Now, the use of 10 Hz frequency in the dynamic tests is further
justified at the light of the impedance arcs. As it is observed in the
figures, the module of the impedance at this frequency is changing
significantly.
Fig. 9 shows the evolution with humidity of the parameters
obtained from the fitting of the equivalent circuit to the impedance
spectra. The resistance of the film decreases around four orders of
magnitude as the humidity increases and the capacitance of the
device is constant as it does not depend on the sensing mechanism.
The results are consistent with the Grotthuss conduction mechanism
[2]. The logarithm of the resistance of the film changes
exponentially as water molecules are adsorbed gradually, from the very first monolayer to the physical adsorption of water
molecules. Even at low humidity levels, it is possible to observe the
hopping of protons in the chemisorbed monolayer of water. As the
humidity increases, the continuous adsorption of water molecules
(physisorbed) likely formed multilayers showing a liquid-like
behavior (Grotthuss mechanism).
The cerium ions lead to a strong electric field around the
surface increasing the ionization of the water molecules and
affecting the deeper physisorbed water [18]. On the other hand
the oxygen vacancies, created due to the gadolinium-doping,
have been recently studied as active sites for the dissociation of
water molecules creating two hydroxyl groups per vacancy [27].
Regarding the humidity sensing mechanism, it is clear that the
main benefit would be for large surface to volume ratio materials.
The here synthesized mesoporous material may play a key
role in the conduction, as the interconnected network with double
gyroid structure could ease the water molecules pass easily through
the regular pores. This can also be in the basis of the previously
observed quick response and recovery.