Synthesis Of Quasi-One-Dimensional TiO₂ Nanomaterials And Investigations Of Their High Pressure Phase Transition

TiO₂ nanomaterial is one of the most important semiconductors have become the most focused material because of their potential applications in numbers of fields, such as: photocatalysis, lithium ion batteries, gas sensors, microelectronics, and photovoltaic cells. Recent years, new phenomena and new rules of quasi-one-dimensional namimaterials are important research subjects in the field of condensed matter physics and nanomaterials. Currently, controlled synthesis of quasi-one-dimensional TiO₂ nanomaterials is still a challenge topic. Meanwhile, high pressure structural phase transition of quasi-one-dimensional TiO₂ nanomaterials is also a new topic for further understanding physical properties of low dimensional nanosystems. In this dissertation, we synthesized various quasi-one-dimensional TiO₂ nanomaterials via a hydrothermal route, and studied the effects of reaction temperatures, reaction times and reactant concentrations on their morphologies and crystal structure. We have further analyzed growth mechanisms for the different morphology of TiO₂ nanomaterials. We have successfully synthesized TiO₂ nanoribbons with high density nanocavities and TiO₂@C core-shell nanoribbons, and investigated their electrochemical properties. We also have systematically studied high pressure structural phase transitions of quasi-one-dimensional TiO₂ nanomaterials, and revealed the effects of morphologies and sizes on their high pressure phase transitions.TiO₂ quasi-one-dimensional nanomaterials with different morphologies and structures were synthesized by modified the reaction time, reaction temperature, and reactant concentration during the hydrothermal process. It was found that anatase TiO₂ nanotubes and TiO₂-B nanoribbons can be obtained through controlling reaction times and reaction temperatures. Reaction times and reaction temperatures can modify and control reaction process and growth velocity. It revealed that the growth mechanism from titanate nanotubes to nanobelts. The filling degrees have no obvious effects on hydrothermal reaction that morphologies and crystallinity of as-prepared TiO₂ nanomaterials are not affected by filling degrees. The reactant concentrations have important effects on morphologies and crystal structures of as-prepared TiO₂ nanomaterials. Through controlling reactant concentrations, we obtained anatase"nano-bamboo raft", anatase/TiO₂-B nanoribbons and TiO₂-B nanoribbons. We also found the crystal structures and crystallinity of titanate play important roles in the calcination process, which determined crystal structures of as-prepared TiO₂ nanomaterials. After calcinations, titanate with low crystallinity converts into anatase TiO₂, while titanate with high crystallinity converts into TiO₂-B.TiO₂-B@C core-shell nanoribbons were synthesized via a simple hydrothermal route, firstly. The nanoribbons are up to tens of micrometers in length and 50-200 nm in width. TiO₂-B@C core-shell nanoribbons shown higher discharge capacity (360 mAh/g) than those of bare TiO₂-B nanowires and nanotubes. It was found that the carbon shell not only benefits to enhance their discharge specific capacity, but also improved their surface conduction properties. This novel material could also be applied in various fields, for example, catalysis, gas sensors, electrode materials. Our study offers an effective method for the preparation of high-quality TiO₂-B@C nanoribbons and provides a better opportunity for the further investigation of their properties and applications. In addition, we further studied the stability of TiO₂-B@C nanoribbons in the range of 500-1000 oC, obtained various crystal structural (TiO₂-B, anatase and rutile) TiO₂@C core-shell nanomaterials. Our results showed that the inner TiO₂-B nanoribbons have similar phase transition series with those of bare TiO₂-B nanowires under high temperatures, but with different phase transition temperatures. The carbon shell can not only preserved TiO₂-B nanoribbons morphology to high temperature, but also decreased the phase transition temperature to rutile phase. It also provides a new approach for preparing rutile TiO₂ nanomaterials under relative low temperatures.TiO₂-B nanoribbons with high density nanocavities were successfully synthesized via a simple hydrothermal route and posttreatment. The as-prepared TiO₂-B nanoribbon is a single crystal grow along the [010] direction, which has a uniform width (30-200 nm) and length (tens of micrometers). The TiO₂-B nanoribbons have high specific surface area (305 m2/g) because of large number of nanocavities inside TiO₂-B nanoribbons. Electrochemical measurements indicated that the TiO₂-B nanoribbons with dense nanocavities showed