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Solid State Sciences 9 (2007) 351e356
www.elsevier.com/locate/ssscie
py
Structural characterization of nanosized silica spheres
co
Francisco Balas a, Montserrat Rodrı́guez-Delgado a,b, Carlos Otero-Arean b,
Fernando Conde a,c, Emilio Matesanz c, Luis Esquivias d, Julio Ramı́rez-Castellanos e,
José Gonzalez-Calbet e, Marı́a Vallet-Regı́ a,*
a
al
Departamento de Quı́mica Inorgánica y Bioinorgánica, Facultad de Farmacia, Universidad Complutense de Madrid,
Pza. Ramon y Cajal, s/n, 28040 Madrid, Spain
b
Departamento de Quı́mica, Facultad de Ciencias, Universidad de las Islas Baleares, 07122 Palma de Mallorca, Spain
c
Unidad de Difracción de Rayos X, Facultad de Farmacia, 28040 Madrid, Spain
d
Departamento de Fı́sica de la Materia Condensada, Facultad de Fı́sica, Universidad de Sevilla, 41012 Sevilla, Spain
e
Departamento de Quı́mica Inorgánica, Facultad de Ciencias Quı́micas, Universidad Complutense de Madrid, 28040 Madrid, Spain
on
Received 27 February 2007; accepted 6 March 2007
Available online 15 March 2007
Abstract
pe
rs
Silica nanospheres with wide applications on chemistry have been synthesized using the Stöber method and have been modified with several
organic functional groups by post-synthesis reaction in anhydrous environments. The surface analysis of the silica nanosized spheres leads to low
surface area values and large skeletal densities, pointing out to dense nanospheres. High-resolution transmission electron microscopy (HRTEM)
and small-angle X-ray scattering (SAXS) prove the microstructure level of the surface porosity that is clearly affected by the organic functionalization.
Ó 2007 Elsevier Masson SAS. All rights reserved.
r's
Keywords: Silica; Nanospheres; Surface area; Functionalization; Electron microscopy; Small-angle X-ray scattering
1. Introduction
Au
th
o
Monodispersed silica spheres having a homogeneous
diameter in the sub-micrometer range can be used as a starting
material for building devices having a wide range of technological applications. Among them, 2D and 3D crystalline lattices made of such nanosized spheres can be used to generate
optical filters and switches [1,2], diffraction gratings [3] and
photonic band gap crystals [4e6]. Ordered arrays of nanosized
silica spheres can also be used as a template for preparing replica organic polymers, metals or semiconductors having tunable pore size [7e9]; useful applications span the fields of
molecular sieving membranes, metallic electrodes for electrochemical sensing and heterogeneous catalysis.
* Corresponding author.
E-mail address: vallet@farm.ucm.es (M. Vallet-Regı́).
1293-2558/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.solidstatesciences.2007.03.004
From a different perspective, sub-micrometer sized silica
spheres can be surface functionalized with different organic
ligands and, by doing so, the field is opened to practical
applications in chromatography, selective separations and
biological immunoassays [10], to mention only a few examples. On the other hand, silica microbeads can easily be
incorporated into microfluidic devices, thus enhancing the
sensitivity for detection of trace amounts of biological
molecules.
For many of the above practical applications, better knowledge of structural details should help improving device design
and performance. The aim of this work was to analyze structural details of: (i) pure silica nanospheres and (ii) nanosized
silica spheres functionalized by covalent attachment of amino
groups; cyano and carboxylic functional groups were also
given some consideration. Main techniques used for structural
characterization were high-resolution transmission electron
microscopy (HRTEM) and small-angle X-ray scattering
F. Balas et al. / Solid State Sciences 9 (2007) 351e356
352
(SAXS), complemented with scanning electron microscopy,
nitrogen adsorption for surface area measurement and infrared
(FTIR) spectroscopy.
