Page 71 - Multifunctional Photocatalytic Materials for Energy

60 Multifunctional Photocatalytic Materials for Energy

photocatalytic performance reported so far. TiO 2 is photochemically stable under
harsh conditions, even in very strong acid or basic electrolytes. TiO 2 ’s shortcoming is
its wide band gaps (3.2 eV for anatase and 3.0 eV for rutile phase), which restricts its
light absorption within the ultraviolent range (<5% of sunlight, poor light absorption
efficiency η abs ) and determines a low STC efficiency of 1.3% for anatase and 2.2% for
rutile when used individually for solar fuel generation [6]. TiO 2 has a very long elec-
tron diffusion length up to micrometer scale, so TiO 2 can act as an electron acceptor
and transporter by constructing a heterojunction with a narrow band gap semicon-
ductor outside for visible light absorption. Doping TiO 2 either with transition metal
cations replacing Ti or with anionic species replacing O can extend its light absorption
to the visible light region, but photoresponse generally is weak in these cases. Please
see a detailed explanation of this topic in Section 4.5.1.

4.4.2.2 Hematite/Fe 2 O 3
Hematite is a representative metal oxide for visible light absorption [13–15]. It has a
band gap of 1.9–2.2 eV, which covers most fractions of the visible light. Theoretically,
based on this band gap energy, the maximum STC efficiency for hematite can reach
2
12.9% with a photocurrent of 10.5 mA/cm . However, practical conversion effi-
ciency is far lower than the theoretical estimate because of hematite's poor hole dif-
fusion behaviors (short diffusion length of 2–4 nm and low mobility on the order of
2
−1
−1
−2
10 cm V S ). Hematite has a large light penetration depth of 118 nm for a 550 nm
photon, which results, however, in a high bulk charge recombination rate given the poor
hole diffusion behaviors [6,13–15,45]. Therefore an ideal hematite photoelectrode is a
nanostructure with a long electron diffusion channel with a suitable thickness compa-
rable to the hole diffusion distance, which enables quick extraction of holes to the sur-
face, and thus suppresses the bulk charge recombination. Another issue for hematite
is its very sluggish water oxidation kinetics on the surface, i.e., low charge injection
efficiency η inj , which causes the accumulation of holes near the surface. Oxygen evo-
lution reaction (OER) co-catalysts such as Co-Pi and Co oxides are always needed.
Sandwich structures with a nanostructured conductive Sb:SnO 2 rod or a porous FTO
as the scaffold, a thin hematite film as the medium absorber layer, and water oxidation
co-catalysts as an outside layer have been developed to partially address η abs , η sep , as
well as η inj . The highest photocurrent achieved recently on high surface area hematite
2
photoelectrode by anodizing ion foil was recorded as up to 5 mA/cm at 1.55 V versus
2
RHE (6.8 mA/cm with co-catalysts) [52].
4.4.2.3 BiVO 4 (BVO)

BVO has two crystal structures: tetragonal and monoclinic. The latter has a smaller
band gap of 2.4 eV for visible light absorption (see Section 4.5.2), which corresponds
to a theoretical maximum STC efficiency of 9.1% and a maximum photocurrent of
2
7.4 mA/cm . In the visible light range (420–530 nm), BVO has a light penetration
depth of around 500 nm, but its electron diffusion length is only on the scale of 10 nm,
which leads to an undesired bulk charge recombination. This issue can be addressed
by increasing the electron diffusion length to ~300 nm by doping BVO with Mo and