Growth Mechanism And Controllable Preparation Of TiO₂ Nanotubes And Mesocrystals

As one of the most important transition metal oxides,nanostructured TiO₂has great applications in various fields including photocatalysis,cosmetics,energy storage and conversion.The electrochemical method is preferred for the fabrication of TiO₂ due to its simple operation and friendliness to large-scale production.TiO₂nanotube arrays and mesocrystalline structures have exhibited excellent application performance because of their high specific surface area,good stability and charge conductivity.However,there are still aspects need to be improved in the understanding of growth mechanism and morphology control for ordered TiO₂ nanotube arrays prepared by electrochemical anodic oxidation.In comparison,there still hasn't been any report on the electrochemical synthesis of TiO₂mesocrystals.Therefore,for TiO₂nanotube arrays and mesocrystalline TiO₂ synthesized by electrochemical method,it has important meaning in theory and practical value to study the in-depth growth mechanism,to optimize their morphology and structure,and to improve their performance in applications such as photocatalysis.This project will study on the preparation and growth mechanism of anodic TiO₂ nanotube arrays with pore size smaller than the current low limit value of 15 nm.In the mean time,we will explore an electrochemical method for the fast and massive synthesis of TiO₂ mesocrystals with controllable morphology and structure and excellent photocatalytic activity.In the following are the main contents of this work:(1)Preparation of TiO₂nanotube arrays with ultra-small pore size by anodization:Be successful at preparation of TiO₂nanotube arrays with pore size less than 10 nm in aggressive and viscous electrolyte under low voltage,breaking the current nanotube aperture limit of 15nm.By adding lactic acid,ammonium hexafluorotitanate or sealing the electrolytic cell,the length of ultra-small pore TiO₂ nanotube array can be effectively extended from more than100 nm to about 1?m.(2)Growth of small pore anodic TiO₂nanotube arrays via the micro breakdown mechanism:The pore size of anodic TiO₂nanotube array did not increase monotonically with anodization voltage under high voltage and high current density,but decreased significantly with elevating voltage.With other conditions being the same,pore size for nanotubes obtained under high current density is smaller,while pore size for nanotubes obtained at low current density is larger.A micro breakdown mechanism was proposed to explain the abnormal experimental fact that small pore anodic TiO₂nanotubes could be grown under high voltage.(3)Electrochemical synthesis of anatase TiO₂ mesocrystals and their application in photocatalysis:Massive mesocrystals of disk-like NH₄TiOF₃?rod-like(NH₄)₂TiOF₄ and octahedral(NH₄)₃TiOF₅ with different morphologies were fabricated with a high rate by an electrochemical method,which were then topotactically transformed to anatase TiO₂mesocrystal with corresponding morphology,showing excellent performance in the photo-degradation of methylene blue.