Fabrication of a self-doped TiO2 nanotube array electrode for electrochemical degradation of methyl orange

Article abstract of DOI:10.1002/jccs.201800456

Self-doped TiO₂ nanotube array (DTNA) electrodes were fabricated through anodic oxidation combined with cathodic reduction. The morphology and structural features of pristine TiO₂ nanotube arrays and DTNA electrodes were studied thro

Full text of DOI:10.1002/jccs.201800456

Received: 27 November 2018
DOI: 10.1002/jccs.201800456
Revised and accepted: 20 January 2019
A R T I C L E
Fabrication of a self-doped TiO2 nanotube array electrode
for electrochemical degradation of methyl orange
Chenghao Mo | Huixian Wei | Tongjun Wang
School of Chemical and Material Engineering, The
Self-doped TiO2 nanotube array (DTNA) electrodes were fabricated through
Key Laboratory of Synthetic and Biotechnological
Colloids, Ministry of Education, Jiangnan
anodic oxidation combined with cathodic reduction. The morphology and struc-
tural features of pristine TiO2 nanotube arrays and DTNA electrodes were studied
through scanning electron microscopy, X-ray diffractometry, and X-ray photoelec-
tron spectroscopy. An accelerated life test was used to test the electrode service
University, Wuxi, Jiangsu, People's Republic of
China
Correspondence
Huixian Wei, School of Chemical and Material
Engineering, The Key Laboratory of Synthetic and
Biotechnologal Colloids, Ministry of Education，
Jiangnan University, 1800 Lihu Road, Wuxi,
Jiangsu 214122, People’s Republic of China.
Email: weihuixian@jiangnan.edu.cn
lifetime and thus the electrode's stability. The service lifetime of the DTNA elec-
trode prepared at constant 40 V for 6 hr was approximately 338.7 hr at constant
1 mA/cm2 in a 1 M NaClO4 solution. Methyl orange (MO) was employed as the
degradation probe for measuring electrochemical oxidation performance. The color
removal rate of 200 mg/L MO of the DTNA electrode (85.2% at 1 mA/cm2) was
greater than that of the Ti/IrO2 electrode (31.1% at 1 mA/cm2). The larger the sur-
face area of the DTNA electrode is, the more conductive the electrode is for the
degradation of organic substances. Organic degradation on the DTNA electrode
occurred primarily through an indirect pathway (producing [ꢀOH]).
Funding information
the Ministry of Housing and Urban-Rural
Development of the People's Republic of China,
Grant/Award Number: 2014ZX07305003
KEYWORDS
accelerated life test, cathodic reduction, self-doped TiO2 nanotube array
To lower the cost of electrode production, this study
investigated the use of Ti metal bases to prepare an active
1
| INTRODUCTION
With the development of modern industry, advanced electro-
chemical oxidation processes have attracted a great deal of
attention for wastewater treatment and have been considered
a promising method for the degradation of organic pollutants
because of their high efficiency, ease of operation, environ-
mental friendliness, and requirement of simple equip-
ment.[1,2] However, the applications of electrochemical
wastewater treatment have been limited by several factors
including foam formation, electrode stability, and cost. The
electrode is the key factor in electrochemical oxidation.
anode for application to wastewater treatment. One-
dimensional titanium dioxide nanotubes have become a pop-
ular research topic in the field of chemistry and materials
due to their large specific surface area, high mechanical
strength, and favorable biocompatibility.[7] Self-doped TiO2
nanotube array (DTNA) electrodes prepared though anodic
oxidation[8–18] combined with cathodic reduction[19–21] have
the advantages of high arrangement regularity and a large
specific surface area. Because DTNAs are prepared directly
on a titanium substrate, the shape and crystal structure of the
nanotubes can be easily controlled, thus allowing for effec-
tive assembly, shortening the process and saving resources.
In recent years, DTNA electrodes have been designed to
exhibit a high electrochemical performance and electrical
conductivity because of the self-doping of Ti3+, and these
electrodes are considered to be promising anode materials.
