Review

セラミックス基盤工学研究センター年報（2011）．Vol．11，33-43

Development of Eco-Friendly Techniques for Preparation of
Porous Nano-/Micro-Size Hollow Silicate Particles
Raymond V.Rivera Virtudazo, Masayoshi Fuji*, Chika Takai, Takashi Shirai
Ceramics Research Laboratory, Nagoya Institute of Technology
3-101-1 Hon-machi, Tajimi, Gifu 507-0033, JAPANDeveloping an eco-friendly approach for the preparation of hollow particles (HP) of varied size/shape with tailored properties
have been an immense scientific and technological importance. Hence, this review intend to mainly focus on the simple ecoapproached done by CRL process group such as core-template, ammonia-hydrothermal template (AMT) and double emulsion
approach for preparing porous novel–type hollow silicate particles (HSP). The our current eco-synthesis showed positive resultsin controlling the size, thickness, and permeability of HSP. Wherein, the potential applications of this porous HSP are also briefly
discussed. The aim of this review is to provide synopsis for our alternative eco-approach for the development of nano-/micro-size
hollow silicate particles (NMHSP).

1.Introduction
With the discovery/creation of fullerenes and carbon
nano-tubes, a lot of research works in the last decade
have been devoted for the development in preparation of
micro/nanostructure hollow particles (M/NHP).1), 2) They
have been attracted tremendous interest because of its
unique properties compared to other solid counterparts,
for instance higher specific surface area, lower densityand better permeation/penetration. With the aid of modern
science, it has been already a common knowledge that an
exceptional properties may occur by downsizing the HP
from micro to nano-size without changing their chemicalcomposition which can be notably influence/affects theperformance of the synthesized materials.3) A typical M/
NHP process are usually suspended in a liquid medium
which generally been the basis for having surprising arrayof material applications for technological scientific value.
If then, functionalize the M/NHP (with facile processing),
it can extend the impact of the M/NHP that may generate
a novel synthetic implications.
This review paper initiates the simple preparation for
the fabrication of M/NHP in an eco-friendly approach.
Due to the broad scope of this topic, all research works
about M/NHP in every eco-processing area cannot
be treated in details. Instead, we focused/outlined ourmajor existing research of M/NHP added with somecurrent improvements in preparation of silicate and otherinorganic M/NHP. A lot of excellent articles have been
published for the progress of silicates and other inorganic

M/NHP with respect to architectural design, synthesis
and bio-related applications (drug release)3), 4) but only
a few deals in establishing a route that would entail
environmentally friendly. This is important task because
taking good care of our environment is a major concern
and if we can eliminate toxic waste by-products it would
be good for human being. Also, a lot of studies involved
in single-step or one-pot eco-synthesis approaches are
becoming available and developing an eco-large–scale
synthesis of HP would be a great challenge to eliminate
environmental problems concerning fabrication of HP
which can find a lot of commercial applications.
This reriew is organized as follows: Firstly, asses thebasic fundamental process which is sol-gel. Then followed
by, the general schematic approaches for M/NHP thatour group has done and other significant reference papers
about HP preparations such as solid-template (solidcores template method), soft-template (double emulsion
method), and facile process like ammonia-hydrothermal
approach (AMT). After then, summarize common
applications of NMHSP. This kind of approach primarily
gears towards sol-gel process; a wet-chemical technique
usually used for the fabrication of M/NHP which cancreate both glassy and ceramic materials.
2.Fundamental process: Sol-gel
Basically, the sol (or solution) evolves gradually
towards the formation of a gel-like network containing
both a liquid phase and a solid phase. The precursors− 33 −

Development of Eco-Friendly Techniques for Preparation of Porous Nano-/Micro-Size Hollow Silicate Particles

used in sol-gel processing consist of a metal or metalloid
element surrounded by various reactive ligands. Metal
alkoxides, such as aluminates, titanates and zirconates,
are the most popular precursors because of their highreactivity towards water. The most widely used non-metal
alkoxides are alkoxysilanes which is mostly discussed
on this section, such as tetramethoxysilane (TMOS)
and tetraethoxysilane (TEOS). A well studied alkoxide
is TEOS. The chemical formula for TEOS is given by:
Si(OC2H5)4, or Si(OR)4 where the alkyl group R = C2H5.
This alkoxides are ideal chemical precursors for solgel synthesis because they react readily with water andsome metal alkoxides (organometallic compounds).
The reaction is called hydrolysis, because a hydroxyl
ion becomes attached to the silicon atom.5) The process
consists of a series of hydrolysis and condensation
reactions of an alkoxide, which is shown in Fig. 1.

