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Initial stages of thallium electrodeposition on iodine-covered Pt(1 1 1)
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 567 (2004) 185–192 www.elsevier.com/locate/jelechem
Initial stages of thallium electrodeposition on iodine-covered Pt(1 1 1) Miguel Labayen 1, David A. Harrington
*
Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6 Received 10 September 2003; received in revised form 10 December 2003; accepted 21 December 2003 Available online 28 February 2004
Abstract pffiffiffi pffiffiffi The initial stages for Tl electrodeposition on Pt(1 1 1) covered with iodine in ( 7 7)R19.1° or (3 3) structures are investigated by cyclic voltammetry, low energy electron diffraction and Auger electron spectroscopy. Estimation of faradaic charges in cyclic voltammetry is facilitated by simultaneous measurement of the double-layer capacitance by ac voltammetry. The first electrodeposition peak corresponds to the deposition of 0.22 pffiffiML ffi pTl ffiffiffi atoms, independently of the structure of the initial iodine adlayer. However, after the electrodeposition of Tl on Pt(1 1 1) ( 7 7)R19.1°-I, the surface undergoes an irreversible rearrangement to a Pt(1 1 1) ½ 43 06 -TlI structure, with two Tl atoms and six I atoms per unit cell. In this rearrangement, thallium and iodine are removed from the surface in a 1:1 ratio. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Underpotential; Electrodeposition; Platinum; Thallium; Iodine; LEED; Auger electron spectroscopy; Ac voltammetry
1. Introduction Underpotential deposition (upd) produces intrinsically interesting monolayers that are thermodynamically stable with respect to the bulk structure. Thallium is known to exhibit upd behavior on Pt(1 1 1) [1–3]. At the most negative potentials, the Tl upd structure formed is a hexagonal close-packed monolayer. However, coad sorbed anions such as HSO 4 and ClO4 have an impact in the Tl upd structure, particularly at low coverages [4,5]. Only a preliminary study about the influence of the halides Cl , Br and I during the electrodeposition of Tl on Pt(1 1 1) has been reported [3]. We report here a detailed study for the electrodeposition of the first stages of on pffiffiffiTl p ffiffiffi Pt(1 1 1), initially covered with iodine in ( 7 7)R19.1°, h ¼ 0:43 ML or (3 3), h ¼ 0:44 ML surface structures, using electrochemical techniques, low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Our results show that Tl upd on Pt(1 1 1)(3 3)-I is a reversible process which produces a *
Corresponding author. Tel.: +1-2507217166; fax: +1-2507217147. E-mail address: [email protected] (D.A. Harrington). 1 Present address: Institut fuer Experimentelle und Angewandte Physik, Leibnizstrasse 19, Universitaet Kiel, Germany. 0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.12.023
variety of surface The first stages of Tl upd pffiffiffi structures. pffiffiffi on Pt(1 1 1) ( 7 7)R19.1°-I are similar to those for Pt(1 1 1)(3 3)-I. However, the surface structure then evolves irreversibly with desorption of Tl and I atoms, producing new features in the voltammogram.
2. Experimental Pt(1 1 1) single crystals were cut with a diamond-wafering saw from a single-crystal boule (1 cm diameter) grown by Metal Oxides and Crystals Ltd. (99.999%). The surfaces were polished with successive grades of diamond paste (Beuhler Ltd.), and oriented within 0.5° of the (1 1 1) plane by X-ray Laue back diffraction. After extensive annealing, high quality surfaces were produced, as judged by reproducing the literature voltammograms in H2 SO4 and HClO4 [6,7]. Before the experiments, all glassware was cleaned with hot chromic or sulfuric acid and then rinsed with Millipore Milli-Q Water. The solutions were made from reagents as purchased (HClO4 BDH, 60%, AnalaR; H2 SO4 Seastar Suprapure, 98% and Tl2 CO3 Alfa AESAR 99.999%), and were degassed for at least 15 min with argon prior to the experiment. The electrochemical
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experiments were carried out by putting the crystal into contact with the solution at a controlled potential, using the hanging meniscus method. The counter electrode was a platinum mesh cleaned with chromic acid prior to the experiment. Either the hydrogen electrode in the same solution (RHE) or hydrogen-charged Pd (+50 mV vs. RHE) was used as the reference electrode, although all potentials are reported vs. RHE. Unless otherwise stated, the peak potentials in cyclic voltammetry were calculated by extrapolating to 0 mV s1 from data at sweep rates in the range 5 to 100 mV s1 . The Pt(1 1 1) surfaces were prepared under either atmospheric or ultra-high vacuum (UHV) conditions. For experiments in which the crystals were cleaned and the iodine deposited at atmospheric pressure, the crystals were flame annealed for 1 min. The (3 3)-I structure was prepared by immersion pffiffiffi pffiffiof ffi the crystal in 1 mM KI solution [8,9]. The ( 7 7)R19.1° iodine monolayer was prepared by cooling the hot crystal in iodine vapor. These protocols were the same as used in an earlier study of silver electrodeposition, and their validity is discussed further in that work [10]. Another set of experiments involved preparation of the crystal surface under UHV conditions. The UHV system in our laboratory has facilities for LEED, AES, thermal desorption spectroscopy (TDS), work function changes, and contamination-free transfer between the UHV and electrochemical environments. I2 was dosed onto the surface using a custom-built doser based on a solid-state electrochemical cell [11]; the detailed procedures for preparing the clean and iodine-covered surfaces have been described elsewhere [12]. The crystal was transferred to the electrochemical chamber and the electrochemical experiment carried out in a custommade electrochemical cell [13]. Following the electrodeposition, the crystal was transferred to a chamber under UHV conditions, and after a few minutes the surface analysis measurements were performed. The pressure during these measurements was never above 5 109 mbar. The cleanliness of the transfer was checked with AES for the presence of impurities on the surface. Measurements of the faradaic charge required accurate correction for the double-layer charging. This was achieved by measuring single-frequency ac voltammograms simultaneously with the dc cyclic voltammograms. Since the cyclic voltammetry peaks for these surface reactions are very narrow, it is essential to use very small amplitude measurements in order for the ac amplitude to be a small perturbation on the reaction, and all measurements reported here were carried out using amplitudes of 0.5 or 0.033 mV rms. A frequency of 1 or 2 kHz was used, and the current and potential ac signals were measured using separate, synchronized lock-in amplifiers (Perkin–Elmer 7265). The potential was measured with the 10 MX differential input of one
lock-in amplifier directly between the reference and working electrodes. The current was similarly measured differentially as a proportional voltage across a measuring resistor, rather than using the potentiostatÕs current-to-voltage converter. In this way, phase or amplitude errors introduced by the potentiostat were corrected for. The double layer capacitance, Cdl , was obtained from the imaginary part of the measured impedance using Cdl ¼ ðxImðZÞÞ1 . This calculation assumes that the equivalent circuit approximates the uncompensated solution resistance in series with Cdl . This was verified in a preliminary set of ac voltammetry experiments over the frequency range 3 Hz to 200 kHz [14]. The validity of these types of experiments has been discussed previously [15]. The equivalent circuit was found to be a faradaic resistor in parallel to the double-layer capacitor, all in series with the solution resistor. At the single frequency used, Cdl agreed within 10% with the Cdl obtained from the full equivalent circuit analysis.
3. Results 3.1. Voltammetry and iodine loss at 0.05 V The voltammogram of thallium upd on (3 3)-I in H2 SO4 is shown in Fig. 1. It is similar to the voltammogram for Tl deposition in I solution [3]. It shows four cathodic peaks (C1–C4) at potentials +0.399, +0.324, +0.160 and +0.086 V, respectively, and the corresponding anodic peaks. This voltammogram was stable to cycling.
