Development of a cyan blue-emitting Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphor for use in dental glazing materials: color tunable emission and energy transfer

Abstract

The Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphors were synthesized by a conventional solid state reaction method. The synthesized phosphor samples are a single phase of Ba₃La₂(BO₃)₄ and showed angular-shaped fine grains with average particle size from 5 μm to 10 μm. The Ba₃La₂(BO₃)₄:Ce³⁺ phosphors showed an asymmetric broad blue emission under excitation at 365 nm and the Ba₃La₂(BO₃)₄:Tb³⁺ phosphors exhibited typical green emission assigned to the 4f–4f transition of Tb³⁺ under excitation at 254 nm. Under near-UV (365 nm) excitation, Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors showed both a blue emission band and green emission peaks due to Ce³⁺ and Tb³⁺, respectively. By optimizing the composition, cyan-blue emission with high color purity (CIE chromaticity coordinate values x = 0.2557 and y = 0.3839) was obtained for the Ba₃La₂(BO₃)₄:0.05Ce³⁺,0.03Tb³⁺ phosphor, and the internal quantum efficiency of the phosphor at the excitation wavelength of 365 nm is estimated to be 50%. The dental glazing paste prepared by mixing organic binder, Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors, and low T_g glass was successfully vitrified when it was heated at 600 and 700 °C, and showed high chemical stability of the luminescence properties in acidic aqueous solution (pH = 4).

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Introduction

Phosphors have been widely applied in displays and lamps and are significantly important materials in our daily life. In particular, rare earth ion doped phosphors have been widely studied for application in a variety of luminescent materials such as general solid state lighting, display devices, electroluminescent devices, optical information storage, X-ray detectors, and TV monitors due to their excellent photoluminescence properties.^1–3 Among these phosphor applications, phosphor-converted light-emitting diodes (pc-LEDs) have great potential in the field of high performance solid-state lighting system such as flashlights, indicating lights, and automobile headlights because of their lower power consumption, long life, high color rending index and conversion efficiency.^4–6

Recently, many investigations have been devoted to the search for new application fields for phosphor materials, such as plant growth,^7–11 optical sensors,^12–17 cosmetics^18,19 anti-counterfeiting²⁰ and road signs/lanes.^21,22 In addition, the phosphors have been used in the dental field to enhance the aesthetics of artificial crowns and restoration.^21–29 Under UV-light irradiation, human natural teeth emit cyan blue color with peak at 450 nm and the photoluminescence (PL) properties of teeth make whiter and brighter for human natural teeth in daylight.³⁰ The artificial crowns and restorative materials can match the shade of the natural teeth to satisfy aesthetic requirements by adding phosphors or pigments.^25,26,31 The studies to develop the paste containing phosphor powder and glass frit, which called the dental glazing, are recently conducted to improve the aesthetics of artificial crowns and restoration.

In the present study, therefore, we have focused on the Ce³⁺ and Tb³⁺ co-doped phosphors to realize a development of the dental glazing paste in accordance with aesthetic requirements for artificial teeth. It is well-known that the Tb³⁺ ion-doped phosphors show the sharp green emission due to the 4f–4f transition of Tb³⁺ under UV excitation and the Ce³⁺ ion-doped phosphors show the broad blue emission due to the 5d–4f transition of Ce³⁺ under UV excitation.^32–37 The PL properties of the Ce³⁺ and Tb³⁺ co-doped phosphors have been investigated by several researchers.^38–45 Although Tb³⁺ ion-doped phosphors have a weak optical absorption in near-UV region, the Ce³⁺ and Tb³⁺ co-doped phosphors show a strong green emission under excitation which is considered to be due to the energy transfer from Ce³⁺ to Tb³⁺. These phosphors exhibit a color tunable emission by changing the concentration of Ce³⁺ and Tb³⁺ ions in the host lattice. These results indicate that the Ce³⁺ and Tb³⁺ co-doped phosphors can be expected as one of the suitable candidate for the cyan blue emission phosphor to achieve aesthetic requirements for teeth application.

