Ultrasonic-assisted solution-phase synthesis and property studies of hierarchical layer-by-layer mesoporous CeO2

Article abstract of DOI:10.1002/bkcs.11398

Hierarchical layer-by-layer quadrangle CeO₂ was prepared through ultrasonic-assisted solution-phase synthesis strategy using cerium oxalate as the precursor. The as-prepared mesoporous CeO₂ displayed a surface area of 98.7 m² g^?1 and pore diameters of 2.0–10.0 nm. The high-resolution TEM image revealed that the layer structures of the CeO₂ were made of numerous nanocrystal particles with the crystallite size of about 13–15 nm. High energy and cavitation of ultrasonic wave assists cerium oxalate precursor in building the layer-by-layer quadrangle staking. UV–vis absorption spectrum showed that the direct allowed transition bandgap energy for the as-prepared CeO₂ was 2.91 eV. Moreover, the CeO₂ exhibited good photocatalytic property for degrading Rhodamine B solution under UV radiation.

Full text of DOI:10.1002/bkcs.11398

Article
DOI: 10.1002/bkcs.11398
BULLETIN OF THE
P. S. Zhao et al.
KOREAN CHEMICAL SOCIETY
Ultrasonic-assisted Solution-Phase Synthesis and Property Studies
of Hierarchical Layer-by-Layer Mesoporous CeO2
†
†
Pu Su Zhao,† Xiu Mei Gao,‡ Feng Xia Zhu,†, Xin Ming Hu, and Li Li Zhang
*
†Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin Normal University,
Huaian 223300, China. *E-mail: zhufengxia501@163.com
‡School of Computer Science and Technology, Huaiyin Normal University, Huaian 223300, China
Received November 8, 2017, Accepted January 15, 2018, Published online February 8, 2018
Hierarchical layer-by-layer quadrangle CeO2 was prepared through ultrasonic-assisted solution-phase syn-
thesis strategy using cerium oxalate as the precursor. The as-prepared mesoporous CeO2 displayed a sur-
face area of 98.7 m2 g−1 and pore diameters of 2.0–10.0 nm. The high-resolution TEM image revealed
that the layer structures of the CeO2 were made of numerous nanocrystal particles with the crystallite size
of about 13–15 nm. High energy and cavitation of ultrasonic wave assists cerium oxalate precursor in
building the layer-by-layer quadrangle staking. UV–vis absorption spectrum showed that the direct
allowed transition bandgap energy for the as-prepared CeO2 was 2.91 eV. Moreover, the CeO2 exhibited
good photocatalytic property for degrading Rhodamine B solution under UV radiation.
Keywords: Mesoporous material, Nanostructure, CeO2, Optical properties, Catalytic properties
Introduction
added. After ultrasonic oscillation for 2 min, 1.0 mL cetyltri-
methyl ammonium bromide (CTAB) aqueous solution
(0.05 g mL−1) and 1.0 mL oxalic acid aqueous solution
(0.35 mM) were added dropwise into the mixture succes-
sively and white precipitates formed immediately. The
above mixture was ultrasonic oscillated for another 5 min,
centrifuged, washed with water, and dried at 70 ꢀС for 12 h.
Afterward, the above precursor was calcined in air at 400 ꢀC
for 4 h to obtain yellow CeO2. Structural analysis of the
powder samples was performed by X-ray diffraction (XRD)
using (ARL XTRA diffractometer, ARL, Switzerland) using
Cu-Kα radiation with 2θ from 5 to 80ꢀ. The TG measure-
ment was operated on an (STA449F3 apparatus,
NETZSCH-Gerätebau GmbH, German) with 10 C min
heating rate. The Brunauer–Emmett–Teller (BET) analysis
was conducted with (3Flex instrument, Mike company,
USA) at −196 ꢀC and the pore-size distributions were calcu-
lated by the Barrett–Joyner–Halenda (BJH) method. The
microstructure investigations were carried out on Quanta
FEG450 field-emission scanning electron microscopy
(SEM, FEI company, USA) and JEOL JEM-2010F trans-
mission electron microscope (TEM). Optical absorption
spectra were obtained from (Shimadzu, Shimadzu Corpora-
tion, Japan) UV-2600 UV–vis spectrophotometer. Photoca-
talytic activity was evaluated through Rhodamine B (RhB)
decomposition under UV irradiation. Typically, CeO2
(0.2 g) was dispersed in 250 mL RhB aqueous solution
(10 mg L−1). The reaction cell was bubbled with air. After
the dispersion was stirred for 60 min in the dark to ensure
adsorption–desorption equilibrium, it was exposed to UV
irradiation. At 20-min interval, 5 mL dispersions were taken
out and centrifuged for measuring the absorbance.
