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Orientational behavior of cyanide on a roughened platinum surface investigated by surface enhanced Raman spectroscopy
2 June 2000
Chemical Physics Letters 322 Ž2000. 561–566 www.elsevier.nlrlocatercplett
Orientational behavior of cyanide on a roughened platinum surface investigated by surface enhanced Raman spectroscopy Bin Ren ) , Xiao-Qin Li 1, De-Yin Wu, Jian-Lin Yao, Yong Xie, Zhong-Qun Tian State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry and Institute of Physical Chemistry, Xiamen UniÕersity, Xiamen 361005, China Received 11 January 2000
Abstract The surface Raman study of the electrode potential effect on the adsorption behavior of cyanide on the Pt surface was performed on a confocal microprobe Raman system with the help of a special surface pretreatment for the platinum electrode. The abrupt change of the dependence of the CNy stretching vibration on the potential at ca. y0.6 V indicates a subtle orientation change of the adsorbed CN. This is supported by the changes of the Pt–C vibration located at ca. 400 cmy1. A detailed discussion is given on the potential dependent adsorption behavior and the frequency shift of the major vibrational bands. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Studies of cyanide ion ŽCNy. adsorption on metal surfaces are of great interest to surface scientists from the theoretical as well as the experimental point of view w1,2x. CNy is a very important complex ion in the electroplating industry for improving the quality of plating w3x. It has been shown to be the oxidation intermediate of amino acids w4x. As an iso-electronic molecule with carbon monoxide ŽCO., it could constitute an interesting comparative case with PtrCO w5x. As a result, CNy has been studied extensively by the conventional electrochemical method. However, it has been found that it can be )
Corresponding author. Fax: 86-592-2085349. Tel: 86-5922181906. Department of Chemistry, Xiamen University, Xiamen, 361005 China. E-mail: [email protected] 1 Present address: Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA.
strongly adsorbed in the whole potential region, so that the detailed investigation into the electrochemical behavior of itself is less informative by the conventional electrochemical methods w6,7x. With the development of spectroelectrochemistry in the 1980’s and thereafter w8x, researchers have been seeking help from spectroelectrochemical methods to reveal the interaction between this ion and metal surfaces. There are some investigations by infrared ŽIR. w9–14x and sum frequency generation ŽSFG. w15,16x techniques to provide insight into the double layer features such as the molecular structure and orientation of the adsorbate under equilibrium conditions. There have been relatively few in-situ IR studies of cyanide on platinum electrodes in electrochemical systems compared with those of CO adsorptions owing to the difficulties in obtaining useful information from the spectral data, which tend to be rather complex, as well as the small IR cross section of CNy compared with CO. Due to the complexity of
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 4 5 5 - 3
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the system studied, in that there are several vibrational modes appearing at roughly the same region, therefore presenting difficulties for IR and SFG in performing low frequency studies, it is very difficult to distinguish the system’s adsorption configuration simply by the n CN from the high frequency region provided by these two techniques w9–16x. Surface enhanced Raman spectroscopy ŽSERS. can readily overcome this problem w17,18x, but its application is limited to Ag, Cu and Au surfaces with a giant surface enhancement effect since normal surface Raman has extremely low sensitivity w19,20x. Platinum, a very widely used electrode material, had been considered for a long time not to be a suitable material to be studied by SERS due to its special dielectric constant w21x. Although the overlayer tactic developed in the last decade has partially overcome the obstacle, but the complexity involved with the composite electrode is still an underlying problem for this technique w22–25x. For instance, recently, Creighton et al. evaporated an ultrathin film over a SERS active Ag film as the SERS substrate to investigate the SERS of CNy on this bimetallic substrate w26x. But the major bands they observed was from the surface complex and with the prolonged acquisition time the interference from substrate Ag became significant. As a result, there is no up to date Raman report on cyanide adsorption at the pure Pt electrode surface where giant surface enhancement is lacking. On the other hand, our group has taken great effort in extending surface Raman studies to pure transition metal electrodes in the past three years w27–30x. The enhancement factors are estimated to be around 10–1000 for Pt, Ni and Fe w28x. The capability of obtaining Raman spectra of hydrogen and CO adsorbed on Pt surfaces stimulates us to perform Raman studies of CNyradsorption at a Pt surface since CNy is a stronger Raman scatterer compared with CO. In this letter, the preliminary surface Raman study of the potential effect on the adsorption behavior of CNy on a roughened Pt surface was conducted.
