Home

SearchCollections

Journals

About

Contact us

My IOPscience

Fast-rate formation of TiO2 nanotube arrays in an organic bath and their applications inphotocatalysis

This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2010 Nanotechnology 21 365603(http://iopscience.iop.org/0957-4484/21/36/365603)
View the table of contents for this issue, or go to the journal homepage for more

Download details:
IP Address: 133.28.19.14
The article was downloaded on 21/08/2010 at 14:35

Please note that terms and conditions apply.

IOP PUBLISHINGNANOTECHNOLOGY

Nanotechnology 21 (2010) 365603 (8pp)

doi:10.1088/0957-4484/21/36/365603

Fast-rate formation of TiO2 nanotube
arrays in an organic bath and theirapplications in photocatalysis
Srimala Sreekantan1,3, Khairul Arifah Saharudin1,
Zainovia Lockman1 and Teoh Wah Tzu2
1

School of Materials and Mineral Resources Engineering, University Sains Malaysia
Engineering Campus, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia
2
Department of Civil and Environmental Engineering, Nagaoka University of Technology,1603-1 Kamitomioka, Nagaoka, Niigata 940-2118, Japan
E-mail: srimala@eng.usm.my

Received 25 March 2010, in final form 26 July 2010
Published 13 August 2010
Online at stacks.iop.org/Nano/21/365603
Abstract
In this work, 18.5 μm titanium oxide (TiO2 ) nanotube arrays were formed by the anodization oftitanium (Ti) foil in ethylene glycol containing 1 wt% water and 5 wt% fluoride for 60 min at
60 V. The fast growth rate of the nanotube arrays at 308 nm min−1 was achieved due to theexcess fluoride content and the limited amount of water in ethylene glycol used for anodization.
Limited water content and excess fluoride in ethylene glycol inhibited the formation of a thickbarrier layer by increasing the dissolution rate at the bottom of the nanotubes. This eased thetransport of titanium, fluorine and oxygen ions, and allowed the nanotubes to grow deep into thetitanium foil. At the same time, the neutral condition offered a protective environment along thetube wall and pore mouth, which minimized lateral and top dissolution. Results from x-ray
photoelectron spectra revealed that the TiO2 nanotubes prepared in ethylene glycol contained
Ti, oxygen (O) and carbon (C) after annealing. The photocatalytic activity of the nanotube
arrays produced was evaluated by monitoring the degradation of methyl orange. Results
indicate that a nanotube with an average diameter of 140 nm and an optimal tube length of
18.5 μm with a thin tube wall (20 nm) is the optimum structure required to achieve highphotocatalytic reaction. In addition, the existence of carbon, high degree of anatase crystallinity,smooth wall and absence of fluorine enhanced the photocatalytic activity of the sample.

fabrication methods, such as electrochemical lithography [2],photoelectrochemical etching [3], sol–gel processing [4],hydrothermal synthesis [5] and template synthesis [6, 7],
have been used to form nanotubes and other 1D TiO2
structures (e.g. nanowires, nanodots and nanorods). Amongthese processes, electrochemical anodization of titanium influorinated electrolytes is a relatively simple method tosynthesize porous or tubular structures. Gong et al [8]were the first to report on the formation of TiO2 nanotubes
using an acidified fluoride aqueous electrolyte under anodic
oxidation. For successful TiO2 formation, the acidified fluoride
solution must have a pH less than 5. However, under thiscondition, oxide dissolution rate is high and only a limited
length (500 nm) of nanotube can be produced. Cai et al [9]used a range of pH for the electrolyte to reduce the dissolution

1. Introduction
Titanium dioxide (TiO2 ) is a semiconductor material thathas been studied extensively in the last few decades due
to its chemical stability, nontoxicity and high photocatalytic
activity [1]. It has been used to eliminate pollutants, asa gas sensor, an electrode in solar cells and a catalyst
support. Due to its photocatalytic activity, TiO2 has also
been used as a photocatalyst in the degradation of organic
compounds that pollute the environment [1]. Owing to the onedimensional (1D) nature of TiO2 nanotubes, which offer a highsurface area for photocatalytic reaction, they provide better
performance compared to nanoparticles. Therefore, numerous
3 Author to whom any correspondence should be addressed.

