It is well known that some metals can oxidise, corrode and form rust when exposed to air, degrading the material. But scientists have found that oxidation could actually be beneficial for metals – at the nanometre scale.
The team in mainland China, Hong Kong and Taiwan developed an oxidised nanomaterial they say is superelastic.
That means the metal can recover from deformation, and according to the researchers it could be used to kill bacteria and viruses, to protect relics, or even for wearable medical devices.
“We experimentally demonstrate that metallic glass nanotubes are superelastic at room temperature, which outperform various superelastic metals and alloys known to date,” they wrote in a paper published in the peer-reviewed journal Nature Materials in December.
“This unique property of the metallic glass nanostructures is useful, which could find many applications in future nanodevices working in harsh environments, such as sensors, medical devices, micro- or nanorobots, springs and actuators.”
The study – a years-long collaboration – was carried out by a team from City University of Hong Kong, the Chinese Academy of Sciences, Beijing Computational Science Research Centre, National Taiwan University of Science and Technology, and Shenzhen University.
Many metals react with oxygen, with silver, platinum and gold being some of the exceptions. For example, iron oxide, commonly known as rust, is produced when iron comes into contact with oxygen and water. So keeping storage areas dry is one way to prevent iron or steel tools from rusting.
Lead author Yang Yong, a professor with CityU’s mechanical engineering department, said the team’s discovery goes against the conventional belief that oxidation only harms metal.
“When metals are made into a form that is dozens of nanometres thick, oxidation triggers the formation of a percolating oxide network which brings superelasticity,” Yang said.
A nanometre is a billionth of a metre. For perspective, human hair is around 80,000 to 100,000 nanometres wide, while the diameter of the Sars-CoV-2 virus is about 80 nanometres.
Yang said there could be applications for the oxidised nanomaterial that make use of different metal properties – such as taking advantage of the antimicrobial properties of copper oxide to kill bacteria or viruses.
“Metals at the nanoscale are more effective in killing germs than at bigger sizes because the nanoparticles are tiny enough to be consumed,” he said.
The material could also be turned into a protective film that could be applied to relics to prevent them from being exposed to air, according to Yang.
He said the material was very stable since it had already been oxidised and an adhesive agent would not be needed to keep the film on a surface. Its oxidation process could be controlled to determine the colour.
“Nanocoppers can come out purplish red or green, just like the Statue of Liberty in the United States,” he said. The statue in New York is made from copper and has turned green over the years because of oxidation.
For medical devices, Yang said the thin material could be applied to the skin, teeth or brain, to assist during surgery or for research purposes.
He said the cost of producing the nanomaterial was low because of the minimal use of raw materials.