- Email: [email protected]
A new approach on the Cu UPD on Ag surfaces
Journal of Electroanalytical Chemistry 621 (2008) 7–12
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
A new approach on the Cu UPD on Ag surfaces K.-O. Thiel a, M. Hintze a, A. Vollmer b, C. Donner a,* a b
Freie Universität Berlin, Institut für Chemie, Takustraße 3, D-14195 Berlin, Germany Berliner Elektronenspeichering-Gesellschaft für Synchrotronstrahlung m. b. H. (BESSY), Albert-Einstein-Straße 15, D-12489 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 16 November 2007 Received in revised form 26 February 2008 Accepted 3 March 2008 Available online 8 March 2008 Keywords: Cu UPD Silver Thymine Au(1 1 1) Cyclic voltammetry Chronocoulometry XPS
a b s t r a c t The Cu UPD on different Ag substrates was investigated by cyclic voltammetry, potential step and X-ray photoelectron spectroscopy (XPS) experiments. Although it is known that a Cu UPD fails on bulk silver substrates, it could be proved that on silver monolayers copper ions are reduced in the underpotential region forming an Au–Ag–Cu sandwich structure. This process occurs only in absence of anions and thymine which are specifically adsorbed on the silver substrate. In this sense the Cu UPD on silver monolayers behaves completely different compared to the Cu UPD on Au(1 1 1) for which the process is supported only in presence of specifically adsorbed anions or thymine. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The deposition of monolayers and submonolayers on foreign metal substrates at potentials positive of the reversible Nernst potential is known as underpotential deposition (UPD). This phenomenon has been investigated by several in situ and ex situ techniques. Excellent overviews over different UPD systems are given in [1–4]. Theoretical descriptions of this phenomenon considering the role of the work function, the potential of zero charge (PZC) and the electronegativity were made by Gerischer and Kolb [5–7] in the middle of the 1970s. At the end of the 1990s Leiva presented an embedded atomic approach [8] to calculate the underpotential shifts of different metal adsorbates on different substrates. Leiva’s theoretical calculations predicted for both Cu UPD systems on Au(1 1 1) and on Ag(1 1 1) negative UPD shifts. In point of fact the Cu UPD occurs experimentally on Au(1 1 1) but not on Ag(1 1 1) surfaces. The UPD of copper on a Au(1 1 1) substrate is one of the experimentally best investigated and well-known UPD system so far [9– 12]. Thereby the deposition behaviour depends sensitively on the electrolyte anions as well as on the adsorbed organics. Specifically adsorbed sulphate anions promote the UPD taking place in a two step process while in presence of not specifically adsorbed perchlorate anions the UPD is strongly inhibited. But adding thymine mol* Corresponding author. Tel.: +49 30 838 53745; fax: +49 30 838 52 717. E-mail address: [email protected] (C. Donner). 0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2008.03.001
ecules into perchlorate solutions, the Cu UPD is pronounced again [13]. The Ag(1 1 1) surface offers nearly the same lattice parameters as Au(1 1 1) but there are considerable differences between these substrates regarding the work functions and PZCs, respectively. Consequently a UPD of Cu on bulk Ag(1 1 1) in sulphate containing electrolytes has not been observed [14] in contrast to the UPD on Au(1 1 1). At this point the question arises whether it is possible to deposit copper in the underpotential range on modified silver substrates. In the present manuscript the influence of different variations of the surface properties of silver on the Cu UPD is discussed, whereby the modification of the surfaces follows two strategies: firstly, the electronic properties of the Ag substrates were varied by depositing Ag mono- and bilayers on Au(1 1 1). It is well known that the electronic properties of metal monolayers covering a foreign substrate differ from the properties of bulk phases due to changed interatomic distances and relaxations [15–17]. The PCZ of Ag shifts from 0.70 V vs. SCE on bulk Ag(1 1 1) [18] to 0.50 V vs. SCE on Ag/Au(1 1 1) [19]. In the same manner the work function changes from 4.74 eV on bulk Ag(1 1 1) [20] to 4.87 eV on Ag/Au(1 1 1) [21]. Secondly, the Ag surface is modified by adsorbed organic molecules. It is known that the metal deposition kinetics can be sensitively influenced due to the different adsorption energies of the organic molecules on different surfaces. Because adsorbed thymine enhances the Cu UPD [13] but inhibits the Ag UPD on Au(1 1 1) [22],
8
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12
the Cu UPD on thymine covered silver substrates will be investigated.
3. After setting up the potential for the copper deposition the electrode was transferred to the copper-containing electrolyte. It was reconnected and immersed under potential control.
