Characterization and photocatalytic activity of Bi3TaO7 prepared by hydrothermal method

Article abstract of DOI:10.1016/j.jssc.2017.08.016

In this paper, Bi₃TaO₇ nanoparticles with visible light response have been prepared via a facile hydrothermal route and solid-state reaction. The photocatalytic performance of as-prepared samples was evaluated by the degradation of Tetracycline hydrochloride (TC). It was observed that the sample obtained by hydrothermal method showed an enhanced photocatalytic activity in contrast to the sample prepared by solid-state method. This could be ascribed to the efficient separation of photogenerated electrons and holes in the sample prepared via hydrothermal process, which is certified by the photoluminescence spectroscopy (PL) and transient photocurrent analysis. Moreover, the result of nitrogen adsorption-desorption isotherm indicated the existence of mesoporous structure in as-prepared sample with hydrothermal process, and specific surface area is significantly higher than the sample obtained by solid-state method. The optimal conditions involved in the photocatalytic reaction, such as TC concentration, catalyst concentration and pH value were investigated.

Full text of DOI:10.1016/j.jssc.2017.08.016

Author’s Accepted Manuscript
Characterization and photocatalytic activity of
Bi TaO prepared by hydrothermal method
3
7
Bifu Luo, Min Chen, Zhengyuan Zhang, Yuanzhi
Hong, Taotao Lv, Weidong Shi
www.elsevier.com/locate/yjssc
PII:
S0022-4596(17)30323-7
http://dx.doi.org/10.1016/j.jssc.2017.08.016
YJSSC19906
DOI:
Reference:
To appear in:
Journal of Solid State Chemistry
Received date: 22 May 2017
Revised date: 9 August 2017
Accepted date: 11 August 2017
Cite this article as: Bifu Luo, Min Chen, Zhengyuan Zhang, Yuanzhi Hong,
Taotao Lv and Weidong Shi, Characterization and photocatalytic activity of
Bi TaO prepared by hydrothermal method, Journal of Solid State Chemistry,
3
7
http://dx.doi.org/10.1016/j.jssc.2017.08.016
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Characterization and photocatalytic activity of
Bi3TaO7 prepared by hydrothermal method
a
a
a
b
a
,*
Bifu Luo , Min Chen , Zhengyuan Zhang , Yuanzhi Hong , Taotao Lv , Weidong
Shi a
,*
a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang
212013, P. R. China.
b School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013,
P.R. China
Corresponding author:
*E-mail address: chenmin3226@ujs.edu.cn (M. Chen)
*E-mail: swd1978@mail.ujs.edu.cn (W.D. Shi)
Abstract: In this paper, Bi3TaO7 nanoparticles with visible light response have been
prepared via a facile hydrothermal route and solid-state reaction. The photocatalytic
performance of as-prepared samples was evaluated by the degradation of Tetracycline
hydrochloride (TC). It was observed that the sample obtained by hydrothermal
method showed an enhanced photocatalytic activity in contrast to the sample prepared
by solid-state method. This could be ascribed to the efficient separation of
photogenerated electrons and holes in the sample prepared via hydrothermal process,
which is certified by the photoluminescence spectroscopy (PL) and transient
photocurrent analysis. Moreover, the result of nitrogen adsorption-desorption
isotherm indicated the existence of mesoporous structure in as-prepared sample with
hydrothermal process, and specific surface area is significantly higher than the sample
obtained by solid-state method. The optimal conditions involved in the photocatalytic
reaction, such as TC concentration, catalyst concentration and pH value were
investigated.
Keywords: Bi3TaO7; Photocatalytic; Hydrothermal; Tetracycline

1. Introduction
Semiconductor photocatalysis has received much attention since it can act as a
potential approach to the energy shortage and for counteracting environmental
degradation [1-3]. Over the past decades, a large number of semiconductor materials
had been studied which mainly apply to hydrogen production and the degradation of
organic pollutants. It was demonstrated that titanium dioxide and titanates are
important materials in photocatalysis due to their superiority with non-toxicity and
photo-chemically stability [4,5]. However, traditional TiO2 photocatalyst suffers from
poor charge transport and high charge recombination rate, which greatly limits its
photocatalytic performance. Hence, it is necessary to explore a new type photocatalyst
for water splitting and degradation of pollutants.
Recently, some alkali tantalate photocatalysts with perovskite-like structure have
garnered considerable attention owing to their high photocatalytic activity for the
removal of organic pollutants from waste water, such as LiTaO3, NaTaO3 and KTaO3
[6-8]. The conduction band of these alkali tantalates mainly attributed to Ta 5d
orbitals, resulting in a more negative reduction potential compared to the conduction
band of Ti 3d orbitals. In addition, the structure of tantalates is based on corner-shared
chains of TaO6 octahedra, which is conducive to the migration of photo-generated
carrier [9]. Therefore, these tantalates exhibit a better photocatalytic activity.
Nevertheless, these catalysts are responsive under UV-light, which occupies only 4-5%
of solar light. It is crucial to explore novel visible light-driven photocatalysts with
high efficiency. Different from these alkali metal tantalates, some other tantalates,
such as Bi3TaO7, InTaO4 and AgTaO3, are visible-light-driven photocatalysts used in

