Sensor Response Under Humid Condition of Surface Modified SnO2 with Zr4+
Authors: Uchino Hotaka , Mizukami Takaharu , Watanabe Ken , Nishibori Maiko , Shimanoe Kengo 
Published: February 2020

Semiconductor gas sensors based on tin dioxide have been widely studied for half a century. Breath monitoring for healthcare using mobile devises has been attracting a great attention as a smart sensing application, because it makes us possible to check our health by ourselves, everywhere as we like, using portable devises.
In order to detect a specific gas in the breath, the sensor should be stable and highly sensitive under humid condition in the range from 0.5 to 3 vol.%. However, many researchers pointed out that SnO2 gas sensor exhibits the response to humidity and water vapor prevents the response to the target gas like H2, CO, ethanol and so on. Thus, there are various discussions about mechanism of sensor response under humid condition, but its answer is still not clear. Yamozoe et al. have proposed that receptor function of semiconductor gas sensors is governed by two adsorbed oxygen, O2- and O-. They demonstrated that drastic change of the sensor resistance is contributed by the formation of O2- in dry condition[1]. In addition, they have also reported that the major adsorbed oxygen change from O2- to O- by switching from dry to humid condition, reducing the sensor response. Recently, Ma et al. have also demonstrated the change of adsorbed oxygen and reduction of sensor response to H2 and CO under humid condition[2]. Thus, reduction of sensor response under humid condition is likely to be related with the change of the major adsorbed oxygen from O2- to O-. These findings strongly suggested that it is key factor to maintain O2-even under humid condition, to achieve the highly sensitive gas sensor under humid condition.
In recent years, we have tried control of oxygen chemisorption species under humid condition by the surface modification with Sb5+and Fe3+[3, 4]. We found that such surface modification is effective to keep O2- even under humid condition. Therefore, we believe that this approach is expected as one of the solution for development of stable and highly sensitive gas sensors under humid condition. However, a further investigation about the surface modification is needed, in order to design the highly sensitive gas sensor under humid condition. Here, in this paper, we focused our research attention on the surface modification on SnO2 with Zr4+, and aimed at revealing the effect of the surface modification.
SnO2 nanoparticle was synthesized by the reported method based on hydrolysis of SnCl4•5H2O and hydrothermal treatment[2]. The obtained precursor was calcined at 700 °C. The calcined powder was added to the mixed solution with HF and zirconium oxynitrate dihydrate. As a comparison, only HF treatment was also applied as the surface modification. The mixed solution is kept standing under stirring, filtrated to collect the Zr-modified SnO2 powder, and washed with deionized water at several times. The resulting powder was calcined at 580 °C for 3h. The sensor films were fabricated on Al2O3substrate by the screen printing method and electrical properties under dry and humid condition were evaluated by the experimental set-up as described elsewhere[2].
In order to investigate the oxygen adsorption species at 350 °C in humid condition, oxygen partial pressure dependence of the sensor resistance was performed. The HF treated SnO2 has liner proportional to PO2
1/2. On the other hand, Zr-modified SnO2 has liner proportional to PO2
1/4. According to the literature[1], when O2- is mainly adsorbed on the SnO2, the electric resistance is linearly proportional to PO2
1/4. Therefore, it was found that the Zr-modification is effective for keeping O2-even under humid condition. In our presentation, we will present these sensing properties and discuss about mechanism of sensing under humid condition in detail.
Reference
[1] N. Yamazoe et al., Sens. Actuators B, 163 128 (2012).
[2] N. Ma et al., ACS Appl. Mater. Interfaces, 7 5863 (2015)
[3] Suematsu et al.,(submitted)
[4] J.-K. Choi et al., Chem. Sens., 29 64 (2013).