Synthesis and Characterization of Uniform Arrays of Copper Sulfide

Dong Un Lee , Debabrata Pradhan , Rola Mouawia , Doo Hwan Oh , Nina F. Heinig , Kam Tong Leung and Eric Prouzet. Chemistry of Materials 2010 22 (1), ...
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Synthesis and Characterization of Uniform Arrays of Copper Sulfide Nanorods Coated with Nanolayers of Polypyrrole Weixin Zhang, Xiaogang Wen, and Shihe Yang* Department of Chemistry and Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received November 6, 2002. In Final Form: February 3, 2003 Isolated and uniform Cu2S nanorods have been prepared in well-aligned large arrays on a copper surface using a modified procedure. Polypyrrole (PPY) has been homogeneously deposited on the Cu2S nanorods via in situ polymerization at the interfacial layer between chloroform and water. For the coating reaction, the pyrrole (PY) monomer and the oxidant (NH4)2S2O8 were dissolved in chloroform and water, respectively. With a PY:oxidant molar ratio of 1.00 and a PY concentration of 0.036 M in chloroform, the PPY coating is smooth, robust, and uniform, extending along the entire length of the Cu2S nanorods. The thickness of the PPY coating could be controlled (20-50 nm) by varying the polymerization time. Scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, Fourier transform infrared spectra, and micro-Raman spectra were used to characterize the core/sheath nanorod arrays of Cu2S/PPY.

Introduction 1

Martin2

et al. pioneered the template Since Ozin and method for the synthesis of nanofibrils and nanotubules of conducting polymers and metals, various nanostructured materials based on this approach have been prepared. The ability to synthesize well-aligned arrays of nanotubes and nanowires of carbon,3 metals,4 conducting polymers,2 semiconductors5 and composites of these materials6,7 has aroused great interest in physics, electronics, optics, material science, chemistry, and biomedical science. In general, nanoscale arrays are believed to have much more attractive optical, electronic, and magnetic properties, which may find applications in sensors, detectors, rechargeable batteries, fuel cells, high-density recording, and storage devices, to name but a few. Surface modification of nanowire or nanorod arrays with conducting polymers is expected to add more functionalities to the system and may even lead to completely new nanocomposite materials.8 In addition, the ability to control the thickness of conducting polymer coated on each * Corresponding author: e-mail [email protected]; Tel (852) 23587362. (1) Ozin, G. A. Adv. Mater. 1992, 4, 612. (2) Martin, C. R. Science 1994, 266, 1961. Menon, V. P.; Lei, J.; Martin, C. R. Chem. Mater. 1996, 8, 2382. De Vito, S.; Martin, C. R. Chem. Mater. 1998, 10, 1738. (3) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701. Hu, W.; Gong, D.; Chen, Z.; Yuan, L.; Saito, K.; Grimes, C. A.; Kichambare, P. Appl. Phys. Lett. 2001, 79, 3083. (4) Nielsch, K.; Wehrspohn, R. B.; Barthel, J.; Kirschner, J.; Go¨sele, U.; Fischer, S. F.; Kronmu¨ller, H. Appl. Phys. Lett. 2001, 79, 1360. Yin, A. J.; Li, J.; Jian, W.; Bennett, A. J.; Xu, J. M. Appl. Phys. Lett. 2001, 79, 1039. (5) Zhang, J.; Zhang, L. D.; Wang, X. F.; Liang, C. H.; Peng, X. S.; Wang, Y. W. Appl. Phys. Lett. 2001, 115, 5714. Li, Y.; Xu, D.; Zhang, Q.; Chen, D.; Huang, F.; Xu, Y.; Guo, G.; Gu, Z. Chem. Mater. 1999, 11, 3433. Zhang, X. Y.; Zhang, L. D.; Chen, W.; Meng, G. W.; Zheng, M. J.; Zhao, L. X. Chem. Mater. 2001, 13, 2511. (6) Chen, J. H.; Huang, Z. P.; Wang, D. Z.; Yang, S. X.; Wen, J. G.; Ren, Z. F. Appl. Phys. A 2001, 73, 129. (7) Cao, H.; Xu, Z.; Sang, H.; Sheng, D.; Tie, C. Adv. Mater. 2001, 13, 121. Liu, J. F.; Yang, K. Z.; Lu, Z. H. J. Am. Chem. Soc. 1997, 119, 11061. Zhang, Z.; Sun, X.; Dresselhaus, M. S.; Ying, J. Y.; Heremans, J. P. Appl. Phys. Lett. 1998, 73, 1589. Thostenson, E. T.; Ren, Z.; Chou, T. W. Comput. Sci. Technol. 2001, 61, 1899.

