Sol-gel synthesis of monodispersed SiO2 nanoparticles in the presence of organic amines

Article abstract of DOI:10.1023/A:1026351321023

Preparation of monodispersed SiO₂ nanoparticles of varied size by hydrolysis of tetramethoxysilane and polycondensation of the hydrolysis products was studied as influenced by the basicity and nucleophilicity of organic amines.

Full text of DOI:10.1023/A:1026351321023

Russian Journal of Applied Chemistry, Vol. 76, No. 6, 2003, pp. 875 878. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 6, 2003,
pp. 904 908.
Original Russian Text Copyright
2003 by Khimich, Zvyagil’skaya, Zhukov, Us’yarov.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Sol Gel Synthesis of Monodispersed SiO2 Nanoparticles
in the Presence of Organic Amines
N. N. Khimich, Yu. V. Zvyagil’skaya, A. N. Zhukov, and O. G. Us’yarov
Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia
St. Petersburg State University, St. Petersburg, Russia
Received December 24, 2002
Abstract Preparation of monodispersed SiO2 nanoparticles of varied size by hydrolysis of tetramethoxy-
silane and polycondensation of the hydrolysis products was studied as influenced by the basicity and
nucleophilicity of organic amines.
In 1967, Stober and Fink [1] showed that hydroly-
Condensation
Si OH + HO Si
Si O Si + H2O.
(5, 6)
sis of silicon alkoxides in the presence of ammonia
yields SiO2 nanoparticles with narrow particle size
distribution. Since that time, suspensions of particles
of this kind became commercially available and found
wide application in electronics and for preparing com-
posites, monolithic glasses for optics, etc. [2, 3]. It
is also known that hydrosols of these particles yield
colloids with alternating structure (colloidal crystals),
which are unique models for fundamental study of
various types of interparticle interactions [4, 5] and
crystallization from solution [6].
Hydrolysis
Hydrolysis of silicon alkoxide yields silanols [re-
action (1)] which can condense by reactions (3) and
(5). When the sol gel process is performed in an al-
kaline solution, reactions of alcoholysis (4) and hy-
drolysis (6), which are responsible for growth of SiO2
particles with a definite size distribution, prevail.
The rate of all of these reactions is acceptable only in
the presence of catalysts. However, the mechanism of
the base catalysis was reduced in the literature and,
in particular, in a comprehensive monograph [11] to
a bimolecular nucleophilic attack (SN2 Si) of the hy-
droxide anion at the silicon atoms of alcoxysilane to
give a five-coordinate intermediate which eliminates
the alcohol molecule ROH (hydrolysis) or the RO an-
ion (condensation). Thus, it is assumed that the sol
gel process involving hydrolysis of a silicon alcoxide
and condensation of the hydrolysis products is con-
trolled exclusively by the concentration of OH an-
ions. Any organic base, in particular a nitrogen-con-
taining compound, can be considered a proton accep-
tor. Its strength can be estimated from the pKa of the
conjugated acid:
Although obvious progress has been achieved in
the synthesis of SiO2 nanoparticles, the results ob-
tained are poorly reproducible [7]. To enhance the re-
producibility, the effect of various factors on the size
of SiO2 nanoparticles was examined. In particular,
the type of the initial alkoxysilane and alcohol used
as the solvent, temperature of the sol gel process,
alkoxysilane concentration, alkoxysilane-to-water ra-
tio, ammonia concentration, and rate of alkoxysilane
addition were varied [1, 7 9]. However, there is one
more factor affecting formation of a SiO2 dispersion
in the course of a sol gel process: nature of the base
catalyst, not examined in previous papers. Let us con-
sider chemical reactions involved in this process [10]:
:NR3 + H2O
HN+R3 + OH .
(7)
Hydrolysis
However, the processes occurring in the sol gel
system incompletely fit in the logical path: (i) increase
in the strength of the organic base, (ii) increase in the
