Efficient photocatalytic dechlorination of chlorophenols over a nonlinear optical material Na3VO2B6O11 under UV-visible light irradiation

Article abstract of DOI:10.1039/c5ta01814e

One type of nonlinear optical crystal Na₃VO₂B₆O₁₁ (NVB) with noncentrosymmetry was synthesized and shows extraordinary UV-visible light driven photocatalytic activity in the dechlorination of 2,4-DCP under UV-vis (λ > 320 nm) light irradiation. The obtained dechlorination efficiency is 90 times higher than that of the commercial P25 TiO₂ catalyst under the same conditions. The noncentrosymmetric structure of NVB gives rise to an intrinsic large polarization effect as evidenced by Kelvin probe force microscopy, and the polarization promotes separation of photogenerated electron-hole pairs, leading to efficient cleavage of chlorophenols into phenol series fragments and dissociative Cl^- anions. A possible reaction pathway for 2,4-DCP dechlorination by NVB upon UV-vis light irradiation is proposed. Hydrodechlorination was found to be the major reaction pathway for 2,4-DCP dechlorination which could react with hydroxyl radicals to produce 1,4-benzoquinone and ocatechol. This work further advances the understanding of nonlinear optical materials, which opens up a new route to the design and synthesis of highly efficient photocatalysts by using nonlinear optical materials.

Full text of DOI:10.1039/c5ta01814e

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Efficient photocatalytic dechlorination of
chlorophenols over a nonlinear optical material
Cite this: DOI: 10.1039/c5ta01814e
Na VO B O under UV-visible light irradiation†
3
2 6 11
a
ab
c
a
a
Xiaoyun Fan,* Kangrong Lai, Lichang Wang, Hengshan Qiu, Jiao Yin,
a
a
a
a
Pengjun Zhao, Shilie Pan, Jinbao Xu and Chuanyi Wang*
3 2 6
One type of nonlinear optical crystal Na VO B O11 (NVB) with noncentrosymmetry was synthesized and
shows extraordinary UV-visible light driven photocatalytic activity in the dechlorination of 2,4-DCP under
UV-vis (l > 320 nm) light irradiation. The obtained dechlorination efficiency is 90 times higher than that
2
of the commercial P25 TiO catalyst under the same conditions. The noncentrosymmetric structure of
NVB gives rise to an intrinsic large polarization effect as evidenced by Kelvin probe force microscopy,
and the polarization promotes separation of photogenerated electron–hole pairs, leading to efficient
ꢀ
cleavage of chlorophenols into phenol series fragments and dissociative Cl anions. A possible reaction
pathway for 2,4-DCP dechlorination by NVB upon UV-vis light irradiation is proposed.
Hydrodechlorination was found to be the major reaction pathway for 2,4-DCP dechlorination which
could react with hydroxyl radicals to produce 1,4-benzoquinone and ocatechol. This work further
advances the understanding of nonlinear optical materials, which opens up a new route to the design
and synthesis of highly efficient photocatalysts by using nonlinear optical materials.
Received 11th March 2015
Accepted 13th April 2015
DOI: 10.1039/c5ta01814e
www.rsc.org/MaterialsA
sites for the photogenerated charges and promote the charge
separation, thus enhancing the quantum efficiency. In addition,
1
. Introduction
Semiconductor materials, as a class of promising photocatalysts some novel catalysts have also been explored, such as, carbon
8
9
10
3 4 3 4
for organic pollutant treatment, have attracted increasing nitrides (g-C N ), Ag PO , graphene-based, organic mole-
1
11
research interest in the past decades. In principle, the overall cules, and so on. Despite the signicant interest and consid-
efficiency of a photocatalytic reaction is determined by all three erable efforts paid to date, the current research on
steps: photo-excitation, charge separation and surface reac- photocatalysts is still far from satisfactory in terms of efficiency,
tions; while in most cases, the overall efficiency is critically stabilities and controlling carrier generation, separation and
limited by the charge separation since about 90% of photo- transportation.
