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Electrochemistry of Cd at (x3×x3)R30°-S/Au(111): Kinetics of Structural Changes in CdS Monolayers Umit Demir and Curtis Shannon* Department of Chemistry, Auburn University, Auburn, Alabama 36849-5312 Received March 11, 1996X Atomic layers of Cd can be adsorbed onto the (x3 × x3)R30°-S/Au(111) surface at underpotential. We have investigated the kinetics of Cd monolayer formation and dissolution at this surface using cyclic voltammetry and chronoamperometry. Scan rate dependent cyclic voltammetry experiments reveal that the peak current in the Cd UPD wave is not a linear function of the scan rate, ν, but scales as ν2/3. Similar behavior is observed when the Cd monolayer is stripped from the surface. These results are characteristic of monolayer formation and dissolution by a nucleation and two-dimensional growth mechanism. Chronoamperometry experiments reveal that both the deposition and stripping of Cd involve two steps: an initial Langmuir-type adsorption-desorption process accompanied by nucleation and two-dimensional growth. CdS monolayer growth kinetics are strongly influenced by the chemisorption of Cd on the S-modified Au(111) substrate, which both increases its rate of adsorption and decreases the rate of lattice growth. The kinetics of monolayer dissolution are characterized by a very large random desorption component which is attributed to formation of CdS.
Introduction II-VI semiconductors are interesting candidates for applications in heterojunction photocells, optoelectronic devices, and catalysis. Furthermore, the chalcogenidebased systems lend themselves well to both chemical and electrochemical growth at ambient temperatures and pressures.1 A number of different approaches have been investigated in the electrochemical growth of CdS thin films;2 however, the morphology of the resulting deposits is often difficult to control. The initial monolayer plays a key role in determining the structure of subsequently deposited layers as well as the solid state properties of the film as a whole; consequently, a great deal of attention has focused on understanding the structure of the first monolayer at the atomic level. A particularly promising method for the growth of structurally well-defined monolayers is electrochemical atomic layer epitaxy (ECALE),3 which is based on the formation of ordered phases through underpotential deposition (UPD).4 UPD generally results in the growth of a well-ordered monolayer that is often stable to emersion into an inert atmosphere, supporting electrolyte, or ultrahigh vacuum. Therefore, sequential UPD of the appropriate elemental atomic layers can be * Author to whom correspondence should be addressed. Telephone: 334-844-6964. Fax: 334-844-6959. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) (a) Lokhande, C. D.; Yermune, V. S.; Pawar, S. H. J. Electrochem. Soc. 1991, 138, 624. (b) Dona, J. M.; Herrero, J. J. Electrochem. Soc. 1992, 139, 2810. (c) Dona, J. M.; Herrero, J. J. Electrochem. Soc. 1994, 141, 205. (d) Froment, M.; Bernard, M. C.; Cortes, R. J. Electrochem. Soc. 1995, 142, 2642. (2) (a) Miller, B.; Heller, A. Nature 1976, 262, 680. (b) Peter, L. M. Electrochim. Acta 1978, 23, 1073. (c) Miller, B.; Menezes, S.; Heller, A. J. Electroanal. Chem. 1978, 94, 85. (d) Fatas, E.; Duo, R.; Herrasti, P.; Arjona, F.; Garcia-Camarero, E. J. Electrochem. Soc. 1984, 131, 2243. (e) Birss, V. I.; Kee, E. L. J. Electrochem. Soc. 1986, 133, 2097. (f) Fatas, E.; Herrasti, P.; Arjona, F.; Garcia-Camarero, E.; Medina, J. A. Electrochim. Acta 1987, 32, 139. (g) Minoura, H.; Takeichi, Y.; Furuta, H.; Suqiura, T.; Ueno, Y. J. Mater. Sci. 1990, 25, 472. (h) Lincot, D.; Borges, R. O. J. Electrochem. Soc. 1992, 139, 1880. (i) de Tacconi, N. R.; Rajeshwar, K. J. Phys. Chem. 1993, 97, 6504. (3) (a) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1991, 300, 543. (b) Villegas, I.; Stickney, J. L. J. Electrochem. Soc. 1992, 139, 686. (c) Suggs, D. W.; Stickney, J. L. Surf. Sci. 1993, 290, 362. (d) Colletti, L. P.; Teklay, D.; Stickney, J. L. J. Electroanal. Chem. 1994, 369, 145. (4) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: Interscience: New York, 1978; Vol. 11, p 125.
