Solar Hydrogen Generation: Photocatalytic and Photoelectrochemical Methods - Hydrogen Generation, Storage, and Utilization - page 44

stoichiometric ratio of the elements could be used to fine-tune the potentials
of valence and conduction bands as well as the bandgap energy. Therefore,
ternary metal oxides show promise for developing high efficient photoelec-
trode for water splitting with suitable bandgap and band edge positions.
As an example, BiVO 4 is a direct bandgap ternary metal oxide semicon-
ductor with a favorable bandgap of 2.3-2.5 eV for solar light absorption.
Furthermore, its conduction band is close to 0V versus RHE at pH = 0, as
a result of the hybridization of empty Bi 6p orbitals with antibonding V 3d-O
2p states, which reduce the need for external bias for PEC water splitting.
However, charge transport and interfacial charge transfer have been found to
be key limiting factors for its PEC performance. A number of methods have
been explored to address these limitations. Element doping with Mo and W
has been found to be effective in increasing the PEC performance of BiVO 4 ,
as M and W introduce shallow donors that improve the separation and trans-
port of photoexcited carriers. In addition, porous electrode was fabricated to
provide a large surface area and shorten the diffusion distance for minority
carriers. For instance, Pilli et al. reported cobalt phosphate (Co-Pi)-modified,
Mo-doped BiVO 4 photoelectrode for solar water oxidation [47]. Mo-doped
BiVO 4 was prepared using a surfactant assisted metal organic decomposition
technique at 500°C. The Mo-doped BiVO 4 film exhibited absorption in the
visible region up to 520 nm. The bandgap was estimated to be around 2.4 eV.
Co-Pi catalyst was deposited on the surface of Mo-BiVO 4 by electrochemi-
cal deposition method. The role of Co-Pi catalyst is used to reduce the
overpotential of BiVO 4 for water oxidation. Importantly, the photocurrent
density of Mo-doped BiVO 4 electrode was enhanced compared with that of
the BiVO 4 electrode. The photocurrent of porous BiVO 4 is higher than non-
porous BiVO 4 . Moreover, significantly enhanced photocurrents were observed
for the Co-Pi catalyzed electrodes in the entire potential range, compared
with unmodified electrodes. The Co-Pi electrodeposited electrode also
exhibited around 150 mV cathodic shift from the onset potential for PEC
water oxidation, as compared with unmodified BiVO 4 .
Although BiVO 4 has favorable bandgap, its relatively poor charge trans-
port properties causes a significant electron-hole recombination loss.
Recently, Hong et al. reported heterojunction BiVO 4 /WO 3 electrodes that
shown enhanced photoactivity for water oxidation [48]. BiVO 4 and WO 3
form a type II junction at the interface, and photogenerated electrons from
the conduction band of BiVO 4 are thermodynamically favorable for transfer-
ring into the WO 3 layer. Meanwhile, the photoexcited holes in WO 3 are also
favorable for transferring to BiVO 4 for water oxidation. The formation
of heterojunction facilitates charge separation and thus suppresses the
electron-hole recombination. The heterojunciton electrodes were fabricated