# Fileset [APT_three phosphors from LRH.pdf](https://mdr.nims.go.jp/filesets/a66df636-0b38-4401-9fd3-b49a05493b5d/download) ## Creator Bowen Wang, Changshuai Gong, Jiantong Wang, Xuejiao Wang, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361) ## Rights [Creative Commons BY-NC-ND Attribution-NonCommercial-NoDerivs 4.0 International](https://creativecommons.org/licenses/by-nc-nd/4.0/) ## Other metadata [Oxysulfate, oxysulfide, and oxide red-phosphors from one single hydroxyl sulfate precursor (Gd,Eu)2(OH)4SO4·nH2O: phase evolution, and photoluminescence](https://mdr.nims.go.jp/datasets/a41d45b7-7927-4644-b6d5-002dc2a98ca3) ## Fulltext 1 Oxysulfate, oxysulfide, and oxide red-phosphors from one single hydroxyl sulfate precursor (Gd,Eu)2(OH)4SO4·nH2O: phase evolution, and photoluminescence Bowen Wang,a Changshuai Gong,a Jiantong Wang,a Xuejiao Wang,a* Ji-Guang Lib* aCollege of Chemistry and Materials Engineering, Bohai University, Jinzhou, Liaoning 121007, China bResearch Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan *Corresponding author Dr. Xuejiao Wang Bohai University Jianzhou, China Tel: +86-416-3400708 E-mail: wangxuejiao@bhu.edu.cn Dr. Ji-Guang Li National Institute for Materials Science Ibaraki, Japan Tel: +81-29-860-4394 E-mail: li.jiguang@nims.go.jp Manuscript Click here to view linked References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 mailto:wangxuejiao@bhu.edu.cnmailto:li.jiguang@nims.go.jphttps://www2.cloud.editorialmanager.com/advpt/viewRCResults.aspx?pdf=1&docID=38708&rev=1&fileID=682908&msid=31b28c0b-1c81-45a8-afa5-d6c620b55dbahttps://www2.cloud.editorialmanager.com/advpt/viewRCResults.aspx?pdf=1&docID=38708&rev=1&fileID=682908&msid=31b28c0b-1c81-45a8-afa5-d6c620b55dba2 Abstract Three kinds of important red phosphors, the monoclinic Ln2O2SO4 (Ln=Gd, Eu), cubic Ln2O3 and hexagonal Ln2O2S were obtained through controlled calcination of single hydroxyl sulfate Ln2(OH)4SO4·nH2O (LLnHs) precursor. The conditions of the hydrothermal synthesis of LGdHs were investigated, and it was found that LGdHs can be obtained in pH range of 7-9. The micro-morphology changes from aggregated spheres to micro plates with increasing pH. Gd2(OH)4SO4·nH2O was transformed into Gd2O2SO4 via removal of hydration water (up to 340 oC) and dehydroxylation (340-800 oC), and finally change into Gd2O3 via desulfuration (800-1230 oC) in the air. Gd2O2S was obtained from Gd2(OH)4SO4·nH2O by calcination in reducing atmosphere (800-1200 oC). Photoluminescence excitation (PLE) studies showed broad and strong O-Eu CT bands at 260, 275, and 267 nm in Ln2O3, Ln2O2SO4 and Ln2O2S, respectively, and extra S-Eu CT band locates at 334 nm was observed in Ln2O2S. The strongest red emission corresponding to the electric dipole 5D0→7F2 transitions of the Eu3+ ions were observed at 613, 618, and 626 nm in Ln2O3, Ln2O2SO4 and Ln2O2S system, respectively. The quenching concentration of Ln2O2S (3%) is smaller than those of Ln2O2SO4 (6%) and Ln2O3 (6%), which is due to the fact that the average Eu3+-Eu3+ distance is smaller in Ln2O2S. The quenching mechanism was investigated, and it was found that the observed luminescence quenching was dominated by exchange interactions among Eu3+ ions. Keywords: Photoluminescence, oxysulfate, oxysulfide, hydroxyl sulfate 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3 1. Introduction Due to the unique physicochemical properties of rare-earth elements, phosphors activated by rare earth are being actively studied for a wide range of applications, including high-performance optical display, luminescence, optoelectronic devices, sensors, and magnets [1-8]. The synthesis route significantly influences the properties of rare earth doped materials, such as morphology, fluorescence intensity, and quantum efficiency. Various methodologies were developed, including solid-state reaction, molten salt method, combustion synthesis method, hydrothermal and so on [9-12]. Amongst, solid-state reaction is the most efficient method for design and synthesis of phosphors, but elevated temperature is always needed. Another widely applied route is hydrothermal method, through which final products were always obtained via calcination of certain intermediate precursors. It has the merits of controlled morphology and low processing temperature, however, in most cases only one product