A. Phase identification and morphology

The crystallinity and phase purity of the products were examined by powder XRD and EDS. The EDS patterns of K₂[Al(B₅O₁₀)]·4H₂O:Eu³⁺ and Al₅BO₉:Eu³⁺ (1300 °C) samples are shown in Figs. 1(a) and 1(b). The elements K, Al, B, O, and Eu exist in Fig. 1(a), and no other elements were found except Au which is from the process of pretreatment (spraying gold for the sample to enhance the conductivity of the sample). The elements Al, B, O, and Eu exist in Fig. 1(b), and no other elements were found except Au.

FIG. 1.

EDS patterns of samples: (a) K₂[Al(B₅O₁₀)]·4H₂O:Eu³⁺ and (b) Al₅BO₉:Eu³⁺.

The powder XRD pattern of K₂[Al(B₅O₁₀)]·4H₂O:Eu³⁺ sample and the simulated pattern (obtained by a software of Diamond Crystal and Molecular Structure Visualization) on the basis of single-crystal structure of K₂[Al(B₅O₁₀)]·4H₂O are shown in Fig. 2. The diffraction peaks on patterns corresponded well in position, which indicated that the Eu³⁺ ions doped K₂[Al(B₅O₁₀)]·4H₂O precursor was prepared successfully, and the doping of Eu³⁺ did not change the structures of the K₂[Al(B₅O₁₀)]·4H₂O.

FIG. 2.

XRD patterns of sample K₂[Al(B₅O₁₀)]·4H₂O:Eu³⁺.

Figure 3 shows the XRD patterns of prepared Al₅BO₉:Eu³⁺ samples by calcining the precursors at various temperatures. It can be seen that the crystallinities of the thermal conversion products gradually improve from 500 °C to 1400 °C, and the Al₅BO₉:Eu³⁺ phosphors with good crystallinity were obtained at 1300–1400 °C. It can be seen that all the diffraction peaks were in good agreement with JCPDS file No. 77-0395 for Al₅BO₉, and no peaks of impurities were observed. From these results, we can conclude that the Eu³⁺ ions doped phosphor Al₅BO₉:Eu³⁺ was prepared successfully by calcining the precursor, and the doping of Eu³⁺ did not change the structure of the product. So, the doping of Eu³⁺ had not changed the XRD patterns of the sample. It is known that the ionic radii (r) of Eu³⁺, Al³⁺, and B³⁺ (CN = 3) are 0.95, 0.54, and 0.21 Å, respectively. As the ionic radius of B³⁺ is too small, it is difficult for Eu³⁺ to replace B³⁺ in the Al₅BO₉. Hence, in this study, it is believed that Al₅BO₉:Eu³⁺ phosphor was generated by the substitution of Eu³⁺ on Al³⁺ in the forming process of K₂[Al(B₅O₁₀)]·4H₂O:Eu³⁺ precursor.

FIG. 3.

XRD patterns of samples calcined at various temperatures: (a) 500 °C, (b) 900 °C, (c) 1000 °C, (d) 1100 °C, (e) 1200 °C, (f) 1300 °C, and (g) 1400 °C.

Figure 4 shows the FT-IR spectra of Al₅BO₉/Al₅BO₉:Eu³⁺ (1300 °C)/Al₅BO₉:Eu³⁺ (1400 °C) samples. It can be seen that they are almost identical. Their absorption bands were assigned referring to the literature.^{Reference Fisch, Armbruster, Rentsch, Libowitzky and Pettke9} The weak band at 1637 cm⁻¹ is the stretching mode of O–H, which may be caused by the little surface adsorption water. The bands at 1427, 1332, 1261, and 893 cm⁻¹ might be asymmetric and symmetric stretching mode of B(3)–O in the triangular BO₃ unit, respectively. The bands at 728 and 671 cm⁻¹ are assigned as out of plane bending of B(3)–O. These assignments are consistent with previously reported structure that it only contains BO₃ unit.^{Reference Fisch, Armbruster, Rentsch, Libowitzky and Pettke9}

FIG. 4.

FT-IR spectra of samples: (a)Al₅BO₉, (b) Al₅BO₉:Eu³⁺ (1300 °C), and (c)Al₅BO₉:Eu³⁺ (1400 °C).

Figure 5 shows the SEM images of Al₅BO₉:Eu³⁺ (1300 °C) and Al₅BO₉:Eu³⁺ (1400 °C) samples. The Al₅BO₉:Eu³⁺ (1300 °C) sample exhibits the uniform tube-like morphology. The Al₅BO₉:Eu³⁺ (1400 °C) sample consists of lots of the uniform stick-like morphology. It can be seen obviously that Al₅BO₉:Eu³⁺ (1300 °C) has better dispersion and more smoother surface than Al₅BO₉:Eu³⁺ (1400 °C).

FIG. 5.

SEM images of samples: (a) Al₅BO₉:Eu³⁺ (1300 °C) and (b)Al₅BO₉:Eu³⁺ (1400 °C).

