engleski

Nanocrystalline anatase derived from modified alkoxide mesostructured gel

A sol prepared by chelation of titanium n- butoxide, Ti(OC4H9)4, with ethyl acetoacetate, C6H10O3, in 1:1 molar ratio was slowly hydrolyzed and the obtained gel was thermally treated in order to produce anatase. The gel and thermally treated samples were investigated by means of Fourier transformed infra-red spectroscopy (FTIR), X-Ray Diffraction (XRD), Differential Thermal and Thermogravimetric Analysis (TGA/DTA), N2 adsorption-desorption isotherms, Field Emission Scanning Electron Microscopy (FESEM) and High Resolution Transmission Electron Microscopy (HRTEM). From the results obtained, gel structure and gel crystallization behavior as well as crystallized sample properties are discussed. It was determined that modified alkoxide is partially hydrolyzed leaving small amount of butoxide and ethyl acetoacetate groups attached to titanium. XRD pattern of gel exhibited broad, low-angle reflection, which indicates the existence of some degree of ordering on the mesostructure scale. HRTEM of the gel also revealed the presence of mesostructure. Observed mesostructure was found to be consisted of inorganic-organic nanoclusters, exhibiting some polydispersity in the size and variations in composition. The random packing of nanoclusters enables development of three- dimensional mesostructured framework. Thermal decomposition of the complex in air up to 1000 °C shows seven decomposition steps: evaporation of water and butanol at ~110 °C, elimination of ethyl acetoacetate at ~180 °C and butoxy groups between 200 and 300 °C, carbonate removal at 380 °C, crystallization of anatase at 420 °C and its transformation to rutile between 655 and 800 °C and, finally, oxidation of carbonaceous species between 600 and 1000 °C. Thermal treatment at 200 °C yielded with collapse of the mesostructure and disappearance of low-angle XRD peak. After thermal treatment of the gels at 350 °C for 2 h, pure nanocrystalline anatase was obtained with average crystallite size of 16.4 nm. A nanocrystalline character of prepared anatase was confirmed through HRTEM micrographs revealing the exposed {;101}; facets. Nitrogen absorption/desorption isotherms and pore size distribution of sample thermally treated at 350 °C is of type IV, typical for porous aggregates of anatase nanocrystals, with a relatively narrow pore size distribution in the range of mesopores. Relatively low specific surface area of 26.09 m2 g-1 was ascribed to the reduction of mesopores volume due to blockage with carbonaceous material.

chelate; sol-gel; titania; thermal analysis

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