Self-induced Preparation Of TiO₂Nanowires By Chemical Vapor Deposition

As a wide band gap semiconductor, TiO2not only has high inclusionperformance of full spectrum deep oxidation for all kinds of organic pollutants, butalso has the characteristics of good chemical stability, abrasion resistance, low-cost,non-toxic, so it has become one of the most potential photocatalysts. With thedevelopment of nano-technology, the research of the titanium oxide nanowires hasentered the people’s vision. Due to huge length-diameter ratio, titanium oxidenanowires have more excellent photocatalytic properties and hydrophilicperformances. Thus, the international academe is pretty serious about the research oftitanium oxide nanowires. In this paper, we used a novel self-induced method byCVD. High-density single-crystalline tetragonal TiO2nanowires have beensuccessfully prepared on the TiSi2layer by chemical vapor deposition, using SiH4andTiCl4as the precursors. The influencing factors, including deposition temperature,deposition time, Si/Ti molar ratio, total gas concentration and heat preservationvacuum time, were investigated respectively to find out their impaction on thestructure, crystalline phase and performance of TiO2nanowires. Results have shownthat：(1)TiO2nanowires have been successfully synthesised on the TiSi2layer. Thenanowires were approximately2μm long and the diameter was about20-40nm. Thetetragonal rutile TiO2nanowires grew from the bottom to top along the direction of[001], with the tip being pushed upwards. This could be defined as a self-inducedgrowth. The self-induction mechanism of TiO2nanowires expanded ourunderstanding about growing nanostructures(2)When the deposition temperature was lower, the nucleation rate of TiO2wasfast but the growth rate was low. The growth rate of TiO2increased as depositiontemperature rose. At700oC, the characteristic peaks of the TiO2showed up from theX-ray diffraction patterns. At720oC, we could find a large amount of titaniumdioxide nanowires on the TiSi2thin films. The length of the nanowires was about2μm and the diameter was about20-40nm. (3) When the depositing time of TiSi2was shorter, TiSi2thin films had a smallercompactness with good roughness of the surface, resulting in higher specific surfacearea. When the depositing time of TiSi2was90seconds, the oxygen atoms had morechance to contact with the titanium atoms, the crystal content and the surfaceroughness of TiSi2thin film were modest which was helpful for titanium dioxidenanowires generation.(4)According to the thermodynamic analysis, when the molar ratio of Si/Ti in thereaction was3, the Gibbs free energy of the reactiont was the lowest, the reaction waseasiest to take place, the TiSi2crystalline phase would be achieved most successfully,which was helpful for titanium dioxide nanowires generation. With the increasing ofthe molar ratio of Si/Ti, the content of TiCl4decreased and the content of SiH4increased, the characteristic peaks of the Si showed up from the X-ray diffractionpatterns.(5) With the total concentration of source gas increasing, the thickness of TiSi2layer and the crystal content increased, which lead to the peak becoming stronger.When the total concentration of source gas was2.7%, the the crystal content and thesurface roughness of TiSi2thin film were modest, the oxygen atoms had more chanceto contact with the titanium atoms. So we could find a large amount of titaniumdioxide nanowires on the TiSi2thin film. The length of the nanowires was about2μmand the diameter was about20-40nm.(6)With the prolongation of heat preservation vacuum time, the generatednanowires would continue to grow in the new locations after adsorbing atoms.However, with the reaction continuing, the Ti and Si source decreased, making littlecontribution on the length change of the nanowires. According to the experimentalresults, the most suited heat preservation vacuum time was120min.(7)Titanium dioxide nanowires were successfully prepared by APCVD with SiH4as the silicon source, TiCl4as the titanium source, N2as the carrier gas and the air asthe oxygen source. When the temperature was720oC, the deposition time was90seconds, the molar ratio of Si/Ti was3, the total concentration of the source gas was2.7%and the vacuumizing time with heat preservation was90minutes, there werelarge numbers of nanowires generated. The length of the nanowires was about2μm and the diameter was about20-40nm.(8)Titanium dioxide nanowires has higher roughness than the films, thehydrophilic contact Angle of TiO2nanowires was13.5°without the ultravioletirradiation, and the water droplets almost laid on the surface of TiO2nanowires afterthe uv irradiation. The titanium dioxide nanowires possessed large surface area, andthe degradation products could be absorbed more easily. On the other hand, theparticle semidiameter of the titanium dioxide nanowires was small, reducing thecompound rate of electron and hole in TiO2and improving the photocatalytic activityof TiO2. When the degradation time was180minutes, the degradation rate of themethylene blue by TiO2nanowires reached79.1%.