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Sulfate Exchange of the Nitrate-Type Layered Hydroxide Nanosheets of Ln2(OH)5NO3·nH2O for Better Dispersed and Multi-color Luminescent Ln2O3 Nanophosphors (Ln = Y0.98RE0.02, RE = Pr, Sm, Eu, Tb, Dy, Ho, Er, and Tm)
© The Author(s). 2016
Received: 20 May 2016
Accepted: 5 July 2016
Published: 12 July 2016
Through restricting thickness growth by performing coprecipitation at the freezing temperature of ~4 °C, solid-solution nanosheets (up to 5-nm thick) of the Ln2(OH)5NO3·nH2O layered hydroxide (Ln = Y0.98RE0.02; RE = Pr, Sm, Eu, Tb, Dy, Ho, Er, and Tm, respectively) were directly synthesized without performing conventional exfoliation. In situ exchange of the interlayer NO3 − with SO4 2− produced a sulfate derivative [Ln2(OH)5(SO4)0.5·nH2O] of the same layered structure and two-dimensional crystallite morphology but substantially contracted d 002 basal spacing (from ~0.886 to 0.841 nm). The sulfate derivative was systematically compared against its nitrate parent in terms of crystal structure and phase/morphology evolution upon heating. It is shown that the interlayer SO4 2−, owing to its bonding with the hydroxide main layer, significantly raises the decomposition temperature from ~600 to 1000 °C to yield remarkably better dispersed oxide nanopowders via a monoclinic Ln2O2SO4 intermediate. The resultant (Y0.98RE0.02)2O3 nanophosphors were studied for their photoluminescence to show that the emission color, depending on RE3+, spans a wide range in the Commission Internationale de l’Eclairage (CIE) chromaticity diagram, from blue to deep red via green, yellow, orange, and orange red.
Y2O3 is a widely used host lattice in the phosphor field, owing to its excellent structure stability, chemical durability, and particularly its ability to accept a substantial amount of various trivalent rare-earth activators for a broad range of optical functionalities. Due to their technical importance in the lighting and display areas, Y2O3-based phosphors are being widely investigated to correlate their luminescent performance with the characteristics of the phosphor powder [1, 2]. Controllable synthesis has always been an active area of phosphor study, and the well-adopted processing technologies may include flux-assisted solid state reaction [3, 4], solution synthesis, combustion , spray pyrolysis [6–8], and gas-phase condensation [9, 10].
Among the aforementioned synthetic strategies, solution processing is of particular interest since it allows a facile manipulation of particle morphology. With this technique, Y2O3-based phosphors that have the various morphologies of zero-dimensional (0D) nanoparticles, monodispersed microspheres [11, 12], 1D nanowires/nanotubes [13–15], 2D nanoplates [16–18], and hierarchical structures [2, 19] have been obtained. As the direct product of solution synthesis is usually a precursor, the final phosphor oxide is thus frequently observed to have properties dependent on the characteristics of its precursor . Layered rare-earth hydroxide (LRH), as a relatively new type of anionic layered compounds , has attracted much attention during the recent years owing to its unique combination of the layered structure and the abundant optical, magnetic, and catalytic properties of the rare-earth elements [21–34]. The crystal structure of Ln2(OH)5A·nH2O LRH (Ln = rare-earth; A = NO3 − or halogen anion; n ~ 1.5) can be viewed as an alternative stacking along the c-axis ( direction) of the positively charged hydroxide main layers containing Ln3+ and exchangeable A anions located in the interlayer for charge balance. The well-established synthetic methodologies of hydrothermal reaction [25–34] and reflux growth [21–24] generally produce platelike LRH crystals of several microns in lateral dimension and tens to hundreds of nanometers in thickness, for which single layer or few-layer thick nanosheets can only be obtained by swelling the pristine crystals via exchange of the interlayer anions with significantly larger ones (such as dodecyl sulfate, DS−), followed by exfoliation in a proper medium (such as formamide) under mechanical agitation [35–39]. Exfoliation, however, is well known to be time consuming, frequently incomplete, and usually accompanied by fragmentation of nanosheets. We previously reported a capped growth technique to synthesize nanometer-thin LRH flakes via one-step hydrothermal reaction , but the batch yield is rather limited. Both the pristine LRH crystals and the exfoliated nanosheets can serve as new precursors for oxide phosphors and phosphor films [35–39], but thick crystallites would not collapse into nanoparticles via calcination and the resultant oxides frequently retain platelike morphologies [40–42].
