Size-Controlled Synthesis and Self-Assembly of Silver Nanoparticles

Dec 5, 2008 - To the best of our knowledge, the synthesis of size-controlled Ag NPs and self-assembled structures of nanochains within a minute has no...
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J. Phys. Chem. C 2009, 113, 134–141

Size-Controlled Synthesis and Self-Assembly of Silver Nanoparticles within a Minute Using Microwave Irradiation Subrata Kundu,* Ke Wang, and Hong Liang* Materials Science and Mechanical Engineering, Texas A&M UniVersity, College Station, Texas 77843-3123 ReceiVed: September 17, 2008; ReVised Manuscript ReceiVed: NoVember 4, 2008

Size-controlled silver (Ag) nanoparticles (NPs) and nanochains were synthesized for the first time in large quantities. This was done in the presence of alkaline 2,7-dihydroxy naphthalene (2,7-DHN) as a new reducing agent under microwave heating for just 60 s in a nonionic surfactant (TX-100) media. Results showed that the Ag NPs were small in size, which could be successfully controlled through varying the TX-100 to Ag(I) molar ratio. The formation of Ag nanochains was enhanced by low TX-100 concentrations and high pH. The synthesized particles are stable for more than 3 months in ambient conditions. The proposed method could be extended for the synthesis of other metal and semiconductor nanomaterials with defined sizes and shapes. These particles could find valuable applications in catalysis, nanoelectronics, and surface-enhanced Raman spectroscopic (SERS) studies. Introduction In recent years, the preparation and characterization of nanostructured materials have become an intense research focus. This is primarily due to their unique properties and potential applications from a technological point of view. The optical,1 electronic,2 magnetic,3 and catalytic4 properties of these materials depend on their sizes and shapes. The high surface-to-volume ratios of the NPs lead to dramatic changes in their properties. Novel metal NPs are particularly interesting due to their close lying conduction and valence bands in which electrons move freely. These free electrons generate surface plasmon bands that depend on the particles size, shape, and surroundings. Similarly, the fascinating color of the noble metal NPs also depends on both the size and shape of the particles as well as the refractive index of the surrounding medium.5 Among the different metals studied to date, silver NPs attract special attention due to their high electrical conductivity, antimicrobial effect, oxidative catalytic functions, and unique Raman spectroscopic behavior.6 Silver is more reactive than other noble metals and undergoes relatively fast oxidation as well as aggregation in solution that complicates its use in the development of sensors and optical instruments. In recent years, a number of chemical approaches have been actively explored to process silver nanostructures. The most well-known and the easiest method is the reduction of silver ions in the presence of the reducing agent NaBH4.7 The other literature methods include chemical reduction,8 polyol process,9 radiolytic process,10 biological,11 and photoreduction.12 Sizecontrolled Ag NPs have been synthesized in the presence of metallic seed particles in polyvinyl pyrrolidone solution.13 However, all of these mentioned procedures were found to be time-consuming and generated mixtures of different shapes with uncontrollable particle size distribution. The removal of the unwanted template or directing agent from the NPs surface requires rather harsh conditions, and consequently increases the difficulty of performing such surface chemistry. * Corresponding author. Phone: (979) 862-2578. Fax: (979) 845-3081. E-mail: [email protected] (S.K.); [email protected] (H.L.).

