Room-temperature preparation of MIL-68 and its derivative In2S3 for enhanced photocatalytic Cr(VI) reduction and organic pollutant degradation under visible light

Article abstract of DOI:10.1016/j.jallcom.2020.155567

MIL-68, a typical In-based MOF, has been studied in many fields due to its excellent performance. Facile preparation of MIL-68, suitable for scalable preparation and industrial applications, is of great significance. In this work, a method for the room-temperature preparation of rod-like MIL-68 at the nano- and micro-scales was developed for the first time, in which water or salts such as NaF, sodium formate, sodium acetate and sodium propionate were used as modulating reagents. It appears that these modulating reagents can promote the deprotonation of terephthalic acid and the hydrolysis of indium salt to accelerate crystal nucleation. The size of MIL-68 can be controlled by changing the modulating reagents. Hollow porous In₂S₃ particles composed of assembled ultrathin nanosheets were obtained via sulfidation treatment using MIL-68 as a self-sacrifice template, and the obtained In₂S₃ exhibited excellent photocatalytic activity toward Cr(VI) reduction and methyl orange degradation under LED visible light irradiation. Furthermore, the photocatalytic mechanism and reusability were studied.

Full text of DOI:10.1016/j.jallcom.2020.155567

Journal of Alloys and Compounds 837 (2020) 155567
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Room-temperature preparation of MIL-68 and its derivative In S for
2
3
enhanced photocatalytic Cr(VI) reduction and organic pollutant
degradation under visible light
*
Huifen Fu , Lin Wu , Jing Hang , Peng Wang , Chen Zhao , Chong-Chen Wang
Beijing Key Laboratory of Functional Materials for Building Structure and Environment Remediation, Beijing Advanced Innovation Centre for Future Urban
Design, Beijing University of Civil Engineering and Architecture, Beijing, 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 January 2020
Received in revised form
MIL-68, a typical In-based MOF, has been studied in many fields due to its excellent performance. Facile
preparation of MIL-68, suitable for scalable preparation and industrial applications, is of great signifi-
cance. In this work, a method for the room-temperature preparation of rod-like MIL-68 at the nano- and
micro-scales was developed for the first time, in which water or salts such as NaF, sodium formate,
sodium acetate and sodium propionate were used as modulating reagents. It appears that these
modulating reagents can promote the deprotonation of terephthalic acid and the hydrolysis of indium
salt to accelerate crystal nucleation. The size of MIL-68 can be controlled by changing the modulating
10 April 2020
Accepted 8 May 2020
Available online xxx
Keywords:
MIL-68
reagents. Hollow porous In
2
S
3
particles composed of assembled ultrathin nanosheets were obtained via
exhibited excellent
sulfidation treatment using MIL-68 as a self-sacrifice template, and the obtained In S
2 3
In
2 3
S
Photocatalysis
photocatalytic activity toward Cr(VI) reduction and methyl orange degradation under LED visible light
irradiation. Furthermore, the photocatalytic mechanism and reusability were studied.
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
[11e13], such as photocatalysis [14,15], adsorption [16,17], gas
separation and storage [18,19]. Some MOFs are reported to display
outstanding photocatalytic performance; however, there are some
limitations to be overcome. First, most MOFs possess micropores,
not mesopores, which are not useful in approaches targeting pol-
lutants, especially organic pollutants with large sizes. Second, the
stability of most MOFs in water is poor, which could cause sec-
ondary pollution because MOFs are built from organic ligands.
The adoption of MOFs as self-sacrificing templates to construct
porous derivatives has received widespread attention [20,21]. For
The problem of organic pollutants and heavy metals in water
has attracted worldwide attention because these pollutants are a
great source of risk to human well-being [1,2]. Photocatalysis, a
sustainable technology using sunlight as an energy source, is a
potential technology to remove pollutants from water [3,4]. How-
ever, developing advanced photocatalysts with excellent perfor-
mance remains a challenge. Porous materials are becoming a hot
topic in photocatalysis due to their abundant pores, large specific
surface areas and many active sites, which could facilitate
improvement of photocatalytic activity [5e7]. The hard-template
method is a common strategy to synthesize porous materials
example, Li’s group [22] successfully prepared PteZnOeCo
3 4
O ,
PteZnSeCoS and PteZn eCoP nanocomposites using ZnCo-ZIF
3 2
P
particles as precursors, and these composites presented excellent
performance toward photocatalytic hydrogen generation. However,
MOFs are generally prepared using the hydrothermal/solvothermal
method [23e25], in which some unique reactors are needed, and
this method is not conducive to large-scale and high-throughput
production. There is a very urgent need to find an inexpensive
preparation method that can be implemented at room temperature
to facilitate scalable preparation and commercial applications of
MOFs.
