Electrospinning-derived Tb₂(WO₄)₃:Eu³⁺ nanowires: energy transfer and tunable luminescence properties

Abstract

One-dimensional Tb₂(WO₄)₃ and Tb₂(WO₄)₃:Eu³⁺ nanowires have been prepared by a combination method of sol–gel process and electrospinning. X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL), low voltage cathodoluminescence (CL) and time-resolved emission spectra as well as kinetic decays were used to characterize the resulting samples. The as-obtained precursor samples present fiber-like morphology with uniform size, and Tb₂(WO₄)₃ and Tb₂(WO₄)₃:Eu³⁺ nanowires were formed after annealing. Under ultraviolet excitation and low-voltage electron beams excitation into WO₄²⁻and the f–f transition of Tb³⁺, the Tb₂(WO₄)₃ samples show the characteristic emission of Tb³⁺ corresponding to ⁵D₄–⁷F_{6, 5, 4, 3} transitions due to an efficient energy transfer from WO₄²⁻ to Tb³⁺, while Tb₂(WO₄)₃:Eu³⁺ samples mainly exhibit the characteristic emission of Eu³⁺ corresponding to ⁵D₀–⁷F_0, 1, 2 transitions due to an energy transfer occurs from WO₄²⁻ and Tb³⁺ to Eu³⁺. The increase of Eu³⁺ concentration leads to the increase of the energy transfer efficiency from Tb³⁺ to Eu³⁺. The PL color of Tb₂(WO₄)₃:x mol% Eu³⁺ phosphors can be tuned from green to red easily by changing the doping concentration (x) of Eu³⁺, making the materials have potential applications in fluorescent lamps and color display fields.

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1. Introduction

Inorganic phosphor materials based on rare earth (RE) compounds and RE ion-doped compounds have been widely used in the fields of high-performance luminescent devices, catalysts, and other functional materials based on their electronic, optical, and chemical characteristics arising from their 4f electrons.¹ Recently, the use of phosphors represents a fast growing industry due to the wide range of applications: light-emitting diodes (LEDs), cathode ray tubes (CRTs), field emission displays (FEDs), vacuum fluorescent displays (VFDs), plasma display panels (PDPs), and X-ray imaging scintillators.² Rare earth oxide phosphors have gained interest due to their better thermal and chemical stability as well as their environmental friendliness, which are currently used in the screens of flat-panel displays (FPDs), VFDs, and FEDs.³ Therefore, more attention has been paid to the improvement of original rare earth oxide phosphors and (or) developing new rare earth oxide phosphors materials with respect to the wide possible applications.

It is well known that the energy transfer plays an important role in luminescent materials both from theoretical and practical points of view.⁴ It has been recognized that the luminescence intensities of various rare earth ions can be enhanced or quenched by the energy transfer from other codoped rare earth ions.^4a,5 For example, the emission intensity of Tb³⁺ is greatly enhanced by an energy transfer from Ce³⁺ in LaPO₄:Ce³⁺,Tb³⁺ phosphor, leading it to be an efficient luminescent material for fluorescent lamps.^4a,c It has also been demonstrated that as a result of the energy transfer phenomena it is possible to reduce the threshold energy of laser oscillation in some solid laser materials.⁶ As the most frequently used activator ions in luminescent materials, the Eu³⁺ and Tb³⁺ mainly show emissions due to transitions of ⁵D₀–⁷F_J (J = 1, 2, 3, 4) in the red region and ⁵D₄–⁷F_J (J = 6, 5, 4, 3) in the green region, respectively.^1c,7 Although the energy transfer phenomena between different rare earth ions have been extensively investigated, little attention has been paid to the case between Tb³⁺ and Eu³⁺, and so far the energy transfer phenomenon from Tb³⁺ to Eu³⁺ have been observed and investigated only in limited hosts.⁸

