Hierarchical BiOI nanostructures supported on a metal organic framework as efficient photocatalysts for degradation of organic pollutants in water

Article abstract of DOI:10.1039/c7dt00459a

Semiconductor-based photocatalysis is a green method for the removal of toxic organic pollutants by decomposition into harmless products. However, traditional single-component semiconductors are unable to reach high degradation efficiencies due to excessive photo charge carrier recombination. The use of hybrid nanocomposite photocatalysts is a promising strategy for overcoming this problem by reducing recombination as well as ensuring that large amounts of solar energy are harvested. Herein, a novel visible-light-active hybrid nanocomposite, BiOI/MIL-88B(Fe), was successfully synthesized through a simple precipitation method. In the BiOI/MIL-88B(Fe) composite, both BiOI and MIL-88B(Fe) have improved charge carrier separation and reduced recombination via a simple Z-scheme mechanism. Photocatalytic degradation of the pollutant RhB was carried out during irradiation of the as-synthesized composites with simulated solar light, and the BiOI/MIL-88B(Fe) (2 wt%) composite was found to exhibit the highest photocatalytic activity among the composites. In addition, colorless phenol and ciprofloxacin (CIP) degradation experiments were also performed to confirm the visible light photocatalytic performance of the BiOI/MIL-88B(Fe) hybrid nanocomposite. Scavenger experiments, PL analysis, NBT transformations, and TA-PL experiments all supported the proposed Z-scheme mechanism of the BiOI/MIL-88B(Fe) composite photocatalyst. Moreover, simple separation from solution provides this 3D composite with good reusability and long-term stability.

Full text of DOI:10.1039/c7dt00459a

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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ARTICLE
Hierarchical BiOI nanostructures supported on a metal organic
framework as an efficient photocatalyst for degradation of
organic pollutants in water
Received 00th
January 20xx,
‡
,a,b
Hyun Kook Kim,‡,a D. Amaranatha Reddy, Yujin Kim, Rory Ma, Heehyun
a
a
a
M. Jahurul Islam,
c
c,
a,
Baek, Joonghan Kim * and Tae Kyu Kim *
Semiconductor‐based photocatalysis is a green method for the removal of toxic organic pollutants by decomposition into
harmless products. However, traditional single‐component semiconductors are unable to reach high degradation
efficiencies due to excessive photo charge carrier recombination. The use of hybrid nanocomposite photocatalysts is a
promising strategy for overcoming this problem by reducing recombination as well as ensuring large amounts of solar energy
are harvested. Herein, a novel visible‐light‐active hybrid nanocomposite, BiOI/MIL‐88B(Fe), was successfully synthesized
through a simple precipitation method. In the BiOI/MIL‐88B(Fe) composite, both BiOI and MIL‐88B(Fe) have improved
charge carrier separation and reduced recombination via a simple Z‐scheme mechanism. Photocatalytic degradation of the
pollutant RhB was carried out during irradiation of the as‐synthesized composites with simulated solar light, and the
BiOI/MIL‐88B(Fe) (2 wt.%) composite was found to exhibit the highest photocatalytic activity of the composites. In addition,
colorless phenol and ciprofloxacin (CIP) degradation experiments were also performed to confirm the visible light
photocatalytic performance of the BiOI/MIL‐88B(Fe) hybrid nanocomposite. Scavenger experiments, PL analysis, NBT
transformations, and TA‐PL experiments all supported the proposed Z‐scheme mechanism of the BiOI/MIL‐88B(Fe)
composite photocatalyst. Moreover, simple separation from solution provides this 3D composite with good reusability and
long‐term stability.
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
removal of organic pollutants from wastewater, most of them
Introduction
6‐8
are expensive and are not used effectively.
Recently,
photocatalysis using semiconductor catalysts has proven to be
a useful green technique for solving energy and environmental
issues by harvesting solar radiation to remove water pollution.
At present, a large number of organic pollutants are
accountable for both environmental as well as water
pollution.1 Organic dyes and common toxic chemical
compounds such as antibiotics and phenol can enter aquatic
environments through industrial discharge, improper removal
of wastewater, intentional disposal of surplus drugs, and human
and animal excretion. These toxic organic pollutants not only
reduce water quality but also threaten the environment and
cause serious health issues.3 Though several physical, chemical,
and biological techniques are commonly applied for the
9
,2
To date, much research has centered on semiconductor‐based
photocatalysts and photocatalytic processes utilizing titanium
10
dioxide (TiO
2
) as a conventional photocatalyst. Although TiO
2
is low cost, reactive, and relatively stable, it only responds to
ultraviolet (UV) radiation, which comprises less than 5% of the
11
solar spectrum. This has inspired researchers to develop novel
‐5
semiconductor materials with reduced band gaps to enable the
use of the abundant solar energy in the rest of the spectrum.
