A. Analyses of crystal phases

Figure 1 shows XRD pattern of the uncalcined sample prepared at 200 °C by the hydrothermal method. Earlandite Ca₃(C₃H₅O₇)₂·4H₂O (JCPDS No. 28-2003) and monoclinic CaCO₃ phase (JCPDS No. 70-0095) may coexist. FTIR spectrum (inset in Fig. 1) shows a band around 3368 cm⁻¹ and a weak dip at 1620 cm⁻¹, assigned to antisymmetric stretching vibration of OH⁻ ions and bending vibration of HOH, respectively. The bands centered at 1418, 1076, and 871 cm⁻¹ can be assigned to asymmetric C–O stretching vibration v ₃, symmetric C–O stretching vibration v ₁, and CO₃ out-of-plane deformation vibration v ₂, respectively. The weak peak at 715 cm⁻¹ can be assigned to OCO in-plane bending vibration v ₄. Moreover, the weak dip at 1801 cm⁻¹ may be due to the combinations v ₁ + v ₄.^{Reference Andersen and Brečević26} Strong absorption peak at 470 cm⁻¹ is attributed to the stretching vibration of the Ti–O bond.^{Reference Last27} Combined with XRD results, C–O bond may be either in carboxylate ion or in CO₃²⁻ ion, in which two ions are from citrate and urea, respectively. Therefore, it is logical to conclude that urea hydrolyzes to form OH⁻ and CO₃²⁻ ions in the sealed Teflon autoclave at 200 °C, and both ions are precipitants for Ca²⁺, Ti⁴⁺, and Zn²⁺ ions. It has been reported that high hydroxide concentration is conducive to the hydrothermal formation of CaTiO₃.^{Reference Croker, Loan and Hodentt28} Indeed, Lencka and Riman^{Reference Lencka and Riman29} have indicated that pH value higher than 14 is prerequisite to obtain high yield of CaTiO₃ at hydrothermal temperature of 150–200 °C in the presence of CaCO₃. According to our hydrothermal process conduced at relatively low pH value (<10), it is hard to form CaTiO₃.

FIG. 1.

XRD pattern of the uncalcined sample prepared by the hydrothermal method. The inset is IR spectrum of the uncalcined sample.

Figure 2 shows XRD patterns of the samples calcined at 800–1200 °C for 3 h, and the corresponding quantitative phase analyses are listed in Table I. All the calcined samples consist of orthorhombic CaTiO₃ phase (JCPDS No. 76-2400) and rhombohedral Ca₂Zn₄Ti₁₆O₃₈ phase (JCPDS No. 85-1102), while CaTiO₃ phase is dominant. Hexagonal ZnTiO₃ phase (JCPDS No. 26-1500) is only observed for the samples calcined below 950 °C, since the phase is decomposed above 945 °C, forming Zn₂TiO₄ and TiO₂.^{Reference Yang and Swisher30,Reference Wang, Hwang, Chang, Ko, His, Huang and Wang31} Thus, hexagonal ZnTiO₃ phase and cubic Zn₂TiO₄ phase coexist in the sample calcined at 950 °C. Above 950 °C, cubic Zn₂TiO₄ (JCPDS No. 73-0578) originated from decomposition of ZnTiO₃ can be observed and its amount increases with elevated calcination temperature. In the case of the sample calcined at 1200 °C, an additional TiO₂ phase is observed aside from the other three phases. Note that the sample calcined at 1150 °C possesses the highest content of CaTiO₃.

FIG. 2.

XRD patterns of Ca_0.8Zn_0.2TiO₃:0.2 mol% Pr³⁺ calcined at different temperatures.

TABLE I.

Phase content for samples calcined at different temperatures (wt%).

B. Micromorphology

SEM micrographs of the samples calcined at different temperatures are shown in Fig. 3. Clearly, uncalcined sample consists of nonuniformed particles with a size range of 3–8 μm [see Fig. 3(a)]. It is interesting that there is a hole in the middle of the sphere for most of the particles, which may be caused by the hydrothermal process. The broken particles clearly show that they are hollow with a shell thickness of 210–480 nm and smaller particles are in the middle of larger ones [shown in the inset of Fig. 3(a)]. The shape of the calcined particles does not change as shown in Figs. 3(b)–3(f). However, elevated calcination temperature leads to increased particle size and simultaneously the surface of particles becomes more smooth. When calcination temperature reaches 1150 °C, tiny particles start to merge and most of them disappear. Note that the morphology of Ca_0.8Zn_0.2TiO₃:Pr³⁺ presented here is different from the results reported previously, wherein, irregular acicular, rectangular,^{Reference Kutty and Viveknandan22} butterfly-like, flower-like microsphere,^{Reference Dong, Zhao, Song, Xu, Zhou and Han23} or microcube-shaped^{Reference Moreira, Paris, Nascimento, Longo, Sambrano, Mastelaro, Bernardi, Andrés, Varela and Longo32} crystallites were obtained. In our case, citric acid is used as a chelator for Ti species and covers the reactants to form microspheres. During the hydrothermal and successive calcination processes, decomposition of organic substances perhaps results in broken particles, and the resultant gases in high temperature calcination further produce hollow spheres. Indeed, the hollow spheres with shell thicknesses of 160 nm [in the inset of Fig. 3(d)] and 230 nm [in the inset of Fig. 3(e)] are observed for the samples calcined at 1000 and 1050 °C, respectively. Nevertheless, the growth mechanism needs to be investigated in detail.

