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One-step growth of triangular silver nanoplates with predictable sizes on a large scale† Cite this: DOI: 10.1039/c4nr00254g
Xiaxia Liu,a Linglong Li,c Yaodong Yang,c Yadong Yinb and Chuanbo Gao*a Received 15th January 2014 Accepted 24th February 2014 DOI: 10.1039/c4nr00254g www.rsc.org/nanoscale
A one-step growth of triangular silver nanoplates on a large scale is developed by a coordination-based kinetically controlled seeded growth method, with their edge length precisely tuned from 150 nm to 1.5 mm, and surface plasmon resonance extends to full near-infrared.
Introduction The past twenty years have witnessed a dramatic progress in colloidal nanostructure synthesis. A great number of synthetic techniques are now accessible for the production of nanostructures with various sizes and shapes from common solid materials. Now a signicant challenge people oen face is how these colloidal nanomaterials can be used in practical applications. Although there are many reasons for this challenge, oen the problems are still in the synthesis. A perfect synthesis expected for practical applications should be simple, reproducible, scalable, and able to generate uniform products with predictable sizes and shapes. Many attempts for applications have failed because of the difficulty in meeting one or more of the requirements. In this regard, perfection of the synthesis is of great importance for the future development of colloidal nanostructures. In this work, we use the synthesis of silver (Ag) nanoplates,1,2 which have been used for a number of applications due to their interesting plasmonic properties,3–5 as an example to report our effort along this direction. More specically, with a robust seeded growth process, we demonstrate a one-step synthesis of high-quality triangular Ag nanoplates with predictable sizes on a large scale. Conventionally, Ag nanoplates can be produced by a photochemical synthesis6–9 or a chemical reduction synthesis,10–14 a
Center for Materials Chemistry, Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710054, China. E-mail: [email protected]. edu.cn
b
Department of Chemistry, University of California, Riverside, California 92521, USA
c
Multi-Disciplinary Materials Research Center, Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710054, China † Electronic supplementary information (ESI) available: Detailed experimental methods and additional data. See DOI: 10.1039/c4nr00254g
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which however involve complicated redox processes controlled by either light irradiation or oxidative additives. Although the size of the Ag nanoplates can be tuned by adjusting a few synthesis parameters,7 the window for the size tuning turns out to be relatively limited, in particular when a large size is targeted, which can be largely attributed to the difficulty in precisely harnessing the redox kinetics in the synthesis and thus the concentration of nuclei with preferred crystalline symmetry for nanoplates.15 For example, Ag nanoplates with an edge length of 1.5 mm can be synthesized by a solvothermal reduction method, which, however, show very limited control over the shape, edge length, thickness and purity of the nanoplates, due to the narrow synthesis window and thus difficulty in controlling the kinetics technically.16 Seeded growth represents a much improved method for the synthesis of noble metal nanostructures in a more controllable manner.17,18 In this case, noble metals grow on preexisting seeds when the reduction rate of the metal salt is sufficiently low such that the concentration of elemental metals resulting from reduction remains lower than their nucleation threshold, which is experimentally achieved typically by slowly introducing a metal precursor into a reaction system.19–21 The seeded growth of Ag nanoplates of various edge lengths and thicknesses can be enabled by slowly injecting a Ag salt into a growth system, with the growth direction of the nanoplates well controlled by selecting an appropriate capping agent.22,23 However, multiple steps are always involved, as only a small size increase can be achieved in each cycle due to the low concentration of the metal precursor, which leads to consumption of a great deal of time, energy and labor, making mass production difficult to achieve. With a clear understanding of the key requirements for seeded growth, we report here a robust synthesis for triangular Ag nanoplates of different sizes (edge length: 150 nm to 1.50 mm) in one step (in addition to the seeding step) by an elaborately designed coordination-based kinetically controlled seeded growth.24–27 In order to effectively suppress the selfnucleation of Ag in a one-step procedure, acetonitrile is employed as a ligand to Ag(I) salt, which helps maintain a low
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concentration of elemental Ag(0) at a high concentration of the Ag(I) salt, as a result of a dramatically decreased reduction potential of the Ag(I) salt due to coordination. Preformed trisodium citrate (TSC)-capped Ag nanoplates (30 nm in size and 5 nm in thickness) containing parallel twin defects were prepared using a previously reported chemical reduction method and then used as the seeds.13,14 To regulate the crystal growth, TSC is applied as a capping agent, which binds the basal {111} facets of the seeds, effectively blocking the growth along the vertical axis and allowing extensive growth along the lateral axis of the seeds. Advantages of this synthesis method include the convenient one-step procedure, wide tunability and stoichiometric determination of the size, high yield of the product, and good scalability of the synthesis.
