ARTICLE pubs.acs.org/JPCC
Electrolyte and Temperature Effects on the Electron Transfer Kinetics of Fe(CN)63/-4 at Boron-Doped Diamond Thin Film Electrodes Doo Young Kim,† Jian Wang,§,† Juchan Yang,‡ Hyoun Woo Kim,‡ and Greg M. Swain*,† † ‡
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322, United States Division of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea ABSTRACT: Cyclic and linear sweep voltammetry were used to investigate the effects of the electrolyte composition and temperature on the electrontransfer kinetics of Fe(CN)63/4 at well-characterized, boron-doped diamond thin-film electrodes. Highly conductive films were employed, which were first cleaned of any adventitious nondiamond carbon impurity by a twostep chemical-oxidation, and subsequently hydrogenated in hydrogen microwave plasma. The apparent heterogeneous electron-transfer rate constant, k°app, depended on the electrolyte concentration and the electrolyte cation type, increasing in order of Liþ < Naþ < Kþ < Csþ. However, the dependence of koapp on the electrolyte cation was less than the dependence observed for other electrodes, like glassy carbon and gold. For example, koapp at the 1.0 M concentration was only a factor of 1.6 greater in KCl than in LiCl for diamond. This is less than the factor of 510 seen for other electrodes, like glassy carbon and gold. The transfer coefficient for the oxidation was largely independent of the temperature and the electrolyte composition with a value ranging from 0.52 to 0.55. The activation energy for electron transfer was found to be 14.3, 15.6, and 16.5 kJ/mol respectively for KCl, NaCl, and LiCl. The results suggest that the electric double layer structure at sp3 diamond may be different from that found at sp2 glassy carbon.
’ INTRODUCTION Electrically conducting microcrystalline (faceted with a grain size of 15 μm) and ultrananocrystalline (nodular with 310 nm nanocrystals) diamond thin-film electrodes are being employed more and more in electrochemical research due to the material’s interesting and, in some cases, unique properties.15 Several factors strongly influence the electrochemical response and behavior of diamond electrodes including: (i) adventitious nondiamond sp2-bonded carbon impurity, (ii) the surface termination (H vs O), (iii) the dopant type, level, and distribution, and (iv) grain boundaries.611 The extent to which one or more of these factors influences the electrode response depends on the reaction mechanism of the particular redox analyte.12,13 Fe(CN)63/4 is a redox system often used to evaluate an electrode’s electrochemical activity. The redox mechanism of this system is complex on both carbon and metal electrodes and does not involve simple electron transfer with the electrode acting solely as a source and sink for electrons. There are several factors known to influence the electrochemical kinetics of this system at metal and sp2 carbon electrodes, and these have been discussed previously.1428 The electrochemical kinetics are also influenced by the physicochemical properties of boron-doped diamond. The cyclic voltammetric ΔEp, which is inversely related to the heterogeneous electron-transfer rate constant, is very sensitive to the surface termination with the lowest ΔEp observed at the clean, hydrogen-terminated surface.10,29,30 After oxygen termination, ΔEp increases by over 125 mV (at 0.1 V/s). The original high activity can be regained by rehydrogenating the surface in a hydrogen plasma to remove the surface oxygen. The strong sensitivity of the electrochemical kinetics to surface oxygen at r 2011 American Chemical Society
diamond is in sharp contrast to the minor effects these functionalities tend to have on the response at sp2 carbon electrodes. Surface oxides tend to have a significant effect at sp2 carbon electrodes only when a thick multilayer film is present.16,3032 For diamond electrodes, the results suggest the direct involvement of a clean, nonoxide surface site when relatively rapid kinetics are observed. We report herein on a detailed study of how the electrolyte composition and concentration, as well as the solution temperature, affect apparent heterogeneous electron transfer rate constant, koapp, for Fe(CN)63/4 at well characterized, boron-doped diamond thin-film electrodes. Such studies have not been reported for diamond but have for other electrodes, such as the several work by Kolthoff and Tomsicek15 on the effect of ionic strength on the oxidation potential, and Peter et al.,25 Kawiak et al.,18 and Noel and Anantharaman17 on electrolyte effects on k0 et at Pt/Au, Au and glassy carbon, respectively. The cyclic voltammetric response for 1 mM Fe(CN)63/4 was investigated in LiCl, NaCl, KCl, and CsCl as a function of the electrolyte concentration and solution temperature. The latter was varied from 20 to 65 °C. The electrodes were cleaned of any adventitious nondiamond sp2 carbon surface impurity, by a two-step chemical treatment, and then hydrogen-terminated by hydrogen microwave plasma treatment.
