Sensors - Hierarchical Nanostructures for Energy Devices - page 250

are defined by the top-down approach and smaller features are fabricated
afterwards by the bottom-up approach. One could obtain a uniform and
regular array of hierarchical structures on a large area by the combinative
method. For example, Cheng et al. 12 prepared an Ag decorated Si/ZnO
nano-tree array as shown in Figure 11.1(a). First of all, Si nanopillars were
fabricated using photolithography on a wafer and ZnO nanorods were grown
by a bottom-up hydrothermal method. Finally, silver nanoparticles were
deposited on Si/ZnO nano-trees by photochemical reduction. Similarly,
Zhang et al. 13 introduced a hierarchical silicon nanowire array with silver
nanoparticles as shown in Figure 11.1(f). The silicon nanowire array was
produced by a chemical etching process on an n-type (100) wafer, and Ag
nanoparticles were grown by galvanic redox reaction. Duan et al. 14 proposed
a novel fabrication method for uniformly distributed surface roughened Au
nanoparticles as shown in Figure 11.1(b). As a result, the hierarchically
roughened Au nanoparticle array showed a strong SERS signal. Con-
sequently, the combinative method is very advantageous in that it can pre-
pare hierarchical nanostructures on a large area, which helps with the
realization of sensors for practical applications.

d n 3 r 4 n g | 4

11.4 Hierarchical Nanostructure Sensors

11.4.1 Gas Sensors
The operation principle of metal oxide sensors is based on the change of the
number of charge carriers (electrons or holes) in the material upon inter-
action with target molecules such as gases and chemical species. There are
two types of metal oxides: n-type (ZnO, SnO 2 ,TiO 2 ,Fe 2 O 3 ) and p-type (NiO,
CoO). The n-type oxides respond to reducing gases such as H 2 ,CH 4 , CO,
C 2 H 5 OH and H 2 S, while the p-type oxides react to oxidizing gases such as O 2 ,
NO 2 and Cl 2 . Figure 11.2 exemplarily depicts a scheme of the sensing
mechanism in n-type semiconductor gas sensors in the case of CO as a target
gas. 15 Oxygen in ambient air traps free electrons from the conduction band
near the surface, resulting in an electron-depletion layer represented by the
white region in Figure 11.2(a). However, once the sensor is exposed to a re-
ducing gas which reacts with oxygen, the trapped electrons are released back
to the bulk leading to a narrower depletion region thereby reduced resistance.

.

1
2 O 2 þ e ! O ð s Þ
R ð g Þþ O ð s Þ! RO ð g Þþ e

where e is an electron from the oxide. R(g) is the reducing gas, and g and s are
the gas and surface, respectively. The change of electrical resistance/
conductance is usually measured by the microfabricated interdigitated
electrodes in Figure 11.2(c). It is noted that these reactions take place at
elevated temperatures, so that the sensing platform should possess an