Novel pathway for the sonochemical synthesis of silver nanoparticles with near-spherical shape and high stability in aqueous media

Abstract

The present study shows the development of a novel sonochemical synthesis pathway of sub-15 nm silver nanoparticles (AgNPs) with quasi-spherical shape and high stability in aqueous suspension. Different analytical techniques such as on-line UV-Vis spectroscopy, Atomic Force Microscopy (AFM), and Transmission Electron Microscopy (TEM) were complementarily used to characterize the evolution of the properties of AgNPs synthesized with this new route. Furthermore, different centrifugation conditions were studied to establish a practical, simple and straightforward purification method. Particle size was determined by TEM employing two different deposition methods, showing that purified AgNPs have a size of 8.1 nm ± 2.4 nm with a narrow dispersion of the size distribution (95% coverage interval from 3.4 to 13 nm). Critical information of the shape and crystalline structure of these sub-15 nm AgNPs, provided by shape descriptors (circularity and roundness) using TEM and high resolution (HR)-TEM measurements, confirmed the generation of AgNPs with quasi-spherical shapes with certain twin-fault particles promoted by the high energy of the ultrasonic treatment. Elemental analysis by TEM-EDS confirmed the high purity of the sub-15 nm AgNPs, consisting solely of Ag. At the optical level, these AgNPs showed a bandgap energy of (2.795 ± 0.002) eV. Finally, the evaluation of the effects of ultraviolet radiation (UVC: 254 nm and UVA: 365 nm) and storage temperature on the spectral stability revealed high stability of the optical properties and subsequently dimensional properties of sub-15 nm AgNPs in the short-term (600 min) and long-term (24 weeks).

Figure 1

Monitoring the evolution of the sonochemical synthesis of AgNPs using AFM and TEM. (a) 4 min, (b) 16 min, (c) 92 min, (d) 140 min.

Figure 2

Particle size and particle size distribution of the raw AgNPs (a–c) TEM microscopies taken in different areas of the grid, (d) Particle size and size distribution of the raw AgNPs (overall result).

Figure 3

TEM images from the liquid suspension (supernatant) and pellet (sediment) obtained after purification of the raw AgNPs using different relative centrifugal fields.

Figure 4

Characterization of the particle size, size distribution, and shape descriptors of the purified AgNPs by TEM. (a) AgNPs deposited using PLL, (b) AgNPs deposited using Alcian blue.

Figure 5

HR-TEM micrographs of the sub-15 nm AgNPs obtained by sonochemistry. (a) AgNPs with near-spherical shape, (b,c) twin-fault AgNPs with near-spherical shape, (d) twin-fault AgNPs containing parallel-twin lamellae.

Figure 6

TEM-EDS analyses of the sub-15 nm AgNPs obtained by sonochemistry (a–d) HR-TEM measurements, (e–h) EDS spectra.

Figure 7

Short-term stability of sub-15 nm AgNPs exposed to UVC (254 nm) and UVA (365 nm) radiation. (a) λ_SPR, (b) FWHM, (c) Abs_MAX, (d) AgNP suspensions after UVC and UVA exposition. Black dots (${}^{\circ }$) represent measurements of AgNPs exposed to UVC radiation, blue dots (${}^{\circ }$) represent measurements of AgNPs exposed to UVA radiation, red dots (${}^{\circ }$) represent measurement of the control sample.

Figure 8

Long-term stability of the sub-15 nm AgNPs stored at different temperature conditions (4 °C and 20 °C). The images illustrate the OLS models of each optical property of the sub-15 AgNPs. The model OLS and t-Student test performed to the slope ($\widehat{\beta }$) is present in all the images.