Synthesis and luminescence properties of a novel Eu³⁺, Tb³⁺ co-doped Al₁₈B₄O₃₃ whiskers by a gel nano-coating method

Abstract

Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ whiskers have been successfully prepared by a simple gel nano-coating method using aluminum isopropoxide as the starting materials. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), photoluminescence (PL), and thermogravimetric analysis (TGA) were used characterize the samples. The results show coexistence of the crystal phase Al₁₈B₄O₃₃, amorphous phase, and Eu³⁺, Tb³⁺ ions of the samples with initial addition Al/B ratios from 3 to 1 are incorporated into the amorphous phase. The Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ whiskers are very straight with an average diameter of 600 nm and lengths ranging from 5 to 10 μm. Under ultraviolet excitation at 365 nm, samples show mainly exhibit the characteristic emission of Eu³⁺ corresponding to $^{ 5} {\text{D}}_{ 0} \to {\text{F}}_{ 1 , 2}$ transitions due to an efficient energy transfer occurs from Tb³⁺ to Eu³⁺.

Introduction

Rare-earth-activated inorganic phosphor materials have been widely used in modern lighting and display fields, such as fluorescent lamps, cathode-ray tubes, field emission displays for their well-defined transitions within the 4f electronic shells [1–5]. As the most frequently used activator ions in luminescent materials, the Eu³⁺ and Tb³⁺ are excellent activator which mainly show emissions due to transitions of ⁵D₀ to ⁷F _J (J = 1, 2, 3, 4) in the red region and ⁵D₄ to ⁷F _J (J = 6, 5, 4, 3) in the green region, respectively [6, 7]. Moreover, it is well-known that an effective energy transfer can take place from Tb³⁺ to Eu³⁺ in several hosts [8–10].

Borates have been proved to be useful in optical materials because of their high stability, low synthesis temperature, and their high ultraviolet (UV) transparency and nonlinear properties [11]. Aluminum borate (Al₁₈B₄O₃₃) is a promising ceramic materials in this family for excellent mechanical properties, chemical inertness, low thermal expansion coefficient, and high-temperature stability [12–15]. In previous study, Al₁₈B₄O₃₃ containing rare earth elements (Ce³⁺, Tb³⁺, Eu³⁺) have been reported [16–18]. However, so far little attention has been paid to synthesis and luminescent properties of Eu³⁺ and Tb³⁺ co-doped Al₁₈B₄O₃₃ and the corresponding energy transfer from Tb³⁺ to Eu³⁺ in Al₁₈B₄O₃₃ glass–ceramic composites has not been realized and reported. On the other hand, luminescence of amorphous materials was paid little attention. This is due to the fact that the quantum yield of an amorphous material is not so high as those of a well-defined crystalline family. However, glasses are well-known in optically pumped laser materials containing Nd³⁺ [19].

Accordingly in this article, we report the synthesis of Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ crystalline-amorphous material by nano-coating method. Comparing with the previous reports [16–18], our present study is very different. We added boric acid to the aluminum isopropoxide alcohol solution to ensure boric acid and aluminum isopropoxide in the isopropyl alcohol react completely. Subsequently, the Al³⁺ in aluminum isopropoxide combines with boric acid during Lewis acid–base reaction to give amorphous aluminum borate. When the deionized water was added to the mixture dropwise, the remnants of aluminum isopropylate which undergo hydrolysis and polycondensation reactions can be utilized to coat a stable layer of alumina on amorphous aluminum borate containing of Eu³⁺ and Tb³⁺. After calcinating at higher temperatures, the glass–ceramic Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ phase is thus obtained. The gel nano-coating method enhances mechanical properties and structural stability of the glass–ceramic Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ and makes rare earth elements uniformly disperse in the final product. Moreover, we investigated the energy transfer property from Tb³⁺ to Eu³⁺ in Al₁₈B₄O₃₃ crystalline-amorphous material.

