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  NANO LETTERS Crystalline Silver Nanowires by Soft Solution Processing Yugang Sun, Byron Gates, Brian Mayers, and Younan Xia* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 Received December 4, 2001 2002 Vol. 2, No. 2 165-168 ABSTRACT This paper describes a soft, solution-phase approach to the large-scale synthesis of uniform nanowires of bicrystalline silver whose lateral dimensions could be controlled in the range of 30−40 nm, and lengths up to ∼50 µm. The
  Crystalline Silver Nanowires by SoftSolution Processing Yugang Sun, Byron Gates, Brian Mayers, and Younan Xia*  Department of Chemistry, Uni V ersity of Washington, Seattle, Washington 98195-1700 Received December 4, 2001 ABSTRACT This paper describes a soft, solution-phase approach to the large-scale synthesis of uniform nanowires of bicrystalline silver whose lateraldimensions could be controlled in the range of 30 − 40 nm, and lengths up to ∼ 50 µ m. The first step of this procedure involved the formationof platinum nanoparticles by reducing PtCl 2 with ethylene glycol heated to ∼ 160 ° C. Due to their close match in crystal structure and latticeconstants, these platinum nanoparticles could serve as seeds for the heterogeneous nucleation and growth of silver that was produced in thesolution via the reduction of AgNO 3 with ethylene glycol. When surfactants such as poly(vinyl pyrrolidone) (PVP) were present in this solution,the silver could be directed to grow into uniform nanowires with aspect ratios as high as ∼ 1000. Measurements of transport property at roomtemperature indicated that these nanowires were electrically continuous with a conductivity of approximately 0.8 × 10 5 S/cm. One-dimensional (1D) nanostructures of metals play animportant role as both interconnects and active componentsin fabricating nanoscale electronic devices. 1 They alsoprovide an ideal model system to experimentally investigatephysical phenomena such as quantized conductance andlocalization effects. 2 Silver nanowires with well-defineddimensions are particularly interesting to synthesize and studybecause bulk silver exhibits the highest electrical (or thermal)conductivity among all metals. Silver has also been used ina rich variety of commercial applications, and the perfor-mance of silver in these applications could be potentiallyenhanced by processing silver into 1D nanostructures withwell-controlled dimensions and aspect ratios. For instance,the loading of silver in a polymeric composite could besignificantly reduced when nanoparticles of silver arereplaced by nanowires having higher aspect ratios. 3 A number of chemical approaches have been activelyexplored to process silver into 1D nanostructures. Forexample, silver nanowires have been synthesized by reducingAgNO 3 with a developer in the presence of AgBr nano-crystallites, 4 or by arc discharging between two silverelectrodes immersed in an aqueous NaNO 3 solution. 5 Silvernanorods have been produced by irradiating an aqueousAgNO 3 solution with ultraviolet light in the presence of poly-(vinyl alcohol). 6 The final products of all these methods are,however, characterized by problems such as low yields,irregular morphologies, polycrystallinity, and low aspectratios. In contrast, the template-directed synthesis offers abetter controlled route to 1D silver nanostructures. A varietyof templates have been successfully demonstrated for usewith this process, and typical examples include channels inmacroporous membranes, 7 mesoporous materials, 8 or carbonnanotubes; 9 block copolymers; 10 DNA chains; 11 rod-shapedmicelles; 12 arrays of calix[4]hydroquinone nanotubes; 13 andsteps or edges on solid substrates. 14 Here we wish to reporta soft (with temperatures < 200 ° C), solution-phase approachthat allows for the production of bicrystalline nanowires of silver with uniform diameters and in bulk quantities.The first step of our synthesis involved the formation of Pt nanoparticles by reducing PtCl 2 with ethylene glycolrefluxed at ∼ 160 ° C. 15 In this so-called polyol process ,ethylene glycol served as both solvent and reducing agent. 