Synthesis and photocatalytic performance of yttrium-doped CeO2 with a hollow sphere structure

Article abstract of DOI:10.1016/j.cattod.2016.06.049

We report a facile and novel approach to prepare yttrium-doped CeO₂ with a hollow sphere hierarchical structure by a simple hydrothermal method with cerium nitrate hexahydrate and yttrium nitrate hexahydrate as original materials and polystyrene microsphere as a soft-template. Through systematic experiments, the effects of experimental parameters such as doping and morphology on photocatalytic characteristics of the ceria were examined in detail. The morphology and element distribution of the as-prepared samples were characterized by field emission scanning electron microscopy and high-resolution transmission electron microscopy. Structure information with Rietveld refined data were obtained by using an X-ray diffractometer. Intrinsic oxygen vacancies and extrinsic oxygen vacancies caused by doping were analyzed from Raman spectra. Analyses of elements and chemical valence analysis were carried out by X-ray photoelectron spectroscopy, and changes in reactive oxygen species were determined by calculation. Based on structural information, element valence states, and the results of photocatalytic decomposition of acetaldehyde, we can draw the conclusion that Y-doped CeO₂ with a hollow sphere hierarchical structure has high photocatalytic activity, attributed to more oxygen vacancies and surface active oxygen species. Yttrium doped ceria with hollow sphere structure has a maximum rate constant of 0.0267?h^?1 and an oxidation quantum efficiency of 0.3610%, which are about 2 times as high as that of pure CeO₂ with octahedral structure.

Full text of DOI:10.1016/j.cattod.2016.06.049

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Synthesis and photocatalytic performance of yttrium-doped CeO2
with a hollow sphere structure
Bin Xua,b,c, Qitao Zhanga,b, Saisai Yuana,b, Sixiao Liu , Ming Zhang
Teruhisa Ohno
a
a
a,c,∗∗
,
b,d,e,f,∗
School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan
Test Center, Yangzhou University, Yangzhou 225002, China
b
c
d
e
f
JST, PRESTO and ACT-C, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
JST, ACT-C, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
Research Center for Advanced Eco-fitting Technology, Kyushu Institute of Technology, Tobata, Kitakyushu 804-8550, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a facile and novel approach to prepare yttrium-doped CeO2 with a hollow sphere hierarchi-
cal structure by a simple hydrothermal method with cerium nitrate hexahydrate and yttrium nitrate
hexahydrate as original materials and polystyrene microsphere as a soft-template. Through systematic
experiments, the effects of experimental parameters such as doping and morphology on photocatalytic
characteristics of the ceria were examined in detail. The morphology and element distribution of the as-
prepared samples were characterized by field emission scanning electron microscopy and high-resolution
transmission electron microscopy. Structure information with Rietveld refined data were obtained by
using an X-ray diffractometer. Intrinsic oxygen vacancies and extrinsic oxygen vacancies caused by dop-
ing were analyzed from Raman spectra. Analyses of elements and chemical valence analysis were carried
out by X-ray photoelectron spectroscopy, and changes in reactive oxygen species were determined by
calculation. Based on structural information, element valence states, and the results of photocatalytic
decomposition of acetaldehyde, we can draw the conclusion that Y-doped CeO2 with a hollow sphere
hierarchical structure has high photocatalytic activity, attributed to more oxygen vacancies and surface
active oxygen species. Yttrium doped ceria with hollow sphere structure has a maximum rate constant
Received 30 March 2016
Received in revised form 25 June 2016
Accepted 25 June 2016
Available online xxx
Keywords:
CeO2
Yttrium-doped
Hierarchical structure
Hollow sphere
Oxygen vacancy
−1
of 0.0267 h and an oxidation quantum efficiency of 0.3610%, which are about 2 times as high as that of
pure CeO2 with octahedral structure.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
and prominent oxygen storage [9,10] and release capacity (OSC) via
facile conversion between Ce4+ and Ce oxidation states [11,12].
3+
Cerium dioxide has been extensively utilized in many practical
Recently, CeO -based materials with different morphologies
2
applications such as polishing materials [1–3], solar cells [4], ultra-
violet light-blocking materials [5,6] and photocatalytic materials.
