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Single phase GdPO4: Dy3+ microspheres blue, yellow and white light emitting phosphor
Accepted Manuscript Single phase GdPO4: Dy phosphor
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microspheres blue, yellow and white light emitting
M. Ferhi, S. Toumi, K. Horchani-Naifer, M. Ferid PII:
S0925-8388(17)31392-0
DOI:
10.1016/j.jallcom.2017.04.193
Reference:
JALCOM 41597
To appear in:
Journal of Alloys and Compounds
Received Date: 11 March 2017 Revised Date:
14 April 2017
Accepted Date: 18 April 2017
3+ Please cite this article as: M. Ferhi, S. Toumi, K. Horchani-Naifer, M. Ferid, Single phase GdPO4: Dy microspheres blue, yellow and white light emitting phosphor, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.193. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The CIE chromatic diagram showing the chromatic coordinates of Gd1-xDyxPO4 emissions
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as function of Dy3+ concentrations and excitation wavelengths.
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Single phase GdPO4: Dy3+ microspheres blue, yellow and white light emitting phosphor
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By
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M. Ferhi*, S. Toumi, K. Horchani-Naifer, M. Ferid
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Physical Chemistry Laboratory of Mineral Materials and their Applications, National Center of Researches in Materials Sciences,
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Technopole de Borj Cedria, BP 73, 8027 Soliman, Tunisia
Corresponding author: Mounir Ferhi
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E-mail: [email protected] Tel.: +216 79 325 280
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Fax: +216 79 325 314
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ACCEPTED MANUSCRIPT Abstract Luminescent microspheres, Gd(1-x)DyxPO4 have been synthesized using simple combustion method for the first time. SEM images show that the obtained powders are formed by dispersed microscaled spheres with diameter ranging from 0.5 to 1 µm. The X-ray diffraction, FTIR and Raman spectroscopy studies reveal that the microparticles annealed at
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800 °C crystallized in a monoclinic phase with space group P21/n. Under host Gd3+ sensitization, Gd(1-x)DyxPO4 emission spectra of Dy3+ show a broad blue band attributed to transitions 4I15/2 + 4I13/2 + 4I11/2 + 4F9/2 → 6H15/2, a sharp yellow band due to 4F9/2→6H13/2 transition, and the little red band of 4F9/2 → 6H11/2 transition. Blue and white emissions have
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been obtained at low and high Dy3+ concentration, respectively due the energy transfer from Gd3+ to Dy3+. Under direct 4f–4f excitation of Dy3+ in GdDPO4, three bands attributed to 4
F9/2→ 6H15/2 , 6H13/2 , 6H11/2 transitions have been observed resulting in a yellow light
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emission. The emission bands intensity are discussed as function of GdPO4 structure. They are in well agreement with crystal structure of the host lattice GdPO4 in contrast to reported works. The decay rates for the 4F9/2 level are found to be deviated from exponential to nonexponential nature with increase of Dy3+ concentration. A promising white-light emission for white LED operating under UV excitation is investigated from single-phased phosphor. The
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obtained results suggest a potential application of GdPO4:Dy3+ microspheres as alternatives to replace bulk materials for many application fields, including light display systems and
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optoelectronic devices.
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Keywords: Luminescence, Combustion synthesis, white LED, Gd(1-x)DyxPO4, microsphere.
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ACCEPTED MANUSCRIPT 1. Introduction Recently, white light-emitting diodes (w-LEDs) have been widely used in various illumination fields. The combination of the UV/near-UV chips with a single-phased phosphor is considered to be an efficient way to obtain white light for solid-state lighting [1, 2]. It is known that the generation of white light from single-phased phosphors is based on codoping
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with two activators, which can emit two distinctive emission bands [3-5]. More importantly, compared to the multiple emitting components of the white LED system, the single-phased white-light-emitting phosphor for a UV-pumped white LED would enable easy fabrication with perfect stability and color [6]. It is also known that the control of the particles size in a
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crystalline system can effect in remarkable modification of physical, chemical, and mechanical properties compared to those of bulk due to the effect of surface-to-volume ratio of particles. The modification of luminescence properties can be brought by reducing the
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particles size and imperfections using different preparation techniques, different capping agents and use of suitable sensitizer, etc. Different types and sizes of luminescent particles are now being synthesized via several physical and chemical methods such as the homogeneous precipitation method, hydrothermal method, solid-state reaction method, laser synthesis, combustion synthesis, sol–gel method and microwave assisted chemical synthesis [7–13]. The
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preparation of particles with specific phases shapes and sizes depend a suitable reaction medium/solvent is important [14]. According to M. Yang et al, the unique properties of the monoclinic LaPO4 microspheres suggest their potential applications as a substitute of bulk countparts for many applications, such as optical displays and other optoelectronic devices.
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Furthermore, microspheres with a more uniform morphology could make better use of the exciting light source and reach a higher efficiency of the phosphor [15]. Among luminescent
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materials, lanthanide monophosphates (LnPO4) are the most useful inorganic host lattice for doping lanthanide ions emitting a wide range of colors [16, 17] due to their interesting properties such as low water solubility [18], high thermal stability [19] and high index of refraction [20]. They have been applied in various fields, such as plasma display panels (PDP), mercury-free lamp and visible lasers [21–24]. For mercury-free lamp, the excitation energy was mainly composed of VUV radiation but VUV energy is mostly absorbed by the host crystal, if the energy can be transferred from host to rare-earth (RE) ions then the rareearth ions can emit visible light. So the host absorption intensity is very important for VUVexcited phosphors applied in mercury-free lamp [25]. According Han et al, monophosphates are promising host materials for application in VUV region [25]. In LnPO4, the high 3
ACCEPTED MANUSCRIPT absorption was observed in VUV region of PO43-. Hence, GdPO4 is an efficient host under VUV excitation [26]. Rare-earth ions have been considered as efficient activators in modern lighting and display fields due to their characteristic emission either from 4f-4f or 5d-4f transitions. Among the RE ions, Dy3+ doped phosphates have drawn much interest for their two-color and
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white light emission [25, 27, 28]. So, Dy3+ ion possesses two intense emissions at blue (486 nm, 4F9/2→6H15/2) and yellow (576 nm, 4F9/2→6H13/2) regions. Further, it is well known that the 4F9/2→6H13/2 transition of Dy3+ ions is hypersensitive and therefore its intensity strongly depends on the nature of the host, whereas the intensity of magnetic-dipole allowed F9/2→6H15/2 transition is less sensitive to the host. Hence, at a suitable environment and by
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adjusting the yellow-to-blue intensity ratio (Y/B), only Dy3+-activated materials would
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generate near-white emission.
In this work we report the combustion synthesis and optical properties of GdPO4:Dy3+ microspheres. The effects of Dy3+ concentration and excitation wavelength on the Y/B ratios and color coordinates have been discussed. The influence of dopant concentration on the lifetimes is presented as well. The utility of the present phosphor for blue, yellow and white
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light emission has also been discussed. 2. Experimental details
2.1. Sample preparation
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Gd(1-x)DyxPO4 (x=0, 0.5, 1, 5, 10%) microspheres were prepared by following three steps. Firstly, the appropriate amounts of lanthanide oxides were dissolved in aqueous
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solution of chlorhydric acid with pH adjusted inferior to 2 and homogenized by stirring. In the second step, a phosphate source (NH4)2HPO4 and the glycin used as fuel in order to provide the ignition of the sample were added. In the third step, the water was evaporated by heating the solution at 120 °C. Further heating, the reaction concentrate led to foaming which was accompanied by evolution of gases formed as a result of GdPO4 formation. After heating on a flame, foaming was followed by ignition, black smoke was observed and black grains were formed then homogenized by grinding. Finally, the powder was stabilized by heating at 800 °C. Stabilization includes crystal growth and removing all residual carbon from the powder. White powders were obtained and referenced below as GP1, GP2, GP3, GP4 and GP5 for 0, 0.5, 1, 5 and 10% Dy3+, respectively. 4
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2.2. Characterization The crystalline phase of the prepared samples have been examined using PANalytical powder diffractometer (X’Pert PRO) using CuKa (1.5405 Å) radiation. The crystalline phases
for Diffraction Data (ICDD)-Powder Diffraction Files (PDF).
