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\title{One-dimensional semiconductor-metal heterostructures for solar-to-fuel
conversion through water splitting}



\author[1]{Jiayi Chen}%
\author[2]{Derek Hao}%
\author[1]{Wei Chen}%
\author[3]{Yazi Liu}%
\author[4]{Zongyou Yin}%
\author[5]{Hsien-Yi Hsu}%
\author[2]{Bing-Jie Ni}%
\author[6]{Aixiang Wang}%
\author[1]{Simon Lewis}%
\author[1]{Guohua Jia}%
\affil[1]{Curtin University}%
\affil[2]{University of Technology Sydney}%
\affil[3]{Nanjing Normal University}%
\affil[4]{Australian National University}%
\affil[5]{City University of Hong Kong}%
\affil[6]{Linyi University}%


\vspace{-1em}



  
  \date{June 07, 2024}


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\selectlanguage{english}
\begin{abstract}
One-dimensional semiconductor-metal heterostructures manifest improved
charge separation capability, tunable band gap and increased surface
area, and have been regarded as promising photocatalysts for
solar-to-fuel conversion through water splitting. We herein review the
synthetic strategies for the rational design and preparation of
one-dimensional semiconductor-metal heterostructures with precise
control in terms of size, shape, composition and function. Approaches
that are capable of improving the photocatalytic performance of
one-dimensional semiconductor-metal heterostructures have been proposed
and analyzed The present work is expected to give the rational design of
one-dimensional semiconductor-metal heterostructures and provide
insights into accomplishing highly efficient photocatalysis based on
such materials%
\end{abstract}%



\sloppy


\textbf{Cite this paper:}\emph{Chin. J. Chem.} \textbf{2023} , \emph{41}
, XXX---XXX. \textbf{DOI: 10.1002/cjoc.202300XXX}

Engineering Colloidal Metal-Semiconductor Nanorods Hybrid Nanostructures
for Photocatalysis

Jiayi Chen,\emph{\textsuperscript{a}} Derek
Hao,\emph{\textsuperscript{b}} Wei Chen,\emph{\textsuperscript{a}} Yazi
Liu,\emph{\textsuperscript{c}} Zongyou
Yin,\emph{\textsuperscript{d}}Hsien-Yi Hsu,*\emph{\textsuperscript{,e}}
Bing-Jie Ni,*\emph{\textsuperscript{,b}} Aixiang
Wang,*\textsuperscript{,\emph{f}} Simon W.
Lewis,\emph{\textsuperscript{a}} and Guohua
Jia*\textsuperscript{,\emph{a}}

\textsuperscript{a} School of Molecular and Life Sciences, Curtin
University, Bentley, Perth WA, 6102, Australia\textsuperscript{b} Centre
for Technology in Water and Wastewater, School of Civil and
Environmental Engineering, University of Technology Sydney, Ultimo, NSW
2007, Australia\textsuperscript{c} School of the Environment, Nanjing
Normal University, Nanjing, 210046 P.R. China \textsuperscript{d}
Research School of Chemistry, The Australian National University, ACT
2601, Australia\textsuperscript{e} School of Energy and Environment \&
Department of Materials Science and Engineering, City University of Hong
Kong, Kowloon Tong, Hong Kong, China, Shenzhen Research Institute of
City University of Hong Kong, Shenzhen, 518057, China\textsuperscript{f}
School of Chemistry and Chemical Engineering, Linyi University, Linyi,
276005, China\textsuperscript{}\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Keywords}\tabularnewline
\midrule
\endhead
Hybrid nanostructure \textbar{} Metal-semiconductor nanorod \textbar{}
Engineering strategy \textbar{} Photocatalysis \textbar{} Hydrogen
production\tabularnewline
\textbf{Comprehensive Summary}\tabularnewline
Emerging engineering strategies of colloidal metal-semiconductor
nanorods hybrid nanostructures spanning from type, size, dimension, and
location of both metal nanoparticles and semiconductors, co-catalyst,
band gap structure, surface ligand to hole scavenger are summarized
symmetrically to rationalize the design of the type of intriguing
materials for efficient photocatalytic applications.\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{longtable}[]{@{}ll@{}}
\toprule
After acceptance, please insert a group photo of the authors taken
recently. & \selectlanguage{ngerman}\vtop{\hbox{\strut Biographical
Sketch.}\hbox{\strut Biographical Sketch.}\hbox{\strut Biographical
Sketch.}\hbox{\strut ···}}\tabularnewline
\midrule
\endhead
\textbf{Left to Right:} Author 1, Author 2, Author 3, ···
&\tabularnewline
\bottomrule
\end{longtable}

