Corrosion mechanisms of TiO₂ photoelectrode coatings in alkaline conditions

Abstract

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.