Template-Free Solvothermal Synthesis of Flower-Like BiOBr Microspheres in Ethanol Medium for Photocatalytic Applications

Article abstract of DOI:10.1134/S0036024418050163

Three-dimensionally (3D) BiOBr microflowers were prepared by a simple solvothermal method, employing Bi(NO₃)₃ · 5H₂O and NaBr as starting reagents in ethanol. The structural, light absorption and morphological properties of as-prepared BiOBr microspheres were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV–Vis diffuse reflectance spectroscopy (DRS), etc. The results showed that ethanol acted not only as a solvent but also as a template in 3D BiOBr preparation. The BiOBr microspheres exhibited superior photocatalytic activity compared with 2D BiOBr nanosheets, far exceeding that of TiO₂ (Degussa, P25). It was found that both superoxide radical (O₂^?-) and holes (h⁺) played a key role in the degradation of RhB by BiOBr microflowers.

Full text of DOI:10.1134/S0036024418050163

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2018, Vol. 92, No. 5, pp. 984–991. © Pleiades Publishing, Ltd., 2018.
PHYSICAL CHEMISTRY OF NANOCLUSTERS
AND NANOMATERIALS
Template-Free Solvothermal Synthesis of Flower-Like BiOBr
Microspheres in Ethanol Medium for Photocatalytic Applications
1
a,b
a,b
b
a,b,
a,b,
Shaoming Hong , Huijun Ren , Yanfeng Fang , Yingping Huang *, and Ruiping Li *
a
College of Hydraulic and Environmental Engineering, China Three Gorges University, Yichang, 443002 China
b
Engineering Research Center of Eco-environment in Three Gorges Reservoir Region, Ministry of Education,
China Three Gorges University, Yichang, 443002 China
*
e-mail: chem_ctgu@126.com
Received July 25, 2017
Abstract—Three-dimensionally (3D) BiOBr microflowers were prepared by a simple solvothermal method,
employing Bi(NO ) · 5H O and NaBr as starting reagents in ethanol. The structural, light absorption and
3
3
2
morphological properties of as-prepared BiOBr microspheres were determined by X-ray diffraction (XRD),
scanning electron microscopy (SEM), and UV–Vis diffuse reflectance spectroscopy (DRS), etc. The results
showed that ethanol acted not only as a solvent but also as a template in 3D BiOBr preparation. The BiOBr
microspheres exhibited superior photocatalytic activity compared with 2D BiOBr nanosheets, far exceeding
•−
2
+
that of TiO (Degussa, P25). It was found that both superoxide radical (O ) and holes (h ) played a key role
2
in the degradation of RhB by BiOBr microflowers.
Keywords: solvothermal method, BiOBr microflowers, ethanol solvent, photocatalysis, RhB
DOI: 10.1134/S0036024418050163
INTRODUCTION
wave-assisted route [19, 23], etc. However, these
methods have some obvious disadvantages, for
instance, using surfactant or ionic liquid as source of
bromine and solvent or template to control the growth
process, or using the complicated preparation process
such as microwave equipment.
As a promising solution to the increasingly serious
environmental pollution, the photocatalytic utiliza-
tion of solar energy on active photocatalysts has drawn
considerable interest worldwide [1–3]. Recently, bis-
muth oxybromide (BiOBr) is emerging as one of the
most attractive visible-light driven photocatalysts
owing to its narrow band gap and unique structure [4,
In this work, 3D BiOBr microflowers were synthe-
sized by a simple template-free solvothermal route in
–
ethanol with NaBr as Bi source. The raw materials
5
]. Because of the strong correlation between the
physical/chemical properties and the microstructure
i.e., particle size, surface area and dimensionality) of
the materials, the construction of BiOBr micro-
used were inexpensive and environmentally-benign.
The phase composition, morphology and optical
properties of 3D BiOBr were investigated by XRD,
SEM, DRS, and PL. The prepared hierarchical BiOBr
microflowers showed excellent activity under visible-
light irradiation in RhB removal.
