Coupling between SAXS and Raman Spectroscopy applied to the
gelation of colloidal zirconium oxide systems
Alban Gossard, Guillaume Toquer*, Stéphane Grandjean, Agnès Grandjean
ICSM-UMR 5257, CEA/CNRS/UM2/ENSCM, BP 17171, 30207 Bagnols sur Cèze, France
guillaume.toquer@cea.fr
Abstract
The colloidal sol-gel transition based on zirconyl nitrate solution systems is investigated in this work.
The different steps occurring in the transition have been identified by coupling small angle X-ray
scattering (SAXS) with Raman spectroscopy and rheology measurements. The main experimental
condition effects on the transition are studied such as the zirconium precursor concentration and the
pH. The precise mechanisms involved during the transition are shown through a well understanding
of the nanostructure of these systems. In particular, the dissolution of zirconium salt leads to the
formation of cyclic tetramers which self-organize into cylinder shape. We clearly show that increasing
pH induces a strong attractive interaction between the cylinders giving rise to a mass fractal
dimension depending on the zirconium concentration. For each system, two characteristic pH values
have been evidenced via the rheological measurements analysis: the gelation is very slow below the
first one whereas the precipitation occurs above the second one. We argue that the complete
description of the quaternary system (ZrO(NO3)2, H2O, acacH, NH4OH) is an efficient formulation
guide for the synthesis of Zr-based ceramics with well-defined morphologies, and further shape by
using a templating route. Our methodology might be transposed to nuclear fuel oxide, as uranium
oxide or thorium oxide.
Keywords: sol-gel transition; SAXS; zirconium, colloidal systems, Raman spectroscopy, rheology.
1. Introduction
Zirconia based ceramics are widely used in many applications such as coating for optical properties or
for thermal barrier refractory materials, oxygen sensors and fuel cell membrane due to the mobility
of oxygen ions through the crystal structure [1]. Zirconia based materials are also used in dentistry
and as knife because of its hardness [2]. Last but not least, zirconia based materials are used in the
nuclear energy field as actinides surrogate and as host for minor actinides in view of their
transmutation. For example, the cubic yttrium stabilized zirconium oxide has been chosen as mixedoxide phases for americium transmutation [3].
The synthesis of nanostructured zirconia particles has received a great increasing attention in recent
years due its wide range of applications. These ceramics can be prepared by using a sol-gel approach
which has some significant advantages [4]. Indeed, it gives the possibility of preparing homogeneous
products at the nanoscale in case of mixing different oxide species and some specifically porous
structure can be obtained by mixing the initial reagent solutions at the sol stage [5]. Different sol-gel
methods could be employed for synthesis zirconia based materials depending on the nature of the
precursors used and then on the nature of the gel obtained. These methods are based on the use of
zirconium alkoxides following by a polymeric gel or mineral zirconium salts following with a colloidal
gel. There are many reports on the polymeric gelation of zirconium alkoxides, including both the
description of new materials obtained [6] and the studies investigating the behavior of the polymeric
systems during the sol-gel transition [7, 8]
Besides, some papers report the synthesis of nanostructured zirconia powder by a colloidal sol-gel
route. In case of colloidal sol-gel route applied to zirconia based materials, many kind of salt
precursors are available. The most widely studied is the zirconyl chloride (i.e. zirconium oxychloride
ZrOCl2), as precursors for new materials, for example to obtained pure tetragonal-ZrO2 nanopowders
[9], metastable tetragonal zirconia with high thermal stability [10] , or as media for studying the
structure and the kinetics of the transition from the colloidal sol to the gel [11] [12-15]. Some authors
propose the use of other salt as the zirconium carbonate in nitric acid [16] or zirconyl nitrate salt as
precursor [3].
An aqueous colloidal sol-gel process results in the conversion of a solution containing the oxide
precursors into an inorganic solid via some polymerization reactions induced by water. A
classification of different colloidal gels has been done in great detail by E. Zaccarelli [17]. This
colloidal sol-gel chemistry is rather complex due to the dual role of water as ligand and solvent and
to the wide number of reaction parameters which have to be controlled. Indeed, colloidal sol-gel
processes consist of the following steps: (i) preparation of a homogeneous solution containing the
inorganic salt as metal precursor;(ii) conversion of this solution into a colloidal suspension named sol
by adding chemical agent (usually acid/base); (iii) aging that leads to the destabilization of this sol
and to the gelation of the system; (iv) shaping and (v) thermal treatment. The formation of the sol
that can be defined as “inorganic polymer” occurs thanks to hydrolysis and condensation reactions
which are the transformation of the salt precursors into highly cross-linked inorganic chains.
