Synthesis and Catalytic Application of Nanostructured Silver Dendrites

16750

J. Phys. Chem. C 2007, 111, 16750-16760

Synthesis and Catalytic Application of Nanostructured Silver Dendrites Md. Harunar Rashid and Tarun K. Mandal* Polymer Science Unit and Centre for AdVanced Materials, Indian Association for the CultiVation of Science, JadaVpur, Kolkata, 700 032, India ReceiVed: June 26, 2007; In Final Form: August 12, 2007

We demonstrate a simple templateless and surfactant-free wet-chemical method of preparing silver (Ag) nanostructures with different dendritic morphologies at room temperature. This has been accomplished by aging the aqueous mixture of AgNO3 and citrate salts, carrying different cations for different time periods. Transmission electron microscopic (TEM) and X-ray diffraction (XRD) studies confirmed the formation of single-crystalline dendritic Ag nanostructures. It has been found that the nature and the concentration of citrate salt have a significant effect on the morphology of the formed Ag nanostructures. A possible formation mechanism has also been discussed on the basis of monitoring the surface plasmon resonance properties and TEM images with time during the course of formation of silver dendrites. The formed dendritic silver nanostructures showed excellent catalytic activity in the borohydride reduction of p-nitrophenol to p-aminophenol compared to the spherical silver nanoparticles.

Introduction Recently, metal nanostructures have attracted steadily growing interest due to their fascinating properties and intriguing applications that are complementary or superior to their bulk counterparts.1 It has been well-established that the physicochemical and optoelectronic properties of metal nanoparticles (MNPs) are size- and shape-dependent and have potential applications in optics,2 catalysis,3,4 information technology,5 SERS,6,7 biological labeling, and imaging.8,9 Therefore, much attention has been focused on the size- and shape-controlled synthesis of MNPs.7,10-16 Among the many different MNPs that have been thoroughly investigated, synthesis and study of silver (Ag) nanoparticles (NPs) seems to be particularly interesting because bulk Ag exhibits the highest electrical and thermal conductivities among all metals.17 Moreover, their electronic and optical properties can also be controlled through tailoring size and shape during synthesis. Accordingly, various shaped Ag NPs such as cubes,18 rods,19 wires,20 disks,21 triangles,7,22 and prisms23 have already been synthesized using variety of techniques. Besides the above-mentioned shapes, dendritic or fractal Ag nanostructures have aroused scientist’s interest recently due to their attractive supramolecular structures, large surface area, and excellent connectivity between the different parts of the structures.24-27 Consequently, several efforts have been made to prepare dendritic Ag nanostructures using variety of synthetic techniques.13,25,28-30 Chen et al. have reported the preparation of dendritic Ag nanostructures in presence of poly(vinyl alcohol) by photo reduction technique.13 Lee et al. have synthesized dendritic Ag nanostructures using ascorbic acid as a reducing agent in presence of sodium polyacrylate.30 Other notable examples include the π-conjugated tetrathiafulvalene reduction route,25 soft solution technique,28 pulse sonoelectrochemical technique,29 etc. However, most of these wet-chemical methods for preparing such dendritic nanostructures were based on the * Corresponding author. Fax: +91-33-2473 2805. E-mail: psutkm@ iacs.res.in.

use of surfactant or polymeric templates, either for controlling the shape or for stabilization of the formed dendritic Ag nanostructures.13,28,30,31 However, the addition of surfactant or polymeric templates may make the synthetic procedure and purification process more complex. In some cases, it is difficult to avoid contamination in the final product due to the formation of undesired byproduct in the reaction medium, which makes them unsuitable for further application. Therefore, the exploration of template-free methods for synthesis of these wellcontrolled Ag nanostructures is a challenging research area. It is known that slow reduction or aging of a metal salt with a reducing agent sometimes results in the formation of anisotropic MNPs.31,32 In such aging process, smaller sized spherical MNPs were formed first, which upon aging gradually converted to nonspherical geometry.31 Such conversion has been confirmed by analyzing the UV-vis spectra as well as TEM images taken in different time periods during the formation of anisotropic MNPs. It has been reported that the reduction of metal salts by sodium citrate at boiling condition usually resulted in the formation of spherical NPs.33,34 So, to achieve MNPs of desired morphology, one must need to carefully control the growth of these NPs during the reduction process, because highly anisotropic MNPs only become favorable when the reduction rate is very slow.32 In our earlier communication, we have reported the preparation of spongy gold (Au) and flowerlike Ag nanocrystals (NCs) by in situ reduction techniques using bismuth ammonium citrate as reducing cum-stabilizing agent.4 In that preliminary study, we have also shown that cationic part of the citrate salt plays an important role in controlling the shape of the formed Au NCs. Furthermore, this method is also advantageous over other chemical approaches because one can control the shape of metal NCs without adding any stabilizers or templates that may facilitate the chances of the formation of undesirable byproducts. These results motivated us to further study the effect of different cations present in the citrate salts on the morphology of Ag nanocrystals (NCs). Thus, in this manuscript, we report a simple aqueous phase room-temperature,

10.1021/jp074963x CCC: $37.00 © 2007 American Chemical Society Published on Web 10/13/2007

Nanostructured Silver Dendrites

J. Phys. Chem. C, Vol. 111, No. 45, 2007 16751

TABLE 1: Reaction Recipe for the Preparation of Ag Nanostructures Using TAC and Their Morphologiesa sample name

[AgNO3] (M)

[TAC] (M)

R ) [TAC]/[AgNO3]

morphology

TAC-Ag-1.0b TAC-Ag-1.4b TAC-Ag-0.7b

7.0 × 10-4 7.0 × 10-4 7.0 × 10-4

7.0 × 10-4 10.0 × 10-4 5.0 × 10-4

1.0 1.4 0.7

corallike dendrite banana leaves-like dendrite banana leaves-like dendrite

a The TEM images were recorded after 72 h of aging. b TAC-Ag represents the triammonium citrate (TAC) and AgNO system, and the numerical 3 value represents the mole ratio (R) of TAC to AgNO3 in the medium.