hydrogen production and the elimination of organic pollutants [10-12]. Specifically,
the band gap energy of Bi3TaO7 is estimated about 2.7-2.9 eV, which is responsive to
the visible-light. Moreover, the valence band (VB) of Bi3TaO7 is composed of O 2p
and Bi 6s orbits and this is propitious to narrow the band gap [13]. Therefore, as a
new type of visible light response photocatalyst, Bi3TaO7 have received great
attention of researchers in recent years. For instance, Yu et al. reported Bi3TaO7
photocatalyst exhibited higher photocatalytic activity for the decomposition of 4BS
azo dye. [14].
Generally, the Bi3TaO7 crystals were synthesized by the traditional
high-temperature solid-state（S-S）method [15], where the size of obtained products
were usually very large (micrometer-scale). In comparison with the S-S method, the
hydrothermal treatment has more advantages in the high purity, good crystal shape
and controllable synthesis, which is widely applied to the preparation of
semiconductor materials [16-18]. The smaller particle sizes increase the specific
surface area of sample, which is helpful to increase the adsorption of target molecules.
On the other hand, the decrease in particle size may give rise to quantum size effect,
which will broaden the band gap of semiconductor hence enhancing the redox ability,
leading to a higher photocatalytic activity. To date, few reports were focused on the
preparation of Bi3TaO7 due to the harsh conditions. Generally, the preparation of
Bi3TaO7 with solid state method need a high temperature, high pressure and a second
calcination process [15].
Herein, Bi3TaO7 photocatalyst was prepared via a facile template-free approach.

The as-prepared Bi3TaO7 presented small nanoparticle with a size about 30-40 nm.
Compared with the sample obtained by solid-state method (S-S), a significantly
improved of specific surface area of sample obtained by hydrothermal route has been
showed, resulting in the higher photocatalytic efficiency for the degradation of
tetracycline (TC). Moreover, the separation efficiency of charge carriers was also
enhanced, which is confirmed by the fluorescence spectroscopy and photocurrent
transient analysis. The cycling experiment revealed that the as-prepared Bi3TaO7 was
a stable photocatalyst.
2. Experimental section
2.1. Preparation of catalyst
Bi3TaO7 photocatalyst was prepared according to the previous work [19]. Typically,
0.242 g of Bi(NO3)3·5H2O was dissolved in 10 mL of ethanol to form a Bi(NO3)3
solution. Meanwhile, 0.107 g of TaCl5 was dissolved in 10 mL of ethanol to generate
a TaCl5 solution. A white suspension was obtained when mixed the above TaCl5
solution into the Bi(NO3)3 solution after continuous stirring 30 min. Then, KOH (7M)
was added dropwise into the solution under magnetic stirring at room temperature
until pH value at 10, and the solution was continuously stirred for 0.5 h to obtain a
pale yellow sediment. Thereafter, the suspension was transferred into a Teflon-lined
stainless steel (50 mL), and the hydrothermal route was carried out at 220 oC for 24 h.
After cooling to the room temperature naturally, the yellow precipitate was collected
by centrifugation and thoroughly washed with deionized water and ethanol several
times, and dried at 60 oC in vacuum for 12 h to obtain pure Bi3TaO7.

