Growth of Tetrahedral Silver Nanocrystals in Aqueous Solution and

Aug 22, 2008 - Silver nanocrystals with tetrahedral shapes and {111} faces have been ... The nanocrystals of Td symmetry were formed under the effect ...
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Langmuir 2008, 24, 10407-10413

10407

Growth of Tetrahedral Silver Nanocrystals in Aqueous Solution and Their SERS Enhancement Ji Zhou,† Jing An,† Bin Tang,† Shuping Xu,† Yanxin Cao,† Bing Zhao,† Weiqing Xu,*,† Jingjing Chang,†,‡ and John R. Lombardi‡ State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun, 130012, P. R. China, and Department of Chemistry, The City College of New York, New York, New York 10031 ReceiVed March 27, 2008. ReVised Manuscript ReceiVed July 5, 2008 Silver nanocrystals with tetrahedral shapes and {111} faces have been synthesized by the light-driven growth method in an aqueous solution. The nanocrystals of Td symmetry were formed under the effect of tartrate and citrate as the structural-directing reagents at the appropriate stages of reaction. Further, the nanocrystals may be assembled through electrostatic interaction to develop large-scale particle surfaces with sharp vertexes, which can generate strong localized electromagnetic field for surface-enhanced Raman scattering (SERS) studies. Benzenethiol was used as the probe to evaluate their SERS enhancement, and enhancement factors of up to 106 are reached. As a kind of promising material, these novel nanocrystals will be applied in surface enhanced spectroscopy and plasmonics field.

1. Introduction Anisotropic nanocrystals have received considerable attention in recent years for the remarkable size and shape dependences of their physical, optical, electronic, magnetic, and catalytic properties. As an example, tetrahexahedral platinum (Pt) nanocrystals enclosed by 24 high-index faces exhibit considerably enhanced catalytic activity, compared with Pt nanospheres and Pt/C catalyst.1 Therefore, the controllable preparation of nanocrystals with different shapes and exposed surfaces is desirable. Recently, among the metallic nanocrystals with well-controlled shapes that have been successfully synthesized are nanoprisms,2 nanodisks,3 nanorods,4 nanowires,5 and nanobelts.6 The platonic solids of favorite geometries feature apparent simplicity, aesthetic beauty, and perfect symmetry and are considered as the ideal models in mathematics and physics. Metallic nanocrystals in platonic shape receive much attention for their anisotropic optical properties, strong localized electromagnetic field, and distinct scattering signatures.7 Four * Author to whom correspondence should be addressed. Tel: +86-43185159383. Fax: +86-431-85193421. E-mail: [email protected]. † Jilin University. ‡ The City College of New York. (1) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (2) (a) Jin, R. C.; Cao, Y. C.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C. Science 2001, 294, 1901. (b) Xiong, Y.; McLellan, J. M.; Chen, J.; Yin, Y.; Li, Z. Y.; Xia, Y. N. J. Am. Chem. Soc. 2005, 127, 17118. (c) Huang, W. L.; Chen, C. H.; Huang, M. H. J. Phys. Chem. C 2007, 111, 2533. (3) (a) Chen, S.; Fan, Z.; Carroll, D. L. J. Phys. Chem. B 2002, 106, 10777. (b) Chen, Y. B.; Chen, L.; Wu, L. M. Inorg. Chem. 2005, 44, 9817. (c) Sun, X.; Jiang, X.; Dong, S.; Wang, E. Angew. Chem., Int. Ed. 2004, 43, 6360. (4) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2002, 13, 1389. (b) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (c) Hu, J. Q.; Chen, Q.; Xie, Z. X.; Han, G. B.; Wang, R. H.; Ren, B.; Zhang, Y.; Yang, Z. L.; Tian, Z. Q. AdV. Funct. Mater. 2004, 14, 183. (5) (a) Qin, L.; Park, S.; Huang, L.; Mirkin, C. A. Science 2005, 309, 113. (b) Sun, Y. G.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. N. Chem. Mater. 2002, 14, 4736. (c) Zhang, D.; Qi, L.; Yang, J.; Ma, J.; Cheng, H.; Huang, L. Chem. Mater. 2004, 16, 872. (6) (a) Yu, D.; Yam, V. W. W. J. Phys. Chem. B 2005, 109, 5497. (b) Pileni, M. P. J. Phys. Chem. C 2007, 111, 9019. (7) Tao, A.; Prasert, S.; Yang, P. D. Angew. Chem., Int. Ed. 2006, 45, 4597.

