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Journal of Colloid and Interface Science 268 (2003) 357–361www.elsevier.com/locate/jcis

Controlling synthesis of silver nanowires and dendrites in mixedsurfactant solutions

Xiuwen Zheng, Liying Zhu, Aihui Yan, Xinjun Wang, and Yi Xie∗

Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026,People’s Republic of China

Received 18 September 2002; accepted 19 September 2003

Abstract

Using a simple wet chemical route, high-yield silver nanowires with an average diameter of 25± 5 nm and length up to several µm and dendrites with a long central backbone and symmetrically ramified secondary branches have been successfulby reducing AgNO3 with L-ascorbic acid (AsA) in the mixed surfactant solutions of cetyltrimethylammonium bromide (CTAB)sodium dodecyl sulfate (SDS). It was found that the architecture of silver nanocrystals was drastically influenced by the ctions of ascorbic acid. At a given high concentration, a nonequilibrium system was easily built, which favored the formation ofWhen the concentration was lowered, one-dimensional silver nanowires were successfully obtained. In addition, the presenctrolyte (NaCl) plays an important role in the preparation of silver nanowires, influencing the silver crystallization process in suways. 2003 Elsevier Inc. All rights reserved.

Keywords: Silver; Mixed-surfactants; Dendrites; Nanowires

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1. Introduction

Recently, much attention has been paid to the deand fabrication of silver nanocrystals with desired siand novel morphologies due to their physical and phophysical properties on the nanoscale [1–5]. Well-defisilver nanowires have been extensively exploited in caysis, electronics, photonics, and photography [5–7]. Ddritic structures, as the result of nonequilibrium aggregagrowth and molecular anisotropy, provide a natural framwork for the study of disordered systems [8,9]. Up to novarious methods have been developed to synthesizeone-dimensional silver nanowires and dendritic structusuch as arc discharge [10], templating from carbon notubes [11], calix[4]hydroquinone nanotubes [1], DNA [1Raney nickel template [13], ultraviolet irradiation photoduction [14], and electrochemical method [15]. To ovcome the shortcomings of low quantity and complicasynthetic procedure, many attempts in recent years

* Corresponding author.E-mail address: yxielab@ustc.edu.cn (Y. Xie).

0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2003.09.021

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been focused on a soft solution-phase approach to synthing uniform silver nanowires. For example, silver nanowihave been successfully fabricated by extraction from zepores [16] as well as by reducing AgNO3 with a devel-oper in the presence of AgBr nanocrystallites [17] or wascorbic acid in a wet chemical route [18]. Recently, Xand co-workers have demonstrated a novel polyol profor the large-scale synthesis of uniform silver nanowiin the presence of poly(vinylpyrrolidone) [19,20]. In thsolution-phase synthesis procedure, it is generally accethat surfactants act as capping agents and are chemabsorbed on growing crystals and kinetically controlgrowth rates by interacting through adsorption and destion [19–21].

In this paper, we report a simple and novel routeprepare high-yield bare silver nanowires with a highpect ratio of ca. 60–70 and beautiful silver dendrites usCTAB and SDS as mixed surfactants under mild conditioThrough adjusting the experimental parameters, such aconcentration of ascorbic acid and the presence or absof electrolyte (NaCl), one-dimensional nanowires and ddrites can also be successfully obtained.

358 X. Zheng et al. / Journal of Colloid and Interface Science 268 (2003) 357–361

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2. Experimental

A typical procedure is as follows:

(1) Procedure for synthesizing silver nanowires: 3.0 g cetrimethylammoniumbromide (CTAB), 3.0 ml 1-butan16.0 ml octane, and 3.0 ml NaCl (1 mol l−1) in aque-ous solution and 3.0 g sodium dodecyl sulfate (SD40.0 ml 1-hexanol, and 4 ml AgNO3 (1 mol l−1) inaqueous solution were added into two different retion vessels, respectively. And then, the solutions wmagnetically stirred for 30–60 min. After that, thetwo different solutions were mixed together and ctinuously stirred for 30 min, then 10 ml ammonia a20 ml ascorbic acid (0.17 mol l−1) were added into thvessel. To ensure the complete reaction, the solutionheated up to 80◦C and then maintained at that tempature for 3 h. The final gray precipitates were collecby centrifugation, washed with absolute ethanol andtilled water several times, and dried at 40◦C in vacuumfor 3 h.