Results and discussion In a typical synthesis, a growth solution is prepared by incorporating acetonitrile, TSC, ascorbic acid and AgNO3 into water, which act respectively as a coordinating ligand, a capping agent for regulating the dimension of the crystal growth, a reducing agent and a Ag(I) source. The coordination between acetonitrile and Ag+ has been conrmed spectrally by NMR (Fig. S1, ESI†). Seeded growth is initiated by injecting a known amount of the seed solution (Fig. S2, ESI†) into the growth solution, indicated by a gradual color change as a result of an evolved surface plasmon resonance, which leads to formation of triangular Ag nanoplates aer the reaction is completed. This synthesis involves only one step besides the seed preparation, which is in clear contrast to conventional syntheses that require stepwise and repetitive seeded growth cycles. Fig. 1 shows the TEM images of the triangular Ag nanoplates obtained from a coordination-based seeded growth. The Ag nanostructures are exclusively nanoplates without discernible presence of particular morphologies, conrming that TSC as a capping agent is effective to guide the growth dimensions of the nanocrystals and give rise to high-purity plate-like Ag nanostructures. The narrow size distributions of the triangular Ag nanoplates indicate that all the seeds grow at the same rate, and that no self-nucleated Ag nanoplates are formed during the whole process at all seeds/Ag(I) salt ratios, endorsing the effect of the Ag(I)–acetonitrile coordination on suppressing selfnucleation events. By adjusting the seed amount, triangular Ag nanoplates of different sizes are readily synthesized, with edge lengths varying from 150 nm to 1.50 mm, which provides a versatile tool for synthesizing triangular Ag nanoplates with widely tunable sizes from a convenient one-step coordinationbased kinetically controlled seeded growth process. It is noteworthy that additional measures are taken to control the reaction kinetics in the synthesis of the triangular Ag nanoplates. Ag nanoplates are obtained only when a slow reaction rate is achieved by controlling the reaction temperature to as low as 5 C and the concentration of each reactant, especially the reducing agent, is lower than a critical value. A seeded growth under other conditions easily produces ower-like nanostructures of Ag with branched architectures (Fig. S3, ESI†), due to a much accelerated reaction and thus an uneven
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TEM images of the triangular Ag nanoplates of different sizes obtained by tuning the volume of the seed solution. Seed volumes are: (a) 12 mL, (b) 6 mL, (c) 4 mL, (d) 2 mL, (e) 1 mL, (f) 200 mL, (g) 100 mL, and (h) 40 mL, respectively. Right to the TEM images: size distributions of the triangular Ag nanoplates. The average sizes are measured to be 150 nm, 179 nm, 220 nm, 310 nm, 432 nm, 708 nm, 1.10 mm and 1.50 mm for (a–h), respectively. Fig. 1
atomic deposition of elemental Ag(0) on the existing nanocrystals. Thanks to the slow reaction kinetics, the Ag nanoplates synthesized in this work possess perfectly triangular shapes with sharp corners, distinct from previous synthesis systems which oen afford alternative shapes such as hexagons. The thickness of the triangular Ag nanoplates is further investigated by atomic force microscopy (AFM) (Fig. S4, ESI†). Ag nanoplates with an edge length of 150 nm have a thickness of 6.9 nm, slightly larger than that of the initial seeds, 5 nm. When the edge length of the nanoplates increases to 432 nm and 1.10 mm, the average thickness increases to 9.5 nm and 14 nm, respectively. However, the rate of increase in the thickness of the nanoplates is much slower than that in the edge length, which conrms that TSC can selectively bind the basal {111} facets of the Ag nanoplates limiting the crystal growth, and that the binding by an organic species on an inorganic facet cannot completely block the crystal growth along this dimension. The change in the edge length of the triangular Ag nanoplates with the seed volume and the evolution of the thickness have been plotted in Fig. 2 for comparison. The coordination-based seeded
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Fig. 2 (a) Relationship between the edge length of the triangular Ag nanoplates and the volume of the seed solution. (b) Changes in the thickness of the Ag nanoplates as a function of edge length. Fig. 3 UV-vis-NIR extinction spectra of the triangular Ag nanoplates. Legend indicates the edge length of each sample.
