Phase and Morphology Control of Hexagonal MoO₃ Crystals via Na⁺ Interactions: A Raman Spectroscopy Study

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, X-ray 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 .