Semiconductor electrodes, such as those composed of
[3,4]
IrO2
and RuO2,[5] are more popular but are a relatively
high cost because of the use of platinum-group metals as pri-
mary ohmic contact materials for transfer of electrons to the
base metal.[6]
© 2019 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2019;1–8.
http://www.jccs.wiley-vch.de
1

2
MO ET AL.
TABLE 1 Scale of DTNA electrodes prepared at constant 40 V at
different times
Time
(hr)
TID (nm)
BID (nm)
TW (nm)
BW (nm)
Length (μm)
6.62 0.15
8.75 0.15
4
86.8 4.0 39.7 3.0 13.6 1.6 42.5 3.0
91.5 4.2 40.2 2.9 12.9 2.1 44.8 3.3
6
FIGURE 1 SEM images of the DTNA electrode prepared through
electrolysis for 6 hr at constant 40 V, (a) top view, (b) bottom view,
(c) cross-sectional view
10
94.5 4.0 40.7 3.0 13.5 1.9 44.7 3.1 11.28 0.13
BID: bottom inner diameter; BW: bottom wall thickness; TID: top inner diame-
ter; TW: top wall thickness.
Yang et al.[22] used a DTNA electrode as the anode to
degrade domestic wastewater, and the removal rate of chem-
ical oxygen demand (COD) exceeded 90%.
In this study, methyl orange (MO) was chosen as the
degradation probe to measure the performance of electro-
chemical oxidation, and the mechanism of organic pollutant
degradation by using DTNA electrodes was explored fur-
ther. Furthermore, the lifetime of a DTNA electrode was
investigated in detail. Ti/IrO2 electrodes were also used to
degrade MO for comparison.
Electrolytic time is one of the crucial factors affecting
the morphology and size of nanotubes, so we can change
electrolytic time to control the size of nanotubes. Table 1
shows the scale of the DTNA electrode prepared at constant
40 V at different times. It demonstrates that the length of the
DTNA electrode increased significantly with the electrolysis
time, whereas the inner diameter and wall thickness of the
DTNA electrode changed only slightly.
Besides, a highly ordered TiO2 layer is formed in the
electric field environment produced under a certain voltage.
Therefore, the electrolytic voltage is another crucial factor
affecting the morphology and size of nanotubes. As shown
in Table 2, the inner diameter, wall thickness, and length
increased with the electrolytic voltage.
2
| RESULTS AND DISCUSSION
2.1 | Structure and morphology of the DTNA electrode
Titanium dioxide is an n-type semiconductor with a for-
bidden bandwidth of approximately 3.2 eV, and the high
density of Ti3+ doping in the reduced state results in a large
decrease in the measured bandgap to 2.4 eV.[23] The TiO2
material can reduce 1% of Ti4+ in the TiO2 layer into Ti3+ in
an aqueous electrolyte. The material formed by this TiO2
doping is almost metallic.[24] Therefore, through cathodic
reduction treatment, Ti4+ in a TNA electrode is reduced to
Figure 1 shows the plane-view and cross-sectional scanning
electron microscopy (SEM) images of the DTNA electrode
prepared at constant 40 V for 6 hr. They indicate that an
anodic titanium oxide layer can grow through self-organized
anodization of titanium under well-controlled conditions.
The crystal structure of the DTNA electrode was anatase
(Figure 3a). EDS analysis was performed to characterize the
elemental composition of the DTNA electrode, and the scan-
ning area is shown in the Figure 2b. The elemental composi-
tion of DTNA electrode was found to be Ti and O
(Figure 2a). The Elemental mappings indicate that Ti and O
elements in the nanotubes were well distributed
(Figure 2c–e).
Ti3+ to increase the conductivity of the electrode[25–28]
:
Ti4 + + e− + H + ! Ti3 + + H +
:
ð1Þ
As illustrated in Figure 3c, the electrochemical imped-
ance of the TNA electrode and DTNA electrode was mea-
sured. The resistance of the TNA electrode was
approximately 2.6 × 105 Ω, whereas that of the DTNA elec-
trode was approximately 60 Ω.
