Available via license: CC BY 4.0
Content may be subject to copyright.
catalysts
Article
Effective Photocatalytic Activity of Mixed Ni/Fe-Base
Metal-Organic Framework under a Compact
Fluorescent Daylight Lamp
Vinh Huu Nguyen 1, Trinh Duy Nguyen 1, * , Long Giang Bach 1, Thai Hoang 2,
Quynh Thi Phuong Bui 3, Lam Dai Tran 2,4 , Chuong V. Nguyen 5, Dai-Viet N. Vo 6and
Sy Trung Do 7 ,*
1
NTT Hi-tech Institute, Nguyen Tat Thanh University, 300A Nguyen Tat Thanh, District 4, Ho Chi Minh City
755414, Vietnam; nguyenhuuvinh3110@gmail.com (V.H.N.); blgiangntt@gmail.com (L.G.B.)
2Institute for Tropical Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau
Giay, Hanoi 10072, Vietnam; hoangth@itt.vast.vn (T.H.); trandailam@gmail.com (L.D.T.)
3
Faculty of Chemical Technology, Ho Chi Minh City University of Food Industry, 140 Le Trong Tan, Tan Phu
District, Ho Chi Minh City 705800, Vietnam; phuongquynh102008@gmail.com
4Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang
Quoc Viet, Cau Giay, Hanoi 10072, Vietnam
5
Department of Materials Science and Engineering, Le Quy Don Technical University, Hanoi 100000, Vietnam;
chuongnguyen11@gmail.com
6
Faculty of Chemical & Natural Resources Engineering, University Malaysia Pahang, Lebuhraya Tun Razak,
Gambang 26300, Malaysia; vietvo@ump.edu.my
7
Institute of Chemistry, Vietnam Academy of Science and Techology, 18 Hoang Quoc Viet, Cau Giay District,
Hanoi 10072, Vietnam
*
Correspondence: ndtrinh@ntt.edu.vn (T.D.N.); dosyvhh@gmail.com (S.T.D.); Tel.: +84-971-275-356 (T.D.N.);
+84-969-937-586 (S.T.D.)
Received: 19 September 2018; Accepted: 15 October 2018; Published: 23 October 2018
Abstract:
Mixed Ni/Fe-base metal-organic framework (Ni/Fe-MOF) with different molar ratios
of Ni
2+
/Fe
3+
have been successfully produced using an appropriate solvothermal router.
Physicochemical properties of all samples were characterized using X-ray diffraction (XRD), Raman,
field emission scanning electron microscopes (FE-SEM), fourier-transform infrared spectroscopy
(FT-IR), N
2
adsorption-desorption analysis, X-ray photoelectron spectroscopy (XPS), ultraviolet-visible
diffuse reflectance spectra (UV-Vis DRS), and photoluminescence spectra (PL). The photocatalytic
degradation performances of the photocatalysts were evaluated in the decomposition of rhodamine
B (RhB) under a compact fluorescent daylight lamp. From XRD, IR, XPS, and Raman results, with
the presence of mixed ion Fe
3+
and Ni
2+
, MIL-88B (MIL standing for Materials of Institut Lavoisier)
crystals based on the mixed metal Fe
2
NiO cluster were formed, while MIL-53(Fe) was formed with the
presence of single ion Fe
3+
. From UV-Vis DRS results, Ni/Fe-MOF samples exhibited the absorption
spectrum up to the visible region, and then they showed the high photocatalytic activity under visible
light irradiation. A Ni/Fe-MOF sample with a Ni
2+
/Fe
3+
molar ratio of 0.3 showed the highest
photocatalytic degradation capacity of RhB, superior to that of the MIL-53(Fe) sample. The obtained
result could be explained as a consequence of the large surface area with large pore volumes and
pore size by the Ni
2+
incorporating into the MOF’s structure. In addition, a mixed metal Fe/Ni-based
framework consisted of mixed-metal cluster Fe
2
NiO with an electron transfer effect and may enhance
the photocatalytic performance.
Keywords:
photocatalytic decomposition of rhodamine B; MIL-53(Fe); Ni/Fe-MOF; visible
light irradiation
Catalysts 2018,8, 487; doi:10.3390/catal8110487 www.mdpi.com/journal/catalysts
Catalysts 2018,8, 487 2 of 20
1. Introduction
Metal-organic frameworks (MOFs), a new class of high surface area and crystalline porous
materials, assemble with metal clusters and organic bridging ligands [
1
]. These materials have
received considerable attention in recent years due to their high resistance, high surface area, large
pore volume, low density, and easily tunable framework. Among the MOFs, MIL-53(Fe) 88B (MIL
standing for Materials of Institut Lavoisier) have attracted extensive interest for applications in gas
storage [
2
,
3
], adsorption and separation of heavy metal [
4
], sensors [
5
], and in the biomedical field
such as for drug delivery [6].
Recently, to eliminate organic dyes, many approaches have been suggested including
adsorption [
7
–
10
] and photodegradation [
11
–
13
]. However, the latter is of interest because this process
could decompose organic dyes to CO
2
, H
2
O, and harmless inorganics, while the adsorption process is
only capable of removing dyes from water media. MIL-53(Fe) as a catalyst carrier or modification of
MIL-53(Fe) as a catalyst for chemical reactions has received research attention [
14
]. MIL-53(Fe) has
the chemical formula of Fe
III
(OH)(O
2
C–C
6
H
4
–CO
2
)
·
H
2
O, which consists of FeO
6
octahedral chains
connected to benzene dicarboxylate (BDC) anions, forming a three-dimensional network with a large
volume and high surface area [
2
,
14
,
15
]. The FeO
6
octahedral chains have the potential to act as a Lewis
acid in many organic reactions [
16
]. Recently, MIL-53(Fe) with the potential use of FeO
6
octahedral
chains has received much attention in photocatalytic degradation of many organic dyes, such as
methylene blue [
11
,
13
,
17
], rhodamine B (RhB) [
14
,
16
,
17
], and p-nitrophenol [
14
], and has given good
decomposition results. Therefore, this is a possible application direction of MIL-53(Fe) in the removal
of organic dyes.
Fe-based MOFs materials have been reported as an effective photocatalyst for decomposition
of organic dyes under visible light irradiation [
18
–
22
]. However, their photocatalytic performance
is not as expected because of the fast recombination of photogenerated holes (h
+
) and electrons
(e
−
), resulting in the lack of h
+
for degradation dyes [
13
]. To address this, various approaches have
been proposed to depress the recombination process. For example, inorganic oxidants (e.g., H
2
O
2
,
KBrO
3
, and (NH
4
)
2
S
2
O
8
), which act as electron acceptors, was introduced in the photocatalytic
processes, significantly enhancing the photocatalytic effect of these materials. According to research
by
Yuan et al. [13]
, H
2
O
2
is an efficient electron acceptor in the photocatalytic decomposition process
of organic pigments by MIL-53(Fe) under visible light irradiation. Another approach that has been
developed to enhance the photocatalytic performance of MiL-53(Fe) is the designed synthesis of
composite photocatalysts containing MOFs materials such as CdS/MIL-53(Fe) [
23
], Ni-MOFs@GO [
24
],
Fe
3
O
4
/MIL-53(Fe) [
14
], and Fe
2
O
3
/MIL-53(Fe) [
25
]. In addition, MIL-53(Fe) that has been doped or
combined with one or more metals have also attracted much attention in recent years [
26
–
29
]. For this
study, Qiao Sun et al. modified the MIL-53(Fe) by adding Mn, Co, and Ni metal into the framework
of MIL-53(Fe) material, which exhibited excellent catalytic performance in liquid-phase degradation
of phenol [
30
]. Various rare-earth or transition metals that modify MOFs structures have recently
been reported such as three-dimensional Ln(III)–Zn(II) heterometallic coordination polymers [
31
], Fe
substituted Cr MIL-101 [
32
], Ag-doped MOF-like organotitanium polymer (Ag@NH2-MOP(Ti)) [
33
],
Ti-doped UiO-66 [34], Eu substituted Fe MIL-53 [35], and Zn-Ln coordination polymers (Ln = Nd, Pr,
Sm, Eu, Tb, Dy) [36].
In this work, we report the synthesis of Ni/Fe-MOF with different Ni
2+
/Fe
3+
molar ratios using
the solvothermal route and their application for the degradation of RhB solution under visible light
irradiation using a 40 W compact fluorescent lamp. To illustrate our method for the synthesis of
Ni/Fe-MOF, we have selected the preparation of the MIL-53(Fe) structure, which consists of FeO
6
octahedral chains connected to BDC anions. Thanks to the presence of Ni
2+
ions in the reaction solution,
MIL-88B crystals were formed with neutral mixed-metal clusters (Fe
2
NiO) connected via BDC anions.
This structure is similar to the MIL-88B structure consisting of the trinuclear oxo-centered iron cluster
(Fe
3
O) [
27
,
28
]. However, our bimetallic metal MOF products were expected to exhibit an excellent
adsorption capacity and photocatalytic activity in comparison to the original single metal MOFs.
Catalysts 2018,8, 487 3 of 20
The advantage of selecting MOF material containing Fe and Ni is due to the low cost, non-toxicity,
and natural abundance of these two transition metal oxides. In addition, the MOF material is also
capable of improving the separation efficiency of electron–hole pairs when Ni is incorporated into
the structure of materials [
37
,
38
]. The structure, morphology, and optical properties of the obtained
photocatalysts have been characterized using X-ray diffraction (XRD), Raman, field emission scanning
electron microscopes and energy-dispersive X-ray spectrometer (FE-SEM/EDS), fourier-transform
infrared spectroscopy (FT-IR), N
2
adsorption-desorption analysis, X-ray photoelectron spectroscopy
(XPS), ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS), photoluminescence (PL) spectra and
nitrogen physisorption measurements (BET). Besides, to obtain the optimal reaction conditions for the
RhB photodecomposition, the effect of the initial RhB concentration and pH on the degradation of RhB
was also investigated in detail.
2. Results and Discussion
2.1. Physical Properties of MIL-53(Fe) and Ni-Doped MIL-53(Fe)
2.1.1. XRD Analysis
Figure 1presents the XRD diffraction patterns of the MIL-53(Fe) and Ni/Fe-MOF samples isolated
from dimethylformamide (DMF) and H
2
O. In patterns of MIL-53(Fe) samples (Figure 1A, curve a),
the main diffraction peaks that appeared at 2
θ
of 9.1
◦
, 9.4
◦
, 14.1
◦
, 16.5
◦
, and 18.8
◦
are similar to those
previously reported for MIL-53(Fe) isolated from DMF [
2
,
11
,
39
]. In patterns of Ni/Fe-MOF samples
(Figure 1A, curves b–e), the main diffraction peaks that appeared around 2
θ
of 7.3
◦
, 8.9
◦
, 9.3
◦
, 9.9
◦
,
16.8◦, 18.7◦, 17.7◦, 20.1◦, and 21.9◦are similar to those previously reported for MIL-88B isolated from
DMF. Notably, the diffraction peak at a 2
θ
of 7.3
◦
observed in the XRD patterns of Ni/Fe-MOF samples
increased in intensity as the molar ratio of Ni
2+
/Fe
3+
increased from 0.1 to 0.7. With the presence
of Ni
2+
in the reaction solution, MIL-88B crystals were made up and the crystallinity of the material
increased. This observation might be attributed to the fact that the structure formation of Ni/Fe-MOF
was significantly influenced by the presence of Ni
2+
in the reaction solution. In addition, no other
diffraction peak associated with nickel oxides, iron oxides, or other impurities could be detected,
demonstrating the high purity of the samples.
XRD patterns of the MIL-53(Fe) and Ni/Fe-MOF samples isolated from H
2
O (Figure 1B) showed
the rugged background and weak intensities; however, the main diffraction peaks still maintained
the same structure as in Reference [
4
]. The difference in XRD patterns of samples isolated from
DMF and water may attribute to the breathing behavior of MIL-53(Fe) and MIL-88B, which has been
well documented by Alhanami et al. [
15
]. Moreover, MIL-53(Fe)
·
H
2
O sample essentially shows a
noncrystalline phase similar to those for MIL-53(Fe)
·
DMF. They can be explained by the effect of the
synthesis temperature on the structure formation of MIL-53(Fe). Pu et al. demonstrated that iron
ion and H
2
BDC could not coordinate successfully under a low temperature (100
◦
C), and therefore
the MIL-53(Fe) crystal structure could not fully develop [
40
]. However, the Ni/Fe-MOF samples still
show a high crystalline phase under low synthesis temperatures. Again, these results indicate that the
presence of a mixed metal ion (Ni
2+
and Fe
3+
ion) did have a significant influence on the formation
of Ni/Fe-MOF crystal structure, in which a Ni
2+
and Fe
3+
ion can coordinate with H
2
BDC to form
MIL-88B crystals instead of MIL-53(Fe) crystals.
