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Inorganic nanocrystals: From molecular design to systematic engineering
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Invited review
Inorganic nanocrystals: From molecular design to systematic engineering夽 Kai Wang, Zhicheng Zhang, Xun Wang ∗ , Yadong Li Department of Chemistry, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 14 March 2014 Accepted 3 April 2014 Keywords: Molecular design Systematic engineering Synthesis Nanocrystal
a b s t r a c t Solution-based routes have been widely applied in the synthesis of nanostructures. It is desirable to choose or design precursors, ligands, and solvent molecules at the molecular level to allow the synthesis of lowdimensional nanocrystals with various shapes and sizes. The increasing requirements for the integration of the properties of nanocrystals have produced a high demand for the rational design and fine control of complex structures, in terms of both the composition and the structure. To meet this demand, researchers have developed new synthetic strategies to produce more complex and more functional nanocrystals. Typical procedures involve systematic engineering, which focuses on the whole synthetic procedure, instead of just the molecular details. This review focuses mainly on the work of Yadong Li’s group over the last decade. © 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precursor molecular design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ligand molecular design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Systematic engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hollow structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A series of strategies for bimetallic catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanostructures with multiple morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rhodium nanosheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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夽 This is an invited review, reporting the ongoing research work on solution-based synthesis of inorganic nanocrystals in the research group of Professor Yadong Li, in Department of Chemistry, Tsinghua University. The review article, written by members of Li’s group, focuses on the strategy—from molecular design for simple nanostructures to systematic engineering for more complex and functional nanostructures, developed in their research practice during the last decade. ∗ Corresponding author. Tel.: +86 10 62792791. E-mail address: [email protected] (X. Wang). URL: http://www.tsinghua.edu.cn/publish/chemen/2141/2011/20110405152005631512897/20110405152005631512897 .html (X. Wang). http://dx.doi.org/10.1016/j.partic.2014.04.002 1674-2001/© 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
Please cite this article in press as: Wang, K., et al. Inorganic nanocrystals: From molecular design to systematic engineering. Particuology (2014), http://dx.doi.org/10.1016/j.partic.2014.04.002
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Introduction In recent decades the nanoscience and nanotechnology fields have developed greatly. Various types of nanostructures with a wide distribution of sizes and shapes have been prepared by chemists and material scientists. Nanocrystals display a variety of outstanding properties, and have been applied in several fields, including biology, medicine (Wang, Bao, Wang, Zhang, & Li, 2006), nanocatalysis (Zhou, Wang, Sun, Peng, & Li, 2005), and assembly (Hu & Wang, 2013; Long et al., 2012; Luo, Wang, Jing, & Wang, 2014; Saleem et al., 2013; Wang, Sun, Ji, Li, & Wang, 2014; Xu et al., 2014). A large number of synthetic techniques have been developed, including hydrothermal, CVD, and sol–gel methods; however, it remains a challenge to precisely control the size, shape, and phase of nanocrystals. Among these methods, solutionbased routes hold unique advantages; they can provide gram-scale samples, even in the laboratory, and can be easily used to investigate general formation mechanisms. Solution-based methods have therefore attracted increasing attention from synthetic scientists. In solution-based routes, molecular design is typically used to design of precursor and/or ligand molecules that are applied for the synthesis of low dimensional nanocrystals. Different precursors—typically including organic and inorganic molecules—have a great influence on the nucleation and growth of these crystals. Organic molecules dissolve well in oils, and are easy to design through organic synthesis. Inorganic molecules, which are very cheap, are widely used in synthesis; however, their low solubility in oils limits their applications. It is therefore desirable to design the ligand molecules and the solvent for a particular reaction to optimize the reaction conditions. To achieve other synthetic routes—especially because of the continuous increases in the requirements in various fields in recent years—it is desirable to use systematic engineering processes to follow the whole crystal growth procedure, rather than simply designing some molecules. If these design and engineering processes can be applied, finally, it may be possible to control the size and shape of nanostructures flexibly and creatively. Yadong Li’s research group has developed a variety of solutionbased routes in recent decades. In 2002 and 2003, Li et al. reported the use of solution-based routes to prepare a variety of rare earth compound nanostructures, including nanowires, and nanotubes (Wang & Li, 2002a, 2003a, 2003b; Wang, Sun, Yu, Zou, & Li, 2003). Besides rare-earth nanocrystals, CdE (E = S, Se, Te) nanorods (Deng, Li, & Li, 2003), Mg(OH)2 nanodrods (Li et al., 2000), MnO2 nanowires (Wang & Li, 2002b), and CaF2 nanocubes (Sun & Li, 2003) were also synthesized before 2003. In 2005, Wang, Zhuang, Peng, and Li reported a general synthetic strategy for the production of various nanocrystals with different chemistries and properties. In recent years, Li’s group has developed solution-based routes for the synthesis of more advanced functional nanostructures. In 2008, Wang, Zheng, Hao, Peng, and Li (2009) developed a synthetic method for a series of transition-metal chalcogenide semiconductor nanocrystals whose size and shape were well controlled. Various bimetallic catalysts (Wu, Cai, Wang, He, & Li, 2012a; Wu, Wang, et al., 2012; Wu et al., 2013) and concave nanomaterials (Li et al., 2013) were then reported, which exhibited excellent catalytic properties. In 2014, Duan et al. reported the synthesis of single-atom-layer rhodium nanosheets, which represented significant progress in the solution-based synthetic methods for the production of nanocrystals. In this paper, we will review the recent progress made in solution-based synthesis strategies, which have been developed from molecular design for producing simple nanostructures to systematic engineering for producing more complex and
Fig. 1. A stable Mg2+ complex (Li et al., 2000).
