Sulfate Exchange of the Nitrate-Type Layered Hydroxide Nanosheets of Ln₂(OH)₅NO₃·nH₂O for Better Dispersed and Multi-color Luminescent Ln₂O₃ Nanophosphors (Ln = Y_0.98RE_0.02, RE = Pr, Sm, Eu, Tb, Dy, Ho, Er, and Tm)

Abstract

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 = Y_0.98RE_0.02; 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 SO₄²⁻ produced a sulfate derivative [Ln₂(OH)₅(SO₄)_0.5·nH₂O] of the same layered structure and two-dimensional crystallite morphology but substantially contracted d₀₀₂ 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 SO₄²⁻, 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 (Y_0.98RE_0.02)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.

Background

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 [5], 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 [2]. Layered rare-earth hydroxide (LRH), as a relatively new type of anionic layered compounds [20], 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 ([001] 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 [32], 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 [001] 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) [43]. 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 SO₄²⁻ exchange for interlayer NO₃⁻ on crystal structure and thermal behavior of the nanosheets and also characteristics and luminescent properties of the derived (Y_0.98RE_0.02)₂O₃ nanophosphors were studied in detail.

Methods

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 = Y_0.98RE_0.02) kept at ~4 °C until pH ~8 to produce Ln₂(OH)₅NO₃·nH₂O nanosheets (30 mmol of Ln₂(OH)₅NO₃·nH₂O per batch) [43]. For anion exchange with SO₄²−100 mL aqueous solution containing 15 mmol of (NH₄)₂SO₄ (one SO₄²⁻ 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.

Characterization Techniques

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 SO₄²⁻ Exchange

The effects of SO₄²⁻ exchange on the crystal structure of LLnHs were analyzed with L(Y_0.98Eu_0.02)H for example. Figure 1a compares XRD patterns of the NO₃⁻-L(Y_0.98Eu_0.02)H and its SO₄²⁻ derivative. The NO₃⁻-type exhibits the characteristic 00l and non-00l diffractions of Ln₂(OH)₅NO₃·nH₂O layered compounds [20–34]. The strong and sharp 220 diffraction, arising from the ab plane (the hydroxide main layer), suggests that the host layers of L(Y_0.98Eu_0.02)H are well crystallized. Compared with the thick LRH crystals synthesized under high temperature [21–31, 33, 34], the 002 diffraction has a substantially lower intensity relative to the 220 one, implying that the primary crystallites are much less developed along the c-axis or rather thin, as also confirmed later by TEM analysis. SO₄²⁻ exchange of NO₃⁻ shortens the interlayer distance, as perceived from the obvious shifting of the 00l diffraction to a higher angle, and the basal spacing (d₀₀₂) calculated from the center of the 002 peak is ~0.886 nm for NO₃⁻-L(Y_0.98Eu_0.02)H and 0.841 nm for the SO₄²⁻ derivative. The values are close to those found for NO₃⁻-LYH and its exchange product, respectively [43]. The sulfate derivative still exhibits quite strong 220 diffraction, indicating that sulfate exchange did not appreciably damage the hydroxide main layers. Shifting of the 220 peak from 2θ ~ 28.86° to 29.02° and decreased intensity of the 400 diffraction by the anion exchange, however, suggests that the intercalated SO₄²⁻ is interacting with the hydroxide layers to deteriorate crystallinity of the sample. The interaction, mostly through hydrogen bonding with the hydroxyls/H₂O in the [Ln(OH)₇H₂O] and [Ln(OH)₈H₂O] polyhedrons that comprise the hydroxide layers [21–24], leads to lattice distortion and thus the slight peak-shifting [43]. The hydrogen bonding is also responsible for the observed interlayer contraction, since it would draw closer the adjacent positively charged hydroxide layers [43].

Fig. 1.