In this study, using Ce³⁺ and Tb³⁺ ions as activators, cyan blue emitting Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors were synthesized by a conventional solid state reaction and the photoluminescence properties were characterized. The dental glazing pastes were prepared by mixing the as-prepared Tb³⁺/Ce³⁺ co-doped phosphor powders, an organic binder and a low T_g glass frits with a composition of (K,Na)₂O–B₂O₃–Al₂O₃–SiO₂. The photoluminescence properties and chemical stability against acid aqueous solution of the dental glazing pastes were investigated.

Experimental

Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphors were synthesized by a conventional solid state reaction method. BaCO₃ (Junsei Chemical Co., Ltd., 99%), La₂O₃ (Daejung Chemical & Metal Co., Ltd., 99.9%), B₂O₃ (Junsei, >95%), CeO₂ (LTS, 99.98%), Tb₄O₇ (LTS, 99.95%) were used as starting materials with no further purification. In Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphor, the amount of Ce³⁺ was adjusted between 1 to 15 mol% and the Tb³⁺ content was varied from 1 to 15 mol%. These raw materials were mixed in a stoichiometric ratio using an agate mortar for 5 times, and then the mixture was calcined at 800 °C for 6 h in a flow of 5% H₂–95% N₂ gas to reduce Ce⁴⁺ to Ce³⁺ and Tb⁴⁺ to Tb³⁺. After the calcination, the samples were reground in a mortar and heated again at 1200 °C for 6 h in a flow of 5% H₂–95% N₂ gas.

To prepare the dental glazing paste, the as-prepared Tb³⁺/Ce³⁺ co-doped phosphor powders mixed with an organic binder and a low T_g glass with a composition of (K,Na)₂O–B₂O₃–Al₂O₃–SiO₂. Then the homogeneous mixture was mixed and aged using wet ball milling process at room temperature for 24 h. The as-prepared dental glazing pastes coated on alumina plate using brushing method and then heated at 300–700 °C for 0.5 h in air atmosphere.

The crystal structure of the resultant powder samples was identified using X-ray powder diffraction (XRD, Bruker D8 advance), and the detailed crystallographic data were obtained by Rietveld analysis using the RIETAN-FP package.⁴⁶ To determine the valence state of Ce and Tb ion in the Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphor, X-ray photoelectron spectroscopy (XPS; JEOL, JPS-9000) was measured at room temperature. The morphology of the samples was characterized using scanning electron microscope (JEOL, JSM6700F). The photoluminescence emission (PL) and excitation (PLE) spectra were measured at room temperature using a fluorescence spectrophotometer (PSI, DARSA PRO 3400). PL spectra of Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphor were obtained for excitation at 365 nm, and PLE spectra were recorded for emission at 450 and 543 nm. The internal quantum yield was measured using a spectrophotometer (PSI, DARSA PRO 3400) with a fluorescence integrating sphere unit (PSI, DARSA PRO 3400) at excitation wavelength of 365 nm. The photoluminescence properties of dental glazing paste were also characterized using a fluorescence spectrophotometer (PSI, DARSA PRO 3400). The time-resolved photoluminescence (TR-PL) was carried out using a confocal microscope (MicroTime-200, Picoquant, Germany). A single-mode pulsed diode laser (375 nm with 30 ps pulse width and average power of ∼1 μW operating in 1 MHz repetition rate) was used as an excitation source. To evaluate the chemical stability of the photoluminescence properties for the dental glazing paste, the acid resistance was investigated with HCl aqueous solution (pH = 4).

