SDBS Mixed-Surfactant

Novel dendritic silver crystals consisting of several branches have been .... In each branch, there are two rows of Ag secondary branches grown on the...
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CRYSTAL GROWTH & DESIGN

Growth of Dendritic Silver Crystals in CTAB/SDBS Mixed-Surfactant Solutions

2008 VOL. 8, NO. 7 2150–2156

Lei Fan and Rong Guo* School of Chemistry and Chemical Engineering, Yangzhou UniVersity, Yangzhou 225002, People’s Republic of China ReceiVed NoVember 6, 2007; ReVised Manuscript ReceiVed March 29, 2008

ABSTRACT: Novel dendritic silver crystals, which consist of several branches with lengths up to 10 µm, have been successfully synthesized by a simple wet chemical route using L-ascorbic acid as reductant in cetyltrimethylammonium bromide (CTAB) and sodium dodecyl benzyl sulfonate (SDBS) mixed surfactant solution at room temperature. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and UV-vis absorption spectroscopy have been used to characterize the obtained silver products. It is found that the architecture of silver crystals is drastically influenced by the molar ratio, concentration of CTAB/SDBS, and the concentration of the precursors. It is revealed that the mixed-surfactant solution might play both capping reagent and dispersion reagent roles. Introduction The morphology control and the patterning of inorganic materials with nanoscale dimensions have been rapidly developed into a promising field in material chemistry.1–3 Especially, many efforts have been focused on the integration of 1D nanoscale building blocks into two- or three-dimensional ordered superstructures or complex functional architectures, which is a crucial step to the realization of functional nanosystems.4–7 For example, multiarmed5 and radially7 aligned semiconductor nanorods were fabricated by solution-growth methods; ordered nanorod arrays were obtained by the self-assembly of preformed uniform nanorods through DNA hybridization8 and a LangmuirBlodgett technique.9,10 Notably, self-organized crystal growth of a variety of novel hierarchical nanostructures made of nanords or nanowires has been achieved; examples include the growth of ZnO dendritic nanostructures, nanobrigdes, and MgO fishbone-like nanostructures by a vapor-transport and condensation technique.11–13 However, most of them are two-dimensional structures. It remains a significant challenge to develop a facile, mild, and effective method for creating three-dimensional hierarchical structures from one-dimensional nanorods or nanobelts. Among various inorganic nanoparticles, silver inorganic nanoparticles are of particular interest because of their wide applications resulting from the unusual properties because of their shape and size.14 Much effort has been devoted to the synthesis of silver nanoparticles with different shapes. This includes zero-dimensional spherical or tetrahedral quantum dots,15 one-dimensional silver nanorods and wires,16–20 twodimensional nanoplates,21–23 and three-dimensional nanocages.24,25 Nevertheless, a few of its novel nanostructures, such as the dendritic Ag nanocrystals, have attracted great attention. Recently, Xie and co-workers have synthesized dendritic silver nanostructures using Raney Ni as template.26 Chen and coworkers have obtained dendritic Ag nanocrystals by irradiating the solution with PVA as capping agent.27 However, the development of a facile, low-temperature synthesis of such novel shape Ag nanocrystals in aqueous solution remains a great challenge. * Corresponding author. E-mail: [email protected]. Fax: 86-514-7311374. Tel: 86-514-7975219.