Powder XRD patterns and FT-IR spectra of the products and the 220 spacing (d ₂₂₀) of the SO₄ ²⁻-L(Y_0.98RE_0.02)H. a Powder XRD patterns and b FT-IR spectra of the NO₃ ⁻-L(Y_0.98Eu_0.02)H and SO₄ ²⁻-L(Y_0.98Eu_0.02)H samples. Part c displays XRD patterns of the rest SO₄ ²⁻-L(Y_0.98RE_0.02)Hs synthesized in this work, and part d is the 220 spacing (d ₂₂₀) of the SO₄ ²⁻-L(Y_0.98RE_0.02)H, as a function of the ionic radius of the RE³⁺ dopant (taken for CN = 8)

Chemical analysis of the NO₃⁻-L(Y_0.98Eu_0.02)H found ~1.65 wt.% of Eu, 49.88 wt.% of Y, and 16.68 wt.% of NO₃⁻, corresponding to Y/Eu and NO₃⁻/Ln (Ln = Y and Eu) molar ratios of 0.98/0.019 and 0.94/2 (close to 1/2), respectively. The results thus confirm that the prescribed Y/Eu atomic ratio (0.98/0.02) has essentially been kept to the product and NO₃⁻-L(Y_0.98Eu_0.02)H has been formed. The SO₄²⁻ exchange product was found to have ~1.87 wt.% of Eu, 51.17 wt.% of Y, 4.50 wt.% of S, and trace NO₃⁻(0.18 wt.%), which lead to Y/Eu and SO₄²⁻/Ln molar ratios of 0.98/0.021 and 0.48/2 (close to 0.5/2), respectively. The outcomes thus indicate that the anion exchange did not appreciably alter the Y/Eu ratio, and SO₄²⁻ exchange of the interlayer NO₃⁻ is virtually complete. The results of the chemical analysis conform to those of FTIR spectroscopy (Fig. 1b). It is clearly seen that the intense NO₃⁻ absorption at ~1385 cm⁻¹ (ν₃ vibration, as of free anion) vanished, and meanwhile the ν₃ (~1105 cm⁻¹) and ν₁ (~982 cm⁻¹) absorptions, being characteristic of SO₄²⁻, appeared from the exchange product. The non-splitting feature of ν₃ suggests that SO₄²⁻ is not directly coordinated to the metal center in the hydroxide layer while the emergence of ν₁ implies that the SO₄²⁻ tetrahedron is distorted owing to the effects of hydrogen bonding [43–45]. It is also owing to the effects of hydrogen bonding that the stretching vibrations of hydroxyls (~3565 cm⁻¹) and the O–H radicals in hydration water (~3370 cm⁻¹) are both substantially enhanced [44, 45]. The twin absorption bands at ~1520 and 1375 cm⁻¹ indicate contamination of the product by CO₃²⁻, mostly from dissolved atmospheric CO₂ during synthesis. All the SO₄²⁻-LLnHs made in this work show almost identical interlayer distances owing to the limited content of RE, but the 220 diffraction successively shifts towards a higher angle along with the decreasing ionic size of the RE³⁺ dopant (Fig. 2c). The 220 spacing (d₂₂₀) is shown in Fig. 2d as a function of RE³⁺ size (for eightfold coordination) [46]. As the 220 diffraction reflects metal-to-metal distance in the hydroxide layer [21–24], the d₂₂₀ value thus monotonically decreases towards a smaller RE³⁺ as expected. The results also provide direct evidence of solid-solution formation.

Fig. 2.

SEM and TEM morphology of the products. SEM micrographs showing overall morphologies of the NO₃ ⁻-L(Y_0.98Eu_0.02)H (a) and SO₄ ²⁻-L(Y_0.98Eu_0.02)H (b) samples. The insets are the TEM morphology and selected area electron diffraction (SAED) pattern of the nanosheets

Figure 2 shows the results of electron microscopy for the two types of L(Y_0.98Eu_0.02)Hs. FE-SEM observation found that the NO₃⁻-L(Y_0.98Eu_0.02)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 d₀₀₂ 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 SO₄²⁻ 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 [43].

Decomposition and Phase/Morphology Evolution of the Nanosheets upon Heating

Thermal behaviors of the nanosheets were analyzed via TG, and the results are compared in Fig. 3 for the NO₃⁻-L(Y_0.98Eu_0.02)H and SO₄²⁻-L(Y_0.98Eu_0.02)H. The NO₃⁻ type clearly decomposes via three well-defined stages as previously observed for thick LRH crystals [25–29, 32–34], with the first one being dehydration to form Ln₂(OH)₅NO₃ (up to ~175 °C), the second one being dehydroxylation to yield an intermediate mass with nominal composition of (Y_0.98Eu_0.02)₂O₂(OH)NO₃ (up to ~325 °C), and the last one being further dehydroxylation and denitration to form oxide (up to ~545 °C). Though the NO₃⁻-LLnH obtained in this work are nanometer thin, it shows a thermal behavior almost identical to that reported for thick crystals [25–29, 32–34]. The sulfate derivative similarly decomposes via three steps, but only up to the significantly higher temperature of ~1135 °C and with a rather sluggish step in the ~175–1005 °C range (stage II).

