1. Introduction

White light emitting diodes (WLEDs) possess numerous advantages, such as high luminescent efficiency, long lifetime, fast response time, small volume and environmental friendliness, therefore, they have become new generation lighting technique [1–6]. Currently, the main commercial WLEDs consist of InGaN blue chip with Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) yellow phosphor, and then the white light is obtained by mixing yellow light from YAG:Ce and residual blue light from InGaN. However, this kind of WLEDs lacks red light component, resulting in low color rendering index and high correlated color temperature, so the obtained white light cannot satisfy the demands in many aspects, for example room illumination [7–10]. Combining (near-)ultraviolet (n-UV) LEDs with tricolor (red, green and blue) phosphors or single phase white light emitting phosphors are recently promising ways to obtain high quality white light [11–14]. The studies to develop n-UV excited multicolor or white light emitting phosphors with excellent luminescent properties are very meaningful. In comparison, silicate materials are excellent hosts of phosphors with advantages of high thermal and chemical stabilities, low cost, and water-resistance. Hence, the present work focuses on the studies about n-UV excited monoclinic Ca₃ZrSi₂O₉ based phosphors. Monoclinic Ca₃ZrSi₂O₉ has a suitable layer structure to act as host materials for phosphors [15]. Layer structure should be resistant to concentration quenching in phosphors, even if a large amount of activators are doped into the host. Back to the research progress, Kim et al. have synthesized single phase Eu³⁺ singly doped Ca₃ZrSi₂O₉:Eu³⁺ phosphors exhibiting typical red light emission assigned to the electron transitions from ⁵D₀ to ⁷F_J (J = 0, 1, 2, 3 and 4) of Eu³⁺ ions, and the internal quantum yield of phosphors Ca_2.83ZrSi₂O₉:0.17Eu³⁺was estimated to be 41% under UV excitation at 268 nm [16]. Further, Eu³⁺ and Al³⁺ co-doped Ca₃ZrSi₂O₉:Eu³⁺,Al³⁺ phosphors were synthesized by the same research group, and the partial substitution of Ca²⁺ site with Eu³⁺ and Zr⁴⁺ site with Al³⁺ maintained the charge balance, relaxed the lattice strain, enhanced the crystallinity of phosphors, and ultimately resulted in the enhancement of the emission intensity of Eu³⁺ ions [17]. Also, the same team synthesized green emitting Ca₃ZrSi₂O₉:Tb³⁺,Al³⁺ phosphors, and the emission intensity of Tb³⁺ ions was effectively enhanced by Al³⁺ doping into the Zr⁴⁺ sites. The quantum yield of phosphors Ca_2.87ZrSi₂O₉:0.13Tb³⁺ was estimated to be 27% under UV excitation at 256 nm, while that of phosphors Ca₂.₉₁Zr₀.₉₅Si₂O₉:0.09Tb³⁺,0.05Al³⁺ was estimated to be 36% [18]. Newly, Zhong et al. introduced sensitizer Bi³⁺ into red emitting phosphors Ca₃ZrSi₂O₉:Eu³⁺,Bi³⁺, and the inner quantum yield of this phosphors was greatly promoted due to the efficient sensitizing of Eu³⁺ with Bi³⁺ ions [19], however, they mainly reported the red emissions from Eu³⁺ ions, and there are not any descriptions about the luminescent properties of Bi³⁺ ions in phosphors Ca₃ZrSi₂O₉:Eu³⁺,Bi₃, while Bi³⁺ ions are also usually used as activators in luminescent materials [20–22]. In the present study, a series of Eu³⁺, Bi³⁺ and Tb³⁺ tri-doped Ca₃ZrSi₂O₉ based phosphors were successfully synthesized by solid state reaction method, and white light was obtained under UV excitation by the mixture of emissions from tricolor luminescent centers (Eu³⁺, Bi³⁺ and Tb³⁺). Otherwise, the energy transfer among Eu³⁺, Bi³⁺ and Tb³⁺ has been deduced, and the thermal stability of this kind of phosphors was also investigated, facing the practical applications of phosphors.

