Synthesis of Triangular Silver Nanoprisms by Stepwise Reduction of

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Synthesis of Triangular Silver Nanoprisms by Stepwise Reduction of Sodium Borohydride and Trisodium Citrate Xinyi Dong, Xiaohui Ji, Jing Jing, Mingyue Li, Jun Li, and Wensheng Yang* State Key Laboratory of Surpramolecular Structures and Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: October 18, 2009; ReVised Manuscript ReceiVed: December 11, 2009

Triangular silver nanoprisms were synthesized by stepwise reduction of silver nitrate with sodium borohydride (NaBH4) and trisodium citrate. In this approach, first small spherical silver nanoparticles were prepared by the rapid reduction of the precursor with NaBH4 at ice-bath temperature. After being heated to 70 °C, further reduction of the precursor contributed to the formation of additional small spherical silver nanoparticles attributed to the catalysis effect of the silver particles formed at the low temperature. The residual precursor after the formation of the small spherical silver nanoparticles is necessary to promote the dissolution of the small nanoparticles and their transformation into the triangular nanoprisms. After consumption of most of the precursor, the nanoprisms became more uniform in shape and size driven by the Ostwald ripening. The triangular nanoprisms prepared by such an approach are expected to be potentially useful in biolabeling since the citrate ligand on the nanoprism surface is ready to be replaced by biomolecules. Introduction Silver nanoparticles have received extensive attentions in recent years due to their application potentials in biological diagnostics,1,2 catalysis,3 electronics,4 and surface-enhanced Raman scattering (SERS).5,6 A variety of silver nanoparticles with different shapes such as nanorod,7 nanodisk,8,9 nanoprism,10-12 nanowire,13 nanosphere,14,15 nanoflower16 etc.17,18 have been successfully prepared to explore the shape dependent optical and electronic properties of silver nanoparticles. Among them, anisotropic triangular nanoprism is one of mostly studied silver nanoparticles due to its interesting optical properties. Photomediated growth, which was first proposed by Mirkin’s group in 2001, has been proven to be an efficient route to prepare triangular nanoprisms.19 In this approach, first small spherical nanoparticles were produced by reduction of silver nitrate (AgNO3) with sodium borohydride (NaBH4), and subsequently the light-induced transformation of the spherical nanoparticles into triangular prisms.20-25 It is documented that the photochemical process involves both the oxidative dissolution of the small spherical silver nanoparticles and the reduction of the silver ions with citrate on the particle surface promoted by photocatalysis of the silver particles.26 Further works showed that the triangular nanoprisms could also be prepared by thermal synthesis approach.27-29 In the thermal approach, it is likely that the rapid reduction of silver precursors, which resulted in the formation of the small silver nanoparticles, and then subsequent reduction of silver ions contributed to the growth of the triangular nanoprisms. Generally, surfactants or capping-agents that can suppress the growth along the (111) plane are necessary to obtain the nanoprisms. In this work, a stepwise reduction method was developed for the preparation of triangular silver nanoprisms in the absence of surfactant or special capping-agent. In this approach, first a part of the precursor, silver nitrate, was reduced rapidly by NaBH4 to produce small spherical silver nanoparticles at ice* To whom correspondence should be addressed. Telephone: +86-43185168185. Fax: +86-431-85168086. E-mail: [email protected].

bath temperature. The remaining precursor was further reduced by citrate under elevated temperature to result in the formation of additional small spherical nanoparticles and induce the transformation of the spherical nanoparticles into the triangular nanoprisms. It is shown that the formation of the triangular nanoprisms is dependent on the molar ratios of NaBH4 and trisodium citrate used in the reactions. A balance between the precursor contributed to the formation of the small spherical particles and that to the transformation of the spherical nanoparticles is critical for the synthesis of the triangular nanoprisms. Experimental Section Materials. Silver nitrate (AgNO3), sodium borohydride (NaBH4), trisodium citrate (Na3C6H5O7), sodium hydroxide (NaOH), nitric acid, sodium acetate, acetic acid, 1,10-phenanthroline(PHEN) and 2,4,5,7-tetrabromofluorescein(TBF) were of analytical grade and were used without further purification. All water was distilled and subsequently purified to Millipore Milli-Q quality. All glassware used was cleaned in a bath of freshly prepared aqua regia solution (HCl/HNO3, 3:1), then rinsed thoroughly with H2O before use. Synthesis of Silver Nanoprisms. A typical reaction was as follows, a 100 mL aqueous solution containing AgNO3 (1 × 10-4 M) and trisodium citrate (3.5 × 10-3 M) was prepared in a 250 mL flask at 4 °C (ice-bath). Definite amount of NaBH4 was added dropwise into the solution under vigorous stirring. After being kept in the ice bath for 30 min, the solution was heated to 70 °C to trigger the reduction of the remaining AgNO3. For all the reactions, the reaction flasks were wrapped by tinfoil and the water-bath was coated by the cover to avoid the light. Characterization. UV-visible adsorption spectra were recorded on a SHIMADZU UV-2450 UV-vis spectrophotometer. Aliquots of the solution were taken out at different times during the reaction, and cooled by ice water to quench the reaction. The concentration of Ag+ was determined by the spectrophotometric method established by Mohamed and Roland,30 and the standard curve was available in our previous work.15 Transmission electron microscopy (TEM) observations were