16 When AgNO 3 and poly(vinyl pyrrolidone) (PVP) were addedto this refluxing solution that contained Pt seeds, silvernanoparticles were formed immediately through the reductionof AgNO 3 by ethylene glycol. 17 After the AgNO 3 solutionhad been added for ∼ 18 min, the reaction mixture turnedturbid with a gray color, and nanorods started to appear inthe reaction mixture when a small portion of the solutionwas taken from the vessel and viewed under an opticalmicroscope. As the reaction mixture was refluxed at ∼ 160 ° C, the number and length of these nanorods increased overa period up to ∼ 60 min.The exact mechanism for the formation of silver nanowiresvia this solution-phase approach is still under investigationby our group. Here is a working hypothesis that agreed wellwith the electron microscopy and spectroscopy studies.Figure 1 gives the electron microscopy images of foursamples that were taken from the refluxing solution afterAgNO 3 and PVP had been added for different periods of time: (A) 10, (B) 20, (C) 40, and (D) 60 min. These imagesindicate the evolution of silver into nanostructures havingdifferent morphologies as the reaction mixture was kept * Corresponding author. E-mail: xia@chem.washington.edu. NANOLETTERS 2002Vol. 2, No. 2165 - 168 10.1021/nl010093y CCC: $22.00 © 2002 American Chemical Society Published on Web 01/03/2002    D  o  w  n   l  o  a   d  e   d   b  y   N   A   T   L   I   B   U   K   R   A   I   N   E  o  n   A  u  g  u  s   t   6 ,   2   0   0   9   P  u   b   l   i  s   h  e   d  o  n   J  a  n  u  a  r  y   3 ,   2   0   0   2  o  n   h   t   t  p  :   /   /  p  u   b  s .  a  c  s .  o  r  g   |   d  o   i  :   1   0 .   1   0   2   1   /  n   l   0   1   0   0   9   3  y  refluxing at ∼ 160 ° C. When AgNO 3 was reduced by ethyleneglycol in the presence of PVP, the initial product was amixture of silver nanoparticles with two major distinctivesizes (Figure 1A): most of them were < 5 nm in diameterthat were formed in the solution through a homogeneousnucleation process, while some of them were 20 - 30 nm indiameter that were formed through heterogeneous nucleationon the platinum seeds. These silver nanoparticles wereprevented from aggregation into larger ones due to thepresence of PVP macromolecules that could chemicallyabsorb onto the surfaces of silver nanoparticles. 18 When thiscolloidal dispersion was constantly refluxed at ∼ 160 ° C, thesmall silver nanoparticles started to dissolve into the solutionand grow onto large nanoparticles of silver via a processknown as Ostwald ripening. 19 With the assistance of PVP,these large silver nanoparticles were able to grow into rod-shaped structures as shown in Figure 1B. The exact role of PVP in this process is still not clear. One possible functionfor PVP was to kinetically control the growth rates of variousfaces by interacting with these faces through adsorption anddesorption. 20 The slow dissolution of the small silver particlesinto the solution might also play a certain role in achievingan anisotropic growth for the large silver particles. Inprinciple, this growth process would continue until all silverparticles with diameters < 5 nm had been completelyconsumed (Figures 1C and 1D). Note that some silverparticles did not develop into the rod-shaped structures(Figure 1B) and kept growing into larger colloids that wereas stable as the wires. These particles could coexist in thesolution with the silver wires (Figure 1D). Both platinumseeds and PVP were critical to the formation of silvernanowires: no wire was generated without the addition of PtCl 2 or PVP to the reaction mixture.Figure 2 shows the UV - visible absorption spectra takenfrom the solutions that were used for electron microscopystudies in Figure 1. The change in optical features observedin these spectra correlates well with the electron microscopyimages. At t  ) 10 min, the appearance of a weak plasmonpeak at ∼ 410 nm indicated the formation of silver nanopar-ticles with diameters of 20 - 30 nm and at a relatively lowconcentration. 