Recently, a microwave heating method has been developed and widely used for the synthesis of nanomaterials at a significantly higher speed than conventional thermal convection. The microwave irradiation generates very fast nucleation sites in the solution, which significantly enhance the reaction rates. Another advantage is that it can heat a substance uniformly and generate homogeneous nucleation sites with higher penetration power as compared to conventional ones. The microwave heating method has been used for the synthesis of metal NPs like Au,14 Ag,15 Cu,16 Pd,17 Pt,18 etc., and semiconductor rods and wires.19 Hu et al.20 synthesized 4-mercaptobenzoic acid (4MBA) capped silver NPs in the presence of amino acid in water. Several years ago, we synthesized silver NPs in silica gel using photochemical methods.12 Recently, we also synthesized highly stable silver nanocubes in the presence of a polymer using the microwave method.21 Liu et al. synthesized Ag NPs in the presence of sodium citrate by adding previously prepared seed particles.22 Almost all other methods for making Ag NPs need either addition of seed particles separately, multiple steps, long reaction time, or generate a mixture of different-shaped particles with lower yields. In the present research, we developed a process using microwave irradiation to synthesize size-controllable Ag NPs in a reaction time of 60 s. The reduction of Ag ions was done in TX-100 (poly oxyethylene isooctyl phenyl ether) micellar media in the presence of a new reducing agent, alkaline 2,7dihydroxy naphthalene (2,7-DHN). The method exclusively produced Ag NPs of variable sizes simply by changing the metal ion-to-surfactant molar ratios, the concentration of 2,7-DHN, and pH of the reaction medium. The method can also be extended to the synthesis of self-assembled silver nanochains in a strongly basic pH medium and at a lower concentration of TX-100. To the best of our knowledge, the synthesis of sizecontrolled Ag NPs and self-assembled structures of nanochains within a minute has not been reported earlier. The yields of the particles with uniform shapes are found to be significantly high (more than 90%), and the particles are extremely stable, that is, more than 3 months in an ambient environment.

10.1021/jp808292s CCC: $40.75  2009 American Chemical Society Published on Web 12/05/2008

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TABLE 1: Concentration of the Reaction Parameters, Absorption Maxima, and Particle Shapes after 60 s MW Irradiation with the Change in pH Values set number

final conc. of TX-100 (M)

final conc. of AgNO3 (M)

final conc. of 2,7-DHN (M)

final conc. of NaOH (M)

pH of the medium

absorption maxima (λmax) (nm)

