Electrochemical Surface-Enhanced Raman Spectroscopic Studies on Nickel Ultramicroelectrode
- 2021年第27卷 
镍（Ni）电极在电化学中应用广泛。原位表征Ni电极表面的吸附物种有益于帮助理解电极反应历程、指导发展高效电催化剂。应用超微电极作为工作电极的电化学表面增强拉曼光谱技术结合了超微电极表面的传质特性和分子水平的高灵敏度表征,是研究Ni电化学的有力手段。本文所述的研究工作通过在金（Au）超微电极表面电吸附具有SERS活性的Au纳米粒子并恒电流沉积金属Ni薄层,制备并表征了具有SERS活性的Ni超微电极。在氢氧化钠溶液中的循环伏安实验和以4-甲基苯硫酚分子作为探针分子的SERS实验结果表明,沉积速率和沉积电量是影响超微电极表面Ni的覆盖度和SERS活性的关键因素。在吸附了直径为55 nm Au纳米粒子的、直径为10 μm Au的超微电极表面,以100 μA·cm-2电流密度电沉积厚度约为5个原子层Ni的条件下,可获得Ni覆盖完好的、具有最强SERS活性的Ni超微电极。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.