(
/
nanomaterials with tailored morphologies and pat-
terns is of great significance for both scientific research
and industrial applications.
To date, various morphologies and structures of
BiOBr materials have been reported, such as two-
dimensional (2D) BiOBr nano-/microstructures
nanoplates [6, 7], nanobelts [8, 9], microspheres [5,
EXPERIMENTAL
Preparation of BiOBr Samples
The flower-like BiOBr microspheres (3D BiOBr)
were prepared via a solvothermal method. Typically,
10–17], etc. Therefore, there are large number of
reports dealing with the design and fabrication of
BiOBr micro/nanostructures with desired morpholo-
gies and enhanced photocatalytic activities. The pub-
lished methodologies include surfactant-assisted sol-
vothermal method [14–16, 18–20], ionic liquid-
0
.002 mol Bi(NO ) · 5H O and 0.002 mol NaBr were
3 3 2
dispersed in 30 mL absolute ethanol under ultrasoni-
cation, respectively. Then the NaBr solution was
added very slowly to the suspension of Bi(NO ) . The
3 3
assisted hydrothermal method [21, 22], and micro- mixture was sonicated for 40 min, and then its pH was
adjusted to 7 with 1 mol/L NaOH. The final suspen-
sion was transferred into 100 mL Teflon-lined auto-
1
The article is published in the original.
984

TEMPLATE-FREE SOLVOTHERMAL SYNTHESIS
985
clave and kept at 160°C for 12 h. After cooling natu- peaks of 3D BiOBr can be indexed to well crystallized
rally to room temperature, the product was collected pure tetragonal phase with lattice constants of a = b =
and washed with ethanol and deionized water 3 times, 3.926 Å, c = 8.103 Å and are in accordance with the
and finally dried at 50°C under vacuum for 4 h. In standard data (JCPDS no. 09-0393) [25, 26], while
order to compare with 3D BiOBr, the BiOBr 2D BiOBr is in good agreement with the literature val-
nanosheets (2D BiOBr) were also prepared by our pre-
viously reported method [24].
ues (JCPDS no. 73-2061) [27]. Both BiOBr samples
showed the strong and sharp diffraction peaks, indi-
cating the high crystallinity of the products. Notably,
obvious preferred orientation of diffraction peaks can
be observed, which would be related to the morpho-
logical properties as shown in Fig. 1. The peak inten-
sity ratio of (012)/(110) for 2D BiOBr is close to that of
the standard value for the tetragonal sample, suggest-
ing there is no preferred orientation of diffraction
peaks for 2D BiOBr. In contrast, the intensity ratio of
Characterization of BiOBr Samples
X-ray diffraction (XRD) (D/max2500, Rigaku,
Japan) was used to determine the phase compositions
of the samples. Scanning electron microscope (SEM,
JSM-7500F, JEOL, Japan) was employed to study the
morphology of the products. UV–Vis diffuse reflec-
tance spectra (DRS) were obtained by Shimadzu UV-
(102)/(110) for 3D BiOBr is remarkably lower than
3
100 spectrophotometer using BaSO as the back-
4
that of the tetragonal phase, implying that the 3D
BiOBr microflowers are growing with preferred [110]
orientation. This is because ethanol molecules can be
preferentially adsorbed on the (102) facets of BiOBr
crystals, thus hindering the growth along [102] direc-
tion [28]. The average crystallite size of 3D BiOBr
microflowers can be calculated according to Scherrer
equation: D = Kλ/βcosθ, here λ is the X-ray wave-
length (1.5405 Å), β and θ are the full width at half
maximum and diffraction angle of an observed peak,
respectively. The average crystal size is about 21.26 nm
based on the diffraction peaks (101) at 25.27°, (110) at
ground. The specific surface area was determined
from N adsorption/desorption isotherms on JW-BK
2
1
12 specific surface area and pore size analyzer (Bei-
jing JWGB Sci. and Tech. Co., Ltd.) and calculated by
the Brunauer–Emmett–Teller (BET) method. The
photoluminescence (PL) spectra were carried out on a
Hitachi F-4500 fluorescence spectrophotometer.
