Formation of nanoscale tungsten oxide structures and colouration characteristics

In this work, pH dependent evolution of tungsten oxide (WO3) nanostructures is being reported along with physical characteristics. The synthesis was carried out via an inexpensive solvothermal cum chemical reduction route, with sodium tungstate (Na2WO4) and cetyl trimethyl ammonium bromide (C19H42NBr) as main reactants. The X-ray diffraction, together with transmission electron microscopic studies have revealed formation of regular polyhedral nanocrystalline structures and fractals as one goes from higher pH (= 5·5) to lower pH (= 2) values. The average crystallite size, as calculated through Williamson–Hall plots, was varied within 2·8–6·8 nm for different pH samples. Fourier transform infrared spectroscopy reveals in-plane bending vibration δ (W–OH), observable at ∼1630 cm − 1 and strong stretching ν (W–O–W) located at ∼814 cm − 1. Raman spectroscopy has divulged WO3 Raman active optical phonon modes positioned at ∼717 and 805 cm − 1. The thermochromic and photochromic properties of the nanoscale WO3 sample prepared at pH = 5·5, are also highlighted.