NH2-MIL-88B (Fe: α In1- α ) mixed-MOFs designed for enhancing photocatalytic Cr(vi) reduction and tetracycline elimination

Article abstract of DOI:10.1039/d0ra07487j

Aiming at solving the issue of wastewater purification, this work synthesized NH2-MIL-88B (FeαIn1-α) photocatalysts by a simple one-pot method, which was employed for photocatalytic reduction of Cr(vi) and oxidation of TC-HCl. Compared with traditional NH2-MIL-88B (Fe) photocatalysts, NH2-MIL-88B (Fe0.6In0.4) displayed excellent photocatalytic performance; the photocatalytic redox rate for Cr(vi) and TC-HCl reached 86.83% and 72.05%, respectively. The good photocatalytic performance might be attributed to the metal-to-metal charge transition (MMCT) between Fe-O clusters and In-O clusters formed by doping In(iii) into NH2-MIL-88B (Fe), which provides effective active sites for the photocatalytic reduction and oxidation routes. Besides, the synergistic effect of the ligand-to-metal charge transition (LMCT) and MMCT expands the separation and transfer of photogenerated carriers and inhibits the recombination of electron-hole pairs, thus effectively improving the photocatalytic performance. Therefore, this work could provide a new method for the construction of mixed metal MOFs for the photocatalytic degradation of pollutants. This journal is

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NH2-MIL-88B (FeaIn1ꢀa) mixed-MOFs designed for
enhancing photocatalytic Cr(VI) reduction and
tetracycline elimination†
Cite this: RSC Adv., 2020, 10, 39080
a
b
Chunhua Xu, Mingjun Bao,b Jiawen Renb and Zhiguang Zhang
*
*
Aiming at solving the issue of wastewater purification, this work synthesized NH2-MIL-88B (FeaIn1ꢀa
)
photocatalysts by a simple one-pot method, which was employed for photocatalytic reduction of Cr(VI)
and oxidation of TC-HCl. Compared with traditional NH2-MIL-88B (Fe) photocatalysts, NH2-MIL-88B
(Fe0.6In0.4) displayed excellent photocatalytic performance; the photocatalytic redox rate for Cr(VI) and
TC-HCl reached 86.83% and 72.05%, respectively. The good photocatalytic performance might be
attributed to the metal-to-metal charge transition (MMCT) between Fe–O clusters and In–O clusters
formed by doping In(III) into NH2-MIL-88B (Fe), which provides effective active sites for the
photocatalytic reduction and oxidation routes. Besides, the synergistic effect of the ligand-to-metal
charge transition (LMCT) and MMCT expands the separation and transfer of photogenerated carriers and
inhibits the recombination of electron–hole pairs, thus effectively improving the photocatalytic
performance. Therefore, this work could provide a new method for the construction of mixed metal
MOFs for the photocatalytic degradation of pollutants.
Received 31st August 2020
Accepted 18th October 2020
DOI: 10.1039/d0ra07487j
rsc.li/rsc-advances
ways to remove TC-HCl, including adsorption,22 advanced
oxidation processes (AOPs),21 biological degradation,23 elec-
trolysis,24 membrane ltration,25 ion exchange26 and photo-
catalytic degradation.27,28 Photocatalytic degradation is widely
used in recent years because of mild reaction conditions, high
efficiency, low cost and greenly.20,29–31
Introduction
Heavy metals and residual antibiotics in wastewater have
become a serious threat to living creatures and the environ-
ment.1–4 Hexavalent chromium (Cr(VI)), as a heavy metal ion
pollutant in wastewater with high toxicity and mobility, has
seriously threatened human health and the natural environ-
ment.5,6 If a large amount of Cr(VI) existed in the drinking water
system, it would cause damage to the human stomach, kidneys,
liver, and retinas and also serious environmental concern.7,8
Therefore, it is urgent to remove Cr(VI) from wastewater. There
are many existing solutions such as chemical precipitation,9
membrane separation,10 ion exchange,11 activated carbon
adsorption12 and photocatalytic reduction.13–15 Among them,
the reduction of Cr(VI) to Cr(III) by semiconductor photocatalysis
is a very effective strategy due to the low toxicity of Cr(III).16–18
Furthermore, tetracycline hydrochloride (TC-HCl), as one of the
most common antibiotics, is widely used in various industries
such as medicine, agriculture, and animal husbandry. TC-HCl
