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A simple method to synthesize triangular silver nanoparticles by light irradiation
Spectrochimica Acta Part A 64 (2006) 956–960
A simple method to synthesize triangular silver nanoparticles by light irradiation Huiying Jia a , Weiqing Xu a , Jing An a , Dongmei Li b,1 , Bing Zhao a,∗ a
Key Laboratory for Supermolecular Structure and Materials of Ministry of Education, Jilin University, Changchun 130012, PR China b National Laboratory for Superhard Materials, Jilin University, Changchun 130012, PR China Received 21 April 2005; accepted 2 September 2005
Abstract We describe a simple method to synthesize triangular silver nanoparticles by photoreducing the silver ions by citrate. A noteworthy difference of the present method as compared with the previous photo-induced methods is that good shape control over the nanoparticles can be realized in the absence of soft templates or polymer directing agents. The formation process of the silver nanoparticles was investigated by UV–vis spectroscopy and transmission electron microscopy (TEM). It was found that the concentration of reactant plays important role in the morphology control of produced silver nanoparticles. As one of the applications of these nanoparticles, they were used as surface-enhanced Raman scattering substrates and 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BVPP) was used as a Raman probe to evaluate the enhancement ability of the triangular silver nanoparticles. © 2005 Elsevier B.V. All rights reserved. Keywords: Silver nanoprisms; Photoinduced; SERS
1. Introduction Considerable attention has been paid to metal nanoparticles due to their remarkable optical, catalytic, electronic and magnetic properties and potential applications. These properties and applications are strongly dependent on the size and shape of the metal nanoparticles. A challenge in colloid chemistry is to control not only the metal nanoparticle size but also the shapes and morphologies of the nanoparticles as well [1]. Silver nanoparticles show remarkable optical properties that depend on nanoparticle size and shape [2–5]. The number of surface plasmon resonance bands and effective spectral ranges for surface-enhanced Raman scattering (SERS) has also been demonstrated to be highly dependent on the morphology exhibited by silver nanostructures [6]. Many techniques have been exploited to prepare shape-controlled silver nanoparticles. Spherical silver nanoparticles with uniform size were prepared by using mercaptoacetic acid as stabilizer [7]. Inorganic templates such as carbon nanotubes [8], mesoporous materials [9], steps on the solid surface [10], or organic templates such as polymer materials [11], or micelles [2,12] were used to pre∗ 1
Corresponding author. Tel.: +86 431 5168473; fax: +86 431 5193421. E-mail address: [email protected] (B. Zhao). Tel: 86 431 5168340; fax: 86 431 5168816.
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.09.004
pare silver nanorods and wires. Recently, several reports specific to prepare triangular silver nanoparticles using directing agents including surfactants and polymers by irradiating with visible light or refluxing an aqueous dispersion of spherical silver seeds were published [13–16]. Although these directing agents play an important role in controlling the shapes of the nanoparticles, it is hard to remove them from the surface of the nanoparticles. For SERS that require analyte adsorption to the nanoparticles surface, the presence of residue from the synthesis on the nanoparticles surface may be a significant interferent [17]. In this paper, we report a simple method to prepare triangular silver nanoparticles with truncated corners by light irradiation without polymer directing agents or soft templates. 2. Experimental 2.1. Silver nanoparticles preparation For the preparation of aqueous solutions, deionized water was deoxygenated by bubbling with nitrogen gas for 30 min before use. The sodium citrate was added to an aqueous solution of silver nitrate with rapid stirring. Then a freshly prepared sodium borohydride solution was added by drop-wise addition to the mixture under vigorous stirring, the solution changed to yellow color immediately. After stirring for 30 s, the reaction
H. Jia et al. / Spectrochimica Acta Part A 64 (2006) 956–960
solution was irradiated with a Sodium lamp (OSRAM NAV-T 70-W λ = 589 nm). 2.2. Instrumentation TEM was measured with a Hitachi H-8100 IV operating at 200 kV. Samples were prepared on formvar copper grids by depositing one drop of the colloids on copper grid and dried naturally in a dissector. UV–vis spectra were recorded on a Shimadzu UV-3100 spectrophotometer. The water was always used as a reference to obtain absorption spectra for the silver colloid. Raman spectra were obtained with a 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. Laser power at the sample position was not more than a few milliwatts. Data acquisition was the results of five 50-s accumulations. 3. Results and discussion The morphology changes of the silver nanoparticles prepared by irradiating the solution containing 1 × 10−4 mol/l silver nitrate, 3 × 10−4 mol/l sodium citrate and 0.5 × 10−4 mol/l sodium borohydride were investigated by transmission electron microscopy (TEM). Fig. 1a was the TEM image of the silver nanoparticles before irradiation, indicating that the initial particles were spherical with diameters in the range of 2 to15 nm. After irradiation for 1 h, the yellow solution turned