generated charge carriers recombine together within very short
Polar materials are featured by the lack of inversion centers
2
time (ꢁps) aer initial photo-excitation. To improve the pho- in the crystal structure. Macroscopic polarization is generated
tocatalytic activity of a photocatalyst, many approaches have when dipolar units are integrated (not compensated) in some
3
12
been adopted, including plasmonic photocatalysts by loading direction. Materials with macroscopic polarization exhibit a
4
5
6
metals (Ag, Au, Cu ) in semiconductor catalysts and multiple variety of special properties, such as piezo-, pyroelectricity and
13
functional heterogeneous photocatalysts such as a z-scheme to second-order nonlinear optical activity. A spontaneous polar-
7
mimic the natural photosynthesis, which can provide trapping ization with directions pointing from the bulk to the surface
+
usually produces a positive charge on the surface (C domain),
and the polarization pointing away from the surface to the bulk
a
Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of
Physics & Chemistry, Key Laboratory of Functional Materials and Devices for Special
Environments, Chinese Academy of Sciences, Urumqi 830011, China. E-mail: xyfan@
ms.xjb.ac.cn; cywang@ms.xjb.ac.cn; Fax: +86 0991 383 8957; Tel: +86 0991 383 5879
ꢀ
14
will generate a negative charge (C domain). The spontaneous
polarization can be screened by free electrons and holes, and/or
by ions or molecules adsorbed on the surface from forming a
1
5
+
b
Stern layer. This accumulation of free electrons on the C
Department of Physics, Changji University, Changji 831100, China
ꢀ
c
Department of Chemistry and Biochemistry, Southern Illinois University, 1245 Lincoln surface and holes on the C surface leads to downward and
Dr Carbondale, IL 62901, USA. E-mail: lwang@chem.siu.edu; Fax: +1 61 8453 6408; upward band bending, respectively. The internal dipolar eld
16
Tel: +1 61 8453 6476
creates charged surfaces that cause photogenerated charge
carriers to move in the opposite direction which separates
†
Electronic supplementary information (ESI) available: Bond valences and dipole
moment calculations, SEM measurements, dechlorinations at NVB under
different pH values, repeated degradation cycles at NVB and NVB XRD pattern
differences with photoreactions. See DOI: 10.1039/c5ta01814e
electron–hole pairs, leading to oxidation and reduction product
17
formation at different locations. Nonlinear optical crystals are
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˚
known as non-concentric materials, however, their efficient c ¼ 12.5697(15) A, which is made of repeated B
6 4
O11, VO units
charge separation and enhanced light absorption as a photo- and Na ions as shown in Fig. 1.
catalyst have rarely been reported.
The basic building units of the NVB are trigonal planar BO3
18
B–O units, such as BO , BO , B O and so on have long been and tetrahedral BO , which form an isolated B O unit. The
3
4
3
6
4
6 11
investigated as nonlinear optical (NLO) materials due to their
B
6
O
11 unit is connected with VO
4
tetrahedron to form innite
19
+
rich structural chemistry. On the other hand, vanadium sheets, whereas Na cations are distributed in the extended
appears to be one of the best alternatives to improve visible framework. The crystal structure of NVB is conrmed by XRD
light-driven photoactivity since it plays an important role in the analysis (Fig. 2a). The UV-vis diffuse reectance spectrum
extending optical absorption region and the high separation (Fig. 2b) indicates that the NVB particles can absorb photons in
20
efficiency of electron–hole pairs. Herein, we demonstrate that wavelength up to 419 nm, and the bandgap of NVB estimated
a simple vanadate–boron semiconductor, Na VO B O (NVB), a from the intercept of the tangent to the inset plot of Fig. 2b is
3
2 6 11
kind of NLO material, can function as an effective photocatalyst 2.95 eV.
for dechlorination under UV-visible light irradiation. Chlor-
ophenols are typical hazardous organic pollutants utilized for
2
.2. Photocatalytic performance of NVB nanoparticles
timber and textile protection worldwide. They are listed as
21
priority contaminants by China and the European Union. In
this work, 2,4-dichlorophenol (2,4-DCP) was selected as a model
pollutant to test the photoactivity of NVB. The dechlorination
efficiency is about 90 times higher than that of the commercial
Experimental runs in the absence of the catalyst with P25 and
NVB under different wavelengths of UV-visible irradiation were
performed separately to evaluate each factor inuencing the
enhancement of photocatalytic degradation by NVB in Fig. 3.
One can observe in Fig. 3a that no obvious degradation took
place in the absence of catalyst under UV-visible light. This is
understandable considering that 2,4-DCP degradation by
oxygen ow is negligible. To explore the photocatalytic activity
of the NVB samples for real applications, the photo-
P25 TiO catalyst. To the best of our knowledge, this is the rst
2
time we report a novel NVB single crystal material as a photo-
catalyst for dechlorination reactions.
