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A determination of copper overlayer structures on Au(111) in the presence of electrolyte additives
Volume200, number 4
CHEMICALPHYSICSLETTERS
11December 1992
A determination of copper overlayer structures on Au ( 111) in the presence of electrolyte additives W. Haiss, D. Lackey, J.K. Sass Fritz-Haber-lnstitut der Max-Planck-Gesellschaji, Faradayweg 4-6, W-1000Berlin 33, Germany
H. Meyer and R.J. Nichols Schering AG, MetalplatingDivision, Postfach 65031 I, W-JO00Berlin 65, Germany
Received2 September 1992;in final form 25 September 1992
Cyclicvoltammetry and scanning tuanelling microscopy@TM) wereused to study the influence of chlorideand crystal violet on the underpotential deposition (upd ) of copperon Au( 111). As previouslyreported,a (3 x fi )R30” copperoverlayerhas been characterizedin the absence of additives and this has been related to the first upd peak. At low chloride concentrations this first upd peak splits into two partially overlapping peaks and a more densely packed copper structure appears alongside the (8 x ,,h )R30” domains. The splitting of this first voltammetric peak disappearsat high chlorideconcentrations and only the more denselypackedcopperoverlayeris observed.The STMimagesare consistent with either a (5 x 5) or ( 1.29XI .29) structure. Crystal violet is a cationic organic dye, with a chloride counterion. It has a marked influence on the voltammetry, with the first upd peak occurring at significantly more negative potentials. Even at a low crystal violet concentration, which correspondedto the lowest chloride concentration (wherejoint domains were always seen), only the (5X 5) (or (1.29X1.29)) structure was observed.This clearly indicates the pronounced effectarising from the adsorption of the organic cation.
1.
Intmduction
The electrochemical formation of the first layer of a metallic deposit on a foreign metal substrate occurs, in many circumstances, at potentials which are more positive than the potential of reversible deposition of the bulk phase [ I ,2 1. This phenomenon is known as underpotential deposition and there has been a great deal of interest in the characterization of the structure of underpotential deposits. One system which has received a great deal of attention is the underpotential deposition of copper on Au ( 1I 1). This is particularly interesting since it has been studied with a variety of surface-structure sensitive techniques [ 3-9 1, as well as with cyclic voltammetry [ 10,111. The voltammetry of copper upd in sulCorrespondence to: D. Lackey,Fritz-Haber-Institut der MaxPlanck-Gesellschaft, Faradayweg 4-6, W-1000 Berlin 33, Germany, and R.J. Nichols,ScheriogAG, MetalplatingDivision, Postfach650311,W-1000Berlin 65, Germany.
phate-containing electrolytes shows two distinct adsorption peaks and it was inferred from such measurements that the copper monolayer formation proceeds through the formation of two ordered adla ers [ IO]. Later, the existence of a (&X J 3)R30° adlayer, at medium copper coverages, was conclusively proved by ex situ electron diffraction experiments [ 3 1. LEED experiments also suggested a ( 1x 1) structure for the full monolayer [ 31. Further evidence for the (1 X 1) structure of the full monolayer was provided by in situ X-ray diffraction studies, with the bond length for the complete monolayer correspondin to that of the Au ( 111) substrate [4]. The (fix $ 3)R30° structure has also been confirmed by Magnussen et al. [ 71 in situ with the STM technique (there are however presently no reports of the ( 1X 1) copper adlayer being resolved with STM, although this structure has been resolved with in situ AM [ $1). However, a second more densely packed structure, not seen previously by ex situ LEED measurements, was seen to slowly de-
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velop at electrode potentials between the two deposition ‘waves [ 71. This slow formation of the (5 x 5) structure was assigned to a slow, diffusioncontrolled coadsorption of trace chloride impurities (which were estimated as less than 10e6 M ) [ 71. Indeed, Hachiya et al. [9], who took special precautions to limit the possibility of chloride contamination, only observed the (J?xfi)R30’ structure, which was stable for long periods of time in the entire potential range between 0.18 and 0.1 V (versus SCE). In this study we have characterized the effect of chloride on the upd of copper on Au( 111) , by systematically varying the chloride concentration. We have limited this work to the first upd peaks in the voltammogram, i.e. those peaks which correspond to the “medium coverage” structures. We have directly correlated the effect of chloride on the voltammetry with the copper adlayers observed by STM. We have also studied the influence of crystal violet, which is a cationic organic dye with a chloride counterion (hexamethylpararosaniline chloride). We have addressed the question of whether adsorbed organic cation plays a structure determining role, or whether the influence of this additive arises solely from the adsorbed chloride counterion.
2. Experimental We have modified a beetle-type STM, which was designed by Besocke [ 12 1, so that we were able to conduct measurements in electrolyte environments. Gold films, prepared by the evaporation of k 2 nm of chromium followed by x200 nm of gold onto tempax glass (Berliner Glas KG. ), were used as working electrodes [ i3,14]. Prior to use these samples were carefully annealed to about 800-1000°C and then quenched in ultrapure water. Electrolytes were prepared from doubly distilled water (from a Fison “Cyclon” distillation system) and H2S04 (Merck, suprapur), CuSO,, (Merck, p.a. ), hydrochloric acid (Merck, suprapur) and crystal violet (Merck), which was purified by recrystallization. The electrolytes were not deaerated prior to the experiment and the in situ STM cell was open to air. The reference electrode consisted of a copper wire which was inserted directly into the cell and all electrode
11DecemberI992
potentials are quoted with respect to the Cu/Cu’+ couple. Iridium wire was mechanically ground to form the tunnelling tip and this was subsequently isolated with Apiezon wax.