0957-4484/10/365603+08$30.00

1© 2010 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 21 (2010) 365603

S Sreekantan et al

Table 1. Formation rate of TiO2 nanotubes produced in various electrolytes. (Note: EG = ethylene glycol.)Anodization condition

Tube length (μm)

Rate of formation (nm min−1 )EG, 60 V, 0.3 wt% NH4 F, 2% H2 O, 5760 min [11]EG, 60 V, 0.4 wt% NH4 F, 2.5% H2 O, 10 080 min [11]EG, 60 V, 0.5 wt% NH4 F, 3% H2 O, 12 960 min [11]DMSO, 60 V, 2 wt% HF, 4200 min [11]EG, 60 V, 0.25 wt% NH4 F, 1% H2 O, 1020 min [12]EG, 60 V, 0.3 wt% NH4 F, 2% H2 O, 1020 min [12]EG, 60 V, 0.25 wt% NH4 F, 2% H2 O, 240 min [13]EG, 60 V, 0.5 wt% HF, 10% H2 O, 1440 min [1]EG, 60 V, 5 wt% NH4 F, 60 min (current work)
EG, 60 V, 5 wt% NH4 F, 1.0% H2 O, 60 min (current work)

380
538
1000
101
139
220
22
15
5
18.5

66.0
53.3
76.7
23.3
136.3
215.6
91.7
10.4
83.3
308.3

rate of the oxide. Nanotubes of about 6 μm were achieved
under such conditions. Recently, we have shown that the useof a neutral bath can obtain up to 3 μm nanotubes, provided
that excess fluoride exists in the bath [10]. However, one of
the major drawbacks of using an aqueous-based electrolyte
is the slow growth of nanotubes. This drawback can beovercome with the use of an organic solvent. Paulose et al[11] introduced the use of organic electrolytes, such as ethylene
glycol (EG), formamide (FA), dimethyl sulfoxide (DMSO) anddimethylformamide (DMF), for the formation of nanotubes
ranging from 10 to 1000 μm in length. In their work, thehighest growth rate achieved was 76.7 nm min−1 (table 1).
However, these experiments are difficult to adopt in a robust
industrial production setting because of the required lengthy
process of anodization. Table 1 shows a summary of the studies
conducted by a few authors on the formation of TiO2 nanotubes
in an organic electrolyte. The conditions adopted for this work
are also presented. In this present work, TiO2 nanotube arrays
were formed in glycerol and ethylene glycol containing 5 wt%
NH4 F. The aim was to produce high aspect ratio nanotubes
in the shortest anodization time possible. A growth rate ashigh as 308.3 nm min−1 was achieved using ethylene glycolwith excess fluoride and limited water content. The findingsof the present study are summarized in table 1. Additionally, a
discussion on the fast-rate formation of nanotubes is presented
in subsequent sections.

voltage is maintained around this value. Finally, the anodized
Ti foil was sonicated in distilled water for 30 s.
The morphologies of the anodized Ti were characterized
by a field emission scanning electron microscope (FESEM)SUPRA 35VP Zeiss operating at a working distance of 4 mmwith an accelerating voltage of 5 kV. To obtain the thicknessof the formed anodic layer, a cross-sectional measurement was
performed on mechanically bent samples. The actual lengthof the nanotubes was estimated by dividing the length on
the micrograph by cos 45◦ . Chemical stoichiometry of thesample was investigated using energy dispersive x-ray (EDX)
analysis. The crystal phases of the TiO2 nanotubes werestudied by x-ray diffraction (XRD) using the Philips model PW
1729, which was operated at 45 kV and 40 mV. Fluorescence
spectra were recorded at room temperature using an LS
55 luminescence spectrometer (Jobin-Yvon HR). Chemical
states were characterized by x-ray photoelectron spectroscopy(XPS) (JEOL JPS-1000SX) with an Mg Kα x-ray source.
Photocatalytic degradation studies were performed by dippinga piece of 3 cm2 Ti foil in 100 ml 30 ppm methyl orange(MO) in a customized photoreactor made of quartz glass.
The sample was left in the reactor for 30 min in a dark
environment to achieve adsorption/desorption equilibrium. It
was photoirradiated at room temperature using TUV 18 W UVC germicidal light, and then 5 ml solution was withdrawn every
1 h from the quartz tube to monitor the degradation of methyl
orange after irradiation. The concentration of the degradedmethyl orange was determined using a UV spectrometer.

