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Electroless formation of gold deposits under positively charged surfactant monolayers
Colloids and Surfaces A: Physicochemical and Engineering Aspects 198– 200 (2002) 401– 407 www.elsevier.com/locate/colsurfa
Electroless formation of gold deposits under positively charged surfactant monolayers Serge Ravaine *, Raphae¨l Saliba, Christophe Mingotaud, Franc¸oise Argoul Centre de Recherche Paul Pascal, C.N.R.S. (CRPP-CNRS), A6enue A. Schweitzer, F-33600 Pessac, France Received 30 August 2000; accepted 22 January 2001
Abstract The electroless formation of gold deposits having a high two-dimensional (2D) character along the surface of aqueous hydrogen tetrachloroaurate solutions coated by a positively charged dimethyldioctadecylammonium (DODA) monolayer is reported. Chronopotentiometry experiments have established the importance of the organization of the DODA molecules at the gas/liquid interface on the formation of the deposits. The addition of chloride anions in the subphase induces a strong modification of the morphology of the gold deposits, from compact structures to finely, ramified patterns. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Gold deposits; Dimethyldioctadecylammonium; Monolayers
1. Introduction Electrochemical deposition of metals plays a fundamental role in several contexts: pure metals recovery, electroplating, electrogrowth, electroshaping, e.g. [1 – 3]. In the last 10 years, a lot of work has dealt with the electrodepostition of metallic aggregates in a quasi two-dimensional (2D) geometry [4 – 17]. These investigations have been prompted by the development of the diffusion-limited aggregation (DLA) model [18] and have been devoted to the understanding of the different transport mechanisms and their morphological consequences. Most of these studies used a * Corresponding author. Tel.: + 33-5-5684-5667; fax: + 335-5684-5600. E-mail address: [email protected] (S. Ravaine).
thin cell geometry, where the deposit was confined to the narrow gap between two plates. Only a few studies investigated the electrodeposition of metals at a fluid interface between two different media [19 –25]. Zhao and Fendler first used organized monolayers of negatively charged surfactants as a template for the electrochemical generation of silver particulate films [26]. We present here a study of the electroless deposition of gold films under organized monolayers of dimethyldioctadecylammonium (DODA) spread at the surface of aqueous solutions of hydrogen tetrachloroaurate. Chronopotentiometry experiments demonstrate the influence of the molecular organization at the gas/liquid interface on the oxydoreduction processes involved in the initial formation of the gold deposits. We also study the influence of the addition of chloride anions in the
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subphase on the growth of the metallic deposits. We show that increasing the Cl− concentration induces a modification of their morphology, from compact patterns to highly ramified ones. A careful optical observation of the growth of the latter ones reveals that they result from the aggregation of particles at the gas– liquid interface.
2. Experimental section
2.1. Reagents and materials Hydrogen tetrachloroaurate, hydrogen dihexadecylphosphate (DHP), zinc sulfate, copper sulfate, potassium chloride, hydrogen chloride, sodium sulphate, octadecyl-trichlorosilane and stearic acid were purchased from Aldrich and DODA (99%) from Kodak. Copper (0.25 mm), zinc (0.5 mm) and platinum (0.2 mm) wires were purchased from Goodfellow (purity higher than 99.99%). A Millipore purification system produced water with a resistivity higher than 18 MV cm.
