Novel n-p-n heterojunction of AgI/BiOI/UiO-66 composites with boosting visible light photocatalytic activities

Article abstract of DOI:10.1002/aoc.6186

Antibiotics, considered as persistent organic pollutants, have harmful effect on the environment and human health. We aimed at designing and fabricating ternary AgI/BiOI/UiO-66 composites and evaluate the photocatalytic activities of tetracycline (TC) in water environment under visible light irradiation. X-ray diffractometer (ХRD), scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), and ultraviolet–visible (UV-vis) diffuse reflectance spectra (DRS) analyses were carried out to characterize the ternary AgI/BiOI/UiO-66 composites. The effects of pristine BiOI, UiO-66, and AgI loading ratios on the AgI/BiOI/UiO-66 composites for TC photodegradation were evaluated to obtain the optimal one (1-ABU), which can remove 90.1% TC with visible light irradiation. In the batch experiment, the influences of pH, ion strength, TC concentration, and dosage of photocatalyst were investigated. 1-ABU exhibited excellent photocatalytic activities and stability including pH and ion strength resistance in certain degree. After recycled five times, 1-ABU still showed good degradation efficiency, indicating the advanced stability of the ternary composites. ·OH and h⁺ active species are the prime, whereas ·O₂^? act as secondary during the TC degradation process. 1-ABU exhibits an n-p-n heterojunction, which can inhibit the recombination of photogenerated electron–hole pairs and generate more active ·OH species. This work highlights a reference for the design and synthesis of heterojunction for excellent photocatalytic performance.

Full text of DOI:10.1002/aoc.6186

Received: 4 November 2020
DOI: 10.1002/aoc.6186
Revised: 6 January 2021
Accepted: 22 January 2021
F U L L P A P E R
Novel n-p-n heterojunction of AgI/BiOI/UiO-66 composites
with boosting visible light photocatalytic activities
1
1
1
1
Min Zhu
| Huimin Chen
| Yu Dai
| Xuanyu Wu
|
2
2
Zhiguo Han
| Yu Zhu
1
Hanlin College, Nanjing University of
Chinese Medicine, Taizhou, China
Antibiotics, considered as persistent organic pollutants, have harmful effect on
the environment and human health. We aimed at designing and fabricating
ternary AgI/BiOI/UiO-66 composites and evaluate the photocatalytic activities
of tetracycline (TC) in water environment under visible light irradiation. X-ray
diffractometer (ХRD), scanning electron microscopy–energy dispersive X-ray
spectroscopy (SEM-EDS), transmission electron microscopy (TEM), Brunauer–
Emmett–Teller (BET), and ultraviolet–visible (UV-vis) diffuse reflectance spec-
tra (DRS) analyses were carried out to characterize the ternary AgI/BiOI/UiO-
2
College of Pharmacy and Chemistry &
Chemical Engineering, Jiangsu Key
Laboratory of Chiral Pharmaceuticals
Biomanufacturing, Taizhou University,
Taizhou, China
Correspondence
Min Zhu, Hanlin College, Nanjing
University of Chinese Medicine, 225300
Taizhou, China.
66 composites. The effects of pristine BiOI, UiO-66, and AgI loading ratios on
Email: zm0811jh@163.com
the AgI/BiOI/UiO-66 composites for TC photodegradation were evaluated to
obtain the optimal one (1-ABU), which can remove 90.1% TC with visible light
irradiation. In the batch experiment, the influences of pH, ion strength, TC
concentration, and dosage of photocatalyst were investigated. 1-ABU exhibited
excellent photocatalytic activities and stability including pH and ion strength
resistance in certain degree. After recycled five times, 1-ABU still showed good
degradation efficiency, indicating the advanced stability of the ternary compos-
Yu Zhu, Jiangsu Key Laboratory of Chiral
Pharmaceuticals Biomanufacturing,
College of Pharmacy and Chemistry &
Chemical Engineering, Taizhou
University, 225300 Taizhou, China.
Email: zhuyu0905@sohu.com
Funding information
Natural Science Foundation of Jiangsu
Province, Grant/Award Number:
BK20190245; Nature Science Research in
Colleges and Universities in Jiangsu
Province, Grant/Award Number:
+
−
ites. ÁOH and h active species are the prime, whereas ÁO act as secondary
2
during the TC degradation process. 1-ABU exhibits an n-p-n heterojunction,
which can inhibit the recombination of photogenerated electron–hole pairs
and generate more active ÁOH species. This work highlights a reference for the
design and synthesis of heterojunction for excellent photocatalytic
performance.
19KJB610021; Qinglan Project of Jiangsu
Province of China; Science and
Technology Support Program (Social
Development) of Taizhou, Grant/Award
Number: SSF20190047; Undergraduate
Training Program for Innovation and
Entrepreneurship of Jiangsu Province,
Grant/Award Number: 202013981005Y
K E Y W O R D S
bismuth oxyhalide, heterojunction, metal organic framework, photocatalyst, silver halide
1
| INTRODUCTION
antibiotics have been considered as persistent organic
[
2,3]
pollutants (POPs) recently.
Various techniques such
Antibiotics, medications that destroy or slow down the
growth of bacteria, have been widely applied for decades
not only for human but also for farming and agricultural
purposes. However, due to the results of resistance
genes, which will finally endanger human health,
as adsorption, membrane separation, and electrochemical
and photocatalytic methods have been applied for antibi-
otics degradation. Photocatalysis based on semiconductor
has received much attention to solve this problem
because of the low cost, high efficiency, and being
[1]
Appl Organomet Chem. 2021;35:e6186.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
1 of 16
https://doi.org/10.1002/aoc.6186