To obtain desired one-dimensional (1D) RE ions doped phosphors, electrospinning is intensively used because it has been developed since 1934 for the synthesis of 1D nanomaterials. Electrospinning is an effective and simple method for preparing a rich variety of materials,⁹ such as polymers, inorganic and hybrid (organic-inorganic) compounds. On the other hand, the sol–gel process has been proven as an efficient way to produce nanoparticles¹⁰ and nanocoatings¹¹ of metal oxide. Sol–gel techniques can be employed to prepare precursor solutions, which have been used to deposit coatings by spinning and dipping.¹² 1D materials fabricated by a combination method of sol–gel process and electrospinning have become important for their exceptionally long length, uniform diameter, diverse composition and large surface areas, which can be applied in biomedical fields, reinforced composites, catalyst supports, sensors, electronic and optical devices, as well as sacrificial templates.¹³

Metal tungstates are an important family of inorganic material that have extensive potential applications in various fields,¹⁴ such as photoluminescence, fluorescent lamps, microwave applications, optical fibers, scintillator materials, X-ray intensified screen. Rare earth tungstates Ln₂(WO₄)₃ (Ln = lanthanides) have been proved to be useful host lattices for the luminescence of rare earth ions.¹⁵Ln₂(WO₄)₃ can be divided into two groups with different structures governed by the ionic radius of Ln³⁺: crystallising in monoclinic for Ln = La–Dy with larger size and orthorhombic for Ln = Ho–Lu with smaller sizes. To the best of our knowledge, there have been few reports about synthesis and luminescent properties of Tb₂(WO₄)₃ and/or RE³⁺-doped Tb₂(WO₄)₃ materials. Especially, there have no reports concerning the preparation of Eu³⁺-doped Tb₂(WO₄)₃ 1D phosphors viaelectrospinning method, and the corresponding energy transfer from WO₄²⁻ and Tb³⁺ to Eu³⁺ in Tb₂(WO₄)₃ host has not been realized and reported. Accordingly, in this paper, we report the preparation of 1D wirelike Tb₂(WO₄)₃ and Tb₂(WO₄)₃:Eu³⁺ phosphors through a combination method of sol–gel process and electrospinning, as well as their photoluminescent and cathodoluminescent properties. Moreover, we investigate the energy transfer property from WO₄²⁻ and Tb³⁺ to Eu³⁺ in Tb₂(WO₄)₃, which results in the tunable emission colors from such phosphor materials.

2. Experimental

Chemicals and materials

Polyvinylpyrrolidone (PVP, M_w = 1 300 000) was purchased from Aldrich, and ammonium (meta) tungstate hydrate (H₂₆N₆O₄₁W₁₂·18H₂O) was purchased from Fluka. Citric acid monohydrate C₆H₈O₇·H₂O (≥99.5%, A. R.), nitric acid HNO₃ (A. R.), hydrogen peroxide (≥30%, A. R.) and ethanol were all purchased from Beijing Fine Chemical Company. Tb₄O₇ and Eu₂O₃ (99.99%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry. Tb(NO₃)₃ and Eu(NO₃)₃ were prepared by dissolving Tb₄O₇ and Eu₂O₃ in dilute nitric acid HNO₃ containing H₂O₂, and evaporating the water in the solutions by heating. All the initial chemicals in this work were used without further purification.

Preparation

1D Tb₂(WO₄)₃:x mol% Eu³⁺ (x = 0–5) samples were prepared by a method of sol–gel process and electrospinning. The doping concertrations of Eu³⁺ are 0.3–5 mol % of Tb³⁺ in Tb₂(WO₄)₃. The stoichiometric amounts of Tb(NO₃)₃, Eu(NO₃)₃, and H₂₆N₆O₄₁W₁₂·18H₂O were dissolved in deionized water, then the solution was mixed with a water–ethanol solution (final volume ratio of water to ethanol is 1 : 4) containing citric acid as a chelating agent for the metal ions. The molar ratio of metal ions to citric acid was 1 : 2. A certain amount of polyvinylpyrrolidone (PVP, M_w = 1 300 000, Aldrich) was added to adjust the viscoelastic behavior of the solution (the weight percentage of PVP is 7% in the water–ethanol solution). The solution was stirred for 4 h to obtain a homogeneous hybrid sol for further electrospinning. The distance between the spinneret (a metallic needle) and collector (a grounded conductor) was fixed at 16 cm and the high-voltage supply was maintained at 15 kV. The spinning rate was controlled at 0.5 mL h⁻¹ by a syringe pump (TJ-3A/W0109-1B, Baoding Longer Precision Pump Co., Ltd, China). The as-prepared hybrid precursor samples were annealed at desired temperature (800 °C) with the heating rate of 2 °C min⁻¹ and held there for 4 h in air.

