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The in-situ preparation of well-defined, single crystal electrodes
221
J. Electroanal. Chem., 230 (1987) 221-231 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
THE IN-SITU PREPARATION ELECTRODES
D. ZURAWSKI, Department
L. RICE, M. HOURANI
of Chemistry,
OF WELGDEFINED,
and A. WIECKOWSKI
SINGLE CRYSTAL
l
Univers@y of Illinois, 1209 W. California Si., Urbana, IL 61801 (U.S.A.)
(Received 18th November 1986; in revised form 3rd February 1987)
ABSTRACT A new method of single crystal preparation for electrochemical application is described in this report. This method does not require the annealing-quenching procedure or ultra-high vacuum methodology. The crystal is armealed and cooled in iodine vapor, then transferred to an electrochemical cell where the chemisorbed iodine is replaced by carbon monoxide present in the electrolyte. The carbon monoxide is subsequently electrooxidized from the surface, leaving the clean, ordered surface exposed to the electrolyte. Thus, the crystal is obtained in situ. The ordered crystal is not subject to the strains imposed by quenching and at no time is the unprotected surface exposed to the atmosphere. The voltammetric evidence presented shows that this procedure is successful in the characterization of a clean and adsorbate-covered Pt (111) surface.
INTRODUCTION
The quality of single crystal electrochemistry results depends on the cleanliness and order of the crystal surface. Thus the crystal preparation technique is an integral part of the experimentation. The presently used techniques are ultra-high vacuum annealing and cleaning [l-3] and the annealing-quenching procedure proposed by Clavilier [4,5]. The annealing-quenching method has been used in many works; however, repeated quenching induces strains in the crystal which may cause polygonization [6]. There is an urgent need for a reproducible, non-destructive single crystal preparation technique not requiring ultra-high vacuum methodology, which is not necessary for many electrochemical applications. The method outlined in this paper fits these requirements. The background for this method was established during our work on lateral modification in platinum electrocatalysis [7], which included studies on the reactivity of coadsorbed species on polycrystalline platinum. We have found that it is
* To whom correspondence should be addressed.
0022-0728/87/$03.50
0 1987 Elsevier Sequoia S.A.
222
possible to replace iodine chemisorbed on platinum by carbon monoxide present in an iodine-free, CO-saturated perchloric acid electrolyte. Evidence for the quantitative replacement of I by CO will be provided in this work. The procedure of cleaning and ordering Pt (111) and Pt (100) single crystals in an iodine-containing atmosphere has already been developed [8,9]. In particular, it has been established that the Pt (111) (J’r X fi)-I, t9i = 0.43 is amenable to identification by the use of its characteristic voltarnrnogram for silver underpotential electrodeposition. As reported in ref. 9, “the fi pattern obtained by heating (the surface resulting from annealing platinum in I, vapor) was strikingly sharp and stable even when compared with the well-resolved fi pattern obtained by vacuum dosing”. As indicated by Auger spectroscopy, the surfaces heated in iodine were virtually contamination-free. In the proposed method, the crystal is annealed and slowly cooled in iodine vapor [8,9]; thus it is not subject to the strains imposed by quenching and it is well-protected from contamination during cooling. The iodine-coated crystal is transferred to the electrochemical cell where the iodine is replaced by carbon monoxide present in the electrolyte. The carbon monoxide is subsequently electrooxidized from the surface, leaving the clean, ordered crystal exposed to the electrolyte. Thus, the crystal surface is obtained in situ. At no time is the annealed crystal exposed to the atmosphere without protection, eliminating all possibility of contamination and oxidation, which can disorder the surface. EXPERIMENTAL
Apparatus
The platinum single crystal (Aremco Products, Inc.) was oriented and polished to produce a Pt (111) surface of 9 mm diameter (the “working surface”). Proper orientation of the crystal was verified by X-ray diffraction. The single crystal and its holder are shown in Fig. 1. Two platinum wires of 0.5 mm diameter were spot welded to the back of the electrode; these wires were used for both heating the crystal during the annealing procedure and for electrical contact during the electrochemical experiments. An ac of 6-8 A was passed through these wires to heat the crystal to 600-700 o C. Crystal temperature was measured by means of a Pt/Pt + 10% Rh thermocouple spot welded to the back of the crystal. These four wires were encased in a quartz piece which fitted into a teflon holder. The quartz piece provided both support for the crystal and protection for the heating wires. The annealing apparatus, shown in Fig. 1, consisted of a glass cell with a nitrogen inlet and a small spoon filled with iodine crystals. The spoon was supported by three glass legs which held it 6-9 mm above the bottom of the cell. This allowed a free flow of nitrogen during the annealing procedure. The Pt crystal was positioned ca. 8 mm above the solid iodine contained in the spoon. A standard two-compartment electrochemical cell was used; one compartment held a Pt wire counter electrode and a Ag/AgCl ([Cl-] = 1 M) reference electrode. The working electrode-electrolyte arrangement is shown in Fig. 2. The cell had two
223
Fig. 1. The single crystal and its holder in the annealing cell. (1) Thermocouple wires (Pt/Pt + 10% Rh). (2) Heating wires (0.5 mm Pt). (3) Nitrogen inlet. (4) Teflon holder. (5) Pt (111) single crystal. (6) Iodine crystals in a glass spoon. Fig. 2. The working electrode-electrolyte arrangement in the electrochemical cell. The electrolyte touches only the “working surface” of the electrode (see text). (1) Thermocouple wires. (2) Heating wires. (3.4) Nitrogen inlets. (5) Pt (111) single crystal. (6) Solution bridge to counter and reference electrodes. (7) Solution outlet. (8,9) Solution inlets. Not shown: nitrogen outlet.
