828
Langmuir 1986, 2, 828-835
H L value. As shown in Figure 5, the shape of the qst curve on OG-400 and OG-25 is very similar to that on OG-700, the only difference being the fact that the break point slightly shifts to higher coverage of H 2 0 from OG-700 to OG-400 and further to OG-25. Admittedly, there is a great difference between the qst curves on OG-700 and OG-1000. The reason why such a difference appears can be attributed to the fact that the CO content on OG-700 is as much as those on OG-25 and OG-400, in contrast to a smaller value on OG-1000 (Table I). Walker and Janov have reported that the CO-desorbing oxides on Graphon act as the sites for adsorption of HzO, which give the heat of adsorption similar to the HL value.22 Thus, the H,O
molecules will be adsorbed on the CO-desorbing oxides of OG-25, OG-400, and OG-700 after the sites, such as the C02- and HpO-desorbing oxides, have been completely covered with H,O.
Acknowledgment. We thank Professor Shigeharu Kittaka of Okayama University of Science for his great help in the electron microscopic observation. We also thank the Ministry of Education (Japan) for the financial support granted for this research. Registry No. H20, 7732-18-5;graphite, 7782-42-5. ( 2 2 ) Walker, P. L., Jr.; Janov, J. J. Colloid Interface SCL.1968,28,449.
Structure and Composition of a Platinum(ll1) Surface as a Function of pH and Electrode Potential in Aqueous Bromide Solutions Ghaleb N. Salaita, Donald A. Stern, Frank Lu, Helmut Baltruschat, Bruce C. Schardt,? John L. Stickney,* Manuel P. Soriaga,§ Douglas G. Frank, and Arthur T. Hubbard* Department of Chemistry, University of California, Santa Barbara, California 93106 Received June 23, 1986 Studies are reported in which surface layers formed by immersion of a well-defined Pt(ll1) electrode surface into aqueous CaBrzsolutions were characterizedwith regard to structure, composition, and reactivity by means of low-energy electron diffraction (LEED), Auger electron spectroscopy, and linear-scan voltammetry. Voltammetry revealed a series of potential-dependent adsorption processes. Comparison of voltammetric curves with surface analysis data (Auger) and structural data (LEED) permitted identification of adsorbed layer structural transitions and adsorption/desorption processes responsible for the electrochemical behavior. Halogen adsorption was stronger from acidic than from alkaline solutions and was stronger at potentials in midrange than at positive or negative extremes. Br atoms were the principal adsorbed form of halogen and underwent reductive desorption to Br- anions beginning at -0.1 V (vs. Ag/AgCl reference). A sharp voltammetric peak occurred at the onset of Br reductive desorption, corresponding to a structural transition within the halogen layer from (3x3) to (4x4) hexagonal close packing. In alkaline Br- solutions adsorption of oxygen species gives rise to a prominent reversible voltammetric peak (oxygen packing density, Bo = 0.5; LEED pattern, P t ( l l l ) ( l x l ) with a diffuse oxygenous overlayer), followed by an irreversible process which disordered the Pt surface. Br was not strongly adsorbed from alkaline solutions due primarily to strong competition from oxygenous adsorbates. Retention of water by the surface from vacuum correlated with Cap+retention and varied from 5 to 15 water molecules per Cap+cation, being largest at alkaline pH. In contrast, K+ ions, being less strongly hydrated, did not retain detectable amounts of water under vacuum. The Pt surface was hydrophilic toward bromide solutions over the full range of pH and potential.
Introduction In a preceding paper1 we reported that ordered layers, adlattices, were formed when a P t ( l l 1 ) surface was immersed into aaueous solutions of KCN, KSCN, KHS, KI, or KBr. Relative affinities of various cations for retention a t such surfaces were explored.2 The p H dependence of ionization of the adlattices derived from KCN or KSCN solutions at Pt(ll1) was found to reflect the structure and intermolecular interactions within the l a t t i ~ e . ~Evidence ,~ * T o whom correspondence should be addressed. Present address: Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221. Present address: Department of Chemistry, Purdue University, West Lafayette, IN 47405. Present address: Department of Chemistry, University of Georgia, Athens, GA 30602. 'Present address: Department of Chemistry, Texas A&M University, College Station, T X 77843.
