CRYSTAL GROWTH & DESIGN
Dendritic Silver Nanostructure Growth and Evolution in Replacement Reaction
2007 VOL. 7, NO. 5 864-867
Jixiang Fang, Hongjun You, Peng Kong, Yan Yi, Xiaoping Song, and Bingjun Ding* School of Science, State Key Laboratory for Mechanical BehaVior of Materials, Xi’an Jiaotong UniVersity, Shann Xi, 710049, People’s Republic of China ReceiVed July 25, 2006; ReVised Manuscript ReceiVed January 10, 2007
ABSTRACT: In this study, a well-defined dendritic silver nanostructure can be large-scale synthesized in AgNO3 (aqueous) at room temperature. The nonequilibrium and anisotropic growth at different silver ion concentrations result in controllable morphologies and morphological evolution. At high silver ion concentrations, a strong anisotropic growth contributes to a fine single crystalline silver dendrite. As the reaction proceeds, the dendritic structure transforms into a thermodynamically stable hexagonal structure. At a relatively low silver ion concentration, a particle-aggregated fractal pattern can be obtained due to relatively small anisotropy. As the reaction time increases, the transition from polycrystalline aggregates to a single crystal during silver dendritic growth can be observed. An oriented attachment mechanism can be used to explain the structural and morphological evolution of silver nanostructures. Silver nanostructures with various morphologies are expected to have significant potential applications in superhydrophobic surfaces, surface-enhanced Raman scattering, and others. Introduction Metal nanostructures have been widely exploited for use in photography, catalysis, biological labeling, photonics, optoelectronics, surface-enhanced Raman scattering (SERS), and formulation of magnetic ferrofluids.1 The intrinsic properties of a metal nanostructure are determined by its size, shape, morphology, composition, and crystallinity.2 To date, much effort has been made to design nanocrystals with well-defined sizes, shapes, and crystallinity. Quite a lot of nanostructures, such as discrete cobalt disks, rods, and cubes, platonic gold nanocrystals and other shapes, silver prisms, cubes, rods, and hexagonal plates, have been synthesized via efficient solution-phase reduction methods.3-5 Self-assembled hierarchical and repetitive superstructures are fascinating because of their promising complex functions. Dendritic patterns are essential phenomena that are observed in nonequilibrium conditions for metallurgic, inorganic, and organic crystal growth.6 Silver dendritic nanostructures have been prepared by an organic reducing agent such as tetrathiafulvalene7 and ascorbic acid,8 ultrasonically assisted templated synthesis,9 ultraviolet irradiation photoreduction,10 plating,11 γ-irradiation route,12 and pulsed sonoelectrochemical methods.13 Because synthetic process organic capping reagents such as poly(vinyl pyrrolidone) are usually introduced,14 subsequent procedures are necessary to obtain pure products. Thus, facile and organic template-free methods are expected. Moreover, a simple morphology-controllable and large-scale synthesis route is still highly desired. A replacement reaction is a basic and simple method to synthesize nanostructured materials in a number of different systems.15 Recently, the Xia group16 reported pioneering work on the synthesis of gold hollow nanostructures with a range of different shapes (e.g., triangular rings, prism-shaped boxes, cubic boxes, spherical capsules, and tubes) by a replacement reaction. The silver-engaged replacement reaction can, in principle, extend to any metal whose redox potential is lower than that of the Ag+/Ag pair. In our previous papers, using Zn vs AuCl4-, a similar dendritic nanostructure and morphology evolution process was observed by means of adjusting the monomer concentration and reaction time.17 This method provides a * To whom correspondence should be addressed. E-mail: bjding@ mail.xjtu.edu.cn.
