J. Phys. Chem. C 2009, 113, 9609–9615
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Electrospinning Preparation and Photoluminescence Properties of Lanthanum Phosphate Nanowires and Nanotubes Lin Xu, Hongwei Song,* Biao Dong, Yu Wang, Xue Bai, Guolei Wang, and Qiong Liu State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin UniVersity, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China ReceiVed: January 31, 2009; ReVised Manuscript ReceiVed: April 29, 2009
In this paper, europium lanthanum phosphate nanowires and nanotubes were fabricated by the electrospinning method for the first time. By changing the ratio of PVP to the inorganic precursors, ultralong, uniform, and size-controllable nanowires/nanotubes were obtained, ranging of 60 to 300 nm. The average wall thicknesses of the nanotubes were ∼20 nm. The structure properties were characterized systematically by FE-SEM, TEM, XRD, FTIR, and ESR techniques. The results indicate that as the ratio of PVP to the inorganic decreased, a phase transition from LaPO4 to La3PO4 gradually occurred and the formation mechanism was proposed. The studies on photoluminescence indicated that the emission intensity, lifetime, and intensity ratio of 5D0-7F2 to 5 D0-7F1 all depended strongly on the structure of the electrospinnig products. This work is important for both the synthesis of novel one-dimensional nanophosphors and understanding of their unique physical properties. I. Introduction As is well-known, many important properties of nanomaterials depend not only on the particle size but also on the morphology and structure of the nanocrystals. Recently, one-dimensional (ID) nanostructures, for example, nanowires (NWs), nanorods, nanotubes (NTs), and nanobelts, have been a subject of intense research due to their unique and fascinating characteristics for huge ratio of diameter to length, superior mechanical toughness, and so on.1-6 Up to now, various techniques have been explored to prepare 1D nanomaterials, such as growth method, solvent evaporation, template technique, vapor deposition, and electrospinning.7-11 Electrostatic fiber formation (electrospinning), which represents a relatively simple, convenient, and versatile method for generating ultrolong and uniform nanofibers, has been rapidly developed since the invention of this technique by Formhals et al.12 The wide use of electrospinning can attributed to its own advantages. First, the diameter of the electrospun nanofibers could be conveniently controlled in the range of less than tens of nanometers to several micrometers. And more, the morphology of the electrospun fibers can be easily altered by modulating the processing variables, such as setting parameters and solution properties. Oxide materials doped with lanthanide ions represent a class of materials with significant technological importance. For most of trivalent lanthanide ions, the 4f electrons of lanthanide ions are shielded by 5s and 5p electrons, so the 4f-4f transitions are not so sensitive to the local environments. Among the dopants we know, the most extensively used probe is the Eu3+ ion, which shows a line spectrum that corresponds to transitions within the manifold of f-electron states of europium.13 In recent years, the synthesis and spectroscopic properties of Eu3+-doped phosphate NWs have attracted considerable interest due to their important role in lighting and display fields. Meyssamy et al.13 first synthesized LaPO4:RE (RE ) Eu3+, Ce3+, Te3+) NWs by the hydrothermal method. Cao et al.14 fabricated the LaPO4: Eu3+ NWs through the hydrothermal microemulsion method and * To whom correspondence should be addressed. E-mail: hwsong2005@ yahoo.com.cn. Phone/fax: 86-431-85155129.
