J. Phys. Chem. C 2007, 111, 18055-18059
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Photoinduced Shape Evolution: From Triangular to Hexagonal Silver Nanoplates Jing An,† Bin Tang,† Xiaohua Ning,† Ji Zhou,† Shuping Xu,† Bing Zhao,† Weiqing Xu,*,† Charlie Corredor,‡ and John R. Lombardi*,‡ Key Laboratory for Supramolecular Structure and Material of Ministry of Education of China, Jilin UniVersity, Changchun, 130012, P. R. China, and Department of Chemistry, The City College of New York, New York, New York 10031 ReceiVed: June 11, 2007; In Final Form: September 14, 2007
Successfully using the solution phase, we have prepared, in large quantities, uniform hexagonal silver nanoplates developed from silver triangular nanoprims by employing a photoinduced technique. The growth process was characterized by ultraviolet-visible (UV-vis) spectroscopy, transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM). The UV-vis spectra showed that three bands of hexagonal silver nanoplates appear at 341 (weak), 368 (medium), and 498 (strong) nm. TEM images showed that hexagonal silver nanoplates had an average edge size of 25.9 nm and thickness of 15.7 ( 1.0 nm. The mechanism of the conversion from triangular to hexagonal nanoplates has also been studied. Triangular silver nanoplates were at first fabricated through seed-mediated growth of silver particles in the presence of trisodium citrate. Subsequently, the truncation of triangular nanoplates led to the formation of hexagonal nanoplates.
Introduction Research in the growing field of nanomaterials has attracted special attention in material science. Nanomaterials, such as silver and gold nanoparticles, have unique chemical and physical properties including optical, electronic, magnetic, and catalytic properties.1 Both the size and shape of anisotropic nanomaterials provide useful control over the properties mentioned above. These nanoparticles exhibit anisotropic optical absorption properties associated with collective oscillations of conduction electrons, which are also known as surface plasmon resonances (SPR).2 For example, Schultz and co-workers has shown that pentagonal nanoparticles display a peak plasmon resonance wavelength in the range 500-560 nm while triangular nanoprisms display resonances in the range 530-700 nm.3 Recently, Mirkin et al. has also successfully prepared triangular nanoprisms that showed a strong in-plane dipole plasmon resonance at 670 nm.4 Over the last two decades, many distinctively shaped nanostructures have been observed or synthesized using various chemical approaches. Due to the potential applications of nanoparticles in optics and interconnection in nanoelectronics,5 many chemical techniques have been employed to synthesize anisotropic nanoparticles. However, further investigations in 1D and 2D nanomaterials, such as nanobeams6 and nanoprisms,7 should be expanded to better understand their behavior. Many synthetic methods have been used in a practical and versatile way8 for production of nanoparticles in the shape of triangular,9 hexagonal,10 square,11 and circular plates.12 However, the preparation of hexagonal silver nanoplates by controlling kinetic growth in liquid solution is very limited and different mixtures of shapes were often obtained from this mixture.13 Thus, * To whom correspondence should be addressed. E-mail: wqxu@ jlu.edu.cn (W.X.);
[email protected] (J.R.L.). Fax: +86-43185193421 (W.X.). † Jilin University. ‡ The City College of New York.
successful synthetic strategies are still a formidable challenge for the preparation of uniform hexagonal silver nanoplates. In this paper, we have successfully prepared uniform hexagonal silver nanoplates using a solution-phase chemical approach, which are generated in large quantities by choosing light with a selected wavelength used to drive the photochemical growth. Our particular interest is that hexagonal silver nanoplates are developed from triangular silver nanoplates by the photoinduced effect. Experimental Section Materials. AgNO3 (99.5%) was obtained from Wako Pure Chemical Industries, Ltd. NaBH4 (98%) was obtained from Sigma Chemical Co. Trisodium citrate (98%) was purchased from Shanghai Chemical Reagent Co., Ltd. All chemicals used were analytic grade reagents without further purification. Instrumentation. Light for the photoinduced effect was obtained by using a 70-W sodium lamp purchased from Shanghai Yaming Co., Ltd. UV-vis spectra were recorded on a Shimadzu UV-3100 spectrophotometer. Transmission electron micrographs (TEM) were measured with a Hitachi H-8100 IV operating at 200 kV. HRTEM images were measured with a JEOL 3010 high-resolution transmission electron microscope operating at 300 kV. Preparation of Hexagonal Silver Nanoplates. A typical experimental procedure was carried out by the following steps: Silver seeds were prepared by dropwise addition of NaBH4 solution (8.0 mM, 1.0 mL) to an aqueous solution of AgNO3 (0.1 mM, 100 mL) in the presence of trisodium citrate (0.1 mM) under vigorous stirring. The yellow silver seeds were then irradiated with a conventional 70-W sodium lamp. A set of color changes for the preparation of hexagonal silver nanoplates was observed during the course of the reaction. Initially, the solution turned green after being irradiated 3.5 h. After 8 h the solution turned purple and finally pink over 10 h (Figure 1). The final pink products consisted of two different size distributions of
10.1021/jp0745081 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/10/2007
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Figure 1. Photograph of the reaction solution at different stages: (I) 0 h; (II) 3.5 h; (III) 8 h; (IV) 10 h.
