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Colloids and Surfafnces A: Physicochemicnl (lnd Engineering Aspects, 87 (1994) 25-31 0927-7757/94/$07.00 0 1994 ~ Elsevier Science B.V. All rights reserved.
25
In situ steric stabilization of titanium dioxide particles synthesized by a sol-gel process
V.J. Nagpal, R.M. Davisa**, J.S. Riffleb
aDepartment of Chemical Engineering, Virginiu Polytechnic Institute & State University, Blacksburg, VA 24061, USA bDepartment of Chemistry, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, USA
(Received 8 February 1993; accepted 3 December 1993)
Abstract
Spherical titanium dioxide particles were synthesized via the hydrolysis of tetraethylorthotitanate (TEOT) in ethanol in the presence of hydroxypropyl cellulose (HPC) which served as an in situ steric stabilizer. This work focused on the effect that water concentration had on the HPC-TiO, interactions leading to steric stabilization. The water concentration varied in the range [H,O]/[TEOT]=5.3-60. The amount of adsorbed HPC increased threefold over this water concentration range. This correlates with measurements of the preferential adsorption of water on TiO, in ethanol. The particle size decreased fivefold in the presence of HPC over the range of water concentrations studied owing to the combined effects of increased HPC adsorption and increased nucleation rates. Mean particle diameters as small as
70 nm were obtained.
Key words: Sol-gel process; Steric stabilization; Synthesis; Titanium dioxide particles
Introduction
The synthesis and processing of fine ceramic
particles less than 100 nm in diameter have received
considerable attention in recent years owing to
their novel optical, electronic and densification
properties [l-5]. Many studies have focused on
particle formation from the hydrolysis and conden-
sation of metal alkoxides [4-61. This paper con-
cerns the effect of water concentration on the
formation of TiO, particles from the hydrolysis
and condensation of tetraethylorthotitanate
(TEOT) in the presence of the steric stabilizer
hydroxypropyl cellulose (HPC). The reactions are
written in abbreviated form [43 as
Ti(OC,H5)4+4H20-+Ti(OH)4+4C2H50H
Hydrolysis
*Corresponding author.
SSDI 0927-7757(93)02735-W
Ti(OH),+TiO,*xHzO + (2 - ?c)H,O
Condensation
The ratio of water molarity to TEOT molarity,
defined as R = [H,OJ/[TEOT], is a crucial parameter controlling the hydrolysis reaction
kinetics and the resulting metal oxide morphology.
Particle formation requires R > 2.5 [ 7,8]. Previous studies of TiO, formation from alkoxides have
focused on how particle growth and the final size
distribution are affected by electrostatic [ 9- 111
and hydrodynamic effects [ 121, as well as by alcohol structure [ 131 and steric stabilization
[14-181. In this last case, the reactions occurred
in a solution of the polymer HPC. The polymer
adsorbed onto the particles as they formed, thus
generating repulsive steric forces that limited fur-
ther particle aggregation and resulted in spherical
particles with relatively narrow size distributions.
26 T/J. Nagpal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31
A significant reduction in particle size occurred
especially above a critical HPC concentration
where the particle size reached a plateau value
corresponding to nearly complete surface coverage
by HPC [17].
With the use of a steric stabilizer, the water
concentration in the reactions of TEOT takes on
added significance since previous theoretical and
experimental studies have shown that solvent com-
position strongly affects the extent of polymer
adsorption and the resulting adsorbed layer struc-
ture which directly affects stabilization [ 19,201.
While the adsorption of HPC on TiO, particles in
anhydrous ethanol was measured by Jean and
Ring to prove that in situ steric stabilization was
the stabilization mechanism [14,17], the effect of
water concentration on HPC adsorption was not
studied. In addition, no particle growth experi-
ments using HPC as a stabilizer have been reported
for water concentrations higher than R = 6.7.
In this paper, we examine the effects of water concentration on HPC adsorption. We demon-
strate for the first time that the amount of HPC
adsorbed onto TiO, increases markedly with water
concentration. This is shown to correlate with the
preferential adsorption of water on TiO, in
ethanol. We also report the effect of water
concentration on the mean diameter of TiO,
particles made over a much broader range of water
concentrations, 5.3
V.J. Nagpal et al.lColloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31 21
were filtered before mixing through a 0.2 urn filter.
