The simultaneous removal of heavy metals and organic contaminants over a Bi2WO6/mesoporous TiO2nanotube composite photocatalyst

Article abstract of DOI:10.1039/d0ra03430d

In this study, Bi₂WO₆/mesoporous TiO₂nanotube composites (BWO/TNTs) were successfully synthesized to remove the heavy metal Cr(vi) and refractory organic compound dibutyl phthalate (DBP) from contaminated water under visible light. Coupling TNTs with BWO can greatly improve the photocatalytic activity of the catalyst for treating Cr(vi)-DBP mixed pollutants because of synergetic effects from Cr(vi) and DBP. Specifically, the visible-light photocatalytic activities of 3percent BWO/TNTs for removing DBP and Cr(vi) from mixed pollutant solutions were 10.8 and 3.8 times higher than those of BWO. Firstly, this system can take full advantage of charge carriers and can spatially separate reduction sites and oxidation sites in the photocatalyst. Secondly, TNTs has a unique multiscale channel structure that can enhance mass transfer and light utilization. These characteristics lead to very obvious photocatalytic activity improvements. In addition, the BWO/TNTs composite photocatalysts exhibited excellent stability and durability under visible and UV light irradiation. This work demonstrated a feasible method for fabricating composite photocatalysts and applied them to the simultaneous removal of heavy metal and refractory organic pollutants from contaminated water.

Full text of DOI:10.1039/d0ra03430d

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PAPER
The simultaneous removal of heavy metals and
organic contaminants over a Bi2WO6/mesoporous
TiO2 nanotube composite photocatalyst†
abc
Cite this: RSC Adv., 2020, 10, 21228
Lei Cheng,a Sijia Liu,a Guangying Hea and Yun Hu
*
In this study, Bi2WO6/mesoporous TiO2 nanotube composites (BWO/TNTs) were successfully synthesized
to remove the heavy metal Cr(VI) and refractory organic compound dibutyl phthalate (DBP) from
contaminated water under visible light. Coupling TNTs with BWO can greatly improve the photocatalytic
activity of the catalyst for treating Cr(VI)–DBP mixed pollutants because of synergetic effects from Cr(VI)
and DBP. Specifically, the visible-light photocatalytic activities of 3% BWO/TNTs for removing DBP and
Cr(VI) from mixed pollutant solutions were 10.8 and 3.8 times higher than those of BWO. Firstly, this
system can take full advantage of charge carriers and can spatially separate reduction sites and oxidation
sites in the photocatalyst. Secondly, TNTs has a unique multiscale channel structure that can enhance
mass transfer and light utilization. These characteristics lead to very obvious photocatalytic activity
improvements. In addition, the BWO/TNTs composite photocatalysts exhibited excellent stability and
durability under visible and UV light irradiation. This work demonstrated a feasible method for fabricating
composite photocatalysts and applied them to the simultaneous removal of heavy metal and refractory
organic pollutants from contaminated water.
Received 17th April 2020
Accepted 19th May 2020
DOI: 10.1039/d0ra03430d
rsc.li/rsc-advances
for treating Cr(VI) polluted water.7 Compared with single-
component pollution, multi-component comprehensive pollu-
1. Introduction
tion consisting of heavy metals and refractory organic
compounds has very negative effects on the environment, and
on human physiology and biochemistry.8
Environmental pollution problems inevitably arise as a result of
economic development. For water pollution problems, multi-
factor combined pollution rather than single-component
pollution is found in contaminated water.1 Both heavy metals
(Cr, Cu, Pb, Cd, etc.) and refractory organic compounds (EDC,
PAHs, etc.) have been detected in wastewater.2,3 Dibutyl phtha-
late (DBP) is a common endocrine disrupting chemical (EDC)
that is widely present in water bodies. It is one of the main
causes of reproductive system diseases, affecting the human
metabolic and endocrine systems.4 Moreover, DBP is relatively
stable in the natural environment and difficult to remove via
traditional water treatment approaches, resulting in low
removal efficiencies being shown by general technologies.5 In
addition, hexavalent chromium (Cr(VI)), a heavy metal widely
found in surface and ground water, exhibits high toxicity, and
mutagenic and carcinogenic effects on human beings.6 The
reduction of Cr(VI) to Cr(III) is a standard and effective method
Heavy metals and refractory organic compounds are
commonly treated separately,9–11 such as through biological
degradation and chemical precipitation approaches. These
methods usually have some limitations, like low removal ratios,
high operating costs, long duration times, etc., due to the
different physicochemical properties of heavy metals and
organic compounds.12 Developing an effective method for
simultaneously removing heavy metals and organic compounds
from wastewater is of great signicance. Zhao et al.13 used
a photo-electrocatalytic (PEC) system to destroy a complex of
Cu-EDTA with a TiO2/Ti electrode. This involved the PEC
oxidation of Cu-EDTA at the anode and the electro-deposition of
Cu2+ ions at the cathode, indicating that spatially separating the
oxidation active sites and reduction active sites could improve
the removal efficiency. However, a bias potential had to be
applied for the removal of Cu-EDTA in the PEC system, and the
efficiency was quite dependent on the current density. Thus, it
is important to develop a more cost-effective and high-efficiency
strategy that can simultaneously remove a Cr(VI)–DBP mixed
pollutant.
