TiO₂and Mn₃O₄Nanocrvstals:Controllable Crystal Growth And Properties

Controlling the morphology of nanocrystals and exploring the relationship between their morphologies and properties are the most interesting topics in material science and technology. Controlling the nanocrystal growth process is essential for its morphology tuning, property designing and functionalizing. In mesoscopic scales, crystal growth process normally follows the classical mechanism "Ostwald ripening" and the non-classical mechanisms, including "Oriented attachment" and "Mesocrystal". Generally the non-classical crystal growth mechanisms are based on the assembly and fusion of primary nanocrystals. Providing innovative pathways and diverse possibility for controlling the morphologies of nanocrystals.Anatase TiO2nanocrystals and hausmannite Mn3O4nanocrystals are two typical transition metal oxide nanomaterials. In this article we investigated the influence of crystal growth mechanism on the morphology of nanocrystals, aiming at solving the related problem on the crystal growth controlling. The potential applications of nanocrystals with specified morphology on photocatalytic degradation of organic pollutants, dye-sensitized solar cells and lithium-ion batteries were investigated. The thesis consists of following contents:1. The TiO2nanocrystals were synthesized by solvethermal method, using halogen ion and oleic acid as surfactants to control the morphology. The two surfactants showed distinct absorption capability on specified facets, tuning the growth rate of different facets. Thus, with the help of surfactants, the morphology of TiO2nanocrystals could be effectively controlled. It was also found that fluoride had stronger tendency to selectively absorb on (001) facet compared with other halogen ions. Using fluoride as surfactant, truncated nanobipyramids were obtained, surfaced by two (001) and eight (101) facets.2. A two step synthetic strategy has been developed to prepare one-dimension chain-like TiO2nanocrystals via oriented attachment process. The primary crystals followed oriented attachment and were able to assemble in solution and fuse along specified facets, forming one-dimension chain-like TiO2nanocrystals with single crystal characteristic. The two-step strategy employed in the experiment effectively separated the Ostwald ripening and oriented attachment growth process. The different absorption capability of fluoride and oleic acid on different facets of TiO2 nanocrystals reduced steric hindrance on (001) facet, facilitating oriented attachment growth. It is worth noticing that relative high temperature drives the crystals to overcome the energy barrier for fusion. Moreover, the hydroxyl in the reaction system has great influence on the length of TiO2nanochains.3. Applying TiO2nanochain to enhance the photoelectric conversion efficiency in dye-sensitized solar cells (DSSCs). TiO2nanocrystal film is the most important component in DSSCs. It has been proved that one-dimensional TiO2nanochains with single crystal structure possessed high electron conductivity. In this work, TiO2nanochains and nanoparticles were mixed to prepare nanocrystal films. TiO2nanochains could reduce the possibility of scattering and annihilation of electrons in the transmission process, thus increasing the conductivity of the film. It was found that the addition of30～40wt.%TiO2nanochains in the film increased the photoelectric conversion efficiency to7.5%, which is a78%increase comparing with nanoparticle films (4.2%).4. Preparation of Mn3O4mesoscopic crystals and mesoporous crystals through non-classical crystallization process. The synthesis was carried out at room temperature via chemical precipitation. Mn3O4nanocrystals with uniform morphology were obtained using hydrazine as precipitation agent. The size and morphology of Mn3O4could be effectively controlled by adjusting experiment conditions. The growth process of mesoporous nanocrystals was monitored. It was found that based on the assembly and fusion of primary nanoparticles, nanocrystals with mesoscopic characteristic were formed, then a ripening process occurred, resulting mesoporous nanocrystals. It was also found that the magnetic properties of Mn3O4nanocrystals were affected by the size and morphology.5.Applying Mn3O4mesoscopic and mesoporous nanocrystals as the cathode for high-performance lithium-ion battery. The high porous rate in nanocrystals could mitigate the volume change cause by insertion of lithium ions. Mn3O4/graphene oxide composites were prepared to compensate the conductivity. Such nanocomposites exhibit high performance in lithium-ion batteries...