TiO2 is one of the most attractive metal oxides because of the excellent chemical and photocatalytic properties. However, a problem in the application of TiO2 is the large band gap energy of 3.2 eV, corresponding to its photocatalytic activity under UV-light irradiation of wavelengths 387 nm. In this work, TiO2 nanoparticles doped with iron were grown on the surface of functionalized carbon nanotubes (TiO2-Fe@CNTs) to expand the photoabsorbance of the nanocomposite materials in the visible light region and improve their photocatalytic activity. TiO2-Fe@CNTs nanocomposite materials were synthesized by hydrothermal route in Teflon-sealed autoclave at 180oC for 10h. The FE-SEM and X-Ray diffraction measurements were taken for morphology and structural analysis of TiO2 nanoparticles doped with Fe coating on CNTs. The effects of the iron and CNTs on the enhanced photocatalytic activity for methylene blue degradation under AM 1.5 illumination of 100 mW.cm−2 were investigated

Chi, Le Ha

Chuc, Nguyen Van

Hong, Le Van

Long, Pham Duy

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 24, No. 4 (2014), pp. 363-369
DOI:10.15625/0868-3166/24/4/5676
HYDROTHERMAL SYNTHESIS AND ENHANCED PHOTOCATALYTIC
ACTIVITY OF TiO2-Fe@CNTs NANOCOMPOSITE FOR METHYLENE BLUE
DEGRADATION UNDER VISIBLE LIGHT IRRADIATION
LE HA CHI, PHAM DUY LONG, NGUYEN VAN CHUC, AND LE VAN HONG
Institute of Materials Science, Vietnam Academy of Science and Technology
E-mail: chilh@ims.vast.ac.vn
Received 26 November 2014
Accepted for publication 12 December 2014
Abstract. TiO2 is one of the most attractive metal oxides because of the excellent chemical and photocatalytic prop-
erties. However, a problem in the application of TiO2 is the large band gap energy of 3.2 eV, corresponding to its pho-
tocatalytic activity under UV-light irradiation of wavelengths < 387 nm. In this work, TiO2 nanoparticles doped with
iron were grown on the surface of functionalized carbon nanotubes (TiO2-Fe@CNTs) to expand the photoabsorbance
of the nanocomposite materials in the visible light region and improve their photocatalytic activity. TiO2-Fe@CNTs
nanocomposite materials were synthesized by hydrothermal route in Teflon-sealed autoclave at 180˚C for 10h. The FE-
SEM and X-Ray diffraction measurements were taken for morphology and structural analysis of TiO2 nanoparticles
doped with Fe coating on CNTs. The effects of the iron and CNTs on the enhanced photocatalytic activity for methylene
blue degradation under AM 1.5 illumination of 100 mW.cm−2were investigated.
Keywords: TiO2, photocatalysis, nanotechnology.
I. INTRODUCTION
Titanium dioxide (TiO2) has been extensively used semiconductor in solar cells, gas sen-
sors, photocatalysis and photochemical processes due to its stability, low cost and non-toxicity
[1]. However, narrow light response range under UV-light irradiation of wavelengths < 387 nm
(corresponding to TiO2band gap energy about 3.2 eV) which accounts only for 4% of the incom-
ing solar energy and low separation probability of the photoinduced electron – hole pairs in TiO2
limits its technological applications. Therefore, doping of TiO2 semiconductor nano-particles with
transition metal ions including Fe, Cu, Ni or nonmetal atoms such as N, S, C to shift the optical
absorption of from UV to the visible light region has been increasingly studied [2,3]. Doping TiO2
with other elements can lead to a lower band gap and therefore improve the sunlight utilization of
TiO2. Additionally, the recombination of electron–hole pairs is suppressed by the introduction of
other elements. Recently, the effect of co-doping TiO2 with both cation and anion on the photocat-
alytic activity, such as Fe and Eu [4], Zn and Fe [5], N and Ni [6], N and S [7], shows apparently
higher photocatalytic activity than that of a single doped TiO2. Among the metals doped TiO2,
Fe doped TiO2 show greatly improved photocatalytic activity compared to crude TiO2. Fe is an
c©2014 Vietnam Academy of Science and Technology
364 HYDROTHERMAL SYNTHESIS AND ENHANCED PHOTOCATALYTIC ACTIVITY OF TiO2-Fe@CNTs ...
abundant element, environmentally friendly and suitably inserted into the TiO2 lattice structures,
because the ionic radius of iron (0.65 A˚) and titanium (0.605 A˚) is similar [8]. In this work, we
demonstrated a simple hydrothermal method to prepare TiO2 nanoparticles doped with iron coat-
ing on the surface of functionalized carbon nanotubes (TiO2-Fe@CNTs) nanocomposite material.
This approach was expected to improve the photoactivity of TiO2 in the visible light range by in-
creasing the surface area, creating defect structures to induce space-charge separation and enlarge
the absorption region. The obtained materials are characterized by different methods such as:
X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), transmission
electron microscopy (TEM), UV–vis spectroscopy. The effects of the Fe and CNTs components
on the properties of the TiO2 were discussed. The photocatalytic activity of the prepared TiO2-
Fe@CNTs composite photocatalysts was evaluated by the degradation of methylene blue under
simulated solar light irradiation.
II. EXPERIMENTAL
II.1. Preparation of nanostructured TiO2 doped with iron
Nanostructured TiO2 doped with iron (TiO2-Fe) powder were prepared by sol-gel method,
using titanium tetra isopropoxide (Ti[OC3H7]4), Fe(NO3)3.9H2O (dopant), ethanol, HNO3 and
de-ionized water. All the chemicals are of highest purity and commercially available. They were
used without further purification. Desired amount of Fe(NO3)3·9H2O (the molar Fe/Ti ratio in the
starting materials is 0.6 at.%) was dissolved in the mixture of 17 ml ethanol, 1.6 ml de-ionized
water and 0.4 ml concentrated HNO3. The mixed solution was then slowly dropped into the
mixture of 34 ml ethanol and 6 ml titanium tetra isopropoxide, and the final solution was stirred
for 30 minutes. The obtained sol was maintained at room temperature for 48 h. The resulting sol
was evaporated at 100 ˚ C for 24 hours to gradually form organic brown-colored gel. The yellow
or pale white TiO2-Fe powders were obtained by calcination at 400 ˚ C for 3 hours. Finally, the
materials were obtained by filtrating and washing with ethanol and de-ionized water for several
times, and then dried at 80 ˚ C for 12 hours.
II.2. Preparation of TiO2-Fe@CNTs nanocomposite material
TiO2-Fe@CNTs composites were prepared by a hydrothermal method as follow. Firstly, the
CNTs need to be treated with strong acid mixture of 1:3 HNO3/H2SO4 (by volume) under reflux
for 12 hours to functionalize surface of CNTs. Due to the inert property of CNTs, this step is
necessary to add carboxylic groups (COOH) on to the surface of the CNTs in order to enable them
to take part in reactions with the nanoparticles. The resulting solution was filtered and washed
thoroughly with water and decantation was used to remove any remaining acid, followed by drying
in an oven at 200˚C. The obtained powder was designated as acid-functionalized multiwalled
carbon nanotubes (CNT–COOH). In the next step, 10 mg of functionalized CNTs was dissolved
in a solution of 40ml de-ionized water and 20ml ethanol mixture using ultrasonic treatment for
2h. Then, 6ml titania precursor, namely titanium tetra isopropoxide (Ti[OC3H7]4) solution was
dissolved in the mixture of 34 ml ethanol, 0.4 ml concentrated HNO3, 1.6 ml de-ionized water and
48.2 mg Fe(NO3)3.9H2O. The mixed solution was then added to the functionalized CNTs solution
and stirred for another 2 hours at 60˚C to get a homogeneous suspension. The suspension was
then placed in a 200 ml Teflon-sealed autoclave and maintained at 180˚C for 10 hours. Finally, the
LE HA CHI, PHAM DUY LONG, NGUYEN VAN CHUC, AND LE VAN HONG 365
 