Catalysts 2018,8, 487 4 of 20
Catalysts 2018, 8, x FOR PEER REVIEW 4 of 20
Figure 1. XRD patterns of as-prepared MIL-53(Fe) and Ni-MIL-53(Fe) crystals isolated from DMF
(A,B) and H2O (C,D): MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d), and
Ni/Fe-MOF-0.7 (e).
2.1.2. FT-IR Spectra
FTIR spectroscopic studies were performed for all samples in the wave range of 400–4000 cm−1,
as shown in Figure 2. As shown in Figure 2A,C, strong vibrational bands around 1657, 1601, 1391,
1017, and 749 cm−1, which are attributed to υ(C=O), υas(OCO), υs(OCO), υ(C–O), and δ(C–H)
vibrations confirms the presence the bridge coordination mode of metal carboxylates in the MOF
structures [4,25,30]. No band at 1700 cm−1 was found, implying no free H2BDC [27]. The band
characteristics of DMF (1657 cm−1) and H2O (3387 cm−1) were present in the samples MIL-53(Fe)·DMF,
Ni/Fe-MOF·DMF, MIL-53(Fe)·H2O and Ni/Fe-MOF-x·H2O, respectively [27].
At lower frequencies (Figure 2B), vibrational bands around 750 cm−1, 690 cm−1, and 660 cm−1
represent the C–H vibration, C=C stretch, OH bend, and OCO bend, respectively, were found,
implying the presence of the vibrations of the organic ligand BDC [27]. Figure 2B also shows that the
strong band at 547 cm−1 in all samples could be attributed to Fe–O vibrations or Ni–O vibrations [41].
The band around 625 cm−1 belongs to the Fe3O vibration, which was observed in MIL-53(Fe) and Ni-
Ni/Fe-MOF-0.1 samples. The weak band around 720 cm−1 is related to the Fe2NiO vibration, which
was observed in Ni/Fe-MOF-x samples [27]. These results reaffirmed that Ni2+ and Fe3+ ions can
coordinate with H2BDC to form MIL-88B crystals.
6 8 10 12 14 16 18 20 22 24 26 28 30 789101112
6 8 10 12 14 16 18 20 22 24 26 28 30 789101112
(D)
(C)
(B)
Intensity (a.u.)
2θ (Degree)
(A)
2θ (Degree)
Intensity (a.u.)
Intensity (a.u.)
(e)
(d)
(c)
(a)
(b)
Intensity (a.u.)
2θ (Degree)
(a)
(b)
(c)
(d)
(e)
MIL-53
2θ (Degree)
(a)
(b)
(c)
(d)
(e)
(e)
(d)
(c)
(a)
(b)
MIL-88B
Figure 1.
XRD patterns of as-prepared MIL-53(Fe) and Ni-MIL-53(Fe) crystals isolated from DMF
(
A
,
B
) and H
2
O (
C
,
D
): MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d), and
Ni/Fe-MOF-0.7 (e).
2.1.2. FT-IR Spectra
FTIR spectroscopic studies were performed for all samples in the wave range of 400–4000 cm
−1
,
as shown in Figure 2. As shown in Figure 2A,C, strong vibrational bands around 1657, 1601, 1391,
1017, and 749 cm
−1
, which are attributed to
υ
(C=O),
υ
as(OCO),
υ
s(OCO),
υ
(C–O), and
δ
(C–H)
vibrations confirms the presence the bridge coordination mode of metal carboxylates in the MOF
structures [4,25,30]
. No band at 1700 cm
−1
was found, implying no free H
2
BDC [
27
]. The band
characteristics of DMF (1657 cm
−1
) and H
2
O (3387 cm
−1
) were present in the samples MIL-53(Fe)
·
DMF,
Ni/Fe-MOF·DMF, MIL-53(Fe)·H2O and Ni/Fe-MOF-x·H2O, respectively [27].
At lower frequencies (Figure 2B), vibrational bands around 750 cm
−1
, 690 cm
−1
, and 660 cm
−1
represent the C–H vibration, C=C stretch, OH bend, and OCO bend, respectively, were found, implying
the presence of the vibrations of the organic ligand BDC [
27
]. Figure 2B also shows that the strong
band at 547 cm
−1
in all samples could be attributed to Fe–O vibrations or Ni–O vibrations [
41
].
The band around 625 cm
−1
belongs to the Fe
3
O vibration, which was observed in MIL-53(Fe) and
Ni-Ni/Fe-MOF-0.1 samples. The weak band around 720 cm
−1
is related to the Fe
2
NiO vibration,
which was observed in Ni/Fe-MOF-x samples [
27
]. These results reaffirmed that Ni
2+
and Fe
3+
ions
can coordinate with H2BDC to form MIL-88B crystals.
Catalysts 2018,8, 487 5 of 20
Catalysts 2018, 8, x FOR PEER REVIEW 5 of 20
Figure 2. FT-IR spectra of as-prepared MIL-53(Fe) and Ni/Fe-MOF crystals isolated from DMF (A,B)
and H2O (C): MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d), and Ni/Fe-
MOF-0.7 (e).
2.1.3. Raman Spectra
Samples were analyzed using Raman spectroscopy using an excitation wavelength at 633 nm
and spectra recorded at a wavenumber range of 100–900 cm−1, as shown in Figure 3. According to
previous studies, the BDC bridge in MOFs has Raman-active modes: the symmetric vibration modes
(s. (COO)) and asymmetric vibration (as (COO)) of the carboxylate group (1445 cm−1 and 1501 cm−1),
the vibration of the C–C bond between the benzene ring and the carboxylate group (1140 cm−1), and
the external plane deformation of the C–H link (865 cm−1 and 630 cm−1) [28]. As seen in Figure 3, the
presence of a BDC linker was also observed in all samples, and no Raman signals corresponding to
nickel oxides, iron oxides, or other impurities were found on any of the samples, which is consistent
with the results of the XRD patterns. Notably, the Raman signal corresponding to the symmetric
vibration (s. (OCO)) of the carboxylate group showed a shift to a lower wavenumber and the peak
split into two peaks corresponding to an increase of the Ni2+/Fe3+ molar ratio. This result was due to
the change in the charge distribution in the organic bridge when they were coordinated with different
metal ions (Figure 3B). Ionic Ni2+ has a smaller nuclear charge and a larger ionic radius than Fe3+
(𝑟 = 0.69 Å and 𝑟 =0.55 Å) [42]. Therefore, Ni2+ creates a weaker coordinated link with the
OCO group on the organic bridge than Fe3+, thus the symmetric vibration (s. (OCO)) of the
carboxylate group when forming coordinated bonds with Ni2+ moves to a lower wavenumber than
Fe3+ [43]. This result is commensurate with the XRD and IR results for Ni/Fe MOF.
4000 3500 3000 2500 2000 1500 1000 500 780 720 660 600 540 4000 3500 3000 2500 2000 1500 1000 500
(e)
(d)
(c)
(b)
ν(OCO)
ν(N-C=O)
ν(OCO)
(C)
(B)
Transmittance (%)
Wavenumber (cm
-1
)
(A)
ν(O-H)
(a)
(a)
(c)
(d)
(e)
(b)
(a)
(c)
(d)
(e)
(b)
Fe
2
NiO
C-H
C-C
OCO
Fe
3
O
Ni-O or Fe-O
Wavenumber (cm
-1
)Wavenumber (cm
-1
)
Figure 2.
FT-IR spectra of as-prepared MIL-53(Fe) and Ni/Fe-MOF crystals isolated from DMF
(
A
,
B
) and H
2
O (
C
): MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d), and
Ni/Fe-MOF-0.7 (e).
2.1.3. Raman Spectra
Samples were analyzed using Raman spectroscopy using an excitation wavelength at 633 nm and
spectra recorded at a wavenumber range of 100–900 cm
−1
, as shown in Figure 3. According to previous
studies, the BDC bridge in MOFs has Raman-active modes: the symmetric vibration modes (vs. (COO))
and asymmetric vibration (vas (COO)) of the carboxylate group (1445 cm
−1
and 1501 cm
−1
), the
vibration of the C–C bond between the benzene ring and the carboxylate group (1140 cm
−1
), and the
external plane deformation of the C–H link (865 cm
−1
and 630 cm
−1
) [
28
]. As seen in Figure 3, the
presence of a BDC linker was also observed in all samples, and no Raman signals corresponding to
nickel oxides, iron oxides, or other impurities were found on any of the samples, which is consistent
with the results of the XRD patterns. Notably, the Raman signal corresponding to the symmetric
vibration (vs. (OCO)) of the carboxylate group showed a shift to a lower wavenumber and the peak
split into two peaks corresponding to an increase of the Ni
2+
/Fe
3+
molar ratio. This result was due to
the change in the charge distribution in the organic bridge when they were coordinated with different
metal ions (Figure 3B). Ionic Ni
2+
has a smaller nuclear charge and a larger ionic radius than Fe
3+
(
rNi2+=
0.69
Å
and
rFe3+=
0.55
Å
) [
42
]. Therefore, Ni
2+
creates a weaker coordinated link with
the OCO group on the organic bridge than Fe
3+
, thus the symmetric vibration (vs. (OCO)) of the
carboxylate group when forming coordinated bonds with Ni
2+
moves to a lower wavenumber than
Fe3+ [43]. This result is commensurate with the XRD and IR results for Ni/Fe MOF.
Catalysts 2018,8, 487 6 of 20
Catalysts 2018, 8, x FOR PEER REVIEW 6 of 20
Figure 3. Raman spectra of as-prepared MIL-53(Fe) and Ni/Fe-MOF crystals isolated from DMF (A):
MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d), and Ni/Fe-MOF-0.7 (e), and
enlarged Raman spectra around 1450 cm−1 (B).
2.1.4. FE-SEM/EDS Analysis
Figure 4 displays SEM images and EDS spectra of the as-prepared MOF samples. As shown in
Figure 4, the morphologies and shapes of MOF samples varied according to the molar ratio of
Ni2+/Fe3+. MIL-53(Fe) sample mostly had amorphous nanoparticles (Figure 4(a1,a2)), which is in good
agreement with the results of XRD patterns with a poor crystallinity. When the molar ratio of Ni2+/Fe3+
was set to 0.1, the crystals of Ni/Fe-MOF-0.1 were not homogeneous with different shapes and sizes
(Figure 4(b1,b2)). A mixture of octahedral and hexagonal bipyramidal shapes, and nanoparticles,
were perceived when the molar ratio of Ni2+/Fe3+ (0.3–0.7) was increased further. However, these
octahedral and hexagonal bipyramidal shapes collapsed with cracks on the crystal surface. These
results, along with the XRD, IR, and Raman results above, indicate that a mixed-metal Ni/Fe-MOF
was successfully synthesized using the solvothermal method.
Moreover, to confirm the molar ratio of Ni2+/Fe3+ in the Ni/Fe-MOF samples in comparison to the
theoretical value, EDS was also conducted. The result from the EDS spectrum of the obtained MIL-
53(Fe) sample (Figure 4(a3)) showed the coexistence of C, O, Fe, and Cl. The presence of Cl may have
been due to the FeCl3 precursor, further confirming that the MIL-53(Fe) crystal structure could not
fully develop at a low temperature (100 °C). The EDS spectra of the Ni/Fe-MOF samples (Figure
4(b3,c3,d3,e3)) revealed that these samples contained C, O, Fe, and Ni. However, the existence of Cl
was still observed in the Ni/Fe-MOF-0.1 sample. The molar ratio of Ni2+/Fe3+ of Ni/Fe-MOF-0.1, Ni/Fe-
MOF-0.3, Ni/Fe-MOF-0.5, and Ni/Fe-MOF-0.7, obtained using EDS analysis, was 0.16, 0.30, 0.48, and
0.66, respectively. In addition, the map of Fe, O, C, and Ni is shown in Figure S1, which indicates that
they were uniformly distributed over the MOF surface.
100 400 800 1200 1600 1300 1400 150
0
δ(C-H) ν
s
(OCO)
ν(C=C)
ν
as
(OCO)
ν
s
(OCO)
δ(C-H)
(e)
(d)
(c)
(b)
Intensity (a.u.)
Wavenumber (cm
-1
)
(a)
ν(C-C)
(B)
(e)
(d)
(c)
(b)
(a)
(A)
Figure 3.
Raman spectra of as-prepared MIL-53(Fe) and Ni/Fe-MOF crystals isolated from DMF (
A
):
MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d), and Ni/Fe-MOF-0.7 (e),
and enlarged Raman spectra around 1450 cm−1(B).
