SpectroElectroChemistry

From the preparation of SERS-active substrates, to methodology and theory, our group aims to expand the breadth and deepen the understanding of electrochemical SERS (EC-SERS) in surface science and material science.

EC-SERS on chemisorptions

For the EC-SERS characterization, our group has invented and developed diverse methods to prepare the electrochemically roughened or nanoparticles assembled film electrodes. Especially, the contributions to expand EC-SERS on transition metal (VIII B) surface of which the original SERS activity is rather limited. With the excellent SERS-active substrates, we have obtained high-quality electrochemical Raman signal of pyridine adsorbed on the coinage metal and transition metal surfaces. Concurrently, to improve our fundamental understanding of the electrode/electrolyte interface, we have successfully observed the first SERS (also the first Raman) signal of surface water on Pt-group metals.

Figure 1. a) SERS spectra of pyridine adsorbed on roughened Ag, Au, Cu and Pt electrodes at open circuit potential (left) and the peak potential (vs. SCE) of the ring breathing mode (right); b) SERS spectra of water adsorbed on Pt, Pd and Au at negative potentials in 0.1 M KClO₄ (right top). The suggested models (right bottom) for the adsorbed water on different electrodes and the influence of potential on metal conduction electron are shown on the left. The suggested models (right bottom) for the adsorbed water on different electrodes and the influence of potential on metal conduction electron are presented.

EC-SERS on reaction processes and electrode kinetics

When combined with electrochemical techniques and theoretical calculation method, in-situ EC-Raman will shed the light on the in-depth understanding of reaction mechanisms in catalysis, electrochemistry, organic chemistry, etc. Take the reduction of benzyl chloride on silver electrode for instance, we carried out an in situ electrochemical surface-enhanced Raman spectroscopic study to characterize various surface species in different electrode potential regions. Corresponding DFT calculation reveals that the benzyl radical and its anionic derivate bonded on a silver electrode are the key intermediates, implying that the pathway could drastically differ from the outer sphere concerted electron reduction at inert electrodes.

Figure 2. a) CV of 5 mM PhCH₂Cl in 0.1 M TEAP + CH₃CN at a Ag electrode with different scan rates; b) Potential dependent SERS spectra of PhCH₂Cl on a Ag electrode; DFT calculated Raman spectra of the possible solvated reaction intermediates: c) free benzyl radical, d) free benzyl anion, f) benzyl radical-Ag₄ adduct, g) benzyl anion-Ag₄ adduct. These are compared with e) the experimental SERS spectrum at -1.4 V vs SCE and h) a 1:5 superposition of the predicted spectra in f and g.

EC-SHINERS for in-situ monitoring reactions on single crystal surfaces

Single crystal surfaces are commonly preferred and used in surface science, because of their well-defined surface state and optic field. However, SERS is seriously limited to roughened or nanostructured surfaces. The electrooxidation processes play the crucial role in electrocatalysis investigations. Herein, electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) is utilized to in situ monitor the electrooxidation processes at Au(hkl) single crystal electrode surfaces. The experimental results are well correlated with our periodic density functional theory calculations and corroborate the long-standing speculation based on theoretical calculations in previous electrochemical studies. The presented in situ electrochemical SHINERS technique offers a unique way for a real-time investigation of an electrocatalytic reaction pathway at various well-defined noble metal surfaces.

Figure 3. a) Schematic diagram of EC-SHINERS at single crystal surface; b) The EC-SHINERS spectra of the electrooxidation processes of Au(111), Au(100), and Au(110) electrodes in 0.1 M NaClO₄ (from top to bottom). The corresponding CVs are presented as well.

References

1. Tian, Z. Q. & Ren, B. Adsorption and reaction at electrochemical interfaces as probed by surface-enhanced Raman spectroscopy. Annu. Rev. Phys. Chem 55, 197–229 (2004).

2. Wu, D., Li, J., Ren, B. & Tian, Z. Electrochemical surface-enhanced Raman spectroscopy of nanostructures. Chem. Soc. Rev . 37, 1025-1041 (2008).

3. Wang, A. et al. In Situ Identification of Intermediates of Benzyl Chloride Reduction at a Silver Electrode by SERS Coupled with DFT Calculations. J. Am. Chem. Soc. 132, 9534–9536 (2010).

4. Li, J. et al. Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature 464, 392-395 (2010).

5. Li, C. Y. et al. In Situ Monitoring of Electrooxidation Processes at Gold Single Crystal Surfaces Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 137, 7648-7651 (2015).

