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Potential-induced migration of top-layer atoms and molecules on Pt(110) electrode surface studied by infrared reflection absorption spectroscopy
Volume 198, number 3,4
CHEMICAL PHYSICS LETTERS
9 October 1992
Potential-induced migration of top-layer atoms and molecules on Pt ( 110) electrode surface studied by infrared reflection absorption spectroscopy Hirohito Ogasawara, Junji Inukai and Masatoki Ito Department of Chemistry, Faculty ofScience and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku. Yokohama 223, Japan
Received 21 May 1992; in final form 23 July 1992
The reconstruction process on a clean Pt ( 110) surface was monitored by infrared reflection absorption spectra of CO on the surface. The adsorbate-induced phase transition from the (1 x2 )-Pt (110) to the ( I x I )-Pt( 110) structure was completely suppressed under negative electrode potentials. The migration of copper ad-atoms on the Pt( 110) surface prepared by underpotential deposition was also controlled by the electrode potential. At negative potentials, the mobility of the copper ad-atoms was depressed strongly, and the copperad-atomsdid not forma uniformfilm.Diffusionof the copperatomsstartedat a relatively positive potential. Thus the reconstruction of a Pt ( 110) surface and the migration of copper ad-atoms did not proceed under highly negative electrode potentials at room temperature,
1.
Introduction
Structures and phase transitions on single-crystal surfaces have recently been studied by several workers. A well-known example is the reconstruction in CO chemisorption on Pt ( 110) under UHV conditions investigated by LEED and vibrational spectroscopy [ I-31. A clean Pt ( 110) surface exhibits a structure in which every second row in the [ 1IO] direction is missing, and removal of this “missing row” structure is induced by CO adsorption [ 11. As for the UHV studies, fhe phase transition proceeds rapidly at temperatures higher than 280 K, but it never occurs at temperatures lower than 160 K [ 21. Recently, static structures as well as phase transitions on a single-crystal electrode in electrochemical processes have also been studied [ 4-61. The structure of CO adsorbed on each electrode surface can be determined by infrared reflection absorption spectroscopy (IRAS). There are at least two or three adsorption sites of CO on each electrode surface. The CO population on each site can be controlled by Correspondence fo: M. Ito, Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-l Hiyoshi, Kohoku-ku, Yokohama 223, Japan.
changing the CO coverage and electrode potential. The reversible CO migration between the on-top and bridge CO sites by application of an electrode potential enables a measurement of time-resolved IRAS or electrode potential modulation IRAS on the surface in an acid solution [ 71, We reported evidence for an electrode-surface phase transition of the Pt(llO)=(lX2) structure to Pt(llO)-(1x1) induced by CO adsorption in an acid solution at room temperature [ 8 1. The phase transition occurred rapidly at positive electrode potentials. However, if a neutral or an alkaline solution is used, a double-layer region can be extended to a more negative potential. Therefore, it was thought important to examine whether or not the phase transition occurs at such negative potential conditions. In the present study we report evidence that the electrode-surface phase transition of the Pt( 1lo)-( 1x2) surface to Pt( I lO)( 1x 1) is totally suppressed under highly negative electrode potentials in a neutral solution. In order to confirm the surface diffusion properties of surface atoms and molecules, we also examined surface diffusion of copper atoms deposited on a Pt( 110) electrode surface. As in the case of CO or platinum atoms on the surface, migration of the copper atoms never occurred at highly negative poten-
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
389
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tials, whereas surface diffusion of the copper atom was accelerated at more positive potentials. Here, we report results on the reconstruction of the Pt(llO)-(1x2) surface to Pt(llO)-(1X1) as well as migration of the underpotentially deposited copper atoms on the Pt( 110) electrode surface as a function of electrode potential by use of IRAS. There have been many reports about surface diffusion of surface atoms and molecules as a function of substrate temperature, in which surface diffusion is normally suppressed at low temperatures [ 9 1. However, there has been no surface diffusion measurement as a function of the electrode potential. This is the first spectroscopic approach to surface atom migration at solid-liquid interfaces.
2. Experimental The measurements were conducted in a 0.5 M NaZS04 or 0.05 M Na2S04 aqueous solution at room temperature. The 0.5 M Na,SO, solution prepared with special grade Na2S04 (Junsei Chemical) and ultrapure water (Milli-Q system, Millipore) was electrolytically purified for 12 h before each measurement. The 0.05 M Na,SO, solution was prepared with suprapure grate Na2S04 (Merk) and ultrapure water. Sample preparation and characterization in these experiments were described elsewhere [ lo]. The in-situ IR cell was attached to a 1720X Fourier transform infrared spectrometer ( Perkin-Elmer Inc. ) with a liquid-nitrogen-cooled InSb detector (Electra-optical SYSTEMS). The spectra were normally obtained by accumulation of 64 scans with a 2 cm-’ resolution. In the time-resolved measurement, the accumulation of 16 scans was performed with 2 cm-’ resolution. Al the electrode potentials are quoted against the normal hydrogen electrode (NHE). Gaseous CO was introduced into solution by the bubbling technique. The copper ad-atoms were underpotentially deposited by immersing the electrode in the solution containing 10 FM of CuS04 until the desired amount of copper was obtained, and the electrode was subsequently rinsed. The coverages of adsorbed CO and deposited copper ad-atoms were determined by the oxidation current densities of CO to CO, and Cue to Cu’+, re390
9 October1992
spectively, after each IRAS measurement.
