III. RESULTS AND DISCUSSION

Figure 1 shows XRD patterns of the products after the hydrothermal reaction, where the diffraction peaks can all be indexed to the sulfate-type (LRH) of Ln₂(OH)₄SO₄·2H₂O in each case. ^{Reference Geng, Ma, Matsushita, Liang, Michiue and Sasaki16} The compound crystallizes in a face-centered monoclinic system (A2/m, no. 12) with α = γ = 90° and the β angle approximately equaling to 90°. The crystal structure can be viewed as an alternative stacking of the corrugated two-dimensional host layers formed by LnO₉ polyhedra and SO₄ ²⁻ layers along the [001] direction (c-axis). ^{Reference Geng, Ma, Matsushita, Liang, Michiue and Sasaki16} The appearances of the (002) and (004) diffractions in the low-angle region are thus indicative of a layered structure while the sharp and strong non-(00l) diffractions [e.g., (111)] confirm that the Ln³⁺-containing host layers are well organized and crystalline. With increasing Eu³⁺ doping both the (00l) and the non-(00l) diffractions shift to larger angles. The calculated lattice spacing of d ₁₁₁ [Fig. S1(a)] accordingly successively decreases due to the incorporation of smaller Eu³⁺ ions into the host layers to form solid solution (for 9-fold coordination, La³⁺ and Eu³⁺ have ionic radii of 0.1216 and 0.1120 nm, respectively). ^{Reference Shannon17} Since the (002) diffraction is closely related to the gallery height (1/2c sin β) of the layered structure, the decreasing d ₀₀₂ value [Fig. S1(b)] along with increasing Eu³⁺ doping thus suggests gradually contracted interlayer distance. This is in accordance with previous studies, ^{Reference Geng, Ma, Matsushita, Liang, Michiue and Sasaki16} which showed that the gallery height tends to shrink with decreasing ionic radius of Ln³⁺, following lanthanide contraction, and is reasonable from the fact that the interlayer SO₄ ²⁻ ions are directly coordinated to the Ln³⁺ centers in the main layers to form Ln–O–S bonds. ^{Reference Geng, Ma, Matsushita, Liang, Michiue and Sasaki16}

FIG. 1.

XRD patterns of the (La_1−x Eu _x )₂(OH)₄SO₄·2H₂O LRHs synthesized at 100 °C and pH of 9.

Typical morphologies of the obtained LRHs are shown in Fig. 2. It is found that the compounds crystallize as nanoplates having lateral sizes of up to 500 nm and thicknesses of 20–30 nm. In addition, Eu³⁺ doping does not obviously change the overall morphology of the LRHs.

FIG. 2.

Typical FE-SEM images of the (La_1−x Eu _x )₂(OH)₄SO₄·2H₂O (x = 0–0.09) powders synthesized at 100 °C and pH of 9, with (a) x = 0, (b) x = 0.09.

(La_0.95Eu_0.05)₂(OH)₄SO₄·2H₂O was chosen as an example to investigate the proper temperature range in which the phase pure (La_0.95Eu_0.05)₂O₂SO₄ can be obtained (Fig. 3). The product calcined at 200 °C exhibits a similar XRD pattern with that of the LRH precursor, suggesting no essential change in chemical composition or crystal structure. While, when treated at 300 °C, the product began to show some diffraction peaks different from the LRH precursor and oxysulfate can be assigned from the results of FTIR as the dehydration product of (La_0.95Eu_0.05)₂(OH)₄SO₄, and crystal structure analysis of which is underway. The targeted oxysulfate, (La_0.95Eu_0.05)₂O₂SO₄, has been to be obtained at the temperature as low as 400 °C, via dehydroxylation of the anhydrous intermediate. With increasing calcinations temperature up to 1200 °C, the individual diffraction peaks become better resolved and the peak intensity gets enhanced, indicating lattice perfection and crystallite growth. Calcination at higher temperatures such as 1300 °C yielded a phase mixture of (La_0.95Eu_0.05)₂O₂SO₄ and (La_0.95Eu_0.05)₂O₃ (labeled with asterisks) due to partial decomposition of the sulfate ions. Therefore, proper heating temperature to produce pure phase of (La,Eu)₂O₂SO₄ should be in the range of 400–1200 °C.