particle shape

1 2 3 4 5 6

7.6 × 10-3 7.58 × 10-3 7.54 × 10-3 7.47 × 10-3 7.40 × 10-3 7.27 × 10-3

3.8 × 10-4 3.79 × 10-4 3.77 × 10-4 3.73 × 10-4 3.70 × 10-4 3.63 × 10-4

1.9 × 10-3 1.89 × 10-3 1.88 × 10-3 1.86 × 10-3 1.85 × 10-3 1.81 × 10-3

9.52 × 10-3 9.47 × 10-3 9.43 × 10-3 9.34 × 10-3 9.25 × 10-3 9.09 × 10-3

10.50 10.80 11.10 11.40 11.54 11.74

425 422 417 415 413 and 592 410 and 582

spherical spherical spherical spherical nanochain nanochain

Experimental Section Reagents. 2,7-Dihydroxy naphthalene (2,7-DHN), 1,2-dihydroxy naphthalene (1,2-DHN), and 2-naphthol (2-N) were purchased from Sigma-Aldrich and used as received. Poly oxyethylene isooctyl phenyl ether (trade name, triton X-100 or TX-100) and cetyltrimethyl ammonium bromide (CTAB, 99%) were purchased from Sigma-Aldrich. The silver nitrate (99%) and sodium hydroxide (NaOH) were also obtained from SigmaAldrich and used as received. Deionized (DI) water was used for the entire synthesis. Instruments. UV-visible (UV-vis) absorption spectra were recorded in a Hitachi (model U-4100) UV-vis-NIR spectrophotometer equipped with a 1 cm quartz cuvette holder for liquid samples. A high-resolution transmission electron microscope (HR-TEM) (ZEOL ZEM 2010) was used at an accelerating voltage of 200 kV. The energy dispersive X-ray spectrum (EDS) was recorded with an Oxford Instruments INCA energy system connected with the TEM. The XRD analysis was done with a scanning rate of 0.020 s-1 in the 2θ range 25-85° using a Bruker-AXS D8 Advanced Bragg-Brentano X-ray powder diffractometer with Cu KR radiation (λ ) 0.154178). The X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos Axis Ultra Imaging X-ray photoelectron spectrometer with monochromatic Al KR line (1486.7 eV). The instrument integrates a magnetic immersion lens and charge neutralization system with a spherical mirror analyzer, which provides realtime chemical state and elemental imaging using a full range of pass energies. The emitted photoelectrons were detected by the analyzer at a passing energy of 20 eV with energy resolution of 0.1 eV. The incident X-ray beam was normal to the sample surface, and the detector was 45° away from the incident direction. The analysis spot on the sample was 0.4 mm × 0.7 mm. A domestic microwave (MW) oven (Gold Star Co., EMZ200S, 1000 W, 60 Hz) was used for MW irradiation for the entire synthesis. Synthesis of Size-Controlled Ag NPs and Nanochains by Microwave Heating. Size selective silver NPs were synthesized by varying the amount of AgNO3 in the solution mixture containing TX-100, 2,7-DHN, and NaOH. For a typical synthesis process, 4 mL of TX-100 (10-2 M) solution was mixed with 200 µL of AgNO3 solution. After that, 1 mL of 2,7-DHN (10-2 M) and finally 50 µL of 1 M NaOH solution were added, and the mixture was stirred for 10 s. Next, the solution mixture was irradiated by MW for about 1 min with an intermittent pause after every 10 s to cool the reaction vessel. For the synthesis of other sized particles, we varied the concentration of Ag(I) ions relative to the concentration of surfactant. The final concentration of the Ag(I) ions was 3.8 × 10-4 M when the average particles size was 4 ( 0.6 nm. When the average particles size was ∼12.5 ( 3 and ∼32 ( 3.5 nm, the final concentration of AgNO3 was 7.33 × 10-4 and 1.061 × 10-3 M, respectively. For the synthesis of Ag nanochains, we increased the pH of the solutions >11.5 and decreased the concentration of TX-

100. The final concentration of the TX-100 was 7.27 × 10-3 M, whereas the concentration of NaOH was 9.09 × 10-3 M (details given in Table 1). In all cases before MW, the solution was stirred for about 5-10 s and then MW irradiated for a total of 60 s. For all of the above cases, the Ag particle formation started after 10 s of MW irradiation as observed from the color change of the reaction mixture as well as from the UV-vis spectrum. After 60 s of MW heating, the resulting solution was kept at room temperature for about 10 min to cool and then centrifuged at 6000 rpm for 20 min and then at 3000 rpm for 10 min to remove excess surfactants and other reactants from the Ag NP solution. Finally, the precipitated yellowish colored Ag NPs were redispersed in DI water for characterization. The solution become light yellowish (for 4 ( 0.6 nm particles), deep yellowish (for 12.5 ( 3 nm particles), and light greenish-yellow (for 32 ( 3.5 nm particles) for three different NP solutions. The color of the Ag NPs was found to be stable after more than 3 months in the dark under ambient condition without change in their optical properties. Preparation of Sample for TEM, EDS, XPS, and XRD Analysis. After centrifugation and redispersion of the Ag NPs in DI water, the solutions were characterized using TEM, EDS, XPS, and XRD analysis. The samples for TEM and EDS were prepared by placing a drop of the corresponding Ag NP solution onto a carbon-coated Cu grid followed by slow evaporation of solvent at ambient condition. For XPS and XRD analysis, silicon (Si) wafers were used as the substrates for the thin film preparation. The wafers were cleaned thoroughly in acetone and sonicated for about 20 min. The cleaned substrates were covered with the Ag NP solution and dried in a vacuum chamber. After the first layer was deposited, subsequent layers were deposited by repeatedly adding more Ag NP solution and drying. Final samples were obtained after eight depositions and then analyzed using XPS and XRD techniques. Results and Discussion Monodispersed spherical silver NPs with variable sizes were synthesized in the presence of alkaline 2,7-DHN using TX-100 as a capping agent in a 1000 W MW oven. Figure 1 shows the UV-vis spectrum of the reaction mixture at different stages of the process. The colorless solution of aqueous silver nitrate (AgNO3) has no specific absorption band in the visible region (curve A, Figure 1). An aqueous solution of TX-100 has an absorption band at ∼278 nm (curve B, Figure 1) due to the presence of the aromatic ring/long chain hydrocarbon moiety. A colorless solution of 2,7-DHN has two distinct absorption bands peaking at 282 and 322 nm (curve C, Figure 1) due to the presence of benzene rings. When TX-100, AgNO3, and 2,7DHN were mixed together, the absorption peak for 2,7-DHN itself slightly shifted and increased absorption value due to the incorporation of Ag(I) in TX-100 or interaction with 2,7-DHN. This is observed from curve D in Figure 1. With the addition of NaOH to the reaction mixture containing TX-100, AgNO3,

