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Electrochemical Preparation of Thiol-Coated Silver Nanostructures on Highly Oriented Pyrolytic Graphite Dorota Romanska and Maciej Mazur* Laboratory of Electrochemistry, Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland Received October 10, 2002. In Final Form: January 13, 2003 We present the study of electrochemical deposition of silver on highly oriented pyrolytic graphite from the aqueous solution containing 3-mercaptopropionic acid. The investigations were conducted by electrochemical methods and scanning tunneling microscopy. It was shown that the presence of 3-mercaptopropionic acid in the solution strongly influences the deposition process of Ag. The immediate adsorption of the thiol on the deposited metal results in formation of nanosized silver islands distributed on the graphite surface. These islands consist of sets of nanorods that are 3-8 nm in diameter and 50-250 nm in length.
Introduction The ability to generate structures with submicrometer feature sizes is central to modern science and technology. Achieving control over the size, shape, and organization of synthetic materials may lead to superior material properties. This might find important applications in optical and electronic devices such as optical filters, switches, and sensors.1 Among many novel nanosized materials, metal nanoparticles have attracted considerable attention recently. These include spherical metal clusters2 as well as one-dimensional structures with nanometer diameter, e.g., nanorods3 and nanotubes.4 The nanostructures can be prepared by generally two approaches: by template synthesis or by self-assembly methods. The first approach entails heterogeneous synthesis of a desired material within the structures, e.g., pores or channels of a template that determine the shape, size, and, in many cases, the orientation of the produced particle. For instance polycarbonate5 or porous alumina membranes6 or carbon nanotubes7 were used to fabricate * To whom correspondence may be addressed. E-mail: mmazur@ chem.uw.edu.pl. (1) Brust, M.; Kiely, C. J. Colloids Surf., A 2002, 202, 175. (2) (a) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Calis, G. H. M.; van der Felden, J. W. A. Chem. Ber. 1981, 114, 3634. (b) Schmid, G.; Lehnert, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 780. (c) Schmid, G.; Lehnert, A.; Kreibig, U.; Adamczyk, Z.; Belouschek, P. Z. Naturforsch. 1990, 45, 989. (d) Schmid, G., Ed. Clusters and Colloids; VCH: Weinheim, 1994. (e) Schmid, G. J. Chem. Soc., Dalton Trans. 1998, 1077. (3) (a) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (b) Zach, M. P.; Ng K. H.; Penner, R. M. Science 2000, 290, 2120. (c) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (d) Mohamed, M. B.; Ismail, K. Z.; Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 9370. (4) (a) Yu, S. F.; Lee, S. B.; Kang, M.; Martin, C. R. Nano Lett. 2001, 1, 495. (b) Lee, S. B.; Martin, C. R. Chem. Mater. 2001, 13, 3236. (c) Martin, C. R.; Nishizawa, M.; Jirage, K.; Kang, M. S.; Lee, S. B. Adv. Mater. 2001, 13, 1351. (d) Kang, M. S.; Martin, C. R. Langmuir 2001, 17, 2753. (e) Martin, C. R.; Nishizawa, M.; Jirage, K.; Kang, M. J. Phys. Chem. B 2001, 105, 1925. (f) Jirage, K. B.; Hulteen, J. C.; Martin, C. R. Anal. Chem. 1999, 71, 4913. (5) Schonenberger, C.; van der Zande, B. M. I.; Fokkink, L. G. J.; Henny, M.; Schmid, C.; Kruger, M.; Bachtold, A.; Huber, R.; Birk, H.; Stauder, U. J. Phys. Chem. B 1997, 101, 5497. (6) (a) Foss, C. A., Jr.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 98, 2963. (b) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075. (c) Al-Rawashdeh, N. A. F.; Sandrock, M. L.; Seugling, C. J.; Foss, A., Jr. J. Phys. Chem. B 1998, 102, 361. (d) van der Zande, B. M. I.; Bohmer, M. R.; Fokkink, L. G. J.; Schonenberger, C. J. Phys. Chem. B 1997, 101, 852.