aSchool of Environment and Energy, South China University of Technology, Guangzhou
510006, P. R. China. E-mail: huyun@scut.edu.cn; Tel: +86-020-3938-0573
bGuangdong Provincial Key Laboratory of Atmospheric Environment and Pollution
Control, Guangzhou 510006, P. R. China
cThe Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters,
Due to its advantages of environmental friendliness and
a high mineralization rate,14 photocatalysis is considered
a promising form of technology in many areas, such as in solar
Ministry of Education, Guangzhou 510006, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra03430d
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cells,15 water splitting, and environmental purication.16,17 synthesis methods. Therefore, Bi2WO6 is a desirable candidate
Among all the used photocatalysts, titanium dioxide (TiO2) has material for coupling with TiO2 to improve the availability of
the advantages of high photoactivity, good stability, non- visible light and the separation of electron–hole pairs.
toxicity, and low cost.18 However, it is hard to neglect the pho-
In this work, Bi2WO6/mesoporous TiO2 nanotube composite
tocatalytic activity limitations of TiO2, which arise from low photocatalysts (BWO/TNTs) were successfully synthesized by
light utilization and quantum yields. The low light utilization is coupling BWO with TNTs, and they were prepared through
caused by its absorption of a limited light region and the a dual-templated solvothermal method. The composite catalyst
aggregation of particles. Via broadening the light response showed excellent photocatalytic activity and superior stability
range, as well as increasing the reection and refraction of light for the synergistic removal of heavy metal and refractory organic
inside a TiO2 material, the light utilization can be improved. pollution from Cr(VI)–DBP mixed solution under UV and visible
The low quantum yields are due to the rapid recombination of light irradiation. The details and mechanism of the synergistic
photogenic carriers.19 Promoting charge separation and effect provide new ideas for the further development and
restraining the recombination of charge carriers can increase application of water treatment methods.
the quantum yields of TiO2. Therefore, it is important to
improve the reection of light, broaden the light response
range, and restrain the recombination of charges to promote
2. Experimental section
2.1. Materials
the photocatalytic activity of TiO2.
Morphology control is one of the more effective strategies for
improving the photocatalytic performance of TiO2.20 TiO2
nanoparticles are widely used for various applications, but they
aggregate easily due to the small particle sizes. Therefore, one-
dimensional TiO2 materials (e.g., nanowires, nanorods, nano-
belts, and nanotubes) have been studied due to their unique
morphologies and properties.21 Podporska-Carroll et al.22
synthesized TiO2 nanotubes via an electrochemical anodization
method, and they showed enhanced antimicrobial photo-
catalytic disinfection properties toward E. coli and S. aureus.
Mushtaq et al.23 fabricated coaxial TiO2-noble metal nanotubes
Multi-walled carbon nanotubes (CNTs) were purchased from
Shenzhen Nanotech Port Co. Ltd. Cetyltrimethylammonium
bromide (CTAB, AR) was purchased from Tianjin Chemical
Reagent Factory. Tetraethyl orthosilicate (TEOS, AR) and tita-
nium butoxide (TBOT, AR) were supplied by Sinopharm
Chemical Reagent Co. Ltd, Shanghai. All chemicals used in this
work were of analytical grade.
2.2. Fabrication of materials
Preparation of m-TiO2NTs. 0.4 g of TiO2@SiO2@CNTs was
lled with nickel nanowires for photocatalytic water remedia- immersed in 50 mL of ethylene diamine solution at pH ¼ 11,
ꢁ
tion with enhanced photocatalytic activity. However, the mate- and the suspension was reuxed at 100 C for 48 h. A sample
rials mentioned above have long channels and opaque walls, was obtained via ltration and was washed with deionized water
preventing the diffusion of guest molecules and the reection of and dried at 80 ꢁC. A predetermined amount of the above
light between the tubes. Weon et al.24 synthesized TiO2 nano- sample wꢀas1 calcined at 600 ꢁC for 4 h at a heating rate of
tubes with open straight channels for the degradation of volatile
1
ꢁC min to remove the CNT template. Next, 0.2 g of the
organic compounds (VOCs). They demonstrated that an open sample was added to 80 mL of 1.0 M NaOH and then autoclaved
channel structure is able to promote mass transfer, thereby at 150 ꢁC for 0.5 h in a 100 mL Teon-lined autoclave. The
preventing photocatalyst deactivation. Therefore, constructing resulting precipitate was washed with 0.1 M HCl, followed by
mesoporous TiO2 nanotubes with multi-scale channels was put several washes with deionized water until the ltrate was
forward as an approach that combines the advantages of neutral. The material was then dried at 80 ꢁC for 10 h and was
nanotubes and mesopores. This unique structure can transform denoted as m-TiO2NTs (referred to as TNTs). Please refer to our
one-dimensional materials into three-dimensional materials. previous work for more details.25
Therefore, it can greatly improve the mass transfer of guest
Preparation of Bi2WO6/TNTs. In a typical synthesis method,
molecules, increase the number of light refraction and reec- Bi2WO6/TNTs was synthesized via a one-step solvothermal
tion channels available in the tubes, and expose more active method. 0.2 mmol of Bi(NO3)3$5H2O was dissolved in 20 mL of
sites.25
ethylene glycol (EG) and stirred at room temperature for 30 min.
