Figure 1 shows the X-ray powder diffraction (XRD) patterns of Ca₁₂Al₁₄O₃₃, Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺. All diffraction patterns accord with JCPDS PDF#09-0413 cards well. No other miscellaneous diffraction peaks are observed, which proves that the three samples are completely transformed into Ca₁₂Al₁₄O₃₃ crystalline phase without generating other types of calcium aluminates. Meanwhile, the sharp and intense diffraction peaks indicate that the as-prepared samples have high crystalline. We used PowderX (Dong, 1999) to execute smooth, deduct back bottom, and isolate K_α2 line diffraction peak of Cu target, seek peak and perform the index treatment of each diffraction peak for the obtained XRD patterns. The results demonstrate that crystal cells of Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺ belong to the cubic crystal system with an I-43d space group, and the crystal cell parameters are a = 11.9895 Å and a = 11.9892 Å, Z = 2, respectively. The crystal cell parameters of two samples are slightly bigger than that of Ca₁₂Al₁₄O₃₃ (11.9820 Å), which means that lattice mild expansion takes place after a small amount of Tb³⁺ or Sm³⁺ ions entering the crystal lattice to replace Ca²⁺ ions in Ca₁₂Al₁₄O₃₃.

The fourier transforming infrared (FT-IR) absorption spectra of Ca₁₂Al₁₄O₃₃, Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺ are shown in Figure 2 which are basically coincide with the results of the reported Ca₁₂Al₁₄O₃₃ (Tas, 1998). The absorption band of condensate and isolation AlO₄ locates at the range of 900–700 cm⁻¹ and 800–650 cm⁻¹, as well as the absorption band of condensate and isolation of AlO₆ locates at the range of 680–500 and 530–400 cm⁻¹, respectively (Yi et al., 2015). As a consequence, the strong broad band absorptions at around 800 cm⁻¹ in Figure 2 are attributed to AlO₄ stretching vibration, which are composed by two absorption peaks at 850.40 and 773.4 cm⁻¹. Those results demonstrate there are two AlO₄ tetrahedral structures in the lattice, which is accordance with the obtained structure in the Ca₁₂Al₁₄O₃₃ unit cell (Boysen et al., 2007). Because AlO₆ octahedral structure is inexistence in the Ca₁₂Al₁₄O₃₃ unit cell and the absorption band located at 400–620 cm⁻¹ shows two group strong peaks in Ca₃Al₂O₆, Ca₁₂Al₁₄O₃₃, CaAl₁₂O₁₉, CaAl₄O₇, CaAl₂O₄, etc. calcium aluminate, the peaks located 617.1, 574.71, and 462.8 cm⁻¹ should derive from characteristic vibration absorption of Al-O bonds. All the above prove that Ca₁₂Al₁₄O₃₃ crystal lattice structure is no more obviously changed except only slight distortion when the Ca²⁺ ions are replaced by Tb³⁺ or Sm³⁺, which is consistent with XRD analysis results.

The light absorption ability of the as-prepared samples is evaluated by the UV-vis diffuse reflectance spectra (DRS). As shown in Figure 3 all samples exhibit strong ultraviolet light absorption characteristics located at 240–320 nm. The steep shapes indicate that the intense absorptions are not due to the transition from the impurity level but band-gap transition (Li et al., 2015, 2017). It is noted that the absorption peak and absorption edge of pure Ca₁₂Al₁₄O₃₃ are located at 263 nm and 309 nm, respectively. However, after doping Tb³⁺ and Sm³⁺ ions, the absorption peaks and absorption edges of Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺ have apparent red-shift compared with that of original Ca₁₂Al₁₄O₃₃ sample, which is red-shift of about 6 and 12 nm toward the longer wavelengths and located at 269 and 321 nm, respectively. The optical absorption change may result from the doping effect of Tb³⁺and Sm³⁺ causing slight lattice expansion of Ca₁₂Al₁₄O₃₃.

