Detection of hydrogen from 5 ppm to 4 % using multi-walled carbon nanotube yarns coated with nanocrystalline Pd and Pd / Pt layered structures

Chemiresistors fabricated from multi-walled carbon nanotube yarns decorated with nanocrystalline Pd (Pd-MWCNT) have been used to detect hydrogen from 20 ppm to 2% in nitrogen at room temperature. In the case of chemiresistors fabricated by seeding a Pd layer with smaller nano islands of Pt (Pd-Pt-MWCNT), the dynamic range was extended further from 5 ppm to 3%. Concentrations of hydrogen above 2 % and 3 % were detectable but the sensitivity saturated above these levels. It is shown that the sensitivity to hydrogen is a resultant of two mechanisms which leads to opposing changes in resistance in the composite yarns. A clear deviation from the Sievert’s law is observed at concentrations above 100 ppm and is attributed to the presence of these competing mechanisms. The sensitivity is reduced for hydrogen when the experiments were carried out in dry air and the lower limit of detection was found to be 2000 ppm for Pd-MWCNT chemiresistor and 400 ppm for Pd-PtMWCNT chemiresistor. Pd-Pt-MWCNT chemiresistor showed excellent response and recovery characteristics in air. For Pd-MWCNT chemiresistor, the nanoscopic gap closing mechanism became prominent at concentrations below ~ 1000 ppm and allowed the detection of hydrogen down to 200 ppm in air using negative changes in