Solar Hydrogen Generation: Photocatalytic and Photoelectrochemical Methods - Hydrogen Generation, Storage, and Utilization - page 40

increased the semiconductor/electrolyte interfacial area, which can improve
the kinetics of H 2 generation. In addition, the silicon nanowire arrays could
function as an antireflective or light trapping layer to minimize reflection of
incident light, and therefore enhancing the light absorption. As shown in
Figure 3.6c, the onset potential on the silicon nanowire photocathode was
0.2 V more positive than that of planar Si, confirming the large surface area
could suppress the surface over-potential of hydrogen generation. In the
presence of Pt electrocatalyst, photocurrent onset potential was further posi-
tively shifted to 0.4 V, which is the lowest onset potential ever reported for
p -type silicon photocathode [36].
To date, the report of p -type semiconductor photoelectrodes for water
splitting is still rare, compared with n -type semiconductors. In addition to
Cu 2 O and silicon discussed earlier, there are several other p -type materials
reported for water splitting, including GaP [37], GaInP 2 [38], and InP [39]
Electrochemical stability and surface kinetics are still the two major issues
for these semiconductor electrodes. The deposition of protective layer and
electrochemical catalysts are two of the best strategies to address these limi-
tations. On the other hand, in order to achieve nonbiased PEC device for
water splitting, it is equally important to develop high performance photo-
anode with photocurrent matching with the photocathode.
3.3.3 Photoanode for Water Oxidation
Various n -type semiconductors, such as silicon [40], metal oxides [11, 13,
25], and metal nitrides [15, 26], have been explored for use as photoanodes.
Among them, metal oxides are of special interest due to their simple syn-
thetic process, excellent chemical stability during water oxidation, and suit-
able band edge positions [4]. These metal oxides can be divided into binary
metal oxides, such as TiO 2 , ZnO, Fe 2 O 3 , and WO 3 , and ternary metal oxides,
such as BiVO 4 , SrTiO 3 , and CuWO 4 . In this section, we will review the recent
studies of metal oxide photoanodes for PEC water oxidation.
TiO 2 and α-Fe 2 O 3 are binary metal oxides commonly used for PEC water
oxidation. TiO 2 has suitable band edge positions for water splitting; however,
its large bandgap energy of 3.0-3.2 eV limits visible light absorption [11,
13]. In contrast, α-Fe 2 O 3 has a favorable bandgap of 2.1-2.2 eV with sub-
stantial visible light absorption, but its conduction band is more positive to
the hydrogen evolution potential. Additionally, α-Fe 2 O 3 has poor electrical
conductivity and short carrier diffusion length, which leads to significant
charge recombination loss [41, 42].
Recently, Wang et al. have developed a general strategy to incorporate
oxygen vacancies into metal oxides, such as TiO 2 , ZnO, and WO 3 via