15468 Phys. Chem. Chem. Phys., 2010, 12, 15468–15475 This journal is c the Owner Societies 2010

Synergistic effect of crystal and electronic structures on the

visible-light-driven photocatalytic performances of Bi2O3 polymorphsw

Hefeng Cheng,a Baibiao Huang,*a Jibao Lu,b Zeyan Wang,a Bing Xu,a

Xiaoyan Qin,aXiaoyang Zhang

aand Ying Dai

b

Received 15th July 2010, Accepted 31st August 2010

DOI: 10.1039/c0cp01189d

Three polymorphs of Bi2O3 were selectively synthesized via solution-based methods. The phase

structures of the as-prepared samples were confirmed by X-ray powder diffraction (XRD) and

X-ray photoelectron spectroscopy (XPS). UV-vis diffuse reflectance spectroscopy was employed to

study the optical properties of Bi2O3 polymorphs, and the band gaps were estimated to be 2.80,

2.48, and 3.01 eV for a-Bi2O3, b-Bi2O3, and d-Bi2O3, respectively. The photocatalytic

performances of the oxides were investigated by decomposing methyl orange and 4-chlorophenol

under visible irradiation at room temperature. It was observed that b-Bi2O3 displayed much

higher photocatalytic performance than N-doped P25. Among the three polymorphs of Bi2O3, the

photocatalytic activities followed the order: b-Bi2O3 > a-Bi2O3 > d-Bi2O3, which was in good

accordance with the photoluminescence spectra measurement results. The synergistic effect of the

crystal and electronic structures on the photocatalytic performances of Bi2O3 polymorphs was

investigated. The much better photocatalytic activity of b-Bi2O3 was considered to be closely

related to its smaller band gap, higher crystallinity and unique tunnel structure.

1. Introduction

The past several decades have witnessed the exponential

increase of studies on semiconductor photocatalysts, which

were employed for energy conversion and environmental

decontamination.1–5 TiO2, one of the most extensively studied

semiconductors, was regarded as an outstanding candidate

and usually used as a reference material for photocatalytic

evaluation owing to its unique photochemical features.2

However, lack of visible absorption hinders the practical

applications of TiO2, which only captures less than 4% of

the sunlight. Therefore, to make the best of the solar energy or

indoor illumination, it is indispensable to exploit visible-light-

driven photocatalysts. To date, substantial efforts have been

devoted to expanding the absorption spectra of the photo-

catalysts into the visible range by energy band engineering and

two approaches have been well developed. One of the

approaches is the modification of TiO2, which involves metal6,7

and/or non-metal ions doping.3,8–10 Nonetheless, the doped

materials usually could not endure thermal stability, and the

dopants always perform as the recombination sites of the

photoinduced electrons and holes.6 Another access is to seek

new semiconductor photocatalysts working under visible

irradiation.11–15 Among them, the Bi-based multimetal oxides

with a 6s2 configuration, such as CaBi2O4,16 BiVO4,

17 have

shown to be active under visible illumination, which can be

ascribed to their fresh-constructed, well-dispersed valence

bands by the hybridization of Bi 6s and O 2p orbitals.

Due to its particular dielectric, optical, and ion-conductive

properties, bismuth trioxide (Bi2O3) has been extensively

applied in gas sensors, optoelectronics devices, and catalysts.18–20

Recently, Bi2O3 as an undoped and single oxide semiconductor

sensitive to visible irradiation has also been found to exhibit

good photocatalytic performance, which originates from its

appropriate band gap.21 Generally, Bi2O3 has four different

polymorphs, denoted as monoclinic a, tetragonal b, body-

centered cubic g, and face-centered cubic d. Among these, the

low-temperature a-phase and the high-temperature d-phaseare stable; while the other two phases are high-temperature

metastable.19 So far, Bi2O3 nano/microstructures have been

prepared by various ways, and different synthetic procedures

could lead to different phases of Bi2O3.22–24 As is known, the

photocatalytic performance of a photocatalyst is closely

related to its corresponding structural and photochemical

features.25,26 For example, as a result of their different band

and crystal structures, monoclinic BiVO4 shows much better

photocatalytic properties than tetragonal BiVO4.17 Since each

polymorph of Bi2O3 possesses a special crystal and electronic

structure, there is good reason to believe that the activities of

Bi2O3 polymorphs will differ from each other. Although the

photocatalytic activities of Bi2O3 have been reported,21,24 to

the best of our knowledge, few studies have been devoted to

exploring the correlation between the crystal structures,

electronic structures, and the photocatalytic properties of the

Bi2O3 polymorphs.