Visible‐light‐active photocatalysts are increasingly being
1
2
used for the degradation of organic pollutants. For example,
aDepartment of Chemistry and Chemical Institute for Functional Materials, Pusan
3
13
14
photocatalysts such as BiOI/La
2
Ti
2
O
7
, BiVO
4
,
3 4 4
g‐C N /BiPO ,
National University, Busan 46241, Republic of Korea.
and BiOX (X = Cl, Br, I)15 have been explored for visible‐light
harvesting and photocatalytic degradation of wastewater
pollutants as well as environmental remediation. Among these
photocatalysts, bismuth oxyhalides (BiOX) have demonstrated
outstanding photocatalytic efficiencies due to their structural
b
Institute of Food and Radiation Biology, AERE, Bangladesh Atomic Energy
Commission, Savar, Dhaka 1349, Bangladesh.
c
Department of Chemistry, The Catholic University of Korea, Bucheon 14662,
Republic of Korea.
*
†
(E‐mail) tkkim@pusan.ac.kr (T.K.K.) and joonghankim@catholic.ac.kr (J.K.)
Electronic Supplementary Information (ESI) available: Experimental section; EDS
element mapping. TEM and EELS spectra; UV‐Vis absorption spectra of RhB, phenol,
CIP aqueous solutions containing BiOI and BiOI/MOF composites; Degradation
efficiency of BM‐2 nanocomposite under UV, Vis and simulated sun light
irradiation; photocatalytic degradation rate in the presence of different
photocatalysts. XRD patterns of BM‐2 nanocomposite before and after
photocatalytic recycling measurement; Mott‐Schottky plot of MIL‐88B (Fe) and BiOI
nanostructures; EPR spectra of BiOI and BiOI‐MOF nanocomposites. See
DOI: 10.1039/x0xx00000x
2+
novelty. BiOX compounds contain [Bi
2
O
2
]
slabs, which have
layered tetragonal structures that interleaf with the double
halogen atom slabs. Due to this arrangement, an internal
electrostatic field is generated perpendicular to each layer,
enhancing the separation of photogenerated electron‐hole
pairs.1 Furthermore, bismuth oxyiodide (BiOI) has a narrow
6,17
‡
These authors have contributed equally.
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band gap, which facilitates visible‐light harvesting.15 However, 5.25,26 MIL‐125 and UiO‐66 have also been demonstrated use as
2
7,28
the use of BiOI as a single semiconductor in photocatalysis has UV photoactive MOFs.
several limitations including the hybridization of the Bi 6s and O response, these MOFs would exhibit limited performance under
p valence orbitals, which is responsible for a large degree of solar light irradiation; however, tuning their optical properties
Due to the lack of visible‐light
2
charge carrier recombination that limits the performance of the by modifying the organic linkers has allowed these materials to
material for the photocatalytic decomposition of organic show higher activities when irradiated with visible light. Gascon
pollutants. Researchers have investigated another strategy for et al. reported that the overall band gap of MOF‐5 could be
2
9
developing stronger redox photocatalysts using the Z‐scheme reduced by modifying the organic linker. In addition, visible‐
1
8
principle, which provides more powerful photocatalytic light‐responsive photocatalytic activity could be introduced in
activities by efficiently separating the photogenerated electron‐ MOFs by substituting the organic linker with an amino
hole pairs. Thus, for effective environmental remediation and group.2
8,30,31
Moreover, Laurier et al. reported a new group of
utilization of the enormous quantity of environmentally friendly visible light photocatalysts using iron(III)‐based MOFs
solar energy, it is essential to develop Z‐scheme visible‐light‐ containing Fe ‐μ
‐oxo clusters.32 Herein, we introduce an
active photocatalytic semiconductors with favorable iron(III)‐based MIL‐88B MOF photocatalyst for the
3
3
19
efficiencies.
immobilization of 3D hierarchical BiOI nanostructures towards
Metal‐organic frameworks (MOFs), which are built by the the synthesis of Z‐scheme BiOI/MOF heterostructured
coordination of metal ions and organic linkers, are promising nanocomposites for the first time.
porous and crystalline organic/inorganic materials,2 showing
0,21
In this work, we report a systematic study of a BiOI/MOF
a variety of configurations including tailorable chemical composite photocatalyst prepared in ethylene glycol for the
2
2‐24
affinities, tunable functionalities, and variable pores sizes.
