Thin-film specialists seek to hone self-cleaning surfaces

A team at University College London (UCL) has devised a method to find the best combination of dopants to achieve certain desired properties.

Thin films of titanium dioxide (TiO₂) are already used in so-called self-cleaning windows. As wide-band semiconductors, these films are excited by sunlight leading to charge separation and the generation of radicals, which kill bacteria and degrade of organic molecules. Furthermore, this photocatalytic process also makes the glass hydroxylated and therefore superhydrophilic, so water spreads evenly on the surfaces to wash away dirt.

Some 10 million square metres of glass is coated with TiO₂ each year, with a worldwide market worth around £50bn. However, there are further inroads to be made as Prof Ivan Parkin of UCL explained to The Engineer.

‘The key issue in the area is that because it’s a wide band-gap semiconductor, titanium dioxide really only functions with the UV portion of sunlight and so the big challenge and where people are trying to move now is to get these films to work efficiently with room lighting conditions, where there is little UV.’

This would open up a much larger market in things such as self-cleaning kitchen units and surfaces inside hospitals and others industrial buildings, he said.

The band gap of regular TiO₂ is around 3–3.2EV, which means it responds to wavelengths of 388–400nm — basically UV. The aim is to push that down to around 2.8EV to start accessing the visible spectrum without compromising other properties such as the superhydrophilicity.

This is done by introducing a dopant and observing the properties, usually in a trial-and-error fashion. Parkin’s team developed a high-throughput screening method using atmospheric pressure chemical vapour deposition (APCVD) where one side is heavily doped and the other side not doped, with grading between.

‘What that means is instead of doing 800 separate experiments we can do one experiment to find where the sweet spot or goldilocks zone is in terms of maximum activity and make that composition over the whole film, and then we can do the full range of testing on that,’ Parkin said.

As well as self-cleaning surfaces, the technology could be used for an anti-microbial catheter, which incorporates the cleaning surfaces and a light source to prevent catheter-acquired infections, a frequent problem in hospitals.