Figure 11 A) Static absorption (solid lines) and
photoluminescence (PL, dashed lines) spectra of CdSe/CdS NRs (black
lines) and CdSe/CdSPt nanorods (red lines). B) Steady state
photocatalytic hydrogen evolution QYs using MUA-capped CdSe/CdS-Pt and
CdS-Pt nanorods, with either
methanol or sodium sulfite as additional sacrificial electron donors.
The insert shows the CdSe seeded CdS nanorod with Pt tip. C) Comparison
of quantum efficiencies for Pt-decorated CdS nanorods using
SO32-, TEA, EDTA4-,
and MeOH as hole scavengers. Data for different ligands (i.e., MPA and
cysteine) are shown. The inset shows the hydrogen evolution as a
function of time for MPA-stabilized nanorods and different hole
scavengers. D) Schematic plot of the energy levels of the electrons of
hole scavengers that are involved in the reduction of the photohole vs.
vacuum and on normal hydrogen electrode (NHE) scale. A-B) Reproduced
with permission.[32] Copyright 2014, American
Chemical Society. C-D) Reproduced with
permission.[33] Copyright 2012, American Institute
of Physics.
3.9. Other strategies nanorods
In addition to the widely studied 1D colloidal metal-semiconductor
nanorod heterostructures and the aforementioned engineering strategies,
there are many other systems and strategies to improve photocatalytic
hydrogen production efficiency. Most of them follow the same principle,
i.e. to achieve better charge separation.[1f,34]Zhang et al. conducted amine-assisted directional growth of CdS
nanords with MoS2 tips (M-t-CdS) (Figure 12A). Results
show that the photocatalytic activity of the M-t-CdS heterostructure
containing 3.5wt% of MoS2 is optimal. The PL emission
of the M-t-CdS nanorods trap states is also significantly reduced which
indicates their longer lifetime (photocatalytic hydrogen production can
be carried out for more than 23 hours). The M-t-CdS heterostructure
provides a large contact area between CdS nanorods and hole scavengers,
thus hindering recombination of photoelectron and hole to achieve
efficient carrier separation (Figure 12B and
12C).[35] Besides, the coating of amorphous
MoS3 surface on
semiconductor nanorods was studied by Tang et al. who used
(NH4)2MoS4 with one-step
heat treatment to deposit amorphous MoS3 film on CdSe /
CdS nanorods (Figure 12D and 12E). The close contact between the
MoS3 surface layer and the nanorods enhances charge
transfer. The system is highly active and can efficiently produce
photocatalytic hydrogen. A drawback of this system is that the surface
coating is relatively easy to dissolve, even in the absence of light.
This thereby reduces the rate of hydrogen generation over time (Figure
12F).[36]
Kameyama et al. studied a special nanocrystals heterostructure of
ZnS-AgInS2 solid solution
((AgIn)xZn2(1-x)S2,
ZAIS). The dumbbell-shaped Zn-Ag-In-S nanocrystals with adjustable
photocatalytic activity were prepared by growing ZAIS nanocrystals at
both ends of ZAIS nanorod (the composition of the rods is
(AgIn)0.24Zn1.52S2). By
regulating the x value of ZAIS nano-ellipsoid
(AgIn)xZn2(1-x)S2(Figure 12G-I), the optimal combination was sought. Since the conduction
band and valence band edge energy levels of ZAIS nanocrystals are
regulated by shape, size and composition, they can be used to design the
band alignment of ZAIS nanocrystal heterojunctions, which will greatly
improve the photocatalytic performance of ZAIS nanocrystals (Figure
12J).[37]