2. Experimental Raman measurements were performed with a confocal microprobe Raman system ŽLabRam I from
Dilor.. The system has an extremely high detecting sensitivity, using a single spectrograph with a holographic notch filter to filter the Rayleigh scattering and a CCD as the detector. The slit and pinhole used were 200 mm and 800 mm respectively. The excitation line was provided by a He-Ne laser at 632.8 nm with a laser power of 8.5 mW delivered at the sample. The microscope objective used in present study is a long-working length objective with 50 magnification. A more detailed description of the confocal Raman microscopic instrumentation will be given elsewhere w29x. The applied potential during Raman measurements was controlled by the use of PAR 173 Potentiostat ŽEG & G.. The square wave was generated by a GFG-8016G function generator ŽGood Will Instrument, Co. Ltd... The working electrode was a polycrystalline Pt disk embedded in a Teflon sheath with a geometric surface area of 0.1 cm2 . The pretreatment and roughening procedure of the electrode will be described elsewhere w29x. Briefly, the smooth Pt electrode was subject to electrochemical cleaning first in 0.5 M H 2 SO4 , then was roughened in the above fresh solution with the square wave potential Žy0.2 to q2.4 V, frequency 1.5 kHz.. After that, the electrode was cycled in the above solution between y0.3 and 1.25 V for several minutes to obtain a stable surface. A large platinum ring served as the counter electrode. The reference electrode was a saturated calomel electrode ŽSCE., thus all the potentials in this paper were quoted versus SCE. All the chemicals used are analytical reagents and the solutions were prepared using MilliQ-water.
3. Results and discussion Fig. 1 gives a set of Raman spectra changing with the electrode potential in a wide frequency range. The roughness of the Pt electrode used was about 50 according to the method described in Ref. w30x, and the solution used was 10y4 M KCN q 0.1 M KNO 3 . At q0.6 V, a broad band at 545 cmy1 could be detected which is from the vibrations of Pt and oxygen containing species w27x. The band at 2120 cmy1 assigned to be n CN is also quite broad. This is a reflection of the complicated surface structure at
B. Ren et al.r Chemical Physics Letters 322 (2000) 561–566
Fig. 1. Surface Raman spectra of CNy adsorbed on a roughened Pt surface in 10y4 M KCN q 0.1 M KNO 3 at different potentials as indicated in the figure. The laser excitation line: 632.8 nm.
this potential. It can be seen from the figure that, upon negative movement of the electrode potential, this band shifts to 2053 cmy1 at y0.6 V, the intensity keeps growing. Further negative movement of the potential to y0.7 V, causes this band frequency to jump back to 2062 cmy1 and it almost remains unchanged with the potential, whilst the intensity of the band decreases. A weak band appears in the potential region from 0 to y0.7 V at ca. 1980 cmy1 . The intensity of the band at about 400 cmy1 changes with potential but the frequency only shifts slightly to higher frequency. The capability to detect the band in the low frequency region shows the advantage of Raman spectroscopy over IR and SFG. The band could be assigned to the Pt–C vibration, which is supported by several experimental facts from other molecular systems and theoretical prediction. In our previous Raman study of CO adsorption on Pt surfaces, we found a band with a negative Stark tuning rate Žd nrd E . at about 480 to 510 cmy1 , which concurrently appeared with the appearance of the band at around 2050 cmy1 , this band was assigned to be from the Pt–C vibration. This assignment is also supported by the SERS result of Zhang and Weaver et al. w31x and the EELS result of Chesters et al. on the COrPt system w32x. Tadjeddine and Flament in their MCSCF calculation on the PtrCNy system
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predicted that the vibrational frequency for the Pt– CNy stretching vibration locates at ca. 393 cmy1 w33x. The value is very close to the band observed by us at about 396 cmy1 at y0.2 V. The frequency of the n CN shifts with the change of the electrode potential with a slope value about 70 cmy1 rV. This is considered to be mainly due to the electrochemical Stark effect w34,35x. In the present CNyrPt system, CNy has one electron sitting on its weak anti-bonding orbital 5s w16,33x. When the potential is moved to a positive value compared to the potential of zero charge Žpzc, for a neutral solution the pzc is at around y0.2 to 0 V. of the Pt electrode, the electron will transfer out of this orbital, thus strengthening the C.N bond, therefore the vibrational frequency blue-shifts. On the contrary, with negative movement of the potential, the metal surface becomes negatively charged, it donates electrons to the CNy, and the electron goes into the weak 5s anti-bonding orbital. This leads to the weakening of the bond, thus the band red-shifts. On the other hand, for the Pt–C bond, it is a single s bond, thus positive movement of the electrode potential pulls the electrons out of the orbital, thus weakening the bond strength. On the other hand, negative movement of the electrode potential injects electrons into the orbital, thus strengthening the bonding, and resulting in the blue shifts of the vibrational frequency of the Pt–C band. This phenomenon cannot be interpreted