Sensors - Hierarchical Nanostructures for Energy Devices - page 252

(t 90% ). The response time is measured when the sensor is exposed to gas
whereas the recovery time is measured when the sensor is released to
ambient air.
The gas sensors exploiting hierarchical metal oxide nanostructures are
summarized in Table 11.2 in terms of material type, hierarchy and
morphology, gas sensing performances, and operation temperatures. The
2-3 flower-like hierarchical SnO 2 microspheres composed of self-assembled
nanosheets were prepared by hydrothermal synthesis of Sn 3 O 4 microspheres
followed by calcination. 16-18 The gas sensor made of hierarchical SnO 2
microspheres in Figure 11.3(a) showed a gas response of 7.7-33.1 in the
range of 10-50 ppm ethanol (C 2 H 5 OH) at 400 1C with a very short response
time of 1-3 s 16 as shown in Figure 11.3(b).
It is evident that a higher gas response was found at each concentration of
gas and a stiffer slope was also observed for nanosheet microstructure
sensors. It is interesting to see that the response time is constant in hier-
archical nanostructure sensors regardless of the concentration of gases,
while compact microspheres showed a decreased response time. Sun et al. 17
also found that the hierarchical nanostructure sensor had a gas response 3-4
times higher than dense microspheres to other gases such as acetone,
butanone, methanol, etc. as well as C 2 H 5 OH by using the same hierarchical
nanostructures as shown in Figure 11.4(a). Furthermore, the response/re-
covery time of sensors was also found to be notably shorter, 0.6 and 11 s,
respectively, than the dense counterpart of a SnO 2 microsphere. 18 The
ethanol selectivity of the sensor was tested with respect to 500 ppm of
various gases (CO 2 ,NO 2 ,CH 4 ,H 2 ,Cl 2 ,C 2 H 4 , and CO) at optimal operation
temperature, resulting in the highest gas response of the tested gases. Qin
et al. 19 synthesized square-shaped single-crystalline SnO 2 nanowires and 1-3
urchin-like hierarchical nanostructures as shown in Figure 11.4(b), suc-
cessfully using the bottom-up hydrothermal approach. The gas response to
20 ppm acetone (CH 3 COCH 3 ) at 290 1C was 5.5 with very low response and
recovery times, 7 and 10 s, respectively. Thong et al. 20 synthesized a 1-2
dendrite SnO 2 hierarchical nanostructure as shown in Figure 11.4(c) by a
two-step thermal evaporation process and conducted gas sensing experi-
ments for liquid petroleum gas (LPG) and ammonia (NH 3 ). The gas response
to 2000 ppm LPG at 350 1C was 20.4, which was almost four times higher
than that in SnO 2 nanowires. Furthermore, they showed operation tem-
peratures of 350 and 200 1C for LPG and NH 3 , respectively, which demon-
strated a similar range of gas response. This suggests that the optimal
operation temperature depends on gas species and the selectivity of sensors
is tunable with operation temperature.
Wang et al. 21 synthesized a 2-3 flower-like a-Fe 2 O 3 hierarchical nano-
structure as shown in Figure 11.4(d) by a simple solvothermal method
combined with a subsequent annealing process. The gas response to 100
ppm C 2 H 5 OH at 280 1C was 37.9 with very short response and recovery times
of 1 and 0.5 s, respectively. They demonstrated that the flower-like a-Fe 2 O 3
hierarchical nanostructure had a higher selectivity for ethanol than other

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.