Photocatalytic Activity Measurement
The photocatalytic experiments were carried out in
an XPA photo-reactor (Xujiang Electromechanical
Plant, Nanjing, China) with a 400 W metal halogen
lamp. The experiments were performed with BiOBr
32.29°, (200) at 46.32°, and (212) at 57.29°.
The SEM images of as-prepared BiOBr samples
samples (5 mg) suspended in RhB solution (40 mL, are presented in Fig. 2. It can be seen that 3D BiOBr
–
5
2
× 10 mol/L) under continuous stirring in the pres- sample is composed of hierarchical flower-like micro-
ence of visible light (>420 nm). Prior to irradiation, spheres assembled by nanosheets, which are beneficial
the suspension was equilibrated under darkness for 2 h for improving the electron-hole separation efficiency
to reach adsorption equilibrium. Aliquots of 2 mL and resulting in high photocatalytic performance [29].
were collected after a certain time of irradiation and
then filtered through 0.22 μm filter. The concentra-
tion of RhB in the filtrate was determined by monitor-
ing the absorbance at wavelength of 554 nm in UV–Vis
The diameter of the microflowers varies in 1–2.5 μm
range. Compared with BiOBr microspheres prepared
–
by EG-based solvothermal process with NaBr as Br
source [30], the obtained BiOBr microspheres pre-
pared in ethanol are smaller, and the nanosheets align
loosely to form hierarchical microspheres. The reac-
tion time and the solvent (ethanol) have a significant
spectra. For comparison, TiO (Degussa, P25) and
2
blank samples were also tested under the same experi-
mental conditions.
In order to determine the main active species influence on the morphology of the obtained powders.
during photodegradation process, hydroxyl radicals In the recent studies undertaken in our laboratory, we
•
2
−
•
+
found that the BiOBr irregular nanosheets (JCPDS
card No. 73-2061) were formed at 145°C for 4.5 h [24]
(
OH), superoxide radical (O ), and holes (h ) were
trapped by 1.0 mM isopropanol (IPA, a quencher of
(
Fig. 2a) while flower-like BiOBr microspheres
•
−
•
OH), benzoquinone (BQ, a quencher of O ) and
2
(JCPDS card no. 78-0348) were produced at 140°C
for 6 h [18] with tetrabutyl-ammonium bromide as the
Br source and ethanol as the reaction solvent. In this
work, BiOBr microflowers (JCPDS card no. 09-0393)
were also formed at 160°C for 12 h in ethanol with
ethylenediaminetetraacetic acid disodium salt
+
(
EDTA, a quencher of h ), respectively [18].
RESULTS AND DISCUSSION
Structure and Morphology of the Prepared BiOBr
Samples
–
NaBr as Br source (Figs. 2b, 2c). These observations
indicated that ethanol possessed the directing ability
which made the BiOBr nanoparticles grew into
Figure 1 showed the XRD patterns of the as-pre- nanosheets and further assembled to form micro-
pared BiOBr samples. In Fig. 1, all of the diffraction spheres with the reaction time prolonging [18, 30].
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 92 No. 5 2018

9
86
SHAOMING HONG et al.
2D BiOBr
3D BiOBr
BiOBr no. 09-0393
BiOBr no. 73-2061
20
30
40
50
60
70
2
80 10
20
30
40
50
60
70
80
θ, deg
2θ, deg
Fig. 1. XRD patterns of the prepared 2D BiOBr and 3D BiOBr samples.
(a)
(b)
(c)
1
μm
1 μm
100 nm
Fig. 2. SEM images of the prepared 2D BiOBr nanosheets (a) and 3D BiOBr mircroflowers (b, c).
Optical Properties of the Prepared BiOBr Samples
BiOBr is apparently weaker than that of 2D BiOBr,
implying lower recombination probability of charge
carriers for 3D BiOBr, as in accord with the previous
studies [5, 32].
