Passivation of nickel nanoneedles in aqueous solutions

Passivation, a typical electrochemical behavior, involves the formation of a thin layer on the surface of a bulky metal component, which has been assumed to be native, dense, and highly protective to the metal from deterioration. In the present paper, nickel was chosen and prepared into a nanoneedle. A duplex layered passive film grown around the surface of Ni nanoneedle in the alkaline solution (0.1 M NaOH) was observed in TEM, which has a thickness of ∼4 nm and consists of NiO inner layer and Ni(OH)₂ outer layer. However, the surface of Ni nanoneedle only appears a single inner NiO layer with a thickness of 3 nm after passivation in a strong acid solution (0.1 M H₂SO ₄). This could be due to the sufficient H⁺ that inhibits the formation of Ni(OH)₂ outer layer. There may be still a mono- or bimolecular Ni(OH)₂ layer beyond the resolution of TEM. The ultrathin NiO inner layer covers the entire nickel nanoneedle and was determined to be crystalline and coherently oriented with Ni substrate.