Tb3+ (Ln = Sm, La, Gd, and Y

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Rapid Production of Ln2O2S:Eu3+/Tb3+ (Ln = Sm, La, Gd and Y) Phosphors by Molten Salt Electrolysis Yuhui Liu, Xiaoyan Jing, Pu Wang, Taiqi Yin, Debin Ji, and Milin Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00304 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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ACS Applied Energy Materials

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Rapid Production of Ln2O2S:Eu3+/Tb3+ (Ln = Sm, La, Gd and Y) Phosphors by

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Molten Salt Electrolysis

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Yuhui Liu,a Xiaoyan Jing,a Pu Wang,a Taiqi Yin,a Debin Ji,a* Milin Zhang,a,b*

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a

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Education, College of Materials Science and Chemical Engineering, Harbin

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Engineering University, Harbin 150001, China

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b

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ABSTRACT: In this paper, a facile one-pot molten salt electrolysis method has been

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proposed for the synthesis of hexagonal sheet-like Ln2O2S:Eu3+/Tb3+ (Ln = Sm, La, Gd

10

and Y) phosphors. The phase, composition, morphology and optical properties of the

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phosphors were characterized by X-ray powder diffraction (XRD), high-resolution

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transmission electron microscope (HRTEM) and photoluminescence. The results

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exhibited that doping Eu3+ or Tb3+ cause no great changes on the crystal structure.

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Under the excitation of 394 nm light, sheet-like Ln2O2S:Eu3+ (Ln = Sm, La, Gd, and Y)

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exhibited predominant red emission at around 627 nm, which corresponds to the

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absorption of host crystals and the energy levels transitions of 5D0 to 7F2 configuration

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of Eu3+ ions. The sheet-like Ln2O2S:Tb3+ exhibited characteristic excitation of Tb3+

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under 364 nm. The emission spectra of Ln2O2S:Tb3+ (Ln = Sm, La, Gd, and Y) were

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measured and those emission lines between 450 and 650 nm corresponded to the

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transitions of 5D4 to 7FJ (J= 0 to 6) ground state energy levels belonged to the

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characteristic emission of Tb3+. The fabrication mechanism of the sheet-like

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Ln2O2S:Eu3+/Tb3+ (Ln = Sm, La, Gd, and Y) was also investigated. This approach

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provides new avenues for the preparation of phosphors for applications in light-storing

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field.

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KEYWORDS: phosphors, rare-earth oxysulfide, energy storage, electrochemistry,

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co-reduction, sheet-like structure

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■ INTRODUCTION

Key Laboratory of Superlight Materials and Surface Technology, Ministry of

College of Science, Heihe University, Heihe 164300, China

ACS Paragon Plus Environment

ACS Applied Energy Materials 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

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Rare-earth (RE) ions have abundant excited energy levels with unique intra 4f

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electronic transitions, which enable them to absorb and emit photons from UV to

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infrared region.1 Among various kinds of optical materials, rare earth oxysulfide

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Ln2O2S (Ln = Sm, La, Gd, and Y) have been investigated as host materials for

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phosphors, which was suitable for synthesizing phosphors with high luminescence

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efficiencies.2 Rare earth oxysulfide exhibits distinguished merits, including high

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thermal stability, as well as high resistance to resisting water.3,4 In the past years, rare

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earth oxysulfides have been widely used in various optical devices such as X-ray

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imaging systems for medical diagnosis,5 long-lasting phosphorescent materials6 and phosphors.7

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up-conversion

So far,

various

synthetic approaches

including

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sulfide-fusion method, sulfidation and the reduction of the rare earth sulfates have been

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employed for the synthesis of rare earth oxysulfides.8–10 However, the sulfide-fusion

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method usually uses strong acids to remove the residual impurities, and the sulfidation

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and sulfide-fusion methods needs H2S and CS2 gas for reduction, which markedly

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restrict the application of these methods. Moreover, highly crystalline single-phase

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rare earth oxysulfides are hard to obtain from these methods.11 To resolve these

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questions, it is highly desired to develop a low energy and environmentally friendly

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synthesis method for the synthesis of rare earth oxysulfide.