Ceria has attracted much attention due to its well-controlled mor-
phology, fluorite-type structure [7,8], remarkable redox properties
have been synthesized by various methods including thermal
evaporation [13], coprecipitation [14], and the sol-gel technique
[15–17]. Previous studies have shown that excellent catalytic per-
formance and easy functionalization of CeO2 materials can be
achieved by controlling their structural properties [18–22]. The
use for morphology-controlled and size-controlled [23–25] metal
and metal oxide composites [26,27] and ionic doping [28–30]
are the most commonly utilized methods for enhancing the pho-
∗ Corresponding author at: Department of Applied Chemistry, Faculty of Engi-
neering, Kyushu Institute of Technology, 804-8550 Kitakyushu, Japan.
tocatalytic properties of CeO -based materials. The keypoint in
2
∗
∗
these approaches is changing the oxygen vacancy concentrations
of products, because oxygen vacancies can act as electron and
hole capture centers and can trap the photogenerated electrons or
Corresponding author at: School of Chemistry and Chemical Engineering,
Yangzhou University, Yangzhou 225002, China.
E-mail addresses: lxyzhangm@yzu.edu.cn (M. Zhang), tohno@che.kyutech.ac.jp
(
T. Ohno).
http://dx.doi.org/10.1016/j.cattod.2016.06.049
920-5861/© 2016 Elsevier B.V. All rights reserved.
0
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holes excited by ultraviolet or visible light [33–36]. Further oxygen
vacancies can effectively restrain the recombination of electron-
hole pairs [37–39], resulting in improvement of photocatalytic
activities [31,32].
with different diameters solutions were added to the aqueous solu-
tion. The mixture was stirred continuously for one hour. The mixed
solution was then sealed in a Teflon-lined autoclave and heated
◦
at 150 C for 24 h. After cooling to room temperature, a light yel-
Ceria with a sphere structure possess high oxygen storage abil-
ity, high thermal stability [41,42], good conductivity [38] and
other good electrochemical performances [39,40]. In the past
few decades, various morphologies of ceria including octahedral,
cubes, wires and rods as well as other special one-dimensional
low precipitate was collected by centrifugation and washed several
times with deionized water and ethanol. Target products were
obtained after drying in the air at 60 C for one day. Yttrium-doped
◦
ceria with hollow structure was obtained by calcination of the as-
◦
prepared precursor in air at 400 C for 4 h.
(
1D) or two-dimensional (2D) morphologies have been investi-
gated. The main fabrication methods are hydrothermal methods
with surfactants and modifying agents. Our group has fabri-
cated yttrium-doped ceria with hedgehog-like [43] and porous
broom-like [44,45] hierarchical structures by using a template-free
hydrothermal process.
2.3. Characterization
An X-ray diffractometer (XRD) (Bruker-AXS, D8 sss) with Cu Ka
radiation (␭ = 1.5406 Å) and a monochromator was used to identify
the crystalline phase. Crystallite parameters and size were cal-
culated by the Rietveld method (TOPAS 4.0). N2 adsorption and
desorption isotherms were recorded at 77 K using a Nova 4200e
instrument. The samples were precisely weighted and degased at
373 K for 3 h. The specific surface area was calculated by the 5 points
Brunauer-Emmett-Teller theory. Raman analysis (Renishaw, In via)
was performed using a 532 nm excitation laser with 5 mW and an
air-cooled CCD detector. Raman peak shifts were determined by
fitting with the Lorentzian and Gaussian composite function. Mor-
phology and size of as-fabricated products were examined by using
a field emission scanning electron microscope (FESEM) (Hitachi,
S-4800, 15KV), high-resolution transmission electron microscope
(HRTEM) and HADDF-STEM (FEI, Tecnai G2 F30 S-TWIN, 300KV).
An inductively coupled plasma atomic emission spectrum (ICP-
AES) (Shimadzu, ICPS-8000) was used to determine stability of the
yttrium in the solution. An x-ray photoelectron spectroscopy (XPS)
experiment was carried using a Shimadzu KRATOS AXIS-NOVA sys-
Hollow spheres have attracted tremendous of potential applica-
tions [46], and various strategies have been used for the fabrication
of hollow spheres, including template-assisted and template-free
strategies. To the best of our knowledge, however, there has been
no report on the use of pure polystyrene (PS) to prepare yttrium-
doped ceria hollow spheres. Herein, we report a facile and feasible
approach to prepare yttrium-doped CeO2 with a hollow sphere
structure by hydrothermal technique using the simple inorganic
salts Ce(NO3)3 and Y(NO3)3 as original materials and polystyrene
as a soft template. The photocatalytic performance of the products
was compared through systematic experiments on acetaldehyde
photocatalytic oxidation.
