Coalescence of Au Nanoparticles in Silica Gel under Electron Beam Irradiation
Heena Sammi, Manish Mohanta, Bhagwati Sharma, Neha Sardana
Department of Metallurgical and Materials Engineering, Indian Institute of Technology,
Ropar Main Campus, Bara Phool, Rupnagar- 140001, Punjab.
*Corresponding Author E-mail: 2018msz0003@iitrpr.ac.in, manish.ph16239@inst.ac.in, bhagwati.mrc@mnit.ac.in, nsardana@iitrpr.ac.in
ABSTRACT:
The coalescence of Au nanoparticles embedded in the silica gel matrix was observed by E-beam irradiation in a transmission electron microscope. 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 ~13nm Au nanoparticles contacted with each other along with the shrinkage of the silica gel 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. This nanoparticle-gel matrix has the potential for applications in sensing, nonlinear optics, and catalysis. This work enhances the understanding of the role of silica gel and E-beam irradiation in directing the coalescence of nanoparticles.
From the mentioned matrices, mesoporous silica aerogel has attracted much attention due to its distinct properties such as facile synthesis, chemical inertness, controlled porosity, biocompatibility, biodegradability as well as low toxicity7-9.
It is well known that transmission electron microscopy (TEM) is generally used for the morphological determination of the NPs due to its high spatial resolution10,11. With the specific benefits, it also has certain limitations, under E-beam exposure the temperature of the sample increases, which damages the thin layer of ligand, due to that NPs become unstable and, as result, particles begin to get closer to each other12,13. On the other hand, it has also been reported that Au NPs on the surface of supported material merge into each other (coalescence) and convert into different shapes of particles when certain kinds of external force such as heating, mechanical compression, and exposure of E-beam is applied14-16.
With the above considerations in this paper, we report the effect of E-beam irradiation on the morphology of Au NPs embedded in the silica gel matrix and discuss the mechanism of phenomenon. We observed that shrinkage of the silica gel occurred during exposure, which pushes adjacent Au NPs. As discussed earlier, the capped NPs behave like bare NPs after E-beam exposure due to the desorption of the surfactant layer, because of that NPs come in contact and subsequent coalescence begins. Although the coalescence of two Au NPs on the surface of SiOx nanowire induced by an E-beam in a TEM is reported17, but after the incorporation in the gel matrix is not studied. In this paper, we demonstrate a novel approach on coalescence of Au NPs in the silica gel environment after E-beam exposure.
MATERIALS AND METHODS:
Materials:
Tetraethyl orthosilicate (TEOS) was purchased from TCI Chemicals, ethanol, ammonia solution (25%) and ammonium fluoride were purchased from Merck, India. Gold (III) chloride trihydrate was purchased from Sigma-Aldrich and trisodium citrate dihydrate was purchased from Fisher scientific chemicals. Carbon-coated TEM copper grids with 200 mesh size were purchased from Agar scientific. All the chemicals were used without further purification and Milli Q water was used throughout the experiments.
Methods of Synthesis of Au NPs:
Au NPs were synthesized following the well-known Turkevich method18. Briefly, 20mL of 1mM HAuCl4 solution was heated to boiling under constant stirring. To this solution, 2mL 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.
Methods of Synthesis of silica aerogel:
The silica aerogel was prepared using the sol-gel method19. 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 1.85g NH4F, 100ml water and 22.8 ml NH4OH (prepared by five times diluting NH3 (25 %) solution). Sol B was prepared by mixing 5ml of TEOS with 11ml ethanol. Sol C was prepared by adding 7ml of Milli Q water to 11ml of ethanol followed by 0.4mL of Sol A. Finally, the silica gel was prepared by mixing Sol B and Sol C.
Characterization of Au NPs and Silica aerogel
UV-visible studies were performed on Shimadzu UV-2600 spectrophotometer. TEM images were recorded using a JEOL JEM-2100 microscope operating at an accelerating voltage of 200 kV. Samples for TEM were made by breaking a piece of gel with a spatula and then dispersing it in water by sonication for 2 min. After that, the suspension was then deposited on a TEM copper grid by a drop-casting method. TEM measurements were recorded after drying the sample at room temperature.