TABLE 2: Reaction Recipe for the Preparation of Ag Nanostructures Using FAC and Their Morphologiesa sample name

[AgNO3] (M)

[FAC] (M)

R ) [FAC]/[AgNO3]

morphology

FAC-Ag-1.0b FAC-Ag-1.4b FAC-Ag-0.7b

7.0 × 10-4 7.0 × 10-4 7.0 × 10-4

7.0 × 10-4 10.0 × 10-4 5.0 × 10-4

1.0 1.4 0.7

mixture of spherical and other shaped cactuslike dendrite 1D chainlike

a The TEM images were recorded after 4 h of aging. b FAC-Ag represents the ferric ammonium citrate (FAC) and AgNO3 system, and the numerical value represents the mole ratio (R) of FAC to AgNO3 in the medium.

TABLE 3: Reaction Recipe for the Preparation of Ag Nanostructures Using TSC and Their Morphologiesa sample name

[AgNO3] (M)

[TSC] (M)

R ) [TSC]/ [AgNO3]

morphology

TSC-Ag-1.0b TSC-Ag-1.4b TSC-Ag-0.7b

7.0 × 10-4 7.0 × 10-4 7.0 × 10-4

7.0 × 10-4 10.0 × 10-4 5.0 × 10-4

1.0 1.4 0.7

fern bush-like dendrites spherical dendritic

a The TEM images were recorded after 96 h of aging. b TSC-Ag represents the trisodium citrate (TSC) and AgNO system, and the numerical 3 value represents the mole ratio (R) of TSC to AgNO3 in the medium.

template-free method to prepare Ag NCs of various dendrtitc morphologies using variety of citrate salts. The morphology of the dendritic Ag NCs can also be tuned by varying the concentration of the citrate salt. In recent years, several research groups have investigated the catalytic reduction of aromatic nitro compounds using a number of noble MNPs.35-40 However, in most of the cases, MNPs were supported either by dendrimer/polymers or resin beads prior to catalysis reaction.35,38-40 But, in our previous studies, we have shown that spongy Au NCs without any polymeric or resinbead support can efficiently catalyze the reduction of pnitrophenol by NaBH4 and the catalytic activity could be a consequence of spongy nature of this NCs.4 Again this result prompted us to study the catalytic reduction of this system using these dendritic Ag nanostructures. Earlier studies of the catalytic activities of Ag NPs are mostly performed using spherical nanoparticles, either bound to polymer/resin bead or in suspension.35,37,40 However, there exists no report on the use of dendritic Ag nanostructures for this purpose. Thus, for the first time, we report the use of dendritic Ag nanostructures as an efficient catalyst for the conversion of p-nitrophenol to paminophenol by NaBH4. Experimental Section Materials. Silver nitrate (AgNO3), sodium borohydride (NaBH4), and ferric ammonium citrate (FAC) (C6H8O7‚xFe‚ xNH3) were purchased from E-Merck, India. Triammonium citrate (TAC) [(NH4)3C6H5O7], p-nitrophenol (4-C6H5NO3), and trisodium citrate dihydrate (TSC) (C6H5Na3O7‚2H2O) were supplied by Loba Chemicals, India, and SRL, India, respectively. All reagents were used without further purification. All solutions were prepared using triple distilled water. Preparation of Silver Nanostructures. The synthesis of Ag nanostructures is based on our earlier protocol of synthesis of spongy Au NCs using bismuth ammonium citrate.4 In a typical synthesis of Ag dendrites, an aliquot (0.225 mL) of 0.01 M triammonium citrate (TAC) solution was added to 3.0 mL of 7.5 × 10-4 M aqueous solution of AgNO3 taken in a labeled glass vial to get a mole ratio value, R, of 1.0 (R ) mole ratio of reductant (TAC) to AgNO3). The reaction mixture was then

kept in static condition (aged) for 5 days at room temperature. Two more similar sets of reactions were also carried out at varying mole ratio (R) value; i.e., the concentration of TAC was varied keeping the concentration of silver salt the same (see Table 1 for the detailed recipe). Similarly, another set of reduction of AgNO3 was also carried out by ferric ammonium citrate (FAC) maintaining the same values of R as used for the TAC reduction, except the aging time (see Table 2 for the detailed recipe). In this case, the reaction mixtures were aged for 2 days at room temperature. Ag dendrites were further prepared by the reduction of AgNO3 using varying concentration of trisodium citrate (TSC) as reductant instead of TAC and FAC under similar reaction conditions as in the above-mentioned two cases (see Table 3 for the detailed recipe). The aging time of these reaction mixtures was 5 days. Studies of the Catalytic Activity of Ag Dendrites. In a representative catalytic reduction reaction, 0.10 mL of a 3.0 × 10-1 M NaBH4 solution was added to 2.9 mL of a 1.03 × 10-4 M p-nitrophenol solution in the presence of 0.004 mg of purified dendritic Ag NCs under magnetic stirring. The progress of the conversion reaction was then followed/monitored by recording the time-dependent UV-vis absorption spectra of the mixture using a spectrophotometer. Two more catalytic reduction of p-nitrophenol were also performed using the same recipe as mentioned above but with different Ag dendrites. Additionally, we have also carried out the catalytic activity studies of spherical Ag NPs (sample TSC-Ag-1.4) maintaining reaction conditions similar to those used in the case of dendritic Ag nanostructures. Characterization. The samples for TEM measurement were prepared by casting a drop of each sample suspension on a carbon-coated copper grid. The excess solvent was then removed by wicking using a tissue paper. The copper grid containing the sample was then dried in air and imaged under JEOL highresolution transmission electron microscope (HRTEM, model JEM 2010 EM) at an accelerating voltage of 200 kV. For field emission scanning electron microscopic (FESEM) studies, the suspension of Ag nanostructures was drop cast on a glass substrates followed by air-drying. The samples were then

