Transformation of novel TiOF2nanoparticles to cluster TiO2-{001/101} and its degradation of tetracycline hydrochloride under simulated sunlight

Article abstract of DOI:10.1039/d0ra08476j

The anatase type cluster TiO2-{001/101} was rapidly generated by a one-step hydrothermal method. The transformation process of coral-like TiOF2 nanoparticles to cluster TiO2-{001/101} was investigated for the first time, and the sensitization between cluster TiO2-{001/101} and tetracycline hydrochloride (TCH) was also discussed. The degradation rate of TCH by cluster TiO2-{001/101} under simulated sunlight was 92.3%, and the total removal rate was 1.76 times that of P25. Besides, cluster TiO2-{001/101} settles more easily than P25 in deionized water. The study showed that cluster TiO2-{001/101} derived from coral-like TiOF2 nanoparticles had a strong adsorption effect on TCH, which was attributed to the oxygen vacancy (Ov) and {001} facets of cluster TiO2-{001/101}. The strong adsorption effect promoted the sensitization between cluster TiO2-{001/101} and TCH, and widened the visible light absorption range of cluster TiO2-{001/101}. In addition, the fluorescence emission spectrum showed that cluster TiO2-{001/101} had a lower luminous intensity, which was attributed to the heterojunction formed by {001} facets and {101} facets that reduces the recombination rate of carriers. It should be noted that cluster TiO2-{001/101} still has good degradation performance for TCH after five cycles of degradation. This study provides a new idea for the synthesis of cluster TiO2-{001/101} with high photocatalytic performance for the treatment of TCH wastewater. This journal is

Full text of DOI:10.1039/d0ra08476j

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PAPER
Transformation of novel TiOF nanoparticles to
2
cluster TiO -{001/101} and its degradation of
2
Cite this: RSC Adv., 2020, 10, 42860
tetracycline hydrochloride under simulated
sunlight
ac
b
ac
ac
a
Yue Jian, Huayang Liu, Jiaming Zhu, Yaqiong Zeng, Zuohua Liu,
b
ac
Chentao Hou * and Shihua Pu*
The anatase type cluster TiO
transformation process of coral-like TiOF
the first time, and the sensitization between cluster TiO
was also discussed. The degradation rate of TCH by cluster TiO
2
-{001/101} was rapidly generated by a one-step hydrothermal method. The
nanoparticles to cluster TiO -{001/101} was investigated for
-{001/101} and tetracycline hydrochloride (TCH)
-{001/101} under simulated sunlight was
2.3%, and the total removal rate was 1.76 times that of P25. Besides, cluster TiO -{001/101} settles
-{001/101} derived from
nanoparticles had a strong adsorption effect on TCH, which was attributed to the
oxygen vacancy (O ) and {001} facets of cluster TiO -{001/101}. The strong adsorption effect promoted
the sensitization between cluster TiO -{001/101} and TCH, and widened the visible light absorption
range of cluster TiO -{001/101}. In addition, the fluorescence emission spectrum showed that cluster
TiO -{001/101} had a lower luminous intensity, which was attributed to the heterojunction formed by
001} facets and {101} facets that reduces the recombination rate of carriers. It should be noted that
cluster TiO -{001/101} still has good degradation performance for TCH after five cycles of degradation.
2
2
2
2
9
2
more easily than P25 in deionized water. The study showed that cluster TiO
2
coral-like TiOF
2
v
2
2
2
2
Received 5th October 2020
Accepted 19th November 2020
{
2
DOI: 10.1039/d0ra08476j
This study provides a new idea for the synthesis of cluster TiO
performance for the treatment of TCH wastewater.
2
-{001/101} with high photocatalytic
rsc.li/rsc-advances
photocatalytic water purication technology is considered to be
1
. Introduction
the most promising water treatment technology due to its
17–20
Antibiotics are widely used around the world. TCH is a typical economic, efficient and clean environment.
broad-spectrum tetracycline antibiotic, which is produced by
TiO is widely used in environmental protection, biomed-
actinomycetes and widely used as a growth promoter in the icine, solar cells and many other elds due to its advantages
18
2
21
22
1
23
treatment of human diseases and animal feeding. However, of simple control, low cost and non-toxic. Especially in the
because the naphthol ring in the TCH structure can't be photocatalytic treatment of pollutants showed excellent
completely metabolized by humans and animals, a large performance. However, it has a wide energy band gap (3.0–
amount of TCH absorbed is excreted into various water bodies 3.2 eV, which means it only reacts to UV) and a high carrier
2
through feces and urine. Drinking water containing TCH for recombination rate, which hinders its application in
2
4,25
a long time will cause a series of diseases such as bacterial sunlight.
So far, many efforts have been made to make up for
26–28
disorders, gastrointestinal reactions and mycoplasma pneu- these shortcomings, such as metal doping,
non-metal
and building heterogeneous
Among these methods, the
3–5
29–31
32–34
monia, and affect the growth of teeth and bones.