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green, the TEM image (Fig. 1b) showed that several small triangular nanoparticles appeared while most of the particles remained spherical in shape. After 3.5 h, the dispersion changed its color from green to blue; TEM image (Fig. 1c) revealed that the product was dominated by truncated triangular nanoparticles whose edge length varied from ∼40 nm to ∼110 nm. Most of the nanoparticles sat flat on the substrate; some assembled into stacks, thus making them appear as rods while viewed from the direction along their basal plane. Measurement of these stacks shows that the triangular nanoparticles thickness is 7 ± 1 nm. If further irradiation was applied (5 h), the corners of the triangular nanoparticles became more truncated, even some of the triangular nanoparticles changed to disklike in shape (Fig. 1d). But the thickness was unchanged. The reason that more truncated triangular nanoparticles are formed is probably by the realignment of silver atoms induced by light irradiation. Similar phenomena (in synthesis of gold nanorods by UV-irradiation) were also observed by Niidome et al. [18]. The above shape changes are also reflected in the UV–vis spectra. Fig. 2 shows the UV–vis spectra changes of silver nanoparticls with different irradiation time. Before irradiation, the silver nanoparticles exhibited a peak at 395 nm (Fig. 2, curve (a), corresponding to the plasmon resonance of small spherical particles. Two distinctive peaks appeared after 1 h of irradiation, respectively at 330 and 687 nm (Fig. 2, curve (b). According to theoretical calculations [13], these peaks could be assigned to the out-of-plane quadruple and in-plane dipole plasmon resonance, indicating that small triangular nanoparticles formed.
Fig. 1. TEM images showing the morphology changes of silver nanoparticles on the copper grid at different irradiation time: (a) 0 h; (b) 1 h (c) 3 h and 5 h (d). The inset in (c) shows that some nanoparticles assembled into stacks.
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Fig. 2. UV–vis spectra of silver nanoparticles obtained in solution at different irradiation time. (a) 0 h; (b) 1 h; (c) 1.5 h; (d) 2.5 h; (e) 3 h and (f) 5 h.
With proceeding irradiation, the UV–vis spectra exhibited one additional peak at 494 nm corresponding to the in-plane quadruple plasmon resonance. The in-plane dipole surface plasmon band grew in intensity and shifted to longer wavelength region until 2.5 h. As we know, the plasmon peak position is mostly sensitive to the particle aspect ratio. For triangular nanoparticles, longer wavelength peaks correspond to a larger lateral size [19]. So the changes in the in-plane dipole surface plasmon band suggested that larger triangular nanoparticles were formed as we saw in the TEM image Fig. 1c. When the irradiation time changed from 3 to 5 h (Fig. 2, curve f), the band at 395 nm completely disappeared. At the same time, the in-plane dipole surface plasmon band blue-shifted to 672 nm, corresponding to the corner of the triangular nanoparticles became more truncated as we saw in the Fig. 1d. The out-of-plane quadruple plasmon peak was almost unchanged due to the constant thickness of the nanoparticles. In this synthesis, citrate molecules play a key role, which act as a capping ligand for the silver particles as well as a photoreducing agent for the silver ions [20]. At the beginning of the reaction, silver seeds formed as we added sodium borohydride to the mixture, the silver seeds then would be capped with citrate. Citrate is known to photoreduce the silver ions on the nanoparticle surface [20,21]. When irritated with light, the reduction speed of citrate at certain crystal faces of silver nanoparticles may be different, leading to the nanoparticles we finally got were not spherical but triangular in shape. The more detailed mechanism of the citrate on the morphology of silver nanoparticles prepared with light irradiation will be investigated further. Several control experiments were done. The concentration of citrate on the morphology of nanoparticles was investigated. It was found that triangular nanoparticles were formed at any concentration, but increasing the citrate concentration was favorable for triangular nanoparticles to form self-assembled structures. Fig. 3 shows the TEM images of the triangular nanoparticles prepared by irradiating the solution containing 1 × 10−4 mol/l silver nitrate, 15 × 10−4 mol/l sodium citrate and 0.5 × 10−4 mol/l sodium borohydride. The triangular nanoparticles were found to assemble into stacks in a large area on the copper grid. It is not very clear whether these self-assembled structures are generated
Fig. 3. TEM image of the triangular nanoparticles prepared by irradiating the solution containing 1 × 10−4 mol/l silver nitrate, 15 × 10−4 mol/l sodium citrate and 0.5 × 10−4 mol/l sodium borohydride.
