Tb-doped LaSrAl3O7 nanophosphors

Tb-doped LaSrAl3O7 nanophosphors

Journal of Alloys and Compounds 549 (2013) 135–140 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepa...

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Journal of Alloys and Compounds 549 (2013) 135–140

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Synthesis, characterization and luminescent properties of Eu/Tb-doped LaSrAl3O7 nanophosphors Sheetal, V.B. Taxak, Mandeep, S.P. Khatkar ⇑ Department of Chemistry, Maharshi Dayanand University, Rohtak 124001, India

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Article history: Received 9 August 2012 Accepted 8 September 2012 Available online 26 September 2012 Keywords: Combustion Nanophosphors Rare-earth Plasma display

a b s t r a c t The synthesis of LaSrAl3O7 doped with rare earth (RE) ions Eu3+ and Tb3+ by a single-step solution combustion method has been demonstrated in the present investigation. The structural, morphological and luminescent properties were characterized by XRD, FT-IR, SEM, TEM and PL spectroscopy. The XRD results suggest that LaSrAl3O7 crystallize in a single phase having tetragonal structure with space group P421m. The transmission electron microscope (TEM) and scanning electron microscope (SEM) analysis show that the sample is made up of tetragonal particles with the size between 38 and 78 nm. The photoluminescence properties of the samples were investigated by excitation and emission spectra. Under UV excitation (265 nm), LaSrAl3O7: Eu3+ shows dominant peak at 619 nm in addition to other characteristic peaks of Eu3+. Under 233 nm excitation, the characteristic emissions corresponding to 5D3–7FJ (J = 4– 6) and 5D4–7FJ (J = 3–6) transitions in LaSrAl3O7: Tb3+ nanophosphors are observed. These results indicate that Eu3+ and Tb3+ activated LaSrAl3O7 nanophosphors can be applied for tunable solid state laser materials and plasma display panels. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Rare-earth ions doped luminescent nanomaterials have attracted much attention of scientists recently, especially Eu3+ and Tb3+, because of 4f electrons intra-configurational transitions. As the electrons are well shielded from neighboring ions so discrete and sharp energy levels are obtained [1–4]. It is well known that nanoparticles exhibit unique chemical and physical properties as compared to their bulk counterparts [5]. Significant efforts have been devoted to synthesize and investigate an important family of inorganic materials having general chemical formula ABC3O7 (A = Ca, Sr, Ba; B = Y, La, Gd; C = Al, Ga) as these materials find various kinds of application in all-solid-state lasers, plasma display panels (PDP) for high definition TV (HDTV), mercury-free high intensity discharge lamps (HID), diode laser pumping and tunable laser generation [6–8]. The compounds have melitite structure and form tetragonal crystals with space group P421m [9–11]. The crystal structure is built up from CO4 tetrahedra to form a tetragonal sheet-like arrangement. The sheet structure consists of five membered rings of CO4 tetrahedra perpendicular to the c-axis and between the layers, A2+ and B3+ ions are distributed randomly in eight coordinated sites with Cs symmetry [12,13]. The conventional method to synthesize the nanophosphors is based on the solid state reaction method. Generally solid state

⇑ Corresponding author. Tel.: +91 9813805666. E-mail address: [email protected] (S.P. Khatkar). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.09.033

method requires high temperature and long calcination time for the preparation. Even at high temperature (1400 °C) pure phase is not obtained and the particles are agglomerated. A large number of melitite structure compounds have been synthesized by solid state [9–11]. In addition, some other methods such as czochralski [14,15] and sol–gel [16–18] have been also used to synthesize phosphors. A non-toxic, low-cost, one-step and fast route combustion method has been employed to synthesized GdCaAl3O7: Eu3+ [7], LaCaAl3O7 doped with variety of ns2 and rare earth activators [12], Er3+/Yb3+ co-doped CaYAl3O7 [19] and CaYAl3O7: Eu3+ [20] phosphors. We have recently employed this combustion method to synthesize melitite structure YCaAl3O7: Eu3+ nanoparticles and studied their photoluminescence characteristics [21]. In the present work, our aim is to prepare an another important member of melitite structure family LaSrAl3O7: Eu3+, Tb3+ by fast route solution combustion method and compare high photoluminescence intensity of nanosized particles doped with different concentrations and calcined at different temperatures, which find potential applications in modern lighting and displays. 2. Experimental 2.1. Powder synthesis The starting reagents were high purity La(NO3)36H2O, Al(NO3)39H2O, Sr(NO3)2, Eu(NO3)36H2O, Tb(NO3)36H2O and urea. La1xSrAl3O7: xRE3+ (RE = Eu, Tb) were synthesized by solution combustion method. The chemical equation for the reactions is:

