ATR-SEIRAS

Figure 6.16 Attenuated total reflection surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS) spectra for the oxidation of 0.1 M HCOOH in 0.5 M H2SO4 on a polycrystaUine electrode. The bands at 2055 -2075 and 1800-1850 cm are assigned to linear- and bridge-bonded CO, whereas the band at 1323 cm corresponds to adsorbed formate. (Reproduced from Samjeske et al. [2006].)...

Delgado JM, Orts JM, Rodes A. 2005. ATR-SEIRAS study of the adsorption of acetate anions at chemically deposited silver thin film electrodes. Langmuir 21 8809-8816. 

Pronkin S, Wandlowski T. 2004. ATR-SEIRAS—An approach to probe the reactivity of Pd-modified quasi-single crystal gold film electrodes. Surf Sci 573 109-127. 

Recently, Shao et al. proposed superoxide (O2) formation as the first step of the ORR on Pt films in alkaline solution (NaC104 with pH=ll) by analyzing spectral data of surface-enhanced infrared reflection absorption spectroscopy with attenuated total reflection (ATR-SEIRAS). This was fuller supported by Zhang and Anderson, who performed DFT cluster calculations for the ORR on Pt(lll), Pt(lOO), and umeconstracted Pt(llO). They... 

We have organized the work herein as follows In Sect. 2 we will discuss briefly two typical electrochemical substrates, Au(l 11) and Au(lOO) in 0.05 M H2SO4, employing in-situ scanning tunnehng microscopy (STM) and surface-enhanced infrared refiection/absorption spectroscopy (ATR-SEIRAS). Self-assembled physisorbed adlayers of various hydrogen-bonded carboxyhc acids will be described in Sect. 3. Then we will focus on selected properties of chemisorbed self-assembled aromatic adlayers based on experiments with... 

The structure and orientation of water molecules on quasi single-crystalline Au(lll -20 nm) thin film electrodes in contact with aqueous sulfuric acid solution were studied by surface-enhanced infrared reflection absorption spectroscopy employing an ATR configuration (ATR-SEIRAS) [22,31]. The spectrum of interfacial water is strongly dependent on electrode potential, ionic strength and pH. Figure 4A shows a series of SEIRA spectra recorded within the potential range of an ideally polarizable Au(lll) electrode in... 

In the following paragraphs we will describe results of a comprehensive electrochemical study employing cychc voltammetry, STM and ATR-SEIRAS on the potential-controlled assembly and on structure transitions of TMA adlayers on Au(lll) electrodes in 0.1 M H2SO4. For comparison we will also discuss data for benzoic acid (BA), isophthalic acid (lA) and terephthalic acid (TA). 

Based on complementary observations from in-situ STM and ATR-SEIRAS experiments we propose the model depicted in Eig. 12C for the adlayer of... 

Fig. 8.1 In-situ monitoring of the electrochemical interface by IRAS (a) and ATR-SEIRAS (b).

This chapter is organized as follows. Eirst, the EM mechanism of SEIRA is briefly described for a better understanding of the technique, and this is followed by experimental aspects of the studies. Then several electrochemical systems studied by ATR-SEIRAS are described. Since SEIRA studies before 1997 have already been summarized in some review articles [8-10], this chapter deals mainly with the work performed after 1998. 

Typical electrochemical cells used for ATR-SEIRAS are shown in Fig. 8.4 [28, 29). The cell can be made of either glass or plastics such as Kel-F. The prism works as the cell window as well as the substrate on which a thin-film electrode is deposited. Infrared-transparent materials with high reflective indices such as... 

Fig. 8.4 Horizontal- [28] and vertical-type [29] spectroelectrochemical cells for ATR-SEIRAS.

The optimum optical conditions (the angle of incidence and polarization of the incident infrared radiation) for ATR-SEIRAS depend on the thickness and morphology of the thin metal film [8-10, 31], Figure 8.5 shows the simulated incident-angle dependence of the band intensity of a model molecule adsorbed on an Ag film for p-polarization, where the metal film was assumed to be a bilayer consisting of a continuous underlayer and a nanoparticle overlayer [11]. 

Fig. 8.5 The incident angle dependence of the band intensity in ATR-SEIRAS for p-polarization (a model calculation). Band intensity is negligibly small for s-polarization [11],...

