WAXS and PDF-Based Analyses of Chromium Doping in Nanocrystalline Titania (Anatase and Brookite)
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0915-R07-02
WAXS and PDF-Based Analyses of Chromium Doping in Nanocrystalline Titania (Anatase and Brookite) Hengzhong Zhang1, Benjamin Gilbert2, Bin Chen1, and Jillian F. Banfield1,3 1 Department of Earth & Planetary Science, University of California Berkeley, 307 McCone Hall, Berkeley, CA, 94720 2 Earth Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road MS 90R1116, Berkeley, CA, 94720 3 Earth Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720
ABSTRACT Chromium-doped (0.5-10 % Cr:Ti molar ratio) nanocrystalline titania (5−6 nm) prepared via sol-gel method was examined by synchrotron-based wide angle x-ray scattering (WAXS) for crystal structure determination. Atomic pair-distribution functions (PDF) for both raw and heattreated samples were obtained by Fourier transforms of the WAXS data. The PDF data were fitted using structural models of nanocrystalline titania that considered phase compositions, lattice parameters, atomic positions and thermal factors. The unit cell of Cr-doped nanocrystalline titania expanded 1-2 % with respect to bulk titania as a consequence of the substitution of Ti by Cr and the generation of oxygen vacancies. We observed a lattice contraction after heat-treatment that may be caused by the redistribution of Cr atoms to nanoparticle surfaces during phase transformation and particle coarsening. INTRODUCTION Doping with chromium changes the band structure of titania (TiO2) and causes red shifts in its UV-vis adsorption edges [1]. As a result, Cr-doped titania finds applications in photocatalysis [2, 3]. Cr doping also creates oxygen vacancies in titania, thus making its physical chemical properties oxygen dependent [4]. As a consequence, Cr-doped titania is a promising oxygen sensor material. The dopant content not only determines the content of oxygen vacancies, but also impacts the crystal structures. This in turn affects the diffusivity of oxygen vacancies and hence the sensor sensitivity. However, when the crystal size of titania is in the nanometer range, the ionic and atomic migrations may be greatly enhanced. Thus, it is essential to understand the influence of Cr content on the structure of titania nanocrystals. In this work we used synchrotron-based wide angle x-ray scattering (WAXS) to study the effects of dopant content and the heat treatment on the crystal structure of Cr-doped nanocrystalline anatase. EXPERIMENT DETAILS Samples of Cr-doped (0.5-10% Cr:Ti molar ratio) nanocrystalline titania were synthesized in a previous work [5, 6]. Briefly, titanium ethoxide and chromium ethoxide were mixed in ethanol and then hydrolyzed in water. The mixture was heated under refluxing at 80 oC. Sol was formed and then evaporated to form gel membranes. Raw samples were obtained by
grounding the membranes. Heat-treated samples were produced by heating portions of the powdered raw samples in air for 2 h at designated temperatures (300-700 oC). The samples were examined by conventional x-ray diffraction (XRD) using a X’Pert PRO x-ray diffractomet
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