1. Introduction

Field emission displays (FEDs) have gained a great interest and been thought as a next-generation flat-panel display for their good performances, such as great brightness, wide horizontal and vertical view angle, high contract ratio, light weight, short response time, wide work temperature range and high efficiency with low power consumption [1–8]. Compared with cathode ray tubes (CRTs), FEDs operate at significantly lower excitation voltages and higher current densities, phosphors used in FEDs must be efficient at low voltages and be resistant to Coulombic aging and saturation at high current densities. Sulfide-based phosphors mostly in CRTs lose efficiency due to the degradation at high current density [3–5]. Thus, oxide phosphors have gained more interests for their better thermal and chemical stability and environmental friendliness compared with sulfides [6,7].

Perovskite CaTiO₃ presents a good conductivity (band gap energy of 3.7 eV) [8], and recently get attention for its biocompatibility [9]. The luminescence properties of CaTiO₃ doped with trivalent praseodymium (Pr) have been increasingly investigated as the CIE coordinates of a red cathodoluminescence signal were measured at x = 0.680 and y = 0.311 [10,11]. Resent years, much work has focused on optimizing suitable phosphors in terms of morphology, particle size, stoichiometry, composition, and surface chemistry [1,2,12–18]. CaTiO₃ has been synthesized by hydrothermal [19] / solvothermal method [20], coprecipitation technique [21], polymeric precursor method [22], and sol-gel method [2,18] instead of solid state method in order to get higher efficiency. One-dimensional (1D) and Quasi-one-dimensional (Q-1D) nanomaterials have attracted great research interest [23,27] because of their specific and fascinating properties, such as high luminescence efficiency, superior mechanical toughness, metal insulator transition, and lowered threshold [24,25,28]. The electrospinning technique has been used to synthesis 1D structure materials since 1934 [26]. And now it has been demonstrated that a variety of materials can be electrospun to form uniform fibers, such as organic, inorganic, and hybrid polymers (organic-inorganic composites) [6,27–29]. The diameter of fibers prepared by the method can range from nanometers to several micrometers.

Accordingly, in the present work, we developed a facile method that combines electrospinning technique with heat treatment to prepare microscale Pr³⁺ doped CaTiO₃ red emission material. Furthermore, we investigated the structure, morphology, and photoluminescence (PL) and cathodoluminescence (CL) properties of the resulting samples in detail.

2. Experimental

CaCl₂ 2H₂O (98.0%, analytical reagent, A. R.) was purchased from Shantou Xilong Chemical Factory. Tetrabutyl titanate [Ti(OC₄H₉)₄, A. R.] was purchased from Tianjin Guangfu Fine Chemical Factory. Poly(vinylpyrrolidone) (PVP, M_w = 1 300 000) was purchased from Aldrich. All of the materials were used without further purification. Ethanol C₂H₅OH (A.R.) was used as solvent.

Ca_1-xTiO₃: xPr³⁺ (x = 0.0005, 0.001, 0.002, 0.003, 0.004, 0.005) were synthesized by electrospinning method succeed with high temperature treatment. First, Pr₆O₁₁ was dissolved in dilute hydrochloric acid HCl (AR) solution under stirring and heating to obtain PrCl₃ solution. Stoichiometric PrCl₃ solution was transferred to a 25 mL beaker which was then heated in an oven to remove the water. After cooling, stoichiometric CaCl₂ · 2H₂O, Ti(OC₄H₉)₄, 7 mL alcohol and 0.6011g PVP were added. The mixture was stirred 3 hours to get viscous solution for further electrospinning. A typical electrospinning setup consists of a syringe through which the solution to be electrospun is forced, a high-voltage power supply, a flat tip needle, and a grounded collector. The above viscous solution was placed in a 10 mL hypodermic syringe. The anode if the high-voltage power supply was clamped to the syringe needle tip and the cathode was connected to the grounded collector plate. The applied voltage was 10 kV, the distance between the needle tip and the collector was 15 cm, and the flow rate of the spinning solution was controlled at 1 mL/h by a syringe pump (TJ-3A/W0109-1B, Boading Longer Precision Pump Co., Ltd., China). The electrospun products were then calcined at 500, 600, 700, 800 °C for 2h with a heating rate of 1 °C /min. In this way, Ca_1-xTiO₃:x Pr³⁺ microfibers were fabricated.

