In the Laboratory
Synthesis and Characterization of Nanocrystalline Y2O3:Eu3+ Phosphor
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An Upper-Division Inorganic Chemistry Laboratory David B. Bolstad and Anthony L. Diaz* Department of Chemistry, Central Washington University, Ellensburg, WA 98926; *
[email protected] Rationale Traditional inorganic synthesis course work has emphasized the preparation of organometallic compounds. However, modern inorganic synthesis in the classroom has come to include a variety of topics, such as polymer chemistry (1) and solid-state chemistry (2). In fact, many of the important topics in solid-state chemistry now appear in laboratory textbooks that can be used at all course levels (3, 4). As a supplement to materials science education literature, we include this laboratory involving the synthesis of nanocrystalline phosphor. This lab is taught in our junior–senior inorganic synthesis course. Interest in nanomaterials continues to grow both industrially and academically, and this experiment provides an opportunity to make some of the concepts associated with this technology more accessible to undergraduate students. The study of solid-state luminescence impacts a wide variety of technologies, including display (CRTs and flat televisions), lighting (fluorescent lamps and Hg-free lamps), and medical imaging (X-ray and tomography). Most solid-state luminescent compounds comprise a host and an activator. The host is usually an optically inert, crystalline oxide, such as Zn2SiO4, LaPO4, or Y2O3. A small percentage of cations within the host are replaced by luminescent transition metal or rare-earth ions, such as Mn2+, Tb3+, or Eu3+. The coordination environment provided by the host lattice can significantly influence both the efficiency of the emission and the wavelengths emitted by the activator. In a practical device, some excitation energy is provided to the luminescent material (called a phosphor), and this energy is then converted to visible light. In a fluorescent lamp, for example, the excitation energy comes from ultraviolet photons generated by a mercury plasma. In this lab we synthesize nanophosphor and study the effects of particle size on crystallinity and luminescence. The phosphor, Y2O3:Eu3+, is the red-emitting component of three-color fluorescent lamps. In this phosphor, about 10% of the Y atoms are replaced by Eu atoms (5). This nanomaterial exhibits interesting photoluminescence properties relative to the bulk material (6, 7, 9) and efficient nanocrystalline Y2O3:Eu3+ could have a variety of technological applications (in high-resolution displays, for example). Nanocrystalline oxides have been synthesized by a variety of methods, including the oxidation of metal nanoparticles, laser ablation, and sol–gel preparations (6 ). These methods are fairly sophisticated and not really suitable for the classroom. Here, we take advantage of a combustion synthesis, as described by Shea et al. (7 ). This preparation involves the explosive reaction of metal nitrates with a fuel such as glycine or urea. The synthesis is very easy to do and is fun for the
students as well as instructive. To our knowledge, although some phosphor synthesis methodology appears in the chemical education literature (8), a solid-state combustion synthesis has not been presented as a college-level laboratory experiment before. The primary methods for characterizing the particle size of nanocrystalline materials are powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) (9, 10). Many universities, particularly small ones, do not have easy access to a TEM. However, X-ray diffraction is so valuable a tool to geology, physics, chemistry, and materials science that most universities today have powder X-ray diffraction capabilities somewhere on campus. The data presented here were acquired using equipment available in the geology department. X-ray line broadening is used to calculate strain and particle-size parameters. This experiment also teaches some of the basics of solidstate luminescence. The efficiency of the phosphor is influenced by the crystallinity of the host and improves dramatically when the nanophosphor is refired. In addition, the energy of the charge-transfer excitation of Eu3+ in Y2O3:Eu3+ increases at very small particle sizes. These phenomena can be measured using luminescence spectroscopy. In this paper, we follow the analysis of Igarashi et al. for both the powder X-ray and the luminescence data (10). Experimental Procedure Details of the preparation and information about our instrumentation are provided in JCE Online.W The procedure we use follows that of Shea et al. (7 ). Urea, Y(NO3)3, and Eu(NO3)3 are dissolved in water in a Pyrex evaporating dish and the dish is placed in a muffle furnace at 500 °C. When a critical concentration is reached the mixture reacts rapidly to form a column of foamy white powder with a volume much larger than that of the original container. A portion of the powder was fired for an additional 24 hours at 900 °C in the muffle furnace. The materials were then analyzed using X-ray diffraction and luminescence spectroscopy. Hazards The highly soluble heavy-metal salts Y(NO3)3 and Eu(NO3)3 are oxidizers. Gloves should be worn when handling these materials, particularly when they are dissolved in water. Because an explosive mixture is being prepared, the students must add the chemicals to the water, rather than diluting a mixture of the solids. The reaction takes place as a rapid combustion. However, under the conditions described here it is an extremely mild and contained event. When the
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Figure 1. Powder X-ray diffraction pattern of as-prepared nanoY2O3:Eu3+.