3. Results and discussion 3.1. CO adsorption on Pt(ll0) electrode surfaces The clean Pt ( I 10) surface exhibits the ( I X 2)-reconstructed missing row structure, and CO adsorption at saturation coverage causes migration of platinum atoms to the ( I X I )-unreconstructed structure. In an acid solution, a phase transition similar to that observed on Pt ( 110) under UHV occurred rapidly [ 8 1. However, such surface reconstruction never occurred on the CO saturated Pt( 110) electrode surfaces in a neutral if the electrode potential was conlined to be highly negative. Fig. la shows the 1RAS of CO adsorbed on the Pt ( 110 ) electrode surface in a 0.5 M Na2S04 solution as a function of the electrode potential (-0.6 to -0.4 V versus NHE). The CO coverages (&,) were estimated to be 00.50 ( ? 0.05) from the CO oxidation currents after the IRAS measurements. We observed a strong absorption band of on-top CO and a weak band of bridgebonded CO at ~2050 and ;z 1860 cm-‘, respectively. The frequencies of both species were shifted continuously to the high-frequency (or low-frequency) side with a positive (or negative) potential change. This phenomenon of the CO interaction with metal atoms indicates charge donation of the CO 50 electrons to the metal with a simultaneous charge backdonation from the metal valence states to the CO 2rc*orbital. The 27r*level of CO is antibonding in character and its occupation leads to a weakening of the carbon-oxygen bond. Therefore, this electron backdonation increases at a negative potential, the frequencies of adsorbed CO being lowered. When the electrode potential changes in the positive direction, the electron backdonation is less predominant, which means a smaller occupation of CO 2x*. The preference of bridge-bonded CO on a highly negative surface has previously been reported for CO/K/ Ni( 111) under UHV conditions [ 111. However, if the small occupation of the bridge-bonded CO (fig. la) is ignored the integrated intensity of on-top CO corresponds approximately to %,,=O.SO (CO saturation value). The metastable surface phase of CO adsorbed on Pt( 1lo)-( 1 x2) at low temperature
Volume 198, number 3,4
9 October I992
CHEMICAL PHYSICS LETTERS
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under UHV conditions has been characterized by Hoffman and King [ 2 1. Adsorption of CO on a ( 1 x 2) surface precooled to 160 K was found to proceed without loss of ( 1x2) reconstruction, in contrast to the adsorption at room temperature. They found that adsorption occured predominantly at the on-top site with a small additional occupation of the
Fig. 1. (a) Electrode-potentialdependent infrared spectra for CO onPt( 110) (-0.6666 -0.4). (b) Time-resolvedIRASofC0 on Pt( 110). At t=O, the electrode potential was stepped up to -0.35 V. (c) Electrode-potential-dependent IRAS of CO on Pt(ll0) (-0.34EC0.1).
bridged site. Heating of the saturated surface to temperatures between 280 and 340 K resulted in irreversible transformation of the bridged species to ontop, with a lifting of the reconstruction of ( 1X 2 ) to ( 1x 1). Thus, the present result in a neutral solution at room temperature is comparable with that observed at low temperatures [ 21. 391
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Our previous report in in an acid solution at POSitive potentials [ 81 revealed that the maximum CO coverage attained was &ozO.95, which means that practically every CO is adsorbed on every platinum atom, by which the ( 1x 1)-unreconstructed phase was created. The present result of eco x 0.50 suggests that (i) the CO saturated Pt( 110) surface still retains the ( 1x2)-phase and that (ii) CO molecules are adsorbed on all the [ 1i 0] ridge platinum atoms and tilted away from the surface normal, which is similar to the low temperature results observed under UHV conditions. The higher frequencies observed support this view, since strong dipole-dipole coupling undergoes a positive frequency shift. The strong intensity (nearly 3%) along with the narrow band width also supports this conclusion. 3.2. Phase transition from (1 X2) to (1 X I) structure Studies at more positive potentials on Pt ( 110) are of particular interest, because one may ask in what potential range the reconstruction of the platinum surface layer occurs. Until -0.40 V the lifting due to CO adsorption did not occur even after prolonged exposure to CO. However, when the electrode potential was changed to the more positive (at -0.35 V versus NHE), the single absorption band at 2058 cm- ’changed in intensity with time, as shown in fig. 1b. The spectra represent the results at every 25 s after the potential change to - 0.3 5 V. The shoulder band started to appear at the low-frequency side five minutes later. The shoulder peak developed gradually and the bands were finally split to 2044 and 2059 cm-’ with equal intensities. The band shape and positions were not changed afterwards. The wavenumber of the peak at the high-frequency side did not change at all in this time sequence, while the intensity was reduced. When the electrode potential was further changed in the positive direction, the high-frequency band increased in intensity, while the low-frequency band decreased, with both bands shifted with the potential change (fig. 1c). Finally, at 0.1 V versus NHE a single absorption band appeared again. The band at the low-frequency side is assignable to on-top CO associated with relatively small dipole-dipole coupling, such as singleton species. This means that very small ( 1x 1) patches or isolated platinum atoms 392
