The reduction of metal oxides is of great importance in a large variety of chemical and materials applications ranging from heterogeneous catalysis to electronic device fabrication. However, the fundamental knowledge of the atomic processes underlying the oxide reduction is still very limited. Several reasons contribute to the lack of this fundamental information including the difficulty of measurements of the atomistic processes of the reaction and the longstanding challenge in identifying the

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... reaction mechanisms in heterogeneous systems. By manipulating the surface roughness of a Fe substrate, we recently demonstrated that various morphologies of iron oxide nanostructures, such as nanowires, nanbelts and nanoblades, can be obtained by thermal oxidation of Fe [1, 2]. We previously showed that the reduction of α-Fe2O3 nanowires proceeds via the ordering of oxygen vacancies followed by the transformation pathway of α-Fe2O3  γ-Fe2O3  Fe3O4 [3]. Here we extend the study to the reduction of α-Fe2O3 nanoblades using an environmental transmission electron microscope (ETEM). The α-Fe2O3 nanoblades samples were prepared by thermal oxidation of sandblasted iron foils. Asprepared α-Fe2O3 nanoblades were transferred onto a Si3N4 membrane TEM window and then loaded into the ETEM. Pure dry hydrogen (99.999 %) was flowed into the ETEM column to achieve a partial pressure of 0.5 Pa. The sample was then heated up to 500 °C. The in situ observations of the oxide reduction were conducted under these conditions by time-resolved (video rate), high-resolution transmission electron microscopy (HRTEM) imaging, nano-diffraction and electron energy loss spectroscopy (EELS). The TEM image in Fig 1a shows the typical morphology of the as-prepared α-Fe2O3 nanoblades. Fig. 1b is a nanodiffraction pattern obtained from the area marked with the red rectangle in Fig. 1a , which shows that the nanoblade has a bicrystal structure with a coincident-site-lattice (CSL) twist boundary (Σ boundary). The CLS twist boundary structure of two stacked crystals can be determined from the nanodiffraction pattern shown in Fig. 1b . The rotation angle of 21.8 ± 1° between the two α-Fe2O3 crystals along the <0001> directions matches with the Σ = 7 boundary structure (theoretical angle: 21.79°), and this CLS boundary structure is dominantly present among all the nanoblades examined. Fig. 1c is a crystal model based on the diffraction pattern in Fig. 1b and used for HRTEM image simulations. The HRTEM image shown in Fig. 1d matches well with the simulated image, with thickness of 4 nm and defocus of -9 nm, as marked by the red rectangle in Fig. 1d . The CLS twist boundary is not directly visible in the HRTEM image (Fig. 1d) because it is perpendicular to the electron beam. Fig. 2a shows an HRTEM image of the nanoblade after 30 min of H2 reduction at 500 °C and Fig. 2b is the nano-diffraction pattern of the same area shown in Fig. 2a. Indexing of the diffraction pattern shows that the parent α-Fe2O3 is partially transformed to γ-Fe2O3, with the rotation angle of 30° between the oxide phases and the <0001> of α-Fe2O3 (hexagonal structure) is parallel to the <111> of γ-Fe2O3 (cubic). Both the {0001} and {111} planes are the close-packed planes for the two Fe oxide phases, 792