Figure 2. Powder X-ray diffraction pattern of nano-Y2O3:Eu3+ after firing at 900 °C for 24 h.
quantities described are used,W the reaction is barely audible above the room noise (air conditioning, hoods, etc.). The reaction is not nearly violent enough, for example, to shake, move, or damage the furnace. The furnace should be large enough to accommodate a column of powder 4–5 inches tall. As a precaution we place our muffle furnace inside the fume hood and close the door of the hood when we do this reaction. We did not attempt to run this reaction under more concentrated conditions or with larger quantities of material; this is not recommended. However, we do note that our laboratory synthesis is done at half the concentration used by Shea et al.
represents the broadening due to strain. Multiplying both sides of eq 1 by cos(θ)/ λ leads to the expression
Results and Discussion
X-ray Diffraction In Figure 1 the powder X-ray diffraction pattern of the as-prepared nano-Y2O3:Eu3+ is shown. This can be compared to the pattern of the same material after firing at 900 °C for 24 h (Figure 2). The broad diffraction lines observed here, particularly for the as-prepared nano material, derive from both particle size and strain effects. A discussion of these phenomena can be found in any X-ray diffraction textbook (for example, refs 11 and 12). In some cases the strain effects can be ignored. Berger et al., for example, have developed an undergraduate experiment in which the broadening of a single diffraction line is used to estimate the particle size of magnetite using Scherrer’s equation (13). Here we provide a more complete analysis of the diffraction data using the simplest application of the methods of Hall (14) and Williamson (15). The analysis of X-ray line broadening begins with the assumption that the total line broadening from size and strain is much larger than the instrumental broadening, so that the line width can be approximated by the equation β=
λ + 4ε tan θ D cos θ
(2)
From eq 2 we can construct a plot of β cos(θ)/ λ versus sin(θ)/ λ that should lead to a straight line with a slope of 4ε and an intercept of 1/D. This is the method of Hall as employed by Igarashi et al. in their analysis of nano-Y2O3:Eu3+ (10). In our analysis, students used the peaks at 29.1, 48.5, and 57.6 degrees 2 θ (Miller indices 222, 440, and 622, respectively) for both the as-prepared and the fired samples. In our case, FWHMs were estimated directly from the raw data, although more sophisticated peak-fitting techniques could also be employed. The Hall plots are shown in Figure 3, and the data are summarized in Table 1. Firing the sample at 900 °C leads to a doubling of the particle size and significant reduction in the lattice strain, consistent with what we would expect from traditional solid-state synthesis. The particle sizes and strain parameters of the student’s samples are consistent with those observed by other researchers for nano-Y2O3:Eu3+ prepared by sol–gel methods (10, 16 ). Unfortunately, Shea et al. do not report the particle size of the Y2O3:Eu3+ they obtained using combustion synthesis. They do report that the Y3Al5O12:Cr3+ they prepared using the same technique had a particle size of 49 nm (7 ).
Luminescence The oxide Y2O3 has a cubic structure when synthesized under moderate conditions (e.g., 1000–1400 °C and atmospheric pressure) (17 ). There are two crystallographically
(1)
Here β is the full width at half maximum (FWHM) in radians, D is the particle diameter, λ is the wavelength of the X-rays (Cu Kα, 1.541 Å, on our instrument), θ is the Bragg angle of the diffraction peak, and ε is the lattice distortion or residual strain. The first term in eq 1 is the Scherrer relation that represents the broadening due to particle size. The second term
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β cos θ 4ε sin θ 1 = + λ λ D
Table 1. Cr ystallographic Data from a Sample of Nano-Y2O3:Eu3+ Sample
FWHM of Miller Reflections/rad 222
440
622
As prepared
0.00977
0.01570
0.01780
Fired at 900 °C
0.00227
0.00288
0.00332
Journal of Chemical Education • Vol. 79 No. 9 September 2002 • JChemEd.chem.wisc.edu
Lattice Crystallite Distortion Size/nm (%) 59.6 109
0.70 0.08
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y = 0.0278x + 0.0168 R 2 = .9888
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Figure 3. Hall’s plots for ( 䊐) as-prepared and ( 䉫) fired nanoY2O3:Eu3+.
