Potential Modulated Techniques

In the case of voltabsorptometry, it is possible to modulate the potential and measure reflectance variations. Indeed, reflectance is related to absorbance by 

In 2001, Nagatani et al. reported a method to analyze ion adsorption-transfer kinetics using PMF [140]. The results just discussed were confirmed, and it was further shown that the PMF response for kinetically controlled adsorption is expressed as a semicircle in the complex plane in which the characteristic frequency of maximum imaginary component is proportional to the adsorption and desorption rate constants. Considering that the potential dependence of adsorption exhibits the opposite sign whether the process takes place from the aqueous or organic phase, the corresponding PMF responses appear in different quadrants of the complex plane. This work therefore confirms that the adsorption at an ITIES can take place on either side of the interface. 

This method was then applied to study pyrene-sulfonate adsorption and dimerization at the Fl20-1,2-DCE interface [141]. More recently, Nagatani et al. extended their PMF spectroscopy investigation to the study of the adsorption and transfer of free-base, water-soluble porphyrins, namely, cationic meso-tetrakis (N-methylpyridyl) porphyrin (H2TMPyP +) and anionic meso-tetrakis(4-sulfonatophenyl)porphyrin (HTPPS ) [142]. The PMF response indicated the presence of an adsorption process for all systems, depending on the 

FIGURE 1.14 Typical PMF spectra for the ion transfer of (a) Ru(bpy)3 % (b) ZnTPPS, and (c) ZnTMPyP +. The solid and dashed lines denote the real and imaginary components. The concentration of dyes was 2.5-10- M, and the potential modulation was 10 mV at 6 Hz. (Reprinted from Nagatani, H., R. A. Iglesias, D. J. Fermin, P. F. Brevet, and H. H. Girault, 2000, J Phys Chem B, Vol. 104, p. 6869. Used with permission.) 

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An example of the application of potential modulation techniques is provided by reduction of Fe4S4(NO)4 between its dianion and trianion forms. At room temperature the dianion has limited stability in dichloromethane and experiments were conducted at lower temperatures in order to diminish the eifects of decomposition. Reversible conversion between the dianion and trianion forms is clearly evidenced by the spectra where the rates of reduction and reoxidation are similar and the waveform for the experiment was symmetric i.e. in Figure 1.12, = 0). 

Figure 1.11 Synchronisation between the collection of FTIR spectra and the potential applied to the working electrode during potential modulation techniques. It is assumed that single-sided interferograms are collected during the forward sweep of the moving mirror (ZPD = zero path difference for the two paths of the interferometer).

In addition, they employ a potential modulation technique (typically at... 

The refined method of analysis of co-dependent ER signal data set to obtain described in the last part of the previous section is in principle the combined use of ac impedance and the modulated spectroscopic signal. A similar calculation was reported by Yamada and Finklea and their colleagues [21, 71]. We describe below in detail the procedure for a concerted use of these two potential modulation techniques in the kinetic analysis. 

Potential modulation techniques are used frequently in electrochemistry. The most well-known potential modulation electrochemical technique is a.c. impedance spectroscopy, in which current modulation caused by a potential modulation is analyzed. The potential modulation technique has also been used for in-situ IR spectroscopy (EMIRS and SNIFTIRS), but its use was aimed only to subtract the solution background and to enhance the S/N ratio of the spectram. If the IR signal caused by a potential modulation is analyzed, some information on electrode dynamics could be obtained as in a.c. impedance spectroscopy. 

Figures. Simplified schematic of the soft X-ray appearance potential spectrometer. S is the sample and F is the filament. Signal is extracted by the potential modulation technique.

Nagatani, H., T. Ozeki, and T. Osakai, Direct spectroelectrochemical observation of interfacial species at the polarized water/l,2-dichloroethane interface by ac potential modulation technique, J Electroanal Chem, Vol. 588, (2006) p. 99. 

