解説

先進セラミックス研究センター年報（2015）．Vol．4，6-11

The Production of Hollow SiO2 Nanoparticles (HSNPs) with a High Monodispersity
Wanghui Chen, Chika Takai, Hadi Razavi Khosroshahi, Masayoshi Fuji, Takashi Shirai
Advanced Ceramics Research Center, Nagoya Institute of Technology
3-101-1, Honmachi, Tajimi, Gifu 507-0033, JAPANWe discuss the state-of-art synthesis of hollow SiO2 nanoparticles with a high monodispersity (m-HSNPs). The template method
using the Stöber process for m-HSNPs is introduced, and the advantages and mechanism of this method are described. Regarding
the selection of templates, we suggest that the monodispersity of the templates should be faithfully pursued, while both positively
and negatively charged templates can be considered as appropriate candidates. Particularly to negatively charged templates, we
discuss their advantages for the dispersity of HSNPs, attributed to the no charge reverse SiO2 coating. Moreover, the influences of
the concentrations of NH3 and H2O in the Stöber process on the SiO2 coating are discussed.
Keywords: hollow nanoparticles, silica, template method, Stöber process, monodispersity

1.   Introduction

non-monodisperse HSNPs to the best of our knowledge.

The structure of hollow SiO2 nanoparticles (HSNPs) can

Deng et al. [5] demonstrated that m-HSNPs can be used

be described as a void interior space surrounded by a SiO2

to build a 2-D colloidal crystal for optical applications.

layer (i.e., shell), with at least one of the following values

Interestingly, such a colloidal crystal showed significant

in nanoscale: shell thickness, sizes of the interior space andFabry−Pérot fringes in its transmission spectrum, even when

the whole particle. Such a unique nanostructure endows theits thickness reached dozens of micrometers.

HSNPs with some physical properties different from solid

Considering the attractive properties and application

SiO2 particles, such as a low density and high specific surface

potential of the m-HSNPs, previous studies on their synthetic

area (SSA). In addition, the HSNPs shows their outstanding

method should to be summarized. This review deals withproperties in the area of optical [1, 2], loading-releasing

the most mature protocol for the synthesis of m-HSNPs, i.e.,ability [3], heat insulation [4], etc. Therefore, considerable

template method. The general process of the template method

attention has been paid to developing facile and efficientis introduced in the next section, with followed by pointing

synthetic methods of high-quality HSNPs. More specifically,out a myth in the principle of the template selection. The

free-standing SiO 2 nanoparticles, aggregated and broken

influences of other parameters on the template method (i.e.,

HSNPs are not supposed to be contained in the high quality

concentrations of NH3 and H2O in the SiO2 coating process)

HSNPs, as these unfavorable species would degrade theare also discussed in this review.

characteristics expression of the hollow structure, and further
reduce the credibility of performance evaluations.

2. Overview of the template method

Obtaining “high-quality” HSNPs can be equal to pursuing

Figure 1 depicts the general process of the template method,

the high monodispersity of the HSNPs to some extent, thatwhich can be further divided into the SiO 2 coating stepis, HSNPs show not only a high dispersity, but also a high(step I) and template removing step (step II). In this process,uniformity in morphology (e.g., shape, particle size, shell

the template@SiO 2 core–shell particles are intermediates,

thickness). There is no doubt that such a homogeneous

whereas they determined the morphology of finally produced

mor phology would make the analysis in ter ms of the

HSNPs to a significant extent, while the step II can be simply

relationship between the structure and performance easier

seen as templates being replaced by void spaces. Therefore,

and more convincing. Furthermore, a high monodispersity

the core issue of the template method is step I, while thecan endow the HSNPs with new attractive properties.

quality of the SiO2 coating directly affects the characteristics

Retsch et al. [1] found a visible Mie scattering phenomenonof the finally produced HSNPs (e.g., size, dispersibility, shellin observing monodispersed HSNPs (m-HSNPs) underthickness and surface roughness).

illumination, which has never been reported when observing－6－

The Production of Hollow SiO2 Nanoparticles (HSNPs) with a High Monodispersity

with the continuous hydrolysis of the silicate esters until itreached a critical concentration for nucleation (cn). Since thecondensation rate was much higher than that of hydrolysis,
csilanol then stopped increasing and soon decreased to below
Figure 1. Schematic diagram of the template method for the

cn. Correspondingly, nucleus would not generate anymore,

synthesis of HSNPs.

thus the generated nucleus started to synchronously grow(via heterogeneous nucleation). The growth procedure of the

