Transition-metal oxides such as WO 3 are of interest because of their photochromic, electrochromic, and photocatatytic properties, which are promising for a variety of applications in mirrors, windows, and gas sensor technologies. These applications require a detailed understanding of the morphology, particle size, and other material characteristics for effective utilization and implementation. We present a correlation between powder particle size, determined from dynamic light scattering, and the bonding characteristics of WO 3 powders, showing that the W−O−W/WO integrated intensity ratio is directly related to the particle size of our powders and not just to the grain size of WO 3 films, as has previously been shown. This correlation can serve as a complementary technique to gauge particle size as well as crystallinity in WO 3 powders. When the WO signal is high and the W−O−W is low, the powders will be of small particle size and/or of lower crystallinity. Thus, this analysis provides a useful approach for obtaining powder particle size in WO 3 powders. The analysis might also prove useful for powders that exhibit Raman behavior similar to that of WO 3 .

We present the effect of sodium ions (Na + ) on the nucleation process and phase selectivity for the formation of hexagonal molybdenum trioxide crystals (h-MoO 3 ). The phase selectivity during the reaction is attributed to the interaction of Na + with the molecules in our precursor solution formed by metallic molybdenum dissolved in a mixture of hydrochloric and nitric acids. The vibrational characteristics of the precursor solutions were studied by Raman spectroscopy in combination with density functional theory modeling, showing the presence of [MoO 2 Cl 3 (H 2 O)] − ions within the solutions. The symmetric stretching vibration of the Mo−O bonds found at 962 cm −1 in [MoO 2 Cl 3 (H 2 O)] − proved that the addition of Na + (in the form of dissolved NaCl) to the precursor solutions resulted only in an electrostatic interaction with the aquo (H 2 O) and chloro (Cl − ) ligands in the complex. After heating the precursor solutions, Xray diffraction, Raman spectroscopy, and scanning electron microscopy of the obtained powders showed that adding NaCl contributed to the phase selectivity of the reaction, with the Na + ions playing a vital role in the formation of h-MoO 3 over other crystalline phases. Based on the nature of the molybdenum complexes found in the precursor solutions and the structural characteristics of the powders, a formation mechanism to obtain h-MoO 3 is proposed. Additionally, the phase stability of h-MoO 3 crystals was studied by calorimetry techniques, showing that h-MoO 3 transforms to α-MoO 3 at ∼649 K. These results provide important insights into phase control to selectively form hexagonal MoO 3 .

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