by the electrostatic field effect. In that model, the negatively charged CNy ion should undergo a decrease of the Pt–C vibration as the charge of the metal surface becomes more negative at lower potentials on the basis of an electrostatic Stark effect arising from the increased Coulombic repulsion occurring under this condition w34x. Thus in this system, the chemical bonding effect dominates the Stark tuning effect. We could find that the frequency of the CN stretching vibration decreases steadily with the potential when it is more positive than y0.6 V. It indicates that in the potential region there is only one species adsorbed at the electrode surface. This is confirmed by the only low frequency band appearing at ca. 400 cmy1 . As has been mentioned above this band is possible due to the Pt–C stretching vibration. Thus, in the potential region more positive than y0.6 V, the CNy is adsorbed by its C end to the Pt
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surface. However, when the potential is set at a value more negative than y0.6 V, the Raman feature changes dramatically. Let us first see about the possible adsorption modes of CNy on the Pt surface. At least five kinds of adsorption configurations of CNy adsorbed on metal surfaces have been proposed, which are given in Fig. 2. The five types are on-top adsorbed CNy with either the C or N end bonded to the surface, bridge-type adsorbed CNy with its C end adsorbed onto two Pt atoms w1x, the semi-bridge type with its C end forming a s bond with a Pt atom and its p bond bonding to the Pt surface w13x, and the last one, the CNy lying flat on the Pt surface with its axis parallel to the electrode surface and interacting with Pt through its p bond w16x. In electrochemical environments, the former four types of adsorption have all been reported, and the last one has been found only in the gasrsolid interface on the Cu surface w16x. Taking these possible orientation configurations in mind, we try to scrutinize the adsorption behavior of CNy on the Pt surface along with some basic knowledge of inorganic chemistry and some of the previous theoretical calculation results. The knowledge of charge distribution in CNy ion will be helpful in understanding the bonding preference. The MSCF calculation results presented by Tadjeddine et al. showed that, the charge in the CNy does not gather totally on any end of the CNy ion, but the C end in CNy ion withholds 56% of the total charge, while the N end possesses the 44% w33x. Thus the C end is slightly more negatively charged than that of the N end. At this point, it is reasonable to imagine that the CNy prefers to adsorb onto the Pt surface through its C end at relatively positive potentials. This is evident by the n Pt – C band appearing at the potential range
Fig. 2. Proposed adsorption models of CNy adsorbed on metal surfaces: Ža. C end linearly adsorbed; Žb. N end linearly adsorbed; Žc. bridge-bonded; Žd. semi-bridge-bonded; Že. flat bonded.
positive compared to y0.6 V. Upon negative movement of the potential, the negatively charged electrode could try to expel the more negatively charged C end away from the surface, whilst the N end tries to get close to the surface. As a consequence, the adsorbed CNy tends to lean slightly towards the Pt surface, and finally lies on the surface when the surface is highly negatively charged. Consequently, the vibrational frequency of C.N should experience a sudden change after the CNy lies flat on the surfaces when the potential crosses the value of y0.6 V, as has been shown in Fig. 1. During the process of CNy leaning onto the surface, the on-top adsorbed CNy could very possibly transfer to adsorbing through its C end to two Pt atoms on the surface, as in the case of Fig. 2Žb.. This is demonstrated by the appearance of the band at 1980 cmy1 , which was assigned to the bridge-bonded CN on the Pt surface by Takamaru et al. in their concentration dependent IR study w12x. The result reported by Ashley et al. about CNy adsorption on Pd surface gave the same assignment w1x. However, at the Pt electrode no clear potential dependent shift of the vibrational frequency could be found compared with that on the Pd surface. With further negative movement of the electrode potential, it is reasonable to expect that the CNy could further lean onto the surface taking very possibly the configuration of semi-bridged type proposed by Korzeniewski et al. for interpreting the abnormally low frequency of the CNy stretching vibrations w13x. A s q p interaction between cyanide and the two metal centers has been proposed to describe this semi-bridged type chemical bonding. Corresponding vibrational spectra of some bi-coordination cyanide complexes indicate that the cyanide coordination environments are very different from those of the mono-coordination compound, as the frequency of CN is shifted to lower energies compared to what it is for the free cyanide in solution. Hence the semi-bridging coordination model may be useful in describing surface cyanide complexation in the case where unexpected frequency downshifts have been observed, as in the negative potential region of this study where as low as 2053 cmy1 can be found. In that model, the C end of the CNy molecular bonds to the Pt atom by a s bond, while the p bond of the CNy ion gets close enough to the surface, as a result
