Photoelectrochemical study of La2NiO4 synthesized using citrate sol gel method—application for hydrogen photo-production

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The semi conducting properties of La2NiO4 synthesized through sol gel method are studied for the first time using photoelectrochemistry. The X-ray diffraction (XRD) shows that the phase is formed at 900 °C in agreement with the thermal analysis (TG-TDA/DSC). The SEM micrographs exhibit spherical and uniform grains with agglomerated nature, confirmed through laser granulometry. The direct optical gap (1.31 eV) comes from the crystal field splitting of Ni2+ octahedrally coordinated. La2NiO4 is chemically stable in the pH range (5–14) and anodic potentials at pH ~ 12 give rise to surface oxidation of La2NiO4 in the diffusion plateau (0.35–0.6 V). The electrochemical oxygen insertion, studied using chrono-amperometry, is slow with a diffusion coefficient of ~ 2.5 × 10−18 cm2 s−1 at 500 mV. The Mott–Schottky characteristic plotted in alkaline solution (NaOH 0.1 M) indicates p-type conductivity due to oxygen over-stoichiometry well-known in this class of compound. A flat band potential of 0.1 VSCE and hole concentration of 1019 cm−3 were determined from the capacitance measurement. Therefore, the oxide has been successfully tested for the H2 evolution under visible light irradiation. The best activity occurs at pH ~ 13 in presence of S2O32− as reducing agent with H2 liberation rate of 23.6 μmol mn−1 (g catalyst)−1 under visible light flux of 29 mW cm−2.