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ð1  xÞLaðNO3 Þ3 þ xREðNO3 Þ3 þ SrðNO3 Þ2 þ 3AlðNO3 Þ3 þ 11:66CH4 N2 OðureaÞ ! La1x SrAl3 O7 : xRE3þ ðsÞ þ gaseous products: According to nominal composition of La1xSrAl3O7: xEu3+ (x = 0.005, 0.05, 0.10, 0.15 and 0.20), a stoichiometric amount of metal nitrates were dissolved in minimum quantity of deionized water in 200 mL capacity Pyrex beaker. Then urea was added in this solution with molar ratio of urea to nitrates based on total oxidizing and reducing valencies of oxidizer and fuel (urea) according to concept used in propellant chemistry [22]. Finally the beaker containing the solution was placed into a preheated furnace at 500 °C. The material undergoes rapid dehydration and foaming followed by decomposition, generating combustible gases. These volatile combustible gases ignite and burn with a flame yielding voluminous solid. Urea was oxidized by nitrate ions and served as a fuel for propellant reaction. Similarly, the La1xSrAl3O7: xTb3+ nanoparticles were synthesized following the same procedure. The powders obtained were again fired at 700–1000 °C for 3 h to increase the brightness. 2.2. Powder characterization methods The crystal phase of LaSrAl3O7: Eu3+ and LaSrAl3O7: Tb3+ powders were characterized by Panalytical X’Pert Pro X-ray powder diffraction with CuKa radiation to record the patterns in the 2h range of 10–80°. The morphology and particle size were evaluated using Jeol JSM-6510 scanning electron microscope (SEM) and FEIMorgagni-268D transmission electron microscope (TEM). The Fourier transform infrared (FT-IR) spectra were recorded using a Perkin–Elmer spectrometer in the spectral range of 4000–400 cm1 following KBr pellet technique. The excitation and emission spectra of the powders in the ultraviolet–visible region were evaluated at room temperature using a Hitachi F-7000 fluorescence spectrophotometer with Xe-lamp as the excitation source.

3. Results and discussion 3.1. X-ray diffraction studies The XRD patterns of La0.9Eu0.1SrAl3O7 and La0.9Tb0.1SrAl3O7 powders, as-prepared and calcined at different temperatures for 3 h, are shown in Figs. 1 and 2 respectively. In strontium–lanthanum aluminate LaSrAl3O7, the frames of the compounds are 5 formed by five membered rings constructed from AlO4 tetrahedral 2+ 3+ and between the layers Sr and La ions are distributed randomly in eight coordinated sites with Cs symmetry [16]. XRD patterns of the as-prepared samples show many additional peaks marked as ‘‘⁄’’ corresponding to those of unreacted Sr(NO3)2 phase (JCPDS card no. 04-0310) and La(NO3)3 phase (JCPDS card no. 24-1112) respectively, in addition to the formation of LaSrAl3O7 phase. At 700 °C, all the peaks characteristic due to LaSrAl3O7 phase appeared, while the peaks due to strontium nitrate and lanthanum nitrate disappeared. At this temperature the characteristic peak intensity at 2h = 30.558 was weak, but on further heating the sample at 900 °C the enhancement of peak intensity is noticed. At 900 °C, all diffraction peaks corresponding to tetragonal crystalline

Fig. 1. XRD patterns of La0.9Eu0.1SrAl3O7 powders calcined at various temperatures and the standard data of LaSrAl3O7 (JCPDS no. 50-1815).