B. Ren et al.r Chemical Physics Letters 322 (2000) 561–566
the bonding between the p bond of CN and the Pt atom on the surface can occur, thus a s q p bonding mode of CNy to the Pt surface can exist. This leads to a further down shift of the n CN . When the potential shifts to relatively negative values, in our case, y0.7 V, the CNy ion completely lies on the Pt surface with its molecular axis parallel to the Pt surface. At this moment, the s bond between the C and Pt atoms breaks and the CNyion interacts with the Pt surface solely through its p electron and p ) antibonding orbital. Under this circumstance, the anticipated back donation of electron density from the Pt surface to CNyp ) anti-bonding orbitals should significantly decrease the frequency of the CN stretching compared with its free ion in solution at 2080 cmy1 . These kind of substantial downshifts in the frequency of the p orbital related vibrational modes have been observed for other molecules. However, in this configuration the interaction between Pt and CNy is not as strong as that with the C end bonded to the surface, the chemical bonding effect, playing a key role in the electrochemical Stark effect greatly affects the CNy vibration. As a consequence, after the threshold potential, the potential dependent frequency shift of the C.N vibration almost amounts to zero. This is consistent with the behavior of flatly adsorbed species. However, for a more detailed interpretation, concentration and immersion potential dependent Raman studies are highly desirable. The quantum chemistry study of the system after considering the potential effect and solvent effect could be helpful in fully understanding the phenomenon. These works are now underway in our lab. In summary, using a highly sensitive confocal microprobe Raman system along with the use of the roughened Pt electrode, we were able for the first time to obtain Raman spectra of CNy adsorption on a pure Pt electrode surface, especially to detect vibrational band reflecting adsorbate and substrate interaction. It has shown that the CNy ion could adsorb on the Pt surface in the whole potential range studied. The electrode potential has a significant influence on the adsorption behavior of CNy adsorption at the Pt surface. At relatively positive potentials CNy adsorbs to the Pt surface through its C end, this is supported by the Pt–C band detected in the potential region more positive than y0.6 V. When the
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potential was made more negative the orientation change was evidenced by the sudden change of the CNy stretching vibration, the variation of the band intensity and the disappearance of the Pt–C band. The appearance of the band at about 1980 cmy1 in the potential region from 0 V to y0.7 V could be a very possible evidence of bridge or semi-bridge adsorption types, which are the intermediates of on-top adsorption to flat adsorption type. The analysis of the potential dependent frequency shifts of the Pt–C and C–N bonds revealed that the chemical bonding effect accounts mainly for the potential dependent band frequency shift–electrochemical Stark effect. This study clearly shows that Raman spectroscopy is a powerful tool for investigating the adsorption process as well as determining the molecular orientation at transition metal surfaces.
Acknowledgements The authors acknowledge the support of the National Natural Science Foundation of China and Ministry of Education under contracts No. 29903009, 29625306 and 99177.
References w1x K. Ashley, M. Lazaga, M.G. Samant, H. Seki, M.R. Philpott, Surf. Sci. 219 Ž1989. L590. w2x A.S. Hinman, R.A. Kydd, R.P. Cooney, J. Chem. Soc.-Faraday Trans. 82 Ž1986. 3525. w3x B. Reents, G. Lacconi, W. Plieth, J. Electroanal. Chem. 376 Ž1994. 185. w4x F. Huerta, E. Morallon, F. Cases, A. Rodes, J.L. Vazquez, A. Aldaz, J. Electroanal. Chem. 431 Ž1997. 269. w5x K. Ashley, F. Weinert, M.G. Samant, H. Seki, M.R. Philpott, J. Phys. Chem. 95 Ž1991. 7409. w6x H. Tamaru, T. Arikado, H. Yoneyama, Y. Matsuda, Electrochim. Acta 19 Ž1974. 273. w7x A. Wieckowski, M. Szklarczyk, J. Electroanal. Chem. 142 Ž1982. 157. w8x R.J. Gale ŽEd.., Spectroelectrochemistry – Theory and Practice, Plenum Press, New York, 1988. w9x K. Kunimatsu, H. Seki, W.G. Golden, Chem. Phys. Lett. 108 Ž1984. 195. w10x F. Kitamura, M. Takahashi, M. Ito, Chem. Phys. Lett. 130 Ž1986. 181. w11x J.J. Zhang, J.T. Lu, C.X. Cha, Z.G. Feng, Acta Phys.-Chim. Sin. 5 Ž1989. 409.
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w12x H. Kawashima, Y. Ikezawa, T. Takamura, J. Electroanal. Chem. 317 Ž1991. 257. w13x V.B. Paulissen, C. Korzeniewski, J. Phys. Chem. 96 Ž1992. 4563. w14x C. Stuhlmann, I. Villegas, M.J. Weaver, Chem. Phys. Lett. 219 Ž1994. 319. w15x A. Tadjeddine, A. Peremans, A. Lerille, W.Q. Zheng, M. Tadjeddine, J.P. Flament, J. Chem. Soc. Faraday Trans. 92 Ž1996. 3823. w16x W. Daum, F. Dederichs, J.E. Muller, Phys. Rev. Lett. 80 Ž1998. 766. w17x M. Moskovits, Rev. Mod. Phys. 57 Ž1985. 783. w18x A. Otto, I. Mrozek, H. Grabhorn, W.J. Akemann, Phys. Condens. Matter. 4 Ž1992. 1143. w19x M. Fleischmann, I.R. Hill, M.E. Pemble, J. Electroanal. Chem. 217 Ž1987. 397. w20x H. Wetzel, H. Gerischer, B. Pettinger, Chem. Phys. Lett. 80 Ž1981. 159. w21x M.P. Cline, P.W. Barber, R.K. Chang, J. Opt. Soc. Am. 319 Ž1991. 403. w22x L.W.H. Leung, M.J. Weaver, J. Am. Chem. Soc. 109 Ž1987. 5113.
