Low Temperature Synthesis of Luminescent RE₂O₃:Eu³⁺ Nanomaterials Using Trimellitic Acid Precursors

Abstract

[RE(TLA)·(H₂O)_n:Eu³⁺] (RE³⁺: Y, Gd and Lu; TLA: trimellitic acid) precursor complexes were synthesized by an one step aqueous co-precipitation method. After annealing for 1 h, RE₂O₃:Eu³⁺ nanophosphors were formed through the benzenetricarboxylate low temperature thermolysis method (500-1000 ºC). The compounds were characterized by using different techniques [elemental analysis (CHN), Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG/DTG), X-ray powder diffraction (XPD) and scanning electron microscope (SEM)]. The XPD data indicated that the Y₂O₃:Eu³⁺ materials have crystallite size range from 11 to 62 nm. The SEM and transmission electron microscopy (TEM) images show that the annealed materials keep morphological similarities with the precursor complexes. The photoluminescence properties were studied based on the excitation and emission spectra, and luminescence decay lifetimes of the ⁵D₀ emitting level of the Eu³⁺ ion. The experimental intensity parameters (Ω_λ), lifetimes (τ), as well as radiative (A_rad) and non-radiative (A_nrad) decay rates were calculated and discussed. The RE₂O₃:Eu³⁺ phosphors (RE: Y³⁺ and Lu³⁺) annealed at 500 to 1000 ºC have emission quantum efficiency (intrinsic quantum yield) values from 60 to 82%, indicating that this material can be potentially used for optical markers applications.

Keywords:
low temperature method; benzenetricarboxylate precursors; rare earth sesquioxides; photoluminescence materials

Introduction

Polycarboxylate ligands have a wide variety of structure providing large range of chemical properties when combined with metal ions. It has been drawing the attention in the areas such as metal framework systems (MOF),^1,2 selective markers for medical applications,³ magnetic materials,⁴ gas storage,⁵ drug delivery,⁶ precursors for materials,⁷ etc.

Rare earth (RE) containing materials show a versatility for application in the areas of science and technology specially in catalysis, permanent magnets in hybrid cars batteries,^8,9 electroluminescent materials, persistent phosphors, structural probes, luminescent markers, display panels, etc.^10-15 Most of those applications are consequence of their intrinsic characteristic: sharp intraconfigurational 4f^N transitions, archiving high monochromatic emission colors and a wide range of emissions, from infrared to ultraviolet,¹⁶ e.g., Nd³⁺, Eu³⁺, Gd³⁺, Tb³⁺ and Tm³⁺ ions which emit in the infrared, red, ultraviolet, green and blue regions, respectively.

One very important feature of the RE³⁺ is their 4f-4f transitions, forbidden by the Laporte's rule. Associated to that, the shielding from the chemical environment by the filled 5s and 5p sub-shells¹⁷ over the 4f electrons lead to a characteristic sharp lines spectra with small absorptivity and emission intensities. Taking into account the RE³⁺ intraconfigurational transitions, these ions can be divided in four groups depending on their spectroscopic features:

(i ) Sc³⁺(3d⁰), Y³⁺(4d⁰), La³⁺ (4f⁰) and Lu³⁺(4f¹⁴) where the 4f electrons are non-optically active due to their completely empty or fully occupied subshells;¹⁶

(ii ) Gd³⁺(4f⁷) is a singular case due to its half-filled 4f layer, and therefore very stable. The energy difference between the lower emitting level (⁶P_7/2) and the fundamental level (⁸S_7/2) is approximately 32000 cm⁻¹ opening the opportunity for its application as inorganic matrices. Due to the chemical similarity with other RE³⁺ ions, it is extensively used to study the emission of the ligands in coordination complexes;

(iii ) Sm³⁺(4f⁵), Eu³⁺(4f⁶), Tb³⁺ (4f⁸) and Dy³⁺(4f⁹): in these ions, the energy gap between the emitting and the lower levels are large enough to reduce the non-radiative decay process and accept energy from the ligands, interconfigurational transitions or charge transfer bands excited levels (Figure 1);

Figure 1
Partial energy diagram of trimellitic acid (TLA) ligand from [RE(TLA)·(H₂O)_n] (RE³⁺: Y, Gd and Lu) precursor (singlet and triplet states), Eu³⁺ ion and RE₂O₃:Eu³⁺ (LMCT) state.