nitrogen inlets: one allowed purging of the electrolyte between experiments and the other provided a protective nitrogen blanket over the solution surface. The electrolyte solutions, held in elevated containers, were delivered to the cell through two inlets and drained through an outlet at the bottom of the cell. Electrochemical
replacement
of chemisorbed
iodine by carbon monoxide
As mentioned earlier, we found it possible to replace iodine chemisorbed on platinum by carbon monoxide present in an iodine-free, CO-saturated perchloric acid electrolyte. The degree of replacement is potential dependent; quantitative replacement is achieved in the hydrogen potential region. In our work with polycrystalline platinum (a platinum wire, Johnson Matthey), iodine was adsorbed at 0.2 V from 1 mM KI in 1 M perchloric acid. The positive-going voltammetric scan, Fig. 3A, was initiated after replacement of the KI-containing electrolyte by clean 1 M HClO,. The chemisorbed iodine is oxidized in a broad peak centered at approximately 1.1 V, as reported previously [lo]. The replacement of chemisorbed iodine by CO was accomplished by exposing the iodine-coated electrode to CO-
-0
2
0.0
0.2
0.4
06
0.6
I.0
1.2
J
1.4
E/V
Fig. 3. (A) Electrooxida&ion of chemisorbed iodine from a polycrystalline platinum electrode in 1 M HClO,. The iodine was adsorbed at 0.2 V from 1 mM KI in 1 M HClO,. The positive-going scan was initiated at 50 mV/s. The dashed trace is the bare Pt voltammogram. (B) The replacement of chemisorbed iodine by carbon monoxide and the subsequent electrooxidation of CO. The iodine was adsorbed as stated above, the potential scanned to -0.2 V, and CO-saturated 1 M HClO, introduced. The positive-going scan was initiated in 1 M HClO, at 50 mV/s. The dashed trace is the bare Pt voltammogram.
saturated electrolyte at -0.2 V. After multiple rinses with the CO-saturated electrolyte, the electrode was rinsed with 1 M HClO, and the positive-going voltammetric scan, Fig. 3B, initiated at 50 mV/s in the clean electrolyte. The complete absence of any oxidation current above the background in the 1.0 to 1.2 V region and the sharp CO electrooxidation peak at 0.6 V indicate quantitative replacement of iodine by carbon monoxide. This quantitative replacement was also seen with a Pt (111) single crystal electrode and was employed in our crystal preparation method. Procedure The following is a description of the procedure used for the preparation of a clean and well-ordered Pt (111) crystal. The oriented and polished single crystal, as well as all the glassware, was cleaned in warm chromic acid prior to each day’s experiments. The whole crystal was subjected to numerous oxidation-reduction voltammetric cycles in 1 M HClO., (ACS reagent grade in 18 MP Millipore water) to clean the crystal and to produce the “disordered” cyclic voltammogram of the Pt
225 -0.2
I
I -0.2
0.0,
0.2
0.4
0.6
0.6
1.0
I
I
I
I
I
I
0.0
Q2
0.4
0.6
0.6
I.0
1.2
I I I.2
E/V
Fig. 4. Cyclic voltammogram of the “disordered” Pt (111) surface obtained after numerous oxidation-reduction cycles. The scan was taken in 1 M HCIO, at 50 mV/s.
(111) working surface, Fig. 4. Cycling was stopped at 1.6 V and the crystal rinsed at open circuit with Millipore water. The clean and oxidized crystal was then transferred via air to the annealing cell, Fig. 1, and placed approximately 8 mm above the iodine surface. We checked that the transfer of such fully oxidized platinum did not result in surface contamination: the air-exposed surface showed a voltammogram identical to that without exposure. These protective properties of the thick Pt oxide film parallel the well-known passive behavior of oxidized platinum in solution. A suitable flow of nitrogen (Linde, oxygen-free) allowed the teflon crystal holder to float free from the sides of the cell. The platinum was heated slowly to 600-700 o C in the iodine-containing nitrogen. After approximately 1 min at this temperature, the heating current was shut off and the crystal allowed to cool to room temperature. Two different iodine-coating methods were employed in this work. In the first program iodine-containing nitrogen flowed during both heating and cooling of the crystal, while in the other method the nitrogen was turned off during cooling of the crystal which allowed iodine vapor to collect in the cell. A visible cloud of iodine vapor surrounded the crystal in this latter approach. As a simple measure of the amount of iodine on the crystal, the hydrophobicity of the working surface was tested by noting visually its ability to repel a drop of water. The extent of the hydrophobicity of the crystal heated and cooled with nitrogen flowing varied between particular experiments while the crystal cooled in an iodine atmosphere was definitely and consistently hydrophobic. We found that the lack of the fully hydrophobic properties of the electrode was well-correlated with the amount of platinum oxide present on the I-covered surface. Since platinum oxidation causes surface disorder, even traces of surface bound oxygen should be avoided. Following the annealing program described above, the iodine-protected surface was safely [8,9] transferred via air to the electrochemical cell containing 1 M HClO,. The potential was then scanned to -0.200 V where CO-saturated 1 M
226
I
I
loOpA
CONSECUTNE
-0.2
0.0
0.2
(x4
0.6
0.8
E/V
Fig. 5. The replacement of chemisorbed iodine by carbon monoxide on the ordered Pt (111) surface and subsequent CO electrooxidation. The replacement took place at -0.2 V from CO-saturated 1 M HClO,. The positive-going voltammetric scan was initiated at 50 mV/s in 1 M HClO,. Consecutive scans were taken in the -0.2 V to 0.2 V region.
