Nanogenerators for electrical energy

Materials World magazine
1 May 2006

Nanogenerators that convert mechanical to electrical energy could be the power source of the future for nano-scale devices.

Researchers at the Georgia Institute of Technology, USA, have developed nanogenerators in the form of nanowires made from zinc oxide. When the wires are bent and released, electricity is produced through the piezoelectric effect.

By building an array of nanowires, Professor Zhong Lin Wang believes that enough power could be produced to power nanoscale devices. 

‘We can build nanodevices that are very small, but if the complete integrated system must include a large power source, that defeats the purpose,' says Wang, who developed the nanowires with graduate student Jinhui Song. 

The team grew arrays of nanowires, and then used an atomic-force microscope (AFM) to deflect the wires, producing stretching on one side and compression on the other. The resulting charge separation - due to the piezoelectric effect - is preserved until the tip of the AFM loses contact with the wire.

As the strain is released, the researchers are able to measure an electrical current. Although the nanowire continues to vibrate through many cycles, the current is only measurable at the very moment of release.

The nanowires could have potential biomedical applications because zinc oxide is non-toxic. They have the added advantage of growing on both crystalline and polymer substrates.

‘You could imagine having these nanogenerators in your shoes to produce electricity as you walk,' Wang says. ‘This could be beneficial to soldiers in the field, who now depend on batteries to power their electrical equipment. As long as the soldiers were moving, they could generate electricity.' 

The nanowires are grown using a vapour-liquid-solid process in a small tube furnace. Using a sapphire substrate, gold nanoparticles are deposited first and serve as catalysts. Zinc oxide powder is heated and flowed into the furnace using an argon carrier. The nanowires then grow beneath the gold particles and range in length from 200-500nm, 20-40nm in diameter, and have a separation distance of approximately 100nm, which is determined by the arrangement of the gold particles. 

By placing the nanowires in acoustic or ultrasonic fields, it might also be possible to produce an electric current.