NANO LETTERS
Transformation of Silver Nanospheres into Nanobelts and Triangular Nanoplates through a Thermal Process
2003 Vol. 3, No. 5 675-679
Yugang Sun, Brian Mayers, and Younan Xia* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 Received March 7, 2003
ABSTRACT Silver has been prepared as triangular nanoplates with sharp corners and thin nanobelts by refluxing an aqueous dispersion of spherical colloids of silver with an average diameter of 3.5 nm. The spherical colloids of silver were, in turn, generated by reducing silver nitrate with sodium borohydride in the presence of poly(vinyl pyrrolidone) (PVP) and sodium citrate. Our studies indicate that the light present in an ordinary chemical laboratory was sufficiently strong to transform some spherical nanoparticles into small triangular nanoplates. Refluxing provided the driving force to facilitate the Ostwald ripening process−growth of these plate-like seeds at the expense of spherical nanoparticles. In addition, refluxing might also selectively disrupt the organic layers on the surfaces of the small triangular nanoplates, driving these plates to assemble into thin nanobelts due to the strong dipole−dipole interaction between adjacent plates. These nanostructures of silver with unique planar shapes might find use in areas that include photonics, optoelectronics, and optical sensing.
Metal nanostructures have been the focus of intensive research in the past several decades due to their potential application in fabricating electronic, optical, optoelectronic, and magnetic devices that may exhibit performance complementary and/or superior to their bulk counterparts.1 The intrinsic properties of a metal nanostructure can be tailored by controlling its size, shape, composition, and crystallinity. Among these parameters, shape-control has been proved to be as effective as size-control in fine-tuning the properties and functions of metal nanostructures. For example, cubic platinum nanoparticles with their surfaces bound by {100} facets prefer to adsorb hydrogen molecules, while carbon monoxide tends to interact strongly with buckyball-shaped platinum nanoparticles bound by {210} facets.2 With regard to optical response, the surface plasmon resonance (SPR) and fluorescence features of gold or silver nanorods have been shown to be highly sensitive to their aspect-ratios.3 The number of SPR bands and effective spectral ranges for surface-enhanced Raman scattering (SERS) have also been demonstrated to be highly dependent on the morphology exhibited by silver nanostructures.4 Despite its fundamental and technological importance, the challenge of synthetically and systematically controlling the shape of metal nanostructures has been met with limited success. This situation did not change until several solution-phase approaches were demonstrated very recently. Different from the gas-phase approach which can only generate metal nanoparticles with well-defined shapes in low * Corresponding author. E-mail:
[email protected] 10.1021/nl034140t CCC: $25.00 Published on Web 04/18/2003
© 2003 American Chemical Society
yields,5 solution-phase methods have the potential to process metals into nanostructures with a range of well-defined morphologies and in bulk quantities. For instance, rod-shaped micelles assembled from cetyltrimethylammonium bromide (CTAB) have been demonstrated as templates to grow nanorods/nanowires of silver (or gold) with controllable diameters and aspect ratios.6 Such one-dimensional (1D) nanostructures and nanocubes could also be synthesized by introducing a capping reagent (small molecules and polymers) that could selectively increase or decrease the growth rates of different crystallographic planes.7,8 Most recently, processing of silver into planar nanoparticles also became an interesting subject since a photoinduced procedure was reported to transform small silver nanospheres into nanoprisms.9 To this end, a number of soft templates such as bilayer systems formed between octylamine and water have been successfully used to synthesize silver nanoplates.10 A micellar system involving CTAB was adopted to generate triangular nanoplates (with highly truncated corners) and circular nanodisks of silver through a seed-mediated growth process.11 Silver nanodisks were also formed by sonicating silver ions and hydrazine dissolved in reverse micelles consisting of di(2-ethyl-hexyl)sulfosuccinate (AOT), isooctane, and water.12 Furthermore, it was demonstrated that silver nanoprisms could be synthesized by boiling silver nitrate dissolved in N,N-dimethyl formamide that contained a polymer such as poly(vinyl pyrrolidone) (PVP).13 The triangular nanoplates obtained from all these solution-phase methods were, however, characterized by shape imperfections such as truncated corners. In addition to nanoplates or
Figure 1. (A) UV-visible-NIR extinction spectra of an aqueous dispersion of silver nanospheres before and after it had been refluxed for 10 h. (B) TEM image of the silver nanoparticles before refluxing, together with their selected-area-electron-diffraction pattern (shown as the inset). (C) TEM and (D) SEM images of this sample after refluxing, indicating the formation of a mixture of triangular nanoplates (with sharp corners) and nanobelts of silver. The arrows in (D) show some nanoplates that were oriented with their triangular faces perpendicular to the supporting substrate.