In the present work, we have prepared three different

polymorphs of Bi2O3 through the solution-based routes by

varying the experimental conditions. The photocatalytic

performances of the Bi2O3 polymorphs were evaluated by

decomposing methyl orange and 4-chlorophenol under visible

a State Key Lab of Crystal Materials, Shandong University,Jinan 250100, People’s Republic of China.E-mail: bbhuang@sdu.edu.cn; Fax: +86-531-8836-5969;Tel: +86-531-8836-6324

b School of Physics, Shandong University, Jinan 250100,People’s Republic of China

w Electronic supplementary information (ESI) available: Adsorptivityof dye on catalysts; SEM images; photocatalytic activity comparison.See DOI: 10.1039/c0cp01189d

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

Publ

ishe

d on

25

Oct

ober

201

0. D

ownl

oade

d by

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

on

27/1

1/20

13 0

6:58

:28.

View Article Online / Journal Homepage / Table of Contents for this issue

This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 15468–15475 15469

light irradiation. A systematical synergistic effect of crystal

and electronic structures on the visible-light-driven photo-

catalytic performances of Bi2O3 polymorphs was discussed.

2. Experimental

2.1 Samples preparation

All the reagents employed in the experiments were analytical

grade and used without any further purification.

(1) a-Bi2O3. In the preparation of a-Bi2O3, Bi(NO3)3�5H2O

(1.94 g, 4 mmol) was first dissolved in dilute HNO3 (1 mol L�1,

20 mL) to give a clear solution. Then NaOH (4 mol L�1,

80 mL) aqueous solution was added with continuous stirring.

The suspension was heated and maintained at 60 1C for 6 h.

The final products were filtered, washed, and dried at 60 1C in

vacuum for several hours.

(2) b-Bi2O3. To synthesize b-Bi2O3, Bi(NO3)3�5H2O was

dissolved in dilute HNO3, then the solution was added

dropwise to the excessive Na2CO3 solution with constant

stirring for 2 h to obtain Bi2O2CO3.27 Then the as-derived

Bi2O2CO3 precipitation was filtered, washed, dried in vacuum,

and finally annealed at 380 1C for 10 min to yield b-Bi2O3.28

(3) d-Bi2O3. The preparation procedure of d-Bi2O3 was

similar to that of a-Bi2O3, except that equal stoichiometric

amount of NH4VO3 was introduced, which modulated the

phase formation of d-Bi2O3.24

2.2 Samples characterization

Powder X-ray diffraction (XRD) patterns were recorded on a

Bruker AXS D8 advance powder diffractometer (Cu-Ka X-ray

radiation, l= 0.154056 nm). X-Ray photoelectron spectroscopy

(XPS) measurements were performed on a Thermo Fisher

Scientific Escalab 250 spectrometer with monochromatized

Al Ka excitation, and C 1s (284.6 eV) was used to calibrate

the peak positions of the elements. The scanning electron

microscopy (SEM) images were obtained on a Hitachi

S-4800 microscope with an accelerating voltage of 7.0 kV. A

Micromeritics ASAP 2020 analyzer was employed to measure

the Brunauer–Emmett–Teller (BET) surface areas of the

samples at liquid nitrogen temperature. The photoluminescence

(PL) was carried out on a Hitachi F-4500 fluorescence spectro-

photometer at room temperature. UV-vis diffuse reflectance

spectra were collected on a Shimadzu UV 2550 recording

spectrophotometer, which was equipped with an integrating

sphere.

2.3 Photocatalytic performance evaluation

The photocatalytic performances of the as-prepared products

were evaluated by decomposition of methyl orange (MO) and

4-chlorophenol (4-CP) under visible light irradiation at room

temperature. A 300 W Xe arc lamp (PLS-SXE300, Beijing

Trusttech Co., Ltd.) was used as the light source and equipped

with an ultraviolet cutoff filter to provide visible light

(l Z 400 nm). The distance between the liquid surface

of the suspension and the light source was set about 10 cm.