was obtainable via such route. We found in this work that Ln2(OH)4SO4·nH2O can serve as an efficient precursor, and through its controlled calcination three kinds of important phosphors, including oxysulfide Ln2O2S, oxysulfate Ln2O2SO4 and oxide Ln2O3 can be obtained. Rare earth doped Ln2O2S is an important kind of phosphors, founding application in many fields, such as lighting, MRI, scintillant and biological labeling [13,14]. Meanwhile, Ln2O2SO4 with a layered structure was reported as a promising material for large volume oxygen storage [15-17] as well as a useful luminescent material [18-20]. The Ln2(OH)4SO4·nH2O crystals have the same Ln:S molar ratio of the above-mentioned two types of phosphors, and thus, they may be directly converted into the target phosphors under proper conditions, without any additional use of hazardous sulfurization reagents. Furthermore, after the decomposition of sulfate ions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4 by calcining LLnHs at higher temperatures in the air, Ln2O3 luminescence materials can be obtained. In this work, (Gd,Eu)2(OH)4SO4·nH2O compounds with tunable morphology were hydrothermally obtained, and through the controlled calcination, three kinds of important red phosphors (Gd,Eu)2O2SO4, (Gd,Eu)2O3 and (Gd,Eu)2O2S were obtained. The influence of pH level on the phase and micro-morphology of hydrothermal products, and photoluminescent properties of the resultant phosphors were system studied based on the analyses of X-ray diffractometry (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetry/differential scanning calorimetry (TG/DSC) and photoluminescence (PL/PLE) results. 2. Experimental Procedure 2.1 Reagents and sample synthesis The reagents for the synthesis are Ln2O3 (Ln = Eu, Gd, 99.99% pure), (NH4)2SO4, NH3·H2O, and HNO3. The rare earth oxide were bought from Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd. (Huizhou, China), and the other reagents were from Shenyang Chemical Reagent Factory (Shenyang, China). All the chemicals were used as received without further purification. Stock solutions of Ln(NO3)3 (Ln=Gd and Eu) were prepared by dissolving Gd2O3 and Eu2O3 in concentrated HNO3 (0.1 M). In a typical synthetic procedure, 6 mmol (NH4)2SO4 was dissolved in 60 ml mixed nitrate solution (6 mmol) under magnetic stirring for 10 min. NH3·H2O was then dropwise added into the mixed solution to reach the designed pH. The resultant mixture was continuously stirred for 10 min before being transferred to a Teflon lined stainless steel autoclave of 100 ml capacity. The autoclave was put in an electric oven preheated to 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5 120 oC for a reaction period of 24 h. After the reaction, the hydrothermal product was collected via centrifugation. The wet precipitate was thoroughly washed with water and ethanol, and it was finally dried in air at 50 oC for 12 h. The (Gd1-xEux)2(OH)4SO4·nH2O compound was calcined in the air and hydrogen atmosphere at 1200 oC to obtain final (Gd1-xEux)2O3 and (Gd1-xEux)2O2S red phosphors (heating rate 10 oC /min), respectively. The (Gd1-xEux)2O2SO4 sulfate was obtained by calcining the LLnH in the air at 1000 oC with the same heating rate. 2.2 Characterization Phase identification was performed via powder XRD (Model PW3040/60, Philips, Eindhoven, The Netherlands) using nickel filtered CuKα radiation operated at 40 kV/40 mA. Morphologies were observed via field emission scanning electron microscopy by an JSM-7001F (JEOL, Tokyo, Japan) operated at 10 kV. Phase decomposition of the LLnH was made in flowing simulated air (heating rate: 10 oC/min) via TG/DSC (Model STI409PC, NETZSCH, Germany) analysis. FTIR analyses (Model 4200, JASCO, Tokyo) were performed by the standard KBr method. Photoluminescence spectra and fluorescence decay kinetics of phosphors were performed at room temperature using an FP-6500 fluorescence spectrophotometer (JASCO, excitation source: 150 W Xe lamp). Results and discussion Fig. 1 shows XRD patterns of products hydrothermally synthesized at 120 oC with different pHs (7.0-10.0). Products synthesized at lower pHs of 7.0-9.0 are indexable to Gd2(OH)4SO4·nH2O (SO42--LLnHs) [21,22] even though the diffraction peak in the same position of the products exhibit some difference in intensity which may be caused by the difference