SURFACE ENHANCED RAMAN SPECTROSCOPIC STUDIES OF SURFACE ORIENTATION OF PYRAZINE ADSORBED ON RHODIUM ELECTRODE Li Cui, Bin Ren, De-Yin Wu, Zhong-Qun Tian* Department of Chemistry, State Key Lab. for Phys. Chem. of Solid Surfaces, Xiamen University, Xiamen 361005, China,
[email protected] Keywords: SERS, adsorption, rhodium, pyrazine, orientation Abstract: Surface enhanced Raman spectroscopic study of pyrazine adsorbed on a roughened rhodium electrode was carried out. The effect of solution pH value, the pyrazine concentration and electrode potential on the adsorption behavior of pyrazine on the Rh surface has been investigated. Surface enhanced Raman scattering (SERS) spectroscopy is a very powerful technique for studying the orientation and bonding of molecules on metal surfaces with its high spectral resolution. As a high symmetric molecule, pyrazine has been extensively used to probe the properties of surface because of its relative simple molecular structure and widely accepted assignment of its vibrational modes. However, it has been found out that the adsorption configurations of pyrazine on different metal surfaces (Ag, Au and Ni) are quite different [1-3]. Therefore, it is worthwhile to investigate the adsorption behavior of pyrazine on other transition metal surfaces, such as Pt and Rh. In our previous study, a surface pretreatment procedure has been developed to obtain SERS-active Rh surfaces, presenting an enhancement of about 3 orders of magnitude [4]. On such kind of substrate, we are able to investigate the adsorption behavior of pyrazine by Raman spectroscopy. Fig.1 shows the SERS spectra of pyrazine adsorbed on a rhodium electrode in 10-3 M pyrazine + 0.1 M NaClO4. Four pairs of overlapping bands are observed , located at about 1570 cm-1 and 1530 cm-1;1212 cm-1and 1236 cm-1;1012 cm-1and 1040 cm-1;628 cm-1and 654 cm-1. The overlapping bands have not been reported in SERS spectra of pyrazine on Ni, Au, Ag, Cu electrode surface. By carefully comparing the change of the relative intensity of these overlapping bands, we will see that the intensities of 1570 cm-1, 1212 cm-1, 1012 cm-1 and 628 cm-1 (denoted as Peak I) present nearly
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Fig.1 Potential dependent SERS spectra of pyrazine adsorbed on a roughened rhodium electrode in a solution containing 0.001 M pyrazine + 0.1 M NaClO4, pH = 7. (a) Potential change from 0 V
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the same changing tendency with potential, so do the other four bands at 1530 cm-1, 1236 cm-1, 1040 cm -1and 654 cm-1(denoted as Peak II). When the potential was moved negatively from the open circuit potential (ocp • 0.3 V) to –0.95 V at which hydrogen evolution occurs (Fig.1a), the intensity of Peak I is higher than Peak II. However, when the potential was moved positively from –0.95 V (Fig.1b), Peak II becomes stronger than Peak I at potentials more positive than –0.8 V. This phenomenon has also been observed in a pyrazine solution with a pH value 9~10 (Fig. 2a). However, in a pyrazine solution with a pH value 2~3, the intensity of peak I is always stronger than Peak II (Fig.2b). By analyzing the vibration modes of the overlapping bands at 1570 cm-1 (8a; νring, Ag) and 1530 cm-1 (8b, δC-H, B2g)[1,2] and employing the SERS selection rule from electromagnetic mechanism [5], we assume that intensity changes are mainly caused by the orientation change of pyrazine with the electrode potential.
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Fig. 2 Potential dependent SERS spectra of pyrazine adsorbed on a roughened rhodium electrode in a solution containing 0.001 M pyrazine + 0.1 M NaClO4.(a) pH = 9~10, potential changes negatively from ocp to -0.95V (hydrogen evolution potential) , then shifts positively to-0.5 V ; (b) pH = 2~3, potential changes negatively from ocp to -0.5 V (hydrogen evolution potential), then shifts positively to -0.2 V. From 0 V to –0.95 V, pyrazine is adsorbed vertically or titledly (end-on) onto the rhodium electrode via N atom. When the potential is more negative than –0.95 V, the adsorbed H generated from H2O +e • Hads reaction will favor the end-on adsorbed pyrazine. However, when the potential shifts positively from –0.95 V (Fig. 1b), the surface concentration of Hads decreases quickly which causes the change of the pyrazine orientation from end-on to edge-on and or tilt. With the latter orientation, the intensity of the 1530 cm-1 band will be mostly enhanced, whereas that of the 1570 cm-1 band will be decreased. Detailed discussion on the adsorption behavior of pyrazine on Rh will be presented on the conference. References: 1. A.G. Brolo, D. E. Irish. J.Electroanal.Chem. 414, 183 (1996). 2. Q. J. Huang, X.F.Lin, Z.L Yang, J.W.Hu, Z.Q.Tian. J. Electroanal. Chem. 563, 121 (2004). 3. A. G. Brolo, D .E. Irish. Langmuir. 14, 517 (1998). 4. B. Ren, X.F.Lin, Z.Q.Tian. J. Phy. Chem. 107, 899 (2003). 5. J. A. Creighton, Spectroscopy of Surfaces. Wiley. New York. 16, 37 (1988).
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