Figure 5 TEM images of A) CdS-Au hybrid nanorods with 1.5±0.2
nm and B) 7.1 ± 0.8 nm Au tip. C)
Hydrogen evolution efficiencies
ratio between small-tipped and large-tipped heterostructures as a
function of excitation fluence presented as peak intensity of the
irradiation beam (red circles)
(insert: normalized hydrogen generation %QY of small-tipped hybrids
(red) and the large-tipped hybrids (blue) as a function of excitation
fluences presented as peak intensity of the irradiation beam. Green
circles represents the corresponding peak intensities used in the TA
experiments. D) Photocatalytic quantum efficiency for the water
reduction of Ni-CdSe@CdS hybrids with different tip size. Experimental
quantum efficiency in dark green bars, and the light green bars are
quantum efficiency corrected for metal absorption. The inserts is the
TEM image of corresponding Ni-CdSe@CdS nanorods, from left to right: 2.3
nm, 3.1 nm, 5.2 nm, 8.9 nm, and 10.1 nm. E) Hydrogen production rate
(blue) and Cd normalized rate (red) curves as a function of Au size
domain in the hybrid nanoparticles. Negligible rates are measured for
the CdS nanorods. The red curve is normalized to total Cd content to
better express the basic metal size effect during hydrogen reduction. F)
Measured QY (black dashed line) along with the non-monotonic kinetic
model behaviour (blue solid line). Green and red dotted lines present
limiting behaviours of the model for zero and infinite metal domain
sizes, respectively. Error bars show the size distribution of the Au tip
and the hydrogen production rate uncertainty. The scale bar is 20 nm.
A-C) Reproduced with permission.[18] Copyright
2018, American Chemical Society. D) Reproduced with
permission.[19] Copyright 2017,
American Chemical Society. E-F)
Reproduced with permission.[20] Copyright 2015,
Springer Nature.
Ben-Shahar et al. found that CdS-Au hybrid nanorods with large Au
tips of a diameter of 7.1 ± 0.8 nm (Figure 5B) can better facilitate the
separation and transference to metal domains of multiple excitons
generated by high excitation fluence than small Au-tipped (1.5 ±0.2 nm)
hybrid nanorods (Figure 5A). Figure 5C compares the quantum yield (QY)
ratio of the large-tip to small-tip hybrid nanorods, indicating that the
small tip favors hydrogen evolution in the single exciton region, and
while in the multi-exciton region this ratio drops sharply to near 1:1.
This is mainly attributed to the transcendence of Auger recombination
over electron transfer from the semiconductor to the metal tip for small
tip hybrid nanorods, resulting in the loss of most of the excited
electrons. Although the increasing the intensity of excitons at the
initial stage of excitation can improve the QY, it eventually reduces
the QY in the small-tip hybrid nanorods. For large-tip hybrid nanorods,
the photocatalytic performance can be enhanced distinctly because the
ultrafast electron transfer plays a dominant role than the Auger
recombination and can extract all the excited
electrons.[18] Nakibli et al. prepared
CdSe@CdS hybrid nanorods with different sizes of Ni tips, and the
characterizations showed that the lowest emission QY, maximum amplitude
value (amplitude reflected charge transfer probability) and the faster
depopulation process of conduction band were obtained on the 5.2
nm-sized Ni tips, which is consistent with the results of the highest
quantum efficiency achieved at this size (Figure
5D).[19] This is reasonable given the fact that
the two processes of Coulomb blockage, which has a greater influence on
the small-tip, and the Schottky barrier height, which affects the
large-tip more, compete with each other, thereby producing an optimum
radius in the middle.[19] Selective small metal
island growth on the CdS nanorods was obtained by varying the
irradiation time and the Au3+/nanorod ratio. Figure 5E
(blue curve) shows the relationship between hydrogen production rate and
Au tip size. The weak dependence observed on the two smallest dimensions
while the photocatalytic performance of the larger Au domain is
significantly reduced. Figure 5 F shows that the non-monotonic
relationship between QY and Au tip sizes which is consistent with the
standardized experiment QY (dotted line connected square). The opposite
behavior of QY at the small and large size of Au domain could be
attribute to the different main factors influencing the charge transfer
rate. For the former, the precipitation of hydrogen QY is mainly
determined by the electron injection rate at the metal tip, while the on
the larger Au tips the water reduction on the metal surface will play
the dominant role. Therefore, the intermediate Au tip size provides the
best balance between charge separation rate and
efficiency.[20]
3.3. Surface location of metal nanoparticles
The place where metal was decorated on the semiconductor nanorods also
affects the catalytic performances of the photocatalysts, owing to the
variation of the rate of charge transfer and effectiveness of
electron-hole seperation. Besides, the loading in different positions
will change the amount of metal requirements, which can affect the cost
of the noble metal-semiconductor
photocatalyst.[21]