Situ IRRAS

Typical in situ refiection cells containing disk electrodes are shown in Figs. 4.45 and 4.46. The edges and back of the electrode are sealed to ensure that 

The foregoing example also shows that, as in transmission measurements, thin-layer cells lead to the thin-layer problem (Section 4.6.1). The diffusion decoupling [420, 363] of the very thin layer between the working electrode and the IR window (retardation of the free exchange of ions with the rest of the elec-frolyte) can result in accumulation/depletion of the reaction products/reactants whose absorption is superimposed on the spectrum of the adsorbed species. Furthermore, the accumulated products of any electrode reaction can distort the spectra measured by IRRAS [412, 421], 

The following technical ways to circumvent this problem have been suggested. 

The barrel-plunger cell design (Figs. 4.45 and 4.46) allows the working electrode to be withdrawn into the bulk electrolyte during the potential step and then returned to the window to record the spectrum. The electrode can be moved manually [422], as shown in Figs. 4.45 and 4.46, or by computer control [423]. Since its development by Pons et al. [424], this technique has been widely applied [356, 425-428]. 

An invasive method such as to use a highly concentrated buffer of an appropriate composition also allows one to avoid significant potential-induced migration [435] (Section 3.7). 

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Fig. 9.8 Optical setup for in-situ IRRAS based on the following windows (a) Cap2 equilateral prism (transmission range 76900-1100 cm" RAir/CaF,=0-03), (b) ZnSe hemisphere (transmission range 22200-700 cm" RAir/znSe=0-18).

The IRRAS spectra of ultrathin films can also be measured in the region of phonon absorption of the substrate. Figure 2.13 shows results of / -polarized ex situ IRRAS of calcite (CaCOs) after adsorption of oleate for 5 min in a 3.3 X 10 M oleate solution at pH 10, measured by Mielczarski and Miel-czarski [41] at different angles of incidence. It is important that although the absorption bands of adsorbed oleate at 1575, 1538, and 1472 cm lie within the region of the reststrahlen band of calcite (1600-1400 cm ), they have sufficient... 

Thus, for both metallic and transparent snbstrates, p-polarization, angles of incidence above cpc, and water layers thinner than 1-2 p,m are preferable. Under these conditions (p > (pc and t/2 < dp, where dp is the penetration depth), the cell windows act as IREs. In other words, the in situ IRRAS geometry under the optimum conditions is in fact the ATR geometry in Otto s configuration (Fig. 236d). 

The above example demonstrates that interference with the solution spectrum can be reduced considerably by use of an appropriate buffer that has high concentration of coadsorbed species and no absorption bands in the spectral region under study (see also Refs. [154, 155]). The solution bands can be distinguished in /7-polarized in situ IRRAS by comparison with the Y-polarized spectra in which only the absorption bands of the species in solution are present (Section 1.8.2). However, since the SSR holds at a distance of 1 p,m out into the solution from the metal surface, this approach can be inefficient. In this case, apart from using other means (vide infra), one can resolve the contribution of bulk solution comparing the dependence of s- and /7-polarized spectra on the buffer concentration [156]. Comparison with the reference spectra of the electrolyte... 

Figure 3.46. In situ IRRAS of poly-(3-methyl thiophene)(CI04 ) film in BU4NCIO4 in acetonitrile at -F0.35 V (with offset of -0.1 reflectance units) and poly-(3-methyl thiophene)(PF6 ) film in BU4NPF6 in acetonitrile at -1-0.65 V. Reference spectrum recorded at -0.35 V. Spectra were recorded using Bruker IFS-113V FTIR spectrometer with MCT detector. Spectral resolution was 4 cm and number of scans for each spectrum was 128. Reprinted, by permission, from E. Lankinen, G. Sundholm, P. Talonen, T. Laltinen, and T. Saario, J. Electroanal. Chem. 447, 135-145 (1998), p. 141, Fig. 6. Copyright 1998 Elsevier Science B.V.

Yakovlev and coworkers [402a, 402b] reported the IR absorption enhancement by mote than one order of magnitude for hydrocarbons adsorbed inside porous silicon, and assigned this effect to photon confinement in the microcavity acting like a multipass (Fabry-Perot type) cell. Recently, Jiang et al. [402c] observed a 50 times enhancement in the in situ IRRAS of CO adsorbed on Pd nanoparticles synthesized in cavities of Y-zeolite, as compared to the cases when the supports were ultrathin Pd films deposited directly on the zeolite or on amorphous alumosiUcate layer. 

Figure 4.42. Physical vacuum deposition apparatus for real-time in situ IRRAS measurements (a) substrate (b) thickness monitor (c) ZnSe windows (d) IR beam (e) shutter (f) mercury lamp (g) crucible. Reprinted, by permission, from M. Tamada, H. Koshikawa, and H. Omichi, Thin Solid Films 292,164-168 (1997), p. 165, Fig. 1. Copyright 1997 Elsevier Science S.A.

SNIFTIRS is able to provide detection limits for in situ IRRAS of 10 -10 " AE/E with a spectral resolution of 8-16 cm" and several hours of data collection [361]. In general, application of this technique is restricted to reversible electrochemical systems. However, flow cell tactics enable one to utilize this method even when examining irreversible Faradaic processes if the... 

Figure 4.53. In situ IRRAS spectra of SCN adsorbed on Cu electrode (a) PM and p) ordinary SPAIRS spectra. Reference is—1.2 V (Ag-AgCI)and sample potentials are marked. Experiments were performed on Mattson RS-1 spectrometer configured with external bench analogous to that shown in Fig. 4.51. Photoelastic modulator was Flinds International ZnSe Series II modulator, operating at 37 kFIz. The PM wavefront was sampled in real time with ATI Instruments real-time sampling accessory. The MCT detector with D of 5 x 10 ° cm W was used. Spectra are represented in absorption depth of PM signal (Aflpm). Reprinted, by permission, from W. N. Richmond, P. W. Faguy, R. S. Jackson, and S. C. Weibel, Anal. Chem 68, 621 (1996), p. 625. Copyright 1996 American Chemical Society.

In situ IRRAS. ATR in Otto s geometry (Section 2.5.4) was applied to the EX-chalcocite system in 13-reflection geometry [330]. The main advantage of... 

In situ IRRAS with a Ge prism window was used by Laajalehto et al. [603] to study the effect of pyrite activation by copper and lead ions at pH 5, 6.5, and 9. It was found that the xanthate interaction with copper-activated pyrite resembles that of chalcopyrite, resulting in adsorption and dixanthogen formation. In similar experiments with lead-activated pyrite, only very weak absorption bands of the adsorbed collector were observed, implying that lead depresses rather than activates pyrite. 

Free OH of ice near the carboxylic acid or methyl terminated Au surface at 80-145 K non situ IRRAS [195a]... 

HOO" (H2O2) on Ti02 in situ IRRAS [655] Water in primary hydration sphere of Li+ in DL in situ IRRAS [156b]... 

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