2. Results and discussion
2.1. Structure and optical absorption
22
Na
3
VO
2
B
6
O
11
1 1 1
(NVB) crystallizes in the space group P2 2 2
˚
˚
with cell dimensions a ¼ 7.7359(9) A, b ¼ 10.1884(12) A, and
Fig. 2 (a) XRD patterns of the NVB powders. Inset: schematic drawing
of the crystal unit. (b) Ultraviolet-visible diffuse reflectance spectrum
2
Fig.
1
Crystal structures of (a) B
6
O
11 units and (b) VO
4
units. of the NVB sample. Inset: plots of (a ꢂ hn) versus energy (hn) for the
(
c) Drawing of the structure of NVB viewed down the a axis.
samples and the photograph of the photocatalyst.
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higher adsorptive capacity of NVB would be contributed by its
surface charge properties other than the surface area.
The pH value is a complex parameter, since it is related to the
state of the material surface as well as their redox potentials,
which affects the adsorption and reaction of DCP on the
material. The effect of the pH value on the photoreaction was
investigated over NVB. The pH of point of the zero charge
(pHpzc) value was measured to be 4.94 for NVB. When the pH is
near the pHpzc of the semiconductor, most of the surface
hydroxyl groups are in a neutral state; nearly no surface charge
25
exists. Considering that the pK of 2,4-DCP is 7.85, it is in its
a
undissociated form at the tested neutral pH values, and the
surface of NVB is more favorable to the adsorption of 2,4-DCP.
As shown in Fig. S2,† the sample has higher degradation activity
in acidic pH than in alkaline pH solution. At pH 3 (adjusted
with HCl), the NVB surface is positively charged which is more
benecial for adsorption of unionized 2,4-DCP molecules
(Fig. S2a†). A similar enhancement of adsorption in the acidic
environment compared to neutral media was also observed in
25
the case of phenol compounds. At pH 11 (adjusted with
NaOH), which is above the pHpzc of NVB, the photocatalyst
surface is negatively charged. Under such conditions water
molecules can block the photocatalyst surface thus giving rise to
low adsorption ability and the dechlorination activity of
2
,4-DCP (Fig. S2b†). The NVB sample retains the photocatalytic
ꢀ1
activity aer four successive cycles of complete degradation of
Fig. 3 (a) Photocatalytic dechlorination of 2,4-DCP (50 mg L
,
1
00 mL) in aqueous dispersions (containing the NVB catalyst, 50 mg). 2,4-DCP under UV-visible irradiation (Fig. S3†). No peak change
ꢀ
(
b) Formation of Cl as a function of irradiation time during the photo-
dechlorination process under UV-vis light, source light wavelength l >
20 nm.
in the XRD patterns of NVB with the JCPDS Card (no. 52-0421)
was observed before and aer the photodechlorination reaction
3
(Fig. S4†), indicating the stability of the NVB catalyst in the
present photocatalytic reaction process.
dechlorination of 2,4-DCP was tested under natural sunshine.
As shown in Fig. 3a, almost 43.85% of 2,4-DCP can be dech-
2
.3. Calculations and electron transfer mechanism
ꢀ2
lorinated by sunlight (intensity ¼ 0.05–0.08 W cm at noon) A bond valence approach has been adopted to evaluate the local
26
within 60 min, and complete dechlorination can be achieved in dipole moments in NVB.
The calculations for trigonal [BO ] and tetrahedral [BO ] give
3
min under UV-visible light irradiation (intensity ¼ 0.15
ꢀ2
3
4
27
W cm ) (Fig. 3a). During the dechlorination, a yellow color values of 0.277–1.482 D and 0.337–1.428 D, respectively. It is
accumulated in the medium. This color is due to 5-chloro-2- noteworthy that [VO ] polyhedra have a large negative charge
4
hydroxymuconic semialdehyde (CHMS), the via-meta cleavage which maintains a large dipole moment of 5.548 D. A complete
23
product of 4-chlorocatechol as also claimed by Farrell and calculation of dipole moments for the constituted polyhedra is
24
Sahinkaya. An interesting observation is that the CHMS listed in Table S1.† These B–O units possess large electron cloud
concentration increases steadily with the time course of overlapping, and prefer to attract holes and repel electrons.
