One Dimensional Aggregates of Silver Nanoparticles Induced by the

J. Phys. Chem. B 2002, 106, 3131-3138

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One Dimensional Aggregates of Silver Nanoparticles Induced by the Stabilizer 2-Mercaptobenzimidazole Yiwei Tan, Lei Jiang, Yongfang Li,* and Daoben Zhu Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China ReceiVed: July 12, 2001; In Final Form: October 21, 2001

Silver colloids stabilized by a specially designated molecule, 2-mercaptobenzimidazole (MBMZ), were prepared in a 50:50 (v/v) aqueous ethanol solution. A number of wire-like configurations impregnated with Ag nanoparticles were identified in the TEM images as the concentration of MBMZ passed beyond a certain value. Moreover, the aggregates of Ag nanocrystals were probed in solution by UV-vis and dynamic light scattering (DLS) measurements. UV-vis and DLS also revealed that the aggregation and aggregate growth of Ag nanocrystals occurred upon aging the aqueous ethanol solutions, which initially contain isolated particles. In addition, two control systems of which control system I and II were, respectively, 2-mercaptobenzothiazole (MBTZ) and [MBMZ‚C2H5]+Br- (see the Experimental Section of this paper) stabilized silver colloids were prepared and studied. The hydrogen bonding (i.e., N-H‚‚‚N interactions) between MBMZ molecules on the surfaces of different Ag nanoparticles is believed to be the driving force for the formation of wire-like aggregates, which is evidenced by comparing the FTIR of MBMZ-stabilized Ag nanoparticles with that of control system II.

Introduction Considerable attention has been paid to assembling nanoparticles into ordered two- or three-dimensional (2D and 3D) superstructures in the past few years due to their fundamental significance and technological implications involved in a wide range of domains.1-4 These novel structures have revealed that the electronic and optical properties are distinctly different from those of individual nanoparticles or their macroscopic equivalents. The ability to synthesize ordered nanoagglomerates (i.e., nanocrystal superlattice structures) will provide a new horizon to study the collective properties stemming from particle interaction and develop future optical,5 nanoscale electronic,6 and information storage devices.7 It is a prerequisite to assemble nanoparticles into well-defined structures for the realization of nanocluster application. Therefore, there have been tremendous research endeavors to explore new avenues to construction of ordered assemblies using nanoparticles as the building blocks. One strategy is to load the nanoparticles on a template to form one-dimensional (1D) and 2D superstructures.8,9 The other is to utilize bifunctional linkers, by which a self-assembled monolayer can be formed with one end adsorbed onto a substrate surface and the other end used to anchor the particles. The latter method may make it possible to self-assemble nanocrystal architectures in solution or on a substrate. A typical example is demonstrated by dithiols with which nanoparticles were bridged to fabricate one-dimensional arrays10 and cross-linked networks.11 Alternatively, a more efficient approach involves adsorbing stabilizer molecules incorporating a receptor site at the surface of a nanocluster, and then this nanocrystal recognizes and selectively binds another at which is adsorbed stabilizer molecules incorporating a complementary substrate site. By this means, * To whom correspondence should be addressed. Fax: 86-10-62559373. E-mail: [email protected].

SCHEME 1: Reaction Routes of the Preparation of Control System II

Fitzmaurice and co-workers achieved the assembly of complex nanocrystal architectures in solutions.12,13 Virtually, the mechanism of the latter strategy of assembling nanoparticles lies in how to establish the force action between “building blocks”. Electrostatic interactions, covalent linkage, and sorptive interactions are all candidates for developing macromaterials on the basis of mesomaterials. Although the organization of isolated nanoparticles into 2D and 3D ordered arrays is booming as extensively described in a flock of documents, only few reports regarding the formation of 1D configurations of nanoparticles have emerged in our line of vision. Notable exceptions are outlined below: The wirelike structures consisting of crystalline SiO2 nanospheres were generated with high-temperature synthesis techniques by Wang et al.14 Recently, Pileni and co-workers performed the organization of well-aligned cobalt ferrite nanoparticles on a very large scale in the presence of an applied magnetic field.15 In addition, the 1D strands of iron oxide particles were used for navigation by magnetotactic bacteria,16 and the 1D CdS chains were observed by Chemseddine et al.17 As far as our knowledge, the report on the 1D assembly of silver nanoparticles is rare until now. Here, we report the self organization of wire-pattern arrays of Ag nanoparticles invoked by aggregation using wet chemical methods. A specially designated molecule, 2-mercaptobenzimidazole (MBMZ), was adopted as the stabilizer in the preparation.