2. Experimental sections

2.1 Synthesis

A series of Ca₃ZrSi₂O₉ based phosphors were designed and synthesized by conventional solid state reaction method, including (1) singly doped Ca_2.83ZrSi₂O₉:0.17Eu³⁺, Ca_2.85ZrSi₂O₉:0.15Bi³⁺ and Ca_2.87ZrSi₂O₉:0.13Tb³⁺, (2) doubly doped Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (y = 0.03, 0.09, 0.15, 0.20), and (3) triply doped Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (z = 0.03, 0.09, 0.13, 0.20, 0.30, 0.40, 0.50). CaCO₃ (analytical pure), ZrO₂ (analytical pure), SiO₂ (spectral pure), Eu₂O₃ (high pure), Bi₂O₃ (analytical pure) and Tb₄O₇ (high pure) are used as raw materials for synthesizing above mentioned phosphors. The used raw materials CaCO₃, ZrO₂, SiO₂ and Bi₂O₃ were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, and raw materials Eu₂O₃ and Tb₄O₇ were purchased from Shanghai Yuelong New Materials Co. Ltd., Shanghai, China. To obtain designed phosphors Ca_2.83ZrSi₂O₉:0.17Eu³⁺, Ca_2.85ZrSi₂O₉:0.15Bi³⁺, Ca_2.87ZrSi₂O₉:0.13Tb³⁺, Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ and Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺, the stoichiometric mole ratio of raw materials according to corresponding chemical formula were thoroughly mixed in an agate mortar, and then were sintered at 1400 °C for 5 h in air atmosphere using the box-type furnace.

2.2 Characterization

The phase composition was evaluated by a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. The photoluminescence (PL) emission and excitation spectra were recorded by a Hitachi F-4600 fluorescence spectrophotometer with 150 W Xe lamp as an excitation source, and the CIE chromaticity coordinates were calculated by the cie software utilizing the measured PL spectra. The excitation and emission slits were set as 1.0 nm and 2.5 nm, respectively, and the scan speed was fixed at 240 nm/min. The above-mentioned Hitachi F-4600 fluorescence spectrophotometer with an in-house made heating apparatus was used to measure the temperature dependent PL spectra to evaluate high temperature stability of photoluminescence. The decay curves were measured on an Edinburgh FLS980 spectrometer with a nanosecond flashlamp nF920 and a microsecond flashlamp μF2 as the excitation source. X-ray photoelectron spectroscopy (XPS) spectra were measured by an ESCAlab250 XPS system (Thermo Fisher Scientific, USA) with an Al Kα source. All measurements, except the temperature dependent PL spectra, were operated at room temperature.

3. Results and discussion

3.1 Crystal phase and luminescent properties of co-doped Ca₃ZrSi₂O₉:Eu³⁺,Bi³⁺ phosphors

3.1.1 Eu³⁺ single doping

At the very beginning of this research, Eu³⁺ singly doped Ca₃ZrSi₂O₉ based phosphor was designed as a foundation. Based on the reported investigation, the highest emission intensity of red light emitting phosphors Ca_3-xZrSi₂O₉:xEu³⁺ was obtained for the composition Ca_2.83ZrSi₂O₉:0.17Eu³⁺ [16], so the optimal Ca_2.83ZrSi₂O₉:0.17Eu³⁺ phosphors were firstly synthesized at 1400 °C for 5 h in this starting work. Under the UV excitation at 270 nm or 392 nm, the emission spectra of Ca_2.83ZrSi₂O₉:0.17Eu³⁺ exhibit typical red light emitting features of Eu³⁺ ions, and the emission peaks at about 577 nm, 591 nm, 610 nm, 652 nm and 705 nm are attributed to the transitions of ⁵D₀-⁷F₀, ⁵D₀-⁷F₁, ⁵D₀-⁷F₂, ⁵D₀-⁷F₃ and ⁵D₀-⁷F₄, respectively [10,16,23], as shown in Fig. 1(a). The excitation spectrum monitoring the emission at 610 nm for Ca_2.83ZrSi₂O₉:0.17Eu³⁺ exhibits a broad band with the maximum at about 270 nm due to charge transfer transition of O^2--Eu³⁺, and several narrow excitation peaks between 350 nm and 550 nm at about 360 nm, 380 nm, 392 nm, 402 nm, 413 nm, 463 nm and 530 nm are attributed to intra-4f transitions of Eu³⁺ ions. Otherwise, under the excitation at 270 nm, the CIE chromaticity coordinates of Ca_2.83ZrSi₂O₉:0.17Eu³⁺ are (0.608, 0.354) (listed in Table 1) and they are located in red light region in CIE chromaticity diagram, as shown in Fig. 2.

 

Fig. 1 (a) Excitation spectrum measured by monitoring the emission at 610 nm, and emission spectra under the excitation at 270 nm, 392 nm for Ca_2.83ZrSi₂O₉:0.17Eu³⁺, (b) excitation spectrum measured by monitoring the emission at 420 nm, and emission spectrum under the excitation at 300 nm for Ca_2.85ZrSi₂O₉:0.15Bi³⁺.