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Figure 1. UV-vis absorption spectra at various stages of Ag NP synthesis. A, B, and C are the absorption spectra of AgNO3, TX-100, and 2,7-DHN in water. D is the absorption spectra of the mixture of AgNO3, TX-100, and 2,7-DHN in water. E is the surface plasmon resonance (SPR) band for spherical Ag NPs. F is the surface plasmon band (SPR) of the same solution after 3 months of aging in ambient atmosphere. Inset shows the two different color Ag NPs solution before (G, left, dark greenish yellow color) and after (H, right, light yellowish color) the centrifugation of the reaction product.

Figure 2. UV-vis absorption spectra of different sizes of Ag NPs solutions. A (λmax at 414 nm), B (λmax at 418 nm), and C (λmax at 426 nm) show the SPR bands for three different sizes of Ag NP solutions. Inset shows the three different color Ag NP solutions corresponding to curves A, B, and C.

and 2,7-DHN and subsequent microwave irradiation, the color of the solution changed to greenish-yellow, which might be due to formation of Ag nanoparticles. After subsequent centrifugation and redispersion, the aqueous solution generates yellowish colored Ag NPs solution (curve E, Figure 1) with the appearance of a new absorption band at 420 nm. This 420 nm peak is due to the well-known surface plasmon resonance (SPR) band of spherical silver nanoparticles.23,24 The synthesized silver NPs solution is remarkably stable with no sign of oxidation or hydroxide formation for at least 3 months of storage in ambient conditions (curve F, Figure 1, which coincides with curve E). The inset of Figure 1 shows the two different colored Ag NPs solutions before (G, left, dark greenish yellow color) and after (H, right, light yellowish color) the centrifugation of the reaction product. Figure 2 shows the UV-vis spectrum of the different Ag NP solutions. Figure 2A-C shows the SPR bands for different sizes of Ag NPs in solution. Figure 2A shows the SPR band maximum (λmax) at 414 nm, whereas Figure 2B and C displays those at 41 and 426 nm. This increase of absorbance value and

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Figure 3. Successive increase of the SPR band of Ag NPs with the increase in MW exposure time from 10 to 60 s (from curve A to F).