(2) As for the synthesis of silver dendrites, the experimeprocess is similar to that of silver nanowires, using 3distilled water instead of sodium chloride aqueous stion (1 mol l−1) to prepare the CTAB/1-butanol/octasolution. Five milliliters of ascorbic acid (1 mol l−1) wasused as reducing agent. To further investigate the inence of the concentration of ascorbic acid and surfacon the formation of silver nanowires and dendrites,controlled experiments were carried out in differentperimental conditions. All the experimental conditioare listed in Table 1.

(3) X-ray powder diffraction (XRD) was carried out oa Rigaku D/max rA X-ray diffractometer with CuKα

radiation (λ = 1.54178 Å). Transmission electron m

Table 1Experimental conditions and results of reaction of AgNO3 with L-ascorbicacid (AsA) in CTAB and SDS mixed solutionsa

No. Concentration AsA Surfactants Electrolyte Morpholog(mol l−1)

1 1 CTAB/SDS Dendrites2 1 CTAB/SDS NaCl Dendritesb

3 0.8 CTAB/SDS Dendritesc

4 0.3 CTAB/SDS NaCl Nanowiresc

5 0.17 CTAB/SDS NaCl Nanowires6 0.17 CTAB/SDS Nanowiresc

7 0.05 CTAB/SDS NaCl Nanoparticle8 1 CTAB NaCl Nanoparticles9 1 SDS NaCl Nanoparticle

10 1(KBH4) CTAB/SDS NaCl Nanoparticles11 1(KBH4) CTAB/SDS Nanoparticles

a Reaction time is 3 h and reaction temperature is 80◦C in all caseslisted.

b A small quantity of filament-like silver nanoparticles exists in the pructs.

c The quantity and quality of the obtained silver dendrites or nanoware poorer than those obtained in experiments 1 or 5.

croscope (TEM) images and field emission scannelectronic microscope (FESEM) images were takena Hitachi Model H-800 and a JSM-6700F field emsion scanning electronic microanalyzer, respectivHigh-resolution transmission electron microscopeages (HRTEM) were taken with a JEOL-2010 transmsion electron microscope.

3. Results and discussion

The X-ray diffraction (XRD) pattern of the as-synthesizsilver nanowires is shown in Fig. 1. All the reflection peacan be indexed to the corresponding (111), (200), (2(311), and (222) planes for pure face-centered cubic pAg without peaks due impurity found. The calculated cparameter isa = 4.084 Å, which is close to the reportedata within the experimental errors (JCPDS 4-783). Judfrom the XRD patterns, the products have very high crtallinity. It should be noted that the XRD pattern of silvnanowires with different morphologies is almost similareach other.

The morphologies and sizes of as-prepared productsobserved by transmission electron microscopy (TEM)field emission scanning electronic microscopy (FESEFigs. 2a and 2b show the images of silver nanowires, wwere obtained by reducing AgNO3 with 0.17 mol l−1 ascor-bic acid. Bulk quantities of bumpy and curly nanowiresformed with relative uniform diameter. Fig. 2a illustratthe panorama of obtained products, which show the buand curly wire-like structure. The nanowires are long, resing in a very large aspect ratio of ca. 60–70. In additiontrace amount of ribbon-like structure also is found. Fig.gives a typical straight single silver nanowire image, frwhich it can be seen that the obtained continuous, unifsilver nanowires are 25 nm in diameter and up to sevµm in length. Judging from the TEM images, the curly astraight silver nanowires both existed in the obtained sples. However, it should be noted that most of the samis bumpy and curly nanowires. Furthermore, selectedelectron diffraction (SAED) is used to confirm the high crtallinity of the as-prepared silver nanowires. Fig. 2c shothe typical SAED pattern of silver nanowires, which w

Fig. 1. The XRD pattern of the obtained silver nanowires.

