Study On Photoelectrocatalytic Capabilities Of Nonmetal Doped And Molecular Imprinted Polymer Modified TiO₂ Nanotube Array Electrodes

Nano titanium dioxide(TiO₂) has long been regarded as the most promising photocatalyst because of its outstanding photocatalytic(PC) activity and finds its wide applications in breaking down many kinds of organic pollutants.TiO₂ nanotube arrays prepared by anodization method possess great specific surface area,prominent light absorption ability and strong mechanical strength,and thus exhibit better PC performance and higher photoelectron conversion efficiency than TiO₂ nano film.Therefore,it is important to further improve its photocatalytic activity by reasonable doping or modification to realize practical application of TiO₂ nanotube arrays.It has been proved that nonmetal doping would be a viable way to improve the photocatalytic activity of TiO₂.Photoelectrocatalytic(PEC) oxidation can prevent the simple combination of photogenerated electrons and holes, consequently improving the PC efficiency of TiO₂ nanotube arrays.Considering what mentioned above,in the present work,nitrogen doping and boron doping TiO₂ nanotube array electrodes were prepared and characterized.Meanwhile,the PEC capabilities of these electrodes were investigated.Additionally,increasing the adsorption of the organic pollutants over TiO₂ nanotube is considered to be an important parameter in enhancing the degradation rates of PEC oxidation.Molecular imprinted polymer(MIP) has unique property of specific affinity for target compound.Therefore,the MIP-modified TiO₂ nanotube array electrode was prepared.Higher PEC activity might be obtained with improved adsorption capability of the electrode.In this dissertation,some works were carried out as follows:(1) The N-doped TiO₂ nanotube array electrodes were prepared by forming nanotube-like TiO₂ film in anodization process on Ti sheets and afterward being annealed under N₂ flow at 500 and 600℃.The electrodes presented compact array configuration with an average pore diameter of approximately 80 nm and the nanotube length of approximately 250 nm.The total nitrogen concentrations for electrodes prepared at 500 and 600℃analyzed by X-ray photoelectron spectroscopy(XPS) were 0.6 at.%and 0.9 at.%,respectively.X-ray diffraction(XRD) indicated that the crystal structure of N-doped electrode prepared at 500℃was pure anatase phase and the other N-doped one was a mixture of anatase and rutile phases. UV-vis absorption spectra(DRS) showed noticeable increase of visible light absorption for TiO₂ nanotube array electrodes due to nitrogen doping.Under visible light irradiation,the PEC degradations of PCP in 6 h on N-doped electrodes were both 8%higher than those on TiO₂ electrodes annealed under 500 and 600℃,respectively,and there was a synergetic effect between PC and electrochemical processes.At last,the possible visible PEC mechanism was discussed.(2) The boron-doped TiO₂ nanotube array electrode was prepared by forming a nanotube-like TiO₂ film in an anodization process on a Ti sheet,followed by chemical vapor deposition treatment,and was characterized by scanning electron microscope(SEM),XPS, XRD and DRS.The highly ordered vertically oriented nanotube arrays were obtained with the length of approximately 800 nm and there were ripples present in the side-walls of the tubes. Analysis by XPS indicated that the introduced boron was probably incorporated into TiO₂ and the chemical environment surrounding boron might be Ti-B-O.The anatase(101) and rutile (110) diffraction peaks were observed in the B-doped electrode and the CVD process led to an increase in crystal size of 10 nm.DRS showed stronger absorption intensity in UV region and the absorption edge shifted 20 nm to a lower energy.The B-doped electrode exhibited higher UV and visible photocurrent densities than TiO₂ electrode,and a notable photoconversion efficiency of 31.5%was achieved under UV light irradiation.Furthermore, the B-doped TiO₂ nanotube electrode exhibited higher PEC activity than non-doped one and contributed more to the UV and visible PEC degradation rate of PCP than PCP degradation efficiency.The boron-doped TiO₂ nanotube arrays were fabricated by potentiostatic anodization of titanium in an aqueous electrolyte containing fluoride ion and sodium fluoroborate(NaBF₄).This electrochemical method provides a one-step way to prepare B-doped electrode,which not only predigests the preparation procedure but also reduces the experimental cost.SEM images showed a barrier layer of approximately 30 nm at the end of the nanotubes and the addition of NaBF₄ had no effect on the morphology of TiO₂ nanotube arrays.XPS data indicated that the boron atoms were successfully incorporated into the TiO₂ matrix,forming Ti-B-O bond in the sample with small amount of boron(1.5 at.%),and the chemical environment surrounding boron was more similar to that in B₂O₃ in the samples with larger amounts of boron(3.1 at.%and 3.8 at.%).The B-doped TiO₂ nanotube arrays with a mixture of anatase phase and very little rutile phase were identified by XRD.Red shifts and enhanced absorption intensities in both UV and visible light regions were observed in the spectra of UV-vis absorption of B-doped samples,especially the B-doped electrode with 3.1 at.%of boron.Under the same experimental conditions,B-doped nanotube array electrodes showed improved photocurrent densities and visible PEC degradation of atrazine.By comparison,the sample with 3.1 at.%of boron exhibited the best PEC performance.(3) The modified TiO₂ nanotube array electrode was prepared by coating a thin layer of molecular imprinted polymer(MIP).Its surface was well structured with compact voids and the average pore diameter decreased from approximately 80 nm to 50 nm.A distinguishable red shift in the absorption spectrum was observed.The maximum of adsorption capacity at equilibrium condition was about 34.88 ng for the MIP-modified sample,which was nearly 1.5-fold of that for the TiO₂ nanotube arrays.Photocurrent was generated on the MIP-modified photoanode and increased with the increase of positive bias potential under simulated solar light irradiation.Due to the better adsorption capability of MIP-modified TiO₂ nanotube array electrode,it increased not only the TC removal efficiency but also the mineralization of TC in the PEC process.In addition,the TC removal on nonimprinted-modified electrode was lower due to its poor adsorption of the target compound, and the thick MIP layer would weaken the light more deeply,which was unfavorable to PEC degradation.