S. CUI et al.: SYNTHESIS, CHARACTERIZATION OF CeO 2@Ag HOLLOW SPHERES ...
537–554
SYNTHESIS, CHARACTERIZATION OF CeO
2
@Ag HOLLOW
SPHERES AND EV ALUATION OF THEIR CATALYST ACTIVITY
FOR THE REDUCTION OF 4-NP
SINTEZA IN KARAKTERIZACIJA VOTLIH KROGLIC CeO
2
@Ag
TER OVREDNOTENJE NJIHOVE KATALITI^NE
AKTIVNOSTI ZA 4-NP
Shu Cui
1,2
, Haixin Zhao
2
, Chengyou Liu
3
, Hai Yu
1
, Nan Li
2*
, Xiaotian Li
2*
1
Tonghua Normal University, School of Physics, Tonghua, Jilin 134002, China
2
Jilin University, College of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education,
2699 Qianjin Street, Changchun 130012, P. R. China
3
Hainan Vocational University of Science and Technology, School of Chemistry and Materials Engineering, Hainan 570000, China
Prejem rokopisa – received: 2023-03-03; sprejem za objavo – accepted for publication: 2023-08-26
doi:10.17222/mit.2023.813
A CeO 2@Ag hollow spherical composite catalyst was synthesized with the template method. Firstly, a SiO 2@CeO 2 core-shell
structure was synthesized using SiO 2 spheres as the template, and then the SiO 2 core was removed by etching to obtain hollow
CeO 2 spheres. The CeO 2 hollow microspheres have a large specific surface area, which can effectively suppress the aggregation
of Ag nanoparticles, leading to CeO 2@Ag with a regular morphology and well dispersed Ag nanoparticles. There is a strong
synergistic effect between CeO 2 and Ag, which is beneficial to improving the catalytic performance. As a result, the CeO 2@Ag
hollow spherical composite catalyst can reduce 4-nitrophenol efficiently.
Keywords: CeO
2
hollow spheres, Ag nanoparticles, composite, catalyst activity
Avtorji so sintetizirali kompozitni katalizator na osnovi srebra in cerijevega oksida (CeO 2@Ag) v obliki votlih kroglic
s{ablonsko metodo. Najprej so za jedro lupinaste strukture SiO 2@CeO 2uporabili SiO 2 kroglice in nato so SiO 2jedro odstranili z
jedkanjem ter tako dobili samo CeO 2mikro kroglice. Te imajo veliko specifi~no povr{ino in presek, ki u~inkovito zavira
skupljanje srebrnih nanodelcev. To je omogo~ilo izdelavo kompozita CeO 2@Ag s pravilno morfologijo in dobro porazdelitvijo
nanodelcev srebra (Ag) na povr{ini kroglic. Pri tem je pri{lo do mo~nega sinergijskega (obojestranskega) delovanja med CeO 2in
Ag, karje mo~no izbolj{alo kataliti~ne u~inke tega kompozita. Avtorji v zaklju~ku povdarjajo, da so izdelali kompozitni
katalizator iz votlih mikrokroglic CeO 2@Ag, ki mo~no zmanj{ujeu~inkovitost 4-nitrofenola (p-nitrofenol ali
4-hidroksinitrobenzen), ki se v industriji uporablja za izdelavo zdravil, fungicidov, insekticidov, barv, za temnjenje usnja itd.
Klju~ne besede: votle kroglice CeO 2, nanodelci srebra, kompozit,kataliti~na aktivnost
1 INTRODUCTION
Noble metal nanomaterials have extensive potential
applications in catalysis, antibacterial, electrochemistry,
sensors, etc. Compared to other noble metal nano-
materials, Ag nanoparticles (AgNPs) have a relatively
low price and distinctive catalytic capability, thus they
have received increasing attention in the field of cataly-
sis, relating to catalytic oxidation,
1,2
catalytic reduction
3,4
or catalytic selection.
5,6
Numerous studies
7–9
showed that
small-sized and monodispersed AgNPs usually exhibit a
high catalytic activity. Nevertheless, individual AgNPs
frequently tend to accumulate during a synthesis process,
reducing the catalytic performance.
10–12
In order to solve
this problem, an effective method is a recombination of
AgNPs on/in nanostructured solid substrates, for in-
stance, silica,
13
carbon,
14
polymer,
15
metal oxides,
16–18
etc.
Among various AgNP supporting materials, ceric ox-
ides have received more attention for their excellent re-
dox properties and high oxygen storage capacity. In
Ag@CeO
2
composite catalysts, the synergistic effect be-
tween CeO
2
and AgNPs can improve the reduction and
oxidization according to some researchers.
19–21
For exam-
ple, Shi et al.