The X-ray powder diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu Kα radiation (λ = 0.15405 nm). Fourier transform infrared spectroscopy (FTIR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer using the KBr pellet technique. Thermogravimetric and differential thermal analysis (TG-DTA) data were recorded with a Thermal Analysis instrument (SDT 2960, TA Instruments, New Castle, DE) with heating rate of 10 °C·min⁻¹. The morphology and composition of the samples were inspected using a field emission scanning electron microscope (FESEM, XL30, Philips) equipped with an energy-dispersive X-ray spectroscope (EDS, JEOL JXA-840). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) micrographs were obtained from a FEI Tecnai G2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV. The photoluminescence (PL) measurements were performed on a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescent (CL) measurements were carried out in an ultrahigh-vacuum chamber (<10⁻⁸ Torr), where the samples were excited by an electron beam at a voltage range of 1-3 kV with different filament currents, and the emission spectra were recorded using an F-4500 spectrophotometer. The luminescence 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). The quantum efficiency of the phosphor samples was performed by the quantum yield measurement system (C9920-02, Hamamatsu Photonics K. K., Japan). All the measurements were performed at room temperature (RT).

3. Results and discussion

3.1 Formation and morphology

The XRD patterns of the CaTiO₃:0.001 Pr³⁺ microfibers annealed from 500 to 800 °C are shown in Fig. 1 . When the precursor sample is calcined at 500 °C (Fig. 1a), there is no obverse diffraction peak, indicating the sample still remains amorphous below this temperature. After annealing at 600 °C (Fig. 1b), well-defined diffraction peaks appear, all of them can be assigned exactly to the orthorhombic phase of CaTiO₃ (JCPDS No. 22-0153). After annealing at 700 (Fig. 1c) and 800 (Fig. 1d) °C, all the diffraction peaks increase in intensity due to the increase of crystallinity. No additional peaks are detected in this dropping level, indicating that the Pr³⁺ can be effectively built into the CaTiO₃ host lattice. The ionic radius of Pr³⁺ is closed to that of Ca²⁺ and far away from that of Ti⁴⁺ [12]. Thus, Pr³⁺ is usually thought to occupy Ca²⁺ site.

 

Fig. 1 X-ray diffraction patterns for CaTiO₃:0.001 Pr³⁺ microfibers annealed at (a) 500, (b) 600, (c) 700, (d) 800 °C, respectively, as well as the JCPDS card 22-0153 of CaTiO₃ for comparison.

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The charge compensation may occur by the formation of intrinsic defects such as negatively charged Ca vacancies, positively charged oxygen vacancies and/or by reduction of Ti⁴⁺ to Ti³⁺. Many works have indicated that the optical performances of Pr³⁺-doped CaTiO₃ depend critically on charge compensation. The defects are undesirable because their presence in the phosphors, even at low concentration, can contribute to quench the Pr³⁺ luminescence [14].

The FT-IR spectra of the as-spun PVP fibers, the as-prepared precursor for CaTiO₃:0.001 Pr³⁺ fibers, and CaTiO₃:0.001 Pr³⁺ fibers annealed at 800 °C are shown in part a, b and c of Fig. 2 , respectively. Although there is a little difference, the broad band at 3421 cm⁻¹ in Fig. 2a and the broad band at 3420 cm⁻¹ in Fig. 2b are both assigned to the symmetric stretching vibration of –OH groups. The FT-IR spectrum (Fig. 2a) of PVP fibers shows a characteristic stretching band corresponding to the amide carbonyl group at 1657 cm⁻¹ (C = O group), whereas there is a red shift of the amide carbonyl group to 1653 cm⁻¹ in Fig. 2b. The bands at 2956, 2955, 1465, 1464, 1424 (Fig. 2a, b) can be attributed to the –CH₂ absorption of PVP. The bands at 1292 cm⁻¹ (Fig. 2a) and 1293 cm⁻¹ (Fig. 2b) can be attributed to the absorption of tertiary amine group of PVP [6]. In Fig. 2c, for the CaTiO₃:0.001 Pr³⁺ microfibers annealed at 800 °C, strong absorption peaks at 556 cm⁻¹ and 443 cm⁻¹ are present, which are attributed to the stretching vibration of the Ti–O bond (from TiO₃ ^2- groups) [30]. The weak –OH vibration can also be observed at 3422 cm⁻¹ in CaTiO₃:0.001 Pr³⁺ fibers due to the absorption of trace of water in the course of measurement.

 

Fig. 2 FT-IR spectra of CaTiO₃:0.001 Pr³⁺ and PVP fibers: (a) as-spun PVP fibers. (b) as-prepared precursor for CaTiO₃:0.001 Pr³⁺ fibers, and (c) CaTiO₃:0.001 Pr³⁺ fibers calcined at 800 °C.

