DOI: 10.1021/cg900066z
Fabrication of Hierarchical Nanostructure of Silver via a Surfactant-Free Mixed Solvents Route
2009, Vol. 9 3941–3947
Yucui Han, Shaohua Liu, Min Han, Jianchun Bao,* and Zhihui Dai* Jiangsu Key Laboratory of Biofunctional Materials, Department of Chemistry, Nanjing Normal University, Nanjing 210097, P. R. China Received January 19, 2009; Revised Manuscript Received June 12, 2009
ABSTRACT: The dendritic Ag nanostructure with ordered branches and a “clean” surface has been successfully prepared via a facile surfactant-free and acetone-based mixed solvents route at room temperature. Experiments and structural characterizations reveal that the dendritic Ag nanostructure is evolved from the initially generated triangular nanoplates by the reaction of AgNO3 with L-ascorbic acid to the dendrites through both the Ostwald ripening and the oriented attachment growth processes. The acetone plays the key role in controlling the nucleation, growth, conversion, and assembly of the Ag nanoparticles. In the absence of acetone, only the polyhedral particles can be obtained. The yield of the dendrites is dependent on the volume ratio of acetone to water. The present work provides an example for the synthesis of a novel metal nanostructure by simply adjusting the solvent components, which is important for the qualitative understanding of the solvent effect on the morphology of nanostructures and the controllable synthesis of desired nanostructures. The dendritic Ag nanostructure possesses surfaceenhanced Raman scattering (SERS) performance similar to that from triangular Ag nanoplates, and they both show much better SERS enhancement ability than that of polyhedral Ag particles which might be relative to their different geometric shapes and microstructures. It is expected that the dendritic Ag nanostructure may find potential applications such as in catalysis, molecular probe, and biological sensing.
*Corresponding author. Tel.: 0086 25 83598260. Fax: 0086 25 83598280. E-mail:
[email protected] (J.B.);
[email protected] (Z.D.).
nanobelts as well as hierarchical nanocolumns consisting of stacked Ag nanoplates were obtained by reducing AgNO3 with ascorbic acid in the absence or presence of acetic acid in aqueous solutions of poly(acrylic acid).8b In most cases, as for the metals with primitive cubic crystal structures, surfactants/ polymers or certain ionic species are required as shape-directing agents by preferential adsorbing on specific crystal planes to form the anisotropic nanostructures. However, for some special applications such as SERS, biosensing, and catalysis, surfactantless synthesis method is better due to the absence of residue from the synthesis on the obtained nanoparticle surface and thereby avoids a significant interference.9 Therefore, development of the surfactantless synthesis method is of great importance. It is well-known that many factors affect the shape of nanostructures. Considerable attempts have been focused on the effects of temperature, concentration, and types of polymers/surfactants, but the study of the solvent remains very limited. Theoretically, the solvent should play an important role in the shape-controlled synthesis of nanostructures because of the variation in solubility, nucleation, and growth of the resultants, reaction kinetics, solution properties, and stability of the particles in different solvent systems, leading the formation of nanostructures with different shapes.10 Recently, ethylene glycol, ethanol, and N,N-dimethylformamide have been applied as the reaction media for the synthesis of novel noble metal nanostructures,11 such as branched Pt and extended Ag, Pd, and Pt nanowires, indicating the important role of solvents in the shapecontrolled synthesis of nanoparticles. To the best of our knowledge, other commonly used organic solvent or mixed solvents have not been used as reaction media to fabricate noble metal nanostructures. Furthermore, the growth mechanisms remain to be clarified. Therefore, exploring new
r 2009 American Chemical Society
Published on Web 07/10/2009