Fig. 1. Cyclic voltammogram for Tl upd on Pt(1 1 1)(3 3)-I in 0.1 M H2 SO4 . [Tlþ ] ¼ 1 mM, sweep rate 20 mV s1 . Inner curve is the doublelayer charging current measured with ac voltammetry, 0.5 mV rms, 2000 Hz.
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pffiffiffi pffiffiffiThe voltammogram of thallium upd on ( 7 7)R19.1°-I in H2 SO4 is shown in Fig. 2. The first sweep of the voltammogram was similar in either H2 SO4 or HClO4 . The main features of the first reduction sweep are similar to the first sweep of the voltammogram of (3 3)-I in Tlþ solution (Fig. 1), showing the same four cathodic peaks (C1–C4), but at slightly different peak potentials: +0.408, 0.329, 0.176 and 0.080 V, respectively. Holding the potential after sweeping part way into these peaks did not show any signs of increasing current, which would be diagnostic for nucleationgrowth-collision (NGC) mechanisms. After the first sweep reversal, the anodic features are already quite different from those in the (3 3)-I, and do not simply reflect reversal of the corresponding cathodic peaks. The peaks continue to evolve with time, and after cycling for at least 30 min, the features of the cyclic voltammogram (Fig. 2(d)) were the same as for the deposition of thallium on unreconstructed Pt(1 1 1) in the absence of iodine, as previously reported [1,4]. pffiffiffi Thus, pffiffiffi although the initial iodine coverages in the ( 7 7)R19.1°-I and (3 3)-I are almost the same (0.43 and 0.44 ML) and the initial cathodic peaks are similar, the stabilities of the upd Tl structures formed are very different: the surfaces formed from (3 3)-I are stable toward cycling, but the
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pffiffiffi pffiffiffi surfaces formed from ( 7 7)R19.1°-I lose iodine readily. In order to estimate the charge required to convert the iodine-covered surface to the thallium-covered, iodine-free surface, a sweep-hold experiment pffiffiffi pwas ffiffiffi undertaken. Starting with the Pt(1 1 1( 7 7)R19.1°-I surface at +0.9 V, the potential was swept in the negative direction down to +0.05 V, and then held at this potential for 3 min until the iodine was completely removed from the surface, Fig. 3. Subsequent cycles showed the upd of thallium on bare Pt(1 1 1), similarly to the voltammogram in Fig. 2(d). Integration of the current transient in Fig. 3(a) corrected for the double layer charging, gave 270 lC cm2 , the equivalent of 1.12 ML of electrons. The double-layer capacitance rises from a value of about 20 lF cm2 before the Tl deposition to values of 75–200 lF cm2 in the region of peaks C1–C4. Traditionally, it is assumed that Cdl ranges from 10 to 40 lF cm2 on Pt surfaces. However, high values of Cdl have also been reported for systems such as hydrogen upd on Pt(1 1 1) in neutral solution [16], or Tl upd on Ag(1 0 0) and Ag(1 1 0) [17].
3.2. Peak C1 and surface rearrangement at 0.4 V pffiffiffi pffiffiffi Although all iodine is lost from the ( 7 7)R19.1°-I surface at 0.05 V, it is possible to form a stable surface without complete iodine loss by restricting the potential to the region including peak C1 and pffiffiffi above, pffiffiffi that is above about 0.4 V. Starting from a ( 7 7)R19.1°-I structure, the cyclic voltammogram with reversal after the first deposition peak C1 is shown in Fig. 4. Peak C1 loses intensity with cycling, and a new pair of peaks, C11 and C12, appears at +0.608 and +0.544 V, respectively. These potentials were calculated by extrapolation of the peak
pffiffiffi pffiffiffi Fig. 2. Cyclic voltammograms for Tl upd on Pt(1 1 1)( 7 7)R19.1°-I þ 1 in 0.1 M H2 SO4 , [Tl ] ¼ 1 mM, sweep rate 20 mV s . (a) 1st cycle, (b) 2nd cycle, (c) 5th cycle and (d) 50th cycle. Arrows indicate the time evolution of the features.
pffiffiffi pffiffiffi Fig. 3. Sweep hold experiment for Tl upd on Pt(1 1 1)( 7 7)R19.1°-I þ in 0.1 M H2 SO4 , [Tl ] ¼ 1 mM. Potential swept at 20 mV s1 from 0.9 V down to 0.05 V, and then (at time zero, vertical dotted line) held at 0.05 V. Dotted curve is double-layer charging current measured by ac voltammetry, 0.5 mV rms, 2000 Hz.
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pffiffiffi Fig. C1 for Tl upd on Pt(1 1 1)( 7 p ffiffiffi 4. Successive cycles around peak 7)R19.1°-I in 0.1 M H2 SO4 , [Tlþ ] ¼ 1 mM, sweep rate 100 mV s1 . Lower potential limit 0.37 V.
potentials to 0 mV s1 for sweep rates from 5 to 100 mV s1 , after the peaks were fully developed. The peaks C11 and C12 did not show NGC behavior, and showed only insignificant differences in peak potentials (<4 mV) in HClO4 rather than H2 SO4 . Charge measurements were made to quantitate the C1 peak and the rearrangement to C11 and C12. The mea2 sured charge under peak C1 is 54 lC cm pffiffiffi for pffiffithe ffi (3 3)2 I surface and 51 lC cm for the ( 7 7) R19.1°-I surface, i.e., these are equal within the measurement error and equivalent to 0.22 ML. If the potential is held at +0.4 V, just after the C1 peak, then for the (3 3)-I surface there is no significant further charge passed. This is consistent with the fact that there is rearpffiffino ffi further pffiffiffi rangement on this surface. For the ( 7 7)R19.1°-I surface, however, a further 14 lC cm2 passes as the potential is held at 0.4 V for 1 min, giving a total charge of 65 lC cm2 (equivalent to 0.27 ML). Subsequent to this hold period, the surface rearrangement is complete and the subsequent voltammetry cycle shows the fully developed C11 and C12 peaks (Fig. 5). 3.3. Rearranged surface pffiffiffi pffiffiffi The ( 7 7)R19.1°-I surface after Tl deposition by holding the potential at 0.4 V or cycling in the C1 region and above will be referred to as the rearranged surface. It was investigated by voltammetry, charge measurements, AES and LEED. The voltammogram of the rearranged surface (Fig. 5(b)) can be divided into three distinct regions, A–C. The narrowness and reversibility of the peaks C11 and C12 imply that these peaks are due to surface phase transitions, and that regions A–C correspond to distinct surface structures.
pffiffiffi Fig. p ffiffiffi 5. (a) Sweep-hold experiment for Tl upd on Pt(1 1 1)( 7 7)R19.1°-I in 0.1 M H2 SO4 around peak C1, (b) Subsequent cycles after holding. Dotted curve is double-layer charging current. Constancy of the solution resistance Rs shows that the double-layer estimate is valid. [Tlþ ] ¼ 1 mM, sweep rates 20 mV s1 .
