J. Phys. Chem. B 2001, 105, 10669-10673
10669
Laser Induced Current Transients Applied to a Au(111) Single Crystal Electrode. A General Method for the Measurement of Potentials of Zero Charge of Solid Electrodes Victor Climent, Barry A. Coles, and Richard G. Compton* Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, United Kingdom ReceiVed: June 25, 2001
The current transients arising after the sudden heating of an electrode by high-power laser irradiation have been studied. The method has been applied to a Au(111) single-crystal electrode in contact with perchloric and sulfuric acid solutions. The current transient can be explained as due to the recovery of the double layer capacity after its decrease due to the sudden rise of the temperature. It has been found that there is a correlation between the sign of the current transient and the sign of the double layer charge, and there is a coincidence between the potential where the transient vanishes and the potential of zero charge. This allows its use as a general method for determination of the potential of zero charge of solid electrodes.
Introduction Determination of the potential of zero charge (pzc) of solid electrodes is a problem that has received much attention in the past decades.1-3 Despite its importance, this issue is not yet completely solved. Several techniques have been applied for the determination of pzc, the interpretation of some of them not being straightforward. The determination of the differential capacity minimum in dilute solution has been the more fruitful method applied to the determination of the pzc of gold and silver electrodes. By using this method, much information has been gathered regarding the dependence of this parameter on properties such as surface structure, temperature, and composition of the solution.1 However, its application is limited to the fulfilment of the conditions required to observe the differential capacity minimum, notably dilute solution and the absence of specific adsorption. This precludes its application to platinum group metals (Pt, Pd, Rh, Ir) where the pseudocapacity due to hydrogen and anion adsorption processes dominates the electrochemical response of these metals.3 Methods based on observation of properties indirectly related to the state of charge of the surface have been proposed, like vibrational properties of water molecules in the interphase,4,5 N2O reduction6 and the electrocapillary properties of the electrode.7-10 Another group of methods relies on the direct measurement of the charge on the electrode or its sign. This can be done by measuring the current transient appearing during the formation11 or the vanishing of the electrical double layer. The quenching of the double layer charge can be achieved by adsorbing CO to saturation. This method has been recently applied to the determination of the pzc of platinum group metals single-crystal electrodes.12-15 Finally, the removal of the electrode charge can also be achieved by renewing the metallic surface. This constituted the scraping method.16 Recently, laser ablation of the electrode surface has been proposed as a similar method for the determination of the pzc and applied to molybdenum electrodes.17 Obviously, the latter two methods cannot be applied to single-crystal electrodes. The illumination of a metallic electrode surface with a high power laser source suddenly increases the temperature of the * To whom all correspondence should be addressed. E-mail: compton@ ermine.ox.ac.uk. Tel.: +44 (0) 1865 275 413. Fax: +44 (0) 1865 275 410.
