Bipolar band

To interpret these features, one needs to recall that the in situ spectra are differential (i.e., are related to the spectrum taken at some potential) (Section 4.6.4). A unipolar, potential-dependent band is obtained in a potential different spectrum if adsorbed species are present at a sample potential Es and absent at the reference Er. If the adsorbed species is present at both potentials and the frequency of its mode is potential dependent, this mode is characterized by a bipolar band. The absence of a band indicates the lack of a measurable change in the population or the parameters of the elementary oscillators of the mode associated with this band. Since the spectra are represented in -AR/R units, features pointing up are due to species gained at the sample potential (Section 4.6.4). 

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It will be clear that EMIRS and SNIFTIRS spectra are difference spectra and can be somewhat complex ( ). Typically they will contain positive absorption bands from species present in excess at potential El compared to potential E2 and negative absorption bands from species whose polulation changes oppositely with potential. In addition, bands which shift with potential will appear as a single bipolar band either with one lobe of each sign, figure 2, (or even more complex structures with three lobes). 

The bond strengths of adsorbed species can be affected by a change of the electric field at the interface. In this case, shifts of the adsorbate vibrational frequencies are also observed [21]. According to Eq. (1.3) the frequency shift gives rise to bipolar bands (i.e., bands exhibiting both negative and positive parts). 

The formation of a bipolar band is due to a shift of the absorption frequency with the potential and... 

The first observations, on platinum roughened by repeated cycling, were made by Beden and co-workers [88] who observed that whilst linearly bonded CO could clearly be seen as a bipolar band in EMIRS in 1 M methanol/0.5 M HC104, the signal due to this species disappeared on dilution to 0.1 M methanol in the same potential range ( = 0.2 V, AE — 0.4 V), suggesting that adsorption of CO was significantly reduced on... 

In fact, these procedures (EMIRS or SNIFTIRS) should be used only when the system under study undergo reversible changes with potential [16]. Bipolar bands are obtained in the difference spectrum for species irreversibly adsorbed, if the band-center frequency is shifted with the potential, e.g., CO adsorbed on platinum (Fig. 8 a). But the situation is problematic when the frequency shift with potential is negligible. Then signals cancel out in the computed spectrum. This question has giv-... 

On-top/bridge site interconversion has been demonstrated for Pt(lOO) by Kitamura et al. [57], who made a careful analysis of intensity changes with the applied potential. The bridged CO species moves to the on-top geometry as the potential is made more positive. This result explains why bridge-bonded CO does not present a bipolar band in EMIRS experiments. 

Besides this theoretical controversy, it is interesting to observe that the band intensity for silver (see Fig. 32a) and copper (Fig. 33) is potential-dependent. This suggests that the interaction of CN with the metal surface presents some electrostatic contribution, as expected for ionic bonding. Contrary to this, a nearly constant intensity of the CN adsorbate band is observed on gold [114] (Fig. 32b) and a bipolar band was observed in SNIFTIRS experiments [113]. These facts suggest a more irreversible adsorption of CN on gold than on Ag and Cu. Irreversible adsorption is expected for bonds with a higher covalent than ionic character. 

Several aspects of this system are not clear for example, what causes a bipolar band in the spectrum with s-polarized light and why has Pons [137] not observed the 2137cm feature A comprehensive survey of the spectroscopic features observed for thiocyanate at electrochemical interfaces was given by Corrigan et al. [113] for gold and silver and by Ashley et al. for gold, silver and platinum [123]. 

The adsorption of cyanate ions was studied on silver [113, 135] and gold electrodes. A bipolar band can be observed for adsorption on a silver electrode [135] (Fig. 45). Corrigan and Weaver [135] have attempted a deconvolution of this band after observing that the intensity of the positive-going part was lower than expected... 

Figure 6b also reveals that the positive lobe of the bipolar band around 1280 cm decreases at potentials above 0.45 V this decline coincides with the onset of the surface oxidation in the voltammetry of Ru(OOOl) (c.f., Fig. 2). Adsorption of the OH species is followed by the desorption of bisulfate and a concurrent increase in the bisulfate species in the double layer. This effect becomes visible in the IR spectrum by the appearance of the positive-going solution-phase bands for the bisulfate anion at 1051 and 1200 cm at potentials equal to, or higher than 0.55 V. The most pronounced feature in the IR spectra above 0.55 V is the negative lobe of the bipolar band centered at 1248 cm , which represents adsorbed bisulfate at the reference potential.