platonic nanocrystals, tetrahedron,8 hexahedron (cube),9 octahedron,10 and icosahedron,11 have been synthesized upon controlling the growth rate of different crystal faces, of which the tetrahedral, octahedral, and icosahedral nanocrystals have {111} faces, while the hexahedral nanocrystal is a single domain with {100} faces. Usually, the tetrahedral shape occurs in covalent bonds of molecules. Tetrahedral clusters of Ta4 have previously been synthesized.12 Truncated tetrahedral Ag nanoparticle arrays may also be formed by nanosphere lithography (NSL).13 Even though the tetrahedron is of the simplest geometry, the preparation of the tetrahedral nanocrystals in large scale is still a great challenge. The light-driven growth method has been proven to be one of the most successful approaches to produce size- and shapecontrolled metallic nanoparticles.14 It was first reported by Mirkin et al.2a that silver nanoprisms could be obtained by photoinduced aggregation of small seeds with visible light. In their subsequent work, they were able to produce monodisperse Ag nanoprisms with controlled edge length in the 30-120 nm range via dual-beam illumination (with a primary and a secondary light) at two appropriate wavelengths.15 For (8) (a) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. D. Angew. Chem., Int. Ed. 2004, 43, 3673. (b) Ng, C. H. B.; Fan, W. Y. J. Phys. Chem. C 2007, 111, 9166. (c) Ko, W. Y.; Chen, W. H.; Tzeng, S. D.; Gwo, S.; Lin, K.-J. Chem. Mater. 2006, 18, 6097. (9) (a) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (b) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 2154. (c) Yu, D.; Yam, W. W.-W. J. Am. Chem. Soc. 2004, 126, 13200. (d) Seo, D.; Park, J. C.; Song, H. J. Am. Chem. Soc. 2006, 128, 14863. (10) (a) Li, C. C.; Shuford, K. L.; Park, Q.-H.; Cai, W. P.; Li, Y.; Lee, E. J.; Cho, S. O. Angew. Chem., Int. Ed. 2007, 46, 3264. (b) Carbo´-Argibay, E.; Rodrı´guez-Gonza´lez, B.; Pacifico, J.; Pastoriza-Santos, I.; Pe´rez-Juste, J.; LizMarza´n, L. M. Angew. Chem., Int. Ed. 2007, 46, 8983. (c) Zhang, J. G.; Gao, Y.; Alvarez-Puebla, R. A.; Buriak, J. M.; Fenniri, H. AdV. Mater. 2006, 18, 3233. (d) He, P.; Shen, X.; Gao, H. J. Colloid Interface Sci. 2005, 284, 510. (11) (a) Zhou, M.; Chen, S.; Zhao, S. J. Phys. Chem. B 2006, 110, 4510. (b) Kwon, K.; Lee, K. Y.; Kim, M.; Lee, Y. W.; Heo, J.; Ahn, S. J.; Han, S. W. Chem. Phys. Lett. 2006, 432, 209. (12) Wang, H. M.; Craig, R.; Haouari, H.; Dong, J. G.; Hu, Z. D.; Vivoni, A.; Lombardi, J. R.; Lindsay, D. M. J. Chem. Phys. 1995, 103, 3289. (13) (a) Zhang, X.; Hicks, E. M.; Zhao, J.; Schatz, G. C.; Van Duyne, R. P. Nano Lett. 2005, 5, 1503. (b) Baia, M.; Baia, L.; Astilean, S. Appl. Phys. Lett. 2006, 88, 143121. (14) Pastoriza-Santos, I.; Liz-Marza´n, L. M. J. Mater. Chem. 2008, 18, 1724. (15) Jin, R.; Cao, Y. W.; Hao, E.; Me´traux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487.

10.1021/la800961j CCC: $40.75  2008 American Chemical Society Published on Web 08/22/2008

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Figure 1. (A) Photographs of reaction solution at different key intermediates with the irradiation time. (B) Temporal UV-vis spectra showing the conversion from silver seeds into TSNCs (a) before irradiation and after (b) 9 h, (c) 13 h, (d) 18 h, and (e) 29 h of irradiation. (C) Plots of the absorbance of the TSNCs at 395 nm (red triangle) and 642 nm (black diamond) as a function of irradiation time.

