JAST 2011 March;2(1):1-6.
Published online 2011 February 09. doi:http://dx.doi.org/10.5355/JAST.2011.1 | | Copyright ⓒ 2010 Journal of Analytical Science & Technology | | Preparation and characterization of Ce-doped ZnO nanofibers by an electrospinning method | | Jong-Seong Bae, Mi-Sook Won, Jang-Hee Yoon, Byoung-Seob Lee, Eun Sick Pak, Hoe-Joo Seo, Jong-Pil Kim* | | Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea | | Corresponding Author: Jong Pil Kim ,Tel: +82-51-510-2993, Fax: +82-51-517-2497, Email: jpkim@kbsi.re.kr |  | ABSTRACT |  | | | ZnO and Ce-doped ZnO Nanofibers on (111) Pt/SiO2/Si substrates were produced using an electrospinning technique. The as-prepared composite fibres were subjected to high-temperature calcination to produce inorganic fibers. After calcining at a temperature of 500 °C, the average diameter of the ZnO and Ce-doped ZnO nanofibers were determined to be 170 nm and 225 nm, respectively. The average grain size of the ZnO and Ce-doped ZnO nanofibers were about 50 nm and 57 nm, respectively. The microstructure, chemical bonding state and photoluminescence of the produced ZnO and Ce-doped ZnO nanofibers were investigated. The Ce-doped ZnO nanofiber can be assigned to the presence of Ce ions on substitutional sites of Zn ions and the Ce3+ state from X-ray photoelectron spectra. Compared with PL spectra of ZnO nanofibers, the peak position of the UV emission of the Ce-doped ZnO nanofibers is sharply suppressed while the green emission band is highly enhanced. | | Keywords: Nanofiber, Ce-doped ZnO, Electrospinning, Chemical bonding state, Photoluminescence | | | Introduction | | | Electrospinning, a top-down nanomanufacturing method, offers a rapid, facile process for creating high-surface-area polymer fibers compared to those produced by most bottom-up methods [1-3]. In addition to its use in the preparation of pure polymer fibers,
electrospinning has also been used to synthesize inorganic oxide fibers from polymer solutions containing inorganic species [4].
It is well known that semiconductors with a low dimensional size may have superior optical properties those of bulk crystals owing to quantum confinement effects. The semiconductor, ZnO is a II-VI semiconductor with a wide band-gap of 3.3 eV and a large exciton binding energy of 60 meV at room temperature. ZnO has attracted considerable attention due to the strong commercial need for blue and ultraviolet (UV) light emitters and detectors, transparent electrodes in solar cells, gas sensors, and piezoelectric transducers. These properties and potential applications have stimulated attempts of prepare and study well-controlled ZnO nanostructures. Rare-earth (RE) doped ZnO is of interest for possible applications in visible emitting phosphors in displays, high power lasers, and other optoelectronic devices. Since CeO2 has a band gap of about 3 eV and shows interesting optical properties such as optical transparency and strong UV absorption similar that of ZnO, Ce-doped ZnO has been investigated for its potential use in highly efficient catalysts or UV filters. Very little attention has been directed toward the incorporation of Ce-doped ZnO nanostructures in polymer systems to form functional micro/nanostructures.
In this study, we report an electrospinning technique for the synthesis of ZnO and Ce-doped ZnO nanofibers, as well as finding related to phase formation, microstructure, chemical bonding state, and optical properties of the resulting fibers.
