pubs.acs.org/Langmuir © 2010 American Chemical Society
Oriented Contraction: A Facile Nonequilibrium Heat-Treatment Approach for Fabrication of Maghemite Fiber-in-Tube and Tube-in-Tube Nanostructures Fangzhi Mou, Jian-guo Guan,* Weidong Shi, Zhigang Sun, and Shuanhu Wang State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China Received May 27, 2010 We present a simple and effective nonequilibrium heat-treatment approach that allows for the facile fabrication of maghemite (γ-Fe2O3) fiber-in-tube and tube-in-tube nanostructures by heat-treating electrospun precursor fibers composed of polyvinylpyrrolidone (PVP) and iron citrate with a carefully devised heating rate (R). In this nonequilibrium heat-treatment procedure, R can be easily utilized to tune the temperature gradient established in the inner portion of the fibers and the difference between the cohesive force and the adhesive force at the interface layer between the inner gel and the dense rigid shell generated in situ by a high R. Therefore, the contraction direction of the precursor nanofibers and the final morphology of the resultant γ-Fe2O3 fibers ranging from a simple tube to a fiber in tube to a tube in tube are realized for control. The nonequilibrium heat-treatment approach reported here can be readily extended to the fabrication of other materials with controllable interior structures by fast heating their corresponding gel precursors, which may be fabricated on the basis of electrospinning techniques and others. The resultant γ-Fe2O3 fiberin-tube and tube-in-tube nanostructures may have important applications in a number of areas, such as magnetic separable catalysts or catalyst supporting materials, sensors, absorbents, microreactors, and so forth, because of their structural characteristics and good magnetic properties.
1. Introduction Multilevel hollow fibers, such as fiber-in-tube, tube-in-tube, multishell, and multichannel hollow fibers, usually exhibit improved electronic, optical, magnetic, catalytic, and mechanical properties compared to those of the simple hollow structures thanks to their inherent characteristics involving isolated interior structures, multiple heterogeneous interfaces, and controllable physicochemical microenvironment.1 This gives them a strong competitive power in nanoelectronic devices,2 molecular separation,3 singleDNA sensing,4 pollutant decomposition,5 hydrogen fuel,6 gas sensors,7 and solar energy conversion devices.8 To obtain multilevel hollow fibers, various approaches such as the most commonly used template method,9 galvanic replacement reaction,10 Kirkendall effect,11 and multifluidic compound-jet electro-hydrodynamic *To whom correspondence should be addressed. E-mail: guanjg@whut. edu.cn. Telephone: 86-27-87218832. Fax: 86-27-87879468. (1) Zhao, Y.; Jiang, L. Adv. Mater. 2009, 21, 3621. (2) Ben Ishai, M.; Patolsky, F. Angew. Chem., Int. Ed. 2009, 48, 8699. (3) Lee, S. B.; Mitchell, D. T.; Trofin, L.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. Science 2002, 296, 2198. (4) Fan, R.; Karnik, R.; Yue, M.; Li, D. Y.; Majumdar, A.; Yang, P. D. Nano Lett. 2005, 5, 1633. (5) Quan, X.; Yang, S. G.; Ruan, X. L.; Zhao, H. M. Environ. Sci. Technol. 2005, 39, 3770. (6) Park, J. H.; Kim, S.; Bard, A. J. Nano Lett. 2006, 6, 24. (7) (a) Varghese, O. K.; Gong, D. W.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. Adv. Mater. 2003, 15, 624. (b) Varghese, O. K.; Gong, D. W.; Paulose, M.; Ong, K. G.; Grimes, C. A. Sens. Actuators, B 2003, 93, 338. (8) (a) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (b) Kang, T. S.; Smith, A. P.; Taylor, B. E.; Durstock, M. F. Nano Lett. 2009, 9, 601. (9) (a) Qin, Y.; Liu, L. F.; Yang, R. B.; Gosele, U.; Knez, M. Nano Lett. 2008, 8, 3221. (b) Qin, Y.; Lee, S. M.; Pan, A.; Gosele, U.; Knez, M. Nano Lett. 2008, 8, 114. (c) Gu, D. F.; Baumgart, H.; Abdel-Fattah, T. M.; Namkoong, G. ACS Nano 2010, 4, 753. (10) (a) Sun, Y. G.; Xia, Y. N. Adv. Mater. 2004, 16, 264. (b) Sun, Y. G.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 3892. (11) Peng, Q.; Sun, X. Y.; Spagnola, J. C.; Saquing, C.; Khan, S. A.; Spontak, R. J.; Parsons, G. N. ACS Nano 2009, 3, 546.
