Nickel Silicide As a Precatalyst for Green Hydrogen

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The development of earth-abundant metal-based catalysts to accelerate the sluggish oxygen evolution reaction (OER) is crucial for the commercial production of green hydrogen. This can be achieved by employing a nickel silicide, which has the potential to serve as a precatalyst for anodic OER due to its high efficiency and stability.

The preparation of nickel silicide involves a two-step procedure. The first step is a chemical dry-clean using ammonium fluorosilicate, which suppresses the formation of NiSi2 from the native oxide on the substrate. The second step is sputtering of nickel, followed by deposition of a titanium nitride cap-film. Both steps are performed in an oxygen-rich atmosphere. This imposes a high energy requirement, which can lead to the agglomeration of Ni, resulting in nickel silicide with an unsatisfactory morphology.

Several approaches have been developed to improve the morphology of nickel silicide, including etchant chemistry and thermal annealing. However, the agglomeration problem is difficult to avoid for very thin (e.g., 10 nm) layers, since it is driven by the surface and interface energy. In addition, the etchant chemistry only delays the agglomeration process and does not completely eliminate unreacted nickel.

In order to fully understand the morphology of nickel silicide, depth-resolved composition and crystallography are required. This can be achieved with a non-invasive probe. X-ray pole figure measurements6,7, transmission electron microscopy (TEM),8,9 and Rutherford backscattering spectrometry (RBS)10 can all provide this information. For example, RBS and TEM can detect the crystal axis and planes in the silicide layer. The resulting patterns are very similar to those of the Si substrate, showing that the silicide layer is epitaxial.

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