CeO2 nanorods and gold nanocrystals supported on CeO2 nanorods as catalyst

Article abstract of DOI:10.1021/jp052978u

The formation mechanism of uniform CeO₂ structure at the nanometer scale via a wet-chemical reaction is of great interest in fundamental study as well as a variety of applications. In this work, large-scale well-crystallized CeO₂ nanorods with uniform diameters in the range of 20-30 nm and lengths up to tens of micrometers are first synthesized through a hydrothermal synthetic route in 5 M KOH solution at 180 ?°C for 45 h without any templates and surfactants. The nanorod formation involves dehydration of CeO₂ nanoparticles and orientation growth along the ???110??? direction in KOH solution. Subsequently, gold nanoparticles with crystallite sizes between 10 and 20 nm are loaded on the surface of CeO ₂ nanorods using HAuCl₄ solution as the gold source and NaBH₄ solution as a reducing agent. The synthesized Au/CeO ₂ nanorods demonstrate a higher catalytic activity in CO oxidation than the pure CeO₂ nanorods. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp052978u

J. Phys. Chem. B 2005, 109, 19169-19174
19169
CeO2 Nanorods and Gold Nanocrystals Supported on CeO2 Nanorods as Catalyst
P. X. Huang,† F. Wu,†,‡ B. L. Zhu,§ X. P. Gao,*,† H. Y. Zhu,*,| T. Y. Yan,† W. P. Huang,§
S. H. Wu,§ and D. Y. Song†
Institute of New Energy Material Chemistry, Department of Materials Chemistry, Nankai UniVersity,
Tianjin 300071, China, School of Chemical Engineering and EnVironmental, Beijing Institute of Technology,
Beijing 100081, China, Department of Chemistry, Nankai UniVersity, Tianjin 300071, China, and Inorganic
Materials Research Program, School of Physical and Chemical Sciences, Queensland UniVersity of
Technology, GPO Box 2434, Brisbane, Qld 4001, Australia
ReceiVed: June 4, 2005; In Final Form: August 23, 2005
The formation mechanism of uniform CeO2 structure at the nanometer scale via a wet-chemical reaction is
of great interest in fundamental study as well as a variety of applications. In this work, large-scale well-
crystallized CeO2 nanorods with uniform diameters in the range of 20-30 nm and lengths up to tens of
micrometers are first synthesized through a hydrothermal synthetic route in 5 M KOH solution at 180 °C for
45 h without any templates and surfactants. The nanorod formation involves dehydration of CeO2 nanoparticles
and orientation growth along the 110 direction in KOH solution. Subsequently, gold nanoparticles with
crystallite sizes between 10 and 20 nm are loaded on the surface of CeO2 nanorods using HAuCl4 solution
as the gold source and NaBH4 solution as a reducing agent. The synthesized Au/CeO2 nanorods demonstrate
a higher catalytic activity in CO oxidation than the pure CeO2 nanorods.
I. Introduction
in fundamental study. Also, it has been reported that CeO2 and
rare earth oxide nanocrystals can significantly increase the
As an important class of nanomaterials, the nanorod has
attracted enormous technological and scientific interest.1 In
synthesizing metal oxide nanorods, such as ZnO,2,3Al2O3,4,5
MgO,6 CuO,7,8 SnO2,9 MnO2,10 Co3O4,11 V2O5,12 and Eu2O3,13
many approaches have been applied in the past several years,
including thermal evaporation, microemulsion method, hydro-
thermal synthesis, template methods, and electrochemical
deposition technique. Hydrothermal reaction under moderate
conditions is an effective approach in synthesizing nanosize
crystals of inorganic oxides and hydroxides. In many cases,
alkaline solution is used in the hydrothermal synthesis in which
the shape and size of the crystals are well-controlled. For
example, anatase and titanate nanotubes and nanorods were
obtained hydrothermally in NaOH solution,14-17 and lanthanide
hydroxide nanowires were synthesized successfully via hydro-
thermal reaction in KOH solution.18 The hydroxide nanorods
or titanate nanorods can be converted into the corresponding
oxide nanorods through a subsequent dehydration process at
high temperature.