Hydrolysis leads to a sol (colloidal particles dispersion), whereas condensation leads to a gel, which is
an interconnected inorganic network enclosing liquid phase. This transformation is called the sol-gel
transition.
All these experimental studies performed on zirconium suspensions obtained indifferently from
organic or from inorganic zirconium precursors are mainly based on (i) scattering techniques, as
Small Angle X-Ray Scattering (SAXS) [8] [18] [12] or Diffusion Light Scattering [16]; (ii) rheological
measurement [13], and (iii) more rarely on spectroscopic analysis as Raman Spectroscopy [14] [16].
It has been established early that the dissolution of zirconium precursors into an aqueous solution
leads to the formation of a cyclic tetramer ([Zr4(OH)8(H2O)16]8+) [19]. According to the previous
studies about the zirconium based sol, it seems that sols of zirconium consisted in zirconia-based
clusters, probably based on zirconia tetrameric building blocks [11] [12] [8], but this structure is still
under debate and depends strongly on the initial system conditions [14]. Depending on the pH and
also the content of zirconium precursors, the structure of the tetramer clusters has been described
as rod-like particles with variable length and constant cross section [16], or as two dimensional
sheets [19]. Moreover, in case chloride precursor, it has been found that these clusters would
contain different number of chloride ions [14].
Nevertheless, despite of these numerous investigations on the behavior of zirconia sol, the sol-gel
transition by increasing the pH is still not well understood. Recently, the structure of zirconium based
sol at different pH has been studied by using Small and Ultra-Small Angle Neutron Scattering. This
study suggests a small increase of the monomer unit of the clusters when increasing pH and a fractal
dimension behavior at higher pH [20, 21]. However, the influence of each species (zirconium
precursor, complexant, water and ammoniac contents) on the nanostructure of the zirconium based
sol, on the gel transition, and also on the reaction kinetics are still unclear. We argue that this
knowledge is crucial to finely control the transition from a sol to a gel in order to be able to shape at
a nanometer scale the final material. Moreover, the recent study about the sol-gel transition are
focused only on the mesostructure of the system and not on the macroscopic behaviour (as
rheological properties), neither on the local structure.
The objective of this paper is to develop experimental methods to follow the transition from a
colloidal nitrate zirconyl solution to the gel state. Here, we propose to describe the sol-gel transition
of quaternary system [ZrO(NO3)2, 6H2O + H2O + acacH + NH4OH] by coupling different experimental
techniques: SAXS, Raman spectroscopy and rheological measurements. A sol-gel route with
zirconium salts instead of using organic precursor is required here because the synthesis of materials
containing minor actinides with a classical sol-gel route is not appropriate, first because actinides
alkoxides are not available but more precisely because the radiolysis of the organic molecular
precursors would have a great negative safety impact (due to the hydrogen release induced by
hydrolysis). In the nuclear energy field, a colloidal sol-gel route is therefore more suitable. Nitrate
zirconium salts have been chosen because nitrate counter ions are known to be most neutral
concerning the cluster complexation compared to other counter ions such as chloride [22].
After a brief description of the experimental tools used for this study, each step of the sol-gel
transition has been clarified and explained. First the solution containing different concentration of
zirconium is described; the effect of the presence of a complexant (here acacH) on the mesostructure
and on the local structure is discussed; the sol formation and structure at different zirconium
concentration and different pH, and finally the sol-gel transition are structurally and macroscopically
studied. The so-obtained description of the entire phase diagram will be very useful to enable the
synthesis of zirconia ceramics with well-defined morphologies, and shape by using a templating
route. This approach could be transposed then to nuclear fuel oxide, as uranium oxide or thorium
oxide.
2. Experimental procedures
2.1. Materials and sample preparation
The different operations and reactions are performed at room temperature and in ambient
atmosphere.
ZrO(NO3)2, xH20 (from Sigma-Aldrich) is used as zirconium precursors and dissolved in de-ionized
water in order to obtain several solutions with different zirconium concentrations: 0.1M, 0.16M and
0.22M.
Acetylacetone (acacH) (from Sigma-Aldrich) is used as a complexing agent.