It is well-known that mixtures of various surfactants, including alkyl amines, alkyl acids, alkylphosphonic acids, and trioctyl phosphine oxide (TOPO), are frequently used as capping agents to tailor the crystal shape in high-temperature solution-phase synthesis. Recently, Qi L. have demonstrated that the lowtemperature synthesis of morphology controlled one-dimensional (1D) BaXO4 (where X ) Cr, Mo,W) nanostructures and their hierarchical architectures can be readily achieved in reversemicelle solutions of mixed cationic/anionic surfactants.28–30 These results have inspired us to explore the low-temperature synthesis of silver nanocrystals with tailored shapes in aqueous solutions of mixed cationic/anionic surfactants. In the present work, we report on the synthesis of dendritic silver crystals in a simple mixed CTAB/SDBS surfactant system. Studies found that some related experimental parameters, including the concentration of the surfactant, the molar ratio of the CTAB and SDBS, and the concentration of the precursor, have great influence on the morphology. Experimental Section Cetyltrimethylammonium bromide (CTAB) and sodium dodecyl benzyl sulfonate (SDBS) were purchased from Alfa-Aesar reagent company. The other reagents were analytically pure and purchased from Shanghai Chemical Co. Ltd. All reagents were used without further purification. A typical synthesis of dendritic silver crystals was as follows: 0.017 g of AgNO3 (0.1mmol) salt and 0.018 g (0.1mmol) of L-ascorbic acid salt were added to a 5 mL CTAB/SDBS solution, respectively. After 10 min of vigorous agitation, equivalent volumes of two separate solutions containing AgNO3 and L-ascorbic acid were mixed rapidly, to give the final concentration of AgNO3 and L-ascorbic acid as 0.01 M and concentrations of 0.3 and 0.7 mM for CTAB and SDBS, respectively. The resulting gray mixture was then laid aside in the dark for 1 h at room temperature. The obtained precipitate was separated, washed several times with distilled water and ethanol, and then dried in a vacuum at 25 °C for 4 h. X-ray powder diffraction (XRD) patterns of the products were recorded with a Bruker (German) AXS D8 ADVANCE X-ray diffractometer with Cu KR radiation (λ ) 1.5418Å) at a scanning speed of 0.4°/min from 30° to 80°. The surface morphology and particle size were examined by scanning electron microscopy (SEM, XL-30E Philips Co., Holland, 20kV) and transmission electron microscopy (TEM, Tecnai-12 Philips Apparatus Co. Holland, 120kV). High-resolution transmission electron microscopy (HRTEM) images were taken on a

10.1021/cg701096g CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008

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Figure 1. (a) SEM image of the obtained dendritic silver crystals. The inset shows a high-magnification image of a dendritic silver. (b) XRD pattern of the obtained dendritic silver crystals. JEOL-2010 electron transmission. UV-visible absorption spectra were recorded at room temperature on a Shimadzu UV-2550 spectrophotometer.

Results and Discussion Figure 1a shows a typical SEM image of dendritic silver structures. The lower magnification image shows the large quantity of such structures. The higher magnification SEM image (inset of Figure 1a) clearly shows that the as-obtained product has several branches, which seem to grow from one point. Also we can find that the branches at bottom are longer than that upside. A typical XRD pattern is shown in Figure 1b, in which all the peaks could be indexed to the corresponding (111), (200), (220), and (311) planes for pure face-centered cubic

phase Ag. The lattice constant calculated from this pattern is 4.087 Å, which is in good agreement with the value in the literature (JCPDS card 04-0783, a ) 4.086 Å). Judging from the XRD patterns, the products have very high crystallinity. It is worth noting that the ratio between the intensities of (111) and (200) peaks exhibits a relatively higher value than the conventional value (the conventional value is 2.5), indicating the enrichment of {111} crystalline planes in the dendritic silver crystals. To further investigate the morphology of the dendritic silver, we show some TEM images in Figure 2. In Figure 2a, we find that a silver tree is composed of several branches. In each branch, there are two rows of Ag secondary branches grown on the major stem, and the angle between the side branch and

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Figure 2. (a) TEM image of the obtained dendritic silver crystals. Inset shows the magnification of a branch. (b) Electron diffraction pattern of the dendritic silver crystals. (c, d) HRTEM images for the stem and the branch of the obtained dendritic silver crystals.

the stem is about 60° (inset in Figure 2a), but they are not exactly symmetrical. The same angle exists in all branches, resulting in the shape of the products being akin to dendrite or trees. These side branches have uniform diameters about 0.5 µm and are evenly spaced on a stem with a regular periodicity of 100-300 nm. In general, the length of the Ag stem along the axis can be as long as 10 µm, as shown in Figure 1a, and the diameter is about 500 nm, whereas the length of the Ag side branch grown on the Ag stem ranges from hundreds of nanometers to 2 µm. Generally, the side branches near the tip are shorter than those near the root. Because of the ultrasonic dispersion, some broken branches can be observed in the TEM image. Images c and d in Figure 2 show typical high-resolution transmission electron microscopy images recorded from the stem and the junction of the dendritic silver crystals. The fringe spacing is determined to be about 0.235nm, which is close to (111) lattice spacing of Ag crystals, indicating that the growth direction of the dendritic silver is preferential in the [111] direction. The HRTEM images of the stem shows that the individual silver dendrite is a single crystal, and the image of the junction shows that the side branch and the stem have identical crystal orientations. The ED pattern (Figure 2b) recorded on the tip of the treelike structure confirms the singlecrystalline nature of the dendritic silver crystals. The chemical composition is analyzed by energy-dispersive X-ray spectrometry (EDX), and the spectra are shown in Figure 3a. The strong peaks from Ag and Au are found in the EDX spectrum. The Au peaks come from the thin gold layer for