For comparison, Bi3TaO7 nanoparticles was also prepared by solid-state method
according to the literature with some modification [15]. In a typical synthesis, Ta2O5
and Bi2O3 were mixed together in a molar ratio of 1:3, and then placed in a corundum
o
crucible. The mixture was sintered at 800 C for 10 h. The as-prepared sample by
solid-state and hydrothermal method are denoted as S1 and S2, respectively.
2.2. Characterization
X-ray diffraction (XRD) measurements were collected using a Bruker D8 Advance
diffractometer (CuKα radiation λ=1.54178Å). X-ray photoelectron spectroscopy (XPS)
analysis was applied to survey and evaluate the surface properties of the
photocatalysts. The morphology and crystalline structure of the photocatalysts were
determined by transmission electron microscopy (TEM) and high-resolution TEM
(HRTEM) on a F20 S-TWIN TEM instrument (Tecnai), using a 200 KV accelerating
voltage. The chemical composition of the samples was determined by Scanning
electron microscope-X-ray energy dispersion spectra (SEM-EDX) with an
accelerating voltage of 25 KV. (SEM, Hitachi, Japan). UV-vis diffused reflectance
spectra of the samples were obtained from an UV-vis spectrophotometer (UV2550,
Shimadzu, Japan); BaSO4 was used as a reflectance standard. Photocurrent and
Mott-Schottky (MS) measurements were carried out using the conventional three
electrode setup connected to an electrochemical station (CHI 760E, Shanghai
Chenhua, China). The nitrogen adsorption isotherms were collected by the
3H-2000PM2 system (Beishide Instrument, Beijing, China). Photoluminescence (PL)
spectra were collected on an Edinburgh FI/FSTCSPC 920 spectrophotometer.

Photocurrent measurements were carried out using the conventional three electrode
setup connected to an electrochemical station (CHI 630E, Shanghai Chenhua, China).
Zeta potential of the photocatalyst was measured with a Zetamaster equipment
2
(Malvern Instrument). Photocatalysts were dispersed in aqueous NaCl solution (10−
M) at different pH values. The EPR signals were obtained on a Bruker model EPR
JES-FA200 spectrometer. ICP measurement was conducted on a ICP spectrometer
(VISTA- MPX, Australia).
2.3. Photocatalysis valuation
Photocatalysis experiments were conducted in a photochemical reactor under visible
light. In a typical run, 0.05 g of Bi3TaO7 catalyst was added to 100 mL of TC aqueous
solution (10 mg L‒1). Prior to irradiation, the suspension was stirred for 60 min in the
dark to reach the absorption equilibrium. Then, the photochemical reactor was
irradiated with a 300W Xe lamp with a UV cutoff filter (λ＞420 nm). The sampling
analysis was conducted in a 30 min interval. The degradation of TC was monitored
with a UV-vis spectrophotometer. For comparison, blank experiment without catalyst
was also carried out. Additionally, the recycle experiments were performed for four
consecutive cycles to test the durability. After each cycle, the catalyst was filtrated
and washed thoroughly with distilled water several times to remove residual dye
impurities and then dried at 60 °C for the next test.
3. Results and discussion
3.1. Phase structures
The XRD patterns of the Bi3TaO7 powders synthesized using hydrothermal route (S2)

and solid-state method (S1) were investigated, as shown in Fig. 1. All of diffraction
peaks of S1 and S2 can be assigned to cubic phase structure of Bi3TaO7 (JCPDS
no.44-0202). It is observed that the peak width of S2 is larger than S1, indicating that
the crystallite size of S2 smaller than S1 according to the Scherrer's equation.
Furthermore, the effect of pH value in the hydrothermal process was also investigated,
as shown in Fig. 1b. When pH was adjusted to 3, most peaks can be ascribed to
monoclinic Bi2O3 (JCPDS no. 02-0498), cubic Bi2O3 (JCPDS no.27-0052) and
tetragonal Ta2O5 (JCPDS no.19-1300), respectively. The formation of Bi2O3 is mainly
caused by decomposition reaction of original Bi(NO3)3 at high temperature, where
Ta2O5 is originated from the hydrolysis of TaCl5. This implies that the decomposition
and hydrolysis reaction occurred and Ta2O5 and Bi2O3 were formed under the strong
acidic condition. Moreover, when pH value was adjusted to 14, the XRD diffraction
peak cannot match with the standard card of Bi3TaO7, whereas the characteristic
peaks can be indexed to Bi3TaO7 while pH value at 10. Therefore, it is essential to
control the pH value in synthesis of Bi3TaO7 by hydrothermal method. Moreover, the
lattice parameters of as-prepared samples by XRD refinement was listed in the table 1.
It is found that the lattice parameters have a slight changes and the density of S1 and
S2 are increased compared to the theoretical value. This phenomenon may be caused
by the formation of surface defects in preparation process.
3.2. Infrared spectrum and EDX analysis
Fig. 2 shows the FT-IR spectra of the samples obtained by different preparation
method. The peaks located at 883 (854) and 1050 cm-1 are mainly derived from the

