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Journal of Physics and Chemistry of Solids 67 (2006) 2501–2505

www.elsevier.com/locate/jpcs

Preparation, characterization and photocatalytic activity ofpolycrystalline Bi2O3/SrTiO3 composite powders

Haitao Zhanga,b,c, Shuxin Ouyanga,b,c, Zhaosheng Lia,b,c, Lifei Liua,c, Tao Yua,c,Jinhua Yea,d, Zhigang Zoua,b,c,�

aDepartment of Physics, Eco-Materials and Renewable Energy Research Center (ERERC), Nanjing University, Nanjing 210093, PR ChinabDepartment of Materials Science and Engineering, Nanjing University, Hankou Road, Nanjing, Jiangsu 210093, PR China

cNational Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR ChinadEcomaterials Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Received 26 January 2006; received in revised form 12 July 2006; accepted 15 July 2006

Abstract

Bi2O3/SrTiO3 composite powders have been prepared and their photocatalytic activities were investigated by photooxidation of

methanol. These powders were characterized by UV–Visible diffuse reflectance spectra, SEM and X-ray diffraction (XRD). The results

revealed that all the Bi2O3/SrTiO3 composite powders exhibited higher photocatalytic activity than pure SrTiO3, Bi2O3 and TiO2 (P25)

under visible light irradiation (l4440 nm). The effects of the Bi2O3 contents on the photocatalytic activities of the composite powders

were examined, the photocatalytic activities increased with the content of Bi2O3 increasing to a maximum of 83% and then decreased

under visible light irradiation. The effects of the calcination temperatures on the photocatalytic activities of the composite powders were

also investigated.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Semiconductors

1. Introduction

The photocatalytic decomposition of the organic pollu-tants by employing semiconductors is one of the promisingmethods [1–4]. It is of great interest to improve thephotocatalytic activity of semiconductors for the degrada-tion of the organic compounds in water and air. However,the fast recombination rate of the photogenerated electron/hole pairs hinders the commercialization of this technology[1]. Among various semiconductors employed, TiO2,SrTiO3 and ZnO are known to be good photocatalyst forthe degradation of several environmental contaminants[2,5–10] due to their high photosensitivity and large band

e front matter r 2006 Elsevier Ltd. All rights reserved.

cs.2006.07.005

ng author. Department of Materials Science and En-

ing University, Hankou Road, Nanjing, Jiangsu 210093,

+8625 83686304; fax: +86 25 83686632.

ss: zgzou@nju.edu.cn (Z. Zou).

gap, which means high driving force for the reduction andoxidation processes, respectively. But for these photocata-lysts, the charge carrier recombination occurs withinnanoseconds [11,12] and the band edge absorption thresh-old does not allow the utilization of the visible light.Coupled semiconductor photocatalysts may increase thephotocatalytic efficiency by increasing the charge separa-tion and extending the visible light adsorption [13]. Lots ofstudies related to the photocatalytic activity of TiO2 orZnO coupled with many different photocatalysts have beencarried out in the field of the photocatalytic pollutantsdegradation [14–18].In this report, the Bi2O3/SrTiO3 composite powders were

firstly prepared and their photocatalytic activities wereevaluated by photooxidation of methanol under visiblelight and UV–Visible light. The effects of the contents ofBi2O3 and the calcination temperatures on the photocata-lytic activities were also investigated.

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a

b

c

d

e

f

g

2θ ((degree))

inte

nsi

ty (

a.u

.)

Fig. 1. A comparison of X-ray diffraction patterns of Bi2O3, SrTiO3 and

Bi2O3/SrTiO3 composite powders. (a) Bi2O3(90wt%)/SrTiO3, (b) Bi2O3

(86wt%)/SrTiO3, (c) Bi2O3(83wt%)/SrTiO3, (d) Bi2O3(75wt%)/SrTiO3,

(e) Bi2O3(50wt%)/SrTiO3, (f) SrTiO3, and (g)Bi2O3.

a

b

c

d

e

f

Ab

sorb

ance

(a.

u.)

