Effect of Eu, Tb codoping on the luminescent properties of multifunctional nanocomposites

Abstract

The synthesis (by a facile two-step co-precipitation process) and characterization, are reported for good dispersed core–shell structured Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Tb³⁺ nanocomposites with luminescent and magnetic properties. XRD patterns show that Fe₃O₄ core has crystallized with a good face-centered cubic structure; SiO₂ layer is amorphous and Y₂O₃ layer is cubic. TEM images show the synthesis of core–shell structured composite microspheres of uniform size with rough surface. For nanocomposites with same host (Y₂O₃), upon excitation with UV irradiation, fluorescent spectra show that there is a strong emission at around 610 nm corresponding to the forced electric dipole ⁵D₀–⁷F₂ transition of Eu³⁺ and at around 545 nm corresponding to ⁵D₄–⁷F₅ transition of Tb³⁺. We speculate that there exists energy transfer from Tb³⁺ to Eu³⁺ ions from excitation spectra. Therefore, the multifunctional nanocomposites are expected to develop many potential applications in biomedical fields.

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1. Introduction

Phosphors have been in the focus of material science researchers because of their remarkable tunable light emission properties. Phosphors exist in various compositions because a wide variety of hosts are available such as Y₂O₃, YVO₄, LaF₃ and ZnS,^1–3 thus their applications are broad.^4–6 One of the most studied ceramics as host matrix for phosphors is yttrium oxide (Y₂O₃). Rare earth ions generally have plenty of well-shielded 4f states that can emit fluorescence covering different wavelength from ultraviolet to infrared. For example, Tb³⁺ can emit green rays and the red-emitting components can be achieved by the emission of Eu³⁺. Phosphors containing sphere, rod, prism and flower shaped particles^7–10 are already available. Among a variety of methods available for the production of nanocrystalline Y₂O₃:Ln (Ln = Eu³⁺ or Tb³⁺) powders, the solvothermal synthesis and homogeneous precipitation method are capable of producing monodisperse spherical particles. The luminescence efficiency from Y₂O₃ based materials strongly depends on the physical properties of the material such as the surface morphology, crystallinity, phase-purity and distribution of activator in the matrix.^11–13 It is observed that by decreasing the grain size of host materials from macroscopic scale to nanoscale leads to many advantages. According to the previous studies, yttria doped with specific rare earth elements shows interesting photoluminescent (PL) and electroluminescent (EL) properties. Y₂O₃:Eu³⁺/Tb³⁺ phosphor emits red emission/green emission and has excellent chemical stability. This phosphor is the only existing red/green phosphor used in three-band fluorescent lamps.^14,15 Co-doping enables the fine-tuning of the excitation and emission spectra of phosphors. When Y₂O₃ is co-doped with Eu³⁺ and Tb³⁺, for example, a strong energy transfer occurs from Tb³⁺ to Eu³⁺ ions, but a back energy transfer from Eu³⁺ to Tb³⁺ is not significant.^16,17 In order to develop luminescent materials with improved luminescence characteristics for using them in various display devices, it will be of interest to synthesize and study the luminescence properties of such nano-phosphors. Recently, Liu et al.¹⁸ investigated the luminescence properties of Y₂O₃:Re (Re = Eu, Tb) and Y₂O₃:Eu³⁺, Tb³⁺ phosphors by changing the concentration of Tb³⁺ and Eu³⁺. Even though there are many studies investigating synthesis and optical properties of doping with two or more rare-earth elements in the same crystal host matrix,^19–22 the effect of magnetic core on these energy transfers has been neglected and the case in which these two ions exist simultaneously also has not been considered. So it is necessary to analyze the influence of magnetic core.

In this paper, Eu³⁺ and Tb³⁺ co-doped Y₂O₃ phosphors are both activator centers have synthesized by co-precipitation method. Additionally to the best of our knowledge, magnetic Fe₃O₄ nanoparticles encapsulated with SiO₂ via the facile method and further functionalized by the Eu³⁺ and Tb³⁺ co-doped Y₂O₃ has not been reported, these magnetic fluorescent multifunctional nanocomposites motivate the authors to pursue the present work.