studied their formation mechanism. In one of our previous papers,15 we observed that in the hydrothermal products of LaPO4:Eu3+ NWs the inner luminescent quantum efficiency of 5 D1-Σ7FJ transitions was considerably more enhanced than that of the corresponding nanoparticles (NPs) and the bulk powders. The photoluminescent properties of nanophosphors should depend strongly on the preparation techniques. Despite LaPO4: Eu3+ nanophosphors having been prepared by the other techniques, the electrospinning preparation has not been reported, to the best of our knowledge.13-16 In this paper, we present the synthesis of lanthanide phosphate NWs and NTs by the electrospinning method using PVP as the template. By changing the ratio of PVP to the inorganic precursor, size-controllable and phase-adjustable LaPO4:Eu3+ and La3PO7:Eu3+ NWs/NTs have been obtained. Their morphology, structure, formation mechanism, and luminescent properties have been systemically studied. II. Experimental Procedures A. Sample Preparation. The following three steps were executed in synthesizing the lanthanide phosphate NWs/NTs: (a) preparation of the precursor solutions through the sol-gel technique; (b) electrospinning the obtained solution to gain the nanofibers; (c) and the following thermal treatment of the polymer to remove the unwanted parts. In the preparation of precursor solutions, La(NO3) · 6H2O, Eu(CH3COO)3 · 5H2O, and C12H27O4P in the molar ratio of 1:0.05:3 were dissolved in a mixed solvent composed of N,N-dimethylformamide (DMF). Then, an appropriate amount of PVP was added into the precursor solutions at different ratios, followed by vigorous stirring for 3 h to obtain the electrospinning solutions. The final solutions then were electrospun to give composite fibers. The schematic diagram of the electrospinning setup is shown in Figure 1. It consists of three major components: a high-voltage power supply, a spinneret (a plastic needle), and a collector plate (a grounded conductor). In a typical electrospinning process, the precursor solution was ejected from the tip of the spinneret under the effect of high voltage that was applied between the spinneret and the collector to form an electrically charged jet
10.1021/jp900916j CCC: $40.75 2009 American Chemical Society Published on Web 05/11/2009
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Figure 1. Schematic diagram of the electrospinning setup.
of solution. The solution jet solidified with accompanying evaporation of solvent and formed a nonwoven fibrous mat on the collector. The precursor fibers were obtained by electrospinning with a collection distance of 15 cm between the spinneret tip and the collector and an applied steady voltage of 15 kV under the protection of a N2 stream. After being dried for 12 h at room temperature under vacuum, the precursor fibers were annealed in a tube furnace with a rising rate of 1 deg/min from room temperature and a keeping time of 3 h at a certain temperature, then they were self-cooled to room temperature again, forming the final products. The slow heating rate was selected to ensure the removal of the organic phase without destroying the nanofibrillar structure and to avoid the disintegration of the ceramic nanofibers.17 B. Measurements and Characterization. The surface morphology of the nanofibers was inspected with a JEOL JSM7500F field emission scanning electron microscope at an accelerating voltage of 15 kV. The transmission electron micrograph (TEM) images and selected area electron diffraction (SAED) were recorded on a JEM-3010 transmission electron microscope under a working voltage of 200 kV. Crystal structures of the fibers were conducted on a Rigaku D/max 2550 X-ray diffractometer, using a monochromatized Cu target radiation resource (λ ) 1.5045 Å). The Fourier-transform infrared absorption (FTIR) spectra were performed on a FTS3000 Bio-Rad spectrophotometer. The electron spin resonance (ESR) spectra were measured by a Bulker ER 200D ESR spectrometer under the protection of pure argon. The general excitation and emission spectra were recorded on a Hitachi F-4500 spectrophotometer equipped with a 980nm laser diode and for comparison of different samples. The emission spectra were measured at a fixed band-pass of 0.2 nm with the same instrument parameters of 2.5 nm for excitation silt, 2.5 nm for emission slit, and 700 V for PMT voltage. In the measurements of time-resolved emission spectra and fluorescence dynamics, the 266-nm light generated from a fourthharmonic generator pumped by a pulsed Nd:YAG laser was used as the excitation source. The Nd:YAG laser had a line width of 0.3 cm-1, pulse duration of 10 ms, and repetition frequency of 10 Hz. A Spex 1403 spectrometer, a photomultiplier, and a boxcar integrator were used for the detection of the spectra. Fluorescence dynamics were recorded by a TEKTRONIX TDS-3052 oscilloscope.
Figure 2. FE-SEM images of the precursor fibers with different weight ratios of PVP to inorganic materials (PVP/In): (a) PVP/In ) 1.5, (b) PVP/In ) 3, (c) PVP/In ) 4.5, (d) PVP/In ) 6, and (e) PVP/In ) 12, precursors dissolved in DMF solution; and (f) PVP/In ) 3, dissolved in ethanol and acetic acid mixed solution. The applied voltage was fixed at 15 kV and the distance between the spinneret tip and the collector was 15 cm.
