Highly effective photocatalyst of TiO2 nanoparticles dispersed on carbon nanotubes for methylene blue degradation in aqueous solution

Cite this paper: Vietnam J. Chem., 2021, 59(2), 167-178 Article 
DOI: 10.1002/vjch.202000091 
167 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH 
Highly effective photocatalyst of TiO2 nanoparticles dispersed on 
carbon nanotubes for methylene blue degradation in aqueous solution 
Nguyen Duc Vu Quyen
1*
, Dinh Quang Khieu
1
, Tran Ngoc Tuyen
1
, Dang Xuan Tin
1
, 
Bui Thi Hoang Diem
1
, Ho Thi Thuy Dung
2 
1
Department of Chemistry, University of Sciences, Hue University, 77 Nguyen Hue Str., Hue City, 
Thua Thien Hue 49000, Viet Nam 
2
Hue Medical College, 01 Nguyen Truong To Str., Hue City, Thua Thien Hue 49000, Viet Nam 
Submitted June 1, 2020; Accepted September 3, 2020 
Abstract 
In the present study, titania nanoparticles are highly dispersed on carbon nanotubes via hydrolysis process of tetra-
isopropyl-orthotitanate Ti[OCH(CH3)2]4 (TPOT). The obtained composite (TiO2/CNTs) is characterized by modern 
methods. The anatase-TiO2 phase is realized based on X-ray diffraction spectrum at different pHs of hydrolysis 
solution. The band gap of TiO2/CNTs (Eg) is calculated by Tauc method using diffuse reflectance spectroscopy (DRS). 
The TiO2/CNTs composite plays as an active photocatalyst for methylene blue (MB) decomposition in aqueous 
solution. The effect of time to photocatalytic ability of TiO2/CNTs composite is described using Langmuir-
Hinshelwood kinetic model. The values of enthalpy variation ( H), entropy change ( S) and Gibbs free energy 
variation ( G) of the decomposition of MB are determined from thermodynamic study. In the range temperature from 
283 K to 323 K, the positive values of H and negative value of G confirms endothermic and spontaneous nature of 
MB degradation. With the increase of temperature, the reaction occurs more easily, which is proved by more negative 
values of Gibbs free energy calculated from Van’t Hoff equation. 
Keywords. TiO2/CNTs composite, hydrolysis of titanium alkoxide, Langmuir-Hinshelwood kinetic, TiO2/CNTs 
photocatalyst. 
1. INTRODUCTION 
Ecosystem is strongly impacted by water 
contamination due to wastewater without treatment 
from industrial factories and household wastewater 
from populous cities in the world. In many big cities 
in Vietnam, numerous rivers and ponds are heavily 
contaminated, that endangers to human life. The 
oustanding pollutants putting negative effects on 
human health are heavy metals, toxic organic 
compounds. Among them, soluble organic pigment 
contributes a large part in household water pollution. 
Therefore, it is essential to study simple methods to 
lighten contamination with the aim of creating a 
fresh environment. Recently, the adsorption, 
biological method and especially, photocatalytic 
decomposition are popularly employed to remove 
organic pigments from aqueous solution. 
At present, the photocatalytic decomposition has 
attracted worldwide interest because of its high 
effectiveness in organic pigments removal. Titania 
(TiO2) is considered as the best photocatalyst for the 
degradation of the pigments from wastewater due to 
its prominent features, such as low cost, high 
chemical stability, environmental friendly and 
efficient photoactivity.
[1-4]
 Especially, the crystalline 
phases of anatase-TiO2 exhibits the strongest 
photocatalytic activity.
[5]
 However, the relatively 
large band gap energy of TiO2 (about 3.2 eV) 
requires high energy for photoactivation, such as 
ultraviolet irradiation.
[1,6]
 In addition, due to non-
porous structure and charged surface, anatase-TiO2 
presents small adsorption capacity for organic 
pollutants which are non-polar.
[6]
 The photocatalytic 
ability of TiO2 is also lessened because of the 
electron/hole pair recombination. These 
disadvantages require the studies on modification of 
TiO2 surface or diffusion of TiO2 on a suitable 
surface.
[7-9]
Carbon nanotubes (CNTs) with very high 
surface area create many active adsorption sites for 
the catalyst surface. CNTs also play as the trap to 
keep electrons transferred from valence band of 
semiconductor for a short time before come to 
Vietnam Journal of Chemistry Nguyen Duc Vu Quyen et al. 
