Anodization of bismuth doped TiO2 nanotubes composite for photocatalytic degradation of phenol in visible light

Article abstract of DOI:10.1016/j.cattod.2016.03.029

Bismuth doped TiO₂ photocatalyst was synthesized in a one-step electrochemical anodization method. Bismuth nitrate Bi(NO₃)₃ was used as a bismuth source. The obtained samples were characterized by FE-SEM, XRD, EDX and XPS. The optimum synthesis conditions for bismuth doping were 1.0 M bismuth nitrate in an ethylene glycol electrolyte with anodization at 40 V for 2 h. Compared with undoped TiO₂ nanotubes, bismuth doped TiO₂ photocatalyst showed a higher photocatalytic activity by a factor of 4.0 for phenol degradation under visible light irradiation. The optimum phenol degradation using a photoelectrocatalytic method was observed at a 0.5 V external bias, and this degradation rate was 5.2 times faster than that observed for undoped TiO₂ nanotubes. The doped bismuth TiO₂ nanotubes are favorable for the separation of photo-induced electrons and holes, reducing the recombination of charges, and promoting the formation of hydroxyl radicals and superoxides that degrade phenol.

Full text of DOI:10.1016/j.cattod.2016.03.029

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Contents lists available at ScienceDirect
Catalysis Today
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Anodization of bismuth doped TiO2 nanotubes composite for
photocatalytic degradation of phenol in visible light
Imran Ali, Seu-Run Kim, Sung-Pil Kim, Jong-Oh Kim∗
Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Bismuth doped TiO2 photocatalyst was synthesized in a one-step electrochemical anodization method.
Bismuth nitrate Bi(NO3)3 was used as a bismuth source. The obtained samples were characterized by
FE-SEM, XRD, EDX and XPS. The optimum synthesis conditions for bismuth doping were 1.0 M bismuth
nitrate in an ethylene glycol electrolyte with anodization at 40 V for 2 h. Compared with undoped TiO2
nanotubes, bismuth doped TiO2 photocatalyst showed a higher photocatalytic activity by a factor of 4.0
for phenol degradation under visible light irradiation. The optimum phenol degradation using a photo-
electrocatalytic method was observed at a 0.5 V external bias, and this degradation rate was 5.2 times
faster than that observed for undoped TiO2 nanotubes. The doped bismuth TiO2 nanotubes are favorable
for the separation of photo-induced electrons and holes, reducing the recombination of charges, and
promoting the formation of hydroxyl radicals and superoxides that degrade phenol.
Received 27 November 2015
Received in revised form 7 March 2016
Accepted 22 March 2016
Available online xxx
Keywords:
Anodization
Bismuth doping
TiO2 nanotubes
Organic pollutant
Phenol
© 2016 Elsevier B.V. All rights reserved.
Photocatalytic degradation
1. Introduction
strategies have been used to modify the TiO2 surface including
metal deposition (e.g., Pt and Fe [11–13]) non-metal doping with N
Phenol is commonly used in many industries as a precursor,
and it is also a representative contaminant in industrial wastew-
ater effluents [1]. Phenol has negative effects on marine life and
aquatic environments when it is discharged in-to streams [2]. Phe-
nol from wastewater can be removed by conventional technologies
like adsorption, coagulation, and enzyme oxidation [3–5]. How-
ever, these conventional technologies do not completely degrade
the organic pollutants and can produce secondary pollutants [6].
In recent years, great progress has been made in the photo-
catalytic degradation of organic pollutants using semiconductor-
based nanomaterials [7]. TiO2-based nanomaterials have been
intensively studied and widely used [8] because of their high pho-
tocatalytic activity, low cost, recyclability and ability to oxidize and
reduce most organic pollutants [9]. TiO2 generates hydroxyl (OH)
radicals, which are more powerful than ozone and other advanced
oxidation processes (AOPs) used to degrade organic pollutants.
However, TiO2 has low photocatalytic efficiency in visible light
because its band gap energy (3.0–3.2 eV) must be activated by ultra-
violet (UV) light, and solar light contains only 5% UV light [10].
Furthermore, TiO2 shows high recombination rate between elec-
tron (e−) and hole (h+) pairs. To overcome these problems, different
and S [14,15], and coupling with other semiconductors CdS, WO3
and MnO2 [16,17].
