Sensitivity of tungsten oxide thin films for nitric oxide and methane gases

Tungsten oxide (WO₃) thin films were investigated to detect trace amount of nitric oxide (NO) and methane gases in air. WO₃ thin films were deposited between micro-patterned gold electrodes and annealed at 500degC in air for 2 hours. The as-deposited films exhibited an amorphous structure and annealed films indicated crystalline WO₃ with the average grain size of 40.3 nm. The crystallinity and surface structure of films were measured by atomic force microscopy (AFM) and X-ray diffraction (XRD) methods. The highest sensitivity for 9 ppm NO gas in air at 250degC was 92.6% and 160% for thin and thick WO₃ films, respectively. The respond and recovery times of thinner films were faster than thicker films. The sensitivity was also measured for high concentration level of methane gas to understand and compare the sensitivity of WO₃ films. The highest methane gas sensitivity was 600% for 6% (60,000 ppm) methane in air at 360degC.