S0743-7463(96)00225-9 CCC: $12.00
used to grow binary compounds, including a number of II-VI semiconductors. We have used this technique to grow monolayers of CdS on the low-Miller-index planes of single-crystal Au as well as on polycrystalline Au surfaces.5 The ability to control film thickness at the monolayer level makes these systems particularly attractive from a spectroscopic point of view. In addition to the possibility of investigating quantum size phenomena, precise control of coverage allows dynamic processes to be investigated spectroscopically.5c The integration of proximal probe and spectroscopic data will also lead to a better understanding of crystal growth mechanisms. Recently, we have investigated the structure of CdS monolayers grown on the (111) and (100) planes of Au in an attempt to template the growth of a desired polymorph. We found that the thermodynamically most stable CdS(111) monolayer formed on both surfaces.5b That is, there appeared to be no kinetic stabilization of the cubic structure on Au(100) in spite of excellent lattice matching. Thus, we were interested in investigating the kinetics of the adsorption and desorption of CdS monolayers in more detail. Potential-step experiments have been used to study the kinetics of UPD processes and other two-dimensional phase transitions on electrode surfaces.6 Recently, there has been renewed interest in the kinetics of Cu UPD on Au(111).7 In this paper, we present the results of a cyclic voltammetric and chronoamperometric study of the growth of Cd monolayers on the (x3 × x3)R30°-S/Au(111) surface. Although analysis of the cyclic voltammetry data suggests that the deposition and stripping of Cd proceed by a nucleation and growth mechanism, potentialstep experiments indicate a more complex mechanism (5) (a) Demir, U.; Shannon, C. Langmuir 1994, 10, 2794. (b) Demir, U.; Shannon, C. Langmuir 1996, 12, 594. (c) Boone, B. E.; Shannon, C. J. Phys. Chem. 1996, 100, 9480. (6) (a) Bewick, A.; Thomas, B. J. Electroanal. Chem. 1975, 65, 911. (b) Budevski, E.; Bostanov, V.; Staikov, G. Annu. Rev. Mater. Sci. 1980, 10, 85. (c) DaSilva Pereir, M. I.; Peter, L. M. J. Electroanal. Chem. 1981, 125, 401. (d) Li, F. B.; Albery, W. J. Langmuir 1992, 8, 1645. (d) Oshaka, K. F.; Tokuda, T. K. J. Electroanal. Chem. 1993, 347, 371. (e) Murugan, K.; Vanmathi, G.; Rangarajan, S. K. J. Electrochem. Soc. 1995, 142, 1770. (f) da Silva Pereira, M. I.; Melo, M. J. B. V.; Peter, L. M. J. Electroanal. Chem. 1989, 273, 215. (7) (a) Holzle, M. H.; Retter, U.; Kolb, D. M. J. Electroanal. Chem. 1994, 371, 101. (b) Holzle, M. H.; Aspel, C. W.; Will, T.; Kolb, D. M. J. Electrochem. Soc. 1995, 142, 3741. (c) Omar, I. H.; Pauling, H. J.; Juttner, K. J. Electrochem. Soc. 1993, 140, 2187.