in solution or formed on the grid during the solvent evaporation. But like the self-assembly of the Au rods, factors such as ionic strength, particle concentration, particle shape and size distribution may influence the self-assembly of the nanoparticles [22]. In our synthesis, one can see that triangular nanoparticles in the self-assembled structures are similar in size, which is a prerequisite for generating these structures [23]. We think that excess citrate in the solution may provide sufficient ionic strength which facilitates the formation of these structures and forms a layer of counterions between two nanoparticles surfaces with the same charges which can stabilize these structures [22]. The self-assembed structure may possess interesting new collective physical properties that are different from those of the original bulk material and expected to be applicable as building blocks in modern nanoelectronics. The concentration of sodium borohydride also plays an important role in the morphology of the produced silver nanoparticles. Studies reveal that only a few triangular silver nanoparticles were obtained if the sodium borohydride concentration was higher than 5 × 10−4 mol/l. Fig. 4a shows the UV–vis spectra of silver nanoparticles synthesized by reducing 1 × 10−4 mol/l silver nitrate in the presence of 3 × 10−4 mol/l sodium citrate and 5 × 10−4 mol/l sodium borohydride, before and after irradiation. Before irradiation, the surface plasmon band appears at 398 nm, which is characteristic of small spherical silver particles. After irradiation, four distinctive peaks appeared which are located at 351, 451, 576 and 630 nm, respectively. Fig. 4b shows the TEM image of the final produce, which indicated the formation of larger silver nanoparticles with different morphologies that included spheres (or disks) and only a few triangular plates. The spherical particles had a broad size distribution that might contribute to the surface plasmon band at 451 nm which are characteristic of the aggregation of spherical particles [23]; the peaks at 351 nm, 630 nm
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Fig. 5. A SERS spectrum of BVPP on the triangular silver nanoparticles (a) and a Raman spectrum of BVPP in a solid state (b).
Fig. 4. (a) TEM image and (b) UV–vis spectra of silver nanoparticles synthesized by reducing 1 × 10−4 mol/l silver nitrate in the presence of 3 × 10−4 mol/l sodium citrate and 5 × 10−4 mol/l sodium borohydride.
could be assigned to plasmon resonance of triangular silver nanoparticles; the peak at 576 nm might be caused by plasmon resonance of silver nanodisks [24]. It was found that decreasing the sodium borohydride was favorable for the formation of the triangular nanoparticles. Optimum results were obtained with the concentration between 0.5 × 10−4 and 10−4 mol/l. The reason may be as follows: at higher concentration, large excess borohydride ions may form a negatively charged layer on the silver nanoparticles surface which might greatly reduce the coordination power of citrate with silver surface, so the most of nanoparticles we finally got were irregular in shape. SERS technique is a useful tool for identifying adsorbed species and exploring their orientation on a metal surface [25]. One of the major goals of the present study is to prepare the SERS substrates that can be useful for investigating the molecules adsorbed on them. 1,4-bis[2-(4-pyridyl)ethenyl]benzene (BVPP) [26] which has a large -bond conjugated system and two active nitrogen atoms has an excellent Raman scattering signal and has been employed as a probing molecule for the exploration of SERS activation on some novel sorts of metal substrates in several literatures [27–30]. So it was selected as a model compound for the SERS measurement. The procedure to modify the colloid with BVPP is as follows. To 0.9 ml of silver colloids, 100 l of the 1 × 10−4 mol/l BVPP solutions was added. The final concentration of the analyte in the sample was 1 × 10−5 mol/l. A glass capillary was used as the sampling device for measuring SERS from the silver colloid substrates. Fig. 5a presents a SERS spectrum of BVPP adsorbed on the triangular silver nanoparticles and a Raman spectrum of BVPP solid powder (Fig. 5b). According to the theoretical calculations
