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Plasmonic Silver Nanobelts via Citrate Reduction in the Presence of HCl and their Orientation-Dependent Scattering Properties Zhiqiang Yang,† KhanhVan T. Nguyen,† Hongyu Chen,† Haijun Qian,‡ Lawrence P. Fernando,§ Kenneth A. Christensen,§ and Jeffrey N. Anker*,† †
Department of Chemistry, Center for Optical Materials Science and Engineering Technology (COMSET), and Environmental Toxicology Program, Hunter Laboratories, Clemson University, Clemson, South Carolina 29634, United States ‡ Electron Microscope Facility, Clemson University, Anderson, South Carolina 29625, United States § Department of Chemistry, Biosystems Research Complex (BRC), Clemson University, Clemson, South Carolina 29634, United States
bS Supporting Information ABSTRACT: Silver nanobelts were synthesized without any external seeds or templates using a facile hydrothermal route via citrate reduction in the presence of hydrochloric acid. Both H+ and Cl ions in HCl are important for the generation of nanobelts. The morphology can be controlled by varying the concentration of HCl and citrate. The obtained nanobelts have 3D orientation-dependent optical properties. By tracking the transportation and rotation of the nanobelts in cells or other biosystems, useful information related to the physical or chemical surroundings may be obtained. SECTION: Nanoparticles and Nanostructures
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anobelts or nanoribbons are special 1D nanostructures with nearly rectangular cross sections. Because of their unique shape-dependent electrical, mechanical, optical, and chemical properties, nanobelts have been synthesized from a variety of materials, such as metals (e.g., gold1 and copper2), metal oxides (e.g., tin dioxide3 and titania4), and metal chalcogenides (e.g., ZnSe5 and CdS6). Silver is a particularly attractive material for electronic and optical applications. Bulk silver has the highest thermal and electrical conductivities among all metals. Furthermore, as a result of localized surface plasmon resonance (LSPR), nanoscale silver particles strongly absorb and scatter light, making them excellent candidates for ultrabright nonbleaching nanolabels and nanosensors.7 One-dimensional silver nanostructures have been prepared and have shown applications in plasmonic waveguiding,8 surface-enhanced Raman spectroscopy (SERS),9 biological/chemical sensing,10 and catalysis.11 Various methods have been exploited to prepare 1D silver nanostructures via wet chemistry. For example, Xia and coworkers have studied the polyol route to prepare Ag nanowires with pentagonal cross section in the presence of poly(vinyl pyrrolidone) (PVP).12 Murphy and coworkers reported the generation of Ag nanowires via citrate reduction by adding NaOH to control pH,13 and Yang and coworkers extended this principle using a simpler method without pH adjustment.14 However, there are very few reports on preparation of Ag nanobelts from solution without using templates. Sun and coworkers generated a mixture of silver nanobelts (5% in yield) and nanoplates by refluxing aqueous suspensions of small spherical r 2011 American Chemical Society
silver nanoparticles.15 Qi et al. synthesized silver nanobelts by using ascorbic acid as the reducing agent with the help of poly(acrylic acid) at 4 °C for 48 h.16 Therefore, a novel and simple route to synthesize silver nanobelts with high yield would be of great value. Herein, we report a facile hydrothermal route to synthesize silver nanobelts using only silver nitrate, sodium citrate, and hydrochloric acid without any external seeds or templates. In this system, sodium citrate is the reducing agent and the only capping agent to direct the anisotropic growth. Hydrochloric acid is applied to control the nucleation and the growth rate. The hydrothermal reactions were carried out at mild temperature 120 °C for 24 h. A high yield of silver nanobelts was obtained without postreaction separation and purification. Furthermore, the Ag nanobelts showed orientation-dependent scattering intensity and color when viewed in dark-field microscopy. Figure 1a shows a typical low-magnified SEM image of the sample obtained using the following experimental conditions: [Ag+] = 0.2 mM, molar ratio of Ag+ to citrate = 1:1, HCl solution (2 N) 12 μL, 120 °C, 24 h in 100 mL of solution. (More experimental details are given in the Supporting Information.) It can be observed that the morphology of the product is wire-like with a length up to tens of micrometers. The wire-like shape is highly dominant, although a small amount of quasi-spherical Received: June 6, 2011 Accepted: June 28, 2011 Published: June 28, 2011 1742
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Figure 1. SEM images of Ag nanobelts with (a) low and (b) high magnification. (c) XRD pattern of silver nanobelts. The diffraction peaks were assigned according to the standard fcc Ag pattern (JCPDS file no. 04-0783). The inset shows an EDX pattern of silver nanobelts. The signals for C and Cu are due to the TEM grids (carbon-coated Formvar copper grids). (d) UV/vis spectrum of Ag nanobelts. The broad peak centered at 392 nm is attributed to the transverse plasmon absorbance of Ag nanobelts. (e) HRTEM image of a single belt indicating the lattice spacing and the growth direction. (f) Crosssectional TEM image of Ag nanobelts.