In addition, constructing a heterostructure between two The resulting solution was denoted as solution A. Meanwhile,
semiconductors is found to be benecial for promoting 0.4 mmol of Na2WO4$2H2O was dissolved in 20 mL of EG at
quantum yields.26–28 In particular, coupling a wide bandgap room temperature and stirred for 30 min. Then a certain
semiconductor with a narrow bandgap one could improve the amount of TNTs was added, and the solution was stirred for
quantum yields and light absorption in the visible region.29 30 min; this was called solution B. Solution A and solution B
Bismuth tungstate (Bi2WO6), with a narrow bandgap (about 2.8 were then mixed and stirred for 60 min to ensure good dis-
eV), has attracted considerable attention due to its special persity. The suspension was transferred to a 100 mL Teon-
physical and chemical properties, such as its unique molecular lined stainless-steel autoclave and then solvothermal
structure, high photostability and non-toxicity.30,31 Bi2WO6 synthesis was performed at 160 C for a reaction time of 16 h.
ꢁ
consists of alternating (Bi2O2)2+ layers and perovskite-like Aer the reaction, the precipitate was collected, washed several
(WO4)2ꢀ sheets,32 demonstrating its structural variability. times with ethanol and deionized water, and then dried at 80 ꢁC
Moreover, the bandgap of Bi2WO6 is adjustable via different for 12 h. Composites with different mass ratios of Bi2WO6 were
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prepared and they are denoted as x% BWO/TNTs, where x was 1, was used as a visible light source, and a 125 W high-pressure
3, 5, and 7. A schematic diagram of the BWO/TNTs composite mercury lamp (Philips, USA) was used as a UV light source.
synthesis is shown in Scheme 1.
The visible light intensity was 150 mW cmꢀ2 and the ultraviolet
Preparation of reference samples. Bi2WO6/TiO2NPs was intensity was 1.5 mW cmꢀ2; these were measured using a UVA
synthesized via a one-step solvothermal method similar to that radiometer (Beijing Normal University Optoelectronic Instru-
used for BWO/TNTs, however the mesoporous TiO2 nanotubes ment Factory). Cr(VI) and DBP were used as representative
were replaced by TiO2 nanoparticles (TiO2NPs). TiO2 nano- pollutants to assess the photocatalytic activities of the prepared
particles were prepared via hydrolysis from TBOT. In detail, samples. The photocatalytic reaction involving Cr(VI) and DBP
a certain amount of deionized water was mixed with 20 mL of on the catalyst was carried out in a homemade reactor. 50 mg of
ethanol, and then a certain amount of TBOT mixed with 30 mL photocatalyst was added to a 100 mL solution containing
of ethanol was added. The mixture was stirred at 40 ꢁC for 24 h. a single contaminant (20 mg Lꢀ1 Cr(VI) or 5 mg Lꢀ1 DBP) or
The precipitate was collected, washed several times with a contaminant mixture (20 mg Lꢀ1 Cr(VI) and 5 mg Lꢀ1 DBP).
ꢁ
ethanol and deionized water, and then dried for 10 h at 80 C. Before irradiation, the suspension was stirred in the dark for
Finally, the samples were calcined for 3 h at a heating rate of 30 min to achieve adsorption–desorption equilibrium. At
ꢀ1
ꢁ
ꢁ
1 C min at 500 C and referred to as TNPs.
a given time interval during photodegradation, 2 mL of
suspension was collected and then ltered with a 0.45 nm
membrane lter for analysis. The concentration of Cr(VI) was
determined via a diphenylcarbazide (DPC) method, using the
maximum absorption band (540 nm), with a UV-visible spec-
trophotometer (Thermo, USA). The DBP concentration was
determined via HPLC (LC-20 at Shimadzu, Japan) with a C18
(250 ꢂ 4.6 mm, Agilent, USA) column. The mobile phase was
a mixture of methanol and water (90 : 10 v/v) at a ow rate of 1.0
mL minꢀ1. The removal rate (h) was calculated via: h (%) ¼ (1 ꢀ
C/C0) ꢂ 100%, where C0 and C are the pollutant concentrations
before and aer the reaction, respectively.
2.3. Characterization
The crystalline structures of the samples were determined via X-
ray diffraction (XRD) (X-ray, PANalytical) using Cu Ka emission
(l ¼ 0.15418 nm). The 2q scan range was from 10 to 80 degrees.
The morphologies of the samples were observed via eld
emission scanning electron microscopy (FE-SEM) (Merlin,
Zeiss). The nitrogen adsorption–desorption isotherms were
measured using an automatic gas adsorption analyzer
(Autosorb-IQ, Quantachrome) at 77 K. X-ray photoelectron
spectroscopy (XPS) studies were performed with a Thermo
Escalab 250Xi spectrometer (Thermo, USA), using a mono-
chromatic Al Ka source. The XPS peaks were calibrated using
the C 1s peak at 284.8 eV of contaminated carbon.