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involving a parallel adsorption-desorption step. Current density-time transients were analyzed using two kinetic models. In both, we model nucleation and growth using the theory developed by Bewick, Fleischmann, and Thirsk,8 while adsorption-desorption is assumed to be Langmuirian. We find that the chemisorption of Cd on S/Au(111) plays a determining role in the monolayer growth kinetics. In the case of monolayer dissolution, on the other hand, the structural changes which occur in the monolayer upon deposition of Cd strongly influence the kinetics. The implications of these findings on the electrosynthesis of high-quality materials are also discussed. Experimental Section The Au(111) electrodes employed in these experiments were prepared as follows.9 A 1.0 mm diameter polycrystalline Au wire (Alfa-Johnson Matthey, 99.995%) was cleaned by dipping into piranha solution (3:1 concentrated H2SO4/30% H2O2) once and rinsing with copious amounts of pure water. The wire was then placed in a H2-O2 flame and allowed to melt until a large, approximately 4 mm diameter bead had formed at the end of the wire. This bead was annealed several times in a cooler (reducing) part of the flame until large facets developed. Facets (visible by eye) were typically located between the equator of the drop and the principal axis of the wire. The facets were composed of two distinct regions: large, flat (111) terraces separated by atomic steps located in the central portion of the facet, and smaller, diamond-shaped (111) terraces with lateral dimensions averaging 150 nm located near the edges of the facet. Immediately after removal from the flame the electrode was submerged in ultrapure water. Due to the large size of the facets, it is possible to coat the polycrystalline regions of the bead with a chemically inert epoxy (Epoxy-Patch), thereby allowing the voltammetric response of the exposed single-crystal facets to be measured. These surfaces, because they have not been mechanically polished, possess an extremely low defect density and are ideal for carrying out kinetic studies. All electrochemical measurements were performed in a singlecompartment, three-electrode Teflon cell using a modified AFRDE-5 bipotentiostat (Pine Instruments), a Hewlett-Packard 6015 B X-Y recorder, and a Nicolet 2090 Digital Oscilloscope. The electrochemical cell was connected to a solution-handling manifold that allowed solutions to be exchanged without exposing the electrode to the laboratory ambient and that allowed all depositions to be carried out under ultrahigh purity (UHP) Ar. All cell and manifold parts that came into contact with the electrolyte were made of Teflon or Kel-F. In all cases, a platinum wire served as the counter electrode and the reference electrode was Ag/AgCl. All voltages are given in reference to this electrode. The solution for the deposition of sulfide was 2 mM Na2S (prepared using Na2S‚9H2O) in 0.1 M CH3COONa/0.01 M KOH. Before adding Na2S‚9H2O, the solution was degassed for 20 min. Cd depositions were performed using 2 mM Cd(CH3COO)2 in 1 mM KOH/2 mM CH3COOH solutions. Oxygen was removed from all solutions by bubbling with UHP Ar for 25 min prior to electrochemical measurements. Millipore-Q purified water was used to make up all solutions. Each electrochemical deposition was preceded by rinsing the working electrode with blank solution.
Results and Discussion Characterization of the Au(111) Electrode. The crystallographic orientation of Au(111) electrodes prepared by flame annealing was verified by scanning tunneling microscopy (STM). Atom-resolved images showed the characteristic sixfold symmetry and interatomic spacing (0.29 nm) expected for this surface.5a In order to measure the voltammetric response of the (111) facets, the polycrystalline portions of the Au bead were (8) (a) Bewick, A.; Fleischmann, M.; Thirsk, H. R. Trans. Faraday Soc. 1962, 58, 2200. (b) Fleischmann, M.; Thirsk, H. R. In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Tobias, C. W., Eds.; Wiley: Interscience: New York, 1963; Vol. 3, p 123. (9) Hsu, T. Ultramicroscopy 1983, 11, 167.
Figure 1. Underpotential deposition of Cu on Au(111). (A) Voltammetric response of an Au(111) electrode immersed in a 1 mM CuSO4/0.05 M H2SO4 solution. The scan rate was 10 mV/s, and the electrode area was 0.0112 cm2. (B) Current density transient for a potential step from 150 to 250 mV.
