Langmuir 2003, 19, 1951-1953
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Electrodeposition of Ti Nanowires on Highly Oriented Pyrolytic Graphite from an Ionic Liquid at Room Temperature I. Mukhopadhyay and W. Freyland* Institute of Physical Chemistry, University of Karlsruhe (TH), Kaiserstrasse 12, D 76128 Karlsruhe, Germany Received November 5, 2002. In Final Form: January 9, 2003 Ti nanowires have been deposited electrochemically, with in situ monitoring by scanning tunneling microscopy, at the step edge of highly oriented pyrolytic graphite (HOPG) at room temperature from 0.24 M TiCl4 in the ionic liquid 1-butyl-3-methyl imidazolium bis((trifluoromethyl)sulfonyl)amide. To our knowledge, this is the first time that a transition metal nanowire has been prepared in this way, which is enabled by the wide electrochemical windows of ionic liquids. The formation of the first nanowire is induced at the step edge of the HOPG substrate at a potential of -1.0 V versus the [Fc]+/[Fc] redox couple; subsequent straight and highly aligned wires grow in parallel to the first. This cooperative nucleation process of nanowires is unusual and has not been reported before. Up to six wires grow at constant potential over a period of about 20 min. The wires exhibit a narrow width distribution of 10 ( 2 nm and have a length of more than 100 nm.
Introduction The discovery of novel nanometer scale phenomena and their potential applications in different areas such as micro- or nanoelectronics has led to a rapid increase in research on functional nanostructures.1 Just recently, various groups have constructed and tested nanoelectronic devices such as field effect transistors or light emitting diodes which they fabricated from metal or semiconductor nanotubes and nanowires.2,3 For preparation of these nanostructures, a variety of methods have been developed, including chemical synthesis and special electrodeposition techniques.4 By use of appropriate templates and with in situ electrochemical scanning tunneling microscopy (ECSTM), nanometer size control of electrodeposited nanostructures has been achieved.5-8 In this letter, we report on the fabrication of Ti nanowires by electrodeposition from a room-temperature ionic liquid on graphite step edges. To monitor the initial steps of nucleation and wire formation, we have performed in situ EC-STM measurements. We have demonstrated the feasibility of such experiments with an ionic electrolyte for several examples before.9-11 In comparison with aqueous electrolytes, these ionic liquids provide a larger electrochemical window which enables the electrodeposition of a large number of light and refractory metals * Corresponding author. Fax: +49-721-6086662. E-mail: werner.
[email protected]. (1) Peercy, P. S. Nature 2000, 406, 1023. (2) Wind, S. J.; Appenzeller, J.; Martel, R.; Derycke, V.; Avouris, Ph. Appl. Phys. Lett. 2002, 80, 3817. (3) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66. (4) Penner, R. M. J. Phys. Chem. B 2002, 106, 3339. (5) Moller, F. A.; Magnussen, O. M.; Behm, R. J. Phys. Rev. Lett. 1996, 77, 3165. (6) Kolb, D. M.; Ullmann, R.; Will, T. Science 1997, 275, 1097. (7) Huang, B. M.; Colletti, L. P.; Gregory, B. W.; Anderson, J. L.; Stickney, J. L. J. Electrochem. Soc. 1995, 142, 3007. (8) Bernadette, M. Q.; Ding, Z.; Moulton, R.; Bard, A. J. Langmuir 2002, 18, 1734. (9) Zell, C. A.; Endres, F.; Freyland, W. Phys. Chem. Chem. Phys. 1999, 1, 697. (10) Zell, C. A.; Freyland, W. Chem. Phys. Lett. 2001, 337, 293. (11) Freyland, W.; Zell, C. A.; Zein Al Abedin, S.; Endres, F. Electrochim. Acta, submitted.
such as Ti, their alloys, and even semiconductors.11 Furthermore, they are characterized by very low vapor pressures and relatively high thermal stability.12 For the preparation of linear and well-aligned nanowires of Ti, we have used the idea of step edge decoration of highly oriented pyrolytic graphite (HOPG) which we have used as a template. The same strategy has been successfully applied in recent studies by Penner and co-workers on the electrodeposition of nanowires of noble metals and conductive metal oxides from aqueous solutions.13 For the nanowire formation and growth, they propose the following scheme: nucleation at the step edges, cluster growth and coalescence forming a “beaded” nanowire, and finally, smoothening of the beaded nanowire at diameters of 1015 nm.13 In this study, we have followed two main objectives. First, we show for the first time that Ti nanowires can be electrodeposited on HOPG from a TiCl4 solution in 1-butyl-3-methyl imidazolium bis((trifluoromethyl)sulfonyl)amide ([BMIm]BTA) at room temperature. Second, we elucidate the growth mechanism of Ti nanowires by in situ STM imaging. Experimental Section The ionic liquid [BMIm][BTA] (98% pure) was obtained from Solvent Innovation, Germany. Prior to use, the liquid was dried under a vacuum at 120 °C and was kept in an argon glovebox (O2 and H2O < 2 ppm). TiCl4 (99.999%) was obtained from Alfa Aceser, Germany, and showed good solubility in the ionic liquid. Although the ionic liquid was extremely hydrophobic, saturated by