Received: December 12, 2010 Revised: April 15, 2011 Published: May 04, 2011 10026
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’ EXPERIMENTAL SECTION Diamond Thin-Film Deposition. The diamond films were deposited by microwave-assisted chemical vapor deposition (CVD) (1.5 kW, 2.54 GHz, Astex Inc., Lowell, MA) on conducting p-Si(100) substrates. The reactive gases (CH4, H2, B2H6) were mixed and a plasma ignited inside a quartz bell jar with all gases present. The substrates (0.1 cm thick 1 cm2 in area) were first pretreated by solvent washing in toluene, methylene chloride, acetone, isopropanol, and methanol. After air drying, the substrates were etched in concentrated hydrofluoric acid for 60 s. After rinsing with ultrapure water and drying, the substrates were sonicated in a diamond powder/acetone slurry (0.1 μm diam., GE Superabrasives, Worthington, OH) for 20 min. The substrates were then rinsed with clean acetone and placed in the CVD reactor. The sonication seeds the surface with nucleation sites (>108 particles/cm2, as estimated by atomic force microscopy). High-quality, polycrystalline diamond thin films were deposited using a methane/hydrogen (C/H) gas mixture at a volumetric ratio of 0.33%, a total flow of 200 sccm, a power of 1000 W, a system pressure of 35 Torr, a substrate temperature of approximately 750 °C (as measured by a disappearing filament optical pyrometer), and a growth time of approximately 10 h. The C/H ratio refers to the composition of the source gases flowing into the reactor and not the final elemental ratio in the films. Ultrahigh purity (99.999%) methane and hydrogen were used as the source gases. The films in this work were doped using solid-state sources: a boron diffusion source (GS-126, BoronPlusTM, Techniglas, Inc., Perrysburg, OH) or a small piece of boron nitride (Goodfellow, Ltd., England), although our normal procedure involves a flow of B2H6. The substrates were placed on top of the diffusion source and adjacent to the piece of boron nitride in the center of the 3 in. diameter gas flow. The film thickness was approximately 5 μm, based on growth rate calculations, and the boron dopant concentration was estimated to be 5 1020 B/cm3 (3000 ppm or 0.3% B/C), based on boron nuclear reaction analysis measurements (Center for Materials Characterization, Case Western Reserve University) of other films grown using identical conditions. Room temperature film resistivity measurements using a tungsten four-point probe (with the diamond attached to the conducting Si substrate) yielded values between 0.1 and 0.01 Ω-cm. After deposition, the CH4 and B2H6 flows were stopped, and the films remained exposed to the hydrogen plasma for an additional 10 min at 35 Torr and 1000 W. Further hydrogen annealing was continued in the following manner: 10 min at 800 W and 25 Torr, 10 min at 600 W and 15 Torr, and 10 min at 200 W and 5 Torr. This procedure cools the samples to approximately 400 °C, or less, in the presence of atomic hydrogen. Postgrowth annealing in atomic hydrogen is essential for gasifying adventitious nondiamond sp2 carbon impurity from the surface, minimizing dangling bonds, and ensuring full hydrogen surface termination. The plasma was then extinguished and the films further cooled to room temperature under a flow of hydrogen. Film Cleaning. The films postgrowth were cleaned by a twopart chemical oxidation procedure to remove adventitious metallic and nondiamond sp2 carbon impurities. This was done in a small 50 mL pyrex breaker with a glass lid to contain the vapor. First, they were gently refluxed in aqua regia (3 HCl: 1 HNO3) for 30 min. The films were then rinsed in distilled H2O and dried.