2. Experimental section The electrochemical experiments were carried out using an Autolab potentiostat PGSTAT 12. The single crystal working Au(1 1 1) electrode (Mateck Jülich, Germany, d = 5 mm) was flame-annealed and cooled down in an argon atmosphere before each experiment. The contact between the Au(1 1 1) plane and the electrolyte was guaranteed by the well known hanging meniscus configuration. The counter electrode was a gold coil and as reference an Ag/Ag+ (0.1 M AgClO4) electrode with a potential of 0.741 V vs. NHE was used. All stated potentials in this work are referred to this reference electrode unless otherwise noted. To avoid contaminations of the electrochemical cell by trace amounts of chlorides, possibly contained in the reference, a salt bridge separates one of each other. The working electrodes used in the XPS experiments consist of a gold layer (200–300 nm thickness) deposited on borosilicate glass. A 1–4 nm chromium layer acts as mediator. The XPS studies were performed with synchrotron radiation at BESSY at the end station SurICat (PM4). The base pressure in the analysis chamber was about 2 10 10 mbar. The spectra were collected with a hemispherical electron energy analyzer (Scienta SES 100) with an energy resolution of 140 meV under normal emission conditions. Core level spectra were fitted using the UNIFIT 2005 program. The background was adjusted by a Shirley-type correction. The layer by layer deposition has been carried out in the following manner:
The transfer procedure between the electrochemical cell and the UHV prechamber was performed as described elsewhere [22] with the only exception that the electrode was rinsed with perchloric acid instead of pure water after emersion of the electrode from the electrochemical cell. In this way the pH value remains unchanged. During the transfer procedures (Ag to Cu-cell and Cu-cell to UHV) air contact was unavoidable. Copper perchlorate (99.999%, Alfa Aesar), silver perchlorate monohydrate (99.999%, Aldrich), perchloric acid (suprapur, Merck), sodium perchlorate (p. A., Merck) and thymine (99%, Aldrich) were used as received. Triply distilled water was used. Prior to each experiment the electrolyte was purged with argon 4.8 (Air Liquid) for at least 20 min to remove the dissolved oxygen. 3. Results and discussion 3.1. Copper deposition on the 1st monolayer of silver The experimental routine to deposit copper on one monolayer silver was applied as described in the Section 2: The silver monolayer has been deposited at 70 mV vs. Ag/Ag+ (0.1 M AgClO4) on the Au(1 1 1) electrode (Fig. 1, point a). At this potential the coverage of silver on the substrate surface has been determined by charge density measurements as 0.94 ± 0.07 ML [23] and was reached after 0.3 s (cf. Fig. 2). Hence, a complete coverage of the Au(1 1 1) substrate with a Ag monolayer is assumed.
1. At first the selected type of the silver layer (1st or 2nd mono layer or bulk layer) was deposited on the Au(1 1 1) single crystal electrode at a certain potential (cf. Fig. 1 points a–c). The deposition time was about 5 min. 2. Thereafter the electrode was removed from the silver electrolyte under potential control (the deposition potential was still applied) to avoid the dissolution of the Ag adlayer. After the emersion the electrode was disconnected and rinsed with 100 mM perchloric acid, dried in an Ar flow.
After the deposition of the Ag monolayer at 70 mV the electrode was transferred into the Cu electrolyte cell and the cyclic voltammogramm (CV) shown in Fig. 3 was recorded. Comparing both the copper UPD on the silver monolayer and on the pure Au(1 1 1) substrate (Fig. 3) the following attracts attention to the Cu/Ag system: Both the deposition and dissolution peaks are shifted into the cathodic direction closer towards the Nernst equilibrium potential of the Cu2+/Cu couple (EN = 0.49 V) (cf. Table 1). Additionally a 2nd dissolution peak arises at about 435 mV.
Fig. 1. CV of the Ag deposition on Au(1 1 1); Electrolyte: 1 mM AgClO4, 60 mM NaClO4 and 100 mM HClO4; Scan rate 2 mV/s; Temperature 20 °C; a: 70 mV (1st ML); b: 100 mV (2nd ML); c: 150 mV (bulk).
Fig. 2. Formation transient of the Ag monolayer on Au(1 1 1); Start: 500 mV; End: 70 mV. Electrolyte: 1 mM AgClO4, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C.
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12
9
ing to approximately only 0.24 ML of a theoretical Cu(1 1 1) monolayer. One can speculate that a stable submonolayer is established on the surface. In Fig. 5 the dissolution transient of the Cu OPD (start potential 700 mV) is shown. This transient is the result of two consecutive processes. In the first step the dissolution of the bulk phase takes place. Thereafter the dissolution of the remaining monolayer of Cu on the Ag ML on Au(1 1 1) substrate occurs. The charge calculated for the 2nd dissolution process is in the same range as the charge obtained during the experiments discussed above for the dissolution of the exclusive UPD layer (Fig. 4). The results of the potential step experiments support definitely the existence of a Cu UPD on the 1st Ag ML on Au(1 1 1) substrate. Repeating the Cu UPD on the 1st Ag ML/Au(1 1 1) system but in sulphuric acid containing electrolytes instead of perchloric acid the cyclic voltammogramms (CV) and also the dissolution transients of the copper deposition are completely changed in their shapes. Both the Cu UPD deposition and dissolution signals recorded in perchloric acid (Fig. 3) disappear completely as shown in Fig. 6. Solely Fig. 3. CVs of the Cu UPD on Au(1 1 1) and on 1st ML of Ag; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Scan rate 2 mV/s; Temperature 20 °C.
The adsorption and the desorption of Cu ions obey a two step mechanism in the UPD region. The different positions of the characteristic peaks in the two systems give evidence that the deposition of Cu proceeds on the Ag surface and not on the bare Au(1 1 1) surface. To enlighten the kinetic mechanism potential step experiments were performed according to the routine described as followed: 1. Deposition of the Ag monolayer on Au (1 1 1) and transfer to the Cu electrolyte as described in the experimental section. 2. Measurement of the deposition transient after applying a potential step between a start potential of 200 mV and a final potential of 480 mV for Cu UPD or analogical 700 mV for Cu OPD. 3. Measurement of the dissolution transient starting at 480 mV for Cu UPD or analogical 700 mV for Cu OPD; the final potential is located at 200 mV.
Fig. 4. Dissolution transient of Cu UPD on 1st ML of Ag; Start: 480 mV; End: 200 mV; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C.
