Design of UiO-66?BiOIO3 heterostructural composites with remarkable boosted photocatalytic activities in removing diverse industrial pollutants

Article abstract of DOI:10.1016/j.jpcs.2020.109903

Novel UiO-66?BiOIO₃-x composites have been successfully designed for the first time. The as-prepared UiO-66?BiOIO₃-x composites exhibit superior photocatalytic activity for the decomposition of rhodamine B (RhB) and tetracycline (TC), compared with the corresponding BiOIO₃. For UiO-66?BiOIO₃-30 composite, approximately 93.2% of RhB and 88.4% of TC were decomposed under a 300 W xenon arc lamp light irradiation. Furthermore, the UiO-66?BiOIO₃-50 composite presents outstanding oxidative removal activity for gaseous Hg⁰. The evident improvement of the photocatalytic property over the UiO-66?BiOIO₃-x composites might be ascribed to the promoted separation efficiency of electron–hole pairs and the larger specific surface area. Finally, the electron-hole migration mechanism over UiO-66? BiOIO₃-50 composite is put forward in the light of trapping experiments. This research strategy will provide outstanding development in the application of photocatalysis.

Full text of DOI:10.1016/j.jpcs.2020.109903

Journal of Physics and Chemistry of Solids 151 (2021) 109903
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: http://www.elsevier.com/locate/jpcs
Design of UiO-66@BiOIO3 heterostructural composites with remarkable
boosted photocatalytic activities in removing diverse industrial pollutants
Xue mei Qi*, Qiang Wu, Xiaojian Wang, Ke xuan Li, Can Liu, Shi ji Li
School of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai, 200090, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
BiOIO3
Novel UiO-66@BiOIO3-x composites have been successfully designed for the first time. The as-prepared UiO-
66@BiOIO3-x composites exhibit superior photocatalytic activity for the decomposition of rhodamine B (RhB)
and tetracycline (TC), compared with the corresponding BiOIO3. For UiO-66@BiOIO3-30 composite, approxi-
mately 93.2% of RhB and 88.4% of TC were decomposed under a 300 W xenon arc lamp light irradiation.
Furthermore, the UiO-66@BiOIO3-50 composite presents outstanding oxidative removal activity for gaseous Hg0.
The evident improvement of the photocatalytic property over the UiO-66@BiOIO3-x composites might be
ascribed to the promoted separation efficiency of electron–hole pairs and the larger specific surface area. Finally,
the electron-hole migration mechanism over UiO-66@ BiOIO3-50 composite is put forward in the light of
trapping experiments. This research strategy will provide outstanding development in the application of
photocatalysis.
UiO-66
Tetracycline
Hg0
Photocatalysis
1. Introduction
adsorption properties for pollutants. It is essential to find a more effec-
tive way to improve the charge separation efficiency and increase the
In modern society, water pollution of antibiotics and organic dyes
has aroused extensive attention [1–3]. Furthermore, as one of heavy
metal contaminants, mercury has done great harm to the human health
and environment [4,5]. At the present various technologies have been
applied for the treatment of antibiotics, organic dyes and mercury pol-
lutants [6–10]. Among these technologies investigated so far, photo-
catalysis is considered to a promising technology for the removal of the
pollutants [11–13].
surface area of the photocatalyst. Various methods have been studied to
improve the charge transfer efficiency of BiOIO3, including element
doping, facet control, polarization, heterojunction formation and defect
engineering [18–22].
Constructing heterojunction structure has been regarded as one of
the important strategies to improve the charge separation efficiency.
Heterostructures integration of MOFs with semiconductor photo-
catalysts has been recognised as an efficient approach to fabricate high
performance photocatalysts. MOFs can supply more pathways which are
favourable for the transfer of photogenerated electrons benefit from
their high pore volumes [23,24]. This would be in favour of the sepa-
ration of photogenerated carriers. In addition, the high surface area of
MOFs makes it difficult for semiconductor nanoparticles to aggregate.
