- Email: [email protected]
Novel flux-assisted synthesis for enhanced afterglow properties of (Ca,Zn)TiO3:Pr3+ phosphor
Accepted Manuscript Novel flux-assisted synthesis for enhanced afterglow properties of (Ca,Zn)TiO3:Pr phosphor
3+
G. Swati, Swati Bishnoi, Paramjeet Singh, B. Rajesh, Gautam Kumar, Pooja Seth, D. Haranath PII:
S0925-8388(16)34245-1
DOI:
10.1016/j.jallcom.2016.12.316
Reference:
JALCOM 40239
To appear in:
Journal of Alloys and Compounds
Received Date: 9 September 2016 Revised Date:
22 December 2016
Accepted Date: 24 December 2016
Please cite this article as: G. Swati, S. Bishnoi, P. Singh, B. Rajesh, G. Kumar, P. Seth, D. Haranath, 3+ Novel flux-assisted synthesis for enhanced afterglow properties of (Ca,Zn)TiO3:Pr phosphor, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2016.12.316. 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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Novel flux-assisted synthesis for enhanced afterglow properties of (Ca,Zn)TiO3:Pr3+ phosphor
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G. Swatia,b, Swati Bishnoia,b, Paramjeet Singha, B. Rajesha,b, Gautam Kumara, Pooja Sethc and D. Haranath∗a,b a
CSIR-National Physical Laboratory, Dr K S Krishnan Road, New Delhi – 110 012, India. Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, New Delhi110012, India c University School of Basic and Applied Science, Guru Gobind Singh Indraprastha University, New Delhi-110078
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Graphical abstract
Present work focuses on the active role of various chemical fluxes in influencing Pr3+ emission, afterglow, structural and morphological properties of the as-synthesized (Ca,Zn)TiO3:Pr3+ afterglow phosphor. There is no specific rule in selecting appropriate flux for the synthesis of the phosphors hence results are important guidelines for the appropriate selection of fluxes. The mechanism involving the possible trap centers in CaZnTiO3 has also been proposed.
∗
Corresponding author email: [email protected] (D HARANATH).
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Novel flux-assisted synthesis for enhanced afterglow properties of (Ca,Zn)TiO3:Pr3+ phosphor
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G. Swatia,b, Swati Bishnoia,b, Paramjeet Singha, B. Rajesha,b, Gautam Kumara, Pooja Sethc and D. Haranath∗a,b a
CSIR- National Physical Laboratory, Dr. K S Krishnan Road, New Delhi – 110 012, India. Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, New Delhi110012, India c University School of Basic and Applied Science, Guru Gobind Singh Indraprastha University, New Delhi-110078
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Abstract
Selection of chemical fluxes for the phosphor synthesis is largely dependent on trial and error, so a detailed understanding of their selection is obligatory. The active role of various fluxes in influencing Pr3+ emission, afterglow, structural and morphological properties of the assynthesized (Ca,Zn)TiO3:Pr3+ phosphor is less explored. Luminescent properties of the
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(Ca,Zn)TiO3:Pr3+ phosphors show a significant enhancement in Pr3+ emission with the addition of small quantities of fluxes. The persistence of the afterglow has also been increased to 15-30 minutes. Flux-dependent luminescent studies suggest that NH4BF4 is the best suitable flux for
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(Ca0.8Zn0.2)TiO3:Pr3+ phosphor among all others. X-ray diffraction studies confirm the
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orthorhombic phase of CaTiO3, in addition, low intensity peaks from cubic ZnTiO3 phase have also been observed. The calculated Commission Internationale de I’Eclairage (CIE) coordinates for the optimized (Ca0.8Zn0.2)TiO3:Pr3+ phosphor sample is found to be (0.66, 0.33), which is close to the ideal red coordinates with color purity of ~96.39%. Spatial distribution of the
∗
Corresponding author email: [email protected] (D HARANATH).
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Electronic supplementary information (ESI) available: Photoluminescent properties of Pr3+-doped (M0.8Zn0.2)TiO3 (M=Ba, Sr, Ca and Mg) phosphors, photoluminescent properties of (Ca1-xZnx)TiO3 Pr3+ phosphor (x varies from 0 to 0.8) and Field Emission-Scanning Electron Microscopic (FE-SEM) results.
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activator ions was investigated using confocal microscopy. Thermoluminescence studies have been carried out to understand the trapping and detrapping behavior of phosphors. This
storage systems, strategic markings, biological staining etc.
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multifunctional phosphor could serve fascinating applications in areas involving photon energy
Keywords: Afterglow; Chemical fluxes; Titanates, Photoluminescence; Trapping; Confocal
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imaging; Thermoluminescence
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1. Introduction
After the development of aluminate-based green and blue emitting long afterglow phosphors, considerable attention has been shifted towards the advancement of the stable and ideal redemitting phosphors [1-3]. The ever-increasing demand for red-emitting afterglow phosphors has resulted in significant thrust for the development of high-quality phosphors with enhanced
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brightness, afterglow times and appreciable color purity. Red-emitting afterglow phosphors are strongly desirable for several applications, such as safety signages, luminescent paints, interior decorations, and as tracer particle for photodynamic therapy [4]. However, only a limited number
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of stable and ideal red-emitting afterglow phosphors have been known till date. Although some
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sulfides and oxysulfides based phosphors such as Y2O2S:Eu3+ and CaS:Eu2+,Tm3+,Ce3+ show red afterglow luminescence, but their poor chemical stability limits their usage for indoor activities only [5-6]. One of the popular choices for stable and ideal red-emitting phosphors is a ternary metal oxide (ABO3) based host lattices. The remarkable feature of this host lattice is that its structure can flexibly accommodate both larger (A site) and smaller (B site) cations by distorting its ideal cubic structure. CaTiO3 is one of the distinguished members of the ABO3 family and is particularly of great importance due to its wide band gap (~3.7 eV), bio-compatibility, 2
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ferroelectric properties, thermal stability, high electrical permittivity, low dielectric loss, temperature-stable resonant frequency, ferroelastic properties and resistance towards electron
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bombardment [7-9]. Several methods have been employed by various researchers to enhance the emission intensity of CaTiO3:Pr3+ phosphors by incorporating the charge compensators, sensitizers, and
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firing conditions [11-13]. Unfortunately, the luminescence characteristics of CaTiO3:Pr3+ phosphors are not yet optimized, which may be due to many interdependent and critical
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parameters that regulate the afterglow characteristics of the phosphors. Some of these critical parameters include the role of co-activator, flux, sintering atmosphere and temperature. Understanding the roles of crucial parameters affecting luminescence properties is indispensable. Recently research reports reveal that blending of boric acid as a chemical flux along with rareearth dopants has arisen as an effective way to enhance the photoluminescence intensity,
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crystallinity and efficiency of the phosphor [14-15]. Ideally, the flux chemicals should have a melting point much below the solid-state reaction temperature, which when melts allow one or more reaction constituents to mix well before attaining the desired solid-state reaction
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conditions. Eventually, these fluxes increase the reactivity of the constituents by dissolving at
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least one of the reactants and provide a favorable medium to incubate the crystallization of the phosphor [16, 17]. The addition of flux has a great influence on the ion diffusions in the solidstate reaction, particle size distribution, growth condition, crystallization process as well as the formation of target product matrix with good crystallinity [18]. Fluxes not only improve the photon conversion efficiency of trivalent rare-earth ions but also play a significant role in the crystal growth. The improved luminescent intensity can be originated from the local crystal field symmetry breaking around rare-earth ions by flux addition [19]. 3
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Frequently used flux chemicals include alkali halides, alkaline halides, boric acid, boron oxide, ammonium halides, etc. The physical parameters of the flux chemicals that have been used in the current study are listed in Table I. However, the selection of flux chemicals is largely dependent
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on trial and error, and there exists no general rule for the selection of specific flux. As shown in the table, the melting point, boiling point/decomposition temperature spans a wide range of temperatures. NH4F and H3BO3 have lower melting and boiling points so they quickly disappear
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from the system whereas ammonium chloride and bromide remain liquid during the solid-state reaction process [20, 21]. So far to the best of our knowledge, there is no research report
(Ca0.8Zn0.2)TiO3:Pr3+ phosphors.
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available on the detailed influence of chemical fluxes in enhancing the photoluminescence of
In this paper, various chemical fluxes blended Pr3+ doped (Ca0.8Zn0.2)TiO3 phosphors were synthesized using a high-temperature solid-state reaction method to develop efficient red-
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emitting afterglow phosphor. To enhance the luminescence intensity, NH4Cl, NH4F, NH4I, NH4Br and H3BO3, NH4BF4 were used as chemical fluxes in the current study. The structural, morphological and luminescent properties were investigated in detail. The mechanism
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responsible for long afterglow has also been discussed. The current study highlights the selection
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of appropriate chemical flux to improve luminescence and long afterglow characteristics of (Ca0.8Zn0.2)TiO3:Pr3+ phosphors.
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2. Experimental 2.1 Preparation of (Ca,Zn)TiO3:Pr3+ phosphors
assisted
high-temperature
reaction
method.
In
a
typical
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(Ca0.8Zn0.2)TiO3:Pr3+ long afterglow phosphor samples were prepared using a novel fluxexperiment,
high
purity
oxides/carbonates of analytical grade namely, Pr6O11 (99.99%), CaCO3 (99.99%), ZnO (99.99%)
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and TiO2 (99.99%) were taken as starting materials. NH4Cl, NH4F, NH4I, NH4Br, NH4BF4 and H3BO3 were used as chemical fluxes and the concentration of each flux was fixed at 10 mol%.
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Doping concentration of the activator (Pr3+) was fixed at 0.01 mol%. Stoichiometric amounts of oxides/carbonates were taken and ground in agate pestle motor using ethanol as a dispersing medium to ensure homogenous mixing of the starting materials. The ground powder was cast into pellets using a hydraulic press and was sintered at 1275oC in a tubular furnace in ambient atmosphere for three hours. Compact and highly sintered pellets were allowed to cool naturally
2.2. Characterization
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and were ground to obtain the powders for further characterization.
