Eroding the Surface of Rare Earth Microcrystals through Vanadate Ions for Considerable Improvement of Luminescence

Developing an efficient approach to improve the luminescence of the phosphors without heating processing is a challenge, but attracts much attention. In the present paper, prismatic microcrystals of RE4O(OH)9NO3 (RE = Y, Eu) were synthesized by a hydrothermal reaction at 180 °C for 24 h. The reaction with VO3− did not change the crystal structure of the microcrystals and VO3− substituting for NO3− anions did not take place. However, it contributed to the formation of amorphous particles containing VO3− on the surface of a prism, which is similar to the surface corrosion of a metal, called “surface eroding”. Therefore, surface modification was successfully achieved by eroding the surface of the microcrystals through the reaction with vanadate ions. As a result of VO3−→Eu3+ energy transfer and the light-harvesting ability of VO3−, the red emission intensity at 617 nm of the modified microcrystals greatly increased. Eroding the surface of rare earth microcrystals recommends a new paradigm for luminescence improvement of rare earth compounds.


Introduction
Directly exciting RE 3+ ions upon irradiation for luminescence is not an efficient method, because the f-f electronic transitions of the rare earth ions (RE 3+ ) are usually forbidden by the spin selection rules [1]. Therefore, doping RE 3+ ions with a low concentration into an appropriate host lattice is widely used to produce solid-solution phosphor and obtain desirable luminescence [2]. In recent years, much attention has been paid to RE 3+doped inorganic materials with a uniform size and specific morphology, because they allow for the attainment of functionalities, not only from the constituent substance but also from the special structure [3][4][5][6]. For rare earth oxide phosphors, calcining the precursors is widely used to obtain the phosphors with a variety of novel structures, since the precursors and their calcined products tend to exhibit generic relationships [3,[7][8][9][10][11]. RE 2 (OH) 5 NO 3 ·nH 2 O layered rare earth hydroxide (LRH), RE 4 O(OH) 9 NO 3 oxy-hydroxyl nitrate, RE(OH) 2.94 (NO 3 ) 0.06 ·nH 2 O hydroxyl nitrate, and RE(OH) 3 hydroxide are the reported products synthesized from the hydrothermal reaction system [7][8][9][10], and their morphology and size could be easily regulated by varying the synthesis conditions, including the pH value, reaction temperature, and reaction time. Therefore, they are the desirable precursors for rare earth oxide phosphors. Therefore, there are a lot of investigations on these precursors. However, abundant water molecules or hydroxyls directly coordinate to the rare earth ions, which result in a serious luminescence quenching [8,9]. Thus, there is the question of how to enhance the emission intensity of the rare earth phosphors. Of course, the thermal decomposition of the precursors is an efficient method to synthesize rare earth oxides, and it could remove the water molecules/dehydroxyl and thus enhance the luminescence intensity [3]. However, morphological damage and crystal-structure collapse would take place during the annealing process, which may significantly affect the final performance [12]. Therefore, developing an efficient approach to improve the luminescence of the phosphors without heating processing is a challenge, but attracts much attention.
Recently, it has been reported that an antenna effect could give rise to an enhanced photoluminescence upon light irradiation, because an effective intramolecular energy transfer from coordinated ligands to the activated RE 3+ ions can enhance the absorption of the optical excitation. This antenna effect effectively promotes the energy transfer from coordinated ligands to the activated RE 3+ ions, and thus enhances the emission intensity [13]. This light-harvesting ability was successfully applied onto the layered rare earth hydroxide nanosheets [14,15], hydroxyl nitrate square nanoplates [16], and rare earth nanoparticles [17,18], through the grafting of organic ligands, including picolinic acid and a rare earth complex, on the surface of the nanocrystals. Indeed, a considerable improvement of luminescence was observed for these rare earth crystals. However, the organic ligands were indirectly linked to the surface of the nanocrystals by hydrogen bonding, indicating that the hybrid phosphors grafted organic ligands were unstable. In addition, the hydrogen bonding connection made the energy transfer from the ligands to the activated RE 3+ ions difficult [7,15], so it is not the most effective way for energy transfer, compared to the directly connecting coordination.
In the present work, prismatic microcrystals of RE 4 O(OH) 9 NO 3 (RE = Y, Eu) were synthesized by the hydrothermal reaction. The surface modification of the microcrystals was successfully achieved by eroding the surface of the microcrystals through vanadate ions. The main means of sample characterization are X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning transmission electron microscopy (STEM), photoluminescence (PL)/photoluminescence excitation (PLE) spectroscopy, and fluorescence decay curve analysis. The reaction with VO 3 − contributed to the formation of amorphous particles containing VO 3 − on the surface of the prism, which lead to a great enhancement of luminescence. The usage of the energy transfer to the activated RE 3+ ions through surface eroding processing, paves a new paradigm for luminescence improvement of the rare earth compounds.

