Synthesis of Bi2O3 architectures in DMF-H 2O solution by precipitation method and their photocatalytic activity

Article abstract of DOI:10.1016/j.jallcom.2014.06.011

Well-crystalline flowerlike α-Bi₂O₃ hierarchical architectures with pineapple-shaped petals have been synthesized by precipitation method at a volume ratio of DMF/H₂O of 5, where DMF and H₂O were used to dissolve Bi(NO₃)₃ and KOH, respectively. If the DMF/H₂O ratio was decreased to 2:1, 1:1 and 0:30, flower-, bundle- and dendrite-shaped α-Bi₂O₃ microcrystallites aggregated by nanorods were formed, respectively. The simple synthetic route and thus obtained Bi₂O₃ architectures of various morphologies provide a basis insight for their formation mechanism. The photocatalytic activity of the as-prepared Bi₂O₃ particles for degradation of Rhodamine B (RhB) under visible-light irradiation was obviously influenced by their morphologies. Bi₂O₃ of nanorod-based microstructures exhibited higher photodegradation activity than nanobrick-based ones, owing to higher light absorption and carrier separation efficiency in one-dimensional (1D) nanostructured materials.

Full text of DOI:10.1016/j.jallcom.2014.06.011

Accepted Manuscript
Synthesis of Bi2O3 architectures in DMF-H2O solution by precipitation method
and their photocatalytic activity
Li-Li Yang, Qiao-Feng Han, Jin Zhao, Jun-Wu Zhu, Xin Wang, Wei-Hua Ma
PII:
S0925-8388(14)01345-0
DOI:
http://dx.doi.org/10.1016/j.jallcom.2014.06.011
JALCOM 31427
Reference:
Journal of Alloys and Compounds
To appear in:
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Accepted Date:
9 March 2014
31 May 2014
2 June 2014
Please cite this article as: L-L. Yang, Q-F. Han, J. Zhao, J-W. Zhu, X. Wang, W-H. Ma, Synthesis of Bi2O3
architectures in DMF-H2O solution by precipitation method and their photocatalytic activity, Journal of Alloys and
Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.06.011
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Synthesis of Bi2O3 architectures in DMF-H2O solution by
precipitation method and their photocatalytic activity
Li-Li Yang, Qiao-Feng Han∗, Jin Zhao, Jun-Wu Zhu, Xin Wang*, Wei-Hua Ma
Key Laboratory for Soft Chemistry and Functional Materials, Ministry of Education,
Nanjing University of Science and Technology, Nanjing 210094, China
ABSTRACT
Well-crystalline flowerlike α-Bi2O3 hierarchical architectures with pineapple-shaped
petals have been synthesized by precipitation method at a volume ratio of DMF/H2O of 5,
where DMF and H2O were used to dissolve Bi(NO3)3 and KOH, respectively. If the
DMF/H2O ratio was decreased to 2:1, 1:1 and 0:30, flower-, bundle- and dendrite-shaped
α-Bi2O3 microcrystallites aggregated by nanorods were formed, respectively. The simple
synthetic route and thus obtained Bi2O3 architectures of various morphologies provide a
basis insight for their formation mechanism. The photocatalytic activity of the as-prepared
Bi2O3 particles for degradation of Rhodamine B (RhB) under visible-light irradiation was
obviously influenced by their morphologies. Bi2O3 of nanorod-based microstructures
exhibited higher photodegradation activity than nanobrick-based ones, owing to higher
light absorption and carrier separation efficiency in one-dimensional (1D) nanostructured
materials.
Keywords: Bi2O3; Precipitation method; Hierarchical architecture; Photodegradation
1. Introduction
* Corresponding authors. Tel. / fax: +86-25-84315054. E-mail: hanqiaofeng@njust.edu.cn; wangx@njust.edu.cn.
1