Figure 4. Emission spectra of Y2O3:Eu3+ samples under 254-nm excitation.
distinct Y sites in this material, of S6 and C2 symmetries, both of which can be occupied by Eu3+, although emission from the C2 site typically dominates the emission spectrum of Y2O3:Eu3+ (18). The emission spectrum is characterized by sharp lines resulting from transitions within the 4f manifold of Eu3+. Excitation of this emission occurs through a charge-transfer transition in which an electron from an O2᎑ in the lattice is transferred to empty excited-state levels of Eu3+ (19). The emission spectra of the as-prepared and fired samples are compared in Figure 4. These spectra exhibit the intense emission band at 611 nm that is typical for this phosphor. They also demonstrate the general observation that solid-state phosphors become more efficient as their crystallinity is improved. In Figure 5 the normalized excitation spectrum of the as-prepared nanomaterial is compared to a spectrum obtained from a commercial sample that has a particle size >5 µm (Y2O3:Eu3+ from Osram Sylvania, type 2342, lot #YCX-625). In the nanomaterial the charge-transfer band has a maximum at 252 nm, whereas in the microcrystalline material the maximum occurs at 259 nm. This shift in the energy of the chargetransfer band is always observed in nano-Y2O3:Eu3+ and has been attributed to a change in the Eu–O bond length (10) and also to surface-strain effects (16 ). Even though the causes for this shift have not been completely identified, the spectra
still provide an opportunity to teach students about spectroscopy and solid-state luminescence phenomena. Conclusions The combustion synthesis of nanocrystalline Y2O3:Eu3+ is easy and interesting for students in an upper-division inorganic preparations course. The entire lab, including data acquisition and analysis, is accomplished in two, 3-hour laboratory sessions. As described, this experiment provides a unique means of teaching solid-state synthesis, X-ray diffraction, luminescence, and nanotechnology concepts. It could also be expanded or modified to emphasize variations in the synthesis conditions or to include more sophisticated fitting of the X-ray diffraction data. We also note that at particle sizes below 8 nm Y2O3 exists in the high-pressure monoclinic phase owing to the Gibbs–Thomson effect (17). Although the synthesis of particles this small is probably too sophisticated for the classroom, a discussion of these effects could supplement the lab or lecture material. Acknowledgments We wish to thank Tony Brown of the Central Washington University Department of Chemistry staff for assistance in the preparation of this experiment for the classroom. We also thank Osram Sylvania for providing a sample of commercial Y2O3:Eu3+ phosphor.
Normalized Intensity
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standard 0.8
Supplemental Material
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Details of the preparation and information about our instrumentation are available in this issue of JCE Online.
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Literature Cited
nanomaterial
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Wavelength / nm Figure 5. Normalized excitation spectra for Y2O3:Eu3+ (emission at 611 nm).
1. Hodgson, S. C.; Orbell, J. D.; Bigger, S. W.; Scheirs, J. J. Chem. Educ. 2000, 77, 745. 2. Zümreoglu-Karan, B.; Yilmazer, E. J. Chem. Educ. 2000, 77, 1207. 3. Ellis, A. B.; Geselbracht, M. J.; Johnson, B. J.; Lisensky, G. C.; Robinson, W. R. Teaching General Chemistry: A Materials Science Companion; American Chemical Society: Washington, DC, 1993.
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In the Laboratory 4. Suib, S. L.; Tanaka, J. Experimental Methods in Inorganic Chemistry; Prentice Hall: Upper Saddle River, NJ, 1999. 5. Blasse, G.; Grabmeier, B. C. Luminescent Materials; Springer: New York, 1994; Chapters 6 and 7. 6. Tissue, B. M. Chem. Mater. 1998, 10, 2837. 7. Shea, L. E.; McKittrick, J.; Lopez, O. A. J. Am. Ceram. Soc. 1996, 79, 3257. 8. Suib, S. L.; Tanaka, J. J. Chem. Educ. 1984, 61, 1099. 9. Li, Q.; Gao, L.; Yan, D. Nanostruct. Mater. 1997, 8, 825. 10. Igarashi, T.; Ihara, M.; Kusunoki, T.; Ohno, K.; Isobe, T.; Senna, M. Appl. Phys. Lett. 2000, 76, 1549. 11. Jenkins, R.; Snyder, R. L. Introduction to Powder X-ray Diffractometry; Wiley: New York, 1996; Chapter 3.
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12. Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: Reading, MA, 1978; Chapter 9. 13. Berger, P.; Adelman, N. B.; Beckman, K. J.; Campbell, D. J.; Ellis, A. B.; Lisensky, G. C. J. Chem. Educ. 1999, 76, 943. 14. Hall, W. H. Proc. Phys. Soc. 1949, A62, 741. 15. Williamson, G. K.; Hall, W. H. Acta Metall. 1953, 1, 22. 16. Tao, Y.; Zhao, G.; Ju, X.; Shao, X.; Zhang, W.; Xia, Sh. Mater. Lett. 1996, 28, 137. 17. Skandan, G.; Foster, C. M.; Frase, H.; Ali, M. N.; Parker, J. C.; Hahn, H. Nanostruct. Mater. 1992, 1, 313. 18. Pappalardo, R.; Hunt, R. B. J. Electrochem. Soc. 1985, 132, 721. 19. Buijs, M.; Meyerink, A.; Blasse, G. J. Lumin. 1987, 37, 9.
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