Nanothick aluminate long-afterglow phosphors using inherited hydrothermal deriving

Journal of Luminescence 206 (2019) 593–602

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Nanothick aluminate long-afterglow phosphors using inherited hydrothermal deriving

T

Chen-Yu Wua, Chien-Ming Leib, Rudder Wuc, Toshiaki Takeid, Chau-Chang Choue, ⁎ Shing-Hoa Wange, Horng-Yi Changa, a

Department of Marine Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan, ROC Department of Chemical & Materials Engineering and Graduate Institute of Nanomaterials, Chinese Culture University, Taipei 11114, Taiwan, ROC c Research Center for Structural Materials, National Institute for Materials Science, Tsukuba 305-0044, Japan d International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan e Department of Mechanical and Mechatronic Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan, ROC b

A R T I C LE I N FO

A B S T R A C T

Keywords: Aluminate Inherited hydrothermal Phosphor Afterglow

A novel strategy of the inherited hydrothermal method was used to prepare SrAl2O4: 0.01Eu2+, 0.01Dy3+ (SAO) without boron (ISAO) and boron-doping (BSAO) via NaAlO2 as an aluminum source and mineralizer at 200 °C for 6–24 h. The investigated optical properties was correlated to phase transformation and phosphor morphology. The main constituting phases of Al2(OOH)2 and SrCO3 transformed to monoclinic SrAl4O7 and mixed monoclinic/hexagonal SrAl2O4 then pure monoclinic SrAl2O4 during different hydrothermal period. The hydrothermally prepared samples exhibited flower-like morphology. The annealed plate phosphors with nanothick single crystals inherited from hydrothermal feature. The shift of photoluminescence (PL) peak wavelength corresponded to the phase transformation from SrAl4O7 to SrAl2O4 with hydrothermal time for ISAO and BSAO. The PL intensity increased with hydrothermal time due to achieving the pure SrAl2O4 phase with preferred orientation geometry and further enhanced by boron-doping. The afterglow of BSAO was significantly enhanced than ISAO, solid-state prepared MSAO and commercial SAO at the initial 100 s after excitation cut-off. The boron doped in the BSAO was scarcely on the grain surface as to small grain size. Thus, the afterglow enhancement should be attributed to the boron in the lattice inducing defect traps. Overall, the inherited hydrothermal and boron-doping derived nanothick SAOs with large surface preferred orientation enhanced the PL and afterglow properties.

1. Introduction Long-afterglow phosphor of doped strontium aluminate (SrAl2O4: Eu2+, Dy3+) is widely applied in several fields of energy-saving and safety installations [1–5]. Aluminates have several superior properties such as high photoluminescence (PL) intensity, color purity, longer afterglow, chemical stability, safety, and no radioactivity compared to classical sulfide phosphorescent phosphors [6]. Spinel SrAl2O4 [7] has a distorted stuffed tridymite structure, in which the crystal framework consists of layers formed from tetrahedral AlO4 linked to form trigonally distorted rings. These layers are stacked and connected by the tetrahedral apices, constructing a three-dimensional structure with open channels, which are occupied by Sr2+ ions [8,9]. SrAl2O4 exhibits polymorphism from monoclinic to hexagonal structure at 650–680 °C reversibly [9,10]. The monoclinic form of SrAl2O4 contains two

⁎

different sites available for Sr2+ ions in the opened channels; both the sites of Sr1 and Sr2, coordinated by nine oxygen ions, are present in equal amounts in the lattice [11]. Europium ions, Eu2+, can substitute the Sr2+ ions, as their ionic radii are similar (1.30 Å and 1.31 Å, respectively) [12]. Thus, the europium ions subjected to two different chemical environments at both the Sr sites result in emission with different spectra: one emits in the blue spectral range only at low temperatures and the other in the green spectral range [13]. The photoluminescence (PL) and afterglow of doped SrAl2O4 phosphors are not only affected by the particle size, crystal size, and geometric structure, but also by the introduction of co-dopants such as Eu, Dy, and B elements [4,14–17]. The afterglow characteristics of doped SrAl2O4 phosphors were explained in terms of trapping and thermal release of charge carriers at various temperatures, wherein Eu is an emitter and Dy serves as a trapping center. The addition of Dy3+

Correspondence to: Department of Marine Engineering, National Taiwan Ocean University, 2, Pei-Ning Rd., Keelung 20224, Taiwan, ROC. E-mail address: [email protected] (H.-Y. Chang).

https://doi.org/10.1016/j.jlumin.2018.10.004 Received 8 May 2018; Received in revised form 24 September 2018; Accepted 7 October 2018 Available online 13 October 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.

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solid-state prepared and commercial SAOs.

does not alter the emission spectrum but considerably enhances the long-afterglow intensity [4,18,19]. Although several long-afterglow luminescence phenomena have already been studied [4,20–24], the luminescence mechanism and energy transfer in doped SrAl2O4 phosphors have not been elucidated and still remain under discussion [5,18,19,23–26]. B2O3 added to doped SrAl2O4 in a solid-state reaction acts as a sintering flux agent [14,15], reduces the synthesis temperature and accelerates grain growth. The enhancement of PL and afterglow by the addition of B2O3 is because liquid-phase sintering occurs at 1350 °C for the penetration of Eu2+ and Dy3+ ions into the SrAl2O4 lattice increases [27]. Some studies reported that the doped B3+ ions (0.11 Å) substitute the Al3+ sites (0.39 Å) in SrAl2O4 to form an O-B-O-Al-O framework, resulting in the enhancement of PL at low concentrations, while the long-afterglow property is suppressed by high concentrations of boron [5,28]. The phosphors used in currently industrial applications are highly crystalline powders with large particle size in range of 20–100 µm. However, submicron particles are more demanded for potential application in solar cells, biological labeling and imaging and security encode [29]. Generally, the final luminescent properties depend on the microstructure, crystallinity and size of powders controlling through the synthesizing conditions. Bulk phosphors prepared by direct solvothermal treatment are transformed into persistent luminescent nanosheets, which may provide large surfaces for illumination sites [30]. Low-energy ball-milling is successfully to reduce commercial strontium aluminate phosphors with several tens of microns size to about 2.8 µm but improve the homogeneity, keep the crystallinity and maintain the luminescence of the particles [31]. Phosphors have been prepared in the nano-phase by employing different techniques such as sol-gel [32–34], co-precipitation [35], combustion [36–38], hydrothermal [39,40], reverse microemulsion [41,42], microwave synthesis [43], template method [44], electrospinning [45], laser synthesis [46] and molten salt route [47]. The laser and molten salt routes can provide liquid environment to enhance the crystal growth like the borate flux providing the dissolution-precipitation process for the formation of SrAl2O4. The wet chemical methods can provide phosphors with controlled size, morphology, and homogeneity, and allow functionalization design as well as optimization of the afterglow property; however, it is difficult to obtain well-defined morphologies. Nanoparticles are suitable for mechano-optical nano-devices, life sciences, and biomedicine [48], especially in printing and labeling technique. To obtain submicron size of long persistent phosphors with low cost and high purity are still a challenge. A high calcination and sintering temperature using solid-state method leads to phosphors with large grain size, poor homogeneity and morphology control. B2O3 addition can lower the preparation temperature of SAOs to 1200 °C but grain growth is inevitable [49]. A nanothick sheet of annealed SAO derived from inherited hydrothermal processing aluminates in our previous study [50] exhibits large sheet grains with a glass layer due to B2O3 addition. The core-shell structure of SAO-B2O3, behaving as a diffusion-assistant medium and environmental protection layer against acid and humidity. Such inherited hydrothermal process has been proved to synthesize nanoparticles with longer persistent duration, smaller particle size, higher homogeneity, and highly controllable morphology. However, the phase transformation mechanism during inherited hydrothermal process is necessary clarification and boron diffusion in SAO lattice to affect optical properties is also required to further investigations. In this study, nanothick SrAl2O4 phosphors with and without borondoping were synthesized by the inherited hydrothermal process using compositional salts without basic mineralizer addition. How the nanothick SrAl2O4 phosphors inheriting the geometry of the aluminum source was revealed and the phase transformation mechanism was under studying in correspondence to optical properties. The superiority of inherited hydrothermal-annealed SAOs was in comparison with the