HClO, was introduced to the cell. The whole crystal was rinsed repeatedly with the CO-saturated electrolyte to ensure removal of iodine from all surfaces. Once all traces of CO were removed from the electrolyte by rinsing with clean electrolyte, the adsorbed CO was removed from the surface by a positive-going voltammetric scan, Fig. 5, which was reversed at 0.65 V. The resulting iodine and CO-free surface was ready for further characterization. To test the cleanliness and order of the crystal at various stages in the preparation procedure, we employed the underpotential electrodeposition (UPD) of silver on the iodine-coated crystal. The silver UPD pattern is very sensitive to the order of the crystal and to the structure of the iodine adlattice [8,9]. Well-resolved underpotential deposition peaks are indicative of a high degree of crystal surface order. To perform the experiments, the iodine-coated working surface was exposed to 1 mM AgClO, (Johnson Matthey, 99.9%) in 1 M HClO, electrolyte at open circuit. The potential was then scanned in the negative direction at 2 mV/s until the onset of silver bulk deposition. At this point the scan direction was reversed and the scan continued to a potential before iodine oxidation. RESULTS AND DISCUSSION
The surface obtained by heating and cooling the crystal in an iodine-containing nitrogen atmosphere was subjected to characterization by silver underpotential electrodeposition; the results are shown in Fig. 6. Comparison of this result with the UHV-characterized results of Wieckowski et al. (Fig. 6A of ref. 9) shows that a voltammogram charcteristic of the fi I-structure was obtained and proves that the
227 0,2
0.4 I
08 I
0.6 I
1.0 I
1.2 I
1.4
L 1:; \t 0.4 I
T
0.6 I
0.6 I
I.0
I.2
I
I
t
START AND
END OF SCAN
Y
START AND END OF SCAN
I
I
4pA
0.2
I 04
0.6
I
4P
I
I
I
J
0.6
I.0
I,2
I.4
!
I
1
I
I
I
0.4
On6
0.6
I.0
I.2
E/V
E/V
. Fig. 6. The underpotential deposition of silver on the iodine-coated P! (111) crystal (J’r X n-1 adlattice). The crystal was annealed and cooled in iodine-containing nitrogen. The scan was taken in 1 mM AgClO, in 1 M HClO, at 2 mV/s. Fig. 7. The underpotential deposition of silver on the iodine-coated Pt (111) crystal. The iodine was adsorbed at 0.2 V from 1 mM KI in 1 A4 HClO, and the scan taken in 1 mM AgClO, in 1 M HClO, at 2 mV/s. See text.
surface is clean and ordered. It was found that a good fi voltammogram was obtained only when the surface was clearly hydrophobic. Silver underpotential electrodeposition was employed again to determine whether the replacement of iodine by carbon monoxide and subsequent CO electrooxidation disordered the surface. The surface obtained after the CO oxidation was immersed in 1 mM KI in 1 M perchloric acid, rinsed, and exposed to the Ag+ electrolyte. The resulting voltammogram, Fig. 7, is essentially the same as the one obtained before the replacement-oxidation, proving that neither process disordered the surface. The alternative method of iodine-coating, cooling the annealed crystal in an iodine-saturated atmosphere, produced a surface with the silver underpotential deposition pattern shown in Fig. 8. This result corresponds well with the 3 X 3-I results obtained previously (Fig. 6B of ref. 9). Carbon monoxide replaces the 3 x 3-I structure as well as the n-1 structure; this was verified in separate experiments. We found that the 3 x 3 iodine adlattice provided better protection of the Pt surface than the fi X fi adlattice, therefore this iodine-coating procedure
0.4
0.4
06
0.8
1.0
0.6
0.8
1.0
E/V
Fig. 8. The underpotential deposition of silver on the iodin~coated Pt (111) crystal (3 X3-I The annealed crystal was cooled in an iodine-saturated atmosphere. The scan was taken AgClO, in 1 M HClO, at 2 mV/s.
-,
0 ,
0.2 I
0.4 I
0.6 I
0.8 1
1.0 I
1.2 1
1.0
1.2
adlattice). in 1 mM
(
=l
3.
5
B 3
,
I
I
I
I
I
0
0.2
0.4
0.6
0.8
E/V
Fig. 9. Cyclic voltammograms scans were taken immediately below. Scan rate: 50 mV/s.
of the clean, ordered Pt (111) single crystal in 1 M HC104. The top two after electrooxidation of CO from the surface. Subsequent scans are shown
229
W I I I I START
0.2
0.4
0.6
0.8
1.0
E/V
Fig. 10. The underpotential deposition of silver on the clean, ordered Pt (111) surface. The scan was taken iu 1 mM AgClO, in 1 M HClO., at 5 mV/s.
