Vibrational double-layer

In situ Fourier transform infrared and in situ infrared reflection spectroscopies have been used to study the electrical double layer structure and adsorption of various species at low-index single-crystal faces of Au, Pt, and other electrodes.206"210 It has been shown that if the ions in the solution have vibrational bands, it is possible to relate their excess density to the experimentally observed surface. 

The EMIRS and SNIFTIRS methods provide the IR vibrational spectra (really the difference spectra - see later) of all species whose population changes either on the electrode surface or in the electrical double layer or in the diffusion layer in response to changing the electrode potential. Spectra will also be obtained for adsorbed species whose population does not change but which undergo a change in orientation or for which the electrode potential alters the Intensity, the position or shape of IR absorption bands. Shifts in band maxima with potential at constant coverage (d nax 6 very common for adsorbed species and they provide valuable information on the nature of adsorbate/absorbent bonding and hence also additional data on adsorbate orientation. 

A major emerging area of research activity in interfacial electrochemistry concerns the development of in-situ surface spectroscopic methods, especially those applicable in conventional electrochemical circumstances. One central objective is to obtain detailed molecular structural information for species within the double layer to complement the inherently macroscopic information that is extracted from conventional electrochemical techniques. Vibrational spectroscopic methods are particularly valuable for this purpose in view of their sensitivity to the nature of intermolecular interactions and surface bonding as well as to molecular structure. Two such techniques have been demonstrated to be useful in electrochemical systems surface-enhanced Raman spectroscopy... 

Electroacoustics — Ultrasound passing through a colloidal dispersion forces the colloidal particles to move back and forth, which leads to a displacement of the double layer around the particles with respect to their centers, and thus induces small electric dipoles. The sum of these dipoles creates a macroscopic AC voltage with the frequency of the sound waves. The latter is called the Colloid Vibration Potential (CVP) [i]. The reverse effect is called Electrokinetic Sonic Amplitude (ESA) effect [ii]. See also Debye effect. 

FIGURE 26.2 Colloid vibration potential caused by the asymmetry of the electrical double layer around the particles. 

Because of the short lifetime of ions in gaseous atmospheres, even at low pressure, gas-phase IR measurements are limited to adsorption of neutral molecules. Electrochemical applications of the IR method offer the interesting possibility of providing data on the adsorption properties of charged particles (Secs. 8 and 9). In the electrochemical environment the applied potential allows ionic adsorbates to be studied under energetically controllable conditions. Otherwise the electrochemical double layer offers exceptional conditions to investigate the Stark effect on vibrational transitions by setting tunable electric fields of the order of 10 V cm at the interface. This phenomenon will be discussed in Sec. 10. 

In electrochemical environments the vibrational spectra are additionally affected by solvation effects, the electric field in the double layer, and the co-adsorption of water and/or ions in the inner Helmholtz plane. 

As demonstrated for the adsorption of sulfate on Pt(l 11), vibrational spectroscopy offers the possibility of investigating the nature of the adsorbed ions on the surface, its site occupancy and the interactions with other ions or molecules at the interface and with the metal substrate. Further spectroscopic data on these systems may contribute greatly to the interpretation of double-layer phenomena. 

It must be stressed that the polarizability gradient da/dQk also appears in the equation for Raman intensities [175], as indicated also by Lambert [176]. Thus, in view of Eq. (25), we can extend the consequences of the static electric field to vibrations which are forbidden by the surface selection rule the high electric field in the double layer can induce a dipole moment component in the direction of the field on permanent dipoles which are parallel to the surface. Thus the effect of orientation due to the electric field is just a manifestation of the Stark effect. 

The electrochemical double layer offers the exceptional possibility of investigating the Stark effect at very high electric fields. Some important progress has been made in the theoretical treatment of the problem. Experimental data of potential effects upon the frequency and/or the intensity of vibrational modes must discriminate between the pure electric field effect and the secondary effect of potential on the coverage and, consequently, on the lateral interactions. 

Carter and coworkers studied how side-chain branching in PFs affects device performance with and without an additional HTL of cross-linkable polymer 2 [ 19]. They found that the device efficiency is affected more by the position of the exciton recombination zone than by variations of polymer morphology induced by side-chain branching, which mainly controls the relative emission between vibrational energy levels and has a minimal effect on polymer charge transport properties. For double-layer devices (ITO/PEDOT PSS/2/3,4, or 5/Ca), a typical brightness of 100 cdm 2 at 0.8 MV cm-1, maximum luminance of 10 000 cd m-2 at 1.5 MV cm x, and device efficiencies between 1.3 and 1.8 cd A 1 for 3 and 5 branching can be achieved. 

Lambert, D.K. (1988) Vibrational Stark effect of CO on Ni(lOO), and CO in the aqueous double layer experiment, theory, and models. Journal of Chemical Physics, 89. 3847-3860. 

Oklejas, V., Sjostrom, C., and Harris, J.M. (2003) Surface-enhanced Raman scattering based vibrational stark effects as a spatial probe of interfacial electric fields in the diffuse double layer. Journal of Physical Chemistry E, 107, 7788-7794. 

Experimental study of the double layer is not limited to thermodynamics. A variety of spectroscopic methods have been applied to determine the structure and composition of the double layer. Two of these, namely, second-harmonic generation and vibrational sum frequency spectroscopy, have already been described in section 8.11. Other important techniques are based on the absorption of electromagnetic radiation when it is transmitted through or reflected at the interface. Finally, the scattering of X-rays and neutrons at interfaces has proven to be a valuable tool for obtaining atomic level information about the interface. In the following section some of these methods are outlined in more detail. 

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