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Fan Li, Sihan Feng, Zhiyuan Pan, Qi Zhu, Xudong Sun, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361)

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[Controlled preparation of Gd2O2SO4:Eu3+ monospheres via hydrothermal precursor engineering for enhanced photoluminescence](https://mdr.nims.go.jp/datasets/275841ab-3313-4136-b290-95a3b5f37e16)

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1  Controlled preparation of Gd2O2SO4:Eu3+ monospheres via hydrothermal precursor engineering for enhanced photoluminescence Fan Li,a Sihan Feng,a Zhiyuan Pan,a Qi Zhu,a Xudong Sun,a Ji-Guang Lib,∗ a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China b Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan          *Corresponding author Dr. Ji-Guang Li National Institute for Materials Science 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:li.jiguang@nims.go.jphttps://www.editorialmanager.com/advpt/viewRCResults.aspx?pdf=1&docID=38506&rev=1&fileID=674184&msid=e77a33a9-1251-402c-afd1-82456f5d8788https://www.editorialmanager.com/advpt/viewRCResults.aspx?pdf=1&docID=38506&rev=1&fileID=674184&msid=e77a33a9-1251-402c-afd1-82456f5d87882  Abstract Rare-earth oxysulfate (RE2O2SO4) attracted attention for large-capacity oxygen storage, low-temperature magnetism and luminescence, whose preparation mostly involves toxic SOx gases and/or complicated procedures. In the phosphor field, monospheres are preferred for a number of applications due to their better luminescence and high packing density. With the simple reactants of RE(NO3)3, Na2CO3 and (NH4)2SO4 for hydrothermal reaction, dispersed monospheres and nanoplates were selectively synthesized for the recently discovered RE2(OH)2CO3SO4nH2O (RE = Gd0.95Eu0.05, REOCSH) layered compound precursor, from which RE2O2SO4 phosphors mostly retaining their precursor morphologies were facilely derived via calcination in air at 800 °C, without involving SOx. Solution pH was found to decisively determine the chemical composition and crystallization kinetics of the initial precipitate, and the possible mechanisms of phase/morphology evolution during hydrothermal treatment were proposed by comparatively investigating the temperature-course and time-course of REOCSH formation under the typical pH values of 6 and 7. It was revealed that (Gd0.95Eu0.05)2O2SO4 monospheres emit ~1.35 times as strong as nanoplates (λex = 257 nm, λem = 617 nm) and may retain as high as ~81% of the room-temperature intensity at 150 °C. The phosphor monospheres were also analyzed to have a fluorescence lifetime of ~1.32 ms at room temperature and an activation energy of ~0.19 eV for the thermal quenching of luminescence.   Keywords: RE2O2SO4; Luminescence; Hydrothermal reaction; Phase/morphology evolution     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 Rare-earth oxysulfates (RE2O2SO4) attracted attention due to their large capacity of oxygen storage (RE = La, Pr, Nd, Sm) [1, 2] and unusual low-temperature magnetism (RE = Gd, Tb, Dy, Ho, Er, Tm) [3-5]. Recent studies also showed that they are promising hosts for luminescence applications (RE = La, Gd, Y) [6-9]. The family of compounds all have a layered crystal structure in the monoclinic system (Space group C2/c), which is formed by alternative stacking of [RE2O2]2+ main layers and inter-layer [SO4]2 along the a-axis [10, 11], as shown in Fig. S1 with La2O2SO4 [10] for example. In such a structure, two opposite O atoms of each [SO4]2 tetrahedron are respectively coordinated with two RE in neighboring [RE2O2]2+ layers [10, 11]. The physicochemical properties of a material are known to be susceptible to morphology, geometric dimension and surface chemistry [12-14]. Therefore, shape-controlled synthesis of multi-dimensional materials is of great significance for novel/enhanced functionalities [12, 15, 16]. For instance, two-dimensional (2D) materials are attractive for application in catalysis and sensors due to their better electron transport properties arising from a large specific surface area and unsaturated surface [17, 18]. Spherical particles, on the other hand, are preferred for luminescence in the field of lighting and display, since a dense phosphor layer may be formed via close packing of the spheres, and such a morphology may minimize surface scattering of the excitation/emission light. Thermal decomposition of RE2(SO4)3nH2O is the common method for RE2O2SO4 preparation. While simple to operate, it is not conducive to morphology control and, particularly, releases toxic SOx gases [1, 19]. To this end, scholars made efforts to develop alternative synthesis techniques, mostly based on liquid-phase processing. For example, with a precursor  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  formed by RE3+ and dodecyl sulfate (C12H25OSO3, DS), Machida et al. [20, 21] obtained ~10 nm sized La2O2SO4 and Pr2O2SO4 crystallites by subsequent calcination at 900 °C. Qu et al. [22] obtained Y2O2SO4 nanofibers with an average diameter of ~90 nm through annealing the precursor fibers produced by electrospinning a solution of Y(NO3)36H2O, thiourea (H2NCSNH2) and polyvinylpyrrolidone (PVP). Liu et al. [6] prepared Y2O2SO4:Eu3+ hollow spheres of ~1 μm by calcining at 650 °C a precursor obtained by a L-cysteine/PVP assisted hydrothermal method and studied their photoluminescence. Wang et al., on the other hand, obtained RE2O2SO4 for the full range of lanthanide elements and Y by calcining in air the RE2(OH)4SO4·nH2O layered hydroxides (SO42-LRH; n = 0 or 2 depending on the type of RE) synthesized via hydrothermal reaction, and the RE-dependent occurrence of RE2O2SO4 was revealed via thermal analysis [23-25]. In our efforts to synthesize Gd2(OH)4SO4·nH2O via hydrothermal reaction of Gd nitrate, ammonium sulfate (SO42 source) and urea (OH and CO32 source) at 140 °C, Gd2(OH)2CO3SO4⋅nH2O was discovered as a new type of layered compound, which transforms to Gd2O2SO4 via dehydroxylation and decarbonation. While such a reaction system allowed to build aligned films through heterogenous nucleation and homogeneous precipitation, pH control appeared as a problem owing to the uncertainties arising from urea hydrolysis (CO(NH2)2 + H2O → NH3∙H2O + CO32). This also prevented us from mechanism understanding of Gd2(OH)2CO3SO4⋅nH2O formation and morphology control of the product. We thus replaced urea with sodium carbonate as the CO32 source for Gd2(OH)2CO3SO4⋅nH2O (doped with Eu3+) synthesis in this work, which produced uniformly dispersed nanoflakes and microspheres as two types of distinctly different products. The temperature-course and time-course of phase and morphology evolution were revealed by  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  regulating solution pH, and the derived Gd2O2SO4:Eu3+ phosphors were comparatively studied for photoluminescence. It was shown that the Gd2O2SO4:Eu3+ microspheres have a significantly better performance and may serve as a red phosphor with good thermal stability.  2. Experimental Section 2.1. Hydrothermal synthesis The synthesis of RE2(OH)2CO3SO4nH2O (RE = Gd0.95Eu0.05) was conducted via hydrothermal reaction of RE nitrate, (NH4)2SO4 (analytical grade, Sinopharm Co., Ltd., Shanghai, China) and Na2CO3 (analytical grade, Sinopharm), where the RE nitrate was prepared by dissolving Gd2O3 and Eu2O3 (99.99% pure, Huizhou Co., Ltd.) with a proper amount of nitric acid (analytical grade, Sinopharm). Ultra-pure water (resistivity  18 Mcm) was used throughout the experiments. In a typical synthesis, 1.5 mmol of (NH4)2SO4 and 1.5 mmol of Na2CO3 were dissolved in a certain amount of water to make a transparent solution, to which 3 mmol of RE nitrate (RE3+:SO42:CO32 = 2:1:1 molar ratio) was added under magnetic stirring, followed by pH adjustment to a designated value (6.0-10) with NH4OH and/or HNO3 while keeping the total volume at 60 mL. The resultant mixture was constantly stirred for 30 min before being transferred into a Teflon lined stainless steel autoclave (100 mL capacity) for 24 h of reaction in an electric oven preheated at a predetermined temperature (100-200 °C). After the reaction, the precipitate was collected via centrifugation, washed with water and ethanol successively, and then dried at 60 °C for 12 h to yield a white precursor powder. (Gd0.95Eu0.05)2O2SO4 was produced by calcining the precursor in stagnant air at 800 °C for 1 h, using a heating rate of 8 °C/min at the ramp stage. In this work, the content of Eu3+ was taken as the optimal value of 5 at.% according to a previous report on La2O2SO4:Eu3+ [26].  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  2.2. Characterization Phase identification was performed via X-ray diffractometry (XRD, SmartLab, Rigaku, Tokyo, Japan) under 40 kV/200 mA, using nickel-filtered Cu-Kα radiation (λ= 0.15406 nm) and a scanning speed of 4°/min for 2. Product morphology was analyzed by field emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo) under an acceleration voltage of 15 kV. Fourier transform infrared spectroscopy (FTIR, Nicolet iS5, Thermo Fisher Scientific, Waltham, USA) was conducted using the standard KBr pellet method. Photoluminescence and fluorescence decay were analyzed with a Model FP-8600 fluorospectrophotometer (JASCO, Tokyo) that is equipped with a 60 mm-diameter integrating sphere and a Model HPC-836 temperature controller (JASCO), using a 150 W xenon lamp for excitation, a scan speed of 100 nm/min, and a slit width of 5 nm.  3. Results and discussion 3.1 The effect of solution pH on phase and morphology evolution of RE2(OH)2CO3SO4nH2O  Fig. 1. XRD patterns of the products obtained at different solution pH, where the diffraction of Gd2(OH)2CO3SO4nH2O (n ~ 1.0) [27] was included for comparison. 