Study of the Underlying Electrochemistry of Polycrystalline Gold

Feb 12, 2008 - School of Bioresources and Technology, King Mongkut's University of Technology, Thonburi, Thailand, School of Chemistry, Monash Univers...
0 downloads 0 Views 435KB Size
Download PDF
2856

Langmuir 2008, 24, 2856-2868

Study of the Underlying Electrochemistry of Polycrystalline Gold Electrodes in Aqueous Solution and Electrocatalysis by Large Amplitude Fourier Transformed Alternating Current Voltammetry Benchaporn Lertanantawong,† Anthony P. O’Mullane,‡ Werasak Surareungchai,† Mithran Somasundrum,† L. Declan Burke,§ and Alan M. Bond*,‡ School of Bioresources and Technology, King Mongkut’s UniVersity of Technology, Thonburi, Thailand, School of Chemistry, Monash UniVersity, Clayton, Victoria 3800, Australia, and Chemistry Department, UniVersity College Cork, Cork, Ireland ReceiVed August 8, 2007. In Final Form: NoVember 5, 2007 Polycrystalline gold electrodes of the kind that are routinely used in analysis and catalysis in aqueous media are often regarded as exhibiting relatively simple double-layer charging/discharging and monolayer oxide formation/ removal in the positive potential region. Application of the large amplitude Fourier transformed alternating current (FT-ac) voltammetric technique that allows the faradaic current contribution of fast electron-transfer processes to be emphasized in the higher harmonic components has revealed the presence of well-defined faradaic (premonolayer oxidation) processes at positive potentials in the double-layer region in acidic and basic media which are enhanced by electrochemical activation. These underlying quasi-reversible interfacial electron-transfer processes may mediate the course of electrocatalytic oxidation reactions of hydrazine, ethylene glycol, and glucose on gold electrodes in aqueous media. The observed responses support key assumptions associated with the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis.

Introduction The concept of surface active sites was introduced into the heterogeneous catalysis area by H. S. Taylor1 in 1925. In a recent discussion of such sites, Ertl2 pointed out that atomic defects are present on all solid surfaces, such as polycrystalline gold, and even those of the well-defined, single crystal plane variety. The atoms at such defect sites exhibit unusual coordination chemistry, and have energetic and kinetic properties that differ from those of terrace atoms on the same surface. According to Ertl, the metal atoms present at these defect sites facilitate bond breaking reactions. According to Somorjai,3 another related feature of such atoms is that their protruding surfaces are unusually electropositive, so that they tend to lose electrons to the bulk phase. Thus, defect atoms exist as positive species which are surrounded by a large local electric field which polarizes and ruptures incoming molecules. The abnormal properties of surface active site atoms are assumed to be a major contributor to the unexpected catalytic properties of oxide-supported gold nanoparticles,4,5 which are currently the subject of widespread investigation. The properties of surface active sites are difficult to investigate as (a) the states involved are unstable and may be of variable character, (b) the coverages involved are usually quite low (in some instances Es), whereas the mediator for reduction can be M* (when E < Es). In the case of gold and other metal surfaces, commonly the interfacial couple is thermodynamically unstable and some element of activated chemisorption (or coordination) of reactants with the active site species may be involved. To date, the main evidence for premonolayer oxidation of gold in aqueous media, and significant electrocatalysis, has been based on data obtained by dc cyclic voltammetry. There are a few reports of analysis of premonolayer oxidation behavior based on spectroscopic, contact electrical resistance, and quartz crystal microbalance data; and extensive work in this area, based on the use of modern surface science techniques, has been carried out in basic media by Doblhofer and co-workers11 for silver electrodes. In the present study, a more sophisticated electrochemical approach, large amplitude Fourier transformed alternating current (FT-ac) voltammetry has been used to explore the behavior of gold electrode surfaces in aqueous media. A significant advantage in analyzing higher harmonic components that are readily accessible from this technique is the ability to achieve almost complete suppression of background capacitive current. This feature of the higher harmonics has been exploited in the study of solution-phase one-electron redox processes12,13 and surfaceconfined systems of biological importance.14-16 Another advantage over dc and other ac techniques is kinetic discrimination. For example, the resolution of overlapping reversible and irreversible electron-transfer processes, such as [Ru(NH)6]3+/2+ reduction in the presence of oxygen17 and the separation of an underlying electron-transfer process from a coupled catalytic chemical reaction in the case of mediated glucose oxidation,18 has been achieved using higher harmonics that are readily available with large amplitude FT-ac voltammetry. This approach is now applied to detect the underlying interfacial redox chemistry at a gold electrode occurring in the presence of an irreversible electrocatalytic reaction of the kind that occurs during the course of hydrazine oxidation. (11) Savinova, E. R.; Zemlyanov, D.; Pettinger, B.; Scheybal, A.; Schlogl, R.; Doblhofer, K. Electrochim. Acta 2000, 46, 175. (12) Sher, A. A.; Bond, A. M.; Gavaghan, D. J.; Harriman, K.; Feldberg, S. W.; Duffy, N. W.; Guo, S.-X.; Zhang, J. Anal. Chem. 2004, 76, 6214. (13) Bond, A. M.; Duffy, N. W.; Guo, S.-X.; Zhang, J.; Elton, D. Anal. Chem. 2005, 77, 186A. (14) Zhang, J.; Guo, S.-X.; Bond, A. M.; Honeychurch, M. J.; Oldham, K. B. J. Phys. Chem. B 2005, 109, 8935. (15) Guo, S.; Zhang, J.; Elton, D. M.; Bond, A. M. Anal. Chem. 2004, 76, 166. (16) Fleming, B. D.; Barlow, N. L.; Zhang, J.; Bond, A. M.; Armstrong, F. A. Anal. Chem. 2006, 78, 2948. (17) Zhang, J.; Guo, S.-X.; Bond, A. M.; Marken, F. Anal. Chem. 2004, 76, 3619. (18) Fleming, B. D.; Zhang, J.; Bond, A. M.; Bell, S. G.; Wong, L.-L. Anal. Chem. 2005, 77, 3502.

