Surface raman spectra of pyridine and hydrogen on bare platinum and nickel electrodes

ELSEVIER

Journal of Electroanalytical

Chemistry

415 (1996)

175-178

Preliminary note

Surface Raman spectra of pyridine and hydrogen on bare platinum and nickel electrodes B. Ren, Q.J. Huang, W.B. Cai, B.W. Mao, F.M. Liu, Z.Q. Tian * State Key Laboratory

for Physical

Chemistry

of Solid Surfaces

Received

Keywords:

Raman

spectroscopy;

Platinum

electrodes;

Nickel

and Department China 27 March

elecbodes;

1. Introduction Surface enhanced Raman spectroscopy is now well established as a means of obtaining detailed information on a wide variety of adsorbates [l-4]. Although many metals, including transition metals such as Pt and Ni, have been predicted to yield the surface enhanced Raman scattering (SERS) effect [5&l, in practice this technique has been restricted almost entirely to Ag, Au and Cu, that have the most pronounced SERS effect. In-situ vibrational spectroscopic studies on Pt and Ni electrodes have, therefore, been limited almost to IR and SFG techniques. Nevertheless, workers have never stopped their efforts to extend Raman spectroscopic studies to Pt, Ni and other transition metals. One important approach involves coating SERS active electrodes with a very thin overlayer (e.g. one to five atomic layers) of Pt [7,&X]or Ni [9,10] by electrochemical deposition. However, the inevitable problem for this approach is the extreme difficulty of eliminating entirely the possibility of the presence of adsorbates bound to exposed substrate or to the boundary of the two metals rather than the overlayer sites. A better and more reliable way would be to obtain surface Raman spectra directly from bare Pt and Ni electrodes. To our knowledge, there have been no reports on surface Raman spectroscopic studies of electrosorption at bare Ni electrodes, while studies on slightly platinized Pt [l 11, a highly rough Pt surface [12] and smooth Pt electrodes [13- 151 have been reported. However, because of the very poor signal-to-noise (S/N) ratio ac h ieved in those works at bare Pt electrodes, detailed studies on the potential dependences of the inten-

* Corresponding

author.

0022-0728/96/$15.00 Copyright PI1 SOO22-0728(96)01004-2

0 1996 Elsevier

Science

of Chemistry,

1996; revised

Pyridine

15 May

adsorption;

Xiamen

University,

Xiamen

361005,

People’s

Republic

of

1996

Hydrogen

adsorption

sity and frequency were impossible [ 1 l-151. As a consequence, there has been no continuous work reported in the past four years, implying the extreme difficulty of practical application. In this note, we report new progress in extending surface Raman spectroscopic studies to bare Pt and Ni electrodes by utilizing a highly sensitive confocal Raman microscope. We will present for the first time good quality potential dependent surface Raman spectra of pyridine at Pt and Ni electrode surfaces based on a special surface treatment, and then demonstrate that the study can be extended further to the commonly interesting hydrogen adsorption process at roughened Pt electrodes. These preliminary results may open a new and promising way to study in detail a variety of chemical or electrochemical processes of practical importance on transition metal surfaces.

2. Experimental Raman measurements were performed with a confocal microprobe Raman system (LabRam I from Dilor) [16]. The single spectrograph with a holographic notch filter and a CCD as the detector has an extremely high detecting sensitivity. The slit and pinhole used were 100 and 400 p,rn respectively. The laser (632.8nm) power delivered at the sample was about 15mW unless stated otherwise. A 50 X long working length objective was used in the present study. A more detailed description of the spectroelectrochemical measurements will be given elsewhere [ 171. The working electrodes were a polycrystalline Pt rod and a polycrystalline Ni rod respectively embedded in a Teflon sheath with a geometric surface area of 0.1 cm2. The smooth Pt electrode was prepared by mechanical polishing

S.A. All rights reserved.

