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

Abstract

Structural evolution of WO x 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 hydroxylated 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 band characteristic of the W O bond was observed at 962 cm −1 in the dried sample, which vanished between 300 and 700 °C, and reappear again after annealing at 800 °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 °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 Na 2WO 4 and Na 2W 2O 7 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 °C, thereafter, it transformed into anatase being stabilized at temperatures as high as 900 °C. At 1000 °C, anatase phase partially converted into rutile. After annealing at 1000 °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 TiO 2 phase.