2. Experimental section

2.1. Materials

Ferrous chloride FeCl₃·6H₂O, ferrous chloride tetrahydrate (FeCl₂·4H₂O), HNO₃ (analytical reagent, A. R., Beijing Fine Chemical Company, China), urea, polyethylene glycol-10 000 (A.R., Beijing Fine Chemical Company, China) ethanol (A.R., Beijing Fine Chemical Company, China), TEOS, ammonia aqueous (25 wt%) (Shanghai Chem. Reagent Co.), Y₂O₃ (C 99.99%, AR), Eu₂O₃ (C 99.99%, AR), Tb₄O₇ (C 99.99%, AR) were all purchased from the Beijing Chemical Reagent Company. All chemicals were of analytical grade and were used as received without further purification. Deionized water was used for all experiments.

2.2. Synthesis of PEG-coated magnetic Fe₃O₄ nanoparticles

PEG-coated magnetic Fe₃O₄ nanoparticles were prepared by the co-precipitation method as described before with modifications. 2.36 g of FeCl₃·6H₂O and 0.86 g of FeCl₂·4H₂O (2 : 1 molar ratio) were added to 40 mL of deionized water and stirred under nitrogen. A total of 5 mL of ammonium hydroxide was added quickly at 90 °C under rapid mechanical stirring. Due to the large surface area-to-volume ratios and magnetic dipole–dipole attraction between magnetic nanoparticles, the magnetic nanoparticles aggregated readily. To get very stable and highly water soluble magnetic nanoparticles, PEG was used as a coating polymer during the co-precipitation process. It is known that PEG is a hydrophilic and biocompatible polymer and its very high surface mobility leads to high steric exclusion. Therefore, 5 mL of PEG aqueous solution was added dropwise, and the suspension was kept at 90 °C for 1 h. The resultant black magnetite nanoparticles were separated magnetically, and washed with deionized (DI) water and ethanol several times, and stored in ethanol.

2.3. Synthesis of Fe₃O₄@SiO₂ nanoparticles

The Fe₃O₄@SiO₂ nanoparticles were prepared via a Stöber sol–gel process. In a typical procedure, 100 mg of obtained Fe₃O₄ particles were dispersed in the mixture solution of 40 mL of ethanol, 10 mL of deionized water by ultrasonication for 30 min. After adding 1.5 mL ammonia water (25 wt%), tetraethyl-orthosilicate (TEOS, 3 mL) was added to the reaction solution. After stirring at 40 °C for 6 h, the obtained particles were separated and washed with ethanol and deionized water.

2.4. Synthesis of Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Tb³⁺ nanocomposites

In a typical procedure, 10 mg of magnetic Fe₃O₄@SiO₂ nanoparticles were dispersed in 50 mL of DI water containing 1.8 M urea and 5 mM Ln(NO₃)₃ (Ln = Y, Eu, Tb; Eu³⁺, Tb³⁺ doping concentration was 5 mol%). The mixture was sonicated for 20 min and then heated to 90 °C under vigorous mechanical stirring. After 4 h, the resultant precursor was separated with a magnet, thoroughly washed with ethanol and water several times, and further dried at 100 °C overnight. Finally, the precursor particles were calcinated at 700 °C for 2 h for complete conversion to the oxide. For comparative tests, we synthesized Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ and Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺ by the above method. We describe synthesis routes of multifunctional nanocomposites with core–shell structure in Scheme 1.

Scheme 1 Illustration for the formation process of the spherical Fe₃O₄@SiO₂@Y₂O₃:Eu, Tb nanocomposites.

2.5. Characterization

The purities of all the samples were checked by X-ray diffraction measurements at room temperature using CuKα radiation (kα = 1.54059 Å). The morphology was characterized with Transmission electron microscopy (TEM, JEOL Jem-1200EX). The vibrating sample magnetometer (VSM) was used for magnetic measurement at room temperature, and a spectrophotometer (Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source) was used for the photoluminescent (PL) measurement.

