Determination of size and ordering of pores in mesoporous silica using small angle neutron scattering

Article abstract of DOI:10.1016/j.physb.2004.04.033

Small angle neutron scattering (SANS) has been used to study the size and ordering of the pore structure of mesoporous silica. Mesoporous silica was synthesised in the presence of a 16 carbon chain surfactant, cetyl-trimethyl ammonium bromide (CTAB) using the sol-gel method. The SANS patterns of the mesoporous silica were measured on samples before and after calcination. Calcination at 600°C for 2h results in removal of the surfactant template. The SANS study revealed the presence of an ordered pore structure with pore spacing of approximately 3.5nm. After calcination the pore spacing decreased. Using the SANS technique it was possible to quantify the pore sizes and pore spacing and then determine the wall thickness.

Full text of DOI:10.1016/j.physb.2004.04.033

ARTICLE IN PRESS
Physica B 350 (2004) 224–226
Determination of size and ordering of pores in mesoporous
silica using small angle neutron scattering
a,
J. Connolly *, M. Singh , C.E. Buckley
b
a
a
Materials Research Group, Department of Applied Physics, Curtin University of Technology, GPO Box U1987, Perth 6845, Australia
b
NanoChemistry Research Institute, Department of Applied Chemistry, Curtin University of Technology, GPO Box U1987,
Perth 6845, Australia
Abstract
Small angle neutron scattering (SANS) has been used to study the size and ordering of the pore structure of
mesoporous silica. Mesoporous silica was synthesised in the presence of a 16 carbon chain surfactant, cetyl-trimethyl
ammonium bromide (CTAB) using the sol–gel method. The SANS patterns of the mesoporous silica were measured on
ꢀ
samples before and after calcination. Calcination at 600 C for 2 h results in removal of the surfactant template. The
SANS study revealed the presence of an ordered pore structure with pore spacing of approximately 3.5 nm. After
calcination the pore spacing decreased. Using the SANS technique it was possible to quantify the pore sizes and pore
spacing and then determine the wall thickness.
r 2004 Elsevier B.V. All rights reserved.
PACS: 61.12.Bt; 81.20.Fw
Keywords: Mesoporous silica; Small angle neutron scattering; MCM-41; Pore size; Pore spacing
1
. Introduction
surfaces of supramolecular surfactant assemblies
2,3]. The most well known of this class of
[
Synthesis of mesoporous silica exhibiting na-
materials is a hexagonal silica MCM-41 prepared
by using a cetyl-trimethyl ammonium bromide
(CTAB) surfactant as the template. On removal of
the template by calcination, a crystalline product is
obtained with hexagonal or cubic arrays [4,5].
This paper focuses on the use of small angle
neutron scattering (SANS) as a technique to study
the pore characteristics of mesoporous silica
produced using different drying methods. Small
angle scattering studies have been used to find the
interplanar spacing between the cylindrical pores.
The size of the pores should also contribute to the
noscale periodicity resulting from pore ordering
has been a major technological advance in the last
decade [1]. The evolution of a nanoscale ordering
in these materials occurs through the formation of
a surfactant/inorganic mesophase during hydro-
lysis and cross-linking of inorganic precursors at
*Corresponding author. Tel.: +61-8-9266-3458; fax: +61-8-
9
266-2377.
E-mail address: J.Connolly@exchange.curtin.edu.au
J. Connolly).
(
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2004.04.033