was used in all subsequent experiments and is recommended for in situ work with Pt single crystals. The voltammogram of the clean, well-ordered Pt (111) surface obtained after the annealing-replacement-oxidation procedure is shown in Fig. 9. Upon oxidation of the carbon monoxide from the surface in the positive-going scan, the clean, well-ordered Pt (111) surface became exposed to the 1 M perchloric acid electrolyte. The scan direction was then reversed and one voltammetric cycle (0.65 V to -0.20 V to 0.65 v) taken to remove possible traces of CO, still present in the interfacial region following CO electrooxidation. The subsequent first and second scans as well as further scans are shown in Fig. 9. The similarity between our data and those obtained with other preparation techniques [4-6,111 (see also papers cited in refs. 6 and 11) is clearly visible. To test the quality of our single crystal preparation method, we studied three additional systems which have been characterized by other workers using the crystal preparation methods mentioned earlier [l-5]. The systems are: the underpotential deposition of silver on Pt (111) [12], the oxidation of adsorbed carbon monoxide [13], and the oxidation of adsorbed ethylene [14]. Following the electrooxidation of carbon monoxide from the ordered surface, the circuit was opened, the electrolyte drained, and 1 mM AgClO, in 1 M HClO, introduced to the Pt surface. The negative-going voltammetric scan was initiated at 5 mV/s and the Ag underpotential deposition voltammogram, Fig. 10, recorded. This result compares favorably with the voltammogram obtained by Omar et al. (Fig. 1 of ref. 12) using the annealing-quenching preparation method [4]. The electrooxidation of carbon monoxide, adsorbed on the clean, ordered Pt (111) crystal is shown in Fig. 11. The carbon monoxide was adsorbed at 0.1 V from
230
-0.2
0.0
0.2
0.4
0.6
0.6
1.0
1.2
1.4
I
10~A
POTENTIAL CF ADSORPTION I
I -0.2
I 0.0
I 0.2 E/V
I OA
I 0.6
I
I
I
I
I
I
I
I
-0.2
a0
0 2
0.4
0 6
0.6
I,0
1.2
1.4
Fig. 11. Electrooxidation of carbon monoxide adsorbed on the ordered Pt (111) surface. The CO was adsorbed at 0.1 V from CO-saturated 1 M HClO,. The positive-going scan was initiated at 5 mV/s in 1 M HClO,. Fig. 12. Electrooxidation of ethylene adsorbed on the ordered Pt (111) surface. The ethylene was adsorbed at 0.35 V from CIH,-saturated 1 M HClO,. The positive-going scan was initiated at 10 mV/s in 1 M HClO,.
CO-saturated electrolyte and the positive-going scan initiated in clean 1 M HClO,. The sharpness of the single electrooxidation peak is indicative of a well-ordered surface. A similar voltammogram was obtained by Lipkowski et al. [13] for CO adsorbed on an annealed-quenched Pt (111) crystal. The additional test of the quality of our preparation method was a comparison of our results with those obtained using a crystal cleaned and characterized in UHV. This comparison was favorable for the underpotential deposition of silver on the iodine-coated surface (Figs. 6 and 8). A study of the electrooxidation of ethylene adsorbed on a UHV-prepared Pt (111) surface was conducted earlier [14]. To simulate the UHV conditions, ethylene was adsorbed on our surface at 0.35 V from a C,H,-saturated electrolyte solution, as in ref. 14. The ethylene-containing electrolyte was replaced with clean 1 M HClO, and the positive-going voltammetric scan, Fig. 12, initiated at 10 mV/s. The voltammogram obtained with our crystal is nearly identical to that obtained with the UHV-prepared crystal (Fig. 6C of ref. 14).
ACKNOWLEDGEMENT
The support of this work by Dow Chemical Illinois is acknowledged.
USA
and by the University
of
231 REFERENCES 1 A.T. Hubbard, Act. Chem. Res., 13 (1980) 177. 2 E. Yeager, J. Electrochem. Sot., 128 (1981) 160C. 3 P.N. Ross and F.T. Wagner in H. Gerischer (Ed.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 13, Wiley, New York, 1984, pp. 69-112. 4 J. Clavilier, R. Fanre, G. Guinet and R. Durand, J. Electroanal. Chem., 107 (1980) 205. 5 J. Clavilier, J. Electroanal. Chem., 107 (1980) 211. 6 D. Aberdam, R. Durand, R. Faure and F. El-Omar, Surf. Sci., 171 (1986) 303. 7 D. Zurawski, K. Chan and A. Wieckowski, J. Electroanal. Chem., 210 (1986) 315. 8 A. Wieckowski, S.D. Rosasaco, B.C. Schardt, J.L.’ Stickney and A.T. Hubbard, Inorg. Chem., 23 (1984) 565. 9 A. Wieckowski, B.C. Schardt, S.D. Rosasco, J.L. Stickney and A.T. Hubbard, Surf. Sci., 146 (1984) 115. 10 R.F. Lane and A.T. Hubbard, J. Phys. Chem., 79 (8) (1975) 808. 11 K. Al Jaaf-Golze, D.M. Kolb and D. Scherson, J. Electroanal. Chem., 200 (1986) 353. 12 F. El-Omar, R. Durand and R. Faure, J. Electroanal. Chem., 160 (1984) 385. 13 B. Love, J. Richer and J. Lipkowski, Fall Meeting of the Electrochemical Society Meeting, San Diego, CA, October 19-24, 1986, Extended Abstracts, Vol. 86-2, the Electrochemical Society, Princeton, NJ, 1986, Abstract No. 479. 14 A. Wieckowski, S.D. Rosasco, G.N. Salaita, A. Hubbard, B.E. Bent, F. Zaera, D. Godbey and G.A. Somojai, J. Am. Chem. Sot., 107 (1985) 5910.
