- Email: [email protected]
Surface exafs of an electrochemical interface
311
J. Electrounul. Chem., 210 (1986) 311-314 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Pdiminary note SURFACE EXAFS OF AN ELECTROCHEMICAL INTERFACE IODINE ON PLATINUM (111)
J.G. GORDON II *, O.R. MELROY, IBM Research Labvrato~, H.D. ABRtiA Department
G.L. EORGBS and D.L. REISNER
Saa Jose, CA 95193 {U.S.A.)
* and P. ~HAND~SEKHAR
of Chemistry, Cornell University, rthucq
NY 14853 (USA)
L. BLUM * Department
vf Physics, P.O. Box AT. Fad@
Rio Piedras, PR
00931
of Nutwd
Sciences, University of Puerto Rico,
(U.S.A)
(Received 14th July 1986)
The structure of the solid/solution interface is a problem of fundamental importance in electrochemistry and other interfacial disciplines, and one which has by-and-large eluded direct experimental study [l]. This is due primarily to the lack of structure sensitive techniques that work in condensed phases. The techniques (e.g. LEED) which have proved so valuable for solid/vacuum interfaces can only be applied ex-situ and transfer of the electrode into UHV raises the very serious question of whether the surface examined has the same structure it did when immersed in solution [2-41. In principle, X-ray based techniques will work in solution, since X-rays are penetrating radiation, but laboratory X-ray sources have insufficient intensity. The advent of synchrotron radiation sources has dramatically changed the situation [S). We report here the observation of the EXAFS spectrum of a monolayer of iodine adsorbed on a platinum single crystal (111) in contact with aqueous electrolyte. Recent experiments have demonstrated the utility of surface EXAFS to obtain information on the structure of monolayers of adsorbed atoms [6]. Most of these studies have relied on electron detection techniques, unsuitable for liquids. Some absorption measurements have been made on thin films [7] but fluorescence is easier and more sensitive. Indeed, Kordesch and Hoffman [8] have monitored fluorescence to obtain the EXAFS spectra of thin passive films on iron immersed in electrolyte
l
To whom wrtespondence should be addressed.
0022-0728/86/$03.50
Q 1986 Elsevier Sequoia S.A.
312
and Heald et al. [9,10] have recently demonstrated, in experiments carried out in air, that at grazing incidence, the technique should be capable of monolayer sensitivity. This was verified by Lairson et al. who used grazing incidence and fluorescence detection to obtain the EXAFS spectrum of bromine adsorbed in Ni (001) in UHV [llJ. Actually, calculations indicate that even at low angles of incidence, the fluorescence signal from a monolayer should be easily detectable. However, Compton and elastic scattering from the substrate are severe interferences. In our initial experiments with X-rays incident at 45”, even with thin electrode films (200 nm) on thin low-z substrates, mica or silicon, scattered radiation obscured the signal from the adsorbate [12]. Now, at X-ray wavelengths the index of refraction of materials is slightly less than 1, so that at a sufficiently high angle (typically > 89”) grazing incidence, total external reflection will occur [lo]. Under this condition, (1) X-rays penetrate only a short distance (2-4 nm) below the surface, greatly reducing the scattered radiation, and (2) the intensity of radiation at the surface is increased, by as much as a factor of four, leading to a simultaneous decrease in background and increase in signal. In preliminary experiments where we measured the iodine fluorescence from an iodine dosed platinum film deposited on a silicon wafer, we observed an order of magnitude increase in signal to noise between 45” and grazing incidence (approx. 89”). Also, the signal from the film substrate was comparable to that obtained from a solid platinum substrate, consistent with the expected small penetration depth. The experimental arrangement was as follows. The sample was a platinum (111) single crystal (10 x 10 X 2 mm) mounted on the end of a lexane holder suspended inside a rectangular lexan cell with mylar* windows which could be filled with electrolyte. The holder was attached to a rotating stage which permitted 360° rotation of the sample with about 2O setting accuracy and finer, about 0.02”, control over about a 5* range. The detector was an energy dispersive high purity germanium detector (EG&G ORTEC) fitted with Soller slits and antimony filters to isolate the characteristic iodine K-edge X-ray fluorescence line at 28.6 keV. The platinum crystal was grown from the melt at the materials preparation facility of Cornell University’s Materials Science Center and after cutting and polishing, was cleaned by argon sputtering, annealed and exposed to iodine vapor while still in the vacuum system. This procedure produces an ordered, strongly chemisorbed iodine monolayer which is relatively insensitive to subsequent contamination ]13]. Data were collected at CHESS using their XOPLUS data collection program and initial analysis was performed at CHESS using Brian K&aid’s EXAFS analysis programs. A well defined iodine K-edge was observed at 33.16 keV from the iodine dosed platinum crystal suspended in nitrogen. Accumulating several spectra yielded analyzable SEXAFS, the Fourier transform of which had a single prominent peak corresponding to a Pt-I distance, after correction for phase shift, of 0.263 mu. (We can assign this confidently to Pt-I and not I-I because the X-ray beam was polarized in the plane of incidence putting the I-I axis perpendicular to the X-ray &field. Thus I-I scattering should not contribute to the observed EXAFS.) Reported Pt-I distances fall between 0.26 and 0.30 nm.