3. Results The gold films were characterized using atomic force microscopy ( AFM), STM and voltammetry. Prior to the first flame annealing the films possessed a granular structure typical of a sputtered metal deposit. However, the surface morphology is dramatically changed upon flame annealing. The surface restructures producing very flat regions, extending over hundreds of nanometers, which are often separated by deep furrows (as we have seen in a preliminary AF’Minvestigation). We have used atomic resolution STM to characterize these terraced regions and have shown that they are predominantly of ( 111) orientation. The voltammetry of the gold films in 0.1 M HzS04 + 1 mM CuS04 is shown in fig. 1A. Although the peaks are not as sharp as for well defined Au( 111) single crystal discs, the voltammetry is consistent with a sample which has predominantly ( 111) orientation. Cyclic voltammograms with the addition of 2.5 x 10m5M and 1 mM HCl are shown in figs. 1B and 1C, respectively. Notice how the first deposition peak is split into two at the lower chloride concentration (fig. 1B ), while no splitting is observed in both the absence of chloride (fig. 1A) and at the higher concentration (fig, 1C). However, the first deposition peaks occurs at significantly more positive potentials at the higher chloride concentration. The addition of crystal violet has a marked effect, leading to a substantial negative shift and a broadening of the first deposition peak (fig. 1D). There is also a very marked asymmetry of this voltammetry, with the first upd peak occurring at 110 mV (peak b in fig. lD), while the corresponding stripping peak (peak b’ in fig. 1D) occurs at 290 mV. We have also analysed the charge associated with the peaks a and b in figs. lA-1D and the results are shown in table 1. There is a 10% margin of error in the determination of the charges, due to the difficulty in subtracting a contribution from the capa-
CHEMICAL PHYSICS LETTERS
Volume200,number 4
3 Ic’
a’
I
lpAcm-2
E/mVusCu/Cd+
Fig. 1. Cyclic voltammograms of Au( 11I ) films recorded at 2 mVs-‘in(A) 0.1 MHzS04+I mMCuSO,,andwiththeaddition of (B) 2.5~10~’ M HCl, (C) 10T3 M HCl and (D) 2.5 X 10e5 M crystal violet.
citive current. Relative charge is also presented in table 1 and is defined as relative charge =
charge associated with observed peak charge expected for a full monolayer ’
1I
December1992
The charge expected for a ( 1x 1) monolayer of copper on Au( 111) is 440 pC cm-‘, assuming that copper has an electroadsorption valency of 2. In agreement with previous in situ STM studies [ 71 we have characterized the (,,bx ,,h) R30” copper overlayer structure on our Au( 111) films, which we have imaged in sulphuric acid/copper sulphate electrolytes. This structure, which is clearly associated with peak a in the voltammogram (fig. IA), is stable in “clean” electrolytes over a long time scale, in the potential range between 70 and 200 mV. Fig. 2 shows an STM image taken at 170 mV in 0.1 M H,SO, + 1 mM CuSO,, with the addition of 2.5~ 10m5M HCl. The coexistence of two types of domains can be clearly seen; (fix 6) R30” domains and other more densely packed domains. At this chloride concentration we have consistently observed such joint domain images, in the potential range between the first upd wave (peaks a and b in fig. 1B ) and the second upd wave (peak c ). Such joint domain structures were rather stable over long periods of time (often more than 1 h), although a gradual extension of the more densely packed domains was noticeable, particularly at lower potentials within this range. We have examined the two structures in more detail and representative images are shown in figs. 3a and 3b. The (fixfi)R30” adlayer, with a nearest neighbour separation of 0.49 nm, is comparable to that observed in chloride free electrolytes. However, we have noticed that there is generally a higher defect density in the presence of chloride (fig. 3a), as compared to “clean” electrolytes (however, the more densely packed structure generally appears to have a lower defect density). The more densely packed structure (fig. 3b) has a nearest neighbour separation of 0.37 Z!Z 0.0 1 nm and also a long-range periodicity of between three to four times this distance. This is consistent with either a (5x5) structure (which has a hexagonal lattice, a nearest neighbour separation of 0.36 nm and a long-range periodicity of four lattice constants) or a ( 1.29 x 1.29) structure (which has a quasi-hexagonal lattice, a nearest neighbour separation of 0.37 nm and a long-range periodicity of three to four lattice constants). The long-range periodicity of both of these structures arises because of a mismatch between the adsorbate and substrate lattices (for structural models of the 345
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CHEMICALPHYSICSLETTERS
Table 1 Evaluation of the chargeassociatedwith peaksa and b in voltammograms 1Ato ID for Au( 1I 1) ftis in the listed electrolytes.Seetext for definition of “relative charge” Electrolyte
Figure
Peak(s)
Charge (pCcm_*)
Relative charge
0.1 M H$O, f 1mM CuSO, (A) (A)t2.5x10-SMHCl
1A 1B
a atb
170 200
0.38 0.45
(A) t 1 mM HCl (A)t2.5x10-SMcrystalviolet
1c 1D
b b
280 240
0.64 0.55
prior to recording this image the potential was cycled across the first upd peak (b’ in fig. 1D) to + 56 mV and the potential was then returned to + 158 mV. The Cu overlayer in fig. 4 is consistent with a (5 x 5) or (1.29 x 1.29) structure. This was the only structure which we observed in the potential range from 50 to 250 mV and this structure disappeared at potentials positive of the stripping peak b’ (fig. 1D) . This is clearly in contrast with the results obtained with a comparable concentration of HCl (2.5 x 10m5 M), where joint domains were observed.