*
0743-7463/86/2402-0828$01.50/0
for strong adsorption of Br- onio polycrystalline Pt, for instance r a d i ~ c h e m i c a l , e~l, l~i p s o m e t r i ~ ,and ~ ~ ~electroChemical d a h g has been reported. (1) Stickney, J. L.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985, I , 66. (2) Rosasco, S. D.; Stickney, J. L.; Salaita, G. N.; Frank, D. G.; Katekaru, J. y.; Schardt, B. C.; Soriaga, M. P.; Stern, D. A.; Hubbard, A. T. Electroanal. Chem. 1985, 188, 95. (3) Schardt, B. C.; Stickney, J. L.; Stern, D. A,; Frank, D. G.; Katekaru, J. Y.; Rosasco, S. D.; Salaita, G. M.; Soriaga, M. P.; Hubbard, A. T. Inorg. Chem. 1985,24, 1419. (4) Frank, D. G.; Katekaru, J. Y.; Rosasco, S. D.; Salaita, G. N.; Schardt, B. C.; Soriaga, M. P.; Stern, D. A.; Stickney, J. L.; Hubbard, A. T. Langmuir 1985, I , 587. ( 5 ) Petrii, 0. A.; Frumkin, A. N.; Shchigorev, I. G. Elektrokhimiya . , , " a
IYIU,
II
,,,A
0,4uu.
(6) Kazarinov, V. E.; Petrii, 0. A.; Topolev, V. V.; Vosev, A. V. Elektrokhimiya 1971, 1365, (7) Hyde, P. J.; Maggiore, C. J.; Redondo, A.; Srinivasan, S.; Gottesfeld. S. J . Electroanal. Chem. 1985. 186. 267. (8) Hyde, P. J.; Gottesfeld, S. Surf. Scl 1985, 149, 601.
0 1986 American Chemical Society
Langmuir, Vol. 2, No. 6, 1986 829
S t r u c t u r e a n d Composition of a P t ( l l 1 ) Surface
In the present article we report studies of the voltammetric behavior of a well-defined Pt(ll1) surface in aqueous CaBr,/KBr/HBr solutions at pH 2,4,6,8,10, and 12. Surface and adsorbed layer structure was examined by means of LEED. Surface elemental composition was monitored with Auger electron spectroscopy. The results of these studies have revealed that ordered layers of halogen are present at Pt(ll1) over a wide range of pH and electrode potential. Br atoms are the principal adsorbed species. Reductive desorption of Br as Br- anions occurs near -0.1 V (vs. Ag/AgCl). Adsorbed Br predominates from acidic solutions, while oxygen species are prevalent from alkaline solutions. Cations are retained only in small amounts at Pt(ll1) in bromide media.
