Design And Catalytic Mechanism Of Bismuth Vanadate Photocatalyst Modified By Carbon Doped And Multi Walled Carbon Nanotubes

Photocatalytic technology has many potential advantages,such as energy saving and environmental protection,so it is always a major research direction for engineers and researchers to hope to apply it to the field of sewage treatment.Based on the analysis of the defects of the new visible-light photocatalytic material BiVO₄,the modification of carbon-doped multi-walled carbon nanotubes?MWCNT?can overcome the existing defects and improve its photocatalytic performance.There are two main deficiencies in the application of BiVO₄ to visible light treatment of organic pollutants.Firstly,the absorption boundary of solar light is about 520 nm,and the response range to visible light needs to be further improved.Moreover,the electron holes are easy to recombine,which limits the utilization of redox properties.Secondly,the crystal surface has limited ability to absorb intrinsic organic matter,and its full contact with pollutants and redox ability need to be improved.Aiming at these two problems,a carbon-doped BiVO₄?C-BiVO₄?composite MWCNT?C-BiVO₄@MWCNT?photocatalytic material with wide visible light response,fast electron hole separation and excellent adsorption and degradation ability for organic pollutants was designed and developed.The main conclusions are as follows:?1?Based on the first principle,the density functional theory was used to design and study the carbon-doped BiVO₄ system.It is found that the valence band of pure monoclinic BiVO 4 VB is mainly composed of O 2p,the density of States is-5.3-0 eV,the Bi 6S orbital and O 2p orbital compounded to form the VB top,and the CB is mainly composed of V 3D orbital,the density of States is 2.4-5 eV,followed by Bi 6p and O 2p orbital.The orbital composition of CB and VB did not change obviously after doping,but the doping level was introduced into VB,and the VB top and CB bottom shifted in varying degrees.After doping,the VB edge of the system increases by 0.52eV,the CB edge increases by 0.27 eV,and the band gap decreases by 0.25 eV.By simulating the variation of lattice constants of the doped system,a direct characterization method for the doping success is established.Three different volumetric expansion cases are found in the theoretical study,i.e.without considering the volume change?C doped BiVO₄?,the in-plane expansion of lattice constants?C doped BiVO₄@in-plane?and the lattice.The XRD spectra of C doped BiVO₄@out-of-plane show different trends,which can be used as a basis for judging whether carbon doping is successful or not.?2?Carbon doped m-BiVO₄ nanosheets were synthesized by one-step hydrothermal synthesis with polyvinylpyrrolidone K-30?PVP?as structure-directing agent and L-cysteine as carbon source.The results show that the carbon doped m-BiVO₄ nanosheets have three-dimensional?3D?layered structure.The results show that the carbon doped bismuth vanadate has good physical and chemical properties.Further analysis showed that L-cysteine doped BiVO₄ lattice as C source and controlled its morphology,while PVP promoted the self-organization of two-dimensional?2D?C-BiVO₄ nanosheets,resulting in the formation of a three-dimensional flower structure with three-dimensional grading.It was found that the diffraction peaks of the XRD?121?plane shifted obviously and the experimental results were consistent with those of the simulated Doped BiVO₄@in-plane.Therefore,it was confirmed that the C-BiVO₄ was successfully prepared and the in-plane expansion of the BiVO₄ lattice was caused by carbon doping.In this study,composite materials?BiVO₄@MWCNT?were prepared by one-step hydrothermal method.The composites were characterized by energy dispersive X-ray analysis,X-ray diffraction,scanning electron microscopy,energy dispersive spectrometer,UV-Vis diffuse reflectance spectroscopy,electron spin resonance?ESR?,X-ray photoelectron spectroscopy and photoluminescence spectroscopy.SEM images show that MWCNT is successfully embedded into BiVO₄.BiVO₄@MWCNT has strong visible light absorption properties,high electron hole separation efficiency and good stability.RhB was degraded under visible light