Stability and Controlled Composition of Hexagonal WO₃

Abstract

This paper discusses the formation of nanosized hexagonal tungsten oxide (h-WO 3 ) during the annealing of hexagonal ammonium tungsten bronze (HATB), (NH 4 ) 0.33-x WO 3-y . This process was investigated by TG/DTA-MS, XRD, SEM, Raman, XPS, and 1 H-MAS NMR analyses. Through adjusting the temperature and atmosphere of annealing HATB, the composition (W oxidation state, residual NH 4 + and NH 3 content) of h-WO 3 could be controlled. The effect of composition on the conductivity and gas sensitivity of h-WO 3 was studied. New structural information was obtained about both HATB and h-WO 3 . It was found that NH 4+ and NH 3 could be situated at three different positions in HATB. Residual NH 4 + and NH 3 in the hexagonal channels seemed to be vital for stabilizing h-WO 3 : when they were completely released, the hexagonal framework collapsed. We propose that the structure of h-WO 3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels.

Room‐Temperature Solid‐State Grown WO3−δ Film on Plastic Substrate for Extremely Sensitive Flexible NO2 Gas Sensors

“…Raman analysis was also carried out to confirm the phases of the GSV‐WO 3–δ film using a laser source with 514.5 nm wavelength, as shown in Figure b. The Raman spectra showed major monoclinic peaks featuring the O–W–O stretching modes at 712 cm −1 and 808 cm −1 , which further corroborates the monoclinic phase of the GSV‐WO 3–δ film . Note that thermal treatment is essential for the fabrication of crystalline m‐WO 3 films in the cases of other deposition processes, yet we achieved crystallinity in the WO 3 by utilizing only the solid‐state GSV process at room temperature.…”

Large-scale synthesis and gas sensing application of vertically aligned and double-sided tungsten oxide nanorod arrays

“…It is well known that hexagonal (h) WO 3 is a metastable phase and can transform into monoclinic (m) WO 3 at high temperature. Recently, Szilágyi et al pointed out that the structure of hexagonal WO 3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels, and thus the existence of strictly stoichiometric hexagonal WO 3 is questionable [22]. X-ray photoelectron spectroscopy (XPS) (Fig.…”

Fabrication of hexagonal/cubic tungsten oxide homojunction with improved photocatalytic activity

“…To maintain the stability of hexagonal and cubic tungsten oxide, some impurities, such as some resident ions or/and molecules, are considered as requisite in the lattice channel to prevent thermodynamic transformation from the metastable phases to stable monoclinic phases [30][31][32][33][34]. Szilagyi et al noted that the Na + cation in the lattice channel is necessary to stabilize the crystal structure of WO 3 ·nH 2 O during a common hydrothermal preparation process [35]. Surface chemical compositions listed in Table 1 give the W/O atom ratio of about 0.4 for the three samples, which shows a nonstoichiometric tungsten oxide with some oxygen vacancies.…”