HUE UNIVERSITY
HUE UNIVERSITY OF SCIENCES

LE THI THANH TUYEN

A RESEARCH OF CeO2/TiO2 NANOTUBES
PREPARATION AND THEIR PHOTOCATALYTIC
DEGRADATION UNDER VISIBLE LIGHT
IRRADIATION

Major: Theoretical Chemistry and Physical Chemistry
Code: 62.44.01.19

PhD DISSERTATION ABSTRACT
HUE, 2019

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The thesis has been completed at Department of Chemistry, Hue University of
Sciences, Hue University.
Supervisors:
1. Prof. Dr. Tran Thai Hoa
2. Dr. Truong Quy Tung

Examiner 1 : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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. . . . . . . . . . . . . . . . . . . Examiner 2 : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Therefore, CeO2 has special properties in the transfer of electrons
and the rise of absorb-light ability.
TiO2 is classified as a semiconductor widely used in photochemical techniques to decompose numerous kinds of toxic organic
contaminants because of its outstanding features. TiO 2 is a low-cost
non-toxic compound with high chemical durability, high photochemical stability and biological inertness. However, its photochemical activity only activates under UV light irradiation due to its
wide band gap (3.2 eV for anatase) and fast recombination of the
photo-generated electron/hole pairs (10 –9 to 10–12 s). Thus, various
approaches have been made to further improve the photo-catalytic
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performance of TiO2 including innovating physical properties of TiO2
like morphology, dimension and crystallite phase or doping/coupling
TiO2 with other metallic elements or oxides. In comparison with
nanoparticles, TiO2–NTs possess photo-catalytic features. Depending
on the synthesizing method utilized, those preeminent features are
massive surface (up to 478 m2/g), great volume of capillary (up to
1,25 cm3/g), capacity of transferring electrons from long distances,
capacity of ion exchange, and noticeable capacity of absorbing light
as a result of the considerable proportion between the length and the
diameter of the tube.
The combination of TiO2 with CeO2 is expected to
significantly improve the catalytic activity that derives from the role
of TiO2 as a mechanical, thermal and chemical stabilizer with the
good dispersion of CeO2 nanoparticles. The adjoining surface of this
oxide system is referred as the unique center (Sui generis) which
exhibits unique chemical properties. Thus, TiO 2 is becoming an
important supporting substance which is investigated to elucidate the
relationship between the structure and catalytic activity of the oxidemixture system.
Although the structure and properties of the CeO 2/TiO2

synthesized CeO2/TiO2-NTs, including four experimental parameters
namely; hydrothermal temperature, hydrothermal time, CeO 2/TiO2
molar ratio and calcination temperature.
Chapter 1. LITERATURE REVIEW
1.1. Overview of photocatalytic reaction
1.2. Overview of TiO2
1.3. CeO2-doped TiO2 nanotubes (CeO2/TiO2-NTs)
3


1.4. Overview of response surface methodology (RSM) in
optimization.
Chapter 2. AIMS, CONTENTS AND EXPERIMENTAL
METHODS
2.1. Aims
Synthesis of TiO2 nanotubes and CeO2/TiO2 nanotubes for
enhancing the photocatalytic degradation in the visible light.
2.2. Contents
2.2.1. Synthesis of TiO2 nanotubes.
2.2.2. Synthesis of CeO2-doped TiO2 nanotubes (CeO2/TiO2-NTs).
2.2.3. Study on the application of CeO2/TiO2-NTs in visible light
photocatalytic degradation of dyes.
2.3. Research methods
2.4. Experimental
Chapter 3. RESULTS AND DISCUSSION
3.1. SYNTHESIS OF CeO2/TiO2 NANOTUBES (CeO2/TiO2-NTs)
3.1.1. Synthesis of TiO2 nanotubes (TiO2-NTs)
3.1.1.1. Effects of hydrothermal temperature
From the SEM image (Figure 3.1), it can be seen that the
obtained TiO2 is shaped like an singe fiber of nanoparticles with the

2

( m /g)

140 °C

Vpore

dpore

Adsorpti

Hysteresis

(cm /g)