 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 168 
conduction band. So, the charge recombination will 
be hampered.
[10,11]
 It is therefore of paramount to 
achieve TiO2/CNTs composite from CNTs and TiO2 
in a controllable way.
[12-17]
In almost of previous studies, CNTs were 
prepared by chemical vapour deposition (CVD) with 
the presence of hydrogen flow as reductant of 
catalyst in form of transition metal oxide.
[18-22]
 In the 
present study, CNTs with high surface area are 
synthesized by CVD without hydrogen. The surface 
area of CNTs is enhanced by oxidation with 
potassium permanganate in order to form oxidized 
CNTs which is dispersed in tetra-isopropyl-
orthotitanate (TPOT) solution. The outstanding 
synthesis method of TiO2/CNTs composite is 
dispering of the resulting CNTs in TiO2 sol. 
However, studies on the formation of anatase phase 
from the mixture of TiO2 sol and CNTs are rarely 
reported, which is investigated here. In addition, 
band gap of the obtained material is determined by 
well-known Tauc method. The composite is applied 
for MB photocatalytic decomposition in aqueous 
solution. The thermodynamic and kinetic of the 
decomposition are clearly studied. 
2. MATERIALS AND METHODS 
2.1. Materials 
The starting CNTs were prepared from LPG 
(Vietnam) via CVD without initial hydrogen flow as 
raw-material. The diameter of carbon tubes were in 
the range from 40 to 50 nm (figure 1A).
[23]
The oxidized CNTs (ox-CNTs) were formed 
with the oxidant of KMnO4 and H2SO4 mixture. 
Upon this functionalization step, the CNTs become 
shorter in long-axis direction, the tubes’ surface is ...  2: First-order kinetic parameters of the 
photocatalytic degradation of MB at different 
temperatures 
Temperature 
(K) 
Correlation 
coefficient (r
2
) 
kT (min
-1
) 
283 0.9771 0.0192 
293 0.9894 0.0266 
303 0.9957 0.0334 
313 0.9852 0.0369 
323 0.9901 0.0463 
Arrhenius and Eyring linear plots are obtained 
from kT at different temperatures shown in Figure 
14. Activation energy value calculated by the 
Arrhenius equation (figure 14A) is 15.94 kJ mol
−1
. 
This value is below 42 kJ mol
−1
 which points out 
that the adsorption of MB molecules is quickly 
occurred onto catalyst surface, the intermediate is 
easily created, as a result of a strong decomposition 
of MB. 
The values of activation parameters shown in 
table 3 are calculated from linear plot of Eyring 
equation (figure 14B). The formation of an 
intermediate or activated complex between MB and 
the catalyst is confirmed again due to the positive 
value of S# (421.40 J mol-1 K-1). The positive value 
of H# (13.43 kJ mol-1) suggests the endothermic 
nature of the formation of the activated complex. 
This intermediate is formed spontaneously and 
Vietnam Journal of Chemistry Highly effective photocatalyst of TiO2  
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 175 
favourable at high temperature because of the large 
negative values of G#. Therefore, MB 
photocatalytic decomposition is spontaneous and 
more favorable at high temperature. This is also 
demonstrated in table 2 that the reaction rate 
constant (kT) of MB degradation increases with 
temperature and table 4 that the Gibbs free energy 
variations (∆Go) of MB degradation at different 
temperatures calculated from equation (8) have 
negative values. 
0.0031 0.0032 0.0033 0.0034 0.0035
-4.0
-3.8
-3.6
-3.4
-3.2
-3.0
ln
k T
1/T
(A) - Arrhenius equation
ln k
T
= -1916.8(1/T) + 2.8698
r
2
 = 0.978
0.0031 0.0032 0.0033 0.0034 0.0035
-9.6
-9.4
-9.2
-9.0
-8.8
1/T
(B) - Eyring equation
ln (k
T
/T)= -1614.8(1/T) - 3.8417
r
2
 = 0.969
ln
 (
k T
/T
)
Figure 14: Arrhenius (A) and Eyring (B) equations 
for MB photocatalytic degradation 
Table 3: Activation parameters for MB 
photocatalytic degradation 
Temperature 
(K) 
 H# 
(kJ mol
-1
) 
 S# 
(J mol
-1
) 
 G# 
(kJ mol
−1
)
283 
13.43 421.40 
-10.58 
293 -11.00 
303 -11.43 
313 -11.85 
323 -12.27 
The enthalpy ( Ho) and entropy ( So) 
parameters were calculated using the Van’t Hoff 
equation (equation (9) and figure 15). 