15580 DOI: 10.1021/la102830p
techniques12 have been developed. However, it is still a great challenge to establish a versatile, economical methodological approach for mass production of these hollow fibers with multilevel interior structures because of their structural complexity and small size. Here, we report a simple and convenient nonequilibrium heattreatment method that allows for the facile fabrication of fiberin-tube and tube-in-tube nanostructures. The formation mechanism of these complex interior structures is deduced from the combination of thermogravity-differentiation scanning calorimeter (TG-DSC) analysis and electronic microscopy observation as oriented contraction determined by the difference between the cohesive force (σco) and the adhesive force (σad) at the interface layer between the inner gel and the dense rigid shell in situ generated by the high heating rate (R). Upon comparison with the multifluidic coaxial electrospinning method, our method has several advantages. First, the formation of the multilevel interior structures of the resultant products is assessed by the nonequilibrium heat-treatment process. Second, this method does not need to rely on the co-electrospinning technique that requires complicate spinnerets and the appropriate precursor solution pairs,12 though the gel precursor fibers used at present are fabricated on the basis of the electrospinning method. Third, the used gel precursors can be either homogeneous or heterogeneous. To elucidate the preparation principle of multilevel hollow fibers obtained by this oriented contraction method based on a nonequilibrium heat-treatment process, we use maghemite (γ-Fe2O3) as an example. The reason why γ-Fe2O3 is selected as an example system is that it shows unique magnetic and catalytic properties, chemical stability, and biocompatibility. γ-Fe2O3 nanostructures with various shapes exhibit interesting shape-dependent (12) (a) Zhao, Y.; Cao, X. Y.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 764. (b) Chen, H.; Wang, N.; Di, J.; Zhao, Y.; Song, Y.; Jiang, L. Langmuir 2010, 26, 11291.
Published on Web 09/14/2010
Langmuir 2010, 26(19), 15580–15585
Mou et al.
Article
Figure 1. SEM images of the electrospun gel fibers (A) and their sections (B).
properties13 and have a wide variety of potential applications, including information storage,14 drug delivery,15 catalysts,16 and water treatment.13e
wavelength of an Arþ laser. The laser power was limited to 0.5 mW to avoid possible phase transition during the laser irradiation. Magnetic properties of the samples were measured by a model 4HF vibrating sample magnetometer (VSM, ADE Co. Ltd.).
2. Experimental Section All the raw materials used to synthesize γ-Fe2O3 hollow fibers were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). In a typical synthesis, 0.75 g of polyvinylpyrrolidone (PVP) with a molecular weight of 50000 was added to 30 mL of a 0.1 g/mL iron citrate aqueous solution. Afterward, the solution was stirred continuously at 60 °C until the concentration of the iron citrate reached 44 wt %. The solution described above was transferred into a syringe connected to a metallic nozzle and fed at a constant rate of 0.3 mL/h through a syringe pump (TJ-1A, Longer Pump). The metallic nozzle was connected to a highvoltage supply (HB-Z303-20AC, Heng Bo High Voltage Power Supply Plant), and an Al board was used as the collector, which was placed 15 cm below the metallic nozzle. Upon application of a high voltage of 25 kV, a fluid jet was ejected from the metallic nozzle. The solvent evaporated, and the iron citrate/PVP composite electrospun gel fibers were deposited on the collector. Then, the as-obtained gel fibers were annealed at 500 °C for 2 h with R values of 100 and 250 °C/min in air. After the fibers were naturally cooled to room temperature, the fiber-in-tube and tubein-tube nanostructures of γ-Fe2O3 were obtained. To understand the formation mechanism of the fiber-in-tube and tube-in-tube nanostructures of γ-Fe2O3, contrast experiments were conducted via adjustment of R to 50 and 1 °C/min to fabricate γ-Fe2O3 hollow and solid fibers, respectively. Scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and energy dispersive X-ray (EDX) analyses were conducted using a Hitachi