5 10 15 20 25 30 35 40 45 50 55 60 2Theta (deg.)Reference 27(001)(110) (011)(111)(020)(210)(002)(012)(112) (221)(310)(031)(311)(212)(320) (222)(113)(041) (411)(213)(313)(114)(004)(224)(043)(520)(611)(115) Intensity (a. u.)pH = 6 pH = 7 pH = 8  pH = 10 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  The reaction system before hydrothermal treatment has been turbid, and reaction at 140 °C for 24 h produced a white precipitate in each case. XRD analysis (Fig. 1) found that solution pH has a pronounced effect on the hydrothermal product, and crystalline RE2(OH)2CO3SO4nH2O (REOCSH; RE = Gd0.95Eu0.05) can only be obtained under pH = 6 and 7, though the pH = 6 product was much better crystallized. FE-SEM analysis (Fig. 2) demonstrated that the pH = 6 sample crystallized as discrete nanoflakes with edge lengths of up to ~400 nm (Fig. 2a and the inset) and the pH = 7 sample consists of monodisperse microspheres of up to ~25 μm in diameter (average size ~17 μm; Fig. 2b). Closer view showed that each of the monospheres has a rough surface and was formed by face-to-face vertical stacking of nanoflakes (edge length ~600 nm and thickness ~35 nm; the inset in Fig. 2b). Though similarly amorphous, the pH = 8 product mostly contains ~50 μm-sized spheres (Fig. 2c) while the pH =10 one is composed of irregular-shaped aggregates (Fig. 2d). In both the latter cases, however, the primary building units are rounded nanoparticles of ~30 nm, as revealed by high magnification FE-SEM observation (the insets in Fig. 2c,d). The results thus imply that crystallization was accompanied by fast 2D development of the crystallites owing to the tetragonal crystal structure of REOCSH.   Fig. 2. FE-SEM morphologies of the products obtained at pH = 6 (a), 7 (b), 8 (c) and 10 (d). The insets are closer views. To clarify the chemical species contained in the hydrothermal products, especially 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 8  amorphous ones, FTIR analysis was performed for the series of samples and the results are shown in Fig. 3. It was found that all the products share similar vibrational absorptions, which are generated by OH− at ~3553 cm−1 (O–H stretching, ν1), H2O at ~3420 cm−1 (O–H stretching, ν1)/1645 cm−1 (H–O–H bending, ν2), CO32− at ~842 cm−1 (ν2, weak) and in the regions of ~1300-1590 cm−1 (ν3, strong)/670-808 cm−1 (ν4, medium strong), and SO42− at ~1010/991 cm−1 (ν1)/600 cm−1 (ν4) and in the regions of ~1025-1270 cm−1 (ν3) [28, 29]. Compared with the amorphous products (pH = 8 and 10), the crystalline ones showed sharper and significantly more split SO42− and CO32− bands, implying that these two kinds of ligands are more tightly coordinated to the RE3+ center ions. Though the chemical species are the same, the content of each differs for the crystalline and amorphous products, since the pH = 10 sample turned into a phase mixture (Fig. S2) of monoclinic Gd2O2SO4 (ICDD No. 04-024-9775) and cubic Gd2O3 (~73 wt%; ICDD No. 12-0797) while the pH = 6 and 7 ones turned into pure Gd2O2SO4 (shown later) by calcination at 800 °C. In view that REOCSH has exactly the same RE3+/SO42− molar ratio of RE2O2SO4, the appearance of RE2O3 indicates that the pH = 10 sample has an off-stoichiometric amount of SO42− when compared with REOCSH.   Fig. 3. FTIR spectra of the hydrothermal products obtained at the different pH values.  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  To understand the effect of solution pH on crystallization and phase formation, the precipitate recovered from the reaction system before hydrothermal treatment was subjected to XRD analysis. As shown in Fig. S3, all the initial precipitates are similarly amorphous irrespective of solution pH. For a better understanding, the amorphous precipitates were calcined in air at 800 °C for 1 h, and XRD analysis indicated that the calcination products are a phase mixture of RE2O2SO4 and RE2O3 in each case (Fig. 4). Rietveld refinement of the XRD patterns with software Jade 7.1 further revealed that the fraction of RE2O3 significantly increases with increasing solution pH, being ~18.7, 25, 68.5 and 75 wt% for pH = 6, 7, 8 and 10, respectively. This implies that a gradually less amount of SO42 was incorporated into the amorphous precipitate at a higher solution pH. Such a result can be understood by considering the solution chemistry of RE3+ and the chemical species existing in the reaction system. In this work, RE3+ would exist as [RE(OH)a(CO3)b(SO4)c(H2O)d]3-a-2b-2c complex ion, where the OH−, CO32−, SO42− species are expected to compete with each other to coordinate RE3+ [30, 31]. Coordination competition is affected by not only the intrinsic coordination ability of the anion ligand but also the concentration of the