nanowire and the size of the intertubular pores is crucial for applications in optoelectronic nanodevices and sensors. This strategy has been demonstrated by the electrochemical coating of aligned MWNTs with a uniform layer of conducting polymer.9 Polypyrrole (PPY) is a typical conducting polymer, which has been extensively investigated by numerous research groups. It offers reasonably high conductivity and has fairly good environmental stability. Thin films of PPY are usually obtained by electrochemical synthesis.6,10 Chemical synthesis of PPY normally gives an intractable and unprocessable bulk powder precipitate.11 Such inherent intractability has prevented PPY from being blended with other materials to form nanocomposites using conventional techniques. Armes and co-workers first prepared colloidal conducting polymers by coating metal oxide nanoparticles with PPY or polyaniline.12 The researchers showed that in the colloidal nanocomposite the encapsulated nanoparticles formed raspberry-shaped domains. Matijevic et al. have synthesized core/shell composite particles through in situ polymerization of PY on the surfaces of inorganic oxide particles utilizing the catalytic activity of the core surfaces.13 These methods, however, often produce PPY precipitates that cannot be easily separated from the core/shell particles. It has been shown that template-synthesized PPY tubules, fibrils, and ultrathin films using chemical meth(8) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. Czerw, R.; Guo, Z.; Ajayan, P. M.; Sun, Y. P.; Carroll, D. L. Nano Lett. 2001, 1, 423. Ajayan, P. M. Chem. Rev. 1999, 99, 1787. Downs, C.; Nugent, J.; Ajayan, P. M.; Duquette, D. J.; Santhanam, K. S. V. Adv. Mater. 1999, 11, 1028. (9) Hughes, M.; Shaffer, M. S. P.; Renouf, A. C.; Singh, C.; Chen, G. Z.; Fray, D. J.; Windle, A. H. Adv. Mater. 2002, 14, 382. Gao, M.; Huang, S.; Dai, L.; Wallace, G.; Gao, R.; Wang, Z. Angew. Chem., Int. Ed. 2000, 39, 3664. (10) Grunden, B.; Iroh, J. O. Polymer 1995, 36, 559. (11) Ames, S. P. Synth. Met. 1987, 20, 365. (12) Armes, S. P.; Gottesfeld, S.; Beery, J. G.; Garzon, F.; Agnew, S. F. Polymer 1991, 32, 2325. Maeda, S.; Armes, S. P. Chem. Mater. 1995, 7, 171. Gill, M.; Mykytiuk, J.; Armes, S. P.; Edwards, J. L.; Yeats, T.; Moreland, P.; Mollett, C. J. Chem. Soc., Chem. Commun. 1992, 108. McCarthy, G. P.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 1997, 13, 3686. (13) Huang, C. L.; Partch, R. E.; Matijevic, E. J. Colloid Interface Sci. 1995, 170, 275. Huang, C. L.; Matijevic, E. J. Mater. Res. 1995, 10, 1327.

10.1021/la020894w CCC: $25.00 © 2003 American Chemical Society Published on Web 04/18/2003

