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J. Phys. Chem. C 2008, 112, 18882–18888
Electrospinning Synthesis and Luminescence Properties of One-Dimensional Zn2SiO4:Mn2+ Microfibers and Microbelts Lili Wang, Xiaoming Liu, Zhiyao Hou, Chunxia Li, Piaoping Yang, Ziyong Cheng, Hongzhou Lian, and Jun Lin* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: July 20, 2008; ReVised Manuscript ReceiVed: August 22, 2008
One-dimensional Mn2+-doped Zn2SiO4 microbelts and microfibers were prepared by a simple and cost-effective electrospinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. The XRD and DTA results show that the Zn2SiO4 phase begins to crystallize at 800 °C and crystallizes completely around 1000 °C. SEM results indicate that the as-prepared microbelts/fibers are smooth, whose diameters decrease with increasing the annealing temperature. The average diameter of the Zn2SiO4:Mn2+ microfibers annealed at 1000 °C is 0.32 µm, and their lengths reach up to several millimeters. The average width and thickness of the Zn2SiO4:Mn2+ microbelts fired at 1000 °C are around 0.48 and 0.24 µm, respectively. Under ultraviolet excitation and low-voltage electron beams (1-3 kV) excitation, the Zn2SiO4:Mn2+ samples show the green emission at 524 nm, corresponding to 4T1-6A1 transition of Mn2+. The luminescence intensity and the lifetime have been studied as a function of the doping concentration of Mn2+ in the Zn2SiO4 samples. 1. Introduction Highly efficient low-voltage cathodoluminescent materials play an important role in flat-panel display devices such as field emission displays (FED), and thin film electroluminescent devices (TFEL).1 Oxide phosphors have gained more interest due to their better thermal and chemical stability and environmental friendliness compared with sulfides.2,3 Therefore, extensive research has been carried out on the improvement of oxide phosphors.4-7 It is well-known that zinc silicate, which possesses a rhombohedral structure and wide energy band gap (5.5 eV), is an ideal host material for transition metal and rare earth ions because of its chemical stability and transparency in the ultraviolet (UV)-visible range.8 Mn2+-doped zinc orthosilicate (R-Zn2SiO4, willemite), as one of the mature green phosphor materials, has been widely used in plasma display panels (PDP), CRTs, field emission displays (FED), and electroluminescence (EL) devices due to its highly saturated color, strong luminescence, and long lifespan.9,10 And so far, there are many methods reported for the synthesis of Zn2SiO4: Mn2+ phosphors, including sol-gel process,11 polymer precursor method,12 combustion method,13 spray pyrolysis method,14 and hydrothermal method.15 Nowadays, with the development of nanotechnology, more attention has been paid to the preparation of one-dimensional (1D) nanostructural materials such as nanofibers, nanowires, and nanorods, which play an important role in fundamental studies and technological applications.16-18 It is well-known that electrospinning is a more simple, convenient, cost-effective, and versatile technique for generating long fibers with diameters * Corresponding author. E-mail:
[email protected].
ranging from tens of nanometers up to micrometers, comparing to other methods of fabricating nanofibers, such as template synthesis and phase separation.19 The electrospinning method was first explored in the 1930s,20 and now it has been demonstrated that a variety of materials can be electrospun to form uniform fibers, such as organic, inorganic, and hybrid polymers (organic-inorganic composites).21-24 The fibers prepared by electrospinning have good orientation, large specific surface area, large aspect ratio, and dimensional stability, which can be applied in sensors, electronic and optical devices, biomedical fields, and catalyst supports.25 In general, the typical electrospinning procedure could be described as follows: (1) preparation of a suitable inorganic sol or a solution containing a polymer together with an alkoxide or salt; (2) electrospinning of the solution to prepare fibers of polymer/inorganic composite; (3) calcination of the composite fibers to obtain the desired micro/nanofibers. As far as we know, the synthesis of one-dimensional Zn2SiO4: Mn2+ phosphor materials via electrospinning process has not been reported. Accordingly in this article, we report the preparation of Mn2+-doped zinc orthosilicate microbelts and microfibers via a simple and cost-effective electrospinning method. Moreover, we also investigate the structure, morphology, and photoluminescence (PL) and cathodoluminescence (CL) properties of the resulting samples in detail. 2. Experimental Section Materials. ZnO (99.0%, analytical reagent, A.R.), MnCl2 · 4H2O (99.0%, A.R.), and hydrochloric acid (HCl, A.R.) were purchased from Beijing Beihua Chemicals Co., Ltd. Tetraethoxysilane [TEOS, Si(OC2H5)4, A.R.] was purchased from Beijing
10.1021/jp806392a CCC: $40.75 2008 American Chemical Society Published on Web 11/06/2008
Zn2SiO4:Mn2+ Microfibers and Microbelts
Figure 1. Schematic diagram of the electrospinning setup.