For the SiO 2 coating step, although there are several

SiO2 particles would finally stop when csilanol decreased to thechemical reactions able to produce SiO2 (e. g., Na 2Si 3O7 +concentration of the SiO2 dissolve balance (cs). Rationally, all

H2SO4 → 3 SiO2 + Na2SO4 + H 2O, SiCl4 + 2H 2 + O2 → SiO2

the resulting SiO2 particles had a similar size, thus achieving+ 4HCl), the Stöber process [6] is believed to be the mosta high monodispersity.

promising and controllable one. In the Stöber process, theTo date, the Stöber process has been the most popularhydrolysis and condensation reactions over silicate esters

protocol for the SiO 2 coating using template method(Si(OR) 4) successively occur by the catalysis of base (e.g.,[10-16], thanks to its high controllability and operative

NH 3) in alkoxide solvent (e.g., ethanol) which contains

under moderate conditions. Nevertheless, adopting thea specific amount of H 2 O. This sol-gel process can be

Stöber process is far from sufficient to achieve the highdescribed as follows,

monodispersity of the HSNPs. Some condition parameters
should be more strictly controlled, e.g., the template particles

Hydrolysis

are supposed to be monodispersed, and the homogeneous
nucleation of SiO2 should be suppressed in the SiO2 coating
to avoid producing of aggregated HSNPs and free–standing
SiO2 nanoparticles.
3. Appropriate template materials for m-HSNPs
If there is no special demand for on the monodispersity of
the HSNPs, the selection of the template particles can justbe based on 1) the surface of the material is easy for SiO2
decorating, and 2) this material can be removed without
destroying SiO2 shells. Accordingly, a number of materials
has been used for the synthesis of HSNPs, e.g., CaCO 3

Condensation

nanoparticles [10], polymer beads [11-13], ceramics powders[14], polymer emulsions [15], and micro-bubbles [16].
Nevertheless, there are only a smaller range of materials
available for the synthesis of m-HSNPs, since the templateparticles should be fabricated to monodisperse.

where R represents the carboxylic chain. The Stöber
process was first developed for producing monodispersed
SiO 2 microspheres as the nucleation and growth of SiO 2
microspheres in this process can be easily controlled bytuning the amount of precursor in the reaction system, as wellas the concentration of H 2O and catalysts [7]. The formationof the monodispersed SiO 2 microspheres in the Stöber
process can be explained by the model of LaMer and Dinegar[8, 9]. Figure 2 depicts the change in the silanol concentration(csilanol) in the reaction system as a function of the reaction

Figure 2. csilanol changes as a function of the reaction time intime. At the beginning of reaction, csilanol gradually increased

the Stöber process for the monodispersed SiO2 microspheres.
－7－

Wanghui Chen, Chika Takai, Hadi Razavi Khosroshahi, Masayoshi Fuji, Takashi Shirai

Taking into account the convenience of fabrication,

shell particles using vinyltrimethoxysilane (VTMS) as themonodispersed polymeric beads (MPBs) have become aprecursor of SiO 2 , and further confirmed the high yieldcommonly used template material for the synthesis of theof the SiO 2 coating on the n-MPBs. Although the authors

m-HSNPs, which can be easily obtained by either soap–freededuced that the coating of SiO2 in this case resulted fromemulsion polymerization or dispersion polymerization [17].

the interaction between the vinyl ligands of VTMS and theBesides, it is also easy to varying the size, hydrophilicityhydrophobic surface of the n-MPBs, they also pointed out

and surface charge (signal and density) of MPBs. Anotherthe negative charges carried by n-MPBs would not suppress

highlight of the MPBs is that they can be removed by

the coating of SiO2. Therefore, for addressing whether or not

calcination in air, which is convenient on operation and

the SiO2 coating on the surface of n-MPBs via the typicalwould have little effect on the SiO2 shell structure. However,

Stöber process is feasible, a further analysis is supposed to be

the disadvantage of using MPBs is also significant—CO 2

conducted based on additional experiments, while more facts

emission in the template-removing step, which means that(e.g., surface charge density of n-MPBs, reaction parameters)

the production of m-HSNPs using MPBs templates is notshould be taken into account for the appropriate conclusion.

very “green”. Based on this issue, a great effort has been

Most recently, Chen et al. [12] directly demonstrated thatmade to find alternative materials of MPBs. Fuji et al. [15]the negatively charged particles can also be appropriate forfabricated SiO2-coated polymer emulsions with an improved

the SiO2 coating, thus can be used as the templates for themonodispersity, and further obtained m-HSNPs by washingsynthesis of m-HSNPs. In that study, the SiO 2 coating via

the composites with water. Wang et al. [14] synthesized

the Stöber process on n-MPBs (i.e., Poly (Styrene-co-acrylic

monodispersed hematite colloidal nanoparticles with highacid) microspheres from soap-free emulsion polymerization)

monodispersity and variable particle shape. They proposed

was studied. Three kinds of n-MPBs with different surface

that these hematite colloidal nanoparticles are appropriate forcharge densities were used, while all the other reaction

the synthesis of m-HSNPs, since SiO2 exhibited high affinity

components were preserved the same as for the typicalto the surface of hematite and such template particles could