Research in visible light response photocatalysts has recently
become popular and has focused on materials like BiVO4, BiOI,
and BaIn2O4 [2,18,19]. Bismuth nanomaterials are used as a photo-
catalyst due to their unique layered and suitable band structure,
non-toxic nature, high chemical stability [2,20] and band gap
energy of 1.8–2.4 eV, which is in the visible light region [21].
Bismuth oxide (Bi2O3) is a new type of visible light responsive
photocatalyst that has a wide range of applications. Composites of
bismuth and TiO2 nanomaterials show good photocatalytic results
as compared to using each alone [22,23]. Bismuth oxide is a p-
type semiconductor having conduction and valance band edges
of +0.336 and +0.130 (vs. NHE), respectively [24]. Bismuth on the
surface of TiO2 can trap photo-generated e−, thus reducing recom-
bination of e- and h+ pairs, and enhance the photocatalytic activity
of TiO2 under visible light irradiation [25].
Anodization is a useful technique for modifying the surface
of a Ti substrate to obtain a nanotubular structure [26]. Many
researchers have used Bi2O3 as a doping material to modify the
surface of anodized TiO2 nanotubes (TNT) with different bismuth
doping strategies like hydrothermal deposition [27], dip-coating
methods [28], and electrochemical deposition [29]. These TNT mod-
ifications were performed in a two-step process beginning with
anodization of TNT followed by bismuth doping. To solve this
∗
Corresponding author.
E-mail addresses: jk120@hanyang.ac.kr, jongohkim1221@gmail.com (J.-O. Kim).
http://dx.doi.org/10.1016/j.cattod.2016.03.029
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: I. Ali, et al., Anodization of bismuth doped TiO2 nanotubes composite for photocatalytic degradation
of phenol in visible light, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.029

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Fig. 1. Anodization equipment setup with power supply and operational parameter control.
Table 1
two-step experimental process, in our study we proposed one-step
synthesis method. This reported synthesis method has high tech-
nical accessibility, low energy consumption, and less time required
for synthesis.
In this study, we fabricated bismuth doped TiO2 nanotube (Bi-
TNT) composites with a new one-step electrochemical anodization
method. To our knowledge, no one has worked on this type of
bismuth modification before. Bismuth was doped on TNT using
different bismuth nitrate concentrations, anodization voltages and
times. Then, Bi-TNT catalyst was used for photocatalytic (PC) and
photoelectrocatalytic (PEC) degradation of phenol under visible
light irradiation. Also, PEC degradation of phenol was monitored
at different external biases (0.1–1.0 V).
Bismuth doping on TNT with different electrolyte concentrations and anodization
parameters.
Order
Electrolyte (M)
Bismuth nitrate
Anodization
Time (h)
Voltage (V)
1
2
3
4
5
6
7
8
9
0.5
0.5
0.5
0.5
0.5
1.0
1.0
1.0
1.5
1
2
3
2
2
2
2
2
2
30
30
30
40
50
30
40
50
30
M = mole.
2. Experimental
2.1. Materials and chemicals
Ti foil (purity of 99.5%, 2 × 5 cm2 in size with a thickness of
0.25 mm) was purchased from Alfa Aesar, Korea. Ethylene glycol
(94.5%) and bismuth nitrate (98.0%) were purchased from Daejung
Chemicals & Metal Co. Ltd., Korea. NH4F (97.0%) was purchased
from Junsei Chemical Co. Ltd., Japan. All chemicals were of ana-
lytical grade and were used as received.
2.2. One-step synthesis of Bi-TNT
Ti foil was sonicated in acetone, ethanol and deionized (DI)
water for 10 min each to remove impurities. The foils were then
dried in N2 gas. We used an electrolyte solution of ethylene gly-
col with 3 wt% NH4F, 5 wt% DI water and different bismuth nitrate
concentrations (0.5–1.5 M) as shown in Table 1. Ti foil was used
as an anode and platinum (Pt) as a cathode. The distance between
two electrodes was kept at 2 cm. Anodization was performed at
different voltages (30–50 V) and times (1–3 h) using a power sup-
ply (N6702A Agilent Technologies, Spain) as shown in Fig. 1. After
anodization, the Ti foil was rinsed in DI water and dried in N2 gas.
Annealing was performed at 550 ◦C for 1.5 h in a furnace to improve
the anatase crystalline structure of the TiO2.
