4428
Langmuir 1995,11,4428-4432
Structures of Iodine on Pt(ll0) Single Crystal Electrode Surfaces '
W. L. DeSimone and J. J. Breen*
Department of Chemistry, Indiana University Purdue University-Indianapolis, 402 North Blackford Street, Indianapolis, Indiana 46202-3274 Received January 27, 1995@ Gas phase dosing of flame-annealed R(110)single crystal electrodes with an iodine/argon vapor results in the formation of ordered iodine adlayer structures on an unreconstructed (110)surface. Electrochemical STM images obtained in 1M HC104 electrolyte reveal a series of adlayer structures approaching a Pt(110)-(2 x 11-1 adlattice structure with increasing exposure to the iodine vapor as corroborated by coulometric measurements following the oxidation of surface bound iodine adatoms. Also evident in the STM images is a surface reconstruction consisting of raised rows of closely packed Pt atoms oriented along the [OOlI direction. These monatomic high raised row features are distributed over the electrode surface. Initial investigations show that the extent of this surface reconstruction is independentof iodine exposure and the surface structure exhibits no observable potential dependent structural dynamics for a limited potential range investigated around 200 mV vs AdAgCl. Finally, Ag underpotential deposition voltammograms reveal a significant degree of overall surface order and a multistep process leading to the deposition of either two successive adlayers or possibly a single high-density Ag adlayer prior to the onset of bulk Ag deposition.
Introduction Many of the basic objectives of electrochemical surface science reflect a n interest in the structural arrangement of atoms and molecules present at the interface between a solid electrode and an electrolyte solution.lS2 While important for fundamental studies of heterogeneous electron transfer, the atomic scale structure of an electrode surface can strongly influence technologically important processes such as the initial stages of metal deposition, the adsorption of neutral and charged species, and electrochemical catalysis. In this paper we report on the structures of iodine atoms adsorbed on a Pt(ll0) single crystal electrode surface. Electrochemical and surface structural studies of iodine modified single crystal electrodes have been prominent in the literature for a number of year^.^-^ This can be attributed to the strong adsorption of iodide ions to electrode su'rfaces such as those of Pt and Au making these surface systems genkrally available and suitable for study under a wide range of conditions. With the advent of the STM and other new in situ surface structure probes, many of these studies have been directed toward understanding the potential dependent adsorption and potential dependent structural transformations in iodine adlayers. The in situ studies of iodine adlayers on Au(lll), -(loo), and -(110) electrodes are especially notable and are outlined in a recent review and other r e p o r t ~ . l , ~For - ~ Pt electrodes, iodine adlayer structures have been investigated using the scanning probe techniques mainly on the Abstract published inAdvanceACSAbstracts, October 1,1995. (1)Weaver, M.J.; Gao, X. Annu. Rev. Phys. Chem. 1993,44,459. (2)Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, 0.;Murray, R.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chkm. 1993,97,7147. ( 3 )Wieckowski, A.; Rosasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem. 1984,23,565. (4)Hubbard, A. T.Acc. Chem. Res. 1980,13,177. (5)Hubbard,A. T.Chem. Rev. 1988,88,633. (6)Wieckowski, A.; Schardt, B. C.; Rosasco, S. D.; Stickney, J. L.; Hubbard, A. T. Surf Sci. 1984,146,115. (7)Gao, X.;Weaver, M. J. J. Phys. Chem. 1993,97,8685. (8)Gao, X.;Edens, G. J.; Weaver, M. J. J. Phys. Chem. 1994,98, 8074. (9)Gao, X.;Weaver, M. J. Ber. Bunsenges. Phys. Chem. 1993,97, 507. @
(110) (1 x 2) Figure 1. Ball model depicting the unreconstructed (1 x 1) and reconstructed (2 x 1)Pt(ll0)surfaces.The light gray circles represent the topmost layer of Pt atoms,the medium gray circles represent the layer of Pt atoms immediatelybelow the surface, and the black circles the next layer of Pt atoms below.
