Environmental Engineering Reference
In-Depth Information
generated i rst by decomposition of molecular precursor at a low tempera-
ture. h e seed particles/catalysts for this kind of reaction are extremely small
and therefore easily activated at low temperatures. h e monomers react with
the nanoseeds to form supersaturated droplets [40, 41]. In the case of self-
assembly, attachment growth is based on the fact that nanoparticles gener-
ated in solution have a large surface-to-volume ratio. To reduce the surface
energy and total system energy, the particles may segregate together. Oriented
attachment is one of the ways for this segregation process. Anisotropic growth
of crystals for formation of the most elongated nanocrystals is induced due
to dif erent surface energies [40, 42, 43]. However, the dif erence in surface
energies (the intrinsic properties of the crystal) is not large enough to cause
highly anisotropic growth of long nanowires. By adding surfactants to the
reaction solution, the ef ective surface energy of a nanocrystal can be modu-
lated, and the surfactant molecules selectively adsorb and bound onto certain
surfaces of the nanocrystal seeds [40, 44, 45]. Fabrication of nanostructured
i lms on a large scale is the main challenge for any industrial application
and solution growth provides an opportunity to grow large-scale nanowire
i lms in a low-cost and facile process. Solution growth has been extensively
explored by various researchers and excellent control on growth location or
spatial growth, orientation, density and various doping has been successfully
achieved [25, 46-48].
16.2.1
Low Temperature Hydrothermal Growth
Low temperature hydrothermal process has advantage that nanowire i lm
growth can be initiated on any arbitrary substrate at er deposition of ZnO
seed layer. A typical synthesis is performed in an aqueous solution of zinc
nitrate and hexamine (hexamethylenetetramine (HMT)), however, zinc
chloride and zinc acetate is also used, and growth is carried out at low
temperature (< 100
C) [29, 49]. Hexamine (HMT) releases hydroxyl ions
in the solution which further react with Zn
2+
ions to form ZnO [50].
h e
following equations explain the reaction of the ZnO growth as proposed
by Vergés
et al.
[38].
°
(CH
2
)
6
N
4
+ 6H
2
O = 6HCHO + 4NH
3
(16.1)
NH
3
+H
2
O = NH
4
+
+ OH
−
(16.2)
2OH
−
+ Zn
2+
= ZnO(s) +H
2
O
(16.3)
It is generally supposed that HMT supplies hydroxyl ions to drive the
precipitation reaction [51]. However, the role of hexamine is not fully
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