2. Material and methods 2.1. Preparation The raw materials used in the inherited hydrothermal process were Sr(NO3)2 (Alfa Aesar, 99%), Eu(NO3)3·6H2O (Alfa Aesar, 99.9%), Dy (NO3)3·5H2O (Alfa Aesar, 99.9%), and NaAlO2 (Showa, 99%). The aluminum source, NaAlO2, was used as the mineralizer. The general strong basic mineralizer was not necessarily utilized in this hydrothermal process. The stoichiometric composition of SrAl2O4: 0.01Eu2+, 0.01Dy3+ (SAO) was prepared using the aforementioned raw materials and dissolved into deionized water under vigorous magnetic stirring. The stoichiometric composition prepared in the inherited hydrothermal process was denoted as ISAO. Further, 0.5 wt% B2O3 (Merck, 99.98%) was dissolved into the SAO solution, and the resultant SAO was denoted as BSAO. The prepared ISAO and BSAO aqueous solutions were poured into a Teflon-lined stainless autoclave, respectively, and heated at 200 °C for 6–24 h. After the system cooled to room temperature naturally, the precursor was collected by centrifugation, washed with deionized water and ethanol repeatedly, and dried at 80 °C. All the powders of ISAO and BSAO were heat-treated at 1100 °C for 4 h in CO reducing atmosphere at a heating rate of 5 °C min−1. Our inherited hydrothermal process is expected to deliver innovative, sustainable and cost effective solutions for the reduction of rare-earth elements and then to employ the nominal concentrations of dopants with 0.01 Eu2+ and 0.01 Dy3+. Actually, the photoluminescence band depends on Eu activator but Dy dopant contributes the afterglow characteristics. The excited electrons of Eu2+ were trapped preferentially by oxygen vacancies in the single doped SrAl2O4: 0.01 Eu2+. The reported 1 mol% Eu2+ ions could provide enough electrons to fill the oxygen vacancy traps to the full. The percentage of deep trap density to total trap density in SrAl2O4 with various concentrations of 0.01–0.03 Dy3+ incorporated with 0.01 Eu2+ is almost the same [51]. Therefore, the nominal concentrations of dopants with 0.01 Eu2+ and 0.01 Dy3+ are utilized in this study. For comparison, solid-state reaction of 0.5 wt% B2O3 (Merck, 99.98%) with SrCO3 (Alfa Aesar, 99.9%), Eu2O3 (Alfa Aesar, 99.9%), Dy2O3 (Alfa Aesar, 99.9%), and Al2O3 (Alfa Aesar, 99.95%) was carried out to prepare SrAl2O4: 0.01Eu2+, 0.01Dy3+ phosphors. The boronmixed products of solid-state reaction were obtained through heattreatment at 1400 °C for 4 h in CO reducing atmosphere at a heating rate of 5 °C min−1, hereafter referred as MSAO. Also, a commercial SAO based phosphor (NCC New Prisematic Enterprise Co., Ltd, Taiwan) was purchased as a reference. 2.2. Characterization The crystal structures were characterized by X-ray diffraction (XRD, BRUKER, D2 Phaser) using Cu-Kα radiation. Morphologies were observed using a Hitachi S-4800 field-emission scanning electron microscope (FESEM) and a JEOL JEM-2100F field-emission transmission electron microscope (FETEM). The functional groups and compositions of the resulting products were identified by Fourier transform infrared (FTIR, BRUKER, Tensor27) spectroscopy using the KBr pellet technique. Particle size distribution of the various SAO powders were determined by laser particle size analyzer (Malvern Mastersizer 2000 with HYDRO 2000S, U.K.). The values of d50 (average particle size) were calculated for comparison. The PL spectra of the samples at room temperature were measured by a HORIBA FluoroMax-4 fluorescence spectrophotometer using a 150 W Xe lamp as the excitation source. Ideally, the standard light source of D65 referred to 6500 K solar light (or CCT = 6504 K) is employed to decay time measurement. For convenient reference to other studies, the 365 nm UV-light was used as irradiating source to excite our SAOs for 10 min. After excitation turned 594

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Fig. 2. XRD patterns of BSAO prepared by different hydrothermal time and annealing at 1100 °C/4 h under CO reducing atmosphere, (a) hy12h, (b) hy24h and (c) solid-state prepared MSAO treated at 1400 °C/4 h under CO reducing atmosphere, and (d) commercial SAO.

hydrothermal process, the NaAlO2 can act more mineralization than the other mineralizers in the chemical solution process [40,52]. In other words, AlO2− can react with Sr2+ ion more actively than other Al3+ ions do. The substitution of a small amount of Sr atoms by the Eu or Dy atoms in the crystal lattice does not have any detectable effects on the XRD spectra; thus, the crystal structure remains unaltered for low Eu/ Dy doping concentrations. Pure monoclinic phase can be obtained without any B2O3 flux and does not require high annealing temperatures by the inherited hydrothermal process. Boron-doped SAOs prepared by hydrothermal process exhibit phase change after 1100 °C/4 h heat treatment, as shown in Figs. 2(a) and 2(b). The SrAl4O7 phase was still observed in BSAO prepared by 12 h of hydrothermal time (Fig. 2(a)). The results are almost the same as those of ISAO (hy12h)-1100 °C/4 h. Further, only monoclinic phase without hexagonal phase was observed in BSAO (hy24h) after 1100 °C/4 h heat treatment (Fig. 2(b)). The reported boron addition caused the formation of impurity phases such as the SrB2O4 and SrAl4O7 [53] that were not observed in the inherited hydrothermal/annealing process. The low content of 0.5 wt% B2O3 doping in our experiments did not affect the stability of the main phase, SrAl2O4. The result was observed in both of high-temperature solid-state reaction and low-temperature hydrothermal boron-doped SAOs (Figs. 2(b) and 2(c)). The doped boron should enter into the lattice of SAO. In comparison, the commercial SAO exhibits lower crystallinity than BSAO and MSAO, and includes secondary phases, as shown in Fig. 2(d). The ISAO (hy24h) annealed at 1100 °C/4 h reveals a slight shift in the diffraction peaks (Fig. 1(e)). This phenomenon has been illustrated by our previous investigations [50]. Considering that the size and valence of Eu2+ are the same as those of Sr2+, it can be supposed that the Sr-sites in SrAl2O4 are partially substituted by the Eu2+ ions. The boron-doped SAOs exhibit no obvious diffraction peaks shift to higher angles, which can be attributed to the small amount of boron doping into the SAOs resulting in slight lattice distortion only that could compensate the lattice distortion affected by Eu2+ and Dy3+. The wet chemical processes can exactly derive nano-particles such as using gel and reverse microemulsion routes [42,54]. On the other hand, the reaction mechanism of Al2O3/SrO is related to the template mechanism formation, where the formation of strontium aluminate phase initiates on the Al2O3 platelet surface. The selected reactive

Fig. 1. XRD patterns of hydrothermal prepared ISAOs without B2O3, (a) 200 °C/ 24 h as-hydrothermal prepared ISAO; and annealing at 1100 °C for 4 h under reducing atmosphere for different hydrothermal time at 200 °C for (b) hy6h, (c) hy12h, (d) hy18h and (e) hy24h.

off, the afterglow of the various SAO phosphors was measured as a function of decay time using a HORIBA fluorescence spectrophotometer. The commercial luminance meter (TES-317, TES Electrical Electronic Corp., Taiwan) was also utilized to measure the brightness (cd/m2) of SAOs after the UV-light excitation.