| | Materials and Methods | | | | The ZnO and Ce(10 mol%)-doped ZnO electrospun fibres were prepared using zinc acetate dehydrate (C4H6O4Zn·2H2O) and cerium acetate hydrate ((C2H3O2)3Ce x H2O) dissolved in a solution of methyl alcohol (CH3OH) and monoethanolamine (NH3CH2CH2OH). Briefly, zinc acetate dihydrate and cerium acetate hydrate was dissolved in the minimum amount of methyl alcohol and monoethanolamine required to achieve complete solubility. The resulting clear solution was mixed with polyvinylpyrrolidone (PVP; Mw ≒ 55,000) and the solution stirred for 10 h. The PVP/inorganics (ZnO and Ce-doped ZnO) fibres were prepared by subjecting the solution to a high electrical potential. The solution was placed in a hypodermic syringe at a fixed distance (15~17 cm) from a metal cathode electrode. The positive (anode) terminal of a variable high-voltage power supply capable of delivering 10 kV was attached to a copper wire inserted into the polymer solution in the hypodermic syringe; the negative terminal (cathode) was attached to the plate target electrode. When the voltage applied between the positive and negative electrodes reached 9.5 kV, dry, centimetre long fibres collected on the surface of the negative electrode((111) Pt/Ti/SiO2/Si). The as-prepared composite fibres were subjected to high-temperature calcinations, resulting in the production of inorganic fibers. The structure of the ZnO and Ce-doped ZnO nanofibers were investigated by means of an X-ray diffractometer (XRD; X’pert PRO MRD, Philips) and transmission electron microscopy (TEM; Jem-2011, Jeol). The microstructure of the fibers was examined by scanning electron microscopy (SEM; S-4200, Hitachi). The chemical bonding states of the nanofibers was determined by X-ray photoelectron spectroscopy (XPS; ESCALAB 250, VG Scientific). In addition, photoluminescence (PL) spectra were obtained at room temperature by means of a luminescence spectrometer (Perkin-Elmer, LS-50B) with a He-Cd laser (325 nm). | | Results and Discussion | | | (Fig.1) shows XRD patterns of the ZnO and Ce-doped ZnO nanofiber deposited a (111) Pt/Ti/SiO2/Si substrate. XRD measurements were performed in both the Bragg (θ-2θ scan) and glancing incidence (GIXRD) geometry. The PVP/ZnO and PVP/Ce-doped ZnO hybrid fibers are essentially amorphous in the assynthesized form.
Following calcination at 500 °C, well defined features appeared, due to the crystallization of zinc oxide. The diffraction peaks could be indexed as a the hexagonal wurtzite structure ZnO (a = 0.3249 nm, c = 0.5206 nm), and diffraction data were in agreement with the JCPDS card of ZnO (JCPDS 36-1451).
The SEM images of in-situ (no calcination) ZnO/Al coils are shown in Fig. 2(a) and transmission electron microscope (TEM) images of PVP/Ce-doped ZnO hybrid fibers (no calcination) are shown in figure 2(b). The image shows that the nanofibers have smooth and uniform surfaces with random orientations and an average diameter of 1.70 μm. The average size of the inorganic particles was about 0.22 μm. The SEM images of a nanofiber/(111)Pt/Ti/SiO2/Si after calcining at a temperature of 500 °C are presented in Fig. 2(c) and Fig. 2(d). When the PVP was selectively removed by calcination in air at 500 °C, the nanofibers remained as continuous structures.
(Fig.3) showns TEM images of the ZnO and Ce-doped ZnO nanofibers after calcining at 500 °C. Fig. 3(a) and 3(b) show a high magnification TEM images of the ZnO and Ce-doped Zno nanofibers, respectively. After calcining ate 500 °C , the average diameter of the ZnO and Ce-doped ZnO nanofibers was determined to be approximately 170 nm and 225 nm, respectively. The average grain size of ZnO and Ce-doped ZnO nanofibers was about 50 nm and 57 nm, respectively. Fig. 3(c) and 3(e) shows high resolution TEM images of the samples. The ZnO and Ce-doped ZnO nanofibers have a clearly visible dislocation free crystal structure and no amorphous layer covering the nanofibers can be detected. Fig. 3(d) and 3(f) exhibit a filtering lattice image corresponding to the (100) and (101) planes of a fast Fourier transformation (FFT) pattern of the white square in Fig. 3(c) and 3(e), respectively. A typical high resolution (HR) TEM image of a nanofiber shows the uniform lattice structure and single crystalline characteristic. It is noteworthy that there are no detectable crystal defects (e.g., microtwins or dislocations). In the ZnO (Fig. 3(d)) and Ce-doped nanofibers (Fig. 3(f)), the (100) and (101) planes are imaged from the hexagonal Wurtzite-type crystal with a lattice spacing of 0.282 nm and 0.248 nm, respectively. The EM images of fibres obtained after calcining at 500 °C exhibits indicates shrinkage, with the diameters being reduced by about 87 % due to decomposition of the PVP component.
(Fig.4) shows XPS spectra of ZnO and Ce-doped ZnO nanofiber after calcinations at 500 °C. High resolution scans of Zn 2p is shown in Fig. 4 (a). The peaks located uniform lattice structure and single crystalline characteristic. It is noteworthy that there are no detectable crystal defects (e.g., microtwins or dislocations). In the ZnO (Fig. 3(d)) and Ce-doped nanofibers (Fig. 3(f)), the (100) and (101) planes are imaged from the hexagonal Wurtzite-type crystal with a lattice spacing of 0.282 nm and 0.248 nm, respectively. The EM images of fibres obtained after calcining at 500 °C exhibits indicates shrinkage, with the diameters being reduced by about 87 % due to decomposition of the PVP component.
