Two examples of phase transition in self-assembled mesostructured hybrid thin

films are reported. The materials have been synthesized using tetraethoxysilane

as the silica source hydrolyzed with or without the addition of methyltriethoxysilane.

The combined use of transmission electron microscopy, small-angle X-ray

scattering and computer simulation has been introduced to achieve a clear

identification of the organized phases. A structural study of the self-assembled

mesophases as a function of thermal treatment has allowed the overall phase

transition to be followed. The initial symmetries of mesophases in as-deposited

films have been linked to those observed in samples after thermal treatment.

The monodimensional shrinkage of silica films during calcination has induced a

phase transition from face-centered orthorhombic to body-centered cubic. In

hybrid films, instead, the phase transition has not involved a change in the unit

cell but a contraction of the cell parameter normal to the substrate

Amenitsch, Heinz

Bello, Valentina

Costacurta, Stefano

Falcaro, Paolo

Guglielmi, Massimo

Innocenzi, Plinio

Kidchob, Tongjit

Malfatti, Luca

Mattei, Giovanni

Two examples of phase transition in self-assembled mesostructured hybrid thin
films are reported. The materials have been synthesized using tetraethoxysilane
as the silica source hydrolyzed with or without the addition of methyltriethoxysilane.
The combined use of transmission electron microscopy, small-angle X-ray
scattering and computer simulation has been introduced to achieve a clear
identification of the organized phases. A structural study of the self-assembled
mesophases as a function of thermal treatment has allowed the overall phase
transition to be followed. The initial symmetries of mesophases in as-deposited
films have been linked to those observed in samples after thermal treatment.
The monodimensional shrinkage of silica films during calcination has induced a
phase transition from face-centered orthorhombic to body-centered cubic. In
hybrid films, instead, the phase transition has not involved a change in the unit
cell but a contraction of the cell parameter normal to the substrate

P. INNOCENZI

L. MALFATTI

T. KIDCHOB

P. FALCARO

H. AMENITSCH

COSTACURTA, STEFANO

GUGLIELMI, MASSIMO

MATTEI, GIOVANNI

BELLO, VALENTINA

Archivio istituzionale della ricerca - Università di Padova

Thermal induced phase transitions in selfassembled mesostructured films studied by small angle X-ray scattering

