"We developed this technology for use as robust and highly efficient early warning sensors," Shankar Rananavare told nanotechweb.org. "Our first prototype device had a sensitivity of around 2 ppm, but we have now extended this performance to 500 ppb. With improved electronic processing we should be able to reach about 100 ppb."

Chlorine is useful in a number of industrial processes, but can be harmful when released into the environment. Levels of 15 ppm cause throat irritation in humans and exposure to concentrations of 1000 ppm can be fatal.

Dual role
As well as enhancing the conductivity of the device, the dopant can improve the specificity of the sensor to a particular gas. Antimony doping at 0.1% gives a material that is not only highly sensitive to chlorine, but also selective against Br2, HCl, NO, NO2, CHCl3, NH3 and H2.

Making the material
To prepare the film, the researchers mix 2 g of Sb-doped SnO2 with 0.5 ml of 2% ethyl silicate 40 in acidified ethanol. Next, the paste is compressed using a pressure of 5463 kg/cm2 to yield a 13 mm diameter pellet with a thickness of 800 µm. The pellet is then heated at 450 °C for 30 min and mounted on to a glass substrate. Finally, copper contacts are attached using conductive silver paint.

The researchers presented their work in Nanotechnology.