Oxidation state and lattice expansion of CeO
2 Àx
nanoparticles as a function of particle size
Lijun Wu, H. J. Wiesmann, A. R. Moodenbaugh, R. F. Klie, Yimei Zhu, D. O. Welch, and M. Suenaga
Materials Science Department, Brookhaven National Laboratory, Upton, New York11973, USA
Received 7 November 2003; published 24 March 2004
Cerium oxide nanoparticles CeO
2-x
3–20 nm in diameter made by a vapor phase condensation method,
have been studied by several methods of transmission electron microscopy TEM: electron energy loss
spectroscopy EELS, high resolution imaging, and electron diffraction. The white-line ratios M
5
/ M
4
of the
EELS spectra were used to determine the relative amounts of cerium ions Ce
3+
and Ce
4+
as a function of
particle size. The fraction of Ce
3+
ions in the particles rapidly increased with decreasing particle size below
15 nm in diameter. The particles were completely reduced to CeO
1.5
at the diameter of 3 nm. This
reduced cerium oxide has a fluorite structure which is the same as that of bulk CeO
2
. Also, EELS spectra taken
from the edge and center of the particle indicated that for larger particles the valence reduction of cerium ions
occurs mainly at the surface, forming a CeO
1.5
layer and leaving the core as essentially CeO
2
. A microme-
chanical model based on linear elasticity was used to explain the lattice expansion of the CeO
2-x
nanopar-
ticles. Comparing our results with previously published works indicates that the amount of CeO
1.5
in CeO
2-x
nanoparticles is a strong function of the particular synthesis methods used to make these particles.
DOI: 10.1103/PhysRevB.69.125415 PACS numbers: 73.22.-f, 68.37.Lp, 79.20.Uv, 61.46.+w
I. INTRODUCTION
Ceria has been widely studied since it is easily reduced or
oxidized and this property is very important for its applica-
tions, such as catalysts in vehicle emissions-control systems
1
and electrolyte materials in solid oxide fuel cells.
2
For appli-
cations as a catalyst, fine particles of ceria are fabricated to
increase surface area in order to enhance the catalytic effi-
ciency. In order to understand the properties of fine ceria
particles, a large number of studies have been carried out
on catalytic,
3,4
electronic,
5
lattice vibrational,
6,7
and
structural
8–12
properties as well as on various synthesis
methods
12–14
for ultrafine ceria particles. However, in spite
of the practical importance of and the scientific interests in
these nanosized particles, no systematic study of properties
as a function of size had been made until a series of articles
by Tsunekawa et al. were published.
5,8,10,13
Using so-called
monodisperse nanoparticles of ceria, which were made by a
hydrothermal process,
13
with different dimensions, they ob-
served that the lattice parameters, as determined by electron
diffraction, of the nanoparticles increased with decreasing
particle size.
8
Based on the analysis of the particle size de-
pendence of the lattice parameter, they suggested that this
expansion was due to the loss of oxygen from the surface
region of CeO
2
particles. Also, they claimed that the particle
of CeO
2 -x
would be fully reduced to CeO
1.5
when the size of
the particles became 1.5 nm and its structure would be the
C-type cubic sesquioxide Ce
2
O
3
. This structure of Ce
2
O
3
has not been observed in bulk ceria although it is a common
structure in some other lanthanide oxides. The structure of
bulk Ce
2
O
3
is an A-type hexagonal sesquioxide.
15
Further-
more, using an x-ray photoelectron spectroscopy technique
XPS they confirmed the existence of Ce
3 +
ions in the small
particles and the increased ratio of Ce
3 +
/Ce
4 +
with decreas-
ing size.
5
In addition, they claimed in this article that these
Ce
3 +
ions are in fact primarily in the surface region of the
particles in agreement with their earlier suggestion. More
recently, in their theoretical study, they attributed the ob-
served lattice expansion to the decrease of the electrostatic
force caused by the valence reduction of Ce ions in the
ceria.
10
Recently, the lattice expansion as a function of particle
size has also been measured by x-ray diffraction measure-
ments for monodispersed particles which were prepared by a
room-temperature precipitation method.
12
Interestingly, as
discussed in detail below, Zhang et al.
12
found much smaller
expansions of the lattice than those found in the study by
Tsunekawa et al.
5
For example, Zhang et al. reported a lat-
tice expansion of 0.3% for 7 nm ceria particles while
Tsunekawa et al.
5
reported 0.8% for similar-sized particles.
Thus, further investigation of the electronic and structural
properties of nanocrystalline CeO
2 -x
as a function of the
particle size is of interest.
In this article, we report the results of an investigation of
the valence of Ce ions in CeO
2 -x
nanoparticles as a function
of their size using electron microscopy techniques: electron
energy loss spectroscopy EELS, high resolution imaging,
and electron diffraction. The particles were prepared by va-
por phase condensation of CeO
2
in an inert gas atmosphere.
The advantage of using EELS in a high resolution transmis-
sion electron microscope is that a single nanoparticle can be
examined, and the size and crystal structure of that indi-
vidual particle may be determined simultaneously. Other
techniques with larger probe sizes, such as XPS, require a
large assembly of particles with unavoidable variations in
size, even when the new synthesis techniques have narrowed
the size distribution
12,13
of the so-called monodisperse nano-
particles. In addition, with the improved spatial resolution of
EELS, it is possible to examine directly the local variations,
such as bulk vs surface regions, in the Ce
3 +
/Ce
4 +
ratios
within a given particle.
II. EXPERIMENTAL PROCEDURE
The cerium oxide nanoparticles were synthesized using
the technique of thermal evaporation of CeO
2
in a helium
PHYSICAL REVIEW B 69, 125415 2004
0163-1829/2004/6912/1254159/$22.50 ©2004 The American Physical Society 69 125415-1