Langmuir, Vol. 24, No. 6, 2008 2857

Experimental Section Chemicals. Aqueous 0.5 M H2SO4 (APS Ajax Finechem) and NaOH (AnalaR, BDH) solutions were made up with deionized water (resistivity of 18.2 MΩ cm) purified by use of a Milli-Q reagent deionizer (Millipore). Hydrazine (Sigma Aldrich), ethylene glycol (Sigma), and glucose (BDH) were used as received. Apparatus and Procedures. A description of the FT voltammetric instrumentation used in this study is available elsewhere.13 Sine waves of frequencies f ) 10 to 30 Hz and amplitudes of ∆E ) 80 to 150 mV were employed as the ac perturbation. dc voltammetric experiments were carried out with this instrumentation by using a zero amplitude perturbation. All voltammetric experiments were undertaken at (20 ( 2)°C in an electrochemical cell that allowed reproducible positioning of the working, reference, and auxiliary electrodes and a nitrogen inlet tube. A 1.5 mm diameter gold electrode (BAS) was used as the working electrode. This electrode was polished with an aqueous 0.3 µm alumina slurry on a polishing cloth (Microcloth, Buehler), sonicated in deionized water for 5 min, and dried with tissue paper (Kimwipe) prior to use. The reference electrode was Ag/AgCl (aqueous 3 M KCl) and the auxiliary electrode was a coiled gold wire. Voltammetric experiments were commenced after degassing the electrolyte solutions with nitrogen for at least 10 min prior to any measurement. When a cathodic polarization procedure was used, the electrolyte solution was continuously degassed with nitrogen.

Results and Discussion Large Amplitude FT-ac Voltammetry at a Gold Electrode in Acidic Solution. Initial studies used a sine wave of frequency f ) 21.46 Hz and amplitude of ∆E ) 150 mV superimposed onto the dc waveform (potential range 0 to 1.5 V vs Ag/AgCl, scan rate (ν) ) 99.34 mV s-1). The separated dc and ac components obtained after the FT-inverse FT sequence are shown in Figure 1 for a polycrystalline gold electrode in 0.5 M H2SO4. The dc component (Figure 1a) is typical of a conventional dc cyclic voltammogram reported for gold in aqueous acid solution.19 Thus, the positive potential sweep response (0 to 1.5 V) is featureless until the onset of the monolayer oxide formation process at ca. 1.06 V. This reaction is sluggish, it continues to the upper limit of the sweep and is represented here in simple terms by eq 1. The subsequent negative potential

2Au + 3H2O ) Au2O3 + 6H+ + 6e-

(1)

direction sweep (1.5 to 0 V) exhibits the expected hysteresis effect in the reduction of the monolayer oxide film19 which is followed by a featureless response until the end of the sweep at 0 V. The data in Figure 1 and all other figures have been plotted in the current-potential format and separated into forward and reverse scans to allow for greater clarity in presentation of the ac harmonic data. In the ac fundamental harmonic response (Figure 1b), a broad peak not seen in the dc component is observed at 0.6 V on both the forward and reverse scans. This potential region of 0.4 to 0.6 V, where fundamental harmonic current is found, corresponds to that where premonolayer responses on gold have been reported after appropriate electrochemical and thermal activation.20,21 The monolayer oxide formation process (eq 1) also gives rise to a broad fundamental harmonic ac component and a much sharper response for the removal of the oxide, analogous to the dc data. In the higher second, third, and fourth ac harmonic components (Figure 1c,d,e), the broad feature at ca. 0.60 V gradually (19) Conway, B. E. Prog. Surf. Sci. 1995, 49, 331. (20) Burke, L. D.; O’Mullane, A. P. J. Solid State Electrochem. 2000, 4, 285. (21) Burke, L. D.; Hurley, L. M.; Lodge, V. E.; Mooney, M. B. J. Solid State Electrochem. 2001, 5, 250.

2858 Langmuir, Vol. 24, No. 6, 2008

Lertanantawong et al.

Figure 1. Large amplitude Fourier transformed ac cyclic voltammograms obtained for the dc (a) and fundamental to fourth harmonics (b-e) for a gold electrode (black) and activated electrode (prepolarized at 0 V for 5 min) (red) in 0.5 M H2SO4. Conditions employed: f ) 21.46 Hz, ∆E ) 150 mV, and ν ) 99.34 mV s-1.

diminishes and is not detectable in the fourth harmonic. In contrast, the monolayer oxide/removal process exhibits a significant response even in the fourth harmonic. The current for the monolayer oxide response for all harmonics begins to decay when a potential of 1.10 V is reached, which is in the potential region for compact monolayer oxide formation.19 However, the higher harmonic data suggest that the initial stages of this process may have a significant level of electrochemical reversibility which is not evident from the dc data. The magnitudes of the currents detected in the fundamental to fourth ac harmonics are larger than that of the dc component. This is predicted for a process that is electrochemically reversible and has been previously demonstrated both experimentally and theoretically for solution and surface-confined processes.15,17,22 The mechanism of oxide formation on gold and other noble metals has been a topic of much discussion and it has

been postulated that the initial stages involve the formation of an electroadsorbed layer of OH which undergoes a place exchange reaction with the metal to form a quasi 3D lattice of AuO.19,23 Upon increasing the anodic potential, a compact oxide containing Au3+ species develops.24 This process is outlined in eqs 2-5.