particles can also be seen (Figure 1a,b). The chemical composition of the nanowires was examined with powder X-ray diffraction (XRD) and energy-dispersive X-ray (EDX). The XRD pattern indicates face-centered cubic (fcc) phase of silver (JCPDS file no. 040783) with no impurities such as AgCl (Figure 1c). The EDX pattern also shows Ag peaks as well as Cu and C peaks from the TEM grid (Figure 1c inset). The UV/vis spectrum shows a broad peak centered at 392 nm, which is attributed to the transverse plasmon absorbance of Ag nanobelts (Figure 1d). The SEM image with higher magnification (Figure 1b) indicates that the width of the nanostructures is (212 ( 47 nm, based on the measurement of 100 nanowires). Figure 1e shows the HRTEM image of a single nanowire. The asmarked lattice spacing is 0.25 nm, which corresponds to the 1/ 3{422} reflection of Ag. By angle relationship, it suggests that the longitudinal direction of the wire may be [110], the same as reported by other researches.15,16 During close-up SEM observation, we found the 1D nanostructures were actually nanobelts. (See Figure S1 of the Supporting Information.) To indicate clearly this feature, a cross-sectional specimen was prepared and viewed by TEM. A typical TEM image (Figure 1f) shows rectangle-like cross sections with thickness from 60 to 120 nm, and width to thickness ratio at 2 to 3. In this stage, the detailed
Figure 2. SEM images when adding different amounts of HCl: (a) 0, (b) 10, (c) 14, and (d) 16 μL. All images have the same magnification.
formation mechanism of the nanobelts is unknown. According to the literature, small spherical Ag nanoparticles could be transferred to Ag nanoplates and nanobelts after heat treatment.15 So the possible shape evolution in our hydrothermal experiments 1743
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The Journal of Physical Chemistry Letters may be similarly related to the formation of initial Ag nuclei, the change from small Ag nanoparticles to Ag nanoplates, and then the further growth of Ag nanoplates to nanobelts. The amount of HCl added to the solution before the hydrothermal reaction greatly influenced the sample’s morphology. Figure 2 shows SEM images of the products obtained when adding 0, 10, 14, 16 μL of HCl solution (2 N) creating a solution with a room-temperature pH of 7.06, 5.37, 5.05, 4.94, and 4.78, respectively. Without adding HCl, only aggregated irregular particles were obtained (Figure 2a). When 10 μL of HCl was added, nanobelts were generated with a relatively broad width distribution, along with a noticeable percentage of irregular particles (Figure 2b). When using 12 μL of HCl, silver nanobelts were generated with high quality. (See Figure 1a,b). Further increasing the amount of HCl to 14 μL resulted in thinner nanobelts with an average width of ∼90 nm (Figure 2c). Therefore, it is possible to adjust the width of the nanobelts (e.g., from 210 to 90 nm) by controlling the amount of HCl added. However, when an even higher concentration of HCl was used, 16 μL, a mixture of microsized cubes and nanobelts was obtained (Figure 2d). According to the EDX analysis, those cubes were attributed to unreacted AgCl. (See Figure S2 of the Supporting Information.) The presence of unreacted reagents indicates an incomplete reaction, implying a greatly reduced reaction rate. The influence of HCl on the final morphology can be explained as follows. (1) Because of the low solubility of AgCl (Ksp = 1.8 1010 at room temperature and 1.0 107 at 120 °C) and the complexation reactions of Ag+ and Cl ions at elevated temperatures,17 the concentration of free Ag+ ions in the solution decreases after adding HCl, which slows down the nucleation process. As the reaction progresses, Ag+ ions are gradually released to the solution, so the initially formed Ag nuclei grow at a relatively slow rate. (2) The increasing HCl concentration decreases the pH, which in turn may reduce the reaction rate of Ag+ with citrate.18 The reaction equation of Ag+ with citrate is given below.
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Figure 3. (a) SEM image of the sample when using the same molarity of HNO3 instead of HCl. (b) SEM image of the sample when using the same molarity of NaCl instead of HCl. (c) Schematic plot indicating the importance of both Cl and H+ ions.
