Hydrothermal synthesis of novel flower-like BiVO4/Bi2Ti2O7 with superior photocatalytic activity toward tetracycline removal

Article abstract of DOI:10.1016/j.apcata.2016.06.025

A novel flower-like BiVO₄ and BiVO₄/Bi₂Ti₂O₇ heterojunction photocatalyst was firstly synthesized by surfactant-free hydrothermal route, which is very convenient, rapid, controllable and environmentally friendly. The crystal structures, chemical composition, morphology, optical and photo-catalytic property of the as-prepared samples were characterized by the techniques of XRD, XPS, SEM, HRTEM, BET, UV–vis DRS, and PL. The BiVO₄/Bi₂Ti₂O₇ heterojunction photocatalyst exhibited higher photocatalytic activity than pure BiVO₄ and Bi₂Ti₂O₇ for the degradation of tetracycline (TC) under xenon lamp irradiation. The enhanced photocatalytic activities of BiVO₄/Bi₂Ti₂O₇ composites were either related to the particular structural features or the mass ratios of BiVO₄ and Bi₂Ti₂O₇ in the composites. The possible photocatalytic mechanism of the BiVO₄/Bi₂Ti₂O₇ composite has been speculated by detecting reactive species based on free radicals trapping experiments. The preliminary removal pathways of tetracycline was proposed on the basis of the identified intermediates using LC-TOF/MS.

Full text of DOI:10.1016/j.apcata.2016.06.025

Accepted Manuscript
Title: Hydrothermal synthesis of novel flower-like
BiVO4/Bi2Ti2O7 with superior photocatalytic activity toward
tetracycline removal
Author: Jinhai Li Mengshu Han Yang Guo Fei Wang Lingjun
Meng Danjun Mao Shanshan Ding Cheng Sun
PII:
S0926-860X(16)30339-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.06.025
APCATA 15921
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
15-12-2015
15-6-2016
17-6-2016
Please cite this article as: Li Jinhai, Mengshu Han, Yang Guo, Fei Wang, Lingjun Meng,
Danjun Mao, Shanshan Ding, Cheng Sun, Hydrothermal synthesis of novel flower-like
BiVO4/Bi2Ti2O7 with superior photocatalytic activity toward tetracycline removal,
Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.06.025
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Hydrothermal synthesis of novel flower-like BiVO4/Bi2Ti2O7 with superior
photocatalytic activity toward tetracycline removal
Jinhai Li, a,b Mengshu Han, a Yang Guo, a Fei Wang, a Lingjun Meng, a Danjun Mao, a
Shanshan Ding, a Cheng Sun a *
a
State Key Laboratory of Pollution Control and Resource Reuse, School of the
Environment, Nanjing University, Nanjing 210023, PR China
b School of chemical engineering, Guizhou University of engineering Science, Bijie
551700, PR China

Graphical abstract

Highlights
● A novel flower-like BiVO4 and BiVO4/Bi2Ti2O7 heterojunction was firstly prepared
via facile and green hydrothermal route.
●The novel BiVO4/Bi2Ti2O7 composites exhibit superior visible-light driven
photocatalytic activity and recycling ability toward tetracycline removal.
●The photocatalytic mechanisms and removal pathways were studied deeply.

Abstract
A novel flower-like BiVO4 and BiVO4/Bi2Ti2O7 heterojunction photocatalyst was
firstly synthesized by surfactant-free hydrothermal route, which is very convenient,
rapid, controllable and environmentally friendly. The crystal structures, chemical
composition, morphology, optical and photo-catalytic property of the as-prepared
samples were characterized by the techniques of XRD, XPS, SEM, HRTEM, BET,
UV–vis DRS, and PL. The BiVO4/Bi2Ti2O7 heterojunction photocatalyst exhibited
higher photocatalytic activity than pure BiVO4 and Bi2Ti2O7 for the degradation of
tetracycline (TC) under xenon lamp irradiation. The enhanced photocatalytic activities
of BiVO4/Bi2Ti2O7 composites were either related to the particular structural features
or the mass ratios of BiVO4 and Bi2Ti2O7 in the composites. The possible
photocatalytic mechanism of the BiVO4/Bi2Ti2O7 composite has been speculated by
detecting reactive species based on free radicals trapping experiments. The
preliminary removal pathways of tetracycline was proposed on the basis of the
identified intermediates using LC-TOF/MS.
Keywords: BiVO4/Bi2Ti2O7, Flower-like, Hydrothermal synthesis, Photocatalytic,
Tetracycline

