CRYSTAL GROWTH & DESIGN
Microwave-Assisted Shape-Controlled Bulk Synthesis of Ag and Fe Nanorods in Poly(ethylene glycol) Solutions
2008 VOL. 8, NO. 1 291–295
Mallikarjuna N. Nadagouda and Rajender S. Varma* Sustainable Technology DiVision, United States EnVironmental Protection Agency, National Risk Management Research Laboratory, 26 West Martin Luther King DriVe, MS 443, Cincinnati, Ohio 45268 ReceiVed May 22, 2007; ReVised Manuscript ReceiVed September 13, 2007
ABSTRACT: Bulk syntheses of silver (Ag) and iron (Fe) nanorods using poly(ethylene glycol) (PEG) under microwave irradiation conditions are reported. Favorable conditions to make Ag nanorods were established and can be extended to make Fe nanorods with uniform size and shape. The nanorods formation depended upon the concentration of PEG used in the reaction with Ag salt. The obtained Ag and Fe nanorods were characterized using scanning electron microscopy, transmission electron microscopy, and UV–visible spectroscopy. Ag and Fe nanorods crystallized in face centered cubic symmetry. The method uses no surfactant or reducing agent and is greener in nature which could open a myriad of applications. Introduction Controlled synthesis of nanostructured materials has been getting more attention lately due to their unique chemical and physical properties that are different from those of the bulk materials.1 Particularly, one-dimensional (1D) nanostructures (rods, wires, and tubes) of silver have received considerable interest from a broad range of researchers because of their wide application to catalysts, scanning probes, and various kinds of electronic and photonic nanodevices.2–5 Various approaches have been reported to prepare 1D nanostructures of Ag, for example, synthesis of Ag nanowires by a hydrothermal route,6 solution phase,7 arc discharge,8 wet chemistry,9 using AgBr nanocrystals,10 polyol process4,11–13 and sono-self-reduction.14 Among these methods, the “polyol process” is a well-known method that uses ethylene glycol (EG) as a reducing agent and solvent.4,11–13 Using EG as a reducing agent with a combination of a capping agent such as poly(vinyl pyrrolidinone) (PVP) Ag cubes, production of irregular Ag rods with spherical particles was reported. All these synthetic methods need a combination of a reducing agent, a capping agent, higher temperature, and prolonged time. Moreover, EG is more toxic when compared with poly(ethylene glycol) (PEG), which is ecofriendly, nontoxic, and biodegradable in nature.15 Apart from this, controlling particle size and shape with bulk morphology is still a challenging proposition to synthetic chemists, and there is renewed interest in applying principles of green chemistry.16,17 Besides noble metals, other transition metals such as Fe nanostructures are gaining interest for several reasons including catalysis,18 magnetic recording,19 and surfaceenhanced Raman-scattering.20 Recently, we have accomplished a shape-selective synthesis of noble nanoparticles and nanowires using vitamin B2 without using any harmful reducing agents, such as sodium borohydride (NaBH4) or hydroxylamine hydrochloride, and/or surfactants.21 Vitamin B2 was used as a reducing agent as well as a capping agent due to its high water solubility, biodegradability, and low toxicity compared with other reducing agents. Microwave (MW) irradiation provides rapid and uniform heating of reagents, solvents, and intermediates, and recently bulk synthesis of shapecontrolled gold (Au) nanostructures with various shapes such * To whom correspondence should be addressed. Fax: (513) 569-7677. E-mail:
[email protected].
Table 1. Composition of PEG Reaction with AgNO3 entry
composition
code
1 2 3 4 5 6 7 8 9 10 11 12
1 mL PEG + 7 mL AgNO3 2 mL PEG + 6 mL AgNO3 3 mL PEG + 5 mL AgNO3 4 mL PEG + 4 mL AgNO3 5 mL PEG + 3 mL AgNO3 6 mL PEG + 2 mL AgNO3 7 mL PEG + 1 mL AgNO3 7.5 mL PEG + 0.5 mL AgNO3 4 mL of PEG + 3 mL of AgNO3 + 1 mL of HAuCl4 4 mL of PEG with 3 mL of AgNO3 and 1 mL of PdCl2 PG-4 nanorods + 4 mL of Na2PtCl6 · XH2O 4 mL of PEG + 4 mL Fe(NO3)3 · XH2O
PG-1 PG-2 PG-3 PG-4 PG-5 PG-6 PG-7 PG-8 PG-9 PG-10 PG-11 PG-12
as prisms, cubes, and hexagons was achieved that occurs via MW-assisted spontaneous reduction of noble metal salts using an aqueous solution of R-D-glucose, sucrose, and maltose.22 The expeditious reaction is completed under MW irradiation in 30–60 s and can be applied to the generation of nanospheres of silver (Ag), palladium (Pd), and platinum (Pt). Furthermore, the use of renewable material such as carboxymethyl cellulose (CMC) in the synthesis of noble metals was also accomplished under MW irradiation conditions at 100 °C, where CMC acts as a capping and reducing agent.23 In continuation of our efforts to develop greener methods to synthesize noble nanostructures, herein we report an environmentally benign approach that provides a facile route to production of Ag and Fe nanorods employing PEG as a reducing and capping agent and that may find widespread technological and medicinal applications. Experimental Procedures In a typical procedure, 4 mL of aqueous silver nitrate (AgNO3) solution (0.1 M) and 4 mL of PEG (molecular weight 300) were mixed in a 10 mL test tube at room temperature to form a clear solution. The reaction mixture was irradiated in a CEM Discover focused MW synthesis system maintaining a temperature of 100 °C (monitored by a built-in infrared sensor) for 1 h with a maximum pressure of 280 psi. The resulting precipitated Ag nanorods were then washed several times with water to remove excess PEG. Control experiments were conducted in an oil bath at 100 °C for 1 h, at the temperature reached in the MW system. Similar experiments were carried out by varying the ratios of PEG, AgNO3, and iron nitrate, Fe(NO3)3 · XH2O, (0.1 M) as shown in Table 1.