C6 H5 O7 3 þ 2Agþ f C5 H4 O5 2 þ Hþ þ CO2 þ 2Ag0 ðC5 H4 O5 2 ¼ OOCCH2 COCH2 COO Þ To study the influence of both Cl and H+ ions, we performed the reactions by replacing HCl (12 μL of 2 N solution) with the same molar concentration of HNO3 or NaCl. In the case of HNO3 addition, only small particles and a low percentage of 1D structures were generated, which demonstrates the importance of Cl (Figure 3a). Also, using NaCl instead of HCl resulted in products with poor quality, demonstrating the importance of H+ (Figure 3b). A schematic plot indicating the significance of both Cl and H+ ions for the morphology development is given in Figure 3c. As both a reducing agent and a capping agent, citrate plays important roles for the generation of Ag nanobelts. The influence of the citrate concentration on the morphology is shown in Figure 4. In those experiments, the molar ratio of Ag+ to citrate was changed while keeping the Ag+ concentration constant. Other experimental conditions were set up unchanged according to the sample in Figure 1. At the molar ratio 1:0.5, no noticeable reaction occurred due to the small amount of citrate. When the ratio changed from 1:1 (Figure 4a) to 1:10 (Figure 4b), the average width of the nanobelts changed from ca. 212 nm to ca. 90 nm. At 1:20 ratio, the average width of the nanobelts further decreased to ∼70 nm (Figure 4c). Also, the shrinkage of the
Figure 4. SEM images of Ag samples obtained by changing the citrate concentration while holding other conditions the same. The molar ratio of Ag+ to citrate is: (a) 1:1, (b) 1:10, (c) 1:20, and (d) 1:100, respectively. All images have the same magnification.
length is noticeable. Further increasing citrate concentration resulted in small quasi-spherical particles as the main product (Figure 4d). This morphology-changing trend can be attributed to the enhanced reaction rate and the improved protecting ability when more citrate was applied. The results also provide another facile morphology-controlling strategy besides the adjustable HCl addition. Nowadays, the potential toxicity of nanoparticles has been receiving increasing attention. Particles with high aspect ratio can be especially toxic (e.g., asbestos fibers), and studying their internal motions can help elucidate the toxic mechanisms. Intracellular rotation of spherical iron oxide microspheres in macrophages has been shown to be dependent on cytoskeletal integrity, and the rotation rate decreases when the cells are exposed to toxic nanoparticles such as asbestos fibers and microglass fibers.19 However, the rotation of the fibers themselves has not been studied. The silver nanobelts we synthesized above have different LSPR modes for light polarized along the 1744
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carefully analyzing these rotation rates during phagocytosis and intracellular transport. In addition, the nanobelts may have useful electronic, thermal, and mechanical applications, which can benefit from large, flat, low resistance areas of contact.
’ ASSOCIATED CONTENT
bS Figure 5. Darkfield photographs of silver nanobelt reorienting, blinking and changing colors in water due to Brownian motion (a,b) and in macrophage cell due to biomechanical transport (c,d).
different cross-section axes, enabling roll orientation to be determined from the resonant scattering color and intensity in dark-field microscopy. Because the nanobelts are tens of micrometers long, well above the diffraction limit, the orientation of the long axis is also evident from the shape of the particles in dark-field microscopy. A paintbrush was wet with a suspension of nanobelts and applied to a glass slide with even strokes to orient the belts in the approximate direction of the stroke. The particles were then viewed with unpolarized light as well as linearly polarized light either parallel or perpendicular to the belt axis (Figure S3 of the Supporting Information). When the light polarization was along the short axis of the belts (Figure S3b of the Supporting Information), many of the belts looked blue because of excitation of a transverse plasmon; when the polarizer was oriented parallel to the intermediate or long axis of the belts, all particles looked gray because the LSPR broadens when the axis becomes significant compared with the wavelength of light due to higher order Mie scattering terms (Figures S3c and S4 of the Supporting Information). In a water