ligand. As consequence of pH increment, the concentration of OH− increases and the content and stability of CO32 would also increase owing to right-hand shift of the HCO3 ↔ CO32 + H+ equilibrium. In view that the ability of inorganic anions to coordinate RE3+ generally follows the order PO43− > CO32− > HPO42− > SO42− > OH− > F− > H2PO4− > NCS− > NO3− > Cl− > ClO4− [30, 31], it can be concluded that gradually more OH and CO32 and meanwhile less SO42 (lower SO42−/RE3+ molar ratio) would be incorporated into the amorphous precipitate with increasing solution pH, which is in compliance with the above experimental results of XRD. The crystallization of REOCSH (pH  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  = 6, 7) from an amorphous mass would proceed via a typical dissolution-reprecipitation (DR) process, which is very similar to the widely studied CaCO3 system [32, 33]. However, the driving force for the dissolution of the initial amorphous mass would be controlled by the nucleation and growth of crystalline REOCSH [34-36]. Since part of the reactant solutes were fixed in the amorphous mass, the part of the solutes remaining in solution would thus dominate the thermodynamics and kinetics of REOCSH crystallization, and the greater the composition difference between the initial amorphous mass and REOCSH the more difficult it is for REOCSH to nucleate and grow. Owing to the small compositional deviation, the amorphous masses precipitated under pH = 6 and 7 underwent a complete DR process during hydrothermal treatment and thus REOCSH was crystallized as a pure phase. Another factor that may significantly affect the kinetics of DR is the solubility product (Ksp) of the initial mass. It is known that, owing to thermodynamic reason, the mass formed under a specific condition of chemical precipitation would have the lowest Ksp under equilibrium and Ksp varies with the condition of reaction. In view that the amorphous masses before hydrothermal reaction are essentially hydroxides, it can be inferred that their Ksp would decrease with increasing solution pH. The lower Ksp values of the pH = 8 and 10 ones may then make the DR process more difficult to occur through limiting the kinetics of solute exchange and, therefore, amorphous products were resulted even after the hydrothermal treatment. The microspheres shown in Fig. 2b,c were apparently formed via a typical aggregation mechanism [37], and the tending to be face-to-face alignment of the nanoflakes (the inset of Fig 2b) is the most efficient way to minimize the total surface energy of the system. It can also be inferred from the aggregation phenomena that the isoelectric point (Eiso) of the precipitates is around 7-8 (closer to 8). 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 11  REOCSH nanoflakes would have positively charged surfaces under pH = 6, which hinders them from aggregation by electric repulsion, and thus discrete nanoflakes were produced (Fig. 2a). Similarly, the pH = 10 product was not formed as spherical aggregates, since the surfaces of the primary particles would be negatively charged under this pH value even though smaller particles have a stronger tendency to undergo aggregation.  Fig. 4. XRD patterns of the products obtained by 800 °C calcination of the initial amorphous masses formed under different pH values. Since thermal field is crucial for the nucleation and growth of the reprecipitated phase, we comparatively investigated the effect of hydrothermal temperature on the crystallization of REOCSH under pH = 6 and 7. XRD analysis (Fig. 5) found that no matter the pH value is 6 or 7, the products obtained at 100-180 °C all exhibited the characteristic diffractions of REOCSH, and diffraction intensity tends to increase with increasing reaction temperature. Noteworthy is that, at each temperatutre, the product of pH = 7 showed a lower diffraction intensity and more noisy diffraction peaks than its pH = 6 counterpart. This indicates a higher crystallization degree of the pH = 6 product, and can be understood by considering that the Ksp under pH = 6 is larger 5 10 15 20 25 30 35 40 45 50  ICDD (Gd2O2SO4): No. 04-024-9775 200110202400-312600-602020-204004310-402-112022220 pH = 6510 pH = 7Intensity (a. u.)pH = 8 pH = 10 2Theta (deg.)ICDD (Gd2O3): No. 12-0797 211222321 400411420332422431521 440433 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  than that under pH = 7, which facilitates the DR process. When the hydrothermal temperature reached 200 °C, monoclinic structured RE(OH)SO4 (ICDD No. 18-2413) [8] appeared as an impurity phase under either pH = 6 or 7 (Fig. 5). Previous study on the La(NO3)3-(NH4)2SO4-NH3∙H2O hydrothermal system showed that the product would transform from layer-structured La2(OH)4SO4∙2H2O (SO42-LLaH) into layer-structured La(OH)SO4 when the temperature reached ~150 °C. The much higher temperature of RE(OH)SO4 formation in this work could possibly be due to the different RE elements, in view of lanthanide contraction, and the interference of CO32 anions. The appearance of RE(OH)SO4 indicates phase separation and a lower content of CO32 in the final product. This occurred owing to right-hand shift of the CO32 ↔ HCO3 ↔ CO2(g) equilibrium, since the solubility of CO2 in water would decrease with increasing temperature, particularly under the slightly acidic/neutral conditions of pH = 6-7. FE-SEM analysis found that both the 100 and 120 °C products of pH = 7 contained amorphous nanoparticles (the insets in Fig. 6a,b), similar to those shown in Fig. S4, and the amorphous content of the former was significantly higher than that of the latter since REOCSH nanoflakes were more readily found in the 120 °C product (the inset in Fig. 6b). This indicates that the DR process was not complete below 140 °C. It is also clearly seen from the FE-SEM images (Fig. 6a-c) that microspheres were formed via coagulation of nano-sized primary particles, and a higher reaction temperature promotes the coagulated cluster to approach an ideal spherical shape. Additionally, the 180 °C product (average particle size ~21 μm; Fig. 6c) has an morphology almost identical to that observed from the 140 °C one (Fig. 2b). At the highest tested temperature of 200 °C, some of the microspheres were disintegrated owing to the formation of giant RE(OH)SO4 plates (Fig. 6d). The above results thus indicate that pure  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  REOCSH is availble in the temperature range of ~140-180 °C under pH = 7. Under pH = 6, on the other hand, the 100 °C product (Fig. 6e) has already been REOCSH nanoplates and no amorphous particles were observed, indicating a much faster DR process owing to the larger Ksp at pH = 6 and the smaller compositional difference between the initial amorphous mass and REOCSH as discussed earlier for the results of XRD analysis (Fig. 5). The nanoplates have a better defined 2D morphology and straighter edges at a higher reaction temperature up to 180 °C, owing to a gradually higher degree of crystallization. Nanoplates should be the intrinsic morphology of REOCSH, since the compound has a layered structure in the tetragonal system and its (001) plane has the lowest surface energy [38, 39], which makes crystal growth along the [001] direction the hardest to proceed according to the Wulff theorem. The 200 °C product (Fig. 6h) consists of nanoplates (REOCSH) and giant plates (RE(OH)SO4), comforming to a mixtutre of the two phases (Fig. 5b). Giant laths with a side length of ~100 μm were also observed for the hydrothermall synthesized La(OH)SO4 and Ce(OH)SO4 compounds [8, 40].   Fig. 5. XRD patterns of the products obtained at different temperatures under pH = 7 (a) and pH = 6 (b). 5 10 15 20 25 30 35 40 45 50 55 60 2Theta (deg.)100 C130 cps(a) 120 C Intensity (a. u.)180 C 200 C ICDD (GdOHSO4):No.18-2413(020)(040)(060)5 10 15 20 25 30 35 40 45 50 55 60 2Theta (deg.)100 C 120 C Intensity (a. u.)180 C 330 cps(b)200 C (020)(040)(060)ICDD (GdOHSO4):No.18-2413 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. 6. FE-SEM morphologies of the products obtained at 100 °C (a, e), 120 °C (b, f), 180 °C (c, g) and 200 °C (d, h) under pH = 7 (a-d) and pH = 6 (e-h). The insets are closer views. To further clarify the effect of solution pH on DR and morphology development, we tracked the time-course morphology evolution by FE-SEM for the products produced by 140 °C of hydrothermal reaction. It is seen from the results obtained under pH = 6 that the 30 min product (Fig. 7a) still maintained the morphology of the initial amorphous nanoparticles (Fig. S4) but a large number underdeveloped REOCSH nanoplates can be observed in the 1 h product (Fig. 7b), and the 3 h product contained only REOCSH nanoplates (Fig. 7c). The results indicate a quite fast phase conversion via DR under pH = 6 and, again, this is due to the relative large Ksp and the small compositional deviation between the initial amorphous mass and REOCSH. Fig. 7d-g show the results obtained under pH = 7, where it is seen that the 30 min product consists of amorphous nanoparticles (Fig. 7d). After 1 h of reaction, in addition to amorphous nanoparticles, a small amount of spherical particles (colloid spheres) with a diameter of ~1 μm and smooth surfaces were observed in the product (Fig. 7e and the inset). This indicates the commencement of nanoparticle aggregation. Observation of the 3 h product (Fig. 7f) found ~10 μm-sized microspheres embedded in a matrix mass, and closer view showed that 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 15  microsphere contains both nanoparticles and nanoplates (the left-hand inset in Fig. 7f) and the matrix consists of nanoparticles