irradiation.The results showed that BiVO₄@MWCNT?K=-0.11894?achieved the highest RhB removal rate under visible light irradiation compared with BiVO₄?K=-0.05657?and P25?K=-0.03227?.The stability of BiVO₄@MWCNT was tested by recovery and reuse experiments.After 5cycles,BiVO₄@MWCNT can still maintain a RhB removal rate of 95.96%.In addition,the growth mechanism of BiVO₄@MWCNT and the degradation mechanism of RhB were also deduced.?3?BiVO₄ composite MWCNT Photocatalyst?BiVO₄@MWCNT?was prepared by one-step hydrothermal method.The composites were characterized by energy dispersive X-ray analysis,X-ray diffraction,scanning electron microscopy,energy dispersive spectrometer,UV-Vis diffuse reflectance spectroscopy,electron spin resonance?ESR?,X-ray photoelectron spectroscopy and photoluminescence spectroscopy.SEM images show that MWCNT is successfully embedded in BiVO₄.BiVO₄@MWCNT has strong visible light absorption performance and high electron hole separation efficiency.To further analyze its growth mechanism,PVP is first adsorbed and wound by MWCNT,thereby reducing the surface energy of the whole system and effectively preventing the agglomeration of MWCNT.After Bi?NO₃?₃ was added into the system,Bi³⁺was selectively adsorbed on the surface of PVP,and there was coordination interaction between metal ions and carbonyl groups of PVP.After adding NH₄VO₃,VO₃^-reacted with Bi³⁺on the PVP to form seed.After transferring the seed to high temperature hydrothermal reactor,the seed grew gradually.Finally,when BiVO₄ matures in a high pressure hydrothermal reactor and PVP is removed by calcination in a muffle furnace at400 C,it can be summarized as five steps:dispersion-adsorption-crystallization-growth-maturation.The effect of carbon source content on photocatalytic activity of photocatalyst was discussed.The results show that the photocatalytic activity follows the order of 0.2C-BiVO₄<0.6C-BiVO₄<1.0C-BiVO₄<1.8C-BiVO₄<1.4C-BiVO₄.It is indicated that the amount of carbon source will affect the photocatalytic performance of C-BiVO₄.Although carbon doping can shorten the band gap of BiVO₄,excessive carbon doping may lead to point defects in monoclinic BiVO₄ lattices,which will reduce the photocatalytic activity when acting as electron-hole composite sites.?4?On the basis of previous studies,C-BiVO₄@MWCNT?C-BiVO₄@MWCNT?was synthesized by two step method.It was found that all the catalysts were monoclinic,carbon elements were successfully doped in BiVO₄ and C-BiVO₄ was successfully coupled with MWCNT.The composite exhibited better catalytic performance than C-BiVO₄ and BiVO₄,and the adsorption of organic compounds was significantly improved compared with the samples without carbon nanotubes.The research and development of the ideal model for the design is realized,and the shortcomings of BiVO₄ are overcome.The research and development process of this photocatalytic system has certain theoretical and practical significance for the research and development of new photocatalytic materials.The effect of carbon source content on Photocatalytic Performance of photocatalysts was discussed.The results showed that the photocatalytic activity followed the order of 0.2C-BiVO₄<0.6C-BiVO₄<1.0C-BiVO₄<1.8C-BiVO₄<1.4C-BiVO₄.It is indicated that the amount of carbon source will affect the photocatalytic performance of C-BiVO₄.Although carbon doping can shorten the band gap of BiVO₄,excessive carbon doping may lead to point defects in BiVO₄ lattice,forming electron-hole composite centers,thus reducing the photocatalytic activity.The photocatalytic degradation mechanism of the process can be divided into 4parts.Finally,recycling experiments show that carbon-doped bismuth vanadate composite multi-walled carbon nanotubes have good stability and recyclability.In summary,based on the two main inherent defects of BiVO₄,the carbon-doped and multi-walled carbon nanotubes modified composite C-BiVO₄@MWCNT photocatalytic material,which can effectively overcome the inherent defects of pure BiVO₄,was successfully developed by combining theoretical design with experimental research and development.The formation and photocatalytic mechanism of C-BiVO₄@MWCNT photocatalytic material were preliminarily studied.