The UV–Vis diffuse reflectance spectra (DRS) of
the as-prepared BiOBr microflowers are shown in
Fig. 3. In comparison with 2D BiOBr, the absorption
edge of 3D BiOBr has a distinct redshift with
enhanced absorption. The redshift in absorption edge
and the enhanced visible light absorption can be
attributed to the improved light harvesting [5] and
larger particle size [31] of 3D BiOBr. It is reported that
BiOBr exhibits an indirect band gap. The inset of
BET Surface Area
Nitrogen adsorption–desorption isotherms were
measured to characterize BET surface area and the
porosity of the obtained 3D BiOBr microflowers. The
typical adsorption–desorption isotherms and pore
size distribution of the sample are presented in Fig. 5.
The isotherm could be categorized as type IV accord-
2
Fig. 3 shows the plots of (αhν) versus the energy of
absorbed light (hν), estimating the band gaps (E ) of
g
the as-prepared BiOBr samples (where α is the absor-
bance coefficient, hν is the discrete photon energy)
ing to IUPAC with the hysteresis loop at about P/P =
0
0
.6–1.0, suggesting the presence of mesoporous struc-
[
32]. The E values of the prepared 2D BiOBr and 3D
g
ture [18, 28]. The inserted pore diameter distribution
mainly revealed the mesoporous diameter of about
BiOBr are estimated to be 2.88 and 2.80 eV from the
2
extrapolated intercepts of the (αhν) –hν plots,
3
.0 nm. The surface area and the pore size of 3D
respectively.
2
BiOBr were calculated to be 17.75 m /g and 5.85 nm,
The recombination of photo-induced electrons respectively. The large surface area and porous archi-
and holes in the semiconductor could be verified by tecture of BiOBr can provide more active sites and dif-
photoluminescence (PL) spectra [32]. Figure 4 pre- fusion pathways for adsorption and transportation of
sented the PL spectra of 2D BiOBr and 3D BiOBr. As reactants and intermediates, which would be benefi-
can be seen in Fig. 4, the PL emission intensity of 3D cial to the photocatalytic process [33, 34].
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TEMPLATE-FREE SOLVOTHERMAL SYNTHESIS
987
1
1
1
4
2
0
8
1
0
0
0
0
.0
.8
.6
.4
.2
0
6 Eg = 2.80 eV 3D BiOBr
4
2
0
BiOBr (3D)
E = 2.88 eV 2D BiOBr
g
2
.0 2.5 3.0 3.5 4.0 4.5
hν, eV
BiOBr (2D)
350
3
00
400
450
500
550
600
Wavelength, nm
Fig. 3. UV–visible DRS and the according band gaps calculated (inset) of the 2D BiOBr and 3D BiOBr samples.
3000
2500
2000
1500
1000
2D BiOBr
3D BiOBr
500
3
75 400 425 450 475 500 525 550 575 600
Wavelength, nm
Fig. 4. PL spectra of the 2D BiOBr and 3D BiOBr samples.
Photocatalytic Activities
while this percentage is only 19% with TiO . The
2
apparent reaction rate constant (k) value for 3D BiOBr
The photodegradation of RhB under visible-light
irradiation (λ > 420 nm) was used as a probe to evalu-
ate the performance of the as-prepared 3D BiOBr.
Figure 6 shows the photocatalytic degradation of RhB
is about 1.7 and 16.6 times that of 2D BiOBr and TiO ,
2
respectively (Fig. 6), which is in agreement with the
PL results. These observations can be attributed to the
advantages of the 3D BiOBr hierarchical microflowers
such as their improved light harvesting, shortened dif-
fusion pathways, faster interfacial charge separation
and more reactive sites [5].
on 2D BiOBr, 3D BiOBr, and TiO (Degussa, P25),
2
presenting the concentration changes of RhB at
5
54 nm as a function of irradiation time during the
degradation process. It can be found that the self-pho-
tolysis of RhB under visible light irradiation is insignif-
Figure 7 presents temporal changes in the UV–Vis
icant within 60 min. However, in the presence of pho- spectrum of RhB solution during adsorption and pho-
tocatalyst, more than 97% of RhB has been photode- tocatalytic degradation over the prepared BiOBr sam-
graded employing 3D BiOBr, and about 88% of RhB ples. For both BiOBr samples, the wavelength of max-
degraded with 2D BiOBr in the same irradiation time, imum absorbance (λmax) gradually shifts from 554 nm
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 92 No. 5 2018

9
88
SHAOMING HONG et al.