metal nanosized tubules or wires. The self-assembly route usually employs the use of stabilizers which control the growth of the material by adsorption on its surface. A variety of molecules can be employed to stabilize forming nanoparticles, e.g., polymers,8 alkylammonium salts,9 and alkanethiols. For the case of preparation of metal nanostructures, alkanethiols are of special interest since they are known to form very stable monomolecular films on surfaces of gold, silver, and copper.10 They are used broadly to synthesize spherical metal nanoclusters by chemical reduction of corresponding cations.11 However, there are probably no reports, so far, on the electrochemical preparation of nanostructures using thiol molecules as stabilizers. In the present paper we report our preliminary results on the synthesis of thiol-coated rodlike silver nanostructers by electrochemical reduction of Ag+ ions in aqueous solution containing 3-mercaptopropionic acid. Experimental Section Chemicals. All chemicals were of the highest quality commercially available: silver nitrate (AgNO3) (Aldrich, 99+%); 3-mercaptopropionic acid (Merck, >98%); lithium perchlorate (LiClO4) (Aldrich, ACS grade); ammonia solution (25% NH3(aq)) (POCh, reagent grade). Aqueous solutions were prepared from water of high purity (Milli-Q). Instrumentation. Electrochemical measurements were conducted with a PC-controlled, custom built potentiostat/galvanostat (KSP Electronics, Poland), using a conventional small volume three-electrode cell with Pt wire as counter electrode and highly oriented pyrolytic graphite crystal (HOPG) as working electrode. All potentials are quoted versus an Ag/AgCl/1 M KCl(aq) reference electrode. A commercially available Nanoscope IIIa from Digital Instruments was used to collect all the scanning tunneling microscope (STM) images presented in this paper. The microscope was run in the constant current mode. Tungsten tips were prepared by electrochemical etching of a 0.25-mm diameter wire (Aldrich, 99.9+%) in 4 M aqueous KOH as described elsewhere.12 (7) Kiang, C. H.; Choi, J. S.; Tran, T. T.; Bacher, A. D. J. Phys. Chem. B 1999, 103, 7449. (8) Naka, K.; Yaguchi, M.; Chujo, Y. Chem. Mater. 1999, 11, 849. (9) (a) Bonnemann, H.; Brinkmann, R.; Neiteler, P. Appl. Organomet. Chem. 1994, 8, 361. (b) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401. (c) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R. Science 1995, 267, 367. (10) Finklea, H. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 110. (11) Bonnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 10, 2455.
10.1021/la026677c CCC: $25.00 © 2003 American Chemical Society Published on Web 04/23/2003
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Figure 1. Cyclic voltammogram of HOPG in aqueous 0.1 M MPA, 10 M NH3, 0.1 M LiClO4. ν ) 50 mV/s.
Figure 2. Cyclic voltammograms of HOPG in aqueous 1 mM AgNO3 in 10 M NH3, 0.1 M LiClO4 containing (a) 0 M MPA, (b) 10-5 M MPA, and (c) 10-3 M MPA. ν ) 50 mV/s.
Preparation of Electrodes. The basal plane surface of a HOPG crystal (SPI Supplies) was cleaved using adhesive tape immediately prior to use.
Results and Discussion The aim of this paper was to prepare silver nanostructures on graphite by reduction of silver ions in the presence of mercaptopropionic acid (MPA) in the solution. Therefore prior to deposition of silver we examined the electrochemical behavior of MPA to check the potential window of its electroinactivity. Figure 1 presents a voltammetric curve of bare graphite in aqueous 0.1 M MPA dissolved in 10 M NH3 in 0.1 M LiClO4 (we used high concentrations of NH3 in order to prevent precipitation of silver mercaptide from the solutions containing Ag+ and MPA13). As it can be seen, the oxidation current above ca. 250 mV is observed, which is due to the oxidation of the thiol to disulfide. One can also see that in the negative potential range there is lack of any electrochemical faradaic processes as the currents are close to zero. Therefore this confirms that the deposition of the metal can be performed in the potential range from -700 to 200 mV without the risk of interference with any other faradaic processes. We studied the voltammetric behavior of silver ions in the solution containing different concentrations of MPA. The results are shown in Figure 2. It can be seen that the addition of MPA to the solution results in a significant decrease of the reduction current of silver ions. While for the concentration of 10-5 M MPA the shape of the voltammogram is similar to that when no MPA is added, the curve for the concentration 10-3 M reveals completely different character, with low magnitude of currents and approximately exponential shape. The obtained results can be explained by adsorption of MPA on the surface of the forming silver, which prevents deposition of larger amounts of the metal. Thiols are known to adsorb strongly on silver and block the access of the species present in the solution to the surface. It is reasonable therefore to assume that after the formation of metal nucleation sites on graphite, the MPA molecules immediately adsorb on the silver surface and make the further growth of the nuclei more difficult. However, the MPA molecules contain only short hydro(12) Heckl, W. M. In Procedures in Scanning Probe Microscopies; Colton, R. J., Engel, A., Frommer, J. E., Gaub, H. E., Gewirth, A. A., Guckenberger, R., Rabe, J., Heckl, W. M., Parkinson, B., Eds.; John Wiley and Sons: Chichester, New York, Weinheim, Brisbane, Singapore, Toronto, 1998; p 76. (13) White, P. C.; Lawrence, N. S.; Davis, J.; Compton, R. G. Electroanalysis 2002, 14, 89.