Bi-O stretching vibration modes [19,20]. The bands that corresponds to water
molecules adsorbed on the surface of the catalysts appeared at about 3429 (3435) and
1628 cm‒1. The vibration of 2970 (2923) cm-1 could be assigned to OH deformation
mode. In more detail, the absorb peak at 2362 cm‒1 attributable to carbon dioxide in
air. Specifically, the characteristic adsorption peak emerged at 661 cm‒1 is caused by
the stretching vibrations of Ta-O and Ta-O-Ta bond [14,21,22]. The characteristic
peak of 1384 cm-1 in the S2 is interpreted as the bridged vibrational mode of adsorbed
nitrate because of residual nitrate in the sample [23-25]. It is noted that some
characteristic peaks in S2 shows some deviations compared to S1, such as the peaks at
3435, 2923 and 854 cm-1. This could be ascribed to the different bond strength with
different preparation methods. And the peak at 1050 cm-1 is not observed in the S1,
which we speculate that it may be due to the destruction of Bi-O bond. In addition, the
elemental composition was investigated by the EDX spectra. As shown in Fig. 2b, the
signals of Bi, Ta and O can be obviously revealed. The atomic ratio for Bi3TaO7
powders synthesized by hydrothermal and solid-state method was also shown in Table
2. The atomic ratio of Bi/Ta is about 2.91 for S1 and 2.35 for S2. Note that the atomic
ratio of Bi/Ta in both S1 and S2 are not the value of 3.0 from EDX result. The reason
might be attributed to the formation of surface defect and therefore the result does not
comply with the theoretical proportion.
3.3. Morphology
The morphology and size of Bi3TaO7 were revealed by SEM, TEM and HRTEM
images. Fig. 3a shows the SEM of Bi3TaO7 obtained by S-S method (S1). The S1

shows irregular morphology in the scale of micron, whereas the S2 exhibits the
aggregated particles, the size of Bi3TaO7 was about 30 nm (Fig 3b). The result
indicates that the hydrothermal synthesis is an effective strategy to reduce the size of
Bi3TaO7 and form a well-defined morphology. The TEM image of S2 also reveals that
the Bi3TaO7 sample mainly consists of 20-30 nm primary particles (Fig. 3c). In Fig.
3d, the periodicity of the fringes obtained from the HRTEM studies were 0.31 and
0.27 nm, corresponding to the (111) and (200) planes of cubic Bi3TaO7, respectively.
The crystal plane angle between (111) and (200) plane was measured to 52° (the
_
theoretical value is 55°) and the crystal orientation can be indexed as [0,2,2] based on
the analysis of Weiss Zone Law .
3.4. X-ray photoelectron spectra (XPS)
The surface chemical states of the S1 and S2 were analyzed by XPS spectra, as shown
in Fig. 4. The survey spectrum indicates that the sample is mainly composed of Bi, Ta,
and O element (Fig. 4a). The peak positions in all of the XPS spectra were calibrated
with C 1s at 284.6 eV. Fig. 4a, the binding energies of 159.1and 164.5 eV,
corresponding to the Bi 4f7/2 and energy values of 159.1 eV, suggesting the presence
of Bi3+ species in both S2 and S1 [26]. Three peak could be obtained after fitting the
Ta 4f spectrum in S1 and S2. Two peaks at 28.9 and 27.5 eV are corresponding and
the other peak at 25.9 eV can be attributed to Ta 4f7/2, which corresponds to the Ta5+
oxidation state [27-29]. The O 1s spectra in Fig. 4d could be fitted to three peaks. The
peaks located at 529.9 eV originates from the crystal lattice oxygen in the structure of
Bi3TaO7, while the others arise from the surface chemisorbed oxygen or physically

adsorbed water [30-32]. Notably, all the peaks show no obvious difference between
S1 and S2，manifesting that it has the same composition in the samples. The XPS
composition of S1 and S2 was shown in Table 2. Similar to the EDX result, the Bi/Ta
ratio was also deviation from the theoretical value of 3.0. This phenomenon may be
caused by the existence of surface defects in the samples.
3.5. UV-vis absorption spectra
UV-vis diffuse reflectance spectra of Bi3TaO7 samples are shown in Fig. 5. It can be
seen that both S1 and S2 have absorption in the UV region visible-light region, and
the absorb band of S1 showed a slight red shift in visible light region compared to S2.
The energy band gap (Eg) could be calculated by using the Kubelka-Munk function:
αhν = A (hν-Eg)n/2
(1)
where hν is the photon energy, α is the absorption coefficient, A is a constant relative
to the material and n is a value that depends on the nature of transition (n = 1 for
direct bandgap semiconductor and n = 4 for indirect band gap semiconductor) [33].
The (αhν)2 versus hν extrapolated to α = 0 gives the absorption band gap energy, as
shown in Fig. 5b. Eg value of S1 and S2 thus determined to be 2.76 eV and 2.87 eV,
respectively. According to the literature, the dependence of particle size on the optical
absorption energy can be recognized based on an active mass approaches using the
equation below [34]：
2
2
2
휋 
1
1
∆퐸 =
[
+
∗ ] − 1.786푒 − 0.248퐸푅∗푦
(2)
∗
2
2푟
푚
푚
휀푟
푒
ℎ
Where ∆E is the band gap of bulk semiconductor, r is the radius of the particle,
푚푒∗ is the electron effective mass, 푚ℎ∗ is the hole effective masses, ɛ is the dielectric

