Controlled growth and up-conversion luminescence of Y₂O₂S : Er³⁺ phosphor with the addition of Bi₂O₃

Abstract

By adding Bi₂O₃ in phosphor raw materials, fine particles (ca. 500 nm in diameter) have been synthesized for the first time in Y₂O₂S : Er³⁺via high-temperature solid-state reaction. The influence of Bi₂O₃ content on the crystal growth and up-conversion (UC) luminescence (excited at 1550 nm) of Y₂O₂S : Er³⁺ phosphors was investigated by varying the Bi₂O₃ concentration from 0 to 10.0 mol%. As Bi₂O₃ concentration was increased, the average particle size was increased and then decreased, and the uniformity of the particle size was descended and then enhanced. The size change of Bi₂O₃ doped Y₂O₂S : Er³⁺ phosphors can be well explained by the liquid phase sintering mechanism. Compared to the phosphors without containing any Bi₂O₃, UC luminescence intensity (excited at 1550 nm) of phosphors containing Bi₂O₃ decreased, but was still sufficiently strong to the naked eye.

---

1. Introduction

Er³⁺ doped up-conversion (UC) materials have recently attracted considerable attention for their valuable applications in solid state visible lasers, color displays, and biomedical imaging.^1–3 It is well known that Y₂O₂S : Er³⁺ is one of the most popular UC phosphors, because it exhibits excellent chemical stability, high luminescent efficiency, and moderate phonon energies (about 520 cm⁻¹).⁴

Currently, the high-temperature solid-state reaction is the main method to prepare rare earth oxysulfide phosphors, which use, e.g. Na₂CO₃, as flux for their low cost and high operability. However, the phosphors prepared via this method exhibit a large particle size in micron range,^5,6 which significantly narrows their applications. Therefore, it is highly desirable to decrease the average particle size.

Several approaches have been developed to prepare fine phosphors, including co-precipitation method, sol–gel method, combustion method, hydrothermal method, etc.^7–9 However, such approaches have been rarely used in commercial production of fine phosphors because of their long manufacturing duration, high cost, and complex operability.

Alternatively, one may use crystal growth inhibitors to lower crystal size. For example, an appropriate liquid phase can be applied to restrain crystal growth. The presence of a liquid phase during sintering can induce the transition from normal to anomalous grain growth. In order to apply liquid phase successfully during sintering of Y₂O₂S and optimize microstructural evolution, the concentration of liquid phase in the sample during sintering must be carefully tuned. Bi₂O₃, which has a low melting point of 700 °C, has been reported to help reduce crystal size of ceramics.^10–12 In this paper, we also choose Bi₂O₃ as a potential crystal growth inhibitor to decrease crystal size. All Y₂O₂S : Er³⁺ samples were prepared by high-temperature solid-state reaction method. The aim of this work is to study the influences of Bi₂O₃ concentration (varying from 0–10.0 mol%) on the crystal growth, crystal size uniformity, and UC luminescence properties of Y₂O₂S : Er³⁺ phosphors.

2. Experimental

2.1 Samples preparation

(Y_0.95Er_0.05)₂O₂S–xBi₂O₃ (x = 0, 1.0, 3.0, 5.0, 7.0, 10.0 mol%) phosphors were prepared by high-temperature solid-state reaction method. High-purity power of Y₂O₃ (99.99%), Er₂O₃ (99.99%) and S (99.999%), Na₂CO₃ and Bi₂O₃ (all analytical reagents) were used as raw materials. Both Na₂CO₃ and S were added from 30.0 to 50.0 wt%. All chemicals were mixed and then sintered at 1100 °C for 3 h using alumina crucibles with alumina covers in carbon reducing atmosphere. The sintered samples were further treated by washing with dilute hydrochloric acid and distilled water several times. The washed powders were subsequently filtered and dried.