[
8e10], and this method has the disadvantage of frequently
requiring multiple complicated steps.
Metal-organic frameworks (MOFs), a kind of porous crystalline
material constructed from metal ions/clusters and organic linkers,
possess great advantages such as porosity and tunable chemical
functionality. MOFs have drawn great attention in many fields
MIL-68, a typical In-based MOF built from In3 and terephthalic
þ
*
Corresponding author.
E-mail address: chongchenwang@126.com (C.-C. Wang).
https://doi.org/10.1016/j.jallcom.2020.155567
925-8388/© 2020 Elsevier B.V. All rights reserved.
0

2
H. Fu et al. / Journal of Alloys and Compounds 837 (2020) 155567
acid (H
using the solvothermal method, Wu’s group [28] prepared NH
mediated MIL-68 particles which exhibited considerable activity for
Cr(VI) reduction. Cheng’s group [29] prepared MIL-68-NH /gra-
2
BDC), has been widely investigated in many fields [26,27]. By
2
particles derived from MIL-68 prepared using H O, sodium
formate, sodium acetate, sodium propionate and NaF as modulating
reagents were named S1, S2, S3, S4 and S5, respectively.
2
-
2
phene oxide composite with good photocatalytic activity toward
amoxicillin degradation. Currently, the derivatives of MIL-68 have
2.5. Characterization
received increasing attention [30]. For example, Wang’s group [31]
X-ray diffraction (XRD) patterns were recorded on a Dandong-
haoyuan DX-2700B diffractometer using Cu Ka radiation. The
II
prepared tube-like In O /Co O
2 3 3 4
using Co -impregnated MIL-68 as a
template, which exhibited excellent sensing performance toward
triethylamine. Fang and coworkers [32] fabricated hollow In by
sulfidation of MIL-68, and the hollow In exhibited high photo-
catalytic activity for the degradation of tetracycline hydrochloride
and methyl orange (MO). MIL-68 was always synthesized by a sol-
morphologies of the samples were observed using scanning elec-
tron microscopy (SEM, SU8020), transmission electron microscopy
(TEM, JEM1200EX) and high-resolution transmission electron mi-
croscopy (HRTEM, FEI Tecnai G2 F30). Textural characteristics and
specific surface areas were characterized using an ASAP 2460
analyzer. The UVeVis diffuse-reflectance spectra (UVeVis DRS)
were obtained on a PerkinElmer Lambda 650S spectrometer. Elec-
2 3
S
2 3
S
vothermal method or oil bath at a relatively high temperature
ꢀ
(
>90 C) [32,33]. To the best of our knowledge, no room-temperature
0
preparation method for MIL-68 has been reported. In this work, rod-
like MIL-68 at the nano- and micro-scales was prepared at room
temperature using H O or salts such as NaF, sodium formate, sodium
2
acetate and sodium propionate as modulating reagents. This method
is good for the scalable preparation and commercial applications of
MIL-68, and we believe that this preparation strategy can be applied
tron spin resonance (ESR) signals of the radicals trapped by 5,5 -
dimethyl-1-pyrroline N-oxide (DMPO) were detected using a JEOL
JES-FA200 spectrometer. Mott-Schottky plots were measured on a
Metrohm Autolab PGSTAT204 electrochemical workstation using a
three-electrode system with In S -coated FTO glass, Pt plate and
2 3
Ag/AgCl as the working electrode, counter electrode and reference
electrode, respectively.
to other MOFs. Furthermore, porous In
2
S
3
was obtained using MIL-
exhibi-
6
8 as a self-sacrificing template, and the as-prepared In S
2 3
ted excellent photocatalytic activity toward Cr(VI) reduction and MO
degradation under visible light.
2.6. Photocatalytic experiment
First, the pH of the treated Cr(VI) solution was adjusted to 6.0.
