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The charge transfer characteristics of metastablephase hexagonal molybdenum oxide (h-MoO 3 ) and stable-phase orthorhombic MoO 3 (α-MoO 3 ) nanocrystals have been investigated for the first time using impedance spectroscopy. The results imply that the metastable phase h-MoO 3 displays a 550-fold increase (at 150 °C) in the electrical conductivity relative to the stable phase α-MoO 3 . The conductivity also increases as the temperature increases from 130 to 170 °C, whereby analysis shows a thermal activation energy (E a ) of ∼0.42 eV. The investigation clearly identifies that the presence of intercalated ammonium ions (NH 4 + ) and crystal water molecules (H 2 O) in the internal structure of h-MoO 3 plays a vital role in enhancing the charge transfer characteristics and showing an ionic conductive nature. Before the impedance investigations, the h-MoO 3 and α-MoO 3 nanocrystals were successfully synthesized through a wet-chemical process.Here, a controlled one-step hydrothermal route was adopted to synthesize stable-phase α-MoO 3 nanocrystals sequentially from metastable-phase h-MoO 3 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 MoO 3 nanocrystals is proposed.

Hexagonal molybdenum oxide (h-MoO 3 ) nanocrystals with a flower-like hierarchical structure have been successfully synthesized by a solution based self assembly route. The as-synthesized h-MoO 3 was recognized as a photocatalyst for the photocatalytic degradation of methylene blue (MB) under various experimental conditions. Initially, the as-synthesized h-MoO 3 was characterized by different physicochemical techniques in order to study and reveal the structural, functional, morphological, and optical properties. The results indicated that the photocatalyst has a hexagonal crystal structure with an average crystallite size of 46 nm. The morphology analysis has proved that the h-MoO 3 comprises one dimensional (1D) rods with a hexagonal cross section. The possible formation mechanism is proposed as a self assembly process for nucleation and an Ostwald ripening mechanism for particle growth. The optical band gap investigation showed that the E g value of h-MoO 3 (2.94 eV) lies in the visible region and can be an appropriate candidate for visible light photocatalytic application. Furthermore, the experimental observations demonstrate an excellent photocatalytic performance of h-MoO 3 in the degradation of MB under visible light irradiation.