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\begin{document}

\title{Synthesis of Monodispersed Hollow Silica Nanospheres with Ultrafine
Grain Size for Optical Polymer}



\author[1]{Xiaofeng Pei}%
\author[1]{Junjie Wang}%
\author[1]{Liangliang Zhang}%
\author[1]{Chao Yang}%
\author[1]{Changwei Cui}%
\author[1]{Xiaofei Zeng}%
\author[1]{Jianfeng Chen}%
\affil[1]{Beijing University of Chemical Technology}%


\vspace{-1em}



  
  \date{May 23, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Hollow silica nanospheres (HSNs) have been widely used as antireflection
coatings. However, it is difficult to incorporate them into optical
polymer matrices because of their larger size and agglomerations
problem, making the haze and transparency worse. Herein, the sub-20 nm
HSNs were firstly synthesized by a reverse microemulsion method and the
scale-up preparation was successfully achieved in a rotating-packed-bed
reactor. The prepared nanocomposites possess super-high transparency and
almost unchanged haze.%
\end{abstract}%



\sloppy


\textbf{Synthesis of Monodispersed Hollow Silica Nanospheres with}

\textbf{Ultrafine Grain Size}

\textbf{for Optical Polymer}

Xiao-Feng Pei\textsuperscript{a,b}, Jun-Jie Wang\textsuperscript{a,b},
Liang-Liang Zhang\textsuperscript{b}*, Chao Yang\textsuperscript{a,b},
Chang-Wei Cui\textsuperscript{b}, Xiao-Fei Zeng\textsuperscript{a,b,}*,
Jian-Feng Chen\textsuperscript{a,b}

a. State Key Laboratory of Organic-Inorganic Composites,

b. Research Center of the Ministry of Education for High Gravity
Engineering and Technology,

Beijing University of Chemical Technology, Beijing 100029, PR China

* Corresponding author:

Xiao-Fei Zeng, Tel: +86-10-64447274; Fax: +86-10-64423474; E-mail:

\emph{zengxf@mail.buct.edu.cn}

Liang-Liang Zhang, Tel: +86-10-64447274; Fax: +86-10-64423474; E-mail:
2016500003@buct.edu.cn

\textbf{Significance}

Hollow silica nanospheres (HSNs) have been widely used as antireflection
coatings. However, it is difficult to incorporate them into optical
polymer matrices because of their larger size and agglomerations
problem, making the haze and transparency worse. Herein, the sub-20 nm
HSNs were firstly synthesized by a reverse microemulsion method and the
scale-up preparation was successfully achieved in a rotating-packed-bed
reactor. The prepared nanocomposites possess super-high transparency and
almost unchanged haze.

Keywords: HSNs, reverse microemulsion, rotating-packed-bed, refractive
index, anti-reflection function

\textbf{Introduction}

With the rapid development of nanotechnology, the emergence of hollow
nano-materials has impelled the application of nanomaterials into a
different fields.\textsuperscript{1-5} Among them, HSNs have been widely
applied in diverse engineering and technological domain including
optical, catalysis and biomedical materials.\textsuperscript{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.\textsuperscript{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.\textsuperscript{12-14} Among
them, the third method is the most well-defined and has been widely
applied in preparation of HSNs.\textsuperscript{12} Jia et
al.\textsuperscript{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.\textsuperscript{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.\textsuperscript{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.

\textbf{Experiment Section}

In this work, the detailed information regarding all of the reagents,
the preparation process for ZnO and HSNs and characterization methods
can be found in the Supporting Information.