2
. Experimental section
2.1. Materials
−
9
tem at room temperature under 10 pa with Al K␣ radiation and
C 1 s peak (83.8 eV) reference.
Yttrium nitrate hexahydrate (Y(NO ) ·6H O), cerium nitrate
3
3
2
hexahydrate (Ce(NO ) ·6H O) polyvinyl pyrrolidone (PVP) and
3
3
2
2.4. Photocatalytic evaluations
azodiisobutyronitrile (AIBN) were of analytical grade and were
used without any further purification. The original materials were
purchased from Wako Co., Ltd. The size and surface electrical prop-
erties of the microsphere were analysis by the zeta potential.
A UV lighter using black light (UVP, XX-15BLB) can be used
to remove possible organic materials adsorbed on the surfaces of
samples more than one week before evaluation of photocatalytic
activity. The photocatalytic activity of as-fabricated samples was
assessed by their ability for decomposition of acetaldehyde. The
instruments used for catalytic activity analysis are shown in Fig.
S1. Twenty milligrams of powder was spread on the bottom of a
glass dish, and the glass dish was placed in a Tedlar bag (AS ONE
2
.2. Preparation
2
.2.1. Preparation of polystyrene (PS) microsphere aqueous
solutions
PVP were dissolved in a solution of alcohol and water, and
Co. Ltd.). Five hundred ppm of acetaldehyde was injected into the
the mixture was poured into a 250 ml four-neck flask with ther-
mometer, mixer and condenser tube. Styrene monomer and AIBN
as an initiator were added into the above mixture during the steer-
ing process. After being swepted with nitrogen 30 min with, the
bag together with 125 cm3 of artificial air. Then the bag was put
in a dark place at room temperature for 2 h for reaching adsorp-
tion equilibrium. A light-emitting diode (LED; Epitex, L365), which
emitted light at wavelengths of ca. 365 nm, was used as a light
◦
four-neck flask was placed in an oil bath at 70 centigrade and
−2
source, and its intensity was controlled at 0.9 mW cm . The con-
the mixture was allowed to react for six hours. A polystyrene
emulsion was prepared by centrifugation. After washing with an
ethanol/water mixed solvent three times, the polystyrene emulsion
was dispersed with water. Finally, a PSt suspension was obtained.
PSt microspheres with different diameter could be obtained by con-
trolling the concentrations of styrene monomer and AIBN initiator.
centration of generated CO2 as a function of irradiation time was
monitored by a gas chromatograph (Shimadzu GC-8A, FID detec-
tor) equipped with a Porapak N-packed column and a methanizer
(GL Science, MT-221).
3. Results and discussion
2.2.2. Yttrium-doped CeO2 preparation
3.1. Morphology characterization
Yttrium-doped CeO2 with a hollow sphere structure was fabri-
cated by the traditional hydrothermal process. First, 0.87 g cerium
The morphology of the products was investigated by using an
FE-SEM (Fig. 1) and HR-TEM (Fig. 2). Fig. 2(a) shows a panoramic
view of fabricated particles. It can be seen that the particles are uni-
form octahedral particles with diameters of approximately 200 nm.
The lattice spacing of the parallel fringes is 0.313 nm, corresponding
to the (111) plane of FCC CeO2 (inset image of Fig. 2(a)). After dop-
ing, there was no obvious change in the morphology of products,
nitrate (Ce(NO ) ·6H O) and 0.07 g yttrium nitrate (Y(NO ) ·6H O)
3
3
2
3
3
2
were dissolved in 100 ml of deionized water with vigorous mag-
netic stirring for 30 min at room temperature. Based on the
optimum dopant concentration determined in previous studies
[
45], we used the optimal mole ratio of 1:0.1 for cerium nitrate and
yttrium nitrate. Then 0.5 ml–2 ml of polystyrene (PS) microsphere
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Fig. 1. FE-SEM images of the products: (a) pure CeO2 with an octahedral structure, (b) yttrium-doped CeO2 with an octahedral structure, (c) yttrium-doped CeO2 with a
sphere structure and (d) yttrium-doped CeO2 with a hollow sphere structure.