H. Zhang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2501–25052502

2. Experimental

All chemicals were of reagent grade and used withoutfurther purification. SrTiO3 was synthesized by the solid-state reaction of SrCO3 and TiO2 at 1373K for 2 h. Thecoupled Bi2O3/SrTiO3 composite powders were preparedas follows: the SrTiO3 and Bi2O3 powders with differentmass ratio were ground and mixed thoroughly in ethanol inan agate mortar. The prepared samples were dried at 333Kovernight and then calcined at 673, 773 and 873K for 5 h inan oven, respectively.

The crystal structure of the samples was determined bythe X-ray diffractometer (JEOL JDX-3500 Tokyo, Japan).The UV–Visible diffuse reflectance spectra of the preparedpowders were obtained by a UV–Visible spectrophot-ometer (Shimadzu UV-2500) at room temperature. TheSEM image was recorded with a JSM-6700F microscope.The surface area of the photocatalysts was determined byBET measurement (Micromeritics-3000, micrometrics,USA) on nitrogen adsorption at 77K after the pretreat-ment at 573K for 2 h.

In the photocatalytic measurement, a 3-g sample wasevenly spread over a dish with a diameter of 6.5 cm in a500mL Pyrex vessel. A 300W Xe lamp was used as thelight source (ILC Technology, CERMAX LX-300), Whichwas focused on window of the reaction cell through a long-pass cutoff filter (HOYA). After the sample was sealed inthe vessel, 1 mL of methanol was injected into the vessel,and then the samples were kept in the dark (the initialconcentration of methanol was about 1100 ppm). The finalproducts of the photocatalytic oxidation of methanol wereCO2 and H2O. To evaluate the photocatalytic activity, theamounts of CO2 were monitored by using gas chromato-graphy (Shimadzu, GC-14B, FID detector).

300 400 500 600 800

g

Wavelength (nm)700

Fig. 2. UV–Vis diffuse spectra of Bi2O3, SrTiO3 and Bi2O3/SrTiO3

composite powders. (a) Bi2O3, (b) Bi2O3(90wt%)/SrTiO3, (c) Bi2O3

(86wt%)/SrTiO3, (d) Bi2O3(83wt%)/SrTiO3, (e) Bi2O3(75wt%)/SrTiO3

(f) Bi2O3(50wt%)/SrTiO3, and (g) SrTiO3.

3. Results and discussion

XRD patterns of the prepared Bi2O3/SrTiO3 compositesamples are shown in Fig. 1. The diffraction peaks can beidentified as SrTiO3 and Bi2O3 with a cubic and monoclinicphase, respectively. The peaks related to bismuthate werenot observed in the prepared samples, which meant that theBi2O3 did not react with SrTiO3. With the increasing of theBi2O3 content, the intensity of the Bi2O3 peaks increased.

Fig. 2 shows the absorption spectra of SrTiO3, Bi2O3 andBi2O3/SrTiO3 composite powders. .In Fig. 2, two absorp-tion edges were observed for the adsorption spectra of theBi2O3/SrTiO3 composite powders. The UV absorptionedge at about 390 nm can be ascribed to the fundamentalabsorption of SrTiO3. The second absorption edge at about460 nm can be attributed to the Bi2O3 absorption. With theincreasing of the concentration of Bi2O3, the adsorption inthe visible light region of the composite powders alsoincreased. As for other composite powders, similar spectrawere observed. These results revealed that the compositepowders could adsorb visible light.