21 The intensity of this plasmon peak changedvery little until ∼ 20 min into the reaction when a new peakdeveloped at ∼ 570 nm. This new peak could be attributedto the longitudinal plasmon resonance of rod-shaped silvernanostructures. 22 As the length of these nanorods grew withtime, the transverse plasmon mode (at ∼ 380 nm) was greatlyincreased in intensity (note the scale change between curvesB and C), while the longitudinal plasmon resonance es-sentially disappeared. At the same time, optical signaturessimilar to those of bulk silver started to appear, as indicatedby a shoulder around 350 nm which could be attributed tothe plasmon resonance of bulk silver film. As the reactionproceeded, the two plasmon peaks at 380 and 350 nm werefurther increased in intensity relative to the plasmon peakpositioned at 410 nm. Nevertheless, the peak around 410nm (this peak slightly shifted to longer wavelengths as theparticles grew in size) still existed, even after the reactionmixture had been refluxed for 60 min or longer. Thisobservation indicates that the final product synthesized underthis particular condition was a mixture of silver nanowiresand nanoparticles (Figure 1D).The silver nanowires could be separated from the nano-particles using centrifugation. In this case, the reactionmixture was cooled to room temperature, diluted with acetone(about 10 times by volume), and centrifuged at 2000 rpmfor 20 min. The nanowires settled down to the bottom of the container under centrifugation while the nanoparticlesthat still remained in the liquid phase were removed using apipet. This separation procedure was repeated several timesuntil nanowire samples essentially free of particles were Figure 1. (A, B) TEM and (C, D) SEM images of fouras-synthesized samples of silver nanowires, showing different stagesof wire growth. The sample was prepared by taking a small portionof solution from the reaction mixture after AgNO 3 and PVP hadbeen added for (A) 10, (B) 20, (C) 40, and (D) 60 min. The solutionwas placed on a TEM grid to let the solvent slowly evaporate in afume hood. Figure 2. UV - vis absorption spectra of the reaction mixture afterAgNO 3 and PVP were added for (A) 10, (B) 20, (C) 40, and (D)60 min. All these solutions were diluted by 30 times of water beforetaking spectra. The absorptions at 350, 380, 410, and 570 nm wereattributed to the plasmon resonance peaks of silver with varioussrcins: long nanowires similar to the bulk silver, transverse modeof nanowires or nanorods, surface plasmon of nanoparticles, andthe longitudinal mode of nanorods. 166 Nano Lett., Vol. 2, No. 2, 2002    D  o  w  n   l  o  a   d  e   d   b  y   N   A   T   L   I   B   U   K   R   A   I   N   E  o  n   A  u  g  u  s   t   6 ,   2   0   0   9   P  u   b   l   i  s   h  e   d  o  n   J  a  n  u  a  r  y   3 ,   2   0   0   2  o  n   h   t   t  p  :   /   /  p  u   b  s .  a  c  s .  o  r  g   |   d  o   i  :   1   0 .   1   0   2   1   /  n   l   0   1   0   0   9   3  y  obtained. Figure 3A shows the SEM image of silvernanowires after three cycles of centrifugation/separation.These nanowires had a mean diameter of  ∼ 38 nm, with astandard deviation of  ∼ 5 nm. Figure 3B shows the TEMimage of several such nanowires, indicating the uniformityin diameter along each wire. Figure 3C shows the selectedarea electron diffraction (SAED) pattern obtained from amassive bundle of silver nanowires. All of these diffractionrings could be indexed to face-center-cubic silver, with alattice constant of  ∼ 4.08 Å. The X-ray diffraction (XRD)pattern taken from a larger quantity of sample also suggestedthat silver nanowires synthesized using this solution-phasemethod existed purely in the face-center-cubic phase. Thecrystal structures of these silver nanowires were furtherstudied using electron microdiffraction and high-resolutionTEM. Previous studies have suggested a low threshold fortwinning parallel to the { 111 } faces for face-center-cubicmetals such as silver and gold. 23 They tend to grow asbicrystals twinned at { 111 } planes. Figure 3D shows theTEM image of the end of a silver nanowire, clearly showingthe [111] twin plane parallel to its longitudinal axis. Theinset