The hydroxide main layer of LRH is a close-packed low-energy plane, and thus, its two-dimensional growth needs lower activation energy than the thickness growth along the  direction. We recently demonstrated that, through suppressing thickness growth by lowering the synthesis temperature to ~4 °C, NO3 −-LRH nanosheets of only ~4-nm thick can be directly crystallized, without exfoliation, for a wide spectrum of single Ln (Ln = Pr-Er, and Y) . With this technique, similarly thin nanosheets were produced in this work for the LRH solid solutions of Y/RE (RE = Pr, Sm, Eu, Tb, Dy, Ho, Er, and Tm) in good batch quantity (0.03 mol of LRH or ~10 g). The effects of SO4 2− exchange for interlayer NO3 − on crystal structure and thermal behavior of the nanosheets and also characteristics and luminescent properties of the derived (Y0.98RE0.02)2O3 nanophosphors were studied in detail.
Freezing Temperature Crystallization of LRH Solid-Solution Nanosheets
The starting rare-earth sources are Pr6O11 (99.96 % pure), Tb4O7 (99.99 % pure), and RE2O3 (99.99 % pure, RE = Y, Sm, Eu, Dy, Ho, Er, and Tm), all were purchased from Huizhou Ruier Rare-Chem. Hi-Tech. Co. Ltd (Huizhou, China). The other reagents of ammonium hydroxide solution (25 %), nitric acid (63 wt.%), and ammonium sulfate are of analytical grade and were purchased from Shenyang Chemical Reagent Factory (Shenyang, China). Nitrate solution of the rare earth was prepared by dissolving the oxide with a proper amount of nitric acid, followed by evaporation to dryness at 95 °C to remove superfluous HNO3 and a final dilution to 1.0 mol/L.
In a typical synthesis, a diluted ammonium hydroxide solution (1.0 mol/L) was slowly dripped (~2.0 mL/min) into 300 mL of a 0.2 mol/L nitrate solution of Ln3+ (Ln = Y0.98RE0.02) kept at ~4 °C until pH ~8 to produce Ln2(OH)5NO3·nH2O nanosheets (30 mmol of Ln2(OH)5NO3·nH2O per batch) . For anion exchange with SO4 2−100 mL aqueous solution containing 15 mmol of (NH4)2SO4 (one SO4 2− would replace two NO3 −) was added into the nanosheets suspension after 20 min of magnetic stirring (in situ anion exchange). The final product was collected via centrifugation after reaction for 1 h, followed by sequential washing with distilled water three times and ethanol one time and then air drying at 50 °C for 15 h. Calcination of the dried nanosheets was performed in flowing oxygen gas (200 mL/min), using a heating rate of 5 °C/min at the ramp stage and a holding time of 4 h. The final phosphor powders are all calcined at 1100 °C, but with the Pr- and Tb-containing samples being subjected to an additional reduction in flowing H2 (200 mL/min) at 1100 °C for 1 h.
Chemical analysis of the products was performed for Ln via inductively coupled plasma (ICP) spectroscopy (Model IRIS Advantage, Jarrell-Ash Japan, Kyoto), for NO3 − via spectrophotometry (Ubest-35, Japan Spectroscopic Co., Ltd., Tokyo), and for S via combustion-infrared absorptiometry (Model CS-444LS, LECO, St. Joseph, MI). The detection limits of these analyses are all 0.01 wt.%. Phase identification was made via X-ray diffractometry (XRD; Model PW3040/60, Panalytical B.V., Almelo, the Netherlands) operated at 40 kV/40 mA, using nickel-filtered Cu-Kα radiation (λ = 0.15406 nm) and a scanning speed of 1.0° 2θ/min. Fourier transform infrared spectroscopy (FTIR; Model Spectrum RXI, Perkin-Elmer, Shelton, CT) was performed by the standard KBr method. Powder morphology was analyzed by transmission electron microscopy under an acceleration voltage of 200 kV (TEM; Model JEM-2000FX, JEOL, Tokyo) and field emission scanning electron microscopy (FE-SEM; Model S-5000, Hitachi, Tokyo) under 10 kV. Thermogravimetry (TG; Model 8120, Rigaku, Tokyo) of the nanosheets was conducted in flowing air (100 mL/min) with a constant heating rate of 10 °C/min. Specific surface area of the oxide phosphor was obtained with an automatic analyzer (Model TriStar II 3020, Micromeritics Instrument Corp., Norcross, GA) using the Brunauer-Emmett-Teller (BET) method via nitrogen adsorption at 77 K. Particle size/size distribution analysis was made with a laser-diffraction particle sizer (Model LA-920, Horiba Scientific, Kyoto), after ultrasonically dispersing the oxide powder in ethanol. Photoluminescence was analyzed at room temperature using an FP-6500 fluorospectrophotometer (JASCO, Tokyo) equipped with a 60-mm-diameter integrating sphere (Model ISF-513, JASCO) and a 150-W Xe-lamp for excitation. Measurements were conducted under identical conditions for all the samples, using a scan speed of 100 nm/min and slit widths of 5 nm for both excitation and emission. Fluorescence lifetime of the luminescence was analyzed with the FP-6500 equipment for Sm3+, Eu3+, and Tb3+, and with a DeltaFlex lifetime fluorescence spectrometer (Horiba Scientific) for the fast decay of Pr3+, Dy3+, Ho3+, Er3+, and Tm3+.
