Applied Surface Science 255 (2008) 2365–2369

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Applied Surface Science

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Synthesis of Bi2O3–TiO2 composite film with high-photocatalytic activityunder sunlight irradiation

Jingjing Xu a,b,c,d, Yanhui Ao a,b,c,d, Degang Fu a,b,c,d,*, Chunwei Yuan a,b

a State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, Chinab School of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, Chinac Key Laboratory of Environmental and Bio-Safety in Suzhou, Research Institute of Southeast University, Dushu Lake Higher Education Town, Suzhou 215123, Chinad Jiangsu Laboratory for Biomaterials and Devices, Nanjing 210096, China

A R T I C L E I N F O

Article history:

Received 17 March 2008

Received in revised form 16 July 2008

Accepted 16 July 2008

Available online 25 July 2008

Keywords:

Bismuth oxide

Titania

Photocatalysis

Solar light

Micro–nano

A B S T R A C T

Bi2O3–TiO2 composite films were synthesized by a sol–gel method under mild condition (i.e. low

temperature and ambient pressure). The as-prepared films were characterized by X-ray diffraction,

scanning electron microscopy, X-ray photoelectron spectroscopy, infrared spectra and fluorescence

spectra. The results showed that Bi2O3–TiO2 composite films were composed of anatase titania and Bi2O3.

TiO2 particles were deposited on the surface of Bi2O3 to form uniform film. Incorporating Bi2O3 with TiO2

leads to increased surface OH group density. All Bi2O3–TiO2 composite films exhibited higher

photocatalytic activity compared with pure TiO2 under solar irradiation, while the film with Bi/Ti

atomic ratio of 1.25% showed the highest photocatalytic activity. Furthermore, the as-prepared films can

be reused with little photocatalytic activity decreasing. Without any further treatment besides rinsing,

the photocatalytic activity of Bi2O3–TiO2 (1.25%) films was still higher than 77% after six-cycle utilization.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Recently, many efforts have been devoted to developing TiO2

heterogeneous photocatalyst due to its promising application inenvironmental remediation and energy conversion [1,2]. As a wideband gap (3.2 eV for anatase) semiconductor [3], TiO2 need to beexcited by an ultraviolet light (UV, wavelength (l) <387 nm)which is less than 5% of the solar irradiance at the Earth’s surface.To prepare TiO2 photocatalysts with visible light responsibility,several strategies have been adopted. One of the approaches iscoupling TiO2 with other semiconductor with appropriate bandgaps. A large number of coupled polycrystallite or colloidalsemiconductor, in which the particles adhere to each other inso-called ‘‘sandwich structures’’ or present a ‘‘core-shell’’ geome-try, have been prepared such as SiO2–TiO2 [4], CdS–TiO2 [5], ZnO–TiO2 [6], SnO2–TiO2 [7], etc.

Bismuth oxide, Bi2O3, due to its high refractive index, dielectricpermittivity, marked photoconductivity and photoluminescence(PL) [8], is used in a variety of areas, such as sensor technology,optical coatings and electrochromic materials [8–10]. Bi2O3 has

* Corresponding author at: State Key Laboratory of Bioelectronics, Southeast

University, Nanjing 210096, China. Tel.: +86 25 85336250; fax: +86 25 83793091.

E-mail addresses: xujj@seu.edu.cn (J. Xu), Fudegang@seu.edu.cn (D. Fu).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.07.095

five main polymorphic forms, denoted by a-, b-, g-, d- and v-Bi2O3

[11,12]. Among them, the band gap of the low-temperature a-phase and high-temperature metastable b-phase are 2.85 eV and2.58 eV, respectively [13]. Kang et al. [14] found that Bi/TiO2

exhibited higher photocatalytic activity in decomposing CH3CHOthan that of pure TiO2 without H2O addition. However, Hong andKang [15] reported that Bi/TiO2 has lower photocatalytic activity indegrading benzene than that of pure TiO2 under UV irradiation.Among those investigations, the photocatalytic activity of Bi2O3/TiO2 composite materials under solar light has not been concerned.

In order to prepare titania photocatalyst with enhancedphotocatalytic activity under solar irradiation, in this paper,Bi2O3–TiO2 films were prepared by a modified sol–gel methodunder low temperature (<100 8C) and ambient pressure. Thephotocatalytic activity of these samples was evaluated bydegradation of azo dyes X-3B in aqueous solution under artificialsolar irradiation. The recycle ability of Bi2O3–TiO2 film was alsoevaluated.