X-ray photoelectron spectroscopy (XPS) was performed
to investigate the change of the surface bonding position of
the DTNA electrode. As shown in Figure 3b, the Ti2p3/2
,
Ti2p1/2, and O1s XPS peaks in the spectrum of the TNA
electrode are centered at binding energies of 458.7, 464.4,
TABLE 2 Scale of DTNA electrodes prepared at different constant
voltages for 6 hr
Voltage
(V)
40
50
60
TID (nm)
BID (nm) TW (nm) BW (nm) Length (μm)
8.75 0.15
91.5 4.2 40.2 2.9 12.9 2.1 44.8 3.3
114.2 5.4 43.5 4.3 16.6 2.3 52.4 4.5 11.71 0.37
133.3 7.2 49.4 5.1 22.5 4.1 58.2 4.5 13.69 0.51
FIGURE 2 EDS analysis of the DTNA electrode, (a) EDS spectrum,
(b) SEM-EDS image, and (c–e) the elemental mappings
BW: bottom wall thickness; BID: bottom inner diameter; TID: top inner diame-
ter; TW: top wall thickness.

MO ET AL.
3
FIGURE 4 (a) Variation in voltage during the life test of the DTNA
electrode; (b) high-resolution XPS spectra of Ti in the TNA and DTNA
electrodes
the DTNA electrode was 4.37%, whereas that in the DTNA
electrode (the electrode after the life test) was 1.69%. The
gradual oxidation of self-doped Ti3+ into Ti4+ in the DTNA
electrode caused an increase in the resistance during the
life test.
The lifetime of the DTNA electrode prepared at constant
40 V for 6 hr was approximately 253 8 min at constant
10 mA/cm2 in a 1 M NaClO4 solution and 70 3 min at
constant 20 mA/cm2. The coefficient n was 1.893 according
to Equation 3, which estimated the lifetime as 338.7 hr at
constant 1 mA/cm2. As shown in Table 4, we discovered
that the lifetime of the DTNA electrode increased with not
only the electrolytic time but also the electrolytic voltage.
This may be related to the surface area of the DTNA elec-
trode. The larger the surface area of the DTNA electrode, the
more self-doped Ti3+ in the nanotube wall after the cathodic
reduction, and thus the longer the lifetime.
⃰
FIGURE 3 (a) XRD patterns of the DTNA electrode, (b) XPS spectra,
(c) EIS spectra, and (d) Mott−Schottky plot of the TNA and DTNA
electrodes
and 529.9 eV, respectively, whereas these respective peaks
for the DTNA electrode are located at 458.6, 464.3, and
529.8 eV. Compared with the spectral peaks of the TNA
electrode, those of the DTNA electrode are slightly nega-
tively shifted, which can be attributed to the increased pres-
ence of Ti3+ and oxygen vacancies.[19,29] Furthermore, as
shown in Figure 13, the color of the DTNA electrode was
darker than that of the TNA electrode; This can be attributed
to the oxygen-vacancy-induced formation of a defect state in
the DTNA electrode bandgap during the cathodic reduction
treatment.[30]
2.3 | Electrochemical degradation of MO
To further characterize the change of the electronic prop-
erties of the TiO2 nanotubes before and after the cathodic
reduction, Mott−Schottky measurements were conducted.
Notably, the donor density of the DTNA electrode was esti-
mated from the Mott−Schottky relationship to be
1.45 × 1020 cm−3, which is more than 113,000 times larger
than that of the TNA electrode (1.28 × 1015 cm−3), indicat-
ing that the cathodic reduction substantially increased the
conductivity of the TiO2 nanotubes.
