Preparation Of Noble Metal Based Composite Photocatalysts And Study Of Their Photocatalytic Property

In recent years, the world faces the challenges of energy shortage, environment pollution. It’s necessary to solve the above problems to guarantee world harmony and peace, as well as realize the sustainable development of our society. Semiconductor-based photocatalysis has been intensively investigated as it is a promising and green technology to solve the problems of worldwide energy crisis and environmental pollution by utilization of sustainable solar energy.TiO₂ and ZnO are the representative of the traditional and most widely investigated photocatalysts. However, their practical applications are greatly restricted due to narrow light-response range and low photogenerated electron-hole pairs separation efficiency. Therefore, the control and optimization of microstructure is the most effective means to improve the performance of the traditional UV-light response and novel visible-light response photocatalysts, and the search for advanced photocatalysts with relatively simple fabrication process and high activity is still underway.Herein, we focused on the improvement of visible light utilization and photogenerated electron-hole pairs separation efficiency, a series of novel metal Au and Ag based composite photocatalysts with different microstructure were fabricated. The as-prepared photocatalysts were characterized in detail by XRD、FT-IR、FESEM、 EDS、TEM、HRTEM、UV-Vis DRS、N₂ adsorption-desorption isotherms, XPS and PL. The photocatalytic activities of the obtained samples were evaluated by the photodegradation of dyes and antibiotics solution. The photocatalytic mechanisms were also investigated and discussed in detail. The main results are summarized as follows:1. Synthesis of Ag/ZnO/C plasmonic photocatalyst with enhanced adsorption capacity and photocatalytic activity to antibioticNovel Ag/ZnO/C plasmonic photocatalyst was synthesized via a facile calcination and photodeposition route with zinc citrate dihydrateas the precursor. The results indicated that the Ag and ZnO nanoparticles sized 5-10 nm were uniformly dispersed on the surface of the carbonaceous layers in Ag/ZnO/C composites. The adsorption capacity and photocatalytic activity were investigated by adsorption and photocatalytic degradation of tetracycline hydrochloride （TC-HCl） in aqueous solution. The results showed that the obtained Ag/ZnO/C sample exhibited higher adsorption capacity and enhanced UV and visible light driven photocatalytic activity to TC-HCl compared to ZnO/C and pure ZnO. Moreover, Ag/ZnO/C showed outstanding photocatalytic stability due to Au and ZnO have close interfacial connection with the carbonaceous layers. With the presence of the Ag nanoparticles and carbonaceous layers incorporating in the structure, the Ag/ZnO/C composites can make use of not only the UV region of sunlight, but also the visible region and efficiently promote photogenerated electron separation and transportation as well as generate more active reaction sites, which synergistically facilitate the photocatalysis process.2. Fabrication of porous g-C3N4/Ag/Fe₂O₃ composites with enhanced visible light photocatalysis performancePorous graphitic carbon nitride （pg-C3N4） synthetized by pyrolysis of urea was hybridized with Ag-doped Fe₂O₃ to form a visible-light-driven photocatalyst pg-C3N4/Ag/Fe₂O₃ via a simple chemical adsorption method. The photocatalytic activities were evaluated by degradation of Rhodamine B （RhB） as a representative organic pollutant under visible light irradiation. The results showed Ag/Fe₂O₃ （5 wt%） modified pg-C3N4 exhibited excellent photocatalytic activity in the degradation of RhB and the degradation rate was 2.74 times higher than that of pure pg-C3N4. Nevertheless, over loading Ag/Fe₂O₃ in the composites may cover the active sites on the pg-C3N4 surface and create recombination centers to increase the recombination rate of photogenerated electron-holes, which is disadvantageous to the photocatalytic activity. The heterostructured coupling of pg-C3N4 with novel metal Ag and semiconductor Fe₂O₃ in the aspect of energy level matching would effectively improve visible-light absorption capability and facilitate photogenerated electron-hole pairs separation, synergistically accounting