## 3 Results and Discussion
### Effects of Fe\({}_{x}\)O Layer and Hydroxyl Groups at the Surface Overlayer on PE
Therefore, we propose that there may exist a non-stoichiometric iron oxide, Fe\({}_{\rm x}\)O, which strongly acts on the TAPE with increasing temperature as a p-type semiconductor, producing the increase in the baseline PE intensity. Figure 17 shows the actions of the two types of chemical species present at the overlayer on the intensity of PE in the Up1 temperature scan: (1) the acid-base interaction of the environment molecule on the surface hydroxyl group (Fe-OH), and (2) the growth of Fe\({}_{\rm x}\)O p-type semiconductor layer as a result of the transfer of electrons, iron ions, and oxygen species from the metal to the oxide film. In the case of the former action, the acid-base interaction causes the formation of an electric dipole at  
Figure 16: Dependence of (**a**) the ratio of XPS O\({}^{2-}\) intensity to XPS Fe 2\(p\) intensity and (**b**) the ratio of XPS O\({}^{2-}\) intensity to XPS Fe 3\(p\) intensity obtained from the O 1\(s\), Fe 2\(p\), and Fe 3\(p\) XPS spectra for 25, 200, and 339 \({}^{\circ}\)C for real iron surfaces on the scratching environments. the outside surface, producing an enhancement of PE for acetone, while it produces a reduction in PE for water, as shown in Figure 17. Furthermore, it is considered that light irradiation for a longer period in the PE measurement increases the electron density of the oxygen atom of the hydroxyl group, which more strongly enhances the basicity of the hydroxyl group [5; 6]. In the case of the latter action, the amount of non-stoichiometric Fe\({}_{\text{x}}\)O with a p-type semiconductor characteristic increases with increasing temperature in the neighborhood of the interface between the base metal and the surface oxide layer. Here, x is a small number with a value less than unity. In Figure 18, a model of the energy diagram for the Fe\({}_{\text{x}}\)O layer is illustrated according to an important concept of the band bending downward and the effective electron affinity used in the band model of the photocathode of the p-type semiconductor [4; 24]. The Fe\({}^{3+}\) ions constitute the positive holes in the oxide, which can move around in the oxide as a result of the transfer of electrons from Fe\({}^{2+}\) ions to Fe\({}^{3+}\) ions. Fe\({}^{3+}\) ions are located at the acceptor level. Two Fe\({}^{3+}\) ions are formed for every three missing Fe\({}^{2+}\) ions to balance the charge. The band structure is bent downward due to the formation of a depletion region that has a negative charge, resulting in the lowering of the effective electron affinity, which enhances the transfer of electrons. Figure 17: Growth of Fe\({}_{\text{x}}\)O p-type semiconductor and the acid–base interaction of the scratching environment molecule at the scratched surface as a result of the transfer of electrons, iron ions, and oxygen species from the metal during Up1 temperature scan. The electron density of the oxygen atom of Fe–OH is considered to become greater during the temperature scan. Figure 18: Band model of the energy diagram proposed for the Fe\({}_{\text{x}}\)O p-type semiconductor only, although the effect of the adjacent oxide layer of Fe\({}_{\text{3}}\)O\({}_{\text{4}}\) is not clear. The symbols \(E_{\text{F}}\), \(E_{t}\), \(E_{\text{vac}}\), \(E_{\text{V}}\), \(E_{\text{C}}\), and \(E_{\text{A}}\) are the Fermi levels of the metal and Fe\({}_{\text{x}}\)O layer, the vacuum level, the top level of the valence band and the bottom level of the conduction band of the Fe\({}_{\text{x}}\)O layer, and the effective electron affinity, respectively. Two types of emitted electrons (hot electrons and thermalized electrons) are indicated [7].