III. Results and Discussion A. Structure and Morphology. Panels a-g of Figure 2 show the field emission scanning electron micrographs (FESEM) of the precursor fibers at the same electrospinning conditions as given above. In Figure 2, panels a-e, the weight ratios of the PVP to the inorganic (PVP/In) were gradually changed, to 1.5, 3.0, 4.5, 6.0, and 12.0, respectively. All the FE-SEM images indicate that ultralong fibers are obtained and they are oriented randomly. As PVP/In ) 1.5, the fibers seem to be dispersed inhomogeneous and a little adhesive. As the content of inorganic materials decreases, the nanofibers become uniform and do not stick to each other anymore. It also can be seen that the diameters of the precursor fibers decrease gradually. The average diameters for the samples of PVP/In ) 1.5, 3.0, 4.5, 6.0, and 12.0 are ∼1000, 350, 300, 240, and 200 nm, respectively. Actually, there are many factors affecting the size of the precursor fibers. According to ours previous results,18 varying the applied voltage and the collection distance had only a little influence on the diameter of fibers, but visible alteration could be seen when the proportion of the inorganic and polymer was changed. In the present experiment, we also compared the precursor fibers which were fabricated from two different precursor solutions by dissolving the same weight of the inorganic and PVP into a certain volume of DMF or the mixed liquid of ethanol and acetic acid. The corresponding FE-SEM images are shown in Figure 2, panels b and f, respectively. The fibers made from the DMF solution have an average diameter of ∼350 nm and those synthesized from the mixture of ethanol and acetic acid have a mean diameter of ∼700 nm. This fact is in accordance with the model proposed by Fridrikh et al.,19 which has predicted the terminal diameter for polymer fibers
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Figure 4. TEM image and SAED pattern (inset) of lp-4.
Figure 3. FE-SEM images of the nanofibers after annealing. (a) PVP/ In ) 1.5, (b) PVP/In ) 3, (c) PVP/In ) 4.5, (d) PVP/In ) 6, and (e) PVP/In ) 12, all calcined up to 750 °C; and (f, g) PVP/In ) 6, the calcined temperature is up to 650 and 850 °C, respectively.
of the whipping jet prepared by electrospinning. In the model the terminal jet radius ht can be depicted as,
(
ht ) γε
2 Q2 2 π(2 ln χ - 3) I
)
1/3
where γ is the surface tension, ε is the dielectric permittivity of air, Q is the liquid jet flow rate, I is the total electric current, and χ is the dimensionless wavelength of the instability responsible for the normal displacements, which is approximately equal to the ratio of the radius of curvature to the jet diameter. Thus, the terminal diameter is dominated by the flow rate, electric current, and the surface tension of the fluid. In the present study, the flow rate and electric current are kept constant, so the prominent change of the diameter in different solutions can be attributed to the influence of surface tension. The morphologies of the ceramic fibers after calcinations are more closely related to the value of PVP/In. As shown in Figure 3a-e, after being annealed, the diameters of the formed ceramic fibers decrease greatly due to the decomposition of organic component from the precursor fibers. The configuration of the fibers seems uniform and ultralong, but rougher than those before annealing. The nanofibers shown in Figure 3a-e, corresponding to the samples in Figure 2a-e, will be named as lp-1, lp-2, lp-3, lp-4, and lp-5 for convenience in the following text. The average diameters of the nanofibers corresponding to samples lp-1 to lp-5 were determined to be ∼300, 120, 90, 70, and 40 nm, respectively. When the inorganic contents were suitable, the NWs (lp-1 to lp-3) even NTs (lp-4) could be obtained, but when the inorganic contents were too small (lp5), the fibers were broken during the annealing and aligned nanoparticles appeared. Actually, the morphology of ceramic fibers was also influenced by the annealing temperature. Panels
Figure 5. XRD patterns of different electrospinning products. The asterisk presents the patterns of the LaPO4 phase and the open circle presents the patterns of the La3PO7 phase.