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The phase purity was checked by X-ray powder diffraction (XRD) using Bruker D-8 X-ray diffractometer with Cu Kα radiation operated at 35 kV and 30 mA with a step size of 0.05o. The
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microstructure of the samples was observed using field-emission scanning electron microscope (FE-SEM, Zeiss, model: Supra 40VP). The morphological observations were studied with SEM (Hitachi, Model: S-3700N). The room temperature photoluminescence (PL) and excitation (PLE) spectra were recorded using an Edinburgh Luminescence Spectrometer (Model: F900) fitted with a xenon lamp with a slit width of 0.5 nm and cut off filter of 395 nm. Decay curves were recorded after exciting the samples for about 10 minutes under UV (325 nm) lamp in a dark
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room. Confocal fluorescence microscope (WITec 300M+) having the objective lens of 50X magnification and excitation of UV (325 nm) excitation was used to image the specific chosen area of 100 µm2 of the phosphor samples. The microscope has lateral and vertical resolutions of
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10-25 µm and 40-120 nm, respectively. Confocal microscopy is a powerful tool for spatial mapping of the distribution of rare-earth dopants in the phosphor samples qualitatively. Colorimetric coordinates, (x, y, z) of the phosphors were calculated using equidistant wavelength
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method and compared to the standard Commission Internationale de I’Eclairage (CIE)
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chromaticity coordinates.
3. Results and discussion
3.1. Powder x-ray diffraction (XRD) analysis
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The powder x-ray diffraction (XRD) studies were carried out on (Ca,Zn)TiO3 lattices to investigate the effect of chemical fluxes on phase formation. Fig. 1(a-g) show the XRD profiles of (Ca0.8Zn0.2)TiO3:Pr3+ samples made with chemical fluxes namely, NH4F, NH4Cl, H3BO3,
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NH4Br, NH4I and NH4BF4, each being 10 mol% in concentration. The XRD profiles were compared to standard JCPDS cards #00-039-0190 and 00-022-0153 for cubic ZnTiO3 and
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orthorhombic CaTiO3, respectively for phase identification. The diffraction peaks attributed to orthorhombic CaTiO3 with the space group Pnma (62) were identified in all the samples. Prominent XRD peaks were indexed and are in good agreement with JCPDS card #00-022-0153 with lattice constants a=5.4405 Å, b= 7.6436 Å, c= 5.3812 Å having interfacial angles α=90.00°, β=90.00°, γ=90.00°. However, some additional low-intensity peaks (marked *) indicate the presence of cubic phase of the ZnTiO3 system when compared to JCPDS card #00-039-0190 originating from the solubility limit for Zn2+ ions into the host of CaTiO3. No spurious peaks 6
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were observed due to the addition of chemical fluxes during phosphor synthesis in the diffraction patterns. The orthorhombic structure includes Ti (IV) centers bonded octahedrally and the Ca2+ centers occupy a cage of 12 oxygen centers. Due to similar ionic radii of Ca2+ (1.14 Å) Pr3+
Ti4+ sites, respectively in (Ca0.8Zn0.2)TiO3 lattice.
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(1.13 Å) and Zn2+(0.74 Å)-Ti4+(0.68 Å); Pr3+ and Zn2+ions most likely substitute for Ca2+ and
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Interestingly, the diffraction curves of all the samples show an additional phase of cubic ZnTiO3 except for the sample, where H3BO3 was used as a flux. This might be elucidated as the
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dual role of boric acid i.e. as fluxing and crystallizing agents. As reported earlier, co-doping of B3+ ions into the CaTiO3 lattice might help Zn2+ ions to easily substitute for Ca2+ site rather than forming additional phases [12]. However, from the density functional theory calculations [22], it is well- documented that presence of Zn-related defects increases the multiple localized states of trap levels so as to favor the charge transfer transitions and 1D2→3H4 transition of Pr3+. Thus,
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incorporated ZnTiO3 phase is expected to enhance the defect density leading to longer afterglow times [23]. NH4BF4 decomposes initially into boric acid and ammonium fluoride, and later on to HF and NH3 at lower temperatures. Further, this increase the diffusion rates of CaCO3, ZnO, and
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TiO2 to form a highly crystalline (Ca0.8Zn0.2)TiO3 phosphor. Moreover, it also quickly disappears
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from the system without forming any impurity. As reported earlier, fluoride compounds have higher reactivity towards reactants such as TiO2 as compared to boric acid [24]. Therefore, fluoride compounds tend to get dissolve into the reactants more easily leading to the enhanced crystallinity of the phosphors, which has a constructive effect on the luminescent properties [24]. NH4Cl, NH4Br, and NH4I however, have similar melting and boiling points and remains in a liquid state during the solid-state reaction. Hence, selection of the chemical flux is a crucial factor which determines structural properties of the as-synthesized phosphors. 7
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3.2. Photoluminescence excitation (PLE) and emission (PL) characteristics As mentioned in the introduction section, there exist many interdependent chemical parameters
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that could alter the afterglow properties of the phosphor. To optimize all these parameters, a systematic study has been done by replacing the cation in the position A with Magnesium (Mg), Calcium (Ca), Strontium (Sr) and Barium (Ba). It is well-known that when a rare-earth ion is
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located at a centrosymmetric site, the electric dipole transition within 4f energy levels is forbidden. As Sr ions occupy the centrosymmetric site in SrZnTiO3, the red photoluminescence
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intensity aroused due to Pr-doping is insignificant. In contrast, the absorption intensities are large for Pr-doped BaZnTiO3 and CaZnTiO3 because of the non-centrosymmetric locations of Ba and Ca sites. Detailed results and discussion has been given in the electronic supplementary information¥. Further, a series of Ca1-xZnxTiO3 (where x varies from 0 to 0.8) phosphors were synthesized for optimizing Ca:Zn ratio and the detailed results and discussion has been given in
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the electronic supplementary information¥. Fig. 2(a-b) shows the room temperature photoluminescence excitation and emission spectra of (Ca0.8,Zn0.2)TiO3 afterglow phosphors prepared via solid-state reaction with 10 mol% of fluxes. Excitation spectrum shows three sets of
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distinct bands that have been labeled as A, B, and C in the Fig. 2(a) at around 330 nm, 370 nm,
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and narrow peaks near 463 nm, respectively. The 330 nm (A) band represents the transition from valence-to-conduction band of the host lattice (from O- 2p levels to Ti-3d levels) followed by energy transfer to the excited 4f levels of Pr3+ ions. Boutinaud et al. [29] have proposed that the band (B) at 370 nm arises out of a charge transfer state (CTS) from Pr3+ to Ti4+ metal ions. The minor peaks between 450 and 495 nm are assigned to 4f→4f transitions of Pr3+ ions. More precisely peaks near 468.3 and 473.6 nm are originated from 3P1→3H4 transitions, and at ~495.2
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nm is due to
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P0→3H4 transition. The phenomenon of observing more than one peak
corresponding to one transition is expected to be produced by crystal field splitting [12].
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Phosphor made with ammonium fluoride (NH4F) as flux shows enhanced B band as a consequence of which there is an enhanced interaction of charge transfer state (CTS) with both the 3P0 and 1D2 states of Pr3+. Thus, all photo-electrons relax directly to the meta-stable excited D2 state whereas, other samples show weak charge transfer related transition (B band). Fig. 2(b)
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shows the emission spectra of the samples made with different fluxes, measured between 560
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and 650 nm. As reported earlier [25], due to the limited substitution of oxygen with fluorine, free electrons could be generated that could reduce titanium ions from Ti4+ to Ti3+ state, which might be beneficial for the luminescence properties. PL emission spectra of (Ca0.8Zn0.2)TiO3:Pr3+ phosphors registered at various excitation wavelengths ranging 325 to 500 nm are shown in Fig. 2(c). For all excitations, the PL emission is centered at 610 nm and the maximum intensity is
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observed for 325 nm excitation. Fig. 2 (d) shows the relative emission intensities of the phosphor samples synthesized using different fluxes. It could be noted that the luminescence intensity of sample made with NH4BF4 as chemical flux significantly enhanced by 2.5 times. Hence, it is
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understood that the chemical flux accelerates the kinetics of the phosphor formation by
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enhancing the diffusion coefficient and out of all fluxes under consideration, NH4BF4 was found to be the most suitable for synthesis of (Ca0.8Zn0.2)TiO3:Pr3+ afterglow phosphors. All the samples show a typical red emission at 610 nm, which originates from the 1D2→3H4 transition of Pr3+. It is clearly seen that the emission intensity of sample made with NH4BF4 as chemical flux has a maximum intensity as compared to other samples. Since 4f→4f transitions of Pr3+ are not dependent on the local host surrounding, no such considerable shift in emission spectrum has
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been observed. Overall a positive response has been observed for all the samples made with various fluxes in terms of improvement in their emission intensities. Internationale
de
I’Eclairage
(CIE)
color
coordinates
of
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(Ca0.8Zn0.2)TiO3:Pr3+ phosphor has been calculated using equidistant wavelength method and is indicated in the CIE chromaticity diagram shown in Fig. 3. These coordinates are highly useful
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in determining the exact emission color and color purity of the samples, as per the CIE chromaticity diagram [26]. The obtained (x=0.66, y=0.33) values are appreciably comparable to
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the ideal red coordinates (x=0.67, y=0.33) as suggested by National Television Standard Committee (NTSC). Also, the color purity of the PL emission has been calculated using the following equation:
( − ) + ( − ) ( − ) + ( − )
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=
where, (xs, ys) and (xi, yi) are the color coordinates of the light source and the CIE illuminant, respectively, and (xd, yd) color coordinates of the dominant wavelength. Calculated results
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indicate that the color purity of the phosphor sample is ~96.39%.
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In addition, the 2D distribution of dopant in host matrix was examined using confocal mapping, recorded under the excitation 325 nm diode laser. Results shown in Fig. 3(a) and (b), indicate the uniform distribution of the dopant throughout the lattice and high CCD counts reveals that the synthesized phosphor is highly luminescent. The inset shows the corresponding optical image of the phosphor for the area under consideration.