3. Results and discussion

TEM images of the prepared samples are represented in Fig. 1.The TEM images confirmed that spherical Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 5% (a), Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺ 4% (b), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 4%, Tb³⁺ 4% (c and d) nanoparticles can be obtained by homogeneous precipitation method. It can be observed from Fig. 1(a) and (b) that the Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺ are all uniform spherical particles with the size in the range of 480–520 nm. The nanocomposites contain a plurality of magnetic micro-goal Fe₃O₄@SiO₂. TEM images of the resulting Fe₃O₄@ SiO₂@Y₂O₃:Eu³⁺, Tb³⁺ composite particles are shown in Fig. 1(c) and (d). The core–shell structural microspheres, like Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ particles, are still uniform and spherical particulates with an average diameter of ca. 450 nm and they have rough surfaces.

Fig. 1 TEM images of Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 5% (a), Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺ 4% (b), Fe₃O₄@SiO₂@ Y₂O₃:Eu³⁺ 4%, Tb³⁺ 4% (c and d).

In order to investigate the structure and composition of the nanocomposites, XRD was employed to analyze the samples. As shown in Fig. 2, the positions of all diffraction peaks match well with the standard JCPDS 89-0688 of Fe₃O₄ powder, which indicates that the Fe₃O₄ particles are single phase and belong to the cubic system. For Fe₃O₄@SiO₂, the broad band at 2θ = 22° to 25.0° can be assigned to the amorphous SiO₂ shell (JCPDS no. 29-0085). From Fig. 2(c-e), we can see that all of the peaks can be indexed to the pure cubic phase of Y₂O₃. The XRD pattern of Y₂O₃:Eu³⁺ crystals indicates five reflection peaks of cubic structure at 2θ = 20.59°, 29.12°, 33.78° 48.50° and 57.63° corresponding to (211), (222), (400), (440) and (622). They are in good accordance with the JCPDS card no. (JCPDS 88-1040), which indicates that there is no impurity phase among all the samples. So we can judge from these experimental results that Eu³⁺, Tb ³⁺ and Eu³⁺, Tb³⁺ codoping ions have been introduced into the Y₂O₃ lattice, and do not cause any change in the cubic structure. It was revealed that the introduction of Eu³⁺, Tb³⁺ ions did not influence the crystal structure of the phosphor matrix.

Fig. 2 XRD patterns of Fe₃O₄ (a), Fe₃O₄@SiO₂ (b), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 5% (c), Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺ 4% (d), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 4%, Tb³⁺ 4% (e).

For a comparative study of the physical and optical properties, samples with different concentrations of Eu³⁺ and Tb³⁺ codoping ions in Y₂O₃ host lattice were synthesized and characterized. Fig. 3 indicates that the XRD patterns of synthesized particles belongs to a cubic phase Y₂O₃. No additional peaks of other phases have been found, indicating the formation of a purely Y₂O₃ cubic phase. It can also be seen that the diffraction peaks of the samples are very sharp and strong, revealing that the final product with high crystallinity can be synthesized by this method. Other diffraction peaks can be attributed to the standard Fe₃O₄ powder diffraction data, and did not change significantly.

Fig. 3 XRD patterns of Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 4%, Tb³⁺ 4% (a), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 5.6%, Tb³⁺ 4% (b), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 0.8%, Tb³⁺ 4% (c), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 2.4%, Tb³⁺ 4% (d).