In recent years, some methods have been used to treat TCH semiconductor composite.
doping,
dye sensitization,
35–38
6,7
8,9
wastewater, such as microbial degradation, adsorption,
electrochemical method, membrane separation method,
photocatalytic degradation.
construction of semiconductor heterojunction seems to be an
effective method to improve charge separation. However, the
10
11,12
13–16
Among these methods, high-quality tight interface requirements required by hetero-
39
junction make the construction more difficult. Although dye
sensitization can improve the response of TiO to visible light
and enhance the photocatalytic activity. However, due to the
, if the dyes
can not timely supplement the lost electrons in the system, the
dyes will fall off from the TiO and form competitive
a
2
Chongqing Academy of Animal Sciences, Chongqing 402460, China. E-mail:
pu88962@126.com
b
College of Geology and Environment, Xi'an University of Science and Technology, weak binding force between organic dyes and TiO
2
Xi'an 710054, China. E-mail: 807484470@qq.com
c
Scientic Observation and Experiment Station of Livestock Equipment Engineering in
2
Southwest, Ministry of Agriculture and Rural Affairs, Chongqing 402460, China
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degradation with the target pollutants, which will reduce the JCPDS no. 01-0490 shows irregular morphology of nano-
40
degradation efficiency of the target pollutants. Doping is the particles. Over the past ten years, some researchers have carried
most commonly used method to enhance the photocatalytic out extensive research on cubic TiOF . For instance, Zhang
2
41–44
59
activity of TiO because of its simple method.
et al. synthesized 3D hollow nanospheres with {001} main
2
It is worth noting that uorine (F) is a special anionic exposure facet using cubic TiOF as a template, which improved
2
3
+
58,60,61
dopant. The existence of uorine forms –Ti–F–Ti– and Ti
the response of TiO
2
to visible light. Zhang et al.
controlled
45
species to replace lattice oxygen to form Ovs
.
The formation of the formation process of TiO
2
-{001} nanosheets under different
3
+
Ti /Ovs is an effective and environmentally friendly strategy to hydrouoric acid dosage, reaction time and reaction tempera-
improve visible light absorption and pollutant adsorption. ture. It is found that cubic TiOF is an important intermediate
Moreover, the existence of O can form an electron trap, which in the hydrothermal synthesis of TiO -{001} nanosheets. Shi
2
v
2
62
greatly reduces the composite rate of electrons and holes, thus et al. studied the transformation process of cubic TiOF2 to
46,47
effectively enhancing the photocatalytic activity of TiO2.
TiO -{001/101} at different calcination temperatures and
2
Furthermore, F, as an end-capping agent, promotes the growth proposed the possible transformation mechanism of TiOF
2
to
nanosheets derived from
are relatively large and easy to aggregate, which
-{001} facets were obtained by hydro- undoubtedly reduces the exposure area of TiO -{001} and leads
48
49
of TiO
titanate and hydrouoric acid as precursors, the F–TiO
sheets with 89% TiO
2
-{001} facet. For instance, Han et al. using tetrabutyl TiO
2 2
-{001/101}. However, anatase TiO
2
nano- cubic TiOF
2
2
2
thermal treatment. Its photocatalytic activity is much higher to a low density of active sites, which inevitably increases the
than that of Degussa P25. Up to now, extensive experimental possibility of carrier recombination. Therefore, to further
and theoretical studies have shown that the {001} facet with improve the photocatalytic activity of anatase TiO , it is neces-
2
high surface energy has higher catalytic activity than the {101} sary to synthesize small-sized TiO with {001} and {101} facet co-
2
5
0,51
facet with thermodynamic stability.
coordination number of exposed atoms and a high density of attention to the TiOF
unsaturated Ti atoms on the {001} facet are in favor of the no. 01-0490. In our previous research, this crystal type of TiOF
This is because a low exposed. Compared with cubic TiOF
2
, few researchers pay
2
nanoparticles corresponding to JCPDS
25
2
52
53
dissociation and adsorption of reactants. Lu et al. demon- nanoparticles were successfully synthesized, and found that it
strated that the {001} facets of anatase TiO nanocrystals can also transform into anatase TiO -{001} in hydrothermal
exhibited much better photocatalytic activity than that of {101} reaction, and the obtained TiO /TiOF hybrid has close inter-
2
2
2
2
facets of anatase TiO2 nanocrystals toward photocatalytic face contact and layered structure. Compared with cubic TiOF2,
oxidation of water and organic compounds with different this kind of TiOF nanoparticles has a large number of meso-
2
functional groups (e.g., –OH, –CHO, –COOH). Besides, Chen porous structures and smaller size. To enrich the research
5
4
et al. found that anatase TiO
exposed {001} facets have strong photocatalytic decomposition of novel TiOF
ability for gaseous styrene. Although the surface has very high TiOF will be expected to transform into smaller TiO
surface energy, maximizing the percentage of exposed {001} nanosheets under certain conditions.
2
microcrystals with highly system of TiOF
in hydrothermal reaction. Moreover, this kind of
-(001/101)
2
, it is necessary to explore the transition process
2
2
2
55
area might not be a good direction. Khalil et al. demonstrated
In this study, the transition process of novel TiOF nano-
2
that an appropriate proportion of exposed {001} and {101} facet, particles to cluster TiO -{001/101} in the hydrothermal reaction
2
which forms a “surface heterojunction”. In the photocatalytic was discussed for the rst time. Under the irradiation of
reactions, photogenerated electrons tend to transfer to the {101} simulated sunlight, the degradation rate of TCH by TiO
2
-{001/
facet with lower energy, and thus accumulating on the {101} 101} was 92.3%. Besides, the sedimentation performance of
facet, while photogenerated holes tend to accumulate on the TiO
2
-{001/101} was investigated and compared with that of
{001} facet with high energy. This feature can effectively Degussa P25. Furthermore, the sensitization between TCH and
promote the separation of photogenerated electron–hole pairs, TiO
thereby improving their photocatalytic performance. There- degradation pathway of TCH was discussed. This study provides
2
-{001/101} was proposed for the rst time, and the possible
56
fore, it is necessary to explore how to synthesize TiO with {001} an idea for the treatment of TCH wastewater.
2
and {101} facet.
