Probing oxygen vacancy-induced mixed-valence states of nickel in LaNiO3 and their influence on electrocatalytic and magnetic properties

Lanthanum nickel oxide (LaNiO3) nanoparticles were synthesized by sol-gel (LNO-SG) and solid state reaction (LNO-SS) methods to investigate the oxygen evolution reaction (OER) activity. Thermal analysis and variable temperature X-ray diffraction of the sol-gel precursor revealed the phase formation temperature of 700 °C. The oxygen non-stoichiometry was found in both LNO-SS and LNO-SG samples through iodometric titrations. A detailed X-ray photoelectron spectroscopic surface and depth profiling analyses revealed the presence of Ni2+ due to oxygen vacancies. The influence of mixed-valence states of nickel (+2 and +3) on the properties of LaNiO3 was demonstrated. A comparison of electrocatalytic oxygen evolution reaction (OER) activity of the two LaNiO3 samples was made in alkaline conditions. LNO-SG exhibited an onset potential of 1.60 V (vs. RHE) and a higher current density than LNO-SS. Both the samples exhibited a weak ferromagnetic behaviour at 300 K and LNO-SS showed a spin glass-like behaviour at low temperatures. These OER activities and unusual magnetic properties of LaNiO3 can be attributed to the oxygen vacancy and mixed-valence states of nickel.