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Room temperature ferromagnetism in undoped ZnO nanofibers prepared by electrospinning
Physica B 448 (2014) 112–114
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Room temperature ferromagnetism in undoped ZnO nanofibers prepared by electrospinning Arnab Kumar Das a,n, Manoranjan Kar b, Ananthakrishanan Srinivasan a a b
Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India Department of Physics, Indian Institute of Technology Patna, Patna 800013, India
art ic l e i nf o
a b s t r a c t
Available online 27 March 2014
We report ferromagnetic behavior in undoped ZnO nanofibers prepared by electrospinning a solution of zinc acetate and poly vinyl alcohol followed by annealing at 550 1C for about 90 min. X-ray diffraction patterns of the heat treated as-spun composite fibers reveal the formation of ZnO nanowires in wurtzite structure with no noticeable impurity phases. ZnO nanowires annealed between 500 1C and 600 1C exhibited room temperature ferromagnetism with decreasing magnetization with increasing annealing temperature. Room temperature ferromagnetism was observed in as-spun fibers annealed in air as well as under vacuum. However, vacuum annealed nanofibers show higher magnetization as compared to air annealed fibers, which indicates that oxygen vacancy is a cause for the observed room temperature ferromagnetism in the ZnO nanofibers. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO Electrospinning Ferromagnetism Defects
1. Introduction Diluted magnetic semiconductors (DMS) are of keen contemporary interest due to their possible application in spintronic devices [1] and their relatively high Curie temperature (TC). Dietl et al. [2] were the first to theoretically predict room temperature ferromagnetism (RTFM) in wide band gap semiconductors. Mn [3] and Co [4] doping in wide band gap oxide semiconductors have been shown to induce room temperature ferromagnetism establishing the existence of DMS. Recently, room temperature ferromagnetism has been observed in undoped wide band gap semiconductors such as TiO2 [5], ZnO [6–13], MgO [14], etc. However, the magnetization of these undoped DMS is very low and the origin of ferromagnetism in these materials is not fully understood. There are some suggestions in the literature that the ferromagnetic interaction in undoped ZnO is due to Zn vacancies [8,15] while others propose O vacancies [16,17] or zinc interstitials [18] as the possible reason for the observed spontaneous magnetization. There are also suggestions that ferromagnetism in ZnO could arise from surface defects and sample preparation conditions [19]. Though no conclusion could be drawn about the origin of ferromagnetism in DMS, it is generally accepted that defects have an important role in RTFM. ZnO is one of the most prominent wide band gap semiconductor with a large exciton binding energy
n
Corresponding author. Tel.: þ 91 361 2582743; fax: þ 91 361 2582749. E-mail addresses: [email protected] (A. Kumar Das), [email protected] (M. Kar), [email protected] (A. Srinivasan). http://dx.doi.org/10.1016/j.physb.2014.03.045 0921-4526/& 2014 Elsevier B.V. All rights reserved.
at room temperature which is extensively used in optoelectronic devices [20]. Undoped nanocrystalline ZnO has been prepared by pulsed laser deposition [7], co-precipitation [12], pulsed electron deposition [13] and mechanical milling [21] routes. Recently, ZnO polyvinylpyrrolidone (PVP) nanofibers have been prepared by electrospinning method [22]. These studies show that the preparative route has distinct influence on the properties of the DMS and the search for the best route of preparation is still on. In this work, we have prepared undoped ZnO/polyvinyl alcohol (PVA) nanofibers by electrospinning route. The objectives of this work are to obtain RTFM in this DMS and to understand the role of defects (Zn or O) in this DMS.
2. Experimental Aqueous polyvinyl alcohol (PVA, MW 80,000) solution (12 wt%) was first prepared by dissolving PVA powder in deionized water. Then, zinc acetate (20 wt%) solution was added slowly to the aqueous PVA solution with continuous stirring of 2 h. The mixture was kept at room temperature for 4–5 h to obtain a viscous solution suitable for electrospinning. The solution was electrospun using a commercial electrospinner (NABOND NEU) under a potential difference of 16 kV, spinneret to collector distance of 12 cm and solution flow rate of 2 ml/h. The composite nanofibers ejected from the tip of the metallic syringe needle (positive electrode) were collected on a grounded collector. As-spun nanofibers, taken either in sealed fused silica ampoules under 10 3 Pa or in an open alumina crucible, were
A. Kumar Das et al. / Physica B 448 (2014) 112–114
annealed for 90 min in an electric furnace to obtain ZnO nanofibers. The annealing temperature (TA) was varied between 500 and 600 1C. The morphology of the as-spun and heat treated nanostructures was imaged using a field effect scanning electron microscopy (FESEM, Sigma, Zeiss). Structural characterization of the nanofibers was carried out by using a powder X-ray diffractometer (Rigaku TTRAX III 18 kW) operating with Cu Kα radiation (λ¼0.1542 nm). A microRaman spectrometer (Seki Technotron Corp. STR 500) was used to obtain structural information and to confirm the presence of oxygen defects in the annealed samples. Magnetic properties were measured using a vibrating sample magnetometer (VSM, Lakeshore model 7410) equipped with a variable temperature sample stage.