Experimental Section T h e procedures employed were as described in ref 1-4. The Pt(ll1) single crystals used for this work were oriented’O and polished” such t h a t all six faces were crystallographically equivalent to the (111) plane. All six faces were cleaned simultaneously by bombardment with Ar+ ions and annealed by resistance heating (about 1000 K) in ultrahigh vacuum. After characterization by means of low-energy electron diffraction (LEED) and Auger electron spectroscopy, the crystal was isolated in an Ar-filled antechamber for immersion into various electrolytic solutions. Electrode potentials and currents were measured and controlled by means of standard electrochemical circuitry based upon operational amplifiers. Potentials were referred to a Ag/ AgCl reference electrode prepared with 1 M KC1. The electrochemical cell was constructed of Pyrex glass and Teflon. After the solution was drained away, a film of liquid about cm thick remained on the crystal. The water layer evaporated a t the start of evacuation of the Ar atmosphere. Solutions employed for the linear scan voltammetric experiments contained a t least 10 mM supporting electrolyte in order to provide adequate conductivity and buffer capacity. However, prior to LEED/Auger experiments the surface was rinsed with a more dilute solution (at the same p H and potential) in order to minimize the contribution due to excess salts present in the liquid film upon removal from solution.’-4 The Pt(ll1) surface was allowed to remain in contact with each solution for 2 min. (i) At p H 2 the scans were obtained in a solution containing 10 mM HBr and 5 mM CaBrz. No rinsing was attempted because, of course, p H 2 cannot be achieved below 10 mM. (ii) Studies a t p H 4 employed 5 m M CaBr,, p H adjusted with HBr. Prior to Auger analysis the surface was rinsed with 0.05 mM CaBr2 adjusted to pH 4 with HBr. In other words, the Br- concentration during the scans was 10 mM but was decreased to 0.1 mM just prior to Auger and LEED analysis. The adsorption profiles thus correspond to 0.1 mM Br- a t the appropriate p H and potential. (iii) Studies a t pH 6 and 8 employed 5 mM KBr or 5 mM CaBr2 buffered with calcium phosphates (saturated, about 0.1 mM) or potassium phosphates; rinsing prior to evacuation was with 0.05 mM CaBrz similarly buffered with phosphate. (iv) The pH 10 solutions contained 5 or 0.05 mM CaBr2 as appropriate, p H adjusted with Ca(OH),. (v) The p H 1 2 scans were obtained in 5 mM C&rz with about 5 mM Ca(OH),; rinsing was not attempted because p H 12 is not possible below 10 mM. All of the commercial reagent grade CaBr, preparations that we tested resulted in a strongly adsorbed nitrogenous impurity a t p H 6-12. This problem was avoided by preparing CaBr2 solutions from high-purity CaO (99.9975% CaO, “Puratronic Grade 1”, Johnson Matthey, Inc., Seabrook, N H 03874) and reagent grade concentrated hydrobromic acid solution (J.T.Baker Chemical Co., Phillipsburg, NJ 08865). Solutions were prepared from water pyrolytically distilled in pure oxygen through a Pt gauze catalyst (800 “C). Studies dealing with halogen adsorption a t Pt electrodes require exceptionally clean techniques in all phases (9) Lane, R. F.; Hubbard, A. T. J . Chem. Phys. 1975, 79, 808. (10) Wood, E. A. Crystal Orientation Manual; Columbia University Press: New York, 1963. (11) Samuels, L. E. Metallographic Polishing by Mechanical Methods; Pitman: London, 1967.
of the experimentation (combined with direct verification of surface cleanliness by a method such as Auger spectroscopy) because a t alkaline p H or negative potential halide adsorption is not sufficiently strong to guard the surface against accidental contamination. Packing densities, Ox, were deduced from Auger spectroscopic data as described in ref 2. Surface analyses such as these are recommended primarily for analytes which are nonvolatile or become attached to the surface through electrostatic interaction or ionic or covalent bonding. Obviously, this approach perturbs the system to some extent (as do most forms of chemical measurement). Just as obviously, evaporation amounts to a smaller perturbation (0.6 kcal/mol) than most of the common analytical probes, such as infrared (4 kcal/mol), visible light (70 kcal/mol), low-energy electrons (lo3 kcal/mol), or X-radiation (lo5 kcal/mol). All of the surface analysis methods apply a t least one perturbation per adsorbate per experiment when operated a t normal sensitivity for a monolayer sample and thus involve comparable risk in this regard. Accordingly, in measurements of adsorption at electrode surfaces involving spectroscopic or diffraction probes, emersion/evacuation is not usually the principal perturbation.
