pubs.acs.org/Langmuir © 2010 American Chemical Society
Electrostatic Field Force Directed Gold Nanowires from Anion Exchange Resin Arun Kumar Sinha, Mrinmoyee Basu, Sougata Sarkar, Mukul Pradhan, and Tarasankar Pal* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India Received June 11, 2010. Revised Manuscript Received August 13, 2010 We have developed a polarization-induced growth process to synthesize gram quantity of gold nanowire (Au NW) on the outer surface of an anion exchange resin matrix. This new, simple, modified hydrothermolysis (MHT) procedure involving resin-bound HAuCl4 produced micrometer long Au nanowire on resin surface. The charged resin matrix responsibly imposes electrostatic field effect (EFF) for 1D growth of Au NWs in the presence of different amines or derivatives of amines. The Au nanowire is separated from resin by sonication. Again, the synthesis of MnO2 nanowire with resin support through similar MHT strengthens the 1D growth proposition, that is, EFF-induced polarization effect.
Introduction One-dimensional gold nanowires (Au NWs) have attracted considerable interest for their high aspect ratio and interesting optical properties. Then, the use of Au NWs would be very interesting to be used in nanoelectronic devices for applications in sensors, photonics, waveguides, piezoelectronics, and memory circuits.1 The general approach for developing gold nanowire can be classified into the following groups: synthesis with soft templates,2 nanowire growth by assembly of nanoparticles,3 and synthesis in templates.4 A number of template-based synthesis strategies have been published in recent years that tailormake the nanowire aspect ratio or length and morphology. Synthesis in hard porous templates is a very popular method. A premade nanoporous template such as an alumina membrane is used as a reaction platform for the electrochemical deposition of a metal.5 The membrane is attached to an electrode for the NW synthesis. To produce free nanorods, the template has to be dissolved, but there the big problem is the production of the high length (50-100 μm) of Au *To whom correspondence should be addressed. E-mail: tpal@chem. iitkgp.ernet.in. (1) (a) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (b) Tong, L.; Gattass, R. R.; Ashcom, J. B.; He, S.; Lou, J.; Shen, M.; Maxwell, I.; Mazur, E. Nature 2003, 426, 816. (c) Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269. (d) McAlpine, M. C.; Friedman, R. S.; Jin, S.; Lin, K.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 1531. (e) Wang, Z. L.; Song, J. Science 2006, 312, 242. (f) Lu, W.; Lieber, C. M. Nat. Mater. 2007, 6, 841. (2) (a) Pileni, M. P.; Ninham, B. W.; Gulik-Kryzwicki, T.; Tanori, J.; Lisiecki, I.; Filankembo, A. Adv. Mater. 1999, 11, 1358. (b) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (c) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 5865. (d) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (3) (a) Maddanimath, T.; Kumar, A.; Arcy-Gall, J. D.; Ganesan, P. G.; Vijayamohanan, K.; Ramanath, G. Chem. Commun. 2005, 1435. (b) Pei, L. H.; Mori, K.; Adachi, M. Langmuir 2004, 20, 7837. (c) Halder, A.; Ravishankar, N. Adv. Mater. 2007, 19, 1854. (4) (a) Edmondson, M. J.; Zhou, W.; Sieber, S. A.; Jones, I. P.; Gameson, I.; Anderson, P. A.; Edwards, P. P. Adv. Mater. 2001, 13, 1608. (b) Govindaraj, A.; Satishkumar, M.; Nath, M.; Rao, C. N. R. Chem. Mater. 2000, 12, 202. (c) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (d) Jiang, X.; Xie, Y.; Lu, J.; Zhu, L.; He, W.; Qian, Y. J. Mater. Chem. 2001, 11, 1775. (e) Wei, G.; Nan, C.; Deng, Y.; Lin, Y. Chem. Mater. 2003, 15, 4436. (5) (a) Forrer, P.; Schlottig, F.; Siegenthaler, H.; Textor, M. J. Appl. Electrochem. 2000, 30, 533. (b) Prieto, A. L.; Sander, M. S.; Martin-Gonzalez, M. S.; Gronsky, R.; Sands, T.; Stacy, A. M. J. Am. Chem. Soc. 2001, 123, 7160. (c) Yin, A. J.; Li, J.; Jian, W.; Bennett, A. J.; Xu, J. M. Appl. Phys. Lett. 2001, 79, 1039. (6) (a) Lisiecki, I.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 4160. (b) Rees, G. D.; Evans-Gowing, R.; Hammond, S. J.; Robinson, B. H. Langmuir 1999, 15, 1993.