2.3.1
| Influence of different DTNA electrodes and initial
concentrations of MO
As plotted in Figure 5a, the DTNA electrode was used to
degrade MO at constant 1 mA/cm2. The color removal rate
of different concentrations of MO increased with electrolytic
TABLE 3 Ti3+ and Ti4+contents of different electrodes
Electrode
TNA
Ti3+ (%)
0.50
Ti4+ (%)
99.50
DTNA
4.37
95.63
DTNA
⃰
1.69
98.31
2.2 | The service lifetime of DTNA electrodes
The electrochemical lifetime of the DTNA electrode was
measured using an accelerated life test. When the cell volt-
age reaches 10 V, the DTNA electrode is considered as
deactivated. Figure 4a depicts the variation of the potential
with time for the DTNA electrode; the voltage increased
slowly between 4.3 and 7.7 V and increased rapidly from
7.7 to 10.0 V. As illustrated in Figure 4b and Table 3, little
Ti3+ content (0.5%) was present in the TNA electrode.
Because of cathodic reduction treatment, the Ti3+ content in
TABLE 4 Lifetime of DTNA electrodes prepared under different
conditions
Voltage (V)
Time (hr)
Lifetime (min)
40
40
40
50
60
4
131
253
6
8
6
10
6
421 14
460 15
645 20
6

4
MO ET AL.
FIGURE 5 (a) Color removal rate and (b) energy consumption per
kilogram of the DTNA electrode for different initial concentrations of MO;
DTNA electrodes prepared (c) for different durations and (d) at different
voltages
FIGURE 6 Color removal rate when using different electrodes to degrade
MO for 6 hr
time. The lower the initial concentration of MO, the higher
was the color removal rate, which can be explained by elec-
trochemical degradation of organic matter being a diffusion-
controlled process.[31–34] Figure 5b illustrates the average
energy consumption per kilogram of MO of different initial
concentrations. The higher the initial concentration of MO,
the lower was the energy consumption. Therefore, in terms
of energy, the electrochemical method is appropriate for the
degradation of organic substances at high concentrations.
Figure 5c and d demonstrate that with a longer electrolytic
time and higher electrolytic voltage, the color removal rate
of MO increased because the surface area gradually
increased. The larger the surface area of the DTNA elec-
trode, the more active and thus more conductive was the site
for the degradation of organic substances.
FIGURE 7 Water droplet on the (a) Ti substrate, (b) DTNA electrode, and
(c) Ti/IrO2 electrode
TABLE 5 Contact angle on different electrode surfaces
Electrode
Ti
DTNA
Ti/IrO2
Contact angle/ꢁ
50.6
5.3
74.4
test the electrochemical performance of the electrodes.
Figure 7 and Table 5 indicate that the water contact angle of
the Ti substrate was 50.6ꢁ whereas that of the DTNA elec-
trodes was 5.3ꢁ. However, the contact angle of the Ti/IrO2
electrode was 74.4ꢁ. The results illustrated that the DTNA
electrode's surface wettability was superior to that of the
Ti/IrO2 electrode, meaning that the DTNA electrode may
have greater hydrophilic properties and absorb more MO on
its surface to quickly oxidize MO and its intermediates.
The oxygen evolution potential of electrodes is crucial
for electrochemical wastewater treatment. Electrodes with
greater oxygen evolution potential are more favorable for
degrading organic matter. As plotted in Figure 8, the oxygen
evolution potential of the DTNA electrode was 3.00 V (ver-
sus the saturated calomel electrode [SCE]), which was
higher than that of the Ti/IrO2 electrode (1.52 V versus the
SCE). To some extent, this proves that the DTNA electrode
is superior for the degradation of MO compared with the
Ti/IrO2 electrode.
2.3.2
| Comparison with the Ti/IrO2 electrode
Figure 6 demonstrates that the color removal rate of
200 mg/L MO electrolyzed by Ti/IrO2 increased slowly with
the degradation time. After 6 hr, the color removal rate was
31.1% at 1 mA/cm2 and 45.0% at 5 mA/cm2. By contrast,
the color removal rate of MO electrolyzed by the DTNA
electrode increased rapidly with the electrolytic time; after
6 hr, the MO removal rate was 85.2% at 1 mA/cm2. There-
fore, the degradation of MO was more effective using the
DTNA electrode than the Ti/IrO2 electrode.
