Porous TiO2 films were prepared by the micro-arc oxidation (MAO) of Ti plates, and their CO sensing properties at low concentrations (5–100 ppm) were investigated as an application for an air quality control sensor. The obtained rutile films exhibited the maximum CO gas response at 350 °C, which is typical for semiconductor-type gas sensors, and the Ra/Rg was ∼1.6 for 10 ppm CO (Ra: resistance in air, Rg: resistance in a sample gas). The magnitude of the gas response increased almost linearly with increasing the CO concentration from 5 to 100 ppm. The CO sensing performance of the TiO2 sensor in the presence of humidity was investigated and compared with that of a SnO2 sensor.
There is an increasing demand for the air quality control in automobile cabins. An automated ventilation system, which requires the gas sensors that can monitor the air quality both inside and outside the vehicle, has been suggested for this purpose [1] and [2]. The major pollutants flowing into the cabin are the exhaust gases such as CO, hydrocarbons (HCs), and NOx emitted from the foregoing cars. At the intake, the pollutant gases are diluted with ambient air. Accordingly, at least, 30 ppm CO and 2 ppm NOx should be detected to find out the presence of gasoline and diesel vehicles ahead [1]. Moreover, the concentration limit of gas detection is being lowered to improve the automated ventilation system.
Tin-dioxide (SnO2)-based semiconductor type gas sensors have been used as air quality control sensors. In the literature, the SnO2-based air quality sensor exhibited a gas response of Ra/Rg ∼ 1.25 for 10 ppm CO and Ra/Rg ∼ 1.6 for 30 ppm CO [2]. However, SnO2-based sensors are known to be quite susceptible to humidity and temperature change accompanying with gas flow rate change, which influence the reference resistance in air (Ra) in addition to the gas response [1], [3] and [4]. For the reliable and reproducible sensor operations, the gas response of SnO2-based sensors should be further improved [5] and [6] or other sensor materials with less susceptibility to humidity are highly required.
TiO2 has been studied as a CO sensor, which works at high temperatures (>400 °C) due to its thermal stability. However, the magnitude of the gas response was generally low (Ra/Rg < 2) even toward several hundreds ppm CO gas [7] and [8]. Several attempts have been made to improve the CO gas sensing performance of the TiO2 sensor, including synthesis of nano-sized powders [8] and [9] and/or incorporation of various metals or metal oxides such as Au, Nb, Ta, Nb2O5, La2O3, CuO [7], [8], [10], [11] and [12]. The CO sensing properties were greatly enhanced by these approaches, but most studies were limited to high CO concentrations (>50 ppm). Recent reports have shown that a hydrogen sensor with an extremely high gas response can be achieved using TiO2 with various nano-dimensional architectures [13], [14], [15] and [16]. Based on these results, it was speculated that the CO sensing performance of the TiO2 sensor at low concentrations (<30 ppm) could be improved by employing these nano-architectures even though this sensor still exhibits the higher H2 sensing performance.
In this study, TiO2 films containing submicron-sized pores were prepared by micro-arc oxidation and their CO sensing properties were characterized at concentrations as low as 5 ppm in order to use them in air quality control sensors. In addition, the effect of humidity on the CO sensing performance was examined and the comparison was made to the SnO2 sensor. Special focus was placed on the low concentration limit of CO detection and signal stability against humidity.
Highly CO-sensitive TiO2 films were prepared by micro-arc oxidation of Ti plates. The sensor with Pd electrode showed a high CO gas response. The optimal operating temperature for CO gas sensing was at 350 °C, and the sensor signal increased almost linearly with the CO concentration from 5 to 100 ppm. The CO gas response to 10 ppm (Ra/Rg) was 1.68, and the sensor signal of the TiO2 sensor was less affected by humidity exposure compared to the SnO2-based sensor. It is expected that this sensor can be used for the reproducible CO gas detection at low concentrations for an air quality sensor application.