Copper Sulfide Nanorods

ods can have electric conductivities that are orders of magnitude higher than those of conventional forms (e.g., powders or normal thin films). The nanopores of zeolites14 and membranes,2 Langmuir-Blodgett films,15 liquid crystalline media,16 surfactant self-assembled monolayers,17 and diblock copolymer micelles18 have proved to be good templates for confined polymerization of pyrrole. Recently, our group has reported the growth of single crystalline Cu2S nanowire arrays on a copper surface by gas-solid reaction under ambient conditions.19 The nanowires are isolated, straight, phase pure, and can therefore be used to template the growth of conducting polymer nanowires. However, it was difficult to grow large area arrays with a uniform density, diameter, and length of the semiconductor nanowires, which are important for practical applications. In the work reported here, we have successfully synthesized isolated and uniform Cu2S nanorods in well-aligned large arrays on a copper surface using a modified fabrication method that involves oxidative pretreatment of the copper surface. This has facilitated the controlled coating of PPY on the nanorod surfaces, which constitutes the main theme of this paper. The homogeneous PPY coating was achieved via in situ polymerization at two levels of confinement. Vertically, it was confined at the interfacial layer of chloroform and water. Horizontally, it was further confined in the interrod spaces of the nanorod arrays. Although the interface of chloroform and water was developed to prepare PPY films with a thickness of 3-4 µm,20 we have first exploited this strategy in our two-level confinement method for the fabrication of ultrathin PPY films on the Cu2S nanorod surfaces. This work has not only demonstrated controlled nanoscale deposition of a conducting polymer PPY on a semiconducting Cu2S nanorod but also achieved fabrication of a uniform, large area array of this core/sheath nanorod. With such a robust core/sheath nanorod array, many possible applications can be envisaged such as sensors, which rely on the advantageous array geometry and the unique core/sheath interfacial electronic properties. Experimental Section Materials. Copper foils (99.98%, Aldrich), with a thickness of 0.15-0.25 mm, oxygen gas (O2, 99.5%, Industrial Gas Ltd.), and hydrogen sulfide (H2S, 99.5%, Special Gas Products Inc.) were used as received. Pyrrole (99%, Acros Organics) was distilled before use. Chloroform (CHCl3, 99.48%, Scharlau) and ammonium persulfate ((NH4)2S2O8, analytical reagent, Guanghua Chemical Factory of Shantou) were used as received. Methanol (CH3OH, 99.99%, Fisher) and deionized water were used as solvents. Growth of Cu2S Nanorod Arrays on a Copper Surface. The preoxidation treatment was carried out by immersing the copper foils in a 0.2 M (NH4)2S2O8 aqueous solution for 12 h, followed by rinsing with deionized water 3-5 times. The gas(14) Bein, T.; Enzel, P. Angew. Chem., Int. Ed. Engl. 1989, 28, 1692. Larsen, G.; Haller, G. L.; Marquez, M. J. Phys. Chem. 1992, 96, 4145. (15) Skotheim, T. A.; Yang, X. Q.; Chen, J.; Hale, P. D.; Inagaki, T.; Samelson, L.; Tripathy, S.; Hong, K.; Rubner, M. F. Synth. Met. 1989, 28, 229. (16) Torres, W.; Fox, M. A. Chem. Mater. 1992, 4, 583. (17) Cho, G.; Glatzhofer, D. T.; Fung, B. M.; Yuan, W. L.; O’Rear, E. A. Langmuir 2000, 16, 4424. Cho, G.; Fung, B. M.; Glatzhofer, D. T.; Lee, J. S.; Shul, Y. G. Langmuir 2001, 17, 456. (18) Selvan, S. T. Chem. Commun. 1998, 351. Selvan, S. T.; Spatz, J. P.; Klok, H. A.; Moller, M. Adv. Mater. 1998, 10, 132. (19) Wang, S.; Yang, S. Chem. Phys. Lett. 2000, 322, 567. Wang, S.; Yang, S. Adv. Mater. Opt. Electron. 2000, 10, 39. Wang, S.; Yang, S. Mater. Sci. Eng., C 2001, 16, 37. Wang, S.; Yang, S.; Dai, Z. R.; Wang, Z. L. Phys. Chem. Chem. Phys. 2001, 3, 3750. Wang, S.; Yang, S. Chem. Mater. 2001, 13, 4794. Wang, N.; Fung, K. K.; Wang, S.; Yang, S. J. Cryst. Growth 2001, 233, 226. Wen, X.; Yang, S. Nano Lett. 2002, 2, 451. (20) Lu, Y.; Shi, G.; Li, C.; Liang, Y. J. Appl. Polym. Sci. 1998, 70, 2169.

