Growth of Ultrafine Single Crystalline WO₃ Nanoparticles Using Flame Spray Pyrolysis

Abstract

Flame spray pyrolysis (FSP) has been employed for the syntheses of single crystalline WO3 nanoparticles using tungsten precursors with tungsten in usual (+6) and unusual (+4 and 0) oxidation states. The WO3 nanoparticles have been characterized by powder X-ray diffraction and the patterns have been refined using the Rietveld and the Le Bail method (space group P21/n, monoclinic system; the averaged lattice parameters of the three WO3: a = 7.3913(4) Å, b = 7.5630(5) Å, c = 7.6615(6) Å, β = 90.78(3)°, Z = 8, V = 423.19 Å3). Because of the very small crystallite sizes, non-Bragg scattering is observed. The crystallite sizes derived from Rietveld or Le Bail analyses were in the range of 6−8 nm, reasonably agreeing with the crystallite sizes (7−10 nm) determined from Brunauer−Emmett−Teller and microscopic analysis where each particle is basically a small single crystal. Well-developed lattice fringes of 3.860−3.994 Å were illustrated by high resolution transmission electron microscopy (HRTEM). Indexing of selected area electron diffraction (SAED) patterns of all WO3 revealed that the crystals are isostructural with the monoclinic phase having pseudocubic lattice parameters. The wide spot HRTEM image additionally examined in reciprocal space by calculating the corresponding power spectrum further proved the single-crystalline nature of the nanoparticles. The development of economic precursors and the systematic studies on the crystallization and assembly behavior of tungsten oxide products presented here now opens the door for the production of ultrafine single crystalline, single phase WO3 nanomaterial.