constant of semiconductor, e is the electric charge and 퐸푅∗푦 is the effective Rydberg
2
energy, 푒4/(2휀2 (푚푒−1 + 푚ℎ−1)). It can be concluded that the band gap will increase
with the reduction of particle sizes. Therefore, the difference of band gap for S1 and
S2 may mainly ascribed to the different particle sizes between S1 and S2. The larger
particle size of S1 makes it has a smaller band gap.
3.6. BET surface areas
As shown in Fig. 6, N2 adsorption-desorption was carried out to analyze the surface
properties of the sample. According to the IUPAC classification, a type IV isotherm
with a H1 hysteresis loop is presented within the sample, corresponding to
characteristic of mesoporous structures [35]. The BET surface area is ca. 39.94 m2 g−1
for S2 sample, which is much higher than S1 (6.2 m2 g−1). Therefore, the higher BET
surface areas of S2 is facilitate to enhance the adsorption ability for the TC molecules,
more active adsorption sites and reaction sites would be provided, which is favorable
for degradation of TC.
3.7. Mott-Schottky plot
To further study the conduction type, the flat band (Efb), and the estimated donor
densities of as-prepared sample, Mott-Schottky (M-S) plot for the Bi3TaO7 was
carried out. The capacitance-potential measurements are presented as an M-S plot
following the equation below [36]:
1
2
=
[(퐸 − 퐸푓푏) − 푘푇
]
(3)
2
퐶
푁 푒휀휀
푒
푑
0
where C is the space charge capacitance in the semiconductor, Nd is the hole carrier
density, e is the elemental charge value, 휀0 is the permittivity of the vacuum of free

space (8.85×10‒14 F cm‒1), 휀 is the relative permittivity of the semiconductor. E is the
applied potential, Efb is the flat band potential, T is the temperature, and k is the
Boltzmann constant. The intercept of the fitted line at C‒2 = 0 and the slope give the
potential Efb of the semiconductor. As shown in Fig. 7, both S1 and S2 are shown a
positive slope of the M-S plot, signifying that Bi3TaO7 nanoparticles is a kind of
n-type semiconductors. Moreover, the flat band potentials of S1 and S2 are measured
to be ca.‒ 0.42 and ‒ 0.37 V vs. Ag/AgCl, respectively.
3.8. Surface defects
It is reported that the surface defects and oxygen vacancies of Bi3TaO7 are due to the
existence of a defect configuration in fluorite-type structure [14]. The surface defects
in as-prepared Bi3TaO7 with different approach was studied in this work. It is noted
that the atomic ratio of Bi/Ta is not the value of 3.0 from EDX and XPS result for S1
and S2 sample. Therefore, inductively coupled plasma resonance spectroscopy (ICP)
measurement was conducted to confirm the amount of Bi and Ta. As illustrated in Fig.
8, the atomic ratio of Bi/Ta was calculated to be 2.94 for S1 and 2.56 for S2, which is
lower than the theoretical value of 3.0. This may be attributed to the existence of
surface defects in as-prepared samples. In this case, electron paramagnetic resonance
(EPR) technique was applied to prove whether there are defects in the as-prepared
samples at room temperature. As represented in Fig 9, the S2 exhibits a very strong
signal with g value of 1.998 in a magnetic field from 2500 to 4000 G, while a
negligible peak is observed for the S1. As reported in the literature, part of Ta5+ would
be reduced to Ta4+ in the solvothermal process when TaCl5 is used as the Ta source

