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Royal Swedish Academy of Sciences
Physica Scripta
Phys. Scr. 89 (2014) 115808 (6pp)
doi:10.1088/0031-8949/89/11/115808
Fabrication of ultrathin In2O3 hollow fibers for UV light sensing Shuai Chen1, Yun-Ze Long1,2,3, Hong-Di Zhang1,2, Shu-Liang Liu1, Ling-Zhi Liu1, Jun-Cheng Zhang1,2, Guo-Xia Liu1,3 and Fu-Kai Shan1,3 1
College of Physics, Qingdao University, Qingdao 266071, People’s Republic of China Key Laboratory of Photonics Materials & Technology in Universities of Shandong (Qingdao University), Qingdao 266071, People’s Republic of China 3 State Key Laboratory Cultivation Base of New Fiber Materials & Modern Textile, Qingdao University, Qingdao 266071, People’s Republic of China 2
E-mail:
[email protected] and
[email protected] Received 22 March 2014, revised 14 September 2014 Accepted for publication 24 September 2014 Published 28 October 2014 Abstract
Ultrathin indium oxide (In2O3) hollow fibers were successfully fabricated by electrospinning poly(vinylidene fluoride) (PVDF) nanofibers, magnetron sputtering of In2O3 on PVDF fibers followed by calcination of In2O3/PVDF composite fibers. The hollow In2O3 fibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD) and UV-Visible spectroscopy. The outer diameter of the hollow fibers was in the range of 700–900 nm, and the inner diameter was about 400–600 nm. The optoelectronic properties of the In2O3 fibers were investigated by the irradiation of UV light with different wavelengths (254, 308 and 365 nm). It was found that the In2O3 hollow nanofibers had a fast and strong response to UV irradiation. The response time was less than 10 s, and the sensitivity (∼102) decreased with the UV light wavelength increasing or the light intensity decreasing. Keywords: indium oxide, hollow fiber, electrospinning, UV light sensing Hollow nanofibers have the potential to find broad applications in many fields, such as gas storage, drug sustained release and sensing devices [9–11]. Until now, a variety of methods has been reported to produce nanofibers with hollow structure [12–15]. For example, through developing new electrospinning techniques such as melt electrospinning [16], dry jet wet spinning [17] and coaxial electrospinning [18] or combining other approaches, such as chemical coating method [19], various kinds of hollow fibers have been prepared. In2O3 is known to be a representative n-type semiconductor material with a broad direct band gap of 3.75 eV and an indirect energy gap of 2.62 eV. Due to its good electrical conductivity, high chemical inertness, low electron affinity and other excellent properties, In2O3 has exhibited promising applications in microelectronic fields, such as semiconductor sensors, flat-panel displays and solar energy conversion [20]. Studies have demonstrated that size, morphology and structural features can easily affect the performance of In2O3 semiconductor devices [21, 22]. In particular,
1. Introduction Electrospinning has been recognized as a convenient and efficient technique to produce ultrathin fibers. Taking advantage of electrospinning, a wide variety of materials including bio-macromolecules, organic polymers, semiconductor ceramics and even metals have been fabricated [1, 2]. Besides polymer fibers, electrospinning can also be used to fabricate inorganic oxide nanofibers from polymer solutions containing inorganic/organic composite ingredients [3]. When the diameters of fibers reduce, their surface-areato-volume ratio becomes larger; therefore, many outstanding properties can be achieved. Thus, the electrospun fibers have many potential applications such as filtration, energy conversion, hydrogen storage, optical devices, electronic devices, sensor devices, etc [4–7]. As the study of nanofibers continues, researchers are paying much attention to some new types of nanofiber structures, such as aligned fiber arrays, three-dimensional stacking fibers [8], coaxial nanofibers and hollow nanofibers. 0031-8949/14/115808+06$33.00
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© 2014 The Royal Swedish Academy of Sciences
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the parameters of gas sensors are obviously dependent on the effective surface area and porosity of In2O3 [23]. Moreover, it has an excellent sensitivity in detecting low concentrations of gas, such as gasoline, alcohol, ammonia [24], butane gas and so on. Meanwhile, In2O3 also has a very high sensitivity for UV detection, which can be used to protect the human body from ultraviolet irradiation. According to recent studies, In2O3 with various morphologies and structures such as powders, nanorods, nanofibers, nanobelts and films have been fabricated by electrospinning, chemical vapor deposition, hydrothermal and solvothermal methods and other strategies [25–28]. However, In2O3 nanofibers fabricated by electrospinning can improve sensitivity and decrease response time due to their higher surface-to-volume ratio and super-dense oxygen vacancies. In particular, nanofibers with a hollow structure are more complicated than nanoparticles and nanowires and may possess more attractive properties that are closely associated with their special structure. Therefore, In2O3 hollow fibers may be a promising and excellent candidate for advanced applications such as solar cells, drug release, biological/gas sensors, advanced catalysis and energy storage. However, there are only a few reports on In2O3 hollow wires mainly through the template-assisted method [29]. Generally, it is difficult to remove the templates without destroying the as-prepared structures and control the remainder without impure components. In addition, outer diameters of the reported In2O3 hollow fibers are usually too large [30]. These problems limit the further development of the ultrathin hollow fibers. In this work, ultrathin In2O3 hollow fibers have been successfully prepared by a new method combining electrospinning, magnetron sputtering and calcination. The In2O3 hollow nanofibers fabricated by this approach have a smaller outer diameter (700–900 nm) and a thinner wall thickness (50–200 nm). Particularly, the In2O3 fibers show fast and strong response to UV irradiation.
