9.3 Titanium Dioxide (TiO)
Figure 1.15 SEM images of TiO2 nanotubes prepared in (A) 0.1 M HF acid solution at 20 V (B) 1.0 M NaHSO4 containing 0.1 M KF at 20 V and (C) ethylene glycol containing 0.25% NH4F at 60 V for 1 h . Reproduced by kind permission from the publisher.Highly ordered transparent TiO2 nanotube arrays produced by electrochemical anodization have been used in DSSCs. It suggests superior electron transport in nanotubular TiO2 based DSSCs . Remarkable photoconversion efficiencies were expected to be obtained with increase in the length of the nanotube arrays. Carboxylated polythiophene derivatives can be self-assembled onto the TiO2 arrays produced by anodizing titanium foils in ethylene-glycol-based electrolytes . Such self-assembled hybrid polymer-TiO2 nanotube array heterojunction solar cells can yield power efficiency of 2.1% under AM 1.5 without dyes. It was found that the formation mechanism of TiO2 nanotubes is similar to the porous alumina case under high electrical field. TiO2 nanotube arrays can be fabricated by anodic oxidation of titanium foil in
different electrolytes. The produced TiO2 nanotube arrays possess large surface area and good uniformity and are ready for enzyme immobilization [165-167], which can be used as biosensors. Furthermore, different length of TiO2 nanotube arrays fabricated by anodic oxidation in different electrolytes were studied for their sensitivities to hydrogen peroxide after co-immobilized horseradish peroxidase (HRP) and thionine chloride. The nanotube arrays fabricated in potassium fluoride solution has the best sensitivity to H2O2 with a detection range from 10-5 to 3×10-3 M .With the use of the template of an AAO, TiO2 nanowires can be also obtained by cathodic electrodeposition [168,169] where the metallic ions are attracted to the AAO cathode electrode and reduced to metallic form. For example, in a typical process, the electrodeposition is carried out in 0.2 M TiCl3 solution with pH = 2 with a pulsed electrodeposition approach, and titanium and/or its compound are deposited into the pores of the AAO. By heating the above deposited template at 500°C for 4 h and removing the template, pure anatase TiO2 nanowires can be obtained. In addition, highly ordered TiO2 single crystalline (pure anatase) nanowire arrays can be fabricated within the pores of anodic aluminum oxide (AAO) template by a cathodically induced sol-gel method . During this electrochemically induced sol-gel process, both the formation of sol particles and the gelation process take place in the AAO pores. Therefore, TiO2 nanowires with very small diameters (less than 20 nm or even smaller) can be obtained by this technique. In addition, the length of the nanowires can be well controlled by varying the deposition time and potential of the working electrode. 1.9.4 Electrochemical Fabrication of Soft Matters in
Nanoscale Using TemplatesConducting polymer nanowires are promising one-dimensional nanostructured materials for application in nanoelectronic devices and sensors [171,172] due to their light weights, large surface areas, chemical specificities, easy processing with low costs, and adjustable transport properties. Nanofiber, nanospheres, and other nanoscales of soft matters such as conducting polymers have been fabricated in traditional chemistry way and/or via self-assembly
[173-182]. Basically, 1D conducting polymer nanostructures can also be synthesized chemically or electrochemically by using “hard” and “soft” template methods. Obviously, the hard-template method (e.g., AAO) is an effective and straightforward route for fabricating conducting polymer nanostructures with diameters determined by the diameter of the pores in the template. Controlled conducting PANI nanotubes and nanofibers have been fabricated in the AAO templates and find promising applications in lithium/PANI rechargeable batteries . Nanotubes and nanowires of conducting polymers, including PANI, polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT), can be synthesized by electrochemical methods using the AAO templates . By changing the doping level, dopant, and template-dissolving solvents, the electrical and optical properties of the nanotubes and nanowires can be controlled. The diameters of the conducting polymer nanotubes and nanowires are in the range 100-200 nm, depending on the diameter of the nanoporous template used. It was found that the polymerization was initiated from the wall-side of the AAO template. The synthesized nanotubes have an open end at the top with the filled end at the bottom. As polymerization time increases, the nanotubes will be filled and nanowires will be formed with the length increased. For example, PPy nanotube can be synthesized by applying current of 2-3 mA for 1 min. When the time is increased to 15-40 min, PPy nanowires will be produced. Conducting polymer nanotube and nanowires prepared by this electrochemical method using AAO templates can be applied in field emitting applications [185,186]. Figure 1.16 shows a uniform layer of polyaniline nanowires produced at constant potential at 1.0 V for 10 min through the AAO template .PPy nanotubes and nanowires can be also electrochemically synthesized through nanoporous AAO template in ionic liquids . The electrolyte consisted of pyrrole monomer, solvent, and room temperature ionic liquid dopant such as 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM][BF4]) and 1-butyl-3methyl imidazolium hexafluorophosphate ([BMIM][PF6]), which is stable in air and moisture. The length and diameter of PPy nanotubes and nanowires were determined by the synthetic conditions such as polymerization time, current, and dopant. The
formation of nanotube and nanowire of PPy sample was confirmed by using field emission scanning electron microscope and transmission electron microscope. Formation of PANI nanotubules in room temperature ionic liquids by means of electrochemical polymerization without any template has also been reported . PANI nanotubules were synthesized electrochemically on a modified ITO glass in [BMIM][PF6] containing 1M trifluoroacetic acid. Tubular structures of PANI with the diameter of ca.120 nm were shown by scanning electron microscopy.