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The Quartz-Crystal Microbalance in an Undergraduate Laboratory Experiment I. Fundamentals and Instrumentation Vladimir Tsionsky School of Chemistry, Tel-Aviv University, Ramat-Aviv 69978, Israel;
[email protected] The physical basis of operation of the quartz-crystal microbalance (QCM) originates in the inverse piezoelectric effect, in which the application of an electric field across a piezoelectric material induces a deformation of the material. The QCM technique has been discussed in several reviews (1–4). Usually, the QCM is in the form of a thin (a few hundred µm) quartz plate, covered on both sides by metallic electrodes, usually gold, of a thickness of about 0.1 µm. The QCM has a resonance frequency, f0, determined by the thickness of the quartz plate. For available QCM modules, f0 varies between 5–10 MHz. With modern devices, this value can be determined with an accuracy of 0.05 Hz or better. This accuracy makes the QCM highly sensitive to any mass rigidly attached to its electrode surface, to the viscosity of the fluid in which it is immersed, and to the roughness of the electrode surface in contact with a viscous medium. Its high sensitivity, especially to mass changes (4 to 20 × 10᎑9 g兾Hz), makes it popular in different branches of both fundamental and applied sciences: studies of sub-monolayer adsorption; as a thickness gauge in thin-layer technology (5); as a sensor, particularly in biochemistry and biotechnology (6); in drug delivery and drug research (7, 8); in situ monitoring of lubricant and petroleum properties (9); and so forth. The number of publications involving the QCM grows every year and exceeded 600 in 2005. We feel that it is time to introduce this technique into student laboratories. We describe two experiments: • the QCM as a viscosimeter and • the QCM as a mass sensor during the electrochemical deposition of metals.
The pedagogical objectives of these experiments consist of acquainting the student with the QCM technique and the study of two different branches of physical chemistry: the influence of molecular interactions on solvent properties and the basic principles of electrochemistry. Background The conductance (admittance) of the QCM to alternating current can be represented by a complex number. A series of typical spectra of the real part of the admittance are presented in Figure 1. In Figure 1A we show a simple case of metal deposition. The QCM acts as a true microbalance in this case. The shape of the admittance spectrum remains unaltered, but the resonance frequency is shifted to lower values as the load increases. This shift, ∆fm, follows the simple equation derived by Sauerbrey (10) ∆fm = −Cm0 f0 2 ∆m 1334
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where the product Cm0f02 is a constant, representing the mass sensitivity, which is related to known properties of quartz and the dimensions of the crystal, and ∆m is the added mass, in units of mass兾unit area. Cm0 is a function of the properties of quartz and has the value of 2.257 × 10᎑7 m2 兾(kg Hz). The effect of viscosity on the admittance spectrum is shown in Figure 1B. Here, the resonance frequency is shifted to lower values as the viscosity increases, but the change in frequency is not caused by mass loading. The shape of the spectrum is quite different; it broadens dramatically with increasing viscosity and density of the liquid. Another aspect of the admittance spectrum is shown in Figure 1C. Here the same metal deposition was conducted as shown in Figure 1A, but the conditions were chosen to produce a very rough surface (by plating at a current density close to the mass-transport-limited value). As the roughness increases, so does the width of the resonance peak. At the same time, the peak frequency shifts to lower values. Consider an ideally flat QCM initially in vacuum, then having one of its surfaces immersed in a liquid. The fundamental frequency of this QCM should change by
∆ fη = −
Cm0 f0
32
2 π
ηρ
(2)
where η and ρ are the viscosity and density of the liquid, respectively. However, all real surfaces have some degree of roughness, hence a QCM shows more complex dependence on viscosity (4, 11). To use the QCM as a viscosimeter one must calibrate it with the use of liquids of known η and ρ. Instrumentation
Frequency Measurements For student experiments, we do not suggest the use of devices that measure the complete resonance curves as shown in Figure 1. A simpler and much less expensive way is to use devices that measure only the resonance frequency (the frequency corresponding to the maximum of the admittance curve). In our laboratory we use “AT-cut” type quartz crystals with a fundamental frequency of about 6 MHz and a Leybold Inficon deposition controller, type XTM兾2, to measure the resonance frequency. The controller is connected to a PC through an RS232 interface. Cell Construction The homemade cylindrical Teflon cell (Figure 2) is closed at the bottom by a quartz-crystal resonator, held between two Viton O-rings. The cell is designed in a way that prevents leakage and avoids overloading the quartz plate (causing
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In the Laboratory
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B
Figure 2. The cross section of the cell: QCM–quartz-crystal resonator disc, 1 and 2–parts of the Teflon cell, between which the QCM disc is held with two Viton O-rings; 3–cell cover; 4–Teflon tube used for gas injection in viscosity experiments; SE and WE–connections to the lower and upper electrodes of the QCM, respectively; RE– reference electrode inserted in a Teflon tube; CE–counter electrode. C
Figure 1. (A) The real part of the admittance vs frequency during deposition of gold on a gold-covered QCM at a current density of 20 µA/cm2. (C) The same at 500 µA/cm2. (B) The response of the QCM immersed in different media: 1–hydrogen, 1 atm; 2–dimethyl ether; 3–water; 4 and 5–40% and 50% aqueous solutions of sucrose, respectively. Line 1 and the inset in this figure show the response of the QCM in the gas phase, where the product of viscosity and density is about four orders of magnitude smaller than in any of the liquids. Correspondingly, the resonance curve is very sharp and the value of the admittance is very high. (Figure 1B is reprinted with permission from ref 11. Copyright 2002 American Chemical Society.)
