4464
Langmuir 2008, 24, 4464-4466
Preparation of Quadrate Crystalline Cu(TCNQ) Microtubes and Assembly of a Novel Copatterned Structure Xin-Hong Zhou, Shu-Jie Wei, and Shu-Sheng Zhang* Key Laboratory of Eco-chemical Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, P. R. China ReceiVed NoVember 4, 2007. In Final Form: February 29, 2008 Crystalline microtubes of functional Cu(TCNQ) were prepared using a facile method of dissolution. XRD, SAED, and EDX characterization showed that they belonged to phase I of Cu(TCNQ), which is important in nanoelectronics and nanodevices. Furthermore, a novel micrometer and nanometer structure co-patterned morphology was assembled, which may have potential applicaton in building nanoscale electrodes or patterning other nanosize functional materials.
Introduction In recent years, there has been a great interest in the control of the shape and orientation of nanoscale/microscale crystallites as well as the ability to align them into large scale threedimensional arrays for the future generation of smart and functional materials. Moreover, numerous applications such as dye-sensitized photovoltaic cells, metal-ion batteries, electrochemical supercapacitors, and hydrogen storage devices require a high-porosity and large surface for improving efficiency and activity. Therefore, designing functional materials with hollow tubular structures contributes to the extension of novel devices for technological applications.1-12 The molecule-based electronic switching effect in Cu(TCNQ) (TCNQ ) 7,7,8,8-tertracyno-p-quinodimethane) has been frequently referred to as a prototype of a molecular electronic device.13-21 It is of significant importance to design Cu(TCNQ) * To whom correspondence should be addressed. E-mail: shushzhang@ 126.com. (1) Adachi, M.; Harada, T.; Harada, M. Langmuir 1999, 15, 7097-7100. (2) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160-3163. (3) Prinz, V. Y.; Selenznev, V. A.; Gutakovsky, A. K.; Chehovskiy, A. V.; Preobrazhenskii, V. V.; Putyato, M. A.; Gavrilova, T. A. Physica E 2000, 6, 828-831. (4) Rothschild, A.; Popovitz-biro, R.; Lourie, O.; Tenne, R. J. Phys. Chem. B 2000, 104, 8976-8981. (5) (a) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222-225. (b) Cui, G.; Xu, W.; Guo, C.; Xiao, X.; Xu, H.; Zhang, D.; Jiang, L.; Zhu, D. J. Phys. Chem. B 2004, 108, 13638-13642. (6) Whangbo, M. H.; Koo, H. J. Solid State Commun. 2000, 115, 675-678. (7) Satisshkumar, B. C.; Govindaraj, A.; Vogl, E. M.; Basumallick, L.; Rao, C. N. R. J. Mater. Res. 1997, 12, 604-606. (8) Niederberger, M.; Muhr, H. J.; Krumeich, F.; Bieri, F.; Gunther, D.; Nesper, R. Chem. Mater. 2000, 12, 1995-2000. (9) Lourie, O. R.; Jones, C. R.; Barlett, B. M.; Gibbons, P. C.; Ruoff, R. S.; Buhro, W. E. Chem. Mater. 2000, 12, 1808-1810. (10) (a) Lijima, S. Nature 1991, 354, 56-58. (b) Cui, G. L.; Zhi, L.; Thomas, A.; Kolb, U.; Lieberwirth, I.; Muellen, K. Angew. Chem., Int. Ed. 2007, 46, 3464-3467. (11) Wang, N.; Tang, Z. K.; Li, G. D.; Chen, J. S. Nature 2000, 408, 50-51. (12) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395-4397. (13) Liu, H. B.; Zhao, Q.; Li, Y. L.; Liu, Y.; Lu, F.; Zhuang, J.; Wang, S.; Jiang, L.; Zhu, D. B.; Yu, D.; Chi, L. J. Am. Chem. Soc. 2005, 127, 1120-1121. (14) Hua, Z. Y.; Chen, G. R. Vacuum 1992, 43, 1019-1023. (15) Potember, R. S.; Pochler, T. O.; Bennson, R. C. Appl. Phys. Lett. 1982, 41, 548-550. (16) Benson, R. C.; Hoffman, R. C.; Potember, R. S.; Bourkoff, E.; Pochler, T. O. Appl. Phys. Lett. 1983, 42, 855-857. (17) Amitsos, E. I.; Risen, W. M. Solid State Commun. 1983, 45, 165-169. (18) Hoshino, H.; Matsushita, S.; Samura, H. J. Jpn. J. Appl. Phys. 1986, 25, L341-342. (19) Liu, S. G.; Liu, Y. Q.; Wu, P. J.; Zhu, D. B. Chem. Mater. 1996, 8, 2779-2787. (20) Potember, R. S.; Pochler, T. O.; Cowan, D. O. Appl. Phys. Lett. 1979, 34, 405-407.
