Sol-gel synthesis of dispersed ZrO2 nanoparticles

Article abstract of DOI:10.1134/S1070427206030025

The influence exerted by the basicity of organic amines on hydrolytic polycondensation of zirconium tetraisopropoxide was studied in order to obtain nanodispersed ZrO₂ particles of various sizes. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S1070427206030025

ISSN 1070-4272. Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 3, pp. 351 355. Pleiades Publishing, Inc., 2006.
Original Russian Text
No. 3, pp. 358 362.
N.N. Khimich, O.V. Semashko, E.N. Khimich, M.G. Voronkov, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79,
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Sol Gel Synthesis of Dispersed ZrO2 Nanoparticles
N. N. Khimich, O. V. Semashko, E. N. Khimich, and M. G. Voronkov
Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia
St. Petersburg State University, St. Petersburg, Russia
Received December 27, 2005
Abstract The influence exerted by the basicity of organic amines on hydrolytic polycondensation of zirconi-
um tetraisopropoxide was studied in order to obtain nanodispersed ZrO2 particles of various sizes.
DOI: 10.1134/S1070427206030025
Hydrolysis of tetraalkoxysilianes, leading to forma-
the formation of ZrO nanoparticles has its distinctive
2
features. This is due, on the one hand, to the tendency
for zirconium atoms to increase their coordination
number to eight and, on the other, to the markedly
higher basicity of the oxygen atom in the Zr OH
group, compared to that in Si OH (electronegativi-
ties of Zr and Si atoms by Pauling are equal to 1.2 and
1.8, respectively). Therefore, zirconium alkoxides have
oligomeric structure in contrast to silicon alkoxides,
which are monomeric. Zirconium alkoxides contain,
as shown in detail for the example of zirconium tetra-
isopropoxide (ZTIP), alkoxy and oxo fragments in
various combinations, with the relative amounts of
these fragments dependent both on the preparation
technique and on the storage duration of the samples
synthesized [11, 12].
tion of SiO nanoparticles with a narrow size distribu-
2
tion, has been studied in sufficient detail [1 5]. The
influence exerted by the structure of the starting alkox-
ysilane and an alcohol used as a solvent, concentra-
tions of the alkoxysilane and water, temperature of
the sol gel process, rate of addition of the alkoxy-
silane, and acidity of the medium has been examined
[1 4]. As a result, suspensions of monodisperse SiO
2
particles have become commercially available and
found wide use for synthesis of monolithic optical
glasses and nanocomposite materials, in optoelectron-
ics, etc. [6, 7]. Hydrosols of particles of this kind
form periodic colloid structures, which are unique
model systems for fundamental studies of interparticle
interaction forces of varied nature [8, 9].
Owing to the increased basicity of the oxygen atom
in the Zr OH group, compared to the Si OH group,
the former is more labile in hydrolysis. As a result,
the equilibrium of reactions (1) and (2) for zirconium
alkoxides is strongly shifted to the right. The con-
densation reactions (3) and (5) are accompanied by
formation of structures that contain a large number of
hydroxy groups, which is quite untypical of silicon
compounds. The reactions of alcoholysis (4) and hy-
drolysis (6) of these condensation products, which are
responsible for the size ordering of oxide particles and
for their growth, occur in the case of zirconium com-
pounds by more complex mechanisms, compared to
silicon compounds.
In contrast to the case of tetraalkoxysilanes, the for-
mation of hydrosols of zirconium dioxide in hydrolyt-
ic condensation of its alkoxides has been studied to
a considerably lesser extent. Let us consider chemical
reactions (1) (6) occurring in this process [10]:
Hydrolysis (1)
Zr(OR)4 + nH2O
Zr OR + HO Zr
Zr OH + HO Zr
Zr(OH)n(OR)4 n + nROH,
Etherification (2)
Condensation (3)
Alcoholysis (4)
Condensation (5)
Hydrolysis (6)
Zr O Zr + ROH,
Zr O Zr + H2O.
Until now, ZrO hydrosols have been synthesized
2
and the corresponding synthesis mechanisms have
been studied in a weakly acidic medium [13 15]. Pre-
sumably, the forming suspensions of particles more
than 1 m in size [14] are poorly stable. It may be
assumed that the sol gel method for synthesis of
Despite the apparent similarity of hydrolysis of Si
and Zr alkoxides [reaction (1)] and of polycondensa-
tion of the forming compounds [reactions (3) and (5)],
351