Fig. 4 (left) shows the excitation and emission spectra of Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ with different Eu³⁺ concentrations. The excitation spectrum was monitored at 610 nm, while the emission spectrum was measured with 254 nm as the excitation wavelength. The excitation spectrum (Fig. 4 left) consists of a broad band with a maximum at 254 nm and some sharp peaks in the longer wavelength region, which is attributed to the CTB from O²⁻ to Eu³⁺. A series of sharp emission lines lying between 500 nm and 700 nm were observed due to transitions from the excited ⁵D₀ to the ⁷F_J (J = 0–3) levels of Eu³⁺ ions. The origin of these transitions (electric dipole or magnetic dipole) from emitting levels to terminating levels depends on the location of Eu ion in the Y₂O₃ lattice, and the type of transition is determined by selection rules. The peaks from ⁵D₀–⁷F₂ (electric-dipole transition) are stronger than those from ⁵D₀–⁷F₁ and ⁵D₀–⁷F₃ (magnetic-dipole transition). As we known that the ⁵D₀–⁷F₂ transition of Eu³⁺ belongs to hypersensitive transitions, which is strongly influenced by crystal field outside surroundings. When the Eu³⁺ is located at a low symmetry local site (without an inversion center), this emission transition is often dominated in their emission spectra and forced-electric dipole transition is often dominant in the emission spectrum. So the strongest ⁵D₀–⁷F₂ transition (610 nm) and nearly all other features in the spectrum are due to the Eu³⁺ on C₂ site. The performance of luminescent materials is influenced by the doping concentration and it is very important to determine the optimum concentration for luminescence applications. Fig. 4 (left inset) is the curve of PL intensity of Y₂O₃:Eu³⁺, which varies with Eu³⁺% (1–7%). The emission intensity increases with increasing in Eu³⁺ concentration up to 5 mol% and then decreases due to concentration quenching phenomena as per the following cross-relaxation mechanism (Fig. 8). This picture reflects that the best doping value of Eu³⁺ is 5%. Fig. 4 (right) shows the excitation and emission spectra of Fe₃O₄@ SiO₂@ Y₂O₃:Tb³⁺ with different Tb³⁺ concentration. The excitation spectra consist of a broad band with a maximum at about 307 nm due to the 4f⁸–4f⁷5d¹ transition of the Tb³⁺ ion. The emission spectrum of Y₂O₃:Tb³⁺ is composed of several sharp lines in the region 470–650 nm (Fig. 4 right) corresponding to the ⁵D₄–⁷F_J transitions where J = 3, 4, 5 and 6. The strongest emission occurs at approximately 545 nm due to the ⁵D₄–⁷F₅ characteristic transition of green emission for Tb³⁺. The relative emission intensity of 545 nm is due to the concentration of Tb³⁺ (Fig. 4). It is found that the relative intensity of 545 nm emission firstly increases up to 4 mol% for Tb³⁺ concentration, and then decreases when Tb³⁺ concentration continuously increases. Thus, the optimum concentration for Tb³⁺ in Y₂O₃ host is 4%.

Fig. 4 Excitation and emission spectra of (left) Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ with different Eu³⁺ concentration: (a) 1%; (b) 3%; (c) 5%; (d) 7%. (Right) Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺ with different Tb³⁺ concentration: (a) 2%; (b) 4%; (c) 6%; (d) 8%.

Fig. 5 shows the excitation (left, λ_em = 610 nm) and emission (right, λ_ex = 266 nm) spectra of the Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ Tb³⁺ microspheres calcined at 700 °C for 2 h as a function of the europium concentration (with a fixed Tb³⁺ concentration of 4 mol%). The PLE analysis of the samples is reported in Fig. 5. From Fig. 5a it is evident that when increasing the Eu content, the intensity of the bands decreases, even if the Tb concentration remains constant, while the excitation spectra collected at 610 nm, Fig. 5 (right), shows an increase of the intensity when increasing the Eu³⁺ content. In the emission spectra (shown in Fig. 5), there are two dominant bands, the green band (545 nm) corresponds to the energy transition ⁵D₄–⁷F₆ and the red band (610 nm) corresponds to the ⁵D₀–⁷F₂ transition. As the Eu³⁺ concentration (≤4 mol%) increases, the intensities of the green band decreases and the red emission increases. The value of Eu³⁺ concentration reaches or exceeds 5.6 mol%, the red emission is quenched. It indicates that the Tb³⁺ ions not only absorb UV radiation and efficiently transfers its energy to Eu³⁺, but also acts as activators and energy acceptors while the concentration of the Eu³⁺ is lower than 5.6 mol%, it also suggests that the energy transfer efficiency from Tb³⁺ to Eu³⁺ depends strongly on their doping concentrations.

Fig. 5 Excitation (left) and emission (right) spectra of Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 0.8%, Tb³⁺ 4% (a), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 2.4%, Tb³⁺ 4% (b), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 4%, Tb³⁺ 4% (c), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ 5.6%, Tb³⁺ 4% (d).

