In the Laboratory
The Radiative Decay of Green and Red Photoluminescent Phosphors: An Undergraduate Kinetics Experiment for Materials Chemistry C. Degli Esposti* and L. Bizzocchi Dipartimento di Chimica “G. Ciamician”, Università di Bologna, 40126 Bologna, Italy; *
[email protected] Kinetic studies of radiative relaxation processes are not frequently included in the undergraduate laboratory of chemical kinetics courses because they generally require specialized instruments, including pulsed lasers and fast acquisition systems (1–4), which are not normally available in student laboratories. General-purpose spectrofluorometers equipped with flashlamps can be also used to perform such studies, provided that the photoluminescent system has a sufficiently long decay time (5–7). Rare-earth compounds are often characterized by fluorescence emissions with long decay times (8), which makes them a suitable choice to perform simple kinetic studies of radiative relaxation processes using non-specialized instrumentation. Commercial fluorescent lamps provide a source of rare-earth photoluminescent materials as they are internally coated with a blend of Eu3+- and Tb3+-activated phosphors, whose red and green emissions (mainly excited by the 254 nm line of mercury), are combined with the blue and green lines of mercury to produce the white light used for general-purpose illumination (9, 10). An article dealing with synthesis and characterization of the red emitting Y2O3:Eu3+ phosphor was published in this Journal (11), but no information was provided on its decay behavior. This article describes a laboratory experiment consisting of the study of the radiative relaxation processes of the green and red phosphors of commercial fluorescent lamps and the determination of the lifetime of the respective emitting states. This experiment has been included in the laboratory related to the chemical kinetics course for the materials chemistry degree at this university. Students learn (i) how to use well-known kinetic equations to describe the time evolution of excited states, in which the typical behavior of an intermediate can be also recognized, (ii) how to perform measurements on a solid-state sample using the different operating modes of a spectrofluorometer, including the time-resolution capability, and (iii) which are the main emission properties of the photoluminescent materials most widely used for artificial lighting. Kinetic Model for the Radiative Decay of a Photoluminescent Phosphor The simplest way to model the decay process occurring upon UV excitation of a phosphor, Phos, is to assume the following first-order relaxation steps, Phos* * Phos*
Phos*
non-radiative
Phos h O
radiative
v1 k1 Phos* * (1) v 2 k2 Phos*
(2)
where Phos** indicates the species directly produced by photoexcitation and Phos* is the light-emitting excited species. The time-dependence of the concentration of the intermediate Phos* during the decay process can be expressed as (12),
Phos*
k1 k2 k1
Phos* * e k1 t 0
Phos*
0
k1 k2 k1
(3) Phos* *
0
e
k2 t
where [Phos**]0 and [Phos*]0 are the concentrations of the two electronically excited species at the beginning of the relaxation process. The number of photons produced per unit time and unit volume is proportional to the concentration of Phos*, so that the intensity of the visible radiation emitted by the phosphor during the decay process changes with time in the following way:
I u k2 Phos*
A1 e k1 t A2 e k2 t
(4)
If k1 >> k2 , as often happens, then eq 4 reduces to a monoexponential decay of the form:
I A2 e k2 t
(5)
The determination of k2 allows one to calculate the lifetime of the emitting state as τ2 = 1∙k2. Actually this simple excitation–relaxation scheme is not exactly fulfilled by the green phosphors employed in fluorescent lamps (Tb3+-activated CeMgAl11O19 or Tb3+/Ce3+-activated LaPO4), because the light emitting species Tb 3+ is excited through an energy-transfer mechanism in which excited Ce3+ ions (produced by absorption at 254 nm) transfer their energy to Tb3+ ions, which finally decay radiatively following the twosteps mechanism mentioned above (9). The energy transfer between excited Ce3+ ions and Tb3+ ions is very fast (ns range; ref 13), so that it can not produce observable effects in the timescale employed in this experiment. Experimental Details Students investigate a sample of photoluminescent powder removed from a compact fluorescent lamp (details in the online supplement). The emission spectra are recorded using a Cary Eclipse spectrofluorometer (Varian). This spectrometer is equipped with solid-state accessories, and it can measure lifetimes in the millisecond range, having as source a Xe flashlamp with a pulse width at half peak height of ca. 2 μs. All the spectra presented in this article have been collected by a single group of students (three persons) employing ca. 2 hours of laboratory time. They represent standard results obtainable by the students following the laboratory instructions (see the online supplement).