The morphologies of the as-prepared samples are observed by photomicrographs measured via scanning electron microscopy (SEM). As shown in Figures 4a–c the images with low magnification of Ca₁₂Al₁₄O₃₃, Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺ present porous and irregular bulk feature, which have the obvious agglomeration gathered by some particles. It may result from high-temperature calcination for a long time. Correspondingly, Figures 4a1–c1 are the high-magnification SEM images of Ca₁₂Al₁₄O₃₃, Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺, respectively. Those samples exhibit honeycomb distribution composed of crystalline granular adhesions with few microns, smooth surface and better crystallization effect. This result can be attributed to the following reasons: in the SPCS process, a lot of gases are released to damage the formation of massive structures owing to urea burning, so that crystal nuclei growth is along to direction to formation of sphere shapes containing the lower surface energy. The shapes of as-prepared samples influence the luminescence performance to some extent and lots of researches have shown spherical surface are conducive to enhancing luminescent intensity (Kang et al., 2000).

The bare Ca₁₂Al₁₄O₃₃ has no luminescence property without doping earth ions. However, when Ca₁₂Al₁₄O₃₃ is doped by Tb³⁺ and Sm³⁺ ions, it will produce characteristic luminescent emission of these two ions. Figure 5 is the luminescent emission intensity of Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺ with different doping amount, where the standard of comparison is evaluated by the strongest energy level transition of ⁵D₄→ ⁷F₅ for Tb³⁺ and ⁴G_5/2→ ⁶H_7/2 for Sm³⁺ ions. Obviously, the luminescent intensity of Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺ present increase first and then decrease with increasing the doping amount of Tb³⁺ and Sm³⁺ ions. When the doping amount of Tb³⁺ and Sm³⁺ is 0.02 of molar fraction, the luminescent emissions reach up the strongest intensity, because the excess rare earth ions usually produce fluorescence quenching effect that results in reduction of luminescent emission intensity.

Figure 6 shows the excitation (λ_em = 545 nm) and emission spectra (λ_ex = 359 nm) of Ca₁₂Al₁₄O₃₃: Tb³⁺. Because the 4f⁷ state of Tb³⁺ ions has a stable semi-filled electron configuration, Tb³⁺ ions can be excited by the relative low energy, whose excitation band is always composed by f → f and f → d transition. Therefore, from the excitation spectrum of Ca₁₂Al₁₄O₃₃: Tb³⁺, we can draw to a conclusion that the excitation band at the short-wave 220–315 nm corresponds to the 4f⁸ → 4f⁷5d¹ transition absorption of Tb³⁺ ions, and the excitation band located at 320–400 nm originates from f → f transition absorption, where the energy level transition of different absorption peaks at 381, 360, 354, 344, 333, 326, and 322 nm may be attributable to the energy level transition absorption of ⁷F₆ → (⁵D₃/⁵G₆), ⁵G₅, (⁵D₂/⁵G₄/⁵L₉), (⁵G₃/⁵L₈/⁵L₇), (⁷F₆→ ⁵L₆/⁵G₂), ⁵D₁, ⁵D₀, respectively (Fu et al., 2010). Furthermore, from the emission spectrum of Ca₁₂Al₁₄O₃₃: Tb³⁺, we can find that the common linear emission peak of Tb³⁺ ion presents wide band distribution, which is different from other common fluorescent materials (Fu et al., 2010; Dong, 2011). The emission peaks located at (495, 511), (529, 547), 597, and 623 nm come from the energy level transition of ⁵D₄→ ⁷F₆, ⁷F₅, ⁷F₄, ⁷F₃, respectively (Fu et al., 2010). Due to the large J-value of the transition, the crystal field will result in the splitting of these energy levels. Meanwhile, to eliminate the parity-forbidden transition of Tb³⁺ ions, the opposite parity energy level of 4f configuration is not the charge transfer band, but is the 4f⁷5d¹ energy level with low energy. ⁵D₄→ ⁷F₆ electric dipole transition of Tb³⁺ ions is not as sensitive to ligand environment as the ⁵D₀→ ⁷F₂ electric dipole transition of Eu³⁺ ions. Therefore, ⁵D₄→ ⁷F₅ magnetic dipole transition is the strongest in the emission spectra, so that the Ca₁₂Al₁₄O₃₃: Tb³⁺ sample emits green light when it is excited under the ultraviolet light. Meanwhile, the stronger emission peaks located at 456, 463, and 482 nm origin from the higher excited states ⁵D₃→ ⁷F₃, ⁷F₂, ⁷F₁ energy level transitions, respectively, which indicates that there is lightly cross relaxation between ⁵D₃ and ⁵D₄ energy levels (Fu et al., 2010; Dong, 2011).