Recently, ultrasonics sonochemical method has been
proved to be
a useful approach to obtain micro-
nanostructual materials.1–5 The cavitation phenomena
derived from ultrasound can induce chemical changes,
which lead to many unique properties in the irradiated solu-
tion and particles suspended in the same.6 However, CeO2
has been widely applied in catalysis,7 oxygen sensors,8
solid fuel cells9,10, UV adsorbents,11 and so on. Among
numerous CeO2, mesoporous CeO2 has attracted much
attention due to their porous structure and high surface area
which can provide more active sites.12 Many methods have
been used to synthesize the mesoporous CeO2, including
sol–gel chelating method,12 electrospinning method,13
microwave-assisted sol–gel method,14 solvothermal and
hydrothermal methods,15,16 biotemplates method,17 and so
on. In this work, we report an ultrasonic-assisted solution-
phase synthetic method to prepare mesoporous CeO2 with
layer-by-layer quadrangle structure. In addition, its optical
and catalytic activities were investigated.
−1
ꢀ
Experimental
All the chemicals and reagents were of analytical grade and
used as received without further purification. First, prepara-
tion of cerium oxalate precursor by using ultrasonic reactor
with working frequency being fixed at 400 Hz and electric
power being controlled at 200 W (KQ5200B; Kunshan Shu
Mei Ultrasonic Instrument Co., Ltd., China). In a water–
glycerol complex solvent (5.0 mL water and 10 mL glyc-
erol), 1.0 mL Ce(NO3)3 (0.15 mM) aqueous solution was
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Article
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KOREAN CHEMICAL SOCIETY
CeO -XRD
2
JCPDF 34-0394
10
20
30
40
50
60
70
80
2θ (degree)
Figure 3. XRD patterns of the as-prepared CeO2.
Figure 1. XRD pattern of the as-made cerium oxalate precursor.
34-0394). Figure 4 presents the N2 adsorption–desorption
isotherms of as-prepared CeO2. The obvious hysteresis loop
ranging from 0.4 to 1.0 at relative pressure suggests the
existence of a mesostructure in the CeO2 particles. The iso-
therms display a type-H4 isotherm and correspond to aggre-
gates of narrow slit-like pores.19 The BET surface area of
the synthesized CeO2 was 98.7 m2 g−1, which was higher
than that of commercial CeO2 (16.3 m2 g−1) and smaller
than that of layer-stacking spindle-like CeO2.12 Based on
BJH method20 and by using adsorption branch of the iso-
therm, pore diameters of the as-prepared CeO2 were calcu-
lated to be 2.0–10.0 nm (inset Figure4). These pores should
be resulted from releasing gaseous H2O and COx during
dehydration and decomposition of cerium oxalate precursor.
Figure 5(a) and (b) displays SEM and TEM images of
the as-prepared cerium oxalate precursor. Figure 5(a) shows
that the precursors are monodisperse and the morphologies
of the precursor are hierarchical layer-by-layer quadrangle
with length of side being about from 2 to 4.5 μm, and
Figure 5(b) further confirms the above microstructures.
Results and Discussion
Figure 1 shows the diffraction peak index of the cerium
oxalate precursor, which can be readily indexed to the
known bulk monoclinic phase cerium oxalate decahydrate
coordination polymer [Ce2(C2O4)3Á10H2O]∞.18
Figure 2 displays the thermal decomposition results of
the precursor, which reveals two weight-loss processes. The
ꢀ
one below 250 C is corresponding to the dehydration of
precursor and another one from 250 to 350 ꢀC is ascribed to
decomposition of cerium oxalate with the residue being
CeO2. The TG experiment suggests that heating cerium oxa-
ꢀ
late precursor above 350 C can obtain CeO2 and the CeO2
obtained may contain voids. In order toꢀensure decomposi-
tion of cerium oxalate precursor, 400 C was selected as
decomposition temperature in the later experiments.
