Langmuir 1985, 1 , 66-71
66 u,., U,Iy
= sin 4 cos 0 = sin 4 sin B
4 where 4 is the angle between z and z', 8 is the angle between the projection of z'in the x y plane, a is the angle of rotation of the x 'y' plane around z' measured by the angle between the intersection of the x y ' ' and x y planes and y', and z is the surface normal direction. u,., = cos
The direction cosine terms vxtz,vg and vztzindicate the orientation of the molecular coor mates relative to the surface normal. To relate a series of n sequential transformations of molecular reference frames Vi to the surface coordinates the overall matrix V is just given by the product of the individual transforms, V = II;=' Vi. Registry No. A1203, 1344-28-1; n-CI6H3,CO2H,57-10-3; CH+2H(CHZ)i,COzH, 53821-23-1; CHzC(CHz)19C02HI 93645-43-3;n-C,$H&O,H, 506-30-9.
Ordered Ionic Layers Formed on Pt( 111) from Aqueous Solutions John L. Stickney, Stephen D. Rosasco, Ghaleb N. Salaita,t and Arthur T. Hubbard* Department of Chemistry, University of California, Santa Barbara, California 93106 Received May 1, 1984 Ordered layers (adlattices) formed spontaneously when the Pt(ll1) surface was immersed into aqueous ionic solutions. On the basis of Auger spectroscopy and LEED, the following salts formed adlattices as indicated: KCN, Pt(ll1) (2d3x2d3)R3O0-KCN; KSCN, Pt(ll1) (2X2)-KSCN; K2S, Pt(ll1) (diffuse lxl)-K,S; KI, Pt(ll1) (3X3)-I; KBr, Pt(ll1) (3X3)-KBr. KI solution yielded a layer of neutral I atoms requiring no cationic counterion, in agreement with previous studies of HI vapor and aqueous electrosorption. All ionic concentrations studied (lo4 to lo-' M) gave similar results. The adsorbed layer of anions functioned as a cation exchanger: K+ ions were quantitatively replaced when the surface was rinsed with M HCl or CaC1,. Exchange of cations did not change the LEED pattern at room temperature; however, reconstruction occurred on heating to about 100 "C in some cases. Auger spectra indicated that hydroquinonesulfonate (KHQS) displayed a packing density transition as a function of concentration, as expected from electrochemicaldata; LEED patterns displayed no fractional-index beams due to the KHQS layer.
Introduction While the vital role played by the electrical double layer a t electrode-solution interfaces' has been recognized for many years,2 experimental difficulties have precluded direct structural characterization of the electrode surface. In particular, the structural arrangement of electrode surface atoms and electrolytic ions is considered to be important on practical3 and theoretical ground^.^ Work leading up to the present study has dealt with hydrogen electrodep~sition,~ halogen chemisorption,6 solvent vapor chemi~orption,~ competitive adsorption of solvent and electrolyte vapor* on Pt(100) and P t ( l l l ) , oriented adsorbed molecule^,^ oxidationlo and reduction of oriented intermediates,l' the influence of microscopic surface roughness on adsorbate orientation and reactivity,12electrodeposition of Cu13and AgI4 onto well-defined surfaces of Pt containing adlattices of iodine atoms, and with preparation and identification of well-defined Pt surfaces a t atmospheric pressure.15 Results of previous work5-15 illustrate the remarkable degree to which surface structure and molecular orientation influence electrochemical and catalytic reactivity although much remains to be explored. Since electrodeposition processes have been found to be ~ ~present '~ highly sensitive to adsorbed layer s t r ~ c t u r e ,the work was undertaken to determine the structures of adlayers formed by treatment of Pt(ll1) with aqueous ionic ' Fulbright scholar, University
of Jordan, Amman, Jordan.