X. Zheng et al. / Journal of Colloid and Interface Science 268 (2003) 357–361 359

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Fig. 2. (a, b) The TEM images of Ag nanowires. (c) The SAED patternsilver nanowires, which indicate the single crystallinity of silver nanowirThe nanowires have grown along the [111] direction. (d) The HRTEMage of silver nanowires. The fringe spacing (0.2356 nm) observed inimage corresponds to the separation between the [111] lattice planes.

obtained by focusing the electronic beam along the [0zone axis of an individual nanowire. Judging from the resof index, the as-prepared silver nanowires have predonantly grown along the [111] direction. The high-resoluttransmission electron microscope (HRTEM) image, shoin Fig. 2d, gives further information on the fine microstruture of the silver nanowires. The interplanar spacing is ab0.2356 nm, which is consistent with the spacing of the (1planes of face-centered cubic silver crystals. This resulveals that the growth of the nanowires is along the [1direction, which further supports the analytical conclusof SAED that the silver nanowires grow preferentially alothe [111] direction. More careful analysis shows thatsilver nanowires contain some dislocations and twin destructures, which may derive from the stacking faults andgrain boundaries during the construction of silver nanowi

To obtain further evidence for the purities and compotions of the as-prepared products, the X-ray photoelecspectra (XPS) were used. The bonding energies obtafrom the XPS analysis was corrected for specimen ching by referencing the C1s to 287.60 eV. Fig. 3a shows th

Fig. 3. XPS spectra of Ag nanowires.

XPS survey spectra of silver nanowires. No obvious imrities (besides trace adsorbent O2) could be detected in thsamples, indicating the products have high purity andlevel of impurities is lower than the resolution limit of XP(1 atm%). The typical high-resolution spectra of Ag3d isshown in Fig. 3b. The binding energy values are 368.62for Ag 3d5/2 and 374.45 eV for Ag3d3/2. All of the observedbinding energy values for Ag3d coincide with the reporteddata within the experimental errors [22].

Fig. 4 displays the TEM and FESEM images of tycal, beautiful dendrites, which were obtained by reducAgNO3 with 1 mol l−1 ascorbic acid. Further observatioshows that each dendrite consists of a long central bbone and very sharp secondary branches, which prefetially grow along two definite directions through crystallgraphic regulation rather than randomly ramified growSurprisingly and interestingly, the secondary branches wemerged at 60◦ angles with respect to the central backbohave uniform spacing and are parallel to each other. Thdendritic structures are usually associated with the diffuslimited particle aggregation process [8]. The SAED patteinset in Fig. 4b, reveal that only hexagonal diffraction sppattern are observed. These high qualities of diffracspots confirm the single crystal feature of the as-prepsilver dendrites.

In the reaction system, the influence of concentrationascorbic acid, which is a prerequisite for controlling syntsis of silver nanocrystals with different morphologies, hbeen investigated in detail. Under the given conditions wdifferent concentrations of ascorbic acid, the obtained si

360 X. Zheng et al. / Journal of Colloid and Interface Science 268 (2003) 357–361

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Fig. 4. TEM and FESEM images of the obtained silver dendrites withferent magnifications. Inset for the corresponding SAED pattern.

crystals have completely different shapes. Under the hiconcentration of ascorbic acid (up to 1 mol l−1), silver den-drites were obtained (Fig. 4) because of the promotiodiffusion-limited growth with increasing reduction rateAg+ ions [23]. However, the bulk quantities of bumpy acurly nanowires were obtained with relatively lower conctration (0.17 mol l−1), as shown in Figs. 2a and 2b. Withe high concentration of ascorbic acid, which accelerthe reduction of AgNO3, the reaction rate is faster than thof the low concentration, resulting in the nonequilibriusystem being easily built, which is beneficial in the formtion of dendrites [8,23]. However, under the relatively lconcentration of ascorbic acid, the rate of nucleation cobe well matched with that of growth. Therefore, thever clusters with larger sizes preferentially grow alongone-dimensional direction at the expense of smaller oand block the deposition on the lateral surface withaid of surfactant molecules. While lowering the conctration to 0.05 mol l−1, the nucleation rate is too low anstop the growth before particles preferential growth becathe smaller the nuclei, the smaller the odds of hittingsticking. In addition, the existences of surfactant molecsurround the nuclei and obstruct the further growth ofnuclei. Therefore, few nanowires are formed. In additionKBH4 (1 mol l−1) solution was used as the reducing aginstead of ascorbic acid solution, and all other proced

were kept constant, the nanowires and dendritic nanoctals are both absent due to that the rate of nucleation is mfaster than that of growth, which leads to the formationnanoparticles.