2. Experimental

2.1. Preparation of Bi2O3–TiO2 sols

a-Bi2O3 powders were purchased from Sinopham ChemicalReagent Co. Ltd., with the size range of 0.5–2 mm, and mainly in

Fig. 1. XRD patterns of TiO2 and Bi–TO-1.25% particles.

Fig. 2. (a) SEM image of Bi–TO-1.25% film; (b) larger magnification micrograph of (a).

J. Xu et al. / Applied Surface Science 255 (2008) 2365–23692366

monoclinic phase. Bi2O3 was firstly added into abundant water,whose pH value was adjusted to 2.0. Then the mixture of Ti(OBu)4

and isopropyl alcohol (i-PrOH) was added dropwise into thesolution. The molar ratio of i-PrOH and water to Ti(OBu)4 were 1.42and 151, respectively. A series of samples with molar ratio of Bi/Tiranged from 0.89% to 1.75% were prepared, and these sampleswere defined as Bi–TO–X%, where X% denotes the Bi/Ti molar ratio.After complete hydrolysis of Ti(OBu)4, the solution was refluxed for20 h at 70 8C, and a Bi2O3–TiO2 sol was formed. For comparison,pure TiO2 sol was also synthesized using the method mentionedabove without adding Bi2O3.

2.2. Preparation of Bi2O3–TiO2 composite films

A glass plate (70 mm � 20 mm) was cleaned in turn by water,dehydrated alcohol and acetone. The cleaned plate was thenimmersed in the sol for 10 min. Afterwards it was drawn out at aspeed of 2 cm/min and dried at 333 K for 1 h. The Bi2O3–TiO2 filmsand pure TiO2 film deposited on glass plate can be obtained withthis method layer by layer.

2.3. Characterization

The as-prepared samples were identified by X-ray diffract-ometer (XRD, XD-3A, Shimadazu Corporation, Japan) usinggraphite monochromatic copper radiation (Cu Ka) at 40 kV,30 mA over the 2u range of 20–808. The morphologies of thefilms were characterized with a scanning electron microscopy(SEM, Sirion200, FEI, The Netherlands). The binding energywas identified by X-ray photoelectron spectroscopy (XPS) withMg Ka radiation (ESCALB MK-II). IR spectrum was recorded asKBr pellets on Shimadzu Fourier transform infrared (FTIR)spectrometer. For each recording of IR spectra, the quantity ofKBr was equal, and the concentration of sample was con-trolled at 10 wt% of the KBr pellet. All the samples were driedin the oven to get rid of water. The photoluminescence emi-ssion spectra of the samples were measured at room tempera-ture by LS-55 (PerkinElmer) illuminated with a 325-nm He–Cdlaser.

2.4. Photocatalytic studies

The photocatalytic activity of Bi2O3–TiO2 films was evalua-ted by degradation of X-3B in aqueous solution, while pure TiO2

film was used for comparison. A piece of glass plate withdeposited catalyst film was put into the cylindrical reactorcontained 10 mL of X-3B solution with an initial concentrationof 50 mg L�1. Prior to photocatalytic reaction, the reactorwas put in the dark for 30 min to reach adsorption–desorption equilibrium. Then the solution was verticallyirradiated from the top by a 250 W halogen lamp (InstrumentalCorporation of Beijing Normal University) which providedartificial solar light. After start of illumination, the solutionwas analyzed at regular intervals of 20 min by UV-Vis spectro-photometer. During all the process, air was pumped into thereactor.

2.5. Reuse of the catalyst film

Recycle experiments on photocatalytic decomposing of X-3B byBi–TO-1.25% and pure TiO2 were designed to examine thephotocatalytic activity in each cycle. After finishing a cycle, thefilm was rinsed by water and dried in the atmosphere, without anyother treatments. The recycle experiment was carried out for sixcycles.

3. Results and discussions

3.1. XRD patterns

Fig. 1 shows the XRD patterns of Bi–TO-1.25% particles and pureTiO2 particles. Pure TiO2 have significant diffraction peaksrepresenting the characteristic of anatase phase, while a smallamount of brookite is also detected. The XRD of Bi–TO-1.25% is verysimilar to that of pure TiO2, which indicates that the structure ofTiO2 was not affected by incorporation of Bi2O3 in the synthesis

Fig. 4. FTIR spectra of pure TiO2, Bi–TO-0.89%, Bi–TO-1.25% and Bi–TO-1.75%

particles.

J. Xu et al. / Applied Surface Science 255 (2008) 2365–2369 2367

process. No observation of Bi2O3 diffraction peaks may be due tothe low content of Bi2O3 in this composite photocatalyst. Thecrystal size of the obtained titania particles determined byScherrer’s equation is estimated to be 5–6 nm.