is abundant in the environment due to its stable chemical
structure and non-biodegradation, which has already imperiled
the ecosystem and human health.19–21 There are many current
Metal–organic frameworks (MOFs), as a class of crystalline
inorganic–organic hybrid materials, are composed of the
interconnection between metal centers and multidentate
organic ligands.32,33 MOFs have been valued by researchers in
the eld of photocatalysis34,35 due to the inherent large specic
surface area, uniform and adjustable pore structure, and
abundant active sites.36 Under the irradiation of incident light,
the photogenerated electrons are excited from the valence band
(VB) and transferred to the conduction band (CB) through the
ligand-to-metal charge transition (LMCT) process. The sepa-
rated photogenerated electrons–holes pairs can perform the
reduction and oxidation reaction with the reactants adsorbed
on the catalyst surface.37 Traditional MOFs existed some
shortcomings, such as small photoresponse range and high
electrons–holes recombination rate.38 It is undoubtedly a good
way to change the photoresponse range and photocatalytic
performance of MOFs by adjusting the central metal or organic
ligands.39 The strategy of combining two or more transition
metals into the same framework to form mixed-metal MOFs has
aroused the interest of researchers.40
aDalian Vocational & Technical College (Dalian Radio and TV University), Dalian
116035, PR China. E-mail: xuchunhua0331@163.com; Tel: +86 411-62614316
bSchool of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian
116029, PR China. E-mail: zgzhang@lnnu.edu.cn; Tel: +86 411-82158309
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra07487j
Mixed-metal organic framework (MM-MOFs), containing two
or more metal centers, are periodically arranged with a single
ligand in the whole framework.41,42 There is a metal-to-metal
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charge transition (MMCT) process between multiple metals in products were named as NH2-MIL-88B (FeaIn1ꢀa), and the
MM-MOFs, which facilitates the transfer of photogenerated synthesis diagram was shown in Scheme 1.
electrons and inhibits the recombination of electrons–holes
pairs. Compared with single-metal MOFs, the synergistic effects
between LMCT and MMCT of MM-MOFs can effectively improve
the photocatalytic activity in various applications.43 Also, MM-
MOFs can effectively control the electronic structure and
bandgap structure while increasing the metal active sites.44–46 For
instance, Vu et al. synthesized Fe-Cr-MIL-101 for dye degradation
by the hydrothermal method. Mixed-metal MOFs showed better
adsorption capacity and higher photocatalytic activity and
stability in comparison with Cr-MIL-101.47 Maite P. et al. studied
the synthesis of multimetal MOFs doped with Zn(II), Ru(III) and
Pd(II) into Cu-MOF and applied to electrochemical reduction of
CO2.48 In our previous report, we successfully synthesized NH2-
MIL-68 (InaFe1ꢀa) that displayed good photocatalytic activity.49
According to reports, NH2-MIL-88B (Fe) has a exible pore
structure and a large open structure channel due to its 3D porous
structure containing hexagonal channels and bipyramidal
cages.50 The wide and exible pore structure facilitated other
metal ions to smoothly enter the 3D structure to replace the
original central metal ions, and can provide adjustment space for
structural changes that may be caused by the introduction of
other central metal ions, thereby improving related performance.
Based on the above analysis, a novel MM-MOFs material
NH2-MIL-88B (FeaIn1ꢀa) was constructed and synthesized via
the one-pot method directly, which used NH2-MIL-88B (Fe) as
the structural framework. Under visible light irradiation, the
activity of photocatalytic treatment of pollutants in water was
tested. The results showed that the efficiency of Cr(VI) reduction
and TC-HCl oxidation of NH2-MIL-88B (FeaIn1ꢀa) was 1.7 and
1.6 times that of NH2-MIL-88B (Fe). Moreover, a possible pho-
tocatalytic mechanism was proposed through characterization
and performance and active species testing. This work provides
an effective way for the construction of MM-MOFs and the
photocatalytic degradation of various pollutants in wastewater.
