© Copyright 1998 by the American Chemical Society
VOLUME 102, NUMBER 33, AUGUST 13, 1998
LETTERS Bromide Adsorption Induced Formation of Thallium Bromide Adlayers with Varying Composition and Structure on the Au(111) Electrode Surface R. R. Adzˇ ic´ * and J. X. Wang Chemical Sciences DiVision, Department of Applied Science, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: January 9, 1998; In Final Form: June 26, 1998
The underpotential deposition of Tl on Au(111) in 0.1 M HClO4 solution containing 1 mM Tl+ and 1 mM Br- has been investigated by using cyclic voltammetry, in situ X-ray scattering, and rotating disk-ring techniques. Three interesting phenomena for the UPD of metals in the presence of adsorbed anions are reported, viz., (i) an increase of the underpotential shift for deposition of Tl on Au(111) in the presence of Br- in solution, (ii) a formation of two mixed adlayers, surface compounds, with the stoichiometry Tl-Br2 and Tl-Br, and (iii) an exceptionally complex structural behavior of this system with five ordered phases. Bromide and thallium form incommensurate rotated-hexagonal close-packed phases at ca. 0.85 and -0.3 V, respectively. In the intermediate potential region three mixed phases exist, identified as 3TlBr2-(x13 × x13), 2TlBr-(3 × x3), and TlBr-c(p × x3).
Introduction Metal monolayers formation on foreign electrode surfaces at potentials positive of the reversible metal deposition, the socalled underpotential deposition (UPD), has been the subject of much interest over the years.1,2 Strongly (specifically) adsorbed anions cause pronounced effects on this phenomenon. It is generally observed for polycrystalline1 and for single-crystal surfaces3,4 that these anions decrease the underpotential shift in proportion to the extent of their specific adsorption. Recent chronocoulometry data, however, show a small shift of the copper UPD to more positive potentials in the presence of coadsorbed bromide.5 Structures of metal adlayers with coadsorbed anions were recently investigated with in situ atomic force microscopy (AFM),6 scanning tunneling microscopy (STM),7 and X-ray scattering (SXS) techniques.8-10 Ultrahigh vacuum techniques were also used to study similar systems ex situ.11 The results of in situ studies demonstrated the effects of anions on ordering of metal adlayers.6 In this work, three interesting phenomena
for the UPD of metals in the presence of adsorbed anions are reported, viz., (i) an increase of the underpotential shift for deposition of Tl on Au(111) in the presence of Br- in solution, (ii) a formation of two mixed adlayers, surface compounds, with the stoichiometry Tl-Br2 and Tl-Br, and (iii) an exceptionally complex structural behavior of this system with five ordered phases. Five ordered phases, formed over a potential interval of 1.3 V, were determined with in situ SXS technique. Bromide and thallium form incommensurate rotated-hexagonal closepacked phases at ca. 0.85 and -0.3 V, respectively. In the intermediate potential region three mixed phases exist, identified as 3TlBr2-(x13 × x13), 2TlBr-(3 × x3) and TlBr-c(p × x3). Experimental Section A full description of the electrochemical SXS technique has been presented elsewhere.12 The rotating disk-ring technique (Au(111) disk and Pt ring) was used to monitor Br- adsorption/ desorption. The solutions were prepared from Tl2CO3 and KBr (Aldrich), HClO4 (Merck), and Millipore QC UV Plus water
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6306 J. Phys. Chem. B, Vol. 102, No. 33, 1998
Figure 1. Voltammetry curve for the UPD of Tl in 0.1 M HClO4 solution containing 1 mM Tl+ and 1 mM Br- and the real space models of the ordered phases (upper panel); diffraction intensities at indicated positions in reciprocal space and adsorbates coverages as a function of potential (lower panel). Open and shaded circles represent Br and Tl, respectively. Sweep rates: 20 and 0.5 mV/s for voltammetry and diffraction intensities curves, respectively.
