The electrochemical quartz crystal microbalance EQCM

Our approach to this problem involves a detailed mechanistic study of model systems, in order to identify the (electro)chemical parameters and the physicochemical processes of importance. This approach takes advantage of one of the major developments in electrochemical science over the last two decades, namely the simultaneous application of /ton-electrochemical techniques to study interfaces maintained under electrochemical control [3-5]. In general terms, spectroscopic methods have provided insight into the detailed structure at a variety of levels, from atomic to morphological, of surface-bound films. Other in situ methods, such as ellipsometry [6], neutron reflectivity [7] and the electrochemical quartz crystal microbalance (EQCM) [8-10], have provided insight into the overall penetration of mobile species (ions, solvent and other small molecules) into polymer films, along with spatial distributions of these mobile species and of the polymer itself. Of these techniques, the one upon which we rely directly here is the EQCM, whose operation and capability we now briefly review. 

The EQCM comprises a quartz crystal oscillator, in which one of the Au exciting electrodes is also exposed to the solution and acts as the working electrode in a conventional (here, three electrode) cell. Provided any surface film is rigidly coupled to the underlying electrode changes in inertial mass (Am) of the electrode result in crystal resonant frequency changes (A/) that are described by the Sauerbrey equation [11]  

In the event that the film is not rigid, the EQCM response is a function of both the film mass and its rheological characteristics. Application of the Sauerbrey equation under these circumstances is inappropriate it underestimates the mass change, to an extent that is dependent on the viscoelastic properties of the film. Under these circumstances, there are two questions to be addressed first, how does one diagnose film (non-)rigidity and, second, how does one interpret responses from a non-rigid film The answers to both questions can be found from crystal impedance measurements. This is a technique in which one determines the admittance (or impedance) of the loaded crystal as a function of frequency in the vicinity of resonance. Effectively, one determines the shape (width and height) and position (on the frequency axis) of the resonance, rather than just its position (as in the simple EQCM technique). 

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The electrochemical quartz crystal microbalance (EQCM) is a powerful tool for elucidating interfacial reactions based on the simultaneous measurement of electrochemical parameters and mass changes at electrode surfaces. The microbalance is based on a quartz crystal wafer, which is sandwiched between two electrodes, used to induce an electric field (Figure 2-20). The field produces a mechanical oscillation... 

The electrochemical quartz crystal microbalance (EQCM) is a very useful technique for detecting small mass changes at the electrode surface that accompany electrochemical processes. In 1880, Jacques and Pierre Curie discovered that when stress was applied to some crystals, such as quartz, it resulted in an electrical potential across the... 

The electrochemical quartz crystal microbalance (EQCM) method was first used to study underpotential deposition in 1988 for Pb, Bi, Cu,... 

A technique for such measurements is the electrochemical quartz crystal microbalance (EQCM figure 14) [71]. Here, the working electrode is part of a quartz crystal oscillator that is mounted on the wall of the electrochemical cell and exposed to the electrolyte. The resonance frequency / of the quartz crystal is proportional to mass changes Am A/ Am. With base frequencies around 10 MHz, the determination of Am in the ng range is possible. 

The Electrochemical Quartz Crystal Microbalance (EQCM) The resonant frequency of a quartz crystal oscillator is perturbed from its base value (f ) by attached overlayers. For thin, rigid films the measured change in resonant frequency (Af) with attached mass (AM) is described by the Sauerbrey equation (10) ... 

The strong dependence of the layer structure on the nature of the contacting electrolyte has been further investigated by using the electrochemical quartz crystal microbalance (EQCM). As discussed above in Chapter 3, this technique is based on the measurement of the frequency with which a coated quartz crystal vibrates, and this frequency can then be related to the mass of this crystal provided that the material attached to the surface is rigid. In this way, the changes that occur in thin films as a result of redox processes can be monitored. 

Kinetic Applications of the Electrochemical Quartz Crystal Microbalance (EQCM)... 

Since the early work of Kanazawa [1] and Bruckenstein in 1985 [2], quartz crystal resonators have been used for more than 12 years in contact with liquids to assess changes in mass during electrochemical surface processes. Extensive use of the electrochemical quartz crystal microbalance (EQCM) has been done in the study of electrode processes with change of mass simultaneous to charge transfer. 

The electrochemical quartz crystal microbalance (EQCM) has emerged as a very powerful in situ technique to complement electrochemical experiments [3-5]. Nomura and Okuhara [15] first used the quartz crystal microbalance (QCM) to detect mass changes at a metal coated quartz resonator immersed in electrolyte during electrochemical experiments. 