2.2 Material characterization

The phosphor samples were characterized by X-ray diffraction (XRD) (Rigaku, D/Max-RB, Cu-Kα radiation, λ = 0.15406 nm). The morphology of the phosphor samples were examined by a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi High-Technologies). Absorption spectra of the samples were measured by a UV-Vis-NIR spectrophotometer (CARY 500, Varian Company, America) equipped with an integrating sphere, and the samples to be measured were put in the same sample pool and then pressed to the same thickness. The UC luminescence spectra were recorded at room temperature on an Edinburgh FLS920P spectrometer, excited by a power tunable 1550 nm semiconductor laser diode (0–500 mW). All measurements were performed under the same conditions.

3. Results and discussion

3.1 Influence of Bi₂O₃ concentration on the crystal growth of Y₂O₂S : Er³⁺ phosphors

The XRD patterns of the as-obtained samples with various Bi₂O₃ concentrations are shown in Fig. 1. According to XRD patterns, the phases of the samples are hexagonal Y₂O₂S (JCPDS # 24-1424) and a small amount of orthorhombic Bi₂S₃ impurity (JCPDS # 65-3884). Increasing Bi₂O₃ concentration from 0 to 10.0 mol% had virtually no effect on the XRD patterns of the samples. In Fig. 1(b)–(c), the XRD patterns of the samples do not reveal the presence of any additional phase other than hexagonal Y₂O₂S. However, in Fig. 1(d)–(g), a very weak peak of orthorhombic Bi₂S₃ phase appears for the samples with x > 1.0 mol% Bi₂O₃ additives. The intensity of Bi₂S₃ phase peak increases slightly with increasing concentration of Bi₂O₃.

Fig. 1 XRD patterns of samples (Y_0.95Er_0.05)₂O₂S–xBi₂O₃ and reference date JCPDS # 24-1424.

The crystal symmetry of Y₂O₂S is hexagonal with space group P-3ml. There is one formula unit per unit cell. The structure is very closely related to the A-type rare-earth oxide structure, with the difference being that one of the three oxygen sites is occupied by a sulfur atom.⁶ The formation of orthorhombic Bi₂S₃ phase indicates that partial Bi ions can participate in the reaction with S ions to form Bi₂S₃ during sintering process. When Bi₂O₃ concentration is low, the amount of Bi₂S₃ impurity is too low to be detected by the XRD measurement. However, as the concentration of Bi₂O₃ increased, the diffraction intensity of the orthorhombic Bi₂S₃ phase increased continually. But overall, the diffraction intensity of the Bi₂S₃ phase peak is very weak compared with the main peak (101) of Y₂O₂S phase, suggesting that the amount of Bi₂S₃ phase is marginal. To confirm the existence of Bi₂S₃ impurity, the x = 10.0 mol% sample was further treated by washing with dilute FeCl₃ solution, and the diffraction peak at 28.5° disappeared, which supports the above hypothesis.

The SEM images of (Y_0.95Er_0.05)₂O₂S–xBi₂O₃ (x = 0, 1.0, 3.0, 5.0, 7.0, 10.0 mol%) samples are shown in Fig. 2. All samples exhibit hexagonal morphology. It is also found that Bi₂O₃ additive influences grain growth. The observed average particle size for Y₂O₂S : Er³⁺ sample without Bi₂O₃ is about 2 μm (Fig. 2(a)). The addition of Bi₂O₃ significantly changes the microstructure. The main feature of the sample with the lowest concentration of Bi₂O₃ was significantly increased grain size (Fig. 2(b)). The addition of a higher concentration of Bi₂O₃ caused a decrease in particle size (Fig. 2(c)–(f)). From Fig. 2(f), one can find that the sample with 10.0 mol% Bi₂O₃ concentration is the best in terms of uniformity and average size (ca. 500 nm in diameter).