high discharge specific capacity (356 mAh/g) and good cycle stability. The discharge specific capacity is higher than those of TiO₂-B nanowires and nanotubes. It was found that the dense nanocavities have an important influence on the electrochemical lithium intercalation properties, which is benefit to improve their electrochemical properties. These TiO₂-B nanoribbons with dense nanocavities also may be of interest for a variety of applications such as gas sensors and photocatalysts.Nanoporous anatase TiO₂ rods and rutile TiO₂ chrysanthemums were successfully synthesized via a simple ethylene glycol-mediated synthesis route. We found a self-assembly growth takes place in the calcinations under vacuum. The possible mechanism: titanium glycolate rods were aggregated and self-assembled grown into chrysanthemums with the dehydration and carbonization of organic groups of titanium glycolate rods in vacuum. The pure rutile TiO₂ chrysanthemums were obtained through calcining TiO₂/C chrysanthemums in air (400 oC). In this process, the organic compositions of titanium glycolate rods and carbon of TiO₂/C chrysanthemums play important roles in the phase transition toward rutile phase.The structural phase transitions of single-crystalline TiO₂-B nanoribbons were investigated by in-situ high pressure synthrotron radiation X-ray diffraction and Raman methods. The morphology changes of samples were observed before compression and after decompression by TEM and HRTEM. It was found that TiO₂-B nanoribbons begin to transform into high density amorphous form upon compression, the high density amorphous form transforms into the low density amorphous form upon decompression. The pressure induced amorphization and polyamorphism were observed in TiO₂ one-dimensional nanomaterials for the first time. It was found that the high density amorphous form and the low density amorphous form have structural relation with baddeleyite andα-PbO₂ structures, respectively. HRTEM images showed that the low density amorphous TiO₂ nanoribbon is a long range disordered and short range ordered structure, in which some short range domains withα-PbO₂ structure randomly dispersive in the body of nanoribbons. We further revealed that the structural relations are originated from these baddeleyite andα-PbO₂ structural nucleus, respectively, the transformation between the high density amorphous form and the low density amorphous form are determined by these nucleus with different crystal structures. We proposed a homogeneous nucleation mechanism to explain the pressure induced phase transitions for the TiO₂-B nanoribbons, and revealed the essence of structural relation between the high/low density amorphous forms and baddeleyite/α-PbO₂ structures. In addition, we also found that the low density amorphous TiO₂ are still remain their pristine nanoribbon morphologies. It also provides a new method for preparing one-dimensional amorphous nanomaterials from crystalline nanomaterials.High pressure structural phase transitions of nanoporous TiO₂ microrods, anatase TiO₂ nanoribbons and nanorods were studied. It was found that nanoporous rutile TiO₂ microrods take on an abnormal property that phase transition pressure is elevated. This abnormal property is associated with their particular nanoporous structure. We suggested that a large volume collapse does not occur during the phase transition from rutile to baddeleyite, so the surface energy becomes the main reason for their phase transition pressure. For the nanoporous anatase TiO₂ microrods, anatase phase is stable up to ¹6 GPa, then transform into amorphous form directly. Upon decompression, the amorphous form transform intoα-PbO₂ structure. These unique phase transition behaviors are also associated with their nanoporous structure. In anatase TiO₂ nanoribbons, the anatase phase is stable up to ¹2 GPa, and transforms to baddeleyite phase directly, then pressure induced amorphization occurs under higher pressure. Upon decompression, the amorphous form transforms intoα-PbO₂ structure under low pressures which remains at ambient pressure. In anatase TiO₂ nanorods, the anatase phase is stable up to ¹6 GPa, and transforms into amorphous form under higher pressure. The low orderedα-PbO₂ structure is obtained after released to ambient pressure. We found that the high pressure phase transition processes of nanoporous TiO₂ microrods, anatase TiO₂ nanorods and nanoribbons are absolutely different from those of the corresponding bulks. We suggested that morphologies and sizes play a crucial role during the high pressure phase transitions, in which high surface energy enhances pristine structural stability, small size effects preclude nucleation and growth of high pressure phases and result in pressure-induced amorphization under high pressures.