4. Discussion
Fig.2.AnSTMimage(22x22nm)ofaAu(lll)filmat170 mV in 0.1 M H2S04 t 1 mM CuSO, with the addition of 2.5x 10ds M HCI. The tip potential was 50 mV and the tunnelIing current 32 nA. This image demonstrates the presence of both ( fi x $)R30” copper domains and also ( 5x 5) domains (or (1.29x 1.29), see text). At this potential such joint domains were observed over long timescales (over 1 h). The arrows show the lattice directions of the underlying substrate.
(5x5) and (1.29x 1.29) adlayers see refs. [7,15], respectively), Since the difference between these two structures is rather small and within the errors of our measurements ( kO.01 nm), we are not able to conclusively decide which of these two structures corresponds to our observed images. Fig. 4 shows an STM image taken at + 158 mV in 0.1 M HzS04 + 1 mM CuS04, with the addition of 2.5 x 10B5 M crystal violet. It should be noted that 346
In agreement with previous studies, it is clear that the ($x fi)R30” structure is the stable structure associated with the voltammetric peak a (fig. 1A), in CuSO,/H,SG, aqueous electrolytes. The addition of chloride to the electrolyte leads to the observation of a second structure, consistent with either a (5 x 5) or ( 1.29X 1.29) copper overlayer. However, it should be noted that ex situ LEED experiments favour the (1.29x1.29) structure [15]. In a previous STM study more densely packed domains (designated as a (5 x 5 ) structure), have been seen to grow slowly into the ($X J?)R30” domains. This was attcibuted to the diffusion-controlled coadsorption of trace chloride contamination ( < 10e6 M). In this study we have observed sizable AX ,,6) domains, which coexist alongside the ( 5 x 5 ) (or ( 1.29~ 1.29) ) domains, over relatively long timescales. The coexistence of these two structures, at a chloride concentration of 10B4M, has also been inferred from ex situ LEED experiments [ 151. The formation of such joint domains correlates well with our voltammetry, since the addition of small amounts of chloride (2.5 x 1Op5
Volume200, number 4
CHEMICALPHYSICSLETTERS
11December 1992
Fig. 3. STM images showingdetails of individual (fix,/‘?)R30” and (5X5) (or (1.29X 1.29)) domains, respectively.The arrows showthe lattice directions of the underlying substrate. The samplewas a Au( I 11)film at 170mV in 0.1 M H$SO,f 1 mM C&O, with the addition of 2.5x lo-” M HCI. In both casesthe tip potential was 30 mV and the tunnelling current 32 nA. (a) An image ( 12x 12) of a (fixfi)R30” copper adlayer.The presenceof chloride leads to less “perfect” (fi~,/?)R30” domains, than those formed in the absence of chloride. (b) The “higher density” domain. This image (8.7 X8.7 nm) clearly showsthe atomic structure as well as the long-rangemodulation of the structure. The uncertainty in measuring the bond length (0.37k 0.01 MI) and the long-rangecorrugation (between three and four lattice constants), makes it diflkult to conclusivelydistinguish betweenthe (5 x 5) or (1.29x 1.29) structures.
M) causes the first deposition peak to split into two partially overlapping peaks (peaks a and b in fig. 1B ) . At the higher chloride concentration of 1 mM, a single peal is once again seen (peak b in fig. lC), and this corresponds with the observation of only the more densely packed overlayer. Consequently, we are able to assign the voltammetric peak a to the (fixfi)R30”structureandpeakbtothe (5x5) (or (1.29~ 1.29)) structure (as labelledin figs. IA1C). Clearly increasing the chloride concentration from 2.5~ 10s5 to 10m3M leads to stabilization of the (5 x 5) (or ( 1.29x 1.29) ) structure as opposed to the ($x ,/?)R30” structure. This implies that there is a strong interaction between adsorbed chloride and copper atoms in the (5 x 5) (or ( 1.29 x 1.29) ) structure. Crystal violet has properties typical of organic dye stuffs which are used in commercial metal plating baths. It has been previously shown that the presence of small quantities of crystal violet in the electrolyte solution markedly influences the deposition of bulk copper [ 141. The present study is concerned with
the effect of this additive on the upd and in particular the respective influence of the organic cation and the chloride counterion. Even at crystal violet concentrations comparable with the lower chloride concentration (2.5 X 10m5 M),onlythe(5~5) (or(1.29~1.29))structurewas observed. There is also a substantial shift of the first upd peak to more negative potentials. These two results show that there is a marked influence of the additive. This action of crystal violet may be intrinsically due to the adsorption of the organic cation, or to enhanced coadsorption of chloride anions induced by adsorbed organic cations. The additive may interact strongly with the uncovered Au( 111) surface and consequently block and inhibit sites for copper deposition. This could explain the significant shift of the first upd peak in the negative direction. Indeed, it has been shown that up to a single monolayer of crystal violet adsorbs strongly on gold, in the potential range from - 600 to + 300 mV [ 16,17 1. The charge analysis of the observed peaks correlates well with the structures observed by STM. The 347
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at the lower chloride concentration a mixture of the ($x,I?)R30” and the higher coverage structure coexists giving a value of the relative charge (0.45) which is intermediate between 0.33 and 0.64.
