Fabrication of Silver Nanorods Controlled by a Segmented Copolymer

A segmented copolymer of waterborne polyurethane (WPU) was used for the first time to synthesize silver nanostructures in the indoor circumstance. The...
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J. Phys. Chem. C 2007, 111, 13673-13678

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Fabrication of Silver Nanorods Controlled by a Segmented Copolymer Qiang Shen,*,† Jinglun Sun,† Hao Wei,‡ Yong Zhou,‡ Yunlan Su,‡ and Dujin Wang§,‡ Key Laboratory for Colloid & Interface Chemistry of Education Ministry, School of Chemistry & Chemical Engineering, Shandong UniVersity, Jinan 250100, China, and State Key Laboratory of Polymer Physics & Chemistry, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100080, China ReceiVed: May 30, 2007

A segmented copolymer of waterborne polyurethane (WPU) was used for the first time to synthesize silver nanostructures in the indoor circumstance. The soft segment of WPU served as a reducing agent of silver ions, whereas WPU hard segments could act as templates for the synthesis of silver nanorods. Silver nanorods and the simultaneously formed nanoparticles were characterized by the combination of scanning electron microscopy (SEM), powder X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and UV-visible spectroscopy, indicating the kinetically controlling mechanism of WPU. However, these imply that a soft, solution-phase approach based on the segmented copolymer represents a simple, novel route to the synthesis of Ag nanorods.

1. Introduction Synthesis and characterization of one-dimensional (1D) nanostructures (wires, rods, belts, and tubes) are the basic efforts on contributing to the fabrication of nanosized devices. The nanostructures of noble metals, such as silver, are of great interest because of their superior electrical, optical, mechanical, and catalytic properties.1,2 In recent years, numerous methods, such as the hydrothermal,3 hard template,4 and soft template of micelles,5 for preparation of 1D Ag nanostructures have been reported in literature. The hydrothermal method using poly(vinylpyrrolidone) (PVP) as an adsorption agent and an architecture soft template is one of the famous results for the synthesis of silver nanowires with the fivefold twinning cross section.3 Because PVP macromolecules interact more strongly with the [100] planes than with the [111] planes of silver, the side surface of Ag nanowires bounded by [100] facets were completely passivated by PVP, leaving only the reactive ends bounded by [111] facets remaining.3a,6 Recently, DNA has been used as an ideal candidate for assisting the bottom-up assembly of nanostructures because of its complexation with silver ions and its self-organized structures.7 However, it is because of the unique selectivity and different catalytic performance for different crystal faces2a,2b that the fabrication of silver nanostructures with specific sizes, structures, and morphologies has still been the focus of intensive research. The segmented copolymer of waterborne polyurethane (WPU), as a nontoxic and nonflammable material, has been used abundantly as an environmental coating and as an adhesive. In recent years, it has been endowed with excellent properties.8 WPU hard segments were formed by the reaction of a lowmolecular-weight difunctional compound (2, 2-bishydroxymethyl-propionic acid, DMPA) with excess toluene dissocyanate * Corresponding author. Address: Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Ji’nan 250100, China. Fax: +86-531-88564750. E-mail: [email protected]. † Key Laboratory for Colloid and Interface Chemistry of Education Ministry. ‡ State Key Laboratory of Polymer Physics & Chemistry. § Additional corresponding author. E-mail: [email protected].