Afterwards the pH of these solutions is adjusted by addition of ammonia drop wise under vigorous
stirring to initiate the hydrolysis reactions. We observe spontaneously the formation of a white
precipitate which is re-dissolved after several minutes. A sol, with a behavior depending on the
system parameters, is then formed and left to stand. The viscosity of this sol increases over time and
the system gets slowly opaque. The gelation of the system occurs after a period depending on the
system parameters (zirconium concentration, acacH concentration, pH).
2.2 Characterizations
2.2.1. Rheology:
Rheological measurements were carried out using an AR1000-N (TA Instruments) rheometer working
in the constant strain module. The used geometry is a vane geometry (see figure 1) and the
measurements were performed in an oscillation mode in order to avoid some perturbation into the
growing tridimensional network. The constant strain was set to 2.10-4 and the oscillation frequency
to 1 rad.s-1 (i.e. 0.16 Hz), allowing to remain in the linear viscoelastic regime. This means that the
storage (G’) and loss (G”) moduli are amplitude independent. These moduli are parts of a complex
modulus G* characteristic of the system. G* is obtained after the measurement of the resultant
stress and the phase angle δ between strain and stress (tan δ=G”/G’).
Figure 1: Vane geometry used for rheological measurements
2.2.2 Raman Spectroscopy:
To probe the local structure of the solution, the sol and the gel samples, Raman spectroscopy
through confocal micro-Raman spectrometer (LabRAM ARAMIS, Horiba-Jobin Yvon) equipped with a
CCD detector was used. The 532 nm line of a coherent Ar+ laser was used as the exciting source. All
spectra were recorded between 300 and 1100 cm-1. After subtraction of a linear baseline, Raman
spectra were normalized to the total integrated intensity contained between 690 and 750 cm-1 which
is assigned to the vibration band of the nitrate[16]. Indeed the nitrate content is unchanged with
time for each composition consisting on a real suitable intern reference.
2.2.3. SAXS:
The Small angle X-ray scattering (SAXS) experiments were conducted using a Guinier-Mering setup
with a 2D image plate detector. The X-ray source was a molybdenum anode which delivered a highenergy monochromatic beam (λ=0.71 Å, E=17.4 keV), providing structural information over scattering
vectors q ranging from 0.01 to 1.5 Å−1. Helium flowed between the sample and the image plate to
avoid air adsorption. The sample acquisition time was 3600 s for sol and gel sample and only 600 s
for dedicated kinetic study. Data correction, radial averaging and absolute scaling were performed
with standard procedures. The image azimuthal average was executed by FIT2D software from ESRF
(France). The scattering curves were fitted to form factor models included in the SASfit software
package[23]. These models are described in detail in the SASfit manual[24]. Several fits have also
been checked via Guinier form factor[25] [ref Scattering Methods Applied to Soft Condensed Matter;
Linder, P., Zemb, T., Eds.; North Holland: Amsterdam, 2002] approximation.
The scattered intensity I(q) is written in the case of a monodisperse system as:
I (q)   V  2 P(q) S (q, )
where q is the scattering vector, V is the scattered object volume,  is the volume fraction of objects,
Δp² is the scattering length density contrast, P(q) is the form factor, and S(q, ) is the structure factor.
In the linear Guinier regime, by assuming no interaction (S(q, )1), the form factor of spherical
shape with radius Rs is approximated as the following:

3Rs 2 
P(q)  exp  
q 
 3 5

For cylindrical shape (elongated anisotropic shape) the geometric radius Rc is obtained from
 Rc 2 2 
qP (q )  exp  
q 
4


3. Results and Discussion
3. 1 Preliminary Study
The study of the colloidal sol-gel transition of the quaternary system [ZrO(NO3)2, 6H2O + H2O + acacH
+NH4OH] firstly requires the adjustment of the different physico-chemical parameters in order to
obtain a controlled gelation. The objective is to achieve a gelation time of a few hours to probe the
different distinguishable steps of the sol-gel transition during this period. The gelation time mainly
depends on different parameters such as the precursor concentration, the value of the [acacH]/[Zr]
ratio and the pH (i.e. the amount of added ammonia). Many experiments were achieved in order to
obtain systems with a well-controlled gelation time. In this way, the [acacH]/[Zr] ratio was fixed at
0.5 for our experiments, allowing the best compromise between the control of the system, that is to
say avoiding the precipitation of powder, and the gelation time.
Three zirconyl nitrate concentrations were chosen (0.1M, 0.16, 0.22M) as well as the pH range
corresponding to the target gelation time. Consequently, a range of systems (see figure 2) has been
defined and then the sol-gel transition has been studied from these concentrations.