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As one of the four systems of the electrodeposition methods, molten salt

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electrolysis is an efficient, energy-efficient and facile one-pot method.12 At present,

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molten salt electrolysis method has been applied for the synthesis of a variety of alloys

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and compounds, two-dimensional material molybdenum disulfide and carbides.13–15

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The advantage of this approach is that it has wide operating temperature ranging from

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150-1050 ºC and diffusion amount of ion increases at higher temperature.16,17 The

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composition, morphology and structure of the sediments can be controlled by the ration

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of the molten salt and the electrodeposition parameters (eg. potential, current and

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electrodeposition modes). Metal chlorides, fluorides and oxides have been used for the

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molten salt system. For example, Yan et al electrodeposited Sm-Co intermetallic

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compounds from LiCl-KCl-SmCl3-CoCl2 melts.18 Ahmad reported that Ta-Cr

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intermetallic

compounds

were

synthetized

from


LiF-NaF-KF

melts

by

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electrodeposition,19 and Nitta and co-workers characterized W and W oxides

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electrodeposited from Li2WO4-Na2WO4-K2WO4 system.20 However, the molten salt

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electrolysis employed for the synthesis of Ln2O2S has rarely been reported.

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In this study, we firstly proposed facile one-pot method to prepare rare earth

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oxysulfide Ln2O2S (Ln = Sm, La, Gd, and Y). EuCl3 and TbCl3 are added during the

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synthesis process to obtain the fluorescence of as-synthesized Ln2O2S phosphors. The

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morphology and crystalline phase of the samples were investigated in detail. The

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photoluminescence properties of Ln2O2S (Ln = Sm, La, Gd, and Y) doped with Eu3+

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and Tb3+ under DC-PL with a 150 W xenon lamp irritation were studied. This

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synthetic route is also suitable for fabricating other luminescent materials.

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■ Experimental

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Materials. AgCl, LiCl, KCl, LiNO3, KNO3, Li2CO3, K2CO3, Sm2O3 (99.99%), YbCl3

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(99.99%), GdCl3 (99.99%), LaCl3 (99.99%), ErCl3 (99.99%), TbCl3 (99.99%) and

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KSCN (from Sinopharm Chemical Reagnet Co., Ltd). All the chemical reagents are of

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analytical grade without purification.

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Electrochemical apparatus and electrodes. The Ln2O2S (Ln=Sm, La, Gd and Y) and

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Ln2O2S:Eu3+/Tb3+ (Ln=Sm, La, Gd and Y) phosphors were synthesized by an Autolab

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potentiostat/galvanostat controlled with the Nova 1.11 software package. A silver wire

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(1 mm in diameter and the silver purity of 99.99 %) was used for the reference electrode,

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which was inserted in an alundum tube filled with LiCl−KCl salt containing AgCl (1

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wt.%). The thickness of alundum tube was about 0.1 mm and polished by a

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metallographic paper. A graphite rod (6 mm in diameter and the purity of 99.99 %) was

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counter electrode. Molybdenum wire (1 mm in diameter and the purity of 99.99%) was

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used as the working electrode, which was polished thoroughly using SiC paper, and

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then cleaned ultrasonically with ethanol prior to use. In addition, all Ag+/Ag couples

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were potential standards.

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Preparation and characterization of phosphors. The Ln2O2S (Ln=Sm, La, Gd and Y)

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and Ln2O2S:Eu3+/Tb3+ (Ln=Sm, La, Gd and Y) phosphors prepared by galvanostatic



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electrolysis were carried out on Mo electrodes in LiCl-KCl-LnCl3(Ln=Sm, La, Gd and

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Y)-KSCN and LiCl-KCl-LnCl3(Ln=Sm, La, Gd and Y)-Ln2O3(Ln=Eu, Tb)-KSCN

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melt, respectively. After electrolysis, the black precipitates washed with distilled water

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and absolute ethanol several times and finally dried under vacuum at 60 °C for 2 h.

