Synthesis of NiO

NiO nanoparticles were synthesized as follows.⁶⁹ 0.5 g of NaOH and 1.66 g of PVP were dissolved in 25 mL of deionized water and 1.45 g of nickel nitrate hexahydrate was dissolved in 10 mL of deionized water, respectively. The nickel nitrate aqueous solution was then added to the NaOH/PVP solution dropwise under stirring. The resulting solution was stirred for 3 hours to obtain the NiO precursor, followed by washing with H₂O and ethanol for several times and drying at 60 °C overnight. Then, the precursor was annealed at 650 °C for 2 hours in air with a ramp rate of 5 °C/minute to form the final NiO nanoparticles.