352
KHIMICH et al.
monodisperse ZrO particles of various sizes by hy-
added was about 20 ml. A total of 12 sol gel systems
were prepared. The molar ratio alkoxide:water:ethan-
ol : amine was 1 : 50 : 200 : X, with the X values
listed below:
2
drolysis of zirconium tetrabutoxide [15] is seemingly
preferable. However, the too complicated experiment-
al procedure and the limited amount of water used for
hydrolysis of zirconium tetrabutoxide cast doubt on
the completeness of hydrolysis, i.e., on the purity of
the oxide synthesized.
Sol gel system
Amine
X
1a
1b
1c
1d
1e
2a
2b
2c
2d
3
I
I
I
I
I
II
II
II
II
III
IV
1.0
5.0
10
20
40
1
2
3
4
1
The influence exerted by the basicity and nucleo-
philicity of a number of organic amines on the hy-
drolysis of tetramethoxysilane (TMOS) and on the
polycondensation of the compounds being formed has
been studied previously in order to obtain monodis-
perse SiO particles of prescribed size [5]. The sim-
2
plicity of this process and of the apparatus involved,
as well as the good reproducibility of the results
obtained, stimulated studies of the hydrolytic poly-
condensation of zirconium alkoxides under similar
conditions. It was intended to study fundamental
4
1
The particle size in the sol gel systems obtained
was determined with an EM-125 electron microscope
at an accelerating voltage of 75 kV. The samples
studied were prepared by applying the suspensions
obtained onto a carbon-reinforced collodion film
substrate.
aspects of formation of monodisperse ZrO nanopar-
2
ticles under similar conditions and to develop optimal
conditions of their synthesis.
EXPERIMENTAL
The choice of compounds I IV is due to their dif-
The hydrolysis of ZTIP and the polycondensation
of the forming compounds were studied under condi-
tions of base catalysis. As catalysts served amines:
pyridine (I), 4-(dimethylamino)pyridine (II), and
ethyldiisopropylamine (III); as well as tetra-n-butyl-
ferent basicities, as characterized by the ionization
constants of conjugated acids, pK :
a
:NR3 + H2O
HN+R + OH .
(7)
+
ammonium hydroxide (IV) (n-Bu) N OH :
At 20 C, the pK values of the compounds used are as
4
a
follows [18]:
i-Pr
N
,
R
,
EtN
i-Pr
Amine
I
II
III
IV
pKa
5.20
9.70
10.50
15.4
I, II
III
All the experiments performed can be convention-
ally divided into three groups. To the first of these
belong experiments involving hydrolytic polyconden-
sation of ZTIP in the presence of a nucleophilic base
of medium strength, 4-(dimethylamino)pyridine (II)
(sol gel systems 2a 2e). The second group includes
experiments with varied concentration of a weak base,
pyridine (I) (sol gel systems 1a 1e). Finally, to the
third group belong experiments associated with hy-
drolysis of ZTIP in the presence of strong organic
bases III and IV.
where R = H (I) and NMe (II).
2
Zirconium tetraisopropxide was synthesized using
the previously described method [16, 17] and purified
by triple recrystallization from isopropanol. Amines I
and III, preliminarily dried over BaO, were purified
by distillation on a 10-TP column; they boiled
within 0.2 C range. Amine II was recrystallized from
ethanol. Hydroxide IV (Aldrich) was used without
additional purification.
The sol gel synthesis of ZrO nanoparticles was
2
performed in a glass reactor equipped with a double
jacket for introduction of thermostating water, a mag-
netic stirrer, a dropping funnel, and a reflux con-
denser. The reactor was charged with ethanol, water,
and an amine, and an isopropanol solution of ZTIP
was added at 70 C under vigorous stirring in the
course of 1 min. The total volume of the solution
Figure 1 shows electron micrographs of particles in
sol gel systems 2a, 2c, and 2e. As the concentration
of a base in the system increases, the forming ZrO
particles become coarser. However, the mechanism of
their growth is apparently different from that in for-
2
mation of SiO particles in hydrolytic polycondensa-
2
tion of TMOS. In the latter case, the coarsening of
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SOL GEL SYNTHESIS OF DISPERSED ZrO2 NANOPARTICLES
353
Fig. 1. Electron micrographs of particles in sol gel systems (a) 2a, (b) 2b, (c) 2c, and (d) 3. Magnification 80000 (125 nm in
1 cm); the same for Figs. 2 and 3.
SiO particles is accompanied by rupture of Si O Si
virtually only siloxane bonds formed within coarsen-
ing particles and silanol groups predominantly situated
on the surface of the particles. Such particles with
a hydroxylated surface show mutual repulsion and,
therefore, cannot aggregate and grow via condensation
2
bonds at places with an excess surface energy, specif-
ically, at sharp projections and irregularities of the
originally formed particles, which results in that they
acquire a regular spherical shape [5]. According to
with molecules of the Si(OH) monomer or siloxane
Fig. 1, the coarsening of ZrO particles is accom-
4
2
oligomers from a true solution [19]. In the case of
hydrolytic polycondensation of zirconium alkoxides,
the pattern is different. First, formation of linear or
panied by aggregation of particles into a kind of
clusters, rather than by growth of each separate par-
ticle. This involves sedimentation of the aggregated
particles and the subsequent coagulation. As a result,
loose nontransparent sediments are formed, with the
rate of their formation being proportional to the con-
centration of the base. On passing from sol gel sys-
tem 2a to system 2e, the lifetime of the sol decreases
from 10 days to several minutes. The difference bet-
ween the processes of hydrolytic polycondensation of
silicon and zirconium alkoxides can be understood as
follows. According to Iler [19], silanols formed in
hydrolysis of silicon alkoxides [reaction (1)] enter into
condensation reactions (3) and (4). Successive elimi-
nation of water or alcohol molecules gives siloxanes
of linear, cyclic, or polycyclic structure. After the
polymerization is complete, particle of the dispersed
cyclic polyzirconoxane bonds (Zr O) is not charac-
n
teristic of zirconium. Second, oxygen compounds of
zirconium are never present in hydroxyl-containing
solvents in the form of monomeric or low-molecular-
weight oligomeric species of the type [Zr(OH) ] .
4 n
Despite that the hydrolysis of the Zr OR bond is
considerably faster than the similar reaction for sili-
con, the ZrO network
2
O
O
Zr
Zr
O
O
Zr
Zr
O
O
O
O
O
Zr
O
Zr
O
phase of SiO are formed and grow in solution. In
2
Zr
Zr
O
the process, coarser particles grow by the condensa-
tion mechanism via dissolution of fine particles, with
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 3 2006