10.1021/cg070473i CCC: $40.75 2008 American Chemical Society Published on Web 12/12/2007
292 Crystal Growth & Design, Vol. 8, No. 1, 2008
Figure 1. Photographic image of (a) precipitated Ag nanorods after microwave irradiation for 2 min; and (b) control reaction of the same reaction composition carried out using oil bath at 100 °C for 1 h.
Figure 2. Reaction profile of PG-4 irradiated at 100 °C for 1 h using MW.
Results and Discussion The formation of Ag nanorods or nanoparticles occurred upon MW irradiation of a solution of aqueous AgNO3 in PEG, depending upon the concentration of the reaction mixtures used (Figure 1a). The control experiments carried out in an oil bath and kitchen microwave oven under similar conditions did not precipitate any Ag nanorods (Figure 1b). Xia et al. have reported Ag nanowires in high yields by reducing AgNO3 with EG heated to 148 °C for 1 hour and a half in the presence of poly(vinyl pyrrolidinone) and a trace amount of sodium chloride in argon atmosphere.24 Similarly, the lead nanowires synthesis is reported in the presence of poly(vinyl pyrrolidinone) under N2 conditions for 1 h.25 However, in the present study formation of Ag nanowires occurred within 2 min of MW irradiation (nanorods yield 90%) and no need of special inert atmosphere. However, a few more minutes are required to get maximum yield. From the reaction profile (Figure 2), it is clear that initially the pressure inside the reaction vial varied a great deal, indicating the reduction of Ag under the influence of microwaves, which
Nadagouda and Varma
Figure 3. Scanning electron microscopy (SEM) images of Ag nanoparticles prepared via MW method using (a) PG-6 and (b) PG-1.
ultimately resulted in the Ag nanorods or nanoparticulate precipitation depending upon the concentration of Ag to PEG. MW irradiation provided rapid and uniform heating of reagents, solvents, and intermediates,26–31 and this homogeneous MW heating also provided uniform nucleation and growth conditions, resulting in the formation of uniform nanomaterials with small sizes. Power dissipation was fairly uniform throughout with “deep” inside-out heating of the solvent, which leads to better crystallinity. It is clear from Figure 1 that Ag nanorods precipitation occurred only under MW conditions and the oil bath did not precipitate any Ag nanorods. The key to the formation of Ag nanorods or wires was simply dependent on the PEG concentration; at higher concentrations, nanoparticles formation was favored (Figure 3a), whereas at low concentrations, nanorods nucleation with nanoparticles was the major product (Figure 3b). However, by modifying the reaction mixture, it was possible to control bulk synthesis of Ag nanorods with various aspect ratios (Figure 4). For example, composition PG-4 yielded bulk morphology of Ag nanorods (Figure 4a). Similarly for PG-5 composition it was possible to control the bulk morphology with a reduction in the size aspect ratio of Ag nanorods. A proposed schematic illustration for the growth process of Ag nanorods and nanoparticles is shown in Scheme 1. Our results clearly demonstrate that the concentration of PEG controlled the final shape and size of the Ag nanostructures, which acts as a reducing agent as well as a capping agent. Once the concentration of Ag has decreased to a threshold value, the sample will start nucleate and grow into particles. The scheme implies that it is the concentration of PEG to AgNO3 that controls the Ag nanostructures growth pattern. Figure 5 shows a representative transmission electron microscopy (TEM) image obtained for Ag nanorods from PG-4 and its corresponding selected area electron diffraction (SAED) pattern. The thickness of nanorods varied from 50 to 200 nm with lengths up to several microns (Figure 5a). Figure 5b shows the SAED pattern obtained from a massive bundle of Ag nanowires. All of these diffraction rings could be indexed to face centered cubic Ag. Plasmon absorption spectroscopy is usually used to examine the size- and shape-controlled nanoparticles because of their
Bulk Synthesis of Ag and Fe Nanorods
Crystal Growth & Design, Vol. 8, No. 1, 2008 293 Scheme 1. Schematic illustrations of experimental mechanisms that generated Ag (a) nanoparticles, (b) nanorods, and (c) nucleated nanorods and nanoparticles