16752 J. Phys. Chem. C, Vol. 111, No. 45, 2007

Rashid and Mandal

Figure 1. TEM images of (A) dendritic Ag nanostructures prepared with TAC at R ) 1.0 (sample TAC-Ag-1.0) and (B) an enlarge view of a single dendritic Ag nanostructure. The micrographs are recorded from the as-prepared samples after 72 h of aging. (C) Enlarge view of a portion of an Ag dendrite. Insets in (C) show the ED pattern (top) and HRTEM image (bottom) taken from a portion of a single dendritic Ag nanostructure. (D) HRTEM image taken from the junction of the two fused spherical Ag NPs as shown in (C).

analyzed on a JEOL field emission scanning electron microscope (model JSM-6700F) operated at an accelerating voltage of 1.0 kV. X-ray diffraction (XRD) analyses of the dried different dendritic Ag NCs, deposited on a microscopic glass slide, were carried out in Seifert XRD 3000P diffractrometer at an accelerating voltage of 35 kV using Cu R (λ ) 1.54 Å) as X-ray source. For UV-vis spectroscopic studies, the desired volume of individual as-prepared colloidal Ag NCs was taken in a quartz cuvette, and the spectra were recorded in the region of 2001000 nm against water as blank using a Hewlett-Packard 8453 diode-array spectrophotometer. The absorption spectra for the kinetic study of the catalytic conversion of p-nitrophenol to p-aminophenol were carried out in the same UV-vis spectrophotometer in kinetic mode operation. Results and Discussion Silver Nanostructures Prepared by Triammonium Citrate (TAC) Reduction. The synthesis of the dendritic Ag nanostructures is simply achieved upon aging the mixture of aqueous AgNO3 and triammonium citrate (TAC) solutions at room temperature. In this aging process, TAC slowly reduces Ag+ ions to Ag metal that eventually grows in the form of a Ag dendrite, and the formation was manifested by a gradual color change from colorless to faint green within the time span of 12 h. The color of this suspension intensified with time and finally became greenish yellow after 5 days. The suspension was stable for more than 2 weeks without any precipitation.

The concentration of TAC in the reaction mixture, as depicted in Table 1, was varied to obtain Ag dendrites of various morphologies. Figure 1 shows the TEM images of the sample TAC-Ag-1.0 (R ) [TAC]/[AgNO3] ) 1.0), which clearly reveal the formation of dendritic Ag nanostructures and are certainly different from the previously reported ones.13,25,26,30 From the enlarged view of a single Ag NC (see Figure 1B), it is apparent that the asprepared samples resemble a coral-like dendritic structure. Some additional TEM images of these Ag nanostructures, showing the same dendritic structure, are provided in Figure S1 in the Supporting Information (SI). The average sizes of these Ag NCs are in the micrometer range. The branches of these Ag dendrites are composed of many fused nanospheres of average diameter ∼35 nm as evident from the high magnification image of the portion of a single dendritic Ag NC shown in Figure 1C. The HRTEM image recorded from the junction of two fused adjacent spherical Ag NPs in dendritic Ag nanostructures (see Figure 1D) indicated that lattice fringes are perfectly aligned between adjacent nanoparticles (as indicated by the solid black line). This result clearly indicates that oriented attachment based assembling of the spherical Ag NPs forms the dendrites. The details of the mechanism of formation of Ag dendrites will be discussed later in this growth mechanism section. For an overview of the dendrites shown in Figure 1A, we have recorded the FESEM image (see Figure S2 in the SI), which also showed similar morphology. When the value of mole ratio (R) of TAC to AgNO3 was increased to 1.4, i.e., the concentration of TAC in the reaction medium was increased from 7.0 × 10-4 to 10.0 × 10-4 M,



Figure 3. XRD patterns of different dendritic Ag NCs samples: (a) TAC-Ag-1.0; (b) TAC-Ag-1.4; (c) TAC-Ag-0.7.

Figure 2. TEM images of the sample prepared with TAC: (A) TACAg-1.4; (B) TAC-Ag-0.7. The micrographs were taken from the asprepared samples after 72 h of aging. The top insets show an enlarged view a single dendritic Ag nanostructures of the respective samples, and the bottom insets show their corresponding ED patterns.

another type of dendritc Ag nanostructure was observed for sample TAC-Ag-1.4 as shown in Figure 2A. The enlarged view of a single Ag dendrite (see top inset in Figure 2A) shows that each dendrite consists of a small number of branches/leaves and as a whole it looks like the branches of a banana tree (for more pictures, see Figure S3 in the SI). The lengths of these leaves/branches are in the range 1.0-4.0 µm. Figure 2B shows the TEM images of the sample TAC-Ag-0.7 prepared by decreasing the value of R from 1.0 to 0.7 ([TAC] ) 5.0 × 10-4 M) in the reaction medium. The images reveal that the morphology of these Ag nanostructures is almost similar to those obtained from sample TAC-Ag-1.4 (see Figure 2A). But the overall sizes as well as the average length of branches of these Ag dendrites are smaller (i.e., 250-600 nm against 1.0-4.0 µm) as compared to those obtained for sample TAC-Ag-1.4 (compare Figure 2A,B). The electron diffraction (ED) patterns of all the abovementioned Ag nanostructures were recorded by focusing the electron beam on a portion of an individual Ag dendrite, and the results are shown in the insets of Figure 1C (top), Figure 2A (bottom), and Figure 2B (bottom) respectively for samples TAC-Ag-1.0, TAC-Ag-1.4, and TAC-Ag-0.7. The diffracted hexagonal spots, as observed in all the ED patterns, clearly indicate that the formed dendritic Ag nanostructures are highly single-crystalline face-centered cubic (fcc) crystal and may have grown preferentially along the (111) lattice plane. This fact was further evidenced from the HRTEM image of the sample TACAg-1.0 shown in Figure 1C (bottom inset). The image clearly shows a lattice fringe with an interplanar distance of 0.23 nm that correspond to (111) lattice plane of fcc Ag (JCPDF Card

