Preparation And Photoelectrocatalytic Avtivity Of Doped TiO₂ Nanotube Photoelectrodes Under Visible Light

Titanium dioxide (TiO₂) has been widely utilized in the environmental protection as the photocatalyst for the degradation of organic pollutants due to its low cost, non-toxicity, excellent stability and without secondary pollution.However, the wide band gap energy of TiO₂ catalyst (3.2 eV) limits this photocatalyst's application in the process of environmental protection. Photoelectrode doping could effectly decrease the band gap energy of TiO₂ by doping with transition metal elements, which could extend the response of TiO₂ to visible light.As one of the typical Environmental Endocrine Disruptors (EEDs), Nonyl Phenol (NP), has attracted great attention owing to their widely used in the manufactures of detergent, textile and papermaking, which are long-standing and bio-refractory organic compounds. It is of significant importance to develop new treatment technologies for the degradation of NP in the environment.In this work, the TiO₂ nanotube(TNT) photoelectrodes were prepared by means of anodic oxidation. W doped TNT photoelectrodes were fabricated via electrochemical deposition, which could extend the response of TiO₂ photocatalyst to visible light. The present work also studied the degradation of endocrine disrupting chemicals NP using W/TNT photoelectrodes.TNT photoelectrodes were prepared by means of anodic oxidation process with post-calcination. The effect of anodization conditions including anodic voltage, anodic time and clacination temperature on the morphology and crystal structure of photoelectrodes were studied. Scanning electronic microscopy (SEM), X-ray diffraction (XRD) and Energy dispersive X-ray detector (EDX) were used to characterize the morphology and crystal structure of photoelectrodes.Photocatalytic activity of the TNT photoelectrodes was evaluated in terms of the degradation of NP in aqueous solution. The effects of the preparation conditions on the photocatalytic activity were investigated in detail. The results indicated that the preparation of TNT electrodes with ideal size and structure were controllable in terms of adjustment of the anodization parameter.The W-doped TNT photoelectrode was prepared on TNT photoelectrode substrate by electro-deposition method. The doping conditions of W/TNT electrodes were optimized. The photocatalytic activity of W/TNT electrodes were evaluated in detail. The results showed that the optimal preparation conditions were deposition voltage 3V, electrode spacing 1cm, (NH4)2WO4 concentration 1.0g/L, deposition time 10min and calcination temperature 550℃.The results of characterization showed that W6+ concentrates on TiO₂ crystal lattice, hindering both the crystal growth and anatase to rutile transition. Meanwhile, the W6+ may be incorporated into the titania lattice and replaced Ti4+ to formW-O-Ti bonds or located at interstitial sites.Significant red-shift in the spectrum of UV-vis absorption was observed. The degradation of Rhodamine B showed the W/TNT photoelectrodes exhibit excellent photoelectrochemical property and photocatalytic activity under visible light, compared with non-doped TNT photoelectrodes.W-doped TNT photoelectrodes were used for the photoelectrocatalytic(PEC) oxidation of endocrine disrupting chemicals——NP. The effects of anodic bias potential, initial pH, and initial concentration of NP on the PEC degradation of NP were investigated. TOC analysis was carried out to evaluate the mineralization of NP on the W/TNT by the PEC treatment.It can be seen that at nearly neutral nature pH, NP shows the highest degradation efficiency. The result indicates that the degradation efficiency is decreased with increasing the initial concentration of NP. It is observed that the degradation efficiency of NP is increased with increasing the bias potential from 0 to 2.0 V. However, when the potential was further increased to higher than 2.0 V, the PEC degradation rate was reduced. The increase of catalyst area was beneficial to the degradation of NP.In the process of NP degradation, the lower TOC removal efficiency means that many NP molecules are actually degraded to intermediates instead of mineralized to CO₂ and H2O.