PYRIDINATION OF POLY(VINYL CHLORIDE) via A HOMOLYTIC PATHWAY

As the next generation of solid-state lighting diodes, white LEDs are believed to trigger a lighting revolution. LEDs have many distinct advantages over traditional incandescent and halogen lamps, such as energy savings, long life, and high efficiency [1-3]. There are many potential applications for white LEDs, such as everyday lighting, LCD backlights, and displays, so white LEDs have received widespread attention in recent years [4, 5]. The earliest commercializedwhite LEDs were developed by Nichia Corporation of Japan using blue light from GaN-based LED chips to excite rare earth phosphor YAG:Ce³⁺ to emit yellow light and combine it into white light [6-8]. However, such a white LED lacks a spectrum of a red light region, resulting in a poor color rendering index and low luminous efficiency. At present, the use of near-ultraviolet chips to excite trichromatic phosphors to achieve white light production has become the focus of currentwhite LEDs industry development [9-11]. This method overcomes the shortcomings of high color temperature and low color rendering index of "yellow-blue" combined white LED. Lumen efficiency and color reproduction performance are greatly affected by problems such as color resorption and ratio adjustment between multi-component phosphors [12]. Therefore, it has become a top priority to develop a new type of phosphor that can overcome the deficiency of mixed phosphors and improve luminescence property.

Up to date, a method of adjusting white light generated by energy transfer (ET) between a sensitizer and an activator has been obtained in a single matrix phosphor, such as Sr₃NaLa(PO₄)₃F:Eu²⁺, Mn²⁺ [13], Ca₄(PO₄)₂O:Dy³⁺, Eu²⁺ [14], and Ca₃YNa(PO₄)₃:Ce³⁺, Mn²⁺ [15]. From the standpoint of the reaction conditions, these reactions need to be carried out strictly under a reducing gas (N₂/H₂). However, single-component phosphors synthesized in the reaction air will be more promising. Due to the transition from Dy^{3+ 4}F_9/2 level to ⁶H_13/2 and ⁶H_15/2, Dy³⁺ ions can produce strong blue and yellow emission bands with peaks at 483 and 574 nm [16, 17]. White light can be produced by appropriately combining the two kinds of light. However, in most cases, the yellow emission of Dy³⁺ is stronger than the blue emission, resulting in yellowwhite light by single-doped Dy³⁺ ions. To overcome this problem, Tm³⁺ ion is introduced because it produces blue emission due to 1D₂-³F₄ transition (about 453 nm) [18, 19]. There are some reports on such white phosphors, such as, BaLaGa₃O₇:Tm³⁺, Dy³⁺ [20], YAl₃(BO₃)₄: Tm³⁺, Dy³⁺ [21], and BaWO₄:Tm³⁺, Dy³⁺ [22]. Therefore, by simultaneously doping Tm³⁺ and Dy³⁺ ions into a single host, a white light of a suitable yellow-blue intensity ratio can be obtained.

Apatite refers to a group of minerals represented by calcium phosphate. Its crystal chemical formula is M₅[XO₄]₃Z, where M is a +1, +2 and +3 valence metal; XO₄ is a variety of acid ions; Z is an additional anions F^-, Cl^-, (OH)^-, O^2-, and so on. The apatite crystal belongs to the hexagonal system, P63/m, and the unit cell is Z = 2 [23, 24]. Hence, the apatite structure is an extraordinary structural model for designing high quality new phosphors. In this work, a series of single host white-emitting Ba₃LaNa(PO₄)₃F:Tm³⁺, Dy³⁺ phosphors were prepared and their luminescence properties were investigated in detail. The results show that this strong coloradjustable BLNPF:Tm³⁺, Dy³⁺ phosphor has great potential as a UVpumped white LED.

Tm³⁺, Dy³⁺ co-doped Ba₃LaNa(PO₄)₃F phosphor was prepared by high temperature solid phase method. Based on the preparation of 1 g of the target product, BaCO₃, La₂O₃, NH₄H₂PO₄, Na₂CO₃, BaF₂, Tm₂O₃, and Dy₂O₃ were weighed according to a molar ratio of 3:(1- x-y)/2:1/2:3:1/2:1/2: x: y. The above raw materials were placed in a mortar and thoroughly ground, and the mixture was placed in an alumina crucible and placed in a high temperature tube furnace. The temperature was raised from room temperature to 1050 ℃ at a heating rate of 10 ℃/min, and kept at 1050 ℃ for 3 h. After being naturally cooled to room temperature, it is sufficiently ground with a mortar to obtain Ba₃LaNa(PO₄)₃F:Tm³⁺, Dy³⁺ phosphor, and the next step is characterized.