No. 04-0783). The ED patterns taken from multiple locations of a single Ag dendrite are essentially identical. Lu et al. have also observed similar type of ED pattern in case of dendritic Ag nanostructures prepared by borohydride reduction of AgNO3 in the presence of p-aminobenzene.41 X-ray diffraction (XRD) was performed to further investigate the crystalline behavior of these dendritic Ag nanostructures. The XRD patterns of all the three samples, as presented in Figure 3, are nearly identical. The peaks observed at 2θ ) 38.18, 44.6, 64.2, and 77.1° are assigned to (111), (200), (220), and (311) lattice planes of fcc metallic Ag (JCPDF Card No. 040783), respectively.41 The cell parameter (“a”) of all the dendritic Ag NCs calculated from the XRD patterns using the following eqs 1 and 2 is 4.07 Å, which is very close to the reported value of 4.08 Å for dendritic Ag nanostructures:27,41

λ ) 2d sin θ

(1)

1/d ) (h2 + k2 + l2)1/2/a

(2)

Here, λ is the wavelength of X-ray source, “d” is the spacing of the planes in the crystal, θ is the angle of diffraction, h, k, and l are the Miller indices, and “a” is the unit cell parameter of Ag. In all the XRD patterns, the peaks belonging to (200), (220), and (311) lattice planes are quite weak compared to that of the (111) plane. These XRD data are also indicative of their crystalline nature and are consistent with the ED data obtained from TEM measurements. It is well-known that the anisotropy in shape of Ag NPs should substantially influence the resulting optical properties such as surface plasmon resonance (SPR) property.2,22 Usually, nonaggregated Ag NPs with a spherical geometry shows only one size-dependent SPR absorption band.42 This optical property changes with the variation of the shape of Ag NPs. For example, nonspherical Ag NPs such as prism and triangle displays three to four SPR absorption bands,7,22 whereas dendrites show two SPR absorption bands in the UV-vis spectrum.30,31 Figure 4 shows a set of UV-vis spectra of the colloidal Ag nanostructures, prepared with varying concentrations of TAC. The spectra of all the samples exhibit two SPR bands that might indicate the formation of nonspherical Ag NCs. Lee et al. have also observed similar UV-vis spectral patterns for dendritic Ag nanostructures prepared with ascorbic acid in presence of sodium polyacrylate.30 The position of the lower wavelength bands in all the samples is identical (420 nm) and is attributed to the


Rashid and Mandal

Figure 4. UV-vis spectra of the as-prepared samples: (a) TAC-Ag1.0; (b) TAC-Ag-1.4; (c) TAC-Ag-0.7. These spectra were recorded after 72 h of aging.

out-of plane dipole resonance in the Ag NCs,22 whereas the second SPR band that appeared at the higher wavelength region might be due to in-plane dipole resonance as reported by Chen et al.22 These bands are known as transverse and longitudinal SPR bands, respectively.2,7 It has been reported that the position of longitudinal SPR band is very sensitive to the shape and aspect ratio (length/breadth) of asymmetric MNPs.2 In our case, we have noticed that the longitudinal plasmon band of the formed Ag NCs either blue-shifted or red-shifted depending on the value of mole ratio, R (R ) [TAC]/[AgNO3]) in the reaction mixture (see Figure 4). For example, when the value of R was maintained at 1.0 ([TAC] ) 7.0 × 10-4 M), the longitudinal SPR band of Ag dendrites in sample TAC-Ag-1.0 appeared at 765 nm (see Figure 4a). However, this band blue-shifted to 710 nm in case of sample TAC-Ag-1.4 upon increasing the value of R from 1.0 to 1.4 ([TAC] ) 10.0 × 10-4 M) (see Figure 4b). This band position is further blue-shifted to 655 nm for sample TAC-Ag-0.7, when the value of R was decreased to 0.7 (see Figure 4c). This shifting in longitudinal band position might be due to the formation of different Ag dendrites of varying size ranges, when prepared with different TAC concentrations. Silver Nanostructures Prepared by Ferric Ammonium Citrate (FAC) Reduction. To study the effect of cation associated with citrate ion on the morphology of the formed Ag NCs, the reduction of AgNO3 was carried out with ferric ammonium citrate (FAC) under reaction conditions similar to that used for TAC reduction. Typically, varying amounts of FAC were added separately to fixed amount of AgNO3 taken in labeled glass vials and the reaction mixtures were aged for 30 h (see Table 2). The changes in the color of the reaction mixtures from orange-yellow (due to the presence of FAC) to gray (within 1 h) and finally to greenish-gray indicate the formation of Ag NCs. In this case, the rate of formation of Ag NCs as evident from the color changes is faster than that with TAC. This might be due to the presence of free Fe3+ ions in the aqueous solution in addition to the NH4+ ions. The presence of Fe3+ ions with strong oxidizing ability might alter the redox potential of FAC facilitating the reaction to proceed at a faster rate compared to TAC having only NH4+ ions. The TEM images of the FACreduced silver sols show the formation of dentritic as well as other shaped Ag NCs (see Figure 5). When the reduction of AgNO3 was performed using FAC with a mole ratio value, R, ) 1.0 (sample FAC-Ag-1.0), mixture of spherical (average diameter 90 nm) and irregular shaped-Ag NCs of various sizes was formed (see Figure 5A). At higher mole ratio value, i.e., R

Figure 5. TEM images of different samples prepared with FAC: (A) FAC-Ag-1.0; (B) FAC-Ag-1.4; (C) FAC-Ag-0.7. The images were recorded from the as-prepared samples after 4 h of reaction. Insets at the top show an enlarged view of a portion of the respective images, and those at the bottom show their respective ED patterns.