5. Conclusions This study has demonstrated the value of the combined use of STM and cyclic voltamrnetry in assessing the influence of electrolyte additives on underpotential deposition. Using STM we have been able to determine copper overlayer structures and relate these results to the observed voltammetry. We expect that such a methodology will play an increasing role in fundamental studies of underpotential deposition. Fig.4.AnSTMimage(6.5x6.5nm)ofagoldf~mat158mVin 0. I M H2SOdt 1 mM &SO, with the addition of 2.5x lo-’ M crystal violet. The tip potential was t 20 mV and the tunnelling current 33 nA. This copper adlayer is consistent with either the (5 x 5) or (1.29x 1.29) structure. The arrows show the lattice directions of the underlying substrate,
charge associated with peak a in the chloride free electrolyte is 170 PC cm-’ and this corresponds to a relative charge of 0.38 (see table 1). The relative charge may be directly related to the coverage of copper if there is a negligible contribution to the charge from anionic adsorption and the electroadsorption valency of copper is 2. The measured relative charge (0.38) for peak a is slightly higher than expected for a (fix$)R30” copper overlayer, which has a coverage of 0.33. This discrepancy may arise from a small charge contribution from the adsorption and desorption of HSO: (or SO:-) anions [91. The charge associated with the first deposition peak increases significantly with the addition of either 1 mM HCI or 2.5 x 10m5M crystal violet to 280 PC cmB2 (relative charge 0.64) and 240 NC cm-2 (relative charge 0.55) respectively. These values are consistent with the STM result, that in the presence of either a high chloride concentration or crystal violet only the (5 X 5) or (1.29 X 1.29) adlayer is observed. The (5x 5) structure has a coverage of 0.64 and the ( 1.29 x 1.29) adlayer a coverage of 0.61. In contrast, 348
Acknowledgement We would like to thank the Fachkonferenz Forschung of Schering and also the Deutsche Forschungsgemeinschaft (Sfb 6) for funding part of this research. We are also grateful to Professor J.H. Block (Fritz-Haber-Institut Berlin) and Professor G. Stock (Schering A.G. ) for their encouragement and support of this work.
Note added We have recently learnt of a study in which a (5 x 5) adlayer, similar to our observation, has also been imaged in the presence of crystal violet [ 181.
References [ 1] D.M. Kolb, in: Advances in electrochemistry and electrochemicalengineering,Vol. 11,eds.H. Gerischerand C.W.Tobias (Wiley,New York, 1978) p. 125. [2]K. Jiittner and W.J. Lorenz, 2. Physik. Chem. NF 122 (1980) 163. [3] Y. Nakai, M.S. Zei, D.M. Kolb and G. Lehmpfuhl, Ber. Bunsenges.Physik.Chem. 88 (1984) 340. [4] O.R. Melroy, M.G. Samant, G.L. Borges,J.G. Gordon II, L. Bhun, J.H. White, M.J. Albarelli,M. McMillanand H.D. Abrufia,Langmuir (1988) 728.
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[5 I L. Blum, H.D. Abrufia, J. White, J.G. Gordon II, G.L. Barges, M.G. Samant and O.R. Melroy, J. Chem. Phys. 85 (1986) 6732. [6] A. Tadjeddine, D. Guay, M. Ladoueeur and G. Tourillon, Phys. Rev. Letters 66 (1991) 2235. [ 71 O.M. Magnussen, J. Hotlos, R.J. Nichols, D.M. Kolb and R.J. Behm, Phys. Rev. Letters 64 ( 1990) 2929. [S] S. Manna, PK. Hansma, J. Massie, V. Elings and A.A. Genvith,Sciencc251 (1991) 183. [ 91 T. Hachiya, H. Honbo and K. Itaya, J. Electroanal. Chem. 315 (1991) 275. [ lo] J.W. Schultze and D. Dickertmann, Surface Sci. 54 (1976) 489. [ 11 ]D.M. Kolb, K. Al Jaaf-Golze and M.S. Zei, Dechema Monographien, Vol. 102 (VCH, Weinheim, 1986) p. 53.
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[ 121K.H. Beaooke, Surface Sci. 181 (1987) 145. [ 131W. Haiss, D. Lackey, J.K Sass and K.H. Besocke, J. Chem. Phys. 95 (1991) 2193.
[ 141R.J. Nichols, W. Beckmann, H. Meyer and N. Batina and D.M. Kolb, I. Electroanal. Chem. 330 (1992) 381. I5 ] R. Michael&, Ph.D. TIesis, Freie Wniwrsitit Berlin ( 1991).
161W.N. Hansen, in: Advances in electrochemistry and electrochemical engineering, Vol. 9, ed. R.H. Muller (Wiley, New York, 1973) p. 1. 171 G.J. Hansen and W.N. Hansen, Ber. Bunsenges. Physik. Chem.91 (1987) 317. 181 N. Batina, T. Will and D.M. Kolb, Faraday Discussions Chests. Sot. 94 (1992), in press.
