Analyzing the Adsorption Behavior of Thiocyanide on Pure Pt and Ni Electrode Surfaces by Confocal Microprobe Raman Spectroscopy

Abstract

Surface vibrational spectroscopies, such as infrared (IR), 1,2 Raman 3,4 and sum frequency generation (SFG), 5 are powerful surface analytical tools for in-situ investigating electrochemical interfaces by providing information at the molecular level. Among these techniques, Raman spectroscopy exhibits several advantages over IR spectroscopy and the SFG technique in analyzing solid/liquid interfaces: 3 (i) easy use of near-UV, visible, or near-IR light sources for the excitation of the Raman process, (ii) transparency of the electrolyte for the light beam, (iii) application of standard electrochemical cells without the need of using thin liquid cells, and hence, (iv) easy combination with electrochemical transient measurements in faradaic and non-faradaic regions, (v) easy obtaining vibrational bands related to the adsorbate-surface interaction in the low-frequency region (<500 cm -1 ). However, the lack of detection sensitivity has been a severe obstacle which had impeded the wide application of Raman spectroscopy in surface science and interfacial electrochemistry. In the absence of any resonance Raman or surface-enhancement processes, the typical differential Raman cross section (dσ/dΩ)NRS is less than 10 -29 cm 2 sr -1 compared with 10 -20 cm 2 sr -1 that of IR. 3 Especially in the analysis of adsorbates on the surface, the molecular quantity present is normally on the order of monolayer, or even submonolayer.Accordingly, the corresponding surface Raman intensities expected for a monolayer adsorbates are less than 1 count per second (cps) when standard Raman spectrometer systems are used. The discovery of surface-enhanced Raman scattering (SERS) in the 1970s, 6-8 that the Raman intensities for pyridine adsorbed at electrochemically roughened Ag electrodes were six orders of magnitude larger than those expected from the known differential Raman cross section for pyridine in solution, changed the situation of surface Raman spectroscopy. These findings stimulated worldwide experimental and theoretical research on SERS. Among the various proposed SERS mechanisms, "electromagnetic mechanism" (EM) 9 and "chemical effect" or "charge transfer" (CT) enhancement 10 have been widely accepted as the two main contributions to the giant surface enhancement. In the past two decades, with its high surface sensitivity and ease detecting vibrations in the lowfrequency region, SERS has been extensively applied in surface science, 3,4,9,10 analytical science 11 and life science. 12 Although it has especially provided a wealth of information in understanding solid/liquid and solid/gas interfacial structures, its application was almost restricted to noble metals, i.e., Ag, Au and Cu. 3,4,9,10 Therefore, it is worth finding other SERS substrates which are extensively used in variety of technologically important processes, such as Pt, Ni and Fe metals. Many researchers have been exploring very hard to attain this goal, but successful cases are few. Among them, Fleischmann's group 13 and Weaver's group 14,15 took advantage of the lo...

An investigation of the adsorption of pyrazine and pyridine on nickel electrodes by in situ surface-enhanced Raman spectroscopy

“…We developed appropriate surface roughening procedures for Ni electrodes with which we could obtain high-quality SERS spectra of pyridine and pyrazine as well as of molecules or ions with small Raman cross sections, such as CO, SCN À , N À 3 [74], etc., by using the highly sensitive confocal microprobe Raman system. Considering the confocal feature, the enhancement factors on roughened Ni electrodes were found to vary from tens to several thousands, which largely depended on the roughening procedures.…”

“…It is desirable to extend the study to transition metals such as Ni for a better understanding of the role of the metal on the activity of the adsorbed pyrazine and pyridine. Recently, we performed a preliminary SERS study on bare Ni electrodes [46,47,53,73,74]. Here, we report a systematic and comparative study of SERS on the interaction and orientation of pyrazine and pyridine on Ni electrode surfaces.…”

Observation of gold electrode surface response to the adsorption and oxidation of thiocyanate in acidic electrolyte with broadband sum-frequency generation spectroscopy

“…Generally, the order of the CÀ ÀN stretching frequency v C-N is bridged, S-bounded, and N-bounded [10]. As potential increases, N-bounded adsorption changes to Sbounded or even bridged [11]. The Stark slopes (dv C-N /dE) of different SCN À orientation are significantly different.…”

Potential dependent thiocyanate adsorption on gold electrodes: a comparison study between SERS and SHINERS

“…Surface‐enhanced Raman scattering (SERS) is a powerful technique for the examination of molecules at electrode surface because of its high sensitivity and fingerprint information in the low wavenumber region, which is difficult to obtain with other vibrational techniques. Several studies have realized the importance of SERS on roughened metal electrodes by explaining the adsorption behavior of target molecules . Interestingly, Weaver et al .…”