Fig. 2. Schematic diagram of photocatalytic reactor prepared in the lab.
analyzed using X-ray diffraction (XRD, D8-Advance, Bruker-AXS
Co., Germany). Elemental analysis and the chemical composition
of the catalyst was examined using energy dispersive X-ray spec-
troscopy (EDX, Noran System 7, Thermo Scientific, USA) and X-ray
photoelectron spectroscopy (XPS, Thermo Fisher Scientific Co.,
USA).
2.4. Photocatalytic test
2.3. Characterization
The photocatalytic performance of the synthesized sample was
investigated towards photodegradation of phenol. The initial phe-
nol concentration was 50 mg L−1 and a 40 mL solution was used
in a reactor (prepared at lab scale) as shown in Fig. 2. The total
catalyst (Ti foil) area used for the photocatalytic test was 32 cm2.
The surface morphology of the synthesized catalyst was
examined using field emission scanning electron microscopy (FE-
SEM, Sigma, Carl Zeiss Co., Germany). The crystal structure was
Please cite this article in press as: I. Ali, et al., Anodization of bismuth doped TiO2 nanotubes composite for photocatalytic degradation
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Fig. 3. FE-SEM images of (a) TNT, (b) side view of TNT, and (c) Bi-TNT.
Table 2
The PC activity test was performed under a visible light source
(Fluorescent lamp 32 W, ␭ = 400–700 nm, Philips Co., Netherland),
and the UV cut-off filter ␭ < 420 nm at room temperature. PEC
activity was performed at different voltages (0.1–1.0 V). Initially,
the samples were kept in the dark for 1 h to achieve adsorp-
tion equilibrium. Later, visible light irradiation was supplied to
perform PC and PEC experiments. The amount of phenol degrada-
tion was determined using a UV–vis spectrophotometer (UV mini
1240, Shimadzu, USA) using the peak at a wavelength of 270 nm
[15,30]. The intermediate products of phenol were analysed using
gas chromatography (GC, Shimadzu, GC-2010, Japan) and mass
spectrometry (MS, Shimadzu GCMS-QP2010, Japan) with a ther-
mal desorber (TD, UNITY II, Markes International, Ltd., UK) system.
The details of the TD-GC-MS method is given in Supplementary
materials and Table 1S.
The element analysis of EDX spectrum of Bi-TNT composite.
Element
Ti
O
C
Bi
At%
Wt%
20.3
41.3
71.6
48.8
6.7
3.4
1.4
6.5
are shown in Supplementary materials Fig. 1S. The FE-SEM clearly
shows that a thin layer of bismuth oxide formed on the TNT surface,
and tube-like structures were still present after bismuth doping.
3.1.2. XRD
The XRD patterns showed that the undoped TNT and Bi-TNT
samples exhibited a crystalline anatase phase after being annealed
at 550 ◦C as shown in Fig. 4(a) and (b), respectively. Bismuth peaks
were observed with Ti peaks, and Ti was converted to an anatase
crystalline phase while Bi was converted to bismuth oxide. The Bi
species were present in a separate phase of bismuth oxide. The
peaks at 27.4, 47.1 and 56.0 two theta were identified as the main
peaks of bismuth oxide [23,25,29].
3. Results and discussion
3.1. Characterization
3.1.1. FE-SEM
Typical FE-SEM images of TNT and Bi-TNT samples are shown
in Fig. 3. It is clear that TiO2 nanotubes were successfully fabri-
cated on Ti foil by anodization, and the average nanotube diameters
were approximately 50 nm, and length was 2000 nm as shown in
Fig. 3(a) and (b), respectively. Most of the nanotubes were uniform
in size and highly oriented. TiO2 nanotubes were well-separated
from each other and tube mouths were open. The optimum syn-
thesis conditions for bismuth doping were found at a bismuth
nitrate concentrations of 1.0 M with anodization at 40 V for 2 h as
shown in Fig. 3(c). The FE-SEM images of other synthesis conditions
3.1.3. EDX and XPS
The quantitative analysis of the Bi-TNT composite was obtained
by EDX spectrum. As shown in Fig. 5(a) the deposit mainly consisted
of Bi [31,32]. The carbon peak observed in the EDX spectrum may
have been due to contamination. The atomic concentrations of Ti, Bi
and O were 20.3, 1.4 and 71.6 at%, respectively as shown in Table 2.