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Table 1. CIE chromaticity coordinates (x, y) and corresponding excitation wavelength for Eu³⁺, Bi³⁺singly and doubly doped Ca₃ZrSi₂O₉ based phosphors

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Fig. 2 CIE chromaticity diagram for Eu³⁺ , Bi³⁺ singly doped Ca_2.83ZrSi₂O₉:0.17Eu³⁺(Eu³⁺ ), Ca_2.85ZrSi₂O₉:0.15Bi³⁺(Bi³⁺ ) phosphors and doubly doped Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (y = 0.03, 0.09, 0.15, 0.20) (1, 2, 3, 4) phosphors, corresponding to Table 1.

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3.1.2 Bi³⁺ single doping

To reveal the luminescent characteristic of Bi³⁺ ions in Ca₃ZrSi₂O₉ host, the Bi³⁺ singly doped phosphor Ca_2.85ZrSi₂O₉:0.15Bi³⁺ was also synthesized at 1400 °C for 5 h. The ground state of Bi³⁺ ions is singlet state ¹S₀ with 6s² electron configuration, and the first excited state with 6s6p electron configuration consists of triplet states ³P₀, ³P₁ and ³P₂ and singlet state ¹P₁ with an increasing energy order. The transitions ¹S₀-³P₁ or ¹P₁ are parity allowed due to spin orbit coupling, while the transitions ¹S₀-³P₀ or ³P₂ are spin forbidden [20,21,24]. Generally, the transition ¹P₁-¹S₀ often occurs in the invisible ultraviolet region, while the transition ³P₁-¹S₀ frequently occurs in the visible region. As shown in Fig. 1(b), under the excitation at 300 nm, the emission spectrum of Ca_2.85ZrSi₂O₉:0.15Bi³⁺ shows a broad band with the maximum at about 420 nm originated from ³P₁-¹S₀ transition of Bi³⁺ ions. Also a broad band with the maximum at about 300 nm appears in the excitation spectrum measured by monitoring the emission at 420 nm for Ca_2.85ZrSi₂O₉:0.15Bi³⁺ and this excitation band originates from ¹S₀-³P₁ transition of Bi³⁺ ions. The CIE chromaticity coordinates (listed in Table 1) of Ca_2.85ZrSi₂O₉:0.15Bi³⁺ are (0.152, 0.049) and they are located in blue light region in CIE chromaticity diagram (shown in Fig. 2).

3.1.3 Eu³⁺ and Bi³⁺ co-doping

It has been reported that, under certain conditions, white light emitting can be obtained in some Eu³⁺, Bi³⁺ co-doped phosphors, for example BaY₂Si₃O₁₀:Bi³⁺,Eu³⁺ [20], Sr₂Y₈(SiO₄)₆O₂:Bi³⁺,Eu³⁺ [21] and Y₂SiO₅:Bi³⁺,Eu³⁺ phosphors [22]. Based on the above researches about singly doped phosphors, Eu³⁺, Bi³⁺ co-doped phosphors, Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (y = 0.03, 0.09, 0.15, 0.20), were synthesized to find out whether the co-doped phosphors can emit white light. The X-ray diffraction (XRD) patterns of Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (y = 0.03-0.20) are shown in Fig. 3(a), and a single phase monoclinic Ca₃ZrSi₂O₉ was successfully obtained for Eu³⁺, Bi³⁺ co-doped phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺. As a general rule, the percentage difference in ion radii (D_r) between the doped ions and substituted host ions should not exceed 30%, and the calculation formula is as follows:

 

Fig. 3 (a) XRD patterns of Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (y = 0.03-0.20), and the standard XRD patterns of monoclinic Ca₃ZrSi₂O₉ (JCPDS 01-083-0365), (b) schematic crystal structure of monoclinic Ca₃ZrSi₂O₉ consisting of Ca(1)O₆ (green), Ca(2)O₆ (dark yellow), Ca(3)O₆ (purple), ZrO₆ octahedron and Si(1, 2)O₄ tetrahedron.

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The emission spectra under the excitation at 270-300 nm and 392 nm for Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (y = 0-0.20) are shown in Fig. 4(a) and 4(b), respectively. Under the excitation at 285-300 nm, the emission spectra of Eu³⁺, Bi³⁺ co-doped phosphors show a broad band with the maximum at ~450 nm due to ³P₁-¹S₀ transition of Bi³⁺ ions, and several sharp peaks at about 579, 587/595, 610/625, 653 and 708 nm attributed to the ⁵D₀-⁷F₀, ⁵D₀-⁷F₁, ⁵D₀-⁷F₂, ⁵D₀-⁷F₃ and ⁵D₀-⁷F₄ transitions of Eu³⁺ ions. It is obvious that the co-doping of Bi³⁺ ions significantly enhances the emission intensity of Eu³⁺ ions in phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ compared with that of Eu³⁺ singly doped phosphor Ca_2.83ZrSi₂O₉:0.17Eu³⁺ (Fig. 4(a)).