It should be noticed that final potentials more positive than 200 mV could not be applied due to the start of the dissolution of the silver layer. The shapes of the deposition transients (not shown here) are exponentially decreasing independent of the start potential. But in contrast the dissolution transients differ from each other as expected depending on the applied formation potentials 480 and 700 mV, respectively. Fig. 4 shows the dissolution transient of the Cu UPD (formation potential 480 mV). The shape is typical for a hole nucleation and growth process accompanied by a Langmuir desorption [24]. The dissolution process provides a charge of 104 lC/cm2 correspond-
Table 1 Potentials of the adsorption and desorption peaks of the Cu UPD on bare Au(1 1 1) and on Ag/Au(1 1 1)
Adsorption/deposition Desorption/dissolution
Au(1 1 1)
Ag/Au(1 1 1)
DE/mV
A1* 315 mV A2* 435 mV D* 295 mV
A1 A2 D1 D2
60 30 70 –
Peak names correspond to Fig. 3.
385 mV 465 mV 365 mV 435 mV
Fig. 5. Dissolution transient of Cu OPD on 1st ML of Ag; Start: 700 mV; End: 200 mV; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C; The inset shows an enlargement of the 2nd dissolution step.
10
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12
Fig. 6. CV of the Cu UPD on 1st ML of Ag; Electrolyte: 1 mM Cu(ClO4)2, 30 mM Na2SO4 and 50 mM H2SO4; Scan rate 2 mV/s; Temperature 20 °C.
the bulk deposition starts at potentials around -480 mV. No desorption peak can be recognized under the applied conditions. This result corresponds very well to the results discussed by Kolb [14] for the copper deposition on bulk silver substrates. Obviously, the Cu UPD on the 1st Ag ML/Au(1 1 1) depends sensitively on the anions being present during the deposition process. In this case the specifically adsorbed anions inhibit the UPD. To enlighten the role of the substrate on the copper deposition bulkier silver electrodes were investigated. 3.2. Copper deposition on the 2nd monolayer/bulk phase of silver As reported earlier [22] the Ag-3d core level shifts of the 2nd monolayer of silver on the Au(1 1 1) surface do not differ from the silver bulk phase. Regarding the published and our own experimental results the same electronic properties of the 2nd monolayer and of the bulk phase of silver can be claimed. Therefore only the results of the copper deposition on the silver bulk phase will be presented and discussed in the following. Actually the results for the copper deposition on Ag bilayers not shown here are the same. The Ag volume phase was deposited at 145 mV onto the Au(1 1 1) electrode (Fig. 1, point c). Subsequently the electrode was transferred to the copper-containing electrolyte and the CV shown in Fig. 7 was recorded as described above. This CV shows neither the deposition nor the dissolution peaks in the underpotential region at all, whereby the Cu UPD occurs neither in perchlorate nor in sulphate electrolytes. The bulk deposition starts on the silver surface at 480 mV and thus the result [7,14] that Cu UPD does not take place on bulk silver substrates is confirmed.
Fig. 7. CV of Cu in the UPD range on bulk Ag; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Scan rate 2 mV/s; Temperature 20 °C.
UPD on thymine modified Au(1 1 1) electrodes is inserted. Thereby it can be recognized that the deposition behaviour differs in its main characteristics depending on the substrate. The 1st Cu deposition peak seen in purely perchloric acid on Ag monolayer at about 357 mV (Fig. 3) is completely suppressed in presence of thymine (Fig. 8). The dissolution peak is shifted about 170 mV into cathodic direction towards the Nernst equilibrium potential (cf. Table 2), indicating that the formed copper layer is less stable in presence of thymine at the surface. The copper deposition proceeds in only one step on thymine modified Ag monolayers in contrast to a two step mechanism on thymine modified Au(1 1 1). The chronoamperometric studies of the dissolution process lead to the following results: Waiting 5 min at the UPD deposition potential at 480 mV and applying afterwards a potential jump to 200 mV only an exponential decaying current transient is recorded (Fig. 9). Surprisingly a neglecting charge value can be calculated by the integration procedure.
3.3. Copper deposition on 1st monolayer of silver in presence of thymine As already mentioned [13] it is known that physisorbed thymine supports the Cu UPD on Au(1 1 1) even in the absence of specifically adsorbed sulphate ions. To answer the question whether thymine supports the Cu UPD on silver monolayers the experiments described above were repeated with the exception that the copper deposition on an Ag monolayer takes place in the presence of thymine in the copper electrolyte. The CV recorded in Cu/ thymine electrolytes are shown in Fig. 8. For comparison the Cu
Fig. 8. CVs of the Cu UPD on Au(1 1 1) and on 1st Ag ML/Au(1 1 1) in presence of thymine each; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Scan rate 2 mV/s; Temperature 20 °C.
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12 Table 2 Potentials of the adsorption and desorption peaks of the Cu UPD on Au(1 1 1) and on Ag / Au(1 1 1) in presence of thymine
Adsorption/deposition Desorption/dissolution
Au(1 1 1) + thymine
Ag/Au(1 1 1) + thymine
DE/mV
A1 305 mV A2 435 mV D 205 mV
A*
435 mV
D*
375 mV
– 0 170
Peak names correspond to Fig. 8.
Otherwise a photoelectron spectroscopy experiment shows for underpotential deposited copper species on thymine modified Ag layers the following result (cf. Fig. 11, spectrum II): Only a very small amount of a Cu species is deposited on the thymine modified Ag ML surface. An identification of the species was not possible due to a weak signal noise ratio.
Fig. 9. Dissolution transient of Cu in the UPD range on 1st Ag ML/Au(1 1 1) in presence of thymine; Start: 480 mV; End: 200 mV; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C.