functional nanostructures, highlighting the contributions of Li’s group. This review is divided into two parts: molecular design, and systematic engineering. The first part reviews precursor molecular design and ligand molecular design, summarizing research into the synthesis of low-dimensional nanomaterials in which the precursors and some nanostructures were chosen or designed for biological applications by designing their ligands to change the surface properties. Molecular design Precursor molecular design Precursors for nanoscale synthesis include inorganic compounds, organometallic coordination compounds, and polyacids, among other compounds. It is important to choose the organic functional groups for organometallic coordination compounds, as well as the solvent for inorganic compounds. We therefore review both the organic and the inorganic cases here. Organometallic methods have been applied relatively successfully in recent decades (Murray, Norris, & Bawendi, 1993; Sun, Murray, Weller, Folks, & Moser, 2000). In these methods, the properties of the precursors typically depend on the organic part. Mai et al. (2006) developed an effective method for the synthesis of high-quality alkali-rare-earth complex fluoride (AREF4 ) nanocrystals, when liquid precipitation methods prevailed at that time. One of the most significant factors influencing their success was that they carefully chose precursors which includes both metal and fluorine elements that made the reaction more controllable. In a typical procedure, Na(CF3 COO) and RE(CF3 COO)3 precursors are heated under an Ar atmosphere to achieve co-thermolysis, producing NaREF4 (RE = Pr to Lu, Y), Yb3+ , and Er3+ /Tm3+ co-doped NaYF4 nanocrystals (Mai et al., 2006). A similar result was reported in the same year (Zhang, Liu, Peng, Wang, & Li, 2006); in this case, the synthesis of nearly monodisperse Cu2 O and CuO nanospheres was performed using a Cu(CH3 COO)2 precursor. In general, inorganic compounds are cheap and easy to obtain, and are much simpler than organometallic coordination compounds; they are therefore widely used as precursors for the synthesis of nanocrystals. Li’s group has successfully produced many nanostructures, including hydroxide and semiconductor nanorods, rare-earth nanorods, and nanowires, by choosing appropriate solvents in which the inorganic compounds formed special precursors with the solvent molecules. Li et al. reported in 2000 that control over the nucleation and growth is important in the synthesis of Mg(OH)2 nanorods; in their study, ethylenediamine was chosen as a solvent in which Mg(OH)2 formed a relatively stable complex (Fig. 1). When the temperature was increased, the stability of complex decreased; hydroxyl groups may coordinate with Mg2+ and gradually replace
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Fig. 2. Formation mechanism of Mg(OH)2 nanorods from complexes (Li et al., 2000).
the bidentate ethylenediamine ligands at high temperatures. With the help of the OH− in the liquid phase, the bonds between the Mg2+ and the N atoms were broken, and, simultaneously, the Mg atoms and the OH− joined together to form one-dimensional Mg(OH)2 nanorods (Fig. 2). The formation mechanism was further verified, showing that the precursor complex was crucial for the formation of the 1D arrangement of the Mg(OH)2 , leading to the synthesis of the nanorod structures. If the ethylenediamine solvent was replaced by an ammonia solution, layer-like or plate-like single crystals were obtained instead of nanorods. CdE (E = S, Se, Te) nanorods were also synthesized in ethylenediamine solvent, via an evolution from CdE 0.5en (en = ethylenediamine) layered precursors (Deng et al., 2003). The ethylenediamine molecules, which served as a structure-director, played a key role in the reaction system, connecting the inorganic molecules together to form inorganic-organic-layered frameworks; the ethylenediamine molecules then escaped from the structure, and the remaining parts were nanocrystallites with expected morphologies. We named this process as the solvent coordination molecular template (SCMT) mechanism. It could not be proven using direct experimental data; nevertheless, the evidence for the formation of CdE 0.5en layer precursors still suggested that it was the SCMT process that had occurred. Characterization indicated that the precursors were mutual isostructures, in which the CdE inorganic layers were bridged by trans-ethylenediamine molecules to produce puckered, honeycomb-like atomic connectivity (Fig. 3). Further exploration of the process from the precursor stage to the production of the nanorods confirmed that it was a refined version of the SCMT mechanism. The layered structure was stabilized by the interlayer ethylenediamine molecules. Various influencing factors, such as high temperature, resulted in the escape of the organic molecules; this always occurred at a position near crystal defects, or near the edge of the platelike precursors. The inorganic slabs became unstable when they lost the joiners. They coupled to form bulk phases, and cracked into nanorods. Fig. 4 provides a simple illustration of this process. Ligand molecular design Although precursor molecular design has been applied to the production of nanocrystals with great effect, significant limitations
Fig. 3. Structural views of the honeycomb-like atomic connectivity of CdE 0.5en down the a, b, and c axes of the orthorhombic cell. The red, cyan, blue, and black balls correspond to Cd, Se, N, and C atoms, respectively; H atoms are omitted for clarity (Deng et al., 2003).
remain. It is desirable to reprocess the nanocrystals, and ligand molecular design is a candidate with significant potential for this task. The life sciences have developed rapidly in the 21st century, and great efforts have been made to apply nanocrystals in the biotechnology field. A variety of semiconductor quantum dots with significant advantages have been synthesized, and have been applied in biological analysis (Bailey, Nam, Mirkin, & Hupp, 2003; Bruchez, Moronne, Gin, Weiss, & Alivisatos, 1998; Chan & Nie, 1998; Goldman et al., 2002; Jaiswal, Mattoussi, Mauro, & Simon, 2003; Rosenthal et al., 2002; Taylor, Fang, & Nie, 2000). A number of methods for the production of magnetic iron oxide nanoparticles were developed in the past; examples of such techniques include ultrasound irradiation technology (Pol, Motiei, Gedanken, Calderon-Moreno, & Mastai, 2003), and coprecipitation and microemulsion methods (Lee et al., 2005; Yi et al., 2005). In addition, different types of iron precursors such as iron carbonyls and iron oleates were investigated by researchers in the period
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Fig. 6. Schematic illustration of the magnetic separation performed in a fluorescence immunoassay using the amine-functionalized magnetite nanoparticles (Wang et al., 2006). Fig. 4. Schematic illustration of the whole process for the formation of CdE nanorods from the layer-like precursor (Deng et al., 2003).
from 1999 until 2005 (Hyeon, Lee, Park, Chung, & Na, 2001; Park et al., 2004; Rockenberger, Scher, & Alivisatos, 1999; Sun et al., 2004; Teng & Yang, 2003; Zeng, Rice, Wang, & Sun, 2004). However, there are intrinsic limitations for all the methods developed before, which limits their application. In brief, it is desirable to increase the effort dedicated to the development of new synthetic strategies such as ligand molecular design. In 2005, Wang, Yan, et al. reported the preparation of a novel biosensor for the detection of trace amounts of avidin, which was realized via fluorescence resonance energy transfer (FRET) between bioconjugated upconversion (UC) nanoparticles and gold particles. Amino groups were introduced to the surface of UC nanoparticles to accomplish the green upconversion phosphors, then the nanoparticles could be applied as fluorescent biological probes. They used the layer-by-layer (LbL) method to design the UC nanoparticles (Fig. 5). The UC nanoparticles were fabricated with three primer polyelectrolyte layers (PAH/PSS/PAH; PAH = poly(allylamine hydrochloride); PSS = poly(styrene sulfonate)), which were formed via the alternate adsorption of PAH and PSS onto the negatively charged UC nanoparticles. Wang et al. (2006) reported for the first time the development of a one-pot template method for the
Fig. 5. Coating of the UC nanoparticles with three polyelectrolyte layers PAH = poly(allylamine hydrochloride); PSS = poly(styrene (PAH/PSS/PAH). sulfonate), EDC = 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide; NHS = Nhydroxysuccinimide (Wang, Yan, et al., 2005).