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CHEMICALPHYSICSLETTERS
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A determination of copper overlayer structures on Au ( 111) in the presence of electrolyte additives W. Haiss, D. Lackey, J.K. Sass Fritz-Haber-lnstitut der Max-Planck-Gesellschaji, Faradayweg 4-6, W-1000Berlin 33, Germany
H. Meyer and R.J. Nichols Schering AG, MetalplatingDivision, Postfach 65031 I, W-JO00Berlin 65, Germany
Received2 September 1992;in final form 25 September 1992
Cyclicvoltammetry and scanning tuanelling microscopy@TM) wereused to study the influence of chlorideand crystal violet on the underpotential deposition (upd ) of copperon Au( 111). As previouslyreported,a (3 x fi )R30” copperoverlayerhas been characterizedin the absence of additives and this has been related to the first upd peak. At low chloride concentrations this first upd peak splits into two partially overlapping peaks and a more densely packed copper structure appears alongside the (8 x ,,h )R30” domains. The splitting of this first voltammetric peak disappearsat high chlorideconcentrations and only the more denselypackedcopperoverlayeris observed.The STMimagesare consistent with either a (5 x 5) or ( 1.29XI .29) structure. Crystal violet is a cationic organic dye, with a chloride counterion. It has a marked influence on the voltammetry, with the first upd peak occurring at significantly more negative potentials. Even at a low crystal violet concentration, which correspondedto the lowest chloride concentration (wherejoint domains were always seen), only the (5X 5) (or (1.29X1.29)) structure was observed.This clearly indicates the pronounced effectarising from the adsorption of the organic cation.
1.
Intmduction
The electrochemical formation of the first layer of a metallic deposit on a foreign metal substrate occurs, in many circumstances, at potentials which are more positive than the potential of reversible deposition of the bulk phase [ I ,2 1. This phenomenon is known as underpotential deposition and there has been a great deal of interest in the characterization of the structure of underpotential deposits. One system which has received a great deal of attention is the underpotential deposition of copper on Au ( 1I 1). This is particularly interesting since it has been studied with a variety of surface-structure sensitive techniques [ 3-9 1, as well as with cyclic voltammetry [ 10,111. The voltammetry of copper upd in sulCorrespondence to: D. Lackey,Fritz-Haber-Institut der MaxPlanck-Gesellschaft, Faradayweg 4-6, W-1000 Berlin 33, Germany, and R.J. Nichols,ScheriogAG, MetalplatingDivision, Postfach650311,W-1000Berlin 65, Germany.
phate-containing electrolytes shows two distinct adsorption peaks and it was inferred from such measurements that the copper monolayer formation proceeds through the formation of two ordered adla ers [ IO]. Later, the existence of a (&X J 3)R30° adlayer, at medium copper coverages, was conclusively proved by ex situ electron diffraction experiments [ 3 1. LEED experiments also suggested a ( 1x 1) structure for the full monolayer [ 31. Further evidence for the (1 X 1) structure of the full monolayer was provided by in situ X-ray diffraction studies, with the bond length for the complete monolayer correspondin to that of the Au ( 111) substrate [4]. The (fix $ 3)R30° structure has also been confirmed by Magnussen et al. [ 71 in situ with the STM technique (there are however presently no reports of the ( 1X 1) copper adlayer being resolved with STM, although this structure has been resolved with in situ AM [ $1). However, a second more densely packed structure, not seen previously by ex situ LEED measurements, was seen to slowly de-
0009-2614/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
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velop at electrode potentials between the two deposition ‘waves [ 71. This slow formation of the (5 x 5) structure was assigned to a slow, diffusioncontrolled coadsorption of trace chloride impurities (which were estimated as less than 10e6 M ) [ 71. Indeed, Hachiya et al. [9], who took special precautions to limit the possibility of chloride contamination, only observed the (J?xfi)R30’ structure, which was stable for long periods of time in the entire potential range between 0.18 and 0.1 V (versus SCE). In this study we have characterized the effect of chloride on the upd of copper on Au( 111) , by systematically varying the chloride concentration. We have limited this work to the first upd peaks in the voltammogram, i.e. those peaks which correspond to the “medium coverage” structures. We have directly correlated the effect of chloride on the voltammetry with the copper adlayers observed by STM. We have also studied the influence of crystal violet, which is a cationic organic dye with a chloride counterion (hexamethylpararosaniline chloride). We have addressed the question of whether adsorbed organic cation plays a structure determining role, or whether the influence of this additive arises solely from the adsorbed chloride counterion.
2. Experimental We have modified a beetle-type STM, which was designed by Besocke [ 12 1, so that we were able to conduct measurements in electrolyte environments. Gold films, prepared by the evaporation of k 2 nm of chromium followed by x200 nm of gold onto tempax glass (Berliner Glas KG. ), were used as working electrodes [ i3,14]. Prior to use these samples were carefully annealed to about 800-1000°C and then quenched in ultrapure water. Electrolytes were prepared from doubly distilled water (from a Fison “Cyclon” distillation system) and H2S04 (Merck, suprapur), CuSO,, (Merck, p.a. ), hydrochloric acid (Merck, suprapur) and crystal violet (Merck), which was purified by recrystallization. The electrolytes were not deaerated prior to the experiment and the in situ STM cell was open to air. The reference electrode consisted of a copper wire which was inserted directly into the cell and all electrode
11DecemberI992
potentials are quoted with respect to the Cu/Cu’+ couple. Iridium wire was mechanically ground to form the tunnelling tip and this was subsequently isolated with Apiezon wax.
