1 23

Journal of Sol-Gel Science andTechnology ISSN 0928-0707 J Sol-Gel Sci TechnolDOI 10.1007/s10971-015-3636-1

Synthesis of silicon dioxide, silicon, andsilicon carbide mesoporous spheres frompolystyrene sphere templates

Lauren S. White, Julia Migenda, XiaonanGao, Dustin M. Clifford, MassimoF. Bertino, Khaled M. Saoud, ChristophWeidmann, et al.

1 23

Your article is protected by copyright and all

rights are held exclusively by Springer Science

+Business Media New York. This e-offprint is

for personal use only and shall not be self-

archived in electronic repositories. If you wish

to self-archive your article, please use the

accepted manuscript version for posting on

your own website. You may further deposit

the accepted manuscript version in any

repository, provided it is only made publicly

available 12 months after official publication

or later and provided acknowledgement is

given to the original source of publication

and a link is inserted to the published article

on Springer's website. The link must be

accompanied by the following text: "The final

publication is available at link.springer.com”.

ORIGINAL PAPER

Synthesis of silicon dioxide, silicon, and silicon carbidemesoporous spheres from polystyrene sphere templates

Lauren S. White • Julia Migenda • Xiaonan Gao •

Dustin M. Clifford • Massimo F. Bertino • Khaled M. Saoud •

Christoph Weidmann • Bernd M. Smarsly

Received: 27 October 2014 / Accepted: 19 January 2015

� Springer Science+Business Media New York 2015

Abstract Amberlite XAD 16N mesoporous polystyrene

spheres were used as a template to create silicon dioxide

(SiO2), silicon, and silicon carbide (SiC) mesoporous

spheres. Polystyrene spheres, infiltrated with either

hydrochloric acid catalyzed tetraethyl orthosilicate or

dimethylethylamine catalyzed tetramethyl orthosilicate,

were heated to 550 �C to induce oxidation and/or decom-

position of the polystyrene template and yielded

SiO2 spheres. To create Si and SiC spheres, SiO2 and SiO2-

infiltrated spherical polystyrene templates, respectively,

were distributed in finely grated magnesium before heating

to 675 and 700 �C each in an argon atmosphere. Mg by-

products in the form of magnesium silicates and residual

SiO2 were removed by washing the spheres with hydro-

chloric acid and hydrofluoric acid, respectively. X-ray

diffraction, Brunauer–Emmet–Teller model specific sur-

face area analysis, Barrett–Joyner–Halenda model pore

diameter analysis, transmission electron microscopy and

scanning electron microscopy were employed to investi-

gate the microstructure and porosity during and after syn-

thesis of the spheres. All three types of spheres maintained

high porosity and their spherical shape throughout the

synthesis. SiO2 spheres had a surface area of 700 m2 g-1,

Si spheres a surface area of 160 m2 g-1, and SiC spheres a

surface area of 215 m2 g-1. SiO2 spheres with dispersed

Ag nanoparticles were also successfully created by adding

AgNO3 to the precursor solution; they had a surface area of

220 m2 g-1. To prove the versatility of this infiltration

method, Dy2O3 spheres were also fabricated, though they

were not porous. This infiltration method is not only ver-

satile, as it is suitable for the preparation of numerous types

of mesoporous spheres, but it is also a simple synthesis

method that guarantees a well-defined spherical shape and

narrow particle size distribution, primarily while main-

taining a high surface area.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10971-015-3636-1) contains supplementarymaterial, which is available to authorized users.

L. S. White � M. F. Bertino (&)

Department of Physics, Virginia Commonwealth University,

Richmond, VA 23284, USA

e-mail: mfbertino@vcu.edu

L. S. White

e-mail: lswhite@vcu.edu

J. Migenda � C. Weidmann � B. M. Smarsly

Institute of Physical Chemistry, Justus-Liebig-Universitat

Giessen, Giessen, Germany

e-mail: julia.migenda@phys.chemie.uni-giessen.de

C. Weidmann

e-mail: c_weidmann@gmx.net

X. Gao � D. M. Clifford

Department of Chemistry, Virginia Commonwealth University,

Richmond, VA 23284, USA

e-mail: gaox6@vcu.edu

D. M. Clifford

e-mail: clifforddm@vcu.edu

K. M. Saoud

Literal Arts and Sciences Program, Virginia Commonwealth

University, Doha, Qatar

e-mail: s2kmsaou@vcu.edu

123

J Sol-Gel Sci Technol

DOI 10.1007/s10971-015-3636-1

Author's personal copy

Graphical Abstract

Keywords Polymer-matrix composites (PMCs) �Porosity/voids � Heat treatment � Sintering � Sol–gel

methods

Abbreviations

TEOS Tetraethyl orthosilicate

TMOS Tetramethyl orthosilicate

XRD X-ray diffraction

BET Brunauer–Emmet–Teller

BJH Barrett–Joyner–Halenda

TEM Transmission electron microscopy

SEM Scanning electron microscopy

1 Introduction

Porous microspheres are very versatile materials for which

a broad assortment of applications has been suggested.