The photodegradation experiments were performed with the

sample powder (100 mg) suspended in MO or 4-CP aqueous

solution (20 mg L�1, 100 mL) with constant stirring. Prior to

irradiation, the suspensions were stirred in the dark for 1 h to

ensure the adsorption/desorption equilibrium. At the given

time intervals, about 5 ml of the suspension was taken for the

following analysis after centrifugation. The MO and 4-CP

Fig. 1 Scheme crystal structures of the Bi2O3 polymorphs: (a) a-Bi2O3, (b) b-Bi2O3 and (c) d-Bi2O3.

Publ

ishe

d on

25

Oct

ober

201

0. D

ownl

oade

d by

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

on

27/1

1/20

13 0

6:58

:28.

View Article Online

15470 Phys. Chem. Chem. Phys., 2010, 12, 15468–15475 This journal is c the Owner Societies 2010

photodegradation were then analyzed at 464 and 225 nm,

respectively, as a function of irradiation time on a UV-vis

spectrophotometer (Shimadzu UV 2550).

2.4 Theoretical calculations

The theoretical calculations in our study were performed by

using the standard CASTEP package, which is a plane-wave

pseudopotential total energy calculation method based on the

density functional theory (DFT). The lattice parameters of the

a-Bi2O3, b-Bi2O3 and d-Bi2O3 models were optimized, and

then these models were used to calculate the ground-state

energy band. The exchange–correlation potential was depicted

by the generalized gradient approximation (GGA) with the

Perdew–Burke–Ernzerhof (PBE) scheme.29 The Brillouin

zones were separately sampled at 5 � 3 � 4, 3 � 3 � 4 and

4� 4� 4Monkhorst–Pack k-points for the a, b and d phase.30

The electronic wave functions were expanded in a plane-wave

basis set up to a 380 eV cutoff, while the self-consistent field

(SCF) tolerance was all 5 � 10�7 eV/atom.

3. Results and discussion

3.1 Structure analysis

Fig. 1 illustrates the scheme crystal structures of the Bi2O3

polymorphs. In a-Bi2O3 structure (see Fig. 1a), the layers of

bismuth atoms are parallel to the (100) crystal plane of the

monoclinic cell, which are separated by the layers of oxygen

ions in a zigzag manner.31,32 In b-Bi2O3 (see Fig. 1b),

tunnels lie in the structure along the crystallographic c axis.33

However, in the fluorite-type structure of d-Bi2O3 (see Fig. 1c), a

large number of oxygen defects result in its high degree of

disorder.32

X-Ray powder diffraction (XRD) is a useful tool to

characterize the phase structure of the materials. Fig. 2 shows

the XRD patterns of the as-prepared Bi2O3 polymorphs. The

identifications of the diffraction peaks of the samples can

be indexed well to the single phases, which are monoclinic

a-Bi2O3 (JCPDS No. 41-1449), tetragonal b-Bi2O3 (JCPDS

No. 27-0050), and cubic d-Bi2O3 (JCPDS No. 52-1007),

respectively. The sharp diffraction peaks of a-Bi2O3 and

b-Bi2O3 indicate their high crystallinity. However, in the case

of d-Bi2O3, the diffraction peaks become much broader and

weaker, which suggests its low crystallinity along with the

decreased crystallites. Furthermore, the lattice parameters

were also deduced from the cell refinement of the compounds

and listed in Table 1.

To elucidate the oxidation states of the Bi2O3 polymorphs,

X-ray photoelectron spectroscopy (XPS) was conducted.

Fig. 3 demonstrates the high-resolution XPS spectra of Bi 4f

and O 1s states of the different polymorphs of Bi2O3 products.

All the three different Bi2O3 polymorphs exhibit narrow

Gauss-shaped symmetrical Bi 4f7/2 and Bi 4f5/2 core spectra

(see Fig. 3a). The characteristic binding energy values of

Bi 4f7/2 for a-Bi2O3, b-Bi2O3, and d-Bi2O3 are 158.5, 158.4,

and 158.6 eV, respectively, which are in agreement with the

reported values.34,35 Moreover, no related peaks of bivalent,

tetravalent, or pentavalent states were found on the Bi 4f7/2shoulder, which indicates that Bi exists as Bi(III) oxidation

state in all the samples.34 However, in the XPS spectra of O 1s

peaks (see Fig. 3b), notable asymmetry and broadening can be

observed. In all the Bi2O3 samples, the O 1s spectra can be

deconvoluted into two peaks, which are the Bi–O in the crystal

lattice with lower binding energy and the absorbed oxygen

(OH� or CO32� groups) on the surface with higher energy.35

The binding energies of Bi–O in a-Bi2O3, b-Bi2O3, and d-Bi2O3

are accordingly found to be 530.1, 530.5, and 530.1 eV, and

this diversity could result from their different crystal

structures.