2. Experimental
co
al
on
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rs
Monodispersed silica spheres (pure SiO2 sample) were synthesized through controlled hydrolysis of reagent-grade tetraethylorthosilicate (TEOS, Aldrich, Milwaukee WI), following
the Stöber and Fink method [11]. To prepare the silica spheres,
TEOS was added dropwise under continuous stirring at room
temperature to a solution of reagent-grade ammonia (Fluka
AG, St. Gallen, Switzerland) and ethanol (Panreac, Barcelona,
Spain) and the mixture was kept under vigorous stirring for
2 h. The nominal molar composition of the synthesis gel
was 1 TEOS/0.1 NH4OH/41.1 C2H5OH/26.5 H2O. The resulting solid was filtered, washed with ethanol and vacuum-dried
in desiccator. The material thus obtained was found to be
amorphous to X-ray diffraction. Amino-functionalized silica
nanospheres were similarly prepared, with the only difference
that 3-aminopropyltriethoxysilane (APTES; ABCR GmbH,
Karlsruhe, Germany) was added to the synthesis gel; which
had the nominal molar composition: 0.8 TEOS/0.2 APTES/
0.1 NH4OH/41.1 C2H5OH/26.5 H2O.
A post-synthesis procedure was used to prepare eCN functionalized SiO2 nanospheres, for which purpose they were
sylilated by refluxing them in hexane with cyanopropyltrichlorosilane (ABCR) under argon. From the cyano-functionalized SiO2 nanospheres, eCOOH functionalized derivatives
were obtained by alkali-catalyzed oxidation with hydrogen
peroxide in a 0.1 M NaOH solution.
For simplicity, the pure SiO2 sample and those functionalized
with amino, cyano and carboxylic groups will be hereafter termed
SiO2, SiO2eNH, SiO2eCN and SiO2eCOOH, respectively.
py
2.1. Sample preparation
Au
th
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r's
Fig. 1. (a) TEM image of SiO2 nanospheres. (b) Higher magnification image of
a SiO2 nanosphere.
Fig. 2. (a) HRTEM image, (b) diffraction pattern, and (c) Fourier filtered image of a pure silica nanosphere. High-contrast dots ordered in crystalline blocks are
marked with a rectangle, and local quasi-crystalline blocks are marked with a circle.
F. Balas et al. / Solid State Sciences 9 (2007) 351e356
py
353
co
Fig. 3. (a) SEM image, (b) HRTEM image, and (c) Fourier filtered image of SiO2eNH nanospheres.
2.2. Sample characterization
al
on
Absorbance (a.u.)
νO-H νN-H
pe
High-resolution transmission electron microscopy was performed in order to analyze the microstructure of SiO2 and
SiO2eNH nanospheres. The low magnification TEM image
corresponding to the pure SiO2 sample (Fig. 1a) shows monodispersed silica spheres, having an average diameter around
250 nm. Increased magnification (HRTEM, Fig. 1b) allows
sphere edge to be observed, showing the characteristic contrast
of an amorphous material.
Fig. 2 shows the Fourier transform (FT) diffraction pattern
of a pure silica nanosphere. The broad and diffused scattering
rings at low angle testify to the amorphous nature of the material. By placing small windows around all fundamental spots in
νC-H
νO-H
4000
3500
3000
2500
2000
1500
1000
500
Frequency / cm-1
(b)
νSi-OSi
νCN
Absorbance (a.u.)
Au
3.1. Electron microscopy
SiO2-NH
SiO2
r's
th
o
3. Results and discussion
νSi-OSi
(a)
rs
High-resolution transmission electron microscopy was performed on a JEOL 300 FEG instrument (JEOL Co., Tokyo,
Japan). Image Fourier filtration was obtained from HRTEM
images by windowing the Fourier Transform. SEM images
were obtained on a JEOL JSM 6335F field emission scanning
microscope operated at an acceleration voltage of 6 kV.
SAXS was performed with an X’Pert PRO MPD diffractometer (Panalytical B.V., Almelo, Netherlands) in transmission
mode on samples mounted in 0.5 mm diameter borosilicate
glass capillaries (Hilgenberg GmbH, Malsfeld, Germany).
The diffractometer was equipped with a hybrid monochromator in the incident beam optics, and with both programmable
anti-scatter and receiving slits working in tunnel mode with
a 0.02 mm opening height in the diffracted beam optics.
Surface area and porosity of SiO2 and SiO2eNH samples
were determined from the corresponding nitrogen adsorptionedesorption isotherms obtained at 77 K using a Micromeritics ASAP 2010C instrument (Micromeritics Co., Atlanta,
GA). For N2 adsorption measurements, samples were previously outgassed for 24 h under a dynamic vacuum. Density
of these samples was determined by He pycnometry (AccuPyc
1330, Micromeritics). Transmission FTIR spectra were recorded at 3 cm1 resolution using a Thermo-Nicolet Nexus
spectrometer (Thermo Electron, Madison, WI).
the FT, a subsequent inverse Fourier transformation strongly
suppresses high-frequency non-periodic noise from the image.