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Second, they were refluxed in a concentrated H2SO4/HNO3/ NaNO3 solution for 30 min. The acid-washed films were then hydrogen plasma treated at ca. 800 °C, 30 Torr, and 1000 W for 30 min to remove surface oxygen and rehydrogenate the surface. After plasma treatment, the samples were slowly cooled in the presence of atomic hydrogen, as described above. The chemical oxidation introduces oxygen functionalities on the surface. Typical XPS atomic O/C ratios after chemical oxidation were near 0.15, decreasing to 0.02, or less, after rehydrogenation. Electrochemical Measurements. The electrochemical measurements were performed in a single compartment glass cell using a CYSY-1090 digital potentiostat (Cypress Systems, Inc., Lawrence, KS). A commercial silver/silver chloride (Ag/AgCl) electrode (4 M KCl) was used as the reference and a large-area carbon rod served as the counter electrode. The diamond film electrodes were pressed against the bottom of the glass cell with the fluid being contained by a Viton O-ring. The exposed geometric area was 0.2 cm2 in all cases. The large-area counter electrode was placed normal to the working electrode. The reference electrode was positioned near the working electrode using a cracked glass capillary. Electrical connection was made to the diamond by scratching a small area on the backside of the Si substrate with a diamond scribe and then coating the area with Ag paste before contacting an Al current-collecting backplate. The Al plate was polished clean prior to contact. All measurements were made in solutions deoxygenated by a nitrogen gas purge. The solution temperature was regulated with heating tape wrapped around the electrochemical cell, and measured with an Hg thermometer in the cell. The mounted diamond film working electrodes were rinsed with ultrapure water (>17 MΩcm, Barnstead Nanopure), soaked for 20 min in isopropyl alcohol distilled and stored over activated carbon, and rinsed with ultrapure water again prior to use. The soaking in isopropyl alcohol is effective at cleaning sp2 carbon electrodes. A three-step cleaning procedure was used for all glassware: washing in a KOH/methanol bath, washing in a liquid detergent (Alconox)/ water bath, and rinsing with ultrapure water. Chemicals. All chemicals were reagent grade quality, or better, and used without additional purification. One millimolar solutions of potassium ferrocyanide (Aldrich) were prepared. The supporting electrolyte for all solutions was either LiCl, NaCl, KCl, or CsCl (Fisher Scientific). All solutions were prepared with ultrapure water from a Barnstead E-Pure purification system (18 MΩ-cm).
’ RESULTS AND DISCUSSION Effect of Electrolyte. Figure 1 shows a series of background cyclic voltammetric iE curves for a diamond film electrode in 0.1 M LiCl, NaCl, KCl, and CsCl at 25 °C. The scan rate for all recordings was 0.1 V/s. The background current in all three electrolytes is flat and featureless between 300 and 800 mV versus Ag/AgCl, and the current is about the same at all potentials. At 0.1 V, the anodic current is ca. 0.2 μA or 1 μA/ cm2 at this scan rate. This current is consistent with a capacitance of ca. 10 μF/cm2. Similar Cdl values have been reported previously for diamond using ac impedance methods.33,34 It is supposed that the background current is primarily capacitive in nature as no significant faradaic surface reactions on diamond are expected in this potential range. In fact, there should be no redoxactive functional groups on diamond as there are at the edgeplane sites of graphitic carbon electrodes; quinine/hydroquinone 10027
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Figure 1. Background cyclic voltammetric iE curves for a diamond film electrode in 0.1 M LiCl, NaCl, KCl, and CsCl at 25 °C. Scan rate = 0.1 V/s.