The typical microstructure information of the Bi2O3

polymorphs was obtained by the SEM images (see Fig. 4).

As Fig. 4a shows, the a-Bi2O3 products are presented as

flower-like architectures, which consist of interlaced micro-

rods that are 2–4 mm in length and about 1 mm in thickness.

The b-Bi2O3 products are composed of the worm-like structures

with numerous pores (see Fig. 4b). In Fig. 4c, plenty of

nanoparticles with an average size of ca. 20 nm are found in

d-Bi2O3 products.

As a significant index to the photocatalytic activity, the

Brunauer–Emmett–Teller (BET) surface areas of the Bi2O3

products were executed and listed in Table 1. The BET surface

area was only 1.10 m2 g�1 for a-Bi2O3, and this was attributed

to its big crystallites. The smaller grain size and the porous

structures enabled b-Bi2O3 to have a bigger BET surface area

of 8.55 m2 g�1. The BET surface area of d-Bi2O3 reached as

high as 39.2 m2 g�1, which stemmed from its much smaller

crystallites.

3.2 Photophysical properties

As the direct recombination consequence of the photogenerated

electrons and holes, photoluminescence emission spectra are

usually employed to determine the separation efficiency of the

charge carriers.36 Fig. 5 shows the room temperature PL

spectra of the Bi2O3 polymorphs with the excitation wavelength

at 300 nm. We can observe that both a-Bi2O3 and d-Bi2O3

show a strong broad emission peak around 460 nm,37 implyingFig. 2 XRD patterns of the a-Bi2O3, b-Bi2O3, and d-Bi2O3 products.

Publ

ishe

d on

25

Oct

ober

201

0. D

ownl

oade

d by

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

on

27/1

1/20

13 0

6:58

:28.

View Article Online

This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 15468–15475 15471

the high recombination rates of the photoinduced carriers.

However, b-Bi2O3 shows a rather lower intensity, which

means the efficient separation rate of the carriers in b-Bi2O3

structure.38 The PL intensity of the samples follows the order:

d-Bi2O3 > a-Bi2O3 > b-Bi2O3, indicating the relative amounts

of trapping sites for the free carriers in the crystal structures of

the Bi2O3 polymorphs.

Fig. 6 displays the UV-vis diffuse reflectance spectra of the

Bi2O3 polymorphs. As Bi2O3 behaves the direct absorption

dependence, its band gap can be extrapolated by the following

equation39 ahn = A(hn � Eg)1/2, where a, n, Eg, and A are

the absorption coefficient, light frequency, band gap, and a

constant, respectively. As shown in the insert, the band gaps

are deduced from the plot (ahn)2 versus (hn) and estimated to

be 2.80, 2.48, and 3.01 eV for a-Bi2O3, b-Bi2O3, and d-Bi2O3,

respectively.

Table 1 The lattice parameters, band gaps and BET surface areas of the Bi2O3 polymorphs

Sample Crystal structure Space group Lattice parameters Eg/eV d/nma SBET /m2 g�1

a-Bi2O3 monoclinic P21/c a = 5.849 A 2.80 80.8 1.10b = 8.165 Ac = 7.509 Ag = 113.01

b-Bi2O3 tetragonal P�421c a = 7.731 A 2.48 47.1 8.55b = 5.628 A

d-Bi2O3 cubic Fm�3m a = 5.520 A 3.01 19.3 39.2

a the mean sizes of the samples obtained according to the Scherrer formula.

Fig. 3 High-resolution XPS spectra of (a) Bi 4f and (b) O 1s states of

the different polymorphs of Bi2O3 products.

Fig. 4 Typical SEM images of (a) a-Bi2O3, (b) b-Bi2O3, and

(c) d-Bi2O3 samples.

Publ

ishe

d on

25

Oct

ober

201

0. D

ownl

oade

d by

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

on

27/1

1/20

13 0

6:58

:28.

