物理化学学报 >> 2017, Vol. 33 >> Issue (3): 530-538.doi: 10.3866/PKU.WHXB201611211

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密度泛函理论研究银上吸附对巯基吡啶的SERS化学增强效应

吴元菲,李明雪,周剑章,吴德印*(),田中群*()   

  • 收稿日期:2016-08-02 发布日期:2017-03-07
  • 通讯作者: 吴德印,田中群 E-mail:dywu@xmu.edu.cn;zqtian@xmu.edu.cn
  • 基金资助:
    国家自然科学基金(21273182);国家自然科学基金(21533006);国家自然科学基金(21373172)

Density Functional Theoretical Study on SERS Chemical Enhancement Mechanism of 4-Mercaptopyridine Adsorbed on Silver

Yuan-Fei WU,Ming-Xue LI,Jian-Zhang ZHOU,De-Yin WU*(),Zhong-Qun TIAN*()   

  • Received:2016-08-02 Published:2017-03-07
  • Contact: De-Yin WU,Zhong-Qun TIAN E-mail:dywu@xmu.edu.cn;zqtian@xmu.edu.cn
  • Supported by:
    the National Natural Science Foundation of China(21273182);the National Natural Science Foundation of China(21533006);the National Natural Science Foundation of China(21373172)

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摘要：

基于密度泛函理论计算和拉曼光谱理论分析，我们研究了对巯基吡啶（4MPY）分子的拉曼光谱和其在银上的表面增强拉曼光谱（SERS），并进一步探讨了SERS与界面吸附结构、异构化、质子化和氢键作用以及低能激发态的关系。首先，我们对两种分子异构体的相对稳定性和拉曼光谱进行了理论分析。在此基础上，进而研究了该分子与不同银簇作用时的拉曼光谱，结果表明，4MPY以巯基硫与银簇作用形成强的Ag-S键，导致拉曼光谱的线型不依赖于所选银簇的大小。接着我们考虑了吡啶氮端作用的两种情况。（1）当4MPY-银簇复合物同时以吡啶氮与水簇或水合质子簇形成氢键时，结果表明吡啶环的部分振动频率随氢键和质子化发生蓝移。（2）当考虑吡啶氮与银簇作用时，吡啶环三角畸变振动发生蓝移。上述情况不仅解释了实验观测的振动频率变化，而且表明了化学环境改变对相对拉曼强度的影响。最后，我们计算了当对巯基吡啶分子以单端或双端与银簇作用，在考虑激发光与低能激发态的能量匹配时，拉曼光谱强度与低能激发态的关系。计算结果表明，在双端吸附构型下，与吡啶氮成键的银簇受激发产生电荷转移态，不仅导致吡啶环v12、v1和v8a振动的拉曼信号增强，而且选择性地增强吡啶环C-H面内对称弯曲振动v9a的拉曼信号。

关键词: 表面增强拉曼光谱, 密度泛函理论, 电荷转移机理, 对巯基吡啶, 银

Abstract:

Surface-enhanced Raman spectroscopy (SERS) is one of the most powerful techniques for obtaining fingerprint information on molecules adsorbed on coinage metal surfaces. Its detection sensitivity has reached the single-molecule level. On the basis of density functional theoretical (DFT) calculations and Raman scattering theory, we investigated the normal Raman spectra of two isomers and surface-enhanced Raman scattering (SERS) spectra of 4-mercaptopyridine (4MPY) adsorbed on silver. The results aided us in uncovering the relationships between normal Raman spectra and SERS spectra and adsorption configuration, tautomerization, protonation, and hydrogen bonding interactions as well as low-lying excited states. First, we compared the relative stability and normal Raman spectra of two isomers of 4MPY in the gas phase and aqueous solution with a solvent model similar to the solvation model of density (SMD). We then studied the Raman spectra of 4MPY interacting with silver clusters. Our results indicate that the Raman spectra were not dependent on the size of the silver clusters, owing to the formation of strong Ag-S bonds. We also considered two cases of Nend interaction in the 4MPY-Ag5 complex. (1) For the hydrogen bond interaction between the nitrogen in 4MPY and water clusters or hydrated proton clusters, the theoretical results indicated that the vibrational frequencies of the pyridine ring increase. (2) For the interaction of the 4MPY-Ag5 complex with a silver cluster Ag4 through the lone-paired orbital in nitrogen of the pyridine ring, the theoretical results further revealed that the vibrational frequency shift is in good agreement with SERS peaks reported in the literature. Finally, our calculated results focused on the relationship between the Raman spectra and the charge transfer mechanism when the excitation photonic energy matches the transition energy of low-lying excited states in single-end and double-end adsorption configuration. Particularly for the case of the double-end adsorption configuration, the charge transfer states from the excitation from the silver cluster binding to the pyridine ring not only enhance the Raman signals of v12, v1, and v8a modes, but also selectively enhance the Raman signal of the v9a mode associated with the symmetric C-H in-plane bending vibration.

Key words: Surface-enhanced Raman spectroscopy, Density functional theory, Charge transfer mechanism, 4-Mercaptopyridine, Silver