Effects of X-ray irradiation on the Eu3+ → Eu2+ conversion in CaAl2O4 phosphors

doi:10.1016/j.optmat.2017.10.016

Optical Materials

Volume 75, January 2018, Pages 122-126

https://doi.org/10.1016/j.optmat.2017.10.016 Get rights and content

Highlights

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Eu doped CaAl₂O₄ produced from proteic Sol-gel route using coconut water.
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Conversion of the Europium emission spectra under X-ray radiation.
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X-ray dose dependence in the radioluminescent emission of the Eu doped CaAl₂O₄.

Abstract

This paper reports structural and luminescence properties of Eu-doped CaAl₂O₄ produced by an alternative sol-gel method using coconut water. Results of differential thermal analysis (DTA), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) allowed us to identify the best synthesis conditions for sample preparation. Simultaneous measurements of X-ray absorption spectroscopy (XAS) and X-ray excited optical luminescence (XEOL) were also performed in the X-ray energy range of the Eu L_III edge. Results from photoluminescence (PL) showed only the characteristic Eu³⁺ emission. However, radioluminescence emission spectra from Eu-doped CaAl₂O₄ shows a process of conversion of Eu³⁺ to Eu²⁺, which is induced by X-ray irradiation and is dependent on the radiation dose energy. X-ray absorption near edge structure (XANES) measurements corroborate Eu reduction due to irradiation, showing that only the Eu³⁺ ion is present in stable form in the CaAl₂O₄.

Introduction

The absorption of ionizing radiation and subsequent conversion to visible light are important factors in the properties that define applications of a phosphor material. Calcium aluminate (CaAl₂O₄) is an inorganic material from the aluminates family that, when doped with rare earth ions, presents luminescent properties [1], [2]. Therefore, it is very important to study the behavior of this material in detail, in order to ensure efficient applications.

CaAl₂O₄ has been successfully produced by several methods, such as the conventional sol-gel route [3], solid state reactions [4], Pechini [5], and combustion [6]. The materials produced by solid state and conventional sol-gel routes usually contain undesirable additional phases of other calcium aluminate such as CaAl₄O₇, Ca₃Al₂O₆, Ca₁₂Al₁₄O_33, and CaAl₁₂O₁₉ [3], [4]. A new alternative sol-gel route, called proteic sol-gel method, use coconut water as a starting solvent [7]; it has been used to successfully produce aluminate compounds such as SrAl₂O₄, Ca₁₂Al₁₄O₃₃ [8], BaAl₂O₄ [9], and Sr₃Al₂O₆ [10], as well as other oxides, for example, ZnO [11], YVO₄:Eu [12], and LiAlSi₂O₆ [13]. Generally, this route allows phosphors production with high crystallinity using lower temperatures than those normally employed in the solid state route.

Eu-doped CaAl₂O₄ has luminescent properties with high intensity emission that depends on the oxidation state of the Eu ion. For Eu doping of CaAl₂O₄, the most favorable oxidation state is Eu³⁺, followed by additional defects as charge compensation. Previous results, obtained via computational simulation, for BaAl₂O₄ [14] and SrAl₂O₄ [15] show a high probability of the incorporation of Eu³⁺ in Ba²⁺/Sr²⁺ sites, compensating by oxygen interstitials (O_i). The similarities between these aluminates with CaAl₂O₄ suggest that the same charge compensation mechanisms may happen.

Wei et al., using a reduction atmosphere, showed that it is possible to get Eu²⁺ in stable form in CaAl₂O₄ [16]. Souza and co-workers reported the use of laser sintering to provide Eu²⁺ in stable form in CaAl₂O₄ using Dy³⁺ as co-dopant to get afterglow characteristics [17]. Nevertheless, there has been no report of investigations of the ionizing radiation effect on the Eu³⁺ → Eu²⁺ conversion in CaAl₂O₄.

In this study, we report the synthesis conditions of Eu-doped CaAl₂O₄ produced using coconut water as solvent to the starting reagents, and also the Eu³⁺ to Eu²⁺ conversion induced by ionizing radiation. Additionally, we also report XANES and PL results to confirm that the Eu conversion is due to X-ray irradiation. Based on these results, we have proposed a mechanism to explain the radioluminescence (RL) in Eu-doped CaAl₂O₄.

Section snippets

Synthesis and structural characterization

Undoped CaAl₂O₄ and Eu³⁺-doped CaAl₂O₄ (Ca_0.97Eu_0.03Al₂O₄) phosphors were produced via a proteic sol-gel route [7]. The samples were prepared by mixtures of stoichiometric amounts of Ca(NO₃)₂·4H₂O-99,0%, Al(NO₃)₃·9H₂O-98,0%, and Eu(NO₃)₃-99,99% dissolved in coconut water (Cocos nucifera), with final concentration of 0.3 mol/L. The final solution was maintained under magnetic stirring at 60 °C for 1 h to form the starting gel, which was then dried at 100 °C for 24 h, resulting in a xerogel.

