Raman Spectroscopy
Raman spectroscopy measures inelastic scattering of laser light by vibrational modes, so band positions report on local bonding (metal-oxygen stretching and bending in oxides) and can distinguish crystalline phases. Because water scatters weakly, it can be run in situ in aqueous electrolyte while a catalyst is under potential control, which makes it a common probe of the working structure of oxide electrocatalysts. An empirical relation due to Hardcastle and Wachs links lower metal-oxygen Raman frequencies to longer bonds, which allows small band shifts to be read as small bond-length changes. It complements X-ray absorption spectroscopy, which probes element-specific oxidation state and coordination. See also Oxygen evolution reaction.
Phase identification in Ni-Fe oxide OER catalysts (2012)
Landon and coworkers used ex situ Raman (442 nm excitation), together with XRD, to show that Fe added to NiO formed a NiFe2O4 spinel phase already at 5 mol % Fe, with no separate iron oxide bands up to 20 mol % Fe (Landon et al. 2012). Summary: landon2012.
Vibrational probes of ion pairing in an ionic liquid (2013)
For [Emim][TCB], scaled B3LYP harmonic frequencies matched the measured Raman and ATR-IR bands with a correlation coefficient of 0.9998, allowing a full mode assignment (Mao et al. 2013). The anion CN stretch (2225 cm-1, the strongest Raman band) lies below that of alkali and ammonium tetracyanoborates and the out-of-plane CH bend is red-shifted by 11 cm-1 relative to [Emim][BF4], both used as markers of weaker cation-anion interaction, while Raman bands near 598, 622 and 704 cm-1 reveal coexisting planar and non-planar cation conformers in the liquid. Summary: mao2013.
Quantifying small band shifts in NiOOH during OER (2015)
Michael and coworkers recorded in situ Raman spectra of NiOOH thin films during linear sweep voltammetry in LiOH and CsOH and fit them to sums of Gaussians so that mean positions of the O-Ni-O bending (near 480 cm-1) and stretching (near 560 cm-1) bands could be compared to a few tenths of a cm-1. CsOH lowered the bands by roughly 1.5 and 5 cm-1 relative to LiOH, interpreted as slightly longer Ni-O bonds that correlated with higher OER activity, while Fe impurities lowered only the bending band by about 1 cm-1. The authors caution that Raman alone could not assign the γ- versus β-NiOOH phase, and they published the fitting code (Michael et al. 2015). Summary: michael2015.