(110) (1x 1)
(111)l0-l2 and the (100)13-15crystal faces while the (110)14J6 crystal face has not been investigated as thoroughly. The unreconstructed Pt(ll0) surface which is schematically illustrated in Figure 1can simply be described as parallel rows of closely packed Pt atoms (0.278 nm spacing) separated by 0.392 nm in the [OOl] direction. Low-energy electron diffraction (LEED) studies under ultrahigh-vacuum (UHV)conditions reveal that clean Pt(110) surfaces prepared under UHV exhibit a missing row type reconstruction in which a row of platinum atoms in the [110] direction is missing.l7-lg Both a (1x 2) and (1 ~
(10)Schardt, B. C.; Yau, S.-L.; Rinaldi, F. Science 1989,243,1050. (11)Yau, S.-L.; Gao, X.; Chang, S.-C.; Schardt, B. C.; Weaver, M.J. J. Am. Chem. SOC.1991,113,6049. (12)Matsumoto,H.; Oda, I.; Inukai, J.;Ito, M. J. EZectrounuZ. Chem. 1993,356,275. (13)Vogel, R.; Baltruschat, H. Ultramicroscopy 1992,42-44,562. (14)Vogel, R.;Kamphausen, I.;Baltruschat,H. Ber. Bunsenges. Phys. Chem. 1992,96,525. (15)Vitus, C. M.;S-C, C.; Schardt, B. C.; Weaver, M. J. J. Phys. Chem. 1991,95,7559. (16)Bittner, A.M.; Wintterlin,J.;Ertl, G.J. EZectrounuZ. Chem.1995, 388,225. (17)F e y , P.; Moritz, W.; Wolf, D. Phys. Rev. B 1988,38,7275. (18)Fenter, P.;Gustafsson, T. Phys. Rev. B 1988,38,10197.
0743-7463/95/2411-4428$09.00/00 1995 American Chemical Society
Iodine on Pt(ll0) Single Crystal Electrode Surfaces x 3) superstructure have been observed, and this type of reconstruction is common to the (110) surfaces of Au and Ir. For Pt(llO), the UHV LEED studies reveaLthat the missing row reconstruction is lifted upon the adsorption of CO, resulting in the (1 x 1)superstructure. Ex situ electrochemical surface science studies utilizing LEED have also revealed that P t ( l l 0 ) crystal surfaces which are flame-annealed, potentiodynamically cycled, and reannealed under UHV exhibit the missing row reconstruction.20 Upon exposure to aqueous electrolytes or to a quarter monolayer of surface oxygen, the surface reverted to the (1x 1)termination of the bulk structure. Other studies have reported that the (1x 2) missing row reconstruction is stable in H&04 electrolyte provided the potential is maintained in the double layer region.21 Upon increasing the potential into the oxygen adsorption region the unreconstructed (1x 1)surface is formed. Early STM imaging experiments of iodine adlayers on Pt(110)crystal surfaces were reported only for conditions resulting in high iodine surface ~0verage.l~ These experiments conducted in both air and under electrolyte following either gas phase (12) or solution phase (I-) iodine dosing reveal a highly disordered atomic scale surface structure and do not reveal with atomic resolution the structure of the initial stages in the evolution of the iodine adlayer. More recently, Ertl and co-workers have reported results from combined STM, LEED, and electrochemical studies of the iodine on the P t ( l l 0 ) system and the effects of adsorbed iodine on the Pt(ll0) (1 x 2) reconstruction.16 In this paper atomic resolution images depicting the structures of iodine adatoms were obtained for a range of iodine surface coverages approaching saturation. The bridged coordination of the iodine adatoms on the (110) surface is clearly observable in the low coverage images where both P t ( l l 0 ) substrate atoms and iodine adatoms are visible. Additionally, the unreconstructed (1 x 1)is also clearly visible in the low coverage images. Coulometric measurements are used to quantify the amount of adsorbed iodine and to compare the experimentally determined surface coverages with the surface coverages of possible surface adlattice structures. In addition, Ag underpotential d e p o ~ i t i o n ~voltammograms ~y~~ obtained for these iodine-treated Pt(110) surfaces give evidence that a considerable amount of surface order is maintained over the whole area of the crystal.
Experimental Section Experiments reported in this paper were conducted using a Pt(ll0) single crystal (9 mm diameter, 2 mm thick) purchased from Aremco Products (Ossining,N.Y.). Handling of the crystal during surfacepreparationand electrochemicalexperiments,was accomplished using a 0.025 in. Pt wire spot welded onto the back of the crystal. The electrode surfaceswere prepared by a method similar to the atmospheric preparation method developed by Hubbard and co-w~rkers.~~~ Briefly, following repeated oxidation-reduction cycles in 1M HC104, the crystal was annealed for 2 min in a hydrogen flame (orange-white color) and cooled by argon flowing from beneath a cup of iodine crystals held in a glass cell 1cm below the sample. Coolingtimes ranging from 1to 10 min were used to attain the different iodine coverages examined in this study. (19)Gritsch, T.;Coulman, D.; Behm, R. J.; Ertl, G. Phys. Rev. Lett. 1989,63,1086. (20)Feddrix, F. H.; Yeager, E. B.; Cahan, B. D. J.Electroanal. Chem. 1992,330,419. (21)Michaelis, R.;Kolb, D. M. J.Electroanal. Chem. 1992,328,341. (22)Kolb, D.M. Physical and Electrochemical Properties of Metal Monolayers on Metallic Substrates. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; John Wiley: New York, 1978;Vol. 11;pp 125. (23)Juttner, K.;Lorenz, W. J. 2.Phys. Chem. NF 1980,122,163.