Langmuir, Vol. 19, No. 10, 2003 4421 solid reaction for the synthesis of Cu2S nanorod arrays on the preoxidized copper foils was performed in a 500 mL homemade reactor consisting of a glass bottle with a rubber plug at the mouth. A two-way cock was installed in the middle of the plug for gas delivery. The glass bottle loaded with four preoxidized copper foils (0.2 cm × 0.4 cm) was evacuated and then filled with 180 mL (1 atm) of H2S and 360 mL (1 atm) of O2 (H2S:O2 ) 1/2). The reactor was kept at room temperature. After 3 h of reaction, the copper foil surfaces became black and fluffy, indicating the formation of Cu2S nanorod arrays. A typical reaction time was 6.0-16.0 h. The nanorod arrays were used directly for the PPY coating. Growth of Ultrathin PPY Films on the Cu2S Nanorod Surfaces. A 100 mL beaker was used to coat the Cu2S nanorods by in situ polymerization of pyrrole at room temperature. A typical synthesis is illustrated in Scheme 1. 0.1 mL of pyrrole was dissolved in 40 mL of chloroform. Then the Cu2S nanorod arrays, freshly grown on copper foil, were immersed in this solution and stirred for 2 h. Afterward, the copper foil surface arrayed with the Cu2S nanorods was raised and positioned right on the surface of the chloroform solution without magnetic stirring. 20 mL of a (NH4)2S2O8 aqueous solution (0.1 M) was added slowly to the chloroform solution, and an interfacial layer could be easily observed between the aqueous solution and the chloroform solution, where the Cu2S nanorod arrays were held. A thin white film was formed at the interface within half an hour, which turned to gray/black within another half an hour. By then the film nearly covered the whole interface. The film became increasingly black with increasing polymerization time. After different polymerization times, such as 1.0, 2.0, 2.5, and 3.5 h, the PPY-coated Cu2S nanorod arrays were carefully taken out and washed with methanol 3-5 times to remove any free PPY and dried at room temperature. Characterization of the Noncoated and PPY-Coated Cu2S Nanorod Arrays. X-ray diffraction (XRD) analysis was performed on a Philips PW 1830 X-ray diffractometer with a 1.5405 Å Cu KR rotating anode point source. The source was operated at 40 kV and 40 mA, and the Kβ radiation was eliminated using a nickel filter. Fourier transform infrared (FTIR) transmission spectra were taken on a Perkin-Elmer 16PC FTIR spectrometer from 4000 to 400 cm-1. Samples of Cu2S/PPY nanorod arrays were scraped from copper foil surfaces and then mixed with KBr powder for FTIR measurements. Background correction was made using a blank KBr pellet as the reference. Raman spectra were obtained on a Renishaw RM 3000 spectrometer using the green line of an Ar+ laser (514.5 nm) in the microscopic configuration (objective ×20). PPY-coated Cu2S nanorods arrayed on copper foils were directly used for microRaman measurements. Transmission electron microscopy (TEM) measurements were conducted with a Philips CM20 transmission electron microscope, using an accelerating voltage of 200 kV. Samples for TEM observation were prepared by putting the nanorod-covered copper foils over copper grids with carbon film and then dragging the copper foils gently along the overlapping surface. The morphologies of noncoated and PPY-coated Cu2S nanorod arrays were observed on a JEOL JSM-6300F scanning electron microscope (SEM) at an accelerating voltage of 10 kV. Sometimes, a thin film of gold was sputtered on the sample surface to prevent charging. A JEOL JSM-6300 scanning electron microscope (SEM) was used for energy-dispersive analysis of X-ray fluorescence (EDAX) of the original samples at an accelerating voltage of 20 kV.

Results and Discussion Growth of Cu2S Nanorod Arrays. The Cu2S nanowires we reported previously were formed from the oxidation of copper, Cu(s) + 1/2O2 ) Cu2O(s), followed by sulfidization, Cu2O(s) + H2S(g) ) Cu2S(s) + H2O(g).19 Although preoxidation of the copper surface with O2 promoted the growth of the Cu2S nanowires, uniform nanowire arrays in large areas have not been obtained until now. The key is to use a stronger oxidant, (NH4)2S2O8, for the pretreatment of the copper surface. Figure 1A shows a SEM image of Cu2S nanowires prepared on a fresh copper foil without any substrate

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Scheme 1. Growth of PPY Nanolayers on the Cu2S Nanorod Arrays at the Interface of Chloroform and Water (1: Cu2S Nanorod Arrays; 2: Copper Foil; 3: Magnetic Bar)

pretreatment. The gas mixture was H2S:O2 ) 1/2, and the reaction time was 8.5 h. As can be seen, the nanowires in certain regions grew longer, thicker, and more luxuriantly than in others. It appears that the copper surface without preoxidation exhibits nonuniform surface activity for the growth of Cu2S nanowires. A SEM image of the Cu2S nanorod arrays obtained with the pretreatment of the Cu surface using an aqueous solution of (NH4)2S2O8 is shown in Figure 1B. Again, the gas mixture for reaction was H2S:O2 ) 1/2. The reaction was continued for 10.0 h. One can see that the Cu2S nanorod arrays exhibit much better order and uniformity over large areas. The preoxidation step probably oxidizes the Cu surface in a homogeneous fashion so that the nucleation of the Cu2S nanorods becomes more synchronized in time and more uniform in space. We believe that only a very thin oxide layer was formed due to the pretreatment step because no detectable oxide phases have been observed by XRD. The preoxidation of the Cu surface not only improved the uniformity of the nanorod arrays significantly but also increased the growth speed of the nanorods markedly. For example, the Cu2S nanorod arrays with tip diameters of ∼140 nm and lengths of ∼1 µm could be formed in less than 3.0 h on the preoxidized Cu surface, whereas it took at least 6.0 h to grow the nanorods of similar diameters and lengths on a fresh Cu surface under otherwise the same reaction conditions. For the substrate preoxidation method, the growth time and the molar ratio of H2S to O2 were also found to be important in the synthesis of the Cu2S nanorod arrays.

