Effect of annealing on tungsten oxide thin films for acetone gas detection

The gas-sensing properties and topology of tungsten oxide thin films deposited by reactive-ion radiofrequency magnetron sputtering were investigated at room temperature. The abnormalities in sensing film behaviour were observed when acetone gas is flowed over surface. The reduction reaction of surface and oxidation reaction of acetone gas were studied. As the gas comes in contact with the surface, the molecules tend to reduce the surface, hence, decreasing the resistance. The sensing film was annealed at different temperatures, viz., 300, 400, 500, 600 and ${700}^{\circ }\text{C}$ for 1 h each for the purpose of finding the optimum annealing temperature to detect acetone gas. Various characterizations, viz., XRD, AFM, FESEM, thickness measurement through surface profiler, gas-sensing characterization for recording resistance changes were performed. The optimum annealing temperature at which the sensing film gives maximum response when it comes in contact with acetone gas was computed to be ${500}^{\circ }\text{C}$. Also, operational optimum temperature for sensing film annealed at ${500}^{\circ }\text{C}$ was computed to be ${260}^{\circ }\text{C}$. A grain size of 7.3 nm was computed through analysis of AFM image and a film thickness of 100 nm was calculated through surface profiler. The SEM image of the film demonstrates that the grains developed on the surface, which increase in size with increase in the annealing temperature. The XRD patterns reveal that the oxide showed up was ${\text{WO}}_2$. It was observed that the response percentage is $\sim$31% for acetone vapour concentration of 20 ppm, $\sim$19% for the concentration of 15 ppm and $\sim$15% for the concentration of 10 ppm. The response time of the sensor is $\sim$5 min and the recovery time is 4 min.