1. Introduction In the past few years, noble metal nanoparticles have attracted extensive interest due to their novel physical and chemical properties and potential applications in catalysis,1 optics,2 biological labeling and imaging,3 sensing,4 and surface-enhanced Raman scattering (SERS).5 Control of the shape of the noble metal nanoparticles has been a major subject of the current research because it provides an alternative means, in addition to size, for tailoring their physicochemical properties.1d,3-6 For example, Ag nanowires with mainly the Ag(100) surface facet exposed are more selective in partial oxidation of ethylene oxide than conventional spherical Ag nanoparticles with mainly the Ag(111) facet exposed.1d Also, the triangular and hexagonal Pd nanoplates exhibit a better SERS enhancement effect than that from the cubooctahedra Pd.5b On the other hand, the hierarchical assembly of nanoscale building blocks into ordered superstructures or complex architectures, such as from one-dimensional nanorods/nanowires to branched nanostructures and from dispersed triangular nanoplates to ordered assemblies with anisotropic orientation, is of great significance because the superstructures/hierarchical assembly can display novel and collective physical and chemical properties which are not found on the level of individual nanoparticles.7a,b They are critical for the success of bottom-up approaches toward integrated and functional nanosystems. To date, a few of strategies have been developed to fabricate various hierarchical/complicated noble metal nanoarchitectures.8 For instance, the Au particles with multipods were prepared by a seed-mediated growth method in aqueous solution of cetyltrimethylammonium bromide (CTAB).8a Also, the Ag
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solvent systems (or combination of solvents) as media to synthesize novel noble metal nanostructures and studying the growth mechanisms are needed and continue to be a challenge in current chemistry and materials science. Herein, we first report the hierarchical ordered dendritic Ag nanostructure prepared in the acetone-based mixed solvents (acetone and water) without using any surfactants at room temperature. The reason we choose acetone as the solvent is that a large difference in the physical properties exists between acetone and water. The properties of the mixed solvents can be adjusted by changing the volume ratio of acetone to water which can be used for the shape-controlled synthesis of nanostructures. The results demonstrate that the dendritic Ag is evolved from the initial formed triangular nanoplates and the acetone plays the key role in the formation of the nanostructures. A possible growth mechanism for the formation of the novel Ag nanostructure is proposed. We also evaluate the performance of the obtained Ag nanostructures (dendritic, triangular, and polyhedral) as SERS substrates by using adenine as a probe. The dendritic and triangular Ag nanostructures display a similar SERS performance which is much better than that from the polyhedral Ag particles.
Han et al.
Figure 1. XRD pattern of the Ag prepared under the typical reaction conditions.
2. Experimental Section Materials. All reagents used in the experiments were of analytical grade and used without further purification. Synthesis of Dendritic Ag Nanostructure. In a typical experiment, 0.034 g (0.2 mmol) of AgNO3 was dissolved in 0.5 mL of deionized water and followed by adding 9.5 mL of acetone to form solution (A). 0.071 g (0.4 mmol) of L-ascorbic acid was dissolved in 0.5 mL of deionized water and followed by adding 9.5 mL of acetone to form solution (B). Then solution (B) was added by dropwise addition to the solution (A) within about 2 min under magnetic stirring at 25 C. During the dropping progress, the color of the solution changed from colorless to violet-blue, then to gray. The resulting solution was allowed to stand for 10 min. Finally, the mixture was separated by centrifugation. The deposit was washed with deionized water and ethanol several times. After vacuum drying, the gray dendritic Ag nanostructure was obtained. Characterization. X-ray diffraction (XRD) measurements were obtained with a D/Max-RA diffractometer equipped with graphite monochromatized Cu KR radiation (λ=0.154056 nm). The morphology and particle sizes of the samples were characterized by JEM-200CX transmission electron microscopy (TEM) and a JEOL 2010 high resolution transmission electron microscopy (HRTEM) all working at 200 kV. A small amount of the sample was dispersed in ethanol, and then a drop of this solution was deposited on an amorphous carbon film on 300 mesh Cu grid for TEM observation. Field emission scanning electron microscopy (FESEM) characterization was carried out using a LEO1530 VP microscopy at an acceleration of 15 KV. Energy dispersive spectroscopy (EDS) was performed on the microscope with a PV9100 scanning electron microanalyzer. UV-vis absorption spectra were obtained on a λ-17 spectrophotometer with a 1-cm optical length quartz cell. The samples for SERS examination were prepared by drying 5 μL of 1.0 mM Ag aqueous sol on a glass substrate. A total of 5 μL of 0.2 M adenine with 0.2 M HCl was then dropped onto the sample on the glass substrate. After 1 h, the substrate was rinsed with deionized water and dried with a stream of air. Raman spectra were recorded with a Labram HR800 model Raman spectrometer by using 10 mW of 514.5 nm laser light. Data acquisition time was 30 s.
