Improved photocatalytic performance of self-assembled Bi/BiOBr square microflowers with square nanopetals

Article abstract of DOI:10.1039/c5ra16571g

Herein we report the first example of the creation of Bi/BiOBr square microflowers with square nanopetals by a one-pot solvothermal process using ethylene glycol as both a solvent and a source of the reductant simultaneously. The square microflower material exhibited good photoelectric response and extremely high photodegradation efficiency for rhodamine B. We believe that this study represents an important advance regarding Bi-based inorganic materials.

Full text of DOI:10.1039/c5ra16571g

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DOI: 10.1039/C5RA16571G
Improved Photocatalytic Performance of SelfꢀAssembled Bi/BiOBr Square Microflowers
with Square Nanopetals†
Qing Shan Wanga, Le Xin Song*a, Yue Tengb, Juan Xiab, Li Zhaoa and Mao Mao Ruana
Received: August 17, 2015
5
Herein we report the first example of the creation of Bi/BiOBr square
microflowers with square nanopetals by a oneꢀpot solvothermal process
using ethylene glycol as both a solvent and a source of the reductant
simultaneously. The square microflower material exhibited good
photoelectric response and extremely high photodegradation efficiency
knowledge, this is the first example of a BBOB composite
constructed by a partial in situ reduction of BiOBr using EG as both a
solvent and a source of the reductant acetaldehyde. Interestingly, the
square microflower crystal represents
a very rare form of
65 microstructures. It is thus to be expected that this would attract much
attention particularly in Biꢀbased semiconductor materials.
10 for rhodamine B. We believe that this study represents an important
advance regarding Biꢀbased inorganic materials.
Fig. 1a shows the XRD pattern of the BBOBꢀ1. All of the diffraction
peaks are in perfect agreement with both the tetragonal phase of BiOBr
[JCPDS, 78ꢀ0348; space group P4/nmm (129); a = 3.923 Å and c = 8.105
70 Å]28 and the hexagonal phase of Bi [JCPDS, 85ꢀ1331; space group R m
(166); a = 4.533Å and c = 11.79Å].29 No impurity was found, and the
sharp diffraction lines indicate that the crystal has a high degree of
crystallinity.
The field emission scanning electron microscope (FEꢀSEM) of the
75 BBOBꢀ1 in Fig. 1b illustrates a large scale and uniform appearance with
almost monodisperse size distribution. The magnified image in Fig. 1c
shows that the particle looks like a square flower (side length, 4~5 ꢁm)
with square nanopetals (side length, 1~2 ꢁm; thickness, 60 nm). Fig. 1d
is the transmission electron microscopy (TEM) image of a single
80 microflower with a regular and clear square structure. The selectedꢀarea
electron diffraction (SAED, Fig. 1e) pattern taken from an edge of the
microflower demonstrated the singleꢀcrystallinity of the BiOBr. As can
be seen in the highꢀresolution TEM (HRꢀTEM, Fig. 1f) pattern from the
edge of the same microflower as the SAED pattern, there are two kinds
85 of lattice fringes with lattice spacings of 0.236 nm and 0.353 nm,
corresponding to the (104) planes of the hexagonal Bi and the (101)
planes of the tetragonal BiOBr, respectively. Thermogravimetry (TG,
Fig. S1) and Xꢀray photoelectron spectroscopy (XPS, Fig. S2) analyses
further confirmed that the molar ratio of Bi to BiOBr (MRBB) is about
90 1:10 in the square microflower material.
In order to elucidate the formation process of the squareꢀflowers, we
investigated the influence of reaction conditions: i) time (t, 3, 9, 12 and
24 h), ii) temperature (T, 363, 413, 433 and 453 K) and iii) initial
concentration ratio (ICR, 1:3, 1:2, 1:1 and 1:0) of Bi to Br. The three
95 series of composite materials consisted of Bi and BiOBr were obtained.
The results are illustrated in Fig. 2.
