Synthesis And Formation Mechanism Of TiO₂/Titanate Nanomaterials With Special Morphologies

TiO2/Titanate nanomaterials with specified crystalline phase, controlled size and special morphology should be considered for extensive applications in optical, electromagnetic, biological, catalytic, and environmental research fields. Chemical reactivity, mechanical and photocatalytic behaviors of titania/titanate nanomaterials should be strongly influenced by the crystalline phase, grain size, morphology, microstructure and other physicochemical characteristics. Because of the importance in many key technologies of titania/titanate nanomaterials, synthesis of highly crystalline titania/titanate nanoparticles with specified crystalline phase, controlled size and special morphology has been an important research focus in the field of materials chemistry. The main research target in this thesis was the innovation of a novel alkalescent approach, in which a systematical investigation of the alkalescent hydrothermal treatment was thus elaborated by regulating the two experimental parameters of reaction temperatures and the concentrations of strong electrolyte of NaCl in order to supervise the phase transition and morphological evolution of titania/titanate nanomaterials. Besides the new alkalescent method, research on the preparation of TiO2/Titanate nanomaterials at alkali wet and near ambient conditions was also consummated in this thesis. Furthermore, the crystalline growth mechanisms of TiO2/Titanate nanomaterials were futher investigated which revealed the hypostasis of phase transition and morphological evolution of titania/titanate nanomaterials in corresponding experimental methods. The details are listed as follows:1. Phase transition and morphological evolution of TiO2/titanate nanomaterials in the novel alkalescent hydrothermal environment.A novel alkalescent route for the synthesis of TiO2/Titanate nanomaterials with specified crystalline phase, controlled size and unique morphology was reported in this work. Protonic titanate nanosheets were synthesized in which aqueous ammonia was used for in-situ regulating of ionic strength under hydrothermal treatment at 100-140℃. Consequently, we investigated the TiO2 phase transition and morphology evolution influenced by the concentration of NaCl at 180℃. Pure brookite TiO2 nanoflowers, which were aggregated by brookite nanorods with a uniform size of about 40 nm, were synthesized when the concentration of NaCl was about 0.25M. Finally, phase transition and morphological evolution of titania/titanate nanomaterials has been systematically investigated in the alkalescent hydrothermal environment, and a systematical morphological phase diagram of titania/titanate nanomaterials was determined by the most important synthesis parameters of reaction temperature and NaCl concentration via such facile one-step alkalescent approach. According to the Ostwald's step rule, a phase transition mechanism was proposed based on our current systematic experimental results, which reveals the phase transitions from layered hydrogen titanate to anatase or brookite under hydrothermal treatment. The brookite-anatase composite exhibited superexcellent photocatalytic activity in the photodegradation experiment of methyl orange, phenol and salicylic acid solution under UV irradiation.2. Preparation and the formation mechanism of TiO2/titanate nanomaterials under alkali wet treatment.Sodium titanate nanosheets and nanofibers were synthesized by regulating the reaction temperatures or durations in high concentrated NaOH solution under hydrothermal treatment. Consequently, hydrogen titanate nanotubes and nanowires were obtained via protonating the as-prepared alkali titanates in dilute acid for hours, respectively. Then the anatase nanoparticles were obtained by calicining hydrogen titanate nanotubes at 400℃, and TiO2-B nanowires were synthesized under the similar post-thermal treatment. The unique morphology of pits and dislocations interspersed on TiO2-B nanowires were firstly characterized and studied by high resolution transmission electron microscopy (HRTEM). Oriented attachment is suggested as an important growth mechanism in the evolvement of pits and dislocations on TiO2-B nanowires. Moreover, the phase transition and morphological evolution from potassium titanate K2Ti8O17 nanowires to anatase nanoparticles were investigated under the post-hydrothermal treatment.3. Preparation and the formation mechanism of potassium titanate nanowires at near ambient condition.Potassium titanate K2Ti8O17 nanowires were synthesized using P25 TiO2 as precursor in high concentrated KOH solution by the refluxing procedure at lower temperatures. According to the Ostwald Ripening Mechanism, the phase transition and morphological evolution from anatase nanoparticles to potassium titanate K2Ti8O17 nanowires were investigated at near ambient condition, while the rutile particles had little change under the treatment above since its phase structure should be more stable than anatase.