S-4800 field-emission scanning electron microscope (Japan). Thermogravity-differentiation scanning calorimeter (TG-DSC) analysis was conducted on a NETZSEC STA-449C thermal analyzer (Germany). The phase purity of the products was examined with an X-ray diffraction (XRD) pattern obtained using a Rigaku D/max-IIIA diffractometer (Japan) at a voltage of 40 kV and a current of 200 mA with Cu KR radiation (λ = 1.5406 A˚), in the 2θ range from 10° to 90° at a scanning step of 0.02°. A micro-Raman study was performed on the Renishaw inVia laser confocal Raman microscope at room temperature and an excitation of 514.5 nm (13) (a) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Bin Na, H. J. Am. Chem. Soc. 2001, 123, 12798. (b) Han, Q.; Liu, Z. H.; Xu, Y. Y.; Chen, Z. Y.; Wang, T. M.; Zhang, H. J. Phys. Chem. C 2007, 111, 5034. (c) Zhan, S. H.; Chen, D. R.; Jiao, X. L.; Liu, S. S. J. Colloid Interface Sci. 2007, 308, 265. (d) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. Adv. Mater. 2006, 18, 2426. (e) Zhao, N.; Gao, M. Y. Adv. Mater. 2009, 21, 184. (f ) Abou-Hassan, A.; Bazzi, R.; Cabuil, V. Angew. Chem., Int. Ed. 2009, 48, 7180. (14) Varanda, L. C.; Goya, G. F.; Morales, M. P.; Marques, R. F. C.; Godoi, R. H. M.; Jafelicci, M.; Serna, C. J. IEEE Trans. Magn. 2002, 38, 1907. (15) Euliss, L. E.; Grancharov, S. G.; O’Brien, S.; Deming, T. J.; Stucky, G. D.; Murray, C. B.; Held, G. A. Nano Lett. 2003, 3, 1489. (16) Du, W. P.; Xu, Y. M.; Wang, Y. S. Langmuir 2008, 24, 175.
Langmuir 2010, 26(19), 15580–15585
3. Results and Discussion To fabricate γ-Fe2O3 fiber-in-tube and tube-in-tube nanostructures, we first prepared gel fibers containing iron citrate and PVP using an electrospinning method. The overall morphology of the electrospun gel fibers is shown in Figure 1A. It can be seen that the gel fibers have a uniform and smooth surface. The length is up to tens of micrometers, and the average diameter is ca. 420 nm. The specific morphology of the sections of gel fibers (Figure 1B) clearly indicates that they are solid in their core. When the electrospun gel fibers are thermally treated at 500 °C for 2 h with an R of 100 °C/min, the resultant fibers (Figure 2A) maintain the continuous features of the electrospun fibers and show a decreasing diameter of ∼320 nm. To obtain the morphological and structural information about the sections, the asobtained fibers were slightly crushed during the preparation of SEM samples. The SEM image of the sections of the resultant fibers is shown in Figure 2B, indicating that the solid precursor fibers have been transformed into fiber-in-tube nanostructures after heat treatment. The fiber-in-tube nanostructures are wellconstructed with some interval spaces between the outer tube and the inner fiber. The wall thicknesses of the outer tube and the diameters of inner fibers are ca. 40 and 160 nm, respectively. The magnified SEM image of the typical section of a fiber-in-tube structure is shown in the inset of Figure 2B, which indicates that the outer tube shows a dense wall while the inner fiber consists of loose compacted nanoparticles. When R is further increased to 250 °C/min, the resultant fibers are fragile and easy to break in the process of SEM sampling, as shown in Figure 2C. From the magnified SEM image of sections (Figure 2D) of resultant fibers, it is interesting to note that the well-constructed tube-in-tube structures are obtained at an R of 250 °C/min. The diameter and wall thickness are ∼400 and ∼40 nm, respectively, for the outer tubes and ∼200 and ∼50 nm, respectively, for the inner ones. The high-magnification SEM image of a typical section (the inset of Figure 2D) indicates that the walls of both the inner tube and the outer tube consist of aggregated nanoparticles. The as-obtained fiber-in-tube and tube-in-tube nanostructures are also clearly revealed by the STEM images in panels E and F of Figure 2. It also can be seen that the fiber-in-tube and tube-in-tube nanostructures are composed of small crystallites with the sizes of ∼10 and ∼25 nm, respectively. The typical intact tips of the asobtained fiber-in-tube and tube-in-tube nanostructures are shown in the insets of panels G and H of Figure 2, respectively, indicating DOI: 10.1021/la102830p
15581
Article
Mou et al.