of Cheng et al. [31]. The peaks at 1630, 1595, 1544, 1420 cm−1 can be assigned to the C=C and C–C stretching mode; the C–H deformation mode at 1332, 1308 and 1220 cm−1 , the Py–C–H deformation at 1180 cm−1 , the ring breathing band appear at 1009, the peak at 965 cm−1 is the Py–C–H out of plane bend. Comparison between SERS and Raman band positions of BVPP in Fig. 5 indicates that the BVPP molecules are chemically adsorbed on the triangular nanoparticles. This can also be confirmed by a band at 1009 cm−1 due to the ring breathing mode, which is characteristic of chemisorbed pyridine molecules [32]. The results are identical with those previously reported for the same compound adsorbed on silver particle films [27]. It was reported that BVPP is chemisorbed on the silver surface through the Ag–N interaction with a nearly perpendicular orientation of BVPP molecules on the silver nanoparticles [33]. Our results for both triangular silver nanoparticles and the silver particle films suggest the same adsorption mode. 4. Conclusion In conclusion, we have synthesized triangular silver nanoparticles by irradiation spherical silver colloids with Sodium lamp in the absence of soft templates or polymer directing agents. The product was dominated by truncated triangular nanoparticles whose edge length varied from 40 to 110 nm. It was found that increasing the citrate concentration was favorable for the triangular nanoparticles to form self-assembled structures. These structures may possess interesting new collective physical properties that are different from those of the original bulk material and are expected to be applicable as building blocks in constructing devices in future nanoelectronics. The triangular silver nanoparticles are also suitable as SERS active substrates. Acknowledgements This research was supported by the National Natural Science Foundation (Grant No. 20473029, 20375014, 20173019, 20273022) of the People’s Republic of China, Program for Changjiang Scholars and Innovative Research Team in Univer-
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sity (PCSIRT), Program for New Century Excellent Talents in University (NCET) and the Innovative Scholars of Jilin University (2004CX035). References [1] Y. Zhou, C.Y. Wang, Y.R. Zhu, Z.Y. Chen, Chem. Mater. 11 (1999) 2310. [2] C.J. Murphy, N.R. Jana, Adv. Mater. 14 (2002) 80. [3] R.M. Dickson, L.A. Lyon, J. Phys. Chem. B 104 (2000) 6095. [4] J.J. Mock, S.J. Oldenburg, D.R. Smith, D.A. Schultz, S. Schultz, Nano Lett. 2 (2002) 465. [5] O. Wilson, G.J. Wilson, P. Mulvaney, Adv. Mater. 14 (2002) 1000. [6] L.A. Dick, A.D. McFarland, C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 106 (2002) 853. [7] X.L. Li, J.H. Zhang, W.Q. Xu, H.Y. Jia, X. Wang, B. Yang, B. Zhao, B.F. Li, Y. Ozaki, Langmuir 19 (2003) 4285. [8] P.M. Ajayan, S. Iijima, Nature 361 (1993) 333. [9] Y.J. Han, J.M. Kim, G.D. Stucky, Chem. Mater. 12 (2000) 2068. [10] H.H. Song, K.M. Jones, A.A. Baski, J. Vac. Sci. Technol. A 17 (1999) 1696. [11] S. Nhattacharrya, S.K. Saha, D. Chakravorty, Appl. Phys. Lett. 76 (2000) 3896. [12] N.R. Jana, L. Gearheart, C.J. Murphy, Chem. Commun. (2001) 617. [13] R.C. Ji n, Y.W. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Science 294 (2001) 1901. [14] S.H. Chen, D.L. Carroll, Nano Lett. 2 (2002) 1003. [15] Y.G. Sun, B. Mayers, Y.N. Xia, Nano Lett. 3 (2003) 675.
