INSTITUTE OF PHYSICS PUBLISHING
NANOTECHNOLOGY
Nanotechnology 17 (2006) S376–S380
doi:10.1088/0957-4484/17/11/S24
In situ growth and characterization of Ag and Cu nanowires X Ding1 , G Briggs2 , W Zhou2 , Q Chen3 and L-M Peng3,4 1 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, PO Box 603, Beijing 100080, People’s Republic of China 2 School of Chemistry, University of St Andrews, St Andrews KY16 9ST, UK 3 Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, People’s Republic of China
E-mail:
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Received 14 December 2005, in final form 23 February 2006 Published 19 May 2006 Online at stacks.iop.org/Nano/17/S376 Abstract Nanowires of Ag and Cu have been produced from metal-loaded zeolites and micropore-containing mesoporous silica SBA-15 in situ inside the electron microscope. The structures of the nanowires have been characterized by using high-resolution electron microscopy and the compositions have been examined by energy dispersive x-ray spectroscopy. The growth of single-crystal nanowires was controlled by the electron beam density. The growth mechanism of the metal nanowires is discussed.
1. Introduction In recent years, investigation of one-dimensional (1D) nanostructural materials, especially metal nanowires, has attracted much attention, because of their interesting chemical, mechanical and physical properties, and potential applications in nanoelectronics and optoelectronics device [1–5]. Metallic nanowires may also provide a unique model system to experimentally probe physical phenomena such as quantized conductance and localization effects [6]. Among these nanowires, Ag and Cu are particularly interesting since they have excellent electrical and thermal conductivity and many significant applications, including interconnects in nanodevices [7–14]. Many methods [15–17], particularly template-assisted methods, have been developed to synthesize 1D nanomaterials of metals, including deposition of metals in carbon nanotubes [18], electrodeposition in nanopores of anodized alumina and polymer templates [19, 20], molecule sieves [21–24] block copolymers [25], Langmuir–Blodgett films [26], DNA molecule templates [27], sodium dodecyl sulfate (SDS) micelles–copolymer gel templates [28] and oleate vesicle templates [29]. Among these methods, fabrication of nanowires from zeolites under electron beam exposure, first reported in 2001 [23], is of particular interest because the length of the nanowires is not limited by a template, although the for4 Author to whom any correspondence should be addressed.
0957-4484/06/110376+05$30.00
mation mechanism is far from being fully understood. According to a recent report, this method can be applied easily for other metal nanowires, such as copper nanowires [30]. It was found by these authors that the nanowires were loose at one end and often vibrated under electron beam exposure. Good atomic images of the nanowires were therefore not recorded and the growth orientation of the singe-crystal nanowires was difficult to investigate. It was also believed [23] that the Ag-loaded zeolite was light sensitive and therefore all the synthetic work was carried out in the dark. In this paper we report the growth of Ag and Cu nanowires inside the electron microscope and detailed in situ characterization of these nanowires. We show that metal nanowires can be produced in various microporous solids and that mesopores are not suitable channels for metal migration, which is the most important step in the formation of the nanowires. We found that the Ag-containing specimens were quite stable in the light and under atmospheric conditions for a period of several months after metal-loading. The synthesis of the Ag-loaded zeolite for growing Ag nanowires was therefore carried out in the light, which greatly simplifies the procedure developed earlier [23].
2. Experimental section The principle of the synthesis procedure is similar to that in the previous report [23]. The precursor materials for the production of silver nanowires were synthesized by a two-stage
© 2006 IOP Publishing Ltd Printed in the UK
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Figure 1. (a) SEM image of silver nanowires (indicated by the arrows) from Ag-loading zeolite A with a beam energy of 30 keV. (b) TEM image of a Ag nanowire from zeolite A with the beam energy corresponding to 200 kV. (c) TEM image of a Ag nanowire using the beam of 200 keV with a plait-like shape. The inset is an enlarged image. (d) Image of a Ag nanowire with a small branch. The arrow indicates the direction of the nanowire growth.
