Structural properties of pure and nickel-modified nanocrystalline tungsten trioxide

The particle size and nickel-doping effect on pure nanocrystalline WO ₃ powders are addressed through X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. A brief review of different structure types of tungsten oxides is also given. Stable and metastable crystallographic structures, resulting from oxygen deficiency, metal doping, or low-temperature synthesis, are discussed. The focus is put on the topology of the structures and notably on the structural features allowing ion intercalation. Small particle size WO ₃ powders were synthesized by two different wet chemical methods to determine the impact of particle size on the crystal symmetry: in the first method, a freeze-drying technique was utilized, whereas the second technique was based on a reverse micelle method. Both methods yielded similar powders with an average size of approximately 10 nm. However, the first method yielded single-phase rhenium oxide structured particles, whereas the latter method produced a mixture of hexagonal tungsten bronze and rhenium oxide structures. In the case of single-phase rhenium oxide structure powders, the crystal symmetry was found to increase from monoclinic P2 ₁/n to orthorhombic Pbcn when particle size decreased below 20 nm. The effect of nickel doping (≈1 wt %) and synthesis conditions on WO ₃ powders were studied. Ni-doped WO ₃ was spatially inhomogeneous: the most abundant phase was monoclinic WO ₃, whereas the minority phase was either perovskite tungsten bronze (annealing temperature below 500 °C) or wolframite (annealing temperature 500 °C or higher) showing that annealing conditions are a way to selectively produce different crystal structures. The wolframite and tungsten bronze structures are very different with different applications. The results are discussed in the context of thin film synthesis and sensor applications.