FIG. 3.

SEM micrographs of Ca_0.8Zn_0.2TiO₃:0.2 mol% Pr³⁺ uncalcined (a) and calcined at (b) 800 °C, (c) 950 °C, (d) 1000 °C, (e) 1050 °C, and (f) 1150 °C for 3 h. The insets are SEM micrographs with higher magnification.

C. The luminescent properties of Ca_0.8Zn_0.2TiO₃:0.2 mol% Pr³⁺

The excitation and emission spectra of Ca_0.8Zn_0.2TiO₃:0.2 mol% Pr³⁺ synthesized at various temperatures are shown in Fig. 4. Upon excitation at wave lengths 330 and 380 nm, the sharp emission peaks located at 612 nm originates from ¹D₂ → ³H₄ of Pr³⁺ ions for the sample calcined at 1150 °C, which is in the red spectral region [see Fig. 4(a)]. The inset of Fig. 4(a) displays the intensity evolution of emissions with the increase of calcination temperature, i.e., the higher the calcination temperature, the higher the intensity of emission. As evidenced by XRD results, elevated calcination temperature leads to increased crystallinity, which, in turn, would result in the stronger interaction between the matrix and Pr³⁺ energy levels and eventually the increase of red emission. As shown in Fig. 4(b), each excitation curve under monitoring at 612 nm mainly consists of two strong broad bands centered at about 330 nm (marked as A) and 380 nm (marked as B) and a weaker broad band ranged from 450 to 500 nm (marked as C). The band A can be related to the transition from the valence band to the conduction band of the host, which is from O²⁻ 2p levels to Ti⁴⁺ 3d levels.^{Reference Diallo, Boutinaud, Mahiou and Cousseins2} The band B can be attributed to a low-lying Pr-metal intervalence charge transfer (IVCT), Pr³⁺/Ti⁴⁺ ↔ Pr⁴⁺/Ti³⁺, by which photoelectrons were nonradiative relaxation from the ³P₀ state to the ¹D₂ state of Pr³⁺ ion. It is generally accepted that the band C are from the 4f → 4f transition of Pr³⁺.^{Reference Diallo, Boutinaud, Mahiou and Cousseins2,Reference Zhang, Xu and Yao8,Reference Boutinaud, Pinel, Dubois, Vink and Mahiou33,Reference Dereń, Pazik, Strek, Boutinaud and Mahiou34} Note that band A slightly shift to a longer wave length with the increase of the calcination temperature, resulting from the increased crystal size that often causes a large average optical path.^{Reference Chang, Xua, Jiang, Mao and Ying35}

FIG. 4.

The emission (a) and excitation (b) spectra of samples fired at different temperatures. The inset a is the relationship between the emission intensity of Ca_0.8Zn_0.2TiO₃:0.2 mol% Pr³⁺ (λ_ex = 330 and 380 nm) and different calcination temperatures and the inset b is the ratio of band B to band A.

In addition, the excitation intensities of these three bands strongly depend on the calcination temperature. The variations of band A, band B, and the normalized excitation spectra are shown in Fig. 5. The intensities of band A and B increase more than three and five times with calcination temperature increasing from 800 to 1150 °C, respectively. From the inset, it is obvious that the intensity ratio of band B to band A passes through a maximum and then decreases with enhanced calcination temperature, and the sample calcined at 1150 °C has the highest intensity ratio.

FIG. 5.

Excitation intensities of peak at 330 nm (a), 380 nm (b) at different calcination temperatures. The inset is normalized excitation curves.

The excitation spectra of Ca_0.8Zn_0.2TiO₃:0.2 mol% Pr³⁺ prepared at 1150 °C by the solid-state and hydrothermal method, as well as CaTiO₃:Pr³⁺ phosphor prepared by the hydrothermal method at the same temperature, are illustrated in Fig. 6. Obviously, Ca_0.8Zn_0.2TiO₃:0.2 mol% Pr³⁺ prepared by the hydrothermal method has stronger excitation intensity than that of its counterpart prepared by the solid-state method at the same calcination temperature, especially the intensity of band B. It has been reported that the relative intensities of the excitation bands and the peaks of the 4f states are dependent on the sample synthesis conditions.^{Reference Jia, Jia, Rodriguez, Evans, Meltzer and Yen36} Moreover, there are only strong band A and very weak band C in the excitation spectrum of CaTiO₃:Pr³⁺, whereas band B is hard to detect.^{Reference Boutinaud, Pinel, Dubois, Vink and Mahiou33,Reference Pan, Su, Xu, Chen, Ge, Yang and Wu37} In this context, the IVCT band from Pr³⁺ to metal ions is dramatically enhanced by the hydrothermal method and Zn incorporation. It is believed that the thermodynamic conditions for IVCT could be attained by oxidizing certain amount of Pr³⁺ to Pr⁴⁺ and that the electronic charge transfer between Pr³⁺ and Ti³⁺ can readily occur at fairly low energy.^{Reference Boutinaud, Pinel, Dubois, Vink and Mahiou33} In other words, the relative concentrations of Pr³⁺, Pr⁴⁺, and Ti³⁺ in the host CaTiO₃ vary with different synthesis methods.^{Reference Kim, Lee and Kim38} Therefore, it is speculated that the hydrothermal process presented here could partly promote IVCT, leading to the significant enhancement of band B.