11
Although a peak pair is seen in the CV (Fig. 8) the low charge values in the transient experiments as well as the result from XPS experiments prevents a definite conclusion about Cu UPD on thymine modified Ag monolayers. At this point further STM experiments are scheduled. Even starting at 700 mV far in the Cu OPD region and applying a potential jump to 200 mV the integration of the dissolution transients leads to only a remarkable low charge density (cf. Fig. 10). About 3–4% of the charge density of the analogous experiment performed in a thymine-free electrolyte (cf. Fig. 5) is calculated. Theoretically this charge density corresponds to only 0.8 ML of Cu deposited pseudomorphic on Ag(1 1 1) adlayers. A XPS investigation clarified whether a monolayer (in the OPD range!) is formed or a Vollmer–Weber growth, forming 3D copper islands, took place
Fig. 10. Dissolution transient of Cu in the OPD range on 1st Ag ML/Au(1 1 1) in presence of thymine; Start: 700 mV; End: 200 mV; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C.
Fig. 11. Cu2p3/2 core levels for different copper species covered by thymine; excitation energy 1150 eV; Spectrum I: deposition of a copper at 0.7 V on a thymine modified 1st Ag ML/Au(1 1 1) electrode, emersion potential 0.7 V; Spectrum II: deposition of a copper monolayer at 0.48 V on a thymine modified 1st Ag ML/Au(1 1 1) electrode, emersion potential 0.48 V.
12
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12
(cf. Fig. 11, spectrum I). For copper species deposited at -700 mV, three peaks assigned to a metallic Cu species (932.8 eV) and two oxidic species (934.7 and 936.0 eV) can be recognized. The peak position of the metallic species is typical for a Cu bulk phase. Hence, a Vollmer–Weber growth forming 3D islands is assumed. For a copper monolayer a shift of around 931.8 eV [25] is expected. 4. Conclusion The Cu UPD on different Ag substrates was investigated in absence and presence of adsorbed thymine. In absence if thymine it is claimed that a Cu UPD on Ag ML/Au(1 1 1) takes place while on bulk silver only copper volume deposition occurs. Thereby the deposition depends strongly on the electrolyte anions. Cu UPD occurs even on the 1st Ag ML only in the exclusive presence of perchlorate ions, while in sulphate containing electrolytes no UPD was observed. On silver adsorbed thymine inhibits the Cu UPD and OPD. In contrast thymine supports the Cu UPD on Au(1 1 1). A possible reason for the differences might be the adsorption state of thymine on silver. At the applied Cu deposition potentials thymine is chemisorbed on the Ag substrate but physisorbed on the Cu substrate. Consequently, the Cu deposition on the Ag substrate would lead to a reorientation and change of the adsorption state of thymine. But a transition from a chemisorbed to a physisorbed state is energetically not favoured. Hence, the Cu UPD and OPD are inhibited on the thymine modified Ag ML/Au(1 1 1) substrate. Acknowledgement Special thanks to Bettina Seidl for her skilful help in the laboratory.
References [1] E. Herrero, L.J. Buller, H.D. Abruna, Chemical Reviews 101 (2001) 1897. [2] E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical Phase Formation and Growth, VCH Verlagsgesellschaft mbH, Weinheim, 1996. [3] G. Staikov, K. Jüttner, W.J. Lorenz, E. Budevski, Electrochimica Acta 39 (1994) 1019. [4] K. Ogaki, K. Itaya, Electrochimica Acta 40 (1995) 1249. [5] D.M. Kolb, H. Gerischer, Surface Science 51 (1975) 323. [6] H. Gerischer, D.M. Kolb, M. Przasnyski, Surface Science 43 (1974) 662. [7] D.M. Kolb, M. Przasnyski, H. Gerischer, Journal of Electroanalytical Chemistry 54 (1974) 25. [8] C.G. Sánchez, M.G. Del Pópolo, E.P.M. Leiva, Surface Science 421 (1999) 59. [9] J.G. Xu, X.W. Wang, Surface Science 408 (1998) 317. [10] Z. Shi, S. Wu, J. Lipkowski, Electrochimica Acta 40 (1995) 9. [11] I.H. Omar, H.J. Pauling, K. Jüttner, Journal of the Electrochemical Society 140 (1993) 2187. [12] M.A. Schneeweiss, D.M. Kolb, Physica Status Solidi A: Applied Research 173 (1999) 51. [13] E. Avci, C. Meyer, C. Donner, Journal of Electroanalytical Chemistry 589 (2006) 24. [14] M. Dietterle, T. Will, D.M. Kolb, Surface Science 342 (1995) 29. [15] P.H. Citrin, G.K. Wertheim, Physical Review Letters 41 (1978) 1425. [16] P. Heimann, J.F. van der Veen, D.E. Eastman, Solid State Communications 38 (1981) 595. [17] D.E. Eastman, F.J. Himpsel, J.F. van der Veen, Journal of Vacuum Science & Technology 20 (1982) 609. [18] G. Valette, A. Hamelin, Journal of Electroanalytical Chemistry 45 (1973) 301. [19] K.A. Soliman, L.A. Kibler, Electrochimica Acta 52 (2007) 5654. [20] C.H.B. Mee, A.W. Dweydari, Physica Status Solidi A: Applied Research 17 (1973) 247. [21] E. Avci, C. Meyer, A. Vollmer, C. Donner, in: 57th Annual Meeting of the ISE, Edinburgh, 2006. [22] A. Vollmer, E. Avci, K.-O. Thiel, C. Donner, Journal of Electroanalytical Chemistry 605 (2007) 15. [23] V. Rooryck, F. Reniers, C. Buess-Herman, G.A. Attard, X. Yang, Journal of Electroanalytical Chemistry 482 (2000) 93. [24] C. Donner, Journal of Electroanalytical Chemistry 550–551 (2003) 209. [25] K.-O. Thiel, A. Vollmer, M. Hintze, E. Avci, C. Donner, Zeitschrift fuer Physikalische Chemie (Munich) 221 (2007) 1255.
