Electrospinning fabrication and luminescence of La₃(BWO₉):Tb³⁺ nanofibers

Abstract

La₃(BWO₉):Tb³⁺ nanofibers were fabricated by an electrospinning process. The XRD patterns of the obtained fibers indicate that the incorporation of Tb³⁺ ions will not change the phase of La₃(BWO₉). The excitation spectrum monitored at 547 nm consists of excitation bands originating from ¹T₁ → ¹A₁ transition of O²⁻ → W⁶⁺ in the WO₆ group and ⁴f⁸ → ⁴f₇5d¹/4f⁸ transitions of Tb³⁺. Upon the excitation at 362 nm, La₃(BWO₉):Tb³⁺ fibers show emission bands originating from the ⁵D₄ → ⁷F_j (j = 6, 5, 4 and 3) transitions of Tb³⁺. The decay characteristics of La₃BWO₉:Tb³⁺nanofibers indicate that Tb³⁺ ions occupy one site in the host lattice.

1 Introduction

The electricity consumption in domestic buildings occupies higher and higher percentage in whole electricity consumption. As a result, white light emitting diodes (WLEDs), having advantages of long lifetime, high luminescence efficiency, low power consumption and environment friendly characteristics, attract considerable research for solid-state lighting applications [1, 2]. In the fabrication of WLEDs, the combination of green and red emitting phosphors with a blue chip leads to the high color rendering and high efficiency [3]. Due to the intense and sharp luminescence originating from the f–f electronic transitions, rare earth ions are used widely as activators in phosphors. Among rare earth ions, Eu³⁺ [4–10] and Tb³⁺ [11–18] ions often act as activators to obtain red and green emissions. The green emission of Tb³⁺ mainly comes from the ⁵D₄ → ⁷F_j (j = 6, 5, 4 and 3) transitions.

Borate and tungstate phosphors for WLED applications have attracted much attention because of the broad and intense ligand-to-metal charge transfer band in the ultraviolet or near ultraviolet region, which is expected to capture the emission from a GaN-based LED in this range [10]. The mixed-anion rare earth compounds in combination with borate and tungstate have been reported in WO₃–Ln₂O₃–B₂O₃ system (Ln = rare earth ion), such as Gd₄B₂WO₁₂ [19], Eu₃BWO₉ [20] and LaBWO₆ [9]. In this work, we report on the luminescent properties of La₃(BWO₉):Tb³⁺ nanofibers fabricated by an electrospinning method. Nanofiber-phosphors have obvious advances owing to their anisotropy, large width-thickness ratio and length-diameter ratio, unique optical, electrical and magnetic properties [21]. Electrospinning technology has been extensively explored as a simple and versatile method for forming inorganic superfine nanofibers using polymer/inorganic composite as the precursor. The morphology of fine products fabricated by electrospinning can be controlled by adjusting experimental parameters, such as the precursor solution viscosity, the structure of spinneret, voltage and the distance between the spinneret and the collector.

2 Materials and methods

A series of La₃(BWO₉):xmol%Tb³⁺ (x = 2, 3, 4, 5 and 6) nanofibers were fabricated by an electrospinning process. La₂O₃ (99.99 %), Tb₄O₇ (99.99 %), H₃BO₃ (99.99 %), (NH₄)₁₀H₂(W₂O₇)₆ (99.95 %) were used as raw materials. Citric acid (99.5 %) and polyvinyl pyrrolidone (PVP, M_w = 90,000) were used as additives. All chemicals were used directly without further purification.

Spinning solution was prepared firstly for fabrication of La₃(BWO₉):Tb³⁺ nanofibers. In a typical preparation, 1.3 mmol of La₂O₃ and 0.1 mmol of Tb₄O₇ were dissolved into 10 mL of hot aqueous nitric acid solution to form the solution A. Then, 1 mmol of H₃BO₃ and 0.083 mmol of (NH₄)₁₀H₂(W₂O₇)₆ were dissolved into 10 mL of deionized water containing citric acid to form the solution B. The molar ratio of La + Tb to citric acid was kept at 1:2. Precursor solution was obtained by mixing solutions A and B under continuous stirring. Finally, 3.0 g of PVP was added to the precursor solution and mixed to form a homogeneous spinning solution. During the process of electrospinning, the spinning solution was loaded into a plastic syringe with a spinneret, and the angle between spinneret and horizon was fixed to 10° for fabricating nanofibers. A flat iron net was used as the collector putting about 18 cm away from the spinneret. The flow rate was kept at 0.5 mL/h. The voltage of direct current adjusted to 15 kV was applied between the spinneret and the collector to generate the stable continuous PVP-based composite nanofibers. The obtained fibers were dried in an oven at 120 °C for 2 h and then heated at 800 °C for 5 h.