of morphology as analyzed later (Fig. 3). Previously, we investigated the hydrothermal synthesis conditions of La2(OH)4SO4·nH2O which can be 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6 hydrothermally obtained at 120 oC and pH of 9.0 - 10.0 [23]. The Ln3+ cations undergo hydration and hydrolysis in an aqueous solution to form six-fold coordinated [Ln(OH)x(H2O)y(SO4)z]3−x−z/2 (x+y+z=6) complex species [24]. Compared with lanthanum, gadolinium has a smaller ion radius (for 9-fold coordination, La3+ and Gd3+ have respective ionic sizes of 0.1216 and 0.1107 nm) [25] which promotes hydrolysis of Gd3+ at same pH thus favoring the formation of Gd2(OH)4SO4·nH2O at lower pHs than La2(OH)4SO4·nH2O. Fig. 1. XRD patterns of the hydrothermal products synthesized at 120 oC under different pHs, with (a) pH=7.0, (b) pH=8.0, (c) pH=9.0, and (d) pH=10.0. Fig. 1d is the product synthesized at higher pH of 10.0 and it conforms well to the layered compound of Gd2(OH)5X·nH2O (X=Cl- or NO3-) [26-29]. Combined the FTIR behavior (Fig.2d) of this product, it can be concluded that the product obtained here is sulfate type, the Ln2(OH)5(SO4)0.5·nH2O. The compound synthesized at pH of 9.0 shows 5 10 15 20 25 30 35 40 45 50 55 60(304)(a)Intensity (a.u.) 2Theta (deg.)(119)(124)(217)(122)(022)(115)(215)(008)(108)(117)(211)(015)(213)(113)(202)(013)(111)(104)(011)(004)(102)(100)(002)(b) (222)(0010)(313)(311)(017)(106)(c) (400)(222)(220)(004)(002)(d) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7 better crystallinity and morphology, and thus, in this work Eu3+ doped Gd2(OH)4SO4·nH2O were synthesized according to the above mentioned hydrothermal route at 120 oC and pH = 9.0. The Eu3+ content in the (Gd1-xEux) combination was varied in the range of x=0.03-0.07 to reveal its effects on optical properties (Fig. S1). Fig. 2. FTIR spectra of the hydrothermal products synthesized at 120 oC under different pHs, with (a) pH=7.0, (b) pH=8.0, (c) pH=9.0, and (d) pH=10.0. Fig. 2 compares the FTIR behaviors of the Gd2(OH)4SO4·nH2O (pH = 7-9 products) and Gd2(OH)5SO4·nH2O (pH = 10 product). The Gd2(OH)4SO4·nH2O obtained at different pHs (Figs. 2a-c) shows similar hydroxides, hydrogen water, and sulfate ions vibrations, however, striking different behaviors were observed in Gd2(OH)5(SO4)0.5·nH2O (Fig. 2d). The fundamental vibrations of SO42- are located at 1104 (ν3), 981 (ν1), 618 (ν4) and 451 cm-1 (ν2), according to [4]. The SO42- vibrations are all clearly observable for the two kinds of LLnHs but with ν3 and ν4 vibrated obviously 4000 3500 3000 2500 2000 1500 1000 5004343(a) 817592668715455100810821130117716483231348221(b) 2111046141445164833543599(d) 3612 (c) Transmittance (a.u.)Wavenumber (cm-1) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 8 different. In Gd2(OH)4SO4·nH2O, ν3 and ν4 modes split into separated sharp peaks indicating the distortion and trans-bidentate ligands chelating of tetrahedron sulfate ion [4]. The distortion of the SO42- tetrahedron in Gd2(OH)4SO4·nH2O was believed to arise from intra-molecular H2O/SO42- interactions via hydrogen bonding. While in Gd2(OH)5(SO4)0.5·nH2O, the ν3 (1104 cm-1) and ν4 (614 cm-1) vibrations are non-splitting, and it indicates that the sulfate ions in Gd2(OH)5(SO4)0.5·nH2O are not directly coordinated to rare earth ions. However, ν1 and ν2 vibrations are observable and imply that the SO42- tetrahedron is not totally free but slightly distorted. The absorptions at ~3354, and 1648 cm-1 are attributed to O-H stretching vibration (ν1 and ν3) and H-O-H bending vibration mode (ν2) of hydration water, and vibration at 3599 cm-1 is arising from hydroxyl (OH-) groups (Fig. 2d) [30-32]. While in spectra (a)-(c), except the similar observed vibrations at 3231 and 1648 cm-1, additional hydroxyl absorption appears at 3612, 817 and 592 cm-1. The reason is that the SO42- interacts with H2O via hydrogen bonding which would distort not only the SO42- but also the molecular structure of H2O [4,21]. Fig. 3. FE-SEM images of the hydrothermal products synthesized at 120 oC under different pHs, with (a) pH=7.0, (b) pH=8.0, (c) pH=9.0, and (d) pH=10.0. The pH level significantly influences the morphology of the hydrothermal products, as shown in Fig. 3. Raising the pH level of the reaction system obviously downsizes the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 9 particles