B. The excitation and emission spectra of Al₅BO₉:Eu³⁺

Figure 6 shows the excitation and emission spectra of Al₅BO₉:Eu³⁺ (1300 °C) and Al₅BO₉ samples. The excitation spectrum (interpolation) consists of a strong broad band with maximum peak at 266 nm in the shorter wave length region due to the charge transfer between O²⁻ and Eu³⁺.^{Reference Lian, Liu, Wang and Sun12} For sample Al₅BO₉:Eu³⁺ (b), the f–f transition lines of Eu³⁺ were observed in the longer wave length region, which were assigned to ⁷F₀–⁵H₃ (318 nm), ⁷F₀–⁵D₄ (361 nm), ⁷F₀–⁵L₇ (380 nm), ⁷F₀–⁵L₆ (391 nm), ⁷F₀–⁵D₃ (412 nm) and ⁷F₀–⁵D₂ (466 nm).^{Reference Parchura and Ningthoujam13} Upon excitation into the f–f transition of Eu³⁺ at 266 nm, the emission spectrum involves the characteristic emission lines originating from ⁵D₀ low energy transfer of Eu³⁺, corresponding to ⁵D₀–⁷F₁ (591 nm), ⁵D₀–⁷F₂ (615 nm), and ⁵D₀–⁷F₄ (705 nm). In general, when the Eu³⁺ ion is located at the crystallographic site without inversion symmetry, its hypersensitive forced electric-dipole transition ⁵D₀–⁷F₂ red emission dominates in the emission spectrum. If the Eu³⁺ site possesses an inversion center, ⁵D₀–⁷F₁ orange emission is dominant. In this, the peak at 615 nm assigned to the electro-dipole transition ⁵D₀–⁷F₂ of Eu³⁺ is the dominant transition, indicating that the Eu³⁺ does occupy the crystallographic site without inversion symmetry in the host lattice according to the Judd–Ofelt theory.^{Reference Parchura and Ningthoujam13}

FIG. 6.

Excitation spectra (λ_em = 615 nm) and the emission spectra (λ_ex = 266 nm) of samples: (a) Al₅BO₉ and (b) Al₅BO₉:Eu³⁺ (1300 °C).

C. The luminescent decay lifetime and quantum efficiency of Al₅BO₉:0.05Eu³⁺

The luminescent decay curve of Al₅BO₉:0.05Eu³⁺ sample prepared by sintering the precursor at 1300 °C for 5.5 h (Fig. 7) can be well fitted into the following double exponential function instead of a single exponential function.

$$I = {I_1}\exp ( - t/{\tau _1}) + {I_2}\exp ( - t/{\tau _2})\quad ,$$

where I ₁ and I ₂ are the intensities at different time intervals, while τ₁ and τ₂ are their corresponding lifetimes. The average luminescent decay lifetime is 0.461 ms, and the luminescent decay curve can be well fitted into the double exponential function, and the goodness of parameter is found to be 0.98305. The value of the parameter goodness is well enough and the double exponential fitting is satisfactory. In addition, the quantum efficiency of this sample was measured up to 40.37%.

FIG. 7.

The luminescent decay curve of Al₅BO₉:0.05Eu³⁺ (1300 °C).

D. Effect of the doping amount of Eu³⁺ on the luminescence properties of Al₅BO₉:Eu³⁺

The luminescent intensity of the phosphor is strongly influenced by the activator concentration.^{Reference Yu, Lin, Wang, Fu, Wang, Zhang and Han14,Reference Li, Cao, Li and Huang15} The luminescent intensity of Al₅BO₉:Eu³⁺ (1400 °C) varies with the amount of doped Eu³⁺ ion, as shown in Fig. 8. The emission intensity increase with the concentration of Eu³⁺ ion when the amount of Eu³⁺ ion is low, and it goes to the maximum when the concentration of Eu³⁺ ion reaches 5 mol%. After that, the intensity goes down. This concentration quench of Eu³⁺ ion in Eu_xAl_5−xBO₉ occurs obviously. The ionic radius of Eu³⁺ ion is a little larger than that of Al³⁺ ion, thus when the amount of Eu³⁺ ion exceeds its critical limits, the lattice structure of Eu_xAl_5−xBO₉ is distorted and unstable. Moreover, when excess Eu³⁺ ion is in the host, the extra Eu³⁺ ion has to stay in the interstitial site of lattice rather than the site of Al³⁺.^{Reference Kim, Kim, Chun, Cho, Jung and Kang16} Therefore, the energy transfer between Eu³⁺ ion in lattice site and Eu³⁺ ion in interstitial site occurs. This energy transfer results in the increase of nonradiative transition between the Eu³⁺ ions due to the electric multiple–multiple interactions, which is distance-dependent according to Dexter's theory,^{Reference Liu, Lin and Lin17} and then leads to the concentration quench of Eu³⁺ ion. Thus, the content of Eu³⁺ is fixed at 5 mol% in the following contrast experiments.

FIG. 8.

Emission spectra of Al₅BO₉:Eu³⁺ (1400 °C) phosphors with various concentrations of Eu³⁺ excited by a 266 nm UV light: (a) 1 mol%, (b) 5 mol%, (c) 9 mol%, and (d) 13 mol%.