Au + H2O f Au - OHads + H+ + eplace exchange reaction

Au - OHads98OH- - Au+

(2) (3)

(22) Gavaghan, D. J.; Bond, A. M. J. Electroanal. Chem. 2000, 480, 133. (23) Woods, R. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol 9, p 27. (24) Juodkazis, K.; Juodkazyte, J.; Jasulaitiene, V.; Lukinskas, A.; Sebeka, B. Electrochem. Commun. 2000, 2, 503.

Polycrystalline Au Electrodes in Aq Solution

Langmuir, Vol. 24, No. 6, 2008 2859

OH- - Au+ f O2- - Au2+ + H+ + e-

(4)

2AuO + H2O f Au2O3 + 2H+ + 2e-

(5)

The FT ac voltammetric data imply that the electron-transfer processes are moderately fast. However, once the compact oxide is formed, the gold electrode is essentially deactivated, which results in a rapid decay in the ac harmonic components at potentials above ca. 1.20 V. Previous studies have shown that cathodic polarization of gold electrodes in the hydrogen evolution region20 or thermal pretreatment21 results in severe disruption of the outer layers of the metal. These treatments therefore provide an unusually active state of gold that can exhibit voltammetric responses in the doublelayer region. Figure 1 also contains dc and ac fundamental to fourth harmonic voltammetric components obtained in 0.5 M H2SO4 after the gold electrode had been subjected to cathodic polarization at 0 V for 5 min. The dc data (Figure 1a) provide evidence of a premonolayer process in the double-layer region at ca. 0.22 V in the forward sweep. However, no significant reduction component is detected on the reverse scan. (It should be noted that the dc component recovered from the inverse FT procedure gave data typical of conventional dc cyclic voltammograms recorded without any sinusoidal perturbation.) The fundamental harmonic component exhibits a new response at 0.22 V as well as the previous identified one at 0.60 V on the forward sweep, but no new features are detected on the reverse sweep. In comparison to the nonactivated gold electrode, the entire response (including monolayer oxide formation/removal) also shows an increase in current magnitude. The new response detected at ca. 0.22 V on the activated gold electrode is attributed20 to the oxidation of active adatoms of gold (Au*) at the surface which have a low lattice coordination number and are hence easily oxidized. The Au adatoms involved in this premonolayer oxidation process are assumed to be in a more active state than those responsible for the process at more positive potential (0.60 V). As for the nonactivated gold electrode case, the feature at 0.60 V decays quite rapidly in the higher ac harmonic components, but the response at 0.22 V remains significant even in the fourth harmonic, which suggests the rate of electron transfer is reasonably fast. However, the lack of a counter process in the reverse sweep indicates that the chemical reversibility of this process is poor and that the product formed in the electron-transfer step is unstable. The product of Au* oxidation has been postulated as being a hydrous oxide species that is anionic in nature7 and represented as [Au2(OH)9]3-ads. Under conditions of low hydroxide ion concentration found in 0.5 M H2SO4, such a species may be quite unstable. As seen in Figure 2, with use of a more cathodic polarization potential of -0.4 V, the magnitude of the premonolayer response is greatly enhanced in the potential region around 0.2 V. The magnitude of the dc component (Figure 2a) is now comparable to that observed for monolayer oxide formation, and the reverse sweep contains evidence of a reduction component. Evidence of more than one process is now found in this potential region. The fundamental to fourth ac harmonic components also demonstrate that the premonolayer oxidation response around 0.2 V dominates the forward sweep in comparison to the monolayer oxide formation process. On the reverse sweep, there is a clear response observed in the same potential region around 0.2 V which is lower in magnitude than that seen in the forward sweep, as expected if the product is unstable in a solution of low pH. Under these more severe activation conditions, the outer layers of the gold lattice are disrupted via hydrogen embrittlement to a