of final products. The reason behind the shrunken particle size is the increasing pH raises the nucleation number. The LLnHs obtained at the lowest pH of ~7.0 show the microsphere morphology (~70-90 μm), which, in fact, is the aggregated small plates (insert in Fig. 3a), and with relatively uniform size. When pH is ~8.0, the aggregated spheres were also obtained but with smaller particle size ~40-60 μm and more loose aggregation. The formation of the aggregated spheres can be explained as followed: the tetrahedral sulfate ions are abundant in negative charges and oxygen ions, and consequently, the sulfate ions attract positive groups. The nucleation and growth of small LLnHs plates are adhered to the sulfate ions, and due to the attraction, the final products are appeared as aggregated microspheres. But with the continued increasing pH, the attraction becomes weaker or even disappears, and so, the crystal can grow freely and show the nature of layered compounds which explains the formation of microplates. The LLnHs compounds were reported to construct by alternative stacking of the bidentated sulfate layer and the corrugated two-dimensional layer formed by LnO9 polyhedra parallel to the (001) plane [22]. When aggregate to the microsphere at pH of 7.0 and 8.0, the edge of the plates is exposured in the surface of the microplates, as shown in the insets. This makes the intensity of the (111) plane much stronger than the (002) plane. When crystallized freely as a microplate, the intensity ratio of the (002)/(111) obviously increases due to the exposure of the plates. In Fig. 1, this explains the peak intensity difference of the same phases obtained at different pHs. Gd2(OH)5(SO4)0.5·nH2O synthesized at pH of 10.0 (image d) is crystallize as thin nano-platelets. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 10 Fig. 4. TG/DSC curves of microplates precursor synthesized at 120 oC and pH of ~9.0. Fig. 4 shows TG/DSC curves of the microplates Gd2(OH)4SO4·nH2O sample obtained in flowing simulated air (5 ml/min, heating rate: 10 oC/min). The compound lost weight in three main separate steps upon heating. The initial removal of hydration water is observed up to 340 oC with a total weight loss of 7.50% and an endothermic peak at 271 oC, from which the hydration number n was derived to be ~2.15. The weight loss of 6.84% took place in the range of ~340-800 oC, accompanied by a sharp endotherm at ~389 oC, which can be attributed to dehydroxylation of the hydroxides layers (calculated weight loss: 6.96%). Compared with the thermal behavior of La2(OH)4SO4·nH2O reported before [33-35], the dehydroxylation reaction which involved the breaking of Gd3+-OH- bonding of Gd2(OH)4SO4·nH2O proceeded at higher temperatures indicating the higher stability in hydroxides layers for smaller lanthanides. The mass stays stable up to ~1030 oC, followed by further decomposition of Gd2O2SO4 to form cubic Gd2O3, accompanied by an endotherm at ~1214 oC (calculated weight loss: 200 400 600 800 1000 1200707580859095100-60-40-2002040608010012015.16%6.84%7.50%EndoExoHeat flow (mw)1214oC389oC271oC235oCDSCTGWeight (%)Temperature (oC) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 11 15.46%; observed weight loss: 15.16%). The weak endotherm at ~235 oC on the DSC curve is due to the evaporation of surface adsorbed water. According to the above analysis, the LGdHs compound decomposes to cubic Gd2O3 according to the following main stages: Gd2(OH)4SO4·2.15H2O→Gd2(OH)4SO4+2.15H2O (up to 340 oC) Gd2(OH)4SO4→Gd2O2SO4+2H2O (340-800 oC) Gd2O2SO4→Gd2O3+SO3 (800-1230 oC) Fig. 5. XRD patterns of the microplate precursor and the products obtained in the air by calcination at different temperatures for 1 h. Phase evolution of the microplates precursor upon calcination in the air is shown in Fig. 5. The calcination at 400-1000 oC produces monoclinic Gd2O2SO4 as a pure phase [16,21]. Narrow peak shapes and improved peak intensities are observed at a 1000 oC owing to high lattice perfection and crystallite growth. The continued calcination at 5 10 15 20 25 30 35 40 45 50 55 60 400 oC(622)(611)(440)(431)(332)(411)(400)(222)(211) 2Theta (deg.)Intensity (a.u.) 1200 oCIntensity (a.u.) precursorJCPDS NO. 00-012-0797 (220)(020)(600)(310)(110)(200)1000 oC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 12 higher temperature 1200 