As the reaction time was increased from 3.0 to 16.0 h while keeping the molar ratio of H2S to O2 (1/2), the nanorod tip diameters decreased from 140 to 50 nm and the lengths increased from 1 to 5 µm. When the reaction time was 10.0 h, the Cu2S nanorods were ∼3 µm long and had tip diameters of ∼100 nm (Figure 1B). With a reaction time of 14.5 h, the nanorods were ∼5 µm long and their tip diameters were reduced to ∼60 nm (Figure 5A-C). For growth beyond 20 h as shown in Figure 1C, the Cu2S nanorods became rather thick, e.g., the tip diameters were ∼500 nm, although the length and morphology were similar. We believe that the initial tip thinning with time is due to the cone growth, and this cone growth pattern is slowed down or stopped, leading to the thickening of the nanorods at long reaction times. It appears from the above discussion that the nanorod morphology was always maintained at different reaction times when the molar ratio H2S to O2 was 1:2. When the molar ratio of H2S to O2 was increased from 1:2 to 1:1, however, uniform Cu2S nanorod arrays could not be obtained even with the preoxidation method. This is revealed in Figure 1D, which shows the SEM image of the Cu2S nanowires prepared on a preoxidized Cu surface at H2S/O2 ) 1:1 and with a reaction time of 36 h. Instead of nanorod arrays, Cu2S nanowires were formed with lengths of up to 100 µm and diameters of tens of nanometers. Most of them are entangled and overlapped with each other and therefore are not good for templated synthesis. For the PPY coating experiment described below, we used

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Figure 1. SEM images of Cu2S nanorods or nanowires: (A) prepared at H2S/O2 ) 1:2 and with a reaction time of 8.5 h on a fresh Cu surface without any substrate pretreatment; (B) prepared at H2S/O2 ) 1:2 and with a reaction time of 10.0 h on a preoxidized Cu surface; (C) prepared at H2S/O2 ) 1:2 and with a reaction time of 24.0 h on a preoxidized Cu surface; (D) prepared at H2S/O2 ) 1:1 and with a reaction time of 36.0 h on a preoxidized Cu surface.

Figure 2. XRD patterns of (A) noncoated Cu2S nanorod arrays, (B) pure PPY film, and (C) PPY-coated Cu2S nanorod arrays.

mainly the Cu2S nanorod samples prepared under the reaction conditions corresponding to Figure 5A-C. PPY Coating by in Situ Polymerization. Typical XRD patterns for the bare Cu2S nanorod arrays, pure PPY film, and PPY-coated Cu2S nanorod arrays on copper substrates are shown in Figure 2. All the three XRD patterns display two peaks at 2θ ) 43.3° and 2θ ) 50.5° due to the copper substrate. The XRD pattern of pure PPY film (Figure 2B) shows a broad feature centered at 2θ ) 25.0°. This can be assigned to the interchain spacings

of PPY.21 In the XRD pattern of the bare Cu2S nanorod arrays (Figure 2A), all the peaks observed, except the two from the copper substrate at 2θ ) 43.3° and 2θ ) 50.5°, can be indexed to the monoclinic Cu2S (JCPDS Card File No. 33-0490). For the PPY-coated Cu2S nanorod arrays (Figure 2C), the XRD pattern shows the characteristic broad features of PPY and also some sharp peaks associated with the monoclinic Cu2S. The diffraction peaks of Cu2S became weaker after the PPY coating. The XRD data show that PPY films were formed that coexisted with the Cu2S nanorods. Figure 3 shows FTIR of liquid pyrrole (A) and the PPYcoated Cu2S nanorod arrays (B) obtained after a pyrrole polymerization time of 3.5 h. The peaks in Figure 3B are all due to the coating because monoclinic Cu2S does not exhibit absorption in this spectral range.22 In addition, most bands in Figure 3B seem to be broader than those in Figure 3A due to more extended conjugation arising from polymerization. Two new bands at 1210 and 925 cm-1 after polymerization (Figure 3B) can be assigned to bipolaron bands.18,23 For conjugated conducting polymers, (21) Fan, J.; Wan M.; Zhu, D.; Chang, B.; Pan, Z.; Xie, S. J. Appl. Polym. Sci. 1999, 74, 2605. Yamaura, M.; Hagiwara, T.; Hirasaka, M.; Demura, T.; Iwata, K. Synth. Met. 1989, 28, 157. Cai, Z.; Lei, J.; Liang, W.; Menon, V.; Martin, C. R. Chem. Mater. 1991, 3, 960. (22) Nyquist, R. A.; Kagel, R. O. Handbook of Infrared and Raman Spectra of Inorganic Compounds and Organic Salts; Academic Press: San Diego, CA, 1997; Vol. 4, p 248. (23) Tian, B.; Zerbi, G. J. Chem. Phys. 1990, 92, 3886. Tian, B.; Zerbi, G. J. Chem. Phys. 1990, 92, 3892. Kostic, R.; Rakovic, D.; Stepanyan, S. A.; Davidova, I. E.; Gribov, L. A. J. Chem. Phys. 1995, 102, 3104.