20
prepared Ag@CeO
2
core-shell nano-
composites through a self-assembly process and investi-
gated the catalytic activity used for the hydrogenation of
4-nitrophenol (4-NP) and 2-nitroaniline (2-NA). The re-
sults demonstrated that the Ag@CeO
2
catalysts have out-
standing catalytic efficiency and cyclic stability. In an-
other study, Wang et al.
21
synthesized Ag@CeO
2
core-shell nanospheres via a facile one-step solvothermal
route using Ce(NO
3
)
3
·6H
2
O, AgNO
3
, poly(N-vinyl-
pyrrolidone) (PVP) and ethanol. The enhanced catalytic
activity is found to be due to the unique oxidized Ag spe-
cies induced by the strong interaction between the core
surfaces of Ag nanospheres and surface defects (oxygen
vacancies) of the CeO
2
shell.
Hollow CeO
2
nanostructures have been used in many
applications, such as photocatalytic water oxidation, cat-
alytic reduction, sensors, etc. A high dispersibility and
Materiali in tehnologije / Materials and technology 57 (2023) 5, 537–554 537
UDK 661.865.5:549.282 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(5)537(2023)
*Corresponding author's e-mail:
xiaotianli@jlu.edu.cn(X. Li)

large specific surface area often result in better perfor-
mance. Despite several studies, a controlled synthesis of
CeO
2
hollow nanostructures for catalysts has still been
limited. In this work, we synthesized CeO
2
hollow
mesoporous spherical supported AgNP catalysts using
the template method. The CeO
2
supports have a large
specific surface area and mesoporous structure, which
can effectively suppress the aggregation of Ag nano-
particles, leading to CeO
2
@Ag with a regular morphol-
ogy and well dispersed Ag nanoparticles. Furthermore,
there is a strong synergistic effect between CeO
2
and Ag,
which is conducive to improving the catalytic perfor-
mance.
2 EXPERIMENT SECTION
2.1 Synthesis
The entire fabrication of the CeO
2
@Ag hollow
microspheres is illustrated in Figure 1 and detailed in-
formation of each step is depicted below.
SiO
2
nanospheres: SiO
2
nanospheres were synthe-
sized with a modified Stöber method following the pro-
cedure reported in the literature.
22
Briefly, 64 mL iso-
propanol and 24 mL deionized water were mixed
uniformly, and 13 mL ammonia hydroxide (25–28 w/%)
were added; they were mixed at 35 °C for 15 min. Then,
0.6 mL tetraethoxysilane (TEOS) was added, and after
mixing for 30 min, 5 mL TEOS were added to the mix-
ture, which was stirred for another 2 h. SiO
2
particles
were centrifuged from the milky mixture, and then dried
at 60 °C for 24 h.
CeO
2
hollow spheres: Primarily, an SiO
2
@CeO
2
core-shell composite was prepared using SiO
2
as the
template.
23
Briefly, 0.277 g cerium nitrate and 0.07 g
methenamine were respectively dissolved in 5 mL
deionized water, forming two aqueous solutions. 1 g
polyvinyl pyrrolidone (PVP, K29/32) was dissolved in
40 mL deionized water, and 0.1 g of the as-prepared SiO
2
nanospheres was introduced and completely dispersed
with ultrasonication, then the mixture was heated to
95 °C in an oil bath. The cerium nitrate solution and
methenamine solution were added dropwise into the
mixture under vigorous stirring. After 2 h, the gray-white
mixture was naturally cooled down, centrifuged, washed
(with ethanol and deionized water) and dried at 80 °C.
Products were calcinated in a tube furnace at 600 °C for
2 h to obtain light yellow SiO
2
@CeO
2
composites. Even-
tually, to achieve hollow mesoporous CeO
2
nanospheres,
the SiO
2
@CeO
2
composites were dispersed in a 40 mL
NaOH solution (0.2 M) and stirred for 24 h to remove
the SiO
2
template.
CeO
2
@Ag hollow spheres: Ag nanoparticles on CeO
2
hollow mesoporous nanospheres were fabricated with a
simple in situ wet chemistry method as described previ-
ously.
24
Briefly, 0.1 g CeO
2
hollow mesoporous nano-
spheres was dispersed into a 10 mL5×1 0
–5
M silver-
ammonia ([Ag(NH
3
)
2
]NO
3
) solution with ultrasonication,
then stirred at 60 °C for 30 min. 12 g ethanol containing
0.1 g PVP were added dropwise into the suspension, and
stirred for 1 h. Finally, the CeO
2
@Ag hollow spheres
were centrifuged and then dried at 80 °C. The obtained
product was denoted as CeO
2
@Ag-1. In control experi-
ments, CeO
2
@Ag-2 and CeO
2
@Ag-3 were prepared
with the same method where the concentrations of
[Ag(NH
3
)
2
]NO
3
w e r e1×1 0
–4
M and 1.5 × 10
–4
M, re-
spectively.
