Zirconia and titania nanoparticles studied by electric hyperfine interactions, XRD and TEM

Article abstract of DOI:10.1016/j.jallcom.2006.08.087

Nanocrystalline ZrO₂ and TiO₂ (n-ZrO₂, n-TiO₂), synthesized in a microwave plasma, have been investigated by X-ray and electron diffraction and by perturbed angular correlation (PAC) measurements of the nuclear electric quadrupole interaction (QI) of the probe nucleus ¹⁸¹Ta residing on the cation site. The microwave synthesis produces zirconia in the cubic/tetragonal phase, titania in the anatase structure. Grain growth and phase transformations have been studied in bare and Al₂O₃-coated zirconia particles. Coating the nanoparticles with an amorphous Al₂O₃ layer obstructs grain growth and may suppress the monoclinic phase.

Full text of DOI:10.1016/j.jallcom.2006.08.087

Journal of Alloys and Compounds 434–435 (2007) 590–593
Zirconia and titania nanoparticles studied by electric hyperfine
interactions, XRD and TEM
a,∗
a
b
c
c
S. Schlabach , D.V. Szab o´ , D. Vollath , P. de la Presa , M. Forker
a
Institut f u¨ r Materialforschung III, Forschungszentrum Karlsruhe GmbH, P.O. Box 3640, D-76021 Karlsruhe, Germany
b
NanoConsulting, Primelweg 3, D-76297 Stutensee, Germany
c
Helmholtz Institut f u¨ r Strahlen- und Kernphysik, University of Bonn, Nussallee 14-16, D-53115 Bonn, Germany
Available online 29 September 2006
Abstract
Nanocrystalline ZrO
2
2 2 2
and TiO (n-ZrO , n-TiO ), synthesized in a microwave plasma, have been investigated by X-ray and electron diffraction
1
81
and by perturbed angular correlation (PAC) measurements of the nuclear electric quadrupole interaction (QI) of the probe nucleus Ta residing on
the cation site. The microwave synthesis produces zirconia in the cubic/tetragonal phase, titania in the anatase structure. Grain growth and phase
transformations have been studied in bare and Al
grain growth and may suppress the monoclinic phase.
2006 Elsevier B.V. All rights reserved.
2 3 2 3
O -coated zirconia particles. Coating the nanoparticles with an amorphous Al O layer obstructs
©
Keywords: Nanostructures; Crystal structure and symmetry; Scanning and transmission microscopy
1
. Introduction
using the Karlsruhe microwave plasma process (KMPP). This process is a gas
phase process capable of producing bare and coated particles [2,3] with a nar-
row particle size distribution [4]. Using a microwave plasma discharge and an
Ar/20 vol.%O2 gas mixture as reaction gas, evaporated water free precursors
The interaction between a nuclear electric quadrupole
moment Q and the tensor of the electric-field gradient (EFG)
at the nuclear site is determined by the charge distribution sur-
(
Ti(OC4H9)4, Zr(OC4H9)4, AlCl3) react to nanosized oxides at T ≤ 900 K. The
relatively low temperatures avoid sintering of the particles and the evolution
of hard agglomerates. The coating of single particles is realized by using two
plasma stages consecutively in a closed system. The synthesized powders are
collected by thermophoresis.
−3
rounding the probe nucleus. Because of the r -dependence of
the EFG the main contributions come from the nearest neigh-
bor charges. Strength and symmetry of the nuclear electric
quadrupole interaction (QI) therefore provides information on
the structure of the solid on a nanometer scale and comple-
ments the structural information on nanoscaled solids obtained
from X-ray and electron diffraction. We are presently engaged
in a program in which structure, grain growth and phase trans-
formations of nanocrystalline (n) ceramics are investigated by
perturbed angular correlation (PAC) measurements [1] of elec-
tric quadrupole interactions, X-ray diffraction and transmis-
sion electron microscopy. In this contribution, we present some
results for n-ZrO2 and n-TiO2.
The particles were characterized in the as-synthesized state by X-ray diffrac-
tion (XRD) and transmission electron microscopy (TEM) (Figs. 1 and 2). XRD
was carried out with a Philips (X’Pert) diffractometer using Cu K␣ radiation at
50 kV and 40 mA on non-compacted powder samples. Grain growth and phase
evolution were studied by room temperature XRD measurements of powders
annealed in air for 6 h at different temperatures (Fig. 3).
Electron microscope and electron diffraction patterns of powder samples on
holey carbon copper grids were recorded with a Philips (CM-30 ST or Tecnai
F20 ST) transmission electron microscope.
The PAC measurements were carried out with the 133–482 keV cascade of
1
81
−
181
Ta which is populated in the ␤ decay of the 45d isotope Hf. In the synthe-
1
80
sis the nanoparticles were doped with the stable isotope Hf by adding about
4
at.% of the corresponding Hf precursor. For the generation of the 1 Hf/ Ta
81
181
probe nuclei, non-compacted powder was enclosed under vacuum into quartz
2
. Experimental
13
2
tubes and irradiated in a flux of thermal neutrons of 5 × 10 n/(s cm ) for times
of the order of 24 h. The PAC spectra were taken with a standard four-detector
Nanosized samples of bare ZrO2 and TiO2 (n-ZrO2, n-TiO2) as well as
setup equipped with fast BaF scintillators as a function of temperature in the
2
Al2O3-coated n-ZrO2 and n-TiO2 with sizes typically <5 nm were synthesized
range 290 K ≤ T ≤ 1500 K. In the study reported here, the samples were cycled
M
between room-temperature and increasing values of TM. In Fig. 4, the room tem-
1
81
perature PAC spectra of Ta in coarse grained ZrO2 and TiO2 are compared
to those of bare and coated nano-oxides. Fig. 5 illustrates the thermal evolution
of the PAC spectra, using n-ZrO2 and n-ZrO2/Al2O3 as examples.
∗
Corresponding author. Tel.: +49 7247 824471; fax: +49 7247 823956.
E-mail address: sabine.schlabach@imf.fzk.de (S. Schlabach).
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.08.087

