## 3 Results
### XPS Analysis of Noble Metal-Decorated In\({}_{2}\)O\({}_{3}\) Particles  
XPS is a surface-sensitive quantitative spectroscopic technique capable of identifying the overall electronic structure and density, chemical states, and electronic states of elements within a substance or that exist on its surface. In particular, XPS is a powerful measurement technique because it shows not only the elements present, but also other combined elements. A more detailed XPS spectrum was performed of Au/In\({}_{2}\)O\({}_{3}\), Pd/In\({}_{2}\)O\({}_{3}\), and Pt/In\({}_{2}\)O\({}_{3}\) to reveal chemical bonding, and the results for the three particles were compared. Figure 7a-c shows the XPS spectra of the metals Au, Pd, and Pt detected in Au/In\({}_{2}\)O\({}_{3}\), Pd/In\({}_{2}\)O\({}_{3}\), and Pt/In\({}_{2}\)O\({}_{3}\) nanoparticles. Figure 7a shows that the high-resolution Au 4f core level spectrum is a dual property of Au/In\({}_{2}\)O\({}_{3}\), with the peaks represented by metallic Au (Au0) centered at 83.7 and 87.4 eV, corresponding to 4f\({}_{7/2}\) and 4f\({}_{5/2}\). The Au 4f peak is relatively sharp and symmetrical, and Au/In\({}_{2}\)O\({}_{3}\) represents the chemical state. According to the reference, the electron binding energies (BEs) of Au 4f\({}_{7/2}\) and 4f\({}_{5/2}\) are commonly 84.0 and 87.7 eV; in addition, the bulk metal Au was 86.4/90.1 eV for low-oxygen (Au\({}^{+}\)) oxidizing, and 85.5/89.2 eV for the high-oxygen species (Au\({}^{3+}\)) [40; 41; 42]. Compared with that of bulk metal Au, the 4f XPS spectrum of Au on Au/In\({}_{2}\)O\({}_{3}\) showed a shift of 0.3 eV at low BE. The negative BE shift is usually due to strong interaction between the metal and In\({}_{2}\)O\({}_{3}\) particles, where an electron transfers from In\({}_{2}\)O\({}_{3}\) to Au. Compared to the Au 4f XPS peaks, the Pd 3d and Pt 4f XPS peaks are slightly wider, meaning that Pd/In\({}_{2}\)O\({}_{3}\) or Pt/In\({}_{2}\)O\({}_{3}\) has several chemical states of the metal, as shown in Figure 6 b,c. The Pd 3d peaks (Pd 3d\({}_{5/2}\) and 3d\({}_{3/2}\)) correspond to 336.9 and 342.2 eV. However, these peaks do  
Figure 5: TEM images of (**a**) Au/In\({}_{2}\)O\({}_{3}\), (**b**) Pd/In\({}_{2}\)O\({}_{3}\), (**c**) Pt/In\({}_{2}\)O\({}_{3}\) nanoparticles. Figure 6: HAADF images of Au/In\({}_{2}\)O\({}_{3}\), Pd/In\({}_{2}\)O\({}_{3}\) and Pt/In\({}_{2}\)O\({}_{3}\) nanoparticles (**a–c**) and their EDS elemental mapping images (**d–l**). not correspond to any valence state of palladium (i.e., Pd\({}^{0}\), Pd\({}^{2+}\) (PdO), and Pd\({}^{4+}\) (PdO\({}_{2}\)) with binding energies of 335.1/340.4 eV, 336.1/341.4 eV, and 337/343 eV [43]. Thus, the Pd particles are considered to coexist in the form of metal Pd, oxide PdO, and/or PdO\({}_{2}\). The Pt 4f peaks are located at 72.4/75.5 eV, which correspond to Pt 4f\({}_{7/2}\) and 4f\({}_{5/2}\), respectively. These peaks can be interpreted as only Pd\({}^{2+}\) (PdO) because they are nearly identical to the binding energy of Pt\({}^{2+}\) (PtO) located at 72.2/75.5 eV, while the binding energy of metallic PtO is located at 71.4/74.6 eV [44] and is very different from PdO. Thus, it is considered that the Pt particles might exist in an oxide form of PtO. From the XPS results, the state of the In\({}_{2}\)O\({}_{3}\) surface was different for Au, Pd, and Pt nanoparticles, and the degree of oxidation of the metal particles was in the order Au < Pd < Pt. Figure 8 exhibits the 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 In3d\({}_{3/2}\) and In3d\({}_{5/2}\). It is interesting to note that the peak positions of their 3d\({}_{5/2}\) are shifted slightly toward the higher binding energy in the order Pt (444.1 eV), Pd (444.4 eV), and Au (444.8 eV), depending on the decorated-metal particles as compared to the metallic In3d\({}_{5/2}\) peak of 443.7 eV or that of indium oxide (443.9 eV).