Hydrothermally Synthesized h-MoO3 and α-MoO3 Nanocrystals: New Findings on Crystal-Structure-Dependent Charge Transport

The charge transfer characteristics of metastable-phase hexagonal molybdenum oxide (h-MoO3) and stable-phase orthorhombic MoO3 (α-MoO3) nanocrystals have been investigated for the first time using impedance spectroscopy. The results imply that the metastable phase h-MoO3 displays a 550-fold increase (at 150 °C) in the electrical conductivity relative to the stable phase α-MoO3. The conductivity also increases as the temperature increases from 130 to 170 °C, whereby analysis shows a thermal activation energy (Ea) of ∼0.42 eV. The investigation clearly identifies that the presence of intercalated ammonium ions (NH4+) and crystal water molecules (H2O) in the internal structure of h-MoO3 plays a vital role in enhancing the charge transfer characteristics and showing an ionic conductive nature. Before the impedance investigations, the h-MoO3 and α-MoO3 nanocrystals were successfully synthesized through a wet-chemical process. Here, a controlled one-step hydrothermal route was adopted to synthesize stable-phase α-MoO3 nanocrystals sequentially from metastable-phase h-MoO3 nanocrystals. The hydrothermal reaction conditions, such as the choice of precipitant, amount of precipitant, reactant solvent medium, reaction time, and reaction temperature, play significant roles in defining the crystal structure, crystallite size, and particle morphology. On the basis of the crystal structure, size, and morphology evolution with respect to the hydrothermal reaction conditions, a possible formation mechanism of MoO3 nanocrystals is proposed.