## 1 Introduction
In\({}_{2}\)O\({}_{3}\) is also well known as a gas-sensitive material excellent for detecting CO gas due to its high oxygen deficiency, which can greatly increase the gas adsorption rate. Therefore, In\({}_{2}\)O\({}_{3}\) has received much attention over the past few decades due to its potential for a variety of applications such as sensors, catalysts, and detectors. An example of the use of an In\({}_{2}\)O\({}_{3}\)-based sensor is in the selective detection of CO gas. One method used for detection is to correct the surface electrical resistance of In\({}_{2}\)O\({}_{3}\) by surface CO adsorption and reaction. Both the adsorption and desorption of gas molecules on the metal oxide surface are thermally activated physicochemical processes that improve sensor response and signal levels at high temperatures; thus, the reaction and recovery times are slower at room temperature. The development of room temperature gas sensors has very important advantages, such as low power consumption, easy system configuration, lower explosion risk, and extended device life. The response of surface chemical species to environmental gases are important for sensors and detection mechanisms and have been improved by work from many approaches, such as morphological control, doping metal oxidation composite doping [19; 20], carbon/graphene-based nanostructured materials [21; 22], and gas sensor operation. The work also includes metal nanoparticle integration [23], UV lighting [24], and microelectromechanical system fabrication [25]. Metal oxide surfaces have been decorated mainly with precious metals such as Au, Pd, Pt, and Ag to reduce their operating temperature and to increase their lower-limit detection, sensor response, and sensitivity [26; 27; 28; 29]. However, such a precious metal-decorated, metal oxide-based sensor is a very complex hybrid system, and the detection performance is subject to many effects on the surface, including the shape of each component, the exposure of the sensing material, and dispersion of the metal detection [30; 31; 32; 33]. Moreover, the mechanism of oxygen adsorption has not been adequately investigated to date when decorating with Au particles that act as active sites. Oxygen is adsorbed directly on the Au particles [34]. Another issue is that oxygen adsorption occurs in metal oxides. [35; 36]. In addition, the adsorption of oxygen released from a metal oxide immediately breaks down to produce lattice oxygen, which can react continuously at the interface with or without oxygen [37]. It will be of great significance to improve detection performance at relatively low temperatures using catalysts such as Au, Pd, and Pt nanoparticles with large surface areas, as well as to understand the accompanying sensitivity mechanisms. In this work, In\({}_{2}\)O\({}_{3}\) nanoparticles decorated with noble metals such as Au, Pd, and Pt were prepared using hexamethylenetetramine and In(NO\({}_{3}\))\({}_{3}\)\(\cdot\)\(\mathrm{Ar}_{2}\)O as raw materials. Nanoparticles such as Au/In\({}_{2}\)O\({}_{3}\), Pd/In\({}_{2}\)O\({}_{3}\), and Pd/In\({}_{2}\)O\({}_{3}\) were obtained by field reductions of HAuCl\({}_{4}\)\(\cdot\)3H\({}_{2}\)O, Cl\({}_{2}\)Pd, and H\({}_{2}\)Cl\({}_{6}\)Pt using NaBH\({}_{4}\). The Au/In\({}_{2}\)O\({}_{3}\) nanoparticles showed better response to low temperature (50 \({}^{\circ}\)C) CO than Pd/In\({}_{2}\)O\({}_{3}\) or Pt/ In\({}_{2}\)O\({}_{3}\). The sensitive effects of noble metals on the sensing properties of CO gas sensors are described using the surface chemical states of Au, Pd, and Pt nanoparticles and their interactions with In\({}_{2}\)O\({}_{3}\) particles. Furthermore, the mechanism by which Au/In\({}_{2}\)O\({}_{3}\) sensors detect carbon monoxide (CO) is discussed in detail.