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these extra IR bands are generated by doping and thus commonly called “doping-induced bands”. Therefore, the observation of the two bipolaron bands signals the formation of PPY in a doped state. The bands at 34003100 and 776 cm-1 (Figure 3B) are readily assigned to the N-H/C-H stretching and C-H ring out-of-plane bending modes, respectively.23,24 After polymerization, the narrow bands of the C-H in-plane deformation vibration of pyrrole (1014-1075 cm-1) were changed to a broad peak at 1119 cm-1, which increased with polymerization time.24,25 The peak at 1697 cm-1, which is normally assigned to the Cd O stretching, suggests some overoxidation of the pyrrole rings.20,26 The CdO groups were also reportedly present in chemically synthesized pure PPY, and they were assumed to be part of the oxidized pyrrole ringspyrrolidinone.13 As such, the PPY polymer nanosheath densely packed around the Cu2S nanorods is probably constructed through mixed linkages of the pyrrole moieties and the CdO bearing pyrrolidinone units. Shown in Figure 4 are Raman spectra of PPY-coated Cu2S nanorod arrays obtained after polymerization for 1.0, 2.0, and 3.5 h. Note that the monoclinic Cu2S has no Raman-active peak in the spectral range 900-1800 cm-1.

Two major broad peaks are observed at ∼1350 and ∼1600 cm-1 in all the spectra, but the intensity increases with increasing polymerization time accompanied by a blue shift of the peaks. The strong and broad peak at ∼1600 cm-1 is assigned to the CdC backbone stretching mode of PPY.23 When the polymerization time is 1.0 h, the CdC backbone stretching mode occurs at 1580 cm-1 (Figure 4A). This mode is shifted to 1600 cm-1 when the polymerization time is beyond 2.0 h (Figure 4B,C), indicating the transformation of PPY from the reduced state to the oxidized state.27 The blue shift is believed to be due to the increased π-electron density induced by charge transfer. This is confirmed by the increasing intensity of the C-H in-plane deformation peaks of PPY at 1053 and 1085 cm-1 with increasing polymerization time (Figure 4B,C).27,28 The broad peak at 1350 cm-1 is assigned to the pyrrole ring stretching mode, which increases and broadens with increasing polymerization time. The peak at 990 cm-1 is attributed to the pyrrole ring in-plane deformation mode, which becomes discernible when the polymerization time is beyond 2.0 h. By changing the polymerization conditions, one can control the redox state of PPY to a certain extent and therefore control the conductivity of the film. In fact, the oxidized form of PPY displays higher conductivity as found by Liu et al., and therefore the oxidation can be regarded as a doping process.28 Scanning electron micrographs (SEM) of the noncoated (A, B, and C) and PPY-coated (A′, B′, and C′; A′′, B′′, and C′′) Cu2S nanorod arrays are shown in Figure 5 at different magnifications. The SEM images A, B, and C correspond to the noncoated Cu2S nanorod arrays. The panels A′, B′, and C′ and A′′, B′′, and C′′ display the SEM images of the PPY-coated Cu2S nanorod arrays prepared with pyrrole polymerization times of 2.0 and 3.5 h, respectively. Although the samples could be imaged using SEM even without an Au overlayer, we still sputter-coated a thin layer of Au on the Cu2S nanorod arrays so as to enhance the SEM image quality. This was normally done after the EDAX analysis. The top three SEM images in Figure 5 (A, A′, and A′′) present the top views of nanorod arrays at low magnifications. Clearly, both the bare and PPY-coated Cu2S nanorod tips form uniform, ordered arrays over large areas. In the middle three SEM images (B, B′, and B′′), the magnifications have been increased. Note that in Figure 5B the surface of the nanorod arrays was modified from the original surface shown in Figure 5A; a crack is evident at the roots of the Cu2S nanorods. This crack might be produced during sample transfer. Nevertheless, this feature allows us to get a better side view of the Cu2S nanorods: they are ∼5 µm long, isolated from each other, and roughly perpendicular to the substrate surface. Interestingly, the nanorod tips have diameters of 50-100 nm, which are much thinner than those in the nanorod roots (droots ) 200-300 nm). This needle-type morphology is preferentially produced under more oxidative conditions, as observed previously.19 Cylindrical nanorods could be produced by controlling the ratio of O2 to H2S. For the PPY-coated Cu2S nanorods, no PPY precipitates can be seen in the gaps between the nanorods in the SEM images B′ and B′′ in Figure 5; the thin PPY layers are closely attached to the Cu2S cores, and the composite nanorods are still well isolated from each other in the form of arrays. At higher magnification, the SEM images C, C′, and C′′