UnissResearch

nanobioscience
734 doi:10.1107/S0909049505014585 J. Synchrotron Rad. (2005). 12, 734–738
Journal of
Synchrotron
Radiation
ISSN 0909-0495
Received 15 October 2004
Accepted 7 May 2005
# 2005 International Union of Crystallography
Printed in Great Britain – all rights reserved
Thermal-induced phase transitions in self-assembled
mesostructured films studied by small-angle X-ray
scattering
Plinio Innocenzi,a* Luca Malfatti,a Tongjit Kidchob,a Paolo Falcaro,b Stefano
Costacurta,b Massimo Guglielmi,b Giovanni Mattei,c Valentina Belloc and
Heinz Amenitschd
aLaboratorio di Scienza dei Materiali e Nanotecnologie, Nanoworld Institute, Dipartimento di
Architettura e Pianificazione, Universita` di Sassari, Palazzo Pou Salid, Piazza Duomo 6, 07041
Alghero (Sassari), Italy, bDipartimento di Ingegneria Meccanica, Settore Materiali, Universita` di
Padova, via Marzolo 9, 35131 Padova, Italy, cDipartimento di Fisica ‘Galileo Galilei’, Universita` di
Padova, via Marzolo 8, 35131 Padova, Italy, and dInstitute of Biophysics and X-ray Structure
Research, Austrian Academy of Sciences, Schmiedelstraße 6, A-8042, Graz, Austria.
E-mail: plinio@uniss.it
Two examples of phase transition in self-assembled mesostructured hybrid thin
films are reported. The materials have been synthesized using tetraethoxysilane
as the silica source hydrolyzed with or without the addition of methyltriethoxy-
silane. The combined use of transmission electron microscopy, small-angle X-ray
scattering and computer simulation has been introduced to achieve a clear
identification of the organized phases. A structural study of the self-assembled
mesophases as a function of thermal treatment has allowed the overall phase
transition to be followed. The initial symmetries of mesophases in as-deposited
films have been linked to those observed in samples after thermal treatment.
The monodimensional shrinkage of silica films during calcination has induced a
phase transition from face-centered orthorhombic to body-centered cubic. In
hybrid films, instead, the phase transition has not involved a change in the unit
cell but a contraction of the cell parameter normal to the substrate.
Keywords: SAXS; self-assembly; mesoporous films; phase transition.
1. Introduction
Synchrotron small-angle X-ray scattering (SAXS) has been
widely used, in transmission and in grazing-incidence config-
urations, to study self-assembled mesostructured films
deposited via dip-coating (Grosso et al., 2002; Soler-Illia et al.,
2002; Cagnol et al., 2003). The technique has revealed to be
particularly effective for studying in situ the self-assembly
mechanisms through which, driven by the fast evaporation of
the solvent (evaporation-induced self-assembly, EISA) (Lu et
al., 1997), the organic micelles assemble to give different
organized mesophases (Klotz et al., 2000; Grosso et al., 2003;
Doshi et al., 2003).
The formation of the final mesostructure can be divided into
two different stages. The first takes place during dip-coating:
the micelles organize, and the solvent evaporation induces a
phase transition connected with the reorganization of the
micelles (Cagnol et al., 2003). In this phase the withdrawal
speed and the relative humidity in the deposition chamber
dramatically influence the spatial disposition of the micelles.
The second stage is observed after the deposition, when drying
or thermal calcination generally induces a phase transforma-
tion (Klotz et al., 2000). This transition, which is due to the
uniaxial shrinkage of the film, results in a loss of symmetry and
the distortion of the unit cell (Besson et al., 2003; Soler-Illia et
al., 2002; Kundu et al., 1998; Falcaro et al., 2004). The extent
and type of this phase transition is very much dependent on
the synthesis parameters, but the templating agent also plays a
very important role. Micelles formed by ionic surfactants, such
as cetyltrimethylammonium bromide (CTAB), have proper-
ties that can be closely compared with ‘hard closed spheres’.
Tri-block copolymer surfactants form instead micelles with the
long polyethylene oxide chains on the surface of a dense core
formed by the hydrophobic polypropylene blocks. The
micelles formed by three-block copolymers are, therefore,
softer and are more easily deformed (McConnell & Gast,
1996). Another important difference is given by the lower
temperature necessary to remove the block copolymers with
respect to ionic surfactants. This is reflected in a lower degree
of condensation of the pore walls and a higher tendency to
deformation. Whilst this effect is well documented in self-
assembled films, a clear identification of the phase involved is
not easy to achieve. In general, the indexing and identification
of the phases is obtained by the distances and the angles of the
observed diffraction spots from SAXS. This can be, however,