There are several Fe‐based MOFs consisting of Fe(III)‐oxide (RhB), phenol, and ciprofloxacin (CIP). Physical and chemical
clusters and organic linkers. Most Fe‐containing MOFs use Fe analyses revealed that the BiOI/MOF nanocomposites were
‐oxo clusters as a structural motif, and their morphologies successfully fabricated, and the photocatalysis results
and pore sizes directly depend on both the organic linkers and demonstrated that the synthesized nanocomposites were more
the synthetic procedures used. Due to the structure of the Fe efficient at decomposing organic pollutants than bare BiOI due
‐oxo clusters, MOFs reduce recombination and enhance to the Z‐scheme mechanism. Scavenger experiments,
degradation of toxic organic pollutants such as Rhodamine B
3
‐
μ
3
3
‐
μ
3
photocatalytic activity. Recently, researchers have become photoluminescence (PL) analysis, nitroblue tetrazolium (NBT)
interested in the use of MOFs for photocatalytic applications. transformations, and terephthalic acid photoluminescence (TA‐
Garcia et al. showed the UV photocatalytic activity of MOF‐ PL) experiments provided significant evidence for the mediator‐
Fig. 1. FESEM images of: (a, b) bare BiOI; (c, d) MOF; and (e, f) BiOI/MOF composite photocatalysts.
2
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free Z‐scheme mechanism of the photocatalyst. The work 88B(Fe) crystals were observed to form irregular spindle‐like
3
4‐36
presented here will be useful for the design, construction, and shapes with average sizes of 100–150 nm,
as displayed in
application of novel visible‐light‐active Z‐scheme photocatalysts Figs. 1(c) and 1(d). As demonstrated in Figs. 1(e) and 1(f), the
with outstanding performances and improved stabilities.
BiOI nanoflower‐like shapes were very closely packed in the
BiOI/MOF composite, with the MOF structures being
interconnected within the hierarchical structure. By observing
the EDS mapping and FESEM images, it was confirmed that the
novel 3D hierarchical microspherical BiOI/MOF composite
consisted of both BiOI and MOF nanostructures. Furthermore,
we performed EDS analysis of the as‐synthesized
nanostructures to measure the composition of the prepared
nanocomposites, and the estimated elemental weight
percentages are provided in Table S1.
Results and discussion
Photocatalyst morphologies
The morphologies and sizes of the as‐synthesized BiOI, Fe MIL‐
8
8B (MOF), and BiOI/MOF were characterized by FESEM
analysis. As can be seen in Figs. 1(a) and 1(b), the BiOI sample
showed a 3D hierarchical flower‐like micro‐spherical structure
consisting of several smooth surface nanopetals. The
nanopetals were connected to one another at their centers to
form 2–3 μm flower‐like microspheres.33 The as‐prepared MIL‐
Detailed microstructural information, elemental distributions,
and interfacial contacts of the as‐synthesized BiOI/MOF
nanocomposite were obtained using field emission transmission
electron microscopy (FETEM), high‐resolution TEM (HRTEM), energy‐
Fig. 2. (a) FETEM image and (b) HRTEM images of BiOI/MoF nanocomposite. (c‐i) HAADF image and elemental mapping of BiOI/MOF nanocomposites showing the
presence of (d) Bi, (e) I, (f) O, (g) Fe, and (h) C. (i) Line spectra of elements.
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dispersive X‐ray spectroscopy (EDS), and electron energy loss
spectroscopy (EELS), with the results being shown in Fig. 2(a–i) and
Fig. S2. The low magnification FETEM image reveals that the
nanocomposite had a 3D hierarchical flower‐like morphology (Fig.
2
(a)). The HRTEM image in Fig. 2(b) reveals that the BiOI
nanostructures had lattice fringe spacing of 0.277 nm,
a
corresponding to the (001) plane of tetragonal‐phase BiOI. The MOF
nanostructures showed lattice fringes corresponding to the (101)
plane of Fe MIL‐88(B). Further, the HAADF‐STEM image and EDS
mapping analysis of the BiOI‐MOF nanocomposites confirm a
homogenous distribution of Bi, O, I, C, and Fe (Fig. 2(c–i)). From
the EELS measurements (Fig. S2), we identified the presence of C,
O, Fe, Bi, and I within as‐synthesized nanocomposite. The above
results clearly confirm that the synthesized nanocomposite
consisted of both BiOI and MOF nanostructures.
Photocatalyst XRD patterns
The crystallographic structures of the as‐synthesized BiOI, MOF,
and BiOI/MOF composites were determined by X‐ray diffraction
(
XRD) analysis, as shown in Fig. 3. The tetragonal phase of the
as‐synthesized BiOI was revealed by XRD patterns indexed to
JCPDS No. 10‐0445.37 The four major peaks at 2θ = 29.7°, 31.7°,
4
5.4°, and 55.2° were assigned to the (102), (110), (200), and
(
212) planes, respectively. The crystallographic structure of the
Fig. 3. XRD patterns of the as‐synthesized BiOI, MOF, and BiOI/MOF composites.
as‐prepared MIL‐88B(Fe) MOF, shown in Fig. 3, revealed well‐
defined diffraction peaks that were in good agreement with the
simulated MIL‐88B structure, indicating the high crystallinity of (BM‐2) composites, suggesting that the BiOI/MOF
nanostructures were both successfully synthesized and highly
3
4,35
the product.
