Synthesis, Assembly, And Properties Of Monodispersed TiO₂ Nanoparticles

For the study of material science, no matter in the field of nanometer or micro even macro areas, there is no doubt that size plays an outstanding role in determining the properties of materials. Nanoparticles have high surface activity due to their small particle size, large specific surface area and the lack of coordinated surface atoms. Agglomeration is often observed in the thermodynamic instability system of nanoparticles. The dispersion with small size and low-agglomerated TiO₂ nanoparticles is the preferred material for the preparation of TiO₂ film. Organic additives do help to the deagglomeration of nanoparticles. But, it can avoid the impurities of the film to prepare monodispersed TiO₂ nanoparticles without adding any organic surfactants, and can also ease the postprocessing and acquire TiO₂ film with high quality and excellent performance. On the other hand, the structures assembled from nanomaterials show excellent collective properties which are better than that of the bulk materials and nanomaterials themselves. It has been a hotspot in the field of nanotechnology.The research strategy of this thesis is the synthesis-assembly of TiO₂ nanoparticles. We study the dispersibility and film-forming ability of the nanoparticles and focus on the assemblies of one-dimensional TiO₂ nanowires, transparent porous objects and TiO₂ nanospheres, whose building blocks was TiO₂ nanoparticles. The main results are described as follows.1. Highly dispersed TiO₂ nanoparticles with small particle size have a higher surface activity, larger specific surface area and superior properties. In this thesis, TiO₂ nanoparticles were prepared by a reflux method at low temperature, and we use the cyclohexane/ethanol as mixed solvent for the first time. The reaction condition was adjusted by usage of concentrated hydrochloric acid, ratio of solvent, type of solvent, reaction time, acid source and titanium source, etc. The crystallite size of as-prepared anatase and rutile TiO₂ was 3.2 nm and 4.1 nm, respectively. The as-prepared anatase TiO₂ nanoparticles had a high dispersibility in water. No cloudy precipitates were observed in the colloid even when the concentration was as high as 1600 g/L. All the colloids were stable for quite a long time（at least six months） without sedimentation or delamination. The excellent dispersibility was ascribed to the electrical double layer at the solid-liquid interface and the water membrane formed on the surface of nanoparticles.2. By spinning coat the TiO₂-H2 O colloid on quartz substrate, the TiO₂ films were achieved. The anatase TiO₂ films exhibited high visible-light transmission and UV shielding property. Durable superhydrophilicity and photocatalysis activity made it to be a self-cleaning material.3. One dimensional structures and porous structures assembled from the highly dispersed TiO₂ anatase nanoparticles were acquired by treating the colloids under different conditions.（1） TiO₂ nanowires with several hundreds of nanometers in length were prepared by taking trometamol（NH2C（CH2OH）3） as the assembling agent.（2） After evaporating the TiO₂-C2H5 OH colloid at room temperature, transparent objects of micropores were formed. The nanoparticles dispersed in ethanol were homogeneously assembled into bulk materials without the formation of inhomogeneous aggregates.（3） Porous TiO₂ nanospheres were obtained through the aggregation of anatase Ti O2 nanoparticles by heating the TiO₂-H2 O colloid.The pore size of TiO₂ nanospheres was thermally adjustable, similar to that of the TiO₂ transparent porous objects. The assembled TiO₂ nanowires and porous TiO₂ nanospheres exhibited enhanced photocatalysis activity compared to TiO₂ nanoparticles. The transparent porous TiO₂ objects had the size of 2-5 times larger than the present products, and realized the fusion of the nanoscale world with that of materials of macro dimensions.