On the figure 2, the dots correspond to different compositions of the system [ZrO(NO3)2, 6H2O + H2O
+ acacH +NH4OH] leading to a gelation in a few hours or less. Stable sols are obtained for low pH
values (i.e. low amount of added NH4OH). These sols will never gelify or otherwise after a very long
time. On the contrary, if the amount of NH4OH is too high, we observe the formation of an
indissoluble white precipitate and the gel state can never be reached.
Figure 2: Range of systems used for the study of the sol-gel transition. Open symbol are the critical
points obtained from rheological experiments (see part 3.4).
Based on the results of this preliminary study, each step of this sol-gel transition has been evidenced.
In a first time, the arrangement of the different species in the initial solution has been studied and
more particularly the influence of the precursor concentration and the presence or not of the
complexant.
3.2. Solution
According to the literature[26], the dissolution of zirconyl salts (nitrate, chloride...) in water consists
first on two consecutive spontaneous hydrolysis followed by an olation reaction. The olation reaction
implies the creation of hydroxy bridges or “ol” bridges between two zirconium atoms. It has been
shown that, in a first step, these reactions (hydrolysis and olation) lead to the formation of a cyclic
tetramer ([Zr4(OH)8(H2O)16]8+ )[19, 27, 28] (figure 3a) assumed to be spheric with a radius of gyration
of roughly 4 Å [26]. In a second step, these tetrameric units self-organize into cylindrical objects[7,
8, 11, 12], which are formed by the stack, one above the other, of (i) different tetramers in a
common plane[12] or (ii) single tetrameric units[8] (see figure 3b).
Figure 3a: Schematic view of the cyclic tetramer
Figure 3b: Stack of cyclic tetramers
3.2.1. Pure zirconyl nitrate solution
SAXS experiments were achieved on zirconyl nitrate solution at different concentrations (figure 4).
The SAXS spectra for the highest zirconyl nitrate concentrated (1.26 M) solution exhibits a pseudo
Bragg peak around 0.37Å-1, which would indicate the signature of a structure factor due to
detectable interactions between the scattering objects. For the lower concentrations, we assume no
specific interaction due to the lack of peaks and then of a significant structure factor. All these
solutions not evolve with the time and their corresponding SAXS experimental spectra have been
fitted by using a monodisperse rod-like particle form factor (see Figures 1 Supplementary
Information SI).
I(q)/(a. u.)
[Zr]
0.03M
0.1M
0.16M
10
0.22M
1.26M
1
0.01
0.1
q (Å-1)
Figure 4: Effect of the Zr precursor concentration in zirconyl nitrate solution on the SAXS data
The geometrical parameters of the scattering objects are extracted from these fits: the radius R fit and
the length L of cylinders (see figure 3b). Note also that the radius of the cylinder Rfit has been
checked to be in a good agreement with the radius obtained via a Guinier approximation of rod-like
particles RGuinier. Figure 5 shows the effect of the Zr precursor concentration on the radius and length
of the cylindrical objects.
Dimension (Å)
40
30
20
10
0.0
0.1
0.2
-1
[Zr] (mol.L )
Figure 5: Influence of precursor concentration on the geometry of the cylindrical structure. Sphere Symbol corresponds to the length of the
cylinder (L); square symbols to the Rguinier and open circle symbols to the Rfit from SAXS data.
Some differences can be observed between the objects as a function of the precursor concentration.
The measured value of the radius of the cylinder for a dilute system (4.9 Å for the solution with
[Zr]=0.03 M) is in the range of the initial tetrameric unit determined by Clearfield[19] (a little bit
smaller). When the zirconyle nitrate concentration increases, a very slow decrease of this radius is
noticed but this variation is not relevant (a few tenths of Angströms). This could be interpreted as the
compression of bond water due to the presence of different anionic species in the system, namely
the salting effect. Furthermore, more the precursor’s concentration is important, smaller is the
length of the cylinder (L decreases). Different hypotheses can be supposed: at high zirconium salt
concentration, either less tetramer is present into the cylindrical stack or this stack is more
compressed. These hypotheses can be plausible simultaneously. In any case, the presence of
cylinders of different sizes depends on their stabilization by electrostatic repulsive interactions into
the solution. We assume that for low zirconium salt concentrations, the tetrameric units are in a
smaller quantity into the solvent and they could have “more space” to self-organize into longer
cylinders before their stabilization.