All prominent diffraction peaks arising from
BiOI and MOF appeared in the XRD pattern of the BiOI/MOF crystalline.
Fig. 4. XPS spectra of BiOI/MOF composite: (a) survey spectra, (b) Bi 4f, (c) I 3d, (d) O 1s, (e) Fe 2p, and (f) C 1s narrow scan spectra.
4
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XPS analysis of the photocatalysts
a lower wavelength of around 650 nm. The MOF exhibited an
absorption edge at approximately 590 nm. These results are
The different valence states of the elements in the BiOI/MOF
nanocomposite were analyzed using X‐ray photoelectron
spectroscopy (XPS), and the survey spectrum in Fig. 4(a) shows
the Bi 4f, I 3d, O 1s, Fe 2p, and C 1s signals of the BiOI/MOF
composite. The narrow scan spectrum showing Bi 4f5/2 and Bi
41,42
consistent with earlier reports.
The BiOI/MOF composite
showed greater absorption intensity in the 450–550 nm region
compared to bare BiOI and MOF (Fig. S3), which is beneficial to
32
the photocatalytic activity of the composite.
The band gap energy is a characteristic property of a
photocatalyst and can be estimated using the following
formula:
4f7/2 reveals binding energy peaks at 164.5 and 159.1 eV,
respectively, as shown in Fig. 4(b). These peaks are shifted to
slightly lower binding energies than would be expected,
3
+ 37
ꢀꢁꢂ ꢃ ꢄꢅꢁꢂ ꢆ ꢇꢈꢉꢊ/ꢋ
indicating reduced charge density on Bi . The high resolution
I 3d spectrum shown in Fig. 4(c) reveals the monovalent
(1)
‐
where α, h, ν, Eg, and
A are the absorption coefficient, Planck’s
oxidation state of iodine (I ) via binding energies of 630.4 and
37
constant, the frequency of light, the band gap energy, and a
6
18.8 eV, which are assigned to I 3d3/2 and I 3d5/2, respectively.
The O 1s narrow scan spectrum of the BiOI/MOF composite constant, respectively.
shown in Fig. 4(d) can be deconvoluted into three peaks at photocatalyst depends on the value of
binding energies of 530.6, 531.6, and 532.5 eV, corresponding transition, = 4 for an indirect transition). For pure BiOI,
to lattice oxygen (O ), defect/vacancy oxygen (O
), and and the band gap energies ( values) of BiOI, MOF, and
1/2 versus the
4
1
The optical transition of
= 1 for a direct
= 4,
a
n (n
n
n
E
g
L
V
), respectively.37 These originate from BiOI/MOF can be derived from a plot of (ꢀꢁꢂ
)
chemisorbed oxygen (O
C
BiOI and the Fe‐containing MOF, with the peak at 530.6 eV photon energy (ꢁꢂ). As shown in Fig. 5(b), the band gap energies
originating from Bi‐O and potentially Fe‐O in MOF, while the of BiOI and MOF are approximately 1.8, and 2.1 eV, respectively,
peak at 531.6 eV corresponds to the vacancy oxygen of BiOI and which are very close to values seen in previous reports.32,42
carbonyl oxygen of MOF. Moreover, the higher binding energy
Quantum chemical calculations
peaks at 532 eV were assigned to hydroxyl and adsorbed
surface oxygen.3
8,39
The high‐resolution spectrum showing Fe
A cluster model was constructed from the experimental crystal
structure of BiOI/MIL‐88B(Fe). The center of the cluster model
2p1/2 and Fe 2p3/2 reveals peaks at 710.3 and 723.6 eV. This
3+
indicates that a small amount of Fe states (2 wt.%) were
present in the BiOI/MOF composite (Fig. 4(e)).3 The narrow
scan spectrum of C 1s can be deconvoluted into three
characteristic peaks at binding energies of 284.6, 285.7, and
3
+
3+
consists of a (Fe )
(μ
3 3
‐O) unit, and each Fe is coordinated to four
8,39
bridging −COO moieties and one water molecule. When the –COO–
moieties were not attached to other Fe3 units in the crystal
structure, they were terminated by H atoms, resulting in −COOH.
Thus, the cluster model contains 139 atoms and its total charge is +1.
HSEH1PBE,4 referred to as the HSE06 hybrid functional, was used
in the density functional theory (DFT) calculations as it has been
reported to show good performance for calculating lattice constants
−
43
+
2
88.5 eV, corresponding to C‐C , C‐O, and O‐C=O, respectively
40
(Fig. 4(f)).