lno CG RT K 
(8) 
ln
o o o
C
G S H
k
RT R RT
(9) 
0.0031 0.0032 0.0033 0.0034 0.0035
2.0
2.5
3.0
3.5
1/T
ln
K
C
Van't Hoff plot
lnK
C
 = -4541.6(1/T) + 17.778
r
2
 = 0.989
Figure 15: Van’t Hoff plot for MB photocatalytic 
degradation 
The endothermic nature of MB decomposition 
and the enhancement of the randomness at the 
liquid-solid interface are proved by the positive 
values of Ho (37.76 kJ mol-1) and So (147.81 
J mol
-1
 K
-1
). 
Table 4: Thermodynamic parameters of MB 
photocatalytic degradation 
Temperature 
(K) 
 Ho 
(kJ mol
-1
) 
 So 
(J mol
-1
) 
 Go 
(kJ mol
-1
)
283 
37.76 147.81 
-4.07 
293 -5.55 
303 -7.03 
313 -8.50 
323 -9.98 
According to plausible mechanism 
recommended by many studies, anatase-TiO2 
exhibits photocatalytic effectivity based on the 
generation of electron-hole pairs.
[32-36]
 The increase 
of MB degradation of TiO2/CNTs composite 
comparing to anatase-TiO2 is explained that CNTs 
play as electron traps and attract MB molecules to 
catalyst surface. The proposed mechanism of MB 
degradation over TiO2/CNTs composite can be 
described as in scheme 2. CNTs may accept the 
electrons (e ) induced by UV irradiation from 
valence band in the TiO2 nanoparticles and then, 
transfer them to the conduction band of TiO2 
nanoparticles. This process forms a positive charged 
hole (h
+
) in valence band of TiO2 nanoparticles. 
These electrons in conduction band may react with 
O2 in the solution to form superoxide radical ion 
Vietnam Journal of Chemistry Nguyen Duc Vu Quyen et al. 
 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 176 
(
2O
 ) and these positive charged hole (h
+
) may react 
with the OH 
derived from H2O to produce 
hydroxyl radical (OH ). Consequently, these groups 
(
2O
 ,OH ) react with MB molecules to form non-
toxic products, such as CO2, H2O, Cl
−
, SO4
2−
, NH4
+
and NO3
−
.
[37]
 Therefore, it can be concluded that the 
appearance of CNTs extends living time of these 
electrons and holes, increases the gathering of 
positive charged MB ions on the surface of catalyst 
due to an active surface containing negative charged 
functional groups (COO
−
, O
−
) created from 
oxidization of CNTs by KMnO4/H2SO4. As results, 
the MB degradation is enhanced. 
Scheme 2: The proposed mechanism of MB degradation over TiO2/CNTs composite 
4. CONCLUSION 
TiO2/CNTs composite was found to be an efficient 
photocatalyst for the degradation of methylene blue 
in aqueous solution. Anatase-TiO2 is favourably 
formed at hydrolysis pH of 8 and highly dispersed 
on carbon nanotubes after 2 hours of ultrasonic 
treatment. More than 95 % of MB with initial MB 
concentration of 20 mg L
-1 
was removed at ambient 
temperature with the catalyst TiO2/CNT at pH of 8 
and catalyst dosage of 1.5 g L
-1
 after 90 min 
irradiation. The photocatalytic degradation 
mechanism of MB on the TiO2/CNTs catalyst 
followed the Langmuir-Hinshelwood model. Kinetic 
study indicates that the intermediate between MB 
and catalyst is formed prior to the decomposition. 
Thermodynamic parameters confirmed the 
spontaneousness and endothermic nature of MB 
degradation. 
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Corresponding author: Nguyen Duc Vu Quyen 
University of Sciences - Hue University 
77, Nguyen Hue Str., Hue City, Thua Thien Hue 49000, Viet Nam 
E-mail: ndvquyen@hueuni.edu.vn 
Tel: +84- 979590971.