Improvement Of The Visible-light Photocatalytic Performance Of Bi₂O₄ Through Interface Modification And Space Confinement

In recent years,enormous attention has been paid to bismuth-based semiconductor photocatalysts because of their unique electronic structure and excellent catalytic performance.Bismuth tetroxide?Bi₂O₄?is a newly emerged visible light responsive semiconductor with narrow band gap of 1.9 to 2.1 eV.However,for pristine Bi₂O₄,there are some disadvantages that limit its photocatalytic activity,such as fast recombination of photogenerated carriers and low utilization of solar light.Therefore,developing new strategies to improve the photocatalytic performance of Bi₂O₄ is of great significance.In this thesis,we developed three ways?PPy encapsulating,surface modification via N-CQDs and spatial confinement of hierarchical TiO₂?to decorate Bi₂O₄,aiming at promoting the separation of photogenerated electron-hole pairs and improving the utilization of solar light,and thus improving the photocatalytic performance.As follows are the main contents of this thesis:Firstly,Bi₂O₄@PPy core-shell structure was constructed via in-situ vapor-phase polymerization method using pyrrole monomer as starting material.Such core-shell structure can not only increase the interfacial contact between Bi₂O₄ and polypyrrole,but also improve the separation efficiency of photogenerated charges carriers due to the formed heterojunction.Moreover,due to the exisit of Pyrrolidine ring,the dye molecular can be enriched on its surface and then improve the photocatalytic performance.When the polymerization duration is 6 h,Bi₂O₄ can be completely coated and form Bi₂O₄@PPy core-shell structure.Moreover,the photocatalytic experimental results show that this sample displays the highest catalytic efficiency towards MO degradation,which is about 2.4 times of pure Bi₂O₄.Secondly,one-step hydrothermal method was carried out to synthesize N-CQDs modified Bi₂O₄ hybrid.Due to the excellent conductivity of N-CQDs,improved charge carrier separation efficiency of Bi₂O₄ was achieved.Moreover,benefitted to the upconvertion effect of N-CQDs,nearly infrared light?NIR?can be converted to visible light,because of which the response range of solar spectrum is broadened,and the utilization ratio of solar light is improved.Thanks to the synergistic effect above,the visible light catalytic activity of Bi₂O₄ is improved and the NIR photocatalytic activity is produced.Photocatalytic performance of N-CQDs modified Bi₂O₄ hybrids were evaluated using MO as the degradation target.The results confirmed that the hybrid with 0.3%N-CQDs showed the best catalytic activity under simulated sunlight,visible light and NIR light and the photocatalytic rates were 3.1,2.3 and 2.9 times that of pure Bi₂O₄,respectively.In addition,the activity capture experiments showed that h⁺and·O₂^-were the main active species in the reaction system.Thirdly,the hierarchical TiO₂ spheres with rich pores were utilized as the framework to confine the growth of Bi₂O₄.Due to this space-confined effect,micro-sized Bi₂O₄ rods were down to nanoscale and create enormous Bi₂O₄/TiO₂ type II heterojunctions.Benefitting from this outstanding structure,optimal photocatalytic activity was achieved when the mass ratio of Bi₂O₄/TiO₂ is 3.Moreover,the rate constant of Bi₂O₄/TiO₂-3 heterojunction is 0.0261 min^-1,which is about 2.9 times of pure Bi₂O₄ under visible light for the degradation of methyl orange.The improved photocatalytic activity mainly derived from the following aspects:1)nanoscaled Bi₂O₄ exposed more active sites that improve the adsorption capacity of dyes due to the increased specific surface areas;2)nanoscaled Bi₂O₄ can shorten the transfer distance of photogenerated charges from the bulk to the interfaces and prevent the recombination of charge carriers;3)nanoscaled Bi₂O₄ greatly enhanced separation efficiency of charge carriers due to the formed heterojunction.