) 1.4 (sample FAC-Ag-1.4), micrometer-sized Ag dendrites were formed (see Figure 5B). The enlarged view of a single Ag dendrite (top inset of Figure 5B) shows that each branch of these dendrites consists of a few subbranches and, as a whole, it resembles a cactuslike structure. However, at lower mole ratio value, i.e., R ) 0.7 (sample FAC-Ag-0.7), predominantly onedimensional (1D) chainlike Ag nanostructures were formed (see Figure 5C). Wei et al. have also observed the formation of similar type of self-organized chainlike Ag nanostructures upon solvothermal reduction of AgNO3 in the presence of poly(vinylpyrrolidone).24 For an overview of the 1D chainlike Ag nanostructures shown in Figure 5C, we have recorded the SEM image (see Figure S4 in the SI) which also showed a similar type of morphology. The ED patterns of the above-mentioned samples were obtained by focusing the electron beam on the respective individual Ag NC during TEM measurement, which are shown



Figure 6. UV-vis spectra of the as-prepared samples prepared with FAC: (a) FAC-Ag-1.0; (b) FAC-Ag-1.4; (c) FAC-Ag-0.7. The spectra were recorded after 4 h of aging.

in Figure 5 (bottom insets). The presence of diffracted hexagonal spots in all the ED patterns again clearly indicates the formation of highly single-crystalline fcc Ag. These ED patterns are similar to those of Ag dendrites prepared with TAC (compare Figure 5 (bottom insets), Figure 1C (top inset), Figure 2A (bottom inset), and Figure 2B (bottom inset)). The XRD patterns of these Ag dendrites show all the characteristic diffraction peaks at 2θ ) 38.18, 44.6, 64.2, and 77.1° corresponding to the (111), (200), (220), and (311) lattice planes, respectively, of fcc Ag (see Figure S5 in the SI). These XRD data are also similar to those of TAC-reduced Ag dendrites (see Figure 3). The XRD data are matching well with the ED data obtained from TEM measurements. As a whole, these results indicate the formation of single-crystalline Ag NCs. The absorption spectra of the as-prepared colloidal Ag NCs suspensions obtained from the reaction between AgNO3 and FAC are shown in Figure 6. A single broad SPR absorption band centered at 480 nm was observed for sample FAC-Ag1.0 (R ) 1.0) (Figure 6a). The appearance of such a broad absorption band might reflect the formation of large-sized Ag NPs of various sizes and shapes. This optical result matches well with the TEM result (see Figure 5A) of that sample, where spherical NPs of average diameter of 90 nm were observed along with some irregular-shaped Ag NPs. Pillai et al. also observed a similar type of broad absorption band in the UV-vis spectra for spherical Ag NPs of diameter ranges from 50 to 100 nm prepared by the citrate reduction technique at high temperature.42 But, both Figure 6b and Figure 6c show two clearly distinguishable SPR absorption bands. The position of the first band is at 480 nm, whereas the second band is at 650 and 690 nm respectively for samples FAC-Ag-1.4 and FAC-Ag-0.7, indicating the formation of nonspherical Ag NPs. These results also agreed well with TEM results, which displayed the dendritic and 1D chainlike Ag NCs for samples FAC-Ag-1.4 and FACAg-0.7, respectively (see Figure 5B,C). Silver Nanostructures Prepared by Trisodium Citrate (TSC) Reduction. Although several other research groups have prepared Ag NPs of various sizes and shapes by the citrate reduction method at high temperature and/or in the presence of a template, it is worthwhile to perform the reduction of AgNO3 under reaction conditions similar to those maintained in the cases of TAC and FAC reduction. In this case also, the amount of added TSC was varied to study the effect of R-values on the morphology of the formed Ag NCs (see Table 3). Figure 7 shows the representative TEM images of the as-prepared

Figure 7. TEM images of different samples prepared with TSC: (A) TSC-Ag-1.0; (B) TSC-Ag-1.4; (C) TSC-Ag-0.7. The images were recorded from the as-prepared samples after 96 h of aging. Insets at the top show an enlarged view of a portion of the respective images, and those at the bottom show the ED patterns of the respective Ag NCs.

colloidal Ag NCs taken after 96 h of aging. These images again reveal the formation of dendritic nanostructures as well as quasispherical Ag NPs depending on the R-values (i.e., concentration of TSC) in the medium. However, these dendrites are completely different from those prepared with TAC and FAC (compare Figures 1, 2, 5, and 7). These results also indicate that the variation of concentration of TSC also affects the morphology of the resultant products as observed in the earlier cases. For sample TSC-Ag-1.0 with R ) 1.0, predominantly two different kinds of Ag dendrites are observed; one type is a large-sized highly branched dendrite, and another type is a less branched dendrite of smaller size (see Figure 7A). The enlarged view of a single large-sized highly branched dendrite indicates that the morphology of these dendrites resemble a fern bush structure (see top inset in Figure 7A) whereas the second type resembles dendritic structure similar to that observed in case of TAC-


