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National Centre for Nanoscience and Nanot

campus, Chennai 600025, India. E-mail

22352494/22353309; Tel: +91-044-2220274

Cite this: RSC Adv., 2013, 3, 23737

Received 1st August 2013Accepted 16th September 2013

DOI: 10.1039/c3ra44085k

www.rsc.org/advances

This journal is ª The Royal Society of

A strategy to fabricate bismuth ferrite (BiFeO3)nanotubes from electrospun nanofibers and their solarlight-driven photocatalytic properties

Sakar Mohan and Balakumar Subramanian*

A strategy has been developed to produce bismuth ferrite (BiFeO3/BFO) nanotubes from electrospun

nanofibers. It involves ramping the temperature to heat the as-spun BFO/PVP composite fibers to their

annealing temperature using different modes, which eventually decides the final morphology of the

BFO. Accordingly, we found that using step-by-step and direct ramping modes to reach the annealing

temperature, yielded nanofibers and nanotubes of BFO, respectively. From the respective XRD patterns,

the average crystallite sizes embedded in the BFO nanofibers and nanotubes were found to be around

15 and 20 nm, respectively. Further, these results were also substantiated through high-resolution

transmission electron microscopy images. The field emission scanning electron microscopy images

showed that the average diameter of the nanotubes and nanofibers was around 100 nm, while the

length varied from one to a few micrometers and the inner diameter of the nanotubes was found to be

around 10 nm. The optical characterization of these BFO nanotubes and nanofibers by UV-visible

absorption and diffuse reflectance spectrometry showed a band gap energy of around 2.38 eV and a

broad UV-visible absorption band between 300 and 500 nm, compared to the BFO bulk particles which

showed absorption only in the UV region. This observation promised visible light driven optical activity

of these 1D BFO nanostructures. As a result, improved photocatalytic activities were observed in both

the BFO nanotubes and nanofibers owing to their quantum efficiency. However, the nanotubes showed

a relatively enhanced photocatalytic activity compared to the BFO bulk particles and nanofibers. This

could be attributed to the fact that the nanotubes might possess more of the catalytic species on the

inner and outer surfaces that degrade the dye molecules. The observed results show that these one-

dimensional BFO nanostructures can become super-efficient solar light-driven photocatalysts for

environmentally benign applications.

1. Introduction

Bismuth ferrite (BiFeO3/BFO) is a single-phase multiferroicmaterial in which ferroelectricity and antiferromagnetismcoexist at room temperature with high antiferromagnetic andferroelectric transition temperatures of around TN 643 K and TC1100 K, respectively.1,2 This makes BFO suitable for applicationsat room temperature in magneto-electric devices, spintronics,data storage, and magnetic sensors.3–5 Besides, BFO shows aweak ferromagnetic behavior owing to its canted spin structurethat evolves through the crystal with an incommensuratewavelength of 62 nm.6,7 Along with its multiferroic properties,BiFeO3 is also of great interest for solar light-based applicationsdue to its photovoltaic effect.8–10 It has been reported that thephotocurrent in ferroelectric materials arises due to the electriceld depolarization, and this depolarization can split up the

echnology, University of Madras, Guindy

: balasuga@yahoo.com; Fax: +91-044-

9

Chemistry 2013

photo-generated charge carriers10–12 and effectively prevent theelectron–hole recombination that is generally observed intypical semiconductor photocatalytic materials. In particular,BiFeO3 has found extensive applications as a visible light-drivenphotocatalyst due to its small bandgap energy (�2.2 eV) andexcellent chemical stability. Accordingly, BFO has been fabri-cated in many forms such as microparticles, nanoparticles,nanocomposites, thin lms and nanobers, and these formshave been studied for their photocatalytic activities undervisible light.13–18 However, recent studies show that the one-dimensional nano-structured materials possess excellent phys-ical and chemical properties compared to the bulk materialsand thin lm materials.19 Consequently, the fabrication of one-dimensional BiFeO3 nanostructures, such as nanotubes,20,21

nanowires,22,23 nanorods,24,25 and nanobers,26,27 has drawnsignicant attention and these structures have displayedenhanced multiferroic and photocatalytic properties.28–30