3. Results and Discussion Characterization of the Dendritic Ag Nanostructure. Figure 1 shows the XRD pattern of the sample prepared by the typical reaction conditions. The diffraction peaks in the range of 30 < 2θ < 85 can be indexed as a face-center-cubic (fcc) structure Ag (111), (200), (220), (311), and (222) and the lattice parameter
Figure 2. TEM image of the dendritic Ag nanostructure (A); SAED pattern of a dendrite (marked with an arrow) in panel A (B); HRTEM images of a tip of a branch (C) and a joint between the shaft and a side branch (D), respectively.
is a = 4.0842 A˚, which all are in good accordance with the ASTM standard (JCPDS 4-0783). The sharpness of the peaks indicates that the products are well crystallized. The TEM images of the sample are shown in Figure 2. As can be seen from Figure 2A, the prepared particles are of dendritic structure. The length and the diameter of the shaft (main branch) are about 1.5 μm and 60 nm, respectively. On the dendritic structure, numerous side branches mainly in the range of 25-50 nm in diameter grow neatly (parallel to each other) along the shaft. The selected area electron diffraction (SAED) pattern of the dendrite of the region marked with an arrow in Figure 2A is shown in Figure 2B. The presence of clear diffraction spots indicates that the dendrite is a single crystal and can be indexed to (111), (200), and (311) reflections of fcc Ag. The further crystal structure information comes from high resolution transmission electron microscopy (HRTEM) analysis. The HRTEM images of a tip of a branch and a joint between a shaft and a side branch are shown in Figure 2, panels C and D, respectively; all clearly reveal the good crystalline and lattice fringes. The fringe spacings of the lattice are about 0.20 and 0.23 nm, which match the interplanar spacings of (200) and (111) of the fcc Ag well, respectively. On the basis of the information from
Article
Figure 3. EDS image of the Ag prepared under the typical reaction conditions.
Figure 4. TEM images of the Ag nanostructures obtained after addition of ascorbic acid then taken out immediately (A) and aging for 3 min (B) in the mixed solvents of acetone/water (v:v 19:1), respectively.
Figure 2B-D, we can conclude that the crystal orientation of the side branch is the same as that of the shaft, and the shaft and side branch grow along Æ100æ and Æ111æ, respectively. In addition, three points are necessary to point out. First, the energy dispersive spectrometer (EDS) spectrum with only peaks (Figure 3) attributed to Ag and without other materials peaks reveals that the products have a “clean” surface. Second, these dendrites which almost all are of two-generation neat dendritic structures are different from those dendrites obtained in the mixed solvents of water and ethanol, which were prepared by reaction of AgNO3 with NaBH4 in the presence of p-aminoazobenzene molecules having random-growing-branch structures or with zinc microparticles having three-generation dendritic structures.7b,c,12 Lastly, the ends of a number of dendrites gather together, implying that these dendrites might grow from the same particle. Growth Mechanism of the Dendritic Ag Nanostructure. Generally, the formation of the anisotropic nanostructure with the highly intrinsic symmetric cubic crystals in solution is difficult when the synthesis is carried out in an “isotropic” medium without using any polymer/surfactant. How does the anisotropic dendritic Ag nanostructure form in the present case? In order to understand the formation process, the time-dependent shape evolution process is carried out by careful examinations of the intermediates. Figure 4A,B shows the TEM images of the Ag particles after addition of ascorbic acid then taken out immediately and aging for 3 min, respectively. As can be seen from the Figure 4A the particles are of triangular plate-like nature, and the thickness of the triangular plate (marked with a solid line arrow) is about 25 nm. Noteworthy, some serrated triangular platelets (marked with a dotted line arrow) are also observed.