S. CUI et al.: SYNTHESIS, CHARACTERIZATION OF CeO 2@Ag HOLLOW SPHERES ...
538 Materiali in tehnologije / Materials and technology 57 (2023) 5, 537–554
Figure 1: Schematic diagram of the fabrication of CeO
2
@Ag hollow microspheres

2.2 Catalyzed reduction of 4-NP
The catalytic activity of the as-prepared catalysts was
studied with 4-NP. A mixture of 0.3 mL 4-NP aqueous
solution (10 mM) and 3 mL fresh NaBH
4
solution
(10 mM) was stirred thoroughly. Then 1 mg CeO
2
@Ag
nanocomposite was added to the mixture, and the cata-
lytic reactions occurred rapidly. The reduction process
was analyzed with a UV-Vis spectrophotometer at 90 s
intervals.
2.3 Characterization
The crystalline nature was studied with a D8 Tools
X-ray diffractometer equipped with Cu K
 ( = 0.15405 nm).
The morphology and microstructure of the synthesized
nanomaterials were investigated with a field-emission
scanning electron microscope (FESEM/EDS, JEOL
JSM-6700F) and transmission electron microscope
(TEM, JEM 3010 and Tecnai G2 F20). The binding ener-
gies of the elements in the synthesized nanomaterials
were measured using X-ray photoelectron spectroscopy
(XPS, VG ADES-400). The porosity of the samples was
analyzed at 77 K based on nitrogen adsorption-
desorption using the Barrett–Joyner–Halenda (BJH)
method on a Quantochrome Autosorb 1 sorption ana-
lyzer. The Brunauer–Emmett–Teller (BET) measurement
was used to determine the specific surface area. UV–Vis
diffuse reflectance spectroscopy (DRUV-VIS) of the re-
sultant samples was performed. A Bws003 spectrophoto-
meter operating in a range of 250–650 nm was used to
record UV–Vis diffuse reflectance spectra (DRS), while
BaSO
4
was used as the reference standard.
3 RESULTS AND DISCUSSION
3.1 Characterization
XRD patterns of SiO
2
@CeO
2
, hollow mesoporous
CeO
2
, CeO
2
@Ag-1, and CeO
2
@Ag-2 are presented in
Figure 2. It can be clearly seen that all the samples have
similar characteristic peaks, recognized to be cubic
CeO
2
(JCPDS 34-0394).
25
Diffraction peaks at 2 =
28.6°, 33.2°, 47.6° and 56.3° can be indexed to the (111),
(200), (220) and (311) crystal faces of cubic CeO
2
. Dif-
fraction peaks are obviously broad, indicating that the
size of CeO
2
crystallites is very small. According to the
Scheler formula, the size of CeO
2
crystallites is about 5.6
nm. For CeO
2
@Ag-1, there is a weak diffraction peak
near 37.9°, corresponding to the face centered cubic
structure of Ag (JCPDS 4-783),
24
indicating that AgNPs
are successfully loaded on CeO
2
hollow spheres. The
characteristic peak of Ag is obviously wide, indicating
that the size of Ag crystallites is very small. In sample
CeO
2
@Ag-2, the characteristic peak of Ag has been en-
hanced, but it is still relatively wide.
Figure 3 shows FESEM images of the prepared
products in each stage. As shown in Figure 3a, the SiO
2
spheres exhibit regular spherical shapes with a uniform
size distribution and good dispersion, and the average di-
ameter is 170 nm. After applying CeO
2
(Figure 3b), the
shape and dispersity of SiO
2
@CeO
2
core-shell compos-
ites did not change obviously, but the surfaces of the
spheres became rough, and the average diameter in-
creased to 185 nm. After the SiO
2
spherical cores are
corroded by NaOH, there are hollow CeO
2
spheres,
shown in Figure 3c. It can be seen that the sample still
maintains a regular spherical morphology. A few broken
spherical shells clearly show the hollow structure, and
the spherical shells are very thin. As shown in Fig-
ures 3d to 3f, the Ag loaded CeO
2
hollow spheres have a
similar spherical morphology, indicating that the CeO
2
shell is relatively stable and the diameter of the
CeO
2
@Ag samples is not changed obviously. However,
the surface of the samples becomes rougher and there are
some nanoparticles of less than 10 nm on the surface.
Based on the above and the previous XRD results, we
can infer that these small particles are AgNPs. For
CeO
2
@Ag-1 (Figure 3d) and CeO
2
@Ag-2 (Figure 3e),
the nanoparticles are uniformly adhered to the surface of
spheres; as the concentration of [Ag(NH
3
)
2
]NO
3
in-
creases from5×1 0
–5
Mt o1×1 0
–4
M there are no distinct
morphological differences between the two samples. But
for CeO
2
@Ag-3 (Figure 3f), when the concentration of
[Ag(NH
3
)
2
]NO
3
increases to 1.5 × 10
–4
M there is a
cuboid particle with a size of 450 nm, marked with a red
dashed circle, and the morphology is obviously different
from the spherical morphology of the CeO
2
@Ag sam-
ples. This cuboid formation could be an Ag particle, in-
dicating that with a high concentration of
[Ag(NH
3
)
2
]NO
3
, the AgNPs tend to agglomerate and
form large-size silver particles, affecting the catalytic ac-
tivity of CeO
2
@Ag-3.