10.1021/jp909964k  2010 American Chemical Society Published on Web 01/20/2010

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Figure 1. TEM images of the silver nanoparticles prepared with different concentrations of NaBH4. The concentrations of silver nitrate and trisodium citrate were set as 1 × 10-4 and 3.5 × 10-3 M, respectively.

carried out under a JEOL JEM-2010 electron microscope with an operating voltage of 200 kV after the samples were dropped onto Formvar-coated copper grids. Results and Discussion In our synthesis, the concentrations of silver nitrate and trisodium citrate were set as 1 × 10-4 M and 3.5 × 10-3 M, respectively, unless stated especially, and the concentration of NaBH4 was changed from 0 to 5 × 10-4 M. First, the solutions of silver nitrate and trisodium citrate were mixed in the flask at 4 °C, and then different amounts of NaBH4 solution was added dropwise into the mixtures. After being kept at the low temperature for 30 min, the solutions were heated to 70 °C and kept for 48 h to get the products. It was identified that the reduction of silver nitrate by NaBH4 was completed in 1 min, and no further reduction of silver nitrate by citrate was observable at the low temperature even after 50 h (see Figure S1 in Supporting Information). Figure 1A-C shows TEM images of three representative products obtained at different concentrations of NaBH4. Spherical and rodlike silver nanoparticles were obtained at low concentration of NaBH4 (2 × 10-7 M, Figure 1A), similar to those prepared by reduction of silver nitrate with citrate.15 At medium concentration of NaBH4 (5 × 10-6 M), triangular silver nanoprisms, accompanied with some spherical silver nanoparticles, were obtained as the product (Figure 1B). At high concentration of NaBH4 (5 × 10-4 M), the product was dominated by small spherical silver nanoparticles (Figure 1C). These results indicated that the formation of the triangular nanoprisms is dependent on the amount of NaBH4 used in the reactions. The reactions were followed by UV-visible spectra to further understand the shape evolution of the silver nanoparticles (Figure 2). For the reaction carried out with the low concentration of NaBH4 (2 × 10-7 M, Figure 2A), almost no silver nanoparticles were formed at the low temperature since no absorption peak was observable in the UV-visible spectrum. After being heated to 70 °C, an absorption peak around 400 nm became observable. Intensity of the absorbance at 400 nm increased with the proceeding of the reaction and kept almost unchanged after 4 h. Temporal evolution of silver ion concentration (Figure 2D) revealed that almost no consumption of the precursor was detectable upon the addition of 2 × 10-7 M NaBH4 at the low temperature. After being heated to 70 °C, the precursor was consumed gradually and almost no precursor was left after 4 h, which was consistent with the UV-visible spectral results. Both spherical and rodlike silver particles were observed in the product due to the poor balance of nucleation and growth of the nanoparticles during the reaction.15 At the medium concentration of NaBH4 (5 × 10-6 M, Figure 2B), an absorption peak at 400 nm became observable after 1 min of the addition of NaBH4 at the low temperature, indicating the formation of spherical silver nanoparticles. After being heated to 70 °C, intensity of the absorbance at 400 nm increased and then began to decrease after 2 h. Simultaneously, the

Figure 2. Temporal evolutions of the UV-visible spectra (A-C) and the precursor concentration (D) of the silver nanoparticles prepared with different concentrations of NaBH4. The concentrations of silver nitrate and trisodium citrate were set as 1 × 10-4 and 3.5 × 10-3 M, respectively.