2. Experimental procedure

3. Results and discussion

Titanium foils (0.1277 mm thickness, 99.6% purity) werepurchased from STREM Chemicals.
Anodization was
performed in a standard two-electrode bath with Ti foil as
the working electrode and platinum as the counter electrode.
Prior to anodization, the Ti foil was sonicated in ethanol for
15 min, rinsed with deionized water and then air-dried. Afterdrying, the foil was exposed to an electrolyte consisting ofeither glycerol or ethylene glycol. The solutions were usedas purchased without any purification. Next, 5 wt% NH4 F
was added. The purity of the glycerol is 85% (with 15%
water) while that of ethylene glycol is 100%. Anodization
voltage was 20 V for the sample prepared in glycerol while 30
and 60 V were used for samples prepared in ethylene glycol.
The conditions are chosen based on our preliminary studies
which indicated that TiO2 nanotubes can be produced when the

3.1. Anodization of titanium in glycerol
Electrochemical anodization was performed in 100 ml glycerolwith the addition of 5 wt% NH4 F at 20 V for 60 min.
The condition was chosen based on our preliminary studies,
which favored the formation of well-aligned TiO2 nanotube
arrays [14]. As shown in figures 1(a) and (b), the anodized
titanium consisted of a nanotubular oxide with a regular andclear pore structure. The pore diameters averaged 90 nm
and the length of the nanotubes was about 0.8 μm. The
inset in figure 1(b) shows the bottom part of the nanotubes.
The so-called barrier layer has a dome-like shape. The
pores are, however, not exactly circular and the ridges on thecircumference of the nanotubes are obvious. This finding is
2

Nanotechnology 21 (2010) 365603

S Sreekantan et al

(a)

(b)

100 nm

100 nm

100 nmFigure 1. FESEM images of TiO2 nanotubes formed in glycerol at 20 V for 60 min.

(a)

(b)

1 μm

200 nm1 μm

200 nm

(c)

2 μm

2 μm

Figure 2. FESEM images of TiO2 nanotubes formed at different anodization times in ethylene glycol electrolyte with 5 wt% NH4 F at 30 V:(a) 60 min, (b) 180 min and (c) 360 min.

use of ethylene glycol allows for the formation of nanotube
arrays. The diameters of the nanotubes are more uniform
and the nanotube walls are smoother. The length of thenanotubes after 60 min was 1.7 μm with a formation rate of
28.3 nm min−1 . Prolonging the anodization time to 180 min
and 360 min resulted in longer nanotubes with lengths of 5 μm
and 13.7 μm, respectively. Nanotubes produced at long times
seemed to accumulate and form a cone-like structure until theoxide layer at their top sections collapsed. Nanotube formation
in a fluoride-containing organic electrolyte has been elaborated
by Prakasam [17]. From here, it is clear that the slow rate of
nanotube formation can be attributed to the insufficient anodic
current flow, which is required to enhance electrochemicaldissolution of Ti metal ions in ethylene glycol, as well as toinduce field-assisted oxidation of Ti metal to form TiO2 .
One way to improve field-assisted dissolution is toincrease the anodization voltage. In this set of experiments,
anodization voltage was increased to 60 V while time wasdifferent compared to the reported work by Schmuki, whoclaimed the formation of nanotubes with a smooth surface inglycerol [15]. The difference might be due to the existing water(15 wt%) in the glycerol, which caused a pH burst, and thus
made the surface of the nanotubes serrated.
3.2. Anodization of titanium in ethylene glycol
The next set of experiments was carried out using ethylene
glycol with 5 wt% fluoride. The anodization voltage was
kept constant at 30 V and three anodization times (60, 180
and 360 min) were investigated. The growth rate of thenanotubes is expected to be high in ethylene glycol because
it is less viscous (η = 16 cP at 25 ◦ C) compared toglycerol (η = 945 cP at 25 ◦ C). This translates to longer
nanotubes in ethylene glycol [16]. Figures 2(a)–(c) show
the FESEM images of anodized titanium foils at 60, 180
and 360 min, respectively. As shown in figure 2(a), the3

Nanotechnology 21 (2010) 365603

S Sreekantan et al

(a)

(b)

200 nm

1 μm

2 μm

(c)