straightforward and robust protocol for preparing complex dendritic silver nanostructures. Herein, we show in detail that silver dendritic nanostructures with various morphologies can be synthesized in AgNO3 aqueous solution at room temperature. The morphological and structural evolution mechanism is also discussed. The synthetic process is very simple; only three ingredients are required in the replacement reaction: silver nitrate, water, and zinc. Materials and Methods The zinc plate (99.9%) was first treated by hydrochloric acid to remove surface contamination and was rinsed with distilled water. The AgNO3(aq) with a certain Ag+ concentration was prepared using analytically pure AgNO3 reagent. The zinc plate was immersed in AgNO3 solution. The whole reaction was performed at room temperature and ambient pressure. Within several seconds, the silver nanostructures with different morphologies were obtained by adjusting the silver ion concentration. The products were peeled carefully from the zinc plate with tweezers and put into a beaker. Then, the products were rinsed using distilled water and ethanol in sequence within an ultrasonic condition. Finally, the product was collected for characterization. The products were characterized by a FEG-JSM 6335 field-emission scanning electron microscope (SEM). Transmission electron microscopy (TEM) images and corresponding selected area electron diffraction (SAED) patterns analysis were performed on a Hitachi model H-800 transmission electron microscope. Samples for the electron microscope were prepared by ultrasonic dispersion of the as-prepared product for more than five times in ethanol to completely remove the impurity. Then, the suspension was dropped onto a conventional carbon-coated copper grid and dried in air before analysis.
Results and Discussion Figure 1a is an X-ray diffraction (XRD) pattern of silver dendrites synthesized at 200 mM silver nitrite(aq) for 10 s of reaction time, indicating that the dendrites are of high crystallinity. The five diffraction peaks can be indexed to diffraction from the (111), (200), (220), (311), and (222) of face-centered cubic (fcc) silver (JCPDS Card File, 4-783). The refined lattice parameter a ) 2.3588 is extracted from the XRD data, which is in good agreement with the literature value of a ) 2.359 Å.18 Figure 1b shows an optical micrograph of silver trees. It exhibits a fractal character and obeys an approximately fractal dimension of D ) 1.75. Figure 1c demonstrates a typical SEM image of silver dendrites (same samples with Figure 1a),
10.1021/cg0604879 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007
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Figure 2. TEM images of a silver dendrite prepared in 200 mM silver nitrite (aqueous), indicating the morphological evolution from dendritic to hexagonal structures for different reaction times: (a) 10, (b) 30, (c) 50, and (d) 70 s.
Figure 1. Silver trees prepared at 200 mM silver nitrate (aqueous) for 10 s of reaction time. (a) X-ray diffraction (XRD), (b) optical micrograph, and (c) SEM images of silver dendrites.
exhibiting a single morphology with uniform featherlike branches. The individual dendrite length is about 5-10 µm and is composed of symmetrical branches and leaves. The growth of the dendritic nanostructure was carefully followed by a time-dependent process. As the replacement reaction proceeded, the initial silver dendrites experienced a morphological evolution and converted to some hexagon nanoplates. The TEM images of Figure 2a-d detail the morphological evolution of silver dendrites that were prepared at 200 mM silver nitrate(aq) for 10, 30, 50, and 70 s of reaction time, respectively. In the first 10 s, the branch tip presented a round shape (Figure 2a). As the reaction proceeded to about 30 s, the tips partly converted their shape from round to hexagonal (Figure 2b). After 50 s, nearly all of the tips became hexagon (Figure 2c), and a well-crystallized separately hexagonal plate was formed (Figure 2d) after 70 s. The crystalline structure of the silver hexagons was studied further and is shown in Figure 3. Figure 3a presents a silver dendrite with a hexagonal shape, and Figure 3b is the corresponding SAED pattern from the red box area in panel a. The electron beam is perpendicular to the flat surface of the noanplate lying on the TEM grid. The circled and boxed spots in the SAED pattern are indexed as {220} and formally forbidden 1/3 {422} Bragg reflections,19 indicating that the flat surface of the disks is parallel to the (111) habit plane, the most stable crystal plane of silver. Figure 3c shows a highresolution (HR) TEM image of the [111] orientated disk with the lattice spacing of 2.502 Å and its flat (111) plane lying on TEM grid, which is consistent with the diffraction pattern given in Figure 3b. Figure 3d illustrates the suggested hexagonal nanocrystals based on the SAED of Figure 3b, where the sixside planes are indexed as {211} faces that are similar to the previous work for the silver nanoinukshuks.3 We believe that the observations here imply an evolution from nonequilibrium to equilibrium. At the early stage (e.g., within 10 s), the reaction
Figure 3. (a) TEM image of a silver dendrite with hexagonal structure prepared in 200 mM silver nitrite (aqueous) for 30 s of reaction time and (b) SAED pattern of the red box area in panel a. The circled and boxed spots in the SAED pattern are indexed as {220} and formally forbidden 1/3 {422} Bragg reflections. (c) HRTEM image of silver nanoplate in the [111] orientation. (d) Indexed hexagonal nanocrystals based on the SAED of panel b.
