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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.

No MeSH data available.


TEM images of the silver nanoparticles prepared with different amounts of NaBH4: a 800 μL, b 900 μL, c 1,000 μL, d 1,100 μL, e 1,200 μL, and f 1,300 μL. Scale bar is 100 nm.
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Figure 1: TEM images of the silver nanoparticles prepared with different amounts of NaBH4: a 800 μL, b 900 μL, c 1,000 μL, d 1,100 μL, e 1,200 μL, and f 1,300 μL. Scale bar is 100 nm.

Mentions: Figure 1 shows the transmission electron microscope (TEM) images of silver nanoparticles prepared with different amounts of NaBH4 while all other parameters remained constant. An extremely high mole ratio was used in this experiment, where the ratio of NaBH4 to AgNO3 is above 8. According to Yang et al. [22], the precursor (Ag ions) can be totally consumed when the mole ratio is 5. These prepared spherical Ag nanoparticles would transform into triangular nanoplates within 24 h at room temperature in dark. As the reaction proceeds, the intensity of the peak at 410 nm decreased and a new peak at a longer wavelength appeared in the NIR–Vis–UV absorption spectrum. This red-shift in energy implies the formation of a triangular structure, which is ascribed to the in-plane dipole resonance mode of silver nanoprisms as shown in Figure 2a. The bisector length of the silver nanoprism prepared with 1,100 μL NaBH4 was found to be 113.4 ± 40.8 nm, as shown in Figure 2b for the histogram. In this study, the out-of plane dipole mode did not show in the spectra, suggesting an inhomogeneous mixture of anisotropic nanoparticles in the solution. From TEM images and absorption spectrum, three features were observed. First, as the concentration of reducing agent (NaBH4) increases, the ratio of nanoprism/nanosphere would also increase. Secondly, the size of the formed silver nanoprisms is almost identical regardless the concentration of NaBH4. Finally, the out-off plane quadruple resonance mode is fixed at the same wavelength (331 nm), which implies that the concentration of the reducing agent would not affect the thickness of nanoprism. Consequently, the extreme excess amount of reducing agent is the crucial factor for the transformation.


Observation of Coalescence Process of Silver Nanospheres During Shape Transformation to Nanoprisms
TEM images of the silver nanoparticles prepared with different amounts of NaBH4: a 800 μL, b 900 μL, c 1,000 μL, d 1,100 μL, e 1,200 μL, and f 1,300 μL. Scale bar is 100 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: TEM images of the silver nanoparticles prepared with different amounts of NaBH4: a 800 μL, b 900 μL, c 1,000 μL, d 1,100 μL, e 1,200 μL, and f 1,300 μL. Scale bar is 100 nm.
Mentions: Figure 1 shows the transmission electron microscope (TEM) images of silver nanoparticles prepared with different amounts of NaBH4 while all other parameters remained constant. An extremely high mole ratio was used in this experiment, where the ratio of NaBH4 to AgNO3 is above 8. According to Yang et al. [22], the precursor (Ag ions) can be totally consumed when the mole ratio is 5. These prepared spherical Ag nanoparticles would transform into triangular nanoplates within 24 h at room temperature in dark. As the reaction proceeds, the intensity of the peak at 410 nm decreased and a new peak at a longer wavelength appeared in the NIR–Vis–UV absorption spectrum. This red-shift in energy implies the formation of a triangular structure, which is ascribed to the in-plane dipole resonance mode of silver nanoprisms as shown in Figure 2a. The bisector length of the silver nanoprism prepared with 1,100 μL NaBH4 was found to be 113.4 ± 40.8 nm, as shown in Figure 2b for the histogram. In this study, the out-of plane dipole mode did not show in the spectra, suggesting an inhomogeneous mixture of anisotropic nanoparticles in the solution. From TEM images and absorption spectrum, three features were observed. First, as the concentration of reducing agent (NaBH4) increases, the ratio of nanoprism/nanosphere would also increase. Secondly, the size of the formed silver nanoprisms is almost identical regardless the concentration of NaBH4. Finally, the out-off plane quadruple resonance mode is fixed at the same wavelength (331 nm), which implies that the concentration of the reducing agent would not affect the thickness of nanoprism. Consequently, the extreme excess amount of reducing agent is the crucial factor for the transformation.

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.