oC yielded cubic Gd2O3 (JCPDS No.00-012-0797) [36]. Thus, in this work, the (Gd1-xEux)2O2SO4 and (Gd1-xEux)2O3 phosphors were obtained by calcining the corresponding L(Gd1-xEux)Hs precursors in the air at 1000 and 1200 oC, respectively (Figs. S2 and S3). The phase transformation of LGdHs to Gd2O2SO4 was viewed as the removal of hydrogen water and hydroxyl groups. The LnO9 polyhedra transformed into LnO4 tetrahedra linked together by sharing edges. The monoclinic Gd2O2SO4, which inherits the layered structure, was built up via alternative stacking of the Gd-O-Gd and SO42- layers along the a-axis [11]. The morphology evolution of the LGdHs calcined in the air at different temperatures is shown in Fig. S4 (Supporting information). The calcination products can mainly preserve the morphology of layered precursor in the calcination process even at so high temperature as 1200 oC. However, cracks and pores were found in the products calcined at 1000 and 1200 oC. Fig. 6. FTIR spectra of the microplates precursor and its products calcined in the air at different temperatures for 1 h. 4000 3500 3000 2500 2000 1500 1000 500(a) precursor (b) 400 oC 4 (c) 600 oC (e) 1000 oC(d) 800 oC 535(f) 1200 oC 4281713164832313482361213 Wavenumber (cm-1)Transmittance (a.u.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 13 To further investigate the change of functional groups, the FTIR results recorded for the precursor and calcination products are shown in Fig. 6. As it can be assumed on the base of spectra shown in Fig. 2 the vibrations of hydroxyl units, water molecule and coordinate sulfate ions are observed in Fig. 6a. Compared with spectrum (a), the absence of hydroxyl vibrations in Figs. 6b-e indicates that the dehydroxylation occurred in this temperature range. However, residual wide bands present at ~3500 cm-1 up to 800 C, this may due to the absorbed water from the ambient atmosphere by the samples. The fundamental ν3 and ν4 modes of sulfate ion with splitting were still observable even after calcination at 1000 oC indicating the similar configuration of sulfate ions in the oxysulfate and LGdHs. However, expansion of the ν3 band from ~1165-1078 to ~1205-1054 cm-1 and obviously sharper split of ν4 band are observed in the oxysulfate. These effects are mainly caused by dehydration and dihydroxylation, which remove the intramolecular SO42-/OH- (H2O) coupling and enhance SO42- coordination with rare earth ions. In Fig. 6f, only the Gd-O vibration (~535 cm-1) can be observed, and it further indicates the decomposition of sulfate ions at a 1200 oC with formation of the pure oxide. The FTIR behaviors observed here correspond well to the thermal behaviors and the phase evolution of the LGdHs (Figs. 4 and 5). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 14 Fig. 7. XRD patterns of the microplates precursor and its products calcined in hydrogen atmosphere at different temperatures for 1 h. The standard diffractions of Gd2O2S (JCPDS No. 03-065-3449) are included as bars for comparison. Fig. 7 exhibits XRD patterns of LGdHs and the products calcined under flowing H2 at different temperatures for 1 h. The 800-1200 oC powders can all be well indexed to hexagonal structured Gd2O2S (JCPDS No. 03-065-3449) [37]. It is reasonable to infer that the LGdHs precursor converted into Gd2O2S through the Gd2O2SO4 as an intermediate phase, as was reported earlier for L(La,Eu)Hs [23]. The phase transformation of Gd2O2SO4 to Gd2O2S can be viewed as the removal of oxide ions surrounding sulfur as a result of the reduction, and thus, the two phase share a similar structure: alternative stacking of a Gd2O22+ layer and a layer of anion groups, i.e. sulfate (SO42-) or sulfide (S2-). As TG/DSC and XRD results showed no phase change of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 15 Gd2O2SO4 in the air up to ~1000 oC, the above observations thus indicate that H2 has a strong capability to take out oxygen atoms from SO42- even at the low temperature of 800 oC. Thus, the (Gd1-xEux)2O2S phosphors were obtained upon annealing the LLnHs precursors in flowing H2 at 1200 °C (Fig. S5). Fig. 8. PL excitation spectra of the (Gd0.97Eu0.03)2O2SO4, (Gd0.97Eu0.03)2O3 and (Gd0.97Eu0.03)2O2S red phosphors converted from the microplate precursors. Fig. 8 shows the photoluminescence excitation spectra of the (Gd0.97Eu0.03)2O2SO4, (Gd0.97Eu0.03)2O3 