Fig. 3.

TG curves of NO₃ ⁻-and SO₄ ²⁻-type L(Y_0.98Eu_0.02)Hs. TG traces for the NO₃ ⁻- and SO₄ ²⁻ type L(Y_0.98Eu_0.02)Hs

To better understand the thermal decomposition of SO₄²⁻-L(Y_0.98Eu_0.02)H, which has not been addressed prior to us, FTIR analysis was performed on the products obtained at various selected temperatures for 4 h (Fig. 4). It is clearly seen that the powders calcined up to 900 °C are characterized by strong SO₄²⁻ absorptions and successively weaker ones of hydroxyls/water. The results may thus imply that the sluggish weight loss observed on the TG curve from ~175 to 1005 °C is mainly owing to successive dehydroxylation rather than desulfuration. The weight loss (~14.4 %) calculated for dehydroxylation of (Y_0.98Eu_0.02)₂(OH)₅(SO₄)_0.5 to the nominal composition of (Y_0.98Eu_0.02)₂O_2.5(SO₄)_0.5 is indeed close to that found via TG (~16.1 %). It is thus plausible to conclude that the slow dehydroxylation of SO₄²⁻-L(Y_0.98Eu_0.02)H is mainly due to the intramolecular hydrogen bonding between SO₄²⁻ and OH⁻ groups. Raising the calcination temperature from 900 to 1000 °C simultaneously eliminates the strong SO₄²⁻ and the already rather weak hydroxyl absorptions, suggesting that the sudden weight loss observed on the TG curve from ~1005 to 1135 °C is dominantly resulted from desulfuration. Again, the weight loss calculated for this thermal event (~14.9 %) is close to the value revealed by TG (~13.5 %). The SO₄²⁻ anions exhibit significantly split ν₃ and ν₄ vibrations for the 800 and 900 °C products, implying that the tetrahedrons of SO₄²⁻ are substantially distorted by direct coordination to Ln³⁺ [44, 45, 47–49].

Fig. 4.

FTIR spectra for the original SO₄ ²⁻-L(Y_0.98Eu_0.02)H and the calcined products. FTIR spectra for the original SO₄ ²⁻-L(Y_0.98Eu_0.02)H and the products calcined from it at various selected temperatures for 4 h

Phase evolution of the SO₄²⁻-L(Y_0.98Eu_0.02)H upon heating was studied via XRD analysis of the products calcined at different temperatures, and the results are displayed in Fig. 5. It is seen that dehydration of the layered compound at 200 °C leads to an amorphous mass, which persists up to ~600 °C despite the already occurrence of dehydroxylation (Figs. 3 and 4). Further removal of hydroxyls at 800 °C produced a phase mixture of poorly crystallized cubic Ln₂O₃ (Ln = Y_0.98Eu_0.02, JCPDS: 00-043-1036) and monoclinic Ln₂O₂SO₄ (JCPDS: 00-053-0168), whose diffraction intensities both remarkably improve for the 900 °C product. As the original layered compound has the approximate composition of Ln₂(OH)₅(SO₄)_0.5·nH₂O, it can thus be said that the 900 °C product is approximately composed of 1/2 mol of Ln₂O₂SO₄ and 1/2 mol of Ln₂O₃. Since the SO₄²⁻ in Ln₂O₂SO₄ is bidentately coordinated to Ln³⁺ [47–49], the significant splitting of the ν₃ and ν₄ IR bands was thus observed once the oxy-sulfate compound was formed (Fig. 4). The Ln₂O₂SO₄ component desulfurates at the higher temperature of 1000 °C, and thus, only cubic structured Ln₂O₃ was found. The results of XRD comply well with those of FTIR (Fig. 4). That is, the ν₃ and ν₄ vibrations of SO₄²⁻ present as single bands for the 200–600 °C products, as split bands for the 800 and 900 °C products, and vanish for the 1000 °C product. Similar phase evolution analysis of the NO₃⁻-L(Y_0.98Eu_0.02)H found that cubic (Y_0.98Eu_0.02)₂O₃ crystallizes at ~600 °C via an amorphous state at lower temperatures. The results are not shown here since they are essentially identical to those previously reported by us for thick NO₃⁻-LRH crystals [33].