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3.3. Surface morphology and compositional analysis Compositional analysis of the phosphors was carried out using energy-dispersive X-ray
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spectroscopy (EDAX), which shows that the as-synthesized samples comprise of Calcium, Zinc, Titanium and Oxygen and no other spurious impurity elements are detected as shown in Fig. 4(a). No peak associated with flux material was detected which again confirms that fluxes do not
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leave any residue during the course of the reaction. Since the doping concentration of praseodymium is very low, its presence was also not detected in EDAX analysis.
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Fig. 4(b) represents the typical Scanning Electron Microscope (SEM) image of (Ca0.8Zn0.2)TiO3:Pr3+ phosphor synthesized using the optimal amount of chemical flux i.e. NH4BF4. Observed SEM image shows an irregular shape and uneven distribution of particles in micrometer (µm) range. Total photoluminescent intensity and afterglow time increases with the
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increase of particle size because the non-radiative recombination centers are reduced monotonically with particle enlargement [27]. SEM image exhibits the highly sintered and melted morphology of clusters of particles. Besides melting point or decomposition temperature,
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other parameters of fluxes such as solubility of raw materials, ionic radii of anion/cation, as well as purity of flux, may also have a significant effect on size, morphology, and thus, on the
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luminescent properties of materials [18]. For higher resolution, Field Emission-SEM studies have also been carried out at a magnification of 12.14 kX, as shown in Fig. 4(c). Aggregated particles of micrometer size with melted morphology have been observed in the sample made with NH4BF4 as chemical flux. Observed particles are not uniform in size and having irregular morphology are mainly due to high-temperature reaction conditions.
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3.4 Afterglow decay curves Fig. 5(a) shows the afterglow spectra of the phosphors synthesized using different chemical
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fluxes, recorded at a constant excitation of 325 nm. The results show that the chemical flux influences the afterglow characteristics of the phosphors to a great extent. Since the intensity remains constant after an initial drop, afterglow curve was recorded for a limited time only.
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Phosphor sample synthesized using NH4BF4 as chemical flux shows longer afterglow time as compared to the other samples, which can be seen from decay curves and the corresponding
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optical image shown in Fig. 5(b). The reason might be that the limited substitution of oxygen with fluorine can change the oxidation state of titanium and/or praseodymium. This might have altered the luminescence decay properties by creating intermediate shallow trap levels and/or increasing the defect density [25]. Also, there are two bivalent cation positions, Ca2+ and Zn2+, in the crystal that can be replaced by Pr3+. Oxygen vacancies arise when trivalent Pr3+ ions are
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substituted in the site of the divalent Ca2+ ions and Zn2+ sites to self-compensate the charge. Due to the low melting point of the fluoride-based flux, they form liquid medium across powders, hence diffusion of the constituent ions became easier across the solid-liquid interface. Thus,
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more Ca2+ and Zn2+ ions in the crystal can be easily replaced by Pr3+ and thereby increase the
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defect density by the generation of more oxygen vacancies due to the self-compensation action of the host lattice to maintain neutrality [13]. Fig. 5(b) shows the optical image showing luminescence pattern of the samples synthesized with different chemical fluxes recorded under 325 nm excitation and corresponding afterglow pattern in dark.
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3.5. Thermoluminescence (TL) measurements The decay characteristics of phosphors are determined by the traps generated within the
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forbidden band gap of the materials. The trap depth and trap density influence the afterglow properties of the phosphors. Fig. 6 represents the thermoluminescence glow curve of the phosphor samples made with different chemical fluxes, recorded after UV (325 nm) irradiation
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for 5 minutes. For any practical application involving long afterglow at room temperature, it is necessary to have suitable trap-creating dopants that could have a TL peak in the region 320–
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380K [28]. Accordingly the as-prepared (Ca0.8Zn0.2)TiO3:Pr3+ phosphors show room-temperature afterglow properties in the above TL region.
Glow curves exhibit a broad peak in the range 350-500 K with peak maxima at 370, 354, 352, 347,350 and 352K for the samples blended with NH4BF4, NH4F, H3BO3, NH4Br, NH4Cl,
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and NH4I, respectively. The results are consistent with the afterglow decay patterns as mentioned in section 3.4. Phosphor made with NH4BF4 as chemical flux shows the glow curve maxima at relatively higher temperature indicating deeper traps and shows longer afterglow times. As
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reported earlier, these traps are ascribed to oxygen vacancies or associations of oxygen vacancies. Pr3+ dopant itself can act as a trapping center and is responsible for a TL signal
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peaking at 343 K in CaTiO3:Pr3+ [29]. However, samples synthesized with NH4Cl and NH4I exhibited weak TL luminescence and peaks at a lower temperature, which represents the formation of shallow traps (surface trapping) inside the material. 3.6 Mechanism of afterglow luminescence A vital role is played by energy levels in the band gap of the host material that has been introduced by point defects in the crystal lattice or via co-doping. These defect related energy 13
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levels are known as trapping centers and are able to capture charge carriers originating from the luminescent ions. These charge carriers remain trapped in these defect related energy levels until they acquire enough thermal energy to escape and recombine at a luminescent center. The
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activation energy required for this is called the trap depth and is supposedly determined by the energy difference between the energy level of the trap and the conduction band or the valence band. If the trap is too shallow, it will result in a very short afterglow time, whereas if the trap is
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too deep then no charge carriers can escape at room temperature. Thus, trap depth is a very crucial parameter in determining the afterglow properties of the phosphor. Deeper traps (still
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shallow enough to be excited at room temperature) and a higher concentration of trap centers display longer afterglow than shallow trap centers.
There are two bivalent cation positions, Ca2+ and Zn2+, in the crystal that can be replaced by Pr3+ to form two different kinds of point defects. Pr3+ ions will occupy Ca2+ ion sites, and due
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to the necessity of charge compensation, two Pr3+ ions will replace three Ca2+ ions, resulting in a Ca2+ vacancy (Vca). The net negative environment at this site is capable of trapping positive charge carriers. Oxygen vacancies are the major contributors to the phosphorescence behavior of
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many oxide-based phosphors. The phosphorescence behavior of the perovskite CaTiO3:Pr3+ is
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attributed to oxygen vacancies that arise when trivalent Pr3+ ions are substituted in the site of the divalent Ca2+ ions [29-31]. These vacancies are attributed to self-compensation of the material, and they have the ability to attract negative carriers. As suggested by Aitasalo et al. [32] when a long afterglow phosphor is excited, the energy is provided directly to the traps of unspecified origin. Electron and hole pairs most likely get self-trapped or acquire the stability at defect centers. The electron is removed from the trap level by thermal energy (kT) and ends up at an positive oxygen vacancy (Vo) related trap states whereas the hole migrates back to valance band 14
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and reaches traps related to negatively charged calcium vacancies (Vca) and as reported Pr3+ can also act as a hole trapping center since position of the 3H4 ground state of Pr3+ ions close to the top of the VB of CaTiO3. It is assumed that the energy released on recombination of the electron
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and the hole was delivered directly to the europium ions, by means of energy transfer (ET). From the literature, trap depth of the point defect generated due to zinc-substituted Pr3+ is deeper than that of defects form by calcium-substituted Pr3+, leading to the longer phosphorescence lifetime
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of (Ca0.8Zn0.2)TiO3:Pr3+ than the CaTiO3:Pr3+ phosphor [21].
4. Conclusions
Since there is no specific rule in selecting appropriate chemical flux for the synthesis of the phosphors, the influence of various fluxes on structural, luminescent, morphological and decay
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properties of the phosphor should be understood. Thus, various chemical flux involved redemitting long afterglow (Ca0.8Zn0.2)TiO3:Pr3+ phosphors have been prepared in the current study. Detailed optical, structural and morphological characterization of the samples has been studied in
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detail. The XRD patterns confirm the pure orthorhombic phase of the (Ca0.8Zn0.2)TiO3:Pr3+ phosphor made with boric acid as flux whereas additional phase of cubic ZnTiO3 was observed
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for rest of the samples. SEM studies reveal that the size of the particles is in the micrometer range with non-uniform and melted morphology. The photoluminescent studies exhibit characteristics emission transitions of Pr3+, i.e. 1D2→3H4 under UV (325 nm) excitation. Fluxdependent luminescent studies suggest that NH4BF4 flux is the best suitable flux for (Ca0.8Zn0.2)TiO3:Pr3+ phase among all other fluxes. The calculated CIE coordinates for the optimized (Ca0.8Zn0.2)TiO3:Pr3+ sample is found to be (0.66,0.33), which are close to the ideal
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red coordinates. Afterglow decay curves indicate that both the initial emission intensity and afterglow time have been enhanced by the addition of flux. A mechanism involving the possible trap centers that might exist in (Ca0.8Zn0.2)TiO3:Pr3+ phase for capturing the charge carriers have
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been studied. Thus, these results are important guidelines for the appropriate selection of fluxes
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in the synthesis of long afterglow phosphors.
Acknowledgments
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The authors (GS and SB) sincerely acknowledge the Council of Scientific and Industrial Research (CSIR), Government of India under the Senior Research Fellowship schemes #31/1(448)/2015-EMR-1 and 31/1(445)/2015-EMR-1, respectively and Academy of Scientific and Innovative Research (AcSIR) to carry out the research work. The author (DH) is grateful for
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the financial support from Board of Research in Nuclear Sciences (BRNS), Government of India for the research project #34/14/16/2016-BRNS/34041. Authors (GS and PS) wish to thank Mr.
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Delhi.
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Birender Singh for assisting in recording the thermoluminescence (TL) results at IUAC, New
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References [1] Koen Van den Eeckhout, Dirk Poelman, Philippe F. Smet, Persistent luminescence in nonEu2+-doped compounds: A review. Materials 6 (2013) 2789-2818.
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[2] S. K. Singh, Near-infrared persistent luminescence nano-probes for bioimaging and targeting applications. RSC. Adv. 4 (2014) 58674-58698.
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[3] Yixi Zhuang, Yumiko Kata Yama, Jumpei Ueda, Setsuhisa Tanabe, A brief review on red to near-infrared persistent luminescence in transition-metal-activated phosphors. Opt. Mater. 36 (2014) 1907–1912.