10.1021/jp012668l CCC: $22.00 © 2002 American Chemical Society Published on Web 03/05/2002

3132 J. Phys. Chem. B, Vol. 106, No. 12, 2002

Tan et al.

Figure 1. TEM images for the as-prepared silver nanoaggregates synthesized by (a) fashion I, (b) fashion II, (c) fashion III, and (d) fashion IV, and (e) the scattered Ag nanoparticles coated with MBTZ on carbon-coated copper grids. The insets in parts b and c represent the corresponding enlarged view of the indicated area in each micrograph. The insets in part d present the SAED pattern (top, showing diffuse diffraction rings) for a small particle in a wire-like configuration and the assignment of the diffractions (bottom). The assignment is based on the International Center for Diffraction Data file no. 03-0931 for FCC silver metal. Experimental conditions were as follows: [AgNO3] ) 0.572 mM, [NaBH4] ) 0.01 M, [MBMZ] ) 0.3125, 1.25, 2.5, and 5.0 mM for fashion I, II, III, and IV, respectively. The molar ratio of NaBH4/AgNO3/MBMZ for fashion I, II, III, and IV is 8:4:0.25, 8:4:1, 4:2:1, and 2:1:1, respectively. Fashions I, II, III, and IV correspond to Samples I, II, III, and IV, respectively. The molar ratio of NaBH4/AgNO3/MBTZ is 8:4:1 in part e.

1D Aggregates of Silver Nanoparticles

Figure 2. Elemental analysis of a single silver nanoparticle in a wirelike configuration by EDS.

Experimental Section Chemicals. The following materials were obtained from Beijing Chemical Reagent Company: AgNO3 (99.8%), NaBH4 (99.0+%), KBr (99.9+%), 2-mercaptobenzothiazole (MBTZ) (98+%), dry ethanol (99.8%), and acetonitrile (99%). 2-Mercaptobenzimidazole (MBMZ) (98+%) was purchased from Aldrich. Sulfuric acid was used as available. All of the chemicals were used as received. All water was 18 MΩ, purified with a Millipore Milli-Q purification system. Aqueous AgNO3 and NaBH4 solutions were prepared using preboiled Millipore water and then degassed by bubbling with dry argon gas for 2 h to well control the growth of nanoparticles as pointed out by Wilcoxon et al.18 A fresh aqueous NaBH4 solution was prepared before every series of experiments because of NaBH4 slowly decomposing for its reaction with water to form hydrogen. Solutions of all of the reagents employed in the synthesis were prepared by standard volumetric techniques. All glassware was washed by ultrasonication in a mixture of Millipore water and nonionic detergent, followed by thorough rinsing with Millipore water and ethanol for many times to get rid of any remnants of nonionic detergent, and dried prior to use. Synthesis of Silver Colloids. Preparation of Ag Sols Using MBMZ as the Capping Agent. The synthesis of monolayer-