Development Co., Ltd, Shenyang, China), a thin layer of In2O3 was deposited on the electrospun PVDF nanofibers, which acted as a hard template. The sputtering parameters were controlled as follows: In2O3 target (target diameter: 100 mm; particle size: 0.3–1.0 μm), sputtering power (90 W), distance between target and substrate (10 cm), sputtering time (1 h), volume ratio of Ar and O2 (10:0.1), room temperature and 1 atm. In order to get a hollow fiber structure, the In2O3/PVDF composite fibers have been calcined for 2 h at different temperatures (600 °C and 800 °C) in a Muffle furnace (Longkou Electric Furnace Factory, China) to remove the PVDF fiber template completely and convert the precursor fibers into polycrystalline In2O3 nanofibers. Then, the furnace was naturally cooled down to room temperature. 2.2. Fabrication of the In2O3 hollow fiber device
In order to study the UV photodetection property of the In2O3 hollow fibers, the In2O3 device was fabricated as follows: First, a square shape of the In2O3 hollow fiber membrane was placed on a glass plate, and then coated with silver paste on both ends of the sample, and each end was linked with a copper conductive wire. 2.3. Characterization of In2O3 hollow fibers
The hollow fibers were characterized by a scanning electron microscope (SEM, FEI DB235 Dual-Beam (FIB/SEM) System), a transmission electron microscope (TEM, FEI Tecnai G20), an x-ray diffraction (XRD, Rigaku D/max-2400) diffractometer, and a TU-1901 Dual-Beam UV-Visible Spectrophotometer. The UV photodetection properties of In2O3 hollow fibers were tested by a UV lamp, which was fixed to different distances (e.g., 1 to 8 cm) to the In2O3 fibers. Three kinds of UV lamps were used: PLS-LAM365 (365 nm UV light) with a power of 10 W, PLS-LAM300 (308 nm UV light) with a power of 10 W, and PLS-LAM254 (254 nm UV light) with a power of 10 W (Beijing Perfectlight Technology Co., Ltd). The photoresponse measurements of the device were carried out with an applied bias voltage of 5 V in room temperature and in air. The radiation intensity was 10.9 mW cm−2 with a lamp-to-sample distance of 1.0 cm, which was measured by an UV radiometer (UVATA-UE500, Shanghai). The sensitivity is defined as Imax/I0, which is calculated automatically by the computer of the electrical measurement system, where Imax is the maximum photocurrent of the sensor under UV irradiation and I0 is the original current of the sensor before UV irradiation.
2. Experimental 2.1. Preparation of ultrathin In2O3 hollow fibers
First, poly(vinylidene fluoride) (PVDF, Sigma-Aldrich) precursor solution was prepared for electrospinning. Next, 2.0 g PVDF was added into 8.0 g of the mixed solvent, containing N,N-dimethyl formamide (analytical reagent, Tianjin Guangcheng Chemical Factory) and acetone (analytical reagent, Laiyang Fine Chemical Industry) with the weight ratio of 1:1 and stirred for 3 h at 60 °C in a water bath. The solution was loaded into a 5 ml plastic syringe with a stainless-steel needle connected to a high-voltage power supply (Tianjin Dongwen, China). A voltage of 13 kV was applied for electrospinning. The distance between the syringe needle and the collector was 10 cm. In the process of electrospinning, the solvent volatilized and PVDF nanofibers were deposited on the collector. Taking advantage of the FJL-560 Ultrahigh Vacuum Magnetron Sputtering Deposition System (SKY Technology
3. Results and discussion Figure 1 shows the SEM and TEM images of the In2O3 hollow nanofibers. It is obvious that the pure PVDF nanofibers are very smooth (shown in figure 1(a)) and the In2O3 particles are coated on the PVDF fibers after magnetron sputtering (figure 1(b)). From figure 1(c) and the inset of 2
Phys. Scr. 89 (2014) 115808
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Figure 1. SEM images of the In2O3 hollow fibers: (a) electrospun PVDF nanofibers, (b) PVDF fibers coated with In2O3 layer before calcination, (c) In2O3 hollow fibers after calcination at 600 °C for 2 h, (d) surface morphology of the In2O3 hollow fibers (inset: TEM image of the In2O3 hollow fibers).
