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Anion Adsorption and Adsorption/Desorption Kinetics onto/from Au(111) Electrodes Studied Using the Indirect Laser-Induced Temperature Jump Technique John F. Smalley* and Yi-Chyi Wu† Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States ABSTRACT: We demonstrate that the potential dependence of the initial change (A/ΔTeq) in the open-circuit potential of Au(111) | (nonelectroactive) electrolyte solution interfaces (effected by the temperature perturbation (ΔTeq) induced by the indirect laser-induced temperature jump (ILIT) technique) is sensitive to the presence of the anion components of the electrolyte adsorbed onto the electrode surfaces for a wide variety of anions. Analysis of the potential dependence of A/ ΔTeq for the ClO4- and SO42- anions, therefore, is used to ascertain (for ClO4-) or estimate (for SO42-) the temperature derivative of the dipole potential (i.e., (dVD)/(dT)) associated with the relevant Au(111) | electrolyte solution interface. The value and any potential dependence of (dVD)/(dT) are suitable probes of the structure of the pertinent electrode | electrolyte interface. The negative value determined here for the (dVD)/(dT) associated with a complete (saturated) layer of ClO4- ions adsorbed onto a Au(111) electrode surface (i.e., -(0.51 ( 0.08) mV K-1) is, for example, consistent with a picture of this layer where the hydrogen atoms of the water molecule constituents of this layer originally (before the temperature perturbation) point, as should be expected, are toward the adsorbed ClO4- ions. Additionally, we demonstrate that studies of the potential dependence of the adsorption/ desorption kinetics of adsorbate ions may also be used as a structural probe of the electrochemical double layer containing the adsorbed ion. Accordingly, as the result of a study of the adsorption/desorption kinetics of Cl- ions adsorbed onto a Au(111) electrode, we speculate that the first layer of water molecules in contact with this surface is compressed.

’ INTRODUCTION In any area of chemistry, structure determines function. This is certainly true for electrochemistry where the relevant structures include the physical arrangements of the ions and dipoles that comprise the electrochemical double layer. We originally developed the indirect laser-induced temperature jump (ILIT) technique to study the kinetics of fast heterogeneous charge-transfer reactions taking place at solid metal electrode | electrolyte interfaces.1,2 However, the size of the initial, open-circuit response of the interface3 to the ILIT temperature perturbation contains information on these arrangements of ions and dipoles.3,4 This information along with knowledge of the potential of zero charge (pzc) of the solid metal electrode5-11 is important for a detailed understanding of the rates and mechanisms of electrochemical reactions (including the adsorption/desorption reactions of ions specifically adsorbed onto electrode surfaces1b,12). Comprehending the chemical kinetic details of these reactions is of increasing importance for the realization and application of a large number of technologies. For example, the practical application of hydrogen fuel cells depends upon a comprehensive understanding of the physical and chemical factors that determine the rates of both the hydrogen oxidation12,13 and oxygen reduction12,14 electrochemical reactions. Additionally, the potential drop across the double layer (which is, of course, caused by the arrangements of ions and dipoles that comprise the double layer4) influences the r 2011 American Chemical Society

kinetics of the interfacial electron-transfer reactions2 that are of importance in biosensor,15 photodiode,16 solar photoconversion,17,18 and molecular electronic19 technologies. We emphasize that the initial, open-circuit response to the ILIT temperature perturbation (denoted as A) is a function of the concentration of charge (σM (C/cm2)) on the surface of the solid metal electrode effected by the initial (before the laser pulse) potential (denoted as Ei) set using the special20 ILIT apparatus potentiostat. (The reason for this is that the temperature derivative of σM (dσM/dT) is, initially, zero2 because there is no time for any reaction to cause a transfer of charge across the double layer and, thereby, change σM.) In the absence of specific adsorption of ions onto the surface of the electrode and if the change in the double layer capacitance caused by the temperature perturbation is very fast1-4 (i.e., much faster than the perturbation), the initial response to this ILIT temperature perturbation (A) is given by1-3 ΔTeq dVD σM d ln½CT A ¼ þ ΔTeq bSoret ð1Þ dT G dT CT where CT (F/cm2) is the total, integral, specific capacitance associated with the double layer at the (working) electrode | electrolyte Received: August 9, 2010 Revised: November 5, 2010 Published: January 25, 2011 2693

dx.doi.org/10.1021/jp1075077 | J. Phys. Chem. C 2011, 115, 2693–2704

The Journal of Physical Chemistry C

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interface,21 and VD is the dipole potential associated with the entire double layer (i.e., the potential induced by the vector sum of the dipoles associated with all of the constituents of the double layer). The quantity ΔTeq is the interfacial temperature change that would be produced by the heat energy absorbed from the laser pulse if none of this absorbed heat were lost to either the dielectric “backing”2-4 of the electrode or the electrolyte solution, and bSoret is a coefficient that describes the Soret potential induced by the temperature difference between the electrode and the bulk of the electrolyte solution.1,23 From the discussion above σ M ¼ CT ðEi - VD Þ so that A ΔTeq

1 dVD d ln½CT - ðEi - VD Þ ¼ þ bSoret G dT dT

ð2Þ

ð3Þ

(Note that VD, dVD/dT, and (d ln[CT])/dT individually can be functions of Ei, and that, from eq 2, VD = Epzc (the potential of zero charge) when Ei = Epzc.) The factor G in eqs 1 and 3 is defined as3 d ln½CT dVD G ¼ 1 þ ðEi - VD Þ ð4Þ dE dE If CT, VD, dVD/dT, and (d ln[CT])/dT do not vary as a function of potential (E), then G = 1 and a plot of A/ΔTeq versus Ei will be linear.2 Also, from eq 3, the potential at which A/ΔTeq = 0 (Ei,pzr)2,4 is given by2 dVD ðEi, pzr Þ þ GðEi, pzr ÞbSoret Ei, pzr ¼ VD ðEi, pzr Þ þ dT ð5Þ d ln½CT ðEi, pzr Þ dT where Ei,pzr refers to the assertion above that VD, dVD/dT, and (d ln[CT])/dT (as well as G) may be functions of potential. Equations 2 and 3 demonstrate that the potential difference between the electrode and the solution as well as the temperature derivative of this potential difference depend on both the charge (σM) separated across the entire double layer and the dipole potential associated with this double layer.1-4,24 Ei,pzr, also, is not simply VD, except in the very unlikely event that [(dVD/dT)(Ei,pzr) þ G(Ei,pzr)bsoret] = 0.25 The specific adsorption of ions (most notably protons12 and the anion components of the electrolyte26a) has a significant impact on the composition, structure, properties, and, consequently, behavior of a solid metal electrode | electrolyte interface.12,26-41 In regard to this behavior, specific adsorption of ions engenders a significant (initial) response of the diffuse double layer to an ILIT temperature perturbation,4 so that21 A 1 dVD d ln½CT - ðEi - VD Þ ¼ ΔTeq G dT dT σ ad d ln½CDL d ln½CT dT dT CDL σad d ln½CO d ln½CT ð6Þ þ bSoret dT dT CO where σad (C/cm2) is the surface concentration of charge associated with the adsorbed ion,42 CO (F/cm2) is the specific integral capacitance associated with the outer layer portion of the electrochemical double layer,21 and CDL (F/cm2) is the integral,

