Preparation of TiO2/Bi2O3Microf i bers and Their Photocatalytic Activity

Preparation of TiO2/Bi2O3Microf i bers and Their Photocatalytic Activity

Zhan-ying Ma∗,Ling-juan Deng,Xiao-bo Li,Guang Fan∗
College of Chemistry and Chemical Engineering,Xianyang Normal University,Xianyang 712000,China

A series of TiO2/Bi2O3heterojunction microf i bers have been fabricated using cotton f i bers as bio-templates,and characterized by XRD,SEM and UV-Vis techniques.Results reveal that Bi2O3in the TiO2/Bi2O3sample is assigned to monoclinic and tetragonal mix-crystal phase.Fibers lengths can reach several micrometers and diameters range from 0.5µm to 3µm.Compared with pure TiO2and Bi2O3,TiO2/Bi2O3samples display better absorption in visible light region.Photocatalytic activity was evaluated by degradation of MB under visible light irradiation.TiO2/Bi2O3microf i bers exhibite much higher activity than pure TiO2and Bi2O3,and 22.84%TiO2/Bi2O3can achieve the decomposition of about 95%MB, which is attributed to synergistic effects of the strong visible-light absorption of TiO2/Bi2O3microf i bers and the heterojunction formed between TiO2and Bi2O3.

TiO2/Bi2O3microf i ber,Visible light,Heterojunction,Photocatalysis

I.INTRODUCTION

During the past decades,semiconductor photocatalytic process has been proven a promising method in the destruction of organic pollutants in wastewater due to its strong oxidation power,moderate operation temperature,and relative“green”f i nal products.Bismuth oxide(Bi2O3)is an attractive semiconductor material with a direct band-gap of 2.8 eV[1],which can be excited by visible light,and is widely used for pollutant decomposing under visible light irradiation[2,3].The band value provides the potential for the generation of reactive radicals,O2·-and OH·,in photocatalytic reactions.However,the photocatalytic activity of Bi2O3is low due to the photocorrosion and recombination of photogenerated electron-hole pairs.So its efficiency under visible light is required to be improved for the costeffective removal of specific organic contaminants.To improve of charge separation,increase charge carrier lifetime,and thus enhance photocatalytic activity due to high efficiency of the interfacial charge transfer from catalyst to adsorbed substrates,coupled semiconductor photocatalysts have been used[4].To date,the most commonly used TiO2have been extensively chosen as potential candidate for forming diverse heterojunctions in photocatalysts because of its non-toxic and stability nature[5].Recently,efforts have been made to look for the possibility of Bi2O3-TiO2combination in a photocatalyst,and its efficiency to degrade methyl orange (MO)has been reported[6].

It is well known that the morphology of materials has an important effect on its catalytic property[7, 8].Therefore,great efforts have been made to combine TiO2and Bi2O3with different morphologies[8,9]. Microf i bers,an important subclass of microstructure materials,have several advantages such as increasing the interactive surface area between photocatalyst material and target pollutant,providing more active sites, and facilitating degradation product molecule dif f usion, which could probably enhance their photocatalytic activity.Photocatalysts with nano-or micro-f i ber structure have been fabricated[10,11].

Template procedures are an ideal way to control material structure,including the outer morphology,size, the inner pore size,and distribution[12].In recent years,natural living things(such as cotton[10,13],silk [14],wood[15],paper[16],etc.),are commonly used as template,which are easily removed with heating procedures.Cotton template has been proven useful for the formation of nanof i bers[10]or microtubes[13].

In this work,TiO2/Bi2O3heterojunction photocatalysts with microf i ber morphology were produced with cotton as the template to enhance their photocatalytic activity.The photocatalytic property of TiO2/Bi2O3heterojunctions was tested by the degradation of methylene blue(MB)under visible light irradiation,and compared with pure Bi2O3and TiO2.22.84%TiO2/Bi2O3heterojunction shows superior visible-light absorption and higher photocatalytic activity in the degradation of MB.Moreover,the possible reasons for the enhanced photocatalytic activity of TiO2/Bi2O3heterojunction were also discussed.