First, we noticed that all the three reaction parameters (t, T and ICR)
can be used to control the molar ratio of Bi to BiOBr based on the
diffraction intensity of Bi and BiOBr. As shown in Fig. 2, the content of
100 Bi in the composite was increased with the increase of t and T as well as
the decrease of ICR. In particular, only metal Bi (Biꢀ1, 0.137 g, 65.6%)
was formed in the absence of CTAB, and only BiOBr (BiOBrꢀ1, 0.294 g,
96.4%) was observed when either the t was decreased to 3 h or the T was
decreased to 363 K (BiOBrꢀ2, 0.296 g, 97.1%). These observations
105 clearly reveal that BiOBr was firstly produced and subsequently further
reduced to Bi, as described in Eqs. 1~4.
Semiconductor materials of metal oxides have attracted great attention
due to their unique photocatalysis.1,2 Conventional metal oxide
15 photocatalysts, such as TiO2 and ZnO, are limited by their wide band
gaps (3.0 eV for rutile, 3.2 eV for anatase and 3.3 eV for ZnO),3−5 only
responding to UV light. Therefore, it is of great significance to develop
visibleꢀlightꢀexcited photocatalysts. It has been revealed that valence
orbitals of some metal ions with nd10 or (n−1)d10ns2 valence electronic
20 configurations, such as Ag(I) 4d10, Sn(II) 4d105s2 and especially Bi(III)
5d106s2, can hybridize with Oꢀ2p orbitals to form a hybridized valence
band,6,7 and thereby to allow oxide semiconductors of the ions to display
a certain absorption in visible light range.
Biꢀbased semiconductors especially bismuth oxyhalides (BiOX, X=
25 Cl~I) are of immense importance for their outstanding optical, electrical
properties and high stability against photocorrosion and photochemical
properties.8−11 It has been reported that BiOBr has an indirectꢀtransition
bandꢀgap which favors to reduce the recombination of photoinduced
electrons and holes,9,10 and that its photoꢀexcited activity is dependent on
30 surface atomic structures, grain sizes and especially morphologies.11,12
Thus, various kinds of morphologies of BiOBr nanoꢀ or microꢀstructures
including sheets, flowers and spheres have been fabricated,13−15 and
various methods were adopted to couple BiOBr with other species, such
as C3N4, ZnFe2O4, Bi2WO4, Pt, Au and Ag,16−23 in order to increase the
35 conversion efficiency of incident photons to electrons for further
improving its photocurrents.24,25 Recently, a few works have reported a
significant enhancement of photocatalytic efficiency due to a composite
of BiOBr with Bi.26,27 However, the Bi/BiOBr (BBOB) composites still
have some problems such as relatively large grain sizes and nonuniform
40 grainꢀsize distributions, irregular shapes, diverse surfaces, and especially
residues from chemical syntheses.
Taken together, these observations encourage us to develop a useful
method coupling Bi with BiOBr to create a novel microstructure for
enhancing photocatalytic efficiency. This is the aim of this paper. To
45 achieve this goal, we adopted a simple oneꢀpot solvothermal synthesis
involving ethylene glycol (EG, 30 mL) as solvent, Bi(NO3)3·5H2O
(0.484 g, 1 mmol) as Bi source, and cetyltrimethylammonium bromide
(CTAB, 0.729 g, 2 mmol) as Br source using a sealed Teflonꢀlined
stainless steel autoclave (50 mL) at 433 K for 12 h. Our results indicated
50 that a grey selfꢀassembled BBOBꢀ1 square microflower crystal with
square nanopetals having highly uniform dimensions was successfully
constructed (Fig. 1), and the square microflowers presented high
photocatalytic selectivity for a group of dyes: rhodamine B (RhB),
methylene blue (MB) and methyl orange (MO). To the best of our
In Eq. 1, Bi(NO3)3 was hydrolyzed into BiONO3 in the presence of
water from the hydrate of the Bi(III) salt.30 Then, the BiONO3 was
reacted with CTAB to generate BiOBr (Eq. 2).31 Meanwhile, some of
110 EG molecules were reduced into acetaldehyde (Eq. 3) at a high
temperature (above 413 K),32 leading to the reduction of the BiOBr
and/or BiONO3 to Bi (Eqs. 4 and 5).33 The HRꢀTEM image in Fig. 1f
clearly shows a tight combination of Bi and BiOBr on the surface of the
BBOBꢀ1, suggesting that the metal Bi came from the in situ reduction of
55 aDepartment of Chemistry, University of Science and Technology of China, Jin
Zhai Road 96, Hefei 230026, P. R. China
Eꢀmail: solexin@ustc.edu.cn
bCAS Key Laboratory of Materials for Energy Conversion, Department of
Materials Science and Engineering, University of Science and Technology of
60 China, Jin Zhai Road 96, Hefei 230026, P. R. China
115

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Fig. 1. The XRD pattern (a), FEꢀSEM images (b and c), TEM image (d), SAED pattern (e) and HRꢀTEM image (f) of the BBOBꢀ1.