[16] Y.G. Sun, Y.N. Xia, Adv. Mater. 15 (2002) 695. [17] K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, Chem. Rev. 99 (1999) 2957. [18] Y. Niidome, K. Nishioka, H. Kawasaki, S. Yamada, Chem. Commun. (2003) 2376. [19] A. Callegari, D. Tonti, M. Chergui, Nano Lett. 3 (2003) 1565. [20] M. Maillard, P. Huang, L. Brus, Nano. Lett. 3 (2003) 1611. [21] A.M. Ahern, R.L. Garrell, Anal. Chem. 59 (1987) 2813. [22] B. Nikoobakht, Z.L. Wang, M.A. El-sayed, J. Phys. Chem. B 104 (2000) 8635. [23] A. Henglein, A. Giersig, J. Phys. Chem. B 103 (2000) 9533. [24] S.H. Chen, Z.Y. Fan, D.L. Carroll, J. Phys. Chem. B 106 (2002) 10777. [25] S.Z. Zou, C.T. Williams, E.K.Y. Chen, M.J. Weaver, J. Am. Chem. Soc. 120 (1998) 3811. [26] V.W.-W. Yam, V.C.-Y. Lau, L.X. Wu, J. Chem. Soc. Dalton Trans. (1998) 1461. [27] X.L. Li, W.Q. Xu, H.Y. Jia, X. Wang, B. Zhao, B.F. Li, Y. Ozaki, Appl. Spectrosc. 58 (2004) 26. [28] X.L. Li, J.H. Zhang, W.Q. Xu, H.Y. Jia, X. Wang, B. Yang, B. Zhao, B.F. Li, Y. Ozaki, Langmnir 19 (2003) 4285. [29] X.L. Li, W.Q. Xu, J.H. Zhang, H.Y. Jia, B. Yang, B. Zhao, Chem. J. Chin. Univ. 11 (2003) 2092. [30] W.Q. Xu, S.P. Xu, Z.C. L¨u, K.X. Wang, B. Zhao, Y.G. Fang, Chem. J. Chin. Univ. 11 (2003) 2083. [31] J.B. Cheng, X.L. Li, W.Q. Xu, G. Zhang, B. Zhao, Chem. Phys. Lett. 4 (405) 344. [32] (a) T. Iliescu, S. Cinta, W. Kiefer, Talanta 53 (2000) 121; (b) A.G. Brolo, D.E. Irish, J. Lipkowski, J. Phys. Chem. B 101 (1997) 3906. [33] T. Sawaguchi, F. Mizutani, I. Taniguchi, Langmuir 14 (1998) 3565.
A simple method to synthesize triangular silver nanoparticles by light irradiation Huiying Jia a , Weiqing Xu a , Jing An a , Dongmei Li b,1 , Bing Zhao a,∗ a
Key Laboratory for Supermolecular Structure and Materials of Ministry of Education, Jilin University, Changchun 130012, PR China b National Laboratory for Superhard Materials, Jilin University, Changchun 130012, PR China Received 21 April 2005; accepted 2 September 2005
Abstract We describe a simple method to synthesize triangular silver nanoparticles by photoreducing the silver ions by citrate. A noteworthy difference of the present method as compared with the previous photo-induced methods is that good shape control over the nanoparticles can be realized in the absence of soft templates or polymer directing agents. The formation process of the silver nanoparticles was investigated by UV–vis spectroscopy and transmission electron microscopy (TEM). It was found that the concentration of reactant plays important role in the morphology control of produced silver nanoparticles. As one of the applications of these nanoparticles, they were used as surface-enhanced Raman scattering substrates and 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BVPP) was used as a Raman probe to evaluate the enhancement ability of the triangular silver nanoparticles. © 2005 Elsevier B.V. All rights reserved. Keywords: Silver nanoprisms; Photoinduced; SERS
1. Introduction Considerable attention has been paid to metal nanoparticles due to their remarkable optical, catalytic, electronic and magnetic properties and potential applications. These properties and applications are strongly dependent on the size and shape of the metal nanoparticles. A challenge in colloid chemistry is to control not only the metal nanoparticle size but also the shapes and morphologies of the nanoparticles as well [1]. Silver nanoparticles show remarkable optical properties that depend on nanoparticle size and shape [2–5]. The number of surface plasmon resonance bands and effective spectral ranges for surface-enhanced Raman scattering (SERS) has also been demonstrated to be highly dependent on the morphology exhibited by silver nanostructures [6]. Many techniques have been exploited to prepare shape-controlled silver nanoparticles. Spherical silver nanoparticles with uniform size were prepared by using mercaptoacetic acid as stabilizer [7]. Inorganic templates such as carbon nanotubes [8], mesoporous materials [9], steps on the solid surface [10], or organic templates such as polymer materials [11], or micelles [2,12] were used to pre∗ 1
Corresponding author. Tel.: +86 431 5168473; fax: +86 431 5193421. E-mail address: [email protected] (B. Zhao). Tel: 86 431 5168340; fax: 86 431 5168816.
