Figure details the nanocrystalline structure at a higher magnification, showing a homogeneous distribution of lattice fringes across the NiFe alloy film. Single-oriented lattices span the crosssectional region of the NiFe alloy layer, indicating that the grain size in the vertical direction is comparable to the NiFe alloy film thickness. The spacing of the resolved lattice fringes in Fig. is around 0.219 nm (Fig. ), corresponding to the (111) plane of NiFe layer as observed above in the FFT pattern of Fig. . Figure details the elemental depth profiles from the top surface to the inside layered structures of the as-fabricated n-Si/SiO x /TiO x /2 nm NiFe oxyhydroxides-alloy photoanode. The NiFe alloy layer and SiO x /TiO x mixed layer are around 2.6 nm and 2.3 nm in depth, respectively, corresponding to the chemical species across the layered structures visualized in former cross-sectional STEM results (Fig. ). The X-ray photoelectron spectroscopy (XPS) survey (Fig. ) shows the co-existence of Ni, Fe, O, and Si elements in the surface layers of photoanodes with 2 nm NiFe, in agreement with the EELS mapping results in Fig. . The XPS overall survey profile of the photoanode presents no appropriate changes after performing 20 h chronoamperometry (CA) measurement under a constant bias of 1.09 V vs. RHE and one sun illumination. Figure reports the grazing incidence X-ray diffraction (GIXRD) pattern of n-Si/SiO x /TiO x /NiFe oxyhydroxides-alloy photoanodes before and after 20 h CA testing with the increasing incident angle of X-ray (ω H , from 0.2 • to 0.8 • ), visualizing the sample phase structures across different depth.