3. Results and discussion 3.1. Phase development in the inherited hydrothermal process For the hydrothermally prepared ISAO, the phases developed with different hydrothermal conditions, as shown in Fig. 1. Mixed phases were observed for the hydrothermal condition of 200 °C/24 h (Fig. 1(a)). The main constituting phases were Al2(OOH)2, SrCO3 and some unidentified phases. Al2(OOH)2 and SrCO3 require further heat treatment to transform into aluminate structures. The intermediate phases in the 200 °C/6 h and 1100 °C/4 h heat-treated sample are monoclinic SrAl4O7, mixed monoclinic/hexagonal SrAl2O4 with dominant diffraction planes (2¯11) (2θ = 28.387°) and (102) (2θ = 29.063°) respectively (Fig. 1(b)). The SrAl4O7 phase was further reduced by increasing the hydrothermal time to 18 h, as shown in Figs. 1(c) and 1(d). The monoclinic and hexagonal phases coexisted in the hydrothermal conditions of 200 °C/6–18 h annealed at 1100 °C/4 h. Pure monoclinic structure was achieved in the hydrothermal condition of 200 °C/24 h after 1100 °C/4 h annealing, as shown in Fig. 1(e). The SrO-Al2O3 system exists large number of polymorphic transformations during the heat treatment because of lack of a definitely well synthesis procedure to provide only one pure phase of compound. In our inherited 595

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Fig. 3. FESEM micrographs of various SAOs, (a) 200 °C/24 h as-hydrothermally prepared ISAO, inset: FETEM image of as-prepared ISAO (hy24h), (b) ISAO annealed at 1100 °C/4 h, (c) BSAO annealed at 1100 °C/4 h, (d) boron-oxide post-added ISAO after 1100 °C/4 h annealing, (e) commercial SAO and (f) particle size distribution of SAOs.

easily anisotropic nucleation and forms lamellar structure at the initial reaction stage. So, the rod-like nanothick plate structure developed from nanosheet in our inherited hydrothermal process is a self-assembly process. Those aforementioned reports also provide a strong evidence to our flower-like ISAOs. The grain size of annealed ISAO and BSAO almost keeps the same without much different, as shown in Figs. 3(b) and 3(c). The nanothickness of rod-like plate of both also maintains the same. A grain growth was observed when the boron oxide was post-added to ISAO [50], which is also shown in Fig. 3(d). The boron oxide added ISAO after 1100 °C/4 h annealing exhibits nanothickness even the grain growth of rod-like plate. Therefore, the nanothickness should be retained for annealed ISAO and BSAO without grain growth. However, the particle size is near 100 µm for commercial SAO, as shown in Fig. 3(e) for reference. The median particle size (d50) by laser particle size analyzer analysis is about 8.5, 9.3, 129 and 68 µm for ISAO, BSAO, MSAO and commercial SAO, respectively, as shown in Table 1 and Fig. 3(f). The particle size distribution exhibits bimodal type indicating seriously agglomerate part of near 10 µm for ISAO and BSAO. Such highly agglomerate results due to using water as dispersion agent in particle size analysis. It still remains to require further evaluation of accurate particle size distribution using effective dispersion method. However, the results show that the particle size of inherited hydrothermal

alumina as a dissolution-precipitation core to grow submicron size of phosphors is also reported by using molten salt method [47,55]. The NaAlO2 was used as both roles of aluminium source and minerlizer in inherited hydrothermal process. The hydrothermally prepared ISAOs possess flower-like structure which is consisted of random sheet clusters and assembly together, as shown in the Fig. 3(a), which also can be referred to our previous study [50]. The random sheets are aggregate layers, as shown in the inset of Fig. 3(a). The large surface-to-thickness ratio provides a large surface area preferred orientation of particles. This characteristic of preferred orientation further leads to the formation of rods or rod-like plates after the ISAO was subjected to 1100 °C/ 4 h heat treatment (Fig. 3(b)). The same trend was also observed in BSAO after treated at 1100 °C/4 h (Fig. 3(c)). The result of flower-like morphology of as-hydrothermally prepared ISAO is also supported by Ji et al. [56]. They synthesize hierarchical micron flower-like γ-AlOOH and γ-Al2O3 superstructures deriving from NaAlO2 of oil shale ash. They report that the morphology formation of 3D flower-like superstructures includes self-assembly of boehmite nanocrystals into leaf-like nanosheets and the assembly of the as-formed nanosheets into 3D flower-like superstructures. The thickness is less than 100 nm for such a self-assembly of leaf-like boehmite. The similar result is also reported by Cai et al. [57]. The layer-shape structure of γAlOOH as a building block to form SrAl2O4 and BaAl2O4 phosphors is also reported by Cheng et al. [44,58], which the layer structure remains 596

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using urea precipitation-hydrothermal process. From the SAED and HRTEM analyses (Figs. 4(b) and 4(c)), it is a single crystal lattice arrangement from the observation of zone axis of [2¯00]. The simulation of atomic arrangement is also shown in the inserted picture of Fig. 4(c). The contour around the rod-like plate surface exhibits an amorphous layer without image of atomic lattice. This phenomenon may be attributed to the effects of moisture exposure, borate flux-segregation or electron beam scattering. It is necessary to be studied further. In the hydrothermal preparation of ISAO, NaAlO2 was used as the Al precursor and a mineralizer. NaAlO2 has flower-like morphology [56]. γ-AlOOH (boehmite) can form by the following reaction during the hydrothermal process:

Table 1 Average particle size (d50) with agglomeration of various SAOs. sample

ISAO

BSAO

MSAO

Commercial SAO

d50 (μm)

