Corrosion mechanisms of TiO2 photoelectrode coatings in alkaline conditions

The crystal structure of TiO₂ has a significant impact on the chemical stability of the protective TiO₂ thin films used in photoelectrochemical conditions. By altering the deposition temperature of the atomic layer deposition method, and by post-deposition annealing treatments, amorphous, microcrystalline anatase, and Ti³⁺-rich nanocrystalline rutile structures can be achieved. In this paper, the chemical stability in alkaline solution and failure mechanisms of ALD grown TiO₂ thin films on Si(100) were studied by SEM, XPS, EIS, and ellipsometry. The results showed that the electrically conductive Ti³⁺-rich nanocrystalline rutile thin film was chemically stable, whereas other samples failed within the first 10 h of the test in 1.0 M NaOH. More detailed analysis in 0.1 M NaOH revealed that the anatase sample experienced sudden failure after NaOH solution penetrated the TiO₂ via grain boundaries, causing the Si substrate to dissolve. In contrast, the amorphous TiO₂ films had more gradual failure as the NaOH solution permeated the TiO₂ film, causing it to swell up to three times the initial thickness of the film.