Synthesis of multi-type rare earth compounds with RE(OH)SO4 as a new sacrificial-template and photoluminescence

ABSTRACT The low OH−/RE3+ molar ratio of hydroxide sulfate RE(OH)SO4 (RE = rare earth) makes it a potential good template for the synthesis of rare earth compounds, which is shown in this work by the direct generation of (La,Eu)F3, (La,Eu)PO4, Na(La,Eu)(WO4)2, and Na(La,Eu)(MoO4)2 without further calcination. The template showed irregular and slightly aggregated micro-plates morphology, while the products derived from the template showed morphologies of nanoparticles (trifluoride), nanorods (phosphate), aggregate spheres (molybdate), and aggregated polyhedrons (tungstate). The photoluminescence of the derived compounds, including excitation/emission spectra, decay kinetics, quantum yields, color coordinates, and color purities were comprehensively investigated, and the results were well correlated with structures and morphologies. Calcination further improved the luminescence intensity and quantum yield of the obtained phosphors. It is believed that the RE(OH)SO4 compound proposed in this work may find applications for the synthesis of other rare earth compounds.


Introduction
Sacrificial phase conversion has been introduced for the controllable synthesis of various compounds [1][2][3][4][5][6][7][8][9][10], where the target compound was formed via reacting a pre-made template with an anion source under appropriate conditions [1][2][3]. Rare earth hydroxides are good templates for the synthesis of other rare earth compounds because the templates themselves can be used as the reactant rare earth sources in the synthesis process. With the reaction proceeding, the template is gradually consumed and plays a decisive role in the reaction process since it often controls reaction kinetics/pathway and determines the yield, chemical composition, and morphology (particle size/shape/dispersion) of the final product. In the strategy, the reaction may proceed via two separate mechanisms: interface chemical transformation and dissolution followed by reprecipitation [5,[11][12][13]. When the template and the target compound have the same crystal structure, the pre-made compound plays a role as both a physical and chemical template, and the final product may well preserve the micro-morphology of the template. One representative example is the synthesis of hexagonal structured β-NaREF 4 from hexagonal RE(OH) 3 via in situ acid corrosion and anion exchange [5]. When the template and final product differ in structure, the dissolution-reprecipitation mechanism may be applied, as exemplified by the synthesis of hexagonal LuBO 3 from monoclinic Lu 4 O(OH) 9 NO 3 via hydrothermal conversion [13]. After a careful literature survey, we summarize the currently used hydroxide templates and their derivatives in Table S1 (in supporting information). It can be seen from Table S1 that a rich family of templates has been investigated, including RE(OH) 3 , RE(OH) 2.94 (NO 3 ) 0.06 ⋅nH 2 O, RE 2 (OH) 5 NO 3 ·nH 2 O, RE 4 O(OH) 9 NO 3 , RE 2 (OH) 4 SO 4 · 2H 2 O, and RE(OH)CO 3 , and so on. [4][5][6][7][8][9][10][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Various rare earth compounds have been successfully prepared through the templates mentioned above, such as YPO 4 :RE 3+ /GdPO 4 phosphate, YVO 4 :RE 3+ vanadate, LaF 3 /YF 3 fluoride, tungstate, and so on. The sacrificial conversion based on hydroxide template will gradually release OH − into the reaction system, which competitively coordinates with rare earth ions with the subsequently added reactant anions such as F − , PO 4

Synthesis of (La,Eu)(OH)SO 4
In a typical synthesis procedure, a certain amount of (NH 4 ) 2 SO 4 was dissolved in 60 ml RE(NO 3 ) 3 stock solution (0.1 M) under magnetic stirring for 10 min. NH 3 ·H 2 O was then dropwise added into the mixed solution to reach a designated pH of ~8. The resultant suspension was either constantly stirred for 24 h at a prescribed temperature (RT or 50°C) under ambient pressure or transferred to a Teflon lined stainless steel autoclave for 24 h of hydrothermal crystallization in an electric oven preheated to 100-200°C (without stirring). The product was collected via centrifugation, followed by washing with distilled water three times to remove byproducts, rinsing with absolute ethanol, and air drying at 70°C for 24 h.

