methods, so as to improve the photocatalytic activity and stability of the Ag3PO4. The structure, morphology, optical properties and photocatalytic properties were characterized by a variety of methods. The relationships among the microstructure of the materials, the composition of the heteroj unctions and photocatalytic properties were revealed. The main content of this paper is summarized as follows:(1) Ag/Ag3PO4composite photocatalyst was prepared by the ion-exchange process and subsequently light-induced reduction route. The decomposition of Ag3PO4in methylene blue solution under visible light has been studied. At the beginning, the content of Ag0on Ag3PO4surface increased with irradiation time. After2hour irradiation, it reached to5.25%(mol%) and then remained stable. Although the decomposition efficiency of Ag/Ag3PO4to MB dye was slightly lower than that of Ag3PO4, it maintained a high level photocatalytic activity. The rate constant (k) over Ag/Ag3PO4was about three times that over Ag/AgBr (0.1459min-1) or Ag/AgCl(0.1067min-1) and dozens of times larger than that over N-TiO2(0.0142min-1). The photocatalyst still maintained high level activity after recycling five times.(2) A high efficient, stable Ag/Ag3PO4composite plasmatic photocatalyst was synthesized by a facile one-pot hydrothermal method assisted by pyridine. The influences of reaction temperature, pH value and pyridine amount on the Ag content of Ag/Ag3PO4composite and morphology of Ag3PO4were investigated. We found that the pyridine not only worked as a coordination agent role of Ag+, which can affect the morphology of Ag3PO4by controlling the release rate of Ag+, but also it acted as reductant for reduce partial Ag+of Ag3PO4to metallic Ag. The Ag content of Ag/Ag3PO4composite can be regulated by changing the amount of added pyridine in the reaction system and hydrothermal temperature. Photocatalytic degradation rate of methyl orange dye with Ag/Ag3PO4was about three times of that with Ag/AgBr or Ag/AgCl under visible light irradiation. Ag/Ag3PO4remained higher activity after recycling5times. The results for photocatalytic degradation of phenol experiments were similar to that above-mentioned.(3) A green synthetic method-phytic acid soft template method was used to prepare Ag3PO4nanocrystals with uniform particle size distribution. The synthetic principle and the impact of the amount of sodium phosphate on generated Ag3PO4size were investigated. It was found that a uniform size Ag3PO4nanocrystals would be obtained unless phytic acid existed, and Ag3PO4particle size could be adjusted in range of40-50nm by changing the amount of Na3PO4. From SEM, and TEM images, we found that silver phosphate nanocrystals were composed of many smaller nanoparticles combined loosely, which had rough surface bringing more photocatalytic active sites. Ag3PO4nanocrystals can absorb the ultraviolet and visible light of wavelength shorter than530nm, and its band gap is2.37eV. The rate constant of degradation of methyl orange for nano Ag3PO4is approximately2times of that for the micron Ag3PO4. Phytic acid micelles soft template method was used to synthesis AgCl and AgBr. The nanocrystals with a uniform particle size～40nm (AgCl) and～50nm (AgBr) were also obtained. As a result, this method has the versatility for the synthesis of silver based nano-crystals. (4) Electrochemical anodic oxidation method was used to one-step synthesis AgCl/Ag3PO4, AgBr/Ag3PO4heteroiunction films. The influence of the electrochemical oxidation potential, time and electrolyte solution composition on the composition and morphology of the precipitation film were discussed. The results show that, Ag3PO4is attached to surface of the large particles of AgCl (of AgBr) as～50nm nano particles in the heterojunction as prepared. We can effectively control the content of Ag3PO4in heterojunction films by changing the oxidation potential. Photocatalytic experiment and photoelectrochemical tests showed the descending order of photocatalytic activity of samples: AgBr-Ag3PO4> AgCl-Ag3PO4> Ag3PO4. This synthetic method is very simple, and can effectively regulate the composition of the heteroj unctions. Moreover, it could be extended to prepare other heterojunction photocatalyst.(5) Fabrication method of Other silver-based heterojunction and its photocatalytic properties were studied. Firstly, AgVO3nanobelts were synthesized by the pyridine-assisted hydrothermal method. Then, AgCl/AgVO3heterojunction was obtained through ion-exchange of AgVO3with NaCl. As a result, cube-shaped AgCl nanoparticles of approximately50-100nm were closely attached to the surface of nanobelts AgVO3. The content of AgCl in heterojunction can be controlled by adjust NaCl concentration in the ion-exchange reaction solution. The absorption edge of AgCl/AgVO3extends to640nm in the visible region and its band gap energy (2.15eV) is slightly lower than that of pure AgVO3(2.2eV). We found that the photocatalytic activity of AgCl/AgVO3heterojunction was far greater than pure AgVO3through photocatalytic degradation of methyl orange solution.