interfacial region. This effect constituted the base of the socalled temperature jump method. This method was developed in the eighties by Benderskii et al.18-20 and applied to the study of the electrochemical double layer between an aqueous solution and a mercury electrode. A very similar approach was used by Feldberg et al.21-23 to investigate the kinetics of electrochemical processes taking place on platinum electrodes. In these studies, it is assumed that the only effect of the laser is the heating of the electrode surface, resulting in a sudden temperature change in the interphase. Because of the relatively high power energy introduced in the system, the temperature change takes place in a very short time scale, allowing the study of fast processes. If the rate of the temperature change is faster than the time needed to reach an equilibrium state (relaxation time) the kinetics of the process can be studied. On the other hand, if the relaxation time is short enough, the system can be considered at equilibrium at each instant, and thermodynamic information can be extracted easily from the experiment. To avoid side effects, such as photoemission of electrons, the energy of the beam has to be selected carefully. The photoemission threshold for typical electrode metals is around 200-300 nm (work function around 4-5 eV). Also, the beam intensity must be chosen in such a way that excessive heating or even melting of the metallic surface is avoided. In the present paper, we present preliminary results on the application of a similar approach to the study of the electrochemical double layer established on a Au(111) single crystal electrode in contact with an acidic solution. We followed an approach slightly different to that in refs 18-23 because the experiments were performed potentiostatically, i.e., the current flowing through the external circuit after the laser pulse was monitored, while the potential of the electrode was kept constant. Perchloric and sulfuric acid were chosen as electrolytes in order to compare the effect of the presence of specifically adsorbed anions. A correlation between the electrochemical response to the laser heating with the position of the pzc was found. Experimental Section The light source employed was a GCR 130 Q-switched Nd:YAG laser (Spectra Physics Lasers, Inc., CA) operating in
10.1021/jp012402e CCC: $20.00 © 2001 American Chemical Society Published on Web 10/02/2001
10670 J. Phys. Chem. B, Vol. 105, No. 43, 2001 frequency doubled mode at a wavelength of 532 nm and pulse duration of 10 ns. A single pulse was shot each time a transient was recorded. The beam diameter obtained directly at the laser output is ca. 7 mm and this was reduced to 3 mm by passing it through a conventional arrangement of lenses. Optics housings and lenses were supplied by the Newport Corporation and mirrors obtained from Comar Instruments (Cambridge, UK). The laser power was determined with a Gentec ED-200L detector head in conjunction with a Gentec SUN Series EM-1 Energy Meter (Gentec, CA). Laser energy of ca. 10-15 mJ per pulse, i.e., 0.15-0.2 J cm-2 was used, well below the damage threshold of the electrode. Experiments were performed in a conventional three-electrode arrangement. The Au(111) single-crystal electrode was prepared at the University of Alicante from a small gold bead (2 mm), oriented, cut and polished to obtain the Au(111) orientation, following Clavilier’s procedure.24 Prior to each experiment the electrode was flame annealed in a butane + air flame, cooled in the atmosphere of the laboratory and then protected with a drop of ultrapure water. A platinum mesh (Goodfellow Cambridge Ltd., Cambridge, UK) was used as a counter electrode and a Pd wire charged with H2 in a separate compartment was used as a reference electrode. The diameter of the gold working electrodes was accurately measured using a travelling microscope. A computer-controlled µ-Autolab potentiostat (Eco-Chemie, Utrecht, Netherlands) was employed to control the potential applied at the working electrode during the control voltammetric experiments. However, during the laser firing, to avoid transient induced disturbance to the potentiostat, a potentiostat of in-house design was employed in which the potential of the counter electrode was locked just before the laser firing. The potential of the working electrode was set to virtual zero. The potential of the reference was monitored during the transient caused by the laser. Since the time scale of the transient was short and the charge involved was small, the potential of the reference electrode was found to remain essentially unchanged. Laser transients were also recorded with the Autolab potentiostat for the sake of comparison and essentially identical results were obtained with both potentiostats. However, the in-house potentiostat was successful in removing the electrical artifacts caused by the laser. The current transients resulting for a laser pulse were recorded with a TDS 220 Tektronix oscilloscope interfaced with a PC computer. A time resolution of 1 µs was usually employed. Solutions were prepared from concentrated sulfuric and perchloric acids (Fisher Scientific, for Trace Metal Analysis) diluted in ultrapure water (resistivity not less than 18 MΩcm) obtained from a Elgastat water purification system (USF Ltd., Bucks., UK). Results Figure 1 shows a cyclic voltammogram corresponding to the Au(111) single-crystal electrode in the 0.1 M H2SO4. A similar voltammogram was recorded before each experiment to confirm the quality of the surface and the cleanliness of the solution. This voltammogram is in accordance to those previously published for this surface25 and only the main features will be described here. The peak at 0.55 V has been attributed to the lifting of the (22 × x3) reconstruction that takes place at the lower potential region,26 whereas two sharp spikes around 1.0 V are explained as due to the formation of an ordered sulfate adlayer. STM experiments have evidenced an (x3 × x7)R19.1°
Climent et al.