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with 0.3 and 0.05 pm alumina powder to a mirror finish on wet polishing cloths followed by ultrasonic cleaning with Mini-Q water. The electrode was then further subjectedto an electrochemical polishing by applying potential cycling between - 0.25 and 1.2V to get a stable surface. The roughening procedure of Pt surfacesfollowed basically the method reported by Arvia and coworkers [18,19]. A fast square-wave of 2kHz with upper and lower potentials of 2.4 and - 0.2 V respectively was applied to the electrodes in 0.5 M H,SO, for 5 to 10 min, then the potential was held at - 0.2V until the completion of surface electroreduction. The electrodes were subjectedfurther to a potential cycling between - 0.25 and 1.2V at 0.5 V s- ’ until all unstable atoms or clusters were removed and a reproducible cyclic voltammogram of hydrogen adsorption/desorption was obtained. The roughening procedure for the Ni electrode was carried out using a chemical etching method, which is similar to that reported by Xue and Dong [20] for roughening Ag and Cu electrodes. The mechanically polished Ni electrode with mirror finish was subjected to chemical etching by immersion into 1M HNO, for about 7 min. The electrode was then rinsed thoroughly and transferred to the spectroelectrochemicalcell for measurement. A large platinum ring served as the counter electrode. All potentials are quoted versus a saturated calomel electrode (SCE). All chemicals used were of analytical reagent grade and the solutions were prepared using MilliQ-water.

3. Results and discussion Fig. 1 showsa set of potential dependentRaman spectra of pyridine adsorbed at the smooth Pt surface. The measurements were carried out stepwise starting from the negative extremity of the potential range and moving towards more positive potentials with a spectral acquisition time of 900 s for each spectrum.The present result appears

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Fig. 1. Potential dependent surface Raman spectra of pyridine a smooth Pt surface in 0.01 M pyridine +O.l M KCI.

adsorbed

at

Chemistry

415 (1996)

175-178

to be the first report of a potential dependent Raman spectra from smooth Pt electrodes, made possible by the advent and use of a confocal microprobe Raman system having significantly higher sensitivity. Furthermore, since the confocal microscope has a good vertical spatial resolution, only a very small amount of the solution is sampled. Therefore, the interference of strong Raman signals from the bulk solution phase can be eliminated markedly, and the difference spectrum method for the in-situ IR measurement is unnecessary. In the present study the electrolyte layer was asthick as about 1 mm. However, it can be seen in Fig. 1 that a weak band of breadth stretching vibration mode located at 1004cm- ’ from the solution bulk pyridine and the broad band of HOH bending vibration mode from bulk water do not obstruct the appearance of the surface Raman signal of the adsorbedpyridine at potentials negative of -0.8 V. Overall, confocal Raman spectroscopy brings a real change to surface Raman spectroscopic studies of a variety of interesting systems and objectives, including monocrystalline electrodes. Since proper surface treatment is always helpful in obtaining good quality surface Raman spectra, a roughening procedure is usually applied to increase the surface area and probably gain substantially higher Raman signals. It has been reported by Arvia and coworkers that Pt electrodes of relatively large surface areascan be obtained in acidic electrolytes via the electroreduction of a previously formed thick hydrous platinum oxide layer by applying a repetitive periodic potential. They claimed that these electrodes behave highly reproducibly over a wide potential range [18,19]. Moreover, they have studied pyridine at such electrode surfacesand unfortunately only a very weak surface signal was detected [12]. In the present work, by modifying slightly the roughening procedure and utilizing the highly sensitive confocal Raman system, we were able to obtain much more intense surface Raman spectra of pyridine, as shown in Fig. 2(a). The large signal for rough surfacescan be attributed mainly to the increasein adsorp tion sites. One striking difference in Fig. 1 and Fig. 2(a) is that the strong pyridine bands that could be observed at - l.OV on a smooth P surface are extremely weak signals at the roughened Pt surface. Moreover, their frequencies also behave differently. These indicate different interactions of pyridine with smooth and roughened surfaces, which will be discussedin detail elsewhere[21]. It is necessaryto emphasizethat the Pt electrodes were found to be surprisingly better than Ag, Cu and Au in terms of stability and reversibility for spectroelectrochemical measurements.The Raman signal could be entirely recovered upon a return to more positive values after a very negative potential excursion. Moreover, the electrode can be reused repetitively provided that it is cleaned electrochemically by cycling in a H,SO, solution prior to each new experiment. This ensures that the vibrational properties of the adsorbateare reasonably representative of the entire surface, which is the essential requirement for

