Rapid Aqueous-Phase Synthesis and Photoluminescence Properties of K_0.3Bi_0.7F_2.4:Ln³⁺ (Ln = Eu, Tb, Pr, Nd, Sm, Dy) Nanocrystalline Particles

Abstract

Trivalent lanthanides (Ln³⁺) doped bismuth-based inorganic compounds have attracted considerable interest as promising candidates for next-generation inorganic luminescent materials. Here, a series of K_0.3Bi_0.7F_2.4 (KBF) nanocrystalline particles with controlled morphology have been synthesized through a low-temperature aqueous-phase precipitation method. Using KBF as the host matrix, Eu³⁺, Tb³⁺, Pr³⁺, Nd³⁺, Sm^3+, and Dy³⁺ ions are introduced to obtain K_0.3Bi_0.7F_2.4:Ln³⁺ (KBF:Ln) nanophosphors. The as-prepared KBF:Ln nanophosphors exhibit commendable photoluminescence properties, in which multicolor emissions in a single host lattice can be obtained by doping different Ln³⁺ ions when excited by ultraviolet light. Moreover, the morphology and photoluminescence performance of these nanophosphors remain unchanged under different soaking times in water, showing good stability in a humid environment. The proposed simple and rapid synthesis route, low-cost and nontoxic bismuth-based host matrix, and tunable luminescent colors will lead the way to access these KBF:Ln nanophosphors for appealing applications such as white LEDs and optical thermometry.

1. Introduction

Inorganic luminescent nanoparticles have aroused tremendous attention compared with their bulk counterparts owing to their unique features such as tunable emission peaks, sharp emission bandwidth, large stokes shift, and excellent luminescence stability against heat and irradiation [1,2,3,4]. As a result, trivalent lanthanides doped inorganic nanoparticles show a rather wide range of applications in areas such as spectroscopic analysis, anti-counterfeiting, biomedicine, and light-emitting diodes (LEDs) [5,6,7,8,9]. In the past two decades, a large number of novel luminescent nanomaterials have been designed and developed to take advantage of their nanoscale size and distinct luminescence properties. Among them, rare earth fluorides are becoming a hot research topic in consideration of their high chemical stability, very low lattice phonon energy, and good accommodation for Ln³⁺ [10,11,12,13,14,15]. Nevertheless, substantial rare earth salts are usually required to synthesize these rare earth fluoride nanomaterials, which greatly limits their further development and application. Therefore, it is urgent and necessary to further explore new and efficient host materials for luminescent lanthanides.

Bismuth-based inorganic materials are considered promising candidates for developing the next-generation inorganic phosphors because of the advantages of the relatively inexpensive, nontoxic, and unique electronic structure of bismuth elements [16,17,18,19]. Among these bismuth-based inorganic hosts, bismuth fluorides have attracted significant attention [20,21]. As a result of extensive research, lanthanide-doped bismuth fluorides have been found numerous applications in photocatalysis (e.g., BiOF:Eu³⁺ and Bi₇O₅F₁₁:Eu³⁺) [22,23], white LEDs (e.g., NaBiF₄:Eu³⁺ and BiF₃:Eu³⁺) [24,25] and optical thermometry (e.g., BiF₃:Yb³⁺,Er³⁺) [26]. Recently, K_0.3Bi_0.7F_2.4 is emerging as a more attractive host matrix for lanthanide doping to realize tunable emissions in a single phosphor. For instance, An et al. synthesized a series of Yb³⁺/Ln³⁺ (Ln = Er, Ho, Tm) co-doped K_0.3Bi_0.7F_2.4: upconverting nanoparticles for dual-modal in vivo imaging through a solvothermal method at 200 °C for 10 h in ethylene glycol [27]. Gao et al. reported a room-temperature chemical precipitation method to synthesize K_0.3Bi_0.7F_2.4:Yb³⁺, Er^3+, and K_0.3Bi_0.7F_2.4:Eu³⁺ nanoparticles by using ethylene glycol as the reaction solvent [28,29]. Du et al. have used a similar precipitation method to prepare Tb³⁺/Eu³⁺-codoped K_0.3Bi_0.7F_2.4 nanoparticles for white LED application [30]. In our previous research, we synthesized the Yb³⁺/Ln³⁺ (Ln = Er, Ho, Tm)-doped K_0.3Bi_0.7F_2.4 nanoparticles by using a very fast (1 min only) method in a water-based system at very low temperature (room temperature−90 °C) [31]. However, rapid synthesis of other lanthanide ions (Pr³⁺, Nd³⁺, Sm³⁺, Dy³⁺, Eu^3+, and Tb³⁺) doped KBF nanocrystals in the water-based system and their photoluminescence properties are rarely reported.