process is dominated by a nonequilibrium condition, and a dendritic morphology is always formed. As the replacement reaction proceeds, the monomer concentration in some areas is depleted by the continuous growth of silver trees. If the reaction time is long enough, the monomer concentration should drop to such a level that the reaction process is dominated by quasiequilibrium or equilibrium conditions. Therefore, a thermodynamically stable hexagonal structure is formed. A drastic change of the silver aggregate patterns is observed when the silver ion concentration is adjusted. Figure 4 demonstrates the transition from fractal to dendritic growth of silver aggregates at silver ion concentration of 5, 30, 60, and 100 mM and a reaction time of 120, 60, 30, and 10 s, respectively. When the silver ion concentration is less than 1 mM, a nanoparticle aggregated pattern is observed. Increasing the silver ion
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Figure 4. TEM images of silver dendrites, demonstrating the morphological transition from fractal to dendritic growth for silver aggregates at different silver ion concentrations and reaction times; the insets are SAED for marked areas: (a) 5 mM and 120 s, (b) 30 mM and 60 s, (c) 60 mM and 30 s, (d) and 100 mM and 10 s.
concentration to 5 mM, the silver aggregate growth presents a fractal character as shown in Figure 4a. The inset in Figure 4a is the SAED pattern of the circle area, demonstrating that the crystallinity is poor. A higher silver ion concentration promotes the transition from fractal to dendritic growth. When the reaction is conducted at about 30 mM silver ion concentration, the transition from fractal to dendritic growth occurs, as shown in Figure 4b. The morphology of a silver branch in Figure 4b suggests the coexistence of fractal and dendritic growth because the top of the silver branch exhibits fractal morphology like that in Figure 4a, while the bottom demonstrates a dendritic pattern. The crystallinity of the fractal and dendritic regions in Figure 4b is also different. The up-right SAED pattern inset in Figure 4b indicates that the top of the silver branch presents polycrystalline character, but the bottom has a single-crystal structure (bottom-center SAED pattern). Figure 4c is the silver aggregation pattern prepared at about 60 mM silver ion concentration, basically indicating a dendrite character. When the silver ion concentration increases to about 100 mM, the nanoplate-aggregated morphology disappears, and an imperfect single crystalline silver dendrite (Figure 4d) can be observed according to the inset SAED pattern in Figure 4d. Above 100 mM, the silver dendrites show a prefect single crystalline structure. These indicate that, by controlling the silver ion concentration in the solution, a silver dendrite with well-defined morphology and crystalline structure could be realized. The mechanism of the transition from polycrystalline aggregates to a single crystal during silver dendritic growth was carefully investigated by HRTEM analysis, and the results are shown in Figure 5. Figure 5a shows three particle-aggregated branches synthesized at 30 mM silver ion concentration for about 5 min of reaction time. It can be seen that the middle branch is composed of four aggregates, and an obvious interface among the aggregates can be found. Figure 5b is a magnified image of an interface area marked by a white rectangle in Figure 5a, demonstrating that the two aggregates share the same crystallographic orientation (marked by a white arrow). So, the silver aggregation branch is a single crystal. On the tip of the single crystal attaches a group of small aggregates with slight misorientation. Figure 5c is a magnified image of the attached region marked by a circle in Figure 5a. The A, B, and C areas in Figure 5c represent the growing grain, interface region, and attached region, respectively. The white arrows present the crystalline plane orientation of the A and C areas. In C areas, two small aggregates have basically completed the self-
Figure 5. HRTEM images of an aggregate-attached leaf synthesized at 30 mM silver ion concentration for about 5 min, demonstrating the transition from polycrystalline aggregates to a single crystal during silver dendritic growth: (a) an aggregate-attached leaf; (b) interface between two aggregates; and (c) small aggregates attached area, showing a process of grain coalescence by an oriented attachment mechanism.