2 μm

Figure 3. FESEM images of TiO2 nanotubes formed in ethylene glycol electrolyte at 60 V with different amount of water added: (a) 0 wt%,(b) 1 wt% and (c) 2 wt%.

kept constant at 60 min. As shown in the FESEM image infigure 3(a), TiO2 nanotubes with an average length of 5 μm
were produced after 60 min of anodization, indicating theenhancement of electrochemical dissolution and the oxidationof Ti at 60 V. The rate of formation is approximatedat 83.3 nm min−1 . It is anticipated that the addition ofwater in ethylene glycol will increase the rate of nanotube
formation [13]. Therefore, another set of experiments was
carried out to investigate the effect of water. Longer nanotubes
with a length of 18.5 μm (figure 3(b)) were produced when
1 wt% water was added to ethylene glycol. The calculated rateof formation was approximately 308 nm min−1 , much higher
compared to values reported so far in the literature (table 1).
The fast rate of formation in this work is mainly attributed
to the degree of dissolution of the anodic oxide, which wascontrolled by two factors: pH and fluoride content. These
factors will be discussed in detail in succeeding sections. Ifwater content is increased further to 2 wt%, the nanotubes
become short (13 μm), as shown in figure 3(c).
During anodization, the dissolution of TiO2 occurs at boththe top (oxide electrolyte interface) and the bottom (inside thechannel) of the nanotubes. To increase the rate of formation,
reducing the dissolution at the top of the nanotubes while
ensuring continual dissolution inside the nanotubes at thebarrier layer is necessary. By maintaining the electrolyte atneutral condition, the dissolution at the top can be suppressed.
To ensure continual dissolution at the bottom of the nanotubes,
two processes can be controlled: the rate of anodic oxidationand the rate of chemical dissolution (figure 4). Inside thenanotubes, chemical dissolution may occur due to H+ and F−
ions. As seen in equation (1), whenever oxidation occurs,
H+ is released; this is followed by an increase in local
acidification at the bottom of the nanotubes. Local acidification

"

(a)

(b)

Slow
etching
H+

TiF6-

O2-

Fast
etching

L0
F

-

H+
O2-

TiF6F-

Ti
Ti

TiO2

+

(1)

[TiF6]2- + 2H2O + 6NH4+(2)

TiO2 + 4H

TiO2 + 6NH4F + 4H+←←

Ti + 2H2O

L1>L0

H+

Figure 4. Schematic diagram illustrating the thinning procedure of
the oxides at the bottom of the nanotubes: (a) less fluoride results in a
thick oxide layer which suppresses the transport of titanium, oxygenand fluorine ions, and (b) excess fluoride results in a thin oxide layer
which enhances the transport of titanium, oxygen and fluorine ions,thus inducing inward growth faster ( L 1 and L o represent the lengthsof the tubes in each aforementioned condition).

in the nanotubes accelerates the chemical dissolution process
in accordance with equation (2). As seen in the equation, theformation of the nanotubes is related to the diffusion of F− ionsthrough the oxide layer and effusion of [TiF6 ]2− . Therefore,
the high rate of formation of TiO2 nanotubes in this work can
be explained by the presence of excess amounts of F− content,
the value of which is needed to accelerate equation (2). The
accelerated rate of etching thinned the barrier layer, which
induced a faster inward growth (figure 4(b)). The higher
the content of F− , the faster the rate of etching of the oxide
4Nanotechnology 21 (2010) 365603

S Sreekantan et al

Figure 6. Photoluminescence spectra of TiO2 nanotubes anodized in(a) ethylene glycol and annealed at 400 ◦ C in argon and (b) glycerol
and annealed at 400 ◦ C in argon.

Figure 5. XRD patterns of titanium: (a) anodized in glycerol and notannealed, (b) anodized in ethylene glycol and not annealed,(c) anodized in glycerol and annealed at 400 ◦ C in argon and(d) anodized in ethylene glycol and annealed at 400 ◦ C in argon.
(: Ti, : anatase).

