Study On TiO₂-Based Nanotube Arrays With Enhanced Visible-Light Activity And ZnFe₂O₄ Nanospheres: Preparation And Their Properties

Photocatalysis has been considered as a promising route for elimination of environment pollutants in this century. The studies on the photocatalytic removal of organic pollutants in waste water and gases have become a growing field to meet the urgent needs of suppressing environmental deterioration for the sustainable development of human civilization. Among the photocatalysts, highly ordered TiO2 nanotube arrays fabricated by anodization have attracted significant interest due to their unique advantages including highly-oriented array structure, larger surface-to-volume ratio, directed charge transfer, and have been explored for applications in solar energy conversion, water splitting for generation of hydrogen and elimination of organic pollutants. However, the practical application of TiO2 nanotube arrays is significantly hindered by relatively low efficiency of light utilization due to its wide band-gap and poor quantum efficiency. A promising solution is to couple TiO2 nanotube arrays with narrow band-gap semiconductors to extend the absorption range of TiO2 nanotube arrays and enhance the charge carrier separation. Another promising solution is the development of novel visible-light-active photocatalysts. Considering what mentioned above, in the present work, Cu2O, ZnFe2O4 and Ag/AgBr loaded TiO2 nanotube arrays were prepared and well characterized. Meanwhile, the photoelectrochemical and photoelectrocatalytic properties were investigated. Additionally, a visible-light-active photocatalyst of ZnFe2O4 nanospheres was developed. In this dissertation, some works were carried out as follows:(1) The Cu2O-loaded TiO2 nanotube array electrode was prepared by electrochemical process of anodization, followed by a photocatalytic reduction method. The UV-Vis absorption edge of the TiO2 nanotube array electrode shifted to lower energy after Cu2O loading. The composite electrode exhibited a higher photovoltage response than the TiO2 powder electrode directly deposited on a Ti sheet. The highest photoconversion efficiencies observed for the Cu2O-loaded TiO2 nanotube array electrode were 17.2% and 0.82% under UV light and visible light irradiation, respectively. The composite electrode showed a higher efficiency than the non-loaded one for the photoelectrocatalytic decomposition of 4-chlorophenol. The improved photoeletrocatalytic activity of the composite electrode was attributed to the synergistic effect between Cu2O nanoparticles and TiO2 nanotube arrays. The byproducts were identified by high-performance liquid chromatography.(2) The ZnFe2O4-loaded TiO2 nanotube array electrode was prepared using a two-step electrochemical process of anodization and a cathodic electrodeposition method followed by annealing. ZnFe2O4 nanoparticles were highly dispersed inside the TiO2 nanotubes but minimized at the tube entrances. The composite electrode displayed a strong photo response in the visible region and low recombination rate of the electron-hole pairs. The dramatically enhanced photoelectrocatalytic activity of the composite electrode was evaluated in the decomposition of 4-chlorophenol and dichloroacetate under visible light irradiation. The improved photoelectrocatalytic activity was derived from the synergetic effect between ZnFe2O4 and TiO2, which promoted the migration efficiency of photogenerated carriers at the interface of the composite and enhanced the efficiency of photon harvesting in the visible region. The degradation of 4-chlorophenol was monitored by measuring Cl-concentrations and analyzing reaction intermediates by liquid chromatography-mass spectroscopy.(3) The ZnFe2O4-loaded high-aspect-ratio TiO2 nanotube array electrode was prepared via electrochemical anodization of Ti foil in NH4F and ethylene glycol organic electrolyte, followed by a cathodic electrodeposition method. The optimal experimental parameters for ZnFe2O4-loaded TiO2 nanotube arrays such as concentration, dipping time, deposition potential and number of repeating cycles were 0.05 M Zn(NO3)2·6H2O and 0.1 M Fe(NO3)3·9H2O,20 min,0.8 V and 10 times, respectively.The deposition of ZnFe2O4 was promoted in the TiO2 nanotubes but minimized at the tube entrances. Thus, pore clogging was prevented. Enhanced absorption in both UV and visible light regions was observed for the composite electrode. The current-voltage curve of ZnFe2O4-loaded TiO2 nanotube arrays revealed a rectifying behavior. The enhanced separation of photoinduced electrons and holes was demonstrated by surface photovoltage and photocurrent measurements. Meanwhile, the photoelectrochemical investigations verified that the ZnFe2O4-loaded TiO2 nanotube array electrode had a more effective photoconversion capability than the TiO2 nanotube array electrode alone.(4) The ZnFe2O4-loaded TiO2 nanotube array electrode was fabricated by a two-step process of anodization and a vacuum-assistant impregnation method followed by annealing. The ZnFe2O4 loading enhanced the probability of photoinduced charge separation and extended the range of the photoresponse of TiO2 nanotube array electrode from the ultraviolet to visible region. The photoluminescence of the TiO2 nanotube array electrode became suppressed, and the surface photovoltage response on the spectrum was significantly enhanced after the introduction of ZnFe2O4 nanoparticles. The transfer dynamics of the photoinduced charges were observed directly by a transient photovoltage measurement, which revealed a fast charge separation at the interface between ZnFe2O4 nanoparticles and TiO2 nanotubes upon light excitation.(5) Ag/AgBr-loaded TiO2 nanotube array electrode was synthesized by a two-step approach including electrochemical process of anodization and an in situ photo-assisted dipping and deposition technique. Ag/AgBr nanoparticles with well dispersion were effectively deposited on both the inside and outside of the TiO2 nanoubes. UV-Vis absorption spectra and surface photovoltage measurement approved that the Ag/AgBr loading enhanced the visible absorption of the TiO2 nanotube array electrode, as well as their separation efficiency of photoinduced electron-hole pairs, which may be due to the nanojunction built between Ag/AgBr and TiO2 in this system. The photoelectrochemical investigations verified that the Ag/AgBr-loaded TiO2 nanotube array electrode showed enhanced photocurrent generation efficiency and had a higher photoelectric conversion efficiency of 0.71% than the aligned TiO2 nanotube array electrode. The dramatically enhanced photoelectrocatalytic activity of the composite electrode was evaluated in the inactivation of E. coli under visible light irradiation.(6) ZnFe2O4 nanospheres photocatalyst with diameters of about 212 nm were synthesized via a one-pot hydrothermal method. The prepared nanospheres had cubic spinel structure and exhibited good size uniformity and regularity. The UV-Vis absorption edge of ZnFe2O4 nanospheres shifted to a higher energy compared with that of ZnFe2O4 nanoparticles. The ZnFe2O4 nanospheres exhibited remarkable surface photovoltage response in the UV and visible region, suggesting the enhanced separation ability of photogenerated electrons and holes. The dramatically enhanced photocatalytic activity of the ZnFe2O4 nanospheres was evaluated in the decomposition of rhodamine B under Xe lamp irradiation.According to these results, the fabricated Cu2O-loaded TiO2 nanotube array electrode, ZnFe2O4-loaded TiO2 nanotube array electrode, Ag/AgBr-loaded TiO2 nanotube array electrode and spherical ZnFe2O4 visible-light-activated photocatalyst can not only extend the absorption range of TiO2 nanotube array electrode but also enhance the separation efficiency of photo-generated electron-hole pairs, therefore improving the photocatalytic ability. The results are beneficial for further promoting the development of photocatalytic technology towards practical application.