Controlled Corrosion—Synthesizing Nickel Nanostructures via pH-Dependent Pitting

Due to its physical and chemical properties, such as high corrosion resistance, excellent thermal and electrical conductivity, and good ductility, Nickel (Ni) is an interesting material for many applications. The fabrication of nickel nanostructures has attracted significant interest due to their potential applications in advanced fields such as catalysis, sensors, and electronic devices [1,2]. This study investigates a novel method for synthesizing nickel nanostructures via pitting corrosion on nickel thin (~60 nm) films deposited by e-beam evaporation. Our objective was to explore the slow corrosion of nickel induced by solutions containing deionized water, glycine and calcium chloride at different pH values. After 48 hours of contact between the solutions and the films, we observed pitting corrosion, with the pits exhibiting approximately circular shapes. Within these pits, nanostructures were formed, varying accordingly to the pH of the solution. At low pH values, very thin planar nanowires with a fractal aspect originated from the center of the pits. At high pH values, in addition to these structures, nanowire-like structures vertically projecting from the center of the pits were also observed. Scanning electron microscopy coupled with Energy dispersive spectroscopy revealed that the central nanostructures are composed of nickel, whereas the planar nanowires can not be resolved by this latter technique. Atomic force microscopy indicated that all pits, regardless of pH, have the same depth, approximately 60 nm, which coincides with the nickel thickness of the deposited nickel thin film. This suggests that the corrosion process occurs at local defects in the Ni film, followed by redeposition due to the radial gradient of dissolved Ni in the solution, particularly when the corrosion reaches the silicon substrate. COMSOL simulations confirmed this possibility, as the Ni removed from the surface flows towards the substrate for large enough pit areas. Additionally, X-ray photoelectron spectroscopy showed an increased presence of calcium on the surface as the solution pH increased, indicating that calcium acts as a corrosion inhibitor [3], as the average pit area decreases with increasing pH. These results suggest a potentially controllable process for synthesizing nickel nanostructures, although further studies are needed to confirm this possibility and explore the applications of this new method.<br/>Acknowledgements: This work was financially supported by the Brazilian funding agencies CNPq and Fapesp (grant number 2019/07616-3).<br/>References:<br/>[1] H. Beitollahi, et al., Biosensors 12 (2022), 872<br/>[2] J. Ding, et al., Nano Letters 16 (2016) 2762-2767<br/>[3] X. Jiang, et al., Corrosion Science 48 (2006), 3091-3108