(figure 6). The emission for the sample made in ethylene glycol
was positioned at 350 (3.66 eV), 400 (3.0 eV) and 527 nm(2.3 eV), while the sample made in glycerol was located at
350 (3.66 eV), 400 (3.0 eV) and 504 nm (2.5 eV). A similarvalue of UV emission at 3.66 and 3.0 eV for both samples
indicates the probability of band-to-band recombination (i.e.
intrinsic states rather than surface states). Similar findings werereported by Daude et al [18], whereby the PL bands at 3.66 and3.0 eV can be assigned to the highest energy direct transitionand lowest energy indirect transition, respectively. On the other
hand, the visible light emission at 2.3 and 2.5 eV were very
close to the shallow trap level, which may be attributable tothe oxygen vacancies giving rise to donor states located below
the conduction band. The values reported in this work areconsistent with those reported by Liqiang et al [19], Withanaand Micha [20] and Shi et al [21].

layer. When the oxide layer is thin enough, the transport oftitanium and oxygen ions occurs easily and assists further in theanodic oxidation reaction. Nevertheless, in ethylene glycol, thedonation of oxygen is more difficult compared to an acidified
solution, thus reducing the tendency to form a thick oxide at thebottom of the nanotubes. In the present study, the amount ofadded water is also very important. As mentioned, in ethylene
glycol, the availability of oxygen ions needed for oxidation isless. When water is used, the water molecules could ensure theoxidation process proceeds easily.
3.3. Effects of heat treatment
It has been well documented that as-made TiO2 nanotubes
are amorphous [14]. To investigate the effects of heat
treatment on the crystal structure of the sample prepared inglycerol, ethylene glycol heat treatment was employed at
400 ◦ C. The crystalline structure was examined using XRD.
Heat treating the nanotubes at 400 ◦ C is expected to result inan anatase phase, hence the nanotubes would present excellent
photocatalytic activity [14]. Figure 5 shows the XRD patternsof the as-anodized sample and those after heat treatment.
Evidently, the as-anodized sample was amorphous; only Ti
peaks were seen. Samples annealed at 400 ◦ C were crystalline
with anatase TiO2 . Although the dominant structure of thesample annealed at 400 ◦ C is anatase, the peak intensity of the(101) plane of the sample made in ethylene glycol is stronger
than that of the sample made in glycerol. This implies thatsamples made in ethylene glycol have better crystallization
than samples made in glycerol.

3.5. Chemical composition
Figures 7(a)–(d) show the high-resolution C 1s, Ti 2p, O 1s andF 1s XPS spectra of the sample anodized in ethylene glycol andglycerol. The Ti 2p spectra of the TiO2 nanotubes are shownin figure 7(a). In addition to the two characteristic peaks of Ti
2p1/2 at 465 eV and Ti 2p3/2 at 459.2 eV, another additional
peak was present at 450.8 eV. According to the literature [13],the peak at 450.8 eV can be assigned to Ti, which may haveoriginated from the substrate because of fractures in the oxide
layer. The C 1s spectra for TiO2 made in ethylene glycol
and glycerol were similar and revealed two peaks at 284.9
and 289.0 eV as well as an additional shoulder at 286.5 eV(figure 7(b)). The strong peak positioned at 284.9 eV is usuallyassigned to adventitious elemental carbon, which cannot be
eliminated. This peak also existed in the case of the pureTiO2 sample [22] and it was close to the position of graphitic
sp2 -hybridized carbon (284.9 eV) [23]. The peak at 286.5 eV
could be ascribed to Ti–C–O bonds that comprise the carbonate
species. This happens when C atoms are incorporated into
the interstitial positions of the TiO2 lattice [24, 25] during
pyrogenation of the organic compound [22]. Meanwhile,
the weak peak at 289.0 eV indicated that a small amountof carbonate species was present on the surface [26, 5, 27].

3.4. Photoluminance
The photoluminance (PL) technique has been widely used
to investigate the energy levels of materials. The room
temperature PL spectra of TiO2 nanotubes annealed in argon
were obtained using an excitation wavelength of 325 nm in therange of 350–600 nm. Typically, the TiO2 nanotube PL spectra
are composed of UV emission and a visible emission band
5

Nanotechnology 21 (2010) 365603

S Sreekantan et al(a)Intensity / a.u

459.2

C1s

465.1

(b)

284.9

Intensity / a.u

Ti2p

286.5

450.8
289.0

450

455 460 465 470
Binding Energy (eV)530.5

Intensity / a.u

O1s

524

475

279

F1s

(c)

531.7

528

532
536
540
Binding Energy (eV)

282

285
288
291
Binding Energy (eV)

294

(d)

684.9

Intensity / a.u

445

544

682

684
686
688
690
Binding Energy (eV)692

Figure 7. XPS high-resolution spectrum of the surfaces: (a) Ti 2p, (b) C 1s, (c) O 1s and (d) F 1s. (Solid line: TiO2 nanotubes formed in
ethylene glycol and annealed at 400 ◦ C in argon; dotted line: TiO2 nanotubes formed in glycerol and annealed at 400 ◦ C in argon.)