2.1.4. FE-SEM/EDS Analysis
Figure 4displays SEM images and EDS spectra of the as-prepared MOF samples. As shown
in Figure 4, the morphologies and shapes of MOF samples varied according to the molar ratio of
Ni
2+
/Fe
3+
. MIL-53(Fe) sample mostly had amorphous nanoparticles (Figure 4(a1,a2)), which is in good
agreement with the results of XRD patterns with a poor crystallinity. When the molar ratio of Ni
2+
/Fe
3+
was set to 0.1, the crystals of Ni/Fe-MOF-0.1 were not homogeneous with different shapes and sizes
(Figure 4(b1,b2)). A mixture of octahedral and hexagonal bipyramidal shapes, and nanoparticles,
were perceived when the molar ratio of Ni
2+
/Fe
3+
(0.3–0.7) was increased further. However, these
octahedral and hexagonal bipyramidal shapes collapsed with cracks on the crystal surface. These
results, along with the XRD, IR, and Raman results above, indicate that a mixed-metal Ni/Fe-MOF
was successfully synthesized using the solvothermal method.
Moreover, to confirm the molar ratio of Ni
2+
/Fe
3+
in the Ni/Fe-MOF samples in comparison to
the theoretical value, EDS was also conducted. The result from the EDS spectrum of the obtained
MIL-53(Fe) sample (Figure 4(a3)) showed the coexistence of C, O, Fe, and Cl. The presence of Cl
may have been due to the FeCl
3
precursor, further confirming that the MIL-53(Fe) crystal structure
could not fully develop at a low temperature (100
◦
C). The EDS spectra of the Ni/Fe-MOF samples
(Figure 4(b3,c3,d3,e3)) revealed that these samples contained C, O, Fe, and Ni. However, the existence
of Cl was still observed in the Ni/Fe-MOF-0.1 sample. The molar ratio of Ni
2+
/Fe
3+
of Ni/Fe-MOF-0.1,
Ni/Fe-MOF-0.3, Ni/Fe-MOF-0.5, and Ni/Fe-MOF-0.7, obtained using EDS analysis, was 0.16, 0.30,
0.48, and 0.66, respectively. In addition, the map of Fe, O, C, and Ni is shown in Figure S1, which
indicates that they were uniformly distributed over the MOF surface.
Catalysts 2018,8, 487 7 of 20
Catalysts 2018, 8, x FOR PEER REVIEW 7 of 20
Figure 4. SEM images (1, 2) and EDS patterns (3) of as-prepared MIL-53(Fe) and Ni/Fe-MOF crystals
isolated from DMF: MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d), and
Ni/Fe-MOF-0.7 (e).
2.1.5. XPS Spectra
To analyze the chemical states of Ni and Fe in the Ni/Fe MOF structure, XPS spectroscopy was
carried out. As illustrated in Figure 5A, the wide-scan XPS spectra of MIL-53(Fe)·H2O possesses the
characteristic peaks of C, O, Fe, and Cl, while Ni/Fe-MOF-0.3.H2O contained C, O, Fe, and Ni. Based
on the XPS analysis, the Ni/Fe-MOF-0.3 had a surface molar ratio of Ni2+/Fe3+ of 0.26, which
approximates the EDS results above. Besides, N was not detected in either sample, indicating that the
DMF solvent was sufficiently eliminated from the MOFs.
Figure 5B shows the C 1s XPS spectra of MIL-53(Fe)·H2O and Ni/Fe-MOF-0.3·H2O samples. Both
spectra were fitted into three peaks at a binding energy (BE) of 285.01, 288.9, and 291.7 eV, which
could be assigned to the carbon components on the phenyl and the carboxylate groups of the BDC
linkers [30,40,44–46]. The O 1s XPS spectra (Figure 5C) could also be fitted into three peaks, which
are (i) the peak at 533.8 eV corresponding to the O components on C=O/H2O, (ii) the peak at 532.3 eV
attributed to the O components on the BDC linkers, and (iii) the peak at 530. 5 eV was assigned to the
Figure 4. SEM images (1, 2) and EDS patterns (3) of as-prepared MIL-53(Fe) and Ni/Fe-MOF crystals
isolated from DMF: MIL-53(Fe) (
a
), Ni/Fe-MOF-0.1 (
b
), Ni/Fe-MOF-0.3 (
c
), Ni/Fe-MOF-0.5 (
d
), and
Ni/Fe-MOF-0.7 (e).
2.1.5. XPS Spectra
To analyze the chemical states of Ni and Fe in the Ni/Fe MOF structure, XPS spectroscopy was
carried out. As illustrated in Figure 5A, the wide-scan XPS spectra of MIL-53(Fe)
·
H
2
O possesses
the characteristic peaks of C, O, Fe, and Cl, while Ni/Fe-MOF-0.3.H
2
O contained C, O, Fe, and Ni.
Based on the XPS analysis, the Ni/Fe-MOF-0.3 had a surface molar ratio of Ni
2+
/Fe
3+
of 0.26, which
approximates the EDS results above. Besides, N was not detected in either sample, indicating that the
DMF solvent was sufficiently eliminated from the MOFs.
Figure 5B shows the C 1s XPS spectra of MIL-53(Fe)
·
H
2
O and Ni/Fe-MOF-0.3
·
H
2
O samples. Both
spectra were fitted into three peaks at a binding energy (BE) of 285.01, 288.9, and 291.7 eV, which
could be assigned to the carbon components on the phenyl and the carboxylate groups of the BDC
linkers [
30
,
40
,
44
–
46
]. The O 1s XPS spectra (Figure 5C) could also be fitted into three peaks, which are
(i) the peak at 533.8 eV corresponding to the O components on C=O/H
2
O, (ii) the peak at 532.3 eV
attributed to the O components on the BDC linkers, and (iii) the peak at 530. 5 eV was assigned to the
Catalysts 2018,8, 487 8 of 20
O components on the Fe–O bonds (for MIL-53(Fe) sample) or Fe
2
NiO clusters (for Ni/Fe-MOF-0.3
sample). These results further confirmed the coordination between the metal ion (Ni
2+
and/or Fe
3+
)
and BDC linkers, which is commensurate with the XRD, IR, and Raman results above.
Catalysts 2018, 8, x FOR PEER REVIEW 8 of 20
O components on the Fe–O bonds (for MIL-53(Fe) sample) or Fe2NiO clusters (for Ni/Fe-MOF-0.3
sample). These results further confirmed the coordination between the metal ion (Ni2+ and/or Fe3+)
and BDC linkers, which is commensurate with the XRD, IR, and Raman results above.
Figure 5. Full scan (A), C1s (B), O1s (C), Fe2p (D), and Ni2p (E) XPS spectra of MIL-53(Fe) and Ni/Fe-
MOF-0.3.
The Fe 2p high-resolution XPS spectrum of MIL-53(Fe) sample (Figure 5D) displays two main
peaks that were indexed to Fe 2p1/2 (712.4 eV) and Fe2p3/2 (726.1 eV). The splitting energy of the 2p
doublet was 13.7 eV, implying that the valence state of Fe was +3 [4,23,44]. Similarly, the valence state
of Fe in the Ni/Fe MOF structure was also +3 because the splitting energy between Fe 2p1/2 (712.9
eV) and Fe 2p3/2 (726.2 eV) was 13.3 eV. To further confirm the valence state of Fe in both of these
1200 1000 800 600 400 200 0
292 288 284 280 536 534 532 530 528
735 730 725 720 715 710 705 885 880 875 870 865 860 855 850
(C)
(B)
MIL(Fe)-53
MIL(Fe)-53
Ni/Fe-MOF-0.3
(A)
Cl2p
Ni2p3
Fe2p
C1s
Intensity (a.u.)
Binding Energy (eV)
Ni/Fe-MOF0.3
MIL-53(Fe)
O1s
C=C
C-C/C-H
HOOCC
6
H
4
COOH
Binding Energy (eV)
C=C
HOOCC
6
H
4
COOH
C-C/C-H
Intensity (a.u.)
Fe-O
C=O/H
2
O
HOOCC
6
H
4
COOH
Binding Energy (eV)
Fe
2
NiO/Ni-O
/Fe-O
C=O/H
2
O
HOOCC
6
H
4
COOH
Ni/Fe-MOF-0.3
Ni/Fe-MOF-0.3
Intensity (a.u.)
MIL(Fe)-53
Fe2p
1/2
Fe2p
3/2
Binding Energy (eV)
(D)
Fe2p
1/2
Fe2p
3/2
Intensity (a.u.)
Ni2p
1/2
Ni2p
3/2
(E)
Ni/Fe-MOF-0.3
Intensity (a.u.)
Binding Energy (eV)
Figure 5.
Full scan (
A
), C1s (
B
), O1s (
C
), Fe2p (
D
), and Ni2p (
E
) XPS spectra of MIL-53(Fe) and
Ni/Fe-MOF-0.3.
The Fe 2p high-resolution XPS spectrum of MIL-53(Fe) sample (Figure 5D) displays two main
peaks that were indexed to Fe 2p1/2 (712.4 eV) and Fe2p3/2 (726.1 eV). The splitting energy of the
2p doublet was 13.7 eV, implying that the valence state of Fe was +3 [
4
,
23
,
44
]. Similarly, the valence
state of Fe in the Ni/Fe MOF structure was also +3 because the splitting energy between Fe 2p1/2
(712.9 eV) and Fe 2p3/2 (726.2 eV) was 13.3 eV. To further confirm the valence state of Fe in both of
these samples, the Fe 2p3/2 peak was fitted into six peaks including Gupta and Sen (GS) multiples,
Catalysts 2018,8, 487 9 of 20
surface structures, and shake-up-related satellites [
28
,
47
,
48
]. The fitting results, as shown in Figures S6
and S7, were indexed well with Fe
3+
GS multiplets, which indicated that the valence state of Fe in the
MIL-53(Fe) and Ni/Fe MOF structure was +3. In the high-resolution XPS spectrum of Ni 2p (Figure 4e),
we observed the BE of the Ni 2p3/2 (857.2 eV) and Ni 2p1/2 (874.8 eV) core-level peaks with the
doublet separation of 17.6 eV, implying that the valence state of Ni was +2 [49,50].
2.1.6. N2Adsorption/Desorption
The specific surface area and porous structure of MIL-53(Fe) and Ni/Fe-MOF crystals isolated
from DMF and H
2
O were determined using N
2
adsorption–desorption isotherms at 77 K. The N
2
adsorption–desorption isotherms, as shown in Figure 6A, displayed an intermediate mode between
type I and type IV, which was associated with mesoporous and microporous materials, respectively [
51
].
The Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore width of MIL-53(Fe)
and Ni/Fe-MOF-0.3 samples are shown in Table 1. The MIL-53(Fe)
·
H
2
O, MIL-53(Fe)
·
DMF,
Ni/Fe-MOF-0.3.H
2
O, and Ni/Fe-MOF-0.3.DMF had specific surface areas of 158, 300, 247, and
480 m
2
/g, respectively (Table 1). The mesopore size distribution curve of samples calculated using the
Barrett–Joyner–Halenda (BJH) model is shown in Figure 6B. The MIL-53(Fe)
·
H
2
O and MIL-53(Fe)
·
DMF
sample was non-porous, whereas Ni/Fe-MOF-0.3
·
H
2
O, and Ni/Fe-MOF-0.3
·
DMF showed a pore
size centered at about 3.8 nm and 21.4 nm, respectively. Therefore, compared with MIL-53(Fe),
Ni/Fe-MOF-0.3 showed a higher value in the specific surface areas. In addition, the higher surface
area and micropore volume for samples isolated from DMF, as compared with samples isolated from
H
2
O, was due to the reversible breathing behavior of these materials, which was dependent on the
molecule present inside their pores, where the pores were opened in the presence of DMF and closed
in the presence of H
2
O [
27
,
28
,
52
]. The formation of porous material for Ni/Fe-MOF-0.3 could be
explained by the formation of Fe
2
NiO cluster in the Ni/Fe-MOF structure, which could affect the
reversible breathing behavior of these materials. MIL-88B(Fe) crystals with trinuclear metal clusters
were known as non-porous materials due to the need for compensating the anion inside their porous
system [
28
,
53
]. Do and coworkers demonstrated that MOF structure with the presence of Fe
2
NiO
cluster as nodes in the MIL-88B framework avoids the compensating anion [
27
,
28
], which results in
the formation of porous material for Ni/Fe-MOF-0.3. In addition, the cracks on the crystal surface of
Ni/Fe-MOF-0.3 (Figure 4) could also partly create the characteristics of microporous or mesoporous
materials for this sample.
Table 1. Specific surface area and porosity of MiL-53(Fe) and Ni/Fe-MOF samples.