(iv ) Ce³⁺(4f¹), Pr³⁺(4f²), Nd³⁺ (4f³), Ho³⁺(4f¹⁰), Er³⁺(4f¹¹), Tm³⁺(4f¹²) and Yb³⁺ (4f¹³): in these ions the energy gap between the emitting and lower levels are small, increasing the non-radiative decay process usually mediated by high energy vibrational modes in ligands (typically water molecules) or matrices (oxycarbonates, hydroxides, etc.). In these cases, the process accounts for the decreasing in the final emission efficiency.

To overcome the small absorptivity coefficients, luminescence sensitizers can be used to absorb and transfer the energy efficiently to the RE ions, keeping their desirable atomic characteristics. This phenomenon is a key feature in design of luminescent materials.^16,18,19

In inorganic matrices such as vanadates, molybdates, tungstates and sesquioxides containing RE³⁺ ion, generally is observed an efficient energy transfer from the ligand metal charge transfer (LMCT) band to the metal ions. In the special case, the Eu³⁺ ion shows a high absorption intensity arising from the allowed LMCT transition, yielding a high intensity luminescence.²⁰

In solid state reactions, typically, is necessary high temperatures and long reaction time periods to prepare luminescent materials. This way to synthesize materials is known as ceramic method, which promotes heterogeneous distribution of the activator ion within the matrix and generate materials with high crystallite and particle sizes. Alternative methods to obtain materials in milder reaction conditions as: sol-gel, combustion or Pechini methods,^21,22 are key to overcome the experimental limitation and improve their properties.

This report demonstrate the synthesis, characterization and optical properties of [RE(TLA):Eu³⁺(x mol%)] complexes (RE³⁺: Y, Gd and Lu; x: 0.1, 0.5, 1.0, and 5.0 mol%) and their low temperature annealing into the high luminescent RE₂O₃:Eu³⁺ phosphors. All the precursor complexes and resulting nanophosphors were characterized by elemental analysis (CHN), Fourier transform infrared (FTIR), thermogravimetry (TG), derivative thermogravimetry (DTG), X-ray powder diffraction (XPD) and scanning electron microscopy (SEM). The photoluminescence properties of the doped materials were studied based on the excitation and emission spectra and luminescence decay curves of the Eu³⁺ ion ⁵D₀ excited level.

Experimental

High purity RE₂O₃ (RE³⁺: Y, Eu, Gd and Lu; CSTARM, 99.99%, China) were used to prepare the respective RECl₃. (H₂O)₆ salts by reaction with concentrated HCl solution until total decomposition (ca. 60-80 ºC) of the solid and final pH close to 6. The trimellitic acid (TLA) (in the form of 1,2,4-benzenetricarboxylic acid 1,2-anhydride or 1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxylic acid; Aldrich, 97%, Germany) was solubilized in water by drop-wise addition of 1 mol L⁻¹ sodium hydroxide up to pH close to 6.

For the preparation of the [RE(TLA):Eu³⁺] complexes, 50 mL of RECl_3(aq) (0.05 mol L⁻¹) was slowly added to a 200 mL solution of Na₃(TLA)_(aq) (0.0125 mol L⁻¹) at 1:1 molar ratio at ca. 100 ºC. The reaction mixture was refluxed for 1 h, the precipitate was filtered and washed four times with distilled water, dried and stored at reduced pressure.

The [RE(TLA):Eu³⁺] complexes obtained are non-hygroscopic, white crystalline powders, stable in air. The RE₂O₃:Eu³⁺ nanophosphors were obtained by annealing the [RE(TLA):Eu³⁺] complexes at 500, 600, 700, 800, 900 and 1000 ºC in a static air atmosphere, resulting in RE₂O₃:Eu³⁺ nanophosphors.

Elemental analyses were performed with a Perkin-Elmer CHN 2400 analyzer. The FTIR were acquired from 400 to 4000 cm⁻¹ in KBr pallets form by using a Bomem MB100 FTIR. Thermogravimetry was performed from 30 to 900 ºC (heating ramp of 5 ºC min⁻¹, synthetic air dynamic atmosphere) in a TA HI-RES TGA 2850 equipment. The XPD patterns were obtained in a Miniflex Rigaku II equipment (CuK_α1) from 5 to 70º (2θ). The SEM micrographs were recorded in a JEOL JSM 7401F field emission scanning electron microscope. The transmission electron microscope (TEM) micrographs were recorded in a JEOL USA JEM-2100 LaB₆ transmission electron microscope.

