- Email: [email protected]
On the alloy formation in the course of upd of Cd on gold
www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 491 (2000) 111– 116
On the alloy formation in the course of upd of Cd on gold G. Inzelt a,*, G. Hora´nyi b b
a Department of Physical Chemistry, Eo¨t6o¨s Uni6ersity, PO Box 32, Budapest 112, H-1518, Hungary Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, PO Box 17, Budapest H-1525, Hungary
Received 6 December 1999; received in revised form 28 January 2000; accepted 27 February 2000 Dedicated to Professor E. Gileadi on the occasion of his retirement from the University of Tel Aviv and in recognition of his contribution to electrochemistry
Abstract The alloy formation in the course of underpotential deposition (upd) of Cd on gold from HClO4 supporting electrolyte was studied by coupled voltammetric and electrochemical microbalance techniques. The experimental results were discussed in the light of data obtained from a radiotracer study of induced adsorption of anions. It has been demonstrated that the alloy formation is well reflected in the results of EQCM measurements and the changes in the induced adsorption of anions. The alloy formation and/or its dissolution is a slow process in comparison with the time scale of the usual voltammetric measurements (sweep rates from 5 to 100 mV s − 1). The results of the long-time polarization experiments (lasting e.g. 30 – 60 min) have provided evidence for the continuous increase of the amount of underpotentially deposited Cd atoms during reduction and the slow dissolution of the embedded Cd accompanied by surface roughening during oxidation. These observations can be explained by a turnover process between the adsorbed Cd and the underlying Au atoms, as well as by a solid-state diffusion of these atoms through the alloyed phase. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Gold electrode; Cadmium; Alloy formation; Underpotential deposition
1. Introduction The underpotential deposition (upd) of Cd on gold has been the subject of several studies [1 – 7]. Especially recent studies of Gewirth et al. [6,7] by using such advanced techniques as in situ scanning tunneling microscopy [6] and a quartz crystal microbalance [7] have provided a deeper insight into the microscopic events occurring at the Au (111) surface during the upd of Cd. However, no attention has been paid to the problem of the alloy formation, although the first observations concerning this phenomenon were reported more than 30 years ago [2]. Since this paper was submitted, an interesting contribution [8] appeared concerning the surface alloying at the Cd Au (100) interface in the upd region. The in situ EC-AFM results have revealed that the dynamics * Corresponding author. Tel.: +36-1-2090555; fax: 2090602. E-mail address: [email protected] (G. Inzelt).
+36-1-
of the alloying depends on polarization conditions. The experimental findings were explained by a turnover mechanism, surface and solid-state diffusion processes. In a recent communication [9], the specific adsorption of 36Cl-labelled Cl− ions and 35S-labelled HSO− 4 ions were studied in 1 mol dm − 3 HClO4 supporting electrolyte in the presence of Cd2 + ions at a polycrystalline gold support over a wide potential range involving the potential regions corresponding to electrodeposition, alloy formation, underpotential deposition of Cd species and an adatom-free surface. The distinct sections found in the potential dependence of the adsorption of anions, together with the potential versus time curves obtained under open circuit conditions were ascribed to the changes in the state of the electrode surface, the dissolution of the bulk Cd phase and the slow elimination of Cd species from the Cd/Au alloy. 2− The induced adsorption of HSO− 4 (SO4 ) species by Cd adatoms was reported more than 20 years ago [10] using radiotracer techniques; however, the possible role of alloy formation in the observed phenomena was not
0022-0728/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 0 ) 0 0 0 9 9 - 1
112
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
discussed at that time. According to our view, the question of alloy formation can be approached easily by using piezoelectric microgravimetry, i.e. by application of electrochemical microbalance (EQCM) techniques in conjunction with the results of radiotracer adsorption studies.
2. Experimental All chemicals were of analytical grade quality and were used without further purification. EQCM experiments were performed as described previously [11 –13]. Gold-coated AT-cut quartz crystals of 10 MHz were used in the form of thin disks (geometrical area, A=0.50 cm2). The Sauerbrey equation with an integral sensitivity of the crystal, Cf =2.26 ×108 Hz cm2 g − 1 was used to calculate the mass change from the frequency change. The EQCM measurements were made in the three-electrode mode using a Pt wire as a
Fig. 1. Simultaneously obtained cyclic voltammetric and EQCM curves for a gold electrode (A= 0.5 cm2) in contact with 0.25 mol dm − 3 HClO4 solution. Sweep rate: 10 mV s − 1.
Fig. 2. Simultaneously detected cyclic voltammetric and EQCM curves for the gold electrode described in Fig. 1 in contact with a solution of 0.25 mol dm − 3 HClO4 + 5× 10 − 3 mol dm − 3 Cd(ClO4)2. Sweep rate: 10 mV s − 1.
counter electrode. Potentials were measured by using a sodium saturated calomel electrode (SCE) as the reference electrode. The radiotracer measurements were carried out in a manner reported elsewhere [14 –17]. In the study, 35S-labelled H2SO4 (specific activity, 185 MBq mmol − 1) and 36 Cl-labelled HCl (specific activity, 26 MBq mmol − 1) were used. All potentials in radiotracer measurements are expressed versus the reversible hydrogen electrode which was immersed in the actual solution investigated (RHE).