Figure 2. SEM images of the as-obtained γ-Fe2O3 fiber-in-tube (A and B) and tube-in-tube (C and D) fibers obtained at 500 °C for 2 h with R values of 100 and 250 °C/min, respectively. STEM images of typical fiber-in-tube (E) and tube-in-tube (F) nanostructures and their corresponding intact tips (G and H). The insets of panels B and D show the magnified SEM morphologies of the sections.
Figure 3. (A) XRD patterns of the γ-Fe2O3 fiber-in-tube and tube-in-tube nanostructures obtained at 500 °C for 2 h with R values of 100 and 250 °C/min, respectively. (B) Raman spectrum of the as-obtained tube-in-tube nanostructures.
that the tips of inner structures are totally separated from the outer tube, and the inner and outer tubes have a close end. Figure 3A shows that the XRD patterns of the as-obtained fiber-in-tube and tube-in-tube nanostructures are in good agreement with that of the pure γ-Fe2O3 (maghemite, JCPDS No. 39-1346). It can also been seen that the intensity of the XRD pattern of the tube-in-tube nanostructures is higher than that of the fiberin-tube nanostructures, indicating that the R of the heat treatment not only affects the morphology of the fibers but also has a positive influence on their crystallinity. The fact that the crystallinity is enhanced with an increase in R is consistent with the STEM observation that the tube-in-tube nanostructures have a larger crystalline size (25 nm) than the fiber-in-tube nanostructures (10 nm). In the Raman spectrum of the as-obtained tubein-tube nanostructures (Figure 3B), the Raman peaks at 358, 507, 659, and 701 cm-1 are observed, which are consistent with the Eg, 15582 DOI: 10.1021/la102830p
T2g, and A1g modes of inverse spinel structure γ-Fe2O3.17 Both XRD and Raman analysis clearly determine that the as-obtained multilevel hollow fibers are γ-Fe2O3, and no impurities such as hematite and magnetite are detected. The EDX analysis (Figure 4) indicates that the as-obtained γ-Fe2O3 fiber-in-tube and tubein-tube nanostructures mainly consist of iron and oxygen along with a small amount of carbon impurity. The proportional relation between the iron and oxygen agrees with the chemical composition of the γ-Fe2O3. The C impurities were calculated to be 5.7 and 3.7 wt % in the as-obtained γ-Fe2O3 fiber-in-tube (Figure 4A) and tube-in-tube (Figure 4B) nanostructures, respectively. The peak of Al in the EDX pattern is caused by the signal generated from the used Al substrate during the SEM test. The (17) (a) Legodi, M. A.; de Waal, D. Dyes Pigm. 2007, 74, 161. (b) Chamritski, I.; Burns, G. J. Phys. Chem. B 2005, 109, 4965.
Langmuir 2010, 26(19), 15580–15585
Mou et al.
Article
Figure 4. EDX analyses of the as-obtained γ-Fe2O3 fiber-in-tube (A) and tube-in-tube (B) nanostructures.
Figure 5. SEM images of the solid (A) and hollow (B) fibers of γ-Fe2O3 obtained at 500 °C for 2 h with R values of 1 and 50 °C/min, respectively.