Here we report the controlled synthesis and multicolor luminescence properties of KBF:Ln nanophosphors synthesized by a low-temperature water-based precipitation method. The composition and morphologies of the obtained KBF:Ln nanophosphors are investigated in detail by varying the synthesis conditions. The obtained KBF:Ln nanophosphors show tunable emission colors and good chemical stability against humidity. The results suggest that the as-prepared KBF:Ln nanophosphors are promising candidates for application in white LEDs and optical thermometry.

2. Experimental Process

BiCl₃ (Macklin, Shanghai, China), KF (99.9%, Macklin, Shanghai, China), HCl (37 wt%, Sinopharm, Shanghai, China), PrCl₃·6H₂O (99.99%, Aladdin, Shanghai, China), NdCl₃·6H₂O (99.9%, Aladdin, Shanghai, China), SmCl₃·6H₂O (99.99%, Aladdin, Shanghai, China), DyCl₃·6H₂O (99.99%, Aladdin, Shanghai, China), EuCl₃·6H₂O (99.99%, Aladdin, Shanghai, China) and TbCl₃·6H₂O (99.9%, Aladdin, Shanghai, China) are used as the starting materials. The pure and doped KBF samples were synthesized by a low-temperature water-based precipitation method in accordance with our earlier study [31]. The detailed synthesis procedure and sample characterization are presented in the Supporting Information.

3. Results and Discussion

3.1. Phase Composition of the Pure and Doped KBF Nanocrystals

The phase composition of the prepared pure KBF nanocrystals is first checked by XRD. XRD patterns of the as-prepared non-doped KBF hosts at room temperature are depicted in Figure 1. When the ratios of F/Bi are 4 and 8, the XRD patterns of the obtained samples can be well matched with the corresponding standard XRD data of the cubic K_0.3Bi_0.7F_2.4 (JCPDS No. 84–0534), as presented in Figure 1a. The narrow and strong XRD peaks reveal that the pure KBF nanocrystalline particles are highly crystalline. However, as the ratio of F/Bi reaches 12 or higher, it is found that the impurity phase starts to appear at around 27°, assigned to the hexagonal KBiF₄ (JCPDS No. 37–0972). Meanwhile, when the F/Bi ratio is set to 8, all the XRD results of the pure KBF nanocrystals obtained at different temperatures (room temperature, 50 °C, and 80 °C) are consistent with the standard pattern of K_0.3Bi_0.7F_2.4 crystal, as shown in Figure 1b, indicating that pure KBF can be synthesized in the water-based system at very low temperature. Furthermore, the full width at half-maximum (FWHM) with the dominant XRD peak at 26° presents a declining trend with the increase of reaction temperature, illuminating that the degree of crystallization becomes better for pure KBF nanocrystalline particles at a high synthetic temperature.

Figure 2 presents the XRD results of the KBF:Ln³⁺ (Ln = Pr, Nd, Sm, Dy, Eu, and Tb) nanocrystalline particles. It can be noted that the XRD patterns of the doped KBF nanocrystals are consistent with the cubic K_0.3Bi_0.7F_2.4 compound, which suggests that introducing different lanthanide ions will not lead to the formation of an impurity phase. Meanwhile, the KBF host matrix can accommodate a high concentration of Ln³⁺ (e.g., 25%Eu³⁺ and 30%Tb³⁺) without changing its crystal structure, indicating that KBF is an attractive host for Ln³⁺ doping to obtain tunable luminescence properties. Furthermore, the average size of KBF:Ln nanocrystalline particles can be estimated by using the Debye Scherrer equation. And the crystallite sizes of KBF:30%Tb, KBF:25%Eu, KBF:5%Dy, KBF:5%Sm, KBF:2%Nd and KBF:5%Pr are calculated to be 20.41, 28.31, 39.75, 45.94, 50.87 and 44.98 nm, respectively.