(TDI), whereas the soft segments are polyethylene glycol (PEG). In our previous work, these synthesized WPUs were used as organic additives in the precipitation processes of calcium carbonate.9 Interestingly, the hard-segmented copolymer could act as a rod-like template to induce the formation of pineconeshaped aggregates of calcite with high aspect ratios because of the binding effectiveness of calcite nuclei onto the template surfaces. When the 50% molar ratio of soft segments (PEGs) was introduced in the polymer backbone through cross-linking copolymerization, the resulting target compound of WPU was used herein for the fabrication of silver nanorods with large aspect ratios. This is based on the possibility that the soft segments of WPU could act as the reducing agent of silver ions, and the hard segments should play a key role in directing the 1D growth of silver nanorods. 2. Experimental Section Materials and Sample Preparation. The segmented copolymer of waterborne polyurethane (WPU) was synthesized according to the procedure described in refs 8a and 10, and its structural formula was proven to coincide with the theoretical value.9 The other chemicals were of A. R. grade and were used without further purification. Doubly deionized water was used to prepare aqueous solutions throughout the experiments. A solution containing 5.0 mL of 1.0 × 104 ppm WPU (pH ≈ 3.52) was prepared first. Then, 20 mL of 125 mM AgNO3 was added and sonicated for 5 min, which gave the final WPU concentration of 2000 ppm and the final AgNO3 concentration of 100 mM. Finally the reaction solution (pH ≈ 3.89) was sealed in the glass bottle and was allowed to stand for a period of time depending upon the measurement of precipitates, at room temperature. Structural Characterization of WPU Aggregates. First, a known volume of pyrene in methanol was put into a test tube; then, after the evaporation of solvent, an aqueous solution of WPU was added into the test tube. The final concentration of pyrene was 1.0 × 10-6 M, and the final concentration of WPU ranged from 50 to 8000 ppm. When these solutions were sonicated for 10 min in an ultrasonic bath, the fluorescence

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13674 J. Phys. Chem. C, Vol. 111, No. 37, 2007 spectra of pyrene were measured at room temperature by means of the Hitachi F-4500 fluorescence spectrophotometer. A narrow bandwidth of 1 nm was used, and a pyrene excitation wavelength of 335 nm was selected. The intensity ratio I3/I1 of the third (385 nm) to the first (374 nm) vibronic peaks in the emission spectra of the solubilized monomeric pyrene was calculated, which has been used as the microenvironment polarity parameter to indicate the formation of hydrophobic cores.11 The self-assembly behavior of WPU in aqueous systems was also conducted by the light-scattering method. The measurements for the sizes and size distribution of WPU aggregates were performed on a DAWN HELEOS instrument equipped with a 60 mw GaAs laser at λ ) 690 nm, with a scattering angle fixed at 90°. Data analysis and size distribution were performed using the DYNALS regularization algorithm and a multi-τ digital recorder (WyattQELS). Finally, the self-assembly behavior of WPU in aqueous systems was carried out by means of freeze fracture replication electron microscopy (FF-TEM). The aqueous solution of 1000 ppm WPU was frozen in liquid nitrogen and was then fractured in a freeze-etch unit (model BAF 400D, Balzers). Prior to being viewed in a transmission electron microscope (Hitachi H-800, 200 kV), the sample was rotary-shadowed at a 45° angle with platinum-carbon and at a 90° angle with carbon. Characterization of Ag Nanostructures. The precipitated products were deposited directly on glass slides and dried in air prior to being characterized by SEM (Hitachi S-4300, field emission, 15 kV) and XRD (Rigaku D/max-2400, Cu KR radiation) methods. The suspended products were deposited directly on a carbon film supported by a copper grid used for the TEM (Hitachi H-800, 200 kV) and HRTEM (Philips Tecnai 20U-Twin, 200 kV) measurements. The diameters of the Ag nanorods and their size distribution were calculated by using an image analysis program (Scion Image-PC version). The optical properties of the reaction solutions were taken on a HP 8453E UV-vis spectrometer with the resolution of 1 nm. 3. Results and Discussion 3.1. Formation of the WPU Self-Assemblies. It has been reported that the analogous waterborne polyurethane, composed of the polyether soft segments and the dissocyanate-based hard segments, can be characterized by a two-phase morphology.12 It is the immiscible phenomenon between the hard and soft phases that leads to the formation of a hard-segmented domain and a soft-segmented matrix. When hydrophobic segments were introduced into the polyurethane molecular backbone,13 the resulting amphiphilic molecules can self-assemble into micelles because of the hydrophobic-hydrophilic interaction. Because the electrostatic interaction between the hydrophilic segments related well to the charge of the pendent head groups, the formation of the polyurethane micelles also depended upon the pH value of the aqueous medium.13 Herein, both the soft segments and hard segments in the synthesized WPU backbone are hydrophilic; however, the phase segregation mode could also be used to explain the formation of WPU aggregates.12 The self-assembly behaviors of WPU at various concentrations were performed by using the fluorescent and light-scattering methods, shown in Figure 1. Generally, the higher polarity of the microenvironment where pyrene was solubilized, the smaller I3/I1 value. For example, the I3/I1 values of pyrene in cyclohexane, isopropyl ether, ethanol, methanol, and water are 1.72, 1.08, 0.87, 0.73, and 0.55, respectively.13b The pH value of the aqueous medium decreased