Langmuir, Vol. 11, No. 11, 1995 4429 15.0
10.0
5.0
0
5:o
10.0
.O 15.0 nw
Figure 2. STM image of an iodine adlayer on Pt(110)in 1M HClOd taken in the current mode at a tunneling current of 4 nA and open circuit potential. A 1min cooling time in iodine/ argon vapor was employed during crystal preparation.
The STM employed in the surface characterization studies was a Nanoscope I11electrochemical STM (DigitalInstruments, Santa Barbara, CA). All imaging was conducted in 1M HC104 with the working electrode surface held at the open circuit potential (%200mV vs Ag/AgCl) followingtransfer of the iodinetreated surface in air to the STM electrochemical cell. Images were attained in both the current mode (small scale)and height mode (large scale) at a tunneling current of 4 nA and a tip bias voltage of -300 mV. Large scale imaging experiments in the height mode were also conducted for potentials within a f400 mV range around the open circuit potential. Platinum and oxidized gold wires were used for the auxiliary and reference electrodes, respectively. Tunneling tips consisted of 0.25 mm tungsten wire electrochemicallyetched in 1MKOH and insulated with either Apiezon wax or thermosetting plastic. The images reported are unprocessed and the surface structure measurements reported are the average of individual measurements recorded mainly from the highly ordered regions in a collection of similar images. The standard deviation is reported as the uncertainty. Potentiodynamiccycling, coulometrymeasurements, and thd Ag underpotential deposition voltammograms were obtained in a closed electrochemical cell using a Cypress Model CYSY-1 potentiostat (CypressSystems,Lawrence, KS). The coulometric measurements quantifying the amount of adsorbed iodine followed the reaction, Iads 3H20 ==103- + 6H+ + 5e- in 1M HC104.24925The Ag underpotential deposition voltammogram was obtained followingiodine treatment of the crystal,immersion in a 1mM AgClOJ1 MHClO4 electrolyteat 975 mV vs Ag/AgCl, and cycling to and from the onset of bulk deposition at 5 mV/s. Boththe coulometry and Ag UPD were conducted via the hanging meniscus method, where only the polished and treated crystal faceis in contact with the electrolyte. All solutionswere degassed using argon and were made using MilliQ water (Millipore, Bedford,MA) and AgC104and HC104,doublydistilledfromVycor (both from GFS Chemicals, Powell, OH).
+
Results Pictured in Figure 2 is an STM image of the P t ( l l 0 ) surface prepared by annealing the crystal in a hydrogen flame followed by cooling in the iodine/argon stream for (24)Lane, R. F.; Hubbard, A. T. J. Phys. Chem. 1975,79,808. (25)Soriaga, M. P.;Hubbard, A. T. J. Am. Chem. Soc. 1982,104, 2742.
4430 Langmuir, Vol. 11,No. 11, 1995
Figure 3. STM image of an iodine adlayer on Pt(110)in 1M HC104 taken in the current mode at a tunneling current of 4 nA and open circuit potential. A 2 min cooling time in iodine/
argon vapor was employed during crystal preparation.
1 min. This results in the lowest iodine coverage investigated in this work. The adsorbed I atoms appear as bright dots in the image, indicating the enhanced tunneling probability at these sites. The iodine adatoms in this image form loosely packed rows on the surface which are parallel to the [OOl] direc.ion and displaced from one another by 0.5 f0.1 in the [110] direction. Along the rows, the iodine-iodine spacings are distributed around the characteristic values associated with multiples of the unit cell lattice constant in the [OOl] direction (0.392 nm). Also visible in the image are the Pt atoms of the underlyingsubstrate. Measurements of the characteristic spacings between the atoms in the substrate are 0.23 f 0.06 and 0.34 f 0.05 nm, which compare well with the established surface lattice parameters of 0.277 and 0.392 nm for the Pt(ll0) surface and clearly reveal both the absence of the missing row reconstruction and the orientation of the crystal surface which is noted in the image. Presented in Figures 3 and 4 are images of the Pt(110) electrode surface after annealing of the crystal in the hydrogen flame and cooling for either 2 or 10 min in the iodine/argon vapor. Reflective of the increasing exposure to the iodine/argon vapor, the density of iodine adatom increases and the rows parallel to the [OOl] direction become more complete. The distance between the rows of iodine adatoms is consistent with electrodes prepared as in Figure 2,0.5f0.1 nm following 2 min exposure and 0.6nm f 0.1 nm following 10 min exposure. Also as in Figure 2,the distance between iodine adatom within rows are distributed about the characteristicvalues associated with multiples of the unit cell lattice constant in the [OOl] direction. Surface iodine coverages of the iodine-treated Pt(110) electrode surfaces were determined from coulometric measurements based on the five-electron oxidation of Iads and are listed in Table l.24*25 These measurements are the average and standard deviation of three individual experiments involving a new preparation of the electrode surface. Comparison values were obtained by counting the number of iodine atoms in a 10 nm x 10 nm region of each of three STM images.