Characterization

The X-ray diffraction (XRD) patterns of the samples were carried out on a D8 Focus diffractometer (Bruker) with use of Cu KR radiation (λ = 0.15405 nm). The morphology of the samples was inspected using a field emission scanning electron microscope (Philips XL 30). The photoluminescence (PL) measurements were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescence (CL) measurements were conducted in an ultrahigh-vacuum chamber (<10⁻⁸ Torr), where the phosphors were excited by an electron beam at a voltage range of 23–25 kV with different filament currents 90–106 mA, and the spectra were recorded using an F-4500 spectrophotometer. The luminescence decay curves and time-resolved photoluminescence spectra 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 (Continuum Sunlite OPO, pumped by a third harmonic of Nd:YAG, 355 nm). All the measurements were performed at room temperature.

3. Results and discussion

Structure and morphology

Fig. 1 shows the XRD patterns of Tb₂(WO₄)₃, Tb₂(WO₄)₃:5 mol % Eu³⁺ annealed at 800 °C as well as the JCPDS card (No. 28–1290) for Tb₂(WO₄)₃, respectively. Both Tb₂(WO₄)₃ and Tb₂(WO₄)₃:5 mol % Eu³⁺ exhibit very similar X-ray diffraction patterns with monoclinic structure [Fig. 1(a), (b)], so that we can expect their full miscibility. Therefore, the whole solid solution Tb₂(WO₄)₃:x mol % Eu³⁺ from x = 0 to 5 is expected to form. When the Tb³⁺ was substituted by the Eu³⁺ with bigger radius, the corresponding XRD peaks shift to lower angle direction based on the Vegard law. No additional peaks of other phases have been found, indicating that Eu³⁺ has been effectively built into the Tb₂(WO₄)₃ host lattice.

Fig. 1 XRD patterns for Tb₂(WO₄)₃ (a), Tb₂(WO₄)₃:5 mol % Eu³⁺ (b) and the JCPDS card 28–1290 of Tb₂(WO₄)₃ for comparison.

Fig. 2 shows the SEM images of the as-formed precursor Tb₂(WO₄)₃:5 mol % Eu³⁺ hybrid fibers, as well as those samples annealed at high temperature (800 °C), respectively. From the low-magnification SEM micrographs of as-formed precursor [Fig. 2(a)] and those annealed at 800 °C [Fig. 2(c)], it can be seen that the samples consist of uniform fibers with length of several tens to hundred micrometres. It also can be observed that the as-formed precursor fibers are smooth with diameters ranging from 300 to 500 nm from the high-magnification SEM micrograph [Fig. 2(b)]. After being annealed at 800 °C, the fiber diameters decrease greatly due to the decomposition of the organic species and the formation of inorganic phase. The diameters of the nanowires annealed at 800 °C for 4 h become 80 to 150 nm [Fig. 2(d)]. It should be mentioned that the key strategy of sol–gel/electrospinning method to obtain 1D inorganic nanomaterials was to form a solution with viscoelastic behavior similar to that of a conventional polymer solution. In our experiments, the purposes of controlling the volume ratio of water to alcohol and the weight percentage of PVP were to adjust the viscoelastic behavior, making the hybrid solution suitable for further electrospinning.

Fig. 2 SEM images for the as-formed precursor for Tb₂(WO₄)₃:5 mol % Eu³⁺ nanowires: images with low magnification (a) and high magnification (b); and those annealed at 800 °C: images with low magnification (c), high magnification (d).