Evaluation of photocatalytic performance
The photocatalytic performance of as-obtained samples was
evaluated by removing Cr(VI) and TC-HCl. A certain amount of
synthesized NH2-MIL-88B (FeaIn1ꢀa) was added into the Cr(VI)
solution and stirring sufficiently. Simultaneously, the pH value
of the Cr(VI) solution was adjusted to 2 with HCl solution. Then
proceed to photocatalytic degradation. During the experiment,
pay attention to the concentration of Cr(VI) in the system. The
suspension was stirred for some time under dark conditions.
When the concentration of Cr(VI) no longer changed, switch to
Xe lamp irradiation. The suspensions taken out regularly are
centrifuged and ltered with a 0.22 mm syringe lter. The
determination of Cr(VI) was realized by the DPC method with
a UV-visible spectrophotometer. The photocatalytic oxidation of
TC-HCl is similar to the above process. Where the pH value of
the TC-HCl solution was adjusted to 9 with NaOH solution and
the determination was realized by the UV-visible spectropho-
tometer method. The removal adsorption reduction/oxidation
efficiency of pollutant is calculated using Lambert Beer's law
and the following formula:
Efficiency (%) ¼ C/C0 ꢂ 100%
(1)
here C and C0 are the concentrations of pollutants in the water
at t and t0.
Results and discussions
Structure and morphology analysis
As revealed in Fig. 1a, the crystalline structure of NH2-MIL-88B
(Fe) and In-doped NH2-MIL-88B (FeaIn1ꢀa) are conrmed by
the XRD pattern. The obvious characteristic peaks of NH2-MIL-
88B (Fe) located at 9.21ꢁ, 10.25ꢁ, 13.07ꢁ, 16.65ꢁ, 18.53ꢁ, 20.69ꢁ,
consistent with the literature reported,15 veried the successful
preparation of NH2-MIL-88 (Fe). The doped amounts of In on
NH2-MIL-88B (Fe) determined by the ICP-AES analysis as shown
in Table S1.† Concomitantly, the main characteristic peaks of
NH2-MIL-88B (FeaIn1ꢀa) appeared in the same position as
NH2-MIL-88B, which preliminarily proved that NH2-MIL-88B
(FeaIn1ꢀa) was successfully prepared. The as-prepared MM-
MOFs mainly exist combination and crystal lattice in the form
Experimental sections
The chemicals and characterization details could be found in
ESI.†
Synthesis of NH2-MIL-88B (Fe)
The NH2-MIL-88B (Fe) sample was prepared via a one-pot
method. 2-Aminoterephthalic acid (NH2-BDC) (0.54 mmol)
and Fe(NO3)3$9H2O (0.48 mmol) were dissolved in DMF
(12 mL), added CH3CN (12 mL) aer mixed well. Then, and the
mixture was transferred into a ask, oil bath at 120 ꢁC for
45 min. The suspension was cooled to room temperature
natꢁurally and washed with methanol. The sample was dried at
70 C for 5 h, then NH2-MIL-88B (Fe) was gained.
Synthesis of In-doped NH2-MIL-88B (Fe)
In-doped NH2-MIL-88B (Fe) was synthesized through the above
similar process, except that pure Fe(NO3)3$9H2O was replaced
Scheme 1 Synthesis diagram of the synthetic processes of NH2-MIL-
with different ratios Fe(NO3)3$9H2O and In(NO3)3$xH2O. The 88B (FeaIn1ꢀa).
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Fig. 1 (a) XRD patterns and (b) FT-IR of NH2-MIL-88B (Fe) and NH2-
MIL-88B (FeaIn1ꢀa) samples.
of NH2-MIL-88B (Fe). As the molar ratio of In(III) ions increases,
Fig. 2 SEM images of (a) NH2-MIL-88B (Fe), (b–e) NH2-MIL-88B
(FeaIn1ꢀa) samples (a ¼ 0.8, 0.6, 0.4, 0.2, respectively), (f) the
the characteristic peak at 9.21ꢁ shis to 9.02ꢁ. The deviation of
the characteristic peak might be due to that the ion size of In(III) compared SEM images of NH2-MIL-88B (FeaIn1ꢀa), (g) TEM image of
NH2-MIL-88B (Fe0.6In0.4
) and (h) EDX image of NH2-MIL-88B
(94.0 pm) in the crystal structure was signicantly larger than the
ion size of Fe(III) (78.5 pm), thereby the size of the trimer
composed of InO6 octahedron was larger than that of FeO6
octahedron in the crystal structure of NH2-MIL-88B (FeaIn1ꢀa).
