Thin-film electrodes, SEIRAS

Figure 9.2 SEIRAS spectra for a Pt thin film electrode in 02-saturated 0.1 M NaC104 + NaOH (pH 11). The reference spectmm at 0.4 V was taken before the potential sweep started. The scan rate was 10 mV/s. (Reproduced with permission from Shao et al. [2006a].)...

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. 

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... 

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... 

Thin-film electrodes that exhibit the SEIRA effect can be deposited on the total-reflecting plane of the prism by vacuum evaporation or electroless plating of the desired metal. Electrochemical deposition cannot be used for non-doped (and thus non-conducting) Si windows, but is possible for Ge. 

Vacuum-evaporated very thin ( 20 run) Au and Ag electrodes suitable for SEIRAS show pale blue to purple colors, while chemically deposited film electrodes show the colors of the corresponding massive metals and are as shiny as a well-polished surface. Both thin-film electrodes have enough conductivity for electrochemistry. These film electrodes are usually polycrystaUine. In the case of Au films,... 

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... 

Fig. 8.11 Comparison of the normal infrared spectrum of (a) 5 M aqueous solution of pyrazine and (b) SEIRA spectrum of pyrazine adsorbed on an Au thin-film electrode at 0.0 V (vs SCE) [71], (c) SER bands of pyrazine adsorbed on a polycrystalline Au electrode taken from Ref [77]. The solid and...

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... 

Fig. 37 Time-resolved SEIRAS-ATR experiment for the dissolution of the chemisorbed uracil monolayer on quasi-Au(in) thin-film electrodes in 0.1 M H2SO4 + 12 mM uracil. The potential was stepped from 1 = 0.60 V (tl = 30 s) to 2 = 0.20 V. (a) original set of time-resolved SEIRAS spectra (step-scan technique) with characteristic uracil vibrations in the C=0 stretching region (For clarity, the loss spectra are plotted with an inverted sign I) (b) Time dependence of the integrated intensity of the uracil vibration uc4=o(1590 cm" ). The solid lines represent fits to an Avrami-type equation (cf Eq. (35)) with m = 1.12.

Chen et al. [95], very recently, prepared Au thin-film electrodes made by electroless deposition for in situ electrochemical attenuated total-reflection surface-enhanced infrared adsorption spectroscopy (ATR-SEIRAS) which consisted of 46 nm Au nanoparticles deposited on a Si infrared window. Very interestingly, they observed that a square-wave treatment of the Au film led to a much enhanced ORR activity (02-saturated 0.1 M HCIO4) as a consequence of the surface reconstruction of the nanoparticle film. Thus, whereas the ORR activity of the initial Au... 

These ions undergo only weak perturbations upon adsorption. Thus, it can be difficult to discriminate species in solution, or in the diffuse doublelayer, from ions at the electrode surface [54]. Better selectivity for adsorbed electrolyte anions has been achieved through use of the surface-enhanced infrared absorption spectroscopy (SEIRAS) technique [22, 54, 89, 90]. Methods for the preparation of quasi-single crystalline thin films are enabling the study of electrolyte adsorption on structurally well-defined surface sites by SEIRAS [22, 89,... 

The (nonelectrochemical) SEIRAS effect was first reported by Hartstein and coworkers in 1980 [110], after which Suetaka and coworkers published a number of papers from 1982 exploiting Kretschmann coupling (Fig. la) to effect SPP excitation and the concomitant enhancement of the infrared absorptions of species adsorbed at the thin metal film electrode surface [69, 111-113], effectively SEIRAS. The authors commented that the short-range enhancement afforded by the technique enabled species at the metal-aqueous solution interfaces to be preferentially observed . Similarly, Neff and coworkers [48] reported the study of water at the electrode-electrolyte interface using SPP excitation. As with the SERS effect, SEIRAS is limited, so far, to the coinage metals [5,10]. 

AIREs, SEIRAS spectra show marked enhancement of the infrared absorptions of adsorbed species, up to 40 x that expected on a smooth, bulk metal electrode. Thus, Sun and coworkers [106] investigated the adsorption of CO at thin Au films evaporated onto a hemispherical Si prism, and observed a 20x enhancement over that expected at a conventional electrode this increased by a further factor of two when the Au film electrode was flame-annealed to generate a highly ordered Au(lll) surface, observed using ex situ scanning tunneling microscopy (STM). 

The object of this chapter is to describe recent methodological developments in SNIFTIRS and PM IRRAS that have made these techniques powerful quantitative tools to measure tilt angles of molecules in thin organic films adsorbed at metal electrode surfaces. This chapter complements a general description of recent advances in IR spectroscopy in Chapter 7 and a review of SEIRAS in Chapter 8. 

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