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Figure 1. (A) Variation of the I3/I1 value with WPU concentration for the fluorescence spectroscopy of monomeric pyrene in aqueous WPU solutions. (B) Dynamic light-scattering analysis for the hydrodynamic radii of the self-assembled aggregates in the aqueous solutions of 2000 ppm WPU.

gradually from 6.19 (50 ppm WPU) to 3.57 (8000 ppm WPU); the results in Figure 1A indicate that weak polar domains formed when WPU concentration was higher than 1000 ppm. Although the pH value of WPU dispersions was adjusted to ∼9.35 by the addition of 0.10 M NaOH solution, the obtained I3/I1 values (0.61 ( 0.10) did not change with WPU concentration. These results suggest the immiscible characterization of the PEG soft segment and the hard segment with pendent -COOH groups in acidic medium. Figure 1B shows the size distribution of the self-assembled aggregates in the aqueous solution of 2000 ppm WPU. The dynamic light-scattering analysis (Figure 1B) indicates that the WPU dispersion comprises two populations of aggregates with an average hydrodynamic radius (Rh) of ∼9.5 nm. When WPU concentrations were fixed at 200, 400, 800, 1000, and 4000 ppm, the corresponding average Rh values were 57.7, 45.6, 37.6, 35.0, and 7.3 nm, respectively. Although these size distributions were polydisperse, the average Rh value decreased with increasing WPU concentration. The break point occurred at a concentration between 1000 and 2000 ppm, suggesting that the main aggregation form of WPU changed from the intermolecular to the intramolecular. In our previous works,9 WPU (therein, Polymer III) had ever been used as the crystal modifier of calcium carbonate. And, the resulting nanocrystals could aggregate to form two classes of vaterite particles, that is, the ∼2.0 µm spheres and the ∼800 nm discoids. This might be used to prove the results illustrated in Figure 1B. Figure 2A presents the FF-TEM micrograph of the aqueous solution containing 1000 ppm WPU. The WPU molecules self-organized in aqueous system, forming a fiberlike structure with a branching and curling morphology (Figure

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Figure 2. (A) FF-TEM micrograph of the aqueous solution of 1000 ppm WPU. (B) Schematic illustration of the self-organized structures of WPU molecules.

2A). By the above results listed, the intramolecular and intermolecular aggregates of WPU could be shown schematically in Figure 2B. 3.2. Fabrication of Ag Nanorods. The reduction of silver nitrate in the presence of WPU molecules was conducted without the addition of reductant, seed, or alkali.6,14 It should be mentioned that, when the AgNO3 solution (100 mM) was sealed in a glass bottle and was allowed to stand for one week, the bulk phase was absolutely clear and no precipitation was observed. Herein, the carboxylate groups in WPU hard segments could coordinate with silver ions, whereas the WPU soft segments of oxyethylene groups were believed to reduce Ag+ ions.5b,15 If the dispersed WPU in aqueous solutions had the aggregation phenomena of phase separation (Figure 2B), then the properties of WPU molecules and their aggregates might be used to examine the argumentative controlling mechanism of metal anisotropic colloids.16 Figure 3A presents the SEM image of silver nanostructures, showing the coexistence of nanorods and nanoparticles after one week of aging at room temperature. The nanorods have a large aspect ratio, with the mean diameter being ∼73 ( 12 nm and the length being ∼1.4 ( 0.4 µm. Part of the rods exhibits curved (or, distorted) morphology (Figure 3A). The crystalline nature of the Ag nanostructures can be partly revealed by the corresponding X-ray diffraction pattern (Figure 3B). The nether diffraction lines in Figure 3B show the standard position and the relative intensities of the synthesized silver with facecentered cubic ( fcc) form (a ) 4.086 Å, JCPDS File 04-0783). The coincidence of the XRD spectrum (Figure 3B) with its nether lines suggests that the synthesized silver exhibits the fcc structure with a calculated lattice constant of ∼4.075 Å. In Figure 3B, the ratio of intensity between [111] and [200] peaks exhibits a relatively high value of 3.6, whereas the theoretical ratio is only 2.5. This means that, when the precipitate was directly deposited for XRD measurements, the Ag nanorods might simply lay down with the [111] planes parallel to the glass surface. Figure 4 shows the typical TEM and HRTEM characterization of silver nanorods obtained at early stage of the rod growth process. The Ag nanorods shown in Figure 4A have 9.0 ( 3.7