In many methods of synthesing TiO
2
, solvothermal reaction
conditions are relatively mild, and it can control the
2
. Experimental
22,28,57
morphology of materials well.
methods, tetrabutyl titanate was used as titanium source,
ethanol as an inhibitor and hydrouoric acid as end-capping Tetrabutyl titanate (TBOT, A. R.), anhydrous ethanol (C
agent. Aer hydrothermal treatment, TiO2 nanosheets with A. R.), hydrouoric acid (HF, A. R.) was purchased from Fuchen
001} and {101} facets co-exposed could be synthesized. Inter- Chemical Reagent Factory, Tianjin, China. Degussa P25 (P25)
estingly, TiOF is sometimes formed during the formation of was purchased from Beijing JiaHeng Technology Co., Ltd.
In the typical synthesis
2.1. Chemicals
2
5
H OH,
{
2
TiO
terization, it can be seen that TiOF
corresponding to no. 08-0060 and no. 01-0490 in JCPDS data- methanol (MT), tert-butyl alcohol (t-BuOH) were purchased
2
-{001}. In the previously reported X-ray diffraction charac- Tetracycline hydrochloride (TCH) was provided by Aladdin
2
has two crystal forms, Industrial Corporation (Shanghai, China), benzoquinone (BQ),
2
5,58
base.
The TiOF
2
corresponding to JCPDS no. 08-0060 shows from Fuchen Chemical Reagent Factory, Tianjin, China. All
corresponding to reagents were used without further purication.
regular cubic morphology, while the TiOF
2
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.2. Synthesis of the photocatalysts
Paper
ꢁ
1
2
solution (10 mg L ) in a measuring cylinder and put it into
a 150 mL photocatalytic reaction tube (Ghx-1, Shanghai Qiao-
feng Industrial Co., Ltd), weigh 30 mg of photocatalyst and add
it into the TCH solution. The photocatalytic reaction tube was
placed in the photocatalytic reactor (Ghx-1, Shanghai Qiaofeng
Industrial Co., Ltd), and a 500 W xenon lamp (GXZ-500,
Shanghai Jiguang Special Lighting Appliance actory) was used
to simulate the degradation of TCH by sunlight. Before the
xenon lamp is turned on, the solution is magnetically stirred in
the dark for 60 min to ensure the adsorption balance, then turn
on the xenon lamp to start the photodegradation test, take out
7 mL suspension every 10 min, and centrifugate at high speed to
remove the catalyst, then analyze the remaining TCH concen-
tration with UV-Vis spectrophotometer at the maximum
absorption wavelength of 357 nm. Aer each photodegradation
experiment, the sample was separated by a high-speed centri-
fuge (Tgl-16c, Shanghai Anting Scientic Instrument Factory),
washed with distilled water, and ultimately dried at 60 C for the
next test. The initial concentration of TCH was changed from
10 mg L to 20 mg L , 30 mg L and 40 mg L to explore the
effect of different TCH concentrations on photocatalytic
degradation. The dosage of the photocatalyst was changed from
0 mg to 10 mg, 20 mg and 40 mg to explore the effect of
photocatalyst dosage on the degradation of TCH. Different
scavengers (BQ, MT and t-BuOH) were used to trap the active
components ($O , h , $OH) in the photocatalytic process. This
test is similar to the photocatalytic degradation test, except that
a certain amount of scavenger was added to the TCH solution,
and then photocatalyst was added.
The experimental water is ultra-pure water. TiO -{001/101} were
2
synthesized by a one-step hydrothermal method. Add 30 mL
anhydrous ethanol and 8 mL hydrouoric acid into 100 mL
polytetrauoroethylene liner (Shanghai Qiuzuo Scientic
Instrument Co., Ltd), stir for 10 min at 25 C, and record the
mixture as solution A. Take 30 mL tetrabutyl titanate, add it to
solution A under stirring at the rate of 2 drops per second to
ꢀ
ꢀ
form a white suspension, and stir the mixture at 25 C for 2 h.
Then, the polytetrauoroethylene was placed in a high-pressure
reactor (Shanghai Qiuzuo Scientic Instrument Co., Ltd) and
ꢀ
reacted at 150 C for 4 h. Aer the system is naturally cooled to
room temperature, the solid products are collected by centri-
fugation and washed with ethanol and ultra-pure water three
times. The samples were dried at 60 C and named T-4h. The
ꢀ
above steps remained unchanged and the reaction time varied
between 0.5 h, 1 h and 3 h to study the transition process of
novel TiOF to TiO -{001/101}. These products were recorded as
ꢀ
2
2
T-0.5h, T-1h, T-3h. Besides, we put the photocatalyst (30 mg)
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
into 100 mL TCH (10 mg L ), stirred for 60 min under dark
ꢀ
conditions, and then centrifuged and dried at 60 C. The ob-
tained samples were recorded as T-0.5h/TCH, T-1h/TCH, T-3h/
TCH, T-4h/TCH and P25/TCH.
3
2
ꢁ
+
2.3. Characterization of the samples
The crystal structure of the samples was observed using X-ray
diffraction (XRD; Bruker D8 Advance X-ray diffractometer) at
36 kV, 20 mA equipped with a Cu anode X-ray tube (Cu Ka X-
˚
rays, l ¼ 1.54056 A). SEM and EDS (JSM7500F, Japan) were
used to record the surface morphology and element distribu-
tion of the samples. Transmission electron microscopy (TEM)
images were achieved with a JEOL JEM-2100 high-resolution
transmission electron microscope. The specic surface area
2
.5. Sedimentation experiment
The settling behavior of T-4h and P25 in deionized water were
studied in glass cylinders (100 mL, diameter 2.8 cm, height
24 cm Fuchen Chemical Reagent Factory, Tianjin, China). T-4h
(SBET) and pore size analysis were performed by Brunauer–
ꢁ1
suspension (1 g L ) was prepared with deionized water. The
obtained T-4h suspension was homogenized by violent shaking
Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)
methods through a micromeritics JW-BK122W. The absorption
characteristics of the samples were determined by UV-visible
diffuse absorption spectroscopy (UV-Vis DRS, Shimadzu UV-
2
600, Japan). Fourier transform infrared spectrum (FT-IR)
ꢁ
1
measurements, recorded in the range of 4000–400 cm , were
performed in KBr pellet by Nicolet IS5 Spectrometer, USA. The
photoluminescence spectrum of the photocatalyst was
measured by a uorescence spectrometer (Shimadzu-RF-6000,
Japan) with the excitation wavelength was 300 nm. The chem-
ical valence states of the samples were analyzed by X-ray
photoelectron spectroscopy (XPS, ThermoFisher K-Alpha).
