Chemical and structural properties of nickel hydroxide xerogels obtained by the sol-gel procedure in the presence of acetic acid

Article abstract of DOI:10.1111/j.1151-2916.1999.tb01868.x

The sol-gel route to glasses and ceramics has attracted an increasing amount of scientific and technological interest recently. In this process, sols with different concentrations are used as precursors for xerogels and to produce materials that consist of fine oxide particles. In the present work, nickel hydroxide gels have been obtained via the hydrolysis of a molecular precursor in the presence of acetic acid. The chemical aspects of the material transformation have been studied by using Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC) for different acetic acid contents and several heat-treatment temperatures. The carboxylic acid acts as a ligand at a molecular level in the precursor, therefore modifying the entire hydrolysis and condensation process. Small-angle X-ray scattering (SAXS) studies and density measurements have been performed for the structural characterization of the xerogels. A denser, final oxide material is obtained when a higher acetic acid concentration is used. The porosity of the dry gels coarsens when they are heat-treated up to a temperature of approximately 400 °C and its density decreases. The material that has been heat-treated up to a temperature of 800 °C densifies and exhibits a finer porosity. The chemical properties at a molecular level satisfactorily explain and are well correlated with the structural characteristics of the studied material.

Full text of DOI:10.1111/j.1151-2916.1999.tb01868.x

J. Am. Ceram. Soc., 82 [4] 1020–24 (1999)
ournal
J
Chemical and Structural Properties of Nickel Hydroxide Xerogels
Obtained by the Sol–Gel Procedure in the Presence of Acetic Acid
Pramod K. Sharma, Hannes Fischer, and Aldo F. Craievich†
Instituto de F ´ı sica, Universidade de S ˜a o Paulo, CP-66318, 05315–970 S ˜a o Paulo, Brazil
The sol–gel route to glasses and ceramics has attracted an
increasing amount of scientific and technological interest
recently. In this process, sols with different concentrations
are used as precursors for xerogels and to produce mate-
rials that consist of fine oxide particles. In the present work,
nickel hydroxide gels have been obtained via the hydrolysis
of a molecular precursor in the presence of acetic acid. The
chemical aspects of the material transformation have been
studied by using Fourier transform infrared (FTIR) spec-
troscopy and differential scanning calorimetry (DSC) for
different acetic acid contents and several heat-treatment
temperatures. The carboxylic acid acts as a ligand at a
molecular level in the precursor, therefore modifying the
entire hydrolysis and condensation process. Small-angle X-
ray scattering (SAXS) studies and density measurements
have been performed for the structural characterization of
the xerogels. A denser, final oxide material is obtained
when a higher acetic acid concentration is used. The poros-
ity of the dry gels coarsens when they are heat-treated up to
a temperature of ∼ 400°C and its density decreases. The
material that has been heat-treated up to a temperature of
way that almost fully hydrolyzed precursors are formed and,
after an adequate heat-treatment procedure, fine powdered
products are obtained.
Several metal oxides—e.g., TiO , SiO , ZrO , Al O , etc.—
2
2
2
2 3
have already been synthesized via the sol–gel method, which
2
–5
leads to materials with a very fine porosity.
Despite the
importance of nickel hydroxide/oxide materials for electro-
chemical reactions and electrochromic devices, only a few in-
vestigations that are related to their preparation procedures
6
,7
have been conducted.
In this paper, we describe a new pathway for the preparation
of nickel hydroxide/oxide materials in the presence of acetic
acid using the sol–gel route. Acetic acid has been used as a
catalyst to better control the rate and extent of the hydrolysis
reaction of the nickel precursor.
Fourier transform infrared (FTIR) spectroscopy and differ-
ential scanning calorimetry (DSC) have been used to investi-
gate the possibility of an additional role of the acetic acetate
anion, namely its action as a ligand directly bound to the nickel
ion. Small-angle X-ray scattering (SAXS) and density mea-
surements have been performed to study the structural charac-
teristics of the porous xerogels. We have investigated the trans-
formations of different xerogels, which have been obtained
from sols with different acetic acid contents, in samples that
have been heat-treated at several temperatures, up to 800°C.
8
00°C densifies and exhibits a finer porosity. The chemical
properties at a molecular level satisfactorily explain and
are well correlated with the structural characteristics of the
studied material.
I. Introduction
II. Experimental Procedure
EW procedures for obtaining glasses and ceramics have
N
been developed recently. These procedures lead to new
A precursor solution was prepared by dissolving a known
materials that are able to meet the challenges of nonclassical
amount of NiCl in butanol. The solution was stirred for 30 min
2
1
applications. Among them, the so-called “sol–gel” technique
and then 0.001–0.1 mol of acetic acid was added. The solution
was refluxed at a temperature of 70°C for 30 min under con-
tinuous stirring to ensure a perfect homogeneity. Then, a con-
trolled amount of water was added to the solution. The final
solution was refluxed again for 3 h more and then cooled to
room temperature. A schematic description of the sol–gel path-
way that has been used in this work is shown in Fig. 1.
The FTIR and DSC chemical studies were performed on
xerogels with different acetic acid concentrations ([C]). The
heat-treatment temperatures range from 200°C to 450°C and
from 60°C to 450°C for the FTIR and DSC studies, respec-
tively. The DSC analysis was performed using a scan rate of
10°C/min.
The SAXS measurements were performed at the SAXS
beamline of the National Synchrotron Light Laboratory
(Campinas, Brazil). A monochromatic X-ray beam with an
almost-pointlike cross section at the focusing plane and a
wavelength ␭ of 1.608 Å was used. A one-dimensional X-ray
position-sensitive detector was used to record the scattering
intensity as a function of the modulus of the scattering vector
q (q ס (4␲/␭) sin ␪, ␪ being one-half of the scattering angle).
The parasitic scattering from air and the beamline windows
was subtracted from the total measured intensities. No math-
ematical desmearing of the experimental results was necessary,
because of the almost-pinhole-like geometry of the X-ray
beam. The bulk density of the sample was measured using a
has a large domain of applicability. In this process, colloidal
particles in liquid solution aggregate, gel, and transform into a
disordered, branched, and continuous network, which is inter-
penetrated by the liquid solvent. Hydrolysis and polyconden-
sation of organometallic precursors in the solution, followed by
gelation and drying, are the main steps of the sol–gel route
toward a highly porous material called xerogel. Subsequent
heat treatment at high temperature leads to fine-grained oxide
powders and, under particular conditions, monolithic and dense
materials.
By varying the initial composition of the solution and other
reaction parameters, wet gels and xerogels (dried gels) with a
large variety of structural characteristics can be obtained. For
example, the type of catalyst, acidic or basic, strongly affects
the rate of hydrolysis and condensation. It has been demon-
strated that acid catalysts control the hydrolysis rate in such a
N. J. Dudney—contributing editor
Manuscript No. 190255. Received April 16, 1998; approved August 17, 1998.
Supported by FAPESP and CNPq.
Author to whom correspondence should be addressed.
†
1020