absorbance at longer wavelength became stronger, indicating the formation of silver nanoparticles with anisotropic structure.31 After 20 h, the peak at longer wavelength became dominant. The broad absorbance in the region of 800-900 nm as well the one at 334 nm observed at 48 h, which corresponded to the in-plane dipole plasmon and out-of-plane quadrupole resonances, respectively, suggested the formation of silver nanoparticles with triangular nanoprism structure at the end of the reaction.19 Temporal evolution of silver ion concentration (Figure 2D) revealed that about 20% of the precursor was consumed after the addition of 5 × 10-6 M NaBH4 at the low temperature, contributing to formation of the spherical silver nanoparticles. After being heated to 70 °C, the precursor was further reduced by citrate, which was consistent with the increase of absorption peak at 400 nm, indicating the continuous formation of small spherical silver nanoparticles. It is noted that the consumption of the precursor became slower in the period of 2-20 h although there was still about 30% of the precursor left in the solution. After 20 h, there was only trace amount of the precursor (about 5%) detectable until the end of the reaction. Thus it is reasonable to attribute the evolution of the UV-visible spectra during the period of 20-48 h to the Ostwald ripening of the silver nanoparticles.27 It is likely that the reaction at the high temperature could be divided into three stages, formation of small spherical particles attributed to the fast reduction of the precursor (0-2 h), formation of anisotropic silver nanoparticles accompanied with slow reduction of the precursor (2-20 h), and finally the shape evolution of the silver nanoparticles dominated by Ostwald ripening (20-48 h). When the concentration of NaBH4 was increased to 5 × 10-4 M, an absorption peak was observed immediately after the addition of NaBH4 at the low temperature, indicating the formation of spherical silver nanoparticles. After being heated

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Figure 3. Temporal evolution of TEM images during the synthesis of triangular silver nanoprisms under the NaBH4 concentration of 5 × 10-6 M.

Figure 4. Temporal evolution of the UV-visible spectra (A-C) and the precursor concentration (D) during the synthesis for the silver nanoprisms with NaBH4 concentrations of 2 × 10-6, 1 × 10-5, and 2 × 10-5 M. The consumption of the precursor for the reaction with NaBH4 concentration of 5 × 10-5 M was also included in (D) for comparison.

to 70 °C, intensity of the absorbance at 400 nm increased slightly and then kept almost unchanged until the end of the reaction. Temporal evolution of silver ion concentration (Figure 2D) revealed that the precursor was consumed almost completely after the addition of 5 × 10-4 M NaBH4 at the low temperature. The slight change in absorbance of the silver nanoparticles should be attributed to the intraparticle ripening of the silver nanoparticles.15 Because of the rapid reduction of the precursor, small spherical silver nanoparticles were dominated in the product, similar to those prepared by conventional NaBH4 reduction method.19 These results indicated that the spherical silver nanoparticles cannot be transformed into triangular nanoprisms by only the intraparticle ripening. TEM observations were taken to follow the shape evolution of the silver nanoparticles for the reaction carried out at NaBH4 concentration of 5 × 10-6 M (Figure 3). Only small spherical silver nanoparticles with average size about 4.7 nm ((29%) were observed after 1 min of the addition of NaBH4 at the low temperature. After being heated to 70 °C for 2 h, the average size of the particles decreased to 3.1 nm ((35%) due to the formation of additional smaller spherical silver nanoparticles. After being heated to 70 °C, the consumption of the precursor during the period of 0-2 h should contribute to the formation of additional small spherical silver naoparticles but not the growth of the particles formed at the low temperature, thus resulting in the increased intensity of the absorbance at 400 nm (Figure 2B). Larger nanoparticles with anisotropic structure became observable at 4 h and then triangular nanoprisms at 6 h of the reaction, while the amount of the spherical nanoparticles decreased. The decreased intensity of the absorbance in Figure 2B during the period of 2-20 h of the reaction at 70 °C was related to the decreased amount of the small nanoparticles. With the proceeded reaction, amount of the triangular nanoprisms increased and became dominant at the end of the reaction. The thickness of the final triangular nanoparisms was estimated to be about 6.2 ( 0.7 nm, and nearly 60% of the nanoprisms presented an edge length of 100 ( 20 nm (see Supporting Information Figure S2). Upon the transformation of the spherical silver nanoparticles into the nanoprisms (2-20 h), it is obvious that the remaining precursor was not enough to supply the growth of the triangular nanoprisms. Therefore the growth of the nanoprisms should be contributed to both the consumption