3. Results and discussion
3.1. XRD analysis
The phases and crystal structures of TNTs, x% BWO/TNTs and
BWO were studied via X-ray diffraction (XRD) analysis. As
shown in Fig. 1, distinct diffraction peaks of TNTs and x%
BWO/TNTs could be found at 25.3ꢁ, 37.8ꢁ, 48.0ꢁ, 53.9ꢁ, 55.1ꢁ
and 62.7ꢁ, and the characteristic peaks of anatase TiO2 (JCPDS
no. 2-11-1272) can assign these to the (101), (004), (200), (105),
(211), and (204) diffraction planes, respectively. The diffraction
peaks of BWO located at 28.3ꢁ, 32.8ꢁ, 47.1ꢁ, 56.0ꢁ, and 58.5ꢁ
correspond to the (131), (200), (202), (133), and (262) planes of
russellite BWO (JCPDS no. 39-0256), respectively. The charac-
teristic peaks of BWO could not be detected in the x%-BWO/
2.4. Photoelectrochemical measurements
Photocurrent measurements were performed at room temper-
ature using a CHI 660B electrochemical analyzer (Chengdu,
China). All experiments were conducted using a standard three-
electrode system consisting of a working electrode (photo-
electrode), a platinum counter electrode, and an Ag/AgCl
reference electrode. The electrode area is approximately 1 ꢂ 1
cm2. The electrolyte was 0.5 M Na2SO4 aqueous solution. The
light source was a 300 W xenon lamp (Perfect Light, China) with
a UV cut lter. The i–t curves are measured at a bias potential of
ꢀ0.1 V.
2.5. Photocatalytic experiments
To assess the photocatalytic activities of the prepared samples,
a 300 W Xe lamp (Perfect Light, China) with a UV cut-off lter
Scheme 1 A schematic illustration of the synthesis of the BWO/TNTs
composites.
Fig. 1 XRD patterns of the samples.
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TNTs composites because of the low content and high disper-
sion of BWO. As the BWO content increased, the intensities of
the TiO2 diffraction peaks weakened gradually without obvi-
ously shiing due to the decrease of the TiO2 content and the
coexistence of BWO. However, the existence of BWO did not
affect the orientation of the TiO2 crystals.33
3.2. Morphologies
Fig. 3 N2 adsorption–desorption isotherms (a) and pore size distri-
butions (b) of TNTs, BWO and 3% BWO/TNTs.
SEM was used to analyze the morphologies of the prepared
samples, as shown in Fig. 2. It is observed from Fig. 2a and
b that the TNTs sample has a distinct tubular structure with
diameters of about 100 nm. TNTs is full of tangles and highly
interconnected; this could facilitate the transport of guest
molecules between the tubes. The walls of TNTs are rough and
porous (Fig. 2b). It can be easily identied as a mesoporous
structure formed aer the removal of the so template. As
shown in Fig. 2c and d, the hierarchical hollow microspheres
are composed of self-assembled BWO nanosheets. The 3%
BWO/TNTs composite was composed of TNTs and BWO, which
presented as identiable nanotubes and scattered sphere frag-
ments, respectively (Fig. 2e). This revealed that aer the
combination of BWO and TNTs, TNTs retained the mesoporous
tube structure well (Fig. 2f). However, BWO was fragmented due
to the presence of TNTs and its low content, which was
consistent with the XRD analysis.
Table 1 The pore structure characteristics of different photocatalystsa
Sample
SBET (m2 gꢀ1
)
Vp (cm3 gꢀ1
)
Dp (nm)
TNTs
256.68
97.90
97.74
95.66
92.23
73.99
0.34
0.24
0.27
0.34
0.33
0.24
5.36
12.64
14.02
17.25
18.36
10.27
1% BWO/TNTs
3% BWO/TNTs
5% BWO/TNTs
7% BWO/TNTs
BWO
a
Vp: pore volume; Dp: pore diameter.
is generally associated with plate-like materials, which agrees
well with the existence of the self-assembled BWO nanosheet
morphology (Fig. 2c and d).35 TNTs shows a narrow pore size
distribution at about 2.5 nm and a wide pore size distribution at
20–60 nm; this is in good agreement with the mesopores of the
walls and the openings of the tubes, respectively. However, 3%
BWO/TNTs shows a wide distribution, due to the presence of
BWO whose distribution is in the range of 2–30 nm. Table 1 also
shows the BET specic surface area, average pore volume and
pore size data from the photocatalysts. The BET specic surface
3.3. BET analysis
Fig. 3 shows the N2 adsorption–desorption isotherms and cor-
responding BJH pore size distribution. The isotherm of TNT can
be classied as type-IV with a H1-type hysteresis loop, indi-
cating the existence of an ordered mesoporous network with
a high specic surface area. The curves of BWO and 3% BWO/
TNTs are type-IV isotherms with H3-type hysteresis loops,
indicating the existence of ssure-like mesoporous holes.34 This
Fig. 2 SEM images of (a and b) TNTs, (c and d) BWO, and (e and f) 3% BWO/TNTs.