April 1999
Properties of Nickel Hydroxide Xerogels Obtained via Sol–Gel
1021
Fig. 1. Schematic diagram of the preparation of the studied xerogels.
standard densitometer for temperatures in the range of 60°–
00°C.
8
III. Results
An X-ray diffractometry (XRD) characterization of the stud-
ied materials indicated the presence of a crystalline Ni(OH)2
phase up to a temperature of ∼ 300°C. Above this temperature,
a Ni O phase starts to appear. A single, crystalline NiO phase
2
3
is obtained when the material is heated to temperatures of
Ն450°C. These phase transformations in materials that have
been treated at increasing temperature involve the classical
processes of nucleation and growth of the new phases. These
processes and the crystallographic-related aspects have been
8
intensively studied in the past.
The morphology of the Ni(OH) particles was established by
2
using scanning electron microscopy (SEM). A micrograph
taken at a temperature of 250°C revealed approximately spheri-
cal particles, with sizes of 100–120 nm.
The FTIR spectra of a sample heat-treated up to a tempera-
ture of 200°C are plotted in Fig. 2 for different [C] values. The
spectrum that corresponds to a powder with [C] ס 0.1 mol
−
1
exhibit a broad absorption peak at 3555 cm and sharper peaks
−
1
in the wavenumber range of 1000–1600 cm . The absorption
−
1
peak at 1570 cm shifts toward a higher wavenumber value
−
1
(
0
i.e., 1600 cm ) as the [C] value increases from 0.001 mol to
Fig. 2. FTIR spectra of xerogels synthesized in the presence of acetic
acid with the indicated acetic acid concentrations ([C]) and heated up
to 200°C. (The units “a.u.” denote arbitrary units.)
−
1
.1 mol. Similarly, the peak at 1440 cm shifts toward 1450
−1
cm .
On the other hand, the absorption peaks in the wavenumber
−
1
range of 1000–1600 cm decrease in intensity as the tempera-
ture increases from 200°C up to 300°C and totally disappear
when the sample is heated at 450°C, as shown in Fig. 3. The
broadening of the peak at 3550 cm decreases as the tempera-
ture increases.
temperature for all the [C] values. An extra endothermic peak
at 168° is apparent in the DSC curve of the xerogel that cor-
responds to the highest [C] value (0.1 mol).
The spectra in Fig. 5 are the SAXS intensities (I(q)) that are
produced by xerogels with different [C] values and a final
heat-treatment temperature of 800°C. All the spectra exhibit an
almost-linear behavior in a log I vs log q representation. The
value of the modulus of the slope ␣ for [C] ס 0.001 mol is
2.23 and decreases down to 1.97 as the value of [C] increases.
−
1
The DSC results are plotted in Fig. 4. The curve that corre-
sponds to the xerogel with an initial [C] value of 0.001 mol
shows three endothermic peaks, at 105°, 202°, and 280°C. The
peaks at 202° and 280°C shift to higher temperatures (i.e., 225°
and 310°C, respectively) when the value of [C] increases from
0
.001 mol to 0.1 mol. The peak at 105°C remains at the same