Moreover, the excitation (λ_em = 604 nm) and emission spectra (λ_ex = 382 nm) of Ca₁₂Al₁₄O₃₃: Sm³⁺ are exhibited in Figure 7 As can be seen from the excitation spectrum, there are five groups of excitation peaks in the range from 320 to 420 nm. It corresponds to the high energy f → f configuration transition absorption of Sm³⁺ ions, where the excitation peaks located at 406, 379, 372, 367, and 348 nm may be belong to the transition absorption of ⁶H_5/2 → (⁴F_7/2/⁴L_13/2),(⁴D_1/2/⁶P_7/2),(⁶H_5/2→ ⁴L_17/2/⁴K_13/2),⁴F_9/2,⁴K_15/2, respectively (Zhang et al., 2010). From the emission spectrum, the emission peaks located at 568 nm, 604 nm, 655 nm and 714 nm come from the energy level transition of ⁴G_5/2→ ⁶H_5/2, ⁶H_7/2, ⁶H_9/2, ⁶H_11/2, respectively (Zhang et al., 2010). P. S. May and coworkers (May et al., 1992) found that, ⁴G_5/2→ ⁶H_5/2 mainly belongs to a magnetic dipole transition, and partly belongs to electric dipole transition; though ⁴G_5/2→ ⁶H_7/2 is magnetic dipole transition, the electric dipole transition plays a predominance function; ⁴G_5/2→ ⁶H_9/2 is assigned to electric dipole transition, but the magnetic dipole transition is forbidden. In addition, according to the results reported by Tamura (Tamura and Shibukawa, 1993), if Sm³⁺ ion mainly occupies the asymmetry center, it can produce typical emission near 650 nm. On the contrary, if Sm³⁺ ion mainly occupies the symmetry center, it can produce typical emission near 602 nm. Therefore, as can be seen from the emission spectrum of Ca₁₂Al₁₄O₃₃: Sm³⁺, the transition emission intensity of ⁴G_5/2→ ⁶H_7/2 is bigger than that of ⁴G_5/2→ ⁶H_9/2, and the transition emission intensity of ⁴G_5/2→ ⁶H_5/2 is bigger than that of ⁴G_5/2→ ⁶H_9/2, which indicate the Sm³⁺ ions mainly occupies the symmetry center in the lattice. Besides, a strong and refined-structure transition emission peak is observed at 450–580 nm, which may come from the transition emission of high energy level of Sm³⁺ ions.