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Then the samples were analyzed by XRD (Rigaku D/max-TTR-III diffractometer)

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using Cu-Kα radiation at 40 kV and 150 mA. X-ray photoelectron spectroscopy (XPS)

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(Thermo EScalab 250 Xi) was performed to investigate the surface composition and

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valence state of the samples. The scanning electron microscope (SEM) was used to

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analyze the microstructure of samples, and energy dispersive spectrometer (EDS)

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(JSM-6480A; JEOL Co., Ltd) was used to analyze the microtone chemical of samples.

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HRTEM (America FEI, 300 KV) was carried out to analyze the crystal structure of

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samples. DC-PL was excited by a 150 W xenon lamp, and the excited spectra were

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recorded from 350 to 450 nm.

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■ RESULTS AND DISCUSSION

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Phase, Structure, and Morphology of Ln2O2S (Ln=Sm, La, Gd and Y)

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A scheme for the electrolysis system is shown in Figure S1. Ln3+ (Ln=Sm, La, Gd,

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and Y) ions are used as a cathode and the cathode discharge electrolysis is conducted

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with the use of a LiCl-KCl melt containing KSCN as sulfur source. Sulfur and oxygen

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ions are used as anodic discharge electrolysis, the discharge is predominantly

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maintained by cation emission form the anode. This cation emission process can be

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applied to metal sulfides particle formation. That is, Ln3+ (Ln=Sm, La, Gd, and Y) ions

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emitted from the cathode can react with sulfur and oxygen ions dissolved in the melt to

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produce Ln2O2S (Ln=Sm, La, Gd, and Y) particles. Martiont et al.21 reported that

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potassium thiocyanate was reacted with oxygen to form sulfur and sulfide in the molten

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salt LiCl-KCl system. The gas of CO, N2 and Cl2 were extracted from the molten salt

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and released at the carbon anode. The process could be expressed as follows:

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2KSCN+O2→K2S+S+2CO+N2 Actually, according to knowledge and original experimental design, we did a parallel


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experiment without the electrolysis. To illustrate this, LiCl-KCl-SmCl3(4 wt.%)

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-KSCN eutectic mixture at 723 K for 1 h act as an example. XRD result (Figure 2S)

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shows that the precipitates are Sm2O3 and SmOCl. In addition, the Ln2O2S (Ln=Sm, Gd,

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La and Y) was synthesized by galvanostatic electrolysis from LiCl-KCl (59:41

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mol%)-LnCl3(4 wt.%) (Ln=Sm, Gd, La and Y)-KSCN(6 wt.%) molten salt at 723 K for

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1 h on Mo electrodes at 1.55 A cm-2 (vs. Ag+/Ag). The XRD patterns of Sm2O2S,

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Gd2O2S, La2O2S and Y2O2S (JCPDS. No. 44-1257, 65-3449, 71-2098 and 24-1424) are

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exhibited in Figures 1A-D, respectively. As shown in Figure 1A, XRD patterns of

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Sm2O2S can be indexed to the standard hexagonal phase with the lattice plane

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perpendicular to the (011) plane, indicating that a pure phase acquired via molten salt

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electrolysis. XRD results (Figure 1B) show that the prepared Gd2O2S are formed as

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hexagonal and the peak corresponding to the (011) lattice plane. Figure 1C shows the

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XRD pattern of a-type rare earth oxide structure and consists of La2O2S, which is

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well-matched with that of La2O2S’s pure hexagonal phase. In addition, the (101) lattice

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plane shows significantly higher intensity. The XRD results (Figure 1D) indicate

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hexagonal of the Y2O2S as phosphors and the lattice plane direction the (101). SEM and

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EDS images of Sm2O2S are shown in Figure 2A and Figure 2B, respectively. A large

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number of thin sheet-like phosphors structures could be clearly observed. The results of

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EDS quantitative analysis indicate that the average atom percentage ration of Sm:O:S is

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2:2:1. The morphology of Sm2O2S was further characterized by HRTEM and selected

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area electron diffraction (SAED). Figure 2C shows the typical HRTEM images, which

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are consistent with the above SEM result. The corresponding HRTEM image exhibits

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obvious lattice fringes with observed d-spacing of 0.301 nm, which is in good

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agreement with the distance between the (110) planes of hexagonal phase Sm2O2S.