8.54

9.31

129.23

68.04

prepared ISAO and BSAO is one order smaller than that of the solidstate prepared MSAO and commercial SAO, effectively. The particle size is then estimated from microstructure of FESEM of 1100 °C/ 4 h annealed ISAO and BSAO in Figs. 3(b) and 3(c). However, the hard agglomeration was still difficult to measure the accurate size. The typical plates of ISAO (hy24h) and BSAO (hy24h) treated at 1100 °C/4 h have nearly 500 nm in plate area size with nanothickness, which was determined by FETEM (Fig. 4(a)) and also by FESEM (Figs. 3(b) and 3(c)). The nanothickness of rod-like plate less than 100 nm is also supported by self-assembly of leaf-like boehmite and nanosheets [56–58]. The plate size was not affected by boron-doping in the hydrothermal process. This result is different from the boron oxide added after inherited hydrothermal process [50]. Grain growth develops while annealing for the boron oxide post-added aluminates, as seen in Fig. 3(d). The boron-doping in BSAO without post-addition of B2O3 limited the grain growth of aluminate. The selected area electron diffraction (SAED) pattern indicates a slight distortion of the SAO lattice (Fig. 4(b)), which might be the effects of dopants and defects resulting from the reducing atmosphere. This is also confirmed from the XRD patterns in Fig. 1(e) and that Fig. 3(b) referred to [50]. It is a single crystal plate with submicron plate size and nanothickness for inherited hydrothermal-annealed aluminates. The single crystal SrAl2O4:Eu2+,Dy3+ nanosheets are also reported [40]

AlO2− + H2O → AlOOH + OH− The flower-like ISAO structure developed from the primary selfassembly of boehmite nanocrystals into leaf-like nanosheets, followed by the formation of flower-like sheets with nanothickness. The γ-AlOOH nanocrystals with plenty of surface OH− groups on the {010} planes with the lowest growth energy reacted with SrCO3 to grow into a sheet structure through dissolution and precipitation [57]. The diffusion mechanism of Sr2+ cations in the hydrothermal process is also reported by the authors for the formation of a SrTiO3 shell on a Ti core and a SrAl2O4 via Al2O3-SrO using core-shell technique [47,55,59,60]. The hydrothermally prepared ISAO precursors then self-aggregated into a flower-like structure retaining the original geometry of γ-AlOOH, as shown in the Fig. 3(a). The precursor consisting of Al2(OOH)2 and SrCO3 (Fig. 1(a)), after annealing at 1100 °C/4 h will transform into monoclinic SrAl4O7, coexisted monoclinic and hexagonal phases of SrAl2O4, as indicated in Figs. 1(b)–1(e). The monoclinic SrAl4O7 and hexagonal SrAl2O4 phases will gradually disappear with increasing

Fig. 4. Analyses of annealed boron-doped BSAO, (a) FETEM image of BSAO particles, (b) SAED pattern and (c) HRTEM image and atomic arrangement picture by simulation. 597

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Fig. 5. FTIR spectra of (a) ISAO and (b) BSAO annealed at 1100 °C for 4 h; all treatment under reducing atmosphere.

hydrothermal time (6–24 h). After hydrothermal treatment at 200 °C/ 24 h and annealing at 1100 °C/4 h, the pure monoclinic phase of SrAl2O4 was obtained, as shown in Fig. 1(e). The transformation during the inherited hydrothermal process was proposed as: 3Al2(OOH)2 + 2SrCO3 → SrAl4O7 + SrAl2O4 + 3H2O + 2CO2 The boron-doping (BSAO) maintained the geometry of ISAO and with nanothick grain size. This implied that irrespective of the amount of Al2(OOH)2 and SrCO3, the doped boron in lattice played an important role in stabilizing the different aluminate phases involving one or more chemical reactions. 3.2. Optical properties of nanothick aluminates at room temperature When B3+ ions replace the Al3+ ions in the SrAl2O4 lattice, Al-O4 may turn into a B-O4 tetrahedral base. B2O3 crystal or glass has B-O3 functional groups, which are very different from the B-O4 group [8]. The FTIR spectra of the phosphors indicate the result of boron incorporation in SAOs, as shown in Fig. 5. As we investigated boronbonding in SAO lattice before [50], the bands at 400–900 cm−1 arises from O-Al-O bond bending and stretching vibrations, which indicate the normal O-Al-O lattice in SAO. Bands of B-O3 and B-O4 bonds appear in the boron-containing sample of BSAO (Fig. 5(b)) excluding ISAO, which did not contain boron. The boron-doping was observed the effects on OAl-O bond vibrations at 400–500 cm−1 and 600–700 cm−1 in the SAO lattice. The bands at 1050 cm−1 represent B-O4 bond, while those at 1450 cm−1 represent B-O3 bond in the boron-doping SAO samples. The B-O4 bond was believed to have formed from the diffusion of boron into the Al-O4 structure to substitute the Al sites. Obviously, ISAO does not contain any boron oxide bonds (Fig. 5(a)). The doped boron in BSAO resulted in the formation of B-O4 bond, with a small amount of B-O3 bond, as shown in Fig. 5(b). This may be the reason for the small grain size of BSAO due to the absence of enough flux outside on the phosphor particles to facilitate the grain growth, compared to MSAO and postadded boron oxide in SAO [50], also shown in Fig. 3(d). The excitation spectra for the various SAOs are shown in Fig. 6(a). It indicates that the SAOs exhibit strong absorption of light energy in the range of 300–430 nm. In Fig. 6(a), the high excitation intensity is observed among the wavelength of 320–380 nm for various SAOs. The peak intensity at 330 nm is enough strongly selected as the excitation wavelength for studied SAOs. Other MAl2O4 (M=Ca, Sr, Ba) nanophosphors prepared by solution-combustion route are reported the excitation by 325 nm [61]. Therefore, it can be inferred that the excitation energy of Dy3+ can be somehow transmitted to Eu2+ eventually in the SrAl2O4: Eu2+, Dy3+ using 330 nm excitation. The excitation intensity of the solid-state prepared MSAO is almost same as the commercial product. We also observe the excitation intensity of hydrothermal-

Fig. 6. Photoluminescence (PL) spectra with wavelength, (a) excitation spectra of various SAOs for 520 nm emission, (b) emission spectra of ISAOs prepared by different hydrothermal time and annealing at 1100 °C/4 h and (c) emission spectra of various processed ISAO, BSAO, MSAO and commercial SAO excited by 330 nm.

annealed BSAO and ISAO higher than MSAO and commercial SAO in Fig. 6(a). The emission PL intensity increased with the hydrothermal time for ISAO under the exciting wavelength of 330 nm, as presented in Fig. 6(b). The ISAO (hy24h) could emit high PL after 1100 °C/4 h heat treatment. If the hydrothermal time was too short, a low PL intensity was obtained, with a shift in the peak wavelength of ISAO (hy6h) from 518 nm to 510 nm. This might be attributed to the low crystallinity and the existence of mixed phases in the ISAO (hy6h), as shown in Fig. 1(b). As the results indicate that the main constituting phases in ISAO (hy6h) are SrAl2O4 and SrAl4O7. The PL peak at 520 nm for SrAl2O4 and that at 480 nm for SrAl4O7 are attributed to single crystals doped with Eu2+ and Dy3+ ions [62]. The obtained emission peak wavelength of ISAO (hy24h) with pure SrAl2O4 phase (Fig. 1(e)) was at 518 nm. The emission peak wavelength of ISAO (hy6h) was at 510 nm. Therefore, the emission peak wavelength shift from 510 to 518 nm shown in Fig. 6(b) corresponded to the phase transformation from SrAl4O7 to SrAl2O4 with hydrothermal time increase. The results then showed that the main phases of the hydrothermally prepared phosphors were dependent on the hydrothermal time at the same processing temperature of 200 °C, irrespective of the addition of the dopants, Eu2+ and Dy3+ ions (Fig. 1). The PL intensity varies with hydrothermal time for SAOs after 1100 °C/4 h annealing: BSAO (hy24h) > BSAO (hy12h), and ISAO (hy24h) > ISAO (hy12h), as shown in Fig. 6(c). The blue shift in the emission peak of BSAO (hy12h) 598

