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 nucleationprocess and phase selectivity for the formation of hexagonal molybdenumtrioxide crystals (h-MoO3). The phaseselectivity during the reaction is attributed to the interaction ofNa(+) with the molecules in our precursor solution formedby metallic molybdenum dissolved in a mixture of hydrochloric andnitric acids. The vibrational characteristics of the precursor solutionswere studied by Raman spectroscopy in combination with density functionaltheory modeling, showing the presence of [MoO2Cl3(H2O)](-) ions within the solutions. Thesymmetric stretching vibration of the Mo-O bonds found at 962cm(-1) in [MoO2Cl3(H2O)](-) proved that the addition of Na+ (in the form of dissolved NaCl) to the precursor solutions resultedonly in an electrostatic interaction with the aquo (H2O)and chloro (Cl-) ligands in the complex. After heatingthe precursor solutions, X-ray diffraction, Raman spectroscopy, andscanning electron microscopy of the obtained powders showed that addingNaCl contributed to the phase selectivity of the reaction, with theNa(+) ions playing a vital role in the formation of h-MoO3 over other crystalline phases. Based onthe nature of the molybdenum complexes found in the precursor solutionsand the structural characteristics of the powders, a formation mechanismto obtain h-MoO3 is proposed. Additionally,the phase stability of h-MoO3 crystalswas studied by calorimetry techniques, showing that h-MoO3 transforms to & alpha;-MoO3 at & SIM;649K. These results provide important insights into phase control toselectively form hexagonal MoO3.