Synthesis Of Oil-Soluble Nano-Titania And Their Formation Mechanism

Up to the present days, nano-sized particles of titania have attracted a great deal of attention, because of their wide application in antibacterial, dyes, food, cosmetics, catalysts, and solar cells. The widespread applications make the preparation extensively studied and a variety of methods have been developed to produce nano-titania, such as hydrothermal method, calcination of amorphous titania, sol-gel technique, and microemulsion method. However, the products obtained by hydrothermal method and sol-gel technique are hydrophilic, limiting the usage of titania in organic environments. The calcination of amorphous titania requires high reaction temperature and the products are easy to aggregate to cause defective sites on their surfaces, resulting in decaying of their optical properties. On the other hand, the products prepared from microemulsion have some excellent properties, such as narrow-size distribution, good dispersity and stability, but the output with a high cost is relatively low. Meanwhile, some oil-soluble titania nanoparticles are prepared by modification of water-soluble titania nanoparticles with coupling agent, which will affect the performance of the nano-particles.Oleophilic titania nanoparticles will greatly extend their application. The integration of the hydrophobic titania nanoparticles to the nylon fabric can improve their tensile properties. The coatings of cars doped with the hydrophobic titania particles will lead to the self-cleaning performance. Also, the hydrophobic titania nanoparticles could avoid certain bacteria breeding when added to the oil spill environment because of the bactericidal properties of titania. In this paper, two methods were adopted to prepare the oil-soluble titania, solvothermal and oil-water interface method. By the two methods, reaction conditions such as the pH value of water phase, reaction temperature, time, and concentration of surfactants were altered to investigate their effects on the properties of titania products. Furthermore, the formation process of titania NPs synthesized by the oil-water interface method was discussed. Additionally, the modification of titania was also studied.The main research results of this paper can be briefly described as follows: (1) With the solvothermal method, tetra-n-butyl titanate (TnBT) and dodecylamine (DDA) were used as titanium source and the surfactant, respectively. And, the cyclohexane was the organic solvent. Titanium hydroxide formed by the hydrolysis of TnBT was the precursor. The reaction conditions were altered to investigate their effects on particle morphology and particle size. The results showed that the mean particle size of titania was7.9-20.4nm when the aging temperature was higher than100℃and the aging time was longer then24h. The particle size was increased with the increasing aqueous solution pH and the reaction time but decreased with the high reaction temperature and the concentration of DDA.(2) With the oil-water interface method, TnBT and the fatty amine were used as titanium source and the surfactant, respectively. The oil phase prepared by dissolving TnBT and fatty amine in cyclohexane and the water phase was diluted by sodium hydroxide or perchloric acid solution to achieve the different pH. The experiments showed that the products were all oil-soluble when the surfactants were n-hexylamine, n-heptylamine, n-octylamine, dodecylamine, and hexadecylamine except that hydrophilic particles were obtained by the use of hexamethylenediamine. And the particle size beccame smaller with the longer carbon chain of the fatty amine. While the concentration range of n-octylamine was0.10-0.25mol/L, the yield of oil phase product was substantially100%, and the particle size was decreased with the increasing of n-octylamine concentration. However, more particles were in the water phase when the n-octylamine concentration was0.05mol/L. By the GC-MS detecting, the adsorption amount of n-octylamine was increasing with its rising equilibrium concentration of the oil-phase. And the adsorption amount was nearly saturated when the n-octylamine concentration was0.25mol/L. When the pH of water phase was from4.0to11.0, the crystal style of particulate was anatase-typed, and the particle size increased with the pH of water phase.(3) This paper preliminarily proposed the formation mechanism of titania, especially that of the oil-water interface method. The pH of water phase affected the concentration of the nucleation. The larger particles were formed with higher pH while the smaller particles were formed with lower pH. These differences were caused by the concentration of titanium hydroxide, more nuclei generated with higher titanium hydroxide concentration while fewer nuclei generated with lower titanium hydroxide concentration. The surfactant adsorption played the decisive role on the products dispersed in the oil phase. High reaction temperature and surfactant concentration could lead to fast adsorption of surfactant obtaining finer particles with narrow-sized distribution. Low reaction temperature and low surfactant concentration would lead to a slow adsorption, obtaining monodispersed particles.(4) Silver stearate was added as raw materials to be doped in titania with the oil-water interphase method, XRD and UV spectra of the products showed that the silver stearate was reduced in the reaction. And triethanolamine was added as stabilizer while the n-octylamine was the surfactant, the role of n-octylamine was inhibited by triethanolamine and larger particles were obtained. Phase transfer experiments proved that the surfactant n-octylamine was adsorbed on the surface of the particles, forming a hydrophobic end around the particles. The hydrophobic end resulted in the stable dispersion of titania particles in the oil phase.