II.EXPERIMENTS

All chemicals were analytical reagent grade and used as received without further purification.In a typicalprocedure,0.485 g of Bi(NO3)3·5H2O and 0.384 g of citric acid were dissolved in nitric acid aqueous solution,the pH was adjusted to 7.0 by dropwisely titration of ammonia solution under stirring,and transparent solution A was formed.Different dosage of n-tetrabutyltitanate(C16H36O4Ti)was dissolved in ethanol at room temperature,thus pale-yellow solution B was formed.The dried and loose cotton f i bers were immersed into the mixture of solution A and B.After immersing for 24 h,the cotton f i bers were taken out,dried at 50°C for 12 h,which were then placed in an alundum crucible and calcined in air at 500°C for 1 h.Finally,pale-yellow TiO2/Bi2O3heterojunction were obtained.These as-synthesized materials were denoted as xTiO2/Bi2O3,x indicates the molar ratio.

FIG.1 XRD patterns of pure TiO2,pure Bi2O3and TiO2/Bi2O3composites.(a)PureTiO2,(b)pure Bi2O3,(c)5.71%TiO2/Bi2O3,(d)11.42%%TiO2/Bi2O3, (e)17.12%TiO2/Bi2O3,(f)22.84%TiO2/Bi2O3,and(g) 28.53%TiO2/Bi2O3.

The phase identification of the as-prepared powders were obtained on a Riggaku D/max-3C X-ray powder X-ray dif f ractometry using Cu Kα radiation (λ=1.5405˚A,40 kV,40 mA).Scanning electron microscopy(SEM)images were observed by Hitachi S-4800 scanning electron microscopy.UV-visible diffuse ref l ectance spectra(UV-DRS)of the samples were recorded on Lambd 950 spectrophotometer using BaSO4as reference.

The photocatalytic activity of TiO2/Bi2O3heterojunctions was evaluated by measuring the degradation of the MB under visible-light irradiation.In a typical process,100 mg TiO2/Bi2O3samples were added to 100 mL of 10 mg/L MB solution and then bubbled in the dark for 20 min,to reach adsorption equilibrium and uniform dispersity.The solution was then exposed to visible light irradiation from a 300 W Xe lamp at room temperature.At given time intervals(20 min), 5 mL of the suspension was withdrawn and centrifuged to remove the photocatalyst powders for analysis.The concentration of MB was monitored using UV-Vis spectrophotometry by measuring the absorbance at 664 nm.

III.RESULTS AND DISCUSSION

A.Crystal structure

XRD is used to determine the phase structure of the samples.Figure 1 shows XRD patterns of pure TiO2,pure Bi2O3,and TiO2/Bi2O3heterojunctions. All the peaks of the pure TiO2sample are assigned to the anatase TiO2(PDF No.21-1272)and those of pure Bi2O3are indexed to two distinct crystal phases monoclinic α-Bi2O3(PDF No.76-1730)and tetragonal β-Bi2O3(PDF No.76-147).In the case of TiO2/Bi2O3composites,all the intensive and sharp ref l ection peaks can match well with pure Bi2O3(mixed crystal phase), and the characteristic peaks observed for α-Bi2O3are assigned to dif f raction from the(002),(041),(104),and (212)planes[17].The characteristic peaks of β-Bi2O3correspond to the dif f raction from the(211),(220), (400),and(402)planes[18].

With increasing the contents of TiO2in the composites,dif f raction peaks of TiO2/Bi2O3composites shift slightly(Fig.1),compared with those of pure Bi2O3, which may be attributed to the formation of p-n heterojunction which leads to the lattice distortion.No TiO2dif f raction peaks are detected in the TiO2/Bi2O3pattern,which may be ascribed to low TiO2content [19].In addition,as shown in Fig.1,no other dif f raction peaks of Bi related impurities are noted,which conf i rm the complete decomposition of bismuth nitrate to bismuth oxide.

B.SEM observation

Morphologies of as-prepared samples are investigated with SEM,and their typical images are displayed in Fig.2.It can be observed that these TiO2/Bi2O3composites were consisted of a large quantity of microf i ber structures.Their lengths can reach several micrometers and diameters range from 0.5µm to 3µm.The surfaces of these microf i bers are coarse and have many microstructures on them.Furthermore,the SEM images obtained with different dosage of TiO2show nonotable differences.All these TiO2/Bi2O3microf i bers present as straight or twisted shapes,which are well consistent with the straight or twisted shapes of cotton template,indicating the formation of biomorphic TiO2/Bi2O3microf i bers via cotton template.