\section*{}

{\label{section}}

Contents

1. Introduction 2

2. Synthetic approaches 2

2.1. Surface nucleation 2

2.2. Materials diffusion 2

2.3. Solution-liquid-solid method 3

3. Engineering strategies 3

3.1. Types of metal nanoparticles and semiconductors 3

3.2. Size of metal domain 4

3.3. Surface location of metal nanoparticles 5

3.4. Length of nanorods with respect to the core 6

3.5. Co-catalyst 6

3.6. Band gap structure 7

3.7. Surface ligands 8

3.8. Hole scavenger 8

3.9. Other strategies nanorods 9

4. Conclusion and outlook 9

1. Introduction

Colloidal semiconductor nanorods manifest size- and shape-depended
properties, large absorption cross-section, and improved charge
transport and separation.\textsuperscript{{[}1{]}} The integration of
metal particles with semiconductor nanorods produces a new type of
materials, i.e. hybrid nanords, with enhanced charge separation
capabilities with relevance to applications in
photocatalysis.\textsuperscript{{[}2{]}} The role of metal-semiconductor
hybrid nanorods in photocatalysis is explained by the light absorption
and the formation of electron-hole pairs in semiconductor domain and by
charge separation on metal-semiconductor
nanojunctions.\textsuperscript{{[}3{]}} Efficient charge separation on
the metal-semiconductor nanorods makes it possible to be served as a
photocatalyst for hydrogen production from water splitting, which is a
hot topic in the research of hybrid
nanomaterials.\textsuperscript{{[}4{]}}

Although the
syntheses,\textsuperscript{{[}5a,5b{]}}architectures,\textsuperscript{{[}5c{]}}
plasmonic properties\textsuperscript{{[}5d{]}} and catalytic
properties\textsuperscript{{[}2a,5e{]}} of hybrid nanocrystals have been
reviewed in a couple of prominent review papers, none of above
specifically targets one-dimensional (1D) metal-semiconductor nanorods
hybrid nanostructures. Furthermore, to the best of our knowledge, the
engineering of the metal-semiconductor nanocrystals hybrid
nanostructures, especially for anisotropic nanorods with elongated
shape, has not been appropriately compared and summarized so far. Thus
it is highly demanded to composite a review paper to outline the
emerging engineering strategies of such hybrid nanostructures with
tailored properties to boost photocatalysis and to provide insightful
perspectives on this stimulating research area. Here we contribute a
timely review to rationalize the design of metal-semiconductor nanorods
hybrid nanostructures through the engineering strategies to maximize
their full potential in photocatysis. This review consists of a brief
introduction to the synthetic approaches of metal-semiconductor nanorods
hybrid nanostructures followed by a broad scope of engineering
strategies, including types of metal and semiconductor components, size
and growth location of metal nanoparticles, dimensional of semiconductor
nanorods, types of co-catalyst, band gap structures, surface ligands,
hole scavengers and other strategies. Finally, the challenges and future
perspectives for metal-semiconductor nanorods hybrid nanostructures are
proposed.

2. Synthetic approaches

The in-depth understanding of the growth mechanisms and state-of-the-art
wet-chemical synthetic methods make it possible to prepare colloidal
metal-semiconductor hybrid nanorods with diverse architectures in a
predefined manner. In this section, three typical synthetic methods such
as surface nucleation (Figure 1A), material diffusion (Figure 2A), and
solution-liquid-solid (SLS) growth (Figure 3A) with representative
examples have been reviewed to provide general background information on
the controlled synthesis of the metal-semiconductor nanorods hybrid
nanostructures.

2.1. Surface nucleation

As a widely used method for the preparation of the metal-semiconductor
nanorods hybrid nanostructures, the process of surface nucleation
usually needs some external inputs such as light illumination or heat in
combination with the presence of concentrated precursors and surface
binding ligands to facilitate the nucleation of metals on semiconductor
nanocrystals. This is because that the energy barrier for the nucleation
of the metal onto the semiconductor nanocrystals could be overcome by an
external input in the form of light illumination or
heat.\textsuperscript{{[}6{]}} Mokari et al. performed site-selective
growth of Au on CdSe nanorods at the room temperature. As the end facets
of nanorods are less passivated by surfactant and have increased surface
energy, Au tends to grow on the tips of
nanorods.\textsuperscript{{[}6b{]}} Dukovic et al. reported the
photodeposition of Pt on colloidal CdS nanorods (Figure
1B).\textsuperscript{{[}6c{]}} They found that Pt randomly distributed
along the nanorod length with no preference for rod ends. This finding
is different from metal-chalcogenide hybrid nanostructures synthesized
by thermal methods wherein metal preferably deposit on the end of
nanorods forming match-like or dumbbell-like heterostructures. Many
nanorods, even some of them are longer than 50 nm, have no indication of
any Pt deposition (Figure 1C). This perhaps because metal deposition
requires the presence of surface defects or incomplete surface
passivation sites on the nanorods as carrier traps. Menagen et al.
studied the effects of light and temperature on the surface nucleation
process of Au on CdS nanorods.\textsuperscript{{[}6d{]}} As shown in
Figure 1D-G, Au grew on CdS nanorods under different conditions. The
results showed that Au can grow on the sulfur-ended facet of the
nanorods when exposed to light, and surface defect growth can be
suppressed at low temperatures (almost completely inhibited in 273 K),
owing to the adsorption of more ligands onto the CdS nanorods and
passivating of the surface, thus preventing defect growth. Therefore,
the size and growth location of metal domain can be easily controlled
through surface passivation and elaboration of synthesis
conditions.\textsuperscript{{[}6d{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 1} A) Schemes of growth of metal phase at different
positions of semiconductor nanoparticles (top) and growth of
semiconductor on the metal nanoparticle (bottom). B) Transmission
electron microscopy (TEM) image of Pt-CdS heterostructure, the scale bar
is 20 nm, C) The number of Pt nanocrystals on a CdS nanorod as a
function of its length. t = 120 min. D-G) Au nanocrystals growth on CdS
nanorods for 1 h: D) 313 K, dark E) 313K, 473 nm laser irradiation, F)
273 K, dark and G) 273 K, 473 nm laser irradiation, the scale bar is 20
nm. A) Reproduced with permission.\textsuperscript{{[}5e{]}} Copyright
2010, Wiley-VCH. B-C) Reproduced with
permission.\textsuperscript{{[}6c{]}} Copyright 2008, Wiley-VCH. D-G)
Reproduced with permission.\textsuperscript{{[}6d{]}} Copyright 2009,
American Chemical Society.