covered with a chemically inert, electrically insulating epoxy. To verify that the observed voltammetry originated from a single-crystal material, we examined the UPD of Cu at the Au(111) facet electrode (Figure 1A). This result is consistent with earlier reports of Cu UPD with respect to the formal potentials, the relative intensities of the voltammetric features, and the width of the UPD peaks. What is more important, the chronoamperometric response of our electrodes, shown in Figure 1B, is very similar to that reported by Kolb et al.7a The potential step from 150 to 250 mV is associated with peak A' in the cyclic voltammogram and has been shown to correspond to the stripping of an ordered Cu monolayer from Au(111). Two components are observed in the current density transient, a rapid initial decay followed by a slower peaked current density function. In fact, the only significant difference between our results and those of previous workers is that the integrated area of the adsorption component (i.e., the initial decay) is significantly less on our annealed surfaces than on conventionally prepared single-crystal electrode surfaces. We believe this indicates that the defect density is substantially less in the case of an annealed surface due to the fact that there is no mechanical polishing of the specimens. Cyclic Voltammetry. A new (x3 × x3)R30°-S/Au(111) adlayer was deposited immediately before each set of Cd deposition and stripping experiments. A freshly prepared Au(111) electrode was immersed at open circuit in a sulfide solution at pH 12. The electrode potential was scanned oxidatively from -1000 to -750 mV to deposit a submonolayer of S atoms (Figure 2A). The details of this procedure have been previously described and will not be elaborated further.5 The (x3 × x3)R30° unit cell for this structure was determined by STM and is consistent with the in situ studies of Weaver and with what is observed on the (111) surface of fcc metals in ultrahigh
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B A
C
Figure 2. Deposition and stripping of Cd atomic layers at (x3 × x3)R30°-S/Au(111). (A) Cyclic voltammetry of a Au(111) electrode immersed in a 2 mM S2- solution showing the underpotential deposition region. The scan rate was 100 mV/s, and the electrode area was 0.0055 cm2. (B) Cyclic voltammetry of a (x3 × x3)R30°-S/Au(111) electrode immersed in a 1 mM Cd2+ solution in the underpotential deposition region. The scan rate was 100 mV/s, and the electrode area was 0.0075 cm2. (C) Underpotential deposition of Cd on a naked Au(111) electrode. The scan rate was 100 mV/s, and the electrode area was 0.0075 cm2.
vacuum.10 The S-modified Au(111) electrode was rinsed with pure electrolyte, after which the solution was exchanged with the Cd-containing solution. The deposition and stripping voltammetry of Cd at a (x3 × x3)R30°-S/Au(111) surface is shown in Figure 2B. For comparison, the cyclic voltammetry of Cd deposition and stripping on a naked Au(111) electrode is shown in Figure 2C. On the naked surface, the UPD region is characterized (10) Gao, X.; Zhang, Y.; Weaver, M. J. Langmuir 1992, 8, 668.
by a broad couple centered at E1/2 ∼ -80 mV (peaks A1 and C1) and is consistent with what has been reported previously.3,11 It should be emphasized that the appearance of the Cd UPD voltammetry is strongly dependent on the electrolyte composition and pH. For example, the behavior observed by Gewirth and Nuzzo in sulfate at low pH is significantly different from what is reported here.12 The electrochemistry of Cd at (x3 × x3)R30°-S/Au(111) is markedly different from the voltammetry at naked Au(111). The most dramatic change is that the areas under the peaks are noticeably different on the two surfaces. If we integrate the cathodic current in peaks C1 and C2, we calculate the total charge densities of 122 and 192 µC cm-2, respectively, for the naked and S-modified surfaces. The origin of this difference has not been investigated in detail, although Cd is known to form an alloy with Au at potentials close to the UPD potential, and this process may compete with UPD at naked Au.10 We note that the charge density for Cd deposition at the S-modified surface compares well with the value of 187 µC cm-2 observed for S monolayer deposition on clean Au(111), suggesting that approximately equal amounts of the two elements are deposited.13 In addition to the differences in coulometry, the deposition and stripping peak potentials shift to -205 and -75 mV, respectively, on the modified surface, and are sharper, with no tailing as in the case of bare Au(111). It is interesting that a single sulfur adlayer (θ ) 1/3) is sufficient to completely inhibit alloy formation, which is not observed on the S/Au(111) surface. Insight into the mechanism of adlayer formation can be obtained from an analysis of the dependence of the current in peak C2 on the scan rate. Although the scan rate dependence appears linear below 150 mV s-1, when a wider range is studied, a distinctly nonlinear behavior is observed (Figure 3A). Furthermore, the data are