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area of the samples was reduced from 256.68 m2 gꢀ1 (TNTs) to corresponding to W–O and Bi–O bonds, respectively.39
73.99 m2 gꢀ1 (BWO) because of the presence of BWO; the Compared with BWO and TNTs, the BWO/TNTs O 1s peak
average pore volume and pore diameter of the composites displays a slight shi toward a lower binding energy, suggesting
increased with increasing BWO content. Coupling with TNTs, that the electron density on BWO/TNTs is increased, therefore
which possesses a multiple channel structure, can enlarge the leading to enhanced photocatalytic activity.40,41 In Fig. 4d, Bi 4f5/
2
surface area, pore volume and pore diameter. As a result, the
and Bi 4f7/2 result in two XPS peaks at 164.5 and 159.2 eV,
mass transfer of guest molecules is promoted, the multiple respectively, indicating the existence of Bi3+ in BWO and BWO/
reection of light increases, and more active sites are exposed. TNT.42 The two BWO/TNTs shoulder peaks at 162.6 and 157.3 eV
Therefore, the BWO/TNTs composites are expected to have can be ascribed to lower valence state Bi, which could arise from
better interfacial reaction properties than BWO.
reduction by oxygen vacancies during the solvothermal
process.43 The two BWO peaks at 37.5 and 35.41 eV can be
assigned to W 4f5/2 and W 4f7/2 from W6+, respectively, as shown
in Fig. 4e. The BWO/TNTs W 4f peak overlapped with the Ti 3p
peak,44,45 which fully demonstrated the coexistence of BWO and
TiO2 in the BWO/TNTs composite. Also, in Fig. 4d and e, both
the Bi 4f and W 4f peaks of BWO/TNTs are offset to lower
binding energies than those of BWO, which may be due to
chemical bonds between BWO and TNTs, indicating that the
combination of BWO and TNTs does not simply involve physical
contact, but also close combination. Intimate contact between
BWO and TNTs with strong chemical bonds, instead of physical
contact, facilitates the transfer of charge carriers, greatly
improving the photocatalytic activity of the material.
3.4. XPS analysis
The surface compositions and chemical states of different
photocatalysts were investigated via XPS analysis, as shown in
Fig. 4. The binding energy of the C 1s peak at 284.8 eV was used
as a reference. As shown in Fig. 4a, the XPS survey spectrum of
3% BWO/TNTs contains the characteristic peaks of Ti, O, Bi, W
and C elements. This suggests that the BWO/TNTs composite
was successfully synthesized through a solvothermal method.
However, the characteristic peaks of the elements Bi and W
cannot be clearly observed in the BWO/TNTs spectrum because
of their low content. High-resolution XPS spectra of the Ti 2p
peaks are shown in Fig. 4b. The XPS peaks at 458.5 and 464.2 eV
can be assigned to Ti 2p3/2 and Ti 2p1/2 36 respectively, indicating
,
the presence of Ti4+ in TNTs and BWO/TNTs. In Fig. 4c, the peak
at about 529.9 eV can be assigned to O 1s, revealing the presence
of O2ꢀ in TNTs, BWO and BWO/TNTs.37 The TNTs O 1s peak at
529.6 eV is attributed to Ti–O bonds.38 The BWO O 1s peak can
be deconvoluted into two peaks at 530.5 and 530.1 eV,
3.5. Photoelectrochemical properties
The photoelectrochemical properties of the samples were
investigated, as shown in Fig. 5. The UV-vis DRS spectra of
TNTs, BWO and x% BWO/TNTs are shown in Fig. 5a. The
optical absorption band of the TNTs presents a steep absorption
edge in the UV region. Aer coupling with BWO, the absorption
edges of the composites show red shis, indicating that the
BWO/TNTs composites achieve a visible light response. Fig. 5b
shows the bandgap values of samples, which were determined
via Tauc plots from UV-vis DRS spectra data.46 The bandgaps of
BWO, TNTs and BWO/TNTs are 2.80, 3.20 and 3.06 eV, respec-
tively. Fig. 5c shows the transient photocurrent curves of TNT,
Fig. 5 UV-vis DRS spectra (a), Tauc plots (b), transient photocurrent
Fig. 4 Survey (a) and high-resolution Ti 2p (b), O 1s (c), Bi 4f (d) and W response curves under visible light irradiation (c), and EIS Nyquist plots
4f (e) XPS spectra for different photocatalysts.
(d) of the samples.