FIG. 3.

XRD patterns of the products calcined from the (La_0.95Eu_0.05)₂(OH)₄SO₄·2H₂O LRH at different temperatures for 1 h in air.

One interesting finding which should be noted is that, though the oxysulfates of Y and Lu are either only stable in a limited temperature range or unstable, ^{Reference Shoji and Sakurai18} the La-based oxysulfate exists in the very wide temperature range of 400–1200 °C. This makes La₂O₂SO₄ a suitable host for luminescence, since high-temperature calcination effectively helps to improve PL intensities. Moreover, crystallite sizes of the phosphors assayed from the Scherrer equation are shown in Table I. Coarsening of the crystallites from ∼15 to ∼45 nm was observed for the products calcined from 400 to 1200 °C.

TABLE I.

Optical properties and crystallite sizes of the (La_0.95Eu_0.05)₂O₂SO₄ red phosphors calcined at different temperatures.

Besides the XRD patterns, the calcination transformation process of (La_0.95Eu_0.05)₂(OH)₄SO₄·2H₂O into (La_0.95Eu_0.05)₂O₂SO₄ can be further confirmed by the FTIR characterization, which is shown in Fig. 4. In spectrum (a), the vibration at ∼3243 cm⁻¹ and the shallow, that shoulder near 1674 cm⁻¹, are assignable to the O–H stretching vibrations (ν₁ and ν₃) and the H–O–H bending mode (ν₂) of hydration water in the structure, respectively, ^{Reference Gadsden19} while the two separated sharp peaks at 3604 and 3478 cm⁻¹ are indicative of hydroxyl (OH⁻) groups. The vibrations of SO₄ ²⁻ (ν₁, ν₂, ν₃, and ν₄) ^{Reference Gadsden19} are all well resolved in spectra (a) and (b) as indicated in the figure. In spectrum (c), the vibration of hydration water at ∼3243 cm⁻¹ is hardly found, suggesting the occurrence of dehydration. The bands of hydroxyls and sulfate are still clearly observable but with vibration behaviors different from the original LRH and the 200 °C product. These phenomena suggest that dehydration has changed coordination spheres of the rare-earth ions. The vibrations of hydroxyls are no longer observable from spectra (d)–(f), indicating complete dehydroxylation to form (La_0.95Eu_0.05)₂O₂SO₄ in this temperature range. The above observations are in accordance with the results of XRD (Fig. 3). Similar vibrations of sulfate ions are observed for the LRH and the oxysulfates, indicating similar configurations of sulfate ions. Expansion of the ν₃ band from ∼1205–1075 to ∼1230–1030 cm⁻¹ and obviously deeper and sharper splitting of the ν₄ band are nonetheless observed for the oxysulfates indicting enhancement of SO₄ ²⁻ coordination to the rare-earth centers after dehydration and dehydroxylation. The 1300 °C product contains La₂O₃ impurity (Fig. 3), which is chemically unstable in air and readily reacts with water vapor to form hydroxide via surface reaction, ^{Reference Imanaka, Masui and Kato20} and hence hydroxyl vibration at 3613 cm⁻¹ appears in spectrum (g).

FIG. 4.

FTIR spectra of the (La_0.95Eu_0.05)₂(OH)₄SO₄·2H₂O LRH and its calcination products.

Morphology evolution of the LRH upon calcination is shown in Fig. 5, where it can be seen that the products calcined at 400 and 600 °C tend to preserve the two-dimensional nanoplate morphologies. Calcination at 800 °C leads to partial collapse of the nanoplates into rounded nanoparticles, and at the higher temperature of 1000 °C, basically all the nanoplates have been disintegrated into rounded nanoparticles of ∼100–200 nm. At the even higher temperature of 1200 °C, the nanoparticles apparently underwent coarsening to generate bigger particles of ∼350–650 nm. Rapid particle growth took place at 1300 °C, where micron-sized (2–5 µm) particles/crystallites with sharper edges were resulted.

FIG. 5.