w23x M. Fleischmann, Z.Q. Tian, L.J. Li, J. Electroanal. Chem. 217 Ž1987. 397. w24x S. Zou, C.T. Williams, E. Chen, M.J. Weaver, J. Am. Chem. Soc. 120 Ž1998. 3811. w25x S. Zou, M.J. Weaver, X.Q. Li, B. Ren, Z.Q. Tian, J. Phys. Chem. B 103 Ž1999. 4218. w26x E. Hesse, J.A. Creighton, Chem. Phys. Lett. 303 Ž1999. 101. w27x Z.Q. Tian, B. Ren, B.W. Mao, J. Phys. Chem. B 101 Ž1997. 1338. w28x Z.Q. Tian, J.S. Gao, X.Q. Li, B. Ren, Q.J. Huang, W.B. Cai, F.M. Liu, B.W. Mao, J. Raman Spectrosc. 29 Ž1998. 703. w29x B. Ren, X.Q. Li, J.L. Yao, F.M. Liu, P.G. Cao, Z.Q. Tian, Anal. Chem., submitted for publication. w30x Q.J. Huang, J.L. Yao, R.A. Gu, B.W. Mao, Z.Q. Tian, Chem. Phys. Lett. 271 Ž1997. 101. w31x Y. Zhang, M.J. Weaver, Langmuir 9 Ž1993. 1397. w32x M.A. Chesters, G.S. McDougall, M.E. Pemble, N. Sheppard, Surf. Sci. 164 Ž1985. 425. w33x M. Tadjeddine, J.P. Flament, Chem. Phys. 240 Ž1999. 39. w34x D.K. Lambert, Electrochim. Acta 41 Ž1996. 623. w35x S. Zou, M.J. Weaver, J. Phys. Chem. 100 Ž1996. 4237.
Chemical Physics Letters 322 Ž2000. 561–566 www.elsevier.nlrlocatercplett
Orientational behavior of cyanide on a roughened platinum surface investigated by surface enhanced Raman spectroscopy Bin Ren ) , Xiao-Qin Li 1, De-Yin Wu, Jian-Lin Yao, Yong Xie, Zhong-Qun Tian State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry and Institute of Physical Chemistry, Xiamen UniÕersity, Xiamen 361005, China Received 11 January 2000
Abstract The surface Raman study of the electrode potential effect on the adsorption behavior of cyanide on the Pt surface was performed on a confocal microprobe Raman system with the help of a special surface pretreatment for the platinum electrode. The abrupt change of the dependence of the CNy stretching vibration on the potential at ca. y0.6 V indicates a subtle orientation change of the adsorbed CN. This is supported by the changes of the Pt–C vibration located at ca. 400 cmy1. A detailed discussion is given on the potential dependent adsorption behavior and the frequency shift of the major vibrational bands. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Studies of cyanide ion ŽCNy. adsorption on metal surfaces are of great interest to surface scientists from the theoretical as well as the experimental point of view w1,2x. CNy is a very important complex ion in the electroplating industry for improving the quality of plating w3x. It has been shown to be the oxidation intermediate of amino acids w4x. As an iso-electronic molecule with carbon monoxide ŽCO., it could constitute an interesting comparative case with PtrCO w5x. As a result, CNy has been studied extensively by the conventional electrochemical method. However, it has been found that it can be )
Corresponding author. Fax: 86-592-2085349. Tel: 86-5922181906. Department of Chemistry, Xiamen University, Xiamen, 361005 China. E-mail: [email protected] 1 Present address: Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA.