concentration of OH anions, and (iii) acceleration of
the hydrolysis and condensation. For example, it is
known that, although F is a weaker base than NH3,
(1, 2)
Si OH + ROH,
Si OR + H2O
Etherification
Condensation
Alcoholysis
Si OR + HO Si
Si O Si + ROH,
(3, 4)
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876
KHIMICH et al.
i-Pr
i-Pr
MeN
RN
VII, VIII
O
O
EtN
IX
VI
Here R = R1 = R2 = H (I); R = R2 = H,
R1 = Me (II); R = H, R1 = Me, R2 = Et (III); R =
Me, R1 = R2 = H (IV); R = R2 = H, R1
NMe2 (V); R = H; (VII); R = Me (VIII).
=
TMOS and amines I IX were distilled on a 10 TP
column at atmospheric and reduced (15 760 mm Hg)
pressure, with fractions boiling in the 0.5 C range
collected. Amine V was recrystallized from alco-
hol. Sol gel synthesis of SiO2 nanoparticles was per-
formed in a glass reactor equipped with a double wa-
ter jacket, magnetic stirrer, dropping funnel, and re-
flux condenser. Appropriate amine, ethanol, and wa-
ter were placed in the reactor. An ethanolic solution
of TMOS (25 ml) was added with vigorous stirring
at 70 C within 3 min. We prepared 13 sol gel sys-
tems with TMOS : water : ethanol = 1 : 25 : 110.
The mole fraction of amines is given below:
Fig. 1. (1) Average size d of SiO particles and (2) gelation
time vs. the 4-(dimethylamino)pyridine V concentration
c in sol gel systems 1 4 and 5b.
2
gelation of tetraethoxysilane solutions in the presence
of KF is substantially faster than in the presence of
the same amount of ammonia [11]. It is believed in
this case that the F anion is close in size to the OH
anion and can increase the coordination of the silicon
atom in alkoxysilane [12]. In other words, not only
proton affinity of the catalyst [Eq. (7)] but also its
nucleophilicity (attack of its lone electron pair at the
silicon atom of alkoxysilane) should be taken into
account when describing the mechanism of the sol gel
process.
Sol gel system
Amine
Mole fraction
1
I
2
3
4
5a 5b 5c
II III IV
V
V
V
The nucleophilicity of a Lewis base depends on
many factors, including the type of the chemical reac-
tion involving this base [13]. In this study, we ana-
lyzed hydrolysis of tetramethoxysilane (TMOS) and
polycondensation of the hydrolysis products in an al-
kaline solution in the presence of amines whose nu-
cleophilicity was varied by changing the steric and
electronic environment of the nitrogen atom.
7.0 1.4 1.4 1.4 0.35 1.4 2.8
Sol gel system
Amine
Mole fraction
5d
V
5e
V
6
VI
7
8
9
VII VIII IX
1.4 1.4 1.4
4.9 7.0 1.4
The particle size was determined with an EM-125
electron microscope at accelerating voltage of 75 kV.
The samples were prepared by applying the suspen-
sion (in some cases, diluted by a factor of 2 3 with
alcohol) to a carbon-reinforced collodion support.
Thus, the present study was concerned with condi-
tions for obtaining monodispersed SiO2 nanoparticles
and development of methods for preparative synthesis
of this material.
The organic amines were chosen so that the acidity
constants of their conjugated acids and their boiling
points varied widely:
EXPERIMENTAL
Amine
bp, C [15] 115
pKa,
20 C [16]
I
II
145
III
IV
143 145
6.77
V
Base hydrolysis of TMOS and polycondenstaion of
the hydrolysis products were studied in the presence of
the following thermally and chemically stable organic
amine cata lysts: pyridine (I), 4-methylpyridine (II),
3-ethyl-4-methylpyridine (III), 2,6-dimethylpyridine
(IV), 4-(dimethylamino)pyridine (V), N-methylmor-
pholine (VI), piperidine (VII), N-methylpiperidine
(VIII), and ethyldiisopropylamine (IX):
198*
5.20
6.02
6.46
9.70
Amine
VI
VII
VIII
106
IX Ammonia
127
bp, C [15] 115 116** 124
pKa,
20 C [16]
7.38
11.3 10.20 10.50 9.25
*
**
R
At 25 C [1, 5].
At 750 mm Hg.
N
R1
All the experiments can be divided into three
groups. In the first group, we varied the concentration
of 4-(dimethylamino)pyridine V (sol gel systems
5a 5d, Fig. 1). In the second, we varied the sub-
R
R2
_
I V
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 6 2003