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The TG-DTA curves of the as-spun composite fibers heat-treated in air with a rate of 10 °C /min is shown in Fig. 3 . TG curve shows three distinct stages of weight loss. The first weight loss (23%) step is observed between 40 and 250 °C due to the evaporation of alcohol. The second weight loss (53%) from 250 to 480 °C can be attributed to the decomposition of PVP. As observed in the DTA curve, the peak at 334 °C of the DTA curve corresponds to the decomposition of nitrates and the degradation of PVP, which has two degradation mechanisms involving both intra- and inter-molecular transfer reactions [31]. The exothermic peak at 448 °C is the result of the oxidation of carbon and carbon monoxide released by the Tetrabutyl titanate and PVP decomposition [6]. The third weight loss (18%) step between 480 and 700 °C accompanied with an exothermic peak at 520 °C in the DTA curve because of the crystallization of the CaTiO₃ phase, basically agreeing with the results of XRD and FT-IR.

 

Fig. 3 TG-DTA curves of the as-spun precursor for CaTiO₃:0.001 Pr³⁺ fibers.

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The morphology and the structure of the samples are investigated by SEM and TEM. Figure 4 shows the SEM micrographs of as-prepared CaTiO₃:0.001 Pr³⁺ samples. From the low-magnification SEM micrograph of the as-formed precursor (Fig. 4a) and those annealed at 800 °C (Fig. 4c), it can be seen that the samples consist of uniform fibers with length of several tens of micrometers. It also can be observed that the as-formed fibers have smooth surface and the diameter range from 630 to 960 nm (Fig. 4b). Due to the decomposition of the organic species and the formation of inorganic phase, the surface become rude and the diameter decrease, ranging from to 230 to 360 nm (Fig. 4d) after being annealed at 800 °C for 2 hours. The EDS was used to characterize the composition of the fibers annealed at 800 °C, as in Fig. 4e. The presence of O, Ti and Ca in Fig. 4e further indicates the formation of calcium titanate in the fibers. The doping concentration of Pr³⁺ is too small to be shown obviously.

 

Fig. 4 SEM images for the as-formed precursor for CaTiO₃:0.001 Pr³⁺ microfibers: image with low magnification (a) and high magnification (b). Those annealed at 800 °C: image with low magnification (c) and large magnification (d). And EDS of the microfibers annealed at 800 °C (e).

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TEM and high-resolution TEM (HRTEM) images of the CaTiO₃:0.001 Pr³⁺ microfibers annealed at 800 °C are shown in Fig. 5 , respectively. In Fig. 5a, it is observed that the fibers are further composed of fine and closely linked nanoparticles which orderly arranged along the axial direction of single fiber. The SEAD pattern in inset of Fig. 5a shows that the microfibers are polycrystalline. From the HRTEM image of CaTiO₃:0.001 Pr³⁺ microfibers (Fig. 5b), we can see crystalline phase (CaTiO₃) with well-resolved lattice fringes. The distance (0.39 nm) between the adjacent lattice fringes just corresponding to the interplanar distance of CaTiO₃ (101) planes, agreeing well with the d (101) spacing of the literature value (JCPDS No. 22-0153). These results further confirm the presence of highly crystalline CaTiO₃ in the fibers after annealing at high temperature, agreeing well with the XRD results.

 

Fig. 5 TEM (a) and HRTEM (b) images of CaTiO₃:0.001 Pr³⁺ microfibers annealed at 800 °C.