Figures 3 and 4 exhibit structural analyses of the sample
decomposed from the precursor at 400 ꢀC in air, which con-
firms above inferences. All the diffraction peaks can be
indexed to the face-centered cubic phase CeO2 (JCPDF
60
50
0
5
10 15 20 25 30 35
Pore Diameter (nm)
40
30
20
CeO Adsorption
2
CeO Desorption
2
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Figure 4. N2 adsorption–desorption isotherms and pore distribu-
tion (inset) of the as-made CeO2.
Figure 2. TG curve of the as-made cerium oxalate precursor.
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Article Ultrasonic-Assisted Synthesis and Property Studies of Layer Mesoporous CeO2 KOREAN CHEMICAL SOCIETY
Figure 5. (a, b) SEM and TEM images of cerium oxalate precursor; (c) SEM, (d) TEM, (e) HRTEM images, and (f ) SAED pattern of
nanoparticles of the as-made CeO2.
SEM image of the as-prepared CeO2 (Figure 5(c)) displays
that the CeO2 particles inherit layer-by-layer quadrangle
morphologies from the precursors with particle sizes being
about 1.5–3.5μm, which are smaller than those of the
precursors.
oxalate precursor, since the as-prepared CeO2 inherits its
morphology from the cerium oxalate precursor. During the
experimental processes, it is found that the ultrasonic wave
could affect the morphology of the final products. In order
to investigate the detailed forming processes of cerium oxa-
late precursor hierarchical architectures, time-dependent
experiments were performed (this “time” means the mixing
time of all reactants), whereby the other reaction parameters
were kept unchanged. Figure 6 shows the effect of ultra-
sonic irradiation time on the morphology and size of the
precursor.
Figure 6(a) displayed the SEM image of cerium oxalate
precursor prepared without the assistance of ultrasonic
wave and only with general stir for 5 min after oxalic acid
aqueous solution were mixed with Ce(NO3)3 aqueous solu-
tion. One could find that the precursor was composed of
abundant rectangular solids with nonuniformity sizes and a
small amount of layer-by-layer quadrangle aggregation with
length of side being about from 2 to 4.5 μm (Figure 6(a)).
When ultrasonic wave condition was employed and after
the reactants were ultrasonic oscillated for 1 to 3 min
For the as-prepared CeO2 particles, TEM and high-
resolution TEM (HRTEM) images are exhibited in Figure 5
(d) and (e), respectively, and the selected area electron dif-
fraction (SAED) pattern is presented in Figure 5(f ). As
shown in Figure 5(d), the magnified part of a layer-like
quadrangle reveals that the layer structures are made of
numerous nanocrystal particles with a crystallite size of
about 13–15 nm. The visible lattice fringes with a spacing
of 0.3136 nm in Figure 5(e) clearly correspond to the spac-
ing of the (1 1 1) planes of CeO2. In Figure 5(f ), electron
diffraction pattern shows bright rings, which are indexed to
the different planes of the cubic CeO2 and in line with the
XRD data.
Based on the above experiments, in order to obtain
layer-by-layer quadrangle mesoporous CeO2, it is important
to obtain the uniform layer-by-layer quadrangle cerium
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Article Ultrasonic-Assisted Synthesis and Property Studies of Layer Mesoporous CeO2 KOREAN CHEMICAL SOCIETY
Figure 6. SEM images of the cerium oxalate precursor prepared from the system under different ultrasonic time: (a) 0 min (only with
5 min general stirring), (b) 1 min, (c) 3 min, (d) 4 min, (e) 5 min, and (f ) 6 min.
(Figure 6(b) and (c)), quadrangle aggregations and lamellar
structures in the cerium oxalate precursors were found.
With an extension of the ultrasonic time to 4 min (Figure 6
(d)), for the precursors, only a few of lamellar structures
left and a large amount of layer-by-layer quadrangle
appeared. When the ultrasonic oscillation time reached
5 min, the lamellar precursors disappeared and all of pre-
cursors became monodispersed layer-by-layer quadrangle
architectures (Figure 6(e)). Further increasing ultrasonic
oscillation time to 6 min, the morphologies of the precur-
sors returned back to the mixture of lamellar and layer-
by-layer quadrangle structures (Figure 6(f )). Optimum
ultrasonic oscillation time for synthesis of fine and uniform
layer-by-layer quadrangle precursor is 5 min.
