Ethanol Response of Semiconductor Gas Sensors Based on SnO2 Layer Prepared from Acidic Solution
Authors: Mori Masami , Sadaoka Yoshihiko , Ueda Tsuyoshi , Mitsuhashi Hirokazu , Nakatani Mikiya 
Published: February 2020

Previously, we examined the response behavior of SnO2 based semiconductor gas sensors prepared from SnO2 powders to volatile organic compounds (VOC). The response to VOC of SnO2 based sensors was strongly influenced with the morphology and the thickness of the sensing layer. In this study, the sensing layer was fabricated from mixed solutions of tin standard acidic solution with zinc or titanium standard acidic solution. The mixed solution was dropped onto the substrate with a pair of Pt electrodes by the micropipette, and then sintered at 600 to 900 oC for 7h. 0.64 ppm ethanol was used as a test gas. The air-balanced 0.64 ppm ethanol was regulated with a diffusion tube put in the gas permeater (GASTEC, PD-1B-2). Resistance of the elements was monitored by an ultra high resistance meter (ADVANTEST R8340). Gas response of the sensors was monitored as S = Gethanol/Gair, where Gethanol and Gair are the resistance in ethanol and in air, respectively.

 Zn-doped SnO2 sensor was prepared from the mixed solution of tin standard solution with zinc standard solution in 1:1.8 molar ratio. The obtained film was heated at 600, 700, 800 and 900 oC for 7h. The grain size increased with the increase in heat treatment temperature by aggregation between particles. The sensor heated at 800 oC showed the highest sensitivity to ethanol. Scanning electron microscopy (SEM) images showed that the Zn/Sn film heated at 800 oC composed of fine particles having 10 nm mean particle diameter. For the sensor heated at 800 oC, we examined the effect of thickness on the sensitivity to ethanol. 2, 4, 6, 8 μl of Zn/Sn mixed solution was dropped on the Pt electrode and heated at 800 oC for 7h. For all sensors, the sensitivity to ethanol increased with the increase in operating temperature from 400 to 700 oC. This result shows that there is a possibility that lattice oxygen involved in the sensor response. The sensor formed with 6, 8 μl of Zn/Sn mixed solution needed to heat at around the operating temperature for 10days before the ethanol sensitivity measurement to obtain stable response. The resistance increased with increasing the thickness of film. The sensitivity to ethanol was in order of 2 μl>8 μl>4 μl>6 μl at 600 to 700 oC under 0 %RH. The sensitivity of Zn/Sn sensor formed with 2μl of Zn/Sn mixed solution was 17 at 700 oC. Zn/Sn sensors showed high sensitivity to ethanol at 0 %RH, but the sensitivity of these sensors was depressed in humidified environment.

 Ti-doped SnO2 sensor was prepared from the mixed solution of tin standard solution with titanium standard solution in Ti/(Sn+Ti)=0, 0.01, 0.05, 0.1, 0.2, 0.4, 0.6, 1.0 volume ratio. The fabricated film was heated at 800 oC for 7h. The sensitivity of Ti/(Sn+Ti)=0.01 sensor was the highest of all the fabricated Ti/Sn sensors. The Ti/(Sn+Ti)=0.01 sensor showed the highest sensitivity at 500 oC under 0 %RH (S=5.5) and at 600 oC under 50 %RH (S=6.5). The sensitivity of Sn sensor was 2.5 at 500 oC, 0 %RH and 3.5 at 600 oC, 50 %RH. By adding 1 vol% of Ti to Sn, the sensitivity to ethanol increased twofold. The response behavior of Ti/Sn sensor in dry air and in humid air was about the same. The sensitivity of Zn/Sn sensor was depressed in humidified environment, while the sensitivity of Ti/Sn sensor was not affected by humidity even if the relative humidity was high. Ti/(Sn+Ti)=0.01 films with the different thickness were prepared from 2, 4, 6, 8 μl of Ti/(Sn+Ti)=0.01 mixed solution, and then heated at 800 oC for 7h. The sensitivity to ethanol was in order of 2 μl>4 μl>6 μl>8 μl. The sensitivity decreased with the increase in thickness of film, and the resistance increased with the increase in thickness of film. The thickness of film formed with 4 μl of Ti/(Sn+Ti)=0.01 mixed solution was about 130 nm.

 The sensitivity of SnO2 sensors prepared from standard solutions was higher than that of SnO2 sensors prepared from SnO2 powders. SnO2 films prepared from SnO2 powders composed of particles with 100 nm mean particle diameter. By using tin standard solution as a material of film, semiconductor films formed of fine particles having 10 nm mean particle diameter was obtained.