Europium(III) Orthophosphates: Synthesis, Characterization, and Optical Properties Cordt Zollfrank,* Hanne Scheel, Sabine Brungs, and Peter Greil Department of Materials Science and Engineering, Glass and Ceramics, UniVersity of Erlangen-Nuernberg, Martensstrasse 5, D-91058 Erlangen, Germany
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 766–770
ReceiVed May 25, 2007
ABSTRACT: Hydrated europium(III) orthophosphates EuPO4 × nH2O (rhabdophane) of a nanocrystalline particle size ranging from 5 to 40 nm were precipitated from aqueous solution at neutral pH. The hexagonal crystal structure of the synthesized phase remained stable up to a calcination temperature of 600 °C according to X-ray diffraction (XRD) analysis. The complete loss of water at temperatures exceeding 600 °C caused the transformation into monoclinic nonhydrated EuPO4 isomorphous to monazite. The typical Eu3+ luminescence emissions excited at 396 nm for hexagonal EuPO4 × nH2O as well as for the monoclinic nonhydrated EuPO4 were attributed to magneticdipole and vibronic as well as forced electric-dipole 5D0 f 7FJ (J ) 1, 2, 3, 4) transitions. If the trivalent europium ion lies on an inversion center, the hypersensitivity is absent. The intensity ratio of the magnetic-dipole 5D0 f 7F1 transition to the electric-dipole 5D0 f 7F2 transition decreased with increasing calcination temperature up to 600 °C, indicating the presence of a hypersensitive, forced electric-dipole 5 D0 f 7F2 transition due to the lack of inversion symmetry sites. The loss of water during heating up to 600 °C was considered to be responsible for the variations in the emission characteristics of the EuPO4 × nH2O. The 4f electron configuration of the lanthanides gives rise to specific electronic, optical, and chemical characteristics of lanthanide compounds, which have been extensively used in magnets, luminescent devices, and time-resolved fluorescence labels for biological detection and medical diagnostics.1–5 Recent work focused on the coordination chemistry of lanthanides and the preparation and properties of new lanthanide-related materials.3 It was shown that the reduction of the grain size to the nanometer regime produced extraordinary electronic or luminescent behavior.6–13 Among the lanthanide (Ln) compounds, the orthophosphates with the general formula LnPO4 × nH2O (n ) 0–3) exhibit a combination of interesting properties such as low solubility products pKsol in water (pKsol ) 25–27)14 and high thermal stability with melting points of up to 2300 °C.15 Lanthanide orthophosphate powders were prepared either by precipitation from aqueous salt solutions or by crystallization from boiling phosphoric acid solutions.16 Crystalline solids of LnPO4 × H2O, where Ln covered the whole lanthanide period, were obtained from highly acidic solutions. Interestingly, the morphology of, for example, EuPO4 × H2O crystals changed from a sphere-like shape when crystallized in 2 M H3PO4 to elongated hexagonal rods in the presence of additional mineral acids.16–18 It could be shown that the crystal size increased with decreasing pH. Single crystalline LnPO4 × H2O nanowires were precipitated from phosphoric acid solution under hydrothermal conditions between 120 and 240 °C.4,19,20 Several different polymorphs of hydrated and nonhydrated orthophosphates LnPO4 × nH2O (n ) 0–3) were described such as hexagonal (rhabdophane), tetragonal (xenotime), orthorhombic, and monoclinic modifications (n ) 0, monazite and weinschenkite).4,21–25 The orthophosphates of some trivalent lanthanide ions (e.g., Ln3+ ) La3+, Ce3+, Eu3+, Nd3+, . . .) were found to exist in the high temperature monoclinic and the low-temperature hexagonal structure.22,25,26 Allotropic phase transition from the hexagonal to the monoclinic form was observed at approximately 700 °C. In the hexagonal structure of LnPO4 × nH2O the Ln3+ is coordinated to eight oxygen atoms in such a manner as to leave open, oxygenlined channels along the hexagonal axis with a diameter of approximately 0.6 nm.22 The presence of water in these channels was claimed to stabilize the structure.21 The photoluminescence of the Ln3+ ions arises from transitions within the 4f shell of the ions. The 4f electrons are shielded from * Corresponding author. E-mail:
[email protected]; phone: +49 9131 8527560.
