NANO LETTERS
Synthesis of Silver Nanoprisms in DMF
2002 Vol. 2, No. 8 903-905
Isabel Pastoriza-Santos and Luis M. Liz-Marza´n* Departamento de Quı´mica Fı´sica, UniVersidade de Vigo, 36200 Vigo, Spain Received June 7, 2002
ABSTRACT Polygonal (mainly triangular) silver nanoprisms were synthesized by boiling AgNO3 in N,N-dimethyl formamide, in the presence of poly(vinylpyrrolidone). Although during the synthesis, a mixture of nanoprisms and nanospheroids is formed, the latter can be removed through careful centrifugation. The UV−visible spectra of the nanoprisms display an intense in-plane dipolar plasmon resonance band, as well as weak bands for in-plane and out-of-plane quadrupolar resonances. The nanoprisms are also stable in other solvents, such as ethanol and water, and solvent exchange leads to strong shifts of the in-plane dipole plasmon band.
The intense study devoted to metal nanoparticles is mainly motivated by their extremely interesting optical properties. The origin of such a special optical behavior can be found in the interaction between incoming light and the free conduction electrons.1 When the wavelength of light couples with the oscillation frequency of the conduction electrons, a so-called plasmon resonance arises, which is manifested as an intense absorption band. The most spectacular cases are found when such plasmon resonance bands exist in the visible wavelength range, since then the dispersions of metal nanoparticles display brilliant colors. The precise wavelength of the plasmon resonance depends on several parameters, among which particle size and shape, surface charge, and the nature of the environment are probably the most important.2 In this communication we describe a novel, simple procedure for the synthesis of anisotropic silver nanoparticles, based on the use of N,N-dimethylformamide (DMF) as a solvent and as a reducing agent, in the presence of the polymer poly(vinylpyrrolidone) (PVP). The formation of spherical silver nanoparticles in DMF has been previously demonstrated,3 and the presence of different additives was shown to induce the formation of core-shell structures, such as Ag@SiO2,4 or
[email protected] A detailed study was also recently carried out on the use of PVP to stabilize silver (and gold) nanoparticles formed in DMF.6 However, in that study only low concentrations of silver salt were employed, and a spherical shape was consistently observed for the obtained nanoparticles. We, however, found that a noticeable increase of silver concentration leads to a dramatic change in the shape of the particles, and in turn to the optical properties of the dispersions. The interest in anisotropic metal nanoparticles is based on the different resonance possibilities that arise for the conduction electrons. It has been found, for instance, that metal nanorods display two distinct dipole * Corresponding author. Fax: 34 9868 12556. Tel: 34 9868 12298. E-mail:
[email protected] 10.1021/nl025638i CCC: $22.00 Published on Web 07/19/2002
© 2002 American Chemical Society
resonances due to transversal and longitudinal oscillation.7-9 A similar behavior has been recently found for gold triangles.10 For silver triangles, Jin et al.11 have shown that the transversal (out-of-plane) dipole is rather weak, while the in-plane dipole band is very intense, but quadrupole resonances are also easily observed. From previous studies on the formation of Ag nanoparticles in DMF, we know that this organic solvent is a powerful reducing agent against Ag+ ions. The chemical reaction that we proposed4 is the following: HCONMe2 + 2Ag+ + H2O f 2Ag0 + Me2NCOOH + 2H+ (1) where a carbamic acid is formed and an acidic environment is generated. Such an acidic medium was important for the polymerization of SiO2 during the deposition of silica shells on the formed silver nanoparticles.4 Although this reaction can proceed at room temperature, the reaction rate dramatically increases as the temperature is raised, so that optimal conditions for the preparation of uniform nanoparticles were found at the boiling point of DMF (156 °C).4 In the case of using PVP as a stabilizer,6 it was also found that the order of addition of the silver salt and polymer solutions affect the morphology of the obtained nanoparticles. In that work it was concluded that mixing of silver salt and PVP prior to addition into boiling DMF yields more stable and monodisperse colloids. Further investigation led us to find that the use of AgNO3 concentrations as high as 0.02 M, while keeping a low PVP (Fluka, MW 40 000) concentration (0.05-0.5 mM) leads to the formation of nanorods and nanoprisms. Similar results were recently reported by Xia and co-workers12 on the synthesis of silver nanowires through the so-called polyol process,13 which is based on the use of ethylene glycol as a solvent and reductant at high temperature, in a similar process to the one described above using DMF. For the synthesis of
Figure 1. Transmission electron micrographs showing the growth of Ag nanosprisms obtained by reduction in DMF in the presence of PVP, at high Ag+ concentration (0.02 M). (a) 5 min; (b) 15 min; (c) 60 min.