Ultrafine CeO2 particles of regular shape and uniform size
are desired in a variety of applications, i.e., ceramic ingredient,
fluorescent powder, polishing agent, catalyst carrier, catalyst
for the exhaust gas treatment from automobiles, and solid
electrolyte for solid oxide fuel cells.19-22 Recently, it has been
reported that the polycrystalline CeO2 nanowires can be
fabricated in anodic alumina membranes (AAM) as tem-
plates,23,24 and crystalline CeO2 nanorods can be formed through
a surfactant assemblage because the surfactant in water can form
a chain structure.25-27 The formation mechanism of the uniform
CeO2 nanorod, via a wet chemical reaction, is of great interest
catalytic activity of dispersed Au nanoparticles for CO
oxidation.28-32 The decoration of Au nanoparticles on the
support of stable CeO2 nanorods could efficiently prevent Au
nanoparticles from the agglomeration. In the present study, we
report a simple approach for the synthesis of large-scale CeO2
nanorods with single crystalline structure in alkaline solution,
without any templates and surfactants. Subsequently, gold
nanoparticles are loaded on the so-fabricated CeO2 nanorods
by using HAuCl4 solution as a gold source and NaBH4 solution
as a reducing agent. The catalytic activity of Au/CeO2 nanorods
in CO oxidation is measured.
II. Experimental Section
1. Sample Preparation and Characterization. In a typical
synthesis, metallic cerium was first dissolved in concentrated
hydrochloride acid to obtain cerium chloride solution, and 5 M
KOH solution was added gradually into the cerium chloride
solution under stirring until the cerium ions precipitate com-
pletely. The precipitates (CeO2‚nH2O) were aged for 30 min in
air and then washed repeatedly with deionized water to remove
Cl- anions in the precipitates. The wet precipitates were then
mixed with 5 M KOH solution (40 mL) using an ultrasonic
bath. The obtained mixture was transferred in a stainless
autoclave with a poly(tetrafluoroethylene) (PTFE) container and
maintained at 180 °C for different reaction durations. The solid
was recovered from the autoclaved mixture, rinsed with water
until a value of about pH 7 was reached. Gold nanoparticles
were loaded on the CeO2 nanorods by dispersing 0.1 g of the
nanorods in 40 mL of 0.1 M HAuCl4 solution. The impregnated
nanorods were separated by centrifugation, and NaBH4 solution
(10 mL, 0.1 M) was added to the solid. The solid was then
recovered, rinsed with deionized water to remove Cl- anions,
and dried at 100 °C. The so-obtained product (CeO2 nanorods)
was further dried at 60 °C in air for 1 day. The microstructure
of the prepared CeO2 powders was characterized by transmission
* To whom correspondence should be addressed. E-mail:
xpgao@nankai.edu.cn (X.P.G.); hy.zhu@qut.edu.au (H.Y.Z.).
† Department of Materials Chemistry, Nankai University.
‡ Beijing Institute of Technology.
§ Department of Chemistry, Nankai University.
| Queensland University of Technology.
10.1021/jp052978u CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/23/2005

19170 J. Phys. Chem. B, Vol. 109, No. 41, 2005
Huang et al.
