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Listening to Nanomaterials: Photoacoustic Spectroscopy Narayanan Kuthirummal Department of Physics and Astronomy, College of Charleston, Charleston, SC 29424;
[email protected] There is always a strong demand for less expensive and more efficient spectroscopic tools for measuring the spectra of materials in a chemistry or physics laboratory. In a typical undergraduate chemistry laboratory, for example, most students are exposed to the conventional absorption and fluorescence spectroscopicbased experiments. Although these spectroscopic techniques provide a great deal of information, they become less efficient when it comes to weakly absorbing samples and highly lightscattering materials such as powders, amorphous solids, gels, and suspensions. Another major challenge faced by conventional spectroscopy is the study of optically opaque samples. A versatile nondestructive spectroscopic tool, photoacoustic spectroscopy (PAS), offers an easy way to obtain the optical absorption spectra of optically opaque samples. This has a great value, especially in nanomaterial characterization, because most of these materials are not only opaque but also scatter light significantly. Basics The photoacoustic (PA) effect was originally discovered by Alexander Graham Bell in 1880 (1). However, the real development came in the 1970s (2, 3). The principles and applications of PAS are well documented in the literature (4–7). Briefly, the sample (e.g., solid powder) is placed in an airtight chamber filled with a nonabsorbing gas such as air. Upon illuminating the sample with monochromatic light, the electrons in the various energy states are excited. Nonradiative decay to the ground state leads to heat generation. In a solid the heat energy appears as the vibrational motion of atoms. Generally, the incident light is intensity-modulated using a standard mechanical chopper. The periodic heat energy, generated as a result of intensitymodulated light, flows from the sample to the surrounding gas and leads to pressure oscillations in the gas. The pressure oscillations are sensed by a microphone, processed, and plotted as a function of wavelength. The strength of the acoustic signal is proportional to the quantity of light absorbed by the sample and there is a close correspondence between the PA spectrum and the conventional optical absorption spectrum. This technique can respond to temperature rises as small as 10‒6 to 10‒5 °C in a sample (4). The main advantage of PAS is that it is immune to scattered, reflected, or transmitted light. The PA signal is also directly related to the incident radiation power. Therefore, the sensitivity can be improved significantly by using a laser light source. Since the sample acts as an electromagnetic radiation detector, a single microphone can detect the PA signals throughout the entire electromagnetic spectral region: there is no need to use different types of expensive light detectors in the UV or IR regions. The Rosencwaig–Gersho (RG) theory (3) provides a relatively comprehensive treatment of PA signal generation in solids. According to RG theory, the main source of the acoustic wave is the repetitive heat flow from the absorbing condensed-phase sample to the surrounding gas. The thickness of the boundary layer of the gas heated as a result of periodic heating is deter-
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mined by the thermal diffusion length, μg,
μg =
2α ω
where α is the thermal diffusivity and ω is the angular frequency at which the light is intensity modulated. For air at room temperature and pressure, this boundary layer is about 1 mm thick when ω = 1000 rad/s. This layer essentially acts as an acoustic piston generating periodic pressure fluctuations inside the closed cell. A detailed theoretical treatment can be found in refs 3 and 4. Photoacoustic Spectroscopy in the Undergraduate Laboratory Although PAS can be introduced in the general undergraduate chemistry curriculum, it would be more appropriate for students in the upper-level courses with some physical chemistry background. Chemical concepts such as energy levels, singlet and triplet states, radiative and nonradiative processes, photochemistry, and dynamics can be studied using PAS. This technique is complementary to fluorescence spectroscopy and it can be typically applied to undergraduate and advanced chemistry laboratories including the traditional core areas of analytical chemistry, inorganic chemistry, organic chemistry, and physical chemistry. In a chemistry laboratory, students are generally asked to synthesize compounds and analyze their products. Students are also given unknown samples and then are allowed to use a variety of spectroscopic tools to identify the samples. In some cases, the products or samples will be opaque powders that are not straightforward to analyze using conventional spectrometers. In such situations, PAS will be helpful. Typically any sample in any form can be studied using PAS. Some of the interesting powder samples to investigate are rare earth holmium oxide (Ho2O3), neodymium oxide (Nd2O3), erbium oxide (Er2O3), and europium oxide (Eu2O3) powders. Rare earth-doped glasses and crystals can be also studied to understand the crystal field splitting of energy levels. Their spectra are well documented in the literature. Other samples include semiconductors with band gaps in the visible region (e.g., CdS), biomolecules, dye molecules, and polymers. In addition to regular spectroscopic analysis, PAS can also be used in microscopy. Here, the incident radiation is focused on a very small spot on the sample. By recording the PA signal at each spot, a complete microscopic image of the sample can be obtained. The details of PA microscopy can be found in this Journal in an article by Mei and Eyring (8). PAS can also be used for investigating the molecular photophysical properties, as well as thermodynamic or kinetic data, for discrete systems as described by Fletcher and Grabowski in their article in this Journal (9). The technique is also suitable for depth profiling applications in layered structures (4).