2.2. Instrumentation and procedures Langmuir experiments were carried out under nitrogen using a commercial KSV mini trough equipped with an optical quartz window at the bottom and illuminated from above with a KL 1500 halogen lamp (Schott; 150 W). The growing structure was observed from below through a microscope. A Hamamatsu CCD video camera was used to monitor the progress of the growth of the deposits. Images were recorded and subsequently analyzed on a Power Macintosh 8200/120 computer using the public domain NIH Image program (available on the Internet at http:// rsb.info.nih.gov/nih-image/). Spreading solutions were prepared from HPLC grade chloroform (Prolabo) and were kept at −18 °C between experiments to limit solvent evaporation. An appropriate amount of the surfactant solution was carefully spread onto HAuCl4 aqueous solutions (concentrations ranging from 10 − 5 to 10 − 3 M, pH varying from 4.7 to 3.1, respectively), and the spreading solvent was
allowed to evaporate for 10 min prior to compression. Surface pressure was measured with a platinum Wilhelmy plate suspended to a KSV microbalance. The Langmuir monolayers were compressed at 209 1 °C using a continuous barrier speed of 2 A, 2 mol − 1 min − 1. The electrode was brought into contact with the liquid surface, as its tip just touched the gas/liquid interface, using a motorized micropositioner (M150 series, Physik Instrumente). The gold deposits were transferred onto glass slides by horizontal lifting through the air/liquid interface for further characterizations by scanning electron microscopy (SEM) and XPS. Care was taken not to disrupt their structure. Scanning electron microscopy analyses were conducted on a JEOL GSM 840 A microscope. XPS measurements were performed with a VG 220 i.XL ESCALAB spectrometer. All spectra were taken using a monochromatized Al Ka source at 1486.6 eV. The spot size was approximatively 250 mm. Typical operating pressure was 10 − 8 Pa. An Autolab PGSTAT 20 potentiostat/galvanostat from Eco Chemie, computer controlled by the General Purpose Electrochemical System software, was used to perform all electrochemical experiments. The zinc (or copper) plating experiments were done according to a previously published procedure [27], in a three-electrode conventional cell, at the ambient laboratory temperature (209 1 °C). The working electrode was a platinum electrode sealed in a glass tube. Before each experiment, the electrode was polished to mirror finish with a diamond suspension (grade down to 0.25 mm). After polishing, the electrode was gently washed with water and acetone. The outer glass tube was made hydrophobic by silanization [28]. The reference and the counter electrodes were a saturated calomel electrode (SCE) and a 0.8 mm diameter platinum wire, respectively. The plating solution contained 0.5 M zinc sulphate (or copper sulphate) and 0.5 M sodium sulphate. As oxygen causes corrosion of the zinc films, nitrogen was bubbled through the solutions for ten minutes before and during the course of the experiments. After plating, the working electrode was rinsed with water and ace-
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tone and then transferred to the experimental trough. These operations were carried out quickly to minimize oxygen contamination. The plating efficiency for zinc deposition was determined chronopotentiometrically. After depositing for 100 s, the plating current was reversed, thus causing the zinc film to dissolve anodically. While zinc metal remains on the inert electrode, the dissolution potential is approximately constant. But when all the zinc is dissolved, the electrode potential increases rapidly. The time, ts, between the reversal of the current and the sharp increase in potential is then related to the current efficiency by: p=(ts/100). Averaging over 20 measurements, the plating efficiency, p, was found to be 0.979 0.01.
3. Results and discussion
3.1. Monolayers at the gas/liquid interface Fig. 1 shows compression isotherms of DODA as a function of the HAuCl4 subphase concentration. We observe that the presence of the gold ions induces a shift of the isotherms toward smaller areas per molecule. Moreover, when the concentration is equal or superior to 10 − 4 M, a drastic decrease of the collapse pressure is observed. Such phenomena have already been observed [29,30] and demonstrate that electrostatic
Fig. 1. Compression isotherms of DODA on (a) pure water and on (b) 10 − 5 M, (c) 10 − 4 M, (d) 5 10 − 4 M or 10 − 3 M HAuCl4 solution at 20 °C.
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interactions occur between the gold anions and the positively charged monolayer.
