Effects of Co-Addition of Au and CuO to Macroporous SnO2 Microspheres on Their Acetone and Toluene Sensing Properties As Thick Film Sensors
Authors: Shimizu Yasuhiro , Torai Soichiro , Ueda Taro , Kamada Kai , Hyodo Takeo 
Published: July 2022

Controlling of meso- and macro-porous structure of semiconductor metal oxide particles used as sensor materials is of primary importance in improving their gas response properties. To date, we could improve H2 and NO2 response properties of SnO2 [1] and In2O3 [2] thick film sensors by the introduction of macropores into these oxide particles, by employing ultrasonic spray pyrolysis (USP) of the oxide precursor solution containing polymethylmethacrylate (PMMA) microspheres as a template. We also demonstrated the usefulness of the Au loading on macroporous In2O3 (pr-In2O3) and the PdOx-CuOx loading on pr-WO3 in improving NO2 [3] and methylmercaptan [4] response properties, respectively.
By the way, toluene and acetone are typical organic solvents used widely in many fields, but are very toxic and hazardous gases to be detected and then eliminated by suitable methods form the viewpoint of our safe and reliable life. In addition, the patients suffering from lung cancer and diabetes are known to release high concentrations of toluene and acetone, respectively. Therefore, the development of gas sensors capable of detecting these gases sensitively and selectively may offer a new and non-invasive diagnostic method of these diseases.
In the present study, attempts were made to improve acetone and toluene response properties of thick film sensors fabricated by pr-SnO2 microspheres prepared by the USP method. Effects of the CuO addition to pr-SnO2 microspheres (pr-(CuO-SnO2)) and the additive methods of different amounts of Au to pr-(CuO-SnO2) microspheres on their response properties to both two gases have been examined.
The aqueous solution containing 5.0×10-3 mol of SnCl4･5H2O and 3.2 g of PMMA microspheres (ca. 70 nm in size) per 100 mL of deionized water was used as the base precursor solution. The precursor mists were carried into the electronic furnace heated at 1,100°C to prepare pr-SnO2 microspheres. In preparing pr-(xCuO-SnO2) and pr-(yAu-xCuO-SnO2) (x and y: the additive amounts in wt% per the SnO2 weight), appropriate amounts of CuCl2･2H2O and HAuCl4･4H2O were added to the base precursor solution. Besides, an appropriate amount of Au was loaded on pr-(xCuO-SnO2) by a conventional impregnation method. Thick film sensors (10 - 15 mm in thick) were fabricated by screen printing of the slurry of thus-prepared pr-SnO2-based microspheres on an alumina substrate equipped with a pair of Pt electrodes, and their sensing properties to 20–100 ppm acetone, toluene and ethanol were measured in the temperature range of 300–500°C in air. The gas response is defined as the ratio of the sensor resistance value in air (Ra) to that in a sample gas (Rg). The sensors thus fabricated are referred to as the abbreviations of sensor materials.
Figure 1 shows FE-SEM photographs of pr-SnO2-based microspheres prepared. All kinds of microspheres are confirmed to show similar porous structure formed by the thermal decomposition and evaporation of PMMA microspheres contained in the mists of the precursor solutions, except for 3.0Au/pr-(2.0CuO-SnO2). The appearance of 3.0Au/pr-(2.0CuO-SnO2) microspheres indicates partial filling of the loaded Au inside of the pores especially for the outermost surface of macroporous microspheres. Figure 2 shows variations in response to 100 ppm acetone of pr-SnO2-based sensors with operating temperature. The order of sensor resistance value in air was pr-(3.0Au-2.0CuO-SnO2) ³ pr-(2.0CuO-SnO2) > pr-(5.0Au-2.0CuO-SnO2) > 5.0Au/pr-(2.0CuO-SnO2) > 3.0Au/pr-(2.0CuO-SnO2) > pr-SnO2 at 350°C. Although the physicochemical states of Au and CuO in pr-(5.0Au-2.0CuO-SnO2) are not clarified yet, the addition of CuO to pr-SnO2 resulted in an increase in sensor resistance in air, due to the formation of p-n heterojunction in the macroporous microspheres. In contrast, the Au addition to or the Au loading on pr-(2.0CuO-SnO2) led to a decrease in sensor resistance. Among the sensors tested, pr-(3.0Au-2.0CuO-SnO2) showed the largest response to 100 ppm acetone at 350°C. Therefore, high sensor resistance in air is one important factor for the largest acetone response, but the added Au in pr-(3.0Au-2.0CuO-SnO2) is considered to accelerate the reaction of acetone with chemisorbed oxygen effectively. The smaller acetone response observed with 3.0Au/pr-(2.0CuO-SnO2) is considered to arise from the decrease in reaction site, as is confirmed by Fig. 1 d). The detailed gas response properties of these sensors, including those to toluene and ethanol, the latter is a possible interference gas in detecting acetone and toluene, will be reported in the presentation.
[1] K. Hieda, et al., Sens. Actuators B, 133(1), 144-150 (2008).
[2] T. Hyodo, et al., Sens. Actuators B, 187, 495-502 (2013).
[3] T. Ueda, et al., Front. Mater., 6, 1-10 (2019).
[4] N. Tammanoon, et al., ACS Appl. Mater. Interfaces, 2020, 12, 41728-41739 (2020).

Figure 1