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3.2 Luminescence properties

Under short wavelength ultraviolet (UV) irradiation, CaTiO₃:0.001 Pr³⁺ microfibers exhibit strong emission band (about 40 nm range) in the range of 590–630 nm with a maximum peak at 612 nm (Fig. 6b ). The characteristic luminescence of the phosphors is determined by the electronic structure of the doped Pr³⁺, while the width and the relative intensity of the spectra frequently depend on the crystal symmetry of the host matrix. The broadening of the spectral line of Pr³⁺ may also arise from the large slits (EX = 5.0 nm. EM = 5.0 nm) during the measurement. The room temperature PL emission spectrum (Fig. 6b) originates from intra 4f ¹D₂-³H₄ transition [10] of Pr³⁺ at the alkaline–earth site by comparison of ionic sizes. The excitation spectrum (Fig. 6a) obtained by monitoring the emission of Pr^{3+ 1}D₂-³H₄ transition at 612nm mainly consists of three broad bands which are located at 325nm (A), 372nm (B), and 277nm (C), respectively. Band C is due to the 4f ²→4f5d excitation of Pr³⁺ [32], and band B is attributed to low-lying Pr-to-metal (Pr³⁺-Ti⁴⁺) intervalence charge transfer state (CTS) [33], and band A arise from the band edge absorption of CaTiO₃ host due to O(2p)-Ti(3d) transition [10]. The weak peaks at 460, 481, 490, and 499 nm in the enlarged curve (in Fig. 6a) are due to the f-f transition of Pr³⁺ from ³H₄ to ³P₂, ³P₁, ¹I₆ and ³P₀ [32], respectively. Jia et al. [32] found that the relative intensity of the three excitation bands and the peaks of the 4f states depended on the sample synthesis conditions.

 

Fig. 6 PL excitation (a) and emission (b) spectra of the CaTiO₃:0.001 Pr³⁺ microfibers annealed at 800 °C.

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Under the 277 nm excitation, there is no 4f5d→4f ² luminescence of Pr³⁺ in CaTiO₃: Pr³⁺, indicating that a nonradiative relaxation from the lowest 4f5d level to the ¹D₂ state occurs. When phosphors are excited at 325 nm, the spectroscopic behavior can be compared to the behavior of large gap semiconductors (e.g., rare-earth-doped ZnS phosphors), since the excitation of the red luminescence id achieved through the conduction band states of host matrix and then transferred to the 4f shell of Pr³⁺ ions [10]. The excitation band at 372 nm is ascribed to a low-lying Pr-to-metal (Pr³⁺-Ti⁴⁺) intervalence charge transfer state, as the final nonradiative relaxation pathway is to the emitting ¹D₂ level in Pr³⁺ doped CaTiO₃ [33].

Figure 7 shows the dependence of the photoluminescence (PL) emission intensity and the quantum efficiency on Pr³⁺ doping concentration (x) in CaTiO₃:x Pr³⁺. It can be found that the PL emission intensity of Pr³⁺ increases with the increase of its concentration (x) first, reaching a maximum value with Pr³⁺ concentration of 0.1 mol %, and then decreases with increasing the Pr³⁺ concentration (x) due to the concentration quenching effect. Thus, the optimum concentration for Pr³⁺ red emission is 0.1 mol % of Ca²⁺ in CaTiO₃ phosphor. Additionally, the quantum efficiency also increases with Pr³⁺ concentration (x) rise firstly, have the largest value with Pr³⁺ concentration of 0.1 mol %, and then decreases with the increase of the Pr³⁺ concentration (x). The concentration quenching effect gives an example how the deficiencies affect the photoluminescence of CaTiO₃:x Pr³⁺ in the present work. In principle, if two Pr³⁺ ions occupy two Ca²⁺ sites, there must generate one Ca vacancies according to charge compensation. More Pr³⁺ doped into the lattice, more Ca vacancies created. The intensity of photoluminescence decreases significantly by the energy transfer process by cross-relaxation between the Pr³⁺ neighbors ions and as well energy transfer with Pr³⁺ and Ca vacancies [34].

 

Fig. 7 PL emission intensity and quantum efficiencies of Pr³⁺ as a function of its concentration (x) in Ca_1-xTiO₃:xPr³⁺ microfibers.

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PL emission spectra of CaTiO₃:x Pr³⁺ microfibers with different Pr³⁺ ion concentrations and the corresponding CIE chromaticity diagram are shown in Fig. 8 . In Fig. 8a, the PL emission intensities have maximum value when the Pr³⁺ doping concentration is 0.1 mol %, which has been shown in Fig. 7. Figure 8b presents the corresponding CIE chromaticity coordinates positions. The CIE chromaticity coordinates change from x = 0.4119, y = 0.2566 (purplish pink) to x = 0.5934, y = 0.3052 (red) when the doping concentration of Pr³⁺ ions varying from x = 0.05 mol % to x = 0.1 mol % in CaTiO₃:x Pr³⁺ samples. The corresponding luminescence color can change from purplish pink to red. However, the corresponding luminescence color turns back to pink when increase the Pr³⁺ doping concentration.

 

Fig. 8 PL spectra of Ca_1-xTiO₃:xPr³⁺ microfibers with different Pr³⁺ ion concentrations (a) and the corresponding CIE chromaticity diagram (b).