The luminescence study was based on the excitation and emission spectra recorded at room (300 K) and liquid nitrogen (77 K) temperatures. The measurements were performed in a SPEX-Fluorolog 3 instrument with double monochromators in front face mode (22.5º) using a 450 W Xenon lamp as excitation source. Luminescence decay curves were obtained by using a 150 W pulsed lamp and recorded in a SPEX 1934D phosphorimeter.

Results and Discussion

Characterization

A combination of elemental and thermogravimetric analysis (Table S1 and Figure 2) suggests an 1:1 molar ratio between the RE³⁺ ion and TLA ligand ([RE(TLA)·(H₂O)_n:Eu³⁺]; n: 4, 4 and 3 for Y³⁺, Gd³⁺ and Lu³⁺, respectively).²³ The TG curves of coordination compounds show a water molecules mass-loss in the temperature interval between 50 and 230 ºC. Although the organic moiety decomposition of the complexes presents only one single-step between 450 and 570 ºC. In this case, it was used annealing temperature of 500 ºC during 1 h, in order to eliminate all the organic part leading to formation of the RE₂O₃:Eu³⁺ luminescent material.

Figure 2
TG and DTG curves (a) [RE(TLA)] and (b) [RE(TLA):Eu³⁺ (5.0 mol%)] (RE³⁺: Y, Gd and Lu).

The infrared absorption spectra (Figure S1) present similar spectral profile for the RE³⁺ complexes and Eu³⁺- doped matrices. The absorption bands between 1300 and 1600 cm^–1 in the FTIR spectra of [RE(TLA)·(H₂O)_n:Eu³⁺] are assigned to the carboxylate symmetric ν_s(C=O) and asymmetric ν_as(C=O) stretching modes, respectively.^19,24,25 The narrow absorption peak around 3070 cm^–1 is assigned to the C–H bond stretching of the [RE(TLA):Eu³⁺] complexes and the broad band between 3100-3700 cm^–1 correspond to the O–H stretching from the water molecules.²⁶

The sharp absorption bands around 510 and 580 cm⁻¹ correspond to the characteristic RE³⁺−O stretching vibration. It is worth mentioning that the broad bands from 1250 to 1600 cm⁻¹ are assigned to stretching mode of oxycarbonate remainder from the decomposition of the organic moistly of TLA and decreases with increasing annealing temperature (Figure S1), due to oxycarbonate decomposition.²³ The broad absorption band located from 2800 to 3700 cm⁻¹ is assigned to the superficial hydroxyl groups in the nanomaterials. Therefore, the RE₂O₃:Eu³⁺ materials originated from the [RE(TLA)] precursor complexes present similar chemical behavior compared to the sesquioxides prepared from the [RE(TMA)] complexes as reported by Silva et al.²³

The X-ray diffraction patterns of the [RE(TLA):Eu³⁺] complexes are similar to the powder diffraction patterns (PDF) for [Gd(TLA)]:Eu³⁺ and [Y(TLA)]:Eu³⁺] (00-056-1733) and [Lu(TLA)]:Eu³⁺] (00-058-1915), Y³⁺ and Gd³⁺ complexes are isomorphs. Consequently, there is no change in position or formation of new diffraction peaks at different concentrations of the dopants. This result is consistent with the Vegard's rule^27,28 which suggests a formation of a solid solution between the Eu³⁺ dopant and the RE³⁺ in the host matrices due to the high similarity in the radii of these RE³⁺ ions.²⁷

The XPD patterns of the annealed materials at 500, 600, 700, 800, 900 and 1000 ºC (Figure 3) reveal a formation of RE₂O₃:Eu³⁺ in a cubic phase crystallization with the Ia3 space group.²⁹ The absence of 2θ shift and reflections of impurities in the patterns of the RE₂O₃:Eu³⁺ indicates the formation of pure RE³⁺ sesquioxides. The XPD data of the Y₂O₃, Gd₂O₃ and Lu₂O₃ matrices (Figure 3) are very similar. Slight differences in the (222) reflection around 28º, moving to higher 2θ values with decreasing of the ionic radius of the RE³⁺ in the matrix, as predicted by Bragg's law.³⁰

Figure 3
XPD patterns of (a) Y₂O₃:Eu³⁺; (b) Gd₂O₃:Eu³⁺ and (c) Lu₂O₃:Eu³⁺ (1.0 mol%) materials annealed for 1 h at different temperatures; reference pattern: PDF: 86-2477 and 86-2475, respectively.