Figure 2. TEM images mapping the morphology changes of the TSNCs (A) before irradiation and after (B) 9 h, (C) 18 h, and (D) 29 h of irradiation.

further mechanism studies, argon laser beams,16 fluorescent tubes combined with different color filters,17 and low-intensity light-emitting diodes17b are used as light sources. It is observed that the final particle size and geometry are controllable by tuning the illumination wavelength. Our research group also reported that the optical properties of the nanoparticles were linearly dependent on the excitation wavelength. A wavelengthdependent self-limiting process governed by the surface plasmon resonance controlling the photochemical reduction of particles is suggested.18 A series of interesting discoveries support the conclusion that light can be used as a major control (16) Maillard, M.; Huang, P.; Brus, L. Nano Lett. 2003, 3, 1611. (17) (a) Callegari, A.; Tonti, D.; Chergui, M. Nano Lett. 2003, 3, 1565. (b) Bastys, V.; Pastoriza-Santos, I.; Rodrı´guez-Gonza´lez, B.; Vaisnoras, R.; LizMarza´n, L. M. AdV. Funct. Mater. 2006, 16, 766. (c) Xue, C.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 2036. (18) Zheng, X. L.; Xu, W. Q.; Corredor, C.; Xu, S. P.; An, J.; Zhao, B.; Lombardi, J. R. J. Phys. Chem. C 2007, 111, 14962.

parameter in metallic nanoparticle growth reactions related to plasmon excitation. Additionally, Rocha and Zanchet19 confirmed that the origin of anisotropic growth in the photoinduced growth process is not related to plasmon excitation but rather to intrinsic aspects of the seeds, such as structural defects or the capping reagent. Bis(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium salt (BSPP) and poly(vinylpyrrolidone) (PVP), which allow one to gain better control over the initial Ag particle size and dispersity, are widely used as the capping reagents in the light-driven method. Moreover, citrate is also proved to be an excellent capping reagent for the shape-control of nanocrystals. Sun and Xia have claimed that citrate anions were the key component in determining the efficiency of the (19) (a) Rocha, T. C. R.; Winnischofer, H.; Westphal, E.; Zanchet, D. J. Phys. Chem. C 2007, 111, 2885. (b) Rocha, T. C. R.; Zanchet, D. J. Phys. Chem. C 2007, 111, 6989.

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Scheme 1. Schematic Diagram of the Proposed Formation of the TSNCs

photoinduced, morphological transformation process. On the basis of the studies mentioned above,20,21 hydroxy acid salts could present a type of promising structural-directing effect in the controllable synthesis of anisotropic nanocrystals through a light-driven growth method. To evolve this approach, another hydroxy acid salt, tartrate, has been explored in the light-driven

growth of silver nanocrystals. Reported here is the preparation of tetrahedral silver nanocrystals (TSNCs) utilizing tartrate and citrate as structuraldirecting reagents through the light-driven growth method in an aqueous solution. The growth process is monitored by ultraviolet-visible (UV-vis) spectroscopy and transmission

Figure 3. SEM images of the single TSNCs-layer film. Images were taken on (A) a horizontal support flat and (B) a tilted 45° angle support.

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Figure 4. Representative XRD pattern of the single TSNCs-layer film.

electron microscopy (TEM). A preliminary growth mechanism of TSNCs is proposed. Furthermore, the morphology and structure of these novel nanocrystals are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron diffraction (ED). A potential application of the TSNCs as a SERSactive material has been probed with the benzenethiol molecule.

