Nanocrystalline Tungsten Oxide Thick Film Sensor for the Detection of H2S Gas
Metal oxide semiconductors (MOS) have been utilized as gas sensing active materials for half a century. One of the most promising solid-state MOS chemo sensors is n-type semiconducting tungsten oxide-based gas sensor. They have demonstrated novel sensing properties such as high sensitivity, fast response time and low operation temperature. In particular, pure or doped tungsten oxide is a promising material for the detection of various substances, e.g., H2,H2S,NOx, NH3 and ethanol.
Scientists have investigated the sensing characteristics of tungsten oxide nanoparticles to H2S in the 7 to 200 ppm range at working temperatures of the range of 100–225 ℃. Semiconductor gas sensors based on nanocrystalline WO3 powders were prepared by acid precipitation method. The thick films of the powder were coated on to glass substrate, annealed at 600 ℃ and its response to different concentration of H2S gas was studied. Sensor behavior is presented in detail for representative concentration of 18ppm. The result showed that WO3 nanoparticles are good candidates for H2S detection at a temperature of 200 ℃.
Sensitive layers of tungsten oxide were prepared by dispersing the prepared tungsten oxide powder in methanol and drop casting on glass substrates followed by overnight annealing at 600 ℃. The obtained crystalline phase of WO3 nanoparticles was triclinic in nature. The structure of sensor was characterized using XRD. The surface morphology and elemental composition were characterized by scanning electron microscopy and energy dispersive X-ray analysis. It was found that the WO3 samples consisted of crystalline aggregates. This was confirmed in TEM results. The particles were spherical in nature. Gas sensing properties of samples were studied for the detection of H2S gas. Resistance of the films decrease upon exposure to gases and attained a saturation value.
Sensor regains its original value after test gas is removed. Sensor exhibit good sensing characteristics to H2S in the concentration range studied, 7 to 200 ppm over the temperature range 100-125 0C. The best results were obtained at operating temperature of 200 ℃ with a sensitivity of 1.3. Response and recovery time of sensor at this optimum temperature was 22 seconds and 4.1 minutes respectively. Lowest measurable concentration is found to be 7ppm. Resistance always returned to its initial value after the test gas is shut off for all concentration studied. Results indicate that response of sensor is reproducible during this test.
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