Surface wettability[35,36] explains the different MO deg-
radation effects of the DTNA and Ti/IrO2 electrodes. Surface
wettability was evaluated through the water contact angle to

MO ET AL.
5
FIGURE 10 Cyclic voltammogram obtained using the electrolyte of 0.1 M
Na2SO4 for the (a) DTNA and (b) Ti/IrO2 electrode
duration, whereas the fluorescence intensity of the DTNA
electrode increased with the electrolytic duration. To further
determine whether direct electron transfer occurred on the
DTNA electrode, CV was performed using benzoic acid
(BA) as a probe. If direct electron transfer occurs, an increase
in current is observed at the same anode potential after the
addition of BA.[41,42] As shown in Figure 10, an increase of
current was not observed for the DTNA electrode but was
observed for the Ti/IrO2 electrode. Therefore, it was mainly
the indirect pathway that led to organic degradation on the
DTNA electrode, whereas the organic degradation on the
Ti/IrO2 electrode resulted mainly from direct electron transfer.
According to previous research, the mechanism through
which a DTNA electrode degrades pollutants (MO) is as fol-
lows (Figure 11). First, adsorbed [ꢀOH] are generated by the
split of H2O on the surface of the DTNA electrode. Then,
pollutant molecules (MO) can be oxidized into other inter-
mediates by the adsorbed [ꢀOH], mineralized H2O, and
finally CO2.[43,44] O2 evolves during the oxidization of
hydroxyl radicals. However, a DTNA electrode with higher
O2 evolution potential (3.00 V versus SVE) can effectively
reduce the rate of O2 evolution and increase the rate of pol-
lutant oxidation.
FIGURE 8 Cyclic voltammogram of DTNA and Ti/IrO2 electrodes
conducted in 0.1 M Na2SO4 aqueous solution
2.3.3
| Electrochemical degradation mechanism
In general, the electrocatalytic oxidation process comprises
two simultaneous oxidation processes—direct and
indirect.[37–41] Direct oxidation means that direct electron
transfer might contribute to organic degradation on the sur-
face of electrodes. By contrast, indirect oxidation refers to
the generation of highly oxidizing hydroxyl radicals on the
surface of the electrode that further oxidize and degrade
organic matter.
To explore the indirect pathway of degradation on the
DTNA and Ti/IrO2 electrodes, fluorescence detection using
TA as a probe was employed to detect hydroxyl radicals. If
the indirect process occurred, [ꢀOH] should be detected, and
the 2-hydroxyterephthalic acid produced could be used to
measure the number of [ꢀOH] generated through the electrol-
ysis reaction. As shown in Figure 9, the fluorescence inten-
sity in the electrolysis system containing the Ti/IrO2
electrode was relatively unaffected by the electrolytic
As plotted in Figure 12, the removal rates of COD
(67.2% in 6 hr), color (85.1% in 6 hr), and MO (91.2% in
6 hr) were all different for the DTNA electrode. The removal
rates of COD and color were lower than that of MO because
the MO molecules were broken down into other smaller
FIGURE 9 Fluorescence spectra of electrolysis lasting different times for
FIGURE 11 Mechanism of DTNA electrode degradation of MO
the (a) DTNA and (b) Ti/IrO2 electrode

6
MO ET AL.
FIGURE 13 DTNA electrode preparation procedure
3.2 | Material characterization
3.2.1
| Physicochemical characterization
The morphology of the DTNA electrodes was characterized
using field-emission SEM (S-4800, Hitachi, Japan), The ele-
mental composition of the DTNA electrode was characterized
using energy dispersive X-ray spectroscopy (EDS, S-4800,
Hitachi, Japan) and the crystalline structure was characterized
using X-ray diffractometry (XRD, D8, AXS, Germany). The
Ti3+ and Ti4+ contents of the electrodes were measured
through X-ray photoelectron spectroscopy using a spectro-
photometer (XPS, Axis supra, Kratos, UK). The contact angle
of the electrodes was measured using a rotating droplet inter-
face tension meter (Texas Model 500, Temco).