and (Gd0.97Eu0.03)2O2S red phosphors converted from the microplate LLnHs precursor, and the PLE spectra recorded for other compositions are shown in Fig. S6. A glance at the three spectra is yielded that all the spectra consist of two parts i.e. the broad and strong charge transfer (CT) excitations and relatively weaker and sharper f-f transitions in the longer wavelengths, as labeled in the spectra [38-42]. In Gd2O2S crystal structure, an additional stronger S-Eu CT excitation band, aside from the O-Eu one, is observed as a broad band center around 344 nm, which makes the oxysulfide compounds show significantly broader CT excitation ranges. While the 4f6 transitions are located at almost the same position even though in Gd2O2S host lattice, some 4f6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 16 transitions are indistinguishable due to the overlapping with much stronger CT band. The reason is that 4f6 electrons are well shielded from the surroundings by completely filled 5s2 and 5p6 orbitals [43,44]. The 8S7/2→6IJ transition of Gd3+ ions is observed at ~276 nm [45] for Gd2O2S and Gd2O3, and for Gd2O2SO4 the 8S7/2→6IJ transition overlaps with the CT band. Fig. 9. PL emission spectra of the (Gd0.97Eu0.03)2O2SO4, (Gd0.97Eu0.03)2O3, and (Gd0.97Eu0.03)2O2S red phosphors converted from the microplates precursors. Fig. 9 displays the photoluminescence emission spectra of the (Gd0.97Eu0.03)2O2SO4, (Gd0.97Eu0.03)2O3 red phosphors excited by the O2−→Eu3+ CTB, and (Gd0.97Eu0.03)2O2S phosphor excited by S2−→Eu3+ CTB. Sharp peaks are observed over the 500-730 nm range, which are associated with the transitions from the excited 5D0 and 5D1 state to the 7FJ (J =0, 1, 2) emission states of Eu3+ [46-48]. The strongest red emission corresponding to the electric dipole 5D0→7F2 transitions of the Eu3+ ions were observed at 613, 618, and 626 nm, as reported before [18,49,50] for (Gd,Eu)2O3, (Gd,Eu)2O2SO4, and (Gd,Eu)2O2S respectively. The (Gd0.97Eu0.03)2O2S phosphors emit red light at 626 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 17 nm, which is closer to the ideal red-color (around 650 nm) than the other two host lattices at around 615 nm. The emission spectra measured for other phosphor compositions are shown in Fig. S7. For each host lattice, the PL bands do not change obviously in their positions but exhibit different intensities with the varying doping concentration. For (Gd1-xEux)2O2SO4 and (Gd1-xEux)2O3 phosphors, the intensity of the strongest emission increases up to ~6% and then decreases, indicating that the optimal Eu3+ content (quenching concentration) is 6 at %. For Gd2O2S host, the PL intensity decreases with the increase of the doping concentration from 3 at% and according to the following analysis, we assume that in this system the optimal doping concentration is 3%. Similar optimal doping concentrations for the three systems were reported before [49,51,52]. The optimal doping concentration observed here corresponds well with the value inferred from the Ozawa’s empirical formula as analyzed later in Fig.10b. Fig. 10. The relationship between log(I/x) and log(x) and the relationship between log(I/x) and log(1-x) for the (Gd1-xEux)2O2SO4, (Gd1-xEux)2O3, and (Gd1-xEux)2O2S red phosphors calcined from the SO42--LGdHs precursors. By analyzing the index of electric multipole (s), the mutual interaction type of luminescence quenching can be inferred. The s value can be obtained according to the equation log(I/x)=(-s/d)logx+logf [53,54], where I is the emission intensity, d is the sample dimension, x is the activator content and f is a constant independent of activator 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 18 content. The s values of 3, 6, 8 and 10 is related to the mechanism of exchange interaction, the dipole-dipole, dipole-quadrupole and quadrupole-quadrupole electric interactions, respectively. The log(I/x)-log(x) plots for the (Gd,Eu)2O2SO4, (Gd,Eu)2O3, and (Gd,Eu)2O2S phosphors are given in Fig. 10a. From the plots, slope (-s/3) of -0.85, -0.87 and -1.41 are derived, yielding s values of around 2.55, 2.61 and 4.23 for the three systems. This indicates that the observed luminescence quenching is dominantly resulted from exchange interactions for the energy transfer among Eu3+ ions, possibly via a phonon-assisted three activated ions