Thermal Effects Associated with the Raman Spectroscopy of WO₃ Gas-Sensor Materials

Abstract

Metal oxides are suitable for detecting, through conductive measurements, a variety of reducing and oxidizing gases in environmental and sensing applications. Metal-oxide gas sensors can be developed with the goal of sensing gases under specific conditions and, as a whole, are heavily dependent on the manufacturing process. Tungsten oxide (WO3) is a promising metal-oxide material for gas-sensing applications. The purpose of this paper is to determine the existence of a correlation between thermal effects and the changes in the Raman spectra for multiple WO3 structures. We have obtained results utilizing Raman spectroscopy for three different structures of WO3 (monoclinic WO3 on Si substrate, nanopowder, and nanowires) that have been subjected to temperatures in the range of 30-160 °C. The major vibrational modes of the WO3:Si and the nanopowder samples, located at ~807, ~716, and ~271 cm(-1), correspond to the stretching of O-W-O bonds, the stretching of W-O, and the bending of O-W-O, respectively; these are consistent with a monoclinic WO3 structure. However in the nanowires sample only asymmetric stretching of the W-O bonds occurs, resulting in a 750 cm(-1) band, and the bending of the O-W-O mode (271 cm(-1)) is a stretching mode (239 cm(-1)) instead, suggesting the nanowires are not strictly monoclinic. The most notable effect of increasing the temperature of the samples is the appearance of the bending mode of W-OH bonds in the approximate range of 1550-1150 cm(-1), which is related to O-H bonding caused by humidity effects. In addition, features such as those at 750 cm(-1) for nanowires and at 492 and 670 cm(-1) for WO3:Si disappear as the temperature increases. A deeper understanding of the effect that temperature has on the Raman spectral characteristics of a metal oxide such as WO3 has helped to extend our knowledge regarding the behavior of metal oxide-gas interactions for sensing applications. This, in turn, will help to develop theoretical models for the identification of specific metal oxide-gas relationships.

Transparent conducting oxide induced by liquid electrolyte gating

“…2G. In the pristine state spectrum, three peaks that are characteristic of monoclinic WO 3 are observed at ∼275, ∼715, and 802 cm −1 (20,21). Based on group theory calculations and Raman selection rules, the first is associated with the bending vibration of W in the O-W-O chain, and the second corresponds to deformation vibrations around the oxygen.…”

Photo-controllable thermoelectric properties with reversibility and photo-thermoelectric effects of tungsten trioxide accompanied by its photochromic phenomenon

“…To date, WO 3 has been applied in visible light-sensitive photocatalysts, photo-/electro-chromic smart windows, and gas/humidity sensors. [10][11][12][13][14] In the present study, we focused on the control of the thermoelectric properties (Seebeck coefficient (S) and electrical conductivity (r)) by light irradiation-induced reversible switching, as well as the photochromic properties, of WO 3 .…”

Visible light catalysis of methyl orange using nanostructured WO3 thin films

“…The presence of high frequency Raman peaks at 715 and 805 cm −1 are due to the W-O-W stretching vibration modes [45]. The chief vibrational modes of the WO 3 sample, located at 805, 715 and 269 cm− 1 , corresponding to the stretching and the bending of O−W−O, respectively, are consistent with a monoclinic structure [46].…”