Low-Cost Nanomaterials for Photoelectrochemical Water Splitting - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 276

air [ 74 ]. Recently, Chen et al. prepared large area and high-quality WO 3 photonic
crystal photoanodes with inverse opal structure [ 6 ]. This 3D-photonic crystal
design is used to enhance light absorption and further increase IPCE of WO 3
photoanodes. The photonic stop-bands of these WO 3 photoanodes can be tuned
experimentally by variation of the pore sizes of inverse opal structures. It was
found that when the red-edge of the photonic stop-band of WO 3 inverse opals
overlapped with the electronic absorption edge of WO 3 at Eg = 2.6-2.8 eV, a
maximum of 100 % increase in photocurrent intensity was observed under visible
light irradiation (k [ 400 nm), in comparison with a disordered porous WO 3
photoanode [ 6 ]. When the red-edge of the stop-band was tuned well within the
electronic absorption range of WO 3 , noticeable but less amplitude of enhancement
in the photocurrent intensity was observed. It was further shown that the spectral
region with a selective IPCE enhancement of the WO 3 inverse opals exhibited a
blue-shift in wavelength under off-normal incidence of light, in agreement with the
calculated stop-band edge locations. (Fig. 9 ) The enhancement was attributed to a
longer photon-matter interaction length as a result of the slow-light effect at the
photonic stop-band edge, thus leading to a remarkable improvement in the light-
harvesting efficiency. This method can provide a potential and promising approach
to effectively utilize solar energy for visible light responsive photoanodes.
Additionally, WO 3 is known to be thermodynamically unstable in electrolyte
solution with pH [ 4 due to OH - -induced chemical dissolution and photocorrosion
induced by the peroxo species created during water oxidation [ 106 ]. In order to
suppress the formation of peroxo species, oxygen evolution catalyst has been used to
modify the surface of WO 3 [ 52 , 78 ]. For example, Seabold et al. demonstrated the
photo-oxidation reactions and photostability of electrochemically prepared WO 3
can be improved by coating with Co-Pi OEC catalyst (Fig. 10 )[ 78 ]. When a bare
WO 3 photoanode was used, a significant portion of the photogenerated holes (ca.
39 %) were used to form peroxo species and dissolve WO 3 ; therefore, the accu-
mulated peroxo species on the WO 3 surface result in a gradual loss of photoactivity.
In contrast, when a thick Co-Pi OEC layer ([1.5 lm) was deposited on the WO 3
electrode, most photon-generated holes were used for O 2 production and suppress
the formation of peroxo species. The suppression of the peroxide species not only
increases O 2 production, but also improves the long-term photostability of WO 3 .
Alternatively, Wang et al. also demonstrated that oxygen-deficient tungsten oxides
are resistive to the peroxo species-induced corrosion [ 99 ]. They showed that the
photostability and photoactivity of WO 3 for water oxidation can be simultaneously
enhanced by self-doping with oxygen vacancies via hydrogen treatment. The
improved photoactivity was believed to be due to the increased donor densities,
which could facilitate charge transport. Importantly, the photocurrent of hydrogen
treated WO 3 was stabilized for at least 7 h without substantial loss, while pristine
WO 3 lost 80 % of initial photocurrent in the first two hours. The results proved that
hydrogen treatment is an effective method to improve and stabilize the photoactivity
of WO 3 .