"Most studies so far either work on the use of ultraviolet light or involve a suspension of nanoparticles," revealed Prof. Joydeep Dutta, director of the Center for Excellence in Nanotechnology at the Thailand-based Asian Institute of Technology (AIT). The AIT research group has dispensed with both. Instead of using a suspension of nanoparticles, which have to be removed later after the water purification process, or relying on UV light, the group demonstrated a system featuring visible light and ZnO nanorods. "The key concept was to incorporate deliberate defects in ZnO nanorods by creating oxygen vacancies and interstitials, which then allows visible light absorption," he explained.

Environmentally friendly approach

Such ZnO nanorods grown on glass were tested on Escherichia coli and Bacillus subtilis bacteria, which are commonly used as model microbes. In the dark, ZnO dissolves slowly releasing zinc ions, which have anti bacterial properties, as it penetrates the bacterial cell envelope thereby thwarting the growth of microbes. Under well lit conditions, the effect is doubled with both photocatalysis and zinc ions playing their part in killing the microbes.

The implications of these experiments are enormous. "Since ZnO has now been tested under solar light, instead of the traditionally used UV light, the potential for commercial applications is huge, particularly since the levels of zinc ions removed from the rods to the water are safe for human consumption," added Dutta.

The team, which also includes Dr Oleg V Shipin, Ajaya Sapkota, Dr Alfredo J Anceno, Sunandan Baruah and Mayuree Jaisai, is continuing its work on photocatalysis for use in water decontamination.

More details can be found in the journal Nanotechnology.