Fabrication of interfaces between carbon nanotubes and catalytic palladium using dielectrophoresis and its application to hydrogen gas sensor

The authors have developed a novel fabrication method of a carbon nanotube (CNT) gas sensor using positive dielectrophoresis (DEP). One advantage of the DEP technique is that one can readily build up interfaces between electrodes and CNT, or between CNT and other nanomaterials. In this paper, the DEP technique was employed to fabricate interfaces between CNT and catalytic palladium (Pd) to realize a CNT-based hydrogen (H₂) gas sensor. Two types of CNT/Pd interfaces were proposed and tested. For one type, CNTs were trapped onto a microelectrode made of Pd so that the CNT/Pd interfaces were formed at both ends of CNTs lying over the Pd electrode surface. The other type of gas sensor was fabricated by simultaneously trapping CNTs and Pd nanoparticles under action of the positive DEP. It was found that both types of the CNT/Pd gas sensors could respond to hydrogen, while the CNT sensor without the Pd functionalization could not. For the hydrogen gas diluted with dry air, the electrical resistance of the CNT/Pd sensors increased at the moment of hydrogen exposure, and then turned to decrease in the later stage. The resistance increase just after the H₂ exposure was probably due to reduction of CNTs by H atoms, which were produced by dissociative adsorption of H₂ molecules on the catalytic Pd surface. In the later stage, the dissociated H atoms might react with dissociated oxygen atoms to create H₂O molecules accompanying heat generation.