3. Results The gold films were characterized using atomic force microscopy ( AFM), STM and voltammetry. Prior to the first flame annealing the films possessed a granular structure typical of a sputtered metal deposit. However, the surface morphology is dramatically changed upon flame annealing. The surface restructures producing very flat regions, extending over hundreds of nanometers, which are often separated by deep furrows (as we have seen in a preliminary AF’Minvestigation). We have used atomic resolution STM to characterize these terraced regions and have shown that they are predominantly of ( 111) orientation. The voltammetry of the gold films in 0.1 M HzS04 + 1 mM CuS04 is shown in fig. 1A. Although the peaks are not as sharp as for well defined Au( 111) single crystal discs, the voltammetry is consistent with a sample which has predominantly ( 111) orientation. Cyclic voltammograms with the addition of 2.5 x 10m5M and 1 mM HCl are shown in figs. 1B and 1C, respectively. Notice how the first deposition peak is split into two at the lower chloride concentration (fig. 1B ), while no splitting is observed in both the absence of chloride (fig. 1A) and at the higher concentration (fig, 1C). However, the first deposition peaks occurs at significantly more positive potentials at the higher chloride concentration. The addition of crystal violet has a marked effect, leading to a substantial negative shift and a broadening of the first deposition peak (fig. 1D). There is also a very marked asymmetry of this voltammetry, with the first upd peak occurring at 110 mV (peak b in fig. lD), while the corresponding stripping peak (peak b’ in fig. 1D) occurs at 290 mV. We have also analysed the charge associated with the peaks a and b in figs. lA-1D and the results are shown in table 1. There is a 10% margin of error in the determination of the charges, due to the difficulty in subtracting a contribution from the capa-
CHEMICAL PHYSICS LETTERS
Volume200,number 4
3 Ic’
a’
I
lpAcm-2
E/mVusCu/Cd+
Fig. 1. Cyclic voltammograms of Au( 11I ) films recorded at 2 mVs-‘in(A) 0.1 MHzS04+I mMCuSO,,andwiththeaddition of (B) 2.5~10~’ M HCl, (C) 10T3 M HCl and (D) 2.5 X 10e5 M crystal violet.
citive current. Relative charge is also presented in table 1 and is defined as relative charge =
charge associated with observed peak charge expected for a full monolayer ’
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The charge expected for a ( 1x 1) monolayer of copper on Au( 111) is 440 pC cm-‘, assuming that copper has an electroadsorption valency of 2. In agreement with previous in situ STM studies [ 71 we have characterized the (,,bx ,,h) R30” copper overlayer structure on our Au( 111) films, which we have imaged in sulphuric acid/copper sulphate electrolytes. This structure, which is clearly associated with peak a in the voltammogram (fig. IA), is stable in “clean” electrolytes over a long time scale, in the potential range between 70 and 200 mV. Fig. 2 shows an STM image taken at 170 mV in 0.1 M H,SO, + 1 mM CuSO,, with the addition of 2.5~ 10m5M HCl. The coexistence of two types of domains can be clearly seen; (fix 6) R30” domains and other more densely packed domains. At this chloride concentration we have consistently observed such joint domain images, in the potential range between the first upd wave (peaks a and b in fig. 1B ) and the second upd wave (peak c ). Such joint domain structures were rather stable over long periods of time (often more than 1 h), although a gradual extension of the more densely packed domains was noticeable, particularly at lower potentials within this range. We have examined the two structures in more detail and representative images are shown in figs. 3a and 3b. The (fixfi)R30” adlayer, with a nearest neighbour separation of 0.49 nm, is comparable to that observed in chloride free electrolytes. However, we have noticed that there is generally a higher defect density in the presence of chloride (fig. 3a), as compared to “clean” electrolytes (however, the more densely packed structure generally appears to have a lower defect density). The more densely packed structure (fig. 3b) has a nearest neighbour separation of 0.37 Z!Z 0.0 1 nm and also a long-range periodicity of between three to four times this distance. This is consistent with either a (5x5) structure (which has a hexagonal lattice, a nearest neighbour separation of 0.36 nm and a long-range periodicity of four lattice constants) or a ( 1.29 x 1.29) structure (which has a quasi-hexagonal lattice, a nearest neighbour separation of 0.37 nm and a long-range periodicity of three to four lattice constants). The long-range periodicity of both of these structures arises because of a mismatch between the adsorbate and substrate lattices (for structural models of the 345
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Table 1 Evaluation of the chargeassociatedwith peaksa and b in voltammograms 1Ato ID for Au( 1I 1) ftis in the listed electrolytes.Seetext for definition of “relative charge” Electrolyte
Figure
Peak(s)
Charge (pCcm_*)
Relative charge
0.1 M H$O, f 1mM CuSO, (A) (A)t2.5x10-SMHCl
1A 1B
a atb
170 200
0.38 0.45
(A) t 1 mM HCl (A)t2.5x10-SMcrystalviolet
1c 1D
b b
280 240
0.64 0.55
prior to recording this image the potential was cycled across the first upd peak (b’ in fig. 1D) to + 56 mV and the potential was then returned to + 158 mV. The Cu overlayer in fig. 4 is consistent with a (5 x 5) or (1.29 x 1.29) structure. This was the only structure which we observed in the potential range from 50 to 250 mV and this structure disappeared at potentials positive of the stripping peak b’ (fig. 1D) . This is clearly in contrast with the results obtained with a comparable concentration of HCl (2.5 x 10m5 M), where joint domains were observed.