3. Results and discussion A comparison of the simultaneous cyclic voltammetric and EQCM responses obtained in 0.25 mol dm − 3 HClO4 (Fig. 1) and 0.25 mol dm − 3 HClO4 + 5×10 − 3 mol dm − 3 Cd(ClO4)2 (Fig. 2), respectively, clearly attests that the underpotential deposition of cadmium on polycrystalline gold starts at ca. − 0.1 V (vs. SCE). In pure supporting electrolyte no mass increase (frequency decrease) can be observed in the potential region below − 0.1 V during the negative potential scan. (The equilibrium potential of the Cd2 + /Cd system in this solution is ca. − 0.7 V vs. SCE; however, it has been demonstrated that the upd shift for the Cd/Au system is 0.5 –0.6 V depending on the anion present and the state of the gold surface [18].) The origin of the frequency decrease in the double layer region is still an unsettled question because several effects can be considered, e.g. the increase of the surface mass due to specifically adsorbed anions, the higher viscosity near the surface due to the surface excess of ions in the diffuse double layer [19], and water adsorption via hydrogen bonding to the submonolayer of hydrous oxide [20]. A closer inspection of the cyclic voltammetric curves reveals other important points. In Ref. [9] it was shown that the induced adsorption of labeled anions, present in very low concentration in comparison with that of supporting electrolyte, may serve as an indicator of changes in the surface state of the Au/Cd system. It was found that, in the potential region ranging from the bulk deposition of Cd to the formation of the surface oxide on gold, four sections can be distinguished in the steady state/equilibrium potential dependence of the anion adsorption as shown by Fig. 3. These sections are as follows: 1. Bulk deposition of Cd. 2. Predominant existence of a Au/Cd alloy. 3. Cd adatoms on gold. 4. Predominant existence of a pure gold surface. Under identical conditions (acid concentration, scan rate) the reduction current of H+-ions is less in the presence of Cd2 + -ions, i.e. the Cd adatoms hinder the
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
Fig. 3. The potential dependence of the adsorption of labelled Cl− ions present in very low concentration (2 × 10 − 5 mol dm − 3) in 1 mol dm − 3 HClO4 + 8 ×10 − 3 mol dm − 3 Cd2 + solution. (Potential is referred to RHE.)
Fig. 4. Simultaneous current and EQCM frequency responses following potential switches in the following order: (1) Open-circuit; (2) −0.5; (3) − 0.45; (4) −0.4; (5) −0.35; (6) − 0.3; (7) − 0.25; (8) − 0.2; (9) − 0.15; (10) − 0.1; (11) − 0.05; and (12) 0 V (SCE). The solution composition was the same that described in Fig. 2.
hydrogen evolution, as expected taking into account the high hydrogen overpotential on the Cd electrode. The maximum coverage of a Cd monolayer is 2.16× 10 − 9 mol cm − 2 [5] which corresponds to a −55 Hz theoretical frequency shift in our case. During cycling the mass increase due to the cadmium deposition depends on the sweep rate and the negative potential limit. Fig. 2 shows the results of a selected experiment where a scan rate of 10 mV s − 1 and − 0.6 V (SCE) negative potential limit were applied. The frequency change (Df ) observed is somewhat higher than that indicated above for a monolayer coverage; however, the value of 1.2 for the roughness factor was calculated from the charge (Q) associated with the reduction of the oxide film formed on gold. (It should be mentioned that the frequency change may be affected by the surface roughness, however, a direct proportionality was found between Df and Q, therefore, this effect can
113
be neglected in the present study.) The roughness of the gold surface became higher and higher, in some cases reaching a value of about 2–3, during the consecutive deposition and dissolution of cadmium on the gold surface. This is most likely to be due to the repetitive alloy formation and dissolution (see below). Another remarkable observation was that during multicycle experiments a gradual, but rather slow frequency decrease has been detected, i.e. the EQCM curves starts and ends at a slightly lower frequency; however, the frequency change during a single cycle remains practically the same. This effect is barely seen in Fig. 2, where the frequency shift between the first and second cycles is less than ca. 0.5 Hz. We thought that this effect was connected with the diffusion of Cd adatoms into the bulk of the gold, i.e. with the alloy formation [2,8,9]. Because it is a rather slow process, potentiostatic or galvanostatic piezoelectric microgravimetry seemed to be a more suitable method to study the rate of the alloy formation. Fig. 4 shows a typical oxidation sequence after the deposition of Cd adatoms at − 0.5 V (SCE). It can be seen that at each potential quasi-steady state current and frequency patterns, respectively, develop. The fast responses after each potential switch are followed by a rather slow increase of the current and decrease of the surface mass. It may be concluded that at each potential a steady state evolves and a more or less defined coverage can be assigned to a given potential value. The actual current is a result of two electrode reactions taking place simultaneously: Cd ? Cd2 + + 2e −
(1)
2H+ + 2e − ? H2
(2)
The rather slow mass decrease after the rapid response might be considered as the slow oxidation of the cadmium –gold alloy, which process needs a solid-state diffusion of cadmium to the surface. A similar explanation can be given for the reduction sequence presented in Fig. 5. The long-time mass increase may be in connection with the diffusion of cadmium-adatoms into the bulk of the gold, i.e. the alloy formation. Its rate increases with decreasing potential. This effect is understandable taking into account when the surface coverage regarding the cadmium atoms is higher at a lower potential. By using the idea given in Ref. [2] for the diffusion of cadmium, the following equation can be derived for the mass increase at long times: Dm= qCdMCdcCd Dt/y
(3)
where qCd is the coverage with respect to Cd at a given potential, MCd = 112.4 is the molar mass of cadmium, cCd is the concentration that can be estimated from the density of cadmium.