Samples Specific Surface
Area (m2/g)
Micropore Volume
(×10−3cm3/g)
Mesopore Volume
(×10−3cm3/g)
Average Pore
Width (nm)
MIL-53(Fe)·DMF 300 128 97 13
MIL-53(Fe)·H2O 158 65 59 11
Ni/Fe-MOF-0.3
·
DMF
480 212 128 8
Ni/Fe-MOF-0.3
·
H
2
O
247 94 271 13
Catalysts 2018,8, 487 10 of 20
Catalysts 2018, 8, x FOR PEER REVIEW 10 of 20
Figure 6. N2 adsorption–desorption isotherms (A) and pore size distributions (B) of the synthesized
samples: MIL-53(Fe)·DMF (a), MIL-53(Fe)·H2O (b), Ni/Fe-MOF-0.3·DMF (c), and Ni/Fe-MOF-0.3·H2O
(d).
2.1.7. UV-Vis Spectra
The light absorption properties of the material were studied through the UV-Vis-DRS spectra.
The UV-Vis-DRS spectrum of the material is shown in Figure 7. For washing samples with DMF
(Figure 7A), MIL-53(Fe)·DMF gave strong absorption bands in the wavelength range of 200 to 400
nm. The strong absorption bands at 256 to 310 nm could be due to the transfer of the charge from the
oxygen center of the organic bridge to the metal center in the octahedral FeO6 structure [17,54]. The
band at 350 to 500 nm was due to the shift of d–d (6A1g → 4A1g + 4Eg (G)) of Fe3+ in the MIL-53(Fe)
structure [14,27]. The main absorption edge (λ, nm) of the MIL-53(Fe)·DMF was 478 nm,
corresponding to the bandgap energy Eg = 2.59 eV (Eg = 1240/λ). This result is in accordance with
previous reports [44,55]. When the MIL-53(Fe) was modified with Ni, the material have the decreased
absorption in the wavelength range from 200 to 500 nm, and the absorption spectrum extended in
the range from 250 to 800 nm, so it was difficult to determine the absorption of the material accurately.
When the material was washed with water (Figure 7B), the modified material had an increased
absorption in the wavelength range from 200 to 400 nm, and the absorption intensity was higher and
broader in the visible light region as compared to the modified sample washed with DMF. As the
material was washed with water, there was a structural change between the large pore and the
narrow pore caused by the “breathing” effect when the material absorbed the water molecules inside
the pore. This phase transformation of the structure led to a change in the electronic structure [56],
and subsequently, a change in the absorption spectrum of the material and decreasing Eg. For Ni/Fe-
MOF-0.1·H2O, Ni/Fe-MOF-0.3·H2O, and Ni/Fe-MOF-0.5·H2O samples, the absorption intensity in the
visible light region and the absorption band of the material shifted to a wavelength longer than for
MIL-53(Fe)·H2O. As absorption in the visible light increased, the visible light energy could be used
more efficiently, thus contributing to the increased photocatalytic efficiency of the material. The
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
250
2.0 2.5 3.0 3.5 4.0 4.5
0
5
10
15
20
25
30
0 10203040
0
5
10
15
20
25
30
Adsorpton
Desorption
(d)
(c)
(b)
Quantity Adsorbed (cm³/g STP)
Relative pressure (p/p
o
)
(a)
(A)
Cumilative Pore Volume (cm
3
/g)
Pore width (nm)
(B)
(a)
(b)
(c)
(d)
Cumilative Pore Volume (cm
3
/g)
Pore width (nm)
Figure 6.
N
2
adsorption–desorption isotherms (
A
) and pore size distributions (
B
) of the synthesized
samples: MIL-53(Fe)
·
DMF (a), MIL-53(Fe)
·
H
2
O (b), Ni/Fe-MOF-0.3
·
DMF (c), and Ni/Fe-MOF-0.3
·
H
2
O (d).
2.1.7. UV-Vis Spectra
The light absorption properties of the material were studied through the UV-Vis-DRS spectra.
The UV-Vis-DRS spectrum of the material is shown in Figure 7. For washing samples with DMF
(Figure 7A), MIL-53(Fe)
·
DMF gave strong absorption bands in the wavelength range of 200 to 400 nm.
The strong absorption bands at 256 to 310 nm could be due to the transfer of the charge from the
oxygen center of the organic bridge to the metal center in the octahedral FeO
6
structure [
17
,
54
].
The band at 350 to 500 nm was due to the shift of d–d (6A1g
→
4A1g + 4Eg (G)) of Fe
3+
in the
MIL-53(Fe) structure [
14
,
27
]. The main absorption edge (
λ
, nm) of the MIL-53(Fe)
·
DMF was 478 nm,
corresponding to the bandgap energy E
g
= 2.59 eV (E
g
= 1240/
λ
). This result is in accordance with
previous reports [
44
,
55
]. When the MIL-53(Fe) was modified with Ni, the material have the decreased
absorption in the wavelength range from 200 to 500 nm, and the absorption spectrum extended
in the range from 250 to 800 nm, so it was difficult to determine the absorption of the material
accurately. When the material was washed with water (Figure 7B), the modified material had an
increased absorption in the wavelength range from 200 to 400 nm, and the absorption intensity
was higher and broader in the visible light region as compared to the modified sample washed
with DMF. As the material was washed with water, there was a structural change between the
large pore and the narrow pore caused by the “breathing” effect when the material absorbed the
water molecules inside the pore. This phase transformation of the structure led to a change in the
electronic structure [
56
], and subsequently, a change in the absorption spectrum of the material and
decreasing E
g
. For Ni/Fe-MOF-0.1
·
H
2
O, Ni/Fe-MOF-0.3
·
H
2
O, and Ni/Fe-MOF-0.5
·
H
2
O samples, the
absorption intensity in the visible light region and the absorption band of the material shifted to a
wavelength longer than for MIL-53(Fe)
·
H
2
O. As absorption in the visible light increased, the visible
Catalysts 2018,8, 487 11 of 20
light energy could be used more efficiently, thus contributing to the increased photocatalytic efficiency
of the material. The absorption edges of MIL-53(Fe)
·
H
2
O, Ni/Fe-MOF-0.1
·
H
2
O, Ni/Fe-MOF-0.3
·
H
2
O,
Ni/Fe-MOF-0.5
·
H
2
O, and Ni/Fe-MOF-0.7
·
H
2
O were 504, 553, 532, 513, and 516 nm (Figure S2),
corresponding to the optical bandgap of 2.46, 2.24, 2.33, 2.42, and 2.40 eV, respectively. These results
provided a potential photoreactivity of MIL-53(Fe) and Ni/Fe-MOF samples in the visible light range.
Catalysts 2018, 8, x FOR PEER REVIEW 11 of 20
absorption edges of MIL-53(Fe)·H2O, Ni/Fe-MOF-0.1·H2O, Ni/Fe-MOF-0.3·H2O, Ni/Fe-MOF-0.5·H2O,
and Ni/Fe-MOF-0.7·H2O were 504, 553, 532, 513, and 516 nm (Figure S2), corresponding to the optical
bandgap of 2.46, 2.24, 2.33, 2.42, and 2.40 eV, respectively. These results provided a potential
photoreactivity of MIL-53(Fe) and Ni/Fe-MOF samples in the visible light range.
Figure 7. UV-Vis DRS spectra of as-prepared MIL-53(Fe) and Ni-MIL-53(Fe) crystals isolated from
DMF (A) and H2O (B): MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d), and
Ni/Fe-MOF-0.7 (e).
2.1.8. PL Spectroscopy
PL spectra of MIL-53(Fe) and Ni/Fe-MOF samples were recorded at room temperature and are
shown in Figure 8. When the MIL-53(Fe) sample was excited by a 320 nm laser, its emission spectrum
showed a strong emission range of 350 to 500 nm and a weak emission range of 570 to 750 nm. In
comparison, the intensity of Ni/Fe-MOF samples was significantly lower than that of the MIL-53(Fe)
sample because of the presence of the Ni2FeO cluster in the structure of the Ni/Fe-MOF crystal. These
results demonstrated that electron–hole recombination could be inhibited in the Ni/Fe-MOF,
resulting in the improvement of photocatalytic performance. PL spectra, along with the UV-Vis DRS
result, could satisfy the prerequisite for visible-light photocatalysis.
Figure 8. PL spectra of as-prepared MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-
MOF-0.5 (d), and Ni/Fe-MOF-0.7 (e).
200 400 600 800 200 400 600 800
(B)
(A)
Absorbance (a.u.)
Wavelength (nm)
(a)
(b)
(c)
(d)
(e)
Wavelength (nm)
(a)
(b)
(c)
(d)
(e)
400 500 600 700 800 900
0
10,000
20,000
30,000
40,000
50,000
400 500 600 700 800
0
200
400
600
800
Intensity (Counts)
Wavelength (nm)
M0
M0.1
M0.3
M0.5
M0.7
Intensity (Counts)
Wavelength (nm)
(a)
(b)
(c)
(d)
(e)
Figure 7.
UV-Vis DRS spectra of as-prepared MIL-53(Fe) and Ni-MIL-53(Fe) crystals isolated from
DMF (
A
) and H
2
O (
B
): MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d),
and Ni/Fe-MOF-0.7 (e).
2.1.8. PL Spectroscopy
PL spectra of MIL-53(Fe) and Ni/Fe-MOF samples were recorded at room temperature and are
shown in Figure 8. When the MIL-53(Fe) sample was excited by a 320 nm laser, its emission spectrum
showed a strong emission range of 350 to 500 nm and a weak emission range of 570 to 750 nm.
In comparison, the intensity of Ni/Fe-MOF samples was significantly lower than that of the MIL-53(Fe)
sample because of the presence of the Ni
2
FeO cluster in the structure of the Ni/Fe-MOF crystal. These
results demonstrated that electron–hole recombination could be inhibited in the Ni/Fe-MOF, resulting
in the improvement of photocatalytic performance. PL spectra, along with the UV-Vis DRS result,
could satisfy the prerequisite for visible-light photocatalysis.
Catalysts 2018, 8, x FOR PEER REVIEW 11 of 20
absorption edges of MIL-53(Fe)·H2O, Ni/Fe-MOF-0.1·H2O, Ni/Fe-MOF-0.3·H2O, Ni/Fe-MOF-0.5·H2O,
and Ni/Fe-MOF-0.7·H2O were 504, 553, 532, 513, and 516 nm (Figure S2), corresponding to the optical
bandgap of 2.46, 2.24, 2.33, 2.42, and 2.40 eV, respectively. These results provided a potential
photoreactivity of MIL-53(Fe) and Ni/Fe-MOF samples in the visible light range.
Figure 7. UV-Vis DRS spectra of as-prepared MIL-53(Fe) and Ni-MIL-53(Fe) crystals isolated from
DMF (A) and H2O (B): MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-MOF-0.5 (d), and
Ni/Fe-MOF-0.7 (e).
2.1.8. PL Spectroscopy
PL spectra of MIL-53(Fe) and Ni/Fe-MOF samples were recorded at room temperature and are
shown in Figure 8. When the MIL-53(Fe) sample was excited by a 320 nm laser, its emission spectrum
showed a strong emission range of 350 to 500 nm and a weak emission range of 570 to 750 nm. In
comparison, the intensity of Ni/Fe-MOF samples was significantly lower than that of the MIL-53(Fe)
sample because of the presence of the Ni2FeO cluster in the structure of the Ni/Fe-MOF crystal. These
results demonstrated that electron–hole recombination could be inhibited in the Ni/Fe-MOF,
resulting in the improvement of photocatalytic performance. PL spectra, along with the UV-Vis DRS
result, could satisfy the prerequisite for visible-light photocatalysis.
Figure 8. PL spectra of as-prepared MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-
MOF-0.5 (d), and Ni/Fe-MOF-0.7 (e).
200 400 600 800 200 400 600 800
(B)
(A)
Absorbance (a.u.)
Wavelength (nm)
(a)
(b)
(c)
(d)
(e)
Wavelength (nm)
(a)
(b)
(c)
(d)
(e)
400 500 600 700 800 900
0
10,000
20,000
30,000
40,000
50,000
400 500 600 700 800
0
200
400
600
800
Intensity (Counts)
Wavelength (nm)
M0
M0.1
M0.3
M0.5
M0.7
Intensity (Counts)
Wavelength (nm)
(a)
(b)
(c)
(d)
(e)
Figure 8.
PL spectra of as-prepared MIL-53(Fe) (a), Ni/Fe-MOF-0.1 (b), Ni/Fe-MOF-0.3 (c), Ni/Fe-
MOF-0.5 (d), and Ni/Fe-MOF-0.7 (e).
