## 3 Results
### XPS Analysis of Noble Metal-Decorated In\({}_{2}\)O\({}_{3}\) Particles
The variation of peak positions in the XPS spectrum is due to the chemical status of the surface elements. Binding energies are determined by the electrostatic interaction between electrons and the nucleus; thus, the removal or addition of electronic charge as a result of changes in bonding will alter the shielding. If the nuclear charge is electrostatically shielded from all the other electrons in the atom (including valence electrons), the binding energy is reduced. Therefore, the removal of a valence electron charge, such as by oxidation, leads to lower electron density and increases the binding, while the addition of an electron charge leads to the higher electron density and decreases the binding energy. When noble metal-decorated In\({}_{2}\)O\({}_{3}\) particles are exposed in air, oxygen is adsorbed to their surfaces and electrons from In\({}_{2}\)O\({}_{3}\) particles are transferred to the surfaces of a noble metal or In\({}_{2}\)O\({}_{3}\) particles, thereby generating depletion in the outer layer of In\({}_{2}\)O\({}_{3}\). This means that the electron density in In\({}_{2}\)O\({}_{3}\) particles is reduced. Therefore, considering the binding energy of the noble-metal decorated In\({}_{2}\)O\({}_{3}\), Au-decorated In\({}_{2}\)O\({}_{3}\) particles would have the largest depletion area free of electrons, followed by Pd-decorated In\({}_{2}\)O\({}_{3}\) particles, and finally by Pt-decorated In\({}_{2}\)O\({}_{3}\) particles. The thickness of the depletion layer is directly related to the sensitivity of a gas sensor based on a metal oxide semiconductor. Figure 9a-c shows the XPS spectra of oxygen (the O1s region) for Au/In\({}_{2}\)O\({}_{3}\), Pd/In\({}_{2}\)O\({}_{3}\), and Pt/In\({}_{2}\)O\({}_{3}\) samples. The peaks can be deconvoluted by three Gaussian curves each. The three XPS peaks for Au/In\({}_{2}\)O\({}_{3}\) are centered at 530.1, 530.6, and 531.9 eV; the peaks for Pd/In\({}_{2}\)O\({}_{3}\) at 529.9, 530.7, and 532.1 eV; and the peaks for Pt/In\({}_{2}\)O\({}_{3}\) at 529.9, 530.8, and 532.1 eV. If the lowest peaks can be assigned to the lattice oxygen (O\({}_{\rm I}\)) in crystalline indium oxide, the middle peak could be associated with O\({}^{2-}\) ions (O\({}_{\rm II}\)) in the oxygen vacancy region (deficient oxygen), and the highest peaks are due to oxygen. Chemical species (O\({}^{2-}\), O\({}^{-}\), or O\({}_{2}\)\({}^{-}\)) (O\({}_{\rm III}\)) are chemically adsorbed not only to the In\({}_{2}\)O\({}_{3}\) surface, but also to the surfaces of hydroxy groups (-OH). Figure 7: (**a**–c**) XPS spectra of metallic Au, Pd and Pt detected from the In\({}_{2}\)O\({}_{3}\)/Au, In\({}_{2}\)O\({}_{3}\)/Pd and In\({}_{2}\)O\({}_{3}\)/Pt particles, respectively. Deficient oxygen (O\({}_{\text{II}}\)) in the oxygen vacancy region and chemisorbed oxygen (O\({}_{\text{III}}\)) species have significant effects on the sensing-reaction performance. Each oxygen vacancy provides two electrons and an active site for the adsorption of oxygen species [45]. An  
Figure 8: XPS spectra of In\({}_{2}\)O\({}_{3}\) particles decorated with noble metals such as Au, Pd and Pt, where the peaks of the electrons are identified as In3d3/2 and In3d5/2. Figure 9: XPS spectra of oxygen in O1s region for Au/In\({}_{2}\)O\({}_{3}\), Pd/In\({}_{2}\)O\({}_{3}\) and Pt/In\({}_{2}\)O\({}_{3}\) samples, where the peaks can be deconvoluted into three Gaussian curves, respectively. increase in the rate of oxygen deficiency promotes the adsorption of oxygen molecules on the surface of the sensing material, increasing the potential for interactions of In\({}_{2}\)O\({}_{3}\) with the target gas, and thereby generating more active oxygen [46]. The peak areas caused by deficient oxygens were centered from 530.6 to 530.8, depending on the metal catalysts decorated, which could reflect the concentration of oxygen vacancies (defects) in the samples [45; 47; 48].