 

Fig. 4 Emission spectra under the excitation at (a) 270-300 nm and (b) 392 nm for Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (y = 0-0.20), the corresponding dependence of emission intensity peaked at 450 nm, 610 nm (λ_EX = 270-300 nm) on the doping value y of Bi³⁺ ions is shown in the inset of (a), and the dependence of emission intensity peaked at 610 nm (λ_EX = 392 nm) on the doping value y of Bi³⁺ ions is shown in the inset of (b).

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On the one hand, the emission spectrum of Bi³⁺ ions (a broad band from 350 to 550 nm with the maximum at about 420 nm) in Ca₃ZrSi₂O₉ host obviously overlaps with the excitation spectrum of Eu³⁺ ions (several narrow peaks at about 360, 380, 392, 402, 413, 463 and 530 nm), combining Fig. 1(a) with 1(b). Dexter’s theory states that the energy transfer rate is proportional to the spectral overlap between the energy donor emission band and the energy acceptor absorption band [27]. On the other hand, as for the Eu³⁺, Bi³⁺ co-doped phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺, under the condition of a fixed doping value 0.17 of Eu³⁺ ions, the dependence of Eu³⁺ (peak at 610) ions’ emission intensity on the co-doping value y of Bi³⁺ ions is similar with that of Bi³⁺ (peak at 450) ions under the excitation at 270-300 nm (as shown in the inset of Fig. 4(a)). The emission intensity of both Bi³⁺ and Eu³⁺ ions first increases with increasing the doping value y of Bi³⁺ ions until y = 0.09, and then decreases with further increasing the doping value y. Only several peaks at about 579, 592, 610, 653 and 705 nm attributed to the ⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4) transitions of Eu³⁺ ions are observed under the excitation at 392 nm in phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (Fig. 4(b)), and the emission intensity of Eu³⁺ ions increases slightly with increasing the doping value y of Bi³⁺ ions (inset of Fig. 4(b)). The dependence of Eu³⁺ ions’ emission intensity (peak at 610, λ_EX = 392 nm) on the doping value y of Bi³⁺ ions is not same with those of both Bi³⁺ (peak at 450) and Eu³⁺ (peak at 610) ions (λ_EX = 270-300 nm), because there should be energy transfer from Bi³⁺ to Eu³⁺ ions under the excitation at 270-300 nm, however, this energy transfer does not exist under the excitation at 392 nm. Therefore, the above two PL spectral features all hint of energy transfer from Bi³⁺ to Eu³⁺ ions in phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺.

The excitation spectra measured by monitoring the emission at 610 nm for Eu³⁺, Bi³⁺ co-doped phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ show a broad band with the maximum at about 300 nm attributed to the charge transfer transition of O^2--Eu³⁺ and ¹S₀-³P₁ transition of Bi³⁺ ions, and several narrow peaks at about 360, 380, 392, 402, 413, 463 and 530 nm due to intra-4f transitions of Eu³⁺ ions are observed (Fig. 5(a)). The appearance of ¹S₀-³P₁ transition from Bi³⁺ ions in the excitation spectra (λ_EM = 610 nm) of Eu³⁺ ions also implies the existence of energy transfer from Bi³⁺ to Eu³⁺ ions. The excitation spectra measured by monitoring the emission at 445 or 450 nm for phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ exhibit a broad band with the maximum at about 320 nm coming from ¹S₀-³P₁ transition of Bi³⁺ ions (Fig. 5(b)).

 

Fig. 5 Excitation spectra measured by monitoring the emission at (a) 610 nm and (b) 445 nm or 450 nm for Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (y = 0-0.20).

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3.1.4 Decay time and energy transfer efficiency

Besides, the decay curves of Bi³⁺ ions in both singly doped Ca_2.85ZrSi₂O₉:0.15Bi³⁺ and doubly doped Ca_2.68ZrSi₂O₉:0.17Eu³⁺,0.15Bi³⁺ phosphors are measured and they are shown in Fig. 6. Generally, Bi³⁺ ions occupy three types of Ca²⁺ sites, and it is reasonable to fit the decay curves by a three-exponential function. However, the luminescent properties of Bi³⁺ ions occupying Ca(1) and Ca(3) sites should be very similar due to the similarity between Ca(1) and Ca(3) sites in coordination environment as stated in section 3.1.3. Therefore, the decay curves of Bi³⁺ ions can be well fitted by a double-exponential function by the following equation [28,29]:

 

Fig. 6 Decay curves of Bi³⁺ ions in both Ca_2.85ZrSi₂O₉:0.15Bi³⁺ (λ_EX = 300 nm, λ_EM = 420 nm) and Ca_2.68ZrSi₂O₉:0.17Eu³⁺,0.15Bi³⁺ (λ_EX = 320 nm, λ_EM = 450 nm).