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In the Laboratory
Hazards A sample of photoluminescent powder is provided to the students at the beginning of the experiment. Commercial luminescent phosphors are not hazardous materials, but for this experiment they are extracted from a compact fluorescent lamp, that also contains few milligrams of mercury. For this reason students prepare the sample suitable for the emission measurements wearing gloves and working in a fume hood. Results Emission Spectra Using the spectrofluorometer in the chemiluminescence mode, students first collect the emission spectrum of a compact fluorescent lamp that illuminates the sample compartment, and then they record the phosphorescence spectrum (150 μs delay) of a sample of the luminescent powder excited with a 263 nm pulse. The phosphorescence mode is necessary to suppress
900
Radiative Decays Having identified the exact position of the main emission peaks of each phosphor, students can record the respective
Hg
800
350
Hg
300
700
Emission Intensity
Emission Intensity
prompt fluorescences that are normally filtered by the inner coating of a lamp. Both spectra are shown in Figure 1, and their comparison allows the students to distinguish phosphor emissions from mercury lines. By comparing the spectrum of the luminescent powder with reference spectra of pure phosphors (9, 11, 14), students are able to identify the emission peaks due to the red, Eu3+-activated phosphor and to the green, Tb3+-activated phosphor. Before performing lifetime measurements, students optimize separately the emissions of the two phosphors to reduce interference effects due to overlap of the bands. Excitation at 240 nm makes the emission of the red Eu3+ phosphor dominant, characterized by the sharp peak at 611.5 nm, whereas 280 nm excitation favors the emission of the green Tb3+ phosphor, whose strongest band is at 541.8 nm. The optimized spectra are shown in Figure 2.
600
Hg
500 400
lamp
300 200
250 200 150 100 50
100
powder
0
0 350
400
450
500
550
600
650
700
0
750
3
6
Figure 1. The emission spectrum of a low-pressure mercury fluorescent lamp (upper trace) compared with the phosphorescence spectrum of the corresponding blend of phosphors (lower trace, excitation 263 nm, delay 150 μs).
700
300
280 nm excitation
200
240 nm excitation
100 0 550
600
650
Wavelength / nm Figure 2. Optimized emission spectra of the Eu3+-activated red phosphor (lower trace, excitation wavelength 240 nm) and Tb3+-activated green phosphor (upper trace, excitation wavelength 280 nm).
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Emission Intensity
Emission Intensity
400
500
15
18
500
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12
Figure 3. The radiative decay of the Tb3+-activated phosphor. Emission wavelength 541.8 nm, delay 10 μs, gate time 30 μs. The solid line corresponds to the fitted mono-exponential decay function.
Eu3á
Tb3á
600
9
Time / ms
Wavelength / nm
400 300 200 100 0 0
1
2
3
4
5
6
Time / ms Figure 4. The radiative decay of the Eu3+-activated phosphor. Emission wavelength 611.5 nm, delay 10 μs, gate time 10 μs. The solid line corresponds to the fitted bi-exponential decay function.
