  In this paper we show that the ionic conduction through surface chemisorbed
ambient moisture leads to the remarkably high room temperature selective
response towards hydrogen gas. The surface adsorbed moisture acts as surface
states and shows ionic conduction, as a result of smaller size of ZnO
nanoparticles of 20 +/- 5 nm. This response is enhanced remarkably i.e. from
10% to 190% for 1200 ppm H2 gas when p-type NiO quasi-nanowires (width ~50 nm)
are mixed with these n-type ZnO nanoparticles to form a homogenous NiO/ZnO
nano-bulk p-n heterostructure. The maximum response is obtained for about 50-50
% composition of NiO/ZnO although it is of still n-type character. The dominant
carrier type reversal from n to p type takes place at rather high NiO content
of about 60-80% in ZnO, depicting dominating contribution of ZnO into the
response. The parallel surface ionic current through chemisorbed moisture
(surface states) has been identified as a primary factor for high sensitivity
at room temperature. Thus, the presence of heterojunction barrier at the
NiO-ZnO interface assisted with the surface ionic conductivity due to adsorbed
moisture results in large, selective response to hydrogen at room temperature.