1. Introduction
To treat bacterial diseases in humans and to promote growth in animal and plant, large
amounts of antibiotics are produced and consumed. Among various antibiotics,
tetracyclines are one kind of the most important antibiotics, which are widely used for
controlling disease and promoting growth due to their excellent efficacy. However,
many antibiotics can be easily entered into the environment because of the poor
absorption, low metabolism, abuse and overuse. Antibiotics in environment could
cause lots of direct toxicities and potential risks (the effects include the development
of super bacteria, direct or indirect toxicity to microorganism and possible risks to
human beings) [1-4].
Over the past decades, antibiotic residues are increasingly found in the water
environment and have attracted much attention. To eliminate these challenges, many
studies were aimed to deal with antibiotic wastewater, such as physical adsorption,
chemical reactions and biological degradation, etc. However, the efficiencies of these
methods are usually very poor as the low concentrations and biochemical rate.
Comparing with the traditional methods, photocatalytic technology is considered a
very promising and effective method of revealing environmental pollutants due to
high mineralization efficiency, no secondary pollution, low energy consumption and
simple process. Often, semiconductor plays a very important role in the photocatalytic
process. Among numerous semiconductor photocatalytic materials, BiVO4 is one of
the newly visible-light-driven photocatalyst owing to its moderate band gap (about 2.4
eV) [5, 6]. BiVO4 has also been widely reported as photocatalyst for water splitting
and the pollutants removal, and so on. However, the commercialization of BiVO4 has
been hindered by poor solar response efficiency. To overcome this obstacle, many
efforts have been tried to improve the photocatalytic activity of semiconductor, such
as element doping and semiconductor coupling, etc. It is well known that the building
of heterojunction photocatalysts by coupling with semiconductors, which is beneficial
to separate of charge carriers and to promote photocatalytic performance. Therefore,
building of heterojunction is a relatively effective and common method of exploiting
for synthesis of new high-efficiency photocatalysts. Furthermore, the physical and

chemical properties of most photocatalyst are also strongly dependent on structure,
morphology and size. These properties of functional materials are closely related to
the synthetic methodologies and preparation conditions. Therefore, selecting an
appropriate method to control preparation of photocatalyst has been paid more and
more attention. Among various morphologies, flower-like architectures are made of
many nanoparticles, which have been given considerable attention due to numerous
advantages in photocatalysts, promoting solar light absorption, providing abundant
exposed interfacial active area and enhancing the separation of photo-generated
charge carriers [7-9].
Recently, synthesized BiVO4 based heterojunction photocatalysts has aroused
significant scientific interest. A lots of BiVO4 based heterojunction photocatalysts
were reported, such as BiVO4/g-C3N4, BiVO4/SnO2/WO3, BiVO4–TiO2 [10-12], etc.
However, BiVO4/Bi2Ti2O7 composite has not been explored. Bi2Ti2O7 has been
confirmed the photocalalytic activity for water splitting and pollutants removal.
However, there are a few practical problems when using Bi2Ti2O7 photocatalysts,
such as low catalytic activity and difficult to recycle after the reaction [13，14], and so
on. These problems could be resolved by constructing the heterojunction of
BiVO4/Bi2Ti2O7.
In this work, a novel flower-like BiVO4 and BiVO4/Bi2Ti2O7 composite catalyst were
firstly successfully prepared by a facile and controllable hydrothermal method. The
structures, morphologies, optical absorption properties, of as-prepared BiVO4 and
BiVO4/Bi2Ti2O7 were carefully studied by relevant tests. The results of tetracycline
removal were investigated under sunlight driven showed that BiVO4/Bi2Ti2O7
heterogeneous catalysts exhibited excellent photocatalytic activity. Moreover, the
possible activity photocatalytic mechanism and removal pathways of tetracycline
have been investigated and proposed in detail.
2. Experimental
2.1. Reagents
Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, AR), trisodium tetraoxovanadate
dodecahydrate (Na3VO4·12H2O, CP), tetrabutyl titanate (C16H36O4Ti, CP) and ethanol

absolute (C2H5OH, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd.
(SCRC, Shanghai, China). All reagents were used without any further purification.
2.2. Preparation
2.2.1 Synthesis of flower-like BiVO4
In a typical experiment, 60 mg Bi(NO3)3·5H2O was dissolved in 40 mL distilled water;
after sonication for 5 min, 100 mg Na3VO4·12H2O was added into. The suspension
was transferred to a sealed Teflon autoclave and was heated at 160℃ for 12 h [15,
16]. Then, the autoclave was cooled to room temperature, and the precipitation was
washed with distilled water and ethanol absolute for three times, respectively.
Subsequently the precipitation was dried in an oven at 60 °C for 3 h.
2.2.2 Synthesis of flower-like BiVO4/Bi2Ti2O7
The fabrication process of flower-like BiVO4/Bi2Ti2O7 via a template method, which
was described in Fig. 3 (Growth process of as-prepared samples). Firstly, the
flower-like BiVO4 (0.1g) was dispersed into 40 mL deionized water by sonication for
5 min. Then, a certain amount of Bi(NO3)3·5H2O and C16H36O4Ti were added into the
supernatant solution. During the process, Bi2Ti2O7 particles were produced and loaded
on the petals of BiVO4. A series of BiVO4/Bi2Ti2O7 samples were obtained by
changed the mass ratio of BiVO4 and Bi2Ti2O7 (2:1, 4:1, 8:1), and the pure Bi2Ti2O7
was prepared by the same method except for addition of BiVO4, for the purpose of
comparison [17, 18].
2.3 Characterization
The crystalline structures of as-prepared samples were determined by powder X-ray
diffraction (XRD) using XRD-6000 X-ray diffractometer (Shimadzu). The chemical
compositions was and determined by X-ray photoelectron spectroscopy (XPS) on a
PHI 5000 Versa Probe spectrometer (ULVAC-PHI, Japan). The morphology
characteristics of the samples were observed by a scanning electron microscope (SEM,
QUANTA FEG 250), the energy dispersive X-ray spectroscopy (EDS). The chemical
mapping was performanced on an energy dispersive X-ray spectroscopy (EDS)
detector, the transmission electron microscopy (TEM) and high-resolution