The wide scan XPS spectrum of the Bi-TNT composite is shown in
Fig. 5(b). Ti 2p and O 1s peaks were observed at binding energies of
about 469 and 530 eV, respectively. The Bi 4f peaks were observed
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at 158 and 163 eV [28,29,33]. Based on these results we confirmed
that bismuth was successfully doped onto the TNT surface.
(a)
Ti
Ti - Titanium
A - Anatase
Ti
3.2. Photocatalytic degradation of phenol
Ti
3.2.1. PC and PEC activity
Ti
Photodegradation of phenol was conducted to evaluate the pho-
tocatalytic activity of Bi doped TiO2 catalyst. Fig. 6(a) and (b) shows
the degradation rate of phenol relative to undoped TNT and Bi-TNT
samples under visible light irradiation for 8 h. No phenol degra-
dation was observed when the experiment was performed under
photolysis or catalysis. This control experiment confirmed that TiO2
requires light source for activation to remove organic pollutant [22].
The phenol photocatalytic degradation rate of the Bi-TNT sample
was higher than that of undoped TNT. The phenol degradation was
4.4, 7.7, 17.5, and 40.3% by TNT PC, TNT PEC, Bi-TNT PC, and Bi-TNT
PEC, respectively [34]. The Bi-TNT sample (optimum synthesis con-
dition of bismuth nitrate 1.0 M, anodization at 40 V for 2 h) showed
a higher phenol degradation rate by a factor of 4.0 as compared with
undoped TNT when operating in PC mode. In terms of PEC activity,
phenol degradation was higher than undoped TNT by a factor of 5.2
at a 0.5 V external bias.
To check the effect of external bias on the degradation of phenol,
PEC activity was performed at different external biases (0.1 ∼ 1.0 V)
as shown in Fig. 7(a) and (b). The phenol degradation was 25.3,
34.4, 40.3, 35.7, and 31.2% at an external voltage of 0.1, 0.3, 0.5, 0.7,
and 1.0 v respectively. The optimum phenol degradation was found
at 0.5 V under visible light irradiation. The PEC activity of phenol
degradation increased with increasing applied voltage upto 0.5 V,
but then decrease thereafter. The decrease in phenol degradation
above 0.5 V was because excess voltage caused a recombination
Ti
Ti
A
Ti
A
A
20
30
40
Ti
50
60
70
80
2ϴ (degree)
Ti - Titanium
A - Anatase
Bi - Bismuth
(b)
A
Ti
Ti
Ti
Ti
Bi
Bi
A
Ti
Bi
Bi
20
30
40
50
60
70
80
2ϴ (degree)
Fig. 4. XRD patterns of (a) TNT and (b) Bi-TNT samples.
Fig. 5. Bi-TNT sample data: (a) EDX spectrum and (b) XPS spectrum.
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1.0
0.9
(a)
1.0
(a)
0.9
0.8
0.8
0.1 V
Photolysis
Catalysis
TNT PC
TNT PEC
Bi-TNT PC
Bi-TNT PEC
0.3 V
0.5 V
0.7 V
1.0 V
0.7
0.6
0.5
0.7
0.6
0.5
0
2
4
6
8
0
2
4
6
8
Irradiation time (h)
Irradiation time (h)
(b)
0.0
-0.2
-0.4
-0.6
0.0
(b)
-0.2
-0.4
-0.6
Photolysis
Catalysis
R² = 0.868
R² = 0.1148
0.1 V
0.3 V
0.5 V
0.7 V
1.0 V
R² = 0.9949
R² = 0.9961
R² = 0.9984
R² = 0.9983
R² = 0.9953
TNT PC
R² = 0.935
R² = 0.9619
TNT PEC
Bi-TNT PC
Bi-TNT PEC
R² = 0.9801
R² = 0.9984
0
2
4
6
8
0
2
4
6
8
Irradiation time (h)
Irradiation time (h)
Fig. 7. Effect of external volt on PEC process for phenol degradation under visible
light (a) C/Co and (b) Ln(C/Co).
Fig. 6. Degradation of phenol under visible light: comparison of PC and PEC process
(a) C/Co and (b) Ln(C/Co).
Table 3
0.5 V had a higher reaction rate constant than other samples, and
this sample showed higher phenol degradation activity.