2
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ZHU ET AL.
environmentally friendly in the process of decomposing
and mineralizing antibiotics.
particles of inorganic semiconductors can be well dis-
persed on MOFs, impacting the exposure between
POPs and active sites.
[4]
Bismuth oxyhalides (BiOX, X = F, Cl, Br, and I)
are a kind of semiconductor photocatalyst containing a
In the present work, a series of binary BiOI/UiO-66
composites have been hydrothermally synthesized and
further coated with AgI to obtain AgI/BiOI/UiO-66 ter-
nary composites. Optimum AgI/BiOI/UiO-66 composite
(1-ABU) exhibited superior photocatalytic activity in
degradation of tetracycline (TC) compared with BiOI,
UiO-66, and BiOI/UiO-66 composites. Besides, a possible
mechanism during the TC photocatalytic degradation
was systematically investigated.
2+
crystal structure with [Bi O ] layers that are inter-
2
2
leaved by slabs containing halogen atoms. Besides the
existence of the strong interlayer bonding and weak
interlayer, van der Waals interactions inside can lead
to the outstanding photocatalytic activity and more
[5,6]
special properties.
Among BiOX photocatalysts,
BiOI with suitable band gap (approximately 1.8 eV)
can be the most promising one because of the more
[7,8]
efficient sunlight-harvesting performance.
However,
BiOI always exhibits poor photocatalytic performance
because of the fast recombination of photogenerated
electrons and holes derived from the narrow band
2 | EXPERIMENTAL
[
9,10]
gap.
To solve this problem, fabricated BiOI with
2.1 | Preparation
semiconductor to construct heterojunctions to
suppressing the recombination of photogenerated
electron–hole pairs has been applied to improve the
2.1.1 | Synthesis of BiOI
BiOI was prepared following a previous reported method
[11–13]
photocatalytic activities of BiOI.
[
28]
Silver halides (AgX, X = Cl, Br, I), a family of photo-
sensitive materials, can absorb visible light with the
with some modifications : Bi(NO ) Á5H O (2.4300 g,
3
3
2
5.0 mmol) and KI (0.8300 g, 5.0 mmol) were dissolved in
40-ml ethylene glycol (EG) with 1-h stirring. The mixture
solution was then transferred into a 50-ml Teflon-lined
[14,15]
absorption edge of about 530 nm.
Recently, AgX
photocatalysts have been reported to show visible-light
photocatalytic activities for the decomposition of various
ꢀ
stainless autoclave. The autoclave was heated at 160 C
[16]
POPs.
Among them, AgI has been proved to be an
for 12 h and cooled to room temperature naturally. The
resulting BiOI was separated by centrifugation, washed
with deionized water and ethanol for three times, and
ideal semiconductor to fabricate with BiOI, which can
construct Type II heterojunction and promote the
[17]
ꢀ
charge-hole separation efficiency.
However, AgI can
dried at 80 C overnight.
be easily decomposed into silver nanoparticles with light
[18]
irradiation,
and recycling AgI/BiOI powder after the
photocatalytic reaction is a challenge because of the well-
2.1.2 | Synthesis of UiO-66
[19]
dispersed reaction suspension.
Metal–organic frameworks (MOFs),
porous and crystalline material, are self-constructed by
[29]
a
kind of
UiO-66 was synthesized according to reported method
:
ZrCl4 (0.233 g, 1.0 mmol) and 1,4-dicarboxybenzene
(0.166 g, 1.0 mmol) were dissolved in 30-ml N,N-
dimethylformamide containing 3.6-ml acetic acid. The
obtained solution was transferred and sealed into a
50-ml Teflon-lined stainless steel autoclave with heating
[20,21]
metal-cluster and organic linkers.
Owning to the
diverse organic linkers, MOFs with high surface areas
and tunable pore shapes and sizes can be achieved for
various applications. Recently, ligand-to-metal charge
transfer (LMCT) transitions in some MOFs have
received attention as a family of potential semiconduc-
ꢀ
in an oven at 120 C for 12 h. After cooling to room
temperature naturally, the resulting white UiO-66
powder was separated by centrifugation, washed with
deionized water and ethanol for three times, and dried
[22]
tors.
With light irradiation, LMCT in MOFs can
emerge because of the transfer of energy with the
HOMO-LUMO gap related closely to the band
ꢀ
at 80 C for 12 h.
[
23,24]
gap.
Fabrication inorganic semiconductors with
MOFs have been proved to be a significant method to
improve the charge carrier separation with at least
three profitable factors: first, more paths for the photo-
induced electrons migration can be taken place with
2.1.3 | Synthesis of x-BiOI/UiO-66
A certain dose of UiO-66 was added into 10-ml EG and
sonicated for 30 min to obtain uniform suspension A. Bi
(NO ) Á5H O(0.4851 g, 2.0 mmol) and KI (0.2078 g,
[25–27]
more semiconductors involved
; second, high sur-
face area of MOFs will lead to the enlargement of
adsorption capacities to POPs; finally, photoactive
3
3
2
2.0 mmol) were dissolved in 20-ml EG and slowly added