Figure 8. UV-vis spectra of the as-prepared suspensions of different samples recorded after 96 h of aging: (a) TSC-Ag-1.4; (b) TSC-Ag1.0; (c) TSC-Ag-0.7.

reduced Ag NCs (compare Figures 7A and 2A). The sizes of the main branches of the large-sized dendrites are in the range 1.4-5.5 µm, but those of smaller dendrites are in the range 0.7-2.3 µm. When the value of R was increased from 1.0 to 1.4, the formed Ag NCs (sample TSC-Ag-1.4) displayed only quasi-spherical structure of average diameter 40 nm (see Figure 7B). However, upon decrease of the value of R from 1.0 to 0.7, the sample TSC-Ag-0.7 again displays a dendritic structure but of slightly different nature (see Figure 7C). The sizes of these Ag dendrites are also in the micrometer range. The ED patterns of the samples TSC-Ag-1.0 and TSC-Ag0.7, having dendritic morphology, show diffracted hexagonal spots (see bottom inset of Figure 7A,C) that again clearly indicate the formation of single-crystalline fcc Ag. However, the ED pattern of sample TSC-Ag-1.4, with quasi-spherical morphology, displays spots superimposed on a ring pattern indicating that the Ag NCs are crystalline but with no preferred orientation. The XRD patterns of all the three samples show all the characteristics diffraction peaks of fcc Ag (see Figure S6 in the SI). These XRD patterns are closely similar to those of the TAC- and FAC-reduced dendritic Ag nanostructures. These results also indicate the formation of crystalline fcc Ag nanostructures when prepared with TSC. However, the intensities of the peaks corresponding to (200), (220), and (311) lattice planes in the sample TSC-Ag-1.4 (see Figure S6b in the SI) seem to be little bit higher than those obtained for samples TSCAg-1.0 (see Figure S6a in the SI) and TSC-Ag-0.7 (Figure S6c in the SI). This might be due to the presence of crystalline quasispherical Ag NCs in the sample TSC-Ag-1.4 formed at higher value of R as mentioned above. The UV-vis spectra of the as-prepared colloidal suspensions of all the samples (see Table 3) prepared with TSC are presented in Figure 8. A high-intensity band at ∼430 nm corresponding to the transverse SPR of Ag NCs was observed for all the three samples. In addition, the sample TSC-Ag-1.4 exhibits a small hump at ∼610 nm (see Figure 8a), which might be due to the formation of aggregated nanostructures of spherical Ag NPs as observed in the TEM images of that sample (see Figure 7B). The aggregation that occurs might be due to the adsorption of citrate ions on the Ag surface thereby reducing the surface charges that results in van der Waals attractive forces between the particles. However, UV-vis spectra of the samples TSCAg-1.0 and TSC-Ag-0.7 show a prominent longitudinal SPR band at 635 and 656 nm, respectively, in addition to the first peak at 430 nm (see Figure 8b,c) indicating the formation of

Rashid and Mandal nonspherical Ag NCs. These optical results are similar to those obtained for as-prepared suspensions of Ag dendrites prepared with TAC and FAC (compare Figures 4 and 6 with Figure 8b,c) and are well supported by the TEM results of the same samples where in both the cases dendritic Ag nanostructures were observed (see Figure 7A,C). Possible Formation/Growth Mechanism of Dendirtic Ag Nanostructures. To obtain an insight into the growth mechanism of dendritic Ag NCs, we have monitored the timedependent absorption spectra of samples TAC-Ag-1.0 (see Figure 9A) and examined the TEM images at different stages of reaction/aging (see Figure 9B-D). The sample aged for 6 h displays a single broad SPR band centered at ∼430 nm [Figure 9A(a)] that might indicate the formation of spherical Ag NPs of broad size distribution. This is consistent with its TEM result (see Figure 9B), which shows the formation of spherical Ag NPs of sizes 9-25 nm. After 23 h of aging, the intensity of the absorption band increases with the appearance of a small hump in the region of 500-700 nm [see Figure 9A(b)]. The corresponding TEM image shows the presence of Ag NCs with some kinds of taillike growth with low contrast from the high-contrast Ag spheres along with a small percentage of spherical NPs of average diameter of 25 nm (see Figure 9C). It looks like that the taillike growth is a consequence of oriented attachment of the individual Ag NPs with each other rather than random aggregation. This assumption was further verified from the HRTEM image of the junction between two adjacent Ag NPs as shown in the inset in Figure 9C. The image clearly shows the lattice fringes of metallic silver indicating the formation of Ag nanostructures. Furthermore, it is clear from the HRTEM image that the lattices fringes are perfectly aligned between adjacent nanoparticles (as indicated by the solid line in the inset in Figure 9C). This result indicates that the Ag nanostructures are formed through the oriented coalescence of spherical Ag NPs. According to the literature report, such type of growth sometimes carries defects (e.g., twins, dislocation, etc.) in the formed nanostructuers.41 We have also observed similar twins formation in the HRTEM image (see Figure S7 in the SI) of taillike Ag nanostructures formed at the early stage of dendrite formation, which again indicated that spherical nanoparticles are combined through oriented attachment. When the aging time was prolonged for 46 h, the absorption spectrum indicates an increase in the intensity of the SPR band of Ag nanostructures with time that accompanied by the appearance of a prominent longitudinal absorption band at ∼600 nm [see Figure 9A(d)]. This result might indicate an increase of the amount of reduced Ag in the reaction mixture. The corresponding TEM image shows the presence of aggregated structure that might form due to the fusion of the individual spherical Ag NCs (see Figure 9D). As the reaction mixture was aged further, both the SPR absorption bands became more intense and the position of the longitudinal SPR absorption band red-shifted to 740 nm after 71 h [see Figure 9A(g)]. The TEM image recorded after 72 h of aging shows that the aggregated Ag NPs ultimately transform into a corallike dendritic structure (see Figure 1A), whose branches are made up of fused individual spherical nanoparticles (see Figure 1C). Again, the HRTEM image of these fused Ag nanostructures (see Figure 1D) clearly supports the oriented attachment-based assembly of Ag NPs in the growth of dendritic Ag nanostructures. Earlier, Lu et al. and Wu et al. have also noticed a similar type of growth mechanism for the formation of dendritic Ag NCs.31,41 Furthermore, the position of the longitudinal SPR band continuously red-shifted with aging time and finally appeared at ∼800 nm after 100 h [see Figure 9A-