UiO-66(Zr) exhibits better thermal and chemical stability compared
with other metal-based MOFs [25,26]. Moreover, UiO-66(Zr) presents
excellent structural stability in water, which will in favour of its appli-
cation in the field of photocatalysis. In this work, UiO-66@BiOIO3-x
composites with different mass ratios (UiO-66 to BiOIO3 are 30:100 and
50:100, respectively) were synthesised for the first time. The photo-
catalytic property of UiO-66@BiOIO3-x compared with BiOIO3 is greatly
enhanced in the decomposition of rhodamine B (RhB) and tetracycline
In the past ten years, Bi-based photocatalysts have been suggested as
outstanding photocatalytic materials due to their typical physico-
chemical properties [14,15]. As a novel bismuth-based material,
BiOIO3 possesses the internal polar and non-centrosymmetrical struc-
ture, which consists of polar (IO3)-groups and the (Bi2O2)2+
hetero-layered structure, resulting in remarkable photocatalytic activity
for the control of contaminants [16,17]. Benefit of its unique crystal
configuration and inter-electric field, BiOIO3 exhibits excellent photo-
induced charge carrier separation efficiency. However, the charge
migration to the catalytic reaction sites of the photocatalyst is still
difficult to achieve. Therefore, the photocatalytic activity is not yet at
acceptable levels for the application. Moreover, the specific surface area
of BiOIO3 photocatalyst is relatively lower which causes weak surface
* Corresponding author.
E-mail address: qixuemei@shiep.edu.cn (X. Qi).
https://doi.org/10.1016/j.jpcs.2020.109903
Received 8 September 2020; Received in revised form 12 November 2020; Accepted 10 December 2020
Available online 29 December 2020
0022-3697/© 2020 Elsevier Ltd. All rights reserved.

X. Qi et al.
Journal of Physics and Chemistry of Solids 151 (2021) 109903
(TC). In addition, UiO-66@BiOIO3-50 composite exhibits appreciable
oxidative removal performance for the gaseous Hg0. The potential re-
action mechanism of increased photocatalytic activity for UiO-66@-
BiOIO3-x composites was investigated by the active species trap
experiments.
2.4. Photocatalytic decomposition of organic pollutants
The decomposition of RhB and TC, at 20 mg/L each with a 300 W
xenon arc lamp (Perfect Light Co., PLS-SXE 300, wavelength range
320–800 nm) irradiation was examined to estimate the photocatalytic
property of the samples. Generally, 100 mg photocatalyst was dispersed
into a 100 mL solution containing one of the pollutants. The solution was
stirred for 0.5h under dark to achieve an adsorption–desorption equi-
librium between the reagents prior to the irradiation. Next, a few mil-
lilitres of suspension were extracted and filtered at regular intervals.
Then the solution was analysed to record the concentration by a
spectrophotometer.
2. Experimental
2.1. Synthesis of UiO-66
UiO-66 was synthesized by solvothermal method in the light of the
previous report method with slightly modification [27]. Briefly, ZrCl4
(0.233 g, 1 mmol) and 1, 4-benzenedicarboxylic acid (0.245 g, 1 mmol)
were dissolved in 60 ml DMF solution. The obtained mixture was then
put into an autoclave and performed the solvothermal reaction at 120 ◦C
for 24 h. The obtained powders were filtered and washed with water and
absolute ethanol several times. Finally, the samples were then dried at
80 ◦C for 12 h in air. In order to eliminate occluded DMF molecules
completely, the obtained white products were immersed into the
methanol solution and shook in constant temperature shaker for 1h.
Then the products were filtered, washed and dried according to the
above procedure.