Electron paramagnetic resonance (EPR) was implemented by
the BrukerA 300 spectrometer operating at room temperature.
2.4. Photocatalytic activity measurement
The photocatalytic activity of the catalyst was tested by the
degradation of TCH. First, at the maximum absorption wave-
length of 357 nm, the initial absorbance of TCH solution
ꢁ
1
(
(
10 mg L ) was measured by a UV-Vis spectrophotometer
Hitachi, UV-3900, Japan). Aer that, take 100 mL of TCH Fig. 1 XRD patterns of the prepared samples.
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ꢀ
ꢀ
ꢀ
and then poured into the cylinder. The settlement behavior of T-
4
For T-4h, the diffraction peaks at 2q ¼ 25 , 37.76 , 47.8 ,
ꢀ ꢀ ꢀ ꢀ ꢀ
h was observed by standing for 2 h, 10 h, 24 h. The settlement 54.86 , 56.6 , 70.3 , 75 , 82.22 are attributed to the {101}, {004},
behavior of P25 was studied under the same conditions.
{200}, {105}, {211}, {220}, {215}, {224} planes of anatase TiO
2
30,32,38
(JCPDS no. 21-1272), respectively.
It can be seen that with
the reaction time from 0.5 h to 4 h, the diffraction peak intensity
of TiO {101} facet increases gradually, while the diffraction
peak of TiOF completely disappears at 4 h, which indicates that
Fig. 1 clearly shows the crystal structure of the prepared TiOF has completely evolved into anatase TiO . It should be
samples. For T-0.5h, the diffraction peaks at 2q ¼ 13.61 , 23.39 , noted that there is an obvious diffraction peak of {004} facet in
3. Results and discussion
2
3.1. Crystal structure analysis
2
2
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
(
7.68 , 47.8 , 53.88 , 59.6 , 69.6 , 74.7 match with TiOF
2
T-4h, which is oen referred to as the existence of {001}
was facet.
25,39
JCPDS no. 01-0490), indicating that the novel TiOF
successfully prepared.
2
25
Fig. 2 The high-magnification FE-SEM images of (a) T-0.5h, (b) T-1h, (c) T-3h, (d) T-4h; the element mapping of (e) Ti, (f) O, (g) F in T-4h.
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Paper
In summary, the chemical reactions of the formation of
Ti(OH) + 2HF / TiOF Y + 3H O
(2)
(3)
4
2
2
TiO -{001/101} nanosheets can be proposed. The rst step is the
2
hydrolysis of TBOT. The reaction of TBOT to form Ti(OH) is
2 2 2
TiOF + H O / TiO Y + 2HF
4
5
4
shown in eqn (1). Secondly, Ti(OH) can react with HF to
4
25
produce TiOF
2
eqn (2). Finally, with the hydrothermal reac-
tion, TiOF
in eqn (3).
2
4
was hydrolyzed to anatase TiO -{001/101}, as shown
2
7
3.2. Morphology analysis of photocatalyst
Field emission scanning electron microscopy (FE-SEM) images
of T-0.5h, T-1h, T-3h, T-4h are shown in Fig. 2. Fig. 2a shows the
T-0.5h image. The novel of TiOF2 shows coral-like structure
Ti(OC H ) + 4H O / Ti(OH) + 4C H OH
(1)
4
9 4
2
4
4
9
Fig. 3 TEM images of as-prepared samples (a) T-0.5h, (b) T-1h, (c) T-3h, (d) T-4h, and HRTEM images of (e and f) T-4h.
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formed by the accumulation of spherical TiOF
2
nanoparticles, 200 nm. High-resolution TEM (HRTEM) image of T-4h (Fig. 3e)
which is completely different from the cubic TiOF that shows that the clear lattice fringes with a spacing of 0.352 nm
2
5
8–61
researchers have focused on before.
The novel TiOF has correspond to the {101} facets, indicating that {101} facet is
2
41
many macroporous structures, which is consistent with the exposed on the surface of T-4h. The T-4h with a lattice spacing
pore size analysis. With the extension of reaction time, the of 0.235 nm is indexed to the {004} planed (Fig. 3f), demon-
45,49
spherical TiOF
to the conversion of some TiOF
further hydrothermal reaction, it can be seen that the TiO
nanosheets derived from the surface of TiOF are closely like TiOF
2
nanoparticles tend to be at, which may be due strating the exposure of {001} facet on the surface of T-4h.
to TiO (Fig. 2b). With the Compared with the TiO with {001} facet exposure reported by
the size of TiO -{001/101} derived from coral-
is smaller. All the above results indicate that the
2
2
2
48,54
2
most studies,
2
2
2
interlaced (Fig. 2c). By XRD analysis, most of the {101} and a few coral-like TiOF can be transformed into cluster TiO -{001/101}.
2
2
{
001} facets of TiO have been co-exposed, and a close interface
2
contact is formed between the TiO -{001/101} and the
2
3.3. BET specic surface area and pore structure
untransformed TiOF
to 4 h, only TiO -{001/101} nanosheets were formed (Fig. 2d).