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the long-afterglow intensity as the large-grained MSAO and commercial SAO did. The reported afterglow of nanosheets decayed more rapidly than that of the commercial powders prepared by solid-state reaction [40]. Our long-afterglow phosphors of BSAO exhibited enough long-afterglow compared to MSAO and commercial SAO, as observed in Figs. 7(a)–(c). The visible afterglow for various SAOs is shown as the photographs in Fig. 7(d). The afterglow intensities of phosphors with boron constituent prepared by hydrothermal and solid-state reaction processes were higher than those of the phosphors without boron addition. This result may be that the B2O3 content caused uniform distortion of the phosphor crystal and generated crystal defects, which can trap the holes generated by the excitation of Eu2+ ions. The B2O3 content in MSAO can cause more of the borate flux to carry out the dissolution-precipitation process for the formation of SrAl2O4, thereby increasing the agglomeration of the resultant particles. Such related discussions are also revealed in our previous investigations [50] and indicated by particle size distribution analysis in Fig. 3(f). Most of the rare-earth ions were dissolved in the grain boundary phase of B2O3 flux, then diffused into lattice. The Eu and Dy contents in the grain boundary phase are about 4 and 16.5 times higher than the Eu and Dy contents in the grains, respectively [66,67]. The afterglow properties of SAOs were enhanced by the addition of B2O3, which promoted the diffusion of rare-earth ions, Eu and Dy, into the SrAl2O4 lattice through rapid grain boundary diffusion. However, the boron was doped in the BSAO and scarcely on the grain surface (Figs. 3(c) and 5(b)). Thus, the afterglow enhancement should be attributed to the boron in the lattice inducing defect traps. This inference is also supported by the increase in the number of traps, and the efficiency of the trapped energy transferred to the vacancies [22]. The afterglow-decay time follows the exponential law, as shown in Fig. 7(a). The afterglow intensity measured at room temperature exhibits an initial fast decay then followed by a slow decay behavior. Afterglow behavior is supposed as the trapping-detrapping mechanism correlating to time and temperature. Such energy corresponding to the afterglow intensity is equal to the trap depth. According to thermodynamic theory, the average capture time τ of charge carrier depends on the trap depth ΔE and temperature T [68,69]:

is greater than that for ISAO (hy12h). The boron effect might play a role on the PL intensity in addition to the low crystallinity and mixed phases of the host. However, the large blue shift affected by boron-doping is still necessarily further study. Eu2+ ions and Dy3+ ions could diffuse into well-crystallized SrAl2O4 lattice efficiently via the assistance of boron oxide-containing regions, referred to Fig. 5(b). Then, the doped boron in BSAO enhances the PL intensity. This result indicated that boron-doping (BSAO) significantly increased the PL with efficiency even in the submicron size with nanothickness. However, the PL intensity of commercial SAO is almost the same as the solid-state prepared MSAO, but PL intensity of both is less than the annealed BSAO in Fig. 6(c). Generally, the absorption of excitation light is reduced because of the strong light scattering by nano-crystals, and thus, the emission strength of nano-sized phosphors will decrease [63,64]. Our hydrothermally prepared SAOs with one dimensionally nano-sized single crystals irrespective of boron-doping did not indicate the PL strength decay, as shown in Fig. 6(c) in comparison with MSAO and commercial SAO. The hydrothermal-annealed SAOs had good crystallinity, singlecrystal structure, and less defects. The PL excitation energy was more effectively absorbed by the plate geometries due to their large surfaceto-volume ratios. Hydrothermally synthesized phosphor of ISAO (hy24h)-1100 °C/4 h demonstrates higher PL intensity than that of MSAO prepared by solid-state reaction and commercial SAO with large grain size. The main PL emission peaks of the hydrothermally prepared phosphors slightly shifted toward the shorter wavelength compared to the PL peaks of phosphors obtained by solid-state reaction. This indicates that the inherited hydrothermal process yielded small crystal size with preferred orientation, with better homogeneously distributed activators, and enhanced the PL intensity without boron-doping (ISAO) and with low annealing temperatures. The increased surface energy of the rod-like plate phosphors may result in the distortion of lattice and the change of crystal field around Eu2+. Therefore, such micro/nanostructured form of submicroscale particles assembled from nanosheet elements can keep away from shortcomings of nano-particles and retain the inherited advantages. For the trapping-detrapping mechanism of afterglow, charge compensation is a key aspect considered for producing traps, possibly from the oxygen interstitial compensation charge defect [65]. In the strontium aluminate host, Dy3+ will induce the formation of hole-trap levels and can prolong the afterglow. The quantitative afterglow intensity decays with time for MSAO, and BSAO, ISAO after 1100 °C/4 h annealing and commercial SAO shown in Fig. 7(a). The decay process of luminescence undergoes an initial fast decay followed by a slow decaying process. The boron-doped SAOs prepared by the inherited hydrothermal process and MSAO prepared by the solid-state reaction exhibit higher afterglow intensities than that of ISAO without boron constituent. The decay curves of log(brightness)-time and log(brightness)-log(time) are shown in Figs. 7(b) and 7(c). It is observed that the brightness decays with the time for all SAOs more distinguishably. The afterglow intensity (brightness) is still above the visibility threshold of the 0.32 mcd/m2 (100 times the light perception of the scotopic vision) even the lowest brightness of annealed ISAO (hy24h) after 3600 s (1 h) for the excitation light cut-off. The BSAO synthesized from inherited hydrothermal method exhibits higher brightness than solid-state prepared MSAO and commercial SAO at the initial time of 100 s after irradiation turned off, as shown in Figs. 7(c) and 7(d). Furthermore, the afterglow of BSAO becomes to be consistent with commercial SAO after 3600 s. The afterglow behaviors of the phosphors prepared by the inherited hydrothermal, solid-state reaction processes and commercial SAO are similar. The difference is that the inherited hydrothermal process required low processing temperature and particles size/morphology control, while the solid-state reaction needed high processing temperature with abnormal grain growth. The small-grained BSAO with nanothickness under the doped boron constraint could still maintain

τ = (1/ f ) exp (ΔE /kT ) where f is the frequency factor for charge carrier such as detrapping electron and k is the Boltzmann constant. The decay process of afterglow intensity can be divided into three parts: fast, medium and subsequent slow decay process at a constant temperature, as shown in Fig. 7(a). Most of the long-afterglow phosphor decay is dominated by the first-order kinetics. If the probability of recapture is not obvious in the early rapid decay stage for multiple independent traps, the afterglow decay with time follows the exponential law including three decay processes.

I (t ) = I1 + A1 exp(−t/ τ1) + A2 exp(−t/ τ2) + A3 exp(−t/ τ3) where I represents the afterglow intensity; I1 is the final intensity, A1, A2 and A3 are constants; t is the decay time; τ1, τ2 and τ3 are the decay constants deciding the decay rate for the rapid, medium and slow exponential decay components, respectively. Our decay time measurement is less than 5 h, the above I(t) equation can be utilized suitably [68]. The fitting results of parameters of τ1, τ2 and τ3 are listed in Table 2. The initial capture time constant τ1 is used to estimate the shallower trap depth for an afterglow. The sequence of τ1 is BSAO > MSAO > commercial > ISAO shown in Table 2. The final afterglow intensity (I1) follows MSAO > BSAO~commercial > ISAO. Such results are also in correspondence with Fig. 7(c) pronouncedly. The shallow detrapping behavior may be resulted from the oxygen vacancy which can trap electron. The oxygen vacancy has been reported that may play a vital role in the persistent luminescence process of the SrAl2O4: Eu2+ and the SrAl2O4: Eu2+, Dy3+ [51]. The 599

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Fig. 7. Afterglow intensity decay with time for various prepared SAOs and commercial SAO after 365 nm UV-light excitation, (a) nominal afterglow intensity with time, (b) log(brightness)-time, (c) log(brightness)-log(time) and (d) photographs of visible afterglow for various SAOs from 10-1000 s.