Phase structure analysis was performed by a Bruker D2 X-ray diffractometer (XRD) using an X-ray source of Cu-K α ( λ = 1.5406 Å), a scan current of 10 mA, a scan voltage of 30 kV, and a scan rate of 0.08×/s, scan range is20×±80×. The excitation and emission spectra were measured using a Hitachi F-4600 fluorescence spectrometer with a 150 W xenon lamp with a slit width of 5 nm and a test voltage of 400 V. The instrument used for fluorescence decay lifetime was a FluoroLog-3 fluorescence spectrometer (HORIBA, France) using a Specral-LED (N-370) with a pulse width of 12 ns. All tests were performed at room temperature.

XRD patterns of the as-prepared phosphors were collected to verify the phase purity. Fig. 1 shows the XRD patterns of Ba₃LaNa(PO₄)₃F, Ba₃La_0.975Na(PO₄)₃F:0.025Tm³⁺, Ba₃La_0.960Na (PO₄)₃F:0.040Dy³⁺, and Ba₃La_0.945Na(PO₄)₃F:0.025Tm³⁺, 0.030Dy³⁺ phosphors. It is clear that according to JCPDS documents 71–1317, all diffraction peaks of these samples can be accurately assigned to the pure hexagonal phase of Ba₃LaNa(PO₄)₃F. The map shows that no significant changes in crystal structure caused by other phases or impurities were detected, demonstrating that Tm³⁺ and Dy³⁺ ions successfully entered the Ba₃LaNa(PO₄)₃F host [25]. In Ba₃LaNa(PO₄)₃F host, Tm³⁺ and Dy³⁺ substitute La³⁺ sites because of the similar ionic radii.

Figure 1

Figure 1.  XRD patterns of (a) Ba₃LaNa(PO₄)₃F; (b) Ba₃La_0.975Na(PO₄)₃F:0.025Tm³⁺; (c) Ba₃La_0.960Na(PO₄)₃F:0.040Dy³⁺; (d) Ba₃La_0.945Na(PO₄)₃F:0.025Tm³⁺, 0.030Dy³⁺ and the standard data Ba₃LaNa(PO₄)₃F (JCPDS No. 71-1317) as a reference.

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The relationship between the emission intensity of Tm³⁺ and its doping concentration is shown in Fig. S1 (Supporting information). Meanwhile, in the emission wavelength range of 400–550 nm, the main emission peak of Tm³⁺ is located at 453 nm, which belongs to the transition of ¹D₂-³H₆. The phosphor exhibits a strong blue emission, revealing its potential value in blue-emitting LED applications. Moreover, another relatively weak blue emission was observed due to the ¹G₄-³H₆ transition of the Tm³⁺ ions [26]. As the concentration of Tm³⁺ ions increases, the emission intensity is positively correlated with the ion concentration until the maximum value is reached at x = 0.025, and concentration quenching occurs.

Fig. S2 (Supporting information) demonstrated the emission and excitation spectra of Tm³⁺ and Dy³⁺ single doped and Tm³⁺/Dy³⁺ co-doped Ba₃LaNa(PO₄)₃F phosphors. Fig. S2a displays the PL and PLE spectra of the Ba₃La_0.975Na(PO₄)₃F:0.025Tm³⁺ phosphor. The PLE spectrum monitored at 453 nm exhibited one peak centered at 358 nm corresponding to the transition from the ³H₆ ground state to the ¹D₂ excited state of Tm³⁺. The band emission spectral characteristic has been discussed above. Fig. S2b shows the PLE and PL emission spectra of the Ba₃La_0.960Na (PO₄)₃F:0.040Dy³⁺ phosphor. The excitation spectrum consists of a series of sharp bands centered around 325, 350, 363, 387, 426, 452 and 477 nm, which are attributed to the 4f-4f transitions of Dy³⁺ ions from the ground state of ⁶H_15/2 to the excited states of ⁴M_17/2, ⁹P_7/2, ⁴P_5/2, ⁴I_13/2, ⁴G_11/2, ⁴I_15/2 and ⁴F_9/2, respectively [27]. It was clear that three main peaks located at 480 nm (⁴F_9/2-⁶H_15/2), 573 nm (⁴F_9/2-⁶H_13/2), and 663 nm (⁴F_9/2-⁶H_11/2) [28]. To further confirm the possible energy transfer process, Fig. S2c illustrates the PLE and PL spectra of Ba₃La_0.945Na(PO₄)₃F:0.025Tm³⁺, 0.030Dy³⁺ phosphor. A notable spectral overlap between the PLE spectrum of Ba₃La_0.960Na(PO₄)₃F:0.040Dy³⁺ and the PL spectrum Ba₃La_0.975Na- (PO₄)₃F:0.025Tm³⁺ is observed. So that the energy transfer from Tm³⁺ and Dy³⁺ ions can be achieved in the Ba₃LaNa(PO₄)₃F host.

