HSNPs have a unique balloon-like structure, which is consisted with inner air cavity and outer silica shell. According to their structures, HSNPs show superior properties than dense silica nanoparticles, such as a high surface area, a low thermal conductivity, a low dielectric constant and so on.
In general, the synthetic routes for HSNPs can be categorized into two methods which are template and non-template method. In the non-template method, it can be divided to the electrostatic atomization method and spray pyrolysis method. The advantage of these method is to be able to mass produce HSNPs. However, the obtained HSNPs are polydispersed. The advantage of the template method is that the shape of HSNPs can be easily controlled by the shape of template. In the template method, silica was first coated on the templates to form the silica shells, followed by removing the templates via thermal or chemical treatments. However, the carbon dioxide gas and waste organic solvent are generated as the byproducts.
One of the eco-friendly template method, there is PAA/ NH4OH emulsion template method. It can be easily removed template by water washing after the silica coating. Therefore, this method doesn’t require thermal and chemical treatment, therefore, this method is very eco-friendly method. In this method, PAA was firstly dissolved in the NH4OH, then the mixture was added to ethanol for obtaining emulsion templates. Secondly, the silica precursor was added into reaction solvent, and then silica coating was the Stöber process, which was directly performed in the ethanol dispersion of the PAA/ NH4OH emulsion templates. This method is useful as not only templates for the HSNPs, but also functional particle containers. Xin et al. reported noble-metal nanoparticles encapsulated HSNPs by using the PAA/ NH4OH emulsions template. In the study, the PAA had dual roles of a template for the HSNPs and as a captor of the metal source in the HSNPs. However, despite the significant advantages of this method, this method is required long reaction time (10 h) for the formation of the silica shells, and the obtained HSNPs were polydispersed because that the template has high mobility.
In this work, due to overcome this disadvantage, the MWs of PAA/NH4OH was adjusted, and the effect on morphology of HSNPs was investigated. When the MWs of polymer increased, the template stability might increase due to increasing template viscosity as the Mark-Houwink-Sakurada equation.
A 0.12 g PAA was dissolved in 2.0 mL of a 25 wt.% NH4OH. Thereafter, 2.0 mL of template solution (PAA/NH4OH) was added to 35.0 mL of ethanol, and then the emulsion templates were formed. Following that, 2.0 mL of TEOS was injected (20μL/min) under vigorous magnetic stirring at room temperature. At the target reaction times (0, 2, 4, 6, 8, 10 h) after injection of the TEOS, the products were centrifuged and washed several times with ethanol and distilled water. Finally, the obtained particles were dried at a vacuum oven at 180 degrees for one day.
It is hard to obtain the emulsion for PAA(250,000)/NH4OH, because precipitation rapidly formed after template solution was added to the ethanol due to the extremely high viscosity. When the template of PAA(5,000)/NH4OH was applied, it seems that the time of the silica coating should be set at 4 h or longer for producing robust HSNPs. Otherwise, HSNPs would be obtained with about 5 nm cracked shells. In addition, all HSNPs samples synthesized with the PAA(5,000)/NH4OH template were polydispersed with sizes varying from 20 nm to 150 nm. When a PAA with a higher MW was used, obviously, it was much easier to obtain robust HSNPs within a short silica coating time. Especially, when PAA(50,000)/NH4OH emulsion templates were used, the HSNPs could be obtained even if the silica coating was stopped at 2 h after adding the TEOS. The size of the obtained HSNPs was also significantly affected by the MW PAA. HSNPs obtained using the templates of PAA(5,000)/NH4OH, PAA(25,000)/NH4OH and PAA(50,000)/NH4OH were 100, 60, and 35 nm in average inner size, respectively.
The particle distribution curves were measured by the dynamic light scattering method. The PAA/NH4OH emulsion templates formed in ethanol, but their sizes were different from each other. The average sizes of PAA(5,000)/NH4OH, PAA(25,000)/NH4OH and PAA(50,000)/NH4OH were 210, 85, and 60 nm, respectively. This result seems that the PAA/NH4OH emulsion templates with a high MW PAA were able to more easily suppress coalescence than those with a low viscosity, and thus reach a stable status with a small size. The obtained HSNPs inner size was almost the same as the template size for each MW PAA. It shows that the PAA/NH4OH emulsion templates can be used as a silica coating template in ethanol.
As the result, with increasing PAA MW, the particle size decreased and the TEOS conversion ratio also increased. In particular, the uniformed 20nm HSNPs could be synthesized by using PAA(50,000)/NH4OH emulsion template in ethanol. In this condition, the TEOS conversion ratio increased to 95%, and the reaction time became shorten (2 hours).
Part of this experiment was supported by the Adaptable and Seamless Technology transfer Program (A-STEP) of the Japan Science and Technology Agency (JST).