The main constituting phases of Al2(OOH)2 and SrCO3 for ISAO transformed to monoclinic SrAl4O7 and mixed monoclinic/hexagonal SrAl2O4 after 1100 °C/4 h annealing. Pure monoclinic SrAl2O4 phase via Al source of NaAlO2 was obtained by hydrothermal treatment at 200 °C/24 h and 1100 °C/4 h annealing irrespective of boron-doping. The boron diffused into the lattice and compensated the induced lattice distortion by Eu2+ and Dy3+ ions in BSAO. The formation of nanothick plates of the annealed ISAO and BSAO resulted from the characteristic flower-like morphology derived by inherited hydrothermal process. The typically derived phosphor is nearly 500 nm in size with nanothickness less than 100 nm, indicating a single crystal plate. The boron doped BSAO had small grain size due to the absence of enough flux outside on the phosphor particles. The photoluminescence excitation energy was more effectively absorbed by the plate geometries due to their large surface-to-volume ratios compared to those of the samples prepared by solid-state reaction and commercial SAO. The surface energy of the plate phosphors increased, resulting in the lattice distortion and changing the crystal field around Eu2+. Boron-doping phosphors had higher excitation and emission intensities than those of the phosphors without boron. The low PL and the blue shift in its peak wavelength from 518 nm to 510 nm for

dysprosium and boron dopants must introduce a large number of new traps so that the energy storage capacity of the SAOs has been enhanced remarkably [51,70]. The results are also indicated by comparison of BSAO and MSAO to ISAO using τ2 and τ3 and I1. The more clearly explanation is still necessary to further investigate.

4. Conclusions This novel inherited hydrothermal bottom-up process contributes lower annealing temperature to synthesize one dimensionally nanosized long-persistent phosphors with enhanced luminescent properties due to homogenizing dopants distribution and well-crystallizing nanostructure with prefer-oriented morphology control. Such micro/nanostructured form of submicron scale particles assembled from nanosheet elements can keep away from shortcomings of nano-particles and retain the inherited advantages. The annealed BSAO exhibits 2.5 times PL higher than MSAO and commercial SAO. The BSAO exhibits higher brightness than solid-state prepared MSAO and commercial SAO at the initial decay time of 100 s after irradiation turned off. The afterglow of BSAO becomes to be consistent with commercial SAO after 3600 s. Table 2 Fitting results of the decay parameters. sample

I1

A1

τ1 (s)

A2

τ2 (s)

A3

τ3 (s)