couple. Because there is no known specific adsorption of electrolyte ions, particularly anions, the magnitude of the background capacitive current is believed to simply be a function of electrolyte concentration or, more correctly, the ionic strength.10 The diamond electrodesolution interface behaves like a capacitor, as described by the GouyChapmanStern model of the electric double layer. The diamond electrode appears to be ideally polarizable in this potential region, based on the shape of the voltammetric iE curves, as there are no peaks present attributable to surface redox reactions.10,35 Prior XPS studies indicated that electrochemical oxidation of the surface does not occur to any appreciable extent until about 1.0 V or so in this medium. Parts AD of Figure 2 show cyclic voltammetric iE curves for 1 mM Fe(CN)63/4 in LiCl, NaCl, KCl, and CsCl as a function of the electrolyte concentration (0.05, 0.1, and 1 M). ΔEp is smallest at the 1 M concentration of all the electrolytes and increases as the electrolyte concentration decreases. The peak currents also increase with an increase in the supporting electrolyte concentration. Both trends are consistent with increasing k°app. We call the rate constant apparent (app) because no correction for any double layer effects was made. A smaller ΔEp reflects a larger koapp, as described in the theoretical treatment of cyclic voltammetric iE curves by Nicholson and Shain.36,37 The decreasing ΔEp with increasing electrolyte concentration is attributed to changes in the potential, φ2, at the outer Helmholtz plane (relative to the bulk solution) or the plane of closest approach the distance at which the electron transfer is expected to occur. The φ2 value is a function of the applied electrode potential (actually the applied potential relative to the point of zero charge, which is unknown for diamond), as well as the composition and concentration (i.e., ionic strength) of the supporting electrolyte. As the electrolyte concentration is increased, the diffuse double layer thickness is compressed and a larger potential drop (i.e., electric field) is experienced by the redox analyte at the plane of closest approach. The magnitude of φ2 influences koapp as shown in the equation k°app ¼ k°true exp ½ðRn zÞFφ2 =RT
Figure 2. Cyclic voltammetric iE curves for 1 mM Fe(CN)63/4 in LiCl, NaCl, and KCl as a function of the electrolyte concentration of (a) 0.05, (b) 0.1, (c) 1 M. Cyclic voltammetric iE curves for 1 mM Fe(CN)63/4 in 0.05, 0.1, and 1 M CsCl are shown in (d). Scan rate = 0.1 V/s.
in which kotrue is the true standard rate constant in absence of any double layer effect, R is the transfer coefficient, n is the 10028
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Figure 3. Correlation between (a) the peak separation (ΔEp) of 1 mM Fe(CN)63/4 and the hydration enthalpy for Liþ, Naþ, Kþ, and Csþ cations and (b) rate constant (koapp) and the hydrated radius for the same cations. Presented peak separations are for 1 M electrolyte concentration. The hydrated radius data for Liþ, Naþ, Kþ, and Csþ were from ref 42.
number of electrons transferred, and z is the formal charge on the analyte. F, R, and T have their usual meanings.38,39 k°app = k°true when φ2 (relative to bulk solution) is zero. More importantly for a particular electrolyte concentration, ΔEp decreases, and ipox increases in order of LiCl < NaCl < KCl < CsCl. In other words, the smallest ΔEp is observed in CsCl and the largest in LiCl at a given electrolyte concentration. This is in agreement with several other observations of electrolyte cation effects on the electrode reaction kinetics for this redox system at Au, Pt, and glassy carbon electrodes.1719,21,27 This is due, at least in part, to the fact that the standard hydration enthalpy, or the hydrated radius, increases in order of Csþ < Kþ < Naþ < Liþ. This means that the approach distance of the analyte molecule will be closest in CsCl, the medium in which ΔEp is smallest. Figure 3 shows the dependence of (A) the ΔEp on the hydration enthalpy and (B) the k°app on the hydrated radius. The reduced hydration enthalpies of Csþ and Kþ and smaller hydrated radius, compared to Naþ and Liþ, make them more effective ion pairing agents. This appears to be important in the charge transfer mechanism for Fe(CN)63/4, as will be discussed below. Figure 4 shows a series of plots of (A) ΔEp versus the electrolyte concentration, (B) koapp versus the electrolyte concentration, and (C) equilibrium potential, Eeq (or Ep/2), versus the electrolyte concentration. Part A of Figure 4 shows a general trend of decreasing ΔEp with increasing electrolyte concentration
Figure 4. Dependence of (a) peak separation (ΔEp) on electrolyte concentration, (b) k°app on electrolyte concentration, and (c) the correlation between equilibrium potential, Eeq, and electrolyte concentration.