ZHU ET AL.
3 of 16
into suspension A with stirring for 2 h, transferred, and
sealed into a 50-ml Teflon-lined stainless steel autoclave
with heating in an oven at 160 C for 12 h. After cooling
2.3 | Photocatalytic test
ꢀ
The photocatalytic activities of the photocatalysts were
evaluated in a GHX-2 photocatalytic reactor (Yangzhou
Kejiao, China) with a 250-W Xenon lamp as simulate
sunlight. Normally, 20 mg of photocatalysts were added
into 100 ml of 10 mg/L of TC solutions in a quartz reactor
for 30 min stirring in dark to ensure adsorption–
desorption equilibrium between the photocatalyst and
TC; 4 ml of suspension was taken out every 5 min once
the reaction was irradiated. The photocatalyst was
removed through centrifugation to obtain the clear solu-
tion, which was further analyzed by a UV-vis spectropho-
tometer at 357 nm (Persee TU-1901, China) to detect the
concentration of TC in the solution.
to room temperature naturally, the resulting orange pow-
der was separated by centrifugation, washed with
deionized water and ethanol for three times, and dried at
ꢀ
8
0 C for 12 h to obtain BiOI/UiO-66 composites. The as-
synthesized BiOI/UiO-66 were denoted as x-BiOI/UiO-66
x = 0.5, 1, 2), where x refers to the theoretical weight
ratios of BiOI to UiO-66 that were 50% (0.5-BiOI/UiO-66,
.5-BU), 100% (BiOI/UiO-66, 1-BU), and 200% (2-BiOI/
UiO-66, 2-BU).
(
0
2.1.4 | Synthesis of AgI/BiOI/UiO-66
A certain dose of 1-BU was dissolved in 30-ml
water with 30-min sonication, and AgNO3 (0.0724 g,
2.4 | Photoelectrochemical
measurement
0
.0042 mol) was added with 5-min sonication in
the dark. KI (0.00707 g, 0.0042 mol) dissolved in 30-ml
water was added dropwise into the suspension above
and stirred for another 1 h. The resulting composite
was separated by centrifugation, washed by
deionized water and ethanol for three times, and dried
The transient photocurrent measurements were per-
formed on a three-electrode system containing ITO
electrode with samples, Pt, and Ag/AgCl electrodes on an
electrochemical workstation (CHI-660E, China). The
BiOI, 1-BU, and 1-ABU films coated on ITO conducting
ꢀ
at 80 C overnight. The as-synthesized composites were
denoted as y-AgI/1-BU (y = 0.2, 0.4, 0.8, 1.0, 1.2),
where y refers to the weight ratios of AgI to 1-BU that
were 20% (0.2-AgI/1-BU, 0.2-ABU), 40% (0.4-AgI/
glass were working electrodes with Na SO4 aqueous
2
solution (0.1 mol/L) as the electrolyte solution. The
Mott–Schottky plots were collected under the same
conditions above without the Xe lamp irradiation.
1
-BU, 0.4-ABU), 80% (0.8-AgI/1-BU, 0.8-ABU), 100%
1-AgI/1-BU, 1-ABU), and 120% (1.2-AgI/1-BU,
.2-ABU).
(
1
3
| RESULTS AND DISCUSSION
2.2 | Characterization
3.1 | Characterizations
X-ray diffractometer (XRD) patterns were measured on a
D8 ADVANCE diffractometer using Cu-Ka radiation
The powder XRD patterns of BiOI, UiO-66, AgI, and
BiOI/UiO-66 and AgI/UiO-66@BiOI composites were
carried out and shown in Figure 1. The detectable diffrac-
(
(
Bruker, Germany). The scanning electron microscopy
SEM) and energy dispersive X-ray spectroscopy (EDS)
ꢀ
tion peaks of tetragonal phase BIOI observed at 29.0 ,
ꢀ
ꢀ
ꢀ
were performed on a QUANTA 250 instrument (Fe-SEM,
USA). Transmission electron microscopy (TEM) were
obtained on a Tecnai G2 20 instrument (Fei, USA).
X-ray photoelectron spectroscopy (XPS) were performed
on a Thermo ESCALAB 250XI instrument (Thermo
Fisher Scientific, USA). The specific surface areas
were measured via Multipoint Brunauer–Emmett–Teller
32.0 , 45.5 , and 55.0 agree well with JCPDS card
no. 73-2062, corresponding to (102), (110), (200), and
[
30]
(212) planes
well consistent with the simulated pattern.
The diffraction pattern of bare UiO-66 is
[
29]
In the pat-
terns of BiOI/UiO-66 composites, the diffraction peaks of
both BiOI and UiO-66 can be observed. In addition, we
can clearly see from Figure 1b the diffraction peaks of
AgI, UiO-66, and BiOI in the patterns of AgI/UiO-66/
BiOI ternary composites, indicating the successful combi-
nation of the three pristine materials.
The structure and morphology of BiOI, 1-BU, and
1-ABU composites were investigated by SEM and TEM.
As depicted in Figure 2a,b, pure BiOI shows hollow
flower-like structure with broad pore sizes. After being
(BET) measurement on Quadrasorb EVO instrument
(Quantachrome, USA). The ultraviolet–visible (UV-vis)
diffuse reflectance spectra (DRS) were performed
on a UV-4100 spectrometer (Hitachi, Japan). Photo-
luminescence (PL) spectra were exhibited by an
Analytical Instruments FLS-980 spectrophotometer
(Edinburgh, UK).