3.2.2. Addition of acacH complexant
As also explained in the section 3.1, the addition of acacH into the initial solutions is used in order to
prevent, by complexation, the fast precipitation of aggregates. When the pH is increased, the acacH
complexants slow down the different kinetic reactions which normally occur into the system and
allow the formation of a monolithic gel. Here, the effect of the addition of acacH into the solution
was followed by Raman spectroscopy coupled with SAXS experiments.
Intensity (a. u.)
The vibration of metal-oxygen bonds are generally found in the 300 – 700 cm-1 range.
300
400
500
600
700
-1
Wavenumber (cm )
Figure 6: Raman spectrum of a zirconyl nitrate solution complexed by acacH. Triangles, square, asterisk and circle correspond respectively
to bands characteristic of tetrameric species, more condensed species, complexation by acacH and nitrates.
The figure 6 displays the Raman spectrum of a zirconyl nitrate solution with acacH having some
characteristic bands which are evidenced. First, the band at 450 cm-1 is typical of the cyclic tetramer
([Zr4(OH)8(H2O)16]8+) defined above (see section 3.2). Furthermore, more condensed species are also
detectable from the bands at 430 cm-1 and 540 cm-1 [16, 29], which would correspond to the
presence of stacks of cyclic tetramers into the solution. These two bands are assigned to Zr-O
vibration within the bridging hydroxy groups between two zirconium atoms. Then the complexation
of zirconium species by the acacH is also noticed on this Raman spectrum. Indeed, the band at
660 cm-1 corresponds to Zr-O vibration bounded to an alkyl chain[30]. Finally, the band at 720 cm-1 is
assigned to nitrate ions in solution, coming from the initial precursor[16].
The SAXS spectra from zirconyl nitrate solutions with and without the acacH complexants (see figures
S2) indicate that the complexation does not significantly modify the geometry of the cylindrical stack
of cyclic tetramer even if a decrease of the scattered intensity at low q in presence of acaH is noticed.
This could be ascribed as a more repulsive system than without complexant. This latter point is
consistent with the specific complexing role played by acacH. After the dissolution of the precursor
and the addition of a complexant, the further step of the sol-gel transition is the formation of a
colloidal dispersion by increasing the pH.
3.3. Sol formation
The addition of ammonia brings hydroxide anions in the system and this increases the pH of the
solution. Then, without complexant into the solution, a neutral cyclic tetramer [Zr4(OH)16(OH2)8]0 is
obtained by hydrolysis which rapidly condenses by olation until the formation of hydroxides
Zr4(OH)16. However, these hydroxides are not stable in solution due to the polarizing feature of
Zr(+IV). It results in dehydration, by oxolation, to form an oxy-hydroxide (ZrO(OH)2), which
spontaneously precipitates in solution[22, 26]. The addition of a complexant as acetylacetone into
the system would prevent this oxolation and then the precipitation of the oxy-hydroxide powder.
Indeed, the formation of bonds between zirconium and acacH, as observed previously on Raman
spectra, avoids some reaction between cylindrical stacks of cyclic tetramer when ammonia is added
and consequently the precipitation of zirconium hydroxides.
Furthermore, acetylacetone acts as a chelating agent on the surface sites of the cylindrical stacks and
creates a steric barrier stabilizing Zr-based colloids[18]. That’s why, by adjusting the ratio of
[acacH]/[Zr], it is possible to tune the hydrolysis which takes place after the ammonia addition. The
stabilization of particles of cylindrical stacks can thus be controlled leading to the formation of a
stable sol of amorphous zirconium oxy-hydroxide ZrO2-x(OH)2x,yH2O.
Besides, SAXS coupled with Raman spectroscopy experiments performed on system at different pH
allow the study of different stable sol of amorphous zirconium oxy-hydroxides.
SAXS experiments have been conducted on system at different pH. As already explained (see section
3.2), the addition of hydroxide anions into the solution containing isolated cylinders allows to the
formation of “ol” bridges between these cylinders and this leads to an size increase of the clusters,
and so their radius of gyration. The corresponding SAXS spectra are interpreted through the
formation of clusters composed of initial cylindrical stacks of cyclic tetramer.
Figure 7: Aggregates of cylinders initially present into the solution after increasing the pH
It is difficult to estimate the precise value of the cluster size when the particles become too
substancial, first because these clusters are certainly associated to a high polydispersity[9, 11, 15]
which gives this measure really complex. Moreover two simultaneous phenomena are present into
the system : the increase of the clusters size with the pH and the apparition of attractive interaction
between them. The contribution of each effect is very difficult to distinguish on SAXS spectra.
