Designed Synthesis And Properties Of Hierarchical Titanium-oxide-based Hollow Materials

Titanium-oxide-based materials have attracted much attention for their excellent physical and chemical properties and applications in the fields ranging from energy storage to catalysis and adsorption.Researchers are aiming to design special titanium-oxide-based materials with superior properties. As is known to all, the properties and functions of materials are closely related to their structures. It is of great significance to design and synthesize the micro/nanomaterials with complicated structures via a simple and effective method and investigate the relationship between their structures and properties. Hierarchical hollow structured material constructed from nanosized building blocks is a newly-developed functional material which has the advantages of both hollow structured material and hierarchical structured material. Moreover, it has showed broad prospects due to its unique large void space, low density and high surface area.In this work, we have concentrated on fabricating hierarchical hollow titanium-oxide-based materials including titanium dioxide and sodium titanate via a novel self-template route and conventional hard-template method. The properties of these materials in lithium-ion batteries, organic dye degradation and pollutant adsorption have been further investigated. The major achievements are described as follows.The first part demonstrated that the spinous hierarchical TiO₂ hollow hexagonal prisms（SHTHPs） composed of nanobelts have been synthesized successfully via a novel self-template route during the solvothermal process by using Ti₂O₃（H₂O）₂（C₂O₄）·H₂O（TC） as the designed precursor. Based on the analysis of SEM, TEM, IR and XRD, a conceivable synthetic route was raised. The successful fabrication of the spinous hierarchical hollow titania material mainly included three steps:（Ι） In the butyl alcohol and ammonia solution, the conversation of the TC was conducted on the surface of the microparticles at the initial stage. The exterior part of the microparticles was converted into titania and exhibited a TC@TiO₂ core-shell structure.（П） The interior part gradually participated and the Kirkendall effect occurred, giving rise to the yolk-shell nanostructure with increased reaction time.（Ш） After reacting 20 h, all the microparticles were converted into spinous hollow hexagonal prisms completely. It is found that ammonia could serve as the shape controller and reaction accelerator in the present synthesis system. The hierarchical hollow material possesses a high surface area of up to 163 m2 g^-1 and this type of morphology is of great interest for lithium-ion batteries, because of its shorter length for Li+ transport and better electrode–electrolyte contact. The results of electrochemical tests also revealed that the hierarchical hollow material demonstrated well cycling performance. In the first cycles under a 1 C(1 C = 168 m A g^-1) rate, the sample showed a discharge capacity of 225 m Ah g^-1 and a specific discharge capacity of 125 m Ah g^-1 could be retained after 40 cycles. Moreover, it exhibited higher lithium storage capacity than of the solid titania at different current density.In the second part, TC was chosen as precursor continued based on the above experimental results and analysis to prepare hollow titania materials with varied morphologies and polymorphs under different synthesis conditions. The concentrations of ammonia have been adjusted first, and hollow structured products could be obtained when the concentrations of ammonia decreased from 0.4 M to 6.7×10^-3 M. Flower-like and dendritic-like anatase hollow materials could be synthesized while at 0.04 M and 0.016 M ammonia solution. Some hexagonal prisms began to crumble when the concentration of ammonia decreased to 6.7×10^-3 M and most of the microparticles have broken into pieces when there was no ammonia in the solution. The analysis of XRD patterns indicated that rutile and brookite titania have emerged in these two samples. We have focused on hierarchical hollow structured materials HHTM^-1（cal） composed of nanosheets and HHTM-2（cal） composed nanorods to investigate their degradation properties of organic dye Acid Red 14 in water. Compared with Degussa P25, the degradation abilities are following in this order: HHTM^-1（cal） > HHTM-2（cal） > P25.The third part demonstrated the synthesis of hierarchical hollow sodium titanate microparticles（HHSTMs） through the reaction of SiO₂@TiO₂ core-shell material with 10 M Na OH solution by using conventional hard-template method. Na OH solution plays a dual role during the hydrothermal process. It could not only remove silica core to obtain the hollow structures, but also reacts with the TiO₂ shell to get the sodium titanate nanowires. The final hierarchical hollow material exhibited excellent adsorption properties for its unique structure and high surface area of 308 m2 g^-1. The max amount of CO₂ uptake for HHSTMs is 42.3 cm³ g^-1 at 273 K and 26.3 cm³ g^-1 at 298 K under 1 bar. HHSTMs could also adsorb of cationic organic dye methylene blue selectively in water and the max adsorption capacity of dye could reach 443 mg g^-1. The reports of the preliminary tests of Pb²⁺ adsorption showed that the concentration of Pb²⁺ can be purified to below 10 ppb（the guideline value of WHO） after treated by HHSTMs.