Mechanical stability enhancement by pore size and connectivity control in colloidal crystals by layer-by-layer growth of oxide

Article abstract of DOI:10.1039/b208805n

Control of the pore size and connectivity of micro-sphere colloidal crystal lattices has been achieved by a layer-by-layer growth of silica using atmospheric pressure room temperature chemical vapour deposition of silica, a method which largely increases the mechanical stability of the lattice without disrupting its long range order.

Full text of DOI:10.1039/b208805n

Mechanical stability enhancement by pore size and connectivity control
in colloidal crystals by layer-by-layer growth of oxide
a
a
b
a
b
Hernán Míguez, Nicolas Tétreault, Benjamin Hatton, San Ming Yang, Doug Perovic and Geoffrey
A. Ozin*
a
a
Materials Chemistry Research Group, Department of Chemistry, University of Toronto, 80 St. George
Street, Toronto, Canada ON M5S 3H6. E-mail: gozin@alchemy.chem.utoronto.ca
Department of Materials Science and Engineering, University of Toronto, 184 College St., Toronto,
b
Canada ON M5S 3E4
Received (in Cambridge, UK) 9th September 2002, Accepted 3rd October 2002
First published as an Advance Article on the web 21st October 2002
Control of the pore size and connectivity of micro-sphere
colloidal crystal lattices has been achieved by a layer-by-
layer growth of silica using atmospheric pressure room
temperature chemical vapour deposition of silica, a method
which largely increases the mechanical stability of the lattice
without disrupting its long range order.
For these reasons, these structures are particularly interesting
for checking the applicability of the method presented herein.
The silica deposition is performed at atmospheric pressure
and room temperature using a chemical vapour deposition
(CVD) process. SiCl
4
(l) is placed in a bubbler through which a
N
2
(g) stream is flown. This flow transports the reactive species
to the reactor where the colloidal crystals are located. A porous
glass septum placed between the bubbler and the reactor favours
the homogeneity of the reactive gas in the sample compartment.
Here, we take advantage of the highly hydroxylated surface of
the silica particles. Due to this, a layer of water, hydrogen
bonded to the silanol groups, covers each particle in the lattice,
Colloidal crystals have attracted the attention of researchers in
numerous fields such as ceramics, photonics,2 chemical
1
3
4
sensing and membranes. The pore size and the connectivity of
these lattices are important parameters that largely affect their
mechanical and optical properties as well as the diffusion of
fluids through them. The approach generally employed to
increase the connectivity of colloidal crystals, is based on the
sintering of the particles from which they are made. In order to
do so, lattices are subjected to thermal annealing at temperatures
between 700 and 1100 °C. This treatment induces the softening
of the silica and the occurrence of viscous mass flow processes,
which, eventually, will provide mass continuity to the structure
and decrease the pore size, which can be gradually varied with
9
as thermogravimetric measurements indicate. This water reacts
with the SiCl
4
(g) flow to form silica. The nucleophilic attack of
O results in the formation of silicic acid Si(OH)
the SiCl by H
4
2
4
and HCl, the latter of which, in turn, may act as a catalyst of the
polycondensation of ·Si–OH bonds to form water and siloxane
bonds.10 Eventually, this gives rise to the formation of a
continuous silica layer that coats each micro-sphere and extends
through the whole lattice, therefore providing mass continuity
to the structure. The pore volume of the structure is determined
by the thickness of the coating, which can be controlled by
5
the sintering temperature. A contraction of the center-to-center
distance between the particles is also observed.
Here we present an alternative method to control the pore size
and, at the same time, increase the connectivity and mechanical
stability of colloidal crystals. Chemical vapour deposition of
silicon tetrachloride, followed by reaction with water hydrogen
bonded to the micro-sphere surface, is performed at room
temperature and atmospheric pressure in a layer-by-layer
process to coat the micro-sphere lattice with a continuous layer
of silica. We show evidence of the accurate control of the
coating thickness and, therefore, pore size achievable by this
procedure.
successive deposition of water and SiCl
Fig. 1.
4
vapours, as depicted in
Scanning electron microscopy (SEM) was used to study the
structural modifications induced by the CVD process. Micro-
graphs shown in Fig. 2 clearly demonstrate that a high
connectivity of the lattice is achieved without disturbing the
long range order of the lattice. The analysis of cleaved edges of
the colloidal crystals allowed the homogeneity of the infiltration
and the degree of micro-sphere overlapping resulting from the
silica coating to be checked. A closer look at the structure
reveals the dramatic changes in pore size resulting from the
layer-by-layer growth of silica. It is worth pointing out that in
order to obtain similar degrees of necking between neighbour-
ing particles by sintering, temperatures higher than 1000 °C
must be employed.11 An accurate control of the final coating
thickness can be attained by the cyclic repetition of the above-
mentioned CVD procedure or by controlling the amount of
water present on the micro-sphere surfaces.
Samples employed in this study were prepared as follows.
Firstly, monodispersed suspensions of colloidal silica particles
of diameter between 200 and 400 nm were prepared following
6
the Stöber method. After that, the particles were crystallized by
convection induced self-assembly using a glass slide or a silicon
7
wafer as substrates, as described in the literature. Colloidal
lattices prepared in such a way present interesting photonic
crystal properties and applications.7 However, they cannot be
sintered without strongly disturbing the order of the lattice. In
addition, in some cases, the low thermal stability of the substrate
does not even allow thermal annealing of the colloidal lattice.
,8
Young modulus, or stiffness, and hardness of the same
sample after different numbers of SiCl
estimated using depth-sensing nanoindentation. Results are
4
CVD treatments were
Fig. 1 Diagram of the atmospheric pressure room temperature silica chemical vapour layer-by-layer deposition process of silicon tetrachloride vapour onto
a colloidal crystal showing explicitly the control over the dimensions of the pores and the connectivity of the lattice.
2
736
CHEM. COMMUN., 2002, 2736–2377
This journal is © The Royal Society of Chemistry 2002