The photooxidation of methanol to CO2 on differentphotocatalysts under visible light (l4440 nm) irradiationfor 3 h was shown in Fig. 3. It can be found that TiO2–P25,pure SrTiO3 and Bi2O3 had little ability to mineralizemethanol under visible light irradiation. All the Bi2O3/SrTiO3 composite powders exhibited higher photocatalyticactivity than that of P25, pure SrTiO3 and Bi2O3. Theincrease of the photocatalytic activity of the compositepowders demonstrated that not only the compositepowders could absorb the visible light, but also electronexchange could occur between the semiconductors. Undervisible light irradiation, the photocatalytic activity of the

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Fig. 3. The photocatalytic activity of Bi2O3, SrTiO3, TiO2–P25 and Bi2O3/

SrTiO3 composite powders under visible light irradiation (l4440nm, 3 h)

(a) TiO2–P25, (b) SrTiO3, (c) Bi2O3, (d) Bi2O3(50wt%)/SrTiO3, (e) Bi2O3

(75wt%)/SrTiO3, (f)Bi2O3(83wt%)/SrTiO3 (g) Bi2O3(86wt%)/SrTiO3,

and (h) Bi2O3(90wt%) SrTiO3.

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CO

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Fig. 4. The photocatalytic activity of Bi2O3, SrTiO3 and Bi2O3/SrTiO3

composite powders under UV–Visible light irradiation (1 h) (a) Bi2O3,

(b) SrTiO3, (c) Bi2O3(50wt%)/SrTiO3, (d) Bi2O3(75wt%)/SrTiO3, (e) Bi2O3

(83wt%)/SrTiO3 (f) Bi2O3(86wt%)/SrTiO3, and (g) Bi2O3(90wt%)

SrTiO3.

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a

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Fig. 5. The photocatalytic activity of Bi2O3(83wt%)/SrTiO3 composite

powders calcined at different temperature under visible light irradiation

(l4440nm, 3 h) (a)573, (b)673 and (c)773K.

H. Zhang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2501–2505 2503

composite powders with the lowest concentration of Bi2O3

(50wt%) showed a little increment, compared to that of thepure SrTiO3, which mainly was due to the less absorptionin the visible light region than other composite powders(Fig. 2). It was also observed that the efficiency of thecomposite powders did not keep constant with theincreasing of Bi2O3 content. Under visible light irradiation,the Bi2O3 (83wt%)/SrTiO3 composite powders showed 4times higher photocatalytic activity than TiO2–P25. How-ever, the composite powders with much higher concentra-tion of Bi2O3 (86 and 90wt%) showed relatively lowerphotocatalytic activity than that of the samples with83wt%. For the composite powders with higher concen-tration of Bi2O3, the surfaces to react with methanol arerelatively fewer. It implied that the surface of thephotocatalyst, which contacts with the contaminant playsan important role in determining the photocatalyticactivity of the composite powders.

The photocatalytic activity of the composite powdersunder UV–Visible light irradiation was also evaluated, andthe results were shown in Fig. 4. The results showed that,under full arc of Xe lamp irradiation, the compositepowders with the lowest concentration of Bi2O3 (50wt%)showed best photocatalytic activity. The Bi2O3 (50wt%)/SrTiO3 composite powders also showed super photocata-lytic activity than pure SrTiO3. But all other compositepowders showed lower photocatalytic activity than pureSrTiO3 under the same condition. With the increment ofBi2O3 concentration, the photocatalytic activity of thepowders decreased. This phenomenon indicated that underUV light irradiation the increment of Bi2O3 concentrationlower the photocatalytic activity of the composite powders.Under the UV–Visible light irradiation, TiO2–P25 showedabout 3 times photocatalytic activity than pure SrTiO3 orthe Bi2O3/SrTiO3 composite powders, which should beattributed to the large surface area of TiO2–P25 (49m2/g).

The effect of the calcinations temperature on thephotocatalytic ability was also discussed. Fig. 5 shows theprofiles of the photooxidation of methanol to of CO2 undervisible light (l4440 nm) irradiation for 3 h by Bi2O3/SrTiO3 composite powders (83wt%) calcined at 573, 673and 773K. The sample calcined at 673K showed higherphotocatalytic activity than the one calcined at 573K. Butwith the increase of calcination temperature to 773K, noobvious increase of the photocatalytic ability was observed.These results revealed that when the sample calcinedrelative lower temperature, the contact between Bi2O3

and SrTiO3 is weaker than that calcined at highertemperature, which made electron exchange between thesemiconductors more difficult. But the increase of thecalcination temperature may not result in a more effectiveelectron exchange. The experiment results revealed that theproper calcination temperature is an important factor in

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Fig. 6. The CO2 concentration on the prepared composite powders

(83wt%, 673K) for 60min irradiation with (a) full arc Xe lamp and (b)

l4440 nm; (c) l4500 nm.