also nonlinear when plotted versus the square root of the scan rate (Figure 3B), ruling out the possibility that diffusion-limited mass transport of Cd to the surface contributes to the current in peak C2. Negative deviations from a linear scan rate dependence at high scan rates may be attributed to slow electron transfer kinetics. However, if the peak current is plotted as a function of the 2/3 power of the scan rate, the plot is linear over the entire range of scan rates we studied (Figure 3C). A ν2/3 scan rate dependence is characteristic of a two-dimensional phase transition or a nucleation and two-dimensional growth mechanism.14,15 The model of Camacho et al.11 suggests an additional criterion for twodimensional nucleation and growth, namely that a loglog plot of the peak separation, ∆Ep, between peaks A2 and C2, and the scan rate should have a slope of 2/5. A plot of log ∆Ep against log ν is shown in Figure 3D. We calculate a value of 0.37 for the slope, which is in excellent agreement with the predictions of this model. Completely analogous scan rate behavior is observed for the dissolution of the films, peak A2, and a plot of the dependence of the peak current on ν2/3 is shown in Figure 4. On the basis of this voltammetric data we conclude that Cd deposition and dissolution at (x3 × x3)R30°-S/Au(111) proceed by a nucleation and two-dimensional growth mechanism. Chronoamperometry. In order to understand the growth of CdS monolayers in more detail, potential-step (11) Schultze, J. W.; Koppitz, F. D.; Lohrengel, M. M. Ber. BunsenGes. Phys. Chem. 1974, 78, 693. (12) Bondos, J. C.; Gewirth, A. A.; Nuzzo, R. G. J. Phys. Chem. 1996, 100, 8617. (13) The following analysis will demonstrate that essentially all of the charge passed in the deposition and stripping of Cd is faradaic. For a discussion of S UPD, see: Demir, U.; Shannon, C. J. Electrochem. Soc., to be published. (14) Sanchez-Maestre, M.; Rodriguez-Amaro, R.; Munoz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 1994, 373, 31. (15) Bosco, E.; Rangarajan, S. K. J. Electroanal. Chem. 1981, 129, 25.
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A
B
Figure 4. Scan rate dependence of the Cd stripping wave. Dependence of the peak current in A2 on ν2/3. The solid line is a least squares fit. Experimental conditions are as described in Figure 2.
A
C
B
D
Figure 5. Current density-time transients for Cd deposition and stripping at (x3 × x3)R30°-S/Au(111). (A) Cd deposition, peaks C2. (B) Cd stripping, peak A2. Potential steps are noted next to each transient.
Figure 3. Scan rate dependence of the Cd deposition wave. (A) Dependence of the peak current in C2 on the scan rate, ν. (B) Dependence of the peak current in C2 on ν1/2. (C) Dependence of the peak current in C2 on ν2/3. (D) Plot of log ∆Ep versus log ν. Note: In parts A and B the solid line is a guide to the eye. In parts C and D, however, the solid line is a least squares fit. Experimental conditions are as described in Figure 2.
experiments were performed for both the deposition and stripping of Cd at the S-modified surface. Typical current density-time transients for adsorption and desorption of Cd at (x3 × x3)R30°-S/Au(111) are presented in Figure 5. Consider first the transient recorded for the deposition step (Figure 5A). After an initial decay during which it decreases to about two thirds of its initial value, the current density levels off and passes through a local maximum that occurs at about 32 ms before decaying monotonically to zero at times longer than about 100 ms. It should be noted that the rate of the initial decay is much too slow to be accounted for by double-layer charging and is more typical of what is expected for an adsorption-desorption process. The appearance of a shoulder is consistent with
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the peaked current density-time behavior expected for two-dimensional nucleation and growth and is also consistent with the observed scan rate dependence of our cyclic voltammetric data. However, due to the relatively large adsorption current, the observed peak is less wellresolved than it would be in the absence of adsorption. Finally, the current density reaches a limiting value of zero at long times, which indicates that diffusion of Cd2+ from bulk solution is not involved in the growth of the atomic layers. Qualitatively, the appearance of the current density-time transients observed for the dissolution of the film is similar to what is seen for adsorption, although the stripping process is noticeably faster overall than the deposition step (Figure 5B). The shape of these current density-time transients is characteristic of nucleation and growth accompanied by a parallel adsorption-desorption step. Adsorptiondesorption accounts for the initial decay of the data, while two-dimensional nucleation and growth gives rise to the shoulders observed at longer times. By comparison of the data shown in Figure 5 with those in Figure 1B, it is also clear that the adsorption component is not due to the presence of crystal defects