degrading 2,4-DCP, as is also conrmed by LC-MS data and the Therefore, NVB can be easily polarized, thus facilitating sepa-
released chloride ion concentration. About 82.7% of the total ration of the photogenerated electron–hole pairs. This in turn
ꢀ
chloride content is converted into Cl anions aer 30 min of enhances the photocatalytic activity.
irradiation (Fig. 3b). The particle size of the crystals is in the
To get a deeper understanding of the high photoactivity of
range of 80–150 nm as measured by using a scanning electron NVB, we have performed density functional theory (DFT)
microscope (SEM) (Fig. S1†). Furthermore, the Brunauer– calculations. Fig. 4 presents the electronic band structure of
Emmett–Teller adsorption analysis shows that the specic NVB in terms of density of states (DOS). The VB has strong O 2p
2
ꢀ1
surface area of the NVB is 1.18 m g (in the powder state). contributions, while the CB has V 3d contributions (Fig. 4a).
Such a small area value clearly indicates a high photo-reactivity There are three types of oxygen atoms in NVB: the O1 atoms
of the present NVB samples in nature. In the dark within belong to the bridge between the VO unit and B O unit, while
4
6 11
2
2
0 min, the adsorption reached equilibrium, and about 15% of the O2 atoms form the VO unit, and the O3 atoms belong to the
,4-DCP was adsorbed onto NVB. Under the same conditions, B6O11 unit. The VB top has O 2p contributions, and O1 and O3
4
about 6% of 2,4-DCP was adsorbed onto P25 TiO
2
. Considering atoms have similar contribution values (Fig. 4b). Therefore,
2
ꢀ1
that P25 TiO has a much bigger surface area (58 m g ), the photoexcitation from the VB top to the CB generates holes at the
2
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Fig. 4 The total and projected DOS plots calculated for (a) NVB and (b)
the different O atoms in NVB; (c) a schematic illustration showing the
internal polarized field enhances the charge separation and the pho-
tocatalytic mechanism of NVB, one of the O atoms is labeled by a
purple circle.
Fig. 5 (a) Ball-and-stick diagram of the NVB structure (orange arrows
indicate the direction of the dipole moments for NVB units). (b)
Schematic illustration of a polar structured NVB photocatalyst.
O 2p states in all O atoms, especially in O2. On the other hand,
the CB bottom has V 3d primary contributions and O3 2p
To evaluate the built-in electric eld induced photocatalytic
activity of NVB, the dechlorination of 2,4-DCP was further carried
out in aqueous dispersions under different wavelengths of light
irradiation aer the adsorption of 2,4-DCP onto the NVB (Fig. 3).
When irradiated with wavelengths l > 254 nm, the rate of 2,4-DCP
dechlorination on NVB is nearly 90 times faster than that on P25
6
secondary contributions from the B O11 unit, thus the photo-
generated electrons can be gathered in the B O unit and cease at
6
11
5+
the p-states of the V sites (Fig. 4c). As described in Fig. 4c, with a
polar electric eld in the crystal structure of NVB, photogenerated
charges at the B
circled in purple) to the adjacent VO
electrons and holes are separated at B
move to the adjacent VO unit by means of the bridging O1 whose
6
O
11 unit can migrate through the O1 atoms
unit, and vice versa. When
11 units, electrons can
2
TiO . Under irradiation of photons with wavelengths l > 320 nm
(
4
from a xenon lamp, in the rst 10 minutes no signicant
6
O
dechlorination occurs on P25 TiO , which hardly absorbs photons
2
4
in these wavelength ranges; while NVB exhibits almost the same
photocatalytic activity as that at l > 254 nm. These results indicate
that the enhanced dechlorination of 2,4-DCP is due to the
internal dipolar eld of NVB nano-particles which facilitate the
excited electron–hole separation. Upon irradiation with photo-
energy above the bandgap, electron–hole pairs are formed, which
causes a restructuring of the conduction and valance band
bending. The resultant electrons and holes then migrate to their
most energetically favored corresponding positions in the system,
where they can accumulate, decay to the ground state, or take part
in a chemical reaction (Fig. 5b). Therefore, internal electric eld is
critical to the enhanced activity of NVB.