growth of noble metal nanoparticles has proven to be a stoichiometric process, with the size of the nanoparticles determined and well predicted by the amount of the seeds.24 In the synthesis of noble metal nanospheres, for example, a relationship of d3 ¼ kV1 is established as a result of an isotropic growth of nanospheres in three dimensions, where d represents the diameter of the nanospheres, V represents the amount of the seeds in terms of volume of the seed solution, and k is a constant.26,27 In the present synthesis, the growth of the Ag nanostructure is anisotropic due to an effective capping effect, with two lateral dimensions growing much faster than the vertical dimension. Therefore, the relationship between the edge length (L) and the volume of the seed solution (V) should be in a form of Ln ¼ kV1, where n is a measure of the degree of anisotropy of the triangular Ag nanoplates. Fig. 2a reveals that the experimental data t this formula very well with n ¼ 2.437, indicating that the one-step seeded growth of triangular Ag nanoplates resembles more a 2-dimensional growth than a 3-dimensional growth. When the thicknesses (d) of the nanoplates are plotted versus their edge lengths (L) (Fig. 2b), it is found that a relationship of d ¼ kL0.437 can be satised, which is consistent with the above L–V equation and states more specically the relevance between the growth rates in the vertical and lateral dimensions of the triangular nanoplates. These correlations conrm that the synthesis of the triangular Ag nanoplates is a well predictable stoichiometric process, making it possible to synthesize nanoplates of an arbitrary size in the range of 150 nm to 1.50 mm by simply introducing a calculated amount of the seeds in the synthesis. Based on these geometry data, a monotone change of the aspect ratio of the triangular Ag nanoplates with the seed volume (or the edge length) is predicted, which gives rise to a continuous shi of their in-plane dipole surface plasmon resonances to longer wavelengths. Fig. 3 shows UV-vis-NIR extinction spectra of the Ag nanoplates, obtained by measuring the deposited nanoplates on a glass slide to avoid overlap with the intense vibrational peak of H2O. The in-plane dipole SPR band is initially at 1102 nm when the edge length of the Ag nanoplates is 150 nm, and readily shis to longer wavelengths as the edge length increases. When the edge length reaches 1.10 mm, the in-plane dipole peak already shis to 3000 nm, which is the limit of our measurement and the upper limit of the nearinfrared region of electromagnetic waves. Therefore, triangular
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Ag nanoplates with surface plasmon resonance extending to a full near-infrared range can be conveniently achieved by our method. Notably, by measuring the UV-vis spectra of the colloids of the triangular Ag nanoplates in water, an in-plane quadrupole peak can be observed in the visible range for nanoplates with an edge length #310 nm, and this peak readily shis to a longer wavelength and eventually to the near-infrared as the size of the nanoplates increases (Fig. S5, ESI†). The clear appearance of quadrupole mode surface plasmon resonance conrms that the triangular Ag nanoplates synthesized by our method are highly uniform in both shape and size. Additionally, this synthesis can be readily scaled up to produce a large quantity of Ag nanoplates due to the robust growth mechanism in our scheme which effectively suppresses self-nucleation events. As a demonstration, 0.3 g of triangular Ag nanoplates with a target edge length of 432 nm have been synthesized in one step by scaling up a typical synthesis in this work for 232 times, which is about 100 times scale of a synthesis of Ag nanoplates of a similar dimension by a conventional procedure that we developed before.22 As scale-up is always a complicated process which introduces new technical factors into the synthesis, the reaction kinetics is slightly accelerated when the synthesis scale is directly enlarged, indicated by a quicker color change during the seeded growth, which has been slowed down by increasing the acetonitrile/water ratio in the
Fig. 4 A large-scale synthesis of the triangular Ag nanoplates. Target edge length: 432 nm; scale: 0.3 g. (a) Digital photograph of the sol of the triangular Ag nanoplates. (b) A typical TEM image. (c) Size distribution of the Ag nanoplates measured by TEM imaging.