quite often a source of error if the data are not properly
processed.
In the present work we show that SAXS, TEM and
computer simulation can be used to define a reliable way of
identifying the porous mesostructure in amorphous inorganic
and hybrid films (Falcaro et al., 2005). An example of meso-
structured films with orthorhombic and tetragonal final
symmetry will be shown.
2. Samples preparation
A stock solution was prepared by adding (in the following
order) ethanol (EtOH), TEOS, water and HCl in the molar
ratios TEOS:EtOH:H2O:HCl = 1:2.78:1.04:56.2  103. The
sol was left to react under stirring for 1 h at room temperature.
A second solution was prepared by dissolving 1.3 g Pluronic
F-127 in 15 cm3 EtOH and 1.5 cm3 acid aqueous solution.
Pluronic F-127 block copolymers (EO106–PO70–EO106) were
obtained from Aldrich and used as received. The final sol was
obtained by adding 7.7 cm3 of the stock sol to the solution
containing the block copolymer. The final solution, with
molar ratios TEOS:EtOH:H2O:HCl:Pluronic F127 = 1:16.3:
5.4:56.5  103:5  103, was reacted under stirring for 24 h
and then was aged at 301 K for one week. The same procedure
was followed using TEOS and MTES in the stock solution.
The final molar ratios of the optimized MTES–TEOS mixture
were set at TEOS:MTES:EtOH:H2O:HCl:F127 = 1:0.56:24.48:
14.70:0.11:7.6  103.
The silicon wafers, previously cleaned with hydrofluoric
acid, water, EtOH and acetone, were dip-coated using the two
different solutions. The dip-coating system was a home-built
model, specifically designed to be employed for in situ
measurements to guarantee high withdrawal stability. The
substrate was mounted on a fixed rod, while the beaker
containing the solution was moved up and down by a move-
able stage connected to an electrical engine. In this way the
sample remained fixed and the solution was raised and
lowered during the dip-coating procedure. The relative
humidity in the deposition chamber was set at 40%; the
withdrawal speed was set at 15 cm min1. After the deposition
the films were maintained in the deposition chamber for
10 min and then heated at 423 K for 1 h in air. The firing
process was carried out in sequence; that is, all of the samples
were fired at 333 K, 473 K, 673 K up to the last thermal step at
873 K. The mesostructure, in films deposited on silicon
substrates, was investigated using the high-flux grazing-inci-
dence small-angle X-ray scattering (GI-SAXS) apparatus at
the Austrian high-flux beamline of the electron storage ring
ELETTRA (Trieste, Italy) (Amenitsch et al., 1998), taking for
each image the average of ten single acquisitions with 3 s of
exposition time. The instrumental grazing angle was set
maintaining an incident X-ray beam (wavelength 1.54 A˚)
smaller than 3. From the recording of the CCD detector
(1024  1024 pixels, Photonic Science) the out of plane
diffraction maxima were observed. The distance between
sample and detector was calibrated with the diffraction
pattern of a salt having a known lattice constant [silver
behenate, H(CH2)21COOAg]: it was possible to obtain this
distance (991 mm) with a good precision (1 mm). The beam
centre position was set from the least-squares fit of a circle of
40 entered coordinates, corresponding to 40 positions in the
silver behenate powder diffraction ring.
Successively, a radial integration of the image intensity was
performed, starting from the beam centre (000). Typically, the
whole image was masked leaving out the peak of interest and
integration was successively made. This procedure allowed us
to minimize the peak position errors caused by the overlap of
information in diffraction patterns. Knowing the distance
between sample and detector, it was possible to calculate 2
and dhkl for each spot and the lattice constants. Supposing that
the radially integrated spot intensities fit a Gaussian curve, the
uncertainty on the d-spacing, calculated from the intensity
maximum, was estimated to be 6% by the FWHM of
the curve. FIT2D was used (A. P. Hammersley, ESRF) to
analyze the SAXS images.
3. Identification of the mesophases in self-assembled
films
Transmission and grazing-incidence SAXS represents a
powerful tool for identifying the porous organized phase in
self-assembled mesoporous films. This technique has been
widely applied both in situ during film formation and to study
the material evolution during thermal processing. The
deposition by evaporation-induced self-assembly of a meso-
structured film is followed by a drying process, generally
performed at low temperature (333 K) to evaporate the
solvent, and a thermal treatment to stabilize the structure. This
treatment can be used at the same time to remove the organic
template from the structure by thermal calcination. After
drying, the pores are in fact still filled with the surfactant
whose removal can cause a partial collapse of the meso-
