Full Length ArticlePhotoelectric properties of Sr2MgSi2O7: Eu2+ phosphors produced by co-precipitation method
Introduction
Long afterglow phosphors have great potential for application in many fields such as lighting [[1], [2], [3]], traffic signs [4,5], watches and clocks [6,7], luminous paints [8], sensors and detectors [9,10], biomedical imaging [11,12], photocatalysis [13,14], and solar cells [15,16]. Among various kinds of phosphors, silicates can be used in a wide range of applications due to their easy preparation [17], low cost [18], stable crystal structure [19], long persistence time [[20], [21], [22]], physical and chemical stabilities [23,24], excellent water resistance [25], environment friendliness [26], varied luminescent colors [27], and strong absorption in the near-UV region.
The family of materials M2MgSi2O7 (M = Ca, Sr, Ba), which are called alkaline earth akermanites, are the most widely studied persistent luminescent silicates [[28], [29], [30], [31], [32], [33], [34]]. Ions of rare earth elements are the most common doped ion to be used as an activator in phosphors. Luminescent Eu2+ ions can emit light from the UV to the infrared region on different host matrices because the involved 5d orbital of a Eu2+ ion is external and strongly influenced by the crystal field. Activator Eu2+ ions are used to make Sr2MgSi2O7 based blue phosphors, and their broadband emission usually consists of transitions from 4f65 d1 to the 4f7 ground state.
The host of Sr2MgSi2O7 (SMSO) has a tetragonal crystal structure. The Sr2+ ion in this crystal structure occupies a position that is connected with eight neighboring O2− ions. Obviously, when Eu2+ is doped into the SMSO crystal, the electronic structure of SMSO is not changed because their ionic radii are almost the same, and no charge compensation is required for the replacement of Sr2+ with Eu2+. Therefore, we adopted the density functional theory (DFT) calculation in the Cambridge Sequential Total Energy Package (CASTEP) software and used a simple primitive crystal model to study how the Eu2+ doping and oxygen vacancies affect the electronic orbitals in the crystal structure.
Among the various synthesis methods, the co-precipitation method [[35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]] is known to produce phosphor powders with uniform, narrow size distribution, and a homogeneous distribution of the activator ions. Therefore, the co-precipitation method was chosen to prepare Sr2MgSi2O7: Eu2+ phosphors with various doping concentrations of Eu2+ ions. According to our experimental results, the optimum Eu2+ doping concentration was 4 mol%, and the optimum sintered temperature and time were 1150 °C and 11 h, respectively.
In addition, the crystal phases and structures of these synthesized phosphors were studied by X-ray wide-angle diffraction (XRD) and field emission scanning electron microscopy (FESEM). The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of these phosphors were studied using a fluorescence spectrophotometer. The binding energies of each surface composition of this phosphor were determined by X-ray photoelectron spectroscopy (XPS). The loss of weight during the synthesis period was studied by thermogravimetric analysis (TGA).
Owing to the advantages of Sr2MgSi2O7: Eu2+ blue phosphor powder such as low cost, ease of synthesis, lack of human toxicity, and environment friendliness, we try to explore the possibility and feasibility of its application to LFP discovery. For this, the pattern identification of LFPs on the surfaces of various materials is evaluated.
Section snippets
Powder synthesis
Sr2MgSi2O7: xEu2+ (x = 0.01, 0.02, 0.03, 0.04, 0.05) phosphors were synthesized using a co-precipitation method. Magnesium nitrate hexahydrate (Mg(NO3)2∙6H2O, 99+%, Acros Organics, Belgium), strontium nitrate (Sr(NO3)2, 99+%, Acros Organics, Belgium), europium(III) acetate hydrate (Eu(C2H3O2)3·xH2O, 99.9%, REacton, Alfa Aesar, U.S.A.), tetraethyl orthosilicate (Si(OC2H5)4, 98%, Acros Organics, Belgium), deionized water, anhydrous alcohol (C2H5OH, 99.5+%, ECHO Chemical Co., Ltd, Taiwan), and
Results and discussion
The X-ray diffraction (XRD) patterns of Sr2MgSi2O7: xEu2+ (x = 0.01, 0.02, 0.03, 0.04, and 0.05) sintered at 1150 °C for 11 h are shown in Fig. 2(a). The measured results are roughly consistent with the standard card JCPDS 75–1736 of Sr2MgSi2O7. All these have a tetragonal crystal structure. As the concentration of Eu2+ ions increases, the distance between Eu2+ ions narrows, and the probability of energy transfer among Eu2+ ions increases. Therefore, the concentration of Eu2+ doping ions must
Conclusion
In this investigation, the blue phosphor powder Sr2MgSi2O7: Eu2+ was successfully prepared using the co-precipitation method. According to our experimental results, the ideal condition for synthesizing the optimum doped concentration of Sr2MgSi2O7: 4mol% Eu2+ phosphor is sintering at 1150 °C for 11 h. The luminescence characteristics of XRD, PLE/PL, and XPS as well as the decay time, CIE color coordinates, and thermal stability of this phosphor were studied. Its external quantum efficiency was
Declaration of competing interest
We confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
Acknowledgments
The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under contract No. MOST 109-2221-E-992-087.
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