J. Electroanal. Chem., 230 (1987) 221-231 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
THE IN-SITU PREPARATION ELECTRODES
D. ZURAWSKI, Department
L. RICE, M. HOURANI
of Chemistry,
OF WELGDEFINED,
and A. WIECKOWSKI
SINGLE CRYSTAL
l
Univers@y of Illinois, 1209 W. California Si., Urbana, IL 61801 (U.S.A.)
(Received 18th November 1986; in revised form 3rd February 1987)
ABSTRACT A new method of single crystal preparation for electrochemical application is described in this report. This method does not require the annealing-quenching procedure or ultra-high vacuum methodology. The crystal is armealed and cooled in iodine vapor, then transferred to an electrochemical cell where the chemisorbed iodine is replaced by carbon monoxide present in the electrolyte. The carbon monoxide is subsequently electrooxidized from the surface, leaving the clean, ordered surface exposed to the electrolyte. Thus, the crystal is obtained in situ. The ordered crystal is not subject to the strains imposed by quenching and at no time is the unprotected surface exposed to the atmosphere. The voltammetric evidence presented shows that this procedure is successful in the characterization of a clean and adsorbate-covered Pt (111) surface.
INTRODUCTION
The quality of single crystal electrochemistry results depends on the cleanliness and order of the crystal surface. Thus the crystal preparation technique is an integral part of the experimentation. The presently used techniques are ultra-high vacuum annealing and cleaning [l-3] and the annealing-quenching procedure proposed by Clavilier [4,5]. The annealing-quenching method has been used in many works; however, repeated quenching induces strains in the crystal which may cause polygonization [6]. There is an urgent need for a reproducible, non-destructive single crystal preparation technique not requiring ultra-high vacuum methodology, which is not necessary for many electrochemical applications. The method outlined in this paper fits these requirements. The background for this method was established during our work on lateral modification in platinum electrocatalysis [7], which included studies on the reactivity of coadsorbed species on polycrystalline platinum. We have found that it is
* To whom correspondence should be addressed.
0022-0728/87/$03.50
0 1987 Elsevier Sequoia S.A.
222
possible to replace iodine chemisorbed on platinum by carbon monoxide present in an iodine-free, CO-saturated perchloric acid electrolyte. Evidence for the quantitative replacement of I by CO will be provided in this work. The procedure of cleaning and ordering Pt (111) and Pt (100) single crystals in an iodine-containing atmosphere has already been developed [8,9]. In particular, it has been established that the Pt (111) (J’r X fi)-I, t9i = 0.43 is amenable to identification by the use of its characteristic voltarnrnogram for silver underpotential electrodeposition. As reported in ref. 9, “the fi pattern obtained by heating (the surface resulting from annealing platinum in I, vapor) was strikingly sharp and stable even when compared with the well-resolved fi pattern obtained by vacuum dosing”. As indicated by Auger spectroscopy, the surfaces heated in iodine were virtually contamination-free. In the proposed method, the crystal is annealed and slowly cooled in iodine vapor [8,9]; thus it is not subject to the strains imposed by quenching and it is well-protected from contamination during cooling. The iodine-coated crystal is transferred to the electrochemical cell where the iodine is replaced by carbon monoxide present in the electrolyte. The carbon monoxide is subsequently electrooxidized from the surface, leaving the clean, ordered crystal exposed to the electrolyte. Thus, the crystal surface is obtained in situ. At no time is the annealed crystal exposed to the atmosphere without protection, eliminating all possibility of contamination and oxidation, which can disorder the surface. EXPERIMENTAL
Apparatus
The platinum single crystal (Aremco Products, Inc.) was oriented and polished to produce a Pt (111) surface of 9 mm diameter (the “working surface”). Proper orientation of the crystal was verified by X-ray diffraction. The single crystal and its holder are shown in Fig. 1. Two platinum wires of 0.5 mm diameter were spot welded to the back of the electrode; these wires were used for both heating the crystal during the annealing procedure and for electrical contact during the electrochemical experiments. An ac of 6-8 A was passed through these wires to heat the crystal to 600-700 o C. Crystal temperature was measured by means of a Pt/Pt + 10% Rh thermocouple spot welded to the back of the crystal. These four wires were encased in a quartz piece which fitted into a teflon holder. The quartz piece provided both support for the crystal and protection for the heating wires. The annealing apparatus, shown in Fig. 1, consisted of a glass cell with a nitrogen inlet and a small spoon filled with iodine crystals. The spoon was supported by three glass legs which held it 6-9 mm above the bottom of the cell. This allowed a free flow of nitrogen during the annealing procedure. The Pt crystal was positioned ca. 8 mm above the solid iodine contained in the spoon. A standard two-compartment electrochemical cell was used; one compartment held a Pt wire counter electrode and a Ag/AgCl ([Cl-] = 1 M) reference electrode. The working electrode-electrolyte arrangement is shown in Fig. 2. The cell had two
223
Fig. 1. The single crystal and its holder in the annealing cell. (1) Thermocouple wires (Pt/Pt + 10% Rh). (2) Heating wires (0.5 mm Pt). (3) Nitrogen inlet. (4) Teflon holder. (5) Pt (111) single crystal. (6) Iodine crystals in a glass spoon. Fig. 2. The working electrode-electrolyte arrangement in the electrochemical cell. The electrolyte touches only the “working surface” of the electrode (see text). (1) Thermocouple wires. (2) Heating wires. (3.4) Nitrogen inlets. (5) Pt (111) single crystal. (6) Solution bridge to counter and reference electrodes. (7) Solution outlet. (8,9) Solution inlets. Not shown: nitrogen outlet.