transmission electron microscopy (HRTEM) were taken over a JEM-200CX
instrument. The Brunauer–Emmett–Teller (BET) surface area of the samples was
determined by nitrogen adsorption on a BET ASAP 2020 Micrometrics instrument.
Both UV–Vis diffuse reflectance spectra (UV-Vis/DRS) and photoluminescence
spectra (PL) were recorded to illustrate the optical properties of the samples by
UV-Visible spetrometry (UV-3600) and Fluorescence Spectrometry (Horiba HJY
FM-4P-TCSPC).
2.4 Photocatalysis
The photocatalytic activities of as-prepared samples were evaluated by the
photocatalytic degradation of tetracycline in aqueous solution under sunlight
irradiation. In a typical process, 50 mg catalyst was added to 50 mL of 10 mg L-1
tetracycline solution. The suspension mixture was stirred in the dark for 30 min to
ensure the establishment of an adsorption–desorption equilibrium between catalyst
particles and tetracycline molecules before illumination. Then the suspension solution
was irradiated by the light originated from a 500W Xe lamp. During the reaction
processes, the suspension mixture was stirred continuously. At given time (10 min)
intervals of light irradiation, about 3 mL sample solution was collected, centrifuged
and then analyzed in the subsequent research. Meanwhile, the cyclic degradation tests
were implemented to investigate the stability and reusability of catalyst. Furthermore,
the active species of photocatalytic reactivity were detected by the trapping
experiments. Ammonium oxalate (AO), isopropanol (IPA) and benzoquinone (BQ)
−
were served as the scavengers to scavenge the reactivity of superoxide (•O2 ) radical
anions, photogenerated holes (h+) and hydroxyl (•OH) radicals, respectively. The
procedures of trapping experiment were similar to the degradation experiment. The
scavengers introduced separately into the aqueous tetracycline solution before
addition of the photocatalyst.
2.5 Analysis
The concentration of tetracycline in the solution was preliminarily determined by
UV−Vis spectroscopy (UV–Vis 750, PerkinElmer)，then was monitored accurately by

an Agilent 1200 high performance liquid chromatography (HPLC) with the UV–Vis
spectrophotometer 2450 and a 4.6 mm×250 mm Zorbax Eclipse XDB-C18 at 25 °C.
The maximum absorption wavelength of tetracycline was 357 nm which was
determined by scanning from 200 to 800 nm using a PerkinElmer UV–Vis 750
spectrophotometer. The mobile phase was mixture of methanol-water (30:70, v/v, 2‰
formic acid containing in water) with a flow rate of 1 mL min-1. The removal rate for
tetracycline was calculated using the formula of (C0 − C)/C0 × 100%, where the C is
the concentration of tetracycline at a given irradiated time and the C0 means the initial
concentration [19-25]. Meanwhile, the mineralization of tetracycline was monitored
by determining the total organic carbon (TOC) by means of a TOC analyser (1030D,
OI).
To analyze the degradation pathway of tetracycline with photocatalysis, the
intermediates were monitored by liquid chromatography-time-of-flight mass
spectrometry (LC-TOF/MS, API 4000 Triple TOF 5600 LCQ Advantage MAX) with
electrospray ionization (ESI) source and a Beta Basic-C18 column (150 mm × 2.1
mm). The mixture of methanol-water (30:70, v/v, 2‰ formic acid containing in water)
was used as mobile phase with a flow rate of 1 mL min -1. The injection volume was
20μl, the vaporizer temperatures were 300°C, the spectra was scanned in mass range
from 50 to 600 m/z with the positive ion mode [19-25].
3. Results and discussion
3.1 Characterizations
3.1.1 Crystal structures
The XRD pattern of as-prepared samples (Fig. 1a) reveal that the diffraction peaks of
pure BiVO4 (as-prepared at 160 °C for 12 h) are in good agreement with the standard
values of monoclinic BiVO4 (JCPDS No. 14-0668). The strongest diffraction peak
located at 2θ value of 28°, which can be indexed to the (121) plane of monoclinic
BiVO4 [26, 27]. The grazing incidence of the diffraction patterns for as-prepared pure
Bi2Ti2O7 sample demonstrates that the synthesized Bi2Ti2O7 diffraction peaks were
well matched to the standard values of Bi2Ti2O7 (JCPDS No. 32-0118). The highest
diffraction peak located at 2θ value of 30°, which can be indexed to the (444) plane