material with novel morphology and in particular, highly porous, well-defined three-dimensional arrays for basic research as well as for various fields of industrial and high-tech applications. However, it is still a great challenge to prepare and assemble three-dimensional microtubes of Cu(TCNQ). Herein, a low-cost and facile method for large-scale preparation of crystalline Cu(TCNQ) microtubes and assembly a novel micrometer and nanometer-scale co-patterned structure was reported. For these novel structures, it is expected they will not only provide a good bridge between nanoscale structures and the outer electrodes of a device but also have the potential to pattern nanoelectrodes or nanosized materials. Experimental Section Preparation of Cu(TCNQ). Electrodeposition of copper was carried out at a potential of -0.1 V(vs SCE) in a solution of CuSO4 (0.5 M) using an EG&G potentiostat/Galvanostat model 283 instrument. The electrodeposition was performed under 25 °C using a gold foil as the working electrode, a platinum wire (φ ) 1 mm) as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. After the gold foil turned wine-colored (the thickness estimated to be 180 nm from the charge), the sample was rinsed with water and acetonitrile under a nitrogen atmosphere. Subsequently, the sample was rapidly immersed in an acetonitrile solution of 0.02 M TCNQ (TCI com) for 30 min. The preparations were carried out under a nitrogen atmosphere unless otherwise indicated. Acetonitrile was dried over 3 Å molecular sieves and distilled, and TCNQ was recrystallized in the acetonitrile solution prior to use. The black-purple films of Cu(TCNQ) were prepared by reacting copper with the TCNQ solution. After periodic, timed dissolution (30 min one time interval, usually 10 h in all) with fresh acetonitrile at a temperature of about -5 °C, the samples were washed, dried, and placed in a vial under anaerobic conditions for later analysis. Characterization of Microtubes. SEM (Jeol JSM 6700F) was employed to assess the topography of the samples. To avoid charging effects while imaging, a thin layer of platinum was sputtered onto the surface, and the images were acquired at electron voltage (3.0 kV). The EDX was measured by EDAX phoenix. XRD results were recorded using a Dmax 2000 spectrometer (Rigaku, Cu KR).
Results and Discussion The “spontaneous electrolysis” technique was used to organize quadrate crystalline microtubes and the copatterned structure on the gold foil under the protection of nitrogen. These microtubes can be synthesized by direct redox reactions of copper films with (21) Iwasa, Y.; Koda, T.; Tokura, Y.; Koshihara, S.; Iwasawa, N.; Saito, G. Appl. Phys. Lett. 1989, 55, 2111-2113.
10.1021/la7034455 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/21/2008
Letters
Langmuir, Vol. 24, No. 9, 2008 4465
Figure 1. SEM image (a) and its enlarged image of microtubes (b).
Figure 2. XRD characterization of Cu(TCNQ) microtubes.
neutral TCNQ in solution. First, a gold film was utilized to deposit a copper film using a potentiostatic method according to the reported literature.22-23 In this method, the copper thickness and topography could be tuned by deposition time. The copper film was then put in an acetonitrile solution of 20 mM TCNQ to produce large-scale microcrystallites with the protection of N2, and it was put in renewal CH3CN solution every 30 min for production of microtubes. The solution gradually turned from achromaticity to green signifying the production of TCNQ- when immersed in CH3CN solution, i.e., part of Cu(TCNQ) dissolved.24 The morphology was characterized with scanning electron microscopy (SEM). From the SEM image, an interesting topography was observed. The overall surface of the gold foil was covered by uniformly distributed hollow tubular structures with open ends (Figure 1a). Quadrate microtubes having crossings or junctions that have (22) Tao, G.; Meng, G. W.; Wang, Y. W.; Sun, S. H.; Zhang, L. D. J. Phy. Condens. Mater. 2002, 14, 355-363. (23) Eugenia, M.; Molares, T.; Buschman, V.; Dobrev, D.; Neumann, R.; Scholz, R.; Schucher, I. U.; Vetter, J. AdV. Mater. 2001, 13, 62-65. (24) Heitz, R. A.; Zhao, H. H.; Ouyang, X.; Grandinetti, G.; Cowen, J.; Dunbar, K. R. Inorg. Chem. 1999, 38, 144-156.
Figure 3. Formation process of Cu(TCNQ) crystalline microtubes at different dissolution times: (a) 0, (b) 0.5, (c) 1, and (d) 3 h.
potentials in microelectronics and nanoelectronics were also be observed.25-27 The widths of the microtubes varied from 0.2 to 0.5 micrometer, wereas lengths were observed in the range of tens to even hundreds of micrometers. SEM images at a higher magnification clearly indicated that the wall thickness of the microtubes was about 80 nm (Figure 1b). The microtubes had rectangularly framed crosssections. Line scanning of energy-dispersive X-ray spectroscopy (25) Menon, M.; Srivastava, D. Phys. ReV. Lett. 1997, 79, 4453-4456. (26) Treboux, G.; Lapstun, P.; Silverbrool, K. Chem. Phys. Lett. 1999, 306, 402-406. (27) Andriotis, A. N.; Menon, M.; Srivastava, D.; Chernozatonskii, L. Phys. ReV. Lett. 2001, 87, 066802-1-066802-4.
