Synthesis of carbon-modified TiO2 nanotube arrays for enhancing the photocatalytic activity under the visible light

Article abstract of DOI:10.1016/j.jallcom.2010.04.059

TiO₂ nanotube arrays modified with carbon were prepared by anodization of Ti foil followed by a hydrothermal treatment in glucose aqueous solution. The morphologies, crystalline structures were characterized by a scanning electron microscope, an energy-dispersive X-ray spectrometer and an X-ray diffraction spectrometer. The optical and photoelectric properties were studied using a UVvis spectrophotometer and an electrochemical workstation. The photocatalytic activities of the samples were evaluated with the degradation of methylene blue under the visible light irradiation. The results revealed that the carbon-modified TiO₂ nanotube arrays exhibit a considerable enhancement of the photocatalytic activity comparing with the pure TiO ₂ nanotube arrays under the visible light.

Full text of DOI:10.1016/j.jallcom.2010.04.059

Journal of Alloys and Compounds 501 (2010) L8–L11
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Synthesis of carbon-modified TiO nanotube arrays for enhancing the
2
photocatalytic activity under the visible light
a
a,b,∗
Han Yang , Chunxu Pan
a
School of Physics and Technology and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, PR China
Center for Electron Microscopy, Wuhan University, Wuhan 430072, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
TiO2 nanotube arrays modified with carbon were prepared by anodization of Ti foil followed by a
hydrothermal treatment in glucose aqueous solution. The morphologies, crystalline structures were
characterized by a scanning electron microscope, an energy-dispersive X-ray spectrometer and an X-ray
diffraction spectrometer. The optical and photoelectric properties were studied using a UV–vis spec-
trophotometer and an electrochemical workstation. The photocatalytic activities of the samples were
evaluated with the degradation of methylene blue under the visible light irradiation. The results revealed
that the carbon-modified TiO2 nanotube arrays exhibit a considerable enhancement of the photocatalytic
activity comparing with the pure TiO2 nanotube arrays under the visible light.
Received 19 January 2010
Received in revised form 5 April 2010
Accepted 7 April 2010
Available online 18 April 2010
Keywords:
Titanium dioxide
Nanotube
Modification
Carbon
©
2010 Elsevier B.V. All rights reserved.
Anodization
Photocatalytic
Visible light
1
. Introduction
comparing with regular TiO2 nano-particles and nano-rods, and
have shown a potential application in photocatalysis [15]. Variant
In recent years, metaloxide semiconductor photocatalyst has
processes have been used for producing TiO nanotubes in the past
2
attracted many attentions in the application in photocatalytic
degradation of organic compounds for the treatment of wastew-
ater and air pollution [1,2]. Among various oxide photocatalysts,
TiO2 is indicated to be an excellent and high efficient photocat-
alyst [3]. However, it still remains many challenges before TiO2
becomes an economically feasible photocatalyst. For example, the
enhancement of solar energy utilization ratio and the suppression
of the recombination of photogenerated electron–hole pairs. A vari-
ety of methods have been developed to increase the photocatalytic
few years, such as template-assistant deposition [16], hydrother-
mal processes [17] and anodization [18]. Among these fabrication
methods, anodization is indicated to be a simple and precisely con-
trollable method for synthesizing highly ordered TiO2 nanotube
arrays in fluoride-based baths [18–21].
Modifying TiO2 with carbon can improve the photocatalytic
activity by improving the transport of photocarriers during photo-
catalysis through the formation of electronic interactions between
TiO2 and carbon, and many approaches [10–12] have been pub-
lished in recent years. To the best of our knowledge, few efforts have
been made for the carbon-modified TiO2 nanotube arrays. In the
present work, TiO2 nanotube arrays prepared by anodization were
modified with carbon from a simple method and exhibited a consid-
erable enhancement of photocatalytic activity comparing with the
pure TiO2 nanotube arrays under the visible light. The mechanism
of the enhancement of photocatalytic activity was investigated sys-
tematically.
efficiency of TiO , such as doping TiO2 with metallic ions [4,5]
2
or nonmetallic ions [6,7] to increase the visible light absorbance;
modifying or coating TiO2 with noble metal [8], semiconductor
[
9], carbon [10–12] or graphite [13] for improving the transport
of photocarriers during photocatalysis.
Enlarging the specific surface area of TiO2 can also improve the
photocatalytic efficiency [3], and many efforts have been made to
obtain nanostructured TiO -based materials with a large specific
2
surface area [14]. TiO2 nanotubes are of larger specific surface area
2
.
Experimental
The synthesis of carbon-modified TiO2 nanotube arrays (C-TNT) consisted of
three steps: (1) anodization of Ti foils to form the pristine TiO2 nanotube arrays
and annealing the pristine TiO2 nanotube arrays in ambient air at 450 C for 3 h to
∗ Corresponding author at: School of Physics and Technology and Key Laboratory
of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University,
Wuhan 430072, PR China. Tel.: +86 27 68752969; fax: +86 27 68752003.
E-mail address: cxpan@whu.edu.cn (C. Pan).
◦
produce crystallized TiO2 nanotube arrays (TNT), (2) hydrothermal treatment of the
◦
pristine TiO2 nanotube arrays in a glucose aqueous solution at 180 C for 4 h, and (3)
◦
annealing the treated TiO2 nanotube arrays in argon (Ar) at 450 C for 3 h.
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.04.059

