## 3 Results
### Gas Sensing Mechanism  
As a rule, semiconducting metal oxide-based sensors work by the adsorption and desorption of gas molecules to and from the oxide surface, which leads to change of the sensor electrical resistance. When a sensing material such as In\({}_{2}\)O\({}_{3}\) is exposed to air, O\({}_{2}\) molecules in the air are adsorbed to the surface. This holds electrons from the oxides carried to form highly chemisorbed oxygen species (O\({}^{2-}\), O\({}^{-}\) or O\({}_{2}\)\({}^{-}\)) [55; 56], as indicated in Equations (1)-(4) due to the strong electron affinity of O\({}_{2}\). The activation temperature decides the present form of oxygen ions. Commonly, O\({}^{2-}\) requires a higher activation temperature to be chemisorbed, while the other two ions appear at lower temperatures. In particular, O\({}^{-}\) was the predominant oxygen species adsorbed at 150-400 \({}^{\circ}\)C [57]. \[\mathrm{O_{2(gas)}\to O_{2(ads)}} \tag{1}\]  
\[\mathrm{O_{2(ads)}+e^{-}\to O^{-}{}_{2(ads)}} \tag{2}\]  
\[\mathrm{O^{-}{}_{2(ads)}+e^{-}\to 2O^{-}{}_{(ads)}} \tag{3}\]  
\[\mathrm{O^{-}{}_{(ads)}+e^{-}\to O^{2-}{}_{(ads)}} \tag{4}\]  
Due to the relatively small band gap of a semiconducting oxide gas sensor, electrons can be thermally excited from the valence band and/or the donor levels to the unoccupied conduction band in the operating temperature from about 50 to 500 \({}^{\circ}\)C, which is needed to overcome the activation energy barrier of surface redox reactions and also increase the electron concentration in the conduction band. If a gas sensor sample is gradually heated, electrons will be thermally activated to the conduction band. If these excited electrons in the conduction band of In\({}_{2}\)O\({}_{3}\) semiconductor move from In\({}_{2}\)O\({}_{3}\) particle to oxygen molecule (O\({}_{2}\)) absorbed on the surface of In\({}_{2}\)O\({}_{3}\) particles inducing chemisorbed oxygen species (O\({}^{2-}\), O\({}^{-}\) or O\({}_{2}\)\({}^{-}\)), electron depletion layers will be created beneath the In\({}_{2}\)O\({}_{3}\) oxide surface, as schematically shown in Figure 12b,c. Figure 11: (**a**–**c**) Sensitivity of the In\({}_{2}\)O\({}_{3}\) nanoparticles sensor as a function of CO concentration and temperature, and (**d**) is the response and recovery time of Au/In\({}_{2}\)O\({}_{3}\) at 50 ppm CO concentration at the optimal temperature of 100 \({}^{\circ}\)C.  
The electron depletion layer works as a potential barrier between neighboring grains, so that the sensor resistance increases in air. When a reducing gas such as carbon monoxide (CO) was released into the test chamber, the oxygen species adsorbed to the oxide surface reacted with CO gas, as indicated in Equations (5) and (6). The electrons caught by oxygen species are released back into the conduction band of In\({}_{2}\)O\({}_{3}\), so that depletion layer thickness becomes thinner, causing a remarkable decrease in resistance of the gas sensor. \[2\text{CO}+\text{O}^{-}{}_{2(\text{ads})}\to 2\text{CO}_{2}+\text{e}^{-} \tag{5}\]  
\[\text{CO}+\text{O}^{-}{}_{(\text{ads})}\to\text{CO}_{2}+\text{e}^{-} \tag{6}\]  
The sensing performance of oxide sensors such as In\({}_{2}\)O\({}_{3}\) was essentially determined by changes in the thickness of the electron depletion layer as the atmosphere changed. One way to improve sensor sensitivity was to decorate the In\({}_{2}\)O\({}_{3}\) surface with metal nanoparticles. This made the change in depth of the electron depletion layer more sensitive, which magnified the change in the resistance of In\({}_{2}\)O\({}_{3}\) sensors. Using Au, Pd, or Pt nanoparticles with photosensizers to decorate the In\({}_{2}\)O\({}_{3}\) surface can trigger two sensitization mechanisms. One mechanism is the electronic sensitization caused by interfacial electronic redistribution, and the other mechanism is the chemical sensitization caused by surface atom transport [55; 56; 57].