4‐50
UV‐visible absorption spectra and band gap calculations
5
1
52
and band gaps. The def2‐TZVP and 6‐31G(d) basis sets were used
The optical properties of the as‐synthesized BiOI, MOF, and for Fe and other atoms (C, O, and H), respectively. All DFT calculations
BiOI/MOF composites were investigated by UV‐visible diffuse were performed using the Gaussian09 program.53 The positions of H
reflectance spectroscopy (UV‐DRS), as shown in Fig. 5(a). The atoms in the cluster were optimized by fixing the coordinates of the
optical absorption edge of BiOI was observed at about 690 nm, other atoms to retain the crystal structure, and the optimized
whereas in the BiOI/MOF composites this was slightly shifted to
Fig. 5. (a) UV‐Vis diffuse‐reflectance spectra, (b) optical conductivity of the photocatalysts, and (c) PL spectra of BiOI, MOF, and different BiOI/MOF composites
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structure of the cluster is shown in Fig. S4. The doublet spin prepared photocatalysts, the photocatalytic activity was
multiplicity of the cluster is the ground state. The molecular orbital determined by monitoring the RhB concentration, i.e.,
(
MO) energy level and frontier MOs of the cluster are shown in Fig. detecting the RhB absorption spectra as a function of time
S5. As can be seen from Fig. S5, the energy gap between the singly‐ exposed to simulated solar light (Figs. S6(a) and (b)). The
occupied MO (SOMO) of an α electron and the lowest‐unoccupied concentration of RhB declined in the presence of the
MO (LUMO) of a β electron is 2.3 eV, which corresponds to the band photocatalysts, with the BiOI/MOF composites resulting in
gap in the solid state. The results of the quantum chemical much quicker degradation than bare BiOI. The characteristic
calculations using the cluster model are in good agreement with the absorption peak of RhB,
max, shifted from 553.8 nm to shorter
experimentally observed value for the band gap (2.1 eV).
wavelengths through stepwise N‐deethylation to produce
’
′
N,N,N ‐triethylated rhodamine (TER, 539 nm), N,N‐diethylated
rhodamine (DER, 522 nm), ‐ethylated rhodamine (MER, 510
Photoluminescence study
N
nm), and rhodamine (498 nm), indicating the breakdown of the
chromophoric structure and elimination of the ethyl groups.
The color changed from pink to yellow/yellowish green, also
suggesting the breakdown of the chromophoric structure, as
To understand the mechanism for the recombination of
photogenerated charge carriers, the PL emission spectra of
BiOI, MOF, and BiOI/MOF composites were measured at room
temperature with an excitation wavelength of 390 nm, with the
results being shown in Fig. 5(c). Bare BiOI shows emission peaks
59‐61
shown in Fig. S7.
The photocatalytic degradation efficiencies
of the RhB dye reached 28% after 120 min in the presence of
bare BiOI, while the BiOI/MOF composites showed higher
photocatalytic performances, as shown in Fig. 6(a). Among the
composites, the BiOI/MOF (2%) exhibited the highest
photocatalytic activity towards the degradation of RhB (88%). It
is believed that the heterostructures of the composite played a
significant role in enhancing the photocatalytic activity. For
comparison, RhB degradation over the pure Fe MIL‐88B MOF
was also carried out under identical conditions. For Fe MIL‐88 B,
there was a photocatalytic degradation rate of 19% due to its
excellent electric conductivity and large specific surface area.
From Fig. S8, it can be seen that the photocatalytic performance
for RhB dye degradation followed pseudo‐first‐order kinetics.62
at approximately 435 and 455 nm, whereas MOF exhibits
emission peaks at around 432 nm.5
4‐56
The BiOI/MOF
nanocomposites exhibit peaks at similar positions, but the
intensity of the composite peaks is higher than that seen for
bare BiOI. In an earlier report, Shifu et al. observed similar
57
results. Usually, the PL intensities of composite materials are
lower compared with those of pure materials. Under our
experimental conditions, the greater intensity of the BiOI/MOF
PL spectrum may indicate more rapid recombination of charge
carriers. The PL spectrum of BiOI/MOF (2%) shows the highest
intensity at 437 nm. This observation is contrary to the usual
expectations of PL experimental results. Generally, higher PL
intensity corresponds to greater recombination rates for
photoexcited electron‐hole pairs. Conversely, lower PL
intensities point to slower recombination of electron‐hole pairs,
suggesting more photo‐excited holes and electrons can
participate in redox reactions, thereby enhancing
The degradation rate constant (
k
) of BiOI/MOF (2%) was 1.6

‐
2
‐1
1
0 min , which was 5 times higher than that of bare BiOI (3.2
‐
3
‐1

10 min ). The photocatalytic activities of the optimized BiOI‐
MOF nanocomposites for the degradation of RhB were also
monitored under visible light (λ ≥ 425 nm) and UV‐light (λ ≤ 425
nm) irradiation, and the results are presented in Fig. S9. The
photocatalytic
performance.3
However,
under
our
experimental conditions higher PL intensity corresponded to
greater photocatalytic activity (as described in the section on
photocatalytic activity). This conclusion is opposed to those
typically seen for PL emission analyses. This may be because
the BiOI/MOF nanocomposites contained surface defects or
oxygen vacancies. Huang et al. reported that inducing oxygen
vacancies in bismuth oxyiodide nanosheets resulted in higher
5
8
photocatalytic performance towards dye pollutants, while
another reason might be that the BiOI/MOF composites
utilized Z‐scheme systems. 57
Photocatalytic activity
Photocatalytic degradation of an organic pollutant, RhB, was
carried out in order to assess the performance of the as‐
prepared nanocomposites under irradiation with simulated
solar light. To attain an adsorption‐desorption equilibrium,
aqueous solutions of RhB containing suspensions of the
desired photocatalysts were stirred for up to 90 min in the
dark before beginning irradiation.59 The self‐photocatalysis of
RhB was not significant, with only 1% photocatalytic ln(C
Fig. 6. (a) Photocatalytic degradation of RhB by different photocatalysts, (b) plot of
0 t
/C ) vs time, (c) recycling of BiOI/MOF nanocomposite, (d) effect of scavengers on
the degradation of RhB in aqueous solutions after irradiation for 120 min.
