Microwave Synthesis of a Fluorescent Ruby Powder - Journal of

Laboratory Experiment pubs.acs.org/jchemeduc

Microwave Synthesis of a Fluorescent Ruby Powder Géraldine Leyral, Laurent Bernaud, Alain Manteghetti, and Jean-Sébastien Filhol* Institut Charles Gerhardt Montpellier UMR 5253 CNRS-UM2-ENSCM-UM1, Université Montpellier 2, Place E. Bataillon 34095 Montpellier Cedex 5, France S Supporting Information *

ABSTRACT: Ruby (aluminum oxide doped with Cr) powders were synthesized using a assisted microwave combustion method in an approach developed to involve students in a research-like experiment. The synthesis is fast (5 min) and efficient. It demonstrates many properties of ruby such as atomic structure, color, and fluorescence as a function of increasing doping content.

KEYWORDS: Second-Year Undergraduate, Inorganic Chemistry, Laboratory Instruction, Inquiry-Based/Discovery Learning, Crystal Field/Ligand Field Theory, Materials Science, Synthesis, Solid State Chemistry, UV−Vis Spectroscopy, X-ray Crystallography

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T

he best way for students to invest their time and interest into learning science is to have them work on amazing materials, such as jewels, with both emotional links and impressive straightforward properties. These properties can be the starting point to a more fundamental understanding of the underlying chemistry and physics. The “Chimie: Science Magique” program was designed at the Université Montpellier 2 (Montpellier, France) for this purpose. The goal for the program is to develop high-impact laboratory experiments by synthesizing appealing materials, while involving undergraduate students in the research process. The “Chimie: Science Magique” program is divided into four parts: in the first part, students learn the basics about laboratory safety and information gathering; in the second part, the bases of synthesis and analysis methods are learned using amazing materials; in the third part (called the “Genius” session), students are challenged under instructor supervision to improve a previous synthesis or to synthesize a new material; and in the last part, students investigate the theoretical tools to explain the observed phenomena, and they present their results to an examination board. Chromium-doped alumina powders present the microscopic structure, color, and appealing red fluorescence of natural ruby jewels.1 Rubies have been synthesized for more than one century using the process of Verneuil.1b,2 In this commonly used synthesis, stoichiometric oxides or carbonates of the constituting metals are heated at a temperature over 2000 °C. The experiment described here is easier and faster because it combines microwave and self-propagating combustion similar to the synthesis of the long lasting phosphor SrAl2O4:Eu:Dy.3 © XXXX American Chemical Society and Division of Chemical Education, Inc.

EXPERIMENTAL OVERVIEW

The goals of this laboratory experiment are for the student to learn how to synthesize, purify, and characterize chromiumdoped corundum powders of various Cr contents by the combustion method; to investigate some fundamental aspects of Cr doping through color change, fluorescence quenching, or structural modification; and finally to apply this information in a new fashion. The characterization allows students to understand why Cr3+ ions can give both the red color of ruby and the green color of emerald by only changing the oxide matrix. The synthesis of ruby powder (Al2O4:Cr) has been performed in the course “Chimie Science Magique: Synthèse de Matériaux à Propriétés Remarquables (GLCHO03)” by about 180 first- and second-year undergraduate students since 2009. In a typical three-hour session, the approximately 15 students are teamed by groups of two or three. Students are invited to collaborate and to identify an optimal organization in particular for repetitive tasks. Each group investigates at least three different chromium contents and compares their results with the other groups. Students meet for 5 three-hour sessions. The final three-hour “Genius” session (6th lab session) is dedicated to applying the skills they have learned, for example, to synthetize a “new” material by changing the dopant to change the color or finding a “new” synthesis to grow larger rubies. About one month later, the students make a 10−15 min oral presentation mainly focused on the results of their “Genius” session, followed by 5−10 min questioning. Students enjoy the chemistry of making fluorescent rubies and then become more responsive to theoretical elements, such as crystal field theory.