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nanowire (Au NW). One-dimensional micelles, microemulsions, and surfactants in solution can also direct the anisotropic growth of the nanomaterials.6 The shape of the 1D micelle promotes the formation of 1D nanomaterials from an ionic precursors. In the case of surfactants, medium-strength binding force of a surfactant to a growing crystal face helps the direct nanorod formation. The aurophilic bonding of organometallic complexes formed from Au(1) halides and coordinating ligands can lead to form the 1D polymeric structure.7 When the Au(1) is converted to Au(0) by slow reduction, the nucleation and growth of Au can be mediated by the 1D polymer strands to generate nanowires. Recently Xia et al.8 prepared Au NWs by the reduction of [(oleylamine)AuCl] complex chains using Ag nanoparticles as a promoting agent. Lieber et al.9 prepared gold nanowire by simple reduction of HAuCl4 in oleic acid and oleylamine at 80 C under vigorous magnetic stirring. Xing et al.10 presented a simple method for preparing ultrathin single crystalline Au NWs from a solution of HAuCl4, oleic acid, and triisopropylsilane at room temperature in a few hours. The process involved the addition of oleic acid as a stabilizer and one-dimensionally grown template and triisopropylsilane as a highly effective reducing agent. Han et al.11 produced a gold nanobelt using sonochemical method, and in this method, HAuCl4 solution with the desired amount of R-D-glucose is exposed to high-intensity ultrasound irradiation (40 kHz, 100 W) for 10-60 min. Murphy et al.12 developed a seed-mediated growth approach for metallic nanorods and nanowires from aqueous solution. There the premade gold or silver spheres serve as seed on which anisotropic nanostructures grows. The seeds are added to a solution containing metal salt, a weak reducing agent (e.g., ascorbic (7) (a) Bachman, R. E.; Fioritto, M. S.; Fetics, S. K.; Cocker, T. M. J. Am. Chem. Soc. 2001, 123, 5376. (b) Onaka, S.; Yaguchi, M.; Yamauchi, R.; Ozeki, T.; Ito, M.; Sunahara, T.; Sugiura, Y.; Shiotsuka, M.; Horibe, M.; Okazaki, K.; Iida, A.; Chiba, H.; Inoue, K.; Imai, H.; Sako, K. J. Organomet. Chem. 2005, 690, 57. (c) Hsu, S. J.; Hsu, K. M.; Leong, M. K.; Lin, I. J. B. Dalton Trans. 2008, 1924. (d) Cao, L. Y.; Jennings, M. C.; Puddephatt, R. J. Inorg. Chem. 2007, 46, 1361. (8) Lu, X.; Yavuz, M. S.; Tuan, H. Y.; Korgel, B. A.; Xia, Y. J. Am. Chem. Soc. 2008, 130, 8900. (9) Wang, C.; Hu, Y.; Lieber, C. M.; Sun, S. J. Am. Chem. Soc. 2008, 130, 8902. (10) Feng, H.; Yang, Y.; You, Y.; Li, G.; Guo, G. J.; Yu, T.; Shen, Z.; Wu, T.; Xing, B. Chem. Commun. 2009, 1984. (11) Zhang, J.; Du, J.; Han, B.; Liu, Z.; Jiang, T.; Zhang, Z. Angew. Chem., Int. Ed. 2006, 45, 1116. (12) Murphy., C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80.
Published on Web 09/28/2010
DOI: 10.1021/la102387x
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Scheme 1. (a) Digital Image of Anion Exchange Resin, (b) AuCl4- Immobilization on Anion Exchange Resin, (c) Pure Gold Nanowire, (d,e) Image of Gold Nanowire Obtained from Renishaw Raman Microscope Connected to Charge-Coupled Device (CCD) Camera
acid), and a rodlike micellar template (cetyltrimethylammonium bromide, CTAB). The seeds serve as nucleation sites for nanorod and nanowire growth. Ramanath13 reported wet chemical approach for assembling Au and Ag nanoparticles into nanowire networks without the use of lithographic templates at room temperature. All of these synthetic approaches usually give Au NWs with low yield, but the greatest problem is the removal of the template to ensure the purity of the synthesized gold nanowires. Purification of pure gold nanowire from solid porous template, surfactant, cyclodextrin is really a time-consuming process. The reported polarization-induced growth approach exclusively fabricates pure metallic Au nanowires (on the surface of the charged resin matrix) from resin-bound HAuCl4 by modified hydrothermal (MHT) procedure. To the best of our knowledge, this is the first report about the synthesis of longest (>100 nm) gold nanowires. The charge of the resin body polarizes the as-obtained gold nanocrystals (NCs) in the solution phase and paves the way for directional growth of Au NWs.
