Detection of hydrogen using multi-walled carbon-nanotube yarns coated with nanocrystalline Pd and Pd/Pt layered structures Original Research Article

Robust and flexible chemiresistors fabricated from multi-walled carbon nanotube yarns decorated with nanocrystalline Pd (Pd-MWCNTs) were used to detect hydrogen from 20 parts per million (ppm) and above in nitrogen at room temperature. With the chemiresistors fabricated by introducing a layer of Pt on Pd (Pt–Pd-MWCNTs), the lower limit of detection (LLD) was found to extend down to 5 ppm. It is shown that the observed response to hydrogen is a resultant of two mechanisms, namely, the formation of the hydrides of Pd and nanoscopic gap closing which leads to opposing changes in resistance in the composite yarns. A deviation from the Sievert’s law is observed at concentrations above 100 ppm and is attributed to the presence of these competing mechanisms. In air, the LLD was found to be 2000 ppm for the Pd-MWCNT chemiresistor and 400 ppm for the Pt–Pd-MWCNT chemiresistor. The Pt–Pd-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 resistance.