specific capacitance of the diffuse double layer. It must be emphasized with respect to eq 6 that VD, dVD/dT, and d ln[CT]/dT (as well as (d ln[CT])/dE and dVD/dE in eq 4) all may be functions of σad. As indicated earlier, the properties and performance of an electrochemical double layer are intimately related to the orientation of the dipoles of the species that comprise this interface. As an example of this, the Epzc of a metal electrode | electrolyte interface may be expressed as7,43 ΦM ΦSHE - ςdl ðEpzc Þ ð7Þ Epzc ðor VD Þ ¼ e e where ΦM is the work function (in eV) of the metal electrode, e is the charge on an electron, ςdl is the potential contribution associated with both any adsorbed ions and the dipoles of the solvent molecule constituents of the metal | electrolyte interface, Epzc refers again to the assertion that VD may be a function of potential, and ΦSHE is the work function (4.5 ( 0.2 eV)7,12,44a of the standard hydrogen electrode.44b Since knowledge of Epzc is of fundamental importance for a detailed understanding of double layer phenomena, electrochemical kinetics, and the adsorption of both charged and neutral species,7,45 any measurement of the parameter VD, as well as molecular dynamics simulations46 that provide information on this parameter, also contributes to our comprehension of the properties and behaviors of the metal electrode | electrolyte interface.38a,38b,38d,47 It is not only the static structure of the solvent dipoles (and, consequently, the sign and magnitude of VD) at the electrode | electrolyte interface that controls the behavior of an electrochemical system, but also the dynamics of dipole reorientation (in response to a change of the electric field across the double layer associated with the interface and/or the temperature of this interface).24,46,48,49 The ILIT technique is, in theory, well suited to measure the dynamics of dipole reorientation at electrochemical interfaces.1,2 However, the time resolution of the ILIT apparatus3 employed in the present study was insufficient to determine the expected subnanosecond time constants associated with these dynamics.49 Nevertheless, this ILIT apparatus was quite capable1 of measuring the kinetics of interfacial ion transfer (i.e., adsorption/desorption) reactions,12,50,51 many of which are relevant to technologically important electrochemical processes.12 In the present paper, we report on measurements of the initial responses (A/ΔTeq) to ILIT temperature perturbations of gold electrodes in contact with various aqueous electrolyte solutions. The crystal facets that comprise the surfaces of the vapor-deposited Au film electrodes employed in these measurements had an approximately 111 cystallographic orientation.2,3 Accordingly, knowledge (obtained using techniques other than ILIT) of the adsorption isotherms of particular ions (i.e., ClO4- and SO42-) on Au(111) surfaces31b,38a,39c can be used to derive (or approximate) values for the VD and dVD/dT associated with an Au(111) electrode | electrolyte interface containing one of these adsorbed anions. We stress that these values of VD and dVD/dT are relevant to the nature and structure of these electrode | electrolyte solution interfaces which, in turn, determine the electrochemical behavior of these interfaces. The unique ability of the ILIT technique to measure the kinetics of very fast interfacial charge-transfer reactions on macroelectrodes1,2 was also employed in the present study to determine the rate constants for the adsorption/desorption reactions of the Cl- ion from (approximately) 111 Au surfaces as a function of potential. Analysis of the potential dependence of the rate 2694

dx.doi.org/10.1021/jp1075077 |J. Phys. Chem. C 2011, 115, 2693–2704


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constants for this charge-transfer reaction can provide information on the potential distribution within and, therefore, the structure of an electrochemical double layer associated with specifically adsorbed Cl- ions.

’ EXPERIMENTAL SECTION The ILIT apparatus, cell, and experimental methods have all been completely described in other publications.1-3,52 The ∼1.0 μm thick Au film electrodes used in the present study were vapor deposited over an approximately 500 Å thick layer of either Cr or Ti which had previously been vapor deposited on a quartz disk. The Cr and Ti layers constituted the “back” sides of these electrodes (see below), and the microcrystallites that comprise the “front” surfaces (the surfaces that were placed in contact with the electrolyte solutions) of these electrodes effect an approximately uniform 111 orientation3,52 for both Cr and Ti. We emphasize that, in ILIT, the temperature perturbation results from an “indirect” excitation of the electrode (i.e., the laser pulse that effects this perturbation impinges onto the back side of the electrode53) rather than a “direct” excitation of the electrode surface (i.e., the front side of the electrode which is in contact with the electrolyte solution) as in other recent studies.24,25,49 Of the many advantages associated with indirect excitation of the electrode,1,2 we stress that, at least for the first microsecond subsequent to the laser pulse for the nominally 1.0 μm thick Au electrodes employed in the present study, the time dependence of the temperature perturbation approximates that of a step function. This slowly decaying temperature perturbation facilitates the analysis54 of the kinetics of interfacial charge-transfer reactions such as the desorption of Cl- ions investigated in the present study.55 If the electrolyte solution did not contain either (or both) a redox couple (see below) or Cl- ions in the present study, the time dependence of an ILIT open-circuit response (ΔVoc(t)) is always well described by ð8Þ ΔVoc ðtÞ ¼ AΔT ðtÞ where

ΔT ðtÞ ¼ ΔTðtÞ=ΔTeq

ð9Þ

and ΔT(t) describes the time dependence of the ILIT temperature perturbation (see eqs 2-6 in ref 3) convoluted with the response function of the ILIT apparatus (see Figure 6 in ref 2). The quantity ΔTeq was measured by placing two large (and equal, i.e., [FeCN63-] = [FeCN64-]) concentrations (typically, one concentration was between 7.0 and 10.0 10-3 M, while the other concentration was between 17.0 and 19.0 10-3 M) of the ferri/ferrocyanide couple into the electrolyte solution at the end of the ILIT experiment and determining the change in the equilibrium potential (ΔVeq, see eq 29 in ref 56)57 associated the ILIT transients with this couple (and effected by ΔTeq) from 0 measured at the formal potential (E0 ) of this couple.56,58 The measured value of ΔTeq then was ΔTeq ¼ ΔVeq =ðbr þ bSoret Þ 00

ð10Þ -1

where br = dE /dT = -(1.86 ( 0.10) mV K for the ferri/ ferrocyanide couple (calculated using the results in ref 56),59 and bSoret was calculated, for each electrolyte solution, using the procedures described in refs 1a and 23 and the data in refs 60 and 61. Aldrich high purity (at least 99.99% pure) potassium fluoride, potassium chloride, potassium sulfate, sodium fluoride, and

Figure 1. Example of an ILIT (open-circuit) response (for ΔTeq = 3.64 K) of a Au(111) electrode (thickness = 0.704 10-4 cm) in contact with a 0.10 M KF aqueous electrolyte solution at Ei = 550 mV versus SCE. The red curve describes a fit of these data to eq 8. The parameter A obtained from this fit is -1.00 mV.

sodium chloride were all used as received. The water used in the present study was purified in a Labconco Water Prodigy. (This purified water was, in all respects, equivalent to that from a Millipore Mill-Q Plus system.) Reagent grade sodium perchlorate (NaClO4 3 H2O) was recrystallized twice from this purified water, and reagent grade potassium ferricyanide and potassium ferrocyanide were both recrystallized three times from this purified water. J. T. Baker Ultrex ultrapure concentrated perchloric acid and sulfuric acid were also used as received. The cyclic voltammograms (CVs) of the electrolyte solutions on the Au film electrodes (taken before each ILIT experiment) were consistent with the extant published CVs of these electrolyte solutions on single crystal Au(111) electrodes, and all of the ILIT experiments were performed at room temperature ((28 ( 2) °C).

’ RESULTS AND DISCUSSION ILIT Measurements of A/ΔTeq versus Ei for Various Aqueous Electrolyte Solutions. Figure 1 contains an example of

the ILIT (open-circuit) responses observed in the present study for all of the electrolyte solutions investigated (except those that contained Cl- ions).62 The most important thing to note about the ILIT transient shown in Figure 1 is that the fit of the ΔVoc(t) versus time (t) data to eq 8 is very good. As mentioned in the Experimental Section, fits of similar quality to eq 8 may be obtained for all of the aqueous electrolyte solutions investigated in the present study (again, with the exception of those containing Cl- ions;see Figure 1 in ref 1b and Figure 7 below). These good fits to eq 8 signify that the rates at which the properties (e.g., the capacitance) of the entire electrochemical double layer respond to the change in temperature are extremely large. Figure 2 shows plots of A/ΔTeq (corrected for the Soret potential coefficient (bSoret)) versus potential (Ei) for all of the aqueous electrolyte solutions included in the present study. (Table 1 contains the values of bSoret determined1,23,60,61 for all of these electrolyte solutions.) In all of the measurements reported in Figure 2, one value of Ei (typically, Ei = -200 mV vs SCE) is 2695



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Figure 2. Plots of A/ΔTeq corrected (shifted) by the relevant Soret coefficient (see Table 1) versus potential (Ei) for various electrolyte solutions (described in the legend) in contact with Au(111) electrode surfaces. For the 1.0 M KF electrolyte solutions, the symbol O refers to a Au electrode “backed” by Ti, and the symbol X refers to a Au electrode backed by Cr (see the text).