Materials and Instrument All chemicals are of analytical grade and are used as received without further purification. Double-distilled water is used throughout the experiment. HAuCl4 and Amberlite IRA-400 (Cl) ion-exchange resin are purchased from Aldrich. Potassium permanganate (KMnO4) is obtained from Merck. Triethylamine (TEA), triethanolamine, diethylamine, ethanolethylamine, and disodium salt of ethylenediaminetetraacetic acid (Na2-EDTA) are purchased from Sisco research laboratories (Mumbai, India). XRD is done in a PW1710 diffractometer (Philips, Holland). The XRD data are analyzed using JCPDS software. The chemical state of the element on the surface was analyzed by a VG Scientific ESCALAB MK II spectrometer (U.K.) equipped with a Mg KR (13) Ramanath, G.; Arcy-Gall, J. D.; Maddanimath, T.; Ellis, A. V.; Ganesan, P. G.; Goswami, R.; Kumar, A.; Vijayamohanan, K. Langmuir 2004, 20, 5583.
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excitation source (1253.6 eV) and a five-channeltron detection system. FESEM analysis is performed with a supra 40 (Carl Zeiss) instrument, and an EDS machine (Oxford link and ISIS 300) attached to the instrument is used to obtain the NC composition. TEM analysis is performed with an instrument H-9000 NAR (Hitachi) using an accelerating voltage of 200 kV. Synthesis. Gold(III) precursor ions (1 10-2 M and 20 mL HAuCl4) are allowed to exchange with Cl- ions of the neat anion-exchange resin beads (0.25 g RþCl-) with occasional shaking and are kept overnight. The resin beads, on which gold precursor ions are immobilized, are washed several times with water to drain out the unexchanged species. Extremely pure Au NC is synthesized by hydrothermolysis14 of resin-bound AuCl4- using TEA as a crystal condensation catalyst as well as a reducing agent. In a typical preparation, resin-bound AuCl4is mixed with a solution containing 3 mL of TEA (0.01 M) in a screw-capped closed reaction vessel (10.5 cm in length and 1.5 cm in diameter). Then, the above mixture is allowed to age for 6 h under heat generated by an illuminated tungsten bulb (200 W) in a closed wooden box (6 inch 6 inch 7 inch). Therefore, there will be no loss of solution during the course of slow heat treatment by the illuminated electric bulb. Here the pressure inside the screw-capped test tube increases to 951.27 mmHg. During the heat treatment, that is, from the MHT, the outer surface of the resin is covered with Au NCs. In the similar way, we have also extended our work with triethanolamine, diethylamine, ethanolethylamine, and disodium salt of ethylenediaminetetraacetic acid (Na2-EDTA) as reducing as well as growth-controlling agent. In an extension of the above synthetic protocol, we have successfully synthesized MnO2 nanowire again on anion-exchange charged resin support. The reaction is quite similar to Au NW preparation, as mentioned above. At first, MnO4- (1 10 -2 M (14) (a) Sinha, A. K.; Jana, S.; Pande, S.; Sarkar, S.; Pradhan, M.; Basu, M.; Saha, S.; Pal, A.; Pal, T. CrystEngComm 2009, 11, 1210. (b) Basu, M.; Sinha, A. K.; Sarkar, S.; Pradhan, M.; Yusuf, S. M.; Negishi, Y.; Pal, T. Langmuir 2010, 26, 5836. (c) Sinha, A. K.; Basu, M.; Pradhan, M.; Sarkar, S.; Pal, T. Chem.;Eur. J. 2010, 16, 7865. (d) Basu, M.; Sinha, A. K.; Pradhan, M.; Sarkar, S.; Negishi, Y.; Gupta, G.; Pal, T. Environ. Sci. Technol. 2010, 44, 6313.