3.2. Morphology Characterization of Pure and Doped KBF Nanocrystalline Particles

The SEM graphs can present the particle size and typical morphology of the as-prepared pure and doped KBF nanocrystals directly and vividly. Figure 3 depicts the representative morphologies of the non-doped KBF nanocrystals synthesized at different ratios of F/Bi. Note that the reaction temperature is room temperature. As presented in Figure 3a,b, when the ratios of F/Bi are determined to be 4 and 8, the obtained pure KBF presents a regular cube shape, and the average size of the cubes decreases as the F/Bi ratio increases. However, as the ratio value of F/Bi increases to 12 or 16, the morphology of the obtained nanocrystalline particles changes from a cubic shape to an irregular polyhedron. These results indicate that the F/Bi ratio significantly affects the morphology of the final products.

Figure 4 presents the SEM graphs of the KBF nanocrystals obtained at different synthetic temperatures. The F/Bi ratio is set to 8. As the reaction temperature increases from room temperature to 80 °C, the cubic morphology of the samples remains almost unchanged, but it is found that the average particle size of the cubic particles increases slightly with an increase in the synthetic temperature.

The SEM images of the KBF:Ln (Ln = Pr, Nd, Sm, Dy, Eu, and Tb) nanocrystalline particles synthesized at room temperature are given in Figure 5. When the ratio of F/Bi is determined to be 8, all the as-prepared KBF:Ln samples consist of cubic particles and irregular nanoparticles attached to cubes after introducing different lanthanide ions into the KBF lattice. Compared with the non-doped KBF, the as-prepared KBF:Ln nanocrystalline particles significantly decrease in size. Moreover, the proportion of nanoparticles shows an increasing trend, and the content of cubic particles gradually decreases with an increase in the doping concentration of Ln³⁺.

3.3. Photoluminescence Properties of KBF:Ln Nanophosphors

The concentration-dependent emission spectra of KBF:Eu and KBF:Tb nanocrystals are presented in Figure S1. And the optimum doping concentration of Eu²⁺ and Tb³⁺ is determined to be 25% and 30%, respectively. The photoluminescence excitation and emission spectra of KBF:25%Eu are given in Figure 6a. The emission spectrum of KBF:Eu consists of a group of sharp lines at 578, 592, 612, 651, and 700 nm upon 393 nm near-UV light excitation, which can be attributed to the characteristic ⁵D₀→⁷F_J (J = 0, 1, 2, 3, 4) electron transitions of Eu³⁺ ions, respectively [32]. Among these emission bands, the most intense one, peaking at 612 nm, originates from the Eu^{3+ 5}D₀→⁷F₂ electric-dipole transition. The excitation spectrum monitored at 612 nm emission is made up of multiple excitation bands centered at 362, 376, 393, and 414 nm due to the electron transitions from the ⁷F₀ ground state to the ⁵D₄, ⁵G₃, ⁵L_6, and ⁵D₃ excited levels. Figure 6d shows the excitation and emission spectra of KBF:30%Tb nanocrystalline particles. The emission spectrum exhibits four distinct peaks at 487, 544, 582, and 622 nm when excited by 368 nm of UV light, resulting from the ⁵D₄→⁷F_J (J = 6, 5, 4, 3) electron transitions of Tb³⁺ ions, respectively [33]. And the most prominent emission peak is located at 544 nm, assigned to the ⁵D₄→⁷F₅ transition of Tb³⁺. The excitation spectrum monitored 544 nm emission comprises excitation bands at 318, 350, and 368 nm due to the ⁷F₆→⁵D_{1, 0}, ⁷F₆→⁵G_6-2, and ⁷F₆→⁵D_2,3 transitions of Tb³⁺ ions. Furthermore, it is found that the photoluminescence intensities of KBF:Eu and KBF:Tb nanocrystals decrease gradually with increasing the test temperature from room temperature to 210 °C, which can be attributed to the increase in the non-radiative transition probability, as shown in Figure S2.

To further reveal the chemical stability of the prepared KBF:Ln nanocrystalline particles, the emission spectra of KBF:25%Eu and KBF:30%Tb phosphor powders immersed in deionized water for different days (0, 15, and 30 days) are shown in Figure 6b,e. It can be noticed that the photoluminescence intensity of both KBF:25%Eu and KBF:30%Tb nanophosphors remains almost unchanged under different soaking times in water, indicating that the as-prepared nanophosphors show excellent chemical stability against water.