nonresonant interaction’ mechanism [54,55]. The luminous intensity of luminescence materials is closely correlated to the activator concentration and depends on the interaction between rare earth ions i.e. the quenching mechanism. According to Ozawa [56], if the luminescence quenching mechanism is governed by exchange interactions of the rare earth ions, the quenching concentration can be estimated according to 1/(1+Z), which can be derived from the following equation: log(I/x)=logb+Zlog(1-x), where I is the emission intensity, x is the Eu3+ content, b is an independent constant and Z represented the number of nearest-neighbor cations surrounding the luminescent center ions. The log(I/x)-log(1-x) plots (Fig. 10b) yield slopes Z=17.49, 17.19, and 28.23 for (Gd1-xEux)2O2SO4, (Gd1-xEux)2O3 and (Gd1-xEux)2O2S phosphors, respectively. It can be obtained that 1/(1+Z) values are approximately 0.054, 0.054 and 0.034 which are nearly consistent with the quenching concentration of Eu3+ observed in the three systems (Fig. 9). The initial increase of the emission intensity is due to the emergence of more luminescence centers with increasing Eu3+ concentration. The further doping of Eu3+ shortens the distances among Eu3+ ions, and thus, the probability of energy transfer among the Eu3+ ions is enhanced which increases the emission intensity. But luminescence quenching occurred with 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 19 higher doped Eu3+ ions the reason of which is Eu- Eu interactions and lattice defects are caused by non-radiative energy transfer among the superfluous Eu3+ activators. Asymmetry factor defined as the intensity ratio of the 5D0→7F2 to 5D0→7F1 transition is widely used as a very sensitive and efficient probe to detect the local structure around Eu3+ [57]. Thus, in this work, the asymmetry factor of luminescence [I(5D0→7F2)/I(5D0→7F1)] was calculated from the PL spectra for the Eu3+ doped phosphors with different doping concentration and host lattice (Fig. S8). The asymmetry factors stay stable at ~6.6 and ~6.9 for hexagonal oxysulfide and monoclinic oxyslufate, respectively, indicating that the variation of doping concentration brings little influence on the distortion of Eu3+ ions. This may be due to the fact that there is only one site for Eu3+ to occupy in the two hosts, and the site symmetry is relatively low. The similarity of the asymmetry factor of Eu3+ in the two host lattices indicates the similar distortion degree of Eu3+ in the two host lattices. Comparatively speaking, for Eu3+ doped cubic oxide, with increasing concentration, the factor is increased from ~9.4 to ~10.8. The reason can be explained as followed: the increasing doping concentration favored the evenly distribution of Eu3+ cations, which allows the Eu3+ activators to move towards their crystallographic positions for Eu3+ substitution (C2 and S6). Since the 5D0→7F2 emission arose from the C2 site while the 5D0→7F1 emission largely arose from the S6 site for Eu3+, the statistically much higher occupancy of the C2 sites (the ratio of C2 to S6 is 3:1) results in an increased asymmetry factor, along with the increasing doping concentration. The higher factor of Gd2O3 than those of Gd2O2S and Gd2O2SO4 indicates a deeper degree of distortion in its host lattice. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 20 Fig. 11. Lifetime of the 5D0→7F2 emission as a function of x value (Eu3+ content) in the (Gd1-xEux)2O2SO4, (Gd1-xEux)2O3 and (Gd1-xEux)2O2S red phosphors. Fluorescence decay kinetics of the 5D0→7F2 transitions for the phosphors were studied, and determined dependences are shown in Fig. S9. The resultant lifetimes of Eu3+ as a function of Eu3+ content in different host lattices are shown in Fig. 11. The exponential fittings yield successive shorter fluorescence lifetimes with increasing Eu3+ concentration for the three systems. The decreasing tendency is gentle in (Gd1-xEux)2O2SO4 and (Gd1-xEux)2O3 and sharp in (Gd1-xEux)2O2S, and it corresponds well to the concentration quenching behaviors observed (Fig. 9). The lifetimes for (Gd1-xEux)2O2SO4, (Gd1-xEux)2O3, and (Gd1-xEux)2O2S obtained in this work generally agree with the previously reported values [50,51,58-60]. The decreased lifetime is mainly due to the formation of a resonance energy transfer network between Eu3+ with continued