The reaction rate constant k, and the associated R2 values of different phenol degra-
dation processes.
The higher photocatalytic activity of Bi doping observed here
may be due to the following reasons. First, Bi doping can reduce
the band gap energy, which significantly improves the absorption
of TiO2 in visible light [37]. Secondly, Bi doped on the surface of TiO2
can trap photogenerated electrons thereby suppressing the recom-
bination of electrons and holes [38]. This promotes the formation of
OH radicals and superoxides. Meanwhile, the specific surface areas
of composite samples are large [39], which favour the adsorption
of organic compounds and provide more accessible active sites.
Process
V
Rate constant k 10−2 h−1
R2
Photolysis
Catalysis
TNT PC
0.05
0.01
0.56
1.00
2.40
3.65
5.26
6.45
5.52
4.67
0.8680
0.1148
0.9350
0.9619
0.9801
0.9949
0.9961
0.9984
0.9983
0.9953
TNT PEC
0.5
Bi-TNT PC
Bi-TNT PEC
Bi-TNT PEC
Bi-TNT PEC
Bi-TNT PEC
Bi-TNT PEC
0.1
0.3
0.5
0.7
1.0
3.2.2. Phenol degradation mechanism
Here we proposed a possible mechanism for the effects of
bismuth, which is illustrated in Fig. 8. This mechanism includes fol-
lowing steps: (i) visible light irradiation on TiO2 surface enhanced
by the bismuth doping, (ii) the excitation of photoinduced elec-
trons from the valence band (VB) to the conduction band (CB) of
TiO2, resulted the generation of holes in the VB of the bismuth (iii)
the created e− and h+ initially reacts with H2O, and (iv) induced
the degradation of organic pollutant [40]. Bismuth enhanced the
photocatalytic activity of TiO2 under visible light irradiation. Con-
sequently, more photons from visible irradiation are utilized to
generate electrons and holes. Moreover, bismuth in the modified
reaction between electrons from the additional applied voltage and
holes formed on TiO2 [35].
The reaction rate constant was calculated according to Eq. (1)
[36],
Ln(C/Co) = −kt
(1)
here C is the concentration at particular time, Co is the initial con-
centration, k is a first order rate constant (h−1), and t is reaction
time in h. Table 3 shows a summary of the reaction rate constant k
and its associated R2 values. The PEC activity of Bi-TNT sample at
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Fig. 8. Mechanism of bismuth doped TiO2 photocatalysis.
TiO2 systems improve the separation efficiency of photogenerated
electrons and holes and produce more OH radicals and superoxides,
which play an important role in the photocatalytic degradation of
phenol [34].
sample. Photoelectrocatalytic activity was higher by a factor of
5.2 at a 0.5 V external bias. The bismuth doped TiO2 photocatalyst
showed efficient separation of photogenerated electrons and holes.
The phenol removal is due to the formation of OH radicals, holes,
and superoxides. We observed an increased photocatalytic activity
of Bi-TNT in visible light due to the bismuth doping.
Possible reactions related to organic pollutants degradation are
shown in Eqs. (2)–(8). Bismuth doped TiO2 photocatalyst under vis-
ible light irradiation generates e− and h+. The generated h+ on the
Bi surface interacts with H2O or hydroxide ions (OH−) produces OH
radicals, and the interaction of e− with O2 generates superoxides
Acknowledgments
(O2 −). These OH radicals, superoxides and h+ degraded organic
•
This study was supported by a National Research Foundation
of Korea (NRF) grant funded by the Korean Government (NRF-
2016R1A2A1A05005388).
pollutants into oxidized products [41].
Bismuth/TiO2 + hv → TiO2(e−) + Bismuth(h+)
H2O + h+ → OH. + H+
OH− + h+ → OH•
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.03.
029.
e
− + O2 → O2
−
•
Organicpollutant + OH• → degradedproducts
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Organicpollutant + O2 − → degradedproducts
•
Organicpollutant + h+ → degradedproducts
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doped sample was higher by a factor of 4.0 than an undoped TNT
Please cite this article in press as: I. Ali, et al., Anodization of bismuth doped TiO2 nanotubes composite for photocatalytic degradation
of phenol in visible light, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.029

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Please cite this article in press as: I. Ali, et al., Anodization of bismuth doped TiO2 nanotubes composite for photocatalytic degradation
of phenol in visible light, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.029