2.5. Photocatalytic oxidative removal of Hg0
The oxidized removal of gaseous Hg0 was chosen to further assess-
ment the activity of the obtained samples at the laboratory system
described in our previous studies [28]. The mercury was generated from
the mercury permeation tube which was immersed into a water bath at
55 ◦C to guarantee an invariable Hg0 permeation rate. The entrance
mercury concentration was kept about 55
μ
g/m3. The concentration of
Hg0 was measured by an on-line mercury analyzer (RA-915 M, Lumex
Ltd., Russia). The device was mainly consisted of three parts: gas system,
photocatalytic system and the online mercury analysis. The compressed
air was split into two streams and employed two mass flow meters
(CS200) to control the velocity of flow. The total velocity was kept at 1.2
L/min and one of the streams with 0.2 L/min run through the Hg0
permeation tube to carry Hg0 vapor to the system. Firstly, the Hg0
stream ran through the bypass. When the concentration of Hg0 reached
to a constant concentration for a certain time, the gas circuit was
changed to the photocatalytic reactor. At the same time, the LED light
was turned on. The photocatalytic test was implemented with the pho-
tocatalyst(50 mg) loaded on a quartz glass plate. Finally, the gas flowed
through the activated carbon system to adsorb the unreacted gaseous
Hg0 and then discharged. The Hg0 removal efficiency can be defined as
follows:
2.2. Synthesis of UiO-66@BiOIO3 composites
UiO-66@BiOIO3 composites with different mass ratio of UiO-66 to
BiOIO3 were fabricated via hydrothermal method. Briefly, 0.485g Bi
(NO3) ⋅5H2O was dissolved in 30 mL deionized water. In order to inhibit
the hydrolysis of bismuth nitrate, 1 ml 67% (w/w) HNO3 solution was
added to the solution. Then 0.12g UiO-66 powder was dispersed into the
above solution to form a homogeneous suspension under the condition
of magnetic stirring. Subsequently, 0.214g KIO3 was dissolved with 30
mL deionized water to another beaker. Then the KIO3 solution was
dropped into the above suspension. A certain amount of NaOH solution
was added to adjust the pH value of the solution. The pH value of the
solution was adjusted to ca.3.02 under the stirring condition. The
mixture was then loaded into a stainless-steel autoclave and heated at
160 ◦C for 16 h. And the following operations are the same as the syn-
thesis of UiO-66. The as-synthesised UiO-66@BiOIO3 composite was
denoted as UiO-66@BiOIO3-x (x = 30, where x refers to the mass ratio of
UiO-66 to BiOIO3 was 30%). Another UiO-66@BiOIO3-50 composite
was also prepared taking the similar process by changing the dosage of
the Bi (NO3) ⋅5H2O and KIO3. Pristine BiOIO3 was obtained via the same
procedure except not added UiO-66 power.
Where, Hg0inlet and Hgo0utlet indicate Hg0 concentration at the inlet and
outlet of the reactor,
μ
g/m3.
3. Results and discussion
3.1. characterizations of the as-prepared samples
2.3. Characterisation
The XRD patterns of the as-prepared samples are illustrated in Fig. 1.
The diffraction pattern of UiO-66 matches well with that reported lit-
eratures and no impurity peaks were observed [25,26]. The main
diffraction peaks at 7.36◦, 8.48◦, 12.04◦, 22.25◦ and 25.68◦ correspond
to the (111), (002), (022), (115) and (224) diffraction planes, respec-
tively. For BiOIO3, the characteristic diffraction peaks match exactly
with the orthorhombic structure of BiOIO3 (ICSD # 262019), indicating
that pure BiOIO3 sample was prepared. For the as-prepared UiO-66@-
BiOIO3-x composites, the main diffraction peaks of UiO-66 can be
detected evidently. In comparison with pristine UiO-66, the main peak
intensity of UiO-66 in UiO-66@BiOIO3-x composites decreased and no
extra peaks were checked, verifying that the UiO-66 maintains its
crystalline structure during the preparation process of the composites.
The diffraction peaks correspond to the (111), (022) and (224) facets of
UiO-66 in the composites. These results demonstrated that the UiO-66
particles have been successfully loaded on the surface of BiOIO3
nanosheets.
The crystal structure of the samples was measured by a diffractom-
eter with a Cu-Kα radiation (D8 ADVANCE, Bruker). The morphology
and microstructure of the samples was characterized by the field emis-
sion scanning electron microscopy (FE-SEM, SU-1500, Hitachi) and
transmission electron microscope (TEM, JEM-2010, Jeol) operated at
200 kV. The optical absorption property of the samples was recorded by
a UV–vis spectrophotometer (UV-2550, Shimadzu). BET specific surface
area was counted via N2 adsorption-desorption with a GeminiVII 2390
instrument. The photoluminescence (PL) spectra was measured using a
fluorescence spectrophotometer (RF-5301, Shimadzu). The spectra were
excited at 360 nm and photoluminescence spectra were recorded in the
range of 400–600 nm. Electron Spin-Resonance spectroscopy (ESR) was
measured by using a Bruker EPR A300-10/12 spectrometer at room
temperature with the signals of radicals trapped by 5,5-dimethyl-1-pyr-
roline nitrogen oxide (DMPO) for •Oꢀ2 and 2,2,6,6- tetramethylpiper-
idine nitrogen oxide (TEMPO) for h+. The data were collected with a
300W xenon lamp to simulate solar irradiation at selected time.
