Improved Stability of Atomic Layer Deposited Amorphous TiO2 Photoelectrode Coatings by Thermally Induced Oxygen Defects

• Published in: Chemistry of materials 2018-02, Vol.30 (4), p.1199-1208
• Main Authors: Hannula, Markku, Ali-Löytty, Harri, Lahtonen, Kimmo, Sarlin, Essi, Saari, Jesse, Valden, Mika
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.7b02938

Amorphous titanium dioxide (a-TiO2) combined with an electrocatalyst has shown to be a promising coating for stabilizing traditional semiconductor materials used in artificial photosynthesis for efficient photoelectrochemical solar-to-fuel energy conversion. In this study we report a detailed analysis of two methods of modifying an undoped thin film of atomic layer deposited (ALD) a-TiO2 without an electrocatalyst to affect its performance in water splitting reaction as a protective photoelectrode coating. The methods are high-temperature annealing in ultrahigh vacuum and atomic hydrogen exposure. A key feature in both methods is that they preserve the amorphous structure of the film. Special attention is paid to the changes in the molecular and electronic structure of a-TiO2 induced by these treatments. On the basis of the photoelectrochemical results, the a-TiO2 is susceptible to photocorrosion but significant improvement in stability is achieved after heat treatment in vacuum at temperatures above 500 °C. On the other hand, the hydrogen treatment does not increase the stability despite the ostensibly similar reduction of a-TiO2. The surface analysis allows us to interpret the improved stability to the thermally induced formation of O– species within a-TiO2 that are essentially electronic defects in the anionic framework.