Table 1. Soret Coefficients (bSoret) for Aqueous Electrolyte Solutions at T = 25 °Cb electrolyte solutiona KF (1.0 M)

a

bSoretb/mV K-1 0.394

KF (0.1 M)

0.428

NaClO4 (1.0 M)

-1.04

HClO4 (0.1 M)

-0.538

K2SO4 (0.5 M)

0.172

NaF (1.0 M)

0.111

NaF (0.99 M) þ NaCl (0.01 M) NaF (0.9 M) þ NaCl (0.1 M)

9.78 10-2 3.87 10-2

KCl (1.0 M)

-6.18 10-3

KCl (0.1 M) þ KF (0.9 M)

0.342 b

The concentration of the electrolyte is in parentheses. Calculated using procedures found in refs 1a and 23 and data found in refs 60 and 61.

designated as a standard, and the value of [(A/ΔTeq) - bSoret] at this Ei is measured several times over the course of a specific ILIT experiment63 (i.e., investigating a specific electrolyte solution). Except for 1.0 M KF (for which all of the data points contain error bars), the scatter in the values of [(A/ΔTeq) - bSoret] measured only at this standard value of Ei is denoted by an error bar. The small sizes of these error bars demonstrate that the characteristics of the electrode | electrolyte solution interfaces do not change during the course of all of these ILIT experiments.63 The first item of note in regard to the data contained in Figure 2 is that, for 1.0 M KF electrolyte solutions, the values of [(A/ΔTeq) - bSoret] (as a function of Ei) measured for electrodes “backed” by either Cr or Ti layers are the same (within experimental error). This observation supports the contention that, at least for the present study, it does not matter whether the

Figure 3. Proposed schematics of (positively charged) metal electrode surface | (1:1) electrolyte solution interfaces: (a) Neither the cation (red-filled ovals) or anion (blue-filled ovals) components of the electrolyte adsorb onto the surface of the electrode; the green-filled ovals represent the solvent molecules, and the arrows inside of these green-filled ovals represent the dipoles associated with these solvent molecules. (As usual, the points of these arrows represent the positive charge associated with these dipoles. Also, note that “IHP” and “OHP” refer to the “inner” and “outer” Helmholtz planes of the interface.) (b) Some of the anions are specifically adsorbed onto the electrode surface. (In each of these schematics, only two of the solvent molecules that constitute the second interfacial layer of solvent molecules (between the IHP and OHP) are pictured. Also, none of the solvent molecules that constitute the solvation spheres of the ions in the bulk of the electrolyte solution are pictured in these schematics.).

(working) electrode is backed by either metal (Cr or Ti). Also, as found in a number of inquiries conducted in other laboratories,5,38a,38b,39,64 the surfaces of all of the carefully prepared65 Au film electrodes used in the present study provide a very good approximation to the surfaces of Au(111) single crystal electrodes. 2696


The Journal of Physical Chemistry C The most important thing to note about the plots in Figure 2 is that, except for the electrolyte solutions containing ClO4anions, all of the values of [(A/ΔTeq) - bSoret] are negative. This observation indicates that the anions of each of the electrolyte solutions described in Figure 2 chemisorb onto the surface of a Au(111) electrode26a because the ClO4- is only weakly chemisorbed,29,38a the diffuse double layer term in eq 6 is generally66 negative for a negatively charged adsorbate, and VD (as well as (dVD)/(dT)) should become more negative in the presence of a negatively charged adsorbate (see below and Figure 3).67 Additionally, and of most consequence for the present study, the results described in Figure 2 (as well as the [SO42-] dependence of (A/ΔTeq);see below) demonstrate that the quantity [(A/ ΔTeq) - bSoret] is sensitive to the presence of adsorbed ions on electrode surfaces.3,56 A further analysis of the data in Figure 2 (along with the results from other ILIT experiments) for the ClO4and SO42- anions will be presented in the following two sections. Adsorption of ClO4- on Au(111) Measured Using ILIT. The data for the aqueous electrolyte solutions containing the ClO4- anion from Figure 2 are replotted in Figures 4a and b. Despite the observation31b,38a,39c that the ClO4- anion is only weakly adsorbed onto Au(111) surfaces at rather positive potentials, the behavior of [(A/ΔTeq) - bSoret] in Figures 4a and b does describe this adsorption (see below). Additionally, the substantial differences between the values of [(A/ΔTeq) - bSoret] observed (at moderately positive potentials) for the NaClO4 and HClO4 electrolyte solutions in Figure 2 can be ascribed to the presence of hydronium ions adsorbed on the surface of the electrode in contact with the acidic solution.38a,38b,69,70 Figure 4a also includes a plot of σM (determined31b for the surface of a (single crystal) Au(111) electrode in contact with a 0.10 M HClO4 electrolyte solution) versus Ei. It is apparent from this plot and the discussion in ref 68 that the ClO4- adsorbs onto the surface of the Au(111) electrode as the potential becomes more positive than ∼300 mV vs SCE, and this adsorption of ClO4- saturates at Ei ∼ 450 mV vs SCE. (The adsorption of ClO4- onto the surface of a Au(111) electrode just described is consistent with the plot of charge (as a function of potential) due to the specific adsorption of ClO4- onto a Au(111) electrode surface featured in Figure 6 of ref 38a) At potentials more positive than 450 mV vs SCE, the plot of σM versus Ei in Figure 4a is linear, which also indicates that the adsorption of ClO4saturates and that the composition and structure of the double layer are stable at these positive potentials. Accordingly, the plot of [(A/ΔTeq) - bSoret] versus Ei is also (within experimental error) linear for Ei > 450 mV vs SCE (i.e., G (defined in eq 4) = 1.0 at these potentials), and the slope of the least-squares regression line (i.e., the black line in Figure 4a) fitted to these data may be identified as (-d ln[CT])/(dT). Additionally, if it is assumed that there is no charge transfer between the adsorbed ClO4- ion and the Au electrode, σad = -1.1 10-5 C cm-2 at these positive potentials.38a The procedures described in ref 4a may then be used to calculate both CDL and (d ln[CDL])/(dT) so that the quantity -(σad/CDL)(((d ln[CT])/(dT)) - ((d ln[CT])/ (dT))) is determined to be -(0.19 ( 0.03) mV K-1 over the potential range of 500-900 mV vs SCE71 (calculated using the value of (-d ln[CT])/(dT) determined in the linear regression analysis that generated the black line in Figure 4a). Since the calculated values of CDL are all larger than CT at these positive potentials (the slope of the blue line in Figure 4a) and the (integral) capacitance of the “inner layer”21 should also be very much larger than CT, we assume that the factor -(σad/CO)(((d ln[CO])/(dT))

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Figure 4. Data from Figure 2 for the aqueous electrolyte solutions containing ClO4-: (a) Plots of both [(A/ΔTeq) - bSoret] (black open circles) and σM31b (blue-filled circles, see the text) versus Ei determined for Au(111) electrodes in contact with 0.10 M HClO4 electrolyte solutions. Black line (ILIT data): least-squares slope ((-d ln[CT])/(dT)) = (1.7 ( 0.1) 10-3 K-1 and intercept (eq 11) = -(0.62 ( 0.06) mV K-1. Blue line (saturated adsorption of ClO4- onto the Au(111) surface; line fitted between Ei = 475 and 900 mV vs SCE): least-squares slope = (42.1 ( 0.2) μF cm-2 and intercept = -(2.96 ( 0.17) μC cm-2, giving Epzc(VD) = (70.5 ( 4.4) mV vs SCE. Red line (no adsorption of ClO4- onto the Au(111) electrode surface; fitted between Ei = 175 and 300 mV vs SCE): least-squares slope = (46.7 ( 2.4) μF cm-2 and intercept = -(13.2 ( 1.9) μC cm-2, giving Epzc(VD) = (283. ( 26.) mV vs SCE. (b) Plot of [(A/ ΔTeq) - bSoret] versus Ei for a Au(111) electrode in contact with a 1.0 M NaClO4 electrolyte solution. See the text for a full discussion of the straight lines in these plots.