contribution to the CB is present, while holes can be trapped in
O3 because of their vertical orientation with the direction of the
internal polar eld or maintained at O3. In a similar manner, the
photogenerated holes at B O units can move through the
6
11
bridging O1 to the VO
the O2 atoms of VO units, while the photogenerated electrons
might end up in V 3d at the CB bottom. In general, electrons and
holes are gathered at B 11 units and VO units, respectively,
4
units and eventually be immobilized on
4
6
O
4
separated along the b axis. Furthermore, for the crystallographic
non-centrosymmetry structure of NVB, with the ordered
arrangement of the units in crystal structures, the direction of the
dipole moment is along the [010] direction, an internal polar eld
is veried to exist and its orientation is shown in Fig. 5a. From the
crystal structure of NVB, the direction of the polar eld is in
2.4. Internal electric eld effect
4
accordance with that of charge transfer in VO , the internal polar
eld promotes their separation along the eld direction in the bc The internal electric eld effect was also studied with a nano-
plane (Fig. 5a), which is expected to function as a driving force for scale resolution by atomic force microscopy (AFM) in conjunc-
the photoexcited holes and electrons to move to their respective tion with Kelvin probe force microscopy (KPFM). In particular,
reduction and oxidation sites. Under the action of the internal KPFM allows quantitative mapping of the electronic properties
polar eld, the photoinduced electron and hole would thereby of nanostructures, i.e., determination of the surface potential of
28
transfer along opposite directions, and facilitate the charge nano-objects. Fig. 6a and c show an AFM topography image of
separation, thus resulting in high photocatalytic activity.
NVB nanoparticles, and Fig. 6b and d show surface potential
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2.5. Enhancement of PC and PEC activity
In order to further conrm that the photon-to-electron conver-
sion efficiency of the NVB particles is affected by the non-centric
structure, the photoelectron chemical (PEC) measurements of
30
the NVB photoanodes have been conducted. The transient
photocurrent responses of P25 TiO and NVB electrodes were
2
recorded via several on–off cycles of irradiation.
It can be seen from Fig. 7a that a prompt generation of
photocurrents occurs with good reproducibility when sample
NVB is irradiated by using UV-visible light. While the light is off,
the photocurrent for sample NVB decreases instantaneously.
This indicates that under light irradiation, most of the photo-
excited electrons at the NVB surface due to internal electric eld
are transported towards the desired reactions. When the bias
potential is 0 V, the anodic photo-current of NVB is about
3.2 times that of P25 TiO2 under the simulated UV-vis light
irradiation. As the bias potential increases, the photocurrents
are enhanced by 1.3, 1.7 and 1.9 times at 0.033, 0.067 and 0.134 V
compared to 0 V, respectively, for the NVB electrode as shown in
Fig. 7b. This suggests any photocurrent enhancement observed
for NVB is more signicant at lower potentials which contributes
to its catalytic ability towards organic pollutant degradation.
Photoelectrochemical impedance spectroscopy (PEIS) anal-
ysis was employed to investigate the charge transfer at the
Fig. 6 AFM images of NVB before (a) and after (c) irradiation and
corresponding surface potential images of NVB before (b) and after
(
(
subtracting the potential value of panel (d) from panel (c) in the white
circled area.
31
d) irradiation. (e) Potential value curve for panel (b) and panel (d). semiconductor interface. A Nyquist plot for NVB and P25 TiO2
f) Surface potential profile along the black line in the inset of (f), by measured at 0.067 V vs. Ag/AgCl under different wavelength
images of NVB particles in the dark and under irradiation. The
surface potential image has been separated into darker and
brighter regions. The contrast is due to the differences in
surface potential for the domains of respective donor rich and
acceptor rich of the material.
In order to quantitatively determine surface potential
differences, the different potential value for Fig. 6b and d of the
NVB is analyzed in Fig. 6e. Upon irradiation with a xenon lamp
light source, the surface potential of the NVB becomes more
positive and the average value increases by about 14.09 mV.
This overall increase in surface potential is due to the lling of
electrons in the conduction band during the light excitation
process. And the cross-section proles with the same area
circled in a white square frame in Fig. 6b and d is shown in
Fig. 6f, from which the surface potential difference between
different faces due to the charge separation is deduced to be
ꢁ
30 mV. The difference is attributed to the light response for
different domains of the material, and aer light irradiation
the charge separation is driven by the internal electric elds.
This result provides direct evidence that the presence of the
internal eld provides a driving force for the separation of
29
photogenerated electrons and holes.
These properties
strongly depend on that the crystal form for NVB is a non-
centric crystal. Thus, the KPFM results demonstrate that the
excellent electron–hole separation of NVB can be signicantly
improved through the enhancement in charge injection and
extraction.