Catalysts 2018,8, 487 12 of 20
2.2. Photocatalytic Activities
2.2.1. RhB Removal by MIL-53(Fe) and Ni-MIL-53(Fe)
The photocatalytic activities of MIL-53(Fe) and Ni/Fe-MOF-x photocatalysts were evaluated in the
liquid-phase photodegradation of RhB dye under visible light irradiation. Figure 9displays the changes
of RhB concentrations via adsorption and photocatalytic degradation under different experimental
conditions. As shown in Figure 9, a negligible degradation of RhB concentrations was observed in the
several blank runs including RhB/H
2
O
2
/Dark, RhB/H
2
O
2
/Light, and RhB/Dark systems, proving
the stability property of RhB under visible light irradiation of compact fluorescent light. Also, as
shown in Figure 9A, after 180 min adsorption (in the dark), 16% and 51% RhB were removed in the
presence of MIL-53(Fe) (MIL-53(Fe)/Dark system) and Ni/Fe-MOF-0.3 (Ni/Fe-MOF-0.3/Dark system),
respectively. The higher adsorption capacity of the Ni/Fe-MOF-0.3 sample was due to its higher
surface area (247 m
2
/g for Ni/Fe-MOF-0.3 and 158 m
2
/g for MIL-53(Fe)). In addition, there was no
significant difference in the removal of RhB concentration in the two adsorption experiments with
the presence of H
2
O
2
(MIL-53(Fe)/H
2
O
2
/Dark and Ni/Fe-MOF-0.3/H
2
O
2
/Dark systems and the
absence of H
2
O
2
(MIL-53(Fe)/Dark and Ni/Fe-MOF-0.3/Dark systems. Therefore, our photocatalytic
experiments do display the presence of a Fenton reaction.
Catalysts 2018, 8, x FOR PEER REVIEW 13 of 20
in the following three processes: (i) successfully migration to the surface of MOFs, (ii) being captured
by the defect sites in bulk and/or on the surface region of semiconductor, and (iii) recombining and
releasing the energy in the form of heat or a photon. Then, the h+ can accept electrons and induce
water molecules to generate hydroxyl radicals (•OH), which exhibit a high oxidation ability to
decompose the organic dyes. However, there is a recombination of excessive electrons and holes,
resulting in the restricted photocatalytic activity of this material. In our study, mixed a metal Fe/Ni-
based framework that consists of a mixed-metal cluster Fe2NiO with electron transfer effect may
enhance the photocatalytic performance [45,61,65].
Besides, a mixed metal Fe/Ni-based framework that consists of the mixed-metal cluster Fe2NiO
possesses large pores and a high surface area, as compared with a single metal Fe-based framework;
therefore, Ni/Fe-MOF exhibited a high adsorption capacity of RhB and high photocatalytic activity
in RhB degradation. XRD patterns of Ni/Fe-MOF-0.3 before and after reactions were shown in Figure
S3 (SI file). As shown in Figure S3, there was no apparent difference in the crystal structure. This
result indicated that the crystal structure of the material did not change after the photocatalytic
reaction.
Figure 9. Adsorption (A) and photodegradation (B) of RhB under different conditions over MIL-
53(Fe) and Ni/Fe-MOF-0.3, and UV–Vis spectral of RhB solution separated from the Ni/Fe-MOF-
0.3/Light/H2O2 catalytic system (C) and MIL-53(Fe)/Light /H2O2 catalytic system (D).
2.2.2. Effect of Initial Dye Concentration, Initial Solution pH, and the Molar Ratio of Ni2+/Fe3+ on the
Degradation of RhB
The effect of initial dye concentration on the degradation of RhB over the Ni/Fe-MOF-
0.3/Light/H2O2 system was evaluated (Figure 10A). As shown in Figure 10A, the degradation
efficiency of RhB was slightly decreased when increasing the initial dye concentration from 1 × 10−5
to 4 × 10−5 M. This was mainly because of the increase of the dye molecules around the active sites
leading to inhibiting the penetration of light to the surface of the catalyst [66].
0 306090120150180
0.0
0.2
0.4
0.6
0.8
1.0
0 30 60 90 120 150 180
0.0
0.2
0.4
0.6
0.8
1.0
300 400 500 600 300 400 500 600
C/C
o
Time (min)
RhB/H
2
O
2
RhB
MIL-53(Fe)/H
2
O
2
MIL-53(Fe)
Ni/Fe-MOF-0.3/H
2
O
2
Ni/Fe-MOF-0.3
(B)
C/C
o
Time (min)
(C)
Absorbance (a.u.)
Wavelength (nm)
(D)
RhB Orignal
Light on 30 min
Light on 60 min
Light on 90 min
Light on 120 min
Light on 150 min
Light on 180 min
Absorbance (a.u.)
Wavelength (nm)
(A)
Figure 9.
Adsorption (
A
) and photodegradation (
B
) of RhB under different conditions over MIL-53(Fe)
and Ni/Fe-MOF-0.3, and UV-Vis spectral of RhB solution separated from the Ni/Fe-MOF-0.3/
Light/H2O2catalytic system (C) and MIL-53(Fe)/Light /H2O2catalytic system (D).
Under visible light irradiation, the presence of MIL-53(Fe) could enhance the degradation
efficiency of RhB up to 81.46% using a photolysis process in MIL-53(Fe)/Light/H
2
O
2
catalytic system
(Figure 9B). For the Ni/Fe-MOF-0.3/Light/H
2
O
2
catalytic system, the degradation efficiency of RhB
was remarkably enhanced where about 91.14% RhB removal was achieved (Figure 9B). The higher
photocatalytic activity of the Ni/Fe-MOF-0.3 sample as compared with MIL-53(Fe) could also be
indicated by the change of the UV-Vis absorption spectra of the solution in the course of the RhB
Catalysts 2018,8, 487 13 of 20
degradation (Figure 9C,D). As seen in Figure 9C,D, the primary absorption band, which could be
attributed to RhB, shifted from 554 to 500 nm in a step-wise manner. This change could be reasonably
assigned to the removal of ethyl groups one by one in this reaction, which is in good agreement
with the previous literature. The photodegradation of RhB over MIL-53(Fe) and Ni/Fe-MOF-0.3
photocatalysts approximately followed a pseudo-first-order kinetics model: ln(C
o
/C) = k
obs
t [
57
–
59
].
The presence of Ni/Fe-MOF-0.3 promoted the photodegradation rate; the rate constants were 8.88
×
10−3min−1for MIL-53(Fe) and 11.15 ×10−3min−1for Ni/Fe-MOF-0.3.
To investigate the role of H
2
O
2
on the photocatalytic performance of MIL-53(Fe) and
Ni/Fe-MOF photocatalysts, the photocatalytic processes with the presence and absence of H
2
O
2
were conducted in parallel (Figure 9B). After 180 min of irradiation, the degradation rate of RhB
over MIL-53(Fe)/Light/H
2
O
2
and MIL-53(Fe)/Light process was 81.46% and 27.60%, respectively. Only
MIL-53(Fe) with the absence of H
2
O
2
exhibits the low efficiency of RhB photodegradation due to the fast
electron-hole recombination, which is in good agreement with the previous literature [
13
,
17
]. For the
MIL-53(Fe)/H
2
O
2
/Light process, H
2
O
2
acted as an electron accepter, resulting in the suppression of
charge recombination; therefore, the rate for RhB decomposition could be significantly enhanced, as
was demonstrated by Du et al. [
13
]. Similarly, Ai et al. also showed that the enhancement of MI-53(Fe)
photocatalytic performance could be due to the synergistic effects of the combination of MIL-53(Fe)
and H
2
O
2
under visible light irradiation [
17
]. Interestingly, the effect of H
2
O
2
on the photocatalytic
performance of the Ni/Fe-MOF photocatalyst showed a considerable difference. The Ni/Fe-MOF sample
could degrade more than 90% of the initial RhB content regardless of the presence or absence of H2O2.
The superior catalytic performance of the Ni/Fe-MOF sample could be explained by the formation
of the mixed metal cluster Fe
2
NiO in the Ni/Fe-MOF framework. According to recent reports, the
Fe-based framework (MIL-101, MIL-100, MIL-88, and MOF-235), containing single metal cluster
Fe
3
-
µ3
-oxo clusters with small particle sizes, are proposed as a visible light photocatalyst [
44
,
60
–
63
].
The reaction mechanism of these materials have been reported based on semiconductor theory and
previous reports [
61
–
64
]. Particularly, when the surface of MOFs material absorbs photons (E
photons ≥
E
g
), the electrons (e
−
) in the valence band (VB) will be excited to the conduction band (CB), leaving
the holes (h
+
) in the VB. These photogenerated e
−
–h
+
pairs may be further involved in the following
three processes: (i) successfully migration to the surface of MOFs, (ii) being captured by the defect
sites in bulk and/or on the surface region of semiconductor, and (iii) recombining and releasing the
energy in the form of heat or a photon. Then, the h
+
can accept electrons and induce water molecules
to generate hydroxyl radicals (
•
OH), which exhibit a high oxidation ability to decompose the organic
dyes. However, there is a recombination of excessive electrons and holes, resulting in the restricted
photocatalytic activity of this material. In our study, mixed a metal Fe/Ni-based framework that
consists of a mixed-metal cluster Fe
2
NiO with electron transfer effect may enhance the photocatalytic
performance [45,61,65].
Besides, a mixed metal Fe/Ni-based framework that consists of the mixed-metal cluster Fe
2
NiO
possesses large pores and a high surface area, as compared with a single metal Fe-based framework;
therefore, Ni/Fe-MOF exhibited a high adsorption capacity of RhB and high photocatalytic activity in
RhB degradation. XRD patterns of Ni/Fe-MOF-0.3 before and after reactions were shown in Figure S3
(SI file). As shown in Figure S3, there was no apparent difference in the crystal structure. This result
indicated that the crystal structure of the material did not change after the photocatalytic reaction.
2.2.2. Effect of Initial Dye Concentration, Initial Solution pH, and the Molar Ratio of Ni
2+
/Fe
3+
on the
Degradation of RhB
The effect of initial dye concentration on the degradation of RhB over the Ni/Fe-MOF-0.3/Light/
H
2
O
2
system was evaluated (Figure 10A). As shown in Figure 10A, the degradation efficiency of RhB
was slightly decreased when increasing the initial dye concentration from 1
×
10
−5
to 4
×
10
−5
M.
This was mainly because of the increase of the dye molecules around the active sites leading to
inhibiting the penetration of light to the surface of the catalyst [66].
Catalysts 2018,8, 487 14 of 20
Catalysts 2018, 8, x FOR PEER REVIEW 14 of 20
0 30 60 90 120 150 180
0.0
0.2
0.4
0.6
0.8
1.0
0 30 60 90 120 150 180
0.0
0.2
0.4
0.6
0.8
1.0
0 306090120150180
0.0
0.2
0.4
0.6
0.8
1.0
(B)
C/C
0
Time (min)
1.10
-5
M
2.10
-5
M
3.10
-5
M
4.10
-5
M
(A)
C/C
0
Time (mi n)
pH = 3
pH = 5
pH = 7
pH = 9
(C)
C/C
0
Time (mi n)
0 % Ni
0.1 % Ni
0.3 % Ni
0.5 % Ni
0.7 % Ni
Figure 10. Effect of initial dye concentration (A), initial solution pH (B), and the molar ratio of Ni2+/Fe3+
(C) on the degradation of RhB.
The effect of the initial pH on the degradation of RhB on the degradation of RhB over Ni/Fe-
MOF/Light/H2O2 system was also investigated. The pH of the initial solution was selected as follows:
3, 5 (acidic), 7 (neutral), and 9 (basic). At different pH conditions, the Ni/Fe-MOF-0.3 remained most
effective when it came to removing RhB. The RhB removal efficiency peaked at the solution pH of 5
and decreased with increasing pH thereafter (Figure 10B). This result could be explained by the fact
that when the pH exceeded the isoelectric point of the material, they were negatively charged. In
addition, the RhB used in this experiment was a cationic color such that the material would absorb
the color gradually from pH 5 to 9. As the adsorption increased, the color molecules would shield the
catalytic surface, which prevented light from irradiating on the catalyst surface, thus decreasing
photocatalytic activity and reducing color removal. The pH at the isoelectric point or point of zero
charge-pzc of the material was an important parameter for evaluation of the acidity/basicity and the
surface charge of the adsorbent in solution. The determination of pHzpc was carried out according
to our previously published study [67–69], as follows: Photocatalysts (20 mg) was added to flasks
containing 100 mL of KCl 0.1 M at different initial pH values (pHi = 2, 4, 6, 8, 10, and 12). The solutions
were shaken in the shaker for 24 h, and then solids were removed from the mixture by centrifugation
at 4000 rpm for 15 min. The final pH of the solution (pHf) is measured using a pH meter. The curve
was plotted via pHf against the pHi, and the pHpzc was calculated at pHi = pHf. As shown in Figure
11A,B, the pHpzc values of the MIL-53(Fe) and Ni/Fe-MOF-0.3 were approximately equal and were
within the pH range of 4.1–4.2.
Figure 11. Measurement of pHzpc: the initial versus final pH plot: pH initial (a), pH initial-MIL-53(Fe)
(b), and pH initial-Ni/Fe-MOF-0.3 (c) (A) and enlarged pH initial from 3 to 5 (B).