and an octahedral structure and monodispersity are well main-
tained (Fig. 1(b)). The particle size was also about 100 nm and the
exposed crystal plane was still (111). However, the inset image of
Fig. 2(b) indicated that the lattice spacing of the parallel fringes is
3.2. Structure characterization
Typical X-ray diffractometer (XRD) patterns of samples obtained
under different conditions are shown in Fig. 4. Characteristic peaks
at about 28.5, 33.0, 47.4 and 56.3 2␪ are attributed to pure cerium
oxide with a cubic fluorite structure (JCPDS No. 34-0394) and
are assigned to the crystal planes (111), (200), (220) and (311),
respectively (Fig. 4(a)). The sharp diffraction peaks from all sam-
ples suggest a high degree of crystallinity of fabricated samples,
and no impurity peaks appeared. The relative intensity of each
peak did not change, suggesting that there is no preferred orien-
tation. The elaborated XRD pattern of the highest peak (Fig. 4(b))
reveals that, compared with pure ceria, the highest peak position
(111) of the products exhibited an obvious shift to a lower angle
0.314 nm, which is larger than that before doping. The composition
of elements and distribution of the as-fabricated products obtained
under different experimental conditions were analyzed by STEM-
HAADF images and EDS mapping images. The image in Fig. 3 shows
the existence of not only cerium but also yttrium, with yttrium
ions being dispersed evenly on the CeO2 octahedral structure sur-
face and in the bulk. Herein we can draw a conclusion that yttrium
enters the lattice and successfully dopes the parts of cerium. The
results for composition of elements and distribution are consistent
with the results of XRD and Raman analysis.
◦
Fig. 1(c) shows that, by adding PS spheres, the morphology
of the product changed to a sphere structure with diameters of
approximately 300 nm. Fig. 2(c) shows that octahedral CeO2 par-
ticles with an average diameter of 30 nm covered the PS sphere
surface through electrostatic adsorption. The negative charge on
the PS surface can easily capture Ce3+ and Y via electrostatic
interaction. Cerium and yttrium ions adsorbed on the PS surface
could not generate a large crystal core due to the existence of a
space steric effect. Therefore, the sizes of particles were smaller
than those without PS assistance.
after yttrium doping and reached about 28.43 (2␪), indicating that
the crystal lattice has expanded conspicuously due to the entry of
yttrium into the crystal structure. At the same time, a PS amor-
◦
phous peak around 20 (2␪) can be detected from the diffraction
pattern of yttrium-doped ceria/ps. Rietveld refinements were also
carried out according to the diffraction lines by varying parame-
ters in order to investigate the actual doping quantity and location
(Fig. S2). It is known that each cerium particle is arranged in a face-
centered cubic surrounded by eight oxygen elements in the cubic
fluorite structure of cerium dioxide. Meanwhile, oxygen elements
occupy all of the tetrahedral positions and each oxygen element
has four ligancy of cerium cations. When the amount of the dopant
was changed without changing the other experimental conditions,
there were significant changes in the lattice parameters. The lat-
tice parameter value (a) changed to 5.4217 Å. It can be clearly seen
that the lattice parameter value (a) after doping is larger than the
bulk CeO2 lattice parameter value a = 5.4178 Å even though the
space group is still Fm–3 m before and after doping. This indicates
that yttrium has partially substituted cerium ions and has been
introduced into the interior lattice, maintaining the ceria cubic
fluorite structure instead of forming a Y O -CeO composite. Con-
3+
Figs. 1 (d) and 2 (d) show that the morphology of products is
◦
maintained as a sphere structure after calcination at 500 C for 4 h.
At same time, the diameters of spheres are about 300 nm, similar
to the diameters before calcination. Thus, removal of the template
does not have an influence on the morphology and size of final
products. A hollow structure can be clearly observed from con-
tact between the dark margins and the pale center in Fig. 2(d).
Moreover, it reveals that the size of hollow is about 250 nm, being
consistent with the PS microsphere diameter. An EDS mapping
image (see Fig. 3) shows that the difference before and after cal-
cination is elemental carbon.