nanowires, PtCl2 was added prior to the silver salt and polymer, to form platinum nanoparticles that would then serve as seeds for the growth of the silver nanowires. In our experiments, the use of seed Pt nanoparticles was not necessary, since Ag seeds were formed through the slow reduction of AgNO3 by PVP before addition to the boiling DMF. When the solution was added before the formation of Ag nuclei, only irregular and polydisperse particles were formed. Although nanowires were also formed under certain conditions ([AgNO3] ) 0.022 M; [PVP] ) 0.06 mM), they were never found to constitute a very high proportion within the sample. Oppositely, experimental conditions can be chosen ([AgNO3] ) 0.022 M; [PVP] ) 0.4 mM) so that a large proportion of (mainly) triangular, and in general polygonal, nanoprisms were formed in solution. Electron microscopy observation indicates that initially small spheres are formed which then assemble and a meltinglike process takes place, which leads to crystalline particles with well-defined shapes.14 The formed nanoprisms become larger with time, and a wider variety of shapes are found for longer boiling times, as shown in Figure 1. The study of the optical properties of these colloids is very interesting because very few reports have been published on the formation of metal nanoprisms,10,11 which allow the experimental confirmation of theoretical calculations.11,15 Such calculations are based on the discrete dipole approximation, so that the particle is divided into small elements that interact with each other through dipole-dipole interactions, and subsequently a global evaluation of absorption and scattering is carried out. Using the discrete dipole approximation, Jin et al.9 calculated the UV-visible spectrum of perfect triangular nanoprisms (thickness 16 nm, lateral dimension 100 nm) and of triangular nanoprisms truncated at each tip. Their results indicate that for the perfect triangles one main peak should be observed at high wavelengths (770 nm), corresponding to the in-plane dipole resonance, and two peaks with much lower intensity at lower wavelengths, corresponding to the in-plane (470 nm) and out-of-plane (340 nm) quadrupole resonances, while the out-of-plane dipole resonance is only observed as a shoulder at 410 nm. In the 904
Figure 2. Time evolution of UV-visible spectra during the formation of Ag nanosprisms in DMF.
case of the truncated triangles, the main difference is a noticeable blue shift of the in-plane dipole resonance down to 670 nm. Figure 2 shows the experimental UV-visible spectra of a silver colloid in DMF during the formation of nanoprisms. Initially, only one band is present, centered at 410 nm, corresponding to the dipole resonance of silver nanospheres. As the reaction proceeds, this band red shifts, and several others gradually show up. After 20 min of boiling, four peaks are observed, which are similar to those predicted by Jin et al., except for an intense peak at 460 nm, which could be easily assigned to the presence of spherical particles within the colloid, as observed by TEM. Since the spheres present in the dispersion are in general noticeably smaller than the nanoprisms, they can be easily separated by centrifugation, with the advantage that the presence of PVP on the surface prevents aggregation during the process. Therefore, two samples prepared by boiling during 10 and 20 min were carefully centrifuged (3000 rpm, 15 min), the sediments were redispersed in ethanol, and the spectra of the resulting stable colloids were measured. The spectra before and after centrifugation/redispersion for both samples are shown in Figure 3. In both cases, the final spectrum basically coincides with the theoretical predictions,11 since the peak at 460 nm is strongly damped, as expected for a pure dispersion of nanoprisms. Nano Lett., Vol. 2, No. 8, 2002
Figure 3. UV-visible spectra of Ag nanosprisms in DMF before (solid lines) and after (dashed lines) centrifugation and redispersion in ethanol. Boiling time is indicated.