3.2. Effect of the nature and the surface density of the surfactant molecules Bringing into contact a copper or zinc electrode with an aqueous solution of tetrachloroaurate such that its tip just touches the liquid induces the formation of a dense, shiny, metallic deposit along the gas/liquid interface. XPS analyses reveal that the elements present in the deposits are C, O, Cl, Au and Cu in the case of a copper electrode. The Au 4f5/2 (88 eV) and Au 4f7/2 (84 eV) spin doublet is always clearly observed, indicating the presence of metallic gold. The presence of copper found in the metal deposit (in an ionized state), suggests some trapping of the copper ions (due to the dissolution of the electrode) during the deposition. As shown in Fig. 2, the metallic aggregate grows concentrically, forming larger and larger disks. Similar experiments carried out in the absence of any surfactant or after spreading of stearic acid or hydrogen dihexadecylphosphate only produced 3D, black and irregular deposits around the electrode. These observations confirm those reported by Fendler et al. [25,26] concerning the elaboration of silver nanoparticulate films. Following the point of view of Zeiri et al. [21], one can propose that the gold ions adsorbed under the DODA monolayer may first be reduced to form of a ‘seed’-deposit along the interface which would then induce a thin, surface-parallel growth of the metal deposit. The effect of the surface density of the DODA molecules on the formation of the gold deposit was investigated by chronopotentiometry. In fact, we have measured the dissolution rate of a known amount of zinc plated onto a platinum electrode which is carefully positioned at the surface of a HAuCl4 aqueous solution covered by a DODA monolayer. The potential of the plated electrode, which corresponds to the mixed potential characteristic of the zinc dissolution, remains unchanged until all zinc has been consumed. At this point, the potential of the inert electrode rises rapidly to some equilibrium potential. A typical chronopotentiogram for the dissolution of zinc can there-
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Fig. 2. Growth of a gold deposit from a 10 − 4 M HAuCl4 solution coated by a DODA monolayer at 70 A, 2 mol − 1. The pictures were taken every 450 s. Electrode: Cu. Scale bar: 0.25 mm.
fore be described as an initial potential plateau followed by a sharp potential rise to a second plateau (see Fig. 3). The dissolution time td is taken to be the time at the half way between the two plateaus [27]. Then, the average dissolution rate, 6, can be calculated from the equation: 6=
pIptp nFAtd
where p is the plating efficiency, Ip is the plating current, tp is the plating time, n is the number of electrons transferred in the plating reaction, F is the Faraday constant and A the electrode area. Plot of the reaction rate as a function of the DODA molecular area is shown in Fig. 4.
It shows that the zinc dissolution is activated by the compression of DODA molecules at the gas/ liquid interface. Following the basic idea of the neutralization of the positive charge density of the organic monolayer by a 2D adsorption of the electroactive anions along the interface, one can assume that compressing the surfactant molecules induces an increase of the number of gold anions associated with the monolayer, and therefore an increase of the number of zinc atoms which can be oxidized per unit of time. Similar results were obtained using a copper electrode even if they were less reproducible, maybe because of the worse reductive character of copper, which induced longer dissolution experiments.
S. Ra6aine et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 401–407
Fig. 3. Chronopotentiograms of zinc dissolution from a 10 − 3 M HAuCl4 solution: (a) covered by a DODA monolayer at 97 A, 2 mol − 1; (b) with a clean surface.