Fig. 2 SEM images for (Y_0.95Er_0.05)₂O₂S–xBi₂O₃ (x = 0, 1.0, 3.0, 5.0, 7.0, 10.0 mol%) samples: (a) x = 0 mol%, (b) x = 1.0 mol%, (c) x = 3.0 mol%, (d) x = 5.0 mol%, (e) x = 7.0 mol%, (f) x = 10.0 mol%.

Bi₂O₃ is a low melting point additive whose melting point is 700 °C. During sintering, it forms a liquid phase and influences the grain growth of Y₂O₂S. Thus, the sintering kinetics would be determined by the liquid phase sintering due to sintering temperature of 1100 °C. It can be postulated that grain growth mechanisms relating to Y₂O₂S were rate controlling, depending on the amount of Bi₂O₃ liquid phase: (i) solution-re-precipitation process—a thin layer of Bi₂O₃-rich liquid phase is needed; (ii) diffusion of atoms through the Bi₂O₃-rich liquid phase.¹³ During the grain growth of Bi₂O₃ added Y₂O₂S, the above two types are clearly identified in the regions with x = 0 or 1.0 mol% and x > 1.0 mol%, respectively.

The microstructural development of Y₂O₂S doped with Bi₂O₃ liquid phase can be discussed by the application of the generally accepted grain growth kinetics represented by the following relation:¹⁴

D_mⁿ − D₀ⁿ = k₀t exp (−Q/kT) (1)

where D_m is the average grain size at time t, D₀ is the initial grain size, n is the kinetic grain growth exponent, and Q is the apparent activation energy. When time and temperature are constants, the final grain size is related to the activation energy Q associated with the specific grain growth. A thin layer of Bi₂O₃ rich liquid phase surrounds all grains (x = 1.0 mol%), thus increasing the grain boundary mobility and the rate of grain growth. It is characterized by rapid grain growth, in which a dramatic decrease in the activation energy must govern the grain growth mechanism and the final grain size. In the sample (x = 1.0 mol%), a dramatic increase in the grain size occurs associated with the formation of grains with overestimated grain size.

The grain growth mechanism of Y₂O₂S doped with 1.0 mol% concentration of Bi₂O₃ can be explained by the application of the well accepted liquid phase sintering mechanism which is a solution-reprecipitation process.¹⁵ During the formation of giant grains, the smallest grains, which are thermodynamically less stable, dissolve in the liquid phase and subsequently precipitate on the larger grains and/or the more curved part of the grain dissolves and precipitates on less curved grain boundaries. The grain growth rate of Y₂O₂S in the presence of liquid phase is controlled either by the solid–liquid phase boundary reaction or by diffusion through the liquid phase from smaller grains to larger grains. Thus, the dramatic decrease of the activation energy for grain growth (dramatic increase of grain size) in the sample (x = 1.0 mol%) and the high solubility of Y₂O₂S in Bi₂O₃ lead to an intense solid-liquid phase boundary reaction, which causes the anomalous grain growth. Moreover, the x = 1.0 mol% sample exhibits a structure of poor uniformity. The discontinuous grain growth probably indicates an inhomogeneous distribution of the liquid phase owing to the low concentration of Bi₂O₃ liquid phase.

The liquid phase layer not only accelerates mass transfer but also forms wetting menisci at the grain boundaries, providing an additional capillary pressure driving force for sintering.¹⁶ Up to a higher concentration (x > 1.0 mol%), the thickness of the liquid phase layer increases with increasing Bi₂O₃ concentration. Thus the sintering mechanism is the diffusion of atoms through the Bi₂O₃-rich liquid phase. The grain growth rate for the liquid phase–boundary reaction controlled process is not dependent on the amount of liquid phase, which can be explained by the following formula:¹⁷

dG/dt = KV_mf/kT (2)