(Fig.4) shows XPS spectra of ZnO and Ce-doped ZnO nanofiber after calcinations at 500 °C. High resolution scans of Zn 2p is shown in Fig. 4 (a). The peaks located at 1021.5 (Zn 2p3/2) and 1044.6 ev (Zn 2p1/2) are attributed to Zn-O bond. The fit result of the O 1s and Ce 3d core level XPS spectra are shown in Fig. 4(b) and 4(c). The O 1s spectrum or ZnO nanofiber consists of the main peak (530.4 eV) and subpeak (532 eV). For the case of the Ce-doped case, O 1s spectrum consists of the main peak (530.4 eV) and two subpeak (529.8 eV, 532 eV). The main O 1s peak (530.4 eV) is attributed to O2- bonded to Zn2+ and the subpeak (532 eV) is attributed to physisorbed O2 and hydrated O species (O-H bond) and the subpeak (529.8 eV) is attributed to Ce-O bond. In the Ce-doped case, the FWHM of spectrum become shift and broden, indicating that the Ce-doped nanofiber contained Zn-O and Ce-O bond than that of pure ZnO nanofiber. Fig 4 (c) shows the core level spectra of Ce 3d for a Ce-doped ZnO nonofiber. The spectra consists of the four peaks at 881.3, 885.9, 899.5, and 904.1 eV, respectively. The spectra show the 3d5/2 and 3d3/2 transitions at 881.3 and 899.5 eV, respectively and the associated satellite peaks at 885.9 and 904.1 eV. The peaks at 881.3 and 899.8 eV are attributed to metal phase for Ce and the peaks at 885.9 and 904.1 eV are attributed to oxide phase for Ce. Using the electrochemical deposition technique, Verma et al., reported that the peaks characteristic of CeO2 (Ce4+) appear at 898.4 eV, while that for Ce2O3 (Ce3+) appear at 885.9 and 904.2 eV [5]. In the present study, the coexistence of metal phase and oxide phase for Ce chemical states in the nanofiber is evident from the peaks at 881.3, 899.5 (3d5/2) and 885.9, 904.1 eV (3d3/2), respectively.
(Fig.5) shows PL spectroscopy of ZnO and Ce-doped ZnO nanofibers calcined for 1 h at the 500 °C. Photoluminescence (PL) spectroscopy using a He-Cd laser (325 nm, 40 mW) has employed to characterize the nanofibers. As seen in (Fig.5), the spectrum is comprised of two main peaks; a narrow peak located at about 380 nm, which corresponds to the near-band edge (NBE) emission of the wide band gap ZnO, and a broad asymmetric band. The emission in the near band edge region (NBE) is attributed to free and bound exciton recombination. The latter is usually referred to as a green emission and is attributed to the singly ionized oxygen vacancy in ZnO where the emission results from the radiative recombination of a photo generated hole with an electron occupying the oxygen vacancy [6]. Oxygen vacancies occur in three different charge states: a neutral oxygen vacancy (VO), a singly ionized oxygen vacancy (VO*), and a doubly ionized oxygen vacancy (VO**) [7]. Vanheusden et al. reported that only singly ionized oxygen vacancies (VO*) are responsible for the green luminescence in ZnO [8]. In comparing the PL spectrum of ZnO nanofibers with the peak for Ce-doped ZnO nanofibers, the UV emission of Ce-doped ZnO nanofibers is sharply suppressed, while the green emission band is sharply enhanced. The peak position of the NBE emission shifts to a higher wavelength region (from 379.0 nm to 379.7 nm). The change in the peak position of NBE is attributed to an increase in the concentration of Ce in ZnO layer [9]. The broadening of this emission (green emission) can be attributed to the Ce dopant introduced into the layer because in nanocrystallites, the deformation potential influences short-range interaction [10]. | | Conclusions | | | | on (111) Pt/SiO2/Si substrates by an electrospinning technique. When the PVP was removed by calcination of the sample in air at 500 °C, the nanofibers remained as continuous structures. The diffraction peaks of the nanofibers can be indexed as the hexagonal wurtzite structure of ZnO, and no detectable crystal defects were observed. The average diameter of the ZnO and Ce-doped ZnO nanofibers was about 170 nm and 225 nm, respectively. The average grain size of the ZnO and Ce-doped ZnO nanofibers was about 50 nm and 57 nm, respectively. In ZnO and Ce-doped ZnO nanofibers, the (100) and (101) planes are imaged as hexagonal Wurtzite-type crystals with a lattice spacing of 0.282 nm and 0.248 nm, respectively. The Ce-doped ZnO nanofiber can be assigned to the presence of Ce ions on substitutional sites of Zn ions and the Ce3+ state from XPS spectra. Compared with PL spectra of ZnO nanofibers, the peak position of the UV emission of the Ce-doped ZnO nanofibers is sharply suppressed while the green emission band is highly enhanced. | | Acknowledgement | | | This work was supported by KBSI Grant (K3008A) to J. P. Kim.
