Introduction
With the rapid development of nanotechnology, the emergence of hollow nano-materials has impelled the application of nanomaterials into a different fields.1-5 Among them, HSNs have been widely applied in diverse engineering and technological domain including optical, catalysis and biomedical materials.6-8 As for the optical materials, HSNs as the anti-reflection agent or coating can effectively reduce the refractive index of materials and enhance their visible transmittance, which have a significant application prospect in the fields of optical transmission, optical storage, and photoelectric display.9-11
The main preparation methods of HSNs are templating strategies, which are further divided into three categories as follow: (ⅰ) self-template, (ⅱ) soft template, and (ⅲ) hard template.12-14 Among them, the third method is the most well-defined and has been widely applied in preparation of HSNs.12 Jia et al.15 synthesized HSNs by using poly (acrylic acid) as template, ethyl orthosilicate as precursor respectively. As antireflective layer, the average transmittance reached 99% in the wavelength from 380 to1600 nm. Zhang et al.8 also prepared poly (acrylic acid)-silica core-shell nanospheres and then obtained HSNs by removing core. The particle size of the product is less than 30 nm with obvious hollow structure, which is suitable for anti-reflection coating on solar glass of photovoltaic modules. However, the as-prepared HSNs are usually used as the thin-film coating to enhance the visible transmittance of materials. Incorporation of HSNs into optical polymer matrices to fabricate optical nanocomposites is rarely reported because it is difficult to disperse them well in matrices with the aggregate size less than 40 nm. According to the Rayleigh scattering principle, particles or particles aggregations with the size over 40 nm in optical materials can cause the light scattering.16, 17 HSNs can theoretically decrease the refractive index of transparent polymer to reduce the surface reflection, whereas they can also increase the light scattering inside the polymer caused by their larger grain size or agglomerations, resulting in that the haze is increased obviously and the total light transmittance may be decreased. So, the synthesis of ultrafine and well-dispersed HSNs is particularly important for the gradient index optical materials.
In this letter, we have firstly synthesized sub-20 nm HSNs by hard template method in water-in-oil system. The prepared HSNs are monodispersed in water and some organic solvents by changing the modifier assortment. The uniformity and output of HSNs can be improved by high-gravity technology in a rotating-packed-bed (RPB). The refractive index of the HSNs is 1.342, which can be well-dispersed in optical polymer matrix to reduce the reflection and therefore boost the transparency remarkably, meanwhile the haze is barely increased.