Stöber process, i.e., using tetraethyl orthosilicate (TEOS)

be easily removed by acid etching.

as the precursor of SiO 2 , NH 3 as the catalyst and ethanol

In previous studies, the sign of charge on the surface

as the solvent. Results in that work indicated when thewas also an important factor to judge whether or not thesecharge density of n-MPBs was high (zeta potential reachedtemplate particles are appropriate for SiO 2 coating. Since

to -45.6mV in water), SiO2 can form a uniform shell on theelectrostatic attraction is believed to be the driving force

surface of n-MPBs rather than yielding free-standing SiO2

for the SiO2 coating, as well as the negatively charged SiO2

particles, Interestingly, the SiO2 coating became much harder

particles came from Stöber process, positively charged

to perform when the surface charge density of the n-MPBs

templates were chronically believed to be much more

was reduced, in other words, being more “positive”, resulting

advantageous for the SiO 2 coating than the negatively

in a considerable number of free-standing SiO 2 particles

charged ones. As reported by [18], when negatively charged

in production (see Figure 3). Since there was no surfactant

monodispersed polymer beads (n-MPBs) were used for

used, i.e., the surface charge density was only adjusted

the SiO 2 coating, the SiO 2 particles generated in the bulk

by varying the content of the poly (acrylic acid) chains insolution instead of depositing on n-MPBs, further leding

n-MPBs, that study directly showed the feasibility of theto the failure in obtaining the m-HSNPs. However, thesen-MPBs using as the template particles for the SiO2 coating

studies may be more reasonable if the surface charge density

and m-HSNPs synthesis. Figure 4 shows the morphologyof the n-MPBs and reaction parameters for the SiO2 coating

and size distribution of the as-obtained m-HSNPs. As noted,

have been varied. In fact, there have been some studies thatthe m-HSNPs were endowed with a considerably highindirectly demonstrated the possibility of a SiO 2 coating

monodispersity as they were uniform in both the size andon n-MPBs. Graf et al. [12] found SiO 2 shell could beshell thickness and there were neither aggregated particles

constructed on the surface of n-MPBs with the assistance ofnor free-standing SiO2 particles produced.

polyvinylpyrrolidone (PVP). Indeed, PVP would not reverse
the charge sign of the n-MPBs, thus in that study the SiO2
shell was successfully constructed on a negative surface,
though the authors did not point out if there was a possibilityof the SiO 2 coating on the n-MPBs without the assistanceof PVP. Deng et al. [5] obtained n-MPB@Vinyl-SiO2 core－8－

The Production of Hollow SiO2 Nanoparticles (HSNPs) with a High Monodispersity

from positive to negative, and the NH4+ cations would not
likely be absorbed on the surface of the template particles
as well. In any case, the reversal of surface charge implies
that there is a certain stage when the apparent surface chargeof the particles was equal to zero (i. e., zeta potential=0).
While, rationally, particles at this stage were under the riskof aggregation. Therefore, Chen et al. [12] believe that the“no-charge reversal” SiO 2 coating process is an important
advantage of using n-MPBs for m-HSNPs and can be seen as
the main reason for the high monodispersity of the resulting
m-HSNPs, besides the high monodispersity of templates.
In summary, the high monodispersity of the template
Figure 3. (a) zeta potential (in water) distribution histograms

particles should be seen as the most important principle forof the as-synthesized n-MPBs with different contents oftemplate selection. Meanwhile, recent work has encouraged

acrylic acid; SEM images of SiO 2 coated n-MPBs withthe application of the negatively charged templates for thelow (b), middle (c) and high (d) surface charge (negative)

synthesis of the m-HSNPs, and thus the surface charge sign

densities; inserts of (b)~(c) are high-magnification SEM

should not be a manipulate criteria for selection.