Generally, these one-dimensional nanostructures are synthe-sized via various methods, such as anodic aluminum oxide(AAO) template,20,21,25 hydrothermal,22,23 precipitation,24 and

RSC Adv., 2013, 3, 23737–23744 | 23737

Fig. 1 Thermal decomposition graph of the as-spun PVP/BFO nanofibers.

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electrospinning methods.26,27 Among these, the electrospinningmethod offers an easy yet effective way to synthesize one-dimensional nanostructured bers in large quantities that are

Fig. 2 FESEM images of: (a) and (b) BFO nanotubes, and (c) and (d) BFO nanofibe

23738 | RSC Adv., 2013, 3, 23737–23744

easy to retrieve and study. Also, it is possible to employ theelectrospinning technique to fabricate nanotubes along withnanobers. Nevertheless, there are no reports where BFOnanotubes have been obtained by the electrospinning method.

Therefore, in this work we demonstrate a strategy to produceBiFeO3 nanotubes from nanobers that are fabricated by theelectrospinning method. It was found that the conversion ofnanobers to nanotubes takes place by modifying the rampingtemperature to its annealing temperature. Further, we have alsoinvestigated the photocatalytic activity of these one-dimen-sional BFO nanostructures under sunlight, on the degradationof organic pollutants such as methylene blue (MB). Theformation kinetics of these one-dimensional nanostructuresand their photocatalytic mechanism have also been discussedwith suitable schematics.

2. Experimental

The nanobers of BiFeO3 were synthesized by the electro-spinning method described below. Bismuth nitrate(Bi(NO3)3$5H2O) and iron nitrate (Fe(NO3)3$9H2O) were dis-solved in glacial acetic acid (C2H4O2) in stoichiometricproportions (1 : 1). Polyvinyl pyrrolidone (PVP) with a molecular

rs.

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weight of 1 300 000 was dissolved in doubly distilled water andthen directly added to the above solution. This was then stirredcontinuously to form a homogeneous BiFeO3 precursorsolution, with the concentration of PVP adjusted to around0.05 g ml�1. The solution was then placed in a plastic syringeequipped with a stainless steel needle and a high voltage (10 kV)was applied to the needle at a ow rate of 0.3 ml h�1. The bercollector plate was kept at a distance of 10 cm from the needle.Consequently, the solution was electrospun and the bers werecollected in a thick aluminum substrate that was spread overthe grounded plate collector. The as-spun bers were trans-ferred into an alumina crucible and annealed at 450 �C for2 hours in normal atmospheric conditions.

Further, it was realized that the formation of BiFeO3 nano-tubes and bers involved mediation of the ramping tempera-ture to heat the as-spun BFO/PVP composite bers to theirannealing temperature as described in the following section.

Fig. 3 Annealing-mediated evolution of BFO tubes and fibers.

Fig. 4 XRD patterns of BFO (a) nanotubes and (b) nanofibers.

3. Results and discussion

The thermal decomposition and combustion of BFO/PVP as-spun composite bers was studied by TGA analysis and theresult is shown in Fig. 1. The rst decomposition step between25 and 150 �C (15%) may be due to the release of adsorbed waterfrom both PVP and BFO.31 The second decomposition stepbetween 150 and 350 �C (40%) is the main decompositiontemperature range of PVP, where the breaking of the sidegroups of PVP (pyrrolidone) took place.32,33 The third decom-position step between 350 and 450 �C (15%) may be due to thecombustion of the carbonized residue of PVP and decomposi-tion of BFO. Finally, the decomposition of the electrospunBFO/PVP composite nanobers ended around 450 �C and nofurther thermal decomposition processes were observed athigher temperatures up to 1000 �C.