and ~1 μm-sized colloid spheres (the right-hand inset in Fig. 7f). After reaction for 6 h (Fig. 7g), the portion of microspheres increased dramatically, which was accompanied by vanishing of the matrix mass. Besides, closer observation (the inset in Fig. 7g) showed that the microspheres formed after 6 h of reaction has much fewer nanoparticles (Fig. 7g) than those formed after 3 h of reaction (the left-hand inset in Fig. 7f) due to more crystallization of the REOCSH phase. The above observations indicated that, under pH = 7, the crystallization of REOCSH started at ~3 h of reaction at 140 °C and that the ~10 μm-sized microspheres were formed via growth of the ~1 μm-sized colloid spheres. The observed evolution of flaky REOCSH crystallites is in accordance with the results of phase analysis via XRD (Fig. S5), and the course of microsphere formation is schematically shown in Fig. 7h.     Fig. 7. FE-SEM morphologies of the products obtained by hydrothermal reaction under pH = 6 (a-c) and pH = 7 (d-g) for 30 min (a, d), 1 h (b, e), 3 h (c, f) and 6 h (g). The insets are closer views and part (h) is a schematic illustration of the growth mechanism of microspheres under pH = 7. 3.2 Derivation and characterization of RE2O2SO4 REOCSH has the same RE/S molar ratio of RE2O2SO4, so it can be used as a precursor to derive RE2O2SO4 by proper calcination without the emission of harmful SOx gases. Our  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  previous study via differential thermal analysis/thermogravimetry (DTA/TG, heating rate 10 °C/min) showed that GdOCSH would decompose into Gd2O2SO4 in air via dehydroxylation and decarbonation up to 800 °C (Gd2(OH)2CO3SO4∙nH2O → Gd2O2SO4 + CO2 + (n + 1)H2O) , followed by further decomposition into Gd2O3 when the temperature is over ~1000 °C (Gd2O2SO4 → Gd2O3 + SO3). We thus selected 800 °C as a proper temperature to derive RE2O2SO4 (RE = Gd0.95Eu0.05) from the REOCSH precursors formed via hydrothermal reaction at 140 °C and pH = 6 and 7. Fig. 8a shows the XRD patterns of the calcination products, where it is seen that phase-pure RE2O2SO4 was obtained in each case since all the diffraction peaks can be well indexed with those of the Gd2O2SO4 (monoclinic, space group C2/c; ICDD No. 04-024-9775). Broadening analysis of the XRD peaks with Scherrer formula [41] found that the RE2O2SO4 powders from the pH = 6 and 7 precursors have average crystallite sizes of ~24 and 27 nm, respectively. Analysis of the XRD patterns with the Jade 6 software found that both the RE2O2SO4 powders have the similar lattice parameters of a ~ 13.6204(2) Å, b ~ 4.1769(4) Å and c ~ 8.1223(3) Å, cell volume V of 441.39(5) Å3, and axis angle β of 107.21(4)°. The cell volume is slightly larger than that of pure Gd2O2SO4 (V = 439.82 Å3; ICDD No. 04-024-9775) due to the incorporation of 5 at.% of larger Eu3+ ions (1.12 Å for Eu3+ and 1.107 Å for Gd3+ under coordination number CN = 9) and indicates the formation of (Gd,Eu)2O2SO4 solid solution. FE-SEM observation indicated that the microspheres of REOCSH (Fig. 8b) essentially maintained their original shapes after 800 °C calcination though cracking occurred due to the stress arising from mass loss (release of H2O and CO2) and intra-sphere sintering. The RE2O2SO4 powder calcined from the REOSCH nanoplates (Fig. 8c) tends to have a nanoplate-like particle morphology, though the original platelets became porous and tend to disintegrate by the mass loss during calcination.   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   Fig. 8. XRD patterns of the products calcined in air at 800 °C (a), and parts (b) and (c) respectively show FE-SEM morphologies of the RE2O2SO4 calcined from REOCSH microspheres and nanoplates. 3.3 Photoluminescence properties of the (Gd0.95Eu0.05)2O2SO4 phosphors  Fig. 9. PLE (a) and PL (b) spectra of the (Gd,Eu)2O2SO4 phosphors shown in Fig. 8b (microspheres) and Fig. 8c (nanoplates). Fig. 9 shows the excitation (PLE) and emission (PL) spectra of the two kinds of (Gd,Eu)2O2SO4 phosphor powders. It is seen that both the nanoplates and microspheres share similar spectral profiles. The PLE spectra (Fig. 9a), taken by monitoring the red emission of Eu3+ at 617 nm, contain a broad and strong band centered at ~275 nm and a few weak and sharp peaks in the longer wavelength region in each case. The broad band largely originates from O2-Eu3+ charge transfer (CT, excitation of electrons from the 2p orbital of O to the 4f orbital 5 10 15 20 25 30 35 40 45 50Intensity (a. u.) pH = 6, 800 C pH = 7, 800 C 2Theta (deg.)