Luminescence decay curves of KBF:25%Eu and KBF:30%Tb samples are depicted in Figure 6c,f. The decay curves can be fitted with a single exponential equation [34]:

I(t) = I₀ exp(−t/τ)

(1)

where I(t) and I₀ represent the photoluminescence intensity at time t and the background intensity for the dark current of the detector, and τ is the lifetime. Therefore, the average lifetimes of KBF:25%Eu were estimated to be 5.74, 5.52, 5.23, 5.08, and 4.81 ms at different testing temperatures, while the average lifetimes of KBF:30%Tb were determined to be 3.14, 3.09, 3.03, 2.99 and 2.84 ms at different temperatures. For both KBF:25%Eu and KBF:30%Tb samples, the lifetimes show a decreasing trend with the increase of the test temperature, which can be attributed to the increase in the probability of non-radiative transition at high temperature [35]. In addition, the internal quantum efficiencies of KBF:25%Eu and KBF:30%Tb nanocrystals are measured to be 42.39% and 13.75%, respectively (Figure S3). The obtained quantum efficiency values of KBF:25%Eu and KBF:30%Tb samples are comparable with that of the previous reported fluorides, such as K_0.3Bi_0.7F_2.4:40%Eu³⁺ (14.9%) [29] and K_0.3Bi_0.7F_2.4:9%Tb³⁺,0.6%Eu³⁺ (50.8%) [30].

The photoluminescence emission and excitation spectra of the KBF:Ln³⁺ (Ln = Pr, Nd, Sm, Dy) nanocrystals are depicted in Figure 7. As presented in Figure 7a, the excitation spectrum of KBF:5%Pr nanocrystal monitored at 606 nm emission is composed of three distinct excitation bands centered at 443, 467, and 481 nm, which are caused by the characteristic ³H₄→³P_J (J = 0, 1, 2) electron transitions of Pr³⁺ ions, respectively [36]. The emission spectrum with a dominant peak at 606 nm in a wavelength range over 500–750 nm can be assigned to the Pr^{3+ 1}D₂→³H₄ transition [37]. The emission spectrum of KBF:2%Nd, upon 808 nm excitation, shows three emission bands at 870, 1054, and 1325 nm. This can be attributed to the characteristic ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 transitions of Nd³⁺, respectively, as shown in Figure 7b, while its excitation spectrum is made up of a group of sharp lines in the visible and NIR regions arising from intraconfigurational f-f transitions of Nd³⁺ [38]. Figure 7c gives the photoluminescence properties of the yielded KBF:5%Sm nanocrystalline particles. The emission spectrum shows several distinct bands centered at 559, 595, 641, and 700 nm ascribed to the ⁴G_5/2→⁶H_J (J = 5/2, 7/2, 9/2, and 11/2) transitions of Sm³⁺ ions upon 399 nm excitation, respectively [36]. The excitation spectrum of KBF:5%Sm monitored at 595 nm emission spans from 300 to 500 nm with a dominant excitation peak at 399 nm ascribed to the ⁶H_5/2→⁴K_11/2 electron transition of Sm³⁺ [39]. Figure 7d depicts the photoluminescence properties of KBF:5%Dy nanocrystals. The excitation spectrum monitored at 572 nm emission is composed of a group of narrow excitation bands in the UV assigned to the electron transitions from ⁶H_15/2 to ⁶P_3/2, ⁶P_7/2, ⁶P_5/2, and ⁴I_13/2 excited levels of Dy³⁺ ions, respectively [40]. Under the excitation of 364 nm, the emission spectrum exhibits four distinct bands with a maximum emission peak of 572 nm due to the Dy^{3+ 4}F_9/2→⁶H_13/2 transition. Overall, the emission colors of the as-prepared nanocrystals can be finely tuned by doping different Ln³⁺ in a single KBF host.

4. Conclusions

In conclusion, we have successfully prepared pure and doped KBF nanocrystalline particles via a rapid water-based precipitation route. With varying the ratios of F/Bi and reaction temperatures, the morphology of samples can be tuned from cubic shape to irregular polyhedron. The as-prepared KBF:Ln (Ln = Pr, Nd, Sm, Dy, Eu, and Tb) nanophosphors exhibit intense characteristic photoluminescence from the electron transitions of the doped trivalent lanthanides. Meanwhile, the morphology and photoluminescence performance of these KBF:Ln nanophosphors remain almost unchanged even when immersed in water for 30 days, indicating the obtained KBF:Ln samples have excellent chemical stability. The merit of multicolor luminescence in these KBF:Ln nanocrystalline particles holds great promise when applied to a wide range of important fields, such as optoelectronic devices and optical thermometry.