Figure 9. (A) Evolution of absorption spectra during the formation of dendritic Ag nanostructures for sample TAC-Ag-1.0 recorded after (a) 6, (b) 23, (c) 30, (d) 46, (e) 53, (f) 61, (g) 71, and (h) 100 h of aging. Also shown are TEM images of the same sample recorded at different stages of aging: (B) 6 h; (C) 23 h; (D) 46 h. Inset in Figure 1C represents the HRTEM image of taillike Ag nanostructures.

Figure 10. Evolution of absorption spectra during the formation of Ag NCs for samples (A) FAC-Ag-1.4 recorded after (a) 30, (b) 75, (c) 120, (d) 165, (e) 210, (f) 240, (g) 1200, and (h) 1800 min and (B) TSC-Ag-1.0 recorded after (g) 6, (h) 23, (i) 30, (j) 46, (k) 53, (l) 61, (m) 71, and (n) 100 h of aging.

(h)] that might reflect an increase of size of the Ag dendrites with aging time. To study the growth mechanism in case of reduction of AgNO3 by FAC, we have also recorded the time-dependent absorption spectra during the formation of Ag NCs, under the reaction condition given for sample FAC-Ag-1.4 (see Table 2) as shown in Figure 10A. The spectral changes are similar to that observed in the case of TAC-reduced Ag NCs formation (compare Figures 10A and 9A). These results might indicate the occurrence of similar formation mechanism as in the case of reduction of AgNO3 by TAC. That is, at early stages of

reduction, spherical Ag NCs are formed, which upon aging converted into dendritic structures by oriented attachment-based aggregation. Similar to the earlier cases, we have also monitored the timedependent absorption spectra of the samples prepared with TSC at R ) 1.0 (sample TSC-Ag-1.0) (see Figure 10B). The nature of changes of absorption spectra with aging time is similar to the changes observed in samples TAC-Ag-1.0 and FAC-Ag1.4 (compare Figures 9A and 10A,B), which again clearly indicate the formation of spherical Ag NPs at early stages of


Rashid and Mandal

Figure 11. Successive UV-vis spectra of p-nitrophenol during its reduction by NaBH4 in the presence of different purified Ag dendrites: (A) TAC-Ag-1.0; (B) TAC-Ag-1.4; (C) TAC-Ag-0.7; (D) TSC-Ag-1.4.

aging. These spherical Ag NPs then might eventually transform into the dendritic structure during the course of reaction. In the citrate reduction method, the following reaction is actually involved in the formation of all types of Ag nanostructures as mentioned above: It has been reported that the citrate

ion, commonly used as mild reductant in metal colloids synthesis, undergoes strong surface interaction with the formed Ag nanocrystallites.42 The slow crystal growth observed as a result of the interaction between the silver surface and the citrate ion makes the reduction process unique compared to other chemical and radiolytic methods. It has also been reported that with citrate most of the reduction is completed at higher citrate concentration; i.e., only a small fraction of Ag+ ions gets reduced at eqimolar concentration and the extent of reaction is strongly influenced by the concentration of citrate ions.42 In our system, the variation of concentration of citrate resulted in the formation of Ag nanostructures of different dendritic morphologies. This might be due to the fact that the differences in concentration of citrate might alter the extent of reduction of Ag+ ions that resulted in the formation of a different number of spherical Ag NCs in a different set of reactions at the early

stage of aging. Depending on the number of initially formed spherical Ag NCs, different types of dendrites might form by the oriented attachment-based aggregation. Adachi et al. have also noticed similar effects of citrate concentration on the morphology of formed gold NPs.43 Additionally, it was observed that different cations associated with citrate salts also influenced the morphology of the resulted Ag nanostructures. In terms of chemical formula, TAC and TSC both have the same structural unit except for the cations, but FAC is composed of citrate, ferric, and ammonium ions. So, the aqueous solution of different citrate salt will obviously provide a different cation whose adsorbing behavior toward the surface of the formed Ag NCs will also be different. As a result, it is expected to have different morphologies of Ag nanostructures with different cations of the citrate salt, because the presence of cations in the medium influenced the morphology of the formed products by altering their growth rate in a particular direction.44 Earlier, we have also reported a similar effect in the case of preparation of Au NCs with citrate salts carrying different cations.4 Thus, we may conclude that the variation of morphologies in different samples was observed due to the effect of different cations associated with the citrate as reported earlier.44 But the exact role of these cations on the morphology of the formed Ag nanostructures is still not known. Catalytic Activity Studies of Dendritic Silver Nanostructures. We have recently reported the catalytic activities of spongy Au NCs prepared with bismuth ammonium citrate, and they were compared with that possessed by spherical Au NPs.4


J. Phys. Chem. C, Vol. 111, No. 45, 2007 16759 10 times/1 order higher in magnitude than the values obtained for spherical Ag NPs as well as that of reported earlier by Esumi et al. and are close to those values reported by Jana et al. for such catalytic conversion using PAMAM/PPI-based Ag nanocomposites or resin bead-coated Ag NPs.35,40 In those reports, the authors also claimed that the dendrimer or polymeric resins that are used as support for Ag NPs effect the diffusion of p-nitrophenol onto the surface of Ag NPs thereby facilitating the catalytic conversion. But, in our case, the dendritic Ag NCs used as catalyst are free of polymeric resins or dendrimers and hence the catalytic activities possessed by such bare Ag NCs are solely due to their hyperbranched structure. Thus, we believe that this is one of the important merits of using these dendritic Ag nanostructures as catalysts. Conclusion

Figure 12. Plot indicating the variation of ln A (measured from Figure 11A-D) versus time for the different samples: (b) TAC-Ag-1.0; (9) TAC-Ag-1.4; (2) TAC-Ag-0.7; (/) TSC-Ag-1.4.

