Schematic showing the synergies between piezoelectric field and photocatalysis in ZnO nanorod arrays on Ni foam.Researchers from the USA and China have come up with a novel nanocomposite that could harness the flow of water to break up organic pollutants [Chen et al., Materials Today (2017), doi: 10.1016/j.mattod.2017.08.027].
The nanocomposite harnesses the piezoelectric effect, which has been widely applied in nanogenerators, sensors and transistors, whereby deformation of the material induces an electric field.
“Recently, the application of the piezoelectric effect to environmental purification has attracted attention,” explains Zhenfeng Bian of Shanghai Normal University.
Bian and colleagues from Beijing Institute of Nanoenergy and Nanosystems and Georgia Institute of Technology designed a composite consisting of arrays of vertical ZnO nanorods, which have excellent piezoelectric and also photocatalytic properties, grown on a three-dimensional Ni foam.
The electric field produced by deformation of the ZnO nanorods is used to separate photoelectrons and holes, reducing the rate at which they recombine and promoting photocatalytic activity.
“We have reported, for the first time, piezo-promoted photocatalysis with enhanced activity for the degradation of organic pollutants in wastewater by using ZnO nanorod arrays vertically grown on a three-dimensional Ni foam substrate,” says Bian.
Until now, external forces such as friction or ultrasonic waves have been used to deform ZnO and produce a piezoelectric field. But the novel nanocomposite can harness other forms of mechanical energy such as water flow, tides, or even wind.
“Taking into account potential applications of photocatalysis for cleaning water and air, flow- or wind-driven piezo-promoted photocatalysis could offer opportunities for practical applications,” points out Bian.
When the new composite is placed in flowing water, which the researchers simulated by magnetically stirring the liquid in a container, turbulence deforms the nanorods, generating a piezoelectric field is at the surface. Simultaneously, eddies in the porous, three-dimensional substrate add to the deformation of the nanorods, further bolstering the piezoelectric effect. Meanwhile, ultraviolet light irradiation of the semiconducting ZnO nanorods generates photoelectrons and holes, which are separated by the field to generate a photocurrent. Increasing the stirring rate intensifies the phototcatalytic effect, boosting the photocurrent further.
The phototcatalytic effect can degrade organic chemicals present in the water such as pollutants like the dye rhodamine B (RhB). The researchers believe their results demonstrate a new way of designing photocatalysts for practical applications like wastewater cleaning.
“We plan to design some small devices that simulate actual wastewater treatment and then to expand the device, with practical application the ultimate goal,” Bian told Materials Today.