Bismuth oxide (Bi2O3), a p-type semiconductor, is widely used as solid electrolyte in
gas sensors [1-3], solid fuel cells [4-7], and photocatalysts [8-12]. As comparison to their
bulk counterparts, nanostructured Bi2O3 have attracted more research interest due to the
high surface area, exposed active lattice facets and thus excellent properties. For example,
nanostructured Bi2O3 is considered as a good visible light photocatalyst candidate due to its
narrow band gap (about 2.8 eV) and deep valence band comparable to that of TiO2 [9, 13].
More recently, nanostructured Bi2O3 has been used as composite photocatalysts and
co-catalysts because Bi2O3 alone shows a low photocatalytic activity resulting from its
more positive potential of the conduction band edge for the single-electron reduction of
−
oxygen (Ecb = 0.33 V; O2 + e− = O2 , E = −0.064 V vs SHE) [14-16].
The design and synthesis of the three dimensional (3D) hierarchical architectures by
self-assembly of nanostructured building components have received considerable attention
in nanoscience and nanotechnology owing to shaped-dependent physical and chemical
properties. Various Bi2O3 hierarchical architectures have been successfully synthesized.
Zhou et al. prepared the Bi2O3 hierarchitectures composed of 2D sheets via a template-free
-
aqueous method in the presence of VO3 at 60-80 ℃, which showed much higher
photocatalytic activity over commercial Bi2O3 under visible light irradiation [8].
Hierarchical rippled Bi2O3 nanobelts were synthesized by an electrodeposition route and
possessed higher specific capacitance and electrochemical stability as supercapacitor
electrodes than nanobelts with smooth surface [17]. α-Bi2O3 microcrystals and microrods
with pyramidal tips have been grown by a vapour-solid method using pure Bi as starting
material [18]. Yu et al synthesized flower-like Bi2O3 superstructures with excellent
2

visible-light-driven photocatalytic activity for degradation of RhB via a hydrothermal
process and subsequent calcination [19]. 3D Bi2O3 hierarchical architectures assembled by
2D nanoplates were fabricated from electrodeposited bismuth (Bi) film, which exhibited a
strong and wide photoluminescence signal [20]. More recently, Wang et al prepared 3D
hierarchical flowerlike Bi2O3 microstructures composed of decahedrons and rods using a
solution precipitation method with the assistance of glycerin [21]. Bouquet-like hierarchical
Bi2O3 photocatalysts with high-density surface oxygen vacancy have been synthesized via
a hydrothermal method by the synergetic control of NaOH and a polyvinyl alcohol (PVA)
stabilizer [22].
Although various methods have been successfully used to synthesize Bi2O3 hierarchical
architectures, the challenge has been remained to explore a simple synthetic route and
understand formation mechanism to achieve the control. Room-temperature precipitation
method is easy and economic for the preparation of Bi2O3 nanoparticles [23, 24]. However,
acid (HNO3 or HCl) was usually used to inhibit hydrolysis of Bi(NO3)3 in order to control
uniform precipitation, so extremely excess NaOH was addded to neutralize the acid and
obtain pure Bi2O3. Additionally, to the best of our knowledge, there is little report on the
preparation of Bi2O3 hierarchitectures via a room precipitation method. Herein, by using
DMF as a solvent to dissolve Bi(NO3)3, pure Bi2O3 microcrystallites in flowerlike shape
assembled by nanobrick-based petals with pineapple surface were easily obtained at room
temperature after 1 h of reaction. Moreover, the Bi2O3 crystals of different shapes including
bundles and dendrites were obtained by varying the ratio of DMF/H2O. The corresponding
mechanism for the formation of various Bi2O3 architectures was proposed. Finally, the
3

influence of Bi2O3 in various shapes on the photocatalytic activity for degradation of RhB
under visible-light irradiation was investigated.
2. Experimental
A homogeneous solution of Bi(NO3)3·5H2O (0.97 g, 2 mmol) in 25 ml DMF was added
into saturated KOH/H2O solution (5 ml distilled water, 5.0 g (89 mmol) KOH), and the
solution changed pale yellow at once. The mixture was stirred for 3 min, and then left still
and aging for about 1 h. The product was collected by filtration, washing several times with
deionized water and ethanol, and drying at 60 ℃ for 12 h. No additional thermal treatment
was needed.
The effect of the DMF/H2O ratio on Bi2O3 morphologies was investigated. The detailed
experimental results are listed in Table 1.
Table 1. DMF/H2O ratio, morphologies, band gap energy and apparent rate constant k of RhB
photodegradation.
Sample
A1
DMF/ml
H2O/ml
Morphology
Bandgap/eV
2.70
k/h-1
0.16
25
20
15
0
5
brick-based flowers
Rod-based flowers
Rod-based bundles
Rod-based dendrites
10
15
30
2.82
0.38
0.53
0.63
A2
2.84
A3
2.88
A4
(* [KOH]/[Bi] = 89/2)
4