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.
Table 1: Methods for the preparation of incorporation of Au NPs within the silica gel
For samples 1, 2 and 5 purple solution was observed immediately as shown in optical images, which gradually changed into a rigid transparent purple gel after 5 minutes, along with that small amount of colloidal solution was separated from the gel at the bottom of the cuvette. Similarly, uniform purple solution appeared for samples 3, 4 and 6, whereas no separation of colloidal solution was observed here. It is widely reported that the colour of the Au NPs depends on the shape, size, and interparticle spacing of the NPs as well as the surrounding environment. But despite this most of the reported literature discusses that the colour change is caused by an increase in particle size, shape, and aggregation of NPs20. Only very few studies are based on interparticle spacing which is also the reason for colour change21.
Au NPs in solution move randomly due to Brownian motion, whereas in the solid matrix their movement is restricted because of the confined structure of the matrix. Due to which the distance between NPs is decreased and particles get closer to each other but not aggregated owing to the surface modification of Au NPs which makes the particle stable22. So, in these cases, the reduction in the interparticle spacing causes the colour change.
Similar is the reason for the colour transformation from wine red to purple in our case. Due to the confined geometry of silica gel, the NPs came close to each other but are not aggregated. Because negatively charged -COO- groups of the citrate molecules are present on the Au NPs surface, which causes the particles to become electrostatically stabilized. As a result, change in the colour of the sample is observed.
To investigate the effect of the silica gel environment on the morphology of Au NPs, two samples sample number 1 and sample number 6 were selected. Because samples 1, 2 and 5 were providing a comparable appearance, and similarly samples 3, 4 and 6 were also showing a similar type of pretension. Therefore, we selected one sample from each group for further studies.
RESULT:
Transmission Electron Microscopy:
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±0.8nm (figure 1a). UV-visible studies carried out on the synthesized Au NPs showed their characteristic localized surface plasmon absorption band at 523nm (inset in figure 1a), 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 1b).
Figure 1: (a) TEM images of colloidal Au NPs (Inset: UV-vis spectrum of colloidal Au NPs showing LSPR band at 523 nm) (b) HRTEM image of Au NPs showing fine lattice spacing.
DISCUSSION:
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.
Figure 2: Shows a TEM image of Au NPs incorporated in silica gel. The inset shows a higher magnification image of dumbbell-shaped particles.
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.
Figure 3. (a) TEM image of sample 2 (Au NPs were added before the formation of gel). The inset shows a higher magnification image. (c) Higher magnification image of another area.
CONCLUSION:
In this paper, the E-beam-induced coalescence of Au NPs embedded in silica matrix was studied. It was examined that the distance between NPs was reduced due to the confined geometry of the silica gel. TEM results revealed that adjacent NPs came in contact with each other due to the shrinkage of silica gel and the desorption of the citrate layer during exposure.
The outcome of the order of mixing to check the effect on Au NPs morphology was also examined. Similar results were observed i.e., particles that were in close proximity approached each other and resulted into different shapes of particles such as dumbbells and chains of particles. These results provide new insights into the E-beam induced coalescence of citrate capped Au NPs upon incorporation into silica gel.
CONFLICT OF INTEREST:
The authors have no conflicts of interest regarding this investigation.
ACKNOWLEDGMENTS:
The authors are grateful to the DST- science and engineering research board (SERB) project funding no. ECR/2016/000150. H. Sammi would like to thank Indian Institute of Technology Ropar for providing PhD fellowship. M.K Mohanta acknowledges IISER Mohali and CSIR, New Delhi for Senior Research Fellowship (File No 09/1129(0007)/2017-EMR-I.
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Received on 23.05.2022 Modified on 08.09.2022
Accepted on 14.11.2022 ©AJRC All right reserved
Asian J. Research Chem. 2023; 16(2):118-122.
DOI: 10.52711/0974-4150.2023.00019