CRYSTAL GROWTH & DESIGN
Control Synthesis of Silver Nanosheets, Chainlike Sheets, and Microwires via a Simple Solvent-Thermal Method Jimin
Du,†
2007 VOL. 7, NO. 5 900-904
Buxing Han,* Zhimin Liu, and Yunqi Liu*
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
Dae Joon Kang* BK21 Physics Research DiVision, Institiute of Basic Science, SKKU AdVanced Institute of Nanotechnology, and Center for Nanotubes and Nanostructured Composites, Sungkyunkwan UniVersity, Suwon 440-746, Republic of Korea ReceiVed October 1, 2006; ReVised Manuscript ReceiVed February 2, 2007
ABSTRACT: In this work, we present a facile method to synthesize silver nanosheets, chainlike sheets, and microwires via decomposition of the AgNO3 in ethanol in the presence of ammonia. The obtained samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected-area electronic diffraction (SAED), and energy dispersive X-ray analysis (EDXA). The experiments show that the concentration of ammonia, reaction temperature, and time are important factors for controlling the morphology of the products. On the basis of our experimental results, we propose an ammonia-tuning Oswald ripening mechanism to elucidate the formation of different morphologies of silver structures such as the nanosheets, chainlike sheets, and microwires. The current-voltage characteristic of the individual silver microwire shows linear behavior, and the electrical conductivity of the silver microwire is found to be comparable to that of the bulk silver materials, indicating that it can serve as interconnect in fabricating the micro/nanodevices. Introduction Recently, much effort has been devoted to the controlled synthesis of nanostructured materials because of their unique chemical and physical properties that are different from those of the bulk materials.1 Particularly, metal nanomaterials have attracted considerable attention because of their unique magnetic, optical, electrical, and catalytic properties and their potential applications in nanoelectronics.2 Among the metal nanomaterials, silver has been intensively studied because of its wide applications including catalysis, electronics, photonics, and photography.3 Furthermore, low-dimensional silver materials may be utilized as interconnects or active components in fabricating micro/nanodevices.4 Therefore, the synthesis and study of silver nanomaterials have been extensively investigated during the past several years. To date, many methods have been developed for preparing silver nanomaterials of various shapes, including nanoparticles,5 disks,6 rods,7 prisms,8 wires,9 hollow structures,10 and nanoprisms.11 For instance, Wehrspohn et al. have successfully synthesized monodisperse silver nanowires with a high aspect ratio via electrochemical plating into monodomain porous alumina templates.12 Meanwhile, Xia et al. also succeeded in transformation from silver nanospheres into nanobelts and triangular nanoplates by refluxing an aqueous dispersion of spherical colloids of silver with an average diameter of 3.5 nm.13 Recently, Pileni et al. tuned silver nanodisks with a similar aspect ratio through the reverse micelle route.14 Well-defined silver dendritic nanostuctures with fractal features were organized via a simple solvent-thermal method using poly(vinyl pyrrolidone) as an adsorption agent and architecture soft template.15 More recently, Mirkin et al. reported * To whom correspondence should be addressed. E-mail:
[email protected] (B.H.);
[email protected] (Y.L.);
[email protected] (D.J.K.). † Also affilliated with SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University.
a new thermal synthetic route to yield silver nanoprisms with modest control over edge length.16 It is well-known that fine control of size and shape simultaneously is quite challenging because of the lack of understanding of the crystallization behavior of nanocrystals. This would hinder the progress on the study of size and the shape control of nanocrystals. Thereby, detailed studies of size and shape control of nanocrystals are necessary for understanding the crystallization mechanism of materials in the nanosized scale and getting insight into quantum-sized phenomena.17 In this work we demonstrate a simple solvent-thermal route for the synthesis of silver nanosheets, chainlike sheets, and mircowires on a large scale. On the basis of our experimental results, the ammonia-tuning Oswald ripening mechanism was put forward to explain the formation of the silver structures. Furthermore, the electrical conductivity of the individual silver microwire was measured, whose value is quite comparable to that of the bulk silver materials. Hence, silver microwires may be used as interconnects in micro/nanodevices. Experimental Section AgNO3, ammonia (25%), and ethanol were supplied by Aldrich Co. Ltd. All of the chemicals were analytical grade and used without further purification. Deionized water was used in the experiments. For the synthesis of silver nanosheets, chainlike sheets, and microwires, 100 mg of AgNO3 was dissolved in 10 mL of ethanol with 0.07 mol/mL, 0.13 mol/mL, and 0.25 mol/mL ammonia concentrations, respectively. The solution mentioned above is transferred into a stainless steel autoclave and maintained at the temperature of 250 °C for 10 h. The solutions are then cooled to room temperature naturally. The product was then washed three times using distilled water and ethanol. Finally, the products were dried at 80 °C in an oven for further characterization. The X-ray diffraction (XRD) patterns of the samples were obtained on a powder X-ray diffractometer (D/Max 2550 V, Rigaku, Japan), using KR radiation (λ ) 1.5418 Å). The scanning electron microscopy (SEM) images were taken on JEOL JSM-6700F SEM. Transmission
10.1021/cg060661t CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
Synthesis of Silver Nanosheets, Chainlike Sheets, and Microwires
Crystal Growth & Design, Vol. 7, No. 5, 2007 901
Figure 1. XRD patterns of the as-prepared silver of (a) nanosheets, (b) chainlike sheets, and (c) microwires.