\textbf{Results and Discussion}

As a template, the prepared ZnO nanoparticles have a spheroid~morphology
(Fig. 1a) with a narrow size distribution, and the average diameter of
nanoparticles is about 9 nm (Fig. 1b), which can be monodispersed in
water to form nanodispersion with high transparency (lower left of Fig.
S1, Supporting Information). Clear and continuous lattice fringes of ZnO
nanoparticles with 0.28 nm lattice spacing in the high resolution
transmission electron microscopy image (the lower right inset in Fig.
1a) correspond to the (100) crystal plane of ZnO, which indicates the
ZnO nanocrystals are well crystallized.\textsuperscript{18}X-ray
diffraction (XRD) patterns of ZnO nanoparticles (Fig. 1c) present all
the peaks of unmodified and modified samples expectedly corresponding to
hexagonal wurtzite crystal form (JCPDS No.361451).\textsuperscript{18,
19} All peaks of the modified sample are slightly broadened and weakened
than unmodified one, because organic modifier molecules coated on the
nanoparticles surface weaken the detection of ZnO crystal
components.\textsuperscript{18} The FTIR spectrum of ZnO nanoparticles
(Fig. 1d) shows that KH550 has been successfully grafted onto the
surface of nanoparticles because its characteristic absorption peaks
appear in the curve of modified ZnO such as -CH- (~2947
cm\textsuperscript{-1}), -CH\textsubscript{2} (~1220
cm\textsuperscript{-1}) and -NH\textsubscript{2} (~1587
cm\textsuperscript{-1}).\textsuperscript{20} Without doubt,
KH550-modified ZnO nanoparticles can be well dispersed in the aqueous
phase, as shown in Fig. 1a. In addition, the absorption peaks at ~3428
cm\textsuperscript{-1} corresponds to the characteristic peaks of -OH
stretching vibration. Thermo- gravimetric analysis (TGA) results
furtherly confirm that ZnO nanoparticles were wrapped by the KH550 and
the loading is 18.6\% (Fig. S1, Supporting Information). It is noted
that surface amination treatment makes ZnO nanoparticles positively
charged up to 34.2 mv, which facilitates coating process of
SiO\textsubscript{2} with negative charge.\textsuperscript{21}

HSNs were prepared by the reverse microemulsion method. As shown in Fig.
2a, the microemulsion is composed of water as the disperse phase and
cyclohexane as the continuous oil phase, Igepal CO-630 as the emulsifier
contributing to the formation of a stable water-in-oil emulsions. ZnO
nanoparticles are monodispersed in water nuclear and methyl
orthosilicate (TMOS) precursor is dissolved in cyclohexane. Moreover,
NH\textsubscript{3}\selectlanguage{ngerman}·H\textsubscript{2}O is selected as hydrolysis
catalyst, and meanwhile as the etchant for template agent. TMOS in oil
phase hydrolyzes at the oil-water interface under the catalytic effect
of NH\textsubscript{3}·H\textsubscript{2}O in water and the hydrolysis
products enter into water nuclear for heterogeneous nucleation on the
surface of ZnO nanoparticles.\textsuperscript{22, 23} The amount of
water in microemulsion is small, resulting in the achievement of the
uniform coating of SiO\textsubscript{2} on ZnO by controlling the
hydrolysis reaction rate of TMOS. At the same time, ZnO nanoparticles
are corroded slowly and gradually by
NH\textsubscript{3}·H\textsubscript{2}O in water. After 2 h reaction,
modifier such as polyethylene pyrro-\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Fig. 1. (a) TEM image and HRTEM image of ZnO nanoparticles (The upper
right corner lies the local enlarged view). (b) Particle size
distribution of ZnO nanoparticles. (c) FTIR spectra of ZnO
nanoparticles. (d) XRD patterns of ZnO nanoparticles.

lidone (PVP) dissolved in water is dropwise added and coated on the
surface of SiO\textsubscript{2} by in-situ modification method. After
forming a firm SiO\textsubscript{2} coating, the ZnO template disappears
and a hollow structure was formed. And thus the modified HSNs are
finally obtained. The whole preparation process is performed in RPB,
which can produce high gravity environment (tens to hundreds of g) by
centrifugal force. The process in the RPB reactor is thus significantly
intensified, helpful to providing a more even emulsion system and
reaction surrounding, which is suitable for the preparation of uniform
nanoparticles at a large-scale.\textsuperscript{24, 25}