The average crystal size of the doped materials was estimated from the powder diffraction data by using the Scherrer's formula (Figure 4).^23,31 The crystallite size of the RE₂O₃ materials increases as function of the RE³⁺ radius and annealing temperature. This behavior can be assigned to the higher reactivity of the Gd₂O₃, with lower melting point (2339 ºC) compared to Y₂O₃ (2410 ºC) and Lu₂O₃ (2427 ºC).³² Therefore, the sintering process is favored for the gadolinium matrix due to the dependence of the partial melting of the nanocrystals.²³

Figure 4
Correlation between the sesquioxides crystallite size and annealing temperature for RE₂O₃:Eu³⁺ (1.0 mol%) materials.

The narrowing of the diffraction peaks of RE₂O₃:Eu³⁺ (1.0 mol%) (RE³⁺: Y, Gd and Lu) phosphors presented in the XPD patterns (Figure 3) as function of the annealing temperature, indicates that the crystallite size increases from 11, 17, 18, 37, 46 and 62 nm as the annealing temperature increases from 500, 600, 700, 800, 900 and 1000 ºC (Y₂O₃), respectively (Figure 4). This behavior is related to the sintering of the nanocrystallites favored at high temperatures. Although the Gd₂O₃:Eu³⁺ annealed at 1000 ºC was also included in this work, the Scherrer's formula is recommended only for crystallite sizes up to 200 nm (Figure 4).

The SEM images of the [RE(TLA):Eu³⁺ (1.0 mol%)] precursors shows rods and a flower like morphologies (stacking of micrometric sheets of the material) for Y³⁺/Gd³⁺ (Figures 5a and 5b) and Lu³⁺ complexes, respectively (Figure 5c). After annealing up to 1000 ºC, the RE₂O₃:Eu³⁺ materials retained the original morphology of the correspond precursor complex (Figures 5d-5f). The nanosesquioxides exhibit higher porosity due to the decomposition of the organic moiety. This property is important for the design of nanomaterials with controlled morphology. Since it is possible to modify the complex morphologies, the desired nanoparticle shapes can be obtained by choosing the suitable synthetic method and reaction conditions.^33,34

Figure 5
SEM images of [RE(TLA):Eu³⁺ (1.0 mol%)] precursor complexes (a; b; c); RE₂O₃:Eu³⁺ (1.0 mol%) phosphor annealed during 1 h (d; e; f) and TEM images of RE₂O₃:Eu³⁺ (1.0 mol%) annelaed during 1 h at 1000 ºC (g; h).

The TEM micrographs (Figures 5g and 5h) show the cubic shape of the crystallites with high crystallinity. The particles retained the shape of the precursor agglomerates, shown in the SEM microscopy. At higher magnification no defects were observed in the crystals (except for the edges and crystallite contact points), suggesting the formation of a solid solution between the Eu³⁺ ions and the host matrices, compatible with the similar RE³⁺ ionic radii and chemical behavior of the Eu³⁺ and RE³⁺ matrices.

Photophysical properties of materials

[RE(TLA):Eu³⁺] precursor complexes

The excitation spectra of [RE(TLA):Eu³⁺ (x mol%)] (RE³⁺: Y, Gd and Lu) compounds were obtained by monitoring the hypersensitive transition ⁵D₀ → ⁷F₂ (619 nm) at 77 K (Figure 6). For all the complexes, the absorption bands are dominated by a high intensity broad TLA ligand band centered at 295 nm assigned to the S₀→ S₁ transition, indicating an efficient energy transfer TLA → Eu³⁺. The sharp peaks are assigned to the absorption of the Eu³⁺ ion originated from the ground state ⁷F₀ to the ⁵L₆ and ⁵D₂ excited levels. The excitation spectra of the [RE(TLA):Eu³⁺ (x mol%)] (RE³⁺: Y and Gd) compounds show similar profiles suggesting that this system presents equivalent chemical environments around RE³⁺ ions and optical behaviors. On the other hand, [Lu(TLA):Eu³⁺ (x mol%)] shows slightly different spectral profile. For all [RE(TLA):Eu³⁺] systems, the ⁷F₀ → ⁵L₆ transition (25445 cm^–1 for [Y(TLA):Eu³⁺ (5.0 mol%)]) exhibits the highest intensity among the intraconfigurational transitions in the excitation spectra.

Figure 6
The (a) excitation spectra of [RE(TLA):Eu³⁺ (5.0 mol%)] (RE³⁺: Y, Gd and Lu), with emission monitored at 616 nm; (b) emission spectra, with excitation at 295 nm, recorded at 77 K and (c) correlation between [RE(TLA):Eu³⁺ (x%)] lifetimes and Eu³⁺-doping concentrations (x: 0.1, 0.5, 1.0 and 5.0 mol%).