Desimone and Breen
Figure 4. STM image of an iodine adlayer on Pt(ll0) in 1M HC104 taken in the current mode at a tunneling current of 4 nA and open circuit potential. A 10 min cooling time in iodine/
argon vapor was employed during crystal preparation.
Table 1. Iodine Coverages for Various Cooling Times Calculated from Coulometric Measurements, STM Images, and (2 x 1) Model Structure
time (mid 1 2
10
coulomet** 1.4 f 0.7 4.7 f 0.4 6;O f 0.8
. countingb*c 1.5 f 0.1 3.7 f 0.2 5.9 f 0.1
model structure* (2 x 1)7.62
a As in refs 24 and 25. * Units: 10-lomol cm-2. From STM images.
Figure 5. Large scale STM image of an iodine adlayer on Pt(110)in 1M HC104 taken in the height mode at a tunneling current of 4 nA and open circuit potential. A 2 min cooling time in iodine/argonvapor was employed during crystal preparation.
Depicted in Figure 5 is a large scale STM image taken in the height mode of a P t ( l l 0 ) electrode annealed and cooled for 2 min in the iodine/argon vapor. Clearly visible in this image is the terraced nature of the modified electrode surface. Measurements of the crystal step heights are clustered about 0.13 f 0.03,0.25f 0.03,and 0.36 f 0.03 nm and are consistent with one, two, and
Iodine on Pt(ll0) Single Crystal Electrode Surfaces
Langmuir, Vol. 11, No. 11, 1995 4431 1
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9.7 pC, corresponding to Ag adatom surface coverages of 2.8 x 10-lo, 2.6 x 10-lo, 2.4 x 10-lo, and 1.9 x mol cm-2, respectively.
h'" 0.3
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Figure 7. Cyclic voltammogramdepicting the underpotential
deposition of Ag onto the iodine-treated Pt(ll0) crystal. The scan was initiated at 0.975 V at a rate of 5 mV s-l, and the electrolyte is 1mM AgC104 in 1 M HC104. three atomic layer steps.26 Also clearly evident in this large scale image are bright lines of raised atoms which are visible in all images of the electrode surfaces '20 nm.2 The density of these raised row features is independent of iodine exposure and they are distributed over the entire electrode surface. Figure 6 is a plot revealing the distribution of neare_stdistances between these raised row features in the [110] direction which is modestly peaked a t around 2 nm. The height of these features, which are oriented along the [OOl] direction, is 0.30 f 0.04 nm, and atomic scale images such as Figure 2 reveal these raised features to be composed of closely packed atoms with an atom to atom spacing of 0.30 f 0.06 nm. Finally, pictured in Figure 7 is a cyclic voltammogram obtained for the underpotential deposition of Ag on an iodine-treated Pt(110) electrode. In this particular experiment the surface was cooled for 1min in the iodine/ argon vapor. The cyclic voltammogram clearly shows at least four discreet surface deposition processes occurring and that overall there is clearly a well-ordered character to the surface. The charge associated with each deposit moving from high to low potentials is 13.8,12.5,11.8, and (26)The STM scanner's z calibration is routinely checked by measuring surface features on the HOPG and iodine-modified Pt(111) surfaces.