4
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ZHU ET AL.
FIGURE 1 (a) The powder X-ray diffractometer (XRD) of BiOI/UiO-66 composites; (b) the powder XRD of AgI/UiO-66/BiOI ternary
composites
decorated with UiO-66, octahedral UiO-66 nanoparticles
can be observed to anchor on the surface of ball-like
BiOI, in which the hollow structure was collapsed
energy peaks are centered at 182.0 and 184.5 eV, respec-
tively, in Figure 4e.
[
31]
Figure 5a illustrated the N2 adsorption–desorption
isotherms of BiOI, 1-BU, and 1-ABU composites and the
corresponding pore size distributions. The Type IV iso-
therms of BiOI displays a Type H3 hysteresis loop at the
high relative pressure from 0.75 to 1.0, indicating the
mesoporous character of BIOI with the pore size of
(
1
(
Figure 2c–e). AgI nanoparticles were then decorated on
-BU, making the surface of 1-ABU a little coarse
Figure 2f,g). In addition, the EDS pattern displays the
composition of Ag, Bi, Zr, and I elements in 1-ABU
Figure 2h). Figure 3 exhibits the TEM images of 1-ABU
(
2
−1.[[32]]
microsphere. UiO-66 nanoparticles can be observed to be
distributed on the surface of BiOI. Besides, many subtle
AgI nanoparticles were anchored on the surface of 1-BU
2.517 nm and BET surface area of 58.74 m Ág
Com-
pared with pure BiOI, 1-BU exhibits Type I isotherm and
2
−1
enlarged BET surface area (432.21 m Ág ) after being
(
Figure 3b). The lattice spacings that belonged to AgI and
BiOI can be observed in the high-resolution TEM
HRTEM) images in Figure 3c,d. The lattice fringes of
fabricated with UiO-66, which possesses the BET surface
2
−1.[[33]]
area of 1276 m Ág
After being fabricated with AgI,
(
the BET surface area of 1-ABU was decreased to
2
−1
BiOI were 0.91 and 0.30 nm, which were assigned to the
crystal planes of (001) and (012), respectively, whereas
the lattice spacings of 0.40, 0.38, and 0.35 nm correspond
to the (100), (002), and (101) planes of AgI, respectively,
indicating successful formation of the ternary composite
with heterojunction structure.
The elements and their valence states in 1-ABU com-
posite were characterized by XPS. The XPS survey spec-
trum of 1-ABU is presented in Figure 4a, which consists
of six elements, that is, Ag, Bi, C, I, O, and Zr. As shown
in Figure 4b, the peaks at 378.0 and 378.1 eV belong to
151.50 m Ág and remain the pore size (1.187 nm) com-
pared with 1-BU. 1-ABU with higher BET surface area
and suitable pore size was favorable for the antibiotics
[
34]
adsorption to boost the photocatalytic activities.
Figure 6a presents the UV-vis adsorption spectra of
pristine BiOI, UiO-66, AgI, 1-BU, and 1-ABU compos-
ites. UiO-66 has an absorption edge at about 350 nm,
whereas AgI has a broader absorption extending to visi-
ble light with an absorption edge of 480 nm. BiOI
exhibited the broadest absorption at 668 nm, exhibiting
the excellent photoabsorption ability under visible light.
After being composited with BiOI, 1-BU and 1-ABU still
maintain the absorption edge at about 668 nm. The
bandgap energies of semiconductor were calculated
Ag 3d5 and Ag 3d3/2, respectively, which are attributed
/2
+
[12]
to Ag coming from AgI.
Figure 4c contain two peaks at 630.3 and 619.1 eV, which
are assigned to I 3d and 3d5/2, respectively, from
The I 3d spectrum in
2
[35]
through the Tauc plots ((αhv) = A (hv − E )).
The
3
/2
g
[20]
AgI.
attributed to Bi 4f7/2 and Bi 4f5/2 of Bi from BiOI.
Besides, two characteristic Zr 3d5/2 and Zr 3d3/2 binding
Two peaks at 164.1 and 158.8 eV in Figure 4d are
band gaps (E ) of photocatalysts were estimated by
g
3+
[30]
1/2
the plot of (αhν) versus photo energy (hν) (Figure 6b).
The band gaps of BiOI, UiO-66, and AgI were calculated