synthesis of amine-functionalized magnetite nanoparticles and hollow nanospheres. The synthesis was achieved using FeCl3 ·6H2 O as the only iron source and 1,6-hexanediamine as the ligand, and the particles obtained had highly magnetic properties, and amino groups on their surface. These particles are useful for biological and biomedical applications, because the amino groups on the outer surface connect easily with different ligands, because of the presence of carboxyl groups (Fig. 6). In addition, their highly magnetic properties and hollow core give them great potential for catalysis and targeted drug delivery. Li, Wang, and Li (2004) also reported the synthesis of double-bond-grafted yttrium hydroxide nanotube core–shell structures using a one-pot hydrothermal approach (Fig. 7). The synthesis was based on rare earth nanostructures, which were produced from aqueous solutions with affluent hydroxyls on their surface; it was simple to introduce the functional groups via a solution-based method. Wang, Zhuang, et al. (2005) developed a strategy to synthesize varieties of nanocrystals, including noble metal, semiconductor, rare-earth, and fluorescent nanocrystals, which formed at the interface of the liquid, solid, and solution phases. This approach was used to generate nanocrystals with different properties simply and conveniently via
Fig. 7. Schematic of the formation of Y(OH)3 nanotubes and the subsequent grafting of carbon–carbon double-bonds (Li et al., 2004).
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A series of strategies for bimetallic catalysts
Fig. 8. Scheme for a liquid–solid–solution (LSS) phase transfer synthetic strategy (Wang, Zhuang, et al., 2005).
a liquid–solid-solution (LSS) phase transfer and separation process (Fig. 8).
Systematic engineering Science and technology is developing constantly, and the requirements for all of the applications in all of the variety of fields cannot be met by just the molecular design process, and one cannot rely entirely on this process to produce multicomponent and multifunctional nanostructures with complex constructions. Composite nanostructures typically meet some specific requirements better than one-component nanostructures. For example, bimetallic nanoparticles exhibit superior catalytic properties compared with monometallic nanoparticles; it is likely that new geometric and electronic structures were produced via the addition of the second metal (Tsang et al., 2008; Wu et al., 2011). Hence, we should focus not only on the precursor or the ligand of the nanostructures, but also on the entire synthetic procedure.
Bimetallic nanocrystals have a very long history. Since Mott and Jones reported in 1936 that the alloy of a group 8 metal with a group 1B metal showed a greater band extent than the pure group 8 metal (Mott & Jones, 1958); the research into bimetallic nanocrystals has been developing constantly. Sun, Mayers, and Xia (2002) reported the production of a series of Au-Ag hollow and framework structures using a galvanic replacement process. In 2012 and 2013, Li’s group developed a number of strategies for the synthesis of different types of metallic nanocrystals based on Pt–Ni bimetallic nanocrystals (Wu, Cai, et al., 2012; Wu, Wang, et al., 2012; Wu et al., 2013). They first synthesized a series of polyhedral nanocrystals (including Pt, Ni monometallic, and bimetallic nanocrystals), and then they designed a concave Pt–Ni alloy using a chemical etching process. Subsequently, they recovered the defects of the nanocrystals with other metals to obtain trimetallic catalysts. This strategy for the synthesis of bimetallic nanostructures allowed the growth of the nanocrystals to be controlled in accordance with the requirements of particular applications. The nanocrystals showed different catalytic performances, and these systems are of great value for the further study of the factors influencing the performance of catalysts. Wu, Cai, et al. (2012) reported a strategy for the synthesis of water-soluble Pt, Ni bimetallic nanocrystals. The shape, composition, and capping agents of the nanocrystals could be changed very conveniently. Octahedral, truncated octahedral, and cubic Pt–Ni alloys (Fig. 11) were produced by adjusting the concentration of agents such as benzoic acid, aniline, and carbon monoxide, which modified the crystal growth. When benzoic acid was used as an agent, Pt and Ptx Ni1−x (0 < x < 1) octahedral nanocrystals were synthesized, whereas truncated octahedral nanocrystals were produced when the benzoic acid was replaced by aniline; to produce cubic structures, it was necessary to introduce potassium bromide. The “x” in Ptx Ni1−x (0 < x < 1) could be adjusted easily by changing the ratio of the Pt and Ni precursors. It was also relatively simple to exchange the capping agent on the surface of the nanocrystals. For example, oleylamine-capped PtNi2 nanocrystals were obtained using a very simple process: the PtNi2 nanocrystals were redispersed with added oleylamine, and the sample was sonicated, refluxed, and finally centrifuged. It is worth noting that all of the nanocrystals that were produced were first capped with poly(vinylpyrrolidone) (PVP), which prevented the catalysts from experiencing any postsynthesis reactions that would result in the loss of the products. The pre-synthesized Pt–Ni alloy nanocrystals were used as raw materials to produce concave structures through a chemical etching process, which was controlled by coordinating complexes at room temperature (Wu, Wang, et al., 2012). The etching was performed in water with an excess of dimethylglyoxime (dissolved in
Hollow structures Peng, Dong, and Li (2003) reported the synthesis of hollow ZnSe microspheres using a convenient and controllable hydrothermal synthetic route. The microspheres were formed via the aggregation of small ZnSe particles (Fig. 9). The interior nanocrystal size could be tuned from 5 to 100 nm by controlling the reaction temperature. Then, Sun and Li (2004) reported the synthesis of a series of monodisperse carbon spheres with a wide size range of 150–1500 nm, using a polymerization and carbonization process (Fig. 10). Au-core and silver-core carbon spheres were produced by encapsulating gold or silver nanoparticles with carbon spheres, and a layered structure with a silver core, a platinum shell, and a carbon interlayer was formed using seed-based encapsulation followed by reflux.