where K is the interface reaction rate, V_m is the mole volume, and f is the driving force. When the amount of Bi₂O₃ increases further, the anomalous microstructure disappears and normal microstructure is developed. The microstructural observations show that in the samples (x = 3.0, 5.0, 7.0, 10.0 mol%) no discontinuous grain growth can be detected. The average grain size for these samples is decreased. The larger amount of Bi₂O₃ increases the diffusion layer and the diffusion of atoms through the Bi₂O₃-rich liquid phase which governs the grain growth rate:¹⁴

dG/dt = 2DSMσ/kTρδ (G/G₀−1) (3)

where D is the diffusion constant of the solid in the liquid, M is mol mass, S is the solubility of a flat surface, σ is the solid–liquid surface energy, ρ is the density of the solid, δ is the thickness of the liquid layers, and G₀ is the critical grain radius below which the grains dissolve and above which they grow. Thus, the grain growth rate and/or the activation energy depend on the thickness of the diffusion layer. The thicker is the liquid phase layer, the lower is the grain growth rate. Therefore, it is not surprising that in samples (x = 3.0, 5.0, 7.0, 10.0 mol%) the average grain size gradually decreases with increasing amount of added Bi₂O₃, due to increasing the thickness of the diffusion layer. However, with increasing Bi₂O₃ concentration from 3.0 to 10.0 mol%, the crystal size uniformity of the samples turns better gradually, because that Bi₂O₃-rich phase are distributed homogeneously over the sample and located mostly at the grain boundaries.

3.2 Influence of Bi₂O₃ concentration on the UC luminescence of Y₂O₂S : Er³⁺ phosphors

Absorption spectra of the samples with various Bi₂O₃ concentrations are shown in Fig. 3. The high-energy bands at wavelength shorter than 240 nm in the absorption spectra resulted from the fundamental absorption of the Y₂O₂S host lattice. The sharp absorption bands can be attributed to the following transitions ⁴G_11/4→⁴I_15/2 (381 nm), ⁴F_7/2→⁴I_15/2 (495 nm), ²H_11/2→⁴I_15/2 (524 nm), ⁴F_9/2→⁴I_15/2 (661 nm), ⁴I_9/2→⁴I_15/2 (808 nm), ⁴I_11/2→⁴I_15/2 (980 nm), ⁴I_13/2→⁴I_15/2 (1550 nm) of Er³⁺, respectively.¹⁸ And, a broad absorption band from 240 to 1100 nm can be observed. It consists of five small broad absorption bands (at ∼ 340, 505, 700, 800, and 1000 nm), which are similar to those reported for Bi-doped glasses with VIS-NIR emission at room temperature.^19–22 Increasing Bi₂O₃ concentration from 0 to 10.0 mol% has little effect on the shape of the absorption spectrum except for their intensities. Introduction of Bi results strong absorption, however, there is no clear trend with regard to the Bi concentration.

Fig. 3 Absorption spectra of (Y_0.95Er_0.05)₂O₂S–xBi₂O₃ (x = 0, 1.0, 3.0, 5.0, 7.0, 10.0 mol%) samples.

Also, in Y₂O₂S : Er³⁺ samples containing Bi₂O₃, we observed 1960 nm absorption band. This band does not exist in other Y₂O₂S : Er³⁺ samples without containing Bi₂O₃. This phenomenon is very interesting, which needs to be further researched. From Fig. 3, we can deduce that a small part of Bi ions may be doped into the Y₂O₂S host.

Excited by a 1550 nm laser at a power of 336 mW, room temperature UC luminescence spectra of the samples with different Bi₂O₃ concentrations are shown in Fig. 4. The inset shows a photograph of the UC emission in the sample (x = 10.0 mol%) under 1550 nm excitation. The emission spectra in the 520–700 nm range are characterized by an intense green band at 546 nm followed by a weaker red band at 671 nm. Although the addition of Bi₂O₃ lowered the luminescence intensity, most samples (x = 7.0 mol% or 10.0 mol%) still exhibited strong emission to the naked eyes, which insure such doped phosphors can still find wide spread applications in some special areas, which require good coating uniformity and resolving capability.