2
3
2
sequently, the crystal lattice would expand due to an increase in
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Fig. 2. HR-SEM images of the products: (a) pure CeO2 with an octahedral structure, (b) yttrium- doped CeO2 with an octahedral structure, (c) yttrium-doped CeO2 with a
sphere structure and (d) yttrium-doped CeO2 with a hollow sphere structure.
cations showing different valence states with Ce4+ and part of the
oxygen would also escape from the lattice to form extrinsic oxygen
vacancies. The relativity intensities of Raman shifts of F2g, Rdopant
and RCe-O can be calculated from Eq. (1) [29]:
oxygen vacancy concentration with an increase in yttrium content,
due to the fact that the ionic radius of yttrium is larger than that of
cerium (1.019 and 0.97 Å, respectively).
Raman spectra of as-fabricated samples under various exper-
imental conditions are shown in Fig. 5. A strong Raman shift
−
1
−1
can be
at ∼460 cm
and a relatively weak shift at ∼600 cm
−
1
detected. The shift at ∼460 cm can be assigned to F2g vibration
of the fluorite-type structure and it can be considered as the sym-
metric stretching mode of oxygen atoms around cerium ions, and
the molecule retains its tetrahedral symmetry throughout. Based
on the results of previous studies and our own studies [43–45], a
weak and less prominent band near ∼600 cm can be attributed
to a nondegenerate longitudinal optical mode caused by a local Ce-
O (RCe-O) bond symmetry stretch. According to previous reports,
ꢀ
ꢁ
AeraR
+ AeraRdopant
Oxygen vacancies
F2g
Ce−O
Aera F2g
=
.
(1)
−
1
The values are summarized in Table 1. It can be seen that the
value of oxygen vacancies rapidly increased compared with that
of pure CeO2 after yttrium doping. This change in the value can
be attributed to the amount of the dopant. When yttrium was
introduced into bulk CeO2, more extrinsic oxygen vacancies were
obtained except intrinsic oxygen vacancies. Therefore, the value of
oxygen vacancies rapidly increased. Meanwhile, the value of oxy-
gen vacancies did not show an obviously change after introduction
of PS microsphere and removal of a soft template. Hence, oxygen
vacancy is mainly influenced by dopant type and concentration.
These results of Raman analysis are consistent with the results of
XPS O1S analysis and activity evaluation.
4+
not all cerium ions show Ce chemical valence in the lattice; small
3
+
cerium ions show Ce tervalence [44.45]. In order to maintain the
particles in an electrically neutral state, the lattice oxygen would
escape from the structure and finally result in the formation of
intrinsic oxygen vacancies. Oxygen vacancies perturb the local Ce-O
−
1
bond symmetry. A new and weak Raman shift at ∼530 cm , which
cannot be detected in Raman spectra of pure CeO , can be detected.
2
It is attributed to extrinsic oxygen vacancies caused by doping. As
a dopant, yttrium enters the cubic fluorite lattice of CeO2 and sub-
stitutes cerium. In order to maintain electronic neutrality, doping
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Fig. 3. HAADF images and element distribution maps of the products with different morphologies before and after doping.
Fig. 4. XRD patterns of as-fabricated samples: (a) whole range and (b) elaborated XRD pattern of the highest peak (111).
Table 1
Rietveld refined data from XRD patterns and oxygen vacancy values calculated by Raman spectra.
2
sample
Lattice value a (Å)
Calculated
Rwp
SBET (m /g)
AreaOxygen vacancies/AreaF2g (%)
grain size (nm)
CeO2/octahedral
5.4178
5.4217
5.4211
5.4215
4.50
4.6
4.52
4.54
5.42
5.63
5.76
5.97
35.35
40.94
78.99
1.1
4.9
5.3
5.8
Y-doped CeO2/octahedral
Y-doped CeO2/PS/sphere
Y-doped CeO2/Hollow sphere
115.34
PS: Area Oxygenvacancies/Area F2g (%) = (Area Rdoped + Area Rce-o)/Area Fag(%).
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Fig. 5. Raman spectra of as-fabricated samples: (a) whole range and (b) elaborated Raman shift.
Fig. 6. Wide scanning XPS spectra of products: (a) before doping and (b) after doping.
Fig. 7. High-resolution XPS spectra of Y3d (a) different morphology and (b) separated peak curve.
3.3. X-ray photoelectron spectroscopy characterization
is assigned to Y3d binding energy, indicating that doping was suc-
cessful (Fig. 6 (b)). Elaborated Y3d XPS spectra of the products after
doping are shown in Fig. 7. After deconvolution and separation, two
peaks located at 156.1ev and 158.2ev, assigned to Y3d5/2 binding
energy, can be found from XPS profiles (Fig. 7(b)). After calculating
the two peaks of Y3d, yttrium concentration in the crystal struc-
ture can be detected (Fig. 7(a)). A similar peak area indicates the
approximate concentration of doping.