H. Yang, C. Pan / Journal of Alloys and Compounds 501 (2010) L8–L11
L9
In the anodization experiments, Ti foils (1 cm × 1 cm in size, 0.25 mm thick,
TiO2 nanotube arrays were obtain by anodization of Ti foils in the
optimized conditions, the diameter of the TiO2 nanotubes were
about 60–70 nm. Generally, the formation of the TiO2 nanotube
arrays is proposed to have the two competitive processes: (1)
electrochemical oxidation, and (2) chemical dissolution [19]. The
electrochemical oxidation can be formulated as Eqs. (1)–(4), and
the chemical dissolution of TiO2 is formulated as Eq. (5):
9
9.5% purity) were degreased by sonicating in acetone, ethanol and deionized (DI)
water followed by drying in ambient air. Glycerol/water/ammonium fluoride elec-
trolyte consisted of a mixture of DI water and glycerol prepared in volumetric
ratio of 1:1 with 0.27 M NH4F, and all chemicals were of analytical grade. A two-
electrode system involving a graphite plate as cathode electrode and a direct current
power supply was used for the electrochemical anodization. The electrochemical
anodization consisted of a voltage ramp from the open-circuit potential (OCP) to
1
5 V, followed by holding this constant voltage during the anodization process. The
anodization process lasted for 3 h, and was carried out at room temperature.
Glucose aqueous solution consisted of 16 ml DI water and 0.1 g glucose was pre-
pared, and was subsequently transferred into a 20-ml autoclave where the pristine
H O → 4H + 4OH−
+
2
(1)
(2)
(3)
(4)
(5)
2
−
−
4OH → 2H + O +4e
2
2
◦
TiO2 nanotube arrays were placed in and hydrothermally treated at 180 C for 4 h.
◦
Anode : Ti + 2O2− → TiO2
4+
After that, hydrothermally treated samples were annealed in Ar at 450 C for 3 h.
The morphologies and chemical compositions of the TNT and the C-TNT sam-
ples were characterized by a scanning electron microscope (SEM, FEI SIRION, The
Netherlands) with an energy-dispersive X-ray spectrometer (EDS). The crystalline
structures were revealed by an X-ray diffraction spectrometer (XRD) (D8 Advanced
XRD, Bruker AXS, Germany) with Cu K␣ radiation (ꢀ = 0.15406 nm) working at 40 mA
and 40 kV.
The UV–vis diffuse reflectance spectra of the TNT and the C-TNT samples were
measured by a diffuse reflectance accessory of UV–vis spectrophotometer (UV-2550,
Shimadzu, Japan).
Cathode : H + 4e− → 2H2
+
TiO2 + 6F− + 4H+ → [TiF6]2− + 2H2O
The C-TNT samples were prepared according to the follow-
ing process: the pristine TiO2 nanotube arrays were hydrothermal
treated in a glucose aqueous solution; and the glucose was
◦
hydrolyzed at high temperature (180 C), therefore, a carbon layer
The photocatalytic activities of the TNT and the C-TNT samples were evaluated
with the degradation of methyl blue (MB) under the visible light at room tempera-
ture. Each sample of the TNT and the C-TNT was placed in a quartz cuvette filled with
was coated on the nanotubes. The EDS measurements indicate
that a carbon peak is observed in the C-TNT sample and the atom
percentage of C is 10.98%, which demonstrates that TiO2 nan-
otube arrays are successfully modified with carbon, as shown in
Fig. 1(d). The modified carbon was probably amorphous because
−5
2
ml MB aqueous solution (1.0 × 10 M) and irradiated with a visible light source
at a distance of 25 cm. A high pressure mercury lamp (160 W) with a cutoff filter
ꢀ > 390 nm) to remove radiation below 390 nm was used as the visible light source.
(
The variation of the MB concentration was evaluated by the absorbance of the solu-
tion at 665 nm, and the absorbance was measured by the UV–vis spectrophotometer
UV-2550, Shimadzu, Japan) for every 60 min.
◦
◦
the hydrothermal treatment (180 C) and anneal (450 C) tempera-
ture were too low to transfer amorphous carbon into graphite-like
carbon.
(
The photocurrent response experiments were carried out under the visible light
pulsed-irradiation using an electrochemical workstation (CHI600C, CH Instruments
Co.) to investigate the photoelectric properties of the TNT and the C-TNT samples.
The visible light source (the same as that in the MB degradation experiments) was
away from the working electrode with a distance of 15 cm, all experiments were
carried out at room temperature.
Fig. 2 shows the high-resolution (HR) TEM images of the C-TNT
sample. The images show that the TiO2 nanotubes are surrounded
by a very thin amorphous carbon layer, which is in good agreement
with our other results. Because of this structure, a good electronic
contact between TiO2 and carbon is achieved.
3
. Results and discussion
The pristine TiO2 nanotube arrays typically exhibit an amor-
phous structure, and can be transformed into anatase phase at
temperatures below 450 C in air [20]. The XRD spectra reveals that
◦
Fig. 1 shows the SEM views and the EDS patterns of the TNT
and the C-TNT samples. It can be seen that the highly ordered
both of the TNT and the C-TNT samples consist of pure anatase
Fig. 1. SEM morphologies and EDS patterns of the (a, c) TNT and the (b, d) C-TNT samples. The insets are the high magnification views.