Experimental

Sample preparation

The starting materials include the following chemicals: H₃BO₃ (99.9%), Eu₂O₃ (99.99%), Tb₂O₃ (99.99%), and aluminum isopropoxide (99.99%). Aluminum isopropoxide was synthesized from high-purity aluminum metal and isopropanol, and then purified by a followed controlled distillation process [20]. The detailed synthesis procedures were described as follows: Tb₂O₃, Eu₂O₃ (molar ratio Eu/(Al + B) = 0.01, Tb/(Al + B) = 0.01) and 0.05 mol aluminum isopropoxide were premixed in 50 mL isopropanol solvent under stirring, followed by the addition of different molar H₃BO₃. The deionized water was added to the vigorously stirred mixture dropwise until aluminum isopropoxide hydrolyzed completely, and then formed a gel precursor. It was dried at 60 °C for 4 h and further calcined for 3 h at 1,250 °C in an alumina crucible.

Characterization

The as-synthesized samples were examined by XRD measurement on a Rigaku-DMax 2400 diffractometer equipped with graphite monochromatized CuKα (λ = 1.5406 Å). Infrared spectroscopy (IR) was measured on a JASCO460 plus spectrophotometer. The excitation and emission spectra were recorded on a JASCO FP-6300 spectrofluorometer. The morphology of the samples was characterized by scanning electron microscopy (SEM, JSM-5600LV, JEOL). The TEM observation was carried out using a Philips TECNAI G² s-twin microscope. The formation of the different crystalline phrases was analyzed by TGA. Thermal analyses were carried out at 10 °C min⁻¹ under air atmosphere on Mettler Toledo apparatuses (TG/DTA 815^e).

Results and discussion

XRD patterns of the samples with initial addition Al/B molar ratio varying from 4.5:1 to 1:1 are shown in Fig. 1. The pattern of the samples with initial addition Al/B = 4.5:1 shows that there are strong diffraction peaks of Al₁₈B₄O₃₃ phase (JCPDS 32-0003) along with weak diffraction peaks of Al₂O₃ (JCPDS 1-1307), EuBO₃ (JCPDS 13-0485) and TbBO₃ (JCPDS 24-1272). Both EuBO₃ and TbBO₃ exhibit very similar X-ray diffraction patterns with hexagonal-vaterite structure. This result indicates that as-synthesized samples are a mixture of Al₁₈B₄O₃₃, Al₂O₃, EuBO₃, and TbBO₃, in which Eu³⁺ and Tb³⁺ ions have not build into the Al₁₈B₄O₃₃ host lattice without change of its surroundings.

Fig. 1

XRD patterns of the Samples with different initial addition Al/B ratios: (a) 4.5:1, (b) 3:1, (c) 2:1, (d) 1:1. The marks indicate diffraction lines of (filled triangle) Al₂O₃ (open diamond) EuBO₃ (open triangle) TbBO₃

As the initial addition Al/B molar ratio from 3:1 to 1:1, the diffraction peaks of Al₂O₃, EuBO₃, and TbBO₃ crystalline phases disappear. Only the crystal phase of Al₁₈B₄O₃₃ is observed. This result indicates that a great change of Eu³⁺ and Tb³⁺ surroundings occurs with an increase of H₃BO₃ concentration.

From the XRD results mentioned above, it seems impossible that Eu³⁺ and Tb³⁺ ions are incorporated into the crystal lattice of Al₁₈B₄O₃₃, because Eu³⁺ and Tb³⁺ ions have a much larger radius, compared with Al³⁺ and B³⁺ ions.

Therefore, it can be inferred that the increase of H₃BO₃ concentration may give rise to an increase of amorphous phase, and Eu³⁺, Tb³⁺ ions dope into amorphous phase. This finding is similar the previously result reported in [16].

SEM micrograph given in Fig. 2a, b shows the morphology of Eu³⁺ and Tb³⁺ co-doped Al₁₈B₄O₃₃ whiskers with initial addition molar ratio Al/B = 1:1. Large amounts of whiskers twist around one another to form a large aggregate (Fig. 2a). A closer examination of the samples indicates that the interweaving whiskers are very straight with an average diameter of 600 nm and lengths ranging from 5 to 10 μm.