The effects of different preparation conditions on the morphology and
dispersibility of HSNs were investigated and the results show that molar
ratio of NH\textsubscript{3}\selectlanguage{ngerman}·H\textsubscript{2}O to ZnO is the most
critical factor to form an obvious hollow structure in the preparation
process. TEM images of SiO\textsubscript{2} nanoparticles prepared in
RPB at different molar ratio of NH\textsubscript{3}·H\textsubscript{2}O
to ZnO (Fig. 2b-e) have presented that SiO\textsubscript{2}nanoparticles
undergo a transition from a solid structure to an inconspicuous hollow
structure, then to an obvious hollow structure and final to a core-shell
structure with the value decreasing from 7.7 to 3.7. More
NH\textsubscript{3}·H\textsubscript{2}O will corrode ZnO rapidly and
SiO\textsubscript{2} nucleates independently. Less
NH\textsubscript{3}·H\textsubscript{2}O cannot corrode ZnO to form
ZnO@SiO\textsubscript{2} nanoparticles with a core-shell structure.
Under the optimum ratio of 6.1, the deposition rate matches the
corrosion rate of template, and the product can be prepared with high
uniformity and well-controlled morphology. In addition, other influence
factors such as reaction temperature (Fig. S2, Supporting Information),
reaction time (Fig. S3, Supporting Information) and types of modifiers
also have important effects on the morphology and dispersion state of
HSNs. The results show that 20°C of reaction temperature and 3 h of
reaction time are necessary to attain ideal products. With the
modification of different kinds of modifiers (PVP,
3-methacryloxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane),
HSNs can be stably monodispersed in corresponding solvents (water,
methyl isobutyl ketone and MeOH) to form the transparent dispersions.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Fig. 2. (a) Reaction mechanism diagram for TMOS in reverse
microemulsion. (b-e) TEM of SiO\textsubscript{2} nanoparticles prepared
in RPB at different molar ratio of NH3\selectlanguage{ngerman}·H\textsubscript{2}O to ZnO. (b)
7.7, (c) 6.9, (d) 6.1, (e) 3.7. (f) TEM of
SiO\textsubscript{2}nanoparticles prepared in STR. (g-h) Histogram of
size distribution for SiO\textsubscript{2} nanoparticles prepared in RPB
and STR. (g) Particle size distribution. (h) Size distribution of inner
cavity.

Processing capacity of RPB reactor for synthesis of such HSNs mentioned
above is 1.5 g per batch. The same scale of preparation in STR were
studied for comparison. The products are solid and hollow
SiO\textsubscript{2} nanoparticles existing simultaneously (Fig. 2f)
under the optimum operation conditions in STR, and further prolongation
of reaction time in STR would not improve their hollow structure. The
particle size distribution of SiO\textsubscript{2}nanoparticles and
their cavity size synthesized in RPB or STR have been analyzed (Fig. 2g
and 2h). The average particle size of SiO\textsubscript{2} nanoparticles
is only 19.8 nm with a narrow distribution from 10 to 30 nm for RPB,
compared with average particle size of 34.0 nm with a broad distribution
from 10 to 50 nm for STR. Furthermore, the average diameter of inner
cavity is about 8 nm for RPB, while it is far less than 8 nm for STR due
to more than half of nanoparticles without the hollow structure. More
importantly, the reaction time in RPB is 3 h as half as that in STR,
resulting from the formation of uniform water-in-oil emulsions with
smaller size of water phase and faster mass transfer at oil-water
interface in RPB.\textsuperscript{25} Moreover, the yields (The mass
percentage of the well-dispersed nanoparticles in the total output) of
HSNs in RPB and STR with the morphology as shown in Fig. 2d and 2f have
a huge difference. The yield in RPB is beyond 70\%, which is twice as
much as that in STR. All in all, the high gravity technology provides
the possibility for the large scale-up preparation of ultrafine HSNs,
which is important for the industrial application of HSNs as
antireflection functional additive in optical polymer matrices.