Silica microspheres have applications in liquid chroma-

tography [1], magnetite–silica microspheres are potential

ultrasound contrast agents [2], oxycarbide and uranium

oxide microspheres are being considered as ceramic pre-

cursors [3, 4], and microspheres of graphite containing iron

oxide nanoparticles are being considered as electrode

materials [5]. Other applications include drug delivery and

catalyst support, for example as described in Refs. [6, 7].

Porous microspheres are considered relevant for so many

applications because they couple a well-defined geometry

with high porosity. This precise geometry, defined as

distinctly spherical particles within a narrow size distri-

bution and each particle being highly porous giving rise to

large surface areas, make the use of spheres quite simple

when compared with materials in powder form. These

geometric considerations are especially relevant for batch

operations. For example, uranium oxide spheres have been

considered as an alternative to powders for the production

of nuclear fuel because they minimize the amount of

radioactive dust spread into the environment during stor-

age, handling and sintering [4]. Furthermore, packed beads

of microspheres can be optimized with respect to pro-

nounced through-transport, which is exploited in high-

performance liquid-chromatography [1]. Recent reviews

that cover the general fabrication principles as well as

applications of porous microspheres in catalysis and drug

delivery are reported in Refs. [6, 7].

Limiting our discussion to inorganic materials, it was

found that, in most cases, microspheres are prepared by

emulsion methods [8], which are variably straightforward

to upscale. Other techniques yield microspheres which are

embedded into the matrix in which they were created [9],

require weeks to grow independently [10], or require

elaborate synthesis methods such as using an optical fibre

drawing technique [11]. Hard templating, an upscalable,

cheap and rapid technique, has been reported for carbon

microspheres, see for example Ref. [12]; infiltration tech-

niques have also been reported that make use of natural

templates such as diatoms or lignin [13–16]. These tech-

niques do not yield materials with a well-defined geometry.

Additionally, it is challenging, as seen during previous

J Sol-Gel Sci Technol

123

Author's personal copy

work done in our group [17], to develop a phase-pure SiC

or Si material from SiO2. It is shown in this study that

a simple method has been developed that yields such

phase-pure materials.

This work describes an infiltration method that uses

readily available polystyrene templates. Subsequent syn-

thesis is based on straightforward, upscalable wet and solid

state chemistry principles. The primary objective of this

study is to develop the synthesis of Si microspheres, a

promising material for Li battery electrodes [18], and of

SiC, a material with potential applications as a superca-

pacitor [19]. Si microspheres could be especially promising

for the fabrication of advanced lithium ion battery anodes,

since the high porosity of such spheres can ensure the

accommodation of mechanical stress upon lithiation and

delithiation cycles [20]. Combined with their spherical

morphology, which allows for a dense packing of material

with retained macroporosity, the material presented herein

could combine superior cycle life with a high volumetric

energy density. Similarly, a straightforward production

method for mesoporous SiC spheres may allow for the next

generation of alternative energy storage and conversion in

the form of environmentally friendly electrochemical

capacitors or supercapacitors [21]. Work has already been

done showing the practical application of SiC nanowires

for microsupercapacitor electrodes [22]. This works well

since the nanowires must be grown directly on the current

collector and have a high surface area of *250 m2 g-1.

For larger supercapacitor electrodes, SiC spheres would

give the same high surface area with applications inde-

pendent from production method. Here we present such a

synthesis method that yields mesoporous SiC spheres with

a comparable surface area of 245 m2 g-1.

Both types of materials can be obtained by a suitable

transformation starting from SiO2 microspheres. A partic-

ular emphasis has been put on the thorough characteriza-

tion of the composition and porosity of the respective

microspheres. The technique developed can also be adap-

ted to the synthesis of microspheres loaded with metal

nanoparticles. To prove this, and with an eye towards

catalysis, SiO2 microspheres loaded with Ag nanoparticles

were synthesized. To further prove the versatility of this

infiltration method, both acid- and base-catalyzed hydro-

lysis methods were successfully attempted.