images; image reprinted from ref. [12].
4. Influence of cNH3 and cH2O in SiO2 coating
During the Stöber process for synthesizing monodispersed
SiO 2 microspheres, the influences of the concentration ofNH3 (cNH3) and the concentration of H 2O (cH2O) in reaction
system have been intensively investigated [7, 19]. It was
found that NH 3 in the Stöber process acted as the catalyst,
since both the hydrolysis and condensation over silicate esters
are nucleophilic reactions, the rate of which would increase
Figure 4. (a) STEM image of m-HSNPs synthesized usingwith the increasing pH of the reaction system. In addition,

n-MPBs; (b) size distribution histograms of templates, SiO2

increasing the concentration of the reactant H 2O would also

coated templates and m-HSNPs; image reprinted from ref.

accelerate the Stöber process. Therefore, adjusting the cNH3

[12].

or cH2O would vary the hydrolysis rate of the silicate esters,
as well as the rate of the following condensation, thus further

As demonstrated by Chen et al. [12] using n-MPBs as

affect the nucleation and growth of the SiO 2 particles. As

the templates is more advantageous for preventing thea highlight, G.H. Bogush et al. [19] systemically studied

aggregation of m-HSNPs, since the mechanism of the SiO2

the inf luence of c NH3 and c H2O, followed by successfully

coating in this case is different from that on positively

proposing an equation to predict the size of the resulting

charged surface. The most important argument for thatmonodispersed SiO2 microspheres.

difference is there was no “surface charge reversal” occurredin the process of SiO2 coating on n-MPBs. In this process, asseen in Figure 5, the zeta potential of subject (i.e., n-MPB/
SiO2 composites) gradually increased from -17.2 mV alongwith the deposition of SiO 2 , then remained constant ataround -36.7 mV after the SiO2 shells formed. Before starting
the Stöber process, the zeta potential of the n-MPBs in
ethanol reduced from -17.2mV to -11.9 mV after ammonia
solution was added. This phenomenon can be attributed
to the adsorption of NH4 + ions on the surface of n-MPBs.
This phenomenon that they observed is opposite that of the

Figure 5. The change in zeta potential (in ethanol) before and

SiO 2 coating on the positively charged surface, in whichin the course of SiO2 coating on n–MPBs; image reprinted

the surface charge of the subject particles would reverse

from ref. [12].

－9－Wanghui Chen, Chika Takai, Hadi Razavi Khosroshahi, Masayoshi Fuji, Takashi Shirai

Although the role of NH 3 in the Stöber process for theof the m-HSNPs in both of these cases. When positive

monodispersed SiO 2 microspheres has been clarified, it istemplates were used for the SiO2 coating, a high cH2O would

still important to discuss the influence of cNH3 on the SiO2

lead to a high growth rate of the SiO2 particles and further

coating in the template method. Yun et al. [11] observed thelead to the occurrence of aggregation and the production of

SiO2 coating on positive MPBs with the varying cNH3 in thefree-standing SiO 2 nanoparticles [11]. On the other hand,

Stöber process. They found that the roughness of the SiO 2

using negative templates, as proposed by [12], a high cH2O

shells increased with the increasing cNH3, which could be

would suffer from the adsorption of NH4 + on the surface

attributed to the fast growth of the SiO 2 particles in theirof the templates, and further result in the homogeneoustransition from the bulk solution to the surface of templates

nucleation of SiO 2 in the bulk solution. Rationally, in thisdue to such a high c NH3. Takai et al. [20] demonstrated

case, only a part of the produced SiO2 could transfer onto thethat the structure of the SiO 2 shell formed on the surface

surface of the templates, thus resulting in an incomplete SiO2

of positive MPBs depended on the c NH3 (i.e., pH in thecoating.

solution). They also observed the effects of c NH3 on the
TEOS conversion rate, shell thickness and dispersity of them-HSNPs. In summary regarding to the SiO2 coating process
on the positively charged templates, cNH3 is supposed to becontrolled at a considerably low level in order to suppress thegeneration of aggregation and free-standing SiO2 particles,

Figure 6. STEM images of m-HSNPs synthesized with low

as well as to reduce the roughness. Nevertheless, since NH3(a), middle (b) and high (c) C NH3; image reprinted from ref.

acts as the catalyst for the SiO2 production, a low cNH3 would[12].

also result in a low conversion rate of the SiO 2 precursor,
thus leading to thin, loose, or even incomplete SiO2 shells on

5. Conclusion

the templates, which would finally result in a low mechanicalThis review deals with the template method for thestrength of the m-HSNPs. Therefore, when positive templates

synthesis of monodispersed HSNPs. The Stöber process is

are used for the m-HSNPs, optimizing experiments for cNH3

the appropriate technique for achieving the SiO2 coating on

are necessary to balance the robustness and monodispersity

templates, thanks to its high controllability and operabilityof the m-HSNPs.

under moderate conditions. Furthermore, to endow theIn the case of using n-MPBs as the templates, rationally,

