The fine-scanning XPS spectra of Fe 2p in Fig. exhibit doublets and satellite peaks similar to Ni 2p. Besides, the Fe 2p 3/2 peak appreciably broadens to the positive side, demonstrating an enhanced oxidation state of Fe after the PEC reaction. The peak of metallic Fe 0 at 706.7 eV partially remains after the PEC reaction, with the ratio of metallic Fe 0 decreasing from 26.9% to 13.8% (Fig. ). Moreover, the intensity ratio of Fe 2+ reduces from 36.7% to 32.9%, while Fe 3+ ratio strongly enhances from 19.6% to 34.2%, demonstrating that after reaction, Fecontaining species mainly exist as Fe 3+ with an enhanced average valence state from +1.59 to +2.08 (Fig. ). We visualize this dynamic changing process in a second set of measurements by employing crosssectional HAADF-STEM images (Fig. ). Figure illustrates the pristine n-Si/SiO x /TiO x /NiFe oxyhydroxides-alloy photoanode with 2 nm NiFe before the PEC reaction, while Fig. reports the image of the 2 nm NiFe photoanode after 20 h CA measurement under constant bias of 1.09 V vs. RHE and one sun illumination. The HAADF-STEM images (Fig. ) show that the NiFederived layer maintains parts of the crystalline structure at the bottom side and in situ generates more amorphous species in the top surface region during the PEC reaction when illuminated with sunlight (schematically illustrated in Fig. ). In this process, the NiFe-evolved oxyhydroxidesalloy layer expands appreciably in thickness (Fig. ) compared with the pristine NiFe alloy layer in Fig. , from 2.47 ± 0.24 nm to 5.85 ± 0.33 nm, with a net 137% increase.