Table 2. Experimental condition to obtain tubes 140 nm in diameter with different lengths.
Nanotube length (μm)
2.6
5
18.5

Electrolyte

Anodization condition−

Ethylene glycol + 5 wt% F
Ethylene glycol + 5 wt% F−
Ethylene glycol + 5 wt% F− + 1 wt% H2 O

pH 7, 60 V, t = 30 min
pH 7, 60 V, t = 60 min
pH 7, 60 V, t = 60 min

photodegradation of MO under UV light irradiation. To
evaluate the effect of nanotube length on its photocatalytic
activity, the degradation of MO was carried out with nanotubes
that had a similar diameter (140 nm) but different lengths(∼2.6, 5 and 18.5 μm). The experimental condition used toachieve such tubes is summarized in table 2.
As shown in figure 8, TiO2 nanotubes with a length of
2.6 μm exhibited a lower degree of MO degradation (70%),
followed by nanotubes with a length of 5 μm (85%) and18.5 μm (90%). The increase in photocatalytic activity for the
longer nanotubes may be attributed to their large surface areas,
which led to better diffusion of reactant inside the nanotubes.
However, beyond 5 μm, there was not much enhancementin photocatalytic activity. This result is in good agreementwith the result reported by Liang et al [32]; in the report,
if the nanotube length is longer than the effective depth oflight penetration, there is difficulty for the lower part to absorb
light. This suggests that the photocatalytic activity cannot be
improved when the nanotube length exceeds a certain range
because of the limited amount of absorbed incident photons.
Figure 9 shows the photocatalytic activity of TiO2
nanotubes prepared in aqueous solution, glycerol and ethylene
glycol. The experimental condition used to achieve suchtubes is summarized in table 3. The TiO2 nanotubes preparedNo peak occurred at around 281 eV, which represents Ti–
C bonds, for the two samples (made in ethylene glycol andglycerol). This suggests that carbon did not substitute foroxygen atoms in the lattice of TiO2 , which usually results inthe formation of O–Ti–C [28, 29]. Because the XRD patternalso did not indicate the formation of Ti–C bonds, Ti–C–O
bonds most likely formed. The O 1s spectra (figure 7(c))
showed a sharp peak at 530.0 eV, which is associated with
TiO2 , and a shoulder at 531.7 eV. The additional shoulder could
be related to the presence of organic compounds with C–O or
C=O bonds [13]. Figure 7(d) shows the F 1s spectra. One
peak existed at 684.9 eV for TiO2 made in glycerol and none
for the sample made in ethylene glycol. The peak positioned at
684.9 eV could be related to Ti–F-type compounds, possibly(NH4 )2 TiF6 [30, 31]. Overall, the XPS results, especially
the C 1s XPS spectra, strongly suggested that the anodizationof Ti in ethylene glycol and glycerol likely resulted in the
introduction of carbon to the crystal structure of TiO2 . It isalso worth noting that the sample made in ethylene glycol was
free from fluoride content.
3.6. Photocatalytic activity
The photocatalytic activity of anodized TiO2 nanotubes
with a different surface structure was evaluated in the6Nanotechnology 21 (2010) 365603

S Sreekantan et al

Table 3. Experimental condition and the corresponding EDX elemental analysis of TiO2 nanotubes with 90 nm diameter and 0.8 μm length
produced in aqueous solution, ethylene glycol and glycerol.
EDX analysis (at.%)
Electrolyte

Anodization condition

Ti

O

C

F

Aqueous solution (1 M Na2 SO4 ) + 3 wt% F
Glycerol + 5 wt% F
Ethylene glycol + 5 wt% F

pH 4, 20 V, t = 60 min
pH 7, 20 V, t = 60 min
pH 7, 30 V, t = 30 min

47.37
64.17
60.31

52.63
31.13
35.11

—
3.84
5.58

—
1.86
—

Figure 8. The photodegradation of MO using TiO2 nanotubes withdifferent lengths: (a) 2.6 μm, (b) 5 μm and (c) 18.5 μm.

