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Observation of Coalescence Process of Silver Nanospheres During Shape Transformation to Nanoprisms

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ABSTRACT

In this report, we observed the growth mechanism and the shape transformation from spherical nanoparticles (diameter ~6 nm) to triangular nanoprisms (bisector length ~100 nm). We used a simple direct chemical reduction method and provided evidences for the growth of silver nanoprisms via a coalescence process. Unlike previous reports, our method does not rely upon light, heat, or strong oxidant for the shape transformation. This transformation could be launched by fine-tuning the pH value of the silver colloidal solution. Based on our extensive examination using transmission electron microscopy, we propose a non-point initiated growth mechanism, which is a combination of coalescence and dissolution–recrystallization process during the growth of silver nanoprisms.

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a High-resolution TEM images of the samples prepared by NaBH4 = 1,100 μL and PVP = 6 mL. The sample was pre-adsorbed on the amino-terminated silica film–coated copper grids to increase the adhesion of anionic nanoprisms. The subplots b–d show the individual nanostructure with a triangular shape. The insets show the electron diffraction pattern taken from the individual nanostructure. e HRTEM image of a silver nanoprism by directing the electron beam perpendicular to the flat face.
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Figure 5: a High-resolution TEM images of the samples prepared by NaBH4 = 1,100 μL and PVP = 6 mL. The sample was pre-adsorbed on the amino-terminated silica film–coated copper grids to increase the adhesion of anionic nanoprisms. The subplots b–d show the individual nanostructure with a triangular shape. The insets show the electron diffraction pattern taken from the individual nanostructure. e HRTEM image of a silver nanoprism by directing the electron beam perpendicular to the flat face.

Mentions: In this study, the "triangular and disk-like agglomerates" were observed as marked in Figure 5a. The surface of silicon film–coated copper grid was modified by the amino-terminate group (APTMS) to immobilize silver nanoparticle, which would reduce the self-assemble of silver nanoparticles during the evaporation. Then, the grid was immersed into the silver colloidal solution. With close examination of the TEM image, the particle–particle adhesion and coalescence by sintering would have a decreased free energy of the system due to the reduction of the interfacial area. Figure 5b–e are the high-resolution TEM images of some nanostructures with triangular shapes. The insets show the electron diffraction pattern taken from an individual nanoparticle indicating a polycrystalline, composite of a single crystal and a polycrystalline structure, and a single crystal, respectively. Figure 5c shows a layer of fused particles with a plate standing on top of the spherical particles. The presence of fused and unfused particles indicates that the nanoprisms were formed not through point initiated vectorial growth but, rather, by the recrystallization of multiple nanospheres in a triangular aggregate then fused gradually into one crystal. From the diffraction pattern, the fused particle was found to be single crystalline structure, which is identical to a silver nanoprism. Polycrystalline structures also exist in the diffraction pattern, which were contributed by the spherical particles or the unfused particles. Therefore, it should be considered as the intermediate structure of the transformation process. By analyzing the electron diffraction patterns, three sets of spots could be identified based on the d-spacing, i.e., the set with a spacing of 1.44 and 0.83 Å could be indexed to the {220} and {422} reflection of f.c.c. silver. Therefore, the top crystal face of the nanoprisms must be {111}. In addition, a set with a spacing 2.48 Å could be indexed to the 1/3{422} reflection of a hexagonal close-packed (h.c.p.) structure due to the atomically flat f.c.c. crystal surface of {111} as shown in Figure 5e. The silver nanoprism was found to be almost like a single crystal with {111} twin planes. These results were consistent with the previous observation of silver [24] and gold nanoprisms [25].


Observation of Coalescence Process of Silver Nanospheres During Shape Transformation to Nanoprisms
a High-resolution TEM images of the samples prepared by NaBH4 = 1,100 μL and PVP = 6 mL. The sample was pre-adsorbed on the amino-terminated silica film–coated copper grids to increase the adhesion of anionic nanoprisms. The subplots b–d show the individual nanostructure with a triangular shape. The insets show the electron diffraction pattern taken from the individual nanostructure. e HRTEM image of a silver nanoprism by directing the electron beam perpendicular to the flat face.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211880&req=5

Figure 5: a High-resolution TEM images of the samples prepared by NaBH4 = 1,100 μL and PVP = 6 mL. The sample was pre-adsorbed on the amino-terminated silica film–coated copper grids to increase the adhesion of anionic nanoprisms. The subplots b–d show the individual nanostructure with a triangular shape. The insets show the electron diffraction pattern taken from the individual nanostructure. e HRTEM image of a silver nanoprism by directing the electron beam perpendicular to the flat face.
Mentions: In this study, the "triangular and disk-like agglomerates" were observed as marked in Figure 5a. The surface of silicon film–coated copper grid was modified by the amino-terminate group (APTMS) to immobilize silver nanoparticle, which would reduce the self-assemble of silver nanoparticles during the evaporation. Then, the grid was immersed into the silver colloidal solution. With close examination of the TEM image, the particle–particle adhesion and coalescence by sintering would have a decreased free energy of the system due to the reduction of the interfacial area. Figure 5b–e are the high-resolution TEM images of some nanostructures with triangular shapes. The insets show the electron diffraction pattern taken from an individual nanoparticle indicating a polycrystalline, composite of a single crystal and a polycrystalline structure, and a single crystal, respectively. Figure 5c shows a layer of fused particles with a plate standing on top of the spherical particles. The presence of fused and unfused particles indicates that the nanoprisms were formed not through point initiated vectorial growth but, rather, by the recrystallization of multiple nanospheres in a triangular aggregate then fused gradually into one crystal. From the diffraction pattern, the fused particle was found to be single crystalline structure, which is identical to a silver nanoprism. Polycrystalline structures also exist in the diffraction pattern, which were contributed by the spherical particles or the unfused particles. Therefore, it should be considered as the intermediate structure of the transformation process. By analyzing the electron diffraction patterns, three sets of spots could be identified based on the d-spacing, i.e., the set with a spacing of 1.44 and 0.83 Å could be indexed to the {220} and {422} reflection of f.c.c. silver. Therefore, the top crystal face of the nanoprisms must be {111}. In addition, a set with a spacing 2.48 Å could be indexed to the 1/3{422} reflection of a hexagonal close-packed (h.c.p.) structure due to the atomically flat f.c.c. crystal surface of {111} as shown in Figure 5e. The silver nanoprism was found to be almost like a single crystal with {111} twin planes. These results were consistent with the previous observation of silver [24] and gold nanoprisms [25].

View Article: PubMed Central - HTML - PubMed

ABSTRACT

In this report, we observed the growth mechanism and the shape transformation from spherical nanoparticles (diameter ~6 nm) to triangular nanoprisms (bisector length ~100 nm). We used a simple direct chemical reduction method and provided evidences for the growth of silver nanoprisms via a coalescence process. Unlike previous reports, our method does not rely upon light, heat, or strong oxidant for the shape transformation. This transformation could be launched by fine-tuning the pH value of the silver colloidal solution. Based on our extensive examination using transmission electron microscopy, we propose a non-point initiated growth mechanism, which is a combination of coalescence and dissolution–recrystallization process during the growth of silver nanoprisms.

No MeSH data available.


Related in: MedlinePlus