The X-ray diffraction patterns (XRD) were obtained on a Bruker D8 Advance X-ray
diffractometer (Cu Kα radiation, λ = 1.5418 Å) at a scan rate of 0.05° 2θ s-1. Transmission
electron microscopy (TEM) and high-resolution TEM (HRTEM) were carried out on a
JEM-2100 (JEOL) and FEI Tecnai T20 microscope equipped with an X-ray energy
dispersive spectrometer (EDS), respectively. Field-emission scanning electron microscopy
(FESEM) was preformed on a HITACHI S-4800 microscope. A Quanta Chrome Nova 1000
analyzer was employed to measure the Brunauer-Emmett-Teller (BET) surface areas of the
samples at liquid nitrogen temperature. X-ray photoelectron spectroscopy (XPS)
measurements were carried out on a PHI Quantera Ⅱ SXM X-ray photoelectron
spectrometer with monochromatized Al Kα as the exciting source, and the results obtained
in the XPS analysis were corrected by referencing the C 1s line to 284.8 eV. The UV-Vis
diffuse reflectance spectra (DRS) of the products were determined on a shimadzu UV 2550
spectrophotometer equipped with an integrating sphere, using BaSO4 as a reference.
The photocatalytic activity of the Bi2O3 microcrystallites was evaluated by Rhodamine
B (RhB) degradation. The Bi2O3 powders (0.05 g) were dispersed in 50 ml RhB aqueous
solution (5 ppm) under continuous stirring, allowing 1 h in the dark to reach the
adsorption-desorption equilibrium between RhB and Bi2O3 particles. Visible light
irradiation using a xenon lamp (500 W) was applied to the suspension with continuous
stirring. At a given time interval, about 5 ml of the suspension was taken out, the catalyst
powders were removed by centrifugation, and residual concentration of the dye in the
solution was obtained by using a UV-Vis spectrophotometer (BRAIC UV 1201).
5

3. Results and discussion
The purity of the product was strongly dependent on the ratio of [KOH]/[Bi]. When the
[KOH]/[Bi] ratio was 89:2, phase pure α-Bi2O3 was obtained. As displayed in Fig. 1a,
XRD pattern of the Bi2O3 microcrystallite obtained at DMF/H2O ratio of 5:1 (A1) can be
indexed to monoclinic α-Bi2O3 (JCPDS No. 41-1449). No peaks of impurities were
detected, revealing the high purity of the product. The reflection peaks are strong and sharp,
which indicates that as-prepared Bi2O3 was well crystalline. The XRD patterns of the
samples A2-A4 obtained at various DMF/H2O ratios also correspond to pure α-Bi2O3 phase
(Supporting Information Fig. S1). If the [KOH]/[Bi] ratio was decreased to 71:2, the XRD
pattern revealed the presence of the impurity BiONO3 (Fig. 1b), which suggests that
sufficiently high ratio of [KOH]/[Bi] was necessary for the formation of phase pure Bi2O3,
different from the result reported by Wu et al [23].
BiONO
*
3
a
*
b *
20
30
40
50
60
A
20
30
40
Two theta (degrees)
50
60
Fig. 1. XRD patterns of the as-prepared Bi2O3 microcrystallites: (a) [KOH]/Bi = 89/2 (A1), (b)
[KOH]/[Bi] = 71/2. Vertical sticks below the patterns represent the standard diffraction data from JCPDS
6

file for bulk Bi2O3 with monoclinic structure (No. 41-1449).
A representative SEM image of the product A1 is shown in Fig. 2a. The flower-like
Bi2O3 hierarchical architectures contain multi-petals. These petals are aligned
perpendicularly to the flower surface with clearly orientation, pointing toward a common
center. The surfaces of petals were rough, analogous to pineapples. A high-magnification
SEM image indicates that the pineapple-shaped petals were composed of nanobricks
attached along certain direction, forming 1D structured petal (Fig. 2b). A TEM image of
A1 further confirms that the Bi2O3 microcrystallites were made up of several individual
petals with the length of about 5 µm and diameter of 2-3 µm (Fig. 2c), and the edges of the
petals look like zigzag stacks of nanobricks (inset in Fig. 2c). These nanobricks were too
thick (∼ 200 nm) to perform HRTEM studies.
The morphologies of Bi2O3 microcrystallites were obviously influenced by the ratio of
DMF/H2O. As shown in Fig. 2d, the flower-shaped Bi2O3 crystals (A2) made up of
microrods with a diameter of about 1 µm were formed when the DMF/H2O ratio was
decreased to 2. With the amount of water increasing further (V DMF/H2O = 1), the rod-based
Bi2O3 bundles (A3) with a rod size of about 200 nm in diameter and 20 to 40 µm in length
were observed (inset in Fig. 2d). If using pure H2O as solvent, typical TEM images indicate
that the dendrite-shaped Bi2O3 microstructures (A4) consisted of dozens of nanorods with a
diameter of about 80 nm extending radially from one root (Fig. 2e and its inset). An
HRTEM image of a typical nanorod shows that the obtained Bi2O3 dendrites are highly
crystalline (Fig. 2f). The measured lattices distance with d-spacings of 0.528, 0.331 and
7