From the knowledge inferred from the TGA results, the as-spun bers were rst directly heated to 450 �C with a rampingrate of 5 �C per minute and then subsequently annealed for 2hours. Then the annealed powder was collected and itsmorphology was investigated using eld emission scanningelectron microscopy (FESEM). The morphology of the BFO wasfound to be a tube-like structure as shown in Fig. 2(a) and (b).The tubes possessed a diameter of around 100 to 300 nm withlengths of 1 to 2 micrometers. Secondly, the as-spun BFO/PVPcomposite nanobers were annealed by ramping the annealingtemperature step by step, that is, the temperature was initiallyraised to 150 �C then to 250 �C then to 350 �C and nally raisedto 450 �C with a uniform heating rate of 5 �C per minute. Aereach step, the particular temperature was kept for 15 minutesand then nally annealed for 2 hours when the temperaturereached 450 �C. Aer this step-wise annealing process, the endpowder was analyzed using FESEM and the brous morphologyof the BFO was observed. The average diameter of the bers wasfound to be 100 nm with lengths of over a few micrometers, asshown in Fig. 2(c) and (d).

The insets of Fig. 2(a) and (c) show photographed images ofthe fabricated BFO nanotubes and nanobers, respectively. Thedirectly annealed product, which contained nanotubes, was

This journal is ª The Royal Society of Chemistry 2013

found to be a powder in nature. However, the step-wiseannealed product, which contained nanobers, was found to besheet or mat like in nature. Interestingly, this sheet/mat likestructure of the BFO nanobers did not turn to powder evenwhen ground using a mortar and pestle.

Further, this sheet/mat like nature of the BFO nanoberscould not be dissolved or destroyed, even when we added it tothe dye solution during the photocatalytic studies (Fig. 9(a)),which is discussed in the following section. Therefore, theseobservations make it clear that the direct annealing and step-wise annealing mostly give homogenous nanotubes and nano-bers of BFO, respectively.

In our experiment, we found that the formation of nanotubesand nanobers was primarily dependent upon the mode oframping used to reach the annealing temperature. Accordingly,a possible mechanism for their formation could be explained asfollows. During the electrospinning of the BFO/PVP compositenanobers, the carbonyl and tertiary amine groups of the PVPmolecules could coordinate with Bi3+ and Fe3+ ions in the

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solution and form a metal–organic skeleton structure. Such astructure may induce stress in the composite bers. Therefore,it is thought that during the step-wise temperature ramping, theas-spun BFO/PVP composite nanobers may have graduallycontracted, and subsequently the stress which is being devel-oped in the as-spun bers might have been released slowlywithout causing any morphological damage to the brousstructure.

On the other hand, when the temperature ramping wasabrupt, the decomposed gaseous species may have diffusedfrom the interior of the BFO/PVP composite nanobers to thesurface, which resulted in the formation of void (hollow) spacesin the bers. More specically, the time period for the removal

Fig. 5 HRTEM images of (a) and (b) BFO nanotubes, and (c) and (d) fringes patter

23740 | RSC Adv., 2013, 3, 23737–23744

of PVP might have varied depending upon the amount of PVPmolecules present in the inter-space region of the two adjacentBFO particulates that resided inside the composite bers. Atthis stage, obviously, the pressure inside the composite berswas larger than the pressure outside. As a result, the BFOparticulates moved toward the outside creating void spacesalong the axial direction, thereby leading to the formation ofhollow bers. Along with this, the induced stress in the bersmight have also been released abruptly and broken the bersinto tube-like structures.

The formation process of the nanobers and nanotubes isillustrated in Fig. 3. However, the size of the inner diameter ofthe tubes is dependent upon various factors such as solvents,

n.

