AC and DC measurements on Langmuir-Blodgett polypyrrole films for selective NH 3 gas detection

Abstract

The Langmuir-Blodgett method has been used to deposit onto quartz substrate polypyrrole film as gas sensitive coating. The AC and DC electrical resistance and capacitance of the sensing device have been investigated under controlled ambient conditions, at room temperature, during gas exposures of NH 3, CH 4, CO in dry air. The AC capacitance of the polypyrrole sensor was not affected by any test gases throughout the examined frequency range (20 Hz to 1 MHz). The AC and DC electrical resistance showed a highly sensitive response towards NH 3 and negligibly small response towards interfering CH 4 and CO: the sensitivity and selectivity of the NH 3 detector increased with the decreasing AC frequency obtaining the optimal working conditions in DC mode. The calibration curve of the sensor towards NH 3 in the gas concentration range 100–10 000 ppm has been performed at different frequencies (DC to 1 MHz) indicating a linear relationship between AC and/or DC resistance and NH 3 concentration. Long-term stability tests have been carried out onto a polypyrrole detector exhibiting fairly reproducible responses towards NH 3 during a six-month period. An efficient recognition of NH 3 as one component of a gas mixture containing CO, CH 4, O 2 in air has been successfully recorded.