3.2. Morphology measurement

The morphology of Bi–TO-1.25% composite film is analyzed bySEM, and the results are given in Fig. 2. It can be seen from Fig. 2(a)that the composite material is composed of micro-scale Bi2O3

particles and sub-micro-scale clusters containing titania nano-particles with an average size of around 20 nm. The large Bi2O3

particles distribute uniformly in the film. Fig. 2(b) shows SEM ofone Bi2O3 particle with TiO2 nano-particles deposited on thesurface. This film with hierarchical structure containing micro,sub-micro and nano-scale elements may be benefit for the achie-vement of higher photocatalytic properties [16]. The observed sizeof TiO2 particles was not consisted with XRD analysis. Thedifference may result from the following reason: the XRD esti-mated value was the size of single crystallites, while measuredvalue in SEM was the size of agglomerates.

3.3. XPS analysis

In order to analyze the chemical composition and purity of theprepared particles, the XPS survey spectrum of Bi–TO-1.25%and the high-resolution XPS spectra of the Bi 4f region areshown in Fig. 3. It shows that Bi–TO-1.25% contains only Ti, O, Biand C elements. The C element can be ascribed to the residualcarbon from precursor, as our samples are all prepared at lowtemperature. The peaks centered at 164.2 eV and 158.9 eV shouldbe assigned to Bi 4f 5/2 and Bi 4f 7/2 region from a trace amountof Bi2O3 species [17]. The results are in good agreement withother investigators [18,19] and indicate that the composite filmconsists of titania and Bi2O3.

3.4. FTIR analysis

The surface hydroxyl groups on titania have been recognized toplay an important role on the photocatalytic reaction since theycan inhibit the recombination of photogeneration charges andinteract with photogenerated holes to product active oxygenspecies. The FTIR transmittance spectra of the samples are shown

Fig. 3. XPS survey spectrum of Bi–TO-1.25% and the high-resolution XPS spectra of

Bi 4f region.

in Fig. 4. The broad and strong peak at 1640 cm�1 is ascribed to thebending vibration absorption of free water, and the peaks at 3200–3600 cm�1 are attributed to the stretching vibration absorption ofhydroxyl function groups (TiO2–OH bonds) which is often believedthat such groups arise from the hydrolysis reaction in the sol–gelprocess [20]. The FTIR investigation confirms that pure titania hasthe lowest surface OH group density. The trend of the surface OHgroup density is as follows, Bi–TO-1.25% > Bi–TO-1.75% > Bi–TO-0.89% > pure TiO2. The larger surface hydroxyl group density willlead to enhancement of the photocatalytic activity. Because theycan interact with photogenerated holes, which gives better chargetransfer and inhibits the recombination of electron–hole pairs.

3.5. PL emission spectra analysis

PL emission spectra have been widely used to investigate theefficiency of charge carrier trapping, immigration, transfer and tounderstand the fate of electron–hole pairs in semiconductorparticles [21]. Fig. 5 shows that the PL spectra of Bi–TO and pureTiO2. The excitonic PL intensity of these samples decreases as thefollowing: pure TiO2 > Bi–TO-0.89% > Bi–TO-1.75% > Bi–TO-1.25%. This indicates that TiO2 incorporating with an appropriateamount of Bi2O3 may slow the radiative recombination process of

Fig. 5. Fluorescence spectra of TiO2, Bi–TO-0.89%, Bi–TO-1.25% and Bi–TO-1.75%

particles.

Fig. 6. Kinetic of X-3B degradation for TiO2, Bi2O3, Bi–TO-0.89%, Bi–TO-1.25% and

Bi–TO-1.75% films, respectively.

Fig. 7. Variations in ln(C0/C) as a function of irradiation time and linear fits of TiO2,

Bi2O3, Bi–TO-0.89%, Bi–TO-1.25% and Bi–TO-1.75% films, respectively.

Fig. 8. Percent of the degrading X-3B over the Bi–TO-1.25% and pure TiO2 films.

J. Xu et al. / Applied Surface Science 255 (2008) 2365–23692368

photogenerated electrons and holes in TiO2. The slower recombi-nation process of photogenerated charges will benefit thephotocatalytic reaction.