Figure 1. Cyclic voltammogram corresponding to a Au(111) electrode in 0.1 M H2SO4 solution. Sweep rate: 50 mV s-1.
Figure 2. Cyclic voltammograms in 0.01 M HClO4 solution corresponding to the following: (A) Au(111) single crystal; (B) Au polycrystalline electrodes. Sweep rate: 50 mV s-1.
structure at potentials more positive than these spikes.27 The presence of these spikes in the voltammogram of Figure 1 is a clear indication of the quality of the surface employed for this study.28 To study transient behavior due to the laser heating of the electrode, a diluted solution of perchloric acid was first examined. Perchlorate anions are characterized by weak adsorption, simplifying the model picture of the interphase. Besides, by choosing a sufficiently dilute concentration, it is possible to observe the minimum in the double layer capacity ascribed to the pzc, according to the Gouy-Chapman-Stern theory of the diffuse double layer, valid in the case of diluted solution in the absence of specific adsorption. Figure 2A shows the voltammogram of the Au(111) electrode in 0.01 M HClO4. It is noteworthy that the minimum in the voltammetric current is around 0.5 V; this has been considered as indicative of the position of the pzc.25 The voltammetric wave at the positive side of the minimum has been attributed to perchlorate adsorp-
Laser Induced Current Transients
Figure 3. Current transient obtained at different potential after laser pulse heating with a Au(111) single-crystal electrode in 0.01 M HClO4 solution. The transients have been displaced vertically for the sake of clarity.
tion.5,29 To study a possible dependence of the laser heating transient behavior on the structure of the surface, experiments with a Au polycrystalline surface were also carried out. Figure 2B shows a typical voltammogram corresponding to this surface in the 0.01 M HClO4 solution. Remarkably, the cyclic voltammogram obtained with this surface in the dilute acid solution shows a minimum current density around 0.25 V, evidencing the position of the pzc corresponding to this surface. The negative shift of the pzc of the polycrystalline surface as compared with the close packed Au(111) surface is wellestablished in the literature and has been explained considering the persistence of the Smoluchowski effect in the metal/aqueous solution interface.30 Figures 3 and 4 show the transient currents obtained after the laser illumination of the Au(111) and polycrystalline gold electrodes, respectively, in the 0.01 M perchloric acid solution. The transients were recorded at different potentials between 0.05 and 1.0 V. The qualitative dependence of the current transients with the electrode potential is similar for both surfaces. At the lower potential region, the current transients are initially in a negative direction. When the potential is increased the magnitude of the current transient decreases and eventually becomes positive. However, the potential where the transient current changes sign (potential of zero transient, pzt) is different for the two surfaces, being around 0.5 V for the Au(111) and around 0.25 V for the polycrystalline electrode. Significantly, the pzt follows the trend observed for the pzc of these surfaces. An unexpected feature of the transient behavior recorded with the Au(111) single-crystal surface is the reverse of the current transient sign at further positive potentials. Figure 5 shows laser transients obtained under similar circumstances but with the electrode in contact with a 0.1 M H2SO4 aqueous solution. Again, a qualitatively similar depen-
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10671
Figure 4. Same as Figure 3 but for a polycrystalline Au electrode in 0.01 M HClO4 solution.
Figure 5. Same as Figure 3 but for a Au(111) single-crystal electrode in 0.1 M H2SO4 solution.
dence of the sign of the current transient with the potential is observed in this case, but the sign change takes place at a different potential. The current transient changes from negative to positive sign around ca. 0.1 V in the sulfuric acid solution. As before, at potentials higher than 0.5 V the current transient becomes negative again.
10672 J. Phys. Chem. B, Vol. 105, No. 43, 2001
Climent et al.