Fig. 5.

XRD patterns of the products calcined from SO₄ ²⁻-L(Y_0.98Eu_0.02)H at the different temperatures. XRD patterns of the products calcined from SO₄ ²⁻-L(Y_0.98Eu_0.02)H at the different temperatures indicated in the figure, where letters C and M denote cubic (Y_0.98Eu_0.02)₂O₃ and monoclinic (Y_0.98Eu_0.02)₂O₂SO₄ phases, respectively

Figure 6 compares XRD patterns of the oxides calcined from SO₄²⁻-L(Y_0.98RE_0.02)H at 1100 °C for 4 h. Only enlarged view of the 2θ = 25–35° region was given to show the effects of RE³⁺ dopant. It is clearly seen that both the (222) and (400) diffractions steadily shift to higher angles along with decreasing ionic radius of RE³⁺, indicating the formation of solid solution. The lattice parameter (a, in angstrom) calculated from the strongest (222) diffraction indeed becomes successively smaller towards a smaller RE³⁺ (Fig. 6). The crystallite size assayed from the (222) diffraction by applying Scherrer equation is ~35 nm for all the oxides, irrespective of the dopant type. The cell parameters determined herein are all larger than the 10.547 Å reported for pure Y₂O₃ (JCPDS: 00-043-1036, Y³⁺ close to Ho³⁺ in radius), possibly owing to the limited crystallite size of the present powders.

Fig. 6.

XRD patterns of the (Y_0.98RE_0.02)₂O₃ powders. XRD patterns of the (Y_0.98RE_0.02)₂O₃ powders calcined from SO₄ ²⁻-L(Y_0.98RE_0.02)H at 1100 °C for 4 h, with the cell parameter a (in angstrom) included

Morphology evolution of the nanosheets during calcination was studied with NO₃⁻-L(Y_0.98Eu_0.02)H and SO₄²⁻-L(Y_0.98Eu_0.02)H for example. Figure 7 exhibits typical FE-SEM morphologies for the powders calcined at some representative temperatures. It is seen that the 800 °C product from NO₃⁻-L(Y_0.98Eu_0.02)H well retained the overall morphology of its precursor, despite that it has been a phase-pure oxide, and the flower-like assemblies (domains) and the individual nanosheets within the domains are clearly observable. Calcination at 900 °C led to substantial collapse of the nanosheets into nanoparticles within each domain, owing to the thermal stress arising from crystallite growth, and the domain boundary is still identifiable. Significant crystallite growth was observed at 1100 °C, together with densification of some of the domains via inter-particle sintering to form dense aggregates. The final powder was found to have a specific surface area of ~8.3 m²/g, corresponding to an average particle size of ~140 nm. The value is significantly larger than that observed from the FE-SEM micrograph (up to ~90 nm) for the primary particles, due to the presence of hard aggregates. The 800 and 900 °C powders from SO₄²⁻-L(Y_0.98Eu_0.02)H show morphologies similar to their counterparts described above, except that the domains in the 900 °C product are much less disintegrated since the Ln₂O₂SO₄ phase (Fig. 5), finely mixed with the Ln₂O₃ portion, significantly restricts crystallite growth. Calcination to 1100 °C causes complete collapse of the domains to yield a substantially better dispersed oxide powder, and the evolution of SO_x gas was believed to promote disintegration of the domains. Accordingly, the final oxide has a much higher specific surface area of ~17.5 m²/g (average particle size ~67 nm). The amount of residual sulfur in the oxide phosphors calcined at 1100 °C was assayed via ICP elemental analysis to be up to 0.18 wt.% in our previous work [50].

Fig. 7.