1022
Journal of the American Ceramic Society—Sharma et al.
Vol. 82, No. 4
Fig. 5. Log–log data of SAXS intensity versus the modulus of the
scattering vector (q) for the different indicated acetic acid concentra-
tion [C] (in mol) corresponding to samples heat-treated at 800°C.
Statistical errors range from 1% at low q values up to 3% at high q
values. (The units “a.u.” denote arbitrary units.)
Fig. 3. FTIR spectra of xerogels corresponding to [C] ס 0.001 mol
and heated to the indicated heat-treatment temperatures. (The units
“
a.u.” denote arbitrary units.)
Fig. 6. Log–log data of SAXS results of xerogels heat-treated at the
indicated temperatures with [C] ס 0.003 mol. Statistical errors range
from 1% at low q values up to 3% at high q values. (The units “a.u.”
denote arbitrary units.)
IV. Discussion
The broad absorption peak at 3555 cm−1 in the spectra in−
Fig. 4. DSC curves of xerogels with [C] ס 0.001 and 0.1 mol.
Arrows indicate the temperature (in °C) of the main peaks.
Fig. 2, for different [C] values, was attributed to the OH
9
−1
stretching mode. The peaks in the range of 1000–1600 cm
−
were attributed to organic-matter bonds and to OH bonds of
1
0
−1
The effect of temperature on the parameter ␣ in samples with
C] ס 0.003 mol is shown in Fig. 6. This figure indicates that
adsorbed water. The peaks at 1450 and 1530 cm were
assigned to C−C and C−O bonds, respectively. The spectra
show a major contribution from organic matter that corre-
sponds to the solvent and catalyst. The separation between the
[
the value of ␣ increases from 2.28 to 3.07 for temperatures
rising from 60°C to 450°C and then decreases down to 2.07 for
the xerogel that has been heated to 800°C. A similar trend of ␣
with temperature, with a maximum at ∼ 400°C, was observed
for all the other [C] values, as shown in Fig. 7.
The effect of different acetic acid concentrations on the den-
sity of the xerogels that have been heat-treated up to 800°C for
−
1
absorption peak of the ␥sym(COO) at 1570 cm and the shoul-
der of ␥asym(COO) at 1440 cm− and 140–170 cm indicates
1
−1
that CH COO acts as a bidentate ligand with the nickel atom
3
7
,11–13
as
1
h is shown in Table I. The density of the powders increases
as [C] increases. The density of the xerogels with [C] ס 0.001
mol are shown in Table II as a function of the heat-treatment
temperature. The density exhibits a minimum value at 450°C.