2 Experimental

2.1 Equipment and chemicals

A Nabertherm open air furnace and a Lindberg/Blue M

tube furnace were obtained for this experiment. Amberlite

XAD 16N mesoporous polystyrene (PS) spheres, with a

size distribution of *560–710 lm and 20–60 mesh, were

purchased from Alfa Aesar and used as a template for SiO2,

Si, SiC, and Dy2O3 spheres. When measured, the actual size

distribution was *500–800 lm. These spheres have a

maximum usage temperature of 249 �C. Reagent-grade tet-

raethyl orthosilicate, hydrochloric acid, ethanol (190 proof),

tetramethyl orthosilicate, and dimethylethylamine were

purchased from Acros Organics and used as received.

Magnesium finely grated powder, dysprosium (III) chloride

hexahydrate, and epichlorohydrin from Fisher Scientific and

inert gas (e.g. argon, nitrogen, etc.) from Airgas were also

used. A Hitachi SU-70 Scanning Electron Microscope, an

X’Pert Pro Panalytical X-ray diffractometer, a Zeiss Libra

120 transmission electron microscope, and five-point BET

and BJH modeled from measurements made on both a Mi-

cromeritics ASAP 2020 Surface Area and Porosity Analyzer

and a Quantachrome Autosorb 6 system were used to char-

acterize the spheres throughout the synthesis process. A

basic flowchart for this synthesis process is shown in Fig. 1.

2.2 Acid-catalyzed synthesis of SiO2 mesoporous

spheres

To synthesize SiO2 mesoporous spheres, PS spheres were

washed with ethanol, dried in air at 80 �C for at least 12 h,

and allowed to cool to room temperature. 0.4 g of washed

PS spheres were submerged in 0.6 ml of tetraethyl ortho-

silicate (TEOS) diluted with 2.0 ml of ethanol. 0.1 ml of

1 mol l-1 hydrochloric acid was then added as a catalyst.

The spheres in solution were immediately put in an ultra-

sonic bath for 5 min to allow the solution to fully penetrate

the spheres during the reaction. The remaining fluid was

drawn from the container using a syringe with a high-gauge

needle. The spheres were again placed in an oven set below

90 �C for 12 h to dry completely.

Dry SiO2-infiltrated spheres were placed in an open air

furnace which was brought to 300 �C over 5 h. This tem-

perature was maintained for 5 h to allow for the complete

oxidation and/or decomposition of the PS template. The

temperature was then brought to 550 �C over 5 h and

maintained for 24. The spheres were allowed to cool to

ambient temperature.

2.3 Base-catalyzed synthesis of SiO2 mesoporous

spheres

Base-catalyzed hydrolysis of alkoxides was also success-

fully attempted [23]. For this method, 0.4 g of washed PS

spheres were submerged in 5 ml of tetramethyl orthosili-

cate (TMOS) diluted with 1 ml of ethanol. 20 ll of dim-

ethylethylamine was then added as a base catalyst. The

same sonication and drying process used for the acid-cat-

alyzed hydrolysis of TEOS was then used.

J Sol-Gel Sci Technol

123

Author's personal copy

Like for the acid-catalyzed synthesis method, dry SiO2-

infiltrated spheres were placed in an open air furnace which

was brought to 300 �C over 5 h. This temperature was

maintained for 5 h to allow for the complete oxidation and/

or decomposition of the PS template. The temperature was

then brought to 550 �C over 5 h and maintained for 24. The

spheres were allowed to cool to ambient temperature.

2.4 Synthesis of SiO2 mesoporous spheres with Ag

nanoparticle dispersion

0.2 g of washed PS spheres were soaked for 12 h in a 3 ml

solution of AgNO3 in ethanol. The solution had a con-

centration of 0.0235 mol l-1 of AgNO3 in ethanol. After

soaking, the PS spheres were infiltrated with SiO2 as

described above.

2.5 Synthesis of Si mesoporous spheres

To create Si mesoporous spheres, preformed

*200–450 lm SiO2 mesoporous spheres were evenly

distributed in a crucible filled with three parts finely grated

magnesium to one part spheres by volume. That dispersion

was then heated to 140 �C over 30 min in a tube furnace

under Ar flow. This temperature was maintained for 1 h.

The temperature was then brought to 675 �C over 7 h and

maintained for 3 h. The resulting spheres were allowed to

cool to ambient temperature. The resulting magnesiother-

mic reaction gives rise to Si and silicate byproducts, which

were removed with subsequent HCl and HF washes.