[28, 29]. The pattern of synthesized BiVO4/Bi2Ti2O7 composite samples indicate the
characteristic diffraction peaks of both BiVO4 and Bi2Ti2O7 crystalline phases. It
shows that the BiVO4/Bi2Ti2O7 composite was prepared successfully.
3.1.2 Chemical composition
The results of XPS (Fig. 1b) analysis provided another insight into the chemical
composition of the as-prepared BiVO4/Bi2Ti2O7 composite. The survey spectra of the
BiVO4/Bi2Ti2O7 sample indicate a carbon peak at a binding energy of about
284.9−285.9 eV, which is derived from the carbon tape used for fixing sample and
from atmospheric CO2 adsorbing on the sample surface [10, 11]. The two strong
peaks are observed at approximately 465.8 eV and 458.3 eV, which are ascribed to
the Ti 2p3/2 and Ti 2p1/2 binding energies, respectively [12, 13]. The dominant peak of
O1s locates at 530 eV [14]. The peaks of 164.7 eV and 159.4 eV were attributed by Bi
4f5/2 and Bi 4f7/2, which correspond to Bi3+ [15, 16]. The two peaks at binding energies
of 524.2 eV and 517.0 eV are the signals of V 2p1/2 and V 2p3/2, which correspond to
the V5+ [17-18]. These XPS detected signals reveal that BiVO4/Bi2Ti2O7 composite
containing these four elements and support the XRD results. In addition, no other
impurity peaks have been identified in the survey spectra, which suggests the high
purity of the obtained BiVO4/Bi2Ti2O7 composite. Furthermore, the SEM-EDS figure
(Fig. 2a) and SEM-EDS mapping (Fig. 2b) confirmed that BiVO4/Bi2Ti2O7 composite
possesses the elements of O, V, Bi and Ti. The highest peak in (Fig. 2a) is the
background peak of aluminum foil for fixed sample. Meanwhile, the SEM-EDS
mappings (Fig. 2b) indicate that the corresponding elements are uniformly distributed
in BiVO4/Bi2Ti2O7 composite.
Fig. 1 (a) XRD patterns of as-prepared samples and (b) XPS figure of
BiVO4/Bi2Ti2O7 (4:1)
Fig. 2 (a) SEM-EDS of BiVO4/Bi2Ti2O7 (4:1) and (b) SEM-EDS mapping of
BiVO4/Bi2Ti2O7 (4:1)
3.1.3 Morphology characteristics
The SEM image Fig. 3a (SEM figure of BiVO4) shows that the as-prepared BiVO4
consists of uniform flower-like morphology. It is also observed that the flower-like

microspheres of BiVO4 are composed of numerous, thin, densely packed “nanosheet
petals”. The irregular Bi2Ti2O7 nanoparticles are revealed in Fig. 3a (SEM figure of
Bi2Ti2O7). Meanwhile, Fig. 3a (SEM figure of BiVO4/Bi2Ti2O7) intuitively indicates
that the nanoparticles of Bi2Ti2O7 were gathered on the surface of “petals” BiVO4
under hydrothermal condition. This result is also confirmed by the results of BET
determination (Fig. 3b), the BET surface area of the BiVO4/Bi2Ti2O7 composite
sample was calculated to be 46m2 g-1, which is much larger than the surface area of
the as-prepared flower-like BiVO4 (16 m2 g-1), and is less than the surface area of the
as-prepared Bi2Ti2O7 nanoparticles (78 m2 g-1).
Fig. 3 SEM figures (a) and BET figures (b) of as-prepared samples
The TEM image of BiVO4/Bi2Ti2O7 Fig. 4a (TEM figure of BiVO4/Bi2Ti2O7) further
indicates that the flower-like BiVO4/Bi2Ti2O7 composite was successfully synthesized.
BiVO4/Bi2Ti2O7 has a diameter of about 800 nm and ultrathin wall thickness of a few
nanometers. The HRTEM image Fig.4a (HRTEM figure of BiVO4/Bi2Ti2O7) clearly
shows that the flower-like BiVO4 possesses fine crystallinity and well agrees with the
size and shape of BiVO4 observed by SEM figures. One of lattice spacing was
calculated to be 0.312 nm, corresponding to the (121) planes of monoclinic BiVO4
[26-27], and another lattice spacing was calculated to be 0.298 nm, corresponding to
the (444) planes of Bi2Ti2O7 [28-29].
3.1.4 Growth process
Generally, various nanoparticle morphologies can be obtained by adding the
surfactants during the synthesis process. However, in the present work, the uniform
flower-like microspherical morphology of BiVO4 and BiVO4/Bi2Ti2O7 was prepared
without addition of any surfactants. As well-known that the absence of surfactants, the
geometrical or individual building components of a material may be regarded as a
route for the formation of flower-like morphology by a self-assembly process [30-33].
Based on the above, the growth process BiVO4 and BiVO4/Bi2Ti2O7 could be
approximately deduced and schematically represented in Fig. 4b. The BiVO4
3−
nanospecies were generated immediately when Bi3+ and VO4 were collided under
the hydrothermal reaction conditions. Subsequently, these BiVO4 particles tend to