Results and discussion

Crystal structure analysis of Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphors

Crystallographic data and structure refinement parameters of the Rietveld refinement of the XRD patterns of the Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphor are summarized in Table 1, and the representative XRD pattern of the Ba₃La₂(BO₃)₄ host material is shown in Fig. 1. The representative XRD patterns of the Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphors are shown in Fig. S1–S3† and the refined structural parameters of the Ba₃La₂(BO₃)₄ host material and Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphors are summarized in Tables S1–S4.† The data of the orthorhombic structure Ba₃La₂(BO₃)₄ from the inorganic crystal structure database (ICSD no. 98-003-9277) were used for the starting model. As shown in Table 1, the final R-factor values, R_wp, R_p, and S, converged to 10.99, 8.10, and 2.36, respectively, which verify the phase purity of the as-prepared sample. The representative XRD pattern of Ba₃La₂(BO₃)₄ was well indexed to the reported data and no impurity phases were detected in the XRD pattern. The final refined results confirm a single phase of the Ba₃La₂(BO₃)₄ that is crystallized in a orthorhombic structure in space group Pnma (no. 62) with refined lattice parameters a = 0.77735(4) nm, b = 1.70989(1) nm, c = 0.90422(3) nm, and V = 1.20187 nm³. A schematic of the Ba₃La₂(BO₃)₄ crystal structure is shown in Fig. 2, produced using VESTA.⁴⁷ According to the crystal structure analysis, Ba²⁺ and La³⁺ ions occupied in three different dodecahedral sites (8d and 4c) and there are three kinds of boron sites (8d and 4c) with 3-fold coordination. The lattice volume of the Ba₃La₂(BO₃)₄ phase decreased with doping the Ce³⁺ and Tb³⁺ in the crystal lattice, which indicates that the small size of Ce³⁺ (ionic radius: 0.1143 nm for 8 coordination⁴⁸) and Tb³⁺ (ionic radius: 0.1040 nm for 8 coordination⁴⁸) are successfully substituted into the larger La³⁺ ions sites (ionic radius: 0.1160 nm for 8 coordination⁴⁸). Fig. 3 shows the XRD patterns of Ba₃La₂(BO₃)₄:Ce³⁺, Ba₃La₂(BO₃)₄:Tb³⁺, Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphor samples with different concentrations. The XRD patterns of all samples were in good agreement with a single phase of highly crystalline orthorhombic structure (ICSD no. 98-003-9277) with space group Pnma (#62). With increasing the Ce³⁺ and Tb³⁺ ion contents in the Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphors, a peak shift to a higher diffraction angle is observed because La³⁺ in the host material is partially substituted with the smaller Ce³⁺ and Tb³⁺ ions to form solid solutions. To investigate the valence state of Ce and Tb ions, binding energy of electrons in Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors was measured using XPS and the result is shown in Fig. S4.† The Ce 3d and Tb 3d XPS spectra are well known to split into two peaks of 3d_3/2 and 3d_5/2 due to the spin–orbital interaction. The XPS spectra of both Ce 3d and Tb 3d state exhibit two clear peaks at 889.9 eV (3d_3/2) and 901.9 eV (3d_5/2), and at 1242.8 eV (3d_3/2) and 1275.9 eV (3d_5/2), respectively, which can be assigned to those of 3+ oxidation state of Ce and Tb ions. In addition, no peaks that would suggest the presence of other oxidation states of Ce and Tb ions were observed. These results support that the Ce and Tb ions successfully substituted in the La³⁺ sites as Ce³⁺ and Tb³⁺ in the Ba₃La₂(BO₃)₄ host lattice. The morphologies of the Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphors were observed by SEM as seen in Fig. 4. The particles of all powder samples were composed of angular-shape fine grain with the average size of around 5–10 μm.