Fig. 2. XRD patterns of La0.9Tb0.1SrAl3O7 powders calcined at various temperatures and the standard data of LaSrAl3O7 (JCPDS no. 50-1815).

structure LaSrAl3O7 (JCPDS card no. 50-1815) with space group P421m are observed. No peaks from other phases can be detected at this temperature, indicating complete phase formation of the nanophosphor. The XRD diffraction peaks of samples calcined at 1000 °C are observed to be of high intensity and narrow line width due to better crystallinity and grain growth. Identical XRD patterns are obtained for Tb3+ doped – LaSrAl3O7 as-prepared, calcined at 700, 900 and 1000 °C showing a crystalline structure composed of LaSrAl3O7 powders with tetragonal phase belonging to space group P421m. The effect of Eu3+ and Tb3+ ions doping on LaSrAl3O7 lattice seems to be negligible as XRD patterns remain the same (Figs. 1 and 2 respectively). The size of the crystallites can be estimated with the help of Scherrer equation, D = 0.89k/bcosh, where D is the average grain size, k is X-ray wavelength (0.15418 nm), and h and b are the diffraction angles and full-width at half-maximum (FWHM, in radian) of an observed peak, respectively [23]. The calculated average particle sizes (D) of LaSrAl3O7: Eu3+ particles are found to be 62, 69 and 77 nm at calcination temperatures 700, 900 and 1000 °C respectively whereas for LaSrAl3O7: Tb3+ particles, the sizes are found to be 37, 44 and 47 nm at calcination temperatures 700, 900 and 1000 °C respectively. Hence, with increase of temperature the crystal size becomes larger. 3.2. Morphological characteristics SEM study is carried out to investigate surface morphology and grain size of the synthesized particles. SEM images of La0.9Eu0.1SrAl3O7 and La0.9Tb0.1SrAl3O7 nanophosphors, as-synthesized, calcined at 700 and 1000 °C are displayed in Fig. 3(a)–(f) respectively. Fig. 3(a and d) depicts very similar unusual morphology of the as-synthesized products i.e. cracks, voids and porous network. This is due to release of a lot of gaseous by-products during combustion. The SEM micrographs shown in Fig. 3(b and e) reveals that the morphology of the samples calcined at 700 °C, have small and coagulated particles of nearly tetragonal shape, with small size distribution and regular surface. Fig. 3(c) displays the smooth surface of europium doped nanophosphors calcined at 1000 °C. It can be noticed from micrograph that particle size of the crystallite is uniform and all the particles are densely packed, thus preventing it from aging. The smooth morphology of the phosphor in nano regime reduces non-radiation and scattering thereby increasing its luminescence efficiency. Fig. 3(f) shows the SEM morphology of Tb doped LaSrAl3O7 calcined at 1000 °C temperature. Smooth surface with non-uniform agglomerated size particles in range of 47–50 nm can be observed. TEM images of both

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Fig. 3. SEM micrographs of La0.9Eu0.1SrAl3O7 (a) as-synthesized, calcined at (b) 700 °C (c) 1000 °C; and of La0.9Tb0.1SrAl3O7 (d) as-synthesized, calcined at (e) 700 °C (f) 1000 °C.

La0.9Eu0.1SrAl3O7 and La0.9Tb0.1SrAl3O7 calcined at 1000 °C (Fig. 4a and b), present nearly tetragonal and loosely agglomerated particles. The average particle sizes were in the range of 70–75 nm for LaSrAl3O7: Eu3+ and 45–47 nm for LaSrAl3O7: Tb3+, agreement with that estimated by Scherrer’s equation. 3.3. Infrared spectroscopy The Fourier transform infra-red (FT-IR) spectra of La0.9Eu0.1SrAl3O7 and La0.9Tb0.1SrAl3O7 powders, as-prepared and calcined at 1000 °C temperature are depicted in Fig. 5(a and b). The spectra reveal intense peaks in the range of 425–915 cm1 attributed to 5 several M–O stretching and bending vibrations of AlO4 group in the melitite structure [20]. This band increases in intensity with in-