To further study the microstructures and AgNP distri-
bution of CeO
2
@Ag hollow spheres, CeO
2
@Ag-2 was
observed with TEM and HRTEM. It can be clearly seen
on Figure 4a that the CeO
2
@Ag microspheres have an
obvious hollow structure. The average diameter of the
S. CUI et al.: SYNTHESIS, CHARACTERIZATION OF CeO 2@Ag HOLLOW SPHERES ...
Materiali in tehnologije / Materials and technology 57 (2023) 5, 537–554 539
Figure 2: XRD patterns of the samples

S. CUI et al.: SYNTHESIS, CHARACTERIZATION OF CeO 2@Ag HOLLOW SPHERES ...
540 Materiali in tehnologije / Materials and technology 57 (2023) 5, 537–554
Figure 4: a, b, c) TEM images of CeO
2
@Ag-2 hollow microspheres, d–f) the elemental mapping of a CeO
2
@Ag-2 hollow monomicrosphere
Figure 3: a) FESEM images of the as-prepared samples: SiO
2
spheres, b) SiO
2
@ CeO
2
core-shell composites, c) CeO
2
hollow microspheres, d)
CeO
2
@Ag-1, e) CeO
2
@Ag-2, f) CeO
2
@Ag-3

hollow spheres is 180 nm, and the thickness of the shell
is about 7 nm, which is consistent with the SEM results.
The light and shade contrast shows that there are some
small and well dispersed nanoparticles on the surface of
the spherical shell, with a size of  10 nm. In the
HRTEM image of Figure 4b, clear lattice fringes can be
seen. After measurement, there are two different lattice
fringes with a distance of 0.24 nm and 0.31 nm that can
be indexed to the (111) plane of the face centered cubic
structure of Ag (JCPDS 4-783) and (111) plane of the
cubic structure of CeO
2
(JCPDS 34-0394).
26
An elemen-
tal mapping analysis (Figures 4e and 4f) of a single
CeO
2
@Ag-2 hollow microsphere (Figure 4d) indicates
the existence of Ce, O and Ag. Figure 4f demonstrates
that AgNPs are homogeneously dispersed in the whole
CeO
2
@Ag-2 hollow microsphere.
To qualitatively determine the surface compositions
and elemental chemical status of the CeO
2
@Ag hollow
microspheres, CeO
2
and CeO
2
@Ag-2 were investigated
using the XPS analysis. Figure 5a shows the full spec-
trum in a range of 0–1200 eV of the two samples; Si, C,
O and Ce signals are observed in both curves. The C sig-
nal results from the carbon calibration.
27
The Si signal is
weak, resulting from the residual of the SiO
2
core in the
sample. The intensity of the Ce signal did not change af-
ter loading AgNPs, indicating that the loading amount of
AgNPs was very small; the coverage area of
nano-AgNPs on the surfaces of the CeO
2
hollow micro-
spheres was very low, and there was still a large area of
the CeO
2
shell exposed on the sample surface. The Na
signal in the curve of the CeO
2
sample is attributed to the
residual NaOH used to remove the SiO
2
core. In the
curve of CeO
2
@Ag-2, there is an obvious Ag signal.
Figure 5b shows the high-resolution XPS spectra of Ag
3d. The two characteristic peaks located at 368.2 eV and
374.2 eV correspond to the binding energies of Ag 3d5/2
and Ag 3d3/2, respectively,
28
indicating that Ag exists as
Ag
0
on the surface. This further confirms the existence of
AgNPs in the CeO
2
@Ag-2 sample. The high-resolution
XPS spectrum of Ce 3d, shown in Figure 5c, has two
characteristic peaks located near 883.8 eV and 902.2 eV
that are attributed to the binding energies of Ce 3d5/2
and Ce 3d3/2,
28
indicating that Ce exists as Ce
4+
. Fig-
ure 5d shows the high-resolution XPS spectrum of O 1s.
The two characteristic peaks located near 529.4 eV and
531.7 eV belong to the lattice oxygen and surface
chemisorbed oxygen O 1s.
29
In order to further clarify the surface areas and
mesoporous structures of CeO
2
and CeO
2
@Ag hollow
spheres, nitrogen sorption experiments were carried out.