nanoparticles, small nanodisks and large hexagonal nanoplates. To separate the hexagonal Ag nanoparticles, we used a centrifugation process at 4000 rpm for a period of 3 min in the presence of trisodium citrate. Results and Discussion Figure 2a shows TEM images of hexagonal silver nanoplates after centrifugation. In Figure 2b clearly we define three curves. The first one, curve a, shows the UV-vis spectrum of silver nanoparticles before centrifuging. The second one, curve b, is the UV-vis spectrum of silver nanoparticles corresponding to Figure 2a, which are located at the bottom layer of the sample after centrifuging. The last one, curve c, shows the UV-vis spectrum of the silver nanoparticles that are located at the top layer of the sample after centrifugation. Three bands of curve b appeared at 341 (weak), 368 (medium), and 498 (strong) nm, which can be assigned to quadrupole, out-of-plane dipole, and in-plane dipole plasmon resonances of hexagonal silver nanoplates, respectively.4 Curve c shows a single peak at 395 nm, indicating an individual presence of silver nanodisks.14 Figure S1 shows the TEM image of the small silver nanodisks. From this image, we can conclude that the small nanoparticles were disklike and not spherical in shape. In the spectrum of the mixture of small silver nanodisks and hexagonal nanoplates, curve a shows three peaks at 341 (weak), 393 (medium), and 509 (strong) nm, respectively. The peak of hexagonal nanoplates at 368 nm which appeares in curve b cannot be observed in curve a because the band of nanodisks at 395 nm is so strong
An et al. that it covers the neighboring weak peaks. Meanwhile, the band at 395 nm exhibited a blue-shift to 393 nm owing to the influence of the band at 368 nm. As well, the band at 509 nm exhibited a blue-shift to 498 nm, which might be due to the change of environment during the redispersion of silver nanoparticles located at the bottom layer of the sample after centrifuging. In the experiment, we observed that the longest wavelength band of silver nanoparticles was sensitive to the environment of silver nanoparticles. Though the slight change of environment caused the band at 509 nm to blue-shift, we think it was reasonable to assign the band at 509 nm to hexagonal silver nanoplates in the centrifugal process. The growth process of hexagonal silver nanoplates was monitored by UV-vis spectra proceeding in time (Figure 3a). For the silver seeds (curve a in Figure 3a), a single absorption band due to the plasmon resonance of small spherical particles of the seed solution appeared at 392 nm. After 3.5 h of irradiation with sodium light, two distinctive peaks appeared at 333 (weak, as a shoulder peak) and 738 (medium) nm, respectively (curve b in Figure 3a). According to Mie’s theory, only a single surface plasmon resonance band could be observed in the absorption spectra of spherical nanoparticles, and anisotropic particles could give rise to two or more surface plasmon resonance bands depending on the shape of the particles.15 These two distinctive peaks which appeared at 333 and 738 nm can be assigned to quadrupole and in-plane dipole plasmon resonances of triangular nanoplates, respectively. During this period, the peak at 392 nm showed a slight red shift to 395 nm while its intensity decreased. As the irradiation continued, the band located at 738 nm showed a significant blue shift to 509 nm (shown in curve d), concomitant with the red shift of the 333nm band to 341 nm (shown in inset of Figure 3a). The 341and 509-nm bands can be assigned to quadrupole and in-plane dipole plasmon resonances of hexagonal nanoplates, respectively. Meanwhile, the location of the peak at 395 nm remained basically unchanged but its intensity increased progressively (shown in curves b-d). We have also noticed the color of the silver seeds turned lighter gradually under room light presence before the irradiation of sodium light, and we observed a series of changes of silver seeds using the UV-vis spectrometer to monitor the process. The 392-nm band showed a progressive decrease in intensity (shown in Figure 3b), which might be due to the fragmentation of silver seeds. This phenomenon was the most obvious when the molar ratio of NaBH4 to AgNO3 was
Figure 2. (a) TEM image of hexagonal silver nanoplates. (b) UV-vis spectra of silver nanoparticles before centrifuging (curve a), the bottom after centrifuging (curve b), and the top after centrifuging (curve c), respectively.