J. Phys. Chem. B, Vol. 106, No. 12, 2002 3133 protected Ag nanoclusters is carried out by the chemical reduction of AgNO3 with NaBH4 as the reductant as follows: Four equivalents of AgNO3 (3.4 mg) were dissolved separately in four 35 mL volumes of 50% (volumetric percentage) aqueous ethanol solution. To each AgNO3 solution, a mixture consisting of 2.0 mL of a freshly prepared aqueous NaBH4 solution (0.02 M) and 2.0 mL of a MBMZ-ethanol solution with different concentration was added dropwise. The concentration of MBMZ was varied from 6.25 × 10-4, 2.5 × 10-3, and 5.0 × 10-3 to 0.01 M to control the size of the Ag particles (corresponding to fashion I, II, III, and IV, respectively). The molar ratio of NaBH4/AgNO3/MBMZ for fashion I, II, III, and IV is 8:4:0.25, 8:4:1, 4:2:1, and 2:1:1, respectively. The samples prepared in fashion I, II, III, and IV are, respectively, named as sample I, II, III, and IV in the following. The reduction reactions were performed under vigorous stirring. The brown Ag sol capped with MBMZ was obtained in the end of each reaction. Control System I. The MBTZ-stabilized Ag nanoparticle dispersion was also synthesized with the same method as fashion II, that is, NaBH4/AgNO3/MBTZ ) 8:4:1. Control System II. Two milliliters of a MBMZ ethanol solution (0.01 M) was added into another aliquot of 35 mL of a AgNO3 (3.4 mg)-50% aqueous ethanol solution prior to the reduction reaction, in which the solution turned a turbid faint yellow for the formation of flocculent silver salt sediment. Then, an equivalent molar amount of KBr (2.4 mg) and two drops of H2SO4 (0.1 M) were put into the solution successively. The solution was refluxed for 2 h at 45 °C in a thermostat.19 Finally, to the solution, 2.0 mL of a freshly prepared aqueous NaBH4 solution (0.02 M) was added dropwise. The whole experiment was carried out under vigorous stirring. The reaction progression is described in Scheme 1. The produced stabilizer will be denoted as [MBMZ‚C2H5]+Br- in this article. Instrumentation. TEM ObserVation and EDS Analysis. Specimens for transmission electron microscope (TEM) observation and energy-dispersive X-ray spectroscopic (EDS) analysis were prepared by drop-casting two drops of a freshly prepared solution containing Ag nanoparticles onto a standard carbonsupported (200-300 Å) 300-mesh copper grid and drying slowly in air naturally. The observations were undertaken with a JEOL-

Figure 3. Powder XRD patterns of the precipitated Ag nanocrystallites. The samples for measurements were obtained by (a) fashion II and (b) fashion III.

3134 J. Phys. Chem. B, Vol. 106, No. 12, 2002 JEM-100 CX II transmission electron microscope operated at 100 kV. The EDS analysis and selected area electron diffraction (SAED) data were acquired on a JEOL-JEM-2010F transmission electron microscope that was equipped with a nanoarea energydispersive X-ray spectroscopic analyzer (EDS) operated at 200 kV. The EDS analysis by using a 0.7 nm diameter electron probe was employed to determine directly the chemical identities of the constituent particles. Only isolated grains were analyzed to avoid simultaneous analysis of two different grains. The measurements were conducted by illuminating electron beams on a whole particle, the middle, and the edge of the particle. XRD Patterns. X-ray diffraction (XRD) measurements were carried out in the reflection mode on a Rigaku D/max-2400 diffractometer operated at 40 kV voltage and a current of 120 mA with Cu KR radiation. Samples for XRD patterns were the precipitates formed following the Ag colloidal solution being left undisturbed for 7-8 h. The X-ray diffraction data were collected at a slow scan rate in the step scan mode, with typical 20 s of data collection time for each step of 0.1°. UV-Vis Spectroscopy. UV-vis absorption spectra were recorded in the range between 240 and 700 nm using a HITACHI U-3010 UV/vis scanning spectrophotometer. Solution spectra were obtained by measuring the absorption of the prepared nanoparticle dispersions in a quartz cell with a path length of 1 cm. The experimental data were corrected for the background absorption of the solvent. DLS Measurements. Samples for dynamic light scattering (DLS) were prepared by diluting a few drops of Ag colloidal solution with 5 mL of a 50% aqueous ethanol solution under vigorous stirring. The DLS measurements were performed on a system assembled by Peking University. The 514.5 nm line of a Spectra-Physics model 2017 argon ion laser was used as the light source (200 mW). The temperature was maintained at 25 ( 0.02 °C throughout with an external circulator. Solutions were centrifuged at 12 000 rpm for 30 min, and the supernant was used for such measurements. The intensity correlation data were analyzed by the CONTIN method. The viscosity for the water/ethanol solution (1:1 v/v) was measured using a Ubbelhode sidearm viscometer and was found to be 2.40 mPa s. Infrared Spectroscopy. Infrared spectra, in the region 40001100 cm-1, were recorded on a Bruker EQUINOX55 infrared spectrophotometer with 4 cm-1 resolution. Pure MBMZ was pressed into a KBr pellet at 1% to obtain IR spectra. Analyses were also performed on solid particles that were deposited on a CaF2 window by evaporation of several drops of the 50% aqueous ethanol solution containing silver nanoparticles. The spectrum obtained was subtracted from a background of a clean CaF2 crystal slice. Results and Discussion Observation of Silver Nanoaggregates by TEM. Samples I, II, III, and IV were examined by transmission electron microscopy (TEM). A number of 1D aggregates of Ag nanoparticles are observed. Figure 1 presents the representative TEM images corresponding to the four samples. In the case of sample I (see Figure 1a), 1D arrays of Ag nanoparticles are observed in the micrographs, in which some larger nanoparticles can be identified clearly. The mean core diameter is calculated to be 9.6 nm by counting more than 150 particles. For sample II, the result displays some relatively straight wire-like configurations consisting of silver nanoparticles. A close examination of the particulate shapes indicates a “rhombus” outline for an observable percentage of the particles with ca. 5.2 nm in average size as shown in the inset of Figure 1b. In contrast to the results of