Photoluminescence properties

The Tb₂(WO₄)₃ sample emits green light ultraviolet (UV) irradiation. Fig. 3 shows the excitation and emission spectra of the Tb₂(WO₄)₃ samples. The excitation spectrum [left of Fig. 3(a) and (b)] of Tb₂(WO₄)₃ monitored with 543 nm emission of Tb³⁺ (⁵D₄–⁷F₅) consists two main features, a broad band with maxima at 280 nm in the range of 200–330 nm, which is ascribed to the charge transfer absorption in WO₄²⁻groups in which an oxygen (O) 2p electron goes into one of the empty tungsten (W) 5d orbital, and the characteristic f–f transition lines within the Tb³⁺ 4f⁸ configuration in the longer wavelength region with maxima at 490 nm (⁵D₄), assigned as the transitions from the ⁷F₆ ground state to the different excited states of Tb³⁺, i.e., 322 nm (⁵D₀), 345 nm (⁵G₂), 356 nm (⁵D₂), 363 nm (⁵L₁₀), 373 nm (⁵G₆) and 382 nm (⁵D₃), respectively.¹³ The presence of the excitation peak of the WO₄²⁻groups in the excitation spectrum of Tb³⁺ indicates that there is an energy transfer from the WO₄²⁻groups to Tb³⁺ ions in the Tb₂(WO₄)₃ nanowires. Upon excitation into WO₄²⁻group at 280 nm, the obtained emission spectrum [Fig. 3(a), right] of Tb₂(WO₄)₃ consists of f–f transition lines within 4f⁸ electron configuration of Tb³⁺, i.e., ⁵D₄–⁷F₆ (488 nm) in the blue region and ⁵D₄–⁷F₅ (543 nm) in the green region, as well as ⁵D₄–⁷F₄ (585 nm), ⁵D₄–⁷F₃ (619 nm) in the red region. The strongest one is located at 543 nm corresponding to ⁵D₄–⁷F₅ transition of Tb³⁺. Compared with the emission of Tb³⁺, the intrinsic blue emission from WO₄²⁻groups is very weak, suggesting that an efficient energy transfer from WO₄²⁻groups to Tb³⁺ has occurred. Excitation into Tb³⁺ at 490 nm yields the emissions spectrum corresponding to the f–f transitions of Tb³⁺, which is also dominated by the green emission ⁵D₄–⁷F₅ transition at 543 nm [Fig. 3(b), right].

Fig. 3 Excitation and emission spectra of the Tb₂(WO₄)₃ upon excitation into WO₄²⁻ (a) and Tb³⁺ (b).

However, the green emission of the Tb₂(WO₄)₃ host material changes greatly when doing Eu³⁺ in it. The Tb₂(WO₄)₃:5 mol % Eu³⁺ sample shows a red emission under UV excitation. Fig. 4 shows the excitation and emission spectra of Tb₂(WO₄)₃:5 mol % Eu³⁺ samples. The excitation spectra [left of Fig. 4] recorded at the 615 nm (⁵D₀–⁷F₂) of Eu³⁺ are composed of the excitation bands of WO₄²⁻ and Tb³⁺, which are identical with the excitation spectrum of Tb³⁺ in Tb₂(WO₄)₃ [Left of Fig. 3], except for some minor f–f transition lines within the Eu³⁺ 4f⁶ configuration (398 nm: ⁷F₀–⁵L₆, 420 nm: ⁷F₀–⁵D₃, and 468 nm: ⁷F₀–⁵D₂). The presence of the excitation bands and lines of WO₄²⁻ and Tb³⁺ in the excitation spectra monitored with Eu³⁺ emission clearly indicates that an energy transfer has occurred from WO₄²⁻ and Tb³⁺ to Eu³⁺ in the Tb₂(WO₄)₃:5 mol % Eu³⁺ samples. Excitation into WO₄²⁻group at 280 nm yields mainly the emission of Eu³⁺ (⁵D₀–⁷F₁ at 592 nm, ⁵D₀–⁷F₂ at 615, ⁵D₀–⁷F₃ at 652 nm, and ⁵D₀–⁷F₄ at 702 nm), together with the emission of Tb³⁺ (488 nm at ⁵D₄–⁷F₆, ⁵D₄–⁷F₅ at 543 nm), as shown in the right of Fig. 4(a). The strongest one is located at 615 nm corresponding to ⁵D₀–⁷F₂ transition of Eu³⁺. Excitation into Tb³⁺ at 490 nm yields the emissions spectrum corresponding to the f–f transitions of Tb³⁺ and Eu³⁺, which is also dominated by the red emission ⁵D₀–⁷F₂ transition at 615 nm [Fig. 4(b), right]. These are further indicative of the energy transfer from WO₄²⁻ and Tb³⁺ to Eu³⁺ in Tb₂(WO₄)₃: 5 mol %Eu³⁺ sample.