Fig. 9. Schematic representation of the formation mechanism for ZnSe microspheres (Peng et al., 2003).
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Fig. 10. Schematic growth model for carbon spheres (in the final step, the carbonized core and the hydrophilic surface formed by dehydration are schematically represented by aromatic rings and polysaccharides, respectively) (Sun & Li, 2004).
ethanol), which could coordinate with NiII to form stable compositions. The octahedron-shaped Pt–Ni alloy nanocrystals turned into nanoparticles with six uniform arms (Fig. 12). The etching process is illustrated in Fig. 13 by the chemical equilibrium Equations, which
indicated that the oxygen in the air contributed to the oxidation of nickel. This was confirmed by replacing the oxygen with nitrogen; in this case, the etching process was almost non-functional. Further research into the relationship between the proportion of Ni in the
Fig. 11. TEM images (a, e, i); HRTEM images (b, f, j, with the top-right and top-middle insets showing the corresponding FFT pattern and the ideal structure model); HAADF-STEM images (c, g, k), with corresponding element maps showing the distribution of Pt (yellow) and Ni (red), and size distributions (d, h, l) of PtNi2 octahedrons, truncated octahedrons and cubes, respectively (HRTEM = high-resolution transmission electron microscopy, FFT = fast Fourier transform, HAADF = high-angle annular darkfield, STEM = scanning transmission electron microscope) (Wu, Cai, et al., 2012).
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Fig. 12. TEM images (a and b, with the inset showing an ideal model of a concave octahedron), HRTEM image of corroded PtNi2 orientated along {1 1 1} direction (c), and HAADF-STEM image and elemental maps for Pt and Ni (d) of the PtNi2 particles (Wu, Wang, et al., 2012).
Pt–Ni alloy nanocrystals and the concavity of the etched nanoparticles was performed. The results showed that the Ni-rich precursors were likely responsible for the increased lattice spacing (Fig. 14). These findings have the potential to enrich the synthetic chemistry of nanocrystals, and to allow the production of different types of etched particles that could be used to investigate the properties of nanocrystals.
The above-mentioned study showed the possibility to expand applications of bimetallic nanoparticles via etching process; it was also shown that it was possible to recover defects to create more new structures. A new strategy was reported in 2013 for the synthesis of trimetallic catalysts via the recovery of etched bimetallic nanoparticles with another metal (Wu et al., 2013). It was found that a solvothermal process in a Ni2+ -rich chemical environment could be used to convert concave Pt3 Ni particles into Pt3 Ni@Ni core–shell structures; these findings agreed with thermodynamic theory, i.e., the {1 1 1} facets have the lowest surface energy. The growth mechanism at work in the shape recovery process was described as “step-induced/terrace-assisted”, based on the computational results of density functional theory (DFT). This mechanism was also suitable for the growth of Au, Ag, Cu, Rh and Fe. Au-coated Pt3 Ni@Au trimetallic core–shell structures were successfully synthesized, and it was easy to adjust the degree of concavity and the surface Au coverage simply by controlling the amount of chloroauric acid. The growth process is shown in Fig. 15, and element maps were taken to confirm the elemental composition of each nanostructure. Nanostructures with multiple morphologies
Fig. 13. Chemical equilibrium equations of etching process. Eqs. (1) and (2) are two half-reactions of an oxidation–reduction reaction, and Eq. (3) is dimethylglyoxime coordination to NiII (Wu, Wang, et al., 2012).
In recent decades, CdSe nanocrystals, a semiconductor material, have been synthesized in a variety of shapes, including rods, arrows, and teardrops (Manna, Scher, & Alivisatos, 2000; Peng et al., 2000); the crystalline structure of the nanocrystals
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Fig. 14. The evolution of the nanoparticle shape as a function of the Ni: Pt mole ratio in the originally prepared Ni-rich alloys (Wu, Wang, et al., 2012).
CdSe nanocrystals (Fig. 16), simply by adjusting the reaction temperature. Later, Zhuang, Lu, Peng, and Li (2010) reported the synthesis of ultrathin water-soluble CdS nanorods with great potential for biological labeling. The CdS nanorods could respond to the alkalinity of the solution, aggregating under ultrahigh alkalinity, and redispersing when the alkalinity was decreased. Polyethyleneimine (PEI) was used as a surfactant for the synthesis, because of its multiple amino groups, which tightly bound the nanocrystals; finally, ultrathin nanostructures were obtained. Rhodium nanosheets
is largely made up of a hexagonal wurtzite phase. Liu et al. (2009) reported the synthesis of a series of zinc blende CdSe nanocrystals with various shapes. They produced distinct cube-shaped, sphere-shaped, tetrahedron-shaped, and branched
Recently, Duan et al. (2014) reported the synthesis of PVPprotected, single-layer rhodium nanosheets. It is difficult for metal atoms to form single-atom-layer structures, because of their propensity to form three-dimensional, close-packed structures. DFT investigations showed that the single-layer Rh nanosheets were stabilized by a unique ␦-bonding framework.
Fig. 15. (a–c) Aberration-corrected HRTEM images of (a) [email protected], (b) Pt3 Ni@Au2 , and (c) Pt3 Ni@Au8 ; (d–f) elemental maps of (d) [email protected], (e) Pt3 Ni@Au2 , and (f) Pt3 Ni@Au8 ; (g) schematic illustration of the evolution from octahedral PtNi3 to Pt3 Ni@M8 (M = Rh, Au, Ag, Cu); and (h) schematic illustration of the growth of Au on the Pt surface (Wu et al., 2013).
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desirable to develop synthetic strategies to meet the new demands on nanocrystals. It is therefore important to consider the whole synthetic procedure, instead of focusing only on molecular design; we call this approach systematic engineering, and we believe it will become a mainstream synthetic strategy for nanocrystals in the near future.
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Fig. 16. TEM images of the CdSe nanocrystals with different shapes: (a, b) sphereshaped; (c, d) cube-shaped; (e, f) tetrahedron-shaped; (g, h) branched. The inset images in parts a, c, d, and g show the SAED patterns of the corresponding samples (SAED = selected area electron diffraction) (Liu et al., 2009).