BiOBr.26, 34ꢀ36 It is worth noting that only metal Bi can be found in the
intensities between the cationic surface active ions from CTAB and the
5
absence of CTAB, implying complete reduction of BiONO3 to Bi. In 40 composites, thereby creating different appearances. Finally, we noted
other words, BiONO3 is an unstable state in EG solvent, having a lower
stability than BiOBr in the light of the composition of the several series
composites (see Fig. 2). Therefore, the MRBB in the composites mainly
depends on a transition from BiOBr to Bi, the latter being a stable state in
10 EG at such a temperature. Therefore, the longer t, the higher T, and the
lower ICR, the greater the proportion of Bi in the composites. This
feature is very significant because it proves that the composition of a
metal and its oxyhalide in their composites can be easily modulated by
adjusting the three reaction parameters.
1)
2)
3)
4)
5)
Bi(NO3)3 + H2O
BiONO3 + C16H33(CH3)3NBr
EG
BiONO3 + 2HNO3
BiOBr + C16H33(CH3)3NNO3
CH3CHO + H2O
3CH3CHO + 2BiOBr + H2O
3CH3CHO + 2BiONO3 + H2O
3CH3COOH + 2Bi + 2HBr
3CH3COOH + 2Bi + 2HNO3
15
Second, we observed that the reaction parameters could produce
completely different effects on the surface structure of the composites
according to the SEM images in Fig. 3. Initially, it can be clearly seen
that the t plays an important role in regulating the selfꢀassembly of small
Fig. 2. Partial XRD patterns of the BiOBr, Bi/BiOBr composites and Bi obtained
at different reaction conditions: t (the first row), T (the second row) and ICR (the
45 third row). The relative amount of Bi in the composites was roughly estimated by
the relative intensity of the characteristic diffraction peak of Bi at 27.2°, which
was denoted by the red arrows. Entire XRD patterns in the range of 10~80° were
shown in Figs. S3ꢀ11.
20 nanoplates. For example, a spontaneous structural transformation occurs
from a compressed tetragonal prism structure for 3 (BiOBrꢀ1) and 9 h
(BBOBꢀ2), to a square flower structure of the BBOBꢀ1 associated with
nanoplates (Fig. 1) for 12 h, and to a multilayer square plate structure for
24 h (BBOBꢀ3). More importantly, the content of metallic Bi in the
25 composites is closely related with the t since the MRBB increases in the
order: BBOBꢀ2 (1:18, Fig. S12) < BBOBꢀ1 (1:10) < BBOBꢀ3 (1:7, Fig.