Fig. 4 Excitation and emission spectra of the Tb₂(WO₄)₃:5 mol % Eu³⁺ upon excitation into WO₄²⁻ (a) and Tb³⁺ (b).

The kinetic decay curves for the representative emission of Tb³⁺ (543 nm, ⁵D₄–⁷F₅) in Tb₂(WO₄)₃ and Eu³⁺ (615 nm, ⁵D₀–⁷F₂) in Tb₂(WO₄)₃:5 mol % Eu³⁺ were measured, as shown in Fig. 5, respectively. All the decay curves can be well fitted into single-exponential function as I = I₀ exp(−t/τ), and the lifetime τ values can be determined. The fluorescence lifetimes for Tb³⁺ (measured at 543 nm) is 0.22 ms in Tb₂(WO₄)₃ nanowires [Fig. 5(a)]. The fluorescence lifetimes for Eu³⁺⁵D₀ state (measured for ⁵D₀–⁷F₂ at 615 nm) is 0.73 ms in Tb₂(WO₄)₃:5 mol % Eu³⁺ nanowires.

Fig. 5 The decay curves for the luminescence of Tb³⁺ in the Tb₂(WO₄)₃ and Eu³⁺ in the Tb₂(WO₄)₃:5 mol % Eu³⁺.

Fig. 6 shows the emission spectra of Tb₂(WO₄)₃:x mol % Eu³⁺ (x = 0–5) under 490 nm excitation (⁷F₆–⁵D₄ transition of Tb³⁺). In the undoped Tb₂(WO₄)₃ (x = 0), only the characteristic emissions of Tb³⁺ are observed. With the doping of Eu³⁺ (x = 0.3), besides Tb³⁺ emissions, we can also observe the characteristic emissions of Eu³⁺. With the increase of Eu³⁺ concentration, the luminescence of Tb³⁺ begins to decrease, and that of Eu³⁺ increases firstly, both due to the enhancing the probability of energy transfer from Tb³⁺ to Eu³⁺ along with the increase of Eu³⁺ concentration. The luminescence of Eu³⁺ reaches the maximum at x = 5, and then begins to decrease due to the concentration quenching effect (figures not shown due to the limited space). The above results indicate an efficient energy transfer from Tb³⁺ to Eu³⁺. Also, such energy transfer behavior shows that the as-synthesized Tb₂(WO₄)₃:Eu³⁺ is not a mixture of Tb₂(WO₄)₃ and Eu₂(WO₄)₃, but a solid solution, in which Eu³⁺ has successfully incorporated into Tb₂(WO₄)₃ lattice. Otherwise, the Tb³⁺ → Eu³⁺ energy transfer can not occur in the separated phases. The PL color can be tuned from green, brown, orange to red by changing the doping concentration of Eu³⁺ ions due to different energy transfer efficiencies at different Eu³⁺concentration. Fig. 7 shows luminescence photographs of Tb₂(WO₄)₃:x mol % Eu³⁺ nanowires ethanol solutions (0.5 mM) upon excitation with 254 nm from a UV lamp, in which tuning of the emission color from green to red can be realized.

Fig. 6 Emission spectra of the Tb₂(WO₄)₃:x mol % Eu³⁺ samples with different concentration (x = 0–5) under 490 nm excitation.

Fig. 7 Luminescence photographs of Tb₂(WO₄)₃:x mol % Eu³⁺ nanowires dispersed in the ethanol solutions, (a) x = 0, (b) x = 0.3, (c) x = 0.7, (d) x = 3, (e) x = 5.