| | | FIGURES | | |  | Fig.1
Figure 1. XRD patterns of (a) ZnO nanofibers and (b) Ce-doped ZnO nanofibers samples calcined at 500°C. | |  | Fig.2
Figure 2. EM images of (a) Ce-doped ZnO fibers/Al-coil (SEM), (b) Ce-doped ZnO (TEM), (c) ZnO nanofibers/(111) Pt, (d) Ce-doped nanofibers/(111) Pt samples. | |  | Fig.3
Figure 3. TEM image of (a) high magnification of ZnO nanofibers, (b) high magnification of Ce-doped manofibers, (c) high resolution image of ZnO nanofibers, (d) filtering lattice image of a (100) fast Fourier transformed pattern that corresponds to the area outlined by the white square in (c), (e) high resolution image of Ce-doped ZnO nanofibers and (f) filtering lattice image of (101) fast Fourier transformed pattern that corresponds to the area outlined by the white | |  | Fig.4
Figure 4. XPS spectra of the ZnO and Ce-doped ZnO nanofibers calcinated at 500°C. (a) Zn 2p, (b) O 1s and (c) Ce 3d | |  | Fig.5
Figure 5. Photoluminescence (PL) spectroscopy of (a) ZnO nanofiber and (b) Ce-doped nanofiber samples calcined at 500°C | | | | | | REFERENCE | | | | 1. | Yoo, J. S.; Dhungel, S. K.; Gowtham, M.; Yi, J.; Lee, J. C.; Kim, S. K.; Yoon, K. H.; Park, I. J.; Song, J. S. Properties of Textured ZnO : Al Films Prepared by RF Magnetron Sputtering for Thin Film Solar Cells. J. Korean Phys. Soc. 2005, 47, S576-S580. | | 2. | Zhu, F.; Zhang, K.; Guenther, E.; Jin, C. S. Optimized indium tin oxide contact for organic light emitting diode applications. Thin Solid Films 2000, 363, 314-317. | |  | | 3. | Kim, H.; Horwitz, J. S.; Kushto, G. P.; Kafafi, Z. H.; Chrisey, D. B. Indium tin oxide thin films grown on flexible plastic substrates by pulsed-laser deposition for organic light-emitting diodes. Appl. Phys. Lett. 2001, 79, 284-286. | | | | 4. | Shan, F. K.; Yu, Y. S. Band Gap Energy of Pure and Al-Doped ZnO Thin Films. J. Eur. Ceram. Soc. 2004, 24, 1869-1872. | | | | 5. | Amita Verma; Karar, M.; Bakhshi, A. K.; Harish Chander; Shivaprasad, S. M.; Agnihotry, S. A. Structural, morphological and photoluminescence characteristics of sol-gel derived nano phase CeO2 films deposited using citric acid. J. Nanoparticle Research 2007, 9, 317-322. | | | | 6. | Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. Mechanisms behind green photoluminescence in ZnO phosphor powders. J. Appl. Phys. 1996, 79, 7983-7990. | | | | 7. | Li, W.; Mao, D.; Zhang, F.; Wang, X.; Liu, X.; Zou, S.; Zhu, Y.; Li, Q.; Xu, J. Characteristics of ZnO:Zn phosphor thin flms by post-deposition annealing. Nucl.Instrum. Methods Phys. Res. B. 2000, 169, 59-63. | | | | 8. | Vanheusden, K.; Seager, C. H.; Warren, W. L.; Tallant, D. R.; Caruso, J.; Hampden-Smith, M. J.; Kodas, T. T. Green photoluminescence efficiency and free-carrier density in ZnO phosphor powders prepared by spray pyrolysis. Journal of Luminescence 1997, 75, 11-16. | | | | 9. | Sofiani, Z.; Derkowska B.; Dalasinski, P.; Wojdyła, M.; Dabos-Seignon, S.; Alaoui Lamrani, M.; Dghoughi, L.; Bała, W.; Addou, M.; Sahraoui, B. Optical properties of ZnO and ZnO:Ce layers grown by spray pyrolysis. Optics Communications 2006, 267, 433-439. | | | | 10. | Huang, M. H.; Wu, Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. Catalytic growth of zinc oxide nanowires by vapor transport. Adv. Mater. 2001, 13, 113-116. | | | | | |