nitrogen inlets: one allowed purging of the electrolyte between experiments and the other provided a protective nitrogen blanket over the solution surface. The electrolyte solutions, held in elevated containers, were delivered to the cell through two inlets and drained through an outlet at the bottom of the cell. Electrochemical
replacement
of chemisorbed
iodine by carbon monoxide
As mentioned earlier, we found it possible to replace iodine chemisorbed on platinum by carbon monoxide present in an iodine-free, CO-saturated perchloric acid electrolyte. The degree of replacement is potential dependent; quantitative replacement is achieved in the hydrogen potential region. In our work with polycrystalline platinum (a platinum wire, Johnson Matthey), iodine was adsorbed at 0.2 V from 1 mM KI in 1 M perchloric acid. The positive-going voltammetric scan, Fig. 3A, was initiated after replacement of the KI-containing electrolyte by clean 1 M HClO,. The chemisorbed iodine is oxidized in a broad peak centered at approximately 1.1 V, as reported previously [lo]. The replacement of chemisorbed iodine by CO was accomplished by exposing the iodine-coated electrode to CO-
-0
2
0.0
0.2
0.4
06
0.6
I.0
1.2
J
1.4
E/V
Fig. 3. (A) Electrooxida&ion of chemisorbed iodine from a polycrystalline platinum electrode in 1 M HClO,. The iodine was adsorbed at 0.2 V from 1 mM KI in 1 M HClO,. The positive-going scan was initiated at 50 mV/s. The dashed trace is the bare Pt voltammogram. (B) The replacement of chemisorbed iodine by carbon monoxide and the subsequent electrooxidation of CO. The iodine was adsorbed as stated above, the potential scanned to -0.2 V, and CO-saturated 1 M HClO, introduced. The positive-going scan was initiated in 1 M HClO, at 50 mV/s. The dashed trace is the bare Pt voltammogram.
saturated electrolyte at -0.2 V. After multiple rinses with the CO-saturated electrolyte, the electrode was rinsed with 1 M HClO, and the positive-going voltammetric scan, Fig. 3B, initiated at 50 mV/s in the clean electrolyte. The complete absence of any oxidation current above the background in the 1.0 to 1.2 V region and the sharp CO electrooxidation peak at 0.6 V indicate quantitative replacement of iodine by carbon monoxide. This quantitative replacement was also seen with a Pt (111) single crystal electrode and was employed in our crystal preparation method. Procedure The following is a description of the procedure used for the preparation of a clean and well-ordered Pt (111) crystal. The oriented and polished single crystal, as well as all the glassware, was cleaned in warm chromic acid prior to each day’s experiments. The whole crystal was subjected to numerous oxidation-reduction voltammetric cycles in 1 M HClO., (ACS reagent grade in 18 MP Millipore water) to clean the crystal and to produce the “disordered” cyclic voltammogram of the Pt
225 -0.2
I
I -0.2
0.0,
0.2
0.4
0.6
0.6
1.0
I
I
I
I
I
I
0.0
Q2
0.4
0.6
0.6
I.0
1.2
I I I.2
E/V
Fig. 4. Cyclic voltammogram of the “disordered” Pt (111) surface obtained after numerous oxidation-reduction cycles. The scan was taken in 1 M HCIO, at 50 mV/s.
(111) working surface, Fig. 4. Cycling was stopped at 1.6 V and the crystal rinsed at open circuit with Millipore water. The clean and oxidized crystal was then transferred via air to the annealing cell, Fig. 1, and placed approximately 8 mm above the iodine surface. We checked that the transfer of such fully oxidized platinum did not result in surface contamination: the air-exposed surface showed a voltammogram identical to that without exposure. These protective properties of the thick Pt oxide film parallel the well-known passive behavior of oxidized platinum in solution. A suitable flow of nitrogen (Linde, oxygen-free) allowed the teflon crystal holder to float free from the sides of the cell. The platinum was heated slowly to 600-700 o C in the iodine-containing nitrogen. After approximately 1 min at this temperature, the heating current was shut off and the crystal allowed to cool to room temperature. Two different iodine-coating methods were employed in this work. In the first program iodine-containing nitrogen flowed during both heating and cooling of the crystal, while in the other method the nitrogen was turned off during cooling of the crystal which allowed iodine vapor to collect in the cell. A visible cloud of iodine vapor surrounded the crystal in this latter approach. As a simple measure of the amount of iodine on the crystal, the hydrophobicity of the working surface was tested by noting visually its ability to repel a drop of water. The extent of the hydrophobicity of the crystal heated and cooled with nitrogen flowing varied between particular experiments while the crystal cooled in an iodine atmosphere was definitely and consistently hydrophobic. We found that the lack of the fully hydrophobic properties of the electrode was well-correlated with the amount of platinum oxide present on the I-covered surface. Since platinum oxidation causes surface disorder, even traces of surface bound oxygen should be avoided. Following the annealing program described above, the iodine-protected surface was safely [8,9] transferred via air to the electrochemical cell containing 1 M HClO,. The potential was then scanned to -0.200 V where CO-saturated 1 M
226
I
I
loOpA
CONSECUTNE
-0.2
0.0
0.2
(x4
0.6
0.8
E/V
Fig. 5. The replacement of chemisorbed iodine by carbon monoxide on the ordered Pt (111) surface and subsequent CO electrooxidation. The replacement took place at -0.2 V from CO-saturated 1 M HClO,. The positive-going voltammetric scan was initiated at 50 mV/s in 1 M HClO,. Consecutive scans were taken in the -0.2 V to 0.2 V region.