Discussion The main phenomena taking place during the illumination of the electrode surface with the laser light is heating of the metallic surface. According to Benderskii et al.,19 the temperature change of the metallic surface due to the laser light absorption, just after the end of the laser pulse, can be calculated according to the following expression
∆T )
I (1 - R)
xπ k c d
( x )x 1+
ks cs d s kcd
-1
t0
(1)
where I is the intensity of the light, R the reflectivity of the surface, k, c, d, and ks, cs, ds are the heat conductivity, specific heat, and density of the metal and solution, respectively and t0 is the duration of the laser pulse. For a gold electrode in contact with an aqueous solution (k ) 3.15 J cm-1 s-1 K-1; c ) 0.129 J g-1 K-1; d ) 19.3 g cm-3; R ) 0.60 at 530 nm; ks ) 0.0059 J cm-1 s-1 K-1; cs ) 4.18 J g-1 K-1; ds ) 1 g cm-3) a temperature change of around 120 K is expected according to expression (1) for the energy employed in the experiments shown in Figure 3 to 5 (I ≈ 17 MW cm-2) This sudden temperature change can be expected to strongly affect the double layer capacity of the electrochemical interface. If we accept, as a simplified model of the interphase, the validity of the Gouy-Chapman-Stern theory31
1 1 1 ) + Cd Ci 2r0z2F21000c RT
(
) ( ) 1/2
zeΦ2 cosh 2kT
(2)
where Cd and Ci are the total and internal differential capacity of the interphase, and Φ2 is the potential at the outer Helmholtz plane, with respect to the solution. From expression 2, it is clear that an increase of the temperature will cause a decrease of the differential capacity of the diffuse layer, due both to the explicit dependence with the temperature and to the implicit dependence of the water dielectric constant (r) with the temperature. This decrease of the differential capacity will cause a decrease of the double layer charge. Then, we can explain, at least in part, the current transient observed after the laser heating shown in Figures 3 to 5, as the recovery of the double layer charge when the temperature of the electrode after the laser pulse falls to the value in the bulk of the solution. In this way, the sign of the current transient would be indicative of the nature of the sign of the double layer charge at the potential of the experiment. According to this explanation the transient should be negligible at potentials close to the pzc, explaining the similar trend followed by the pzt and pzc, observed in the comparison of Figures 3 and 4. One feature that deserves further discussion is the second change of sign of the current transient observed with Au(111) electrode in both electrolytes in the higher potential region. To explain this observation it is worth recalling the structure of the interphase in this potential region. This is better known in the case of sulfuric acid solution. The second change in sign takes place at the onset of the sulfate adsorption process. Sulfate is known to form a (x3 × x7)R19.1° at potentials more positive than the spike at ca. 1.0 V in the corresponding voltammogram. Two maxima have been observed in the STM images corresponding to this structure27,32 that have been considered indicative of the coadsorption of sulfate with another species. Some controversy is found in the literature in the explanation of the nature of this second species, which has been assigned to either coadsorbed water or hydronium ions. As pointed out by Edens
et al.,27 knowledge of the nature of this species is important when quantifying the electrical charge lying in the interphase. The charge on the metallic side of the interphase after the completion of the sulfate adlayer has been measured for this system by chronocoulometric methods resulting in a value around +65 µC cm-2 33. A similar charge value is obtained from the integration of the voltammogram. The charge corresponding to the sulfate adlayer, according to the (x3 × x7)R19.1°structure is ca. -90 µC cm-2. If the second coadsorbed species is water, then an excess of ca. +15 µC cm-2 should lie in the diffuse part of the double layer, i.e., superequivalent