2.2. Materials diffusion

The driving force of Ostwald ripening comes from the interface energy
which tends to reach its lowest value under thermodynamic conditions. As
the larger particles grow at the expense of the smaller particles, the
specific interface energy per unit mass decreases, while the total free
energy of the system is reduced.\textsuperscript{{[}5e{]}} This
mechanism can be used in the design and fabrication of
metal-semiconductor hybrid nanostructures. Bala \emph{et al.} used this
mechanism for surface diffusion growth. The phase transfer of
AuCl\textsubscript{4}\textsuperscript{-} and subsequent metal reduction
were carried out using a water-soluble HAuCl\textsubscript{4} precursor
and octylamine, and Au was grown to the tip of the CdSe nanorod.
Initially, as shown in Figure 2B, several small Au particles were found
on CdSe nanorods like gold islands, not only at the tip but also on the
body of the rod. After 4 hours, it was found that all the small gold
islands on the CdSe were aggregated at one end of the nanorods and
became bigger (Figure 2C). The metal tip size can be adjusted through
controlling the nucleation time and growth rate of the gold islands at
different locations during the growth process. Under the Ostwald
ripening mechanism, when the size of each Au nanoparticle reaches
thermodynamic stability, the smaller tip will gradually be oxidized and
dissolved into the solvent, and electrons will be transferred from the
surface of the nanorod to the Au-tip, thereby continuing to produce Au
and makes the nanoparticles grow at one
end.\textsuperscript{{[}7{]}}Therefore, the metal-semiconductor hybrid
structure with diverse metal domain size and growth location can be
achieved.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 2} A) Schemes of surface diffusion (middle) or inward
diffusion (right) of the metal phase after surface nucleation. B)
Scanning transmission electron microscopy (STEM) image of the CdSe-Au
heterostructure after 1 h of reaction. C) TEM image of CdSe-Au
heterostructure after 4 h of reaction. The scale bar is 20 nm. A)
Reproduced with permission.\textsuperscript{{[}5e{]}} Copyright 2010,
Wiley-VCH. B-C) Reproduced with permission.\textsuperscript{{[}7{]}}
Copyright 2013, Tsinghua University Press and Springer-Verlag Berlin
Heidelberg.

2.3. Solution-liquid-solid method

SLS method is a facile synthetic approach for hybrid nanorods using the
pre-synthesized metal nanoparticles. In this process, metal
nanoparticles with low melting points introduced in the organic solvent
are transformed into liquid metal clusters as a catalyst on which
nanocrystals are precipitated, and the growth of crystals on metal
droplets occurs only at the interface between
them.\textsuperscript{{[}1f,8{]}} Being different from the methods of
surface nucleation and materials diffusion which require careful control
of synthesis conditions, the ease and robustness of this method open new
opportunities for the preparation of high quality 1D hybrid
nanostructures with relevance in photocatalytic
applications.\textsuperscript{{[}1f, 8d{]}} Shen \emph{et al.} prepared
Ag-ZnS nanorods by the SLS method (Figure 3B, 3C) by optimizing the
diameter of Ag nanocrystals, the concentration of Zn precursor, reaction
time, reaction temperature and other conditions to effectively control
the diameter and length of Ag-ZnS nanorods hybrid
nanostructures.\textsuperscript{{[}9{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{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.\textsuperscript{{[}1f{]}} Copyright 2019, Wiley-VCH. B-C)
Reproduced with permission.\textsuperscript{{[}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.\textsuperscript{{[}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 \emph{et al.} prepared pristine CdS nanorods, hybrid Au-CdS and
Pt-CdS heterostructures and compared their photocatalytic activities
(Figure 4C and D).\textsuperscript{{[}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 \emph{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 \selectlanguage{greek}μ\selectlanguage{english}mol
h\textsuperscript{-1}g\textsuperscript{-1} from 80.92 \selectlanguage{greek}μ\selectlanguage{english}mol
h\textsuperscript{-1}g\textsuperscript{-1} when Pt metal is introduced
into the CdSe/CdS nanorods that forms Pt-CdSe/CdS hybrid
nanoparticles.\textsuperscript{{[}12{]}} Like other noble metals with
excellent hydrogen evolution performances, Pd is considered to be an
efficient catalyst for hydrogen production in previous
research.\textsuperscript{{[}10h{]}} However, Aronovitch \emph{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.\textsuperscript{{[}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.\textsuperscript{{[}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 \emph{d} band, and the synergistic
effect.\textsuperscript{{[}14{]}} Aronovitch \emph{et al.} synthesized
Au@Au/Pd tip CdS/CdSe nanorods (Figure 4F), which greatly improved
hydrogen generation efficiency.\textsuperscript{{[}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.\textsuperscript{{[}13{]}}Kalisman \emph{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.\textsuperscript{{[}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 \emph{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.\textsuperscript{{[}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 \textsuperscript{-}OH ion concentrations with the system
prevented photooxidation at a high pH are
achieved.\textsuperscript{{[}4d{]}} Maynadi\selectlanguage{ngerman}é \emph{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.\textsuperscript{{[}16{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 4} A) Scheme of the potentials of hydrogen evolution and
oxygen evolution, bandgap of CdS semiconductor, and Fermi levels of each
metal. B) Scheme of hole shuttle mechanism for the photocatalytic
evolution of H\textsubscript{2}, nickel nanoparticles growth on the
cysteine-stabilized CdS nanorods for photocatalytiv hydrogen generation,
the photoexcited holes oxidize hydroxyl anions which act as a
radical-carry away the positive charges, and in turn oxidize ethanol to
acetaldehyde. The blue and red arrows show the movement of the species
and a redox reaction respectively. TEM images of C) Au-CdS (2.5\selectlanguage{ngerman}±0.6 nm),
and D) Pt-CdS (1.9±0.5 nm) hybrid nanorods, respectively. The dimensions
of CdS nanorods are 48±5 nm×3.3±0.5 nm. E) TEM images of Pd-CdS hybrids.
F) TEM image of Au@Alloy-CdS hybrid nanorods (insert: from left to right
are EDS elemental maps of Au, Pd, and an overlay of Cd, S, Au, and Pd).
G) TEM image of bimetallic tips Au-Pt-CdSe@CdS hybrid nanorods (insert:
EDS elemental maps of the bimetallic tip). H) TEM images of Co-CdSe
hybrids after 24 h reaction at 80\selectlanguage{english}. The scale bar of TEM images above
are 20 nm. A) Reproduced with permission.\textsuperscript{{[}10i{]}}
Copyright 2018, The Royal Society of Chemistry. B) Reproduced with
permission.\textsuperscript{{[}4d{]}}Copyright 2014, Macmillan
Publishers Limited. C-D) Reproduced with
permission.\textsuperscript{{[}11{]}} Copyright 2018, Wiley-VCH. E-F)
Reproduced with permission.\textsuperscript{{[}13{]}} Copyright 2016,
American Chemical Society. G) Reproduced with
permission.\textsuperscript{{[}15{]}} Copyright 2015, The Royal Society
of Chemistry. H) Reproduced with
permission.\textsuperscript{{[}16{]}}Copyright 2009, Wiley-VCH.