significantly greater extent than when cathodic polarization is applied at 0 V. In this process, the entry of hydrogen into the metal generates strain in the lattice which results in the expulsion of gold atoms from bulk metal to the surface.25 The insertion of energy into the system (assumed to be related to the cathodic polarization potential) is retained to some degree by metal atoms which have converted from a low to a high energy state with a low lattice coordination number. The resulting Au* or MMS state readily undergoes oxidation as seen by the large current at potential around 0.2 V. The electron-transfer step is most likely quasi reversible according to the magnitude of the current in the higher harmonic components. The magnitude of the current in the reverse sweep indicates that the stability of the hydrous oxide species is enhanced when a higher surface coverage of the initial MMS state is achieved by using a cathodic polarization potential of -0.4 V. In Figure 3, the influence of the standard electron-transfer rate constant for a simple Asurf + e- a Bsurf surface-confined oneelectron transfer process (the Au* to hydrous oxide transition is assumed to be surface-confined) is shown via simulation using the Butler-Volmer formulation with electron-transfer rate constants of k ) 1 × 1010, 10, and 1 s-1 with a charge-transfer coefficient of 0.5. Other parameters included in the simulation, and which closely match experimental conditions, were as follows: surface coverage ) 8 × 10-11 mol cm-2 (calculated from the charge passed for the response at 0.22 V), sweep rate ) 100 mV s-1, f ) 21.46 Hz, ∆E ) 150 mV, uncompensated resistance ) 80 Ω, E0 ) 0.20 V, and the potential limits were -0.20 to 0.60 V. Simulations that mimic the potential dependent double-layer capacitance are not yet available and hence the background current contributions are not included in the model. When the process is fully reversible, well-defined ac harmonic components are achieved (Figure 3a), whereas when much slower kinetics are involved (Figure 3b, c), the symmetry of the response is lost, particularly in the second to fourth harmonics. Full details of the simulation can be found in ref 15. Quantitative agreement between simulated and experimental data is not expected, given that the Au*/Au hydrous oxide transition is a far more complex than that simulated, since the experimental case involves an unstable product and a surface coverage of Au* species that is extremely difficult to estimate because the activity of the surface is likely to be dependent on factors such as electrode history, potential cycling rate, and potential limits. Furthermore, the gold species have been speculated as having a level of mobility on the surface, which according to Kolb26 provides imaging problems during STM experiments. Recently, it was demonstrated that the atomic structure of gold is not static in the presence of weakly absorbing molecules even at temperatures as low as 80 K.27 Therefore, simulations are only used qualitatively to illustrate that the Au*/Au hydrous oxide transition is likely to have a significant degree of electrochemical reversibility. Cathodic polarization in the hydrogen evolution region is not the only means of activating a gold surface. Even mild abrasion of the electrode surface with emery paper results in the generation of active gold as illustrated by the FT-ac harmonic responses in Figure S1. In the dc component, a rise in anodic current is detected from 0.76 V until the onset of monolayer oxide formation (at 1.06 V). Significant responses in the forward sweep for each of the ac harmonics are also seen at ca. 0.95 V. Analogous behavior was recently reported in dc cyclic voltammograms in the double(25) Cobden, P. D.; Nieuwenhuys, B. E.; Gorodetskii, V. V.; Parmon, V. N. Platinum Met. ReV. 1998, 42, 141. (26) Kolb, D. M.; Schneeweiss, M. A. Electrochem. Soc. Interf. 1999, 8, 26. (27) Baber, A. E.; Jensen, S. C.; Iski, E. V.; Sykes, E. C. H. J. Am. Chem. Soc. 2006, 128, 15384.

2860 Langmuir, Vol. 24, No. 6, 2008

Lertanantawong et al.

Figure 2. Large amplitude Fourier transformed ac cyclic voltammograms obtained for the dc (a) and fundamental to fourth harmonics (b-e) for an activated gold electrode (prepolarized at -0.40 V for 20 min) in 0.5 M H2SO4. Conditions employed: f ) 21.46 Hz, ∆E ) 150 mV, and ν ) 102.95 mV s-1.

layer region after use of either conventional polishing procedures or chemical treatment in piranha solution.28 The second and higher harmonic components of a large amplitude FT-ac experiment are extremely effective in discriminating against background capacitive current and hence in emphasizing faradaic currents of processes involving fast electrontransfer kinetics, as shown in studies of surface-confined redox active biological moieties such as azurin.15 In the present example, the background capacitive current dominates the dc response, but its influence is essentially absent in the fourth harmonic. Consequently, the existence of significant fourth harmonic currents, in a potential region often regarded as purely capacitive, supports the view that the origin of these responses is faradaic, and that electron-transfer kinetics are fast.

Large Amplitude FT-ac Voltammetry at a Gold Electrode in the Presence of Hydrazine in Acidic Solution. Hydrazine oxidation at a gold electrode in acidic solution was chosen as an example of a chemically irreversible29 electrocatalytic process. The proposed mechanism is given in eq 6. Previous studies using

(28) Rafaela Fernanda Carvalhal, R. S. F., Lauro Tatsuo Kubota,, Electroanalysis 2005, 17, 1251.

(29) Gomez, R.; Orts, J. M.; Rodes, A.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1993, 358, 287.

NH2 - NH3+ f N2 + 5H+ + 4e-

(6)

the FT-ac technique have shown that the higher harmonic components almost completely discriminates between reversible and nonreversible components, e.g., [Ru(NH)6]3+/2+ reversible reduction in the presence of oxygen17 and the ferrocene mediated oxidation of glucose.18 In these examples, the dc current is dominated by the oxygen reduction reaction and the catalytic

Polycrystalline Au Electrodes in Aq Solution

Langmuir, Vol. 24, No. 6, 2008 2861

Figure 3. Simulated large amplitude Fourier transformed ac cyclic voltammograms obtained for the fundamental to fourth harmonics for a surface-confined reversible electron-transfer process at different values of k ) (a) 1 × 1010, (b) 10, and (c) 1 s-1. Other parameters included: surface coverage of 8 × 10-11 mol cm-2, ν ) 100 mV s-1, ∆E ) 150 mV, f ) 21.46 Hz, uncompensated resistance ) 80 Ω, E0 ) 0.20 V. See reference 15 for further details.

