We report the electrical responses of water vapour and O
2
adsorption onto
macroscopic multi-walled carbon nanotube (MWCNT) ropes, and compare
the results with mats of acid-treated MWCNTs on SiO
2
substrates in order to
investigate the importance of oxygen-containing defects on CNTs. In the
outgassed state both carbon nanotube (CNT) materials exhibit rapid changes
in electrical resistance when exposed to dry air, humid air or water vapour at
standard temperature and pressure (STP). The measured electrical responses
are highly reversible at STP when cycled between humid air, vacuum and dry
air. We report a decrease in resistance for the CNT materials in dry air,
attributed to O
2
p-type doping of the CNTs, whereas there is an increase in
resistance when exposed to a humid environment. This latter effect is
attributed to the formation of hydrogen bonding from the polar water
molecules with the oxygen-containing defects on the CNTs. Our
observations indicate that the increase in electrical resistance upon water
absorption affects a reduction of the electron-withdrawing power of the
oxygen-containing defect groups, thus leading to a reduced hole carrier
concentration in the p-type nanotubes.
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