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synthesis. Fig. 4 shows a typical TEM image of the Ag nanoplates and a digital photo of a sol of the nanoplates. It is clear that the products obtained are exclusively triangular nanoplates that are almost identical to those obtained from a typical-scale synthesis (Fig. 1e), and the size of the triangular Ag nanoplates (447 nm) is close to the target size, which conrm a successful scale-up of the synthesis without loss of stoichiometry and overall quality. Therefore, this synthesis is capable of providing a large quantity of Ag nanoplates in a convenient way, which is highly desirable for practical applications.
Conclusions In conclusion, we have successfully synthesized triangular Ag nanoplates of uniform size in the range of 150 nm to 1.5 mm through a one-step coordination-based seeded growth. This synthesis takes advantages of the coordinating effect of acetonitrile to a Ag(I) salt which suppresses self-nucleation of Ag during the synthesis, and the selective capping effect of TSC on specic facets of the Ag nanocrystals which allows anisotropic growth of the Ag nanostructures. By tuning the amount of the seeds, the size of the triangular Ag nanoplates has been conveniently tailored in a stoichiometric way and the in-plane dipole surface plasmon resonance extends to full near-infrared. This synthesis can be conveniently scaled up while retaining the high quality of the products. Therefore, this method provides a simple yet very robust, scalable, and reproducible route to triangular Ag nanoplates for many practical applications. The strategy demonstrated in this work is also potentially extendable to seeded growth of noble metal nanoparticles of many other morphologies and compositions with similar advantages.
Acknowledgements This work was supported by the start-up fund for C. Gao and operational fund for the Center for Materials Chemistry from Xi'an Jiaotong University (XJTU). Y. Yin acknowledges the support from U. S. National Science Foundation (CHE-1308587). The authors thank Prof. X. Zeng at Center for Organic Chemistry, Frontier Institute of Science and Technology, XJTU, for help with NMR.
Notes and references 1 J. E. Millstone, S. J. Hurst, G. S. M´ etraux, J. I. Cutler and C. A. Mirkin, Small, 2009, 5, 646–664. 2 I. Pastoriza-Santos and L. M. Liz-Marz´ an, J. Mater. Chem., 2008, 18, 1724–1737. 3 H. Ko, S. Singamaneni and V. V. Tsukruk, Small, 2008, 4, 1576–1599.
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4 C. B. Gao, Z. D. Lu, Y. Liu, Q. Zhang, M. F. Chi, Q. Cheng and Y. D. Yin, Angew. Chem., Int. Ed., 2012, 51, 5629– 5633. 5 K. A. Homan, M. Souza, R. Truby, G. P. Luke, C. Green, E. Vreeland and S. Emelianov, ACS Nano, 2011, 6, 641– 650. 6 R. C. Jin, Y. W. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz and J. G. Zheng, Science, 2001, 294, 1901–1903. 7 R. C. Jin, Y. C. Cao, E. C. Hao, G. S. Metraux, G. C. Schatz and C. A. Mirkin, Nature, 2003, 425, 487–490. 8 A. Callegari, D. Tonti and M. Chergui, Nano Lett., 2003, 3, 1565–1568. 9 Y. G. Sun, B. Mayers and Y. N. Xia, Nano Lett., 2003, 3, 675– 679. 10 Y. G. Sun and Y. N. Xia, Adv. Mater., 2003, 15, 695–699. 11 S. H. Chen and D. L. Carroll, Nano Lett., 2002, 2, 1003– 1007. 12 S. H. Chen and D. L. Carroll, J. Phys. Chem. B, 2004, 108, 5500–5506. 13 G. S. M´ etraux and C. A. Mirkin, Adv. Mater., 2005, 17, 412– 415. 14 Q. Zhang, N. Li, J. Goebl, Z. D. Lu and Y. D. Yin, J. Am. Chem. Soc., 2011, 133, 18931–18939. 15 V. Germain, J. Li, D. Ingert, Z. L. Wang and M. P. Pileni, J. Phys. Chem. B, 2003, 107, 8717–8720. 16 T. You, S. Sun, X. Song and S. Xu, Cryst. Res. Technol., 2009, 44, 857–860. 17 K. R. Brown, D. G. Walter and M. J. Natan, Chem. Mater., 1999, 12, 306–313. 18 N. R. Jana, L. Gearheart and C. J. Murphy, Langmuir, 2001, 17, 6782–6786. 19 J. Rodr´ıguez-Fern´ andez, J. P´ erez-Juste, F. J. Garc´ıa de Abajo and L. M. Liz-Marz´ an, Langmuir, 2006, 22, 7007– 7010. 20 C. Ziegler and A. Eychm¨ uller, J. Phys. Chem. C, 2011, 115, 4502–4506. 21 N. G. Bastus, J. Comenge and V. Puntes, Langmuir, 2011, 27, 11098–11105. 22 Q. Zhang, Y. X. Hu, S. R. Guo, J. Goebl and Y. D. Yin, Nano Lett., 2010, 10, 5037–5042. 23 J. Zeng, X. H. Xia, M. Rycenga, P. Henneghan, Q. G. Li and Y. N. Xia, Angew. Chem., Int. Ed., 2011, 50, 244–249. 24 C. B. Gao, J. Goebl and Y. D. Yin, J. Mater. Chem. C, 2013, 1, 3898–3909. 25 C. B. Gao, Q. Zhang, Z. D. Lu and Y. D. Yin, J. Am. Chem. Soc., 2011, 133, 19706–19709. 26 C. B. Gao, J. Vuong, Q. Zhang, Y. D. Liu and Y. D. Yin, Nanoscale, 2012, 4, 2875–2878. 27 X. X. Liu, Y. D. Yin and C. B. Gao, Langmuir, 2013, 29, 10559– 10565.