3.3. Dependence of the morphology of the deposits on the presence of chloride ions in the subphase The addition of potassium chloride in the subphase significantly modified the morphology of the gold deposits. As shown in Fig. 5, these ones are ramified structures with an outer boundary that remains circularly symmetric during the entire growth period. Experiments carried out in presence of hydrogen chloride instead of potassium chloride gave similar results, indicating the crucial role played the Cl− ions during the deposition. Meanwhile, the pH of the subphase (in the range 1–3.1) does not seem to have any profound effect on the morphology of the deposits. A care-
405
ful observation of the growth of the ramified structures allows us to note that microparticles first form around the electrode. These objects aggregate at the gas/liquid interface to form large stringy clusters, which diffuse and finally irreversibly stick to one of the growing branches. Scanning electron microscopy experiments have confirmed that the intimate structure of the deposits is constituted of aggregated particles, which size ranged from 0.3 to 1 mm. The exact mechanisms which govern such a growth remain unclear. Nevertheless, one may assume that the reduction of AuCl− 4 , which takes place at the surface of the electrode, induces the formation of particles which are expelled towards the bulk solution. These objects should present an excess of negative charges on their surface due to the large amount of anions in the subphase. Some of the particles can be trapped by surface-tension forces at the gas–liquid interface by the ‘flotation’ mechanism, where they aggregate to form stringy clusters. This aggregation process should be related to the study of Hurd and Schaefer, who investigated the aggregation of 0.3 mm silica microspheres at the air– water interface [31]. These authors observed irreversible clustering and attributed the bonding to van der Waals forces. The final sticking of the clusters to the growing deposit may be partially explained by attractive capillary forces, which may become significant as the size of the clusters increases.
4. Conclusions
Fig. 4. Plots of the surface pressure (continuous line) and the dissolution rate of a zinc electrode ( ) against surface areas of a DODA monolayer on a 10 − 3 M HAuCl4 solution.
The careful positioning of a copper or zinc electrode at the surface of aqueous HAuCl4 solutions coated by a DODA monolayer results in the electroless formation of gold deposits with a high 2D character. The crucial role of the surfactant molecules has been shown since their organization at the gas/liquid interface strongly influences the dissolution rate of the zinc (or copper) electrode. Highly ramified metallic deposits have been generated by adding chloride ions in the subphase. Our present aim concerns the complete comprehension of the mechanisms occurring during the deposition. We are therefore conducting a new series of
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Fig. 5. Growth of a gold deposit from a 10 − 3 M HAuCl4 +0.25 M Cl− solution. The pictures were taken every 2 min. Electrode: Cu. Scale bar: 0.1 mm.
experiments, varying the nature and the concentration of the additive in the subphase.
References [1] J.O.’M. Bockris, A.K.N. Reddy, Modern Electrochemistry, vol. 2, Plenum, New York, 1977. [2] B.E. Conway, J.O.’M. Bockris, in: A.R. Despic, K.I. Popov (Eds.), Modern Electrochemistry, vol. 7, Plenum, New York, 1972. [3] C.J. Hurle, in: P. Ramasany (Ed.), Handbook of Crystal Growth, vol. 1a, North-Holland, Amsterdam, 1993.
[4] D. Grier, E. Ben-Jacob, R. Clarke, L.M. Sander, Phys. Rev. Lett. 56 (1986) 1264. [5] Y. Sawada, A. Dougherty, J.P. Gollub, Phys. Rev. Lett. 56 (1986) 1260. [6] O. Zik, Phys. A 224 (1996) 338. [7] F. Argoul, E. Freysz, A. Kuhn, C. Leger, Phys. Rev. E 53 (1996) 1777. [8] A. Kuhn, F. Argoul, J. Electroanal. Chem. 371 (1994) 93. [9] A. Kuhn, F. Argoul, Phys. Rev. E 49 (1994) 4298. [10] J.M. Costa, F. Sagues, M. Vilarrasa, Phys. Rev. A 43 (1991) 7057. [11] M.-Q. Lo`pez-Salvans, P.P. Trigueros, S. Vallmitjana, J. Claret, F. Sagues, Phys. Rev. Lett. 76 (1996) 4062. [12] J. De Bruyn, Phys. Rev. Lett. 74 (1995) 4843.