doping. The formed energy transfer network will advance the energy transfer from the inside to the radiation center of the surface and therefore a shorter lifetime is resulted. The comparatively shorter lifetime in (Gd1-xEux)2O2S may be due to the shorter 0.03 0.04 0.05 0.06 0.070.80.91.01.11.21.31.41.51.6 (Gd1-xEux)2O2SO4 (Gd1-xEux)2O3 (Gd1-xEux)2O2SLifetime (ms)The x value 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 21 Ln-Ln distance in this host lattice, which enhances the energy transfer between the rare earth ions, as analyzed before, and the higher calcination temperature. The lifetimes in (Gd1-xEux)2O3 are slightly smaller than those in (Gd1-xEux)2O2SO4, and it is mainly due to the fact that the higher calcination temperature would favor the elimination of lattice defects and increase of crystal perfection. Conclusions The conditions of hydrothermal synthesis of the sulfate-typed LGdHs was investigated, and the LGdHs microplates were obtained at 120 oC with pH = 9.0. Three kinds of important red phosphors (Gd1-xEux)2O2SO4, (Gd1-xEux)2O3, and (Gd1-xEux)2O2S were obtained by controlled calcination of the LLnHs precursor. The influences of doping concentration and host lattice on the Eu3+ photoluminescence were studied in detail, and the main conclusions are as followed: (1) Through adjusting the hydrothermal pH, the compounds Gd2(OH)4SO4·nH2O and Gd2(OH)5(SO4)0.5·nH2O can be obtained. Under lower pH 7.0-9.0, the former compound is obtainable and the particle can be downsized by raising the pH of the reaction system. Higher pH of 10.0 favors the formation of Gd2(OH)5(SO4)0.5·nH2O. The calcination of LGdHs in the air from 400 to 1000 oC produced Gd2O2SO4 via the removement of hydration water and dehydroxylation, and continued calcination to 1200 oC yielded cubic Gd2O3. The calcination of LGdHs from 800 oC to 1200 oC in hydrogen atmosphere leads to the formation of Gd2O2S. (2) The PLE spectra consisted of broad and stronger CT bands and sharper and weaker lines attributed to the f-f transitions. The CT behavior was significantly influenced by the host lattice, and broader CT band was observed in the (Gd1-xEux)2O2S due to 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 22 additional stronger S-Eu CT excitation band, aside from the O-Eu one exists in the host. The f-f transitions are located at almost the same positions due to the well-shielded 4f shells. The strongest red emissions were found at 613, 618, and 626 nm for the (Gd1-xEux)2O2SO4, (Gd1-xEux)2O3 and (Gd1-xEux)2O2S red phosphors, respectively, which corresponds to the 5D0→7F2 transition of Eu3+. (3) The optimal doping concentration of Eu3+ in Gd2O3 and Gd2O2SO4 is 6% and 3% in Gd2O2S, and the quenching was governed by exchange interactions. The fluorescence kinetics analysis shows that the 5D0→7F2 red emissions decay in a single exponential manner irrespective of host lattice and doping concentration, and successively shortened lifetimes were yielded with the increasing doping concentration. Acknowledgements This work is supported by the Natural Science Foundation of Liaoning Province (Grant No. 2020-MS-286). References [1] P.F. Du, L.X. Song, J. Xiong, Z.Q. Xi, D.L. Jin, L.C. Wang, Preparation and the luminescent properties of Tb3+-Doped Gd2O3 fluorescent nanofibers via electrospinning, Nanotechnology 22(3) (2011) 035602. [2] J. Xu, J. Luo, L. Zeng, Y. Tao, G. Li, C. Li, J. Liu, L. Zhou, S. Hu, J. Yang, F. Lin, J. Tang, Comparative study of red-emitting pyrophosphate phosphors: Site-selective replacement of Eu3+ and luminescent properties, Ceram. 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Mater. 36(6) (2014) 1083-1091. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Supplemental DataClick here to access/downloadSupplemental DataSupporting Information.docxhttps://www2.cloud.editorialmanager.com/advpt/download.aspx?id=682998&guid=a0f233da-63b2-4219-b781-35e5386538d8&scheme=1Declaration of interests □√ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Conflict of Interest Statement Click here to view linked Referenceshttps://www2.cloud.editorialmanager.com/advpt/viewRCResults.aspx?pdf=1&docID=38708&rev=1&fileID=682999&msid=31b28c0b-1c81-45a8-afa5-d6c620b55dbahttps://www2.cloud.editorialmanager.com/advpt/viewRCResults.aspx?pdf=1&docID=38708&rev=1&fileID=682999&msid=31b28c0b-1c81-45a8-afa5-d6c620b55dba