ISAO BSAO MSAO commercial

4068 179,721 368,100 170,588

1,544,660 8,524,030 6,204,640 7,534,730

8.1 25.3 23.7 13. 3

699,963 6,632,880 5,945,740 5,128,700

45.9 118.8 125.9 77.6

122,107 2,532,240 2,372,070 1,527,810

301.6 645.3 751.0 545.9

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SrAl2O4:Eu2+, Rad. Meas. 38 (2004) 727–730. [24] D.S. Kshatri, A. Khare, Optical properties of rare earth doped strontium aluminate (SAO) phosphors: a review, Opt. Spectr. 117 (2014) 769–783. [25] F.C. Palilla, A.K. Levine, M.R. Tomkus, Fluorescent properties of alkaline earth aluminates of the type MAl2O4 activated by divalent europium, J. Electrochem. Soc. 115 (1968) 642–644. [26] J. Botterman, J.J. Joos, P.F. Smet, Trapping and detrapping in SrAl2O4:Eu,Dy persistent phosphors: influence of excitation wavelength and temperature, Phys. Rev. B90 (2014) 085147. [27] S.D. Han, K.C. Singh, T.Y. Cho, H.S. Lee, D. Jakhar, J.P. Hulme, C.H. Han, J.D. Kim, I.S. Chun, J.H. Gwak, Preparation and characterization of long persistence strontium aluminate phosphor, J. Lumin. 128 (2008) 301–305. [28] J. Niittykoski, T. Aitasalo, J. Holsa, H. Jungner, M. Lastusaari, M. Parkkinen, M. Tukia, Effect of boron substitution on the preparation and luminescence of Eu2+ doped strontium aluminates, J. Alloy. Compd. 374 (2004) 108–111. [29] R.E. Rojas-Hernandez, F. Rubio-Marcos, M.A. Rodriguez, J.F. Fernandez, Long lasting phosphors: SrAl2O4:Eu, Dy as the most studied material, Renew. Sustain. Energy Rev. 81 (2018) 2759–2770. [30] H.R. Zhang, Z.P. Xue, B.F. Lei, H.W. Dong, H.M. Zhang, S.Q. Deng, M.T. Zheng, Y.L. Liu, Y. Xiao, A top-down method to fabricate SrAl2O4:Eu2+,Dy3+ nanosheets from commercial blocky phosphors, Opt. Mater. 36 (2014) 1802–1807. [31] R.E. Rojas-Hernandez, F. Rubio-Marcos, E. Enríquez, M.A. De La Rubia, J.F. Fernandez, A low-energy milling approach to reduce particle size maintains the luminescence of strontium aluminates, RSC Adv. 5 (2015) 42559–42567. [32] I.C. Chen, T.M. Chen, Sol-gel synthesis and the effect of boron addition on the phosphorescent properties of SrAl2O4:Eu2+,Dy3+ phosphors, J. Mater. Res. 16 (2001) 644–651. [33] X.Y. Chen, Z. Li, S.P. Bao, P.T. Ji, Porous MAl2O4:Eu2+(Eu3+), Dy3+ (M=Sr, Ca, Ba) phosphors prepared by Pechini-type sol–gel method: the effect of solvents, Opt. Mater. 34 (2011) 48–55. [34] P. Huang, Q. Zhang, C.E. Cui, J. Li, Influence of excitation wavelengths on luminescent properties of Sr3Al2O6:Eu2+, Dy3+ phosphors prepared by sol–gel-combustion processing, Opt. Mater. 33 (2011) 1252–1257. [35] L. Xiao, S. Meng, Z. Junying, W. Tianmin, Effect of mixing process on the luminescent properties of SrAl2O4:Eu2+, Dy3+ long afterglow phosphors, J. Rare Earths 28 (1) (2010) 150–152. [36] J.B. Montes-Frausto, K. Juarez-Moreno, B. Can-Uc, G.A. Hirata-Flores, Synthesis and cytotoxic effects of SrAl2O4 persistent luminescence nanoparticles co-doped with Eu2+/Dy3+ ions, Opt. Mater. Express 6 (2016) 1488–1499. [37] B.M. Mothudi, O.M. Ntwaeaborwa, J.R. Botha, H.C. Swart, Photoluminescence and phosphorescence properties of MAl2O4:Eu2+,Dy3+ (M=Ca, Ba, Sr) phosphors prepared at an initiating combustion temperature of 500 °C, Physica B 404 (2009) 4440–4444. [38] N.M. Son, L.T.T. Vien, L.V.K. Bao, N.N. Trac, Synthesis of SrAl2O4:Eu2+ Dy3+ phosphorescence nanosized powder by combustion method and its optical properties, J. Phys.: Conf. Ser. 187 (2009) 012017. [39] Q. Xiao, L. Xiao, Y. Liu, X. Chen, Y. Li, Synthesis and luminescence properties of needle-like SrAl2O4:Eu,Dy phosphor via a hydrothermal co-precipitation method, J. Phys. Chem. Solids 71 (2010) 1026–1030. [40] Y.F. Xu, D.K. Ma, M.L. Guan, X.A. Chen, Q.Q. Pan, S.M. Huang, Controlled synthesis of single-crystal SrAl2O4:Eu2+,Dy3+ nanosheets with long afterglow phosphorescence, J. Alloy. Compd. 502 (2010) 38–42. [41] D. Jia, J. Zhu, B. Wu, Luminescence and energy transfer in CaAl4O7:Tb3+,Ce3+, J. Lumin. 93 (2001) 107–114. [42] C.H. Lu, S.Y. Chen, C.H. Hsu, Nanosized strontium aluminate phosphors prepared via a reverse microemulsion route, Mater. Sci. Eng. B Solid-State Mater. Adv. Technol. 140 (2007) 218–221. [43] J. Geng, Z. Wu, Synthesis of long afterglow SrAl2O4:Eu2+,Dy3+ phosphors through microwave route, J. Mater. Synth. Proc. 10 (2002) 245–248. [44] B.C. Cheng, H.J. Liu, M. Fang, Y.H. Xiao, S.J. Lei, L. Zhang, Long-persistent phosphorescent SrAl2O4:Eu2+, Dy3+ nanotubes, Chem. Commun. 8 (2009) 944–946. [45] G.P. Dong, X.D. Xiao, L.L. Zhang, Z.J. Ma, X. Bao, M.Y. Peng, Q.Y. Zhang, J.R. Qiu, Preparation and optical properties of red, green and blue afterglow electrospun nanofibers, J. Mater. Chem. 21 (2011) 2194–2203. [46] R. Aroz, V. Lennikov, R. Cases, M.L. Sanjuán, G.F. de la Fuente, E. Muñoz, Laser synthesis and luminescence properties of SrAl2O4:Eu2+, Dy3+ phosphors, J. Eur. Ceram. Soc. 32 (2012) 4363–4369. [47] R.E. Rojas-Hernandez, F. Rubio-Marcos, R.H. Gonçalves, M.Á. Rodriguez, E. Véron, M. Allix, C. Bessada, J.F. Fernandez, Original synthetic route to obtain a SrAl2O4 phosphor by the molten salt method: insights into the reaction mechanism and enhancement of the persistent luminescence, Inorg. Chem. 54 (2015) 9896–9907. [48] C.Z. Li, Y. Imai, Y. Adachi, H. Yamada, K. Nishikubo, C.N. Xu, One-step synthesis of luminescent nanoparticles of complex oxide, strontium aluminate, J. Am. Ceram. Soc. 90 (2007) 2273–2275. [49] P. Escribano, M. Marchal, M.L. Sanjuán, P. Alonso-Gutiérrez, B. Julián, E. Cordoncillo, Low-temperature synthesis of SrAl2O4 by a modified sol–gel route: XRD and Raman characterization, J. Solid State Chem. 178 (2005) 1978–1987. [50] C.Y. Wu, C.M. Lei, T. Takei, Rudder Wu, S.H. Wang, C.C. Chou, H.Y. Chang, Structure and luminescence of core-shell phosphor aluminate-boron oxide, J. Alloy. Compd. 743 (2018) 506–513. [51] P. Zeng, X.T. Wei, M. Yin, Y.H. Chen, Investigation of the long afterglow mechanism in SrAl2O4:Eu2+/Dy3+ by optically stimulated luminescence and thermoluminescence, J. Lumin. 199 (2018) 400–406. [52] J.T. Chen, F. Gu, C.Z. Li, Influence of precalcination and boron-doping on the initial photoluminescent properties of SrAl2O4:Eu,Dy phosphors, Cryst. Growth Des. 8 (2008) 3175–3179.

ISAO (hy6h) might be attributed to the low crystallinity and mixed phases of SrAl2O4 and SrAl4O7 in it that also affected the crystal field. The submicron-grained BSAO could maintain the long-afterglow intensity, as did the several tens microns large-grained MSAO and commercial SAO. The afterglow of the nanothick phosphors prepared by inherited hydrothermal process and 1100 °C/4 h annealing (ISAO and BSAO) should also follow defects trapping-detrapping mechanism; however, the PL behavior of nanothick phosphors depended on wellcrystallized pure phase possessing preferred-surface energy and fully activators homogenization significantly. Acknowledgements The authors acknowledge the funding supports from Grant no. NTOU-105-6 of National Taiwan Ocean University and Grant no. MOST 106-2622-E-019-004-CC3 of the Ministry of Science and Technology, Taiwan, ROC. The financial support from the program of pilot overseas internships (2016), sponsored by the Ministry of Education, Taiwan, ROC, is also thankfully. References [1] Y. Li, M. Gecevicius, J.R. Qiu, Long persistent phosphors - from fundamentals to applications, Chem. Soc. Rev. 45 (2016) 2090–2136. [2] J. Botterman, P.F. Smet, Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution, Opt. Express 23 (15) (2015) A868–A881. [3] J. Kaur, R. Shrivastava, B. Jaykumar, N.S. Suryanarayana, Studies on the persistent luminescence of Eu2+ and Dy3+ - doped SrAl2O4 phosphors: a review, Res. Chem. Intermed. 40 (2014) 317–343. [4] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, A new long phosphorescent phosphor with high brightness, SrAl2O4:Eu2+, Dy3+, J. Electrochem. Soc. 143 (1996) 2670–2673. [5] A. Nag, T.R.N. Kutty, Role of B2O3 on the phase stability and long phosphorescence of SrAl2O4:Eu, Dy, J. Alloy. Compd. 354 (2003) 221–223. [6] G. Blasse, A. Bril, Fluorescence of Eu2+-activated alkaline-earth aluminates, Philips Res. Rep. 23 (1968) 201–206. [7] F.P. Glasser, L.S. Dent Glasser, Crystal chemistry of some AB2O4 compounds, J. Am. Ceram. Soc. 46 (1963) 377–380. [8] H.D. Megaw, Crystal Structure: A Working Approach, W.B. Saunders, London, 1973. [9] K. Fukuda, K. Fukushima, Crystal structure of hexagonal SrAl2O4 at 1073 K, J. Solid State Chem. 178 (2005) 2709–2714. [10] S. Ito, S. Banno, K. Suzuki, M. Inagaki, Phase transition in SrAl2O4, Z. Phys. Chem. 105 (1977) 173–178. [11] S.H.M. Poort, W.P. Blokpoel, G. Blasse, Luminescence of Eu2+ in barium and strontium aluminate and gallate, Chem. Mater. 7 (1995) 1547–1551. [12] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. A32 (1976) 751–767 (Shannon Radii - Atomistic Simulation Group), 〈http://abulafia.mt.ic.ac.uk/shannon/ptable. php〉. [13] J. Bierwagen, S. Yoon, N. Gartmann, B. Walfort, H. Hagemann, Thermal and concentration dependent energy transfer of Eu2+ in SrAl2O4, Opt. Mater. Express 6 (2016) 793–803. [14] D. Haranath, V. Shanker, H. Chander, P. Sharma, Tuning of emission colours in strontium aluminate long persisting phosphor, J. Phys. D: Appl. Phys. 36 (2003) 2244–2248. [15] G.M. Qiu, Y.G. Chen, J.Q. Cui, X.J. Geng, H. Wang, B. Song, Synthesis of long afterglow phosphors doped B SrA12O4: Eu2+, Dy3+ and its luminescent properties, J. Rare Earths 25 (2007) 86–89. [16] Y.L. Chang, H.I. Hsiang, M.T. Liang, F.S. Yen, Phase evolution and thermal behaviors of the solid-state reaction between SrCO3 and Al2O3 to form SrAl2O4 under air and CO2-air atmospheres, Ceram. Int. 38 (2012) 2269–2276. [17] Y.H. Wang, X.Y. Song, S.H. Zhang, Effect of B2O3 on long-last phosphorescence of SrAl2O4:Eu2+,Dy3+, Mater. Sci. 610–613 (2009) 403–407. [18] P. Dorenbos, Mechanism of persistent luminescence in Eu2+ and Dy3+ codoped aluminate and silicate compounds, J. Electrochem. Soc. 152 (7) (2005) H107–H110. [19] F. Clabau, X. Rocquefelte, S. Jobic, P. Deniard, M.H. Whangbo, A. Garcia, T. Le Mercier, Mechanism of phosphorescence appropriate for the long afterglow phosphors Eu2+ doped with codopants Dy3+ and B3+, Chem. Mater. 17 (15) (2005) 3904–3912. [20] E. Nakazawa, T. Mochida, Traps in SrAl2O4:Eu2+ phosphor with rare-earth ion doping, J. Lumin. 72–74 (1997) 236–237. [21] W. Jia, H. Yuan, L. Lu, H. Liu, W.M. Yen, Phosphorescent dynamics in SrAl2O4: Eu2 + , Dy3 + single crystal fibers, J. Lumin. 76–77 (1998) 424–428. [22] J. Hölsä, H. Jungner, M. Lastusaari, J. Niittykoski, Persistent luminescence of Eu2+ doped alkaline earth aluminates, MAl2O3:Eu2+, J. Alloy. Compd. 323 (2001) 326–330. [23] T. Aitasalo, J. Hölsä, H. Jungner, J.C. Krupa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, Effect of temperature on the luminescence processes of