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.09.004
pare silver nanorods and wires. Recently, several reports specific to prepare triangular silver nanoparticles using directing agents including surfactants and polymers by irradiating with visible light or refluxing an aqueous dispersion of spherical silver seeds were published [13–16]. Although these directing agents play an important role in controlling the shapes of the nanoparticles, it is hard to remove them from the surface of the nanoparticles. For SERS that require analyte adsorption to the nanoparticles surface, the presence of residue from the synthesis on the nanoparticles surface may be a significant interferent [17]. In this paper, we report a simple method to prepare triangular silver nanoparticles with truncated corners by light irradiation without polymer directing agents or soft templates. 2. Experimental 2.1. Silver nanoparticles preparation For the preparation of aqueous solutions, deionized water was deoxygenated by bubbling with nitrogen gas for 30 min before use. The sodium citrate was added to an aqueous solution of silver nitrate with rapid stirring. Then a freshly prepared sodium borohydride solution was added by drop-wise addition to the mixture under vigorous stirring, the solution changed to yellow color immediately. After stirring for 30 s, the reaction
H. Jia et al. / Spectrochimica Acta Part A 64 (2006) 956–960
solution was irradiated with a Sodium lamp (OSRAM NAV-T 70-W λ = 589 nm). 2.2. Instrumentation TEM was measured with a Hitachi H-8100 IV operating at 200 kV. Samples were prepared on formvar copper grids by depositing one drop of the colloids on copper grid and dried naturally in a dissector. UV–vis spectra were recorded on a Shimadzu UV-3100 spectrophotometer. The water was always used as a reference to obtain absorption spectra for the silver colloid. Raman spectra were obtained with a 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. Laser power at the sample position was not more than a few milliwatts. Data acquisition was the results of five 50-s accumulations. 3. Results and discussion The morphology changes of the silver nanoparticles prepared by irradiating the solution containing 1 × 10−4 mol/l silver nitrate, 3 × 10−4 mol/l sodium citrate and 0.5 × 10−4 mol/l sodium borohydride were investigated by transmission electron microscopy (TEM). Fig. 1a was the TEM image of the silver nanoparticles before irradiation, indicating that the initial particles were spherical with diameters in the range of 2 to15 nm. After irradiation for 1 h, the yellow solution turned
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green, the TEM image (Fig. 1b) showed that several small triangular nanoparticles appeared while most of the particles remained spherical in shape. After 3.5 h, the dispersion changed its color from green to blue; TEM image (Fig. 1c) revealed that the product was dominated by truncated triangular nanoparticles whose edge length varied from ∼40 nm to ∼110 nm. Most of the nanoparticles sat flat on the substrate; some assembled into stacks, thus making them appear as rods while viewed from the direction along their basal plane. Measurement of these stacks shows that the triangular nanoparticles thickness is 7 ± 1 nm. If further irradiation was applied (5 h), the corners of the triangular nanoparticles became more truncated, even some of the triangular nanoparticles changed to disklike in shape (Fig. 1d). But the thickness was unchanged. The reason that more truncated triangular nanoparticles are formed is probably by the realignment of silver atoms induced by light irradiation. Similar phenomena (in synthesis of gold nanorods by UV-irradiation) were also observed by Niidome et al. [18]. The above shape changes are also reflected in the UV–vis spectra. Fig. 2 shows the UV–vis spectra changes of silver nanoparticls with different irradiation time. Before irradiation, the silver nanoparticles exhibited a peak at 395 nm (Fig. 2, curve (a), corresponding to the plasmon resonance of small spherical particles. Two distinctive peaks appeared after 1 h of irradiation, respectively at 330 and 687 nm (Fig. 2, curve (b). According to theoretical calculations [13], these peaks could be assigned to the out-of-plane quadruple and in-plane dipole plasmon resonance, indicating that small triangular nanoparticles formed.