Fig. 4 UC luminescence spectra of (Y_0.95Er_0.05)₂O₂S–xBi₂O₃ (x = 0, 1.0, 3.0, 5.0, 7.0, 10.0 mol%) samples excited by a 1550 nm laser (336 mW). The inset is the photograph of the UC emission of the sample (x = 10.0 mol%) excited by a 1550 nm laser (336 mW).

The UC process in the Er³⁺ system is well understood and is briefly explained in the energy level diagram in several reports.^23–30 For our case, the up-conversion mechanisms can be explained on the basis of various processes such as three photon absorption, excited state absorption and cross relaxation from energy transfer process. Emission bands observed at 671 nm, 546 nm, and 524 nm are assigned to the transitions ⁴F_9/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, ²H_11/2→⁴I_15/2 of Er³⁺, respectively. The emission intensity decreased with the doping of Bi. Furthermore, the relationship of luminescence intensity with the Bi dopant, is inversely related to the absorption intensity in VIS (green-red) region, which is due to the absorption of the Er³⁺ emission at the visible range by Bi ions. The decrease of UC luminescence intensity, in comparison with the Bi-free sample, can be explained by two reasons. On one hand, the emission intensity declines as grain size decreases. As reported in the literature,³¹ typically the larger the phosphor particle size, the higher the emission intensity. Well crystallized large phosphors have high luminous efficiency. Phosphors with a small particle size typically have strong scattering and low emission intensity. In addition, small phosphor particles typically contain more surface defects owing to incomplete crystallization, which can largely reduce the UC emission intensity. On the other hand, in samples containing Bi₂O₃, the Y₂O₂S host is doped with a small amount of Bi ions (shown in Fig. 3). The samples exhibit strong absorption in the VIS (green-red) region, and the emissions are absorbed by themselves. Even with strong absorption, the UC emissions of samples containing Bi₂O₃ are still sufficiently strong to the naked eyes excited by a 1550 nm laser (336 mW).

4. Conclusions

Herein we reported controlled growth of Y₂O₂S : Er³⁺ particles for the first time via high-temperature solid-state reaction, by addition of Bi₂O₃ in phosphor raw materials. The addition of Bi₂O₃ greatly influenced the grain growth of Y₂O₂S. The mechanisms relating to Y₂O₂S grain growth were rate controlling, depending on the amount of Bi₂O₃ liquid phase due to the low melting point of Bi₂O₃ (700 °C): (i) solution-re-precipitation process; (ii) diffusion of atoms through the Bi₂O₃-rich liquid phase. The UC luminescence properties were dependent on the microstructure and Bi₂O₃ concentrations. Optimum addition of Bi₂O₃ led to fine phosphor grains and high UC luminescence. The fine Y₂O₂S : Er³⁺ UC phosphors, which possess good coating uniformity and resolving capability, offers several potential applications such as color displays and biomedical imaging.

Acknowledgements

This work is financially supported by the Natural Science Foundation of Shanghai (11ZR1409300, 12ZR1407600) , the Fundamental Research Funds for the Central Universities (WD1014035), and the Shanghai Leading Academic Discipline Project (B502). L.S. would like to thank the U.S. Department of Agriculture (2011-38422-30803), the ACS Petroleum Research Fund, and Research Corporation for Science Advancement for partial support for this research.

