Bimuth-containing Oxide Composite Photocatalytic Materials:Preparation And Their Application Of Organic Pollutants’ Degradation

With the global energy crisis and environmental pollution being more and more serious, environmental protection and sustainable development have become an important issue for human must to care. Photocatalytic technology is a new kind of efficient environmental pollution control technology, which has the advantages of environmental friendship, thorough decomposition of contaminants. However, the traditonal photocatalyst with TiO₂ as the representative material exhibits two inherent defects which limit its practical application:（1） Titanium dioxide has a wide band gap of 3.2 e V, which can only be activated by UV light with wavelength less than 386.5 nm. More than 90% of solar light energy cannot be utilized.（2） The rapid recombination of photoinduced electrons and holes reduces the quantum yield, which greatly influences the photocatalytic activity. In recent years, the studies found that the bismuth-based oxides semiconductor materials, such as Bi₂O₃, Bi₂O₂CO₃, BiPO₄ and BiOX（X=Br,I）have the advantages of sensitive visible-light response and high quantum yield. They have attracted much attention in the field of photocatalysis and exhbibt a promising application prospect.Fast recombiniation of photoinduced electron-hole pairs is generally happened on semiconductor photocatalysts with single component, which limits the improvement of photocatalytic efficiency. In order to meet the demands of practical application, constructing silver-based photocatalysts with high photocatalytic activity and good stability has great scientific significance. In this work, taking bismuth-containing oxide semiconductor materials as research basis, and empolying different synthetic methods, a series of bismuth-containing oxide composite photocatalytic materials such as Ag₂O/Bi₂O₃, Ag₂CO₃/Bi₂O₂CO₃, Ag/Ag₃PO₄/BiPO₄ and Ag/AgX/BiOX（X=Br, I） were prepared using different technological means. The structure, composition, morphology and optical absorption properties of these materials were studied through a variety of characterization methods. The photocatalytic activities of as-prepared catalysts were evaluated by degradation of different organic pollutants under visible light irradiation, which revealed the relationship between microscopic structures and catalytic performances. The details of research work in this paper are summarized as follows:（1） The Ag₂O/Bi₂O₃ composite microspheres were synthesized by one-step hydrothermal method. By changing the molar content of the raw materials AgNO₃ in the precursor solutions, the Ag₂O/Bi₂O₃ with different molar percentage of Ag₂O（x=0, 0.1, 0.2, 0.3, 0.5） were prepared. The photocatalytic activities of the Ag₂O/Bi₂O₃ composite microspheres were evaluated by decomposition of rhodamine B under visible-light irradiation（λ≥420nm）, and the effect of the molar percentage of Ag₂O on the photocatalytic performance of the composite materials was further discussed. The results demonstrated that the Ag₂O/Bi₂O₃ composites with 0.3 mole fraction of Ag₂O exhibited the highest photocatalytic activity. The degradation efficiency of rhodamine B is up to 97% within 60 min, which is 6 times that of N-TiO₂ and 4 times that of pure Bi₂O₃. This is mainly due to the interlaced and matched energy bands of Ag₂O and Bi₂O₃, which enhances the visible-light absorption, reduces the recombiniation probability of photo-induced electrons and holes and improves the photocatalytic activities of the Ag₂O/Bi₂O₃ composite microspheres（in chapter 2）.（2） Hierarchical Ag₂CO₃/Bi₂O₂CO₃ microflowers were first fabricated by a lowtemperature wet chemical method using water as solvent. The structure, composition, morphology and optical absorption properties of products were systematically characterized. According to the observation of the intermediate product and the crystal structure, the growth process mechanism of the Ag₂CO₃/Bi₂O₂CO₃ microflowers was proposed. The photocatalytic activity and stablity were evaluated by photodegradation of rhodamin B（Rh B）, methyl orange（MO）, methylene blue（MB） and the mixed dye under visible-light irradiation（λ≥420nm）. The Ag₂CO₃/Bi₂O₂CO₃ composites showed much higher photocatalytic activity for dyes degradation than Ag₂CO₃ and Bi₂O₂CO₃. The as-prepared Ag₂CO₃/Bi₂O₂CO₃ microflowers have universal applicability, good reusability and stability for many organic dyes, and there is no apparent loss of activity after 6 cycles It provides a simple, green, fast and universal way for the preparation of bismuth-containing non-metal oxides photocatalyst（in chapter 3）.（3） Novel ternary nanomaterials Ag/Ag₃PO₄/BiPO₄ was prepared by ions coprecipitation and hydrothermal method. The effect of experimental parameters such as solvent, temperature, duration of hydrothermal reaction and the molar percentage of Ag/Ag₃PO₄ on the photocatalytic degradation efficiency of MO were systematically studied in the work. In comparison with the pure BiPO₄ and Ag/Ag₃PO₄, the Ag/Ag₃PO₄/BiPO₄ nanocomposites showed much higher photocatalytic degradation efficiency for the degradation of MO and 2,4-dichlorophenol. The enhanced photocatalytic activity of Ag/Ag₃PO₄/BiPO₄ nanocomposites is mainly attributed to the enhanced visible-light response arised from the surface plasmon resonance effect of metal Ag0, the improved separation efficiency of photogenerated electrons and holes as well as the scattering of light among nanoparticles with different particle size（in chapter 4）.（4） The three-dimensional orderly flower-like（X=Br, I） microspheres were prepared by microwave-assisted solvothermal route combined with in situ photo-assisted reduction. The Ag/AgX/BiOX displays three-dimensional rose-like microspheres architectures, which are assblemed by a stack of nanosheets with the thickness of about 50 nm. Many Ag/AgX nanoparticles with the diameter of 10²0 nm are uniformly anchored on the nanosheets, which can serve as active sites for the photocatalytic reaction. The resulting composite showed much better visible-light photocatalytic activity than the pure BiOX and Ag/AgX in the degradation of p-nitrophenol. It can be ascribed to the porous and unsmooth surface of the Ag/AgX/BiOX composite, which is favorable for visible-light absorption and thus enhance the amount of photogenerated charges. Moreover, the Ag/AgX/BiOX flower-like microspheres combined the synergetic effect of Ag/AgX surface plasmon photocatalytic materials and AgX/BiOX heterojunction composite photocatalytic materials, realizing the enhancement of the visible-light absorption and the fast transfer of photogenerated carriers. As a result, the Ag/AgX/BiOX flower-like microspheres exhibit the excellent photocatalytic performance（in chapter 5）.（5） The adsorption and photocatalytic degradation performance towards MB of Ag/AgI/BiOI composite photocatalyst were studied by using the simulated solar light as the light source. The influence of operational variables, such as catalyst dosage, p H, reaction temperature and the initial concentration of the pollutants on the photocatayical activity were systematically investigated. The results showed that the optimal conditions of the photocatalytic degradation of MB by the Ag/AgI/BiOI were the temperature of 25℃, the catalyst dosage of 1.0 g/L, initial solution concentration of 10 mg/L, the p H of 7.5,and the irradiation time of 120 min. Under these conditions, the degradation efficiency of MB is almost 100%. According to the LangmuirHinshelwood equation, the photocatalytic decolorization reaction of MB followed pseudo-first-order reaction kinetics model in low concentration. Moreover, the study on testing the specific reactive species such as h+, ?OH and O2?- in the photocatalytic process of MB by photocatalyst were carried out. By combining the interface effects between metal and semiconductor contact as well as the special properties of semiconductor photocatalytic materials, the catalytic mechanism of Ag/AgI/BiOI composite was ascribed to the photocatalytic degradation reaction mechanism based on the surface plasmon resonance effect（in chapter 6）.