Metal
oxide semiconductor gas sensors have been widely studied
for the selective detection of various gases with trace concentrations.
The identification of the reaction scheme governing the gas sensing
response is crucial for further development; however, the mechanism
of ethanol (EtOH) gas sensing by ZnO is still controversial despite
being one of the most intensively studied target gas and sensing material
combinations. In this work, for the first time, the detailed mechanism
of EtOH sensing by ZnO is studied by using a bulk single-crystalline
substrate, which has a well-defined stoichiometry and atomic arrangement,
as the sensing material. The sensing response is substantial on the
ZnO substrate even with a millimeter-size thickness, and it becomes
larger with resistance of the substrate. The large sensing response
is described in terms of the adsorption/desorption of the oxygen species
on the substrate surface, namely, oxygen ionosorption. The valence
state of the ionosorbed oxygen involved in EtOH sensing is identified
to be O
2–
regardless of the temperature. The increase
in the sensing response with the temperature is attributed to the
enhanced oxidation rate of the EtOH molecule on the surface as analyzed
by pulsed-jet temperature-programmed desorption mass spectrometry,
which has been newly developed for analyzing surface reactions in
simulated working conditions.