Fig. 2. SEM and TEM images of the as-prepared Bi2O3: (a-c) A1, and the inset in (c) is a high-resolution
TEM image of an individual petal; (d) A2, and the inset is a TEM image of A3; (e) A4, and the inset
showing its nanorod-like building units; (f) An HRTEM image of an individual from A4, and the upper
8

left and lower right insets are its SAED and FFTs patterns, respectively.
0.276 nm corresponds to (011), (111), and ( 211) planes of α-Bi2O3, respectively. The
selected area electron diffraction (SAED) patterns could be indexed to monoclinic structure
with zone axis of [01 1] (upper left inset in Fig. 2f), which is consistent with the hexagonal
lattice in the FFT diffractograms (lower right inset in Fig. 2f). The calculated zone axis and
the fringes in the HRTEM image indicate that the Bi2O3 nanorods grow along the [001]
orientation.
A time-dependent experiment was conducted to track the morphological evolution of
the nanobrick-based flowerlike Bi2O3 microcrystallites (A1). When Bi(NO3)3/DMF
solution was mixed with saturated KOH/H2O solution, pale yellow Bi2O3 precipitation
merged at once, indicating that precipitation reaction was very fast and a large amount of
Bi2O3 nuclei were produced in a short time. TEM image of the product with 10 seconds of
reaction indicates that Bi2O3 nuclei grew into bricks and a small amount of flower-like
structures were formed (Supporting Information Fig. S2a). After 3 min of reaction, a large
amount of the Bi2O3 flower buds were observed (Fig. 3a). XRD pattern shows that the
flower buds are well crystalline and all the diffraction peaks can be indexed to the
monoclinic α-Bi2O3 (Supporting Information Fig. S3a). When the reaction time was
prolonged to 30 min, the flower petals grew longer (Supporting Information Fig. S4a). We
proposed that 1D aggregation of nanobricks should be owing to oriented attachment
mechanism, in which attachment occurs by sharing the same crystallographic orientation,
analogous to the fusion of high active facets, as displayed in Scheme 1A. The shape and
9

Fig. 3. (a)-(d) SEM and TEM images of Bi2O3 prepared after 3 min of reaction at various DMF/H2O
ratio: (a) 5:1; (b) 2:1, and the inset showing the presence of flower-like morphology; (c) 1:1; (d) 0:30,
and the inset is a high-magnification TEM image. TEM images of Bi2O3 crystals obtained at DMF/H2O
10

ratio of 5: (e) with 1 mmol of Bi(NO3)3, and the inset showing crossed petals; (f) using NaOH instead of
KOH.
size of Bi2O3 crystals did not show evident changes even when the ageing time was
extended to 24 h (Supporting Information Fig. S4b), which suggests that Ostwald ripening
did not occur in this case. The absence of Ostwald ripening means that the current
precipitation was not a kinetically controlled reaction but a thermodynamically controlled
one [23, 25, 26].
The effect of ageing time on morphology of the products (A2-A4) has also been studied.
After Bi(NO3)3/DMF solution was mixed with KOH/H2O solution at a DMF/H2O ratio of 2
and 1 (A2 and A3) for 10 seconds, white BiONO3 from the hydrolysis of Bi(NO3)3
suspended in the solution except for yellow Bi2O3 precipitates. TEM images of the yellow
products show a small amount of rod-based flowers and bundles together with individual
rods (Supporting Information Fig. S2b and c). While, yellow precipitates did not appear in
one minute without the addition of DMF (A4), which suggests that the reaction between
BiONO3 and KOH was a little slower than that of Bi(NO3)3/DMF and KOH/H2O. Some
dendritic structures could be observed 1 min later (Supporting Information Fig. S2d). After
reacted for 3 min, a considerable amount of rod-based Bi2O3 flowers, bundles and dendrites
were formed (Fig. 3b-d). The particles in light yellow together with white revealed that the
products were not pure. XRD patterns show that the intensity of diffraction peaks of
BiONO3 was increased with the H2O amount increasing (Supporting Information Fig. S3).
When the H2O amount was increased, some Bi(NO3)3 may strongly hydrolyze into
11