3.2. Size of metal domain

As an important component to separate excitons, extract electrons and
undergo reduction reaction, the size of the metal domain is one of the
most important parameters for obtaining the optimal catalytic activity
in the metal-semiconductor hybrid structure. Early studies show the
Fermi level shifting of metal domain is caused by size decreasing which
could affect catalytic performance of hybrid materials directly, the
quantum size effect will also bring the adjustable semiconducting
properties when the size of metal tip becomes tiny clusters. On the
other hand, the undesired radiative recombination might occur on the
metal clusters as well as the non-radiatively exciton recombination need
to be considered in the hybrid structure. \textsuperscript{{[}17{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{Figure 5} TEM images of A) CdS-Au hybrid nanorods with 1.5\selectlanguage{ngerman}±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.\textsuperscript{{[}18{]}} Copyright 2018, American Chemical
Society. D) Reproduced with permission.\textsuperscript{{[}19{]}}
Copyright 2017, American Chemical Society. E-F) Reproduced with
permission.\textsuperscript{{[}20{]}} Copyright 2015, Springer Nature.

Ben-Shahar \emph{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.\textsuperscript{{[}18{]}} Nakibli \emph{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).\textsuperscript{{[}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.\textsuperscript{{[}19{]}} Selective small metal
island growth on the CdS nanorods was obtained by varying the
irradiation time and the Au\textsuperscript{3+}/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.\textsuperscript{{[}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.\textsuperscript{{[}21{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{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
Na\textsubscript{2}SO\textsubscript{3}/0.25 M Na\textsubscript{2}S
aqueous solution by using Pt-CdSe and Au-CdSe hybrids with single or
double tips. A-B) Reproduced with
permission.\textsuperscript{{[}22{]}}Copyright 2015, American Chemical
Society. C-D) Reproduced with permission.\textsuperscript{{[}23{]}}
Copyright 2016, American Chemical Society. E-G) Reproduced with
permission.\textsuperscript{{[}24{]}}Copyright 2012, American Chemical
Society.

Nakibli \emph{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.\textsuperscript{{[}22{]}} They hypothesized that the first step
in the water redox half reaction is that the bonding between
H\textsuperscript{+} and catalyst lead to the formation of
intermediates. In the second step, the H\textsubscript{2} released by
the combination of either two intermediates, or an intermediate with an
H\textsuperscript{+} 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 H\textsubscript{2} 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.\textsuperscript{{[}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.\textsuperscript{{[}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.
Bang\emph{et al.} attributed the the different effects of loading metals
at different locations to the geometric effect of the metal on the
nanorods.\textsuperscript{{[}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).\textsuperscript{{[}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.\textsuperscript{{[}23,25{]}}

Amirav \emph{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.\textsuperscript{{[}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.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{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.\textsuperscript{{[}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\textsuperscript{-} 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.\textsuperscript{{[}26{]}}