of the remaining precursor and the dissolution of the small silver nanoparticle (Ostwald ripening). In the following stage of the reaction (20-48 h), the nanoprisms became more uniform in shape and size driven by the Ostwald ripening. Further experimental results indicated that the triangular nanoprisms were only observable in the product in the presence of a suitable concentration of NaBH4 (2 × 10-6 to 2 × 10-5 M, see Supporting Information Figure S3). UV-visible spectra and temporal evolution of the Ag+ concentration of all the reactions produced the triangular nanoprisms presented the same characters (Figure 4). First, addition of the NaBH4 at the low temperature induced the rapid reduction of silver nitrate and the formation of small silver nanoparticles. After being heated to 70 °C, further reduction of the precursor by citrate resulted in the continuous formation of small silver particles but not growth of the small particles formed at the low temperature. Since the energy needed to reduce Ag ions and deposit Ag0 on existing small silver particles should be lower than the nucleation of new particles by using mild reducing agent citrate, the formation of additional small silver is likely to be related to the existence of reduced Ag0 or (Ag0)n clusters besides the silver nanoparticles after the reduction of silver nitrate by NaBH4 at low temperature.32,33 It is supposed that the Ag0 or (Ag0)n clusters promote the reduction of silver nitrate and formation of additional small silver particles at the high temperature since the reduction was very slow even under the evaluated temperature for the reaction carried out without NaBH4 (see Supporting Information Figure S4). After 2 h of the reaction, the amount of the small silver particles began to decrease and triangular nanoprisms were formed accompanied by the slow consumption of the precursor. Although the concentration of the precursor remained before heating was dependent on the concentration of NaBH4 used, similar amount of the precursor remained (around 30%) after 2 h of the reactions proceeded at 70 °C when the absorbance at 400 nm reached the maximum. These results means that although the concentration of NaBH4 used was different, similar amount of the precursor contributed to the formation of small spherical particles for all the reactions which can produced the nanoprisms. Control experiments showed if the precursor was consumed rapidly by adding 5 × 10-4 M NaBH4 at 2 h of the reactions proceeded at 70 °C, no transformation of the spherical particles into the nanoprisms

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Figure 5. Temporal evolution of the UV-visible spectra (A-E) and the precursor concentration (F) during the synthesis for the silver nanoparticles prepared with different trisodium citrate (Na3Cit) concentrations from 0 M to 3.5 × 10-3 M. The concentrations of silver nitrate and NaBH4 were set as 1 × 10-4 and 5 × 10-6 M, respectively.

were observable (see Supporting Information Figure S5). It is proposed that the concentration of the remained precursor at 2 h is a little lower than the solubility of the small spherical particles, thus promoting their dissolution and transformation into the nanoprisms. During the period of 20-48 h, a few amount of the precursor was still detectable for all the reactions (about 5%), indicating the subsequent improvement in shape and size distribution of the nanoprisms was dominantly driven by the Ostwald ripening but not the intraparticle ripening. It is noted that the in-plane dipole plasmon resonance of the nanoprisms shifted experienced blue-shift and thus decreased average edge length of the nanoprisms with increased concentration of NaBH4, which was consistent with the TEM observations. The role of citrate was also investigated by changing the concentrations of citrate while fixing the concentrations of silver nitrate and NaBH4. The concentrations of silver nitrate and NaBH4 were set as 1 × 10-4 and 5 × 10-6 M, respectively, for all the reactions. Temporal evolutions of the UV-visible spectra were recorded to follow the formation and growth of the silver nanoparticles (Figure 5). Without the use of citrate, only weak absorbance was detected due to rapid precipitation of the silver nanoparticles (Figure 5A). When the citrate concentration was low (5 × 10-5 M), the final product was dominated by spherical silver nanoparticles. The absorption peak experienced a red shift after 6 h of the reaction at 70 °C (Figure 5B), indicating the growth of the spherical particles. When the concentration of citrate was 1 × 10-4 M (Figure 5C), silver nanoparticles with anisotropic structure were formed. When the concentration of citrate increased to 5 × 10-4 and 3.5 × 10-3 M (Figure 5D,E), the out-of-plane quadrupole around 334 nm and in-plane dipole plasmon resonance (600-900 nm) were observed for the final products. Temporal evolutions of the Ag+ concentrations as shown in Figure 5F revealed that the precursor could not be consumed efficiently when the concentration of citrate was lower than 1 × 10-4 M, implying both the roles of citrate as stabilizer and reductant are vital important to promote transformation of the spherical silver nanoparticles into the nanoprisms. When the concentration of citrate increased to 5 × 10-4 and 3.5 ×