2. Experimental Section 2.1. Materials. Silver nitrate (99.5%), trisodium citrate (98%), disodium tartrate (99%), H2SO4, and 30% H2O2 were obtained from Beijing Chemical Plant. Sodium borohydride (98%) was obtained from Sinopharm Chemical Reagent (Shanghai) Co., Ltd. PVP (Mw ) 30 000) was obtained from Beijing Dingguo Co. Poly(diallydimethylammonium chloride) (PDDA) (Mw ) 200 000-350 000) was purchased from Aldrich. Benzenethiol was purchased from Fluka. All chemicals are analytic grade reagents and were used without further purification. Deionized water was obtained from Hangzhou Wahaha Co. 2.2. Preparation of TSNCs. In the process of preparation of TSNCs, two steps were involved, i.e., nucleation and growth. First, silver seeds solution was prepared as follows: NaBH4 solution (1 mM, 1.0 mL) was added dropwise to the aqueous solution (99 mL) composed of AgNO3 (0.1 mM) and tartrate (2 mM) under virgous stirring for 5 min. The freshly prepared solution of silver seeds (100 mL) was irradiated under a 70-W sodium lamp for 9 h. Then a small amount of PVP (0.0033 g) was added to control the size and dispersity of tartrate-capped silver nanoparticles. Subsequently, citrate (0.0029 g) was added to the above solution and the resulting mixture was continuously illuminated for 20 h more. The solution turned to a turbidly greenish color, indicating that the reaction is completed. The molar concentration of the final colloid is 1 × 10-4 M (with respect to the initial molar concentration of Ag+). The as-prepared TSNCs colloid stabilized by tartrate and citrate has been found to be stable for at least 2 weeks without any deterioration of the spectral property. 2.3. Characterization of TSNCs. A 70-W sodium lamp was supplied by Osram Co. UV-vis extinction spectra were recorded on a Shimadzu UV-3100 spectrophotometer, using 1 cm path length quartz cuvettes. TEM images and corresponding electron diffraction (ED) were taken on a Hitachi H-8100 IV TEM operated at 200 kV, while SEM images were obtained using a Philips XL30 ESEM-FEG operated at 15.0 kV. XRD patterns were recorded on a Rigaku D/max2550 diffractometer using Cu KR (40 kV, 200 mA) radiation. Samples for TEM were prepared by dripping a drop of the TSNCs colloid onto the carbon-coated copper grids. The samples for SEM and XRD were prepared by assembling the TSNCs colloid on a glass (20) (a) Jia, H. Y.; Xu, W. Q.; An, J.; Li, D. M.; Zhao, B. Spectrochim. Acta A 2006, 64, 956. (b) An, J.; Tang, B.; Ning, X. H.; Zhou, J.; Xu, S. P.; Zhao, B.; Xu, W. Q.; Corredor, C.; Lombardi, J. R. J. Phys. Chem. C 2007, 111, 18055.

Zhou et al. slide. Raman spectra were measured with a confocal microscopy Raman spectrometer (Renishaw 1000 model) equipped with a CCD detector and a holographic notch filter. Radiation of 514.5 nm from an air-cooled argon ion laser (Spectra-Physics model 163-C4260) was used for excitation. SERS-active substrates were prepared as following: silicon slides were first submerged in a 70 °C piranha solution consisting of a 3:1 (v/v) mixture of concentrated H2SO4 and 30% H2O2 for 30 min and rinsed copiously with H2O. Then they were immersed in a 0.5 wt % PDDA solution for about 30 min, rinsed copiously with H2O, and dried under a gentle stream of nitrogen. Finally, the derivatized slides were placed into the TSNCs colloid and allowed to sit for 24 h to form single nanoparticle-layer substrates. The substrates were then immersed into 1 × 10-6 M benzenethiol in ethanol for 1 h and washed with ethanol to remove the physically adsorbed molecules for the SERS measurement. To measure the Raman spectra of the neat benzenethiol, a glass capillary was used as the sampling container. The laser beam penetrates the capillary wall and focused on the liquid. Data acquisition was the result of a single 10-s accumulation.