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have been determined by comparison of the registered patterns with the International Center
Infrared spectra were recorded on a Perkin Elmer spectrometer (FTIR2000 using KBr pellets in the region of 4000–400 cm–1. Raman scattering spectra have been recorded using
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HORIBA Scientific (lab RAM HR) spectrometer equipped with Laser source (632 nm) and CCD detector. The surface morphology of the samples was studied scanning electron microscopy (SEM) (FEI QUANTA 200). For laser granulometry measurement, we have used
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a Microtrac S3500 Series Particle Size Analyzer with Tri-laser Technology in a range from 0.02 to 2800 µm and solid laser of 780 nm. The excitation, emission spectra and luminescence decay time curves have been done by a Perkin-Elmer spectrophotometer (LS 55) with Xenon lamp (200-700 nm). The emission spectra recorded under excitation with Laser source (325 nm) have been done using the HORIBA Scientific (lab RAM HR) spectrometer. All these
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analysis have been made at room temperature.
3. Results and discussion
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3.1. Powder characterization
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The XRD patterns of Gd(1-x)DyxPO4 (x=0, 0.5, 1, 5, 10%) prepared by combustion synthesis show well crystallized monoclinic phase (space group P21/n (N°14)) with lattice parameters after refinement a=0.6812(2) nm, b=0.7049(2) nm, c=0.6484(2) nm and β=103.6°. All picks in the XRD patterns were indexed to a single phase based on the International Center for Diffraction Data (ICDD)-Powder Diffraction Files (PDF) (ICDDPDF 032-0386) [29] (Fig. 1). Learning XRD patterns in more detail, we revealed that the diffraction peaks were shifted towards the higher 2θ angle side with increasing Dy3+ concentration, which exhibited a shrink of host lattice. This shrink is due to the smaller ionic radii of Dy3+ (1.083 Å) compared to that of substituted Gd3+ (1.107 Å) in the nine-fold coordination environment which facilitates the substitution and the incorporation of Dy3+ in the GdPO4 matrix [30]. 5
ACCEPTED MANUSCRIPT The average sizes of the crystallites were estimated by the Scherrer’s equation: . λ
= θ where D is the average crystallite size, λ is the wavelength of the Cu Kα line, θ is the
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Bragg angle and β is the full-width at half maximum (FWHM) in radians. The strongest peak at 29° was used to estimate the crystallite size by the Scherrer equation [31]. Using this procedure an average crystallite size of about 0.5µm was obtained for the prepared phosphor particles. According to P. Scherrer, when parallel monochromatic radiation falls on a random oriented mass of crystals, the diffracted beam is broadened when
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the particle size is small [31]. The SEM of phosphor particles shows spherical shape with an average particle size of 1µm (Fig. 2). The particle-size distributions related to the percentage
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of volume obtained by laser granulometry measurement are close for all samples and show a multimodal or heterogeneous distribution (0.5–10µm) as shown in figure 3.a. The histogram of sizes distribution for GP1 shows a domination of particles with diameter less than 1µm (Fig. 3.b). So, the average size of the phosphor particles observed from SEM images is close
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to average particles sizes obtained from XRD data and by laser granulometry measurement.
The infrared spectra of Gd(1-x)DyxPO4 have been investigated and shown in Fig. 4. All IR active vibrations are related to stretching and bending of the phosphate group of monazite [32]. A broad band between 1150–1000 cm−1 and a sharper band at 975–950 cm−1 can be
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assigned to ν3 mode (P–O asymmetric stretching) and ν1 mode (P–O symmetric stretching), respectively. The ν4 mode (O–P–O asymmetric bending) are located between 650–521 cm−1,
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while the ν2 mode (O–P–O symmetric bending) only appears at about 492 cm−1 as a small shoulder of ν4 [33-35]. Within experimental resolution, none of the IR modes show a dependence on Dy3+ concentration.
The Raman spectra show the vibrations modes of the PO4 species of phosphates in monazite between 390 and 1100 cm−1 and lattices vibrations between 200 and 300 cm−1 (Fig. 5) [33, 34, 36]. For the monoclinic phosphates the spectra are rich especially in the ν2 mode (O–P–O symmetric bending) and ν4 mode (O–P–O asymmetric bending) situated at 390–490
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ACCEPTED MANUSCRIPT and 520–630 cm−1, respectively. The most intense vibration mode ν1 (P–O symmetric stretching) was observed at 980 cm−1 and was situated at 967 cm−1 for lanthanum and at 971 cm−1 for cerium [34]. The ν3 mode (P–O asymmetric stretching) is composed with four bands situated at 995, 1040, 1060 and 1082 cm−1 [33, 34]. All scattering lines are clearly shifted and broadened towards the higher frequency. In previous works; phenomenon that the Raman
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scattering lines are shifted and broadened to higher wavenumber side when doping crystals with impurities was attributed to the coupling interaction between phonons and carrier Plasmon [35, 36]. According to Lacomba-Perales et al. [37], the shift and broadening of Raman modes were induced by a decrease of lattice parameter and bond length of compound
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under pressure. In our case, the radius of Dy3+ ion is smaller than that of Gd3+ ion. So, as indicated by T. Lien et al. [38], the replacement of the Gd3+ ions with the Dy3+ ions causes a distortion of host lattice due the decrease of lattice parameters and bond lengths; and as a increasing Dy3+-dopant concentration.
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result of this, the vibrational modes are shifted and broadened to higher wavenumber with
Based on data from infrared and Raman spectroscopy provided for other isotopic monophosphates, the bands positions, shapes and intensities of samples are characteristic of a
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monoclinic structure of monazite (P21/n) [39, 40].
3.2. Photoluminescence study
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The UV-visible excitation spectra of Dy3+ luminescence in GdPO4 with different concentrations (x = 0.5, 1, 5 and 10%) monitored at 573 nm are shown in figure 6. A broad band ranging from 260 to 286 nm was observed. A slight shift to lower energy and a decrease
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of integral intensity of this band was observed as function of Dy3+ concentration. It is known that in this energy range a competition between the low charge transfer band absorption (CTB) of a weakly doped ion (Dy3+) and the strong excitation bands of a fully concentrated ion (Gd3+) was expected. This makes the relative evolution of both transition intensities complicated [41]. A number of excitation peaks observed at around 250 nm, 273 nm and 311 nm are attributed to the 8S7/2→ 6DJ, 6IJ, 6PJ transitions within Gd3+ ions respectively [42-44]. The existence of these peaks indicates the energy transfer process from Gd3+ to Dy3+ electronic levels in this sample [6, 45]. Furthermore the probability of energy transfer Gd3+→Dy3+ which also depend on the Dy3+ concentration make the analysis difficult. Anyway, it seems that the excitation of Dy3+ via the CTB of Gd3+ is the major process [46]. 7
ACCEPTED MANUSCRIPT The 4f-4f shell transitions from the ground state (6H15/2) to different excitation levels of the Dy3+ ions are also observed in the excitation spectrum curves which are attributed as follow [47, 48]: (1) 4K15/2+6P3/2 at 324 nm, (2) 4M15/2+ 6P7/2 at 350 nm, 4I11/2 (3) at 363 nm, (6) 4
M21/2+ 4M19/2+ 4K17/2 at 386 nm, 4G11/2 (7) at 426 nm, 4I15/2 (8) at 451 nm, and 4F9/2 (9) at 474
nm, respectively as indexed in the figure 6.