BiONO3 except that those reacted directly with KOH to generate Bi2O3. BiONO3 would
dissolve and re-crystallize into Bi2O3 due to high KOH concentration. These
newly-generated Bi2O3 may nucleate heterogeneously attaching to the original Bi2O3 nuclei
from the reaction of Bi(NO3)3/DMF and KOH, and grew along preferred direction, forming
Bi2O3 rod-based microstructures due to its strong 1D growth habit [23]. The nucleation and
growth processes at the beginning stage are morphology-determinant. The rod-based Bi2O3
architectures presented various morphologies including flower (A2), bundle (A3) and
dendrite (A4), should be owing to the competition between nucleation and growth rate
from various DMF/H2O ratios. With the reaction time extending longer, no obvious change
in shapes was observed except that the size of the rods became bigger. The possible
formation processes of Bi2O3 architectures are illustrated in Scheme 1B. The more detailed
growth mechanism is not very clear because of rapid precipitation reaction.
(A)
12

(B)
Scheme 1. (A): a schematic (a) and SEM image (b) of the sample A1; (B): proposed formation
mechanism of Bi2O3 architectures.
The effect of the other preparation conditions on the morphology of nanobrick-based
flowerlike Bi2O3 microcrystallites (A1) has been investigated. If the amount of
Bi(NO3)3·5H2O was decreased to 1 mmol keeping other conditions unchanged, TEM
observation shows that the crossed flowerlike Bi2O3 microcrystallites with an average petal
length of 5 µm constituted 60% of all particles except for multi-petal microstructures (Fig.
3e). While the number of petals increased if the amount of Bi(NO3)3·5H2O was increased
to 4 mmol (Supporting Information Fig. S4c). The concentration of Bi(NO3)3·5H2O
affected the nucleation rate and amount of the Bi2O3 nuclei, which controlled the number of
the petals. If NaOH was used instead of KOH as OH- source, the rod-based bundle-shaped
Bi2O3 architectures developed (Fig. 3f), possibly because of weaker basic strength of
NaOH and thus slower reaction and nucleation rate than growth. When the solvent or
bismuth source was changed to DMSO or BiCl3 and bismuth citrate, the Bi2O3 flower-like
13

morphology could be observed but became poorly uniform (Supporting Information Fig.
S4d-f).
Cu
Bi
Cu
a
b
Bi
Bi
Bi
Cu
Cu
Cu
Cu
O
O
0
2
4 6
Energy (keV)
8
10
12
0
2
4
6
Energy (keV)
8
10
12
Fig. 4. EDS spectra of the Bi2O3 microcrystallites: (a) A1 and (b) A3.
The EDS spectra of the Bi2O3 microcrystallites (A1 and A3) show that the as-prepared
products consisted of O and Bi elements (Fig. 4). C and Cu peaks in the spectra are due to
carbon-coated TEM Cu grid [27].
O1s
Bi 4f7/2
a
b
Bi 4f5/2
Bi-O
absorbed oxygen
535
534
533
532
531
530
529
528
170 168 166 164 162 160 158 156 154
Binding energy (eV)
Binding energy (eV)
Fig. 5. XPS spectra of the samlpe A1: (a) Bi 4f; (b) O 1s.
14

The surface composition and chemical state of the product A1 were analyzed by using
XPS (Fig. 5). The high-resolution XPS spectrum of Bi 4f shows two peaks at 158.6 and
163.9 eV, corresponding to Bi 4f7/2 and 4f5/2, respectively (Fig. 5a). As shown in Fig. 5b,
the deconvolution of O 1s spectrum indicates two contributions of lattice oxygen O2- at
530.5 eV and surface adsorbed oxygen at 531.5 eV [28, 29].
3.5
0.6
a
b
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
A1
A2
A3
A4
A1 2.70 eV
A2 2.82 eV
A3 2.84 eV
A4 2.88 eV
2.0
2.2
2.4
2.6
hυ (eV)
2.8
3.0
3.2
400
500
Wavelength (nm)
600
700
800
Fig. 6. (a) DRS spectra and (b) the plot of (αhv)2 as a function of photon energy hv of the samples
A1-A4.
The UV-Vis absorption spectrum is one of the important methods for researching the
optical properties and band structure of semiconductor nanomaterials. The optical
properties of the as-prepared Bi2O3 microcrystallites were measured by using DRS spectra,
and the results are shown in Fig. 6. The absorbance edge of the samples A3 and A4 located
at approximately 450 nm, which are in accordance with their yellowish color. The samples
A1 and A2 show absorbance throughout the entire visible region. The energy of the band
gap (Eg) was calculated following Tuac’s equation [30] αhν = A(hν - Eg)n/2, where α, h, ν,
and A are the absorption coefficient, Plank’s constant, light frequency, and a constant,
15