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dx.doi.org/10.1021/ed3008389 | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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Laboratory Experiment

EXPERIMENTAL DETAILS Chromium-doped corundums are prepared by mixing different ratios of aluminum nitrate, Al(NO3)3·9H2O, with chromium nitrate, Cr(NO3)3·9H2O, in a beaker and dissolving it completely in a minimum volume of distilled water (see Supporting Information for details). The combustion reaction is sensitive to the area/volume ratio of the container so the beaker volume may have to be optimized depending on the microwave cavity geometry. Nitrates can be mixed without danger, but no urea should be added until all of the nitrates are completely dissolved. Then, urea is added to the solution of nitrates and stirred until the urea is completely dissolved. The resulting solution is covered with a thick watch glass. As a large quantity of heat is produced in a few seconds, a protective refractory layer should be placed between the bottom of the beaker and the floor of the microwave oven to protect it as shown in Figure 1. The refractory stone should not

built-in safety features, such as an antiexplosion door, temperature control, and switches preventing microwave emission with an open door. As some nitrogen oxides and organic byproducts are produced, the microwave oven should have a working fume extraction system and should be cleaned after each use. UV light irradiation is potentially harmful to retina and skin, so skin should be covered and anti-UV coated goggles should be worn for added safety. Students should always wear appropriate gloves and clothing and safety goggles and limit both alumina and nitrate powder exposure.

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RESULTS AND DISCUSSION

The Microwave Self-Propagating Combustion Method

In the microwave oven, once the water was boiled off, the combustion reaction between the nitrates and urea ignited and reached an extremely high local temperature necessary to get reproducible crystalline materials with excellent properties.3 The classical equation of a self-propagating combustion reaction between nitrates and urea, where ε is the fraction of chromium, in a high temperature oven4 is (10 − ε)Al(NO3)3(s) + εCr(NO3)3(s) + 25(NH 2)2 CO(s) → 40N2(g) + 25CO2(g) + 50H 2O(g) + 5Al 2(1 − ε)O3:Cr2ε(s)

(1a)

In the experiment carried out in a microwave NO(g) formation was observed, and, therefore, the equation for this reaction was assumed to be (10 − ε)Al(NO3)3(s) + εCr(NO3)3(s) + 9(NH 2)2 CO(s) → 48NO(g) + 9CO2(g) + 18H 2O(g) +

Figure 1. Schematic outline and photo of the experimental setup.

5Al 2(1 − ε)O3:Cr2ε(s)

absorb microwave radiation (alumina or silica based refractory). To avoid splattering, a large upturned, clean, and dry beaker (greater than 1 L) can also be added. The solution is put in a microwave oven at 1000 W power for 4 to 5 min until all of the water is evaporated and the combustion reaction ignites. When the combustion is finished, the microwave oven is shut down, the door is opened, the hot watch glass and protecting beaker are removed using hightemperature gloves, and the beaker, which is very hot, is allowed to cool to room temperature. The solid material is crushed and washed with both distilled water and ethanol in a Büchner funnel under vacuum. On average, the whole procedure is completed in less than 30 min. Then, under the instructor’s supervision, students prepare samples for X-ray diffraction from their dry products, record UV−vis spectra using an integration sphere, and test their powder under UV light to investigate structure, color, and fluorescence of the ruby powders.

(1b)

However, both equations may be possible, simultaneously. Sufficient energy was released by the reaction to be selfpropagating and sustained for a few seconds until all of the reagents were consumed and the desired product was formed. The resulting powders obtained by students are shown in Figure 2. Students modified the experimental conditions by increasing the fraction of chromium nitrate or decreasing the weight of urea. Pure alumina was white (Figure 2A). A pink powder was obtained with 0.8% Cr (Figure 2B). An increasing chromium content led first, at 4% Cr, to a darker pink color

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HAZARDS The toxicities of Cr(III) or Al(III) nitrates and urea are moderate (see the Supporting Information for details). As the nitrate compounds are strong oxidizers, they should never be mixed in the solid state with a reducing agent, such as urea, because of the risk of fire or explosion. Urea should always be added only after full dissolution of the solid nitrates in water. Students should use a laboratory microwave oven that includes

Figure 2. Colors of the solids: (A) pure alumina Al2O3, (B−F) increasing Cr content in Al2−2εCr2εO3 for a chromium fraction, ε, of (B) 0.8%, (C) 4%, (D) 25%, (E) 30%, (F) 50%, (G) low urea synthesis, and (H) 0.8% Cr content under UV light. B




