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]