Conclusions This article provides an overview of the progress made in Li’s group in the development of solution-based synthetic routes, from precursor and ligand molecular design to systematic engineering. Molecular design is an important traditional method for the synthesis of nanocrystals, and it can be used to control the size, shape, and phase of crystals; it has been successfully applied to produce various types of nanocrystals in recent decades. As the requirements from different fields become more demanding, it is
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Invited review
Inorganic nanocrystals: From molecular design to systematic engineering夽 Kai Wang, Zhicheng Zhang, Xun Wang ∗ , Yadong Li Department of Chemistry, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 14 March 2014 Accepted 3 April 2014 Keywords: Molecular design Systematic engineering Synthesis Nanocrystal
a b s t r a c t Solution-based routes have been widely applied in the synthesis of nanostructures. It is desirable to choose or design precursors, ligands, and solvent molecules at the molecular level to allow the synthesis of lowdimensional nanocrystals with various shapes and sizes. The increasing requirements for the integration of the properties of nanocrystals have produced a high demand for the rational design and fine control of complex structures, in terms of both the composition and the structure. To meet this demand, researchers have developed new synthetic strategies to produce more complex and more functional nanocrystals. Typical procedures involve systematic engineering, which focuses on the whole synthetic procedure, instead of just the molecular details. This review focuses mainly on the work of Yadong Li’s group over the last decade. © 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precursor molecular design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ligand molecular design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Systematic engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hollow structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A series of strategies for bimetallic catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanostructures with multiple morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rhodium nanosheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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夽 This is an invited review, reporting the ongoing research work on solution-based synthesis of inorganic nanocrystals in the research group of Professor Yadong Li, in Department of Chemistry, Tsinghua University. The review article, written by members of Li’s group, focuses on the strategy—from molecular design for simple nanostructures to systematic engineering for more complex and functional nanostructures, developed in their research practice during the last decade. ∗ Corresponding author. Tel.: +86 10 62792791. E-mail address: [email protected] (X. Wang). URL: http://www.tsinghua.edu.cn/publish/chemen/2141/2011/20110405152005631512897/20110405152005631512897 .html (X. Wang). http://dx.doi.org/10.1016/j.partic.2014.04.002 1674-2001/© 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
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Introduction In recent decades the nanoscience and nanotechnology fields have developed greatly. Various types of nanostructures with a wide distribution of sizes and shapes have been prepared by chemists and material scientists. Nanocrystals display a variety of outstanding properties, and have been applied in several fields, including biology, medicine (Wang, Bao, Wang, Zhang, & Li, 2006), nanocatalysis (Zhou, Wang, Sun, Peng, & Li, 2005), and assembly (Hu & Wang, 2013; Long et al., 2012; Luo, Wang, Jing, & Wang, 2014; Saleem et al., 2013; Wang, Sun, Ji, Li, & Wang, 2014; Xu et al., 2014). A large number of synthetic techniques have been developed, including hydrothermal, CVD, and sol–gel methods; however, it remains a challenge to precisely control the size, shape, and phase of nanocrystals. Among these methods, solutionbased routes hold unique advantages; they can provide gram-scale samples, even in the laboratory, and can be easily used to investigate general formation mechanisms. Solution-based methods have therefore attracted increasing attention from synthetic scientists. In solution-based routes, molecular design is typically used to design of precursor and/or ligand molecules that are applied for the synthesis of low dimensional nanocrystals. Different precursors—typically including organic and inorganic molecules—have a great influence on the nucleation and growth of these crystals. Organic molecules dissolve well in oils, and are easy to design through organic synthesis. Inorganic molecules, which are very cheap, are widely used in synthesis; however, their low solubility in oils limits their applications. It is therefore desirable to design the ligand molecules and the solvent for a particular reaction to optimize the reaction conditions. To achieve other synthetic routes—especially because of the continuous increases in the requirements in various fields in recent years—it is desirable to use systematic engineering processes to follow the whole crystal growth procedure, rather than simply designing some molecules. If these design and engineering processes can be applied, finally, it may be possible to control the size and shape of nanostructures flexibly and creatively. Yadong Li’s research group has developed a variety of solutionbased routes in recent decades. In 2002 and 2003, Li et al. reported the use of solution-based routes to prepare a variety of rare earth compound nanostructures, including nanowires, and nanotubes (Wang & Li, 2002a, 2003a, 2003b; Wang, Sun, Yu, Zou, & Li, 2003). Besides rare-earth nanocrystals, CdE (E = S, Se, Te) nanorods (Deng, Li, & Li, 2003), Mg(OH)2 nanodrods (Li et al., 2000), MnO2 nanowires (Wang & Li, 2002b), and CaF2 nanocubes (Sun & Li, 2003) were also synthesized before 2003. In 2005, Wang, Zhuang, Peng, and Li reported a general synthetic strategy for the production of various nanocrystals with different chemistries and properties. In recent years, Li’s group has developed solution-based routes for the synthesis of more advanced functional nanostructures. In 2008, Wang, Zheng, Hao, Peng, and Li (2009) developed a synthetic method for a series of transition-metal chalcogenide semiconductor nanocrystals whose size and shape were well controlled. Various bimetallic catalysts (Wu, Cai, Wang, He, & Li, 2012a; Wu, Wang, et al., 2012; Wu et al., 2013) and concave nanomaterials (Li et al., 2013) were then reported, which exhibited excellent catalytic properties. In 2014, Duan et al. reported the synthesis of single-atom-layer rhodium nanosheets, which represented significant progress in the solution-based synthetic methods for the production of nanocrystals. In this paper, we will review the recent progress made in solution-based synthesis strategies, which have been developed from molecular design for producing simple nanostructures to systematic engineering for producing more complex and
Fig. 1. A stable Mg2+ complex (Li et al., 2000).
