Electrochemical-SERS Investigation for Cationic Hydrophilic Effects on the Adsorption Behaviors of Water at Ionic Liquid/Metal Interface

doi:10.6023/A24090260

Abstract

Ionic liquid has emerged as ideal electrolyte due to its unique properties. However, the previous studies have shown that ionic liquid was very easy to absorb water, and the water in the ionic liquid is trended to enrich at the electrode interface, resulting the significant influence on the performance of ionic liquid. In this paper, an electrochemical surface-enhanced Raman spectroscopy (EC-SERS) combined with probe reaction technique was developed to study adsorption behavior of water at ionic liquid/metal interface. The electrochemical reduction reaction of p-dimercaptoazobenzene (DMAB), which is highly sensitive to interfacial water, was explored as a probe. The Au nanoparticle monolayer film (Au MLF) with good uniformity and reliability was transferred to the surface of glassy carbon electrode as SERS film electrode to improve the reliability of horizontal comparison of spectral result. The interfacial water adsorption behavior in ionic liquid with different cations was studied accordingly. The activity and homogeneity of the SERS substrate have emerged as pivotal in both fundamental researches and practical applications. Herein, Au MLF was successfully fabricated through a self-assembly process at the air-water interface, with the aid of polyvinylpyrrolidone (PVP) as an auxiliary agent. It was found that the reduction efficiency of DMAB increased significantly with the increase of water content. The reduction efficiencies of DMAB in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF₄) and 1-butylpyridinium tetrafluoroborate ([BPy]BF₄) were almost the same in the solution with the water content less than 1% (volume fraction). It was mainly contributed by the balance in the water adsorption between the anions and cations of bulk phase and the interfacial cations. With the increase of water content to higher than 1%, the reduction efficiency of DMAB in [BMIm]BF₄ is higher than that in [BPy]BF₄, which is consistent with the hydrophilic ability of cations. The results demonstrate that the water adsorption behavior of the ionic liquid/electrode interface is critically dependent on the hydrophilicity of the ionic liquid cation. More hydrophilicity of [BMIm]⁺ allowed the water molecules in [BMIm]BF₄ to reach at the interface with more molecules than that in the [BPy]BF₄ system in the negative potential range. It produced a more significant effect on the water-sensitive interfacial reaction. The present study provides a new approach for the determination of interfacial water and its related behaviors.

Key words: ionic liquid, surface-enhanced raman spectroscopy, probe reaction, interfacial water, hydrophilicity

Cite this article

Qianjun He, Chenjie Zhang, Minmin Xu, Yaxian Yuan, Jianlin Yao. Electrochemical-SERS Investigation for Cationic Hydrophilic Effects on the Adsorption Behaviors of Water at Ionic Liquid/Metal Interface[J]. Acta Chimica Sinica, 2024, 82(12): 1202-1208.

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Fig. & Tab. 5

Figure 1. The SERS spectra of DMAB (Ⅰ) and PATP (Ⅱ) on Au MLF/GC in 0.1 mol?L-1 aqueous KCl

Figure 2. Potential dependent SERS spectra from the PATP adsorbed on Au MLF/GC in 0.1 mol?L-1 aqueous KCl (a), “dry” [BMIm]BF4 (b) and [BPy]BF4 (c). Potential dependent relative intensity of the band at 1140 cm-1 to 1073 cm-1 (d)

Figure 3. Potential dependent SERS spectra from the PATP adsorbed on Au MLF/GC in [BMIm]BF4 with different volume fraction of water, 0.1% (a), 0.5% (b), 1% (c) and 2% (d), potential dependent relative intensity of the band at 1140 cm-1 to 1073 cm-1 (e)

Figure 4. Potential dependent SERS spectra from the PATP adsorbed on Au MLF/GC in [BPy]BF4 with 0.1% (a), 0.5% (b), 1% (c) and 2% (d) volume fraction of water

Figure 5. Potential dependent relative intensity of the band at 1140 cm-1 to 1073 cm-1 in [BPy]BF4 (a), comparison histogram of the minimum value of I1140 cm-1/I1073 cm-1 in two ionic liquids with different volume fraction of water (b)

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