Self-induced Preparation And Performance Of TiO₂ Nanowires By Chemical Vapor Deposition On Glass Substrates

High-density single-crystalline TiO2 nanowires were successfully fabricated on Ti5Si3 layer by APCVD, using SiH4 and TiCl4 as the precursors, and N2 as diluted gas and protective gas. The produce included two steps. First, Ti5Si3 layer was deposited on glass substrates. Then, TiO2 nanowires formed on on Ti5Si3 layer by a self-induced thermal oxidation method. The correlations between the technological parameters(deposition tempeature, deposition time, total flow rate and molar ratio of Si/Ti) and the characterization of TiO2 nanowires were investigated systematically. The formation and growth of TiO2 nanowires were also clarified. The results showed that:(1)High-density single-crystalline TiO2 nanowires were successfully synthesized on glass substrates by self-induced APCVD. The inducing Ti5Si3 layer was deposited in a hexagonal structure and the nanowires with lengths of 0.5-2μm, diameters of about 20-40 nm and length-diameter ratios of over 25 were obtained. Ti O2 nanowires grew along the [001] direction of the tetragonal rutile TiO2 crystal from the bottom, with the tip being pushed upwards. This growth process can be defined as self-induced growth.(2)The crystal structure and surface morphology of TiO2 nanowires formed on Ti5Si3 layer were greatly influenced by technological parameters(deposition tempeature, deposition time, total flow rate and molar ratio of Si/Ti). With the deposition tempeature rising continuously, the nucleation and growth rates of TiO2 nanowires increased with higher crystallinity, higher density, larger length and bigger length-diameter ratio. The optimum deposition temperature was 720 oC, when the content of TiO2 crystalline phase was maximum and dense nanowires fabricated on Ti5Si3 layer with lengths of 0.5-2μm and diameters of about 20-40 nm.(3)The deposition time mainly affected the thickness and crystallinity of Ti5Si3 layers and then influenced the formation of TiO2 nanowires. With the deposition time increasing continuously, the Ti5Si3 layer had increased thickness, decreased grain size and smooth and compact surface, resulting in higher specific surface area. The reaction probability of Ti and O atoms became higher, which was help for TiO2 nanowires’ generation.(4)The variance of total flow rates influenced the flow speed from nozzle and the concentration of SiH4 and TiCl4, thus the crystallinity and surface morphology of the Ti5Si3 layer and self-induce formation of TiO2 nanowires were affected. With the increase of total flow rates, gas flow rate increased with higher deposition and atoms’ diffusion speed. Therefore, the Ti5Si3 layer had enough time to crystallize and dense surface, which was helpful to increase the content of TiO2 crystalline phase and form higher density and length-diameter ratio TiO2 nanowires.But when the total flow rate was too large(2000sccm), the flow speed was too fast and a turbulence region was formed between the nozzle and the glass substrate, which was not conducive to the nucleation of Ti5Si3. The sample was amorphous without TiO2 nanowires.(5)The molar ratio of Si/Ti was the key factor to control the formation of Ti5Si3 crystalline phase on the inducing layer, thereby affecting self-induced generation of TiO2 nanowires. The optimal molar ratio of Si/Ti was 1, when the Ti5Si3 layer had higher crystallinity and content. And we fabricated long and dense TiO2 nanowires.(6)The experiments showed that photocatalysis and hydrophilicity of TiO2 nanowires changes due to its surface morphology, grain size(specific surface area), structure and content of crystalline phase. TiO2 nanowires with higher density, crystallinity, specific surface area and content of crystalline phase showed best photocatalytic activity and almost 80% of methylene blue was finally degraded. Meanwhile, the nanowires favored the drop spreading and exhibited better hydrophilicity.(7)The synthesized composite film system of TiO2/Ti5Si3 exhibited excellent electrical properties: low square resistance and good electrical conductivity. The minimum of the square resistance was 0.723?/sq. The sample square resistance depended not only on the content and crystalline phase of TiO2 and Ti5Si3, but also on the surface morphology(bulk density, particle size and distribution) of samples. The higher density with uniform distribution and small particle size, the more TiO2 and Ti5Si3 crystalline phase per thickness formed in samples, resulting in better electrical conductivity.In conclusion, plenty of nanowires could be synthesized efficiently on various solid substrates with low facility request, which solves the immobilization of nanowires. By changing technological parameters, TiO2 nanowires with different morphology and composition could be obtained. When the deposition temperature was 720 oC, the deposition time was 60 s, the molar ratio of Si/Ti was 1, the total concentration of SiH4 and TiCl4 was 2.7% and total flow rate was 1000 sccm, TiO2 nanowires exhibited best crystalline, density and length-diameter ratio. Moreover, it exhibits best photocatalytic and hydrophilic performance, the degradation of methylene blue reached almost 80% after UV-irradiation, and the contact angle was only 0o.