Other Applications - Hierarchical Nanostructures for Energy Devices - page 278

previously reported by Barthlott and Neinhuis. 91 It was believed that this
unique property is based on surface roughness caused by micrometre-scale
papillae and epicuticular wax. 92 Zhai et al. 93 reported micro- and nanoscale
hierarchical structures on the surface of a lotus leaf, i.e., branch-like nanos-
tructures on top of the micropapillae. These structures can induce super-
hydrophobic surfaces with a large contact angle and a small sliding angle.
Hierarchical nanostructures on a solid surface are very important for
superhydrophobicity, which can induce a high contact angle. A number of
methods have been explored to fabricate hierarchical structures including
nanomolding and micromolding, direct growth of organic or inorganic
structures, and lithography patterning of surfaces. 94 Li et al. 95,96 reported the
preparation of a densely packed aligned carbon nanotube film with pure
nanostructures. As an extension to this work, Ko et al. 74 introduced a simple
and robust method to make hierarchical fibrillar arrays based on hybrid
organic-inorganic material systems on mechanically flexible substrates. The
hybrid structures are beneficial for multifunctional materials due to the
mixed and synergetic functionalities of organic and inorganic components as
shown in Figure 12.6. 74 The structures consist of polymer micropillar (mPLR)
arrays decorated with ZnO nanowires. The polymer mPLR arrays are fabricated
by replica molding on microfabricated silicon templates containing hex-
agonal micropore arrays. Subsequently, the ZnO nanowires are grown on the
surface of mPLRs by a low-temperature, solution-based growth method, re-
sulting in hierarchical microfibrillar and nanofibrillar arrays. The super-
hydrophobic surface properties of hierarchical mPLR/NW arrays with potential
applications in self-cleaning smart surfaces, microfluidics, and biomedical
devices, were demonstrated. For these applications, the mechanical flexibility
of the polymeric support substrate is advantageous over traditional super-
hydrophobic surfaces with rigid silicon or glass support substrates. 74
Guo et al. 75 reported the synthesis of a ZnO/CuO hetero-hierarchical
nanotree array based on a CuO nanowire array via a simple hydrothermal
approach combined with a thermal oxidation method as shown in Figure
12.7. Also, an as-prepared ZnO/CuO nanotree array after silanization pre-
sented remarkable superhydrophobic performance, which is attributed to
the trapped air and hierarchical roughness. Furthermore, wettability of the
ZnO/CuO hierarchical nanotree could be manipulated by the morphologies
of hierarchical ZnO nanorods. By adjusting the reaction conditions, such as
Zn(NO 3 ) 2 concentration and reaction time, the density, diameter and length
of the hierarchical ZnO nanorod branch could be tuned. After silanization,
the surface containing the ZnO/CuO hetero-hierarchical nanotree array ex-
hibit impressive superhydrophobic behavior. Changing the structure from a
CuO nanowire array to a ZnO/CuO nanotree array at the surface causes an
obvious increase in static contact angle, which then decreases. These results
demonstrate the switch between a Wenzel and Cassie-Baxter model. When
in contact with such a solid surface, a water drop can fully bounce like a
balloon. As a potential application study, the nanotree array surface is found
to be self-cleaning. 75

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