4466 Langmuir, Vol. 24, No. 9, 2008
Letters
microtubes, the mechanism of Cu(TCNQ) tubes can also be regarded as a dissolution process. The center of one crystal with an unsatisfactory crystal preferentially dissolved. The surrounding one with a good crystal was difficult to dissolve. Therefore, it was easy to form the tubelike microcrystal by tuning the process. This topography may correlate with a diffusion-limited aggregation (DLA) process.27-31 In fact, two opposite processes occurred in the solution.32 On one hand, TCNQ molecules reacted with Cu to form Cu(TCNQ). On the other hand, Cu(TCNQ) molecules dissolved into acetonitrile again.32 The topography of Cu(TCNQ) can be controlled by tuning either process. Following the above mechanism, a novel micrometer- and nanometer-scale copatterned structure was also assembled by controlling partial dissolution of the Cu(TCNQ) microcrystals and subsequent growth of nanocrystals. Shown in Figure 4a are nanoislandlike arrays in the rectangle-frame openings of every quadrate microcrystal. As can be seen from the enlarged images of Figure 4b, the nanoislands were inlaid in the openings of the microcrystals, with a tip that ranged from 10 to 40 nm. The distance between tips ranged from 50 to 120 nm. This novel structure confirmed the aforementioned mechanism. The copatterned structure not only can act as a template for replicating nanoelectrodes but also provide a possible bridge to connect nanostructures with the outside world through microstructures.
Conclusion Figure 4. Typical image (a) and its enlarged image (b) of microand nanosized copatterned structures of Cu(TCNQ).
(EDX) along the microtubes was conducted to confirm that all the microstructures had the same element composition (C, N, and Cu). It was revealed that the atomic ratio of Cu to N in our samples was equal to 4:1. This result showed that all of the microtubes were the same composition of Cu(TCNQ). Consistent with the EDX results, the XRD method also indicated that the microtubes belonged to phase I (Figure 2).24 Phase I of the Cu(TCNQ) nanostructures has a potential application in organic nanodevices, for example, as field emitters.13 It is significant to control the size of Cu(TCNQ) nano- and microstructures and to pattern surfaces with these structures. These functional microtubes may be used to prepare special hybrid or junction materials by induction of other functional molecules such as fullerene into the tubelike pores. In order to understand the formation mechanism, the growth process was tracked in our experiments (Figure 3). From SEM images, it can be clearly observed that the center of quadrate crystal microtubes preferentially dissolved and deepened gradually into the depth of the crystal, but the wall had no apparent changes. A similar phenomenon was observed in ZnO crystalline microtubes reported by Vayssieres and co-workers.12 The center of the ZnO microtubes dissolved preferentially owing to an unsatisfactory crystallinity. Similar to the formation of ZnO
In conclusion, a facile method for large-scale preparation of crystalline Cu(TCNQ) microtubes and assembly a novel micrometer- and nanometer-scale co-patterned structure was reported. The characterization of XRD and EDX showed that microtubes belonged to phase I of crystalline Cu(TCNQ). These novel structures are expected to provide a good connection between the nanoscale structures and the outer electrodes of a device, which may have the potential to pattern nanoelectrodes or nanosized materials. Acknowledgment. This work was supported by the Doctoral Found of Qingdao University of Science and Technology, the National Natural Science Foundation of China (No. 20775038), and the National High-tech R&D Program (863 Program, No. 2007AA09Z113). Supporting Information Available: The typical TEM image and its SAED pattern of Cu (TCNQ) microtubes and the typical SEM image of dendrite Cu(TCNQ) microtube arrays. This material is available free of charge via the Internet at http://pubs.acs.org. LA7034455 (28) Witten, T. A.; Sander, L. M. Phys. ReV. Lett. 1981, 47, 1400-1403. (29) Witten, T. A.; Sander, L. M. Phys. ReV. B. 1983, 27, 5686-5697. (30) Meakin, P. Phys. ReV. Lett. 1983, 51, 1119-1122. (31) Hwang, R. Q.; Schro¨der, J.; Gu¨nther, C.; Behm, R. J. Phys. ReV. Lett. 1991, 67, 3279-3282. (32) Liu, Y. L.; Ji, Z. Y.; Tang, Q. X.; Jiang, L.; Li, H. X.; He, M.; Hu, W. P.; Zhang, D. Q.; Jiang, L.; Wang, X. K.; Wang, C.; Liu, Y. Q.; Zhu, D. B. AdV. Mater. 2005, 17, 2953-2957.