and ethanol as solvent, which leads to the self-doping of Ta4+ in the sample [37]. This
result indicates that the defects are present in the S2. The sample obtained by
solid-state method reveals a fewer defect in contrast to the sample treated by
hydrothermal process. It is expected that photocatalytic performance of the S2 would
be higher than that of S1. Because the surface defects could be served as trapping sites
for electrons and it is accordingly promoting charge-carrier separation [38-40].
3.9. Photocatalytic degradation of TC
The photocatalytic activity of the prepared samples was studied by comparing
degradation rate of tetracycline (TC) in aqueous solutions under visible light
irradiation, and the results is plotted in Fig. 10. It was found that the photocatalytic
performance of S2 sample shows a remarkable increase, which is about 2-fold higher
than that of S1. The result reveals that the sample obtained by hydrothermal method
was contributed to improve the photocatalytic activity in comparison to S-S method.
The decrease of the particle size can not only be conducive to the reduction of
diffusion length for photogenerated electron-hole pairs, but also broaden the band gap,
thereby enhancing the redox ability of photocatalyst. In addition, with the decrease of
nanopowders for Bi3TaO7, which is beneficial to reducing the distance of
photogenerated carriers migrate to the surface active sites, resulting in the
improvement of the charge separation efficiency. As a result, a higher photocatalytic
efficiency for the degradation of TC can be achieved [41-43].
3.10. Effects of operational conditions on photocatalytic performance of Bi3TaO7
It is very important to explore the effect of the pollutant concentration on

photocatalytic decomposition efficiency [44]. Firstly, the effect of initial TC
concentration on the photocatalytic degradation was studied, and the results have been
shown in Fig. 11a. The photocatalytic efficiency decreased gradually with the TC
concentration ranged from 10 mg L‒1 to 50 mg L‒1, clearly demonstrating that the low
concentration of TC is beneficial to the improvement of the photocatalytic activity.
This is mainly due to the increased concentration of TC may result in more adsorption
of TC onto the Bi3TaO7 nanoparticles, which can cause inhibitive effect on the further
photocatalytic degradation of TC due to decreased adsorption sites on the
photocatalyst. Similar results have been reported by previous literatures [45-48].
In addition, the effect of catalyst concentration was investigated by varying the initial
concentration range of 0.4-1.0 g L‒1 under constant pH 6.8. As shown in Fig.11c, the
photocatalyst exhibited a highest degradation rate (DR) when catalyst initial
concentration was 0.6 g L‒1. Above the optimal concentration, the turbidity of the
suspension increase leads to light penetration decreases, which result in the reduction
of active species in the photocatalytic system. As the result, the photocatalytic activity
of catalyst for the degradation of TC is decreased.
The initial pH was also a significant factor in photocatalytic degradation of organic
pollutants. Fig. 11e shows the effect of initial pH evolved in the photodegradation of
TC. It can be seen that the photocatalytic process strongly depends on the pH of the
TC solution. As seen in Fig. 11f, the degradation rate of TC is only 27.9% at pH 2.
While pH increased to 4, the photocatalyst exhibits a highest photocatalytic efficiency
(74.9%), which is 2.5-fold higher than that of pH 2. After which it decreased with the

increase of pH value [49]. This indicates that the as-prepared photocatalyst has a good
adsorption capacity of protonated and monomeric species of TC under acidic
environment. It is known that the pKa values of TC molecule were 3.3, 7.7 and 9.7,
respectively. According to the similar report, all TC molecules have no net charge, so
that they have almost no electrostatic attraction or repulsion due to the presence of TC
zwitterion at the pH value of nearly 7. It is unstable and easy to be photolyzed in the
alkaline condition [50]. In addition, the degradation of TC may be ascribed to the
strong nonelectrostatic interaction at pH between 5 and 9 [51]. The adsorption of TC
may reduce with the increase of pH, resulting in the decrease of the photocatalytic
efficiency.
Fig. 12 shows the plots of the zeta-potential as a function of pH for S2 in the presence
of NaCl (10‒2 M). It can be seen that the point of zero charge (PZC) is 5.13. Therefore,
the surface of the catalyst with a positive charge would facilitate to the dispersion of
catalyst in TC aqueous solution when the pH value is less than 5.13. When the pH
value larger than that of PZC, the adsorption would decrease owing to the existence of
repulsive force between catalyst and TC aqueous solution. It should be noted that
the strong acidic condition would cause the protonation of TC because of the presence
of amino groups in TC molecules. A good adsorption would be achieved between
catalyst and TC aqueous solution when pH was near to 4. In this case, Bi3TaO7
nanoparticles provide the TC molecules with stronger surface adsorption ability and
more surface active sites for the photocatalytic degradation of TC owing to the
photocatalytic reaction occurs on the surface of the catalyst.
3.11. Photoluminescence and photocurrent
It is well known that photoluminescence (PL) spectra and transient photocurrent
analysis is a powerful technique to survey the separation efficiency of the