Yili Fine Chemical Co., Ltd. Poly(vinylpyrrolidone) (PVP, Mw ) 1 300 000) was purchased from Aldrich. All of the materials were used without further purification. Ethanol C2H5OH (A.R.) and deionized water were used as solvents. Preparation. The Zn2SiO4:Mn2+ microfibers was prepared by an electrospinning process. The doping concentrations of Mn2+ are 1-10 mol % of Zn2+ in Zn2SiO4. First, the stoichiometric amounts of ZnO and MnCl2 · 4H2O were dissolved in diluted hydrochloric acid (HCl) under stirring and heating. Then, a suitable amount of water-ethanol solution (v/v ) 1:1) and a stoichiometric amount of tetraethoxysilane (TEOS) were added to the above solution under magnetic stirring for 1 h, resulting in the formation of a transparent solution. Finally, 8 wt % PVP ethanol solution was dropped slowly into the above solution under stirring for 10 h to obtain the viscous solution for electrospinning. A typical electrospinning setup consists of a syringe through which the solution to be electrospun is forced, a high-voltage power supply, a flat tip needle, and a grounded collector, as shown in Figure 1. The above viscous solution was placed in a 10 mL hypodermic syringe. The anode of the highvoltage power supply was clamped to the syringe needle tip, and the cathode was connected to the grounded collector plate. The applied voltage was 20 kV, the distance between the needle tip and the collector was 16 cm, and the flow rate of the spinning solution was controlled at 1 mL/h by a syringe pump (TJ-3A/ W0109-1B, Boading Longer Precision Pump Co., Ltd., China). The electrospun products were then calcined at 800, 900, and 1000 °C for 3 h with a heating rate of 1.5 °C/min. For the preparation of Zn2SiO4:Mn2+ microbelts, the solution for electrospinning was prepared under the same experimental conditions as stated above except for the different ratio water-ethanol, which was adjusted to 2:3 by volume. The distance between the needle tip and the collector was changed to 17 cm, and the applied voltage was tuned to 15 kV. The flowing rate was also controlled by the syringe pump. The asprepared precursor samples were annealed at desired temperature (800-1000 °C) for 3 h with the heating rate of 1.5 °C/min. In this way, Zn2SiO4:Mn2+ microfibers and microbelts were fabricated. The detailed experimental parameters are summarized in Table 1. For comparison, Zn2SiO4:Mn2+ bulk powders were prepared by drying the above electrospun solutions at 100 °C followed by heating at 1000 °C for 3 h. Characterization. The X-ray powder diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu KR radiation (λ ) 0.154 05 nm). FTIR spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. The morphology and composition of the samples were inspected using a field
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18883
Figure 2. XRD patterns of the Zn2SiO4:Mn2+ fibers annealed at different temperatures: (a) 800, (b) 900, and (c) 1000 °C, and the standard card of Zn2SiO4 (JCPDS no. 37-1485) as a reference (* denoted as the ZnO phase).
emission scanning electron microscope (FESEM, XL30, Philips) equipped with an energy-dispersive X-ray spectroscope (EDS, JEOL JXA-840). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) micrographs were obtained from a FEI Tecnai G2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV. The photoluminescence (PL) measurements were performed on a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescent (CL) measurements were carried out in an ultrahigh-vacuum chamber (