XPS characterization was applied to determine the CeO2
nanoparticles before and after yttrium doping. Wide scanning XPS
spectra of the product is shown in Fig. 6. The elements Ce, O, and C
can be detected in Fig. 6(a), and they are assigned to Ce3d, O1s and
C1s binding energies, respectively. Not only the above elements
but also yttrium element can be detected from the spectra, and it
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Fig. 8. High-resolution XPS spectra of Ce3d (a) different morphology and (b) separated peak curve.
Fig. 9. High-resolution XPS spectra of O1s (a) different morphology and (b) separated peak curve.
Fig.
8
shows Ce 3d electron core level XPS spectra for
face active oxygen (Osur), respectively. After doping, the Olat and
Osur binding energy obviously shifted to a high energy, while the
effects on Olat and Osur were different. The Osur’s binding energy
value shows a change of 0.8 eV. However, the Olat’s binding energy
value shows a change of only 0.2 ev. Therefore, doping of yttrium
mainly affects the ceria’s surface oxygen binding energy, improv-
ing the surface oxygen binding energy and enhancing the activity of
surface oxygen. After deconvolution and multiple-peak separation,
we could obtain the surface oxygen ratio from the Eq. (3):
as-fabricated samples and changes in Ce 3d and Ce 3d5/2 compo-
3/2
nents depending on the Ce3+ and Ce oxidation states. Five pairs of
4+
doublets, (u, v), (u , v ), (u , v ), (u , v ) and (u , v ), can be decom-
posed from the origin region, where u and v come from Ce 3d3/2 and
Ce 3d5/2 states, respectively. We observed that (u, v), (u , v ) and
1
1
2
2
3
3
0
0
2
2
9
9
2
4
1
(
2
u , v ), which were attributed to Ce 3d 4f O 2p , Ce 3d 4f O
3 3
p
and Ce 3d 4f O 2p6 final states, respectively, all belonged to
5
9
0
4
+
the Ce oxidation state. The two pairs of doublets (u , v ) and (u ,
1
1
0
v0) originated from the Ce 3d 4f O 2p4 and Ce 3d 4f O 2p final
9
1
9
1
5
[
Osur]
area (Osur)
total area
3+
states, respectively, which correspond to the Ce oxidation state.
The relative amount of cerium in the tervalent oxidation state can
be calculated from Eq. (2):
=
]
.
(3)
[
Osur + Olat
The [Osur] concentration after doping is larger than that without
3+
ꢂ
ꢃ
ꢀ
ꢁ
doping. There was a similar phenomenon with [Ce ], although the
morphology had changed from an octahedral structure to a hollow
sphere, [Osur] and [Olat] concentrations were closed to each other
after doping. The cerium trivalence and activity oxygen concentra-
tions of the products can directly reflect the amount and activity
of oxygen vacancies and suggest that yttrium-doped CeO2 with a
hollow structure should have higher catalytic activity.
Ce3
+
area V , V , U0,U
0
1
1
ꢂ
ꢃ =
.
(2)
3
+
4+
totalarea
Ce + Ce
The fittied data demonstrated that the as-fabricated samples
exhibit variation of [Ce3+] concentration. The [Ce ] concentra-
tion after doping was larger than that without doping. However,
although the morphology changed from an octahedral structure to
3+
3
+
a hollow sphere, [Ce ] concentrations were similar after doping.
Results for cerium trivalence ions and surface active oxygen are
summarized in Table 2.
Fig. 9 shows O1s electron core level XPS spectra. One peak
around 529.0 eV with a shoulder around 531.5 eV can be clearly
seen, and they can be attributed to lattice oxygen (Olat) and sur-
3.4. Photocatalytic activity for acetaldehyde decomposition
The photocatalytic activities of the prepared samples were
evaluated by CO2 liberation from photocatalytic degradation
of acetaldehyde. Fig. 10(a) shows liberation from acetaldehyde
decomposition of as-fabricated samples. Compared with pure
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Table 2
Calculated [Ce3+] and [Osur] concentrations of as-fabricated samples from the XPS spectrum.