Therefore, the peak corresponding to the characteristic peak in
the spectrum could shi to a lower angle due to the increase of
the unit cell parameters.51
To further determine the chemical structure and investigate the
doped of In(III) of the prepared samples, the FT-IR spectrum is
employed to analyze. As presented in Fig. 1b, the FT-IR spectrum
of NH2-MIL-88B (Fe) is consistent with the previous literature.52 By
comparison, aer indium doping, a new characteristic absorption
peak generation, which could be attributed to the In–O bond
appears at 663 cmꢀ1, indicating that In(III) is successfully intro-
duced and the In–O bond is formed. Except for the appearance of
new characteristic absorption peaks, the characteristic absorption
peaks corresponding to other chemical bonds have not been
changed, indicating that the introduction of In(III) does not affect
the chemical structure of NH2-MIL-88B (Fe), NH2-MIL-88B
(FeaIn1ꢀa) was successfully prepared.
Fig. 2a–f displays representative SEM images and length
comparison of NH2-MIL-88B (Fe) and In-doped NH2-MIL-88B
(FeaIn1ꢀa). As shown in Fig. 2a, NH2-MIL-88B (Fe) performed
a standard spindle with uniform morphology, in which the
length is 300–500 nm. Additionally, the spindle is continuously
elongated in the longitudinal direction with the increase of
In(III) ion content as shown in Fig. 2b–f. However, the overall
morphology of NH2-MIL-88B (FeaIn1ꢀa) is unchanged, forming
a six-sided double pyramid with a length in the range of
500–1000 nm. The cause of the morphological change may be
ascribed to In(III) ions replacing Fe(III) into the NH2-MIL-88B (Fe)
lattice, which increases the lattice spacing in the crystal.51 The
BET specic surface area test results showed that the BET
specic surface areas of NH2-MIL-88B (Fe0.6In0.4) is 204 m2 gꢀ1
less than that of NH2-MIL-88B (Fe), as Table S2† in details. The
pore structure of NH2-MIL-88B (Fe) was narrowed aer the
introduction and the specic surface area was relatively reduced
because of the large ion size of In(III).
(FeaIn1ꢀa).
Fig. 2g, which is the same as the result obtained in the SEM
image. HRTEM displayed that NH2-MIL-88B (Fe0.6In0.4) has an
ordered pore structure, where is difficult to nd nanoparticles
or metal oxide clusters outside the catalyst. Thence, In(III) has
been incorporated into the framework of NH2-MIL-88B (Fe)
instead of being supported on the surface. As shown in Fig. 2h,
In and Fe elements were approved in the EDX images, which
even demonstrated the conformation of MM-MOFs NH2-MIL-
88B (FeaIn1ꢀa).
XPS analysis
The XPS spectra of NH2-MIL-88B (Fe0.6In0.4) composite is shown
in Fig. 3a. There are ve characteristic peaks in the survey
spectrum, which corresponding to the C 1s, N 1s, In 3d, O 1s,
and Fe 2p electron orbitals, respectively. These elements
conrm the results of the EDX analysis as displayed in Table
S3.† Fig. 3b is the characteristic peak of C, all of them are
derived from organic ligands. The characteristic peaks at 284.4,
286.2, and 288.2 eV could be attributed to C]C, C–N and C]O
respectively, which correspond to various carbon-containing
covalent bonds in NH2-BDC.53 In Fig. 3c, the XPS spectrum
of N 1s can be divided to 396.8 and 397.9 eV, which represents
the –N]+ and –NH–+ respectively. It belonged to the amino
group extending or protruding into the cavity and the positively
charged. There are four characteristic peaks in the XPS spec-
trum of O 1s shown in Fig. 3d. The peaks at 531.7 and 530.8 eV
can be attributed to –OH and C]O of the organic ligand. Also,
529.9 and 529.1 eV correspond to the characteristic peaks of In–
O and Fe–O respectively. The appearance of In–O clusters shows
that In(III) has been doped into the catalyst and connected to the
organic ligands, thus successfully constructing NH2-MIL-88B
(FeaIn1ꢀa), which is consistent with the results of TEM.