Results and discussion

Fig. 1(a) shows the results of thermal analysis of the xerogel of the sample. The curves can be divided into three temperature regions containing important events: I - A strong endothermic peak, denoting weight loss, is observed approximately between 120 and 180 °C. This variation may correspond to the loss of structural water and H₂O absorbed from the ambient; II - The events between 300 and 600 °C correspond to organic solvent evaporation and nitrate decomposition [18]; III - The small

Conclusions

In summary, Eu³⁺-doped CaAl₂O₄ was successfully produced using an alternative sol-gel method. XRD confirmed the formation of monoclinic CaAl₂O₄. The RL measurements resulted in characteristic emissions from Eu³⁺, and a broad band around 550 nm corresponding to the 5d4f⁶ – 4f⁷ transition from the Eu²⁺ ion, although only Eu³⁺ was used as a starting reagent. This, combined with PL results showing only Eu³⁺ emission and Eu³⁺ characteristic spectra in the XANES curves, lead us to conclude that Eu

Acknowledgments

The authors gratefully acknowledge the Brazilian funding agencies FINEP, CAPES, CNEN, INAMI-CNPq, and LNLS for financial support. Synchrotron radiation measurements were obtained at the Brazilian Synchrotron Light Laboratory, National Center for Research in Energy and Materials, Ministry of Science, Technology and Innovation (LNLS/CNPEM/MCT) under proposal number XAFS1 #13512/12.