24681012
2
4
6
8
10
12
3.0 3.5 4.0 4.5 5.0
3.0
3.5
4.0
4.5
5.0
(a)
(b)
(c)
(a)
(b)
(c)
pH final
pH initial
4.11
(B)
(A)
4.17
pH final
pH initial
Figure 10.
Effect of initial dye concentration (
A
), initial solution pH (
B
), and the molar ratio of Ni
2+
/Fe
3+
(C) on the degradation of RhB.
The effect of the initial pH on the degradation of RhB on the degradation of RhB over
Ni/Fe-MOF/Light/H
2
O
2
system was also investigated. The pH of the initial solution was selected as
follows: 3, 5 (acidic), 7 (neutral), and 9 (basic). At different pH conditions, the Ni/Fe-MOF-0.3 remained
most effective when it came to removing RhB. The RhB removal efficiency peaked at the solution pH
of 5 and decreased with increasing pH thereafter (Figure 10B). This result could be explained by the
fact that when the pH exceeded the isoelectric point of the material, they were negatively charged.
In addition, the RhB used in this experiment was a cationic color such that the material would absorb
the color gradually from pH 5 to 9. As the adsorption increased, the color molecules would shield
the catalytic surface, which prevented light from irradiating on the catalyst surface, thus decreasing
photocatalytic activity and reducing color removal. The pH at the isoelectric point or point of zero
charge-pzc of the material was an important parameter for evaluation of the acidity/basicity and the
surface charge of the adsorbent in solution. The determination of pHzpc was carried out according
to our previously published study [
67
–
69
], as follows: Photocatalysts (20 mg) was added to flasks
containing 100 mL of KCl 0.1 M at different initial pH values (pH
i
= 2, 4, 6, 8, 10, and 12). The solutions
were shaken in the shaker for 24 h, and then solids were removed from the mixture by centrifugation at
4000 rpm for 15 min. The final pH of the solution (pH
f
) is measured using a pH meter. The curve was
plotted via pH
f
against the pH
i
, and the pHpzc was calculated at pH
i
= pH
f
. As shown in Figure 11A,B,
the pH
pzc
values of the MIL-53(Fe) and Ni/Fe-MOF-0.3 were approximately equal and were within
the pH range of 4.1–4.2.
Catalysts 2018, 8, x FOR PEER REVIEW 14 of 20
0 30 60 90 120 150 180
0.0
0.2
0.4
0.6
0.8
1.0
0 30 60 90 120 150 180
0.0
0.2
0.4
0.6
0.8
1.0
0 306090120150180
0.0
0.2
0.4
0.6
0.8
1.0
(B)
C/C
0
Time (min)
1.10
-5
M
2.10
-5
M
3.10
-5
M
4.10
-5
M
(A)
C/C
0
Time (mi n)
pH = 3
pH = 5
pH = 7
pH = 9
(C)
C/C
0
Time (mi n)
0 % Ni
0.1 % Ni
0.3 % Ni
0.5 % Ni
0.7 % Ni
Figure 10. Effect of initial dye concentration (A), initial solution pH (B), and the molar ratio of Ni2+/Fe3+
(C) on the degradation of RhB.
The effect of the initial pH on the degradation of RhB on the degradation of RhB over Ni/Fe-
MOF/Light/H2O2 system was also investigated. The pH of the initial solution was selected as follows:
3, 5 (acidic), 7 (neutral), and 9 (basic). At different pH conditions, the Ni/Fe-MOF-0.3 remained most
effective when it came to removing RhB. The RhB removal efficiency peaked at the solution pH of 5
and decreased with increasing pH thereafter (Figure 10B). This result could be explained by the fact
that when the pH exceeded the isoelectric point of the material, they were negatively charged. In
addition, the RhB used in this experiment was a cationic color such that the material would absorb
the color gradually from pH 5 to 9. As the adsorption increased, the color molecules would shield the
catalytic surface, which prevented light from irradiating on the catalyst surface, thus decreasing
photocatalytic activity and reducing color removal. The pH at the isoelectric point or point of zero
charge-pzc of the material was an important parameter for evaluation of the acidity/basicity and the
surface charge of the adsorbent in solution. The determination of pHzpc was carried out according
to our previously published study [67–69], as follows: Photocatalysts (20 mg) was added to flasks
containing 100 mL of KCl 0.1 M at different initial pH values (pHi = 2, 4, 6, 8, 10, and 12). The solutions
were shaken in the shaker for 24 h, and then solids were removed from the mixture by centrifugation
at 4000 rpm for 15 min. The final pH of the solution (pHf) is measured using a pH meter. The curve
was plotted via pHf against the pHi, and the pHpzc was calculated at pHi = pHf. As shown in Figure
11A,B, the pHpzc values of the MIL-53(Fe) and Ni/Fe-MOF-0.3 were approximately equal and were
within the pH range of 4.1–4.2.
Figure 11. Measurement of pHzpc: the initial versus final pH plot: pH initial (a), pH initial-MIL-53(Fe)
(b), and pH initial-Ni/Fe-MOF-0.3 (c) (A) and enlarged pH initial from 3 to 5 (B).
24681012
2
4
6
8
10
12
3.0 3.5 4.0 4.5 5.0
3.0
3.5
4.0
4.5
5.0
(a)
(b)
(c)
(a)
(b)
(c)
pH final
pH initial
4.11
(B)
(A)
4.17
pH final
pH initial
Figure 11.
Measurement of pHzpc: the initial versus final pH plot: pH initial (a), pH initial-MIL-53(Fe)
(b), and pH initial-Ni/Fe-MOF-0.3 (c) (A) and enlarged pH initial from 3 to 5 (B).
Catalysts 2018,8, 487 15 of 20
The degradation results of the different molar ratios of Ni
2+
/Fe
3+
in the samples are shown in
Figure 10C, where the best performance was obtained with the Ni/Fe-MOF-0.3 sample, followed by
the Ni/Fe-MOF-0.1 and Ni/Fe-MOF-0.7 samples. The Ni/Fe-MOF-0.5 sample showed the lowest
catalytic activity among all the Ni/Fe-MOF catalysts. This result indicated that the different molar
ratio of Ni
2+
/Fe
3+
had a significant impact on the photocatalytic performance of Ni/Fe-MOF samples,
which may be conducive to the structure and morphology formation of Ni/Fe-MOF.
3. Experimental
3.1. Materials
1,4-Benzenedioic acid (H
2
BDC, 98%) and RhB (
≥
95%) were purchased from Sigma-Aldrich
(St. Louis, MO, USA). Iron(III) chloride hexahydrate (FeCl
3·
6H
2
O, 99%), nickel(II) nitrate hexahydrate
(Ni(NO
3
)
2·
6H
2
O, 99%), N,N-dimethylformamide (DMF, 99%), ethanol, and hydrogen peroxide (H
2
O
2
,
30%) were obtained from Xilong Chemical Co., Ltd. (Guangzhou, China). All reagents were used as
received without further purification.
3.2. Preparation of Catalysts
Ni/Fe-MOF samples were synthesized using a solvothermal router similar to MIL-53(Fe),
according to the previous literature [
39
]. In a typical synthesis, 9 mmol of H
2
BDC, 6 mmol of
FeCl
3·
6H
2
O, and a certain amount of Ni(NO
3
)
2·
6H
2
O were dissolved in 60 mL DMF. The obtained
mixture was vigorously stirred for 30 min before being transferred into a 100 mL hydrothermal
synthesis autoclave reactor 304 stainless steel high-pressure digestion tank with PTFE lining
(Baoshishan Co., Ltd., Shanghai, China). The autoclave was heated at 100
◦
C in an oven (Memmert
UN110, Schwabach, Germany) with a heating rate of 5
◦
C/min for three days. After being cooled to
room temperature in air, the remaining H
2
BDC was removed using a distillation method with DMF
solvent for 24 h at 100
◦
C with a heating rate of 5
◦
C/min. The obtained suspension was centrifuged at
6000 rpm for 30 min, and the orange precipitates located at the bottom of the tube were washed with
DMF (three times) and water (three times), respectively. Finally, the product was dried for 24 h at 60
◦
C.
The obtained MOFs samples with corresponding Ni concentration were denoted as Ni/Fe-MOF-x (x is
the molar ratio of Ni
2+
/Fe
3+
, and was chosen as 0, 0.1, 0.3, 0.5, and 0.7). The specific description is
shown in Table S1 and the flow chart of the synthesis method is described in Figure S4. The sample
was washed with DMF and water to obtain Ni/Fe-MOF-x
·
DMF and Ni/Fe-MOF-x
·
H
2
O, respectively.
For comparison, MIL(53) also was prepared using a similar method above without the presence of
Ni(NO3)2·6H2O in the reaction solution mixture.
3.3. Catalyst Characterization
Powder X-ray diffraction (XRD) patterns were conducted on a D8 Advance Bruker powder
diffractometer with a Cu K
α
source (
λ
= 0.15405) at a scan rate of 0.04
◦
/s with 2
θ
= 2 to 30
◦
.
The surface morphologies and particle size of Ni/Fe-MOF samples were observed using field emission
scanning electron microscope (FESEM, JEOL JSM-7600F, Peabody, MA, USA) equipped with an energy
dispersive X-ray spectroscope (EDS, Oxford instruments 50 mm
2
X-Max, Abingdon, UK). FT-IR
spectra were recorded on an EQUINOX 55 spectrometer (Bruker, Germany) using the KBr pellet
technique. Raman spectroscopy was carried out on the HORIBA Jobin Yvon spectrometer with a
laser beam of 633 nm. To examine the existence of Ni and Fe in the samples, X-ray photoelectron
spectra (XPS) of the samples was measured using MultiLab 2000 spectrometer (Thermo VG Scientific,
Waltham, MA, USA). The optical absorption characteristics of the photocatalysts were determined
using ultraviolet-visible (UV/Vis) diffuse reflectance spectroscopy (UV/Vis DRS, Shimazu UV-2450,
Kyoto, Japan) in the range 200–900 cm
−1
. PL spectroscopy was performed using a Hitachi F4500
Fluorescence Spectrometer (Schaumburg, IL, USA) with the Xe Lamp Power range (700–900 V) at
room temperature. The specific surface area and pore distribution of MIL-53(Fe) and Ni/Fe-MOFs
Catalysts 2018,8, 487 16 of 20
were determined using the Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–Halenda (BJH)
method, respectively (TriStar 3000 V6.07, Micromeritics instrument corporation, Norcross, GA, USA).
The samples were kept at 200
◦
C for 5 h to degas. The pH value was measured using a pH meter
(Consort-C1010, Turnhout, Belgium) at room temperature.
3.4. Photocatalytic Test
The photocatalytic activities of Ni/Fe-MOF photocatalysts were evaluated using the
photodegradation of RhB under visible light irradiation with a 40 W compact fluorescent lamp
(Philips) in the open air and at room temperature (Figure S5). The intensity and wavelength of the light
source was 4400 lm and >400 nm, respectively (Figure S6 and Table S2). Therefore, it was suggested
that the photocatalytic processes in our experiments were mainly due to the action of the visible
light range [
70
–
72
]. In each run, a mixture of RhB aqueous solution (3.10
−5
mol/L, 100 mL), the
given catalyst (20 mg), and H
2
O
2
(10
−5
mol/L) was magnetically stirred in the presence or absence
of light. Five milliliters of the suspension was withdrawn at the same intervals and immediately
centrifuged to separate the photocatalyst particles for 15 min. The concentration of RhB was analyzed
using a UV-visible spectrophotometer (Model Evolution 60S, Thermo Fisher Scientific, Waltham, MA,
USA) at a maximum absorbance wavelength of
λ
= 554 nm. In addition, the effect of parameters
including initial dye concentration and initial solution pH on the photodegradation of RhB over
Ni/Fe-MOF photocatalysts was also investigated. pH levels of 3, 5, 7, and 9 were selected, whereas the
concentrations of RhB were increased from 1.10−5M to 4.10−5M.
4. Conclusions
In summary, we have successfully prepared mixed Ni/Fe-base MOF with different molar ratios of
Ni
2+
/Fe
3+
via a direct solvothermal approach. The structure characterization results from XRD, Raman,
XPS, and FT-IR confirmed that with the presence of mixed ionic Fe
3+
and Ni
2+
, MIL-88B crystals based
on the mixed metal Fe
2
NiO cluster was formed, while MIL-53 (Fe) was formed with the presence of
a single ion Fe
3+
. The photocatalytic performance of the obtained photocatalysts was evaluated in
the decolorization of RhB dye. The results indicated that the obtained Ni/Fe-MOF samples exhibited
high photocatalytic activity in comparison to MIL-53(Fe). The degradation rate of Ni/Fe-MOF-0.3
could reach the highest (91.14%) after 180 min of visible light irradiation. These results suggest that the
Ni/Fe-MOF, which consist mixed-metal cluster Fe
2
NiO with electron transfer effects, might enhance
the photocatalytic performance.