Journal of Chemical Education • Vol. 85 No. 6 June 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
decay curves, which are then analyzed through Origin graphics software (Microcal). A delay time of 10 μs is adopted to avoid significant perturbations from the excitation pulse. Figure 3 shows the decay curve of the 541.8 nm phosphorescence signal of Tb3+-activated phosphor, excited with 280 nm radiation. The initial rise of the signal is very small, because k1 >> k2 , so that a mono-exponential decay function corresponding to eq 5 can be fitted to the experimental points. The least-squares analysis of the decay curve shown in Figure 3 yields k2 = (3.209 ± 0.007) × 102 s‒1, corresponding to the lifetime τ2 = 3.12 ms for the 5D4 emitting state of Tb3+ (15). This value is close to that determined in Ce3+/Tb3+-coactivated LaPO4 nanowires (2.82 ms, from ref 13). The decay curve of the 611.5 nm phosphorescence signal of Eu3+-activated phosphor, excited with 240 nm radiation, is shown in Figure 4. In this case the decay is faster, and the corresponding curve exhibits a pronounced maximum, so that the bi-exponential decay function given by eq 4 must be used to accurately reproduce the experimental data. The least-squares analysis of the decay curve shown in Figure 4 yields k1 = (1.81 ± 0.03) × 104 s‒1 and k2 = (9.67 ± 0.02) × 102 s‒1, corresponding to the lifetime τ2 = 1.03 ms for the 5D0 emitting state of Eu3+ (16). This value is in excellent agreement with that reported in the literature for the Y2O3 : Eu3+ phosphor (1.04 ms, from ref 16). Conclusions This article describes a laboratory experiment that allows the students to apply fundamental kinetic equations to the study of the relaxation processes that involve the green (Tb3+-activated) and red (Eu3+-activated) photoluminescent phosphors from commercial fluorescent lamps. Students verify that a simple first-order kinetic model is sufficient to reproduce the radiative decay observed for the green phosphor, whereas a bi-exponential function must be used to fit the faster decay of the red phosphor. The latter result allows the students to understand that the relaxation process of a photoluminescent phosphor is the result of (at least) two consecutive steps, the first of which, non-radiative, is generally much faster than the subsequent radiative decay. The analysis of the decay curves allows the students to determine the lifetimes of the emitting states of Tb3+ and Eu3+, which are considerably different. The decay time is an important parameter for phosphors employed in luminescent displays because it determines the persistence of the moving images. This laboratory experiment has been designed mainly to provide new experimental activities for a chemical kinetics course because it allows the students to apply concepts and formulas that are generally explained to describe first-order consecutive reactions. In addition this experiment allows the students to acquire technical abilities in fluorescence measurements for solids and new knowledge in the important field of
photoluminescent materials. The phosphors of fluorescent lamps are common materials, but their properties are scarcely known by students, whose interest and curiosity are greatly stimulated by the applicative nature of this experiment. Acknowledgments The Faculty of Mathematical, Physical and Natural Sciences of the University of Bologna is gratefully acknowledged for the financial support provided to buy the spectrofluorometer used in this experiment. Literature Cited 1. Muenter, J. S.; Deutsch, J. L. J. Chem. Educ. 1996, 73, 580–585. 2. Van Dyke, D. A.; Pryor, B. A.; Smith, P. G.; Topp, M. R. J. Chem. Educ. 1998, 75, 615–620. 3. Masiello, T.; Vulpanovici, N.; Nibler, J. W. J. Chem. Educ. 2003, 80, 914–917. 4. Gutow, J. H. J. Chem. Educ. 2005, 82, 302–305. 5. Lisensky, G. C.; Patel, M. N.; Reich, M. L. J. Chem. Educ. 1996, 73, 1048–1052. 6. Roalstad, S.; Rue, C.; LeMaster, C. B.; Lasco, C. J. Chem. Educ. 1997, 74, 853–854. 7. Fister, J. C.; Harris, J. M.; Rank, D.; Wacholtz, W. J. Chem. Educ. 1997, 74, 1208–1212. 8. Xiao, M.; Selvin, P. R. Rev. Sci. Instrum. 1999, 70, 3877–3881. 9. Srivastava, A. M.; Ronda, C. R. Interface 2003, 12 (2), 48–51. 10. Spectrum of a fluorescent lamp with line identification. http:// en.wikipedia.org/wiki/Fluorescent_lamp#Phosphors_and_the_spectrum_of_emitted_light (accessed Jan 2008). 11. Bolstad, D. B.; Diaz, A. L. J. Chem. Educ. 2002, 79, 1101–1104. 12. Steinfeld, J. I.; Francisco, J. S.; Hase, W. L. Chemical Kinetics and Dynamics; Prentice-Hall: Englewood Cliffs, NJ, 1989. 13. Yu, L.; Song, H.; Liu, Z.; Yang, L.; Lu, S. J. Phys. Chem. B 2005, 109, 11450–11455. 14. Emission spectra of commercial phosphors. http://www.stanfordmaterials.com/ph.html (accessed Jan 2008). 15. Tonooka, K.; Nishimura, O. J. Lumin. 2000, 87–89, 679–681. 16. Serra, O. A.; Cicillini, S. A.; Ishiki, R. R. J. Alloys Comp. 2000, 303–304, 316–319
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