Based on the above time-dependent analysis, the ultra-
sonic wave plays important roles to help forming layer-by-
layer quadrangle cerium oxalate precursor. This phenomenon
is maybe mainly ascribed to the high-energy and cavitation
phenomena of ultrasonic wave.1,21–23 By comparing
Figure 6(a) and (e), it can be deduced that accumulation of
the precursor to layer-by-layer quadrangle structure needs a
certain amount of energy. Although the general stirring can
provide some energy, which lead to the aggregation of the
precursors, the total amount of energy is not enough, so only
a small number of layer-by-layer quadrangle structures
formed in Figure 6(a). While the ultrasonic oscillation can
generate higher energy than that of general stir, which leads
the precursor particles not only to growing larger but also
taking place regular layer-by-layer stacking (Figure 6(e)). In
addition, seen from Figure 6(b)–(f ), with the prolonging of
ultrasonic time, for the precursor particles, high-energy ultra-
sonic first help lamellar nuclei gradually become wider and
longer. At the same time, the grown lamellar precursor heap
together to reach a relative stable layer-by-layer quadrangle
structure (Figure 6(e)). Then, this relative stable layer-by-
layer quadrangle configuration collapses and disappears.
Obviously, the collapses is also caused by high-energy and
cavitation of ultrasonic wave.
Figures 7 and 8 show the UV–vis absorption spectra and
Tauc plot for the as-prepared cerium oxalate precursor and
CeO2, respectively. In Figure 7, the as-prepared CeO2
exhibits adsorption band from 200 to 360 nm, which is
broader than that of the precursor. From the Tauc plot, one
can find that the direct allowed transition bandgap energy
of CeO2 is 2.91 eV (Figure 8), which is smaller than that
of the well-known semiconductor CeO2 (3.37 eV)24 and
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1.0
(a)
1.5
0.8
0.6
0.4
0.2
0.0
Dark
1.0
0.5
0.0
Precursor
CeO
2
Light on
200 300 400 500 600 700 800
Wavelength (nm)
-60
-40
-20
0
20
40
60
80
100
Time(min)
Figure 9. The concentration of RhB with time.
Figure 7. UV–vis absorption spectra of the as-prepared cerium
oxalate precursor and CeO2.
k (min−1) being degradation rate constant. Here, the k is
determined as 0.0647 min−1. Comparing to the mesoporous
subuliform CeO2 with k being 0.0125 min−1, BET surface
area being 92.4 m2 g−1 and direct allowed transition band-
gap energy being 3.0 eV,11 this layer-like quadrangle CeO2
has bigger BET surface area, smaller bandgap energy and
better photocatalytic activity. This situation supports the
results reported earlier.27,28
larger than that of corresponding precursor. Also, this band-
gap value is smaller than those of the nano CeO2 with aver-
age diameters being 1 μm (3.03 eV) and 5–10 nm
(3.44 eV).11 This phenomenon is in according with the pre-
vious reports that bandgap decreases with the increasing
sizes of CeO2.25,26
For the layer-like quadrangle CeO2, Figures 9 and 10
show the photocatalytic degrading RhB results. Degrada-
tion rate is defined as: degradation (%) = (1 − Ci/
C0) × 100%, where C0 and Ci are RhB concentration at the
initial adsorption equilibrium and at different illumination
intervals, respectively.
As seen from Figure 9, in the presence of CeO2, UV irra-
diation of the RhB aqueous solution leads to absorbance
decrease. A linear relationship between ln(C0/Ci) and reac-
tion time (t) is shown in Figure 10, revealing that the degra-
dation reaction follows pseudo-first-order kinetics and the
corresponding kinetic equation is: lnC0/Ci = kt with
Conclusion
An ultrasonic-assisted solution-phase synthetic method was
applied to fabricate hierarchical layer-by-layer mesoporous
CeO2. The layer-like quadrangle CeO2 are assembled
numerous nanocrystal particles. Time-dependent experi-
ments indicate that ultrasonic wave can help to preparation
layer-by-layer cerium oxalate precursor. For the as-prepared
CeO2, its higher surface area and smaller bandgap energy
lead to its better photocatalytic activity for degrading RhB.
5
Precursor
CeO
30
20
10
0
2
R = 0.99903
2
4
3
2
1
0
SD = 0.1043
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
Bandgap (eV)
0
20
40
60
80
Time (min)
Figure 8. Tauc plots for the as-made cerium oxalate precursor
Figure 10. A linear relationship between ln(C0/Ci) and
and CeO2.
reaction time.
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Acknowledgments. This research was supported by the
Natural Science Foundation of Jiangsu Provincial Depart-
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