References

1. J. W. Zhao, M. Dong, Y. G. Zhao and X. G. Kong, Adv. Mater. Res., 2012, 510, 609.
2. J. H. Chung, J. H. Ryu, J. W. Eun, J. H. Lee, S. Y. Lee, T. H. Heo, B. G. Choi and K. B. Shim, J. Alloys Compd., 2012, 522, 30.
3. X. D. Qi, C. M. Liu and C. C. Kuo, J. Alloys Compd., 2010, 492, L61.
4. G. F. J. Garlick and C. L. Richards, J. Lumin., 1974, 9, 432.
5. P. D. Han, L. Zhang, L. X. Wang and Q. T. Zhang, J. Rare Earths, 2011, 29, 849.
6. G. A. Kumar, M. Pokhrel, A. Martinez, R. C. Dennis, I. L. Villegas and D. K. Sardar, J. Alloys Compd., 2012, 513, 559.
7. L. Liu, Y. X. Wang, X. R. Zhang, K. Yang, J. Li, G. Liu and Y. L. Song, J. Lumin., 2012, 132, 1483.
8. C. Y. Cao, X. M. Zhang, M. L. Chen, W. P. Qin and J. S. Zhang, J. Alloys Compd., 2010, 505, 6.
9. G. S. Yi, H. C. Lu, S. Y. Zhao, Y. Ge, W. J. Yang, D. P. Chen and L. H. Guo, Nano Lett., 2004, 4, 2191.
10. S. Altin, M. A. Aksan, E. Altin, Y. Balci and M. E. Yakinci, J. Supercond. Novel Magn., 2011, 24, 331.
11. T. Ohkura, Y. Fujimoto, M. Nakatsuka and S. Young-Seok, J. Am. Ceram. Soc., 2007, 90, 3596.
12. Y. F. Wei, C. H. Kao, C. F. Yang, H. H. Huang and C. J. Huang, Mater. Lett., 2007, 61, 4643.
13. M. Drofenik, A. Znidarsic and D. Makovec, J. Am. Ceram. Soc., 1998, 81, 2841.
14. S. Hingorani and D. O. Shah, J. Mater. Res., 1995, 10, 461.
15. M. L. S. Teo, L. B. Kong, Z. W. Li and Y. B. Gan, J. Alloys Compd., 2008, 459, 557.
16. K. Sun, Z. W. Lan, Z. Yu, L. Z. Li, J. M. Huang and X. N. Zhao, J. Phys. D: Appl. Phys., 2008, 41, 235002.
17. C. Wagner, Elektrochem., 1961, 65, 581.
18. E. P. Chukova, Moscow: Sovetskoe Radio, 1980.
19. C. H. An, S. T. Wang and Y. Q. Liu, Mater. Lett., 2007, 61, 2284.
20. M. Y. Peng, J. R. Qiu, D. P. Chen, X. G. Meng and C. S. Zhu, Opt. Express, 2005, 13, 6892.
21. T. Suzuki and Y. Ohishi, Appl. Phys. Lett., 2006, 88, 191912.
22. Y. Arai, T. Suzuki, Y. Ohishi, S. Morimoto and S. Khonthon, Appl. Phys. Lett., 2007, 90, 261110.
23. G. A. Kumar, M. Pokhrel and D. K. Sardar, Mater. Lett., 2012, 68, 395.
24. J. Guo, F. Z. Ma, S. N. Gu, Y. Shi and J. J. Xie, J. Alloys Compd., 2012, 523, 161.
25. M. Glauco, S. R. Nikifor, F. Michael, C. S. Isabel and C. P. B. Carlos, Appl. Phys. Lett., 2006, 89, 081109.
26. S. Ivanova and F. Pellé, J. Opt. Soc. Am. B, 2009, 26, 1930.
27. G. Y. Chen, T. Y. Ohulchanskyy, A. Kachynski, H. Agren and P. N. Prasad, ACS Nano, 2011, 5, 4981.
28. T. Trupke, A. Shalav, B. S. Richards, P. Würfel and M. A. Green, Sol. Energy Mater. Sol. Cells, 2006, 90, 3327.
29. X. B. Chen, Opt. Commun., 2004, 242, 565.
30. P. Xie and S. Rand, J. Opt. Soc. Am. B, 1994, 11, 901.
31. Y. S. Chen, W. He, H. H. Wang, X. L. Hao, Y. C. Jiao, J. X. Lu and S. E. Yang, J. Lumin., 2012, 132, 2404.

---