Fig. 1. Synthetic scheme for the preparation of TiO2-Fe@CNTs nanocomposite material
resulting powder was recovered by filtration, rinsed by de-ionized water several times, and dried
at 100˚C for 12 hours.
II.3. Sample characterization
The surface morphology of samples was investigated by using a “Hitachi S-4800” field
emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM,
JEOL JEM1010). The crystalline phase was identified by X-ray diffraction (XRD) using a
SIEMENS D5000 powder X-ray diffractometer (CuKα as radiation source, λ=0.15406 nm). The
ultraviolet–visible (UV–vis) absorption spectra were performed by using a Jasco V-670 UV-VIS
spectrometer.
II.4. Evaluation of photocatalytic performance
The photoreactor system used in the present work was measured under 1 sun illumina-
tion (100 mW.cm−2, AM 1.5). For a typical experiment, 10 mg of catalyst was put into 20 ml
of 0.01 g/l methylene blue (MB) aqueous solution with stirring in darkness for 30 min to reach
the adsorption–desorption equilibrium of MB on the catalyst surface prior to illumination. The
suspension was then exposed to light with continuous magnetic stirring. Then suspensions were
collected and centrifugally separated every 20 min, and the filtrate was returned to the vial for mea-
surement of the concentration MB degradation using a UV–vis spectrophotometer at a wavelength
of 664 nm.
III. RESULTS AND DISCUSSION
III.1. Structure and morphology
Morphological characterization of TiO2-Fe@CNTs nanocomposite material was carried out
by FE-SEM (Fig. 2a) and TEM (Fig. 2b). As shown in Fig. 2a, FE-SEM image of TiO2-Fe@CNTs
nanocomposite material exhibits the presence of nanoparticles on the surface of carbon nanotubes.
Some clusters of 100 nm nanoparticles were either randomly immobilized on the surface of CNTs
or densely collected among CNTs bundles. TEM image in Fig. 2b reveals that granular TiO2-Fe
nanoparticles with 7 nm in diameter was intimately bound to the surface of CNTs.
Fig. 3 shows the XRD characterization of the TiO2-Fe nanoparticles and TiO2-Fe@CNTs
nanocomposite material. The XRD pattern also shows that intensities of 0.6 at.% Fe doped TiO2
(Fig. 3a) were slightly higher than TiO2-Fe@CNTs (Fig. 3b). As seen from Fig. 3b, the strong
diffraction peak for the TiO2-Fe@CNTs at the angle (2θ) of 25.8o can be assigned to the C (002)
reflection of the hexagonal graphite structure. There are intensity peaks corresponding to anatase
366 HYDROTHERMAL SYNTHESIS AND ENHANCED PHOTOCATALYTIC ACTIVITY OF TiO2-Fe@CNTs ...
 