Results and Discussion
Characteristics of the NO3 −-LLnH Nanosheets and the Effects of SO4 2− Exchange
Figure 2 shows the results of electron microscopy for the two types of L(Y0.98Eu0.02)Hs. FE-SEM observation found that the NO3 −-L(Y0.98Eu0.02)H is composed of 3D flower-like assemblies of nanoflakes having lateral dimensions up to ~300 nm (Fig. 2a, coated with 10-nm-thick tungsten for electrical conduction), while TEM analysis found entangled nanosheets of up to ~5-nm thick (the inset). Calculated from the d 002 basal spacing of ~0.886 nm, each single nanosheet would have only ~5–6 stacking repetitions along the c-axis. Selected area electron diffraction (SAED) yielded a well-arranged spot-like pattern (the inset) for the hydroxide layer, indicating that the individual nanosheets are primary of single crystalline and are well crystallized. Anion exchange with SO4 2− did not incur any appreciable morphology change to either the overall flower-like assemblies or the individual nanosheets (Fig. 2b), in compliance with our previous observations on NO3 −-LYH .
Decomposition and Phase/Morphology Evolution of the Nanosheets upon Heating
Photoluminescent Properties of the (Y0.98RE0.02)2O3 Nanophosphors
Optical properties of the (Y0.98RE0.02)2O3 nanophosphors
Main PLE band (nm)
Main PL band (nm)
CIE coordinates (x,y)
280, 4f2 → 4f15d1
645,1D2 → 3H4
160 ± 13 ns
407, 6H5/2 → 4K11/2
609, 4G5/2 → 6H7/2
1.52 ± 0.01 ms
250, CTB (O2- → Eu3+)
613, 5D0 → 7F2
2.71 ± 0.02 ms
275, 4f8 → 4f75d1
545, 5D4 → 7F5
3.08 ± 0.02 ms
350, 6H15/2 → 6P7/2
573, 4F9/2 → 6H13/2
229 ± 12 ns
449, 5I8 → 5F1
551, 5S2 → 5I8
126 ± 9 ns
380, I15/2 → 4G11/2
564, 4S3/2 → 4I15/2
246 ± 15 ns
360, 3H6 → 1D2
453, 1D2 → 3F4
170 ± 13 ns
It is shown in this work that coprecipitation at the freezing temperature of ~4 °C can directly produce, without exfoliation, solid-solution nanosheets of the nitrate-type layered hydroxides of Ln2(OH)5NO3·nH2O (NO3-LLnH, Ln = Y0.98RE0.02, and RE = Pr, Sm, Eu, Tb, Dy, Ho, Er, and Tm). Replacement of the interlayer NO3 − with SO4 2− via in situ anion exchange was achieved to produce the sulfate derivative of SO4 2−-LLnH. Detailed characterizations of both the types of layered materials and their calcination products via the combined techniques of XRD, FTIR, DTA/TG, FE-SEM/TEM, BET, particle sizing, and photoluminescence spectroscopy have led to the following main conclusions: (1) anion exchange did not bring about any appreciable change to the layered structure and the two-dimensional crystallite morphology, but induces a basal-spacing contraction from ~0.886 to 0.841 nm, (2) the interlayer SO4 2− significantly raises the decomposition temperature of the nanosheets from ~600 to 1000 °C to yield oxide via a monoclinic-structured Ln2O2SO4 intermediate phase, and (3) the (Y0.98RE0.02)2O3 powders from SO4 2−-LLnH are much better dispersed and finer than those from NO3-LLnH, and exhibit emission colors, depending on RE3+, covering a wide range in the CIE chromaticity diagram, from blue to deep red via green, yellow, orange, and orange red.
The work is supported in part by the National Natural Science Foundation of China (Grants No. 51172038, 51302032, and 51402059), the Fundamental Research Funds for the Central Universities (Grants N140204002 and N130810003) and Grants-in-Aid for Scientific Research (KAKENHI, No.26420686).
JGL conceived the project and drafted the manuscript. XLW and WGL carried out the experiments. QZ, XDL and XDS were involved in sample characterization and results discussion. All the authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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