The corresponding set of spectra obtained by ATR-SEIRAS with a vacuum-evaporated Au(lll) thin-film electrode and 1 mM pyridine-i-0.1 M NaClO. solution in H2O is shown in Fig. 8.8b [55]. The vacuum-evaporated film electrode was subjected to flame annealing before use to give a (111) quasi-single crystal surface. The spectra were recorded with unpolarized radiation and by a simple potential difference tactic with the reference potential of -0.8 V. These spectra are represented in the absorbance units [A=-log(R/i o)]. and thus the up-going bands correspond to pyridine adsorbed on the electrode. Largely different from... 

Recently, the redox reaction of cytochrome c on Au electrodes modified with mercaptopropionic acid (MPA), mercaptoethanol, 4,4 -dithiodipyridine, and L-cys-teine were directly observed with ATR-SEIRAS [98,99]. Figure 8.16a shows the... 

Fig. 8.17 Redox difference spectra of cytochrome c on the MPA-modified Au electrode recorded with the ATR-SEIRA technique at (a) +0.20, (b) +0.15,...

ATR-SEIRAS is more convenient than IRAS for this purpose because of its higher sensitivity and its ability to probe the electrode surface without the disturbance by H2 gas evolution. Figure 8.21A shows a set of SEIRA spectra on a Pt thin-film electrode measured in 0.5 M H2SO4 at the various potentials indicated with respect to the reference potential of 0.6 V [109]. All of the potentials shown are the values after correcting ohmic drops. A band appears at 2079-2096 cm and grows as the potential is made less positive. Since this band was shifted to around 1500 cm in D2O, it is undoubtedly ascribed to adsorbed hydrogen species. The potential dependence of the band intensity is shown in Fig. 8.21 B (closed circles). The open circles were taken from the IRAS study [108] and multipHed by 5. The SEIRAS measurement is consistent with the IRAS study, but it should be noted that the S/N of the SEIRA spectrum is much... 

ATR-SEIRAS has been employed successfully to identify the reactive intermediate in methanol oxidation [118]. Figure 8.23a shows a series of SEIRA spectra of a Pt thin-fibn electrode in 0.1 M HClO4-t0.5 M methanol acquired during a potential sweep from 0.05 to 1.3 and back to 0.05 V (vs RHE) at a sweep rate of 5 mV s. Two strong bands attributed to CO molecules linearly and bridge-bonded to the Pt electrode surface are observed at 2060 and... 

The reaction scheme of methanol electro-oxidation proposed from the ATR-SEIRAS study is totally different from those proposed earlier, but is similar to that in methanol oxidation on Pt(lll) covered with molecular oxygen in ambi-... 

The combination of the high sensitivity of SEIRAS and a rapid-scan FT-IR spectrometer enables the spectral collection simultaneously with electrochemical measurements such as cyclic voltammetry and potential-step chronoamperome-try. The time-resolved measurement can give some information on electrode kinetics and dynamics, as has been shown in Fig. 8.24. Figures 8.25 and 8.26 represent another example of millisecond time-resolved ATR-SEIRAS study of current oscillations during potentiostatic formic acid oxidation on a Pt electrode [123]. At a constant applied potential F of 1.1 V, the current oscillates as shown in Fig. 8.25 a. Synchronizing with the current oscillations, the band intensities of linear CO and formate oscillate as shown in Fig. 8.26 (and also in Fig. 8.25 c). 

As has been discussed in the preceding section, most of the recent ATR-SEIRAS measurements are carried out simultaneously with electrochemical... 

In addition to the expansion of time resolution, the expansion of spectral range to far IR is also desirable, since metal-adsorbate vibrations and surface vibrational modes occur in this range. The available spectral range is now limited by the prism (>900 cm" for Si and >700 cm" for Ge), the low intensity of the light source, and the low sensitivity of detectors. Since the absorption coefficient of Si fortunately decreases in the far-IR region, ATR-SEIRAS in the far-IR wiU be possible with the use of a synchrotron far-IR facility and a helium-cooled bolometer [142]. 

Shao MH, Adzic RR (2005) Electrooxidation of ethanol on a Pt electrode in acid solutions in situ ATR-SEIRAS study. Electrochim Acta 50 2415-2422... 

Ataka and co-workers [171] have presented the following interpretation of the absorption bands of water in the ATR-SEIRA spectra of the Au 0.5 M HCIO4 interface measured in the DL region from - -0.1 to -1-1.3 V (SHE) (Fig. 3.41). 

Analyzing the phase delay between the stimulus and the resulting infrared signal as a function of the modulation frequency, the kinetics of the modulated process may be deduced, provided that the rate constants of the forward and reverse processes are equal [268]. Using the ATR-SEIRA method, Ataka and co-workers [265] measured the rate constant as large as 10 s for the charge transfer between an adsorbate and an electrode (Section 7.6). 

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