<i>In Situ</i> Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy Study of Nickel Surface Oxides Reveals Previously Unseen Surface Chemical Dynamics

Potential-dependent shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is applied to study the surface oxide/hydroxide formation and reduction on a planar polycrystalline Ni electrode surface in contact with alkaline aqueous solutions. A new Raman amplifier, stable in alkaline aqueous solutions, is fabricated by introducing a thin layer of ZrO2 on the surface of Au nanoparticles (NPs). The thickness of the ZrO2 layer is controlled to be 2–3 nm, so the resulting ZrO2-modified Au NPs selectively enhanced the Raman signals of the surface oxide/hydroxide species, with the amplifiers remaining spectroscopically silent. In situ SHINERS measurements were combined with electrochemical measurements. Potential-dependent Raman signals from the thin layer of surface oxides/hydroxides, namely, α-Ni(OH)2, β-Ni(OH)2, β-Ni(O)OH, γ-Ni(O)OH, and Ni-OO– species, are successfully identified in the potential range between the onsets of the hydrogen and oxygen evolution reactions (HER, OER). The combined SHINERS and electrochemical measurements shed new light on the chemical identity of Ni surface oxide/hydroxide species and their potential-driven interconversions. In particular, the transformations of surface species, such as α-Ni(OH)2 to β-Ni(OH)2 and γ-Ni(O)OH to β-Ni(O)OH, which had not been described in voltammograms, were identified in the spectral behavior. The measurements also enable the identification of the precursor of the OER; thus SHINERS can be useful in establishing a correlation between the chemical state of the Ni electrode and the OER activity, the slowest electrode process occurring in alkaline water electrolyzers, emphasizing the importance of this technique to the development of electrocatalysts. The SHINERS and electrochemical results are supported by density functional theory (DFT) calculations. The outcome of this research results in an updated Bode scheme for the electrochemical oxidation products of Ni materials in aqueous alkaline media.