Synthesis
Preparation of oxy-hydroxyl nitrate microcrystals. The starting rare earth sources are rare earth oxides, which are Y 2 O 3 and Eu 2 O 3 , all 99.99% pure products from Huizhou Ruier Rare-Chem, Hi-Tech, Co., Ltd. (Huizhou, China). Dissolving the rare earth oxide in hot nitric acid yielded a rare earth nitrate solution, which was then treated by evaporation at 90 • C to remove the excess acid. The synthesis of Y/Eu binary microcrystals (Y:Eu molar ratio of 0.95:0.05) was conducted via a hydrothermal reaction at 180 • C for 24 h with a pH value of 10-11, which was described in our previous work [3].
Erosion of the microcrystal surface. In typical processing, 2 mmol of RE 4 O(OH) 9 NO 3 microcrystals was dispersed in 60 mL of water containing a proper amount of NH 4 VO 3 and NH 4 OH with the pH value of 10-11. After being stirred for 30 min, the resultant suspension was transferred into a Teflon-lined stainless-steel autoclave of 100 mL capacity. The autoclave was tightly sealed and was put in an electric oven preheated to 200 • C. After a 12 h reaction, the autoclave was left to cool naturally to room temperature, and then the product was collected via centrifugation. The wet precipitate was washed with distilled water 3 times, rinsed with absolute ethanol, and was finally dried in air at 60 • C for a day. The oxy-hydroxyl nitrate microcrystals and the microcrystals eroded by VO 3 − are denoted as MC and MC-RV hereafter, with the R-fold VO 3 − in the reaction system.

Characterization Techniques
Phase identification was performed by X-ray diffraction (XRD, Model SmartLab, Rigaku, Tokyo, Japan), operating at 40 kV/40 mA using nickel-filtered Cu Kα radiation and a scanning speed of 6.0 • 2θ/min. The product morphology was analyzed by field emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, Model JEM-2000FX, JEOL, Tokyo, Japan). Elemental mapping was performed using scanning transmission electron microscopy (STEM, Model JEM-2000FX, JEOL, Tokyo, Japan). Fourier transform infrared spectroscopy experiments (FT-IR, Nicolet iS5, Thermal Fisher Scientific, NY, USA) were under taken using the standard KBr method. Photoluminescence (PL), photoluminescence excitation (PLE), and fluorescence decay were analyzed at room temperature using an FP-8600 fluorospectrophotometer (JASCO, Tokyo, Japan) equipped with a Φ60 mm integrating sphere and a 150-W Xe-lamp. The slit widths were 5 nm for both the excitation and emission sides. The spectral responses were corrected in the range 220-850 nm with a Rhodamine-B solution (5.5 g/L in ethylene glycol, for 220-600 nm range) and with a standard light source unit (ECS-333, JASCO, for 350-850 nm) as references. The external quantum efficiency (ε ex , the total number of emitted photons divided by the total number of excitation photons) and the internal quantum efficiency (ε in , the total number of emitted photons divided by the number of photons absorbed by the sample) of the phosphors are derived from the following equations [19], using the built-in analysis software: where E(λ)/hv, R(λ)/hv, and P(λ)/hv are the number of photons in the excitation, reflectance, and emission spectra of the samples, respectively. The reflection spectrum of the spectral on diffusive white standards was used for calibration.