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One-step growth of triangular silver nanoplates with predictable sizes on a large scale† Cite this: DOI: 10.1039/c4nr00254g
Xiaxia Liu,a Linglong Li,c Yaodong Yang,c Yadong Yinb and Chuanbo Gao*a Received 15th January 2014 Accepted 24th February 2014 DOI: 10.1039/c4nr00254g www.rsc.org/nanoscale
A one-step growth of triangular silver nanoplates on a large scale is developed by a coordination-based kinetically controlled seeded growth method, with their edge length precisely tuned from 150 nm to 1.5 mm, and surface plasmon resonance extends to full near-infrared.
Introduction The past twenty years have witnessed a dramatic progress in colloidal nanostructure synthesis. A great number of synthetic techniques are now accessible for the production of nanostructures with various sizes and shapes from common solid materials. Now a signicant challenge people oen face is how these colloidal nanomaterials can be used in practical applications. Although there are many reasons for this challenge, oen the problems are still in the synthesis. A perfect synthesis expected for practical applications should be simple, reproducible, scalable, and able to generate uniform products with predictable sizes and shapes. Many attempts for applications have failed because of the difficulty in meeting one or more of the requirements. In this regard, perfection of the synthesis is of great importance for the future development of colloidal nanostructures. In this work, we use the synthesis of silver (Ag) nanoplates,1,2 which have been used for a number of applications due to their interesting plasmonic properties,3–5 as an example to report our effort along this direction. More specically, with a robust seeded growth process, we demonstrate a one-step synthesis of high-quality triangular Ag nanoplates with predictable sizes on a large scale. Conventionally, Ag nanoplates can be produced by a photochemical synthesis6–9 or a chemical reduction synthesis,10–14 a
Center for Materials Chemistry, Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710054, China. E-mail: [email protected]. edu.cn
b
Department of Chemistry, University of California, Riverside, California 92521, USA
c
Multi-Disciplinary Materials Research Center, Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710054, China † Electronic supplementary information (ESI) available: Detailed experimental methods and additional data. See DOI: 10.1039/c4nr00254g
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which however involve complicated redox processes controlled by either light irradiation or oxidative additives. Although the size of the Ag nanoplates can be tuned by adjusting a few synthesis parameters,7 the window for the size tuning turns out to be relatively limited, in particular when a large size is targeted, which can be largely attributed to the difficulty in precisely harnessing the redox kinetics in the synthesis and thus the concentration of nuclei with preferred crystalline symmetry for nanoplates.15 For example, Ag nanoplates with an edge length of 1.5 mm can be synthesized by a solvothermal reduction method, which, however, show very limited control over the shape, edge length, thickness and purity of the nanoplates, due to the narrow synthesis window and thus difficulty in controlling the kinetics technically.16 Seeded growth represents a much improved method for the synthesis of noble metal nanostructures in a more controllable manner.17,18 In this case, noble metals grow on preexisting seeds when the reduction rate of the metal salt is sufficiently low such that the concentration of elemental metals resulting from reduction remains lower than their nucleation threshold, which is experimentally achieved typically by slowly introducing a metal precursor into a reaction system.19–21 The seeded growth of Ag nanoplates of various edge lengths and thicknesses can be enabled by slowly injecting a Ag salt into a growth system, with the growth