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Figure 1. FESEM image gold nanowire on the surface of resin bead and the gold nanowires are easily separable. (a) Au nanowire deposited on anion exchange resin, (b) Au nanowire separated from anion exchange resin by sonication, and (c) high magnification image of Au nanowire.
Figure 2. FESEM image of (a) gold nanowire on the surface of resin bead and gold nanowire at different magnification (b) low, (c) medium, and (d) high. and 20 mL of KMnO4) is immobilized onto the anion exchange resin beads (0.25 g). After thorough washing with water, the MnO4- immobilized resin beads are taken in a 15 mL screw cap test tube with 3 mL of TEA (0.01 M). Then, heat treatment of the reaction mixture is done by irradiation as usual using a 200W bulb under the MHT condition for 8-12 h.
Result and Discussion Immobilization of HAuCl4 on Anion Exchange Resin and Reduction. Anion exchange resin has a polymer backbone with quaternary ammonium groups as an integral part of the polymer lattice with electric charges that are neutralized by the charges carried by the counterions. The counterions are generally chloride, hydroxyl, or sulfate, as the situation may be. SERALITE-SRA-400, a cross-linked polystyrene containing quaternary ammonium groups as an integral part, is supplied in the chloride form with an ion exchange capacity of 3.5 mmol/ g gold precursor ions. The yellow AuCl4- complex bears unit negative charge, so the complex is effectively exchanged with the chloride form of the anion exchange resin moiety. Therefore, AuCl4- is immobilized on the resin beads by the electrostatic field force (EFF). Rþ Cl - ðsolidÞ þ Hþ ½AuCl4 - ðsoln Þ a Rþ ½AuCl4 - ðsolidÞ þ Hþ Cl - ðsoln Þ Langmuir 2010, 26(22), 17419–17426
After AuCl4- exchange onto the resin matrix, the color is changed from yellow to red (Scheme 1), but under modified hydrothermal condition (MHT), Au(III) is selectively reduced to metallic gold by amine and gets deposited on the surface of resin body in succession. The AuNWs are easily separated from the resin surface (figure 1) by sonication. FESEM and TEM Analysis of 1D Gold Nanowire. Figure 2 shows representative field-emission scanning electron microscopic (FESEM) images of the as-synthesized gold products at different magnification. Large amounts of hairlike materials are shown. The longest wire observed in the sample is >100 μm. They are flexible and easily bend without breaking. The bent part of the gold nanowire is shown in Figure 3a. The product contains 100% nanowire, and no other morphology has been detected, which reveals that the pure Au with nanowire structure is morphologically pure. The typical width of the nanowires lies in the range of 50-100 nm. To further analyze the characteristics of the nanowires, high-magnification FESEM images are given in Figure 3b. Typical nanowires have smooth surface. Figure 3b shows the tip surface of a typical nanowire, in which a round shape is observed. The detailed microstructure of the gold nanowires is further characterized by transmission electron microscopy (TEM). The TEM images also reveal that the width of the products is in the range 50-100 nm and that the length can reach several micrometers. DOI: 10.1021/la102387x
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Figure 4. XRD pattern of gold nanowire produce from triethylamine as reducing agent.
Figure 3. FESEM image of (a) gold nanowire showing bending part and (b) “Y” junction and tip of the gold nanowire. (c,d,e) TEM image of gold nanowire at different positions and (f) HRTEM image.
A high-resolution TEM image of a typical nanowire is shown in Figure 3c-e. HRTEM has been obtained with the incident electron beam perpendicular to the wide surface of the nanowire. The measured interplanar spacing for all lattice fringes is 0.231 nm, which corresponds to the (111) lattice plane of facecentered-cubic (fcc) gold (shown in Figure 3f). The overgrowth of Au nanowires is clearly shown in Figure 3b,c. It looks like a “Y” junction. The “Y” junction is also shown in the FESEM image, and the Y junction is extended through several Au nanowires and forms a network structure, and the cross-linked image is shown in Figure 3c (inset). X-ray Diffraction, X-ray Photoelectron Spectroscopy, and Energy-Dispersive X-ray Spectroscopy Analysis. X-ray diffraction pattern (XRD) gives further support to the phase structure of the gold nanowires. The diffraction peaks shown in Figure 4 correspond to the (111), (200), (220), and (311) diffraction peaks of metallic gold.15 Note that the ratio of the intensity between the (200) and (111) diffraction peaks is much lower than the standard file (JCPDS; 0.061 versus 0.249). These observations confirm that our nanowires are primarily dominated by (111) facets, and thus their (111) planes tend to be preferentially oriented parallel to the surface of the supporting (15) Sun, X.; Dong, S.; Wang, E. Angew. Chem., Int. Ed. 2004, 43, 6360.