Figure 3. SEM image (A) and X-ray diffraction data (B) of Ag nanostructures obtained after one week of aging in the indoor circumstance.

nm in diameter and 169 ( 33 nm in length. Figure 4B illustrates that part of these rods have no uniform size in diameter, especially for the ends. The further magnified TEM pictures (Figure 4C and the inset in it) show that parts of these rods have a ridge in the middle and along the longitudinal axis. Although a HRTEM image taken the side edge of a rod from the top (Figure 4D) and the corresponding Fourier Transform pattern (the inset in Figure 4D) clearly show the lattice fringes for single-crystalline silver, the ridged characteristics (Figure 4C) suggest that the nanorods should possess a multiply twinned structure. By focusing the electron beam on the side surfaces of silver nanorods, both the [111] (Figure 4E) and [100] (Figure 4F) planes of fcc silver could be detected. The typical selected area electron diffraction (SAED) patterns (Figure 4E and F) further suggest that even the seemingly tabular silvers are the faceted rods. For the fcc metals, the stability of crystalline faces decreases in order [111] < [100] < [110]. So, the growth rate of [110] facets should be much faster than those of the other two facets. This possibly induced the 1D growth along [110], generating the nanorods with the side surfaces of [111] and [100]. Figure 5 shows the triangular and hexagonal nanoplates of silver sampled at the aging time of 25 h. The SAED pattern (the inset in Figure 5) taken perpendicular to the nanoplate corresponds to the [111]-type crystal face, indicating the strong binding capability of WPU hard segments on active planes. According to Sigmund’s opinion,16 the internal structure and growth pattern of the silver 2D particles closely follow the silver halide model. The initial formation of Ag nuclei, followed by the slow generation of adatoms, caused the formation of platelets. Then, the changing of the silver supersaturation in solution encouraged the 1D growth of twinned particles. 3.3. Formation Mechanism of Ag Nanorods. It is generally accepted that neither the physical-constriction nor the surfacemodification mechanisms could be used to explain the simultaneous formation of nanorods and nanoparticles in a homoge-

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Figure 4. TEM image (A) and the magnified pictures (B and C) of the growing Ag nanorods obtained after 23 h of aging at room temperature. The arrows and the inset in panel C indicate that parts of the nanorods have a ridge along the longitudinal axis. (D) HRTEM image taken on the side surface of Ag nanorods; the inset is the corresponding Fourier-Transform pattern. The typical SAED patterns (E and F) of Ag nanorods show that both the [111] (E) and [100] (F) zones could be detected on the side surfaces.

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Figure 6. (A) UV-visible absorption spectra of the reaction system recorded at the aging time of 2 (a), 26 (b), and 51 h (c), respectively. (B) SEM images of Ag nanostructures sampled at the aging time of 120 h.

Figure 5. (A) TEM images of Ag nanoplates sampled at 25-h incubation time. The inset is a representative SAED pattern taken perpendicular to the main faces of the platelets.

neous reaction system.16 To prove the kinetically directing mechanism of WPU, we measured the optical properties of the reaction solutions and show them in Figure 6A. Actually, there is no UV-visible absorption for the aqueous solution of 2000 ppm WPU during the wavelength range from 300 to 800 nm. For the reaction system, as well as the “pure” aqueous solution of 100 mM AgNO3 (data omitted), the UVvisible absorption spectra (Figure 6A) showed one adsorption peak at 302 nm, indicating the formation of metallic silver under ultraviolet irradiation. When the reduction reaction proceeded from 2 (curve a in Figure 6A) to 51 h (curve c in Figure 6A), the absorbance peak at 302 nm gradually experienced a large increase in intensity and a slight blue-shift in energy. These also suggest the gradual reduction of silver ions and the consequent binding of metallic silver in the bulk phase.7a,17 As