in the Au(111) substrate but is a property of the (x3 × x3)R30°-S adlattice. To confirm this conclusion, we performed potential-step experiments on Au(111) surfaces possessing higher defect densities than the bead electrodes. When a polished single-crystal disk was used as the substrate, for example, adsorption currents in Cu UPD experiments typically accounted for about 10% of the total current density, whereas for the bead electrodes, the adsorption component typically accounted for less than 5% of the total charge. In the case of Cd deposition on the S-modified surface, however, the adsorption current was approximately 50% of the total charge for both substrates. This indicates that, although some adsorption at surface defects undoubtedly occurs, the adsorption component we observe in the potential step experiments is due to Cd adsorption on (x3 × x3)R30°S/Au(111). Several different models have been proposed to account for the chronoamperometric response of systems in which adsorption-desorption competes with nucleation and growth. The simplest model is to consider the current density as the sum of two independent terms, a Langmuirtype adsorption-desorption term and a nucleationgrowth term. This approach can be rationalized on the basis that adsorption is typically a rapid phenomenon, while nucleation and growth develop more slowly due to kinetic limitations.7a If the adsorption-desorption rate constant follows the Butler-Volmer relation, this model can also be interpreted as surface diffusion control of monolayer nucleation and growth.16 In cases where the two processes are not decoupled, a more complex model, such as that of Bosco and Rangarajan, is required.17 In the following discussion, we refer to these two cases as decoupled and coupled kinetics, respectively. In the decoupled case (i.e., when the adsorptiondesorption step is rapid and/or when the lattice incorporation step is slow), the total current density is given by
jtot ) jads + jnucl/gr
(1)
The current density for a Langmuir-type adsorptiondesorption process can be modeled using a simple exponential of the form.18 (16) Armstrong, R. D.; Harrison, J. A. J. Electrochem. Soc. 1969, 116, 328. (17) Bosco, E.; Rangarajan, S. K. J. Chem. Soc., Faraday Trans. 1 1981, 77, 1673. See also: Bhattacharjee, B.; Rangarajan, S. K. J. Electroanal. Chem. 1991, 302, 207. (18) Barradas, R. G.; Bosco, E. J. Electroanal. Chem. 1985, 193, 23.
jads ) k exp(-k’t)
(2)
The problem of the kinetics of monolayer formation by two-dimensional growth has been solved by several authors; our treatment follows that of Bewick, Fleischmann, and Thirsk.8 We consider the two well-known limiting cases: instantaneous nucleation, when the nucleation rate is large and the maximum number of nuclei are formed as soon as the potential is stepped; and progressive nucleation, when the nucleation rate is small and remains essentially constant on the chronoamperometric time scale. The expected current densities for these two cases are
jinst ) at exp(-bt2)
(3)
for the instantaneous case, and
jprog ) ct2 exp(-dt3)
(4)
for the progressive case. In these equations, the preexponential terms give the current density into the step edges of the independently growing nuclei (i.e., the asymptotic behavior at short times). The exponential terms, on the other hand, correct for the decreased step edge length due to the overlap of adjacent nuclei at longer times (i.e., the Avrami theorem). In a potential step experiment a, b, c, and d are constants; explicit formulas can be found in the review by Budevski.19 In the decoupled model, then, the total current density transient for instantaneous kinetics is
jtot ) k exp(-k’t) + at exp(-bt2)
(5)
and that for progressive kinetics is
jtot ) k exp(-k’t) + ct2 exp(-dt3)
(6)
In the coupled case (i.e., if adsorption-desorption and nucleation-growth occur on similar time scales), Bosco and Rangarajan have shown that the current density responses for instantaneous and progressive kinetics are
jtot/qmL ) Gµ exp(-µt) + 2β1t exp(-β1t2) -
∫0µt2β1z exp(µz - β1z2) dz
Gµ exp(-µt)
(7)
and
jtot/qmL ) Gµ exp(-µt) + 3β2t2 exp(-β2t3) -
∫0µt3β2z exp(µz - β2z3) dz
Gµ exp(-µt)
(8)
respectively.17 In these equations, qmL is the total charge due to nucleation and growth (i.e., lattice incorporation), µ is a time scaling factor which is proportional to the rate of the adsorption-desorption step, β1 and β2 are the rates of lattice growth for instantaneous and progressive growth, respectively, and G is a dimensionless parameter that describes the relative importance of adsorption-desorption relative to lattice growth. The terms containing the integrals on the right hand side of eqs 7 and 8 can be thought of as the coupling terms, since, as these terms become vanishingly small, eqs 5 and 6 are recovered. The magnitude of the coupling terms is determined principally by the values of β1, β2 and µ. (19) Budevski, E. B. In Comprehensive Treatise of Electrochemistry; Conway, B. E., Bockris, J. O’M., Yeager, E., Kahn, S. U. M., White, R. E., Eds.; Plenum Press: New York, Vol. 7, p 399-450.