in all three electrolytes. This is attributable to solution resistance and double layer effects, as described above. There is also a progressive decrease in the relative difference in ΔEp for the three electrolytes at a given concentration as the concentration is increased. The largest change occurs as the concentration is increased from 0.01 to 0.5 M. There is still a trend of decreasing ΔEp with increasing electrolyte concentration from 0.5 to 2.0 M but the magnitude of the decrease is much smaller. The ΔEp 10029
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Table 1. Comparison of k°app Values for Fe(CN)63/4 at ∼25 °C for Diamond, Glassy Carbon, Pt, Au, and HGC Electrodes electrode
electrolyte
ko (cm/s)
refs
Pt
1 M LiCl
0.07
21
Pt
1 M LiCl
1 M KCl
0.21 0.02
1 M KCl
0.09
Au
1 M LiCl 1 M KCl
0.013 0.070.10
GC
0.1 M NaCl 0.1 M KCl
GC Diamond HGCa
0.0013
27 17
0.0066
0.1 N Na2SO4
0.0026
0.1 N K2SO4
0.013
1 M LiCl
18
0.008
1 M KCl
0.012
0.1 M NaCl 0.1 M KCl
0.031 0.038
17 this work 30
a
Hydrogenated glassy carbon (HGC) electrode was prepared by exposing polished GC to a hydrogen microwave plasma for 6 h.29,30
increases in the order of CsCl < KCl < NaCl < LiCl for all concentrations except 2.0 M, a medium in which all values are approximately the same. Part B of Figure 4 shows how koapp varies with the electrolyte concentration. There is a quasi-linear dependence of koapp on the electrolyte concentration. For a given electrolyte, the apparent rate constant increases with the concentration. There is also a significant cation effect as the largest koapp values are observed in CsCl and the smallest values in LiCl at all concentrations. The widest disparity in the values occurs at concentrations less than 2.0 M. For example, koapp at 1.0 M is a factor of ca. 1.6 larger in KCl than in LiCl. The dependence of koapp on the cation concentration and type is well-known in the literature.1719,21,27 The largest koapp values for this redox analyte are observed in the Kþ salt. The increase in rate constant with increasing concentration of electrolyte cation indicates an involvement of the cation in the activated complex.19 Part C of Figure 4 demonstrates that Eeq shifts positive with increasing electrolyte concentration, and the most positive value for a given concentration is in CsCl and the most negative in LiCl. These trends can be understood by considering the Nernst equation for the reaction, which describes how the electrode potential varies with the activities, a, of Fe(CN)63 and Fe(CN)64. 0
E ¼ E° þ
aðFeðCNÞ3 0:059 6 Þ log n aðFeðCNÞ4 6 Þ
The trend of increasing potential with increasing electrolyte concentration results because the activity ratio increases proportionally as the activity of Fe(CN)64 decreases with increasing electrolyte concentration. The Fe(CN)64 species is the most highly charged form of the couple, and therefore, it is expected to complex more strongly with the cations. Thus, the activity of the reduced species decreases more than the oxidized species with increasing electrolyte concentration. The activity diminishes because the activity
coefficient decreases, as described in the DebyeH€uckel relationship -log γ µ μ1=2 where γ is the activity coefficient for a particular ion and μ is the ionic strength of the solution. The activity coefficient for the reduced species decreases in order of Csþ > Kþ > Naþ > Liþ. This trend is directly related to the enthalpy of hydration of these ions. Values of 520, 406, 321, and 276 kJ/mol (25 °C) are reported for Liþ, Naþ, Kþ, and Csþ, respectively. Thus, small ions with high charge have the largest enthalpy decrease on hydration because they attract the solvent so strongly. Kolthoff and Tomsicek observed a profound effect of the electrolyte cation on the electrode potential for the Fe(CN)63/4 redox system. 