S13). These results signify that both the shape and the composition of the
grains can be easily controlled by adjusting the t. In this way, the
structure transformation could be envisaged as a ′′grain coarsening′′
30 process, in which the formation, composition and structure of the square
flowers is regulated. Further, by comparing the shape of the composites
obtained using different ICRs, we noticed that CTAB can act as a
morphologyꢀdirecting agent facilitating selfꢀassembly of the
nanoparticles (ICR, 1:3) to generate the square flowers (ICR, 1:2),
35 microspheres (ICR, 1:1) and irregular particles (ICR, 1:0), as seen from
Fig. 3. We suggest that the role of CTAB may be associated with the
formation of surface active ionic liquids in EG solvent. Different
concentrations of CTAB are likely to lead to different interaction
50 Fig. 3. SEM images of the BiOBr, Bi/BiOBr and Bi materials obtained at
different reaction conditions: t (the red arrow), T (the blue arrow) and ICR (the
green arrow). Large scale SEM images were shown in Figs. S3ꢀ11
.

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DOI: 10.1039/C5RA16571G
a
b
c
d
e
f
no catalyst
Bi-1
BiOBr-1
BBOB-3
BBOB-2
BBOB-1
99.1%
100
A
B
C
1.6
1.2
0.8
0.4
0.0
1.2
0.9
0.6
0.3
0.0
69.5%
2.0
1.5
1.0
0.5
0.0
ξ
50
27.2%
a
f
0
i
j
MO MB RhB
g
h
300
400
500
600
300
450
600
750
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Fig. 4. (A) UV–Vis absorption spectra of the RhB solutions (10 mg·L−1) after being treated by the BBOBꢀ1 (20 mg) after 0 (a), 25 (b), 50 (c), 75 (d), 100 (e) and 125 (f)
min of sunlight irradiation. The inset shows a colour change of the solutions from a to f; (B) UV–vis absorption spectra of the MO solutions (10 mg·L−1) after being treated
by the BBOBꢀ1 (20 mg) after 0 (g) and 125 min (h) and the MB solutions (10 mg·L−1) after being treated by the BBOBꢀ1 (20 mg) after 0 (i) and 125 min (j) of sunlight
irradiation. The inset shows the ξ of MO, MB and RhB at the same conditions (concentration: 10 mg·L−1; the amount of BBOBꢀ1: 20 mg; sunlight irradiation time: 125
min); (C) UV–Vis absorption spectra of the RhB solutions (10 mg·L−1) after being treated by the catalysts (20 mg) after 125 min of sunlight irradiation.
5
that the equilibrium of such a form based on the interaction type could be 50 degradation process of the series solutions, but also provide information
easily disturbed even at either slightly higher (453 K) or lower (363 and
10 413 K) T, resulting in the generation of irregular surfaces.
about the complete photodegradation of RhB in the presence of the
BBOBꢀ1 catalyst. The photodegradation degree (ξ, %) of RhB was
determined based on Eq. 6.41
The surface area and porous structure of the BBOBꢀ1 were studied by
gas adsorption/desorption measurements in liquid nitrogen. The sorption
isotherm (Fig. S14) exhibits a similar type IV pattern37 with a small
hysteresis loop at a relative pressure range of 0.43~0.96, implying some
15 mesoporous characteristics. Also, our data show that the BBOBꢀ1 has a
Brunauer–Emmett–Teller (BET)38 specific surface area of 9.75 m2·gꢀ1
and an average pore diameter of 15.9 nm. The UVꢀvis diffuse reflectance
spectroscopy (Fig. S15) was used to examine the electronic structure of
this material. Its optical bandgap was determined to be 2.44 eV (Fig. S16)
20 in terms of the Kubelka–Munk function,39 which is the same as that of
Sn/Bi/BiOBr,26 but much lower than those obtained for BiOBr10,14 and
especially for Bi/BiOCl34,35 (Table S1). It indicates that the presence of
Bi may be used as a means to reduce the bandgap of BiOBr, thereby
giving an absorption in a large range of wavelengths.
To demonstrate this, a timeꢀdependent photocurrent response was
performed (Fig. S17). It was shown that upon illumination the
photocurrent of the square microflower material rapidly increases to a
maximum of 893.9 nA (light current) and keeps unchanged with the
increasing time. Moreover, when the light is turn off, the current
30 immediately decreases to a minimum of 142.1 nA (dark current). Such a
large light current, which is much higher than that of Bi/BiOCl,34 has
been achieved under a low bias voltage of 0.5 V, suggesting a low
recombination rate of electrons and holes. This high ratio of light current
to dark current (6.3) is an indication of an effective photoelectron
35 transfer. Additionally, the photocurrent response is reversible and stable
under successive cycling, signifying that the photocurrent could easily
be reproduced for each irradiation.