HClO, was introduced to the cell. The whole crystal was rinsed repeatedly with the CO-saturated electrolyte to ensure removal of iodine from all surfaces. Once all traces of CO were removed from the electrolyte by rinsing with clean electrolyte, the adsorbed CO was removed from the surface by a positive-going voltammetric scan, Fig. 5, which was reversed at 0.65 V. The resulting iodine and CO-free surface was ready for further characterization. To test the cleanliness and order of the crystal at various stages in the preparation procedure, we employed the underpotential electrodeposition (UPD) of silver on the iodine-coated crystal. The silver UPD pattern is very sensitive to the order of the crystal and to the structure of the iodine adlattice [8,9]. Well-resolved underpotential deposition peaks are indicative of a high degree of crystal surface order. To perform the experiments, the iodine-coated working surface was exposed to 1 mM AgClO, (Johnson Matthey, 99.9%) in 1 M HClO, electrolyte at open circuit. The potential was then scanned in the negative direction at 2 mV/s until the onset of silver bulk deposition. At this point the scan direction was reversed and the scan continued to a potential before iodine oxidation. RESULTS AND DISCUSSION
The surface obtained by heating and cooling the crystal in an iodine-containing nitrogen atmosphere was subjected to characterization by silver underpotential electrodeposition; the results are shown in Fig. 6. Comparison of this result with the UHV-characterized results of Wieckowski et al. (Fig. 6A of ref. 9) shows that a voltammogram charcteristic of the fi I-structure was obtained and proves that the
227 0,2
0.4 I
08 I
0.6 I
1.0 I
1.2 I
1.4
L 1:; \t 0.4 I
T
0.6 I
0.6 I
I.0
I.2
I
I
t
START AND
END OF SCAN
Y
START AND END OF SCAN
I
I
4pA
0.2
I 04
0.6
I
4P
I
I
I
J
0.6
I.0
I,2
I.4
!
I
1
I
I
I
0.4
On6
0.6
I.0
I.2
E/V
E/V
. Fig. 6. The underpotential deposition of silver on the iodine-coated P! (111) crystal (J’r X n-1 adlattice). The crystal was annealed and cooled in iodine-containing nitrogen. The scan was taken in 1 mM AgClO, in 1 M HClO, at 2 mV/s. Fig. 7. The underpotential deposition of silver on the iodine-coated Pt (111) crystal. The iodine was adsorbed at 0.2 V from 1 mM KI in 1 A4 HClO, and the scan taken in 1 mM AgClO, in 1 M HClO, at 2 mV/s. See text.
surface is clean and ordered. It was found that a good fi voltammogram was obtained only when the surface was clearly hydrophobic. Silver underpotential electrodeposition was employed again to determine whether the replacement of iodine by carbon monoxide and subsequent CO electrooxidation disordered the surface. The surface obtained after the CO oxidation was immersed in 1 mM KI in 1 M perchloric acid, rinsed, and exposed to the Ag+ electrolyte. The resulting voltammogram, Fig. 7, is essentially the same as the one obtained before the replacement-oxidation, proving that neither process disordered the surface. The alternative method of iodine-coating, cooling the annealed crystal in an iodine-saturated atmosphere, produced a surface with the silver underpotential deposition pattern shown in Fig. 8. This result corresponds well with the 3 X 3-I results obtained previously (Fig. 6B of ref. 9). Carbon monoxide replaces the 3 x 3-I structure as well as the n-1 structure; this was verified in separate experiments. We found that the 3 x 3 iodine adlattice provided better protection of the Pt surface than the fi X fi adlattice, therefore this iodine-coating procedure
0.4
0.4
06
0.8
1.0
0.6
0.8
1.0
E/V
Fig. 8. The underpotential deposition of silver on the iodin~coated Pt (111) crystal (3 X3-I The annealed crystal was cooled in an iodine-saturated atmosphere. The scan was taken AgClO, in 1 M HClO, at 2 mV/s.
-,
0 ,
0.2 I
0.4 I
0.6 I
0.8 1
1.0 I
1.2 1
1.0
1.2
adlattice). in 1 mM
(
=l
3.
5
B 3
,
I
I
I
I
I
0
0.2
0.4
0.6
0.8
E/V
Fig. 9. Cyclic voltammograms scans were taken immediately below. Scan rate: 50 mV/s.
of the clean, ordered Pt (111) single crystal in 1 M HC104. The top two after electrooxidation of CO from the surface. Subsequent scans are shown
229
W I I I I START
0.2
0.4
0.6
0.8
1.0
E/V
Fig. 10. The underpotential deposition of silver on the clean, ordered Pt (111) surface. The scan was taken iu 1 mM AgClO, in 1 M HClO., at 5 mV/s.