View Article Online

15472 Phys. Chem. Chem. Phys., 2010, 12, 15468–15475 This journal is c the Owner Societies 2010

3.3 Photocatalytic properties

The photocatalytic performances of the different Bi2O3

polymorphs were investigated by decomposing MO and

4-CP under visible irradiation (l Z 400 nm). Fig. 7 represents

the photodegradation of MO as a function of irradiation time

over different photocatalysts. As shown, after irradiation for

90 min, the photodegradation efficiency of MO on a-Bi2O3

was 13.9%, which was negligible for d-Bi2O3. However, for

b-Bi2O3, the photodegradation efficiency could reach 87.3%

when subjected to illumination only for 60 min. As the MO

photodegradation follows the pseudo-first-order reaction, it is

worth noting that the MO photodegradation rate over b-Bi2O3

(1.7505 h�1) was almost 16 times faster than that of a-Bi2O3

(0.1098 h�1). The photocatalytic activity of the Bi2O3 polymorphs

followed the order: b-Bi2O3 > a-Bi2O3 > d-Bi2O3. For

comparison, after irradiation for 90 min, N-doped P25 displayed

a low degradation rate of 9.8%.

As a precondition for photocatalytic performance, the

adsorptivity of MO dye on catalysts was studied after the

adsorption/desorption equilibrium in the dark. As shown in

the ESI (see Fig. S1),w the bar plot illustrated the remaining

concentration of MO over different photocatalysts. Among

the photocatalysts, b-Bi2O3 showed the best adsorption

capacity, and the remaining concentration of MO dye was

ca. 83%. Since the BET area of b-Bi2O3 is far lower than that

of d-Bi2O3 or N-doped P25, the adsorptivity of MO dye would

not merely stem from the simple physical absorption. The

better adsorptivity of MO dye on b-Bi2O3 is believed to be

largely associated with the selective absorption of aromatic

dye.40

Given that MO has considerable absorption in the visible

light region, we have also tested the photodecomposition

of 4-CP under visible irradiation to exclude the possible

influential factors such as dye sensitization. Fig. 8 shows the

photodecomposition of 4-CP in the presence of the different

photocatalysts. As an aromatic compound to show resonance

stability, 4-CP was difficult to decompose. After visible

irradiation for 90 min, about 12.3% of 4-CP molecules were

decomposed over a-Bi2O3, and the photodecomposition

efficiency of d-Bi2O3 was only 6.5%. In particular, b-Bi2O3

displayed much higher decomposition efficiency, which could

Fig. 5 PL spectra of the different Bi2O3 polymorphs (lexc = 300 nm).

Fig. 6 UV-vis diffuse reflectance spectra of the Bi2O3 polymorphs:

(a) a-Bi2O3, (b) b-Bi2O3, and (c) d-Bi2O3; Insert: the corresponding

plots of (ahn)2 versus energy (hn).

Fig. 7 Photodegradation of MO with time over different photo-

catalysts under visible irradiation.

Fig. 8 Photodegradation of 4-CP with time over different photo-

catalysts under visible irradiation.

Publ

ishe

d on

25

Oct

ober

201

0. D

ownl

oade

d by

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

on

27/1

1/20

13 0

6:58

:28.

View Article Online

This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 15468–15475 15473

reach 80% after irradiation for 90 min. The photocatalytic

performances of the Bi2O3 polymorphs also obeyed the

sequence b-Bi2O3 > a-Bi2O3 > d-Bi2O3. Besides, about

21.7% of the 4-CP molecules were decomposed over N-doped

P25 sample under the same conditions. It was found that the

4-CP photodecomposition rate over b-Bi2O3 (0.8543 h�1) was

much faster than that of a-Bi2O3 (0.0783 h�1) and N-doped

P25 (0.0816 h�1) by a factor of about eleven.

As the BET area of b-Bi2O3 (8.55 m2 g�1) is ca. 8 times

larger than that of a-Bi2O3 (1.10 m2 g�1), one could conceive

that larger BET area of a-Bi2O3 will lead to higher photo-

catalytic performance. To elucidate the influences of BET

surface area on the photocatalytic performance, we also

prepared a-Bi2O3 by calcination treatment and the product

was labeled as a-Bi2O3 (CT). Firstly, Bi(NO3)3�5H2O was

dissolved in dilute HNO3, then NH3�H2O was introduced

dropwise to the solution and the pH value was adjusted to

7.0. The white suspension was filtered, washed, dried, and

finally calcined at 500 1C for 1 h. The as-prepared a-Bi2O3

(CT) products consist of irregular nanoparticles with an

average diameter of about 1 mm (see Fig. S2 in the ESI).wThe BET area of a-Bi2O3 (CT) products (6.53 m

2 g�1) is about

6 times larger than that of a-Bi2O3, however, the corresponding

photocatalytic activity improved slightly a little (see Fig. S3

in the ESI).w The above results suggest that the dramatic

difference in the photocatalytic activities of a-Bi2O3 and

b-Bi2O3, under the maintenance of similar crystallinity and

BET area, can be attributed to their crystal and electronic

structure differences..

