1. Introduction

Recently, spectra modification has been investigated deeply due to their potential applications in lamp phosphors, solar cells, photosynthesis and so on [1–5]. Typically, the spectra modification relies on energy transfer (ET) and luminescence processes from luminescent centers. Rare earth (RE) and transition metal (TM) ions possess abundant excited states, which enable them to absorb and emit photons from ultraviolet (UV) to infrared region [6–11].

Dy³⁺ ions doped samples usually exhibit two intense emission bands at around 484 and 573 nm, corresponding to ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions, respectively. Interestingly, the ⁴F_9/2→⁶H_15/2 transition is a magnetic dipole one while the ⁴F_9/2→⁶H_13/2 transition is an electric dipole one. The intensity of electric dipole transition depends on the nature of the host greatly [4]. White emission can be obtained in Dy³⁺ ions doped materials [4]. Consequently, Dy³⁺ is one of the best suitable RE ions for analyzing the luminescence properties in different hosts. Since the absorption efficiency of f-f transition of Dy³⁺ is low due to the spin- and parity-forbidden electric dipole transition, sensitizing ions (Eu²⁺, Ce³⁺, Ag⁺, etc.) are often used to enhance the absorption of Dy³⁺ ions by means of ET [8].

While for sensitizer, Sb is a promising low-cost and environmentally-friendly element [6]. Generally, Sb₂O₃ is used as a fining agent in glass melting [12]. In addition, luminescence of Sb³⁺ ions comes from 5s5p→5s² transition [6]. Sb³⁺ ions usually present broad excitation at UV band (220-320 nm) and wide emission at visible (VIS) band (300-700 nm) that originated from ³P₁→¹S₀ transition, which are very sensitive to surrounding environment [6]. Many TM and RE ions, such as Mn²⁺, Eu³⁺ and so on, have characteristic excitation peaks in the region of 300-500 nm [12–15]. There exist an overlap between the excitation spectra of these ions and the emission spectra of Sb³⁺ ions. Therefore, Sb³⁺ ions could be a proper sensitizer for many activator ions [6].

Among the different matrix materials, luminescent phosphate based glasses present excellent optical properties, better thermal resistance, lower production cost and simpler elaboration procedures [13, 15, 16]. Hence, the phosphate glasses are ideal hosts for RE and TM ions. To our best knowledge, there is no report on ET and tunable emission based on Sb³⁺ and Dy³⁺ ions co-doped magnesium sodium-phosphate glasses.

In this work, the preparation and luminescent properties of Sb³⁺ and Dy³⁺ ions doped 60P₂O₅-35MgO-5Na₂O glasses are presented. The ET process from Sb³⁺ to Dy³⁺ ions was investigated systematically. Results indicated that blue emission in Sb³⁺ doped glasses have potential application in photosynthesis.

2. Experimental

The composition of the glass matrix is 60P₂O₅-35MgO-5Na₂O (in mol%). Samples G-host (pure host without doping), GSx (doped with xSb₂O₃ only, x = 0.20, 0.35, 0.50, 1.00, 1.50, 2.00), GSDz (doped with 1Sb₂O₃ and zDy₂O₃, z = 0.25, 0.50, 1.00, 1.50, 2.00, 2.50), GD (doped with 1Dy₂O₃ only) were prepared by melt-quenching method. P₂O₅, MgO, Na₂CO₃, Sb₂O₃ (A.R., all from Sinopharm Chemical Reagent Co., Ltd., China) and high purity Dy₂O₃ (99.99%, from AnSheng Inorganic Materials Co., Ltd.) were used as starting materials. The weighted raw materials (15 g) were mixed thoroughly, then transferred to an alumina crucible and melted at 1200 °C for 1 h in air atmosphere. After melting, the glass melts were poured onto a preheated copper plate and pressed by another plate quickly for quenching. Finally, the glasses were sliced and polished with a thickness of 2 mm for optical measurements.

Transmission spectra were recorded by a U-3900 ultraviolet-visible (UV-VIS) spectrophotometer (Hitachi). The excitation, emission spectra and quantum yield were measured on an Edinburgh FS5 spectrofluorometer equipped with a continuous wave 150 W Xe lamp and SC-30 Integrating Sphere. Lifetime measurement was acquired on an Edinburgh FLS920 spectrofluorometer equipped with a nanosecond flashlamp (nF900) as excitation sources. All measurements were carried out at room temperature.

3. Results and discussion

Figure 1 presents the UV-VIS transmission spectra of all samples. All samples are colorless and high transparence (higher than 85%), which will benefit their practical application. There is only weak absorption band around 200-300 nm in the transmission spectra of G-host. While compared with G-host sample, all GSx samples [Fig. 1(a)] show a distinct absorption from 240 to 320 nm attributed to ¹S₀→ ³P₁ transition of Sb³⁺. With increasing of Sb³⁺ content, red shift of absorption band can be observed.