E. Effect of calcination time of precursor on the luminescence properties of Al₅BO₉:Eu³⁺

Theoretically, the PL intensity increases with prolonged reaction time because the longer the reaction interval, the better the crystallization and the luminescence properties.^{Reference Lei, Yan, Chen and Zhao18} It can be seen from Fig. 9 that the luminescence intensity enhances with the increasing reaction time and reaches a maximum at 5.5 h, however the intensity decreases dramatically when the reaction prolongs to 8 h. The reason is that the long calcination time results in the agglomeration, which leads to the increase of surface defects and decrease of exciting surface area.^{Reference Wang, Liang, Huang and Liu19} So the best calcination time is determined to be 5.5 h.

FIG. 9.

Emission intensity of Al₅BO₉:Eu³⁺ prepared by calcining the precursor at 1300 °C for various times: (a)3.5 h, (b)5.5 h, and (c)8 h.

F. Effect of calcination temperature of precursor on the luminescence properties of Al₅BO₉:Eu³⁺

Figure 10 shows the emission spectra of Al₅BO₉:Eu³⁺ phosphors prepared at 1300 and 1400 °C, respectively. Although the major peak positions in the emission spectra are identical to each other, the luminescence intensity relatively decreases with the increase of calcination temperature, which might be resulted from the more dispersive, more smoother surface, and larger particle size of prepared Al₅BO₉:Eu³⁺ (1300 °C) sample compared with Al₅BO₉:Eu³⁺ (1400 °C) sample, as shown in Fig. 5.

FIG. 10.

Emission spectra (λ_ex = 266 nm) of Al₅BO₉:Eu³⁺ prepared by calcining the precursor at different temperatures: (a) 1300 °C and (b) 1400 °C.

G. Comparing study of Al₅BO₉:Eu³⁺ prepared by precursor method and solid-state reaction method

Figure 11 shows the XRD pattern and the SEM image of Al₅BO₉:Eu³⁺ (1300 °C, S) sample prepared by conventional solid-state reaction method at 1300 °C for 5.5 h, respectively. It can be seen that the Al₅BO₉:Eu³⁺ (1300 °C, S) sample exhibits collective block-like morphology, and its XRD pattern is almost the same as that of prepared by precursor method at 1300 °C.

FIG. 11.

SEM image (a) and XRD pattern (b) of Al₅BO₉:Eu³⁺ sample prepared by conventional solid-state reaction method at 1300 °C.

From Fig. 12, we can see that the PL of Al₅BO₉:Eu³⁺ (1300 °C) and Al₅BO₉:Eu³⁺ (1300 °C, S) samples show different profiles though their main emission peak positions are almost the same. This observation may relate to the different symmetries around Eu³⁺ in the hosts, which resulted from their different prepared methods.

FIG. 12.

Emission spectra (λ_ex = 266 nm) of Al₅BO₉:Eu³⁺ prepared by different methods: (a) precursor method at 1300 °C and (b) conventional solid-state reaction method at 1300 °C.

We can see that the Al₅BO₉:Eu³⁺ (1300 °C, S) phosphor prepared by conventional solid-state reaction method has the dominant transition at 591 nm assigned to ⁵D₀–⁷F₁ of Eu³⁺, indicating the Eu³⁺ occupy the crystallographic site with inversion symmetry in the host lattice site^{Reference Parchura and Ningthoujam13}; while the Al₅BO₉:Eu³⁺ (1300 °C) phosphor prepared by present precursor reaction method has the dominant transition at 615 nm assigned to ⁵D₀–⁷F₂ of Eu³⁺, indicating that the Eu³⁺ does occupy the crystallographic site without inversion symmetry in this host lattice.^{Reference Parchura and Ningthoujam13} In Al₅BO₉ crystal structure, there exists three different kinds of Al³⁺ coordinated surroundings, namely AlO₆ octahedron, AlO₅ bipyramid, and AlO₄ tetrahedron.^{Reference Fisch, Armbruster, Rentsch, Libowitzky and Pettke9,Reference Gatta, Rotiroti, Fisch and Armbruster20} Bond distances and angles show that AlO₆ octahedra and AlO₄ tetrahedra are not regular, but bipyramidal units are strongly distorted.^{Reference Gatta, Rotiroti, Fisch and Armbruster20} It is obvious that AlO₅ bipyramid has the inferior symmetry. So, we deduce that the Eu³⁺ cations in Al₅BO₉:Eu³⁺ phosphor prepared by conventional solid-state reaction method occupied the AlO₆ and/or AlO₄ sites, while the Eu³⁺ cations in Al₅BO₉:Eu³⁺ phosphor prepared by present precursor method occupied the AlO₅ position.

From Fig. 12 inset for the red-orange ratio of samples, it is obvious that the color purity is prominently enhanced by the present precursor method, indicating that this product could serve as a new potential red phosphor. The improvement of R/O value might be ascribed to the lower preparation temperature of precursor and its uniform tube-like morphology with better dispersion and more smoother surface.