The emission spectra of the [RE(TLA):Eu³⁺ (x mol%)] complexes (RE³⁺: Y, Gd and Lu), were recorded under excitation in the TLA ligand band (ca. 295 nm) at 77 K, to reduce the vibronic coupling compared to the room temperature case. The emission energy levels of ⁵D₀ → ⁷F_J transitions (J = 0-4) of the Eu³⁺ ion, can be attributed as the following (in cm–1): ⁷F₀ (17270); ⁷F₁ (16920); ⁷F₂, (16210); ⁷F₃ (15337) and ⁷F₄ (14350), based at the [Y(TLA):Eu³⁺ 5.0 mol%)]. The efficient energy transference TLA → Eu³⁺ ion is evidenced by the absence of ligand broad emission band in the emission spectra in the spectral range from the 400 to 700 nm.

Using the optical data obtained from the emission spectra, it is possible to calculate the radiative rates (A_0→J) from the ⁵D₀ → ⁷F_J transitions using equation 1:^16,17

where σ_0→1 and σ_0→2,4 correspond to the energy barycenter of the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F_2,4 transitions, respectively. The S_0→1 and S_0→J are the areas calculated under the emission of the spectral curve corresponding to the ⁵D₀→⁷F₁ and ⁵D₀→⁷F_J transitions, respectively.³⁵ Since the magnetic dipole ⁵D₀ → ⁷F₁ transition is almost insensitive to changes with the chemical environment around the Eu³⁺ ion, the A_0→1 rate can be used as an internal standard to determine the A_0→J coefficients for Eu³⁺ containing compounds.¹⁶

The lifetime (τ) of the luminescent compounds were obtained from the luminescence decay curve using a first order exponential decay, with excitation at the ⁷F₀ → ⁵L₆ band. The emission quantum efficiency (η, or intrinsic quantum yield, ³⁶ of the ⁵D₀ emitting level is determined according to equation 2:, as it has been defined by Bünzli)

where the total decay rate, A_tot = 1/τ = A_rad + A_nrad and the A_rad = ∑_J A_0→J. The A_rad and A_nrad quantities are the radiative and non-radiative rates, respectively. Table 1 shows the experimental values of the radiative (A_rad), non-radiative (A_nrad) rates and ⁵D₀ emitting level emission quantum efficiency (η).

Table 1
Experimental values of intensity parameters (Ω_λ), radiative (A_rad) and non-radiative (A_nrad) rates, emission lifetimes and emission quantum efficiencies of the ⁵D₀ emitting level determined for the [RE(TLA):Eu³⁺ (x mol%)] (RE³⁺: Y, Gd and Lu) phosphors based on the emission spectra recorded at 77 K

[RE(TLA):Eu3+ / (x mol%)]	Ω2 / (10–20 cm2)	Ω4 / (10–20 cm2)	Arad / s−1	Anrad / s−1	Atot / s−1	τ / ms	η / %
Y3+
0.1	6.4	1.6	264	2492	2756	0.363	10
0.5	6.2	1.7	258	2642	2900	0.345	9
1.0	6.1	1.7	257	2475	2732	0.366	9
5.0	6.1	1.7	257	2948	3205	0.312	8
Gd3+
0.1	4.7	1.7	216	1936	2151	0.465	10
0.5	4.8	1.8	219	1862	2081	0.481	11
1.0	5.1	1.7	225	1928	2153	0.464	10
5.0	4.7	1.8	217	1870	2087	0.479	10
Lu3+
0.1	5.5	1.3	234	3587	3821	0.262	6
0.5	5.4	1.4	229	3693	3922	0.255	6
1.0	5.4	1.3	229	3641	3870	0.258	6
5.0	5.4	1.3	229	3792	4021	0.249	6

• Ω_λ: experimental values of intensity parameters; A_rad: radiative rate; A_nrad: non-radiative rate; A_tot: total decay rate; τ: lifetime; η: quantum efficiency.

The [RE(TLA):Eu³⁺ (x mol%)] lifetime values (Table 1 and Figure 6c) show higher values for Gd³⁺ and Y³⁺ containing complexes when compared to the Lu³⁺ ion case. On the other hand, there are no changes in the lifetime behavior doping with an increasing concentration from 0.1, 0.5, 1.0 and 5.0 mol%, within the same system.

The ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transitions can be used to estimate the experimental intensity parameters (Ω_λ, λ = 2 and 4). The Ω₆ intensity parameter is not included in this study since the ⁵D₀ → ⁷F₆ transition was not observed for these systems. The coefficient of spontaneous emission, A, is given by equation 3:

where, χ = n (n + 2)²/9 is the Lorentz local field correction and n is the refractive index of the medium (refractive index used: 1.5 for all [RE(TLA):Eu³⁺] complexes and between 1.5 and 1.6 for RE₂O₃:Eu³⁺ materials). The squared reduced matrix elements 〈⁷F_J||U^(λ)||⁵D_J〉² are 0.0032 and 0.0023 calculated for J = 2 and 4, respectively.^35,37

The Ω_λ parameters depend mainly on the local geometry, bonding atoms and polarizabilities in the first coordination sphere of the RE³⁺ metal ion, and are governed by both forced electric dipole (FED) and dynamic coupling (DC) mechanisms. Moura et al.³⁸ reported that the Ω₂ parameter values are very sensitive to small angular changes in the local coordination geometry (much more than the Ω_4,6 parameters). This spectroscopic behavior is associated with the hypersensitivity of certain 4f-4f transitions, to changes in the chemical environment, that are usually ruled by the Ω₂ intensity parameter. On the other hand, the Ω₄ and Ω₆ values are most sensitive the chemical bond distances to the ligating atoms around the lanthanide ion. Indeed, as concluded by Moura et al. ,³⁸ covalency in the ion-ligand bonding becomes more important with the increasing rank of the Ω_λ, supporting the idea that the Ω₄ and Ω₆ parameters are better probes then Ω₂ to quantify covalency in these compounds.

The Ω_λ (λ = 2 and 4) parameter values for the [RE(TLA):Eu³⁺(x mol%)] compounds (x = 0.1, 0.5, 1.0 and 5.0 mol%) are presented in Table 1. The Ω₂ values (ca. 6 × 10^–20 cm2) found for these doped complexes are systematically larger than the [RE(TMA):Eu³⁺] (RE³⁺: Y and Lu) anhydrous complexes (ca. 2 × 10^–20 cm²) values [RETMA] reported by Silva et al .³⁹ reflecting the higher hypersensitive character of the ⁵D₀ → ⁷F₂ transition.^23,40,41

The emission quantum efficiency values of the [RE(TLA):Eu³⁺ (x mol%)] are lower for the complexes containing Y³⁺ and Gd³⁺ (η ca. 10%) and Lu³⁺ (η ca. 6%) ions, which indicate a strong non-radiative decay pathway mediated by water molecules (Table 1). It is also observed that increasing the Eu³⁺ concentration from 0.1 to 5.0 mol% produces no change in the emission quantum efficiency values, suggesting that the luminescence quenching concentration effect is not operative for these systems.

RE₂O₃:Eu³⁺ materials

The excitation spectra of RE₂O₃:Eu³⁺ annealed phosphors (RE³⁺: Y, Gd and Lu) were recorded at 77 K in the spectral range from 200 to 590 nm, with the emission monitored at 613 nm (Figure 7 and Figure S3). They show the presence of a broad absorption band centered around (ca. 39000 cm–1) assigned to the O²⁻(2p) → Eu³⁺(4f6) LMCT transition. Besides, the narrow absorption bands arisen from 4f-4f transitions from the RE³⁺ ion (ca. 17000 to 34000 cm^–1) are observed.

The excitation spectra recorded at 300 K (Figure S4) show the presence of the overlapped ⁷F₀ → ⁵D₁ and ⁷F₁→⁵D₁ transitions (ca. 19000 cm–1) allowed by magnetic-dipole mechanism (∆J = 0, ±1, but 0 ↔ 0 is forbidden) for both the C₂ and S₆ symmetries. This optical results are due to the thermal population of the ⁷F₁ level that are in agreement with the results previously reported for RE₂O₃:Eu³⁺.^42,43 The absorption bands assigned to the ⁷F₀ → ⁵D₂ transition allowed by induced electric dipole and dynamic coupling mechanisms were observed from 21500 to 21900 cm^–1. In addition, a weak absorption band around 24100 cm^–1 is assigned to the forbidden ⁷F₀ → ⁵D₃ transition (by ∆J selection rules) as a result of the relaxation of the selection rule due to the J-mixing effects in the ⁷F_J manifolds. Moreover, the other absorption bands (Figure 7) originated from 4f-4f transitions of the Eu³⁺ ion were observed such as (in nm): the ⁷F₀ → ⁵L₆ (394), ⁵G₂₋₆ (387), ⁵L_7,8 (376), ⁵D₄ (363), ⁵H_J', ⁵F_J', ⁵I_J' and ³P₀ (between 286 and 335).

