Probing the Structure of D2O Ice Layers on ALD-grown ZrO2, Al2O3 and TiO2 Thin Films by Sum Frequency Generation (SFG) Spectroscopy

Abstract

Sum frequency generation (SFG) spectroscopy was applied to investigate D₂O adsorption on atomic layer deposition (ALD)–grown Al₂O₃, ZrO₂, and TiO₂ films at 94 ± 1 K. Film composition and thickness were characterized by ellipsometry and X-ray photoelectron spectroscopy (XPS). Additional SFG measurements were conducted on the SiO2/Si wafer and on a CoO film prepared by oxidizing Co foil. At D₂O exposure below 3 000 L, the spectra were dominated by interfacial features originating from the ice-oxide interface. These spectra exhibited a weak, broad O–D stretching band (OD₃) centered at 2650 cm⁻¹, attributed to water molecules hydrogen-bonded to the oxide surface; this assignment was supported by the absence of the OD3 feature on the SiO2/Si substrate. A sharp peak at 2730 cm⁻¹ was also observed and assigned to the “free” O–D stretch (non-hydrogen-bonded with any neighboring molecule) of surface D2O molecules pointing into the vapor phase. Upon increasing D2O exposure, both the OD3 and “free” OD bands decreased in intensity and were replaced by weakly hydrogen-bonded OD₂ and strongly hydrogen-bonded OD₁ modes associated with the ice-vapor interface. As the exposure increased further, the OD₂ and OD₁ bands shifted to lower wavenumbers (2310 to 2284 cm-1) and became stronger, with the OD1 mode exhibiting a larger red shift and more pronounced intensity enhancement. No significant differences in water structure were observed on the Al₂O₃, ZrO₂, and CoO films at the ice-vapor interfaces, apart from an approximately fivefold reduction in intensity on CoO, which is attributed to signal scattering from the rough CoO film/Co foil surface. However, when D2O exposure reached ≥30 000 L, the OD1 band on the TiO2 surfaces decreased substantially in intensity and shifted to much lower wavenumbers (2065 cm-1 at 30 000 L; 2030 cm-1 at 102 000 L) than on Al2O3 (2283 cm-1 at 90 000 L), ZrO2 (2293 cm-1 at 30 000 L), and CoO (2284 cm-1 at 900 000 L), indicating specific hydrogen-bonding interactions on the TiO₂ surface.