Tan et al.

Figure 4. The color changes of the MBMZ-stabilized Ag colloidal solution (sample II) that is left after the following periods: (a) 0 h (freshly prepared colloidal solution), (b) 2 h, (c) 4 h, and (d) 6 h.

sample II, both samples III and IV exhibit similar characteristics in architecture. Two striking features are noticeable for samples III and IV (see Figure 1c,d). First, a few larger particles of which the shape is close to a sphere are linked together by numerous smaller Ag particles, which are revealed by EDS as shown below. The average size of the Ag nanoparticles is ca. 3.1 nm. The shape of the smaller nanocrystals, though not clearly identifiable within the TEM resolution, appears rhombic. Second, each larger particle is encircled by a layer of smaller particles as shown in the inset of Figure 1c, which gives out an enlarged image of a selected area. It is well-known that the concentration and species of capping materials as well as the concentration of cations and the reaction time play important roles in the morphology control of the particles.20 In our case, the variance in the amount of MBMZ determines the size and morphology of the Ag nanoparticles. In addition, the rate of dropping reductant and stabilizer may affect the size and


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Figure 5. Time dependence of the variation of the absorption spectra of the MBMZ-stabilized Ag colloidal solution (sample II): (a) 0 h (freshly prepared colloidal solution); (b) 2 h; (c) 4 h; (d) 6 h.

morphology of Ag nanoparticles. As we use 2-mercaptobenzothiazole (MBTZ) as the stabilizer to replace MBMZ, only discrete silver nanoparticles instead of wire-like configurations were discerned in the TEM patterns as shown in Figure 1e. The chemical composition of 1D arrays of nanoparticles was determined with energy-dispersive X-ray spectroscopy (EDS). The EDS analysis suggests that the wire-like aggregates are composed of Ag nanoparticles by a 0.7 nm diameter electron probe being scanned along a line in the wire-like aggregates across the cores of four particles. The EDS spectrum obtained from a randomly selected nanoparticle from sample III (see Figure 2) shows the peaks of Ag and S together with peaks of C and Cu. The C and Cu peaks are due to the carbon-coated copper grid used for TEM measurements. Although the N peaks are not detected because of its low contents and small atomic number, the appearance of the S peak indicates that for the asprepared Ag nanoclusters the products are coated with MBMZ molecules. At the same time, the analytical results manifest that the silver/sulfur atomic ratio in all parts of the particle is nearly identical, that is, 3.2. The polycrystalline Ag electron diffraction pattern obtained from a small particle in a wire-like configuration from sample IV is presented in the inset of Figure 1d. Crystalline features of small Ag particles (fcc structured) were confirmed by fine discrete spot patterns overlapped with a ring pattern from carbon. This result further indicates the existence of Ag clusters in the wire-like configurations. A powder X-ray diffraction (XRD) pattern of a sample, unlike TEM, probes a large number of crystallites that are statistically oriented. Figure 3 shows the typical wide-angle X-ray diffraction patterns of the Ag nanocrystallites of samples II and III. Those of samples I and IV are elliptical to avoid verbosity of the text beacuse the mean size of Ag nanocrystallites of sample I can be unambiguously obtained from the TEM images and the mean size of sample IV is almost identical with that of sample III judging from the TEM images. As expected, the XRD peaks of the nanocrystallites are considerably broadened compared to those of the bulk Ag because of the finite size of these crystallites. Nevertheless, both sizes of the nanocrystallites have characteristic features matching the bulk cubic Ag pattern. These diffraction features appearing at about 38.1°, 44.1°, 64.4°, 77.3°,