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SAED image (Figure 2D) indicates that the samples are hexagonal with single crystal

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nature. The features of integration and steadiness of the sheet-like Sm2O2S were

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revealed by the crystal characterisation measurements and the microstructure images.

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The evidence for the oxidation states of the sample were provided from XPS analysis.

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As shown in Figure 3A, there are two strong peaks located at 1110.9 eV and 1082.9 eV,

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which can be assigned to Sm 3d5/2 and 3d3/2 binding energies, respectively, and ACS Paragon Plus Environment


1

indicate that Sm(III) is the dominant oxidation state. In addition, the S 2s peaks

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detected at 162.3 eV and 161.1 eV can be indexed to S 2p3/2 and S 2p1/2 binding

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energies, respectively, which indicate that S(II) is the dominant oxidation state (Figure

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3B). The peak of O 1s is observed at 531.2 eV, which revealed that O(II) is the

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dominant oxidation state (Figure 3C). (011)

(B)

Sm2O2S

(011)

Gd2O2S

Intensity/a.u.

Intensity/a.u.

(A)

Ga2 O2S( PDF#65-3449)

Sm2O2S( PDF#44-1257)

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2θ/degree

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La2O2S

La2O2S(PDF#71-2098)

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2θ/degree

7

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(101)

(D)

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Y2O2S

Intensity/a.u.

(C)

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2θ/degree

Intensity/a.u.

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Y2O2S(PDF#24-1424)

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70

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2θ /degree

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Figure 1. Measured XRD pattern for the samples obtained by galvanostatic electrolysis

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on Mo electrode in the LiCl-KCl-LnCl3(4 wt.%) (Ln=Sm, Gd, La and Y)-KSCN(6

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wt.%) melt at 723 K for 1 h at 1.55 A cm-2 (A) Sm2O2S, (B) Gd2O2S, (C) La2O2S and (D)

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Y2O2S.


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Figure 2. (A) SEM, (B) EDS, (C) HRTEM and (D) SAED image of the Sm2O2S sample

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by galvanostatic electrolysis on Mo electrode in the LiCl-KCl-SmCl3(4 wt.%)-KSCN(6

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wt.%) melt. 4

8.0x10

(A)

4

7.5x10

1082.9 ev

Sm3d 1110.9 ev

4

Count/s

7.0x10

4

6.5x10

4

6.0x10

4

5.5x10

4

5.0x10

1120

1115

1110

1105

1100

1095

1090

1085

1080

1075

Binding energy/ev

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6.4x103 2.6x104

(B)

6.2x103

O 1s 531.2 ev

2.4x104

5.8x103

2.0x104

Count/s

3

1.8x104 1.6x104

5.6x10

5.4x103 5.2x103

1.4x104

5.0x103

1.2x104

4.8x103

1.0x104

4.6x103

8.0x103

4.4x103 538

6

S 2p 162.3 ev 161.1 ev

(C)

6.0x103

2.2x104

Count/s

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


536

534

532

530

528

526

524

175

Binding energy/ev

170

165

160

Binding energy/ev

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Figure 3. XPS of the Sm2O2S sample by galvanostatic electrolysis on Mo electrode in

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the LiCl-KCl-SmCl3(4 wt.%)-KSCN(6 wt.%) melt, (A) Sm 3d lines from Sm2O2S, (B)

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O 1s lines form Sm2O2S, (C) S 2p lines form Sm2O2S.