601

Journal of Luminescence 206 (2019) 593–602

C.-Y. Wu et al. [53] Y.T. Yu, B.G. Kim, Effects of adding B2O3 as a flux on phosphorescent properties in synthesis of SrAl2O4:(Eu2+, Dy3+) phosphor, Korean J. Chem. Eng. 20 (5) (2003) 973–976. [54] Z. Tang, F. Zhang, Z. Zhang, C. Huang, Y. Lin, Luminescent properties of SrAl2O4:Eu,Dy material prepared by the gel method, J. Eur. Ceram. Soc. 20 (2000) 2–5. [55] R.E. Rojas-Hernandez, F. Rubio-Marcos, A. Serrano, A.D. Campo, J.F. Fernandez, Precise tuning of the nanostructured surface leading to the luminescence enhancement in SrAl2O4 based core/shell structure, Sci. Rep. 7 (2017) 462 (9 pages). [56] G.J. Ji, M.M. Li, G.H. Li, G.M. Gao, H.F. Zou, S.C. Gan, X.H. Xu, Hydrothermal synthesis of hierarchical micron flower-like γ-AlOOH and γ-Al2O3 superstructures from oil shale ash, Powder Tech. 215–216 (2012) 54–58. [57] W.Q. Cai, J.G. Yu, M. Jaroniec, Template-free synthesis of hierarchical spindle-like γ-Al2O3 materials and their adsorption affinity towards organic and inorganic pollutants in water, J. Mater. Chem. 20 (2010) 4587–4594. [58] B.C. Cheng, L.T. Fang, Z.D. Zhang, Y.H. Xiao, S.J. Lei, BaAl2O4:Eu2+, Dy3+ nanotube synthesis by heating conversion of homogeneous coprecipitates and afterglow characteristics, J. Phys. Chem. C 115 (2011) 1708–1713. [59] H.Y. Chang, S.Y. Cheng, C.I. Sheu, Controlling interface characteristics by adjusting core-shell structure, Acta Mater. 52 (18) (2004) 5389–5396. [60] H.Y. Chang, S.Y. Cheng, C.I. Sheu, Y.H. Wang, Core–shell structure of strontium titanate self-grown by a hydrothermal process for use in grain boundary barrier layers, Nanotechnology 14 (2003) 603–608. [61] F.B. Dejene, M.A. Kebede, M. Redi-Abshiro, B.V. Kgarebe, Structural and photoluminescence properties of Dy3+ co-doped and Eu2+ activated MAl2O4 (M=Ba, Ca,

Sr) nanophosphors, Opt. Mater. 35 (2013) 1927–1931. [62] J.T.C. van Kemenade, G.P.F. Hoeks, in: The electrochemical society extended abstracts, , San Francisco, CA, vol. 83-1, Abstract 607, May 8–13 1993, p. 914. [63] H. Yamada, W.S. Shi, C.N. Xu, Observation of orientational disorder in the hexagonal stuffed tridymite Sr0.864Eu0.136Al2O4 by the maximum-entropy method, J. Appl. Cryst. 37 (2004) 698–702. [64] T.Y. Peng, H.P. Yang, X.L. Pu, B. Hu, Z.C. Jiang, C.H. Yan, Combustion synthesis and photoluminescence of SrAl2O4:Eu,Dy phosphor nanoparticles, Mater. Lett. 58 (2004) 352–356. [65] B.F. dos Santos Jr, M.V. dos Santos Rezende, P.J.R. Montes, R.M. Araujo, M.A.C. dos Santos, M.E.G. Valerio, Spectroscopy study of SrAl2O4:Eu3+, J. Lumin. 132 (2012) 1015–1020. [66] X.X. Luo, W.H. Cao, Z.G. Xiao, Investigation on the distribution of rare earth ions in strontium aluminate phosphors, J. Alloy. Compd. 416 (2006) 250–255. [67] G.H. Lee, C.S. Yoon, S.H. Kang, Role of flux in the production process of red phosphors for white LEDs, J. Mater. Sci. 43 (2008) 6109–6115. [68] T. Jiang, H. Wang, M.M. Xing, Y. Fu, Y. Peng, X.X. Luo, Luminescence decay evaluation of long-afterglow phosphors, Phys. B Condens. Matter 450 (2014) 94–98. [69] W.M. Yen, S. Shionoya, H. Yamamoto, Phosphor Handbook, Second edition, CRC Press, Boca Raton, London, New York, 2006. [70] I.C. Chen, K.K. Chen, H.S. Chen, J.K. Du, T.J. Lin, S.S. Lin, T.M. Chen, T.Y. Shieh, Investigations into thermoluminescence and afterglow characterization of strontium aluminates with boron-modification and reductions via sol-gel route, J. Rare Earths 30 (2012) 972–978.

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