3416
J . Phys. Chem. 1992, 96, 3416-3419
I n Situ Monitoring of Electrochemically Induced Roughening with the Crystal Truncation Rod Technique G. M. Bommarito, D.Acevedo, and H. D. AbruBa* Department of Chemistry, Baker Laboratory, Cornell University, Ithaca. New York 14853-I301 (Received: September 25, 1991)
We have employed the crystal truncation rod technique to monitor (in situ) the electrochemically induced roughening of a Pt( 11 1) electrode pretreated with a chemisorbed layer of iodine. We find that for the electrode as prepared (both in air and in contact with an electrolyte solution at the rest potential) the surface is best described by one that has an rms roughness of 3.30 f 0.3 A and where the atoms that are displaced from a perfectly truncated lattice still occupy lattice positions. Reductive desorption of the iodine adlayer at -0.90 V does not alter the interfacial roughness. However, the application of a potential of +1.0 V for 15 min results in a significantly rougher interface. In this case the roughness due to displaced atoms occupying lattice positions is 3.35 f 0.34 A, and there is also a second contribution to the roughness that can be described in terms of a Debye-Waller factor and which contributes an additional roughness of 2.05 f 0.35 A.
Introduction The ubiquitous presence of solid/liquid interfaces1in areas of great fundamental and technological importance has spurred the development of techniques capable of yielding surface structural information on these systemse2 Of particular importance and value have been techniques based on X-ray photons as probes3 since their short wavelengths and weak interaction with matter (thus significant penetration) make X-rays ideal probes of the surface structure of solid/liquid interfaces. As a result, techniques such as surface extended X-ray absorption fine structure (EXAFS),"6 X-ray standing waves (XSW),9-" and surface diffractionf2*I3have been used in structural studies of solid/liquid in( I ) (a) Sparnaay, M. J. In The International Encyclopedia of Physical Chemistry and Chemical Physics; Pergamon: Glasgow, 1972; Vol. 14. (b) Bockris, J. OM.; Conway, B. E.; Yeager, E. Comprehensive Trearise of Electrochemistry; Plenum: New York, 1980; Vol. 1. (2) (a) Furtak, T. E., Kliewar, K. L., Lynch, D. W., Eds. Proceedings on the International Conference on Non-Traditional Aooroaches to the Studv of the Solid Electrolyte Interface; Sur/. Sci. 1980,'iOl. (b) See also: j . Electroanal. Chem. 1983, 150. (3) (a) Abrufia, H. D.; White, J. H.; Albarelli, M. J.; Bommarito, G. M.; Bedzyk, M. J.; McMillan, M. J. Phys. Chem. 1988,92,7045. (b) Abruiia, H. D. Adu. Chem. Phvs. 1990. 77. 255. (c) Abruiia. H. D.. Ed. Electrochemical Interfaces: Godern Techniques for In-Situ Characterization; VCH Publishers: Deerfield, FL, 1991. (4) (a) Blum, L.; Abrufia, H. D.; White, J. H.; Albarelli, M. J.; Gordon, J. G.; Borges, G.; Samant, M.; Melroy, 0. R. J . Chem. Phys. 1986,85,6732. (b) Melroy, 0. R.; Samant, M. G.;Borges, G.C.; Gordon, J. G.; Blum, L.; White, J. H.; Albarelli, M. J.; McMillan, M.; Abruiia, H. D. Longmuir 1988, 4,728. ( 5 ) White, J. H.; Albarelli, M. J.; AbruRa, H. D.; Blum, L.; Melroy, 0. R.; Samant, M.; Borges, G.; Gordon, J. G. J. Phys. Chem. 1988,92,4432. (6) Samant, M. G.;Borges, G. L.; Gordon, J. G.; Melroy, 0. R.; Blum, L. J. A m . Chem. Soc. 1987,109,5970. (7) Albarelli, M. J.; White, J. H.; Bommarito, G. M.; McMillan, M.; Abrufia, H. D. J. Electroanal.'Chem. 1988,248,77. (8) (a) Tourillon, G.;Guay, D.; Tadjeddine, A. J . Elecfroanal. Chem. 1990,289,263. (b) Tadjeddine, A.; Guay, D.; Ladoucer, M.; Tourillon, G. Phys. Rev. Lett. 1991, 66,2235. (9) Bedzyk, M. J.; Bilderback, D. H.; White, J. H.; Abruiia, H. D.; Bommarito, G.M. J. Phys. Chem. 1986,90,4926. (IO) Bedzyk, M. J.; Bilderback, D. H.; Bommarito, G. M.; Caffrey, M.; Schildkraut, J. J. Science 1988,241, 1788. ( I 1) Abrufia, H. D.; Bommarito, G. M.; Acevedo, D. A. Science 1990.250, 69. (12) (a) Samant, M.G.; Toney, M. F.; Borges, G. L.; Blum, L.; Melroy, 0.R. Surf. Sci. Lert. 1988,193, L29. (b) Samant, M. G.; Toney, M. F.; Borges, G.L.; Blum, L.; Melroy, 0. R. J . Phys. Chem. 1988,92,220. (c) Melroy, 0.R.; Toney, M. F.; Borges. G. L.; Samant, M. G.; Kortright, J. B.; ROSS,P. N.; Blum, L. Phys. Reo. E 1988,38, 10962. (d) Melroy, 0. R.; Toney, M. F.; Borges, G. L.; Samant, M. G.; Kortright, J. B.; Ross, P. N.; Blum, L. J. Elecfroanal. Chem. 1989,258,403. (13) (a) Fleischmann. M.; Graves, P.; Hill, 1.; Oliver, A.; Robinson, J. J . Electroanal. Chem. 1983,150, 33. (b) Fleischmann, M.; Oliver, A.; Robinson, J. Electrochim. Acta 1986,31. 