disks, another class of planar nanostructures (e.g., nanobelts, or nanoribbons) has also been intensively studied in the past two years because they may represent a good system for examining dimensionally confined transport phenomena.14 Many oxides have already been processed into nanobelts by evaporating the desired material in the powder form.15 To our knowledge, there has been no report about solution-phase synthesis of metal nanobelts. Here we describe a thermal process that enabled us to transform spherical colloids of silver (with diameters 5 nm. This observation implies that a silver nanosphere will be more reactive and less stable when its dimension becomes smaller. As a result, the natural light with a weaker intensity could only induce the transformation of silver nanospheres smaller than 5 nm into triangular seeds while it did not work for silver colloids larger than 5 nm. Kotov et al. recently reported that small CdTe quantum dots with partially protective shells of an organic stabilizer could self-assemble into nanowires as driven by a strong dipole-dipole interaction between adjacent particles.23 The silver nanobelts observed in the present work might also be formed through a similar process, in which small silver nanoplates self-organized (antiparallel to each other, similar to the configuration shown in Figure 2A) into 1D nanostructures and then recrystallized to form single crystalline nanobelts without changing their flat surfaces. The PVP and citrate species adsorbed on the surfaces of edges might be selectively removed when their dispersion was heated.24 Because the triangular nanoplates were extremely low in aspect-ratio (i.e., thickness relative to lateral dimensions), the resultant products were nanobelts with rectangular crosssections rather than wire-like nanostructure with circular, hexagonal, or square cross-sections. In summary, triangular nanoplates of silver with sharp corners have been synthesized by refluxing aqueous disper678
sions of spherical silver colloids (smaller than 5 nm in diameter) under the ambient condition of a chemical laboratory. Single crystalline nanobelts with lateral dimensions in the range of 7-30 nm were also formed at low yields of ∼5%. The presence of natural light and the small size of original silver colloids were believed to be the essential factors responsible for the generation of triangular seeds. The thermal treatment provided a mild driving force to facilitate the transport of atoms from the spherical colloids to the anisotropic plates in a process similar to the Ostwald ripening. The high extinction coefficient of these triangular nanoplates in the NIR region might make them useful in fabricating photonic devices such as optical sensors and NIR absorbers. The silver nanobelts with relatively thin lateral dimensions might provide a good model system to systematically study the morphological and dimensional dependence for electronic and thermal transport properties. Acknowledgment. This work has been supported in part by a Career Award from the National Science Foundation (DMR-9983893), and a Fellowship from the David and Lucile Packard Foundation. Y.X. is a Camille Dreyfus Teacher Scholar (2002) and an Alfred P. Sloan Research Fellow (2000). B.M. thanks the Center for Nanotechnology at the UW for an IGERT Fellowship Award supported by the NSF (DGE-9987620). We thank Thurston Herricks for discussions on TEM and electron diffraction measurements. We also thank Professor D. Gamelin at the UW for allowing us to use the UV-vis-NIR spectrometer in his research group. References (1) See, for example: (a) Halperin, W. P. ReV. Mod. Phys. 1986, 58, 533. (b) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (c) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (d) Murray, C. B.; Sun, S.; Doyle, H.; Betley, T. Mater. Res. Soc. Bull. 2001, 26, 985. (e) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (f) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (2) (a) Shi, A.-C.; Masel, R. I. J. Catal. 1989, 120, 421. (b) Falicov, L. M.; Somorjai, G. A. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 2207. (3) (a) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (b) ElSayed, M. A. Acc. Chem. Res. 2001, 34, 257. (4) (a) Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. Phys. ReV. B 2002, 64, 235402. (b) Dick, L. A.; McFarland, A. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2002, 106, 853. (5) (a) Harris, P. J. F. Nature 1986, 323, 792. (b) Graoui, H.; Giorgio, S.; Henry, C. R. Surf. Sci. 1998, 417, 350. (6) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (c) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (d) Yu, Y.-Y.; Chang, S.-S.; Lee, C.L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (e) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (7) (a) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (b) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833. (c) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z. Y. AdV. Mater. 1999, 11, 850. (8) (a) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (b) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (9) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (10) Yener, D. O.; Sindel, J.; Randall, C. A.; Adair, J. H. Langmuir 2002, 18, 8692. (11) (a) Chen, S.; Carroll, D. L. Nano Lett. 2002, 2, 1003. (b) Chen, S.; Fan, Z.; Carroll, D. L. J. Phys. Chem. B 2002, 106, 10777. Nano Lett., Vol. 3, No. 5, 2003
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