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[4] Wei Zeng, Yuhua Wang, Shaochun Han, Wenbo Chen, Gen Li, Yanzhao Wang, Yan Wen, Design, synthesis and characterization of a novel yellow long-persistent phosphor: Ca2BO3Cl:Eu2+,Dy3+. J. Mater. Chem. C. 1 (2013) 3004-3011. [5] Yang Li, Mindaugas Gecevicius, Jianrong Qiu, Long persistent fundamentals to applications. Chem. Soc. Rev. 45 (2016) 2090-2136.
phosphors—from
[6] Diana C. Rodríguez Burbano, Suchinder K. Sharma, Pieter Dorenbos, Bruno Viana, John A. Capobianco, Persistent and photostimulated red emission in CaS:Eu2+,Dy3+ nanophosphors. Adv. Opt. Mater. 3 (2015) 551–557.
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[7] Chong Peng, Zhiyao Hou, Cuimiao Zhang, Guogang Li, Hongzhou Lian, Ziyong Cheng, Jun Lin, Synthesis and luminescent properties of CaTiO3:Pr3+ microfibers prepared by electrospinning method. Opt. Express 18 (2010) 7543-7553.
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[8] Yinhui Zhu, Xiaokang Wang, Yilong Zhou, Cuilian Zhao, Jianmin Yuan, Zhenjun Wu, Song Wu, Shuangyin Wang, In situ formation of bioactive calcium titanate coatings on titanium screws for medical implants. RSC Adv. 6 (2016) 53182-53187.
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[9] Bo Wang, Hang Lin, Ju Xu, Hui Chen, Zebin Lin, Feng Huang, Yuansheng Wang, Design, preparation, and characterization of a novel red long persistent perovskite phosphor: Ca3Ti2O7:Pr3+. Inorg. Chem. 54 (2015) 11299−11306. [10] David R Lide, CRC Handbook of Chemistry, 88 Ed. (2007) pages 4-43 to 4-100. [11] Ailing Zhu, Jianchuan Wang, Yong Du, Dongdong Zhao, Qiannan Gao, Effects of Zn impurities on the electronic properties of Pr-doped CaTiO3.Physica. Physica. 5 (2011) 849–854. [12] Huang Ping, Wu Yaqiao, Cui Cai’e, Guo Jin, Effect of different dopants on the long lasting phosphorescence behavior of CaTiO3:Pr3+. J. Rare Earths 4 (2011) 340-344.
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[13] L. L. Noto, S. S. Pitale, M. A. Gusowki, J. J. Terblans, O. M. Ntwaeaborwa, H. C. Swart Afterglow enhancement with In3+ codoping in CaTiO3:Pr3+ red phosphor. Powder Technol. 237 (2013) 141–146.
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[14] D. Haranath, P. Sharma, H. Chander, A. Ali, N. Bhalla, S. K. Halder, Role of boric acid in the synthesis and tailoring the properties of calcium aluminate phosphor. Mater. Chem. Phys. 101 (2007) 163–169.
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[15] Songhak Yoon, Jakob Bierwagen, Matthias Trottmann, Bernhard Walfort, Nando Gartmann, Anke Weidenkaff, Hans Hagemann, Simone Pokrant, The influence of boric acid on improved persistent luminescence and thermal oxidation resistance of SrAl2O4:Eu2+. J. Lumin. 167 (2015) 126–131.
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[16] Hyun Ju lee, Kyung Pickim, Gun Young Hong, Jae Soo Yoo, The effect of flux materials on the physical and optical properties of Eu3+ activated yttrium oxide phosphors. J. Lumin. 130 (2010) 941-946. [17] Haoyi Wu, Yihua Hu Wei Zhang, He Duan, Li Chen, Xiao Juan Wang, Flux's influence on the luminescent properties of europium and dysprosium codoped melilite long afterglow phosphor. J. Non-Cryst. Solids 358 (2012) 2734–2740.
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[18] Amit K. Vishwakarma, M. Jayasimhadri, Significant enhancement in photoluminescent properties via flux-assisted Eu3+ doped BaNb2O6 phosphor for white LEDs. J. Alloys Compd. 683 (2016) 379-386.
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[19] Cássio C. S. Pedroso, José M. Carvalho, Lucas C. V. Rodrigues, Jorma Hölsä, Hermi F. Brito, Rapid and energy-saving microwave-assisted solid-state synthesis of Pr3+, Eu3+, or Tb3+ doped Lu2O3 persistent luminescence materials, ACS Appl. Mater. Interfaces 8 (2016) 19593– 19604.
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[20] Han Bin, Wang Yi-Fei, Liu Qian, Huang Qing, Role of fluxes in the synthesis and luminescence properties of Ba3Si6O12N2:Eu2+ oxynitride phosphors by microwave sintering. Int. J. Inorg. Mater. 31 (2016) 652-659. [21] Lihong Liu, Rong-Jun Xie, Chenning Zhang, Naoto Hirosaki, Role of fluxes in optimizing the optical properties of Sr0.95Si2O2N2:0.05Eu2+ green-emitting phosphor. Materials 6 (2013) 2862-2872. [22] Shixun Lian, Yuan Qi, Chunying Rong, Liping Yu, Ailing Zhu, Dulin Yin, Shubin Liu, effectively leveraging solar energy through persistent dual red phosphorescence: preparation, characterization, and density functional theory study of Ca2Zn4Ti16O38:Pr3+. J. Phys. Chem. C 114 (2010) 7196–7204.
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[23] Sung Mook Chung, Seung-Youl Kang, Jae-Heon Shin, Woo-Seok Cheong, Chi-Sun Hwang, Kyoung Ik Cho, Seung Jae Lee, Young Jin Kim, The effects of zinc on the characteristics of (Zn,Ca)TiO3:Pr3+ phosphors. J. Cryst. Growth 326 (2011) 94–97.
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[24] Pengpeng Dai, Xintong Zhang, Panpan Sun, Jikai Yang, Lingling Wang, Shili Yan, Yichun Liu, Influence of flux on morphology and luminescence properties of phosphors: A case study on Y1.55Ti2O7:0.45Eu3+ J. Am. Ceram. Soc. 95 (2012) 1447–1453.
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[25] Songhak Yoon, Eugenio H. Otal, Alexandra E. Maegli, Lassi Karvonen, Santhosh K. Matam, Stefan G. Ebbinghaus, Bernhard Walfort, Hans Hagemann, Simone Pokrant, Anke Weidenkaff, the Improved persistent luminescence of CaTiO3:Pr by fluorine substitution and thermochemical treatment. J. Alloys Compd. 613 (2014) 338–343. [26] J. Kelmer, Luminescent Screens: Photometry and Colorimetry, Iliffe, London, (1969) p.118.
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[27] Xianmin Zhang, Jiahua Zhang, Xia Zhang, Meiyuan Wang, Haifeng Zhao, Shaozhe Lu, Xiao-Jun Wang, Size manipulated photoluminescence and phosphorescence in CaTiO3:Pr3+ nanoparticles. J. Phys. Chem. C 111 (2007) 18044-18048. [28] Xigang Wang, Yanlin Huang, Hyo Jin Seo, A new long-lasting phosphor Zr4+ and Eu3+ codoped SrMg2(PO4)2. Sens. Actuators, B: Chemical 158 (2011) 171-175.
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[29] P. Boutinaud, L. Sarakha, E. Cavalli, M. Bettinelli, P. Dorenbos, R. Mahiou, About red afterglow in Pr3+ doped titanate perovskites J. Phys. D: Appl. Phys. 42 (2009) 045106 (7pp). [30] P. Boutinaud, E. Pinel, M. Dubois, A. P. Vink, R. Mahiou, UV-to-red relaxation pathways in CaTiO3:Pr3+. J. Lumin. 111 (2005) 69-80.
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[31] P. J. Deren, R. Pazik, W. Strek, P. Boutinaud, R. Mahiou, Synthesis and spectroscopic properties of CaTiO3 nanocrystals doped with Pr3+ ions. J. Alloys compd. 451 (2008) 595-599.
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[32] T. Aitasalo, P. Deren, J. Hölsä, H. Jungner, J.C. Krupa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, W. Strek, Persistent luminescence phenomena in materials doped with rare-earth ions. J. Solid State Chem. 171 (2003) 114–122.
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Table 1: Physical parameters of the flux materials used in the present work [10] Melting point (oC)
Boiling point (oC)
Density (g/cm3)
H3BO3
171
300
1.5
2.
NH4F
238
Decomposition
1.0
3.
NH4BF4
230
Decomposition
1.8
4.
NH4Cl
338
520
1.5
5.
NH4Br
235
452
6.
NH4I
551
405
5.7
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1.
85.3
25.83 39.5
2.4
78.3
2.5
172.0
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Solubility in water at 20oC (g/100 ml)
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S. No. Flux
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Fig. 1: X-ray diffraction patterns of (Ca0.8Zn0.2)TiO3:Pr3+ phosphors made using various
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chemical fluxes namely (a) NH4I, (b) NH4Cl, (c) NH4Br, (d) NH4BF4, (e) NH4F, (f) H3BO3, (g) standard JCPDS card no. 00-039-0190 of ZnTiO3 and (h) standard JCPDS card no. 00-
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022-0153 of CaTiO3.
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Fig. 2: (a) Photoluminescence excitation and (b) emission spectra of the phosphor samples prepared with different fluxes. (c) Emission spectra recorded at different excitation wavelengths and (d) relative emission intensities of the phosphors
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Fig. 3: (a) CIE chromaticity diagram indicating the color coordinates and (b) confocal microscopy image of the optimized phosphor made with NH4BF4 flux, the inset shows the
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optical image of the sample at the scale of 10 µm.
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Fig. 4: (a) EDAX pattern showing compositional analysis, (b) SEM image and (c)FE-SEM
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image depicting the microstructure of as-synthesized (Ca0.8Zn0.2)TiO3:Pr3+ phosphor.
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Fig. 5: (a) Afterglow decay curves of the phosphors made with different fluxes and excited
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under UV (325 nm) and (b) the digital photographs of samples taken under UV (325 nm)
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excitation and their afterglow at different time intervals.
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Fig. 6: Thermoluminescence glow curves of the (Ca0.8Zn0.2)TiO3:Pr3+ phosphors made with
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different fluxes and exciting the samples at UV (325 nm).