Supplementary Materials:
The following are available online at http://www.mdpi.com/2073-4344/8/11/487/s1,
Figure S1: EDS mapping of Ni/Fe-MOF-0.3 sample, Figure S2: UV-vis DRS spectra of as-prepared MIL-53(Fe) and
Ni-MIL-53(Fe) crystals isolated from H
2
O, Figure S3: XRD patterns of Ni/Fe-MOF-0.3 before and after reactions,
Table S1: Synthetic parameters of MIL-53(Fe) and Ni/Fe-MOF samples, Figure S4: The flow chart of the synthesis
method, Figure S5: Illustration of the utilized photocatalytic test system, Figure S6. The spectral distribution of
a 40 W compact fluorescent lamp, Figure S7: Background-subtracted Fe 2p
3/2
spectrum from Ni/Fe-MOF-0.3,
Figure S8: Background-subtracted Fe 2p
3/2
spectrum from MIL-53(Fe), Table S2: Product data of a 40 W compact
fluorescent lamp.
Author Contributions:
T.D.N. proposed the concept and supervised the research work at Nguyen Tat Thanh
University. V.H.N. and Q.T.P.B. designed the experiments and performed the experiments. T.H. and L.D.T.
performed XPS and FT-IR analyses. C.V.N. performed SEM and EDS analyses. D.-V.N.V. contributed to the
revision of the manucript. L.G.B. and S.T.D. analyzed the data and wrote the paper.
Funding:
This research was funded by NTTU Foundation for Science and Technology Development under grant
number 2017.01.13/HĐ-KHCN.
Conflicts of Interest: The authors declare no conflict of interest.
Catalysts 2018,8, 487 17 of 20
References
1.
Tranchemontagne, D.J.; Mendoza-Cortés, J.L.; O’Keeffe, M.; Yaghi, O.M. Secondary building units, nets and
bonding in the chemistry of metal-organic frameworks. Chem. Soc. Rev.
2009
,38, 1257–1283. [CrossRef]
[PubMed]
2.
Hamon, L.; Serre, C.; Devic, T.; Loiseau, T.; Millange, F.; Férey, G.; De Weireld, G. Comparative study
of hydrogen sulfide adsorption in the MIL-53(Al, Cr, Fe), MIL-47(V), MIL-100(Cr), and MIL-101(Cr)
metal-organic frameworks at room temperature. J. Am. Chem. Soc.
2009
,131, 8775–8777. [CrossRef] [PubMed]
3.
Devic, T.; Salles, F.; Bourrelly, S.; Moulin, B.; Maurin, G.; Horcajada, P.; Serre, C.; Vimont, A.; Lavalley, J.C.;
Leclerc, H.; et al. Effect of the organic functionalization of flexible MOFs on the adsorption of CO
2
.
J. Mater. Chem. 2012,22, 10266–10273. [CrossRef]
4.
Vu, T.A.; Le, G.H.; Dao, C.D.; Dang, L.Q.; Nguyen, K.T.; Nguyen, Q.K.; Dang, P.T.; Tran, H.T.K.; Duong, Q.T.;
Nguyen, T.V.; et al. Arsenic removal from aqueous solutions by adsorption using novel MIL-53(Fe) as a
highly efficient adsorbent. RSC Adv. 2015,5, 5261–5268. [CrossRef]
5.
Jia, J.; Xu, F.; Long, Z.; Hou, X.; Sepaniak, M.J. Metal–organic framework MIL-53(Fe) for highly selective and
ultrasensitive direct sensing of MeHg+. Chem. Commun. 2013,49, 4670. [CrossRef] [PubMed]
6.
Gao, X.; Zhai, M.; Guan, W.; Liu, J.; Liu, Z.; Damirin, A. Controllable synthesis of a smart multifunctional
nanoscale metal-organic framework for magnetic resonance/optical imaging and targeted drug delivery.
ACS Appl. Mater. Interfaces 2017,9, 3455–3462. [CrossRef] [PubMed]
7.
Bach, L.G.; Van Tran, T.; Nguyen, T.D.; Van Pham, T.; Do, S.T. Enhanced adsorption of methylene blue onto
graphene oxide-doped XFe
2
O
4
(X = Co, Mn, Ni) nanocomposites: Kinetic, isothermal, thermodynamic and
recyclability studies. Res. Chem. Intermed. 2018,44, 1661–1687. [CrossRef]
8. Nhung, N.T.H.; Quynh, B.T.P.; Thao, P.T.T.; Bich, H.N.; Giang, B.L. Pretreated Fruit Peels as Adsorbents for
Removal of Dyes from Water. IOP Conf. Ser. Earth Environ. Sci. 2018,159, 012015. [CrossRef]
9.
Kim, D.W.; Bach, L.G.; Hong, S.S.; Park, C.; Lim, K.T. A Facile Route towards the Synthesis of
Fe
3
O
4
/Graphene Oxide Nanocomposites for Environmental Applications. Mol. Cryst. Liq. Cryst.
2014
.
[CrossRef]
10.
Van Thuan, T.; Ho, V.T.T.; Trinh, N.D.; Thuong, N.T.; Quynh, B.T.P.; Bach, L.G. Facile one-spot synthesis
of highly porous KOH-activated carbon from rice husk: Response surface methodology approach.
Carbon Sci. Technol. 2016,8, 63–69.
11.
Trinh, N.D.; Hong, S.-S. Photocatalytic Decomposition of Methylene Blue Over MIL-53 (Fe) Prepared Using
Microwave-Assisted Process Under Visible Light Irradiation. J. Nanosci. Nanotechnol.
2015
,15, 5450–5454.
[CrossRef] [PubMed]
12.
Yang, S.J.; Im, J.H.; Kim, T.; Lee, K.; Park, C.R. MOF-derived ZnO and ZnO@C composites with high
photocatalytic activity and adsorption capacity. J. Hazard. Mater. 2011,186, 376–382. [CrossRef] [PubMed]
13.
Du, J.J.; Yuan, Y.P.; Sun, J.X.; Peng, F.M.; Jiang, X.; Qiu, L.G.; Xie, A.J.; Shen, Y.H.; Zhu, J.F. New photocatalysts
based on MIL-53 metal-organic frameworks for the decolorization of methylene blue dye. J. Hazard. Mater.
2011,190, 945–951. [CrossRef] [PubMed]
14.
Zhang, C.; Ai, L.; Jiang, J. Solvothermal synthesis of MIL-53(Fe) hybrid magnetic composites for
photoelectrochemical water oxidation and organic pollutant photodegradation under visible light. J. Mater.
Chem. A 2015,3, 3074–3081. [CrossRef]
15.
Alhamami, M.; Doan, H.; Cheng, C.H. A review on breathing behaviors of metal-organic-frameworks (MOFs)
for gas adsorption. Materials 2014,7, 3198–3250. [CrossRef] [PubMed]
16.
Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. Commercial metal-organic frameworks as heterogeneous
catalysts. Chem. Commun. 2012,48, 11275–11288. [CrossRef] [PubMed]
17.
Ai, L.; Zhang, C.; Li, L.; Jiang, J. Iron terephthalate metal-organic framework: Revealing the effective
activation of hydrogen peroxide for the degradation of organic dye under visible light irradiation. Appl. Catal.
B Environ. 2014,148–149, 191–200. [CrossRef]
18.
Lionet, Z.; Kamata, Y.; Nishijima, S.; Toyao, T.; Kim, T.-H.; Horiuchi, Y.; Lee, S.W.; Matsuoka, M. Water
oxidation reaction promoted by MIL-101(Fe) photoanode under visible light irradiation. Res. Chem. Intermed.
2018,44, 4755–4764. [CrossRef]
19.
Qu, L.-L.; Wang, J.; Xu, T.-Y.; Chen, Q.-Y.; Chen, J.-H.; Shi, C.-J. Iron(iii)-based metal–organic frameworks as
oxygen-evolving photocatalysts for water oxidation. Sustain. Energy Fuels 2018,2, 2109–2114. [CrossRef]
Catalysts 2018,8, 487 18 of 20
20.
Wang, D.; Li, Z. Iron-based metal–organic frameworks (MOFs) for visible-light-induced photocatalysis.
Res. Chem. Intermed. 2017,43, 5169–5186. [CrossRef]
21.
Wang, D.; Albero, J.; García, H.; Li, Z. Visible-light-induced tandem reaction of o-aminothiophenols and
alcohols to benzothiazoles over Fe-based MOFs: Influence of the structure elucidated by transient absorption
spectroscopy. J. Catal. 2017,349, 156–162. [CrossRef]
22.
Horiuchi, Y.; Toyao, T.; Miyahara, K.; Zakary, L.; Do Van, D.; Kamata, Y.; Kim, T.-H.; Lee, S.W.; Matsuoka, M.
Visible-light-driven photocatalytic water oxidation catalysed by iron-based metal–organic frameworks.
Chem. Commun. 2016,52, 5190–5193. [CrossRef] [PubMed]
23.
Hu, L.; Deng, G.; Lu, W.; Pang, S.; Hu, X. Deposition of CdS nanoparticles on MIL-53(Fe) metal-organic
framework with enhanced photocatalytic degradation of RhB under visible light irradiation. Appl. Surf. Sci.
2017,410, 401–413. [CrossRef]
24.
Zhou, Y.; Mao, Z.; Wang, W.; Yang, Z.; Liu, X. In-Situ Fabrication of Graphene Oxide Hybrid Ni-Based
Metal-Organic Framework (Ni-MOFs@GO) with Ultrahigh Capacitance as Electrochemical Pseudocapacitor
Materials. ACS Appl. Mater. Interfaces 2016,8, 28904–28916. [CrossRef] [PubMed]
25.
Panda, R.; Rahut, S.; Basu, J.K. Preparation of a Fe
2
O
3
/MIL-53(Fe) composite by partial thermal
decomposition of MIL-53(Fe) nanorods and their photocatalytic activity. RSC Adv.
2016
,6, 80981–80985.
[CrossRef]
26.
Lou, X.; Hu, H.; Li, C.; Hu, X.; Li, T.; Shen, M.; Chen, Q.; Hu, B. Capacity control of ferric coordination
polymers by zinc nitrate for lithium-ion batteries. RSC Adv. 2016,6, 86126–86130. [CrossRef]
27.
Vuong, G.T.; Pham, M.H.; Do, T.O. Synthesis and engineering porosity of a mixed metal Fe2Ni MIL-88B
metal-organic framework. Dalt. Trans. 2013,42, 550–557. [CrossRef] [PubMed]
28.
Vuong, G.-T.; Pham, M.-H.; Do, T.-O. Direct synthesis and mechanism of the formation of mixed metal
Fe2Ni-MIL-88B. CrystEngComm 2013,15, 9694. [CrossRef]
29.
Pham, M.-H.; Dinh, C.-T.; Vuong, G.-T.; Ta, N.-D.; Do, T.-O. Visible light induced hydrogen generation using
a hollow photocatalyst with two cocatalysts separated on two surface sides. Phys. Chem. Chem. Phys.
2014
,
16, 5937. [CrossRef] [PubMed]
30.
Sun, Q.; Liu, M.; Li, K.; Han, Y.; Zuo, Y.; Chai, F.; Song, C.; Zhang, G.; Guo, X. Synthesis of Fe/M (M = Mn,
Co, Ni) bimetallic metal organic frameworks and their catalytic activity for phenol degradation under mild
conditions. Inorg. Chem. Front. 2017,4, 144–153. [CrossRef]
31.
Díaz-Gallifa, P.; Fabelo, O.; Pasán, J.; Cañadillas-Delgado, L.; Lloret, F.; Julve, M.; Ruiz-Pérez, C.
Two-dimensional 3d-4f heterometallic coordination polymers: Syntheses, crystal structures, and magnetic
properties of six new Co(II)-Ln(III) compounds. Inorg. Chem. 2014,53, 6299–6308. [CrossRef] [PubMed]
32.
Vu, T.A.; Le, G.H.; Dao, C.D.; Dang, L.Q.; Nguyen, K.T.; Dang, P.T.; Tran, H.T.K.; Duong, Q.T.; Nguyen, T.V.;
Lee, G.D. Isomorphous substitution of Cr by Fe in MIL-101 framework and its application as a novel
heterogeneous photo-Fenton catalyst for reactive dye degradation. RSC Adv.
2014
,40, 41185–41194. [CrossRef]
33.
Zhu, W.; Liu, P.; Xiao, S.; Wang, W.; Zhang, D.; Li, H. Microwave-assisted synthesis of Ag-doped MOFs-like
organotitanium polymer with high activity in visible-light driven photocatalytic NO oxidization. Appl. Catal.
B Environ. 2015,172–173, 46–51. [CrossRef]
34.
