Figure 7 A) TEM images of Pt decorated and CdSe seeded CdS
nanorods (average length≈27nm). The insert is the STEM image of Pt tip,
the scale bar is 20 nm. B) Photocatalytic hydrogen production quantum
efficiency of Pt-tipped unseeded CdS hybrids (yellow) and seed diameters
of 3.1 (red) or 2.3 nm (green) Pt-tipped seeded CdS hybrids. The size
underneath are the corresponding average length of sample. C) Effect of
incident light wavelength on apparent quantum yield of hydrogen
production. (D) Effect of the light intensity (white light, 0-1.3 W) on
the hydrogen generation rate. A-D) Reproduced with
permission.[25a] Copyright 2010, American Chemical
Society.
3.5. Solution-liquid-solid method
In a typical metal tipped semiconductor heterostrucutre using for
hydrogen production reaction, the reduction reaction will occur in the
metal domain while the hole scavenger needs to be introduced to remove
OH- left in the semiconductor domain, otherwise the
catalyst will be damaged because lattice sulfide or the surface ligands
for passivating with thiol functional group might be oxidized, and
charge transfer will be slow down as the recombination of electron and
hole might also happens. To bi-functionalized the catalyst for effective
redox reaction to get both hydrogen and oxygen at the same time, the
co-catalysts are involved into this system.[26]
Figure 8A shows a schematic diagram of a photocatalyst with
co-catalyst.[26a] Wolff et al. synthesized
Pt-tipped CdS nanorods and employed a ruthenium complex (a derivative of
Ru (tpy)(bpy) Cl2) anchored on the surface of CdS
nanorods as an oxidation catalyst to further improve the efficiency of
hole and electron separation and hole transport for subsequent water
reduction and oxidation reactions, respectively. The photocatalytic
hydrogen evolution rate of CdS nanorods with or without supported
oxidation catalyst was tested. In the CdS-Pt system, triethanolamine
(TEOA) is used as the hole scavenger and the decoration of Pt and use of
TEOA obviously improve the charge seperation rate thus shows excellent
hydrogen evolution performance (Figure 8B and 8C). When hybrid nanorods
are decorated with the oxidation catalyst of RuDTC, the formation rate
of hydrogen is only reduced by 25% compared with the CdS-Pt with hole
scavengers. This suggests that the oxidation catalyst effectively
removes holes throughout the system, and the rate of oxygen generation
increases with the number of oxidation catalyst molecules anchored to
the nanorods. The characterization results showed that the introduction
of oxidation catalyst caused the transfer of ultra-fast hole to RuDTC in
300 fs, which is three orders of magnitude faster than the transfer into
hole scavenger. Even though the amount of oxygen generated is smaller
than the stoichiometric ratio towards hydrogen evolution, this means
there are a small number of holes that may not be able to be removed
from the system and the degradation rate of catalysts is significantly
diminished, thus demonstrating the superiority of the
co-catalysts.[26a]