2
. Experimental section
2 3
Then, 10 mg as-prepared In S was dispersed in 50 mL Cr(VI) so-
lution with the help of ultrasonication. After stirring in the dark for
1 h to achieve adsorption-desorption equilibrium, the suspension
was irradiated under a 350 mW LED visible light source (PCX50A,
Beijing Perfect Light Technology Co., Ltd.). At a set interval, 1.5 mL
solution was removed and centrifuged, and the clear supernatant
was measured by the diphenylcarbazide method using Auto
Analyzer 3 (Seal, Germany). The process of MO photodegradation is
similar to that of Cr(VI) photoreduction, and the residual MO con-
centration was determined on a Laspec Alpha-1860 UVeVis
spectrometer.
2
.1. Materials
All chemicals were commercially available and used without
3 3 2 2
further purification. In(NO ) $xH O and H BDC were purchased
from Aladdin Reagent Co. Ltd. Thioacetamide (TAA) and p-benzo-
quinone (BQ) were purchased from J&K Scientific Ltd. NaF, sodium
acetate and t-butanol (TBA) were purchased from Beijing Chemical
Works. Sodium formate was purchased from Tianjin Guangfu Fine
Chemical Industry Research Institute. Sodium propionate was
purchased from Shanghai Sanaisi Reagent Co., Ltd.
3. Results and discussion
2
.2. Room-temperature preparation of MIL-68 using H
2
O as the
modulating reagent
3 3 2
MIL-68 was always prepared using In(NO ) and H BDC as
precursors and DMF as the solvent via an oil bath or solvothermal
routine [32,33]. In this study, rod-like MIL-68 was obtained at room
H
2
BDC (2 mmol) and In(NO
5 mL DMF and 5 mL H O, respectively. Subsequently, the aqueous
solution of In(NO $xH O was added to the DMF solution con-
taining H BDC at room temperature under stirring. After 24 h, the
3 3 2
) $xH O (2 mmol) were dissolved in
2
2
temperature by adopting a DMF/H
solvent (Fig. 1aeb). However, no significant precipitate was ob-
tained when only DMF was used as the solvent, indicating that H
might trigger the reaction between In(NO and H BDC at room
temperature. Additionally, to produce MIL-68 at room temperature,
a DMF/H O mixture rather than H O alone was used as the solvent
because H BDC cannot be sufficiently dissolved in water. It can be
2
O (V/V, 25/5) mixture as the
3
)
3
2
2
2
O
precipitate was obtained after centrifugation and washed with
ethanol twice.
3
)
3
2
2
2
2
.3. Room-temperature preparation of MIL-68 using salts as
2
modulating reagents
seen from Fig. 1b that the prepared MIL-68 presented a rod-like
shape, which was similar to the MIL-68 prepared using the re-
ꢀ
NaF was added to 25 mL DMF containing 2 mmol H
2
BDC. Sub-
ported method at 90 C (Fig. S1). This revealed that H
2
O could
sequently, 5 mL DMF containing 2 mmol In(NO
3
)
3
$xH
2
O was added
accelerate the formation of MIL-68 and exert a minor influence on
its morphology and size. Wang’s group [34] reported that a series of
zirconium-metalloporphyrinic MOFs were successfully synthesized
under stirring. After 24 h, a precipitate was obtained after centri-
fugation and washed twice with ethanol. The targeted products can
also be obtained when NaF is replaced by sodium formate, sodium
acetate and sodium propionate.
by introducing H
agents, in which H
2
O and monocarboxylic acid as modulating re-
O was considered to accelerate the hydrolysis of
2
zirconium salt and facilitate the deprotonation of acid to promote
crystal nucleation. Subsequently, they synthesized zirconium- and
hafnium-based MOF nanocrystals under mild conditions in the
presence of water and acetic acid [35]. In our case, it appears that
2
.4. Preparation of porous hollow In
2 3
S
First, 0.5 g as-prepared MIL-68 was dispersed in 75 mL ethanol
containing 2 g TAA, which was transferred to a 100 mL Teflon-lined
stainless-steel Parr bomb and heated at 120 C for 8 h. After cooling
H
2
O can promote the deprotonation of terephthalic acid and the
hydrolysis of indium salt to accelerate crystal nucleation. Consid-
ering that the formula of MIL-68 is In(OH) (O CeC eCO )$
ꢀ
to room temperature, porous hollow In
S
2 3
was obtained. The In
S
2 3
2
6
H
4
2

H. Fu et al. / Journal of Alloys and Compounds 837 (2020) 155567
3
2
Fig. 1. (a) XRD patterns and (bef) SEM images of MIL-68 prepared with the aid of (b) H O, (c) sodium formate, (d) sodium acetate, (e) sodium propionate and (f) NaF.