conductive coating. Traces of carbon, oxygen, and nitrogen (from L-ascorbic acid and nitrate) are not found. This indicates the high purity of the product. Figure 3b shows the UV-vis absorption spectrum of the dendritic silver crystals, which is obtained by dispersing the dendritic silver in water under sonication. The spectrum exhibits a broad plasmon peak centered at 380 nm. It is a well-known surface plasmon resonance of silver, the transverse plasmon bands, which are expected for anisotropic metallic nanoparticles including silver nanorods.9 But the expected longitudinal plasmon band does not appear. These results are consistent with the SEM results: the dendritic silver consists of large amount of branches, which seems like nanorods. In the present work, it is found that a mixing molar ratio of CTAB to SDBS near 3:7 is favorable for the formation of dendritic silver crystals. If only CTAB is used in solution and the other conditions are kept consistent, Ag quasi-spheres can be obtained (Figure 4a). If the mixing ratio of CTAB to SDBS changes to 7:3, it usually results in irregular nanoparticles and nanorods (Figure 4b); a mixing ratio of 5:5 results in irregular sheet and nanorods (Figure 4c); and with only SDBS solution, the results is curly Ag wires lengths to several micrometers (Figure 4d). It is apparent that the mixed CTAB/SDBS provide significant control over the nucleation and directional aggregation growth of dendritic silver crystals in the crystallization process. The dendritic silver crystals cannot be obtained when only one kind of surfactant is used. But the mixed surfactants are not likely to act as templates, because it has been documented that vesicles are generally the predominant struc-

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Figure 3. (a) EDX spectrum for the dendritic silver; (b) UV-vis spectrum of dendritic silver crystals.

Figure 4. SEM images of Ag particles obtained in different molar ratios of CTAB/SDBS solutions: (a) 10:0, (b) 7:3, (c) 5:5, and (d) 0:10.

tures in solutions of CTAB/SDBS mixtures.31,32 So the surfactant molecules probably act as capping agents and they are chemically absorbed onto the surface of sliver nanoparticles to form compact and uniform double layers in tail-tail mode.33 In addition to the effect of the ratio of CTAB to SDBS, the morphology of the resulting silver products also depends strongly on the concentration of the AgNO3 and L-ascorbic acid aqueous solution. When the other conditions keep constant, if the final AgNO3 concentration is lower than 2.5 mM and the ratio between AgNO3 and L-ascorbic acid remains at 1:1, only

tightly cohered silver grains are observed (Figure 5a). When a 5 mM AgNO3 solution is used, silver clusters are orderly assembled but the branch seems not to have grown well (Figure 5b). As the AgNO3 concentration increases to 20 mM, some neighboring branches coalesce together to form dentritic structures (Figure 5c). When a 40 mM AgNO3 solution is used, larger size dentritic structures are formed (Figure 5d). It is a litter different from the dentric structures obtained before. It looks much more like leaves, because the width of the branch is much greater than before and they are tightly arranged. So the concentration of precursor takes an important effect on the

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Figure 5. SEM images of Ag particles obtained in different concentration of AgNO3: 2.5, 5, 20, and 40 mM.

Figure 6. SEM images of Ag particles obtained in different concentrations of 3:7 CTAB/SDBS solutions: 1 × 10-4, 5 × 10-4, 5 × 10-3, and 1 × 10-2 M.