The Fourier filtered image thus obtained (Fig. 2c) shows a complex ultra-structure. Different areas showing high-contrast dots
SiO2-COOH
2350
νH-OC
νO-H
2300
2250
2200
2150
νC=O
νC-H
νO-H
SiO2-CN
νC-H
νCN
νO-H
4000
3500
SiO2
3000
2500
2000
1500
1000
500
Frequency / cm-1
Fig. 4. (a) FTIR spectra of SiO2 and SiO2eNH samples; (b) FTIR spectra
of silica nanospheres after functionalization with cyano (eCN) groups and
further oxidation to carboxylic acid (eCOOH) groups.
F. Balas et al. / Solid State Sciences 9 (2007) 351e356
354
spectrum of SiO2eNH; for comparison, the spectrum of
SiO2 is also given. The broad IR absorption band covering
from about 3700 to 3100 cm1 can be assigned to hydrogenbonded and partially hydrated silanols [15]. The spectrum of
amine-functionalized silica nanospheres also shows an IR
absorption band around 3100 cm1, that corresponds to the
NeH stretching mode, together with weaker bands around
2900 cm1 assigned to eCH and eCHe stretching modes
of the alkylamine chains. The sharp IR absorption band at
w1100 cm1 in the spectrum of pure SiO2 should be assigned
to the asymmetric SieOeSi stretching mode; the shoulder at
a slightly higher frequency comes from distorted SiO4 tetrahedra (expected to occur till some extent in amorphous silica).
For the amine-functionalized sample the band at w1100 cm1
broadens and the high-frequency shoulder becomes more
prominent; thus suggesting a further distortion (involving
more SiO4 tetrahedra) upon functionalization.
Infrared spectra of SiO2eCN and SiO2eCOOH samples are
shown in Fig. 4b, together (for comparison) with the spectrum
of non-functionalized SiO2. For SiO2eCN, the weak IR absorption band around 2250 cm1 is assigned to the CeN stretching
mode. The inset in Fig. 4b shows an expansion of the spectral
area where such bands usually can be observed. Also, CeH
stretching bands are seen around 3000 cm1 confirming the
presence of propyl chains together with the cyano functional
groups. The oxidizing treatment with H2O2 in alkaline solution
transforms cyano groups into eCOOH groups; as shown by the
disappearance of the CeN stretching band, and simultaneous
appearance of a shoulder at about 3600 cm1, which corresponds to the OeH stretching mode of the carboxylic group
(top spectrum in Fig. 4b). Comparison with the spectrum of
non-functionalized SiO2 shows that broadening of the Sie
OeSi asymmetric stretching mode occurs upon functionalization, as already pointed out in the case of SiO2eNH
samples.
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rs
on
al
co
py
ordered in crystalline blocks (marked by a rectangle) can be observed; the average distance between dots is 0.51 nm. On the
other hand, local quasi-crystalline blocks (marked by circles)
also appear, showing distances of around 0.46 nm between
contrast dots. Chains connecting these different blocks can
also be observed. Local crystalline nanodomains were also observed by Mackenzie et al. [12], who described the structure of
SiO2ePDMS related phases as a SiO2 network formed by
a three-dimensional model of silica glass including linear
chains of PDMS, which break up the continuity of the SiO2
block network. A tentative structural model was proposed for
this hybrid material in terms of a block framework composed
of [SiO4
4 ] tetrahedra, involving a disordered distribution of
crystalline nanodomains, alternating with non-crystalline
domains, giving rise to a random distribution of SiO2 clusters
connected by e(Si(CH3)2O)ne chains of different lengths, as
previously observed by Vallet-Regi et al. [13].
Fig. 3a shows the SEM image of the SiO2eNH sample.
Nanosphere size is more variable than in the case of nonfunctionalized SiO2 nanospheres (Fig. 1a). However, most
of the particles are still spherical and have a diameter of
about 210 nm. At higher magnification (Fig. 3b) the HRTEM
image shows similar contrast as previously observed for the
non-functionalized sample. However, lower distances (0.35 nm)
were measured between high-contrast dots (Fig. 3c); in agreement with a previous report for similarly functionalized materials [14]. The smaller distance between high-contrast dots
suggests a slight shortening of (some) SieOeSi bonds brought
about by eNH2 functionalization; this bond shortening would
involve distortion of SiO4 tetrahedra.