crease in temperature due to improvement of M–O bonding leading to enhancement of crystallinity in LaSrAl3O7 host lattice. The peaks at 3435, 1635 are observed due to O–H stretching and H– O–H bending vibrations of water or moisture from air physically absorbed on the sample surface and the peak at 1384 cm1 related to the residual NO 3 in the sample. These bands decrease as the annealing temperature increases, indicating reduction of undesired impurities. The FT-IR results obtained are in accordance with XRD analysis. 3.4. Luminescent properties The photoluminescence excitation spectrum of La0.9Eu0.1SrAl3O7 calcined at 1000 °C, kem=618 nm, is shown in Fig. 6. The excita-

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Fig. 4. TEM images of (a) La0.9Eu0.1SrAl3O7 and (b) La0.9Tb0.1SrAl3O7 calcined at 1000 °C.

tion spectrum includes a dominant broad region (200–300 nm) with a maximum at about 265 nm. The broad peak is ascribed to charge transfer band (CTB) corresponding to an electron transfer from an oxygen 2p orbital to an empty 4f orbital of europium ion (O2 ? Eu3+). This charge transfer band at 265 nm finds applications in Hg discharge lamps [20]. The excitation peaks in the range above 350 nm are intra-configurational 4f–4f transitions of Eu3+ in the host lattice, peak with maximum at 395 nm (7F0 ? 5L6) being the dominating [24,25]. Fig. 7 depicts the typical red photoluminescence from Eu3+ ions in La0.9Eu0.1SrAl3O7 calcined at 1000 °C due to 5D0 ? 7FJ (J = 1–4) transitions when rooting the excitation wavelength at 265 nm. In particular, the most intense emission peak at 618 nm in LaSrAl3O7: Eu3+ corresponds to 5D0 ? 7F2 transition and occurs through the forced electric dipole, while 5D0 ? 7F1

band at 588 nm is the magnetic dipole transition. The emission spectrum exhibits the well known hypersensitive transition 5 D0 ? 7F2 due to europium ion, which is situated at low symmetry local site in LaSrAl3O7 crystal lattice [26,27]. The inset of Fig. 7 shows the relative PL intensity of La0.9Eu0.1SrAl3O7 as a function of temperature at kex = 265 nm. All the spectra show similar shape with a strong red emission at 618 nm corresponding to 5D0 ? 7F2 transition of Eu3+ ions. It is visible that the PL intensity of the dominant peak (5D0 ? 7F2) increased rapidly with the increase in temperature showing the maximum intensity at 1000 °C. The PL intensity dependence on temperature is mainly due to the high crystallinity and non-agglomeration of the particles at higher temperature. Fig. 8 shows the excitation spectrum of La0.9Tb0.1SrAl3O7 calcined at 1000 °C and the inset includes the expansion spectrum in the range 300–390 for the same sample, kem=544 nm. The excitation spectrum consist of a broad excitation band in the range from 200 to 260 nm with a maximum at about 233 nm and a series of sharp peaks between 300 and 400 nm with very low intensity in comparison to broad band. The high intensity band at 233 nm is assigned to spin-allowed 4f8–4f75d1 transitions of Tb3+ ions in the LaSrAl3O7 host lattice. The peaks from 300 to 390 nm are assigned to Tb3+ intra-4f (4f8–4f8) transitions from the ground state to higher energy levels (shown in inset of Fig. 8). The dominant excitation peaks at 317, 349, 369 and 379 nm can be attributed to 7F6–5D1, 7 F6–5D2, 7F6–5D3 and 7F6–5L10 transitions of Tb3+, respectively

Fig. 5. FT-IR spectra of (a) La0.9Eu0.1SrAl3O7 and (b) La0.9Tb0.1SrAl3O7 at different temperatures.

Fig. 6. Excitation kem = 618 nm.

spectrum

of

La0.9Eu0.1SrAl3O7

sample

calcined

1000 °C,

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Fig. 7. Emission spectra of La0.9Eu0.1SrAl3O7 calcined at 1000 °C and the inset shows the relative PL intensity at 618 nm of La0.9Eu0.1SrAl3O7 as a function of temperature, kex = 265 nm. Fig. 10. Variation of emission intensity with Eu3+ concentrations in La1xEuxSrAl3O7 and Tb3+ concentrations in La1xTbxSrAl3O7 samples calcined at 1000 °C.