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Therefore, both the PL spectra and decay curves have shown that energy transfer exists from Bi³⁺ to Eu³⁺ ions in Eu³⁺, Bi³⁺ co-doped phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺. The schematic energy level and energy transfer diagram of Eu³⁺, Bi³⁺ ions in Ca₃ZrSi₂O₉ host are depicted in Fig. 7. The energy transfer efficiency (η_T) can be calculated using the following expression [29]:

 

Fig. 7 Schematic energy level and energy transfer (ET) diagram of Eu³⁺, Bi³⁺ and Tb³⁺ ions in Ca₃ZrSi₂O₉ host.

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The CIE chromaticity coordinates of Eu³⁺, Bi³⁺ co-doped phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ are listed in Table 1, and the corresponding CIE chromaticity diagram are shown in Fig. 2. The CIE chromaticity coordinates of phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ are located in orange and pink region, and it is obvious that their CIE chromaticity coordinates are nearly on the line between the CIE chromaticity coordinates of phosphors Ca_2.83ZrSi₂O₉:0.17Eu³⁺ and Ca_2.85ZrSi₂O₉:0.15Bi³⁺.

3.2 Crystal phase and luminescent properties of tri-doped Ca₃ZrSi₂O₉:Eu³⁺,Bi³⁺,Tb³⁺ phosphors

3.2.1 Tb³⁺ single doping

As mentioned above, white light emitting is not obtained in Eu³⁺, Bi³⁺ co-doped phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ because of lacking green light emitting. In considering that Tb³⁺ ion is commonly used as a kind of green light emitting activator, therefore, the Tb³⁺ single-doped phosphors, Ca_2.87ZrSi₂O₉:0.13Tb³⁺, were synthesized, and their PL spectra are shown in Fig. 8(a). The visible PL emission peaks of Tb³⁺ ions are mainly due to the ⁵D₃ and ⁵D₄ to ⁷F_J’ transitions. Under the excitation at 244 nm or 281 nm, the emission spectra of Ca_2.87ZrSi₂O₉:0.13Tb³⁺ exhibit several peaks locating at 381 nm, 414 nm, 437 nm, 488 nm, 543 nm, 585 nm and 622 nm belong to ⁵D₃-⁷F₆, ⁵D₃-⁷F₅, ⁵D₃-⁷F₄, ⁵D₄-⁷F₆, ⁵D₄-⁷F₅, ⁵D₄-⁷F₄ and ⁵D₄-⁷F₃ transitions of Tb³⁺ ions, respectively [30]. The excitation spectrum measured by monitoring the emission at 543 nm consists of a broad band with the first and second maximum at 244 and 288 nm respectively. The ground state of Tb³⁺ ion with 4f⁸ electron configuration is at the ⁷F₆ level, and its 4f⁷5d¹ excitation levels have the spin forbidden ⁹D_J and spin allowed ⁷D_J states. So Tb³⁺ ions show two groups of f-d transitions in a specific host, in which one group is spin allowed with high energy and another group is spin forbidden with low energy [13]. Therefore, the excitation band of Ca_2.87ZrSi₂O₉:0.13Tb³⁺ with the first and second maximum at 244 and 288 nm is due to the transitions from ground state level ⁷F₆ to spin allowed ⁷D_J and spin forbidden ⁹D_J excitation levels of Tb³⁺ ions, respectively. The CIE chromaticity coordinates (listed in Table 2) of Ca_2.87ZrSi₂O₉:0.13Tb³⁺ are (0.291, 0.551) and they are obviously located in green region in CIE chromaticity diagram (shown in Fig. 9).

 

Fig. 8 (a) Excitation spectrum measured by monitoring the emission at 543 nm, and emission spectra under the excitation at 244 nm, 281 nm for Ca_2.87ZrSi₂O₉:0.13Tb³⁺, (b) XPS spectra of Tb 3d core levels for Tb₄O₇ raw materials and Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors, and the arrow points to the position of Tb⁴⁺ satellite. The insets of (b) are corresponding photos taken in daylight for Tb₄O₇ raw materials and Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors.