Phase-conversion synthesis of multi-type rare earth compounds using (La,Eu)(OH)SO 4 as the template
In a typical procedure for sacrificial phase conversion, 2 mmol of the above synthesized (La,Eu)(OH)SO 4 was dispersed in 60 ml distilled water, followed by the addition of a certain amount of NH 4 F, NH 4 H 2 PO 4 , Na 2 MoO 4 ⋅2H 2 O, or Na 2 WO 4 ⋅2H 2 O. The obtained suspension was stirred at a designated temperature (below 100°C) or subjected to hydrothermal reaction at different temperatures (above 100°C) for a certain period of time. After natural cooling, the product was collected via centrifugation, followed by washing with distilled water three times to remove by-products, rinsing with absolute ethanol, and air drying at 70°C for 24 h. Calcination was performed in the air at 300-800°C for 1 h, using a heating rate of 5°C/min at the ramp stage.

Characterization
Phase identification was performed via XRD (Model  Figure 1 shows the XRD patterns of the products crystallized at various temperatures (from RT to 200°C). It was found that the products obtained in the RT-50°C range are amorphous. The peaks recorded from the products synthesized in the 100-200°C range can all be indexed to (La,Eu)(OH)SO 4 (JCPDS No. 00-045-0750), which is verified by Rietveld refinement for the typical sample ( Figure S1(a)). The peak intensity noticeably increased with increasing reaction temperature. It was also noticed that the intensity ratio of the (020) to (111) diffractions increased from 0.86 to 4.31 as the synthesis temperature increased from 100°C to 200°C (Table S2). This phenomenon is originated from the growth habit of (La,Eu)(OH)SO 4 and is also closely related to morphology change of the crystallites in response to the synthesis temperature ( Figure S2). (La,Eu)(OH)SO 4 crystallizes in a monoclinic unit cell by alternative stacking of sulfate anions and the [LaO 9 ] polyhedra containing two-dimensional hydroxide layers along the b-axis ( Figure S1(b)). Thus, the non-(0k0) diffractions (such as (111)) reflect the structural features of the host layer, while the (0k0) diffractions reflect the stacking of the host layer along the b-axis.

Synthesis of (La,Eu)(OH)SO 4 compound
A higher reaction temperature produced thicker and bigger crystallite plates for the product, and thus substantially stronger (020) diffraction was observed. The effects of RE 3+ :SO 4 2ratio on the formation of La(OH)SO 4 were also investigated, and the results are shown in Figure S3.

