Page 203 - Multifunctional Photocatalytic Materials for Energy

Metal-based semiconductor nanomaterials for photocatalysis 189

9.3 Photocatalyst requirements

Choosing the photocatalyst is a crucial point, as a number of requirements should be
addressed. First of all, according to the principle of microscopic reversibility, the cat-
alytic material promotes both the direct (water dissociation) and the reverse reaction
(hydrogen-oxygen recombination).
Second, photogenerated electron-hole pairs can be involved in three main processes:
migration to the surface of semiconductor;
●
capture by defect sites located in the bulk and/or on the surface of the photocatalyst; and
●
parasitic recombination releasing energy as heat or photons (luminescence or phosphorescence).
●
In order to (i) suppress hydrogen-oxygen recombination, (ii) prevent the electron-hole
−9
recombination process (timescale: c.10 s), and (iii) promote the migration of photo-
generated charge carriers to the surface where they may react with the water molecules
−1
−8
or other reagents (timescale: 10 –10 s), the following strategies can be adopted:
band gap engineering of the photocatalytic materials (water photosplitting and photoreforming);
●
purging the reactive system with an inert gas (water photosplitting and photoreforming); and
●
use of suitable sacrificial agents (photoreforming).
●
Third, because water molecules or organics are involved in redox reactions with
photogenerated electrons and holes, it is very crucial that the energy value of the band
gap and the energetic levels of the conduction/valence bands are consistent with the
standard reduction potentials of the half-redox reactions. In particular, in water photo-
splitting, H 2 O molecules are simultaneously reduced by photogenerated electrons and
oxidized by photogenerated holes:
hn
Semiconductor ® e + h +
-
cb vb
+
+
HO 2 h ® 0 5 O + 2 H + E = 123 V
.
.
2 vb 2 HO O 2
2 /
+
-
2H + 2e ® H 2 E H / H 2 = 0V
+
cb
Therefore the bottom level of the conduction band of the photocatalyst has to be
+
more negative than the redox potential of the H /H 2(g) semi-couple (0 V vs. NHE at
pH = 0), whereas the top level of the valence band should be more positive than the re-
dox potential of O 2(g) /H 2 O (l) (1.23 V vs. NHE at pH = 0). Overall, in order to efficiently
carry out the previous semi-reactions, an overpotential (i.e., the redox potential of the
+
H /H 2(g) semi-couple is −0.421 V at pH = 7.0) for the band-edges of the semiconductor
is required. The redox potentials (U redox , V vs. NHE) of the conduction and valence
+
levels, referenced to H /H 2(g) semi-couple (0 V), can be related to the electronic energy
levels (E redox , eV in absolute scale), which are referenced to the energy level of an
electron in vacuum (0 eV) through the following relation:
-
E =-45 eV U
.
redox redox