Fig. 2. FE-SEM (a) and TEM (b) images of the TiO2-Fe@CNTs nanocomposite material
phase of TiO2 for all samples without the appearance of trace rutile phase. No Fe peak was
observed for either Fe doped TiO2 or TiO2-Fe@CNTs composite. This indicates that doping Fe
ions with small percentages did not affect the crystalline structure of TiO2 nanoparticles and no
precipitate formation of doped metal such as iron oxides or FexTiOy. As noted previously [8,9],
Fe3+ can be suitably inserted into the TiO2 lattice structures and substituted Ti4+ site because the
ionic radius of Fe3+ and Ti4+ is similar.
 
Fig. 3. XRD patterns of the TiO2-Fe nanoparticles(a) and TiO2-Fe@CNTs nanocompos-
ite material (b).
LE HA CHI, PHAM DUY LONG, NGUYEN VAN CHUC, AND LE VAN HONG 367
III.2. Optical properties
To study the optical properties of Fe doped TiO2 and TiO2-Fe@CNTs samples, the UV–
vis absorption spectra were measured, as shown in Fig. 4a. Compared with pure anatase TiO2
that can only absorb λ 6387 nm UV light, the light absorption edges of Fe doped TiO2 and
TiO2-Fe@CNTs nanocomposites move remarkably with a red shift to the visible range, as well as
reduction in the bandgap energy.
 
Fig. 4. UV–visible spectra (a) and plots of (αhν)1/2 as a function of photon energy (b)
of the TiO2-Fe nanoparticles and TiO2-Fe@CNTs nanocomposite material.
The band gap energy Eg and absorption coefficient α are related by the following equation
[10]:
αhν = A(hν - Eg)m
Where α is the absorption coefficient, hν is the photon energy in eV, and Eg is the band
gap energy in eV. A is a constant related to the effective mass of the electrons and holes and m
being equal to 0.5 for allowed direct transition and 2 for an allowed indirect transition. Plots
of between (αhν)1/2and photon energy (hν) for the TiO2-Fe nanoparticles and TiO2-Fe@CNTs
nanocomposite material shown in the Fig. 4b. The energy gap Eg of the samples was evaluated
from the intercept of the linear portion of the each curve with the hν in X-axis. The straight lines
imply that the TiO2-Fe and TiO2-Fe@CNTs nanocomposite samples have energy band gaps of 2.6
eV and 2.1 eV, respectively. The red shift in absorption spectrum and reduction in bandgap are
attributed to the transfer of 3d-electrons from Fe to the conduction band of TiO2.
III.3. Photocatalytic degradation of methylene blue
η(%) = Co−CtCo ×100The photocatalytic degradation efficiency (η%) could be calculated as
the following equation:
where Co is the initial absorbance of the MB solution and Ct , its absorbance at different irradia-
tion times. All C values were obtained by the maximum absorption at 664 nm in the absorption
spectrum in order to evaluate the degradation efficiency as shown in Fig. 5.
368 HYDROTHERMAL SYNTHESIS AND ENHANCED PHOTOCATALYTIC ACTIVITY OF TiO2-Fe@CNTs ...
 
Fig. 5. Degradation indicated by absorption spectrum against TiO2-Fe@CNTs photo-
catalyst (a) and the percentage degradation of methylene blue solutions under simulated
solar light irradiation (b).
 