suspension, the nanobelts spontaneously rotate because of Brownian motion and appear to blink when rotating. The belt strongly scatters blue light when oriented with the short axis parallel to the incident light polarization (Figure 5a) and appears to be a dim gray color when oriented orthogonal to the incident light polarization (Figure 5b). The blinking and colorchanging were also observed in J774 mouse microphages due to intracellular transport after incubation with the Ag nanobelts (Figure 5c,d and Figure S5 of the Supporting Information). In addition to observing roll from the color/intensity changes, we can determine pitch and yaw from the angle and length of the long axis. By taking a series of images in time, we can measure the rotation rates along all three axes. These rotation rates provide useful information related to the physical or chemical surroundings, for example, the local viscosity, with slower rotation in more viscous samples.20 Future work involves calculating the autocorrelation of the fluctuating optical intensity to determine rotation rates from Brownian and intracellular transport motion in cells. These measurements will be important for monitoring disruption in transport and phagocytosis due to damage of the actin cytoskeleton from reactive oxygen species generation on the nanoparticle surface. In summary, we present a facile method to prepare silver nanobelts using only citrate, AgNO3, and HCl. At certain concentration of HCl, the particles have a nearly rectangular cross-section with an aspect ratio of 2 to 3 and length up to tens of micrometers. Both H+ and Cl ions in HCl are significant for the nanobelt generation. The morphology can be controlled by adjusting the addition of HCl and citrate. The noncircular crosssection provides an interesting opportunity to study orientationdependent properties such as scattering. Future work involves
Supporting Information. Experimental details and additional characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by a startup package from Clemson University to J.N.A. and a fellowship from the Vietnam Education Foundation to K.V.T.N. ’ REFERENCES (1) Zhang, J.; Du, J.; Han, B.; Liu, Z.; Jiang, T.; Zhang, Z. Sonochemical Formation of Single-Crystalline Gold Nanobelts. Angew. Chem. 2006, 118, 1134–1137. (2) Huang, T.; Cheng, T.; Yen, M.; Hsiao, W.; Wang, L.; Chen, F.; Kai, J.; Lee, C.; Chiu, H. Growth of Cu Nanobelt and Ag Belt-Like Materials by Surfactant-Assisted Galvanic Reductions. Langmuir 2007, 23, 5722–5726. (3) Lu, N.; Wan, Q.; Zhu, J. Surface Structure of Zigzag SnO2 Nanobelts. J. Phys. Chem. Lett. 2010, 1, 1468–1471. (4) Wang, D.; Zhao, H.; Wu, N.; Khakani, M.; Ma, D. Tuning the Charge-Transfer Property of PbS-Quantum Dot/TiO2-Nanobelt Nanohybrids via Quantum Confinement. J. Phys. Chem. Lett. 2010, 1, 1030–1035. (5) Liu, S. Y.; Choy, W. C. H.; Jin, L.; Leung, Y. P.; Zheng, G. P.; Wang, J.; Soh, A. K. Triple-Crystal Zinc Selenide Nanobelts. J. Phys. Chem. C 2007, 111, 9055–9059. (6) Zhang, J.; Jiang, F.; Zhang, L. Fabrication of Single-Crystalline Semiconductor CdS Nanobelts by Vapor Transport. J. Phys. Chem. B 2004, 108, 7002–7005. (7) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7, 442–453. (8) Li, Z.; Hao, F.; Huang, Y.; Fang, Y.; Nordlander, P.; Xu, H. Directional Light Emission from Propagating Surface Plasmons of Silver Nanowires. Nano Lett. 2009, 9, 4383–4386. (9) Tao, A. R.; Yang, P. Polarized Surface-Enhanced Raman Spectroscopy on Coupled Metallic Nanowires. J. Phys. Chem. B 2005, 109, 15687–15690. (10) Lu, J.; Yang, L.; Xie, A.; Shen, Y. DNA-Templated PhotoInduced Silver Nanowires: Fabrication and Use in Detection of Relative Humidity. Biophys. Chem. 2009, 145, 91–97. (11) Christopher, P.; Linic, S. Engineering Selectivity in Heterogeneous Catalysis: Ag Nanowires as Selective Ethylene Epoxidation Catalysts. J. Am. Chem. Soc. 2008, 130, 11264–11265. (12) Korte, K. E.; Skrabalak, S. E.; Xia, Y. Rapid Synthesis of Silver Nanowires through a CuCl- or CuCl2-Mediated Polyol Process. J. Mater. Chem. 2008, 18, 437–441. (13) Caswell, K. K.; Bender, C. M.; Murphy, C. J. Seedless, Surfactantless Wet Chemical Synthesis of Silver Nanowires. Nano Lett. 2003, 3, 667–669. (14) Yang, Z.; Qian, H.; Chen, H.; Anker, J. N. One-Pot Hydrothermal Synthesis of Silver Nanowires via Citrate Reduction. J. Colloid Interface Sci. 2010, 352, 285–291. 1745
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