FIGURE 12 Removal rate of MO, color, and COD for the DTNA
electrode and 200 mg/L MO
molecules and not completely mineralized into H2O
and CO2.
3
| EXPERIMENTAL
3.2.2
| Electrochemical experiment
3.1 | Experimental method
Cyclic voltammetry (CV) and electrochemical impedance
spectroscopy (EIS) measurements of the electrodes were
conducted on the electrochemical workstation (IM6, Zahner,
Germany) with a traditional three-electrode system. A SCE
and a platinum electrode served as the reference and counter
electrodes, respectively. The measurements were conducted
in 0.1 M Na2SO4 aqueous solution at room temperature. The
scan speed for CV was 50 mV/s. The frequency range of
EIS was 1 Hz to 100 kHz, and the amplitude of the sinusoi-
dal wave was 5 mV. A Mott–Schottky plot was drawn from
the EIS measurements obtained using a solution of 0.1 M
KH2PO4 (pH = 7.2) with an alternating current potential of
10 mV and a frequency of 100 Hz. The doping level (ND)
indicates the donor density of a semiconductor and can be
calculated using the Mott–Schottky equation:
Highly oriented TiO2 nanotube arrays (TNA) were formed
on titanium sheets through anodic oxidation with a direct
current power supply (MS1003DH, Dongguan Maihao,
Dongguan, China). All anodization was conducted in a water
bath with a constant temperature of 25ꢁC by using a two-
electrode system, with the titanium sheet as the anode and
platinum as the cathode. The distance between the titanium
sheet and counter electrode was 2.5 cm. The electrolytic time
and voltage were modified to obtain different TNA elec-
trodes. First, TNA electrodes were prepared under a constant
curtain voltage for several hours in an ethylene glycol elec-
trolyte (AR, China Pharmaceutical Chemical Reagent,
China) containing 0.25 wt% NH4F (AR, Aladdin Biochemi-
cal Technology, China) and 2 wt% H2O. After anodization
in the electrolyte, the samples were ultrasonically cleaned
with ethanol for 2 min and then dried under vacuum for
1 hr, followed by calcination in air at 450ꢁC for 1 hr by
using a muffle furnace (KSF series, Yixing Qianjin, China).
To obtain the DTNA electrode, the TNA electrode was used
as a cathode for 10 min at 5 mA/cm2 in a 1 M NaClO4 solu-
tion. The DTNA electrode preparation procedure is illus-
trated in Figure 13. Unless noted otherwise, the TNA and
DTNA electrodes tested below were prepared at constant
40 V for 6 hr.
ꢀ
ꢁꢀ
ꢁ
1
C2SC
2
kBT
e
=
E − EFB
−
,
ð2Þ
eε0εNDA2
where CSC is the space charge layer capacity (F), e is the ele-
mentary electron charge (1.6 × 10−19 C), ε0 is the vacuum
permittivity (8.86 × 10−10 F/cm), ε is the dielectric constant
of anatase TiO2 (31),[45] ND is the donor state density
(cm−3), A is the electrode area (cm2), E is the applied bias
potential (V), EFB is the flat band potential (V), kB is the

MO ET AL.
7
Boltzmann constant (1.38 × 10−23 J/K), and T is the temper-
spectrometry using a spectrophotometer (TU-1901, Persee,
China) and calculated as follows:
−2
ature (298 K). A plot of C
against E should yield a
SC
straight line, of which ND is the slope.
A0 − At
A0
Color removal efficiencyð100%Þ =
× 100%, ð4Þ
3.2.3
| Accelerated life test
The electrochemical lifetime of the DTNA electrodes was
measured using the direct current power supply with a two-
electrode system where the DTNA electrodes and platinum
served as the working and counter electrodes, respectively.
The DTNA electrodes were used as an anode at 10 mA/cm2
in a 1 M NaClO4 solution to determine the service lifetime.