(a)200110-202310-402-112-312004220-204020600-602510ICDD (Gd2O2SO4):No. 04-024-9775022400200 250 300 350 400 4500100020003000400050006000Intensity (a. u.)Wavelength (nm) Nanoplates Microspheres275 nm lem=617 nm395 nm (7F05L6) 379 nm (7F05L7) 363 nm (7F05D4) 415 nm (7F05D3) (a)312 nm Gd3+(8S7/26PJ) 319 nm (7F05H4)  500 550 600 650 700 7500100020003000400050006000Intensity (a.u.)Wavelength (nm) Nanoplates Microspheres617 nm (5D07F2)702 nm (5D07F4)702 nm (5D07F3)596 nm (5D07F1)580 nm (5D07F0)lex=275 nm(b) 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  of Eu) [42], which is overlapped with the 8S7/2  6IJ (~275 nm) and 8S7/2  6PJ (~312 nm) intra-4f7 excitation transition of Gd3+ [43, 44], while the sharp peaks correspond to intra-4f6 transitions of Eu3+ as labelled in the figure. Under excitation with the peak wavelength of the CT band (275 nm), both the (Gd,Eu)2O2SO4 phosphors exhibited the typical 5D0  7FJ (J = 0-4) emissions of Eu3+, with the 5D0  7F2 red one (~617 nm) being the most prominent (Fig. 9b). Noteworthy is that the 617 nm emission of the microspheres is ~1.35 times as strong as that of the nanoplates, since the microspheres have a much larger particle size and the spherical particle shape reduced surface scattering of the excitation light (stronger excitation absorption, Fig. S6). Analysis of the PL spectra with the built-in software of the spectrophotometer found that the microspheres and nanoplates have quantum efficiencies of ~40.8% and 32.1%, respectively. Analyzing the fluorescence decay curves with equation I = Aexp(-t/) + B, where  is fluorescence lifetime, t is delay time, I is emission intensity and A and B are constants, found that the 617 nm main emission (λem = 617 nm, λex = 275 nm) decayed in a single exponential manner (Fig. 10a) and that the microspheres and nanoplates have  values of ~1.32 and 2.43 ms, respectively. Though the lifetimes fall in the range of 0.71-4.37 ms reported for the red emission of Eu3+ in some oxide/oxysulfide hosts [45-48], the obviously longer lifetime of the nanoplates could be due to smaller particle size. It is known that  can be correlated with the effective refractive index of the material (neff) via equation τ ~ [(λ02/f(ED)]/[neff(neff2+2)2], where f(ED) is the oscillator strength for electric dipole transition and λ0 is the wavelength in vacuum [49]. The neff in the equation can be further expressed as neff = x∙nc + (1-x)∙nmed, where x is the filling factor, that is, the fraction of space occupied by the particle in the surrounding medium (air in this work), and nc and nmed are the refractive indices of the bulk material and the surrounding medium, respectively. neff roughly equals to nmed for extremely small particles,  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  since the x value approaches 0. For the intermediately sized particles of this work, neff is affected by particle size and decreases for smaller particles. This may account for the longer fluorescence lifetime of the (Gd,Eu)2O2SO4 nanoplates, since they are much smaller than the microspheres. Despite the different particle morphologies, both the types of (Gd,Eu)2O2SO4 phosphors have almost the same Internationale de L’Eclairage (CIE) chromaticity coordinates of around (0.653, 0.345), typical of a red color (Fig. 10b).    Fig. 10. Fluorescence decay behaviors of the 617 nm emission (a) and a CIE chromaticity diagram (b) for the two types of (Gd,Eu)2O2SO4 phosphors. To evaluate the thermal stability of luminescence, temperature-dependent PL spectra were taken for the microspheres under 275 nm excitation. It is seen from the results displayed in Fig. 11a that raising the measurement temperature up to 300 oC did not produce new emission and did not appreciably alter the position of the existing emission peaks, but successively weakened the luminescence of Eu3+ due to thermal quenching. It is seen from the normalized intensity (Fig. 11b) that the 617 nm emission retained ~81% and 53% of its room-temperature (RT) intensity at 150 and 300 oC, respectively, showing a good thermal stability of luminescence. The value at 150 oC, a temperature frequently used to evaluate whether a phosphor is suitable for high-power LED application, is higher than that of Y2O2S:Eu (42.8%) [50] and is almost 0.010.11  (Gd,Eu)2O2SO4 nanoplates  Linear fitting data  Relative intensity (a.u.)lex = 275 nm lem = 617 nm = 2.43±0.01 msχ2 = 1.00lex = 275 nm lem = 617 nm = 1.32±0.01 msχ2 = 1.00(a)  (Gd,Eu)2O2SO4 microspheres  Linear fitting data 8 10 12 14 16 18 20 22Decay time (ms) 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  the same as that of Y2O3:Eu (82%) [51]. The activation energy (Ea) of thermal quenching can be derived with the Arrhenius equation [52]: 𝐼 =𝐼01 + 𝑐𝑒𝑥𝑝(−𝐸a𝑘𝑇)                                                            (1) where I0 and I are the initial emission intensity and the emission intensity at a given temperature T (in Kelvin), respectively, c is a pre-exponential constant, and k is Boltzmann constant (8.617×10-5 eV). ln[(I0/I-1)] vs 1/(kT) transformation of the experimental data for the measured temperature range