- ((d ln[CT])/(dT))) is negligible. The intercept (Ib = -(0.62 ( 0.06) mV K-1) of the black line in Figure 4a, therefore, becomes d ln½CT dVD σad d ln½CDL d ln½CT þ Ib ¼ VD ð11Þ dT dT dT dT CDL From eq 2 (which describes the blue line in Figure 4a), VD for a 2697


The Journal of Physical Chemistry C Au(111) electrode (where the coverage of adsorbed ClO4- ions is completely saturated) is determined to be (70.5 ( 4.4) mV vs SCE. Using eq 11, (dVD)/(dT) then becomes -(0.51 ( 0.08) mV K-1. The negative value for (dVD)/(dT) determined above is consistent with the schematic of an electrochemical interface (double layer) containing an electrode surface-adsorbed anion in Figure 3b and the discussion in ref 68 In the presence of a specifically adsorbed anion, therefore, an increase in the random thermal collisions caused by the higher temperature should oppose the inclination72 of the dipoles of the solvent (water) molecule constituents of the double layer to align with the electric field associated with the adsorbed anions.11 In other words, the oxygen atoms in these water molecules are more likely to point toward the (positively charged) electrode at the higher temperature.11 Alternatively, in the absence of specific adsorption (of anions) on Au(111) electrode surfaces (i.e., at potentials negative of the pzc in electrolyte solutions containing only ClO4- anions), an increase in temperature apparently effects an increase in VD (see Figure 4b).73,74 Since it has been confirmed38a that the first layer of water molecules exists in an “oxygen up” (i.e., the oxygen atoms in these molecules point away from the electrode surface) configuration on Au(111) surfaces at potentials negative of the pzc, the observation of a positive value of (dVD)/(dT) at74 or negative of the pzc seems to contradict the above conclusion that an increase in temperature should counteract the tendency of the dipoles associated with the solvent molecule constituents of a double layer to align with the extant (interfacial) electric field.75 However, both the configuration and properties of the water molecules coadsorbed38b,76 with an anion (or a cation, or a neutral species24) on a metal surface should be different than those of a neat layer (or layers) of adsorbed water.37,38a,38b,69 It is, therefore, not surprising that the response of this pure layer (or layers) to a change in temperature is unlike that of a mixed layer (or layers) of adsorbate ions and water molecules. Nevertheless, a complete description of the causes of these different responses to an increase in temperature requires more data (e.g., from experiments performed in different solvents). Adsorption of SO42- on Au(111) Measured Using ILIT. Because Au electrode surfaces are minimally electrocatalytic and, therefore, are ideally polarizable over a broad range of potentials, the adsorption of even strongly chemisorbed species is reversible.31a The sulfate (or bisulfate) anion is one such strongly chemisorbed species whose structure and adsorption isotherms on both polycrystalline Au and Au(111) surfaces have been extensively studied.26,31,36,37 Accordingly, Figure 5 shows the effect (on the initial ILIT response (A) as a function of Ei) of the addition of small amounts of sulfuric acid to a 0.10 M HClO4 electrolyte solution77 in contact with a Au(111) electrode. (The quantity [SO42-] in Figure 5 is the sum of the sulfate and bisulfate concentrations in the electrolyte solution. However, it has been determined31b that SO42- is the adsorbed species on Au(111) (even if HSO4- is the predominant species in the electrolyte solution).) It is apparent from the data in Figure 5 that, at positive values of Ei, the addition of sulfate/bisulfate effects a negative change in A/ΔTeq, and, at a particular [SO42-], the size of this negative change increases as Ei becomes more positive. Additionally, the behavior of A/ΔTeq shown in Figure 5 is consistent with the adsorption isotherms (inferred (as a function of potential) from the data in refs 31a and 31b) of the sulfate anion onto Au(111). As was done for chemisorbed ClO4- (see the discussion related to Figure 4a above), the adsorption isotherms inferred for SO42- provide the opportunity to calculate the diffuse double

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Figure 5. Effect (on A/ΔTeq) of the addition of small amounts of SO42- to a 0.10 M HClO4 electrolyte solution in contact with a Au(111) electrode measured at various values of Ei (see the legend).

layer contribution (i.e., -(σad/CDL)(((d ln[CDL])/(dT)) - ((d ln[CT])/(dT))) = Z(Ei)) to Y(Ei) (also defined in Figure 5) as a function of [SO42-]. If it is assumed that there is minimal charge transfer between the adsorbed SO42- ions and the Au(111) electrode, then σad = -2FΓSO42-;where ΓSO42- is the surface concentration of adsorbed SO42- (determined in ref 31b). For the current calculation of Z(Ei), it may also be assumed that (-d ln[CT])/(dT) is the same as that (i.e., (1.6 ( 0.1) 10-3 K-1) determined in Figures 4a and b. As seen in Figure 6a, the values of Z(Ei = 900 mV vs SCE) calculated with these assumptions (as well as Z(Ei = 900 mV vs SCE) corrected for the diffuse double layer contribution effected by an assumed loss of the adsorbed ClO4- (the green points in Figure 6a)) is uniformly (at all values of [SO42-]) more negative than the corresponding value of Y(Ei = 900 mV vs SCE). The comparison between Z(Ei = 900 mV vs SCE) (or (Z(Ei = 900 mV vs SCE) þ 0.19 mVK-1)) and Y(Ei = 900 mV vs SCE) in Figure 6a, therefore, is consistent with the expected67,68,78 change (more negative) in VD caused by an increase in the surface coverage of a specifically adsorbed anion.79 Additionally (see Figure 3b and the schematic model of an electrode | electrolyte interface containing adsorbed SO42- ions in Figure 4c of ref 37), the first layer of water molecules in a Au(111) surface | adsorbed SO42- | aqueous electrolyte interface orients with all of its hydrogen atoms pointing toward the surface. Accordingly, as discussed above in reference to the adsorption of the ClO4- ion, (dVD)/(dT) for this interface may be very negative. In this case (see eq 678), both Y(Ei) and Z(Ei) would behave as shown in Figures 5 and 6a. However, early scanning tunneling microscopy studies of the adsorption of sulfate species (from H2SO4 electrolyte solutions) onto Au(111)26b indicate both that the adsorbed anion is bisulfate and that the adsorbed anion is effectively discharged. Measurements33b of both the electrosorption valency and the Esin-Markov coefficient of SO42- adsorbed onto Au(111) indicate that the charge retained by an adsorbed sulfate anion is significantly more positive than -2. Accordingly, Figure 6b depicts values of Z(Ei = 900 mV vs SCE) 2698



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Figure 7. Points: Example of an ILIT (open-circuit) response (for ΔTeq = 1.93 K) of a Au(111) electrode (thickness = 1.05 10-4 cm) in contact with a 1.0 M KCl aqueous electrolyte solution at Ei = -200 mV vs SCE. Curve: Fit of these data to eqs 25-29 in ref 1b resulting in A = -1.12 mV, ΔVeq,a/d = 0.87 mV, P75,a/d = 1.78 10-4 s1/2, and km,a/d = 1.54 107s-1.