ARTICLE IN PRESS
J. Connolly et al. / Physica B 350 (2004) 224–226
225
scattering pattern in the small angle limit. The
contribution to the scattering intensity from the
size and shape of the pores can be modelled
numerically using known models. The size, shape
and ordering of the pores are important para-
meters in the design of mesoporous materials and
the use of the SANS technique to investigate these
factors is discussed.
Facility (AUSANS). The q range of this instru-
ꢁ1
(
ment is 0.013–0.23 A
with the detector in an
offset position. The data was reduced and radially
averaged using standard methods. The data was
converted to an absolute scale by calibration with
a standard silica sample of known differential
cross-section.
2.3. Modelling of data
2
. Materials and methods
The data was modelled with the addition of a
power-law model and a cylindrical form factor
with a hard sphere interaction structure factor.
The scattering intensity then become:
2
.1. Production of silica
Mesoporous silica was prepared by the sol–gel
I ¼ Pðq; r; LÞSðq; rhs; ZhsÞ þ Aqꢁ þ b:
m
ð1Þ
route. An aqueous solution of quaternary ammo-
nium surfactant cetyl-trimethyl ammonium bro-
mide (CTAB) was added dropwise to a mixture of
tetraethyl orthosilicate (TEOS) and ethanol at a
pH in the range of 1–2 (HCl), with vigorous
Here, Aqꢁ is the power-law scattering, b
m
is the background scattering, Sðq; rhs; Z Þ is
hs
the hard sphere structure factor for spheres of
radius r and volume fraction, Z : Pðq; r; LÞ ¼
ꢀ
hs
hs
stirring at room temperature (20 C). The molar
2
2
2
p
ZF ðq; r; LÞDr V where Fðq; r; LÞ is the form
ratios
.0:0.27:10.0:9.09, respectively. Hydrolysis of
TEOS gave a clear sol of silica. The resulting sol
of
TEOS:CTAB:H O:ethanol
were
2
factor for scattering from a cylinder, Dr is the
scattering length density difference between the
1
ꢀ
ꢀ
pore and the silica, V is the pore volume and Z is
p
was aged at 40 C for 12 h and then fired at 600 C
for 2 h to remove the surfactant and to achieve
final sintering.
An overview of pore sizes and spacings of silica
powders studied is given in Table 1. Pore forma-
tion occurs after calcination. The micelles that are
present before calcination are not true pores but
will be referred to as pores in this paper.
the volume fraction of pores. The modelling was
performed with the Igor Pro software package
with model macros provided by NIST [6]. The
macros were modified to allow rhs to be linearly
related to the cylinder radius.
A cylindrical form factor was chosen as previous
electron microscopy studies have shown that the
pores formed in MCM-41 silica are cylindrical and
ordered [7]. This is believed to be due to the
surfactant micelle forming a hexagonal array of
micellar rods in solution [8]. The hard sphere
structure factor is used to describe the spacing
2
.2. Small angle neutron scattering
SANS experiments were performed at the
Australian Small Angle Neutron Scattering
between the pores. The radius, r , is the distance
hs
Table 1
Diameters and spacing of pores in mesoporous silica powders
as obtained from modelling of SANS data
of closest approach of the pores. In this model, rhs
is a multiple of the pore radius to account for the
presence of a regular silica wall between the pores.
Sample
Surfactant
Drying
temperature
Pore
diameter
(
(A)
Pore
spacing
(
(A)
ꢀ
(
C)
3. Results
0C
0D
2C
2D
—
—
600
40
—
—
—
—
A broad peak was observed at approximately
ꢁ
CTAB
CTAB
600
40
15.070.3
14.470.2
3471
3971
1
q=0.2 A for the silica produced in the presence
(
ꢁ
1
(
of a surfactant molecule (Fig. 1). At qo0.5 A

ARTICLE IN PRESS
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J. Connolly et al. / Physica B 350 (2004) 224–226
Calcination of the powders to remove the
2
organic matter appears to decrease the silica wall
thickness between the pores. This is most likely
due to densification and sintering of silica. In
addition, the hard sphere interaction factor may
not fully describe interaction potential between
pore volumes. As it neglects the length of the pore
completely, it can only be an approximation to the
true interaction between the pores. This may have
an effect on the pore spacing calculated using the
model.
1
08
6
4
2
18
6
4
2
5. Conclusions
0
.1
2
3
4
5
6
1
)
7
8 9
0
2
The diameter of the pore and the pore spacing
.1
-
can be determined using modelling of SANS data.
Pore wall thickness can be calculated from these
values. However, the technique needs to be further
developed to be a reliable method for accurately
finding the pore characteristics.
q (Å
Fig. 1. Radially averaged SANS data from calcined (X) and
uncalcined (’) mesoporous silica powders. The lines through
the data are the modelled fit to the data.
the SANS patterns of all the silica powders
displayed power law behaviour.
Acknowledgements
The authors are grateful to the Australian
Institute of Nuclear Science and Engineering for
access to the AUSANS facility.
3
.1. Effect of calcination of pore ordering
ꢀ
Calcining the silica at 600 C for 2 h is expected
to completely remove any organic material from
the silica. The pore spacing decreases after the heat
treatment (Table 1). There also appears to be an
increase in pore size after calcination. From the
results of the modelling of the SANS data, as
shown in Table 1, the pore wall thickness was
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(
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4
. Discussion
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