;We used reflective high energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) to investigate the structure and electronic state of iridium oxide layers grown by oxidizing Ir(001) at 775 K with oxygen pressure in between high vacuum to 5 torr. The result shows the surface only forms 2×1 oxygen adsorbed Ir(001) for oxygen dosage smaller than 1.8×〖10〗^7 L, indicating that Ir(001) is hard to be oxidized under this environment. The structure transforms to IrO2(001) with 3D structure form on the surface, where the oxide state can be observed in the valence band structure as the oxygen dosage increase to 1.8×〖10〗^8 L. Further increasing the oxygen dosage to 6×〖10〗^8 L results in the formation of the double-phase IrO2(102) & (102 ̅) surfaces, which has a huge oxide state signal in the valence band and shows the 3D structure with two symmetry phases coexisting on the surface. The IrO2(110) surface can be obtained if the pressure is larger than one torr. A flat surface coexisting with the 3D structure indicates that high surface-adsorbate interaction happens, which is counterintuitive to the general concept. The phenomenon can be elucidated as the strong surface oxide-adsorbate interaction, in which the dendritic island and stripe can form on the surface shown in the STM image. The XPS spectrum shows IrO2(110) eventually saturated at 18 Å with a dramatic enhancement of the oxide state. According to the second law of thermodynamics, the oxidation of clean Ir(001) could be an irreversible process so that the work of the intermediate state might determine the structure it will be transformed. Further oxidation on the oxide surface transfers to a reversible reaction so that the surface becomes equilibrium, eventually. On the other hand, the transformation from double-phase IrO2(102) & (102 ̅) to IrO2(110) is limited. This study demonstrated the step-by-step growth mechanism of iridium oxide under different oxygen dosages and pressure. The structure of iridium dioxide is very sensitive to oxygen pressure in the near ambient pressure environment. We hope this study can shed light on the study of organic compound reactions on stoichiometric IrO2.