FE-SEM micrographs of the products derived from (La_0.95Eu_0.05)₂(OH)₄SO₄·2H₂O by calcination at (a) 400, (b) 600, (c) 800, (d) 1000, (e) 1200, and (f) 1300 °C.

Figure 6 shows photoluminescence excitation (PLE) and PL spectra of the (La_0.95Eu_0.05)₂O₂SO₄ red phosphors calcined at different temperatures. The PLE spectra consist of the broad and strong band (centered at 284 nm) which arises from electronic transition from the completely filled 2p orbital of O²⁻ to the partially filled 4f orbital of Eu³⁺ (CT), and the sharper peaks are attributed to the intra-4f ⁶ electronic transitions of Eu³⁺ at longer wavelengths (350–500 nm). (Gd,Eu)₂O₂SO₄ and (Y,Eu)₂O₂SO₄ red phosphors were reported to have CT bands centered at ∼270 nm, ^{Reference Lian, Sun, Liu, Yu and Li6,Reference Song, Shao, Du, Cao, Hui, Xing and Xiong9} which is shorter than the ∼284 nm observed for (La_0.95Eu_0.05)₂O₂SO₄ in this work. CT transitions occur when a valence electron is transferred from the ligand to an empty or incompletely filled orbital of the lanthanide ions. ^{Reference Li and Zhang21} Higher covalency of the Ln–O bond means that the electrons spread out over wider orbitals, which reduces the interactions among the electrons and diminishes the electronegativity difference between the constituting ions. ^{Reference Đorđević, Antićn, Nikolić and Dramićanin22} This in turn lowers the energies needed for electronic transition between energy levels. Red-shift of the CT transition in La₂O₂SO₄ is thus understandable from the lower electronegativity of La³⁺. Under CT band excitation, the characteristic ⁵D₀ → ⁷F _J (J = 0–4) transitions of Eu³⁺ were observed with the strongest one located at 617 nm as labeled in the figure. Owing to the shielding by 5s ²5p ⁶ electrons, the 4f ⁶ energy levels of Eu³⁺ are hardly affected by crystal field ^{Reference Li and Sakka23} and thus the position of Eu³⁺ emission would theoretically not be affected by host lattice. The oxysulfate phosphor obtained in this work, nonetheless, shows a 7 nm red-shift of the main ⁵D₀ → ⁷F₂ emission of Eu³⁺ when compared with the classic red phosphor of Y₂O₃:Eu³⁺ (emission at 610 nm). ^{Reference Li, Li, Sun and Ishigaki11} The Wyckoff position of Ln³⁺ was reported to be solely of 8f in monoclinic La₂O₂SO₄ while 8b and 24d in cubic Y₂O₃. ^{Reference Machida, Kawano, Eto, Zhang and Ikeue2,Reference Dhananjaya, Nagabhushana, Nagabhushana, Rudraswamy, Shivakumara and Chakradhar24} Both the lattices have relatively low site symmetries for Ln³⁺, and thus the so called J-mixing effect is particularly applicable. ^{Reference Souza and Couto Dos Santos25} The higher symmetry of O–Ln–O in monoclinic La₂O₂SO₄ than in cubic Y₂O₃ produces less J-mixing effect in the former and thus red-shifts the ⁵D₀ → ⁷F₂ emission if one assumes that Eu³⁺ exclusively replaces Ln sites. ^{Reference Song, Shao, Du, Cao, Hui, Xing and Xiong9} Increasing temperature of calcination did not appreciably change peak positions of the PL bands but led to successively stronger PL intensities. A 160% increase in the intensity of the 617 nm emission was observed from 400 to 1200 °C. The external and internal quantum efficiencies and the percentage of excitation absorption (η, η = (ε_ex/ε_in) × 100%) of the phosphors under O–Eu CT excitation are tabulated in Table I, where it is seen that raising the synthesis temperature from 400 to 1200 °C effectively improved the external and internal quantum efficiencies from ∼16 to ∼40% and from ∼33 to ∼54%, respectively. Notably, an over 16% increment in the external quantum efficiency was observed from 1000 to 1200 °C, which is understandable from the combined contribution of two aspects, that is, improved internal quantum efficiency (10%, Table I) by lattice perfection and the significantly increased excitation absorption (∼21%, Table I) by reduced surface scattering along with obvious crystallite/particle growth (Fig. 5). Commission International de L'Eclairage (CIE) color coordinates of the phosphors (Table I) calcined at different temperatures were found to be stable at around (0.65, 0.35), corresponding to a vivid red color. Since the ⁵D₀ → ⁷F₂ electric dipole transition is hypersensitive to the Eu³⁺ local environment while the parity-allowed ⁵D₀ → ⁷F₁ magnetic dipole transition is not, the intensity ratio ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ has been widely adopted to probe the local symmetry of Eu³⁺. ^{Reference Ji, Huang, Xia, Molokeev, Jiang, Lin and Atuchin26} We analyzed the I ₆₁₇/I ₅₉₅ values for the (La_0.95Eu_0.05)₂O₂SO₄ phosphors calcined at different temperatures and the results are shown in Table I. The ratio was found to slowly decrease from ∼9.7 to ∼7.7 with increasing calcination from 400 to 1200 °C, which is mainly due to the decreased number of lower-symmetric surface Eu³⁺ ions along with particle/crystallite growth.