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phosphor.
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Fig. 7: Probable energy band model depicting afterglow mechanism of (Ca0.8Zn0.2)TiO3:Pr3+
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Highlights of this work: 1. Facile technique to produce red-emitting afterglow phosphor using different fluxes. 2. Active role of each flux in influencing Pr3+ emission and afterglow was studied. 3. Achieved efficient red PL with spectral color purity of ~96.39%.
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4. Achieved afterglow time variation of 15-30 minutes depending on flux used.
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G. Swati, Swati Bishnoi, Paramjeet Singh, B. Rajesh, Gautam Kumar, Pooja Seth, D. Haranath PII:
S0925-8388(16)34245-1
DOI:
10.1016/j.jallcom.2016.12.316
Reference:
JALCOM 40239
To appear in:
Journal of Alloys and Compounds
Received Date: 9 September 2016 Revised Date:
22 December 2016
Accepted Date: 24 December 2016
Please cite this article as: G. Swati, S. Bishnoi, P. Singh, B. Rajesh, G. Kumar, P. Seth, D. Haranath, 3+ Novel flux-assisted synthesis for enhanced afterglow properties of (Ca,Zn)TiO3:Pr phosphor, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2016.12.316. 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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Novel flux-assisted synthesis for enhanced afterglow properties of (Ca,Zn)TiO3:Pr3+ phosphor
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G. Swatia,b, Swati Bishnoia,b, Paramjeet Singha, B. Rajesha,b, Gautam Kumara, Pooja Sethc and D. Haranath∗a,b a
CSIR-National Physical Laboratory, Dr K S Krishnan Road, New Delhi – 110 012, India. Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, New Delhi110012, India c University School of Basic and Applied Science, Guru Gobind Singh Indraprastha University, New Delhi-110078
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b
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Graphical abstract
Present work focuses on the active role of various chemical fluxes in influencing Pr3+ emission, afterglow, structural and morphological properties of the as-synthesized (Ca,Zn)TiO3:Pr3+ afterglow phosphor. There is no specific rule in selecting appropriate flux for the synthesis of the phosphors hence results are important guidelines for the appropriate selection of fluxes. The mechanism involving the possible trap centers in CaZnTiO3 has also been proposed.
∗
Corresponding author email: [email protected] (D HARANATH).
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Novel flux-assisted synthesis for enhanced afterglow properties of (Ca,Zn)TiO3:Pr3+ phosphor
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G. Swatia,b, Swati Bishnoia,b, Paramjeet Singha, B. Rajesha,b, Gautam Kumara, Pooja Sethc and D. Haranath∗a,b a
CSIR- National Physical Laboratory, Dr. K S Krishnan Road, New Delhi – 110 012, India. Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, New Delhi110012, India c University School of Basic and Applied Science, Guru Gobind Singh Indraprastha University, New Delhi-110078
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b
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Abstract
Selection of chemical fluxes for the phosphor synthesis is largely dependent on trial and error, so a detailed understanding of their selection is obligatory. The active role of various fluxes in influencing Pr3+ emission, afterglow, structural and morphological properties of the assynthesized (Ca,Zn)TiO3:Pr3+ phosphor is less explored. Luminescent properties of the
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(Ca,Zn)TiO3:Pr3+ phosphors show a significant enhancement in Pr3+ emission with the addition of small quantities of fluxes. The persistence of the afterglow has also been increased to 15-30 minutes. Flux-dependent luminescent studies suggest that NH4BF4 is the best suitable flux for
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(Ca0.8Zn0.2)TiO3:Pr3+ phosphor among all others. X-ray diffraction studies confirm the
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orthorhombic phase of CaTiO3, in addition, low intensity peaks from cubic ZnTiO3 phase have also been observed. The calculated Commission Internationale de I’Eclairage (CIE) coordinates for the optimized (Ca0.8Zn0.2)TiO3:Pr3+ phosphor sample is found to be (0.66, 0.33), which is close to the ideal red coordinates with color purity of ~96.39%. Spatial distribution of the
∗
Corresponding author email: [email protected] (D HARANATH).
¥
Electronic supplementary information (ESI) available: Photoluminescent properties of Pr3+-doped (M0.8Zn0.2)TiO3 (M=Ba, Sr, Ca and Mg) phosphors, photoluminescent properties of (Ca1-xZnx)TiO3 Pr3+ phosphor (x varies from 0 to 0.8) and Field Emission-Scanning Electron Microscopic (FE-SEM) results.
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activator ions was investigated using confocal microscopy. Thermoluminescence studies have been carried out to understand the trapping and detrapping behavior of phosphors. This
storage systems, strategic markings, biological staining etc.
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multifunctional phosphor could serve fascinating applications in areas involving photon energy
Keywords: Afterglow; Chemical fluxes; Titanates, Photoluminescence; Trapping; Confocal
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imaging; Thermoluminescence
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1. Introduction
After the development of aluminate-based green and blue emitting long afterglow phosphors, considerable attention has been shifted towards the advancement of the stable and ideal redemitting phosphors [1-3]. The ever-increasing demand for red-emitting afterglow phosphors has resulted in significant thrust for the development of high-quality phosphors with enhanced
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brightness, afterglow times and appreciable color purity. Red-emitting afterglow phosphors are strongly desirable for several applications, such as safety signages, luminescent paints, interior decorations, and as tracer particle for photodynamic therapy [4]. However, only a limited number
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of stable and ideal red-emitting afterglow phosphors have been known till date. Although some
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sulfides and oxysulfides based phosphors such as Y2O2S:Eu3+ and CaS:Eu2+,Tm3+,Ce3+ show red afterglow luminescence, but their poor chemical stability limits their usage for indoor activities only [5-6]. One of the popular choices for stable and ideal red-emitting phosphors is a ternary metal oxide (ABO3) based host lattices. The remarkable feature of this host lattice is that its structure can flexibly accommodate both larger (A site) and smaller (B site) cations by distorting its ideal cubic structure. CaTiO3 is one of the distinguished members of the ABO3 family and is particularly of great importance due to its wide band gap (~3.7 eV), bio-compatibility, 2
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ferroelectric properties, thermal stability, high electrical permittivity, low dielectric loss, temperature-stable resonant frequency, ferroelastic properties and resistance towards electron
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bombardment [7-9]. Several methods have been employed by various researchers to enhance the emission intensity of CaTiO3:Pr3+ phosphors by incorporating the charge compensators, sensitizers, and
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firing conditions [11-13]. Unfortunately, the luminescence characteristics of CaTiO3:Pr3+ phosphors are not yet optimized, which may be due to many interdependent and critical
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parameters that regulate the afterglow characteristics of the phosphors. Some of these critical parameters include the role of co-activator, flux, sintering atmosphere and temperature. Understanding the roles of crucial parameters affecting luminescence properties is indispensable. Recently research reports reveal that blending of boric acid as a chemical flux along with rareearth dopants has arisen as an effective way to enhance the photoluminescence intensity,
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crystallinity and efficiency of the phosphor [14-15]. Ideally, the flux chemicals should have a melting point much below the solid-state reaction temperature, which when melts allow one or more reaction constituents to mix well before attaining the desired solid-state reaction
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conditions. Eventually, these fluxes increase the reactivity of the constituents by dissolving at
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least one of the reactants and provide a favorable medium to incubate the crystallization of the phosphor [16, 17]. The addition of flux has a great influence on the ion diffusions in the solidstate reaction, particle size distribution, growth condition, crystallization process as well as the formation of target product matrix with good crystallinity [18]. Fluxes not only improve the photon conversion efficiency of trivalent rare-earth ions but also play a significant role in the crystal growth. The improved luminescent intensity can be originated from the local crystal field symmetry breaking around rare-earth ions by flux addition [19]. 3
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Frequently used flux chemicals include alkali halides, alkaline halides, boric acid, boron oxide, ammonium halides, etc. The physical parameters of the flux chemicals that have been used in the current study are listed in Table I. However, the selection of flux chemicals is largely dependent
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on trial and error, and there exists no general rule for the selection of specific flux. As shown in the table, the melting point, boiling point/decomposition temperature spans a wide range of temperatures. NH4F and H3BO3 have lower melting and boiling points so they quickly disappear
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from the system whereas ammonium chloride and bromide remain liquid during the solid-state reaction process [20, 21]. So far to the best of our knowledge, there is no research report
(Ca0.8Zn0.2)TiO3:Pr3+ phosphors.
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available on the detailed influence of chemical fluxes in enhancing the photoluminescence of
In this paper, various chemical fluxes blended Pr3+ doped (Ca0.8Zn0.2)TiO3 phosphors were synthesized using a high-temperature solid-state reaction method to develop efficient red-
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emitting afterglow phosphor. To enhance the luminescence intensity, NH4Cl, NH4F, NH4I, NH4Br and H3BO3, NH4BF4 were used as chemical fluxes in the current study. The structural, morphological and luminescent properties were investigated in detail. The mechanism
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responsible for long afterglow has also been discussed. The current study highlights the selection
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of appropriate chemical flux to improve luminescence and long afterglow characteristics of (Ca0.8Zn0.2)TiO3:Pr3+ phosphors.
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2. Experimental 2.1 Preparation of (Ca,Zn)TiO3:Pr3+ phosphors
assisted
high-temperature
reaction
method.
In
a
typical
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(Ca0.8Zn0.2)TiO3:Pr3+ long afterglow phosphor samples were prepared using a novel fluxexperiment,
high
purity
oxides/carbonates of analytical grade namely, Pr6O11 (99.99%), CaCO3 (99.99%), ZnO (99.99%)
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and TiO2 (99.99%) were taken as starting materials. NH4Cl, NH4F, NH4I, NH4Br, NH4BF4 and H3BO3 were used as chemical fluxes and the concentration of each flux was fixed at 10 mol%.
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Doping concentration of the activator (Pr3+) was fixed at 0.01 mol%. Stoichiometric amounts of oxides/carbonates were taken and ground in agate pestle motor using ethanol as a dispersing medium to ensure homogenous mixing of the starting materials. The ground powder was cast into pellets using a hydraulic press and was sintered at 1275oC in a tubular furnace in ambient atmosphere for three hours. Compact and highly sintered pellets were allowed to cool naturally
2.2. Characterization
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and were ground to obtain the powders for further characterization.