ZHU ET AL.
5 of 16
FIGURE 2 Scanning electron microscopy images of BiOI (a,b), 1-BU (c–e), 1-ABU (f,g); (h) the corresponding energy dispersive X-ray
spectroscopy elemental mapping of 1-ABU
to be 1.71, 3.52, and 2.76 eV, respectively. The character-
istic band gap of pristine BiOI was slightly changed with
the coating of UiO-66 and AgI, which will be a benefit to
photocatalyst. When induced UiO-66, BiOI, 0.5-BU,
1-BU, and 2-BU as photocatalysts, 33.3%, 83.6%, 70.5%,
62.1%, and 61.3% of TC can be removed, respectively
(Figure 7a). The high removal efficiency can be attributed
to the strong adsorption ability that originated from hol-
low flower-like structure of BiOI.
[12]
the optical adsorption ability of pure BiOI.
3
.2 | Photocatalytic activity evaluation
In order to investigate the photocatalytic ability of
these compounds more accurately, the pseudo-first-order
kinetic model has been applied to obtain the
The photocatalytic performance of AgI/UiO-66/BIOI
photocatalysts were evaluated by degradating a selected
antibiotic TC under the visible-light irradiation
[
36]
corresponding degradation rate constants
through the
formula (ln(C /C ) = kt). C is the TC concentration at
0
t
0
(Figure 7); 3.4% of TC was self-degraded without any
0 min, whereas C is the concentration at t min. k is
t

6
of 16
ZHU ET AL.
FIGURE 3 (a,b) Transmission electron microscopy image of 1-ABU; (c,d) high-resolution transmission electron microscopy image
of 1-ABU
obtained from the slope of ln(C /C ) versus t as the
compared with reported photocatalysts. Although degra-
dation rate constant ranking second in Table 1, however,
less dose of 1-ABU and weaker intensity of light have
been involved in the TC degradation. In a word, 1-ABU
displays outstanding photocatalytic performance to
degrade TC.
0
t
degradation rate constant of the photocatalyst. The
−
1
degradation rate constant k of BiOI is 0.00988 min , and
−
1
it reaches to 0.1313 min after being fabricated with
UiO-66 with equal theoretical quality (Figure 7b).
Besides, the suitable weight ratio of UiO-66 and BiOI
plays an important role on the enhancement of photo-
The recyclability and stability play a significant role
on evaluating photocatalysts for further application. The
recycle experiment was performed on 1-ABU for TC deg-
radation (Figure 8). 1-ABU was refreshed by centrifuga-
tion, washed for three times with water and ethanol, and
[37]
catalytic activities.
Furthermore, after being fabricated with AgI on the
surface of 1-BU, the photocatalytic activities of AgI/UiO-
6
6/BIOI nanocomposites were enhanced in varying
degrees compared with that of 1-BU and the pure AgI
Figure 7c). The rate constants of AgI, 0.2-ABU,
.4-ABU, 0.8-ABU, 1-ABU, and 1.2-ABU were obtained
ꢀ
dried overnight in vacuum at 80 C. After five runs, the
(
0
removal of TC decreased from 90.1% to 84.6% with only
5.5% loss, demonstrating the high recyclability and stabil-
ity of 1-ABU.
−1
to be 0.0133, 0.0199, 0.0253, 0.02982, and 0.0245 min ,
respectively (Figure 7d). The degradation constant of
pH is considered to be an essential affecting the TC
degradating efficiency via the change of surface charges
on the catalyst and the pollution and the their interac-
1
-ABU was calculated to be about 6.6 times than that of
UiO-66, 3.0 times than that of BiOI, and 1.9 times than
that of AgI. The summarization of BiOI-based photo-
catalysts is shown in Table 1, indicating that 1-ABU dis-
plays the highest removal efficiency of TC degradation
[
48,49]
tion.
As shown in Figure 9a, TC removals at differ-
ent pH remain nearly the same at the range from 86.3%
to 90.1%. However, Figure 9b presents that the

ZHU ET AL.
7 of 16
FIGURE 4 (a) Survey-scan X-ray photoelectron spectroscopy (XPS) spectra of 1-ABU; (b) Ag 3d XPS spectra; (c) I 3d XPS spectra; (d) Bi
f XPS spectra; (e) Zr 3d XPS spectra
4
−
2−
degradation rate constants attained the highest at pH = 4
and 7. At strong acid (pH = 1) or alkaline condition
pH = 10 is TCH and TC , whereas at pH = 4 and 7, the
predominant TC specie should be TCH with small
2
[50]
+
−
(
pH = 10), the degradation rate constants were decreased.
amount of TCH3 and TCH respectively.
As a result,
+
As reported, TC specie at pH = 1 is TCH3 and at
1-ABU exhibits more TC adsorption in dark at pH = 1