 

Fig. 1 Transmission spectra of (a) G-host and GSx samples, (b) GD and GSDz samples.

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Aside from the broadband absorption of Sb³⁺, GSDz samples [Fig. 1(b)] present sharp absorption peaks centered at 294, 324, 350, 364, 386, 424, 453, 750 and 800 nm, which can be assigned to ⁶H_15/2 → ⁴D_7/2, ⁶P_3/2, ⁶P_7/2, ⁶P_5/2, ⁴I_13/2, ⁴G_11/2, ⁴I_15/2, ⁶H_1/2 and ⁶H_3/2 transitions of Dy³⁺, respectively [17–19]. And it is obvious that the absorption peaks of Dy³⁺ become more distinct with the increase of Dy³⁺ concentration.

Figure 2(a) depicts excitation and emission spectra of GSx samples. It can be seen that the excitation spectra (λ_em = 420 nm) for all samples covers a broad UV wavelength region (225-320 nm) with a maximum at about 266 nm, which is assigned to ¹S₀ → ³P₁ transition of Sb³⁺ ions [6]. At the same time, with the increase of Sb₂O₃ content, the excitation peaks experience red shift from 258 to 270 nm also. Such phenomenon is similar to the red shift of absorption band in Fig. 1(a). Both phenomena can be explained as that the values for the optical band gap undergo an obvious decrease with increasing Sb³⁺ content [16, 20].

 

Fig. 2 (a) Excitation spectra of GSx samples and emission spectra of GSx and G-host samples. (b) Excitation spectra of GD sample and emission spectra of GD and G-host samples.

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Upon 266 nm excitation, no luminescence can be observed in G-host notably. While GSx samples exhibit a broadband emission ranging from 300 to 700 nm, which is related to ³P₁ → ¹S₀ transition of Sb³⁺ ions [6]. With increasing Sb³⁺ content, the emission intensity increases rapidly at first and reaches the maximum value for GS1.00 sample, and then remarkably decreases due to concentration quenching. Interestingly, the main absorption of chlorophylls (380-480 nm) matches well with the blue emission of Sb³⁺ ions (marked by blue dashed square in Fig. 2(a)), which means that Sb³⁺ ions can convert UV light into blue light to promote the photosynthesis of plant [1]. Thereby, the Sb³⁺ ions doped magnesium sodium-phosphate glasses are promising materials for greenhouse application.

Figure 2(b) shows excitation and emission spectra of GD sample. The excitation spectrum (λ_em = 573 nm) mainly exhibits nine bands peaking at 293, 323, 336, 349, 364, 386, 423, 451 and 476 nm, corresponding to the transitions from ⁶H_15/2 ground level to higher energy levels of Dy³⁺ [17–19, 21]. Additionally, the weak broad excitation band around 250 nm comes from charge transfer band (CTB) of O^2-→Dy³⁺ [21]. For the emission spectra (λ_ex = 349 nm), the sharp peaks at 484 nm, 573 nm and 662 nm are related to ⁴F_9/2→⁶H_J (J = 15/2, 13/2 and 11/2) transitions of Dy³⁺ ions, respectively [4]. There is no luminescence in G-host sample, too. Besides, excited by characteristic excitation peaks of Sb³⁺ ions (λ_ex = 266 nm), almost no emission is detected for GD sample. Hence, 266 nm is not ideal excitation wavelength for Dy³⁺ ions.

By carefully comparing emission spectra of GSx samples and excitation spectrum of GD sample given in Fig. 2(a) and 2(b), respectively, it can be clearly seen that there is a strong overlap between the emission of Sb³⁺ ions and the excitation of Dy³⁺ ions in the range of 300 to 500 nm. This overlap indicates that an efficient ET process may occur from Sb³⁺ to Dy³⁺.

Figure 3(a) presents emission spectra of GS1.00 and GSDz samples. The inset is enlarged spectra of emission peaks at 573 nm. Excited by 266 nm light, the optimal excitation wavelength for Sb³⁺, but not for Dy³⁺, as expected, emission spectra of the GSDz samples not only includes blue emission from Sb³⁺ ions but also contains emission peaks of Dy³⁺ ions. With increasing Dy³⁺ ions doping content, the emission intensity of Sb³⁺ ions decreases continuously. While the intensity of Dy³⁺ emission increases rapidly first and reaches a maximum at z = 1.00 and then remarkably decreases when Dy³⁺ content is further increased due to concentration quenching. Such phenomena indicate that the quenching concentration of Dy³⁺ is 2 mol% (z = 1.00). In addition, the emission band of Sb³⁺ presents several little dips, which is due to weak radiative re-absorption process from Dy³⁺ ions.