functional nanostructures, highlighting the contributions of Li’s group. This review is divided into two parts: molecular design, and systematic engineering. The first part reviews precursor molecular design and ligand molecular design, summarizing research into the synthesis of low-dimensional nanomaterials in which the precursors and some nanostructures were chosen or designed for biological applications by designing their ligands to change the surface properties. Molecular design Precursor molecular design Precursors for nanoscale synthesis include inorganic compounds, organometallic coordination compounds, and polyacids, among other compounds. It is important to choose the organic functional groups for organometallic coordination compounds, as well as the solvent for inorganic compounds. We therefore review both the organic and the inorganic cases here. Organometallic methods have been applied relatively successfully in recent decades (Murray, Norris, & Bawendi, 1993; Sun, Murray, Weller, Folks, & Moser, 2000). In these methods, the properties of the precursors typically depend on the organic part. Mai et al. (2006) developed an effective method for the synthesis of high-quality alkali-rare-earth complex fluoride (AREF4 ) nanocrystals, when liquid precipitation methods prevailed at that time. One of the most significant factors influencing their success was that they carefully chose precursors which includes both metal and fluorine elements that made the reaction more controllable. In a typical procedure, Na(CF3 COO) and RE(CF3 COO)3 precursors are heated under an Ar atmosphere to achieve co-thermolysis, producing NaREF4 (RE = Pr to Lu, Y), Yb3+ , and Er3+ /Tm3+ co-doped NaYF4 nanocrystals (Mai et al., 2006). A similar result was reported in the same year (Zhang, Liu, Peng, Wang, & Li, 2006); in this case, the synthesis of nearly monodisperse Cu2 O and CuO nanospheres was performed using a Cu(CH3 COO)2 precursor. In general, inorganic compounds are cheap and easy to obtain, and are much simpler than organometallic coordination compounds; they are therefore widely used as precursors for the synthesis of nanocrystals. Li’s group has successfully produced many nanostructures, including hydroxide and semiconductor nanorods, rare-earth nanorods, and nanowires, by choosing appropriate solvents in which the inorganic compounds formed special precursors with the solvent molecules. Li et al. reported in 2000 that control over the nucleation and growth is important in the synthesis of Mg(OH)2 nanorods; in their study, ethylenediamine was chosen as a solvent in which Mg(OH)2 formed a relatively stable complex (Fig. 1). When the temperature was increased, the stability of complex decreased; hydroxyl groups may coordinate with Mg2+ and gradually replace
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Fig. 2. Formation mechanism of Mg(OH)2 nanorods from complexes (Li et al., 2000).
the bidentate ethylenediamine ligands at high temperatures. With the help of the OH− in the liquid phase, the bonds between the Mg2+ and the N atoms were broken, and, simultaneously, the Mg atoms and the OH− joined together to form one-dimensional Mg(OH)2 nanorods (Fig. 2). The formation mechanism was further verified, showing that the precursor complex was crucial for the formation of the 1D arrangement of the Mg(OH)2 , leading to the synthesis of the nanorod structures. If the ethylenediamine solvent was replaced by an ammonia solution, layer-like or plate-like single crystals were obtained instead of nanorods. CdE (E = S, Se, Te) nanorods were also synthesized in ethylenediamine solvent, via an evolution from CdE 0.5en (en = ethylenediamine) layered precursors (Deng et al., 2003). The ethylenediamine molecules, which served as a structure-director, played a key role in the reaction system, connecting the inorganic molecules together to form inorganic-organic-layered frameworks; the ethylenediamine molecules then escaped from the structure, and the remaining parts were nanocrystallites with expected morphologies. We named this process as the solvent coordination molecular template (SCMT) mechanism. It could not be proven using direct experimental data; nevertheless, the evidence for the formation of CdE 0.5en layer precursors still suggested that it was the SCMT process that had occurred. Characterization indicated that the precursors were mutual isostructures, in which the CdE inorganic layers were bridged by trans-ethylenediamine molecules to produce puckered, honeycomb-like atomic connectivity (Fig. 3). Further exploration of the process from the precursor stage to the production of the nanorods confirmed that it was a refined version of the SCMT mechanism. The layered structure was stabilized by the interlayer ethylenediamine molecules. Various influencing factors, such as high temperature, resulted in the escape of the organic molecules; this always occurred at a position near crystal defects, or near the edge of the platelike precursors. The inorganic slabs became unstable when they lost the joiners. They coupled to form bulk phases, and cracked into nanorods. Fig. 4 provides a simple illustration of this process. Ligand molecular design Although precursor molecular design has been applied to the production of nanocrystals with great effect, significant limitations
Fig. 3. Structural views of the honeycomb-like atomic connectivity of CdE 0.5en down the a, b, and c axes of the orthorhombic cell. The red, cyan, blue, and black balls correspond to Cd, Se, N, and C atoms, respectively; H atoms are omitted for clarity (Deng et al., 2003).
remain. It is desirable to reprocess the nanocrystals, and ligand molecular design is a candidate with significant potential for this task. The life sciences have developed rapidly in the 21st century, and great efforts have been made to apply nanocrystals in the biotechnology field. A variety of semiconductor quantum dots with significant advantages have been synthesized, and have been applied in biological analysis (Bailey, Nam, Mirkin, & Hupp, 2003; Bruchez, Moronne, Gin, Weiss, & Alivisatos, 1998; Chan & Nie, 1998; Goldman et al., 2002; Jaiswal, Mattoussi, Mauro, & Simon, 2003; Rosenthal et al., 2002; Taylor, Fang, & Nie, 2000). A number of methods for the production of magnetic iron oxide nanoparticles were developed in the past; examples of such techniques include ultrasound irradiation technology (Pol, Motiei, Gedanken, Calderon-Moreno, & Mastai, 2003), and coprecipitation and microemulsion methods (Lee et al., 2005; Yi et al., 2005). In addition, different types of iron precursors such as iron carbonyls and iron oleates were investigated by researchers in the period
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Fig. 6. Schematic illustration of the magnetic separation performed in a fluorescence immunoassay using the amine-functionalized magnetite nanoparticles (Wang et al., 2006). Fig. 4. Schematic illustration of the whole process for the formation of CdE nanorods from the layer-like precursor (Deng et al., 2003).
from 1999 until 2005 (Hyeon, Lee, Park, Chung, & Na, 2001; Park et al., 2004; Rockenberger, Scher, & Alivisatos, 1999; Sun et al., 2004; Teng & Yang, 2003; Zeng, Rice, Wang, & Sun, 2004). However, there are intrinsic limitations for all the methods developed before, which limits their application. In brief, it is desirable to increase the effort dedicated to the development of new synthetic strategies such as ligand molecular design. In 2005, Wang, Yan, et al. reported the preparation of a novel biosensor for the detection of trace amounts of avidin, which was realized via fluorescence resonance energy transfer (FRET) between bioconjugated upconversion (UC) nanoparticles and gold particles. Amino groups were introduced to the surface of UC nanoparticles to accomplish the green upconversion phosphors, then the nanoparticles could be applied as fluorescent biological probes. They used the layer-by-layer (LbL) method to design the UC nanoparticles (Fig. 5). The UC nanoparticles were fabricated with three primer polyelectrolyte layers (PAH/PSS/PAH; PAH = poly(allylamine hydrochloride); PSS = poly(styrene sulfonate)), which were formed via the alternate adsorption of PAH and PSS onto the negatively charged UC nanoparticles. Wang et al. (2006) reported for the first time the development of a one-pot template method for the
Fig. 5. Coating of the UC nanoparticles with three polyelectrolyte layers PAH = poly(allylamine hydrochloride); PSS = poly(styrene (PAH/PSS/PAH). sulfonate), EDC = 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide; NHS = Nhydroxysuccinimide (Wang, Yan, et al., 2005).