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
A new approach on the Cu UPD on Ag surfaces K.-O. Thiel a, M. Hintze a, A. Vollmer b, C. Donner a,* a b
Freie Universität Berlin, Institut für Chemie, Takustraße 3, D-14195 Berlin, Germany Berliner Elektronenspeichering-Gesellschaft für Synchrotronstrahlung m. b. H. (BESSY), Albert-Einstein-Straße 15, D-12489 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 16 November 2007 Received in revised form 26 February 2008 Accepted 3 March 2008 Available online 8 March 2008 Keywords: Cu UPD Silver Thymine Au(1 1 1) Cyclic voltammetry Chronocoulometry XPS
a b s t r a c t The Cu UPD on different Ag substrates was investigated by cyclic voltammetry, potential step and X-ray photoelectron spectroscopy (XPS) experiments. Although it is known that a Cu UPD fails on bulk silver substrates, it could be proved that on silver monolayers copper ions are reduced in the underpotential region forming an Au–Ag–Cu sandwich structure. This process occurs only in absence of anions and thymine which are specifically adsorbed on the silver substrate. In this sense the Cu UPD on silver monolayers behaves completely different compared to the Cu UPD on Au(1 1 1) for which the process is supported only in presence of specifically adsorbed anions or thymine. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The deposition of monolayers and submonolayers on foreign metal substrates at potentials positive of the reversible Nernst potential is known as underpotential deposition (UPD). This phenomenon has been investigated by several in situ and ex situ techniques. Excellent overviews over different UPD systems are given in [1–4]. Theoretical descriptions of this phenomenon considering the role of the work function, the potential of zero charge (PZC) and the electronegativity were made by Gerischer and Kolb [5–7] in the middle of the 1970s. At the end of the 1990s Leiva presented an embedded atomic approach [8] to calculate the underpotential shifts of different metal adsorbates on different substrates. Leiva’s theoretical calculations predicted for both Cu UPD systems on Au(1 1 1) and on Ag(1 1 1) negative UPD shifts. In point of fact the Cu UPD occurs experimentally on Au(1 1 1) but not on Ag(1 1 1) surfaces. The UPD of copper on a Au(1 1 1) substrate is one of the experimentally best investigated and well-known UPD system so far [9– 12]. Thereby the deposition behaviour depends sensitively on the electrolyte anions as well as on the adsorbed organics. Specifically adsorbed sulphate anions promote the UPD taking place in a two step process while in presence of not specifically adsorbed perchlorate anions the UPD is strongly inhibited. But adding thymine mol* Corresponding author. Tel.: +49 30 838 53745; fax: +49 30 838 52 717. E-mail address: [email protected] (C. Donner). 0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2008.03.001
ecules into perchlorate solutions, the Cu UPD is pronounced again [13]. The Ag(1 1 1) surface offers nearly the same lattice parameters as Au(1 1 1) but there are considerable differences between these substrates regarding the work functions and PZCs, respectively. Consequently a UPD of Cu on bulk Ag(1 1 1) in sulphate containing electrolytes has not been observed [14] in contrast to the UPD on Au(1 1 1). At this point the question arises whether it is possible to deposit copper in the underpotential range on modified silver substrates. In the present manuscript the influence of different variations of the surface properties of silver on the Cu UPD is discussed, whereby the modification of the surfaces follows two strategies: firstly, the electronic properties of the Ag substrates were varied by depositing Ag mono- and bilayers on Au(1 1 1). It is well known that the electronic properties of metal monolayers covering a foreign substrate differ from the properties of bulk phases due to changed interatomic distances and relaxations [15–17]. The PCZ of Ag shifts from 0.70 V vs. SCE on bulk Ag(1 1 1) [18] to 0.50 V vs. SCE on Ag/Au(1 1 1) [19]. In the same manner the work function changes from 4.74 eV on bulk Ag(1 1 1) [20] to 4.87 eV on Ag/Au(1 1 1) [21]. Secondly, the Ag surface is modified by adsorbed organic molecules. It is known that the metal deposition kinetics can be sensitively influenced due to the different adsorption energies of the organic molecules on different surfaces. Because adsorbed thymine enhances the Cu UPD [13] but inhibits the Ag UPD on Au(1 1 1) [22],
8
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12
the Cu UPD on thymine covered silver substrates will be investigated.
3. After setting up the potential for the copper deposition the electrode was transferred to the copper-containing electrolyte. It was reconnected and immersed under potential control.