Discussion The atomicscale STM images presented in Figures 2-4 clearly reveal the presence of ordered structures on a Pt(110)electrode surface resulting from annealing the crystal in a hydrogen flame and cooling the hot crystal for various times in an iodine/argon vapor. As expected, the amount of adsorbed iodine appearing in the surface images increases with increasing exposure to the iodine/argon vapor. This is corroborated by the electrochemical measurements presented in Table l. Images obtained a t low iodine surface coverage reveal both the iodine adatoms and the underlyingPt atoms of the electrode surface. These low-coverage images reveal the bridged coordination of the iodine adatoms, the unreconstructed Pt(ll0) (1 x 1) substrate, and the orientation of the crystal in the image and with respect to the other prominent surface features. Depicted in Figure 8 is a plot of the distribution of distances measured between iodine adatoms in the rows paralleling the [OOl] direction. Approxiamtely 200 atomatom distances were measured from three different highresolution images and a t least two electrode preparations. The measured distances were grouped about multiple values of the 0.392 nm lattice spacingin the [OOl] direction. At low iodine coverages the distribution is rather broad, but with increasing iodine coverage the distribution becomes quite narrow. At the highest coverages examined the most probable iodine-iodine separation in a row of iodine atoms is 0.392 nm. The distance between rows of iodine adatoms is similar for all three preparation conditions examined. Consequently,we propose that the iodine adlayer, which our STM images reveal exhibits some local order a t low coverage, evolves into a (2 x 1) iodine adlattice as the exposure to the iodine vapor increases. The proposed Pt( 110)-(2 x 1)-I structure, mol cm-2, is which has a surface coverage of 7.62 x consistent with the experimentally determined iodine adatom coverage, 6.0 f 0.8 x mol cm-2, obtained from electrochemical measurements of the charge passed to oxidatively desorbed the iodine adatoms. Very apparent in both the atomic and large-scaleimages of the iodine-treated Pt( 110) electrode surfaces are the raised row features which are distributed over the entire electrode surface. The features appear in all images of the iodine-treated Pt(110) electrodes prepared in our laboratory and the density of these features appears to be independent of iodine exposure. The appearance of these
Desimone and Breen
4432 Langmuir, Vol. 11, No. 11, 1995 features is also reported in the recent study of Ertl and co-workers for electrodes prepared similarly to those in this study.16 These features, which are oriented along the [OOl] direction, are 0.30 i 0.04 nm high and are composed of closely packed atoms spaced every 0.30 f 0.06 nm apart, which is smaller than the closest iodineiodine distance observed, 0.5 nm and in good agreement with the characteristic 0.277 nm distance for closely packed Pt atoms. Consequently we attribute these features to a surface reconstruction consisting of rows of raised closely packed Pt atoms. While it is possible that these raised row features are a manifestation of the atmospheric preparation method used for these electrodes, such an unevenly distributed surface feature would be difficult to detect by many other surface science techniques. No potential dependence in the number, spacings, and orientations of these raised row features was observed in large scale imaging experiments conducted over a range o f f 400 mV around the open circuit potential. Finally, in Figure 7 a voltammogram depicting the UPD ofAg on an iodine-treated Pt(ll0) electrode is shown. Since UPD measurements are very sensitive to the atomic scale structure of an electrode surface, this voltammogram reveals a significant degree of overall order in the surface s t r u ~ t u r e In . ~ addition, ~ ~ ~ ~ the presence of a t least four distinct surface deposition processes are evident from the features in the voltammogram. The charge associated with the features in the voltammogram are indicative of the deposition of Ag adatoms with a surface coverage of 10 x mol/cm2. The UPD voltammogram also reveals the oxidative stripping of the Ag deposit occurring in two steps. Tentatively we attribute the features in the voltammogram to either the successive deposition of two (1x 2) metal monolayers (each would correspond to a Ag surface coverage of 7.62 x mol/cm2)or possibly the formation of a higher density adlatice prior to the onset
of bulk Ag deposition. Additional Ag UPD and STM imaging experiments examining the surface structure in relation to observed voltammetric features and the dependence of these features in the cyclic voltammogram as a function of iodine exposure are in progress.
Conclusions Gas phase dosing of flame-annealed Pt(110) single crystal electrodes with a n iodinelargon vapor results in the formation of ordered adlayer structures. Electrochemical STM images obtained in 1M HC104 electrolyte reveal low surface coverage adlayer structures which evolve into a (2 x 1)adlattice structure with increasing iodine exposure. Also observed in the STM images is a surface reconstruction consisting of raised rows of closely packed Pt atoms oriented along the [OOl] direction and distributed over the electrode surface. Investigations show that the extent of this surface reconstruction is independent of iodine exposure and exhibits no observable potential dependent dynamics for a limited potential range around the open circuit potential. Finally, Ag underpotential deposition voltammograms reveal a significant degree of overall surface order and a multistep process leading to the deposition of either a high-density Ag adlayer or possibly two successive adlayers prior to the onset of bulk Ag deposition.
Acknowledgment is made to the IUPUI Faculty Development Office, the Purdue Research Foundation, and The Petroleum Research Fund, administered by the ACS, for support of this research. The authors also wish to thank Professor F. A. Schultz for the loan of the Cypress potentiostat. W.L.D. also thanks the Department of Education for fellowship support ( G A A " ) . LA9500610