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The spectra registered under excitation at the 4K15/2+6P3/2 levels (325 nm) showed a strong yellow emission band centering at 573 nm which corresponds to the hypersensitive transition 4F9/2→ 6H13/2 (Fig. 7). It’s well known that its intensity is strongly influenced by the surrounding environment around the Dy3+. Another strong emission band in the blue region at
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478 nm corresponds to the 4F9/2→ 6H15/2 transition, which is less sensitive to the host. It is also known that the Dy3+ emission around 478 nm is of magnetic dipole and 573 nm is of electric
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dipole origin. Generally, when Dy3+ is located at a low symmetry (without an inversion center), the yellow emission is dominant whereas the blue emission is stronger when Dy3+ is located at a high symmetry (with an inversion center) [48]. The integrated emission intensity ratios (termed as asymmetric ratio) of electric to magnetic transitions (Y/B) for Dy3+ activated GdPO4 phosphor are around 1.6 while Cao et al mentioned that the yellow emission is equivalent to the blue emission of Dy3+ in the same host matrix GdPO4 phosphor prepared by
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solid state reaction. The registered intensity ratios suggest that Dy3+ is located at a low symmetry and the ligand field don’t present an inversion symmetry character in this matrix. This is in agreement with the crystal structure of GdPO4 in which Dy3+ ion occupies 4e symmetry site of space group P2l/n (monoclinic phase of LaPO4). Every Dy3+ ion is
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surrounded by 9 oxygen ions to form DyO9 polyhedron, in which there are different Ln-O bond lengths and there by DyO9 polyhedron is highly asymmetric. In addition, a weak blue 6
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emission at 455 nm and red emissions at 660 nm are assigned to the 4I15/2→ 6H15/2 and 4F9/2→ H11/2 transitions, respectively. The emission bands don't show a regular dependence on Dy3+
concentration.
The emission spectra of the Dy3+ doped GdPO4 microspheres recorded after exciting
through the Gd−Dy energy-transfer band (ETB) (λex = 274nm) and normalized considering the yellow band maximum intensity of
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F9/2→ 6H13/2 transition (I/IY) are shown in figure 8.
They show an additional broad emission band of Dy3+ centered at 396 nm compared to the spectra registered under excitation with 325 nm. This indicates that energy can be transferred efficiently from the host Gd3+ to the activator Dy3+. This band was not observed by Cao et al under excitation with 275 nm of the same luminophore synthetisized by solid state reaction 8
ACCEPTED MANUSCRIPT [48]. The additional blue band was attributed to 4I11/2 + 4I13/2 + 4I11/2 → 6H15/2 transitions. According literature, these two levels 4I15/2 and 4I13/2 can be thermally populated at the expense of 4F9/2 at thermal equilibrium [49, 50, 51]. But, in this case the population of excited levels 4
I11/2, 4I13/2 and 4I15/2 by energy transfer from Gd3+ to the activator Dy3+ was probably more
efficient and can be proved by three observations:
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• The recovery of the three emission bands from the excited levels 4I11/2, 4I13/2 and 4I15/2 to 6H15/2 due to their close energies.
• The gradual disappearance of these bands and the population of the 4F9/2 level as function of Dy3+ concentration.
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• The immense decrease of intensity as function of Dy3+ concentration. This intensity evolution indicates that at high concentration the transition from the 4F9/2 level was
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more pronounced than from the 4I11/2, 4I13/2 and 4I15/2 levels due to their nearest energies.
The emission integrated intensity ratio of yellow to blue emission bands in Dy3+ doped GdPO4 microspheres under excitation in the ETB still larger then unity and show an increase from 1.1 to 1.3. This indicates that Dy3+ was located at a site with low symmetry and the
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symmetry changes slightly with Dy3+ doping concentration. These results are in agreement with values ratio obtained under excitation at 325 nm and with crystal structure of GdPO4. Also G. Han et al. reported that ratio of yellow to the blue are above 1 under excitation at 147 nm (band attributed to the host absorption of phosphate) of Dy3+-doped LaPO4 synthesized by
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hydrothermal method [25]. Also G. Han et al indicated that the ratio of yellow to blue was larger than one in Ba3Gd(PO4)3 under excitation with 172 nm which indicating the location site of Dy3+ has a low symmetry [43]. While, Cao et al reported that the ratio of the yellow to
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the blue emission bands of Dy3+ in GdPO4 synthetisized by solid state are all less than 1 [48]. Also, Phaomei et al reported that (Y/B) intensity ratio is less than unity (≈ 0.5) in Ce3+ sensitized LaPO4:Dy3+ nanorods prepared by chemical route at relatively low temperature [52] and in dispersed and rice-shaped nanoparticles of diameter 5–10 nm, length of 13–37 nm for Dy3+-doped LaPO4 [53]. They reveal that this contradictory nature is due to the contribution of polarization ability from the surrounding environment in LaPO4 system [54, 55]. According to M. Yang et al, the special morphology of microscaled spheres enhances luminescent properties. However, both microspheres and the bulk material exhibited an almost difference in excitation and emission spectra [15].
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ACCEPTED MANUSCRIPT 3.3. Lifetime study The concentration dependent decay rates of fluorescence originating from the 4F9/2 level of the Dy3+ ion in GdPO4 are obtained by exciting at 326 nm and monitoring the 4F9/2 →6H13/2 (573 nm) emission transition and are shown in semi-logarithmic scale in figure 9. At low concentration of 0.5% and 1%, the decay rates exhibit single exponential behavior and
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are measured by the following function:
=
where I(t) is the total intensity at time t, I0 is intensity at t=0 and τ is lifetime.
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As the concentration of Dy3+ increases above 1%, the decay rates exhibit nonexponential shape and the decay curves have been fitted to the following bi-exponential function:
= +
where I1 and I2 are intensities at different times. τ1 and τ2 are their corresponding lifetimes. The average (τav) lifetime in case of a non-exponential with two components can be calculated using the following equation [56, 57]: + +
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=
The registered life time τ for 0.5% and 1% Dy3+ in GdPO4 are 1.64 and 1.97 ms, respectively. The average life time values for 5 and 10 % of Dy3+ are 0.38 and 0.27 ms, respectively. The life time increases with an increase in concentration of Dy3+ up to 1 at% and
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then decreases with a further increase in the Dy3+ concentration known as luminescence quenching. Luminescence quenching with an increase of Dy3+ concentration is a typical
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property of lanthanide-doped systems where the distance between the neighboring Dy3+ ions decreases with the increase of Dy3+ ions. This can be explained on the basis of the crossrelaxation among the lanthanide ions when the mean distance between them is less than a critical value [58]. Cao et al suggest that in the Dy3+ activated GdPO4, the optimal doping concentration is 1 mol % for host Gd3+ ion sensitization [48]. The registered life time value at low concentration are high then the average lifetime values for Dy3+ in Ce3+ co-doped LaPO4 (0.87 ms) and LaPO4 (0.57 ms) synthesized by simple chemical route [52, 53] and the reported lifetime value of Dy3+ doped YVO4 (0.21ms) [59].
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ACCEPTED MANUSCRIPT To evaluate the materials' performance on color luminescent emission, CIE chromaticity coordinates (x,y) were determined from the emission spectra, and they are plotted in the chromaticity diagram in figure 10. Under excitation at 325 nm the chromaticity coordinates are close and independent of Dy3+ concentration. They are situated in the yellow region of the chromaticity diagram (x=0.36; y=0.40). Wile, the chromaticity coordinates
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obtained from the emission spectra registered with excitation in the ETB show a dependence on the Dy3+ concentration. At low concentration, 0.5% and 1% Dy3+, the chromaticity coordinates are (x=0.28; y=0.29) and (x=0.287; y=0.308), respectively. As shown in the figure 10, these coordinates are situated in the blue region. For the high Dy3+ concentration, 5
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and 10 %, the chromaticity coordinates are (x=0.317; y=0.328) and (x=0.314; y=0.336) which are located in the white light region. The obtained results are similar to those obtained for
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Dy3+ in KLa(PO3)4 [26].