Figure 7
The (a) excitation spectra of Y₂O₃:Eu³⁺ (1.0 mol%), with emission monitored at 613 nm; (b) emission spectra, with excitation at 260 nm, recorded at 77 K.

It is worth mentioning that the excitation spectra of the Gd₂O₃:Eu³ present the characteristic strong absorption (nm): ⁸S_7/2 → ⁶P_7/2 (313), ⁸S_7/2 → ⁶P_5/2 (307) and ⁸S_7/2→⁶P_3/2 (302) transitions, indicating efficient energy transfer from the Gd³⁺ to the Eu³⁺ ion upper levels.⁴⁴ The ⁸S_7/2→⁶I_J(J = 7/2,9/2,17/2) (276) transitions overlap with the LMCT band. This high intensity absorption band indicates an efficient Gd³⁺ to Eu³⁺ energy transfer.⁴⁵

The luminescent materials prepared by the benzenetricarboxylate method present comparable excitation features, indicating the reproducibility of the method even when using different benzenetricarboxylate (BTC) ligands.²³

The emission spectra of the RE₂O₃:Eu³⁺ (RE³⁺: Y, Gd and Lu) annealed at temperatures from 500 to 1000 ºC were recorded at 77 K from 400 to 750 nm, under excitation in the LMCT band at 260 nm (Figure 7). All the spectra exhibit only the sharp lines arising from the ⁵D_0,1,2,3→⁷F_0-6, transitions of the Eu³⁺ ion. All materials show only one emission line assigned to ⁵D₀ → ⁷F₀ transition (ca. 17270 cm^–1) of the C₂ site of the cubic C-type. The ⁵D₀ → ⁷F₁ transition is present in both sites in the region of 16666, 16846 and 17015 cm^–1 as well at 16770 and 17165 cm^–1 originating from the C₂ and S₆ sites.^42,43

As reported by Boyer et al.⁴⁶ and Meltzer et al. ,⁴⁷ the refractive index (n) of the bulk RE₂O₃:Eu³⁺ is around 1.9 and the ⁵D₀ lifetime (τ) of europium ion is 1.0 ms. On the other hand, these values can be different in the case of the RE₂O₃ nanostructured materials, with average sizes around 20-30 nm (crystallite size inferior to the wavelength of exciting radiation). Moreover, the morphology and surface/volume ratio of the nanoparticles may play a role in the profile of the decay curves.

The radiative rate (A₀₁) of the ⁵D₀ → ⁷F₁ transition of Eu³⁺ ion (allowed by the magnetic dipole mechanism) is formally insensitive to the ligand field environment. Therefore it can be used as a reference transition whose value is 50 s^–1 assuming a refractive index equal to 1.6.^17,48,49 Based on this value, the refractive indices were determined and compared to the lifetime and crystallite size values reported previously.^46-48 The experimental intensity parameters (Ω_2,4) and lifetimes (0.8-1.9 ms) values were obtained using the effective refractive index values between 1.5 and 1.6. The values of the experimental intensity parameters (Ω_2,4) the radiative (A_rad) and non-radiative (A_nrad) rates and emission quantum efficiencies (η) of the ⁵D₀ emitting level of the RE₂O₃:Eu³⁺ are presented in Table 2.

Table 2
Experimental values of intensity parameters (Ω_λ), radiative (A_rad) and non-radiative (A_nrad) rates, emission lifetimes and emission quantum efficiencies of the ⁵D₀ emitting level determined for the RE₂O₃:Eu³⁺ (1.0 mol%) (RE³⁺: Y, Gd and Lu) phosphors, annealed for 1 hour, based on the emission spectra recorded at 77 K

RE2O3:Eu3+ (1.0 mol%) / °C	Ω2 / (10–20 cm2)	Ω4 / (10–20 cm2)	Arad / s−1	Anrad / s−1	Atot / s−1	τ / ms	η / %
Y2O3:Eu3+
500	12.8	2.4	470	264	734	1.362	64
600	12.6	2.3	461	207	668	1.496	69
700	12.6	2.3	462	181	643	1.555	72
800	12.5	2.2	459	113	571	1.751	80
900	12.1	2.2	446	107	553	1.808	81
1000	11.8	2.1	435	111	546	1.830	80
Gd2O3:Eu3+
500	12.6	1.9	458	788	1246	0.803	37
600	12.3	1.9	447	632	1079	0.927	41
700	12.0	1.8	438	428	866	1.155	51
800	12.0	1.8	438	265	702	1.424	62
900	11.7	1.8	430	195	625	1.600	69
1000	11.4	1.7	419	166	585	1.709	72
Lu2O3:Eu3+
500	11.5	2.8	434	284	719	1.391	60
600	11.5	2.7	435	253	688	1.454	63
700	11.4	2.7	431	155	588	1.701	73
800	11.5	2.6	433	107	541	1.850	80
900	11.6	2.6	434	95	529	1.890	82
1000	11.2	2.5	422	104	526	1.900	80