The following empirical formula was used to predict the life-
time of water oxidation electrodes:
where A0 and At are the absorbance values at 464 nm before
electrolysis and after electrolysis of time t, respectively. The
MO concentration during anodic oxidation was monitored
through ultrahigh-performance liquid chromatography
(UHPLC, UltiMate 3000 RS, Thermo Fisher Scientific). In
UHPLC, the MO was detected using an ultraviolet detector
at 350 nm. Ultrapure water and methanol were used as the
mobile phase at a flow rate of 0.5 mL/min, and the injection
volume was 20 μL. In the gradient elution, methanol was ini-
tially 20% and gradually increased to 100% after 40 min.
T1 × in1 = T2 × in2,
ð3Þ
where T1 is the lifetime obtained at high current density i1;
T2 and i2 are the actual lifetime and current density in the
operational mode, respectively; and n is a coefficient.
4
| CONCLUSIONS
Anodic oxidation combined with cathodic reduction was
employed to prepare highly oriented DTNA electrodes. The
service lifetime of the DTNA electrodes and electrochemical
degradation of MO were studied. The results revealed that
because of cathodic reduction treatment, the Ti3+ content of
the DTNA electrode was 4.37% compared with 0.5% for the
TNA electrode. The lifetime of the DTNA electrode pre-
pared at constant 40 V for 6 hr was approximately 338.7 hr
at constant 1 mA/cm2 in a 1 M NaClO4 solution. Compared
with the Ti/IrO2 electrode, a color removal rate of 200 mg/L
MO was achieved using the DTNA electrode, which was
substantially higher. A longer electrolytic duration and
greater electrolytic voltage increased the MO removal rate
because the surface area gradually increased with these
parameters. Electrochemical degradation occurred along the
indirect pathway (producing [ꢀOH]), which mainly led to
organic degradation by the DTNA electrode. Clearly, the
DTNA electrode can be suggested as a potentially effective
anode material for organic wastewater treatment.
3.2.4
| Hydroxyl radical detection
Terephthalic acid (TA) molecules were used as probe mole-
cules for measuring the number of hydroxyl radicals using a
fluorescence spectrophotometer (CARY Eclipse, Varian) with
a two-electrode system in which the DTNA electrode and plat-
inum served as working and counter electrodes, respectively.
The 2-hydroxyterephthalic acid generated by TA and hydroxyl
radicals was detected at excitation and emission wavelengths
of 311 and 415 nm, respectively (Figure 14). The DTNA and
Ti/IrO2 electrodes were taken in an electrolyte containing
0.5 mM TA, 4.0 mM NaOH, and 0.25 M Na2SO4 at a current
density of 10 mA/cm2, and each sample was taken for 10 min
and diluted twice for fluorescence detection.
3.2.5
| Electrochemical degradation test
Electrochemical oxidation of MO was conducted in a cylin-
drical cell equipped with a magnetic stirrer using a two-
electrode system. The DTNA electrodes and Ti/IrO2 elec-
trodes (Suzhou Shuertai Industrial Technology Co., Ltd.,
China) served as the anodes, and the cathode was a titanium
plate with the same dimensions. The areas of all electrodes
were 4 cm2, and the gap between the electrodes was 2.5 cm.
Electrochemical degradation tests were conducted at room
temperature and at a constant current in a curtain concentra-
tion MO aqueous solution containing 0.05 M Na2SO4 as the
supporting electrolyte. Samples were withdrawn from the
compartment at regular time intervals and subsequently ana-
lyzed for their contaminant concentration. The MO color
removal efficiency was measured through ultraviolet–visible
ACKNOWLEDGMENTS
This study was supported by the Ministry of Housing and
Urban–Rural Development of the People's Republic of
China (2014ZX07305003).
ORCID
Huixian Wei https://orcid.org/0000-0001-6137-7854
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How to cite this article: Mo C, Wei H, Wang T.
Fabrication of a self-doped TiO2 nanotube array elec-
trode for electrochemical degradation of methyl
orange. J Chin Chem Soc. 2019;1–8. https://doi.org/
10.1002/jccs.201800456