4. Discussion
Fig.2.AnSTMimage(22x22nm)ofaAu(lll)filmat170 mV in 0.1 M H2S04 t 1 mM CuSO, with the addition of 2.5x 10ds M HCI. The tip potential was 50 mV and the tunnelIing current 32 nA. This image demonstrates the presence of both ( fi x $)R30” copper domains and also ( 5x 5) domains (or (1.29x 1.29), see text). At this potential such joint domains were observed over long timescales (over 1 h). The arrows show the lattice directions of the underlying substrate.
(5x5) and (1.29x 1.29) adlayers see refs. [7,15], respectively), Since the difference between these two structures is rather small and within the errors of our measurements ( kO.01 nm), we are not able to conclusively decide which of these two structures corresponds to our observed images. Fig. 4 shows an STM image taken at + 158 mV in 0.1 M HzS04 + 1 mM CuS04, with the addition of 2.5 x 10B5 M crystal violet. It should be noted that 346
In agreement with previous studies, it is clear that the ($x fi)R30” structure is the stable structure associated with the voltammetric peak a (fig. 1A), in CuSO,/H,SG, aqueous electrolytes. The addition of chloride to the electrolyte leads to the observation of a second structure, consistent with either a (5 x 5) or ( 1.29X 1.29) copper overlayer. However, it should be noted that ex situ LEED experiments favour the (1.29x1.29) structure [15]. In a previous STM study more densely packed domains (designated as a (5 x 5 ) structure), have been seen to grow slowly into the ($X J?)R30” domains. This was attcibuted to the diffusion-controlled coadsorption of trace chloride contamination ( < 10e6 M). In this study we have observed sizable AX ,,6) domains, which coexist alongside the ( 5 x 5 ) (or ( 1.29~ 1.29) ) domains, over relatively long timescales. The coexistence of these two structures, at a chloride concentration of 10B4M, has also been inferred from ex situ LEED experiments [ 151. The formation of such joint domains correlates well with our voltammetry, since the addition of small amounts of chloride (2.5 x 1Op5
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Fig. 3. STM images showingdetails of individual (fix,/‘?)R30” and (5X5) (or (1.29X 1.29)) domains, respectively.The arrows showthe lattice directions of the underlying substrate. The samplewas a Au( I 11)film at 170mV in 0.1 M H$SO,f 1 mM C&O, with the addition of 2.5x lo-” M HCI. In both casesthe tip potential was 30 mV and the tunnelling current 32 nA. (a) An image ( 12x 12) of a (fixfi)R30” copper adlayer.The presenceof chloride leads to less “perfect” (fi~,/?)R30” domains, than those formed in the absence of chloride. (b) The “higher density” domain. This image (8.7 X8.7 nm) clearly showsthe atomic structure as well as the long-rangemodulation of the structure. The uncertainty in measuring the bond length (0.37k 0.01 MI) and the long-rangecorrugation (between three and four lattice constants), makes it diflkult to conclusivelydistinguish betweenthe (5 x 5) or (1.29x 1.29) structures.
M) causes the first deposition peak to split into two partially overlapping peaks (peaks a and b in fig. 1B ) . At the higher chloride concentration of 1 mM, a single peal is once again seen (peak b in fig. lC), and this corresponds with the observation of only the more densely packed overlayer. Consequently, we are able to assign the voltammetric peak a to the (fixfi)R30”structureandpeakbtothe (5x5) (or (1.29~ 1.29)) structure (as labelledin figs. IA1C). Clearly increasing the chloride concentration from 2.5~ 10s5 to 10m3M leads to stabilization of the (5 x 5) (or ( 1.29x 1.29) ) structure as opposed to the ($x ,/?)R30” structure. This implies that there is a strong interaction between adsorbed chloride and copper atoms in the (5 x 5) (or ( 1.29 x 1.29) ) structure. Crystal violet has properties typical of organic dye stuffs which are used in commercial metal plating baths. It has been previously shown that the presence of small quantities of crystal violet in the electrolyte solution markedly influences the deposition of bulk copper [ 141. The present study is concerned with
the effect of this additive on the upd and in particular the respective influence of the organic cation and the chloride counterion. Even at crystal violet concentrations comparable with the lower chloride concentration (2.5 X 10m5 M),onlythe(5~5) (or(1.29~1.29))structurewas observed. There is also a substantial shift of the first upd peak to more negative potentials. These two results show that there is a marked influence of the additive. This action of crystal violet may be intrinsically due to the adsorption of the organic cation, or to enhanced coadsorption of chloride anions induced by adsorbed organic cations. The additive may interact strongly with the uncovered Au( 111) surface and consequently block and inhibit sites for copper deposition. This could explain the significant shift of the first upd peak in the negative direction. Indeed, it has been shown that up to a single monolayer of crystal violet adsorbs strongly on gold, in the potential range from - 600 to + 300 mV [ 16,17 1. The charge analysis of the observed peaks correlates well with the structures observed by STM. The 347
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at the lower chloride concentration a mixture of the ($x,I?)R30” and the higher coverage structure coexists giving a value of the relative charge (0.45) which is intermediate between 0.33 and 0.64.
