I’ve mentioned numerous times that UV light is a double-edged sword, capable of doing both good (UV curing) or bad (UV degradation). A special subset of UV reactions involves photocatalysis, in which the reactions occur on a solid surface exposed to the light. I’ve run into these reactions a couple of times here at my current employment: as a “self-cleaning” window coating and as a source of unintended but extreme UV degradation of a polymer (a white pigment caused absolute havoc in an olefin polymer). I also drive on a regular basis past a “sculpture” that has such a coating:
The curved surfaces are sculptures on both ends of the new 35W bridge. The surfaces of the sculptures are coated with TiO2 which reduces pollution in the immediate vicinity. Not enough to do any real good of course.
I’m not going to attempt to explain the reaction mechanism of photocatalysis – it involves a good deal of solid state physics (something I have no formal training in) and is an area still being actively studied. Inconsistent results are sign (in any field for that matter) that everything is still not understood or in control.
This is all prelude to a report that came out a few months ago about a new photocatalytic surface that not only uses visible light (that in itself is not new) but that the surface is capable of still running after the light is removed. TiO2 normally need UV light to activate it, but when doped with nitrogen, visible light works just fine. In this case, they also added palladium oxide nanoparticles to the surface. The nanoparticles appear to work as a trap for the activated electrons which are then released in the dark.
The curious part of this entry is that all of these are degradation reactions – I’ve yet to see a photocatalytic reaction that is used in a curing or other building reaction. Just degradation. I led into this entry with the idea that UV light can be both productive and destructive, but I’m not so sure that this is the case with photocatalysis.