ꢁ
3þ
eCO-
1
.0DMF$zH
2
O [36], it can be speculated that H
2
O can facilitate the
H
2
BDC to form BDC2 (-O
2
CeC
6
H
4
2
) and further accelerate the
3
þ
ꢁ
2ꢁ
combination of In and OH and further promote the formation of
MIL-68.
As depicted in Fig. 1aef, rod-like MIL-68 was successfully pre-
pared using only DMF as the solvent at room temperature when
NaF was added as the modulating reagent, indicating that NaF can
also result in the quick formation of MIL-68 at room temperature. It
reaction between In and BDC . It was reported that DMF can be
decomposed under solvothermal conditions to yield dimethyl-
amine, which acts as a Brønsted base, leading to the deprotonation
of H
BDC or metal-BTC composites [37,38]. At room temperature, H
cannot be deprotonated by DMF alone, while the addition of NaF
promoted the deprotonation of H BDC, accelerating the formation
2
BDC or H
3
BTC and finally resulting in the formation of metal-
2
BDC
ꢁ
could be speculated that F could promote the deprotonation of
2
Fig. 2. (a) XRD patterns of S1, S2, S3, S4 and S5. SEM images of (b) S1, (c) S2, (d) S3, (d) S4 and (f) S5.

4
H. Fu et al. / Journal of Alloys and Compounds 837 (2020) 155567
Fig. 3. TEM and HRTEM images of (a) S1, (b) S2, (c) S3, (d) S4 and (d) S5.
of MIL-68. Therefore, MIL-68 can be obtained under mild condi-
tions at room temperature with the help of NaF. As shown in Fig. 1a,
all diffraction peaks of MIL-68 prepared in the presence of NaF
which is imperative to advance the development of MIL-68.
As shown in Fig. 2a, the perfect match of characteristic XRD peaks
of In
MIL-68 particles were completely transformed into In
seen from Fig. 2bef and Fig. 3 that all In exhibited hollow struc-
tures. Moreover, all these In particles were composed of ultrathin
S
2 3
and the absence of XRD peaks of MIL-68 revealed that these
ꢁ
matched well with the simulated results [36], implying that F
2 3
S . It can be
3
þ
could not influence the coordination environment of In in MIL-
8. This was further confirmed by EDS determination, in which
2 3
S
6
2 3
S
no F was detected in MIL-68 (Fig. S2).
In addition to NaF, three other alkali salts, sodium formate, so-
dium acetate and sodium propionate, were selected as modulating
reagents to prepare MIL-68 at room temperature. It can be
observed from Fig. 1ae1c-1e that rod-like MIL-68 materials were
also obtained, which affirmed that these salts played important
roles in the room-temperature synthesis. It is worth noting that the
sizes of the three MIL-68 prepared in the presence of sodium
formate, sodium acetate, and sodium propionate were smaller than
the size of the MIL-68 prepared with the help of NaF, which may be
attributed to the carboxyl group (-COOH) in the three modulating
reagents (sodium formate, sodium acetate, sodium propionate)
bonding on the specific surface of MIL-68, hindering the growth of
the crystal plane and leading to the smaller size [39]. This room-
temperature preparation of MIL-68 can be facilely accomplished
in beakers rather than special reactors, such as high-temperature
and high-pressure reactors, and large-scale synthesis can be
easily realized by using larger beakers or other general reactors,
2 3
Fig. 5. UVeVis spectra and plots of (ahv) [2] versus photoenergy of different In S .
2 2 3
Fig. 4. (a) N adsorption-desorption isotherms and (b) pore size diameter distributions of different as-prepared In S samples.

H. Fu et al. / Journal of Alloys and Compounds 837 (2020) 155567
5
Fig. 6. (a) Photocatalytic conversion curves and (d) kinetic constants of different photocatalysts for Cr(VI) reduction. Reaction conditions: photocatalyst ¼ 10 mg, Cr(VI) ¼ 10 mg/L,
0 mL, pH ¼ 6.0. (b) Photocatalytic conversion curves and (e) kinetic constants of S5 toward Cr(VI) reduction in lake/rain/tap/pure water system, (c) photocatalytic conversion
curves and (f) kinetic constants of S5 toward Cr(VI) reduction in the presence of different scavengers. Reaction conditions: photocatalyst ¼ 10 mg, Cr(VI) ¼ 20 mg/L, 50 mL, pH ¼ 6.0.