The morphology structure of the obtained samples was detected by
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X. Qi et al.
Journal of Physics and Chemistry of Solids 151 (2021) 109903
were investigated by N2 adsorption-desorption measurement and the
corresponding results are presented in Table 1 and Fig. 4. Compared
with pure UiO-66, UiO-66@BiOIO3-x samples obtain a decreased sur-
face area and total pore volume. With the increasing of UiO-66 in the
composites, the BET value of the UiO-66@BiOIO3-x composites
increased (Table 1). It needs to be known that the BET specific surface
area of UiO-66@BiOIO3-50 was much higher than that of BiOIO3, but
lower than that of UiO-66. The total pore volume of UiO-66@BiOIO3-x
composites was higher than that of BiOIO3.The N2 adsorp-
tion–desorption isothermal of the UiO-66@BiOIO3-x samples had a type
IV loop at a relative pressure range from 0.05 to 1 (Fig. 4). UiO-66
sample possesses the type IV isotherms curves with a H3 typical hys-
teresis loop. It shows high adsorption at relatively high p/p0 range,
which implying the existence of accumulation pores. It can safely
deduce that the increased surface area of UiO-66@BiOIO3-x and the
porous structure compared to BiOIO3 may favour the transfer of pho-
togenerated electrons and thus promote its photocatalytic activity to-
wards the pollutants’ decomposition or oxidation.
The UV–vis diffuse reflectance spectra (DRS) of UiO-66, BiOIO3 and
UiO-66@BiOIO3-x composites are shown in Fig. 5. It shows that UiO-66
sample exhibits strong absorption in the UV light region of 200–350 nm,
with an absorption edge of about 350 nm. The BiOIO3 and UiO-
66@BiOIO3-x samples possess similar absorption edge and the value is
ca. 400 nm, and therefore the bandgap can be estimated to be about
3.10 eV. Moreover, the band gap energy of BiOIO3 and UiO-66 can be
estimated by the following formula:
Fig. 1. XRD patterns of BiOIO3, UiO-66, and UiO-66@BiOIO3-x composites.
the FE-SEM and TEM technologies and the results are displayed in Fig. 2
(a–d) and Fig. 3(a–d). The UiO-66 sample exhibits a nanocube structure
with the particle size of ca. 200 nm, which is consistent with the results
previously reported [29,30]. It can be observed that the BiOIO3 sample
was composed of irregular nanoplates. The SEM micrograph of
UiO-66@BiOIO3-x composites (Fig. 2(c), (d)) displays that UiO-66 par-
ticles are dispersed on the BiOIO3 nanoplates surface. TEM results (Fig. 3
(c)) clarify that the big nanosheets with the smooth surface are BiOIO3,
and the small particles loaded on the BiOIO3 surface are taken as UiO-66
particles. The interface between BiOIO3 and UiO-66 was observed
clearly, which provide the evidence of the UiO-66@BiOIO3-x hetero-
structure was fabricated. Additionally, the related elemental mapping
images of UiO-66@BiOIO3-50 composite state the existence of all ex-
pected elements (Bi, I, O, Zr and C) and the uniform distribution in the
composite together. As a result, it is proved that the UiO-66 grains were
successfully loaded on the surface of BiOIO3 nanosheets to form a het-
erojunction, which was beneficial to the transfer of photogenerated
charge carriers.
n/2
α
hν
= A(h
ν
ꢀ Eg)
(2)
where Eg,
α, h,
ν
, n and A mean the bandgap energy, coefficient of ab-
sorption, Planck constant, frequency of light, and a constant, respec-
tively. The n is a constant related to semiconductor transfer properties.
BiOIO3 is an indirect semiconductor and therefore n is taken as 4 while n
is taken as 1 for UiO-66 because of its direct transfer feature [31,32].
According to the above equation, the Eg of the pristine BiOIO3 and
UiO-66 are estimated to be 3.13 eV and 3.50 eV (Fig. 6 and Fig. 7), which
is similar to the literatures reported previously [33,34].