Fig. 1 Typical Sol-gel reaction scheme. 10)

The mechanisms of hydrolysis and condensation,and the factors that bias the structure toward linear or
branched structures are the most critical issues of sol-gel
science and technology.6), 7) Hence, sol-gel process has
been frequently employed in coating the colloidal coretemplates followed by forming M/NHP or NMHSP byremoving templates. This is a simple reaction that does
not require unusual materials or expensive deposition
equipment. Likewise, sol-gel reactions do not employ
extreme reaction conditions. The reactions can takeplace at room temperature and require only moderatetemperatures to ‘cure’ the gel which can easily removing
the excess water/alcohol that the reaction generates.
Also, the properties of the M/NHP prepared using sol
gel approaches can easily be modify by utilizing anorganically modified alkoxide or a variable metalloid (forexample, an alkoxyborate instead of an alkoxysilane).
Mostly HPs are obtained through colloidal-templatesynthesis. 8), 9) In template route, the inner diameter/
shape of HP is only determined by the dimensions of

the kind of template (organic10), 11), inorganic12), 13) andbiomolecules14)) used. Predominantly, these templates
syntheses are divided into two parts namely hard and soft
template syntheses in fabricating hollow particles withhomogeneous, dense shell wall layers. The next section
discuss on the preparation of NMHSP with some example
of inorganic HP via solid-template (solid-cores template
method), soft-template (double emulsion method) andfacile route like AMT.
3． Fabrication and common approaches informing hollow inorganic/silicates particles
Extensive studies had been done to evaluate an idealand convenient synthesis for M/NHP especially under
mild eco-conditions. These common features such as thesolid template, soft template and ammonia-hydrothermal
approach (AMT) are basically discussed in this paper
especially on our present eco-preparation in forming
NMHSP.
3.1 Solid- Core Template Method
Colloid core-template particle is probably an effective
and universal method for the preparation of HP, especially
if required a narrow size distribution, for example, selfassembly and photonic crystals 15), 16). In the solid-templateassisted synthesis, poly(styrene acrylic acid) (PSA),
polystyrene (PS) latex, silica spheres and calcium carbonate(CaCO3) particles are commonly used as colloidal coretemplates because of their readily available in a wide
range of sizes.4), 17) Technically, inorganic precursor (metal
or non-metal alkoxides) is coated over the core-template
either by physical or chemical reaction to provide anintermediate called core-shell (hybrid materials) may
contain organic-inorganic or inorganic-inorganic core shellstructure. The core-template particles were subsequently
removed by selective dissolution in an appropriate solvent
or by calcination at elevated temperature in air to generate
M/NHP.18), 19) Majority of these M/NHP are formed onouter surfaces of core-templates through sol–gel, 20), 21)hydrothermal treatment, 22) layer-by-layer 2), 23) and direct
chemical deposition.4)
For instance, the research works done by Li et al24)
use these surface-modified PS spheres as sacrificial
template to fabricate silica-coated PS and titaniacoated PS composites by co-condensation betweenhydroxyl groups with tetraethyl orthosilicate (TEOS) andtitanium(IV) isopropoxide (TIPP) in a sol-gel process,respectively. The HSS and THS can be generated by thesubsequent removal of the PS core using tetrahydrofuran.

− 34 −

Raymond V.Rivera Virtudazo, Masayoshi Fuji, Chika Takai, Takashi Shirai

Although the above mentioned research studies were very
interesting, the preparation processes seem to be timeconsuming, expensive and not eco-friendly.

of acid to dissolve the core templates. This is a facile ecoroute in synthesizing nano-/micro-size hollow silicate
particle (NMHSP) with tunable shell thickness and uniqueanisotropic hollow shape by employing inorganic particles(Ca-HAp and CaCO3) as template in concurrence withthe sol gel method. The results showed relatively stableanisotropic hollow shape with uniform silicate shell wall
thickness. In addition, at relatively high concentration ofsilicate precursor (TEOS), the shell thickness and surface
roughness increase. As expected, unique anisotropic
shape and size of the NMHSP depend on the inorganic
template used as illustrated in Fig. 3.