In order to study the energy transfer process in Tb₂(WO₄)₃:Eu³⁺ in more detail, time-resolved emission spectra of Tb₂(WO₄)₃:5 mol % Eu³⁺ were measured by excitation into the WO₄²⁻ band with a 285 nm laser. The emission spectra collected at different delay times (t_d) are shown in Fig. 8. When t_d = 10 ns, the spectrum exhibits two types of emission, namely, the characteristic line emission from Tb³⁺ and Eu³⁺. Tb³⁺ and Eu³⁺ emission are present because the excitation energy of WO₄²⁻ has been transferred to Tb³⁺ and Eu³⁺ within this short time, respectively. When t_d = 500 ns, the emission of Tb³⁺ starts to decrease accompanied by the increasing of Eu³⁺ emissions intensity due to the energy transfer from Tb³⁺ to Eu³⁺. The emission of Tb³⁺ decreases gradually with further increase of the delay time, while that of Eu³⁺ increases due to transfer of more excitation energy from Tb³⁺ to Eu³⁺. When t_d = 50 μs, the emission intensity of Tb³⁺ is very weak, and the emission spectrum contains mainly that of Eu³⁺. When t_d > 50 μs, the Eu³⁺ emission also begins to decay gradually and disappears finally due to the depopulation of the excited states (figures not shown due to the limited space).

Fig. 8 Time-resolved PL spectra of the Tb₂(WO₄)₃:5 mol % Eu³⁺ sample (λ_ex = 285 nm laser).

A summary of the emission and energy transfer process in Tb₂(WO₄)₃:Eu³⁺ is shown schematically in Fig. 9. The emission process for WO₄²⁻ and the energy transfer from WO₄²⁻ to Tb³⁺ and Eu³⁺ are shown in Fig. 9(a). Upon UV (280 nm) excitation, an electron in the ground state (¹A₁) is excited into the ¹B (¹T₂) level of WO₄²⁻, where the electron either relaxes to its lowest excited ¹B (¹T₂) level of WO₄²⁻, producing the emission by the transition to the ¹A₁ level, or the excitation energy transfers to Eu³⁺⁵D₃ or higher levels by resonance process, from which the energy nonradiatively relaxes to the ⁵D₀ level by multiphonon relaxation. Then the characteristic emission of ⁵D₀–⁷F_J (J = 0, 1, 2, 3, 4) occurs. The excitation energy of WO₄²⁻ mainly transfers to ⁵D₃ or higher levels of Tb³⁺. The relaxation from ⁵D₃ to ⁵D₄ cannot happen by a multiphonon relaxation process due to the large energy difference between these two levels.¹⁶ In view of the high concentration (5 mol %), the relaxation from ⁵D₃ to ⁵D₄ of Tb³⁺ mainly occurred by the cross-relaxation process. For Tb³⁺, the energy difference between ⁵D₃ and ⁵D₄ is equal to that between ⁷F₀ and ⁷F₆. The emission process for Tb³⁺ and the energy transfer between Tb³⁺ and Eu³⁺ are shown in Fig. 9(b).⁸ Firstly, electrons on Tb³⁺ ions are excited from the ground state (4f⁸) to the excited state (4f⁷5d) by UV light. Subsequently, these electrons relax to the lowest excited state ⁵D₄ through multiphonon relaxation then either return to the ground state to produce the Tb³⁺ emissions (⁵D₄–⁷F_{6, 5, 4, 3}), or transfer their excitation energy from ⁵D₄ (Tb³⁺) level to the higher excited energy levels of Eu³⁺ (4f⁶) through cross relaxation, which relax to the ⁵D₀ (Eu³⁺) level, where the red-orange emission (⁵D₀–⁷F_{0, 1, 2, 3, 4, 5, 6}) takes place. Since the ⁵D₄–⁷F_{6, 5, 4, 3} emission of Tb³⁺ is effectively overlapped with the ⁷F_0, 1–⁵D_0, 1, 2 absorption of Eu³⁺, the energy transfer from Tb³⁺ to Eu³⁺ is very efficient.¹⁷

Fig. 9 Schematic diagram for the energy transfer from WO₄²⁻ to Tb³⁺ and Eu³⁺ as well as the emission process from Ln³⁺ (a), and the energy transfer from Tb³⁺ to Eu³⁺ (b).