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The phase purity was checked by X-ray powder diffraction (XRD) using Bruker D-8 X-ray diffractometer with Cu Kα radiation operated at 35 kV and 30 mA with a step size of 0.05o. The
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microstructure of the samples was observed using field-emission scanning electron microscope (FE-SEM, Zeiss, model: Supra 40VP). The morphological observations were studied with SEM (Hitachi, Model: S-3700N). The room temperature photoluminescence (PL) and excitation (PLE) spectra were recorded using an Edinburgh Luminescence Spectrometer (Model: F900) fitted with a xenon lamp with a slit width of 0.5 nm and cut off filter of 395 nm. Decay curves were recorded after exciting the samples for about 10 minutes under UV (325 nm) lamp in a dark
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room. Confocal fluorescence microscope (WITec 300M+) having the objective lens of 50X magnification and excitation of UV (325 nm) excitation was used to image the specific chosen area of 100 µm2 of the phosphor samples. The microscope has lateral and vertical resolutions of
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10-25 µm and 40-120 nm, respectively. Confocal microscopy is a powerful tool for spatial mapping of the distribution of rare-earth dopants in the phosphor samples qualitatively. Colorimetric coordinates, (x, y, z) of the phosphors were calculated using equidistant wavelength
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method and compared to the standard Commission Internationale de I’Eclairage (CIE)
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chromaticity coordinates.
3. Results and discussion
3.1. Powder x-ray diffraction (XRD) analysis
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The powder x-ray diffraction (XRD) studies were carried out on (Ca,Zn)TiO3 lattices to investigate the effect of chemical fluxes on phase formation. Fig. 1(a-g) show the XRD profiles of (Ca0.8Zn0.2)TiO3:Pr3+ samples made with chemical fluxes namely, NH4F, NH4Cl, H3BO3,
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NH4Br, NH4I and NH4BF4, each being 10 mol% in concentration. The XRD profiles were compared to standard JCPDS cards #00-039-0190 and 00-022-0153 for cubic ZnTiO3 and
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orthorhombic CaTiO3, respectively for phase identification. The diffraction peaks attributed to orthorhombic CaTiO3 with the space group Pnma (62) were identified in all the samples. Prominent XRD peaks were indexed and are in good agreement with JCPDS card #00-022-0153 with lattice constants a=5.4405 Å, b= 7.6436 Å, c= 5.3812 Å having interfacial angles α=90.00°, β=90.00°, γ=90.00°. However, some additional low-intensity peaks (marked *) indicate the presence of cubic phase of the ZnTiO3 system when compared to JCPDS card #00-039-0190 originating from the solubility limit for Zn2+ ions into the host of CaTiO3. No spurious peaks 6
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were observed due to the addition of chemical fluxes during phosphor synthesis in the diffraction patterns. The orthorhombic structure includes Ti (IV) centers bonded octahedrally and the Ca2+ centers occupy a cage of 12 oxygen centers. Due to similar ionic radii of Ca2+ (1.14 Å) Pr3+
Ti4+ sites, respectively in (Ca0.8Zn0.2)TiO3 lattice.
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(1.13 Å) and Zn2+(0.74 Å)-Ti4+(0.68 Å); Pr3+ and Zn2+ions most likely substitute for Ca2+ and
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Interestingly, the diffraction curves of all the samples show an additional phase of cubic ZnTiO3 except for the sample, where H3BO3 was used as a flux. This might be elucidated as the
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dual role of boric acid i.e. as fluxing and crystallizing agents. As reported earlier, co-doping of B3+ ions into the CaTiO3 lattice might help Zn2+ ions to easily substitute for Ca2+ site rather than forming additional phases [12]. However, from the density functional theory calculations [22], it is well- documented that presence of Zn-related defects increases the multiple localized states of trap levels so as to favor the charge transfer transitions and 1D2→3H4 transition of Pr3+. Thus,
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incorporated ZnTiO3 phase is expected to enhance the defect density leading to longer afterglow times [23]. NH4BF4 decomposes initially into boric acid and ammonium fluoride, and later on to HF and NH3 at lower temperatures. Further, this increase the diffusion rates of CaCO3, ZnO, and
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TiO2 to form a highly crystalline (Ca0.8Zn0.2)TiO3 phosphor. Moreover, it also quickly disappears
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from the system without forming any impurity. As reported earlier, fluoride compounds have higher reactivity towards reactants such as TiO2 as compared to boric acid [24]. Therefore, fluoride compounds tend to get dissolve into the reactants more easily leading to the enhanced crystallinity of the phosphors, which has a constructive effect on the luminescent properties [24]. NH4Cl, NH4Br, and NH4I however, have similar melting and boiling points and remains in a liquid state during the solid-state reaction. Hence, selection of the chemical flux is a crucial factor which determines structural properties of the as-synthesized phosphors. 7
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3.2. Photoluminescence excitation (PLE) and emission (PL) characteristics As mentioned in the introduction section, there exist many interdependent chemical parameters
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that could alter the afterglow properties of the phosphor. To optimize all these parameters, a systematic study has been done by replacing the cation in the position A with Magnesium (Mg), Calcium (Ca), Strontium (Sr) and Barium (Ba). It is well-known that when a rare-earth ion is
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located at a centrosymmetric site, the electric dipole transition within 4f energy levels is forbidden. As Sr ions occupy the centrosymmetric site in SrZnTiO3, the red photoluminescence
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intensity aroused due to Pr-doping is insignificant. In contrast, the absorption intensities are large for Pr-doped BaZnTiO3 and CaZnTiO3 because of the non-centrosymmetric locations of Ba and Ca sites. Detailed results and discussion has been given in the electronic supplementary information¥. Further, a series of Ca1-xZnxTiO3 (where x varies from 0 to 0.8) phosphors were synthesized for optimizing Ca:Zn ratio and the detailed results and discussion has been given in
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the electronic supplementary information¥. Fig. 2(a-b) shows the room temperature photoluminescence excitation and emission spectra of (Ca0.8,Zn0.2)TiO3 afterglow phosphors prepared via solid-state reaction with 10 mol% of fluxes. Excitation spectrum shows three sets of
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distinct bands that have been labeled as A, B, and C in the Fig. 2(a) at around 330 nm, 370 nm,
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and narrow peaks near 463 nm, respectively. The 330 nm (A) band represents the transition from valence-to-conduction band of the host lattice (from O- 2p levels to Ti-3d levels) followed by energy transfer to the excited 4f levels of Pr3+ ions. Boutinaud et al. [29] have proposed that the band (B) at 370 nm arises out of a charge transfer state (CTS) from Pr3+ to Ti4+ metal ions. The minor peaks between 450 and 495 nm are assigned to 4f→4f transitions of Pr3+ ions. More precisely peaks near 468.3 and 473.6 nm are originated from 3P1→3H4 transitions, and at ~495.2
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nm is due to
3
P0→3H4 transition. The phenomenon of observing more than one peak
corresponding to one transition is expected to be produced by crystal field splitting [12].
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Phosphor made with ammonium fluoride (NH4F) as flux shows enhanced B band as a consequence of which there is an enhanced interaction of charge transfer state (CTS) with both the 3P0 and 1D2 states of Pr3+. Thus, all photo-electrons relax directly to the meta-stable excited D2 state whereas, other samples show weak charge transfer related transition (B band). Fig. 2(b)
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shows the emission spectra of the samples made with different fluxes, measured between 560
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and 650 nm. As reported earlier [25], due to the limited substitution of oxygen with fluorine, free electrons could be generated that could reduce titanium ions from Ti4+ to Ti3+ state, which might be beneficial for the luminescence properties. PL emission spectra of (Ca0.8Zn0.2)TiO3:Pr3+ phosphors registered at various excitation wavelengths ranging 325 to 500 nm are shown in Fig. 2(c). For all excitations, the PL emission is centered at 610 nm and the maximum intensity is
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observed for 325 nm excitation. Fig. 2 (d) shows the relative emission intensities of the phosphor samples synthesized using different fluxes. It could be noted that the luminescence intensity of sample made with NH4BF4 as chemical flux significantly enhanced by 2.5 times. Hence, it is
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understood that the chemical flux accelerates the kinetics of the phosphor formation by
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enhancing the diffusion coefficient and out of all fluxes under consideration, NH4BF4 was found to be the most suitable for synthesis of (Ca0.8Zn0.2)TiO3:Pr3+ afterglow phosphors. All the samples show a typical red emission at 610 nm, which originates from the 1D2→3H4 transition of Pr3+. It is clearly seen that the emission intensity of sample made with NH4BF4 as chemical flux has a maximum intensity as compared to other samples. Since 4f→4f transitions of Pr3+ are not dependent on the local host surrounding, no such considerable shift in emission spectrum has
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been observed. Overall a positive response has been observed for all the samples made with various fluxes in terms of improvement in their emission intensities. Internationale
de
I’Eclairage
(CIE)
color
coordinates
of
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(Ca0.8Zn0.2)TiO3:Pr3+ phosphor has been calculated using equidistant wavelength method and is indicated in the CIE chromaticity diagram shown in Fig. 3. These coordinates are highly useful
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in determining the exact emission color and color purity of the samples, as per the CIE chromaticity diagram [26]. The obtained (x=0.66, y=0.33) values are appreciably comparable to
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the ideal red coordinates (x=0.67, y=0.33) as suggested by National Television Standard Committee (NTSC). Also, the color purity of the PL emission has been calculated using the following equation:
( − ) + ( − ) ( − ) + ( − )
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=
where, (xs, ys) and (xi, yi) are the color coordinates of the light source and the CIE illuminant, respectively, and (xd, yd) color coordinates of the dominant wavelength. Calculated results
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indicate that the color purity of the phosphor sample is ~96.39%.
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In addition, the 2D distribution of dopant in host matrix was examined using confocal mapping, recorded under the excitation 325 nm diode laser. Results shown in Fig. 3(a) and (b), indicate the uniform distribution of the dopant throughout the lattice and high CCD counts reveals that the synthesized phosphor is highly luminescent. The inset shows the corresponding optical image of the phosphor for the area under consideration.