9 October1992
evolved on the surface. A recent STM study [ 121 has revealed that the phase transition starts by the evolution of homogeneous nucleation of small characteristic (1 x 1) patches (“nucleation and growth mechanisms”). Therefore, the phase transition from the (lx2)structureto (1x1)-Pt(llO)startsatthe potential of - 0.35 V. The equal intensities of the two absorptions indicate that the phase transition is not completed at this potential. The surface is composed of small domains of the ( 1x2) and ( 1X 1) phases. In order to complete the phase transition, a more positive potential is needed. Unreconstructed ( 1X 1) structure is completed at the positive potential of 0.1 V, where the single absorption on the ( 1x 1) surface is seen. At increased potentials, the enhanced mobility of the platinum atoms gives rise to the formation of the ( 1x 1) domains at the expense of dissolution of the smaller islands. Thus, we conclude that the phase transition never occurs at negative potentials but is accelerated at positive potentials. 3.3. Copper ad-atom migration on the Pt(ll0) electrode surface induced by electrode potential In previous sections we indicated that the migration of platinum atoms as well as CO molecules on the surface were controlled by the electrode potential. Now we report migration of copper ad-atoms underpotentially deposited on the Pt ( 110) surface. The copper electrode position was carried out in a 10m5M CuS04+0.05 M Na,SO, solution. From the current densities of the CV peaks (160 FLCcme2), t&=0.65 was obtained. Fig. 2a shows the IRAS of CO adsorbed on a copper-modified Pt ( 110) surface in a 0.05 M Na2S04 solution as a function of the electrode potential. CO was introduced at -0.6 V versus NHE, where the ( 1x 2) phase was exposed on the surface, and the potential was changed stepwise in the positive direction. We observed two kinds of CO absorptions in the on-top region at x 2 100 and ~2050 cm-‘. Potential-dependent frequency shifts were also seen here for both species. The band at 2100 cm-’ decreased gradually in intensity with a positive potential change and disappeared at 0 V. The other band initially appearing at 2015 cm-’ exhibited a very broad absorption with a tail at the low-frequency side, and the peak intensity at the high-frequency side de-
Volume 198, number 3,4
CHEMICAL PHYSICS LETTERS
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Fig. 2. Electrode potential-dependent IRAS of CO on Pt( 110) modified by underpotentialiy deposited copper atom. CO was introduced at -0.6 V (a) and -0.3 V (b).
9 October 1992
veloped gradually with a positive potential change. The former band around 2 100 cm- ’is assignable to CO on irregularly deposited copper atoms. CO adsorption on a copper surface has been reported by many workers under UHV conditions [ 131. CO adsorption on a high-indexed (stepped) copper surface or a polycrystalline copper surface at a low temperature reveals that that CO absorption band appears at wavenumbers higher than 2 100 cm-‘, whereas CO on an atomically smooth Cu( 111) or Cu ( 100) surface appears at a lower wavenumber, x 2085 cm-’ [ 131. In our previous study [ 141 of CO adsorption on a copper-modified Pd( 111) or Pd( 100) electrode surface in an acid solution, we attributed the band at 2115 cm- ’to on-top CO on irregularly deposited copper atoms. A similar absorption has also been reported recently on copper-modified Pt ( 100) and Pt ( 110) electrode surfaces by Weaver [ 151. On the other hand, the high-frequency component of the latter band could be assigned to the on-top CO adsorbed on the [ 1iO] ridge platinum atoms as described before. The remarkable suppression of on-top CO on the [ 1TO] ridge platinum atoms at highly negative potentials indicates that the CO sites are originally blocked by the irregularly deposited copper ad-atoms. However, the CO sites on the platinum atoms become available at more positive potentials (2040-2060 cm- ’). When CO adsorption was carried out at a less negative potential ( -0.3 V), we observed different results as shown in fig. 2b. IRAS measurements were started at -0.3 V after introduction of CO at this potential. The electrode potential was first