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It is beneficial to reduce thickness of high-purity of c-Si wafer for cost-effective water splitting system based on silicon photoelectrode. However, a thin Si absorber causes insufficient absorption of light and it should be needed to solve problems. Here, a 21-µm-thin Si photoanode for which a NiOx-coated silicon nanopyramid (SiNP) is employed is presented as a solution. The NiOx/n-SiNP structures improved the light absorption as well as the total number of catalytic sites to enable photo-oxidation reactions. A photocurrent of 7.7 mA/cm² was generated at the equilibrium potential of the OER (EOER = 1.23 V vs. a reversible hydrogen electrode in a 1 M KOH solution). Also, the photoanode did not show any degradation over the 24 hr of a photoelectrochemical water-oxidation process. [ABSTRACT FROM AUTHOR]