Conclusion
Dy3+ doped GdPO4 microspheres particles phosphors have been synthesized by combustion route. The characterization of the obtained powders by X-ray diffraction (XRD),
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FTIR and Raman spectroscopy reveal that the microspheres particles annealed at 800 °C crystallized in a monoclinic phase with space group P21/n. The excitation spectra show a strong energy transfer band in the vacuum ultraviolet range indicating that energy can easily be transferred from Gd3+ to the energy levels of Dy3+ ion. The effects of Dy3+ concentration
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and excitation wavelength have been studied. Blue and white light have been obtained under excitation in the ETB at low and high Dy3+ concentration, respectively. A yellow light have
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been obtained under excitation with 325 nm. Therefore, a single phosphor with three emitting color blue, yellow and white based on Dy3+ in GdPO4 microspheres have been obtained. The luminescent intensity of Dy3+ in GdPO4 microspheres has been found to be in well agreement with crystal structure of the host matrix compared to bulk materials under identical conditions. The decay rates for 4F9/2 level are found to be deviated from exponential to nonexponential nature with increase in Dy3+ ion concentration. These experimental results indicate that the Dy3+ activated GdPO4 microsphere has potential applications in lighting technology and is a promising white light material.
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ACCEPTED MANUSCRIPT [41] S. Hachani , B. Moine, A. El-akrmi , M. Férid, Luminescent properties of some orthoand pentaphosphates doped with Gd3+–Eu3+: Potential phosphors for vacuum ultraviolet excitation, Opt. Mater. 31 (2009) 678–684. [42] W. Qin, C. Cao, L.Wang, J. Zhang, D. Zhang, K. Zheng, Y. Wang, G. Wei, G. Wang, P. Zhu, R. Kim, Ultraviolet upconversion fluorescence from 6DJ of Gd3+ induced by 980 nm
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ACCEPTED MANUSCRIPT tunable blue to white light emission from Eu3+ co-activated, LaPO4:Dy3+, Ce3+, J. Lumin. 134 (2013) 649–656 [53] G. Phaomei, W.Rameshwor Singh , R.S. Ningthoujam, Solvent effect in monoclinic to hexagonal
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nanoparticles:
Photoluminescence study, J. Lumin. 131 (2011) 1164–1171.
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Redispersible Tb3+-Doped GdPO4 Nanoparticles Prepared by an Ethylene Glycol Route, Eur. J. Inorg. Chem. (2010) 2682.
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[56] N.S. Singh, R.S. Ningthoujam, M.N. Luwang, S.D. Singh, R.K. Vatsa, Luminescence, lifetime and quantum yield studies of YVO4: Ln3+ (Ln3+ = Dy3+, Eu3+) nanoparticles: concentration and annealing effects, Chem. Phys. Lett. 480 (2009) 237. [57] B. Grobelna, A. Synak, P. Bojarski, K. Szczodrowski, B. Kuklinski, S. Raut, I. Gryczynski, Synthesis and luminescence characteristics of Dy3+ ions in silica xerogels doped with Ln2−xDyx(WO4)3, Opt. Mater. 35 (2013) 456.
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[58] Q. Su, J. Lin, B. Li, A study on the luminescence properties of Eu3+ and Dy3+ in M2RE8(SiO4)6O2 (M = Mg, Ca; RE = Y, Gd, La), J. Alloys Compd., 225 (1995) 120. [59] G. Jia, Y. Song, M. Yang., Y. Huang, L. Zhang, H. You, Uniform YVO4:Ln3+ (Ln=Eu,
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31 (2009) 1032.
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Dy, and Sm) nanocrystals: Solvothermal synthesis and luminescence properties, Opt. Mater.
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ACCEPTED MANUSCRIPT Figure captions Fig.1. X-ray powder diffraction patterns of Gd(1-x)DyxPO4 Fig.2. SEM image of GdPO4 synthesized by combustion route
Fig.4. FTIR spectra of Gd1-xDyxPO4 microspheres Fig.5. Raman spectra of Gd1-xDyxPO4 microspheres Fig.6. Excitation spectra of Gd1-xDyxPO4 (λem=573nm).
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Fig.7. Emission spectra of Gd1-xDyxPO4 (λexc= 325 nm)
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Fig.3. Particle-size distributions related to the % volume (a) and distribution histogram of Gd(1-x)DyxPO4.
Fig.8. Emission spectra of Gd1-xDyxPO4 under excitation in the energy transfer band (λexc= 274 nm).
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Fig.9. Decay profiles for 4F9/2 level of Dy3+ doped GdPO4 (λexc=326 nm, λem=573nm). Fig.10.The CIE chromatic diagram showing the chromatic coordinates of Gd1-xDyxPO4
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emissions as a function of Dy3+ concentrations and excitation wavelengths.
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140 123 40-2 410
02-3 32-2 132
212
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GP1: 0% Dy
301 23-1
03-1
112 220
20-2
01-2
210
020
11-1
101
01-1
110
10-1
0,5
31-1 221
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200 120
1,0
0,0
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0,5
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GP3:1% Dy
0,5 0,0
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Fig. 1. X-ray powder diffraction patterns of Gd(1-x)DyxPO4
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Fig. 2. SEM image of GdPO4 synthesized by combustion route
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GP1 GP2 GP3 GP4 GP5
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Diameter (µm)
5
6
7
Fig.3. Particle-size distributions related to the % volume (a) and distribution histogram of Gd(1-x)DyxPO4.
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GP1: 0%Dy
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GP2: 0.5%Dy 3+
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GP5: 10%Dy
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ν 1
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Fig. 4. FTIR spectra of Gd1-xDyxPO4 microspheres
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ν1
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ν2
ν3
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Fig.5. Raman spectra of Gd1-xDyxPO4 microspheres
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3+ 6
Gd : IJ
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λem = 573 nm
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GP2: 0.5% Dy 3+
GP3: 1% Dy (4M21/2+4M19/2+4K17/2)
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(4I11/2)
6 (4M 15/2+ P7/2)
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( I15/2) 4
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0,6
(Gd3+: 6DJ)
Intensity (a.u)
0,8
380
400
420
440
460
Wavelenght (nm)
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Fig. 6. Excitation spectra of Gd1-xDyxPO4 (λem=573nm).
480
500
4
6
F9/2 (558-593) H13/2
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GP2: 0.5% Dy
λex =325 nm
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F9/2(465-495) 6H
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15/2
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GP3: 1% Dy
F9/2 (660)
4
550
Wavelenght (nm)
600
650
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Fig. 7. Emission spectra of Gd1-xDyxPO4 (λexc= 325 nm)
6
H11/2 700
λex= 274 nm
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F9/2 (573)
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F9/2 (658) 6H 11/2
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Fig. 8. Emission spectra of Gd1-xDyxPO4 under excitation in the energy transfer band (λexc= 274 nm).
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GP2:0,5% Dy τ = 1.64 ms
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τ = 1.97 ms
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GP3:1% Dy GP4:5% Dy
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2
τav = 0.27 ms
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Fig. 9. Decay profiles for 4F9/2 level of Dy3+ doped GdPO4 (λexc=326 nm, λem=573nm).
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Fig. 10. The CIE chromatic diagram showing the chromatic coordinates of Gd1-xDyxPO4 emissions as a function of Dy3+ concentrations and excitation wavelengths.
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Highlights - Combustion synthesis of Gd(1-x)DyxPO4 microspheres phosphors for the first time - Identification of phosphors by different techniques
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Single-phase phosphor emitting blue, yellow and white light Effects of Dy3+ concentration and excitation wavelength on Dy3+ emission A potential single-host phosphor for white LEDs
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-
3+
microspheres blue, yellow and white light emitting
M. Ferhi, S. Toumi, K. Horchani-Naifer, M. Ferid PII:
S0925-8388(17)31392-0
DOI:
10.1016/j.jallcom.2017.04.193
Reference:
JALCOM 41597
To appear in:
Journal of Alloys and Compounds
Received Date: 11 March 2017 Revised Date:
14 April 2017
Accepted Date: 18 April 2017
3+ Please cite this article as: M. Ferhi, S. Toumi, K. Horchani-Naifer, M. Ferid, Single phase GdPO4: Dy microspheres blue, yellow and white light emitting phosphor, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.193. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The CIE chromatic diagram showing the chromatic coordinates of Gd1-xDyxPO4 emissions
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as function of Dy3+ concentrations and excitation wavelengths.