Figure 6. (a) Plots of Y(Ei) and Z(Ei) (both defined in the figure), as well as Z(Ei) corrected for the assumed loss of adsorbed ClO4- (i.e., Z(Ei) þ 0.19 mV/K) obtained (by the additions of small amounts of SO42- to 0.10 M HClO4) at 900 mV vs SCE. The blue points are Y(Ei), the pink points are Z(Ei), and the green points are Z(Ei) þ 0.19 mV/K. The calculation of Z(Ei) is based on the assumption that σad = -2FΓSO42- (see the text and ref 31b). (b) The same as (a), but σad = -0.6FΓSO42- (where the factor -0.6 is determined from the reciprocal of the Esin-Markov coefficient33b of the adsorbed sulfate moieties).

(and (Z(Ei = 900 mV vs SCE) þ 0.19 mV K-1)) calculated using σad = -0.6FΓSO42- (where the factor “-0.6” is the charge on an adsorbed sulfate moiety determined from the reciprocal of its Esin-Markov coefficient33b). The plots in Figure 6b indicate that, if there is considerable charge transfer between the adsorbed sulfate ions and the Au(111) surface, the change in Y(Ei = 900 mV vs SCE) is (approximately) due only to diffuse double layer

effects. This means either that the change in both VD and (dVD)/(dT) effected by adsorbed sulfate moiety is minimal or that the effect (on Y(Ei = 900 mV vs SCE)) of any change in VD is almost exactly counteracted by the change in (dVD)/(dT) . In either case (or if there is minimal charge transfer between an adsorbed SO42- ion and the Au(111) electrode), experiments beyond the scope of the present study are required to definitely determine both VD and (dVD)/(dT) as a function of ΓSO42-. Kinetics of Adsorption/Desorption of Cl - onto/from Au(111). Because of their simple structure and technological utility,26a,27,28a,30,33a-33c,34-36,39a,39b adlayers formed by the specific adsorption of halide ions onto various metal electrode surfaces have been the subject of many thermodynamic and structural studies. The adsorption/desorption kinetics of adsorbed halides have also been the subject of a few investigations.51,80 As demonstrated in ref 1, the ILIT technique is quite well suited to the study of these adsorption/desorption kinetics. An example of the data from such an ILIT study (on the adsorption/desorption kinetics of Cl- ions) is shown in Figure 7. The measured rate constants (km,a/d) that are obtained from fits of ILIT data such as that shown in Figure 7 to eqs 25-29 in ref 1b are defined by1b " ! F 2 z2ion S km, a=d ¼ kad ðEi Þ c Ra=d ð1 - θeq Þ RTCO ion ! # cS bV ion þ 1Γad, Max CO " ! # F 2 z2ion bV þ kde ðEi Þ Γad:Max ð1 - Ra=d Þθeq þ 1 RTCO CO ð12Þ and the mass-transfer parameters (P75,a/d) obtained from these fits are defined by1b P75, a=d ¼ 2699

bc ðka =km, a=d Þ 1=2

zion FDion

ð13Þ



ARTICLE

In eqs 12 and 13, zion is the charge on the adsorbate ion and F is 3 the Faraday constant. In eq 12, cS* ion (mol/cm ) is the bulk, solution concentration of the adsorbate ion, Γad,Max (mol/cm2) is the maximum surface concentration of the adsorbate ion, and Ra/d is the transfer coefficient for the ion transfer (adsorption/ desorption) reaction. The parameter θeq in eq 12 is the equilibrium coverage of the adsorbate ion on the surface of the electrode (at Ei), defined by ð14Þ θeq ¼ Γad, eq =Γad, Max where Γad,eq (mol/cm2) is the equilibrium surface concentration of the adsorbate ion (at Ei), and the parameter bV in eq 12 is defined by1b bV ¼

δqad, eq δVO

ð15Þ

where qad,eq is the equilibrium value of the adsorbate charge density on the surface of the electrode and VO is the voltage drop across the outer layer portion of the electrochemical double layer.21 (Since bV is always negative, the factor (1 - (bv/CO)) in eq 12 is always positive.) The rate constants (kad(Ei) and kde(Ei)) in eq 12 are defined by1b F S 0 kad ðEi Þ ¼ kad ðT, cion , E00 Þ exp - Ra=d zion Ei ð16Þ RT

Figure 8. Measured (see Figure 7) values of P75,a/d (defined in eq 13) for Au(111) electrodes in contact with 0.10 M KCl/0.90 M KF (red data points) and 1.0 M KCl (black data points) electrolyte solutions versus potential (Ei).

and

F kde ðEi Þ ¼ kad0 ðT, cS , E Þ exp ð1 R Þz Ei a=d ion ion 00 RT ð17Þ

where E00 is the standard equilibrium potential for the iontransfer reaction.12,81 In eq 13, the rate constant ka is defined by1b ! cS ion ka ¼ kad ðEi Þ ð18Þ þ kde ðEi Þ Γad, Max and the parameter bc is defined by1b bc ¼

δqad, eq δcSion

ð19Þ

S where cion is a generalized solution concentration of the adsorbate ion. (Note that the sign of bc is always the same as that of zion so that P75,a/d is always positive.1b) Figure 8 demonstrates that P75,a/d is uniformly (i.e., at all values of Ei) much larger for the ILIT experiments that investigated the adsorption/desorption reaction kinetics associated with the electrolyte solutions containing 0.10 M Cl- and 0.90 M F- anions than for the ILIT experiments that investigated these kinetics for the electrolyte solutions containing 1.0 M of the Clanion alone. This observation indicates that the coverage of the Cl- anion on the Au electrode in contact with a 1.0 M KCl electrolyte solution is essentially complete (i.e., θeq ∼ 1.0) because bc ∼ 0 (and, therefore, P75,a/d ∼ 0) when θeq ∼ 1.0.82 Also, (probably) because the adsorption/desorption reactions of the Fanion occur on a much faster time scale than those of the Clanion51b (i.e., the adsorption/desorption reactions of the F- anion are too fast to be observed using the present ILIT apparatus;see Figure 1), the reactions observed in the ILIT experiments performed on the 0.10 M KCl/0.90 M KF electrolyte solutions are those associated with Cl- anions.83 However, the adsorption/

Figure 9. Logarithms (both natural and common) of km,a/d (defined in eq 12) measured (as a function of potential (Ei) using ILIT (see Figure 7)) for Au(111) electrodes in contact with 0.10 M KCl/0.90 M KF (red data points) and 1.0 M KCl (black data points) electrolyte solutions. The black line describes the result of a linear least-squares fit of the 1.0 M KCl data for -400 mV vs SCE e Ei e 400 mV vs SCE, where, for the natural logarithm data (see eq 17 and the text), the slope is -(4.54 ( 0.18) 10-3 mV-1 and the intercept is 15.66 ( 0.05.

desorption reaction kinetics (see Figure 9) of the chloride ion constituents of an 0.10 M KCl/0.90 M KF electrolyte solution may very well be influenced (e.g., as a function of potential) by the presence of adsorbed F- anions on the Au(111) electrode surface (which is, in turn, demonstrated by the F- anion data shown in 2700