313
% lwJ4$3;r
6.6961-
7 2430 s : 9 55696 -
3.9367-
2.2836 i.; 32.9146 33.1255
33.3353 33.5472
33.7580 33.SSSS
E/k&
Fig. 1. EXAFS spectrum (iodine K-edge) for iodine adsorbed on a Pt (111) single crystal electrode which was in contact with a 0.1 M aqueous KCI solution and at an applied potential of 0.0 V vs. A&A&l.
The electrochemically controlled experiments were done by immersing the crystal in 0.1 M NaClO, and then partially withdrawing it so that it was still in in contact with the electrolyte and the X-ray beam passed through the meniscus. (The electrolyte film must be thin to minimize attiuation and scattering.) The signal amplitude decreased by lo-196 and the signal to noise by a comparable amount. A well defined K-edge was observed as shown in Fig. 1.
"0 124434 F
7.7566 a e %
5.4163
cl.7315 0.0000
2.6626
4.6639
6.0059
6.0678
10.66SS
lORdl~nu/"~
Fig. 2. Fourier Transform of EXAFS spectrum from Fig. 1.
314
Analysis of the spectrum produced results in good agreement with the ex situ measurements. The Fourier transform of the EXAFS spectrum, after background subtraction and multiplication by k2, is shown in Fig. 2. It has a single prominent peak for the nearest neighbor shell, which yields a Pt-I distance of 0.264 nm. Although the present example represents a case of irreversible adsorption, it demonstrates that it is possible to obtain in situ EXAFS spectra of mono-atomic layers at a metal/liquid interface. We are now pursuing studies of reversibly adsorbed systems and we are confident that the expected evolutionary improvements in technique will make this a very powerful in situ structural probe for the study of solid/liquid interfaces. ACKNOWLEDGEMENTS
J.G.G., O.R.M., G.L.B., D.L.R. and L.B. acknowledge support by the Office of Naval Research. H.D.A., P.C. and M.A. acknowledge support by the Materials Science Center at Cornell University. We are very grateful to Dr. Brian M. K&aid (AT&T Bell Labs) for his advice and for providing us with a copy of his EXAFS data analysis program as well as to the staff and operators at CHESS for their assistance. REFERENCES 1 M.J. Sparnaay, The Electrical Double Layer, Tbe International Encyclopedia of Physical Chemistry and Chemical Physics, Vol. 14, Peqmnon Press, Glasgow, 1972. 2 A.T. Hubbard, Act. Chem. Res., 13 (1980) 177. 3 AS. Homa, E Yeager and B.D. C&m, J. Electroanal. Chem., 125 (1981) 237. 4 F.T. Wagner and P.N. Ross, J. Electroanai. Chem., 150 (1983) 141. 5 H. Winick and S. Don&h (&is.), Synchrotron Radiation Research, Plenum Press, New York, 1980. 6 P. Eisenberger and B.M. Kin&d, Science, 200 (1978) 1441; A. Bianconi, Appt. Surf. Sci.. 6 (1980) 392; P.H. Citrin, P. Eisenberger and R.C. Hewitt, Phys. Rev. Lett., 45 (1980) 1948; P.H. Citrin. P. EisenbetBer and J.E. Rowe, Phys. Rev. Lett., 48 (1982) 802; P.H. Citrin and J.E. Rowe, Surf. Sci., 132 (1983) 205. 7 L. Bosio, R. Cortes, A. Refrain, M. Froment and A.M. Lebrun, J. Electroanal. Chem., 180 (1984) 265. 8 ME. Kordesh and R.W. Hoffman, Nucl. Instrum. Meth. Phys. Res., 222 (1984) 347. 9 S.M. Heald, E. Keller and E.A. Stem, Phys. Lett., 103A (1984) 155. 10 E.A. Stem and S.M. Heald. Rev. Sci. Instrum., 50 (1979) 1579. 11 B. Lairson, T.N. Rhodin and W. Ho, Solid State Commun., 55 (1985) 925. 12 J.G. Gordon and L. Blum, SSRL Report 83/01. Project 717M (1982); D.E. Reisner, O.R. Melroy, J.G. Gordon II, D.A. Buttry, G.L. Barges and L. Blum, SSRL Report 85/01, 52 (1985). 13 T.E. Felter and A.T. Hubbard, J. Eiectroanal. Chem., 100 (1979) 473.