Figure 7. TEM images of Ag nanostructures sampled at the various incubation times of 7 (A), 11 (B and C), and 120 h (D), respectively.

the reaction proceeded, one shoulder peak at 384 nm appeared due to the formation of nanorods (curve b in Figure 6A).3a According to Xia’s opinion, when the dispersed Ag nanorods accumulate to a critical concentration, another shoulder peak

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Figure 8. Schematic illustration of the growth process of Ag nanorods.

at ∼353 nm appeared in curve c of Figure 6A.3a At 51 h (curve c in Figure 6A), the higher absorption band at 420 nm could be assigned to the dispersed Ag nanoparticles.17 In fact, without the irradiation of UV light both nanoparticles and nanorods could be obtained, shown in Figure 6B. The relatively high peak intensity of 420 nm compared to that of 353 nm (curve c in Figure 6A) indicates that the major nanoparticles and the minor nanorods are due to the UV-light acceleration reduction of silver ions. So, we infer that both the reduction rate of the silver ions and the growth rate of the silver nanorods should be considered for the formation of silver nanorods.18 The time-course experimental results for the formation process of Ag nanorods in the indoor circumstance are shown in Figure 7. Without the irradiation of UV light, the functional groups of WPU hard segments could act as nucleation sites. The slowly deoxidized Ag+ ions by WPU soft segments favored the growth of Ag0 nuclei, generating the nanoparticles with size polydispersity (Figure 7A). Figure 7B showed that at the aging time of 11 h for average-sized nanoparticles was bigger than that of particles sampled previously. Here, we would like to cite the synthesis of Ag nanorods on the λ-DNA network:7b by adjusting the concentration of DNA and reduction time, the diameter and aspect ratio of Ag nanorods can be controlled effectively. According to Li’s results,7b if the WPU hardsegmented domain functionalized as a substrate for the absorption of Ag+ ions, then it should act as an organic precursor for the fabrication of nanorods.7b Interestingly, at the aging time of 11 h, parts of the Ag nanoparticles exhibited the embedded fibers, one of which was magnified and inserted in Figure 7B. Simultaneously, the nanoparticles with the relatively high aspect ratios were also obtained (Figure 7C). These indicate the controllable effectiveness of WPU hard-segmented domains on the 1D growth of Ag nanoparticles. So, the grown Ag nanorods could be observed at the aging time of 120 h (Figure 7D). It should also be emphasized that the edge dislocations of nanorods might be due to the further imperfect oriented attachment.19 According to Liz-Marza´n’s results,15 ethoxylated surfactants were proven to induce the formation of Ag nanoparticles and to stabilize these particles in the bulk phase for weeks; nevertheless, no anisotropic particles was observed. Alternatively, for the reaction systems of 2000 ppm WPU + 100 mM AgNO3, it is the introduction of functional hard segments in the polymer backbone that led to the anisotropic 1D growth of Ag nanorods at room temperature. The evolution process of Ag nanorods is illustrated schematically in Figure 8. From Figure 8, the one-week formation process could be divided into the following five stages: (1) the Ag+-WPU complexes were formed; (2) Ag+ ions were partly reduced by WPU soft segments to form the metallic silver bound onto the surfaces of WPU hard segments; (3) the reduction of Ag+ ions and the adsorption of metallic silver continued to form nanoparticles; (4) the growth of Ag nanoparticles would come to form the youthful rods possessing multiple facets; and (5) the 1D growth