degradation being observed (Fig. 6(a)). In the presence of as‐
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results demonstrate that the photocatalytic efficiency was BiOI‐MOF nanocomposite system before and after five
much higher under simulated sunlight irradiation compared to successful photocatalytic cycles was measured using nitrogen
other light sources.
adsorption‐desorption curves. The nitrogen adsorption‐
Degradation of a colorless pollutant, phenol, using the desorption curves demonstrate that the surface area of the
2
BiOI/MOF composite was also monitored to examine whether BiOI‐MOF (109.8 m /g) nanostructures was slightly decreased
2
dye sensitization was occurring under light irradiation. The after recycling (92.5 m /g). This could be attributed to the
degree of photocatalytic phenol degradation under simulated adsorption of oxidation products during the photocatalytic
solar irradiation was determined using the absorption peak at reaction.
4
2
70 nm (Figs. S6(c) and (d)), and samples were withdrawn at
regular time intervals. A decrease in the intensity of the
absorption peak indicated photocatalytic decomposition of
phenol, which increased with longer irradiation times.
BiOI/MOF was found to exhibit more effective photocatalytic
degradation of phenol than bare BiOI (Fig. 6(b)). From this
result, it was confirmed that the degradation of RhB dye was
related to the photocatalytic activity of BiOI/MOF, rather than
dye sensitization.61
Proposed mechanism for the BiOI/MOF composite
Several reactive species may be involved in photocatalytic redox
reactions for the degradation of organic pollutants, including
+
‐
2
holes (h ), ∙O , and ∙OH. Trapping experiments were conducted
in the presence of the BiOI/MOF (2 wt.%) composite to evaluate
which reactive species were responsible for the degradation of
‐
1
RhB. Ethylenediaminetetraacetic acid (EDTA, 5 mmol∙L ),
For further confirmation of the photocatalytic behavior of
the BiOI/MOF composite, another colorless pollutant, CIP, was
used to perform a photocatalytic degradation experiment
under irradiation with simulated solar light. As with the RhB and
phenol degradation experiments, adsorption‐desorption
equilibration was performed for up to 90 min in the dark before
beginning irradiation. The characteristic absorption peak of CIP
occurred at 276 nm, and the relative intensity of the UV‐Vis
spectra decreased over time, indicating the photocatalytic
degradation of CIP. Again, the BiOI/MOF composite
demonstrated higher degradation efficiency than bare BiOI (Figs.
‐
1
benzoquinone (BQ, 1 mmol∙L ), and t‐butyl alcohol (TBA, 5
‐
1
mmol∙L ) were used in the photocatalytic trapping experiments
+
‐
42, 54
as scavengers for h , ∙O
2
, and ∙OH, respectively.
From Fig.
6
(d), it can be seen that the addition of BQ had a minimal
influence on the photocatalytic degradation of RhB, indicating
‐
that ∙O
2
had little impact on the degradation reaction. On the
other hand, when EDTA was added there was a strong decrease
in the degradation rate, suggesting that h+ was the main
reactive species responsible for the photocatalytic process.
After addition of TBA, the photocatalytic degradation rate was
partially reduced, indicating that ∙OH also contributed to the
degradation reaction.
Photocatalytic activity is mainly governed by the recombination
rate of photogenerated electrons and holes, which is rapid in
pure BiOI due to the narrow band gap of the material. However,
due to the band matching of BiOI and MOF in BiOI/MOF
5
S6(e) and (f)). The photocatalytic degradation rate of CIP was
significantly higher than that observed for phenol
decomposition (Fig. 6(b)). In an earlier report, it has been
explained that CIP degradation was promoted by an oxidative
63
mechanism,
which is consistent with our proposed
mechanism, meaning that this could be a potential reason for
the increased degradation of CIP. These results confirm that the
degradation of RhB dye was related to the photocatalytic
activity of BiOI/MOF rather than dye sensitization, because the
colorless phenol and CIP could not sensitize photocatalysis
under visible light irradiation.4
,5
Recycling of the photocatalysts
Regeneration, stability, and reusability are significant factors for
the practical application of photocatalysts to dye degradation.