5. Conclusions This study has demonstrated the value of the combined use of STM and cyclic voltamrnetry in assessing the influence of electrolyte additives on underpotential deposition. Using STM we have been able to determine copper overlayer structures and relate these results to the observed voltammetry. We expect that such a methodology will play an increasing role in fundamental studies of underpotential deposition. Fig.4.AnSTMimage(6.5x6.5nm)ofagoldf~mat158mVin 0. I M H2SOdt 1 mM &SO, with the addition of 2.5x lo-’ M crystal violet. The tip potential was t 20 mV and the tunnelling current 33 nA. This copper adlayer is consistent with either the (5 x 5) or (1.29x 1.29) structure. The arrows show the lattice directions of the underlying substrate,
charge associated with peak a in the chloride free electrolyte is 170 PC cm-’ and this corresponds to a relative charge of 0.38 (see table 1). The relative charge may be directly related to the coverage of copper if there is a negligible contribution to the charge from anionic adsorption and the electroadsorption valency of copper is 2. The measured relative charge (0.38) for peak a is slightly higher than expected for a (fix$)R30” copper overlayer, which has a coverage of 0.33. This discrepancy may arise from a small charge contribution from the adsorption and desorption of HSO: (or SO:-) anions [91. The charge associated with the first deposition peak increases significantly with the addition of either 1 mM HCI or 2.5 x 10m5M crystal violet to 280 PC cmB2 (relative charge 0.64) and 240 NC cm-2 (relative charge 0.55) respectively. These values are consistent with the STM result, that in the presence of either a high chloride concentration or crystal violet only the (5 X 5) or (1.29 X 1.29) adlayer is observed. The (5x 5) structure has a coverage of 0.64 and the ( 1.29 x 1.29) adlayer a coverage of 0.61. In contrast, 348
Acknowledgement We would like to thank the Fachkonferenz Forschung of Schering and also the Deutsche Forschungsgemeinschaft (Sfb 6) for funding part of this research. We are also grateful to Professor J.H. Block (Fritz-Haber-Institut Berlin) and Professor G. Stock (Schering A.G. ) for their encouragement and support of this work.
Note added We have recently learnt of a study in which a (5 x 5) adlayer, similar to our observation, has also been imaged in the presence of crystal violet [ 181.
References [ 1] D.M. Kolb, in: Advances in electrochemistry and electrochemicalengineering,Vol. 11,eds.H. Gerischerand C.W.Tobias (Wiley,New York, 1978) p. 125. [2]K. Jiittner and W.J. Lorenz, 2. Physik. Chem. NF 122 (1980) 163. [3] Y. Nakai, M.S. Zei, D.M. Kolb and G. Lehmpfuhl, Ber. Bunsenges.Physik.Chem. 88 (1984) 340. [4] O.R. Melroy, M.G. Samant, G.L. Borges,J.G. Gordon II, L. Bhun, J.H. White, M.J. Albarelli,M. McMillanand H.D. Abrufia,Langmuir (1988) 728.
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[5 I L. Blum, H.D. Abrufia, J. White, J.G. Gordon II, G.L. Barges, M.G. Samant and O.R. Melroy, J. Chem. Phys. 85 (1986) 6732. [6] A. Tadjeddine, D. Guay, M. Ladoueeur and G. Tourillon, Phys. Rev. Letters 66 (1991) 2235. [ 71 O.M. Magnussen, J. Hotlos, R.J. Nichols, D.M. Kolb and R.J. Behm, Phys. Rev. Letters 64 ( 1990) 2929. [S] S. Manna, PK. Hansma, J. Massie, V. Elings and A.A. Genvith,Sciencc251 (1991) 183. [ 91 T. Hachiya, H. Honbo and K. Itaya, J. Electroanal. Chem. 315 (1991) 275. [ lo] J.W. Schultze and D. Dickertmann, Surface Sci. 54 (1976) 489. [ 11 ]D.M. Kolb, K. Al Jaaf-Golze and M.S. Zei, Dechema Monographien, Vol. 102 (VCH, Weinheim, 1986) p. 53.
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[ 121K.H. Beaooke, Surface Sci. 181 (1987) 145. [ 131W. Haiss, D. Lackey, J.K Sass and K.H. Besocke, J. Chem. Phys. 95 (1991) 2193.
[ 141R.J. Nichols, W. Beckmann, H. Meyer and N. Batina and D.M. Kolb, I. Electroanal. Chem. 330 (1992) 381. I5 ] R. Michael&, Ph.D. TIesis, Freie Wniwrsitit Berlin ( 1991).
161W.N. Hansen, in: Advances in electrochemistry and electrochemical engineering, Vol. 9, ed. R.H. Muller (Wiley, New York, 1973) p. 1. 171 G.J. Hansen and W.N. Hansen, Ber. Bunsenges. Physik. Chem.91 (1987) 317. 181 N. Batina, T. Will and D.M. Kolb, Faraday Discussions Chests. Sot. 94 (1992), in press.
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