for the enhancement of photocatalytic activity. Furthermore, the trapping experiment with scavenger investigation suggested that ·O₂- and h+ are the main active species in the current pg-C3N4/Ag/Fe₂O₃ photocatalytic degradation system.3. Facile synthesis of Ag2O/N-doped helical carbon nanotubes with enhanced visible-light photocatalytic activityNovel Ag2O/N-doped helical carbon nanotubes （Ag2O/N-HCNTs） were successfully synthesized via a simple coprecipitation method. N-HCNTs were prepared with the template of C14-L-Glu and have a relatively high specific surface area. Nitrogen atoms included in N-HCNTs could create more active sites for anchoring functional Ag+ and the support role of N-HCNTs made the deposited Ag2O have small size, and the introduction of N-HCNTs also can decrease the usage of novel metal. The photocatalytic activities were evaluated in the degradation of methylene blue （MB） aqueous solution. The results showed that Ag2O nanoparticles sized 3-10 nm were highly anchored on the surface and inner tubes of the N-HCNTs support, and significantly enhanced the visible-light photocatalytic activity compared to bare Ag2O. In particular, the rate of degradation of the as-prepared Ag2O/N-HCNTs was 3.9 times faster than that of using bare Ag2O nanoparticles under visible light irradiation. It was attributed to the combined effects, including highly dispersed smaller Ag2O particles, improved visible light utilization efficiency and higher charge separation efficiency. In addition, the Ag2O/N-HCNTs could also degrade MB dye in different water sources like Changjiang river water and tap water with high efficiency as well as in deionized water.4. Facile photochemical synthesis of Au/Pt/g-C3N4 with plasmon-enhanced photocatalytic activity for antibiotic degradationA novel plasmonic photocatalyst Au/Pt/g-C3N4 was prepared by a facile calcination-photodeposition technique. The g-C3N4 power was synthesized by one-step polymerization of melamine, and Au and Pt NPs with size of 7-15 nm were deposited on the layered g-C3N4 by photodeposition process. The Au/Pt co-decorated g-C3N4 heterostructure displayed enhanced photocatalytic activity for antibiotic tetracycline hydrochloride （TC-HCl） degradation and the degradation rate was 3.4 times higher than that of pure g-C3N4 under visible light irradiation. The enhancement of photocatalytic activity could be attributed to the surface plasmon resonance （SPR） effect of Au, electron-sink function of Pt nanoparticles and the excited g-C3N4, which improve the optical absorption property, promote photogenerated charge carriers separation of g-C3N4, as well as higher conductivity of g-C3N4 for the rapid transport of the electrons, synergistically facilitating the photocatalysis process.5. Au-Ioaded porous graphitic C3N4/graphene layered composite as a ternary plasmonic photocatalyst and its visible-light photocatalytic performanceA novel ternary plasmonic photocatalyst, Au-loaded porous graphitic C3N4/graphene layered composite （Au/pg/C3N4/GR）, was fabricated by a facile sonication-photodeposition technique. In this hybrid structure, polymeric semiconductor pg-C3N4 was immobilized on the surfaces of graphene sheets to form a layered composite with Au nanoparticles sized of 10-15 nm uniformly deposited on it. The photocatalytic performance of the as-prepared Au/pg-C3N4/GR composite was evaluated by degradation of methylene blue （MB） and ciprofloxacin （CIP） as representative dye pollutant and antibiotic pollutant under visible light irradiation, respectively. The degradation rates of MB and CIP over Au/pg-C3N4/GR photocatalyst were 4.34 and 3.05 times higher that of porous g-C3N4 （pg-C3N4）, respectively, and even 7.42 and 6.09 times higher than that of pure g-C3N4, respectively. The results indicated that a improved photocatalytic efficiency was obtained when Au nanoparticles and graphene sheets co-incorporated in porous g-C3N4. The porous structure within the samples is advantageous to the adsorption capacity. The surface plasmon resonance （SPR） effect of Au and electron-acceptor role of graphene, which would improve the visible light harvesting ability, facilitate photogenerated charge carriers separation, as well as create more active reaction sites, synergistically contribute to the enhancement of photocatalytic activity.