Figure 8A shows a schematic diagram of a photocatalyst with
co-catalyst.\textsuperscript{{[}26a{]}} Wolff \emph{et al.} synthesized
Pt-tipped CdS nanorods and employed a ruthenium complex (a derivative of
Ru (tpy)(bpy) Cl\textsubscript{2}) 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.\textsuperscript{{[}26a{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Figure 8} A) Scheme of synthesis method for Pt-CdS hybrids
(yellow) by synthesis in organic media, selective growth of Pt
nanoparticles (grey) at the tips, and phase transfer into aqueous medium
with cysteine and anchoring of the RuDTC catalyst to the side surfaces
of the nanorods. B) Hydrogen evolution by bare and tip-decorated CdS
nanorods. C) Comparison of average H\textsubscript{2} and
O\textsubscript{2} generation rates over the first hour of illumination
calculated from at least two measurements for each system. Error bars
represent s.d. of measured rates. Sacrificial agent (TEOA) was used only
in the first experiment (leftmost blue bar). A-C) Reproduced with
permission.\textsuperscript{{[}26a{]}} Copyright 2018, Springer Nature.

3.6. Band gap structure

In the metal-semiconductor heterostructures, a photocatalyst with
excellent performance can be designed by the regulation strategy of the
energy band engineering. This is because the maximized synergistic
effect can be reached by blending the appropriate components to enhance
its catalytic effect.\textsuperscript{{[}27{]}}

Zhuang \emph{et al.} constructed a unique 1D binary
{[}ZnS-CdS{]}-ZnS-{[}ZnS-CdS{]}-ZnS structure, a hetero-structure
nanorod with multi-nodal sheath CdS, which can achieve better absorbance
and charge transport continuity and provide a smaller band gap
semiconductor - CdS node sheath.\textsuperscript{{[}28{]}} Through
selective growth of the metal on the node sheath, the structure of
ternary hybrid {[}ZnS-(CdS/M){]}-ZnS-{[}ZnS-(CdS/M){]}-ZnS is obtained.
This unique structure enables the transfer of photoexcited electrons
from the node sheath of CdS to the metal surface and the ZnS nanorod,
and thus can improve the charge separation efficiency. Research on the
band gap alignment of materials shows that the binary multi-node sheath
nanostructures can be regarded as the connection between ZnS(111)
nanorods and ZnS(\(\overline{\overset{\overline{}}{\overline{1}}10}\))/CdS(\(\overline{1}10\)) hybrid
nodal sheath, and the energy gap of
ZnS(\(\overline{\overset{\overline{}}{\overline{1}}10}\))/CdS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)) is located within
ZnS(111). Therefore, the binary structure forms a periodic straddling
gap alignment (type I heterojunction) (Figure 9B), but there might be an
accumulation of electrons and holes in CdS. Therefore, the ternary
multi-node sheath with metal loaded nanostructures are investigated
then. The calculated workfunction of CdS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)) is larger
than the (\(\overline{\overset{\overline{}}{\overline{1}}10}\)) surface of Au, Pd and Pt, so the
difference of Fermi energy level/workfunction propels free electrons
from Au, Pd or Pt to CdS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)), thereby bending the CdS
band so that it can be staggered with ZnS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)) and
ZnS(111), forming type II heterojunction (Figure 9C). Thus,
photogenerated electrons will be transferred to both the metal domain
and ZnS- (111). In addition, this structure can also highly suppress the
recombination between photoexcitation electron and hole. Tests on the
catalytic effect show that the ternary multi-node sheath nanostructures
have the highest hydrogen production rate than other structure (Figure
9A), and the best effect is achieved when Pt replaces Au in the ternary
structure, which is consistent with the excellent performance of the Pt
decorated hybrid nanostructures described in section
3.1.\textsuperscript{{[}28{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

\textbf{Figure 9} A) Comparison of H\textsubscript{2} evolution rates
under visible-light irradiation using different photocatalysts: ZnS
nanorods, CdS nanorods, binary ZnS-CdS hetero-nanorods, ternary
ZnS-(CdS/Au) hetero-nanorods(ZCAu), ternary ZnS-(CdS/Pd)
hetero-nanorods(ZCPd), ternary ZnS-(CdS/Pt) hetero-nanorods(ZCPt). The
insert is the representative geometric model of ternary hetero-nanorods
with four CdS node sheaths. B) The energy-band alignment of a ZnS(111)
nanorod attached by the ZnS()/CdS() hybrid in the periodic binary
heterojunction. C) The energy-band alignment of the periodic ternary
heterojunction with CdS()/Au hybrids attached to a ZnS(111) nanorod. D)
Absorption spectra of the as-prepared samples. a) CIS nanorods, b) ZCIS
nanorods, c) ZCIS--Pt hybrid nanorods, and d) ZCIS--Pd\textsubscript{4}S
hybrid nanorods. E) Photocatalytic hydrogen production under
visible-light illumination by CIS nanorods, ZCIS nanorods, ZCIS--Pt
hybrid nanorods, and ZCIS--Pd\textsubscript{4}S hybrid nanorods from an
aqueous solution containing
0.25\textsubscript{M}Na\textsubscript{2}SO\textsubscript{3} and
0.35\textsubscript{M}Na\textsubscript{2}S. Measurements were taken every
hour for 6 h. A-C) Reproduced with permission.\textsuperscript{{[}28{]}}
Copyright 2015, Wiley-VCH. D-E) Reproduced with
permission.\textsuperscript{{[}29a{]}}Copyright 2015, Wiley-VCH.