 

Fig. 3 (a) Emission spectra of GS1.00 and GSDz samples, inset is enlarged spectra of emission peaks at 573 nm. (b) Excitation spectra of GS1.00 and GSDz samples (c) Excitation spectra of GS1.00, GD and GSDz samples. (d) Luminescence decay curves of GS1.00 and GSDz samples.

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By monitoring 420 nm emission of Sb³⁺ ions in GS1.00 and GSDz samples, the excitation intensity of Sb³⁺ ions decreases continually with increasing Dy³⁺ content (as shown in Fig. 3(b)). The excitation spectra of GSDz samples (λ_em = 573 nm), GS1.00 (λ_em = 420 nm) and GD sample (λ_em = 573 nm) are presented in Fig. 3(c). By contrast with the excitation spectrum of GD sample, the excitation spectra of GSDz samples present an additional excitation band located at 266 nm. Significantly, this excitation band is similar to excitation band of Sb³⁺ emission of GSx samples. Excitation and emission spectra in Fig. 3 indicate the high light output of Dy³⁺ by the excitation of UV light is due to the ET process from Sb³⁺ to Dy³⁺ (major in non-radiative ET process and minor in radiative re-absorption process).

Given this, the ET efficiency (η) from Sb³⁺ to Dy³⁺ is estimated according to the following equation [22, 23]:

The luminescent quantum yield was measured and the values are 50.5, 43.5, 37.5, 32.9, 25.9, 19.6, 14.0% for GS1.00, GSD0.25, GSD0.50, GSD1.00, GSD1.50, GSD2.00 and GSD2.50 samples, respectively.

In order to give a convincing evidence for the presence of ET from Sb³⁺ to Dy³⁺, decay curves of Sb³⁺ 420 nm emission in GS1.00 and GSDz samples were systemically investigated and are illustrated in Fig. 3(d). The lifetimes are characterized by average lifetime ($\overline{\tau }$) [22]:

The energy-level diagram of Sb³⁺ and Dy³⁺ ions, the possible ET process is schematically demonstrated in Fig. 4(a). Upon 266 nm excitation, Sb³⁺ is excited from ¹S₀ ground state to ³P₁ excited state. Then, a part of Sb³⁺ ions will relax to ground state and then exhibit the broadband emission at 300-700 nm. At the same time, the emission of Sb³⁺ ions can be re-absorbed by Dy³⁺ ion at the ground state.

 

Fig. 4 (a) Energy level diagrams of Sb³⁺ and Dy³⁺ and feasible ET mechanism. (b) CIE chromaticity diagram corresponding to the emission of GS1.00 and GSDz samples, the inset shows the luminescent photos of GS1.00 and GSDz samples excited by 266 nm light in dark.

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Because the excited state of Sb³⁺ is close to ⁶P_7/2, ⁶P_5/2 and other levels of Dy³⁺, other part of excited Sb³⁺ ions relax from ³P₁ excited state to ground state non-radiatively and transfer the energy to neighboring Dy³⁺ ions, promoting them from ⁶H_15/2 ground state to ⁶P_7/2, ⁶P_5/2 and other excited states. Afterwards, Dy³⁺ ions at ⁶P_7/2, ⁶P_5/2 and other levels non-radiatively relax (NR) to ⁴F_9/2 excited state and give strong emissions at 484, 573 and 662 nm of Dy³⁺ (⁴F_9/2→⁶H_J, J = 15/2, 13/2 and 11/2).

Finally, luminescent colors of GS1.00 and GSDz (z = 0.25, 0.50, 1.00, 1.50, 2.00 and 2.50) samples excited by 266 nm light are characterized by the Commission International Eclairage (CIE) chromaticity diagram and shown in Fig. 4(b). The inset of Fig. 4(b) gives the photos of GS1.00 and GSDz samples taken under the 266 nm excitation in dark, respectively. It is apparent that the chromaticity coordinates and emitting color of the GS1.00 and GSDz samples gradually move from blue to white with increasing z.

4. Conclusion

In summary, a series of Sb³⁺ and Dy³⁺ ions doped magnesium sodium-phosphate glasses with high transparence have been prepared at low-melting temperature. Sb³⁺ single doped glasses exhibit a broadband emission with luminescent quantum yield of 50.5% for GS1.00 sample, indicating that Sb³⁺ ions doped magnesium sodium-phosphate glasses are promising material for greenhouse application. White-light emission is obtained under deep UV (266 nm) excitation by combining the blue emission of Sb³⁺ and the yellow emission of Dy³⁺ due to efficient energy transfer process from Sb³⁺ to Dy³⁺. Our result shows a method to convert deep UV light into visible band in luminescent magnesium sodium-phosphate glasses.