## 1 Introduction  
Environmental pollution has attracted growing attention due to its serious harm to human health. Carbon monoxide (CO) is one of the most harmful gas contaminants and readily reacts with hemoglobin and damages the human body. CO is considered a major threat to environmental safety and human health, even at the very low concentration of 35 ppm. CO is an odorless, colorless, toxic gas with mild effects such as headaches, dizziness, disorientation, and fatigue at low exposure levels. However, greater exposure to carbon monoxide (CO) often causes death in everyday life. Therefore, it is essential to develop high-sensitivity room temperature CO gas sensors capable of rapid responses and sensitive enough to detect parts per million concentrations when applied in domestic and industrial sectors. Available CO gas sensors have several issues, including low sensitivity, high working temperature, and slow response or recovery time [1, 2, 3, 4, 5]. Nanostructures of oxidized semiconductors such as SnO\({}_{2}\), ZnO, WO\({}_{3}\), TiO\({}_{2}\), and In\({}_{2}\)O\({}_{3}\) are widely used as gas sensors, especially for CO detection [6, 7, 8, 9, 10, 11, 12]. Metallic oxide semiconductor (MOS)-based CO sensors have drawn considerable attention because they can be applied in microfilms or nano thin films. The advantage of MOS-based CO sensors is that they are easier to fabricate, smaller, and more cost-effective than other types of detection systems. The sensitivity of metal oxide semiconductor (MOS) gas sensors is largely influenced by the electron transfer between metal oxide particles, including metal catalysts and chemically adsorbed oxygen ions, which is dependent on the physical andchemical properties of the metal oxide semiconductor particles and the catalysts on them, such as particle size, specific surface area, microstructure of sensing layer, band gap energy and chemical state, etc. [13; 14; 15; 16]. However, most of the current MOS sensors are limited by long response times, high operating temperatures, and high detection limits. In the meantime, considerable efforts have been made to improve the performance of MOS sensors through doping or decoration. For example, In\({}_{2}\)O\({}_{3}\)-ZnO composite nanotubes, Ce\({}^{-}\) doped SnO\({}_{2}\) hollow, Ni-doped and W-doped NiO hierarchies have been reported. In addition, nano-sized precious metals have widely been used to improve the sensing properties of MOS due to their excellent catalytic activity. The most common method used to increase the sensitivity of a metal oxide sensor is to decorate it with metal catalysts. At present, various metal nanoparticle catalysts are being actively developed as metal oxide materials, including catalysts that are highly sensitive to certain gases. These catalysts can be broadly classified into two types. One type of catalyst is based on a chemical sensitization method in which metal catalysts such as Au, Pd, Pt or Pd are used to increase the concentration of gases participating in the surface reaction. Another type of catalyst is based on an electron sensitization method that improves sensitivity by taking advantage of the change in the number of oxidations caused by forming metal oxides such as In\({}_{2}\)O\({}_{3}\), NiO, Co\({}_{2}\)O\({}_{3}\) and Ag\({}_{2}\)O, with Ni, Co, and Ag. Existing studies on such oxides as In\({}_{2}\)O\({}_{3}\), NiO, Co\({}_{2}\)O\({}_{3}\), and Ag\({}_{2}\)O have reported ways to apply catalysts, improve sensitivity, or improve selectivity to oxides to reduce operating temperatures. The n-type semiconductor, which has a direct band gap energy of 3.55-3.75 eV, is most commonly recognized as a sensing material [17; 18]. Indium oxide (In\({}_{2}\)O\({}_{3}\)) has a relatively good electrical conductivity and is chemically and thermally stable, even in environments with a mixture of various toxic gases.