The TEOT solutions were handled in a nitrogen
glove bag to prevent premature hydrolysis of the
ethoxide. The water concentrations in these experi-
ments varied over the range 5.3
28 1/J. Nagpal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31
o.5 I
I . M = B&M Kc/mole v MT = 1150 Kg/male
0.4 T 1
z .i 0.3 - E
E,z-\
s
;; \ 1 .z
0.1 - k- $
1 0.0 / I I I I I
0 10 20 30 40 50 60
R=[Water]/[TEOT]
Fig. 2. TEM mean diameter (urn) as a function of water concen- tration R ( = [H*O]/[ TEOT]) for [ TEOT] = 0.075 M, Cn,c = 1.7 g 1-r and different HPC molecular weights. The bars represent 10.
decreased from 60 to 43% as the HPC concen-
tration C,rc increased from 0.42 to 1.7 g 1-r. The
extent of conversion after 24 h increased with water
concentration, rising from 35% at R= 5.3 to over 99% for R>30 and was independent of HPC.
The reduction in particle size with water concen-
tration in the reaction mixture at a fixed polymer
and TEOT concentration can be due to the effect
of water concentration on (i) the kinetics of the
hydrolysis and condensation reactions, (ii) the
adsorption of HPC on TiO,, and (iii) the solubility
of HPC in the reaction medium. The effect of water
concentration on electrostatic stabilization of the
particles was not considered important here since
particles grown without HPC formed large, irregu-
lar aggregates at all water concentrations. The
effect of water concentration on the kinetics of the reactions of TEOT is well-documented [4].
Factors (ii) and (iii) will be discussed later.
E#ect of water concentration on HPC adsorption
The mass of adsorbed HPC per gram TiO,, r nPC, increased threefold as the water mole fraction
X, increased from 0.01 (R=5.3) to 0.12 (R=60)
at a fixed HPC molecular weight and concentration
as seen in Fig. 3(A). The error bars represent 50%
(2a/3) confidence limits. The adsorbed amount
l- nPc also increased with HPC molecular weight
since the longer chains have more segments to
adsorb onto the TiO,. This agrees with previous
theoretical and experimental studies of homo-
polymer adsorption [ 19,201. For C,= 1.7 g 1-l and
X,=0.01 (R=5.3), FHpC=5 x lop4 (g adsorbed
HPC E) (g TiO,)-, about ten times lower than
values for HPC E at 1.7 gl- in pure ethanol
reported by Jean and Ring [ 171. The discrepancy
Mw=l150 Kg/mole
c 006
/
Mw=68.5 Kg/mole,
coo1 1 T/ J ;--? Yw=68.5 Kg/mole. Cp=0.24 g/l
i l
n rlnn l I u VU
0.000 0 025 0.050 0.075 0100 Cl25 0 150
(A) R=5 3 Mole fraction water, X_ R=60
Fig. 3. (A) HPC adsorption isotherm, F (grams adsorbed HPC per gram TiO,) vs. mole fraction water X,, as a function of HPC concentration and molecular weight. The bars represent 2a/3. (B) Proposed water bridging between the TiO, surface and HPC via hydrogen bonding. Adsorption of an ethanol molecule at a Ti-OH site blocks adsorption by HPC.
V.J. Nagpal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31 29
could be due to the difference in the powder
preparation technique. That previous study used
TiO, powders which were dried prior to the experi-
ments whereas the powder in the present study
was never dried in order to replicate as closely as
possible the conditions of adsorption of HPC on
the growing TiO, particles. The surface chemistry
of TiO, powder can change significantly depending
upon the washing and the drying conditions [7]
which, in turn, might have a significant affect on
the polymer adsorption isotherm. Figure 4 shows
that water preferentially adsorbed on TiO, in
ethanol. This effect has not been reported before.
The plateau in the water adsorption curve occurs
at approximately the same water concentration
where the plateaux in the HPC adsorption iso-
therms occur in Fig. 3(A).
Solubility of HPC
The effect of water concentration on the solubil-
ity of HPC in the reaction medium is important
as this can affect the adsorbed amount FHpC and
the thickness of the adsorbed layer on TiO,.
Table 1 shows that the hydrodynamic diameter D,,
for HPC H in a mixture of ethanol and water
I / I I / , I
600 c 4
-0 _ 500 - i x
-& 2
400 -
\ B
300 - ;;/-t I
0.000 0.025 0.050 0.075 0.100 0.125 0.150
R=5.3 Mole fraction water, X_ R=60
Fig. 4. Adsorption isotherm of water on TiOz in ethanol as a function of mole fraction water X, (no added HPC). The bars represent 2a/3.
Table 1 Dependence of hydrodynamic diameter of HPC H on water concentration
Mole fraction water, X,
0.0 0.01 (R = 5.3) 0.12 (R=60) 1.00
M,z 1150 kg mol-.
& (nm?
70 72 78
210
bFrom dynamic light scattering measurements at 25C and a scattering angle of 90 using the second cumulants method
~241.
increased by approximately 7% over the range of
water concentration 5.3
30 V.J. Nagpal et ul.jColloids Surfaces A: Physicochem Eng. Aspects 87 (1994) 25 ~31
particle size owing to the combined effects of
increased HPC adsorption and nucleation rates.