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Single phase GdPO4: Dy3+ microspheres blue, yellow and white light emitting phosphor
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By
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M. Ferhi*, S. Toumi, K. Horchani-Naifer, M. Ferid
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Physical Chemistry Laboratory of Mineral Materials and their Applications, National Center of Researches in Materials Sciences,
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Technopole de Borj Cedria, BP 73, 8027 Soliman, Tunisia
Corresponding author: Mounir Ferhi
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E-mail: [email protected] Tel.: +216 79 325 280
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Fax: +216 79 325 314
1
ACCEPTED MANUSCRIPT Abstract Luminescent microspheres, Gd(1-x)DyxPO4 have been synthesized using simple combustion method for the first time. SEM images show that the obtained powders are formed by dispersed microscaled spheres with diameter ranging from 0.5 to 1 µm. The X-ray diffraction, FTIR and Raman spectroscopy studies reveal that the microparticles annealed at
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800 °C crystallized in a monoclinic phase with space group P21/n. Under host Gd3+ sensitization, Gd(1-x)DyxPO4 emission spectra of Dy3+ show a broad blue band attributed to transitions 4I15/2 + 4I13/2 + 4I11/2 + 4F9/2 → 6H15/2, a sharp yellow band due to 4F9/2→6H13/2 transition, and the little red band of 4F9/2 → 6H11/2 transition. Blue and white emissions have
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been obtained at low and high Dy3+ concentration, respectively due the energy transfer from Gd3+ to Dy3+. Under direct 4f–4f excitation of Dy3+ in GdDPO4, three bands attributed to 4
F9/2→ 6H15/2 , 6H13/2 , 6H11/2 transitions have been observed resulting in a yellow light
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emission. The emission bands intensity are discussed as function of GdPO4 structure. They are in well agreement with crystal structure of the host lattice GdPO4 in contrast to reported works. The decay rates for the 4F9/2 level are found to be deviated from exponential to nonexponential nature with increase of Dy3+ concentration. A promising white-light emission for white LED operating under UV excitation is investigated from single-phased phosphor. The
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obtained results suggest a potential application of GdPO4:Dy3+ microspheres as alternatives to replace bulk materials for many application fields, including light display systems and
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optoelectronic devices.
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Keywords: Luminescence, Combustion synthesis, white LED, Gd(1-x)DyxPO4, microsphere.
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ACCEPTED MANUSCRIPT 1. Introduction Recently, white light-emitting diodes (w-LEDs) have been widely used in various illumination fields. The combination of the UV/near-UV chips with a single-phased phosphor is considered to be an efficient way to obtain white light for solid-state lighting [1, 2]. It is known that the generation of white light from single-phased phosphors is based on codoping
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with two activators, which can emit two distinctive emission bands [3-5]. More importantly, compared to the multiple emitting components of the white LED system, the single-phased white-light-emitting phosphor for a UV-pumped white LED would enable easy fabrication with perfect stability and color [6]. It is also known that the control of the particles size in a
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crystalline system can effect in remarkable modification of physical, chemical, and mechanical properties compared to those of bulk due to the effect of surface-to-volume ratio of particles. The modification of luminescence properties can be brought by reducing the
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particles size and imperfections using different preparation techniques, different capping agents and use of suitable sensitizer, etc. Different types and sizes of luminescent particles are now being synthesized via several physical and chemical methods such as the homogeneous precipitation method, hydrothermal method, solid-state reaction method, laser synthesis, combustion synthesis, sol–gel method and microwave assisted chemical synthesis [7–13]. The
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preparation of particles with specific phases shapes and sizes depend a suitable reaction medium/solvent is important [14]. According to M. Yang et al, the unique properties of the monoclinic LaPO4 microspheres suggest their potential applications as a substitute of bulk countparts for many applications, such as optical displays and other optoelectronic devices.
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Furthermore, microspheres with a more uniform morphology could make better use of the exciting light source and reach a higher efficiency of the phosphor [15]. Among luminescent
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materials, lanthanide monophosphates (LnPO4) are the most useful inorganic host lattice for doping lanthanide ions emitting a wide range of colors [16, 17] due to their interesting properties such as low water solubility [18], high thermal stability [19] and high index of refraction [20]. They have been applied in various fields, such as plasma display panels (PDP), mercury-free lamp and visible lasers [21–24]. For mercury-free lamp, the excitation energy was mainly composed of VUV radiation but VUV energy is mostly absorbed by the host crystal, if the energy can be transferred from host to rare-earth (RE) ions then the rareearth ions can emit visible light. So the host absorption intensity is very important for VUVexcited phosphors applied in mercury-free lamp [25]. According Han et al, monophosphates are promising host materials for application in VUV region [25]. In LnPO4, the high 3
ACCEPTED MANUSCRIPT absorption was observed in VUV region of PO43-. Hence, GdPO4 is an efficient host under VUV excitation [26]. Rare-earth ions have been considered as efficient activators in modern lighting and display fields due to their characteristic emission either from 4f-4f or 5d-4f transitions. Among the RE ions, Dy3+ doped phosphates have drawn much interest for their two-color and
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white light emission [25, 27, 28]. So, Dy3+ ion possesses two intense emissions at blue (486 nm, 4F9/2→6H15/2) and yellow (576 nm, 4F9/2→6H13/2) regions. Further, it is well known that the 4F9/2→6H13/2 transition of Dy3+ ions is hypersensitive and therefore its intensity strongly depends on the nature of the host, whereas the intensity of magnetic-dipole allowed F9/2→6H15/2 transition is less sensitive to the host. Hence, at a suitable environment and by
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adjusting the yellow-to-blue intensity ratio (Y/B), only Dy3+-activated materials would
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generate near-white emission.
In this work we report the combustion synthesis and optical properties of GdPO4:Dy3+ microspheres. The effects of Dy3+ concentration and excitation wavelength on the Y/B ratios and color coordinates have been discussed. The influence of dopant concentration on the lifetimes is presented as well. The utility of the present phosphor for blue, yellow and white
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light emission has also been discussed. 2. Experimental details
2.1. Sample preparation
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Gd(1-x)DyxPO4 (x=0, 0.5, 1, 5, 10%) microspheres were prepared by following three steps. Firstly, the appropriate amounts of lanthanide oxides were dissolved in aqueous
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solution of chlorhydric acid with pH adjusted inferior to 2 and homogenized by stirring. In the second step, a phosphate source (NH4)2HPO4 and the glycin used as fuel in order to provide the ignition of the sample were added. In the third step, the water was evaporated by heating the solution at 120 °C. Further heating, the reaction concentrate led to foaming which was accompanied by evolution of gases formed as a result of GdPO4 formation. After heating on a flame, foaming was followed by ignition, black smoke was observed and black grains were formed then homogenized by grinding. Finally, the powder was stabilized by heating at 800 °C. Stabilization includes crystal growth and removing all residual carbon from the powder. White powders were obtained and referenced below as GP1, GP2, GP3, GP4 and GP5 for 0, 0.5, 1, 5 and 10% Dy3+, respectively. 4
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2.2. Characterization The crystalline phase of the prepared samples have been examined using PANalytical powder diffractometer (X’Pert PRO) using CuKa (1.5405 Å) radiation. The crystalline phases
for Diffraction Data (ICDD)-Powder Diffraction Files (PDF).
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have been determined by comparison of the registered patterns with the International Center
Infrared spectra were recorded on a Perkin Elmer spectrometer (FTIR2000 using KBr pellets in the region of 4000–400 cm–1. Raman scattering spectra have been recorded using
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HORIBA Scientific (lab RAM HR) spectrometer equipped with Laser source (632 nm) and CCD detector. The surface morphology of the samples was studied scanning electron microscopy (SEM) (FEI QUANTA 200). For laser granulometry measurement, we have used
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a Microtrac S3500 Series Particle Size Analyzer with Tri-laser Technology in a range from 0.02 to 2800 µm and solid laser of 780 nm. The excitation, emission spectra and luminescence decay time curves have been done by a Perkin-Elmer spectrophotometer (LS 55) with Xenon lamp (200-700 nm). The emission spectra recorded under excitation with Laser source (325 nm) have been done using the HORIBA Scientific (lab RAM HR) spectrometer. All these
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analysis have been made at room temperature.