electron microscopy (TEM, JEM 2010, JEOL, Japan) was performed to observe the microstructure of the composites. Selected area electronic diffraction (SAED) and energy dispersive X-ray analysis (EDXA) were also taken on the same apparatus. The electrical property of theindividual silver microwire was measured using Probe Station (Wentworth Company MP1008) and Semiconductor Parameter Analyzer (HewlettPackard 4140B) at room temperature in the air.
Results and Discussions Silver nanosheets, chainlike sheets, and microwires were synthesized by heating 100 mg of AgNO3 in 10 mL of ethanol with of 0.07 mol/mL, 0.13 mol/mL, and 0.25 mol/mL ammonia at 250 °C for 10 h. Figure 1 shows the XRD patterns of the as-prepared samples. All the diffraction peaks can be indexed to the (111), (200), (220), and (311) planes of the face-centered cubic (fcc) silver.18 It was observed that the intensity of the diffraction peak (111) is slightly stronger than other peaks, indicating that the silver sample is mainly dominated by crystal facet (111), and thus the crystal growth direction is favorably oriented parallel to the [111] direction.19 The lattice constant (a) calculated from (111) patterns of samples a, b, and c according to the crystal fringe distances are 4.087, 4.086, and 4.087 Å, respectively, which are in excellent agreement with the standard value of 4.086 Å (JCPDS no. 4-783). No impurities can be detected from the pattern, indicating that pure silver was obtained under the present synthesis conditions. Similarly, only Ag peaks could be observed in the EDXA patterns of the products (not shown). SEM image shows that the silver products obtained with 100 mg of AgNO3 in the solution of 10 mL of ethanol with 0.07 mol/mL ammonia at 250 °C for 10 h are nanosheets with lateral dimension of up to 15 micrometers (Figure 2a). Most silver nanosheets have hexagonal and truncated triangular shapes. High-magnification SEM image (inset in Figure 2a) indicate that the thickness of the individual silver nanosheet is about 50 nm. In addition to nanosheets, irregularly silver particles can also be observed, suggesting that the silver nanosheets may be formed by the assembly of the small silver nanoparticles during the heat-treatment process. Moreover, the typical TEM image also exhibited a truncated triangular shape (Figure 2b). It was also found that many small silver nanoparticles are attached to the edge of the silver lamellas, indicating that silver nanosheets may be formed by the adsorption of small silver nanoparticles via Oswald ripening mechanism.20 To investigate the crystalline orientation of individual sheet, we used HRTEM to measure the distance of the lattice planes. The HRTEM image (Figure 2c) reveals parallel fringes with a space of 0.23 nm, which is consistent with the space of (111) lattice planes.21 Clearly, the parallel fringes of the nanosheet display loose and discontinuous
Figure 2. (a) SEM (inset, high-magnification SEM), (b) TEM, (c) HRETM images, and (d) SAED patterns of the silver nanosheets.