fact that the electrospun precursor fibers are evolved into γ-Fe2O3 instead of R-Fe2O3 after being heat-treated at 500 °C for 2 h in air is possibly attributed to the reducing effect of its carbonaceous residues generated from the organic components including PVP during the heat-treatment procedure.18 The carbonaceous residues may cause partial reduction of Fe3þ ions to produce magnetite (Fe3O4), which transforms into γ-Fe2O3 of the same spinel structure by reoxidation in air.18,19 To understand the influence of R on the interior structures of the resultant fibers, we systematically investigated the morphologies of annealing electrospun solid gel fibers. It is known that directly annealing electrospun solid gel fibers at an elevated temperature usually results in solid metal oxide fibers and sometimes produces simple hollow fibers if a short-time low-temperature rapid-heating pretreatment procedure is introduced because of the occurrence of in situ-generated dense shell-engaged Ostwald ripening.20 Figure 5 depicts the SEM images of the fibers obtained at 500 °C for 2 h with R values of 1 and 50 °C/min. As expected, γ-Fe2O3 fibers derived from the heat treatment with a low R (1 °C/min) are solid in their core and have a diameter of ca. 240 nm, as shown in Figure 5A. However, when R is increased to 50 °C/min, the resultant fibers have a typical hollow structure with an average outer diameter and wall thickness of ∼280 and ∼80 nm, respectively (Figure 5B). From these contrast (18) (a) Khaleel, A. A. Chem.;Eur. J. 2004, 10, 925. (b) Bourlinos, A. B.; Simopoulos, A.; Petridis, D. Chem. Mater. 2002, 14, 899. (19) Saito, S. Fine Ceramics; Elsevier: New York, 1988. (20) (a) Wu, H.; Zhang, R.; Liu, X. X.; Lin, D. D.; Pan, W. Chem. Mater. 2007, 19, 3506. (b) Mou, F. Z.; Guan, J. G.; Sun, Z. G.; Fan, X. A.; Tong, G. X. J. Solid State Chem. 2010, 183, 736.
Langmuir 2010, 26(19), 15580–15585
experiments, one can reasonably conclude that the R of calcination plays a key role in the formation of multilevel interior hollow fibers. With an increase in R, the as-obtained nanostructures are evolved from solid fibers, via simple hollow fibers, to fiber-in-tube and even tube-in-tube nanostructures. To further elucidate the key role of R in the interior structure evolution of electrospun gel fibers during the calcination, we have conducted the TG-DSC analysis of the electrospun precursor fibers (Figure 6). It indicates that the electrospun gel precursor fibers have three differentiated steps of weight loss in the temperature range of 40-1000 °C. The first weight loss occurs around 40-120 °C, accompanied by a broad endothermic peak at 78 °C. This is ascribed to the evaporation of water. The second weight loss of 47.8% appears around 120-325 °C, corresponding to a sharp exothermic peak at ∼265 °C. This may be attributed to the decomposition of organic salts, including iron citrate and the degradation of the side chain of PVP through the intermolecular cross-linking reaction.21 The third step around 325-450 °C involves 11.9% weight loss and accompanies a sharp exothermic peak at 361 °C in the DSC curve, which could be attributed to the oxidation and decomposition of the main chain of PVP. At a temperature higher than 450 °C, no weight loss is observed in the TG curve. This indicates that the organic components have completely decomposed before 450 °C. The electrospun gel fibers undergo intensive weight loss (as much as 74%) caused by the decomposition of iron citrate and PVP in the process of calcination, leading to significant volume shrinkage and the formation (21) Remiro, P. M.; Cortazar, M. M.; Calahorra, M. E. J. Mater. Sci. 1999, 34.
DOI: 10.1021/la102830p
15583
Article
Mou et al.
Figure 7. Magnetic hysteresis loops of the as-obtained γ-Fe2O3 fiber-in-tube and tube-in-tube nanostructures.
Figure 6. TG-DSC analyses of the electrospun gel fibers.