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Table 2. CIE chromaticity coordinates (x, y) and corresponding excitation wavelength for Eu³⁺, Bi³⁺, Tb³⁺ singly doped, Eu³⁺, Bi³⁺ doubly doped and Eu³⁺, Bi³⁺, Tb³⁺ triply doped Ca₃ZrSi₂O₉ based phosphors

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Fig. 9 CIE chromaticity diagram for Eu³⁺, Bi³⁺, Tb³⁺ singly doped Ca_2.83ZrSi₂O₉:0.17Eu³⁺ (Eu³⁺), Ca_2.85ZrSi₂O₉:0.15Bi³⁺ (Bi³⁺), Ca_2.87ZrSi₂O9:0.13Tb³⁺ (Tb³⁺) phosphors, Eu³⁺, Bi³⁺ doubly doped Ca_2.74ZrSi₂O9:0.17Eu³⁺,0.09Bi³⁺ (Eu³⁺,Bi³⁺)) phosphors and Eu³⁺, Bi³⁺, Tb³⁺ triply doped Ca_2.74-zZrSi2O9:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (z=0.03, 0.09, 0.13, 0.20, 0.30, 0.40, 0.50) (1, 2, 3, 4, 5, 6, 7) phosphors, corresponding to Table 2.

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There is a mixture of Tb³⁺and Tb⁴⁺for Tb₄O₇ (50% Tb³⁺, 50%Tb⁴⁺) raw materials used to synthesize Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors. In order to verify whether there is Tb⁴⁺ existing in phosphors Ca_2.87ZrSi₂O₉:0.13Tb³⁺, the XPS spectra of Tb 3d core levels for Tb₄O₇ raw materials and Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors are measured, and they are shown in Fig. 8(b). The main XPS peaks of Tb 3d for Tb₄O₇ raw materials appear at 1241.7 eV and 1276.5 eV for 3d_5/2 and 3d_3/2, respectively. In addition to the main peaks, a relatively intense satellite appears at 1252.0 eV. Blanco et al. have reported that the higher the relative intensity of the satellite, the higher the Tb⁴⁺ content in the sample [31]. However, only main XPS peaks appear at 1241.6 eV and 1277.1 eV for Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors, and the satellite is not observed. So the XPS results of Tb₄O₇ raw materials and Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors show that no Tb⁴⁺ is observed in Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors. Besides, if there is Tb⁴⁺ existing, the materials usually show brown color under daylight. The photos taken in daylight for Tb₄O₇ raw materials and Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors are also shown in the insets of Fig. 8(b). It is obvious that the Tb₄O₇ raw materials are deep brown powders, but the Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors are white powders with a little yellow. So the photos taken in daylight for Tb₄O₇ raw materials and Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors also show that no Tb⁴⁺ is observed in Ca_2.87ZrSi₂O₉:0.13Tb³⁺ phosphors.

3.2.2 Eu³⁺, Bi³⁺ and Tb³⁺ triple doping

As mentioned in section 3.1.3, the emission intensity of optimal phosphor Ca_2.74ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺ is the highest in Eu³⁺, Bi³⁺ co-doped phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ (y = 0.03-0.20) (Fig. 4(a)), therefore, based on this fact, Tb³⁺ ions were further introduced into the phosphor Ca_2.74ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺ to form Eu³⁺, Bi³⁺, Tb³⁺ triply doped phosphors Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺, and their crystal structure and PL properties were studied.

XRD patterns of phosphors Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (z = 0.03-0.50) are measured and they are shown in Fig. 10. Ca_2.71ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,0.03Tb³⁺ phosphor with low doping value z = 0.03 of Tb³⁺ ions possesses a single monoclinic Ca₃ZrSi₂O₉ phase. Since the ion radius of doped ion Tb³⁺(CN = 6) is 0.923 Å, calculated with formula (1), the D_r between doped Tb³⁺(CN = 6) ion and host cations Ca²⁺(CN = 6), Zr⁴⁺(CN = 6), Si⁴⁺(CN = 4) is 7.70%, −28.19%, −255.00%, respectively. Similarly, there are no available ion radius data for Tb³⁺ ion with CN = 4, so the data of CN = 6 is used as a reasonable approximation in the calculation of D_r between Tb³⁺ and Si⁴⁺ ions. Obviously, the ion radius of doped Tb³⁺ ion is close to that of host cation Ca²⁺, while it has relatively large difference to those of other host cations Zr⁴⁺ and Si⁴⁺. Therefore, the doped Tb³⁺ ions should occupy Ca²⁺ sites in Ca₃ZrSi₂O₉ host. A second phase of hexagonal Ca₂Tb₈Si₆O₂₆ appears in Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ with higher doping value z ≥ 0.09, and the larger the doping value z is, the stronger the diffraction peaks of second phase Ca₂Tb₈Si₆O₂₆ become. Under UV excitation, Ca₂Tb₈Si₆O₂₆ can emit green light attributed to the 4f-4f transitions of Tb³⁺ ions [32,33], and the green light is helpful for the obtainment of white light, so the appearance of second phase Ca₂Tb₈Si₆O₂₆ has no negative effects to our purpose of obtaining white light emission in phosphors Ca_2.74-zZrSi₂O₉:0.17Eu³⁺, 0.09Bi³⁺, zTb³⁺.