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B
C
Figure 6. Fits of the current density-time transients shown in Figure 5 to the decoupled and coupled kinetics models. Cd depostion transient fit using the decoupled model (A) and the coupled model (B). Cd stripping transient fit using the coupled model (C). Experimental points are indicated by the filled diamonds.
In Figure 6 we analyze the potential-step data shown in Figure 5 using the two kinetic models described above. Parts A and B of Figure 6 show fits of the Cd deposition data to the decoupled and coupled kinetics models, respectively. Both models give an excellent fit and indicate that the deposition of Cd on the (x3 × x3)R30°-S/Au(111) surface is best described as adsorption accompanied by instantaneous nucleation and two-dimensional growth of the CdS monolayer. Both the relatively large adsorption current and fact that terrace growth proceeds by instantaneous kinetics (i.e., formation of a large number of small nuclei as soon as the potential is stepped cathodically) are consistent with the strong chemisorption of Cd on this surface. Figure 6C shows a fit of the Cd stripping data to the coupled kinetic model. Using the decoupled model, it was not possible to fit the data using either instantaneous or progressive kinetics. In fact, the data appeared
Demir and Shannon
to follow instantaneous kinetics at short times and progressive kinetics at longer times. This interpretation is unphysical, however, since by definition instantaneous kinetics means that the maximum number of nuclei have formed at t ) 0. Thus, the inability to fit the data must be due to the fact that random desorption and dissolution of the CdS lattice proceed with comparable rates, such that the assumptions of the decoupled model are no longer valid. This view is confirmed by the excellent fit of the data obtained using the coupled kinetics model (Figure 6C). The coupled kinetics model indicates that Cd stripping proceeds by a two-step mechanism in which random desorption of Cd atoms occurs in parallel with progressive dissolution of the Cd adlattice. Coulometric data and the kinetic parameters obtained from fits of the chronoamperometric transients are given in Table 1. In the case of Cd adsorption, it is interesting to note that both kinetic models give an identical fit to the experimental data. This can be seen more clearly if the fit parameters obtained using the decoupled model are employed to calculate values for qmL, β1, µ, and G (i.e., the fit parameters in the coupled model). The relationship among the variables can be determined straightforwardly by setting the coupling term in eq 7 to zero and equating the coefficients of the two remaining terms on the right hand side of eqs 5 and 7. We find that the calculated fit parameters (Table 1) are identical to the values obtained for the best fit using the coupled kinetics model, indicating that the coupling term in eq 7 is vanishingly small. The fraction of the total charge passed that is due to twodimensional lattice growth can be calculated from qmL. Integrating the chronoamperometric transient (157 µC cm-2) and using the value of 97.3 µC cm-2 found for qmL, we calculate that 62% of the total charge passed is due to lattice growth. Assuming that the random adsorption step is purely faradaic, we calculate a value of 59.7 µC cm-2 (38%) for qmA, the charge due to random adsorption. This is about six times larger than what we observed in the case of Cu UPD. In the case of strong adsorption, qmA can be calculated from G ) qmA/qmL, which gives a value of 51.1 µC cm-2, in excellent agreement with the previous estimate. Finally, β1, which is a measure of the rate of lattice growth, is quite small in this case. This can be seen by comparing β1 for Cd on S/Au(111) to the value measured by Kolb et al. for Cu UPD on Au(111).7a The β1 value for Cu on the naked Au(111) surface was a factor of 40 larger than that measured here. We now consider the Cd stripping data. In this instance, of course, a direct comparison of the two kinetic models cannot be made. As in the case of Cd adsorption, we find that the rate of lattice growth (i.e., β2) is small. However, the most striking thing about the desorption data is the low value of qmL. Only 38% of the total stripping charge is due to lattice dissolution. We believe that this is the result of a lattice compression that occurs upon deposition of Cd onto the (x3 × x3)R30°-S adlattice. Our previously reported