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BWO and 3% BWO/TNT for studying the separation efficiencies
Fig. 6c and d exhibits the visible-light photocatalytic activi-
of photoinduced electron–hole pairs. Compared with TNT and ties of TNTs, BWO and x% BWO/TNTs for the removal of Cr(VI)
BWO, the photocurrent response of 3% BWO/TNTs is the and DBP from Cr(VI)–DBP mixed pollutants. TNTs removed 9%
highest, most stable, and reproducible in the intermittent Cr(VI) and 15% DBP via adsorption, while BWO removed 32%
irradiation switching period, which means the charge carrier Cr(VI) and 43% DBP. The BWO/TNTs composites displayed
separation efficiency is the highest. The expected improved enhanced photocatalytic activities for removing Cr(VI)–DBP
current response is related to the enhanced photocatalytic mixed pollutants. The Cr(VI) removal efficiency increased from
performance.47
32% to 76% and that of DBP increased from 43% to 100% on
In order to study the charge transfer properties and interfa- 3% BWO/TNTs, which showed the highest photocatalytic
cial reaction capacities of the samples, EIS Nyquist plots were activities. Furthermore, the presence of DBP enhanced the
measured, as shown in Fig. 5d. The arc radius of 3% BWO/TNTs removal of Cr(VI) signicantly. The Cr(VI) removal rate using 3%
is much smaller than those of TNTs and BWO under dark BWO/TNTs increased from 51% to 76%. Similarly, the presence
conditions. This result conrms that the resistance of the of Cr(VI) also improved the removal efficiency of DBP, which was
composite is much lower than that of the other samples, indi- 99% degraded within 3 h. These results indicated that Cr(VI)
cating the high charge transfer and separation efficiency. All could be reduced by eꢀ, while DBP could be oxidized by h+. The
these photoelectrochemical properties show that the separation process can make full use of the advantages of eꢀ and h+,
efficiency and interface reaction ability of charges are remark- effectively inhibiting recombination and thereby greatly
ably improved in the BWO/TNTs composites.
improving the photocatalytic efficiency.48
Fig. S1† shows the photocatalytic performances of the
different catalysts for treating Cr(VI) and DBP single pollutant
solutions and Cr(VI)–DBP mixed pollutants under visible light
irradiation. As a reference sample, BWO/TNPs was prepared by
coupling BWO with traditional TiO2 nanoparticles (TNPs). The
composites, both BWO/TNPs and BWO/TNTs, showed remark-
able enhancements in photocatalytic activity compared with
TNPs, TNTs and BWO. The photocatalytic reaction rate
constants (k) of the photocatalysts are summarized in Fig. 7. For
the removal of Cr(VI) from Cr(VI)–DBP pollutant solution, the k
values of BWO and BWO/TNTs are 1.47 ꢂ 10ꢀ3 and 5.61 ꢂ
10ꢀ3 minꢀ1, respectively; in the case of DBP removal, the k
values are 2.35 ꢂ 10ꢀ3 and 24.26 ꢂ 10ꢀ3 minꢀ1. Accordingly,
the photocatalytic activities of BWO/TNTs are 3.8 and 10.8 times
higher than those of BWO for the removal of Cr(VI) and DBP,
respectively.
All the catalysts showed higher photocatalytic activities in
Cr(VI)–DBP mixed solution than in Cr(VI) or DBP single pollutant
solutions. For the removal of Cr(VI) from Cr(VI) single pollutant
solution, the k value of BWO/TNTs is 2.51 ꢂ 10ꢀ3 minꢀ1, while it
is 5.61 ꢂ 10ꢀ3 minꢀ1 from Cr(VI)–DBP mixed solution. In the
case of DBP, the values are 14.98 ꢂ 10ꢀ3 and 24.26 ꢂ
10ꢀ3 minꢀ1, respectively. The photocatalytic efficiency of BWO/
TNTs for Cr(VI) reduction in the presence of DBP is 2.2 times
higher than in its absence. Similarly, the oxidation efficiency of
DBP in the presence of Cr(VI) was 1.6 times higher than without
Cr(VI). This enhancement demonstrated that Cr(VI) was an
3.6. Photocatalytic performance
Fig. 6 shows the photocatalytic performances of different pho-
tocatalysts under visible light irradiation. Fig. 6a and b shows
the visible light photocatalytic activities of TNTs, BWO and x%
BWO/TNTs towards Cr(VI) and DBP in single pollutant solu-
tions. TNTs only removed a small amount of Cr(VI) and DBP
because it showed no visible light response. BWO removed only
22% Cr(VI) and 38% DBP in 4 h, while the BWO/TNTs
composites greatly improved the Cr(VI) and DBP photocatalytic
removal efficiencies. The highest photocatalytic activity was
achieved using 3% BWO/TNTs, on which 51% Cr(VI) and 98%
DBP were removed. These results show that the photocatalytic
activities of the BWO/TNTs composites were enhanced, due to
charge separation improvements. These results are consistent
with the results from studies of the photoelectrochemical
properties (Fig. 5).
Fig. 6 The photocatalytic removal of (a) Cr(VI) and (b) DBP from single
pollutant solutions, and (c) Cr(VI) and (d) DBP from Cr(VI)–DBP mixed Fig. 7 The photocatalytic reaction rate constants of different catalysts
pollutant solutions using different catalysts under visible light for the removal of (a) Cr(VI) and (b) DBP from Cr(VI)-only, DBP-only and
irradiation.
Cr(VI)–DBP mixed solutions under visible light irradiation.