Fig. 1. TEM images showing the morphology changes of silver nanoparticles on the copper grid at different irradiation time: (a) 0 h; (b) 1 h (c) 3 h and 5 h (d). The inset in (c) shows that some nanoparticles assembled into stacks.
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Fig. 2. UV–vis spectra of silver nanoparticles obtained in solution at different irradiation time. (a) 0 h; (b) 1 h; (c) 1.5 h; (d) 2.5 h; (e) 3 h and (f) 5 h.
With proceeding irradiation, the UV–vis spectra exhibited one additional peak at 494 nm corresponding to the in-plane quadruple plasmon resonance. The in-plane dipole surface plasmon band grew in intensity and shifted to longer wavelength region until 2.5 h. As we know, the plasmon peak position is mostly sensitive to the particle aspect ratio. For triangular nanoparticles, longer wavelength peaks correspond to a larger lateral size [19]. So the changes in the in-plane dipole surface plasmon band suggested that larger triangular nanoparticles were formed as we saw in the TEM image Fig. 1c. When the irradiation time changed from 3 to 5 h (Fig. 2, curve f), the band at 395 nm completely disappeared. At the same time, the in-plane dipole surface plasmon band blue-shifted to 672 nm, corresponding to the corner of the triangular nanoparticles became more truncated as we saw in the Fig. 1d. The out-of-plane quadruple plasmon peak was almost unchanged due to the constant thickness of the nanoparticles. In this synthesis, citrate molecules play a key role, which act as a capping ligand for the silver particles as well as a photoreducing agent for the silver ions [20]. At the beginning of the reaction, silver seeds formed as we added sodium borohydride to the mixture, the silver seeds then would be capped with citrate. Citrate is known to photoreduce the silver ions on the nanoparticle surface [20,21]. When irritated with light, the reduction speed of citrate at certain crystal faces of silver nanoparticles may be different, leading to the nanoparticles we finally got were not spherical but triangular in shape. The more detailed mechanism of the citrate on the morphology of silver nanoparticles prepared with light irradiation will be investigated further. Several control experiments were done. The concentration of citrate on the morphology of nanoparticles was investigated. It was found that triangular nanoparticles were formed at any concentration, but increasing the citrate concentration was favorable for triangular nanoparticles to form self-assembled structures. Fig. 3 shows the TEM images of the triangular nanoparticles prepared by irradiating the solution containing 1 × 10−4 mol/l silver nitrate, 15 × 10−4 mol/l sodium citrate and 0.5 × 10−4 mol/l sodium borohydride. The triangular nanoparticles were found to assemble into stacks in a large area on the copper grid. It is not very clear whether these self-assembled structures are generated
Fig. 3. TEM image of the triangular nanoparticles prepared by irradiating the solution containing 1 × 10−4 mol/l silver nitrate, 15 × 10−4 mol/l sodium citrate and 0.5 × 10−4 mol/l sodium borohydride.
in solution or formed on the grid during the solvent evaporation. But like the self-assembly of the Au rods, factors such as ionic strength, particle concentration, particle shape and size distribution may influence the self-assembly of the nanoparticles [22]. In our synthesis, one can see that triangular nanoparticles in the self-assembled structures are similar in size, which is a prerequisite for generating these structures [23]. We think that excess citrate in the solution may provide sufficient ionic strength which facilitates the formation of these structures and forms a layer of counterions between two nanoparticles surfaces with the same charges which can stabilize these structures [22]. The self-assembed structure may possess interesting new collective physical properties that are different from those of the original bulk material and expected to be applicable as building blocks in modern nanoelectronics. The concentration of sodium borohydride also plays an important role in the morphology of the produced silver nanoparticles. Studies reveal that only a few triangular silver nanoparticles were obtained if the sodium borohydride concentration was higher than 5 × 10−4 mol/l. Fig. 4a shows the UV–vis spectra of silver nanoparticles synthesized by reducing 1 × 10−4 mol/l silver nitrate in the presence of 3 × 10−4 mol/l sodium citrate and 5 × 10−4 mol/l sodium borohydride, before and after irradiation. Before irradiation, the surface plasmon band appears at 398 nm, which is characteristic of small spherical silver particles. After irradiation, four distinctive peaks appeared which are located at 351, 451, 576 and 630 nm, respectively. Fig. 4b shows the TEM image of the final produce, which indicated the formation of larger silver nanoparticles with different morphologies that included spheres (or disks) and only a few triangular plates. The spherical particles had a broad size distribution that might contribute to the surface plasmon band at 451 nm which are characteristic of the aggregation of spherical particles [23]; the peaks at 351 nm, 630 nm
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Fig. 5. A SERS spectrum of BVPP on the triangular silver nanoparticles (a) and a Raman spectrum of BVPP in a solid state (b).