8
of 16
ZHU ET AL.
FIGURE 5 (a) Nitrogen adsorption–desorption isotherm and (b) corresponding pore size distributions of BiOI, 1-BU, and 1-ABU
FIGURE 6 (a) Ultraviolet–visible diffuse reflectance spectra and (b) calculated band gap energies of UiO-66, BiOI, AgI, 1-BU,
and 1-ABU
and 10 mainly because that TC species possess charges at
strong acid or alkaline condition, which will enhance the
adsorption capacity of the composite through electro-
static interaction between the surface of photocatalyst
existence of NaCl has little influence on the photo-
catalytic activity of 1-ABU. The degradation rate con-
−
1
stants decreased slightly to 0.024 min with 0–30 mg/L
NaCl added compared with 0.02982 min− without NaCl,
indicating that 1-ABU can maintain the photocatalytic
activity within a certain ion strength. It provides a possi-
bility for semiconductor composites such as 1-ABU to
treat the industrial wastewater with salts.
1
[51]
and TC species.
Ion strength is another important factor affecting the
TC degradation when involved in industrial wastewa-
[52]
ter.
The effects of NaCl concentration on TC degrada-
tion with 1-ABU has been carried out and presented in
Figure 10. Apparently, ionic strength has an obvious
influence on the TC adsorption in the dark. The adsorp-
tion capacity of 1-ABU increases when increasing the
NaCl concentration from 0 to 30 mg/L, which may be
attributed to the fact that the electrostatic interaction
emerged after adding NaCl to enhance the TC adsorp-
Figure 11 represents the effect of initial TC concentra-
tion on the photocatalytic activity with the initial concen-
trations of 5, 10, 20, and 40 mg/L. It is obvious that initial
TC concentration plays a significant role on both the
[
54]
adsorption and photocatalytic processes.
To the
adsorption process, 53.2%, 28.8%, 16.3%, and 11.1% of TC
can be removed using 20 mg 1-ABU with the initial TC
concentration of 5, 10, 20, and 40 mg/L, respectively.
Meanwhile, the degradation rate constants of 1-ABU
[53]
tion.
The results of TC adsorption enhancement are
also consistent with influence of pH. In addition, the

ZHU ET AL.
9 of 16
FIGURE 7 Photocatalytic performance of (a) pristine and binary (c) AgI/BiOI/UiO-66 composites; degradation rate constant of
(
b) pristine and binary and (d) AgI/BiOI/UiO-66 composites
TABLE 1 Comparison with reported BiOI based photocatalysts for degradation of TC
1
Sample
Catalyst (mg)
50 (0.5 g/L)
25 (0.5 g/L)
50 (1.7 g/L)
50 (1 g/L)
TC (mg/L)
20 (100 ml)
10 (50 ml)
20 (30 ml)
10 (50 ml)
10 (50 ml)
10 (50 ml)
30 (150 ml)
20 (100 ml)
20 (100 ml)
20 (50 ml)
10 (100 ml)
Removal (%)
92.1 (210 min)
90.0 (30 min)
91.6 (120 min)
63 (240 min)
90 (180 min)
96 (180 min)
48 (50 min)
k (min−
)
Light source
300 W Xe lamp
300 W Xe lamp
300 W Xe lamp
500 W Xe lamp
500 W Xe lamp
500 W Xe lamp
300 W Xe lamp
300 W Xe lamp
300 W Xe lamp
300 W Xe lamp
250 W Xe lamp
Reference
[
38]
AgI-BiOI/CNFs
BP/BiOI
Zhang et al.
[
39]
0.0767
0.0205
Jing et al.
[
40]
BiOI/g-C
Bi /g-C
BiOI nanosheets
Bi -BiOI
LaCoO /BiOI
0%Ag@AgI//BiOI
BiVO /BiOI
3
N
4
/CeO
2
Jiang et al.
[
41]
x
O
y
I
z
3
N
4
Huang et al.
[
42]
50 (1 g/L)
Bai et al.
Bai et al.
[
43]
2
O
3
50 (1 g/L)
[
44]
3
50 (0.33 g/L)
30 (0.3 g/L)
50 (0.5 g/L)
20 (0.4 g/L)
20 (0.2 g/L)
0.0161
0.0317
0.0134
0.0189
Yao et al.
[
45]
1
86.40 (60 min)
69.57 (60 min)
87.20 (80 min)
Yang et al.
[
46]
4
Chen et al.
[
47]
ZnMoO4/BiOI
Yan et al.
This work
AgI/BiOI/UiO-66
90.1 (60 min)
95.6 (60 min)
0.02982
0.3689
40 (0.4 g/L)
Abbreviation: TC, tetracycline.