114
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
Fig. 5. Simultaneous current and EQCM frequency responses following potential switches in the following order: (1) 0; (2) − 0.1; (3) −0.2; (4) − 0.3; and (5) − 0.4 V (SCE). The same solution was used as that described in Figs. 2 and 4.
Fig. 6. Current and frequency responses after stepping the potential from − 0.35 to 0.65 V (SCE). Other conditions as in Fig. 5.
Fig. 7. Current and frequency responses following consecutive potential switches. The curves measured at potentials (1) − 0.5; (2) − 0.55; (3) − 0.6 and (4) −0.35 V (SCE). Other conditions as in Fig. 2.
Fig. 8. Increase of the adsorption of labeled sulfate species in the course of alloy dissolution at E = −360 mV (RHE) in the presence of a solution containing 2 ×10 − 4 mol dm − 3 H2SO4 +1 × 10 − 2 mol dm − 3 Cd2 + +1 mol dm − 3 HClO4.
From the linear section of the Dm − t 1/2 plots at long times and by considering cCd = 0.07 mol cm − 3, as well as the actual q value estimated from the mass change at a given potential, D= (190.5)× 10 − 16 cm2 s − 1 can be derived for the solid-state diffusion coefficient of cadmium which is in good accordance with the value determined earlier [2]. The diffusion of cadmium into the bulk gold phase and the roughening of the surface in connection with the deposition/dissolution cycles of the alloy cause a pronounced hysteresis in the mass response. Fig. 6 shows the current and mass responses after a potential step from −0.35 to + 0.65 V (SCE) for the electrode used in the experiments presented in Figs. 4 and 5. After many, prolonged potential switches in the potential interval between −0.65 and + 0.7 V, the mass of the electrode increased by ca. 245 ng, however, the characteristics of the response remained practically identical. It can be seen in Fig. 6 that even after 1200 s the surface mass still decreases, most likely due to the slow dissolution of cadmium that had embedded inside the gold. (After ca. 1000 s the frequency change becomes rather small but it is still much higher than the thermal frequency drift observed for the same crystal.) In Fig. 7 the results of a long-time polarization experiment are presented. (The same electrode was used in the experiments shown in Figs. 1 and 2. After intensive use, i.e. after many deposition/dissolution cycles, the frequency decreased due to the imperfect removal of Cd from the bulk alloy phase.) It can be seen that the slow mass increase during reduction at longer times — that is probably in connection with the alloy formation — depends on the potential, because the actual surface coverage is higher at lower potentials. The slowness of the dissolution of Cd from the Au/Cd alloy is clearly demonstrated by the slowness of the increase in the specific adsorption of sulfate ions at
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
115
a potential where the dissolution of the alloy takes place (Fig. 8). This phenomenon can be understood considering the potential dependence of the sulfate adsorption (see Fig. 3), i.e. the amount of sulfate ions on the surface will increase until the complete elimination of the Cd by dissolution. Fig. 9 presents the results of an experiment when fast cathodic current pulses were applied and then the current circuit was interrupted. It can be seen that after deposition of the cadmium, at open-circuit a relatively fast corrosion process occurs according to reactions (1) and (2) that can be written in the form: Cd +2H+ Cd2 + + H2 Fig. 9. Galvanostatic experiment with alternative cathodic pulses and open-circuit sequences. (1) open-circuit; (2) − 50 mA; (3) open-circuit; (4) − 50 mA; (5) open-circuit; (6) −40 mA; (7) open-circuit; (8) − 15 mA; (9) open-circuit. The solution composition was the same as in Fig. 2. (Potential is referred to SCE.)
Fig. 10. The change of the potential and frequency responses after a current step from open-circuit (1) to −40 mA (2) and open-circuit (3). Other conditions as in Fig. 9. (Potential is referred to SCE.)
Fig. 11. The final part of a galvanostatic experiment. (1) 5 × 10 − 6; (2) 8× 10 − 6; (3) 1 × 10 − 5 and (4) 1.5 ×10 − 5 mA. Other conditions as in Fig. 9.
