Page 204 - Multifunctional Photocatalytic Materials for Energy

190 Multifunctional Photocatalytic Materials for Energy

In other words, the standard potential reduction of the proton solution (0 V) corre-
sponds to −4.5 eV on an absolute scale [13].
This means that to meet the requirements of the energetic levels of valence/con-
duction bands, the photocatalyst should have a band gap greater than 1.6–1.8 eV.
Moreover, with the aim to efficiently use natural solar light radiation, the value of the
band gap should be less than about 2.2 eV (Fig. 9.2):
On the contrary, in catalytic photoreforming, the sacrificial agent is oxidized by
photogenerated holes forming protons, which are reduced by photogenerated elec-
trons to generate hydrogen gas (Fig. 9.3).
In this case, the top of the valence band should be more positive than the oxidation
potential of the sacrificial species to be oxidized. However, the band gap of the photocat-
alyst represents the minimum energy thermodynamically required to drive the process.
In addition, the lifetime of the photogenerated electron-hole pair should be as long
as possible in order to react with oxidant and reductant species rather than recombine.

NHE (V)
Conduction band
<0
+
H +e − 0.5 H 2 0
1.6−1.8 eV ≤∆E ≤ 2.2 eV
g
½ H O + h + ¼ O +H +
2
2
1.23
>1.3
Valence band
Photocatalyst
Fig. 9.2 Band gap and potential edges for solar-driven photocatalytic water photosplitting.

H + H 2

e −
H + e − e −
h +
h +
CO 2
h +

Sacrificial
agents
Fig. 9.3 Schematic representation of photocatalytic reforming.