TiO nanosheets with an average diameter of about 200 nm are
interlaced with each other to form clusters, which is conducive
to the migration and transformation of electrons and holes
between {001} and {101} facets. To further describe the element
distribution of T-4h, elemental mapping analysis (Fig. 2e–g) of
T-4h was performed. It is proved the existence of Ti, O, F
elements and the mapping image matches well with the SEM
image.
More morphological details on prepared samples were
further obtained using transmission electron microscopy (TEM)
2
images. It can be seen from Fig. 3a that the TiOF nanoparticles
are tightly packed together, which is consistent with the results
observed by SEM. With the hydrothermal reaction, a small part
2
. When the reaction was further extended
The specic surface area and pore size distribution of T-0.5h, T-
h, T-3h, T-4h were analyzed by nitrogen adsorption–desorption
2
1
2
technology. As shown in Fig. 4, according to the classication of
IUPAC, T-0.5h and T-1h show a typical IV type adsorption–
desorption isotherm with H1 type hysteresis ring, which
represents that the samples have a mesoporous structure with
close-packed spherical particles and matches the image pre-
sented by SEM. T-3h, T-4h and P25 show a typical IV type
adsorption–desorption isotherm with H3 type hysteresis ring,
which represents that the samples have a mesoporous structure
22
with cracks. Table 1 shows the specic surface area, pore
volume and average pore size of all samples.
The specic surface areas of T-0.5h, T-1h, T-3h, T-4h are
2
ꢁ1
3
3.44, 92.4, 31.75, 50.06 m g , respectively. The specic
2
surface area of these samples is larger than that of P25 (21.98 m
of TiOF was transformed into TiO , showing a loose stacking
2
2
ꢁ1
g
). According to the corresponding BJH pore size distribution
structure (Fig. 3b). With further reaction, it can be seen that
TiOF nanoparticles are more dispersed, and more TiO nano-
sheets with the size of 100–200 nm are closely linked with TiOF
diagram (Fig. 4 inset), the average pore size of T-4h is 13.82 nm,
which is well-matched with the size of TC (1.41 nm in length,
2
2
2
0.46 nm in width and 0.82 nm in height).
(Fig. 3c). In Fig. 4d, it can be seen that the TiOF
2
nanoparticles
This is consistent with the strong removal rate of the dark
have been completely transformed in TiO
2
nanosheets with
reaction stage in the subsequent photocatalytic degradation
experiment. Besides, these uniform and small pore size meso-
pores are conducive to the absorption of light and multiple
reections inside the material and provide an effective trans-
{001} and {101} facets interlaced, with an average size of
3
mission path for photogenerated carriers.
3.4. FT-IR and EPR analysis
Fig. 5a shows the FT-IR spectra of photocatalysts prepared at
different hydrothermal reaction times. The stretching and
bending vibrations due to absorption by H O and the Ti–OH
2
ꢁ
1
group on the sample surface are found at 3100–3500 cm and
ꢁ
1
25,47
1628 cm
.
T-0.5h, T-1h, T-3h have strong peaks at
Table 1 Specific surface area, volume, mean pore size for the tested
catalysts
Surface area
Pore volume
(cm g )
Average pore
size (nm)
2
ꢁ1
3
ꢁ1
Samples
(m g
)
T-0.5h
T-1h
T-3h
33.44
92.40
31.75
50.06
21.98
0.22477
0.28203
0.09572
0.12127
0.04630
37.39
18.88
17.78
13.82
12.67
Fig. 4 Nitrogen adsorption and desorption isotherms of T-0.5h, T-1h, T-4h
T-3h, T-4h and P25. The inset is the corresponding pore size P25
distribution.
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Fig. 5 FT-IR (a) and EPR (b) spectra of different samples.
ꢁ
1
9
22 cm , which is attributed to Ti–F vibration in TiOF . T-4h strength of the Ti–O bond in TiO compared with the Ti–F bond
2
2
ꢁ1
58
has an only a slight peak at 922 cm , which can be attrib- in TiOF2 nanoparticles. Through Gauss tting, these peaks
uted to the Ti–F vibration formed by adsorbed F and TiO . The centered at 459.31 and 464.21 eV were ascribed to Ti species,
peak at 534 cm is attributed to Ti–O vibration or Ti–O–Ti while peaks at 459.88 and 465.26 eV were ascribed to Ti which
vibration. It should be noted that the peak of T-4h shis indicate that the Ti has formed in the reduction process. The
negatively from 534 cm , which may be attributed to the relative content of Ti increases gradually with the increase of
v v
formation of Ti and O . The increased number of O in the hydrothermal reaction time. The asymmetric O1s spectra were
50
3+
2
ꢁ
1
4+
52
3+
25
ꢁ1
3+
3
+
lattice structure changes the number of Ti atom surrounding tted to four different types of O, as shown in Fig. 6c. Aer
the O atom, and the electron cloud density around a Ti atom Gauss tting of all samples, the peak of the binding energy of
59
decreased. This causes the stretching vibration absorption each sample from small to large corresponds to lattice oxygen,
peak of a Ti–O bond to shi. Additional information on oxygen hydroxyl oxygen, O and oxygen in adsorbed water, which can be
v
58,63,64
v v
vacancy (O ) was provided by EPR spectra collected at room considered as convincing evidence for the existence of O .
temperature. As displayed in Fig. 5b, the signal at g ¼ 2.002
Fig. 6d shows the core-level F1s XPS spectra. The binding
It can be seen that all samples show the energy of F1s electrons shied from 684.81 eV for T-0.5h to
63
corresponds to O
signal of O
01}, the signal of O
v
.
v
, and with the transformation of TiOF
2 2
to TiO -{001/ 684.76 eV for T-4h. The slight decrease in the F1s binding
1
v
in T-4h is the strongest, which indicates energy indicated the shiing of the Ti–F bond in the bulk TiOF
2
that there is a lot of O in T-4h, which will effectively inhibit the nanoparticles to the surface Ti–F bonds on the TiO2
v
2
recombination of electrons and holes, and improve the photo- nanosheets.
catalytic activity.