10 of 16
ZHU ET AL.
FIGURE 8 Cycling experiments of
tetracycline (TC) degradation in
presence of 1-ABU
FIGURE 9 (a) Photocatalytic performance and (b) degradation rate constants of 1-ABU at different pH
decreased from 0.03381, 0.02982, 0.00936, and
with the same TC concentration (10 mg/L). The removals
−1
0
.00307 min , respectively. It is remarkable that the deg-
of TC are almost the same and reach to about 95% with
the concentration more than 0.4 g/L (Figure 12a). With
more 1-ABU added, more TC can be adsorbed by the
porous 1-ABU, which means adsorption plays the
dominant role in TC removal. To explore the degradation
efficiency of 1-ABU with different concentrations, the
first-order kinetics was then studied in Figure 12b. The
optimum dose for the removal of TC by 1-ABU was
0.4 g/L with the best degradation rate constants
radation rate constants dropped sharply when the TC
concentration increased more than 20 mg/L. It can be
explained as follows: with increased TC concentration,
more TC molecules are agglomerated around 1-ABU,
preventing photons of the visible light from being
exposed to the surface, further resulting in less possibility
to the generation of active species involving in the photo-
[55]
catalytic process.
−1
Dose of photocatalyst on the TC degradation has also
(k = 0.03689 min ). However, the decreasing of degrada-
been studied with 1-ABU amount from 0.1 to 0.8 g/L
tion rate constants can be observed with further

ZHU ET AL.
11 of 16
FIGURE 10 (a) Photocatalytic performance and (b) degradation rate constants of 1-ABU with different quantity of NaCl added
FIGURE 11 (a) Photocatalytic performance and (b) degradation rate constants of 1-ABU at different tetracycline (TC) initial
concentrations
FIGURE 12 (a) Photocatalytic performance and (b) degradation rate constants of 1-ABU with different doses of 1-ABU

12 of 16
ZHU ET AL.
increasing dose of 1-ABU, attributing to reduce of the
strongly enhanced compared with the pristine BiOI. In
addition, 1-ABU exhibited higher photocurrent densities
than 1-BU, demonstrating the inhibited recombination of
photogenerated electron–hole pairs and more effective
light penetration and dispersion with overdosed
[56,57]
photocatalysts.
The duration of well-performed photocatalysts is
another important parameter for its widespread applica-
tions. The powder XRD of 1-ABU after five recycle runs
and photocatalytic degradation in NaCl, pH = 1, pH = 4,
pH = 10 solutions were carried out to evaluate the stabil-
ity of 1-ABU (Figure 13). The theta peaks remain nearly
the same with no obvious deactivation or transformation,
indicating the excellent stability of 1-ABU.
[
59]
mobility of charge carriers in 1-ABU.
The photocur-
rent results agree with the results of the PL spectra and
TC photodegradation presented in Figure 7a. In either
Interestingly, induced UiO-66 possessing high specific
surface area into the nanocomposites cannot enhance the
adsorption of TC obviously in the reaction. The improve-
ment of the photocatalytic activities should be attributed
to the fast separation of photogenerated electrons and
holes of the nanocomposite. PL spectroscopy and photo-
current were usually used to investigate the separation
efficiency of charge carriers in photocatalysts. Figure 14
shows the PL spectra of BiOI, 1-BU, and 1-ABU with the
excitation wavelength of 437 nm. The PL intensity of
1
-BU was a little weaker than that of BiOI. Meanwhile,
PL emission of 1-ABU is quenched obviously with the
lowest intensity, indicating the restraining recombination
of electron–hole pair, which can make contributions to
[58]
the higher photocatalytic performance.
Figure 15
shows the photoelectrochemical results of BiOI, 1-BU,
and 1-ABU. The photocurrent intensity of 1-BU was
FIGURE 14 The photoluminescence responses of BiOI, 1-BU,
and 1-ABU
FIGURE 13 The powder X-ray
diffractometer of 1-ABU after five recycle runs
and photocatalytic degradation in NaCl,
pH = 1, pH = 4, pH = 10

ZHU ET AL.
13 of 16
case, microporous structure of 1-ABU that originated
from UiO-66 can increase the interaction efficiency
between TC molecules and the nanocomposite compared
with the mesoporous structure of 1-BU, further enhanc-
ing the photocatalytic activity. Mott–Schottky analysis
has been used to specify the type of semiconductors and
determine the type of heterojunctions. It is reported that
slopes of the Mott–Schottky plot for pure BiOI, AgI, and
UiO-66 are positive, negative, and negative, indicating
The Mott–Schottky plots of 1-BU and 1-ABU were fur-
ther measured in Figure 16. The Mott–Schottky plots of
1-BU with different frequencies clearly show a positive
and a negative slope, confirming the formation of the n-p
type heterojunction between BiOI and UiO-66. After
being decorated with n-type AgI, the positive and nega-
tive slopes in the Mott–Schottky plot of 1-ABU indicate
the formation of n-p-n type heterojunction.
[
60]
[61]
[62]
the p-type of BiOI,
n-type of AgI,
and UiO-66.
3.3 | Photocatalytic mechanism
The trapping experiments for reactive radicals were per-
formed with 1-ABU in TC degradation reaction. Iso-
propanol (IPA) (2 mM), p-benzoquinone (BQ) (0.2 mM),
and sodium oxalate (SO) (1 mM) were selected to be the
−
+
scavenger of ÁOH, ÁO , and h . As shown in Figure 17,
2
the degradation efficiencies of TC are strangely inhibited
when BQ and IPA are added to the reaction, indicating
+
that ÁOH and h are the prime active species in the
−
photocatalytic process. Besides, ÁO radical also affects
2
the degradation efficiency after adding SO, which means
−
that ÁO2 should act as a secondary active specie during
the degradation process.
Based on results presented above, a possible photo-
catalytic mechanism of 1-ABU with n-p-n type hetero-
junction for TC degradation under visible light
irradiation is depicted in Figure 18. The conduction band
(CB) and valence band (VB) potentials of BiOI, UiO-66,
FIGURE 15 The photocurrent spectra of BiOI, 1-BU,
and 1-ABU
[63,64]
and AgI are presented from the reported literature.
FIGURE 16 Mott–Schottky plots of 1-BU
and 1-ABU at two frequencies of 0.5 and
1
.5 kHz

