Environmental Engineering Reference
In-Depth Information
diffusion to the cell interior (via the channels; Fig. 12.2), can occur without changing
the electrode/optical window configuration. As the working electrode is supported by
a spacer, the solution gap is uniform. Therefore, the electrode may be cycled at a scan
rate (up to 5 mV/s) producing state-of-the-art, ohmic-drop-free voltammetric data
characteristic of the average properties of the full surface. During the scan, the surface
is investigated by BB-SFG, providing BB-SFG (vibrational) - electrochemistry
characteristics, as already reported [Lu et al., 2005; Lagutchev et al., 2006].
SFG also has clear advantages compared with IRAS [Kunimatsu et al., 1985a, b].
In IRAS, the signal is proportional to the IR intensity exiting the optical cell, after it
has double-passed the electrolyte layer, whereas in SFG, the signal is proportional to
the product of IR times visible at the electrode. The IR has only to single-pass the elec-
trolyte layer, and the visible is barely attenuated by the electrolyte (Figs. 12.1 and
12.2). Although the effects of electrolyte IR absorption can be mitigated in both tech-
niques by increasing the IR intensity, SFG has advantages in this regard as well. IRAS
uses a thermal IR source whose brightness is limited by the meltdown temperature of
the blackbody. Since the source is continuous, high intensities will result in high temp-
eratures in the electrolyte and on the electrode surface. In SFG, the IR is generated by a
laser, and although the optical physics are rather complicated, the IR pulse energy is
approximately linear in the pump laser energy and is thereby limited mostly by how
much one chooses to spend on the pump laser. Since the IR pulses are a low
duty cycle pulsed source, where the laser is typically “on” for a short period every
1 ms, even with high intensity pulses the steady-state rises in electrolyte and surface
temperatures are minimal.
In the earliest SFG experiments [Tadjeddine, 2000; Guyot-Sionnest et al., 1987;
Hunt et al., 1987; Zhu et al., 1987], a first-generation data acquisition method was
used, and, because of the limited signal-to-noise ratios, IR attenuation by the elec-
trolyte solution was a substantial handicap. So, in earlier SFG studies, as in IRAS
studies, measurements were performed with the electrode pressed directly against
the optical window [Baldelli et al., 1999; Dederichs et al., 2000]. With the in-contact
geometry, the electrolyte was a thin film of uncertain and variable depth, probably of
the order of 1 mm. However, the thin nonuniform electrolyte layers can strongly distort
the potential/coverage relationship and hinder the ability to study fast kinetics.
In our first BB-SFG experiments, we used a second-generation SFG spectrometer
based on the scheme developed by Richter et al. [1998]. More recently, we have devel-
oped a compact new apparatus that features greatly enhanced user functionality and
improved signal-to-noise ratios (the third-generation SFG). Since the original appar-
atus has been described in detail previously [Lu et al., 2005; Lagutchev et al., 2005,
2006], Section 12.2 below will focus on the new apparatus. The combination of
BB-SFG with a TLE cell having a known controlled electrolyte gap might seem, at
first glance, to be a mere technical improvement, but in actuality it represents a
major advance [Guyot-Sionnest, 2005], as demonstrated in our recent publications
[Lu et al., 2005; Lagutchev et al., 2006]. The SFG apparatus can obtain spectra in
200 ms or less, with a signal-to-noise ratio . 100 : 1. The thin electrolyte film and
rapid spectral acquisition rate minimize the possibilities for chemical contamination.
They also enable a variety of investigations involving measurements of chemical
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