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The SEM and EDS images of as-prepared Ln2O2S (Ln=Gd, La and Y) are



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presented in Figures S(3-5)A and S(3-5)B (Supporting Information) which show a

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dominant formation of sheet-like structures. The results of EDS show that the

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percentage ration of element of Ln (Ln=Gd, La and Y), O and S is 2:2:1 in rectangle

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zone. As Figures S(3-5)C shows, the HRTEM characteristic indicates that the sheet-like

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structure shows a crystalline nature with interplanar distances of 0.298, 0.314 and 0.293

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nm, corresponding to the (011), (101) and (101) lattice planes of Ln2O2S (Ln=Gd, La

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and Y), respectively. The symmetric SAED pattern shown in Figures S(3-5)D indicate

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the crystalline state of sheet-like are hexagonal structure. Based on HRTEM and SAED

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pattern analysis, the crystal growth of the plates preferentially occurs in the (011), (101)

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and (101) direction, respectively. To investigate the valence state of Ln2O2S (Ln=Gd,

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La and Y), the samples were examined by XPS in the region of 0-1300 ev. The survey

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scanning of XPS (Figures S6(A-C), Supporting Information) show Ln (Ln=Gd 3d, La

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3d and Y 3p) binding energies, respectively. Moreover, the S 2s peaks detected at 160.3

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eV and 141.5 eV can be indexed to S 2p3/2 and S 2p1/2 binding energies, respectively.

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The peak of O 1s is observed at 530.3 eV.

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Phase, Structure, and Morphology of Ln2O2S:Eu3+/Tb3+ (Ln=Sm, La, Gd, and Y)

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The Ln2O2S:Eu3+/Tb3+ (Ln=Sm, La, Gd, and Y) phosphors were synthesized by

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galvanostatic electrolysis, and were carried out in a LiCl-KCl (59:41 mol%)-LnCl3 (4

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wt.%) (Ln=Sm, Gd, La and Y)-KSCN(6 wt.%)-EuCl3/TbCl3 (0.05 wt.%) molten salt

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system at 723 K for 1 h on Mo electrodes at 1.55 A cm-2 (vs. Ag+/Ag). In Figures 4(A-D),

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the XRD patterns of the fabricated samples of Ln2O2S:Eu3+ (Ln=Sm, La, Gd, and Y) are

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presented, the standard pattern of Ln2O2S (Ln=Sm, La, Gd, and Y) (JCPDS. No.

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44-1257, 65-3449, 71-2098 and 24-1424) are shown for comparison. In comparison with

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the XRD pattern of Ln2O2S (Ln=Sm, La, Gd, and Y), there are no extra diffraction peaks,

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indicating the formation of a homogeneous Ln2O2S:Eu3+ (Ln=Sm, La, Gd, and Y) solid

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solution. The results can be attributed to the small structural difference between the

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hexagonal phase Ln2O2S (Ln=Sm, La, Gd, and Y) and Eu3+ ions.22,23 In addition, the

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XRD patterns of samples prepared with Tb3+ ions doped into Ln2O2S (Ln=Sm, La, Gd,

29

and Y) are also shown in Figure S7 (Supporting Information). The structure and


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composition of the samples transforms from hexagonal Ln2O2S (Ln=Sm, La, Gd, and Y)

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to Ln2O2S: Tb3+ (Ln=Sm, La, Gd, and Y) were investigated. The results and conclusions

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of the XRD pattern analysis confirmed that Tb3+ ions were successfully doped into

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Ln2O2S (Ln=Sm, La, Gd, and Y). The morphology of samples was further characterized

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by SEM-EDS, HRTEM and SAED. The SEM-EDS images of Sm2O2S:Eu3+ are shown

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in Figure 5I-IV(A, B), a large number of sheet-like could be clearly observed. The EDS

7

test results show that the concentration percentage of Eu element in the fleet-like

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Ln2O2S:Eu3+ (Ln=Sm, La, Gd, and Y) are 10-15 wt.%. Figure 5I-IV(C) shows the

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typical HRTEM and SAED images of Ln2O2S:Eu3+ (Ln=Sm, La, Gd, and Y), the

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corresponding HRTEM image of Ln2O2S (Ln=Sm, La, Gd, and Y) with doping of Eu3+