3.4 Synergistic effects of the crystal and electronic structures

on the photocatalytic performances

In general, the photocatalytic capacity of a semiconductor

photocatalyst is mainly dependent on two factors, which are

the separation efficiency of the photoinduced carriers and the

light absorption range. Among the Bi2O3 polymorphs, their

photocatalytic activities for decomposing MO and 4-CP both

follow the order b-Bi2O3 > a-Bi2O3 > d-Bi2O3. The crystal

structures have an important effect on the corresponding

photocatalytic performances.2,25,26 d-Bi2O3 exhibits much

structural disarrangement, which could readily become the

recombination sites of the carriers. In addition, the rather low

crystallinity of d-Bi2O3 gives rise to many defects on the

surface or in the volume, which leads to its high recombination

rate of the electron–hole pairs and the negligible photocatalytic

activity. This was also confirmed by its highest PL intensity of

d-Bi2O3 among the Bi2O3 polymorphs. In contrast, the high

crystallinity of a-Bi2O3 and b-Bi2O3 could be propitious to

decreasing the recombination sites of the free carriers, which

results in a higher photocatalytic efficiency. Whereas the dramatic

differences in the photocatalytic activities of a-Bi2O3 and

Fig. 9 Electronic structures of (a) a-Bi2O3 and (b) b-Bi2O3.

Publ

ishe

d on

25

Oct

ober

201

0. D

ownl

oade

d by

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

on

27/1

1/20

13 0

6:58

:28.

View Article Online

15474 Phys. Chem. Chem. Phys., 2010, 12, 15468–15475 This journal is c the Owner Societies 2010

b-Bi2O3 still remain, and these could be associated with their

specific crystal structures. The tunnels in b-Bi2O3 can provide

the channels for the transfer of the photogenerated electrons

and holes to prevent the excessive recombination of them,

which could enable more free carriers to participate in the

photodecomposition process. Since the formed internal

electric field between the two layers benefits the transfer of

the photoinduced carriers, the layered compounds could

effectively improve the separation efficiency.13 Nonetheless,

for a-Bi2O3, its zigzag-type configuration increases the

recombination rates of the carriers, leading to its higher PL

intensity than that of b-Bi2O3. Therefore, b-Bi2O3 shows much

higher photocatalytic performance than a-Bi2O3. The results

confirmed that while maintaining similar crystallinity and

BET surface areas, b-Bi2O3 also exhibited much better photo-

catalytic property than a-Bi2O3, and this could be explained by

their difference in crystal structure.

Fig. 9 illustrates the band structure and density of states of

monoclinic a-Bi2O3 and tetragonal b-Bi2O3. As is known, due

to the deep energy position of O 2p orbital in valence band,

TiO2 has a wide band gap only responsive to UV light.4 For

the Bi(III)-based oxides, the hybridized valence band (VB)

composed of Bi 6s and O 2p orbitals is thought to narrow

the band gaps into the visible region, and its large dispersivity

is favorable to the mobility of the photogenerated carriers.15–17 It

can be observed that in both a-Bi2O3 and b-Bi2O3, the top of

VBs is mainly comprised of O 2p and Bi 6s orbitals, which

makes a-Bi2O3 and b-Bi2O3 sensitive to visible light irradiation,

while the bottom of conduction bands (CBs) is dominantly

constructed by Bi 6p orbital. It is noted that b-Bi2O3 exhibits a

more dispersive band structure than a-Bi2O3, which is more

suitable for the transfer of the photoinduced electrons and

holes. In addition, The VBs of a-Bi2O3 and b-Bi2O3 are

similar, yet the bottom of CB of b-Bi2O3 shifts to a more

negative potential, resulting in Eg(b-Bi2O3) o Eg(a-Bi2O3).