Fig. 7 (a) P25 TiO
xenon lamp (150 mW cm ) by the removal of 2,4-DCP (with an initial
concentration of 50 mg L ) in 0.1 M Na
0 V, 0.033 V, 0.067 V and 0.134 V of NVB vs. Ag/AgCl.
2
and NVB light on–off experiment using a 300 W
ꢀ2
ꢀ1
2 4
SO solution. (b) Electrode at
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irradiations is shown in Fig. 8. With wavelength l > 320 nm light intermediates as well as end products are less toxic than that
irradiation, the impedance arcs of the two electrodes are large, of 2,4-DCP and can be readily biodegraded into harmless
indicating that the electrons going through the electrode and compounds by microorganisms under both aerobic and
3
4
electrolyte interface are few. However, arcs obtained from NVB anaerobic conditions.
have much smaller radius than from P25 TiO , which indicates The enhanced photo-dechlorination of chlorinated
2
that NVB has a more effective separation of photogenerated compounds by NVB is illustrated in Scheme 1. The NVB pho-
electron–hole pairs resulting in faster interfacial charge trans- tocatalysts can be photo-excited by UV-visible (l > 320 nm) light
32
fer. Under UV-vis light irradiation (l > 254 nm), the arc radius irradiation to generate electron–hole pairs, while the strong
on the EIS Nyquist plot of NVB is also smaller than that of P25 built-in electric eld in the NVB structure inhibits the recom-
TiO . It can be deduced that UV-vis light irradiation facilitates bination of holes with electrons, leading to the enhancement of
2
photogenerated hole transfer from the valence band of NVB to the oxidizing capability of NVB. This process would signicantly
the solution, reducing the recombination of photogenerated accelerate the simultaneous photodechlorination rates of
electron–hole pairs and improving the photocurrent density. chlorinated compounds.
The efficiency of NVB is higher compared to that of P25 TiO
2
To further explore the photocatalytic reaction pathway of
electrodes. This could be attributed to the improved electrode– NVB, the formation of active hydroxyl radicals (cOH) upon
electrolyte interfacial area due to the presence of polarity in NVB irradiation was monitored, which are generally considered as
arisen from its noncentric symmetry structure.
the most important oxidative intermediate in photocatalytic
35
reactions. Terephthalic acid (TA) was used as a uorescence
probe which can react with cOH in basic solution to generate
2-hydroxy terephthalic acid (TAOH), emitting a unique uores-
2.6. Possible reaction mechanism for DCP
photodechlorination
36
cence signal at around 426 nm.
3
3
As reported by Doong et al. , the reaction pathway for 2,4-DCP
dechlorination could involve adsorption, dechlorination and
cleavage of the benzene ring and several intermediates
including 2-chlorophenol, 4-chlorophenol, phenol, and
chlorocatechol. To elucidate the reaction pathways of DCP by
NVB, LC-MS was used to identify the intermediates and end
products. Degradation intermediates including 2-chlorohy-
droquione, 4-chlorophenol and catechol were identied,
while 1,4-benzoquinone and catechol were detected as the
end products during the course of 2,4-DCP photo-
dechlorination (see ESI Fig. S5 and Table S2† for details).
Therefore, a possible reaction pathway for 2,4-DCP dechlori-
nation by NVB upon UV-vis light irradiation is proposed
As shown in Fig. 9, intense uorescence signals associated
with 2-hydroxy terephthalic acid (TAOH) are generated upon
irradiation of NVB suspended in terephthalic acid (TA) solution
within 320–780 nm with different irradiation durations. The
quasi-linear relationship between uorescence intensity and
irradiation time (inset of Fig. 9) conrms the stability of NVB.
Taking all together, the dechlorination processes of 2,4-DCP at
NVB are proposed in eqn (1)–(6) as follows:
ꢀ
+
NVB / NVB (e + h )
(1)
(2)
(3)
(4)
+
+
h + H O (surface) / cOH (surface) + H
2
ꢀ ꢀ
2 2
e + O (surface) / O c
(
Fig. S6†). As shown in Fig. S6,† 2,4-DCP can react with photo-
generated hydroxyl radicals to form 2-chlorohydroquinone
and then 1,4-benzoquinone. In addition, 2,4-DCP can
undergo reductive dechlorination to generate 4-chlorophenol
ꢀ
O c + H / HOOc / cOH
2
+
cOH + C
H
6 4
Cl
2
O (2,4-DCP) /
ClO

rst and then reacts with hydroxyl radicals, leading to the
C
6
H
5
2
(2-chlorhydroquinone)
(5)
(6)
formation of catechol and phenol. Although 2,4-DCP cannot
be completely mineralized by NVB, the degradation
ꢀ
e
+ C
H
6 4
Cl
2
O (2,4-DCP) /
C H ClO (4-chlorophenol) + Cl
ꢀ
6
4
Scheme 1 The enhanced photodechlorination efficiency of 2,4-DCP
2
and NVB electrodes under UV by NVB materials upon irradiation of UV-visible light (l > 320 nm) under
Fig. 8 EIS Nyquist plots of the P25 TiO
and UV-visible-light irradiation. The bias potential is 0.067 V.