sample
[Ce3+]/[Ce3+ + Ce4+] %
[Osur]/[Osur+Olat] %
CeO2/octahedral
12.60
21.38
23.06
23.43
28.53
33.81
39.18
39.48
[Osur ]
[Osur + O ]
lat
Y-doped CeO2/ octahedral
Y-doped CeO2/ PS/sphere
Y-doped CeO2/ Hollow sphere
3+
[
Ce
]
area(V0,V1,U0,U1
)
area(Osur )
total area
=
=
Ce
3
++ Ce4+
]
total area
[
Table 3
Pseudo-first-order rate constant for acetaldehyde photocatalytic oxidation under different photocatalysts and oxidation quantum efficiency of as-fabricated samples.
sample
The first order kinetic equation
K(h−1)
R2
oxidation quantum
efficiency
CeO2/octahedral
−ln(C/C0) = 0.0145t
−ln(C/C0) = 0.0145t
−ln(C/C0) = 0.0199t
−ln(C/C0) = 0.0267t
0.0145
0.0145
0.0199
0.0267
0.9963
0.9976
0.9956
0.9945
0.2043%
0.2842%
0.3115%
0.3610%
Y-doped CeO2/octahedral
Y-doped CeO2/PS/sphere
Y-doped CeO2/Hollow sphere
Yttrium doped ceria with hollow sphere structure has a maximum
oxidation quantum efficiency of 0.3610%, which are about 2 times
as high as that of pure CeO2 with octahedral structure (0.2042%).
The oxidation quantum efficiency values are summarized in Table 3.
Fig. 10(a) also shows that yttrium doped-CeO2 with differ-
ent morphologies exhibited different photocatalytic activities.
Yttrium-doped CeO with a hollow sphere structure possess higher
2
photocatalytic activities than CeO2 with an octahedral structure,
due to the fact that small particles were fabricated in the spherical
structure and a higher specific surface area was obtained. A higher
specific surface area and small nanoparticles can provide more
activity spots where redox reaction takes place. At the same time,
yttrium dopant can provide more oxygen vacancies than without
doping. When fabricated samples are irradiated by UV light, elec-
trons are excited from the O2p valence band to the conduction band
Fig. 10. (a)Time course of CO2 liberation from acetaldehyde decomposition of as-
fabricated samples and (b) kinetic fit for the degradation of acetaldehyde with as-
fabricated samples.
(Ce4f), resulting in the formation of hole and electron pairs. More
surface oxygen vacancies exist on the surface of the products, and
the surface oxygen-deficient site can interact strongly with exoteric
−
−
oxygen molecules to form O2 , O2 is an efficient electron scav-
enger which can trap the photogenerated electrons and then form
CeO , the amount and liberation rate were significantly improved
2
after yttrium doping, indicating that photocatalytic activities were
enhanced. These results are attributed to that more oxygen vacan-
cies and trivalent cerium produced in the ceria crystal structure.
The higher the concentrations of oxygen vacancies and trivalent
cerium are, the higher are the photocatalytic activities of products.
From Fig. 10(a), we also can see that this kind of increase in activity
was caused by yttrium doped and porous structure rather than the
error of measurement. After measured CO2 liberation amount, we
calculated the oxidation quantum efficiency for synthesis products.
−
−
two O . The formation of O is considered to be very important for
photocatalytic oxidation process. Meanwhile, the holes easily get
trapped on the oxygen ions. When the number of oxygen vacancies
is larger, the oxygen ion mobility becomes higher. Transportation
mobility of lattice oxygen ions is beneficial for the separation of
photogenerated electrons and holes [43–45]. In another word, the
recombination of photogenerated electrons and holes is restricted
at a higher oxygen vacancy. Therefore, yttrium-doped CeO2 with
Fig. 11. Cycling runs of photocatalytic activity evaluation of yttrium-doped CeO2 with a hollow sphere hierarchical structure.
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a hollow sphere structure exhibit higher photocatalytic activities
than ceria with other traditional structures.
The photocatalytic degradation kinetics of acetaldehyde was
investigated, and the results were shown in Fig. 10(b). From
Fig. 10(b), we can see that the changes of acetaldehyde concentra-
tion vs reaction time over the products followed pseudo-first-order
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Appendix A. Supplementary data
10000–10002.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.06.
049.
Please cite this article in press as: B. Xu, et al., Synthesis and photocatalytic performance of yttrium-doped CeO2 with a hollow sphere
structure, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.06.049