shifting of absorption maximum in the higher wavelength region is due to the increase of Ag ions in the solution and subsequent formation of larger sized particles. The inset of Figure 2 shows the three different colored Ag NPs solutions corresponding to curves A, B, and C. The SPR band of Ag NPs monotonically increases with the MW exposure. It saturates after 60 s, indicating completion of the reaction. The formation of Ag NPs starts after 10 s of MW exposure and completes after 60 s as shown in Figure 3 (curves A-F). Figure 4 illustrates the transmission electron microscopy (TEM) images of the different sizes of Ag NPs prepared with the above method (details given in the Experimental Section) after 60 s of MW irradiation. Figure 4A and B shows the lowand high-magnified images of the Ag NPs corresponding to curve A in Figure 2. All of the particles are monodisperse and spherical with an average particle size of ∼4 ( 0.6 nm. From the higher magnified image in Figure 4B, the lattice spacing in different crystal planes is 0.2272 nm. The inset of Figure 4B shows the corresponding selected area electron diffraction (SAED) pattern and confirms that the particles are single crystals. Figures 4C and D shows the low- and high-magnified TEM images corresponding to curve B in Figure 2. Here, the average particles size is ∼12.5 ( 3 nm, and the corresponding lattice spacing calculated from Figure 4D is ∼0.2083 nm. The inset of Figure 4D shows the SAED pattern, which signifies the single crystalline nature of the particles. Figure 4E and F presents TEM images where the average particles size is ∼32 ( 3.5 nm, corresponding to curve C in Figure 2. All of the particles are monodisperse, and the lattice spacing calculated from the higher magnified image (Figure 4F) is 0.1724 nm. The inset of Figure 4F shows the SAED pattern of the crystalline particles. From the above analysis, it is clear that the lattice spacing is decreasing with increasing particle size. Similar types of results were also observed by Bai et al. for the synthesis of FePt-C nanocrystals.25 Figure 5 presents the results obtained from the energy dispersive X-ray spectroscopy (EDS) analysis to determine the chemicals present in the reaction product. The spectrum consists of different peaks for Ag, C, Cu, and Cr. The large intense Ag peak came from the Ag NPs, the C and Cu peaks came from the C-coated Cu TEM grids, and the weak Cr peak came from the sample holder. Figure 6A shows the overall survey and Ag (3d) XPS spectra of a self-assembled Ag NP film. The survey spectrum contains the characteristic peaks of Na (1s) at 1070 eV, O (1s) at 530

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Figure 4. Transmission electron microscopy (TEM) images for the synthesis of Ag NPs after 60 s of MW irradiation with variation of AgNO3 concentrations. A and B are the low- and high-magnified images of the Ag NPs with an average particles size ∼4 ( 0.6 nm when AgNO3 concentration was 3.80 × 10-4 M. Similarly, C,D and E,F are the low- and high-magnified images of the Ag NPs with an average particles size ∼12.5 ( 3 and ∼32 ( 3.5 nm, respectively, where the AgNO3 concentration was 7.33 × 10-4 and 1.061 × 10-3 M, respectively. Inset of B, D, and F shows the selected area electron diffraction (SAED) pattern of the corresponding particles.

eV, Na (Auger KL23L23) at 495 eV, and C (1s) at 284.5 eV. The Ag (3d) region is characterized by a doublet, which arises from spin-orbit coupling (3d5/2 and 3d3/2) as shown in Figure 6B. The Ag 3d3/2 and Ag 3d5/2 peaks appear at a binding energy of 373.8 and 367.9 eV, which indicate the formation of Ag NPs. The positions of the peaks are in agreement with the literature.26 We do not observe any oxide formation (i.e., Ag2O) peaks, indicating the high stability of the Ag NPs. Other small features at the lower binding energy are also identified. The peaks at 99.5 and 150.5 eV are attributed to the Si (2p) and Si (2s), which

came from the Si substrate used. The peak at 63 eV belongs to Na (2s). The O peak came from the TX-100 bound to the surface of the Ag NPs as a capping agent, and the Na peak most likely came from the NaOH used during the reaction. Figure 7 shows the X-ray diffraction pattern of the Ag NPs. The diffraction peaks originated from the (111), (200), and (222) planes of silver NPs (JCPDS card number 4-0783). These diffraction peaks confirm the presence of a face-centered cubic (fcc) structure of crystalline silver nanoparticles.20 Other small peaks were also observed, which mostly came from the

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Figure 7. Powder X-ray diffraction (XRD) pattern of the Ag NPs. Figure 5. Energy dispersive X-ray spectrum (EDS) of the Ag NPs.