3.3.1. Thallium coverage Integration of the deposition peaks after subtraction of the double layer charging gave charges of 11 lC cm2 (0.045 ML of Tl) for peak C11, and 8 lC cm2 (0.033 ML of Tl) for peak C12. Here the assumption is made that all the faradaic charge gives Tl, with one Tl atom per electron. The total Tl coverage deposited under peaks C11 and C12 is therefore 0.078 ML. (Linear or flat double-layer charging baselines increase the estimated coverage by 25% or 50%, respectively.) These coverages were confirmed using AES (Fig. 6), using as a reference the intensity of the Tl peak at 89 eV at the known coverage of 0.66 ML (+0.05 V in the absence of iodine, e.g., leftmost potential in Fig. 2(d)) [1,4]. AES did not show the presence of Tl on the surface in region A of the voltammogram. The Tl coverage calculated from AES in region B of the voltammogram was 0.039 ML, and in region C was 0.087 ML. Since the errors in
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oxidation at +1.4 V includes the charge for the oxidation of platinum to platinum oxide. After the oxidation at +1.4 V, the potential was swept down to +0.4 V and the charge to reduce the Pt oxide was measured. This oxide charge was subtracted from the total oxidation charge to give the charge for iodine oxidation, which was found to be 330 lC cm2 . The iodine oxidation is a five-electron process according to Eq. (1): þ IðadsÞ þ 3H2 OðlÞ ! IO 3 ðaqÞ þ 6H ðaqÞ þ 5e
ð1Þ
so that the iodine coverage on the rearranged surface was 0.27 ML. This methodology was checkedpby ffiffiffi meapffiffiffi suring the iodine coverage on the initial ( 7 7) R19.1°-I surface, which gave 520 lC cm2 , or 0.43 ML, as expected for the ideal surface structure.
Fig. 6. Auger electron spectra of the rearranged surface emersed at potentials in regions A–C. Spectrum X is the reference spectrum of an iodine-free surface with 0.66 ML Tl. Beam energy 3 keV. Modulation 10 V pp at 4.7 kHz. Spectra normalized to a beam current of 10 lA.
AES coverages are larger than those from charge measurements, the coverages from the charges are used in subsequent calculations. 3.3.2. Iodine coverage The desorption of iodine from the surface was independently assessed by charge measurements of iodine oxidation. Oxidation of adsorbed iodine to IO 3 (aq) occurs at +1.3 V under the conditions studied. Oxidation of Tlþ (aq) to Tl3þ (aq) starts at +1.2 V. Therefore, owing to the overlap of the oxidation peaks, the oxidation of iodine was carried out after transferring the electrode to a different electrochemical cell, which had electrolyte but not Tlþ (aq). Emersion and immersion from one cell to another was done at a potential of +0.9 V, where there is no Tl adsorbed on the surface (Region A in Fig. 5(b)). The total charge passed during iodine
3.3.3. Surface structures by LEED The surface structures in the three regions A–C of the voltammogram in Fig. 5(b) were determined using LEED. The open-circuit potential was measured in order to ensure that the surface structure did not change during the transfer from atmospheric to UHV conditions. However, there was a gradual change of the open potential, which stabilized at +1.15 V, signaling a potential difficulty with the transfer. The shift of the open-circuit potential was slow, taking of the order of 5 min to shift from +0.4 to +1.15 V. Therefore the time for emersion and initial pumpdown was minimized, though total pumpdown and transfer time was still 10–15 min. Emersion experiments were carried out at +400, +388 and +385 mV (region C), +590, +592 and +575 mV (region B), and +815 and +727 mV (region A). Since the LEED diffraction patterns were consistent within each region, and different between regions, it is reasonable to assume that the emersion procedure was fast enough that the emersed structures were the same as those existing at the emersion potential. AES showed that Tl was retained during the pumpdown process, and no traces of electrolyte were detected. This suggests that the analyzed surface was the same as the in situ surface, though structural rearrangements cannot be rigorously excluded.
Fig. 7. LEED patterns at (a) 35 and (b) 70 eV of the rearranged surface in region C.
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Fig. 8. Computer simulation of the kinematic LEED pattern for a structure on a fcc(1 1 1) surface showing the three domains of the unit cell, (a) for no special symmetry of the unit cell, (b) for p2gg symmetry of the unit cell showing the systematic absences (circles).
The LEED pattern for region C (Fig. 7) shows 12 triangles of three closely spaced spots spaced at 30° intervals around the pattern. This indicates a square or rectangular unit cell structure, lying on the substrate (which has threefold symmetry) in three rotational domains. Spot measurements and the computer pattern simulator LEEDpat v 1.0 [18] were used to show that the 4 0 structure has a ½ 3 6 unit cell. The simulated LEED pattern for this unit cell (Fig. 8(a)) correctly shows the 12 triangles, and larger inverted triangles between every second triangle. However, a rectangle of spots with a fifth spot at its centre is expected just inside the inverted triangles and the observed pattern fails to show the fifth spot (Figs. 7(b) and 8(b)). This corresponds to systematic absences (h; 0), h odd and (0; k), k odd, which implies a p2gg symmetry of the superlattice, for which the simulated pattern (Fig. 8(b)) agrees with the observed pattern. LEED pattern pffiffiin ffi region pffiffiffi B (Fig. 9) was a mixed pffiffiThe ffi p ffiffiffi ( 7 7)R19.1° + ( 3 3)R30° pattern, indicating patches of the two surface structures. LEED for region pffiffiffi p ffiffiffi A showed a simple ( 7 7)R19.1° pattern.
4. Discussion 4.1. Iodine loss The most noteworthy feature of the Tl deposition is the marked difference in reactivity of the two iodine surfaces: pffiffiffi pffiffithe ffi (3 3)-I surface is stable to cycling, but the ( 7 7)R19.1°-I loses iodine, and at the most negative potentials all iodine is removed from the surface. The loss of iodine appears to occur concomitantly with the thallium deposition, since the pattern of reduction peaks is the same on both surfaces, with the exception of small changes in peak potentials. That is, no separate peaks can be identified with the iodine loss. This is also seen in the sweep-hold data of Fig. 3, where the additional charge passed in the hold part of the experiment is small and cannot account for the iodine loss. The 1.12 ML charge passed in this sweep-hold experiment may be as follows: The initial state pffiffiinterpreted ffi pffiffiffi is the Pt(1 1 1)( 7 7)R19.1°-I surface without thallium and the final state has no iodine, but has electrodeposited Tl. The premoval ffiffiffi pffiffiffi of the 0.43 ML I atoms existing on the ( 7 7)R19.1°-I surface requires consumption of 0.43 ML electrons, assuming that it is lost as iodide ions (Eq. (2)). The remaining reduction charge must therefore be due to the Tl upd reaction, Eq. (3), IðadsÞ þ e ! I ðaqÞ
ð2Þ
so that the Tl coverage may be estimated as 1.12–0.43 ¼ 0.69 ML. Tlþ ðaqÞ þ e ! TlðadsÞ
Fig. 9. LEED pattern at 73 eV of the rearranged surface in region B.