oxidation of glucose respectively, which obscured the underlying reversible electron transfer chemistry. Illustrated in Figure 4 are the dc and ac fundamental to fourth harmonic components obtained at a gold electrode in 0.5 M H2SO4 containing 10 mM hydrazine. The dc component exhibits a broad oxidation response on both the forward and reverse sweep, with the current commencing and terminating at a potential of ca. 0.52 V. The reverse sweep does not show any reduction component below the latter value, as expected for an irreversible catalytic oxidation process. From the dc data, it cannot be ascertained if there is any underlying interfacial redox cycling of the active state. However, the presence of significant currents in both the forward and reverse sweeps in the fundamental to fourth harmonic components implies that this is the case. The onset potential of ca. 0.52 V for hydrazine oxidation coincides with the potential region for Au*/Au hydrous oxide transition observed for an unactivated and moderately activated gold electrode (Figure 1) in acid only. There is also a large enhancement in the ac harmonic response over the potential range of 0.6 to 0.8 V in the presence of hydrazine compared to gold in acid alone, presumably because the oxidized state, rather than remaining as the hydrous oxide, is rapidly reduced back to the active metal state by hydrazine, thereby inducing a repetitive redox cycle like that illustrated in Scheme 1. Recent numerical simulations30 for a mechanism where a surface-confined one-electron-transfer process is reversible and the coupled catalytic reaction involves a dissolved substrate is pseudo first order show that enhancement in the magnitude of the ac harmonic components is predicted when the bimolecular rate constant is very fast. A somewhat analogous scenario prevails (30) Zhang, J.; Bond, A. M. J. Electroanal. Chem. 2007, 600, 23.

in the present case, since it is assumed that the surface-confined Au*/Au hydrous oxide electron-transfer process is coupled to the catalytic reaction involving dissolved hydrazine. The change in shape of the harmonic components also suggests that the mechanism of the underlying redox process is possibly affected by the presence of hydrazine. Thus, the response on unactivated gold in this potential region is broad, whereas in the presence of hydrazine, the ac responses become better defined in the higher harmonics. It should be noted that in the dc and fundamental to fourth ac harmonic components that currents are detected around 0.2 V. This is due to the gold electrode remaining in an active state as a result of previous electrochemical activation experiments. It is often difficult to completely remove the active state response for a gold electrode in aqueous solution, which again highlights the importance of such phenomena when using this material in electrochemical experiments. Large Amplitude FT-ac Voltammetry at a Gold Electrode in Basic Solution. Illustrated in Figure 5 are the dc and fundamental to fifth ac harmonic components for both the positive and negative potential scan directions (ν ) 51.48 mV s-1, f ) 21.46 Hz, ∆E ) 150 mV, potential range ) -1.2 to 0.7 V) for a polycrystalline gold electrode in 0.5 M NaOH. The dc data (Figure 5a) show, as in the case for gold in acid solution, that the positive sweep is featureless until the onset of monolayer oxide formation at 0.06 V. On the subsequent negative potential direction sweep, the monolayer oxide is removed and is followed by a featureless response until the end of the sweep at -1.20 V. However, in each of the fundamental to fifth harmonic responses, there is a well-defined premonolayer oxidation response seen in the forward sweep at -0.15 V which does not decay significantly with respect to current magnitude in the higher harmonics. Equally

2862 Langmuir, Vol. 24, No. 6, 2008

Lertanantawong et al.

Figure 4. Large amplitude Fourier transformed ac cyclic voltammograms obtained for the dc (a) and fundamental to fourth harmonics (b-e) for a gold electrode in 0.5 M H2SO4 containing 10 mM hydrazine. Conditions employed: f ) 21.46 Hz, ∆E ) 150 mV, and ν ) 104.31 mV s-1.

well-defined responses are observed on the reverse sweep. In comparison with unactivated gold electrode in acidic solution, the ac harmonics are larger in magnitude, are present on the reverse sweep, and are better defined. In particular, the shape of the fourth and fifth harmonics exhibit features close to those predicted for a fully reversible surface-confined electron-transfer process (Figure 3). The monolayer oxide formation/reduction processes also exhibit significantly larger harmonic responses than detected in acidic solution. This is consistent with a reversible electrontransfer process occurring during the early stages of oxide formation. The mechanism for gold oxide formation in base is assumed31 to occur via the scheme outlined in eqs 7-8.

Au + 3OH- f Au(OH)3 + 3epost electrochemical step

2Au(OH)3 98 Au2O3 + 3H2O

(7) (8)

(31) Burke, L. D.; Cunnane, V. J.; Lee, B. H. J. Electrochem. Soc. 1992, 139, 399.

However, oxide formation is also possible via

2Au + 6OH- f Au2O3 + 3H2O + 6e-

(9)

These well-defined ac harmonic responses for gold electrodes in basic media at ca. -0.15 V can be expected if the product of the Au*/Au hydrous oxide transition is a species such as [Au2(OH)9]3-ads, which is stable under conditions of high hydroxide ion concentration. Cathodic polarization at -1.2 V was employed to activate the surface of the polycrystalline gold electrode. In this case, the dc data do not show any evidence of an active gold response (Figure 5) but the higher ac harmonic components exhibit current enhancement at potentials around -0.15 V, consistent with an increase in Au* coverage of the surface. The enhancement is particularly pronounced in the fourth and fifth ac harmonics where the responses now mimic behavior close to that expected for a fully reversible surface-confined electron-transfer process (compare Figure 5e with Figure 3a). Under conditions achieved with cathodic polarization, the monolayer oxide formation/ removal processes are only slightly perturbed compared to