Results and Discussion Cyclic voltammograms (first cycle, starting from the open-circuit potential) of Pt(ll1) in 5 mM CaBr, solutions at various pH are shown in Figure 1. The voltammetric behavior was simplest at pH 2 (Figure 1A): From the open-circuit potential (0.59 V vs. Ag/AgCl reference) to 0.0 V only double-layer charging occurred. However, a sharp, reversible current spike, peak 2, was observed at -0.1 V. The LEED pattern prior to this spike was (3X3), similar to that shown in Figure 2A, while immediately following peak 2 the LEED pattern was (4x4) such as in Figure 2B. Peak 2 was present near -0.1 V at each pH from 2 through 10. Evidently, the positions of peak 2 and the broad peak associated with peak 2 are independent of pH, Figures 1A-E, except that these peaks were absent altogether at pH 12, Figure 1F. Auger spectra prior to or following immersion showed only peaks attributable to Pt, CaBr2, or HzO, Figure 3. As can be seen from the adsorption profiles, Figure 4 and Tables I and 11, peak 2 corresponded to reductive desorption of Br. That is, the spike in peak 2 of Figures 1A-E is due to an adsorbed layer structural transition involving reductive desorption of Br and reconstruction of the Br adlattice from (3x3) to (4x4) Pt(111)(3x3)-Br Pt(111)(4X4)-Br
+ xe- + Pt(111)(4x4)-Br + xBr+ (1- x)e- +
(1)
P t ( l l l ) ( l X l ) + (1 - x)Br- ( 2 )
followed by c o m p l e t e removal of Br from the surface at or before the negative potential limit. The packing density of Br revealed a maximum of about 0.48 (Br per surface Pt), Figure 4A, consistent with the van der Waals radius of Brl, and comparable to results for adsorption from gaseous HBr.13 Of the possible hexagonal close packed (3x3) structures (‘/g, 4/9, 7 / 9 ) , the experimental value is closest to 8Br = 4/9 = 0.444.13 The measured packing density in the (4x4) structure was not very different from that in the (3x3) structure. Of the possible packing densities for hexagonal packing of Br in a (4x4) adlattice (‘/6, 3 /16, 7/16, and 9/1s), 7/16 = 0.4375 is closest to the experimental value. Accordingly, the ideal packing density for the (4x4) structure was probably 7/16. That is the ideal value of x in eq 3 is ( 4 / 9 - 7/,6)/(4/9) = 0.0155. Models of these (3x3) and (4x4) adlattices are shown in Figure 5A,B. (12) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornel1 University Press: Ithaca, New York, 1960. (13) Garwood, G. A.; Hubbard, A. T. Surf. Sci. 1982, 112, 281.
830 Langmuir, Vol. 2, No. 6, 1986 I
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The general trends in packing density of Br, 0, and Ca or K are perhaps easier to visualize in Figure 6. The potential dependence of Br at Pt(ll1) from CaBr, (pH 4) as measured by means of Auger spectroscopy is compared with data for NaBr (pH 3) at platinized Pt obtained by a radiotracer method6 in Figure 7. The radiotracer data indicated an increase in Br with increasing potential, from the hydrogen region* to the data based upon Auger spectroscopy. However, the
packing densities obtained by the radiochemical method are less than half as large as would be expected from the Auger data. Since Auger data of this type have been confirmed by independent electrochemical measurements and correlated with LEED data,1-4J3J4J7the discrepancy (14)Stern, D. A.; Baltruschat, H.; Martinez, M.; Stickney, J. L.; Song, D.; Lewis, S. K.; Frank, D. G.; Hubbard, A. T. J.Electroanul. Chem., in
press.