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ions exhibit obvious lattice fringes with 0.301, 0.298, 0.314 and 0.293 nm, respectively,

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which is in good agreement with the distance between the (011), (011), (101) and (101)

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planes of hexagonal phase Ln2O2S (Ln=Sm, La, Gd, and Y). The successful doped of the

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Eu3+ ions into Ln2O2S (Ln=Sm, La, Gd, and Y) were confirmed by SAED (Figure

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5I-IV(D)). As Figures 5I-IV(D) show, diffraction spots for crystals can be indexed by

16

the body-centered hexagonal lattice. Their structure must result from partial ordering of

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sulfur-oxygen vacancies and the lanthanoid metal. The appearance of strong and weak

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superlattice reflections in this figure suggests that electrons are chemically doped into

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the Ln2O2S (Ln = Sm, La, Gd, and Y) by the doping of Eu3+ ions. On the basis of the

20

above results，the present invention prepares rare earth hybridized luminous material

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with homogeneously distributed Eu element and no change the crystal form of Ln2O2S

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(Ln=Sm, La, Gd, and Y). The SEM, HRTEM and SAED images of the samples are

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shown in Figure S8. These results indicate that the sheet-like structure consist of

24

hexagonal single-crystalline Ln2O2S:Tb3+ (Ln=Sm, La, Gd, and Y). Lattice fringes

25

associated with (011), (011), (101) and (101) planes appear along the sheet-like,

26

respectively. Compositional analysis of an individual sheet-like by EDS reveals the

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presence of the doped elemental Tb, and the concentration percentage of Tb element are

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0.281, 0.507, 0.933 and 1.725 wt.%, respectively.



(A)

(011)

3+

(011)

(B)

Gd2O2S:Eu3+

Intensity/a.u.

Intensity / a.u.

Sm2O2S:Eu

Sm2O2S( PDF#44-1257)

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Ga2O2S( PDF#65-3449)

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(D)

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Y2O2S:Eu

Intensity / a.u.

(101)

Intensity/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

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La2O2S(PDF#71-2098)

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Y2O2S(PDF#24-1424)

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2θ / degree

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Figure 4. Measured XRD pattern for the samples obtained by galvanostatic electrolysis

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on Mo electrode in the LiCl-KCl-LnCl3(4 wt.%) (Ln=Sm, Gd, La and Y)-KSCN(6

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wt.%)-EuCl3(0.05 wt.%) melt at 723 K for 1 h at 1.55 A cm-2 (A) Sm2O2S:Eu3+, (B)

6

Gd2O2S:Eu3+, (C) La2O2S:Eu3+ and (D) Y2O2S:Eu3+.


Page 11 of 18 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


1 2

Figure 5. (A) SEM, (B) EDS, (C) HRTEM and (D) SAED image of the Ln2O2S:Eu

3

(Ln=Sm, Gd, La and Y) sample by galvanostatic electrolysis on Mo electrode in the

4

LiCl-KCl-LnCl3(4 wt.%) (Ln=Sm, Gd, La and Y)-KSCN(6 wt.%)-EuCl3(0.05 wt.%)

5

melt at 723 K for 1 h at 1.55 A cm-2 (I) Sm2O2S:Eu3+, (II) Gd2O2S:Eu3+, (III)

6

La2O2S:Eu3+ and (IV) Y2O2S:Eu3+.

7

Luminescence Properties of Eu3+ Doped Ln2O2S (Ln=Sm, La, Gd, and Y).

8

Figures 6(A-D) show the excitation (600-800 nm) and emission (350-450 nm)

9

spectra of Ln2O2S: Eu3+ (Ln=Sm, La, Gd, and Y) at room temperature, respectively.

