Hierarchical Nanostructures for Photo-Electro-Chemical Cells - Hierarchical Nanostructures for Energy Devices - page 190

conductivity. TiO 2 /Si heterostructures have also been demonstrated as a fully
integrated system for PEC solar hydrogen production (Figure 8.12(b)). 28
Rutile TiO 2 nanowire branches serving as oxygen-generating photoanodes
were hydrothermally grown from the p-Si backbone nanowires that com-
prised the hydrogen-generating photocathodes (Figure 8.12(c)). The Si
nanowires were fabricated by a reactive-ion etching (RIE) process on pat-
terned single crystalline Si wafers. Iridium nanoparticles and platinum
nanoparticles were loaded on the TiO 2 and Si nanowires to diminish the
reaction overpotentials. These results confirmed solar water splitting with-
out applied bias on the Si/TiO 2 heterostructure, apparently due to the opti-
mized and integrated design of the two materials since water splitting could
not be observed on the constituent materials or their composites without
integration. Kargar et al. 71 reported the synthesis of 3-D Si/ZnO branched
nanowire photoelectrodes consisting of Si cores and ZnO branches as shown
in Figure 8.12(d). The core p-Si nanowires were fabricated by nanoimprint-
ing followed by dry etching and ZnO nanowire branches were grown sub-
sequently by the hydrothermal growth method. The lack of stability of the Si/
ZnO nanowire structures could be enhanced by corrosion-resistant thin
(

d n 3 r 4 n g | 4

20 nm) atomic layer deposition (ALD)-coated TiO 2 and a platinum co-
catalyst. The Si/ZnO heterostructures exhibit photoanodic (p 1 -Si/TiO 2 )or
photocathodic (p-Si/TiO 2 ) characteristics, depending on the doping con-
centration of the Si cores, which is attributed to the different depletion re-
gion position. The photocurrent density obtained from the Si/ZnO-NW
heterostructure has been enhanced up to 80 times compared to Si/ZnO-seed
(no branches). The enhancement is attributed to the promoted charge car-
rier separation as well as the significantly augmented surface area facili-
tating ecient chemical reactions.
Recently, combinations of other materials to build heterostructures are
increasingly being reported. Some of them with their respective applications
are listed in Table 8.1 in section 8.6.

B

.

8.5 Useful Parameters for Hierarchical
Nanostructures
One of the main advantages of hierarchical structures for PEC applications
is that they can realize electrodes with extremely large surface areas
within a limited cell size. Therefore, it is convenient to define parameters
expressing the density of the hierarchical structure and thereby the antici-
pated benefit in surface area that can be achieved from the given

configuration.
The most widely used parameter is the ''roughness factor (RF)'', which is a
dimensionless parameter defined as the ratio of the total surface area of the
nanostructure to the projected substrate area 10,32 as illustrated in
Figure 8.13. If N backbone nanowires (diameter: D, length: H) are grown on
the substrate (area ¼ a b) and n cylindrical branches (diameter: d, length: h)