Cathodoluminescence properties

Similar to the emission under UV excitation, the Tb₂(WO₄)₃ and Tb₂(WO₄)₃:5 mol % Eu³⁺ samples also exhibit strong green and red light, respectively, upon excitation with an electron beam (Tb₂(WO₄)₃: accelerating voltage = 4 kV, filament current = 103 mA; Tb₂(WO₄)₃:5 mol % Eu³⁺: accelerating voltage = 4 kV, filament current = 100 mA). Typical emission spectra are shown in Fig. 10(a) and (b), respectively. For the undoped Tb₂(WO₄)₃ and Tb₂(WO₄)₃:5 mol % Eu³⁺, the CL emission spectrum is basically identical with their PL spectra [Fig. 3(a) and 4(a)].

Fig. 10 Typical cathodoluminescence spectra of the Tb₂(WO₄)₃ (a) and Tb₂(WO₄)₃:5 mol % Eu³⁺ (b).

The CL intensity for the Tb₂(WO₄)₃ and Tb₂(WO₄)₃:5 mol % Eu³⁺ samples were investigated as a function of the accelerating voltage and the filament current, as shown in Fig. 11, respectively. The filament current of Tb₂(WO₄)₃ [Fig. 11(a)] is fixed at 103 mA and that of Tb₂(WO₄)₃:5 mol % Eu³⁺ [Fig. 11(b)] is fixed at 100 mA, the CL intensity increases with the accelerating voltage from 3 to 5 kV, respectively. Similarly, when the accelerating voltage is fixed at 4 kV, the CL intensity also increases with raising the filament current from 93 to 106 mA of Tb₂(WO₄)₃ [Fig. 11(c)] and from 90 to 104 mA of Tb₂(WO₄)₃:5 mol % Eu³⁺ [Fig. 11(d)]. The increase in CL brightness with an increasing electron energy and filament current can be attributed to deeper penetration of electrons into the phosphors and the larger electron beam current density. The electron penetration depth can be estimated using the empirical formula L/Å = 250(A/ρ)(E/Z^1/2)ⁿ, where n = 1.2/(1 − 0.29log10Z), A is the atomic or molecular weight of the material, ρ is the bulk density, Z is the atomic number or the number of electrons per molecule in the case compounds, and E is the accelerating voltage (kV).¹⁸ For cathodoluminescence, the Ln³⁺ ions are excited by the plasma produced by the incident electrons. The deeper the electron penetration depth, the more the plasma will be produced, which resulted in more Ln³⁺ ions being excited and thus the CL intensity increased.

Fig. 11 The cathodoluminescence intensity of Tb₂(WO₄)₃ and Tb₂(WO₄)₃:5 mol % Eu³⁺ nanowires as a function of accelerating voltage (a, b) and filament current (c, d).

4. Conclusions

In summary, one-dimensional Tb₂(WO₄)₃ and Tb₂(WO₄)₃:Eu³⁺ nanomaterials have been successfully synthesized by means of electrospinning technique in conjunction with sol–gel process. The as-form hybrid precursor materials present uniform fiberlike morphology. After annealing the precursors at high temperature, the as-prepared samples with wire morphology are well crystallized with the monoclinic structure of Tb₂(WO₄)₃. Due to an efficient energy transfer from the WO₄²⁻group to Tb³⁺ ions, Tb³⁺ ions show its characteristic strong emissions in the 1D Tb₂(WO₄)₃ nanowires upon excitation into the WO₄²⁻group at 280 nm and Tb³⁺ at 490 nm. Due to an efficient energy transfer from the WO₄²⁻group to Tb³⁺/Eu³⁺ ions and from Tb³⁺ ions to Eu³⁺ ions, Eu³⁺ ions show its characteristic strong emissions in the Tb₂(WO₄)₃:5 mol % Eu³⁺ 1D nanowires upon excitation into the WO₄²⁻group at 280 nm and Tb³⁺ at 490 nm. Under low-voltage electron beam excitation, Tb₂(WO₄)₃ and Tb₂(WO₄)₃:5 mol % Eu³⁺ phosphors show green emission of Tb³⁺ and red emission of Eu³⁺, respectively. These studies indicate that electrospinning is a facile and novel route for the development 1D luminescent nanomaterials that are useful in many types of lighting and color display fields.

Acknowledgements

This project is financially supported by National Basic Research Program of China (Grant Nos. 2007CB935502, 2010CB327704), and the National Natural Science Foundation of China (Grant Nos. NSFC 50702057, 50872131, 20921002).

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