It can be seen from Figure 6 that the shape of both iso-
therms is a typical type IV curve, with an obvious H1
S. CUI et al.: SYNTHESIS, CHARACTERIZATION OF CeO 2@Ag HOLLOW SPHERES ...
Materiali in tehnologije / Materials and technology 57 (2023) 5, 537–554 541
Figure 5: a) XPS survey of CeO
2
and CeO
2
@Ag-2, b) XP spectra of CeO
2
@Ag-2: Ag 3d, c) Ce 3d, d) O 1s

hysteresis loop, indicating that CeO
2
and CeO
2
@Ag-2
have mesoporous properties, which mainly arise from
the accumulation of CeO
2
nanoparticles. The hysteresis
loops of CeO
2
and CeO
2
@Ag-2 are almost the same. Ac-
cording to the results of SEM and TEM, Ag is mainly
distributed on the surfaces of CeO
2
hollow microspheres,
so it has little effect on the accumulation of CeO
2
grains
in the shell. The specific surface areas of CeO
2
and
CeO
2
@Ag-2 are 122.4 cm
2
/g and 117.0 cm
2
/g, respec-
tively, resulting from the hollow structures of the sam-
ples. This result is much higher than in similar cases re-
ported in the literature, which are listed in Table 1. Such
a high specific surface area is conducive to the adsorp-
tion and catalytic reaction of pollutants.
Table 1: Comparison of specific surface areas based on various CeO
2
hollow nanostructures
Catalyst
Specific surface
area
References
CeO
2
hollow spheres 67.16 (cm
2
/g)
30
CeO
2
hollow spheres 67.1 (cm
2
/g)
31
CeO
2
hollow spheres with
a tunable pore structure
32.266  54.668 (cm
2
/g)
32
CeO
2
hollow spheres 122.4 (cm
2
/g) This work
CeO
2
@Ag-2 117.0 (cm
2
/g) This work
3.2 Catalytic activity for the catalytic reduction of
4-NP
4-NP is an organic pollutant widely existing in indus-
trial wastewater. It is highly soluble and difficult to be
naturally degraded. Nobel metal catalysis is an effective
green method for dealing with this problem. Therefore,
we chose 4-NP as the degradation target to evaluate the
catalytic activity of as-prepared CeO
2
@Ag, and the re-
sults are shown in Figure 7. Figure 7a shows the ab-
sorption spectrum of 4-NP. Two obvious absorption
peaks can be seen, one is near 400 nm and the other is
near 300 nm, corresponding to 4-NP and 4-aminophenol
(4-AP). Generally, the conversion of aromatic nitro com-
pounds into aromatic amino compounds is an important
step in organic chemical synthesis and industrial produc-
tion.
33
With a catalytic time from 0 to 450 s, a strong ab-
sorption peak corresponding to 4-NP decreases rapidly,
while the absorption peak corresponding to 4-AP in-
creases gradually, indicating that 4-NP was rapidly re-
duced to 4-AP under the effect of the CeO
2
@Ag-2 cata-
lyst. The catalytic reduction rate was 86 % at 450 s,
indicating that the CeO
2
@Ag-2 catalyst is very effective
for the catalytic reduction of 4-NP.
The catalytic reduction curves of C/C
0
versus cata-
lytic time for the CeO
2
@Ag samples are plotted in Fig-
ure 7b where C
0
is the initial concentration of 4-NP and
C is the real-time concentration. After 450 s, the cata-
lytic reduction efficiencies for CeO
2
@Ag-1,
S. CUI et al.: SYNTHESIS, CHARACTERIZATION OF CeO 2@Ag HOLLOW SPHERES ...
542 Materiali in tehnologije / Materials and technology 57 (2023) 5, 537–554
Figure 7: a) Absorption spectra of 4-NP in the presence of
CeO
2
@Ag-2, b) catalytic reduction of 4-NP by different CeO
2
@Ag
samples, c) corresponding curves between ln(C/C
0
) and reduction time
Figure 6: Nitrogen adsorption–desorption isotherm of CeO
2
and
CeO
2
@Ag-2

CeO
2
@Ag-2 and CeO
2
@Ag-3 were (62, 86 and 42) %,
respectively. Thus, CeO
2
@Ag-2 exhibits the best perfor-
mance. Figure 7c shows the corresponding ln(C/C
0
)
curves. All the curves of the three CeO
2
@Ag samples
show the Langmuir-Hinshelwood first-order kinetics
model.
34
As calculated, the rate constants are 0.120,
0.278 and 0.074 for CeO
2
@Ag-1, CeO
2
@Ag-2 and
CeO
2
@Ag-3. Obviously, with the increase in the Ag
loading, the catalytic performance of the series of sam-
ples increased first and then decreased; CeO
2
@Ag-2
showed the best catalytic performance.
To further analyze the difference in the catalytic per-
formance of CeO
2
@Ag catalysts, we carried out a
UV–Vis diffuse reflectance test, and the result is shown
in Figure 8. It can be seen that all the samples have an
obvious absorption edge near 400 nm, corresponding to
the band gap width of CeO
2
.