20
16
12
8
4
0
0.006
0.005
0.004
0.003
0.002
0.001
0
0
10 20 30 40 50 60 70
Pore diameter, nm
Adsorbed
Desorption
0
0.2
0.4
0.6
0.8
1.0
P/P
0
Fig. 5. N adsorption–desorption isotherms and corresponding pore size distribution curve (inset) of the 3D BiOBr micro-
2
flowers.
1
0
0
0
0
.0
.8
.6
.4
k = 0.0035 min−
1
Blank
TiO
BiOBr2D
2
k = 0.035 min−
1
BiOBr3D
.2
dark light on
15
k = 0.058 min−
1
0
−
30
−15
0
30 45
60
Irradiation time, min
Fig. 6. Photodegradation plots and photocatalytic reaction kinetics constants of RhB solution with 2D BiOBr, 3D BiOBr, and
P25 powders.
to shorter wavelength during visible light irradiation, among which rhodamine (λmax = 498 nm) is the com-
accompanied by a gradual decrease in maximum pletely deethylated product of RhB [16, 35, 36].
absorbance. Generally, a blue shift of λ
can be Figure 7 clearly showed that during the process of deg-
max
observed during oxidation process of RhB using radation, RhB has been photodegraded into different
BiVO [35] or BiOBr photocatalyst [16, 36], as is intermediates for 2D BiOBr and 3D BiOBr samples.
4
ascribed to progressive removal of N-ethyl groups In case of 2D BiOBr, λmax after irradiation for 60 min
during degradation of RhB (N,N,N',N'-tetraethyl is observed at 522 nm, which indicates the presence of
rhodamine). The previous studies have identified that N,N'-diethylrhodamine. For 3D BiOBr, the final deg-
the characteristic λ
for RhB and deethylated radation product (λ = 506 nm) of RhB is N-ethyl-
max
max
rhodamine species are 554 (RhB), 539 (N,N,N'-tri- rhodamine. In our study, the overall photocatalytic
ethylrhodamine), 522 (N,N'-diethylrhodamine), 510 degradation process of RhB in the observed time range
(
N-ethylrhodamine) and 498 nm (rhodamine), is the deethylation process of RhB (i.e., λmax shifts pro-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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No. 5
2018

TEMPLATE-FREE SOLVOTHERMAL SYNTHESIS
989
1
1
0
0
0
0
.2
.0
.8
.6
.4
1.2
BiOBr2D
BiOBr3D
λ
max
λ
max
origin
origin
0 min
5 min
15 min
25 min
35 min
554 nm
0
5
1
2
3
min
1.0
0.8
0.6
0.4
0.2
554 nm
554 nm
552 nm
546 nm
540 nm
534 nm
506 nm
554 nm
552 nm
549 nm
545 nm
537 nm
522 nm
min
5 min
5 min
5 min
60 min
60 min
.2
0
0
3
50
400
450
500
550
600
650 350
400
450
500
550
600
650
Wavelength, nm
Wavelength, nm
Fig. 7. Temperal spectral evolution during photocatalytic degradation of RhB over the prepared BiOBr samples.
gressively from 554 to 506 nm), and the mineralization
rate of RhB is far lower than the decolorization rate
(Fig. 8).
+
vb
−
holes (h ) can be generated in valence band and elec-
trons (ecb) can transfer into conduct band in BiOBr
crystal:
+
vb
−
cb
Photodegradation Mechanism
In order to determine the active species during the
BiOBr + hν → h + e .