From the FT-IR spectra of HSNs (Fig. 3a), it can be seen that unmodified
HSNs has no C-H (\selectlanguage{english}~2969cm\textsuperscript{-1}), C=O
(\selectlanguage{english}~1644cm\textsuperscript{-1}) and C-N (\selectlanguage{english}~1398cm\textsuperscript{-1})
peaks (characteristic absorption peak for PVP), which appear in the
modified ones, indicating PVP are successfully wrapped on HSNs. However,
there is no new characteristic absorption peak, illustrating only
existence of physical interaction between PVP and HSNs in modified HSNs.
Moreover, there are a large number of --OH on the surface of HSNs,
resulting in the appearance of -OH stretching vibration peaks at \selectlanguage{english}~3384
cm\selectlanguage{english}\textsuperscript{-1}. XRD patterns of HSNs (Fig. 3b) exhibits only one
wide diffraction peak at about 25deg, indicating that the HSNs present an
amorphous state. In the TGA curves of HSNs (Fig. 3c), the weight loss of
unmodified and modified HSNs are 5.9 wt\% and 21.8 wt \% at 800degC
respectively. Combined with the results of FT-IR, they are mainly due to
elimination of -OH bonds and decomposition of PVP on the surface of
nanoparticles respectively, and the coating amount of PVP is about 15.9
wt\%.

The refractive index (n) of HSNs aqueous dispersions has a linear
relationship with the HSNs volume fraction (V\%) in water (Fig. 3d),
which follows equation (1).\textsuperscript{16, 26}

\(n_{t}=n_{1}V\%+n_{2}\left(1-V\%\right)=\left(n_{1}-n_{2}\right)V\%+n_{2}\ \)(1)

Where \(n_{t},n_{1}\text{\ and\ }n_{2}\) are refractive index of HSNs aqueous
dispersion, HSNs and H\textsubscript{2}O. V\% is volume fraction of
HSNs. As described in Fig. 3d, \(n_{2}\) is about 1.333 and the
slope of the line is 0.009. The function of \(n_{t}\) and V\%
is fitted linearly (R\textsuperscript{2}=0.999), namely
\(n_{t}\)=0.009V\%+1.333. So the refractive index of HSNs
(\(n_{1}\)) is the sum of 1.333 and 0.009, namely 1.342.
Moreover, the refractive index of HSNs also can be calculated from their
size, following the equation (1) too, but\(n_{t},n_{1}\text{\ and\ }n_{2}\) are
refractive index of HSNs, solid SiO\textsubscript{2} and air. As we know
n\textsubscript{1} and n\textsubscript{2} are about 1.46 and 1
respectively. The shell of HSNs has high porosity. According to the
literatures, \textsuperscript{27, 28} pore volume of the amorphous
SiO\textsubscript{2} is generally about 0.12 cm\textsuperscript{3}/g.
And the ratio of pore volume of the HSNs shell to the whole volume of
HSNs can be calculated at 17\%, based on the specific volume of
0.73cm\textsuperscript{3}/g (density of 1.37 g/cm\textsuperscript{3})
for HSNs. Considering inner cavity volume fraction of 7 \%, the total
porosity of HSNs is about 24\% and theoretical refractive index is
1.350, which shows the theoretical calculation result is in good
agreement with experimental one. This also indicates that cavity of HSNs
is fully filled with air instead of water. Otherwise, the theoretical
and measured value of refractive index for HSNs should be 1.430. In
addition, it is satisfactory that the modified HSNs in water are endowed
with excellent dispersion stability, still retaining transparent without
settlement after placed at half month, due to strong charge repulsion
effect among HSNs (-19.2 mv).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Fig. 3. (a) FT-IR spectra of HSNs. (b) XRD patterns of HSNs. (c) TGA
curves of HSNs. (d) The refractive index of aqueous HSNs dispersion.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