IR spectroscopy was used to obtain insights on the carbon monoxide absorption and oxidation mechanism on Pt-Ru electrocatalysts. Figure 17 shows the SNIFTIR spectra of CO on submonolayer Pt deposits on Ru(OOOl). Two bipolar bands are clearly visible at potentials from 0.1 to 0.8 V. Analyses of IR spectra (vide supra) attributed the bipolar band at lower frequencies to bhie-sliifted CO (i.e., moved to higher frequency) on polycrystalline Ru, whereas the higher-frequency bipolar band represents red-sliifted CO onPt(lll). ... 

The first in-situ PM study was reported by Russell et al. in 1982 [63] and was concerned with the adsorption of CO on Pt. As was discussed above, the first EMIRS paper on this subject [62] had concluded that the dominant poison in the electro-oxidation of MeOH at Pt under aqueous acid conditions was —C = 0, which exists at high coverage, and another CO species thought at that time to be >C = 0. The C = 0 feature appeared as a bipolar band, indicating that its frequency was potential-dependent. Russell and coworkers commented upon the indirect nature of the information derived... 

Transition metal electrodes prepared by sputtering and electrochemical deposition often show derivative-hke (i.e., bipolar) [17, 34—38] or negative absorption (anti-absorption) bands [46-50]. Spectral features change from normal absorption to anti-absorption through bipolar shapes with increasing amount of the metal deposited [17]. The bipolar band shape was ascribed to a Fano-type resonance of electronic interactions between molecular vibrations and the metal [34, 35]. The anti-... 

Carbon monoxide has a large attenuation coefEcient for absorption of IR radiation. Therefore, the SNIFTIRS spectra of CO adsorbed at a Pt electrode surface are a convenient standard to test the S/N of a spectroelectrochemical setup. The left panel in Fig. 9.11 shows SNIFTIRS spectra of a monolayer of CO at Pt electrode surface recorded using a cell equipped with a Cap2 prism [91]. The right panel shows similar spectra recorded using a ZnSe hemispherical window. The IR bands of CO adsorbed at Pt are significantly Stark-shifted when the electrode potential is modulated between -200 and -f200 mV versus SCE. Consequently, the potential difference spectrum displays bipolar bands. Clearly a much better S/N for these bands is achieved when a ZnSe hemisphere is used as the window. 

Because of the subtraction, SNIFTIRS spectra are devoid of the common background signal from the aqueous electrolyte. However, they represent the difference between absorbances of organic molecules at potentials Ei and E2 (or precisely the difference multiplied by 2.3). In order to facilitate interpretation of such spectra, it is convenient to choose a value of the base potential Fi at which the molecules are totally desorbed into the bulk of the thin-layer cavity. The sample potential E2 corresponds then to the adsorbed state of the film. Consequently, SNIFTIRS spectra plot a difference between the absorbances of molecules desorbed into the thin-layer cavity at potential Ei and those adsorbed at the electrode surface at potential 2- Thus, positive bands (or positive lobes in bipolar bands) are due to absorbance by desorbed molecules and negative bands (or negative lobes in bipolar bands) are the result of absorbance by adsorbed molecules. 

Secondary redox storage must be ac-compKshed at sufficiently low potentials to prevent losses due to simultaneous (undesired) solvent electrolysis. However, bipolar band gap photoelectrochemistry imposes large photopotentials. These are avoided through the inverted band gap configuration, as exemplified in Fig. 3(d), in which the photopotentials generated in the respective small and wide band gap portions of the tandem cell, and Vg drive two separate electrochemical storage processes ... 

A bipolar gap direct ohmic photoelectro-chemical system comprises either a bipolar band gap pnpn/electrolyte ohmic photo-electrochemical cell, with reduction occurring at the photoelectrode-electrolyte interface and regenerative oxidation occurring at the electrolyte-counter electrode (anode) interface or alternately a npnp/electrolyte bipolar band gap with oxidation occurring at the semiconductor-electrolyte interface and regenerative reduction occurring at the electrolyte-counter electrode interface. In the bipolar gap direct ohmic photoelectro-chemical system, direct refers to the direct contact between semiconductor and solution, and ohmic indicates this interface is an ohmic rather than a Schottky junction. This facilitates study of several characteristics of bipolar multiple band gap systems, without the added complication of simultaneous parameterization of a direct Schottky barrier at the electrolyte interface. 

Fig, 6 Photocurrent stability in several aqueous electrolytes ofthe bipolar band gap... 

Bipolar gap indirect ohmic photoelectrochemistry comprises either a bipolar band gap pnpn/electrolyte ohmic photoelectrochemical cell, with reduction occurring at the semiconductor-electrocatalyst-electrolyte interface and regenerative oxidation occurring at the electrolyte-counter electrode (anode) interface or alternately an npnp/electrolyte cell, with oxidation occurring at the semiconductor-electrocatalyst-electrolyte interface and regenerative reduction occurring at... 

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