Figure 6. X-ray photoelectron spectrum (XPS) of the Ag NPs. A is the overall spectrum, and B is the spectrum of Ag NPs only.

substrate. The size of the Ag NPs was measured by X-ray diffraction peak line width broadening using the Debye formula for small crystalline spheres. The mean diameter of the particles is consistent with the result of TEM measurements in Figure 4. The broad nature of the diffraction peak could be attributed to the nanosize scale of the particles. The shape of any fcc nanoparticle is mainly determined by the ratio of growth rates

along the 〈100〉 and 〈111〉 directions as suggested by Wang et al.27 As we used TX-100 as a stabilizer, the interaction between the TX-100 and the different crystal facets of Ag NPs reduced the growth rate along the 〈100〉 direction, thus enhancing the reaction rate along the 〈111〉 direction. In the proposed reaction, we varied the concentration of the reagents such as TX-100, AgNO3, 2,7-DHN, and NaOH and other reaction parameters like MW heating time. The best results we obtained were given in the Experimental Section. By changing the concentration of Ag(I) from 3.8 × 10-4 to 1.061 × 10-3 M and keeping the other parameters fixed, we obtained different sizes of Ag particles from 4 to 35 nm. When the Ag ion concentration was increased to ∼3.3 × 10-4 M, there was a yellowish white precipitate formed after adding NaOH, which might be due to the formation of hydroxyl compounds of silver. Similar types of phenomena were reported by Dai et al.28 When the Ag ion concentration is very low (9.8 × 10-5 M), no particles were formed in the experimental time scale. When we increased the TX-100 concentration from 10-2 to 10-1 M, more time was needed to form the Ag particles. In addition, when we decreased the concentration to below 10-3 M, Ag NPs tended to aggregate to form chain structures. With the changing of the concentration of 2,7-DHN (like 10-1 or 10-3 M), polydispersity of the samples was confirmed from TEM images (not shown here). We also examined the variation of MW heating time and observed that 60 s was sufficient for the formation of the particles in our reaction conditions. The TEM images of the particles synthesized after 20 and 40 s of MW heating are shown in the Supporting Information. We also tested our reaction in a conventional heating process instead of MW irradiation. In the conventional heating process, we used a small vial containing the solutions in a hot water bath during the reaction. After the reaction, we kept the vial in a cold water bath to cool the reaction vessel. We repeated the “hot water bath” and “cold water bath” experiment several times during the experiment. From the TEM images, we observed that bigger size aggregated, highly polydisperse Ag particles are formed after 20-30 min without any specific shapes. We observed similar types of results by keeping the same reaction conditions that we used to synthesize the nanochains. The TEM images for these conventional heating processes are shown in the Supporting Information. From our experiment, we concluded that MW heating is essential to obtain the monodisperse spherical Ag NPs and the nanochains. We tested our reaction with another positively charged surfactant, CTAB, but it did not produce Ag NPs in our experimental time scale. Instead, a white-colored precipitate formed, which might be due to formation of its insoluble salts.

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Figure 8. UV-vis absorption spectra with variation of pH values. A, B, C, D, E, and F indicate the SPR bands of Ag NPs when the pH was 10.5, 10.8, 11.1, 11.4, 11.54, and 11.74, respectively. Inset shows the different color Ag NPs formed with different pH values.