3. Results and Discussion 3.1. Growth of TSNCs. The growth of TSNCs was conducted in two steps (Figure 1A): (1) a solution of silver seeds stabilized by tartrate was irradiated under a sodium lamp for 9 h and (2) citrate was then added to the above solution and the resulting mixture was continuously irradiated for another 20 h. Photographs of several intermediates in the light-driven process are shown in Figure 1A. A set of color changes, from light yellow to turbidly green, was observed during the light-driven process. Figure 1B depicts the UV-vis spectra subsequently taken at different stages of the continuous conversion process. The surface plasmon band for silver seeds appeared at 404 nm (curve a). Nine hours later, this peak in the spectrum underwent a blue shift to 395 nm (curve b, species 1). The intensity of this peak was found to increase; the full width at half-maximum (fwhm) appeared narrower. These changes imply that the seeds rearranged, and spherical nanoparticles were formed during this period. After adding some citrate, the spectra of the sample underwent a notable transformation: the plasmon peak at 395 nm slightly weakened, broadened, and was red-shifted to 414 nm, while the longer wavelength peak slightly increased in intensity and was blueshifted to 642 nm (curve e, species 2). The peak at 642 nm turns narrower, higher, and more symmetrical compared with the peak at 673 nm (curve d). These spectral changes illustrated that the growth of TSNCs approached completion. It is noticed in Figure 1C that the growth of species 2 was at the expense of species 1. Finally, the intensities of two species no longer changed, implying the evolution of silver nanocrystals from spherical to tetrahedral ones. The turbidly greenish color of the end product reflects the various multipolar plasmon modes. According to the calculations of Schatz et al.,22 there is a strong peak centered at 642 nm (in-plane dipole resonance), a medium one at 414 nm (out-of-plane dipole resonance), and a weak one at 341 nm (outof-plane quadrupole resonance). TEM results also clearly show that the TSNCs evolve from the initial seeds (Figure 2). During the initial growth period, the species shaped in spheres, disks, and tetrahedrons can be seen (Figure 2B). The tetrahedral ones exhibit edge lengths ranging from 20 to 50 nm. After adding citrate, both the size (about 90 nm) and population of the TSNCs increase with irradiation time, whereas the population of spherical particles decreases concomitantly (Figure 2C). Twenty-nine hours later, it is found that large amounts of initial spherical silver seeds are completely converted into TSNCs. The TSNCs (Figure 2D) were separated from the smaller particles by centrifuging (6000 rpm, for 5 min).

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Figure 5. (A) TEM image of an individual TSNC. (B) The electron diffraction pattern of the TSNC. Table 1. Summary of Products Obtained under Different Experimental Conditions reaction conditions molar ratioa trial 1 2 3 4

(TSNCs) (state Ι) (state Π) (state III) a

+

products

Ag

tartrate

citrate

added order

time

color

shape

1 1 1 1

20 0 20 20

1 1 0 1

tartrate first, citrate later

∼29 h, slow ∼ 5 h, fast ∼18 h, slow ∼ 10 h, fast

turbidly greenish blue turbidly yellow bluish-purple

tetrahedral truncated triangular and discal polyhedral and tetrahedral triangular, discal, tetrahedral, and polyhedral

simultaneously

In the final mixture solution, [Ag+] was always kept at 10-4 M for each trial.

Table 2. Assignment and the SERS Enhancement Factors (EF) of Several Raman Peaks of Benzenethiola shift (cm-1) assignment

NR

SERS

EF

S-H bending + in-plane ring def In-plane ring def C-S stretching + in-plane ring def C-C stretching

999 1024 1092 1583

997 1021 1072 1572

1.75 × 106 3.05 × 106 1.02 × 107 8.97 × 106

a Normal Raman (NR) peaks are from neat benzenethiol and SERS peaks are from 1 × 10-6 M benzenethiol on the single-layer TSNCs film. Data and assignments obtained from ref 25.

Figure 6. Representative Raman spectra: (a) the SERS spectrum for 1 × 10-6 M benzenethiol on the single TSNCs-layer film and (b) the normal Raman (NR) spectrum for neat benzenethiol.

In the light-driven process, the tartrate-capped silver seeds underwent a rearrangement to spherical nanoparticles, which is shown through the UV-vis spectra (curves a and b). Further, the nanoparticles coalesced to each other to form the rudimental species in truncated tetrahedral shapes under the effect of tartrate as the structural-directing reagent (Figure S1, Supporting Information). After 9 h, citrate was added to the reaction system as the capping reagent for the silver particles as well as the

photoreducing reagent for the silver ions.16 During this period, particle edge-sharpening occurred concomitantly with particle growth. Meanwhile, the smaller seeds were dissolved and larger particles grew more, which is typical of Ostwald ripening.23 Finally, the shape and size focusing may improve the growth of TSNCs. The whole light-driven process is depicted in Scheme 1, summarized as nucleation (rudiment formation), growth, and shape and size focusing. Thus, the formation of TSNCs suggests that the TSNCs observed here result from the kinetically limited reaction equilibrium. 3.2. Morphology and Structural Characterization. Since TEM images display only the projection of the particles onto the (21) Sun, Y. G.; Xia, Y. N. AdV. Mater. 2003, 15, 695. (22) (a) Schatz, G. C.; Van Duyne, P. R. In Handbook of Vibrational Spectroscopy; Chalmers, J. M.; Griffiths, P. R., Eds.; John Wiley & Sons Ltd.: Chichester, UK, 2002. (b) Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6269. (23) Hoang, T. K. N.; Deriemaeker, L.; La, V. B.; Finsy, R. Langmuir 2004, 20, 8966, and references therein.