Fig. 6 illustrates the CIE chromaticity diagram of the Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Tb³⁺ codoped nanocomposites for different Tb³⁺/Eu³⁺ molar ratios. The CIE (1931) xy chromaticity coordinates²³ for the fluorescence of Eu³⁺, Tb³⁺ codoped Y₂O₃ under various excitation wavelength conditions can be calculated using the following formulae , , , where x, y, and z are the three tristimulus values. The tristimulus values for the colour with a spectral power distribution P(λ) are given by.²³

where λ is the wavelength of the equivalent monochromatic light. x̄(λ), ȳ(λ) and z̄(λ) are the three colour-matching functions. The colour coordinates are calculated to be (0.5123, 0.4814) and (0.2611, 0.4870) corresponding to 254 and 307 nm UV excitation conditions as marked in Fig. 6. The colour coordinates traverse a wide range from green to the extreme red region on varying the Tb³⁺/Eu³⁺ molar ratios. The desirable white fluorescence is likely to be achieved by adding a green component to adjust the fluorescence colour. Near white light was obtained from the phosphor for a Tb³⁺/Eu³⁺ molar ratio of 1 : 5 with good CRI for solid-state lighting applications. The values of CIE parameters for the different codoped phosphors are summarized in Table 1.

Fig. 6 CIE chromaticity coordinates of Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ (a), Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺ (b), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Tb³⁺, (c) 0.8% Eu³⁺, 4%Tb³⁺, (d) 2.4% Eu³⁺, 4%Tb³⁺, (e): 4%Eu³⁺, 4%Tb³⁺, (f): 5.6% Eu³⁺, 4%Tb³⁺.

Table 1 CIE parameters

(Y1−x−yTbxEuy)2O3	Colour coordinates		Em (color)
x : y	x	y
1 : 5	0.3338	0.3589	Yellow/white
3 : 5	0.3264	0.3196	Orange red
1 : 1	0.3627	0.3250	Red
7 : 5	0.3315	0.3114	Red

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Magnetic measurement shows that the saturation magnetization values of Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Tb³⁺ and Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺ is 5.90, 1.43, and 1.02 emu g⁻¹, respectively. It should be noted that the multifunctional nanocomposite still shows good magnetism, which suggests its suitability for magnetic separation and targeting. The magnetic hysteresis loops confirm the magnetic features of all the samples (Fig. 7). Moreover, the inset of Fig. 7 shows fast response (60 s) to the external magnetic field because their good saturation magnetization values can quickly respond to the external magnetic field and quickly redisperse once the external magnetic field is removed. The saturation magnetization of Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Tb³⁺ reaches a saturation moment of 1.43 emu g⁻¹, which is smaller than that of the Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ due to the decreased proportion of Fe₃O₄ in the nanocomposites. The activator Tb³⁺ varies the remanence and coercivity. The decrease of saturation magnetism leads to the decrease of the remanence. On the one hand the increasing coercivity demonstrates the reinforced magnetic anisotropy, which can be attributed to the Tb³⁺ disturbing the original stable crystal lattice arrangement. On the other hand, the energy from Tb³⁺ to the Eu³⁺ results in the intrinsic characters variety.

Fig. 7 The magnetic hysteresis loops of Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺ (a), Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Tb³⁺ (b), Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺ (c) at 300 K.

Fig. 8 Energy level scheme representing the energy transfer and cross-relaxation mechanism in the Y₂O₃ as synthesized powders, activated with Tb³⁺ and Eu³⁺. The dashed lines represent cross-relaxation processes.

Conclusion

We synthesize Fe₃O₄@SiO₂@Y₂O₃:Eu³⁺, Tb³⁺ nanocomposites by a simple homogeneous precipitation method. The results show that the tuning of emission colour is possible by the change in the Tb³⁺/Eu³⁺ molar ratio of Fe₃O₄@SiO₂@Y₂O₃:Tb³⁺, Eu³⁺. When Tb³⁺/Eu³⁺ is 1 : 5, the nanocomposites may be white fluorescent. Furthermore, we give a possible mechanism of energy transfer from Tb³⁺ to Eu³⁺. The above conclusion has been examined and verified by various characterization techniques. Therefore, the multifunctional nanocomposites are expected to develop many potential applications in biomedical fields.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (NSFC).

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