varied in the negative direction until -0.6 V, and then swept in the positive direction as before. Two important results were noted: (i) The intensity of the band at 2 100 cm- I was very weak (the band even disappeared completely when CO was introduced at a potential higher than -0.3 V). (ii) The intensity of the low-frequency band (2014-2054 cm-‘) did not change at all, except in the region of a higher positive potential, where oxidation occurred. This is in remarkable contrast with the result for CO introduction at - 0.6 V (fig. 2a), where the low-frequency band developed gradually at a potential higher than - 0.3 V with a simultaneous decrease in the band at ~2100 cm-‘. The CO frequencies in these spectra at the same potential shown in fig. 2b are lower by 393
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CHEMICAL PHYSICS LETTERS
‘Y10 cm-’ than those. This means that the CO coverage in the former case (fig. 2a) was larger than the latter (fig. 2b). Therefore, it is clear that the CO adsorbed on irregularly deposited copper atoms moves to the on-top CO site on the [ Ii01 ridge platinum atoms at a potential higher than -0.3 V. This suggests that aggregated copper atoms blocking the [ 1501 platinum ridge sites migrate to other sites, possibly to trough sites so that CO can occupy the ridge platinum site. Even when the 0,” values were changed in the range of 0.3 to 6.0,we were able to detect two kinds of CO absorption around 2100 and 2000 cm-’ if a highly negative electrode potential was chosen. This means that both the band at z 2 100 cm-’ and the low-frequency tail at the other absorption, z 2000 cm-’ can be assigned to CO on copper atoms. Furthermore, the former band appeared only under the highly negative electrode potentials. This is important since CO adsorption on a copper electrode surface has never been reported. We will discuss this problem in section 3.4.
9 October 1992
tentials (about 1 V higher than the negative potential regions covered in the present study) [ 141. Strong backdonation to CO 2x* at a negative potential strengthens the metal-carbon bond. This is the reason why adsorbed CO hardly migrates to other sites at highly negative potential conditions. Likewise, top layer platinum or underpotentially deposited copper ad-atoms are likely to stay there at a negative potential since the increased electron density between these top-layer atoms and the electrode surface can strengthen the bonds due to increased metalto-metal overlap.
Acknowledgement The present research was supported by the Japan Private School Promotion Foundation, to which the authors’ thanks are due.
References [ 1] CM. Comrie and R.M. Lambert, J. Chem. Sot. Faraday
3.4. Migration mechanism on the Pt(l10) surface Copper films deposited under UHV conditions at a low temperature take irregular polycrystalline structure, and such aggregated copper atoms migrate easily into an atomically smooth surface at room temperature. At highly negative potentials the underpotentially deposited copper atoms on the present Pt ( 110) electrode retain defect-rich structures similar to that formed in the UHV measurement at low temperatures, possibly because the surface diffusion of copper ad-atoms is suppressed completely at negative electrode potential. At relatively positive potentials, migration of the copper atoms is accelerated to form a smooth surface. Thus, the mobility of top layer atoms or molecules is easily controlled by electrode potentials. It is clear that at highly negative conditions the electron density is excessively distributed on top-layer platinum atoms, and a negatively charged surface is created. This differs intrinsically from a UHV surface, which corresponds to extremely positive po-
394
Trans. 72 (1976) 1659. [2 ] P. Hoffman, S.R. Bare and D.A. King, Physica Scripta T4 (1983) 118. [3] S.R. Bare, P. Hoffman and D.A. King, Surface Sci. 144 (1984) 347. [4] E. Morallon, J.L. Vazquez, A. Aldaz and J. Clavilier, J. Electroanal. Chem. 3 16 ( I991 ) 263. [ 5 ] P. Skouda and D.M. Kolb, Surface Sci. 260 ( 1992) 229. [61X. Gao, A. Hamelin and M.J. Weaver, Phys. Rev. B 44 (1991) 10983. [ 7 ] M. Nakamura, H. Ogasawara, J. Inukai and M. Ito, Surface Sci. (1992), to be published. [ 81 Y. Kinomoto, S. Watanabe, M. Takahashi and M. Ito, Surface Sci. 242 (1991) 538. [ 91 J.E. Reutt-Robey, D.J. Doren, Y.J. Chabal and S.B. Christman, J. Chem. Phys. 93 (1990)9113. [ IO] S. Watanabe, Y. Kinomoto, M. Takahashi and M. Ito, J. Electron Spectry. 54/55 (1990) 1205. [ 1I ] K.J. Uran, L. Ng and J.T. Yates Jr., Surface Sci. 177 ( 1986) 253. 121T. Gritsch, D. Coulman, R.J. Behm and G. Ertl, Phys. Rev. Letters 10 (1989) 1086. 131J. Pritchard, T. Catterick and R.K. Gupta, Surface Sci. 53 (1975) 1. 14] K. Yoshioka, F. Kitamura, M. Takahashi and M. Ito, Surface Sci. 227 (1990) 90. IS] S.-C. Chang and M.J. Weaver, Surface Sci. 241 ( 199I) 11.