354
KHIMICH et al.
does not commonly appear in pure form (in contrast
to the case of the siloxane bond). In all cases, poly-
charged aqua structures, e.g., species of the type
8+
[Zr (OH) (H O) ] [20], are formed in the course of
4
8
2
16
polymerization. Because of the large number of water
molecules in the coordination sphere of a zirconium
atom, the content of hydroxy groups on the surface of
growing particles is insignificant, which allows their
successful aggregation to give jelly-like sediments of
hydrated ZrO .
2
The course of the hydrolytic polycondensation of
ZTIP is, undoubtedly, strongly affected by the pro-
nounced nucleophilicity of the amine used (amine II).
In hydrolytic polycondensation of ZTIP in the pres-
ence of base III, which has low nucleophilicity for
steric reasons, but is considerably stronger, the degree
of particle aggregation decreases (Fig. 1d).
The degree of aggregation of ZrO particles and
2
their size markedly decrease in the course of a sol gel
process catalyzed by a weak base I. The rate of their
formation remains equally high, in contrast to the case
of hydrolytic polycondensation of tetramethoxysiliane
in the presence of pyridine, when many hours’ boiling
of the reaction mixture is necessary for hardly dis-
Fig. 2. Electron micrographs of particles in sol gel systems
(a) 1a and (b) 1d.
cernible SiO particles to be formed. The growth of
2
aggregates of ZrO particles is presumably hindered
2
by competition between the nitrogen atoms of pyri-
dine and water molecules for the coordination sphere
of a zirconium atom. An increase in the amount of
pyridine in the reaction mixture does not make larger
the size of the forming aggregates (Fig. 2), but only
decreases the time in which a transparent, slightly
opalescent gel is formed from tens of days to 1
2 days.
The formation of comparatively spherical ZrO
2
nanoparticles 100 150 nm in diameter (Fig. 3a) was
only observed when a very strong base IV was used to
catalyze the sol gel process. In contrast to analogous
SiO particles, these nanoparticles have a rather loose
2
structure (Fig. 3a) and, in a short time, disintegrate to
give a gel-like structure (Fig. 3b).
CONCLUSIONS
(1) The mechanisms of formation and growth of
nanodispersed ZrO and SiO particles synthesized by
2
2
hydrolytic polycondensation of zirconium and silicon
alkoxides in the presence of organic bases are dif-
ferent.
(2) The rate of formation of ZrO particles in the
sol gel process of hydrolytic polycondensation of
Fig. 3. Electron micrographs of particles in sol gel system
4 (a) before and (b) after gelation.
2
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SOL GEL SYNTHESIS OF DISPERSED ZrO2 NANOPARTICLES
355
zirconium tetraisopropoxide is independent of the
basicity of the catalysts used.
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2
formed occurs via aggregation of finer particles. This
makes impossible synthesis of ensembles of monodis-
perse ZrO particles in a protonic medium in the
2
presence of organic amines.
ACKNOWLEDGMENTS
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