2. Experimental section The electrochemical experiments were carried out using an Autolab potentiostat PGSTAT 12. The single crystal working Au(1 1 1) electrode (Mateck Jülich, Germany, d = 5 mm) was flame-annealed and cooled down in an argon atmosphere before each experiment. The contact between the Au(1 1 1) plane and the electrolyte was guaranteed by the well known hanging meniscus configuration. The counter electrode was a gold coil and as reference an Ag/Ag+ (0.1 M AgClO4) electrode with a potential of 0.741 V vs. NHE was used. All stated potentials in this work are referred to this reference electrode unless otherwise noted. To avoid contaminations of the electrochemical cell by trace amounts of chlorides, possibly contained in the reference, a salt bridge separates one of each other. The working electrodes used in the XPS experiments consist of a gold layer (200–300 nm thickness) deposited on borosilicate glass. A 1–4 nm chromium layer acts as mediator. The XPS studies were performed with synchrotron radiation at BESSY at the end station SurICat (PM4). The base pressure in the analysis chamber was about 2 10 10 mbar. The spectra were collected with a hemispherical electron energy analyzer (Scienta SES 100) with an energy resolution of 140 meV under normal emission conditions. Core level spectra were fitted using the UNIFIT 2005 program. The background was adjusted by a Shirley-type correction. The layer by layer deposition has been carried out in the following manner:
The transfer procedure between the electrochemical cell and the UHV prechamber was performed as described elsewhere [22] with the only exception that the electrode was rinsed with perchloric acid instead of pure water after emersion of the electrode from the electrochemical cell. In this way the pH value remains unchanged. During the transfer procedures (Ag to Cu-cell and Cu-cell to UHV) air contact was unavoidable. Copper perchlorate (99.999%, Alfa Aesar), silver perchlorate monohydrate (99.999%, Aldrich), perchloric acid (suprapur, Merck), sodium perchlorate (p. A., Merck) and thymine (99%, Aldrich) were used as received. Triply distilled water was used. Prior to each experiment the electrolyte was purged with argon 4.8 (Air Liquid) for at least 20 min to remove the dissolved oxygen. 3. Results and discussion 3.1. Copper deposition on the 1st monolayer of silver The experimental routine to deposit copper on one monolayer silver was applied as described in the Section 2: The silver monolayer has been deposited at 70 mV vs. Ag/Ag+ (0.1 M AgClO4) on the Au(1 1 1) electrode (Fig. 1, point a). At this potential the coverage of silver on the substrate surface has been determined by charge density measurements as 0.94 ± 0.07 ML [23] and was reached after 0.3 s (cf. Fig. 2). Hence, a complete coverage of the Au(1 1 1) substrate with a Ag monolayer is assumed.
1. At first the selected type of the silver layer (1st or 2nd mono layer or bulk layer) was deposited on the Au(1 1 1) single crystal electrode at a certain potential (cf. Fig. 1 points a–c). The deposition time was about 5 min. 2. Thereafter the electrode was removed from the silver electrolyte under potential control (the deposition potential was still applied) to avoid the dissolution of the Ag adlayer. After the emersion the electrode was disconnected and rinsed with 100 mM perchloric acid, dried in an Ar flow.
After the deposition of the Ag monolayer at 70 mV the electrode was transferred into the Cu electrolyte cell and the cyclic voltammogramm (CV) shown in Fig. 3 was recorded. Comparing both the copper UPD on the silver monolayer and on the pure Au(1 1 1) substrate (Fig. 3) the following attracts attention to the Cu/Ag system: Both the deposition and dissolution peaks are shifted into the cathodic direction closer towards the Nernst equilibrium potential of the Cu2+/Cu couple (EN = 0.49 V) (cf. Table 1). Additionally a 2nd dissolution peak arises at about 435 mV.
Fig. 1. CV of the Ag deposition on Au(1 1 1); Electrolyte: 1 mM AgClO4, 60 mM NaClO4 and 100 mM HClO4; Scan rate 2 mV/s; Temperature 20 °C; a: 70 mV (1st ML); b: 100 mV (2nd ML); c: 150 mV (bulk).
Fig. 2. Formation transient of the Ag monolayer on Au(1 1 1); Start: 500 mV; End: 70 mV. Electrolyte: 1 mM AgClO4, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C.
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12
9
ing to approximately only 0.24 ML of a theoretical Cu(1 1 1) monolayer. One can speculate that a stable submonolayer is established on the surface. In Fig. 5 the dissolution transient of the Cu OPD (start potential 700 mV) is shown. This transient is the result of two consecutive processes. In the first step the dissolution of the bulk phase takes place. Thereafter the dissolution of the remaining monolayer of Cu on the Ag ML on Au(1 1 1) substrate occurs. The charge calculated for the 2nd dissolution process is in the same range as the charge obtained during the experiments discussed above for the dissolution of the exclusive UPD layer (Fig. 4). The results of the potential step experiments support definitely the existence of a Cu UPD on the 1st Ag ML on Au(1 1 1) substrate. Repeating the Cu UPD on the 1st Ag ML/Au(1 1 1) system but in sulphuric acid containing electrolytes instead of perchloric acid the cyclic voltammogramms (CV) and also the dissolution transients of the copper deposition are completely changed in their shapes. Both the Cu UPD deposition and dissolution signals recorded in perchloric acid (Fig. 3) disappear completely as shown in Fig. 6. Solely Fig. 3. CVs of the Cu UPD on Au(1 1 1) and on 1st ML of Ag; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Scan rate 2 mV/s; Temperature 20 °C.
The adsorption and the desorption of Cu ions obey a two step mechanism in the UPD region. The different positions of the characteristic peaks in the two systems give evidence that the deposition of Cu proceeds on the Ag surface and not on the bare Au(1 1 1) surface. To enlighten the kinetic mechanism potential step experiments were performed according to the routine described as followed: 1. Deposition of the Ag monolayer on Au (1 1 1) and transfer to the Cu electrolyte as described in the experimental section. 2. Measurement of the deposition transient after applying a potential step between a start potential of 200 mV and a final potential of 480 mV for Cu UPD or analogical 700 mV for Cu OPD. 3. Measurement of the dissolution transient starting at 480 mV for Cu UPD or analogical 700 mV for Cu OPD; the final potential is located at 200 mV.
Fig. 4. Dissolution transient of Cu UPD on 1st ML of Ag; Start: 480 mV; End: 200 mV; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C.
It should be noticed that final potentials more positive than 200 mV could not be applied due to the start of the dissolution of the silver layer. The shapes of the deposition transients (not shown here) are exponentially decreasing independent of the start potential. But in contrast the dissolution transients differ from each other as expected depending on the applied formation potentials 480 and 700 mV, respectively. Fig. 4 shows the dissolution transient of the Cu UPD (formation potential 480 mV). The shape is typical for a hole nucleation and growth process accompanied by a Langmuir desorption [24]. The dissolution process provides a charge of 104 lC/cm2 correspond-
Table 1 Potentials of the adsorption and desorption peaks of the Cu UPD on bare Au(1 1 1) and on Ag/Au(1 1 1)
Adsorption/deposition Desorption/dissolution
Au(1 1 1)
Ag/Au(1 1 1)
DE/mV
A1* 315 mV A2* 435 mV D* 295 mV
A1 A2 D1 D2
60 30 70 –
Peak names correspond to Fig. 3.