ð3Þ
This calculation of the Tl coverage at +0.05 V agrees with the saturation 0.66 ML thallium measured at this potential by in situ X-ray diffraction experiments of Tl upd on iodine-free Pt(1 1 1) [3,4]. The agreement shows
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that the double-layer correction procedure used in this work is reliable and gives coverages accurate to about 5%. A flat double-layer charging assumption would have increased the measured charge to 1.18 ML, and the estimated Tl coverage would have been 0.75 ML, 14% away from the literature value. reactivity of the (3 3)-I and pffiffiThe ffi pffiffidifferent ffi ( 7 7)R19.1°-I surfaces toward Tl electrodeposition is in sharp contrast to their behavior toward Ag electrodeposition. In that case, the initial deposition structures are the same, and neither surface exhibits iodine loss after deposition [19]. The coverage of the iodine on the two surfaces is essentially identical (ideal coverages of 0.43 and 0.44 ML). Furthermore, the partial charges of the iodine atoms are the same on the two surfaces, since it is possible in a UHV experiment pffiffiffi ptoffiffiffi heat the surface through the (3 3)-I to ( 7 7)R19.1°-I phase transition without observing any measurable change in work function [20]. One possibility is that the different types of iodine atoms on the surface have different reactivity because of their different bond energies. the known structures pffiffiffi pffiffiFrom ffi of the (3 3)-I and ( 7 7)R19.1°-I surfaces [12] and thermal desorption studies [20,21], the strength of the iodine–platinum interaction increases in the following sequence of sites: twofold, atop, hcp threefold and fcc threefold. Since the twopmost bonded iodine ffiffiffi pstrongly ffiffiffi types occur only on the ( 7 7)R19.1°-I surface, this surface would be expected to be the least reactive, in contrast to observation. We suggest that the (3 3) arrangement of iodine atoms is closer to the arrangement required in the TlI structure, so that a type of templatingpeffect ffiffiffi pffiffifacilitates ffi deposition into this structure. For the ( 7 7)R19.1°I surface, more extensive rearrangement of the iodine atoms is required, and this initiates iodine loss. In support of this, kinetic studies of silver upd on these two surfaces show is more facile on (3 3)-I pffiffiffithatpdeposition ffiffiffi than on ( 7 7)R19.1°-I [22], though the final structures are probably different from those in the thallium case. pffiffiffi pffiffiffi The rearrangement of the ( 7 7)R19.1°-I surface formed by Tl deposition at 0.4 V or by cycling in the C1 region and above shows partial iodine loss. Conceptually, this process can be considered in two stages: (1) deposition of 0.27 ML Tl (from sweep-hold charge measurements) without loss of any of the 0.43 ML iodine, and (2) loss of Tl and I to give the rearranged surface with 0.08 ML Tl and 0.27 ML iodine. Interestingly, the amounts of Tl and I lost are nearly the same (0.19 and 0.16 ML, respectively). 2 The above process could be concerted rather than occur in two stages, but 2
If the iodine coverage of 0.25 ML corresponding to the ideal structure deduced by LEED (see below) is used rather than the 0.27 ML measured by iodine oxidation, then the iodine loss is 0.18 ML.
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the 1:1 stoichiometry of the loss suggests that each iodine might leave the surface accompanied by a thallium atom. 4.2. Surface structures The experiments show that the surface structure on the rearranged surface at 0.4 V (region C, Fig. 5) has a 4 0 ½ 3 6 unit cell with p2gg symmetry, Tl coverage of 0.08 ML and I coverage of 0.27 ML. There are 24 Pt surfacelayer atoms in this unit cell and therefore there must be 0:08 24 ¼ 1:9 2 Tl atoms and 0:27 24 ¼ 6:5 6 or 7 I atoms in the unit cell. The locations of the twofold rotation axes and glide planes within the unit cell are shown in Fig. 10, together with the probable structure. The p2gg symmetry has two types of special positions (both on twofold rotation axes) each of multiplicity two, and general positions of multiplicity four. Therefore, there cannot be an odd number of atoms in the unit cell, so that there must be six and not seven iodine atoms. Furthermore, four of these iodine atoms must be in general positions and the other two on one of the special positions. The two thallium atoms must be on one of the special positions. The structure shown in Fig. 10 is the only chemically reasonable structure consistent with these restrictions. Strictly, the p2gg symmetry of the superlattice is incompatible with the p3m1 symmetry of the Pt(1 1 1) substrate. However, the first Pt layer is compatible, and the incompatibility only arises if the second Pt layer is considered. The intensity of the superlattice diffraction spots is due largely to the adsorbate, and the influence of the second Pt layer is expected
Fig. 10. Unit cell of the proposed Pt(1 1 1)-TlI3 structure for the rearranged surface in region C. Dashed lines and filled symbols are the glide planes and twofold rotation axes of the p2gg symmetry.
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to be small. The apparent systematic absences must actually be spots that are too faint to be detected. The assigned surface structure has the two thallium atoms per unit cell occupying twofold bridge sites. There are two types of iodine atoms: four of the iodine atoms per unit cell are adsorbed on displaced atop sites, while the other two are in twofold bridge sites. In this structure, each Tl atom is surrounded by six iodine atoms. The stoichiometry of Tl and I corresponds to TlI3 . Adzic and co-workers [23] have shown that many other Tl upd structures with coadsorbed halogens are also mixed monolayers with simple stoichiometries, and have suggested that such structures are principally ionic. There is insufficient pffiffito ffi assign pffiffiffi structures pffiffiffi pffiffiffi information to the mixed ( 7 7)R19.1° + ( 3 3)R30° LEED pffiffiffi pffiffiffi pattern of region B or the ( 7 7)R19.1° pattern of pffiffiffi p ffiffiffi region A. Note that well-ordered ( 7 7)R19.1° structures must have of >1/7 ML for each pffiffifficoverages pffiffiffi type of atom, and ( 3 3)R30° structures must have coverages >1/3 ML, but the Tl coverages in regions B are less than either of these values. This indicates that the ordering implied by LEED arises from patches with locally high Tl coverages or from local ordering of I atoms induced by small numbers of Tl atoms. Similar patch structures of iodine must be present in region A, since I atoms alone at the observed coverage of 0.25 ML are disordered in UHV.
5. Conclusions The observations and discussion above lead to the following picture of the first 0.3 ML of pffiffiTl ffi deposition. pffiffiffi The C1 peak on either the (3 3)-I or ( 7 7)R19.1° surface corresponds to deposition of 0.22 ML Tl. For the (3 3)-I surface, the voltammetry is unchanged on cycling, so that thallium is retained on the surface in a stable structure pffiffithat ffi phas ffiffiffi a Tl:I ratio of 0.22:0.44 ML ¼ 1:2. For the ( 7 7)R19.1° surface, the initially deposited TlI2 structure is metastable. A further 0.05 ML Tl deposits (at 0.4 V) and then a rearrangement occurs in which Tl and I are lost in a 1:1 ratio leaving a surface 4 0 with a ½ 3 6 unit cell of p2gg symmetry and TlI3 stoichiometry (2/25 ML Tl, 6/25 ML I). The structure is a mixed thallium-iodine monolayer. It is stable to cycling between 0.4 and 0.9 V and exhibits two sharp voltammetry peaks. Use of simultaneous ac voltammetry to estimate and correct for the double-layer charging baseline in cyclic voltammetry was validated by comparison with a Tl surface of known coverage. It is essential for charge
measurements of low coverage structures (such as the rearranged surface) where the double-layer charge is a significant fraction of the total charge.
Acknowledgements The authors thank the Natural Sciences and Engineering Research Council of Canada and the University of Victoria for financial support of this research. M.L. thanks the University of Victoria for a fellowship.