Han, Y.; Liu, M.; Li, K.; Sun, Q.; Zhang, W.; Song, C.; Zhang, G.; Conrad Zhang, Z.; Guo, X. In situ synthesis
of titanium doped hybrid metal-organic framework UiO-66 with enhanced adsorption capacity for organic
dyes. Inorg. Chem. Front. 2017,4, 1870–1880. [CrossRef]
35.
Vinh, N.H.; Long Giang, B.; Sy Trung, D.; Thuong, N.T.; Trinh, N.D. Photoluminescence Properties of
Eu-Doped MIL-53(Fe) Obtained by Solvothermal Synthesis. J. Nanosci. Nanotechnol. 2019,19, 1148–1150.
36.
Chi, Y.X.; Niu, S.Y.; Jin, J. Syntheses, structures and photophysical properties of a series of Zn-Ln coordination
polymers (Ln = Nd, Pr, Sm, Eu, Tb, Dy). Inorg. Chim. Acta 2009,362, 3821–3828. [CrossRef]
37.
Tahir, M. Synergistic effect in MMT-dispersed Au/TiO
2
monolithic nanocatalyst for plasmon-absorption
and metallic interband transitions dynamic CO
2
photo-reduction to CO. Appl. Catal. B Environ.
2017
,219,
329–343. [CrossRef]
38.
Tahir, M. Ni/MMT-promoted TiO
2
nanocatalyst for dynamic photocatalytic H
2
and hydrocarbons production
from ethanol-water mixture under UV-light. Int. J. Hydrogen Energy 2017,42, 28309–28326. [CrossRef]
39.
Haque, E.; Khan, N.; Park, H.J.; Jhung, S.H. Synthesis of a metal-organic framework material, iron
terephthalate, by ultrasound, microwave, and conventional electric heating: A kinetic study. Chem. A
Eur. J. 2010,16, 1046–1052. [CrossRef] [PubMed]
Catalysts 2018,8, 487 19 of 20
40.
Pu, M.; Guan, Z.; Ma, Y.; Wan, J.; Wang, Y.; Brusseau, M.L.; Chi, H. Synthesis of iron-based metal-organic
framework MIL-53 as an efficient catalyst to activate persulfate for the degradation of Orange G in aqueous
solution. Appl. Catal. A Gen. 2018,549, 82–92. [CrossRef] [PubMed]
41.
Feng, X.; Chen, H.; Jiang, F. In-situ ethylenediamine-assisted synthesis of a magnetic iron-based metal-organic
framework MIL-53(Fe) for visible light photocatalysis. J. Colloid Interface Sci.
2017
,494, 32–37. [CrossRef]
[PubMed]
42.
Shannon, R.D. Revised effective ionic radii and systematic studies of interatomic distances in halides and
chalcogenides. Acta Crystallogr. Sect. A 1976,32, 751–767. [CrossRef]
43.
Nakamoto, K. Applications in Organometallic Chemistry. In Infrared and Raman Spectra of Inorganic and
Coordination Compounds; John Wiley & Sons, Inc.: Hoboken, NJ, USA; pp. 275–331, ISBN 9780470405888.
44.
Gao, Y.; Li, S.; Li, Y.; Yao, L.; Zhang, H. Accelerated photocatalytic degradation of organic pollutant over
metal-organic framework MIL-53(Fe) under visible LED light mediated by persulfate. Appl. Catal. B Environ.
2017,202, 165–174. [CrossRef]
45.
Vu, T.A.; Le, G.H.; Vu, H.T.; Nguyen, K.T.; Quan, T.T.T.; Nguyen, Q.K.; Tran, H.T.K.; Dang, P.T.; Vu, L.D.;
Lee, G.D. Highly photocatalytic activity of novel Fe-MIL-88B/GO nanocomposite in the degradation of
reactive dye from aqueous solution. Mater. Res. Express 2017,4, 035038. [CrossRef]
46.
Islam, M.R.; Bach, L.G.; Vo, T.S.; Tran, T.N.; Lim, K.T. Nondestructive chemical functionalization of MWNTs
by poly(2-dimethylaminoethyl methacrylate) and their conjugation with CdSe quantum dots: Synthesis,
properties, and cytotoxicity studies. Appl. Surf. Sci. 2013. [CrossRef]
47.
Grosvenor, A.P.; Kobe, B.A.; Biesinger, M.C.; McIntyre, N.S. Investigation of multiplet splitting of Fe 2p XPS
spectra and bonding in iron compounds. Surf. Interface Anal. 2004,36, 1564–1574. [CrossRef]
48. Molchan, I.S.; Thompson, G.E.; Skeldon, P.; Lindsay, R.; Walton, J.; Kouvelos, E.; Romanos, G.E.; Falaras, P.;
Kontos, A.G.; Arfanis, M.; et al. Microscopic study of the corrosion behaviour of mild steel in ionic liquids
for CO2capture applications. RSC Adv. 2015,5, 35181–35194. [CrossRef]
49.
Lian, K.K.; Kirk, D.W.; Thorpe, S.J. Investigation of a “Two-State” Tafel Phenomenon for the Oxygen
Evolution Reaction on an Amorphous Ni-Co Alloy. J. Electrochem. Soc. 1995,142, 3704–3712. [CrossRef]
50.
Siang, T.J.; Bach, L.G.; Singh, S.; Truong, Q.D.; Ho, V.T.T.; Huy Phuc, N.H.; Alenazey, F.; Vo, D.V.N. Methane
bi-reforming over boron-doped Ni/SBA-15 catalyst: Longevity evaluation. Int. J. Hydrogen Energy
2018
.
[CrossRef]
51.
Sing, K.S.W. Reporting physisorption data for gas/solid systems with special reference to the determination
of surface area and porosity (Recommendations 1984). Pure Appl. Chem. 1985,57, 603–619. [CrossRef]
52.
Horcajada, P.; Serre, C.; Maurin, G.; Ramsahye, N.A.; Balas, F.; Vallet-Regí, M.; Sebban, M.; Taulelle, F.;
Férey, G. Flexible porous metal-organic frameworks for a controlled drug delivery. J. Am. Chem. Soc.
2008
,
130, 6774–6780. [CrossRef] [PubMed]
53.
Blake, A.B.; Yavari, A.; Hatfield, W.E.; Sethulekshmi, C.N. Magnetic and spectroscopic properties of some
heterotrinuclear basic acetates of chromium(III), iron(III), and divalent metal ions. J. Chem. Soc. Dalt. Trans.
1985, 2509–2520. [CrossRef]
54.
Zhang, Z.; Li, X.; Liu, B.; Zhao, Q.; Chen, G. Hexagonal microspindle of NH2-MIL-101(Fe) metal-organic
frameworks with visible-light-induced photocatalytic activity for the degradation of toluene. RSC Adv.
2016
,
6, 4289–4295. [CrossRef]
55.
Nguyen, M.T.H.; Nguyen, Q.T. Efficient refinement of a metal-organic framework MIL-53(Fe) by UV-vis
irradiation in aqueous hydrogen peroxide solution. J. Photochem. Photobiol. A Chem.
2014
,288, 55–59.
[CrossRef]
56.
Ling, S.; Slater, B. Unusually Large Band Gap Changes in Breathing Metal-Organic Framework Materials.
J. Phys. Chem. C 2015,119, 16667–16677. [CrossRef]
57.
Hou, Y.; Yuan, H.; Chen, H.; Feng, J.; Ding, Y.; Li, L. Preparation of La
3+
/Zn
2+
-doped BiVO4 nanoparticles
and its enhanced visible photocatalytic activity. Appl. Phys. A 2017,123, 611. [CrossRef]
58.
Dutta, D.P.; Ballal, A.; Chopade, S.; Kumar, A. A study on the effect of transition metal (Ti
4+
, Mn
2+
, Cu
2+
and
Zn2+)-doping on visible light photocatalytic activity of Bi2MoO6nanorods. J. Photochem. Photobiol. A Chem.
2017,346, 105–112. [CrossRef]
59.
Jin, A.Z.; Dong, W.; Yang, M.; Wang, J.; Wang, G.; Jin, Z.; Dong, W.; Yang, M.; Wang, J.; Wang, G. One-pot
Preparation of Hierarchical Nanosheet-Constructed Fe
3
O
4
/MIL-88B (Fe) Magnetic Microspheres with High
Efficiency Photocatalytic Degradation of Dye. ChemCatChem 2016,22, 3510–3517. [CrossRef]
Catalysts 2018,8, 487 20 of 20
60.
Liang, R.; Luo, S.; Jing, F.; Shen, L.; Qin, N.; Wu, L. A simple strategy for fabrication of Pd@MIL-100(Fe)
nanocomposite as a visible-light-driven photocatalyst for the treatment of pharmaceuticals and personal
care products (PPCPs). Appl. Catal. B Environ. 2015,176–177, 240–248. [CrossRef]
61.
Laurier, K.G.M.; Vermoortele, F.; Ameloot, R.; De Vos, D.E.; Hofkens, J.; Roeffaers, M.B.J. Iron(III)-based
metal-organic frameworks as visible light photocatalysts. J. Am. Chem. Soc.
2013
,135, 14488–14491.
[CrossRef] [PubMed]
62.
Martínez, F.; Leo, P.; Orcajo, G.; Díaz-García, M.; Sanchez-Sanchez, M.; Calleja, G. Sustainable Fe-BTC catalyst
for efficient removal of mehylene blue by advanced fenton oxidation. Catal. Today
2018
,313, 6–11. [CrossRef]
63.
Gao, C.; Chen, S.; Quan, X.; Yu, H.; Zhang, Y. Enhanced Fenton-like catalysis by iron-based metal organic
frameworks for degradation of organic pollutants. J. Catal. 2017,356, 125–132. [CrossRef]
64.
Liu, Q.; Liu, Y.; Gao, B.; Chen, Y.; Lin, B. Hydrothermal synthesis of In
2
O
3
-loaded BiVO4 with exposed
{010}{110} facets for enhanced visible-light photocatalytic activity. Mater. Res. Bull.
2017
,87, 114–118.
[CrossRef]
65.
Sotnik, S.A.; Polunin, R.A.; Kiskin, M.A.; Kirillov, A.M.; Dorofeeva, V.N.; Gavrilenko, K.S.; Eremenko, I.L.;
Novotortsev, V.M.; Kolotilov, S.V. Heterometallic coordination polymers assembled from trigonal trinuclear
Fe
2
Ni-pivalate blocks and polypyridine spacers: Topological diversity, sorption, and catalytic properties.
Inorg. Chem. 2015,54, 5169–5181. [CrossRef] [PubMed]
66.
Jiang, Q.; Håkansson, M.; Suomi, J.; Ala-Kleme, T.; Kulmala, S. Cathodic electrochemiluminescence of
lucigenin at disposable oxide-coated aluminum electrodes. J. Electroanal. Chem.
2006
,591, 85–92. [CrossRef]
67.
Van Thuan, T.; Quynh, B.T.P.; Nguyen, T.D.; Ho, V.T.T.; Bach, L.G. Response surface methodology approach
for optimization of Cu
2+
, Ni
2+
and Pb
2+
adsorption using KOH-activated carbon from banana peel.
Surf. Interfaces 2017,6, 209–217. [CrossRef]
68.
Van Tran, T.; Bui, Q.T.P.; Nguyen, T.D.; Le, N.T.H.; Bach, L.G. A comparative study on the removal efficiency
of metal ions (Cu
2+
, Ni
2+
, and Pb
2+
) using sugarcane bagasse-derived ZnCl2-activated carbon by the
response surface methodology. Adsorpt. Sci. Technol. 2017,35, 72–85. [CrossRef]
69.
Van Tran, T.; Bui, Q.T.P.; Nguyen, T.D.; Thanh Ho, V.T.; Bach, L.G. Application of response surface
methodology to optimize the fabrication of ZnCl
2
-activated carbon from sugarcane bagasse for the removal
of Cu2+.Water Sci. Technol. 2017,75, 2047–2055. [CrossRef] [PubMed]
70.
Dinh, C.-T.; Nguyen, T.-D.; Kleitz, F.; Do, T.-O. Large-scale synthesis of uniform silver orthophosphate
colloidal nanocrystals exhibiting high visible light photocatalytic activity. Chem. Commun.
2011
,47,
7797–7799. [CrossRef] [PubMed]
71.
Yu, X.; Cohen, S.M. Photocatalytic metal-organic frameworks for the aerobic oxidation of arylboronic acids.
Chem. Commun. 2015,51, 9880–9883. [CrossRef] [PubMed]
72.
Lam, S.M.; Sin, J.C.; Abdullah, A.Z.; Mohamed, A.R. Efficient photodegradation of resorcinol with
Ag
2
O/ZnO nanorods heterostructure under a compact fluorescent lamp irradiation. Chem. Pap.
2013
,
67, 1277–1284. [CrossRef]
©
2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