This latter point was evidenced by the time
required for the onset of turbidity. For R = 5.3 at
all HPC concentrations, the typical time required
for the reaction mixtures to become turbid after
the addition of water was greater than 10 min
whereas, for R 230, the mixtures became turbid within 5 s.
Previous experimental and theoretical studies of
polymer adsorption have shown that the amount
of adsorbed polymer, Fp, can be described in terms
of the mean field lattice theory of Scheutjens and
Fleer as a function of two parameters: (i) the
polymer segmental adsorption free energy xskT
which reflects attractive interactions between
polymer segments and surface; (ii) the Flory
polymer-solvent interaction parameter 31 which
characterizes the polymer solubility [ 19,201. For a
fixed value of 1, Ip increases with the segmental
adsorption free energy XskT. For a fixed value of
ilskT> Fp increases with decreasing solubility
(increasing 71). Given the observation that the
solubility of HPC increases with water concen-
tration, the increase in FHpC with water concen-
tration can be explained qualitatively as being due
to an increase in xskT. The preferential adsorption
of water on TiO, in ethanol suggests a possible
mechanism for the enhanced adsorption of HPC.
It is possible that water may preferentially
adsorb since water forms a more polar and hence
stronger hydrogen bond with Ti-OH sites than
does ethanol. A definitive explanation for this will
require further work. The adsorbed water may
enhance HPC adsorption through the formation
of hydrogen bond bridges between Ti-OH surface
sites and the hydroxyl groups on the HPC as
depicted in Fig. 3(B). By contrast, ethanol cannot
participate in hydrogen bond bridge formation
between the Ti-OH surface and an HPC chain.
Adsorption of an ethanol molecule at a Ti-OH
site would block an HPC chain from forming a
hydrogen bond with that site. Whatever the precise
mechanism of enhanced adsorption, it is clear that
the effect of solvent composition on polymer
adsorption must be considered when choosing
reaction conditions. It is interesting to note that
the preferential adsorption of water on TiO, may
be related to the short range repulsive hydration
force proposed by Look and Zukoski to account
for TiO, particle formation [ 11,121.
A model for particle growth that predicts particle
size distribution requires an accurate calculation
of the particle pair interaction energy [ 11,121. An
accurate estimation of the steric interaction energy
in the present case is precluded by the polydisper-
sity of the polymer and by the fact that homo-
polymer adsorption typically occurs in trains, tails
and loops [ 191. Homopolymer adsorption theories
at present provide only qualitatively accurate pre-
dictions of the adsorbed layer thickness and seg-
ment density profile. An attractive alternative to
HPC for in situ steric stabilization experiments is
a block copolymer with narrow molecular weight
distribution where the anchor block and tail block
structures and molecular weights are chosen so
that the adsorbed copolymer forms a well-defined
brush layer [ 271. This would permit an accurate
calculation of the steric interaction energy.
Conclusions
TiOz particles were formed from the hydrolysis
and condensation of TEOT in the presence of an
in situ steric stabilizer, HPC. The mean particle
diameter when grown in the presence of HPC
decreased about fivefold as the water concentration
increased from R= 5.3 to R= 60 owing to the
combined effects of increased HPC adsorption and
increased nucleation rates. Mean diameters as
small as 70 nm were obtained. The amount of
adsorbed HPC increased threefold as the water
concentration R( = [ H,O]/[TEOT]) increased
from 5.3 to 60. This correlated with the preferential
adsorption of water on TiO, in ethanol which has
not been reported previously. A mechanism for
enhanced hydrogen bonding of the HPC to Ti-OH
surface sites was proposed in which the adsorbed
water might form hydrogen bond bridges between
Ti-OH sites and hydroxyl groups on HPC. Future
V.J. Nagpal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31 31
work will probe further the effect of solvent com-
position on the adsorption of HPC and related
polymers on TiO,. The difficulties connected with
estimating the steric interaction energy for the
HPC-TiO, system may be overcome by using
well-defined block copolymers. Future work will
involve in situ steric stabilization experiments
using block copolymers where the anchor block
and tail block structures and molecular weights
are chosen so that the adsorbed copolymer forms
a well-defined brush layer.
Acknowledgments
This work was partially supported by the
National Science Foundation under grant number
DMR-9005148-02. Support for V.J.N. was pro-
vided by Hercules Incorporated and the Virginia
Center for Innovative Technology. The authors
wish to thank Drs. C.F. Zukoski, J.L. Look and
M.T. Harris for helpful discussions and for provid-
ing preprints prior to publication. The authors
wish to thank Dr. M. Konas for performing the
gel permeation chromatography analyses of HPC.
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