photo-generated electron-hole pairs in a semiconductor [52,53]. Fig. 13a presents the
PL spectra of Bi3TaO7 photocatalysts. It can be seen that the sample obtained by the
hydrothermal method exhibits much lower emission intensity than that of the sample
obtained by the S-S method, suggesting that the recombination of photogenerated
electrons and holes is inhibited greatly in the Bi3TaO7 obtained by the hydrothermal
method. Fig 13b shows the photocurrent densities of the Bi3TaO7 photocatalysts
prepared by hydrothermal method and S-S method conducted in a three-electrode
electrochemical cell in 0.5 M Na2SO4 solution for a 350 s period in an on-and-off
cycle mode under visible light. It is observed that the S2 sample exhibites a
remarkable enhancement of photocurrent higher than the S1, indicating that the
hydrothermal method is more effective to separate the electron-hole. The PL and
photocurrent result reveal that the sample obtained by hydrothermal method reduced
the recombination rate of the photogenerated electrons and holes, resulting in a higher
photodegradation activity.
3.12. Stability of catalyst
To investigate the stability and durability of catalyst, the cycling experiments were
also carried out, as shown in Fig. 14. After four cycles for photodegradation of TC,
the catalyst does not show significant loss, which indicates that the Bi3TaO7 has good
reusability and stability (Fig. 14a). In Fig. 14b, the XRD pattern revealed no
difference on the Bi3TaO7 catalyst before and after use, suggesting that Bi3TaO7
catalyst is stable for the degradation of TC. These results indicate that the as-prepared
Bi3TaO7 is expected to be used as a promising material for the applications in

pollutant removal.
4. Conclusion
In summary, visible light-driven Bi3TaO7 photocatalyst was synthesized by
hydrothermal method and solid-state reaction. The as-prepared Bi3TaO7 photocatalyst
by hydrothermal method exhibited highly visible light photocatalytic activity toward
tetracycline degradation. With the decrease of nanopapowders for Bi3TaO7, which is
beneficial to reducing the distance that photogenerated carriers migrate to the surface
active sites. Moreover, the formation of surface defects could be served as trapping
sites for photo-induced electrons, resulting in the improvement of the efficiency of
charge separation, which could be confirmed by the PL and transient photocurrent
spectra. The prepared Bi3TaO7 photocatalyst also shows good photo-stability under
visible light and high recyclability for degradation of TC in aqueous solution. The
as-prepared Bi3TaO7 could be used as a visible light-driven photocatalyst for TC
degradation.
Acknowledgements
The authors are grateful for the National Natural Science Foundation of China
(21477050, 21301076 and 21303074), the Excellent Youth Foundation of Jiangsu
Scientific Committee (BK20140011), the Open Project of State Key Laboratory of
Rare Earth Resource Utilizations (RERU2014010), the Henry Fok Education
Foundation (141068), Six Talents Peak Project in Jiangsu Province (XCL-025), and
the Chinese-German Cooperation Research Project (GZ1091).

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Scheme. 1. A schematic illustration of the preparation of Bi3TaO7 nanoparticles by
hydrothermal method.
Fig. 1. XRD patterns of obtained samples (a) by hydrothermal method and S-S
method; (b) in various pH condition by hydrothermal method.
Fig. 2. (a) The FT-IR absorption spectra of the S1 and S2 samples; (b) EDX spectra of
S1 and S2 samples.
Fig. 3. (a) SEM images of S1 and (b) S2 sample; (c) TEM image of S2; (d) HRTEM
image of S2 sample.
Fig. 4. The XPS spectra of the Bi3TaO7, (a) survey spectrum; (b) XPS spectrum of Bi;
(c) Ta and (d) O.
Fig. 5. (a) UV-vis diffuse reflectance spectra of Bi3TaO7; (b) the calculated band gap
energy by the plot of (αhν)2 vs hν for Bi3TaO7.
Fig. 6. Nitrogen adsorption-desorption isotherms of Bi3TaO7 obtained by
hydrothermal method; inset is pore size distribution curve.
Fig. 7. Mott-Schottky curves of S1 and S2 at 1 kHz, 2 kHz and 3 kHz b in 0.5 M
Na2SO4 solution (pH = 7).
Fig. 8. The atomic ratio of Bi/Ta in S1and S2 tested by ICP.
Fig. 9. EPR spectra of S1 (a) and S2 (b).
Fig. 10. photodecomposition of TC under visible irradiation (λ>420 nm) over various
photocatalysts; inset is the chemical structure of TC.
Fig. 11. (a) Effect of initial TC concentration on the photocatalytic degradation