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observed plane, SEM measurements were performed to investigate all the crystal faces of the TSNCs. It is found in Figure 3 that a large proportion (typically around 90%) of the particles have regular tetrahedral symmetry, with most of the remaining particles having the shape of triangular prisms and flat-topped tetrahedrons. To intensively investigate the three-dimensional morphology, a tilted (45°) support was adopted. Figure 3B shows that the TSNCs are “standing” with one of the planes on the support, as “hills” on a plain. The average edge length is about 118 ( 18 nm by statistics of more than 200 particles (Figure S2, Supporting Information). It is known that the TSNCs have Td symmetry in which three triangular faces meet at each corner. The nanostructures with the well-defined facets and corners are predicted to have distinct scattering signatures, and the sharp vertexes can generate strong localized electromagnetic field for applications of sensing studies such as surface-enhanced Raman scattering.7 In TEM and SEM experiments, only a small portion of the products is tested, while XRD measures the bulk of colloid. There is a sharp and intense peak at 2θ ) 38.2° (Figure 4), which is the reflection of the (111) plane of face-centered cubic (fcc) lattice (JCPDS File 04-0783), indicating that the {111} faces tend to be preferentially oriented parallel to the surface of the supporting substrate. Figure 5A displays the morphology of an individual TSNC, while Figure 5B is the electron diffraction (ED) pattern, which is taken with the face attached to Cu grid being perpendicular to the incident electron beam. The clear spots and their regular arrangement (hexagonal) in ED image indicate that the particle is of single crystal structure. Calculations give the spacing of 1.44 Å for (220) planes. Further indexing reveals the bottom crystal face as (111), which is consistent with XRD results. 3.3. Effect of Capping Reagent. To separate the roles of tartrate and citrate in this light-driven, morphology-transforming process, three contrasting experiments have been further designed (Figure S3, Supporting Information). Table 1 summarizes the products obtained under different experimental conditions: (Ι) When citrate was used only as the capping reagent ([Ag+]:[citrate] ) 1:1, state Ι), the color of the colloid changed from yellow to blue after 5 h irradiation, and nanoprisms with strong plamson peak at 665 nm were produced. We have already reported detailed research concerning the effect of citrate in the photoinduced system.20b (II) When only tartrate was used ([Ag+]: [tartrate] ) 1:20, state II), the color of the solution turned to turbidly yellow after 18 h irradiation, and only a shoulder peak (around 600 nm) appeared. A large number of polyhedral nanoparticles and several tetrahedral ones were generated. The tetrahedrons exhibited edge lengths between 30 and 80 nm. At this moment, the growth process stopped for the weak photoreduction of tartrate, and no uniform TSNCs can be obtained. (III) If both tartrate and citrate were used simultaneously ([Ag+]: [tartrate]:[citrate] ) 1:20:1, state III), as the color of solution turned to bluish-purple after 10 h irradiation, a product mixture of prisms, disks, tetrahedrons, and polyhedrons was obtained. On the basis of the results mentioned above, the effect of tartrate and citrate on the growth of TSNCs can be divided into two stages: (1) in the early stage, the addition of tartrate helps silver seeds nucleate into tetrahedral rudiments under irradiation of a sodium lamp as the result of face-selective adsorption; (2) following the above stage, citrate is added to the reaction system as a photoreducing reagent for the silver ions to promote the growth of the TSNCs.16,24 The resulting nanocrystals become larger and are of more regular shape (tetrahedral). In addition, citrate ions are preferentially adsorbed on the {111} planes of

