Page 39 - Multifunctional Photocatalytic Materials for Energy

28 Multifunctional Photocatalytic Materials for Energy

was obtained, corresponding to a ten-fold increase with respect to pristine Fe 2 O 3 . At
variance with the latter, TiO 2 -containing samples did not display any saturation up to
1.80 V versus RHE, a feature that, along with the photocurrent increase, was traced
back to the formation of Fe 2 O 3 -TiO 2 heterojunctions [14], promoting a more efficient
charge separation than hematite. In addition, titania overlayers exerted a protective ac-
tion against α-Fe 2 O 3 photocorrosion [29]. The present performances correspond to the
highest ones ever reported for similar materials, especially at high applied potentials.
Based on these results, bare Fe 2 O 3 and Fe 2 O 3 -TiO 2 (H) photoelectrodes were tested us-
ing simulated seawater solutions as electrolytes, an important technological target for
real-world applications, since 97.5% of the overall water supply on Earth corresponds
to salt water [83,84]. Even under these conditions, the TiO 2 -containing photoelec-
−2
trode enabled to obtain higher J values than bare Fe 2 O 3 (≈0.4 versus 0.2 mA × cm
at 1.23 V versus RHE). Despite the performances are lower than those of NaOH solu-
tions, these preliminary results are very promising in view of sustainable energy gen-
eration starting from abundant and renewable natural resources through cost-effective
nanoheterostructured devices.

3.3.2 WO 3 -based materials

Tungsten(VI) oxide has been investigated for water splitting applications since the
early 1970s [85,86]. WO 3 is considered as a promising photoanode material, thanks
to its low cost, stability in aqueous solutions under O 2 evolution [19], band gap
(E G = 2.5−2.8 eV) [15,41,43,44] enabling Vis light harvesting, good electron transport
properties, and more favorable hole diffusion length (≈150 nm) than hematite [2,33].
On the other hand, WO 3 presents sluggish kinetics for photoproduced holes and rapid
electron-hole recombination, limiting its functional performances. To increase ma-
terial photoactivity, efforts have been dedicated to the preparation of nanostructured
WO 3 systems, with enhanced light absorption, high active area, and optimized carrier
transport properties [2,19,21]. To this aim, various synthetic routes have been adopted,
including anodization, sol–gel, sputtering, as well as electrochemical, solvothermal,
and hydrothermal processes [19,33]. In the latter context, Feng et al. [2] reported on
sandwich-structured WO 3 nanoplatelet arrays supported on FTO substrates. For these
materials, X-ray diffraction (XRD) revealed a direct dependence of the phase compo-
sition on annealing temperature [33]. As-prepared specimens contained only orthor-
hombic WO 3 ·0.33H 2 O, whereas after treatment at 400°C, the pattern was dominated
by hexagonal WO 3 . In a different way, upon annealing at 475°C, reflections from
monoclinic WO 3 were detected, and were also revealed at 500°C and 600°C. This
structural evolution was accompanied by appreciable morphological variations. For
as-prepared systems (Fig. 3.6A), multilayer nanoplatelets were obtained, and their
surface increased in roughness after annealing at 400°C (Fig. 3.6B). Upon treatment
at 475°C, a convex morphology at the nanoplatelet edges became evident (Fig. 3.6C),
whereas at 500°C the nanoplatelets showed a sandwich morphology with a thick mid-
dle layer and two thin side layers (Fig. 3.6D) and were almost perpendicular to the
substrate surface (Fig. 3.6F). Upon harsher annealing, polycrystalline aggregate arrays
were detected (Fig. 3.6E) [2].