stress) and minimizes breakage of the thin (0.2 mm) resonator plate. Between the quartz plate and the meniscus of the electrolyte, it is useful to place a disk, the diameter of which is slightly smaller than that of the cell (20 mm). This helps to prevent erratic response of the QCM, resulting from interaction of the vibrating QCM surface with the meniscus of the liquid (see ref 12 and Appendix 3 in ref 4 ). In the electrochemical part of the experiments, the platinum counter electrode can also serve as a screen between the QCM and the meniscus, as shown in Figure 2. The cell covers are difwww.JCE.DivCHED.org
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ferent for experiments on viscosity and on metal deposition: In the first case, only a Teflon tube for gas flow is used, while in the second case, the cell cover includes reference (RE) and counter (CE) electrodes. For these electrodes we use a copper rod and platinum plate, respectively. It has been our experience that, over the course of a semester, the total duration of the experiments with a given QCM resonator is about 50 hours and there is no need to renew it. Several types of commercially available cells for QCM techniques can also be used successfully. Conclusion We have described the fundamentals of the QCM techniques and instrumentation successfully used in our student laboratory of physical chemistry. We believe that this instrumentation can be applied to different systems, suited to particular teaching programs, and will be sufficiently simple to be carried out in an undergraduate laboratory. The next two articles will present two experiments: the QCM as a viscosimeter to measure the viscosity of liquids with H-bonds and the QCM as a mass sensor during the electrochemical deposition and dissolution of copper. These experiments introduce students to the QCM technique and, at the same time, provide an idea of its scope. The QCM response can reflect not only mass changes, as implied by its name, but also changes in other properties of the system studied, among them viscosity and roughness. Acknowledgments Financial support for this work by the Israel Science Foundation (grant 174兾05) is gratefully acknowledged. I
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thank my colleagues S. Cheskis, E. Gileadi, and M. Urbakh and J. Penciner for their support and help. W
Supplementary Material
Principles of operation of the QCM, software, description of equipment, and a student quiz with solutions are available in this issue of JCE Online. Literature Cited 1. Buttry, D. A. Application of the Quartz Crystal Microbalance to Electrochemistry. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker Inc.: New York, 1991; Vol. 17, Chapter 1. 2. Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355–1371. 3. Hillman, A. R. The Electrochemical Quartz Crystal Microbalance. In Encyclopedia of Electrochemistry; Bard, A. J., Ed.; Wiley-VCH Verlag GmbH&Co. KGaA: Weinheim, Germany, 2003; Chapter 2.7.
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4. Tsionsky, V.; Daikhin, L.; Urbakh, M.; Gileadi, E. Looking at the Metal/Solution Interface with the Electrochemical QuartzCrystal Microbalance. Theory and Experiment. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker, Inc.: New York, 2003; Vol. 22, Chapter 1. 5. Benes, E.; Groschl, M.; Burger, W.; Schmid, M. Sensors and Actuators A-Physical 1995, 48, 1–21. 6. Minunni, M.; Mascini, M.; Guilbault, G. G.; Hock, B. Analytical Letters 1995, 28, 749–764. 7. Marx, K. A. Biomacromolecules 2003, 4, 1099–1120. 8. Yu, D. H.; Blankert, B.; Vire, J. C.; Kauffmann, J. M. Analytical Letters 2005, 38, 1687–1702. 9. Ash, D. C.; Joyce, M. J.; Barnes, C.; Booth, C. J.; Jefferies A. C. Meas. Sci. Technol. 2003, 14, 1955–1962. 10. Sauerbrey, G. Z. Phys. 1959, 155, 206–222. 11. Daikhin, L.; Gileadi, E.; Katz, G.; Tsionsky, V.; Urbakh M.; Zagidulin, D. Anal. Chem. 2002, 74, 554–561. 12. Lin, Z.; Ward, M. D. Anal. Chem. 1995, 67, 685–692.
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