5
nanosheets, which was beneficial for increasing the specific surface
area. During the formation process of In , TAA may first react with
the superficial In3 of MIL-68 to form nanosheet-like In
on the
surface of MIL-68, producing an In @MIL-68 core-shell structure.
Then, BDC2 ligands could be released and result in empty spaces
The optical properties were measured by UVeVis DRS, and the
values were calculated according to Eq. (1).
S
2 3
E
g
þ
2 3
S
S
2 3
ahv ¼ K(hv-E
g
)1/n
(1)
ꢁ
2ꢁ
3þ
originally occupied by BDC in MIL-68. Subsequently, the In in-
side would diffuse outward to react with TAA to form In . Over
time, porous hollow In particles composed of assembled ultrathin
nanosheets were finally formed.
adsorption-desorption isotherms and pore size distributions
are shown in Fig. 4. All In samples exhibited type IV N
adsorption-desorption isotherms, revealing the mesoporous
structures [40]. The pore diameters of these In were distributed
where a, h, v and K are the diffuse absorption coefficient, Plank
constant, light frequency and a constant, respectively. n is deter-
mined by the type of optical transition of a semiconductor. For the
2 3
S
2 3
S
2 3
direct bandgap of In S , the value of n is 2. It can be seen from Fig. 5
N
2
that the E values of S1, S2, S3, S4 and S5 are 2.41, 2.43, 2.38, 2.36
g
2
S
3
2
and 2.44 eV, respectively, indicating that these In
2
S
3
could display
photocatalytic performances under visible light.
2
S
3
The photocatalytic performances of these In S toward Cr(VI)
2 3
in a wide range from 2 nm to 50 nm. The specific surface areas of S1,
S2, S3, S4 and S5 are 122.7, 170.4, 207.2, 133.9 and 186. 9 m /g,
reduction at pH 6.0 were estimated under visible light irradiation.
First, the adsorption abilities of In samples toward Cr(VI) were
tested in the dark, and Fig. S3a shows that all the porous In
photocatalysts accomplished <20% Cr(VI) removal after adsorption-
2
2 3
S
respectively, which are much higher than those of other In
samples reported in the literature [41,42].
2
S
3
2 3
S
Fig. 7. ESR spectra of (a) ꢂOH and (b) ꢂO2ꢁ active radicals trapped by DMPO on S5.

6
H. Fu et al. / Journal of Alloys and Compounds 837 (2020) 155567
Fig. 8. (a) Photocatalytic conversion curves and (c) kinetic constants of S5 toward MO degradation in different systems, (b) photocatalytic conversion curves and (d) kinetic
constants of S5 toward MO degradation in the presence of different scavengers. Reaction conditions: photocatalyst ¼ 10 mg, MO ¼ 10 mg/L, 50 mL.
desorption equilibrium within 1 h. The control experiment man-
ifested that no obvious self-photoreduction was observed under
visible light irradiation (Fig. S3b). However, nearly 100% Cr(VI)
photoreduction efficiencies were achieved within 30 min with S1, S2,
S3 and S5 as photocatalysts (Fig. 6a and d). S5 exhibited the best
photocatalytic activity toward Cr(VI) reduction, followed by S1, S3, S2
and S4. As reported, the photoreduction activity of Cr(VI) decreased
with increasing pH and generally reached its highest value at pH ¼ 2
photoreduction efficiency in natural water than in ultrapure water.
Moreover, the different photoreduction efficiencies in different
natural water systems could be attributed to the different inhibi-
tory effects and promotional effects.