figure 1(d), we can see that the PVDF fiber template has been burned out, and that In2O3 hollow fibers have been formed after calcination under 600 °C. The outer diameter of the hollow nanofibers is broadly distributed in the range of 700–900 nm, and with an inner diameter of 400–600 nm. The wall thickness of the hollow fibers is approximately 50–200 nm. After calcining, the deposited In2O3 particles become more compacted. The crystal structure of the In2O3 nanofibers after calcining at 600 °C and 800 °C was examined by XRD. The XRD patterns proved that the In2O3 nanofibers have a cubic structure with diffraction peaks of (222), (400) and (440). As for 800 °C, all diffraction peaks became much higher and sharper compared to 600 °C while the positions remained unaltered, which demonstrates that with the increase of the calcination temperature, the crystallization becomes better and crystalline grain grows much bigger. All the diffraction peaks of the sample in figure 2 could be indexed to In2O3 (JCPDS No. 06-0416). There are no impurity peaks obviously in the XRD patterns, so the sample is In2O3 of cubic structure in high purity. Figures 3(a)–(c) show the time-dependent photocurrent curves when the UV light was turned on and turned off periodically, and a total of four cycles of data were recorded under UV illumination with different wavelengths at a distance of 1.0 cm. The conductance of the sample was observed
Figure 2. XRD patterns of the In2O3 hollow fibers after calcination at different temperatures (600 °C and 800 °C) for 2 h.
to increase rapidly when the UV light was turned on, and it decreased gradually when the UV light was turned off. The sensitivity defined as Imax/I0 is as high as 102. The response time was obtained to be less than 10 s, but the recovery time was up to 500 s. From figure 3, it can be observed that the device exhibits good reversibility after repeated exposure to the light. Figure 3(d) shows the photoresponse and I-V 3
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Figure 3. (a)–(c) Photoresponse of the In2O3 hollow fibers in dark and under UV illumination with different wavelengths of (a) λ = 254 nm, (b) λ = 308 nm and (c) λ = 365 nm. (d) Photoresponse of the In2O3 hollow fibers to sequential UV illumination with wavelengths of 254 nm, 308 nm and 365 nm. The applied voltage was 5.0 V. 3.0
3.5
427 μA and 367 μA under UV illumination with wavelengths of 254 nm, 308 nm and 365 nm, respectively. The calculated sensitivities are 100.6, 82.1 and 70.6 with UV wavelengths of 254 nm, 308 nm and 365 nm, respectively, and we can see that the sensitivity decreases with the increase of UV wavelength. The results have a large improvement compared with the performance of thin-film ultraviolet photodetector fabricated from In2O3 nanoparticles [31, 32]. For the thin-film photodetector with a maximum responsivity of 11 A W−1 at 340 nm, the response time and recovery time were up to 1100 s and 3200 s, respectively, under illumination by a 335 nm UV LED with intensity of 31.65 mW cm−2. Moreover, the sensitivity was only at the magnitude of 10°, which is lower than that of the device fabricated from In2O3 hollow fibers (101 ∼ 102). Even though the performance of the thinfilm photodetector could be improved to some extent by coating a polymer layer to reduce surface defects, the response time and sensitivity of our present device based on In2O3 hollow fibers are still competitive. Because the photon energy of UV light is near the forbidden bandwidth of many metal oxide semiconductors, UV radiation energy could be absorbed by the semiconductors effectively. In2O3 nanofibers are known to have a direct band
2.5 Absorbance (a.u.)
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Wavelength (nm) Figure 4. UV-Visible absorption spectra of the In2O3 hollow fibers. Inset: Plot of absorbance versus photon energy for In2O3 hollow fibers.