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In addition, broad emission peak located at 425 nm may indicate the presence of Eu2+ ions which is paramagnetic in nature attributed to 4f65 d1 → 4f7 transition. The literature has reported a peak at 425 nm corresponding to Eu2+ ions in KCaF3 host (Gomes et al., 2018; Liua and Shi, 2000). The recorded radioluminescence spectrum exactly resembles the photoluminescence emission spectrum of Eu3+ doped KCaF3 with coexistence of Eu2+ peak confirming that the synthesized KCaF3:Eu3+ phosphor can be used for X-ray detection.
Europium doped KCaF₃ phosphors (KCaF₃:Eu³⁺) were prepared using various concentrations of Eu³⁺ by conventional solid-state reaction process. The X-ray diffraction (XRD) studies confirmed the formation of orthorhombic structured KCaF₃:Eu³⁺ phosphors. Scanning Electron Microscopy (SEM) image of the synthesized phosphor exhibits agglomerated particles with irregular shapes. The composition of the synthesized sample was determined by Energy Dispersive Spectroscopy (EDS) spectrum and elemental mapping showed the homogeneous dispersion of Eu³⁺ ions into the synthesized KCaF₃:Eu³⁺ phosphor. The emission peak intensity at 594 nm from photoluminescence (PL) spectra was found to increase with the increase of Eu³⁺ concentrations from 0.02 mol% to 0.06 mol% and decreased with the further increase of Eu³⁺ concentration up to 0.1 mol%. CIE1931 chromaticity diagram coordinates (x, y) of KCaF₃:(0.06 mol%) Eu³⁺ phosphors were positioned in the reddish-orange region (x = 0.5736, y = 0.4224). Photoluminescence property confirms the suitability of KCaF₃:Eu³⁺ phosphors for Solid state lighting application. X-ray induced luminescence (radioluminescence, RL) is recorded for KCaF₃:Eu³⁺ phosphors showing the characteristic emission of Eu²⁺ and Eu³⁺. ESR study on KCaF₃:Eu³⁺ phosphors confirm the presence of Eu²⁺ ions. Beta irradiated thermoluminescence (TL) glow curve of Eu³⁺ doped KCaF₃ phosphors is measured and deconvoluted using Gaussian fitting. TL kinetic parameters like activation energy (E_a) and frequency factor (s) are calculated for all the deconvoluted peaks using peak shape method which shows the synthesized KCaF₃:Eu³⁺ phosphors is suitable for dosimetry application.
Co-existence and unique co-emission properties of Eu3+/Eu2+ and Sm3+/Sm2+ in LiSrB<inf>9</inf>O<inf>15</inf> host lattice
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Su et al. also summarized the causes of self reduction in SrB4O7 [19]. In fact, there are many matrix materials with this rigid structure, including phosphate [20–22], silicate [23–25] and aluminate [26–28] matrix materials. Later, in various kinds of borate phosphors with different groups, it was also reported that different valence rare earth ions coexist in borate matrix phosphors.
“Eu³⁺/Eu²⁺ and Sm³⁺/Sm²⁺ co-doped LiSrB₉O₁₅ phosphors” were synthesized by high temperature solid-state method in air atmosphere, and the XRD, crystal structure and photoluminescence properties were studied. The photoluminescence spectra show that the self reduction phenomenon of Eu³⁺ to Eu²⁺ and Sm³⁺ to Sm²⁺ do exist in LiSrB₉O₁₅:Eu and LiSrB₉O₁₅:Sm prepared in air atmosphere. Under the excitation of 248 nm, LiSrB₉O₁₅: Eu sample shows two bands of peaks. The sharp peaks in the range of 550–750 nm belong to the 4f-4f transition of Eu³⁺, and the wide peak from 350 to 450 nm centered on 367 nm belongs to the 4f-5d transition of Eu²⁺. Under 401 nm excitation, LiSrB₉O₁₅:Sm sample also shows two bands of peaks. The peaks in the range of 550–670 nm belong to the transition of Sm³⁺, and the peaks in the range of 680–780 nm belong to the transition of Sm²⁺. Then, we analyzed the lattice occupation of Eu and Sm ions in LiSrB₉O₁₅ matrix with the method of theoretical calculation, and explained why the self-reduction phenomenon can occur. Moreover, by changing the excitation wavelength of LiSrB₉O₁₅:Eu and LiSrB₉O₁₅:Sm samples, the luminescence color can be adjusted. The energy transfer from Eu²⁺ to Eu³⁺ was also found in LiSrB₉O₁₅:Eu³⁺/Eu²⁺ due to the spectral overlap. Finally, we found that Eu²⁺ has unique near ultraviolet luminescence in the range of about 367 nm. The near ultraviolet light in this range can just be used to trap and kill pests. This ultraviolet light can also promote plant growth and apply to ultraviolet medical treatment. Therefore, LiSrB₉O₁₅:Eu phosphor is a potential candidate in the preparation of ultraviolet LED lighting.
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The most intensive 4f-4f excitation/emission bands are located at Eexc/Eemi = 3.14/2.03 eV (394/610 nm) and are shown in Figs. 7 and 8. The positions of the 4f-4f bands coincide with those presented in the literature data devoted to the Eu3+ doped CaAl2O4 [12,47–51]. The excitation spectrum exhibits thirteen sharp bands while the emission spectrum reveals at least nine bands.
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In light matter interaction, the phenomenon of absorption and successive emission of light by materials is known as luminescence. These materials are categorized as phosphors. Designing a phosphor for a particular application requires appropriate choice of a host and an activator ion. Rare-earth ions are widely used activators having deeply embedded, optically active, 4f shell electrons. Due to the shielding nature of the outer shell electrons in these ions, emission spectra of the rare-earth doped phosphors is sharp. Choice of host material is equally important and calcium silicate-based phosphors are considered important because of their chemical and thermal stability. Ca₂SiO₄, CaSiO₃, and Ca₃Si₂O₇ are the most common calcium silicates that are known to provide suitable matrix for synthesizing luminescent materials. Ca₃Si₂O₇ exists in monoclinic crystal structure. However, CaSiO₃ exhibits two structurally different forms which are the high temperature α-wollastonite and low-temperature β-wollastonite. In comparison to Ca₃Si₂O₇ and CaSiO₃, Ca₂SiO₄ exhibits five polymorphic crystalline forms, namely hexagonal (α), orthorhombic (α_H’,α_L’,γ) and monoclinic (β), at different temperatures. Research indicates that the addition of certain dopants affects the stabilization of these crystalline phases at room temperature. These calcium silicate hosts are being doped by various rare-earth ions to synthesize visible light-emitting phosphors. Rare-earth doped phosphors are known to find use in opto-electronics, bio-imaging, and optical sensors. This chapter covers the structural and luminescent properties of different rare-earth doped calcium silicates being developed for different lighting applications.
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Nanostructured Eu₂O₃:Polyphenylacetylene (Eu₂O₃:PPA) complexes prepared at different lanthanide content were synthesized as active ligand–to–metal charge–transfer (LMCT) systems. These phosphors were embedded into mesoporous a–SiO₂ sonogel hosting networks at different dopant ratios to obtain luminescent (Eu₂O₃:PPA):SiO₂ monolith glasses. The electroactive sonochemical medium of the colloidal sol–phase activates massive $E u^{3 +} \to E u^{2 +}$ self–reduction processes along the polycondensation reaction of the doped emulsions, giving rise to sharp violet (407 nm) and blue (470 nm) PL–emission bands (FWHM ≤18 nm). The unusual violet–lines arise from the 4f ⁶5d¹→4f⁷ transitions of Eu²⁺ ions to produce intense ⁶P_7/2 → ⁸S_7/2 emissions, while the blue–lines are ascribed to overlapping ⁵D₂→⁷F_j (j = 0, 1) transitions of remanent Eu³⁺ ions assisted by LMCT and interionic energy transfers. The CIE coordinates of these composites lay within the light violet/blue region and the bandgap energy was reduced up to 2.16 eV for increasing Eu₂O₃ concentration. Our experimental approach shows that the $E u^{3 +} \to E u^{2 +}$ process occurs at room temperature material processing, without the use of reducing atmospheres, stimulated UV photoreduction, or post–doping functionalization. Results suggest that guest–host redox interactions and Eu/SiO₂ surface–assisted mechanisms are responsible for accomplishing a homogeneous self–reduction and the structuring of steady Eu²⁺–silicates. Extensive morphological, structural, linear and NLO spectroscopic characterizations, CIE–chromaticity, and energy bandgap evaluations, were performed to explore these novel nanostructured composites' physicochemical and photophysical properties, pointing out potential applications in tunable phosphor devices.
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Citation Excerpt :
Its characteristic luminescence due to the 2P3/2(1)→2P1/2 transition might be useful for either LED applications (when positioned in the red part of the spectrum) or in bio-imaging (when located in the infrared) [42]. The calcium aluminates are of great interest in a variety of areas, especially as construction materials [46] and luminescent materials [47,48]. Various crystalline compounds existing in the CaO–Al2O3 systems, CaAl2O4, CaAl4O7, Ca3Al2O6, CaAl12O19, Ca12Al14O33, have been employed as host materials for transition metal and rare-earth doped phosphors [47,48].
In the present work, bismuth-doped CaAl₄O₇ phosphor powders were obtained via modified Pechini citrate process and their photoluminescence, radioluminescence and thermally stimulated luminescence properties were studied down to 20 K. At room temperature excitation in UV led to broad-band ³P_0,1 → ¹S₀ Bi³⁺ luminescence with three maxima located in blue-green part of spectrum as well as Bi²⁺ emission around 760 nm assigned to the ²P_3/2 → ²P_1/2 transition. At 150 K an additional Bi³⁺ emission band peaking at 370 nm appeared and its intensity increased significantly upon further cooling. The presence of a few luminescent centers of Bi³⁺ in CaAl₄O₇ was assigned to the presence of defects disturbing the local symmetry of activator ions. The radioluminescence spectra showed only two bands, an intense one at 760 nm clearly related to Bi²⁺ and a very weak component at 555 nm associated with one of the Bi³⁺ sites. These two emissions were also observed in the thermoluminescence of the material following its exposure to X-rays. The Bi³⁺ luminescence is suitably positioned to contribute the cyan color to white-LEDs. Its quantum yield is satisfactory, close to 40%. The Bi²⁺ 720–850 nm emission is attractive for bioimaging as it appears in the I^st biological window.