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Figure 2).36,39b Accordingly, since θeq ∼ 1.0 for [Cl-] = 1.0 M at all of the potentials studied, the preexponential factors in eq 12 may be independent of potential for the 1.0 M KCl electrolyte solution data in Figure 9. In that case, if kad(Ei)[(cS* ion)/(Γad,Max)][1 - (bv/C0)], kde(Ei)[((F2z2ion)/(RTCO))(Γad,Max) þ (1 (bv/C0))] (i.e., only the desorption reaction is observable in these ILIT experiments;which is consistent with the positive values of ΔVoc(t) shown in Figure 7 for t > 0.2 μs), a plot of ln[km,a/d] versus Ei should be linear with a negative slope (because zion = 1;see the definition of kde(Ei) in eq 17). This is exactly what is observed in regard to the [Cl-] = 1.0 M data in Figure 9 for -400 mV versus SCE e Ei e þ400 mV versus SCE.84 The value of Ra/d computed from the slope of the least-squares line fitted to these data is 0.88 ( 0.01 (see eq 17). Because the reaction coordinate for these ion-transfer reactions corresponds to the movement of the ion through the electrochemical double layer, Ra/d need not be ∼1/2.12 The value of Ra/d determined above indicates that the maximum of the potential energy curve associated with these adsorption/ desorption reactions is quite close to the surface of the electrode. Schmickler has proposed12,15 that this potential energy curve is effected by the displacement of solvent molecules (from both the solvation sphere of the adsorbate ion and the electrode surface) and that its maximum is at a distance from the surface of the electrode that is (approximately) congruent with the diameter of the solvent molecules in the first layer adsorbed (directly) onto the electrode surface (see Figure 3). If, as suggested by the work of Toney et al. for Ag(111) electrodes,47a,47b the first layer of water molecules in contact with a polarized Au(111) electrode surface is compressed, our observation that Ra/d ∼ 1.0, therefore, is consistent with the theory for ion-transfer reactions discussed by Schmickler in refs 12 and 50. However, further consideration of this theory (as well as the structure of the electrochemical double layer elucidated by studies of adsorption/desorption kinetics) must await additional ILIT experiments performed at other chloride ion concentrations (in electrolyte solutions containing no other anion except Cl-), at other temperatures,12 and on electrolyte solutions containing other adsorbates (e.g., the bromide anion51b).

temperature perturbation, were measured as a function of potential (Ei). The first conclusion that may be drawn from these measurements (Figure 2) is that the ILIT technique may be employed to demonstrate that all of the anions studied here specifically adsorb onto Au(111) electrode surfaces.3 The data shown in Figure 2 may, therefore, be combined with other studies of the adsorption of these anions onto Au(111) surfaces in order to deduce various properties (i.e., VD and, especially, (dVD)/(dT)) of the interfaces between the gold electrodes and (weakly adsorbing) ClO4- containing electrolyte solutions as well as (strongly adsorbing) SO42- containing electrolyte solutions. These evaluations (for ClO4-) and estimations (for SO42-) of VD and, in particular, (dVD)/(dT) may be used to derive various features of the structure of the relevant electrode | electrolyte interface. For the adsorbed ClO4- and SO42- anions, these features are not only found to be consistent with the schematic of an electrode | electrolyte interface containing a specifically adsorbed anion proposed in Figure 3b, but they are also consistent with many of the results of previous studies of the adsorption of ClO4-31b,38a,39c and SO42-26,31,36,37 onto Au surfaces. ILIT measurements of A/ΔTeq as a function of potential, therefore, may be used to provide information on the composition and structure of an electrode | electrolyte interface, which are determining factors for the kinetics of charge-transfer reactions that take place at (or across) this interface. Accordingly, since the composition and structure of an electrode | electrolyte interface determine12 the kinetics of the adsorption and desorption reactions of an ion chemisorbed at this interface, studies of these kinetics provide an additional26,30,37,38,47,69 structural probe for these interfaces.85 Analysis of the potential dependence of the measured, adsorption/desorption rate constants (km,a/d) for a Au(111) electrode | 1.0 M KCl (aqueous) electrolyte solution interface observed in the present study, therefore, leads us to speculate that the structure of the first layer of water molecules directly adsorbed onto the surface of this polarized electrode (where θeq ∼ 1.0 for the adsorbed Cl- ions) may be compressed compared to the structure of bulk water.47a,47b We further suggest that studies of adsorbate ion adsorption/desorption kinetics (along with extant49 and future studies of the rates or reorientation (in response to a temperature perturbation) of the dipolar constituents of electrochemical double layers) will greatly contribute to a detailed understanding of the processes responsible for the performance of many important electrochemical technologies.12-14

’ CONCLUSIONS We have used the ILIT method to elucidate the characteristics of a variety of Au(111) | (nonelectroactive, aqueous) electrolyte solution interfaces. The initial changes (A/ΔTeq) in the opencircuit potentials of these interfaces, induced by the ILIT

’ APPENDIX

Table A.1. Glossary symbol

units

definition

A

mV (or V)

open-circuit potential response produced by a temperature perturbation (i.e., ΔTeq;see below) in the absence

bSoret

mV K-1 (or V K-1)

of any charge- transfer reaction (see eq 1) coefficient that describes the Soret potential produced by the temperature difference between the electrode and the bulk of the electrolyte solution1,23

-1

bc

C mol

br

mV K-1 (or V K-1) -2

cm -1

see eq 19 and accompanying discussion 0

coefficient that describes the temperature derivative of the formal potential of a redox couple (i.e.,dE0 /dT) see eq 15 and accompanying discussion

bV

C cm

cSion

mol cm-3

generalized solution concentration of an adsorbate ion

cS* ion

mol cm-3

bulk solution concentration of an adsorbate ion

CDL CO

F cm-2 F cm-2

specific (integral) capacitance of the “diffuse layer” component21 of the complete electrochemical double layer specific (integral) capacitance of the “outer layer” component21 of the complete electrochemical double layer

V

2701



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Table A.1. Continued symbol

units

definition

CT

F cm-2

specific (integral) capacitance of the complete electrochemical double layer21

Dion Ei

cm2 s-1 V (or mV)

diffusion coefficient of an adsorbate ion potential of the working electrode just before the temperature perturbation

E0

V (or mV)

formal potential of a redox couple

E00

V (or mV)

standard equilibrium potential for an ion-transfer reaction12,81

0

F

-1

C mol

-1

Faraday’s constant (96 485 C mol-1)

kad(Ei)

cm s

rate constant for an interfacial adsorption reaction as a function of potential (Ei;see eq 16)

kde(Ei)

s-1

rate constant for an interfacial desorption reaction as a function of potential (Ei;see eq 17)

ka

s-1

see eq 18

km,a/d P75,a/d

s-1 s1/2

ILIT measured rate constant for an adsorption/desorption reaction (see eq 12) mass-transfer parameter for an ion adsorption/desorption process (see eq 13)

qad,eq

C cm-2

equilibrium value of adsorbate charge density (concentration) on the surface of an electrode

R

J K-1 mol-1

ideal gas constant (8.313 4 J K-1 mol-1)

T

K

temperature

ΔTeq

K

interfacial temperature change that would be produced in an ILIT experiment by the heat energy absorbed from the

t

s

time

VO ΔVoc(t)

mV (or V) mV (or V)

voltage drop across the “outer layer” of the double layer open-circuit potential change of an electrode (as a function of time (t) subsequent to an ILIT perturbation)

ΔVeq,a/d

mV (or V)

change in the equilibrium potential (associated with an adsorption/desorption reaction) of an electrode effected by ΔTeq

zion

-

charge on an adsorbate ion

laser pulse if none of this absorbed heat were lost to either the dielectric “backing” of the electrode or the electrolyte solution

Ra/d

-

transfer coefficient for an ion-transfer reaction

Γad,eq

mol cm-2

equilibrium surface concentration of an adsorbate ion

Γad,Max

mol cm-2

maximum surface concentration of an adsorbate ion

θeq

-

equilibrium value of the coverage of an adsorbate ion on the surface of an electrode (at Ei)

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses †

Seagate Technology, Fremont, CA 94538. E-mail: yi-chyi.c.wu@ seagate.com.