Fig. 2

SEM images of the Samples with Al/B = 1:1 a high-density Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ whiskers twisted together; b magnified image of Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ whiskers

The microscopic structure of Eu³⁺ and Tb³⁺ co-doped Al₁₈B₄O₃₃ whiskers with initial addition molar ratio Al/B = 1:1 is further characterized by the TEM technique accompanied by SAED. TEM observations (Fig. 3a) showed that there are two types of particles A and B. Particle A has thin film structure, while particle B shows thick structure. The selected area electron diffraction patterns taken from a white square were found to have diffraction points and diffuse rings. It is strongly suggested that amorphous phase and crystalline phase coexist in the samples and Eu³⁺ and Tb³⁺ are doped into the amorphous phase. It should be noted that there are weak diffraction rings in the amorphous particle A, indicating that a small of crystallites exist.

Fig. 3

a HRTEM image of Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ whiskers. b Selected area diffraction image taken from a white square (inseta)

Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ whiskers with initial addition molar ratio Al/B = 1:1 show a red–orange emission under UV excitation. Figure 4 shows the excitation and emission spectra of the sample. The excitation spectrum of samples monitored at 613 nm is composed of two excitation bands of Eu³⁺ and Tb³⁺. The f–f transitions within Eu³⁺ 4f⁶ electron configuration (main peaks at the 365, 383, 396, 406, 468 nm) is corresponding to the electron transitions from the ⁷F₀ ground state to ⁵D₄, ⁵L₇, ⁵L₆, ⁵D₃, ⁵D₂), with $^{ 5} {\text{F}}_{ 0} \to {}^{5}{\text{L}}_{ 6}$(396 nm) transition as most intense peak [21]. The characteristic f–f transition lines within the Tb³⁺ 4f⁸ configuration is assigned as the transitions from the ⁷F₆ ground state to the different excited states of Tb³⁺, that is, 374 nm (⁵G₆) 490 nm (⁵D₄), respectively [22]. The emission spectrum of samples (Fig. 4b) under 365 nm excitation consists of four bands. Besides Eu³⁺ emissions, we can also observe the characteristic emissions of Tb³⁺. The peaks at about 489, 541 nm are due to Tb³⁺ transitions from ⁵D₄ to ⁷F _J (J = 6, 5), and the other emission peaks at about 591, 613 nm are from the ⁵D₀ to ⁷F _J (J = 1, 2). Among these luminescence emission peaks, the emission spectrum is dominated by the hypersensitive red transition at 613 nm. The most prominent emission line at 613 nm is due to hypersensitive electronic dipole transition of $^{ 5} {\text{D}}_{ 0} \to {}^{2}F_{7}$, induced by the lack of inversion symmetry at Eu³⁺ local sites, while the emission near 591 nm is the magnetic dipole transition owing to the $^{ 5} {\text{D}}_{ 0} \to {}^{7}F_{1}$ states, which obey the selection rule (Δj = 0, ±1), and its intensity hardly varies with evolution of the surroundings [18].

Fig. 4

Excitation (a) and emission (b) spectra of the Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ whiskers with initial addition Al/B ratios 1:1

Figure 5 shows the emission spectra of Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ with initial addition Al/B varying from 4.5 to 1 under 365 nm excitation. With the increase of H₃BO₃ concentration, the intensity of Eu³⁺ and Tb³⁺ emissions become stranger. Meanwhile, Eu³⁺ and Tb³⁺ emission lines become inhomogeneous and broadened. This characteristic feature of Eu³⁺ and Tb³⁺ emission spectra is quite similar to that of Eu³⁺ and Tb³⁺ in an amorphous has a characteristic features: inhomogeneous broadening of luminescence band [23]. When initial addition Al/B ratio changes from 4.5 to 1, this change of luminescent properties reveals a variation of Eu³⁺ and Tb³⁺ surroundings, indicating that Eu³⁺ and Tb³⁺ of the samples dope into an amorphous phase with the increase of H₃BO₃ quantity. Moreover, the emission from the ⁵D₃ level of Tb³⁺ has not been observed in this sample, which can be explained by the quenching of the efficient multiphonon de-excited with lattice vibration due to the high energy of phonons in the borates [17].

