Controllable Self-assembly Of The Nanostructured Titanium Dioxide And Their Performances

The micro/nano-structure design of self-assembled materials and the precise control of self-assembly behavior is one of the great themes in the research field of functional materials. The synergy of weak interaction between subunits will lead to the crystallographically oriented assembly, and therefore mesocrystalline materials with defined structure can be obtained. We can regulate the material transport, energy transmission and chemical conversion by this way. Furthermore, the system functional design can be realized. Mesocrystal is a new class of unique superstructure material, which can be regarded as the assemblies of crystallographically oriented nanocrystals. In general, they have single-crystal-like structures but with much higher porosities than conventional single crystals. Titanium dioxide (TiO2), as one of the most promising semiconductor materials, has exhibited wide application prospects in photocatalysis, dye-sensitised solar cells and lithium ion batteries applications owing to its unique band structure and chemical stability. In this paper, the research is carried out based on the line of hierarchical architecture-controllable preparation of mesocrystals-structure control-performance optimization. This paper will reveal the key factors that influence the self-assembly of nanocrystals and the formation of mesocrystals, clarify the role of surfactants in tuning the morphology, size and exposed facets of TiO2 mesocrystals and deepen the relationship between self-assembled structure and properties. The main research contents and results are as follows:1. A facile method of the preparation of TiO2 hierarchical structures by controlling the assembly behavior of nanocrytalline subunits under different reaction solvents has been proposed using titanium sulfate as the Ti-source, and the resultant TiO2 hierarchical materials showed excellent electrochemical and photoelectric performances. In the Ti(SO4)2-HAc system, TiO2 nanocrystals can be spontaneously assembled to form spherical hierarchical structure under the interaction between SO42-and acetic acid. In addition, we realized size-controllable synthesis of hierarchical TiO2 just by tuning reaction time and Ti(SO4)2 concentration. When used as the anode in Li-ion batteries, these TiO2 hierarchical structures present much higher specific capacity and better rate performances than TiO2 nanoparticles, which are attributed to the stability of the secondary-structure during the charge and discharge processes. The capacity the hierarchical TiO2 is 170 mAhg"1 after 50 cycles at 1C, double than that of TiO2 nanoparticle. Based on the inside-out Ostwald ripening process, the TiO2 hollow hierarchical microspheres (HTS) were synthesized via a new one-pot template-free method in the Ti(SO4)2-EtOH-HCl reaction system. By investigating the growth mechanism, we found that the formation of hollow structure was determined by the relative rates of the amorphous particles dissolution and the crystalline phase nucleation, and ethanol solvent was selected to balance the two rates. HTS exhibited amazing structure stability even at high temperature calcinations of 800℃. As a new functional material in the DSSC application, HTS simultaneously offer incompatible features such as a high specific surface area and a pronounced light-scattering effect, and hence an energy conversion efficiency of 7.3% can be achieved.2. The key to fabricate TiO2 mesocrystals is to precisely control the raction activity of Ti-sources and the assembly behavior of TiO22 nanocrystalline subunits by the contrastive analysis of growth process in the two systems, TBT-HAc and TiCl4-HAc. In the TBT-HAc:the complexation reaction between TBT and HAc reduces the reactivity of Ti-source, the hydrolysis rate is controlled by esterification reaction because no hydrolysis agent is added into the reaction system, the primary nanocrystals attached to the organic matrix which can induce the oriented self-assembly. However, in the TiCl4-HAc, the hydrolysis rate is controlled by TiCl4 concentration. The controllable synthesis of TiO2 mesocrystals (MCs) can be realized just by keeping the TiCl4 concentration a liitele more than that of the preparation condition of TiO2 single crystal (SCs). In addition, if the angle of orientation mismatches between nanocrystals is small, the polycrystals (PCs) can be further transferred into mesocrystals by increasing reaction time. Owing to the unique structures constructed from crystallographically oriented nanocrystals, MCs offer larger specific surface area than SCs and more effective electron transport compared with PCs. As anticipated, MCs will exhabits outstanding photoelectric property, the photoelectric conversion efficiency of MCs is about 1.5 times of P25 and the rate constant is almost 3.6 times of the SCs.3. The structure control of TiO2 mesocrystals including morphology, particle size and exposed facets can be realized with the assistant of surfactants, and the as-prepared TiO2 mesocrystals with various architectures exhibited fascinating lithium storage and photocatalytic performances. In the condition of benzoic acid, the morphology of TiO2 mesocrystals evolved from spindle to sphere owing to that the selective adsorption of carboxyl, steric hindrance effect and π-π interaction can suppress the growth rate of{001} facets. The spherical mesocrystals showed good cycle stability and rate performance, the capacity retation is about 86.5%at 1 C. The good electrochemical performance is ascribed to the short transport distance of Li+, the large specific surface area and the stable structure of spherical anatase mesocrystals. When using NH4F as the surfactant, TiO2 mesocrystals (MC-H) exposed by highly reactive facets can be obtained, and the percentage of exposed {001} facet is calculated to be 52%. By adjusting the ratio of fluorine to titanium (Rf), the structure of TiO2 products can be accurately regulated. The sheet-like single crystals (SCs),{001}-mesocrystals and{101}-mesocrystals were prepared at various RF (RF=4,2,0). Owing to the mesocrystalline structure and exposure of highly active facets, the MC-H exhibited superior photocatalytic activity than the{101}-MC and SCs. The rate constant of MC-H is almost 4 times of the SCs, and 1.5 times that of the {101}-MC. TiO2 mesocrystals (MSCs) with diverse constructions in a controllable fashion are successfully fabricated by a new template-free method using oxalic acid (OA) as the morphological controller. The morphology can be tuned from discoid to cubic, the size varies from 500 nm to 80 nm, and the percentage of exposed{001} facet tailors from 71.4% to 33%. Moreover, the TiO2 MSCs shows substantial improvement in lithium specific capacity from 0.2 C to 10 C in comparison to the referenced TiO2 nanoparticle (NP). After 200 cycles, the TiO2 MSCs electrode still delivers a discharge capacity of 156.3 mAhg-1, which is 82% retention of the second cycle. It indicates a slow capacity fading of only 0.09% per cycle. MSCs/GO composites were prepared to further optimize the high-rate performance. The capacity of MSCs/GO at 5 C is 1.5 times of MSCs and the capacity at 10 C is nearly double than MSCs. Even at a high current rate of 20 C, a reversible capacity of 90 mAhg’1 could still be delivered.