S. Schlabach et al. / Journal of Alloys and Compounds 434–435 (2007) 590–593
591
Fig. 1. Electron micrograph (a) and X-ray diffraction (b) of bare n-ZrO2. For comparison h k l-values of c-ZrO2 (JCPDS card no. 27-0997) are given in (b).
Fig. 2. Electron diffraction (a) and X-ray diffraction (b) of bare n-TiO2. For comparison d-values of reference anatase (JCPDS card no. 21-1272) and rutile (JCPDS
card no. 21–1276) structure are given.
3
. Results and discussion
structure. This reverse phase stability at very small particle
sizes is described by several authors [for TiO2 see e.g. 5–7]. It
can be attributed to the fact that the possible phases differ in the
surface contribution to the free energy. As the relative surface
The electron and X-ray diffraction data (Figs. 1 and 2)
clearly show that the structure of the as-synthesized powders
differs from that usually observed in the bulk material at room
temperature: n-ZrO2 is in the cubic or tetragonal rather than the
monoclinic phase, n-TiO2 has the anatase rather than the rutile
Fig. 3. Room temperature X-ray diffraction pattern of n-ZrO2 as synthesized
and after annealing for 6 h at the temperatures given. The insert shows the grain
size as a function of the annealing temperature.
Fig. 4. 181Ta PAC spectra in coarse-grained (cg-) and nanocrystalline n-ZrO2
and n-TiO at 300 K.
2

