Evolution of titania nanotubes-supported WOx species by in situ thermo-Raman spectroscopy, X-ray diffraction and high resolution transmission electron microscopy

Structural evolution of WO, species on the surface of titania nanotubes was followed by in situ thermo-Raman spectroscopy. A total of 15 wt% of W atoms were loaded on the surface of a hvdroxylated titania nanotubes by impregnation with ammonium metatungstate solution and then, the sample was thermally treated in a Linkam cell at different temperatures in nitrogen flow. The hand characteristic of the W = O bond was observed at 962 cm(-1) in the dried sample, which vanished between 300 and 700 degrees C, and reappear again after annealing at 800 degrees C. along with a broad band centered at 935 cm(-1). attributed to the v(1) vibration of W = O in tetrahedral coordination. At 900 and 1000 degrees C. the broad band decomposed into four bands at 923, 934, 940 and 950 cm(-1) corresponding to the symmetric and asymmetric vibration of W = O bonds in Na2WO4 and Na2W2O7 phases as determined by X-ray diffraction and High resolution transmission electron microscopy (HRTEM). The structure of the nanotubular Support was kept at temperatures below 450 degrees C. thereafter. it transformed into anatase being stabilized at temperatures as high as 900 degrees C. At 1000 degrees C, anatase phase partially converted into rutile. After annealing at 1000 degrees C, a core-shell model material was obtained, with a shell of ca. 5 nm thickness, composed of sodium tungstate nanoclusters. and a core composed mainly of rutile TiO2 phase. (C) 2007 Elsevier Inc. All rights reserved.