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decreased dramatically at + 1.OV and a broad band of Pt-0 stretch at ca. 578 cm-’ emerged, revealing the formation of the oxide film). This may indicate that pyridine could interact strongly with platinum, and that a change of adsorption orientation might be involved in the two potential regions [21]. However, in the case of Ni, pyridine adsorption could only be observed in certain potential regions (from - 0.6 to - 1.4V) and a slight frequency shift could be detected, characterizing the weaker interaction for the nickel system. Although a systematic study of the frequency and intensity dependences on potential and electrolyte is required in order to come to a definite conclusion, the high quality potential dependent Raman spectra provide a good reason to be optimistic that surface Raman spectroscopy will be extended successfully to more detailed studies on transition metallsolution interfaces. There have been several IR and SFG studies on hydrogen adsorption at transition metal electrodes, especially at Pt surfaces [24-281. However, there have been no Raman studies on hydrogen adsorption at Pt surfaces. mainly due to the lack of sufficient detecting sensitivity. In the present work, with the aid of a special surface pretreatment procedure and the confocal Raman spectroscopy as described above, we obtained surface Raman spectra of hydrogen adsorption at a roughened Pt electrode for the first time. Fig. 3(a) shows the surface Raman spectra of adsorbed H at a roughened Pt surface at different potentials. It can be seen that when the potential was extended to that of hydrogen evolution, an adsorption band at 2088 cm- ’ was observed, corresponding to the adsorption of a hydrogen atom singly coordinated on top of a surface metal atom [24]. The band at 1644cm-’ is assigned to a HOH bend-

Fig. 2. Potential dependent surface Raman spectra of pyridine adsorbed at (a) a roughened Pt surface and (b) a roughened Ni surface in 0.01 M pyridine + 0.1 M KCI.

surface Raman spectroscopy to become a general and reliable technique for wide practical applications. A systematic study of the effects of surface pretreatment, surface morphology and laser excitation line has been carried out, and detailed results will be presented elsewhere [21]. Fig. 2(b) shows the potential dependent Raman spectra obtained from pyridine at the roughened Ni electrode. The laser (5 14.5 nm) power delivered at the sample was about 30mW. There are at least two significant differences in comparison with the Pt electrode. First, the spectral intensity is maximized at - 0.6 V for the Pt, but at - 1.O V for the Ni electrode. This can be explained by the difference in the surface charge density at the same potential because the potentials of zero charge of Pt [22] and Ni 1231 are about +0.16 and -0.33 V respectively. Second, for the Pt electrode the frequency v, of the most intense band remains almost constant in the potential range negative of -0.6V but shifts considerably in the potential range from -0.4 to +0.8V with a slope of IOcm- V- ’ (this band

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Fig. 3. Potential dependent surface Raman spectra of (a) hydrogen adsorption in 0.5 M H, SO, + H,O and (b) deuterium adsorption in 0.5 M H,SO, + D,O at roughened Pt surfaces.

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ing vibration mode from bulk water. More confirmed evidence for the observed band arising from a hydrogen adsorbate is provided by the spectra obtained using a D,O + H,SO, electrolyte, see Fig. 3(b). The 2088cm-’ band in the H,O solution is shifted by a factor of 1.39 to 1496cm-’ in the D,O solution, exactly as expected for the isotopic effect and the influence of anharmonicity. When the electrode potential was made more positive, both bands from H,O and D,O disappeared completely. These results coincide well with those from SNIFTIR measurements [24]. Further studies of the influence of electrolyte ion and pH on hydrogen adsorption are in progress. Our preliminary results demonstrate the applicability of surface Raman spectroscopy in more interesting and practical systems.

Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China and the State Education Committee of China.

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