(4)
This reaction results in the removal of Cd-atoms from the surface. After a fast increase, the potential tends to the open-circuit value characteristic for the system. It is a mixed-potential due to the presence of traces of oxygen. The slow potential increase at longer times may be in connection with the change of the surface state since Cd atoms slowly diffuse from the bulk to the surface. Then these Cd atoms are removed by an oxidation reaction with the participation of oxygen. The complete elimination of Cd atoms from the gold surface leads to the increase in the sulfate adsorption, therefore — in accordance with the results of the radiotracer experiments — a mass increase can be detected. If a relatively high cathodic current is applied, beside the upd process, a local formation of visible cadmium deposit (alloy?) can be observed on the gold surface that is unstable and dissolves. This behavior is illustrated in Fig. 10, which shows a mass decrease after a fast deposition process while the relatively high cathodic current flows still. At low currents, relatively stable potentials were established as expected and are seen in Fig. 11. Closer inspection of the curves, however, reveals that the potential increased slowly with a simultaneous slow increase in mass. It proves that the cadmium on the surface hinders the discharge of the H+-ions, i.e. more and more of the current is used to reduce Cd2 + -ions. (This effect manifests itself as a slow decrease of the current in the course of potentiostatic experiments.) This experiment attests that a nearly full coverage can be reached even at potentials as high as − 0.27 V (SCE), because the corrosion reaction gradually slows down and the galvanostatic current can be maintained only by the bulk deposition of cadmium. As a consequence, the potential suddenly drops to a low value corresponding to the bulk Cd deposition and a rapid mass increase can be observed due to this process as seen in Fig. 11. It should also be mentioned that adding H2SO4 to HClO4 solutions (final concentration: 2×10 − 2 mol dm − 3 H2SO4) caused a 4–8 Hz decrease in frequency
116
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
both in pure and Cd2 + -ion containing solutions in the potential region from 0.3 to 0.7 V (SCE). This observation is in accordance with the data presented in Fig. 3. In the presence of Cd2 + -ions, a frequency decrease of a similar magnitude was also observed below − 0.1 V (SCE); however, no such effect has been detected in pure HClO4 solutions. It means that the induced specific adsorption of sulfate ions in the upd potential region can be detected also by EQCM in accordance with the results of radiotracer [9,10] and previous EQCM [7] studies. The consequence of the alloy formation/dissolution processes is the roughening of the surface as mentioned above. This is reflected not only by an increase in the oxide reduction peak (considered as a simple indication of changes in real surface area) but also by an increase in the total amount of adsorbed anions and adatoms resulting in frequency changes. Thus it can be stated that in the case of continuous cycling the observed results are influenced by the roughening effect as well.
4. Conclusions It follows, from the results presented above, that the Au – Cd alloy is formed not only in the course of bulk deposition but also with the participation of Cd adatoms in the upd region. The solid-state diffusion plays an important role in both the alloy formation and the stripping of the Cd from the electrode surface. The latter process leads to roughening of the surface. Our results support the model introduced in Ref. [8], i.e. the amount of underpotentially deposited Cd atoms is not limited to one monolayer but increases with polarization time. The Au atoms beneath the adsorbed Cd atoms move to the topmost surface by a turnover process providing available sites for further adsorption of Cd atoms. This turnover process acts as a continuous source for alloy growth.
.
Acknowledgements This work was supported by the Hungarian Scientific Research Fund (OTKA) under grants T023056, T031703 (G.H.) and T014928 (G.I.).
References [1] E. Schmidt, H.R. Gygax, J. Electroanal. Chem. 12 (1966) 300. [2] J.W. Schultze, F.D. Koppitz, M.M. Lohrengel, Ber. Bunsenges. 78 (1974) 693. [3] B.W. Gregory, M.L. Norton, J.L. Stickney, J. Electroanal. Chem. 293 (1990) 85. [4] K. Takamura, F. Watanabe, T. Takamura, Electrochim. Acta 26 (1981) 979. [5] M.R. Deakin, O. Melroy, J. Electroanal. Chem. 239 (1988) 321. [6] J.C. Bondos, A.A. Gewirth, R.G. Nuzzo, J. Phys. Chem. 100 (1996) 8617. [7] B.K. Niece, A.A. Gewirth, Langmuir 13 (1997) 6302. [8] R. Vidu, S. Hara, J. Electroanal. Chem. 475 (1999) 171. [9] G. Hora´nyi, J. Solid State Electrochem. 4 (2000) 177. [10] G. Hora´nyi, E.M. Rizmayer, P. Joo´, J. Electroanal. Chem. 152 (1983) 211. [11] J. Ba´cskai, K. Martinusz, E. Czirok, G. Inzelt, P.J. Kulesza, E. Malik, J. Electroanal. Chem. 385 (1995) 241. [12] S. Pruneanu, E. Csaho´k, V. Kerte´sz, G. Inzelt, Electrochim. Acta 43 (1998) 2305. [13] G. Inzelt, V. Kerte´sz, G. La´ng, J. Phys. Chem. 97 (1993) 6104. [14] G. Hora´nyi, J. Solt, F. Nagy, J. Electroanal. Chem. 31 (1971) 87. [15] G. Hora´nyi, Rev. Anal. Chem. 14 (1995) 1. [16] K. Martinusz, G. Inzelt, G. Hora´nyi, J. Electroanal. Chem. 404 (1996) 143. [17] G. Hora´nyi, Electrochim. Acta 25 (1980) 43. [18] D. Kolb, in: H. Gerischer, C.W. Tobias (Eds.), Advances of Electrochemistry and Electrochemical Engineering, vol. 11, Wiley, New York, 1978, p. 125. [19] V. Tsionsky, L. Daikhin, E. Gileadi, J. Electrochem. Soc. 143 (1996) 2240. [20] J.S. Gordon, D.C. Johnson, J. Electroanal. Chem. 365 (1996) 267.