process. Zeolite A, zeolite Mazzite, STA-7, ZSM-5, SBA15 and MCM-41 were synthesized through the established hydrothermal procedures and ion-exchanged with an aqueous solution of 0.5 M AgNO3 at 35 ◦ C overnight. After filtering and drying, the resulting Ag+ ion-exchanged molecular sieves were then mechanically ground with excess solid silver nitrate (its decomposition temperature is 450 ◦ C) and heated to 280 ◦ C for 10 h to further introduce argentous ions into the pores by the process of salt occlusion. The samples were then washed with a minimum amount of water to remove excess silver salt from the surface of the crystallites. To grow the copper nanowires, zeolite Y was stirred in a 0.1 M aqueous solution of copper (II) acetate for 5 h and then washed with water until the filtrate was clear. The resulting pale turquoise powder was dried overnight and ground with solid copper (I) chloride. The mixed powder was pressed into a pellet (1 cm in diameter), heated to 460 ◦ C in N2 for 10 h (ramp rate of 5 ◦ C min−1 ), and then allowed to cool. The product was washed with water until the filtrate ran clear, and then dried overnight. The resulting powder was dark green. Zeolite A was also used for loading copper. In this case, the precursor material was heated at 260 ◦ C in argon gas for 4 h. Electron microscopic studies were performed on a Philips XL30 scanning electron microscope (SEM) operating at 10– 30 kV and JEOL JSM-5600; a FEI Tecnai F20 transmission electron microscope (TEM) operated at 200 kV and a JEOL JEM-2011 electron microscope also operated at 200 kV. For TEM, samples were loaded onto a specimen grid, coated with a holey carbon film, and then transferred into the TEM.
3. Results and discussions Both the TEM and the SEM were used to draw nanowires from the Ag/zeolite A precursor materials, with both the conventional W-filament and field emission guns. The applied energy of the electron beam ranged from 10 to 200 keV
Figure 2. (a) TEM images of a single-crystal Ag nanowire from zeolite A. (b) HRTEM image of the silver nanowire. Two principal d -spacing are marked.
(figure 1). It was found that the effect of the energy of the electron beam on the nanowire growth is not significant, although the speed of nanowire growth is slower with the lower accelerating voltage. The template zeolite A used in the present work has two types of morphology, namely cubic and orbicular (figure 1(a)). It was found that silver nanowires can be drawn from the cubic crystallites easily. But it was relatively difficult to produce nanowires from orbicular particles. Since the levels of Agloading in these two types of particles are the same as those detected by energy dispersive x-ray spectroscopy (EDX), the possible reason for the fast extraction of the Ag nanowires from cubic crystallites is their higher crystallinity which offers better channel structures for silver migration. In most cases, there was only one nanowire extracted from one zeolite particle as shown in figure 1(b). Therefore, the nanowires grew continuously with a very long length. The nanowires show three basic shapes. One is straight and has uniform diameter (figure 2(a)). The second is curved, also with a uniform diameter (figure 1(b)). The last one looks like a plait (figure 1(c)). The morphology of the nanowires depends on the rate of crystal growth. When the crystal grows very quickly, e.g. 10 to 30 s for the whole growth process, with an electron beam intensity of 300 to 400 pA cm−2 , the nanowires are curved and polycrystalline. If the crystal grows slowly, e.g. more than 2 min, with a beam intensity of about 20 pA cm−2 , the nanowires are single crystals, as proved by HRTEM images and selected area electron diffraction (SAED) patterns, and their morphology is straight. When the rate of crystal growth changed periodically, a plait-like morphology formed, although we do not fully understand why the rate changes periodically under the same beam exposure conditions. The length of the nanowires can be controlled via switching on and off the electron beam and the rate of crystal growth can be varied by changing the density of the electron beam. Occasionally, Ag nanowires with a small branch were also observed (figure 1(d)). This phenomenon can be explained as follows. When the large nanowire grew, a small nanowire formed at a close, but not confined, location. When the small nanowire grows towards the large one and breaks through the silica network between these two adjacent nanowires, it eventually joins the large nanowire and its crystal growth stops. S377
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Figure 2(a) shows a low magnification image of a typical single-crystal Ag nanowire grown from a zeolite A particle when the growth speed was very slow; the HRTEM image of a part of the nanowire is shown in figure 2(b). A measurement of interplanar distances indicates that the single-crystal silver nanowire possesses a face centered cubic (fcc) structure with ¯ planes of 0.23 nm, measured d -spacings of the (111) and (111) which are consistent with the standard lattice constant of the Ag crystal structure. The HRTEM image was taken along ¯ zone axis, and from this image it can be seen that the [011] the growth direction was along [211], indicated by the arrow in figure 2(b). However, different crystal growth orientations were observed from different nanowires. It is also noticed that the surface of the nanowire is very clean, without an amorphous coating layer. If the nanowires do not join together, more than one nanowire might be produced. In some cases, a bunch of nanowires was extracted from a zeolite particle (figure 3). It is very interesting to see that nanowires were never extruded randomly from the whole surface of the zeolite particles. Instead, they always came out from the same area and therefore were parallel to each other. This indicates that when a nanowire forms in a near-surface area of a zeolite particle, silver cations migrate from everywhere in the crystallite towards the nanowire. The silver concentration near the nanowires becomes higher and the chance to form new crystal seeds increases. We now discuss how the Ag+ cations are reduced. The framework of a zeolite is very stable and unlikely to act as an electron donor. EDX study indicates that the resource of electrons for Ag-reduction might be NO− 3 . Figure 4 is two typical EDX spectra, from Ag-loaded zeolite A and from the Ag nanowires only. It was confirmed that nitrogen exists in the precursor materials before the nanowire growth (figure 4(a)), while no nitrogen was detected either in the zeolite particles or in the silver nanowires (figure 4(b)) after the nanowire growth. Consequently, based on the repeated observations in our experiments, a reasonable explanation of the nanowire formation is that the decomposition of NO− 3 provides electrons for the reduction of the Ag+ cations. The relevant reactions are as follows:
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Figure 5. TEM image of a single-crystal Ag nanowire produced from SBA-15.