The calculated finding turns out to be in accordance with the

measured band gaps of b-Bi2O3 (2.48 eV) and a-Bi2O3

(2.80 eV), which demonstrates that b-Bi2O3 can absorb more

visible light than a-Bi2O3. The mentioned two points above

give an electronic structure explanation why b-Bi2O3 shows a

much better photocatalytic performance than a-Bi2O3.

However, with regard to d-Bi2O3, we failed to optimize the

semiconductor electronic structure on account of its high ionic

conductivity.32,41 Nonetheless, derived from the wide band

gap of d-Bi2O3 (3.01 eV), it could only respond to visible

light in a small proportion, which also resulted in its low

photocatalytic efficiency.

4. Conclusions

In this work, we have synthesized three different phases of

Bi2O3 through the solution-based routes. The band gaps of the

Bi2O3 polymorphs were estimated to be 2.80, 2.48, and

3.01 eV for a-Bi2O3, b-Bi2O3, and d-Bi2O3, respectively. In

the photocatalytic experiments of decomposing methyl orange

and 4-chlorophenol, it was observed that b-Bi2O3 displayed

much higher photocatalytic performance than N-doped P25.

Furthermore, the photocatalytic activities of Bi2O3 polymorphs

followed the order: b-Bi2O3 > a-Bi2O3 > d-Bi2O3, which was

in good accordance with the photoluminescence spectra

measurement results. The layered structure of a-Bi2O3 and

tunnel structure of b-Bi2O3 favor the transfer of the photo-

induced carriers, while a mass of defects in d-Bi2O3 increase

the recombination rates of the carriers. Moreover, deduced

from the electronic structures calculations, b-Bi2O3 has smaller

band gap than a-Bi2O3, and this is in good agreement with the

measured values. The higher crystallinity, smaller band gap,

and tunnel structure are believed to be associated with the

excellent photocatalytic activity of b-Bi2O3. Our study reveals

b-Bi2O3 as a highly efficient visible-light-driven photocatalyst,

and appropriate microstructure modulation may lead to

higher photocatalytic performance. In addition, this work

provides theoretical and experimental evidence for the

synergistic effects of the crystal and electronic structures on

the photocatalytic properties, which could be applied to other

semiconductor photocatalysts with polymorphs, such as WO3,

In2O3, CdS and so on.

Acknowledgements

This work was financially supported by a research Grant from

the National Basic Research Program of China (973 Program,

Grant 2007CB613302), the National Natural Science

Foundation of China under Grants (Nos. 20973102,

50721002 and 10774091), and China Postdoctoral Science

Foundation funded project (20090461200).

References

1 A. Fujishima and K. Honda, Nature, 1972, 238, 37.2 A. L. Linsebigler, G. Q. Lu and J. T. Yates, Chem. Rev., 1995, 95,735.

3 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science,2001, 293, 269.

4 Y. Bessekhouad, D. Robert and J. V. Weber, J. Photochem.Photobiol., A, 2004, 163, 569.

5 F. E. Osterloh, Chem. Mater., 2008, 20, 35.6 W. Choi, A. Termin and M. R. Hoffmann, J. Phys. Chem., 1994,98, 13669.

7 D. Dvoranova, V. Brezova, M. Mazur and M. A. Malati, Appl.Catal., B, 2002, 37, 91.

8 S. Sakthivel and H. Kisch, Angew. Chem., Int. Ed., 2003, 42, 4908.9 W. Zhao, W. Ma, C. Chen, J. Zhao and Z. Shuai, J. Am. Chem.Soc., 2004, 126, 4782.

10 N. Serpone, J. Phys. Chem. B, 2006, 110, 24287.11 Z. B. Lei, W. S. You, M. Y. Liu, G. H. Zhou, T. Takata, M. Hara,

K. Domen and C. Li, Chem. Commun., 2003, 2142.12 H. G. Kim, D. W. Hwang and J. S. Lee, J. Am. Chem. Soc., 2004,

126, 8912.13 W. F. Yao, X. H. Xu, H. Wang, J. T. Zhou, X. N. Yang, Y. Zhang,

S. X. Shang and B. B. Huang, Appl. Catal., B, 2004, 52, 109.14 H. F. Cheng, B. B. Huang, Y. Dai, X. Y. Qin and X. Y. Zhang,

Langmuir, 2010, 26, 6618.15 H. F. Cheng, B. B. Huang, Y. Dai, X. Y. Qin, X. Y. Zhang,

Z. Y. Wang andM. H. Jiang, J. Solid State Chem., 2009, 182, 2274.16 J. Tang, Z. Zou and J. Ye, Angew. Chem., Int. Ed., 2004, 43, 4463.17 A. Kudo, K. Omori and H. Kato, J. Am. Chem. Soc., 1999, 121,

11459.18 A. Cabot, A. Marsal, J. Arbiol and J. R. Morante, Sens. Actuators,

B, 2004, 99, 74.19 L. Leontie, M. Caraman, M. Delibas and G. I. Rusu, Mater. Res.