Fig. 5

Emission spectra of the Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ samples with different initial addition Al/B ratios: (a) 4.5:1, (b) 3:1, (c) 2:1, (d) 1:1 under 365 nm excitation

The emission spectrum of Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺, Al₁₈B₄O₃₃:Eu³⁺, and Al₁₈B₄O₃₃:Tb³⁺ (initial addition molar ratio Al/B = 1:1) under 365 nm excitation are presented in Fig. 6 for comparison. In the Al₁₈B₄O₃₃:Tb³⁺ samples, only the characteristic emissions of Tb³⁺ are observed. With the doping of Eu³⁺ and Tb³⁺ in the Al₁₈B₄O₃₃, besides Tb³⁺ emission, we can also observe the characteristic emissions of Eu³⁺. With the addition Eu³⁺, the luminescence of Tb³⁺ decrease (Fig. 6), and that of Eu³⁺ increases, both due to the enhancing the probability of energy transfer from Tb³⁺ to Eu³⁺. All results above indicate an efficient energy transfer behavior shows that Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ is not a mixture of TbBO₃ and EuBO₃, but aluminoborate glass phase, in which Eu³⁺ and Tb³⁺ has been incorporated the aluminoborate glass phase at certain H₃BO₃ concentration. Otherwise, the ${\text{Tb}}^{ 3+ } \to Eu^{3 + }$energy transfer can not occur in the separated phases [10]. A summary of emission and energy transfer process of Eu³⁺ and Tb³⁺ in Al₁₈B₄O₃₃ is described in below. First, 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 emissions (${}^{ 5}{\text{D}}_{ 4} \to {}^{7}F_{6,5,4,3}$) 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 (${}^{ 5}{\text{D}}_{ 0} \to {}^{7}F_{0,1,2,3,4,5,6}$) takes place [24]. Because the ${}^{ 5}{\text{D}}_{ 4} \to {}^{7}F_{6,5,4,3}$ emission of Tb³⁺ is effectively overlapped with the ${}^{ 7}{\text{F}}_{ 0 , 1} \to {}^{5}D_{0,1,2}$ absorption of Eu³⁺, the energy transfer from Tb³⁺ to Eu³⁺ is very efficient in general.

Fig. 6

Emission spectra of samples (a) Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺, (b) Al₁₈B₄O₃₃:Eu³⁺, (c) Al₁₈B₄O₃₃:Tb³⁺

Figure 7 represents the TGA analysis of amorphous aluminum borate containing of Eu³⁺ and Tb³⁺. The TGA curves present two stages of weight loss and the total weight loss of about 31.5%. The initial weight loss between 30 and 210 °C results from desorption of isopropoxide alcohol and boric acid from the surface of amorphous precursors. During this process, H₃BO₃ will decompose into B₂O₃ at about 185 °C, and then B₂O₃ will melt at about 430 °C. There is no structural change till 750 °C. It is assigned to the glass–ceramic Al₄B₂O₉:Eu³⁺, Tb³⁺ without any weight variation. The weight loss was detected at around 1,150–1,400 °C, which corresponds to melting of Al₄B₂O₉ phase and a yield of Al₁₈B₄O₃₃ and liquid [25].

Fig. 7

Thermogravimetry and differentiate thermogravimetry trace of the as-synthesized Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ samples

Conclusion

In summary, we have demonstrated that a simple and mild gel nano-coating method for synthesis of the glass–ceramic Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ whiskers. The results reveal coexistence of the crystal phase Al₁₈B₄O₃₃ and aluminoborate glass phase and Eu³⁺, Tb³⁺ ions are incorporated into the aluminoborate glass phase at certain H₃BO₃ concentration. Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ exhibits a red–orange emission spectrum, consisting of four emission bands peaking at 489, 541, 591, and 613 nm. With initial addition Al/B molar ratio change from 4.5 to 1, Eu³⁺ and Tb³⁺ emission intensity increase. An efficient energy transfer can occur from Tb³⁺ to Eu³⁺ in the glass–ceramic Al₁₈B₄O₃₃:Eu³⁺, Tb³⁺ whiskers, which are ascribed to the energy overlap between Eu³⁺ and Tb³⁺. Further study is under way to modify luminescent color by controlling the doping concentration of Eu³⁺ and Tb³⁺ ions, for different composition of emissions of Eu³⁺ and Tb³⁺ resulted from energy efficiency at different doping concentration of Eu³⁺ and Tb³⁺.