Results and Discussion
In the present study, the RE 4 O(OH) 9 NO 3 (RE = Y, Eu) microcrystals (termed as MC) was chosen as an example for surface modification. As reported in the literature [3], Y 4 O(OH) 9 NO 3 is a monoclinic crystal structure, which is a three-dimensional framework with one-dimensional channels containing NO 3 − . The NO 3 − is indirectly linked to Y 3+ rather than forming a direct connection. There are 4 trivalent yttrium ions in the asymmetric unit, with 3 in a 7-coordinated environment with a capped trigonal prismatic geometry and 1 in a 9-coordinated environment with a tricapped trigonal prismatic geometry. The trivalent yttrium ions are linked through hydroxide anions forming the framework around the channels. In the channel, the nitrate ion is indirectly linked to Y 3+ through the hydrogen bonding. As a result of the one-dimensional channels, Y 4 O(OH) 9 NO 3 always crystallizes into prismatic and wire-like crystallites [3]. Here, the incorporation of Eu 3+ in Y 4 O(OH) 9 NO 3 does not significantly affect its crystal structure, because the diffractions of MC are indexed to the monoclinic Y 4 O(OH) 9 NO 3 (JCPDS File no. 79-1352), except for slight spectral shifts to the lower angle side (Figure 1c). The replacement of Y 3+ with larger Eu 3+ ions (for 8-fold coordination, r Y 3+ = 0.1019 nm, r Eu 3+ = 0.1066 nm) contributed to the lattice expansion, thus resulting in the diffraction shifts [20]. Figure 1d shows the FE-SEM image of MC, and pure hexagonal prisms with a diameter of~0.3-0.5 µm and a length of 1.5-2.5 µm are found in the observation.   Figure 3). However, the surface of MC-RV (microcrystals reacted with VO 3 − , with R-fold VO 3 − in the reaction system) is not smooth, with nano-sized crystals on the prism surface. Increasing the R value from 0.25 to 1.5 contributed to a rougher particle surface for MC-RV. Since the above phenomenon is similar to that for the surface corrosion of metal, in the present paper, it is called "surface eroding" for the reaction with VO 3 − . However, there are not any other impurity phases in the XRD patterns, indicating that the nano-sized crystals are amorphous.   with intense absorptions at 3401, 3487, and 3615 cm −1 , which arise from hydroxyl (OH − ) groups [22,23]. This is in compliance with the derived chemical formula of RE 4 O(OH) 9 NO 3 . The absorption peaks around 1364 and 1407 cm −1 are assignable to the v 3 vibration mode of NO 3 − and the v 4 asymmetric stretch of O-NO 2 , respectively [22,23]. It is clearly seen that the absorptions of NO 3 − in MC are different from those of the interlaye-free NO 3 − in layered rare earth hydroxide. After the reaction with VO 3 − , NO 3 − still exists in the FT-IR spectra and is not significantly affected by the reaction. However, the absorption for VO 3 − at 792 cm −1 appeared after the reaction with VO 3 − [22][23][24], and more intense absorption is found at a higher R value.  Figure 5 shows the elemental mapping distribution of MC, and the results indicate that MC is a homogeneous solid solution, because all the elements of Y, Eu, and N are distributed among the particles. In addition, the close observation of MC through the TEM image found that MC is well crystallized, with sharp edges and corners, and the surface is smooth. Selected area electron diffraction (SAED) found that the MC is a single crystalline. The calculated planar spacings of~0.352 nm and~0.268 nm correspond well with the (230) and (-141) planes of Y 4 O(OH) 9 NO 3 . After the reaction with VO 3 − , V is distributed among the particles, except for the original component elements of Y, Eu, and N (Figures 6 and 7). The TEM images of MC-RV confirmed that there are nano-sized crystals on the surface of the prisms, and they grow up and tend to dendritic growth. Evidently, the materials needed for growth are obtained from the dissolution of the MC surface, similar to the surface corrosion of metal. The diameter of the prisms became slimmer at a higher R value, further presenting direct evidence.  . 79-1352). Evidently, amorphous diffraction circular patterns were found in the SAED patterns, confirming the existence of amorphous particles on the surface of the prism. Collectively, the results from these analyses confirmed that the reaction with VO 3 − contributed to the formation of amorphous particles containing VO 3 − on the surface of the prism, which is similar to the surface corrosion of metal, called "surface eroding". Therefore, it can be said that the surface modification of MC was successfully achieved by eroding the surface of MC through vanadate ions.    