The X-ray powder diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu Kα radiation (λ = 0.15405 nm). The morphology and structure of the samples were inspected by an FESEM-4800 field emission scanning electron microscope (SEM, Hitachi). The luminescence was performed on a HitachiF-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The decay curves were obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as excitation source (Continuum Suncite OPO).

3 Results and discussion

The phase of La₃(BWO₉):xmol%Tb³⁺ (x = 2, 3, 4, 5 and 6) after the heating at 800 °C are confirmed by XRD patterns. La₃(BWO₉) crystallizes in a hexagonal structure with the P6₃ space group. W atoms in the structure are surrounded by six oxygen atoms, forming a trigonal-prismatic coordination, which shares edges with three rare-earth polyhedrons. La atoms adopt ninefold coordination, of which six O atoms are derived from three WO₆ groups by sharing one edge with a WO₆ trigonal prism and the other three O atoms from BO₃ groups. LaO₉ polyhedrons connect each other by sharing the corner O atoms of WO₆ and BO₃ groups, forming the framework of La₃BWO₉. As shown in Fig. 1, the XRD patterns of all samples are identical and match well with that of the pure La₃BWO₉ with a hexagonal structure (JCPDS No. 77-1040). No other impurity peaks can be detected. These results suggest that Tb³⁺ ions have doped into the La₃BWO₉ host lattice entirely and form the complete solid solution. Due to the same valence state and similar ionic radii, Tb³⁺ ions will substitute La³⁺ ions in La₃BWO₉. The diffraction peaks of these samples are broadened remarkably, which reveals their nanocrystalline structure. The average crystallite sizes of samples can be calculated by the Scherrer equation:

$$ {\text{D}} = \frac{{{\text{K}}\uplambda}}{{\cos\uptheta\sqrt {\upbeta^{2} -\upbeta_{0}^{2} } }} $$

(1)

where D is the average grain size, K = 0.9 is a characteristic for spherical objects, λ is the X-ray wavelength, θ and β are the diffraction angel and full-width at half-maximum of an observed peak. On the basis of diffraction peak of (201), the average grain sizes are calculated to be about 20 nm.

Fig. 1

XRD patterns of La₃(BWO₉):Tb³⁺ nanofibers and standard XRD data of La₃(BWO₉)

To show the morphology and size of the samples, the SEM images are obtained. Figure 2 shows the SEM images of La₃(BWO₉):5 mol%Tb³⁺ after the heating at 800 °C. As shown in Fig. 2a, a large number of uniform fibers with diameters about 150 nm are obtained. The enlarged SEM image (Fig. 2b) shows that these fibers are rough and comprised of nanoparticles with sizes about 20 nm. The rough surface is induced by the removal of the residual organic solvent and PVP, plus the agglomeration of nanoparticles to form nanofibers during the electrospinning and heating processes. All samples have the same morphology and size. The morphology and diameter of the electrospun samples depend on the intrinsic properties of the solution and the operational conditions. In order to obtain fibers with perfect uniform morphology, the key is searching a balance point of various electrospinning parameters, such as the volume ratio of deionized water to citric acid, the weight percentage of PVP, the spinning rate, the strength of the electric field, and the distance between the spinneret and the collector.

Fig. 2

SEM images of La₃(BWO₉):5 mol%Tb³⁺ nanofibers

Figure 3 shows the excitation and emission spectra of La₃(BWO₉):5 mol%Tb³⁺ nanofibers. The excitation spectrum consists of two parts by monitoring the emission at 547 nm. The broad band maximized at 296 nm is ascribed to charge transfer from the ligand to the metal, derived from the ¹T₁ → ¹A₁ transition of O²⁻ → W⁶⁺ in the WO₆ group [10] and the ⁴f⁸ → ⁴f₇5d¹ transition of Tb³⁺ ions [22]. Those excitation bands in the range of 300-400 nm come from the 4f⁸ → 4f⁸ transitions from the ground ⁷F₆ level to excited ^2S+1L_J of Tb³⁺ ions [23]. There is a weakness of the interconfigurational ⁴f⁸ → ⁴f⁷5d¹ transition comparing to the 4f⁸ → 4f⁸ transitions although the former is parity allowed and thus usually much stronger, because that the ⁴fⁿ → ⁴fⁿ⁻¹5d¹ transitions within the conduction band will lose the superior intensity as has been shown for the Ce³⁺-doped Y₃Al₅O₁₂ single crystals [24]. Upon the excitation at 362 nm, four typical emission peaks at 489, 547, 589 and 623 nm originating from ⁵D₄ → ⁷F_j (j = 6, 5, 4 and 3) transitions of Tb³⁺ ions can be observed. Among them, the green emission band at 547 nm originating from the ⁵D₄ → ⁷F₅ transition is strongest.