ξ
=
[(C0- C)/C0] × 100%
6)
55
In this equation, C0 and C is the initial concentration of RhB and its
concentration after irradiation, respectively. Our data indicate that the ξ
is 99.1% after 125 min of irradiation in the presence of the BBOBꢀ1, and
especially it can still reach 96.9% over the first five cycles (Fig. S18).
Moreover, recycling experiments reveal that the catalyst exhibits a good
60 structural stability because no change in crystal structure and surface
morphology was observed after consecutive cycles (Figs. S19 and 20).
Thus these observations give us a strong indication that the BBOBꢀ1 has
high photocatalytic activity, stability and sustainability for the
degradation of RhB. The present results are comparable to the best and
65 most recent results obtained from synthesized materials.14,26.
Further, we found that a low ξ (69.5%) for MB and a very low ξ
(27.2%) for MO were obtained when RhB was replaced by MB and MO
after 125 min of irradiation (Fig. 4B). It implies an exceptionally high
selectivity of the catalyst for the dyes. This is a very significant and
70 interesting phenomenon and is extremely important in the structural
recognition of different organic dyes. Of these pollutants, RhB and MB
are cationic dyes,42,43 whereas MO is an anionic dye.44 Therefore, the
catalytic selectivity toward cationic dyes probably owing to favorable
electrostatic interactions. This should be a clear indication that the
75 surface of the BBOBꢀ1 square flowers present partially negative charges.
Finally, we compared the catalytic activities of free Bi and BiOBr
with those of the BBOB materials under the same photocatalytic
conditions. As shown in Fig. 4C, there are two important features to
note.
25
The large surface area, high porosity and related mesoporous
microarchitectures, combined with the wellꢀorganized arrays of highly
40 monodisperse microparticles with a relative low bandgap, should
considerably improve the photocatalytic performance of BiOBr.
Fig. 4A indicates the UV–Vis absorption spectra of the RhB solutions
(10 mg·L−1) after being treated by the BBOBꢀ1 (20 mg) under the natural
sunlight irradiation. It is obvious that the characteristic maximum
45 absorption band of RhB at 554 nm40 was significantly decreased with the
increase of irradiation time. Most importantly, this band almost
completely disappeared after 125 min of irradiation. Furthermore, we
have also observed a gradual colour change from purple to colourless by
a photograph (the inset of this figure.). These results not only reflect the
80
First, the catalytic activities of both the BBOBꢀ2 tetragonal prisms (ξ,
84.7%) and the BBOBꢀ3 multilayer square plates (ξ, 80.6%) are much
lower than that of the BBOBꢀ1 square flowers for the photodegradation
of RhB, with an increase order BBOBꢀ3 < BBOBꢀ2 < BBOBꢀ1. The
most probable explanation is that the result is associated with the
85 composition and surface structure of the materials. A moderate ratio of
Bi to BiOBr in the materials would be beneficial to charge separation
and transfer, which have an advantageous effect on the photocatalytic
property. This result is in agreement with the report of other studies.36,45
Further, our data demonstrated that the BET specific surface areas of the
90 BBOBꢀ2 (8.07 m2·gꢀ1, Fig. S21) and BBOBꢀ3 (5.12 m2·gꢀ1, Fig. S22) are

RSC Advances
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smaller than that of the BBOBꢀ1, having the same order as the catalytic
coordination sites exposed to the ambient and an easier transfer of
activities. The larger surface area may be one reason that the BBOBꢀ1 55 electrons and holes. These may be why the square flower material
gives a stronger photocurrent response (Fig. S23) and a higher visible
absorption (Fig. S15). For instance, the ratios of light current to dark
current and the optical bandgaps of the BBOBꢀ1 and BBOBꢀ2 are 6.3
and 3.7, and 2.44 and 2.62 eV (Fig. S16), respectively. It is worthy of
note that the BBOBꢀ3 exhibits a very low ratio (1.8) of light current to
dark current, and its optical bandgap could not be determined. These
results allow us to conclude that the relatively large surface area plays a
exhibits improved performance in photocatalytic degradation of dyes.49
+
)
hv
O2
h+
+ e
7)
8)
BiOBr
e
+
+
+
+
BiOBr
O2
(
h
5
RhB+
RhB
O2
9)
RhB+
10)
CO2 + H2O
10 crucial role in improving photocurrent responses and lowering bandgaps
(leading to a lower Schottky barrier height and an output current
increase)46 in the photoelectrochemical process, thereby enhancing
catalytic efficiency.