atomic resolution STM study of the CdS-Au(111) system showed that the interatomic spacing in the Cd-S monolayer is 0.43 nm compared to 0.50 nm observed in the (x3 × x3)R30°-S adlattice, a lateral compression of about 14%.5a This structural change leads to a more polycrystalline adsorbed CdS layer and, hence, a larger random desorption component. The growth and dissolution of the monolayer can now be understood completely as follows. In the formation of the monolayer, the large initial current density is due to rapid adsorption of Cd atoms on (x3 × x3)R30°-S/Au(111), presumably at the threefold hollow sites. The fact that Cd is strongly chemisorbed acts to lower its surface mobility (i.e., relative to its value on the naked Au(111) surface); therefore, a relatively smaller fraction of the
Electrochemistry of Cd at (x3×x3)R30°-S/Au(111)
Langmuir, Vol. 12, No. 25, 1996 6097
Table 1: Coulometric and Kinetic Parameters for Cd Deposition and Stripping experiment
model
β1 (s-2)
Cd deposition
decoupleda,b
4.20 × 4.20 × 10-4
Cd stripping
coupledc decoupled coupledd
β2 (s-3)
10-4 2.16 × 10-5
G
µ (s-1)
qmL (µC cm-2)
qtot (µC cm-2)
qmA (µC cm-2)
0.525 0.525
10-2
8.35 × 8.35 × 10-2
97.3 97.3
157 157
59.7 51.1
2.27
5.60 × 10-2
57.4
151
93.6
a
Calculated using eqs 5 and 7. See text for details. b Mean deviation of fit, 0.1140; standard errors for each parameter were less than 5%. c Mean deviation of fit, 0.1140; standard errors for each parameter were less than 5%. d Mean deviation of fit, 1.38; standard errors for each parameter were less than 14%.
lattice grows by two-dimensional nucleation and growth than is the case in a typical UPD process. In the case of monolayer dissolution, on the other hand, the most interesting finding is the unexpectedly low value for qmL, which indicates that most of the Cd desorbs randomly from the surface rather that by step propagation (i.e., lateral expansion of ordered pits which nucleate in the Cd adlattice). Scanning probe microscopy reveals that a significant lattice compression accompanies the adsorption of Cd and implies that the CdS monolayer is composed of numerous relatively small CdS(111) domains from which Cd atoms randomly desorb. Conclusions In summary, we have investigated the electrochemistry of Cd on (x3 × x3)R30°-S/Au(111) in order to understand the kinetics of CdS monolayer growth. Our most important finding is that the formation and dissolution of the Cd monolayer proceeds by a two-step mechanism involving Langmuir adsorption accompanied by nucleation and twodimensional growth. In the case of Cd deposition, these two steps are temporally decoupled. Nucleation and growth of the CdS monolayer, which accounts for about 62% of the total charge passed, occurs by the rapid initial formation of a large number of small nuclei which subsequently grow by radial expansion (step propagation) in competition with random adsorption of Cd. The relatively large adsorption component is a result of the high sticking probability of Cd as well as its low mobility on (x3 × x3)R30°-S/Au(111). On the other hand,
stripping of Cd occurs by a progressive mechanism with a characteristic timescale comparable to that of random desorption. In contrast to what was observed in the case of Cd adsorption, a significantly higher fraction of the Cd desorbs randomly than by dissolution of the ordered phase. This is due to structural changes in the adlattice that accompany CdS formation and result in a monolayer that is composed of small domains of CdS(111). Growth of a well-ordered initial monolayer is key to achieving high-quality electrosynthesized materials and thin films. Our results suggest that much more careful attention must be paid to electrodeposition conditions such as the applied overpotential. Specifically, in the future, monolayer quality may be significantly improved by using a series of voltage pulses to prepare a controlled number of nuclei on the surface and allowing the monolayer to form at very low overpotentals. Finally, we have presented a method for preparing single-crystal electrodes that are characterized by extremely low defect densities. These surfaces are simple to fabricate and are especially promising for use in studies of the kinetics of surface electrochemical processes. Acknowledgment. The financial support of this research by the National Science Foundation (Grant OSR9553348), the Society of Analytical Chemists of Pittsburgh, and Auburn University is gratefully acknowledged. LA960225G