33
After loading AgNPs, the
absorption of CeO
2
@Ag catalysts in the visible region is
enhanced, as described in the literature,
33,35
which can be
generally attributed to the interaction between electrons
in CeO
2
and AgNPs, leading to more oxygen vacancies.
CeO
2
@Ag-2 has the strongest absorption in the visible
region, so we infer that the synergistic effect between
CeO
2
and AgNPs is also the strongest.
According to the previous SEM results, with the con-
centration of [Ag(NH
3
)
2
]NO
3
solution increasing from 5
×1 0
–5
Mt o1×1 0
–4
M, AgNPs can be well dispersed on
the surfaces of CeO
2
hollow spheres and the grain size is
less than 10 nm. The synergistic effect between AgNPs
and CeO
2
benefits the catalytic activity of AgNPs in the
catalytic process, so the CeO
2
@Ag-2 sample with the
higher Ag loading amount exhibits the best performance.
When the concentration of the [Ag(NH
3
)
2
]NO
3
solution
increases to 1.5 × 10
–4
M, the concentration of silver ions
is too high; they are rapidly reduced to AgNPs in the so-
lution and then agglomerated to large-size silver particles
which have a poor catalytic ability. As a result, the
amount of the silver ions adsorbed on the CeO
2
hollow
spheres is reduced, and the synergistic effect between
AgNPs and CeO
2
is weakened; both reasons lead to a de-
creased catalytic performance of the CeO
2
@Ag-3 sam-
ple. On the one hand, for our prepared CeO
2
@Ag cata-
lysts, the CeO
2
hollow sphere framework provides a
carrier with a large specific surface area for AgNPs,
which is conductive to controlling the size of AgNPs and
increasing the effective area of AgNPs in the catalytic
process. On the other hand, the synergistic effect be-
tween CeO
2
and AgNPs is affected by the loading
amount of Ag. Excessive loading will not only cause a
waste of raw materials, but also reduce the catalytic effi-
ciency. As a result, the CeO
2
@Ag hollow composite cat-
alysts can show the best catalytic performance only at
the optimum loading amount of Ag.
4 CONCLUSIONS
In summary, we successfully prepared a CeO
2
@Ag
hollow spherical composite catalyst using the template
method. The specific surface area of CeO
2
hollow
microspheres is up to 122.4 cm
2
/g, which is higher than
that of similar CeO
2
hollow microspheres found in the
literature. The mesoporous structure of CeO
2
hollow
microspheres plays a key role in controlling the size and
dispersion of AgNPs. The grain size of the AgNPs on the
surfaces of CeO
2
hollow microspheres is less than 10
nm, and the specific surface area of the CeO
2
@Ag-2 cat-
alyst is 117.0 cm
2
/g. In the study of the catalytic perfor-
mance for the reduction of 4-NP, the as-prepared
CeO
2
@Ag-2 sample exhibits the best catalytic efficiency
of 86 % at 450 s. This excellent catalytic performance is
attributed to the small grain size of AgNPs controlled by
the CeO
2
hollow mesoporous structure and the synergis-
tic effect between CeO
2
and AgNPs. The preparation
method for metal@oxide hollow spheres can be used in
the synthesis of many other materials, and this
CeO
2
@Ag composite material can also be used in many
other application fields.
Acknowledgements
This work was supported by the National Natural
Science Foundation of China (No. 21076094), the Edu-
cational Commission of the Jilin Province of China (No.
JJKH20210527KJ) and the Natural Science Foundation
of the Jilin Province (No. 20210101407JC).
References
1
S. J. Zhao, Z. Li, Z. Qu, N. Q. Yan, W. J. Huang, W. M. Chen, H. M.
Xu, Co-benefit of Ag and Mo for the catalytic oxidation of elemental
mercury, Fuel, 158 (2015), 891–897
2
M. Özacar, A. S. Poyraz, H. C. Genuino, C. H. Kuo, Y . T. Meng, S.
L. Suib, Influence of silver on the catalytic properties of the
cryptomelane and Ag-hollandite types manganese oxides OMS-2 in
the low-temperature CO oxidation, Appl. Catal. A, 462–463 (2013),
64–74
S. CUI et al.: SYNTHESIS, CHARACTERIZATION OF CeO 2@Ag HOLLOW SPHERES ...
Materiali in tehnologije / Materials and technology 57 (2023) 5, 537–554 543
Figure 8: UV-Vis absorption spectra of the samples