anoxic conditions.
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Quantachrome Autosorb-2 apparatus at 77.35 K. The concen-
tration of chlorophenol was measured by UV-vis spectroscopy
(Shimadzu UV-1800) and using a high-performance liquid
chromatograph (Ultimate 3000; Dionex) with a Supelco PAH
column (4.6 ꢂ 250 mm). A mixture of acetonitrile, water, and
acetic acid [60/39.75/0.25 (v/v/v)] was used as an effluent, and
ꢀ
1
the ow rate was 1.0 mL min . The intermediate products
during 2,4-DCP degradation were qualitatively analyzed by a
liquid chromatography-mass spectrometer (LC-MS, Agilent
1290). The size of the sample loop was 20 mL. The wavelength of
the detector was set at 282 nm. To determine the concentration
ꢀ
of chloride ions (Cl ), an ion chromatograph (ICS2000, Dionex,
Fig. 9 Hydroxyl radical cOH trapping photoluminescence (PL) spectra
of NVB in TA solution under UV-visible-light irradiation. Inset: the plot
of the induced PL intensity (at 426 nm) vs. irradiation time.
Germany) equipped with an analytical column (4 mm ꢂ 250
mm) and a guard column (4 mm ꢂ 50 mm) was used
2 3
throughout the experiment. The eluent was 3.5 mM Na CO and
ꢀ1
1.0 mM NaHCO
3
with a ow rate of 1.0 mL min .
Atomic force microscopy (AFM) combined with Kelvin probe
force microscopy (KPFM) measurements were performed at
room temperature with an atomic force microscope (AFM)
3
. Experimental
3.1. Synthesis of the NVB nanoparticles
(Bruker Multimode 8). Every scan took approximately 20 min
Nanoparticles of NVB were synthesized by a high temperature
from top to bottom, which is the time scale to refer to in this
experiment. Images were obtained in the tapping mode with a
scan rate of 1 Hz by using conductive Pt–Ir tips (SCM-PIT) with a
resonance frequency of 60–100 kHz and an AC mode for a scan
size of 1 mm ꢂ 1 mm. In the experiment, NVB nanoparticles were
solid-state reaction method. Stoichiometric amounts of Na
2
CO
Tianjin Benchmark Chemical Reagent Co., Ltd, 99.8%), V
Shanghai Shanpu Chemical Co., Ltd, 99.5%), and H BO
3
5
3
(
(
(
2
O
3
Tianjin Baishi Chemical Industry Co., Ltd, 99.5%) were ground
together and then placed in an alumina crucible and heated at

rst dispersed in water and then sprayed on a Pt substrate to
ꢃ
6
00 C for 10 h in air in a tube furnace followed by natural
form a lm. Before measurements, the sample was baked in a
ꢃ
cooling.
dry oven at 400 C for 2 h. For the light irradiation experiment, a
xenon lamp with a wavelength (l) of >320 nm and a convex lens
with a focusing spot size (d) of 1.5 cm was used to introduce
focused UV light onto the measurement region between the tip
of the atomic force microscope and sample.
3
.2. Photoactivity evaluation
Photocatalytic activity tests for all samples were carried out at
room temperature. During the experiment, 50 mg of the NVB
sample was dispersed in 100 mL of 2,4-DCP solution (50 mg
The photocurrents of UV-vis light on and off studies were
performed on a CHI660E electrochemical system (Shanghai,
China) using a standard three-electrode cell with a working
electrode (20 mm ꢂ 20 mm), a platinum plate as a counter
electrode, and a standard calomel electrode (SCE) as a reference
electrode. PEC measurements were performed using a three-
electrode conguration by the removal of 2,4-DCP (with an
ꢀ
1
L
) in a reactor and stored for 20 min with stirring in the dark
to attain adsorption equilibrium. The reactor was then illumi-
nated by using a 300 W xenon lamp with a wavelength range of
3
Samples were withdrawn periodically from the reactor. The
percentage of residual 2,4-DCP solution at a selected time of
irradiation is given by C/C
ꢀ
2
20–780 nm (the intensity at the test samples ¼ 0.15 W cm ).