Fig. 2 General schematic illustration of anisotropic HP using
inorganic template approach.

In this manner, simple eco-friendly step processes
are needed for the synthesis of M/NHP. Our group havebeen successfully reported this process (Fuji et al.,25), 26)),
via template sol-gel process by utilizing and removing
the nano-core CaCO 3 particles through dissolutionin acid etching followed by the formation of nanosize hollow silica particles (NSHSP). In this method,aqueous ammonia solution was used as the catalyst
and medium. The naturally positively charged CaCO3
nanoparticles were dispersed into the ethanol (EtOH)
solution followed by sonicating for 10 min. Thus, ensures
the generation of silicate sol from the hydrolysis andthe condensation of TEOS. This rapidly captured by
CaCO3 particles via electrostatic interaction in aqueous
ammonia solution either in room or elevated temperature.
Under acid conditions, CaCO3 particles were “dissolved”
subsequently and even synchronously to directly
formed NSHSP, as shown in Fig.2. The formation of
the inorganic silicate shells and the dissolution of core
template particle occurred during acid dissolution; neither
additional toxic chemicals or calcination process was used
to remove the CaCO3 core.26), 27)
With this concept, we are able to utilized otherinorganic core templates such as colloidal calcium richhydroxyapatite (Ca-HAp)28) nanoparticles and micro-size
CaCO3 particles for the formation of anisotropic NSHSP
to micro-size hollow silicate particle (MSHSP).29) These
core-templates are recyclable and only mild concentration

Fig. 3 Schematic illustration and typical images of theanisotropic template process to form NMHSP 36), 37) using
colloidal nano-size CaHAp (a.1, a.2, a.3) and micro-size CaCO3
particles (b.1, b.2)

With solid template (hard), refilling the hollow interior
with functional species or in situ encapsulations of quest
molecules during the formation of shells, though possible,
is still demanding. Thereby, fabricating a macroporousin the shell wall of hollow microsphere particles (HMP)
is a remedy, but it is still tough. These difficulties haveprompted to synthesize a more simple approach for
producing HMP with macroporous shell that can easily
encapsulate and release quest species. For instance,
templating against soft (liquid or gaseous) template
has been significant especially for the developmentof emulsion method. The next section will confer the
essential concept of soft template and discuss the basic
concept of the double emulsion method. The next section− 35 −

Development of Eco-Friendly Techniques for Preparation of Porous Nano-/Micro-Size Hollow Silicate Particles

re-established the potential of the soft template method
to create an interior spaces and for a creating a functionalshell structure through a one-pot synthesis.
3.2 Soft Template Method
The soft template synthesis is another simple andgeneral method for obtaining hollow particles. Somehollow particles with nanometer to micrometer diameter
have been successfully fabricated using double emulsion
process usually composed of water-in-oil-in-water (W1/O/
W2).30)Generally, (W1/O/W2) double-emulsion system consistsof individual oil globules that contain smaller droplets of
the internal aqueous phase and are dispersed in an externalaqueous phase. Because of their special internal structure,
double emulsions have found significant applications inmany areas, such as pharmaceuticals, foods, cosmetics
and separation. 31), 32) Double emulsions are typicallyformed through two-emulsification processes, i.e. by firstemulsifying the inner droplets in the middle fluid, andthen by undertaking a second emulsification step for thedispersion.4), 32) This is simply illustrated in Fig. 4.