anion adsorption takes place in this case. On the other hand, if hydronium ions are coadsorbed with sulfate, an excess of negative charge should lie in the diffuse layer in order to compensate the inner charge (this has been called subequivalent anion adsorption27). The experiment shown in Figure 5 can help in discriminating between the two possibilities. Hence, the negative current transient observed in the potential region where sulfate adsorption takes place agrees better with the water coadsorption hypothesis. Within this hypothesis the positive charge in the diffuse layer would lead to a negative transient (negative charge coming to the metal) during the recovery after the laser pulse. Implicit in this conclusion is the assumption that the effect of the laser on the anion adlayer is less important than on the diffuse part of the double layer. Preliminary experiments indicate that the laser intensity employed in this work is not able to remove specifically adsorbed anions. The observation of a similar behavior (negative current transient at high potentials) in the perchloric acid solution can be understood considering a similar mechanism implying superequivalent perchlorate adsorption. It is noteworthy that perchlorate adsorption has been described in the potential region where this sign reversal takes place. However, no structural data is available about the perchlorate adlayer to support this hypothesis. Another feature of the current transients shown in Figures 3 to 5 is the rate of decay of the current after the laser pulse. According to ref 34, the temperature of the metallic surface falls as follows
x
1 ∆T(t) ) ∆Tm 2
t0 t
(3)
where ∆Tm is the maximum temperature change achieved at the end of the laser pulse. For the parameters given above, eq 3 indicates that the temperature on the metal surface 10 µs after the laser pulse is only 2 degrees above the equilibrium temperature. However, the experimental current transients have time constants on the order of ms. To explain this apparent inconsistency, one must calculate the time constant of the cell, according to an equivalent circuit composed of the double layer capacitance and the solution resistance in series. This equivalent circuit predicts an exponential current decay after any perturbation of the equilibrium state of the system. The product of the capacitance and the resistance of the cell gives the time constant of the system. To check the validity of the previous model, logarithmic plots of the current transients obtained after the laser pulse at 0.05 V in perchloric acid solutions of three different concentrations are shown in Figure 6. Clearly, good linear relations are observed in all the cases. The time constants are inversely proportional to the perchloric acid concentration, as expected. The two experiments shown for the 1 mM HClO4 solution corresponds to two different laser intensities, indicating that the time constant is independent of the laser intensity. From
Laser Induced Current Transients
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10673 under the European Community program “Improving Human Research Potential and the Socio-economic Knowledge Base” under Contract No. HPMFCT-2000-00529. We thank Prof. J. Feliu for providing the Au(111) single crystal. References and Notes
Figure 6. Semilogarithmic plots of the current transients at 0.05 V in perchloric acid aqueous solution of three different concentrations: (A) 0.1 M; (B) 0.01 M; (C) 0.001 M. The two lines in graph (C) correspond to two different laser intensities: 13 mJ/pulse (lower curve) and 15 mJ/pulse (upper curve).