It was reported that the active species for photocatalytic Cr(VI)
ꢁ
ꢁ
reduction were generally e and $O
2
[52e55]. To understand the
ꢁ
photocatalytic mechanism, BQ, a trapping agent of $O
2
, was
introduced during the photocatalytic process. As shown in Fig. 6c
[
43e45]. In our case, S5 achieved nearly 100% Cr(VI) reduction at pH
.0. Cr(VI) reduction under neutral conditions is cost-saving and
sustainable, as a lower pH would consume a considerable amount of
acid, such as H SO and HCl. Compared to the photocatalysts re-
and f, the photocatalytic activity of S5 was inhibited with the
ꢁ
6
addition of BQ, indicating that $O
2
played an important role in
Cr(VI) reduction. N
remove dissolved oxygen (DO) and avoid the formation of $O
2
was bubbled into the reaction system to
ꢁ
2
4
2
. It
ported in the literature, S5 presented superior photocatalytic per-
formance toward Cr(VI) reduction (Table S1) [46e51].
can be seen from Fig. 6c that the photocatalytic activity was also
The initial Cr(VI) concentration can affect the photocatalytic
performance. Fig. S4 shows that the photocatalytic activities
decreased with increasing Cr(VI) concentration. To further inves-
tigate the photocatalytic performance and practical application of
2 3
the obtained In S , a photocatalytic experiment was conducted in
simulated wastewater samples containing Cr(VI) prepared with tap
water, rainwater and lake water. As illustrated in Fig. 6b and e, slight
decreases in the photoreduction of Cr(VI) in different natural water
samples were observed, probably due to the inhibitory effect of
ꢁ
ꢁ
3ꢁ
2ꢁ
4
in natural water
coexisting ions such as Cl , NO
3
, PO
4
and SO
[
44]. For the three natural water systems, the photoreduction ef-
ficiencies of Cr(VI) in lake water and rainwater were superior to the
photoreduction efficiency in tap water. It was determined that
dissolved organic matter (DOM) existed in lake water and rainwater
þ
as organics could deplete h , leading to the effective separation of
ꢁ
photoinduced electron-hole pairs and the formation of more e or/
ꢁ
and $O
2
. Although the DOM in rainwater and lake water could
þ
Fig. 9. Photocatalytic performance of Cr(VI) reduction and MO degradation over S5 in
consume h , the inhibitory effects from the inorganic ions may
outweigh the promotional effect from the DOM, leading to a lower
a
single-component system and
a
mixed system. Reaction conditions:
photocatalyst ¼ 10 mg, MO ¼ 10 mg/L, Cr(VI) ¼ 20 mg/L, 50 mL.

H. Fu et al. / Journal of Alloys and Compounds 837 (2020) 155567
7
2 3
Fig. 10. (a) Mott-Schottky plots of S5 and (b) mechanism diagram of In S for photocatalytic Cr(VI) reduction and MO degradation.
ꢁ
restrained, further demonstrating that $O
2
might be a main active
responsible for the photoreduction of Cr(VI).
species for Cr(VI) reduction. In addition, the photoreduction of
The photocatalytic performance toward MO degradation over S5
was also studied. The saturation adsorption of MO with approxi-
mately 45% removal efficiency was achieved after 1 h in the dark
(Fig. S5a). It can be seen from Fig. S5b that self-degradation was not
obvious. Nearly 100% MO with an initial concentration of 10 mg/L
was removed within 9 min upon irradiation with visible light
(Fig. 8a). To prove the excellent photocatalytic performance, photo-
catalytic experiments were conducted in simulated wastewater
samples containing MO prepared with lake water, rainwater and tap
water. As exhibited in Fig. 8a, a slight decline in the photocatalytic
Cr(VI) still occurred in the presence of BQ or with a bubble of N
suggesting that e could also be significant for Cr(VI) reduction.
2
,
ꢁ
Electron spin resonance (ESR) was performed to test whether
ꢁ
$O
2
radicals existed under visible light irradiation. Fig. 7a shows
ꢁ
that there was no DMPO-$O
2
signal in the dark. However, four
intensive peak signals with a ratio of 1:1:1:1 occurred under visible
light irradiation, and these peaks became stronger with time,
ꢁ
indicating that more $O
2
radicals were generated [56,57]. From the
ꢁ
ꢁ
discussions above, it can be concluded that $O and e could be
2
Fig. 11. (a) Reusability of S5 for the photoreduction of Cr(VI), (b) XRD patterns, and (c, d) TEM images of S5 (c) before and (d) after catalysis.