3
.6. The optical absorption and PL analysis
Fig. 7a shows the light absorption performance of different
The surface compositions and chemical states of pure T-0.5h, T- photocatalysts and complexes aer the adsorption of TCH. All
h, T-3h and T-4h are displayed in Fig. 6. The survey spectra samples show different absorption in the UV region without
reveal signals of Ti, O, F, and C on all samples that match their adsorption of TCH. The amount of light absorbed in the UV
respective signals from the individual spectra of TiOF and TiO region increased in the order T-1h < P25 < T-0.5h < T-3h < T-4h.
Fig. 6a), which is in accordance with the element mapping However, in the visible region, only T-0.5h shows visible light
results. The existence of the peaks of element C at 284.8 eV can response, which is attributed to the narrow band gap energy of
3.5. XPS analysis
1
2
2
(
2
2,34
ascribe to surface adventitious reference carbon, which is TiOF
2
(2.91 eV), as shown in Fig. 7b. Many researchers
have
and dyes, but only a few
and anti-
samples (Fig. 6b). The binding energies of both Ti2p3/2 (from biotics. Hasan et al. believed that the LMCT effect formed
59.48 to 459.18 eV) and Ti2p1/2 (from 465.28 to 464.88 eV) between sulfonamides and TiO , because sulfonamides did not
25
unavoidable during the XPS measurement.
studied the sensitization between TiO
2
From the high-resolution XPS spectrum of Ti2p in the researchers focus on the sensitization between TiO
2
65
4
2
decreased slightly from TiOF
2
nanoparticles to cluster TiO
2
-
absorb visible light. Interestingly, TCH has strong absorption in
{001/101}. This could be attributed to the decreased bond the visible and UV regions as shown in Fig. 7b, which is
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Fig. 6 (a) Survey XPS spectra of the samples; (b, c and d) high-resolution XPS data of Ti2p, O1s and F1s for samples respectively.
attributed to TCH molecule possesses a delocalized p bond not show the best photocatalytic activity in the degradation test
connected with –OH group, leading to a small energy gap of TCH, which shows that the response-ability of photocatalyst
between highest occupied molecular orbital (HOMO) and to visible light is not the only factor to evaluate the photo-
g
lowest unoccupied molecular orbital (LUMO) and thus avail- catalytic activity. The band gap energy (E ) of T-0.5h, T-1h, T-3h,
66
ability for visible light absorption. When the photocatalyst T-4h and P25 was calculated according to the Tauc curve. The
adsorbs TCH, it shows different degrees of absorption in the results are shown in Fig. 7b. The band gap energies of T-0.5h, T-
66
visible light region. Wu et al. believed that the electron excited 1h, T-3h, T-4h and P25 are 2.91 eV, 3.00 eV, 3.00 eV, 2.97 eV and
by visible light from HOMO to LUMO of TCH and further 3.05 eV.
transferred to the conduction band of TiO2 was a kind of
PL measurements are effective to examine the separation
sensitization. P25/TCH has only weak light absorption in the UV efficiency and recombination processes of photo-generated
and visible light region. The amount of light absorbed in the carriers because increased photo-generated electron–hole pair
visible light region increased in the order P25/TCH < T-4h/TCH recombination results in stronger luminescence intensity.
<
T-3h/TCH < T-1h/TCH < T-0.5h/TCH. The novel TiOF
2
seems Three peaks were observed in the spectra.
Fig. 8 representatively shows the PL spectra of the T-0.5h, T-
to be more easily sensitized with TCH, showing the strongest
absorption in the visible region, which may be attributed to the 1h, T-3h, T-4h and P25 samples. Three peaks were observed in
formation of intermolecular hydroxyl groups, which makes the the spectra. The broad emission bands centered at 390 nm were
55
p orbitals of TCH more easily form electronic coupling with the ascribed to the interband transition of TiO2. The two peaks at
4
+
3+
3
d orbitals of Ti and Ti , resulting in the formation of tight 454 and 469 nm are attributed to the O and two trapped elec-
v
surface complexes between TCH and TiOF . Although the T- trons. The peak of O also appears in P25, which is attributed to
2
v
0.5h/TCH has a strong response to visible light, T-0.5h does the mixture of rutile and anatase phases, which increases the
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1/2
Fig. 7 (a) UV-Vis absorbance spectra of the samples and (b) the corresponding plots of (ahv) versus hv.
simulate the solar photocatalytic degradation test. The results
are shown in Fig. 9a. The photocatalysis system reacts in the
dark for 60 minutes to reach the adsorption–desorption equi-
librium before the sunlight. TCH has little self-degradation
under sunlight. It can be seen that the samples of T-0.5h, T-
1h, T-3h and T-4h have good adsorption properties. When the
adsorption–desorption equilibrium was reached, the removal
rates of TCH were 40%, 47%, 47% and 55%, respectively. T-4h
sample has the strongest adsorption capacity for TCH. Large
specic surface area, suitable pore structure and high exposure
{
001} facets make TCH easy to adsorb on the surface of T-4h,
which lays a foundation for the subsequent photocatalytic
degradation. Although the adsorption capacity of T-0.5h for
TCH is stronger than that of T-1h, the total removal rate (60%) is
not higher than that of T-1h (62%). This shows that the
adsorption performance of photocatalyst for TCH can not
dominate its ability to degrade TCH. Surprisingly, the total
degradation rate of TCH by T-4h reached 83.2% only aer
Fig. 8 The photoluminescence spectra with the excitation wave-
length lex ¼ 300 nm for T-0.5h, T-1h, T-3h, T-4h and P25.
