Figure 6 A) Schemes of the formation of hydrogen on a single
(a), double (b) metal catalyst. B) Photocatalytic hydrogen evolution
quantum efficiency of Pt-CdSe@CdS hybrids with single, double, or
multiple tips. C) Scheme of the metal domain decoration and its phase
transfer to water. The red and blue background representative the
organic and aqueous environment, respectively. D) Photocatalytic
Hydrogen generation in neutral aqueous solution contains sulfite ions by
using Pt-CdSe@CdS hybrids with zero, single or multiple tips. E)-F) TEM
images of E) single, and F) double Pt-CdSe hybrids, the scale bar is 10
nm. G) Time course of hydrogen generation in a 0.35 M
Na2SO3/0.25 M Na2S
aqueous solution by using Pt-CdSe and Au-CdSe hybrids with single or
double tips. A-B) Reproduced with permission.[22]Copyright 2015, American Chemical Society. C-D) Reproduced with
permission.[23] Copyright 2016, American Chemical
Society. E-G) Reproduced with permission.[24]Copyright 2012, American Chemical Society.
Nakibli et al. proposed a general rule for the establishment of a
catalytic system, that is, in the catalysis of multi-electron reactions,
the use of a catalyst having a single active site would achieve better
results.[22] They hypothesized that the first step
in the water redox half reaction is that the bonding between
H+ and catalyst lead to the formation of
intermediates. In the second step, the H2 released by
the combination of either two intermediates, or an intermediate with an
H+ and electrons. Therefore, the two electrons
generated in the semiconductor must be transferred to the same metal
domain in order for the second step proceeding smoothly (Figure 6A). To
verify this hypothesis, CdS nanorod with asymmetrically embedded CdSe
quantum dots were prepared, and Pt nanoparticle was grown onto CdS
nanorods to form hybrid structures with single, double or multiple
reduction sites, respectively. Figure 6B shows that hybrid nanorods with
a single metal decoration achieve the best quantum efficiency (QE). The
release of H2 associated with the absorption of two
photons for a photocatalyst occurs on a single reduction site. Due to
the influence of the Coulomb repulsion and multiple electrons tend to
flow to different active sites when photocatalysts have more than one
reduction locations. In such a case, the intermediate must wait for
another photon to be absorbed to the same position to complete the
reaction, prolonging the reaction time and decreasing the reaction
rate.[22] Figure 6C illustrates two types of
metal-CdS hybrid nanorods, one with Pt grown at one end and the other
was randomly decorated with multiple Pt
clusters.[23] Good contact between the nanorods
and Pt particles ensures the possible transfer of the photoexcited
electron to the metal domain. Since the distance from the exciton to the
Pt domain is much shorter in a multiple-Pt-cluster sample, the electron
transfer rate and efficiency are also significantly increased than
Pt-tip sample. However, the structure of randomly decorated nanorods
shows worse hydrogen production effect than that of the tip decorated
structure. They believe that although the speed at which photoexcited
electrons transfer to the metal domain increased in the
multiple-Pt-cluster sample, it does not guarantee an increase in the
amount of electrons involved in the water redox half reaction. In fact,
this shortened distance leads to faster charge recombination, which
exceeds the advantageous impact of faster electron transfer. It can also
be noted in Figure 6D that the catalytic efficiency of samples adorned
by multiple Pt clusters gradually decreases over time, possibly due to
photooxidation of the hole. This work clearly shows that fewer metal
domains with diminishing electron-hole recombination are beneficial in
achieving high photocatalytic performance in hydrogen production. Banget al. attributed the the different effects of loading metals at
different locations to the geometric effect of the metal on the
nanorods.[24] Figure 6E and F show that CdSe
nanocrystals with a single Pt-tip and double Pt-tips (dumbbell shape)
are formed on heterogeneous surface through defect-mediated growth,
respectively. The average size of the Pt particles in these two hybrid
structures is almost the same. In the single Pt-tipped structure, the
CdSe nanorod on the other end where has more activity is in direct
contact with the solution, which could facilitates the hole transfer and
removal by scavenger after electron transfer to the Pt-tip. In reverse,
the holes left can only be transferred and neutralized by the scavenger
through the less active, strongly surfactant passivated and defect-free
side facets of the dumbbell shape structure which decorated by Pt
nanoparticles on both tips, thus decreases the charge separation
efficiency (Figure 6G).[24]
3.4. Length of nanorods with respect to the core
In metal-semiconductor hybrid nanostructured photocatalysts, the metal
domain promotes charge separation of excitons, causing electrons to
migrate to the metal domain and then participate in the water splitting
half reaction. However, because the recombination of electrons and holes
may outweigh the advantages of the metal domain, this promotion of
charge separation is not necessarily effective. Therefore, the change in
the nanorod lengths, as an influential parameter, will lead to different
distances between exciton and reduction site, which will consequently
affects the photocatalytic efficiency of hybrid
nanostructures.[23,25]
Amirav et al. decorated the same size Pt nanoparticles on one of
the two end of CdS nanorods embedded with CdSe seeds (Figure 7A) to
prepare spatially controlled nano-heterostructures with different
lengths of CdSe@CdS nanorods, the relation between nanorod length and
their catalytic performance is studied here. The results show that the
increase in the length of the CdSe@CdS nanorods provides a higher
activity for the same CdSe seed size, resulting in a greatly increased
hydrogen production capacity (Figure 7B and 7C). In addition, it was
found that the hydrogen generation rate is linear with the light
intensity (Figure 7D). This finding indicates that the intermediate of
the reduction reaction is stable in this system. Therefore, the physical
separation of the reaction sites by elongating the nanorod length
appears to be benefit to the hydrogen generation from
photocatalysis.[25b] However, blindly increasing
the length of nanorods may against electrons transferring to metal part
and increase the probability of electron hole recombination, the length
matter still needs to be investigated further.