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Optical Materials

Highlights

Abstract

Introduction

Section snippets

Synthesis and structural characterization

Results and discussion

Conclusions

Acknowledgments

References (33)

Phys. B Condens. Matter

Opt. Mater. (Amst)

Opt. Mater. (Amst)

J. Alloys Compd.

Mater. Lett.

J. Electron Spectros. Relat. Phenom.

J. Lumin.

Opt. Mater. (Amst)

Radiat. Phys. Chem.

Solid State Sci.

Radiat. Meas.

Opt. Mater. (Amst)

Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms

J. Alloys Compd.

Mater. Chem. Phys.

J. Rare Earths

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Co-existence and unique co-emission properties of Eu<sup>3+</sup>/Eu<sup>2+</sup> and Sm<sup>3+</sup>/Sm<sup>2+</sup> in LiSrB<inf>9</inf>O<inf>15</inf> host lattice

Effect of reducing and oxidizing atmosphere on photoluminescence of undoped and Eu doped nanostructured CaAl<inf>2</inf>O<inf>4</inf>

Photoluminescent properties of rare-earth doped perovskite calcium silicates and related systems

Sharp luminescent violet- and blue-emitting stable (Eu<sup>3+</sup> → Eu<sup>2+</sup>: PPA):SiO<inf>2</inf> sonogel hybrid glasses: Synthesis, structural and overall photophysical characterizations

Luminescences of Bi<sup>3+</sup> and Bi<sup>2+</sup> ions in Bi-doped CaAl<inf>4</inf>O<inf>7</inf> phosphor powders obtained via modified Pechini citrate process