2.6 Synthesis of SiC mesoporous spheres

SiC mesoporous spheres were prepared using the infiltra-

tion process with subsequent acid-catalyzed hydrolysis

used for the synthesis of SiO2 mesoporous spheres. Once

the infiltrated spherical templates were completely dry,

they were poured into a ceramic crucible and heated in a

tube furnace in inert atmospheric conditions to decompose

the PS template and leaving spheres of SiO2. The furnace

was placed under Ar flow and the temperature was brought

to 425 �C over 3 h. This temperature was maintained for

10 h, after which the spheres cooled to ambient tempera-

ture. The resulting spheres were mixed with finely grated

Mg powder in a ratio of 1:3 v/v and placed back into the

ceramic crucible. The crucible was repositioned in the

furnace, and the furnace was again placed under Ar flow.

The furnace was brought to 700 �C over 7 h and the

temperature maintained for 12 h, after which the spheres

cooled to ambient temperature. This synthesis method

required the two-phase heating process.

2.7 Synthesis of mesoporous Dy2O3 spheres

To synthesize Dy2O3 spheres, PS spheres were washed with

ethanol, dried in air at 80 �C for at least 12 h, and allowed to

cool to room temperature. 0.5 g of washed PS spheres were

submerged in 0.5 g of dysprosium (III) chloride hexahy-

drate diluted with 15.0 ml of ethanol. The spheres in solu-

tion were immediately put in an ultrasonic bath for 5 min to

allow the solution to fully penetrate the spheres during the

reaction. The spheres still submerged in solution were then

placed in the freezer for 120 min. This is to slow the reaction

rate upon the addition of the epichlorohydrin catalyst. After

the 2 h of cooling time, 0.05 ml of epichlorohydrin was

added to the solution and the spheres in solution immedi-

ately put into an ultrasonic bath for 5 min. The remaining

fluid was drawn from the container using a syringe with a

high-gauge needle. The spheres were again placed in an

oven set below 90 �C for 12 h to dry completely.

Fig. 1 Flowchart of fabrication

basics for SiO2, SiO2 ? Ag

nanoparticles, Si, and SiC

mesoporous spheres as well as

for solid Dy2O3 spheres

J Sol-Gel Sci Technol

123

Author's personal copy

Dry infiltrated spheres were placed in an open air fur-

nace which was brought to 300 �C over 10 h. This tem-

perature was maintained for 5 h to allow for the complete

calcination of the PS template. The temperature was then

brought to 600 �C over 6 h and maintained for 24 h. The

spheres were allowed to cool to ambient temperature.

3 Results and discussion

SiO2 spheres, created using either an acid- or base-cata-

lyzed hydrolysis method, can be used to produce either Si

or SiC mesoporous spheres. The atmosphere under which

the silica-infiltrated PS spheres are heated determines if the

PS template is completely calcinated or degraded into

residual carbon. The SiO2 spheres or SiO2 spheres with PS-

derived carbon give rise to either Si or SiC spheres,

respectively, with the addition of Mg and further heating in

an inert atmosphere. The addition of AgNO3 to the initial

sol–gel solution can be used to produce SiO2 ? Ag nano-

particle mesoporous spheres. The surface areas of these

spheres calculated from nitrogen physisorption measure-

ments are provided in Table 1. Beginning from the first

step of the synthesis methods, a better understanding of this

infiltration method may be gained. First, Amberlite poly-

styrene (PS) spherical templates with a diameter size dis-

tribution between about 500 and 800 lm were washed with

ethanol, which increased the specific surface area of the

templates from 810 to 924 m2 g-1. The associated iso-

therm plots are shown Figure S1. SEM analysis indicated

that washing did not change the sphere morphology (Fig-

ures S2.a, S2.b) but did increase the porosity (Figures S2.c,

S2.d), which can likely be attributed to the removal of

oligomers.

Polystyrene spheres were infiltrated with an ethanol

solution of a silicon alkoxide, water and an acid or base

catalyst, as described in the experimental section. At this

Table 1 Measured surface areas and average pore diameter of the

mesoporous spherical materials in the course of the synthesis method

beginning with unwashed polystyrene mesoporous spheres and fin-

ishing with Si, SiC, SiO2, and SiO2 ? Ag nanoparticle mesoporous

spheres

Material Surface area

[m2 g-1]

Average pore

size [nm]

Unwashed PS 810 9

Washed PS 924 9

Acid-catalyzed SiO2:

infiltrated PS

109 N/A

Base-catalyzed SiO2:

infiltrated PS

33 N/A

SiO2 698 28

SiO2 ? Ag nanoparticles 219 28

Si 160 4

SiC 245 23

Fig. 2 Nitrogen physisorption isotherm of SiO2 spheres synthesized

using acid catalysis. According to the BET model, a specific surface

area of 698 m2 g-1 was calculated

Fig. 3 SEM image of SiO2 mesoporous spheres produced using the

acid-catalysis infiltration method. a Low magnification image shows

the size distribution is between *200 and 450 lm. b High magni-

fication image shows that the surface of the spheres is porous

J Sol-Gel Sci Technol

123

Author's personal copy

point, the specific surface area of the infiltrated spherical

templates was found to be 109 and 33 m2 g-1 for acid- and

base-catalyzed reactions, respectively. The corresponding

isotherm plots are reported in Figure S3. SEM images were

taken of infiltrated spheres to compare them to the washed

and unwashed polystyrene spheres. The spheres’ mor-

phology was not altered by the infiltration process, as

shown in Figure S4.