the surroundings by the valence electrons, which causes sharp emission transition lines. The parity does not change in such transitions resulting in long life times of the excited state (∼10-3 s).1,8 The emission of Eu3+ usually consists of several lines in the red spectral region, which corresponds to transitions from the excited 5D0 to 7FJ (J ) 0–6) level, of the 4f6 electron configuration.1 The 5D0 f 7FJ transitions are extremely sensitive to the local symmetry of the Eu3+ ion coordination environment in the crystal lattice. The electric-dipole transitions are strictly forbidden because of the parity selection rules, if the Eu3+ occupies a lattice site with inversion symmetry. They can only occur as magnetic-dipole transition that obey the selection rule ∆J ) 0, ( 1 (but 5D0 f 7F0 is forbidden) or as a vibronic electric-dipole transition. The dominant 5D0 f 7F1 emission located at 590 nm originates from a magnetic-dipole transition.4 The other transitions appear weak and broad, since they are vibronic transitions lacking bands of electronic origin. If there is no inversion symmetry present in the crystal, electric-dipole transitions may occur. If the initial level has J ) 0, forced electric-dipole transitions are forbidden to levels with an uneven J and to the 7F0 level. In this case, the hypersensitive, forced electric-dipole 5D0 f 7F2 transition emission at 614 nm dominates the spectra. Even small deviations from the local symmetry, however, might exhibit large effects on the emission spectra with respect to the intensity ratio of emission transitions.1 In this paper, we describe the emission behavior of Eu3+ in hydrated and calcined europium(III) orthophosphate. EuPO4 × 2H2O precipitated from aqueous salt solutions. The emission spectra of the synthesized phosphors are discussed in terms of the hypersensitivity of the different local symmetries of Eu3+ in hydrated and dehydrated EuPO4 × nH2O. Experimental Procedures. All chemicals were used as received without further purification. A total of 2.93 g of Eu(III)Cl3 × 6H2O (8 mmol, Aldrich) was dissolved in 10 mL of water (Chromasolv, Riedel de Haën, Germany) in a glass beaker. Then, 2.11 g of (NH4)2HPO4 (16 mmol, Fluka, Germany) was added. A white precipitate EuPO4 × nH2O (2.19 g, 7.4 mmol, 92.5% yield) immediately appeared. The suspension was vigorously stirred for 4 h. The white precipitate was repeatedly centrifuged and resuspended in water to remove the byproduct NH4Cl. The white solid powder was dried at 80 °C and ground using an agate mortar and pestle. The obtained EuPO4 × nH2O powder was calcined from 200 to 1000 °C in an electrically heated furnace in air with a heating rate of 10 °C/min and dwell time of 4 h at the peak temperature.
10.1021/cg070483j CCC: $40.75 2008 American Chemical Society Published on Web 01/30/2008
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Figure 1. (a) X-ray diffraction patterns of EuPO4 × nH2O at room temperature, after calcination up to 600 °C and transformation into monoclinic EuPO4 at 800 and 1000 °C; (b) calculated crystallite sizes of EuPO4 × nH2O and EuPO4 as a function of calcination temperature.
Figure 2. HRTEM micrographs of the (a) as-synthesized EuPO4 × nH2O at 25 °C and after calcination at (b) 200 °C, (c) 600 °C, and (d) 800 °C, respectively.