Fig. 6. Schematic diagram of the separation of generated electrons and holes on the in-
terface of TiO2-Fe@CNTs photocatalyst under simulated solar light irradiation.
Enhanced activity of the TiO2-Fe@CNTs photocatalyst in the visible light region as seen
in the Fig. 5 is attributed to the formation of appearance of localized electronic gap states ascribed
to Fe d–d or “localized charge transfer” transitions simultaneously. In addition, Ti–C and Ti–O–C
defect sites in the TiO2-Fe@CNTs photocatalyst act as trapping sites for photogenerated charges.
When the catalyst is irradiated by photons, electrons (e−) are excited from the valence band (VB)
to the conduction band (CB) of Fe doped TiO2 nanoparticles creating holes (h+) in the VB. The
hole (h+) on the metal oxide nanoparticles oxidize hydroxyl groups to form hydroxyl radical (OH)
which can decompose the methylene blue (MB). While CNTs act as a photo-generated electron
LE HA CHI, PHAM DUY LONG, NGUYEN VAN CHUC, AND LE VAN HONG 369
acceptor to promote interfacial electron transfer process and reduce the electron-hole recombina-
tion [11]. The adsorbed oxygen molecules on the surface of CNTs react with the electrons forming
very reactive superoxide radical ion (O−2 ) which oxidize methylene blue (MB). The proposed pho-
tocatalytic methylene blue degradation mechanisms were described in the Fig. 6 and the reactions
could be expressed as follows:
IV. CONCLUSIONS
TiO2-Fe@CNTs nanocomposite photocatalyst was successfully prepared via hydrother-
mal synthesis. The structure, morphology and optical properties were investigated. The TiO2-
Fe@CNTs nanocomposite photocatalysts showed a high efficiency in the degradation of methy-
lene blue under visible light. A tight interface between CNTs and TiO2 doped with Fe was created.
Doping TiO2 with Fe ions can also expand its light absorption range to visible light through the
formation of mid-gap states. On the other hand, CNTs with good adsorption capacity was success-
fully bonded with TiO2 to reduce the electron-hole recombination rate and prevent the particles
agglomeration. The higher visible light photocatalytic activity for methylene blue degradation of
the TiO2-Fe@CNTs nanocomposite was attributed to the increased adsorption capacity, enhanced
light absorption and better charge separation. This semiconductor heterogeneous photocatalysis
based on the advanced oxidation process (AOP) has enormous potential for the treatment of a wide
range of organic contaminants in water.
ACKNOWLEDGMENT
The authors thank National Key Laboratory, Institute of Materials Science, Vietnam Acad-
emy of Science and Technology for giving support in using experimental facilities. One of the
authors (Le Ha Chi) undertook this work with the support of Fundamental Research Funds for the
Young Researchers Programme.
REFERENCES
[1] T.L. Thompson and J.T. Yates, Chem. Rev. 106 (2006) 4428–4453.
[2] N. Serpone, J. Phys. Chem. B 110 (2006) 24287–24293.
[3] T.P. Ang, J.Y. Law and Y.F. Han, Catalysis Letters 139 (1-2) (2010) 77–84.
[4] L. Diamandescu, F. Vasiliu, D.T. Mihaila, M. Feder, A.M. Vlaicu, C.M. Teodorescu, D. Macovei, I. Enculescu,
V. Parvulescu and E. Vasile, Materials Chemistry and Physics 112 (2008) 146–153.
[5] Z.H. Yuan, J.H. Jia and L.D. Zhang, Materials Science Communications 73 (2002) 323–326.
[6] J. Fan, E.Z. Liu, L. Tian, X.Y. Hu, Q. He and T. Sun, J. Env. Eng. ASCE 37 (2011) 171–176.
[7] F.Y. Wei, L.G. Ni and P. Cui, Journal of Hazardous Materials 156 (2008) 135–140.
[8] K. C. Christoforidis, A. Iglesias-Juez, S. J. A. Figueroa, M. Di Michiel, M. A. Newtonb and M. Fernandez-Garcı,
Catal. Sci. Technol. 3 (2013) 626-634.
[9] S.Wang, J.S. Lian,W.T. Zheng and Q. Jiang, Applied Surface Science 263 (2012) 260–265.
[10] R. Suryanarayanan, V.M. Naik, P. Kharel, P. Talagala and R. Naik, Solid State Commun 133 (2005) 439–443.
[11] F. K. Mohamad Alosfur, M. H. Haji Jumali, S. Radimana, N. J. Ridha and A. Ali Umar, Procedia Technology 12
(2014) 264 – 270.


Hydrothermal Synthesis and Enhanced Photocatalytic Activity of  TiO\(_{2}\)-Fe@CNTs Nanocomposite for Methylene Blue Degradation under Visible  Light Irradiation

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Hydrothermal Synthesis and Enhanced Photocatalytic Activity of TiO $_{2}$ -Fe@CNTs Nanocomposite for Methylene Blue Degradation under Visible Light Irradiation