Preparation And Application Of Environmental Functional Nanomaterials Based On Tio₂ Materials

Environmental pollution and energy shortage are seriously global problems. Applications of nanomaterials in purifying the contaminated water and energy exploitation are highly efficient and promising due to the unique electric, optic and catalytic properties. In this study, anodic TiO₂ nanotube (NT) arrays with large surface area, controllable pore size and length as well as the high orientation were prepared and designed with specific functions. The functionalized TiO₂ NT arrays were employed in the following applications: (i) anode materials of direct methanol fuel cells, (ii) photocatalytic degradation of organic pollutants and photocatalytic reduction of Cr(VI), (iii) sensors for detecting toxic heavy metal ions or persistent organic pollutants through taking advantages of the electric and optic properties of TiO₂ NTs. The details are summarized as follows:(1) Investigations on the self-organized growth of TiO₂ NT arrays by anodic oxidization. The formation mechanism of a thin film of self organized TiO₂ NT arrays prepared by anodic oxidization of a pure titanium sheet in electrolyte solutions containing potassium fluoride and sulfate was investigated through near-real time monitoring the anode mass, the current density, and the surface topography during the anodization. Energy dispersive x-ray spectrometry was used to monitor the surface composition change during the anodization. The titanium surface was first electrochemically oxidized to form a layer of dense oxide under which NTs were originated. With the protection of the oxide layer, long NTs could be formed in electrolyte solutions with relatively high pH. The surface composition analysis indicates that the NTs were not totally oxidized to TiO₂. However, no other elements but Ti and O were found in the oxide film. This work reveals a way to fabricate long NTs with defined sizes (in chapter 2).(2) Size-controllable fabrication of noble metal nanonets using TiO₂ template. Uniform NTs with defined size are obtained by controlling the anodic potential. Gold nanonets are prepared by electrodepositing Au on the template and then dissolving the TiO₂ template in a 0.2 M HF solution. The pore size of the Au nanonet is determined by the TiO₂ NT hole size which is determined by the anodic potential. Au nanonets with 50, 90, 140 nm in pore size were obtained using the TiO₂ NT templated anodized at 10, 15, and 25 V, respectively. Such nanonets are expected to have widely applications in chemo/biosensing, catalysis, molecule sieve and etc. due to their uniform nanonet structure, unique physical/ chemical properties, and perfect biocompatible characteristics (in chapter 3).(3) Carbon nanotube-guiding perpendicular growth of gold shrubs on TiO₂ NT arrays. A simple potential-static electrodeposition method was developed to fabricate shrub-like gold nanostructures on TiO₂ NT arrays assisted by acidified carbon NTs (CNTs). Au crystals were electro-stacked along the electric field lines that are perpendicular to the TiO₂ substrate, constructing beautiful shrubs on TiO₂ NT arrays. In this process the negatively charged CNTs attract Au3+ and work as primers guiding the Au growth along the direction of the electric field. The Au shrubs are single crystalline with intertwined symmetrical nanostructures. The as-prepared Au shrub-TiO₂ NTs exhibited more sensitive response in sensing highly toxic As3+ than Au films due to the higher surface area and unique three dimensional structures (in chapter 4).(4) Fabrication and catalytic properties of Co-Ag-Pt nanoparticle decorated titania NT arrays. In this work Co-Ag-Pt nanoparticles, approximately 20 nm in diameter, are electrochemically deposited inside the NTs and the catalytic properties of the resultant architecture examined using Mn2+/Mn³⁺ as the probe. The effect of Co, Ag and Pt loading content on the catalytic activity is investigated. In comparison to a Pt electrode, the oxidation potential of the Co-Ag-Pt/titania NT array electrode is negatively shifted by 93 mV and the peak current increased by a factor of nine (in chapter 5). PtAu nanoparticles were modified in TiO₂ NTs using the same electrodeposition technique. The resulting PtAu/TiO₂ NTs were applied as the anode electrode in direct methanol fuel cells and exhibited high catalytic activities in methanol oxidation reaction in alkaline media, which was attributed to the high affinity of the Au nanoparticles and titania NTs to the abundant -OH groups, and the synergistic catalytic effects of platinum-gold nanoparticles (in chapter 6).