TABLE 1: Absorption Peaks and Full Width at Half-Maximum (fwhm) of UV-Vis Spectra of Ag Colloidal Dispersion in 50% Aqueous Ethanol Solution Left for the Times Indicated time elapsed (hour)

peak positions (nm)

fwhm (nm)

0 2 4 6

406 434 452

109 281 335

and 81.6° correspond to the (111), (200), (220), (311), and (222) planes of the cubic phase of Ag, respectively. The average size of the silver nanoparticles was determined from the width of the reflection according to the Debye-Scherrer equation.21

D)

0.9λ β cos θ

(1)

where β is the full width at half-maximum (fwhm) of the peak, θ is the angle of diffraction, and λ is the wavelength of the X-ray radiation. For very broad diffraction lines, a previous deconvolution of the signals in two components, the (111) and (200) reflections, was carried out to obtain a more accurate fwhm measurement. For curve a in Figure 3, a couple of broadenings of 2.0° and 2.3° (fwhm) are, respectively, for the (111) and the (200) reflection of Ag. For curve b in Figure 3, two broadenings of 1.4° and 1.8° are for the (111) and the (200) reflection of Ag, respectively. By virtue of the average value of D calculated from the (111) and the (200) reflection of Ag, respectively, with λ ) 0.1542 nm for Cu KR radiation, a mean diameter of approximately 5.4 and 4.0 nm was found for Ag nanoparticles of samples II and III, respectively. This is consistent with, although slightly larger than, the mean size of 5.2 and 3.1 nm for Ag particles of samples II and III, respectively, as determined by the TEM measurements. Aggregation of MBMZ-Modified Nanoparticles in Solution. To gain a better understanding of the mechanism of forming silver 1D arrays, we investigate the aggregation of the MBMZ-modified Ag colloids. The detailed results are only given out to sample II because all samples I-IV have similar aggregation behavior in solution.


Tan et al.

Figure 6. Plots of the hydrodynamic radius distribution of Ag colloids protected by MBMZ in 50% aqueous ethanol solution at 25 °C, which were obtained by DLS after the sample being left for (a) 0 h, (b) 2 h, (c) 4 h, and (d) 6 h.

TABLE 2: The Mean Hydrodynamic Radius (Rm) of Each Peak in Figure 6a peaks in panel a Rm (nm) percent a

peaks in panel b

peaks in panel c

peaks in panel d

1

2

3

1

2

3

1

2

3

1

3.51 2.7

14.98 18.49

112.8 35.16

1.09 0.35

14.15 16.99

155.7 39.81

6.36 3.50

18.73 10.07

212.7 43.73

11.46 8.51

2

3 265.8 48.86

The data were analyzed with the CONTIN method.