Dendritic Ag Nanocrystals in Mixed-Surfactant Solution

product morphology, and the size of the branch will increase with the precursor concentration. Furthermore, it is found that a suitable CTAB/SDBS concentration (e.g., 1 × 10-3 M) is essential to the formation of dendritic silver crystals in tubes. If we keep the molar ratio of CTAB/SDBS at 3:7 and change the total concentration, as shown in Figure 6a, aggregated small silver particles precipitate at a lower CTAB/SDBS concentration (1 × 10-4 M), whereas netlike silver crystals precipitate at a higher CTAB/SDBS concentration (5 × 10-4M) (Figure 6b). If the concentration increases to 5 × 10-3 M, branchlike Ag nanoribbons rather than dendritic silver crystals can be obtained (Figure 6c). And silver wires will be precipitated at a higher CTAB/SDBS concentration (1 × 10-2 M) (Figure 6d). It is indicated that a minimum concentration of CTAB/SDBS is required for the heterogeneous nucleation and growth of dendritic silver crystals but excessive CTAB/SDBS would considerably inhibit heterogeneous nucleation of silver. It might be rationalized by considering that at a lower surfactant concentration, the surface of the nucleate is not well-covered by the adsorbed surfactant molecules, so the nucleate grow irregularly to all direction. But in higher surfactant concentration, the nucleate is largely covered by the adsorbed surfactant molecules, impairing the direct adsorption of Ag+ ions on the nucleate surface, which can lead to poor heterogeneous nucleation sites. Possible Formation Mechanism From the above analysis, it can be seen that the morphologies of the final products are greatly dependent on the experimental conditions, such as the ratio of CTAB to SDBS, surfactant concentration, and precursor concentration. To gain control over the synthesis, it is necessary to understand the mechanism by which the dendritic silver crystal is formed. Like most facecentered crystal metals, metal silver has a cuboctahedral qurilibrium shape dominated by the {111} faces and the {100} faces, because these faces have the lowest energies. A general order of the surface energies for different faces of the facecentered cubic metals may hold, γ{111} < γ{100} < γ{110}.34 That means more energy is released by adding a silver atom to the {100} faces or the {110} faces than to {111} faces during crystal growth.35 And these facets, which have a higher surface energy compared to the others, have great tendency to bind to surfactant. CTAB and SDBS may act as capping agents in a tail-tail mode. 23 First, Ag clusters may be covered with CTAB, most likely, with its (CH3)4-N+ headgroup bound to the silver surface, and its long alkyl hydrophobic chain toward the outside.36 This may act as a bridge for the adsorption of SDBS in the SDBS and CTAB mixed-surfactant solution. There is a strong hydrophobic interaction between long alkyl chains of SDBS and CTAB, which is called tail-tail interaction and leads to a strong synergism, which favors the formation of wormlike micelles and directs the growth of the one-dimensional nanostructure. Furthermore, the mixed CTAB/SDBS may work as a dispersant. From the above experiments, we can find that the Ag nanoparticles aggregated together in lower concentration of the surfactant solution but dispersed Ag nanoribbons can be found in higher concentration. Maybe at lower surfactant concentrations, the small Ag nanoparticles grown at first aggregate easily in order to decrease the energy of the system. However, at higher surfactant concentrations, the seeds are completely covered by the surfactant molecules, so they can be dispersed well in surfactant solutions, and then grow along a given orientation. So a proper surfactant concentration is important for the

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formation of dendritic silver crystals. A proper concentration of surfactant is beneficial to the seeds formed at first to aggregate partially, and then the seeds can grow into branches in every orientation to form dendritic silver crystals. Hence, the mixed CTAB/SDBS surfactants may play the role of both capping agent and dispersant in the nucleation and growth of Ag crystals, leading to the gradual growth of dendritic silver crystals. A detailed time course study is expected to provide direct evidence for such a speculation. Unfortunately the experiments show the dendritic silver crystals are formed quickly, which prevented the direct observation of their detailed formation process. However, the exact mechanism for the morphological control of Ag crystals by mixed cationic/anionic surfactants is worthy of further investigation. Conclusions In summary, dendritic silver crystals have been successfully synthesized via a solution-growth route in CTAB/SDBS mixture surfactants. The HRTEM images and ED pattern indicate that both the stem and secondary branches grow predominantly along the [111] direction. It is also found that the concentration and molar ratio of CTAB/SDBS have largely influenced the morphology of the product. A suitable total concentration (1 × 10-3molL-1) and a proper molar ratio (3:7) of CTAB/SDBS are favorable for the formation of dendritic silver crystals. The concentration of the precursor also has great influence on the size and the morphology of the product. The obtained dendritic silver crystals may find potential applications in catalysis or microelectronic devices. Acknowledgment. This work is supported by the National Natural Scientific Foundations of China (20633010 and 20773106).

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