Recent experiments indicate that water molecules adsorbed on carbon nanotub
e tips significantly enhance field-emission current. Through first-principl
es density-functional theory calculations we show that the water-nanotube i
nteraction is weak in zero electric field. However, under emission conditio
ns large electric field present at the tube tip: (a) increases the binding
energy appreciably, thereby stabilizing the adsorbate; and (b) lowers the i
onization potential (IP), thereby making it easier to extract electrons. Lo
wering of IP is enhanced further through the formation of a water cluster o
n the nanotube tip.