3. Results and discussion
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3.1. Powder characterization
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The XRD patterns of Gd(1-x)DyxPO4 (x=0, 0.5, 1, 5, 10%) prepared by combustion synthesis show well crystallized monoclinic phase (space group P21/n (N°14)) with lattice parameters after refinement a=0.6812(2) nm, b=0.7049(2) nm, c=0.6484(2) nm and β=103.6°. All picks in the XRD patterns were indexed to a single phase based on the International Center for Diffraction Data (ICDD)-Powder Diffraction Files (PDF) (ICDDPDF 032-0386) [29] (Fig. 1). Learning XRD patterns in more detail, we revealed that the diffraction peaks were shifted towards the higher 2θ angle side with increasing Dy3+ concentration, which exhibited a shrink of host lattice. This shrink is due to the smaller ionic radii of Dy3+ (1.083 Å) compared to that of substituted Gd3+ (1.107 Å) in the nine-fold coordination environment which facilitates the substitution and the incorporation of Dy3+ in the GdPO4 matrix [30]. 5
ACCEPTED MANUSCRIPT The average sizes of the crystallites were estimated by the Scherrer’s equation: . λ
= θ where D is the average crystallite size, λ is the wavelength of the Cu Kα line, θ is the
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Bragg angle and β is the full-width at half maximum (FWHM) in radians. The strongest peak at 29° was used to estimate the crystallite size by the Scherrer equation [31]. Using this procedure an average crystallite size of about 0.5µm was obtained for the prepared phosphor particles. According to P. Scherrer, when parallel monochromatic radiation falls on a random oriented mass of crystals, the diffracted beam is broadened when
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the particle size is small [31]. The SEM of phosphor particles shows spherical shape with an average particle size of 1µm (Fig. 2). The particle-size distributions related to the percentage
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of volume obtained by laser granulometry measurement are close for all samples and show a multimodal or heterogeneous distribution (0.5–10µm) as shown in figure 3.a. The histogram of sizes distribution for GP1 shows a domination of particles with diameter less than 1µm (Fig. 3.b). So, the average size of the phosphor particles observed from SEM images is close
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to average particles sizes obtained from XRD data and by laser granulometry measurement.
The infrared spectra of Gd(1-x)DyxPO4 have been investigated and shown in Fig. 4. All IR active vibrations are related to stretching and bending of the phosphate group of monazite [32]. A broad band between 1150–1000 cm−1 and a sharper band at 975–950 cm−1 can be
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assigned to ν3 mode (P–O asymmetric stretching) and ν1 mode (P–O symmetric stretching), respectively. The ν4 mode (O–P–O asymmetric bending) are located between 650–521 cm−1,
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while the ν2 mode (O–P–O symmetric bending) only appears at about 492 cm−1 as a small shoulder of ν4 [33-35]. Within experimental resolution, none of the IR modes show a dependence on Dy3+ concentration.
The Raman spectra show the vibrations modes of the PO4 species of phosphates in monazite between 390 and 1100 cm−1 and lattices vibrations between 200 and 300 cm−1 (Fig. 5) [33, 34, 36]. For the monoclinic phosphates the spectra are rich especially in the ν2 mode (O–P–O symmetric bending) and ν4 mode (O–P–O asymmetric bending) situated at 390–490
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ACCEPTED MANUSCRIPT and 520–630 cm−1, respectively. The most intense vibration mode ν1 (P–O symmetric stretching) was observed at 980 cm−1 and was situated at 967 cm−1 for lanthanum and at 971 cm−1 for cerium [34]. The ν3 mode (P–O asymmetric stretching) is composed with four bands situated at 995, 1040, 1060 and 1082 cm−1 [33, 34]. All scattering lines are clearly shifted and broadened towards the higher frequency. In previous works; phenomenon that the Raman
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scattering lines are shifted and broadened to higher wavenumber side when doping crystals with impurities was attributed to the coupling interaction between phonons and carrier Plasmon [35, 36]. According to Lacomba-Perales et al. [37], the shift and broadening of Raman modes were induced by a decrease of lattice parameter and bond length of compound
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under pressure. In our case, the radius of Dy3+ ion is smaller than that of Gd3+ ion. So, as indicated by T. Lien et al. [38], the replacement of the Gd3+ ions with the Dy3+ ions causes a distortion of host lattice due the decrease of lattice parameters and bond lengths; and as a increasing Dy3+-dopant concentration.
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result of this, the vibrational modes are shifted and broadened to higher wavenumber with
Based on data from infrared and Raman spectroscopy provided for other isotopic monophosphates, the bands positions, shapes and intensities of samples are characteristic of a
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monoclinic structure of monazite (P21/n) [39, 40].
3.2. Photoluminescence study
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The UV-visible excitation spectra of Dy3+ luminescence in GdPO4 with different concentrations (x = 0.5, 1, 5 and 10%) monitored at 573 nm are shown in figure 6. A broad band ranging from 260 to 286 nm was observed. A slight shift to lower energy and a decrease
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of integral intensity of this band was observed as function of Dy3+ concentration. It is known that in this energy range a competition between the low charge transfer band absorption (CTB) of a weakly doped ion (Dy3+) and the strong excitation bands of a fully concentrated ion (Gd3+) was expected. This makes the relative evolution of both transition intensities complicated [41]. A number of excitation peaks observed at around 250 nm, 273 nm and 311 nm are attributed to the 8S7/2→ 6DJ, 6IJ, 6PJ transitions within Gd3+ ions respectively [42-44]. The existence of these peaks indicates the energy transfer process from Gd3+ to Dy3+ electronic levels in this sample [6, 45]. Furthermore the probability of energy transfer Gd3+→Dy3+ which also depend on the Dy3+ concentration make the analysis difficult. Anyway, it seems that the excitation of Dy3+ via the CTB of Gd3+ is the major process [46]. 7
ACCEPTED MANUSCRIPT The 4f-4f shell transitions from the ground state (6H15/2) to different excitation levels of the Dy3+ ions are also observed in the excitation spectrum curves which are attributed as follow [47, 48]: (1) 4K15/2+6P3/2 at 324 nm, (2) 4M15/2+ 6P7/2 at 350 nm, 4I11/2 (3) at 363 nm, (6) 4
M21/2+ 4M19/2+ 4K17/2 at 386 nm, 4G11/2 (7) at 426 nm, 4I15/2 (8) at 451 nm, and 4F9/2 (9) at 474
nm, respectively as indexed in the figure 6.
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The spectra registered under excitation at the 4K15/2+6P3/2 levels (325 nm) showed a strong yellow emission band centering at 573 nm which corresponds to the hypersensitive transition 4F9/2→ 6H13/2 (Fig. 7). It’s well known that its intensity is strongly influenced by the surrounding environment around the Dy3+. Another strong emission band in the blue region at
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478 nm corresponds to the 4F9/2→ 6H15/2 transition, which is less sensitive to the host. It is also known that the Dy3+ emission around 478 nm is of magnetic dipole and 573 nm is of electric
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dipole origin. Generally, when Dy3+ is located at a low symmetry (without an inversion center), the yellow emission is dominant whereas the blue emission is stronger when Dy3+ is located at a high symmetry (with an inversion center) [48]. The integrated emission intensity ratios (termed as asymmetric ratio) of electric to magnetic transitions (Y/B) for Dy3+ activated GdPO4 phosphor are around 1.6 while Cao et al mentioned that the yellow emission is equivalent to the blue emission of Dy3+ in the same host matrix GdPO4 phosphor prepared by
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solid state reaction. The registered intensity ratios suggest that Dy3+ is located at a low symmetry and the ligand field don’t present an inversion symmetry character in this matrix. This is in agreement with the crystal structure of GdPO4 in which Dy3+ ion occupies 4e symmetry site of space group P2l/n (monoclinic phase of LaPO4). Every Dy3+ ion is
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surrounded by 9 oxygen ions to form DyO9 polyhedron, in which there are different Ln-O bond lengths and there by DyO9 polyhedron is highly asymmetric. In addition, a weak blue 6
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emission at 455 nm and red emissions at 660 nm are assigned to the 4I15/2→ 6H15/2 and 4F9/2→ H11/2 transitions, respectively. The emission bands don't show a regular dependence on Dy3+
concentration.