However, it is possible to obtain an estimation of the evolution of the clusters size present into the
different sol by using Guinier plots[15] under sphere assumption. This is justified by the fact that
reactions between cylinders might occur in each direction of the space as schematically shown on
figure 7. In this way,the figure 8 shows the SAXS data with Guinier fits for a zirconium concentration
of 0.16M at different pH. These fits have also been done for the other zirconium concentration and
show a similar behaviour when pH is increased has been observed. The more the pH is increased, the
more the Guinier plots are curved which is characteristic of the apparition of a higher polydispersity.
At high q, these different curves well superimpose which reveals the presence of the same primary
Ln(I(q))
units into the different systems namely the cylindrical stacks of zirconium tetramer. At low q, the rise
of the scattering intensity for higher pH shows both the increase of the gyration radius of the clusters
and the attractive interactions between them.
2
2
0
0
pH=5.1
pH=4.5
pH=3.5
pH=2.5
0.000
0.005
0.010
0.015
-2
-4
0.0
0.1
0.2
0.3
q2 (A-2)
Figure 8: Guiniers plot of SAXS spectra of a sol a different pH ([Zr] =0.16M. The inserted graph is a zoom at low q.
Due to the difficulty to obtain accurate measurement of cluster radii because of the presence of
these both simultaneous phenomena (increase of the clusters size and apparition of attractive
interaction between them), a specific fractal dimension can be useful to characterize these sols at
different pH. Indeed, the apparition of a fractal dimension is characteristic of the formed clusters and
gives information about the structuration within them.
According to the literature, if there exists a power law relation between I(q) and q (i.e. I(q) ∝ q-α),
then the parameter is defined as a fractal dimension in the range 1/Rg < q < 1/ξ where Rg is the
radius of gyration of the aggregates and ξ the radius of a primary particle[11]. But the fractal
structure can then be distinguished in two cases: mass and surface fractal model which depends on
the value of the fractal dimension α:
-
1<α<3, the fractal dimension Dm is a mass dimension and Dm=α.
3<α<4, the fractal dimension Ds is a surface dimension and Ds=6-α.
In our case, the primary particles are the cylindrical stacks of cyclic tetramer of zirconium. Then
thanks to the previous study of the different initial solutions with different content of precursor, the
radius of the primary particles (ξ) has been evaluated.
Thus, the curves I(q) as a function of q have been fitted with a fractal low in the range 1/Rg < q < 1/ξ .
The figure 9 shows the log-log plot of I(q) versus q for the system with a zirconium concentration of
0.16M at different pH. The same study has been done for several zirconium concentrations and a
similar trend has also been observed.
pH=5.1
Inorm (a. u.)
pH=4.5
1 pH=2.5
pHi
0.1
0.01
0.1
q(A-1)
Figure 9: SAXS data of initial solution containing [Zr]=0.16M and of different sol at different pH obtained from the same initial solution. The
black lines for curve obtained from solution at pH=4.5 and pH=5.1 correspond to linear fit from the fractal model.
pH
Dm
4.5
1.5
5.1
1.8
Table 1: mass fractal dimension of two “sol” at different pH from the same initial solution containing [Zr]=0.16M
In a first step, up to a pH of 2.5, no power-law regime is observed. But comparing the spectra done at
higher pH with the one obtained from the same initial system at pH=2.5 an increase of the intensity
of Inorm at low q is observed, which is interpreted as more attractive interactions and then to a weak
aggregation of the primary particles, i.e. the cylinder stacks of cyclic tetramer of zirconium. Then at
higher pH, the apparition of a power-law regime characteristic of a mass fractal dimension (see Table
1) is observed. Indeed, higher is the pH, higher is the fractal dimension Dm. This means that by
increasing pH, the clusters become more compact. This behavior would come from the formation of
“ol” brigdes between and within the cylinders.
In order to follow this formation of “ol” bridges and to validate this hypothesis, the formation of a sol
of fractal aggregates has been chemically studied by Raman spectroscopy (figure 10). Increasing the
pH of the solution leads to a significant shift of the band assigned to the Zr-O. The band at 430 cm-1,
assigned to the Zr-O vibration for condensed species, becomes prominent even though the band at
450 cm-1, characteristic of the tetrameric form, disappears. This shift clearly indicates some structural
changes of the species observed and the creation of Zr-O bonds formed by condensation of the
cylinders between and within them.
Inorm (a. u.)