Fig. 4. (a) TEM image and (b) UV–vis spectra of silver nanoparticles synthesized by reducing 1 × 10−4 mol/l silver nitrate in the presence of 3 × 10−4 mol/l sodium citrate and 5 × 10−4 mol/l sodium borohydride.
could be assigned to plasmon resonance of triangular silver nanoparticles; the peak at 576 nm might be caused by plasmon resonance of silver nanodisks [24]. It was found that decreasing the sodium borohydride was favorable for the formation of the triangular nanoparticles. Optimum results were obtained with the concentration between 0.5 × 10−4 and 10−4 mol/l. The reason may be as follows: at higher concentration, large excess borohydride ions may form a negatively charged layer on the silver nanoparticles surface which might greatly reduce the coordination power of citrate with silver surface, so the most of nanoparticles we finally got were irregular in shape. SERS technique is a useful tool for identifying adsorbed species and exploring their orientation on a metal surface [25]. One of the major goals of the present study is to prepare the SERS substrates that can be useful for investigating the molecules adsorbed on them. 1,4-bis[2-(4-pyridyl)ethenyl]benzene (BVPP) [26] which has a large -bond conjugated system and two active nitrogen atoms has an excellent Raman scattering signal and has been employed as a probing molecule for the exploration of SERS activation on some novel sorts of metal substrates in several literatures [27–30]. So it was selected as a model compound for the SERS measurement. The procedure to modify the colloid with BVPP is as follows. To 0.9 ml of silver colloids, 100 l of the 1 × 10−4 mol/l BVPP solutions was added. The final concentration of the analyte in the sample was 1 × 10−5 mol/l. A glass capillary was used as the sampling device for measuring SERS from the silver colloid substrates. Fig. 5a presents a SERS spectrum of BVPP adsorbed on the triangular silver nanoparticles and a Raman spectrum of BVPP solid powder (Fig. 5b). According to the theoretical calculations
of Cheng et al. [31]. The peaks at 1630, 1595, 1544, 1420 cm−1 can be assigned to the C=C and C–C stretching mode; the C–H deformation mode at 1332, 1308 and 1220 cm−1 , the Py–C–H deformation at 1180 cm−1 , the ring breathing band appear at 1009, the peak at 965 cm−1 is the Py–C–H out of plane bend. Comparison between SERS and Raman band positions of BVPP in Fig. 5 indicates that the BVPP molecules are chemically adsorbed on the triangular nanoparticles. This can also be confirmed by a band at 1009 cm−1 due to the ring breathing mode, which is characteristic of chemisorbed pyridine molecules [32]. The results are identical with those previously reported for the same compound adsorbed on silver particle films [27]. It was reported that BVPP is chemisorbed on the silver surface through the Ag–N interaction with a nearly perpendicular orientation of BVPP molecules on the silver nanoparticles [33]. Our results for both triangular silver nanoparticles and the silver particle films suggest the same adsorption mode. 4. Conclusion In conclusion, we have synthesized triangular silver nanoparticles by irradiation spherical silver colloids with Sodium lamp in the absence of soft templates or polymer directing agents. The product was dominated by truncated triangular nanoparticles whose edge length varied from 40 to 110 nm. It was found that increasing the citrate concentration was favorable for the triangular nanoparticles to form self-assembled structures. These structures may possess interesting new collective physical properties that are different from those of the original bulk material and are expected to be applicable as building blocks in constructing devices in future nanoelectronics. The triangular silver nanoparticles are also suitable as SERS active substrates. Acknowledgements This research was supported by the National Natural Science Foundation (Grant No. 20473029, 20375014, 20173019, 20273022) of the People’s Republic of China, Program for Changjiang Scholars and Innovative Research Team in Univer-
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