The dye methylene blue (MB) is typical organic pollutant, which is usually used as target molecule to evaluate the photocatalytic ability of the photocatalytic materials (Dong et al., 2014c). Figures 8A, 9A show the degradation dynamic curves of dye MB over the Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺ samples, respectively. After running 15 min, two samples all show the high degradation rates is more than 98% for removing dye MB, respectively, whose photocatalytic activities are obviously higher than that of bare Ca₁₂Al₁₄O₃₃. Moreover, the kinetic curves of dye MB degradation can be approximated as the pseudo-first-order process (Dong et al., 2013, 2014a,b,c; Li et al., 2014, 2016). By plotting the ln(c₀/c) vs. time and making linear fitting for dynamic curves in Figures 8B, 9B the removal rate constants (k) of dye MB are estimated to be 0.186 and 0.167 min⁻¹, respectively, which is distinctly higher than that of bare Ca₁₂Al₁₄O₃₃ (0.131 min⁻¹). Moreover, according to the absorbance variations of dye MB solutions in Figures 8C, 9C in the photocatalytic reaction process, there are no shifting of the maximum absorption position of dye MB solution at 664 nm. In addition, the absorption peak at 293 nm in the ultraviolet range also vanishes, which implies that the benzene/heterocyclic rings of dye MB molecule may be completely decomposed, leading to the thorough mineralization of dye MB (Dong et al., 2013, 2014a,b,c; Li et al., 2014, 2016). Meanwhile, in order to investigate the reusability of the Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺ samples, the circle degradation experiments of dye MB solution over the Ca₁₂Al₁₄O₃₃: Tb³⁺ and Ca₁₂Al₁₄O₃₃: Sm³⁺ samples are all performed. As shown in Figures 8D, 9D the experimental results indicate the photocatalytic ability of two samples does not show obviously loss after four recycles, indicating that they have superior stability and reusability.

The possible transfer behavior of charge carriers, luminescent emission and photocatalytic mechanism are shown in Figure 10 Under the UV-vis light excitation, the Ca₁₂Al₁₄O₃₃ host and Tb³⁺ and Sm³⁺ ions are all excited at the same time. Electrons in the VB of Ca₁₂Al₁₄O₃₃ host transfer into the corresponding CB, as well as electrons in the ground state ⁷F₆ of Tb³⁺ and ⁶H_5/2 of Sm³⁺ ions transfer into the (⁵D₃/⁵G₆), ⁵L₁₀, ⁵G₅, (⁵D₂/⁵G₄/⁵L₉), (⁵G₃/⁵L₈/⁵L₇), (⁵L₆/⁵G₂), ⁵D₁, ⁵D₀ states and (⁴F_7/2/⁴L_13/2), (⁴D_1/2/⁶P_7/2), (⁴L_17/2/⁴K_13/2), ⁴F_9/2, ⁴K_15/2 states of them, respectively. At the luminescent process, the electrons in the excitation states ⁵D₄ and ⁵D₃ return to ⁷F states of Tb³⁺ ions to generate luminescence, such as ⁵D₄→ ⁷F₆, ⁷F₅, ⁷F₄, ⁷F₃ and ⁵D₃→ ⁷F₃, ⁷F₂, ⁷F₁ transition emission. It should be pointed out that parts of electrons in the CB of Ca₁₂Al₁₄O₃₃ host and the high energy levels of Tb³⁺ can transfer into ⁵D₄ and ⁵D₃ states by means of multi-phonon assisted relaxation effect to enhance luminescent performance. Similarly, the electrons in the excitation state ⁴G_5/2 return to ⁶H states of Sm³⁺ ions to generate luminescence, such as ⁴G_5/2→ ⁶H_5/2, ⁶H_7/2, ⁶H_9/2, ⁶H_11/2 transition emission. The parts of electrons in the CB of Ca₁₂Al₁₄O₃₃ host and the high energy levels of Sm³⁺ also can transfer into ⁴G_5/2 state by means of multi-phonon assisted relaxation effect to enhance luminescent performance. In the photocatalytic degrading MB process, parts of the electrons in the CB of the calcium aluminate host migrate to Ca₁₂Al₁₄O₃₃ host surface and are captured by O₂ molecules in water to yield superoxide radicals (•O2-). The superoxide radicals may react with H⁺ ions to further transform into hydroxyl radicals (•OH). Finally, the superoxide radicals, hydroxyl radicals and holes all decompose MB dye molecules. In MB degradation process, the optical absorption increase may result from slight lattice expansion of Ca₁₂Al₁₄O₃₃ host owing to the doping effect of Tb³⁺ and Sm³⁺ ions, which may be the main reason for the improved photocatalytic performance.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.