Basing from Fujiwara et al33), 34) in the case of hollow
silicate microspheres (HSMP), the reaction of sodium
silicate with a precipitant is utilized. W1/O emulsion ismade of W1 (consist of water and sodium silicate) and O(oil phase) with surfactant for stabilizing emulsion. Then
W1/O is added to another aqueous solution of a precipitant
W 2, forming W 1/O/W 2 emulsion system. During theelimination of the oil phase (O) between the two waterphases (W1 and W2), these two aqueous solution are
mixed to form silica precipitate along the emulsioninterface. The sodium silicate of W1 was providing
interfaces forming silicate particles (precipitate). Aftercomplete formation of the precipitate on the interfaces,
all sodium silicate is consumed and the inside of the
microcapsule was filled with only water. This water canbe readily removed by drying treatment thru the poresof the silica shell. Finally HSMP with vacant inside are
obtained. In this process no other process to remove thecore compound are necessary.
With some innovations, water-soluble polymer (sodium
polymethacrylate, Na-PA) was added into the aqueous
solution of W1, connotes as W1P-X. Thereby, formed
water 1+polymer/oil/water 2 (W 1P-X/O/W 2) emulsion
system. Consequently, the parameters were controlled for
instance emulsification rotational speed constant, fixedvolume ratio, fixed surfactant ratio, modified (set-up)pressurized by N2 filtration and calcination. Then, formed
hollow silicate microspheres with meso/macroporous
shell (HSMSP) 35) were successfully prepared. The
prepared HSMSP exhibits unique three-dimensional
hierarchical architectures and demonstrated significantly
improved physico-chemical properties as shown at theSEM images (inset) of Fig. 5.

Fig. 4 Basic illustration of double emulsion method (W1OW2)To develop simple, effective, controllable, andenvironmentally benign methods to form hollow
microspheres by double emulsions remains challenging
and is of great importance. In this section, the researcher
utilized the works of Fujiwara to fabricate macroporous
hollow silicate microspheres (HSMSP) with additionof water soluble polymer and some innovation intersof pressure filtration for faster processing. This method
has some unusual advantages. Such as, it is very simple,
green, and the formation of double emulsions can becontrolled the micro-size through emulsion rate speed.

Fig. 5 Conceptual scheme of the porous hollow silicate
microspheres (HSMSP) by W1P-X/O/W2 interfacial reaction upon
addition of Na-PA and calcinations. Inset: SEM images of a usual
HSMSP with macroporous shell.46)− 36 −

Raymond V.Rivera Virtudazo, Masayoshi Fuji, Chika Takai, Takashi Shirai

Most of the double emulsions reported are macroemulsions of which the inner and outer droplets are bothin micron scale. But recently Deming et al.36) reported anano-scale double emulsion was formed, in which bothoil droplet and internal aqueous droplet are in nano-meter
size. The ultrasonic and micro-fluidic homogenization
was utilized to induce the formation of nano-scale double
emulsion, which was stabilized by specially designed
amphiphilic diblock copolypeptide surfactants. This opens
up the way to prepare double nano-emulsion. Anotherapproach such as micro-fluidic device 37), 38) and electricfield 39), 40) were also developed to make double emulsions.
Stability of the shell wall is generally the mostcommon problems especially for template-sol-gel
process. This is due to that template sol–gel process
involves uncontrollable fast hydrolysis and condensation,and resulted to the formation of unstable shell wall ofamorphous inorganic oxides.41), 42) Calcination of the coreshell is commonly done for this problem to stabilize theshell walls, generally above 500℃ usually required totransform amorphous oxides into crystalline ones. But, on
the other hand, the high temperature thermal calcinations
would seriously affect the particle size, surface structureand even would result in a collapse of the hollow
structure.8), 17) Hence, next section briefly discussed ourinnovative method via ammonia-hydrothermal approach
which is one way to eliminate this problem and enhances
stability of the shell wall.
3.3

Ammonia-Hydrothermal-Template (AMT,
facile approach)Hybrid hollow particles of crystalline metal oxides
can be synthesized in a simple one-pot synthesis via
hydrothermal approach. Adding metal salts directly
to the carbohydrate solutions in water, followed by ahydrothermal treatment and then calcination, hollow
particles of various metal oxides, such as Fe2O3, Ni2O3,
Co3O4, CeO2, MgO, and CuO, were obtained.43), 44) But
with ammonia-hydrothermal treatment (AMT) provides
a facile and convenient method for refining the structuralorder and pore size uniformity. The AMT intensify
transformation of silica wall (S)–interactions within the
micro/nanostructure from a weaker (X) hydrogen bonding(S+X−I0) to the stronger electrostatic (S+I−) interactions
which can be illustrated in Figure 6d. This treatment
resulted in an increase of both thermal and hydrothermal
stability. Recently, our research group introduced AMT
to improve the stability of template NSHSP using CaCO3
nanoparticles while preserving the original morphology.45)