Structure and Composition of a Pt(ll1) Surface
,
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Langmuir, Vol. 2, No. 6, 1986 831
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Figure 3. Auger spectra: (A) clean Pt(ll1); (B) Pt(lll)(lXI) (by immersion into 0.05 mM CaBq (pH 4.1) at 4.30 V); (C) Pt(111)(3X3)-Br (by immersion into0.05 mM CaBr, (pH 4.1) at +0.40 V); (D) Pt(lll)/(DIFFUSE)(by oxidation of the surface in 0.05 mM CaBr. lnH 4.1) at +0.8 V for 3 min). Exnerimental conditions the &dent heam was 1 0 PA a1 2 0 0 e-V. angle of incidence from the surface plane was 1 I o
Figure 2. IXED patterns: ( A ) Pt(lll)(3X3)-Br (hy immersion into 0.05 mM CaHr, (pH 4); electrode potential, 0.1 V vs. Ag/AgCCI; beam energy, 62 eV); ( R ) Pt(111)(4x4)-Br (hy immersion into 0.05 mM CaRr, (pH 4); -0.04 V; SO eV).
is probably due to a combination of assumptions and approximations involved in the radiochemical measurements. Some of those problems were described in connection with studies of aromatic adsorption a t Pt surfaces.Is Auger spectra obtained following immersion at electrode potentials negative of peak 2 and its accompanying shoulder (less than about -0.2 V) indicated that Br was absent from the surface, and only K (or Ca and 0)and Pt signals were present, Figures 3 and 4. Reductive desorption of Br was complete at -0.2 V. That is, adsorption from Br- solutions is a redox process, analogous to the behavior of I- and HI?J3 Since the Br-reduction peak 2 is well separated from neighboring peaks a t pH 7.8 M Br-), Figure lD, the (15) Benziger, J. B.; Paaeale, F. A.; Bernasek, S. L.; Soriaga, M. P.; Hubbard. A. T. J . Efeetroanal. Chem. 1985,198.65. (16) &lahay. P.Double Layer ond Electrode Kinetics, 2nd ed.; W h y : New York. 1965. (17) (a) Sehoeffel, J. A,; Hubbard, A. T. A w l . Chem. 1917.49.2330. (b) Stiekney. J. L.; Rosasco, S.D.;Song. D.A,; Soriags. M. P.; Hubbard, A. T. Surf. Sei. 1983, 130. 326. (18) (a) White, J. H.; Soriags. M. P.; Hubbard, A. T. J. Eleetroowl. Ckem. 1984,177.89. (b) Soriaga. M. P.; Hubbard, A. T. J. Phys. Chem. 1984,88. 1089. (e) Soriaga, M. P.;Hubbard, A. T. J . E l e e t r w w f .Chem. 1984.167.79. (d) Song, D.; Soriaga, M.P.;Vieira, K. L.; Hubbard, A. T. J . Efeetroonaf.Chem. 1985, 184, 171.
coulometric charge in this peak has been evaluated 99.7 &/cm2. On the basis of the stoichiometry stated in eq 2, this corresponds to a Br packing density of 0.42, which is close to the ideal packing density of the (3x3) adlattice, 0.44. This suggests that reductive desorption of Br is not directly coupled to adsorption of H; however, the adsorbed-Br-reduction peak (peak 2) and the H-desorption peaks (peaks 3.4, and 5) heavily overlap when the pH is less than about 8. Voltammograms a t pH 10 and 12.1, Figure 1E,F, clearly display a reversible feature, peak 1, resulting from adsorption of an oxygen-containing species. A similar reversible adsorption/desorption process was found in aqueous KCI a t P t ( l l l ) , where i t was clearly associated with the emergence of strong Auger emission due to oxygen, signaling the onset of surface oxidation." Peak 1 emerged earlier in chloride than in bromide media, pH 4 instead of pH 10, due apparently to the fact that chloride adsorbs less strongly than bromide and therefore competes less effectively with the oxygen species. This oxygen species is apparently the same as that observed in perchlorate media and in sodium hydroxide a t polycrystalline Pt hy means of infrared reflection absorption spectroscopy.I5 In that work the oxygen species was identified as adsorbed O H
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;
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