Table 1 Crystallographic data of Ba₃La₂(BO₃)₄ and Ce³⁺/Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphors

Formula	Ba3La2(BO3)4	Ba3(La,Ce)2(BO3)4	Ba3(La,Tb)2(BO3)4	Ba3(La,Ce,Tb)2(BO3)4
Crystal system	Orthorhombic	Orthorhombic	Orthorhombic	Orthorhombic
Space group	Pnma	Pnma	Pnma	Pnma
a (nm)	0.77735(4)	0.77692(5)	0.77708(2)	0.77610(2)
b (nm)	1.70989(1)	1.71059(12)	1.70686(5)	1.70481(4)
c (nm)	0.90422(3)	0.90271(4)	0.90220(2)	0.90225(1)
Rwp (%)	10.99	11.36	10.00	10.64
Rp (%)	8.10	8.40	7.31	7.84
S (%)	2.36	2.65	2.57	3.08

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Fig. 1 Rietveld refinement result for X-ray powder diffraction data of Ba₃La₂(BO₃)₄. Red symbol: measured pattern, green solid line: calculated pattern, blue solid line: difference between their intensities.

Fig. 2 A schematic of the Ba₃La₂(BO₃)₄ crystal structure.

Fig. 3 XRD patterns of (a) Ba₃La₂(BO₃)₄:Ce³⁺, (b) Ba₃La₂(BO₃)₄: Tb³⁺, (c) Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors doped with different concentrations and Ba₃La₂(BO₃)₄ reference (ICSD 98-003-9277).

Fig. 4 SEM images of (a) Ba₃La₂(BO₃)₄:Ce³⁺, (b) Ba₃La₂(BO₃)₄: Tb³⁺, (c) Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors.

Photoluminescence properties of Ce³⁺ doped Ba₃La₂(BO₃)₄ phosphor

The photoluminescence emission (PL) and excitation (PLE) spectra of Ba₃La₂(BO₃)₄:Ce³⁺ phosphors with different concentration of Ce³⁺ ions are presented in Fig. 5. The PLE spectra recorded by monitoring the emission at 470 nm exhibited a broad excitation band from 270 to 400 nm, which was attributed to 4f → 5d transition of Ce³⁺ ions. The PL spectra of Ba₃La₂(BO₃)₄:Ce³⁺ phosphors recorded under 365 nm excitation showed a broad non-symmetry blue emission band from 400 nm to 650 nm with a maximum peak at 470 nm due to 5d → 4f transition of Ce³⁺ ions. The emission of Ce³⁺ usually includes two bands of the transitions of 5d-excited state to ²F_7/2 and ²F_5/2 states, and the emission spectra seem to consist of two of them in a single band. The emission peak wavelength of the Ce³⁺ 5d → 4f transition shifts to the longer wavelength (lower energy) direction with increasing Ce³⁺ concentration for the sample, which can be attributed to the change of the crystal field strength around Ce³⁺ in the host lattice. The crystal field strength increases with increasing the amount of Ce³⁺ substitution for La³⁺ in the Ba₃La₂(BO₃)₄ lattice because the average bond length of Ce³⁺–O²⁻ becomes progressively shorter by lattice shrinkage.⁴⁹ The emission peak intensity of Ba₃La₂(BO₃)₄:Ce³⁺ phosphors was increased with the Ce³⁺ concentration up to 0.05 mol%, and then decreased probably due to the concentration quenching effect. The internal quantum yield of the 0.05 mol% Ce³⁺ doped Ba₃La₂(BO₃)₄ phosphor at the excitation wavelength of 365 nm was estimated to be 50%. Thus, the optimum concentration of Ce³⁺ ions was confirmed to be 0.05, and it was kept as a constant value to prepare Ce³⁺/Tb³⁺ co-doped Ba₃La₂(BO₃)₄ phosphors.

Fig. 5 The (a) excitation (λ_em = 470 nm) and (b) emission (λ_ex = 365 nm) and spectra of Ba₃La₂(BO₃)₄:Ce³⁺ phosphors with different concentrations of Ce³⁺.

