Corresponding Author

Yi-Fan Huang(huangyf@shanghaitech.edu.cn)


Nickel (Ni) electrodes are widely used in electrochemical researches. Understanding electrochemical processes on Ni electrodes through in-situ characterization of adsorbed species on their surfaces is helpful for rational optimization and application of Ni electrochemistry. Microelectrochemical surface-enhanced Raman spectroscopy (μEC-SERS) combines the mass transfer feature of ultramicroelectrode with high-sensitivity characterizations of molecular structures, which is a powerful method for studying Ni electrochemistry on polarization and non-equilibrium conditions. The key point of performing μEC-SERS is to make a SERS-active Ni ultramicroelectrode.
Here, we demonstrate a method of preparing a SERS-active Ni ultramicroelectrode through electrochemical deposition of several atomic layers of metallic Ni onto a SERS-active gold (Au) ultramicroelectrode. Firstly, a SERS-active Au ultramicroelectrode was made through electrochemical adsorption of Au nanoparticles. A smooth polycrystalline Au ultramicroelectrode with a diameter of 10 μm was made by sealing a Au wire into a glassy tube. The Au nanoparticles of 55 nm in diameter were adsorbed from Au sol onto the Au ultramicroelectrode under an electrochemical polarization at 1.8 V. The scanning electron microscopic (SEM) images showed that Au nanoparticles aggregated on surface.
On the prepared Au ultramicroelectrode adsorbed by Au nanoparticles, a thin and compact Ni layer was deposited by using galvanostatic method in 5 mmol·L-1 Ni(NO3)2 electrolyte. The thickness of Ni layer was controlled via the charge. The voltammograms of the prepared SERS-active Ni ultramicroelectrode in 0.1 mol·L-1 NaOH showed the characters of polycrystalline Ni electrode. Since the SERS activity decreased as a result of the increase in the thickness of Ni layer, the SERS measurements of 4-methylthiophenol in air were carried out for evaluating SERS activity. The comparisons in the intensity of the band at 1077 cm-1 from the 4-methylthiophenol adsorbed on the ultramicroelectrode made by using 10 μA·cm-2, 50 μA·cm-2, 100 μA·cm-2, 500 μA·cm-2 and 1000 μA·cm-2 indicated that the rate and charge of deposition are key in determining the coverage status of Ni layer and the SERS activity. An optimized SERS activity on a compact Ni was obtained by electrodepositing 5 atomic layers of Ni at a current density of 100 μA·cm-2.
To demonstrate the application of Ni ultramicroelectrode in the in-situ μEC-SERS measurement, the molecule of 4-methylthiophenol, employed as a probe, was adsorbed onto the prepared Ni ultramicroelectrode through spontaneous adsorption in the methanol solution of 4-methylthiophenol. The obtained SERS spectra showed characteristic bands of 4-methylthiophenol. In addition, stark effect of the bands was observed, indicating the successful application of Ni ultramicroelectrode in the in-situ μEC-SERS measurement.
The preparation methodology of SERS-active ultramicroelectrode enables the in-situ μEC-SERS measurement on Ni under electrochemical polarization or non-equilibrium reaction conditions, which exhibits a good potential application in studying Ni electrochemistry.

Graphical Abstract


nickel, ultramicroelectrode, surface-enhanced Raman spectroscopy, electrochemical deposition

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