L10
H. Yang, C. Pan / Journal of Alloys and Compounds 501 (2010) L8–L11
Fig. 2. High-resolution (HR)–TEM images of the C-TNT sample.
◦
phase due to the annealing temperature (450 C), which shows the
potential for photocatalytic application, as shown in Fig. 3.
Fig. 4 illustrates the optical properties of the TNT and the C-
TNT samples probed with UV–vis diffuse reflectance spectroscopy
(
DRS). The TNT sample shows a spectrum with its fundamental
absorption sharp edge rising at 380 nm, while the C-TNT sample
shows a higher absorption in the visible light region because of the
presence of carbon.
The photocatalytic activities of the TNT and the C-TNT samples
were evaluated with the degradation of MB under the visible light
irradiation, as shown in Fig. 5. The results indicate that (1) the
blank test (without any sample) shows only a small amount of MB
degradation; (2) the amount of MB degraded by the TNT sample is
only 40% after 5 h; (3) the amount of MB degraded by the C-TNT
sample was almost 85%. The C-TNT sample exhibits a considerable
enhancement of photocatalytic activity comparing with the TNT
sample under the visible light irradiation.
The photocatalytic activity of semiconductor oxide is mainly
governed by surface area, crystalline structure. The TNT and C-
TNT samples have the similar surface area and crystalline structure,
hence, the interaction between carbon and TiO nanotube arrays is
2
Fig. 4. UV–vis diffuse reflectance spectra (DRS) of the (a) TNT and the (b) C-TNT
samples.
considered to be an important factor for the photocatalytic activity
of the C-TNT sample. Therefore, the photocurrent response exper-
iments under the visible light were carried out to investigate the
electronic interaction of carbon and TiO2 nanotube arrays.
Fig. 5. The concentration change of methyl blue as a function of irradiation time
Fig. 3. XRD patterns of the (a) TNT and the (b) C-TNT samples.
under the visible light for the TNT and the C-TNT samples.

H. Yang, C. Pan / Journal of Alloys and Compounds 501 (2010) L8–L11
L11
4. Conclusions
In summary, highly ordered TiO2 nanotube arrays were pre-
pared by anodization of Ti foils in the glycerol/water/ammonium
fluoride electrolyte and then hydrothermal treated in glucose
aqueous solution for modifying with carbon. MB degradation
experiments revealed that the carbon-modified TiO2 nanotube
arrays exhibited a considerable enhancement of the photocatalytic
activity comparing with the pure TiO2 nanotube arrays under the
visible light irradiation. Photocurrent experiments indicated that
modifying TiO2 nanotube arrays with carbon improved the sepa-
ration of photoinduced electrons and holes through the electronic
interaction between carbon and TiO2 nanotube arrays, therefore,
enhanced the photocatalytic activity under the visible light irradi-
ation.
Acknowledgement
This work was supported by National Basic Research Program
of China (973 Program) (No. 2009CB939705).
Fig. 6. Photocurrent response of the (a) TNT and the (b) C-TNT samples under the
visible light.
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