direction of the nanoplates well controlled by selecting an appropriate capping agent.22,23 However, multiple steps are always involved, as only a small size increase can be achieved in each cycle due to the low concentration of the metal precursor, which leads to consumption of a great deal of time, energy and labor, making mass production difficult to achieve. With a clear understanding of the key requirements for seeded growth, we report here a robust synthesis for triangular Ag nanoplates of different sizes (edge length: 150 nm to 1.50 mm) in one step (in addition to the seeding step) by an elaborately designed coordination-based kinetically controlled seeded growth.24–27 In order to effectively suppress the selfnucleation of Ag in a one-step procedure, acetonitrile is employed as a ligand to Ag(I) salt, which helps maintain a low
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concentration of elemental Ag(0) at a high concentration of the Ag(I) salt, as a result of a dramatically decreased reduction potential of the Ag(I) salt due to coordination. Preformed trisodium citrate (TSC)-capped Ag nanoplates (30 nm in size and 5 nm in thickness) containing parallel twin defects were prepared using a previously reported chemical reduction method and then used as the seeds.13,14 To regulate the crystal growth, TSC is applied as a capping agent, which binds the basal {111} facets of the seeds, effectively blocking the growth along the vertical axis and allowing extensive growth along the lateral axis of the seeds. Advantages of this synthesis method include the convenient one-step procedure, wide tunability and stoichiometric determination of the size, high yield of the product, and good scalability of the synthesis.
Results and discussion In a typical synthesis, a growth solution is prepared by incorporating acetonitrile, TSC, ascorbic acid and AgNO3 into water, which act respectively as a coordinating ligand, a capping agent for regulating the dimension of the crystal growth, a reducing agent and a Ag(I) source. The coordination between acetonitrile and Ag+ has been conrmed spectrally by NMR (Fig. S1, ESI†). Seeded growth is initiated by injecting a known amount of the seed solution (Fig. S2, ESI†) into the growth solution, indicated by a gradual color change as a result of an evolved surface plasmon resonance, which leads to formation of triangular Ag nanoplates aer the reaction is completed. This synthesis involves only one step besides the seed preparation, which is in clear contrast to conventional syntheses that require stepwise and repetitive seeded growth cycles. Fig. 1 shows the TEM images of the triangular Ag nanoplates obtained from a coordination-based seeded growth. The Ag nanostructures are exclusively nanoplates without discernible presence of particular morphologies, conrming that TSC as a capping agent is effective to guide the growth dimensions of the nanocrystals and give rise to high-purity plate-like Ag nanostructures. The narrow size distributions of the triangular Ag nanoplates indicate that all the seeds grow at the same rate, and that no self-nucleated Ag nanoplates are formed during the whole process at all seeds/Ag(I) salt ratios, endorsing the effect of the Ag(I)–acetonitrile coordination on suppressing selfnucleation events. By adjusting the seed amount, triangular Ag nanoplates of different sizes are readily synthesized, with edge lengths varying from 150 nm to 1.50 mm, which provides a versatile tool for synthesizing triangular Ag nanoplates with widely tunable sizes from a convenient one-step coordinationbased kinetically controlled seeded growth process. It is noteworthy that additional measures are taken to control the reaction kinetics in the synthesis of the triangular Ag nanoplates. Ag nanoplates are obtained only when a slow reaction rate is achieved by controlling the reaction temperature to as low as 5 C and the concentration of each reactant, especially the reducing agent, is lower than a critical value. A seeded growth under other conditions easily produces ower-like nanostructures of Ag with branched architectures (Fig. S3, ESI†), due to a much accelerated reaction and thus an uneven