By changing the relative concentrations of co-doped Tm³⁺ and Dy³⁺, the energy transfer process between Tm³⁺→Dy³⁺ ions can be further studied. It can be seen from Fig. 2 that the emission spectrum after co-doping is composed of Tm³⁺ and Dy³⁺, and the emission intensity of Tm³⁺ is significantly reduced as the Dy³⁺ doping concentration increases. This is due to the continuously enhanced energy transfer process between Tm³⁺ and Dy³⁺. Later, due to the concentration quenching between Dy³⁺ and Dy³⁺, the Dy³⁺ emission peak intensity increases first and then decreases and reaches a maximum at y = 0.030.

Figure 2

Figure 2.  Dependence of the PL spectra of Ba₃La_0.975-yNa(PO₄)₃F:0.025Tm³⁺, yDy³⁺ samples on the Dy³⁺ concentrations. The inset shows the relative intensity of Tm³⁺ (453 nm) and Dy³⁺ (573 nm) ions emission as a function of the doped content of Dy³⁺ ions.

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In order to further verify the energy transfer from Tm³⁺ and Dy³⁺, we synthesize a series of samples Ba₃LaNa(PO₄)₃F: xTm³⁺, 0.040Dy³⁺. Fig. S3 (Supporting information) shows the PL spectra of Ba₃LaNa(PO₄)F: xTm³⁺, 0.040Dy³⁺. The inset shows the emission intensity trends of Tm³⁺ (453 nm) and Dy³⁺ (573 nm). At 350 nm excitation, the shape of the PL spectrum is very similar to the 0.025Tm³⁺, yDy³⁺ sample. It is apparent from the inset that as the Tm³⁺ doping concentration increases, the emission intensity at 453 nm gradually increases. However, the intensity of luminescence at Dy³⁺ (573 nm) increased firstly, reached its maximum when x = 0.015, then decrease.

Fig. 3 illustrates the energy level model of the energy transfer process between Tm³⁺-Dy³⁺ in the BLNPF host. When Ba₃LaNa (PO₄)₃F:Tm³⁺, Dy³⁺ phosphor is excited by 358 nm, the ground state electron (⁶H_15/2) of Dy³⁺ ion is pumped to the excited state ⁴I_11/2 state by energy transfer from Tm³⁺ ion. This is due to the difference between the similar energy gaps of the two ions of Tm³⁺ (¹D₂-³F₄) and Dy³⁺ (⁶H_15/2-⁴I_15/2). Radiation transitions from ⁴F_9/2 to ⁶H_15/2, ⁶H_13/2, and ⁶H_11/2 can cause the emission main peaks to be centered at 480, 573, and 663 nm, respectively.

Figure 3

Figure 3.  Energy level scheme of Ba₃LaNa(PO₄)₃F:Tm³⁺, Dy³⁺phospor with electronic transitions and energy transfer process.

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By changing the relative intensities of co-doped Tm³⁺/Dy³⁺ ions, the formula for calculating the energy transfer efficiency ( η_ET) of Tm³⁺→Dy³⁺ is [29]:

here, I_S and I_S0 are the emission intensities of Tm³⁺ ions when undoped and doped with Dy³⁺ ions, respectively. It can be visually seen from the Fig. S4(Supporting information)that the η_ET value increases with the increase of Dy³⁺ ion concentration.When y = 0.04, the maximum η_ET value is about 72.6%.