FIG.2 SEM images of TiO2/Bi2O3microf i bers obtained with different dosage of TiO2.(a)5.71%TiO2/Bi2O3, (b)11.42%TiO2/Bi2O3,(c)17.12%TiO2/Bi2O3,(d)22.84%TiO2/Bi2O3,and(e)28.53%TiO2/Bi2O3.

FIG.3 (a)UV-Vis dif f use ref l ectance spectra and(b)the band gap energies(Eg)of the pure TiO2,Bi2O3,and TiO2/Bi2O3composites.

C.UV-Vis DRS analysis

Figure 3(a)demonstrates the UV-Vis DRS spectra of the pure Bi2O3,pure TiO2and TiO2/Bi2O3heterostructures.It is shown that,pure TiO2displays no absorption of visible-light.While Bi2O3and a series of TiO2/Bi2O3heterostructures display prominent photoabsorption ability in the visible region,implying that they have the potential to be efficient visiblelight-driven photocatalysts.In addition,among them, 22.84%TiO2/Bi2O3heterostructure present a better visible light absorption performance.

Based on the DRS data,as shown in Fig.3(b), thebandgapsofpureTiO2,pureBi2O3and 22.84%TiO2/Bi2O3composites are estimated to be 3.20,2.89 and 2.55 eV based on the following formula [20]:

The change in the band gap values is ascribed to the introduction of TiO2into Bi2O3.The formation of heterojunction between Bi2O3and TiO2narrows the band gap and extends the optical absorption range.Thus the results further indicate that TiO2/Bi2O3composite photocatalyst has a wider photoabsorption range and more suitable band gap for photocatalytic applications.

D.Photocatalytic properties

1.Photocatalytic activities

FIG.4 Visible-light photocatalytic activities of pure TiO2, pure Bi2O3and TiO2/Bi2O3composites for the degradation of MB.

The photocatalytic activities of as-prepared photocatalysts were evaluated for degradation of MB under visible light.Figure 4 exhibited the plots of ct/c0versus irradiation time for different samples,where c0and ctis the MB concentration before and after irradiation.As can be seen from Fig.4,TiO2exhibits no visible-light-driven photocatalytic activity, but all the TiO2/Bi2O3heterojunctions and pure Bi2O3exhibit considerably high photocatalytic activities in the decomposition of MB under visible light irradiation.What’s more,the photocatalytic activities of TiO2/Bi2O3heterojunctions are all higher than that of pure Bi2O3.22.84%TiO2/Bi2O3exhibits the highest photocatalytic activity,which can achieve the decomposition of about 95%MB upon visible light irradiation for 160 min.

There are two possibile machanisms,that is photocatalytic decomposition of MB,and production of the doubly reduced form of MB,leuco-MB[21].With the addition of AgNO3into the system,the solution was not recovered to blue,indicating that the MB was not reduced to leuco-MB,which can be reversibly oxidized to MB by AgNO3[22].Figure 5 shows the evolution of MB absorption spectra,from which it could be seen that the absorption peak of MB at 664 nm drops rapidly in the presence of 22.84%TiO2/Bi2O3.This indicates that the MB dye is photocatalytically decomposed by 22.84%TiO2/Bi2O3.The color of the dispersion almost disappears after 160 min of irradiation,indicating that the chromophoric structure of the dye was destroyed.

2.Photocatalytic mechanism

In order to investigate the mechanism of the enhanced photocatalytic activity of heterojunction,the relative band position of the two semiconductors should be confi rmed,because the band-edge potential positions play an important part in determining the fl owchart of photoinduced charge carriers in the heterojunction structure[23,24].The band edge positions of conduction band(CB)and valence band(VB)of the semiconductor at the point of zero charge can be calculated by the empirical equation[25,26]:

FIG.5UV-Vis absorption of the MB aqueous solutions under visible light irradiation in the presence of 22.84%TiO2/Bi2O3for different time.

where χ is the absolute electro-negativity of the semiconductor(χ is 5.90 and 6.09 eV for TiO2and Bi2O3,respectively[9]).ECis the energy of free electrons on the hydrogen scale(4.5 eV),and Egis the band-gap of the semiconductor.Therefore,according to the Eq.(2),the calculated CB and VB of TiO2are-0.2 and 3.00 eV, and those of Bi2O3are 0.18 and 2.99 eV,respectively.