5

D3–7F4, 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3 transitions of Tb3+ ions, respectively [26,30]. With increasing Tb3+ concentration, the emission intensity of the 5D4–7FJ (J = 3–6) transitions increases whereas that of the 5D3–7FJ (J = 4–6) transitions decreases due to cross relaxation process which depends on the interaction between adjacent Tb3+ ions [31]. In Tb3+ ions, the energy gap between the 5 D4 and 5D3 levels is close to that between the 7F0 and 7F6 levels. This cross-relaxation process between 5D3–5D4 and 7F0–7F6 results in rapid population of the 5D4 level at the expense of the 5D3 level and is described as: 3þ







Tb ð5 D3 Þ þ Tb ð7 F6 Þ ! Tb ð5 D4 Þ þ Tb ð5 F0 Þ Fig. 8. Excitation spectrum of La0.9Tb0.1SrAl3O7 calcined at 1000 °C and the inset shows the expansion spectrum in the range 300–390 nm for the same sample, kem = 544 nm.

Fig. 9. Emission spectra of La1xTbxSrAl3O7 (x = 0.005, 0.10) samples calcined at 1000 °C and the inset shows the relative PL intensity at the dominant peak 5D4–7F5 (544 nm) of La0.9Tb0.1SrAl3O7 as a function of temperature, kex = 233 nm.

[28,29]. The emission spectrum of La1xTbxSrAl3O7 (x = 0.005, 0.1) on 233 nm excitation wavelength shows several peaks from 380 to 625 nm as shown in Fig. 9. The peaks centered at 381, 415, 438, 490, 544, 589 and 622 nm attributed to 5D3–7F6, 5D3–7F5,

The relative PL intensity of the dominant 5D4–7F5 transition of La0.9Tb0.1SrAl3O7 as a function of temperature, at kex = 233 nm, is also shown in inset of Fig. 9. The inset shows clearly the increasing PL intensity of the dominant peak (5D4–7F5) with increase in temperature. The maximum intensity is observed at 1000 °C temperature. The luminescence intensities of nanoparticles is dependent on the dopant concentration, hence the emission intensity at 618 nm of Eu3+ and at 544 nm of Tb3+ are investigated as a function of dopant concentrations. The variation of emission intensity with Eu3+ concentrations in La1xEuxSrAl3O7 and Tb3+ concentrations in La1xTbxSrAl3O7, where x = 0.005 to 0.20, are depicted in Fig. 10. It is found that the PL emission intensity of LaSrAl3O7: Eu3+ increases with increase in Eu3+ concentration, reaching a maximum value when x = 0.10 and after this concentration the emission intensity decreases. Usually, an over-doping concentration results in the enhancement of non-radiative relaxation between the neighboring Eu3+ ions which indicates the concentration quenching [32]. Similarly in case of Tb3+, the emission intensity first increases with the increasing Tb3+ concentration reaching a maximum at x = 0.10, then it decreases with further increase in Tb3+ concentration because of mutual Tb3+–Tb3+ interactions. 4. Conclusions This paper presents lanthanum strontium aluminate doped with different concentrations of rare earth ions (Eu3+, Tb3+) synthesized by solution combustion method. The PL spectroscopic characterization shows the effect of rare earth ions on the luminescent properties of host lattice. The emission spectrum of LaSrAl3O7: Eu3+ nanophosphor shows a dominant peak at 618 nm

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(red) wavelength under 265 nm excitation, while the Tb3+ doped LaSrAl3O7 shows a dominant peak at 544 nm (green) wavelength on excition at 233 nm. Furthermore, the phosphors characterized by XRD, SEM and TEM reveal nanosized particles. These nanophosphors find potential applications in the field of lasers and plasma display panels (PDP). Acknowledgement One of the authors Sheetal gratefully acknowledges the financial support in the form of SRF (UGC) New Delhi. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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