Figure 1. SEM images of CeO2 nanorods synthesized at different reaction conditions: (a) before the hydrothermal treatment, (b) 5 M KOH at 180
°C for 4 h, (c) 5 M KOH at 180 °C for 24 h, (d) 5 M KOH at 180 °C for 45 h, (e) 5 M KOH at 150 °C for 45 h, (f) 10 M KOH at 150 °C for
45 h, and (g) 10 M KOH at 180 °C for 45 h.
electron microscopy (TEM; FEI Tecnai 20) and X-ray diffrac-
tion (XRD, Rigaku D/max-2500) with a Cu KR radiation. Since
the as-synthesized nanoparticles were suspended in solution, a
droplet of it was also deposited on a Cu grid covered with a
carbon film for TEM analysis. X-ray photoelectron spectra
(XPS) analyses were investigated using PHI-5300 ESCA in
order to assess the chemical state and element distribution on
the catalyst surface.
sand. The reaction gas mixture consisting of 1% CO balanced
with air is passed through the catalyst bed at a total flow rate
of 33.6 mL/min. The reactant and product composition were
analyzed on-line by a GC-508A gas chromatograph equipped
with a thermal conductivity detector (TCD).
III. Results and Discussion
2. Catalytic Activity. The catalytic activity measurements
of the catalysts for CO oxidation were carried out in a fixed
bed flow microreactor (7 mm i.d.) under atmospheric pressure
using 50 mg of catalyst powder doped with 2 g of fine quartz
Scanning electron microscopy (SEM) images of CeO2 na-
norods synthesized at different hydrothermal reaction conditions
are shown in Figure 1. The big agglomerated particles are
obtained before the hydrothermal treatment. It can be seen that

CeO2 Nanorods and Gold Nanocrystals
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19171
Figure 2. XRD patterns of the CeO2 nanorods synthesized at 180 °C
for 45 h and CeO2‚nH2O nanoparticles.
CeO2 nanorods can be formed at the hydrothermal temperature
of 180 °C for 4 h though some nanoparticles still coexisted.
Longer reaction duration of the hydrothermal treatment improves
of the purity of CeO2 nanorods and enhances the aspect ratio
of CeO2 nanorods. The alkaline concentration and hydrothermal
temperature are observed to have a strong effect on the structure
and morphological features of the resulting CeO2 nanorods. The
morphological transformation from particles to nanorods is not
complete when the hydrothermal temperature is below 150 °C
at 5 or 10 M KOH solution, as shown in Figure 1e,f. It reveals
that the hydrothermal temperature is a key parameter that
influences the nucleation and crystal growth of CeO2 nanorods,
which is also found for the preparation of titanate nanorods
under the hydrothermal condition in NaOH solution.17 However,
the shape of the hydrothermal products is irregular when the
alkaline concentration is higher than 5 M at 180 °C. In particular,
the large quantity of thicker rods attached with thin nanorods
is formed in 10 M KOH solution at 180 °C, indicating that some
nanorods further grow and become thicker and longer at the
expense of thinner nanorods being dissolved in the higher
alkaline concentration.
Figure 3. Ce3d (a) and O1s (b) core level spectra observed for CeO2
nanorods synthesized at 180 °C for 45 h.