The pure PVA and HSNs/PVA nanocomposites with 10 wt\% HSNs have been
successfully prepared by the solution blending method and they have the
similar transparency in the visible region (Fig. S4, Supporting
Information). UV-vis transmittance spectra (Fig. 4a) present optic

Fig. 4 (a) UV-visible transmittance spectra with and without HSNs (b)
Transmittance and haze of HSNs/PVA nanocomposites at 580 nm. (c) TEM
image of HSNs/PVA nanocomposites with 22 wt\% HSNs. (d) Refractive index
of HSNs/PVA nanocomposites at 580 nm.

performance of neat PVA and HSNs/PVA nanocomposites. The transmittance
of visible light is increased remarkably with the increase in HSNs
content, which illustrates that HSNs indeed have an excellent
antireflection effect on the nanocomposite films. The visible light
transmittance and haze of them at 580 nm are clearly presented in Fig.
4b. When HSNs content increases from 0 to 33 wt\%, the visible light
transmittance increases from 91.5\% to 97.9\%, because that the
well-dispersed HSNs can decrease surface reflectivity and boost total
luminous flux of nanocomposites due to their low refractive index.
However, as we know, the existence of nanoparticles in polymer matrix
can change the optical path more or less and cause the light diffuse
scattering, leading to the increase in reflectivity on the contrary.
Although the total reflectivity of nanocomposites may be declined and
anti-reflection effect may be realized, the haze of nanocomposite must
be increased remarkably, which is a huge obstacle for the application of
antireflection transparent composite films.\textsuperscript{9, 16} When
the particles size is bigger and particles aggregation is more, the haze
of composites is raised more obviously. To our surprise, the haze of
prepared HSNs/PVA nanocomposite is increased slightly with the addition
of HSNs, indicating the excellent dispersion state of HSNs in PVA
matrix. As shown in Fig. 4c, HSNs with 22 wt\% content have a
homogeneous dispersion in PVA matrix without any aggregation. Light
diffuse scattering (Rayleigh scattering) does not happen because the
particles have the size of much less than 40 nm and are monodispersed in
optical polymer matrix basically. \textsuperscript{16} In this way, it
gives full play to the function of anti-reflection and remarkably
improves optical performance of nanocomposites without diffuse
reflection. Moreover, HSNs can reduces refractive index of organic
matrix obviously (Fig. 4d). It is decreased nonlinearly from 1.535 to
1.458 with increased HSNs volume content from 0 to 21\%, which is not
consistent with the theoretical calculation
results,\textsuperscript{26}because the addition of inorganic
nanoparticles would reduce the crystallinity of the organic
substrate.\textsuperscript{29} All these indicate that the HSNs/PVA
composite materials have outstanding optical performance with
considerable application prospect. Of course, by changing the types of
modifiers, the HSNs can be incorporated into the different organic
substrates to form nanocomposites with excellent optical properties,
which has the widespread application prospect in many different domains.

\textbf{Conclusion}

In this experiment, the sub-20 nm HSNs with the inner cavity diameter of
8 nm can be scaled up in RPB, and they show uniform particles size and
excellent monodispersity. As the anti-reflection additive agent, they
can be incorporated into optical polymer matrices to reduce the
refractive index and improved transparency of polymer effectively. Most
importantly, the haze of polymer is unchanged basically by adding HSNs,
indicating that HSNs with ultrafine grain size are well-dispersed in
polymer matrix. All in all, this work provides a simple and effective
method to stably synthesize the ultrafine HSNs, which can promote the
development of the technology for the large-scale preparation and
industrial application of anti-reflection materials.

\textbf{Acknowledgement}

This work was financially supported by National Natural Science
Foundation of China 21776016.

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