Page 239 - Multifunctional Photocatalytic Materials for Energy

222 Multifunctional Photocatalytic Materials for Energy

e – Increasing Type-I Type-I 1/2 Type-II
–
0.00 V energy of CB –
electrons –
E
+ + +
1.23 V VB
h + Increasing
energy of
holes
Potential
v/s NHE
(A) (B)
Fig. 10.6 (A) The redox potentials for water splitting occur at 0 and 1.23 V with respect to
NHE. A semiconductor with straddling gap greater than 1.23 eV can be employed to carry out
redox reactions. (B) Core-shell nanoparticles can manifest limiting charge carrier localization
regimes as shown [14].
Reproduced from reference CdM. Donegá, Synthesis and properties of colloidal
heteronanocrystals. Chem. Soc. Rev. 40 (2011) 1512–1546, with permission of the
Royal Society of Chemistry. http://pubs.rsc.org/en/content/articlelanding/2011/cs/
c0cs00055h#!divAbstract

Thus the holes in the valance band can carry out oxidation whereas electrons in
the conduction band can carry out reduction. The ideal straddling condition required
by a semiconducting material to carry out spontaneous water splitting is shown in
Fig. 10.6A. If voltages are measured with respect to the normal hydrogen electrode
+
(NHE), then the conduction band must lie higher (more negative) than the H /H 2 (0 V
versus NHE) reduction potential, and the valence band must lie lower (more positive)
than the H 2 O/O 2 (1.23 V versus NHE) oxidation potential. Fig. 10.4B shows semicon-
ductors with band gaps and their corresponding redox potentials for water splitting.
Popular semiconductors such as Fe 2 O 3 or TiO 2 have band gaps ranging from 2.2 to
3.3 eV, which lie in the near-UV or UV region. In order to meet the conditions for re-
dox reactions involved in water splitting, the minimum band gap of the photocatalyst
must be 1.23 eV, which lies in the visible region of the electromagnetic spectrum.
Therefore, for semiconductors with band gaps lying in the near UV or UV range, it
is required that the band gap be suitably engineered. Besides having a suitable band
gap, the charge carriers in the semiconductor must have lifetime of about a few milli-
seconds to participate in the redox reaction. For this, a sufficient number of electrons
from the bottom of the conduction band must be transferred to the surface of the
semiconducting nanoparticle for hydrogen evolution, and therefore the lifetime of
electrons must be greater than the recombination time. Similarly, in order to oxidize
the organic contaminants, holes from the valance band of the semiconductor nanopar-
ticle must exist and be transferred to its surface. Different approaches have been
followed for charge separation, such as coupling semiconductors with staggered band
gaps, straddling type, discontinuous type nanocomposites of metal/semiconductors
where metals can serve as sink for electrons/holes, or by adding a hole (or electron)
scavenger species. Semiconducting heterostructures with semiconductors with stag-
gered band gaps, straddling type and discontinuous type, are depicted in Fig. 10.6B.