## 3 Results
### Gas Sensing Mechanism
In the electron sensitivity process, electrons are products of In\({}_{2}\)O\({}_{3}\) particles, and due to the difference in work function, they move to the precious metal nanoparticles. Thereby, the precious metal nanoparticles become rich in electrons, resulting in an increase in the electron depletion layer. An In\({}_{2}\)O\({}_{3}\) sensor decorated with noble metal will become more sensitive to change in the external atmosphere because the potential barrier height between neighboring grains rises. Chemical sensitization of noble metals occurs by the transport of atoms in the interfacial region, known as the spillover effect, and has been used to describe the catalytic process of metal-supported catalysts [24; 25; 26]. In this process, the loaded metal particles such as Pt, Pd and Au will have more active sites than In\({}_{2}\)O\({}_{3}\) particles for the adsorption of molecular  
Figure 12: (**a**,**b**) Schematic illustration of gas sensing mechanism and (**c**,**d**) schematic energy band diagram of the sensing materials (In\({}_{2}\)O\({}_{3}\), Au/In\({}_{2}\)O\({}_{3}\)). oxygen and target gas (carbon monoxide). This will lead to much more adsorption and easier dissociation of these molecules owing to the high catalytic or conductive nature of the noble metal particles. In air, chemically adsorbed ionic oxygen species on the metal particles will move toward the In\({}_{2}\)O\({}_{3}\) particles, causing the enlarged depletion region near the outer surface region of the In\({}_{2}\)O\({}_{3}\) particles. In the target gas atmosphere, the target gas is easily adsorbed to these metal nanoparticles and is rapidly oxidized by ionic oxygen species densely present near the metal nanoparticles. This spillover effect of metal nanoparticles may significantly improve the number and speed of electron transfer, which is directly related to the sensitivity and response speed of the gas sensor. The effectiveness of noble metals depends greatly on their location and chemical state. As revealed in XPS analysis, Au nanoparticles decorating In\({}_{2}\)O\({}_{3}\) particles exist mainly as metal Au0 and exhibit much stronger electronic interactions with In\({}_{2}\)O\({}_{3}\) particles than with Pd or Pt. This means that the sensing properties of Au/In\({}_{2}\)O\({}_{3}\) for CO gas are superior to those of pristine In\({}_{2}\)O\({}_{3}\), Pd/In\({}_{2}\)O\({}_{3}\), and Pt/In\({}_{2}\)O\({}_{3}\), because Pd or Pt nanoparticles decorating In\({}_{2}\)O\({}_{3}\) are partially oxidized or exist in the form of oxides. Oxidation of Pd or Pt particles reduces the effective active area of metal Pd or Pt particles for gas adsorption, weakening the electron interaction between the metal and In\({}_{2}\)O\({}_{3}\), and greatly affecting the spillover of the target gas to the metal nanoparticle surface. In addition to the chemical state, the amount and dispersion of noble metal nanoparticles decorating the In\({}_{2}\)O\({}_{3}\) surface have a great influence on the sensitization effect of a sensor's sensing properties [25; 57]. To identify the effects of noble metals on In\({}_{2}\)O\({}_{3}\) sensors, the responses of Pt/In\({}_{2}\)O\({}_{3}\), Pd/In\({}_{2}\)O\({}_{3}\), and Au/In\({}_{2}\)O\({}_{3}\) sensors with CO gas were compare, as shown in Figure 12a-d. The response of the four In\({}_{2}\)O\({}_{3}\)-based sensors to carbon monoxide (CO) followed the order Au/In\({}_{2}\)O\({}_{3}\) > Pd/In\({}_{2}\)O\({}_{3}\) > Pt/In\({}_{2}\)O\({}_{3}\) > In\({}_{2}\)O\({}_{3}\). This means that Au/In\({}_{2}\)O\({}_{3}\) is better than Pd/In\({}_{2}\)O\({}_{3}\) and Pt/In\({}_{2}\)O\({}_{3}\), and that this is directly related to the catalytic activity of Au nanoparticles. Nanoparticles are well known because their catalytic functions can serve as effective adsorption sites that bind oxygen molecules. Thus, more of the absorbed oxygen species diffuse to the surface of the sensing material, resulting in a thick layer of electron depletion layers forming a conduction band of In\({}_{2}\)O\({}_{3}\).