899. (c) Fleischmann, M.; Rao, B. W. J . Elecrroanal. Chem. 1987, 229, 125. (d) Fleischmann, M.; Rao, B. W. J . Elecrroanal. Chem. 1988, 247, 297. (e) Fleischmann. M.; Rao, B. W. J . Elecfroanal. Chem. 1988,247,3 1 1,
0022-365419212096-3416$03.00/0
terfaces. Each of these techniques can be employed to provide information that is complementary to the other two. For example, EXAFS provides short-range order around an absorbing atom and can also yield information on the identity of the near neighbors. The XSW technique provides information on the distance (coherent position) and distribution (coherent fraction) of an overlayer normal to a surface. Finally, surface diffraction provides information on long-range order and great accuracy in the determination of lattice spacings and atomic positions. Within the general area of diffraction, two techniques have been shown to be particularly valuable for the determination of surface structural information, and these are the total external reflection Bragg diffraction (TERBD) and the crystal truncation rod (CTR) techniques. The TERBD technique (sometimes referred to as grazing incidence X-ray scattering) involves conventional Bragg diffraction under conditions of total external reflection. As originally described by Eisenberger and Marra,I4 the angle of incidence is kept below the critical angle for the material under study so that the X-ray beam undergoes total external reflection. This has two important consequences. First of all, since only an evanescent wave penetrates the substrate, the sampling depth is very shallow and of the order of 10-20 A. In addition, there is an enhancement in the reflected intensity by a factor that can be as large as 4. Because of the small angle of incidence, the scattering vector lies predominantly on the plane of the surface, and as a result, this technique is most sensitive to in-plane scattering. Because of this, however, this technique does not provide information on atomic correlations normal to the surface plane. This technique has been recently employed in the study of the structure of electrochemically deposited monolayers on gold and silver electrodes.I0 The CTR is based on the fact that the abrupt termination of a crystal lattice at a sharp boundary (i.e., a surface) gives rise to two-dimensional diffraction features termed crystal truncation rods which connect Bragg points in reciprocal space. This technique is very sensitive to surface and interface roughness and can be very useful in the determination of crystallographic phase information. The most important feature of CTR is the characteristic decay of the scattered intensity, which for a perfectly terminated lattice is given by (14) (a) Marra, W. C.; Eisenberger, P.; Cho, A. Y. J. Appl. Phys. 1979, 50,6927. (b) Eisenberger, P.; Marra, W. C. Phys. Rev. Lett. 1981.46,1081. (c) Marra, W. C.; Fuoss, P. H.; Eisenberger, P. E. Phys. Reo. Lett. 1982,49, 1169. (15) (a) Robinson, I. K. Phys. Reo. B 1986,33, 3830. (b) Robinson, I. K. In Handbook on Synchrotron Radiation; Moncton, D., Brown, G. S.,U s . ; North Holland: Amsterdam, 1988. (16) (a) Robinson, 1. K.; Waskiewicz, W. K.; Tung, R. T.; Bohr, J. Phys. Reo. Leu. 1986,57, 2714. (b) Mochrie, S.G. J. Phys. Reu. Lett. 1987,59, 304. (c) Gibbs, D.; Ocko, B. M.; Zehner, D. M.; Mochrie, S.G. J. Phys. Rev. B 1988,38,7303. (d) Ocko, B. M.; Mochrie, S.G. J. Phys. Reu. B 1988,38, 7378.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 3417
Roughening of a Pt( 111) Electrode 1 IF(2~h,2ak,q,)(~ = Na2Nb2 2 sin2 y2qc
(1)
