As discussed
before, AuNPs increase kOxy due to catalytic effect on oxygen
adsorption reaction. Then, the dissociation of oxygen molecules
in air leads to oxygen ions adsorption by capturing electron to
become oxygen ion species, O2−
[24]. ZnO nanostructure loses more
conduction electrons at the surface ([e]) due to AuNPs spillover
effect which causes a spillover zone at the neighborhood of AuNPs.
Therefore, the depletion layer width is enlarged with suppression
of the underlying conduction channel [34]. The larger depletion
layer width (w1) results in the enhancement of sensor response as
seen in Eq. (16). Moreover, the larger depletion layer width results
in increase height of the potential barrier at the contacts among
the nanowires and consequently leads to resistance increase of the
Fig. 13. Schematic diagrams of gold adsorbed on ZnO surface, (a) without gold, (b)
with gold nanoparticle and (c) with large amount of gold nanoparticles.
nanowire sensors. It should be noted that the resistance of sensor
coated with AuNPs (S10–S100) in air atmosphere showed higher
resistance than pure sensor (see in Fig. 11) even though the sensor
has a large amount of AuNPs on the surface.
Finally, at higher loading amounts of AuNPS, the over amount
of AuNPs on the ZnO surface results in an overlap with the other
AuNPs on ZnO surface as seen in Fig. 13