e, c, and f of Figure 3 depict the SEM images of annealing the same precursor nanofibers at 650, 750, and 850 °C, respectively. As exhibited, the NWs with average diameter of ∼70 nm were obtained at 650 °C, the NTs with diameter of ∼70 nm were obtained at 750 °C, and the necklace-like NWs with diameter of ∼60 nm were obtained at 850 °C. This result indicates that the NTs were only formed under an extremely rigorous condition. To our knowledge, the lanthanide phosphates NTs have not been fabricated by the other techniques yet. The TEM image and SEAD of sample lp-4 are given in Figure 4 to further testify the morphology and the structure of the NTs. It can be seen that NTs consisting of uniform nanoparticles appeared, with an average diameter of ∼70 nm and wall thickness of ∼20 nm. The grain sizes of the nanoparticles were less than 20 nm. The SEAD indicates the polycrystalline nature of the NWs/NTs. The XRD patterns of the ceramic nanocrystals lp-1 to lp-5 are depicted in Figure 5. It can be seen that all the samples are well crystallized, and all the strong peaks in samples lp-1 and lp-2 could be readily indexed to the monoclinic monazite LaPO4 phase (JCPDS no. 43-0493); however, a very small part of the extra phase was formed in sample lp-1 (at ∼25.9°), which was attributed to the presence of a small quantity of La4(P2O7)3. This was further proved by the FTIR spectra. The crystal structures of samples lp-4 and lp-5 all crystallized in the pure monoclinic La3PO7 phase (JCPDS no. 49-1023) and no additional diffraction peaks were formed. The crystal structure of sample lp-3 originated from the mixture of monoclinic LaPO4 and La3PO7 phase and the ratio of the highest peak of LaPO4 to La3PO7 was calculated to be 1.35. For a clearer expression, we summarized the details of the structures of various samples and listed them in Table 1. The change of the crystal phase in samples lp-1 to lp-5 indicates that the content of PVP can modify the crystal structure in the electrospinning preparation of lanthanide phosphate NWs/NTs. The FTIR spectra of samples lp-1 to lp-5 were studied in the range of 4000 to 500 cm-1. As shown in Figure 6, the broad
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TABLE 1: A List of the Phase Compositions and the Content of Eu, La, and P in Different Samples element (mean concn) sample name lp-1 lp-2 lp-3 lp-4 lp-5
host of Eu
3+
LaPO4/La3(P2O7)4 LaPO4 LaPO4/La3PO7 La3PO7 La3PO7
temp (°C)
crystal phase
JCPDS no.
750 750 750 750 750
monolinic monolinic monolinic monolinic monolinic
32-0493 32-0493
band in the region of 3700-3400 cm-1 is due to the OHstretching vibration and the corresponding bending vibration locates at 1650 cm-1. By comparing the strengths of the OHbond in different samples it can be concluded that the La3PO7 sample adsorbed much water in contrast to the LaPO4 one. In all the samples a weak peak appears at 2900 cm-1, corresponding to the C-H stretching bond. This probably originates from the contamination of organic materials in the KBr disks. The prominent peaks around 1040 cm-1 are due to the phosphate P-O stretching bonds, which are typical features for the lanthanum phosphate in the monoclinic phase. In the monoclinic form the tetrahedral phosphate groups are distorted in the 9-fold coordination of La atoms.20,21 The peaks at 952.6 cm-1 for samples lp-1 to lp-4 and peak at 773.8 cm-1 for samples lp-1 and lp-2 are attributed to symmetric (νs) vs. asymmetric (νas) PsOsP vibrations, respectively.22 The peaks around 620 and 550 cm-1 correspond to the OdPsO bending and OsPsO bending modes.20 Moreover, the appearance of the vibration bond at 1265-1267 cm-1 in sample lp-1 further shows the presence of a small amount of pyrophosphate.23 The ESR experiments were performed on samples lp-2 to lp-4 to identify their unpaired electrons, as shown in Figure 7. It can be clearly seen that all the examined samples have unsymmetrical and broad differential ESR signals, appearing in the range of 200-500 G. Their peaks appear around 300 G, while valleys appear around 350 G. In sample lp-2, the strength of the ESR signal is much lower than those in samples lp-3 to lp-4. According to the magnetization and magnetic susceptibility analysis in some works,24-26 we suggest that unpaired electrons of Eu2+ account for the presence of the ESR signals described
Figure 6. FTIR spectra of the different electrospinning products.