1
6
0 min irradiation under simulated sunlight, and 92.3% aer
0 min irradiation, which was much higher than that of P25
(
52%).
2
5
defect density in TiO
nescence intensity of T-3h is signicantly lower than that of T-
.5h and T-1h, which can be attributed to the formation of suit-
able heterojunction between residual TiOF and TiO , which
greatly reduces the recombination rate of electrons and holes. T-
h shows the lowest luminescence intensity, much lower than
that of P25, this may be due to the surface heterojunction formed
2
lattice. It is worth noting that the lumi-
The degradation kinetic model was studied and the photo-
catalytic properties of the samples were further analyzed. Fig. 9b
shows that the linear relationship between degradation time (t)
and ln(C /C) is almost a straight line, which proves that the
0
degradation process of TCH conforms to pseudo-rst-order
reaction. The rate constants k of T-0.5h, T-1h, T-3h, T-4h and
P25 were 0.00518, 0.00722, 0.01372, 0.02514 and 0.01123 min
respectively. The results showed that the degradation rate of
TCH by T-3h photocatalyst was signicantly higher than that of
T-0.5h and T-1h, which may be due to the appropriate ratio of
0
2
2
4
ꢁ1
,
56,59
on {001} and {101} facets and the highest O concentration.
v
3
.7. Photocatalytic activity
2 2
residual TiOF and TiO in T-3h sample, and the synergistic
To study the photocatalytic performance of samples, TCH effect of the two substances promoted the photodegradation.
solution was used as a typical antibiotic pollutant in water to
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ꢁ1
Fig. 9 (a) Photodegradation of TCH solution (10 mg L , 100 mL) using different samples under simulated sunlight; (b) kinetic linear simulation
curves of TCH solution photodegradation; (c) the effect of different concentration of TCH solution on the photocatalytic performance of T-4h
ꢁ1
(
30 mg) under simulated sunlight; (d) the effect of different dosage of T-4h photocatalyst on degradation of TCH solution (10 mg L , 100 mL)
under simulated sunlight.
The maximum rate constant of T-4h was 2.24 times that of aggregation of excessive catalysts, which makes the solution
P25. Besides, the effects of different concentrations of TCH turbid and leads to photon scattering, thus reducing the pho-
16
solution on the photocatalytic performance of the T-4h sample tocatalytic rate. The stability and reusability of photocatalyst
were studied (Fig. 9c). T-4h had the best degradation effect on are important factors affecting its practical application. Five
ꢁ
1
the TCH solution of 10 mg L . With the increase of TCH consecutive cyclic photodegradation tests were carried out on
solution concentration, the photocatalytic performance of T-4h sample T-4h, and the results are shown in Fig. 10a. Aer ve
gradually decreased. It may be that the photocatalyst is not to cycles, the degradation rate of sample T-4h in simulated
provide enough active reaction sites to deal with so many TCH sunlight decreased from 92.3% to 84.5%, indicating that
molecules. Fig. 9d shows the degradation curve of TCH solution sample T-4h has a high reuse rate and photocatalytic
ꢁ
1
ꢁ1
ꢁ1
with different T-4h dosage (0.1 g L , 0.2 g L , 0.3 g L
0
,
performance.
ꢁ1
ꢁ1
.4 g L ). When the amount of catalyst is 0.3 g L , the
degradation effect of TCH is the best.
As shown in Fig. 10b, aer ve cycles, the peak intensity of T-
4h photocatalyst decreased, but the position of diffraction peak
ꢁ
1
However, when the dosage of T-4h reached 0.4 g L , the was the same as that before reaction, indicating that T-4h has
photocatalytic effect decreased which may be due to the high chemical stability.
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Fig. 10 (a) Photo stability tests over T-4h for TCH degradation; (b) XRD patterns of T-4h before and after five cycling runs; (c) effect of different
ꢁ1
scavengers on the degradation of TCH (10 mg L , 100 mL) efficiencies over T-4h.
Besides, the settlement performance of T-4h and P25 were of sedimentation. This shows that T-4h is more conducive to the
further studied. The digital images of P25 and T-4h suspensions practical application of wastewater treatment than P25.
in deionized water at pH 7.0 were taken at different sedimen-
tation times, as shown in Fig. 11. P25 nanoparticles were well
dispersed in water and settled very slowly. The supernatant was
still very turbid aer 24 h of sedimentation. The settling In general, hydroxyl radicals ($OH), superoxide radicals ($O
3
.8. Photocatalysis mechanism
2ꢁ
)
+
behavior of T-4h was distinctly different from that of P25. Most and holes (h ) can be found in the photocatalytic degradation
of the composites was settled in 4 h leaving a small fraction of T- process. For this reason, many scavengers, such as benzoqui-
ꢁ
4
h suspended in the supernatant, which became clear aer 24 h none (BQ, $O
2
), tert-butyl alcohol (t-BuOH, $OH), methanol
+
(
MT, h ) and so on, were introduced to carry out free radical
trapping experiments.
Fig. 11 Photographs of P25 suspension and T-4h suspension in deionized water at pH 7.0 at different settling times.
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mechanism of outstanding photocatalytic activity of T-4h was
proposed (Fig. 13). Firstly, the {001} facets of TiO -{001/101}
2
has a strong adsorption capacity for TCH.
Furthermore, the p orbital of TCH may form electronic
4
+
coupling with the 3d orbital of Ti , resulting in a surface
67
2
complex between TCH and TiO -{001/101}.
TCH was degraded simultaneously in two pathways. In the
rst path, electrons are excited from the HOMO level of TCH

to LUMO level under simulated sunlight, and then rapidly
injected into the conduction band maximum (CBM) position
3
9
of TiO -{001} facets. However, TCH lost its electrons and
2
+
remained a stable product (TCH ). Because the CBM of TiO -
2
{
001} facets is more negative than that of TiO
2
-{101} facets.