3
H. Mao, C. G. Ji, M. H. Liu, Z. Q. Cao, D. Y . Sun, Z. Q. Xing, X.
Chen, Y. Zhang, X. M. Song, Enhanced catalytic activity of Ag
nanoparticles supported on polyacrylamide/polypyrrole/graphene ox-
ide nanosheets for the reduction of 4-nitrophenol, Appl. Surf. Sci.,
434 (2018), 522–533
4
A. Ahmad, F. Ali, Z. A. Alothman, R. Luque, UV assisted synthesis
of folic acid functionalized ZnO–Ag hexagonal nanoprisms for effi-
cient catalytic reduction of Cr
+6
and 4-nitrophenol, Chemosphere,
319 (2023), 137951
5
K. Sawabe, T. Hiro, K. Shimizu, A. Satsuma, Density functional the-
ory calculation on the promotion effect of H 2 in the selective cata-
lytic reduction of NO x over Ag–MFI zeolite, Catal. Today, 153
(2010) 3–4, 90–94
6
S. Kundu, W. Dai, Y . Y . Chen, L. Ma, Y . Yuan, M. Sinyukov Alexan-
der, L. Hong, Shape-selective catalysis and surface enhanced Raman
scattering studies using Ag nanocubes, nanospheres and aggregated
anisotropic nanostructures, J. Colloid Interface Sci., 498 (2017),
248–262
7
T. Gan, Z. K. Wang, Z. X. Shi, D. Y . Zheng, J. Y . Sun, Y . M. Liu,
Graphene oxide reinforced core–shell structured Ag@Cu 2O with tun-
able hierarchical morphologies and their morphology–dependent
electrocatalytic properties for bio-sensing applications, Biosens.
Bioelectron., 112 (2018), 23–30
8
Y . G. Wang, X. Y . Wang, B. Sun, S. C. Tang, X. K. Meng, Concen-
tration-Dependent Morphology Control of Pt-coated-Ag Nanowires
and Effects of Bimetallic Interfaces on Catalytic Activity, J. Mater.
