Photocatalytic Properties Of Modified ZnO And Ag₃PO₄ Nano-photocatalysts

With environmental pollution worsening, it is a hot topic to efficiently remove the organic pollutants from wastewater. Traditional wastewater treatments, such as adsorption, coagulation and membrane separation, possess high operating costs, and even generate the secondary pollutants. Semiconductor photocatalysis has been deemed to be one of the most promising technologies for eliminating environmental pollution. The technology can efficiently degrade a wide range of organic pollutants into biodegradable or harmfulless organic compounds, and some inorganic CO₂, H₂O, NO₃^- and PO₄^3- ions under the natural sunlight irradiation. Howerer, the traditional photocatalysts exist some critical issues such as the limited light absorption and fast charge recombination, which have restricted their practical applications. Therefore, it has an important practical significance to explore highly-efficient visible light photocatalyts.In the present work, based on the wide band gap ZnO and narrow band gap Ag₃PO₄, we prepared Ag2SO₄/ZnO, Ag/Ag2SO₄/ZnO and Ag2MoO₄/Ag₃PO₄ heterojunctions, as well as SO₄^2--doped Ag₃PO₄ catalysts by various methods. The morphologies, structures, optical properties and photocatalytic activities of as-prepared photocatalysts were analyzed by a variety of characterizations. Meanwhile, the relationships between structure and photocatalytic performance have been been thoroughly researched. The photocatalytic mechanisms for different photocatalysts were also provided. The main results are summarized as follows:Novel nano-sized Ag2SO₄/ZnO composites were successfully synthesized by a facile solvothermal method. The results showed that Ag2SO₄ nanoparticles were uniformly dispersed on ZnO naoflakes. The Ag2SO₄/ZnO composites exhibited enhanced photocatalytic activities for rhodamine B （RhB） and methylene blue （MB） degradation under UV-vis light irradiation compared with pure ZnO, Ag2SO₄ and TiO₂ （Degussa P25）. According to the optimization of the amount of AgNO₃ during the preparation, the as-prepared 3ASZ displayed the highest photocatalytic performance, and its degradation rate constant was more than 4 times higher than that of pure ZnO. The UV-vis diffuse reflectance spectra （DRS） showed that the loading of Ag2SO₄ could contribute to the visible-light-response enhancement of ZnO. Furthermore, the possible photocatalytic mechanism has been systematically explored.Highly-efficient visible-light-driven Ag/Ag2SO₄/ZnO （AZ） photocatalysts were prepared via a microwave-assisted hydrothermal reaction. The results revealed that Ag/Ag2SO₄ nanoparticles were uniformly dispersed on the surface of ZnO plates. The AZ-2 photocatalyst obtained by adjusting the amounts of thiourea and AgNO₃ with the molar ratio of 1:1 exhibited the highest photocatalytic efficiency, and RhB could be decolorized within 35 and 20 min under visible light （λ≥> 420 nm） and natural sunlight irradiation, respectively. The enhanced photocatalytic performances of AZ composites were attributed to the positive synergistic effects of three components （Ag, Ag2SO₄ and ZnO） in favor of the enhancement of visible-light absorption capability and efficient separation of photogenerated electron-hole pairs.Ag2MoO₄/Ag₃PO₄ （AgMoP） composites were synthesized via a simple precipitation method. The results revealed that AgMoP composites displayed much higher photocatalytic activities than those of pure Ag2MoO₄ and Ag₃PO₄ for RhB degradation. The composite with 5 wt% Ag2MoO₄ showed a 2.8-fold enhancement in the degradation rate constant compared with pure Ag₃PO₄. Moreover,5% AgMoP composite still maintained a stable photocatalytic property even after four recycles. In addition, the types of the substrate, the initial RhB concentration and pH value, catalyst dosage could affect the photocatalytic activity. The radical trapping experiments suggested that holes were the primary active species in the photocatalytic process. In addition, the possible photocatalytic mechanism were also proposed.Sulfate-doped Ag₃PO₄ photocatalysts were synthesized via a simple precipitation method. XRD and XPS confirm that SO₄^2- ions were incorporated into the lattice of Ag₃PO₄ by replacing PO₄^3-. The crystalline structure and optical absorption behavior of Ag₃PO₄ remained unchanged after SO₄^2- doping. However, S042--doped Ag₃PO₄ catalyst with 0.50 at% SO₄^2- concentration ratio exhibited remarkably enhanced photocatalytic activity, and completely decomposed RhB, MB and methyl orange （MO） in 5 min under visible light irradiation, respectively. Its degradation rate constants for the degradation of three types of dyes were 5.6,5.4 and 10.7 times higher than that of pristine Ag₃PO₄, respectively. The high photocatalytic performance was attributed to the improved the separation efficiency of photogenerated electron-hole pairs and hindered their recombination by doping SO₄^2- into Ag₃PO₄ lattice. In addition, the density functional theory （DFT） calculations indicated that SO₄^2- substitution could effectively tune the electronic structures of Ag₃PO₄, thus resulting in the high photocatalytic activity under visible light irradiation.