Highly Sensitive Acetone Gas Sensors Using Nanocomposites of CuO Hollow Nanocubes Decorated on TiO2

Abstract

Introduction There has been considerable effort to detect volatile organic compounds (VOCs) by metal oxide semiconductor gas sensors for their high sensitivity, low cost, easy production, and compact size [1]. In recent times, acetone in exhaled breath have studied for biomarkers more than hazardous air pollutants. According to the previous studies, breath acetone may be correlated with fat burning [2-3] not only a specific breath marker for type-1 diabetes [4]. However, there are many drawbacks of metal oxide based acetone sensors to apply diet monitoring since the concentration of breath acetone for healthy humans are from 300 to 900 ppb, which require to develop highly sensitive sensing materials and systems. From this point of view, addition of noble metals for catalytic effect, formation of p-n heterojunction and synthesis of hierarchical and hollow nanostructures for high surface area are suggested to improve the sensing performances. In this study, we describe the novel nanostructure of hollow CuO nanocubes decorated on hollow TiO2 nanosphere and investigate their acetone gas response properties. Method TEM images of the nanoparticles were obtained by FEI Tecnai G2 F30 S-Twin (300 kV, KAIST). For the preparation of gas sensing devices based on TiO2-CuO nanocomposites, interdigitated electrodes on 8-inch oxidized silicon wafers were formed using conventional photo lithographic techniques. A sub micrometer layer of the photoresist was spin-coated onto the silicon oxide wafer at 5000 rpm and baked at 90 ℃ for 3 min before ultraviolet (UV) exposure in a photo mask aligner. After the exposure, the wafer was developed for 30 s in a developing solution. The Au electrode with 10 μm gaps on silicon substrate were successfully patterned with thin films (10 nm Cr/100 nm Au) deposited by e-beam evaporation and a metal lift-off process in acetone. Then, TiO2-CuO dispersed in ethanol spin-coated onto interdigitated electrodes. The dynamic sensing responses were measured using a data-acquisition system consisting of Agilent 34970A and BenchLink Data logger program, and the sensor devices were located in a temperature-controlled chamber. Dry air was used as a balance gas at a flow rate of 1000 cc/min. In addition, the acetone gas was used as an analyte (obtained from RIGAS Co. in Korea) at a concentration of 10 ppm/100ppm and blended with a balance gas to achieve the desired analyte concentrations of 20 ppb to 10 ppm by using mass-flow controllers. The gas response was measured by allowing the concentration-controlled acetone gas to flow into the reaction chamber, and then allowing only the balance gas to flow into the chamber for recovering the gas sensor. Results and Conclusions Figure 1 shows the TEM images of the formation of TiO2-CuO hollow nanocubes. First, 300 nm of hollow TiO2 nanosphere was synthesized as shown in figure 1 (a). Then, Cu2O nanocubes were decorated on TiO2 (figure 1 (b)) and diameter of nanocubes was approximately 20 nm. For the TiO2-CuO hollow nanocubes, TiO2-Cu2O need additional thermal oxidation process and the oxidized CuO hollow nanocubes are shown in figure 1 (c-d). Acetone-sensing properties of TiO2-CuO hollow nanocubes were studied at optimized operating temperature, 200 ℃, under a dry condition. Figure 2 shows the response curve of TiO2-CuO hollow nanocubes for acetone 1 ppm and the response value is estimated to 12.1. Here, TiO2-CuO hollow nanocubes shows the p-type semiconductor properties even TiO2 is typical n-type semiconductor because p-type CuO hollow nanocubes are covered the surface of TiO2 hollow nanosphere so that the ratio of p-p contact becomes higher and therefore, its electrical property is dominated by p-type. Figure 3 shows the abbreviation curve of sensitivity as a function of acetone concentration from 20 ppb to 500 ppb. The response values were increased with increasing acetone concentration and limit of detection was calculated as 2.23 ppb using the noise of base resistance [5]. And the responses of TiO2-CuO hollow nanocubes toward other gases existing in VOCs was also examined. As shown in figure 4, response of 1 ppm acetone, formaldehyde, benzene and ammonia gas at 200 ℃ were measured and TiO2-CuO showed the highest response to acetone gas among the tested gases.