features, indicating that the edge of the silver nanosheets provides active growth areas to attach small silver nanoparticles along the (111) planes. The corresponding SAED spots (Figure 2d) was obtained by focusing the electron beam along the [11h1h] zone axis on the silver nanosheet, corroborating that silver nanosheet is single crystal. These diffraction spots can be indexed to (202), (220), (022h) of the fcc silver. This is consistent with the XRD results as well. Our experimental results demonstrate that the morphology of the Ag products is dependent on the concentration of the ammonia used at the same reaction conditions. The SEM image shows that when the concentration of the ammonia was increased to 0.13 mol/mL, silver chainlike sheets could be formed as shown in Figure 3a. Meanwhile, the TEM image (Figure 3b) also shows the formation of silver chainlike sheets. As the concentration of ammonia is further increased to 0.19 mol/mL, well-defined silver chainlike sheets about 1 µm in width and several tens of micrometers in length are obtained, some of which exhibit the fractal structure with chainlike and sheetlike morphologies, as can be seen from SEM image (Figure 3c). Moreover, the TEM image (Figure 3d) hints that the silver chainlike nanosheets were formed from the newly produced nuclei, which adhered to the edges of the existing sheets during the reaction process.22 In addition, the chainlike nanosheets have straight edges with sharp corners, suggesting that they are terminated by faceted crystallographic planes. It should be mentioned that the silver chainlike sheets could be not destroyed with lengthy ultrasonic treatment, indicating that the networks were formed via a chemical bond connection. When the concentration of ammonia is increased to 0.25 mol/ mL with the same experimental conditions, the silver samples obtained consist of a large quantity of microwires with diameter of about 2 µm and length of up to hundreds of micrometers (Figure 4a). Interestingly, it is clearly seen from the TEM image (Figure 4b) that individual silver microwires typically have many irregular silver particles attached to the surface of the silver microwires, indicating that the microwires may be formed via an Oswald ripening mechanism. Furthermore, the HRTEM image (Figure 4c) shows that the regular spacing of the observed
902 Crystal Growth & Design, Vol. 7, No. 5, 2007
Du et al.
Figure 5. SEM images of (a) silver microspheres and (b) irregular silver particles,.
Figure 3. (a, c) SEM and (b, d) TEM images of the chainlike silver sheets obtained with 100 mg of AgNO3 in a solution of 10 mL of ethanol with (a, b) 0.13 mol/mL and (c, d) 0.19 mol/mL ammonia at 250 °C for 10 h.
along the [100] direction.24 The corresponding SAED diffraction dots (Figure 4d) indicate that the silver microwires has singlecrystal structure, which can be indexed to (111), (200), (11h1h) of the fcc silver, in agreement with the XRD results. To understand the formation mechanism of the nanostructured silver described above, we carried out the control experiments in 10 mL of the pure aqueous ammonia (1.47 mol/mL) in the absence of ethanol at 250 °C for 10 h. In this case, silver microspheres (Figure 5a) were obtained with an average size ranging from 1 to 3 micrometers. This may be due to the high coverage of ammonia on all faces of silver nucleation, leading to an isotropic growth to form silver microspheres. The Ag products were also prepared in 10 mL of the pure ethanol, and the irregular silver particles (Figure 5b) were formed. Hence the blank experimental results verify that the ammonia concentration plays an important role for the morphology of the silver nanomaterials. According to our experiments, silver sheets, chainlike sheets, and microwires can be prepared selectively in the presence of ammonia via the simple solvent-thermal route. Even though the detailed mechanism for the formation of each silver structure is not so clear yet, we would like to propose an ammonia-tuning Oswald ripening mechanism to explain the morphology variety of silver materials in the following.
Ag+ + xNH3 f Ag(NH3)x+ 2Ag(NH3)x+ + C2H5OH f Ag + 2NH4+ + CH3CHO
(1) (2)
+
Ag(NH3)x + CH3CHO + H2O +NH3 f Ag + CH3COONH4 + 2NH4+ (3) nAg f (Ag)n
Figure 4. (a) SEM, (b) TEM, and (c) HRETM images; (d) SAED patterns of the silver microwires.
lattice planes is 0.20 nm, which is consistent with the distance of Ag (200) lattice planes.23 The fringes of the silver microwires can be characterized as discontinuous crystal planes, showing that the fringes are active growth areas via dissolving silver small particles with high surface energy and atoms in the solution. The preferable longitudinal and latitudinal growth direction of the silver microwires is along the [111] and [100] directions, brought about by the formation of silver microwires due to the higher growth speed along the [111] orientation than
(4)
Because of the presence of ammonia, the silver must be coordinated with NH3 with an existing state of Ag(NH3)x+, as shown in reaction 1. Silver atoms (reactions 2 and 3) were then produced and aggregated to nuclei (reaction 4) by heating the ethanol solution of silver nitrate in the presence of ammonia because of the strong temperature-dependent reducing power of ethanol, similar to that put forward by Chen et.al.25 Finally, the silver nuclei can form silver nanostructures via the ammoniatuning Oswald ripening mechanism. When there is a low concentration of ammonia in the reaction solution, the resulting silver atoms tend to assemble to the silver nuclei to form larger particles, which are inclined to absorb ammonia on the lowest energy (111) facet of the particles and reduce the growing rate of these facets. As a result, other facets adsorbed by fewer ammonia molecules grew more quickly than (111) facets, which can be favorable to grow silver nanosheets primarily bound by
Synthesis of Silver Nanosheets, Chainlike Sheets, and Microwires
Crystal Growth & Design, Vol. 7, No. 5, 2007 903
Figure 7. Current-voltage (I-V) characteristics of an individual silver microwire measured at room temperature.