solutions, including constituents commonly found in electroplating baths16 and other practical electrolytes. (1)(a) Delahay, P. "Double Layer and Electrode Kinetics"; Interscience: New York, 1965. (b) Overbeeck, J. T. G. Pure Appl. Chem. 1965, 10,359. (c) Spamaay, M.J. "The International Encyclopedia of Physical Chemisty and Chemical Physics"; Everett, D. H., Ed.; Pergamon Press: New York, 1972;Vol. 4, Topic 14. (d) Hurwitz, N. D. "Electrosorption"; Gileadi, E., Ed.; Plenum Press: New York, 1967. (2)(a) von Helmholtz, H. L. F. Ann. Phys. (Leipzig) 1853,89(2),211. von Helmholtz, H. L. F. Ann. Phys.(Leipzig) 1879,7 (3),337. (b) Gouy, G. J. Phys.Radium 1910,9(4),457. Gouy, G. C. R. Hebd. Seances Acad. Sci. 1910,149,654. (c) Chapman, D. L. Philos.Mag. 1913,25 (6), 475. (c) . , Stern. 0.Z.Elektrochem. 1924. 30. 508. (3)Hubbard, A. T. Acc. Chem. Res.' 1980,13,1977.Hubbard, A. T. J. Vac. Sci. Technol. 1980,17,49. (4)Barlow, C. A.,Jr.; MacDonald, J. R. J. Chem. Phys.1964,40,1535. (5)Ishikawa, R. M.; Katekaru, J.; Hubbard, A. T. J . Electroanab Chem. 1978,86,27:. (6)(a) Felter, T.E.; Hubbard, A. T. J. Electroanal. Chem. 1979,100, 473. (b) Garwood, G. A., Jr.; Hubbard, A. T. Surf. Sci. 1980,92,617.(c) Garwood. G. A., Jr.: Hubbard. A. T. Surf. Sci. 1982,112,281. (7)Garwood, G.'A., Jr.; Hubbard, A,' T. Surf. sci. 1982, 118, 223. (8)Katekaru, J. Y.; Hershberger, J.; Garwood, G. A., Jr.; Hubbard, A. T.Surf. Sci. 1982,121,396. (9)Soriaga, M.P.; Hubbard, A. T. J. Am. Chem. SOC.1982,104,3937. (10)(a) Soriaga, M.P.; Stickney, J. L.; Hubbard, A. T. J. Electroanal. Chem. 1983,144,207.(b) Stickney, J. L.; Hubbard, A. T. J. Mol. Catal. 1983,21, 211. (11)Soriaga, M.P.; Hubbard, A. T. J.EZectroanaL Chem. 1983,159, 101. (12)White, J. M.;Soriaga, M. P.; Hubbard, A. T. J . Electroanal. Chem., in press. (13)Stickney, J. L.;Rosasco, S. D.; Hubbard, A. T. J.Electrochem. SOC.1984,131,260.
0743-7163/85/2401-0066801.50/0 0 1985 American Chemical Society
Langmuir, Vol. 1, No. 1, 1985 67
Ordered Ionic Layers Formed on P t ( l l 1 )
Table I. Auger Spectroecopy a n d LEED D a t a for Ionic Layers Auger suectroscouv0 ionic compd KCN KSCN
KZS KHQSb KI
concn, M
BC
ON
10-4 10-1 10-4 10-1
0.6 0.7
0.7 0.8 0.6 0.6
0.4 0.5
90
0.5 0.5
1.0 1.6
0.4
0.6
0.07 0.11
OBr
4
0.16 0.18 0.10 0.10 0.04 0.04 0.04 0.09
10-1
0.00 0.00 0.00
lo-'
0.05
10-4 KBr
OK
0.18 0.25
10-4 10-1 10-4 10-1 10-5
OS
0.43 0.44 0.44 0.44c
LEED (2v'/3X2v'3)R3Oo (2v'/3X2v'/3)R3Oo (2x21 (2x2) (diffuse 1x1) (diffuse 1x1) (diffuse 1x1) (diffuse 1Xl) (v'/?xv'7)R19.lo (3x3) (3x3) (3x3)
Packing densities were obtained by integrating Auger spectra 88 described in ref 14b, without correction for self-scattering. *Potassium hydroquinonesulfonate. Estimated from ref 6c.
Experimental Section The procedures employed here have been described.&ls In the present work, all six faces of a Pt single crystal were oriented and polished to the (111)or equivalent plane. Cleaning was ale0 carried out on all six faces simultaneously by argon ion sputtering, with subsequent annealing by resistance heating. After initial characterization by Auger spectroscopy and LEED,the crystal was isolated in an Ar-filled antechamber and immersed into aqueous solution. After reevacuation, the surface was characterized again by Auger spectroscopy and LEED. A cylindrical mirror analyzer (Varian 981-2707) was employed, with a glancing incidence electron gun ( l l o from the surface plane), for Auger spectroscopy.
Results and Discussion The Pt(ll1) surface employed for these experiments displayed the (1x1) LEED pattern observed by other workers (Figure 1A). The Auger spectrum of clean Pt(ll1) contained only transitions assignable" to Pt, in agreement with previous studieda (Figure 2, solid lines). LEED patterns observed following immersion of Pt(ll1) into aqueous solutions of a variety of common salts are shown in Figure 1. A range of concentrations of each electrolyte were examined from 10"' to lo-' M, followed in all cases by a f i a l rinse with lo4 M KC1. On the basis of the Auger spectra, the rinse with 10-4 M KC1 was found to remove excess salts from the crystal surface while allowing a majority of the K+ counterions to remain (Figure 2). Chloride ions, being the most weakly adsorbed of those studied, did not replace the preadsorbed anions, based upon the absence of detectable Cl Auger signal (ecl