3.2. Photocatalytic activity
The specific surface area and porosity of the as-prepared samples
RhB and TC are chosen as contaminants to assess the photocatalytic
activities of the samples and the results are shown in Fig. 8. For RhB, the
blank experiment exhibited slight photolysis, and only approximately
4.4% of RhB was decomposed after 18 min light irradiation. After
adsorption for 30 min, the as-prepared UiO-66@BiOIO3-x composites
and UiO-66 showed similar adsorption capacity, and approximately
20% of the RhB molecules were adsorbed. However, the BiOIO3 sample
presents lower adsorption capacity. Pure UiO-66 shows weak photo-
catalytic activity and finally ca. 30% of RhB are discoloured on the basis
of its higher adsorption. The as-prepared UiO-66@BiOIO3-x composites
demonstrate remarkable photocatalytic activity for RhB. The final
discoloration rate can reach ca. 94% after adsorption and photode-
composition. The photocatalytic performance can be promoted by the
addition of UiO-66 and RhB was almost completely discoloured within
15 min for all UiO-66@BiOIO3-x composites system. UiO-66@BiOIO3-x
composites exhibit relatively higher adsorption ability for RhB
compared to the BiOIO3 sample, which might be favourable for the
photocatalytic process.
It was found that TC can barely be decomposed under light irradia-
tion in the absence of photocatalyst. UiO-66 and UiO-66@BiOIO3-x
composites demonstrated higher adsorption capacity for the TC mole-
cules (ca. 50%) compared to RhB molecules. Compared to BiOIO3 and
UiO-66@BiOIO3-x composites, UiO-66 exhibited relatively weak pho-
tocatalytic activity for the decomposition of TC. The decomposed rate
could reach ca. 89.3% for the UiO-66@BiOIO3-30 composite, which is
much higher than that of BiOIO3 (69.1%) within 24 min light
Fig. 2. SEM images of UiO-66(a), BiOIO3 (b) and UiO-66@BiOIO3–x compos-
ites (c, x = 30, d, x = 50).
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X. Qi et al.
Journal of Physics and Chemistry of Solids 151 (2021) 109903
Fig. 3. TEM images of UiO-66(a), BiOIO3 (b), UiO-66@BiOIO3–50 composite (c, bright field; d, dark field scan) and the elemental mapping images of UiO-
66@BiOIO3–50 composite.
irradiation. It can be inferred that the adhering amount of UiO-66 plays
a significant role in the photodecomposition process.
89%, respectively. Note that UiO-66@BiOIO3-50 composite provides
remarkable photocatalytic oxidative removal efficiency for gaseous Hg0.
These results verify that UiO-66@BiOIO3-x composites could be used in
the field of mercury pollution control. It can be inferred that UiO-66 can
be used as a suitable substrate for photocatalysts in pollution control,
and it should create the chance for the development of various MOF-
based photocatalysts in the future.
In order to further evaluate the photocatalytic performance of the
UiO-66@BiOIO3-x composites, the Hg0 oxidative removal performance
was implemented under 24 W LED light irradiation (Fig. 9). The results
showed that the UiO-66@BiOIO3-50 composite possessed relatively
higher photocatalytic activity compared with BiOIO3 and UiO-66. UiO-
66 exhibits much lower removal efficiency, and its highest removal ef-
ficiency was ca. 30%. It was note that the highest removal efficiency for
BiOIO3 and UiO-66@BiOIO3-50 composites could reach ca. 54% and ca.
The reusability and stability of the UiO-66@BiOIO3-50 composite
was investigated by three consecutive photocatalytic discoloration runs
for RhB (Fig. 10). There was no obvious reduction in the
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X. Qi et al.
Journal of Physics and Chemistry of Solids 151 (2021) 109903
Table 1
The BET surface areas and the photocatalytic decomposition rate of the as-
prepared samples.
Sample
Total pore volume
BET surface areas
Decomposition
rate
(cm3.gꢀ 1
)
(m2.gꢀ 1
)
RhB
TC
BiOIO3
UiO-66
UiO-
0.03051
0.4197
0.1144
9.5054
736.92
108.87
88.2%
33.5%
95.2%
69.1%
61.8%
88.4%
66@BiOIO3-
30
UiO-
0.1293
144.28
94.8%
89.5%
66@BiOIO3-
50
Fig. 5. UV–vis diffusion reflection spectra (DRS) of BiOIO3, UiO-66, and
various UiO-66@BiOIO3-x composites.
