An efficient titania photocatalyst that continues to work when the lights go out gives continuous protection against bacteria, say scientists in the US.  

Photocatalysts create electron-hole pairs which generate free radicals able to undergo secondary reactions such as killing bacteria. Most current visible-light photocatalytic disinfection materials are based on anion-doped titanium oxide, explains Jian-Ku Shang at the University of Illinois at Urbana-Champaign. But they only operate in the light when electron and hole pairs are produced. 
 
Shang and colleagues added palladium oxide PdO nanoparticles to nitrogen-doped titania. The nanoparticles act as a photoelectron trap and under visible light illumination, electrons flow from the titania to the PdO nanoparticles. When the light is switched off a catalytic memory effect makes the PdO nanoparticles release the electrons back slowly. The phocatalyst continues to operate in the dark, limiting the growth of bacteria for up to 24 hours.

As well as operating in the dark, Shang says that the photocatalyst is also more efficient than before in the light as the electrons stored in the PdO nanoparticles prevent recombination with holes, allowing them to be more effective in photocatalytic oxidation of bacteria. 

'This is very significant progress in the area,' says Max Lu an expert in functional nanomaterials at the University of Queensland, Brisbane, Australia. 'It has been a challenge to develop effective visible light photocatalysts with nitrogen-doped titania, as the doping always creates recombination centres for the electrons and holes. But the most interesting thing is the memory effect when the electrons are released in the dark,' he adds. 

Shang says that he hopes to study the charge transfer process in more detail, so that he can optimise the efficiency of the individual charge transfer steps. He indicates that the solar driven materials could have many environmental antimicrobial systems. In particular, he says that it could be a cheaper and more environmentally-friendly alternative to water disinfection by chlorination, which produces harmful disinfection by-products. 

Fay Nolan-Neylan 

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