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and 600 1C for 90 min each. The hysteresis loops establish that all the samples have spontaneous magnetization (or ferromagnetism) at room temperature after subtracting the diamagnetic effect. The M–H curves indicate that saturation in magnetization is attained within an applied field of 10 kOe. It is clearly seen that with increasing TA, saturation magnetization (Ms) decreased. Maximum Ms of 0.039 emu/g was obtained for the composite nanofibers heat treated at 500 1C, while the sample heat treated at 600 1C exhibited the lowest Ms of 0.007 emu/g. These values are comparable to the values of 0.047 emu/g and 0.0054 emu/g reported for mechanically milled [21] and co-pecipitated [12] undoped ZnO. Inset in Fig. 4 shows variation of Ms with TA in the range of study. Though RTFM has been reported by other research groups in
3. Results and discussion The morphology of the as-spun and annealed composite nanofibers was characterized by FESEM (cf. Fig. 1). It is clearly seen from the images that the as-spun fibers are smooth and bead free because of the amorphous nature of the zinc acetate/PVA composite. But, after annealing, the surface of the nanofibers turns rough and the fiber diameter decreases as a consequence of the removal of PVA and the conversion of the Zn salt to ZnO. Typical X-ray diffraction (XRD) patterns of the composite nanofibers annealed at 500 1C and 550 1C for 90 min are shown in Fig. 2. Here, all the peaks could be indexed to hexagonal wurtzite structure of ZnO (JCPDS file # 36-1451) which establishes that ZnO exists as a pure single (wurtzite) phase structure without any other impurity phases. ZnO/PVP nanofibres synthesized earlier by electrospinning method [22] also crystallized in the same hexagonal structure. The average crystallite size was estimated from the most intense XRD peak [(1 0 1)] to be about 33 nm by using Scherrer's formula [23]. Fig. 3 shows the Raman spectra of ZnO nanofibers annealed at 500 1C and 550 1C for 90 min. The strong Raman peak near 437 cm 1 corresponds to Ehigh phonon mode of wurtzite ZnO and 2 the peak located at 385 cm 1 corresponds to A1-TO mode. The peak located at 581 cm 1 (E1-LO) has been attributed to O-vacancy defects present in the crystal [24]. The star marked peaks in the figure are due to multiple phonon scattering processes. It can be noticed that except the E2-high peak, the position of the rest shift slightly with increase in annealing temperature. The decrease in the intensity and the broadening of the E2-high peak with increasing temperature indicate a variation in defect concentration with temperature as proposed earlier [24]. It is evident that annealing at higher temperatures reduces the defect concentration in these nanostrucures. The Raman spectra also confirm the existence of a single hexagonal wurtzite phase with no other impurity phase as depicted by the XRD patterns. The as-spun sample exhibited diamagnetic nature and those annealed in air up to 400 1C did not show ferromagnetic behavior. Fig. 4 shows the room temperature magnetization versus magnetic field (M–H) curves of ZnO/PVA nanofibers annealed between 500
Fig. 2. XRD patterns of electrospun ZnO/PVA nanofibers annealed at 500 1C and 550 1C for 90 min in air.
Fig. 3. Raman spectra of ZnO nanofibers annealed at 500 and 550 1C.
Fig. 1. FESEM images of ZnO/PVA nanofibers (a) as-spun and (b) annealed at 550 1C in air.
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4. Conclusion
Fig. 4. Room temperature M–H curves of ZnO nanofibers annealed in air at 500 1C, 550 1C and 600 1C for 90 min.
