3.2. Humidity sensing capabilities of a mesoporous CGO film
The response to humidity of the fabricated films is evaluated.
Sinusoidal excitation signals of different frequencies are imposed
and the impedance is measured. Fig. 4 shows the tendency of
the impedance at different frequencies for several humidity levels
at a controllable fixed room temperature (T = 30 ◦C). In the low
frequency region (f = 1–10 Hz) the device presents the highest sensitivity,
10 Hz is a good tradeoff between measurement velocity and
selectivity, and will be the frequency used in the following.
A detailed analysis of the impedance response of the sensors is
presented in the next section. There,the choice ofthe low frequency
range for on line monitoring of the sensor response is justified. The
impedance does not change at higher frequencies (f = 104–105 Hz)
where the response of the sensor becomes independent of the
humidity. This indicates that, in this higher range of frequencies,
it is short-circuit by the parallel resistance.
The relative resistance (R0/RRH) versus the relative humidity
(RH) at room temperature (100% RH = 3% H2O) is presented in Fig. 5
for the here-fabricated mesoporous CGO humidity sensor and compared
to different cerium oxides based humidity sensors extracted
from the literature [18–20]. In this figure, R0 indicates the resistance
of the sensors at 25% RH and R(RH) the resistance values
at the corresponding humidity level. Dense bulk cerium oxides
show almost no change in resistance on the whole range of relative
humidities, while for nanostructured materials sensitivity starts
to become significant. The here-proposed CGO mesoporous material
sensor shows an exponential decrease of the logarithm of the
resistance with the increase of humidity. This behavior has been reported as arising from an ionic type humidity sensing mechanism
[3,22,23]. Only complex nanostructured configurations like CeO2
nanowires present a sensitivity comparable to the here presented
material.