Ye \emph{et al.} studied the photocatalytic effect of an alloyed
semiconductor of
ZnS-CuInS\textsubscript{2}(ZCIS).\textsuperscript{{[}29a{]}} Actually,
the band gap energy of CuInS\textsubscript{2} (CIS) and ZnS are 1.5 eV
and 3.7 eV respectively, which are not suitable for visible light
induced photocatalysis to generate hydrogen from water splitting.
However, previous studies by Ye\emph{et al.} have shown that the band
gap of ZCIS is highly correlated with alloy
composition,\textsuperscript{{[}29b{]}} so it is possible to regulate
the band gap by alloying, and adjust the ratio of CIS to ZnS to absorb
visible light. Compared with the absorption spectrum of CIS, a blue
shift occurred for ZCIS, indicates the widen of band gap by introduction
of ZnS (Figure 9D). The ZCIS band gap measured was 2.23 eV, which was
between the band gap of ZnS and CIS. Based on this, Pt and
Pd\textsubscript{4}S were decorated on ZCIS to form hybrid nanorods
nanostructures, and their photocatalytic properties were tested with
Na\textsubscript{2}S and Na\textsubscript{2}SO\textsubscript{3} as hole
scavengers, as shown in Figure 9E, ZCIS-Pt and ZCIS-Pd\textsubscript{4}S
hybrid nanorods nanostructures showed better photocatalytic performance
than naked ZCIS nanorods. This indicates that by constructing
heterogeneous structures to regulate the band gap, the materials may
possess unprecedented properties that are not available previously,
which expands the selection of photocatalyst materials. Compared with
traditional photocatalytic materials, it is more feasible to prepare and
control heterogeneous catalysts with excellent performance, low price
and environmental friendliness.\textsuperscript{{[}29a{]}}

3.7. Surface ligands\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

\end{center}
\end{figure}

\textbf{Figure 10} A) Kinetic hydrogen evolution measurements by CdS-Au
hybrid nanorods for different surface coatings. Straight lines represent
the linear fits from which the \%QY was extracted. B) Apparent
photocatalysis \%QY values for a wide range of surface coatings
including thiolated alkyl ligands, GSH, and polymer coating. PEI
exhibits the highest QY. A-B) Reproduced with
permission.\textsuperscript{{[}30{]}} Copyright 2014, Wiley-VCH.

Surface ligand plays an important role in the construction of catalysts.
It is used to provide colloidal stability to the photocatalyst in water
while maintaining good ability to be close to the active surface to
transfer photo-generated excitons. In addition, the passivation of
surface defect achieved by coating the ligands can also eliminate trap
state thus increasing the photocatalytic properties.

Ben-Shahar and co-workers prepared CdS-Au and CdSe/CdS hybrid nanorods,
respectively, with thiolated-alkyl ligands and polymer coating to study
the influence of surface ligands on the catalytic performances. Firstly,
hydrophobic phosphonic acid and alkylamine ligands were exchanged to
different types of thiolated alkyl ligands, including mercaptocarboxylic
acids of different chain lengths (mercaptoundecanoic acid (MUA),
mercaptohexanoic acid (MHA) and mercaptopropionic acid (MPA),
L-glutathione (GSH)) and thiol bound ligands (mercaptosulfonic acid
(MSA) and O-(2-carboxyethyl)-O'-(2-mercaptoethyl) heptaethylene glycol
(S-PEG)). Another type is polymer encapsulation, which generally uses
polyethylenimine (PEI). PEI binds to the surface of nanorods with amine
groups through ligand exchange, and poly(styrene-co-maleic anhydride)
(PSMA), which can encapsulate the hybrid nanorods to maintain the
surface coating of the original nanorods with a hydrophilic end toward
the solution. CdS-Au catalysts with different surface coatings were used
for photocatalysis. The results of hydrogen production rate and QE are
shown in Figure 10A, 10B. Thiolated-alkyl ligands show very low hydrogen
production rates, while polymer-coated hetero-structures offer quite
high rates, with the hydrogen production rates of PEI coated hybrid
nanorods being even an order of magnitude higher than that of MUA coated
hetero-structures. In addition, the fastest charge transfer kinetics and
particle passivation, and the longer effective fluorescence lifetime of
surface ligands can be obtained with PEI coating. Stone and colleagues
used a similar surface encapsulation strategy in which the PEI coated
nanocrystals achieved excellent catalytic performance than thiolated
alkyl ligands.\textsuperscript{{[}11{]}}Through the passivation of
surface defects and decreasing of available hole trapping sites, the
effective charge transfer across the semiconductor-metal interface can
be realized.\textsuperscript{{[}30{]}}

3.8. Hole scavenger

It is well known that electrons and holes are generated when
semiconductor nanorods are irradiated with sunlight. The photoexcited
electrons are transferred to the metal domain to reduce water to
hydrogen, while the photogenerated holes require the addition of
scavenger to remove. The introduction of hole scavenger not only
suppresses electron-hole recombination but also prevents photo-corrosion
of the metal sulfide semiconductor. Therefore, the addition of
sacrificial agent is very important in photocatalytic reaction. In
addition, different hole scavenger will bring different degrees of
influence and improper scavenger may even reduce the performance of the
catalyst.\textsuperscript{{[}31{]}}