CHEMICAL PHYSICS LETTERS
9 October 1992
Potential-induced migration of top-layer atoms and molecules on Pt ( 110) electrode surface studied by infrared reflection absorption spectroscopy Hirohito Ogasawara, Junji Inukai and Masatoki Ito Department of Chemistry, Faculty ofScience and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku. Yokohama 223, Japan
Received 21 May 1992; in final form 23 July 1992
The reconstruction process on a clean Pt ( 110) surface was monitored by infrared reflection absorption spectra of CO on the surface. The adsorbate-induced phase transition from the (1 x2 )-Pt (110) to the ( I x I )-Pt( 110) structure was completely suppressed under negative electrode potentials. The migration of copper ad-atoms on the Pt( 110) surface prepared by underpotential deposition was also controlled by the electrode potential. At negative potentials, the mobility of the copper ad-atoms was depressed strongly, and the copperad-atomsdid not forma uniformfilm.Diffusionof the copperatomsstartedat a relatively positive potential. Thus the reconstruction of a Pt ( 110) surface and the migration of copper ad-atoms did not proceed under highly negative electrode potentials at room temperature,
1.
Introduction
Structures and phase transitions on single-crystal surfaces have recently been studied by several workers. A well-known example is the reconstruction in CO chemisorption on Pt ( 110) under UHV conditions investigated by LEED and vibrational spectroscopy [ I-31. A clean Pt ( 110) surface exhibits a structure in which every second row in the [ 1IO] direction is missing, and removal of this “missing row” structure is induced by CO adsorption [ 11. As for the UHV studies, fhe phase transition proceeds rapidly at temperatures higher than 280 K, but it never occurs at temperatures lower than 160 K [ 21. Recently, static structures as well as phase transitions on a single-crystal electrode in electrochemical processes have also been studied [ 4-61. The structure of CO adsorbed on each electrode surface can be determined by infrared reflection absorption spectroscopy (IRAS). There are at least two or three adsorption sites of CO on each electrode surface. The CO population on each site can be controlled by Correspondence fo: M. Ito, Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-l Hiyoshi, Kohoku-ku, Yokohama 223, Japan.
changing the CO coverage and electrode potential. The reversible CO migration between the on-top and bridge CO sites by application of an electrode potential enables a measurement of time-resolved IRAS or electrode potential modulation IRAS on the surface in an acid solution [ 71, We reported evidence for an electrode-surface phase transition of the Pt(llO)=(lX2) structure to Pt(llO)-(1x1) induced by CO adsorption in an acid solution at room temperature [ 8 1. The phase transition occurred rapidly at positive electrode potentials. However, if a neutral or an alkaline solution is used, a double-layer region can be extended to a more negative potential. Therefore, it was thought important to examine whether or not the phase transition occurs at such negative potential conditions. In the present study we report evidence that the electrode-surface phase transition of the Pt( 1lo)-( 1x2) surface to Pt( I lO)( 1x 1) is totally suppressed under highly negative electrode potentials in a neutral solution. In order to confirm the surface diffusion properties of surface atoms and molecules, we also examined surface diffusion of copper atoms deposited on a Pt( 110) electrode surface. As in the case of CO or platinum atoms on the surface, migration of the copper atoms never occurred at highly negative poten-
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
389
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tials, whereas surface diffusion of the copper atom was accelerated at more positive potentials. Here, we report results on the reconstruction of the Pt(llO)-(1x2) surface to Pt(llO)-(1X1) as well as migration of the underpotentially deposited copper atoms on the Pt( 110) electrode surface as a function of electrode potential by use of IRAS. There have been many reports about surface diffusion of surface atoms and molecules as a function of substrate temperature, in which surface diffusion is normally suppressed at low temperatures [ 9 1. However, there has been no surface diffusion measurement as a function of the electrode potential. This is the first spectroscopic approach to surface atom migration at solid-liquid interfaces.
2. Experimental The measurements were conducted in a 0.5 M NaZS04 or 0.05 M Na2S04 aqueous solution at room temperature. The 0.5 M Na,SO, solution prepared with special grade Na2S04 (Junsei Chemical) and ultrapure water (Milli-Q system, Millipore) was electrolytically purified for 12 h before each measurement. The 0.05 M Na,SO, solution was prepared with suprapure grate Na2S04 (Merk) and ultrapure water. Sample preparation and characterization in these experiments were described elsewhere [ lo]. The in-situ IR cell was attached to a 1720X Fourier transform infrared spectrometer ( Perkin-Elmer Inc. ) with a liquid-nitrogen-cooled InSb detector (Electra-optical SYSTEMS). The spectra were normally obtained by accumulation of 64 scans with a 2 cm-’ resolution. In the time-resolved measurement, the accumulation of 16 scans was performed with 2 cm-’ resolution. Al the electrode potentials are quoted against the normal hydrogen electrode (NHE). Gaseous CO was introduced into solution by the bubbling technique. The copper ad-atoms were underpotentially deposited by immersing the electrode in the solution containing 10 FM of CuS04 until the desired amount of copper was obtained, and the electrode was subsequently rinsed. The coverages of adsorbed CO and deposited copper ad-atoms were determined by the oxidation current densities of CO to CO, and Cue to Cu’+, re390
9 October1992
spectively, after each IRAS measurement.