Photoluminescence properties of Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphor

Fig. 6 shows the PL and PLE spectra of Ba₃La₂(BO₃)₄:Tb³⁺ phosphors with different concentration of Tb³⁺ ions. The PLE spectra recorded by monitoring the emission at 543 nm, there are a broad band from 225 to 300 nm, corresponding to the spin-allowed energy transition from the 4f⁸ to 4f⁷5d configuration of Tb³⁺. A number of small absorption peaks at 300–400 nm correspond to the spin-forbidden 4f–4f transition (⁷F₆ → ⁵L_1,5,10, ⁷F₆ → ⁵D₃). Under 254 nm excitation, the Ba₃La₂(BO₃)₄:Tb³⁺ phosphors exhibited a well-known characteristic Tb³⁺ green emission, and no self-activated emission was observed in the undoped samples. The emission peaks of Ba₃La₂(BO₃)₄:Tb³⁺ phosphors at 490, 547, 585, and 623 nm correspond to the ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃ transitions of Tb³⁺ ions, respectively.^50–52 The emission intensity of Ba₃La₂(BO₃)₄:Tb³⁺ phosphors was increased with increasing the Tb³⁺ concentration up to 0.10, and then decreased due to the concentration quenching effect. In particular, the emission intensity of 0.15 showed a dramatically decrease. The concentration quenching of luminescence is generally observed in phosphor materials in which a large amount of luminescence ion is doped into the host lattice.^53,54 By increasing the luminescence ion concentration in host lattice, the distance between two adjacent luminescence ions is decreased and the interaction between the luminescence ions is enhanced, which usually results in the concentration quenching due to the nonradiative energy transfer between two luminescence ions.^53,54 The internal quantum yield of the 0.10 mol% Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphor at the excitation wavelength of 365 nm was estimated to be 60%.

Fig. 6 The (a) excitation (λ_em = 543 nm) and (b) emission (λ_ex = 254 nm) spectra of Ba₃La₂(BO₃)₄ phosphors doped with different concentrations of Tb³⁺.

Photoluminescence properties of Ce³⁺ and Tb³⁺ doped Ba₃La₂(BO₃)₄ phosphor

To develop the cyan blue-emitting phosphors with similar to that of human teeth, Ce³⁺ and Tb³⁺ co-activated Ba₃La₂(BO₃)₄ phosphors synthesized and their optical properties were characterized. In Ce³⁺/Tb³⁺ co-activated Ba₃La₂(BO₃)₄ phosphors, the concentration of Ce³⁺ was fixed at the optimal value of 0.05 mol% and the concentration of Tb³⁺ was varied in the range of 0.01–0.15 mol%. The PLE and PL spectra of Ce³⁺/Tb³⁺ co-activated Ba₃La₂(BO₃)₄ phosphors are shown in Fig. 7. The PLE spectra monitored at 547 nm (Tb^{3+ 5}D₄–⁷F₅ transition emission) consists of two strong broad bands with peaks at about 247 nm and 330 nm due to the 4f⁸ → 4f⁷5d transition of Tb³⁺ and 4f¹ → 4f⁰5d transition of Ce³⁺, respectively. These PLE spectra are evidence of energy transfer from Ce³⁺ to Tb³⁺ in Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors. The intensities of the optical absorption bands due to 4f⁸ → 4f⁷5d transition of Tb³⁺ and 4f¹ → 4f⁰5d transition of Ce³⁺ were effectively enhanced by increasing the Tb³⁺ ion contents in Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors. Under the excitation of 365 nm, the emission spectra exhibit a broad band of Ce³⁺ ions in the blue light region and a series of strong emission lines at 490, 545, 585, and 623 nm due to the ⁵D₄–⁷F_J (J = 6, 5, 4, and 3) transitions of Tb³⁺ ions in which the green emission line at 542 nm from ⁵D₄–⁷F₅ transitions dominate. Although the emission spectra of the phosphors were recorded for excitation at 365 nm (optical absorption due to Ce³⁺), the Ce³⁺ emission intensity increased with increasing the concentration of Tb³⁺ in Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors, whereas Tb³⁺ emission effectively enhanced. The energy-transfer efficiency of Ce³⁺ → Tb³⁺ in Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors increased remarkably with increasing the Tb³⁺ contents. These results are evidence of energy transfer from Ce³⁺ to Tb³⁺ in the Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors. As shown in Fig. 7 inset, it can be found that color-tunable blue-to-green emission can be obtained with increasing Tb³⁺ content due to Ce³⁺ → Tb³⁺ energy transfer. Additionally, the CIE coordinates for the Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors could be changed from the (x = 0.2368, y = 0.3388; blue) to (x = 0.3006, y = 0.4914; green) by controlling the different emission compositions of the Ce³⁺ and Tb³⁺ contents. Among the different amount of Ce³⁺/Tb³⁺ co-doped Ba₃La₂(BO₃)₄ phosphors, the emission color of the Ba₃La₂(BO₃)₄:0.05Ce³⁺,0.03Tb³⁺ phosphor is very close to cyan light of commercial materials for use in dental glassing.