S. Ra6aine et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 401–407 [13] S.N. Atchison, R.P. Burford, C.P. Whitby, D.B. Hibbert, J. Electroanal. Chem. 399 (1995) 71. [14] D.P. Barkey, D. Watt, Z. Liu, S. Raber, J. Electrochem. Soc. 141 (1994) 1206. [15] V. Fleury, J.-N. Chazalviel, M. Rosso, Phys. Rev. Lett. 68 (1992) 2492. [16] P. Garik, D. Barkey, E. Ben-Jacob, E. Bochner, N. Broxholm, B. Miller, B. Orr, R. Zamir, Phys. Rev. Lett. 62 (1989) 2703. [17] D.B. Hibbert, J.R. Melrose, Phys. Rev. A 38 (1988) 1036. [18] T.A. Witten, L.M. Sander, Phys. Rev. Lett. 47 (1981) 1400. [19] O. Younes, L. Zeiri, S. Efrima, M. Deutsch, Langmuir 13 (1997) 1767. [20] L. Zeiri, S. Efrima, M. Deutsch, Langmuir 12 (1996) 5180. [21] L. Zeiri, O. Younes, S. Efrima, M. Deutsch, J. Phys. Chem. B 101 (1997) 9299. [22] S. Nakabayashi, R. Aogaki, A. Karantonis, U. Iguchi, K. Ushida, M. Nawa, J. Electroanal. Chem. 473 (1999) 54.
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[23] Z. Tai, G. Zhang, X. Qian, S. Xiao, Z. Lu, Y. Wei, Langmuir 9 (1993) 1601. [24] S. Ravaine, C. Breton, C. Mingotaud, F. Argoul, Mater. Sci. Eng. C 8-9 (1999) 437. [25] N.A. Kotov, M.E. Zaniquelli, F.C. Meldrum, J.H. Fendler, Langmuir 9 (1993) 3710. [26] X.K. Zhao, J.H. Fendler, J. Phys. Chem. 94 (1990) 3384. [27] J. Zheng, M. Khan, S.R. La Brooy, M. Ritchie, P. Singh, J. Appl. Electrochem. 26 (1996) 509. [28] A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir – Blodgett to Self-Assembly, Academic Press, New York, 1991, p. 109. [29] C. Mingotaud, C. Lafuente, J. Amiell, P. Delhaes, Langmuir 15 (1999) 289. [30] M. Clemente-Leon, B. Agricole, C. Mingotaud, C.J. Gomez-Garcia, E. Coronado, P. Delhaes, Langmuir 13 (1997) 2340. [31] A.J. Hurd, D.W. Schaefer, Phys. Rev. Lett. 54 (1985) 1043.
Electroless formation of gold deposits under positively charged surfactant monolayers Serge Ravaine *, Raphae¨l Saliba, Christophe Mingotaud, Franc¸oise Argoul Centre de Recherche Paul Pascal, C.N.R.S. (CRPP-CNRS), A6enue A. Schweitzer, F-33600 Pessac, France Received 30 August 2000; accepted 22 January 2001
Abstract The electroless formation of gold deposits having a high two-dimensional (2D) character along the surface of aqueous hydrogen tetrachloroaurate solutions coated by a positively charged dimethyldioctadecylammonium (DODA) monolayer is reported. Chronopotentiometry experiments have established the importance of the organization of the DODA molecules at the gas/liquid interface on the formation of the deposits. The addition of chloride anions in the subphase induces a strong modification of the morphology of the gold deposits, from compact structures to finely, ramified patterns. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Gold deposits; Dimethyldioctadecylammonium; Monolayers
1. Introduction Electrochemical deposition of metals plays a fundamental role in several contexts: pure metals recovery, electroplating, electrogrowth, electroshaping, e.g. [1 – 3]. In the last 10 years, a lot of work has dealt with the electrodepostition of metallic aggregates in a quasi two-dimensional (2D) geometry [4 – 17]. These investigations have been prompted by the development of the diffusion-limited aggregation (DLA) model [18] and have been devoted to the understanding of the different transport mechanisms and their morphological consequences. Most of these studies used a * Corresponding author. Tel.: + 33-5-5684-5667; fax: + 335-5684-5600. E-mail address: [email protected] (S. Ravaine).