J. Electrounul. Chem., 210 (1986) 311-314 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Pdiminary note SURFACE EXAFS OF AN ELECTROCHEMICAL INTERFACE IODINE ON PLATINUM (111)
J.G. GORDON II *, O.R. MELROY, IBM Research Labvrato~, H.D. ABRtiA Department
G.L. EORGBS and D.L. REISNER
Saa Jose, CA 95193 {U.S.A.)
* and P. ~HAND~SEKHAR
of Chemistry, Cornell University, rthucq
NY 14853 (USA)
L. BLUM * Department
vf Physics, P.O. Box AT. Fad@
Rio Piedras, PR
00931
of Nutwd
Sciences, University of Puerto Rico,
(U.S.A)
(Received 14th July 1986)
The structure of the solid/solution interface is a problem of fundamental importance in electrochemistry and other interfacial disciplines, and one which has by-and-large eluded direct experimental study [l]. This is due primarily to the lack of structure sensitive techniques that work in condensed phases. The techniques (e.g. LEED) which have proved so valuable for solid/vacuum interfaces can only be applied ex-situ and transfer of the electrode into UHV raises the very serious question of whether the surface examined has the same structure it did when immersed in solution [2-41. In principle, X-ray based techniques will work in solution, since X-rays are penetrating radiation, but laboratory X-ray sources have insufficient intensity. The advent of synchrotron radiation sources has dramatically changed the situation [S). We report here the observation of the EXAFS spectrum of a monolayer of iodine adsorbed on a platinum single crystal (111) in contact with aqueous electrolyte. Recent experiments have demonstrated the utility of surface EXAFS to obtain information on the structure of monolayers of adsorbed atoms [6]. Most of these studies have relied on electron detection techniques, unsuitable for liquids. Some absorption measurements have been made on thin films [7] but fluorescence is easier and more sensitive. Indeed, Kordesch and Hoffman [8] have monitored fluorescence to obtain the EXAFS spectra of thin passive films on iron immersed in electrolyte
l
To whom wrtespondence should be addressed.
0022-0728/86/$03.50
Q 1986 Elsevier Sequoia S.A.
312
and Heald et al. [9,10] have recently demonstrated, in experiments carried out in air, that at grazing incidence, the technique should be capable of monolayer sensitivity. This was verified by Lairson et al. who used grazing incidence and fluorescence detection to obtain the EXAFS spectrum of bromine adsorbed in Ni (001) in UHV [llJ. Actually, calculations indicate that even at low angles of incidence, the fluorescence signal from a monolayer should be easily detectable. However, Compton and elastic scattering from the substrate are severe interferences. In our initial experiments with X-rays incident at 45”, even with thin electrode films (200 nm) on thin low-z substrates, mica or silicon, scattered radiation obscured the signal from the adsorbate [12]. Now, at X-ray wavelengths the index of refraction of materials is slightly less than 1, so that at a sufficiently high angle (typically > 89”) grazing incidence, total external reflection will occur [lo]. Under this condition, (1) X-rays penetrate only a short distance (2-4 nm) below the surface, greatly reducing the scattered radiation, and (2) the intensity of radiation at the surface is increased, by as much as a factor of four, leading to a simultaneous decrease in background and increase in signal. In preliminary experiments where we measured the iodine fluorescence from an iodine dosed platinum film deposited on a silicon wafer, we observed an order of magnitude increase in signal to noise between 45” and grazing incidence (approx. 89”). Also, the signal from the film substrate was comparable to that obtained from a solid platinum substrate, consistent with the expected small penetration depth. The experimental arrangement was as follows. The sample was a platinum (111) single crystal (10 x 10 X 2 mm) mounted on the end of a lexane holder suspended inside a rectangular lexan cell with mylar* windows which could be filled with electrolyte. The holder was attached to a rotating stage which permitted 360° rotation of the sample with about 2O setting accuracy and finer, about 0.02”, control over about a 5* range. The detector was an energy dispersive high purity germanium detector (EG&G ORTEC) fitted with Soller slits and antimony filters to isolate the characteristic iodine K-edge X-ray fluorescence line at 28.6 keV. The platinum crystal was grown from the melt at the materials preparation facility of Cornell University’s Materials Science Center and after cutting and polishing, was cleaned by argon sputtering, annealed and exposed to iodine vapor while still in the vacuum system. This procedure produces an ordered, strongly chemisorbed iodine monolayer which is relatively insensitive to subsequent contamination ]13]. Data were collected at CHESS using their XOPLUS data collection program and initial analysis was performed at CHESS using Brian K&aid’s EXAFS analysis programs. A well defined iodine K-edge was observed at 33.16 keV from the iodine dosed platinum crystal suspended in nitrogen. Accumulating several spectra yielded analyzable SEXAFS, the Fourier transform of which had a single prominent peak corresponding to a Pt-I distance, after correction for phase shift, of 0.263 mu. (We can assign this confidently to Pt-I and not I-I because the X-ray beam was polarized in the plane of incidence putting the I-I axis perpendicular to the X-ray &field. Thus I-I scattering should not contribute to the observed EXAFS.) Reported Pt-I distances fall between 0.26 and 0.30 nm.