’ ACKNOWLEDGMENT The authors thank the Fundamental Interactions Branch, Office of Basic Energy Science of the U.S. Department of Energy, for support through Contract No. DE-AC02-98CH10886. J.F.S. also thanks Dr. Marshall D. Newton, Chemistry Department, Brookhaven National Laboratory, for many helpful discussions. ’ REFERENCES (1) (a) Smalley, J. F.; Krishnan, C. V.; Goldman, M.; Feldberg, S. W.; Ruzic, I. J. Electroanal. Chem. 1988, 248, 255–282. (b) Smalley, J. F. J. Electroanal. Chem. 2010, 640, 68–74. (2) Feldberg, S. W.; Newton, M. D.; Smalley, J. F. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 2003; Vol. 22, pp 101-180. (3) Smalley, J. F.; Geng, L.; Feldberg, S. W.; Rogers, L. C.; Leddy, J. J. Electroanal. Chem. 1993, 356, 181–200. (4) (a) Smalley, J. F.; Chalfant, K.; Feldberg, S. W.; Nahir, T. M.; Bowden, E. F. J. Phys. Chem. B 1999, 103, 1676–1685. (b) Smalley, J. F. Langmuir 2003, 19, 9284–9289. (5) Hamelin, A.; Vitanov, T.; Sevastyanov, E.; Popov, A. J. Electroanal. Chem. 1983, 145, 225–264 and references therein. (6) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986, 31, 929–936.

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The Journal of Physical Chemistry C trigger pulse.2 The potentiostat effectively changes into a galvanostat for the ∼5 ms subsequent to the trigger pulse. (21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; John Wiley & Sons: New York, 2001; pp 12-14. In this reference, the double layer is described as being made up of three layers in series:22 an inner layer containing solvent molecules and (sometimes) specifically adsorbed ions and/or molecules, an outer layer ending at the locus of electrical centers (called the outer Helmholtz plane) of the solvated electrolyte ions that are closest to the electrode, and a diffuse layer. The total, specific, integral double layer capacitance (CT, F/cm2) of the double layer, therefore, is a series combination of the specific, integral capacitance associated with an each of these layers;i.e., CI for the inner layer, CO for the outer layer, and CDL for the diffuse layer.1b (22) Newton, M. D.; Smalley, J. F. Phys. Chem. Chem. Phys. 2007, 9, 555–572. (23) Smalley, J. F.; MacFarquhar, R. A.; Feldberg, S. W. J. Electroanal. Chem. 1988, 256, 21–32. Equation 1 does not include a coefficient (bj) which describes the temperature-induced potential change associated with the thermal junctions in the electrode | electrolyte system because this coefficient is always negligible (ref 2). (24) Yamakata, A.; Osawa, M. J. Phys. Chem. C 2008, 112, 11427–11432. (25) (a) Climent, V.; Coles, B. A.; Compton, R. G. J. Phys. Chem. B 2002, 106, 5258–5265. (b) Climent, V.; Coles, B. A.; Compton, R. G. J. Phys. Chem. B 2002, 106, 5988–5996. (c) Climent, V.; Garcia-Araez, N.; Compton, R. G.; Feliu, J. M. J. Phys. Chem. B 2006, 110, 21092– 21100. (d) Garcia-Areaz, N.; Climent, V.; Feliu, J. M. J. Am. Chem. Soc. 2008, 130, 3824–3833. (e) Garcia-Araez, N.; Climent, V.; Feliu, J. J. Phys. Chem. C 2009, 113, 9290–9304. (26) (a) Magnussen, O. M. Chem. Rev. 2002, 102, 679–725. (b) Magnussen, O. M.; Hageb€ock, J.; Hotlos, J.; Behm, R. J. Faraday Discuss. 1992, 94, 329–338. (27) Takamura, T.; Takamura, K. J. Electroanal. Chem. 1971, 29, 279–291. (28) (a) Larkin, D.; Guyer, K. L.; Hupp, J. T.; Weaver, M. J. J. Electroanal. Chem. 1982, 138, 401–423. (b) Edens, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357–366. (29) (a) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. Electrochim. Acta 1986, 31, 1051–1061. (b) AngersteinKozlowski, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429–453. (30) (a) Wang, J.; Ocko, B. M.; Davenport, A. J.; Isaacs, H. S. Phys. Rev. B 1992, 46, 10321–10338. (b) Magnussen, O. M.; Ocko, B. M.; Adzic, R. R.; Wang, J. X. Phys. Rev. B 1995, 51, 5510–5513. (31) (a) Zelenay, P.; Rice-Jackson, L. M.; Wieckowski, A. J. Electroanal. Chem. 1990, 283, 389–401. (b) Shi, A.; Lipkowski, J.; Gamboa, M.; Zelenay, P.; Wieckowski, A. J. Electroanal. Chem. 1994, 366, 317–326. (c) Mrozek, P.; Han, M.; Sung, Y.-E.; Wieckowski, A. Surf. Sci. 1994, 319, 21–33. (32) Marinkovic, N. S.; Marinkovic, J. S.; Adzic, R. R. J. Electroanal. Chem. 1999, 467, 291–298. (33) (a) Shi, Z.; Wu, S.; Lipkowski, J. J. Electroanal. Chem. 1995, 384, 171–177. (b) Lipkowski, J.; Shi, Z.; Chen, A.; Pettinger, B.; Bilger, C. Electrochim. Acta 1998, 43, 2875–2888. (c) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1996, 403, 225–239. (d) Lipkowski, J.; Wieckowski, A. J. Electroanal. Chem. 2001, 504, 230–234. (34) Kolics, A.; Thomas, A. E.; Wieckowski, A. J. Chem. Soc., Faraday Trans. 1996, 92, 3727–3736. (35) (a) Pajkossy, T. Solid State Ionics 1997, 94, 123–129. (b) Pajkossy, T.; Kolb, D. M. Electrochem. Commun. 2007, 9, 1171–1174. (36) Vasiljevic, N.; Trimble, T.; Dimitrov, N.; Sieradzki, K. Langmuir 2004, 20, 6639–6643. (37) Nihonyanagi, S.; Ye, S.; Uosaki, K.; Dressen, L.; Humbert, C.; Thiry, P.; Peremans, A. Surf. Sci. 2004, 573, 11–16. (38) (a) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664–10672. (b) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951– 959. (c) Nakata, K.; Okubo, A.; Shimazu, K.; Yamakata, A.; Ye, S.; Osawa, M. Langmuir 2008, 24, 4352–4357. (d) Nakata, K.; Kayama, Y.; Shimazu, K.; Yamakata, A.; Ye, S.; Osawa, M Langmuir 2008, 24, 4358– 4363.