The PS template was removed through a calcination

process in air, as described in the experimental section.

After heating in air, the resulting spheres were translucent

and showed an average shrinkage of 50 %. X-ray diffrac-

tion (XRD) was taken of the translucent spheres, and the

spectra exhibited a single broad reflex centered around

2h = 22� as seen in Figure S5, which is consistent with

amorphous SiO2. Though the spheres did shrink, the sur-

face area of the SiO2 spheres was determined to be

698 m2 g-1 using the acid-catalyzed method, as calculated

using the BET model from the isotherm shown in Fig. 2.

Scanning electron microscopy (SEM) revealed that the

spherical geometry was not altered by the fabrication, as

seen in Fig. 3a, and revealed a particle size distribution of

Fig. 4 Nitrogen physisorption analysis of SiO2 ? Ag nanoparticle

spheres synthesized using base catalysis. The specific surface area

according to the BET model was calculated to be 219 m2 g-1. a Plot

of the isotherm as measured during adsorption and desorption of N2

into and from the sample. b Plot of the pore size distribution (BJH

model from the desorption branch) showing a mean pore diameter of

28 nm

J Sol-Gel Sci Technol

123

Author's personal copy

between *200 and 450 lm. SEM higher magnification

imaging of the surfaces of the spheres revealed that the

spheres were mesoporous, as seen in Fig. 3b. As a test to

see if an oxygen atmosphere was necessary to produce

SiO2 spheres, infiltrated spheres were heated under Argon

flow instead of air, resulting in the production of shrunken

spheres with a surface area below the limit of detection of

our instrument (*5 m2 g-1). Figures S6.a and S6.b are

SEM images of these shrunken spheres, and the associated

isotherm plot is provided in Figure S7.

To show how our infiltration method could be applied to

the preparation of supported catalysts, SiO2 mesoporous

spheres were synthesized with Ag nanoparticles dispersed

within the SiO2 matrix using the method described in the

experimental section. These infiltrated spheres were light

grey, turning glassy and light brown upon subsequent sub-

limation. When compared to the bare SiO2 microspheres, the

surface area was found to be reduced from 698 to

219 m2 g-1 with a mean pore diameter of ca. 28 nm (BJH

method), as mentioned in Table 1 and shown in Fig. 4. To

confirm the existence of Ag nanoparticles within our spheres,

transmission electron microscopy (TEM) was used to take

diffraction, bright field, and dark field images of some of the

spheres ground to a powder. Bright field imaging, an

example of which may be seen in Fig. 5a, revealed dense

areas that were found to consist of crystalline material, as

shown by selected area diffraction (Fig. 5b). The dark field

image, taken from the (111) and (200) fcc [24] reflections, is

shown in Fig. 5c and shows the presence of Ag crystalline

aggregates.

To create Si mesoporous spheres, preformed

*200–450 lm SiO2 mesoporous spheres were heated in a

dispersion of magnesium, as described in the experimental

section. The resulting spheres were opaque and colored a

b Fig. 5 SiO2 ? Ag nanoparticle spheres crushed to a powder for

TEM analysis. a Bright field image of crushed spheres shows dark,

dense spots from Ag aggregates. b Diffraction ring spacing of crushed

spheres reveals fcc material [24] is present. c Dark field image taken

from the circled area of two innermost diffraction rings, which

correspond to the (111) and (200) Ag aggregates [24], shows bright

spots corresponding to the Ag nanoparticles

Fig. 6 XRD pattern from Si mesoporous spheres before and after

washing with HCl and HF. When compared with the PANalytical

X’Pert HighScore Plus XRD Si reference file 00-005-0565, these

reflexes are a near perfect match. Any small reflexes produced by the

unwashed spheres correspond to magnesium silicide (triangle, Mg2Si)

and magnesium silicates, specifically enstatite (plus sign, MgSiO3) or

forsterite (asterisk, Mg2SiO4) with JCPDS reference cards 00-034-

0673, 00-001-0773 and 00-001-1290 respectively

Fig. 7 Nitrogen physisorption analysis of Si spheres after washing.