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Figure 5. XPS pattern of gold nanowire produce from triethylamine as reducing agent.
substrate. X-ray photoelectron spectroscopy (XPS) is generally employed as a surface-monitoring technique to support the XRD results further. Figure 5 represents the XPS profile of as-synthesized gold nanowire. The binding energies of Au 4f7/2 and Au 4f5/2 are measured at approximately 83.89 and 87.58 eV, respectively. The energies for gold are in good agreement with the literature values for the binding energies of pure metallic gold. The gold nanowires are additionally characterized by energy-dispersive X-ray spectroscopy (EDS) which confirmed that the nanowires are pure gold (Figure 6a,b). Sidewise, we have also characterized the uncoated resin surface by energydispersive X-ray spectroscopy as a point scan, which implies that some Au(III) ion is still present in the resin matrix (shown in Figure 6c,d). Growth Mechanism of 1D Gold Nanowire Formation. The gold precursor on the anion exchange resin reacts with triethylamine [(ethyl)3N] under modified hydrothermal condition wherefrom 1D growth is observed on the resin surface. We propose a new pathway on the growth mechanism for Au nanowire formation. Here (ethyl)3N acts as a reducing agent for Au(III) ion present in RþAuCl4-. Again, in turn, (ethyl)3N acts as a growthcontrolling agent providing support for the generation of Au nanowires under EFF. Simple reduction of RþAuCl4- by BH4would enable one to obtain Au nanoparticle impregnated resin beads. Langmuir 2010, 26(22), 17419–17426
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Figure 6. (a) EDS pattern of gold nanowire and (b) the corresponding scan area. (c) EDS pattern on uncoated resin surface and (d) the corresponding point scan. Scheme 2. Time-Dependent Growth of Gold Nanowire on the Resin Surface
However, during the course of reaction actually in practice, we see short Au nanowires on the resin surface and then long wires separate out, leaving aside the resin moiety (Scheme 2). The reduction of AuCl4 - sets free Cl- ions (for one Au(III) ion four Cl - ions) in the solution, which Langmuir 2010, 26(22), 17419–17426
then expels AuCl 4- from the solid RþAuCl4 - matrix. Higher concentrations of Cl - ions make it possible. These free ions with their increased randomness in solution phase easily encounter (ethyl)3 N favorably, and their reduction by the amine becomes thermodynamically facile in DOI: 10.1021/la102387x
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Figure 7. FESEM image of gold nanowire produced from different reducing and growth controlling agent di sodium salt of (a) ethylenediaminetetraacetic acid, (b) diethylamine, (c) triethanolamine, and (d) ethanolethylamine.
solution. The MHT reaction equation is summarized as follows: modified hydrothermal(MHT) reaction: outer surface of resin: reduction
Rþ ½AuCl4 - ðsolidÞ s f
Rþ Auð0ÞCl - ðsolidÞ polarized Auð0Þ deposited on resin surface
þ 3Cl - ðsoln Þ
inner matrix of resin: Rþ ½AuCl4 - ðsolidÞ þ 3Cl - f Rþ Cl - ðsolidÞ þ 3½AuCl4 - ðsoln Þ 1D growth of Au nanowire: reduction
i: 3½AuCl4 - ðsoln Þ s f 3Auð0Þðsoln Þ ðpolarizedÞ þ 12Cl - ðsoln Þ
ii: 3½Auð0Þðsoln Þ þ RAuð0ÞCl - ðsolidÞ f Rþ ½Auð0Þ¥ Auð0Þ¥ Auð0Þ¥ Auð0ÞCl -
where ¥ represents deposition by dipole-dipole interaction. Because of the high surface energy, the budding Au(0) grows in size and is eventually polarized by the charged resin matrix and longitudinally aligned on the already polarized Au(0) present in the resin matrix. During the progress of the reaction, the nanowires grow in length. Here (ethyl)3N plays its role as a 1D growth directing agent that is not observed with cetyltrimetyl ammonium bromide. Slowly, the whole resin surface is covered with the compact array of Au nanowires. A time-dependent information of the growth mechanism has been shown in Scheme 2. The Au nanowire from the surface of resin matrix is separated best by sonication. We have also extended our work with other growth controlling or reducing agents such as triethanolamine, diethylamine, ethanolethylamine, and disodium salt of ethylenediaminetetraacetic acid (Na2-EDTA). In all cases, micrometer long nanowires are obtained, and the morphology of the Au nanowire is shown in Figure 7. The as-synthesized Au nanowires depict fcc 17424 DOI: 10.1021/la102387x
Figure 8. XRD pattern of gold nanowire produce from different reducing and growth controlling agent (a) diethylamine, (b) ethanolethylamine, (c) triethanolamine, and (d) di sodium salt of ethylenediaminetetraacetic acid.