continued to form mature nanorods with large aspect ratios, showing the distorted morphology. 3. Conclusions In summary, the wet chemical method that utilizes a segmented copolymer of waterborne polyurethane for the synthesis of silver nanorods has been developed. The soft segments of PEGs served as the reducing agents of Ag+ ions, whereas the hard segments of (DMPD-TDI) could act as the substrates for the absorption of metallic silver and as the templates for the synthesis of Ag nanorods. In addition, the possible five-stage process was speculated to illustrate the formation mechanism of silver nanorods. Furthermore, the synthesis of Ag nanostructures was conducted by the direct mixing of aqueous WPU and AgNO3 solutions and by the silent deposition in the indoor circumstance. Aside from these, it does not need a seed, a reducing agent, an alkali, or any other organic additive, suggesting a very-simple method for the polymer-directed growth of Ag nanorods. Acknowledgment. The financial support from the National Natural Science Foundation of China (20471064) is gratefully acknowledged. References and Notes (1) (a) Bockrath, M.; Liang, W.; Bozovic, D.; Hafner, J. H.; Lieber, C. M.; Tinkham, M.; Park, H. Q. Science 2001, 291, 283. (b) Cui, Y.; Wei, Q. Q.; Park, H. Q.; Lieber, C. M. Science 2001, 293, 1289. (2) (a) Torres, D.; Lopez, N.; Illas, F.; Lamber, R. M. J. Am. Chem. Soc. 2005, 127, 10774. (b) Williams, F. J.; Bird, D. P. C.; Palermo, A.; Santra, A. K.; Lambert, R. M. J. Am. Chem. Soc. 2004, 126, 8509. (c) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R. R.; Sun, Y. G.; Xia, Y. N.; Yang, P. D. Nano Lett. 2005, 3, 1229. (3) (a) Sun, Y. G.; Yin, Y. D.; Mayers, B. T.; Herricks, T.; Xia, Y. N. Chem. Mater. 2002, 14, 4736. (b) Wei, G.; Nan, C. W.; Deng, Y.; Lin, Y. H. Chem. Mater. 2003, 15, 4436. (c) Zhang, S. H.; Jiang, Z. Y.; Xie, Z. X.; Xu, X.; Huang, R. B.; Zheng, L. S. J. Phys. Chem. B 2005, 109, 9416. (4) (a) Behrens, S.; Wu, J.; Habicht, W.; Unger, E. Chem. Mater. 2004, 16, 3085. (b) Zong, R. L.; Zhou, J.; Li, Q.; Du, B.; Li, B.; Fu, M.; Qi, X. W.; Li, L. T. J. Phys. Chem. B 2004, 108, 16713. (c) Wu, Y. Y.; Livneh, T.; Zhang, Y. X.; Cheng, G. S.; Wang, J.; Tang, J. F.; Moskovits, M.; Stucky, G. D. Nano Lett. 2004, 4, 2337. (5) (a) Jana, N. R.; Gearheart, H.; Murphy, C. J. Chem. Commun. 2001, 617. (b) Zhang, D. B.; Qi, L. M.; Ma, J. M.; Cheng, H. M. Chem. Mater. 2001, 13, 2753. (c) Zhang, J. L.; Liu, Z. M.; Han, B. X.; Jiang, T.; Wu, W. Z.; Chen, J.; Li, Z. H.; Liu, D. X. J. Phys. Chem. B 2004, 108, 2200. (6) (a) Sun, Y. G.; Mayers, B.; Herricks, T.; Xia, Y. N. Nano Lett. 2003, 3, 955. (b) Jiang, P.; Li, S.-Y.; Xie, S.-S.; Gao, Y.; Song, L. Chem.s Eur. J. 2004, 10, 4817. (7) (a) Berti, L.; Alessandrini, A.; Facci, P. J. Am. Chem. Soc. 2005, 127, 11216. (b) Wei, G.; Zhou, H. L.; Liu, Z. G.; Song, Y. H.; Wang, L.; Sun, L. L.; Li, Z. J. Phys. Chem. B 2005, 109, 8738. (8) (a) Liu, Z.; Wu, X.; Yang, X.; Liu, D.; Jun, C.; Sun, R.; Liu, X.; Li, F. Biomacromolecules 2005, 6, 1713. (b) Madbouly, S. A.; Otaigbe, J. U.; Nanda, A. K.; Wicks, D. A. Macromolecules 2005, 38, 4014. (9) Wei, H.; Shen, Q.; Wang, H. H.; Gao, Y. Y.; Zhao, Y.; Xu, D. F.; Wang, D. J. J. Cryst. Growth 2007, 303, 537. (10) Nomula, S.; Cooper, S. L. Macromolecules 2001, 34, 925. (11) (a) Lianos, P.; Viroit, M.-L.; Zana, R. J. Phys. Chem. 1984, 88, 1098. (b) Shen, Q.; Li, G. Z.; Huang, Y. Z.; Yie, J. P. Acta Phys.-Chem. Sin. 1999, 15, 216.

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