Polycrystalline Au Electrodes in Aq Solution

Langmuir, Vol. 24, No. 6, 2008 2863

Figure 5. Large amplitude Fourier transformed ac cyclic voltammograms obtained for the dc (a) and fundamental to fifth harmonics (b-f) for a gold electrode (black) and activated electrode prepolarized at -1.20 V for 5 min (red) and 20 min (blue) in 0.5 M NaOH. Conditions employed: f ) 21.46 Hz, ∆E ) 150 mV, and ν ) 51.48 mV s-1.

unactivated gold, in both the dc and ac harmonics (Figure 5). The FT-ac technique highlights underlying processes not observable in dc assessments, which show only a capacitive double-layer charging region. Given the ability of the FT-ac technique to discriminate against capacitive current in the fourth and fifth harmonics, it can assumed that the process observed at -0.15 V is faradaic in nature. Increasing the cathodic polarization time from 5 to 20 min has an extreme effect on the voltammetry of a gold electrode in basic media (Figure 5, blue line). Under these conditions, the dc response shows a large increase in current just prior to the switching potential of 0.7 V at the positive end of the sweep. This has been observed previously and attributed to the catalytic evolution of oxygen.31 The subsequent negative potential sweep contains the usual response due to monolayer oxide removal as

well as a new and larger peak due to the reduction of a Au hydrous oxide species formed during the oxygen evolution process.32 The fundamental to fifth ac harmonic responses now exhibit much larger currents at -0.15 V than found when using the milder activation conditions used in the previous experiment (Figure 5). Indeed, in the fourth and fifth harmonic ac components the response in this potential region is dominated by the Au*/Au hydrous oxide transition and little evidence for the monolayer oxide formation/removal process is observable. It can also be seen that there are minor process present at ca. -0.45 and -0.91 V attributed to the oxidation of more active Au adatoms whose coverage is much lower than those that are oxidized at more (32) Burke, L. D.; Moran, J. M.; Nugent, P. F. J. Solid State Electrochem. 2003, 7, 529.

2864 Langmuir, Vol. 24, No. 6, 2008

Lertanantawong et al.

Figure 6. Large amplitude Fourier transformed ac cyclic voltammograms obtained for the dc (a) and fundamental to fifth harmonics (b-f) for a gold electrode in 0.5 M NaOH containing 10 mM hydrazine. Conditions employed: f ) 21.46 Hz, ∆E ) 150 mV, and ν ) 52.15 mV s-1.

(10)

-0.5 V. The reverse sweep contains a large plateau region from 0.20 to -0.50 V but no evidence of reduction. The fundamental to fifth harmonic responses show evidence of two processes occurring at ca. -0.45 and -0.18 V. The magnitude of the latter response is comparable to that achieved when a gold electrode in base alone was severely activated (Figure 5), whereas the response at the more negative potential is greatly enhanced. As is the case in acidic media, the magnitude of the catalytic response in basic media is significantly greater than that seen for unactivated gold in base as the oxidized hydrous oxide state is rapidly reduced back to the active adatom state, facilitating a reversible redox cycle. The presence of hydrazine also enhances the process at -0.45 V which was only slightly detectable even after severe activation. The ac harmonic response at -0.18 V is assumed to be electrochemically quasi reversible on the basis

The forward positive potential direction sweep contains a welldefined catalytic peak for hydrazine oxidation at a potential at

(33) Abu-Rabi, A.; Jasin, D.; Mentus, S. J. Electroanal. Chem. 2007, 600, 364.

positive potential (-0.15 V). Related premonolayer responses also have been observed recently by dc cyclic voltammetry at ca. -0.55 and -0.25 V (vs SCE) in basic solution after a gold electrode had been cathodically polarized at -1.6 V for 5 min.33 Large Amplitude FT-ac Voltammetry at a Gold Electrode in Basic Solution in the Presence of Hydrazine. Figure 6 contains the dc and ac fundamental to fifth harmonic components for both the positive and negative potential scan directions for a polycrystalline gold electrode in 0.5 M NaOH containing 10 mM hydrazine (ν ) 52.15 mV s-1, f ) 21.46 Hz, ∆E ) 150 mV). The absence of a dc reduction response, as seen in acidic solution, indicates that hydrazine oxidation is an overall chemically irreversible process that occurs according to eq 10.

N2H4 + 4OH- f N2 + 4H2O + 4e-

Polycrystalline Au Electrodes in Aq Solution

Langmuir, Vol. 24, No. 6, 2008 2865

Figure 7. Large amplitude Fourier transformed ac cyclic voltammograms obtained for the dc (a) and fundamental to fifth harmonics (b-f) for a gold electrode in 0.5 M NaOH containing 10 mM ethylene glycol. Conditions employed: f ) 21.46 Hz, ∆E ) 150 mV, and ν ) 52.15 mV s-1.

of comparison to the simulation provided in Figure 3. This study illustrates the effectiveness of the FT-ac technique in probing underlying interfacial electron-transfer chemistry in the presence of a large irreversible catalytic process where almost total discrimination of the two processes is achieved in the higher harmonics. Large Amplitude FT-ac Voltammetry at a Gold Electrode in Basic Solution in the Presence of Ethylene Glycol. Illustrated in Figure 7 are the dc and ac harmonic components (ν ) 52.15 mV s-1, f ) 21.46 Hz, ∆E ) 150 mV) for a polycrystalline gold electrode in 0.5 M NaOH, but now containing 10 mM ethylene glycol. The dc response (Figure 7a) reveals that oxidation of ethylene glycol commences at ca. -0.20 V until it reaches a peak at 0.18 V where monolayer oxide commences. At more positive potentials, the gold surface is deactivated, and the electrocatalytic reaction shuts off completely. On the reverse

sweep a small cathodic peak is observed at 0.20 V which is due to the removal of the oxide layer after which there is a large increase in anodic current due to the recommencement of ethylene glycol oxidation. The absence of a dc reduction response for the remainder of the reverse sweep indicates an irreversible reaction which can be represented by eq 11. The fundamental to fifth