3. Results and discussion 3.1. CO adsorption on Pt(ll0) electrode surfaces The clean Pt ( I 10) surface exhibits the ( I X 2)-reconstructed missing row structure, and CO adsorption at saturation coverage causes migration of platinum atoms to the ( I X I )-unreconstructed structure. In an acid solution, a phase transition similar to that observed on Pt ( 110) under UHV occurred rapidly [ 8 1. However, such surface reconstruction never occurred on the CO saturated Pt( 110) electrode surfaces in a neutral if the electrode potential was conlined to be highly negative. Fig. la shows the 1RAS of CO adsorbed on the Pt ( 110 ) electrode surface in a 0.5 M Na2S04 solution as a function of the electrode potential (-0.6 to -0.4 V versus NHE). The CO coverages (&,) were estimated to be 00.50 ( ? 0.05) from the CO oxidation currents after the IRAS measurements. We observed a strong absorption band of on-top CO and a weak band of bridgebonded CO at ~2050 and ;z 1860 cm-‘, respectively. The frequencies of both species were shifted continuously to the high-frequency (or low-frequency) side with a positive (or negative) potential change. This phenomenon of the CO interaction with metal atoms indicates charge donation of the CO 50 electrons to the metal with a simultaneous charge backdonation from the metal valence states to the CO 2rc*orbital. The 27r*level of CO is antibonding in character and its occupation leads to a weakening of the carbon-oxygen bond. Therefore, this electron backdonation increases at a negative potential, the frequencies of adsorbed CO being lowered. When the electrode potential changes in the positive direction, the electron backdonation is less predominant, which means a smaller occupation of CO 2x*. The preference of bridge-bonded CO on a highly negative surface has previously been reported for CO/K/ Ni( 111) under UHV conditions [ 111. However, if the small occupation of the bridge-bonded CO (fig. la) is ignored the integrated intensity of on-top CO corresponds approximately to %,,=O.SO (CO saturation value). The metastable surface phase of CO adsorbed on Pt( 1lo)-( 1 x2) at low temperature
Volume 198, number 3,4
9 October I992
CHEMICAL PHYSICS LETTERS
(4
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-0.3%’ vs NHE
vs
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1
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under UHV conditions has been characterized by Hoffman and King [ 2 1. Adsorption of CO on a ( 1 x 2) surface precooled to 160 K was found to proceed without loss of ( 1x2) reconstruction, in contrast to the adsorption at room temperature. They found that adsorption occured predominantly at the on-top site with a small additional occupation of the
Fig. 1. (a) Electrode-potentialdependent infrared spectra for CO onPt( 110) (-0.6666 -0.4). (b) Time-resolvedIRASofC0 on Pt( 110). At t=O, the electrode potential was stepped up to -0.35 V. (c) Electrode-potential-dependent IRAS of CO on Pt(ll0) (-0.34EC0.1).
bridged site. Heating of the saturated surface to temperatures between 280 and 340 K resulted in irreversible transformation of the bridged species to ontop, with a lifting of the reconstruction of ( 1X 2 ) to ( 1x 1). Thus, the present result in a neutral solution at room temperature is comparable with that observed at low temperatures [ 21. 391
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Our previous report in in an acid solution at POSitive potentials [ 81 revealed that the maximum CO coverage attained was &ozO.95, which means that practically every CO is adsorbed on every platinum atom, by which the ( 1x 1)-unreconstructed phase was created. The present result of eco x 0.50 suggests that (i) the CO saturated Pt( 110) surface still retains the ( 1x2)-phase and that (ii) CO molecules are adsorbed on all the [ 1i 0] ridge platinum atoms and tilted away from the surface normal, which is similar to the low temperature results observed under UHV conditions. The higher frequencies observed support this view, since strong dipole-dipole coupling undergoes a positive frequency shift. The strong intensity (nearly 3%) along with the narrow band width also supports this conclusion. 3.2. Phase transition from (1 X2) to (1 X I) structure Studies at more positive potentials on Pt ( 110) are of particular interest, because one may ask in what potential range the reconstruction of the platinum surface layer occurs. Until -0.40 V the lifting due to CO adsorption did not occur even after prolonged exposure to CO. However, when the electrode potential was changed to the more positive (at -0.35 V versus NHE), the single absorption band at 2058 cm- ’changed in intensity with time, as shown in fig. 1b. The spectra represent the results at every 25 s after the potential change to - 0.3 5 V. The shoulder band started to appear at the low-frequency side five minutes later. The shoulder peak developed gradually and the bands were finally split to 2044 and 2059 cm-’ with equal intensities. The band shape and positions were not changed afterwards. The wavenumber of the peak at the high-frequency side did not change at all in this time sequence, while the intensity was reduced. When the electrode potential was further changed in the positive direction, the high-frequency band increased in intensity, while the low-frequency band decreased, with both bands shifted with the potential change (fig. 1c). Finally, at 0.1 V versus NHE a single absorption band appeared again. The band at the low-frequency side is assignable to on-top CO associated with relatively small dipole-dipole coupling, such as singleton species. This means that very small ( 1x 1) patches or isolated platinum atoms 392