385 mV 465 mV 365 mV 435 mV
Fig. 5. Dissolution transient of Cu OPD on 1st ML of Ag; Start: 700 mV; End: 200 mV; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C; The inset shows an enlargement of the 2nd dissolution step.
10
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12
Fig. 6. CV of the Cu UPD on 1st ML of Ag; Electrolyte: 1 mM Cu(ClO4)2, 30 mM Na2SO4 and 50 mM H2SO4; Scan rate 2 mV/s; Temperature 20 °C.
the bulk deposition starts at potentials around -480 mV. No desorption peak can be recognized under the applied conditions. This result corresponds very well to the results discussed by Kolb [14] for the copper deposition on bulk silver substrates. Obviously, the Cu UPD on the 1st Ag ML/Au(1 1 1) depends sensitively on the anions being present during the deposition process. In this case the specifically adsorbed anions inhibit the UPD. To enlighten the role of the substrate on the copper deposition bulkier silver electrodes were investigated. 3.2. Copper deposition on the 2nd monolayer/bulk phase of silver As reported earlier [22] the Ag-3d core level shifts of the 2nd monolayer of silver on the Au(1 1 1) surface do not differ from the silver bulk phase. Regarding the published and our own experimental results the same electronic properties of the 2nd monolayer and of the bulk phase of silver can be claimed. Therefore only the results of the copper deposition on the silver bulk phase will be presented and discussed in the following. Actually the results for the copper deposition on Ag bilayers not shown here are the same. The Ag volume phase was deposited at 145 mV onto the Au(1 1 1) electrode (Fig. 1, point c). Subsequently the electrode was transferred to the copper-containing electrolyte and the CV shown in Fig. 7 was recorded as described above. This CV shows neither the deposition nor the dissolution peaks in the underpotential region at all, whereby the Cu UPD occurs neither in perchlorate nor in sulphate electrolytes. The bulk deposition starts on the silver surface at 480 mV and thus the result [7,14] that Cu UPD does not take place on bulk silver substrates is confirmed.
Fig. 7. CV of Cu in the UPD range on bulk Ag; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Scan rate 2 mV/s; Temperature 20 °C.
UPD on thymine modified Au(1 1 1) electrodes is inserted. Thereby it can be recognized that the deposition behaviour differs in its main characteristics depending on the substrate. The 1st Cu deposition peak seen in purely perchloric acid on Ag monolayer at about 357 mV (Fig. 3) is completely suppressed in presence of thymine (Fig. 8). The dissolution peak is shifted about 170 mV into cathodic direction towards the Nernst equilibrium potential (cf. Table 2), indicating that the formed copper layer is less stable in presence of thymine at the surface. The copper deposition proceeds in only one step on thymine modified Ag monolayers in contrast to a two step mechanism on thymine modified Au(1 1 1). The chronoamperometric studies of the dissolution process lead to the following results: Waiting 5 min at the UPD deposition potential at 480 mV and applying afterwards a potential jump to 200 mV only an exponential decaying current transient is recorded (Fig. 9). Surprisingly a neglecting charge value can be calculated by the integration procedure.
3.3. Copper deposition on 1st monolayer of silver in presence of thymine As already mentioned [13] it is known that physisorbed thymine supports the Cu UPD on Au(1 1 1) even in the absence of specifically adsorbed sulphate ions. To answer the question whether thymine supports the Cu UPD on silver monolayers the experiments described above were repeated with the exception that the copper deposition on an Ag monolayer takes place in the presence of thymine in the copper electrolyte. The CV recorded in Cu/ thymine electrolytes are shown in Fig. 8. For comparison the Cu
Fig. 8. CVs of the Cu UPD on Au(1 1 1) and on 1st Ag ML/Au(1 1 1) in presence of thymine each; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Scan rate 2 mV/s; Temperature 20 °C.
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12 Table 2 Potentials of the adsorption and desorption peaks of the Cu UPD on Au(1 1 1) and on Ag / Au(1 1 1) in presence of thymine
Adsorption/deposition Desorption/dissolution
Au(1 1 1) + thymine
Ag/Au(1 1 1) + thymine
DE/mV
A1 305 mV A2 435 mV D 205 mV
A*
435 mV
D*
375 mV
– 0 170
Peak names correspond to Fig. 8.
Otherwise a photoelectron spectroscopy experiment shows for underpotential deposited copper species on thymine modified Ag layers the following result (cf. Fig. 11, spectrum II): Only a very small amount of a Cu species is deposited on the thymine modified Ag ML surface. An identification of the species was not possible due to a weak signal noise ratio.
Fig. 9. Dissolution transient of Cu in the UPD range on 1st Ag ML/Au(1 1 1) in presence of thymine; Start: 480 mV; End: 200 mV; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C.
