Sum frequency generation (SFG) spectroscopic techniques are used to investigate the molecular orientation of adsorbed acetonitrile on rutile TiO2 (110) at the solid-vapor interface. Generally, most molecular orientation analyses using SFG have been performed on dielectric substrates, to avoid the spectral interference between resonant and the near-resonant background signal. Although rutile crystal can be treated as a dielectric substrate, its electronic state contributes to the intensity and interferes with the resonant signal when the SFG frequency is close to its band gap energy. In addition, the rutile crystal is a uniaxial birefringent material, and the (110) surface is anisotropic, which further complicates the spectral analysis. In this study, various SFG measurement techniques were applied, and quantitative analytical methods were established to interpret the surface orientation of an adsorbed molecule. SFG vibrational spectra of acetonitrile on rutile TiO2 (110) surface have been measured using distinct polarization combinations, polarization mapping, and null angle method. By varying the polarization combinations of SFG, the magnitude and shape of the spectra undergo substantial change, which originate from the interference between the near-resonant signal from the rutile substrate and the resonance signal from the acetonitrile. Theory, simulation, and analytical methods for obtaining quantitative orientation information of a molecule on an anisotropic semiconductor substrate in the presence of a near-resonant signal are presented.

A procedure for wet chemical preparation of TiO 2 single crystal surfaces is detailed. The potential of this procedure is demonstrated through application to rutile-TiO 2 (110) and rutile-TiO 2 (011) substrates. Characterization with atomic force microscopy, low energy electron diffraction, auger electron spectroscopy, and vibrational sum frequency spectroscopy indicate that flat, well-ordered, carbon-free surfaces can be generated. Notably, in contrast to the (2x1) low energy electron diffraction pattern observed for TiO 2 (011) prepared in ultrahigh vacuum, wet chemical preparation results in a (4x1) unit cell; wet chemically prepared TiO 2 (110) displays an unreconstructed (1x1) surface.