April 28, 2010 – Researchers from the US and Europe say they have created devices with carbon nanotube (CNT) that can act as membranes for air filters far more effective than current ones.

The showerhead-resembling devices were created by chemical vapor deposition (CVD) of silicon dioxide templates, with laser-created holes; after 30min in the furnace the holes fill up with carbon nanotubes, through which only nanoscale objects can pass.

Nanotube-infused microdevices, with forests of carbon nanotubes grown inside pores, can act as filters or as a carrier for improved catalysts. (Source: Rice U.)

"The basic idea is you have this carbon nanotube forest," explains Robert Vajtai, a Rice faculty fellow in mechanical engineering and materials science, in a statement. "The gas flows through, and because of the very small distance between the tubes, gas atoms have to hit many of them before they get out the other side."

This interaction lays the "scaffolding" for a catalyst template. When the CNTs are functionalized with catalytic chemicals, particles entering on one side of the filter come out the end in a different form — e.g., like an automobile catalytic converter, which turns carbon monoxide into carbon dioxide/nitrogen/water. In tests, Rice researchers deposited palladium onto the CNTs and used them to turn propene into propane (a benchmark test for catalysis), finding that the activated membranes "showed excellent and durable activity."

As a filter, the CNT-enabled membrane achieved 99% extraction of <1μm particles, removing about 100× more nanoparticles from laboratory air than the material used in high-efficiency particulate-absorbing (HEPA) filters, they note. The length of the CNTs (and thus density in the membrane) determines the filters’ permeability.

Results of the work were published in the journal ACS Nano.

Nanotubes grown in holes in silicon dioxide wafers have the potential to outperform currently available filters for many uses. (Source: Rice U.)