References [1] J. Clavilier, J.-P. Ganon, M. Petit, J. Electroanal. Chem. 265 (1989) 231. [2] I. Oda, Y. Shingaya, H. Matsumoto, M. Ito, J. Electroanal. Chem. 409 (1996) 95. [3] D.R. Wheeler, J.X. Wang, R.R. Adzic, J. Electroanal. Chem. 387 (1995) 115. [4] R.R. Adzic, J.X. Wang, O.M. Magnussen, B.M. Ocko, J. Phys. Chem. 100 (1996) 14721. [5] N.S. Marinkovic, W.R. Fawcett, J.X. Wang, R.R. Adzic, J. Phys. Chem. 99 (1995) 17490. [6] J. Clavilier, K. Achi, A. Rodes, Chem. Phys. 141 (1990) 1. [7] J. Clavilier, J. Ganon, M. Petit, J. Electroanal. Chem. 265 (1989) 231. [8] N. Shinotsuka, K. Sashikata, K. Itaya, Surf. Sci. 335 (1995) 75. [9] F. Lu, G.N. Salaita, H. Baltruschat, A.T. Hubbard, J. Electroanal. Chem. 222 (1987) 305. [10] M. Labayen, E. Herrero, J.M. Feliu, D.A. Harrington, J. Electroanal. Chem. 488 (2000) 32. [11] S.A. Furman, D.A. Harrington, J. Vac. Sci. Technol. A 14 (1996) 256. [12] M. Saidy, K.A.R. Mitchell, S.A. Furman, M. Labayen, D.A. Harrington, Surf. Rev. Lett. 6 (1999) 871. [13] M. Labayen, D.A. Harrington, M. Saidy, K.A.R. Mitchell, Surf. Sci. 490 (2001) 256. [14] M.M. Labayen de Inza, Ph.D Thesis, University of Victoria, 2002. [15] D.A. Harrington, J. Electroanal. Chem. 355 (1993) 21. [16] T. Pajkossy, D.M. Kolb, Electrochim. Acta 46 (2001) 3063. [17] M. Klimmeck, K. Juttner, Electrochim. Acta 27 (1982) 83. [18] K. Hermann, M.A. Van Hove (LBL), Lawrence Berkeley National Laboratory, Berkeley, and University of California, Davis, USA. Available from. [19] A. Wieckowski, B.C. Schardt, S.D. Rosasco, J.L. Stickney, A.T. Hubbard, Surf. Sci. 146 (1984) 115. [20] S.A. Furman, Ph.D. Thesis, University of Victoria, 1998. [21] M. Labayen, S.A. Furman, D.A. Harrington, Surf. Sci. 25 (2003) 149. [22] N.C. Gibson, P.M. Saville, D.A. Harrington, J. Electroanal. Chem. 318 (1991) 271. [23] J.X. Wang, I.K. Robinson, J.E. DeVilbiss, R.R. Adzic, J. Phys. Chem. B 104 (2000) 7951.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 567 (2004) 185–192 www.elsevier.com/locate/jelechem
Initial stages of thallium electrodeposition on iodine-covered Pt(1 1 1) Miguel Labayen 1, David A. Harrington
*
Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6 Received 10 September 2003; received in revised form 10 December 2003; accepted 21 December 2003 Available online 28 February 2004
Abstract pffiffiffi pffiffiffi The initial stages for Tl electrodeposition on Pt(1 1 1) covered with iodine in ( 7 7)R19.1° or (3 3) structures are investigated by cyclic voltammetry, low energy electron diffraction and Auger electron spectroscopy. Estimation of faradaic charges in cyclic voltammetry is facilitated by simultaneous measurement of the double-layer capacitance by ac voltammetry. The first electrodeposition peak corresponds to the deposition of 0.22 pffiffiML ffi pTl ffiffiffi atoms, independently of the structure of the initial iodine adlayer. However, after the electrodeposition of Tl on Pt(1 1 1) ( 7 7)R19.1°-I, the surface undergoes an irreversible rearrangement to a Pt(1 1 1) ½ 43 06 -TlI structure, with two Tl atoms and six I atoms per unit cell. In this rearrangement, thallium and iodine are removed from the surface in a 1:1 ratio. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Underpotential; Electrodeposition; Platinum; Thallium; Iodine; LEED; Auger electron spectroscopy; Ac voltammetry
1. Introduction Underpotential deposition (upd) produces intrinsically interesting monolayers that are thermodynamically stable with respect to the bulk structure. Thallium is known to exhibit upd behavior on Pt(1 1 1) [1–3]. At the most negative potentials, the Tl upd structure formed is a hexagonal close-packed monolayer. However, coad sorbed anions such as HSO 4 and ClO4 have an impact in the Tl upd structure, particularly at low coverages [4,5]. Only a preliminary study about the influence of the halides Cl , Br and I during the electrodeposition of Tl on Pt(1 1 1) has been reported [3]. We report here a detailed study for the electrodeposition of the first stages of on pffiffiffiTl p ffiffiffi Pt(1 1 1), initially covered with iodine in ( 7 7)R19.1°, h ¼ 0:43 ML or (3 3), h ¼ 0:44 ML surface structures, using electrochemical techniques, low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Our results show that Tl upd on Pt(1 1 1)(3 3)-I is a reversible process which produces a *
Corresponding author. Tel.: +1-2507217166; fax: +1-2507217147. E-mail address: [email protected] (D.A. Harrington). 1 Present address: Institut fuer Experimentelle und Angewandte Physik, Leibnizstrasse 19, Universitaet Kiel, Germany. 0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.12.023
variety of surface The first stages of Tl upd pffiffiffi structures. pffiffiffi on Pt(1 1 1) ( 7 7)R19.1°-I are similar to those for Pt(1 1 1)(3 3)-I. However, the surface structure then evolves irreversibly with desorption of Tl and I atoms, producing new features in the voltammogram.
2. Experimental Pt(1 1 1) single crystals were cut with a diamond-wafering saw from a single-crystal boule (1 cm diameter) grown by Metal Oxides and Crystals Ltd. (99.999%). The surfaces were polished with successive grades of diamond paste (Beuhler Ltd.), and oriented within 0.5° of the (1 1 1) plane by X-ray Laue back diffraction. After extensive annealing, high quality surfaces were produced, as judged by reproducing the literature voltammograms in H2 SO4 and HClO4 [6,7]. Before the experiments, all glassware was cleaned with hot chromic or sulfuric acid and then rinsed with Millipore Milli-Q Water. The solutions were made from reagents as purchased (HClO4 BDH, 60%, AnalaR; H2 SO4 Seastar Suprapure, 98% and Tl2 CO3 Alfa AESAR 99.999%), and were degassed for at least 15 min with argon prior to the experiment. The electrochemical
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experiments were carried out by putting the crystal into contact with the solution at a controlled potential, using the hanging meniscus method. The counter electrode was a platinum mesh cleaned with chromic acid prior to the experiment. Either the hydrogen electrode in the same solution (RHE) or hydrogen-charged Pd (+50 mV vs. RHE) was used as the reference electrode, although all potentials are reported vs. RHE. Unless otherwise stated, the peak potentials in cyclic voltammetry were calculated by extrapolating to 0 mV s1 from data at sweep rates in the range 5 to 100 mV s1 . The Pt(1 1 1) surfaces were prepared under either atmospheric or ultra-high vacuum (UHV) conditions. For experiments in which the crystals were cleaned and the iodine deposited at atmospheric pressure, the crystals were flame annealed for 1 min. The (3 3)-I structure was prepared by immersion pffiffiffi pffiffiof ffi the crystal in 1 mM KI solution [8,9]. The ( 7 7)R19.1° iodine monolayer was prepared by cooling the hot crystal in iodine vapor. These protocols were the same as used in an earlier study of silver electrodeposition, and their validity is discussed further in that work [10]. Another set of experiments involved preparation of the crystal surface under UHV conditions. The UHV system in our laboratory has facilities for LEED, AES, thermal desorption spectroscopy (TDS), work function changes, and contamination-free transfer between the UHV and electrochemical environments. I2 was dosed onto the surface using a custom-built doser based on a solid-state electrochemical cell [11]; the detailed procedures for preparing the clean and iodine-covered surfaces have been described elsewhere [12]. The crystal was transferred to the electrochemical chamber and the electrochemical experiment carried out in a custommade electrochemical cell [13]. Following the electrodeposition, the crystal was transferred to a chamber under UHV conditions, and after a few minutes the surface analysis measurements were performed. The pressure during these measurements was never above 5 109 mbar. The cleanliness of the transfer was checked with AES for the presence of impurities on the surface. Measurements of the faradaic charge required accurate correction for the double-layer charging. This was achieved by measuring single-frequency ac voltammograms simultaneously with the dc cyclic voltammograms. Since the cyclic voltammetry peaks for these surface reactions are very narrow, it is essential to use very small amplitude measurements in order for the ac amplitude to be a small perturbation on the reaction, and all measurements reported here were carried out using amplitudes of 0.5 or 0.033 mV rms. A frequency of 1 or 2 kHz was used, and the current and potential ac signals were measured using separate, synchronized lock-in amplifiers (Perkin–Elmer 7265). The potential was measured with the 10 MX differential input of one
lock-in amplifier directly between the reference and working electrodes. The current was similarly measured differentially as a proportional voltage across a measuring resistor, rather than using the potentiostatÕs current-to-voltage converter. In this way, phase or amplitude errors introduced by the potentiostat were corrected for. The double layer capacitance, Cdl , was obtained from the imaginary part of the measured impedance using Cdl ¼ ðxImðZÞÞ1 . This calculation assumes that the equivalent circuit approximates the uncompensated solution resistance in series with Cdl . This was verified in a preliminary set of ac voltammetry experiments over the frequency range 3 Hz to 200 kHz [14]. The validity of these types of experiments has been discussed previously [15]. The equivalent circuit was found to be a faradaic resistor in parallel to the double-layer capacitor, all in series with the solution resistor. At the single frequency used, Cdl agreed within 10% with the Cdl obtained from the full equivalent circuit analysis.