plotted in Fig. 3, in which it can be seen that the stiffness
increases from 1.5 GPa in the as-grown film up to 15 GPa after
4
CVD treatments, while the hardness changes from 0.01 GPa
to 0.8 GPa. Typical values of these parameters for dense silica
glass are 70 and 5.5 GPa, respectively. CVD necking has a
dramatic effect on the mechanical properties of the colloidal
crystal.
A measure of the fine tuning of pore size achievable by the
method herein presented can be obtained by analysing the
evolution of the optical properties of the colloidal crystals. It is
well known that the 3D ordering of sub-micrometer size
colloids gives rise to diffraction of visible and near infrared light
at wavelengths determined by the refractive index of the
particles and the geometry of the arrangement. These optical
properties are extremely sensitive to disorder or lack of
homogeneity. Therefore, the uniformity of the silica coating
should reflect in the quality of the optical spectra of the silica-
infiltrated crystals. In Fig. 4 we show three spectra correspond-
ing to a planarized crystal prior and after different degrees of
silica infiltration by CVD. The spectral maximum detected
corresponds to the diffraction coming from the (111) colloidal
crystal planes of the sample, while the side fringes are a
consequence of the finite crystal size of the sample. As we grow
Fig. 4 Optical reflectance spectra of a planarized colloidal crystal prior to
silica infiltration (solid line) and after two different silica infiltration
treatments (dotted and dashed line). From the optical analysis we can
estimate that 30 and 70% of the pore volume is infiltrated, respectively.
where d(111) is the interplanar distance along the [111] direction
and åeÅ is the volume averaged dielectric constant of the
composite:
åeÅ = ffSiO2
e
s
+ (1 2 ffSiO2)e
b
(2)
the
where ffSiO2 is the filling fraction of silica, and e
s
and e
b
dielectric constant of the micro-spheres and the background,
respectively. Thus we are gradually increasing the average
refractive index in the structure as we increase the filling
fraction of silica and close the air pores, keeping the lattice
constant of the system unaltered. Formulas (1) and (2)
combined explains the red shift in the position of the reflectance
maximum detected and allows us to estimate the infiltrated pore
volume (see Fig. 4). In addition, the fine structure, that is, the
secondary minima resulting from finite crystal size effects,
remains after the CVD infiltration. The good optical quality of
the infiltrated samples are proof of the uniformity of the silica
coating achieved by the CVD process proposed herein.
silica on the micro-spheres, a clear red-shift of the main peak is
observed, which can be explained by the formula:12
1
⁄
2
l
(111) = 2d(111)åeÅ
(1)
In conclusion, we report here a new method to control the
pore size and enhance the connectivity of silica colloidal
crystals. A chemical vapour deposition process performed at
room temperature allows the growth of a continuous silica layer
of controlled thickness interconnecting all the micro-spheres
while maintaining unaltered the order and unit cell dimensions
of the lattice. This method represents an alternative way to
increase the mechanical stability of a colloidal crystal when
sintering is not possible. Also, it is not limited to silica CVD on
silica micro-spheres and can be extended to micro-spheres and
CVD precursors of other compositions.
Fig. 2 SEM characterization of planarized colloidal crystals made of silica
micro-spheres after being infiltrated with silica using the coating process
described in the text. (a) and (b) show details of the external surface prior to
and after a few silica coating cycles, respectively. (c) Cleaved edge showing
that the treatment does not disturb the long range order of the planar
structure. (d) Detail of a cleaved edge corresponding to an internal {111}
plane of the structure. The closing of the external pores occurs when the
coating thickness reaches the value 1.155D, where D is the diameter of the
spheres. However, at this value the internal pores of the structure remain
open although not interconnected any more.
G. A. O. is Government of Canada Research Chair in
Materials Chemistry. The Natural Sciences and Engineering
Research Council of Canada and the University of Toronto
generously provided financial support for this work.
Notes and references
1
2
3
4
5
M. D. Sacks and T. Y. Tseng, J. Am. Ceram. Soc., 1984, 67, 526.
V. L. Colvin, MRS Bulletin, 2001, 26, 637.
K. Lee and S. A. Asher, J. Am. Chem. Soc., 2000, 122, 9534.
B. Gates, Y. D. Yin and Y. N. Xia, Chem. Mater., 1999, 11, 2827.
J. S. Moya, C. Baudín and P. Miranzo, Encycl. Phys. Sci. Technol.,
1
983, 15, 263.
W. Stöber, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, 26,
2.
P. Jiang, J. F. Bertone, K. S. Hwang and V. L. Colvin, Chem. Mater.,
999, 11, 2132.
Y. A. Vlasov, X-Z Bo, J. C. Sturm and D. J. Norris, Nature, 2001, 414,
89.
6
7
8
9
6
1
2
F. García-Santamaría, H. Míguez, M. Ibisate, F. Meseguer and C.
López, Langmuir, 2002, 18, 1942.
1
0 C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press,
London, 1990, ch. 3.
1
1 H. Míguez, F. Meseguer, C. López, A. Blanco, J. S. Moya, J. Requena,
A. Mifsud and V. Fornés, Adv. Mater., 1998, 10, 480.
Fig. 3 Young modulus (open circle) and hardness (solid circles) of a
planarized colloidal crystal versus the number of CVD cycles, as obtained
by depth-sensing nanoindentation.
12 M. Born and E. Wolf, Principles of Optics, Cambridge University Press,
Cambridge, UK, 1997.
CHEM. COMMUN., 2002, 2736–2377
2737