was used in all subsequent experiments and is recommended for in situ work with Pt single crystals. The voltammogram of the clean, well-ordered Pt (111) surface obtained after the annealing-replacement-oxidation procedure is shown in Fig. 9. Upon oxidation of the carbon monoxide from the surface in the positive-going scan, the clean, well-ordered Pt (111) surface became exposed to the 1 M perchloric acid electrolyte. The scan direction was then reversed and one voltammetric cycle (0.65 V to -0.20 V to 0.65 v) taken to remove possible traces of CO, still present in the interfacial region following CO electrooxidation. The subsequent first and second scans as well as further scans are shown in Fig. 9. The similarity between our data and those obtained with other preparation techniques [4-6,111 (see also papers cited in refs. 6 and 11) is clearly visible. To test the quality of our single crystal preparation method, we studied three additional systems which have been characterized by other workers using the crystal preparation methods mentioned earlier [l-5]. The systems are: the underpotential deposition of silver on Pt (111) [12], the oxidation of adsorbed carbon monoxide [13], and the oxidation of adsorbed ethylene [14]. Following the electrooxidation of carbon monoxide from the ordered surface, the circuit was opened, the electrolyte drained, and 1 mM AgClO, in 1 M HClO, introduced to the Pt surface. The negative-going voltammetric scan was initiated at 5 mV/s and the Ag underpotential deposition voltammogram, Fig. 10, recorded. This result compares favorably with the voltammogram obtained by Omar et al. (Fig. 1 of ref. 12) using the annealing-quenching preparation method [4]. The electrooxidation of carbon monoxide, adsorbed on the clean, ordered Pt (111) crystal is shown in Fig. 11. The carbon monoxide was adsorbed at 0.1 V from
230
-0.2
0.0
0.2
0.4
0.6
0.6
1.0
1.2
1.4
I
10~A
POTENTIAL CF ADSORPTION I
I -0.2
I 0.0
I 0.2 E/V
I OA
I 0.6
I
I
I
I
I
I
I
I
-0.2
a0
0 2
0.4
0 6
0.6
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1.4
Fig. 11. Electrooxidation of carbon monoxide adsorbed on the ordered Pt (111) surface. The CO was adsorbed at 0.1 V from CO-saturated 1 M HClO,. The positive-going scan was initiated at 5 mV/s in 1 M HClO,. Fig. 12. Electrooxidation of ethylene adsorbed on the ordered Pt (111) surface. The ethylene was adsorbed at 0.35 V from CIH,-saturated 1 M HClO,. The positive-going scan was initiated at 10 mV/s in 1 M HClO,.
CO-saturated electrolyte and the positive-going scan initiated in clean 1 M HClO,. The sharpness of the single electrooxidation peak is indicative of a well-ordered surface. A similar voltammogram was obtained by Lipkowski et al. [13] for CO adsorbed on an annealed-quenched Pt (111) crystal. The additional test of the quality of our preparation method was a comparison of our results with those obtained using a crystal cleaned and characterized in UHV. This comparison was favorable for the underpotential deposition of silver on the iodine-coated surface (Figs. 6 and 8). A study of the electrooxidation of ethylene adsorbed on a UHV-prepared Pt (111) surface was conducted earlier [14]. To simulate the UHV conditions, ethylene was adsorbed on our surface at 0.35 V from a C,H,-saturated electrolyte solution, as in ref. 14. The ethylene-containing electrolyte was replaced with clean 1 M HClO, and the positive-going voltammetric scan, Fig. 12, initiated at 10 mV/s. The voltammogram obtained with our crystal is nearly identical to that obtained with the UHV-prepared crystal (Fig. 6C of ref. 14).
ACKNOWLEDGEMENT
The support of this work by Dow Chemical Illinois is acknowledged.
USA
and by the University
of
231 REFERENCES 1 A.T. Hubbard, Act. Chem. Res., 13 (1980) 177. 2 E. Yeager, J. Electrochem. Sot., 128 (1981) 160C. 3 P.N. Ross and F.T. Wagner in H. Gerischer (Ed.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 13, Wiley, New York, 1984, pp. 69-112. 4 J. Clavilier, R. Fanre, G. Guinet and R. Durand, J. Electroanal. Chem., 107 (1980) 205. 5 J. Clavilier, J. Electroanal. Chem., 107 (1980) 211. 6 D. Aberdam, R. Durand, R. Faure and F. El-Omar, Surf. Sci., 171 (1986) 303. 7 D. Zurawski, K. Chan and A. Wieckowski, J. Electroanal. Chem., 210 (1986) 315. 8 A. Wieckowski, S.D. Rosasaco, B.C. Schardt, J.L.’ Stickney and A.T. Hubbard, Inorg. Chem., 23 (1984) 565. 9 A. Wieckowski, B.C. Schardt, S.D. Rosasco, J.L. Stickney and A.T. Hubbard, Surf. Sci., 146 (1984) 115. 10 R.F. Lane and A.T. Hubbard, J. Phys. Chem., 79 (8) (1975) 808. 11 K. Al Jaaf-Golze, D.M. Kolb and D. Scherson, J. Electroanal. Chem., 200 (1986) 353. 12 F. El-Omar, R. Durand and R. Faure, J. Electroanal. Chem., 160 (1984) 385. 13 B. Love, J. Richer and J. Lipkowski, Fall Meeting of the Electrochemical Society Meeting, San Diego, CA, October 19-24, 1986, Extended Abstracts, Vol. 86-2, the Electrochemical Society, Princeton, NJ, 1986, Abstract No. 479. 14 A. Wieckowski, S.D. Rosasco, G.N. Salaita, A. Hubbard, B.E. Bent, F. Zaera, D. Godbey and G.A. Somojai, J. Am. Chem. Sot., 107 (1985) 5910.