CeO2‚1.1H2O, and the synthesis process of CeO2 nanorods can
be described as follows:
4Ce3+ + 12OH - + O2 + (4n - 6)H2O f 4(CeO2‚nH2O)
CeO2‚nH2O f CeO2 + nH2O
The XRD patterns of CeO2 nanorods synthesized at 180 °C
for 45 h and CeO2‚nH2O nanoparticles are illustrated in Figure
2. All the diffraction peaks for the products before and after
hydrothermal treatment in alkaline solution can be indexed to
(111), (200), (220), (311), (222), (400), (331), and (420) phases,
corresponding to a face-centered cubic (fcc) fluorite structure
for CeO2 (JCPDS Card No. 43-1002). There are not any other
impurities of the hexagonal structure of Ce(OH)3 or Ce(OH)-
CO3 detected. It is well-known that the Ce(III) oxidation state
is unstable as compared with the Ce(IV) oxidation state in
alkaline solution and in the presence of air. From the Ce3d core
level peak in the XPS spectra shown in Figure 3, it is clear that
the cerium exists as the Ce(IV) oxidation state (882.8 eV),
without any impurity of the Ce(III) oxidation state. Also, the
XPS peak centered at 529.6 eV corresponds to the O2-
contribution.33 Thus, the XPS-detected binding energies of
Ce3d and O1s are in agreement with the standard CeO2. Ther-
mal gravimetrical analysis (TGA) of CeO2‚nH2O nanopart-
icles indicates a mass loss of about 10.7 wt % below 600 °C,
due to the loss of water from the sample. Both nanopart-
icles and nanorods exhibit a light yellow color, similar to the
standard CeO2. Therefore, the nanoparticles are deduced to be
The XRD peaks for CeO2‚1.1H2O nanoparticles are obviously
broader, compared with those of the nanorods, indicating that
the crystallites of the nanoparticles are smaller than those in
the CeO2 nanorods. The hydrothermal treatment in alkaline
solution at 180 °C results in a remarkable improvement in
crystallinity. As can be seen clearly from TEM images of the
nanorods synthesized at 180 °C for 45 h and nanoparticles
(Figure 4), we obtained the straight CeO2 nanorods of a high
purity with uniform diameters in the range of 20-30 nm and
lengths up to tens of micrometers (Figure 4a). It is also
confirmed by energy-dispersive X-ray spectra (EDS, Figure 4b)
that there are not any impurities, indicating that sodium ions
and chlorides are completely removed during washing. Cu and
C signals are coming from the Cu grid and carbon film
supporting the specimen in TEM observation. The selected area
electron diffraction (SAED) pattern (insert in Figure 4c)
indicates the single crystalline nature of the CeO2 nanorods with
a face-centered cubic structure, and this is consistent with the
XRD results. The calculated interference fringe spacing in the
HRTEM image is about 0.31 nm, which is consistent with the

19172 J. Phys. Chem. B, Vol. 109, No. 41, 2005
Huang et al.
Figure 4. TEM images (a) and EDS (b) of the CeO2 nanorods synthesized at 180 °C for 45 h. The HRTEM images of the CeO2 nanorods and
CeO2‚nH2O nanoparticles are shown in c and d. The inserts in c and d are SAED of individual CeO2 nanorod and CeO2‚nH2O nanoparticles,
respectively.
interplanar distance of the (111) plane in the cubic fluorite
structure. This suggests that the preferred orientation growth
of the individual CeO2 nanorod is along the 110 direction,34
similar to that of the CeO2 nanorods obtained by PEG surfactant-
assisted preparation.26 The HRTEM image and SAED pattern
also confirm that the precursor nanoparticles have a polycrys-
talline structure with smaller crystallite sizes of 6-9 nm (Figure
4d). Therefore, the hydrothermal treatment in alkaline solution
is essential to the formation of large-scale CeO2 nanorods via
the orientation growth.
As shown in Figure 5a, the gold nanoparticles are dispersed
effectively on the surface of the CeO2 nanorods synthesized at
180 °C for 45 h and are not agglomerated. The gold nanopar-
ticles on the CeO2 nanorods have crystallite sizes between 10
and 20 nm. The interplanar spacing of the gold nanoparticle in
HRTEM images (Figure 5b) is ca. 0.24 nm, corresponding to
the interplanar distance of the (111) plane of fcc Au nanocrystal
as found in Au/TiO2 nanotubes.36,37 The energy-dispersive X-ray
spectrum (EDS, as inserted in Figure 5a) confirms the presence
of Au element. The observation of the diffractions peaks of
metallic gold (JCPDS Card No. 04-0784) in the XRD pattern
(Figure 6) confirms that gold exists as metallic particles. It is
noted that the surface of the CeO2 nanorods, after the decoration
of gold nanoparticles, is roughened as compared to that of the
pure CeO2 nanorods, indicating a strong interaction at the
interface of Au nanoparticles and CeO2 nanorods during the
reduction in HAuCl4 and NaBH4 solution. Moreover, the surface
of a gold nanoparticle is covered by a thin discrete layer of the
cerium oxides (1-2 nm) attributed to migration of the oxide
onto the gold surface.38 The interference fringe of the (111)
plane of inner CeO2 nanorods is clearly observed in the HRTEM
image (Figure 5b), demonstrating a good crystallinity and a high
stability of CeO2 nanorods after the decoration of Au nanopar-
ticles. Similarly, gold nanoparticles supported on CeO2 nano-
particles are also found with the preferential (111) plane of fcc
Au nanocrystal (interplanar spacing is ca. 0.24 nm as shown in
Figure 5d). However, the dispersed state of gold nanoparticles
on CeO2 nanoparticles is relatively poor as compared with that
on CeO2 nanorods.