FIG. 6.

Comparison of excitation intensities of samples prepared at 1150 °C by the solid-state (dashed line) method and hydrothermal method (solid line and dotted line), respectively.

In our work, there are four phases including CaTiO₃, Ca₂Zn₄Ti₁₆O₃₈, ZnTiO₃, and Zn₂TiO₄. As a major phase, the content of CaTiO₃ increases with elevated calcination temperature, thus its variation results in the enhancement of band A, B, and C. From Ref. Reference Lian, Qi, Rong, Yu, Zhu, Yin and Liu39, the major excitation peak of Ca₂Zn₄Ti₁₆O₃₈: Pr³⁺, Na⁺ is located at 380 nm, and the peak at 330 nm often shrinks to undetectable levels. We note that the content of Ca₂Zn₄Ti₁₆O₃₈ has a maximum value at 950 °C. Thus, Ca₂Zn₄Ti₁₆O₃₈ is not the main reason for the remarkable enhancement of band B. Kang et al.^{Reference Kang, Chung, Kang, Lee and Kim40} have proven that the incorporation of ZnO would lead to the enhanced excitation band around 370 nm. The incorporated Zn species may form defect levels below the conduction band of the CaTiO₃ host, which can capture excited electrons, leading to the red emission.^{Reference Lian, Lin and Su12} Indeed, similar results have been obtained in Mg²⁺- or Zn²⁺-doped SrTiO₃:Pr³⁺ phosphors.^{Reference Ryu, Singh, Bartwal, Brik and Kityk41} Hexagonal ZnTiO₃ is a thermodynamically stable compound between 800 and 945 °C, above which it transforms into cubic Zn₂TiO₄.^{Reference Yang and Swisher30} From the results of XRD, it is reasonable that the increased content of Zn₂TiO₄ is helpful for the improvement of band B. According to the density functional theory calculation,^{Reference Zhu, Wang, Du, Zhao and Gao42} the presence of Zn defects increases the multiple localized states of trap levels so as to favor the charge transfer transitions (i.e., band B) and ¹D₂ → ³H₄ transition of Pr³⁺.

The decay curves for the samples prepared by the hydrothermal and solid-state method are shown in Fig. 7. The decay curves for the ¹D₂ → ³H₄ emission of Pr³⁺ can be fitted by a double-exponential function: I(t) = A ₁exp(−t/τ₁) + A ₂exp(−t/τ₂), where I is the luminescent intensity, A ₁ and A ₂ are constants, t is the time, τ₁ and τ₂ are the decay time for the exponential components, respectively. The corresponding fitted results are listed in Fig. 7. The average lifetime values of the phosphors prepared by the solid-state and hydrothermal method for Pr³⁺ luminescence are 52.83 ms (a) and 77.03 ms (b), respectively, calculated by the formula τ = (A ₁τ₁² + A ₂τ₂²)/(A ₁τ₁ + A ₂τ₂). It is evident that the phosphor of Ca_0.8Zn_0.2TiO₃:0.2 mol% Pr³⁺ prepared by the hydrothermal method has longer afterglow.

FIG. 7.

Decay curve of the sample prepared by the solid-state method (a) and hydrothermal method (b) calcining at 1150 °C, monitoring at 612 nm, excited at 330 nm.

The red emission of Ca_0.8Zn_0.2TiO₃:Pr³⁺ can further be confirmed by the CIE (Commission Internationale de l’Eclairage 1931) coordinations from their emission spectra. As shown in Fig. 8, upon excitation at 330 nm, the CIE chromaticity coordination of Ca_0.8Zn_0.2TiO₃:0.2 % Pr³⁺ shifts from (x = 0.593, y = 0.399) to (x = 0.636, y = 0.360) with the increase of calcination temperatures. It is closer to the chromaticity coordinate of the standard red light,^{Reference Chadha, Smith, Vecht and Gibbons1} indicating that Ca_0.8Zn_0.2TiO₃:0.2% Pr³⁺ calcined at 1150 °C is a potential red phosphor for the near-UV-pumped white LED.

FIG. 8.

The corresponding CIE chromaticity of diagram of sample calcinated at 800 °C (a), 900 °C (b), and 1150 °C (c).