Structural investigations of nanocrystalline TiO2 samples

Two kinds of nanocrystalline TiO2 samples were synthesized by the sol-gel method: iron doped TiO2 (containing iron as a solid solution and poly(ethylene)glycol) and undoped TiO2 The structural parameters and the phase composition of the synthesized samples were investigated applying the Rietveld refinement of X-ray diffraction (XRD) patterns in conjunction with the Rietveld refinement of selected area electron diffraction (SAED) patterns. Both methods gave similar values of structural parameters, indicating a possibility of use of SAED in quantitative structural analysis. Significant differences of lattice parameters, bond lengths and angles were found between undoped and iron doped samples. The average grain size and lattice microstrain were determined through the line-broadening analyses using the Rietveld method based on the single-line approach and on the Warren-Averbach method. The grain size dependence of the lattice parameters, microstrain and unit-cell volume of anatase was determined. The lattice parameter a was found to increase while the parameter a decreased with a decreasing of the grain size, keeping the unit-cell volume almost constant. The variations of the lattice parameter deviation from the ICDD values with the average grain size can be well correlated with the brain size dependence of the corresponding microstrains. Upon annealing at elevated temperature the grain growth occurred and lattice microstrain decreased, due to the annihilation of dislocations. The results from the exhaustive XRD analysis were confirmed with transmission electron microscopy (TEM). In addition, fine details in material characterization were observed. The most probable microstructural model of investigated nanocrystalline TiO2 is a model in which defects are uniformly distributed throughout individual grains but whose concentration is proportional to the inverse gram size. (c) 2005 Elsevier B.V. All rights reserved.