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where N , and N b represent the number of unit cells sampled in the x and y directions, respectively, and q, is the scattering vector in the z direction (normal to the surface). For surfaces that are not perfectly terminated (Le,, rough) the intensity will decay faster than predicted by this expression, and this can be used as a measure of the surface or interface roughness. In essence, the net effect of surface roughness is to effectively increase the transmission of X-rays through the crystal, thus significantly reducing the reflected portion of the incident intensity. In this study we have employed the CTR technique to study (in situ) the electrochemically induced roughening of a Pt( 11 1) electrode pretreated with a chemisorbed layer of iodine.
Experimental Section Experiments were performed at the X-22C beam line of the National Synchrotron Light Source at Brookhaven National Laboratory. The incident wavelength of 1.25 8, was selected with the use of a Si(ll1) single-crystal monochromator. One-degree Soller slits were employed in the beam collimation. Incident and reflected intensities were monitored with NaI(T1) scintillators. The electrochemical cell (described below) was mounted on a six-circle Huber diffractometer driven by subroutines from the program FOUR-C. The electrochemical cell, similar to that previously described," was machined from Teflon and was provided with connections for the addition/withdrawal of electrolyte solution (via Teflon syringes) as well as for electrical connection to the electrodes. The working electrode was a 1-cm-diameter X 2mm-thick Pt( 111) single-crystal disk grown from the melt at the Materials Preparation Facility of Cornel1 University. The electrode was polished to a mirror finish on both faces. The rocking curve width of this electrode was measured to be 0.2". A platinum wire and a Ag/AgCl electrode served as auxiliary and reference electrodes, respectively. The reference electrode was isolated from the cell through a porous Vycor plug. The cell window consisted of 6-pm-thick Mylar film held in place through the use of a Viton O-ring and an aluminum flange that was secured to the body,of the cell by six screws. All solutions were prepared with pyrolytically distilled water (PDW) and ultrapure reagents. Prior to use, solutions were degassed with high-purity helium (which was also passed through an oxysorb cartridge) for at least 30 min. Electrolyte solutions consisted of pH 7.0 phosphate buffer with 0.1 M sodium sulfate. Electrochemical experiments were carried out with a Princeton Applied Research Model 173 potentiostat, Model 175 universal programmer, and Model 179 digital coulometer. Electrochemical data were recorded on a Soltec X-Y recorder. Electrode Pretreatment. Initially, the Pt single-crystal electrode was electroplished in hot (80 "C)concentrated HCl for 1-4 h at a current density of 10 pA/cm*. The electrode was rinsed with PDW and immersed in hot (80 "C) concentrated nitric acid for 1 h (which generated a protective oxide layer) followed by rinsing with PDW. Afterward, the electrode was placed inside a quartz tube furnace and heated to 700 "C for 10 min in the presence of iodine vapor, which was carried through the furnace by prepurified (oxygen-free) nitrogen. The electrode was allowed to cool in this flowing iodine vapor to about 150 "C and was then reheated to 430 "C for 10 min with only nitrogen (Le., no iodine vapor) flowing through. After the crystal was allowed to cool to room temperature in the flowing nitrogen, it was placed in the cell where electrical connection was made through a thin piece of Pt wire. This pretreatment procedure is a modification of that reported by Wieckowski and Hubbardigwhich was shown to produce ordered adlattices of iodine on Pt( 111). The electrochemical response of such a pretreated electrode was featureless over the entire double-layer region. ( 1 7 ) White, J . H.;AbruTta, H. D. J . Phys. Chem. 1988, 92, 7131. (18) Wieckowski, A.; Rosasco, S. D.; Schardt, B. C.; Stickney. J. L.; Hubbard, A. T.Inorg. Chem. 1984, 23, 5 6 5 .