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(39) (a) Lei, H.-W.; Uchida, H.; Watanabe, M. J. Electroanal. Chem. 1996, 413, 131–136. (b) Lei, H.-W.; Uchida, H.; Watanabe, M. Langmuir 1997, 13, 3523–3528. (c) Uchida, H.; Ikeda, N.; Watanabe, M. J. Electroanal. Chem. 1997, 424, 5–12. (40) Arenz, M.; Broekmann, P.; Lennartz, M.; Vogler, E.; Wandelt, K. Phys. Status Solidi A 2001, 187, 63–74. (41) Wan, L.-J.; Yau, S.-S.; Itaya, K. J. Phys. Chem. 1995, 99, 9507– 9513. (42) Note that σad may be either positive or negative and that the adsorbed ion may carry only a partial charge12 (i.e., the chemisorption reaction may include a (partial or complete) charge transfer between the adsorbed ion and the electrode). (43) Trasatti, S. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1977; Vol. 10, pp 213-321. (44) (a) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2006, 110, 16066–16081.(b) Hence Epzc should be expressed versus SHE. (45) Frumkin, A. N.; Petrii, O. A.; Damaskin, B. B. In Comprehensive Treatise of Electrochemistry; Bockris, J. O’M., Conway, B. E., Yeager, E., Eds.; Plenum Press: New York, 1980; Vol. 1, pp 221-285. (46) For example: (a) Nagy, G.; Heinzinger, K. J. Electroanal. Chem. 1990, 296, 549–558. (b) Heinzinger, K. Pure Appl. Chem. 1991, 63, 1733–1742. (c) Nagy, G.; Heinzinger, K. J. Electroanal. Chem. 1992, 327, 25–30. (d) Nagy, G.; Heinzinger, K.; Spohr, E. Faraday Discuss. 1992, 94, 307–315. (47) (a) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Nature 1994, 368, 444–446. (b) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Surf. Sci. 1995, 335, 326–332. (c) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. W.; Yee, D.; Sorensen, L. B. Phys. Rev. Lett. 1995, 75, 4472–4475. (48) Kubota, J.; Wada, A.; Domen, K.; Kano, S. Chem. Phys. Lett. 2002, 362, 476–482. (49) Yamakata, A.; Uchida, T.; Kubota, J.; Osawa, M. J. Phys. Chem. B 2006, 110, 6423–6427. (50) Schmickler, W. Chem. Phys. Lett. 1995, 237, 152–160. (51) (a) Pajkossy, T.; Wandlowski, T.; Kolb, D. M. J. Electroanal. Chem. 1996, 414, 209–220. (b) Kerner, Z.; Pajkossy, T. Electrochim. Acta 2002, 47, 2055–2063. (52) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141–13149. (53) The laser energy absorbed from this “indirect” excitation of the electrode is rapidly (within ∼1 ps) degraded to heat, and this heat quickly moves through the electrode to the “front” surface causing a small (2-5 °C) change in the temperature of the electrode | electrolyte interface. (54) Smalley, J. F.; Newton, M. D.; Feldberg, S. W. Electrochem. Commun. 2000, 2, 832–838. (55) We also note here that, because the response to the ILIT temperature perturbation is a change in the open-circuit potential, the time dependence of this response (as well as its magnitude) is unaffected by uncompensated solution resistance.2 (56) Smalley, J. F.; Geng, L.; Chen, A.; Feldberg, S. W.; Lewis, N. S.; Cali, G. J. Electroanal. Chem. 2003, 549, 13–24. (57) The factor ΔTeq is missing from the first0 term on the right0 hand side of eq 29 in ref 56 so that ΔVeq = ΔTeq[(dE0 /dT) - (E0 /T)] þ (ΔTeq/T)Ei (where Ei is the initial (before the laser pulse) open-circuit potential poised by the redox couple). (58) These transients determine the interfacial electron-transfer kinetics of the ferri/ferrocyanide couple in the electrolyte solutions investigated in the present study.56 (59) 0Alternatively, the absolute single ion entropy data in ref 60 gives br = dE0 /dT = -1.89 10-3 V K-1 for the ferri/ferrocyanide redox couple. (60) Breck, W. G.; Lin, J. Trans. Faraday Soc. 1965, 61, 2223–2228. (61) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Academic Press Inc.: New York, 1959; pp 491-503. 2703


The Journal of Physical Chemistry C (62) These ILIT responses were, of course, measured before the addition of the ferri/ferrocyanide couple to the electrolyte solutions. (63) The plots of [(A/ΔTeq) - bSoret] versus Ei in Figure 2 are reproducible over a number of independent ILIT experiements. (64) Tao, N. J.; Lindsay, S. M. J. Appl. Phys. 1991, 70, 5141–5143. (65) The quality of the surfaces of the vapor-deposited film electrodes used in the present study (especially those “backed” by Ti)3 is comparable to Au films epitaxially grown on mica.64 (66) Note that the factor ((dln[CDL])/(dT) - (d ln[CT])/)dT)) is almost always negative. (67) This is a manifestation of the Esin-Markov effect68 which, for example, causes Epzc (and, therefore, VD) to become more negative in the presence of an adsorbed anion. (68) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; John Wiley & Sons: New York, 2001; pp 555-556. (69) Osawa, M.; Tsushima, M.; Mogami, H.; Samjeske’, G.; Yamakata, A. J. Phys. Chem. C 2008, 112, 4248–4256. (70) For example, because (d ln[CT])/(dT) is generally negative and the adsorption of cations effects a more positive value of VD,67,68 the quantity VD((d ln[CT])/(dT)) (see eq 6) should be more negative in the presence of adsorbed hydronium ions. (71) The error bounds ((0.03 mV K-1) describe the range in the values of -(σad/CDL)[(d ln[CDL])/(dT) - (d ln[CT])/(dT)] over this potential range. (72) Moore, W. J. Physical Chemistry, 3rd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1962; p 557. (73) On the other hand, for example, VD (for a Au(111) electrode in contact with 1.0 M NaClO4 between -200 and 0.0 mV vs SCE) could decrease as a function of potential. However, the value of (d ln[CT])/(dT) calculated from the slope of the line in Figure 4b is quite reasonable and very similar to the value of (d ln[CT])/(dT) determined from the slope of the line in Figure 4a;both of which imply that VD is not a function of potential between -200 and 0.0 mV vs SCE. (74) Additionally, differential capacitance measurements in 0.01 M HClO4 determine that (dEpzc)/(dT) = þ2.3 mV K-1 (where Epzc(T = 25 °C) = þ280mV vs SCE) for an Au(111) electrode.8 (75) In the absence of specific ion adsorption, the separation of charge across the double layer is the sole cause of this electric field. (76) Funtikov, A. M.; Stimming, U.; Vogel, R. J. Electroanal. Chem. 1997, 428, 147–153. (77) The [Hþ] and [ClO4-] remain constant (to within ( 2%) over the course of the experiment described in Figure 5. (78) We assume that the factor G ∼ 1 in this discussion of the effect of SO42- adsorption on the initial response of a Au(111) electrode to an ILIT temperature perturbation. (79) This increase is, in turn, caused by the weak solvation of SO42-.26a (80) Pajkossy, T.; Kolb, D. M. Electrochim. Acta 2008, 53, 7403– 7409. 3 (81) If cs* ion is unity (e.g., 1.0 mol/cm ), the overall rate of the ion transfer reaction (i.e., the rate of adsorption minus the rate of desorption;see eq 1 in ref 1b) is by definition zero at E00. (82) In regard to the data plotted in Figure 8, an analysis of eqs 12 and 17 shows that 0.05 < ka/km,a/d < 1.0 for reasonable values of CO, Ra/d, Γad,Max, and θeq so that P75,a/d can only be approximately zero if bc is very close to zero. Additionally, the conclusion that θeq = 1.0 at [Cl-] = 1.0 M and at all of the potentials studied in the experiment, the results of which are described in Figures 8 and 9, is consistent with thermodynamic measurements of chloride ion adlayers.26a,33a-33c (83) Furthermore, the magnitudes of the measured rate constants (km,a/d) plotted in Figure 9 (for both [Cl-] = 0.10 and 1.0 M) are consistent with the observation reported in ref 51b that the rates of the adsorption and (by implication) desorption reactions of the Cl- ion onto/off of Au(111) electrode surfaces are too fast to be measured using the impedance spectroscopy technique employed in the study described in this reference. (84) A neat chloride ion adlayer should undergo an order/disorder phase transition at the two most positive of the potentials which effect

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the data plotted in Figure 9.30b (Both chloride and bromide ions form ordered, incommensurate, hexagonal-close-packed monolayers at very positive potentials while even (approximate) monolayers of these anions are disordered at less positive potentials.30b) This may be the reason why the points for Ei = 500 and 600 mV versus SCE ([Cl-] = 1.0 M) are not on the extrapolation of the least-squares line in Figure 9. (85) The solute-solvent dynamics in the first solvation shell of a reactant ion are also a major determining86 factor for the rates of both adsorption/desorption reactions12 and (solution-phase as well as interfacial) adiabatic, outer-sphere electron-transfer reactions.86 (86) Maroncelli, M.; MacInnis, J.; Fleming, G. R. Science 1989, 243, 1674–1681.

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