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.[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.[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.[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.[4]
Although the syntheses,[5a,5b]architectures,[5c] plasmonic
properties[5d] and catalytic
properties[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.[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.[6b] Dukovic et al. reported the
photodeposition of Pt on colloidal CdS nanorods (Figure
1B).[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.[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.[6d]