the time constant and the average capacity measured from the voltammogram, values of the cell resistance ranging from 70 to 6000 Ω for the 0.1 to 0.001 M HClO4 solution are obtained. These values are in accordance with the resistances measured by impedance and potential step experiments. Unfortunately, this result precludes the possibility of studying kinetics of the relaxation processes if they are faster than the relaxation time of the cell. It is worth pointing out that the relatively high solution resistance values reported above are mainly a consequence of the meniscus configuration necessary to handle the single-crystal electrodes. Conclusions The study of the current transient obtained after the potentiostatic heating of a gold electrode surface has revealed a correlation between its sign and the sign of the double layer charge. This allows its use as a general method for determination of pzc. Additionally, a selective displacement of the diffuse double layer charge has been observed, allowing discrimination between the inner and the diffuse parts of the double layer. This is a promising result, which application to metals adsorbing hydrogen and anions, such as platinum, would allow the determination the potential of zero free charge, independently of the potential of zero total charge. Acknowledgment. V.C. gratefully acknowledges the European Commission for the award of a Marie Curie Fellowship
(1) Hamelin, A. In Modern Aspects of Electrochemistry; Conway, B. E., White, R. E., Bockris, J. O. M., Eds.; Plenum: New York, 1985; Vol. 16; pp 1-101. (2) Trasatti, S.; Lust, E. In Modern Aspects of Electrochemistry; White, R. E., Bockris, J. O. M., Conway, B. E., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; Vol. 33, pp 1-215. (3) 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: New York, 1980; Vol. 1; pp 221-289. (4) Iwasita, T.; Xia, X. H. J. Electroanal. Chem. 1996, 411, 95-102. (5) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10 664-10 672. (6) Attard, G. A.; Ahmadi, A. J. Electroanal. Chem. 1995, 389, 175190. (7) Valincius, G. Langmuir 1998, 14, 6307-6319. (8) Haiss, W.; Nichols, R. J.; Sass, J. K.; Charle, K. P. J. Electroanal. Chem. 1998, 452, 199-202. (9) Ibach, H.; Bach, C. E.; Giesen, M.; Grossmann, A. Surf. Sci. 1997, 375, 107-119. (10) Chen, J. H.; Si, S. H.; Nie, L. H.; Yao, S. Z. Electrochim. Acta 1997, 42, 689-695. (11) Hamm, U. W.; Kramer, D.; Zhai, R. S.; Kolb, D. M. J. Electroanal. Chem. 1996, 414, 85-89. (12) Alvarez, B.; Climent, V.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 2000. (13) Alvarez, B.; Climent, V.; Feliu, J. M.; Aldaz, A. Electrochem. Commun. 2000, 2, 427-430. (14) Go´mez, R.; Climent, V.; Feliu, J. M.; Weaver, M. J. J. Phys. Chem. B 2000, 104, 597-605. (15) Climent, V.; Go´mez, R.; Feliu, J. M. Electrochim. Acta 1999, 45, 629-637. (16) Andersen, T. N.; Perkins, R. S.; Eyring, H. J. Am. Chem. Soc. 1964, 86, 6. (17) Nakamura, K.; Ohno, M.; Umemoto, K.; Hinoue, T. Chem. Lett. 2000, 1050-1051. (18) Benderskii, V. A.; Babenko, S. D.; Krivenko, A. G. J. Electroanal. Chem. 1978, 86, 223-5. (19) Benderskii, V. A.; Velichko, G. I. J. Electroanal. Chem. 1982, 140, 1-22. (20) Benderskii, V. A.; Velichko, G. I.; Kreitus, I. J. Electroanal. Chem. 1984, 181, 1-20. (21) Smalley, J. F.; Geng, L.; Feldberg, S. W.; Rogers, L. C.; Leddy, J. J. Electroanal. Chem. 1993, 356, 181-200. (22) Smalley, J. F.; Krishnan, C. V.; Goldman, M.; Feldberg, S. W.; Ruzic, I. J. Electroanal. Chem. 1988, 248, 255-82. (23) Smalley, J. F.; Newton, M. D.; Feldberg, S. W. Electrochem. Commun. 2000, 2, 832-838. (24) Clavilier, J.; Armand, D.; Sun, S.-G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. (25) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1-11. (26) Kolb, D. M. Prog. Surf. Sci. 1996, 51, 109-173. (27) Edens, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357-366. (28) Dretschkow, T.; Wandlowski, T. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 749-757. (29) Angersteinkozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. Electrochim. Acta 1986, 31, 1051-1061. (30) Lecoeur, J.; Andro, J.; Parsons, R. Surf. Sci. 1982, 114, 320-330. (31) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications; John Wiley & Sons: New York, 1980. (32) Cuesta, A.; Kleinert, M.; Kolb, D. M. Phys. Chem. Chem. Phys. 2000, 2, 5684. (33) Shi, Z.; Lipkowski, J.; Gamboa, M.; Zelenay, P.; Wieckowski, A. J. Electroanal. Chem. 1994, 366, 317-326. (34) Benderskii, V. A.; Efimov, I. O.; Krivenko, A. G. J. Electroanal. Chem. 1991, 315, 29-64.
