Page 34 - Multifunctional Photocatalytic Materials for Energy

Metal oxide electrodes for photo-activated water splitting 23

3.3 Relevant case studies for photoanode development

In this section, as already mentioned in the introduction, attention will be dedicated
to four different types of metal oxide photoanodes, starting from Fe 2 O 3 , one of the
most studied systems, through WO 3 , ZnO, and BiVO 4 . In particular, Fe 2 O 3 , WO 3 , and
BiVO 4 are among the few stable n-type semiconducting oxides capable of harvesting a
significant sunlight fraction [9,13,41], whereas ZnO is an amenable candidate because
of its excellent electron mobility, as well as its favorable environmental compatibility
[28,34]. In each of the target cases, selected recent examples are provided not only
for single-phase oxide photoelectrodes, but also for composites, nanoheterostructures,
and systems doped or functionalized with oxygen evolution catalysts (OECs), with
emphasis on the design and tailoring of material chemico-physical properties as a
powerful tool to attain improved functional performances.

3.3.1 Fe 2 O 3 -based materials
Iron(III) oxide, and, in particular, hematite (α-Fe 2 O 3 ), its most stable polymorph, is
one of the most important functional materials for PEC water splitting photoanodes
[5,8,11,18], thanks to its abundance, nontoxicity, economic viability, stability in aque-
ous mixtures, and suitable E G (≈2.0 eV) to absorb a significant solar spectrum fraction
[11,29,69]. Unfortunately, these properties are adversely affected by various disadvan-
tages, among which: (i) an unfavorable conduction band edge position with respect to
water reduction potential (see Fig. 3.2) [11]; (ii) sluggish kinetics of OER reaction re-
quiring a large overpotential for water oxidation [1,11]; (iii) a limited majority carrier
conductivity; (iv) a relatively low absorption coefficient; and (v) a short hole diffusion
length (L D ≈ 2–4 nm) [1,14,29,33,69], much lower than for other oxides (for instance,
4
L D up to 10 and 150 nm for TiO 2 and WO 3 , respectively [1]), due to ultrafast carrier
recombination [69]. The combination of these features is responsible for performance
−2
losses, resulting in photocurrents much lower than the maximum 12.6 mA × cm pre-
dicted for an ideal hematite photoanode [1]. As a result, several studies have concen-
trated on tailoring Fe 2 O 3 photoelectrode properties in order to improve their functional
behavior. In particular, drawback (iii) was circumvented by adding impurity dopants
to obtain higher conductivity values, depending on the dopant type and concentration
(e.g., Ti(IV), Sn(IV), Zr(IV), among the most used) [1,5].
Various efforts have demonstrated that: (i) optimizing hematite electrode nano-
organization can increase photocurrent density, and (ii) surface functionalization can
lower the required overpotential. The combination of these strategies can enable to
approach the performances of an ideal α-Fe 2 O 3 photoanode (Fig. 3.3A) [70]. In this
regard, Graetzel et al. [69] reported on the fabrication of hematite photoelectrodes
on FTO substrates via a solution-based route followed by air annealing. As shown
in Fig. 3.3B, an increase in the annealing temperature from 400°C to 800°C resulted
in the obtainment of larger feature sizes and in a porosity enhancement (Fig. 3.3C).
Correspondingly, variations in the optical absorption coefficient (α) spectra took place
(Fig. 3.3D). In particular, whereas the absorption band located at ≈540 nm was almost
unchanged by the processing temperature, the one at λ ≈ 380 nm underwent a red