ꢀ
1
initial concentration of 50 mg L ) in an aqueous solution
containing 0.1 M Na SO . The transient photocurrent responses
0 0
, where C is the concentration of the
2
4
2,4-DCP solution at the initial stage, i.e., right before irradia-
of P25 TiO and NVB electrodes were recorded via several on–off
2
tion, and C is the concentration at selected irradiation times of
time ¼ 0, 1, 2, 3, 4 and 5 min. For comparison, parallel exper-
iments were conducted with P-25. Adopting the same proce-
dures, a 500 W mercury lamp with wavelength l > 254 nm was
also employed for comparison.
cycles of irradiation. Electrochemical impedance spectroscopy
(EIS) measurements carried out by using an electrochemical
workstation (Zahner Im6ex) were conducted for the working
electrode in a frequency range of 100 kHz to 0.01 Hz with an ac
signal amplitude of 5 mV at open circuit potential in different
aqueous electrolytes.
3.3. Characterization
Fluorescence spectra of 2-hydroxyterephthalic acid were
Phase identication was performed on a Bruker D8 ADVANCE measured on a Hitachi uorescence spectrophotometer F-7000.
X-ray diffractometer equipped with a diffracted-beam mono- The cOH radical trapping experiments were carried out using
˚
chromator set for Cu Ka radiation (l ¼ 1.5418 A). The the following procedure: terephthalic acid (TA) (16.6 mg) was
ꢀ
3
morphologies of the samples were observed on a scanning rst dissolved in 200 mL of dilute NaOH solution (2 ꢂ 10 M),
electron microscope (SEM) using a ZEISS SUPRA55VP appa- followed by the addition of 100 mg of photocatalysts, and stir-
ratus. UV-visible diffuse reectance spectra were recorded on a red for 30 min in the dark. The suspension was irradiated by
Shimadzu UV-3600 UV-vis spectrophotometer using BaSO
4
as a using a 300 W xenon lamp at wavelengths in the range of
reference. BET measurements were conducted by using a 320–780 nm for 30 min. The uorescence emission spectrum of
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the solution was measured every 5 min during irradiation. At a 201401) of Chinese Academy of Sciences, the “CAS Action Plan
dened time interval, the concentration of solution in the for the Development of Western China” (no. KGZD-EW-502), the
system was analyzed by PL (excited at 312 nm).
National Nature Science Foundation of China (no. 21373267,
1173261), the “One Hundred Talents Project Foundation
2
Program” of Chinese Academy of Sciences, the CAS/SAFEA
International Partnership Program for Creative Research
Teams, the “Youth Technology Innovation Talents Culture
Engineering” of Xinjiang Uygur Autonomous Region of China
3
.4. Computational details
The rst-principles calculations were performed in the frame-
work of functional theory with the projector augmented wave
37
(PAW) pseudopotential method using Vienna Ab initio Simu-
(no. 2013721045), and the “Cross-Cooperation Program for
38
lation Package (VASP). The generalized gradient approxima-
tion (GGA) in the scheme of the Perdew–Burke–Ernzerhof
Creative Research Teams”.
39
40
(PBE) was used for the exchange-correlation functional. The
Notes and references
electronic wave functions were expanded into a basis set of
plane waves with a kinetic energy cutoff of 400 eV, and a
Monkhorst–Pack k-point mesh of 3 ꢂ 2 ꢂ 2 was used for
geometry optimization and electronic property calculations,
which was found to be sufficient to reach convergence for bulk
super cell calculations. The PAW potentials with the valence
states 2s and 2p for B and O, 3s and 2p for Na, 3p, 4s, and 3d for
V, were employed. Both the atomic positions and cell parame-
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each ion converged to be smaller than 0.02 eV A˚ , and the
convergence threshold for self-consistence-eld iteration was
set at 10
employed as the starting points of relaxation, and subsequent
calculations were conducted using the relaxed atomic positions.
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Acknowledgements
This work was supported by the International Science & Tech-
nology Cooperation Program of Xinjiang Uygur Autonomous
Region (no. 20146005), the “Western Light” Program (no. YBXM
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