Preparation And Properties Of TiO₂ Nanotube Arrays And Nanowires

TiO₂,with a wide direct band gap 3.0eV(rutile) and 3.2eV(anatase), is an important n-type semiconducting material. High-aspect-ratio TiO₂ nanotubes and nanowires who possess favorable photocatalyse and sensing properties, are attracting broad attention. In this thesis,we thesis,we prepare TiO₂ nanotubes films and investigate their optical absorption and humidity sensing properties; synthesize TiO₂ and Na₂Ti₃O₇ nanowires and study their humidity sensing properties. A series of important results are obtained.In this paper, high-ordered P-doped TiO₂ nanotube arrays films with the tube diameters of 20-100nm have been prepared by electrochemical anodize method in the mixture electrolyte of HF and H3PO4. Through the measurements of TiO₂ nanotubes films by scanning electron microscopy(SEM) and X-ray diffraction, it was found that the TiO₂ nanotubes we prepared is uniformed and ordering tubes arrays. The tubes vertically growth on the Ti plate, and parallel each other. FESEM micrographs show that the tube diameters are increased with the increasing voltages. The diameters of tubes formed in 1% H3PO4 and 0.5% HF, at 10V, 15V, 20V and 25V for 2h, are about 20nm, 70nm, 100nm and110nm. When the voltages increase to 27.5V and 30V, the nanotubes morphologies are destroyed. The TiO₂ nanotubes formed in 1% H3PO4 and 0.5% HF at 20V voltage for 2h are uniform and arranged orderly, so it is the perfect condition for the synthesization of TiO₂ nanotubes. The unannealed nanotubes is amorphous, the sample annealed at 300℃is anatase and the rutile appears in the sample annealed at 600℃.With the increasing the amount of H3PO4 from 1% to7%, the primary peak of anatase shifts from 25.43°to 25.3°.The studies on optical absorptions of the TiO₂ nanotubes indicate many conclusions. The optical absorption edge of the sample formed in 1% H3PO4 and 0.5% HF shows t red-shift comparing with that of the sample formed in 0.5% HF. This phenomenon indicates that the use of H3PO4 could change of band gap of TiO₂ and further induce the red-shift of the optical absorption edge. With the increasing voltage, the optical absorption edge also show obvious red-shift. The possible reason is the applied voltage could influence the dopant in the TiO₂, and induce the optical absorption edge shifting to longer wavelength. The absorption in ultraviolet area is obviously enhanced with more H3PO4 being used.The studies on the humidity sensing properties of the sensor made of TiO₂ nanotube arrays show the conclusion as follow: The novel sensor based on TiO₂ nanotube arrays is effective. The sensors based on the samples annealed at 400℃, 500℃and 600℃show different response to the change of relative humidity. The sample annealed at 600℃show the highest sensitivity with the two roders change in resistance at 11-95 % RH.TiO₂ and Na₂Ti₃O₇ nanowires were synthesized by a simple hydrothermal method. Through the measurements of FESEM and TEM, it is found that the nanowires are of typical lengths ranging from several micrometers to several tens of micrometers. The average diameter of the nanowires is found to be about 100 nm, some nanowires form bundles of～500 nm in diameter and some of the nanowires lay close to each other to form bundles. The XRD patterns show the direct production of hydrothermal reaction is Na₂Ti₃O₇ nanowires, after treating by dilute HCl, the H+ replace the Na+ to form the H2Ti3O7 nanowires. The H2Ti3O7 nanowires which is annealed at high temperature would change to TiO₂ nanowires.We fabricated the humidity sensors based on TiO₂ and Na₂Ti₃O₇ nanowires.and investigated the humidity sensing properties of these sensors. It is found that both the two sensor show high sensitivity to the change of relative humidity from 11 to 95% at low frequencese. The response time of TiO₂ and Na₂Ti₃O₇ nanowires sensors is 9s and 5s, the recovery time is 13s and 6s. It indicates that the Na₂Ti₃O₇ nanowires shows more rapid response to the humidity change than TiO₂ nanowires. There are not obvious change on sensitivity, response and recovery time of Na₂Ti₃O₇ nanowires sensors, which indicate its humidity sensing properties is better than TiO₂ nanowires sensor. It can be noted that the sensing element exhibits a narrow hysteresis loop during cyclic humidity operation. The maximum humidity hysteresis is around 3% RH under about 80% RH, <5%, is up to the mustard of the normal humidity sensor. In the test of humidity response of the sensor at different temperatures, the curves of resistance vs. humidity show parallel shift. The average temperature coefficient between 15 and 35 oC is about 0.2 % RH / oC in the humidity range of 11–95 % RH. But there is no obvious change on the sensitivity, response and recovery time.