On the alloy formation in the course of upd of Cd on gold G. Inzelt a,*, G. Hora´nyi b b
a Department of Physical Chemistry, Eo¨t6o¨s Uni6ersity, PO Box 32, Budapest 112, H-1518, Hungary Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, PO Box 17, Budapest H-1525, Hungary
Received 6 December 1999; received in revised form 28 January 2000; accepted 27 February 2000 Dedicated to Professor E. Gileadi on the occasion of his retirement from the University of Tel Aviv and in recognition of his contribution to electrochemistry
Abstract The alloy formation in the course of underpotential deposition (upd) of Cd on gold from HClO4 supporting electrolyte was studied by coupled voltammetric and electrochemical microbalance techniques. The experimental results were discussed in the light of data obtained from a radiotracer study of induced adsorption of anions. It has been demonstrated that the alloy formation is well reflected in the results of EQCM measurements and the changes in the induced adsorption of anions. The alloy formation and/or its dissolution is a slow process in comparison with the time scale of the usual voltammetric measurements (sweep rates from 5 to 100 mV s − 1). The results of the long-time polarization experiments (lasting e.g. 30 – 60 min) have provided evidence for the continuous increase of the amount of underpotentially deposited Cd atoms during reduction and the slow dissolution of the embedded Cd accompanied by surface roughening during oxidation. These observations can be explained by a turnover process between the adsorbed Cd and the underlying Au atoms, as well as by a solid-state diffusion of these atoms through the alloyed phase. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Gold electrode; Cadmium; Alloy formation; Underpotential deposition
1. Introduction The underpotential deposition (upd) of Cd on gold has been the subject of several studies [1 – 7]. Especially recent studies of Gewirth et al. [6,7] by using such advanced techniques as in situ scanning tunneling microscopy [6] and a quartz crystal microbalance [7] have provided a deeper insight into the microscopic events occurring at the Au (111) surface during the upd of Cd. However, no attention has been paid to the problem of the alloy formation, although the first observations concerning this phenomenon were reported more than 30 years ago [2]. Since this paper was submitted, an interesting contribution [8] appeared concerning the surface alloying at the Cd Au (100) interface in the upd region. The in situ EC-AFM results have revealed that the dynamics * Corresponding author. Tel.: +36-1-2090555; fax: 2090602. E-mail address: [email protected] (G. Inzelt).
+36-1-
of the alloying depends on polarization conditions. The experimental findings were explained by a turnover mechanism, surface and solid-state diffusion processes. In a recent communication [9], the specific adsorption of 36Cl-labelled Cl− ions and 35S-labelled HSO− 4 ions were studied in 1 mol dm − 3 HClO4 supporting electrolyte in the presence of Cd2 + ions at a polycrystalline gold support over a wide potential range involving the potential regions corresponding to electrodeposition, alloy formation, underpotential deposition of Cd species and an adatom-free surface. The distinct sections found in the potential dependence of the adsorption of anions, together with the potential versus time curves obtained under open circuit conditions were ascribed to the changes in the state of the electrode surface, the dissolution of the bulk Cd phase and the slow elimination of Cd species from the Cd/Au alloy. 2− The induced adsorption of HSO− 4 (SO4 ) species by Cd adatoms was reported more than 20 years ago [10] using radiotracer techniques; however, the possible role of alloy formation in the observed phenomena was not
0022-0728/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 0 ) 0 0 0 9 9 - 1
112
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
discussed at that time. According to our view, the question of alloy formation can be approached easily by using piezoelectric microgravimetry, i.e. by application of electrochemical microbalance (EQCM) techniques in conjunction with the results of radiotracer adsorption studies.
2. Experimental All chemicals were of analytical grade quality and were used without further purification. EQCM experiments were performed as described previously [11 –13]. Gold-coated AT-cut quartz crystals of 10 MHz were used in the form of thin disks (geometrical area, A=0.50 cm2). The Sauerbrey equation with an integral sensitivity of the crystal, Cf =2.26 ×108 Hz cm2 g − 1 was used to calculate the mass change from the frequency change. The EQCM measurements were made in the three-electrode mode using a Pt wire as a
Fig. 1. Simultaneously obtained cyclic voltammetric and EQCM curves for a gold electrode (A= 0.5 cm2) in contact with 0.25 mol dm − 3 HClO4 solution. Sweep rate: 10 mV s − 1.
Fig. 2. Simultaneously detected cyclic voltammetric and EQCM curves for the gold electrode described in Fig. 1 in contact with a solution of 0.25 mol dm − 3 HClO4 + 5× 10 − 3 mol dm − 3 Cd(ClO4)2. Sweep rate: 10 mV s − 1.
counter electrode. Potentials were measured by using a sodium saturated calomel electrode (SCE) as the reference electrode. The radiotracer measurements were carried out in a manner reported elsewhere [14 –17]. In the study, 35S-labelled H2SO4 (specific activity, 185 MBq mmol − 1) and 36 Cl-labelled HCl (specific activity, 26 MBq mmol − 1) were used. All potentials in radiotracer measurements are expressed versus the reversible hydrogen electrode which was immersed in the actual solution investigated (RHE).