The nanorods of titanate,16,17 lanthanide hydroxide,18 and
CuO8 have been synthesized hydrothermally in a concentrated
alkaline solution without any templates and surfactants. It
appears that there are common features and a probably common
mechanism for the formation of these metal oxides and hydrous
oxides. We believe that the Ostwald ripening process, in which
larger crystallites grow at the expense of smaller crystallites
being dissolved,35 is involved in the structure transformation
from the polycrystalline CeO2‚1.1H2O nanoparticles to the single
crystalline CeO2 nanorods during the hydrothermal treatment
at 180 °C for 45 h. The above formation mechanism is different
from the polycrystalline or nanocrystalline CeO2 nanowires
prepared by the AAM method.24,25 Also the OH- concentration
in the reaction system could be the key factor controlling the
growth rate of different crystal faces and leading to the formation
of a rodlike morphology. In the hydrothermal process, the
nanosize CeO2 crystallinates grow along the 110 direction in
the concentrated KOH solution by consuming the CeO2‚1.1H2O
precipitates, yielding the rodlike structure.

CeO2 Nanorods and Gold Nanocrystals
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19173
Figure 5. TEM and HRTEM images of Au/CeO2 nanorods (a and b) and Au/CeO2 nanoparticles (c and d). The EDS spectrum of Au/CeO2
nanorods is inserted in a.
Figure 6. XRD pattern of Au/CeO2 nanorods.
Figure 7. Catalytic activities of CeO2 nanorods and Au/CeO2 nanorods
(solid) as compared with CeO2 nanoparticles and Au/CeO2 nanoparticles
(open) at different reaction temperatures.
Catalytic activities of CeO2 nanorods and gold nanoparticles
on the CeO2 nanorods as a function of reaction temperature are
shown in Figure 7. It is noted that the CO conversion increases
with increasing reaction temperature for all samples. An 81%
CO conversion is achieved at 220 °C for Au/CeO2 nanorods as
catalyst, while only 20-22% CO conversion is obtained at the
same temperature for pure CeO2 nanorods and nanoparticles as
catalyst. The catalytic activity of Au/CeO2 nanorods and Au/
CeO2 nanoparticles is much higher than that of pure CeO2
nanorods and nanoparticles. There could be a synergistic
interaction between gold and CeO2 nanorods, which is respon-
sible for the high activity. It should be noted that the catalytic
activity of Au/CeO2 nanoparticles is measured to be superior
in all samples. Recently, Venezia found that the reactive oxygen
on the surface of nanocrystalline CeO2 and the strong interaction
between cationic gold and ceria may determine the high activity
for CO conversion.32,39 The smaller CeO2 nanoparticles may
supply more active sites and contribute to the strong interaction
between gold and nanocrystalline CeO2. Therefore, the catalytic
activity of Au/CeO2 nanoparticles is better as compared to that
of Au/CeO2 nanorods. However, the thermal stability of the Au/

19174 J. Phys. Chem. B, Vol. 109, No. 41, 2005
Huang et al.
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Acknowledgment. This work is supported by the NCET
(Grant 040219), the NSFC (Grants 90206043 and 20271028),
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