FIG. 6.

PLE (a) and PL (b) spectra of the (La_0.95Eu_0.05)₂O₂SO₄ red phosphors calcined from (La_0.95Eu_0.05)₂(OH)₄SO₄·2H₂O at different temperatures. The PLE spectra are obtained by monitoring the 617 nm emission, and the PL spectra are measured under UV excitation at 284 nm.

Luminescence decay kinetics of a phosphor can be described by the Inokuti–Hirayama formula, ^{Reference Inokuti and Hirayama27} as follows:

where τ₀ is the intrinsic decay time, t the decay time, I the relative intensity. The s values of 3, 6, 8, and 10 correspond to exchange, dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The energy transfer parameter Q is defined as:

where Γ(x) is the gamma function, N ₀ the concentration of acceptor, R ₀ the critical distance defined as donor–acceptor separation. In this work, the s value was analyzed to be 3, as shown later in Fig. 8(b), and thus Eq. (3) can be reduced to a single exponential. Fitting the fluorescence decay data of the strongest red emission with the single exponential equation indeed yielded good linear relationship for the semilog (log I–t) plots [Figs. S2(a) and S2(b)], and the derived lifetime values are summarized in Table I. It can be seen that the lifetime stays stable at 2.0 ms for the 400–800 °C products and decreases to 1.88 and 1.17 ms for the 1000 and 1200 °C products, respectively. Shortened lifetime at a higher temperature of synthesis was also previously observed by other researchers. ^{Reference Lu, Li and Sakka28}

Figure 7 illustrates XRD patterns of the oxysulfate phosphors calcined at the same temperature of 1200 °C with a holding time of 1 h but with different Eu³⁺ concentrations. All the products can be indexed to the monoclinic La₂O₂SO₄ phase. The diffraction peaks continuously shift to larger angles with increasing Eu³⁺ content, indicating contracted cell dimensions. The cell parameters derived from the software Cellcalc are shown in Table SI, where it is seen that the constants a, b, and c all decrease with increasing Eu³⁺ doping and follow Vegard's law (Fig. S3), implying a successful incorporation of Eu³⁺ into the host lattice to form solid solution. The crystallite sizes calculated from the Scherrer equation are similarly 50 nm for the x = 0.01–0.09 powders.

FIG. 7.

XRD patterns of the (La_1−x Eu _x )₂O₂SO₄ red phosphors calcined at 1200 °C for 1 h.