In summary, ZnO nanofibers were prepared via electrospinning and annealing in air or vacuum. XRD data show that there is no impurity phase in the wurtzite ZnO nanofibers. Raman data indicates the presence of defects in the wurtzite ZnO nanofibers. Room temperature ferromagnetism is observed in all the samples with maximum saturation magnetization value in samples annealed at 550 1C. Higher magnetization exhibited by vacuum annealed ZnO nanofibers establishes the role of O vacancies in the RTFM observed in this wurtzite ZnO nanostructure. The present studies not only provide evidence for the role of O vacancies in the observed RTFM in ZnO nanofibers but also provides a means to tailor the O vacancies in ZnO nanostructures by controlling the ambient during the heat treatment of electrospun ZnO/PVA composites.
Acknowledgment Financial support from the Board of Research in Nuclear Sciences, Department of Atomic Energy, Government of India, vide Project no. 2010/34/54/BRNS is gratefully acknowledged. References
Fig. 5. M–H curves of ZnO nanofibers annealed in air and vacuum at 550 1C for 90 min.
semiconducting oxides, there is doubt whether this is an intrinsic or extrinsic property of these materials. As pointed out earlier, though there seems to be a general consensus about the role of defects in this phenomenon, it is not clear whether one can attribute it to the metal ion vacancies [14] or O [5,25]. XRD and Raman data presented in the earlier part of the paper clearly establish that ZnO exists in single (wurtzite) phase in 1-d form without any other impurity phases. Our Raman studies show that defects are present in the single phase wurtzite ZnO nanofibers which confirm the role of defects in the observed RTFM in ZnO nanofibers. In order to understand the influence of O vacancies on RTFM, we performed heat treatment of the as-spun nanofiber composite in air and under vacuum ( 10 3 Pa) at 550 1C for 90 min and recorded their M–H loops. Fig. 5 shows the M–H curves of air and vacuum annealed samples which show that both samples are ferromagnetic at room temperature. It is also clearly noticeable that the vacuum annealed sample exhibits higher magnetization (Ms ¼0.056 emu/g) than the air annealed sample (Ms ¼0.022 emu/g). Since the oxygen available in the two annealing procedures are different and the vacuum annealed samples have more O vacancies, the influence of O vacancies is obvious in the higher magnetization exhibited by the vacuum annealed ZnO nanofibers.
[1] C. Liu, F. Yun, H. Morkoc, J. Mater. Sci. Mater. Electron. 16 (2005) 555. [2] T. Dietl, H. Ohno, F. Matsujura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [3] M. Diaconu, H. Schmidt, H. Hochmuth, M. Lorenz, G. Benndorf, D. Spemann, A. Setzer, P. Esquinazi, A. Poppl, H. Von Wenckstern, K.W. Nielsen, R. Gross, H. Schmid, W. Mader, G. Wagner, M. Grundmann, J. Magn. Magn. Mater. 307 (2006) 212. [4] Y. Belghazi, G. Schmerber, S. Colis, J.L. Rehspringer, A. Dinia, A. Berrada, Appl. Phys. Lett. 89 (2006) 122504. [5] N.H. Hong, J. Sakai, N. Poirot, V. Brize, Phys. Rev. B 73 (2006) 132404. [6] H. Pan, J.B. Yi, L. Shen, R.Q. Wu, J.H. Yang, J.Y. Lin, Y.P. Feng, J. Ding, L.H. Van, J.H. Yin, Phys. Rev. Lett. 99 (2007) 127201. [7] Q. Xu, H. Schmidt, S. Zhou, K. Potzger, M. Helm, H. Hochmuth, M. Lorenz, A. Setzer, P. Esquinazi, C. Meinecke, M. Grundmann, Appl. Phys. Lett. 92 (2008) 082508. [8] M. Khalid, M. Ziese, A. Setzer, P. Esquinazi, M. Lorenz, H. Hochmuth, M. Grundmann, D. Spemann, T. Butz, G. Brauer, W. Anwand, G. Fischer, W.A. Adeagbo, W. Hergert, A. Ernst, Phys. Rev. B 80 (2009) 035331. [9] J.B. Yi, C.C. Lim, G.Z. Xing, H.M. Fan, L.H. Van, S.L. Huang, K.S. Yang, X.L. Huang, X.B. Qin, B.Y. Wang, T. Wu, L. Wang, H.T. Zhang, X.Y. Gao, T. Liu, A.T.S. Wee, Y.P. Feng, J. Ding, Phys. Rev. Lett. 104 (2010) 137201. [10] S. Banerjee, M. Mandal, N. Gayathri, M. Sardar, Appl. Phys. Lett. 91 (2007) 182501. [11] P. Zhan, W. Wang, C. Liu, Y. Hu, Z. Li, Z. Zhang, P. Zhang, B. Wang, X. Cao, J. Appl. Phys. 111 (2012) 033501. [12] D. Gao, Z. Zhang, J. Fu, Y. Xu, J. Qiand, D. Xue, J. Appl. Phys. 105 (2009) 113928. [13] P. Zhan, Z. Xie, Z. Li, W. Wang, Z. Zhang, Z. Li, G. Cheng, P. Zhang, B. Wang, X. Cao, Appl. Phys. Lett. 102 (2013) 071914. [14] C.M. Boubeta, J.I. Beltran, Li Balcells, Z. Konstantinovic, S. Valencia, D. Schmitz, J. Arbiol, S. Estrade, J. Cornil, B. Martinez, Phys. Rev. B 82 (2010) 024405. [15] W. Yan, Z. Sun, Q. Liu, Z. Li, Z. Pan, D. Wang, Y. Zhou, X. Zhang, Appl. Phys. Lett. 91 (2007) 062113. [16] S. Ramachandran, J. Narayan, J.T. Prater, Appl. Phys. Lett. 88 (2006) 242503. [17] A.M.A. Hakeem, J. Magn. Magn. Mater. 