Figure 7. ESR spectra of samples lp-2 to lp-4.
49-1023 49-1023
Eu
La
P
0.171 0.151 0.127
5.05 4.15 3.41
7.68 11.3 10.2
ratio of P to La 1.52 2.72 2.99
above. The lower strength of the ESR signal in sample lp-2 can be attributed to the lower concentration of Eu2+, which is in accordance with the following discussion. Besides, an additional ESR signal appears on the valley of the broad one, for all three samples; however, their contributions in different samples are quite different. Here, we cannot distinguish its origin strictly. Usually, the sharper the ESR signal is, the weaker the exchange interaction of the local moment and the conduction electron is.27 B. Formation Mechanism of the Lanthanide Phosphate NWs/NTs. A fundamental understanding of the formation mechanism of the obtained various lanthanide phosphate products is crucial. Note that in our previous works, when the ratios of polymer (PVP or PVA) to inorganic were changed in the electrospinning preparation of YBO3 or YVO4, no phase change occurred. To understand the formation processes of the samples, a basic question should be answered: When did the structure change occur, in the electrospinning preparation of the precursor fibers or in the process of annealing? To resolve this doubt, the inductively coupled plasma (ICP) analysis was performed on samples lp-2 to lp-4 which contained the hosts of LaPO4, LaPO4/La3PO7, and La3PO7, respectively, and the obtained data are listed in Table 1. When electrospinning solutions were prepared, the same ratio of inorganic starting materials was added to form the final inorganic solutions; however, the contents of the materials in the precursor products differed. The content of the Eu element was almost unchanged and the contents of La and P altered obviously. The ratios of P to La were calculated to be 1.52, 2.72, and 2.99, respectively, corresponding to samples lp-2 to lp-4. Obviously, in the precursor fibers the P element was superfluous for the forming of LaPO4 in all the samples. Therefore, we suggest that the phase transition from LaPO4 to La3PO7 related not only to the electrospinning process, but also to the annealing process. As we all know, PVP is one kind of water-soluble polymer, in which the monomer is N-vinylpyrrolidone. Since a large amount of PVP was added into the samples, during the annealing the carbon- and hydrogen-containing products should be produced with the decomposition of PVP. Especially, the possible existence of C or CO could create a reduction environment and promote the formation of products with less oxygen (La3PO7). And the surface of the NWS and the inside of the NWS were in different environments during annealing, which might induce the formation of different phases. As the PVP content was high enough, the reduction atmosphere became dominant, and the pure La3PO7 phase was formed. Actually, the reduction atmosphere can be proved by the appearance of Eu2+ in the annealed electrospinning samples, which has been identified by ESR spectra. Now let us further discuss the formation mechanism of the NWs/NTs morphologies. As the temperature was elevated, some lanthanide phosphate primary nanoparticles were first formed in the precursor NWs with the removal of PVP, from outside to inner. The nanoparticles are composed of a very loose nanowire, which contained a large number of holes. At a
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Figure 8. Room temperature excitation (λem ) 611 nm) and emission (λex ) 253 nm) spectra for Eu3+ ions in samples lp-1 to lp-5.