structure, and the XRD pattern of the synthesized nanowires are shown in Figure 8. The main reduction reaction occurs in aqueous solution containing RþAuCl4 - (resin immobilized Au(III) species) and (ethyl)3N under MHT conditions. The assembly of Au particles has directional property and is induced by the charged resin surface. To designate the importance of the charged resin matrix, we carried out the same reaction in the absence of resin beads, and we obtained spherical particles only (Figure 9). During the nucleation and growth of AuNWs, at first a Au particle is deposited on the surface of resin matrix and polarized by the charged polymeric resin. This polarized particle initiates the chain propagation and becomes the driving force for the synthesis of Au nanowire. As the reaction proceeds, the incoming Langmuir 2010, 26(22), 17419–17426
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Figure 9. FESEM image of Au nanoparticle when HAuCl4 is hydrothermolysed in the presence of (ethyl)3N in the absence of charge resin.
Figure 10. FESEM image of MnO2 nanowire on the outer surface anion exchange resin at different magnification. Scheme 3. (a) Growth Mechanism of Au Nanowire on the Surface of Charged Resin via Dipole-Dipole Attraction and (b) One Directional Arrangement of Pins by Magnet As Model for Growth Mechanism of Au or MnO2 Nanowire Formation on Outer Surface of Charged Resin Matrix
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Figure 11. TEM, HRTEM, SAED, and EDS profiles of MnO2 nanowire (obtained from resin surface) are shown in a-d, respectively.
Au nanoparticles are produced and become attached one by one by the EFF. The growth continues by the dipole-dipole attraction (DDA) through the oriented attachment mechanism, and the building blocks are template-free Au nanoparticles only (Scheme 3a). Therefore, the EFF is the cause, polarization of AuNPs is the observed phenomenon, and DDA is the result, giving nanowires as the product. A demonstration has been given as a model: pins under the influence of magnetic field are arranged like the polarized Au nanoparticles (Scheme 3b). (Ethyl)3N is a strong base, and at its higher concentration (0.1 M), a faster reaction takes place, as a result of which spherical particles of Au(0) are obtained and 1D growth does not propagate at all. The novel implication of the EFF has been validated and comes true in the case of oxide material but on the resin surface. In that case, also no nanowire formation is observed in the absence of resin support. We have successfully synthesized MnO2 nanowire again on anion-exchange resin surface. The procedure remains the same as that mentioned for AuNW formation. Out of the similar MHT condition, RþMnO4- (R represents the resin) undergoes a reduction reaction forming MnO2 nuclei on the resin matrix. The as-obtained MnO2 nanoparticles (on the surface of resin) are polarized by charged resin surface resulting nanowires
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(Scheme 3) through an oriented attachment mechanism. Onedirectional growth of the oxide materials is shown in the FESEM image in Figure 10. The authentication of MnO2 nanowire comes from TEM, SAED, and EDS analysis, as represented in Figure 11. Therefore the EFF may be considered to be a general strategy for nanowire formation out of different metal and metal oxide but on the charged resin surface.
Conclusions We have demonstrated a simple wet chemical approach for making gold nanowire (Au NW) on the outer surface of charged resin matrix by MHT. The process has the following advantages: (1) Micrometer long, extremely pure Au NW formation and its isolation from the resin matrix. (2) A new mechanism of Au NW formation through EFF-induced polarization method. This synthetic strategy is extendible to other oxide materials to bear 1 D nanostructure. (3) Lastly, the procedure certifies a cheap homemade tungsten-light-fitted setup as a deliverable for performing the MHT reaction. Acknowledgment. We thank CSIR, UGC, DST, New Delhi and Indian Institute of Technology, Kharagpur, India.
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