structure if the pore walls are not thick enough or if their
degree of condensation is still low. In Fig. 1 the FTIR
nanobioscience
J. Synchrotron Rad. (2005). 12, 734–738 Plinio Innocenzi et al.  Thermal-induced phase transitions 735
Figure 1
FTIR absorption spectra of MTES–TEOS films after drying at 333 K and
after thermal calcination at 473 K.
absorption spectra of self-assembled films after drying (333 K)
and calcination at 473 K in air are shown. This temperature is
high enough to remove most of the surfactant, because block
copolymers are easily degraded even at relatively low
temperatures. In the wavenumber range of Fig. 1, the typical
fingerprint of C—H vibrations in Pluronic is observed (Su et
al., 2002). The reduction in absorption area at 473 K indicates
that around 85% of the organic template has been removed.
At this temperature, however, the pore walls are not rigid
enough to sustain the porous structure. Therefore mono-
dimensional shrinkage follows calcination, because the
changes in the directions parallel to the substrates are
hindered by the adhesion of the film (Yu et al., 2003). This
effect not only causes a pure shrinkage of the films, with a
reduction in thickness and distortion of pores shape; the
transformation induced by the thermal treatment causes, in
fact, a distortion of the network structure that allows the
identification of a new porous phase. Whilst the formation of
the mesophase during EISA is ruled by a complex interaction
of several parameters, and epitaxic relationships have been
supposed to explain the formation of two-dimensional hexa-
gonal phases from cubic phases (Grosso et al., 2003; Yao
et al., 2000), the transitions from dried to calcined films
are restricted to those allowed by group theory (Falcaro
et al., 2004).
The identification of the mesophase is, however, quite
difficult to achieve even in the dried and calcined films, where
a stationary condition is reached. The difficulty is essentially
arisen by the limited number of diffraction spots, splitting of
spots owing to grazing-incidence geometry that induces
instrumental effects and a lower organization of the film. Fig. 2
shows the SAXS images obtained from films dried at 333 K
and calcined at 873 K. Two different sets of samples are
shown, the first one obtained from pure TEOS sols which give
inorganic (SiO2) films after thermal treatment (Figs. 2a and
2b), and the second one from MTES–TEOS sols which give
hybrid organic inorganic films (Figs. 2c and 2d). All the images
are in grazing incidence with the exception of Fig. 2(a) which is
taken in transmission mode. This is, in fact, the only instru-
mental configuration that allows the identification of a cubic
phase. The silica sample (Figs. 2a and 2b) shows a lower degree
of organization, with porous domains within the structure, as
also observed by TEM.
Fig. 3 shows the TEM bright-field image of a cross section of
a silica film (TEOS samples). The film was calcined at 773 K
and the pores, owing to the uniaxial shrinkage, exhibit an
elliptical shape. The thickness of the film was 385  5 nm and
the measured pore dimensions were 9.3 nm (main axis) and
4.2 nm. The image shows that the periodicity of matter is given
by organized porous domains as opposed to crystalline
materials where the periodicity is given by the ordered
disposition of atoms in a lattice.
The SAXS images were used to identify the mesophases in
the dried films (333 K) and after calcination (873 K). The
phase transitions observed in the TEOS and MTES–TEOS
films are shown in Fig. 4. The TEOS samples show a thermal-
induced phase transformation from cubic, Im3m (in the space
group), to orthorhombic, Fmmm, whilst the hybrid MTES–
TEOS films exhibit a transition from body-centered tetra-
gonal, I4/mmm (with the cell parameter c orthogonal to the
substrate, so that a < c), to tetragonal, I4/mmm, with a > c. It is
interesting to observe that the substitution of a part of the
silica precursor (tetraethoxysilane) with methyltriethoxysilane
nanobioscience
736 Plinio Innocenzi et al.  Thermal-induced phase transitions J. Synchrotron Rad. (2005). 12, 734–738
Figure 2
SAXS images of self-assembled films obtained from TEOS (a) and (b) or
MTES–TEOS sols (c) and (d). The figure shows GISAXS images of films
calcined at 873 K (b) and (d), or after drying at 333 K (a) and (c). The
image (a) is shown in transmission to allow identification of the cubic
phase. In the image (d) the diffraction spots corresponding to the 040, 202
and equivalent label have been added from a measure obtained
increasing the exposition time.
Figure 3
TEM bright-field image of a cross section of a silica film after calcination
at 773 K (TEOS samples).
(which contains a non-hydrolyzable CH3 group covalently
bonded to the Si atom) yields a different mesophase and
different phase transitions.
The attribution of these phases has been obtained taking