Photoinduced Shape Evolution
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Figure 3. (a) UV-vis spectra showing the conversion of silver seeds to hexagonal nanoplates: before irradiation (curve a) and after 3.5 (curve b), 8 (curve c), and 10 (curve d) h of irradiation. Inset: Enlarged UV-vis spectra between 315 and 365 nm. (b) UV-vis spectra of silver seeds placed before irradiation for different times: (1) 0 min; (2) 10 min; (3) 20 min; (4) 30 min; (5) 40 min; (6) 50 min. (c) UV-vis spectra of silver seeds irradiated with a sodium lamp for different times: (7) 10 min; (8) 20 min; (9) 30 min; (10) 40 min.
Figure 4. TEM images showing the conversion of silver seeds to hexagonal nanoplates: (a) before irradiation and after (b) 3.5, (c) 8, and (d) 10 h of irradiation. Inset: Enlarged photos are showing triangular (e) and hexagonal (f) silver nanoplates stacks.
1:1. After irradiation with a sodium lamp, its intensity increased until triangular nanoplates began to appear (Figure 3c). Correlated with the UV-vis spectroscopic observations, the TEM images showed that the initial silver seeds (Figure 4a) were first converted into triangular silver nanoplates (Figure 4b) and, subsequently, they turn to hexagonal plates under irradiation of sodium light (Figure 4d). During the initial stages of growth (after 3.5 h, Figures 4b and 5a), silver nanoprisms with edge length 76.2 nm (designated as type 1) were the dominant shape of the solution, while some small silver nanoparticles (designated as type 2) with diameter of about 7.6
nm were also observed adjacent to the triangular particles. With increase of the irradiation time (Figure 4c), the type 1 particle changed from a prism to a hexagon shape and the diameter of the type 2 particle increased corresponding to the enhancement of the peak at 395 nm (curves b-d in Figure 3a). After 10 h, all of the silver prisms were completely converted into hexagonal silver nanoplates with edge length 25.9 nm (Figures 4d and 5b). Meanwhile, the particles of type 2 changed to nanodisks with the diameter of 11.2 nm. Although, the average edge lengths for triangular and hexagonal nanoplates are different, their thickness is essentially unchangeable (14.1 ( 1.7 nm at 3.5 h vs 15.7 ( 1.0 nm at 10 h, Figure 4e,f). Figure S3 shows the electron diffraction pattern taken from an individual nanoplate by directing the electron beam perpendicular to one of its flat faces. The electron diffraction pattern indicates that each nanoplate was a single crystal. The hexagonal symmetry of these patterned spots implied that the two flat faces of each nanoplate were bounded by the (111) planes and the (111) basal planes were not changed during this shape evolution process. Furthermore, Figure S4 shows the HR-TEM images of the triangular and hexagonal nanoplates recorded perpendicular to flat faces of an individual nanoplate. The fringes of the triangular silver nanoplate are separated by 2.49 Å, which can be ascribed to the (1/3) {422} reflection that is generally forbidden for an fcc lattice. The fringe spacing of the hexagonal nanoplate is also measured as 2.49 Å, which is the same as the triangular nanoplate. To gain insight into the mechanism of this unusual and remarkably efficient conversion of triangular to hexagonal nanoplates, a set of experiments were further carried out by changing the amount of reagent. Triangular silver nanoplates (type 1) and small silver nanoparticles (type 2) were obtained when the molar ratio of Na3C6H5O7 and AgNO3 was at the range 0.8-2. The triangular silver nanoplates (type 1) could only be observed when their molar ratio was at the range of 3-50. The
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Figure 5. Histograms of silver nanoparticles after 3.5 (a) and 10 (b) h of irradiation illustrating the particle size distributions (the radii for type 1 and the edge lengths for type 2) as bimodal.