(CH2OH)2 + 10OH- f (CO2)22- + 8H2O + 8e-

(11)

harmonic responses provide evidence of two processes at ca. -0.15 and 0.20 V on both sweep directions. The process at the latter is again assumed to be due to a reasonably fast electrontransfer process associated with the initial stages of monolayer oxide formation. The process at -0.15 V, as seen in the hydrazine case, is assigned to the repetitive cycling of the Au*/Au hydrous oxide transition induced by the presence of ethylene glycol, noting

2866 Langmuir, Vol. 24, No. 6, 2008

Lertanantawong et al.

Figure 8. Large amplitude Fourier transformed ac cyclic voltammograms obtained for the dc (a) and fundamental to fifth harmonics (b-f) for a gold electrode in 0.5 M NaOH containing 10 mM glucose. Conditions employed: f ) 21.46 Hz, ∆E ) 150 mV, and ν ) 51.48 mV s-1.

that an unactivated gold electrode in base gave a much smaller response at this potential (Figure 5). From the magnitude and shape of the harmonics it can be concluded that this underlying interfacial redox process is close to reversible as is the case with a highly activated gold electrode in basic solution (Figure 5). However, the current magnitude of this response is less than that observed in the hydrazine case (Figure 6) even though equal concentrations were used. This is possibly due to the fact that ethylene glycol reacts less rapidly with the Au hydrous oxide state than hydrazine which is a very efficient reductant.34 Large Amplitude FT-ac Voltammetry at a Gold Electrode in Basic Solution in the Presence of Glucose. In the case of large amplitude FT-ac voltammetry at a polycrystalline gold electrode in 0.5 M NaOH containing 10 mM glucose, the dc current (Figure 8a) shows that glucose oxidation commences at -0.70 V. At -0.30 V a further increase is detected until 0.18

V where monolayer oxide formation deactivates the gold surface and the electrocatalytic reaction ceases. On the reverse sweep, a small cathodic peak at 0.20 V associated with the removal of the oxide layer, is followed by a large anodic current region due to recommencement of oxidation of glucose with a broad minor shoulder centered at ca. -0.60 V also present. The ac harmonics (Figure 8b,c,d,e) all exhibit three distinct processes on both the forward and reverse sweeps centered at ca. -0.45, -0.15, and 0.20 V. The latter response, as noted previously, is associated with monolayer oxide formation/removal processes. The two processes at -0.45 and -0.15 V indicate the possibility of two different mediating species being involved in the oxidation of glucose. Such behavior has been postulated35 on the basis of dc cyclic voltammetry. (34) Djokic, S. S. J. Electrochem. Soc. 1997, 144, 2358.

Polycrystalline Au Electrodes in Aq Solution

As in the ethylene glycol case, the current magnitude of the premonolayer responses is lower than observed in the hydrazine case (but greater than for unactivated gold) presumably because glucose does not interact as rapidly with the oxidized state of Au*. However, the shape and magnitude of the ac components indicates that there are underlying quasi-reversible interfacial redox processes that take place during the electrocatalytic oxidation of glucose. Comparison of the Behavior of Gold Electrodes in Aqueous Acidic and Basic Media. FT-ac voltammetric studies reveal that the extent of premonolayer oxidation is low at an unactivated gold electrode in aqueous solution. A small broad fundamental harmonic minor response was found in acid solution at 0.60 V (Figure 1b). However, in basic media a significant Au* oxidation response was detected at -0.20 V in all five ac harmonics examined. The electron-transfer component of the response in base is assumed to be rapid on the basis of comparison with simulated FT-ac voltammetry for a fully reversible surface-confined electron-transfer process. The dc response in either 0.5 M H2SO4 or 0.5 M NaOH showed no evidence of any faradaic process in the so-called gold electrode double-layer region. This highlights the sensitivity of the FT-ac technique to faradaic processes that involve fast electron-transfer kinetics. The possibility of a capacitive effect being the sole origin of processes found by ac voltammetry also can be ruled out as this technique is extremely effective in achieving very high faradaic to capacitive current ratios, particularly in the fourth and fifth harmonics. Interestingly, a substantial current response also is found in the fundamental and higher ac harmonics in the potential region involving monolayer oxide formation and its removal in both acidic and basic solutions. It has been postulated that the initial stages of monolayer oxide formation at a gold electrode involves a fast electron-transfer process, outlined in eqs 4 and 7, before conversion to the compact anhydrous Au2O3 oxide layer. The shapes of the fourth and fifth ac harmonics indicate that the electron-transfer step associated with monolayer oxide formation in base is more reversible than in acid, as also is the case for the premonolayer oxidation responses found at 0.60 V in acid and -0.45 V in base. The latter is related to the enhanced stability of hydrous oxide species when a high concentration of hydroxide ion are present in solution. Cathodic polarization of gold in the hydrogen evolution region results in three premonolayer oxidation processes at different potentials which were identified from FT-ac voltammetry in both acid (0.22, 0.60, and 0.95 V) and base (-0.91, -0.45, and -0.15 V) whose magnitude depend on the applied potential and duration (compare Figures 1 and 2). A dc oxidative response (0.22 V) from an activated gold electrode in acid was detected but no reductive counterpart was in evidence in dc cyclic voltammograms. After a related form of activation pretreatment in base, a significant enhancement in the oxidative response at -0.15 V in the ac fundamental to fifth harmonics was observed, along with a significant reductive counterpart. However, the dc data showed no evidence of a premonolayer oxidation response. It is clear that cathodic pretreatment (Figure 2) generates significant FT-ac responses within the double-layer region in both the forward and reverse sweeps. The assumption here is that the active site is invariably present (at a very low coverage) at the metal surface as demonstrated for nonactivated electrodes (Figure 1, black line). Its coverage and hence response as detected by the FT-ac technique is temporarily enhanced by appropriate electrochemical pretreatment. (35) Burke, L. D.; Ryan, T. G. Electrochim. Acta 1992, 37, 1363.