Figure 3. Voltammograms of HOPG in aqueous 1 mM AgNO3 in 10 M NH3, 0.1 M LiClO4 containing (a) 0 M MPA, (b) 10-5 M MPA, and (c) 10-3 M MPA. ν ) 1 mV/s.
carbon chains and the monomolecular film formed on the metal deposit surface is relatively thin. As a consequence, the thiol monolayer is not a very high barrier for electron transfer and the further growth of the deposit is not completely inhibited. The same tendency is observed for oxidation currents of the deposited Ag. While for the case of the deposition from 10-5 M MPA solution the oxidation peak (although smaller than that for 0 M MPA) is still observable, for thiol concentrations of 10-3 M no oxidation current associated with anodic dissolution of Ag can be observed. Analyzing of the values of charge released during oxidation of Ag and consumed for silver ion reduction, we could calculate the percent of Ag deposit that is not removed (oxidized) from the graphite surface during voltammetric scanning (r ) Qox/Qred, where Qox is the oxidation charge and Qred is the reduction charge). The r value for the case when no MPA is added to the solution is close to 47%, which means that applying of positive potential to the electrode results in oxidation of nearly half of the Ag deposit. When the experiment is performed in 10-5 M MPA, only 37% of the deposit is dissolved in the oxidation process. When the concentration of MPA increases further (10-3 M), no Ag is dissolved (r ) 0), as the protecting layer of the thiol completely prevents its oxidation. Our further investigation involved electrochemical deposition of silver at slower sweep rates and subsequent visualization of the deposits by microscopic techniques.
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Figure 4. STM images of silver deposited on HOPG at ν ) 1 mV/s in 10-3 M MPA (Q ) 0.93 mC/cm2).
Figure 3 presents voltammetric curves of silver deposition from the solutions containing different concentrations of MPA, at 1 mV/s sweep rate. The reduction currents, similarly as for the previous case, decrease with an increase of the MPA concentration. The only difference is the presence of multiple reduction peaks (0 M MPA), which can be explained by the formation of metal on energetically distinct sites at already deposited silver (this was not observed for faster sweep rates).14 These peaks disappear as the MPA molecules are present in the solution and adsorb on forming Ag. The above observations are in agreement with previous conclusions. The nature of the deposit (10-3 M MPA) was studied by STM. Figure 4a shows that the deposit consists of nonuniformly distributed islands of the diameter ranging from ca. 50 to 250 nm. These islands (Figure 4b) consist of sets of nanometer-sized rods (3-8 nm in diameter). The length of the rods varies from 50 to 250 nm. The detailed mechanism of the formation of nanorods is not known at this stage; however, it seems that the adsorption of MPA facilitates growth of the metal in some preferential directions. This is probably due to different affinity of thiol molecules to various crystallographic planes of silver. We did not find systematic studies of MPA adsorption on various low index facets of Ag in the literature; therefore we can only speculate on the possible scenarios of nanorods growth. For instance, we can assume that the nanorod’s side walls are formed of Ag(111) facets, and on the basis of the value of the angle between these planes (ca. 109°), the geometry of the nanowire close to pentagonal prism can be proposed, with capping faces of, e.g., (110) or (100) planes. However, according to the literature, the adsorption of thiols on Ag(111) seems to be (14) Harrison, J. A.; Thirsk, H. R In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1971; Vol. 5, p 67. (15) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (16) Mohtat, N.; Byloos, M.; Soucy, M.; Morin, S.; Morin, M. J. Electroanal. Chem. 2000, 484, 120. (17) Hatchett, D. W.; Stevenson, K. J.; Lacy, W. B.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1997, 119, 6596.
weaker than that on (110) or (100) facets. Porter at al. reports that the (100) surface can accommodate a closely packed array of alkanethiol molecules spaced 4.1 Å apart, whereas the most densely packed structures on (111) facets result in distances of 5 Å.15 Morin at al. observed the increased stability of buthanethiol toward electroreductive desorption on Ag(110) when compared to Ag(111).16 Additionally, it is believed that thiol molecules occupy two energetically distinct sites on Ag(111), which results in formation of low- or high-coverage phases depending on the potential applied to the electrode.17 It is probable therefore that MPA molecules adsorbing on the (111) facet of the growing nanorod form a low-coverage two-dimensional phase which does not block strongly silver deposition on this plane. Hence, we may suggest that the growth of the nanowire occurs at (111) facets, while the rod’s side walls form a prism (e.g., hexagonal) consisting of (110) and (100) planes. However, in fact, one can imagine also different geometries of the nanorods, so the problem of the details of the growth mechanism obviously needs additional thorough studies. Conclusions The electrochemical deposition of silver is strongly affected by 3-mercaptopropionic acid present in the solution. The adsorption of the thiol on the surface of the forming silver deposit decreases significantly the rate of metal deposition and effects in formation of thiol-coated nanosized islands of the metal on the electrode (graphite) surface. The islands consist of sets of silver nanorods that are 3-8 nm in diameter and 50-250 nm in length. Acknowledgment. The authors are very grateful to Dr. Pawel Krysinski for helpful comments and valuable discussions. The funding for this work was provided by Grant 3 T09A 117 19 from the State Committee for Scientific Research. Part of the funding (D.R.) was provided by the Department of Chemistry, University of Warsaw, within project BST-761/16/2002. LA026677C