of γ-Fe2O3 rigid fibers at ∼500 °C. The gel is viscoelastic and easy to shrink in the temperature range of 160-325 °C along with partial mass loss (33.4% according to TG curves) due to the decomposition of iron salt and degradation of side chains of PVP;19 the temperatures of 160 and 325 °C represent the glass transition temperature and the starting temperature of degradation of the main chain of PVP,22 respectively. At a low R, the electrospun solid gel fibers are homogeneously heated from the surface to the center and shrink into a solid structure along with the mass loss of the organic component. However, with an increasing R, a dense rigid shell, which is believed to play a key role in the formation of various hollow structures, will be first generated on the eletrospun gel fiber because of the existence of a large temperature gradient (ΔT ) along the radial direction.20b It is reasonable to believe that the dense shell can be regarded as a framework to prevent further contraction of the outer diameter while the inner viscoelastic gel will continue to undergo intensive shrinkage due to the loss of organic components in the subsequent calcination. Namely, oriented contraction occurs. In this case, it is imaginable that the interface layer between the inner gel and the dense shell simultaneously receives two forces,
a cohesive force (σco) coming from the inner gel and an adhesive force (σad) with a direction opposite to that of the former. When σco e σad, the inner gel will shrink outward to the preformed iron oxide shell along with the loss of organic components, resulting in a hollow structure at the R of 50 °C/min (Figure 5B). Further increasing R from 50 to g100 °C/min reinforces ΔT and reduces σad, resulting in σco g σad. Therefore, with the calcination continuing, the inner gel fiber will contract inward and separate from the dense rigid shell. The as-generated inner gel fiber will clone the above forming process of solid or hollow structure depending on the remaining ΔT in its radial direction. It is reasonable to suppose that ΔT decreases from the surface to the inner core. When R = 100 °C/min, ΔT is too small to induce another dense shell for the separated inner core. Thus, the inner gel core shrinks into a solid structure, and a fiber-in-tube nanostructure is obtained (Figure 2B, E). When R = 250 °C/min, ΔT is still large enough to generate another dense shell but σco e σad for the separated inner gel fiber. Thus, the inner gel fiber is evolved into a hollow structure, as if the forming process of the hollow fiber is duplicated in the further calcination. Consequently, a tube-in-tube nanostructure is finally generated (Figure 2D,F). The detailed formation mechanism of the fibers of various structures ranging from solid and hollow to fiber-in-tube and tube-in-tube nanostructures is depicted in Scheme 1. This suggests that the applicability of the nonequilibrium heat-treatment method to the fabricattion of multilevel hollow fibers does not limit the use of the electrospun gel precursors as raw materials, though the precursor fibers used here are fabricated on the basis of the electrospinning method. In addition, the in situgenerated dense shell-engaged Ostwald ripening may also be responsible for the formation of the tube wall of the resultant products.20b The oriented contraction, the direction of which is determined by the differentiation of σco and σad, rather than the Ostwald ripening process decides the formation of the middle voids caused by the separation of the dense rigid shell and the inner gel fiber, which is an important and indispensable step for the formation of multilevel hollow fibers. Figure 7 displays the room-temperature hysteresis loops, which are calibrated with carbon impurity detected by EDX analysis (Figure 4). The nonlinear hysteresis loops with non-zero remnant magnetization and coercivity indicate that the fiber-in-tube and tube-in-tube nanostructures of γ-Fe2O3 have a pronounced ferromagnetic property. The saturation magnetization (Ms) values are 55.2 and 56.3 emu/g, respectively; both are lower than that of the bulk ferromagnetic γ-Fe2O3 (74 emu/g)23 because of
(22) Feldstein, M. M.; Lebedeva, T. L.; Shandryuk, G. A.; Kotomin, S. V.; Kuptsov, S. A.; Igonin, V. E.; Grokhovskaya, T. E.; Kulichikhin, V. G. Polym. Sci. Ser. A 1999, 41, 854.
(23) Martinez-Boubeta, C.; Simeonidis, K.; Angelakeris, M.; Pazos-Perez, N.; Giersig, M.; Delimitis, A.; Nalbandian, L.; Alexandrakis, V.; Niarchos, D. Phys. Rev. B 2006, 74.
Scheme 1. Formation of the Solid Fibers (A), Hollow Fibers (B), and Fiber-in-Tube (C) and Tube-in-Tube (D) Nanostructures through the Nonequilibrium Heat-Treatment Procedurea
a R can be easily utilized to tune the temperature gradient (ΔT ) established in the inner portion of the fibers and the difference between the cohesive force (σco) and the adhesive force (σad) at the interface layer between the inner gel and the dense rigid shell generated in situ by a high R. Therefore, the contraction direction of the precursor nanofibers and the final morphology of the resultant hierarchical nanofibers are realized for control.
15584 DOI: 10.1021/la102830p
Langmuir 2010, 26(19), 15580–15585
Mou et al.