Photographs in Figure 4 exhibit the color changes of MBMZderivatized Ag colloids from dark brown to yellow over a 6 h period, which may imply the variation in colloidal structures. The brown flocculent precipitation is observed following about 7-8 h upon completion of the reduction of AgNO3 in 50% aqueous ethanol solution with MBMZ as the capping agent. In contrast, the resulting solutions of control systems I and II are extremely stable. No precipitation proceeds in both control systems even over half a year period. As to weak competitive solvent such as acetonitrile (relative to 50% aqueous ethanol solution), a more rapid formation of a brown solid is observed, within 1-2 h. With respect to this phenomenon, a question arising is whether there exists a state of aggregation of Ag particles in solution, that is, fractal structure. It is well-known that the position and magnitude of the surface plasmon absorption band are intensively dependent on the degree of aggregation.22 Figure 5 shows the variation of the UV-vis absorption of MBMZ-modified Ag colloidal solution with time elapsed. The broad absorption band at ca. 400 nm of the freshly prepared Ag colloid is probably due to the interparticle interaction in solution. Yet, the onset of significant aggregation, which has been evident in a “tail” extending into the red, is shown.23 The intensity of this band gradually reduces, moreover, the band is broadened significantly and shifted to red with time

evolution, which implies that the aggregation is more prominently featured.24 The positions of the absorption bands and the values of fwhm are summarized in Table 1. In contrast, apparent changes in the absorption spectra can hardly be observed within several days for the provided control systems I and II aforementioned. Shown in Figure 6 are the hydrodynamic radii measured by DLS after the sample was left for different periods. The distribution of hydrodynamic radius of the scattering entity was calculated by the CONTIN method according to the autocorrelation function of the intensity of the scattering light. For the freshly prepared Ag colloids, the plots of size distribution indicate the existence of double structures in the colloids, namely, single nanoparticles as well as aggregates. The increase in average hydrodynamic radius suggests the continual growth of aggregates with time elapsed. These results are quite consistent with the conclusions obtained by the UV-vis spectra. Table 2 gives out the average hydrodynamic radius of each peak in Figure 6 calculated by the CONTIN method. In addition, the hydrodynamic radius distribution of control system II, which was left for 5 days, is given out in Figure 7 for comparison, in which few large aggregates are probed. The Mechanism of Ag Nanoparticle Aggregation. Kogel et al.25 and Ohara et al.26 proposed that, unlike micron-size hard


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Figure 7. Hydrodynamic radius distribution of control system II ([MBMZ‚C2H5]+Br--stabilized Ag colloids in 50% aqueous ethanol solution), which was left for 5 days, measured by DLS at 25 °C.

SCHEME 2: Diagram Depicting the Possible Linkage Pattern of 1D Arrays of Silver Nanoparticles, in Which the Formula Represent (a) a Pair of Tautomers of MBMZ, (b) the Linkage of MBMZ by Hydrogen Bond, and (c) the Linkage of Ag Nanoparticles by the Hydrogen Bondinga

a Although we give the assumption of the shape of small Ag particles being rhombic, for convenience, we still replace the shape of particles with a spherical outline.

spheres, self organization of nanocrystals on a substrate is not simply entropy-driven. Instead, interactions between particles and between the particle and the substrate also play an important role in determining the packing morphology of the superlattices. In our case, the formation mechanism of the silver nanoaggregates may originate from specific hydrogen-bonding interactions between MBMZ molecules on the surfaces of Ag particles, although it is somewhat not easy to probe this interaction in aqueous ethanol solution. For MBMZ, it is commonly known that, whether it is dissolved or in condensed state, a strong hydrogen bond exists between two molecules as shown in Scheme 2. Part a in Scheme 2 represents a pair of tautomers of MBMZ. Part b exhibits a long MBMZ chain connected by the hydrogen bonding. This connection manner probably exists between two silver particles as shown in part c of Scheme 2. However, the possibility of MBMZ forming a hydrogen bond on single Ag particles is negligible because organic molecules densely assemble on the surface of particles.27

Figure 8. Comparison of the IR spectra of (a) pure MBMZ, (b) MBMZ-derivatized Ag nanoparticles, and (c) control system II.