 

Fig. 10 XRD patterns of Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (z = 0.03-0.50), with the standard XRD patterns of monoclinic Ca₃ZrSi₂O₉ (JCPDS 01-083-0365) and hexagonal Ca₂Tb₈Si₆O₂₆ (JCPDS 00-029-0386), the symbol “*” marking peaks of the second phase Ca₂Tb₈Si₆O₂₆ appeared in Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (z ≥ 0.09).

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Under the excitation at 323 nm or 320 nm, a broad emission band with the maximum at about 450 nm originated from ³P₁-¹S₀ transition of Bi³⁺ ions, several emission peaks locating at 488 nm and 543/550 nm belong to ⁵D₄-⁷F₆ and ⁵D₄-⁷F₅ transitions of Tb³⁺ ions, respectively, and several emission peaks at about 579 nm, 587/595 nm, 610/625 nm, 653 nm and 708 nm attributed to the ⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4) transitions of Eu³⁺ ions all appear in the Eu³⁺, Bi³⁺, Tb³⁺ tri-doped phosphors Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (Fig. 11(a)). The emission intensity of Tb³⁺ ions (peak at 543) increases, while the emission intensity of both Bi³⁺ (peak at 450) and Eu³⁺ (peak at 610) ions decreases with increasing the doping value z of Tb³⁺ ions (Fig. 11(b)). The co-doping of green light emitting Tb³⁺ ions makes the CIE chromaticity coordinates of Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (listed in Table 2) gradually moving to white light region, as shown in the CIE chromaticity diagram of Fig. 9. The excitation spectra obtained by monitoring the emission at 450 nm or 445 nm for Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ consist of a broad band with the maximum at about 320 nm originated from ¹S₀-³P₁ transition of Bi³⁺ ions (Fig. 11(c)). The choice of excitation wavelength in Fig. 11(a) is based on these excitation spectra in Fig. 11(c). The excitation spectra obtained for λ_EM = 543 nm contain a broad band with the maximum at about 244 nm from 4f⁸ - 4f⁷5d¹ transition of Tb³⁺ ions, another broad band with the maximum at about 320 nm due to ¹S₀-³P₁ transition of Bi³⁺ ions, and two peaks at 376 and 483 nm from intra 4f transitions of Tb³⁺ ions (Fig. 11(d)). Further, the excitation spectra obtained for λ_EM = 610 nm show a broad band with the maximum at about 310 nm attributed to the charge transfer transition of O^2--Eu³⁺ and ¹S₀-³P₁ transition of Bi³⁺ ions, and several narrow peaks at about 360 nm, 380 nm, 392 nm, 402 nm, 413 nm, 463 nm and 530 nm from intra-4f transitions of Eu³⁺ ions (Fig. 11(e)). For the purpose of intuitive comparison between the PL spectra, the excitation spectra (λ_EM = 450 nm, 543 nm and 610 nm) and emission spectrum (λ_EX = 323 nm) of the representative Ca_2.24ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,0.50Tb³⁺ phosphor are depicted together as indicated in Fig. 11(f). It is obvious that the excitation spectrum (λ_EM = 450 nm) measured for Bi³⁺ ions consists of a broad band with the maximum at about 320 nm due to ¹S₀-³P₁ transition of Bi³⁺ ions, at the same time, the excitation spectra for Tb³⁺ and Eu³⁺ ions obtained by λ_EM = 543 nm and 610 nm respectively also contain the broad band at about 320 nm from ¹S₀-³P₁ transition of Bi³⁺ ions, which indicates the energy transfer from Bi³⁺ to both Tb³⁺ and Eu³⁺ ions, as deduced and shown in the schematic energy level and energy transfer diagram of Fig. 7.

 

Fig. 11 (a) Emission spectra under the excitation at 323 nm or 320 nm, (b) the dependence of emission intensity peaked at 450 nm, 543 nm, 610 nm (λ_EX = 323 nm or 320 nm) on the doping value z of Tb³⁺ ions, and excitation spectra measured by monitoring the emission at (c) 450 nm or 445 nm, (d) 543 nm and (e) 610 nm for Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (z = 0.03-0.50), (f) emission spectrum under the excitation at 323 nm and excitation spectra measured by monitoring the emission at 450 nm, 543 nm and 610 nm for Ca_2.24ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,0.50Tb³⁺.