It is apparent that the mixed surfactants provide signcant control over the nucleation and directional aggregagrowth of silver nanocrystals in the crystallization procewhich can be proved by the contrast experiments thatone kind of surfactant was used and the nanowires anddrites could not be obtained. The new silver clusterssorb both surfactant molecules and halide ions strongly fthe solution. The surfactant molecules probably act asping agents and are chemically absorbed onto the suof silver nanoparticles to form compact and uniform dble layers in tail–tail mode [24–26]. On the other hand,coadsorbed Br− ions (form Ag10Br− cluster) act as a bridgfor the adsorption of surfactants (CTA+) [26]. In the mixedcationic–anionic (catanionic) surfactant solutions, the strinteraction between headgroups (–SO−

4 and R4N+) medi-ated by tail–tail interactions leads to a strong synergism [which favors the formation of worm-like micelles and drects the growth of one-dimensional nanostructure [28,The initial formation of silver clusters and their directionaggregation growth are under the influence of preferesurfactant binding to form long linear aggregates. Theseplay a similar function to PVP, which kinetically controls tgrowth rates of various faces by interacting with these fathrough adsorption and desorption [19,20].

In addition, it is worth pointing out that the presenceelectrolyte (NaCl) also has important influence on themation of 1D nanowires. A given experiment was mato test the role of electrolyte (NaCl), namely using 3NaCl (1 mol l−1) aqueous solution instead of distilled wter during the preparation of CTAB/1-butanol/octanelution and other experimental conditions are the samthose of the preparation of silver dendrites. The results sthe coexistence of a small quantity of filament-like silvnanoparticles and dendrites. In contrast, keeping the eximental conditions the same as in the preparation of snanowires and only using distilled water instead of N(1 mol l−1) aqueous solution, albeit the silver nanowicould also be obtained, the quantity and quality of thetained silver nanowires are poorer than those of the wmentioned above (Fig. 2). These results imply that the pence of electrolyte is favorable for the formation of silvnanoparticles with 1D structure and increases the quaof silver nanowires. It is generally accepted that the pence of a high concentration of NaCl salts out the surtants to some extent and promotes the formation of flexwormlike microstructures, which perhaps have a greatpact on the crystallization process of noble metal nanoctals and are beneficial for the aggregation of nanocrysinto threadlike one-dimensional structures [30–32]. In ation, in the salt-dependent solution, the ionic strength efmay be lead to more preferential adsorption of surfacmolecules onto different crystal faces, which perhaps

X. Zheng et al. / Journal of Colloid and Interface Science 268 (2003) 357–361 361

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ther promotes the preferential growth of silver nanowirHowever, the advantageous formation of silver nanowin the mixed CTAB and SDS solution with electrolyte isvery complex process, which needs further investigatiothe future. Judging from the comparison experiments,can see that the proper concentrations of reducing agenmixed surfactants presented in the reaction system areprerequisites for the controlling synthesis of silver nanoctals with one-dimensional and dendritic structures. Howeelectrolyte (NaCl) also has an important influence on themation of silver nanowires in surprising and unknown wa

4. Conclusions

In summary, a simple but practical method has beenveloped to control the synthesis of silver nanowires and ddrites. Particularly, the growth of nanowires and dendrcan be well controlled through altering the concentratof reducing agent. It is found that both the reducing agand mixed surfactants play important roles in controllingsynthesis of silver nanoparticles with different morphogies. Under a high reducing agent concentration (1 mol l−1),dendrites have been obtained. In contrast, under relatlow concentration of reducing agent (0.17 mol l−1), sil-ver nanowires have been obtained. Although the presof electrolyte is not the prerequisite for the formationnanowires, but it could favor preferential growth andcrease of the quantity of silver nanowires. From a techlogical point of view, these obtained silver nanowires adendrites may have important applications in catalysis,croelectronic devices or nanometer-scale electrodes.

Acknowledgment

Financial support from the Chinese National Foundaof National Sciences and Chinese Ministry of Educationgratefully acknowledged.

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