The electrons on CBM of TiO -{001} facets can migrate to
2
CBM of TiO
2
-{101} facets, and eventually react with O
2
ꢁ
adsorbed on the TiO
2
-{101} facet to form $O
2
to oxidize TCH
Fig. 12 Effect of different scavengers on the degradation of TCH
ꢁ1
+
(
10 mg L , 100 mL) efficiencies over T-4h.
and TCH .
3
+
In the second path, the doping of Ti /O can form new states
v
at the bottom of the TiO -{001/101} CBM. Under simulated
2
sunlight irradiation, the {001} and {101} facets of TiO produce
2
+
It can be seen from Fig. 12 that under simulated sunlight
irradiation, the introduction of BQ drastically decreased
3
electron–hole pairs respectively. The CBM and Ti /O
potentials of TiO -{001} facets are negative than that of TiO
{101} facets, respectively, allowing the holes of TiO -{101} CBM
v
state
2
2
-
photodegradation efficiency of TCH from 92.3 to 67.4%,
ꢁ
2
suggesting that $O
2
played a key role in the photo-
2
partly transfer to that of TiO -{001} CBM, and the electrons in
degradation. When MT was added, the total degradation rate
3
+
3+
+
v 2 v
Ti /O states of TiO -{001} facets transfer to the Ti /O states of
decreased by 5.65%, indicating that h was the secondary
67
TiO -{101} facets. This process resulted in efficient space
2
substance affecting the photocatalytic degradation. Aer
adding t-BuOH, the photocatalytic degradation was almost
not inhibited, and the total removal rate decreased by 2%,
indicating that $OH did not play a major role in the degra-
dation. Based on the experimental results, the possible
separation of photo-induced charge carriers to suppress
recombination. Similar to the rst pathway, electrons in the
3
+
ꢁ
Ti /O
v
states can interact with O
2
to form $O
2
which can
+
oxidize TCH and TCH . The remaining holes can react with H
to form $OH and oxidize TCH with $O
2
O
ꢁ
2
.
Fig. 13 Schematic illustration of the photocatalytic mechanism for TCH removal over the T-4h photocatalyst.
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1
3 Y. Wang, L. Rao, P. Wang, Z. Shi and L. Zhang, Appl. Catal.,
B, 2020, 262, 118308.
4
. Conclusions
In summary, coral-like TiOF
hydrothermal method, and with the extension of hydro-
thermal reaction time, cluster TiO
-{001/101} with the smaller 15 J. Xu, C. Liu, J. Niu and M. Chen, Sep. Purif. Technol., 2020,
size can be obtained. The prepared cluster TiO -{001/101}
230, 115861.
exhibits excellent TCH degradation activity under simulated 16 W. Li, Q. Chen, X. Lei and S. Gong, RSC Adv., 2019, 9, 5100–
sunlight. The sensitization between TCH and cluster TiO -{001/
5109.
01} can induce strong visible light absorption. It is worth 17 Q. Zhang, J. Bai, G. Li and C. Li, J. Solid State Chem., 2019,
2
can be synthesized by a one-step 14 C. Hou, J. Xie, H. Yang, S. Chen and H. Liu, RSC Adv., 2019, 9,
37911–37918.
2
2
2
1
noting that the photocatalyst has good stability, repeatability,
270, 129–134.
and sedimentation, which is very important in practical appli- 18 C. Hou, B. Hu and J. Zhu, Catalysts, 2018, 8, 575.
cation. This work opens up a new way to explore the TiOF
provides a new idea for the degradation of TCH.
2
and 19 Z. Yu, H. Moussa, B. Chouchene, M. Liu, R. Schneider,
W. Wang, M. Moliere and H. Liao, Nanotechnology, 2019,
30, 045707.
2
0 S. Zhong, C. Lv, M. Shen, L. Wu and C. Li, J. Mater. Sci.:
Mater. Electron., 2019, 30, 4152–4163.
Conflicts of interest
There is no conict of interests exiting in the manuscript 21 V. Madhubala, A. Pugazhendhi and K. Thirunavukarasu,
submission, and it is approved by all of the authors for publi-
Process Biochem., 2019, 86, 186–195.
cation. All the authors listed have approved the manuscript to 22 V. M. Ramakrishnan, N. Muthukumarasamy, P. Balraju,
be enclosed.
S. Pitchaiya, D. Velauthapillai and A. Pugazhendhi, Int. J.
Hydrogen Energy, 2020, 45, 15441–15452.
2
3 M. Srinivasan, M. Venkatesan, V. Arumugam, G. Natesan,
N. Saravanan, S. Murugesan, S. Ramachandran,
R. Ayyasamy and A. Pugazhendhi, Process Biochem., 2019,
80, 197–202.
Acknowledgements
This work was supported by Special Project for Performance
Guidance of Scientic Research Institution in Chongqing
(
19237), Chongqing Special Financial Funds Project (19511), the 24 Y. Wang, L. Rao, P. Wang, Z. Shi and L. Zhang, Appl. Catal.,
Earmarked Fund for Modern Agro-industry Technology
B, 2020, 262, 118308.
Research System (CARS-35) and Scientic Observation and 25 C. Hou and W. Liu, R. Soc. Open Sci., 2018, 5, 172005.
Experiment Station of Livestock Equipment Engineering in 26 X. Wei, H. Cai, Q. Feng, Z. Liu, D. Ma, K. Chen and Y. Huang,
Southwest, Ministry of Agriculture and Rural Affairs.
Mater. Lett., 2018, 228, 379–383.
27 M. Malakootian, N. Olama and A. Nasiri, Int. J. Environ. Sci.
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