从 SERS 活性基底的制备，到方法学和理论研究，本课题组致力于拓展电化学 SERS（EC-SERS）在表面科学和材料科学中的研究广度，并加深对其机理的理解。

化学吸附过程中的 EC-SERS

在 EC-SERS 表征方面，本课题组发明并发展了多种方法，用于制备电化学粗糙化电极或纳米粒子组装膜电极。特别是，我们对将 EC-SERS 扩展到本征 SERS 活性十分有限的过渡金属（VIII B）表面作出了贡献。依托优良的 SERS 活性基底，我们获得了吡啶吸附在币族金属和过渡金属表面上的高质量电化学拉曼信号。同时，为提高对电极/电解液界面的基础理解，我们成功观察到了 Pt 族金属表面水的首个 SERS 信号，也是首个 Raman 信号。

图 1. a) 吡啶在粗糙化 Ag、Au、Cu 和 Pt 电极上于开路电位下的 SERS 光谱（左）以及环呼吸振动峰位电位（vs. SCE）（右）；b) 0.1 M KClO₄ 中负电位下水吸附在 Pt、Pd 和 Au 上的 SERS 光谱（右上）。左侧显示了不同电极上吸附水的建议模型（右下）以及电位对金属导带电子的影响。不同电极上吸附水的建议模型（右下）以及电位对金属导带电子的影响也在图中给出。

反应过程和电极动力学中的 EC-SERS

当与电化学技术和理论计算方法结合时，原位 EC-Raman 将有助于深入理解催化、电化学、有机化学等领域中的反应机理。以氯化苄在银电极上的还原为例，我们开展了原位电化学表面增强拉曼光谱研究，以表征不同电极电位区间中的各种表面物种。相应的 DFT 计算表明，键合在银电极上的苄基自由基及其阴离子衍生物是关键中间体，这意味着其反应路径可能与惰性电极上的外层协同电子还原过程显著不同。

图 2. a) 5 mM PhCH₂Cl 在 0.1 M TEAP + CH₃CN 中于 Ag 电极上不同扫描速率下的 CV；b) PhCH₂Cl 在 Ag 电极上的电位依赖 SERS 光谱；可能溶剂化反应中间体的 DFT 计算 Raman 光谱：c) 游离苄基自由基，d) 游离苄基阴离子，f) 苄基自由基-Ag₄ 加合物，g) 苄基阴离子-Ag₄ 加合物。它们与 e) -1.4 V vs SCE 下的实验 SERS 光谱以及 h) f 和 g 预测光谱按 1:5 叠加的结果进行比较。

用于单晶表面反应原位监测的 EC-SHINERS

由于单晶表面具有明确的表面状态和光场，表面科学研究通常偏好并使用单晶表面。然而，SERS 严重受限于粗糙化或纳米结构化表面。电氧化过程在电催化研究中起着关键作用。在这里，电化学壳层隔绝纳米粒子增强拉曼光谱（EC-SHINERS）被用于原位监测 Au(hkl) 单晶电极表面的电氧化过程。实验结果与我们的周期性密度泛函理论计算高度相关，并证实了以往电化学研究中基于理论计算提出的长期推测。所展示的原位电化学 SHINERS 技术为在多种结构明确的贵金属表面实时研究电催化反应路径提供了独特方法。

图 3. a) 单晶表面 EC-SHINERS 示意图；b) Au(111)、Au(100) 和 Au(110) 电极在 0.1 M NaClO₄ 中电氧化过程的 EC-SHINERS 光谱（自上而下）。相应的 CV 也一并给出。

参考文献

1. Tian, Z. Q. & Ren, B. Adsorption and reaction at electrochemical interfaces as probed by surface-enhanced Raman spectroscopy. Annu. Rev. Phys. Chem 55, 197–229 (2004).

2. Wu, D., Li, J., Ren, B. & Tian, Z. Electrochemical surface-enhanced Raman spectroscopy of nanostructures. Chem. Soc. Rev . 37, 1025-1041 (2008).

3. Wang, A. et al. In Situ Identification of Intermediates of Benzyl Chloride Reduction at a Silver Electrode by SERS Coupled with DFT Calculations. J. Am. Chem. Soc. 132, 9534–9536 (2010).

4. Li, J. et al. Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature 464, 392-395 (2010).

5. Li, C. Y. et al. In Situ Monitoring of Electrooxidation Processes at Gold Single Crystal Surfaces Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 137, 7648-7651 (2015).