gather to reduce their surface energies, forming the BiVO4 nanosheet. With the
temperature increasing, the petal-like morphology of BiVO4 starts to grow from the
nanospecies, the single petal-like nanosheet petals can be clearly observed in the SEM
and TEM figures. Then, the continuous hydrothermal reaction could lead to the
self-assembly of these petals into the uniform flower-like morphology of BiVO4. In
the process of BiVO4/Bi2Ti2O7 synthesized, the newly generating nanoparticles of
Bi2Ti2O7 were loaded on the petals of BiVO4 and formed the heterojunction of
BiVO4/Bi2Ti2O7 by using flower-like BiVO4 template.
Fig. 4 (a) TEM figures of BiVO4/Bi2Ti2O7 (4:1) and (b) growth process of as-prepared
samples
3.1.5 Optical properties
The optical properties of the photogenerated electron–hole pairs are recognized as the
key factors determining the catalytic activity of photocatalysts. Herein, the optical
properties of as-prepared samples were studied by a UV–Vis/DRS and PL. The
diffuse reflectance UV–vis spectra (UV–vis DRS) of the samples were recorded on an
UV-visible spetrometry (UV-3600) in the wavelength range of 200-800 nm with
BaSO4 powders as the reference. The results of UV–Vis/DRS were showed in Fig. 5a.
It indicates that the absorption spectrum of BiVO4/Bi2Ti2O7 composite photocatalyst
lies between the ones of BiVO4 and Bi2Ti2O7, which could be attributed to the
heterojunction effect in BiVO4/Bi2Ti2O7 composite. Usually, these data can also be
converted to the band gap (Eg) to measure light absorption ability of a photocatalyst.
The optical absorption near band edge can be evaluated by the equation (αhv) = A(hv
– Eg)1/2, where α, h, A, v, and Eg are the absorption coefficient, Planck's constant,
constant, light frequency and band gap, respectively [10-18]. According to this
equation, the band gap of as-prepared samples were calculated and showed in the
insert of Fig. 5a (insert the figures of DRS). The band gap of BiVO4/Bi2Ti2O7
composite photocatalyst is 2.75 eV, which is larger than that of flower-like BiVO4
(2.4 ev) and less than that of Bi2Ti2O7 (2.9 eV). Thus it can be seen that the light
utilization efficiency could be promoted by building BiVO4/Bi2Ti2O7 heterojunction.
Furthermore, the charge transfer efficiency can be observed by PL spectrum, the PL

spectrum is mainly photogenerated electron–hole pairs when semiconductor being
irradiated by light, and the lower emission often reveals the higher separation
efficiency of the charge carriers. The PL spectra of as-prepared samples (Fig. 5b)
show that a strong emission peak of pure Bi2Ti2O7 centered at about 370 nm was
observed, the maximum emission peak of BiVO4 located at approximately 430 nm
presents in the spectra, and the lowest emission peak belongs to BiVO4/Bi2Ti2O7. It
indicates that the BiVO4/Bi2Ti2O7 composite has the highest separation efficiency of
photogenerated electron–hole pairs. Therefore, the optical properties of UV–Vis/DRS
and PL implied that the photocatalytic activity of BiVO4/Bi2Ti2O7 heterojunction
could be enhanced.
Fig. 5 DRS figure (a) and PL figure (b) of as-prepared samples (the insert shows a
plot of (Ahυ)2 vs. energy hυ)
3.2 Photocatalytic activity
According to the formula of (C0 − C)/C0 × 100% and analytical concentration of
tetracycline during the process of photocatalytic experiments, the removal rates of
tetracycline were calculated and summarized in Fig. 6a. Further, the kinetics of the
photocatalytic degradation of tetracycline were studied. It was been reported that
degradation of tetracycline followed pseudo first order kinetics [34-35]. According to
the following equation, the Ln (C0/Ct) values are obtained and graphically presented
in Fig. 6b.
Ln (C0/C) = kapp·t
(1)
Where C0 is initial concentration of tetracycline, and C is the concentration of
tetracycline at a given time ‘t’. Fig. 6b clearly showed that the experimental data were
consistent with the pseudo first order kinetics model. The results also suggested that
reducing the concentration of tetracycline the kapp values increase in the photocatalytic
reaction system. Therefore, the low initial concentration of tetracycline is found to be
efficient.
The result of the blank test (without the photocatalyst) revealed that the tetracycline
molecule was very stable in the aqueous solution. Because the photodegradation of
the tetracycline was very slowly. The removal rate of pure BiVO4 (53%) and pure