Figure 4. (a) TEM micrograph showing the geometry of truncated triangles observed in most of the samples prepared. (b) Electron diffraction pattern from a single nanoprism. Careful analysis shows that it corresponds to a single crystal with atomically flat surface.
It should also be mentioned that the in-plane resonance band is in general placed at wavelengths lower than expected, which can be explained on the basis of the deviations of the shape of the nanoparticles with respect to perfect triangles (see Figures1 and 4), especially at short boiling times. After centrifugation and redispersion, only a residual amount of small spheres is observed under TEM, as shown in Figure 4. In the same figure, an electron diffraction pattern obtained from a single nanoprism is shown, whose careful indexation indicates the morphology of an atomically flat silver surface, in the same fashion as was shown by Jin et al.11 Atomic force microscopy images (not shown) also demonstrate the flat nature of the nanoprisms, with a thickness of around 30 nm and lateral dimensions of some 200 nm for a sample boiled during 45 min. One further demonstration of the interest of the optical properties of this system is the large sensitivity toward the refractive index changes in the surrounding medium. In the case of PVP-protected silver nanospheres, it was observed (Figure 7 of ref 6) that a change in the solvent from DMF to water (refractive indexes of 1.426 and 1.333, respectively) leads to a blue shift of the plasmon band of 5 nm. However, when a similar experiment is performed on a dispersion of nanoprisms with an average lateral dimension of 80 nm (Figure 5), solvent exchange from DMF to water leads to a blue shift of almost 40 nm in the in-plane dipole resonance, while the out-of-plane quadrupole only shifts by 2 nm. This means that the plasmon oscillation over large distances is extremely sensitive to environmental changes and can thus Nano Lett., Vol. 2, No. 8, 2002
Figure 5. UV-visible spectra of Ag nanosprisms with an average lateral dimension of 80 nm, in DMF, ethanol, and water. The maximum position of the in-plane dipole resonance band is indicated for each solvent.
be used for sensor applications. Further experiments on the influence of surface charge and chemical modification are in progress, and preliminary results also show more spectacular effects than in the case of spheres.16 In summary, we have devised a simple procedure for the synthesis of Ag nanoprisms in DMF using PVP as a stabilizer. The size of the nanoprisms can be controlled through the reaction time at reflux, and the optical properties show a qualitative coincidence with theoretical predictions, though further calculations are still needed for a more accurate comparison. Acknowledgment. This work has been supported by the Spanish Xunta de Galicia, (Project no. PGIDT01PXI30106PR) and Ministerio de Ciencia y Tecnologı´a (Project no. BQU2001-3799). The authors are indebted to J. B. Rodrı´guez-Gonza´lez (CACTI, Universidade de Vigo) for his assistance with electron diffraction measurements. References (1) Kreibig, U.; Vollmer, M. Optical Properties Of Metal Clusters, Springer-Verlag: Berlin, 1995. (2) Mulvaney, P. Langmuir 1996, 12, 788. (3) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Pure Appl. Chem. 2000, 72, 83. (4) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Langmuir 1999, 15, 948. (5) Pastoriza-Santos, I.; Koktysh, D.; Mamedov, A. A.; Giersig, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 2000, 16, 2731. (6) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Langmuir 2002, 18, 2888. (7) van der Zande, B. M. I.; Bo¨hmer, M. R.; Fokkink, L. G.; Scho¨neberger, C. Langmuir 2000, 16, 451. (8) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (9) Chang, S. S.; Shih, C. W.; Chen, C. D.; Lai, W. C.; Wang, C. R. C. Langmuir 1999, 15, 701. (10) Malikova, N.; Pastoriza-Santos, I.; Schierhorn, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 2002, 18, 3694. (11) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (12) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (13) Fievet, F., Lagier, J. P., Figlarz, M. MRS Bull. 1999, December, 29. (14) Pastoriza-Santos, I.; Liz-Marza´n, L. M.; Giersig, M., manuscript in preparation. (15) Schatz, G. C. J. Mol. Struct. (THEOCHEM) 2001, 573, 73. (16) Ung, T., Giersig, M., Dunstan, D., Mulvaney, P. Langmuir 1997, 13, 1773.
NL025638I 905