thin cell geometry, where the deposit was confined to the narrow gap between two plates. Only a few studies investigated the electrodeposition of metals at a fluid interface between two different media [19 –25]. Zhao and Fendler first used organized monolayers of negatively charged surfactants as a template for the electrochemical generation of silver particulate films [26]. We present here a study of the electroless deposition of gold films under organized monolayers of dimethyldioctadecylammonium (DODA) spread at the surface of aqueous solutions of hydrogen tetrachloroaurate. Chronopotentiometry experiments demonstrate the influence of the molecular organization at the gas/liquid interface on the oxydoreduction processes involved in the initial formation of the gold deposits. We also study the influence of the addition of chloride anions in the
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S. Ra6aine et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 401–407
subphase on the growth of the metallic deposits. We show that increasing the Cl− concentration induces a modification of their morphology, from compact patterns to highly ramified ones. A careful optical observation of the growth of the latter ones reveals that they result from the aggregation of particles at the gas– liquid interface.
2. Experimental section
2.1. Reagents and materials Hydrogen tetrachloroaurate, hydrogen dihexadecylphosphate (DHP), zinc sulfate, copper sulfate, potassium chloride, hydrogen chloride, sodium sulphate, octadecyl-trichlorosilane and stearic acid were purchased from Aldrich and DODA (99%) from Kodak. Copper (0.25 mm), zinc (0.5 mm) and platinum (0.2 mm) wires were purchased from Goodfellow (purity higher than 99.99%). A Millipore purification system produced water with a resistivity higher than 18 MV cm.
2.2. Instrumentation and procedures Langmuir experiments were carried out under nitrogen using a commercial KSV mini trough equipped with an optical quartz window at the bottom and illuminated from above with a KL 1500 halogen lamp (Schott; 150 W). The growing structure was observed from below through a microscope. A Hamamatsu CCD video camera was used to monitor the progress of the growth of the deposits. Images were recorded and subsequently analyzed on a Power Macintosh 8200/120 computer using the public domain NIH Image program (available on the Internet at http:// rsb.info.nih.gov/nih-image/). Spreading solutions were prepared from HPLC grade chloroform (Prolabo) and were kept at −18 °C between experiments to limit solvent evaporation. An appropriate amount of the surfactant solution was carefully spread onto HAuCl4 aqueous solutions (concentrations ranging from 10 − 5 to 10 − 3 M, pH varying from 4.7 to 3.1, respectively), and the spreading solvent was
allowed to evaporate for 10 min prior to compression. Surface pressure was measured with a platinum Wilhelmy plate suspended to a KSV microbalance. The Langmuir monolayers were compressed at 209 1 °C using a continuous barrier speed of 2 A, 2 mol − 1 min − 1. The electrode was brought into contact with the liquid surface, as its tip just touched the gas/liquid interface, using a motorized micropositioner (M150 series, Physik Instrumente). The gold deposits were transferred onto glass slides by horizontal lifting through the air/liquid interface for further characterizations by scanning electron microscopy (SEM) and XPS. Care was taken not to disrupt their structure. Scanning electron microscopy analyses were conducted on a JEOL GSM 840 A microscope. XPS measurements were performed with a VG 220 i.XL ESCALAB spectrometer. All spectra were taken using a monochromatized Al Ka source at 1486.6 eV. The spot size was approximatively 250 mm. Typical operating pressure was 10 − 8 Pa. An Autolab PGSTAT 20 potentiostat/galvanostat from Eco Chemie, computer controlled by the General Purpose Electrochemical System software, was used to perform all electrochemical experiments. The zinc (or copper) plating experiments were done according to a previously published procedure [27], in a three-electrode conventional cell, at the ambient laboratory temperature (209 1 °C). The working electrode was a platinum electrode sealed in a glass tube. Before each experiment, the electrode was polished to mirror finish with a diamond suspension (grade down to 0.25 mm). After polishing, the electrode was gently washed with water and acetone. The outer glass tube was made hydrophobic by silanization [28]. The reference and the counter electrodes were a saturated calomel electrode (SCE) and a 0.8 mm diameter platinum wire, respectively. The plating solution contained 0.5 M zinc sulphate (or copper sulphate) and 0.5 M sodium sulphate. As oxygen causes corrosion of the zinc films, nitrogen was bubbled through the solutions for ten minutes before and during the course of the experiments. After plating, the working electrode was rinsed with water and ace-
S. Ra6aine et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 401–407
tone and then transferred to the experimental trough. These operations were carried out quickly to minimize oxygen contamination. The plating efficiency for zinc deposition was determined chronopotentiometrically. After depositing for 100 s, the plating current was reversed, thus causing the zinc film to dissolve anodically. While zinc metal remains on the inert electrode, the dissolution potential is approximately constant. But when all the zinc is dissolved, the electrode potential increases rapidly. The time, ts, between the reversal of the current and the sharp increase in potential is then related to the current efficiency by: p=(ts/100). Averaging over 20 measurements, the plating efficiency, p, was found to be 0.979 0.01.