10

The excitation spectra of Eu3+ ions of the La2O2S:Eu3+(Ln=Sm, La, Gd, and Y)

11

compound arise from the transition of Eu3+, and the peaks is around 394 nm. The

12

emission lines around 627 nm corresponded to transitions of 5D0 to 7F2 energy levels,

13

where the emission peak at around 627 mm was strongest, which is due to Eu3+ ions

14

possessed the lattice positions of an asymmetric center in the crystal.24-28 The results

15

show that the Eu3+ doped with four kinds of rare earth luminescent material of Ln2O2S

16

(Ln=Sm, La, Gd, and Y), and both can emit red light with good monochromaticity and

17

bigger intensity. This result found that the composite substrate material is more

18

advantageous to doped with Eu3+ and its luminous property is more excellent.



3.0x105

(A) Sm2O2S:Eu

3+

627

(B) Gd2O2S:Eu3+

7x104

394

2.0x105

(a)

1.5x105

(b)

5

1.0x10

4x104

(a)

3x104 4

excitation spectra

1x104

350

400

450

550

600

650

700

750

800

300

350

400

Wavelength (nm)

1

450

550

600

650

700

750

800

Wavelength (nm) 5x104

(C) La2O2S:Eu3+

(D) Y2O2S:Eu3+

627

394

394 8.0x103

4

4x10

3

(b)

(a) 4.0x103

2.0x103

excitation spectra

emission spectra

0.0

Intensity (a.u.)

627 6.0x10

emission spectra

0

0.0

1.0x104

(b)

2x10

emission spectra

excitation spectra

5.0x104

3x104

(a)

(b)

2x104

1x104

excitation spectra

emission spectra

0 300

2

627

5x104

Intensity (a.u.)

Intensity (a.u.)

394

6x104

2.5x105

Intensity (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

Page 12 of 18

350

400

450

550 600 650 700 750 800 850

300

350

Wavelength (nm)

400

450

550

600

650

700

750

800

Wavelength (nm)

3

Figure 6. (a) Excitationand (b) emission spectra of samples (A) Sm2O2S:Eu3+, (B)

4

Gd2O2S:Eu3+, (C) La2O2S:Eu3+ and (D) Y2O2S:Eu3+.

5

Luminescence Properties of Tb3+ Doped Ln2O2S (Ln=Sm, La, Gd, and Y).

6

Figure 7 shows the excitation and emission spectra of Ln2O2S:Tb3+ (Ln=Sm, La,

7

Gd, and Y). Those excitation spectra of compounds consist of a strong broad band from

8

360-400 nm, which corresponds to the absorption of host material and the energy

9

transition from the 4f-4f configuration of Tb3+. In contrast, those emission spectra arose

10

from the transitions of 5D4 energy levels to 7FJ (J= 0 to 6) ground state energy levels

11

belong to the characteristic emission of Tb3+ ions, where the emission lines between

12

the 450 and 700 nm corresponded to the transitions of 5D4 → 7FJ (J= 0 to 6). The

13

characteristic emission of Tb3+ ions belong to the f-f transitions in the 4f shell layer,

14

while it is shielded by 5s and 5p shell layers.29,30 As shown in Figure 7A-D, sheet-like

15

Ln2O2: Tb3+ (Ln = Sm, La, Gd, and Y) show the characteristic green UC emission

16

spectra, with emissions of 450-700 nm.


Page 13 of 18

3.5x104

552 540

364

1.2x105

Intensity (a.u.)

4.0x104

(A) Sm2O2S:Tb3+

4

8.0x10

(a)

6.0x10

(b)

4

4.0x10

excitation spectra

emission spectra

2.0x104 1.5x104

(a)

300

excitation spectra

350

500

600

700

300

800

350

1

7x10

3

(C) La2O2S:Tb3+

2.5x104

Intensity (a.u.)

364 6x103

552 540

5x103 4x10

3

3x10

3

2x10

3

emission spectra

(a)

500

(b)

excitation spectra

700

800

(D) Y2O2S:Tb3+ 540 552

4

2.0x10

364

1.5x104

1.0x104

(a)

(b)

excitation spectra

emission spectra

5.0x103

emission spectra

1x103

600

Wavelength (nm)

Wavelength (nm) 3

(b)

1.0x104

0.0

0.0

0

0.0

300

2

364

2.5x104

5.0x103

2.0x104

8x10

552 540

3.0x10

1.0x105

4

(B) Gd2O2S:Tb3+

4

Intensity (a.u.)