The specific surface area according to the BET model was calculated

to be 160 m2 g-1. a Plot of the isotherm as measured during

adsorption and desorption of N2 into and from the sample. b Plot of

the pore size distribution (BJH model from the desorption branch)

showing a mean pore diameter of 4 nm

J Sol-Gel Sci Technol

123

Author's personal copy

heterogeneous red-brown, as is expected of Si. It was

expected that residual Mg by-products would be present in

the Si spheres, as is common for magnesiothermic reac-

tions [25]; however, X-ray diffraction (XRD) revealed a

diffraction pattern dominated by signals originating from

Si (Fig. 6), indicating that residual Mg compounds were

likely amorphous or present in minute amounts. The XRD

reflexes for present Mg compounds most closely matched

JCPDS reference reflexes for magnesium silicates as

opposed to magnesium oxides. The spheres were purified

by washing with 1 mol l-1 HCl to remove magnesium

silicates and were then washed with pure HF to remove

residual SiO2. After washing, the spheres were colored a

homogeneous red-brown. XRD performed after the wash-

ings showed that all detectable by-products had been

removed, leaving mesoporous Si spheres. BET analysis of

the resulting Si spheres (Fig. 7) provided a surface area

value of 160 m2 g-1, and BJH model calculations deter-

mined a mean pore size of 4 nm. SEM was used to

determine that the Si material had maintained its spherical

geometry, as seen in Fig. 8a, and no additional shrinkage

had occurred. Some spheres were intentionally broken

during sample preparation to determine if the infiltration

process had fully penetrated the template. It can be seen

that the centers of the spheres are uniform which confirms

that this process does work. When taking a closer look at

the surface of the spheres, their mesoporous nature can be

observed, as seen in Fig. 8b.

SiC mesoporous spheres were created using the SiO2

infiltration process, heated in an Ar atmosphere, and then

mixed with Mg and reheated in an Ar atmosphere, as

detailed in the experimental section. The resulting spheres

were opaque and colored a heterogeneous dark brown, as is

expected of SiC. XRD showed that the spheres consisted of

SiC and Mg by-products such as forsterite and enstatite

[25], as seen in Fig. 9. To remove the Mg by-products, the

spheres were washed with 1 mol l-1 HCl. XRD showed

that HCl washing did not completely remove the enstatite

phase, as seen in Fig. 9. The spheres were then washed in

hydrofluoric acid, which removed all residual Mg species

from the sample, as shown in Fig. 9. BET and BJH ana-

lysis of the resulting SiC spheres was used to determine

that the surface area was 215 m2 g-1 and the mean pore

size was 23 nm. The associated isotherm plot and pore size

distribution are shown in Fig. 10. Scanning electron

microscopy (SEM) revealed that SiC particles had spheri-

cal morphologies, as seen in Fig. 11a. The spheres were

mesoporous, as seen in Fig. 11b. During experimentation, a

test was performed where Mg was added to the infiltrated

Fig. 8 SEM image of Si mesoporous spheres. a Low magnification

image shows the size distribution is between *200 and 450 lm.

b High magnification image shows surface of the spheres is porous

Fig. 9 XRD of SiC mesoporous spheres before and after washing

with HCl and then HF. When compared with the PANalytical X’Pert

HighScore Plus XRD SiC reference file 00-001-1118, the diffraction

pattern of the washed spheres is near perfect match. Before washing,

there are extra reflexes in the pattern that correspond to magnesium

silicates, specifically enstatite (plus sign, MgSiO3) or forsterite

(asterisk, Mg2SiO4) with JCPDS reference cards 00-001-0773 and

00-001-1290, respectively

J Sol-Gel Sci Technol

123

Author's personal copy

spheres and the spheres not cooled between 425 and

700 �C. Mg began reacting with the spheres before the

polystyrene had sublimated, resulting in brittle spheres of

an inhomogeneous material, as can be seen in Figure S8.

While Si, SiO2 and SiC mesoporous spheres were suc-

cessfully synthesized, dysprosium oxide (Dy2O3) meso-

porous spheres were not successfully synthesized

regardless of many variations of method. While Dy2O3

spheres were eventually achieved using the method

described in the experimental section, the spheres lacked

significant porosity. As had previously been done in the

synthesis of aerogels, epichlorohydrin was used as the

catalyst to create Dy2O3 spheres [26]. One of the major

challenges of creating these spheres was the short reaction

time, inhibiting full penetration of the PS spherical tem-

plates. The initial result was spherical shells, as determined

from SEM (see Figure S9). By cooling the solution of

ethanol diluted DyCl3, the reaction time was sufficiently

slowed upon the addition of epichlorohydrin to fully

infiltrate the PS spherical templates, yielding solid spheres

with a surface area beyond the lower detection limit of our

porosity analysis instrument (*1 m2 g-1). XRD was

taken, and the nearly solid spheres were determined to be

comprised of Dy2O3 with no strongly detectible byprod-

ucts, as shown in Figure S10.

