对－硝基苯硫酚在银膜上的表面增强红外光谱研究*

 

何林涛   Peter R. Griffiths

Department of Chemistry, University of Idaho, Moscow, ID83844-2343, USA

(*已发表在光谱学与光谱分析，18，1，1998)

 

    自从Hartstein[1]等人在1980年发现了表面增强红外吸收（SEIRA）现象后，SEIRA的模型化合物－－对-硝基苯甲酸被广泛地研究报导。理论上，具有活泼氢的官能团(-COOH, -OH, -SH等)能被吸附在一些金属(如金、银、铜)表面上。在表面增强红外光谱中，金属表面的形成主要有两种方法：1. 以真空热蒸发法形成若干纳米厚度的金属薄膜 2. 化学电极的表面。本文以显微透射模式测量表面增强红外光谱来研究对-硝基苯硫酚(PNTP)在银膜上的化学作用。

    当PNTP的丙酮或甲醇溶液点到ZnSe晶体上时，通过观察其红外光谱~2550cm-1附近S-H振动峰变化，我们发现PNTP会发生氧化反应变成它的二聚体。Wakefield[2]等人利用薄层色谱与质谱分析PNTP时，也观察到PNTP的二聚反应：

 

2O2N-C6H4-SH + 1/2O2               O2N-C6H4-S-S-C6H4-NO2 + H2O

 

       根据化合物在金属表面的相对位置，表面可分为“上层表面”（Overlayer Surface）---金属表面覆盖在化合物之上，和“下层表面”（Underlayer Surface）---金属表面在化合物之下，两种结构[3]。PNTP二聚体的表面增强红外光谱分别在“上层表面”和“下层表面”两种实验方式测量。实验表明PNTP二聚体的下层表面增强红外光谱与其本身的正常红外光谱非常相似。但是，PNTP二聚体的上层表面增强红外光谱是非常不同于相应的下层表面增强光谱，前者的增强倍数更大。我们推断：以真空热蒸发法形成银上层表面的过程中，高能量的气态银粒子沉积在PNTP二聚体上时，二聚体的S-S化学键断裂，并吸附在银表面上形成S-Ag键，因而它的上层表面增强效应更强。图1显示了PNTP二聚体在银膜表面的SEIRA光谱特征。

        另一方面，我们利用Bio-Rad TracerTM GC/FT-IR冷阱接口来研究PNTP单体在Ag下层表面(Underlayer Surface)的表面增强红外光谱。PNTP单体通过GC毛细管色谱柱沉积在5nm厚银/ZnSe表面上，表面的温度是可以调节变化的。以这种实验方式得到的PNTP单体表面增强红外光谱(SEIRA)与PNTP二聚体的银上层表面(Overlayer Surface)的SEIRA光谱非常相似，表明了PNTP单体离解了活泼氢，并通过S-Ag键化学吸附在Ag表面上。我们还发现PNTP与银表面之间的化学反应可以发生在低达-30oC,但不能发生在-40oC之下。图2是PNTP单体在GC/FT-IR冷阱接口实验中的SEIRA光谱行为。

 

 

Fig.1. (Above) SEIRA Spectra of PNTP dimer measured on an Ag overlayer and (middle) on an Ag underlayer SEIRA. (Below) normal IR spectrum of PNTP dimer on ZnSe plate.

Fig. 2. SEIRA spectra of PNTP monomer on 5 nm film of Ag on ZnSe plate mounted in the Bio-Rad Tracer at pressure of  9x 10-5 torr. The sample was deposited at -90oC, and the temperature of the plate was allowed to increase to -100C (above) and +40oC (moddle). (Below) normal GC-Tracer IR sepectrum of PNTP monomer.

 

Surface-Enhanced Infrared Absorption Studies of

p-Nitrothiophenol on Silver

 

Lin-Tao HE and Peter R. Griffiths

Department of Chemistry, University of Idaho, Moscow, ID 83844-2343, USA

(* Published in Proc. 11th Int. Conf. Fourier Transform Spectrosc.,Am. Inst. Phys. Conf. Proceedings 590, 1997)

 

   Since the discovery of the phenomenon of surface-enhanced infrared absorption (SEIRA) by Hartstein et al, the SEIRA spectrum of the model compound, p-nitrobenzoic acid, has been reported in a number of publications. In theory, the functional groups with acidic protons, e. g., -COOH, -OH and -SH, can chemisorb on the surface of metals such as silver, copper and gold. In this work, SEIRA spectra measured in the microtransmission mode have  been used to investigate the chemistry of p-nitrothiophenol (PNTP) on silver films.

   By monitoring the -S-H stretching band at ~2550 cm-1, we demonstrated that PNTP dimerizes oxidatively when applied to bare ZnSe substrate or Ag surfaces from acetone and methanol solutions under ambient conditions. This conclusion is consistent with the observations by Wakefiel et al., who used thin-layer chromatography and mass spectrometry to show that the following reaction occurs:

2 O2N-C6H4-SH + ½ O2 ------> O2N-C6H4-S-S-C6H4-NO2 + H2O

   SEIRA spectra of the PNTP dimer were measured on both silver underlayer and overlayer surfaces by casting the films of the dimer from solution.  The SEIRA spectrum of the PNTP dimer on an Ag underlayer is very similar to its normal infrared spectrum. However, the SEIRA spectrum of  PNTP dimer on the Ag overlayer is very different and more enhanced than the corresponding underlayer spectrum. We conclude that the S-S bond of the dimer is broken and Ag-S bonds are formed when energetic silver atoms are vapor deposited on a film of the PNTP dimer already on ZnSe plate.

   In a separate experiment, we used the Bio-Rad Tracer direct deposition GC/FT-IR interface to obtain the SEIRA spectrum of PNTP monomer on an Ag underlayer. PNTP monomer was passed through the GC column and deposited on a 5-nm Ag layer on ZnSe plate the temperature of which could be varied. The SEIRA spectrum of PNTP monomer measured in this way is similar to the  SEIRA spectrum of PNTP dimers measured with an Ag overlayer, indicating that the active proton of the PNTP monomer dissociates its active proton and the PNTP chemisorbs on the Ag surface via an S-Ag bond. We also found that the chemical reaction between PNTP monomer and the Ag surface can occur  at temperature as low as -30oC, but no lower than -40oC.

 

参考文献

 

1. Hartstein, A., Kirtley, J. R., Tsang, T. C., Phys Rev.  Lett.  45,  201 (1980).

2. Wakefield, C. J., Waring, D. R., J. Chromatogr. Sci., 15, 82 (1977).

3. Nishikawa, Y., Fujiwara, K., Ataka, K-I., and Osawa, M.,  Anal. Chem. 65, 556 (1993)