Sensitive Detection of Methane By Indium Oxide Nanoparticles for Environmental Monitoring System
Authors: Zheng Qian , Lee Jun Ho , Kim Seong-Jun , Jeong Hwaebong , Lee Wooyoung 
Published: May 2020

Introduction and Motivation
With the development of science and technology, the process of industrialization is getting faster and faster, and people's living standards have also increased. However, the emission of gases such as methane has led to the greenhouse effect, which is 25 times [1] that of CO2 with equal molar volume. In addition, methane, as one of the main components of natural gas, is also flammable and explosive. If the gas concentration in the underground operation is too high (>5% [2]), it will be prone to explosion, resulting in casualties and property losses. Therefore, the development of high-performance methane gas sensors is particularly important.
In the past few decades, sensors prepared using metal oxide semiconductors (MOS) have been widely used in various application with the advantages of low-cost, easy fabrication and integration, good safety and long service life. Up to now, many kinds of MOS have been reported to detect methane, TiO2 [3], SnO2 [4], VO2 [5] and so on. However, there is little literature on the detection of methane by using indium oxide. Indium oxide (In2O3) has been extensively applied to the field of gas sensor as a gas sensing material due to its good catalytic activity and high electric conductivity.
In this paper, In2O3 NPs were prepared by co-precipitation method, and their morphology, nanostructure and sensing properties to methane gas were investigated.
Experiment
Indium oxide (In2O3) NPs were prepared by the co-precipitation method. Ammonia hydroxide (NH4OH), ethanol and indium nitrate (In(NO3)2·9H2O) were used as raw materials to obtain the precursor, which was calcined in a furnace at 300°C for 2h to get yellow powder. The as-prepared In2O3 NP powder was dissolved in methanol and dropped on Pt interdigitated electrode, which was fabricated with photolithographic techniques. After that, the sensor was annealed at 600℃ for 30 minutes.
Surface morphology and structural properties of In2O3 powder were investigated by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopes (TEM) and X-ray diffraction (XRD). The sensing properties of In2O3 NPs toward methane gas were investigated at various operating temperatures (200-500℃). The response of the sensor was defined as R = (Ra-Rg)/Rg, where Ra and Rg represent the resistance of the sensor in air and CH4 gas, respectively.
Results and Conclusions
The SEM image shows that indium oxide is composed of the spherical nanoparticles and the TEM image (Figure 1) confirms its spherical structure with a diameter of about 20 nm. The XRD result reveals that all the diffraction peaks are in good agreement with the cubic structure In2O3 (JCPD 06-0416). No impurity peaks were found, and the diffraction peaks had high diffraction intensity, which indicated that In2O3 had high purity and crystallinity.
The response of the In2O3 NP sensor increased and reached a maximum (R=6.81) at 350 ℃ and decreased with the increasing temperature (Figure 2). Therefore, this temperature is optimized and used for subsequent experiments.
The higher response compared to other groups means that the prepared sensor has a good potential for the detection of methane. Limit of detection, long-term stability, repeatability and selectivity are also important parameters for a successful sensor. Therefore, we will conduct a number of follow-up experiments to further analyze the performance of In2O3 NP sensor.
References
[1] D.P. Xue, Zh. Zhang, et al. Hydrothermal synthesis of methane sensitive porous In2O3 nanosheets. Materials Letters 252 (2019) 169–172
[2] S. Basu, P.K. Basu, et al. Nanocrystalline metal oxides for methane sensors: role of noble metals. J. Sens. 2009 (2009)
[3] B. Comert, N. Akin, et al. Titanium Dioxide Thin Films as Methane Gas Sensors. IEEE SENSORS JOURNAL, VOL.16, NO.24, DECEMBER 15, 2016
[4] L.P. Yang, Zh. Wang, et al. Synthesis of Pd-loaded mesoporous SnO2 hollow spheres for highly sensitive and stable methane gas sensors. RSC Adv., 2018, 8, 24268
[5] W.J. Li, J.R. Liang, et al. Synthesis and room temperature CH4 gas sensing properties of vanadium dioxide nanorods. Materials Letters 173 (2016) 199–202

Figure 1