313
% lwJ4$3;r
6.6961-
7 2430 s : 9 55696 -
3.9367-
2.2836 i.; 32.9146 33.1255
33.3353 33.5472
33.7580 33.SSSS
E/k&
Fig. 1. EXAFS spectrum (iodine K-edge) for iodine adsorbed on a Pt (111) single crystal electrode which was in contact with a 0.1 M aqueous KCI solution and at an applied potential of 0.0 V vs. A&A&l.
The electrochemically controlled experiments were done by immersing the crystal in 0.1 M NaClO, and then partially withdrawing it so that it was still in in contact with the electrolyte and the X-ray beam passed through the meniscus. (The electrolyte film must be thin to minimize attiuation and scattering.) The signal amplitude decreased by lo-196 and the signal to noise by a comparable amount. A well defined K-edge was observed as shown in Fig. 1.
"0 124434 F
7.7566 a e %
5.4163
cl.7315 0.0000
2.6626
4.6639
6.0059
6.0678
10.66SS
lORdl~nu/"~
Fig. 2. Fourier Transform of EXAFS spectrum from Fig. 1.
314
Analysis of the spectrum produced results in good agreement with the ex situ measurements. The Fourier transform of the EXAFS spectrum, after background subtraction and multiplication by k2, is shown in Fig. 2. It has a single prominent peak for the nearest neighbor shell, which yields a Pt-I distance of 0.264 nm. Although the present example represents a case of irreversible adsorption, it demonstrates that it is possible to obtain in situ EXAFS spectra of mono-atomic layers at a metal/liquid interface. We are now pursuing studies of reversibly adsorbed systems and we are confident that the expected evolutionary improvements in technique will make this a very powerful in situ structural probe for the study of solid/liquid interfaces. ACKNOWLEDGEMENTS
J.G.G., O.R.M., G.L.B., D.L.R. and L.B. acknowledge support by the Office of Naval Research. H.D.A., P.C. and M.A. acknowledge support by the Materials Science Center at Cornell University. We are very grateful to Dr. Brian M. K&aid (AT&T Bell Labs) for his advice and for providing us with a copy of his EXAFS data analysis program as well as to the staff and operators at CHESS for their assistance. REFERENCES 1 M.J. Sparnaay, The Electrical Double Layer, Tbe International Encyclopedia of Physical Chemistry and Chemical Physics, Vol. 14, Peqmnon Press, Glasgow, 1972. 2 A.T. Hubbard, Act. Chem. Res., 13 (1980) 177. 3 AS. Homa, E Yeager and B.D. C&m, J. Electroanal. Chem., 125 (1981) 237. 4 F.T. Wagner and P.N. Ross, J. Electroanai. Chem., 150 (1983) 141. 5 H. Winick and S. Don&h (&is.), Synchrotron Radiation Research, Plenum Press, New York, 1980. 6 P. Eisenberger and B.M. Kin&d, Science, 200 (1978) 1441; A. Bianconi, Appt. Surf. Sci.. 6 (1980) 392; P.H. Citrin, P. Eisenberger and R.C. Hewitt, Phys. Rev. Lett., 45 (1980) 1948; P.H. Citrin. P. EisenbetBer and J.E. Rowe, Phys. Rev. Lett., 48 (1982) 802; P.H. Citrin and J.E. Rowe, Surf. Sci., 132 (1983) 205. 7 L. Bosio, R. Cortes, A. Refrain, M. Froment and A.M. Lebrun, J. Electroanal. Chem., 180 (1984) 265. 8 ME. Kordesh and R.W. Hoffman, Nucl. Instrum. Meth. Phys. Res., 222 (1984) 347. 9 S.M. Heald, E. Keller and E.A. Stem, Phys. Lett., 103A (1984) 155. 10 E.A. Stem and S.M. Heald. Rev. Sci. Instrum., 50 (1979) 1579. 11 B. Lairson, T.N. Rhodin and W. Ho, Solid State Commun., 55 (1985) 925. 12 J.G. Gordon and L. Blum, SSRL Report 83/01. Project 717M (1982); D.E. Reisner, O.R. Melroy, J.G. Gordon II, D.A. Buttry, G.L. Barges and L. Blum, SSRL Report 85/01, 52 (1985). 13 T.E. Felter and A.T. Hubbard, J. Eiectroanal. Chem., 100 (1979) 473.