the intensive surface effect because of their small nanocrystallites (10 and 25 nm, respectively) and hollow porous structure.24 However, they are both higher than that of γ-Fe2O3 nanoparticles (27.99 emu/g),25 nanotubes (46 emu/g),26 and hollow nanoparticles (50.8 emu/g).27 The inset of Figure 7 indicates that the coercivity (Hc), remanence (Mr), and calculated squareness (S, S = Mr/Ms) are 78 Oe, 5.5 emu/g, and 0.1, respectively, for the as-prepared γ-Fe2O3 fiber-in-tube nanostructures obtained at an R of 100 °C/min and increase to 206 Oe, 15.9 emu/g, and 0.28, respectively, for the γ-Fe2O3 tube-in-tube nanostructures obtained at an R of 250 °C/min. This can be reasonably explained by the fact that the as-obtained tube-in-tube γ-Fe2O3 nanostructures show an increasing crystalline size and enhanced crystallinity compared with the fiber-in-tube ones, as Hc, Mr, and S all increase with increasing crystalline size when the crystalline size is less than the single-domain critical size (the typical value is 30-50 nm for γ-Fe2O3).28 It is also consistent with the results obtained for MgFe2O4,29 CuFe2O4,30 and Co0.5Zn0.5Fe2O431 nanofibers prepared by electrospinning with subsequent heat treatment. (24) (a) Punnoose, A.; Magnone, H.; Seehra, M. S.; Bonevich, J. Phys. Rev. B 2001, 6417. (b) Lu, A. H.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 1222. (c) Liang, X.; Wang, X.; Zhuang, J.; Chen, Y. T.; Wang, D. S.; Li, Y. D. Adv. Funct. Mater. 2006, 16, 1805. (d) Song, O.; Zhang, Z. J. J. Am. Chem. Soc. 2004, 126, 6164. (e) Li, L.; Yang, Y.; Ding, J.; Xue, J. M. Chem. Mater. 2010, 22, 3183. (25) Jing, Z. H.; Wu, S. H. J. Solid State Chem. 2004, 177, 1213. (26) Wang, J. H.; Ma, Y. W.; Watanabe, K. Chem. Mater. 2008, 20, 20. (27) Gao, J. H.; Liang, G. L.; Cheung, J. S.; Pan, Y.; Kuang, Y.; Zhao, F.; Zhang, B.; Zhang, X. X.; Wu, E. X.; Xu, B. J. Am. Chem. Soc. 2008, 130, 11828. (28) (a) O’Handley, R. C. Modern magnetic materials: Principles and applications; Wiley: New York, 2000. (b) Spaldin, N. A. Magnetic materials: Fundamentals and device applications; Cambridge University Press: Cambridge, U.K., 2003. (c) Dou, Y. W.; Luo, H. L. Magnetic Record Materials; Electron Technology Publishing: Beijing, 1992; pp 206 (in Chinese). (29) Maensiri, S.; Sangmanee, M.; Wiengmoon, A. Nanoscale Res. Lett. 2009, 4, 221. (30) Ponhan, W.; Maensiri, S. Solid State Sci. 2009, 11, 479. (31) Shen, X. Q.; Xiang, J.; Song, F. Z.; Liu, M. Q. Appl. Phys. A: Mater. Sci. Process. 2010, 99, 189.
Langmuir 2010, 26(19), 15580–15585
Article
Via combination of the fine magnetic properties and the structural characteristics, including isolated inner voids, multiple interfaces, and porous walls, of the as-prepared γ-Fe2O3 multilevel hollow fibers, they can be regarded as a promising potential candidate for magnetic separable catalysts or catalyst supporting materials, absorbents, microreactors, etc. In summary, we have presented a very facile and effective nonequilibrium heat-treatment approach to fabricating fiberin-tube and tube-in-tube nanostructures. By this method, we can intentionally control the contraction direction of the precursor nanofibers during the heat-treatment process by only adjusting R of the calcination as R can be easily utilized to tune the temperature gradient established in the inner portion of the fibers and the difference between the cohesive force and the adhesive force at the interface layer between the inner gel and the dense rigid shell in situ generated by a high R. Thus, the final products can be tuned to be solid fibers, hollow fibers, fiberin-tube and tube-in-tube nanostructures, and other even more complex interior hollow nanostructures. The oriented contraction approach reported here can be readily extended to fabricate other metal oxide hollow fibers and particles with controllable interior structures by fast heating their gel precursors, which are not limited in the ones fabricated by electrospinning techniques. The resultant γ-Fe2O3 fiber-in-tube and tube-in-tube nanostructures may have a number of applications that involve magnetic separable catalysts or catalyst supporting materials, sensors, absorbents, microreactors, etc. Acknowledgment. This work was supported by the National high-technology Research and Development Program of China (2006AA03A209), the Fundamental Research Funds for the Central Universities (2010-IV-006), the Ministry of Education (NCET-05-0660 and PCSIRT0644), and the China Postdoctoral Science Foundation (20070420169).
DOI: 10.1021/la102830p
15585