Sci. Technol., 32 (2016) 1, 41–47
9
C. R. Zhang, L. F. Wang, C. U. Dong, W. J. Zhu, W. H. Ge, Prepara-
tion of hollow Ag/Pt heterostructures on TiO 2 nanowires and their
catalytic properties, Appl. Catal. B, 180 (2016), 344–350
10
D. Biswas, S. Mondal, A. Rakshit, A. Bose, S. Bhattacharyya, S.
Chakraborty, Size and density controlled Ag nanocluster embedded
MOS structure for memory applications, Mater. Sci. Semicond. Pro-
cess., 63 (2017), 1–5
11
E. Ramírez-Meneses, V . Montiel-Palma, M. A. Domínguez-Crespo,
M. G. Izaguirre-Lope, E. Palacios-Gonzalez, H. Dorantes-Rosales,
Shape- and size-controlled Ag nanoparticles stabilized by in situ gen-
erated secondary amines, J. Alloys Compd., 643 (2015), s51–s61
12
H. Tigger-Zaborov, G. Maayan, Nanoparticles assemblies on de-
mand: Controlled aggregation of Ag(0) mediated by modified
peptoid sequences, J. Colloid Interface Sci., 508 (2017), 56–64
13
Z. Chen, D. S. Jia, Y . Zhou, J. Hao, Y . Liang, Z. M. Cui, W. G. Song,
In situ generation of highly dispersed metal nanoparticles on two-di-
mensional layered SiO 2 by topotactic structure conversion and their
superior catalytic activity, Appl. Surf. Sci., 434 (2018), 1137–1143
14
Y . Chi, L. Zhao, Q. Yuan, X. Yan, Y . J. Li, N. Li, X. T. Li, In situ
auto-reduction of silver nanoparticles in mesoporous carbon with
multifunctionalized surfaces, J. Mater. Chem., 22 (2012),
13571–13577
15
H. Mao, C. G. Ji, M. H. Liu, Z. Q. Cao, D. Y . Sun, Z. Q, Xing, X.
Chen, Y. Zhang, X. M. Song, Enhanced catalytic activity of Ag
nanoparticles supported on polyacrylamide/polypyrrole/graphene ox-
ide nanosheets for the reduction of 4-nitrophenol, Appl. Surf. Sci.,
434 (2018), 522–533
16
X. G. Liao, L. Zheng, Q. He, G. Li, L. Zheng, H. M. Li, T. Tian, Fab-
rication of Ag/TiO 2 membrane on Ti substrate with integral structure
for catalytic reduction of 4-nitrophenol, Process Saf. Environ. Prot.,
168 (2022), 792–799
17
S. Mohammadzadeh, M. E. Olya, A. M. Arabi, A. Shariati, M. R.
Khosravi Nikou, Synthesis, characterization and application of
ZnO-Ag as a nanophotocatalyst for organic compounds degradation,
mechanism and economic study, J. Environ. Sci., 35 (2015), 194–207
18
M. K. Kim, P. S. Kim, J. H. Baik, I. S. Nam, B. K. Cho, S. H. Oh,
DeNO x performance of Ag/Al 2O 3 catalyst using simulated diesel
fuel–ethanol mixture as reductant, Appl. Catal. B, 105 (2011) 1–2,
1–14
19
S. Liu, X. D. Wu, D. Weng, R. Ran, Ceria-based catalysts for soot
oxidation: a review, J. Rare Earths, 33 (2015) 6, 567–590
20
Y . Shi, X. L. Zhang, Y . M. Zhu, H. L. Tan, X. S. Chen, Z. H. Lu,
Core–shell structured nanocomposites Ag@CeO 2 as catalysts for hy-
drogenation of 4-nitrophenol and 2-nitroaniline, RSC Adv., 6 (2016),
47966–47973
21
Y. Y. Wang, Y. Shu, J. Xu, H. Pang, Facile one-step synthesis of
Ag@CeO 2 core–shell nanospheres with efficient catalytic activity for
the reduction of 4-nitrophenol, CrystEngComm, 19 (2017), 684–689
22
B. Y . Guan, L. Yu, J. Li, X. W. Lou, A universal cooperative assem-
bly-directed method for coating of mesoporous TiO 2 nanoshells with
enhanced lithium storage properties, Sci. Adv. Mater., 2 (2016),
1501554
23
Z. H. Wang, H. F. Fu, Z. W. Tian, D. M. Han, F. B. Gu, Strong
metal–support interaction in novel core–shell Au–CeO 2 nano-
structures induced by different pretreatment atmospheres and its in-
fluence on CO oxidation, Nanoscale, 8 (2016), 5865–5872
24
Y . Chi, Q. Yuan, Y . J. Li, J. C. Tu, L. Zhao, N. Li, X. T. Li, Synthesis
of Fe 3O 4@SiO 2-Ag magnetic nanocomposite based on small-sized
and highly dispersed silver nanoparticles for catalytic reduction of
4-nitrophenol, J. Colloid Interface Sci., 383 (2012), 96–102
25
J. S. Fang, Y. W. Zhang, Y. M. Zhou, S. Zhao, C. Zhang, H. X.
Zhang, X. L. Sheng, In-situ formation of supported Au nanoparticles
in hierarchical yolkshell CeO 2/mSiO 2 structures as highly reactive
and sinter-resistant catalysts, J. Colloid Interface Sci., 488 (2017),
196–206
26
C. Lee, J. Park, Y . G. Shul, H, Einaga, Y . Teraoka, Ag supported on
electrospun macro-structure CeO 2 fibrous mats for diesel soot oxida-
tion, Appl. Catal. B, 174–175 (2015), 185–192
27
J. H. Yang, J. Wang, X. Y . Li, D. D. Wang, H. Song, Synthesis of ur-
chin-like Fe 3O 4@SiO 2@ZnO/CdS core–shell microspheres for the
repeated photocatalytic degradation of rhodamine B under visible
light, Catal. Sci. & Technol., 6 (2016), 4525–4534
28
L. Yu, R. S. Peng, L. M. Chen, M. L. Fu, J. L. Wu, D. Q. Ye, Ag sup-
ported on CeO 2 with different morphologies for the catalytic oxida-
tion of HCHO, Chem. Eng. J., 334 (2018), 2480–2487
29
L. Ma, D. S. Wang, J. H. Li, Ag/CeO 2 nanospheres: efficient cata-
lysts for formaldehyde oxidation, Appl. Catal. B, 148–149 (2014),
36–43
30
W. J. Deng, D. H. Chen, L. Chen, Synthesis of monodisperse CeO 2
hollow spheres with enhanced photocatalytic activity, Ceram. Int., 41
(2015), 11570–11575
31
W. W. Zhang, D. H. Chen, Preparation and performance of CeO 2 hol-
low spheres and nanoparticles, J. Rare Earths, 34 (2016) 3, 295–299
32
J. Zhang, M. Gong, C. Tian, C. A. Wang, Facile synthesis of well-de-
fined CeO 2 hollow spheres with a tunable pore structure, Ceram. Int.,
42 (2016), 6088–6093
33
Y. Y. Wang, Y. Shu, J. Xu, H. Pang, Facile one-step synthesis of
Ag@CeO 2 core–shell nanospheres with efficient catalytic activity for
the reduction of 4-nitrophenol, CrystEngComm, 19 (2017), 684–689
34
K. Vasanth Kumar, K. Porkodi, F. Rocha, Langmuir–Hinshelwood
kinetics – A theoretical study, Catalysis Communications, 9 (2008)
1, 82–84
35
L. E. Wu, S. M. Fang, L. Ge, C. C. Han, P. Qiu, Y . J. Xin, Facile syn-
thesis of Ag@CeO 2 core–shell plasmonic photocatalysts with en-
hanced visible-light photocatalytic performance, J. Hazard. Mater.,
300 (2015), 93–103
S. CUI et al.: SYNTHESIS, CHARACTERIZATION OF CeO 2@Ag HOLLOW SPHERES ...
544 Materiali in tehnologije / Materials and technology 57 (2023) 5, 537–554