Wu \emph{et al.} used MUA-capped CdSe/CdS-Pt and CdS-Pt hybrid nanorods
to study the performance of methanol (MeOH) and sulfite as hole
scavengers for photocatalytic hydrogen generation (Figure 11A and B).
Both CdSe/CdS-Pt and CdS-Pt nanorods have achieved efficient charge
separation and long-life charge separation states, the hole transfer
rate is positively correlated with hydrogen generation QE. Combining the
characterization results and calculation data, the use of MeOH scavenger
on CdS-Pt nanocrystals exhibit the lowest rate in the hole transferring.
The hole transfer rate from CdSe/CdS-Pt to the MUA and MeOH is about 4.9
times faster than that of CdS-Pt. The combination of MUA and highly
reductive sulfites provides a greater driving force for hole removal,
making the effect of this group better than the group using MeOH as an
electron donor. The interaction of the hole acceptor with the
surface-capping ligand, the accessibility of the surface sites, and the
strong reducibility of the sulfite cause more holes in the surface trap
of the CdS-Pt nanorod to be removed, which is reflected in the fact that
the hole transfer time from CdS-Pt to sulfite is about 2.5 times faster
than that of CdSe/CdS-Pt.\textsuperscript{{[}32{]}} In addition, Berr
\emph{et al.}also studied the catalytic performances of four hole
scavengers for Pt-CdS nanorods such as sodium sulfite
(SO\textsubscript{3}\textsuperscript{2-}), disodium
ethylenediaminetetraacetic acid (EDTA\textsuperscript{4-}),
triethanolamine (TEA) and MeOH,
respectively.\textsuperscript{{[}33{]}}As can be seen from Figure 11C,
SO\textsubscript{3}\textsuperscript{2-}achieves the best results with a
QE that can exceed an order of magnitude of the worst one such as MeOH.
They believe that this is the result of different redox potentials of
hole scavengers. As shown in Figure 11D,
SO\textsubscript{3}\textsuperscript{2-} has the strongest reducibility
and is more easily oxidized by the combination of holes. Besides, the
morphology of the catalyst after photocatalysis using different hole
scavengers indicates that the nanorods using MeOH as hole scavenger are
shortened and aggregated while the nanorods using
SO\textsubscript{3}\textsuperscript{2-} can remain their morphology
after catalysis due to the electron donor timely cleared the holes
captured by the surface traps of the nanorods, which preventing the
photooxidation of catalyst.\textsuperscript{{[}33{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image15/image15}

\end{center}
\end{figure}

\textbf{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 SO\textsubscript{3}\textsuperscript{2-},
TEA, EDTA\textsuperscript{4-}, 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.\textsuperscript{{[}32{]}} Copyright
2014, American Chemical Society. C-D) Reproduced with
permission.\textsuperscript{{[}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.\textsuperscript{{[}1f,34{]}}Zhang \emph{et al.} conducted
amine-assisted directional growth of CdS nanords with
MoS\textsubscript{2} tips (M-t-CdS) (Figure 12A). Results show that the
photocatalytic activity of the M-t-CdS heterostructure containing
3.5wt\% of MoS\textsubscript{2} 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).\textsuperscript{{[}35{]}} Besides, the coating of amorphous
MoS\textsubscript{3} surface on semiconductor nanorods was studied by
Tang \emph{et al.} who used
(NH\textsubscript{4})\textsubscript{2}MoS\textsubscript{4} with one-step
heat treatment to deposit amorphous MoS\textsubscript{3} film on CdSe /
CdS nanorods (Figure 12D and 12E). The close contact between the
MoS\textsubscript{3} 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).\textsuperscript{{[}36{]}}

Kameyama \emph{et al.} studied a special nanocrystals heterostructure of
ZnS-AgInS\textsubscript{2} solid solution
((AgIn)\textsubscript{x}Zn\textsubscript{2(1-x)}S\textsubscript{2},
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)\textsubscript{0.24}Zn\textsubscript{1.52}S\textsubscript{2}). By
regulating the \emph{x} value of ZAIS nano-ellipsoid
(AgIn)\textsubscript{x}Zn\textsubscript{2(1-x)}S\textsubscript{2}(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).\textsuperscript{{[}37{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image16/image16}