Figure 8 displays the effects of Eu³⁺ content on PL of the oxysulfate phosphors. The PL [Fig. 8(a)] and PLE (Fig. S4) spectra are essentially identical to those shown in Fig. 6, that is, the PLE spectra are dominated by the CT band centered at ∼284 nm and the PL spectra dominated by the red emission of Eu³⁺ at ∼617 nm. The PL intensity increases with increasing Eu³⁺ content until 5 at.% and then decreases, revealing an optimal concentration of ∼5 at.% (x = 0.05). The value agrees well with that reported before ^{Reference Lian, Liang, Wang and Liu7} and also the theoretical value analyzed later [Fig. 8(c)]. The PLE spectra also imply that the phosphors can be effectively excited by the longer UV light of ∼395 nm (the ⁷F_0,1 → ⁵L₆ intra-f transition of Eu³⁺) and the blue light of ∼465 nm (the ⁷F_0,1 → ⁵D₂ intra-f transition of Eu³⁺, also around the wave length of blue LED chips), though the emission efficiency would be lower than that under CT excitation. The CIE color coordinates calculated from the PL spectra (Table II) are all around (0.65, 0.35). The fluorescence lifetime was also obtained by applying single-exponential fitting [Fig. S5(a)], as also seen from the good linear relationships between log I and t on the semilog plots [Fig. S5(b)]. The resultant lifetime tends to decrease with increasing Eu³⁺ content (Table II). This is basically due to the gradually shortened separation distance among the Eu³⁺ activators, which may result in the formation of a resonant energy transfer network to advance energy transfer from the inside to the nonradiative centers on particle surfaces and thus leads to a shorter lifetime. The lifetime obtained in this work also agrees with the values of 1.2–1.6 ms reported before. ^{Reference Wang, Li, Zhu, Li, Sun and Sakka13} The internal quantum efficiency (ε_in, Table II) reached its maximum of ∼53.9% at 5 at.% of Eu³⁺, in accordance with the PL intensity [Fig. 8(a)]. The percentage of excitation absorption (η, Table II) tends to steadily increase with increasing Eu³⁺ doping, as expected, and the external quantum efficiency (ε_ex, Table II), interplayed from η and ε_in, is ∼40% for x = 0.05. The I ₆₁₇/I ₅₉₅ intensity ratio remains almost constant at 7.6 ± 0.2 for the phosphors of different Eu contents (Table II).

FIG. 8.

PL spectra of the (La_1−x Eu _x )₂O₂SO₄ red phosphors calcined at 1200 °C (a, excitation at 284 nm), where the inset is the relative intensity of the 617 nm emission as a function of the Eu³⁺ content; (b) and (c) show the plots of log(I/c) against log(c) and log(1 − c), respectively, where the number in bracket represents standard deviation.

TABLE II.

Optical properties of the (La_1−x Eu _x )₂O₂SO₄ red-emitting phosphors.

The mutual interaction type of luminescence quenching in solid-state phosphors can be concluded by analyzing the s value in the equation, ^{Reference Lian, Liang, Wang and Liu7,Reference Huang and Lou29,Reference Dai, Song, Wang, Bai, Dong, Qin, Qu and Zhang30}

where I is the emission intensity, c is the Eu³⁺ content, d is the sample dimension (d = 3 for energy transfer among the activators inside particles), f is a constant independent of activator concentration. The s values of 3, 6, 8, and 10 are for the mechanisms of exchange interaction, dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole electric interactions, respectively. The log(I/c)–log(c) plot is given in Fig. 8(b) for the phosphors, from which a slope (−s/3) of −0.86 is derived via linear fitting, leading to an s value of 2.58 (close to 3). This indicates that the observed luminescence quenching is dominantly resulted from exchange interactions for the energy transfer among Eu³⁺ ions.

According to Ozawa, ^{Reference Ozawa31} the starting concentration of luminescence quenching via exchange interactions can be approximated as 1/(1 + Z), where the Z value can be derived from the equation

where I is the emission intensity, c the activator content, b an independent constant, and Z the nearest lattice sites for the activator. The equation proved to be widely applicable by previous studies. ^{Reference Zhang, Lu, Yang, Han, Xu and Zhang32–Reference Fouassier, Saubat and Hagenmuller34} In this work, the Z can be obtained from the following transformed logarithm:

Fitting the log(I/c)–log(1 − c) plot [Fig. 8(c)] according to Eq. (5) yields a slope of Z = 21.99 and thus an optimal Eu³⁺ concentration of ∼4.3 at.% [1/(1 + Z) = 0.043]. The value is nearly identical to that (5.0 at.%) experimentally determined in this study which further indicate that the exchange interaction is responsible for the quench observed [Fig. 8(a)].