3. Results and discussion
3.1. Monolayers at the gas/liquid interface Fig. 1 shows compression isotherms of DODA as a function of the HAuCl4 subphase concentration. We observe that the presence of the gold ions induces a shift of the isotherms toward smaller areas per molecule. Moreover, when the concentration is equal or superior to 10 − 4 M, a drastic decrease of the collapse pressure is observed. Such phenomena have already been observed [29,30] and demonstrate that electrostatic
Fig. 1. Compression isotherms of DODA on (a) pure water and on (b) 10 − 5 M, (c) 10 − 4 M, (d) 5 10 − 4 M or 10 − 3 M HAuCl4 solution at 20 °C.
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interactions occur between the gold anions and the positively charged monolayer.
3.2. Effect of the nature and the surface density of the surfactant molecules Bringing into contact a copper or zinc electrode with an aqueous solution of tetrachloroaurate such that its tip just touches the liquid induces the formation of a dense, shiny, metallic deposit along the gas/liquid interface. XPS analyses reveal that the elements present in the deposits are C, O, Cl, Au and Cu in the case of a copper electrode. The Au 4f5/2 (88 eV) and Au 4f7/2 (84 eV) spin doublet is always clearly observed, indicating the presence of metallic gold. The presence of copper found in the metal deposit (in an ionized state), suggests some trapping of the copper ions (due to the dissolution of the electrode) during the deposition. As shown in Fig. 2, the metallic aggregate grows concentrically, forming larger and larger disks. Similar experiments carried out in the absence of any surfactant or after spreading of stearic acid or hydrogen dihexadecylphosphate only produced 3D, black and irregular deposits around the electrode. These observations confirm those reported by Fendler et al. [25,26] concerning the elaboration of silver nanoparticulate films. Following the point of view of Zeiri et al. [21], one can propose that the gold ions adsorbed under the DODA monolayer may first be reduced to form of a ‘seed’-deposit along the interface which would then induce a thin, surface-parallel growth of the metal deposit. The effect of the surface density of the DODA molecules on the formation of the gold deposit was investigated by chronopotentiometry. In fact, we have measured the dissolution rate of a known amount of zinc plated onto a platinum electrode which is carefully positioned at the surface of a HAuCl4 aqueous solution covered by a DODA monolayer. The potential of the plated electrode, which corresponds to the mixed potential characteristic of the zinc dissolution, remains unchanged until all zinc has been consumed. At this point, the potential of the inert electrode rises rapidly to some equilibrium potential. A typical chronopotentiogram for the dissolution of zinc can there-
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Fig. 2. Growth of a gold deposit from a 10 − 4 M HAuCl4 solution coated by a DODA monolayer at 70 A, 2 mol − 1. The pictures were taken every 450 s. Electrode: Cu. Scale bar: 0.25 mm.
fore be described as an initial potential plateau followed by a sharp potential rise to a second plateau (see Fig. 3). The dissolution time td is taken to be the time at the half way between the two plateaus [27]. Then, the average dissolution rate, 6, can be calculated from the equation: 6=
pIptp nFAtd
where p is the plating efficiency, Ip is the plating current, tp is the plating time, n is the number of electrons transferred in the plating reaction, F is the Faraday constant and A the electrode area. Plot of the reaction rate as a function of the DODA molecular area is shown in Fig. 4.