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heating rate, etc., which is a phenomenon that is still to beexplored. In our experiment, we observed that the inner diam-eter of the tubes was very small in the order of around 10 nm,which was evident from the HRTEM images discussed in a latersection.

X-ray diffraction analysis was carried out to examine thecrystalline structure of the fabricated BFO nanotubes andnanobers, and the results are shown in Fig. 4(a) and (b),respectively. The diffraction peaks were well matched with therhombohedral perovskite structure of BiFeO3 (JCPDS card no.20-0169). The average grain size was calculated using Scherrer'sformula and it was found to be around 15 and 20 nm for the

Fig. 6 (a)–(d) HRTEM images of BFO nanofibers.

This journal is ª The Royal Society of Chemistry 2013

nanobers and nanotubes, respectively. It is known that phasepure BFO is hard to synthesize and secondary phases oen formin the material. Consequently, some low-intensity peaks werefound that represent the presence of a small amount ofBi2Fe4O9 secondary phase. It is suggested that the slowdecomposition of the as-spun BFO/PVP composite nanobersmight have been favored for the formation of uniform bers butnot for the formation of pure phase BFO. However, suchsecondary phases can be reduced by annealing in a protectiveenvironment such as an Ar atmosphere.26,27,34,35

High-resolution transmission electron microscopy (HRTEM)was employed to analyze the crystalline structures of the BFO

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Fig. 8 Photocatalytic degradation graphs of BFO (a) particles, (b) nanofibers and(c) nanotubes, and (d) the photocatalytic efficiency graph.

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nanotubes and nanobers, and the images are shown inFig. 5(a)–(d) and Fig. 6(a)–(d). From the HRTEM images, it wasobserved that the BFO tubes were composed of dense nano-crystalline grains with sizes around 15–20 nm (inset of Fig. 5(a)).This can also be substantiated from the XRD studies. Similarly,the rhombohedral structure of the BFO tubes was alsoconrmed by HRTEM analysis from the measured latticedistance of 0.396 nm along the (012) plane (Fig. 5(c) and (d)).These observed values were consistent with the reported latticedistance values of rhombohedral-structured perovskite BiFeO3

systems.26–30 As previously mentioned, we observed that theinner diameter of these nanotubes was very small in the order of10 nm, as shown in Fig. 5(c) and (d). Further, Fig. 6(d) and itsmagnied image (inset) clearly reveal that the formed BFOnanobers are not hollow in nature.

Further, we have also synthesized BFO particles as describedin our previous report,36 in order to show the dimensiondependent optical properties of BFO particles, nanobers andnanotubes. Accordingly, the optical properties, such as theabsorption and reectance characteristics of the BFO particles,nanotubes and nanobers, were measured by UV-visibleabsorption and diffuse reectance spectroscopy and the resultsare revealed in Fig. 7. The broad absorption band in the range of300–500 nm indicated that the fabricated BFO nanotubes andnanobers absorbed substantial quanta of both UV and visiblephotons. On the other hand, the absorption band for the BFOparticles appeared only in the UV region.

A possible reason for the improved visible photon absorptionof these one dimensional BFO structures is the electronictransitions from the valence band to the conduction band (O2�

2p / Fe3+ 3d) in the BiFeO3 lattices. Unlike in three-dimen-sional materials (BFO particles), the electronic properties ofone-dimensional materials are modulated either along its radialor axial directions, and therefore the manifestation of signi-cant quantum effects in these materials is observed.37 Subse-quently, the photocatalytic activities of the fabricated bulk BFO

Fig. 7 UV-visible absorption and diffuse reflectance spectra of the BFO particles,nanofibers and nanotubes.

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particles and the one-dimensional nanostructures of BFO wereexamined on MB under solar light irradiation. The degradationand C/C0 ratio graphs are shown in Fig. 8(a)–(d). It was foundthat the bulk BFO particles and the nanobers degraded around55% and 70% of the dye, respectively, aer 4 hours, while thenanotubes degraded around 82% of the dye in the same time.