(nm)

on type

type

282

1,29

17,56

IV


0,06

18,53

IV

H3

3

The Table 3.1 shows that the increase in hydrothermal
temperature leads to the decrease in the specific surface area and the
volume of capillary as well as the change in capillary diameters.
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When the hydrothermal temperature was 140 °C and 160 °C, the
TiO2-NTs had a very large surface area, much larger than the P25 (50
m2/g). At 180 °C, the SBET of the TiO2-NTs declines by nearly six
times, compared to that of hydrothermals at 160 °C. Meanwhile,
samples of the hydrothermal at 200 °C had S BET less than 16 times
that of 160 °C. More importantly, at over 160 °C, the S BET of the
material is smaller than that of P25.
3.1.1.2. Effects of hydrothermal time
From the Figure 3.2, it can be seen that the main phase
composition of the obtained TiO2 samples is in amorphous form
along with the presence of Ti 9O17 and Na2Ti3O7 crystals in low
diffraction intensity, which shows the poor crystallinity. The presence
of Na2Ti3O7 in hydrothermally synthesized TiO2 nanotubes has been
reported in numerous published studies. Anatase and rutile crystals

In addition, a quite unclear diffraction peak near the position of 43°
which can be assigned to the rutile (210) crystal plane was detected,
revealing the remaining of the rutile phase in TiO 2–NTs. This fact
might serve as an evidence of the transformation of rutile to anatase
during calcination. Therefore, it is undeniable that, after being
calcined at high temperature, there is an improvement in the anatase
crystallinity and the anatase to rutile phase transformation.
The face centered cubic CeO 2 found in the synthesized
CeO2/TiO2–NTs sample is based on the presence of (101) and (200)
characteristic peaks. The existence of the anatase phase in
CeO2/TiO2–NTs at the same position as in TiO 2–NTs 550 and the face
centered cubic CeO2 phase is a firm proof for the separation of these
oxides during the synthesis process; in other words, the synthesized
materials exist as CeO2–TiO2 composites. As a result, ceria could
disperse mainly on the surface of the TiO 2 nanotubes in the form of
grains creating the boundaries in the synthesized composites (Figure
3.4b).

Figure 3.4. TEM images of (a) TiO2–NTs, (b) CeO2/TiO2–NTs, (c)
HRTEM image
of CeO2/TiO2–NTs and (d) EDX spectrum of CeO2//TiO2–NTs
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As shown in Figure 3.4a and 3.4b, the aggregated CeO2
nanoparticles ranging from 5 to 10 nm in size on the surface of the
TiO2 nanotubes could be clearly observed. The TiO 2 nanotubes were
hollow and open-ended with an average inner diameter of 4 nm,
average outer diameter of 10 nm, and about 200 nm in length. Ceria
dispersed mainly on the surface of TiO 2 nanotubes and, therefore,

mesoporous structure of materials, and the type H3 hysteresis loop
suggesting the presence of slit-shaped pores. The synthesized TiO 2NTs presents a higher BET surface area (247 m 2/g) than both TiO2NTs 550 (64 m2/g) and CeO2/TiO2-NTs (66 m2/g).
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Figure 3.5. Nitrogen adsorption/desorption isotherms of TiO2–NTs,
TiO2–NTs 550
and CeO2/TiO2-NTs@0,1.
3.1.2.1. Effects of calcination temperature
It can be seen from the XRD spectra the calcination
temperature had a strong effect on crystal structure and phase
components of the catalysts. The introduction of CeO 2 in TiO2-NTs
under proper calcination temperature (below 600 °C) did not change
the nanotubes structure of starting material. At 550 °C, the
crystallization of the anatase phase obtained was the most perfect
with the sharpest diffraction peak at 25.3°. The CeO 2 crystal structure
found in the synthesized products at this temperature was also more
complete with the clear characteristic peak at 28.5°.The surface area
SBET decreased as the calcination temperature increased, especially
when the temperature changed from 400 °C to 550 °C led to the
decrease in SBET from 149 m2/g to 65 m2/g.
3.1.2.2. Effects of doped ratio CeO2/TiO2
Compared to

bare TiO 2 nanotubes and bare CeO2,

CeO2/TiO2-NTs@X clearly exhibits a broader absorption in the
visible region (wavelength = 400-600 nm), displays a slight red shift
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6