322 (2010) 709. [18] D.A. Schwartz, D.R. Gamelin, Adv. Mater. 16 (2004) 2115. [19] R. Podila, W. Queen, A. Nath, J.T. Arantes, A.L. Schoenhalz, A. Fazzio, G.M. Dalpian, J. He, S.J. Hwu, M.J. Skove, A.M. Rao, Nano Lett. 10 (2010) 1383. [20] U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005) 041301. [21] T.L. Phan, Y.D. Zhang, D.S. Yang, N.X. Nghia, T.D. Thanh, S.C. Yu, Appl. Phys. Lett. 102 (2013) 072408. [22] S.S. Mali, H. Kim, W.Y. Jang, H.S. Park, P.S. Patil, C.K. Hong, ACS Sustain. Chem. Eng. 1 (2013) 1207. [23] H.P. Klug, L.E. Alexander (Eds.), X-ray Diffraction Procedures: For Polycrystalline and Amorphous Materials, Wiley, New York, 1962. [24] A.K. Pradhan, K. Zhang, G.B. Loutts, U.N. Roy, Y. Cui, A. Burger, J. Phys. Condens. Matter 16 (2004) 7123. [25] C.W. Zhang, S.S. Yan, J. Appl. Phys. 106 (2009) 063709.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Room temperature ferromagnetism in undoped ZnO nanofibers prepared by electrospinning Arnab Kumar Das a,n, Manoranjan Kar b, Ananthakrishanan Srinivasan a a b
Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India Department of Physics, Indian Institute of Technology Patna, Patna 800013, India
art ic l e i nf o
a b s t r a c t
Available online 27 March 2014
We report ferromagnetic behavior in undoped ZnO nanofibers prepared by electrospinning a solution of zinc acetate and poly vinyl alcohol followed by annealing at 550 1C for about 90 min. X-ray diffraction patterns of the heat treated as-spun composite fibers reveal the formation of ZnO nanowires in wurtzite structure with no noticeable impurity phases. ZnO nanowires annealed between 500 1C and 600 1C exhibited room temperature ferromagnetism with decreasing magnetization with increasing annealing temperature. Room temperature ferromagnetism was observed in as-spun fibers annealed in air as well as under vacuum. However, vacuum annealed nanofibers show higher magnetization as compared to air annealed fibers, which indicates that oxygen vacancy is a cause for the observed room temperature ferromagnetism in the ZnO nanofibers. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO Electrospinning Ferromagnetism Defects
1. Introduction Diluted magnetic semiconductors (DMS) are of keen contemporary interest due to their possible application in spintronic devices [1] and their relatively high Curie temperature (TC). Dietl et al. [2] were the first to theoretically predict room temperature ferromagnetism (RTFM) in wide band gap semiconductors. Mn [3] and Co [4] doping in wide band gap oxide semiconductors have been shown to induce room temperature ferromagnetism establishing the existence of DMS. Recently, room temperature ferromagnetism has been observed in undoped wide band gap semiconductors such as TiO2 [5], ZnO [6–13], MgO [14], etc. However, the magnetization of these undoped DMS is very low and the origin of ferromagnetism in these materials is not fully understood. There are some suggestions in the literature that the ferromagnetic interaction in undoped ZnO is due to Zn vacancies [8,15] while others propose O vacancies [16,17] or zinc interstitials [18] as the possible reason for the observed spontaneous magnetization. There are also suggestions that ferromagnetism in ZnO could arise from surface defects and sample preparation conditions [19]. Though no conclusion could be drawn about the origin of ferromagnetism in DMS, it is generally accepted that defects have an important role in RTFM. ZnO is one of the most prominent wide band gap semiconductor with a large exciton binding energy
n
Corresponding author. Tel.: þ 91 361 2582743; fax: þ 91 361 2582749. E-mail addresses: [email protected] (A. Kumar Das), [email protected] (M. Kar), [email protected] (A. Srinivasan). http://dx.doi.org/10.1016/j.physb.2014.03.045 0921-4526/& 2014 Elsevier B.V. All rights reserved.