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TEM images of the triangular Ag nanoplates of different sizes obtained by tuning the volume of the seed solution. Seed volumes are: (a) 12 mL, (b) 6 mL, (c) 4 mL, (d) 2 mL, (e) 1 mL, (f) 200 mL, (g) 100 mL, and (h) 40 mL, respectively. Right to the TEM images: size distributions of the triangular Ag nanoplates. The average sizes are measured to be 150 nm, 179 nm, 220 nm, 310 nm, 432 nm, 708 nm, 1.10 mm and 1.50 mm for (a–h), respectively. Fig. 1
atomic deposition of elemental Ag(0) on the existing nanocrystals. Thanks to the slow reaction kinetics, the Ag nanoplates synthesized in this work possess perfectly triangular shapes with sharp corners, distinct from previous synthesis systems which oen afford alternative shapes such as hexagons. The thickness of the triangular Ag nanoplates is further investigated by atomic force microscopy (AFM) (Fig. S4, ESI†). Ag nanoplates with an edge length of 150 nm have a thickness of 6.9 nm, slightly larger than that of the initial seeds, 5 nm. When the edge length of the nanoplates increases to 432 nm and 1.10 mm, the average thickness increases to 9.5 nm and 14 nm, respectively. However, the rate of increase in the thickness of the nanoplates is much slower than that in the edge length, which conrms that TSC can selectively bind the basal {111} facets of the Ag nanoplates limiting the crystal growth, and that the binding by an organic species on an inorganic facet cannot completely block the crystal growth along this dimension. The change in the edge length of the triangular Ag nanoplates with the seed volume and the evolution of the thickness have been plotted in Fig. 2 for comparison. The coordination-based seeded
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Fig. 2 (a) Relationship between the edge length of the triangular Ag nanoplates and the volume of the seed solution. (b) Changes in the thickness of the Ag nanoplates as a function of edge length. Fig. 3 UV-vis-NIR extinction spectra of the triangular Ag nanoplates. Legend indicates the edge length of each sample.
growth of noble metal nanoparticles has proven to be a stoichiometric process, with the size of the nanoparticles determined and well predicted by the amount of the seeds.24 In the synthesis of noble metal nanospheres, for example, a relationship of d3 ¼ kV1 is established as a result of an isotropic growth of nanospheres in three dimensions, where d represents the diameter of the nanospheres, V represents the amount of the seeds in terms of volume of the seed solution, and k is a constant.26,27 In the present synthesis, the growth of the Ag nanostructure is anisotropic due to an effective capping effect, with two lateral dimensions growing much faster than the vertical dimension. Therefore, the relationship between the edge length (L) and the volume of the seed solution (V) should be in a form of Ln ¼ kV1, where n is a measure of the degree of anisotropy of the triangular Ag nanoplates. Fig. 2a reveals that the experimental data t this formula very well with n ¼ 2.437, indicating that the one-step seeded growth of triangular Ag nanoplates resembles more a 2-dimensional growth than a 3-dimensional growth. When the thicknesses (d) of the nanoplates are plotted versus their edge lengths (L) (Fig. 2b), it is found that a relationship of d ¼ kL0.437 can be satised, which is consistent with the above L–V equation and states more specically the relevance between the growth rates in the vertical and lateral dimensions of the triangular nanoplates. These correlations conrm that the synthesis of the triangular Ag nanoplates is a well predictable stoichiometric process, making it possible to synthesize nanoplates of an arbitrary size in the range of 150 nm to 1.50 mm by simply introducing a calculated amount of the seeds in the synthesis. Based on these geometry data, a monotone change of the aspect ratio of the triangular Ag nanoplates with the seed volume (or the edge length) is predicted, which gives rise to a continuous shi of their in-plane dipole surface plasmon resonances to longer wavelengths. Fig. 3 shows UV-vis-NIR extinction spectra of the Ag nanoplates, obtained by measuring the deposited nanoplates on a glass slide to avoid overlap with the intense vibrational peak of H2O. The in-plane dipole SPR band is initially at 1102 nm when the edge length of the Ag nanoplates is 150 nm, and readily shis to longer wavelengths as the edge length increases. When the edge length reaches 1.10 mm, the in-plane dipole peak already shis to 3000 nm, which is the limit of our measurement and the upper limit of the nearinfrared region of electromagnetic waves. Therefore, triangular