(24) Kojima, T.; Takaku, H.; Urata, Y.; Gotoh, K. J. Appl. Polym. Sci. 1993, 48, 1395. (25) Yakovleva, A. A. Russ. J. Electrochem. 2000, 36, 1275. (26) Pfluger, P.; Krounbi, M.; Street, G. B. J. Chem. Phys. 1983, 78, 3212. Novak, P. Electrochim. Acta 1992, 37, 1227.

(27) Liu, Y. C.; Hwang, B. J.; Jian, W. J.; Santhanam, R. Thin Solid Films 2000, 374, 85. Liu, Y. C.; Hwang, B. J. Synth. Met. 2000, 113, 203. (28) Liu, Y. C. Langmuir 2002, 18, 174. Demoustier-Champagne, S.; Stavaux, P. Y. Chem. Mater. 1999, 11, 829.

Figure 3. FT-IR spectra of (A) liquid pyrrole and (B) PPYcoated Cu2S nanorod arrays obtained with a pyrrole polymerization time of 3.5 h.

Figure 4. Micro-Raman spectra of PPY-coated Cu2S nanorod arrays obtained with a pyrrole polymerization time of (A) 1.0, (B) 2.0, and (C) 3.5 h.

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Figure 5. (A-C) SEM images of noncoated Cu2S nanorod arrays; (A′-C′) SEM images of PPY-coated Cu2S nanorod arrays obtained with a pyrrole polymerization time of 2.0 h; (A′′-C′′) SEM images of PPY-coated Cu2S nanorod arrays obtained with a pyrrole polymerization time of 3.5 h. The images from top to bottom panels are at different magnifications.

in Figure 5 reveal more clearly the surface morphology. The PPY coating appears to cover the Cu2S nanorods along their entire lengths. As the polymerization time increases, the PPY/Cu2S composite nanorods become thicker, and the surfaces of the PPY sheaths become somewhat rougher, although the nanorods still retain their individual identities. In addition, the needle-shaped Cu2S nanorods become more cylindrical after being coated with PPY, suggesting that the rate of PPY deposition is faster at the nanorod tips than that at the roots; this is due perhaps to better access to the pyrrole monomer. TEM images were recorded to examine the effect of the pyrrole polymerization time on the quality and thickness of the PPY coating layer enclosing the Cu2S nanorod core. All the samples for this measurement were prepared under the same polymerization conditions, except with different polymerization times. Figure 6 presents TEM micrographs of the samples prepared with polymerization times of 1.0 (A), 2.0 (B), 3.5 (C), and 5.0 h (D). When the polymerization time was 1.0 h, the PPY coating covered the entire length of the Cu2S nanorods (Figure 6A). However, this PPY coating layer is still thin and not quite uniform. For pyrrole polymerization times of 2.0-3.5 h, a very uniform PPY film was seamlessly attached to the Cu2S nanorods. In Figure 6B, corresponding to the pyrrole polymerization time of 2.0 h, the Cu2S nanorod core has a diameter of 65 nm and the PPY coating is 20 nm thick. A longer polymerization time of 3.5 h resulted in a thicker PPY coating layer; as Figure 6C shows, a 62.5 nm diameter Cu2S nanorod dovetailed into a PPY nanotube with a wall