Langmuir, Vol. 24, No. 6, 2008 2867

The existence and the amount of coverage of active sites has been the most widely accepted explanation for the variability observed in the performance of gold catalysts36 even though the identification of such active gold sites at room temperature using surface science techniques such as LEED, STM, and AFM is extremely difficult.37 Practical surfaces such as polycrystalline gold used in catalytic systems and often in analytical applications are far from ideal and are not totally defect free. The clearest analogy to the present work is the use of surface enhanced raman spectroscopy (SERS) to explore the behavior of gold (plus silver and copper) electrode surfaces. To observe a SERS response for these electrodes in aqueous media, it is necessary to roughen the surface by oxidation-reduction cycling (ORC) pretreatment.38 The present FT-ac voltammetry approach in combination with other surface techniques offer unique insights into the electrochemistry of polycrystalline gold which is quite complex. As highlighted in a recent review of the electrochemical behavior of gold in aqueous media, virtually all electrocatalytic reactions on this surface occur within the so-called double-layer region at potentials values where Au* to Au hydrous oxide transitions have been postulated to occur, in the absence of reagent.7 Therefore, gold in aqueous media may behave analogously to a chemically modified electrode. In the latter case, a relatively inert electrode material such as carbon or indium tin oxide is modified (to enhance its electrocatalytic activity) by attaching, for example, a layer of redox active material. For a gold electrode, bulk gold or EMS atoms function as the inert support while the Au* or MMS atoms act as the redox mediators. A mediated electrocatalytic process is then established as outlined in Scheme 1 which is the basis of the IHOAM model. In most previous studies, only the dc catalytic current was available to provide evidence for such a scheme. The FT-ac technique offers the advantage of being able to identify mediated electrocatalytic schemes by probing underlying reversible electron-transfer processes. The higher harmonics effectively discriminate between electron transfer and coupled chemical processes. A substantial enhancement of the magnitude of the premonolayer responses for gold electrodes in acidic and basic media was detected in the presence of hydrazine, ethylene glycol, and glucose compared to an unactivated electrode from the fundamental to fourth harmonics. Analogous behavior was predicted recently by theoretical studies of large amplitude alternating current voltammetry for a reversible surface-confined electron-transfer process coupled to a pseudo first-order electrocatalytic process where significant enhancement of the magnitude of the ac harmonics was seen when the bimolecular reaction rate constant for the catalytic reaction was very fast. However, the enhancement in the magnitude of the ac responses seen for the ethylene glycol and glucose case in basic media was not as significant as seen for the hydrazine case suggesting that the regeneration of the Au* state is not as effective. The results from this study using FT-ac voltammetry support the IHOAM model of electrocatalysis and offer further insights into the nature of active sites.

Conclusions The large amplitude FT-ac technique readily reveals the presence of significant Faradaic processes at potentials within the so-called double-layer region of gold electrodes in acidic and (36) Menegazzo, F.; Manzoli, M.; Chiorino, A.; Boccuzzi, F.; Tabakova, T.; Signoretto, M.; Pinna, F.; Pernicone, N. J. Catal. 2006, 237, 431. (37) Burke, L. D.; Hurley, L. M. Electrochim. Acta 1999, 44, 3451. (38) Desilvestro, J.; Weaver, M. J. J. Electroanal. Chem. 1986, 209, 377.

2868 Langmuir, Vol. 24, No. 6, 2008

basic media, where the current has often been assumed to be purely capacitive in nature. The origin of these faradaic processes has been related to an active state model in which protruding or low coordination adatoms (Au*) on the gold surface are present at low coverages and are oxidized in the double-layer region to form Au hydrous oxide species. During the course of overall irreversible electrocatalytic reactions, the FT-ac technique has revealed that there exists one or in some cases two underlying interfacial electron-transfer process that are fast and extremely effective in mediating the reactions. Although only qualitative, the FT-ac results emphasize that the electrochemical behavior of polycrystalline gold electrodes in aqueous solution is complex in the double-layer region where most electrocatalytic processes occur.

Lertanantawong et al.

Acknowledgment. Funding from the Australian Research Council in support of this project is gratefully acknowledged. B.L. acknowledges the National Center for Genetic Engineering and Biotechnology and King Mongkut’s University of Technology Thonburi for providing funding to participate in this work in Australia. Supporting Information Available: Figure S1 shows large amplitude Fourier transformed ac cyclic voltammograms obtained for an activated (mildly abraded) gold electrode in 0.5 M H2SO4. This material is available free of charge via the Internet at http://pubs.acs.org. LA702454K