We have studied in detail the effect of pH in our reaction. The pH values of TX-100 (10-2 M), TX-100, and AgNO3 mixture, and a mixture of TX-100, AgNO3, and 2,7-DHN, are 6.6, 6.4, and 6.46, respectively. With the addition of 50 µL of 1 M NaOH to the reaction mixture containing TX-100, AgNO3, and 2,7-DHN, the pH changed to 10.5. We checked the effects of increasing pH values using UV-vis spectrum and TEM analysis. Table 1 lists the different parameters showing the effects of pH on absorption maxima and particles shapes. From Table 1, it is clear that with increasing pH, there is a blue shift of the λmax value. This is consistent with reports by other researchers.29 According to Mie’s theory,30 only a single SPR band is expected in the absorption spectra of spherical NPs, whereas anisotropic particles can give rise to two or more SPR bands depending on the shape of the particles. Figure 8 shows the UV-vis spectra recorded with different pH values. The λmax value shifted gradually toward the blue region with the increase in pH values. For pH 10.5, the λmax is 425 nm (curve A, Figure 8), whereas for pH 11.74, the λmax is 410 nm (curve F, Figure 8). The absorption spectra for other pH values like 10.8, 11.1, 11.4, and 11.54 are labeled by curves B, C, D, and E in Figure 8. The reducing tendency of any hydroxyl compound strongly depends on the pH of the reaction medium. In our reaction, no silver NPs were formed in the absence of NaOH, whereas Ag NPs were formed in the presence of NaOH. The blue shifting in the UV-visible spectrum and the formation of smaller sized particles (confirmed from TEM images, not shown here) at higher pH should be due to the enhanced reduction power of 2,7-DHN due to deprotonation caused by reduction. With the further increase in pH, the solution was precipitated immediately due to the formation of hydroxyl compounds of Ag in highly alkaline medium.28 From the UV-visible spectrum in Figure 8, it was observed that when the pH value increased beyond 11.4, a new peak at the higher wavelength region was observed, which was very prominent at pH 11.74. Two distinct λmax’s were observed, one at 410 nm and another at 582 nm. This new peak in the higher wavelength region might be due to the formation of chain-like/aggregated nanostructures. Formation of nanochain structures was also observed with lower TX-100 concentrations. The inset of Figure 8 shows the different colored Ag NPs formed with different pH values. Figure 9 shows the TEM images of the Ag nanochain structures with a lower TX-100 concentration

Figure 9. TEM images of the Ag nanochain structure at low magnification (A) and at high magnification (B).

but higher pH value. Figure 9A and B shows the low-magnified and the high-magnified TEM images, respectively, confirming the formation of the nanochain structure. The formation of different sized particles and the nanochain structures is schematically shown in Scheme 1. The formation of size-controlled Ag NPs as well as nanochain structures was caused by the reduction of Ag(I) ions in alkaline 2,7-DHN in the presence of MW heating. It is reported in the literature that phenolic compounds such as polyvinyl alcohol,31 ascorbic acid,32 dendrimer,33 benzophenone,34 etc., act as reducing agents and undergo photolytic cleavage to phenoxy radicals in the presence of UV-light or at high temperatures. In our study, it is believed that some radical species or solvated electrons are formed during the MW heating of the reaction mixture containing 2,7-DHN that enables the reduction of Ag(I) to Ag(0). The reduction of Ag(I) to Ag(0) leads to the formation of small spherical nuclei. These small spherical nuclei subsequently grow to form Ag atoms and aggregate together to form small crystalline Ag NPs. With the increasing amount of Ag(I) ions in the solution, more Ag nuclei are formed, which generate larger sized Ag particles as shown in Figure 4 and Scheme 1. The nonionic surfactant TX-100 molecules are attached to the surface of the Ag NPs to prevent unwanted growth and aggregation. The TX-100 is adsorbed on particular facets on the Ag NPs and induced 3D growth. The formation of nanochain structures at lower concentrations of TX-100 might be due to

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SCHEME 1: Schematic Representation for the Formation of Different Sizes of Ag NPs and the Nanochain Using MW Irradiation

