Sensors - Hierarchical Nanostructures for Energy Devices - page 247

building block is the unit to formulate the final structure. It may be classi-
fied into 0 (nanoparticle, nanocrystal), 1 (nanowire, nanorod) and
2 (nanosheet) dimensions. 5 The final structure might be in 2D or 3D with
various types of morphology such as flower-like, urchin-like, cactus-like, etc.
For example, a 1-3 flower-like hierarchical nanostructure is a three-dimen-
sional flower-like structure composed of 1D nano building blocks such as
nanowires or nanorods. Table 11.1 summarizes the hierarchical nano-
structures that have been used in sensing applications. Particularly, struc-
tures with a red background were used as biosensors, a green one as SERS
sensors, and a blue one as gas sensors. This will be discussed in detail in
section 11.4.
Usually, a 3D hierarchical structure is more suitable for sensors than a 2D
structure due to its higher surface-to-volume ratio. Particularly, in chemical
sensors where the analyte sample is in either gas or liquid phase, 3D
structures are preferable. In the case of SERS sensors, however, a 2D struc-
ture might be used if directionality is needed upon interaction with light. On
the other hand, mechanical sensors such as force and strain sensors, even
though they will not be considered in this chapter, prefer 1D or 2D types of
final morphology. In this chapter, 3D hierarchical nanostructures are the
focus for chemical sensing applications.

d n 3 r 4 n g | 4

11.3 Preparation of Hierarchical Nanostructures for
Sensors

11.3.1 Hierarchical Metal Oxide Nanostructures
Metal oxides have the electrical properties that are explained using a well-
established semiconductor theory. Since metal oxides are intrinsically
semiconductors, they can be applied to electronic devices such as field-effect
transistors (FETs). Nanostructures allow more room to be utilized for sen-
sors due to the electrical behavior that is sensitive to external physical or
chemical quantities such as ultraviolet illumination, 6 oxidizing/reducing
agents, 7 and charge carrier doing/trapping species. 2,3 For this reason, metal
oxides are widely used for electrochemical sensing applications suitable for
chemical sensors such as gas sensors and biosensors. In this section,
methods that have been used for the preparation of hierarchical metal oxide
nanostructures for sensing applications are summarized and discussed.
Mostly they have been prepared either (a) by evaporation and condensation
in the gas phase or (b) by hydrothermal and solvothermal growth in the
liquid phase.

.

11.3.1.1 Evaporation and Condensation
This method is based on the vapor phase growth that has been extensively
used for producing 1D nano building blocks such as nanowires or nanorods.
The

vaporized source elements condense resulting in directional