respectively. For Bi2O3, the value of n is 1 [23]. The band gap energies of the as-prepared
Bi2O3 samples are found to be about 2.70-2.88 eV (inset in Fig. 6b and Table 1). The band
gap energy decreased with increasing rod diameter, whereas the petal-based Bi2O3 crystal
with the largest thickness and width has the smallest band gap energy, which may be
caused by the size effect [4, 23].
3.5
blank
A1
1.0
0.8
0.6
0.4
0.2
0.0
a
b
3.0
2.5
2.0
1.5
1.0
0.5
0.0
A2
A3
A4
blank
A1
A2
A3
A4
0
1
3
4
5
2 t (h)
0
1
2
4
5
t (h) 3
Fig. 7. (a) Photocatalytic degradation efficiency of RhB and (b) kinetic linear simulation curves of RhB
degradation over the samples A1- A4.
The photocatalytic activity of the as-prepared Bi2O3 microcrystallites were detected by
degradation of RhB. Fig. 7a shows the photodegradation efficiency of RhB as a function of
irradiation time over different photocatalysts under xenon lamp irradiation, where C0 is the
initial concentration of the RhB after adsorption-desorption equilibration, and C is the
concentration of RhB at different irradiation time, which was measured by UV-Vis spectra
(Supporting Information Fig. S5). After 5 h of irradiation, the photodegradation rate
reached 93% and 96% over the rod-based bundle-like (A3) and dendrite-like (A4) Bi2O3,
respectively, higher than the nanobrick-based flower-like (A1) Bi2O3 (about 53%). On the
16

other hand, the maximum peak obviously shifted from 554 nm to 500 nm, which can be
ascribed to the formation of N-deethylated RhB in the photocatalytic process (Supporting
Information Fig. S5). The time-dependent decomposition of RhB follows the first-order
kinetics, -ln(C/C0) = kt, where t is the irradiation time and k is the apparent rate constant
[29]. The calculated curves tangent values (k) for the samples A1-A4 are 0.16, 0.38, 0.53
and 0.63 h-1, respectively (Fig. 7b), indicating that rod-based Bi2O3 architectures (A2-A4)
present higher photocatalytic rate than A1 upon visible-light irradiation. As well known,
many factors influence photocatalytic activity including crystallinity, surface area, band
gap and so on [31, 32]. BET surface area was also measured and it is found to be 1.28 and
1.24 m2/g for the samples A1 and A3, respectively. Therefore, in the present work, the
morphology and microstructures of the catalysts are the main factors to influence
photodegradation activity. The photocatalytic activity of various Bi2O3 structures or
morphology reported by the literatures and this work is listed in Table 2, where 1D
needle-like Bi2O3 was reported to exhibit better photocatalytic activity over Bi2O3
polyhedrons and plates [23]. We proposed that the higher photocatalytic efficiency for the
nanorod-based Bi2O3 architectures may be attributed to higher light absorption and carrier
separation efficiency in 1D nanostructured materials [33]. Additionally, the average
diameter of the building units (microrods or nanorods) for the samples A2-A4 is 1 µm, 200
nm, and 80 nm, respectively. For a given catalyst amount, a smaller size means a higher
specific surface area, and so offers more active reaction centers. The photocatalytic activity
of the samples from A2 to A4 increased gradually, which should result from the decrease
of the rod diameter and increase of the surface area. As displayed in Fig. 7b, the apparent
17