The key determining factors involved in this treatment
include hydrothermal temperature and aging time.46), 47)
Then at elevated temperature (120℃), hollow calcium
silicate hydrate nanoparticles (HCSHNP) formed. This
simple process for the formation of a unique HCSHNP(< 100 nm), was successfully prepared via the hydrolysis
and condensation of TEOS, ammonia water (NH4OH)
and CaCO3 as template and then AMT. To appreciate
the formation of HCSHNP, the temperature reaction wasvaried at room temperature (RT), 90 and 120℃ . Theneach reaction temperature was varied in aging time for 3 h,
9 h, 24 h and 10 d.

Fig. 6 General Illustration of stabilization and formationof hollow calcium silicate hydrate particles by ammoniahydrothermal template process.64)

In Fig. 6 shows the typical (Fig. 6a) XRD pattern ofsemi-amorphous HCSHNP materials. The additional
sharp peak at ~18.1 (2θ), corresponds to overlapped
diffractions of Ca(OH) 2 48), 49) and low crystalline
CSH.49) While no visual peaks (amorphous phase) wereobserved for samples synthesize at room temperatureand 90℃ . Hence, semi-amorphous CSH with Ca(OH)2
may contained in the shell wall of the nano-size hollow
particles (Fig. 6b and 6c). Thus, we successfully develop
a simple-direct process via AMT, followed by acidetching; formed HCSHNP ranging from (60 to 100)
nm. In future works, we can further functionalize thisto allow more dispersed particles and fabricate hollow
hybrid bio-glass particles. Due to its exceptional physicochemical properties, it may find wide use for nano-cement
materials, nano-biomedical material and coating additives
as thermal insulating materials.
4．Developing a facile route and ecoinnovative way for hollow silicate particles(HSP).

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Development of Eco-Friendly Techniques for Preparation of Porous Nano-/Micro-Size Hollow Silicate Particles

A lot of significant achievements have been made inthis area, as reviewed in this paper. Nevertheless, there arestill some challenges that need to be fulfilled. Synthesesof complicated structure such as controlling the size/
shape, sphere-in-ellipsoidal and cage-like surfaces for
HSP are just in its initial stage. It is anticipated that solid
template and soft template/free template techniques willplay a greater role in future eco-fabrication of complicatedstructure. As mention above, functional use of hollow
structures is closely correlated with their morphologicalproperties such as the exterior shape, the interior space,
and the shell structure. To increase the permeability
and create the ability to store bio-macromolecules andnanoparticles, it is desirable to build meso/macro-holes on
the shells of inorganic hollow structures but needs proper
proportioning inorder not to deteriorate the mechanicalstrength of the HSP. Thus, simultaneous achievement ofa robust hollow architecture and improved large molecule
permeability in a environmentally friendly approach isstill remains a challenge
For this reason, NMHSP particles are interesting not
only because of their superior interior void structures,
but also for a few more important reasons as mention
previously. Generally, the successful creation ofthese micro/nano-scopic materials itself is a proud
representative of the advancement of modern synthetic
technology. Thus, it is very helpful to deepen in theunderstanding the mechanism for the formation of micro/
nanostructure HSP, which will encourage designing
more and more novel structure. Also, compared withbulk micro/nano-material, the micro/nano-scopic hollow
interior spaces with multiphase interfaces might cause anumber of variations in the physico-chemical properties,
reinforce interfacial effects and control the local chemical
micro-environment that can bring a lot of potential
applications.50), 51)
5． Potential applications of hollow inorganic/
silicate particles
Since NMHSP possess characteristics such as low
density, high surface-to volume ratio, low coefficient ofthermal expansion and refractive index which makes it
attractive for widespread potential applications ranging
from chemical reactors, drug delivery, catalyst support,antireflection surface coating, lightweight materials,
rechargeable batteries and various new application fields.8)
Next section, will briefly summarized NMHSP common
applications into four parts which simply illustrated inFig. 7.

Fig. 7 Actual images of the typical-results of NMHSP
fabricated via an eco-friendly process and basic summary of thepotential application of the NMHSP.