Ag nanowires can also be produced from all other zeolites used in the present work, such as zeolite Mazzite, STA-7, ZSM-5, etc. Figure 3(b) shows a bunch of Ag nanowires extruded from zeolite Mazzite. These zeolites have significantly different structures. However, all of them have networks with three-dimensional channels with bottleneck diameters of about 0.4 to 0.7 nm, corresponding to 4-member rings up to 12-member rings. Therefore, these zeolites provide three-dimensional micropores for the migration of Ag+ cations (with an atomic diameter of about 0.25 nm). Both MCM-41 and SBA-15 have ordered one-dimensional mesopores. However, the former has no micropores, while the silica framework of the latter normally contains a large amount of micropores in the wall. It was observed that silver nanowires can be extruded from SBA-15 (figure 5), but not from
In situ growth and characterization of Ag and Cu nanowires
Figure 6(b) indicates that the single-crystal copper nanowire possesses the face centred cubic (fcc) structure; the image of ¯ zone axis, which was the copper nanowire was taken the [111] taken from the frame of figure 6(a). The FFT image inset in figure 6(b) also verified that the nanowire was just copper ¯ nanowire. The nanowire growth orientation is along the [110] direction. Unlike single-crystal Ag nanowires (figure 2(b)) where a clean surface was observed, an amorphous surface coating layer (thinner than 1 nm) can be seen from the surface of the Cu nanowires (figure 6(b)). EDX analysis indicates that this coating layer contains Cl and O (figure 6(c)). Carbon comes from the carbon film on the grid, and the copper element of the specimen grid contributes a little to the Cu element peak. Figure 6(d) shows a Cu nanowire from zeolite Y. It is polycrystalline and its diameter is not uniform. However, the SAED pattern from a small part of the nanowire shows a single-crystal pattern. EDX detected Cl from this nanowire. This indicates that oxidation of Cl− is much more difficult in comparison with NO− 3 . In this case, the resource of electrons for the reduction of Cu+ is probably from Cu+ itself:
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Figure 6. (a) TEM and (b) HRTEM images of a single-crystal Cu nanowire from zeolite A. The atomic planes are indexed according to the fcc cubic Cu unit cell. (c) EDX spectrum of a surface area of the Cu nanowire shown in (b). (d) A polycrystalline Cu nanowire from zeolite Y.
MCM-41. Anderson and co-workers have also gained similar results using SBA-15 as a precursor [31]. We also found that the silver nanowires were not parallel to the mesopores in SBA-15. Consequently, existing mesopores in molecule sieves cannot be used as templates for nanowire growth. They are too large to be suitable channels for the migration of silver atoms. Silver atoms can only migrate in micropores in solids such as zeolites and SBA-15 mentioned above, although in the latter, the micropores are neither regular in shape nor ordered. For growing copper nanowires, zeolite A and zeolite Y were used as support materials. Cu(NO3 )2 were initially loaded in zeolite A. No copper nanowires were produced in these Cu2+ -containing materials. The copper nanowires were indeed produced from CuCl-containing zeolites. We believe that the main reason for the above results is that Cu+ cations are much easier to be reduced under the electron irradiation. Typical TEM and HRTEM images of copper nanowires from zeolite A are shown in figures 6(a) and (b). Figure 6(a) shows that the diameter of the nanowire is uniform and over 100 nm.
In summary, electron beam induced growth of silver and copper nanowires from molecule sieves, such as microporous zeolite A, zeolite Mazzite, zeolite STA-7, zeolite Y and mesoporous SBA-15, has been achieved. The structure and growth orientations of the nanowires were investigated, and the growth mechanism of the metal nanowires discussed.
Acknowledgments This work was support by the National Science Foundation of China (Grant No 10434010), the Chinese Ministry of Education (Grant Nos 10401 and 20030001071), and the National Center for Nanoscience and Technology of China. WZ thanks SHEFC for financial support.
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