Toluene-Sensing Properties of Solid-Electrolyte Gas Sensors Using CeO2-Added Au Thin-Film As Sensing Electrodes

Abstract

Introduction The exhaled breath of patients contains higher concentrations of specific gases than those of healthy people. For example, the patients suffering from lung cancer, diabetes and periodontitis release high concentrations of toluene, acetone and methylmercaptan, respectively. Among various kinds of gas sensors, solid-electrolyte gas sensors have shown enhanced sensing properties to volatile organic compounds (VOCs) by optimizing the microstructure as well as composition of the sensing electrode (SE) [1]. We have also reported that the addition of CeO2 to the Au SE of yttria-stabilized zirconia (YSZ)-based gas sensors increased the toluene response [2, 3]. In this study, thin-film CeO2-added Au SEs (thickness: 30‒100 nm) were fabricated by a spin-coating method, and their sensing properties to toluene were examined at 500°C. 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. Response 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 500°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 8CeO2/Au(5) SE was very thin (ca. 30 nm), and the thickness increased with the number of spin-coating cycles. The SE 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.Figure 1 shows response transients of the nCeO2/Au(x) sensors to 50 ppm toluene at 500°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. ΔE of the 8CeO2/Au(x) sensors largely decreased with an increase in the thickness of SEs. Generally, the VOC-sensing mechanism of solid-electrolyte gas sensors can be explained on the basis of the mixed potential theory [1‒3]. 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 [2]. 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 the SE thickness, which resulted in the largest toluene response of the 8CeO2/Au(5) sensor among the 8CeO2/Au(x) sensors. However, ΔE of the 24CeO2/Au(x) sensors increased with an increase in the thickness of SEs, and the 24CeO2/Au(15) sensor showed the largest ΔE (ca. 199 mV) among the sensors examined. The details of this phenomena will be discussed in the presentation.