Study On Preparation Of Modified TiO₂Nanobelts And Photocatalytic Degradation Of Levofloxacin In Water

TiO2photocatalysis, which can eliminate contaminants from environment, is a promising technology for environment cleanup. To improve the quantum efficiency and utilization efficiency for visible light of TiO2photocatalyst, the surface microstructure and crystal structure of TiO2photocatalyst were optimized and the transfer of photogenerated carriers and utilization efficiency for visible light were enhanced by changing preparation method and deposition. The prepared TiO2photocatalysts were researched by a series of characterization methods. In addition, the application of prepared TiO2photocatalysts in removing emerging contaminants was assessed by degrading levofloxacin. This work aims to promote the practical application of TiO2photocatalytic technology in water treatment.The photocatalytic activity of TiO2photocatalysis can be significantly influenced by its microstructure. Therefore, the immobilized TiO2nanobelts (TiO2NBs) photocatalysts were prepared through modifying the electrolyte composition, oxidation voltage, oxidation time of anodic oxidation process and calcination temperature. The optimal synthesis parameters for TiO2NBs were determined, that is, the electrolyte is ethylene glycol containing ammonium fluoride (NH4F,0.5wt%) and water (5vol%), the oxidation voltage is60V, the oxidation time is3h and calcination temperature is550℃. A series of characterizations were conducted to study the physicochemical properties of prepared samples, such as, TEM, SEM, XRD, XPS, UV vis/DRS, Raman, PL, FT IR and photoelectrochemical detection. Results show that the as synthesized samples are typical belt like structure, the single nanobelt is about50nm in width, several micrometres in length and several nm in thickness. The crystal structure of TiO2NBs is single crystalline anatase, and the major exposed surfaces of TiO2NBs is (101) facet. Furthermore, the TiO2NBs exhibits enhanced accessible surface area. The as prepared TiO2NBs exhibited significant enhancement in photocatalytic activity under simulated solar irradiation. The formation machenism of TiO2NBs was studied in depth, results show that the formation of TiO2NBs is the combined result of field assisted etching and chemical dissolution based on the nanotubes. Due to the presence of F and H+and higher voltage at the mouth of NTs, field assisted etching occurs at the statistically distributed ‘‘breakdown sites”. As the anodization time increasing, the NTs split along the vertical tube axis to form NBs from the top. The lateral deflection was formed owing to the action of capillary forces between adjacent NBs, leading to deflections of NBs and bundled NBs. The crystal structure, light absorption, PL, FT IR and photoelectrochemical performance are effected by calcination, leading to the change in photocatalytic property, the sample calcined at 550℃showed the highest photocatalytic performance.Semiconductor coupling is an effective method to improve the photocatalytic activity of TiO2. The CuS and CdS CuS composite nanostructure were decorated on the surface of TiO2NBs by successive ionic layer adsorption and reaction method to improve the utilization efficiency of TiO2NBs for visible light and enhance the photostability of CdS. The operating parameters were optimized: the concentration of Cu(NO3)2, Cd(NO3)2and Na2S is1mM, deposition time is10min, cycle index is3for CuS and5for CdS CuS. The modified TiO2photocatalysts were detected by TEM, SEM, XRD, XPS, UV vis/DRS, PL and photoelectrochemical measurement. In addition, the photocatalytic performance of TiO2photocatalysts was evaluated. Results show that the absorption of visible light and transfer of photogenerated electron holes are significantly improved by decoration of CuS, consequently, resulting in the enhancement in photocatalysis degradation efficiency of TiO2NBs. The gradient band gap structure was formed after the deposition of nanocomposites of CdS CuS on the surface of TiO2NBs, which further improve the utilization efficiency for visible light and transfer of photogenerated electron holes, thereby, leading to further improvement in photocatalytic activity. Moreover, the deposition of CdS CuS nanocomposites boosts obviously the stability of CdS. Compared with TiO2NBs, the photocatalytic efficiency of CuS/TiO2NBs and CdS CuS/TiO2NBs was increased by5%and10%, respectively.In order to further improve the separation efficiency of photogenerated carriers, utilization efficiency for visible light and stability of TiO2NBs, the Au and Au Pd deposition were carried out by electrochemical deposition, and the operating parameters were optimized: the concentration of HAuCl4and PdCl2is0.1mM, applied voltage is0.2V, deposition time is10s for Au and5s for Pd. The as prepared samples were researched by TEM, SEM, XRD, UV vis/DRS, PL and photoelectrochemical detection. Results show that the Au and Au Pd nanoparticles are uniformly distributed on the surface of TiO2NBs, and the particle diameter of Au and Au Pd nanoparticles is approximately310nm. The metal Pd particles were interspersed on the surface of metal Au particles to form composite morphology. It is clearly that the spectrum of Au/TiO2NBs has a broad absorption peak centered at570nm, which is assigned to the characteristic surface plasmon absorption of Au nanoparticles. The absorption for visible light and transfer of photogenerated electrons are improved by Au and Au Pd nanoparticles modification, therefore, the photocatalytic property is enhanced. Compared to TiO2NBs, the photocatalytic efficiency of Au/TiO2NBs and Au Pd/TiO2NBs was increased by12%and18%, respectively. Moreover, the enhancement mechanism of photocatalytic property was analysed.Finaly, in order to assess the application of the synthetic photocatalysts in water cleanup, emerging contaminants—levofloxacin—were choosed as the target pollutant for photocatalytic degradation by optimal Au Pd/TiO2NBs photocatalysts. Results show that approximately97.1%of the levofloxacin molecules were decomposed by Au Pd/TiO2NBs after irradiation of60min under simulated sunlight, and the decomposition rate can reach94.0%after100min visible light irradiation. The degradation of LEV was primarily driven by OH and O2. The influence factors on photocatalytic degradation, such as the common anion in natural water, initial concentration and initial pH, were researched. In addition, the photocatalytic efficiency of Au Pd/TiO2NBs decreased approximately by3%after5cycles.