(Millipore Inc.). A reversible hydrogen electrode was used as a reference electrode. SXS measurements were carried out at the National Synchrotron Light Source (NSLS) at beam line X22A with λ ) 1.20 Å. For Au(111), it is convenient to use a hexagonal coordinate system as described in ref 12. The detector resolution was determined by a 2 × 2 mm slit, located 650 mm from the sample. Results and Discussion Figure 1 shows the results obtained with several techniques. The upper panel displays the voltammetry curve for the UPD of Tl on Au(111) in relation to the also shown real space models of the ordered adlayers derived from the in-plane X-ray diffraction measurements. The lower panel shows the diffraction intensities for the three mixed phases at the corresponding positions in reciprocal space as a function of potential. The adsorbate coverages, calculated for ordered phases from the adlayer lattices, are also given in this plot. A rarely observed multitude of reversible voltammetry results from the potentialdependent coadsorption of Tl+ and Br- and phase transitions in the adlayers. The increase of the UPD shift contrasts the usual behavior of these systems,1,3,4,8 but it is similar to the shift observed for the UPD of Cu on Au(111) with coadsorbed Br.5 In the absence of Br-, Tl adsorption is negligible at potentials above 0.6 V, while in the presence of Br- it commences at 0.86 V (Figure 1). An incommensurate, rotated-hexagonal close-packed Br adlayer was found between the potential of Au dissolution and the peak at 0.86 V. The same Br structure exists, however, down to 0.6 V in the absence of Tl.13 No Br- desorption could be detected by rotating disk-ring technique in the potential region of the UPD of Tl between 0.94 and 0.49 V (vide infra). The adsorption of Tl causes a disordering of the Br adlayer at 0.86 V, and no ordered phase is found up to a sharp peak B
Letters
Figure 2. Rotating disk-ring experiment with Au(111) disk and Pt ring showing disk currents (upper panel) and ring current (lower panel) as a function of the disk potential. Ring potential, ER ) 1.3 V; collection efficiency, N ) 0.21; sweep rate, 20 mV/s; rotation rate, 900 rpm; unshielded ring current, IR∞ ) 24 µA.
at 0.58 V. This peak in cathodic direction is associated with formation of a commensurate 3TlBr2-(x13 × x13) adlayer, which exists in the potential region between 0.58 and 0.47 V. The peak B in anodic direction causes a vanishing of this phase. The Tl coverage determined from the voltammetry curve by integrating the charge in the potential region between the peaks A and B is θTl ) 0.24 ()53/220). This is in excellent agreement with θTl ) 0.23 obtained from the adlayer unit cell determined from the SXS data (see Figure 1 and the text given below). The positive shift of the UPD of Tl at Au(111) covered by Br-, compared with the Au surface in nonadsorbing electrolytes, can be at least partly due to the electrostatic interactions between Tl+ and the incompletely discharged Br-. This view is supported with the observation of the coadsorption of Cs+, a cation that does not undergo the UPD, with I- adsorbed on Au(110).14 A similar explanation has been given for the Cu-Br coadsorption on Au(111).5 The peak C is associated with a very small Br- desorption and with the formation of the 2TlBr-(3 × x3) commensurate phase. This phase vanishes with the peak D when the incommensurate c(p × x3) phase is formed upon further increase of Tl coverages. A large Br- desorption occurs in the region of the peak E, where the c(p × x3) phase vanishes. At potentials negative of the peak E, Tl forms an incommensurate, rotated-hexagonal phase, as in the absence of Br-.13 Differences in diffraction intensities between the anodic and cathodic sweeps (Figure 1) are due to a slow formation of ordered phase, which lags the adsorption/desortion processes. As for the (x13 × x13) phase, the Tl coverage for the (3 × x3) phase calculated from voltammetry curve, θTl ) 0.36 ) (81/220), is in a good agreement with the 1/3 coverage obtained from the unit cell. Such an agreement is not obtained for the c(p × x3) and Tl rotated-hexagonal phases in the potential ranges where Brdesorption takes place. This is a consequence of the addition of the currents for Tl+ adsorption and Br- desorption, which cannot be separated to calculate partial currents and coverage for either of the two of adsorbates. Figure 2 shows the results of the rotating disk-ring measurements used to monitor the Br- adsorption/desorption throughout the potential region employed in Figure 1. The upper panel
Letters
Figure 3. (a) In-plane θ rocking scans at the (2/13, 5/13, 0.2) and (9/13, 3/13, 0.2) positions measured at 0.55 V for the first and secondorder diffractions from the 3-(x13 × x13) phase. Inset: The in-plane diffraction peaks from the adlayer (open circles) and the Au(111) substrate (filled circles). The size (area) of the open circles is proportional to the measured structure factor intensities. (b) Model of the first adlayer containing three Tl (light shade, 2.98 Å) and three Br (heavy shade, 3.92 Å) atoms in the (x13 × x13) unit cell. (c) Model of the TlBr2 adlayer derived from (a).