To evaluate the reusability of the BiOI/MOF (2%) composite, a
recycling experiment involving RhB dye degradation under
simulated solar irradiation was carried out, as shown in Fig. 6(c).
In this procedure, after every degradation reaction the
BiOI/MOF photocatalyst was centrifuged, washed, dried for 8 h
at 80 °C, and then reused for RhB degradation at the same dye
concentration. As can be seen in Fig. 6(c), the BiOI/MOF (2%)
photocatalyst exhibited excellent reusability for up to five
consecutive cycles. Further, the structural stability and surface
area of the recycled photocatalyst was confirmed by XRD
analysis, which showed no structural changes in BiOI‐MOF after
recycling (Figure S10). Further, the specific surface area of the Scheme 1. Proposed mechanism of photocatalysis using the BiOI/MOF
nanocomposite involving Z‐scheme and oxygen vacancy states
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Journal Name
composites, photogenerated carriers can easily migrate These results are supported by the results of the scavenger
between the two. The migration of photogenerated electrons experiments. On the other hand, the CB of the MOF is slightly
‐
and holes is dependent on the potential. The potentials of the more positive than the standard redox potential of E°(O
2
/∙O
2
=
conduction band (CB) and valence band (VB) energies of BiOI ‐0.046 V vs. NHE), hence less electrons participate in the
and MOF nanostructures were determined using reduction reaction. Moreover, some of the electrons in the CB
Mott−Schoꢀky plots (Fig. S11) The estimated flat‐band of the MOF are transferred to the CB of BiOI, where they
potentials of the MIL‐88B(Fe) MOF and BiOI are 0.13 and 0.71 V recombine with holes in the VB of the MOF through the O
v
state,
vs. NHE respectively.34 However, it is well known that the which can delay recombination and thus enhance
63
conduction band edge potential of a semiconductor is about photocatalytic activity. An et al. explained the CIP degradation
.10 eV below the flat‐band potential.64 Therefore, the CBs mechanism in detail, finding it to be oxidative in agreement with
LUMOs) of MIL‐88B(Fe) and BiOI can be estimated to be 0.03 our results. Therefore, it can be seen that the inter‐constituent
0
(
and 0.61 V vs. NHE. According to the band gaps of 2.1 eV for charge recombination improves charge separation in the
MOF and 1.8 eV for BiOI as described above, the VBs (HOMOs) BiOI/MOF composites, significantly increasing their
of MIL‐88B(Fe) and BiOI are 2.13 and 2.41 V vs. NHE, performance for the photocatalytic degradation of organic
respectively.
Further,
pollutants under irradiation with simulated solar light.
electron
paramagnetic
response
(EPR)
spectroscopy was used to monitor the separation and migration
of electron‐hole pairs in the BiOI‐MOF nanocomposites and
Evidence for the mechanism
pure BiOI nanostructures (Fig. S12). The EPR resonance peaks, The photoluminescence spectrum of a photocatalyst can be
associated with trapped electrons, were clearly more intense in utilized to identify the charge carrier recombination mechanism,
the BiOI‐MOF nanocomposite than in the bare BiOI with the intensity generally originating from the recombination
nanostructures, confirming that the nanocomposite favors the of the photogenerated charge carriers. Hence, higher intensity
separation and migration of photo‐induced charge carriers.
PL signals indicate higher rates of charge carrier
67‐70
Based on the trapping experiment results, PL emission recombination
. Generally, binary composites show lower PL
spectra, EPR, band gap energies, and potential positions of BiOI intensities than pure ones. Thus, the PL intensities of the
and MOF, a Z‐scheme type mechanism for photocatalysis using BiOI/MOF composites would be expected to be lower than that
the BiOI/MOF composite is tentatively proposed in Scheme 1. of bare BiOI, because photoexcited charge carriers are
Under the present experimental conditions, holes are the main separated more effectively in the composites, which should
reactive species for the degradation of RhB, and the higher lead to decreased recombination rates and thus lower PL
photocatalytic activities of the BiOI/MOF composites are intensities. However, our experimental results (Fig. 5(c)) show
consistent with their increased PL intensities. Under these that the PL intensities of the BiOI/MOF composites were higher
circumstances, the traditional electron‐hole separation process than that of bare BiOI, indicating that the photoinduced
(
i.e., electron migrates from CB of MOF to the CB of BiOI while electrons and holes recombined more quickly in the BiOI/MOF
holes migrate from VB of BiOI to the VB of MOF) cannot composites in accordance with the Z‐scheme mechanism
describe the mechanism of action of the BiOI/MOF composites. presented in Scheme 1. Presumably, the highly effective
However, the enhanced charge carrier separation is explained recombination of the electrons in BiOI with the holes in MOF
in the mechanism proposed in Scheme 1. If charge carrier was related to the appropriate potential difference between
separation followed the conventional route, then the
photogenerated electrons in the CB (LUMO) of the MOF (+0.03
V) would migrate to the CB of BiOI (+0.61 V). The accumulated
electrons in the CB of BiOI would be unable to undergo efficient
reduction because the potential of the CB is lower than the
‐
65, 66
standard redox potential of E°(O
2
/∙O
2
) = ‐0.046 V.