The emission spectra of the Dy3+ doped GdPO4 microspheres recorded after exciting
through the Gd−Dy energy-transfer band (ETB) (λex = 274nm) and normalized considering the yellow band maximum intensity of
4
F9/2→ 6H13/2 transition (I/IY) are shown in figure 8.
They show an additional broad emission band of Dy3+ centered at 396 nm compared to the spectra registered under excitation with 325 nm. This indicates that energy can be transferred efficiently from the host Gd3+ to the activator Dy3+. This band was not observed by Cao et al under excitation with 275 nm of the same luminophore synthetisized by solid state reaction 8
ACCEPTED MANUSCRIPT [48]. The additional blue band was attributed to 4I11/2 + 4I13/2 + 4I11/2 → 6H15/2 transitions. According literature, these two levels 4I15/2 and 4I13/2 can be thermally populated at the expense of 4F9/2 at thermal equilibrium [49, 50, 51]. But, in this case the population of excited levels 4
I11/2, 4I13/2 and 4I15/2 by energy transfer from Gd3+ to the activator Dy3+ was probably more
efficient and can be proved by three observations:
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• The recovery of the three emission bands from the excited levels 4I11/2, 4I13/2 and 4I15/2 to 6H15/2 due to their close energies.
• The gradual disappearance of these bands and the population of the 4F9/2 level as function of Dy3+ concentration.
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• The immense decrease of intensity as function of Dy3+ concentration. This intensity evolution indicates that at high concentration the transition from the 4F9/2 level was
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more pronounced than from the 4I11/2, 4I13/2 and 4I15/2 levels due to their nearest energies.
The emission integrated intensity ratio of yellow to blue emission bands in Dy3+ doped GdPO4 microspheres under excitation in the ETB still larger then unity and show an increase from 1.1 to 1.3. This indicates that Dy3+ was located at a site with low symmetry and the
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symmetry changes slightly with Dy3+ doping concentration. These results are in agreement with values ratio obtained under excitation at 325 nm and with crystal structure of GdPO4. Also G. Han et al. reported that ratio of yellow to the blue are above 1 under excitation at 147 nm (band attributed to the host absorption of phosphate) of Dy3+-doped LaPO4 synthesized by
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hydrothermal method [25]. Also G. Han et al indicated that the ratio of yellow to blue was larger than one in Ba3Gd(PO4)3 under excitation with 172 nm which indicating the location site of Dy3+ has a low symmetry [43]. While, Cao et al reported that the ratio of the yellow to
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the blue emission bands of Dy3+ in GdPO4 synthetisized by solid state are all less than 1 [48]. Also, Phaomei et al reported that (Y/B) intensity ratio is less than unity (≈ 0.5) in Ce3+ sensitized LaPO4:Dy3+ nanorods prepared by chemical route at relatively low temperature [52] and in dispersed and rice-shaped nanoparticles of diameter 5–10 nm, length of 13–37 nm for Dy3+-doped LaPO4 [53]. They reveal that this contradictory nature is due to the contribution of polarization ability from the surrounding environment in LaPO4 system [54, 55]. According to M. Yang et al, the special morphology of microscaled spheres enhances luminescent properties. However, both microspheres and the bulk material exhibited an almost difference in excitation and emission spectra [15].
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ACCEPTED MANUSCRIPT 3.3. Lifetime study The concentration dependent decay rates of fluorescence originating from the 4F9/2 level of the Dy3+ ion in GdPO4 are obtained by exciting at 326 nm and monitoring the 4F9/2 →6H13/2 (573 nm) emission transition and are shown in semi-logarithmic scale in figure 9. At low concentration of 0.5% and 1%, the decay rates exhibit single exponential behavior and
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are measured by the following function:
=
where I(t) is the total intensity at time t, I0 is intensity at t=0 and τ is lifetime.
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As the concentration of Dy3+ increases above 1%, the decay rates exhibit nonexponential shape and the decay curves have been fitted to the following bi-exponential function:
= +
where I1 and I2 are intensities at different times. τ1 and τ2 are their corresponding lifetimes. The average (τav) lifetime in case of a non-exponential with two components can be calculated using the following equation [56, 57]: + +
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=
The registered life time τ for 0.5% and 1% Dy3+ in GdPO4 are 1.64 and 1.97 ms, respectively. The average life time values for 5 and 10 % of Dy3+ are 0.38 and 0.27 ms, respectively. The life time increases with an increase in concentration of Dy3+ up to 1 at% and
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then decreases with a further increase in the Dy3+ concentration known as luminescence quenching. Luminescence quenching with an increase of Dy3+ concentration is a typical
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property of lanthanide-doped systems where the distance between the neighboring Dy3+ ions decreases with the increase of Dy3+ ions. This can be explained on the basis of the crossrelaxation among the lanthanide ions when the mean distance between them is less than a critical value [58]. Cao et al suggest that in the Dy3+ activated GdPO4, the optimal doping concentration is 1 mol % for host Gd3+ ion sensitization [48]. The registered life time value at low concentration are high then the average lifetime values for Dy3+ in Ce3+ co-doped LaPO4 (0.87 ms) and LaPO4 (0.57 ms) synthesized by simple chemical route [52, 53] and the reported lifetime value of Dy3+ doped YVO4 (0.21ms) [59].
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ACCEPTED MANUSCRIPT To evaluate the materials' performance on color luminescent emission, CIE chromaticity coordinates (x,y) were determined from the emission spectra, and they are plotted in the chromaticity diagram in figure 10. Under excitation at 325 nm the chromaticity coordinates are close and independent of Dy3+ concentration. They are situated in the yellow region of the chromaticity diagram (x=0.36; y=0.40). Wile, the chromaticity coordinates
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obtained from the emission spectra registered with excitation in the ETB show a dependence on the Dy3+ concentration. At low concentration, 0.5% and 1% Dy3+, the chromaticity coordinates are (x=0.28; y=0.29) and (x=0.287; y=0.308), respectively. As shown in the figure 10, these coordinates are situated in the blue region. For the high Dy3+ concentration, 5
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and 10 %, the chromaticity coordinates are (x=0.317; y=0.328) and (x=0.314; y=0.336) which are located in the white light region. The obtained results are similar to those obtained for
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Dy3+ in KLa(PO3)4 [26].
Conclusion
Dy3+ doped GdPO4 microspheres particles phosphors have been synthesized by combustion route. The characterization of the obtained powders by X-ray diffraction (XRD),
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FTIR and Raman spectroscopy reveal that the microspheres particles annealed at 800 °C crystallized in a monoclinic phase with space group P21/n. The excitation spectra show a strong energy transfer band in the vacuum ultraviolet range indicating that energy can easily be transferred from Gd3+ to the energy levels of Dy3+ ion. The effects of Dy3+ concentration
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and excitation wavelength have been studied. Blue and white light have been obtained under excitation in the ETB at low and high Dy3+ concentration, respectively. A yellow light have
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been obtained under excitation with 325 nm. Therefore, a single phosphor with three emitting color blue, yellow and white based on Dy3+ in GdPO4 microspheres have been obtained. The luminescent intensity of Dy3+ in GdPO4 microspheres has been found to be in well agreement with crystal structure of the host matrix compared to bulk materials under identical conditions. The decay rates for 4F9/2 level are found to be deviated from exponential to nonexponential nature with increase in Dy3+ ion concentration. These experimental results indicate that the Dy3+ activated GdPO4 microsphere has potential applications in lighting technology and is a promising white light material.