synthesis of amine-functionalized magnetite nanoparticles and hollow nanospheres. The synthesis was achieved using FeCl3 ·6H2 O as the only iron source and 1,6-hexanediamine as the ligand, and the particles obtained had highly magnetic properties, and amino groups on their surface. These particles are useful for biological and biomedical applications, because the amino groups on the outer surface connect easily with different ligands, because of the presence of carboxyl groups (Fig. 6). In addition, their highly magnetic properties and hollow core give them great potential for catalysis and targeted drug delivery. Li, Wang, and Li (2004) also reported the synthesis of double-bond-grafted yttrium hydroxide nanotube core–shell structures using a one-pot hydrothermal approach (Fig. 7). The synthesis was based on rare earth nanostructures, which were produced from aqueous solutions with affluent hydroxyls on their surface; it was simple to introduce the functional groups via a solution-based method. Wang, Zhuang, et al. (2005) developed a strategy to synthesize varieties of nanocrystals, including noble metal, semiconductor, rare-earth, and fluorescent nanocrystals, which formed at the interface of the liquid, solid, and solution phases. This approach was used to generate nanocrystals with different properties simply and conveniently via
Fig. 7. Schematic of the formation of Y(OH)3 nanotubes and the subsequent grafting of carbon–carbon double-bonds (Li et al., 2004).
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A series of strategies for bimetallic catalysts
Fig. 8. Scheme for a liquid–solid–solution (LSS) phase transfer synthetic strategy (Wang, Zhuang, et al., 2005).
a liquid–solid-solution (LSS) phase transfer and separation process (Fig. 8).
Systematic engineering Science and technology is developing constantly, and the requirements for all of the applications in all of the variety of fields cannot be met by just the molecular design process, and one cannot rely entirely on this process to produce multicomponent and multifunctional nanostructures with complex constructions. Composite nanostructures typically meet some specific requirements better than one-component nanostructures. For example, bimetallic nanoparticles exhibit superior catalytic properties compared with monometallic nanoparticles; it is likely that new geometric and electronic structures were produced via the addition of the second metal (Tsang et al., 2008; Wu et al., 2011). Hence, we should focus not only on the precursor or the ligand of the nanostructures, but also on the entire synthetic procedure.
Bimetallic nanocrystals have a very long history. Since Mott and Jones reported in 1936 that the alloy of a group 8 metal with a group 1B metal showed a greater band extent than the pure group 8 metal (Mott & Jones, 1958); the research into bimetallic nanocrystals has been developing constantly. Sun, Mayers, and Xia (2002) reported the production of a series of Au-Ag hollow and framework structures using a galvanic replacement process. In 2012 and 2013, Li’s group developed a number of strategies for the synthesis of different types of metallic nanocrystals based on Pt–Ni bimetallic nanocrystals (Wu, Cai, et al., 2012; Wu, Wang, et al., 2012; Wu et al., 2013). They first synthesized a series of polyhedral nanocrystals (including Pt, Ni monometallic, and bimetallic nanocrystals), and then they designed a concave Pt–Ni alloy using a chemical etching process. Subsequently, they recovered the defects of the nanocrystals with other metals to obtain trimetallic catalysts. This strategy for the synthesis of bimetallic nanostructures allowed the growth of the nanocrystals to be controlled in accordance with the requirements of particular applications. The nanocrystals showed different catalytic performances, and these systems are of great value for the further study of the factors influencing the performance of catalysts. Wu, Cai, et al. (2012) reported a strategy for the synthesis of water-soluble Pt, Ni bimetallic nanocrystals. The shape, composition, and capping agents of the nanocrystals could be changed very conveniently. Octahedral, truncated octahedral, and cubic Pt–Ni alloys (Fig. 11) were produced by adjusting the concentration of agents such as benzoic acid, aniline, and carbon monoxide, which modified the crystal growth. When benzoic acid was used as an agent, Pt and Ptx Ni1−x (0 < x < 1) octahedral nanocrystals were synthesized, whereas truncated octahedral nanocrystals were produced when the benzoic acid was replaced by aniline; to produce cubic structures, it was necessary to introduce potassium bromide. The “x” in Ptx Ni1−x (0 < x < 1) could be adjusted easily by changing the ratio of the Pt and Ni precursors. It was also relatively simple to exchange the capping agent on the surface of the nanocrystals. For example, oleylamine-capped PtNi2 nanocrystals were obtained using a very simple process: the PtNi2 nanocrystals were redispersed with added oleylamine, and the sample was sonicated, refluxed, and finally centrifuged. It is worth noting that all of the nanocrystals that were produced were first capped with poly(vinylpyrrolidone) (PVP), which prevented the catalysts from experiencing any postsynthesis reactions that would result in the loss of the products. The pre-synthesized Pt–Ni alloy nanocrystals were used as raw materials to produce concave structures through a chemical etching process, which was controlled by coordinating complexes at room temperature (Wu, Wang, et al., 2012). The etching was performed in water with an excess of dimethylglyoxime (dissolved in
Hollow structures Peng, Dong, and Li (2003) reported the synthesis of hollow ZnSe microspheres using a convenient and controllable hydrothermal synthetic route. The microspheres were formed via the aggregation of small ZnSe particles (Fig. 9). The interior nanocrystal size could be tuned from 5 to 100 nm by controlling the reaction temperature. Then, Sun and Li (2004) reported the synthesis of a series of monodisperse carbon spheres with a wide size range of 150–1500 nm, using a polymerization and carbonization process (Fig. 10). Au-core and silver-core carbon spheres were produced by encapsulating gold or silver nanoparticles with carbon spheres, and a layered structure with a silver core, a platinum shell, and a carbon interlayer was formed using seed-based encapsulation followed by reflux.
Fig. 9. Schematic representation of the formation mechanism for ZnSe microspheres (Peng et al., 2003).