40 The equilibrium electrode potential, Eeq, shifted positive with increasing ionic strength, with the potential in CsCl being more positive than in LiCl for the same ionic strength. Table 1 shows a comparison of the measured koapp values (ca. 25 °C) for Fe(CN)63/4 at diamond with reported values at glassy carbon, Pt, and Au electrodes.17,19,21,27 Data for 0.1 or 1 M KCl and LiCl (or NaCl) are compared. First, koapp for diamond is within an order of magnitude of the rate constant for freshly activated glassy carbon. This reflects the high stability and activity of the hydrogen-terminated surface. Second, the trends are the same for diamond, glassy carbon, Pt, and Au electrodes the largest koapp is observed in KCl. What is noteworthy is the relative difference in koapp for the two electrolytes at a given electrode. For Au, the difference between the rate constants in the Liþ and Kþ electrolytes is a factor of 10, whereas for Pt the difference is 3 to 4.5.18,21,27 For glassy carbon, the difference is a factor of 5.17 In contrast, the difference for sp3-diamond is only a factor of 1.6. Previously, minor cationic effects (a factor of about 1.2) on koapp were reported for hydrogenated glassy carbon (HGC).30 The origin of the lesser cation effect at diamond and HGC, as compared to Au, Pt, and glassy carbon, is not intuitively clear. It is likely related to the double layer structure. Future work will focus on characterizing the double layer structure of diamond. Effect of Temperature. Parts AC of Figure 5 show a series of slow scan, linear sweep voltammetric iE curves for 1 mM Fe(CN)63/4 in 0.1 M LiCl, NaCl, and KCl. The scan rate was 0.010 V/s. The rising part of the oxidation wave is shown at temperatures from 20 to 65 °C. In all cases, the current increases with temperature, as expected. For example, the currents at 150 mV in LiCl are 2.23, 3.23, 4.15, and 6.54 respectively for 20, 40, 50, and 65 °C. The currents at 150 mV in both NaCl and KCl are similar at these temperatures but both are larger than in LiCl. For example, in NaCl they are 3.00, 3.85, 4.77, and 7.39, and in KCl they are 3.00, 3.64, 4.64, and 7.37 respectively for temperatures of 25, 40, 50, and 65 °C. Tafel plots were constructed from the voltammetric data according to the equation log i ¼ log io þ RA nFη=2:3RT where i is the measured current, io is the exchange current density at the equilibrium potential, RA is the transfer coefficient for the oxidation reaction, η is the overpotential (Eappl Eeq) and the other terms have their usual meaning. The quotient, RAnF/ 2.3RT, is Tafel slope, β, for a plot of log i versus η. The RA values were calculated from the Tafel slopes at each temperature, and 10030
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Figure 6. Plots of the natural log of the exchange current density (io) versus 1/T.
with the electrode reaction mechanism being similar at all temperatures and in all electrolyte compositions. For a given electrolyte, io increases with temperature, and for a given temperature, io increases in order of LiCl < NaCl < KCl. Figure 6 shows plots of the natural log of the exchange current density (io) versus 1/T. The slope of these linear plots is the activation energy for the electrode reaction. The large and specific effects of the cations on the rate of electron transfer for Fe(CN)63/4 suggest the direct involvement of cations in the reaction mechanism. This interaction may involve ion association between the cation in the double layer, Mþ, and the highly charged ferri and ferrocyanide molecules. The increased rate with ion association might result from a reduction in the Coulombic repulsion of the ferri and ferrocyanide molecules in the transition state or activated complex. Campion et al. studied homogeneous electron exchange between Fe(CN)64 and Fe(CN)63 and found a large and specific cation effect. They postulated that cation association with the Fe(CN)64 gives ion pairs and these ion pairs undergo electron transfer more rapidly, perhaps because of greater electronic coupling.41 Figure 5. Slow scan, linear sweep voltammetric iE curves for 1 mM Fe(CN)63/4 in 0.1 M (a) LiCl, (b) NaCl, and (c) KCl at temperatures of 25, 40, 50, and 65 °C. Scan rate = 0.010 V/s.
