A new method for cleaning tarnished metals using a UV-activated photocatalyst ink has been developed by scientists in the UK. The simple UV-activated, TiO2-based film or ink can remove thin oxide or sulfide layers from metal surfaces by reductive photocatalysis, allowing for corrosion to be removed more easily and cheaply than current approaches, and which could offer a range of applications for industry.
The study, which was published in Chemical Communications [Mills, A., Hazafy, D., Chem. Commun., (2012) DOI: 10.1039/C1CC15774D], showed how to clean metal surfaces with an ink or coating merely by turning a light on and without the need for the usual aggressive and highly reactive chemical acids and chelating agents.
The formation of an oxide layer on the surface of metals is a well known reaction. The problem is, that as the metal oxides get thicker, contamination from unwanted corrosive products can occur. This new research applied a photocatalyst to the corroded metal, before a UV light was shone onto it, producing conduction band electrons and valence band holes. There are electron donor species within the ink that reacts with the holes, so that the photogenerated electrons react with absorbed metal ions. This allowed the layer of metal oxide layer to be removed using water.
Although such light-absorbing species are commonly dyes, the team preferred semiconductor materials, such as titania, as photosensitisers, and they are relatively inexpensive and sturdy, and also extremely versatile. Such semiconductor materials can act as a driver of solar to chemical energy conversion reactions, such as the reduction of water to hydrogen using a sacrificial electron donor (SED).
When they examined this reaction using titania films on stainless steel, the researchers found that stains could be removed by the overlying titania film. This led them to realise titania (along with most other semiconductor photocatalysts) have the potential to remove this film as long as a SED was also present. This breakthrough led them to the idea of a photo-driven, tarnish-removing ink.
It is hoped that the photo-induced reduction reaction combined with the concomitant oxidation of a SED will have commercial possibilities, and that more interest in semiconductor-sensitised photoreduction reactions will follow from the findings. As co-author Andrew Mills, from Queen's University Belfast, points out, “These reactions underpin the growing commercial applications of semiconductor photocatalysis, such as: self-cleaning glass, tiles, concrete, fabrics and paint; photo-disinfection and photo-induced antifogging activity.”
The next step will be to improve the reaction by increasing its speed and to use visible light if possible, as well as by investigating its efficacy for a range of other metal tarnishes.