Metastable monoclinic molybdenum trioxide (β-MoO 3 ) nanosized particles were synthesized by pulsed wire discharge in oxygen and nitrogen gases. The collected samples were characterized by X-ray diffraction to identify the phases, and field emission scanning electron microscopy was also used for the analysis of the particle size and its distribution. Different results were obtained by applying various conditions of the charged capacitor voltage (V c ) at 6, 4, and 3 kV and total gas pressures (P) of 100, 50, and 25 kPa, where the oxygen partial pressure of 25 kPa was always constant, and nitrogen gas was added to acquire the expected total pressure. The higher volume fraction of β-MoO 3 was obtained at lower V c and P (3 kV, 25 kPa). Inversely, the amount of α-MoO 3 and Mo increased at higher V c and P. The geometric mean diameter (D 1 ) of nanosized particles at every condition was determined under 50 nm.

Pulsed wire discharge was used to prepare nanoparticles of molybdenum and its carbides from Mo wires in a gas mixture of argon and kerosene at pressures of 100, 50, and 25 kPa. The different pressures affected the carburization process and particle formation. The most effective pressure was 25 kPa, where the volume fraction of MoC was identified by Rietveld refinement of X‐ray diffraction data to be 98.4%. The particle size distribution was also obtained from transmission electron microscopy measurements, and the smallest geometric mean diameter was determined tobe 24.3 nm for the sample prepared at 25 kPa.

β-MoO 3 is a monoclinic phase of MoO 3 ; it has been shown to be a promising material that can replace α-MoO 3 in chemical, optical, electronic, and electrochromic applications. However, the difficulty in synthesizing β-MoO 3 with a one-dimensional (1D) morphology has limited its use in applications requiring a large specific surface area. In the present work, β-MoO 3 whiskers were prepared by thermally evaporating α-MoO 3 powder in a tube furnace at temperatures (T f ) from 750 to 1000 • C and under flowing O 2 gas. The collected samples were identified as mainly β-MoO 3 by X-ray diffraction measurements, and the highest purity β-MoO 3 was obtained at T f = 1000 • C. Scanning and transmission electron microscopy observations showed that whiskers with a width of 10 nm were successfully synthesized by this method. The whiskers were confirmed to be β-MoO 3 via lattice image analysis. Measurements of the temperature distribution in the tube furnace and comparisons with the Mo-O phase diagram led to the conclusion that the whiskers formed via a vapor-solid route. Prepared β-MoO 3 whiskers were compared with α-MoO 3 powder via the X-ray photoelectron spectroscopy characterization method. By elucidating the β-MoO 3 whisker synthesis mechanism, this research provides guidance for the large-scale production of β-MoO 3 whiskers.

β-MoO 3, the molybdenum oxide phase with the highest catalytic activity, is a promising material in optical, chemical, and electronics applications as a replacement for α-MoO 3 . α-MoO 3 1D nanostructures such as whiskers and fibers have been investigated and used in applications. However, difficulties in the synthesis of onedimensional β-MoO 3 have obstructed researchers in the study of its properties. In this research, β-MoO 3 nanowhiskers were synthesized from core/shell molybdenum/molybdenum oxide wires by pulsed wire discharge in mixed oxygen and argon gases. X-ray diffraction analysis identified the main phase in the obtained samples as β-MoO 3 . Electron microscopy observations revealed whiskers with an average length and width of 216 nm and 23 nm, respectively. Transmission electron microscopy lattice imaging confirmed the successful synthesis of β-MoO 3 nanowhiskers. Various models were considered to explain the formation of β-MoO 3 nanowhiskers, including mechanical fragmentation of α-MoO 3 layers as well as vapor-liquid-solid and vaporsolid mechanisms. The phase transformation from α to β-MoO 3 under a nucleation process was explained based on the Mo-O phase diagram. To the best of our knowledge, this is the first report of β-MoO 3 nanowhisker synthesis.