Fig. 3e shows the characteristic peaks of Fe 2p, where the
satellite peak of Fe is at 715.2 eV. Among the peaks of Fe 2p1/2
and Fe 2p3/2, 724.0 eV and 710.0 eV represent Fe3+, and 722.2 eV
The morphology of the NH2-MIL-88B (Fe0.6In0.4) in the TEM
image is a hexagonal prism with sharp ends, as shown in
2+
and 709.0 eV represent Fe2+
.
The appearance of Fe is due
54–56
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has the strongest absorption intensity of visible light, and
NH2-MIL-88B (Fe0.6In0.4) has a slight enhance in visible light
absorption intensity. Comparing with NH2-MIL-88B (Fe), both
have signicantly enhanced visible light absorption. Thereby,
this indicates that a certain proportion of In(III) doping, which is
benecial to improve the visible light absorption capacity of
NH2-MIL-88B (Fe).
The corresponding bandgap energy (Eg) values could be
calculated by following the eqn (1)
(ahv)2 ¼ A(hv ꢀ Eg)
(2)
here, a, h, v, A, Eg corresponds to the absorption coefficient, the
Planck's constant, light frequency, constant, the bandgap energy,
respectively. As displayed in Fig. 4b, the Eg values of NH2-MIL-88B
(Fe), NH2-MIL-88B (Fe0.8In0.2), NH2-MIL-88B (Fe0.6In0.4),
NH2-MIL-88B (Fe0.4In0.6) and NH2-MIL-88B (Fe0.2In0.8) were esti-
mated to be 2.62, 2.48, 2.58, 2.66 and 2.69 eV, respectively. The
size of the bandgap energy is related to the difficulty of electronic
excitation. Under visible light irradiation, the lower bandgap
energy can allow more electrons to excite the transition to CB,
promote the effective separation of electrons and holes, thereby
improving the photocatalytic activity. Among them, NH2-MIL-88B
(Fe0.8In0.2) and NH2-MIL-88B (Fe0.6In0.4) displayed the lower Eg
and the stronger visible light response-ability, which may
perform better photocatalytic performance.
Fig. 3 XPS spectra of NH2-MIL-88B (Fe0.6In0.4) composite (a). Survey;
(b) C 1s; (c) N 1s; (d) O 1s; (e) Fe 2p; (f) In 3d, respectively.
As shown in Fig. S1,† the uorescence intensity of NH2-MIL-
88B (Fe0.6In0.4) is signicantly lower than that of NH2-MIL-88B
(Fe), which indicates that the addition of In(III) inhibits the
recombination of photogenerated eꢀ–h+, thereby allowing more
photogenerated carriers to participate photocatalytic reaction.57,58
to the combination of photogenerated electrons and Fe3+
,
reducing Fe3+ to Fe2+. The peak at 450.6 and 443.1 eV can be
veried to In 3d3/2 and In 3d5/2 as presented in Fig. 3f, respec-
tively. This once again proves that In is successfully doped in
NH2-MIL-88B (FeaIn1ꢀa). Contacted with the characterization
results of XRD, IR, EDS, and XPS, it can be further conrmed
that the In-doped NH2-MIL-88B (FeaIn1ꢀa) bimetallic material
has been successfully prepared.
Photocatalytic activities
For verifying the photocatalytic performance of NH2-MIL-88B
(FeaIn1ꢀa) obtained from the characterization results, we con-
ducted experiments on photocatalytic Cr(VI) reduction under
different catalytic conditions. As displayed in Fig. 5a, the Cr(VI)
concentration was unaltered signicantly with visible light
irradiation and no photocatalyst, at the same time, the small
range change of Cr(VI) concentration was due to adsorption with
photocatalyst and no light, which indicated that visible light
irradiation played an important role in the photocatalytic
reduction of Cr(VI). The photocatalytic reduction efficiency of
NH2-MIL-88B (Fe0.6In0.4) for reducing Cr(VI) is 86.83%, which is
signicantly higher 1.7 times than that of NH2-MIL-88B (Fe) of
50.96%, thereby, which demonstrated that the doping of In(III)
effectively improves the photocatalytic reduction performance.