SOL GEL SYNTHESIS OF MONODISPERSED SiO2 NANOPARTICLES
stituients in the pyridine ring (sol gel systems 1 4 and
877
5b). Electron micrographs of SiO2 particles formed
in these systems are shown in Fig. 2. In experiments
of the third group, we used strong organic bases
V IX.
Prior to discussing the results obtained, the follow-
ing should be noted. First, the resolution of the micro-
scope used does not allow determination of particles
smaller than 10 nm in diameter. Second, separate SiO2
nanoparticles can be obtained only when TMOS is
strongly diluted with ethanol and water, which pro-
motes hydrolysis and alcoholysis of the Si O Si bonds
in the sol gel systems [Eqs. (4), (7)] [17]. Otherwise
gelation of the sol is complete in less than 10 min in
the presence of pyridine catalyst and is almost instan-
taneous in the presence of bases V VIII.
The size of silica particles prepared by hydrolysis
of TMOS and polycondensation of the hydrolysis
products in the presence of 4-(dimethylamino)pyridine
grows linearly with increasing amine concentration
1
(Fig. 1). At amine concentration of 0.35 g-equiv l ,
the particle size is close to the detection limit, where-
1
as at amine concentration of 7 g-equiv l , it is 210
240 nm. Since particles formed in alkaline solution
are negatively charged and repel one another [18],
they grow by the condensation mechanism through
dissolution of smaller particles. These smaller par-
ticles are not perfectly spherical, with pointed projec-
tions and irregularities (Fig. 2b). It is these projections
and irregularities that have an excess energy and
initiate, in the first place, rupture of the Si O Si
bonds. Hence, as silica particles grow, their shape
becomes more spherical (Figs. 2c, 2d).
Fig. 2. Electron microgaphs of SiO particles prepared
in sol gel systems (a) 5a, (b) 5b, (c) 5c, (d) 5d, (e) 5e,
(f) 1 and (g) 2. Magnification 100 000 (100 nm in 1 cm);
the same for Fig. 3.
2
after the onset of hydrolysis of TMOS and polycon-
densation of the hydrolysis products, are shown in
Figs. 2a 2e. Such a high rate is provided by high ba-
sicity and nucleophilicity of 4-(dimethylamino)pyri-
dine. When amines I IV having a similar structure
and lower basicity are used as catalysts, all these
processes are strongly decelerated. Hardly visible par-
ticles are formed on heating at 70 C for 4 h in the
presence of 4-methylpyridine (Fig. 2g). Silica particles
generated in the presence of 3-ethyl-4-methylpyridine
III within the same time are slightly larger. When the
least basic amine, pyridine, was used, visible particles
(Fig. 2f) were formed only after keeping the sol gel
system at 70 C for 8 h and then at room temperature
for 8 days. The behavior of sol gel systems 1 3 cor-
relates with an increase in the amine basicity. How-
ever, no visible particles appear when sol gel sys-
tem 4 containing 2,6-dimethylpyridine is heated for 4
h, although the basicity of this amine is substantially
higher than that of amines I III.
It should be noted that the dissolution and conden-
sation to form an ensemble of spherical particles com-
pete with polymerization gelation. Each of these pro-
cesses prevails at a definite base concentration. Heat-
ing of sol gel system 5b for 30 min does not cause
particle coarsening but accelerates the gelation. Thus,
at low catalyst content, small silica particles suscep-
tible to polycondensation with inclusion of a solvent
and to gelation are formed owing to insufficiently fast
rupture formation of Si O Si bonds. At high cata-
1
lyst content (Fig. 2e, 7 g-equiv l ), large particles are
formed. In this case, the sol degrades by sedimenta-
tion and subsequent coagulation. Sols prepared at
1
a catalyst content of about 0.5 g-equiv l are stable
for many days (Figs. 2d, 2d).
It should be noted that all the above processes of
silica nanoparticle formation in sol gel systems 5a
5e are fast. Electron micrographs taken immediate-
ly after the reaction completion, i.e., several minutes
Thus, the mechanism of formation of silica nano-
particles in the presence of organic amines must in-
volve an attack on the silicon atoms by the lone elec-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 6 2003

878
KHIMICH et al.
ACKNOWLEDGMENTS
We are grateful to I.A. Drozdova for performing
electron microscopic studies.
This study was supported financially by the Rus-
sian Foundation for Basic Research (project no. 02-
03-32730).
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CONCLUSIONS
(1) The procedure developed for preparative syn-
thesis of monodispersed SiO2 nanoparticles gives
well-reproducible results and requires simple equip-
ment.
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