Bi2Ti2O7 (60%) were lower than that of BiVO4/Bi2Ti2O7 composites. Furthermore, to
examine practical value of BiVO4/Bi2Ti2O7, the comparative experiment with TiO2
was carried out, and the removal rate of TiO2 was about 30%. It indicated that the
photoactivity of TiO2 was lower than that of BiVO4/Bi2Ti2O7 composites. It showed
that the as-synthesized BiVO4/Bi2Ti2O7 photocatalysts can efficientlyenhance the
photocatalytic activity in compared with the pure BiVO4 and pure Bi2Ti2O7.
Meanwhile, the mass ratio of BiVO4 and Bi2Ti2O7 in the composite plays an important
role for improving the photocatalytic activity of BiVO4/Bi2Ti2O7 compound catalyst.
With increasing the mass ratio of BiVO4, the photocatalytic activities first increase
and then decrease. While the mass ratio of BiVO4 and Bi2Ti2O7 is 4:1 in the composite,
BiVO4/Bi2Ti2O7 catalyst demonstrates the highest photocatalytic activities for
degradation tetracycline. The removal rate approximated 98% after 60 min irradiation
under sunlight. It maybe that the suitable mass ratio of BiVO4 and Bi2Ti2O7 could be
conducive to form the more heterojunctions in BiVO4/Bi2Ti2O7 composite. The
heterojunctions can efficiently facilitate charge separation, delay the recombination of
photogenerated electron–hole pairs and broad the scope of visible light response.
However, with increasing BiVO4 or Bi2Ti2O7 could lead some particles of BiVO4 or
Bi2Ti2O7 couldn’t be contacted closely. Hence, the less heterojunctions were
constituted effectively and the photocatalytic activity were also reduced. Furthermore,
the significantly enhanced photocatalytic activity of flower-like BiVO4/Bi2Ti2O7
composite could also be attributed to the physical properties such as the synergic
effect of heterogeneous structure between BiVO4 and Bi2Ti2O7, morphology effect,
crystal size, BET surface area, and so on. In particular, the large (diameter of about
800 nm) and ultrathin petals wall with a few nanometer thickness could be more
beneficial to promote light absorption and the separation of photogenerated
electron–hole pairs.
Usually, it is highly desirable to completely degrade or mineralize pollutants.
However, many pollutants can’t be completely degraded or mineralized in most of
pollution treatment process, and high degradation rate doesn’t mean high
mineralization rate. To investigate the effective removal rate of tetracycline, the TOC

disappearance was examined during the photocatalytic degradation experiments. It
showed that the TOC value of tetracycline decreased from 3.38 mg/L to 1.05 mg/L,
and the corresponding mineralization rate was approximately 70% for the
BiVO4/Bi2Ti2O7 (4:1) under the experimental conditions. It suggested that most of
tetracycline molecules could be completely mineralized in the photocatalytic process.
To study the stability of heterojunction photocatalyst, the circulation experiments of
the as-synthesized BiVO4/Bi2Ti2O7 (4:1) sample were invetisgated and the results
were presented in Fig. 6c. It showed that the photocatalytic activity of the
BiVO4/Bi2Ti2O7 (4:1) sample had no apparent deactivation after three successive
recycles for the degradation of tetracycline under visible light irradiation, which
revealed that the as-synthesized BiVO4/Bi2Ti2O7 heterojunction photocatalyst
possessed high stability for the practical application. Moreover, the XRD pattern and
SEM figure of the BiVO4/Bi2Ti2O7 (4:1) sample were also been researched after 3th
run cycle (Fig. 6d). They clearly indicated that the phase and structure of the
BiVO4/Bi2Ti2O7 sample was not been unchanged, which revealed that the
BiVO4/Bi2Ti2O7 sample was very stable.
Fig. 6 (a) results of photocatalytic activity, (b) the kinetics of photocatalytic
degradation, (c) results of cycling experiments, (d) XRD patterns of cycling
BiVO4/Bi2Ti2O7 (4:1) and SEM figure of cycling BiVO4/Bi2Ti2O7 (4:1)
3.3 Photocatalytic mechanism
It is well known that the photocatalytic activity of a heterojunction photocatalyst is
directly related to its energy band structure and the fate of the photoinduced
electron–hole pairs, and the photocatalytic removal of pollutants occurs mainly
−
through the involvement of the active species (such as h+, •OH, and •O2 ). To
investigate the photocatalytic reaction mechanism of the BiVO4/Bi2Ti2O7
heterojunction, the energy band structure of BiVO4/Bi2Ti2O7 and the trapping
experiments of the active species were studied. According to the references [10-18],
the conduction band of BiVO4 and Bi2Ti2O7 are 0.31 eV and -0.24 eV, respectively.
And the valence band of BiVO4 and Bi2Ti2O7 are 2.78 eV and 2.66eV, respectively.
Bi2Ti2O7 has more negative CB and VB than BiVO4, which could result in transfer of