It shows that the zinc dissolution is activated by the compression of DODA molecules at the gas/ liquid interface. Following the basic idea of the neutralization of the positive charge density of the organic monolayer by a 2D adsorption of the electroactive anions along the interface, one can assume that compressing the surfactant molecules induces an increase of the number of gold anions associated with the monolayer, and therefore an increase of the number of zinc atoms which can be oxidized per unit of time. Similar results were obtained using a copper electrode even if they were less reproducible, maybe because of the worse reductive character of copper, which induced longer dissolution experiments.
S. Ra6aine et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 401–407
Fig. 3. Chronopotentiograms of zinc dissolution from a 10 − 3 M HAuCl4 solution: (a) covered by a DODA monolayer at 97 A, 2 mol − 1; (b) with a clean surface.
3.3. Dependence of the morphology of the deposits on the presence of chloride ions in the subphase The addition of potassium chloride in the subphase significantly modified the morphology of the gold deposits. As shown in Fig. 5, these ones are ramified structures with an outer boundary that remains circularly symmetric during the entire growth period. Experiments carried out in presence of hydrogen chloride instead of potassium chloride gave similar results, indicating the crucial role played the Cl− ions during the deposition. Meanwhile, the pH of the subphase (in the range 1–3.1) does not seem to have any profound effect on the morphology of the deposits. A care-
405
ful observation of the growth of the ramified structures allows us to note that microparticles first form around the electrode. These objects aggregate at the gas/liquid interface to form large stringy clusters, which diffuse and finally irreversibly stick to one of the growing branches. Scanning electron microscopy experiments have confirmed that the intimate structure of the deposits is constituted of aggregated particles, which size ranged from 0.3 to 1 mm. The exact mechanisms which govern such a growth remain unclear. Nevertheless, one may assume that the reduction of AuCl− 4 , which takes place at the surface of the electrode, induces the formation of particles which are expelled towards the bulk solution. These objects should present an excess of negative charges on their surface due to the large amount of anions in the subphase. Some of the particles can be trapped by surface-tension forces at the gas–liquid interface by the ‘flotation’ mechanism, where they aggregate to form stringy clusters. This aggregation process should be related to the study of Hurd and Schaefer, who investigated the aggregation of 0.3 mm silica microspheres at the air– water interface [31]. These authors observed irreversible clustering and attributed the bonding to van der Waals forces. The final sticking of the clusters to the growing deposit may be partially explained by attractive capillary forces, which may become significant as the size of the clusters increases.
4. Conclusions
Fig. 4. Plots of the surface pressure (continuous line) and the dissolution rate of a zinc electrode ( ) against surface areas of a DODA monolayer on a 10 − 3 M HAuCl4 solution.
The careful positioning of a copper or zinc electrode at the surface of aqueous HAuCl4 solutions coated by a DODA monolayer results in the electroless formation of gold deposits with a high 2D character. The crucial role of the surfactant molecules has been shown since their organization at the gas/liquid interface strongly influences the dissolution rate of the zinc (or copper) electrode. Highly ramified metallic deposits have been generated by adding chloride ions in the subphase. Our present aim concerns the complete comprehension of the mechanisms occurring during the deposition. We are therefore conducting a new series of
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Fig. 5. Growth of a gold deposit from a 10 − 3 M HAuCl4 +0.25 M Cl− solution. The pictures were taken every 2 min. Electrode: Cu. Scale bar: 0.1 mm.
experiments, varying the nature and the concentration of the additive in the subphase.
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