8

10

12

-0.5
pH

Figure 3.7. the pHPZC estimated using pH drift method of CeO2/TiO2NTs.
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The point of zero charge (pHPZC) of CeO2/TiO2-NTs
estimated by the pH drift method is approximate 3.97. At pH < 3.97,
the surface of the CeO2/TiO2-NTs is charged positively due to
protonation and is charged negatively when pH > 3.97.
3.2.1.2. The adsorption kinetics of MB on CeO2/TiO2-NTs
The experimental data of the first order kinetic model with
the high coefficient of determination (R2 = 0.912-0.963) implies that
the adsorption process of MB on CeO2/TiO2-NTs followed the
pseudo- first order kinetic model. This means that the adsorption
process was controlled by a physical adsorption and the rate-limiting
step in this case involved a diffusion of MB to the surface of TiO 2
nanotubes.
3.2.1.3. The isotherm adsorption of MB on CeO2/TiO2-NTs
The experimental data are analyzed according to the linear form
of Langmuir and Freudlich model. The both R2 values are high and

CeO-/TiO2-NTs shows that the maximum absorption at 664 nm
(electron transfer –* in MB structure) decrease with an increase in
light illumination time. To confirm the mineralization of MB over
CeO2/TiO2-NTs catalyst, the COD of reaction products were carried
out. The initial COD was 28.2 mg·L–1, and its decrease became faster
as the illumination time increased reaching 10.68 mg·L –1 after 120
minutes. These results confirmed the effectiveness of CeO 2/TiO2–
NTs as a photo-catalyst for MB degradation under visible light. After
120 minutes of illumination, the total decolorization of MB
practically occurred, while around 61.1 % of COD reduction was
obtained. The difference between the degradation and mineralization
could be attributed to the existence of intermediate products.
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Figure 3.8. The concentration changes of MB solution during visible
light illumination over TiO2-NTs, P25, CeO2, CeO2/TiO2-NTs and
Blank.
3.2.2.2. Effects of different parameters on photodegradation of MB
a. Effects of initial MB concentration
The results showed that the photodegradation of MB
decreased from 97.61% to 47.54 when the MB initial concentration
increased from 10 ppm to 30 ppm.
b. Effects of solution pH
It is found that the MB degradation efficiency increased
sharply when pH was increased from 3 to 4, then continued to
increase slowly when the pH changed from 4 to 8 but then decreased
sharply when the pH increased to 12. The point of zero charge
(pHPZC) of CeO2/TiO2-NTs estimated by the pH drift method is
approximate 4. At pH < 4, MB molecule is neutral (pKa = 3.8) and

increase in doped ratio CeO2/TiO2 and the highest MB degradation
efficiency was obtained if CeO2/TiO2 ratio of 0.1. However, the
photodegradation efficiency decreased with further increase of Ce.
3.2.2.3. The mechanism of free radical formation
The free radical generation by photo-induced electron-hole
pairs were confirmed by the fluorescence emission spectrum. Figure
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3.9a shows the induction of fluorescence from 5·10 −4 M terephthalic
acid solution in 2·10−3M NaOH. The increase in fluorescence
intensity against illumination time at 425 nm was observed. The
fluorescence intensity by UV light illumination in terephthalic acid
solutions increased almost linearly against time. Consequently, we
can conclude that OH. radicals formed at the CeO 2/TiO2–NTs
interface are in proportional to the light illumination.
In order to confirm the formation of hydroxyl radicals on the
catalytic surface with visible light radiation the tert-butanol is used as
the free radical scavenger (Figure 3.9b). In the presence of tertbutanol, a marked reduction in the MB photo-chemical catalytic
activity was observed, and the decomposition efficiency decreased
sharply when increasing the amount of tert-butanol. Specifically,
when adding 0.1 mL of tert-butanol, after 120 minutes of light
illumination, the degradation efficiency of MB decreases by more
than 30 % compared to the absence of tert-butanol (64.6 % vs. 97
%). The presence of 0.2 mL tert-butanol decays to 58.3 %. It is clear
that the reaction is constrained by the presence of free radical
scavenger. The occurrence is related to the mechanism of free radical
formation.

