Gas-sensing
characteristics of thick films prepared using NiO nanoparticles
(NPs) have been investigated. NiO NPs were synthesized via a facile
and cost-effective chemical coprecipitation method. NPs of different
sizes were produced by varying growth parameters such as stirring
and annealing temperatures. These nanomaterials were investigated
by several techniques to determine their morphology, crystal structure,
phase purity, and composition. A dilute solution of hexamethyldisilazane
(HMDS)-functionalized NiO NPs in propan-2-ol was used to prepare the
films on polycrystalline alumina substrates with predeposited interdigitated
gold electrodes. Gas-sensing studies of NiO nanoparticulate films
illustrate that, among various toxic gases, they have a fast response
to H2S. However, this response toward H2S was
found to be anomalous. Upon exposure to H2S, the resistance
of NiO films increases from a base value, which is then followed by
an immediate and unpredicted decrease before saturation. To investigate
this anomalous behavior, H2S gas-sensing studies were carried
out as a function of the operating temperature, gas concentration,
and particle size. The increase in the resistance upon exposure to
gas is observed to increase with the concentration (range 1–100
ppm) at a fixed temperature. However, this anomalous response was
observed in all cases. To identify the mechanism responsible for this
unusal behavior, NiO films were characterized using X-ray photoelectron
spectroscopy, X-ray diffraction, and Raman spectroscopy, before and
after H2S exposure. It was found that this strange occurrence
is due to a combined competitive response from two different kinds
of kinetics occurring simultaneously, i.e., the formation of NiS along
with the typical oxidation of H2S by NiO, where the oxidation
reaction leads to an increase in the resistance and the sulfide formation
leads to a decrease in the resistance because of the metallic nature
of NiS.