Low-Cost Nanomaterials for Photoelectrochemical Water Splitting - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 266

closed system such as autoclave filled with precursor solution. By heating the
autoclave, the chemical reaction can be carried out under controlled temperature
(typically \300 C) and pressure. The reaction temperature, pressure, and con-
centration of reactants play important roles in determining the material properties
of the products such as dimension, morphology, and crystal phase. For example,
Greene et al. developed a facile seed-mediated hydrothermal method for the
preparation of vertically aligned ZnO nanowire arrays [ 24 ]. As shown in Fig. 3 ,
the entire 4-inch wafer can be covered with a highly uniform and densely packed
array of ZnO nanowires. Importantly, this is a general strategy to grow ZnO
nanowire arrays on virtually any substrates. The seed-mediated hydrothermal
method has also been used to grow other metal oxide materials. For example,
Hoang et al. prepared vertical rutile TiO 2 nanowire arrays on FTO substrates by a
simple seed-assistant hydrothermal method. These single-crystalline TiO 2 nano-
wires have average diameter of *5 nm and length up to *4.4 lm[ 31 ]. Grimes
et al. fabricated WO 3 nanowire arrays directly on FTO substrate by a seed-med-
iated solvothermal method [ 85 ]. WO 3 nanowires and nanoflakes with hexagonal
and monoclinic structure could be readily obtained by adjusting the precursor
composition. Moreover, hydrothermal methods have also been used to grow metal
oxide nanomaterials without the need of seed layer [ 33 ]. For instance, Lee et al.
synthesized WO 3 nanocrystals using a hydrothermal process as a precursor and
studied the effect of annealing temperature on their morphology and photocatalytic
performance [ 33 ]. Zhong et al. utilized a solvothermal method to synthesize 3D
flower-like hematite nanostructure by using an ethylene glycol-mediated self-
assembly process [ 125 ]. In comparison to other gas-phase synthetic methods such
as chemical vapor deposition, atomic layer deposition, solution-based hydrother-
mal methods are simpler, cheaper, and more scalable, which offer significant
advantages for large scale production of photoelectrodes for water splitting.
2.2 Sol-gel Methods
Sol-gel methods typically involve either dip-coating or drop-coating colloidal
precursors directly onto conductive substrates, followed by calcination at various
temperatures. They are well-developed growth techniques and are known for
convenience and environmental friendliness. For example, the sol-gel spin-coating
process provides a simplified fabrication route for nanolayers, as it eliminates the
need of vapor-phase deposition equipment.
Various metal oxide thin films with different morphologies, have been prepared
using sol-gel techniques. Laberty-Robert et al. fabricated transparent a-Fe 2 O 3
mesoporous films via a template-directed sol-gel method coupled with the dip-
coating approach, followed by thermally annealing at various temperatures from
350 to 750 C in air [ 26 ]. The crystallite size is about 14 nm at 400 C and
becomes two times larger at 500 C (30 nm) due to the thermal aggregation. It has
been found that the heat treatment has an obvious effect on the optical property of