J. Phys. Chem. C 2009, 113, 19397–19403
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ARTICLES ZnO Hollow Nanofibers: Fabrication from Facile Single Capillary Electrospinning and Applications in Gas Sensors Zhenyi Zhang, Xinghua Li, Changhua Wang, Liming Wei, Yichun Liu, and Changlu Shao* Center for AdVanced Optoelectronic Functional Materials Research, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China, and Key Laboratory of UV Light-Emitting Materials and Technology, Ministry of Education, Peoples Republic of China ReceiVed: July 24, 2009; ReVised Manuscript ReceiVed: September 28, 2009
In this work, ZnO hollow nanofibers with diameters of 120-150 nm were successfully fabricated by electrospinning the precursor solution consisting of polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), and zinc acetate composite through a facile single capillary, followed by thermal decomposition for removal of the above polymers from the precursor fibers. The as-prepared nanofibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), resonant Raman spectra, thermal gravimetric and differential thermal analysis (TG-DTA), and Fourier transform infrared spectroscopy (FT-IR) spectra, respectively. The results indicated that, during the electrospinning process, there occurred phase separation between the electrospun composite materials, while the obtained precursor nanofibers of PAN, PVP, and zinc acetate composite might possess a core-shell structure (PAN as the core and PVP/zinc acetate composite as the shell). Furthermore, the composite nanofibers with core/shell structure could play a structural directing template role for preparing ZnO hollow nanofibers during the calcination process. The ZnO hollow nanofibers exhibited excellent sensing properties against ethanol due to their special one-dimensional nanostructural properties. 1. Introduction As a specific one-dimensional (1D) morphology, tubular nanostructures are very useful in a wealth of applications that include catalysis, fluidics, purification, gas storage, energy conversion, drug release, sensing, and so on.1-5 For this reason, a rich variety of methods such as metal-organic chemical vapor deposition, hydrothermal synthesis, template methods, and so forth have been demonstrated to prepare nanotubes with walls made of metals, ceramics, polymers, and carbon.6,7 Notably, electrospinning, a remarkably simple and versatile technique, has been exploited for many years to process polymers and related materials into 1D structural fibers with controllable compositions, diameters, and porosities for a variety of applications, including tissue engineering, drug delivery, catalyst supports, gas storage, sensors, and so on.8-12 This method provides a means to bridge the dimensional and property gap between nano- and macroscale engineering materials and structures. Satisfactory, by using this method, 1D tubular nanostructures could also be fabricated. As we know, essentially three main strategies have been used until now: (1) The fiber template method is based on use of the electrospun nanofibers as templates to prepare polymer, polymer-metal hybrid, metal nanotubes, and so on.13-16 In this case, polymer nanofibers are produced by electrospinning and then coated with a precursor material, from which the nanotubes are to be prepared by various deposition methods. Subsequently, the inner section of electrospun nanofiber is removed by selective dissolution or thermal * To whom correspondence should be addressed at Northeast Normal University. E-mail:
[email protected]. Phone: 864315098803.
degradation, and the nanotubes can be obtained. This approach, however, only works best for relatively short structures because the overlapping or entanglement between long, flexible templates (such as nanofibers) inevitably causes interconnections between the resulting nanotubes. (2) With the coaxial electrospinning method two different solutions are spun simultaneously, using a spinneret with two coaxial capillaries, to produce core/shell nanofibers. The core of the above fibers is then selectively removed, and hollow nanofibers as a kind of long nanotubes are formed. By using this method, hollow nanofibers with walls made of ceramic or carbon could be obtained.17-20 However, there are still some problems in obtaining hollow nanofibers from coaxial electrospinning, such as how to select a suitable inner solvent and how to accurately control electrospinning parameters, and so forth. (3) The phase separation coelectrospinning process is a simple way in which, based on phase separation, the core/shell nanofibers are fabricated by coelectrospinning a blend of two-phase separation solutions through a single nozzle. Then, after removal of the core of the above nanofibers, the hollow nanofibers can be easily obtained.21-24 Recently, Yarin’s group found that the carbon hollow fibers could be prepared by electrospinning a PMMA and PAN mixture polymer solution through a single nozzle, followed by heat treatment of the electrospun core/shell fibers of a PMMA/ PAN mixed polymer.25 Likewise, our group also reported that the polymer hollow nanofibers could be prepared by a facile one-step electrospinning with a single nozzle, instead of coaxial electrospinng.26 These works imply that it might be possible to fabricate the ceramic hollow nanofibers by calcination of the
10.1021/jp9070373 CCC: $40.75 2009 American Chemical Society Published on Web 10/16/2009