3. Results and discussion A comparison of the simultaneous cyclic voltammetric and EQCM responses obtained in 0.25 mol dm − 3 HClO4 (Fig. 1) and 0.25 mol dm − 3 HClO4 + 5×10 − 3 mol dm − 3 Cd(ClO4)2 (Fig. 2), respectively, clearly attests that the underpotential deposition of cadmium on polycrystalline gold starts at ca. − 0.1 V (vs. SCE). In pure supporting electrolyte no mass increase (frequency decrease) can be observed in the potential region below − 0.1 V during the negative potential scan. (The equilibrium potential of the Cd2 + /Cd system in this solution is ca. − 0.7 V vs. SCE; however, it has been demonstrated that the upd shift for the Cd/Au system is 0.5 –0.6 V depending on the anion present and the state of the gold surface [18].) The origin of the frequency decrease in the double layer region is still an unsettled question because several effects can be considered, e.g. the increase of the surface mass due to specifically adsorbed anions, the higher viscosity near the surface due to the surface excess of ions in the diffuse double layer [19], and water adsorption via hydrogen bonding to the submonolayer of hydrous oxide [20]. A closer inspection of the cyclic voltammetric curves reveals other important points. In Ref. [9] it was shown that the induced adsorption of labeled anions, present in very low concentration in comparison with that of supporting electrolyte, may serve as an indicator of changes in the surface state of the Au/Cd system. It was found that, in the potential region ranging from the bulk deposition of Cd to the formation of the surface oxide on gold, four sections can be distinguished in the steady state/equilibrium potential dependence of the anion adsorption as shown by Fig. 3. These sections are as follows: 1. Bulk deposition of Cd. 2. Predominant existence of a Au/Cd alloy. 3. Cd adatoms on gold. 4. Predominant existence of a pure gold surface. Under identical conditions (acid concentration, scan rate) the reduction current of H+-ions is less in the presence of Cd2 + -ions, i.e. the Cd adatoms hinder the
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
Fig. 3. The potential dependence of the adsorption of labelled Cl− ions present in very low concentration (2 × 10 − 5 mol dm − 3) in 1 mol dm − 3 HClO4 + 8 ×10 − 3 mol dm − 3 Cd2 + solution. (Potential is referred to RHE.)
Fig. 4. Simultaneous current and EQCM frequency responses following potential switches in the following order: (1) Open-circuit; (2) −0.5; (3) − 0.45; (4) −0.4; (5) −0.35; (6) − 0.3; (7) − 0.25; (8) − 0.2; (9) − 0.15; (10) − 0.1; (11) − 0.05; and (12) 0 V (SCE). The solution composition was the same that described in Fig. 2.
hydrogen evolution, as expected taking into account the high hydrogen overpotential on the Cd electrode. The maximum coverage of a Cd monolayer is 2.16× 10 − 9 mol cm − 2 [5] which corresponds to a −55 Hz theoretical frequency shift in our case. During cycling the mass increase due to the cadmium deposition depends on the sweep rate and the negative potential limit. Fig. 2 shows the results of a selected experiment where a scan rate of 10 mV s − 1 and − 0.6 V (SCE) negative potential limit were applied. The frequency change (Df ) observed is somewhat higher than that indicated above for a monolayer coverage; however, the value of 1.2 for the roughness factor was calculated from the charge (Q) associated with the reduction of the oxide film formed on gold. (It should be mentioned that the frequency change may be affected by the surface roughness, however, a direct proportionality was found between Df and Q, therefore, this effect can
113
be neglected in the present study.) The roughness of the gold surface became higher and higher, in some cases reaching a value of about 2–3, during the consecutive deposition and dissolution of cadmium on the gold surface. This is most likely to be due to the repetitive alloy formation and dissolution (see below). Another remarkable observation was that during multicycle experiments a gradual, but rather slow frequency decrease has been detected, i.e. the EQCM curves starts and ends at a slightly lower frequency; however, the frequency change during a single cycle remains practically the same. This effect is barely seen in Fig. 2, where the frequency shift between the first and second cycles is less than ca. 0.5 Hz. We thought that this effect was connected with the diffusion of Cd adatoms into the bulk of the gold, i.e. with the alloy formation [2,8,9]. Because it is a rather slow process, potentiostatic or galvanostatic piezoelectric microgravimetry seemed to be a more suitable method to study the rate of the alloy formation. Fig. 4 shows a typical oxidation sequence after the deposition of Cd adatoms at − 0.5 V (SCE). It can be seen that at each potential quasi-steady state current and frequency patterns, respectively, develop. The fast responses after each potential switch are followed by a rather slow increase of the current and decrease of the surface mass. It may be concluded that at each potential a steady state evolves and a more or less defined coverage can be assigned to a given potential value. The actual current is a result of two electrode reactions taking place simultaneously: Cd ? Cd2 + + 2e −
(1)
2H+ + 2e − ? H2
(2)
The rather slow mass decrease after the rapid response might be considered as the slow oxidation of the cadmium –gold alloy, which process needs a solid-state diffusion of cadmium to the surface. A similar explanation can be given for the reduction sequence presented in Fig. 5. The long-time mass increase may be in connection with the diffusion of cadmium-adatoms into the bulk of the gold, i.e. the alloy formation. Its rate increases with decreasing potential. This effect is understandable taking into account when the surface coverage regarding the cadmium atoms is higher at a lower potential. By using the idea given in Ref. [2] for the diffusion of cadmium, the following equation can be derived for the mass increase at long times: Dm= qCdMCdcCd Dt/y
(3)
where qCd is the coverage with respect to Cd at a given potential, MCd = 112.4 is the molar mass of cadmium, cCd is the concentration that can be estimated from the density of cadmium.