3. Results 3.1. Voltammetry and iodine loss at 0.05 V The voltammogram of thallium upd on (3 3)-I in H2 SO4 is shown in Fig. 1. It is similar to the voltammogram for Tl deposition in I solution [3]. It shows four cathodic peaks (C1–C4) at potentials +0.399, +0.324, +0.160 and +0.086 V, respectively, and the corresponding anodic peaks. This voltammogram was stable to cycling.
Fig. 1. Cyclic voltammogram for Tl upd on Pt(1 1 1)(3 3)-I in 0.1 M H2 SO4 . [Tlþ ] ¼ 1 mM, sweep rate 20 mV s1 . Inner curve is the doublelayer charging current measured with ac voltammetry, 0.5 mV rms, 2000 Hz.
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pffiffiffi pffiffiffiThe voltammogram of thallium upd on ( 7 7)R19.1°-I in H2 SO4 is shown in Fig. 2. The first sweep of the voltammogram was similar in either H2 SO4 or HClO4 . The main features of the first reduction sweep are similar to the first sweep of the voltammogram of (3 3)-I in Tlþ solution (Fig. 1), showing the same four cathodic peaks (C1–C4), but at slightly different peak potentials: +0.408, 0.329, 0.176 and 0.080 V, respectively. Holding the potential after sweeping part way into these peaks did not show any signs of increasing current, which would be diagnostic for nucleationgrowth-collision (NGC) mechanisms. After the first sweep reversal, the anodic features are already quite different from those in the (3 3)-I, and do not simply reflect reversal of the corresponding cathodic peaks. The peaks continue to evolve with time, and after cycling for at least 30 min, the features of the cyclic voltammogram (Fig. 2(d)) were the same as for the deposition of thallium on unreconstructed Pt(1 1 1) in the absence of iodine, as previously reported [1,4]. pffiffiffi Thus, pffiffiffi although the initial iodine coverages in the ( 7 7)R19.1°-I and (3 3)-I are almost the same (0.43 and 0.44 ML) and the initial cathodic peaks are similar, the stabilities of the upd Tl structures formed are very different: the surfaces formed from (3 3)-I are stable toward cycling, but the
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pffiffiffi pffiffiffi surfaces formed from ( 7 7)R19.1°-I lose iodine readily. In order to estimate the charge required to convert the iodine-covered surface to the thallium-covered, iodine-free surface, a sweep-hold experiment pffiffiffi pwas ffiffiffi undertaken. Starting with the Pt(1 1 1( 7 7)R19.1°-I surface at +0.9 V, the potential was swept in the negative direction down to +0.05 V, and then held at this potential for 3 min until the iodine was completely removed from the surface, Fig. 3. Subsequent cycles showed the upd of thallium on bare Pt(1 1 1), similarly to the voltammogram in Fig. 2(d). Integration of the current transient in Fig. 3(a) corrected for the double layer charging, gave 270 lC cm2 , the equivalent of 1.12 ML of electrons. The double-layer capacitance rises from a value of about 20 lF cm2 before the Tl deposition to values of 75–200 lF cm2 in the region of peaks C1–C4. Traditionally, it is assumed that Cdl ranges from 10 to 40 lF cm2 on Pt surfaces. However, high values of Cdl have also been reported for systems such as hydrogen upd on Pt(1 1 1) in neutral solution [16], or Tl upd on Ag(1 0 0) and Ag(1 1 0) [17].
3.2. Peak C1 and surface rearrangement at 0.4 V pffiffiffi pffiffiffi Although all iodine is lost from the ( 7 7)R19.1°-I surface at 0.05 V, it is possible to form a stable surface without complete iodine loss by restricting the potential to the region including peak C1 and pffiffiffi above, pffiffiffi that is above about 0.4 V. Starting from a ( 7 7)R19.1°-I structure, the cyclic voltammogram with reversal after the first deposition peak C1 is shown in Fig. 4. Peak C1 loses intensity with cycling, and a new pair of peaks, C11 and C12, appears at +0.608 and +0.544 V, respectively. These potentials were calculated by extrapolation of the peak
pffiffiffi pffiffiffi Fig. 2. Cyclic voltammograms for Tl upd on Pt(1 1 1)( 7 7)R19.1°-I þ 1 in 0.1 M H2 SO4 , [Tl ] ¼ 1 mM, sweep rate 20 mV s . (a) 1st cycle, (b) 2nd cycle, (c) 5th cycle and (d) 50th cycle. Arrows indicate the time evolution of the features.
pffiffiffi pffiffiffi Fig. 3. Sweep hold experiment for Tl upd on Pt(1 1 1)( 7 7)R19.1°-I þ in 0.1 M H2 SO4 , [Tl ] ¼ 1 mM. Potential swept at 20 mV s1 from 0.9 V down to 0.05 V, and then (at time zero, vertical dotted line) held at 0.05 V. Dotted curve is double-layer charging current measured by ac voltammetry, 0.5 mV rms, 2000 Hz.
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pffiffiffi Fig. C1 for Tl upd on Pt(1 1 1)( 7 p ffiffiffi 4. Successive cycles around peak 7)R19.1°-I in 0.1 M H2 SO4 , [Tlþ ] ¼ 1 mM, sweep rate 100 mV s1 . Lower potential limit 0.37 V.
potentials to 0 mV s1 for sweep rates from 5 to 100 mV s1 , after the peaks were fully developed. The peaks C11 and C12 did not show NGC behavior, and showed only insignificant differences in peak potentials (<4 mV) in HClO4 rather than H2 SO4 . Charge measurements were made to quantitate the C1 peak and the rearrangement to C11 and C12. The mea2 sured charge under peak C1 is 54 lC cm pffiffiffi for pffiffithe ffi (3 3)2 I surface and 51 lC cm for the ( 7 7) R19.1°-I surface, i.e., these are equal within the measurement error and equivalent to 0.22 ML. If the potential is held at +0.4 V, just after the C1 peak, then for the (3 3)-I surface there is no significant further charge passed. This is consistent with the fact that there is rearpffiffino ffi further pffiffiffi rangement on this surface. For the ( 7 7)R19.1°-I surface, however, a further 14 lC cm2 passes as the potential is held at 0.4 V for 1 min, giving a total charge of 65 lC cm2 (equivalent to 0.27 ML). Subsequent to this hold period, the surface rearrangement is complete and the subsequent voltammetry cycle shows the fully developed C11 and C12 peaks (Fig. 5). 3.3. Rearranged surface pffiffiffi pffiffiffi The ( 7 7)R19.1°-I surface after Tl deposition by holding the potential at 0.4 V or cycling in the C1 region and above will be referred to as the rearranged surface. It was investigated by voltammetry, charge measurements, AES and LEED. The voltammogram of the rearranged surface (Fig. 5(b)) can be divided into three distinct regions, A–C. The narrowness and reversibility of the peaks C11 and C12 imply that these peaks are due to surface phase transitions, and that regions A–C correspond to distinct surface structures.
pffiffiffi Fig. p ffiffiffi 5. (a) Sweep-hold experiment for Tl upd on Pt(1 1 1)( 7 7)R19.1°-I in 0.1 M H2 SO4 around peak C1, (b) Subsequent cycles after holding. Dotted curve is double-layer charging current. Constancy of the solution resistance Rs shows that the double-layer estimate is valid. [Tlþ ] ¼ 1 mM, sweep rates 20 mV s1 .