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3.3. Surface morphology and compositional analysis Compositional analysis of the phosphors was carried out using energy-dispersive X-ray
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spectroscopy (EDAX), which shows that the as-synthesized samples comprise of Calcium, Zinc, Titanium and Oxygen and no other spurious impurity elements are detected as shown in Fig. 4(a). No peak associated with flux material was detected which again confirms that fluxes do not
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leave any residue during the course of the reaction. Since the doping concentration of praseodymium is very low, its presence was also not detected in EDAX analysis.
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Fig. 4(b) represents the typical Scanning Electron Microscope (SEM) image of (Ca0.8Zn0.2)TiO3:Pr3+ phosphor synthesized using the optimal amount of chemical flux i.e. NH4BF4. Observed SEM image shows an irregular shape and uneven distribution of particles in micrometer (µm) range. Total photoluminescent intensity and afterglow time increases with the
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increase of particle size because the non-radiative recombination centers are reduced monotonically with particle enlargement [27]. SEM image exhibits the highly sintered and melted morphology of clusters of particles. Besides melting point or decomposition temperature,
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other parameters of fluxes such as solubility of raw materials, ionic radii of anion/cation, as well as purity of flux, may also have a significant effect on size, morphology, and thus, on the
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luminescent properties of materials [18]. For higher resolution, Field Emission-SEM studies have also been carried out at a magnification of 12.14 kX, as shown in Fig. 4(c). Aggregated particles of micrometer size with melted morphology have been observed in the sample made with NH4BF4 as chemical flux. Observed particles are not uniform in size and having irregular morphology are mainly due to high-temperature reaction conditions.
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3.4 Afterglow decay curves Fig. 5(a) shows the afterglow spectra of the phosphors synthesized using different chemical
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fluxes, recorded at a constant excitation of 325 nm. The results show that the chemical flux influences the afterglow characteristics of the phosphors to a great extent. Since the intensity remains constant after an initial drop, afterglow curve was recorded for a limited time only.
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Phosphor sample synthesized using NH4BF4 as chemical flux shows longer afterglow time as compared to the other samples, which can be seen from decay curves and the corresponding
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optical image shown in Fig. 5(b). The reason might be that the limited substitution of oxygen with fluorine can change the oxidation state of titanium and/or praseodymium. This might have altered the luminescence decay properties by creating intermediate shallow trap levels and/or increasing the defect density [25]. Also, there are two bivalent cation positions, Ca2+ and Zn2+, in the crystal that can be replaced by Pr3+. Oxygen vacancies arise when trivalent Pr3+ ions are
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substituted in the site of the divalent Ca2+ ions and Zn2+ sites to self-compensate the charge. Due to the low melting point of the fluoride-based flux, they form liquid medium across powders, hence diffusion of the constituent ions became easier across the solid-liquid interface. Thus,
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more Ca2+ and Zn2+ ions in the crystal can be easily replaced by Pr3+ and thereby increase the
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defect density by the generation of more oxygen vacancies due to the self-compensation action of the host lattice to maintain neutrality [13]. Fig. 5(b) shows the optical image showing luminescence pattern of the samples synthesized with different chemical fluxes recorded under 325 nm excitation and corresponding afterglow pattern in dark.
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3.5. Thermoluminescence (TL) measurements The decay characteristics of phosphors are determined by the traps generated within the
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forbidden band gap of the materials. The trap depth and trap density influence the afterglow properties of the phosphors. Fig. 6 represents the thermoluminescence glow curve of the phosphor samples made with different chemical fluxes, recorded after UV (325 nm) irradiation
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for 5 minutes. For any practical application involving long afterglow at room temperature, it is necessary to have suitable trap-creating dopants that could have a TL peak in the region 320–
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380K [28]. Accordingly the as-prepared (Ca0.8Zn0.2)TiO3:Pr3+ phosphors show room-temperature afterglow properties in the above TL region.
Glow curves exhibit a broad peak in the range 350-500 K with peak maxima at 370, 354, 352, 347,350 and 352K for the samples blended with NH4BF4, NH4F, H3BO3, NH4Br, NH4Cl,
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and NH4I, respectively. The results are consistent with the afterglow decay patterns as mentioned in section 3.4. Phosphor made with NH4BF4 as chemical flux shows the glow curve maxima at relatively higher temperature indicating deeper traps and shows longer afterglow times. As
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reported earlier, these traps are ascribed to oxygen vacancies or associations of oxygen vacancies. Pr3+ dopant itself can act as a trapping center and is responsible for a TL signal
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peaking at 343 K in CaTiO3:Pr3+ [29]. However, samples synthesized with NH4Cl and NH4I exhibited weak TL luminescence and peaks at a lower temperature, which represents the formation of shallow traps (surface trapping) inside the material. 3.6 Mechanism of afterglow luminescence A vital role is played by energy levels in the band gap of the host material that has been introduced by point defects in the crystal lattice or via co-doping. These defect related energy 13
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levels are known as trapping centers and are able to capture charge carriers originating from the luminescent ions. These charge carriers remain trapped in these defect related energy levels until they acquire enough thermal energy to escape and recombine at a luminescent center. The
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activation energy required for this is called the trap depth and is supposedly determined by the energy difference between the energy level of the trap and the conduction band or the valence band. If the trap is too shallow, it will result in a very short afterglow time, whereas if the trap is
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too deep then no charge carriers can escape at room temperature. Thus, trap depth is a very crucial parameter in determining the afterglow properties of the phosphor. Deeper traps (still
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shallow enough to be excited at room temperature) and a higher concentration of trap centers display longer afterglow than shallow trap centers.
There are two bivalent cation positions, Ca2+ and Zn2+, in the crystal that can be replaced by Pr3+ to form two different kinds of point defects. Pr3+ ions will occupy Ca2+ ion sites, and due
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to the necessity of charge compensation, two Pr3+ ions will replace three Ca2+ ions, resulting in a Ca2+ vacancy (Vca). The net negative environment at this site is capable of trapping positive charge carriers. Oxygen vacancies are the major contributors to the phosphorescence behavior of
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many oxide-based phosphors. The phosphorescence behavior of the perovskite CaTiO3:Pr3+ is
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attributed to oxygen vacancies that arise when trivalent Pr3+ ions are substituted in the site of the divalent Ca2+ ions [29-31]. These vacancies are attributed to self-compensation of the material, and they have the ability to attract negative carriers. As suggested by Aitasalo et al. [32] when a long afterglow phosphor is excited, the energy is provided directly to the traps of unspecified origin. Electron and hole pairs most likely get self-trapped or acquire the stability at defect centers. The electron is removed from the trap level by thermal energy (kT) and ends up at an positive oxygen vacancy (Vo) related trap states whereas the hole migrates back to valance band 14
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and reaches traps related to negatively charged calcium vacancies (Vca) and as reported Pr3+ can also act as a hole trapping center since position of the 3H4 ground state of Pr3+ ions close to the top of the VB of CaTiO3. It is assumed that the energy released on recombination of the electron
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and the hole was delivered directly to the europium ions, by means of energy transfer (ET). From the literature, trap depth of the point defect generated due to zinc-substituted Pr3+ is deeper than that of defects form by calcium-substituted Pr3+, leading to the longer phosphorescence lifetime
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of (Ca0.8Zn0.2)TiO3:Pr3+ than the CaTiO3:Pr3+ phosphor [21].
4. Conclusions
Since there is no specific rule in selecting appropriate chemical flux for the synthesis of the phosphors, the influence of various fluxes on structural, luminescent, morphological and decay
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properties of the phosphor should be understood. Thus, various chemical flux involved redemitting long afterglow (Ca0.8Zn0.2)TiO3:Pr3+ phosphors have been prepared in the current study. Detailed optical, structural and morphological characterization of the samples has been studied in
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detail. The XRD patterns confirm the pure orthorhombic phase of the (Ca0.8Zn0.2)TiO3:Pr3+ phosphor made with boric acid as flux whereas additional phase of cubic ZnTiO3 was observed
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for rest of the samples. SEM studies reveal that the size of the particles is in the micrometer range with non-uniform and melted morphology. The photoluminescent studies exhibit characteristics emission transitions of Pr3+, i.e. 1D2→3H4 under UV (325 nm) excitation. Fluxdependent luminescent studies suggest that NH4BF4 flux is the best suitable flux for (Ca0.8Zn0.2)TiO3:Pr3+ phase among all other fluxes. The calculated CIE coordinates for the optimized (Ca0.8Zn0.2)TiO3:Pr3+ sample is found to be (0.66,0.33), which are close to the ideal
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red coordinates. Afterglow decay curves indicate that both the initial emission intensity and afterglow time have been enhanced by the addition of flux. A mechanism involving the possible trap centers that might exist in (Ca0.8Zn0.2)TiO3:Pr3+ phase for capturing the charge carriers have
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been studied. Thus, these results are important guidelines for the appropriate selection of fluxes
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in the synthesis of long afterglow phosphors.
Acknowledgments
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The authors (GS and SB) sincerely acknowledge the Council of Scientific and Industrial Research (CSIR), Government of India under the Senior Research Fellowship schemes #31/1(448)/2015-EMR-1 and 31/1(445)/2015-EMR-1, respectively and Academy of Scientific and Innovative Research (AcSIR) to carry out the research work. The author (DH) is grateful for
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the financial support from Board of Research in Nuclear Sciences (BRNS), Government of India for the research project #34/14/16/2016-BRNS/34041. Authors (GS and PS) wish to thank Mr.
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Delhi.
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Birender Singh for assisting in recording the thermoluminescence (TL) results at IUAC, New
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References [1] Koen Van den Eeckhout, Dirk Poelman, Philippe F. Smet, Persistent luminescence in nonEu2+-doped compounds: A review. Materials 6 (2013) 2789-2818.
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[2] S. K. Singh, Near-infrared persistent luminescence nano-probes for bioimaging and targeting applications. RSC. Adv. 4 (2014) 58674-58698.
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[3] Yixi Zhuang, Yumiko Kata Yama, Jumpei Ueda, Setsuhisa Tanabe, A brief review on red to near-infrared persistent luminescence in transition-metal-activated phosphors. Opt. Mater. 36 (2014) 1907–1912.