TABLE 4: Values of Rate Constant of the Catalytic Conversions of p-Nitrophenol Using TAC-Reduced Dendritic Ag NCs sample name

morphology

rate const (K) (s-1)

TAC-Ag-1.0 TAC-Ag-1.4 TAC-Ag-0.7 TSC-Ag-1.4

corallike dendrite banana leaves-like dendrite banana leaves-like dendrite spherical

5.19 × 10-3 1.65 × 10-3 2.78 × 10-3 3.64 × 10-4

This study prompted us to check the catalytic activities of these Ag dendrites with the expectation that these dendrites with higher surface area/hyperbranched structure would exhibit better catalytic activity than their spherical counterparts. Hence, we have examined the performance of these dendritic Ag NCs, prepared with different concentrations of TAC, as catalysts for reduction of p-nitrophenol by NaBH4 as a model reaction. The details of the catalytic experiment have been provided in the Experimental Section. In the absence of dendritic Ag NCs, the mixture of p-nitrophenol and NaBH4 shows an absorption band at λmax ) 400 nm corresponding to the p-nitrophenolate ion under alkaline conditions. This peak remains unaltered with time, which suggest that the reduction did not take place in the absence of a catalyst as reported earlier.37 However, the addition of a small amount (0.004 g) of purified dendritic Ag NCs to the above reaction mixture with magnetic stirring causes fading and ultimate bleaching of the yellow color of the reaction mixture in quick succession. Time-dependent UV-vis spectra of this reaction mixture show the disappearance of the peak at 400 nm that was accompanied by a gradual development of a new peak at 300 nm, which corresponds to the formation of p-aminophenol (see Figure 11A-C). These results indicate that the dendritic Ag NCs can catalyze the reduction reaction. For comparison, we have also studied the catalytic activity of the spherical Ag NPs (sample TSC-Ag-1.4; for the TEM image, see Figure 7B), and the spectral change of p-nitrophenol is shown in Figure 11D. Several research groups have reported similar kinds of spectral changes of p-nitrophenol using Ag NPs either supported by dendrimer/resin bead or Ag NPs in suspension as catalyst.35,37,40 The values of rate constants of these catalytic reactions in the presence of different dendritic Ag as well as spherical Ag NPs were calculated from the plot of ln A (A ) absorbance at 400 nm) versus time (see Figure 12) and are given in Table 4. The differences in rate constant values of the above reactions in the presence of different Ag dendrites might be due to the differences in their morphologies as reported earlier by Narayanan et al.3 However, these values are almost

In conclusion, dendritic Ag nanostructures have been prepared by wet-chemical reduction of AgNO3 using citrate salts carrying different cations at room temperature. This is a templateless and seedless method. The concentration and nature of citrate salt used for the reduction has a significant effect on the morphology of the formed Ag dendrites. This method can be further extended to prepare other noble metal nanoparticles of various morphologies. The formed Ag dendrites show excellent catalytic activity toward the conversion of p-nitrophenol to p-aminophenol by NaBH4 compared to the spherical Ag nanoparticles. Acknowledgment. Md.H.R. thanks the CSIR, Government of India, for providing a Fellowship. This research was supported by grants from the CSIR, New Delhi. Thanks are also due to the partial support from DBT and the Nanoscience and Nanotechnology Initiatives, DST, New Delhi. Supporting Information Available: Additional TEM images and FESEM images of Ag nanostructures (Figures S1S4), XRD patterns of Ag nanostructures prepared with FAC and TSC (Figures S5 and S6), and HRTEM image of taillike Ag nanostructures (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schulz, J.; Roucoux, A.; Patin, H. Chem. ReV. 2002, 102, 37573778. (2) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729-7744. (3) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2004, 126, 7194-7195. (4) Rashid, M. H.; Bhattacharjee, R. R.; Kotal, A.; Mandal, T. K. Langmuir 2006, 22, 7141-7143. (5) Peyser, L. A.; Vinson, A. E.; Bartko, A. P.; Dickson, R. M. Science 2001, 291, 103-106. (6) Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106. (7) Zhang, J.; Li, X.; Sun, X.; Li, Y. J. Phys. Chem. B 2005, 109, 12544-12548. (8) Schultz, S.; Smith, D. R.; Mock, J. J.; Schultz, D. A. Proc. Natl. Acad. Sci. 2000, 97, 996-1001. (9) Cao, Y. C.; Jin, R.; Mirkin, C. A. Science 2002, 297, 1536-1540. (10) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5-147. (11) Sun, Y.; Xia, Y. Science 2002, 298, 2176-2179. (12) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80-82. (13) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z. Y. AdV. Mater. 1999, 11, 850-852. (14) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635-645. (15) Callegari, A.; Tonti, D.; Chergui, M. Nano Lett. 2003, 3, 15651568. (16) Teng, X.; Yang, H. Nano Lett. 2005, 5, 885-891.

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Synthesis and Catalytic Application of Nanostructured Silver Dendrites

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