rate constant k is increased from 0.38 to 0.63 h-1 for the samples A2 to A4. A decrease in
rod diameter will shorten the route for photogenerated electrons to migrate to the surface
and therefore enhance the photodegradation rate [32].
Table 2. Photocatalytic activity of Bi2O3 in various structures or morphology
Morphology
Pollutants
Irradiation time (h) Photodegradation rate
Rod-based flowers [34]
Plates-based flowers [8]
Needles [23]
MO a
RhB
MB b
MB
1.5
2.5
3
13.9%
87%
28%
38%
11%
14%
53%
85%
93%
96%
Bundles of needles [23]
Plates [23]
3
MB
3
Polyhedrons [23]
MB
3
Brick-based flowers (A1)
Rod-based flowers (A2)
Rod-based bundles (A3)
Rod-based dendrites (A4)
RhB
RhB
RhB
RhB
5
5
5
5
(a MO = Methyl orange; b MB = Methyl blue)
4. Conclusions
In summary, by selecting proper DMF/H2O ratio, the morphologies of the Bi2O3
hierarchical architectures can be easily tuned via precipitation method. The amount of H2O
18

influences the hydrolysis degree of the precursor Bi(NO3)3, thus the amount of initial Bi2O3
nuclei, and finally determines the morphology of the building units of the Bi2O3
architectures. The facile strategy to fabricate Bi2O3 crystals provides a basis for gaining
mechanistic insights into shape control of Bi2O3 hierarchical architectures.
Acknowledgments
The work is financially supported by National Natural Science Foundation of China
(51372117, 21276127), Natural Science Foundation of Jiangsu Province (BK20131347)
and Jiangsu Funds for Distinguished Young Scientists (BK2012035).
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Figure Captions:
Table 1. DMF/H2O ratio, morphologies, band gap energy and apparent rate constant k of RhB
photodegradation.
Fig. 1. XRD patterns of the as-prepared Bi2O3 microcrystallites: (a) [KOH]/Bi = 89/2 (A1), (b)
[KOH]/[Bi] = 71/2. Vertical sticks below the patterns represent the standard diffraction data from JCPDS
file for bulk Bi2O3 with monoclinic structure (No. 41-1449).
Fig. 2. SEM and TEM images of the as-prepared Bi2O3: (a-c) A1, and the inset in (c) is a high-resolution
TEM image of an individual petal; (d) A2, and the inset is a TEM image of A3; (e) A4, and the inset
showing its nanorod-like building units; (f) An HRTEM image of an individual from A4, and the upper
left and lower right insets are its SAED and FFTs patterns, respectively.
Fig. 3. (a)-(d) SEM and TEM images of Bi2O3 prepared after 3 min of reaction at various DMF/H2O
ratio: (a) 5:1; (b) 2:1, and the inset showing the presence of flower-like morphology; (c) 1:1; (d) 0:30,
and the inset is a high-magnification TEM image. TEM images of Bi2O3 crystals obtained at DMF/H2O
ratio of 5: (e) with 1 mmol of Bi(NO3)3, and the inset showing crossed petals; (f) using NaOH instead of
KOH.
Scheme 1. (A): a schematic (a) and SEM image (b) of the sample A1; (B): proposed formation
mechanism of Bi2O3 architectures.
Fig. 4. EDS spectra of the Bi2O3 microcrystallites: (a) A1 and (b) A3.
Fig. 5. XPS spectra of the sample A1: (a) Bi 4f; (b) O 1s.
Fig. 6. (a) DRS spectra and (b) the plot of (αhv)2 as a function of photon energy hv of the samples
A1-A4.
Fig. 7. (a) Photocatalytic degradation efficiency of RhB and (b) kinetic linear simulation curves of RhB
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degradation over the samples A1- A4.
Table 2. Photocatalytic activity of Bi2O3 in various structures or morphology
22

Fig. 1

Graphic abstract:
Synthesis of Bi2O3 architectures by precipitation method in
DMF-H2O solution and their photocatalytic activity
Li-Li Yang, Qiao-Feng Han, Jin Zhao, Jun-Wu Zhu, Xin Wang, Wei-Hua Ma
Flowerlike α-Bi2O3 architectures assembled by nanobrick-based petals with pineapple
surface were firstly synthesized by precipitation method at room temperature in DMF-H2O
solution.
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Highlights:
> Nanobrick-based flowerlike Bi2O3 crystals with pineapple surface were synthesized by
precipitation method.
> Good solubility of Bi(NO3)3 in DMF played a crucial role in the growth of flowerlike
Bi2O3.
> The growth mechanism of Bi2O3 microcrystallites has been explained in detail.
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