5.1 Lithium-Ion Batteries (LIBs)
Macro-porous HSP is a fine example of good electrodematerials for lithium ions.52), 53) Fundamentally, Silicon(Si) believed to be the highest known lithium storage
capacity of 4200 mA h g-1, which makes it very attractive
as a negative electrode materials for LIBs. But, Si
nanoparticles based anodes are often found to have lesssatisfactory cycle life compared to other materials (e.g Snbased anodes) again due to ultra large (>300%) volume
changes during Li-Si alloying/dealloying processes.
Recently, modified nestlike NSHSP exhibits much
improved cycle life and rate capacity. These observations
clearly verify the advantages of the NMHSP (nestlike
Si hollow morphology)54) as compared for solid micro/
nanoparticles of the same composition.
5.2 Biomedical Applications
With extensive development of NMHSP synthesis,
it greatly helps tune the optical properties andsuperparamagnetic behaviour.55) As mention, NMHSP
are good for storage and charge carriers.56) Since, silicate
particles is an accepted materials for drug delivery, forthe reason that of its non-toxicity. So, HSP has been
extensively studied for biomedical applications technicallybecause of its biocompatibility and well-established interms of bioconjugation methods using silane chemistry.57)
Generally, it has the ability for adsorption and releaseof sensitive materials such as fluorescent and drug
markers aside from well-known catalytic applications.51)
Moreover, drug molecules can be loaded into the cavity
and on the surface of the hollow structure and typicallyrelease over living cells wherein it can control the amountrate of drugs by altering the pore dimensions and wall

− 38 −

Raymond V.Rivera Virtudazo, Masayoshi Fuji, Chika Takai, Takashi Shirai

structures.58) Significantly, it was demonstrated that drug
molecules can be stably encapsulated during the synthesis
of HSP and the release can be triggered by ultrasound or
controlling the pH.59), 60)
5.3 Waste treatment application
Wastewater is universally acknowledged as health andenvironmental problems. The complexity of wastewatercomposition led to intricate treatment process with highcosts, which are impractically given the large volumesof waste produced by both domestic use and industry.
Develop an effective and cheap decontamination method
is urgent need. One remedy is for the using HSP as
conventional adopted method for adsorption and ion
exchanged. Some current studies confirm that these as
prepared HSP showed an effective adsorbent; remove
organic pollutants and removal of heavy metals ions in
wastewater.61), 62)
5.4 Catalysis/sensor
Inorganic/silicate porous particles has been as usedin extensively as catalysis, the scientific curiosity ofthis materials led to numerous applications. By only
manipulating the particle range from nano- to microscale
can directly represent a promising type of structures forenhancing their catalytic ability. The high surface-volumeratio enhances the fraction of low coordination metal sites(e.g vertices, edges and kinks), and promotes adsorptionof reactants. The accessible micro/meso/macropores athollow surfaces afford ample channels between inner andouter surfaces, which facilitate diffusion of solvent, andreactants, thus accelerate chemical reactions.63) Recently,
a unique structure, in which the shell can effectively
prevent the core particles from aggregation, hollow
spheres incorporated with catalytic metal NPs (metal
NP yolk/shell particles), has potential applications as
nano-reactors for catalysis.64) Taking Au/SiO2 yolk/shellparticles as an example, they have been widely tested forthe catalytic reaction of p-nitrophenol by NaBH4.65) In
addition to their catalytic property, the confined growthand Oswald ripening of Au NPs in HSP have also beenreported.66) Greatly enhance photocatalytic activity andalso by creating mesoporous silicates in the shell walls.
Most importantly, the confined reaction on the surface ofthe core NPs, or transformation of the NPs themselves,provides an alternative mode for producing many otherfunctional yolk/shell particles.
On the other hand, the high surface areas of NMHSP
are also advantage for chemical/gas sensing. The HSP