114
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
Fig. 5. Simultaneous current and EQCM frequency responses following potential switches in the following order: (1) 0; (2) − 0.1; (3) −0.2; (4) − 0.3; and (5) − 0.4 V (SCE). The same solution was used as that described in Figs. 2 and 4.
Fig. 6. Current and frequency responses after stepping the potential from − 0.35 to 0.65 V (SCE). Other conditions as in Fig. 5.
Fig. 7. Current and frequency responses following consecutive potential switches. The curves measured at potentials (1) − 0.5; (2) − 0.55; (3) − 0.6 and (4) −0.35 V (SCE). Other conditions as in Fig. 2.
Fig. 8. Increase of the adsorption of labeled sulfate species in the course of alloy dissolution at E = −360 mV (RHE) in the presence of a solution containing 2 ×10 − 4 mol dm − 3 H2SO4 +1 × 10 − 2 mol dm − 3 Cd2 + +1 mol dm − 3 HClO4.
From the linear section of the Dm − t 1/2 plots at long times and by considering cCd = 0.07 mol cm − 3, as well as the actual q value estimated from the mass change at a given potential, D= (190.5)× 10 − 16 cm2 s − 1 can be derived for the solid-state diffusion coefficient of cadmium which is in good accordance with the value determined earlier [2]. The diffusion of cadmium into the bulk gold phase and the roughening of the surface in connection with the deposition/dissolution cycles of the alloy cause a pronounced hysteresis in the mass response. Fig. 6 shows the current and mass responses after a potential step from −0.35 to + 0.65 V (SCE) for the electrode used in the experiments presented in Figs. 4 and 5. After many, prolonged potential switches in the potential interval between −0.65 and + 0.7 V, the mass of the electrode increased by ca. 245 ng, however, the characteristics of the response remained practically identical. It can be seen in Fig. 6 that even after 1200 s the surface mass still decreases, most likely due to the slow dissolution of cadmium that had embedded inside the gold. (After ca. 1000 s the frequency change becomes rather small but it is still much higher than the thermal frequency drift observed for the same crystal.) In Fig. 7 the results of a long-time polarization experiment are presented. (The same electrode was used in the experiments shown in Figs. 1 and 2. After intensive use, i.e. after many deposition/dissolution cycles, the frequency decreased due to the imperfect removal of Cd from the bulk alloy phase.) It can be seen that the slow mass increase during reduction at longer times — that is probably in connection with the alloy formation — depends on the potential, because the actual surface coverage is higher at lower potentials. The slowness of the dissolution of Cd from the Au/Cd alloy is clearly demonstrated by the slowness of the increase in the specific adsorption of sulfate ions at
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
115
a potential where the dissolution of the alloy takes place (Fig. 8). This phenomenon can be understood considering the potential dependence of the sulfate adsorption (see Fig. 3), i.e. the amount of sulfate ions on the surface will increase until the complete elimination of the Cd by dissolution. Fig. 9 presents the results of an experiment when fast cathodic current pulses were applied and then the current circuit was interrupted. It can be seen that after deposition of the cadmium, at open-circuit a relatively fast corrosion process occurs according to reactions (1) and (2) that can be written in the form: Cd +2H+ Cd2 + + H2 Fig. 9. Galvanostatic experiment with alternative cathodic pulses and open-circuit sequences. (1) open-circuit; (2) − 50 mA; (3) open-circuit; (4) − 50 mA; (5) open-circuit; (6) −40 mA; (7) open-circuit; (8) − 15 mA; (9) open-circuit. The solution composition was the same as in Fig. 2. (Potential is referred to SCE.)
Fig. 10. The change of the potential and frequency responses after a current step from open-circuit (1) to −40 mA (2) and open-circuit (3). Other conditions as in Fig. 9. (Potential is referred to SCE.)
Fig. 11. The final part of a galvanostatic experiment. (1) 5 × 10 − 6; (2) 8× 10 − 6; (3) 1 × 10 − 5 and (4) 1.5 ×10 − 5 mA. Other conditions as in Fig. 9.