Fig. 6. BiOIO3 of plots of (
αh
ν)
1/2 versus photon energy (h
ν).
Fig. 4. N2 adsorption-desorption isotherms of UiO-66 and UiO-66@BiOIO3-
x composites.
Fig. 7. UiO-66 of plots of (
α
hν
)
2 versus photon energy (h
ν).
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X. Qi et al.
Journal of Physics and Chemistry of Solids 151 (2021) 109903
Fig. 10. Photocatalytic recycling runs(three) for RhB over UiO-66@BiOIO3-50
composite under the xenon lamp irradiation.
photodiscoloration of RhB, even after three recycles, suggesting the
sample owns excellent recyclability in the decomposition reaction.
3.3. Possible photocatalytic mechanism
Trapping experiments were implemented to ascertain the main
active radicals during the photodegradation of RhB with the addition in
turn of benzoquinone (BQ), isopropyl alcohol (IPA), and ethyl-
enediaminetetraacetic acid disodium salt (EDTA-2Na) to quench •Oꢀ2 ,
•OH and h+, respectively. As can be seen from Fig. 11, a significant
reduce in the discoloration of RhB was observed when EDTA-2Na was
used as the scavenger compared with no scavenger. It could be safely
concluded that h+ is the significant active species. In contrast, employ-
ing IPA as the scavenger, there was notable decrease in the discoloration
of RhB. Moreover, the discoloration rate was also reduced in the pres-
ence of BQ. It is suggested that all the oxidative species containing h+,
•OH and •Oꢀ2 play an obvious role for the photodiscoloration of RhB.
To further prove the above inference, the ESR spin-trapping tech-
nique was performed to explore the essence of the reactive oxygen
Fig. 8. Photocatalytic activities of the blank (direct photolysis) and samples of
BiOIO3, UiO-66 and UiO-66@BiOIO3-x composites for the degradation of RhB
(a) and TC(b) under 300W xenon lamp full spectrum irradiation.
Fig. 9. Removal efficiency of Hg0 for UiO-66, BiOIO3 and UiO-66@BiOIO3-50
Fig. 11. Photocatalytic activities of UiO-66@BiOIO3-50 composite for the
degradation of RhB in the presence of different scavengers under the xenon
lamp irradiation.
composites under 24W LED light irradiation.
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X. Qi et al.
Journal of Physics and Chemistry of Solids 151 (2021) 109903
means the free electrons energy on the hydrogen scale (ca. 4.5 eV) and
Eg means the bandgap energy. The Eg value for BiOIO3 is about 3.13 eV.
Consequently, the VB and CB for BiOIO3 are estimated to 4.10 eV and
0.97 eV (vs NHE), respectively. According to the same way, the LUMO
potential and the HOMO potential of UiO-66(Zr) is determined as ꢀ 0.6
eV and 2.9 eV, respectively. The Eg is estimated to 3.50 eV, which are
consisted with the previous report [34,36]. The photogenerated holes
may possess higher oxidizing ability as result of the relatively positive
level VB potential [33]. It should be noted that the CB potentials of
UiO-66 and BiOIO3 are ꢀ 0.6 eV and 0.97 eV (vs NHE), correspondingly.
On the basis of Nernstian behaviour in aqueous solution, the band edge
positions were plotted as a function of the pH solution vs. NHE [37]. The
calculated band-edge position for BiOIO3 and UiO-66 perhaps shift to
more negative potential (0.56 eV and ꢀ 1. 01eV) under the photo-
catalytic reaction experiment (pH ~7). The CB potential of UiO-66 was
more negative than the standard reduction potential of O2/•Oꢀ2 ((E0
(O2/•Oꢀ2 ) = -0.33 eV vs. NHE); the excited electrons of UiO-66 can
directly reduce molecular oxygen to •Oꢀ2 . The VB potential of BiOIO3
(4.10 eV) may be shift to 3.69eV (pH ~7), which was more positive than
the standard oxidation potential of •OH/H2O (E0 (H2O/•OH) = 2.4 eV
vs. NHE) [31]; the excited holes can oxidize the adsorbed H2O molecules
to •OH. Also, the induced h+ in the VB of BiOIO3 can directly oxidize
RhB.
