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.