1'
I
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Perpendicular Momentum Transfer, Q I (reciprocal lattice units) Figure 1. (A) Intensity vs perpendicular momentum transfer for (I 11) CTR for a Pt( 111) electrode surface under the following conditions: (0) with an iodine adlayer and in contact with air (Le., ex situ); (m) with an iodine adlayer, in contact with an electrolyte solution and at the rest potential; (A)same as (m) except after electroreductivestripping of the iodine adlayer; ( 0 )same as (m) except after holding the potential at +1.0 V for 15 min. (e) Depiction of part of the reciprocal space map for an fcc crystal. (C) Transverse (6)scan employed in subtraction of thermal diffuse scattering.
Sample Alignment. Initially, the sample height was adjusted optically by the use of a theodolite. Two bulk reflections (e&, (1 11) and (202)) were employed in order to align the crystal and set the orientational vector. Afterward, scans on surface reflections (e.g., (1 10) and (220)) as well as on C T R s were obtained. Scans along the (377) and (71 1) CTR's were obtained. (See Figure 1B for a depiction of the reciprocal lattice map.) At all points along the rod, transverse scans were made in order to subtract out the background scatter which was predominantly due to thermal diffuse scattering (TDS). One such representative scan is shown in Figure 1C. Results and Discussion Figure 1B presents part of the reciprocal lattice map for an fcc crystal. We will focus on scans across the (11 1) CTR, data for which are presented in Figure 1A. Data are presented for four experimental conditions: Pt( 11 1) with an iodine adlayer (Pt(1 1l)/I) examined ex situ, that is in contact with air; Pt( 1l l ) / I in situ (Le., in contact with electrolyte solution) and at the rest potential; Pt( 111) in situ and at the rest potential after electroreductive desorption of the iodine adlayer at -1 .O V; Pt( 111) in situ at the rest potential after applying a potential of + 1.O V for 15 min. As can be ascertained from Figure lA, the profiles for the electrode examined in situ and ex situ are virtually superimposable. (The small shifts could be due to the fact that the electrode was annealed in iodine vapor, as described in the Experimental Section, between these two runs.) In addition, electroreductive stripping of the iodine adlayer resulted in very small changes. These data indicate that the surface remained virtually unchanged in the three cases.
3418 The Journal of Physical Chemistry, Vol. 96, No. 8,1992 B
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Figure 2. Depiction of roughening of an electrochemical interface. (A) Ideal case where all interfaces are perfectly terminated as depicted by the straight lines representing the various interfaces (bulk/surface; sur-
These results are to be contrasted with the profile obtained after holding the potential at +1.0 V for 15 min. In this case, there are very significant variations, especially in the trailing part of the profile. Qualitatively, this indicates that the oxidative electrochemical treatment results in the roughening of the surface since the intensity drops off much more rapidly in this case than in the previous three. In order to get a more quantitative estimate of the interfacial roughness, we have tried to fit the experimental profiles before and after electrochemical roughening to two models which describe two different types of roughness and which are depicted in Figure 2. In principle, an ideally truncated lattice (Figure 2A) can be made rough by two general mechanisms. In the first, the displaced surface atoms are moved so as to still occupy lattice positions (Figure 2B). In the second case, the displaced atoms are randomly distributed so as to make a largely amorphous layer (Figure 2C). We have employed these two models to fit the experimental profiles. In the first case, we begin by considering an ideal density profie (Figure 2D). The interface is then made rough by changing the perfectly sharp density profile to one described by the error function (Figure 2E) as shown in eq 2. From the shape of the = '4 eXP(-kQ,m,2)
