Structure, Optical, and Catalytic Properties of Novel Hexagonal Metastable h-MoO₃ Nano- and Microrods Synthesized with Modified Liquid-Phase Processes

Abstract

Single crystalline h-MoO 3 nano-and microrods were successfully synthesized using modified liquid-phase processes with concentrated HNO 3 and H 2 SO 4 . Their X-ray powder diffraction (XRD) data were unambiguously indexed based on a hexagonal structure with the lattice constants a ≈ 10.57 and c ≈ 3.72 A ˚instead of a = 10.53 and c = 14.98 A ˚(JCPDS 21-0569) usually adopted. Rietveld refinements of the XRD data were pioneeringly performed based on the (Na 3 2H 2 O)Mo 5.33 -[H 4.5 ] 0.67 O 18 structure with the space group of P6 3 /m regardless of H þ and Na þ . Nanorods synthesized under different conditions show different sizes and aspect ratios. Annealing at 300 °C for 3 h significantly improves the crystallinity and phase purity of as-synthesized h-MoO 3 rods, which is evidenced by sharpening of peaks in micro-Raman spectra with no shift. An irreversible transition from h-MoO 3 to R-MoO 3 occurring between 413 and 436 °C can be triggered by irradiation of either electrons or laser with high energies or powers as well. The turning points on both differential thermal analysis (DTA) and thermogravimetry (TG) curves show presence of water molecules interacted differently with the lattice which escape at different temperatures. h-MoO 3 rods reduce the temperatures of soot oxidation to 482-490 °C, much higher than its structural transition temperatures. This makes it simply suitable for catalyzing reactions taking place at temperatures lower than the transition temperatures, say, as the catalyst of the selective oxidation of methanol.

Nanostructured MoO3 for Efficient Energy and Environmental Catalysis

“…The tunnel structure in h-MoO 3 exhibits considerably higher sensitivity, coloration efficiency, and faster response in comparison with the orthorhombic α-MoO 3 . It is mainly attributed to the h-MoO 3 of tunnel structure with a higher structure openness degree, giving rise to an accelerated electron-hole separation in photochromism and a facile Li + ion insertion/extraction in electrochromism [36].…”

Thermal transformations of the composition and structure of hexagonal molybdenum oxide

“…The tunnels are 1D cavities with a diameter of ∼3 Å formed by the adherence of 12 octahedra towards the с axis. At present, the physicochemical characteristics of the h-MoO 3 phase have been extensively studied for optical, catalytic, electrochemical, and sensory applications [4,[6][7][8][9]. However, a wide practical use of this metastable phase is largely refrained by a weak development of the synthesis basis in which the empirical approach prevails, and both reasons for its formation and factors providing the stability of the metastable state remain unclear.…”

“…After the critical analysis of the literature data, the method of chemical deposition from aqueous solutions in an open system excluding high vapor pressures of mother liquor components at a maximum possible temperature of 90 °C seems to be promising [4,7,[14][15][16][17][18][19][20]. The advantage of this method of obtaining h-MoO 3 crystals consists in the use of products with the minimum content of hard-to-remove impurities inevitably coming from the solutions.…”

Aqueous Crystallization Strategy for Metastable h‐MoO3 Crystals with Polyvinylpyrrolidone Induction

“…Even though h ‐MoO 3 has promising performance for potential applications, the development of an easy synthetic strategy is still a challenge, partly because of the complications in stabilizing the metastable phase 28–32. The 1D tunnels in h ‐MoO 3 crystals were commonly thought to form through the loss of Mo cations from these positions, and the unstable structure makes the preparation of h ‐MoO 3 crystals unsatisfactory.…”