Study on Controlled Synthesis of WO3·nH2O Micro/Nanomaterials and Their Properties
|School||Shaanxi University of Science and Technology|
|Course||Materials Physics and Chemistry|
|Keywords||tungsten oxide hydrates hydrothermal method film morphology control photocatalytic properties electrochemical properties|
The past couple of decades have witnessed an exponential growth ofactivities in this field worldwide, driven both by the excitement of understandingnew science and by the potential hope for applications and economic impacts.The largest activity in this field at this time has been in the synthesis of newnanoparticles of different sizes and new shapes. Besides giving us new properties,creating novel nanostructures requires new understanding of the properties oftheir surfaces, which has been in the center of attentions from many scientificresearchers. As one kind of well-known semiconductor materials, tungsten oxideand its hydrates （WO3·nH2O, n=02） have been extensively researched andemployed in various application fields. Because many of their nanostructuresand morphologies are inextricably bound to properties in several fields, scientistshave been concentrating on the research of the preparation ofmicro/nanostructures of these materials. However, there still remains furtherinvestigations on the exploration of developing facile, green and economicaltechniques when controlling these interesting promising structures, which arebelieved to be very necessary for expanding the application fields of tungstenoxide hydrates materials. In this dissertation, a facile hydrothermal technique hasbeen developed to prepare micro/nanostructured tungsten oxide hydratesmaterials （particles and films） with/without the assistant of templates andmicrowave heating. The following are the main results and conclusions:（1） Tungsten oxide hydrates crystallites with a diversity of phases andmorphologies were successfully controlled by using tungsten powders as startingmaterials via a facile and additive-free hydrothermal process. Various micro/nanostructures of products, including cubic blocks,“snowflakes”, roundangular blocks, hexagonal “gears”, cuboid rods, hexagonal plates and nanoflakes,were obtained by simply changing the reaction conditions. Results show that thenucleation, crystallization and diffusion of the growth units were found to playkey roles in different conditions to vary the products phases and morphologies.（2） By employing different solvents and microwave-assisted heating, theas-prepared tungsten oxide hydrates crystallites were controlled to exhibitvarious phases and morphologies. The growth units of these crystallites werefound to follow different nucleation and growth orientation process, leading to avariety of micro/nanostructures, such as nanowires, nanosheets, urchins, holedhexagonal sheets, hexagonal bipyramid, hexagonal plates and snowflakes.Results show that different solvent polarity and solvent molecular structures mayhave significant effects on the two dimensional growth orientation of tungstenoxide hydrates and particle sizes to change the thickness of the as-obtainedhexagonal sheets. The comparison between conventional heating and microwaveheating reveals that microwave heating may influence the crystal growth processin three different ways, which are selectively heating of crystal faces, changingthe surface conditions of the templated crystallites and reducing the solventspolarity. The photocatalytic experiment of these tungsten oxide hydrates particleswere carried out under visible light. The connection between theirmicrostructures and photocatalytic performance were discussed. It is revealedthat the proton affinity constants and charge of the surface groups in differentcrystal faces of tungsten oxide hydrates is crucial to the product photocatalyticproperties.（3） Tungsten oxide hydrates films materials were successfully prepared onITO substrate by using a facile, one-step hydrothermal reaction withperoxo-polytungstic acid as the precursor solution. By adjusting experimentalparameters, cubic and orthorhombic tungsten oxide hydrates phases with varioussurface morphologies were obtained, including nanowires, granules, verticallycrossed hexagonal sheets and hierarchically layered triangle sheets.Results show that solvents polarity is crucial to the film growth on the ITOsubstrate. The reaction temperature and precursor concentration were found tohave main influences on crystal structure and crystallinity of the tungsten oxide hydrates films. Corresponding electrochemical performances of these films showthat the crystallinity plays a important role in adjusting the electrochromicresponse time and ion diffusion process. High crystallinity were found to havehigher ion diffusion coefficient but relavtively low sensitivity （long responsetime） to the changes of applied voltage. On the contrary, lower crystallinity leadsto short response time with low ion diffusion coefficient, showing a highsensitivity to applied voltage with weak ion diffusion in the as-prepared filmstructures.