(5) Fabrication and characterization of Pt/C-TiO₂ NT arrays as anode materials for methanol electrocatalytic oxidation. Carbon-modified TiO₂ NT (C-TiO₂ NT) array is fabricated by depositing carbon in TiO₂ NTs. Well dispersed Pt nanoparticles (NPs) are electrochemically deposited on the C-TiO₂ NTs. The performances of the as-prepared NT array electrode in methanol oxidation reaction (MOR) as an anode are investigated. The result present in this study highlights such a finding: depositing partly graphitized carbon on the inside of TiO₂ NTs can significantly enhance the catalytic efficiency. An optimum forward oxidation peak current density (Ipf) of 71.6 mA cm^-2 is obtained from the Pt/C-TiO₂ NT anode at a low Pt loading of 23μg cm^-2. The achieved Ipf is almost 27 times that achieved on Pt modified TiO₂ NTs without carbon modification. The enhanced catalytic efficiency is mainly attributed to the superiorly electrical conductivity of the deposited carbon, which facilitates the well dispersion of Pt NPs, charge transfer during MOR, and removal of the by-product CO-like species (in chapter 7).(6) Graphitized carbon nanotubes form in TiO₂ NT arrays: A novel functional photocatalys with tube-in-tube nanostructure. Carbon layers were deposited in 8μm long TiO₂ NTs by the same way mentioned in chapter 7 except for the prolonging carbonization duration, resulting in the novel composites. Compared with unmodified TiO₂, coupled C-TiO₂ photocatalyst shows an enhanced efficiency of photodecomposing methyl orange process due to the increasing carrier rate and stronger adsorbability as well as the unique mechanical nanostructure. Furthermore, the transition from anatase to rutile was suppressed by carbon, resulting in a high content of the photoactive anatase, which also benefits the high catalytic activity of C-TiO₂ photocatalyst (in chapter 8).(7) Photocatalytic reduction of Cr(VI) on WO3 doped long TiO₂ NT arrays in the presence of citric acid. WO3 doped TiO₂ NT arrays were fabricated by annealing anodic titania NTs that were preloaded with peroxotungstic acid. Driven by electrostatic force, the negatively charged peroxotungstic acid sol uniformly clung to the positively charged titania NT arrays, causing a solid solution of WO3 and TiO₂ after annealing. The WO3/TiO₂ NTs were applied in the photocatalytic reduction of Cr(VI) in the presence of citric acid which served as sacrificial electron donor to deplete the photogenerated holes from photocatalysts. The net Cr(VI) reduction rate on WO3/TiO₂ NT arrays is 3.76 times that on the unmodified TiO₂ NTs. Since both WO3 and TiO₂ can be excited by UV light, the enhanced photoactivity of WO3/TiO₂ is attributed to the increased probability of charge-carrier separation and the extended photo-response spectrum in visible light region due to the doping of WO3 (in chapter 9).(8) Fabrication and application of ultra-fine Cu₂O nanowires modified TiO₂ NT arrays. Firstly, Cu particles were electrodeposited in the calcined TiO₂ NTs and on the arrays top surface. After that, the Cu/TiO₂ NTs were anodized in alkaline media, resulting in the ultra-fine Cu₂O nanowires modified TiO₂ NTs. The Cu₂O nanowires with less than 5 nm in diameter were connected together, constructing a network with huge surface area which facilitated the transfer of the photogenerated carriers. In comparison with the unmodified TiO₂ NT arrays, the Cu₂O/TiO₂ NT arrays exhibited enhanced photocatalytic activities in decomposing p-Nitrophenol due to the extended absorption in visible light region due to the narrow band gaps Cu₂O (2.1 eV) and the huge network construction (in chapter 10).(9) Fabrication and photocatalytic application of CdSe nanoparticles sensitized long TiO₂ NT arrays under green monochromatic light. CdSe nanoparticles with well dispersion were decorated on inner and outer surfaces of 4μm long TiO₂ NTs through a simple direct current electrotechnique, resulting in a composite functional material with a perfect construction. The CdSe/TiO₂ composite NT arrays exhibit high absorption in the visible light region due to the narrow band gap of CdSe, and depict sensitive photoelectrochemical response under visible light illumination. Photocatalytic degradation of anthracene-9-carbonxylic acid (ACA), one of the derivants of persistent organic pollutants (POPs), was successfully achieved on CdSe/TiO₂ NTs when exposed to the 550 nm green monochromatic light. Furthermore, CdTe nanoparticles with 10 nm diameter were electrodeposited on TiO₂ NT arrays, constructing an ideal fluorescent sensor. In the light of fluorescence resonance energy transfer, CdTe/TiO₂ NTs were applied in sensing benzopyrene, one of the persistent organic pollutants. The linearity range of the developed fluorescent sensor for the detection of benzopyrene observed is from 3.96×10^-7 to 3.96×10-12 M (in chapter 11).(10) Fabrication of Bi2X3 (X=Se, S) crystals with controllable morphology modified TiO₂ NTs and their applications. Single-crystalline, Bi₂Se₃ and Bi₂S₃ crystals with various morphologies were electrodeposited by applying the conductively organic electrolytes containing BiCl3 and selenum source. In addition, the mesoporous Bi₂S₃ crystals have secondary structure of small needles with 10 70 nm in length, 5 25 nm in width were electrodeposited on TiO₂ NTs. The Bi₂S₃/TiO₂ NT arrays showed enhanced photocatalytic efficiency in the co-detoxifcation of 2,4-diclorophenoxy acetic acid/Cr(VI) contaminated water specimen (in chapter 12).