Buttry [47] measured the admittance around resonance of a quartz crystal coated with polynitrostyrene and related those measurements to the rheological changes due to film swelling. Muramatsu et al. used the resonant resistance in addition to the resonant frequency of the electrochemical quartz crystal microbalance (EQCM) as a criterion to evaluate the film non-rigidity for several electroactive polymer systems [6], including... 

PPy films modified by platinum catalyst particles were also considered for electrocatalytic reactions (oxygen reduction and methanol oxidation) by Hepel et al. [41], The incorporation of a PtCl anion was performed during the electropolymerization of pyrrole and monitored by the electrochemical quartz crystal microbalance (EQCM) technique, allowing us to evaluate the amount of platinum obtained after reduction of the PPy/PtCl film. 

A system developed recently sheds further light on these dynamic processes. The technique is the electrochemical quartz crystal microbalance (EQCM), wherein the polymer is deposited on a gold-coated quartz crystal. Changes in polymer mass, as the polymer is electrochemically reduced or oxidized, can then be monitored in situ,144 145 For example, as the polymer is reduced, anion removal is indicated by the change in mass observed, as shown in Figure 1.23b. This technique has proved particularly useful for the study of complex systems, e.g., those containing polyelectrolytes, wherein cation movement rather than anion predominates, and this is reflected in increases in mass at negative potentials. 

Second harmonic generation has also been used to study the semiconductor/ solution interface during the deposition of gold on Si(lll) [777]. In a setup combining SHG and the electrochemical quartz crystal microbalance (EQCM), the underpotential deposition of copper on a polycrystalline gold surface has been studied a decrease of the SHG signal by 60% upon formation of the upd-layer was found [778]. A study of the electrochemical liquid/liquid interface between two immiscible solutions where adsorption of surfactants occurred has been reported [779]. 

The application of combinations of electrochemical methods with non-electro-chemical techniques, especially spectroelectrochemistiy (UV-VIS, FITR, ESR), the electrochemical quartz crystal microbalance (EQCM), radiotracer methods, probe beam deflection (PBD), various microscopies (STM, AFM, SECM), ellipsometiy, and in situ conductivity measurements, has enhanced our understanding of the nature of charge transport and charge transfer processes, stmcture-property relationships, and the mechanisms of chemical transformations that occur during charg-ing/discharging processes. 

Calvo EJ, Etchenique RA (1999) Kinetic applications of the electrochemical quartz crystal microbalance (EQCM). In Compton RG, Hancock G (eds) Comprehensive chemical kinetics, vol. 37. Elsevier, Amsterdam, pp 461 87 Lucklum R, Hauptmann P (2000) Electrochim Acta 45 3907 Ortega JM (1998) Synth Met 97 81... 

The details of the deposition and dissolution processes of the DAB-dend-(NHCOFc) in tetra n-butyl ammonium perchorate (TBAP)/CH2Cl2 solution were investigated using the electrochemical quartz crystal microbalance (EQCM) technique as well as admittance measurement of the quartz crystal resonator by Takada etal. [77]. It was found that the oxidized form of the dendrimers deposited onto the Pt electrode likely due to the low solubility of the salt composed of the oxidized dendrimer (ferricenium form) and C104 anions. On the other hand, the reduced form of the dendrimers easily redissolved except for the first monolayer, which appeared to be strongly adsorbed. Further, the mass-transfer process, during the redox reaction of the adsorbed dendrimers in an AN solution, was found to be of the anion exchange type. The resistance measurements of the quartz crystal resonator based on the admittance also supported the results obtained by EQCM. 

The active element of the electrochemical quartz crystal microbalance (EQCM) was a gold-coated quartz crystal whose oscillation frequency was equal to 5 or 6 MHz. It was plated in a similar way, but the metal coatings were thinner (0.2-0.4pm). The constant relating the variations in quartz crystal mass with its oscillation frequency was determined by special calibration using EQCM data obtained in acid CuSO solutions at a controlled current density. 

Complex transmittances relevant to corrosion phenomena have been introduced [36,38,39]. Examples are given in several parts of the chapter. Two of them deserve a particular interest. The case of techniques pertaining to the rotating ring disk electrode (RRDE) is dealt with here. Electrogravimetric transmittance, a frequency-resolved technique based on the electrochemical quartz crystal microbalance (EQCM), was presented in a paper in 1996 [55]. 

All of these redox reactions are accompanied by ion movement into or out of the CEP membrane. These processes were studied using the electrochemical quartz crystal microbalance (EQCM) technique [132]. EQCM allows small changes in mass of the polymers to be monitored in situ as electrical stimuli are applied. Information... 

In the electrochemical quartz crystal microbalance (EQCM), the QCM surface in contact with the external medium is simultaneously used as a working electrode in an electrochemical circuit (see Fig. 14.5). This enables frequency changes and... 

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