Theimprovedperformanceobservedoverthe TiO2/Bi2O3composite compared to pure TiO2and Bi2O3can be ascribed to formation of the p-n junction.When p-type Bi2O3and n-type TiO2are contacted,the Fermi level of p-type Bi2O3moves up,while in the meantime,that of n-type TiO2moves down until the equilibrium state is formed.Consistent with the moving of the Fermi level,the whole energy band of p-type Bi2O3is raised up,while that of n-type TiO2is descended.An inner electric field from n-type TiO2to p-type Bi2O3is thus established.The activity enhancement of Bi2O3was ascertained owing to this high efficient separation mode for TiO2/Bi2O3heterojunction[27].According to the schematic diagram in Fig.6, under visible light irradiation,only Bi2O3could be activated and induce the generation of electron-hole pairs. The produced electrons on the CB of Bi2O3are transferred to that of TiO2,whereas h+still remains in the VB of Bi2O3.Thus,the separation of e-and h+in Bi2O3can be promoted,and accordingly their recombination is reduced.The more efficient separation of e-and h+can increase their lifetimes and enhance the efficiency of their transfer to the adsorbed substrates. Similar mechanism was presented for BiOI/TiO2[28] and Bi2WO6/TiO2[29].Hence,it is reasonable that Bi2O3/TiO2composites can exhibit higher photocatalytic efficiencies than pure TiO2under visible light irradiation.

FIG.6 Schematic diagram of separation of electron-hole pairs over TiO2/Bi2O3p-n junction under visible light irradiation.

With regard to the difference in the photocatalytic activities of TiO2/Bi2O3heterojunctions,it should be attributed to the combined action of many factors,such as composition,size,speci fic area,and adsorption capacity for MB,etc.Since all the other in fl uencing factors(e.g.,size,speci fic surface area and adsorption capacity,etc.)are also dependent on the composition. It is believed that the composition of Bi2O3/TiO2heterojunctions should play a dominant role in their photocatalytic activities.Bi2O3possesses relatively high visible-light-driven photocatalytic activity in the degradation of MB,whereas TiO2has no photocataltic activity under visible light irradiation.When TiO2content is too low(e.g.,5.71%TiO2/Bi2O3,11.42%TiO2/Bi2O3), Bi2O3cannot contact“enough amount”of TiO2or the surface of Bi2O3is insufficiently covered with TiO2, which restricts the efficient interfacial electron transfer from Bi2O3to TiO2.On the other hand,too much TiO2possibly blocks the incident light irradiation on Bi2O3. Therefore,when TiO2/Bi2O3heterojunctions have a too high content of TiO2(e.g.,28.53%TiO2/Bi2O3), they cannot accomplish the highest photocatalytic efficiency[30].Therefore,there must be an optimum composition for TiO2/Bi2O3heterojunctions to achieve the highest photocatalytic efficiency.In this work, 22.84%TiO2/Bi2O3with a suitable content of TiO2exhibits the highest photocatalytic efficiency among a series of TiO2/Bi2O3heterojunctions.

IV.CONCLUSION

A heterojunction photocatalyst with microberstructure is prepared using cotton bers as bio-templates.Introduction of TiO2to Bi2O3don’t change the crystalline structure of Bi2O3,but extend the photoabsorption region apparently.Photocatalytic experiments show that TiO2/Bi2O3heterojunction exhibites enhanced photocatalytic activity compared with pure Bi2O3,and 22.84%TiO2/Bi2O3exhibited the highest photocatalytic activity towards the degradation of MB, and 95%MB can be degraded within 160 min.Based on the UV-Vis DRS data,a possible photocatalytic mechanism is proposed.The enhanced photocatalytic activity of 22.84%TiO2/Bi2O3is mainly ascribed to the effective separation of photoinduced electron-hole pairs at the interface of heterojunction.Besides,the wider photoabsorption range and better crystallinity also favor its excellent photocatalytic performance.This work provides a new insight for developing novel composite catalyst,as well as offering high efficient visible-lightdriven photocatalysts for water purification and environmental remediation.

V.ACKNOWLEDGEMENTS

This work was supported by the Scientific Research Program Funded by Shaanxi Provincial Education Department(No.2013JK0690),and the Shaanxi Province Natural Science Foundation(No.2013JM2013),the NationalNaturalScienceFoundationofChina (No.21203160),and the Special Research Fund of Xianyang Normal University(No.11XSYK204).

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ceived on March 12,2014;Accepted on May 8,2014)

∗Authors to whom correspondence should be addressed.E-mail：fanguang2004@163.com,mazhanying@163.com,Tel.：+86-29-33720371