(Figure 6a), while the SPR peak of the longitudinal band for Ag nanorods and nanowires disappeared from the visible regime due to their aspect ratios of greater than five.25 It is important to mention here that the formation of Ag nanorods precipitation in the presence of MW takes place within a few minutes. For example, PG-4 yielded a mixture of nanorods and particulate morphology within 2 min of MW irradiation (Figure 6b). We believe to get uniform morphology it is necessary to carry out the reaction for an extended period of time as mentioned in the experimental section. Detailed studies are needed to understand the formation of particle morphology with reaction time. Furthermore, it is possible to reduce the size of the Ag nanorods using gold salts. For example, PG-9 composition can reduce the Ag nanorods thickness to length ratios dramatically (Figure 7). However, this strategy does not work with other noble metals such as Pd (PG-10) and completely alters the Ag morphology (Figure 8). The Ag nanorods can be used to make Pt nanocubes. In PG11, the PG-4 composition underwent metal displacement reaction with Pt salt to yield Pt nanocubes on the surface of Ag nanorods (see Figure 9).
Figure 4. SEM images of Ag nanorods synthesized via MW irradiation for 1 h using (a) PG-4, (b) PG-5, (c) PG-3, and (d) PG-2 compositions.
optical properties in aqueous suspensions related to this property.24 For example, the UV–visible spectrum for the colloidal solution of spherical Ag nanoparticles prepared shows a peak at approximately 408 nm. This broad extinction peak potentially results from inhomogeneity in morphology for the sample.24 However, the plasmon absorption peak of the nanorods showed a more complex absorption pattern due to the absorption of visible light both along the length of the nanorods (the longitudinal plasmon band) and along the width of the nanorods (the transverse plasmon band). As the aspect ratio increases, the longitudinal surface plasmon resonance (SPR) band (λ1) should be red-shifted significantly, whereas the transverse SPR (λT) band should only show a slight blue-shift.24 Figure 6a shows the UV–visible absorption spectra taken from the solutions that were used for the electron microscopy studies. As the shapes of Ag nanoparticles changed from sphere to rod and wire, the plasmon absorption peak for λT blue-shifted from 408 to 325
Figure 5. TEM images of Ag nanorods from (a) PG-4 under MW conditions, and (b) its SAED pattern obtained from a bundle of Ag nanorods randomly deposited on the TEM grid.
294 Crystal Growth & Design, Vol. 8, No. 1, 2008
Nadagouda and Varma
Figure 8. SEM image of Ag-Pd composite (PG-10) prepared using MW irradiation at 100 °C for 1 h.
Figure 9. TEM image of Pt nanocubes decorated on Ag nanorods by metal displacement reaction using PG-11 composition.
Figure 6. (A) UV spectra of (a) PG-8, (b) PG-4, and (c) PG-1, and (B) SEM image of mixture of Ag nanorods and particles prepared from PG-4 for 2 min under MW irradiation.
Figure 7. SEM image of Ag nanorods prepared via MW irradiation for 1 h using PG-9 composition.
The desired conditions for making Ag nanorods can be extended to prepare Fe nanorods. PG-12 composition yielded uniform Fe nanorods with micron sizes in length (see Figure 10). However, the reaction time taken for completion of Fe nanorods preparation was as low as 5 min. The obtained nanorods were uniform in size and crystallized in face centered cubic symmetry (see Figure 10b inset). Conclusions In summary, bulk synthesis of Ag nanorods can be achieved in PEG medium using MW irradiation. The nanorods formation depends upon the concentration of PEG used in the reaction with Ag salt. The method can be extended to make Fe nanorods
Figure 10. SEM images of Fe nanorods obtained from PG-12 composition under MW conditions. The inset shows the corresponding SAED pattern.
with uniform size and shape. The method uses no surfactant or reducing agent and is greener in nature, which could open a myriad of applications, such as energy storage system, capacitors, catalysis, fuel cell membranes, and nanodevices. Acknowledgment. M.N.N. was supported, in part, by the Postgraduate Research Program at the National Risk Management Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency between U.S. Department of Energy and the U.S. EPA. We are thankful to Cristina Bennet-Stamper for assistance with TEM alignment.
Bulk Synthesis of Ag and Fe Nanorods
Crystal Growth & Design, Vol. 8, No. 1, 2008 295
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