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 Bi 2 O 3 polymorphsw Hefeng Cheng, a Baibiao Huang,* a Jibao Lu, b Zeyan Wang, a Bing Xu, a Xiaoyan Qin, a Xiaoyang Zhang a and Ying Dai b Received 15th July 2010, Accepted 31st August 2010 DOI: 10.1039/c0cp01189d Three polymorphs of Bi 2 O 3 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 Bi 2 O 3 polymorphs, and the band gaps were estimated to be 2.80, 2.48, and 3.01 eV for a-Bi 2 O 3 , b-Bi 2 O 3 , and d-Bi 2 O 3 , 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-Bi 2 O 3 displayed much higher photocatalytic performance than N-doped P25. Among the three polymorphs of Bi 2 O 3 , the photocatalytic activities followed the order: b-Bi 2 O 3 > a-Bi 2 O 3 > d-Bi 2 O 3 , 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 Bi 2 O 3 polymorphs was investigated. The much better photocatalytic activity of b-Bi 2 O 3 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 TiO 2 , 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 TiO 2 , 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 TiO 2 , which involves metal 6,7 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 6s 2 configuration, such as CaBi 2 O 4 , 16 BiVO 4 , 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 (Bi 2 O 3 ) has been extensively applied in gas sensors, optoelectronics devices, and catalysts. 18–20 Recently, Bi 2 O 3 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, Bi 2 O 3 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-phase are stable; while the other two phases are high-temperature metastable. 19 So far, Bi 2 O 3 nano/microstructures have been prepared by various ways, and different synthetic procedures could lead to different phases of Bi 2 O 3 . 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 BiVO 4 shows much better photocatalytic properties than tetragonal BiVO 4 . 17 Since each polymorph of Bi 2 O 3 possesses a special crystal and electronic structure, there is good reason to believe that the activities of Bi 2 O 3 polymorphs will differ from each other. Although the photocatalytic activities of Bi 2 O 3 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 Bi 2 O 3 polymorphs. In the present work, we have prepared three different polymorphs of Bi 2 O 3 through the solution-based routes by varying the experimental conditions. The photocatalytic performances of the Bi 2 O 3 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: [email protected]; 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: Adsorptivity of dye on catalysts; SEM images; photocatalytic activity comparison. See DOI: 10.1039/c0cp01189d PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics Published on 25 October 2010. Downloaded by University of Hong Kong Libraries on 27/11/2013 06:58:28. View Article Online / Journal Homepage / Table of Contents for this issue
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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
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: [email protected]; 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
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View Article Online / Journal Homepage / Table of Contents for this issue
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.
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.