Origin of Instability of Titanicone Grown by Molecular Layer Deposition Using TiCl₄ and Ethylene Glycol

Titanicone, obtained through molecular layer deposition (MLD) using TiCl₄ and ethylene glycol (EG), is often regarded as a thin film of titanium ethylene glycolate [Ti(OCH₂CH₂O)₂]. Nevertheless, titanicone exhibits a distinct vulnerability to moisture, while single crystals of Ti(OCH₂CH₂O)₂ remain stable, even in the presence of water. To elucidate the origin of instability, we investigated the pathway of chemical degradation for titanicone using in situ and ex situ analytical methods such as Fourier transform infrared spectrometry, quartz crystal microbalance, quadrupole mass spectrometry, and X-ray photoelectron spectroscopy. Our analyses unveiled that the instability of the MLD-grown titanicone film in the presence of water can be primarily attributed to the high chlorine content present as Ti–Cl species and the coexistence of single-reacted EG species as well as double-reacted EG species, unlike in Ti(OCH₂CH₂O)₂ crystals. Water molecules react with the Ti–Cl species, leading to the formation of Ti–OH species and the release of HCl gas. Furthermore, the single-reacted EG species undergo an intramolecular cyclization reaction catalyzed by HCl, resulting in the formation of Ti–OH and the liberation of ethylene oxide. Consequently, when exposed to water, the MLD-grown titanicone turns into a water-stable mixed film composed of TiO₂ and Ti(OCH₂CH₂O)₂.