Fig. 7 The (a) excitation (λ_em = 547 nm) and (b) emission (λ_ex = 365 nm) spectra of Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors with different concentrations of Tb³⁺.

To determine the energy transfer from Ce³⁺ to Tb³⁺ in Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors, the luminescence lifetime of the phosphors were recorded by confocal microscope. Fig. 8 illustrates the decay curves of Ba₃La₂(BO₃)₄:0.05Ce³⁺ and Ba₃La₂(BO₃)₄:0.05Ce³⁺,0.13Tb³⁺ phosphors monitored at 470 nm for Ce³⁺ emission. The decline of lifetime further demonstrated the presence of energy transfer from Ce³⁺ to Tb³⁺. Additionally, efficiency of energy transfer (η_ET) between Ce³⁺ and Tb³⁺ can be calculated by following equation:⁵⁵

η_ET = 1 − τ/τ₀

where τ and τ₀ represent fluorescence lifetime of phosphors with and without Tb³⁺ doping. The average lifetime of Ba₃La₂(BO₃)₄:0.05Ce³⁺,0.13Tb³⁺ phosphors with and without Tb³⁺ doping can be calculated as 14 ns and 11 ns, respectively. It obviously shows that the average lifetime of Ce³⁺ ions decreases with Tb³⁺ ions doping. The efficiency of energy transfer from Ce³⁺ to Tb³⁺ in the Ba₃La₂(BO₃)₄:0.05Ce³⁺,0.13Tb³⁺ phosphors was estimated to be approximately 21.5%.

Fig. 8 The decay curves of Ba₃La₂(BO₃)₄:0.05Ce³⁺ and Ba₃La₂(BO₃)₄:0.05Ce³⁺,0.13Tb³⁺ phosphors.

Characterization of dental glazing paste using Ce³⁺/Tb³⁺ co-doped Ba₃La₂(BO₃)₄ phosphor

The dental glazing paste using the Ce³⁺/Tb³⁺ co-doped Ba₃La₂(BO₃)₄ phosphor was prepared by mixing with an organic binder and a low T_g glass with a composition of (K,Na)₂O–B₂O₃–Al₂O₃–SiO₂. The as-prepared dental glazing pastes coated on alumina plate using brushing method and then heated at 300–700 °C for 0.5 h in air atmosphere. Fig. 9 shows the photograph of dental glazing pastes coated on alumina plate and the PL spectra of the dental glazing pastes heated at 300–700 °C for 0.5 h. The pastes heated at 300–500 °C was observed no vitrification, whereas, in the case of the pastes heated at 600 and 700 °C, vitrification was successfully observed. The emission peak intensity of the dental glazing pastes decreases with increasing the heating temperature (Fig. 9(a)). The reduction of the emission intensity of the Ce³⁺ or Eu²⁺ doped phosphors owing to re-heating is well known in phosphor-in-glass (PiG) application.^56–62 The reduction of the emission intensity is well known to be due to the oxidation of Ce³⁺ into Ce⁴⁺ (or Eu²⁺ into Eu³⁺) in the phosphors, and the oxidation of luminescence ions usually results in the thermal degradation of the photoluminescence efficiency of the phosphors.^63,64

