Page 69 - Multifunctional Photocatalytic Materials for Energy

58 Multifunctional Photocatalytic Materials for Energy

aims to extend the light absorption range by introducing new internal band energy
levels or by narrowing the band gap.
η sep stands for the separation of the photoexcited electrons and holes, and their
following transport to the current collector and the electrolyte interface [44]. η sep is
in reverse proportion to the recombination either in bulk or at internal interfaces. The
intrinsic electronic features such as charge carrier mobility and diffusion length deter-
mine the η sep .
η inj represents the efficiency of charge transfer and injection at the photoelectrode/
electrolyte interface, and is decreased by the surface recombination and poor OER/
HER kinetics [44,45]. High values of η inj have been achieved either by coating a
passivation layer on the surface to suppress surface recombination or by depositing
co-catalysts to improve the surface reaction kinetics (oxygen evolution reaction/OER
catalysts such as Co-Pi, FeOOH, and NiOOH on photoanode or hydrogen evolution
reaction/HER catalysts such as MoS 2 and Pt on photocathode) [44–49].
In practical devices, particularly for metal oxides, the value of the product of η abs
and η sep (i.e., η abs × η sep ) always remains low, which is a challenge because η abs and η sep
are coupled together inversely: increasing the thickness of photoelectrode, increases
η abs but decreases η sep , and vice versa. For example, hematite has a very short hole dif-
fusion length of 2 nm to 4 nm but a 180 nm penetration length of 550 nm. This means
most photogenerated charge carriers in thick hematite film recombine during the sep-
aration process and cannot contribute to surface reactions [13–15].

4.4 Metal oxide photocatalysts

Compared with the conventional inorganic semiconductors, metal oxide semiconduc-
tors demonstrate a different electronic band structure that endows them with new op-
toelectronic properties and design concepts, and even novel functions [7,18]. In this
section, we briefly review the electronic energy band structure of metal oxide semi-
conductors and list the most typical benchmark metal oxide photocatalysts applied in
solar fuel generation.

4.4.1 Electronic energy band of metal oxide photocatalysts

Each semiconductor's electronic energy band has three regions (Fig. 4.1). If we ex-
amine further, we see that the VB of metal oxides is composed mainly of the O 2p
character, and the CB mainly comes from the metal s character and/or d character
depending on the electronic configurations of the metal ions [50,51]. Fig. 4.6 shows
the electronic structures of anatase and rutile TiO 2 . It is clear that the VB-edge of both
materials is dominated by O 2p and that the CB-edge is formed from Ti 3d. The O 2p
character has a very positive potential as well as a high ionic metric, which generates a
large separation between band edges and consequently leads to the wide band gap for
most metal oxide semiconductors [50]. Seen another way, these electronic states are
very sensitive to changes in the surrounding environment, which provides viable pos-
sibilities for tuning the electronic band structure and thus the light absorption for metal