10-3 M, the precursor was consumed almost completely after 3 h of the reaction at 70 °C. It is noted that almost the same amount of the precursor (∼5%) remained until the end of the reaction although the concentration of citrate was different, similar to those observed for the reactions carried out with different concentration of NaBH4 while fixing the concentration of citrate as shown in Figure 4. It is also noted that the triangular nanoprisms prepared at citrate concentration of 3.5 × 10-3 M presented the in-plane dipole plasmon resonance at shorter wavelength than those prepared at citrate concentration of 5 × 10-4 M, suggesting the decreased edge length of the nanoprisms with increased concentration of citrate. The role of O2 was also studied as it plays an important role during the photochemical approach described by Mirkin’s group. It was found that the nanoprisms could be obtained even in the solution degassed by N2 (see Supporting Information Figure S6). In the photochemical approach, O2 is necessary to oxidatively dissolve some Ag nanoparticles and generate Ag+, leading to a Ag+/Ag nanoparticles copresence environment and thus the growth of the nanoprisms.26 It is reasonable that O2 is not necessary in such stepwise reduction approach since there was suitable amount of residual Ag+ during the growth of the nanoprisms. Conclusion In summary, a stepwise reduction method was developed for the synthesis of triangular silver nanoprisms. In this approach, the strong reductant, NaBH4, was employed to induce the reduction of silver nitrate and formation of small spherical silver nanoparticles at the low temperature, and then the weak reductant, citrate, was employed to induce the further reduction of the precursor and formation of additional small spherical silver nanoparticles and their transformation into the nanoprisms at the high temperature. The ratio of two reductants (NaBH4 and citrate) was found to be critical to result in the formation of small spherical particles and then their transformation into the triangular nanoprisms. The residual silver ions after the formation of the spherical nanoparticles are necessary to promote

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their dissolution and transformation into the triangular nanoprisms. Uncompleted consumption of the silver ions even at the end of the reaction contributed to the improved shape and size uniformity of the nanoprisms by the Ostwald ripening process. Such an approach provides a facile way to prepare silver nanoprisms capped only by citrate ligands, which can be readily replaced by a variety of other ligands including biomolecules. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20803029, 20773053, 50825202), the National Research Fund for Fundamental Key Project (2009CB939701), and the Program for NCET in University of Chinese Ministry of Education. Supporting Information Available: Supporting results mentioned in the text, including Figures S1-S6. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mulvaney, S. P.; Musick, M. D.; Keating, C. D.; Natan, M. J. Langmuir 2003, 19, 4784. (2) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547. (3) Xiong, Y. J.; Wiley, B.; Xia, Y. N. Angew. Chem., Int. Ed. 2007, 46, 7157. (4) Tominaga, J.; Mihalcea, C.; Buchel, D.; Fukuda, H.; Nakano, T.; Atoda, N.; Fuji, H.; Kikukawa, T. Appl. Phys. Lett. 2001, 78, 2417. (5) Munro, C. H.; Smith, W. E.; Garner, M.; Clarkson, J.; White, P. C. Langmuir 1995, 11, 3712. (6) Yang, Y.; Matsubara, S.; Xiong, L. M.; Hayakawa, T.; Nogami, M. J. Phys. Chem. C 2007, 111, 9095. (7) Wiley, B.; Sun, Y. G.; Xia, Y. Acc. Chem. Res. 2007, 40, 1067. (8) Maillard, M.; Giorgio, S.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 2466. (9) Maillard, M.; Huang, P. R.; Brus, L. Nano Lett. 2003, 3, 1611. (10) Millstone, J. E.; Hurst, S. J.; Metraux, G. S.; Cutler, J. I.; Mirkin, C. A. Small 2009, 5, 646.

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