4 Conclusions

This paper describes the methods of synthesizing Si, SiO2,

SiO2 ? Ag nanoparticles, and SiC mesoporous spheres

using commercially available polystyrene templates based

on straightforward wet and solid state chemistry principles.

An infiltration method for the fabrication of mesoporous

spheres with a well-defined size distribution has been

shown. In addition, nearly solid Dy2O3 spheres were also

fabricated. With added Ag nanoparticles, these spheres

may be useful for the production of catalysts. In general,

the foundation for many applications of Si, SiO2, and SiC

Fig. 10 Nitrogen physisorption analysis of SiC spheres after wash-

ing. The specific surface area according to the BET model was

calculated to be 215 m2 g-1. a Plot of the isotherm as measured

during adsorption and desorption of N2 into and from the sample.

b Plot of the pore size distribution (BJH model from the desorption

branch) showing a mean pore diameter of 23 nm

Fig. 11 SEM image of SiC mesoporous spheres. a Low magnifica-

tion image shows the size distribution is between *200 and 450 lm.

b High magnification image shows surface of the spheres is porous

J Sol-Gel Sci Technol

123

Author's personal copy

mesoporous spheres has already been laid, and this simple

method of template infiltration provides an avenue by

which the implementation of these new technologies may

be made a reality. The Si, SiO2 and SiC mesoporous

spheres that were created must now be tested for their

electrical properties.

Acknowledgments This publication was made possible by an

NPRP award [NPRP 6-892-1-169] from the Qatar National Research

Fund (a member of The Qatar Foundation). We also thank the US

National Science Foundation for an instrumentation award (CHE-

1337700) which was used to purchase the Raman spectrometer used

for preliminary characterization. The statements made herein are

solely the responsibility of the authors.

References

1. Kirkland JJ (1976) Porous silica microspheres for high-performance

size exclusion chromatography. J Chromatogr 125:231–250. doi:10.

1016/S0021-9673(00)93822-6

2. Zhang M, Itoh T, Abe M (1997) Ultrasonic visualization of still

and flowing waters using contrast agents of magnetite-encapsu-

lated porous silica microspheres. Jpn J Appl Phys 36:243–246.

doi:10.1143/JJAP.36.243

3. Ye C, Chen A, Colombo P, Martinez C (2010) Ceramic micro-

particles and capsules via microfluidic processing of a preceramic

polymer. J R Soc Interface 7:S461–S473. doi:10.1098/rsif.2010.

0133.focus

4. Remy E, Picart S, Delahaye T, Jobelin I, Dugne O, Bisel I,

Blanchart P, Ayral A (2014) Fabrication of uranium dioxide

ceramic pellets with controlled porosity from oxide microspheres.

J Nucl Mater 448:80–86. doi:10.1016/j.jnucmat.2014.01.017

5. Liu R, Li T, Han F-D, Bai Y-J, Qi Y-X, Lun N (2014) Thermal

formation of porous Fe3O4/C microspheres and the lithium stor-

age performance. J Alloy Compd 597:30–35. doi:10.1016/j.jall

com.2014.01.218

6. Fan J-B, Huang C, Jiang L, Wang S (2013) Nanoporous micro-

spheres: from controllable synthesis to healthcare applications.

J Mater Chem B 1:2222–2235. doi:10.1039/c3tb00021d

7. Horikoshi S, Serpone N (2009) Photochemistry with microwaves:

catalysts and environmental applications. J Photochem Photobiol

C Photochem Rev 10(2):96–110. doi:10.1016/j.jphotochemrev.

2009.06.001

8. Huo Q, Feng J, Schuth F, Stucky GD (1997) Preparation of hard

mesoporous silica spheres. Chem Mater 9(1):14–17. doi:10.1021/

cm960464p

9. Comorettoa D, Marabellib F, Socib C, Gallib M, Pavarinib E,

Patrinib M, Andreanib LC (2003) Morphology and optical

properties of bare and polydiacetylenes-infiltrated opals. Synth

Met 139:633–636. doi:10.1016/S0379-6779(03)00324-2

10. Jongh P, Eggenhuisen T (2013) Melt infiltration: an emerging

technique for the preparation of novel functional nanostructured

materials. Adv Mater 25:6672–6690. doi:10.1002/adma.201301912

11. Gumennik A, Wei L, Lestoquoy G, Stolyarov AM, Jia X, Re-

kemeyer PH, Smith MJ, Liang X, Grena BJ-B, Johnson SG,

Gradecak S, Abouraddy AF, Joannopoulos JD, Fink Y (2013)

Silicon-in-silica spheres via axial thermal gradient in-fibre cap-

illary instabilities. Nature Commun. doi:10.1038/ncomms3216

12. Cheng J, Wang Y, Teng C, Shang Y, Ren L, Jiang B (2014)

Preparation and characterization of monodisperse, micrometer-

sized, hierarchically porous carbon spheres as catalyst support.

