Page 77 - Multifunctional Photocatalytic Materials for Energy

66 Multifunctional Photocatalytic Materials for Energy

band gap semiconductor, whereas the highest occupied molecular orbital (HOMO) of
the dye (the VB of the narrow band gap semiconductor) should lie between the VB
of the wide band gap semiconductor and the oxidation reaction potential [6,80,81].
Formation of heterojunction not only results in improved light absorption but also im-
proves the charge transfer at the interface [81]. Stability is the issue facing dye sensi-
tization for photocatalytic application. An effective electron donor must be employed
to regenerate and stabilize the excited dye [80]. Many narrow band gap semiconduc-
tors have been candidates for obtaining this sensitization, with the most popular ones
being colloidal quantum dots because of their high light absorption coefficient. The
challenge in heterojunction structure design is to precisely control the interfacial states
between two semiconductors, which applies to all designs having an interface [82,83].

4.5.4 Plasmonic photocatalysts

Incorporating a semiconductor with a plasmonic metal nanostructure is an alternative
strategy to enable the visible light absorption of wide band gap semiconductors or to
extend the absorption capability for those semiconductors whose band gap already
falls within the visible light absorption range [84–90]. Plasmonic metal nanostructure
is the carrier for surface plasmon resonance (SPR), which describes the interaction
between a metal nanostructure and the light and that occurs when the frequency of
the incoming light matches the oscillation frequency of the conduction electrons on
a metal surface. Two forms of SPR exist: (i) surface plasmon polariton (SPP) propa-
gating along the metal/dielectric interface (mainly occurring in a periodic structure),
and (ii) localized SPR (LSPR) with collective oscillation of the surface electrons in
the very vicinity of metal nanostructures. Owing to its nature regarding interaction
with light, SPR mainly works for enhancing light absorption, but also influences the
charge separation and recombination of even chemical reactions, because it is a kind
of electromagnetic field and may generate a huge amount of heat. Moreover, when
bringing metal together with a semiconductor, the Schottky junction between them
benefits the charge separation, and also metal itself is most of the time catalytically
active for some chemical reactions. All of these functions may co-contribute to the
overall enhancement of the performance. To distinguish each contribution is of great
importance for understanding the enhancement mechanism and designing efficient de-
vices [45]. In this chapter we mainly describe three pathways of plasmonic nanostruc-
tures for the light absorption enhancement in an adjacent semiconductor: (i) photonic
enhancement, (ii) hot electron injection, and (iii) plasmon-induced resonance energy
transfer (PIRET).

4.5.4.1 Photonic enhancement
Upon resonance, plasmonic metal nanostructures strongly concentrate and local-
ize the upcoming light into the vicinity of the nanostructures. As a result, the opti-
cal electric field near the surface of plasmonic metal nanostructures is substantially
enhanced and generates a high degree of absorption and scattering cross sections at
the resonant wavelength, generally several orders of magnitude greater than those for