Coalescence of Au Nanoparticles in Silica Aerogel under Electron Beam Irradiation

Abstract

Background: The coalescence of Au nanoparticles embedded in the silica gel matrix was observed by E-beam irradiation in a transmission electron microscope. Background: The coalescence of Au nanoparticles embedded in the silica gel matrix was observed by E-beam irradiation in a transmission electron microscope. Method: It was examined that interparticle spacing between nanoparticles was reduced after incorporation into the matrix and particles came close to each other. TEM studies have shown that during E-beam irradiation ~13 nm Au nanoparticles contacted with each other along with the shrinkage of the silica aerogel or as well as the removal of surfactant layer, and transformed into different shapes of particles such as dumbbell and chain-like particles as per the interparticle gap. Results: This nanoparticle-aerogel matrix has the potential for applications in sensing, nonlinear optics, and catalysis. method: Synthesis of Au NPs Au NPs were synthesized following the well-known Turkevich method [18]. Briefly, 20 mL of 1 mM HAuCl4 solution was heated to boiling under constant stirring. To this solution, 2 mL of 1% trisodium citrate dihydrate was rapidly added under stirring, which resulted in a colour change from light yellow to colourless, followed by dark black to wine red. The still stirring solution was then cooled to room temperature. Synthesis of silica aerogel The silica aerogel was prepared using the sol-gel method [19]. For the preparation of silica gel, a stock solution (Sol A), an alkoxide solution (Sol B) and a catalyst solution (Sol C) were first prepared. Sol A was prepared by adding 1.85 g NH4F, 100 ml water and 22.8 ml NH4OH (prepared by five times diluting NH3 (25 %) solution). Sol B was prepared by mixing 5 ml of TEOS with 11 ml ethanol. Sol C was prepared by adding 7 ml of Milli Q water to 11 ml of ethanol followed by 0.4 mL of Sol A. Finally, the silica gel was prepared by mixing Sol B and Sol C. Incorporation of Au NPs within the silica gel matrix The silica gel was prepared by the method discussed earlier. The ratios of stock solution were fixed so that complete gelation of the whole volume completes in just a couple of minutes. The order of mixing of Au NPs w.r.t. Sol C and Sol B was changed to examine the effect of silica gel on the morphology of Au NPs. The rate of mixing was very fast, the whole process was completed in 2 minutes. In this work, six different samples were analysed, which were prepared by varying the order of addition of Sol B, Sol C and Au NPs in the ratio of 2: 2: 1 as shown in table 1. In the first two samples, Au NPs were added at last, by then the gelation process has started. Similarly, in the next two samples, Au NPs were added in the middle, and in the last two samples, Au NPs were added before the gelation process began. Conclusion: This work enhances the understanding of the role of silica aerogel and E-beam irradiation in directing the coalescence of nanoparticles. result: The Au NPs were synthesized using the Turkevich method, as mentioned in the experimental section. The average size and morphology of the NPs were studied using TEM. The TEM images showed the formation of monodispersed spherical Au NPs with an average diameter of 13.7 ± 0.8 nm (figure 1(a)). UV-visible studies carried out on the synthesized Au NPs showed their characteristic localized surface plasmon absorption band at 523 nm (inset in figure 1(a)), which corresponds to the visible complementary dark-red colour of the colloidal Au NPs. The high-resolution TEM image of the Au NPs showed clear lattice spacing of approximately 0.24 nm, corresponding to the (111) plane of Au (figure 1(b)). In order to examine the effect of E-beam on the morphology of citrate capped Au NPs after incorporation into silica gel, TEM was performed. Figure 2 shows a TEM image of sample 1. Silica gel is highlighted by red arrows demonstrating NPs are entirely surrounded by silica gel. The higher magnification image in the inset clearly indicates the coalescence of Au NPs occurred due to the shrinkage of the silica gel or the removal of surfactant layer against the high-energy electron beam. Due to the stripping of the citrate layer from the surface, Au NPs become unstable and at the same time shrinkage of silica gel induces a driving force for coalescence of the particles. The NPs that are in close proximity, start to come in contact with each other and their centre-to-centre distance begins to decrease. During this step of contact, particles of different shapes are formed depending upon the interparticle distance. It is observed that in some places, only two NPs are connected to form dumbbell-shaped particles (upper left side) it is because NPs preferred to connect with the nearest particles leading to the coalescence of only two NPs. Whereas in another area, coalescence of various NPs is observed on the lower right side, it is seen as a chain of particles because multiple NPs are in close proximity. These dumbbell-shaped particles have an average size of 38.4 nm and chains have an average size of 112.8 nm. In order to investigate the effect of the mixing order on the morphology of Au NPs, TEM of the second sample was also recorded. This sample also follows a similar trend as for sample 1, as shown in figure 3. The Au NPs preferred to attach with the nearest particles and coalescence of multiple NPs occurred due to the shrinkage of silica gel or as well as due to the removal of surfactant layer during the E-beam irradiation. other: Only this much information I have.