Modifications in the vibrational properties of a single microparticle of LiMn2O4 induced by extraction and
subsequent injection of Li+ into the lattice have been monitored in situ via simultaneous acquisition of Raman
scattering spectra and cyclic voltammetry data in 1 M LiClO4 solutions in ethylene carbonate (EC):diethyl
carbonate (DEC) mixtures (1:1 by volume). Statistical analyses of the spectra in the range 15 < SOD <
45%, where SOD represents the state of discharge (in percent) of the nominally fully charged material, i.e.,
λ-MnO2, were found to be consistent with the coexistence of two distinct phases of lithiated metal oxide in
agreement with information derived from in situ X-ray diffraction (XRD) measurements involving more
conventional battery-type electrodes (J. Electrochem. Soc.
2002, 149, A1164).
Highly detailed Raman spectra from a single KS-44 graphite microflake electrode as a function of the applied potential have been collected in situ using a Raman microscope and a sealed spectroelectrochemical cell isolated from the laboratory environment.
A method is herein described for performing simultaneous in situ normal incidence reflectance spectroscopy (DeltaR/R, lambda = 633 nm) and probe beam deflection (PBD) measurements at solid electrodes in aqueous electrolytes, while scanning the potential linearly between two prescribed limits. Results obtained for Au in 0.1 M HClO4 and for Pt in both 0.1 M HClO4 and 0.1 M NaOH were found to be in excellent agreement with those reported in the literature for each individual spectroelectrochemical technique under otherwise similar conditions. Data collected for Pt electrodes in CO-saturated 0.1 M HClO4 revealed rather sudden changes in both DeltaR/R and PBD signals in the voltammetric region where the characteristic sharp peak associated with the oxidation of adsorbed CO occurs. This behavior was ascribed, respectively, to oxide formation (DeltaR/R) and to changes in the electrolyte composition in region neighboring the electrode, involving predominantly the acid concentration (PBD). In contrast, CO oxidation on Pt in 0.1 M NaOH yielded a PBD response consistent with formation of solution-phase carbonate via the reaction of the product, CO2, with hydroxyl ion. The exquisite sensitivity of DeltaR/R and PBD to interfacial phenomena was further illustrated using a monolayer of hemin irreversibly adsorbed on glassy carbon surfaces in 0.1 M Na2B4O7 (pH approximately 9.2). For this system, DeltaR/R was found to be proportional to the relative fractions of hemin and its reduced counterpart, while the PBD signal could be correlated with corresponding variations in the electrolyte concentration induced by the surface-bound redox process.
Modifications in the electrochemical and spectral properties of hemin (Hm) adsorbed on roughened Ag
electrodes in an aqueous electrolyte (phosphate buffer, pH 3) induced by brief exposure to mildly acidic
solutions containing NO have been examined by in situ surface-enhanced Raman scattering (SERS) using Q
band excitation (λexc = 532 nm). Two lines of evidence support the formation of the adsorbed nitrosyl adduct
of Hm (NO−Hm) under the conditions employed for these experiments: the complete disappearance of the
characteristic voltammetric redox peaks of Hm centered at ca. −0.25 V versus SCE, and a significant drop
in the intensity, I, of the ring-based ν30 mode at 1165 cm-1 in the in situ SERS spectrum compared to that
of adsorbed Hm under the same conditions. Such a decrease in the intensity of the I(ν30) mode was also
found in the solution phase Raman spectrum of Hm in THF and DMF upon NO exposure. Notably absent
from the SERS spectrum were features attributable to vibrational modes due to NO and Fe−NO, which are
clearly discernible for NO iron macrocyclic adducts in solution phase using Soret band excitation (λexc = 413
and 406 nm). Scanning to sufficiently negative potentials led to the reemergence of the Hm redox peaks and
restored the I(ν30) mode in the SERS spectrum recorded at potentials that were positive enough for the reduced
form of Hm to undergo full oxidation. These observations clearly indicate that, to the level of sensitivity of
these measurements, the formation and subsequent reduction of the adduct can be effected without
compromising the integrity of Hm. Careful inspection of the data showed that the SERS spectrum of the
NO−Hm adduct resembles that of adsorbed (axially uncoordinated) Hm, rather than its reduced counterpart,
measured under otherwise identical conditions, suggesting that the electronic environment within the ring in
the NO−hemin adduct and the (NO-free) oxidized Hm are similar. This behavior is in agreement with
information derived from Mossbauer effect spectra for NO−Fe macrocyclic adducts.
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