(Bi3TaO7 dosage = 0.5 g/L, pH 6.8); (b) the degradation rate in various initial TC
concentration; (c) Effect of photocatalyst dosage on photocatalytic
decomposition of TC (TC concentration = 10 mg/L, pH 6.8); (d) the degradation
rate in various photocatalyst concentration; (e) Effect of initial TC concentration
on the photocatalytic degradation of TC (Bi3TaO7 dosage = 0.5 g/L, TC
concentration = 10 mg/L); (f) the degradation rate in various pH condition.
2
Fig. 12. Zeta potential of S2 in aqueous NaCl solution (10− M) at different pH
values.
Fig. 13. (a) Photoluminescence spectra and (b) photocurrent transient responses of
selected Bi3TaO7 samples prepared by different method.
Fig. 14. (a) The cycling experiment for degration of TC with the Bi3TaO7 prepared by
hydrothermal method; (b) XRD patterns of Bi3TaO7 before degradation and after
degradation for the four cycles.

Scheme. 1. A schematic illustration of the preparation of Bi3TaO7 nanoparticles by
hydrothermal method.
Bi2O3 (JCPDS:02-0498)
Bi2O3 (JCPDS:27-0052)
Ta O (JCPDS:19-1300)

.
(a)
(b)
(111)
.

.

2
5




(200)




.
.
(220)
(311)
pH=3.0
S1
(222)
(400)
pH=14.0
pH=10.0
S2
Bi3TaO7
10
20
30
40
50
60
70
10
20
30
40
50
60
70
2 Theta (degree)
2 Theta (degree)
Fig. 1. XRD patterns of obtained samples (a) by hydrothermal method (S2) and
solid-state method (S2); (b) in various pH condition by hydrothermal method (S1 =
solid-state synthesized, S2 = hydrothermally synthesized).

(a)
Bi
Bi
(b)
S1
Ta
Au
Au
2923
2362
2362
854
3435
O
O
1628
1628
S1
S2
Ta
Ta
Ta
Bi
Bi
Bi
Bi
S2
661
Ta
O
2970
3000
1050
1384
3429
883
Ta
8
O
661
0
4
12
16
4000
3500
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Energy (keV)
Fig. 2. (a) The FT-IR absorption spectra of the S1 and S2 samples; (b) EDS spectra of
S1 and S2 samples (S1 = solid-state synthesized, S2 = hydrothermally synthesized).

(c)
Fig. 3. (a) SEM images of S1 and (b) S2 samples; (c) TEM image of S2; (d) HRTEM
image of S2 samples (S1 = solid-state synthesized, S2 = hydrothermally synthesized).

159.1 eV
(a)
S2
S1
(b)
Bi 4f
Bi 4f
164.5 eV
O 1s
Bi 4d
Bi 4d
Ta 4f
Ta 4d
C 1s
S2
Bi 4f
O 1s
Ta 4d
Ta 4f
C 1s
S1
168
166
164
162
160
158
156
600
500
400
300
200
100
0
Binding energy (eV)
Binding energy (eV)
25.9
529.9
O 1s
Ta 4f
(c)
(d)
27.5
531.2
532.5
28.9
S2
S2
529.9
25.9
28.9
27.5
531.3
532.5
S1
S1
536
534
532
530
528
526
32
30
28
26
24
22
Binding energy (eV)
Binding energy (eV)
Fig. 4. The XPS spectra of the Bi3TaO7, (a) survey spectra; (b) XPS spectra of Bi; (c)
Ta and (d) O.

(a)
S2
S1
(b)
S1
S2
2.76 eV
2.87 eV
200
300
400
500
600
700
2
3
4
5
hν (eV)
Wavelength (nm)
Fig. 5. (a) UV-vis diffuse reflectance spectra of Bi3TaO7; (b) the calculated band gap
energy by the plot of (αhν)2 vs hν for Bi3TaO7.

350
300
250
200
150
100
50
0.010
0.008
0.006
0.004
0.002
0.000
0
10
20
30
40
50
60
70
Pore size (nm)
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
Fig. 6. Nitrogen adsorption-desorption isotherms of Bi3TaO7 obtained by
hydrothermal method; inset is pore size distribution curve.

5
4
3
2
1
0
(a)
(b)
5
4
3
2
1
0
3 kHz
2 kHz
1 kHz
3 kHz
2 kHz
1 kHz
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
-0.4
-0.2
0.0
0.2
0.4
Potential (V vs Ag/AgCl )
Potential (V vs Ag/AgCl )
Fig. 7. (a) Mott-Schottky curves of S1 and (b) S2 at 1 kHz, 2 kHz and 3 kHz b in 0.5
M Na2SO4 solution (pH = 7).

5
4
3
2
1
0
3.00
2.94
2.56
S2
Nominal amount
S1
Samples
Fig. 8. The atomic ratio of Bi/Ta in S1and S2 tested by ICP.