(Figure 2C), at 25%, to gray (Figure 2D), and then, at 30%, to a green color that turned darker and darker at 50% (Figure 2E and 2F). Decreasing the weight of urea below 1.0 g led to the formation of a yellow powder (Figure 2G) that certainly corresponded to polyaluminum, which has the same color. Structure of the Al2O3:Cr

The X-ray diffraction (XRD) pattern of the pink ruby powder (0.8%) (Figure 3) was compared with the XRD database and

Figure 4. UV−visible spectra of 0.8% (red solid line), 30% (dashed light green line), and 50% (dashed dotted green line) Cr-doped Al2O3. The electronic transitions are given. A color scale of the visible light was added.

contents. The fluorescence spectrum of ruby was previously investigated8 and showed a strong peak at 694 nm associated with the red emission. Genuis Session

The synthesis was modified by students in the “Genius” sessions to investigate other corundums, such as rare-earth metal-doped sapphires that present an interesting fluorescence linked with f−f transitions or blue sapphires where color is due to charge transfer between Ti and Fe dopants. Other syntheses were also investigated to grow millimeter-sized ruby crystals.

Figure 3. Powder XRD pattern of a 0.8% Cr-doped α-Al2O3 corundum structure using Cu Kα; only main reflections are indexed (JCPDS 46-1212). Inset: Corundum structure; oxygen atoms are shown as red balls.

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was found to be similar to pure α-alumina (JCPDS 46-1212). α-Alumina has a corundum crystalline structure associated with a trigonal unit cell with Al atoms occupying the center of oxygen octahedra. Cr3+-dopants substitute for Al3+ ions.5 As shown in the Supporting Information, the increase of Cr content shifted the reflections to lower angles, coherently with the larger ionic radius of Cr3+ (0.755 Å) versus Al3+ (0.675 Å). A scanning electron microscopy (SEM) image of the powder (0.8%) is shown in the Supporting Information and presented a specific morphology associated with the microwave process. The yellow powders obtained for low urea did not show an XRD pattern, suggesting a noncrystalline product consistent with amorphous polyaluminum.

ACHIEVEMENT OF THE GOALS This teaching approach with undergraduate students proved to be positive for both laboratory periods and the final oral presentations. As ruby synthesis was performed in less than 30 min, the XRD sample preparation in 5 min, and the UV−vis spectra in 20−25 min, many compositions were synthesized and analyzed during a typical three-hour lab period. This enabled a careful monitoring of the lab skills, such as weighing reactants, using the microwave oven, washing and filtering powders using a Büchner funnel, and preparing samples for UV−vis spectroscopy and X-ray diffraction. Furthermore, the following “Genius” sessions permitted the evaluation of a student’s ability to transpose the experimental knowledge gained from preparing and characterizing ruby powders to the synthesis of other oxide materials, such as sapphires, fluorescent materials, or long lasting phosphors. The ability to communicate, to understand experimental setups, to characterize products, and to analyze spectroscopic changes with increasing Cr content were evaluated during a final oral presentation and followed by questioning from an external board composed of high school and university teachers and scientists. The board assessment was that nearly all of the students achieved satisfactory grades and a majority of students obtained results beyond expectations by both producing high standard presentations and showing an important personal investment in the theoretical understanding of the materials.

Color of Ruby

Ruby powder showed two large intense peaks at 390 and 560 nm (Figure 4) and a small sharp peak at 694 nm. Only the longer wavelengths were not adsorbed, coherently with the observed pink color. The two main peaks were shifted to higher wavelength by increasing Cr content. This color change was due to the perturbation of the electronic structure of the chromium atoms by the surrounding crystal field.6 The full analysis of the spectra and the color change from pink to green with the increase of Cr content is given in the instructor notes in the Supporting Information. Fluorescence

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Another important property of low Cr content rubies is their fluorescence,7 which was used to build one of the first working lasers. Cr ruby powder (0.8%) exhibited a strong red fluorescence (Figure 2H) under UV irradiation (standard, λ = 365 nm, power is 8 W) that was quenched at higher Cr

CONCLUSION This laboratory experiment was a direct consequence of developing new materials using the “Chimie Magique” C




Haase, M.; Jho, Y. D. Electron paramagnetic resonance and photoluminescence properties of alpha-Al2O3:Cr3+ phosphors. Appl. Phys. B: Lasers Opt. 2012, 107, 489−495. (9) Prezzi, D.; Eberlein, T. A. G.; Filhol, J. S.; Jones, R.; Shaw, M. J.; Briddon, P. R.; Oberg, S. Optical and electrical properties of vanadium and erbium in 4H-SiC. Phys. Rev. B 2004, 69, 193202-1−193202-4.

approach. The synthesis of ruby powders engaged undergraduate students to learn the synthesis of doped matrices and to understand the origin of color and fluorescence in materials.9 It also was used to investigate complex effects, such as the crystal field and color modification associated with different dopant concentrations. The experiments can be readily adapted from undergraduate to graduate student materials laboratory courses. They help students at all levels to gain interest in solidstate chemistry and to test theories on practical materials.