strongly adsorbed in the whole potential region, so that the detailed investigation into the electrochemical behavior of itself is less informative by the conventional electrochemical methods w6,7x. With the development of spectroelectrochemistry in the 1980’s and thereafter w8x, researchers have been seeking help from spectroelectrochemical methods to reveal the interaction between this ion and metal surfaces. There are some investigations by infrared ŽIR. w9–14x and sum frequency generation ŽSFG. w15,16x techniques to provide insight into the double layer features such as the molecular structure and orientation of the adsorbate under equilibrium conditions. There have been relatively few in-situ IR studies of cyanide on platinum electrodes in electrochemical systems compared with those of CO adsorptions owing to the difficulties in obtaining useful information from the spectral data, which tend to be rather complex, as well as the small IR cross section of CNy compared with CO. Due to the complexity of
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 4 5 5 - 3
562
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the system studied, in that there are several vibrational modes appearing at roughly the same region, therefore presenting difficulties for IR and SFG in performing low frequency studies, it is very difficult to distinguish the system’s adsorption configuration simply by the n CN from the high frequency region provided by these two techniques w9–16x. Surface enhanced Raman spectroscopy ŽSERS. can readily overcome this problem w17,18x, but its application is limited to Ag, Cu and Au surfaces with a giant surface enhancement effect since normal surface Raman has extremely low sensitivity w19,20x. Platinum, a very widely used electrode material, had been considered for a long time not to be a suitable material to be studied by SERS due to its special dielectric constant w21x. Although the overlayer tactic developed in the last decade has partially overcome the obstacle, but the complexity involved with the composite electrode is still an underlying problem for this technique w22–25x. For instance, recently, Creighton et al. evaporated an ultrathin film over a SERS active Ag film as the SERS substrate to investigate the SERS of CNy on this bimetallic substrate w26x. But the major bands they observed was from the surface complex and with the prolonged acquisition time the interference from substrate Ag became significant. As a result, there is no up to date Raman report on cyanide adsorption at the pure Pt electrode surface where giant surface enhancement is lacking. On the other hand, our group has taken great effort in extending surface Raman studies to pure transition metal electrodes in the past three years w27–30x. The enhancement factors are estimated to be around 10–1000 for Pt, Ni and Fe w28x. The capability of obtaining Raman spectra of hydrogen and CO adsorbed on Pt surfaces stimulates us to perform Raman studies of CNyradsorption at a Pt surface since CNy is a stronger Raman scatterer compared with CO. In this letter, the preliminary surface Raman study of the potential effect on the adsorption behavior of CNy on a roughened Pt surface was conducted.
2. Experimental Raman measurements were performed with a confocal microprobe Raman system ŽLabRam I from
Dilor.. The system has an extremely high detecting sensitivity, using a single spectrograph with a holographic notch filter to filter the Rayleigh scattering and a CCD as the detector. The slit and pinhole used were 200 mm and 800 mm respectively. The excitation line was provided by a He-Ne laser at 632.8 nm with a laser power of 8.5 mW delivered at the sample. The microscope objective used in present study is a long-working length objective with 50 magnification. A more detailed description of the confocal Raman microscopic instrumentation will be given elsewhere w29x. The applied potential during Raman measurements was controlled by the use of PAR 173 Potentiostat ŽEG & G.. The square wave was generated by a GFG-8016G function generator ŽGood Will Instrument, Co. Ltd... The working electrode was a polycrystalline Pt disk embedded in a Teflon sheath with a geometric surface area of 0.1 cm2 . The pretreatment and roughening procedure of the electrode will be described elsewhere w29x. Briefly, the smooth Pt electrode was subject to electrochemical cleaning first in 0.5 M H 2 SO4 , then was roughened in the above fresh solution with the square wave potential Žy0.2 to q2.4 V, frequency 1.5 kHz.. After that, the electrode was cycled in the above solution between y0.3 and 1.25 V for several minutes to obtain a stable surface. A large platinum ring served as the counter electrode. The reference electrode was a saturated calomel electrode ŽSCE., thus all the potentials in this paper were quoted versus SCE. All the chemicals used are analytical reagents and the solutions were prepared using MilliQ-water.
3. Results and discussion Fig. 1 gives a set of Raman spectra changing with the electrode potential in a wide frequency range. The roughness of the Pt electrode used was about 50 according to the method described in Ref. w30x, and the solution used was 10y4 M KCN q 0.1 M KNO 3 . At q0.6 V, a broad band at 545 cmy1 could be detected which is from the vibrations of Pt and oxygen containing species w27x. The band at 2120 cmy1 assigned to be n CN is also quite broad. This is a reflection of the complicated surface structure at
B. Ren et al.r Chemical Physics Letters 322 (2000) 561–566
Fig. 1. Surface Raman spectra of CNy adsorbed on a roughened Pt surface in 10y4 M KCN q 0.1 M KNO 3 at different potentials as indicated in the figure. The laser excitation line: 632.8 nm.
