Adsorption and photon-driven charge transfer of pyridine on a cobalt electrode analyzed by surface enhanced Raman spectroscopy and relevant theories

doi:10.1016/S0022-0728(03)00318-8

Journal of Electroanalytical Chemistry

Volumes 554–555, 15 September 2003, Pages 417-425

In memory of Michael Weaver

https://doi.org/10.1016/S0022-0728(03)00318-8 Get rights and content

Abstract

Surface enhanced Raman spectroscopy (SERS) has been applied to study the interaction of pyridine with a cobalt electrode surface. A set of good quality SERS spectra was obtained by using an appropriate surface-roughening procedure and a highly sensitive confocal Raman microscope. The surface enhancement factor was about three orders of magnitude calculated from experimental data and analyzed by the relevant theories. The electromagnetic contribution accounts for two out of the three orders of magnitude mainly through the lightning-rod effect. The remaining enhancement originates from the photon-driven charge transfer mechanism. The SERS intensity-potential profile shows the existence of two charge transfer processes. One is the excitation of the metal 4s orbital to the mixing orbital of the metal 4p_x and π-type 3b₁ of pyridine, and the other is from the metal 3d orbital to the π-type 2a₂ orbital of pyridine. Furthermore, the spectral difference for the adsorbed pyridine on cobalt and silver surfaces indicates that the chemical interaction of pyridine with the former is considerably stronger than the latter.

Introduction

As one of the most important transition metals, cobalt (Co) and its compounds have been widely used in many technological applications including electrochemistry [1], [2], [3]. In order to have a thorough knowledge of the properties of the metal, including its related processes and mechanisms, a wide variety of spectroscopic techniques have been employed to study the processes and reactions on Co electrodes [4], [5], [6], [7], [8], [9], [10]. Compared with the conventional electrochemical techniques, these techniques have some advantages in monitoring in situ the surface and interfacial processes at the molecular/atomic level. Surface enhanced Raman spectroscopy (SERS), with its very high surface sensitivity and selectivity, can detect the surface species and minimize the interference from the bulk [11], [12], [13], [14], [15], [16]. As an in situ diagnostic probe, SERS also has the capacity to provide bonding information between adsorbed molecule and substrate. It has been applied to a variety of surface systems, including electrochemical, biological and other ambient interfaces [17], [18], [19]. The fact that only Ag, Au and Cu produce the largest surface enhancements severely limits the use of other metallic materials as the SERS substrate. Extending the range of SERS applications to other metallic and non-metallic surfaces has been a long-term issue in the communities of surface science and spectroscopy [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31].

In order to extend SERS to Co and other materials, a strategy based on ‘borrowing SERS’ was proposed and developed by both the Weaver and Fleischmann groups independently [24], [25]. This concept was realized by depositing an ultra-thin Co film over the highly SERS-active Ag substrate [25]. With the aid of the long-range effect of the electromagnetic (EM) enhancement created by the SERS-active substrate underneath, weak SERS spectra of adsorbate on the transition metal over-layer can be obtained. Since the strong EM field generated on the SERS-active substrate is damped significantly by the coated film, the film must be ultra-thin, normally only several atomic layers. Originally, it was very difficult to cover the rough substrate completely with such a thin film. In the mid-1990s, Weaver and coworkers made significant progress in solving this problem. They reported a series of work on ‘pinhole-free’ transition metal layers over the SERS-active Au surface, which was accomplished by electrochemical atomic-layer epitaxial growth using constant-current deposition at a low current density [32], [33], [34] or by redox replacement of under-potential-deposited metals on Au [35]. It has been shown that this method is very promising if one can prepare the ‘pinhole-free’ ultra-thin film for different materials with good stability in a wide range of potentials and/or temperatures. In addition to studying surface adsorption and reaction, the over-layer method has been used to characterize the fine structure of the ultra-thin film itself. This includes oxides [27], semiconductors [36] and polymers [37]. It makes SERS a versatile tool for studying various material surfaces of practical importance.

Another strategy, not involving ‘borrowing’, is to generate SERS directly from the massive Co metal, since the ultra-thin Co films may have different crystalline structures and other chemical and physical properties compared to the Co bulk phase commonly formed by metallurgy. However, this strategy is much more challenging as it contradicts the widely accepted notion that transition metals are not SERS-active. Since the early days of SERS, several groups have attempted to obtain SERS signals from adsorbates on roughened Pt and Rh electrodes [28], [29], or porous Ni, Pd, Pt, Ti and Co films [23]. However, surface Raman signals from these studies were typically too weak to be investigated as a function of the electrode potential or temperature, some results could even not be repeated by other groups. The results were not strongly supportive of SERS studies on transition metals, and pointed to a gloomy future in this direction. Recently, with the aid of new generation Raman instruments with high sensitivity and the development of surface-roughening procedures, high-quality SERS signal has been obtained from a series of massive transition metals such as Pt, Fe, Ni and Rh [38], [39], [40], [41], [42], [43], [44]. Very recently, we reported a study of SERS from bare Co electrodes [45]. Here, we present a systematic study of SERS and the relevant theoretical calculations on the interaction of pyridine with Co electrode surfaces.

Section snippets

Experimental

Raman spectra were obtained using two confocal microprobe Raman Instruments: LabRam I (Dilor) and R1000 (Renishaw). The excitation wavelength of 632.8 nm (LabRam I) was used from an internal He–Ne laser with a power of 3 mW at the electrode surface; the 514.5 nm (R1000) was from an external Ar-ion laser with a power of 5 mW at the surface.

Various surface-roughening procedures were employed, which will be described in the following sections. Prior to the surface pretreatment, the Co electrode

Results and discussion

Fig. 1 illustrates the impact of different surface-roughening procedures on the surface Raman intensity of pyridine adsorbed on Co electrode surfaces. The peaks marked with asterisks are from the solution. After being well polished and cleaned with triply distilled water, the Co electrode was chemically etched in 1 M HNO₃ in the sonication bath for 10 min. As can been seen in Fig. 1a, the surface Raman signal is quite weak; the signal for the strongest band (ν₁, ring-breathing mode) of pyridine

Conclusion

In summary, using an appropriate surface-roughening procedure for Co electrodes and a highly sensitive confocal microprobe Raman system, we have been able to obtain good-quality SERS spectra of pyridine adsorbed at massive Co electrodes over a wide potential range. The remarkable difference in the SERS spectra of pyridine adsorbed on Co and Ag can be explained by the stronger chemical interaction between pyridine and the Co than that with Ag. The SERS generated from the cobalt surface result

Acknowledgements

The authors gratefully acknowledge the financial support from the Natural Science Foundation of China under contract Nos. 20003008, 20021002 and 90206039.

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¹: Present address: Department of Chemistry, Purdue University, West Lafayette, IN, 47907-1393 USA.

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Adsorption and photon-driven charge transfer of pyridine on a cobalt electrode analyzed by surface enhanced Raman spectroscopy and relevant theories

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgements

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Surf. Sci.

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J. Electroanal. Chem.

J. Electroanal. Chem.

Chem. Phys. Lett.

Chem. Phys. Lett.

Electrochim. Acta

J. Electroanal. Chem.

Surf. Sci.

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Phys. Lett.

Surf. Sci.

Chem. Phys. Lett.

Chem. Phys. Lett.

J. Mol. Struct. (THEOCHEM)

J. Mater. Chem.

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J. Electrochem. Soc.

Phys. Rev. B

J. Appl. Electrochem.

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Chem. J. Chin. Univ.

J. Am. Chem. Soc.

Surface Enhanced Raman Scattering

Rev. Mod. Phys.

Chem. Rev.