Journal of Chemical Education • Vol. 86 No. 10 October 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
Hazards
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Rare Earth Nanopowders The PA technique offers a unique method to study the spectra of powdered rare earth materials. Rare earth materials have interesting spectroscopic properties. Their spectra are atomic in nature due to the shielding of the electrons in the unfilled f orbitals by a closed electronic 5s25p6 shell (10). As an example, the PA spectrum of Eu2O3 nanopowder (~60 nm diameter, 99.99% pure, American Elements) is shown in Figure 1. Well-defined peaks corresponding to transitions from the ground state (7F0) to various excited states are obtained, and the observed peaks are assigned based on the data available in the literature (11–13). Although some transitions such as 5D1 ← 7F0 and 5D3 ← 7F0 are not allowed, the proximity of 7F1 level to the 7F level causes mixing of levels making these transitions allowed 0 (11). This is an excellent experiment to introduce students to rare earth spectroscopy. High-resolution spectra using a laser source can provide detailed information about the crystal field Stark splitting. Semiconducting Nanowires Cadmium sulfide (CdS) is a direct band gap semiconductor. The PA spectra of bulk and as-prepared CdS nanowires are shown in Figure 2. The band gap of bulk CdS powder occurs at 2.39 ± 0.04 eV, which agrees with the literature value of 2.42 eV (14). The absorption edge of CdS nanowires is much steeper and occurs at a slightly larger value of 2.49 ± 0.04 eV. These data show that there is no significant contribution from quantum-confinement effects because the average diameter of the nanowires was about 50 nm, which is much larger than the calculated Bohr radius of 2.8 nm (15). The increased steepness might be attributed to the relatively well-ordered structure and size distribution. Quantum Rods As discussed previously, when the chopping frequency changes, the length through which the thermal energy penetrates into the sample changes. The length within the sample that the entire heat energy is dissipated is known as the thermal diffusion length (μs). The sample is thermally thin if l μs (4). Upon changing the chopping frequency, there will be a crossover from the thermally thick to the thermally thin case. The frequency at which such a crossover occurs is known as the characteristic frequency, fc. The thermal diffusivity, α, can be calculated using the relationship
α = fc l 2
As an example, the PA studies on the CdSe quantum rods, carried out by El-Brolossy et al. (16), are shown in Figure 3. ElBrolossy and co-workers prepared five sets of CdSe quantum rods with average lengths of about 37 nm and diameters of
Eu3∙ ground state: 7F0
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Wavelength / nm Figure 1. PA absorption spectrum of Eu2O3 nanopowders.
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Results and Discussion
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nano CdS
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Energy / eV Figure 2. PA spectra of bulk and CdS nanowires.
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UV light is harmful to eyes and skin. Europium(III) oxide and cadmium sulfide can be irritating to the eyes, skin, and respiratory system. Cadmium sulfide is a suspected human carcinogen.
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Figure 3. Variation of PA signal amplitude with inverse chopping frequency for CdSe nanorods. (Reproduced with permission from ref 16.)
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 10 October 2009 • Journal of Chemical Education
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In the Laboratory
about 5.8 nm. The PA studies were conducted by compressing the powder nanorods into a disc of 1 mm thickness. The signal shows a distinct change in the slope where the crossover takes place and is labeled as fc in Figure 3. The calculated value of thermal diffusivity of CdSe quantum rod is 0.49 cm2/s. Once the thermal diffusivity is known, the thermal conductivity, κ, can also be calculated κ = αρCp
where ρ is the density and Cp is the specific heat capacity of the material (17). Summary The potential of PAS in studying powdered nanomaterials has been described. Since PAS is easy to set up and does not require any sample preparation, it is an ideal technique in a general undergraduate lab or undergraduate (graduate) research lab to study the spectral and thermal properties of as-received samples. A simple photoacoustic spectrometer using a visible light source (tungsten–halogen lamp) may be set up for $25,000. Acknowledgment This work was supported by the Nanotechnology Undergraduate Education (NUE) program of the National Science Foundation (Award No.EEC-0634142). Thanks are also due to Apparao Rao and Jason Reppert at Clemson University for providing the CdS nanowires. Literature Cited 1. Bell, A. G. Am. J. Sci. 1880, 20, 305–324.
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2. Kreuzer, L. B. J. Appl. Phys. 1971, 42, 2934–2943. 3. Rosencwaig, A.; Gersho, A. J. Appl. Phys. 1976, 47, 64–69. 4. Rosencwaig, A. Photoacoustics and Photoacoustic Spectroscopy; Wiley: New York, 1980. 5. Rosencwaig, A. Ann. Rev. Biophys. Bioeng. 1980, 9, 31–54. 6. Schmid, T. Anl. Bioanl. Chem. 2006, 384, 1071–1086. 7. Ball, D. W. Spectroscopy 2006, 21, 14–16. 8. Mei, E. H.; Eyring, E. M. J. Chem. Educ. 1981, 58, 812–813. 9. Fletcher, B.; Grabowski, J. J. Chem. Educ. 2000, 77, 640–645. 10. Dieke, D. H. Spectra and Energy Levels of Rare Earth Ions in Crystals; Wiley: New York, 1968. 11. Mahato, K. K.; Rai, S. B.; Rai, A. Spectrochim. Acta A 2004, 60, 979–985. 12. Wakefield, G.; Keron, H. A.; Dobson, P. J.; Hutchison, J. L. J. Col. Int. Sci. 1999, 215, 179–182. 13. Lu, H.; Ballato, J. J. Am. Ceram. Soc. 2006, 89, 3573–3576. 14. Rosencwaig, A. Phy. Today 1975, 28, 23–30. 15. Ghosh, P. K.; Maiti, U. N.; Chattopadhyay, K. K. Mat. Lett. 2006, 60, 2881–2885. 16. El-Brolossy, T. A.; Abadalla, S.; Abadalla, T.; Awad, H.; Mohamed, M. B.; Negm, S.; Talaat, H. Eur. Phys. J. Special Topics 2008, 153, 369–372. 17. Raji, P.; Sanjeeviraja, C.; Ramachandran, K. Cryst. Res. Technol. 2004, 39, 617–622.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Oct/abs1238.html Abstract and keywords Full text (PDF) with links to cited JCE articles Supplement Details about the experimental setup
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