3.2. FTIR spectroscopy
r's
Fourier-transform infrared (FTIR) spectroscopy was used
to study functionalized samples. Fig. 4a shows the IR
30
30
SiO2
25
25
20
20
Au
VADS (cm3/g)
th
o
SiO2-NH
15
15
10
10
5
5
0
0
0,0
0,5
P/P0
1,0
0,0
0,5
P/P0
Fig. 5. N2 adsorptionedesorption isotherms at 77 K on SiO2 and SiO2eNH samples.
1,0
F. Balas et al. / Solid State Sciences 9 (2007) 351e356
Table 1
Textural data for SiO2 and SiO2eNH samples
107
VmP 103
(cm3/g)a
r
(g/cm3)b
12.5
11.4
3.8 0.3
4.2 0.6
0.6 0.1
0.7 0.2
2.15 0.03
1.77 0.04
SiO2
SiO -NH
106
2
5
10
a
Micropore volume (VmP) estimated after applying the t-plot model to the
adsorption branch of the isotherm between t ¼ 0.3 nm and t ¼ 0.5 nm.
b
Density obtained by He pycnometry for nanospheres dried at 333 K for
24 h.
104
107
106
105
104
103
102
101
103
2
10
101
3.3. Surface area and porosity
Fig. 6 shows the intensity of the radiation scattered by the
SiO2 and SiO2eNH samples in a double logarithmic scale, as
a function of the scattering vector q.
10-1
100
10-2
q
10-1
(nm-1)
100
101
al
Porod’s limiting law for SAXS, Eq. (1),
IðqÞq4
S
1
¼
lim
V pfð1 fÞ
Q0
on
ð1Þ
enables the quantity S/V to be calculated irrespective of the
geometric distribution of the phases even for not well defined
particles [20]. Q0 is the integrated intensity given by Eq. (2)
pe
r's
th
o
Au
3.4. Small-angle X-ray scattering
m = -3.02
Fig. 6. Small-angle X-ray scattering curves for SiO2 (dashed line) and SiO2e
NH (continuous line) samples. Inset shows the q interval along which the
curves If qDs . They were deliberately shifted for the sake of clarity.
rs
Fig. 5 shows nitrogen adsorptionedesorption isotherms (at
77 K) on SiO2 and SiO2eNH samples. These isotherms are typical of basically non-porous materials, although a small amount
of microporosity cannot be discarded (see below). The aminemodified sample shows a nitrogen adsorption isotherm very
similar to that of non-functionalized SiO2. BET surface areas,
derived from analysis of the isotherms (in the 0.05 p/
p0 0.2 range) resulted to be 12.5 and 11.4 m2 g1 for SiO2
and SiO2eNH samples, respectively. For the pure silica sample,
the BET surface area is slightly larger than the external (geometrical) surface area (10.8 m2 g1) calculated assuming SiO2
spheres having 250 nm in diameter, and taking the approximate
value of 2.2 g cm3 for the density of amorphous silica [16].
This discrepancy can be explained in terms of a contribution
from internal surface area due to microporosity. In fact application of the t-method [17] showed that microporosity has a small
contribution (about 5%) of the total volume of adsorbed nitrogen. Presence of microporosity in silica nanospheres obtained
by the Stöber method was also described by several authors
[18,19]. For the SiO2eNH sample no comparison is made between BET and geometric (external) surface area, because the
observed higher dispersion of nanosphere diameter (Fig. 3a)
renders calculation of geometric surface area very imprecise.
Helium pycnometry showed for SiO2 nanospheres a density
of 2.15 g cm3, which is close to the theoretical value of
2.2 g cm3 [16] for non-porous amorphous SiO2. For the
SiO2eNH sample a significantly smaller value (1.77 g cm3)
was obtained, as expected (because of the lower density of the
aliphatic chain, as compared to SiO2). In both the cases a density
value smaller than 2.2 g cm3 also suggests some microporosity
which, in part, could not be accessible to helium. Table 1 summarizes textural data for SiO2 and SiO2eNH samples.
100
10-3
m = -2.94
py
VT 103
(cm3/g)
co
SBET
(m2/g)
I(q) (a.u)
SiO2
SiO2eNH
355
Q0 ¼
ZN
I q q2 dq
ð2Þ
0
and S and V are, respectively, the surface and the volume of
one of the two phases of volume fraction f. The materials under study exhibit a positive deviation from Porod’s law, due to
electronic density fluctuation at the poreesolid interface. Positive deviation from Porod’s law leads to a non-constant, but
linear, relationship of Iq4 as a function of the scattering angle:
lim I q q4 ¼ A þ Bq4
ð3Þ
where A is the Porod constant, and B the corresponding intensity fluctuation. This parameter is a measure of the deviation
from Porod’s law, associated with electronic density fluctuations due to the solid phase microporosity. On the other
hand, the parameter A is strongly related to surface roughness
of the solidepore interface network.