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The photoluminescence decay curve of CaTiO₃:0.001 Pr³⁺ fibers is shown in Fig. 9a . The decay curve for the ¹D₂-³H₄ of Pr³⁺ fit into a double exponential function as I = A₁ exp(-t/τ₁) + A₂ exp(-t/τ₂), and the fitting results are shown inside of Fig. 9a. Two lifetimes, a slow one, τ₁ = 155.10 μs, and a fast one, τ₂ = 37.17 μs, have been obtained for the 612 nm emission of Pr³⁺. The average lifetime determined to be by the formula τ = (A₁τ₁ ² + A₂τ₂ ²)/ (A₁τ₁ + A₂τ₂) [35]. In Fig. 9b, the lifetime of Pr³⁺ in CaTiO₃:x Pr³⁺ microfibers increases with the increase of its doping concentration (x) first and then decreases with the increase of its concentration (x), although there is a little increase when x = 0.3 mol % compared with x = 0.2 mol %.

 

Fig. 9 Decay curve (a) for the luminescence of CaTiO₃:0.001 Pr³⁺ microfibers and lifetime of Ca_1-xTiO₃:xPr³⁺ samples as a function of the concentration (x) of Pr³⁺.

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The CaTiO₃:x Pr³⁺ microfibers also exhibit a strong red emission under low voltage excitation. The CL spectrum and photograph for CaTiO₃:0.003 Pr³⁺ under the excitation of electric beam (accelerating voltage = 3 kV, filament current = 100 mA) are shown in Fig. 10a . Different from the PL intensity, the CL intensity of CaTiO₃:x Pr³⁺ have largest value at x = 0.003, it may be because the different excitation mechanisms under UV excitation and low-voltage electron beam. The CL intensity for the CaTiO₃:0.003 Pr³⁺ microfibers phosphor have been investigated as a function of the accelerating voltage and the filament current, as shown in part b and c of Fig. 10, respectively. When the filament current is fix at 100mA, the CL intensity increases with the increase of the accelerating voltage from 1.0 to 3.0 kV (Fig. 10b). Similarly, under a 3.0 kV electron beam excitation, the CL intensity also increases with increasing the filament current from 88 to 100 mA (Fig. 10c). The increase of CL intensity with increasing electron energy and filament current are attributed to the deeper penetration of the electron into the phosphor's body and the larger electron beam current intensity. The electron penetration depth can be estimated by the empirical formula L[Å] = 250(A/ρ)(E/Z ^1/2)ⁿ, where n = 1.2/(1 – 0.29 log₁₀ Z), A is the atomic or molecule weight of the material, ρ is the bulk density, Z is the atomic number or the number of the electrons per molecule in the case compounds, and E is the accelerating voltage (kV) [36]. For cathodoluminescence, the Pr³⁺ ions are exited by the plasmons produced by the incident electrons. The deeper the electron penetration depth, the more plasmons will be produced, which results in more Pr³⁺ ions being excited and thus the CL intensity increase. The corresponding CIE chromaticity diagram (accelerating voltage = 3 kV, filament current = 100 mA) is shown in Fig. 10d. Although the CIE chromaticity coordinates (x = 0.4885, y = 0.3362) locates at yellowish pink, the digital photograph for CaTiO₃:0.003 Pr³⁺ (inset of Fig. 10a) indicate the luminescence is red, same as eye can see.

 

Fig. 10 The typical cathodoluminescence spectrum of CaTiO₃:0.003 Pr³⁺ microfibers under an electron beam excitation (3 kV) (a), the cathodoluminescence intensity of CaTiO₃:0.003 Pr³⁺ as a function of accelerating voltage (b) and filament current (c) and the corresponding CIE chromaticity diagram (3 kV, 100 mA) (d).

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4. Conclusions

A simple and versatile electronspinning method was explored to prepare single-distributed one-dimensional CaTiO₃:x Pr³⁺ microfibers. The diameter of the obtained microfibers ranges from 230 to 360 nm. Under ultraviolet excitation and low-voltage electron beam exaltation, the CaTiO₃:x Pr³⁺ phosphor shows a red (612 nm) emission corresponding to ¹D₂-³H₄ transition of Pr³⁺, and the CL intensity increases with increasing accelerating voltage and filament current. The optimum doping concentration of Pr³⁺ is different under ultraviolet excitation and low-voltage electron beam excitation, which is 0.1 mol % and 0.3 mol % of Ca²⁺ in CaTiO₃ microfibers, respectively. The phosphors we synthesized show potential in FED applications.