Adsorption Structure and Coverage-Dependent Orientation Analysis of Sub-Monolayer Acetonitrile on TiO2(110)

Abstract

The adsorption structure and orientation of acetonitrile on TiO2(110) have been investigated by temperature-programmed desorption (TPD) and high-resolution broadband sum frequency generation vibrational spectroscopy (HR-BB-SFG-VS) in combination with ab initio molecular dynamics (AIMD) simulation. Sub-monolayer, monolayer, and multilayer states of acetonitrile on TiO2(110) have been unambiguously distinguished in the TPD spectra. The in situ HR-BB-SFG vibrational spectra of acetonitrile on TiO2(110) at 100 K not only show the symmetric stretching mode of the methyl group and the nitrile group, similar to those of vapor acetonitrile on oxide surfaces, but also present an intense antisymmetric stretching mode of the methyl group as well as the sum resonance mode between symmetric C–C stretching and symmetric CH3 deformation. Besides, two adsorption forms for acetonitrile on TiO2(110), including interactions with five-coordinated titanium (Ti5c) and bridge-bonded oxygen (Obr) vacancies, have also been resolved and identified in our SFG vibrational spectra. By the combination of SFG polarization analyses and AIMD simulations, we further found that the tilt angle of CH3CN on TiO2(110) decreases as the coverage increases from sub-monolayer to monolayer. Considering the down shifts of desorption temperature, vibrational frequency, and adsorption energy for acetonitrile on TiO2(110) at higher coverage, we propose that the intermolecular repulsion plays a major role in the coverage-dependent adsorption configuration. Our results not only provide a detailed insight into the adsorption states of acetonitrile on TiO2(110) at a low temperature but also demonstrates the capabilities of high-resolution SFG-VS for investigating the complicated structure and orientation of adsorbates on single-crystal metal oxides.