3.3.1. Thallium coverage Integration of the deposition peaks after subtraction of the double layer charging gave charges of 11 lC cm2 (0.045 ML of Tl) for peak C11, and 8 lC cm2 (0.033 ML of Tl) for peak C12. Here the assumption is made that all the faradaic charge gives Tl, with one Tl atom per electron. The total Tl coverage deposited under peaks C11 and C12 is therefore 0.078 ML. (Linear or flat double-layer charging baselines increase the estimated coverage by 25% or 50%, respectively.) These coverages were confirmed using AES (Fig. 6), using as a reference the intensity of the Tl peak at 89 eV at the known coverage of 0.66 ML (+0.05 V in the absence of iodine, e.g., leftmost potential in Fig. 2(d)) [1,4]. AES did not show the presence of Tl on the surface in region A of the voltammogram. The Tl coverage calculated from AES in region B of the voltammogram was 0.039 ML, and in region C was 0.087 ML. Since the errors in
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oxidation at +1.4 V includes the charge for the oxidation of platinum to platinum oxide. After the oxidation at +1.4 V, the potential was swept down to +0.4 V and the charge to reduce the Pt oxide was measured. This oxide charge was subtracted from the total oxidation charge to give the charge for iodine oxidation, which was found to be 330 lC cm2 . The iodine oxidation is a five-electron process according to Eq. (1): þ IðadsÞ þ 3H2 OðlÞ ! IO 3 ðaqÞ þ 6H ðaqÞ þ 5e
ð1Þ
so that the iodine coverage on the rearranged surface was 0.27 ML. This methodology was checkedpby ffiffiffi meapffiffiffi suring the iodine coverage on the initial ( 7 7) R19.1°-I surface, which gave 520 lC cm2 , or 0.43 ML, as expected for the ideal surface structure.
Fig. 6. Auger electron spectra of the rearranged surface emersed at potentials in regions A–C. Spectrum X is the reference spectrum of an iodine-free surface with 0.66 ML Tl. Beam energy 3 keV. Modulation 10 V pp at 4.7 kHz. Spectra normalized to a beam current of 10 lA.
AES coverages are larger than those from charge measurements, the coverages from the charges are used in subsequent calculations. 3.3.2. Iodine coverage The desorption of iodine from the surface was independently assessed by charge measurements of iodine oxidation. Oxidation of adsorbed iodine to IO 3 (aq) occurs at +1.3 V under the conditions studied. Oxidation of Tlþ (aq) to Tl3þ (aq) starts at +1.2 V. Therefore, owing to the overlap of the oxidation peaks, the oxidation of iodine was carried out after transferring the electrode to a different electrochemical cell, which had electrolyte but not Tlþ (aq). Emersion and immersion from one cell to another was done at a potential of +0.9 V, where there is no Tl adsorbed on the surface (Region A in Fig. 5(b)). The total charge passed during iodine
3.3.3. Surface structures by LEED The surface structures in the three regions A–C of the voltammogram in Fig. 5(b) were determined using LEED. The open-circuit potential was measured in order to ensure that the surface structure did not change during the transfer from atmospheric to UHV conditions. However, there was a gradual change of the open potential, which stabilized at +1.15 V, signaling a potential difficulty with the transfer. The shift of the open-circuit potential was slow, taking of the order of 5 min to shift from +0.4 to +1.15 V. Therefore the time for emersion and initial pumpdown was minimized, though total pumpdown and transfer time was still 10–15 min. Emersion experiments were carried out at +400, +388 and +385 mV (region C), +590, +592 and +575 mV (region B), and +815 and +727 mV (region A). Since the LEED diffraction patterns were consistent within each region, and different between regions, it is reasonable to assume that the emersion procedure was fast enough that the emersed structures were the same as those existing at the emersion potential. AES showed that Tl was retained during the pumpdown process, and no traces of electrolyte were detected. This suggests that the analyzed surface was the same as the in situ surface, though structural rearrangements cannot be rigorously excluded.
Fig. 7. LEED patterns at (a) 35 and (b) 70 eV of the rearranged surface in region C.
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Fig. 8. Computer simulation of the kinematic LEED pattern for a structure on a fcc(1 1 1) surface showing the three domains of the unit cell, (a) for no special symmetry of the unit cell, (b) for p2gg symmetry of the unit cell showing the systematic absences (circles).
The LEED pattern for region C (Fig. 7) shows 12 triangles of three closely spaced spots spaced at 30° intervals around the pattern. This indicates a square or rectangular unit cell structure, lying on the substrate (which has threefold symmetry) in three rotational domains. Spot measurements and the computer pattern simulator LEEDpat v 1.0 [18] were used to show that the 4 0 structure has a ½ 3 6 unit cell. The simulated LEED pattern for this unit cell (Fig. 8(a)) correctly shows the 12 triangles, and larger inverted triangles between every second triangle. However, a rectangle of spots with a fifth spot at its centre is expected just inside the inverted triangles and the observed pattern fails to show the fifth spot (Figs. 7(b) and 8(b)). This corresponds to systematic absences (h; 0), h odd and (0; k), k odd, which implies a p2gg symmetry of the superlattice, for which the simulated pattern (Fig. 8(b)) agrees with the observed pattern. LEED pattern pffiffiin ffi region pffiffiffi B (Fig. 9) was a mixed pffiffiThe ffi p ffiffiffi ( 7 7)R19.1° + ( 3 3)R30° pattern, indicating patches of the two surface structures. LEED for region pffiffiffi p ffiffiffi A showed a simple ( 7 7)R19.1° pattern.
4. Discussion 4.1. Iodine loss The most noteworthy feature of the Tl deposition is the marked difference in reactivity of the two iodine surfaces: pffiffiffi pffiffithe ffi (3 3)-I surface is stable to cycling, but the ( 7 7)R19.1°-I loses iodine, and at the most negative potentials all iodine is removed from the surface. The loss of iodine appears to occur concomitantly with the thallium deposition, since the pattern of reduction peaks is the same on both surfaces, with the exception of small changes in peak potentials. That is, no separate peaks can be identified with the iodine loss. This is also seen in the sweep-hold data of Fig. 3, where the additional charge passed in the hold part of the experiment is small and cannot account for the iodine loss. The 1.12 ML charge passed in this sweep-hold experiment may be as follows: The initial state pffiffiinterpreted ffi pffiffiffi is the Pt(1 1 1)( 7 7)R19.1°-I surface without thallium and the final state has no iodine, but has electrodeposited Tl. The premoval ffiffiffi pffiffiffi of the 0.43 ML I atoms existing on the ( 7 7)R19.1°-I surface requires consumption of 0.43 ML electrons, assuming that it is lost as iodide ions (Eq. (2)). The remaining reduction charge must therefore be due to the Tl upd reaction, Eq. (3), IðadsÞ þ e ! I ðaqÞ
ð2Þ
so that the Tl coverage may be estimated as 1.12–0.43 ¼ 0.69 ML. Tlþ ðaqÞ þ e ! TlðadsÞ
Fig. 9. LEED pattern at 73 eV of the rearranged surface in region B.