displays a voltammetry curve of the disk electrode rotated at 900 rpm, which is in good agreement with the curve obtained for a stationary electrode (Figure 1). The ring electrode was potentiostated at 1.3 V where Br- oxidation is diffusion-limited. No interference from the Tl+/Tl3+ redox reaction or O2 evolution is observed at this potential. Figure 2 shows that there is no observable Br- desorption associated with formation of the (x13 × x13) phase. A minute desorption appears to be associated with the peak C in formation of the (3 × x3) phase. A small current observed in the potential region between 0.9 and 0.5 V is not due to the Br adsorption/desorption. It appears to be caused by small drifts in the electronic circuits, since the difference between the cathodic and anodic currents depends on potential where the sweep was initiated. The negligible desorption of Br- desorption upon formation of the (3 × x3) phase where the Tl:Br ratio is 1:1 means that the excess Brremains adsorbed on the surface but is not ordered. Thus, it does not affect the diffraction intensities associated with the (3 × x3) phase. Bromide desorption, however, occurs in the potential regions of the formation of the c(p × x3) and Tl rotated-hexagonal phases. The change in the Br coverage, ∆θBr ) 0.13, has been calculated by using the standard procedure.15 This means that approximately 1/3 of the Br coverage still remains as a disordered adlayer over the Tl adlayer. Similar observations of the incomplete Br- desorption were made in the case of Br and Cu coadsorption on Pt(111)9 and Au(111).5 Figure 3a shows the θ-rocking curves through the first (2/ 13, 5/13) and the second (9/13, 3/13) order in-plane diffraction peaks for the (x13 × x13) phase observed at 0.55 V. Integrated intensities were measured up to sixth order at 10 positions shown by the open circles in the insert to Figure 3. The size (area) of each circle represents the integrated intensity after correction for Lorentz factor and instrumental resolution. Two Br per Tl are required to account for the more than double intensity at the (9/13, 3/13) position, compared to that at the (2/13, 5/13) position. A detailed analysis will soon be re-
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Figure 4. Upper panel: In-plane θ rocking scans (L ) 0.2) through the three lowest order diffractions from the 2TlBr-(3 × x3) adlayer at 0.25 V. Lower panel: Observed adlayer diffractions, shown with open circles with the area proportional to the structure factor intensities, and the model for the (3 × x3) phase with two Tl and two Br in the unit cell.
ported.16 One of the two bromides and one Tl atom can be coadsorbed in the first layer on the Au(111) surface, as illustrated Figure 3b. This satisfies the minimal steric requirements since the separation between the Tl and Br is 3.47 Å, which is very close to the sum of the ionic radii of Tl+ (1.49 Å) and Br- (1.96 Å). The other Br is likely to be positioned in the hollow site between three Tl and three Br (Figure 3c). It is farther apart from the Au surface owing to the repulsion from the bromides in the first adlayer. Thus, the atomic coverages in this phase are 0.23 ()3/13) and 0.46 ()0.23 × 2) for Tl and Br, respectively. The latter is close to the Br coverage of 0.48 in the rot-hex phase at 0.9 V. This is consistent with the rotating disk-ring result, which shows that no significant desorption of Br- occurs between 0.9 and 0.5 V. The 1:2 ratio of Tl and Br can be rationalized by a positive partial charge on Tl larger than the negative partial charge on Br at this very positive potential. At potentials negative of the peak C, Tl+ becomes more discharged, and the ratio of Tl to Br decreases to 1:1 in the 2TlBr-(3 × x3) phase with a quasi-square symmetry. The model for this phase and the diffraction data supporting it are shown in Figure 4. The calculated intensities based on this model agree with the data for the three lowest order diffractions. Surface rod measurements and layer-spacing determination will be reported elsewhere.16 At potentials below 0.0 V, the (3 × x3) phase vanishes and the only one very weak and broad peak at the (0.468, 0.468, 0.2) position is found between the voltammetry peaks D and E (Figure 1). On the basis of this peak and other observations, the centered (2.14 × x3) unit cell (p ) 2.14), which has two Tl and two Br atoms, is proposed (Figure 1). Other low-order peaks have lower intensity than the peak at (0.468, 0.468, 0.2) because the diffractions from Tl and Br are out-of-phase for the former. The fact that only the lowest order in-phase diffraction can be observed, with significantly broader peak width than those for other two phases, suggests that the adlayer in this phase orders in patches. Therefore, the coverages calculated from the lattice may differ from the average value. Five ordered adlayers, two with single and three with two adsorbates, observed on the electrode surface as a function of
6308 J. Phys. Chem. B, Vol. 102, No. 33, 1998 potential, represent an unusual chemical system that may offer interesting possibilities for surface modification with other various metal atoms and two-dimensional (2D) compound formation. Its reversibility facilitates easy investigations of some fundamental questions of surface chemistry and physics, such as the kinetics of 2D mixed adlayer formation, phase transitions, and the solution-phase effects on ordering of adsorbates. The shift of the UPD of Tl to positive potentials in the presence of adsorbed anions adds a new aspect to this phenomenon. Acknowledgment. The authors thank B. Ocko, I. K. Robinson, and S. Feldberg for useful discussions. This research was performed under the auspices of the U.S. Department of Energy, Division of Chemical Sciences and Materials Sciences Division, Office of Basic Energy Sciences under Contract No. DE-AC02-98CH10886. References and Notes (1) Kolb, D. M. In AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; John Wiley: New York, 1978; Vol. 11, pp 125-271. (2) Adzic, R. R. In AdVances in Electrochemistry and Electrochemical Engineering, Gerischer, H., Tobias, C. W., Eds.; John Wiley: New York, 1984; Vol. 13, pp 159-260.
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