3 research outputs found
Electrochemical ATR-SEIRAS Using Low-Cost, Micromachined Si Wafers
Thin, micromachined
Si wafers, designed as internal reflection
elements (IREs) for attenuated total reflectance infrared spectroscopy,
are adapted to serve as substrates for electrochemical ATR surface
enhanced infrared absorption spectroscopy (ATR-SEIRAS). The 500 μm
thick wafer IREs with groove angles of 35° are significantly
more transparent at long mid-IR wavelengths as compared to conventional
large Si hemisphere IREs. The appeal of greater transparency is mitigated
by smaller optical throughput at larger grazing angles and steeper
angles of incidence at the reflecting plane that reduce the enhancement
factor. Through use of the potential dependent adsorption of 4-methoxypyridine
(MOP) as a test system, the microgroove IRE is shown to provide relatively
strong electrochemical ATR-SEIRAS responses when the angle of incident
radiation is between 50 and 55°, corresponding to refracted angles
through the crystal of ∼40°. The higher than expected
enhancement is attributed to attenuation of the reflection loss of
p-polarized light and multiple reflections within the wafer-based
IRE. The micromachined IREs are shown to outperform a 25 mm radius
hemisphere in terms of S/N at wavenumbers less than ca. 1400 cm<sup>–1</sup> despite the weaker signal enhancement derived from
the steeper angle incident on the IRE/sample interface. The high optical
transparency of the new IREs allows the spectral observation of displaced
water libration bands at ca. 730 cm<sup>–1</sup> upon solvent
replacement by adsorbed MOP. The results are highly encouraging for
the further development of low-cost, Si wafer-based IREs for electrochemical
ATR-SEIRAS applications
Spatial Mapping of Methanol Oxidation Activity on a Monolithic Variable-Composition PtNi Alloy Using Synchrotron Infrared Microspectroscopy
The
use of synchrotron-sourced infrared radiation to map the electrochemical
activity of a binary metal (Pt and Ni) alloy is demonstrated. The
alloy is created in such a way that its metal concentration varies
along one of its dimensions thus creating a continuum of electrocatalyst
compositions on a single electrode. Localized methanol oxidation activity
is determined spectroscopically by measuring the rate of CO<sub>2</sub> production at variable positions along the alloy concentration gradient
using an infrared microscope. Numerical simulations of the kinetically
controlled reaction demonstrate that qualitative assessment of relative
reaction rates is possible as long as the reaction is followed on
time scales smaller than those that lead to diffusional broadening.
Characterization of the alloy before and after electrochemical experiments
reveals significant levels of base metal leaching. Highly dealloyed
regions of the sample show the highest rates of methanol activity
and have a final Ni atomic composition of approximately 5%. Surface
roughening from the dealloying process is shown to be at least partially
responsible for enhanced activity
Femtomole Infrared Spectroscopy at the Electrified Metal–Solution Interface
Characterization
of surface adsorbed species using infrared (IR)
spectroscopy provides valuable information concerning interfacial
chemical and physical processes. However, <i>in situ</i> infrared studies of surface areas approaching the IR diffraction
limit, such as micrometer scale electrodes, require a hitherto unrealized
means to obtain high signal-to-noise (S/N) spectra from femtomole
quantities of adsorbed molecules. A major methodological breakthrough
is described that couples the high brilliance of synchrotron-sourced
infrared microscopy with attenuated total reflection surface enhanced
infrared spectroscopy (ATR-SEIRAS). The method is shown to allow the
spectral measurement of a monolayer of 4-methoxypyridine (MOP) adsorbed
on a surface enhancing gold film electrode under fully operational
electrochemistry conditions. A factor of 15 noise improvement is achieved
with small apertures using synchrotron IR relative to a thermal IR
source. The very low noise levels allow the measurement of high quality
IR spectra of 2.5 fmol of molecules confined to a 125 μm<sup>2</sup> beam spot