relatively low annealing temperature (650 °C), with the removal of PVP the nanoparticles outside gradually shrunk, and the little nanoparticles formed a whole nanowire. As the temperature increased (750 °C), the reaction rate increased, and the nanoparticles formed quickly, both outside and in inner sites. In this case, the outside particles shrunk, while the inner particles expanded due to large strain strength. As the thermaldynamic balance was reached, NTs were formed. As the temperature increased further (850 °C), very large nanoparticles were formed; the assembly of large nanoparticles could only form a necklacelike structure due to limited space. Note that in the other electrospinning products, similar temperature-dependent formation behavior of NWs/NTs has also been observed, implying that such a formation mechanism is universal.29-31 C. Excitation and Emission Spectra. Figure 8 (left part) shows the excitation spectra at 611 nm in different samples. In the excitation spectra, the wide band extending from 200 to 300 nm is associated with the charge transfer (CT) transition of Eu3+, while the weak lines are assigned to the f-f shell transitions of Eu3+. The peak locations of the CT band in samples lp-1 to lp-5 are around 251, 254, 264, 267, and 268 nm, respectively. This implies that as the host material varied from the phase of LaPO4 to that of La3PO7, the CT band shifted to red. It is well-known that the CT band is closely related to the covalency between O2- and Eu3+ and the coordination environment around Eu3+. Generally, the Eu3+, the adjacent O2-, and the subadjacent La3+ can form Eu3+-O2--La3+ pairs and the electrons transfer from the 2p orbital of O2- ions to the 4f 6 orbital of Eu3+ to form the CT band.15 So, how much energy is needed for the migration of the p electrons depends on the potential field acting on the O2- ions, which are derived from the surrounding ions. The red-shift of the CT band indicates that the potential field coming from the La3+ ions has less effect on the O2-, in other words, less energy is needed to cause the electrons to migrate. At this time, the bond distance between Eu3+ and O2- becomes shorter and the mixture of wave function of Eu3+ and O2- is enhanced.32-34 Figure 8 (right part) depicts the emission spectra under the 253-nm excitation. The red 5D0-7FJ (J ) 0-4) transitions are observed, as labeled in the figure. In samples lp-1 and lp-2, all the peaks of 5D0-7FJ (J ) 0-4) transitions can be clearly observed and the 5D0-7F1 transitions are dominant among these lines. In samples lp-3 to lp-5, the peaks of 5D0-7FJ (J ) 0-2) can be seen obviously, and the peaks of 5D0-7F3 and 5D0-7F4 are too weak to be detected; the 5D0-7F2 transition is the strongest in comparison to any others. The spectra also show that the luminescent intensities for Eu3+ in the LaPO4 samples are stronger than those in the pure La3PO4 and the LaPO4 and La3PO7 coexisting samples, which could be attributed to less water adsorption on the surface, as displayed in the FTIR spectra of the samples. The intensity ratios of I(5D0-7F2)/I(5D0-7F1) were
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Figure 9. Room temperature emission spectra for Eu2+ (λex ) 335 nm) ions in samples lp-2 to lp-4.
deduced to be 0.53, 0.55, 1.52, 2.19, and 3.02, corresponding to samples lp-1 to lp-5, respectively. As we all know, the 5D07 F1 is the magnetic-dipole transition, while the 5D0-7F2 is the electric-dipole one. In terms of the Judd-Felt theory, the magnetic-dipole transition is permitted. The electric-dipole transition is allowed only on the condition that the europium ion occupies a site without an inversion center and is sensitive to local symmetry. Subsequently, when Eu3+ ions occupy inversion center sites, the 5D0-7F1 transitions should be relatively strong, while the 5D0-7F2 transitions should be relatively weak. The results above indicate that Eu3+ ions mainly occupy the site with an inversion center in the phase of LaPO4, while they occupy the site without an inversion center in the pure La3PO4 and LaPO4 and La3PO7 coexisting samples. It is worth mentioning that the emission in lp-3 is similar to that in the La3PO7 phase. Figure 9 shows the emission spectra under the excitation of λex ) 335 nm in lp-2 to lp-4. In the spectra, a wide band ranging from 380 to 570 nm was observed, which was ascribed to the emission of Eu2+.35-37 The broad bands can be decomposed into two Gaussian components, A and B. The location of peak A is around 440 nm, while that of peak B is around 513 nm. The two peaks have a little shift (
