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3 Low-Cost Nanomaterials for PEC Water Splitting
Metal oxide semiconductors such as TiO
2
, ZnO, Fe
2
O
3
and WO
3
are the most
common materials used for photocatalytic and PEC water splitting, due to their
excellent chemical stability, low cost, and suitable band edge positions; however,
each metal oxide has its own limitations as photoelectrode for PEC water splitting.
Various techniques have been developed, such as morphology engineering, ele-
ment doping, and surface modification in order to solve these limitations. In this
section, we will review the recent research progress on these popular and low-cost
metal oxide nanomaterials for PEC water splitting and the strategies have been
used to solve their limitations.
3.1 TiO
2
and ZnO Nanomaterials for PEC Water Splitting
Since the first demonstration of PEC water splitting on TiO
2
by Honda and
Fujishima in 1972 [
17
], TiO
2
has been widely studied as photocatalyst [
2
,
49
] and
photoelectrodes [
101
,
117
] for water splitting. In comparison to bulk materials,
nanostructures could provide larger surface area and further facilitate charge
separation at the interface between semiconductor and electrolyte [
42
]. Various
nanostructured TiO
2
such as nanoparticle films [
14
,
110
], nanowire arrays [
16
,
101
,
108
], branched nanowire arrays [
7
], and nanotube arrays [
43
,
127
] have been
developed and implemented as photoelectrodes for water splitting. For instance,
Shankar et al. used electrochemical anodization method to synthesize TiO
2
nanotube arrays and applied them for PEC water splitting [
80
]. They systemati-
cally studied the effects of the anodization voltages, times, nanotube lengths, and
annealing temperatures on the PEC performance [
70
,
71
,
80
]. Rutile TiO
2
nano-
wire arrays grown on FTO glass have also been used for PEC water splitting [
16
,
101
]. Compared to the polycrystalline TiO
2
nanotube arrays, the single crystal
nanowires could have better charge transport property. Besides, the rutile TiO
2
has
a relative smaller band-gap of 3.0 eV than anatase TiO
2
(3.2 eV), which allows it
to utilize longer wavelength light in the solar spectrum. The large band-gap energy
is still the major limitation for TiO
2
materials for solar energy conversion. The
theoretical solar energy conversion efficiency for rutile TiO
2
should be *2.5 %,
depends on its band-gap energy [
62
]. However, the reported STH conversion
efficiencies were much lower than this theoretical value. For example, *0.7 % of
STH conversion efficiency was obtained on pristine TiO
2
nanowire arrays [
16
]. It
suggests that the conversion efficiency of TiO
2
is also limited by another factor,
which is believed to be the rapid electron-hole recombination.
To reduce the electron-hole recombination loss, the charge separation and
collection efficiencies should be improved. Wang et al. reported that hydrogen
thermal treatment improves charge transport of TiO
2
nanowire arrays by con-
trolled incorporation of oxygen vacancies [
101
]. Figure
6
shows the PEC and
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