5
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S. Schlabach et al. / Journal of Alloys and Compounds 434–435 (2007) 590–593
Fig. 6. The thermal evolution of the monoclinic and the tetragonal phase in bare
1
81
and Al2O3-coated ZrO2 nanoparticles, observed by
The inserts show the m ↔ t transitions of bulk ZrO2.
Ta PAC spectroscopy.
Fig. 5. PAC spectra of 181Ta in bare and Al2O3-coated ZrO2 nanoparticles at
different temperatures.
non-periodic oscillation of monoclinic ZrO2 and an admixture
of the periodic pattern of tetragonal ZrO2 is observed only at
T ≥ 1400 K. In coated n-ZrO2/Al2O3, it is the tetragonal phase
which first appears upon heating, the monoclinic phase is sup-
pressed practically up to T ∼ 1350 K. The relative intensities of
the monoclinic and the tetragonal phase in the spectra of Fig. 5
have been extracted using standard PAC analysis. The results
are collected in Fig. 6 and compared (see inserts) to the phase
transformations in the bulk material. In non-coated n-ZrO2 the
monoclinic phase develops gradually. The transformation to the
tetragonal phase occurs at about the same temperature and is as
sharp as in cg-ZrO2. An interesting result which asks for fur-
ther investigation is the observation that upon cooling to 300 K
the relative intensity of the monoclinic phase is significantly
reduced. This reduction implies that at 300 K a part of the probe
nuclei is subject to another unresolved high frequency QI. The
fact that this reduction does not occur in the bulk material sug-
gests a relation of this phenomenon to the interfacial region of
the nanoparticles.
part increases with decreasing particle size, the phase stability
in the nm-range may differ from that of the coarse grained (cg-)
material [see e.g. 8,9]. In the TEM micrographs which basically
reflect the cation sub-lattice, one recognizes a fairly well estab-
lished long range order. Fig. 3 illustrates the changes of the XRD
spectra of n-ZrO2 upon annealing. The annealing process leads
to (i) the transformation of the initially c/t-phase to monoclinic
ZrO2 and (ii) to grain growth, shown by the decrease of the line
width of the X-ray peaks with increasing annealing temperature
TA. The insert shows the grain size determined from the
line width using the Scherrer-formula [10]. The exponential
increase of the grain size with increasing TA corresponds to
a grain growth activation enthalpy of QA ∼ 30 kJ/mol, in fair
agreement with the result of Siu et al. [11] for n-ZrO2 prepared
by the hydrothermal method. Similar annealing experiments
for Al2O3-coated ZrO2 and n-TiO2 are under way.
In Fig. 4 we compare the 1 Ta PAC spectra of n-ZrO2, bare
and Al2O3-coated n-TiO2 to those of the corresponding coarse-
grained material. In the case of cg-ZrO2 and cg-TiO2 the time
spectrum of the anisotropy shows the well-known non-periodic
oscillatory structure typical for the axially asymmetric QI of
m-ZrO2 [12] and rutile TiO2 [13], respectively. In the nanocrys-
talline powders, however, the oscillations of the anisotropy are
completely wiped out. This is a clear indication that the probe
nuclei no longer experience a single well-defined electric field
gradient (EFG), but are subject to a broad distribution of dif-
81
A comparison between the PAC spectra of 181Ta in n-ZrO2
and n-ZrO2/Al2O3 (Fig. 5) and the corresponding relative phase
intensities (Fig. 6) clearly shows that a coating of the parti-
cles with amorphous Al2O3 strongly affects the phase transfor-
mations in nanocrystalline zirconia. In contrast to non-coated
zirconia, at T > 700 K the content of tetragonal ZrO2 increases
continuously, while the monoclinic phase is almost entirely sup-
pressed. The difference can be attributed to the volume expan-
sion of ∼4.5%, which accompanies the tetragonal-monoclinic
transformation in zirconia. In particles mechanically confined by
an Al2O3 coating, this expansion leads to compressive stresses
which stabilize the tetragonal phase. Akin to the obstructed
phase transformations, the grain growth of the coated particles
is hindered, too. The coated material exhibits ∼40% of the par-
ticle size of theirs uncoated counterparts after annealing under
similar conditions.
−
3
ferent QI’s. Because of the r dependence of the EFG, the
measurements of the QI mainly sample the charge distribution
of the nearest-neighbor environment of the probe nucleus. In
ZrO2 and TiO2, oxygen ions are the nearest neighbors of the
metal sites. The broad QI distribution seen by 181Ta on the Zr-
or Ti-site therefore implies a high degree of disorder of the oxy-
gen sublattice which contrasts with the long-range order of the
cation sublattice seen in TEM micrographs.
Upon annealing, the broad QI distribution characteristic for
the nanoparticles in the as-synthesized state evolves towards to
a well defined QI. This gradual transition is illustrated in Fig. 5
for n-ZrO2 and n-ZrO2/Al2O3. In bare n-ZrO2, one first finds the
4. Conclusion
The combination of electron and X-ray diffraction with mea-
surements of electric quadrupole interactions allows comple-

S. Schlabach et al. / Journal of Alloys and Compounds 434–435 (2007) 590–593
593
mentaryinsightsintostructure, phasetransitionandgraingrowth
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grained oxides. With increasing temperature, grain growth and
phase transformations have been observed. For n-ZrO2 the grain
growth activation enthalpy has been determined. Coating the
particles with amorphous Al2O3, suppresses the monoclinic
phase in zirconia and also obstructs grain growth.
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