Vibrational Sum Frequency Spectroscopy Study of Methanol Adsorption on Thin Film TiO₂ at Ambient Pressure and Temperature

Abstract

Surface-sensitive vibrational sum-frequency spectroscopy (vSFS) has been utilized to study the adsorption behavior of methanol on anatase TiO2 thin films under ambient temperature and pressure condition. The vSF spectra in the C–H region establish the presence of both molecular methanol and dissociated methoxy adsorbates. Spectra for the isotopologues of methanol also support the deprotonatation process at the surface. Furthermore, by a systematic study of the vSF spectra of methanol isotopologues in the C–H and C–D stretching regions, we clarify that in the CH spectral region the molecular signal coherently interferes with the near-resonant one from hydrogen bonded OH. The latter is due to adsorption from the water vapor present in the background and its interaction with surface OH.

Vibrational Sum Frequency Spectroscopy Study of Alcohol Adsorption on Thin-Film TiO2 at Ambient Pressure and Temperature

“…Figure presents the vSF spectra obtained for TiO 2 exposed to methanol utilizing all three useful polarization combinations. The spectra have been discussed in detail in a previous publication . In concordance with the earlier work, − the vSF spectra in the C–H region establish the presence of both molecular methanol and its dissociation product methoxy as adsorbates.…”

“…In previous work, we investigated the interaction of methanol with anatase TiO 2 thin films under ambient temperature and pressure conditions using vibrational sum frequency spectroscopy (vSFS) . The results confirmed the general picture reported in earlier studies.…”

“…The deprotonation of a part of the adsorbed methanol is mediated by surface defects which are naturally present in large abundance at thin-film TiO 2 surfaces. The red shift of the MeOx lines with respect to the MeOH ones by 40 cm –1 is expected as removing the proton from the −OH group reduces the overall electron density around the CH 3 group . The lines in the ppp spectrum clearly exhibit dispersive shapes due to the presence of a strong non-resonant background, the origin of which is discussed in detail in our earlier report .…”

“…The red shift of the MeOx lines with respect to the MeOH ones by 40 cm –1 is expected as removing the proton from the −OH group reduces the overall electron density around the CH 3 group . The lines in the ppp spectrum clearly exhibit dispersive shapes due to the presence of a strong non-resonant background, the origin of which is discussed in detail in our earlier report . Briefly, coadsorbed H 2 O from the water vapor, which is present to some extent under near-ambient conditions, forms hydrogen bonds at the hydroxylated surface.…”

“…The detailed experimental setup is described elsewhere . Briefly, 150 nm TiO 2 thin films were prepared in the local optical workshop by evaporating Ti 2 O 3 on glass slides.…”