Page 36 - Multifunctional Photocatalytic Materials for Energy

Metal oxide electrodes for photo-activated water splitting 25

The improvement of material photoactivity could be traced back to the increase in
porosity and in light harvesting, of crucial importance in determining the recorded J
values, and to the diffusion into the film of Sn(IV) centers from the substrate, acting
as donor impurities [72]. The latter phenomenon, which took place only at 800°C,
exerted a beneficial role on material conductivity and photoactivity [69].
Further efforts were devoted to the surface functionalization of α-Fe 2 O 3 systems by
OECs, with the aim of facilitating the complex four-electron water oxidation, over-
coming the already mentioned OER kinetic issues [11,14,18]. To this aim, rutheni-
um(IV) and iridium(IV) oxides have proved to be effective [33,73], but Ru and Ir
are precious metals with limited commercial potential. Among noble metal-free OER
catalysts, cobalt phosphate (CoPi), exploiting the cyclic valence change of Co ions
between Co(II)/Co(III) and Co(III)/Co(IV), has been successfully integrated with var-
ious photoanodes, including hematite-based ones [10,13,14,27,62,65,72]. A viable,
though less explored, alternative is offered by the use of Mn oxide-based catalysts,
thanks to the Mn multi-oxidation states, that play an important role in enhancing the
local hole transport [11]. In a recent work, α-Fe 2 O 3 nanorod arrays were fabricated by
a hydrothermal process on FTO substrates and functionalized via spin coating with
preformed MnO nanoparticles (NPs) [11]. For comparison, CoPi catalysts were pho-
toelectrodeposited onto hematite nanosystems under controlled conditions. Structural
and optical analyses on pristine, MnO-loaded, and CoPi-treated hematite nanomateri-
als revealed no significant variations and a very similar gap value (E G ≈ 2.1 eV). SEM
images highlighted the presence of nanorod-like hematite nanostructures on the FTO
substrate, and functionalization with MnO resulted in the formation of a very thin top
layer (Fig. 3.4A). CoPi-treated systems showed similar morphology and thickness.
Fig. 3.4B sketches the mechanism of charge separation at the MnO-hematite interface,
whereas the current density/potential curves obtained upon simulated sunlight irradi-
ation are shown in Fig. 3.4C. As can be seen, J values at 1.23 V versus RHE increased
−2
−2
from 1.21 mA × cm , for the pristine hematite electrode, up to 1.45 and 2.06 mA × cm
after functionalization with CoPi and MnO, respectively. Additionally, both CoPi and
MnO introduction produced a negative shift of the onset potential. Current density
versus time measurements revealed a comparable stability of the three photoanodes,
an important prerequisite for practical utilization. Further studies [11] pointed out that,
whereas the beneficial MnO effect was attributed to the higher charge injection into
the electrolyte, CoPi mainly exerted a surface passivation effect. The possibility of im-
proving hematite PEC performances through passivation of surface states has also been
explored in the formation of oxide-Fe 2 O 3 nanoheterostructures via the introduction
of various overlayers, among which Al 2 O 3 , Ga 2 O 3 , Fe x Sn 1-x O 4 , and TiO 2 [31,74–79].
Additional benefits of such strategies include the corrosion protection of the underly-
ing iron oxide deposit and proper tailoring of charge transfer processes between the
constituent phases. Nevertheless, carrying out PEC water splitting more efficiently
than with actual state-of-the-art hematite photoanodes [80] remains an open challenge.
In this regard, a viable approach has involved coating Fe 2 O 3 nanostructures prepared
via plasma enhanced-chemical vapor deposition (PE-CVD) by atomic layer deposi-
tion (ALD) TiO 2 overlayers with different thickness [29], taking advantage of ALD
repeatability, conformality, and precise thickness control [14,81,82]. Fig. 3.5A and B