The loss of water due to heating was determined by weighing 100 mg of powder for each temperature. The phase composition, the crystalline structure, and the crystallite size of the specimens were determined by X-ray diffractometry (XRD) using monochromatic Cu KR radiation at a scan rate of 0.75° min-1 over a 2θ range of 10–70° (D 500, Siemens, Germany). The X-ray crystallite sizes were evaluated by means of the Scherrer equation using the (200) reflection for the hexagonal EuPO4 × nH2O (according to the JCPDS card No: 20-1044) and the (200) peak for the monoclinic EuPO4 (JCPDS card No: 83-
0656). The powders were examined in a high-resolution transmission electron microscope (HR-TEM, CM 300 UT, Philips, The Netherlands) operated at 300 kV. The as-synthesized and the calcined powders were suspended in isopropanol and ground using an agate mortar and pestle. The resulting suspension was ultrasonicated at a frequency of 50 KHz for 1 min, and 5 µL of this suspension was transferred onto a carbon coated copper TEM-grid (Plano, Germany) and allowed to dry. Photoluminescence (PL) spectra were recorded at room temperature with a fluorescence spectrometer (J&M, TIDAS MMS/16, Germany) equipped with
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Figure 3. (a) Emission spectra of the EuPO4 powders at an excitation wavelength of 396 nm in the as-synthesised state and after calcination, (b) intensity ratio I(5D0f7F1)/I(5D0f7F2) at an excitation wavelength of 396 nm.
Figure 4. Crystal stucture representation of the [001] zone axis of the hexagonal hydrated EuPO4; the maximum of 4–5 water molecules is shown schematically in the ∼0.6 nm wide, oxygen-lined channels along the hexagonal axis.
fiber optics for excitation and detection of the emission. The powdered samples were placed on a flat substrate and illuminated at 396 nm excitation wavelength under an angle of 45° and a distance of approximately 1 cm from the powder surface. The emission was detected at an angle of 45° relative to the substrate surface between 320 and 750 nm with an integration time of about 500 ms at a distance of approximately 5 mm. The data sets were evaluated with the Fluoroscan software FL 3095 (J&M, Germany) and corrected for the intensity of the Si detecting array using correction files provided by the software. The emission spectra were normalized to the incident excitation peak (396 nm), which was set to 100%. Results and Discussion. Synthesis of lanthanide-based phosphates reported in the literature ususally starts from the corresponding lanthanide oxide (Ln2O3), which is dissolved in concentrated phosphoric acid.17,18 A restriction of this route is the limited quantitative dissolution of the Ln2O3 in the phosphoric acid, which is difficult by control of the temperature and pH, often resulting in nonstoichiometric products.16 Alternatively, we used the halogenide educt (EuCl3), and the precipitation reaction by adding (NH4)2HPO4 was carried out at neutral pH 7, eq 1.
2EuCl3 + 3(NH4)2HPO4 + 4H2O a 2EuPO4 × 2H2O V + 6NH4Cl + H3PO4 (1) The formation of phosphoric acid during the reaction could result in a slight decrease of the pH. As a consequence, dissolution of the precipitated EuPO4 × 2H2O might occur. This was prevented by adding an excess of (NH4)2HPO4 with respect to the Eu3+ concentration, where the additional HPO42- ions act as a buffer to the reaction solution. The as-synthesized precipitate was identified as hexagonal rhabdophane (EuPO4 × nH2O) by XRD analysis, Figure 1a.4,18,21,22,26,27 Figure 1b shows the variation of the crystallite size derived from line-broadening measurements of the (200) peak of the hexagonal and monoclinic modifications. The crystallite size slightly decreased from 10 nm for the as-received powder sample to 8 nm after calcination at 600 °C. The decrease of coherence length might be correlated to the destabilization of the hexagonal crystal structure, when water evaporates from the oxygen-lined channels along the c-axis with a diameter of approximately 0.6 nm.21,22 While the literature reports on the thermogravimetric analysis performed on similar materials showed a significant weight loss occurred at 150
