For employing as sensing materials, multi-walled carbon nanotubes-tungsten oxide (MWCNT-WO₃) composites were synthesized by using acid precipitation method following by calcination under air atmospheres. The research procedure was divided into 3 steps including investigation of WO₃ synthesis, synthesis of MWCNT-WO₃ composites, and investigation of gas sensing ability of as-prepared sensing materials. In the step of synthesis of tungsten oxide nanoparticles, it was found that the suitable condition with the highest percent yield of 71.4% was obtained from precipitation of 5.3 mM tungsten salt and 10 M nitric acid at 80°C for 30 min and then calcine at 400°C for 1 hour. In the second step, MWCNT-WO3 composites were then synthesized with different mass ratio of MWCNTs to tungsten salt according to the suitable precipitation and calcination condition. The actual mass ratio (MWCNTs: WO₃) is in a range of 0.4:100 to 8.5:100. In the last step, three sensing materials including WO₃ nanoparticles, MWCNT-WO₃ composites, pristine MWCNTs were employed to prepare in-house fabricated thick film gas sensors by facile drop coating method. Sensing ability of all fabricated sensor were tested by exposure to CO₂ with concentration of 500 ppm at room temperature to 200°C. Gas sensors prepared from WO₃ calcined at 400°C would exhibit higher sensitivity to CO₂ than that calcined at 300 °C or 600°C. For the composites as sensing materials, the fabricated sensor with the smallest ratio of MWCNTs to WO₃ (0.4:100) could provide three times sensitivity higher than the sensors prepared from WO₃ nanoparticles. With the optimum ratio of MWCNTs to WO₃ (1.5:100), the sensors could also respond to CO₂ at the lowest operating temperature of 50°C with high sensitivity of 41.0% and the shortest response time of 19.1 seconds. It means that the operating temperature could be reduced four times from 200°C to 50°C when the composites at this ratio (1.5:100) were used instead of WO₃ nanoparticles.