(4)
This reaction results in the removal of Cd-atoms from the surface. After a fast increase, the potential tends to the open-circuit value characteristic for the system. It is a mixed-potential due to the presence of traces of oxygen. The slow potential increase at longer times may be in connection with the change of the surface state since Cd atoms slowly diffuse from the bulk to the surface. Then these Cd atoms are removed by an oxidation reaction with the participation of oxygen. The complete elimination of Cd atoms from the gold surface leads to the increase in the sulfate adsorption, therefore — in accordance with the results of the radiotracer experiments — a mass increase can be detected. If a relatively high cathodic current is applied, beside the upd process, a local formation of visible cadmium deposit (alloy?) can be observed on the gold surface that is unstable and dissolves. This behavior is illustrated in Fig. 10, which shows a mass decrease after a fast deposition process while the relatively high cathodic current flows still. At low currents, relatively stable potentials were established as expected and are seen in Fig. 11. Closer inspection of the curves, however, reveals that the potential increased slowly with a simultaneous slow increase in mass. It proves that the cadmium on the surface hinders the discharge of the H+-ions, i.e. more and more of the current is used to reduce Cd2 + -ions. (This effect manifests itself as a slow decrease of the current in the course of potentiostatic experiments.) This experiment attests that a nearly full coverage can be reached even at potentials as high as − 0.27 V (SCE), because the corrosion reaction gradually slows down and the galvanostatic current can be maintained only by the bulk deposition of cadmium. As a consequence, the potential suddenly drops to a low value corresponding to the bulk Cd deposition and a rapid mass increase can be observed due to this process as seen in Fig. 11. It should also be mentioned that adding H2SO4 to HClO4 solutions (final concentration: 2×10 − 2 mol dm − 3 H2SO4) caused a 4–8 Hz decrease in frequency
116
G. Inzelt, G. Hora´nyi / Journal of Electroanalytical Chemistry 491 (2000) 111–116
both in pure and Cd2 + -ion containing solutions in the potential region from 0.3 to 0.7 V (SCE). This observation is in accordance with the data presented in Fig. 3. In the presence of Cd2 + -ions, a frequency decrease of a similar magnitude was also observed below − 0.1 V (SCE); however, no such effect has been detected in pure HClO4 solutions. It means that the induced specific adsorption of sulfate ions in the upd potential region can be detected also by EQCM in accordance with the results of radiotracer [9,10] and previous EQCM [7] studies. The consequence of the alloy formation/dissolution processes is the roughening of the surface as mentioned above. This is reflected not only by an increase in the oxide reduction peak (considered as a simple indication of changes in real surface area) but also by an increase in the total amount of adsorbed anions and adatoms resulting in frequency changes. Thus it can be stated that in the case of continuous cycling the observed results are influenced by the roughening effect as well.
4. Conclusions It follows, from the results presented above, that the Au – Cd alloy is formed not only in the course of bulk deposition but also with the participation of Cd adatoms in the upd region. The solid-state diffusion plays an important role in both the alloy formation and the stripping of the Cd from the electrode surface. The latter process leads to roughening of the surface. Our results support the model introduced in Ref. [8], i.e. the amount of underpotentially deposited Cd atoms is not limited to one monolayer but increases with polarization time. The Au atoms beneath the adsorbed Cd atoms move to the topmost surface by a turnover process providing available sites for further adsorption of Cd atoms. This turnover process acts as a continuous source for alloy growth.
.
Acknowledgements This work was supported by the Hungarian Scientific Research Fund (OTKA) under grants T023056, T031703 (G.H.) and T014928 (G.I.).
References [1] E. Schmidt, H.R. Gygax, J. Electroanal. Chem. 12 (1966) 300. [2] J.W. Schultze, F.D. Koppitz, M.M. Lohrengel, Ber. Bunsenges. 78 (1974) 693. [3] B.W. Gregory, M.L. Norton, J.L. Stickney, J. Electroanal. Chem. 293 (1990) 85. [4] K. Takamura, F. Watanabe, T. Takamura, Electrochim. Acta 26 (1981) 979. [5] M.R. Deakin, O. Melroy, J. Electroanal. Chem. 239 (1988) 321. [6] J.C. Bondos, A.A. Gewirth, R.G. Nuzzo, J. Phys. Chem. 100 (1996) 8617. [7] B.K. Niece, A.A. Gewirth, Langmuir 13 (1997) 6302. [8] R. Vidu, S. Hara, J. Electroanal. Chem. 475 (1999) 171. [9] G. Hora´nyi, J. Solid State Electrochem. 4 (2000) 177. [10] G. Hora´nyi, E.M. Rizmayer, P. Joo´, J. Electroanal. Chem. 152 (1983) 211. [11] J. Ba´cskai, K. Martinusz, E. Czirok, G. Inzelt, P.J. Kulesza, E. Malik, J. Electroanal. Chem. 385 (1995) 241. [12] S. Pruneanu, E. Csaho´k, V. Kerte´sz, G. Inzelt, Electrochim. Acta 43 (1998) 2305. [13] G. Inzelt, V. Kerte´sz, G. La´ng, J. Phys. Chem. 97 (1993) 6104. [14] G. Hora´nyi, J. Solt, F. Nagy, J. Electroanal. Chem. 31 (1971) 87. [15] G. Hora´nyi, Rev. Anal. Chem. 14 (1995) 1. [16] K. Martinusz, G. Inzelt, G. Hora´nyi, J. Electroanal. Chem. 404 (1996) 143. [17] G. Hora´nyi, Electrochim. Acta 25 (1980) 43. [18] D. Kolb, in: H. Gerischer, C.W. Tobias (Eds.), Advances of Electrochemistry and Electrochemical Engineering, vol. 11, Wiley, New York, 1978, p. 125. [19] V. Tsionsky, L. Daikhin, E. Gileadi, J. Electrochem. Soc. 143 (1996) 2240. [20] J.S. Gordon, D.C. Johnson, J. Electroanal. Chem. 365 (1996) 267.