this potential. It can be seen from the figure that, upon negative movement of the electrode potential, this band shifts to 2053 cmy1 at y0.6 V, the intensity keeps growing. Further negative movement of the potential to y0.7 V, causes this band frequency to jump back to 2062 cmy1 and it almost remains unchanged with the potential, whilst the intensity of the band decreases. A weak band appears in the potential region from 0 to y0.7 V at ca. 1980 cmy1 . The intensity of the band at about 400 cmy1 changes with potential but the frequency only shifts slightly to higher frequency. The capability to detect the band in the low frequency region shows the advantage of Raman spectroscopy over IR and SFG. The band could be assigned to the Pt–C vibration, which is supported by several experimental facts from other molecular systems and theoretical prediction. In our previous Raman study of CO adsorption on Pt surfaces, we found a band with a negative Stark tuning rate Žd nrd E . at about 480 to 510 cmy1 , which concurrently appeared with the appearance of the band at around 2050 cmy1 , this band was assigned to be from the Pt–C vibration. This assignment is also supported by the SERS result of Zhang and Weaver et al. w31x and the EELS result of Chesters et al. on the COrPt system w32x. Tadjeddine and Flament in their MCSCF calculation on the PtrCNy system
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predicted that the vibrational frequency for the Pt– CNy stretching vibration locates at ca. 393 cmy1 w33x. The value is very close to the band observed by us at about 396 cmy1 at y0.2 V. The frequency of the n CN shifts with the change of the electrode potential with a slope value about 70 cmy1 rV. This is considered to be mainly due to the electrochemical Stark effect w34,35x. In the present CNyrPt system, CNy has one electron sitting on its weak anti-bonding orbital 5s w16,33x. When the potential is moved to a positive value compared to the potential of zero charge Žpzc, for a neutral solution the pzc is at around y0.2 to 0 V. of the Pt electrode, the electron will transfer out of this orbital, thus strengthening the C.N bond, therefore the vibrational frequency blue-shifts. On the contrary, with negative movement of the potential, the metal surface becomes negatively charged, it donates electrons to the CNy, and the electron goes into the weak 5s anti-bonding orbital. This leads to the weakening of the bond, thus the band red-shifts. On the other hand, for the Pt–C bond, it is a single s bond, thus positive movement of the electrode potential pulls the electrons out of the orbital, thus weakening the bond strength. On the other hand, negative movement of the electrode potential injects electrons into the orbital, thus strengthening the bonding, and resulting in the blue shifts of the vibrational frequency of the Pt–C band. This phenomenon cannot be interpreted by the electrostatic field effect. In that model, the negatively charged CNy ion should undergo a decrease of the Pt–C vibration as the charge of the metal surface becomes more negative at lower potentials on the basis of an electrostatic Stark effect arising from the increased Coulombic repulsion occurring under this condition w34x. Thus in this system, the chemical bonding effect dominates the Stark tuning effect. We could find that the frequency of the CN stretching vibration decreases steadily with the potential when it is more positive than y0.6 V. It indicates that in the potential region there is only one species adsorbed at the electrode surface. This is confirmed by the only low frequency band appearing at ca. 400 cmy1 . As has been mentioned above this band is possible due to the Pt–C stretching vibration. Thus, in the potential region more positive than y0.6 V, the CNy is adsorbed by its C end to the Pt
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surface. However, when the potential is set at a value more negative than y0.6 V, the Raman feature changes dramatically. Let us first see about the possible adsorption modes of CNy on the Pt surface. At least five kinds of adsorption configurations of CNy adsorbed on metal surfaces have been proposed, which are given in Fig. 2. The five types are on-top adsorbed CNy with either the C or N end bonded to the surface, bridge-type adsorbed CNy with its C end adsorbed onto two Pt atoms w1x, the semi-bridge type with its C end forming a s bond with a Pt atom and its p bond bonding to the Pt surface w13x, and the last one, the CNy lying flat on the Pt surface with its axis parallel to the electrode surface and interacting with Pt through its p bond w16x. In electrochemical environments, the former four types of adsorption have all been reported, and the last one has been found only in the gasrsolid interface on the Cu surface w16x. Taking these possible orientation configurations in mind, we try to scrutinize the adsorption behavior of CNy on the Pt surface along with some basic knowledge of inorganic chemistry and some of the previous theoretical calculation results. The knowledge of charge distribution in CNy ion will be helpful in understanding the bonding preference. The MSCF calculation results presented by Tadjeddine et al. showed that, the charge in the CNy does not gather totally on any end of the CNy ion, but the C end in CNy ion withholds 56% of the total charge, while the N end possesses the 44% w33x. Thus the C end is slightly more negatively charged than that of the N end. At this point, it is reasonable to imagine that the CNy prefers to adsorb onto the Pt surface through its C end at relatively positive potentials. This is evident by the n Pt – C band appearing at the potential range
Fig. 2. Proposed adsorption models of CNy adsorbed on metal surfaces: Ža. C end linearly adsorbed; Žb. N end linearly adsorbed; Žc. bridge-bonded; Žd. semi-bridge-bonded; Že. flat bonded.