0.4
Sol
0.2
Solution
300
400
500
600
700
Wavenumber (cm-1)
Figure 10: Raman spectrum of zirconyl nitrate solution complexed by acacH and sol of aggregates ([Zr]=0.16M and pHsol=5.3)
3.4 Sol-gel transition
After the formation of a sol of fractal aggregates, a phase of aging is necessary to obtain a gel. If the
pH of the sol is high enough, the system is sufficiently destabilized to observe a percolation of the
clusters. As interconnexions between them occur [31, 32], a 3-D lattice is formed and the viscosity of
the system increases leading to the gelation of the structure.
Raman spectra of the sol and the corresponding gel are quite similar (figure 11). The increase of
condensed species, observed on Raman spectra after normalization of the intensity, corresponds to
the apparition of new Zr-O bonds between and within the clusters. Furthermore, SAXS data from a
sol and the corresponding gel are given on figure 12 and are also very similar. Mass fractal dimension
Dm are measured from these data. With a Dm equal to 1.8 for the sol, we finally obtain a gel with
Dm=2. This increase of Dm is characteristic of a compaction of the structure[15]. These
characterizations prove that the objects inside the gel and the sol are still the same (clusters of initial
cylinders) and, with aging, they interconnect each other, creating more condensed and compact
species.
0.30
10
 (Zr-O condensed species)
sol pH=5.1
Gel
Fits
0.25
Inorm (a. u.)
1
Inorm (a. u.)
Gel
0.20
Sol
0.1
0.15
0.01
0.10
400
500
600
Wavenumber (cm-1)
700
0.1
q (A-1)
Figure 11 : Raman spectra of a sol of aggregates Figure 12: SAXS data of a sol (pH=5.1) and final
and a final gel ([Zr]=0.16M and pH=5.3)
gel for [Zr]=0.16M
Rheological experiments have finally been performed to follow this sol-gel transition. It is a suitable
tool to give a comparative and quantitative understanding of time of gelation (tg) and to describe the
mascroscopic behavior of the system during the sol-gel transition. The complex modulus G* is
defined into two distinct parts:
-
-
The storage modulus G’ (real part of G*) which represents the accumulated and stored
energy, in the material. This is the elastic potential energy and it can be recovered when the
system get back to the initial point.
The loss modulus G” (complex part of G*) which represents the dissipated energy because of
the viscous frictions and lost into calorific energy.
According to some previous works[33, 34], the gelation time can be described, in a first
approximation, as the time when the storage and the loss modulus are equal. After this time, the
storage modulus, characteristic of the gel, becomes predominant in comparison with the loss
modulus, characteristic of the liquid state. This theory is not fully exact as reported by Winter and
Chambon[35]. The loss and storage moduli depend actually on the frequency of the experiment
oscillation (unless both G’ and G” are proportional to the square root of the frequency). These
authors proved that, at the gelation time, the ratio G”/G’ is independent from the frequency. We can
thus determine the gelation time as the intersection of the different curves G”/G’ versus time for
different frequencies[36].
Note that in our case, a unique single frequency (1 rad.s-1) is used for all experiments. A weak
approximation is therefore done on the results. However, we consider that the assessment of the
gelation time using the intersection of both moduli G’ and G”, is accurate enough for this study.
Indeed, the aim of these rheological measurements is not the determination of an exact gelation
time but only the description of the macroscopic behaviour of the system during the sol-gel
transition depending on the pH and the precursor concentration and the comparison of the different
systems.
An example of the evolution of G’ and G” as a function of time is shown on the figure 13. The general
shape of this curve is representative of the system evolution and is always the same for all the
different studied systems. For t lower than tg, the loss modulus G” is higher than the storage modulus
G’ (which is close to zero) and remains approximately constant. The system is still a sol and has the
rheological behavior of a liquid. Near to the gelation point, G’ and G” begin to increase and an elastic
component appears within the system. Finally both moduli increase quickly and, as G’ increases
much faster than G”, the storage modulus becomes predominate over the loss modulus. Visually, this
corresponds to a clouding of the solution associated to an increase of the viscosity. The gelation is
considered reached when G’=G” and from this point, the gel contribution prevails. The
interconnexions between clusters become also sufficient to support a load elastically.