at room temperature which is extensively used in optoelectronic devices [20]. Undoped nanocrystalline ZnO has been prepared by pulsed laser deposition [7], co-precipitation [12], pulsed electron deposition [13] and mechanical milling [21] routes. Recently, ZnO polyvinylpyrrolidone (PVP) nanofibers have been prepared by electrospinning method [22]. These studies show that the preparative route has distinct influence on the properties of the DMS and the search for the best route of preparation is still on. In this work, we have prepared undoped ZnO/polyvinyl alcohol (PVA) nanofibers by electrospinning route. The objectives of this work are to obtain RTFM in this DMS and to understand the role of defects (Zn or O) in this DMS.
2. Experimental Aqueous polyvinyl alcohol (PVA, MW 80,000) solution (12 wt%) was first prepared by dissolving PVA powder in deionized water. Then, zinc acetate (20 wt%) solution was added slowly to the aqueous PVA solution with continuous stirring of 2 h. The mixture was kept at room temperature for 4–5 h to obtain a viscous solution suitable for electrospinning. The solution was electrospun using a commercial electrospinner (NABOND NEU) under a potential difference of 16 kV, spinneret to collector distance of 12 cm and solution flow rate of 2 ml/h. The composite nanofibers ejected from the tip of the metallic syringe needle (positive electrode) were collected on a grounded collector. As-spun nanofibers, taken either in sealed fused silica ampoules under 10 3 Pa or in an open alumina crucible, were
A. Kumar Das et al. / Physica B 448 (2014) 112–114
annealed for 90 min in an electric furnace to obtain ZnO nanofibers. The annealing temperature (TA) was varied between 500 and 600 1C. The morphology of the as-spun and heat treated nanostructures was imaged using a field effect scanning electron microscopy (FESEM, Sigma, Zeiss). Structural characterization of the nanofibers was carried out by using a powder X-ray diffractometer (Rigaku TTRAX III 18 kW) operating with Cu Kα radiation (λ¼0.1542 nm). A microRaman spectrometer (Seki Technotron Corp. STR 500) was used to obtain structural information and to confirm the presence of oxygen defects in the annealed samples. Magnetic properties were measured using a vibrating sample magnetometer (VSM, Lakeshore model 7410) equipped with a variable temperature sample stage.
113
and 600 1C for 90 min each. The hysteresis loops establish that all the samples have spontaneous magnetization (or ferromagnetism) at room temperature after subtracting the diamagnetic effect. The M–H curves indicate that saturation in magnetization is attained within an applied field of 10 kOe. It is clearly seen that with increasing TA, saturation magnetization (Ms) decreased. Maximum Ms of 0.039 emu/g was obtained for the composite nanofibers heat treated at 500 1C, while the sample heat treated at 600 1C exhibited the lowest Ms of 0.007 emu/g. These values are comparable to the values of 0.047 emu/g and 0.0054 emu/g reported for mechanically milled [21] and co-pecipitated [12] undoped ZnO. Inset in Fig. 4 shows variation of Ms with TA in the range of study. Though RTFM has been reported by other research groups in
3. Results and discussion The morphology of the as-spun and annealed composite nanofibers was characterized by FESEM (cf. Fig. 1). It is clearly seen from the images that the as-spun fibers are smooth and bead free because of the amorphous nature of the zinc acetate/PVA composite. But, after annealing, the surface of the nanofibers turns rough and the fiber diameter decreases as a consequence of the removal of PVA and the conversion of the Zn salt to ZnO. Typical X-ray diffraction (XRD) patterns of the composite nanofibers annealed at 500 1C and 550 1C for 90 min are shown in Fig. 2. Here, all the peaks could be indexed to hexagonal wurtzite structure of ZnO (JCPDS file # 36-1451) which establishes that ZnO exists as a pure single (wurtzite) phase structure without any other impurity phases. ZnO/PVP nanofibres synthesized earlier by electrospinning method [22] also crystallized in the same hexagonal structure. The average crystallite size was estimated from the most intense XRD peak [(1 0 1)] to be about 33 nm by using Scherrer's