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Fig. 10. Schematic growth model for carbon spheres (in the final step, the carbonized core and the hydrophilic surface formed by dehydration are schematically represented by aromatic rings and polysaccharides, respectively) (Sun & Li, 2004).
ethanol), which could coordinate with NiII to form stable compositions. The octahedron-shaped Pt–Ni alloy nanocrystals turned into nanoparticles with six uniform arms (Fig. 12). The etching process is illustrated in Fig. 13 by the chemical equilibrium Equations, which
indicated that the oxygen in the air contributed to the oxidation of nickel. This was confirmed by replacing the oxygen with nitrogen; in this case, the etching process was almost non-functional. Further research into the relationship between the proportion of Ni in the
Fig. 11. TEM images (a, e, i); HRTEM images (b, f, j, with the top-right and top-middle insets showing the corresponding FFT pattern and the ideal structure model); HAADF-STEM images (c, g, k), with corresponding element maps showing the distribution of Pt (yellow) and Ni (red), and size distributions (d, h, l) of PtNi2 octahedrons, truncated octahedrons and cubes, respectively (HRTEM = high-resolution transmission electron microscopy, FFT = fast Fourier transform, HAADF = high-angle annular darkfield, STEM = scanning transmission electron microscope) (Wu, Cai, et al., 2012).
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Fig. 12. TEM images (a and b, with the inset showing an ideal model of a concave octahedron), HRTEM image of corroded PtNi2 orientated along {1 1 1} direction (c), and HAADF-STEM image and elemental maps for Pt and Ni (d) of the PtNi2 particles (Wu, Wang, et al., 2012).
Pt–Ni alloy nanocrystals and the concavity of the etched nanoparticles was performed. The results showed that the Ni-rich precursors were likely responsible for the increased lattice spacing (Fig. 14). These findings have the potential to enrich the synthetic chemistry of nanocrystals, and to allow the production of different types of etched particles that could be used to investigate the properties of nanocrystals.
The above-mentioned study showed the possibility to expand applications of bimetallic nanoparticles via etching process; it was also shown that it was possible to recover defects to create more new structures. A new strategy was reported in 2013 for the synthesis of trimetallic catalysts via the recovery of etched bimetallic nanoparticles with another metal (Wu et al., 2013). It was found that a solvothermal process in a Ni2+ -rich chemical environment could be used to convert concave Pt3 Ni particles into Pt3 Ni@Ni core–shell structures; these findings agreed with thermodynamic theory, i.e., the {1 1 1} facets have the lowest surface energy. The growth mechanism at work in the shape recovery process was described as “step-induced/terrace-assisted”, based on the computational results of density functional theory (DFT). This mechanism was also suitable for the growth of Au, Ag, Cu, Rh and Fe. Au-coated Pt3 Ni@Au trimetallic core–shell structures were successfully synthesized, and it was easy to adjust the degree of concavity and the surface Au coverage simply by controlling the amount of chloroauric acid. The growth process is shown in Fig. 15, and element maps were taken to confirm the elemental composition of each nanostructure. Nanostructures with multiple morphologies
Fig. 13. Chemical equilibrium equations of etching process. Eqs. (1) and (2) are two half-reactions of an oxidation–reduction reaction, and Eq. (3) is dimethylglyoxime coordination to NiII (Wu, Wang, et al., 2012).
In recent decades, CdSe nanocrystals, a semiconductor material, have been synthesized in a variety of shapes, including rods, arrows, and teardrops (Manna, Scher, & Alivisatos, 2000; Peng et al., 2000); the crystalline structure of the nanocrystals
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Fig. 14. The evolution of the nanoparticle shape as a function of the Ni: Pt mole ratio in the originally prepared Ni-rich alloys (Wu, Wang, et al., 2012).
CdSe nanocrystals (Fig. 16), simply by adjusting the reaction temperature. Later, Zhuang, Lu, Peng, and Li (2010) reported the synthesis of ultrathin water-soluble CdS nanorods with great potential for biological labeling. The CdS nanorods could respond to the alkalinity of the solution, aggregating under ultrahigh alkalinity, and redispersing when the alkalinity was decreased. Polyethyleneimine (PEI) was used as a surfactant for the synthesis, because of its multiple amino groups, which tightly bound the nanocrystals; finally, ultrathin nanostructures were obtained. Rhodium nanosheets
is largely made up of a hexagonal wurtzite phase. Liu et al. (2009) reported the synthesis of a series of zinc blende CdSe nanocrystals with various shapes. They produced distinct cube-shaped, sphere-shaped, tetrahedron-shaped, and branched
Recently, Duan et al. (2014) reported the synthesis of PVPprotected, single-layer rhodium nanosheets. It is difficult for metal atoms to form single-atom-layer structures, because of their propensity to form three-dimensional, close-packed structures. DFT investigations showed that the single-layer Rh nanosheets were stabilized by a unique ␦-bonding framework.
Fig. 15. (a–c) Aberration-corrected HRTEM images of (a) [email protected], (b) Pt3 Ni@Au2 , and (c) Pt3 Ni@Au8 ; (d–f) elemental maps of (d) [email protected], (e) Pt3 Ni@Au2 , and (f) Pt3 Ni@Au8 ; (g) schematic illustration of the evolution from octahedral PtNi3 to Pt3 Ni@M8 (M = Rh, Au, Ag, Cu); and (h) schematic illustration of the growth of Au on the Pt surface (Wu et al., 2013).
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desirable to develop synthetic strategies to meet the new demands on nanocrystals. It is therefore important to consider the whole synthetic procedure, instead of focusing only on molecular design; we call this approach systematic engineering, and we believe it will become a mainstream synthetic strategy for nanocrystals in the near future.
References
Fig. 16. TEM images of the CdSe nanocrystals with different shapes: (a, b) sphereshaped; (c, d) cube-shaped; (e, f) tetrahedron-shaped; (g, h) branched. The inset images in parts a, c, d, and g show the SAED patterns of the corresponding samples (SAED = selected area electron diffraction) (Liu et al., 2009).
Conclusions This article provides an overview of the progress made in Li’s group in the development of solution-based synthetic routes, from precursor and ligand molecular design to systematic engineering. Molecular design is an important traditional method for the synthesis of nanocrystals, and it can be used to control the size, shape, and phase of crystals; it has been successfully applied to produce various types of nanocrystals in recent decades. As the requirements from different fields become more demanding, it is
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Please cite this article in press as: Wang, K., et al. Inorganic nanocrystals: From molecular design to systematic engineering. Particuology (2014), http://dx.doi.org/10.1016/j.partic.2014.04.002