As shown in Fig. 5b, the different In(III) doped amount of
NH2-MIL-88B (FeaIn1ꢀa) was carried out to discuss the capa-
bility of photocatalytic Cr(VI) reduction. With the doping of In,
the photocatalytic reduction effect of the catalyst gradually
increases. Among them, NH2-MIL-88B (Fe0.6In0.4) has the
highest photocatalytic reduction rate. By contrast, NH2-MIL-88B
(Fe0.8In0.2) is the lowest one. With the addition of more In(III)
content, the catalytic effect gradually decreases. This may be
due to more In(III) doping, resulting in lower visible light
absorption. And higher bandgap energy is not conducive to the
Optical properties
The UV-vis DRS spectra of the samples were used to demon-
strate the optical absorption properties in Fig. 4a. As reduced,
the absorption intensity of NH2-MIL-88B (FeaIn1ꢀa) gradually
decreases in the visible light region. NH2-MIL-88B (Fe0.8In0.2
)
Fig. 4 (a) UV-vis DRS spectrum and (b) plots of (ahn)2 versus photon
energy (hn) of the NH2-MIL-88B (Fe) and the NH2-MIL-88B (FeaIn1ꢀa
samples.
)
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Fig. 5 Photocatalytic reduction of Cr(VI) (a) blank group comparison,
(b) NH2-MIL-88B (FeaIn1ꢀa), (c) Cr(VI) on different concentrations
reduction performance, and (d) the fitting of the pseudo-first-order
linear line for the photocatalytic reaction. (Reaction conditions: 0.5 g
Fig.
6
Photocatalytic degradation of TC-HCl (a) blank group
comparison, (b) NH2-MIL-88B (FeaIn1ꢀa), (c) TC-HCl on different
concentrations degradation performance and (d) the fitting of the
pseudo-first-order linear line for the photocatalytic reaction. (Reaction
conditions: 0.1 g Lꢀ1 photocatalyst, RT).
L
ꢀ1 photocatalyst, 3 mL ethanol, RT, pH ¼ 2).
excitation of electrons on VB, thereby reducing the photo-
catalytic activity of photocatalytic reduction of hexavalent
chromium. Meanwhile, Fig. 5c shows that the photocatalytic
Cr(VI) on different concentrations reduction performance of
NH2-MIL-88B (Fe0.6In0.4) was investigated. With increasing the
amount of catalyst, the photocatalytic performance gradually
increases, reaching the maximum at 10 mg Lꢀ1. However, an
excess of the catalyst leads to lower photocatalytic reduction
efficiency. This may be due to excessive catalyst hindering the
light irradiation of the Cr(VI) solution. Fig. 5d displays that the
photocatalytic reduction efficiency has a good linear relation-
ship with the reaction time, which conforms to the rst-order
reaction kinetics. The pseudo-rst-order rate constant k is ob-
tained by the following formula:
degradation rate gradually decreases, and when the content is
10 mg, NH2-MIL-88B (Fe0.6In0.4) displays the best degradation
effect. The relationship between ln(C0/C) and time shows that the
photocatalytic degradation efficiency has a good linear relation-
ship with the reaction time, as presented in Fig. 6d, which is in
line with the rst-order reaction kinetics. The value of k and R2 of
NH2-MIL-88B (Fe) and NH2-MIL-88B (FeaIn1ꢀa) was displayed in
Tables S6 and S7.†
To detect the reactive oxygen species generation during the
photocatalytic degradation of TC-HCl by NH2-MIL-88B
(FeaIn1ꢀa), electron spin resonance (ESR) technique is
employed. Fig. 7 depicts the conspicuous characteristic peak
ꢀ
signals of DMPO-cO2 and DMPO-cOH, which conrms the
ꢀ
generation of cO2 and cOH radicals in the experiment. The
peak intensity ratio of the quadruple characteristic peak signal
of DMPO-cOH is about 1 : 2 : 2 : 1.59 NH2-MIL-88B (FeaIn1ꢀa) by
the photoexcitation generates eꢀ under visible light irradiation,
which can activate dissolved oxygen in the solution that
ln(C0/C) ¼ kt
(3)
here, C0, C, k, and t corresponds to the initial concentrations of
