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 3000 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, tentatively attributed to (dissociated) 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.