Chem Eng J 242:285–293. doi:10.1016/j.cej.2013.12.089

13. Sandhage KH (2010) Materials ‘‘alchemy’’: shape-preserving

chemical transformation of micro-to-macroscopic 3-D structures.

J Met 62:32–43. doi:10.1007/s11837-010-0085-8

14. Bao Z, Weatherspoon MR, Shian S, Cai Y, Graham PD, Allan

SM, Ahmad G, Dickerson MB, Church BC, Kang Z, Abernathy

HW III, Summers CJ, Liu M, Sandhage KH (2007) Chemical

reduction of three-dimensional silica micro-assemblies into

microporous silicon replicas. Nature 446:172–175. doi:10.1038/

nature05570

15. Chen K, Bao Z, Shen J, Wu G, Zhou B, Sandhage KH (2012) Free-

standing monolithic silicon aerogels. J Mater Chem 22:16196–16200.

doi:10.1039/C2JM31662E

16. Sieber H, Hoffmann C, Kaindl A, Greil P (2000) Biomorphic

cellular ceramics. Adv Eng Mater 2:105–109. doi:10.1002/

(SICI)1527-2648(200003)2:3\105:AID-ADEM105[3.0.CO;2-P

17. Sonnenburg K, Adelhelm P, Antonietti M, Smarsly B, Noske R,

Strauch P (2006) Synthesis and characterization of SiC materials

with hierarchical porosity obtained by replication techniques.

Phys Chem Chem Phys 8:3561–3566. doi:10.1039/B604819F

18. Wen CJ, Huggins RA (1981) Chemical diffusion in intermediate

phases in the lithium-silicon system. J Solid State Chem

37:271–278. doi:10.1016/0022-4596(81)90487-4

19. Tsai W-Y, Gao P-C, Daffos B, Taberna P-L, Perez CR, Gotosi Y,

Favier F, Simon P (2013) Ordered mesoporous silicon carbide-

derived carbon for high-power supercapacitors. Electrochem

Commun 34:109–112. doi:10.1016/j.elecom.2013.05.031

20. Yao Y, McDowell MT, Ryu I, Wu H, Liu N, Hu L, Nix WD, Cui Y

(2011) Interconnected silicon hollow nanospheres for lithium-ion

battery anodes with long cycle life. NanoLetters 11:2949–2954.

doi:10.1021/nl201470j

21. Kim M, Kim J (2014) Redox desorption of birnessite-type man-

ganese oxide on silicon carbide microspheres for use as superca-

pacitor electrodes. ACS Appl Mater Interfaces 6:9036–9045.

doi:10.1021/am406032y

22. Alper JP, Kim MS, Vincent M, Hsia B, Radmilovic V, Carraro C,

Maboudian R (2013) Silicon carbide nanowires as highly robust

electrodes for micro-supercapacitors. J Power Sources 230:298–302.

doi:10.1016/j.jpowsour.2012.12.085

23. Wingfield C, Franzel L, Bertino M, Leventis N (2011) Fabrication

of functionally graded aerogels, cellular aerogels and anisotropic

ceramics. J Mater Chem 21:11737–11741. doi:10.1039/C1JM1

0898K

24. Fultz B, Howe J (2002) Transmission electron microscopy and

diffractometry of materials. Springer, Berlin, New York

25. Franzel L, Wingfield C, Bertino M, Mahadik-Khanolkarb S, Leventis

N (2013) Regioselective cross-linking of silica aerogels with mag-

nesium silicate ceramics. J Mater Chem A 1(19):6021–6029. doi:10.

1039/C3TA90165C

26. Leventis N, Vassilaras P, Fabriziob EF, Dassc A (2007) Polymer

nanoencapsulated rare earth aerogels: chemically complex but

stoichiometrically similar core-shell superstructures with skeletal

properties of pure compounds. J Mater Chem 17:1502–1508.

doi:10.1039/b612625a

J Sol-Gel Sci Technol

123

Author's personal copy