Another important parameter that can be tested is the mean
chord length given by
Table 2
Structural parameters for SiO2 and SiO2eNH samples, calculated from Figs. 7 and 8 according to the relations explained in the text
Q0 (nm3)
SiO2
SiO2eNH
191.3
45.9
lim[I(q)q4]
4
160.94 þ 3.40q
11.30 þ 3.89q4
Fs
V/S (nm)
V/SBET (nm)
S0 (m2/g)
SBET (m2/g)
lc (nm)
ls (nm)
lp (nm)
0.12
0.33
3.58
5.84
407.9
189.7
1074
294.8
12.5
11.4
1.51
5.17
1.72
7.71
12.6
15.7
F. Balas et al. / Solid State Sciences 9 (2007) 351e356
356
1000
700
q vs SiO2
q- vs SiO2-NH
600
q vs SiO2
q- vs SiO2-NH
800
160.94 + 3.40q4
Q0 (SiO2) = 191.3
300
0
1
2
q (nm-1)
3
0
4
Fig. 7. q2I(q) vs. q plot employed for calculating the integrated intensities (Q0)
for silica nanospheres.
4Q0
pA
Au
th
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120
molecule. For spherical particles the ratio V/R is R/3, R being
the radius of the sphere. Table 2 abridges the structural parameters for SiO2 and SiO2eNH samples, calculated from Figs. 7
and 8 according to the relations explained above. It can be
noted in Table 2 that V/S from BET model data turn out to
be three times larger than that expected from the size observed
by electron microscopy. This agrees with a rough surface of
the particles, whether pure SiO2 or SiO2eNH, although the
former seems to present a finer structure on its surface.
al
pe
Going towards the low-q region, the scattering patterns present
an increasing intensity, related to an open structure at this level
(Fig. 6). They take values significantly similar up to q z
0.2 nm1. This range corresponds to the contribution of the
less fine parts of the structure. The difference is a shoulder
in the SiO2 sample pattern at q z 0.7 nm1 that does not appear in the amine-functionalized sample, undoubtedly caused
by the mono-sized silica particle distribution.
Both curves present linear regime of slope m w 3 along
one decade (inset in Fig. 6) allowing this effect to be described
in terms of surface fractality of dimension Ds w 3. They differ
in the q value where this condition is accomplished: 0.02e
0.2 nm1 for the SiO2 sample and 0.06e0.6 nm1 for the
SiO2eNH sample. This difference manifests a finer structure
of the scatterers (whether pore or solid) of the SiO2eNH sample. In the case of the SiO2 sample, from a resolution length
larger than 50 nm and shorter than 5 nm no autosimilarity is
observed in its surface structure. At the same time, in the
SiO2eNH sample surface autosimilarity is only observed for
a resolution length between 17 nm and 1.7 nm. That means
that the surface of this sample is rougher than the former
one. The data account for microporosity that cannot be easily
deduced from electron microscopy. Curve analysis leads to
a mean chord length that should represent a fine structure
with pore size of tenths of nanometer.
Specific surface area calculated from N2 adsorption is two
orders of magnitude smaller than that resulting from SAXS
data analysis. This is a consequence of the fine rugosity of
the surface of scatterers, which is out of the resolution of
adsorptionedesorption isotherms, limited by the size of N2
View publication stats
80
rs
ð5Þ
q4 (nm-1)
on
1 1 1
¼ þ
lc ls lp
40
Fig. 8. Porod plot (q4I(q) vs. q4) employed for calculating the Porod constant
(A) and intensity fluctuations (B) for silica nanospheres. See text for details.
ð4Þ
This parameter represents the harmonic average of the pore
and solid chord which may be considered as the weighted average of both phases:
0
co
200
100
lc ¼
11.30 + 3.89q4
400
Q0 (SiO2-NH) = 45.9
200
0
600
py
400
q4*I(q)
Iq2 (nm-2)
500
Acknowledgments
Authors thank MAT-2005-01486 and S-0505/MAT/0324
research projects for financial support.
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