Table 2. Exchange Current Density (io) at the Equilibrium Potential and the Transfer Coefficient (rA) for the Oxidation Reaction at 25, 40, 60, and 80 °C RA
io (μA/cm2)a temp. (°C)
a
KCl
NaCl
LiCl
KCl
NaCl
LiCl
b
25
21.3
17.4
12.6
0.53
0.54
0.55
40
26.9
21.8
19.0
0.53
0.53
0.53
50
33.8
27.5
23.9
0.52
0.54
0.52
65
41.6
36.2
30.8
0.52
0.52
0.53
ΔH‡ (kJ/mol)
14.3
15.6
16.5
Geometric area of electrode is 0.2 cm2 b Measurement at 20 °C
are listed in Table 2. The values are all near 0.5, irrespective of the temperature or the electrolyte composition. This is consistent
’ CONCLUSIONS The effects of the electrolyte composition and temperature on the electron-transfer kinetics of Fe(CN)63/4 at boron-doped diamond thin-film electrodes were investigated. This redox system can be categorized as an inner-sphere type with koapp values that are extremely sensitive to surface microstructure, surface chemistry and surface cleanliness of sp2 carbon electrodes. Interestingly, the effect of the electrolyte cation on koapp at diamond was significantly less than the effect seen at other metal and carbon electrodes. The k°app value increased with the electrolyte concentration, as expected, because of a compression of the diffuse layer thickness. k°app was also dependent on the cation type, increasing in order of Liþ < Naþ < Kþ < Csþ. This is attributed to the decrease in cation hydration sphere, as the cation size increases. The largest koapp (0.0135 cm/s) was observed in CsCl, not in KCl, as is typical for other carbon and metal electrodes. The most interesting observation was the reduced cation effect seen for diamond as compared to other electrodes, like glassy carbon and Au. For example, koapp at the 1.0 M concentration was only a factor of 1.6 greater in KCl than 10031
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The Journal of Physical Chemistry C in LiCl for diamond. This is less than the factor of approximately 10 observed at glassy carbon and gold. The transfer coefficient for the oxidation was relatively independent of the temperature and the electrolyte composition with a value ranging from 0.52 to 0.55. The exchange current density in each of the electrolytes increased linearly with the temperature between 25 and 65 °C with the largest values seen in KCl. The activation enthalpy for the electron transfer reaction decreased in the order of Liþ > Naþ > Kþ with values of 16.5, 15.6, and 14.3 kJ/mol. Overall, the results are suggestive of a different interfacial/double layer structure at hydrogen-terminated diamond electrodes as compared to the interfacial structure formed at sp2 carbon electrodes. While speculative at this point, there is at least one way the double layer structure at diamond is possibly different. The diamond surface when hydrogen terminated is hydrophobic so it is likely that the organization of water molecules is quite different from the organization at hydrophilic (oxygenterminated) sp2 carbon electrodes. A significant body of work has been conducted by others to learn how hydrophobic surfaces are hydrated. The water layer at such surfaces is expanded, less dense and more ordered to maximize hydrogen bonding.43 If this occurs at hydrophobic diamond surfaces, then this might cause a slightly lower potential gradient, φ2, at the plane of closest approach than what would occur at an sp2 carbon electrode under the same conditions. Future electrolyte-effect studies at chemically modified diamond electrodes should provide greater insight on the similarities and differences in the double layer structure at diamond compared to sp2 carbon electrodes (e.g., glassy carbon).
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses §
Present address: Tyco Electronics, Research and Development, Menlo Park, CA.
’ ACKNOWLEDGMENT The research was generously supported by the Department of Energy-Office of Science (DE-FG03-95ER14577), and the National Science Foundation (CHE-0616730 and CHE-0911383). ’ REFERENCES (1) Swain, G. M. Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; 2003; Vol. 22, pp 181. (2) Park, J.; Quaiserova-Mocko, V.; Patel, B. A.; Novotny, M.; Liu, A.; Bian, X.; Galligan, J. J.; Swain, G. M. Analyst 2008, 133, 17. (3) Ay, A.; Swope, V. M.; Swain, G. M. J. Electrochem. Soc. 2008, 155, B1013. (4) Dai, Y.; Proshlyakov, D. A.; Zak, J. K.; Swain, G. M. Anal. Chem. 2007, 79, 7526. (5) Show, Y.; Witek, M.-l. A.; Sonthalia, P.; Swain, G. M. Chem. Mater. 2003, 15, 879. (6) Knigge, D.; Kaur, P.; Swain, G. M. Encyclopedia of Electrochemistry 2007, 11, 236. (7) Bennett, J. A.; Wang, J.; Show, Y.; Swain, G. M. J. Electrochem. Soc. 2004, 151, E306. (8) Chen, Q.; Gruen, D. M.; Krauss, A. R.; Corrigan, T. D.; Witek, M.; Swain, G. M. J. Electrochem. Soc. 2001, 148, E44. (9) Wang, S.; Swain, G. M. J. Phys. Chem. C 2007, 111, 3986.
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