Bull., 2001, 36, 1629.20 D. Kulkarni and I. E. Wachs, Appl. Catal., A, 2002, 237, 121.21 L. S. Zhang, W. Z. Wang, J. Yang, Z. G. Chen, W. Q. Zhang,

L. Zhou and S. W. Liu, Appl. Catal., A, 2006, 308, 105.

Publ

ishe

d on

25

Oct

ober

201

0. D

ownl

oade

d by

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

on

27/1

1/20

13 0

6:58

:28.

View Article Online

This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 15468–15475 15475

22 J. C. Yu, A. W. Xu, L. Z. Zhang, R. Q. Song and L. Wu, J. Phys.Chem. B, 2004, 108, 64.

23 Y. F. Qiu, D. F. Liu, J. H. Yang and S. H. Yang, Adv. Mater.,2006, 18, 2604.

24 L. Zhou, W. Z. Wang, H. L. Xu, S. M. Sun and M. Shang,Chem.–Eur. J., 2009, 15, 1776.

25 Y. D. Hou, L. Wu, X. C. Wang, Z. X. Ding, Z. H. Li and X. Z. Fu,J. Catal., 2007, 250, 12.

26 S. X. Ouyang, Z. S. Li, Z. Ouyang, T. Yu, J. H. Ye and Z. G. Zou,J. Phys. Chem. C, 2008, 112, 3134.

27 C. Greaves and S. K. Blower, Mater. Res. Bull., 1988, 23, 1001.28 H. F. Cheng, B. B. Huang, K. S. Yang, Z. Y. Wang, X. Y. Qin,

X. Y. Zhang and Y. Dai, ChemPhysChem, 2010, 11, 2167.29 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996,

77, 3865.30 H. J. Monkhorst and J. D. Pack, Phys. Rev. B: Solid State, 1976,

13, 5188.31 H. A. Harwig, Z. Anorg. Allg. Chem., 1978, 444, 151.32 H. A. Harwig and J. W. Weenk, Z. Anorg. Allg. Chem., 1978, 444,

167.

33 V. P. Zhukov, V. M. Zhukovskii, V. M. Zainullina andN. I. Medvedeva, J. Struct. Chem., 1999, 40, 831.

34 H. T. Fan, S. S. Pan, X. M. Teng, C. Ye, G. H. Li andL. D. Zhang, Thin Solid Films, 2006, 513, 142.

35 D. Barreca, F. Morazzoni, G. A. Rizzi, R. Scotti and E. Tondello,Phys. Chem. Chem. Phys., 2001, 3, 1743.

36 J. G. Yu, H. G. Yu, B. Cheng, X. J. Zhao, J. C. Yu and W. K. Ho,J. Phys. Chem. B, 2003, 107, 13871.

37 J. M. Xie, X. M. Lu, M. Chen, G. Q. Zhao, Y. Z. Song andS. S. Lu, Dyes Pigm., 2008, 77, 43.

38 D. P. Volanti, L. S. Cavalcante, E. C. Paris, A. Z. Simoes,D. Keyson, V. M. Longo, A. T. De Figueiredo, E. Longo,J. A. Varela, F. S. De Vicente and A. C. Hernandes, Appl. Phys.Lett., 2007, 90, 261913.

39 M. A. Butler, J. Appl. Phys., 1977, 48, 1914.40 H. Zhang, X. J. Lv, Y. M. Li, Y. Wang and J. H. Li, ACS Nano,

2010, 4, 380.41 A. Walsh, G. W. Watson, D. J. Payne, R. G. Edgell, J. H. Guo,

P. A. Glans, T. Learmonth and K. E. Smith, Phys. Rev. B:Condens. Matter Mater. Phys., 2006, 73, 235104.

Publ

ishe

d on

25

Oct

ober

201

0. D

ownl

oade

d by

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

on

27/1

1/20

13 0

6:58

:28.

View Article Online