Figure 9. Photodegradation of MO using TiO2 nanotubes producedin three different electrolytes: (a) aqueous solution (Na2 SO4 ),(b) glycerol and (c) ethylene glycol.

in ethylene glycol produced the greatest degradation, which
was 40% faster than that of TiO2 nanotubes prepared in an
aqueous solution. The TiO2 nanotubes made in glycerol were
17% better than those made in aqueous solution. Because
the wall thickness, diameter, length and crystal structure of
all three samples were almost similar (thickness: 20 nm;
diameter: 90 nm; length: 800 nm; crystal structure: anatase)
it is believed that the greater degradation by TiO2 nanotubes
made in glycerol and ethylene glycol is due to carbon doping.
Based on the EDX elemental analysis (not shown), the carbon
content of the sample anodized in aqueous solution was 0 at.%,whereas the sample prepared in glycerol and ethylene glycol
had a carbon content of 3.84 and 5.58 at.%, respectively.
XPS results further supported the presence of carbon in TiO2
crystal structures prepared in glycerol and ethylene glycol.
Enhancement of MO degradation with carbon doping may
be further explained on the basis of the bandgap energy ofTiO2 . Because the bandgap is reduced by oxygen vacancies,
photogenerated electrons and holes gain extra energy to reachthe conduction and valence band, respectively, and carbon
acts as an inhibitor for the electron–hole recombination. This
leads to the formation of more oxidation radicals, resulting
in better degradation of the reactants. Comparing TiO2
prepared in glycerol and ethylene glycol, nanotubes preparedin ethylene glycol exhibited a higher degradation. The
high degree of crystallinity, smooth wall and the absenceof fluoride species are believed to be responsible for theenhancement of photoactivity of the TiO2 nanotubes preparedin ethylene glycol. With regard to the smoothness of thewall, ridges on the circumference of the nanotubes produced
in glycerol might have resulted in a light scattering point. This
limited the amount of light penetrating to the bottom of thenanotubes, leading to poor degradation. In contrast, the smoothwall of TiO2 nanotubes made in ethylene glycol enhanced

light penetration, resulting in higher photodegradation. The
XRD analysis showed that samples made in ethylene glycol
had better crystallization of the anatase phase than samples
made in glycerol, resulting in enhanced photodegradation.
Furthermore, XPS results showed that TiO2 prepared in
ethylene glycol was free from fluoride content, but this was
not the case for glycerol. Fluoride was detrimental to MO
photodegradation because it exchanged with the hydroxyl
groups at the TiO2 surface. The hydroxyl group was essential
to increase the overall efficiency of the MO degradation
reaction [33].

4. Conclusions
We have investigated the formation of well-aligned nanotubes
in fluorinated glycerol and ethylene glycol electrolytes. Long
tubes with a smooth surface and a regular structure wereobtained in ethylene glycol. In contrast, short tubes with avery rough and irregular structure were obtained in glycerol.
In ethylene glycol, nanotubes can be grown up to 18.5 μm
under optimized conditions of a voltage of 60 V, fluoride
content of 5 wt%, water content of 1 wt% and anodization
time of 60 min. The well-aligned layers are amorphous innature, which can be transformed into anatase upon annealing
at 400 ◦ C. Based on the photocatalytic results, anodic TiO2
with a length of 18.5 μm was deemed efficient for MO
degradation. At the same time, photocatalytic activity was not
significantly enhanced when the tube length exceeded 5 μm.
Based on this study, we inferred that, other than the TiO2
nanotube morphology and structure, other factors (e.g. oxygen
vacancies, degree of crystallinity, wall texture, and presence ofcarbon and fluoride) also play important roles in determining
the photocatalytic activity of TiO2 nanotubes.
7