ð3Þ
This calculation of the Tl coverage at +0.05 V agrees with the saturation 0.66 ML thallium measured at this potential by in situ X-ray diffraction experiments of Tl upd on iodine-free Pt(1 1 1) [3,4]. The agreement shows
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that the double-layer correction procedure used in this work is reliable and gives coverages accurate to about 5%. A flat double-layer charging assumption would have increased the measured charge to 1.18 ML, and the estimated Tl coverage would have been 0.75 ML, 14% away from the literature value. reactivity of the (3 3)-I and pffiffiThe ffi pffiffidifferent ffi ( 7 7)R19.1°-I surfaces toward Tl electrodeposition is in sharp contrast to their behavior toward Ag electrodeposition. In that case, the initial deposition structures are the same, and neither surface exhibits iodine loss after deposition [19]. The coverage of the iodine on the two surfaces is essentially identical (ideal coverages of 0.43 and 0.44 ML). Furthermore, the partial charges of the iodine atoms are the same on the two surfaces, since it is possible in a UHV experiment pffiffiffi ptoffiffiffi heat the surface through the (3 3)-I to ( 7 7)R19.1°-I phase transition without observing any measurable change in work function [20]. One possibility is that the different types of iodine atoms on the surface have different reactivity because of their different bond energies. the known structures pffiffiffi pffiffiFrom ffi of the (3 3)-I and ( 7 7)R19.1°-I surfaces [12] and thermal desorption studies [20,21], the strength of the iodine–platinum interaction increases in the following sequence of sites: twofold, atop, hcp threefold and fcc threefold. Since the twopmost bonded iodine ffiffiffi pstrongly ffiffiffi types occur only on the ( 7 7)R19.1°-I surface, this surface would be expected to be the least reactive, in contrast to observation. We suggest that the (3 3) arrangement of iodine atoms is closer to the arrangement required in the TlI structure, so that a type of templatingpeffect ffiffiffi pffiffifacilitates ffi deposition into this structure. For the ( 7 7)R19.1°I surface, more extensive rearrangement of the iodine atoms is required, and this initiates iodine loss. In support of this, kinetic studies of silver upd on these two surfaces show is more facile on (3 3)-I pffiffiffithatpdeposition ffiffiffi than on ( 7 7)R19.1°-I [22], though the final structures are probably different from those in the thallium case. pffiffiffi pffiffiffi The rearrangement of the ( 7 7)R19.1°-I surface formed by Tl deposition at 0.4 V or by cycling in the C1 region and above shows partial iodine loss. Conceptually, this process can be considered in two stages: (1) deposition of 0.27 ML Tl (from sweep-hold charge measurements) without loss of any of the 0.43 ML iodine, and (2) loss of Tl and I to give the rearranged surface with 0.08 ML Tl and 0.27 ML iodine. Interestingly, the amounts of Tl and I lost are nearly the same (0.19 and 0.16 ML, respectively). 2 The above process could be concerted rather than occur in two stages, but 2
If the iodine coverage of 0.25 ML corresponding to the ideal structure deduced by LEED (see below) is used rather than the 0.27 ML measured by iodine oxidation, then the iodine loss is 0.18 ML.
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the 1:1 stoichiometry of the loss suggests that each iodine might leave the surface accompanied by a thallium atom. 4.2. Surface structures The experiments show that the surface structure on the rearranged surface at 0.4 V (region C, Fig. 5) has a 4 0 ½ 3 6 unit cell with p2gg symmetry, Tl coverage of 0.08 ML and I coverage of 0.27 ML. There are 24 Pt surfacelayer atoms in this unit cell and therefore there must be 0:08 24 ¼ 1:9 2 Tl atoms and 0:27 24 ¼ 6:5 6 or 7 I atoms in the unit cell. The locations of the twofold rotation axes and glide planes within the unit cell are shown in Fig. 10, together with the probable structure. The p2gg symmetry has two types of special positions (both on twofold rotation axes) each of multiplicity two, and general positions of multiplicity four. Therefore, there cannot be an odd number of atoms in the unit cell, so that there must be six and not seven iodine atoms. Furthermore, four of these iodine atoms must be in general positions and the other two on one of the special positions. The two thallium atoms must be on one of the special positions. The structure shown in Fig. 10 is the only chemically reasonable structure consistent with these restrictions. Strictly, the p2gg symmetry of the superlattice is incompatible with the p3m1 symmetry of the Pt(1 1 1) substrate. However, the first Pt layer is compatible, and the incompatibility only arises if the second Pt layer is considered. The intensity of the superlattice diffraction spots is due largely to the adsorbate, and the influence of the second Pt layer is expected
Fig. 10. Unit cell of the proposed Pt(1 1 1)-TlI3 structure for the rearranged surface in region C. Dashed lines and filled symbols are the glide planes and twofold rotation axes of the p2gg symmetry.
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to be small. The apparent systematic absences must actually be spots that are too faint to be detected. The assigned surface structure has the two thallium atoms per unit cell occupying twofold bridge sites. There are two types of iodine atoms: four of the iodine atoms per unit cell are adsorbed on displaced atop sites, while the other two are in twofold bridge sites. In this structure, each Tl atom is surrounded by six iodine atoms. The stoichiometry of Tl and I corresponds to TlI3 . Adzic and co-workers [23] have shown that many other Tl upd structures with coadsorbed halogens are also mixed monolayers with simple stoichiometries, and have suggested that such structures are principally ionic. There is insufficient pffiffito ffi assign pffiffiffi structures pffiffiffi pffiffiffi information to the mixed ( 7 7)R19.1° + ( 3 3)R30° LEED pffiffiffi pffiffiffi pattern of region B or the ( 7 7)R19.1° pattern of pffiffiffi p ffiffiffi region A. Note that well-ordered ( 7 7)R19.1° structures must have of >1/7 ML for each pffiffifficoverages pffiffiffi type of atom, and ( 3 3)R30° structures must have coverages >1/3 ML, but the Tl coverages in regions B are less than either of these values. This indicates that the ordering implied by LEED arises from patches with locally high Tl coverages or from local ordering of I atoms induced by small numbers of Tl atoms. Similar patch structures of iodine must be present in region A, since I atoms alone at the observed coverage of 0.25 ML are disordered in UHV.
5. Conclusions The observations and discussion above lead to the following picture of the first 0.3 ML of pffiffiTl ffi deposition. pffiffiffi The C1 peak on either the (3 3)-I or ( 7 7)R19.1° surface corresponds to deposition of 0.22 ML Tl. For the (3 3)-I surface, the voltammetry is unchanged on cycling, so that thallium is retained on the surface in a stable structure pffiffithat ffi phas ffiffiffi a Tl:I ratio of 0.22:0.44 ML ¼ 1:2. For the ( 7 7)R19.1° surface, the initially deposited TlI2 structure is metastable. A further 0.05 ML Tl deposits (at 0.4 V) and then a rearrangement occurs in which Tl and I are lost in a 1:1 ratio leaving a surface 4 0 with a ½ 3 6 unit cell of p2gg symmetry and TlI3 stoichiometry (2/25 ML Tl, 6/25 ML I). The structure is a mixed thallium-iodine monolayer. It is stable to cycling between 0.4 and 0.9 V and exhibits two sharp voltammetry peaks. Use of simultaneous ac voltammetry to estimate and correct for the double-layer charging baseline in cyclic voltammetry was validated by comparison with a Tl surface of known coverage. It is essential for charge
measurements of low coverage structures (such as the rearranged surface) where the double-layer charge is a significant fraction of the total charge.
Acknowledgements The authors thank the Natural Sciences and Engineering Research Council of Canada and the University of Victoria for financial support of this research. M.L. thanks the University of Victoria for a fellowship.
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