Figure 6. SEM images of (a) irregular silver nanoparticles, (b) silver nanobelts, and (c, d) microwires, respectively.
(111) facts. When the ammonia amount increases, the silver chainlike sheets are formed, whereas other reaction conditions are kept the same. During the reaction process, new nuclei can be continually produced, which may adhere to the edge of the big particles because of diffusion, which is limited in the presence of ammonia.24 NH3 can then kinetically control the growth rates of various facets of silver crystals by interacting with these facets through adsorption and desorption to produce silver chainlike sheets. Further increasing the ammonia amount in the reaction system, it should be obvious that NH3 will interact more strongly with (100) facets than the (111) facets known from the XRD results discussed above, resulting in the anisotropic growth of the particles. Once the rod-shaped structures have been formed, they can readily grow into longer nanowires, because its side surfaces are tightly passivated by NH3 and its ends are largely uncovered and remain reactive toward new silver atoms. Thus, thicker and longer silver microswires are generally grown. A complete coverage of NH3 on the surfaces of nanoplates and nanoparticles favors an isotropic growth for all different facets, resulting in nanospheres, as indicated in Figure 5a. It is well-known that the Oswald ripening mechanism has a close relation with the reaction temperature and time, which can be described by an equation (D - D0 ) Kt1/n, where k is a temperature-dependent material constant and t is reaction time) for Ostwald ripening.26 Therefore, the reaction temperature and time should have an effect on the motif of the silver materials discussed above. When the experiment was carried out using 100 mg of AgNO3 in 10 mL of ethanol with 0.07 mol/mL ammonia at 200 °C for 10 h, the samples obtained display irregular silver particles (Figure 6a). On the other hand, when the reaction temperature was increased up to 300 °C, the products exhibit silver belts motif (Figure 6b). The relationship of the silver materials morphologies with the reaction temperature also supports the Oswald ripening mechanism discussed above. To further corroborate our possible growth mechanism, we performed similar experiments with the concentration of ammonia up to 0.25 mol/mL at 250 °C for 3 and 30 h, respectively. Both results show that silver microwires with diameters of about 1.3 and 5 µm can be synthesized as shown
in images c and d of Figure 6, respectively, whose results are also in agreement with the Oswald ripening mechanism. Electronic Properties. The quest for nanometer-scale electronics has prompted us to study transport properties of metallic nanowires.27 In this work, the electrical conductivity of an individual silver microwire was measured using a probe station (Wentworth Company MP1008) and semiconductor parameter analyzer (Hewlett-Packard 4140B) at room temperature in the air. In this case, a silver microwire (length of 10 µm) was aligned on a 500 nm thick thermal oxidized silicon surface between Ti/Au pads. Using a probe point extension focused-ion-beam (FIB) system, the surface was visualized using a low-beam current (4 pA) searching for the silver microwires. Two Pt leads were connected to the two ends of silver microwires. The SEM image of the individual silver microwire is shown by the inset in Figure 7. The obtained I-V curve is linear, as demonstrated in Figure 7, showing the metallic characteristics. By fitting the data in the Figure 7, the equation I(A) ) 0.0324 V (V) + 8.7 × 10-5 was obtained and the electrical conductivity calculated for the silver microwries is 0.3 × 105 S/cm, which is found to be comparable to that of the bulk value of silver materials (6.2 × 105 S/cm).28 The conductivity of silver microwire suggests that they can be used as interconnects in fabricating micro/nanodevices. Conclusions In conclusion, we have presented a simple route to prepare silver nanosheets, chainlike sheets, and microwires by heating AgNO3 in ethanol in the presence of ammonia. It is shown that ammonia concentration in the solution plays a key role in controlling the morphology of the silver materials. In addition, the reaction temperature and time also affect the morphology of products. On the basis of our experimental results, the ammonia-tuning Oswald ripening mechanism is carefully proposed to explain the formation of the silver nanosheets, chainlike sheets, and microwires. Our electrical measurement shows that the electrical conductivity of the individual silver microwire is found to be 0.3 × 105 S/cm, showing that it can be used as an interconnect in fabricating micro/nanodevices. Acknowledgment. This work is supported in part by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-005-J11903) and in part by the SRC program (Center for Nanotubes and Nanostructured Composites) of the Ministry of Science and Technology of Korea/Korea Science and Engineering Foundation. We also thank the National Natural Science Foundation of China (50472096, 20633080, 90206049, 20472089), the Major State
904 Crystal Growth & Design, Vol. 7, No. 5, 2007
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