14 of 16
ZHU ET AL.
The CB and the VB potential for BiOI are +0.585 and
2.295 eV; those for AgI are +2.245 and −0.545 eV and
Type I heterojunction means the poor efficiency of
electron–hole separation, which is consistent with the
slight improvement of degradation efficiencies when fab-
ricated UiO-66 with BiOI in Figure 7a. When turning to
another n-p heterojunction between AgI and UiO-66,
both the CB and VB potentials of AgI are more negative
than those of BiOI and the Type II heterojunction with
the electrons and holes transferring to each other can be
achieved. Type II heterojunction has been considered as
the optimal structure for high separation rate of
+
for UiO-66 are −0.60 and +2.90 eV, respectively. The n-p
heterojunction between UiO-66 and BiOI was first cre-
ated. The CB edge potential of BiOI is more positive than
that of UiO-66, indicating that the electrons on the CB of
UiO-66 will migrate to the CB of BiOI. Meanwhile, the
VB potential of UiO-66 is more positive, and the holes on
the VB of UiO-66 will transfer to the VB of BiOI. How-
ever, the same direction of the hole and electron as the
[
65]
photogenerated electron holes.
After being decorated
with AgI, degradation rate constant k of the 1-ABU can
highly improve compared with the pristine and 1-BU
composites via facilitating the separation of electron–hole
[
66]
pairs.
In any event, UiO-66 plays an important role on
boosting the photocatalytic activities for two reasons.
First, ÁOH has been proved to be one of the prime active
species in photocatalytic degradation according to the
trapping experiments. Because the VB of UiO-66 (2.90 eV
−
vs. NHE) is more positive than the potential of •OH/OH
(
2.38 eV vs. NHE), the holes on VB of UiO-66 can oxidize
−
OH to generate •OH species, leading to the TC
oxidization. Second, as an excellent porous material,
UiO-66 can enlarge the BET surface area of the ternary
composites and improve the photocatalytic performance
by generating highly dispersed active sites and catalytic
contact probability. As a result, the n-p-n heterojunction
can promote the electrons-hole transport and reduce the
recombination of charge carriers with more active species
generated, making 1-ABU a photocatalytic material with
and potential applications.
FIGURE 17 The species trapping experiments of different
scavenger for tetracycline degradation under visible light
over 1-ABU
FIGURE 18 Proposed
photocatalytic mechanism of
1
-ABU for tetracycline
TC) degradation under visible
light irradiation
(

ZHU ET AL.
15 of 16
4
| CONCLUSION
Xuanyu Wu https://orcid.org/0000-0002-8820-8257
Zhiguo Han https://orcid.org/0000-0002-7146-2937
Yu Zhu https://orcid.org/0000-0003-2477-8717
In summary, a series of n-p heterojunctions as BiOI/
UiO-66 have been constructed successfully and further
modified with AgI to obtain the AgI/BiOI/UiO-66 via
a facile solvothermal method. The optimal AgI/BiOI/
UiO-66 (1-ABU) exhibits highest degradation efficiency
of TC (90.1%) compared with pristine BiOI, UiO-66,
and series of BiOI/UiO-66 composites. The trapping
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Min Zhu: Conceptualization; project administration;
supervision. Huimin Chen: Investigation. Yu Dai: For-
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How to cite this article: Zhu M, Chen H, Dai Y,
Wu X, Han Z, Zhu Y. Novel n-p-n heterojunction
of AgI/BiOI/UiO-66 composites with boosting
visible light photocatalytic activities. Appl
[
46] Y. Y. Chen, Y. G. Liu, X. Xie, C. Li, Y. S. Si, M. H. Zhang, Q. S.
Yan, J. Mater. Sci.: Mater. Electron. 2019, 30, 9311.
Organomet Chem. 2021;35:e6186. https://doi.org/
10.1002/aoc.6186
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Zhang, J. Mater. Sci.: Mater. Electron. 2019, 30, 19069.