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ASSOCIATED CONTENT

* Supporting Information S

Undergraduate and advanced laboratory student handouts and instructor notes. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*J.-S.F.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This solid-state synthesis was optimized for the “Chimie Science Magique” program (www.chimiemagique.fr) and sponsored by the Université Montpellier 2, CNRS and Institut Charles Gerhardt. The authors also thank Odile Eisenstein for the useful discussion.

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REFERENCES

(1) (a) Nikolskaya, L. V.; Terekhova, V. M.; Samoilovich, M. I. Origin of Natural Sapphire Color. Phys. Chem. Miner. 1978, 3, 213− 224. (b) Wade, F. B. Man-made gems. J. Chem. Educ. 1931, 8, 1015− 1026. (c) Emmett, J. L.; Scarratt, K.; McClure, S. F.; Moses, T.; Douthit, T. R.; Hughes, R.; Novak, S.; Shigley, J. E.; Wang, W. Y.; Bordelon, O.; Kane, R. E. Beryllium diffusion of ruby and sapphire. Gems Gemol. 2003, 39, 84−135. (2) (a) Alexander, A. E. Genuine and synthetic rubies and sapphires. J. Chem. Educ. 1946, 23, 418−422. (b) Wade, F. B. The cause of color in precious stones. J. Chem. Educ. 1944, 21, 133−134. (3) Filhol, J. S.; Zitoun, D.; Bernaud, L.; Manteghetti, A. Microwave Synthesis of a Long-Lasting Phosphor. J. Chem. Educ. 2009, 86, 72−75. (4) Patil, K. C.; Hegde, M. S.; Rattan, T.; Aruna, S. T. In Chemistry of Nanocrystalline Oxide Materials - Combustion Synthesis, Properties and Applications; World Scientific Publishing Company Incorporated: Hackensack, NJ, 2008; ISBN 978-981-279-314-0. (5) Feret, F. R.; Roy, D.; Boulanger, C. Determination of alpha and beta alumina in ceramic alumina by X-ray diffraction. Spectrochim. Acta, Part B 2000, 55, 1051−1061. (6) (a) Gaudry, É.; Sainctavit, P.; Juillot, F.; Bondioli, F.; Ohresser, P.; Letard, I. From the green color of eskolaite to the red color of ruby: an X-ray absorption spectroscopy study. Phys. Chem. Miner. 2006, 32, 710−720. (b) Reinen, D. Ligand-field spectroscopy and chemical bonding in Cr3+-containing oxidic solids. In Structure and Bonding; Jørgensen, C. K., Neilands, J. B., Nyholm, R., Reinen, D., Williams, R. J. P., Eds.; Springer: Berlin Heidelberg: 1969; Vol. 6, pp 30−51. (7) (a) Ishizaka, T.; Kurokawa, Y. Optical properties of rare-earth ion (Gd3+, Ho3+, Pr3+, Sm3+, Dy3+ and Tm3+)-doped alumina films prepared by the sol-gel method. J. Lumin. 2000, 92, 57−63. (b) Ishizaka, T.; Kurokawa, Y.; Makino, T.; Segawa, Y. Optical properties of rare earth ion (Nd3+, Er3+ and Tb3+)-doped alumina films prepared by the sol-gel method. Opt. Mater. 2001, 15, 293−299. (8) (a) Esposti, C. D.; Bizzocchi, L. Absorption and Emission Spectroscopy of a Lasing Material: Ruby. J. Chem. Educ. 2007, 84, 1316−1318. (b) Singh, V.; Chakradhar, R.; Rao, J.; Al-Shamery, K.; D


Microwave Synthesis of a Fluorescent Ruby Powder - Journal of

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