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evolved on the surface. A recent STM study [ 121 has revealed that the phase transition starts by the evolution of homogeneous nucleation of small characteristic (1 x 1) patches (“nucleation and growth mechanisms”). Therefore, the phase transition from the (lx2)structureto (1x1)-Pt(llO)startsatthe potential of - 0.35 V. The equal intensities of the two absorptions indicate that the phase transition is not completed at this potential. The surface is composed of small domains of the ( 1x2) and ( 1X 1) phases. In order to complete the phase transition, a more positive potential is needed. Unreconstructed ( 1X 1) structure is completed at the positive potential of 0.1 V, where the single absorption on the ( 1x 1) surface is seen. At increased potentials, the enhanced mobility of the platinum atoms gives rise to the formation of the ( 1x 1) domains at the expense of dissolution of the smaller islands. Thus, we conclude that the phase transition never occurs at negative potentials but is accelerated at positive potentials. 3.3. Copper ad-atom migration on the Pt(ll0) electrode surface induced by electrode potential In previous sections we indicated that the migration of platinum atoms as well as CO molecules on the surface were controlled by the electrode potential. Now we report migration of copper ad-atoms underpotentially deposited on the Pt ( 110) surface. The copper electrode position was carried out in a 10m5M CuS04+0.05 M Na,SO, solution. From the current densities of the CV peaks (160 FLCcme2), t&=0.65 was obtained. Fig. 2a shows the IRAS of CO adsorbed on a copper-modified Pt ( 110) surface in a 0.05 M Na2S04 solution as a function of the electrode potential. CO was introduced at -0.6 V versus NHE, where the ( 1x 2) phase was exposed on the surface, and the potential was changed stepwise in the positive direction. We observed two kinds of CO absorptions in the on-top region at x 2 100 and ~2050 cm-‘. Potential-dependent frequency shifts were also seen here for both species. The band at 2100 cm-’ decreased gradually in intensity with a positive potential change and disappeared at 0 V. The other band initially appearing at 2015 cm-’ exhibited a very broad absorption with a tail at the low-frequency side, and the peak intensity at the high-frequency side de-
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(4
-0.3 -0.4 -0.5 -0.6
2047
0.1
2n44 0 2041 -0.1 2031
-0.2 2024
-9
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Fig. 2. Electrode potential-dependent IRAS of CO on Pt( 110) modified by underpotentialiy deposited copper atom. CO was introduced at -0.6 V (a) and -0.3 V (b).
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veloped gradually with a positive potential change. The former band around 2 100 cm- ’is assignable to CO on irregularly deposited copper atoms. CO adsorption on a copper surface has been reported by many workers under UHV conditions [ 131. CO adsorption on a high-indexed (stepped) copper surface or a polycrystalline copper surface at a low temperature reveals that that CO absorption band appears at wavenumbers higher than 2 100 cm-‘, whereas CO on an atomically smooth Cu( 111) or Cu ( 100) surface appears at a lower wavenumber, x 2085 cm-’ [ 131. In our previous study [ 141 of CO adsorption on a copper-modified Pd( 111) or Pd( 100) electrode surface in an acid solution, we attributed the band at 2115 cm- ’to on-top CO on irregularly deposited copper atoms. A similar absorption has also been reported recently on copper-modified Pt ( 100) and Pt ( 110) electrode surfaces by Weaver [ 151. On the other hand, the high-frequency component of the latter band could be assigned to the on-top CO adsorbed on the [ 1iO] ridge platinum atoms as described before. The remarkable suppression of on-top CO on the [ 1TO] ridge platinum atoms at highly negative potentials indicates that the CO sites are originally blocked by the irregularly deposited copper ad-atoms. However, the CO sites on the platinum atoms become available at more positive potentials (2040-2060 cm- ’). When CO adsorption was carried out at a less negative potential ( -0.3 V), we observed different results as shown in fig. 2b. IRAS measurements were started at -0.3 V after introduction of CO