HSNPs with a high monodispersity, the following principles

c NH3 also significantly affects the conversion rate of theare supposed to be maintained for selecting the appropriate

SiO 2 precursor, and thus a high c NH3 is advantageousmaterials of the template particles: (i) the surface of thisfor constructing thick SiO 2 shells on the templates [12].

material easily undergoes a SiO2 decorating; (ii) this material

However, different from the case using positive templates,

can be removed without destroying SiO2 shells; (iii) it is easy

the increasing c NH3 would not likely cause the increasing

to obtain highly monodispersed particles of this material.

surface roughness of the m-HSNPs. As seen in Figure 6,

According to previous studies, monodisperse polymer beads,

all the obtained m-HSNPs with using the same n-MPBs

polymer emulsions and monodisperse hematite colloidal

but applying different c NH3 value show similar surface

nanoparticles are all competitive materials for the templateroughnesses, whereas the shell thickness increases withparticles, while the latter two can also lead to the “green”

the increasing cNH3. Figure 6 also shows that the m-HSNPs

production of the m-HSNPs. In terms of the surface charge

synthesized with a high cNH3 contained neither aggregatesof the templates, we suggest both negatively and positively

nor SiO 2 nanoparticles. This phenomenon is believed tocharged templates can be taken into consideration. Specificbe another significant difference between using negative

to negatively charged templates, such templates show theirtemplates and using positive templates for the synthesis ofadvantages towards the dispersity of the m-HSNPs, possibly

the m-HSNPs.

attributed to the fact that the corresponding SiO2 coating on

As already discussed, the influence of cNH3 towards thethem is a “no-charge reversal” process. However, it seems

morphology of the obtained m-HSNPs depends on the chargethat the negative templates are suitable for the SiO2 coating

signal of the template surface, possibly due to the SiO 2

only when their surface charge density is considerably high.

coating mechanisms on the positive or negative templates are

We also discussed the influence of cNH3 and cH2O in the SiO2

different from each other. On the contrary, an increasing cH2O

coating process on the morphology of the m-HSNPs. As

would always exert a negative influence on the morphology

discussed, both a high and low cNH3 are unfavorable for SiO2

－ 10 －

The Production of Hollow SiO2 Nanoparticles (HSNPs) with a High Monodispersity

Physicochem. Eng. Aspects 481 (2015) 375–383.

coating on the positively charged templates, as the former
would lead to a rough surface of the SiO2 shells, as well as[13] Asep Bayu Dani Nandiyanto, Yui Akane, Takashi Ogi,

producing aggregates and free-standing SiO2 nanoparticles,

Kikuo Okuyama, Langmuir 28 (2012) 8616–8624.

while the latter would reduce the mechanical strength of[14] Yong Wang, Xiaowen Su, Panshuang Ding, Shan Lu,

the SiO2 shells. On the other hand, for the SiO2 coating on

and Huaping Yu, Langmuir, 29 (2013) 11575–11581.

negatively charged templates, a high cNH3 is advantageous[15] M Fuji, C Takai, H Imabeppu, X Xu, Journal of Physics:
Conference Series 596 (2015) 012007.

for constructing thick SiO2 shells, while unfavorable phasesof SiO2 (i.e., aggregates and free-standing particles) would[16] Yongsheng Han, Masayoshi Fuji, Dmitry Shchukin,

not likely to be produced in this case. On the contrary, a highHelmuth Möhwald, Minoru Takahashi, Cryst. GrowthcH2O is always undesirable for the SiO2 coating, no matter on

Des. 9 (2009) 3771–3775.
[17] Lijuan Zhang, Maria D’Acunzi, Michael Kappl, Gunterthe positively or negatively charged templates.
Considering the attractive properties of the HSNPs and their

K. Auernhammer, Doris Vollmer, Carlos M. van Kats,

additional characteristics due to their high monodispersity,

and Alfons van Blaaderen, Langmuir 25 (2009) 2711–

we anticipating there will be a significant improvement in the2717.

synthetic protocol of m-HSNPs. We also suggest some issues[18] Yu Lu, Joe McLellan, Younan Xia, Langmuir 20 (2004)
3464–3470.

not considered in this review, such as the hydrophilicity
and mass concentration of the templates in the SiO2 coating[19] G.H. Bogush, M.A. Tracy, C.F. Zukoski IV, Journal of
Non–Crystalline Solids 104 (1988) 95–106.

system, need to be addressed in the future.

[20] Chika Takai, Hideo Watanabe, Takuya Asai, Masayoshi
Fuji, Colloids and Surfaces A: Physicochem. Eng.

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