Toluene-Sensing Properties of Mixed-Potential Type YSZ-Based Gas Sensors Attached with Au-Based Electrodes Prepared By a Spin-Coating Method

Abstract

Introduction Exhaled breath of patients contains a higher concentration of specific gases than that of healthy people. For example, the patients suffering from lung cancer, diabetes and periodontitis release a high concentration of toluene, acetone and hydrogen sulfide and/or methyl mercaptan, respectively [1, 2]. Among various kinds of gas sensors, solid-electrolyte gas sensors have shown enhanced sensing properties to volatile organic compounds (VOCs) by optimizing the composition and/or microstructure of the sensing electrode (SE) [3]. We also reported that sensors attached with thin-film CeO2-added Au SEs (thickness: 30‒100 nm) fabricated by a spin-coating method showed large response to toluene at 500°C. In addition, the toluene response of the sensors attached with a 24 wt% CeO2-added Au SE increased with an increase in the SE thickness, and the sensor attached with the thickest SE showed the largest response [4]. In this study, effects of the change in the additive amount of CeO2 to Au SE, operating temperature, and toluene concentration on the sensing properties of the sensors were evaluated, and their sensing mechanisms were discussed. Experimental CeO2-added Au SEs were fabricated by a spin-coating method. The coating solution was prepared by the addition of Ce(NO3)3ˑ6H2O and polyvinyl alcohol into a 0.1 M HAuCl4 aqueous solution. The solution was dropped on the YSZ substrate, and it was spun at 3000 rpm for 30 s, followed by the heat treatment at 300°C. This process was repeated multiple times to increase the film thickness. Then, they were annealed at 700°C for 2 h in air. The obtained SEs were denoted as nCeO2/Au(x) SEs (n: an additive amount of CeO2, 8‒24 (wt%), x: the number of spin-coating cycle, 5–15). A Pt paste was screen-printed on the backside of the YSZ as the counter electrode (CE), and it was annealed at 700°C for 2 h in air. Sensing properties of the fabricated sensors (nCeO2/Au(x) sensors) to 50 ppm toluene in dry air were measured in a flow apparatus (gas-flow rate: 100 cm3 min−1) at 450‒550°C. The SE was exposed to a sample gas, while the CE was always exposed to dry synthetic air during gas response measurements. The open-circuit voltage between the two electrodes (E, mV) of the sensors as a sensing signal was measured with a digital electrometer, and the response (ΔE) was defined as the difference in E between in dry air and in toluene balanced with dry air. The microstructure of the sensors was observed by scanning electron microscopy (SEM; JEOL Ltd., JSM-7500F). Results and Discussion The thickness of the SEs increased with the number of spin-coating cycles, and the thickness was not affected by the additive amounts of CeO2 in the examined number of spin-coating cycles (x). Therefore, the SE thickness was controlled from 30 to 100 nm only by changing the x value [4]. Figure 1 shows response transients of the nCeO2/Au(x) sensors (n: 8, 24, x: 5, 10, 15) to 50 ppm toluene at 450 and 550°C in dry air. The E values of all the sensors negatively shifted upon exposure to toluene, but ΔE was largely affected by the SE thickness and the additive amount of CeO2 at both temperatures. ΔE of the 8CeO2/Au(x) sensors largely decreased with an increase in the thickness of SE, and the values observed at 550°C was much smaller than that at 450°C. Generally, the VOC-sensing mechanism of solid-electrolyte gas sensors can be explained on the basis of the mixed potential theory [3]. Based on the theory, the electrochemical oxidation of toluene and electrochemical reduction of oxygen simultaneously proceed at triple phase boundaries (TPBs) at the interface between SE and YSZ, when toluene diffuses in SE and reaches TPBs [5]. In addition, a certain amount of toluene is catalytically oxidized during the diffusion in SE. Therefore, the actual concentration of toluene at TPBs decreased with an increase in both the SE thickness and the operating temperature, which resulted in the largest toluene response of the 8CeO2/Au(5) sensor at 450°C among the 8CeO2/Au(x) sensors. However, ΔE of the 24CeO2/Au(x) sensors increased with an increase in the thickness of SEs at both temperatures. In addition, ΔE of the 24CeO2/Au(5) sensor at 550°C was larger than that at 450°C, while ΔE of the 24CeO2/Au(x) sensors (x: 10, 15) at 550°C was smaller than that at 450°C. This behavior probably arises from the increase in the number of active sites for electrochemical toluene oxidation in the CeO2-added Au SE. The details of the response properties and gas-sensing mechanism of these sensors will be discussed in the presentation.