at this potential. The electrode potential was first varied in the negative direction until -0.6 V, and then swept in the positive direction as before. Two important results were noted: (i) The intensity of the band at 2 100 cm- I was very weak (the band even disappeared completely when CO was introduced at a potential higher than -0.3 V). (ii) The intensity of the low-frequency band (2014-2054 cm-‘) did not change at all, except in the region of a higher positive potential, where oxidation occurred. This is in remarkable contrast with the result for CO introduction at - 0.6 V (fig. 2a), where the low-frequency band developed gradually at a potential higher than - 0.3 V with a simultaneous decrease in the band at ~2100 cm-‘. The CO frequencies in these spectra at the same potential shown in fig. 2b are lower by 393
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‘Y10 cm-’ than those. This means that the CO coverage in the former case (fig. 2a) was larger than the latter (fig. 2b). Therefore, it is clear that the CO adsorbed on irregularly deposited copper atoms moves to the on-top CO site on the [ Ii01 ridge platinum atoms at a potential higher than -0.3 V. This suggests that aggregated copper atoms blocking the [ 1501 platinum ridge sites migrate to other sites, possibly to trough sites so that CO can occupy the ridge platinum site. Even when the 0,” values were changed in the range of 0.3 to 6.0,we were able to detect two kinds of CO absorption around 2100 and 2000 cm-’ if a highly negative electrode potential was chosen. This means that both the band at z 2 100 cm-’ and the low-frequency tail at the other absorption, z 2000 cm-’ can be assigned to CO on copper atoms. Furthermore, the former band appeared only under the highly negative electrode potentials. This is important since CO adsorption on a copper electrode surface has never been reported. We will discuss this problem in section 3.4.
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tentials (about 1 V higher than the negative potential regions covered in the present study) [ 141. Strong backdonation to CO 2x* at a negative potential strengthens the metal-carbon bond. This is the reason why adsorbed CO hardly migrates to other sites at highly negative potential conditions. Likewise, top layer platinum or underpotentially deposited copper ad-atoms are likely to stay there at a negative potential since the increased electron density between these top-layer atoms and the electrode surface can strengthen the bonds due to increased metalto-metal overlap.
Acknowledgement The present research was supported by the Japan Private School Promotion Foundation, to which the authors’ thanks are due.
References [ 1] CM. Comrie and R.M. Lambert, J. Chem. Sot. Faraday
3.4. Migration mechanism on the Pt(l10) surface Copper films deposited under UHV conditions at a low temperature take irregular polycrystalline structure, and such aggregated copper atoms migrate easily into an atomically smooth surface at room temperature. At highly negative potentials the underpotentially deposited copper atoms on the present Pt ( 110) electrode retain defect-rich structures similar to that formed in the UHV measurement at low temperatures, possibly because the surface diffusion of copper ad-atoms is suppressed completely at negative electrode potential. At relatively positive potentials, migration of the copper atoms is accelerated to form a smooth surface. Thus, the mobility of top layer atoms or molecules is easily controlled by electrode potentials. It is clear that at highly negative conditions the electron density is excessively distributed on top-layer platinum atoms, and a negatively charged surface is created. This differs intrinsically from a UHV surface, which corresponds to extremely positive po-
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Trans. 72 (1976) 1659. [2 ] P. Hoffman, S.R. Bare and D.A. King, Physica Scripta T4 (1983) 118. [3] S.R. Bare, P. Hoffman and D.A. King, Surface Sci. 144 (1984) 347. [4] E. Morallon, J.L. Vazquez, A. Aldaz and J. Clavilier, J. Electroanal. Chem. 3 16 ( I991 ) 263. [ 5 ] P. Skouda and D.M. Kolb, Surface Sci. 260 ( 1992) 229. [61X. Gao, A. Hamelin and M.J. Weaver, Phys. Rev. B 44 (1991) 10983. [ 7 ] M. Nakamura, H. Ogasawara, J. Inukai and M. Ito, Surface Sci. (1992), to be published. [ 81 Y. Kinomoto, S. Watanabe, M. Takahashi and M. Ito, Surface Sci. 242 (1991) 538. [ 91 J.E. Reutt-Robey, D.J. Doren, Y.J. Chabal and S.B. Christman, J. Chem. Phys. 93 (1990)9113. [ IO] S. Watanabe, Y. Kinomoto, M. Takahashi and M. Ito, J. Electron Spectry. 54/55 (1990) 1205. [ 1I ] K.J. Uran, L. Ng and J.T. Yates Jr., Surface Sci. 177 ( 1986) 253. 121T. Gritsch, D. Coulman, R.J. Behm and G. Ertl, Phys. Rev. Letters 10 (1989) 1086. 131J. Pritchard, T. Catterick and R.K. Gupta, Surface Sci. 53 (1975) 1. 14] K. Yoshioka, F. Kitamura, M. Takahashi and M. Ito, Surface Sci. 227 (1990) 90. IS] S.-C. Chang and M.J. Weaver, Surface Sci. 241 ( 199I) 11.