Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 457

nanowire i lms. h e top surface of the nanowire is smooth and hexagonal
in shape (Figure 16.1a). Nanowires are perpendicular to substrate surface
and around 6 μm long, as shown in Figure 16.1b and 16.1c; however, vari-
ation of growth temperature leads to slightly rough surface of nanowire
(Figure 16.1d) [56]. Hydrothermal methods provide various opportuni-
ties to control the structural, morphological and doping properties of ZnO
nanostructures [57-59].

16.2.1.1 Shape, Size, Orientation and Density Controlled Growth

h e hydrothermal growth process has shown great potential to achieve
excellent control of shape, size and growth orientation on a large scale in
an inexpensive and facile way. h e size of ZnO nanowire ranges from a
few nanometers (~20 nm) to hundreds of nanometer in diameter and sev-
eral micrometers in length. Oriented growth of ZnO nanowire in a verti-
cal as well as lateral direction of substrates is performed [60]. Precursor
concentration has been shown to be a major factor to control the den-
sity of nanowire in seeded and non-seeded substrates [61-65]. Selective
adsorption of citrate anions on a ZnO basal plane results in a shape selec-
tive growth and controls aspect ratio, morphology and oriented nanocol-
umns [60]. Pretreatment of substrates, seeding and annealing af ects both
diameter and orientation of the nanowire [66]. A vapor-phase hydrother-
mal method has been recently demonstrated to grow ZnO nanotube and
nanorod array i lms on both sides of a zinc foil under ammonia vapor [67].
Ultra-thin ZnO nanowire growth is observed in the alkali solution of zinc
prepared by zinc nitrate and NaOH mixed solution in deionized water
followed by ethanol [68]. Recently, Kawai et al. stated that each plane of
ZnO crystal has their critical concentration for nucleation, which plays an
important role in the anisotropic growth of hydrothermal ZnO nanowires
[69]. h e Shin group showed that Au nanodots on Al-doped ZnO seed
layer can also be used for density control growth of nanowire [70].

A lateral growth of ZnO nanowire on substrates was shown, and it was
proposed that surface-induced variations in the local Zn 2+/ OH - concentra-
tion ratio within the reactive solution are responsible for this orientation
[71]. h e growth of highly dense and ultra-long ZnO nanowires is per-
formed by introducing an adequate content of ammonia into the nutri-
ent solution on seedless substrates [61]. h eoretically, predicted growth
conditions/growth statistics are implemented and it was observed that the
aspect ratio of nanowire can be change from 10 to 23 by using optimum
growth condition [72]. Recently, growth of “nanoforest” composed of
high-density, long-branched “treelike” multigeneration hierarchical ZnO