more available unoccupied surface sites for the adsorption of surfactants on the Ag nuclei. At higher concentrations of TX100, the particles are well separated from each other (Figure 4), whereas at lower concentrations of TX-100 (Figure 9), the particles are in close proximity to form nanochain structures. In our present study, we found that the MW heating is essential, as we do not observe any Ag particles formation in our experimental time scale by conventional heating. The bigger sized aggregated particles only formed after 20-30 min with no specific shapes, as shown in the Supporting Information. So the interaction of the dielectric of the MW with the solution materials to generate the radical species is very important for the formation of Ag NPs. During the growth stage, the free silver ions in solution are adsorbed over the growing nuclei previously formed and reduced again to silver and produced bigger size particles with time, as observed from the TEM images in the Supporting Information. Similar types of observations were also found with the nanochains formation. This increase in particle size with the increase in MW irradiation time is consistent with the UV-vis spectrum in Figure 3. The present Ag NPs growth mechanism found similarity to the formation of Au nanoprisms14 by MW heating. The present method can find various important applications due to the presence of TX-100 on the NPs surface. It is known that the TX-100 has been used as an efficient molecular spacer for SERS study of free-base porphyrins.35 So application of our TX-100 encapsulated Ag NPs in SERS spectroscopy studies is also possible. We tested our reaction with other hydroxylated compounds (like 2-N, 1,2-DHN) having structures similar to that of 2,7-DHN. The preliminary result reveals the formation of Ag NPs, but a detailed study is in progress. In summary, we demonstrated that size-controlled Ag NPs and nanochains could be synthesized within 60 s of MW heating in the presence of a new reducing agent, alkaline 2,7-DHN. The synthesized particles were found to be extremely stable for at least 3 months in ambient conditions without any indication of oxide formation. The proposed method is extremely time efficient, straightforward, and scalable. With changes in the Ag(I) ion-to-TX-100 molar ratio, the particle size is successfully tuned. Change in TX-100 concentration and pH

value leads to the formation of well-developed nanochain structures. The present method would lead to the fast manufacturing process for the synthesis of other metallic and semiconductor nanomaterials and might find applications in catalysis, nanoelectronics, and SERS studies. Acknowledgment. This research was in part sponsored by the NSF-0506082; the Department of Mechanical Engineering, Texas A&M University; and the Texas Engineering Experiments Station. We wish to acknowledge Mr. Sean Lau from Texas A&M University for proofreading the manuscript. Support for TEM and EDS by Dr. Zhiping Luo at the Microscopy Imaging Center (MIC), Texas A&M University, was greatly appreciated. Supporting Information Available: Transmission electron microscopy (TEM) images with other conditions. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Kundu, S.; Liang, H. AdV. Mater. 2008, 20, 826. (3) Mandal, M.; Kundu, S.; Sau, T. K.; Yusuf, S. M.; Pal, T. Chem. Mater. 2003, 15, 3710. (4) Fang, H.; Wu, Y.; Zhao, J.; Zhu, J. Nanotechnology 2006, 17, 3768. (5) Yuan, B.; Wicks, D. A. J. Appl. Polym. Sci. 2007, 105, 446. (6) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (7) Prochazka, M.; Stepanek, J.; Turpin, P. Y.; Bok, J. J. Phys. Chem. B 2002, 106, 1543. (8) Frens, G. Nature 1972, 20, 241. (9) Silvert, P. V.; Urbina, R. H.; Elhsissen, K. T. J. Mater. Chem. 1997, 7, 293. (10) Marignier, J. L.; Belloni, J.; Delcourt, M. O.; Chevalier, J. P. Nature 1985, 317, 344. (11) Shankar, S. S.; Rai, A.; Ahmad, A.; Shastry, M. J. Colloid Interface Sci. 2004, 275, 496. (12) Kundu, S.; Mandal, M.; Ghosh, S. K.; Pal, T. J. Colloid Interface Sci. 2004, 272, 134. (13) Liu, F.-K.; Huang, P.-W.; Chu, T.-C.; Ko, F.-H. Mater. Lett. 2005, 59, 940. (14) Kundu, S.; Peng, L.; Liang, H. Inorg. Chem. 2008, 47, 6344. (15) Tsuji, M.; Nishizawa, Y.; Matsumoto, K.; Miyamae, N.; Tsuji, T.; Zhang, X. Colloids Surf., A: Physicochem. Eng. Aspects 2007, 293, 185. (16) Nakamura, T.; Tsukahara, Y.; Sakato, T.; Mori, H.; Kanbe, Y.; Bessho, H.; Wada, Y. Bull. Chem. Soc. Jpn. 2007, 80, 224.

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