Fig. 9 (a) The photograph of dental glazing pastes coated on alumina plate and the PL spectra of the dental glazing pastes heated at 300–700 °C for 0.5 h, (b) the emission spectra of the dental glazing paste before and after pH 4 test.

To evaluate the chemical stability of the photoluminescence efficiency of the as-prepared dental glazing pastes using the Ce³⁺/Tb³⁺ co-doped Ba₃La₂(BO₃)₄ phosphor, the acid resistance of the as-prepared dental glazing pastes was carried out in HCl aqueous solution (pH = 4). The pH values of drink such as an orange juice, coke, ade and etc. is about 3–4. The dental glazing pastes coated on alumina plate, which prepared by heating at 600 °C, was treated with HCl aqueous solution and soaked for 48 h, and then washed with water and dried at 80 °C for 24 h. After the HCl aqueous solution treatment for 48 h, which is very hard condition, the emission intensity of the dental glazing paste was decreased to 85% of the non-treatment sample as shown in Fig. 9(b). This result evidences that the dental glazing paste possesses high resistance to the acid solution.

Conclusions

In this study, we demonstrated the possibility of the Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors for use in dental glazing paste. The Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors were synthesized in a single phase form by a conventional solid state reaction method. The Ba₃La₂(BO₃)₄:Ce³⁺ phosphors showed a asymmetric broad blue emission with maximum peak at 470 nm under excitation at 365 nm due to the 5d → 4f transition of Ce³⁺, and the maximum emission peak intensity was obtained for the Ba₃La₂(BO₃)₄:0.05Ce³⁺. The Ba₃La₂(BO₃)₄:Tb³⁺ phosphors exhibited typical green emission assigned to the 4f–4f transition of Tb³⁺ under excitation at 254 nm, and the highest green emission intensity was obtained for the Ba₃La₂(BO₃)₄:0.10Tb³⁺. Under near-UV (365 nm) excitation, Ba₃La₂(BO₃)₄:Ce³⁺,Tb³⁺ phosphors showed both blue emission band and green emission peaks due to the Ce³⁺ and Tb³⁺, respectively, and the emission color of the phosphors can be tuned from blue to green by changing Ce³⁺/Tb³⁺ ratio. By optimization of the composition, cyan-blue emission with high color purity was obtained for the Ba₃La₂(BO₃)₄:0.05Ce³⁺,0.03Tb³⁺ phosphor, and the CIE chromaticity coordinate values for the phosphor were x = 0.2557 and y = 0.3839 under excitation at 365 nm, which is close to the cyan-blue emission for natural teeth. The internal quantum efficiency of the phosphor at the excitation wavelength of 365 nm is estimated to be 50%. To prepare the dental glazing paste, the Ce³⁺/Tb³⁺ co-doped Ba₃La₂(BO₃)₄ phosphor powders mixed with an organic binder and a low Tg glass with a composition of (K,Na)₂O–B₂O₃–Al₂O₃–SiO₂. For the samples heated at 600 and 700 °C, the dental glazing paste showed clear vitrification and they showed a high chemical stability of the luminescence properties for acid aqueous solution (pH = 4). These results indicate that the Ce³⁺/Tb³⁺ co-doped Ba₃La₂(BO₃)₄ phosphor are a suitable candidate as a luminescent material to produce the dental glazing paste with a high aesthetic.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Technology Development Program (S2800580) funded by the Ministry of SMEs and Startups (MSS, Korea).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra04384f

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