photoinduced electron–hole pairs under light irradiation, and the electron hole
recombination could also be successfully impeded.
Meanwhile, the trapping experiments of active species involved in the photocatalytic
reaction and the enhanced catalytic activity of BiVO4/Bi2Ti2O7 were invested. The
effects of different scavengers on the removal efficiency of tetracycline were shown
in Fig. 7a. The results indicated that the removal ratio of tetracycline was significantly
decreased to approximate 60%, 30%, 40% after the addition of IPA, BQ, and AO,
respectively. Comparing with 98% in the absence of the scavengers, these results
−
reveal that the main reactive species of h+, •OH, and •O2 participated in the
−
photocatalytic degradation processes, and h+ and •O2 radicals play more important
role in the photocatalytic degradation reaction.
Furthermore, the photoelectrochemical characterization was conducted on a CHI760E
electrochemical workstation (Shanghai, China) with a standard three-electrode system.
The prepared samples were loaded onto ITO electrode (1cm*2cm squares) and served
as the working electrode. The counter and reference electrodes were Pt plate and
Ag/AgCl electrode and 0.2 M Na2SO4 was used as electrolyte. A 500 W xenon lamp
was used to provide light source. The photocurrent was recorded and shown in Fig. 7b.
It indicate that the BiVO4/Bi2Ti2O7 (4:1) heterojunction exhibited a higher transient
photocurrent density (about 0.6–1.1 µA/cm2) than that of pure BiVO4 and Bi2Ti2O7,
which suggested that the separation efficiency of photogenerated electrons and holes
was significantly enhanced at heterojunction interface.
Based on the obtained results and the above discussion, the probable mechanism
diagram for the photocatalytic removal of tetracycline using BiVO4/Bi2Ti2O7
heterojunction under sunlight irradiation maybe proposed and described in Fig. 7c.
Under the light illumination, the electrons (e-) in the CB of Bi2Ti2O7 are excited and
transferred to the CB of BiVO4, and the generated holes in the VB of BiVO4 are
synchronously excited and transferred to the VB of Bi2Ti2O7. Then the electrons in the
−
CB of BiVO4 could be trapped by surface chemisorbed O2 to generate •O2 .
Meanwhile, the h+ in VB of Bi2Ti2O7 can directly oxidize the organic compounds or
react with surface-adsorbed H2O to produce •OH. Therefore, the photogenerated

charge carriers are effectively separated by heterojunction between BiVO4 and
Bi2Ti2O7, and the photocatalytic activity is also improved.
Fig. 7 (a) results of active species detection, (b) The photocurrent responses of
as-prepared samples for each switch-on/off event under sunlight irradiation and (c)
possible photocatalytic mechanism
3.4 Removal pathways
To identify the degradation products generated during the photocatalytic process and
deduce degradation pathways, the degradation intermediates of tetracycline were
monitored by LC-TOF/MS. In the reaction process of BiVO4/Bi2Ti2O7 heterojunction
photocatalysis of tetracycline, numerous intermediates produced are presented in
Table 1. Based on these results and previous studies [19-25], the proposed removal
pathways were speculated in Fig. 8.
Table 1 The identified intermediate products of tetracycline degradation by using
LC-TOF/MS
No.
TC
1st
[M + H]+ measured
Molecular formula
C22H24N2O8
C22H25N2O9
C20H22N2O9
C19H23NO8
C19H21NO7
C11H12O4
Chemical structure (symbol in Fig. 8)
445
461
434
393
375
208
171
416
402
357
191
172
TC
p (1)
p (2)
p (3)
p (4)
p (5)
p (6)
p (7)
p (8)
p (9)
p (10)
p (11)
2nd
3rd
4th
5th
6th
C8H13NO3
7th
C20H20N2O8
C20H20NO8
C19H17O7
8th
9th
10th
11th
C11H11O3
C8H12O4
It could be observed that the peak with m/z of 445 was identified, which was regarded
as the tetracycline molecule. Meanwhile, a lot of extra peaks appeared along with the