Figure 3.9. a) Fluorescence spectra observed during illumination of

65

k (min-1)

R2

p

0,011
0,013
0,017
0,022
0,03

0,968
0,988
0,990
0,934
0,977

< 0,001
< 0,001
< 0,001
< 0,001
< 0,001

The MB photodegradation rate was found to increase quickly
with increasing temperature. Particularly, the photodegradation rate
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Ea

R2

(kJ.mol-1)

Transition state theory

H #

S #

21,12

Ea(t)

R2

(kJ.mol-1) (kJ.mol- (kJ.mol- (kJ.mol-1)
1

25
35
45
55
65

G #

0,982

present study, the experimental design of Box–Behnken was
employed to determine the optimum levels of the significant
variables. The number of experiments (N) required for the
development of this design is defined as N = 2k(k-1) + C0, where k is
the factor number and C0 is the replicate number of the central point.
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Thus a total of 27 runs performed for optimizing these four variables
in the current Box– Behnken design.
High level

New
High
D
Cur
0.00000 Low

Optimizati
on
Low level
conditions

X1
180.0
[163.2787]
140.0

X2
600.0

The profile for predicted values in the MINITAB–16 is
employed for the optimization process. The optimization design
matrix

(Figure

1)

represents

the

maximum

photo-catalytic

decolorization (92.9 % for MB) at conditions set as: hydrothermal
temperature (163 °C), calcination temperature (557 °C) hydrothermal
time (20 h) and doped ratio (0.1 mol·mol –1). The reliability of this
prediction was examined by performance of five similar experiment
at optimization conditions. The experimental decolorization yield for
were 93 %; 96 %; 94.5 %; 95 % and 94.2 %. The one-sample t-test
show non-significant difference with respective value presented by
model (t (4) = –2.32, p = 0.08). Therefore, the synthetic conditions
were used to synthesize the CeO2/TiO2–NTs for further experiments.
3.2.2.6. Photocatalytic kinetics of MB degradation
Kinetics of adsorption and photo-catalytic degradation of
MB over CeO2/TiO2–NTs is shown by Figure 3.12.
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0.15
0.15
0.13

ln (C0 a C )

R2
0.989
0.979
0.970
0.967
0.911
0.958
0.956

p
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001

vs. t yields the kapp. The high

2

coefficient of determination, R (0.911–0.989) confirms that the MB
photo-catalytic degradation fitted well with the L–H first-order

surface areas of 246.65 m2/g.
2.

CeO2/TiO2 nanotubes

(CeO2/TiO2-NTs)

were

successfully

synthesized by the impregnation of CeO2 on hydrothermally
synthesized TiO2-NTs. The effects of cerium doping content and
calcination temperature on structure, morphology, composition, and
visible light absorption property of CeO2/TiO2-NTs were also
studied. The results show that CeO 2/TiO2–NTs basically maintained
the TiO2 the anatase crystal structure and a mixture of Ce 4+/Ce3+
oxidation states exists on the surface of the synthesized CeO 2/TiO2–
NTs catalyst. Compared to bare TiO 2 nanotubes and bare CeO2,
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CeO2/TiO2–NTs clearly exhibit broader absorption in the visible
region, displaying a slight red shift in the optical adsorption. This red
shift of the adsorption edge indicates the enhanced ability of the
CeO2/TiO2–NTs hybrid catalyst to adsorb visible light. CeO 2-doped
TiO2 nanotube is stable and potential as a visible-light photo-catalyst
for organic substances degradation in aqueous solutions.
3. The photo-catalytic degradation of dyes over CeO2/TiO2-NTs in
the visible region was systematically investigated. The enhanced

equations were used to study the photocatalytic degradation of MB
by CeO2/TiO2-NTs in visible light. The results show that the
photocatalytic degradation of MB by CeO2/TiO2-NTs was controlled
by diffusion and free hydroxyl radical reaction.
5. The effective variables on the preparation of CeO 2/TiO2–NTs
catalysts for photo-catalytic performance were optimized utilizing
the Box–Behnken design (BBD) of response surface methodology
(RSM) to find out the optimum conditions for obtaining the
maximum photo-catalytic yield and the ability of obtained a catalyst
to photo-degrade methylene blue (MB) under visible light. This is the
first time the Box Behnken design of the response surface
methodology was employed to optimize synthesis conditions for the
photocatalytic degradation of MB over the synthesized CeO 2/TiO2NTs, including four experimental parameters namely: hydrothermal
temperature, hydrothermal time, CeO2/TiO2 molar ratio and
calcination temperature. Optimization results show that maximum
removal yield (92.9 %) was obtained at the optimum synthesis
conditions: hydrothermal temperature of 163 °C; calcination
temperature of 557 °C; hydrothermal time of 20 hours and
CeO2/TiO2 molar ratio of 0.1, The obtained results clearly
demonstrated that response surface methodology (RSM) with a Box–
Behnken design was one of the reliable methods for modeling and
optimization of the synthesis variables.

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