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ACCEPTED MANUSCRIPT tunable blue to white light emission from Eu3+ co-activated, LaPO4:Dy3+, Ce3+, J. Lumin. 134 (2013) 649–656 [53] G. Phaomei, W.Rameshwor Singh , R.S. Ningthoujam, Solvent effect in monoclinic to hexagonal
phase
transformation
in
LaPO4:RE
(RE=Dy3+,
Sm3+)
nanoparticles:
Photoluminescence study, J. Lumin. 131 (2011) 1164–1171.
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[54] G. Phaomei, R.S. Ningthoujam, W. Rameshwor Singh, Naorem Shanta Singh, M. Niraj Luwang, R. Tewari, R.K. Vatsa, Low temperature synthesis and luminescence properties of re-dispersible Eu3+ doped LaPO4 nanorods by ethylene glycol route, Opt. Mater. 32 (2010) 616.
[55] N. Yaiphaba, R.S. Ningthoujam, N.R. Singh, R.K. Vatsa, Luminescence Properties of
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Redispersible Tb3+-Doped GdPO4 Nanoparticles Prepared by an Ethylene Glycol Route, Eur. J. Inorg. Chem. (2010) 2682.
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[56] N.S. Singh, R.S. Ningthoujam, M.N. Luwang, S.D. Singh, R.K. Vatsa, Luminescence, lifetime and quantum yield studies of YVO4: Ln3+ (Ln3+ = Dy3+, Eu3+) nanoparticles: concentration and annealing effects, Chem. Phys. Lett. 480 (2009) 237. [57] B. Grobelna, A. Synak, P. Bojarski, K. Szczodrowski, B. Kuklinski, S. Raut, I. Gryczynski, Synthesis and luminescence characteristics of Dy3+ ions in silica xerogels doped with Ln2−xDyx(WO4)3, Opt. Mater. 35 (2013) 456.
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[58] Q. Su, J. Lin, B. Li, A study on the luminescence properties of Eu3+ and Dy3+ in M2RE8(SiO4)6O2 (M = Mg, Ca; RE = Y, Gd, La), J. Alloys Compd., 225 (1995) 120. [59] G. Jia, Y. Song, M. Yang., Y. Huang, L. Zhang, H. You, Uniform YVO4:Ln3+ (Ln=Eu,
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31 (2009) 1032.
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Dy, and Sm) nanocrystals: Solvothermal synthesis and luminescence properties, Opt. Mater.
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ACCEPTED MANUSCRIPT Figure captions Fig.1. X-ray powder diffraction patterns of Gd(1-x)DyxPO4 Fig.2. SEM image of GdPO4 synthesized by combustion route
Fig.4. FTIR spectra of Gd1-xDyxPO4 microspheres Fig.5. Raman spectra of Gd1-xDyxPO4 microspheres Fig.6. Excitation spectra of Gd1-xDyxPO4 (λem=573nm).
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Fig.7. Emission spectra of Gd1-xDyxPO4 (λexc= 325 nm)
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Fig.3. Particle-size distributions related to the % volume (a) and distribution histogram of Gd(1-x)DyxPO4.
Fig.8. Emission spectra of Gd1-xDyxPO4 under excitation in the energy transfer band (λexc= 274 nm).
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Fig.9. Decay profiles for 4F9/2 level of Dy3+ doped GdPO4 (λexc=326 nm, λem=573nm). Fig.10.The CIE chromatic diagram showing the chromatic coordinates of Gd1-xDyxPO4
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emissions as a function of Dy3+ concentrations and excitation wavelengths.
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140 123 40-2 410
02-3 32-2 132
212
03-2
GP1: 0% Dy
301 23-1
03-1
112 220
20-2
01-2
210
020
11-1
101
01-1
110
10-1
0,5
31-1 221
3+
200 120
1,0
0,0
3+
GP2:0.5% Dy
0,5
3+
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GP3:1% Dy
0,5 0,0
3+
GP4: 5% Dy
0,5 0,0
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Intensity (a. u)
0,0
3+
GP5: 10% Dy
0,5 0,0 20
30
2θ (°)
40
50
60
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Fig. 1. X-ray powder diffraction patterns of Gd(1-x)DyxPO4
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Fig. 2. SEM image of GdPO4 synthesized by combustion route
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6
4
3
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% Volume
5
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1
0
Diameter (µm)
1
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40
30
20
10
100
(b)
GP1
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Frequency
(a)
GP1 GP2 GP3 GP4 GP5
7
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10
0
0
1
2
3
4
Diameter (µm)
5
6
7
Fig.3. Particle-size distributions related to the % volume (a) and distribution histogram of Gd(1-x)DyxPO4.
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3+
GP1: 0%Dy
1,5
3+
GP2: 0.5%Dy 3+
GP3: 1%Dy 1,2
3+
GP4: 5%Dy
3+
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Intensity (a,u)
GP5: 10%Dy
0,9
0,6
ν 2
0,0
1400
1200
ν
ν 1
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Fig. 4. FTIR spectra of Gd1-xDyxPO4 microspheres
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ν1
3+
GP1: 0% Dy
1,2
3+
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GP2: 0.5% Dy 3+
1,0
GP3: 1% Dy
3+
GP5: 10% Dy
0,6
ν2
ν3
ν4
Externel mode
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0,2
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GP4: 5% Dy
600
800
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Fig.5. Raman spectra of Gd1-xDyxPO4 microspheres
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3+ 6
Gd : IJ
1,0
λem = 573 nm
3+
GP2: 0.5% Dy 3+
GP3: 1% Dy (4M21/2+4M19/2+4K17/2)
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(4I11/2)
6 (4M 15/2+ P7/2)
3+
GP5: 10% Dy
4
( I15/2) 4
( F9/2)
4
( G11/2)
0,0 260
280
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320
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0,6
(Gd3+: 6DJ)
Intensity (a.u)
0,8
380
400
420
440
460
Wavelenght (nm)
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Fig. 6. Excitation spectra of Gd1-xDyxPO4 (λem=573nm).
480
500
4
6
F9/2 (558-593) H13/2
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GP2: 0.5% Dy
λex =325 nm
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4
F9/2(465-495) 6H
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GP5: 10% Dy
15/2
0,4
0,2
3+
GP4: 5% Dy
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4
Intensity (a.u)
GP3: 1% Dy
F9/2 (660)
4
550
Wavelenght (nm)
600
650
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Fig. 7. Emission spectra of Gd1-xDyxPO4 (λexc= 325 nm)
6
H11/2 700
λex= 274 nm
4
F9/2 (573)
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H15/2
6
H15/2
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(420)
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400
450
500
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GP3: 1% Dy
6
6
I15/2
F9/2
4
I11/2 (364)
4
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0,4
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GP4: 5% Dy
3+
GP5: 10% Dy
4
F9/2 (658) 6H 11/2
550
600
650
700
Wavelenght (nm)
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Intensity (a. u)
0,6
H13/2
GP2: 0.5% Dy
4
4
(398) I13/2
6
0,8
H15/2
H15/2
1,0
6
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Fig. 8. Emission spectra of Gd1-xDyxPO4 under excitation in the energy transfer band (λexc= 274 nm).
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GP2:0,5% Dy τ = 1.64 ms
0,1
3+
τ = 1.97 ms
3+
τav = 0.38 ms
GP3:1% Dy GP4:5% Dy
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1
3+
GP5:10% Dy
0
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0,01
1
2
τav = 0.27 ms
3
Time (ms)
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Fig. 9. Decay profiles for 4F9/2 level of Dy3+ doped GdPO4 (λexc=326 nm, λem=573nm).
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Fig. 10. The CIE chromatic diagram showing the chromatic coordinates of Gd1-xDyxPO4 emissions as a function of Dy3+ concentrations and excitation wavelengths.
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Highlights - Combustion synthesis of Gd(1-x)DyxPO4 microspheres phosphors for the first time - Identification of phosphors by different techniques
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Single-phase phosphor emitting blue, yellow and white light Effects of Dy3+ concentration and excitation wavelength on Dy3+ emission A potential single-host phosphor for white LEDs
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