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When aging for 3 min, the rudimentary dendritic structures form from the triangular plates and many of them form around the corners of the triangle plates (Figure 4B). In addition, some smaller triangular plates are also observed in the image (marked with arrows in Figure 4B). Further prolonging the aging time to 10 min, the perfect dendritic Ag nanostructures are obtained (Figure 2A). These intermediates clearly show that the dendritic Ag nanostructure does not form directly, but is evolved from the triangular nanoplates formed at the early stage of the synthesis. This is different from the cases in which the dendrites resulted directly from the assembly of primary particles and growth.7b,c,12,13 In addition, it should be pointed out that about 30% of the triangular nanoplates are truncated in the present case. Previous work reported that triangular Ag nanoplates synthesized using a self-seeding coreduction method in the presence of citrate acid and NaAOT could gradually convert to the intermediate of plates and discs, and finally to circular plates in aqueous media at room temperature.14 Also, truncated triangular Ag nanoplates synthesized by reduction of AgNO3 with ascorbic acid on Ag seeds in the alkaline solution of highly concentrated CTAB micelles could transfer to disklike or spherical shape at a higher temperature.15 The present novel morphology conversion from triangular to dendritic structure has not been reported so far, which results from the acetone-based mixed solvents. Several groups reported the fabrication of triangular Ag nanoplates by photoinduced conversion of nanospheres or spherical seed-mediated growth in the presence of different types of capping agents (e.g., PVP, PAM, citrate).6a,14,16 According to the literature reports, in order to obtain the nanoplates, the formation of Ag atoms must be kept at a slow rate. When the reduction is fast, sufficient metal atoms can be added to the surface of seeds for continuous growth so they tend to take a thermodynamically favored shape such as cuboctahedra and spherical particles. When the reduction rate is considerably slowed, the concentration of metal atoms is low. The atoms tend to aggregate into small clusters, and then into nanoparticles.16b,17 This synthesis is often known as a kinetically controlled process and the nanoparticles will be of shapes deviating from the thermodynamically favored one such as platelike particles. To achieve kinetically controlled synthesis of metal nanoplates, some strategies, such as the coupling of reduction with oxidative etching, the use of an extremely mild reducing agent, and the competing reduction reaction have been successfully developed.5b,6a,16b In the present case, the Ag nanoplates are obtained in a very short time (within 2 min), suggesting that the formation rate of the Ag nanoplates is very fast. The nanoplates possibly form in the stage as early as nucleation, rather than through an evolutional path from other shapes during the growth process. Archer et al. reported the synthesis of hyperbranched gearlike Ag nanocrystals by the reaction of silver nitrate with ascorbic acid in the trisodium citrate-containing aqueous solution.8c They believed that during the earliest stages of the reaction, ascorbic acid rapidly reduced the silver nitrate precursor to yield disklike (or possibly nanoprism) Ag bodies sandwiched between citrate ions. However, this suggestion seems not to be operative in our case, since no citrate ions or surfactant exists in the reaction system. To get further insight into the formation mechanism of the triangular plates, some control experiments are carried out. When using ethanol instead of acetone and keeping other typical conditions
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Figure 5. TEM (A) and SEM (B) images of the polyhedral Ag nanostructure obtained by using water as the solvent and keeping other typical conditions constant. TEM images of Ag nanostructures obtained under different volume ratios of acetone/water: (C) v:v 3:1; (D) v:v 9:1.
constant, only little (