gives the electron diffraction pattern obtained byfocusing the electron beam ( ∼ 40 nm in diameter) onto thisnanowire. The assignment of this pattern suggests [2 1 h 1 h ]and [01 1 h ] as the growth directions for these two twinnedregions, respectively. The HRTEM images shown in Figures3E and 3F indicate that each portion of this twinned nanowirewas single crystalline, each with a well-resolved interferencefringe spacing.The dimensions of these bicrystalline silver nanowireswere found to strongly depend on the reaction conditionssuch as temperature and the concentration of the seedingsolution. When the reaction temperature was higher or lowerthan 160 ° C, the lengths of silver nanowires decreasedsignificantly (from ∼ 50 to ∼ 2 µ m). Very few silvernanowires were formed if the reaction temperature was lowerthan 100 ° C. Figure 4A shows the TEM image of relativelyshort nanowires (nanorods) of silver that were grown at ∼ 185 ° C. These nanorods had a mean diameter of 39 ( 3 nm andaverage length of 1.9 ( 0.4 µ m. Figure 4B shows the TEMimage of a sample that was synthesized using a proceduresimilar to that used for the sample shown in Figure 3, exceptthat the concentration of PtCl 2 was increased by 10 times.In this case, the diameter of these silver nanowires wasreduced from ∼ 40 to ∼ 30 nm due to an increase in seednumber. These experimental results suggest that it would bepossible to control the dimensions of silver nanowires byvarying the experimental conditions. Figure 3. (A, B) SEM and TEM images of silver nanowires thatwere separated from a product containing silver nanoparticlesthrough centrifugation. (C) The SAED pattern obtained from abundle of silver nanowires randomly deposited on the TEM grid.(D) The TEM image of a silver nanowire, showing the bicrystal-linity and corresponding growth directions that are characteristicsof silver nanowires synthesized using the present method. The [111]twin plane in the middle of this nanowire is indicated by an arrow.The inset gives the microdiffraction pattern recorded by focusingthe electron beam (with a diameter of  ∼ 40 nm) onto this wire,with the red-colored spots indexed to the [2 1 h 1 h ] growth direction,and the green-colored spots to the [01 1 h ]. The blue spots are sharedby the two different sides of this twinned crystal. (E, F) High-resolution TEM images taken from each edge of this bicrystallinenanowire of silver, indicating the single crystallinity of each side. Figure 4. TEM images of two as-synthesized samples of silvernanowires, showing the variation of wire dimensions when reactionconditions were changed. These two samples were preparedusing a procedure similar to that of Figures 1 - 3, except that (A)the ethylene glycol was refluxed at 185 ° C when AgNO 3 and PVPwere added, and (b) the concentration of PtCl 2 was increased by10 times. Nano Lett., Vol. 2, No. 2, 2002 167    D  o  w  n   l  o  a   d  e   d   b  y   N   A   T   L   I   B   U   K   R   A   I   N   E  o  n   A  u  g  u  s   t   6 ,   2   0   0   9   P  u   b   l   i  s   h  e   d  o  n   J  a  n  u  a  r  y   3 ,   2   0   0   2  o  n   h   t   t  p  :   /   /  p  u   b  s .  a  c  s .  o  r  g   |   d  o   i  :   1   0 .   1   0   2   1   /  n   l   0   1   0   0   9   3  y  We have tested the electrical continuity of these silvernanowires by measuring the resistance of an individualnanowire at room temperature using the four-probe method.In this case, a silver nanowire of  ∼ 40 nm in diameter and ∼ 20 µ m in length was aligned across four gold electrodesthat had been patterned on a glass slide. Currents weremeasured as a range of DC voltages that were applied tothese gold electrodes. A linear I - V curve was obtained, fromwhich an electrical conductivity of  ∼ 0.8 × 10 5 S/cm wascalculated for this nanowire. This value is very reasonablefor such a thin nanowire (the conductivity of bulk silver is6.2 × 10 5 S/cm), and strongly indicates that the bicrystallinenanowires of silver synthesized using the present chemicalapproach are electrically continuous.In summary, we have demonstrated a practical approachto the large-scale