One-dimensional metal oxide nanostructured semiconductor films such as ZnO, In2O3, and SnO2 can be exploited to make gas sensors for detecting toxic and dangerous gas thanks to their large surface activity and excellent electron collection efficiency arising from their one-dimensional morphologies. Most metal oxides are classified as n-type semiconductors. Their surface resistance decreases in reducing gases (e.g. CO, NH3) and increases in oxidizing gases (e.g. O3, NO2) due to the adsorption and desorption of surface oxygen.

In a recent gas-sensing study, which was published in the journal Nanotechnology, researchers have fabricated a gas sensor using arrays of hydrothermally synthesized ZnO nanotubes that exhibit abnormal N-P transition behaviour. The resistance of the ZnO nanotube film decreases with increasing NO2 concentration at 30 °C – a typical response of p-type semiconductor oxides. The gas sensor is sensitive to NO2 concentrations as low as 500 ppb at this temperature.

This behaviour disappears when the nanotube arrays are exposed to higher temperature and higher gas concentrations. The transition is attributed to the changes in mobile carrier density at the surface and the high surface to volume ratio associated with the tubular ZnO nanostructures.

The researchers, based at Nanyang Technological University, Singapore, and the City University of Hong Kong, grow their nanotube arrays on a silicon substrate with an insulated silicon dioxide layer using a low-temperature hydrothermal technique.

"This method is a simple and efficient way of fabricating nanotube arrays on a large scale," Jianxiong Wang told "Looking at the behaviour in more detail, we believe that the N-P transition of ZnO nanotubes could open up interesting possibilities for tailoring the electronic properties of nanostructured devices."