(1)
However, the conduction band position of 3D BiOBr
calculated on the basis of E is +0.55 V vs. NHE,
photodegradation process of RhB over BiOBr micro-
g
spheres, various trapping agents of the reactive species which is more positive than the reduction potential of
were added to the reaction system. As shown in Fig. 9,
the RhB degradation was significantly suppressed after
• −
2
− •
2
O (E°(O /O ) = –0.33 V vs. NHE, E°(O /HO ) =
2
2
2
‒
0.55 V vs. NHE) [37, 38]. That is to say, the photo-
•
2
−
+
trapping O by the addition of BQ and h by EDTA, generated electrons cannot be reduced by oxygen. But,
•
respectively. When trapping OH with IPA, the photo-
degradation rate exhibited much weaker inhibiting
• −
2
the O radicals are the predominant oxidizing species
during RhB oxidation process, which maybe resulted
•
2
−
+
effect. Thus, the O radical and h were the main from the injected electron from the excitation of RhB
active species during the photodegradation process.
Here the plausible photodegradation mechanism is
proposed. After BiOBr is irradiated by visible light,
by visible light radiation:
dye + hν → dye*,
(2)
(3)
+
•
dye* + BiOBr → dye + BiOBr (e).
The injected electron reduces the surface-adsorbed O
2
•
−
60
50
40
30
20
10
0
molecules to generate O radicals:
2
•
2
−
1
2
BiOBr (e) + O → BiOBr + O .
(4)
2
On the other hand, the valence band position (3.35 V
vs. NHE) of 3D BiOBr illustrates that H O can be oxi-
2
•
dized to form OH:
+
•
+
3
4
h + H O → OH + H .
2
(5)
1
2
3
•
Nevertheless, OH is not the main oxidation species
owing to the inconsiderable deactivation in photodeg-
radation rate in the presence of IPA. Finally, RhB
molecules will be oxidized into some other small mol-
1
2
3
4
4
Blank
3DBiOBr
P25
• −
2
+
vb
ecule byproducts mainly by O or by h directly:
−
2
•
+
• −
2
Fig. 8. TOC removal rates (W) of RhB as a function of irra-
diation time ((1) 50, (2) 35, (3) 25, (4) 15 min) over 3D
BiOBr and P25.
O (h or O ) + dye
(6)
→ degraded or mineralized products.
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SHAOMING HONG et al.
1
0
0
0
0
.0
.8
.6
.4
Blank
BQ
IPA
EDTA
.2
dark light on
30
0
−
30
0
60
90
120
Irradiation time, min
Fig. 9. Degradation activities of the BiOBr microflowers for RhB with the addition of various trapping agents.
CONCLUSIONS
This work developed a simple and surfactant-free
approach for the synthesis of hierarchical flower-like
BiOBr microspheres via solvothermal method in the
presence of ethanol. The solvent after preparation can
be easily recycled by rotary evaporation, which is an
environmentally safe route in actual applications. The
6. D. Mao, X. Lü, Z. Jiang, J. Xie, X. Lu, W. Wei, and
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. K. Natarajan, H. C. Bajaj, and R. J. Tayade, J. Ind.
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. H. Li, J. Liu, T. Hu, N. Du, S. Song, and W. Hou,
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3
D BiOBr under visible-light irradiation in compari-
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2
−
Z. Chen, and M. Ye, Chem. Eng. J. 297, 139 (2016).
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1
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1
ACKNOWLEDGMENTS
1
6. B. Chai, H. Zhou, F. Zhang, X. Liao, and M. Ren,
This work was supported by the National Natural
Science Foundation of China (nos. 21577077,
Mater. Sci. Semicond. Process. 23, 151 (2014).
2
1677086, 21577078, and 21377067) and the Natural 17. A. C. Mera, H. Váldes, F. J. Jamett, and M. F. Melén-
drez, Solid State Sci. 65, 15 (2017).
Science Foundation for Innovation Group of Hubei
Province, China (no. 2015CFA021).
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