The observed difference between the degradation efficiencyof the BFO nanotubes and nanobers was essentially attributedto their dispersing nature and the effective catalytic activities ofthe inner and outer surfaces with the dye molecules.

There are essentially two possibilities that would enhancethe catalytic activity in the tube-like structures. They are, (1) it ispossible for the excited photocatalytic species to reach both theinner and outer surfaces of the tubes, and (2) it is also possiblefor the dye molecules to get into the tube channels and reactwith the catalytic species that are excited to the inner and outersurfaces of the tubes. However, for the BFO bers the excited

Fig. 9 Dispersing nature of the (a) nanofibers and (b) nanotubes in the MB dyesolution – photocatalytic studies.

This journal is ª The Royal Society of Chemistry 2013

Fig. 10 Mechanism of the photocatalytic process occurring in the BFO nano-tubes and nanofibers.

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catalytic species will only be available at the outer surfaces andthey could degrade only the dye molecules that are in contactwith the surface of the bers. Accordingly, in our experiment weobserved the excellent disperse nature of the nanotubescompared to nanobers, as shown in Fig. 9(a) and (b). Thiswould allow the dye molecules to get into the tube channels andtherefore get degraded more easily.

Nevertheless, the relative photocatalytic enhancement ofthese BFO nanotubes and nanobers could be essentiallyattributed to their one-dimensional structures. It has beenreported that in one-dimensional structures, the average timefor photo-excited carriers to diffuse from the interior to thesurface (�10�12 to 10�10 s) is signicantly decreased withrespect to the electron-to-hole recombination times (�10�9 to10�8 s). This signies that the surface photo-generated redoxreactions occur with quantum yields close to unity, especiallywhen the reactants arrive at the surfaces rapidly enough andwithout limiting the interfacial charge transfer rates. Moreover,this fast diffusion rate of electrons–holes to the surface of ananostructured material provides signicant enhancements tothe optical activity of the material.38,39 On the basis of our study,Fig. 10 illustrates the optical- and photo-induced catalyticprocess occurring in the one-dimensional nanostructured tubesand bers.

4. Summary and conclusion

We have reported a strategy to obtain BiFeO3 nanotubes fromelectrospun nanobers. It was observed that the formation ofnanotubes from the nanobers was dependent upon theramping temperature mode used to anneal the as-spun BFO/PVP composite nanobers. Correspondingly, we obtainednanobers of BFO while performing the annealing process byramping the temperature step-by-step and nanotubes wereobtained while ramping the temperature abruptly. This wasessentially because of the uncontrolled release of stress fromthe as-spun BFO/PVP composite nanobers that facilitated theformation of nanotubes, while the controlled release of stressduring the step-wise annealing resulted in the formation of

This journal is ª The Royal Society of Chemistry 2013

uniform nanobers of BFO. The photocatalytic activities ofthese one-dimensional nanostructures and bulk particles werestudied under the sunlight exposure. As a result, the nanotubesof BFO showed an enhanced photocatalytic activity compared tothe bulk particles and nanobers. Such an enhancement mightbe due to the increased number of induced catalytic species onboth the inner and outer surfaces of the nanotubes that maxi-mise the degradation of the dye molecules present in andaround the nanotubes. However, the overall photocatalyticenhancement of BFO nanotubes and nanobers, compared tobulk BFO, can be attributed to their one-dimensional structuressince the catalytic species can be rapidly accelerated to thesurface where they signicantly reduce the fast recombinationrate of electrons–holes pairs. Such a slow recombination effectcould make more electrons available on the surface of thematerial, thereby enhancing the redox reaction and degradingthe dye molecules very efficiently as observed in thepresent study.

Acknowledgements

The authors gratefully acknowledge the Council of Scienticand Industrial Research (CSIR), Govt. of India for funding andfellowship to carry out this research work.

Notes and references

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