X-ray diffraction studies, in that the entire spectrum is collected simultaneously. Therefore, for a given signal-to-noise ratio, data collection times are reduced, time-resolved studies become possible, and potential modulation techniques are facilitated. Electrochemical studies using this configuration are only just beginning but early results appear very promising. 

Another technique consists of MC measurements during potential modulation. In this case the MC change is measured synchronously with the potential change at an electrode/electrolyte interface and recorded. To a first approximation this information is equivalent to a first derivative of the just-explained MC-potential curve. However, the signals obtained will depend on the frequency of modulation, since it will influence the charge carrier profiles in the space charge layer of the semiconductor. 

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

Electrochemical impedance spectroscopy leads to information on surface states and representative circuits of electrode/electrolyte interfaces. Here, the measurement technique involves potential modulation and the detection of phase shifts with respect to the generated current. The driving force in a microwave measurement is the microwave power, which is proportional to E2 (E = electrical microwave field). Therefore, for a microwave impedance measurement, the microwave power P has to be modulated to observe a phase shift with respect to the flux, the transmitted or reflected microwave power APIP. Phase-sensitive microwave conductivity (impedance) measurements, again provided that a reliable theory is available for combining them with an electrochemical impedance measurement, should lead to information on the kinetics of surface states and defects and the polarizability of surface states, and may lead to more reliable information on real representative circuits of electrodes. We suspect that representative electrical circuits for electrode/electrolyte interfaces may become directly determinable by combining phase-sensitive electrical and microwave conductivity measurements. However, up to now, in this early stage of development of microwave electrochemistry, only comparatively simple measurements can be evaluated. 

Another technique for flatband determination is based on the measurement of potential-modulated microwave conductivity signals and is described further in the next section. 

The surface actlve/surface inactive difference between p-polarlsed/ s-polarised radiation has enabled an alternative modulation technique, polarisation modulation, to be developed (15,16). In electrochemical applications, it allows surface specificity to be achieved whilst working at fixed potential and without electrochemical modulation of the interface. It can be implemented either on EMIRS or on SNIFTIRS spectrometers and can be very valuable in dealing with electrochemically irreversible systems however, the achievable sensitivity falls well short of that obtained with electrochemical modulation. It should also be noted that its "surface specificity" is not truly surface but extends out into the electrolyte with decreasing specificity to about half a wavelength. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

Polarisation modulation infrared rejiection-absorption spectroscopy (PM-IRRAS or JRRAS). Potential modulation IR studies rely on switching the potential at a reflective electrode between rest and active states, generating difference spectra. However, the EMIRS technique has several drawbacks the relatively fast potential modulation requires that only fast and reversible electrochemical process are investigated the absorption due to irreversibly chemisorbed species would be gradually eliminated by the rapid perturbation. Secondly, there is some concern that rapid modulation between two potentials may, to some extent, in itself induce reactions to occur. 

In order to employ a lock-in detection technique, as in EMIRS, the modulation frequency of the potential at the electrode would have to be at least an order of magnitude greater than F(v). Thus, the potential modulation would have to be c. 100 kHz too great to allow sufficient relaxation time for most electrochemical processes to respond. Instead, a slow modulation or single-step approach is employed, as follows ... 

Although the detection of COads by in situ IR was accepted as not ruling out the existence of other adsorbed species (particularly since the experiments were not quantitative in terms of coverage and the potential-modulation aspect of the technique could render it blind to adsorbed species that do not exhibit a potential-dependent absorption frequency), it was generally accepted that the EMIRS data had ended the long controversy over the nature of the poison derived from methanol. 

Conductance relaxation Is also shown to be critically dependent upon aggregation equilibria affecting non-conducting (ion-pairs) as well as Ionic species. The relaxation behavior In the presence of quadrupoles (ion-pair dimers) and triple Ions Is thoroughly analyzed. The experimental results show the potential of the field modulation techniques as a method for the Investigation of ionization processes, independent of conductance measurements. 

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