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the TSNCs.21 The selective capping of {111} facets by citrate ions and the relatively fast growth of other facets lead to the formation of TSNCs bound by {111} facets, which is consistent with the structure characterization results of XRD and ED. Overall, introducing tartrate and citrate at the appropriate stages may lead to the formation of stable tetrahedral nanocrystals. 3.4. SERS Application. It is clear that there exist a large number of sharp vertexes for the single TSNCs-layer film. According to previous theoretical and experimental investigations,7 the TSNCs film can be expected to exhibit intense local electromagnetic (EM) field enhancement behavior and serve as a SERS material for molecular sensing. Figure 6A shows a typical SERS spectrum of benzenethiol adsorbed on the TSNCs film. Figure 6B demonstrates a normal Raman (NR) spectrum of the neat benzenethiol. The SERS spectrum consists of several observable bands at 1572, 1072, 1021, 997 cm-1, which is intrinsic to benzenethiol. Detailed peak frequency assignments25 are given in Table 2. It is found that a few remarkable spectral changes occur upon the adsorption of benzenethiol on TSNCs. The C-S stretching mode (1072 cm-1) shows the most dramatic change from NR to SERS. Not only was its wavenumber downshifted by 20 cm-1 in SERS but its intensity was more strongly enhanced. The C-C stretching mode (1572 cm-1) also shows a large downshift and a huge enhancement in SERS. Most prominently, the S-H bending mode (913 cm-1) in the NR spectrum disappeared in the SERS spectrum. Together these changes indicate that benzenethiol is chemisorbed on TSNCs surface as benzenethiolate. The average SERS enhancement factors (detailed calculation in Supporting Information) of the TSNCs are estimated up to 106 (Table 2). Previous results show that the optimal SERS enhancements occur when localized plasmon resonances of the single TSNCs-layer film are present at both the excitation wavelength and Raman scattered wavelength.26 We believe the SERS enhancement factor of the TSNCs can be further improved by choosing the appropriate laser excitation wavelength.

Conclusions In summary, successfully utilizing tartrate and citrate as structural-directing reagents, we have prepared, in large scale, regularly tetrahedral silver nanocrystals developed from silver spherical seeds through a light-driven method. The shape of the special nanocrystals evolves sequentially through nucleation, growth, and shape and size focusing. Tartrate helps silver seeds nucleate in the shape of rudimental tetrahedrons. Citrate promotes the growth of the TSNCs. The subsequent addition of tartrate and citrate leads to the formation of the TSNCs. For the regular geometry, TSNCs display a striking beauty and show their intriguing optical properties for the research fields such as surface enhanced spectroscopy, as well as plasmonic materials. We are also interested in the assembly of TSNCs into large-scale metallic architectures and in studies of applications to nanodevices. Other unknown physical and chemical properties of such kind nanocrystals remain to be explored. Acknowledgment. This work was supported by the National Natural Science Foundation of China (NSFC 20773045, 20627002, 20573041). This work was partly carried out while (24) Ahern, A. M.; Garrell, R. L. Anal. Chem. 1987, 59, 2813. (25) (a) Han, S. W.; Lee, S. J.; Kim, K. Langmuir 2001, 17, 6981. (b) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979. (26) (a) Cintra, S.; Abdelsalam, M. E.; Bartlett, P. N.; Baumberg, J. J.; Kelf, T. A.; Sugawara, Y.; Russell, A. E. Faraday Discuss. 2006, 132, 191. (b) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 11279.

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one of us (J.C.) was an exchange visiting graduate student at the City College of New York. We are indebted to the National Institute of Justice (Department of Justice Award #2006-DNBX-K034) and the City University Collaborative Incentive program (#80209). This work was also supported by the National Science Foundation under Cooperative Agreement No. RII-9353488, Grant Numbers CHE-0091362 and CHE0345987 and Grant Number ECS0217646 and by the City University of New York PSC-BHE Faculty Research Award Program.

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Supporting Information Available: SERS enhancement factors calculation; figures showing the separated experiments: TEM image of the silver colloid after 9 h of irradiation; histograms of the edge lengths of TSNCs illustrating the size distributions; the contrasting UV-vis spectra and TEM images of the following three states (state Ι, [Ag+]: [citrate] ) 1:1; state II, [Ag+]:[tartrate] ) 1:20; state III, [Ag+]:[tartrate]: [citrate] ) 1:20:1, added simultaneously). (The other reaction reagents and the preparation process are in the same condition.) This material is available free of charge via the Internet at http://pubs.acs.org. LA800961J