FE-SEM micrographs of the products calcined from NO₃ ⁻- and SO₄ ²⁻-L(Y_0.98Eu_0.02)Hs at the different temperatures. FE-SEM micrographs showing morphologies of the products calcined from NO₃ ⁻- and SO₄ ²⁻-L(Y_0.98Eu_0.02)Hs at the temperatures for (a) 800 °C, (b) 900 °C and (c) 1100 °C

Figure 8 shows the particle size/size distribution of the two kinds of (Y_0.98Eu_0.02)₂O₃ powders calcined at 1100 °C. It is clearly seen that the powder from NO₃⁻-L(Y_0.98Eu_0.02)H exhibits a bimodal size distribution owing to the presence of hard aggregates (Fig. 7). The finer portion (~56.5 vol.%) has an average particle (cluster) size of ~320 ± 43 nm while the coarser part has a value of ~6.21 ± 0.53 μm. A unimodal size distribution was observed for the (Y_0.98Eu_0.02)₂O₃ powder from SO₄²⁻-L(Y_0.98Eu_0.02)H, and the average particle size was analyzed to be ~219 ± 94 nm. The above results are in agreement with morphology observations (Fig. 7), and further confirm that SO₄²⁻ exchange of the interlayer NO₃⁻ is beneficial to the derivation of finer and better dispersed oxide powders.

Fig. 8.

Particle size/size distribution analysis of the (Y_0.98Eu_0.02)₂O₃ powders. Particle size/size distribution analysis of the (Y_0.98Eu_0.02)₂O₃ powders calcined at 1100 °C for 4 h from a the NO₃ ⁻-L(Y_0.98Eu_0.02)H and b the SO₄ ²⁻-L(Y_0.98Eu_0.02)H. The size distribution is given in cumulative volume (right axis)

Photoluminescent Properties of the (Y_0.98RE_0.02)₂O₃ Nanophosphors

Figure 9 shows photoluminescence excitation/emission spectra for the (Y_0.98RE_0.02)₂O₃ nanophosphors calcined at 1100 °C, with the excitation and emission wavelengths used for the measurements indicated in each part of the figure. The origins of these main bands [19, 51–53] are summarized in Table 1, together with the chromaticity coordinates of emission and the fluorescence lifetime. The origins of the other PLE/PL bands in each part of Fig. 9 are well documented and can be found in the literature [19, 51–53]. It is seen from the Commission Internationale de l’Eclairage (CIE) chromaticity diagram that the phosphors synthesized in this work span a wide range of emission colors, from blue (Tm³⁺) to deep red (Pr³⁺) via green (Tb³⁺, Ho³⁺, and Er³⁺), yellow (Dy³⁺), orange (Sm³⁺), and orange red (Eu³⁺).

Fig. 9.

PLE and PL spectra for the (Y_0.98RE_0.02)₂O₃ nanophosphors. PLE (left) and PL (right) spectra for the (Y_0.98RE_0.02)₂O₃ nanophosphors calcined from SO₄ ²⁻-LLnH at 1100 °C for 4 h, together with the CIE chromaticity diagram for the emissions

Table 1.

Optical properties of the (Y_0.98RE_0.02)₂O₃ nanophosphors

RE	Main PLE band (nm)	Main PL band (nm)	CIE coordinates (x,y)	Emission color	Lifetime
Pr	280, 4f2 → 4f15d1	645,1D2 → 3H4	(0.639,0.303)	Deep red	160 ± 13 ns
Sm	407, 6H5/2 → 4K11/2	609, 4G5/2 → 6H7/2	(0.538,0.414)	Orange	1.52 ± 0.01 ms
Eu	250, CTB (O2- → Eu3+)	613, 5D0 → 7F2	(0.640,0.329)	Orange red	2.71 ± 0.02 ms
Tb	275, 4f8 → 4f75d1	545, 5D4 → 7F5	(0.321,0.551)	Green	3.08 ± 0.02 ms
Dy	350, 6H15/2 → 6P7/2	573, 4F9/2 → 6H13/2	(0.457,0.492)	Yellow	229 ± 12 ns
Ho	449, 5I8 → 5F1	551, 5S2 → 5I8	(0.308,0.657)	Green	126 ± 9 ns
Er	380, I15/2 → 4G11/2	564, 4S3/2 → 4I15/2	(0.281,0.675)	Green	246 ± 15 ns
Tm	360, 3H6 → 1D2	453, 1D2 → 3F4	(0.194,0.206)	Blue	170 ± 13 ns

A summary of optical properties of the (Y_0.98RE_0.02)₂O₃ nanophosphors

Conclusions

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 = Y_0.98RE_0.02, and RE = Pr, Sm, Eu, Tb, Dy, Ho, Er, and Tm). Replacement of the interlayer NO₃⁻ with SO₄²⁻ via in situ anion exchange was achieved to produce the sulfate derivative of SO₄²⁻-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 SO₄²⁻ 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 (Y_0.98RE_0.02)₂O₃ powders from SO₄²⁻-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.