April 1999
Properties of Nickel Hydroxide Xerogels Obtained via Sol–Gel
1023
was observed in Guinier plots (log I vs q2),15 we concluded that
the pores have a rather wide size distribution. The linear be-
havior in log–log plots of the SAXS intensity (Figs. 5 and 6)
implies that the q dependence of the SAXS intensity is given by
I(q) ϰ q−
␣
(1)
This behavior is expected for the intensity produced by a sys-
tem that is composed of spheroidal particles (or pores) that are
embedded in a homogeneous matrix, with a potential function
−
␥ 16
for the radius distribution of g(R) ϰ R . It has been demon-
strated that, under the above-mentioned specified condition,
␥
ס 7 − ␣ for three-dimensional systems. In real porous ma-
terials, g(R) is generally defined in a range of Rmin < R < ϱ,
where Rmin is a minimum threshold value of the pore radius.
Under these assumptions for the size-distribution function, the
average pore radius size 〈R〉 is given by
ϱ
ϱ
−
␥+1
ming(R)R dR
minR
dR
͐R
͐R
〈
R〉〉
ס
(2)
(3)
ϱ
ϱ
−
␥
͐Rming(R) dR ͐RminR
dR
Fig. 7. Parameter ␣ versus temperature for the indicated acetic acid
concentrations [C] (in mol). Error bars are smaller than the symbols.
Consequently, the quotient 〈R〉/Rmin is
͗ ͘
R
1 − ␥ 6 − ␣
Table I. Effect of Acetic Acid Concentration on the Density
〉
〉
and Average Pore Size of Xerogels Heat-Treated at 800°C
Rmin 2 − ␥ 5 − ␣
3
Acetic acid concentration (mol)
Density (g/cm )
〈R〉/Rmin (a.u.)
Equation (3) implies that the value of 〈R〉 increases as the value
of ␣, which is the modulus of the slope in linear log–log plots
of the SAXS intensity function, increases.
Our SAXS results that are presented in Fig. 5 indicate that ␣
decreases as [C] increases. Therefore, by applying Eq. (3), we
concluded that samples with a higher [C] value promote a
more-compact structure that contains smaller pores. The SAXS
result is consistent with that obtained via FTIR spectroscopy:
higher concentrations of ligand acetate ions would promote the
formation of materials with a finer porosity.
0
0
0
.001
.005
.1
6.35
6.42
6.55
1.36
1.34
1.33
Table II. Effect of Temperature on the Density
and Average Pore Size of Xerogels with an
Acetic Acid Concentration of 0.001 mol
3
Temperature (°C)
Density (g/cm )
〈R〉/Rmin (a.u.)
The maxima of parameter ␣ (Figs. 6 and 7) imply, indepen-
dent of the [C] value, that the material exhibits a coarser po-
rosity (a larger 〈R〉 value) when it is treated at an intermediate
temperature (∼ 400°C) than when treated to lower or higher
temperatures. The increase in 〈R〉 when the samples are heated
from 60°C up to 250°C is due to the evaporation of physically
and chemically absorbed water (as was also observed via
DSC). The additional increase in pore size when the samples
are heated to ∼ 450°C is a consequence of the evaporation of
organic compounds. This observation was also experimentally
verified independently via FTIR spectroscopy on the sample
with [C] ס 0.001. The decreasing value of 〈R〉 when the tem-
perature is increased from ∼ 400°C up to 800°C suggests a
structural rearrangement, which leads to a densification of the
inorganic matrix that is attributed to the startup of a sintering
process.
The results that have been reported in Table I indicate an
increase in the density of xerogels when the value of [C] is
increased from 0.001 mol to 0.1 mol. This observation can be
understood as a consequence of the formation of a more-
compact structure that is composed of smaller particles and
pores, for higher acetic acid contents. This result is in good
qualitative agreement with our SAXS results (average pore size
in Table I) and is consistent with the conclusion from FTIR and
DSC measurements: the increase in density would be a direct
consequence of the increase in the number of nickel–ligand
bonds.
6
0
5.85
5.36
6.36
1.34
1.39
1.36
4
8
50
00
−
1
Moreover, the shifts of the ␥sym(COO) peak at 1570 cm
−
1
toward a higher wavenumber (i.e., 1600 cm ) as the value
of [C] increases from 0.001 to 0.1 mol, and, similarly, the
␥asym(COO) shoulder shift toward 1450 cm− are expected,
because the increase in the value of [C] increases the number
of strong metal–ligand bonds. Therefore, the FTIR spectra
that are presented in Fig. 2 clearly demonstrate that the acetate
anions act as ligands that modify the precursor at a molecular
level through a controlled hydrolysis and condensation during
the evolution of the aggregation process.
The decrease in the intensity of the absorption peaks in the
wavenumber range of 1000–1550 cm and the reduction of
broadening of the OH stretching peak at 1600 cm with in-
creasing treatment temperature (Fig. 3), are due to the removal
of organic matter during heating.
The two endothermic peaks at 105° and 202°C in the DSC
curves (Fig. 4) were attributed to adsorbed water on the surface
and chemisorbed water, respectively. The endothermic peak at
1
1
4
7
−
1
−
−1
2
80°C for low [C] values (0.001 mol) was associated with the
transformation of nickel hydroxide to nickel oxide (still not
stoichiometric). The shift of this peak toward a higher tem-
perature (310°C) for a higher [C] value (0.1 mol) was attrib-
uted to a strengthening effect on the xerogel structure that was
produced by the increasing number of chemical bonds between
the ligand and nickel. This observation is in good agreement
with the results of our FTIR study.
We considered the observed SAXS intensity, as produced by
the interparticle and intraparticle nanopores and micropores
that exist in xerogel samples. Because no clear linear behavior
Table II reports a decrease in density when xerogels are
heated from 60°C to 450°C. This observation can be attributed
to the increase in pore size that is due to the evaporation of
organic matter, as also suggested by our SAXS results (average
pore size in Table II) and FTIR studies. Table II also indicates
that, when the temperature increases from 450°C to 800°C,
xerogels densify again, as a consequence of the correlated re-
duction in pore size.

1024
Journal of the American Ceramic Society—Sharma et al.
Vol. 82, No. 4
V. Conclusion
Acetic acid is used as a catalyst in the sol–gel route toward
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2
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act as strong bidentate ligands with nickel ions. Acetic acid
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The increase in the temperature of transformation of nickel
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