Figure 8 shows the PLE and PL spectra for MC. By monitoring the 5 D 0 → 7 F 2 emission at 617 nm, a series of sharp lines in the PLE spectrum ranging from 300 nm to 500 nm can be ascribed to the transitions within the Eu 3+ 4f 6 electronic configuration. Different from other rare earth hydroxide precursors, MC exhibited O 2− -Eu 3+ charge transfer (CT) transitions at~255 nm, as were commonly found for Eu 3+ -activated Y 2 O 3 [7][8][9][10]. Upon excitation at 395 nm (intra-4f 6 transition of Eu 3+ ), the PL spectrum displayed the typical 5 D 0 → 7 F J (J = 0-4) transitions of Eu 3+ . The relative intensities of the transitions to different J levels depended on the symmetry of the Eu 3+ environment and can be described in terms of the Judd-Ofelt theory [7]. The Judd-Ofelt parity law predicts that the magnetic dipole 5 D 0 → 7 F 1 transition is permitted while the electric dipole 5 D 0 → 7 F 2 transition is forbidden, and the latter is allowed only on the condition that the Eu 3+ ions occupy the asymmetric site [7][8][9]. The MC, having the composition of (Y 0.95 Eu 0.05 ) 4 O(OH) 9 NO 3 , has a monoclinic structure and 2 kinds of Eu 3+ ions, which are 7-coordinated Eu 3+ ions in C 2v non-centrosymmetric sites and 9-coordinated Eu 3+ ions in D 3h centrosymmetric sites [3]. Since the molar ratio of C 2v occupancy to that of D 3h is 3, most Eu 3+ ions occupy the asymmetric site, and thus the 5 D 0 → 7 F 2 transition at 617 nm is stronger than the 5 D 0 → 7 F 1 transition at 595 nm [3]. However, the MC did not output a strong red light, mainly due to the fact that the hydroxyls directly coordinated to the rare earth ions Eu 3+ , which resulted in serious luminescence quenching [24].  Figure 9 shows the PLE and PL spectra for MC-RV, which was eroded by vanadate ions. The excitation spectrum consisted of a strong and broad absorption band ranging from 200 to 350 nm, which was assigned to the energy transfer from VO 3 − to Eu 3+ . The overlapped excitation was from 2 individual bands located at~275 nm and~323 nm, which corresponded to the transitions of 1 A 2 ( 1 T 1 )→ 1 E ( 1 T 2 ) and 1 A 2 ( 1 T 2 )→ 1 A 1 ( 1 E) of V 5+ , respectively [25,26]. The band at~255 nm was the contribution of the O 2− -Eu 3+ charge transfer [7][8][9], while the very weak transitions of 7 F 0,1 → 5 L 6 and 7 F 0,1 → 5 D 2 at 395 nm and 463 nm for Eu 3+ were observed in the excitation spectra [7][8][9]. Since the strongest excitation was located at~323 nm, the excitation wavelength was chosen as 323 nm ( 1 A 2 ( 1 T 2 )→ 1 A 1 ( 1 E) transition of V 5+ ). Upon UV excitation at 323 nm, the PL spectra displayed strong emissions at 540 nm, 590 nm, 617 nm, 650 nm, and 702 nm, which were assigned to 5 D 1 → 7 F 0,1 , 5 D 0 → 7 F 1 , 5 D 0 → 7 F 2 , 5 D 0 → 7 F 3 , and 5 D 0 → 7 F 4 transitions of Eu 3+ , respectively. The emission at 617 nm attained the dominate role. This further confirmed the existence of VO 3 − →Eu 3+ energy transfer. Interestingly, the emission intensity at 617 nm was greatly enhanced by increasing the R value from 0 to 0.25, indicating that the VO 3 − →Eu 3+ energy transfer contributed to the improved luminescence. The external/internal quantum efficiencies for R = 0 were 6 ± 1%/11 ± 1%, and the external/internal quantum efficiencies for R = 0.25 were 36 ± 1%/65 ± 1%, directly confirm the great enhancement of luminescence. However, a higher R value induced a rougher particle surface, which contributed to the light scattering and quenching of the luminescence. Therefore, increasing the R value further resulted in the luminescent decay. The external/internal quantum efficiencies for R = 0.5, R = 1.0, and R = 1.5 were 25 ± 1%/49 ± 2%, 18 ± 2%/43 ± 1%, and 10 ± 1%/37 ± 2%, respectively. However, the emission intensity for MC-RV is evidently stronger than that for MC on the whole, indicating that the erosion of the surface of rare earth microcrystals through vanadate ions can contribute to the considerable improvement to luminescence ( Figure 10). The decay kinetics of the 5 D 0 → 7 F 2 transition at 617 nm for MC and MC-RVre a investigated in Figure 11. All the fluorescence decay curves can be fitted to single exponentials. The average lifetimes of the MC, MC-0.25V, MC-0.50V, MC-1.0V, and MC-1.5V samples determined in this work are~0.67 ms,~1.38 ms,~1.21 ms,~1.14 ms, and~1.12 ms, respectively. Evidently, the lifetime for MC-RV is longer than that for MC, since the energy transfer of VO 3 − to Eu 3+ is more time consuming.

Conclusions
In the present study, prismatic microcrystals of RE 4 O(OH) 9 NO 3 (RE = Y, Eu) were synthesized by a hydrothermal reaction (180 • C for 24 h, pH = 10-11). The surface modification of the microcrystals was successfully achieved by eroding the surface of the microcrystals through vanadate ions. The main means of sample characterization were XRD, TEM, SAED, STEM, PLE/PL spectroscopy, and fluorescence decay curve analysis. The reaction with VO 3 − contributed to the formation of amorphous particles containing VO 3 − on the surface of the prism, which was similar to the surface corrosion of metal, called "surface eroding". Upon UV excitation at 323 nm ( 1 A 2 ( 1 T 2 )→ 1 A 1 ( 1 E) transition of V 5+ ), the modified microcrystals displayed a strong red emission at 617 nm ( 5 D 0 → 7 F 2 transition of Eu 3+ ). As a result of the VO 3 − →Eu 3+ energy transfer, the emission intensity at 617 nm for the modified microcrystals increased greatly after modification by vanadate ions. Additionally, due to the VO 3 − →Eu 3+ energy transfer, the modified microcrystals exhibited longer lifetimes. The usage of the energy transfer to the activated RE 3+ ions through surface eroding processing for rare earth compounds recommends a new paradigm for luminescence improvement. Data Availability Statement: Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.