The direct evidence for hydrogen bonding between MBMZ molecules on the surface of Ag nanoparticles can be obtained using FTIR spectroscopy. Figure 8 exhibits the FTIR spectra of free MBMZ, a cast film of MBMZ-stabilized Ag nanoparticles, and a cast film of control system II. By comparison, we found the similarity of MBMZ to the MBMZstabilized Ag nanoparticles with the exception of stretch vibration of S-H at 2573 cm-1 vanishing in spectrum b. The broad NH stretch (∼3125 cm-1) indicates the strong hydrogenbonding interactions between MBMZ molecules. Whereas, pronounced distinctions are observed between the MBMZstabilized Ag nanoparticles and control system II. First, the broad N-H stretch band in spectrum b converts to a narrow, relatively weak, and blue-shifted N-H stretch band (∼3252 cm-1) in spectrum c, which implies the hydrogen-bonding interactions have weakened and even disappeared. Second, another striking feature in spectrum c is the presence of new bands at 2950, 2921, and 2850 cm-1. These bands are assigned to the methyl (-CH3) and methylene (-CH2-) stretches. In addition, the vibration bands of the benzene ring arise in the range of 16001400 cm-1.


Tan et al. UV-vis and DLS techniques suggest that Ag nanoaggregates also exist in solutions. The aggregation of MBMZ-stabilized Ag nanoparticles goes on with time elapsed so as to lead to the formation of precipitation. But extremely stable Ag colloids passivated by MBTZ and [MBMZ‚C2H5]+Br-, respectively, are obtained. It is shown that the hydrogen-bonding interaction between MBMZ molecules is the factor in causing the aggregation of Ag nanoparticles by analysis of the distinction of FTIR spectra between MBMZ-stabilized Ag nanoparticles and control system II. In summary, we have presented a facile chemical reaction strategy to fabricate the silver nanoaggregates. Thus, we may extend the method for the preparation of other metal nanoaggregates.

Figure 9. TEM micrograph of the MBMZ-stabilized Ag nanoparticles showing the direction of wire-like structures determined by large Ag particles.

Acknowledgment. This work was supported by “973” project of the Ministry of Science and Technology of China and The Chinese Academy of Sciences.

We assume that a plausible mechanism for the formation of wire-like configurations lies in the production of large particles. Because the hydrogen bonding is comparably weak, the nanoaggregates on the substrate will be liable to be affected by evaporation of the solvent. The large nanoparticles are heavyweights so that they cannot move easily during evaporation. Thus, they dictate the direction of 1D arrays of Ag nanoparticles as illustrated in Figure 9. The large particles appear like “nails” driven in “laths”. As we can see, although the mean diameter of Ag nanoparticles tends to decrease with increasing the concentration of the stabilizer, the large particles remain in the TEM images all the way within the scope of the variance in MBMZ concentration in our experiments. Two factors are expected to be favorable for generating large spherical particles. Silver ions were reduced immediately in the initial stage when the reductant and MBMZ were added into a AgNO3 aqueous ethanol solution. But, on one hand, at this moment, the MBMZ molecules at low concentration could not diffuse to adsorb on the surfaces of small Ag embryos rapidly because more energy is required for MBMZ molecules diffusing out of the cages of solvent molecules (ethanol) caused by the hydrogen-bonding interactions between MBMZ and ethanol molecules. On the other hand, MBMZ is insoluble in water, which limits the diffusion of MBMZ onto the surfaces of those Ag nuclei surrounded by water molecules. Thus, these two factors result in enough time left for the silver nuclei growing to form large clusters in the initial stage of reaction. Subsequently, with increasing the concentration of MBMZ, the size of particles may decrease because of an increase in the probability of MBMZ molecules encountering Ag nuclei. The significance of large particles lies in that the direction of 1D arrays of Ag nanoparticles can be controlled in the future if large particles are confined to certain sites. For instance, let a porous template with the pore size of ca. 30 nm be impregnated with a MBMZstabilized Ag colloidal solution, and evaporate the solvent, then the expected 1D arrays can be obtained.