Nanotechnology 21 (2010) 365603

S Sreekantan et al

Acknowledgments[14] Sreekantan S, Hazan R and Lockman Z 2009 Thin Solid Films
518 16–21[15] Macak J M and Schmuki P 2006 Electrochim. Acta 52 1258–64[16] Yin H, Liu H and Shen W Z 2010 Nanotechnology 21 035601[17] Prakasam H E, Shankar K, Paulose M, Varghese O K and
Grimes C A 2007 J. Phys. Chem. C 111 7235–41[18] Daude N, Gout C and Jounin C 1977 Phys. Rev. B 15 3229–35[19] Liqiang J, Xiaojun S, Weimin C, Zili X, Yauguo D and
Honggang F 2003 J. Phys. Chem. Solids 64 615–23[20] Withana S and Micha T 1982 J. Electrochem. Soc.
129 1240–45[21] Shi J, Chen J, Feng Z, Chen T, Lian Y, Wang X and Li C 2007
J. Phys. Chem. C 111 693–9[22] Hu X, Zhang T, Jin Z, Zhang J, Xu W, Yan J, Zhang J,
Zhang L and Wu Y 2008 Mater. Lett. 62 4579–81[23] Zhang Y, Xiao P, Zhou X, Liu D, Garcia B B and Cao G 2009
J. Mater. Chem. 19 948–53[24] Ren W J, Ai Z H, Jia F L, Zhang L Z, Fan X X and Zou Z G
2007 Appl. Catal. B 69 138–44[25] Sakthivel S and Kisch H 2003 Angew. Chem. Int. Edn
42 4908–11[26] Yang X X, Cao C D, Ericksona L, Hohna K, Maghirang R and
Klabunde K 2008 J. Catal. 260 126–33[27] Li Y, Hwang D S, Lee N H and Kim S J 2005 Chem. Phys.
Lett. 404 25–9
[28] Gu D E, Lu Y, Yang B C and Hu Y D 2008 Chem. Commun.
21 2453[29] Wu Z, Dong F, Zhao W, Wang H, Liu Y and Guan B 2009
Nanotechnology 20 235701[30] Regonini D, Jaroenworaluck A, Stevensa R and Bowena C R
2010 Surf. Interface Anal. 42 139–44[31] Macak J M, Aldabergerova S, Ghicov A and Schmuki P 2006
Phys. Status Solidi a 203 67–9[32] Liang H and Li X 2008 J. Hazard. Mater. 162 1415–22[33] Han X, Kuang Q, Jin M, Xie Z and Zheng L 2009 J. Am. Chem.
Soc. 131 3152–3

The authors would like to thank University Sains Malaysia
Research University grant no. 811073, USM-RU-PGRS grantno. 8043045 and USM Fellowship for sponsoring this work.

References[1] Mohapatra S K, Misra M, Mahajan V K and Raja K S 2007
J. Phys. Chem. C 111 8677–85[2] Chu S Z, Inoue S, Wada K, Hishita S and Kumshima K 2005
Adv. Funct. Mater. 15 1343–9[3] Masuda H, Kanezawa K, Nakao M, Yokoo A, Tamamura T,
Sugiura T, Minoura H and Nishio K 2003 Adv. Mater.
15 159–61[4] Wijnhoven J E G J and Vos W L 1998 Science 281 802–4[5] Wang X, Meng S, Zhang X, Wang H, Zhong W and Du Q 2007
Chem. Phys. Lett. 444 292–6[6] Hoyer P 1996 Langmuir 12 1411–3[7] Cheng L, Zhang X, Liu B, Wang H, Li Y, Huang Y andDu Z 2005 Nanotechnology 16 1341–5[8] Gong D, Grimes C A, Varghese O K, Hu W, Singh R S,
Chen Z and Dickey E C 2001 J. Mater. Res. 16 3331–4[9] Cai Q, Yang L and Yu Y 2006 Thin Solid Films 515 1802–6[10] Lockman Z, Ismail S, Sreekantan S, Schmidt-Mende L and
MacManus-Driscoll J L 2010 Nanotechnology 21 055601[11] Paulose M, Shankar K, Yoriya S, Prakasam H E, Varghese O K,
Mor G K, Latempa T A, Fitzgerald A and Grimes C A 2006
J. Phys. Chem. B 110 16179–84[12] Shankar K, Mor G K, Prakasam H E, Yoriya S, Paulose M,
Varghese O K and Grimes C A 2007 Nanotechnology
18 065707[13] Raja K S, Gandhi T and Misra M 2007 Electrochem. Commun.
9 1069–76

8