• Ω_λ: experimental values of intensity parameters; A_rad: radiative rate; A_nrad: non-radiative rate; A_tot: total decay rate; τ: lifetime; η: quantum efficiency

The values for Ω₂ (ca. 12) and Ω₄ (ca. 2-3) are very similar in the same matrix (Table 2) for different annealing temperatures as shown in the spectral profiles (Figure 7b).⁵⁰ These results are a reflection of the observed emission intensity variations of the ⁵D₀ → ⁷F₂ transition of the Eu³⁺ ion. This optical behavior demonstrates that the Eu³⁺ ion acts as efficient luminescence probe even for the samples annealed at different temperatures. In addition, Ω₂ and Ω₄ values are also comparable changing the RE³⁺ matrix, due to the similarity in the radii in the lanthanide series.

The experimental intensity parameter values for the phosphors using the TLA ligand as precursor are smaller for all the systems, as compared to those originated from the TMA ligand, especially for the of Gd³⁺ matrix.¹⁹

According to Table 2, the RE₂O₃:Eu³⁺ phosphors present an emission quantum efficiency values varying from 37 to 82% with the annealing temperature of 500-1000 ºC. Among the materials, the Lu₂O₃:Eu³⁺(1.0 mol%) with annealing at 900 ºC present the highest emission quantum efficiency (η = 82%). This phenomenon is probably associated to the removal of oxycarbonate from the matrices with increasing the annealing temperature. It is important to mention that the RE₂O₃:Eu³⁺ phosphors prepared by the benzenetricarboxylate method using the TLA ligand is cheaper than compared with the TMA ligand.

The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates generated from the emission spectra of Eu³⁺ doped RE₂O₃ (Figure 8) are x: 0.650 and y: 0.335.⁴⁹ The color coordinates show virtually no change for different sesquioxide matrices, concentration or annealing temperature. The phosphors containing Gd³⁺, Y³⁺, Lu³⁺ ions exhibit the same characteristic nearly monochromatic emission. The images of the Y₂O₃:Eu³⁺ (1.0 mol%) nanomaterials under UV irradiation show identical strong red emission for all the phosphors annealed at temperatures from 500 to 1000 ºC.

Figure 8
CIE diagram (center) and images and of [RE(TLA):Eu³⁺] (left) and RE₂O₃:Eu³⁺ (right), excitated at 254 nm.

Conclusions

[RE(TLA):Eu³⁺] complexes present low total decomposition temperature of the organic moistly producing the high luminescent RE₂O₃:Eu³⁺ materials at 500 ºC. The benzenetricarboxylate method is reliable, efficient and reproducible for the synthesis of phosphors at low temperature. The red emission of the RE₂O₃:Eu³⁺ materials (RE³⁺: Y³⁺, Gd³⁺ and Lu³⁺) arise mainly from the C₂ symmetry site. The large values of the Ω₂ experimental parameters corroborates with the high intensity of the ⁵D₀→⁷F₂ transition. Besides, these materials can act as efficient red light conversion devices in the studied Eu³⁺-concentration range. Finally, the RE₂O₃:Eu³⁺ phosphors prepared by the benzenetricarboxylate method using the [RE(TLA):Eu³⁺] present lower emission quantum efficiency (η close to 80%) than from the [RE(TMA):Eu³⁺] precursor complexes (η close to 90%). However they are cheaper, becoming an efficient and more economically viable system potentially usable as optical markers.

• FAPESP has sponsored the publication of this article.
• Supplementary Information
  Supplementary information is available free of charge at http://jbcs.sbq.org.br as PDF file.

Acknowledgments

The authors acknowledge financial support from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).

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Data availability

https://minio.scielo.br/documentstore/1678-4790/vc8JVNJN9DMgSZj8gW8zwGg/2a1da50d82ebf82930e768e0d3580bc772c84ac3.pdf

Publication Dates

• Publication in this collection
  Dec 2015

History

• Received
  26 Aug 2015
• Accepted
  16 Nov 2015