References and Notes

Conclusion Silver colloids protected with a series of stabilizers including MBMZ, MBTZ, and [MBMZ‚C2H5]+Br- have been prepared by the reduction of AgNO3 in 50% aqueous ethanol solutions. TEM images show that the self-assembly of MBMZ molecules on Ag nanoparticles results in the formation of 1D aggregates. The length of the wire-like configurations ranges from 200 to 450 nm. The composition and dimension of the Ag nanoparticles are studied by EDS and XRD, respectively. Further studies with

(1) For example: (a) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reinfenberger, R. Science 1996, 272, 1323. (b) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (c) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. H. Science 2000, 290, 1131. (2) (a) Taleb, A.; Petit, C.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 2214. (b) Thomas, P. J.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 2000, 104, 8138. (c) Martin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2000, 104, 9475. (3) (a) Shenton, W.; Davis, S. A.; Mann, S. AdV. Mater. 1999, 11, 449. (b) Yin, J. S.; Wang, Z. L. AdV. Mater. 1999, 11, 469. (4) Sandhyarani, N.; Resmi, M. R.; Unnikrishnan, R.; Vidyasagar, K.; Ma, S. G.; Antony, M. P.; Selvam, G. P.; Visalakshi, V.; Chandrakumar, N.; Pandian, K.; Tao, Y. T.; Pradeep, T. Chem. Mater. 2000, 12, 104. (5) (a) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601. (b) Kamar, A.; Whitesides, G. M. Science 1994, 263, 601. (6) (a) Mullen, K.; Ben-Jacob, E.; Jaklevic, R. C.; Schuss, Z. Phys. ReV. B 1988, 37, 98. (b) Grabert, H., Devoret, M. H., Eds.; Single Charge Tunneling; Plenum Press: New York, 1992. (7) Jin, B. Y.; Ketteren, J. B. AdV. Phys. 1988, 38, 189. (8) Lordi, V.; Yao, N.; Wei, J. Chem. Mater. 2001, 13, 733. (9) Schmid, G.; Ba¨umle, M.; Beyer, N. Angew. Chem., Int. Ed. 2000, 39, 181. (10) Marinakos, S. M.; Brousseau, L. C., III; Jones, A.; Feldheim, D. L. Chem. Mater. 1998, 10, 1214. (11) (a) Chen, S. AdV. Mater. 2000, 12, 186. (b) Brust, M.; Berthell, D.; Schiffrin, D. J.; Kiely, C. J. AdV. Mater. 1995, 7, 795. (12) Fullam, S.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 2000, 104, 6164 and references therein. (13) Ryan, D.; Rao, S. N.; Rensmo, H.; Fitzmaurice, D.; Preece, J. A.; Wenger, S.; Stoddart, J. F.; Zaccheroni, N. J. Am Chem. Soc. 2000, 122, 6252 and references therein. (14) Gole, J. L.; Stout, J. L.; Rauch, W. L.; Wang, Z. L. Appl. Phys. Lett. 2000, 76, 2346. (15) Ngo, A. T.; Pileni, M. P. AdV. Mater. 2000, 12, 276. (16) Mann, S. J. Chem. Soc., Dalton Trans. 1993, 1. (17) Chemseddine, A.; Jungblut, H.; Boulmaaz, S. J. Phys. Chem. 1996, 100, 12546. (18) Wilcoxon, J. P.; Williamson, R. L.; Baughman, R. J. Chem. Phys. 1993, 98, 993. (19) Anal. Calcd. for the flocculent sediments following reflux (C9H10N2SBrAg): C, 29.53; H, 2.75; N, 7.65; S, 8.76; Br, 21.83. Found: C, 26.28; H, 2.81; N, 6.76; S, 6.92; Br, 23.45. (20) Zhou, Y.; Wang, C. Y.; Zhu, Y. R.; Chen, Z. Y. Chem. Mater. 1999, 11, 2310. (21) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: Menlo Park, CA, 1978. (22) Mulvaney, P. Langmuir 1996, 12, 788. (23) Wang, W.; Chen, X.; Efrima, S. J. Phys. Chem. B 1999, 103, 7238. (24) Zhang, Z. Q.; Patel, R. C.; Kothari, R.; Johnson, C. P.; Friberg, S. E.; Aikens, P. A. J. Phys. Chem. B 2000, 104, 1176 and references therein. (25) Korgel, B. A.; Fitzmaurice, D. Phys. ReV. Lett. 1998, 80, 3531. (26) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. ReV. Lett. 1995, 75, 3466. (27) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436.

One Dimensional Aggregates of Silver Nanoparticles Induced by the

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