Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 78

nanowires can be grown by repeating the hydrothermal growth in a fresh
aqueous precursor solution by the LG mode. A dramatic change in nano-
wire structure could occur by heating the first generation nanowire at 350 1C
(10 minutes), adding seed nanoparticles and then hydrothermal growth.
Instead of LG, highly BG of nanowires on the first generation nanowire
sidewall could be observed. The combination of multiple LG and BG steps
can be applied for more complex hierarchical nanowire structures.
A single hydrothermal reaction LG process can grow vertically aligned
nanowires 2-8 mm long (130-200 nm diameter). Multiple LG growth steps
can grow nanowires 40-50 mm long of a high aspect ratio (4100). The length
of the higher generation LG nanowires becomes shorter upon each growth
step. A vertically aligned long nanowire forest grown by multiple LG can be
used as the backbone of a hierarchical branched nanowire forest. High
quality hierarchical branched nanowire forests can be grown only after
both (1) removal of polymer (HMTA, PEI) by heating the nanowire and (2)
coating with seed nanoparticles on the backbone nanowire surface. HMTA
and PEI hinder only the lateral growth but allow axial growth of the
nanowires in solution to yield high aspect ratio nanowires. The polymers
can be removed by heating the nanowire at 350 1Cfor10minutes.Oncethe
polymer was removed from the backbone nanowire, branched growth (BG)
nanowires on the sidewall, which had been suppressed by HMTA and PEI
for a regular LG, could be induced. In addition to random and sparsely
branched nanowire growth on the side wall, the diameter of the first
generation backbone nanowire also increased due to lateral growth after
the removal of the HMTA and PEI polymer layer. After polymer removal,
seed nanoparticle coating on the first generation backbone nanowire
can grow densely packed higher order generation BG nanowires and could
also suppress the diameter increase of the first generation backbone
nanowire.
A seed nanoparticle coating on the backbone nanowire without polymer
removal could grow sparsely branched nanowires while high quality hier-
archical branched nanowire forests could be achieved with a seed nano-
particle coating on the backbone nanowire after polymer removal. Nanowire
growth from the seed nanoparticle on HMTA and PEI polymer may be less
favorable than on the nanowire surface without polymer. This signifies that
both polymer removal and seed layer are important for high density hier-
archical branched nanowire forest growth.
The ''nanoforest'' of the hierarchical branched ZnO nanowire could be
easily grown on a large area by a low cost, all solution processed hydro-
thermal method.
Hierarchical mesoporous films can also be formed by the interconnected
power of different dimensions by various self-assembly methods.
A hierarchical porous material should exhibit at least a bimodal pore size
distribution. The properties of the material and its capability of carrying
out a specific function are directly dependent on the distribution of size,
shape, and organization of the pores. Furthermore, while the pores can be

d n 3 r 4 n g | 9

.