Figure 3 A) Scheme of 1D nanocrystals growth by the SLS method.
B-C) TEM images of B) Ag nanoparticles (the inset is its high
magnification image), C) Ag-ZnS hybrids (the inset is its
high-magnification image). The scale bar is 20 nm. A) Reproduced with
permission.[1f] Copyright 2019, Wiley-VCH. B-C)
Reproduced with permission.[9] Copyright 2012, The
Royal Society of Chemistry.
3. Engineering strategies
Metal-semiconductor hybrid nanoparticles are composed of at least two
components made from different materials and exhibit not only unique
characteristics that are intrinsic to each component but also new
properties that are not possessed by each component due to synergistic
effects. State-of-art synthetic approaches enable the precise control of
the size, shape, composition and spacial location of metal-semiconductor
hybrid nanorods so one can regulate their functionalities through the
engineering strategies to realize their full potential in
photocatalysis. At present, engineering strategies for leveraging the
photocatalytic activity of metal-semiconductor hybrid nanoparticles
mainly include changing metal types, adjusting the metal domain size,
growing metal nanoparticles in different locations, adjusting the length
of nanorods, introducing of co-catalyst, regulating band gap structure,
using various type of surface ligands or hole scavenger and so on. In
this section, we will discuss and summarize the emerging engineering
strategies.
3.1. Types of metal nanoparticles and semiconductors
Metal-semiconductor hybrid nanorods can be used as efficient catalysts
for photocatalytic hydrogen evolution processes due to the enhanced
charge separation capability and the reduction of the activation energy
for the formation of hydrogen in a photocatalytic water splitting
reaction. In the hybrid nanorods, the photoexcited electron generated by
the light absorption of semiconductor is transferred to the metal domain
to promote the water reduction reaction of hydrogen evolution. Each type
of metals has different Fermi energy levels with diverse capabilities in
separating charges and facilitating electrons to be transferred to the
metal domain, so that metal-semiconductor hybrid nanorods may have
diverse photocatalytic activities in hydrogen
production.[10] Figure 4A is a schematic diagram
of the Fermi energy levels of metals and band alignments of CdS. As
shown in the Figure 4A, platinum has a relatively good reductive
activity among these metals for catalytic water splitting reaction and
has been widely studied.
Stone et al. prepared pristine CdS nanorods, hybrid Au-CdS and
Pt-CdS heterostructures and compared their photocatalytic activities
(Figure 4C and D).[11] The kinetics of hydrogen
generation measured by gas chromatography showed that the hydrogen
evolution efficiency of Pt-CdS hybrid nanorods was higher than that of
Au-CdS hybrid nanorods, and both of them were larger than the bare CdS
nanorods which is consistent with our previous discussion. Besides, they
revealed that hydrogen peroxide and hydroxyl radical are produced more
effectively by Au-tipped hybrid nanorods, which can be explained by
d-band theory. Compared to the fully occupied d-band of Au metal, Pt
tends to form strong Pt-O bond thus improves the dissociation of O-O
bond. Tongying et al. performed hydrogen production by using
CdSe/CdS and Pt-CdSe/CdS hybrid nanoparticles. Under broadband
(UV/Visible) illumination,
an
obvious improvement of hydrogen production efficiency of Pt-CdSe/CdS
hybrid nanoparticles can be clearly seen, and the hydrogen production
rate is increased to 434.29 μmol h-1g-1 from 80.92 μmol h-1g-1 when Pt metal is introduced into the CdSe/CdS
nanorods that forms Pt-CdSe/CdS hybrid
nanoparticles.[12] Like other noble metals with
excellent hydrogen evolution performances, Pd is considered to be an
efficient catalyst for hydrogen production in previous
research.[10h] However, Aronovitch et al.shows that the efficiency of Pd-CdS/CdSe photocatalyst for hydrogen
production is not such satisfactory, which is even worse than the
Au-tipped hybrid nanorods due to the severe etching of nanorods
(shortened about 21%) during Pd deposition process in its deposition
process.[13] In this process, partial substitution
of Cd ions by Pd ions in the nanorods produces more defect sites on the
surface of the nanorods and consequently reduces the photocatalytic
efficiency (Figure 4E). Furthermore, this type of hybrid nanorods is
more likely to break down in the photocatalytic
process.[13]
Bimetallic or polymetallic components sometimes can achieve better
catalytic activity than single metal-based hybrid nanorods due to the
increased adjustability of the catalyst properties, optimized width and
energy position of the surface d band, and the synergistic
effect.[14] Aronovitch et al. synthesized
Au@Au/Pd tip CdS/CdSe nanorods (Figure 4F), which greatly improved
hydrogen generation efficiency.[13] This is
attributed to a more favorable hydrogen release at the bimetallic tip.
On the other hand, Pd tends to form oxidation layer on the surface which
increases the time of light induction, while more inert Au can reduce
the formation of it. In addition, Au reduces migration of Pd by forming
a physical barrier, making the structure of hybrid nanorods less
susceptible to change than Pd tip nanorods.[13]Kalisman et al. prepared a combination of the same nanorods with
different metal tips (Figure 4G). The results show that when the Pt is
coated on the outside of the Au tip to form the Au@Pt core-shell
structure, the hydrogen generation rate is twice as high as that of pure
Pt-tipped hybrid nanorods. This can probably attributed to the fast
injection of photoexcited electrons to Au and the ability of fully
extraction of photoexcited electrons of Pt. When photodeposition of Au
and Pt is carried out simultaneously on the nanorods, a gold core
decorated with a platinum island is formed, which has a hydrogen
generation rate four times higher than that of the pure Pt tip. It is
believed that many exposed interfaces introduced between the two phases
can achieve improved hydrogen evolution
activity.[15]
Generally speaking, noble metals can better extract charge for redox
reaction at metal domain. However, it is difficult to achieve high-scale
production due to the high cost of precious metals. Therefore, it is a
trend of current research to seek abundant metal elements in the earth’s
crust as the metal components for metal-semiconductor hybrid nanorods.
The non-noble metal elements that are currently studied are nickel,
cobalt, etc. Simon et al. firstly used the Ni decorated CdS
nanorods for visible light photocatalysis and they proposed a two-step
mechanism to promote hole transfer from CdS to the hole scavenger by a
hydroxyl anion/radical redox shuttle to improve the efficiency of
hydrogen evolution.[4d] The basic principle of the
process is shown in the Figure 4B. In Ni-CdS hybrid nanorods, an
external quantum yield of up to 53% and internal quantum yield up to
71% at high -OH ion concentrations with the system
prevented photooxidation at a high pH are
achieved.[4d] Maynadié et al. reported an
example of heterogeneous growth of Co on CdSe nanorods (Figure 4H). With
a Fermi level similar to Au, the band alignment of such hybrid nanorods
will allow the transfer of electrons from the excited semiconductor to
the metal tip within a shortened time period, which is consistent with
the PL quenching observations in the photophysical studies. Due to
ferromagnetism of Co, this type of hybrid nanorods are more commonly
used in biological labelling and optoelectronic
devices.[16]