Fig. 7. Typical SEM micrograph taken from the particles of the

Bi2O3(83wt%)/SrTiO3 composite. The indexed result showed that the

composite was composed of Bi2O3 and SrTiO3.

e-

e-CB

VB

VB

Bi2O3

CB

OH-/O2

-3

-4

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Ene

rgy

repe

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uum

, eV

-5

H+/H2

SrTiO3h+

e-

Fig. 8. Energetic diagrams of Bi2O3/SrTiO3 heterojunction.

H. Zhang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2501–25052504

the preparation of the composite powders with a highphotocatalytic activity.

The photocatalytic ability of the composite powders(83wt%, calcined at 673K) was evaluated by usingdifferent cut-off filter. The wavelength dependence of thephotocatalytic reaction is used to prove if the reaction isreally driven by light irradiation, and the results wereshown in Fig. 6. It could be find that the photocatalyticactivity decreased with increasing wavelength of light,when a 500 nm cut-off filter was used almost no CO2

increment could be observed. This indicates that thisreaction is driven by light and that the absorption propertyof the photocatalyst governs the photocatalytic activity.

The surface area of Bi2O3/SrTiO3 composite pow-ders(83wt%,) calcined at 573, 673 and 773K was 1.0, 0.8and 0.7m2/g, while the surface area of TiO2–P25 is 49.1m

2/g,which showed that much higher photocatalysis efficiency ofBi2O3/SrTiO3 composite powders for the could be expectedif the surface area increased.

The SEM image of the composite powders (83wt%) wasshown in Fig. 7. From the SEM image, we could find thatthe SrTiO3 and Bi2O3 were well alloyed so that an ohmiccontact was likely to form. The well contact will facilitateinterparticle electron transfer between SrTiO3 and Bi2O3.

Fig. 8 depicts the flat band potentials [19] of the valenceand conduction bands at pH 0 for Bi2O3 and SrTiO3 withtheir band gap energy. As is shown, the conduction band ofBi2O3 is located at about �4.83 eV. This value is morenegative than the conduction band potential of SrTiO3

(�3.24 eV). The valence band of Bi2O3 and SrTiO3 locatedat �7.63 and �6.64 eV, respectively. It is clear thatinterparticle electron transfer is effective between bothsemiconductors, as demonstrated by the profiles ofphotocatalytic activity as shown in Fig. 3.

4. Conclusion

Bi2O3/SrTiO3 composite powders were prepared and thephotocatalytic activity of prepared samples was evaluated.

UV–Visible spectra showed that the Bi2O3/SrTiO3 compo-site powders were able to absorb visible light. The electrontransfer occurring inside the Bi2O3/SrTiO3 compositepowders was demonstrated by performing photocatalyticdecomposition of methanol under visible light andUV–Visible light irradiation. The composite powdersexhibited higher photocatalytic activity towards methanoldegradation than pure Bi2O3, SrTiO3 and TiO2–P25 undervisible light irradiation. In the composite powders, thecontent of Bi2O3 is an important factor in determining thephotocatalytic activity. The calcination temperature is alsoan important factor in obtaining high photocatalyticactivity. The discussions could give useful information forthe exploring of other composite powders.

Acknowledgments

The authors would like to acknowledge the financialsupport from the National Natural Science Foundation ofChina (Nos. 20373025 and 20528302), SRF for ROCS,SEM of China, the Jiangsu Provincial Natural ScienceFoundation of China (Nos. BK2006718 and BK2006127),and the Jiangsu Provincial High Technology Research

ARTICLE IN PRESSH. Zhang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2501–2505 2505

Project (No. BG2006030). One of the authors (ProfessorZou) would like to thank the Talent Project of Jiangsuprovince.

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