provide facile light guiding for quantum cascade laser(QCLs) over several meters and, hence can be used
for remote gas sensing or IR light delivery in medical
applications.67) Chemical/gas sensors are also widelyused for industrial process control and are experiencing
growing use in security applications.68) Sensing withthese materials are performed through measurements ofchanges in electrical conductance produced by adsorptiondesorption of a targeted analyte on the oxide surfaces.
6．Summary and Outlook
Considerable eco-synthesis of NMHSP has been madesignificantly for this past decade. In this review paper,
eco-synthetic approach for the fabrication of micro/
nano-size porous HSP was briefly summarize the basic
concept base on the actual results done especially on thefollowsing: (1) core-templating (using nano-size CaHAp
and micro-size CaCO3 particles); (2) double emulsion
method (using water soluble polymer as additive for(W1OW2); and (3) Ammonia-hydrothermal templating(using coreshell-CaCO3/SiO2 nanoparticles for fabricating
HCSHNP). This review is likely to contribute/re-evaluate
the fabrication of HSP, which presents a simple, innovate
way and an eco-friendly process for the formation of
NMHSP.
Evidently, advantage of template using (solid)
CaCO3 particles and CaHAp are possibly the greenest
approach, clearly effective, and most facile technique
for synthesizing NMHSP. Aside from that CaCO3 or
CaHAp particles are technically abundant in nature andlow cost material, they generally require no or minimal
addition of surface functionalization and shell formation
is guaranteed by chemical reactions. However the generaldisadvantages on this approach rely on achieving highproduct yields from this simple synthetics process,difficulty in forming a uniform coating around surfaces
with large variation of curvature and lack of structuralrobustness of the shells upon template removal. But these
difficulties can be partly overcome by monitoring the acidconcentration during acid etching, used of surfactant andexposed the core-shell particles through hydrothermal
approach for structural stability.
While for double emulsion approach (W1OW2) for
fabricating macroporous hollow microspheres (PHSM),
a key advantage of using double emulsion method forthe preparation of PHSM is that the core removal stage isvery easily eliminated. The liquid template, liquid cores(W1) can be easily removed by low-stress generatingprocesses such as evaporation, filtration or dissolutions− 39 −

Development of Eco-Friendly Techniques for Preparation of Porous Nano-/Micro-Size Hollow Silicate Particles

with common solvent such as ethanol/water after theshell formation, final hollow product cannot be altered
even it was calcined and has low parental toxicity
because of the used of water–soluble polymer. However,
the relatively low stability and the polydispersity of
double emulsion approach may limit their application.
Mostly controlling the microspheres, uniformity, andmacroporous distribution into the shell still remainchallenge. Moreover, the water-soluble polymer cannot
be easily removed into the shell and liquid removal (W1)
is an energy-intensive process. But this problems can be
partly overcome by calcination, pressure-filtration andcontrolling/monitoring the emulsion speed rate.
In our present results for the fabrications of NMHSP,
we offer an innovative method for eco-synthetic strategy
to form unique NMHSP with stable shell wall poresystems and also provide further understanding for the
future mechanism on composite hollow materials which
can be furtherly explained through extensive researchworks.
The core-template approach can be easily implemented.
However, the capability of constructing complicatedstructure, such as macro-through-holes connecting theinner and outer spaces of hollow structures, is limitedby the availability of a template. As an alternative, it isimportant to develop a range of comprehensive template
free methods, confidently having the same flexibility
as the existing template-assisted techniques, to meet
new technological requirements.20) Hopefully, based
our innovate processes; another researcher can develop
and synthesize hollow particles with more complicatedstructure in a simple eco-approach.
Finally based on the above findings, it is concluded
that an eco-friendly approached in fabricating of NMHSP
can be done. In general, by controlling the parameters
enhance the surface morphology, porosity of the shell andstability of the HSP. The insights obtained from this nontoxic alternative mechanism will allow other researcher
to gain better controls of the HSP especially in exploring
the dispersion of the HSP. We are hopeful that a moreversatile and powerful eco-method of preparation of HSP
will be developed from both fundamental and practical
viewpoints and develop more novel physicochemical
properties. Thus, this sample eco-techniques presented inthis review provides a good foundation for eco-synthesis
of NMHSP especially for template and double emulsion
approached and applications will be explored in the near
future.

ACKNOWLEGEMENT
This research was partially supported by the
Ministry of Education, Science, Sports and Culture,
MONBUKAGAKUSHO Scholarship Program and
Grant-in-Aid for Scientific Research (B), 22310066"Clarification of mechanism of super-thermal insulationof hybrid thin film between nano-size hollow silica andpolymer" 2010-2012.
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