degradation of tetracycline (such as m/z = 461, 434, 393, 375, 208, 171, 416, 402, 357,
191, 172). Combining with these detection results and the literature [19-25], the
degradation removal process could be proposed as two main pathways. The first
pathway showed that the intermediates (p1) with the m/z of 461 was caused from a
cycloaddition of •OH locating the C11a–C12 double-bond, and the maximum
adsorption band of tetracycline at the 357 nm would be disappeared by this effect.
−
With continuous irradiation and attack of the active species (such as •OH, •O2 , and
h+), tetracycline molecules and the intermediates were broken down into secondary
degradation products with lower molecular weight. The compound (p2) with m/z of
434 was produced by losing of two methyls from the tertiary amine; the product (p3)
with m/z of 394 was dissociated off a formamid. The intermediate product (p4) m/z of
375 was removal of a hydroxyl from p3. Further degraded products with m/z of 172
and 209 (p5 and p6) resulted from the breaking of the C5a–C6 bond and C11a–C12
bond of p4. The second pathway should be considered as the two methyls locating at
the tertiary amine from the tetracycline molecules were left firstly and produced the
m/z of 416 (p7). Following, the amino connecting with the C5 was removed with m/z
of 402 (p8), then a formamid was abandoned with m/z of 358 (p9). Two additional
ions at m/z 191 and 173 (p10 and p11) were obtained due to broking of the C5a–C6
bond and C11a–C12 bond.
Thus, it can be seen that tetracycline had been gradually disintegrated, and the
relevant degradation intermediates were produced and then decomposed, finally these
+
substances were converted into CO2, H2O, NH4 and the other degradation products.
In the removal process, these intermediate products mainly stemmed from tetracycline
degradation after losses of some groups from the ring or wreck of bridge ring, and the
positions were vulnerable to be attacked by the active species often locating at the
oxhydryl, methyl, third amine, formamid and the band of C5a–C6 and C11a–C12, etc.
However, the photocatalytic removal of tetracycline is a very complex process, and
about which further studies should be carried out.
Fig. 8 The proposed pathways for removal of tetracycline
4. Conclusion

A novel flower-like BiVO4 was firstly synthesized by a facile surfactant-free
hydrothermal process, and a novel flower-like BiVO4/Bi2Ti2O7 heterojunction
photocatalyst was successfully prepared by the template method. The growth process
of flower-like BiVO4 and BiVO4/Bi2Ti2O7 composite could be considered to be a
self-assembly process. The optical properties of DRS and PL indicate that the
as-synthesized BiVO4/Bi2Ti2O7 heterojunction catalyst sample can enhance
photoresponse and exhibit an excellent separation ability of photogenerated
electron–hole pairs. The as-synthesized BiVO4/Bi2Ti2O7 heterojunction exposed a
superior photocatalytic efficiency and stability for removal of tetracycline in aqueous
solution. The improvement in photocatalytic activity displayed by flower-like
BiVO4/Bi2Ti2O7 composite catalyst could be attributed to the heterojunction structure,
morphology effect, crystal size, BET surface area, and so on. The results of free
−
radicals trapping experiments reveal that h+ and •O2 radicals play more important
role. The LC-TOF/MS analysis showed that the degradation intermediates of
tetracycline mainly stemmed from tetracycline degradation after losses of some
groups from the ring or wreck of bridge ring. This work will not only contribute to the
shape control about synthesis of uniform BiVO4 hierarchical structures and excellent
BiVO4 composite heterojunction photocatalyst, but also offer a valuable insights into
purifying polluted water resources and environmental governance.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No.
21307103), the funds of Southwest University (No. XDJK2010E002), the Institute of
Circular Economy of Bijie and Guizhou University (No. ZK005), the Guizhou
Provincial Science and Technology Supporting Program (No. LKB [2013] 08, No. LH
[2014] 7518), the Guizhou Provincial Education Department Supporting Program
(No. KY [2015] 406), the National Natural Science Foundation of China (No.
21307103).
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Figure captions
Fig. 1 (a) XRD patterns of as-prepared samples and (b) XPS figure of
BiVO4/Bi2Ti2O7 (4:1)

Fig. 2 (a) SEM-EDS of BiVO4/Bi2Ti2O7 (4:1) and (b) SEM-EDS mapping of
BiVO4/Bi2Ti2O7 (4:1)

Fig. 3 SEM figures (a) and BET figures (b) of as-prepared samples

Fig. 4 (a) TEM figures of BiVO4/Bi2Ti2O7 (4:1) and (b) growth process of as-prepared
samples

Fig. 5 DRS figure (a, the insert shows a plot of (Ahυ)2 vs energy hυ) and PL figure (b)
of as-prepared samples

Fig. 6 (a) results of photocatalytic activity, (b) the kinetics of photocatalytic
degradation,
(c) results of cycling experiments, (d) XRD patterns of cycling BiVO4/Bi2Ti2O7 (4:1)
and SEM figure of cycling BiVO4/Bi2Ti2O7 (4:1)

Fig. 7 (a) results of active species detection, (b) The photocurrent responses of
as-prepared samples for each switch-on/off event under sunlight irradiation and (c)
possible photocatalytic mechanism

Fig. 8 The proposed pathways for removal of tetracycline