positive compared to y0.6 V. Upon negative movement of the potential, the negatively charged electrode could try to expel the more negatively charged C end away from the surface, whilst the N end tries to get close to the surface. As a consequence, the adsorbed CNy tends to lean slightly towards the Pt surface, and finally lies on the surface when the surface is highly negatively charged. Consequently, the vibrational frequency of C.N should experience a sudden change after the CNy lies flat on the surfaces when the potential crosses the value of y0.6 V, as has been shown in Fig. 1. During the process of CNy leaning onto the surface, the on-top adsorbed CNy could very possibly transfer to adsorbing through its C end to two Pt atoms on the surface, as in the case of Fig. 2Žb.. This is demonstrated by the appearance of the band at 1980 cmy1 , which was assigned to the bridge-bonded CN on the Pt surface by Takamaru et al. in their concentration dependent IR study w12x. The result reported by Ashley et al. about CNy adsorption on Pd surface gave the same assignment w1x. However, at the Pt electrode no clear potential dependent shift of the vibrational frequency could be found compared with that on the Pd surface. With further negative movement of the electrode potential, it is reasonable to expect that the CNy could further lean onto the surface taking very possibly the configuration of semi-bridged type proposed by Korzeniewski et al. for interpreting the abnormally low frequency of the CNy stretching vibrations w13x. A s q p interaction between cyanide and the two metal centers has been proposed to describe this semi-bridged type chemical bonding. Corresponding vibrational spectra of some bi-coordination cyanide complexes indicate that the cyanide coordination environments are very different from those of the mono-coordination compound, as the frequency of CN is shifted to lower energies compared to what it is for the free cyanide in solution. Hence the semi-bridging coordination model may be useful in describing surface cyanide complexation in the case where unexpected frequency downshifts have been observed, as in the negative potential region of this study where as low as 2053 cmy1 can be found. In that model, the C end of the CNy molecular bonds to the Pt atom by a s bond, while the p bond of the CNy ion gets close enough to the surface, as a result
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the bonding between the p bond of CN and the Pt atom on the surface can occur, thus a s q p bonding mode of CNy to the Pt surface can exist. This leads to a further down shift of the n CN . When the potential shifts to relatively negative values, in our case, y0.7 V, the CNy ion completely lies on the Pt surface with its molecular axis parallel to the Pt surface. At this moment, the s bond between the C and Pt atoms breaks and the CNyion interacts with the Pt surface solely through its p electron and p ) antibonding orbital. Under this circumstance, the anticipated back donation of electron density from the Pt surface to CNyp ) anti-bonding orbitals should significantly decrease the frequency of the CN stretching compared with its free ion in solution at 2080 cmy1 . These kind of substantial downshifts in the frequency of the p orbital related vibrational modes have been observed for other molecules. However, in this configuration the interaction between Pt and CNy is not as strong as that with the C end bonded to the surface, the chemical bonding effect, playing a key role in the electrochemical Stark effect greatly affects the CNy vibration. As a consequence, after the threshold potential, the potential dependent frequency shift of the C.N vibration almost amounts to zero. This is consistent with the behavior of flatly adsorbed species. However, for a more detailed interpretation, concentration and immersion potential dependent Raman studies are highly desirable. The quantum chemistry study of the system after considering the potential effect and solvent effect could be helpful in fully understanding the phenomenon. These works are now underway in our lab. In summary, using a highly sensitive confocal microprobe Raman system along with the use of the roughened Pt electrode, we were able for the first time to obtain Raman spectra of CNy adsorption on a pure Pt electrode surface, especially to detect vibrational band reflecting adsorbate and substrate interaction. It has shown that the CNy ion could adsorb on the Pt surface in the whole potential range studied. The electrode potential has a significant influence on the adsorption behavior of CNy adsorption at the Pt surface. At relatively positive potentials CNy adsorbs to the Pt surface through its C end, this is supported by the Pt–C band detected in the potential region more positive than y0.6 V. When the
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potential was made more negative the orientation change was evidenced by the sudden change of the CNy stretching vibration, the variation of the band intensity and the disappearance of the Pt–C band. The appearance of the band at about 1980 cmy1 in the potential region from 0 V to y0.7 V could be a very possible evidence of bridge or semi-bridge adsorption types, which are the intermediates of on-top adsorption to flat adsorption type. The analysis of the potential dependent frequency shifts of the Pt–C and C–N bonds revealed that the chemical bonding effect accounts mainly for the potential dependent band frequency shift–electrochemical Stark effect. This study clearly shows that Raman spectroscopy is a powerful tool for investigating the adsorption process as well as determining the molecular orientation at transition metal surfaces.
Acknowledgements The authors acknowledge the support of the National Natural Science Foundation of China and Ministry of Education under contracts No. 29903009, 29625306 and 99177.
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