3.6. Photocatalytic activity

In order to investigate the photocatalytic activity of the as-prepared films, degradation experiments of X-3B were studiedunder solar light irradiation and the results are shown in Fig. 6. Theblank experiment without catalysts indicated that the merelyphotolysis can be ignored as it is about 0.9% after be illuminated for80 min. As shown in the figure, the photocatalytic activity of all theBi–TO films were higher than that of pure titania film. Thedegraded X-3B was 80.2%, 66.8%, 51.8% and 24.9% for Bi–TO-1.25%,Bi–TO-1.75%, Bi–TO-0.89% and pure titania, respectively. Forcomparison, we also study the photocatalytic activity of Bi2O3

film, the degradation ratio is just 5.5%.

Table 1kapp and R2 data for each sample

Samples Pure TiO2 Bi–TO-0.8

Apparent rate constant, kapp (min�1) 0.006 0.009

Regression relative coefficient, R2 0.996 0.998

The apparent rate constant (kapp) has been chosen as the basickinetic parameter for the different photocatalysts, since it enablesone to determine a photocatalytic activity independent of theprevious adsorption period in the dark and the concentration of X-3B remaining in the solution. The apparent first order kineticEq. (1) is used to fit experimental data (Fig. 6):

lnC0

C

� �¼ kappt (1)

where C is the concentration of X-3B remaining in the solution atirradiation time of t, and C0 is the initial concentration at t = 0 [22].The variations in ln(C0/C) as a function of irradiation time are givenin Fig. 7. The calculated kapp data for Bi–TO films and pure titaniaare exhibited in Table 1. It confirms that kapp enhanced byincorporating TiO2 with Bi2O3.

3.7. Recycle of catalysts

Regeneration of TiO2 photocatalyst was one of key steps tomake heterogeneous photocatalysis technology for practicalapplications. The degradation ratio of X-3B for Bi–TO-1.25% andpure titania films in each cycle are shown in Fig. 8. Thephotocatalytic activity of the composite film exhibits a littledecline after six cycles, and the degradation ratio was still higherthan 77%. While for pure titania, 15% decrease was observed. Theseresults illustrated that Bi–TO film has higher stability.

4. Photocatalytic mechanism of Bi2O3–TiO2 under solarirradiation

There are two response ways in X-3B/Bi–TO system undervisible illumination to induce photocatalytic reactions. One is X-3Bdye sensitization process [23] (as shown in Fig. 9(a)). It involves theexcitation of dye molecules by absorbing visible light photons andsubsequent electron injection from excitation state dye tocondition band (CB) of TiO2 and Bi2O3. Then the cation radicalformed at the surface quickly undergoes degradation reaction.

9% Bi–TO-1.25% Bi–TO-1.75% Bi2O3

0.020 0.014 0.0007

0.994 0.995 0.973

Fig. 9. Photocatalytic mechanism of Bi2O3/TiO2 film under visible light (a) X-3B dye

sensitization process; (b) Bi2O3 assisted photocatalytic process.

J. Xu et al. / Applied Surface Science 255 (2008) 2365–2369 2369

Another path is Bi2O3 assisted photocatalytic process (as shownin Fig. 9(b)). As the band gap of a-Bi2O3 is 2.85 eV, it can be excitedby light with wavelength less than 435 nm [24,25]. However, thephotocatalytic activity of Bi2O3 is very low (Figs. 6 and 7), this canbe ascribed to the high electron–hole recombination rate in Bi2O3

[26]. Because the valence band of Bi2O3 is lower than that of titania[27], the Bi2O3–TiO2 heterojunctions formed in the composite filmwill promote the photo generated holes in bismuth oxide to betransferred to the upper lying valence bands of titania (as shown inFig. 9). This process is thermodynamically feasible [28]. Therefore,the recombination rate of photo-induced electron–hole pairs wasreduced and much more holes were captured to induce photo-catalytic reactions. As a result, the photocatalytic activity of Bi2O3–TiO2 composite film enhanced a lot compared to the single phasefilms (pure Bi2O3 or TiO2).

5. Conclusion

In summary, Bi2O3–TiO2 composite films were synthesized by amodified sol–gel method under mild condition. The composite filmexhibited higher surface hydroxyl group density. All the Bi–TOfilms showed higher photocatalytic activity than pure TiO2 undersolar irradiation. The photocatalytic ratio increased as follows:pure TiO2 < Bi–TO-0.89% < Bi–TO-1.75% < Bi–TO-1.25%. Theenhanced photocatalytic activity was contributed to the specialfilm structure, increasing of OH group density and narrow bandgap of Bi2O3. The recycle of composite film was also investigated.The degradation ratio decreased only 3% after six cycles.

Acknowledgements

This work is financial supported by the National Natural ScienceFoundation of China (No. 60121101) and Joint project of GuangdongProvince and Education Department (No. 2007A090302018).

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