Viljami Pore, Mikko Ritala and co-workers discuss their hot paper.

Could you explain the significance of your article to the non-specialist? And what has motivated you to conduct this work?
Photocatalytic thin films can break down organic dirt, kill bacteria and destroy harmful molecules in water and air by using light as the energy source. Such films can be used in self-cleaning windows, as sterilizing and deodorizing surfaces and in water and air purification systems. In this work, we have succeeded in the preparation of highly efficient photocatalytic TiO₂ films by using a modified atomic layer deposition (ALD) process. With ALD, objects of any shape or porosity can be easily coated with a uniform thin film, which is a very difficult if not impossible task for all the other conventional thin film deposition methods.

What has motivated you to conduct this work?
The work was a part of project: 'Clean Surfaces 2002-2006' funded by the Finnish Funding Agency for Technology and Innovation, TEKES, Finland . The main objective of the program was to gain deeper knowledge of fouling phenomena and clean surfaces through multidisciplinary research, and photocatalysis had an important role. Our task (at Laboratory of Inorganic Chemistry, University of Helsinki) was to study the ALD of photocatalytic thin films. ALD is currently a very hot topic in the semiconductor research and industry, and through the increase of awareness of the benefits of the method, other application areas are likely to adapt ALD as well. Photocatalysis is possibly one of these areas and our new modified TiO2 process is worth considering if highly active photocatalytic thin films by ALD are desired.

Where do you see this work developing in the future?
Optimization and scale-up are the next steps if this process is chosen for further development in a specific application. To improve the photocatalytic activity of the films even further, metallization with noble metals such as silver or platinum could be studied. Adding silver or copper to improve the antimicrobial properties of the films would also be interesting. In addition, the photoelectrochemical properties of the films could be studied in detail to evaluate their performance as an electrode in a photoelectrochemical cell. In fact, preliminary studies conducted in our laboratory have shown that quite high photocurrents can be achieved with these films. The films could thus be good candidates for photoelectrochemical splitting of water to produce hydrogen.

Are there any particular challenges facing future research in this area?
Challenges are focused in discovering the right application areas where ALD would be the best solution. ALD is a relatively slow method, and thereby more expensive than many of its competitors. In general, ALD should be applied only if the cheaper/faster methods fail or there is a significant improvement in the properties of the films. One such application could be the coating of porous, high surface area substrates for photocatalytic purification of air or water. Another challenge is the introduction of ALD and its capabilities to the photocatalysis community so that the special characteristics of ALD would be fully taken into account in novel system designs that have not been possible without ALD.