into account the allowed phase transitions from group theory
and simulating, through an iterative process, the structure
using the programme CMPR (http://www.ncnr.nist.gov/
programs/crystallography/software/cmpr). Fig. 5 shows an
example of the correspondence between the simulated and the
measured GISAXS images in MTES–TEOS (Figs. 5a and 5b)
and TEOS films (Figs. 5c and 5d) calcined at 473 K. The use of
different precursors in the starting solution (with different
kinetics of policondensation) involves the formation of two
phases with different space groups. Moreover, Fig. 5 clearly
shows how the presence of MTES–TEOS influences not only
the mesostructure but also the dimension of the ordered
domains: the spots in Fig. 5(b) have a less diffuse and more
regular shape. The comparison of SAXS and GISAXS images
with the simulated structures has also shown to be a reliable
procedure for phase identification for mesostructured mate-
rials. Its application, as confirmed by TEM analysis, has
allowed an unambiguous description of the mesophase
observed in self-assembled films.
4. Conclusions
In this article we have presented an application of transmis-
sion and grazing-incidence small-angle X-ray scattering to
characterize the phase transformation in self-assembled films.
These transitions have been unambiguously identified
combining SAXS analysis with computer simulation and
transmission electron microscopy; these analytical tools are
usually employed to study crystalline structures instead of
amorphous materials with ordered spatial distribution of
pores such as mesostructured porous films.
FIRB Italian projects are acknowledged for financial
support (FIRB contract no. RBNE01P4JF). Professor
Roberto Paroni (University of Sassari) is gratefully acknowl-
edged for helpful discussions. Michele Rossi and Ing. Michele
Ingrassi are acknowledged for the support given to fabricate
the dip-coating system.
References
Amenitsch, H., Rappolt, M., Kriechbaum, M., Mio, H., Laggner, P. &
Bernstorff, S. (1998). J. Synchrotron Rad. 5, 506–508.
Besson, S., Ricolleau, C., Gacoin, T., Jacquiod, C. & Boilot, J.-P.
(2003). Microp. Mesop. Mater. 60, 43–49.
Cagnol, F., Grosso, D., Soler-Illia, G. J. A. A., Crepaldi, E. L.,
Babonneau, F., Amenitsch, H. & Sanchez, C. (2003). J. Mater.
Chem. 13, 61–66.
Doshi, D. A., Gibaud, A., Goletto, V., Lu, M., Gerung, H., Ocko, B.,
Han, S. M. & Brinker, C. J. (2003). J. Am. Chem. Soc. 125, 11646–
11655.
Falcaro, P., Costacurta, S., Mattei, G., Amenitsch, H., Marcelli, A.,
Cestelli Guidi, M., Piccinini, M., Nucara, A., Malfatti, L., Kidchob,
T. & Innocenzi, P. (2005). J. Am. Chem. Soc. 127, 3838–3846.
Falcaro, P., Grosso, D., Amenistch, H. & Innocenzi, P. (2004). J. Phys.
Chem. B, 108, 10942–10948.
Grosso, D., Babonneau, F., Sanchez, C., Soler-Illia, G. J. A. A.,
Crepaldi, E. L., Albouy, P. A., Amenitsch, H., Balkenende, A. R. &
Brunet-Bruneau, A. (2003). J. Sol-Gel Sci. Technol. 26, 561–565.
Grosso, D., Babonneau, F., Soler-Illia, G. J. A. A., Albouy, P. A. &
Amenitsch, H. (2002). Chem. Commun. pp. 748–749.
Klotz, M., Albouy, P. A., Ayral, A., Me´nager, C., Grosso, D., Van der
Lee, A., Cabuil, V., Babonneau, F. & Guizard, C. (2000). Chem.
Mater. 12, 1721–1728.
Kundu, D., Zhou, H. S. & Honma, I. (1998). J. Mater. Sci. Lett. 17,
2089–2092.
nanobioscience
J. Synchrotron Rad. (2005). 12, 734–738 Plinio Innocenzi et al.  Thermal-induced phase transitions 737
Figure 5
Correspondence between the simulated structure (calculated using the
programme CMPR) and the measured GISAXS images in MTES–TEOS
(a) and (b), and TEOS films (c) and (d), calcined at 473 K.
Figure 4
Drawing of the phase transitions observed in dried (333 K) TEOS (a) and
MTES–TEOS (c) films and upon calcination at 873 K (b) and (d),
respectively.
Lu, Y., Gangli, R., Drewien, C. A., Anderson, M. T., Brinker, C. J.,
Gong, W., Guo, Y., Soyez, H., Dunn, B., Huang, M. H. & Zink, J. I.
(1997). Nature (London), 389, 364–368.
McConnell, G. A. & Gast, A. P. (1996). Phys. Rev. E, 54, 5447–5455.
Soler-Illia, G. J. A. A., Crepaldi, E. L., Grosso, D., Durand, D. &
Sanchez, C. (2002). Chem. Commun. pp. 2298–2299.
Su, Y.-L., Wang, J. & Liu, H.-Z. (2002). Macromolecules, 35, 6426–
6431.
Yao, N., Ku, A. Y., Nakagawa, N., Lee, T., Saville, D. & Aksay, I. A.
(2000). Chem. Mater. 12, 1536–1548.
Yu, K., Wu, X., Brinker, C. J. & Ripmeester, J. (2003). Langmuir, 19,
7282–728.
nanobioscience
738 Plinio Innocenzi et al.  Thermal-induced phase transitions J. Synchrotron Rad. (2005). 12, 734–738


Thermal-induced phase transitions in self-assembled

mesostructured films studied by small-angle X-ray

scattering

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Thermal-induced phase transitions in self-assembled mesostructured films studied by small-angle X-ray scattering

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