11
Although a peak pair is seen in the CV (Fig. 8) the low charge values in the transient experiments as well as the result from XPS experiments prevents a definite conclusion about Cu UPD on thymine modified Ag monolayers. At this point further STM experiments are scheduled. Even starting at 700 mV far in the Cu OPD region and applying a potential jump to 200 mV the integration of the dissolution transients leads to only a remarkable low charge density (cf. Fig. 10). About 3–4% of the charge density of the analogous experiment performed in a thymine-free electrolyte (cf. Fig. 5) is calculated. Theoretically this charge density corresponds to only 0.8 ML of Cu deposited pseudomorphic on Ag(1 1 1) adlayers. A XPS investigation clarified whether a monolayer (in the OPD range!) is formed or a Vollmer–Weber growth, forming 3D copper islands, took place
Fig. 10. Dissolution transient of Cu in the OPD range on 1st Ag ML/Au(1 1 1) in presence of thymine; Start: 700 mV; End: 200 mV; Electrolyte: 1 mM Cu(ClO4)2, 60 mM NaClO4 and 100 mM HClO4; Temperature 20 °C.
Fig. 11. Cu2p3/2 core levels for different copper species covered by thymine; excitation energy 1150 eV; Spectrum I: deposition of a copper at 0.7 V on a thymine modified 1st Ag ML/Au(1 1 1) electrode, emersion potential 0.7 V; Spectrum II: deposition of a copper monolayer at 0.48 V on a thymine modified 1st Ag ML/Au(1 1 1) electrode, emersion potential 0.48 V.
12
K.-O. Thiel et al. / Journal of Electroanalytical Chemistry 621 (2008) 7–12
(cf. Fig. 11, spectrum I). For copper species deposited at -700 mV, three peaks assigned to a metallic Cu species (932.8 eV) and two oxidic species (934.7 and 936.0 eV) can be recognized. The peak position of the metallic species is typical for a Cu bulk phase. Hence, a Vollmer–Weber growth forming 3D islands is assumed. For a copper monolayer a shift of around 931.8 eV [25] is expected. 4. Conclusion The Cu UPD on different Ag substrates was investigated in absence and presence of adsorbed thymine. In absence if thymine it is claimed that a Cu UPD on Ag ML/Au(1 1 1) takes place while on bulk silver only copper volume deposition occurs. Thereby the deposition depends strongly on the electrolyte anions. Cu UPD occurs even on the 1st Ag ML only in the exclusive presence of perchlorate ions, while in sulphate containing electrolytes no UPD was observed. On silver adsorbed thymine inhibits the Cu UPD and OPD. In contrast thymine supports the Cu UPD on Au(1 1 1). A possible reason for the differences might be the adsorption state of thymine on silver. At the applied Cu deposition potentials thymine is chemisorbed on the Ag substrate but physisorbed on the Cu substrate. Consequently, the Cu deposition on the Ag substrate would lead to a reorientation and change of the adsorption state of thymine. But a transition from a chemisorbed to a physisorbed state is energetically not favoured. Hence, the Cu UPD and OPD are inhibited on the thymine modified Ag ML/Au(1 1 1) substrate. Acknowledgement Special thanks to Bettina Seidl for her skilful help in the laboratory.
References [1] E. Herrero, L.J. Buller, H.D. Abruna, Chemical Reviews 101 (2001) 1897. [2] E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical Phase Formation and Growth, VCH Verlagsgesellschaft mbH, Weinheim, 1996. [3] G. Staikov, K. Jüttner, W.J. Lorenz, E. Budevski, Electrochimica Acta 39 (1994) 1019. [4] K. Ogaki, K. Itaya, Electrochimica Acta 40 (1995) 1249. [5] D.M. Kolb, H. Gerischer, Surface Science 51 (1975) 323. [6] H. Gerischer, D.M. Kolb, M. Przasnyski, Surface Science 43 (1974) 662. [7] D.M. Kolb, M. Przasnyski, H. Gerischer, Journal of Electroanalytical Chemistry 54 (1974) 25. [8] C.G. Sánchez, M.G. Del Pópolo, E.P.M. Leiva, Surface Science 421 (1999) 59. [9] J.G. Xu, X.W. Wang, Surface Science 408 (1998) 317. [10] Z. Shi, S. Wu, J. Lipkowski, Electrochimica Acta 40 (1995) 9. [11] I.H. Omar, H.J. Pauling, K. Jüttner, Journal of the Electrochemical Society 140 (1993) 2187. [12] M.A. Schneeweiss, D.M. Kolb, Physica Status Solidi A: Applied Research 173 (1999) 51. [13] E. Avci, C. Meyer, C. Donner, Journal of Electroanalytical Chemistry 589 (2006) 24. [14] M. Dietterle, T. Will, D.M. Kolb, Surface Science 342 (1995) 29. [15] P.H. Citrin, G.K. Wertheim, Physical Review Letters 41 (1978) 1425. [16] P. Heimann, J.F. van der Veen, D.E. Eastman, Solid State Communications 38 (1981) 595. [17] D.E. Eastman, F.J. Himpsel, J.F. van der Veen, Journal of Vacuum Science & Technology 20 (1982) 609. [18] G. Valette, A. Hamelin, Journal of Electroanalytical Chemistry 45 (1973) 301. [19] K.A. Soliman, L.A. Kibler, Electrochimica Acta 52 (2007) 5654. [20] C.H.B. Mee, A.W. Dweydari, Physica Status Solidi A: Applied Research 17 (1973) 247. [21] E. Avci, C. Meyer, A. Vollmer, C. Donner, in: 57th Annual Meeting of the ISE, Edinburgh, 2006. [22] A. Vollmer, E. Avci, K.-O. Thiel, C. Donner, Journal of Electroanalytical Chemistry 605 (2007) 15. [23] V. Rooryck, F. Reniers, C. Buess-Herman, G.A. Attard, X. Yang, Journal of Electroanalytical Chemistry 482 (2000) 93. [24] C. Donner, Journal of Electroanalytical Chemistry 550–551 (2003) 209. [25] K.-O. Thiel, A. Vollmer, M. Hintze, E. Avci, C. Donner, Zeitschrift fuer Physikalische Chemie (Munich) 221 (2007) 1255.