engleski

, Rietveld refinement of electron diffraction data obtained on nanocrystalline TiO2

The Rietveld method is well-known powder-pattern fitting method, which consists of adjusting theoretical diffraction pattern, calculated on the basis of the model of its crystal structure (structural parameters) with experimental recorded data. The method is preferably applicable to XRD data, synchrotron and neutron diffraction data. There are tendencies to apply also to electron powder pattern diffraction data [Th. E. Weirich et al., Ultramicroscopy 81 (2000) 263-270]. In this work four representative TiO2 samples (dominant anatase) were chosen: two samples denoted with PEG contained iron (13% as a solid solution) and poly(ethylene) glycol, and two TiO2 samples (denoted with FET) without iron and poly(ethylene)glycol, but with small amount of brookite. Every group of samples contained as-prepared TiO2 sample and annealed at 5000C TiO2 sample. The preparation procedure was sol-gel route. A nanocrystalline samples (average grain size 3 nm for as -prepared and 6 nm for annealed samples) were characterised by selected area electron diffraction in transmission electron microscope operated at 200 kV (JEOL JEM 2010), Cs=0.5 mm having point resolution of 0.19 nm. Radially integrated intensities were extracted from digitised photographic films [J. Labar : "ProcessDiffraction", Proceedings of EUREM 12, Brno, 2000, pp. I379-I380] and used in the course of structure refinements by program FULLPROF [J. Rodriguez-Carvajal, FULLPROF- A program for Rietveld Refinement, Laboratoire Leon Brillouin, CEA-Saclay, France] for Rietveld analysis. The structure was refined in space group I41/amd. The background was adjusted by polynomial function of fifth order. A pseudo-Voigt profile function was employed for modelling the shape of the Bragg reflections since this function gave the best fit between the experimental and calculated data. Very interesting results are obtained by refinement of unit cell parameters a and c of anatase and partial atomic co-ordinates z of oxygen. We obtained relative increase of unit cell parameter c in PEG samples in comparison to FET samples of amount 3.2% for as prepared and 3.4% for annealed samples. This could be explained by influence of iron solid solution in titania matrix and by distribution of poly(ethylene) glycol within anatase [Kato et al., J. Am. Ceram. Soc., 79 [6] (1996) 1483-88]. With simple geometrical relations, we have calculated chemical bond lengths (Ti-O, Ti-O`, O-O) and corresponding angles (O-Ti-O, O-Ti-O`, Ti-O-Ti) [J. K. Burdett et al. J. Am. Chem. Soc. 109 (1987) 3639-3646]. In FET samples Ti-O` bonds are longer than Ti-O bonds. Contrary to that in PEG samples Ti-O` bonds are shorter than Ti-O bonds. The similar situation is with angles: in FET samples O-Ti-O angle is larger than in PEG samples. According to these results we can draw conclusion that titanium octahedron in samples with iron and poly(ethylene) glycol is inverted (inversion of bond lengths and angles distortion) compared to samples without iron and poly(ethylene) glycol.

nije evidentirano

nije evidentirano