formula [23]. Fig. 3 shows the Raman spectra of ZnO nanofibers annealed at 500 1C and 550 1C for 90 min. The strong Raman peak near 437 cm 1 corresponds to Ehigh phonon mode of wurtzite ZnO and 2 the peak located at 385 cm 1 corresponds to A1-TO mode. The peak located at 581 cm 1 (E1-LO) has been attributed to O-vacancy defects present in the crystal [24]. The star marked peaks in the figure are due to multiple phonon scattering processes. It can be noticed that except the E2-high peak, the position of the rest shift slightly with increase in annealing temperature. The decrease in the intensity and the broadening of the E2-high peak with increasing temperature indicate a variation in defect concentration with temperature as proposed earlier [24]. It is evident that annealing at higher temperatures reduces the defect concentration in these nanostrucures. The Raman spectra also confirm the existence of a single hexagonal wurtzite phase with no other impurity phase as depicted by the XRD patterns. The as-spun sample exhibited diamagnetic nature and those annealed in air up to 400 1C did not show ferromagnetic behavior. Fig. 4 shows the room temperature magnetization versus magnetic field (M–H) curves of ZnO/PVA nanofibers annealed between 500
Fig. 2. XRD patterns of electrospun ZnO/PVA nanofibers annealed at 500 1C and 550 1C for 90 min in air.
Fig. 3. Raman spectra of ZnO nanofibers annealed at 500 and 550 1C.
Fig. 1. FESEM images of ZnO/PVA nanofibers (a) as-spun and (b) annealed at 550 1C in air.
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4. Conclusion
Fig. 4. Room temperature M–H curves of ZnO nanofibers annealed in air at 500 1C, 550 1C and 600 1C for 90 min.
In summary, ZnO nanofibers were prepared via electrospinning and annealing in air or vacuum. XRD data show that there is no impurity phase in the wurtzite ZnO nanofibers. Raman data indicates the presence of defects in the wurtzite ZnO nanofibers. Room temperature ferromagnetism is observed in all the samples with maximum saturation magnetization value in samples annealed at 550 1C. Higher magnetization exhibited by vacuum annealed ZnO nanofibers establishes the role of O vacancies in the RTFM observed in this wurtzite ZnO nanostructure. The present studies not only provide evidence for the role of O vacancies in the observed RTFM in ZnO nanofibers but also provides a means to tailor the O vacancies in ZnO nanostructures by controlling the ambient during the heat treatment of electrospun ZnO/PVA composites.
Acknowledgment Financial support from the Board of Research in Nuclear Sciences, Department of Atomic Energy, Government of India, vide Project no. 2010/34/54/BRNS is gratefully acknowledged. References
Fig. 5. M–H curves of ZnO nanofibers annealed in air and vacuum at 550 1C for 90 min.
semiconducting oxides, there is doubt whether this is an intrinsic or extrinsic property of these materials. As pointed out earlier, though there seems to be a general consensus about the role of defects in this phenomenon, it is not clear whether one can attribute it to the metal ion vacancies [14] or O [5,25]. XRD and Raman data presented in the earlier part of the paper clearly establish that ZnO exists in single (wurtzite) phase in 1-d form without any other impurity phases. Our Raman studies show that defects are present in the single phase wurtzite ZnO nanofibers which confirm the role of defects in the observed RTFM in ZnO nanofibers. In order to understand the influence of O vacancies on RTFM, we performed heat treatment of the as-spun nanofiber composite in air and under vacuum ( 10 3 Pa) at 550 1C for 90 min and recorded their M–H loops. Fig. 5 shows the M–H curves of air and vacuum annealed samples which show that both samples are ferromagnetic at room temperature. It is also clearly noticeable that the vacuum annealed sample exhibits higher magnetization (Ms ¼0.056 emu/g) than the air annealed sample (Ms ¼0.022 emu/g). Since the oxygen available in the two annealing procedures are different and the vacuum annealed samples have more O vacancies, the influence of O vacancies is obvious in the higher magnetization exhibited by the vacuum annealed ZnO nanofibers.
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