Fig. 3

Excitation and emission spectra of La₃(BWO₉):5 mol%Tb³⁺ nanofibers

The dependence of emission intensity on Tb³⁺ doping concentration is shown in Fig. 4. Upon the excitation of 362 nm, La₃(BWO₉):xmol%Tb³⁺ (x = 2, 3, 4, 5 and 6) nanofibers show emission bands originating from ⁵D₄ → ⁷F_j (j = 6, 5, 4 and 3) transitions of Tb³⁺ ions and the increasing concentrations of Tb³⁺ ions bring no obvious alteration in the shapes of emission spectra. The almost complete absence of emission bands originating from ⁵D₃ → ⁷F_j transitions means that the emission from the ⁵D₃ level is quenched and the excitation energy ends to the ⁵D₄ level instead. The weakness of the ⁴f⁸ → ⁴f⁷5d¹ transitions in the excitation spectrum offers a plausible explanation for the absence of ⁵D₃ emission: the ⁵D₃ level locates in the conduction band of La₃BWO₉, and the ⁵D₃ emissions are quenched to the lowest emitting level, ⁵D₄, via the conduction band. The emission intensity increases with the increasing concentrations of Tb³⁺ and reaches the maxima for La₃(BWO₉):5 mol%Tb³⁺, then decreases with the further increase of Tb³⁺ concentration. The increases of emission intensity with the increasing concentrations of Tb³⁺ is attributed to a large number of luminescent centers up to a critical concentration, beyond which a decrease in the emission intensity occurs due to the concentration quenching effect. Quenching can originate from efficient energy transfer among the dopant ions, because of the matching of their energy levels, followed by the eventual energy transfer from the dopant ions to a defect where non-radioactive decay occurs. The occurrence of nonradiative energy transfer may be caused by exchange interaction, radiation reabsorption, or multipole–multipole interaction [25]. The exchange interaction is a short-distance interaction and the typical critical distance is about 5 Å. The critical transfer distance (R_c) can be calculated by the formula of

$$ {\text{R}}_{\text{C}} \approx 2\left( {3{\text{V/}}4\uppi{\text{XN}}} \right)^{{\frac{1}{3}}} $$

(2)

where X is the critical concentration of Tb³⁺, V is the volume of the unit cell, and N is the number of available sites of the dopant in the unit cell. For La₃BWO₉:Tb³⁺nanofibers, V = 371.05 Å, X = 0.05 and N = 6, the critical transfer distance of Tb³⁺ in La₃BWO₉ is calculated to be 13.32 Å. And the mechanism of radiation reabsorption comes into effect only when there is a broad overlap of the emission spectrum of the sensitizer and the excitation spectrum of the activator. Since the critical transfer distance is larger than 5 Å and there is no overlap between the excitation and emission spectra of La₃BWO₉:Tb³⁺, the energy transfer process should be controlled by the multipole–multipole interaction.

Fig. 4

Emission spectra of La₃(BWO₉):Tb³⁺ nanofibers

To obtain additional information on the luminescent properties of the Tb³⁺ ions in La₃(BWO₉) host, the decay curves of the Tb³⁺ emission at 547 nm corresponding to the ⁵D₄ → ⁷F₅ transition upon 362 nm excitation for La₃(BWO₉):xmol%Tb³⁺ nanofibers are measured, as shown in Fig. 5. All of curves demonstrate almost perfect single-exponential function:

$$ {\text{I}} = {\text{A}}\exp \left( { - {\text{t}}/\uptau} \right) $$

(3)

where I is the emission intensity at time of t, A is a constant, t is the time, τ is the decay time. This suggests that Tb³⁺ ions occupy one site in the host lattice. The decay times for La₃(BWO₉):xmol%Tb³⁺ (x = 2, 3, 4, 5 and 6) are 2.896, 2.821, 2.608, 2.448, 2.365 and 2.290 ms, respectively.

Fig. 5

Decay curves of La₃(BWO₉):Tb³⁺ nanofibers

4 Conclusion

La₃BWO₉:Tb³⁺nanofibers have been fabricated via an electrospinning process. The phase structure, size and morphology and luminescent properties have been studied as a function of Tb³⁺ concentration. The results suggest that the Tb³⁺ concentration in La₃BWO₉ host will not change the phase structure and morphology of La₃BWO₉:Tb³⁺nanofibers, but has obvious influence on the luminescent properties. The emission intensity increases with the increasing concentrations of Tb³⁺ and reaches the maximum for La₃(BWO₉):5 mol%Tb³⁺, then decreases with the further increase of Tb³⁺ concentration because of the concentration quenching effect. The decay characteristics of La₃BWO₉:Tb³⁺nanofibers indicate that Tb³⁺ ions occupy one site in the host lattice.