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3.2.3 Decay time and energy transfer efficiency

The decay curves of Bi³⁺ ions in the triply doped phosphors Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ are measured under conditions of λ_EX = 320 nm and λ_EM = 450 nm or 445 nm, and they are shown in Fig. 12. The average decay time of Bi³⁺ ions in Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ phosphors with z = 0, 0.03, 0.09 and 0.13 is 320.08 ns, 318.98 ns, 302.59 ns and 281.83 ns, respectively. The content of second phase Ca₂Tb₈Si₆O₂₆ is obvious in triply doped phosphors Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ with z ≥ 0.20, so the decay curves of these phosphors are not measured. It can be seen from Fig. 12 that the decay time of Bi³⁺ ions gradually decreases with increasing the doping value z of Tb³⁺ ions in Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (z = 0-0.13), which also manifests the existence of energy transfer from Bi³⁺ to Tb³⁺ ions. According to Eq. (4), the energy transfer efficiency from Bi³⁺ to Tb³⁺ ions is calculated to be 0.34%, 5.46% and 11.95% in Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ with z = 0.03, 0.09 and 0.13, respectively.

 

Fig. 12 Decay curves of Bi³⁺ ions (λ_EX = 320 nm, λ_EM = 450 nm or 445 nm) in Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (z = 0-0.13).

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3.2.4 Luminescence thermal stability

The PL thermal stability is an important parameter of phosphors when considering their application in WLED. The temperature dependent emission spectra of the representative phosphor Ca_2.24ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,0.50Tb³⁺ under 323 nm excitation are measured and they are shown in Fig. 13(a). The emission intensity monotonously decreases with increasing the temperature from 25°C to 275°C because of thermal quenching. A widely accepted thermal quenching mechanism is the electron transition through the intersection between the ground and excited states, and the energy gap between the intersection and the excited state is defined as activation energy [29]. The activation energy E_a can be quantitatively calculated from the Arrhenius formula to evaluate the ability of resisting thermal quenching [34], expressed as

 

Fig. 13 (a) Emission spectra under the excitation at 323 nm at different temperatures from 25 °C to 275 °C, (b) Arrhenius plot of the total emission intensity under 323 nm excitation and corresponding fit using the Arrhenius formula, (c) relative total emission intensity and relative emission intensity peaked at 450 nm, 543 nm, 610 nm (λ_EX = 323 nm), respectively, as a function of temperature for Ca_2.24ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,0.50Tb³⁺.

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Table 3. CIE chromaticity coordinates (x, y) and correlated color temperature (CCT) (λ_EX = 323 nm) at different temperatures from 25 °C to 275 °C for Ca_2.24ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,0.50Tb³⁺

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Fig. 14 CIE chromaticity diagram at different temperatures from 25 to 275 °C (25 (1), 50 (2), 75 (3), 100 (4), 125 (5), 150 (6), 175 (7), 200 (8), 225 (9), 250 (10), 275 (11)) for Ca_2.24ZrSi₂O₉:0.17Eu³⁺ ,0.09Bi³⁺ ,0.50Tb³⁺, corresponding to Table 3.

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4. Conclusions

A kind of white light emitting phosphors Ca₃ZrSi₂O₉:Eu³⁺,Bi³⁺,Tb³⁺ has been successfully synthesized by conventional solid state reaction method. Eu³⁺ ions emit typical red light and Bi³⁺ ions show broad band blue light in Ca₃ZrSi₂O₉ host. Under UV light excitation, the blue light of Bi³⁺ ions and the red light of Eu³⁺ ions are simultaneously observed in co-doped phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺, and the co-doping of Bi³⁺ ions significantly enhances the emission intensity of Eu³⁺ ions. The spectral overlap exists between the emission spectrum of Bi³⁺ ions and the excitation spectrum of Eu³⁺ ions, the ¹S₀-³P₁ transition of Bi³⁺ ions appears in the excitation spectra (λ_EM = 610 nm) of Eu³⁺ ions for Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺, and the Bi³⁺ ions’ decay time decreases in Eu³⁺, Bi³⁺ co-doped phosphor Ca_2.68ZrSi₂O₉:0.17Eu³⁺,0.15Bi³⁺ compared with that in Bi³⁺ single-doped phosphor Ca_2.85ZrSi₂O₉:0.15Bi³⁺. Therefore, there is energy transfer from Bi³⁺ to Eu³⁺ ions in co-doped phosphors Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺. But white light is not obtained in Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺ because of lacking green light. So typically green light emitting Tb³⁺ ions are introduced, and white light is finally obtained in triply doped phosphors Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺. The ¹S₀-³P₁ transition of Bi³⁺ ions also appears in the excitation spectra (λ_EM = 543 nm) of Tb³⁺ ions for Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺, and the Bi³⁺ ions’ decay time decreases with increasing the doping value z of Tb³⁺ ions in Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺ (z = 0-0.13), so energy transfer also exists from Bi³⁺ to Tb³⁺ ions. The PL thermal stability of the representative phosphor Ca_2.24ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,0.50Tb³⁺ is acceptable for practical application, because the total emission intensity at 100 °C remains about 83% of the initial value at room temperature, and the CIE chromaticity coordinates are always in white light region with temperature increasing from 25 °C to 275 °C.