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1.2 2.0 Perpendicular Momentum Transfer, k (Reciprocal Lattice Units) Figure 4. Experimental data (0,annealed electrode; 0, after electrochemical oxidation) and fits employing a Debye-Waller model of roughness where the amplitudes were (a) 0 (ideally truncated lattice), (b) 1, (c) 3, and (d) 5 A. 0.4
annealed electrode (and for the case of the electrode in contact with electrolyte solution at the rest potential as well as after electroreductive desorption of iodine) the interface can be best described by one where the roughness is largely due to displaced surface atoms that still occupy lattice positions as depicted in Figure 2B. In the case of randomly distributed surface atoms, we assume a model analogous to a Debye-Waller factor (e-2M)where M =
(2)
profile one can extract an rms value that describes the roughness directly. This model is conceptually similar to one based on fractional occupancy of surface layers which has been previously employed by Robinson13 in fitting CTR profiles. We prefer the model based on variations in the density profile as a more physically realistic one. The fits and experimental profiles for this model are presented in Figure 3. In here, curve a represents the profile for an ideally truncated surface whereas curves b, c, and d are for rms roughness values of 1.5,3.3, and 225 A, respectively. We note that the profile for the annealed electrode is reasonably well fit for an rms value of 3.3 f 0.3 A, whereas for the electrochemically oxidized surface there were gross discrepancies at all rms values attempted in the fits and of particular note was the fact that the downward curvature could not be reproduced regardless of the magnitude of the rms roughness (e.g., 225 A). This suggests that for the
,
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Perpendicular Momentum Transfer, k (Reciprocal Lattice Units) Figure 3. Experimental data ( 0 ,annealed electrode; 0, after electrochemical Oxidation) and fits employing a roughening model described by a density profile following the error function. Rms roughness values were (a) 0.0 (ideally truncated lattice), (b) 1.5, (c) 3.3, and (d) 225 A.
face/solution). The hatched lines depict the perfect periodicity of the lattice from the bulk to the surface. (B) Roughened surface where the displaced surface atoms still occupy lattice positions as depicted by the continuous hatched lines. (C) Roughened surface where the displaced surface atoms (depicted by the dotted area) are randomly displaced so that they no longer occupy lattice positions. (D) Density profile for an ideally terminated surface. (E) Density profile for a roughened surface.
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and where 6k is (k-keraeg)(as mentioned before, k is the perpendicular momentum transfer) and ( us2)represents the mean square displacement.t9 Figure 4 presents the experimental profiles as well as fits employing such a model. Curve a represents the profile for a perfectly terminated lattice, and again we notice that the data for both experimental conditions are below this with the one corresponding to the electrochemically oxidized surface beiig the lowest. Curves b, c, and d represent fits using displacements of 1, 3, and 5 A. There were significant discrepancies in the tits to the data for the annealed surface at all values, implying that, for this case, the contribution to the roughness due to randomly displaced atoms is essentially insignificant. For the electrochemically oxidized ~~
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(19) Warren, B. E. X-Ray Diffraction; Addison-Wesley: Reading, MA, 1969; Chapter 1 I .