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efficient scavenger of photogenerated electrons, while DBP was Fig. S3.† When the pH value was 10 or 13, the removal of Cr(VI)
a scavenger of photogenerated holes. Thus a synergistic mech- was negligible. When the pH value was lower than 7, the
anism in Cr(VI)–DBP mixed pollutant solution can be proposed. removal efficiency was enhanced signicantly, showing that the
In the photocatalytic reaction on the BWO/TNTs composites, removal of Cr(VI) is pH-dependent.49
Cr(VI) captures eꢀ and then is reduced, and DBP captures h+ and
The photocatalytic reduction performance of BWO/TNTs
then is oxidized. As a result, the photocatalytic reaction can toward different heavy metals was studied. As shown in
make full use of the redox process and effectively inhibit the Fig. S4,† Cr(VI), Cu(II), and Pb(II) could be reduced. The Cr(VI)
recombination of electron–hole pairs.
reduction efficiency was the highest due to the low reduction
BWO/TNPs was used as a reference sample to study the role potential (1.33 V), while the catalyst reduced just 31% Pb(II),
of the unique mesoporous TiO2 nanotubes. In a Cr(VI)–DBP whose reduction potential is rather high (ꢀ0.126 V).48–50
mixture, the k values for removing Cr(VI) and DBP are 2.29 ꢂ
10ꢀ3 minꢀ1 and 3.78 ꢂ 10ꢀ3 minꢀ1, respectively. Therefore, the
3.7. Photocatalytic mechanism
photocatalytic activities of BWO/TNTs for the removal of Cr(VI)
DMPO spin-trapping ESR spectra were measured to detect free
and DBP were 2.5 and 6.7 times higher than those of BWO/
TNPs, respectively. The photocatalytic performance of BWO/
TNTs, involving the composite coupling of BWO with TNTs,
was signicantly enhanced. This can be ascribed to the multi-
channel structure of the mesoporous TiO2 nanotubes, which
can greatly enhance the mass transfer of pollutants, support the
multiple reection of light, and expose more active sites. In
Cr(VI)–DBP mixed solution, under UV irradiation, Cr(VI) was
radicals in the reaction system.51 The hydroxyl radicals ($OH)
and superoxide radicals ($O2ꢀ) generated by BWO/TNTs were
separately detected, as shown in Fig. 9. Fig. 9 shows weak ESR
signals in the dark, but signicant $OH and $O2ꢀ response
signals are seen aer visible light irradiation. The signal
intensities become higher as the irradiation time is prolonged,
and the intensity of $OH is much higher than that of $O2ꢀ. This
indicates that the BWO/TNTs composites can generate $OH and
photocatalytically reduced and DBP was oxidized in order to
study the UV photocatalytic performance of the catalyst
(Fig. S2†). The BWO/TNTs composites also show the highest
$O2ꢀ, and that $OH makes a greater contribution than $O2ꢀ in
the reaction.
photocatalytic activities, consistent with the results of visible
light experiments.
To evaluate the recyclability and stability of BWO/TNTs,
BWO/TNTs and BWO were reused several times in Cr(VI)–DBP
mixed solutions. As shown in Fig. 8, the photocatalytic perfor-
mance of BWO/TNTs showed negligible decay aer six cycles.
The Cr(VI) and DBP degradation efficiencies were 76% and 100%
during the rst cycle, and the efficiencies dropped slightly to
68% and 97% aer the sixth cycle. In contrast, the photo-
catalytic activity of BWO greatly decreased aer the rst cycle,
indicating that it is relatively unstable and shows poor photo-
catalytic activity over multiple cycles. The photocatalytic prop-
erties of the BWO/TNTs composites hardly changed, indicating
that the optical absorption abilities of the catalysts aer long-
Fig. 9 DMPO spin-trapping ESR spectra of 3% BWO/TNTs for the
term photocatalytic durability experiments did not change
signicantly. These results showed that the BWO/TNTs
detection of $OH and $O2ꢀ under visible light irradiation.
composites were signicantly more stable and durable than
BWO.
The photocatalytic reduction of Cr(VI) on BWO/TNTs at
different pH values from 1 to 13 was investigated, as shown in
Fig. 8 Recyclability of BWO and BWO/TNTs for the removal of (a)
Cr(VI) and (b) DBP from Cr(VI)–DBP mixed pollutant solutions under Fig. 10 The photocatalytic degradation of DBP on 3% BWO/TNTs with
visible light irradiation.
the addition of different scavengers.
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retained during the recovery of the catalyst. These results
indicated that the BWO/TNTs composite is durable and effec-
tive for photocatalytic reduction and mineralization.
The measured XPS spectra of fresh and used BWO/TNTs are
shown in Fig. S6.† An obvious Cr 2p peak can be seen in the
used BWO/TNTs spectrum due to the deposition of Cr elements
aer the reaction. In order to study the chemical state changes
of Cr elements during the photocatalytic process, catalysts were
analyzed aer different reaction times via high-resolution XPS
spectroscopy, as shown in Fig. S7.† As shown in Fig. S7a,† Cr3+
2p1/2 and Cr3+ 2p1/2 are assigned to the Cr 2p peaks at 577.3 and
586.8 eV, respectively. The peaks at 580.3 and 589.7 eV corre-
spond to Cr6+ 2p3/2 and Cr6+ 2p1/2, respectively, indicating the
presence of Cr(III) and Cr(VI).56,57 Aer 8 h of reaction, two new
peaks at 573.9 and 584.5 eV appeared, belonging to Cr0 2p3/2
and Cr0 2p1/2, respectively.58–60 This indicated that an amount of
Cr(III) was reduced to Cr(0) between 4 and 8 h of reaction time.