On the
+
other hand, the photogenerated holes (h ) in the VB of BiOI
(+2.41 V) would migrate to the VB (HOMO) of the MOF (+2.13
V), where the accumulated holes would be unable to undergo
oxidation because the potential of the VB of the MOF is more
‐
negative than the standard redox potential of E°(OH /∙OH) = 2.4
66
V.
According to the mechanism proposed in Scheme 1, the
photogenerated electrons in the CB of BiOI (+0.61 V) are
transferred to the VB of the MOF, where they rapidly recombine
with holes in the VB of the MOF (2.13 V) through the Z‐scheme.
In addition, oxygen vacancy states situated below the CB of BiOI
further enhance the recombination process. As a result, the
holes in the VB of BiOI (+2.41 V) participate in the oxidation
reactions either directly or through the creation of ∙OH radicals.
Fig. 7 Spectra of NBT transformations with (a) BiOI and (b) BiOI/MOF composite.
Fluorescence spectra of TAOH solutions with (c) BiOI and (d) BiOI/MOF
composite.
8
| J. Name., 2012, 00, 1‐3
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ARTICLE
the CB of BiOI and the VB of the MOF, which enabled the Z‐
Conclusions
scheme system to work efficiently. Hence, the electrons in the
‐
CB of the MOF formed ∙O
2
by a reduction process, whereas the In this study, we have successfully synthesized a 3D hierarchical
holes in the VB of BiOI participated in oxidation reactions either BiOI/MOF composite through a simple precipitation method.
directly or through the formation of ∙OH radicals. This Z‐scheme The BiOI/MOF composite showed higher photocatalytic activity
significantly enhanced the photocatalytic activity of the than pure BiOI for the degradation of RhB, phenol, and CIP. The
material for the degradation of RhB under simulated solar light enhanced performance of the BiOI/MOF composite may have
irradiation.
originated from the more effective separation of
To provide further confirmation that the BiOI/MOF photogenerated electron‐hole pairs within the composite
composites used the Z‐scheme mechanism, nitroblue through a Z‐scheme model that increased the charge transfer
tetrazolium (NBT) transformation experiments were carried out efficiency. Scavenger experiments, PL analysis, NBT
‐
to allow quantitative detection of ∙O
2
radical production during transformations, and TA‐PL experiments all supported the
65
the photocatalytic reaction of the BiOI/MOF composite. As proposed Z‐scheme mechanism of the composite
shown in Figs. 7(a) and (b), the BiOI/MOF composite showed a photocatalyst. Therefore, it may be concluded that the
higher percentage of NBT transformation than bare BiOI, BiOI/MOF composite would be a promising heterostructured
‐
indicating more ∙O
2
radicals were produced in the presence of photocatalyst for efficient environmental remediation.
the BiOI/MOF composite. In fact, the CB potential of BiOI (+0.61
V
E°(O
vs. NHE) is more positive than the standard redox potential of
‐
Acknowledgements
2
/∙O
2
= ‐0.046 V vs. NHE), while the CB (LUMO) of MIL‐
8
8B(Fe) can be estimated to be 0.03 V vs. NHE, which is nearer
/∙O ‐
This work was financially supported by National Research
Foundation of Korea (NRF) grants funded by the Korean
to the standard redox potential of E°(O
2
2
). For this reason,
the photogenerated electrons in the CB of BiOI quickly
recombined with the holes in the VB of the MOF during the
photocatalytic reaction of the BiOI/MOF composite, while
simultaneously most of the electrons in the CB of the MOF
government
(MSIP)
(2014R1A4A1001690
and
2
016R1E1A1A01941978). This research was also supported in
part by the Max Planck POSTECH/KOREA Research Initiative
Program [Grant No. 2016K1A4A4A01922028] through the
MEST's NRF funding.
‐
formed ∙O
2
radicals. As a result, the BiOI/MOF composite
showed higher photocatalytic activity through the Z‐scheme
mechanism.
In addition, the terephthalic acid photoluminescence
technique (TA‐PL) was used to estimate the amount of ∙OH
radicals formed during photocatalysis during simulated solar
light irradiation. In this process, terephthalic acid (TA) reacts
with ∙OH radicals to form the highly fluorescent 2‐hydroxy
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