| Summary: | The surface plasmon resonance (SPR) effect and the hetero-junction structure play crucial roles in enhancing the photocatalytic performances of catalysts for the water-splitting reaction. In this study, a series of double perovskites LaFe<sub>1−x</sub>N<sub>ix</sub>O<sub>3</sub> was synthesized. LaFe<sub>1−x</sub>N<sub>ix</sub>O<sub>3</sub> particles were then decorated with sea urchin-like Au nanoparticles (NPs) with the average size of approximately 109.83 ± 8.48 nm via electrophoresis. The d-spacing became narrow and the absorption spectra occurred the redshift phenomenon more when doping increasing Ni mole concentrations for the raw LaFe<sub>1−x</sub>N<sub>ix</sub>O<sub>3</sub> samples. From XPS analysis, the Ni atoms were inserted into the lattice of the matrix, resulting in the defect of the oxygen vacancy, and NiO and Fe<sub>2</sub>O<sub>3</sub> were formed. This hybrid structure was the ideal electrode for photoelectrochemical hydrogen production. The photonic extinction of the Au-coated LaFe<sub>1−x</sub>Ni<sub>x</sub>O<sub>3</sub> was less than 2.1 eV (narrow band gap), and the particles absorbed more light in the visible region. According to the Mott–Schottky plots, all the LaFe<sub>1−x</sub>N<sub>ix</sub>O<sub>3</sub> samples were the n-type semiconductors. Moreover, all the band gaps of the Au-coated LaFe<sub>1−x</sub>N<sub>ix</sub>O<sub>3</sub> samples were higher than 1.23 eV (H<sup>+</sup>/H<sub>2</sub>). Then, the hot electrons from the Au NPs were injected via the SPR effect, the coupling effect between LaFe<sub>1−x</sub>N<sub>ix</sub>O<sub>3</sub> and Au NPs, and the more active sites from Au NPs into the conduction band of the semiconductor, improving the hydrogen efficiency. The H<sub>2</sub> efficiency of the Au-coated LaFe<sub>1−x</sub>Ni<sub>x</sub>O<sub>3</sub> measured in ethanol was approximately ten times larger than the that of Au-coated LaFe<sub>1−x</sub>Ni<sub>x</sub>O<sub>3</sub> measured in 1-butanol at any testing temperature because ohmic and kinetic losses occurred in the latter solvent. Thus, the activation energies of ethanol at any testing temperature were smaller. The maximum real H<sub>2</sub> production was up to 43,800 μmol g<sup>−1</sup> h<sup>−1</sup> in ethanol. The redox reactions among metal ions, OH*, and oxides were consecutively proceeded under visible light illumination.
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