8
H. Fu et al. / Journal of Alloys and Compounds 837 (2020) 155567
activity was observed, and the conversion efficiency reached
approximately 95% within 18 min. To understand the photocatalytic
mechanism, TBA was introduced during the photocatalytic process
to trap $OH radicals. It can be observed from Fig. 8b that the pho-
todegradation efficiency was inhibited, implying that $OH could be
responsible for the degradation of MO. The photocatalytic perfor-
mance toward MO degradation was improved in the presence of
formate, sodium acetate and sodium propionate as modulating
reagents for the first time. H O could promote the hydrolysis of
indium salt and the deprotonation of terephthalic acid to accelerate
crystal nucleation, and the salts could also promote the deproto-
nation of terephthalic acid to accelerate the formation of MIL-68.
This preparation method is suitable for large-scale production
and industrial applications of MIL-68. Hollow porous In S particles
2 3
with assembled ultrathin nanosheets were obtained via sulfidation
treatment using these MIL-68 as self-sacrificing templates. The
2
þ
EDTA acting as a h trapping reagent, which could be attributed to
ꢁ
the increased $O
2
production upon separation of photoinduced
was bubbled into the
electron-hole pairs. Furthermore, when N
2
obtained In
photodegradation of MO. $O
photoreduction of Cr(VI), and both $O
2
S
3
possessed excellent photoreduction of Cr(VI) and
ꢁ
ꢁ
reaction system to remove DO, the photocatalytic activity was
2
and e could be responsible for the
ꢁ
ꢁ
restrained, further implying that $O
2
could also play an important
2
and $OH were the dominant
role in the degradation of MO. The ESR results (Fig. 7b) showed that
there was no DMPO-$OH signal in the dark, while four intensive
peak signals with a ratio of 1:2:2:1 occurred under visible light
irradiation. Furthermore, these four peaks became stronger with
time, revealing that more $OH radicals were generated [58]. From
active species in the photodegradation of MO. Furthermore, the
reusability of In was good, and there was no obvious decline in
photocatalytic performance after five runs of operation.
2 3
S
Declaration of competing interest
the discussions above, it can be concluded that the dominant active
ꢁ
species for MO photodegradation were $O
2
and $OH radicals.
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Considering that both Cr(VI) and organics can exist in real
wastewater, the photocatalytic performances of Cr(VI) reduction
and MO degradation in a mixed solution of Cr(VI) and MO were
measured. It can be observed from Fig. 9 that the Cr(VI) photore-
duction and MO photodegradation efficiencies in the mixed system
both decreased compared to those in the single-component sys-
CRediT authorship contribution statement
Huifen Fu: Data curation, Investigation, Visualization, Writing -
original draft. Lin Wu: Data curation, Methodology, Software. Jing
Hang: Visualization, Software. Peng Wang: Resources. Chen Zhao:
Validation, Software. Chong-Chen Wang: Conceptualization,
Funding acquisition, Supervision, Project administration, Writing -
review & editing.
tems, which could be ascribed to the competitive consumption of
ꢁ
$O
2
between Cr(VI) reduction and MO degradation.
To further understand the photocatalytic mechanism, Mott-
Schottky plots of S5 were measured. As shown in Fig. 10a, the flat
(n-type semi-
band potential is ꢁ1.00 eV vs. Ag/AgCl. For In
2 3
S
conductor), the minimum value of the CB is 0.1 eV higher than that
of the flat band potential [59], and the CB value should be ꢁ1.10 eV
vs. Ag/AgCl, which can be converted to ꢁ0.90 eV vs. NHE using Eq.
Acknowledgements
(
g
2). The E value is 2.44 eV, therefore the VB value is 1.54 eV.
This work was supported by the National Natural Science Foun-
dation of China (21806008, 21876008, 51878023), Beijing Natural
Science Foundation (8202016), Beijing Talent Project (2019A22),
Great Wall Scholars Training Program Project of Beijing Municipality
Universities (CIT&TCD20180323).
ENHE ¼ EAg/AgCl þ 0.2 eV
(2)
The mechanism diagram is shown in Fig. 10b. In the photo-
catalytic reduction of Cr(VI), the main active species were e and
ꢁ
ꢁ
ꢁ
2
$O
2
, while $OH and $O
were the main active species for the
ꢁ
2
was generated as described by Eq. (3),
Appendix A. Supplementary data
degradation of MO. $O
while $OH was formed as described by Eqs. (4) and (5) rather than
by Eq. (6) because the VB potential of S5 was not positive enough to
produce $OH [60].
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2020.155567.
ꢁ
ꢁ
O þ e / ꢂ O
(3)
(4)
(5)
(6)
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