A Combinatorial Study Investigating the Growth of Ultrasmall Embedded Silver Nanoparticles upon Thermal Annealing

Abstract

Ultrasmall nanoparticles (NPs) with a high active surface area are essential for optoelectronic and photovoltaic applications. However, the structural stability and sustainability of these ultrasmall NPs at higher temperatures remain a critical problem. Here, we have synthesized the nanocomposites (NCs) of Ag NPs inside the silica matrix using the atom beam co-sputtering technique. The post-deposition growth of the embedded Ag NPs is systematically investigated at a wide range of annealing temperatures (ATs). A novel, fast, and effective procedure, correlating the experimental (UV−vis absorption results) and theoretical (quantum mechanical modeling, QMM) results, is used to estimate the size of NPs. The QMM-based simulation, employed for this work, is found to be more accurate in reproducing the absorption spectra over the classical/modified Drude model, which fails to predict the expected shift in the LSPR for ultrasmall NPs. Unlike the classical Drude model, the QMM incorporates the intraband transition of the conduction band electrons to calculate the effective dielectric function of metallic NCs, which is the major contribution of LSPR shifts for ultrasmall NPs. In this framework, a direct comparison is made between experimentally and theoretically observed LSPR peak positions, and it is observed that the size of NPs grows from 3 to 18 nm as AT increases from room temperature to 900 °C. Further, in situ grazing-incidence small-& wide-angle X-ray scattering and transmission electron microscopy measurements are employed to comprehend the growth of Ag NPs and validate the UV + QMM results. We demonstrate that, unlike chemically grown NPs, the embedded Ag NPs ensure greater stability in size and remain in an ultrasmall regime up to 800 °C, and beyond this temperature, the size of NPs increases exponentially due to dominant Ostwald ripening. Finally, a three-stage mechanism is discussed to understand the process of nucleation and growth of the silica-embedded Ag NPs.

Investigating the thermal stability of ultra-small Ag, Au and AuAg alloy nanoparticles embedded in a silica matrix

“…35 Recently, our group highlighted the significance of embedded nanocomposites (NCs) for the structural stability of Ag/SiO 2 . 36 We showed that Ag NPs remain in the ultra-small size region up to 800 °C, beyond which the size of the NPs increases rapidly due to the Ostwald ripening process. Therefore, the NCs of the embedded NPs are found to be best suited to ensure the stability of ultra-small NPs, owing to their higher active surface area as compared to the NPs of size greater than 10 nm, which is required in industry for improved activity and application efficiency.…”

“…44 Similar to AuAg/SiO 2 NCs, Ag/SiO 2 NCs also showed an overall red-shift in the LSPR peak position with an increase in the AT, attributed to an increase in the size of the embedded NPs as previously reported. 36…”

“…A similar behaviour in the temperature-dependent growth mechanism has been demonstrated for Ag/SiO 2 NCs. 36…”

“…A schematic of the setup is shown in ref. 36. To minimize the effect of radiation damage, the sample position was shifted by 2 mm after each measurement.…”

“…44 Similar to AuAg/SiO 2 NCs, Ag/SiO 2 NCs also showed an overall red-shift in the LSPR peak position with an increase in the AT, attributed to an increase in the size of the embedded NPs as previously reported. 36 For the elemental compositions of Au (55%) and Ag (45%), the absorption spectrum of the AuAg/SiO 2 NCs at 900 °C was compared with those of Au/SiO 2 and Ag/SiO 2 NCs as shown in Fig. 1d.…”