thickness of 50 nm. With further increases in the polymerization time, e.g., 5.0 h, the PPY film became even thicker (Figure 6D). It is noteworthy that when the polymerization time was 5.0 h or longer, the PPY coating became rough again on the outer surface although the inner joint between the Cu2S core and the PPY sheath remained tight. The surface roughness of the PPY film may be caused by PPY precipitation; many PPY pieces are lumped together on the surface of the original uniform PPY film. This can be severe when the PPY film becomes too thick. Selected area electron diffraction (SAED) from a PPY coated nanorod shown in the inset of Figure 6B indicated that the single crystalline structure of the inner core of Cu2S remained unchanged. This is consistent with the XRD data given in Figure 2C. In addition, the PPY-coated Cu2S nanorod samples have been characterized by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics PHI 5600 multitechnique system with a monochromatic Al KR X-ray source. Before measurements, the samples were first washed with methanol to remove free pyrrole and then with deionized water to take way (NH4)2S2O8. Cu 2p3/2 XPS and Cu LMM Auger spectra show characteristics of Cu(I) of Cu2S. N 1s XPS exhibits a pronounced peak at 400.8 eV due to N-H in PPY.29 (29) Perruchot, C.; Chehimi, M. M.; Delamar, M.; Cabet-Deliry, E.; Miksa, B.; Slomknowski, S.; Khan, M. A.; Armes, S. P. Colloid Polym. Sci. 2000, 278, 1139. Radhakrishnan, S.; Adhikari, A.; Awasthi, D. K. Chem. Phys. Lett. 2001, 341, 518. Kang, E. T.; Neoh, K. G.; Tan, K. L. Adv. Polym. Sci. 1993, 106, 135.

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PPY or even failed to have any PPY coating. Second, the pyrrole polymerization should be slow enough so that the nanorod surface polymerization is preferred to the bulk process. The polymerization is likely a stepwise reaction rather than a chain reaction.30-32 The polymerization is initiated by the generation of pyrrole radical cations with the oxidant (NH4)2S2O8, which are then propagated by the stepwise incorporation of the pyrrole monomer units until termination. Therefore, the molar ratio of PY to oxidant and the concentrations of PY in chloroform influenced the kinetics and the outcome of polymerization. When the ratio of PY to oxidant was 1:1, the appropriate pyrrole concentration in chloroform was found to be ∼0.036 M, which is so dilute that only surface polymerization occurs and is therefore ideal for the nanolayer growth on the Cu2S nanorods. When the pyrrole concentration in chloroform was >0.036 M and the molar ratio of PY to oxidant was 1:1, low-quality particulate films were obtained due to incomplete polymerization.

Figure 6. TEM images of PPY-coated Cu2S nanorod arrays obtained with a pyrrole polymerization time of (A) 1.0, (B) 2.0, (C) 3.5, and (D) 5.0 h. Inset of (B): SAED of the single PPYcoated Cu2S nanorod.

The interfacial layer technique was reported by Lu et al. for the synthesis of PPY thin films with a thickness of 3-4 µm at the interface between the chloroform phase with dissolved pyrrole (PY) and the aqueous phase with dissolved oxidant (NH4)2S2O8.20 We have extended the application of this technique to the growth of ultrathin PPY films on the Cu2S nanorods arrayed on a copper surface. Such in situ polymerization confined at the chloroform-water interface and the interrod pores allowed us to assemble PPY on the nanorod surfaces. Because the height of Cu2S nanorod arrays was only ∼5 µm, the nanorods could be wrapped completely with PPY when they were introduced into the interfacial layer for the coating. Several points are worth noticing for the growth of uniform PPY films on the Cu2S nanorod arrays. First, PY should be dissolved in chloroform completely before coating. To accelerate dissolution, magnetic stirring was used. When magnetic stirring was for less than 1.0 h, some of the Cu2S nanorods were coated with too much PPY, while others were not completely encapsulated with

Conclusions In summary, we have synthesized isolated and uniform Cu2S nanorods in well-aligned large arrays on a copper surface using a modified procedure. By using an interfacial polymerization technique, the Cu2S nanorods were successfully coated with ultrathin, homogeneous, and welladhered layers of a conducting polymer PPY with a thickness of 20-50 nm. The PPY film growth could be controlled at the interfacial layer of chloroform and water by the polymerization time, the PY concentration, and the PY-to-oxidant ratio. The method presented in this paper may be extended to synthesize other one-dimensional nanoscale composites. The PPY layer is expected to improve the stability and mechanical properties of the Cu2S nanorods, which are otherwise environmentally sensitive and mechanically brittle. We anticipate that the successful coating with the conducting polymer will facilitate the application of the Cu2S nanorod arrays in nanoelectronics, batteries, solar cells, and sensors. Acknowledgment. This work was supported by an RGC grant administered by the UGC of Hong Kong. We thank Dr. Suhua Wang for his help during the initial experiment and Dr. Yesha Zheng and MCPF of HKUST for assistance in sample characterization. LA020894W (30) Chandrasekhar, P. Conducting Polymers, Fundamentals and Applications: A Practical Approach; Kluwer Academic Publishers: Boston, 1999; Chapter 1, p 21. (31) Ayad, M. M. J. Appl. Polym. Sci. 1994, 53, 1331. (32) Castillo-Ortega, M. M.; Inoue, M. B.; Inoue, M. Synth. Met. 1989, 28, C65.