Temperature-dependent interfacial behaviour of Au@SiO2 core shell nanoparticles on Si3N4 support film
Graphical abstract
Introduction
The stability of low dimensional noble metal nanostructures is of utmost importance for their various intended applications [1]. Desirable properties of these nanostructures will show their optimum efficiency only when their structural, chemical and thermodynamic stabilities are ensured [2,3]. Several techniques are being followed to prepare such metal nanoparticles including physical routes like Physical Vapour Deposition and sputtering techniques, and a variety of solution-based chemical techniques. The latter methods are most sought after for their cost-effective and mass production traits. Many chemical routes like microemulsion and template-based techniques use organic surfactants to maintain the individuality of nanoparticles and avoid agglomeration [[4], [5], [6]]. Some of the chemical methods have inherent properties of chemical reagents used, which help in sustaining the overall stability of the nanostructures. But at higher temperatures, most of these isolating traits cease to work. This might act as a hindrance to many of their intended applications. It is also well documented that thermodynamic properties of pure noble metal nanostructures drastically change as compared to their bulk counterparts. Melting point reduces considerably below certain size limits [[7], [8], [9]]. Especially in the microelectronic industry, it is very important to ensure the thermal stability of metallic contacts and circuitry at higher temperature regimes where the organic surfactants/supports wear off [3,10,11].
SiO2, Si3N4, Al2O3 are some of the support materials that are being used in the microelectronics industry [[12], [13], [14], [15]]. Metallic nanoparticles covered with any of these materials have been known to alter the thermodynamic, optical, and electrical properties of the system [[16], [17], [18]]. But individual metal nanoparticle species are very selective when it comes to pairing with available shell/support materials [[19], [20], [21], [22], [23]]. Each metallic species reacts with such dielectric materials to varying degrees at various temperatures, despite their high melting points. For example, Au@SiO2 core-shell nanoparticle is a very promising combination which finds potential application in various fields like dye-sensitized solar cells, sensors, as a catalyst and also in the medical field [[24], [25], [26], [27]]. Gold tends to diffuse through SiO2 and interact with any available unbound silicon to form a variety of gold silicides [[28], [29], [30], [31], [32], [33]]. This happens at much lower temperatures called eutectic temperatures, compared to their melting points [[31], [32], [33]]. Similar interactions also lead to thermal decomposition of support materials.
Alternatively, proper knowledge of these interactions could also be used for constructive purposes. Gold particles deposited on substrates such as Si3N4, and SiO2 have been shown to form nanopores across the substrate material at high temperatures [34]. So, it is very important to study such systems and combinations in every possible detail as a matter of both fundamental and technological interests.
In this work, we have fabricated a multi-component system consisting of gold, SiO2, and Si3N4. Au@SiO2 core-shell particles have been prepared using a solvothermal method [35,36] and dispersed on a commercially available 50 nm Si3N4 TEM grid. Effect of ex-situ annealing on the shape, size, elemental composition, and distribution has been analysed using Scanning Transmission Electron Microscopy - High Angle Annular Dark Field (STEM-HAADF) coupled with Energy Dispersive X-ray Spectroscopy (EDX) elemental mapping, High-Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction analysis (SAED).
All the chemicals used for the synthesis are of analytical grade. Throughout the experiment ultrapure water of type 1 obtained from Milli-Q has been used. Chloroauric Acid (HAuCl4), Cetrimonium Bromide (CTAB), 2-methylamino ethanol, and Tetraethoxysilane (TEOS) were procured from Sigma-Aldrich. Gold-SiO2 core-shell nanoparticles were synthesized using a modified method of facile heating of an alcoholic-aqueous solution using a reflux condenser [35,36].
Section snippets
Synthesis of Au@SiO2 nanoparticles
For a typical synthesis of Au@SiO2 core-shell nanoparticles, 38 ml of water and 6 ml of ethanol were taken in a 100 ml round bottom flask. Then, 70 mg of CTAB, 2 ml of 0.133 M 2-methylamino ethanol, 2 ml of 8.14 mM chloroauric acid solution, and 0.1 ml of TEOS was added and mixed thoroughly in an ultrasonicator for about 5 min. The colour of the solution changed from light yellow to wine red. Then the solution was refluxed using an oil bath maintained at 80 °C for 30 min. The mixture was
Results and discussion
Fig. 1 shows the Au@SiO2 core-shell nanoparticles and their crystalline characteristics. Randomly shaped gold nanoparticles are isolated by a network of SiO2 shells as shown in Fig. 1(a). A slightly magnified image shows that the SiO2 shell is highly porous (Fig. 1(b)). These bright field images show central core material to be of high Z and the surrounding shell is of lower atomic number. The presence of gold, silicon, and oxygen is confirmed by EDX analysis as shown in the inset in Fig. 1(a).
Conclusion
Porous Au@SiO2 core-shell nanoparticles were prepared using a reported solvothermal method and were dispersed on a 50 nm Si3N4 TEM grid. The whole system was subjected to high-temperature annealing at 900 °C in an N2 atmosphere. Structural and chemical changes were studied using BFTEM, SAED, STEM-HAADF, and EDX elemental mapping techniques before and after annealing.
- (i)
The average gold nanoparticle size increases considerably from 23 nm to 39 nm.
- (ii)
Individual SiO2 shells isolating gold nanoparticles
Credit author statement
Susheel Kumar Gundanna: Formal analysis, Investigation, Data curation, Writing - original draft. Arijit Mitra: Resources, Methodology. Lakshminarayana K G Bhatta: Conceptualization, Visualization, Writing - review & editing. Umananda M Bhatta: Conceptualization, Methodology, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is funded by UGC -DAE-CSR-KC/CRS/15/IOP/MS/01 and YSS2014/0001555 (SERB -DST) research projects. The authors acknowledge the TEM Facility, funded by a TPF Nano mission, GoI project at Centre for Nano and Soft Matter Sciences, Bengaluru. Authors would also like to acknowledge Prof PV Satyam for providing the TEM facility at IOP, Bhubaneswar.
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