3419
J . Phys. Chem. 1992, 96, 3419-3423 Pt( 1 1 1)
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Perpendicular Momentum Transfer, k (Reciprocal Lattice Units) Figure 5. ldeal CTR profile and fits to experimental data (a,annealed electrode; 0,after electrochemical oxidation) employing contributions from both types of roughening. surface, there were also some discrepancies. However, it is also clear that the fit was greatly improved over that for the model where the only contribution to the roughness arme from displaced atoms still occupying lattice positions. Especially notable was the fact that, in this case, the downward curvature in the data was reproduced (Figure 4c). Finally, we attempted fits to the data using a weighted combination of both models of roughness, and the rsults are presented in Figure 5 . For the annealed surface the roughness can be described as being due entirely to displaced atoms occupying lattice positions. From the fits to the data, we obtain a value for the roughness of 3.3 f 0.3 A. On the other hand, for the electrochemically oxidized electrode we find significant contributions from both types of roughness. From the fits to the data we obtain values of 3.35 f 0.34 and 2.05 f 0.35 A for roughness due to displaced atoms occupying lattice positions and randomly displaced atoms, respectively. These measurements suggest that although the surface, as prepared, has some roughness, it is essentially due to displaced
atoms occupying lattice positions so that the surface is relatively well ordered. Exposure of the electrode to an electrolyte solution and the application of the rest potential did not affect the interfacial structure. Similarly, the reductive desorption of the iodine adlayer (at -1.0 V) did not alter the surface structure. On the other hand, the effects of applying a potential of t 1.O V for 15 min were dramatic. The surface becomes significantly rougher, and in addition, there is a significant degree of roughness due to randomly displaced atoms. It is interesting to note, however, that the roughness due to displaced atoms occupying lattice positions was not significantly changed. This suggests that as the outermost layers of the surface are amorphized by the electrochemical treatment, atoms from the region proximal to the surface are displaced so as to maintain asentially the same degree of fractional occupancy in these layers. This might suggest a roughening mechanism where as the outermost layers amorphize, the subsequent layers largely retain the structural features of the original surface. Thus, the net effect of the electrochemical roughening is to, in essence, cover the surface with a largely amorphous layer of Pt atoms. We are continuing our studia on the use of the CTR technique to study electrochemically induced roughening in the presence of other strongly chemisorbed species.
Conclusions We have demonstrated the applicability of the CTR technique to study electrochemically induced roughening of an iodine-pretreated Pt( 11 1) electrode. We find that the annealed surface is best described as one where surface atoms have been displaced but still occupy lattice positions whereas electrooxidative roughening gives rise to a largely amorphous surface layer. Acknowledgment. This work was carried out in collaboration with Dr. B. Ocko at Brookhaven National Laboratory whose help is greatly appreciated. Our work was generously funded by the Office of Naval Research, the Army Research Office, and the National Science Foundation. H.D.A. acknowledges support by the Presidential Young Investigator program at N S F and the A. P. Sloan Foundation. D.A. acknowledges support by the MARC Fellowship Program a t NIH. NSLS is supported by the Department of Energy.
The Local Structure of B12,2Srl,gCul, Fe, 0, Single Crystals Determined by Scanning Tunneling Microscopy Chunming Niu and Charles M. Lieber* Department of Chemistry and Division of Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 (Received: October 7, 1991; In Final Form: November 26, 1991) Scanning tunneling microscopy (STM) has been used to characterize the surface structure of cleaved single crystals of Bi2,2Srl,8CuOy and Bi, Sr,&u,#~.,O,. Images of Bi2,2Srl,8Cu0,exhibit a one-dimensionalsuperstructure with a modulation The atomic structure in these images is ordered and shows no evidence of local distortions due to period of 25.1 f 0.3 Sr2+substitution and/or extra oxygen in the BiO layer. Substitution of Fe in the CuO layer of this material caused substantial changes in the superstructure and the BiO layer atomic structure. The superstructure period in Bi2,2Srl,8Cu,,,F%,,OYdetermined from the analysis of STM images and two-dimensional Fourier transform power spectra, 22.8 f 1.O was smaller and less regular than that observed for Bi2,2Sr1,8C~Oy samples. In addition, the atomic structure of the Fe-substituted material exhibited significant disorder. These results are discussed in terms of lattice mismatch and bonding between the BiO and the Cu(Fe)O layers of this material.
1.
1,
Introduction The bismuth-based copper oxide superconductors, B i 2 S r 2 C a ~ l C ~ n 0 2(n n +=4 1-3), comprise a structurally and chemically rich class of materials that exhibit a wide range of superconducting properties (Figure One Unique feature Of Author to whom correspondence should be
addressed.
these bismuth-based materials that distinguishes them from other copper oxide superconductors is a strong, one-dimensional ( 1 ) Michel, C.; Hervieu, M.; Borel, M. M.; Crandin, A.; Deslandes, F.; Provost, J.; Raveau, B. Z. Phys. 1%7, B68, 421. (2) Maeda, H.; Tanaka, Y.; Fukutomi, M.; Asano, T. Jpn. J. Appl. Phys. 1988, 27, L209.
0022-3654/92/2096-3419$03.00/00 1992 American Chemical Society