(La,Eu)(OH)SO 4 as a sacrificial-template for the synthesis of various rare earth compounds
(La,Eu)(OH)SO 4 was used for the first time as a sacrificial-template for the synthesis of rare earth fluoride, and the XRD results of the temperaturedependent fluorination products under a fixed reaction time of 24 h are shown in Figure 2. An immediate increase in pH was observed after F − was added to the template containing suspension, indicating that the reaction is fast. It is encouraging to find from the XRD results that all the diffraction peaks of the reaction product are in good agreement with the standard card of LaF 3 (JCPDS No. 01-074-2415). No impurity can be identified from the XRD patterns even after reaction at room temperature. With increasing temperature, the diffraction peaks gradually became sharper and the (002) and (110) peaks gradually separate, which implies increased crystallization. When NH 4 F is dissolved in water, the following dynamic equilibrium exists: According to Pearson's hard and soft acid and base (HSAB) theory [30], La 3+ and Eu 3+ are hard Lewis acids, and OH − , SO 4 2-, and F − are hard bases. The basicity of the three types of anions decreases following the order of F − > SO 4 2-> OH − [31]. (2) Figure 3 shows the micro-morphologies of the (La,Eu)F 3 obtained at different reaction temperatures. Though the (La,Eu)F 3 product basically retained the template profile, enlarged views found that nanoplates were actually formed and are the true morphology units of the product (Figure 3(c)). It was found that the nanoplates became bigger and thicker with increasing reaction temperature, collapsed into nanoparticles above 120°C, and became bigger and smoothly rounded nanoparticles at 200°C (Figure 3 (d-g)).
The influence of the reaction time was also examined at room temperature, and the XRD/FT-IR results are shown in Figure S4-5. The results indicated that pure products can be obtained after 30 min of reaction. The reaction (RT/30 min) is much faster than that reported for LaF 3 formation from La(OH)CO 3 template that also has low OH − /RE 3+ of 1 (50°C/3 h) [26] and is faster than those for LaF 3 formation from RE 2 (OH) 4 [14,22]. Furthermore, the product yield is much higher than that from the direct reaction of La(NO 3 ) 3 /Eu(NO 3 ) 3 with NH 4 F for the same time of 0.5 h ( Figure S6-7). It can also be seen from the XRD result ( Figure S6) that the direct product of the latter case has a high amorphous background and poor crystallinity. The effects of the F − :RE 3+ ratio on sacrificial phase conversion were also investigated, and the results are shown in Figure S8-9. It can be concluded from the above results that (La,Eu)F 3 can In view that a higher reaction temperature would better crystallize the product, the synthesis of other rare earth compounds was conducted at 200°C for 24 h. It is seen from Figure 4(a) Figure S10) and pure products were obtained under proper conditions Figure 4(c,d)   The absence of SO 4 2vibrations in the FT-IR spectra ( Figure S11) of the products further confirmed a complete consumption of the template and the formation of pure phases. Based on the above results, it can be concluded that four different types of compounds can be successfully synthesized by reacting the per-made (La 0.95 Eu 0.05 )(OH)SO 4 template with the corresponding anion sources under proper reaction conditions. Figure 5 shows the FE-SEM morphologies of the (La 0.95 Eu 0.05 )(OH)SO 4 template and the four types of rare earth phosphors converted from the template via reaction at 200°C for 24 h. It is seen that the (La 0.95 Eu 0.05 )(OH)SO 4 template has micro-plate-like crystallite morphologies (Figure 5 Figure 5(e)). The quite different morphologies between the template and each product imply that dissolution-reprecipitation is the dominant mechanism of phase conversion and the final morphology is primarily determined by the intrinsic crystal structure of the product.