On account of the above mentioned, a plausible photocatalytic
mechanism of the UiO-66@BiOIO3-50 composite is proposed and dis-
played in Fig. 13. Under light irradiation, excited electrons can be
migrated from the CB of UiO-66 to that of BiOIO3. At the same time, the
electrons retained on the CB of UiO-66 can reduce the oxygen to form
•Oꢀ2 . Meanwhile, the holes generated by BiOIO3 VB can be transferred to
the VB of UiO-66. Water molecules also trap the holes to produce •OH.
In summary, both •Oꢀ2 and h+, as well as •OH, are all favourable for the
decomposition of RhB. The enhanced photocatalytic property of the
UiO-66@ BiOIO3-x composites is attributed to the existence of energy
band cross between UiO-66 and BiOIO3, improving the transfer effi-
ciency of electrons and holes at the interface and then enhance the
photocatalytic performance. This conclusion could be further verified by
the photoluminescence (PL) spectra of the as-prepared samples at 360
nm excitation wavelength (Fig. 14). Obviously, the PL emission intensity
of UiO-66@ BiOIO3-x composites was weaker than that of pure BiOIO3
and UiO-66, which indicates the higher separation efficiency for the
UiO-66@ BiOIO3-x composites. This result demonstrates that the sepa-
ration efficiency of the electron-hole pairs can be promoted with the
formation of heterojunction structure.
Fig. 12. ESR signals of the TEMPO-h+ and DMPO-⋅O2ꢀ for UiO-66@BiOIO3- 50
in the dark and under 300W xenon lamp full spectrum irradiation.
species produced for UiO-66@BiOIO3-50 composite under the 300 W
xenon lamp and the results are presented in Fig. 12. As shown, the
typical peaks of the DMPO-•O2ꢀ spin adducts were detected in the
methanol dispersion under light irradiation. Furthermore, no signals
could be detected without light irradiation. It suggests that the genera-
tion of •Oꢀ2 under the light irradiation. The obvious characteristic
consecutive 1:1:1 triplet signal was observed whether in dark or light
condition. The triplet signal originates from the addition of trapping
agent TEMPO. There is an obvious reduce in triplet ESR signal strength
under light irradiation compared with dark condition. This is because
the photogenerated holes react with the electrons that originate from
TEMPO molecule. It causes the decrease of triplet signal under the light
irradiation and the results are accordance with the literature [35].
Combining the analysis of the trapping experiments with ESR charac-
terisation, it can be safely come to a conclusion that h+, •OH and •O2ꢀ
are the main active species in the photocatalytic process.
4. Conclusion
UiO-66@BiOIO3-x composites have been successfully prepared by a
hydrothermal route. The photocatalytic performance of the as-prepared
UiO-66@BiOIO3-x composites was estimated by the decomposition of
RhB and TC as well as the oxidative removal of gaseous Hg0. Impor-
tantly, UiO-66@BiOIO3-x composites (x = 30 and 50) exhibited similar
photocatalytic performance and displayed excellent activity, which is
higher than that of UiO-66 and BiOIO3 samples. The improved photo-
catalytic activity could be attributed to the boosted separation efficiency
of the photogenerated carriers. The results prove that UiO-66 can be
used as an excellent substrate in the photocatalysis. This study provides
a new route for exploring excellent efficient and stable photocatalysts in
many fields.
The band structures of the photocatalyst can be calculated according
to the following equation:
Credit author contribution statement
Xuemei Qi: Conceptualization, Methodology, Writing-original draft,
Investigation.
EVB = X ꢀ Ee + 0.5Eg
(3)
Qiang Wu: Data curation, Reviewing and Editing.
Xiaojian Wang: Supervision.
ECB = EVB ꢀ Eg
(4)
Where, X is the absolute electronegativity of the semiconductor, Ee
Kexuan Li: Supervision, Validation.
7

X. Qi et al.
Journal of Physics and Chemistry of Solids 151 (2021) 109903
Fig. 13. Schematic illustration of the proposed photocatalytic mechanism.
Acknowledgments
This work was sponsored by National Natural Science Foundation of
China (52076126) and Shanghai Natural Science Foundation
(18ZR1416200).
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