growing process of triangular silver nanoplates can be attributed to the transformation of small nanoparticles into other particle geometries via the Ostwald ripening process.16 The results indicate that, due to the photoinduced effect, the vertices of triangular nanoplates gain higher energy and begin to be truncated as reactive dots (shown in Figure 4c). In this process, the effect of heat is obvious. The triangular silver nanoplates will change to nanodisks rather than hexagonal nanoplates when the reactive system is above 80 °C. Hexagonal nanoplates cannot be changed from triangular nanoplates when the molar ratio of Na3C6H5O7 and AgNO3 is at the range 5-50. A detailed mechanism for this process has been suggested in previous articles on the self-limiting photoinduced growth of Ag nanoplates.4,9a We believe the mechanism here is similar. Meanwhile, the effect of heat is also limited in this process. The final products are triangular silver nanoplates (Figure S2). The results suggest that the vertices of triangular nanoplates are fixed by excess citrate ions in this case. The thickness values of triangular and hexagonal silver nanoplates are essentially similar (14.1 ( 1.7 nm vs 15.7 ( 1.0 nm), implying that the (111) basal plane is not changed during the growth process. The result shows that the anisotropic growth of silver mainly occurrs at the edge (110), (101), and (011h) planes of the triangular silver nanoparticles.17 It is reasonable to suppose that
SCHEME 1: Schematic Diagram of Photoinduced Shape Evolution of Triangular to Hexagonal Silver Nanoplates
TABLE 1: Aspect Parameters (nm) of Silver Nanoparticles Corresponding to Scheme 1 colloids scheme part b d
type 1 a1 ) 76.2 a2 ) 25.9
h1 ) 14.1 ( 1.7 h2 ) 15.7 ( 1.0
type 2 R1 ) 22.0 R2 ) 22.4
2r1 ) 7.6 2r2 ) 11.2
the citrate molecule plays a key role in this anisotropic growth, which acts as a capping agent for the silver particles and a photoreducing agent for the silver ions.18 During the truncation process, the (111) plane of the silver triangular particle was protected by the well-defined self-assembled layer of citrate ions and further change of the (111) plane was prevented. Combining the information from the UV-vis spectra and TEM images, we can further explore the mechanism of how light influences
Photoinduced Shape Evolution the interconversion and growth of hexagonal silver nanoplates (Scheme 1 and Table 1). The UV-vis spectra showed the 738nm peak was blue-shifted and the 333-nm peak was red-shifted, which caused the in-plane shape transformation from triangle to hexagon. TEM images exhibiting triangular and hexagonal nanoplates possess the similar radii of inscribed circles (22.0 nm vs 22.4 nm), which indicates that hexagonal nanoplates were derived from triangular nanoplates by truncation. Conclusion We have shown that uniform hexagonal silver nanoplates developed from silver nanoprisms have been successfully prepared by using a solution-phase chemical approach. These are generated in large quantities by choosing a sodium lamp with the appropriate wavelengths used to drive the photochemical growth. In this method, triangular silver nanoplates are fabricated first through seed-mediated growth of silver particles in the presence of trisodium citrate. Subsequently the truncation of triangular nanoplates leads to the formation of hexagonal nanoplates. The growth process has been characterized by UVvis spectroscopy, TEM, and HRTEM. TEM images show that hexagonal silver nanoplates have an average edge size of 25.9 nm and thickness of 15.7 ( 1.0 nm (Scheme 1 and Table 1). The UV-vis spectra show that three bands of hexagonal silver nanoplates appear at 341 (weak), 368 (medium), and 498 (strong) nm. Acknowledgment. This work is supported by the National Natural Science Foundation of China (NSFC20573041, -20773045, and -20627002). We are also indebted to the National Institute of Justice (Department of Justice Award No. 2006-DN-BXK034) and the City University Collaborative Incentive program (No. 80209). This work was also supported by the National Science Foundation under Cooperative Agreement No. RII9353488, Grant Nos. CHE-0091362, CHE-0345987, and ECS0217646, and by the City University of New York PSCBHE Faculty Research Award Program. Supporting Information Available: Additional TEM and UV-vis images. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (b) Creighton, J. A.; Eadon, D. G. J. Chem. Soc.,
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