Communications °C,4 only a weight loss of 6.2 wt% was observed in our case at 200 °C. However, remaining water was fully eliminated after calcination at 800 °C resulting in a total weight loss of 15.0 wt%, which roughly corresponds to 2 mol of H2O. Above 800 °C a phase transition into the monoclinic, monazite-like EuPO4 structure was observed (Figure 1a), as it was shown for similar lanthanide orthophosphates (LnPO4).28 After the transition into the monoclinic EuPO4 at 800 °C a pronounced grain growth started, and the crystallite size increased to 25 nm, Figure 1b. Figure 2 shows HRTEM micrographs of the nanoparticle microstructure suggesting the c-direction as the preferred growth direction of the hexagonal EuPO4 × nH2O phase. A similar crystal habitus was described for lanthanide orthophosphate nanowires such as TbPO4 × 0.5H2O20 and other hydrated lanthanide orthophosphates19 prepared from solution precipitation under hydrothermal conditions. A recent HRTEM analysis of nanocrystalline lanthanide orthophosphates, which were prepared via the phosphoric acid precipitation route in the presence of a chelating ligand, also showed the formation of elongated and round-shaped nanocrystalline particles.29 After the phase transition to the monoclinic EuPO4 above 600 °C the habitus of the crystallites changed to sphere-like particles with an average particle size of 20–40 nm. The HRTEM analysis corroborated the results evaluated from the XRD measurements. Figure 3a shows the emission spectra of the EuPO4 × nH2O as-synthesized and after various calcination temperatures. The assynthesized EuPO4 × nH2O exhibited the typical emission lines of Eu3+ at 595 nm (5D0 f 7F1), 614 nm (5D0 f 7F2), and 695 nm (5D0 f 7F4). The 5D0 f 7F3 transition located at 653 nm was not detected for the EuPO4 × nH2O as-synthesized; however, a weak, broad emission was observed up to a calcination temperature of 600 °C. Calcination of the as-synthesized EuPO4 × nH2O up to 600 °C resulted in a characteristic shift of the peak intensities. The strong increase of the peak intensities from 25 to 200 °C calcination temperature might be attributed to the removal of water that adhered to the surfaces of the nanosized particles. However, the intensity ratio I(5D0 f 7F1)/I(5D0 f 7F2) plotted as a function of temperature revealed a linear decrease with increasing calcination temperature from 1.2:1 (25 °C) to 0.8:1 (600 °C), Figure 3b. Yan et al. reported that the dominant 5D0 f 7F1 emission for Eu3+ ions in the hexagonal environment of LnPO4/Eu × H2O nanowires originated from a magnetic-dipole transition.4 In contrast, the 5D0 f 7F2 dominated the emission spectrum of hexagonal YBO3/Eu nanocrystals, which were calcined at 650 °C.30 The 5D0 f 7F1 transition was the most intense both for bulk and submicron-sized particles, whereas the 5D0 f 7F2 transition dominated the spectrum for nanosized particles only (30–50 nm). For nanocrystals of LaPO4/ Eu with a mean particle size of 5 nm the luminescence lines were found at identical positions to those of the bulk material.6,31 The I(5D0 f 7F1)/I(5D0 f 7F2) ratio was approximately 1:1, which is in good agreement with our results. We found that the observed slight decrease of the crystallite size up to 600 °C was accompanied by a linear increase of the intensity of the 5D0 f 7FJ (J ) 1, 2) transitions in all spectra. Simultaneously, the intensity of the 5D0 f 7F4 transition first decreased and subsequently showed a slight increase. Since electronic transitions are strictly forbidden for Eu3+ ions at a lattice site with an inversion symmetry, they can only occur as a magnetic-dipole or vibronic electric-dipole transition.1,32,33 In the lattice of hexagonal lanthanide phosphates (e.g., LnPO4 × nH2O19), the Ln3+ ions are coordinated by eight oxygen atoms.22,26 Hence, the observed change in the intensity ratio I(5D0 f 7F1)/ I(5D0 f 7F2) should be related to a change in the coordination sphere. Intensity ratios lower than 1 indicate the lack of an inversion symmetry of the local crystal symmetry. The ratio of the intensity of 5D0 f 7F1 to 5D0 f 7F2 transitions for LaPO4 nanowires doped with Eu3+ was smaller than that of bulk LaPO4/Eu.19 Considering that the 5D0 f 7F2 transition is hypersensitive to the symmetry of the crystal field, its intensity will be strong if the symmetry of the crystal field is low. We propose that the water molecules in hydrated hexagonal EuPO4 exert a strong influence on the emission proper-