of 323-573 K displayed a nearly straight line (the inset in Fig. 11b), and linear fitting yielded an Ea value of ~0.19±0.01 eV. It is seen from Table S1 that the color coordinates of Eu3+ luminescence slightly drifted from (0.653, 0.345) to (0635, 0.361) with increasing temperature from 25 to 300 °C, corresponding to color changes toward the orange red region of the CIE chromaticity diagram (Fig. S7). The extent of chromaticity shift (E) can be evaluated by applying the equation [53]: ∆𝐸 = √(𝑢𝑓′ − 𝑢𝑖′)2 + (𝑣𝑓′ − 𝑣𝑖′)2 + (𝑤𝑓′ − 𝑤𝑖′)2                               (2) where u = 4x/(3-2x+12y), v = 9y/(3-2x+12y) and w = 1- u- v, x and y are the CIE coordinates, and f and i refer to the measurement temperature and initial temperature (25 °C), respectively. The results of calculation (Table S1) indicated that E gradually increased with increasing temperature, which is ~0.9% at 150 °C and ~3.8% at 300 °C. The continuous color drift (Fig. S7) implies a gradually stronger emission of lights shorter than 617 nm. We thus examined the intensity variations of the 5D0,1  7F0,1,2 transitions. Taking 617, 596, 580, 556 and 533 nm as the typical emission wavelengths of 5D0  7F2 (Fig. 9b), 5D0  7F1 (Fig. 9b), 5D0  7F0 (Fig. 9b), 5D1  7F2 (Fig. S8) and 5D1  7F1 (Fig. S8), respectively, it was found that the I617/I596 and I617/I580 intensity ratios only slightly decreased while the I617/I556 and I617/I533 ones were  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  dramatically reduced with increasing temperature up to 300 °C (Table S1). This clearly indicates that the color drift of luminescence was mainly caused by gradually stronger emissions from the 5D1 level, which can be understood by considering that the 5D1/5D0 energy levels of Eu3+ are thermally coupled and, therefore, the excited electrons at 5D0 can be promoted to the higher lying 5D1 level by thermal activation.  Fig. 11. Temperature-dependent PL spectra (a) and relative intensity of the 617 nm emission as a function of the measurement temperature (b) for the (Gd,Eu)2O2SO4 microspheres under 275 nm excitation. The inset in (b) is the ln(I0/IT − 1) versus 1/(kT) plot for determination of the activation energy for thermal quenching of luminescence.  4. Conclusion RE2(OH)2CO3SO4nH2O layered compound (RE = Gd0.95Eu0.05, REOCSH) was obtained via hydrothermal reaction at 140 °C for 24 h, and its particle morphology was successfully regulated from microsphere (~25 μm in diameter) to nanoplate (~400 nm in length) by adjusting the solution pH from 7 to 6. Solution pH substantially affected the chemical composition and crystallization kinetics of the initial amorphous precipitate and, therefore, influenced the crystallization of REOCSH via dissolution-reprecipitation during hydrothermal treatment. Investigation of the temperature-course and time-course of phase/morphology evolution showed that the microspheres were formed via a typical aggregation process under pH = 7. 500 550 600 650 700 750 800020040060080010001200140016001800Intensity (a. u.)Wavelength (nm) 25 C 50 C 75 C 100 C 125 C 150 C 175 C 200 C 225 C 250 C 275 C 300 C(a)617 nm (5D0  7F2)0 50 100 150 200 250 3000.30.40.50.60.70.80.91.018 20 22 24 26 28 30 32 34 36 38-3.2-2.8-2.4-2.0-1.6-1.2-0.8-0.40.00.4 Experimental data Fitting lineln(I0/IT−1)1/(kT)Ea = 0.19±0.01 M % (2)Relative Intensity (a. u.)Temperature (C)100%80.8%52.5%(b) 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  Calcining the two types of REOCSH in air at 800 °C produced (Gd0.95Eu0.05)2O2SO4 monospheres and nanoplates, and the former showed a luminescence intensity ~1.35 times that of the latter (λex = 275 nm, λem = 617 nm). The 617 nm red emission of Eu3+ in the monospheres has a lifetime of ~1.32 ms and retained ~81% of its room temperature intensity at 150 °C (activation energy of thermal quenching ~0.19 eV). Despite the different particle morphologies, both the types of (Gd,Eu)2O2SO4 have chromaticity coordinates of around (0.653, 0.345) for their luminescence at room temperature. A gradual chromaticity drift was observed for the luminesce of the monospheres up to the highest measurement temperature of 300 °C, which was analyzed to be largely due to stronger emissions from the 5D1 level of Eu3+. Acknowledgements This work was supported in part by the National Natural Science Foundation of China (Grant No. 52172112 and 51972047). References [1] M. Machida, K. Kawamura, K. Ito, K. Ikeue, Large-capacity oxygen storage by lanthanide oxysulfate/oxysulfide systems, Chem. Mater., 17 (2005) 1487-1492. [2] K. Ikeue, M. Eto, D. Zhang, T. Kawano, M. 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Interfaces, 6 (2014) 2709-2717.   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.dochttps://www.editorialmanager.com/advpt/download.aspx?id=674187&guid=66200bd3-5c1e-4d1c-9fff-723e07022a84&scheme=1