Page 80 - Multifunctional Photocatalytic Materials for Energy

Energy band engineering of metal oxide for enhanced visible light absorption 69

height of the Schottky barrier formed at the metal/semiconductor interface, the hot
electron could inject into the CB of the adjacent semiconductor and thus improve pho-
tocatalytic performance. The unique character for the hot electron injection is that the
enhancement exactly follows the LSPR spectrum [86,92–96]. Strong evidences were
observed in Au-TiO 2 heterojunctions, where photocatalytic activity appeared in the Au
LSPR region, but that region was inert for bare TiO 2 or Au alone. This process works
like photosensitization in Section 4.5.3. Therefore these metallic nanoparticles are
also named plasmonic photosensitizer [94]. However, their advantage over traditional
photosensitizers is that plasmonic hot electron injection can take place with the LSPR
energies either below or above the band gap of the adjacent semiconductor without
mandatory requirement for the thermodynamic band alignment that ensures down-
ward electron transfer [6,94,96]. This breakthrough offers the flexibility needed for
full utilization of the sunlight spectrum with plasmonic nanostructures whose LSPR
can be tuned with material, size, shape, and surroundings. As shown in Fig. 4.12B,
the sunlight spectrum from UV to infrared region can be fully covered by LSPR [86].
Theoretically, any hot electron that has enough energy to overcome the Schottky bar-
rier could transfer to a semiconductor and improve efficiency. However, the efficiency
of hot electron injection in practical devices is still lower than expected. Hidden path-
ways causing energy losses during hot electron transfer may exist and need to be
explored in order to design efficient plasmonic photocatalysts by fully utilizing plas-
monic hot electrons.

4.5.4.3 Plasmon-induced resonance energy transfer (PIRET)
In addition to the hot electron injection, another pathway for plasmonic energy trans-
fer is PIRET. This energy transfer process was found in, but not limited to, a plas-
monic Au@SiO 2 @Cu 2 O sandwich structure, where plasmon-enhanced photocatalytic
activities appeared even with an electron-insulating SiO 2 layer (Fig. 4.12C) [93,95].
It was believed that PIRET proceeds via a nonradiative transfer from plasmonic
metal to semiconductor because of the dipole-dipole coupling between the LSPR of
the plasmonic metal and the semiconductor, which could excite the semiconductor
and generate charge carriers [96]. Advantageous to hot electron transfer, the dipole-
dipole interaction does not require an intimate contact between the plasmonic metal
and the semiconductor. PIRET allows the charge separation in the semiconductor at
energies above or below the band gap; however, transfer efficiency depends on the
spectral overlap between the semiconductor's absorption band and the LSPR band of
the plasmonic metal, as well as the distance between them [6,96]. Different plasmonic
enhancement mechanisms (photonic enhancement, hot electron transfer, and PIRET)
may coexist in one plasmon/semiconductor heterojunction [91,94].

4.5.5 Multijunctional systems
To date, numerous semiconductors have been studied as photoelectrode materials
for PEC water splitting in a half-cell configuration, as shown in Fig. 4.5B and C.
However, a half cell has difficultly driving overall water splitting. The half cell must