Crystal Growth & Design, Vol. 8, No. 3, 2008 769 ties. Estimating the number of water molecules (volume ∼ 0.07 nm3), which can fill the hexagonal open pore channels (∼0.6 nm3) per unit cell, a maximum of eight water molecules might be introduced into the channel respecting packing arguments. Here, we found approximately two water molecules per formula unit, Figure 4. The eight-coordinate Eu3+ ions in the hydrated EuPO4 × nH2O structure might be stabilized by the presence of water molecules in the channels along the hexagonal axis.21,22 With increasing calcination temperature, water is released from the channels along the hexagonal axis, resulting in a destabilization of the hexagonal crystal structure.21,22 Because of the loss of the inversion symmetry, the electric-dipole transitions are no longer strictly forbidden, and an increase of the hypersensitive, forced electric-dipole 5D0 f 7F2 transition was observed after 400 °C. Above 600 °C the hexagonal phase transformed into the monoclinic phase, and the intensity of the 5D0 f 7F1 transition increased compared to the 5D0 f 7F2 and the 5D0 f 7F4 transitions, Figure 3a. Simultaneously, a weak and broad 5D0 f 7F3 transition was observed, and the evaluation of the I(5D0 f 7F1)/I(5D0 f 7F2) ratio rose steeply, Figure 3b. The investigation of the luminescence properties of Eu3+ in monoclinic monazite-type lanthanide orthophosphates (La0.98Eu0.02PO4 and EuPO4) which were fired up to 1300 °C exhibited a strong emission derived from 5D0 f 7F1 transition, while the other transitions were of a considerably lower intensity.28 In a different study, the emission spectra of Eu3+-doped dual lanthanide oxyhalides calcined at 700 °C showed the 5D0 f 7FJ (J ) 2, 3, 4) transitions in addition to a strong 5D0 f 7F1 emission.34 A similar emission behavior was observed for hexagonal YBO3/Eu nanocrystals.30 Interestingly, the literature data with respect to our results suggested that local lattice sites with an inversion symmetry for the Eu3+ are likely to be formed in a monoclinic, monazite-like crystal structure, giving a strong argument for attributing the dominating peaks to the magnetic-dipole 5 D0 f 7F1 and vibronic electric-dipole 5D0 f 7FJ transitions (J ) 2, 3, 4). Conclusions. Nanosized hydrated EuPO4 × nH2O powders were prepared by precipitation from aqueous halogenide salt solutions at neutral pH. A gradual loss of water was observed upon calcination up to 600 °C and at 800 °C a phase transformation from the hexagonal EuPO4 × nH2O to monoclinic EuPO4 was observed. The magnetic-dipole 5D0 f 7F1 and the vibronic electric-dipole transitions dominated the emission of Eu3+ in the water-stabilized hexagonal modification. In association with the loss of water upon calcination up to 600 °C, the intensity of the hypersensitive, forced electric-dipole transition I(5D0 f 7F2) increased, resulting in a I(5D0 f 7F1)/I(5D0 f 7F2) ratio smaller than one. After the transformation into the monoclinic EuPO4 structure, the magnetic-dipole 5D0 f 7F1 and vibronic electric-dipole 5D0 f 7FJ transitions (J ) 2, 3, 4) were the dominating peaks in the emission spectra. To our knowledge the observed correlation between water and the optical characteristics of EuPO4 × nH2O is reported for the first time. The results might be elaborated toward the design of novel, oxidation resistant gas sensor materials based on lanthanide orthophosphates in a wide temperature range.
Acknowledgment. The authors are indebted to Dr. Miroslaw Batentschuk, Institute of Electrical Engineering Materials, University of Erlangen-Nuernberg, and to Dr. Edda Stern and Alfons Stiegelschmitt, both at Institute of Glass and Ceramics, University of Erlangen-Nuernberg, for many helpful discussions. The use of the electron microscopes at the Central Facility for Electron Microscopy of the Friedrich-Alexander-University of ErlangenNuernberg is gratefully acknowledged.
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