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core/shell structural fibers with one polymer as the core and another kind of polymer and metal salt composite as the shell through a phase separation coelectrospinning process. In this paper, we give a successful attempt to fabricate ZnO hollow nanofibers by electrospinning the polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), and zinc acetate composite solution via a facile single capillary, then using heat treatment of the as-electrospun composite nanofibers to elimination of above two polymers. And the hollow nanofibers display an excellent ethanol sensing characteristic due to their hollow 1D nanostructure and high surface property. 2. Experimental Section 2.1. Materials. Polyacrylonitrile (PAN) (Mw ) 150 000), polyvinylpyrrolidone (PVP) (Mw ) 1 300 000), N,N-dimethylformamide (DMF), and zinc acetate (Zn(CH3COO)2 · 2H2O) were all purchased from Shanxi Chemical Co. Ltd. All materials were directly used as received without further purification. 2.2. Preparation of ZnO Hollow Nanofibers. First, 2.25 g of PAN was dissolved in 13 mL of DMF solution, then 4.5 g of PVP and 1.2 g of zinc acetate were dissolved in 26 mL of DMF solution, and then the two solutions were mixed for half an hour, thus obtaining the precursor solution of PAN/PVP/ zinc acetate. Subsequently, the above precursor solutions were drawn into a hypodermic syringe. The positive terminal of a variable high-voltage power supply was connected to the needle tip of the syringe while the other terminal was connected to the collector plate. The positive voltage applied to the tip was 10 kV and the distance between the needle tip and the collector was 10 cm, resulting in a dense web of electrospun composite nanofibers of PAN/PVP/zinc acetate being collected on the aluminum foil. Afterward, the above composite nanofibers were calcined at a rate of 25 deg/h and remained for 2 h at 650 °C. Thus, ZnO hollow nanofibers were obtained. 2.3. Characterization. The morphologies of the as-prepared nanofibers were observed by a scanning electron microscope (SEM; XL-30 ESEM FEG, Micro FEI Philips) and transmission electron microscopy (TEM; Hitachi 600). X-ray diffraction (XRD) measurements were carried out using a D/max 2500 XRD spectrometer (Rigaku) with Cu KR line of 0.1541 nm. Resonant Raman spectra were taken on a Jobin-Yvon HR800 micro-Raman spectrometer, using the 325 nm line of a He-Cd laser as the excitation source at room temperature. TG-DTA analysis was carried out on a NETZSCH STA 449C thermoanalyzer in air. Fourier transform infrared spectroscopy (FT-IR) spectra were obtained on a Magna 560 FT-IR spectrometer with a resolution of 1 cm-1. The viscosity measurements were performed with a HANG PING SNB-1 viscometer at 60 rpm. The ethanol gas sensing properties were measured by the gas sensing measurement system of a RQ1 intelligent test meter (China), with ethanol concentrations varying from 10 to 5000 ppm. 3. Results and Discussion 3.1. Morphology of the As-Prepared Nanofibers. Figure 1a-d showed the SEM images of the as-prepared fiber samples. From Figure 1a, it could be observed that these randomly oriented nanofibers had a smooth and uniform surface due to the amorphous nature of the PAN/PVP/zinc acetate composite nanofibers. Meanwhile, Figure 1b displayed the corresponding SEM image with higher magnification, and the diameters of the above composite nanofibers ranged from 200 to 250 nm. Figure 1c showed SEM images of the ZnO long continuous hollow nanofibers, the diameters of which ranged from 120 to
Zhang et al. 150 nm, and the nanofibers exhibited shrinkage resulting from the decomposition polymers template of PAN and PVP. Likewise, a higher magnification SEM image of the ZnO hollow nanofibers in Figure 1d showed that their surfaces are rough. The ruptured sections in Figure 1c,d clearly showed the hollow structure of ZnO nanofibers. In addition, Figure 1e,f showed the typical TEM images of the above ZnO hollow nanofibers taken at low and high magnification, respectively. As observed in Figure 1e, a large quantity of ZnO hollow nanofibers with nearly uniform diameters could be clearly seen. Accordingly, from Figure 1f, it was seen that the thickness of the hollow nanofiber wall was about 24 nm. Those TEM images further confirmed the as-prepared ZnO nanofibers possessed 1D hollow fibers structures and were uniform in size. Furthermore, the insert of Figure 1f shows the corresponding selected area electron diffraction (SAED) pattern, indicating the polycrystalline configuration of the zinc oxide hollow nanofiber, which consisted of the following XRD result. Energy-dispersive X-ray (EDX) spectroscopy composition analysis in Figure 1g showed that only the peaks associated with Zn and O atoms (the molar ratio of Zn to O was nearly 1) were detected, leading to the obvious fact that the hollow nanofibers were indeed ZnO material. 3.2. XRD Patterns of the ZnO Hollow Nanofibers. Figure 2 showed the X-ray diffraction pattern of ZnO hollow nanofibers. Nine reflection peaks appeared at 2θ ) 31.9° (100), 34.6° (002), 36.5° (101), 47.7° (102), 56.8° (110), 63.1° (103), 66.5° (200), 68.1° (112), and 69.2° (201), respectively, and all diffraction peaks could be indexed as the hexagonal wurtzite structure of ZnO, which were consistent with the values in the standard card (JCPDS 36-1451). No diffraction peaks from any other impurities were detected. Thus, the results clearly showed that the product was pure ZnO hollow nanofibers. 3.3. Resonant Raman Scattering Spectra of the ZnO Hollow Nanofibers. Figure 3 showed the measured resonant Raman scattering spectra of the ZnO hollow nanofibers. In our experiment, the UV resonant Raman scattering at room temperature was performed to investigate the vibrational properties of the hollow nanofibers. The energy of the He-Cd laser line (325 nm) was 3.82 eV, which was higher than the band gap of ZnO (3.37 eV). As observed in Figure 3, the significant peaks centered at around 577, 1140, 1723, and 2297 cm-1 were observed, respectively, which were attributed to the 1-4 Raman longitudinal optical (LO) phonon mode of nanosized ZnO.27 3.4. TG-DTA of the Nanofibers of PAN//PVP/Zn(CH3COO)2 Composites. Figure 4 showed the thermal behavior of the precursor nanofibers of PAN//PVP/Zn(CH3COO)2 composite, which indicated that most of the organic belonged to PAN, PVP, and the CH3COO group of zinc acetate and other volatiles was removed at temperature