Photoluminescence properties of rare earth compounds derived from (La,Eu)(OH)SO 4
The photoluminescence properties of Eu 3+ have long been reported, and this work mainly studies the performance of Eu 3+ in correlation with crystal structure and crystallite morphology. The PL excitation and emission spectra of derived Na(La 0.95 Eu 0.05 )(WO 4 ) 2 , Na(La 0.95 Eu 0.05 )(MoO 4 ) 2 , (La 0.95 Eu 0.05 )F 3 , and (La 0.95 Eu 0.05 )PO 4 phosphors are shown in Figure 6, with the quantum yield (QY), color coordinates (CIE), and color purity (CP) of luminescence being inserted in the corresponding figure. As can be seen from Figures 6(a,b) the excitation and emission spectral features of Na(La 0.95 Eu 0.05 )(WO 4 ) 2 and Na(La 0.95 Eu 0.05 )(MoO 4 ) 2 are similar. The PLE spectra of the two compounds consist of a broad band in the UV region of 200-350 nm and a series of sharp peaks in 350-500 nm region. The broad band is overlapped from WO 4 2-/MoO 4 2excitation and O 2-→ Eu 3+ charge transfer (CT), while the sharp peaks are arising from intra-4f 6 excitation transitions of Eu 3+ . The PL spectra of Na(La 0.95 Eu 0.05 )(WO 4 ) 2 and Na(La 0.95 Eu 0.05 )(MoO 4 ) 2 consist of sharp peaks located at 590, 615, 650, and 700 nm, which are assigned to 5 D 0 → 7 F J (J = 1, 2, 3, 4) transitions of Eu 3 + ions. The peak at ~615 nm is the strongest, which is well consistent with the fact that Eu 3+ has a C 1 symmetry, which is distorted from D 3d due to the statistical distribution of Na + and RE 3+ ions in the two hosts.
Considering that the two hosts have similar crystal structure and site symmetry of luminescence centers Figure 7(a,b) their similar photoluminescence behaviors can be well understood. For (La,Eu)F 3 , the excitation spectrum consists of a series of sharp peaks in the range of 280-500 nm, with the 397 nm excitation ( 7 F 0 → 5 L 6 ) being the strongest. F − → Eu 3+ CT was not observed because the large bandgap of LaF 3 (~6.2 eV) makes it occur below 200 nm. Under 397 nm excitation, the 5 D 0 → 7 F J (J = 1, 2, 3, 4) transitions of Eu 3+ were produced with the 5 D 0 → 7 F 1 one (590 nm) being the strongest. This is different from that observed from Eu 3+ in the tungstate and molybdate hosts, where the 5 D 0 → 7 F 2 transition is the strongest. This, however, is consistent with the fact that Eu 3+ is at the higher symmetric C 2v site in hexagonal LaF 3 . Besides, low symmetry surface-site Eu 3 + may contribute to the strong 5 D 0 → 7 F 2 and 5 D 0 → 7 F 4 transitions for the tungstate and molybdate phosphors, and this corresponds well with the biexponential decay behaviors of the ~615 nm main emission ( Figure S12).
For (La 0.95 Eu 0.05 )PO 4 , the excitation spectrum consists of a strong and broad O 2-→ Eu 3+ CT band in the 200-300 nm range (centered at ~268 nm) and a series of sharp peaks arising from intra-4f 6 transitions of Eu 3+ . It should also be noted that the position of the CT for (La 0.95 Eu 0.05 )PO 4 appeared at a shorter wavelength than those observed for Na(La 0.95 Eu 0.05 )(WO 4 ) 2 (~285 nm) and Na(La 0.95 Eu 0.05 )(MoO 4 ) 2 (~291 nm). This is due to the fact that the position of CT is strongly dependent on the length of (La/Eu)-O bond, and the longer the bond would red-shift the CT band [33,34]. The positions of the CT bands observed in this work  corresponds well with the (La/Eu)-O bond lengths in the three types of compounds, as summarized in Table 1. Under the excitation of 395 nm, strong and split 5 D 0 → 7 F 1 , 5 D 0 → 7 F 2 , and 5 D 0 → 7 F 4 transitions were observed for the (La 0.95 Eu 0.05 )PO 4 phosphor, with 5 D 0 → 7 F 4 being the strongest. A stronger 5 D 0 → 7 F 1 transition was expected since Eu 3+ would replace the La 3+ in LaPO 4 to inherit a symmetric D 2 point symmetry. The abnormally higher intensities of the 5 D 0 → 7 F 2,4 emissions may be due to the rod-like crystallite morphology ( Figure 5(e)), which gives rise to distortion of the Eu 3+ site, as demonstrated by one-dimensional YPO 4 :Eu 3+ and Sr 3 Ga 2 O 5 C l2 :Eu 3+ ,Bi 3+ phosphors [9,35], and the existence of low symmetric surface site. The decay kinetics of the main emissions, quantum yields, and color coordinates of the four phosphors are shown in Figures S12-S14 and Tables S3-S4.
It is known that a higher crystallinity benefits the luminescence of a phosphor. Calcination was thus carried out for the four phosphors of this work. The XRD analysis indicated that calcination at 300-800°C has no influence on phase purity for Na(La,Eu)(WO 4 ) 2 and Na(La,Eu)(MoO 4 ) 2 ( Figure S15). (La,Eu)PO 4 undergoes phase transformation from hexagonal to monoclinic during calcination. 300-500°C calcination improved the crystallinity of (La,Eu)F 3 , but a trace amount of (La,Eu)OF appeared in the 800°C product ( Figure S15). As seen from Figure S16, luminescence intensity successively increases with increasing temperature of calcination, which is due to crystal perfection/growth and the elimination of luminescence quenching centers such as surface absorbed water and surface dangling bonds [9]. The quantum yields ( Figure S17)

Conclusions
(La,Eu)(OH)SO 4 was used as a novel sacrificial-template in this work for direct synthesis Na(La,Eu)(WO 4 ) 2 , Na(La,Eu)(MoO 4 ) 2 , (La,Eu)F 3 , and (La,Eu)PO 4 phosphors. The synthesis of the template, the phase conversion process for each type of the phosphors, and the photoluminescence properties of the products were systematically investigated. The main conclusions are as follows: (1) The (La,Eu)(OH)SO 4

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This work is supported by the Natural Science Foundation of Liaoning Province (Grant No. 2020-MS-286).