G'
G"
1.0
600
500
t(gel) (min)
G' and G" (Pa)
[Zr] = 0.1M
[Zr] = 0.16M
[Zr] = 0.22M
700
1.2
0.8
400
0.6
300
0.4
200
0.2
100
0.0
0
20
40
60
time (min)
80
100
120
0
4.5
5.0
5.5
6.0
6.5
7.0
pH
Figure 13: Characteristic evolution of G’ and G”
in function of time (example for [Zr]=0.16M and Figure 14: Variation of the gelation time as a
function of the pH for different precursor
pH=5.5)
concentrations
The gelation time has been measured as a function of the pH for different precursor concentrations
and is reported on figure 14. For each precursor concentration, the influence of the pH is obvious
and two regimes can be distinguished. At higher pH, the gelation time is short (lower than 60min)
and from a specific pH (pHs), characteristic of the precursor concentration (pHs=5 for [Zr]=0.22M,
pHs=5.5 for [Zr]=0.16M, pHs=6.1 for [Zr]=0.1M), the gelation becomes very much slower.
Furthermore, we observe that, for concentrated systems, the gelation pH is lower than for diluted
systems. Indeed, the aggregates tend to interconnect faster because of the presence of more
precursors in solution and consequently, the addition of less hydroxide is necessary to trigger the
gelation.
From these experimental data of gelation time, we can define a critical pH when gelation time is
close to zero which depends drastically on the zirconyl nitrate concentration. It is difficult to measure
this critical pH (pHc), but according to the variation of the gelation time as a function of pH, we can
consider that for [Zr]=0.1M pHc = 6.7, for [Zr]=0.16M pHc = 6.1, and for [Zr]=0.22M pHc = 5.3. This
range of critical pH has been reported onto the pseudo phase diagram from preliminary study (see
figure 2). The values are in a good agreement with the domain in which the system precipitate into
oxy-hydroxyde powder. Furthermore, it is clear that beyond a certain value of the pH, the time of
gelation is very low or even zero. Thus, we can observe that the different areas defined into the
preliminary study are well confirmed by the rheological study.
4. Conclusion
The mechanisms involved in the chemical reactions which take place in different zirconium colloidal
systems have been clarified. Each step, from the solution to the sol-gel transition has been
scrutinized by using different characterization techniques. These steps are the following:

The dissolution of zirconyl salts (nitrate, chloride...) leads to the formation of cyclic tetramer
([Zr4(OH)8(H2O)16]8+) units which then self-organize into cylindrical objects. The length of


these cylinders decreases when the zirconium concentration increases. This can be explained
either by a compaction within the cylinder or by a decrease of the amount of cyclic tetramer
units within the cylinders.
The addition of hydroxide anions, by increasing pH, into the solution containing isolated
cylinders allows the formation of some clusters composed of initial cylindrical stacks of cyclic
tetramer. Increasing the pH of zirconyl nitrate solution containing a complexant (acacH) leads
to the aggregation of the cylinder stacks of the cyclic tetramer of zirconium, with a specific
fractal dimension. A mass fractal dimension can be extracted from SAXS experiment.
Moreover higher is the pH, higher is the fractal dimension meaning a compaction of the
clusters. This behaviour has been interpreted by the creation of “ol” bridges between and
within the cylinder through Raman spectroscopy analysis.
After the formation of a sol of fractal aggregates, a phase of aging is necessary to obtain a
gel. Rheological experiments have been performed to follow precisely the sol-gel transition.
The effect of pH on the sol-gel transition is obvious and two regimes have been then
distinguished. At higher pH, the gelation time is short (<60min), with a critical pH (pHc) when
gelation time is close to zero, and below a specific pH (pHs) the gelation becomes very much
slower. Both pHc and pHs depend strongly on the zirconyl nitrate concentration. We observe
that, for concentrated systems, these latter parameters are lower than for diluted systems.
Indeed, at high zirconium salt concentration, the addition of less hydroxide is necessary to
trigger the gelation due to the presence of more precursors.
This study provides the keys to explore and understand a colloidal system based on zirconium salts
and the associated mechanisms which occur during the sol-gel transition, for different
concentrations of precursor and as a function of pH.
Figure 15: Monolithic gel of ZrO2-x(OH)2x,yH2O obtained after the sol-gel transition
Consequently, the control of this sol-gel transition enables the synthesis of zirconia ceramics with
well-defined morphologies, and suggests new opportunities to shape zirconium oxide monoliths by
using a templating route coupled to this sol-gel route. As for example, a picture of monolithic gel of
ZrO2-x(OH)2x,yH2O obtained after the sol-gel transition is shown on figure 15.
Acknowledgement
Financial support from CEA-DEN and CNRS (needs matériaux) are greatly acknowledged. The authors
are grateful to Dr Causse and Dr Zemb for interesting discussions.
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