Cr(VI), the concentrations of Cr(VI), the pseudo-rst-order rate
constant, and time, respectively. The value of k and R2 of
NH2-MIL-88B (Fe) and NH2-MIL-88B (FeaIn1ꢀa) was displayed in
Tables S4 and S5.†
ꢀ
promotes oxygen molecules to become cO2 species and can
To investigate the oxidation performance of the prepared
photocatalyst, the photocatalytic degradation of TC-HCl by
NH2-MIL-88B (FeaIn1ꢀa) under different catalytic conditions was
performed. Similar to the photocatalytic reduction effect of Cr(VI),
the test result of Fig. 6a shows that the light and photocatalyst are
crucial in the reaction system. As shown in Fig. 6b, the photo-
catalytic degradation efficiency of NH2-MIL-88B (Fe0.6In0.4) on
TC-HCl under visible light irradiation conditions is 72.05%,
which is signicantly higher 1.6 times than that of NH2-MIL-88B
(Fe) of 44.68%. Fig. 6c demonstrated the effect of different
concentrations photocatalytic degradation efficiency of TC-HCl.
Among them, as the content of TC-HCl increases, the
Fig. 7 ESR spectra of DMPO-cO2ꢀ and DMPO-cOH adducts generated
NH2-MIL-88B (FeaIn1ꢀa) after 60 seconds under visible light irradiation
(l > 400 nm) during the photocatalytic process.
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also interact with water molecules to generate cOH species, mainly attributed to the more reactive sites when In(III) doped to
simultaneously.
NH2-MIL-88B (Fe). Simultaneously, the synergistic effect of
LMCT and MMCT of NH2-MIL-88B (FeaIn1ꢀa effectively
)
expands the separation and transfer of photogenerated carriers
and inhibits the recombination of electrons–holes pairs,
thereby enhancing the performance of photocatalytic reduction
of Cr(VI) and oxidative degradation of TC-HCl. This work may
provide an insight to design and develop mixed-metal MOFs as
photocatalysts for the wastewater treatment.
Mechanism
Based on the above results, the possible mechanism of
NH2-MIL-88B (FeaIn1ꢀa) photocatalytic treatment of Cr(VI) and
TC-HCl in water is proposed. First, the electrons transfer
process of NH2-MIL-88B (FeaIn1ꢀa) photocatalytic reduction of
Cr(VI) is as follows. Under visible light irradiation, the organic
ligand of NH2-MIL-88B (FeaIn1ꢀa) can be excited to generate
electrons and holes. A part of the photogenerated electrons
transfers to the surface of the catalyst through the In–O clus-
ters through the LMCT to reduce Cr(VI) adsorbed directly.
Then, the In–O clusters transfer another electron to the Fe–O
cluster through MMCT, which promotes the separation of
electrons and holes effectively.49 The excitation and transfer of
photogenerated carriers of photocatalytic oxidation of TC-HCl
are similar to the process of reduction of Cr(VI). The photo-
generated electrons transfer to the surface of the catalyst
combine with O2 and H2O in the solution to produce cO2ꢀ and
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Thanks to the 2020 School-level Scientic Research Project of
Dalian Vocational & Technical College (Dalian Radio and TV
University) and the Key Project of Natural Science Foundation of
Liaoning Province (No. 20170540578).
cOH. Both the holes generated by NH2-MIL-88B (FeaIn1ꢀa
)
References
excited by visible light and cO2ꢀ/cOH can interact with TC-HCl
directly and eventually generate to CO2 and H2O. Anyway, the
MMCT in the NH2-MIL-88B (FeaIn1ꢀa) expands the active sites
of photocatalytic oxidation-reduction. Also, the synergistic
process of LMCT and MMCT effectively inhibits the recombi-
nation of electrons and holes, thereby enhancing the perfor-
mance of the NH2-MIL-88B (FeaIn1ꢀa) (Scheme 2).
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