1.4x105

Intensity (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


350

500

600

700

800

300

350

Wavelength (nm)

500

600

700

800

Wavelength (nm)

3

Figure 7. (a) Excitation and (b) emission spectra of samples (A) Sm2O2S:Tb3+, (B)

4

Gd2O2S:Tb3+, (C) La2O2S:Tb3+ and (D) Y2O2S:Tb3+.

5

■ CONCLUTIONS

6

In summary, sheet-like Ln2O2S (Ln=Sm, La, Gd, and Y) were fabricated by molten salt

7

electrolysis. XRD results showed the sheet-like oxy-sulfide Ln2O2S (Ln=Sm, La, Gd,

8

and Y) are pure hexagonal phase. The morphology of phosphors is sheet-like structure

9

and the thickness of sheet-like is several nanometers. Doping Eu3+ and Tb3+ did not

10

cause remarkable changes in crystal structure. The crystal structure of sheet-like

11

Ln2O2S: Eu3+/Tb3+with hexagonal structures were studied by HRTEM and SAED

12

analysis. In addition, UV measurements demonstrated the strongest red emission

13

centering peak is at around 627 nm, originating from the 5D0 to 7F2 transition of the

14

Eu3+ ions, indicating that the Eu3+ ions occupy a site center in the sheet-like

15

Ln2O2S:Eu3+ (Ln=Sm, La, Gd, and Y). In addition, the emission of Ln2O2: Tb3+ (Ln =



1

Sm, La, Gd, and Y) is the transition of 5D4 → 7FJ (J = 0 to 6). This is considered to

2

be the exchange of the environment around the Tb3+ ion in the host lattice by the

3

molten salt electrolysis treatment. The molten salt electrolysis method proposed here is

4

great significance. This technique can be employed to prepare luminescent materials

5

with various morphologies, such as rare earth oxysulfide, phosphate, silicate and the

6

like.

7

■ ASSOCIATED CONTENT

8

Supporting Information

9

The Supporting Information is available free of charge on the ACS Publications

10

website at

11

Scheme of an experimental cell for cathode/anodic discharge electrolysis; XRD of

12

LiCl-KCl-SmCl3(4 wt.%))-KSCN(6 wt.%) melt without electrolysis; SEM, EDS,

13

HRTEM and SAED images of the Gd2O2S, La2O2S and Y2O2S; XPS survey scanning

14

of Gd2O2S, La2O2S and Y2O2S; XRD of Sm2O2S:Tb3+, Gd2O2S:Tb3+, La2O2S:Tb3+

15

and Y2O2S:Tb3+; SEM, EDS, HRTEM and SAED images of the Sm2O2S:Tb3+,

16

Gd2O2S:Tb3+, La2O2S:Tb3+ and Y2O2S:Tb3+. (Word)

17

■ AUTHOR INFORMATION

18

Corresponding Authors

19

*E-mail: [email protected] (Mi-Lin Zhang).

20

*E-mail: [email protected] (De-Bin Ji).

21

ORCID

22

Yuhui Liu: 0000-0003-2689-6338

23

■ ACKNOWLEDGMENTS

24

The work was financially supported by the China Scholarship Council, the National

25

Natural Science Foundation of China (91226201, 91326113, 51574097, 21790373

26

and 51774104), and Project funded by China Postdoctoral Science Foundation

27

(2017M621244). ACS Paragon Plus Environment

Page 14 of 18

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1

Rapid Production of Ln2O2S:Eu3+/Tb3+ (Ln = Sm, La, Gd and Y)

2

Phosphors by Molten Salt Electrolysis

3 4

Yu-Hui Liu,a Xiao-Yan Jing,a Pu Wang,a Tai-Qi Yin,a De-Bin Ji,a Mi-Lin Zhang,a, b *

5 6 7

8 9


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Tb3+ (Ln = Sm, La, Gd, and Y

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