In general, the resonant energy transfer process mainly has two types of exchange interaction and electric multipole interaction. When the exchange interaction is dominant, the critical distance between the sensitization center and the activation center should be less than 5 Å. The critical distance ( R_c) from Tm³⁺ to Dy³⁺ energy transfer can be calculated by the formula given by Blasse [30, 31]:

where V is the volume of the unit cell; X_C is the critical concentration; N is the number of cations in the unit cell. For the BLNPF host, V = 626.735 Å3, N = 2, X_C = 0.055, so the critical distance R_C is determined to be 22.16 Å. Therefore, the energy transfer mechanism in this system is limited by electric multipole interaction.

The following relationship can be obtained by analyzing the energy transfer type between the sensitizer ions Tm³⁺ and the activator ions Dy³⁺ in Dexter's multipole interaction energy transfer expression and Reisfeld approximation [32]:

where η₀ and η_S are the luminescence quantum efficiencies of Tm³⁺ ions in the absence and presence of Dy³⁺ ions, respectively; C is the total doping concentration of Tm³⁺ and Dy³⁺ ions. When n = 6, 8, and 10, they correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interaction, respectively. The value of η₀/η_S can be approximately replaced by the luminous intensity ratio I_S0/I_S. Fig. S5 (Supporting information) shows relationship betweenIS0/IS and C^6/3, C^8/3 and C^10/3 of Tm³⁺ ions. It can be found that when n = 6, the best linear fit is obtained. The analysis shows that energy transfer from Tm³⁺ to Dy³⁺ occurs via a dipole-dipole mechanism.

The decay curves of Ba₃La_0.975-yNa(PO₄)₃F:0.025Tm³⁺, yDy³⁺ phosphors measured at 453 nm emission with 358 nm excitation were also detected, and the decay curves are shown in Fig. S6 (Supporting information). It is found that all the decay curves can be fitted well with a second-order exponential decay, which can be obtained using the equation [33, 34]:

where I is the luminescence intensity, t is the time, A₁ and A₂ are constants, and τ₁ and τ₂ are rapid and slow decay times for the exponential components, respectively. According to the parameters in Eq.5, the average lifetime t^* can be obtained using the formula:

Based on the second-order exponential function and the decay curve, Ba₃La_0.975-yNa(PO₄)₃F:0.025Tm³⁺, yDy³⁺ at 358 nm excitation. The fluorescence decay timet at 453 nm decreases with increasing Dy³⁺ doping concentration, and the corresponding Tm³⁺ fluorescence decay time τ is 7.96, 7.78, 6.56, 6.34 and 6.12ms, respectively. Therefore, the energy of Tm³⁺ in the co-doped sample can be transferred to Dy³⁺ ions.

The emission spectrum of Ba₃La_0.975-yNa(PO₄)₃F:0.025Tm³⁺, yDy³⁺ phosphors were converted to the CIE 1931 chromaticity using the photoluminescence data and the CIE software diagram as shown in Fig. 4. It can be seen from the figure that as the concentration of Dy³⁺ increases, the CIE chromaticity coordinates change from (0.2830, 0.2992) to (0.3109, 0.3503), and the corresponding illuminating color gradually changes from blue to white. Obviously, by adjusting the doping concentration of Dy³⁺ in Ba₃LaNa(PO₄)₃F:Tm³⁺, Dy³⁺, warm white light can be produced for different practical applications.

Figure 4

Figure 4.  The CIE chromaticity diagram for Ba₃La_0.975-yNa(PO₄)₃F:0.025Tm³⁺, yDy³⁺ phosphors.

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In summary, we synthesized a series of Ba₃LaNa(PO₄)₃F:Tm³⁺, Dy³⁺ phosphors by high temperature solid phase reaction. Under the excitation of UV light, the characteristic peaks of Tm³⁺ and Dy³⁺ ions appeared in Ba₃LaNa(PO₄)₃F: xTm³⁺ and Ba₃LaNa(PO₄)₃F: yDy³⁺ phosphors, respectively. In addition, it is matched to commercial UV LED chips when excited at 358 nm. By fixing the Tm³⁺ ions concentration and changing the ions concentration of Dy³⁺, the luminescence color of Ba₃LaNa(PO₄)₃F:0.025Tm³⁺, yDy³⁺ phosphor can be changed from blue to white. The analysis shows that the dipole-dipole interaction should be the reaction mechanism of the transferof energyfromTm³⁺toDy³⁺ions. These resultsindicate that Ba₃LaNa(PO₄)₃F:Tm³⁺, Dy³⁺ may serve as a white light-emitting phosphor for white light LED.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.04.046.