characteristic curves of the hollow fibers exposed to sequential UV illumination with different wavelengths of 254, 308 and 365 nm. The photocurrents increase from initial current (background current under bias of 5.0 V) 5.2 μA to 523 μA, 4
Phys. Scr. 89 (2014) 115808
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Figure 5. (a) Photoresponse of the In2O3 hollow fibers at different distances from the UV lamp to In2O3 fibers under a bias voltage of 5.0 V. (b) The curves of photocurrent versus 1/distance of the In2O3 hollow fibers. (c) I–V characteristic curves of the In2O3 hollow fibers exposed to UV light with different wavelengths. (d) I–V characteristic curves of the In2O3 hollow fibers at different lamp-to-sample distances.
gap of 3.75 eV at k = 0 and an indirect energy gap of 2.62 eV [33]. Figure 4 shows absorption spectrum of In2O3 hollow fibers under the UV-Visible wavelength region. The plot of absorbance versus photon energy (hν) for direct transitions to determine the band gap of the sample is shown in the inset of figure 4. The absorption edge depicts that the sample has a direct band gap that can be obtained by extrapolating the linear portion of the plot to the photon energy axis, and the direct band gap has been obtained to be about 3.41 eV (363 nm in figure 4), which is lower than the widely quoted value of 3.75 eV. When illuminated by the 254 nm UV light, the sample can absorb the photon sufficiently, which has an energy of 4.9 eV, enough to excite electrons directly from the valence band to the conduction band. We also know that the same material shows different absorption ability for different wavelengths of light. According to previous studies, the In2O3 absorption coefficient is decreased with the increase of UV wavelength [34]. For the 308 nm UV light, which has an energy of 4.03 eV, though the energy is higher than the In2O3 direct band gap, the absorbance is lower compared to that for the 254 nm UV light, and that the sensitivity of In2O3 fibers under 308 nm UV irradiation is lower than that under 254 nm UV irradiation. For the 365 nm UV light, which has a photo energy of 3.40 eV, its energy is below the In2O3 direct band
gap but above the indirect energy gap. This is true even though the electrons can be excited from the valence band to the indirect energy band, and the direct transition of electrons from the valence band to the conduction band is prohibited. The sensitivity is also decreased in comparison with the sensitivity under 308 nm UV irradiation. Figure 5(a) shows the response of the In2O3 hollow nanofibers at different distances from the UV lamp to the electrode under the same UV light of 254 nm. The curves of photocurrent versus 1/distance of the In2O3 hollow fibers are shown in figure 5(b) and the I–V characteristic curves of the In2O3 hollow nanofibers with different lamp-to-sample distances are shown in figure 5(d). We can see that the curves showed in figure 5(b) is parabolic and when the distance is larger than 1.5 m, the illumination intensity becomes very weak, and the photocurrent keeps unchanged. When the electrode is unlimitedly close to the lamp, the photocurrent will tend to reach a constant value due to its rated power. From these figures, we can observe that the sensitivity is decreased with the distance increasing. Furthermore, the decrease trend slows down with increasing distance. While the distance was 1.0 cm, the current could be brought to 508 μA from background current of 5.2 μA. However, while the distance was 8 cm, the current only could be brought to 5
Phys. Scr. 89 (2014) 115808
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196 μA. Since the illumination intensity on the sample is proportional to the power of point light source and inversely proportional to the square of the light-source-to-sample distance, the increase of this distance will lower the detectable illumination intensity on the sample. In addition, the number of photons that can excite electrons directly from the valence band to the conduction band is less, leading to the lower sensitivity. However, compared with the initial current (background current) without UV irradiation, the photocurrent has increased greatly.
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4. Conclusion In2O3 hollow nanofibers were successfully synthesized by a new method combining electrospinning with magnetron sputtering and calcination. From the SEM and TEM images, we could observe the hollow structures clearly. We have investigated the photoresponse of the In2O3 fibers under UV illumination with different wavelengths and found that the In2O3 fiber device could enhance the conduction significantly when exposed to UV light. The sensitivity is as high as 102. We also studied the relationship between distance and sensitivity, and demonstrated that the sensitivity decreases with the increase of the lamp-to-sample distance. Further study on the gas-sensing property of these electrospun In2O3 hollow nanofibers is now in progress.
Acknowledgments This work was supported by National Natural Science Foundation of China (51373082, 11074138 and 11004114), Natural Science Foundation of Shandong Province (China) for Distinguished Young Scholars (JQ201103), Taishan Scholars Program of Shandong Province (ts20120528), National Key Basic Research Development Program of China (973 special preliminary study plan, 2012CB722705), and Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province, China.
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