The relative amounts of different chemical states of Cr were
obtained using the peak areas of the Cr 2p peaks. As shown in
Fig. 12, the content of Cr(VI) decreased, while those of Cr(III) and
Cr(0) increased with reaction time within 16 h. This was
attributed to the photocatalytic reduction of Cr(VI) to Cr(III) and
then to Cr(0). Aer 16 h, the relative content values of different
chemical states of Cr were basically unchanged. According to
the long-term experiment (Fig. S6†) results, there was not
enough organic material in the solution to act as an oxidation
scavenger. Therefore, the Cr deposited on BWO/TNTs achieved
a balance between reduction and oxidation, which resulted in
negligible changes between the different chemical states of Cr.
Based on the above results, a synergistic photocatalytic
mechanism involving the BWO/TNTs composites was proposed,
as shown in Scheme 2. Under visible light irradiation, photo-
generated eꢀ was excited to the CB from the VB of BWO, then eꢀ
in the CB of BWO was transported to the CB of TNTs due to the
conduction band offset. Therefore, the photogenerated eꢀ and
h+ were separated spatially by the heterostructure, thus
restraining the recombination of charges. Photo-generated eꢀ
arriving at TNTs can effectively reduce Cr(VI) to Cr(III), while eꢀ
on BWO can further reduce Cr(III) to Cr(0). Meanwhile, posi-
tively charged h+ can accumulate in the valence band of BWO
and reach the surface, oxidizing DBP to CO2 and H2O. Hence,
contaminants could also be spatially separated via using
different active sites. Making full use of eꢀ and h+ and spatially
Fig. 11 Mott–Schottky plots of (a) TNTs and (b) BWO.
As shown in Fig. 10, the effects of free radical scavengers on
DBP photocatalytic oxidation were investigated. In this case,
triethanolamine (TEOA), isopropanol (IPA), and KBrO3 are used
as scavengers for h+, $OH, and $O2ꢀ 52 The addition of TEOA led
.
to the most obvious decline in the degradation efficiency, which
indicates that h+ is the main active species. Compared with
KBrO3, the activity loss caused by the addition of IPA is more
obvious, indicating that $OH is more important than $O2ꢀ in
the reaction, which is consistent with the ESR results.
Mott–Schottky plots are used to measure the at band
potential (VFB) of a semiconductor.53 As shown in Fig. 11, the
slopes of the TNT and BWO plots are positive, suggesting that
they are n-type semiconductors.54 The VFB values of TNT and
BWO are ꢀ0.19 and ꢀ0.69 V vs. NHE, respectively, following
correction from a saturated calomel electrode (SCE) potential
(0.24 V vs. NHE). The conduction band (CB) edge of an n-type
semiconductor is more negative (0.1–0.2 eV) than its VFB
,
which is equal to the Fermi level generally.55 Therefore, the ECB
values of TNT and BWO are ꢀ0.29 and ꢀ0.79 eV vs. NHE.
The states of Cr(VI) and DBP were studied during long-term
photocatalytic experiments. Fig. S5† shows the removal of
Cr(VI) and TOC from Cr(VI)–DBP mixed pollutant solution using
the BWO/TNTs composite for 24 h under visible light irradia-
tion. The catalyst reduced almost all Cr(VI) within 16 h, and
mineralized all DBP within 12 h. It can be seen from Fig. S5a†
that aer 20 h, Cr(VI) is completely reduced. However, it can still
be detected based on the measurements of the proportions of
different chemical states present (Fig. 12) and via high-
resolution XPS spectra (Fig. S7†); this is due to the presence
of a small amount of Cr(VI) on the catalyst that is adsorbed and
Scheme 2 The synergistic photocatalytic mechanism for Cr(VI)–DBP
Fig. 12 The proportions of the different chemical states of Cr present mixed pollutant removal on the BWO/TNTs composite under visible
over 24 h of irradiation when using 3% BWO/TNTs.
light irradiation.
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separating the reduction and oxidation of different reactants
allowed the BWO/TNTs composite to synergistically remove
Cr(VI) and DBP from mixed pollutants. Meanwhile, for the as-
synthesized materials, hierarchical porosity composed of mes-
opores connected via macropores (open channels of macro-
porous size) could be observed from the N2 adsorption–
desorption isotherms and SEM observations. In the literature,
the presence of mesopores favors the multiple scattering/
reection of light, resulting in the enhanced harvesting of the
excitation light and, thus, improved photocatalytic activity.61,62
In addition, hierarchical porosity composed of mesopores
connected via macropores (or larger mesopores) facilitates fast
mass transport, resulting in improved performance.63–67 The
unique mesoporous tubular structure of TNTs could greatly
facilitate the mass transfer of guest molecules, signicantly
improve light utilization in the interior of the tubes, and
remarkably promote the exposure of active sites for reactions.
Hence, the BWO/TNTs composite achieved excellent photo-
catalytic performance for the synergistic removal of mixed
pollutants.
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