In summary,
a novel monodisperse Bi/BiOBr square flower
60 microstructure was successfully fabricated through a facile oneꢀstep
solvothermal route utilized EG as both a solvent and a source of the
reductant simultaneously. Importantly, the composition and structure of
Bi and BiOBr in the composites can be readily tuned by adjusting any
one of the reaction parameters: t, T and ICR. Further, the square flower
65 material with a relatively large surface area and a moderate MRBB
seems to exhibit an improved photocatalytic activity for RhB. Overall,
this work represents an important step toward the structural functional
characterization of Biꢀbased semiconductor materials and the
development of controllable synthesis of micro/nanostructures.
70
Second, both Biꢀ1 and BiOBrꢀ1 exhibit much lower catalytic activities
15 than the BBOB materials for the photodegradation of RhB at the same
conditions. For instance, the values of ξ in the cases of the Biꢀ1 and
BiOBrꢀ1 are 20.9 and 70.9%, respectively, as seen from Fig. 4C. This
result strongly supports the suggestion that such a composite between Bi
and BiOBr has greatly improved the catalytic performance of BiOBr
20 since metallic Bi only presents a very low catalytic ability. Similar
phenomena have been observed in several recent studies.26, 27, 34ꢀ36.
Additionally, a series of trapping experiments were carried out to
demonstrate the active species during the photocatalytic process (Fig.
S24). Our results implied that the holes (h+) and superoxide radical ions
Acknowledgements
−
This project was supported by NSFC (No. 21071139) and Natural
25 (·O2 ) are the main active species for the BBOBꢀ1 to degrade RhB in
Science Foundation of Anhui Province (No. 1508085MB30).
aqueous solution under visible light irradiation.
75
Here, we would like to present a possible degradation process of the
dyes to explain the improved photocatalytic activity in terms of our data.
As illustrated in Fig. 5, under visible light irradiation, the BiOBr in the
30 composite was photoexcited to generate electrons in the conduction band
(CB) and h+ in the valence band (VB, Eq. 7). A longꢀdistance
intervalence electron transfer can occur from the excited BiOBr to Bi,26
thereby causing electron accumulation at Bi interfaces. This process is
beneficial to the separation of electronꢀhole pairs. Highly active oxidants
35 such as ꢂO2− were produced from oxygen by the photoꢀinduced electrons
on the Bi (Eq. 8). The dye molecules were first reacted with h+ to
generate corresponding cation radicals (like ·RhB+, Eq. 9). Then, the
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Fig. 5. Possible Schematic illustration of the photocatalytic selectivity and
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5
Graphical Abstract
10
Improved Photocatalytic Performance of SelfꢀAssembled Bi/BiOBr Square Microflowers
with Square Nanopetals
Qing Shan Wang, Le Xin Song, Yue Teng, Juan Xia, Li Zhao and Mao Mao Ruan
15
The Bi/BiOBr square microflowers with square nanopetals were successfully constructed by a simple and facile solvothermal route
using ethylene glycol as both a solvent and a source of reductant acetaldehyde simultaneously. The square microflower material
exhibited good photoelectric response and extremely high photodecomposition efficiency for rhodamine B. We believe that this study
represents an important advance regarding Biꢀbased inorganic materials.
20