\end{center}
\end{figure}

\textbf{Figure 12} A) Schematic representation of photocatalytic
H\textsubscript{2} generation on MoS\textsubscript{2}-CdS nanorods. B)
H\textsubscript{2} evolution rates of MoS\textsubscript{2}-CdS nanorods
with different MoS\textsubscript{2} content. C) Comparison of
H\textsubscript{2} evolution rates between CdS nanorods, 1 wt.\% Pt/CdS
nanorods, 3.5 wt.\% MoS\textsubscript{2} /CdS nanorods, 3.5 wt.\%
MoS\textsubscript{2}-CdS nanorods, and 3.5\% Pt/CdS nanorods. D)
Bright-field TEM image of CdSe-seeded CdS nanorods (length 60 nm). E)
MoS\textsubscript{3} deposition on a CdSe-seeded CdS nanorod, with
photocatalytic hydrogen production in the visible range using
triethanolamine (TEOA) as a sacrificial reductant. F) A typical gas
chromatogram observed for a MoS\textsubscript{3}-coated CdS/CdSe nanorod
using 450 nm light with an induction period of approximately 50 min.
0.07 nmol of rods were used with 5.0 mL 0.1m tris buffer (pH 7.0) and
0.20 mL TEOA. The inset shows the measurement over a period of four
hours. The activities are derived from the maximum rate of
H\textsubscript{2} produced, as indicated by the arrows. G)-H) Wide-area
TEM images of ZAIS nanodumbbells prepared when x\textsubscript{p}=0.5
and corresponding ZAIS nanoellipsoids simultaneously formed as a
byproduct. ZAIS nanocrystals of each shape were separated from
as-prepared mixture nanocrystals by a size-selective precipitation
technique. I) Schematic illustration of the formation mechanism of ZAIS
nanodumbbells and nanoellipsoids. J) Dependence of R(H\textsubscript{2})
on the kind of ZAIS nanocrystals used. Samples were (i) ZAIS(x/0.24/x)
nanodumbbells, (ii) ZAIS(x) nanoellipsoids, (iii) ZAIS(0.24) nanorods,
and (iv) mixtures of ZAIS(0.24) nanorods and ZAIS(x) nanoellipsoids
(1:2). The scale bar is 20 nm. A-C) Reproduced with
permission.\textsuperscript{{[}35{]}} Copyright 2016, Wiley-VCH. D-F)
Reproduced with permission.\textsuperscript{{[}36{]}} Copyright 2011,
Wiley-VCH. G-J) Reproduced with
permission.\textsuperscript{{[}37{]}}Copyright 2018, American Chemical
Society.

3. Conclusion and outlook

This Review presents a variety of strategies to engineer the
metal-semiconductor nanorods hybrid nanostructures so that their
electronic properties could be tuned to perform optimal photocatalytic
functions in solar-driven water splitting. While the scope of this
engineering is broad and the photocatalytic properties of derived hybrid
nanomaterials are impressive, this area of research still faces
significant challenges before the full potential of the hybrid
nanomaterials is realized. One challenge is to expand these engineering
strategies from cadmium-based 1D hybrid structures to additional
semiconductor materials, in particular to heavy-metal-free earth
abundant systems. This will require further development of synthetic
routes in combination with emerging engineering strategies which may
lead to less or even non-toxic ``green'' photocatalysts for solar-driven
fuel production through water splitting but also may provide an avenue
to sustainable and environmental friendly clean fuels. On the other
hand, the construction of multifunctional heterostructure seems to be a
promising approach for water splitting with the realization of high
hydrogen evolution efficiency and prevention from the degradation
induced by holes accumulation.

It is worth noting that this Reviews is mainly focused on
metal-semiconductor nanorods hybrid nanostructures with elongated shape.
Two-dimensional (2D) metal-semiconductor nanoplatelets hybrid
nanostructures with excitons being confined in their thickness direction
are less studies and not well developed. As the catalytic reactions are
more likely to take place on the edge atoms, selectively depositing the
metal atoms, especially the noble metal atoms, onto the edges of
semiconductor nanoplatelets will increase their exposure, which can
reduce their amount of deposition and cost. Furthermore, exploration of
the engineering strategies to this type of intriguing hybrid materials
are meaningful and exciting because studies into this area may lead to
unexpected observations and unprecedented photocatalytic performances.

To summarize, metal-semiconductor hybrid nanoparticles have significant
potential in solar-driven photocatalysis for clean fuel production. The
remarkable advances in the engineering strategies of these materials
alongside with the in-depth understanding the physico-chemical
principals that govern the photocatalytic reactions provide a solid
basis for their photocatalytic applications.

Acknowledgement

This work is supported by the Australian Research Council (ARC) Future
Fellowship Scheme (FT210100509), ARC Discovery Project (DP220101959),
the Hebrew University of Jerusalem - Zelman Cowen Academic Initiatives
(ZCAI) Joint Projects 2021, the Innovation and Technology Commission
(grant no. MHP/104/21), Shenzhen Science Technology and Innovation
Commission (grant no. 20210324125612035), and City University of Hong
Kong (grant no. 9360140).

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\end{enumerate}\selectlanguage{english}
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(The following will be filled in by the editorial staff)\tabularnewline
Manuscript received: XXXX, 2023 Manuscript revised: XXXX, 2023
Manuscript accepted: XXXX, 2023 Accepted manuscript online: XXXX, 2023
Version of record online: XXXX, 2023\tabularnewline
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\textbf{Entry for the Table of Contents}\selectlanguage{english}
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\textbf{Engineering Colloidal Metal-Semiconductor Nanorods Hybrid
Nanostructures for Photocatalysis} Jiayi Chen, Derek Hao, Wei Chen, Yazi
Liu, Zongyou Yin, Hsien-Yi Hsu*, Bing-Jie Ni*, Aixiang Wang*, Simon W.
Lewis, and Guohua Jia* \emph{Chin. J. Chem.} \textbf{2023}, \emph{41},
XXX---XXX. \textbf{DOI: 10.1002/cjoc.202200XXX}\tabularnewline
Emerging engineering strategies of colloidal metal-semiconductor
nanorods hybrid nanostructures spanning from type, size, dimension, and
location of both metal nanoparticles and semiconductors, co-catalyst,
band gap structure, surface ligand to hole scavenger are summarized
symmetrically to rationalize the design of the type of intriguing
materials for efficient photocatalytic applications.\tabularnewline
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