assembling process by a proposed grain rotation and realignment. The lattice spacing of fringes in C area and A area are also nearly the same, corresponding to 2.338 and 2.331 Å, respectively. The inset in Figure 5c (magnified image of B area) shows some misorientations or defects originating from the imperfect assembling among several small aggregates. The white lines in the inset highlight the presence of a series of edge dislocations. Dislocations formed at self-assembling interfaces are in agreement with earlier studies of “oriented attachment” on other systems reported by Banfield et al.20 After the selfassembling process, the former interface between two aggregates (marked by black arrows) nearly disappears, and the two aggregates share the same single crystallographic orientation. As a result, the polycrystalline aggregates transform to a single crystal. The present experiments provide strong evidence that oriented attachment mechanism is a major path for the transition from polycrystalline aggregates to a single crystalline silver dendrite. The driving force for the spontaneous oriented attachment is the elimination of the high-energy surfaces, which will lead to a substantial reduction in the surface free energy
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particle-aggregated dendrite can further transform to a single crystalline dendrite. An oriented attachment mechanism can be used to explain the transition from polycrystalline aggregates to the single crystal during dendritic growth of the silver nanostructure. We expect that this inexpensive, simple, and fast route can be possibly extended to some other metals to synthesize controllable dendritic superstructures. Incorporating this method into the morphology-controlled growth of other metals, controlling growth on heterogeneous substrate, and application in the superhydrophobic surface26 and SERS16 will be the subject of future study.
Figure 6. Schematic representation of the proposed morphological and structural evolution during silver dendritic growth.
viewpoint.21
from the thermodynamic The initial randomly attached aggregates can align epitaxially with the substrate, which was explained as the rotation of the aggregates between adjacent surfaces.22 From a view of thermodynamics, the silver dendrite, by virtue of its extended surface, has a considerably increased surface energy in contrast to the equilibrium shape.23 The aggregation process of the dendritic silver, in a growth situation far from equilibrium, may be much faster than the relaxation process of the small grains,24 so the small grains may aggregate with a hemicycle tip. As the reaction proceeds, the silver ion concentration around previous grains drops to a certain level, the small grains may have sufficient time to relax and to minimum energy position by the proposed oriented attachment mechanism, and thus, the product exhibits the transition from dendrite to a more compact hexagonal structure; that is, each small grain may have six nearest neighbors. Washio et al. also reported that the thermodynamically favorable shapes, for a facecentered cubic (fcc) noble metal, are truncated nanocubes and multiple twinned particles.25 In these regards, at a higher silver ion concentration, the reaction is dominated by the kinetic factor; while at a low silver ion concentration, thermodynamic factor is favorable. In addition, one may find that there are some small attached aggregates on the lateral of silver nanopaltes as shown in Figure 5a. These small attached aggregates will also contribute to the formation of single-crystal silver hexagonal nanoplates. For a higher silver ion concentration, the reaction also experiences an oriented attachment process as shown in Figure S1 of the Supporting Information. Figure 6 gives a schematic representation of the proposed growth mechanism, including the aggregates attachment, grain rotation, grains relaxation, and final formation of the silver nanoplate. Conclusion To conclude, dendritic silver nanostructures with controlled morphology were obtained by adjusting the silver ion concentration in the replacement reaction at room temperature. The nonequilibrium and anisotropic growth at different silver ion concentrations result in controllable morphologies and morphological evolution. At high silver ion concentrations, a strong anisotropy contributes to a fine single crystalline silver dendrite. As the reaction proceeds, the dendrite structure transforms into thermodynamically stable hexagonal structure. At relatively low silver ion concentrations, the fractal pattern can be observed due to relatively small anisotropy. The silver fractal pattern can transform to dendritic structure spontaneously, and the nano-
Acknowledgment. This work was supported by the National Science Foundation of China (no. 50471033) and the Doctoral Foundation of Xi’an Jiaotong University (no. DFXJTU200512). We also thank Professor Bingbo Wei, Zhengxin Lu, and Yaping Wang for their helpful discussions. Supporting Information Available: The silver dendrite prepared at 100mM silver ion concentration. This material is available free of charge via the Internet at http://pubs.acs.org.
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