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[4] Wei Zeng, Yuhua Wang, Shaochun Han, Wenbo Chen, Gen Li, Yanzhao Wang, Yan Wen, Design, synthesis and characterization of a novel yellow long-persistent phosphor: Ca2BO3Cl:Eu2+,Dy3+. J. Mater. Chem. C. 1 (2013) 3004-3011. [5] Yang Li, Mindaugas Gecevicius, Jianrong Qiu, Long persistent fundamentals to applications. Chem. Soc. Rev. 45 (2016) 2090-2136.
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[7] Chong Peng, Zhiyao Hou, Cuimiao Zhang, Guogang Li, Hongzhou Lian, Ziyong Cheng, Jun Lin, Synthesis and luminescent properties of CaTiO3:Pr3+ microfibers prepared by electrospinning method. Opt. Express 18 (2010) 7543-7553.
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[8] Yinhui Zhu, Xiaokang Wang, Yilong Zhou, Cuilian Zhao, Jianmin Yuan, Zhenjun Wu, Song Wu, Shuangyin Wang, In situ formation of bioactive calcium titanate coatings on titanium screws for medical implants. RSC Adv. 6 (2016) 53182-53187.
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[9] Bo Wang, Hang Lin, Ju Xu, Hui Chen, Zebin Lin, Feng Huang, Yuansheng Wang, Design, preparation, and characterization of a novel red long persistent perovskite phosphor: Ca3Ti2O7:Pr3+. Inorg. Chem. 54 (2015) 11299−11306. [10] David R Lide, CRC Handbook of Chemistry, 88 Ed. (2007) pages 4-43 to 4-100. [11] Ailing Zhu, Jianchuan Wang, Yong Du, Dongdong Zhao, Qiannan Gao, Effects of Zn impurities on the electronic properties of Pr-doped CaTiO3.Physica. Physica. 5 (2011) 849–854. [12] Huang Ping, Wu Yaqiao, Cui Cai’e, Guo Jin, Effect of different dopants on the long lasting phosphorescence behavior of CaTiO3:Pr3+. J. Rare Earths 4 (2011) 340-344.
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[13] L. L. Noto, S. S. Pitale, M. A. Gusowki, J. J. Terblans, O. M. Ntwaeaborwa, H. C. Swart Afterglow enhancement with In3+ codoping in CaTiO3:Pr3+ red phosphor. Powder Technol. 237 (2013) 141–146.
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[14] D. Haranath, P. Sharma, H. Chander, A. Ali, N. Bhalla, S. K. Halder, Role of boric acid in the synthesis and tailoring the properties of calcium aluminate phosphor. Mater. Chem. Phys. 101 (2007) 163–169.
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[16] Hyun Ju lee, Kyung Pickim, Gun Young Hong, Jae Soo Yoo, The effect of flux materials on the physical and optical properties of Eu3+ activated yttrium oxide phosphors. J. Lumin. 130 (2010) 941-946. [17] Haoyi Wu, Yihua Hu Wei Zhang, He Duan, Li Chen, Xiao Juan Wang, Flux's influence on the luminescent properties of europium and dysprosium codoped melilite long afterglow phosphor. J. Non-Cryst. Solids 358 (2012) 2734–2740.
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[18] Amit K. Vishwakarma, M. Jayasimhadri, Significant enhancement in photoluminescent properties via flux-assisted Eu3+ doped BaNb2O6 phosphor for white LEDs. J. Alloys Compd. 683 (2016) 379-386.
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[19] Cássio C. S. Pedroso, José M. Carvalho, Lucas C. V. Rodrigues, Jorma Hölsä, Hermi F. Brito, Rapid and energy-saving microwave-assisted solid-state synthesis of Pr3+, Eu3+, or Tb3+ doped Lu2O3 persistent luminescence materials, ACS Appl. Mater. Interfaces 8 (2016) 19593– 19604.
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[20] Han Bin, Wang Yi-Fei, Liu Qian, Huang Qing, Role of fluxes in the synthesis and luminescence properties of Ba3Si6O12N2:Eu2+ oxynitride phosphors by microwave sintering. Int. J. Inorg. Mater. 31 (2016) 652-659. [21] Lihong Liu, Rong-Jun Xie, Chenning Zhang, Naoto Hirosaki, Role of fluxes in optimizing the optical properties of Sr0.95Si2O2N2:0.05Eu2+ green-emitting phosphor. Materials 6 (2013) 2862-2872. [22] Shixun Lian, Yuan Qi, Chunying Rong, Liping Yu, Ailing Zhu, Dulin Yin, Shubin Liu, effectively leveraging solar energy through persistent dual red phosphorescence: preparation, characterization, and density functional theory study of Ca2Zn4Ti16O38:Pr3+. J. Phys. Chem. C 114 (2010) 7196–7204.
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[23] Sung Mook Chung, Seung-Youl Kang, Jae-Heon Shin, Woo-Seok Cheong, Chi-Sun Hwang, Kyoung Ik Cho, Seung Jae Lee, Young Jin Kim, The effects of zinc on the characteristics of (Zn,Ca)TiO3:Pr3+ phosphors. J. Cryst. Growth 326 (2011) 94–97.
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[24] Pengpeng Dai, Xintong Zhang, Panpan Sun, Jikai Yang, Lingling Wang, Shili Yan, Yichun Liu, Influence of flux on morphology and luminescence properties of phosphors: A case study on Y1.55Ti2O7:0.45Eu3+ J. Am. Ceram. Soc. 95 (2012) 1447–1453.
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[25] Songhak Yoon, Eugenio H. Otal, Alexandra E. Maegli, Lassi Karvonen, Santhosh K. Matam, Stefan G. Ebbinghaus, Bernhard Walfort, Hans Hagemann, Simone Pokrant, Anke Weidenkaff, the Improved persistent luminescence of CaTiO3:Pr by fluorine substitution and thermochemical treatment. J. Alloys Compd. 613 (2014) 338–343. [26] J. Kelmer, Luminescent Screens: Photometry and Colorimetry, Iliffe, London, (1969) p.118.
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[27] Xianmin Zhang, Jiahua Zhang, Xia Zhang, Meiyuan Wang, Haifeng Zhao, Shaozhe Lu, Xiao-Jun Wang, Size manipulated photoluminescence and phosphorescence in CaTiO3:Pr3+ nanoparticles. J. Phys. Chem. C 111 (2007) 18044-18048. [28] Xigang Wang, Yanlin Huang, Hyo Jin Seo, A new long-lasting phosphor Zr4+ and Eu3+ codoped SrMg2(PO4)2. Sens. Actuators, B: Chemical 158 (2011) 171-175.
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[29] P. Boutinaud, L. Sarakha, E. Cavalli, M. Bettinelli, P. Dorenbos, R. Mahiou, About red afterglow in Pr3+ doped titanate perovskites J. Phys. D: Appl. Phys. 42 (2009) 045106 (7pp). [30] P. Boutinaud, E. Pinel, M. Dubois, A. P. Vink, R. Mahiou, UV-to-red relaxation pathways in CaTiO3:Pr3+. J. Lumin. 111 (2005) 69-80.
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[31] P. J. Deren, R. Pazik, W. Strek, P. Boutinaud, R. Mahiou, Synthesis and spectroscopic properties of CaTiO3 nanocrystals doped with Pr3+ ions. J. Alloys compd. 451 (2008) 595-599.
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[32] T. Aitasalo, P. Deren, J. Hölsä, H. Jungner, J.C. Krupa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, W. Strek, Persistent luminescence phenomena in materials doped with rare-earth ions. J. Solid State Chem. 171 (2003) 114–122.
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Table 1: Physical parameters of the flux materials used in the present work [10] Melting point (oC)
Boiling point (oC)
Density (g/cm3)
H3BO3
171
300
1.5
2.
NH4F
238
Decomposition
1.0
3.
NH4BF4
230
Decomposition
1.8
4.
NH4Cl
338
520
1.5
5.
NH4Br
235
452
6.
NH4I
551
405
5.7
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1.
85.3
25.83 39.5
2.4
78.3
2.5
172.0
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Solubility in water at 20oC (g/100 ml)
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S. No. Flux
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Fig. 1: X-ray diffraction patterns of (Ca0.8Zn0.2)TiO3:Pr3+ phosphors made using various
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chemical fluxes namely (a) NH4I, (b) NH4Cl, (c) NH4Br, (d) NH4BF4, (e) NH4F, (f) H3BO3, (g) standard JCPDS card no. 00-039-0190 of ZnTiO3 and (h) standard JCPDS card no. 00-
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Fig. 2: (a) Photoluminescence excitation and (b) emission spectra of the phosphor samples prepared with different fluxes. (c) Emission spectra recorded at different excitation wavelengths and (d) relative emission intensities of the phosphors
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Fig. 3: (a) CIE chromaticity diagram indicating the color coordinates and (b) confocal microscopy image of the optimized phosphor made with NH4BF4 flux, the inset shows the
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optical image of the sample at the scale of 10 µm.
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Fig. 4: (a) EDAX pattern showing compositional analysis, (b) SEM image and (c)FE-SEM
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image depicting the microstructure of as-synthesized (Ca0.8Zn0.2)TiO3:Pr3+ phosphor.
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Fig. 5: (a) Afterglow decay curves of the phosphors made with different fluxes and excited
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under UV (325 nm) and (b) the digital photographs of samples taken under UV (325 nm)
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excitation and their afterglow at different time intervals.
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Fig. 6: Thermoluminescence glow curves of the (Ca0.8Zn0.2)TiO3:Pr3+ phosphors made with
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different fluxes and exciting the samples at UV (325 nm).
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phosphor.
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Fig. 7: Probable energy band model depicting afterglow mechanism of (Ca0.8Zn0.2)TiO3:Pr3+
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Highlights of this work: 1. Facile technique to produce red-emitting afterglow phosphor using different fluxes. 2. Active role of each flux in influencing Pr3+ emission and afterglow was studied. 3. Achieved efficient red PL with spectral color purity of ~96.39%.
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4. Achieved afterglow time variation of 15-30 minutes depending on flux used.