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It is said that VOCs (volatile organic compounds) generated in house break human’s health due to long term exposure, so called sick house syndrom and that typical VOCs have bad influence are aldehydes (folmaldehyde, acetaldehyde), aromatics (toluene, xylene, ethylbenzene, styrene), and so on. These days, high sensitivity gas sensors for the detection of VOCs are incresingly demanded in order to control an autoventilation system in a highly airtight house. For this purpose, the sensors are needed to detect dilute VOCs in ppb-level. There was almost no report on the detection of dilute aromatic VOCs in ppb level. The high sensitivity aromatic VOC gas sensor was developed with the WO3 thin film deposited on the Au interdigitated microelectrode. The Au interdigitated microelectrode was fabricated by means of MEMS techniques (photolithography and RIE) (Fig. 1). Three different interdigitated electrodes were fabricated with the teeth distance of 2, 3, and 5 μm and fixed number of teeth (50) in order to investigate the effect of teeth distance. The H2WO4 suspension prepared from (NH4)10W12O41・5H2O was dropped on the Au interdigitated microelectrode, dried, and calcined at 500 °C for 3h in air to be the WO3 microsensor. The sensor resistance was measured in air (Ra) and in aromatic VOC containing air (Rg) at 400-550°C. The sensor response (S) was defined as Ra/Rg. Figure 2 shows the sensor responses to 1.5 ppm xylene of the WO3 thin film microsensors with interdigitated electrode of various teeth distance (2, 3, and 5 μm) as a function of operating temperature. The sensor response was increased with decreasing teeth distance. This behavior was also observed for toluene detection. In the WO3 microsensor with teeth distance of 2 μm, the highest sensor response was obtained. It was as high as 100 to 1.5 ppm xylene at 550 °C. It is noteworthy that the high sensor response is attained only WO3 sensing layer without any noble metal sensitizer such as Pd. The similar effect to the micro gap effect was observed as the teeth distance effect in the interdigitated microelectrode, suggesting that the sensor response at interface between oxide and electrode was higher than that at grain boundary in the aromatic VOCs sensing using WO3 sensor. This indicates that the high sensitivity aromatic VOC sensor can be developed with the electrode design of submicron level. Figure 3 depicts the sensor response of the WO3 microsensor with 2 μm interdigitated electrode to dilute aromatic VOCs at 500 °C as a function of gas concentration. The sensor responses to 0.01 ppm aromatic VOCs were sufficiently high (higher than 3), particularly the sensor response was as high as 6 to 0.01 ppm xylene. The WO3 microsensor with 2 μm interdigitated electrode exhibited the excellent response-recovery characteristics, i.e., the 90% response time of 10 sec and the 90% recovery time of 35 sec to 1.5 ppm xylene at 500 °C. This suggests that the WO3 microsensor can be used as the high sensitivity VOC sensor.

Detection of Dilute Aromatic VOCs Using WO3 Thin Film Sensors Equipped with Au Interdigitated Microelectrode