Research On Synthesis And Properties Of Novel Nano-photocatalytic Materials

Since twenty-first century,fossil energy depletion and environmental deterioration has become the biggest problem that affects the development of human society,which needs to be solved urgently.Photocatalysis holds great promise for addressing global energy and environmental problems.TiO2 is one of the most promising photocatalysts,and has been generally used in water and air purification and hydrogen production.However,TiO2 can only absorb UV light due to its intrinsic wide bandgap,and has low quantum efficiency because of high recombination of photoexcited electrons and holes.The two disadvantages highly limit its practical application in a large scale.Therefore,extensive studies have been devoted to exploiting highly-efficient semiconductor photocatalysts that are photoactive in H2 evolution and organic pollutant degradation under visible light irradiation.Some organic polymers and multicomponent metal oxide semiconductors show the characteristics of the visible light response.Nevertheless,the lower specific surface area and higher recombination rate of photogenerated carriers seriously affect their photocatalytic activity.Thus,we selects the organic polymer semiconductor photocatalyst of graphite carbon nitride?g-C₃N₄?and multi metal oxide lead niobate?Pb₃Nb₂O₈?as the research object,aiming at the deficiency of the material itself,design template free and soft template method to improve the specific surface area of the material,and the low activity Pb₃Nb₂O₈ through the surface load of silver nanoparticles to further improve its photocatalytic activity.The details are as follows:1.An artful and simple synthetic strategy for fabricating low carbon residual porous g-C₃N₄ with enhanced visible-light photocatalytic properties.The low carbon residual and porous g-C₃N₄ was prepared by employing acetic acid mediated melamine as precursor,accompanied with a designed “two-step” thermal treatment process.That is to say,first step is heating the acetic acid treated melamine at N2 atmosphere,thus keeping carbon residue from the composition of templating exist in the g-C₃N₄,then further calcination in air to remove the carbon in air and obtain large specific surface area g-C₃N₄.The phase transformation process,composition,structure,specific surface area and pore size distribution,morphology,and light absorption ability of the samples were characterized by TG-DSC,XRD,N2 adsorption-desorption isotherms,SEM,TEM,XPS,EA and UV-vis.The possible formation mechanism of porous g-C₃N₄ was also discussed.The specific surface area of the g-C₃N₄ increased from 9 to 126 m2 g-1.In addition,acetic acid treated melamine as precursor can inhibiting effect of the crystal growth,which lead the band gap increase of 0.08 eV in comparison with pristine g-C₃N₄.The larger band gap by shifiing conduction and valence bands in opposite directions can contribute to a stronger redox ability of charge carriers and retarding the recombination of photogenerated charge carriers.The porous g-C₃N₄ towards degradation of 2-propanol under visible light is higher than the pristine g-C₃N₄.The photocatalytic activity of the porous g-C₃N₄ is 5.4 times of that of the pristine g-C₃N₄.The enhancement of the photocatalytic activity of the catalysts can be ascribed to their improved specific surface area.The higher specific surface area of porous g-C₃N₄ enhances the adsorption towards 2-propanol,provides more active sites and short bulk diffusion length as well,thus higher photocatalytic activity was achieved.2.Facile transformation of low cost melamine-oxalic acid into porous graphitic carbon nitride nanosheets with high visible-light photocatalytic performanceThe porous g-C₃N₄ nanosheet was prepared by using a long strip-like structure of melamine-oxalic acid?MO?as a starting product.The effects of the ratio of melamine and oxalic acid on the structure,morphology,specific surface area and photocatalytic properties of g-C₃N₄ were investigated systematically.The phase transformation process,composition,structure,specific surface area and pore size distribution,morphology,and light absorption ability of the samples were investigated.The possible formation mechanism of porous g-C₃N₄ nanosheets was also discussed.The addition of oxalic acid can influence the thermal condensation model and inhibit grain growth of g-C₃N₄.Meanwhile,the synergism of MO-4 precursor and specific heating process can effectively adjust the porosity and number of layers.The synthesized porous g-C₃N₄ nanosheets possess large specific surface areas?203 m2 g-1?,a widened band gap 2.98 e V,and improved electron transport ability,exhibit high photocatalytic activities as much as 8 times than primative g-C₃N₄ for the photodegradation of gas pollute under visible light?? > 420 nm?.3.Ag-loaded mesoporous Pb₃Nb₂O₈ photocatalysts with enhanced activity under visible-light irradiation.Mesoporous Pb₃Nb₂O₈ photocatalysts were synthesized by using F127 as soft-template with the evaporation-induced self-assembly?EISA?method.The phase transformation process,composition,structure,specific surface area and pore size distribution,and light absorption ability of the samples were characterized by TG?DSC,XRD,N2 adsorption-desorption isotherms,HR-TEM and UV-Vis spectroscopy.The results revealed that MPNO-400?calcination at 400 oC?has the largest specific surface area and crystalline pore walls.The specific surface area of the MPNO-400 was approximately 26.5 times larger than that of Pb₃Nb₂O₈ prepared by the conventional solid state reaction,and the photocatalytic activity of MPNO-400 was increased by 19.1 times.Moreover,Ag nanoparticles were used as a co-catalyst to further improve the photocatalytic activity of mesoporous Pb₃Nb₂O₈.The content of Ag to mesoporous Pb₃Nb₂O₈ was studied to determine the optimal loading conditions corresponding to the highest photocatalytic activity.The results revealed that 1% Ag-loaded content is the optimal loading condition,which corresponds to the highest photocatalytic activity?increased 41.6 times than that of PNO-SSR?.In addition,Ag loaded mesoporous Pb₃Nb₂O₈ were further characterized by TEM?XPS?UV-vis and PL spectroscopy.The results revealed that the loaded Ag should be metal Ag,the average size of Ag particles was less than 10 nm,and no large Ag particles were formed.The UV-vis results revealed that the existence of Ag nanoparticles leads to the formation of surface plasmon resonance absorption.The PL results indicated that the Ag co-catalyst decreased the recombination of photo-generated charge carriers.Finally,the photocatalytic degradation of acetaldehyde was carried out to prove that the Ag-loaded mesoporous Pb₃Nb₂O₈ is a general photocatalyst and is useful for use in the degradation of other kinds of gas pollutants.The best activity was achieved on MPNO-400/Ag-a,and the photocatalytic activity of MPNO-400/Ag-a could reach 80.4% after illumination for 1 h under visible light.Based on this research,it is believed that Pb₃Nb₂O₈ is a promising material that possesses good potential for use in photocatalytic fields if some techniques,such as increasingthe surface area and loading co-catalyst,are used to improve its photocatalytic activity.