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Ag nanoplates with surface plasmon resonance extending to a full near-infrared range can be conveniently achieved by our method. Notably, by measuring the UV-vis spectra of the colloids of the triangular Ag nanoplates in water, an in-plane quadrupole peak can be observed in the visible range for nanoplates with an edge length #310 nm, and this peak readily shis to a longer wavelength and eventually to the near-infrared as the size of the nanoplates increases (Fig. S5, ESI†). The clear appearance of quadrupole mode surface plasmon resonance conrms that the triangular Ag nanoplates synthesized by our method are highly uniform in both shape and size. Additionally, this synthesis can be readily scaled up to produce a large quantity of Ag nanoplates due to the robust growth mechanism in our scheme which effectively suppresses self-nucleation events. As a demonstration, 0.3 g of triangular Ag nanoplates with a target edge length of 432 nm have been synthesized in one step by scaling up a typical synthesis in this work for 232 times, which is about 100 times scale of a synthesis of Ag nanoplates of a similar dimension by a conventional procedure that we developed before.22 As scale-up is always a complicated process which introduces new technical factors into the synthesis, the reaction kinetics is slightly accelerated when the synthesis scale is directly enlarged, indicated by a quicker color change during the seeded growth, which has been slowed down by increasing the acetonitrile/water ratio in the
Fig. 4 A large-scale synthesis of the triangular Ag nanoplates. Target edge length: 432 nm; scale: 0.3 g. (a) Digital photograph of the sol of the triangular Ag nanoplates. (b) A typical TEM image. (c) Size distribution of the Ag nanoplates measured by TEM imaging.
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synthesis. Fig. 4 shows a typical TEM image of the Ag nanoplates and a digital photo of a sol of the nanoplates. It is clear that the products obtained are exclusively triangular nanoplates that are almost identical to those obtained from a typical-scale synthesis (Fig. 1e), and the size of the triangular Ag nanoplates (447 nm) is close to the target size, which conrm a successful scale-up of the synthesis without loss of stoichiometry and overall quality. Therefore, this synthesis is capable of providing a large quantity of Ag nanoplates in a convenient way, which is highly desirable for practical applications.
Conclusions In conclusion, we have successfully synthesized triangular Ag nanoplates of uniform size in the range of 150 nm to 1.5 mm through a one-step coordination-based seeded growth. This synthesis takes advantages of the coordinating effect of acetonitrile to a Ag(I) salt which suppresses self-nucleation of Ag during the synthesis, and the selective capping effect of TSC on specic facets of the Ag nanocrystals which allows anisotropic growth of the Ag nanostructures. By tuning the amount of the seeds, the size of the triangular Ag nanoplates has been conveniently tailored in a stoichiometric way and the in-plane dipole surface plasmon resonance extends to full near-infrared. This synthesis can be conveniently scaled up while retaining the high quality of the products. Therefore, this method provides a simple yet very robust, scalable, and reproducible route to triangular Ag nanoplates for many practical applications. The strategy demonstrated in this work is also potentially extendable to seeded growth of noble metal nanoparticles of many other morphologies and compositions with similar advantages.
Acknowledgements This work was supported by the start-up fund for C. Gao and operational fund for the Center for Materials Chemistry from Xi'an Jiaotong University (XJTU). Y. Yin acknowledges the support from U. S. National Science Foundation (CHE-1308587). The authors thank Prof. X. Zeng at Center for Organic Chemistry, Frontier Institute of Science and Technology, XJTU, for help with NMR.
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