synthesis of silver nanowires that haveuniform diameters in the range of 30 - 40 nm and lengthsup to ∼ 50 µ m. In this process, the evolution of silver intoanisotropic nanostructures within an isotropic medium wasdetermined mainly by two factors: (i) the formation of platinum seeds in the solution that could effectively separatethe subsequent nucleation and growth steps of silver; (ii) theuse of a polymer surfactant that could kinetically control thegrowth rates of various planes of face-center-cubic silver.Since the polyol process we used here has been previouslyapplied to the synthesis of colloidal particles from a broadrange of metals, 24 we believe the approach we demonstratedhere could also be extended to these metals. The onlyrequirement seems to be the selection of a proper seedingsolid and an appropriate polymer surfactant that couldchemically absorb onto the surfaces of these metals. Acknowledgment. This work has been supported in partby the ONR, a Fellowship from the David and LucilePackard Foundation, and a Research Fellowship from theAlfred P. Sloan Foundation. B.G. and B.M. thank the Centerfor Nanotechnology at the UW for the IGERT fellowshipfunded by the NSF (DGE - 9987620). We also thank Mrs.Yu Lu, Mr. Yadong Yin, Mr. Hanson Fong, and Dr. BrianReed for their help with XRD, electron microdiffraction, andHRTEM studies. References (1) (a) Prokes, S. M.; Wang, K. L. MRS Bull. 1999 , 24 (8), 13. (b) Cui,Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001 , 293 , 1289.(2) (a) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999 , 32 ,435. (b) Zhang, Z.; Sun, X.; Dresselhaus, M. S.; Ying, J. Y. Phys. Re V . B 2000 , 61 , 4850. (c) Bockrath, M.; Liang, W.; Bozovic, D.;Hafner, J. H.; Lieber, C. M.; Tinkham, M.; Park, H. Science 2001 , 291 , 283.(3) Carmona, F.; Barreau, F.; Delhaes, P.; Canet, R. J. Phys. Lett. 1980 , 41 , L-531.(4) Liu, S.; Yue, J.; Gedanken, A. Ad  V . Mater. 2001 , 13 , 656.(5) Zhou, Y.; Yu, S. H.; Cui, X. P.; Wang, C. Y.; Chen, Z. Y. Chem. Mater. 1999 , 11 , 545.(6) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. 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H.; Jones, K. M.; Baski, A. A. J. Vac. Sci. Technol. A 1999 , 17  , 1696. (b) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000 , 290 , 2120.(15) In a typical synthesis, 2 × 10 - 5 g of PtCl 2 (Aldrich, 99.99 + %) wasdissolved into 0.5 mL ethylene glycol (Aldrich, 99.8%), and thissolution was added to 5 mL ethylene glycol (heated to 160 ° C) undercontinuous magnetic stirring. After ∼ 4 min, 2.5 mL ethylene glycolsolution of AgNO 3 (0.0500 g, Aldrich, 99 + %) and 5 mL ethyleneglycol solution of poly(vinyl pyrrolidone) (PVP, 0.200 g, Aldrich,  M  W ≈ 40 000) were added to the ethylene glycol containing platinumseeds. This reaction mixture was then constantly heated at 160 ° Cfor various periods of time up to ∼ 60 min. The nanowires wereseparated from particles using centrifugation. In this case, the reactionmixture was cooled to room temperature, diluted with acetone (about10 times by volume), and centrifuged at 2000 rpm for 20 min. Thenanowires settled down to the bottom of the container undercentrifugation while the nanoparticles that still remained in the liquidphase were removed using a pipet. This separation procedure wasrepeated several times until nanowire samples essentially free of particles were obtained. The SEM images were obtained on a fieldemission microscope (FSEM, JEOL-6300F, Peabody, MA) operatingat an accelerating voltage of 15 kV. The TEM images andmicrodiffraction patterns were taken on a JEOL-1200EX II micro-scope (80 kV). High-resolution TEM images were obtained on aTOPCON 002B microscope operated at 160 kV. XRD measurementswere obtained with a Philips PW1710 diffractometer (Cu - K R radiation).(16) Fievet, F.; Lagier, J. P.; Figlarz, M. MRS Bull. 1989 , December  , 29.(17) Ayyappan, S.; Subbanna, G. N.; Gopalan, R. S.; Rao, C. N. R. Solid State Ionics 1996 , 84 , 271.(18) Zhang, Z.; Zhao, B.; Hu, L. J. Solid State Chem. 1996 , 121 , 105.(19) Roosen, A. R.; Carter, W. C. 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