Seed Mediated Growth Technique

Seed mediated growth mechanism is one of the most common and traditional techniques for the synthesis of anisotropic bimetallic nanostructures. As bimetallic nanostructures involve two metal precursors, the first step is the reduction of one metal using a preferential choice of reducing agent and surfactant to form seeds. Then, the other metal precursor is added in the presence of the seed with surfactant and reducing agent for the controlled overgrowth of the second metal on the well-defined surface of the seeds of the first metals. Physical parameters have played a major role in the structural growth of the BNS. Lattice match, correlation of surface interfacial energies, electro negativities between the two metals, and reduction potentials are the crucial factors which are responsible for the nucleation and growth of bimetallic nanocrystals (Mahmoud and El-Sayed, 2014). Reducing and capping agents have also been considered as vital factors that govern the growth of nanostructures. The reduction potential of the two metals in bimetallic determine the formation of an alloy or core-shell. Metals having very close reduction potentials follow the seed mediated co-reduction (SMCR) technique and reduce together to form an alloy. If two metals have a significant difference in their reduction potentials, they form a preferentially core-shell structure. Pd-Au bimetallic nanocubes (Lim et al., 2010), Pd-Cu diverse nanostructures (Kunz et al., 2017), Au-Pd bimetallic nanorods (Sun et al., 2017), and Au-Pd nano rings (Wang W. et al., 2016) were some examples synthesized using the seed mediated method.

Au and Pd have been investigated more extensively compared to other BNS owing to their excellent catalytic performance compared to their monometallic counterparts. Both Au and Pd have a face-centered cubic (fcc) crystalline arrangement with slight mismatch (~4%) and can form materials with a wide range of stoichiometry which provides a path to designing diverse Au-Pd nanostructures. Regardless of differences in their redox potential they may reduce together to form alloys via controlling the diffusion environment where the surface deposition occurs faster than diffusion of reactants to the surface of nucleus. Pd-Au core-shell nanocrystals are synthesized using seed mediated method reported elsewhere (Lim et al., 2010). The thickness of the Au shell depends on the concentration of a gold (III) chloride (HAuCl₄) precursor. Initially, Pd nanocube seed was synthesized using L-ascorbic acid (AA) and bromide ions as the reducing and capping agents, respectively, and favors the growth in (100) direction. Further, the accumulation of Au on the surface of cubic Pd seeds by the reduction of HAuCl₄ in presence of mild reducing agent, AA was carried out. Different concentrations of HAuCl₄ were added to Pd nanocubes for the controlled overgrowth of different thicknesses of Au on Pd as shown in Figures 2A–C. TEM image confirmed the overgrowth of Au as a shell of thickness 1–2 nm on the Pd nanocubes. The shape of the Pd core retained its original cubic shape, indicating that there is no loss of Pd during the accumulation of an Au shell.

(A–C) TEM images of Pd-Au core-shell nanocrystals at different concentrations of HAuCl₄ (A) 0.53, (B) 0.80, and (C) 1.33 mM. (D) HAADF-STEM image of Pd-Au core-shell nanocrystals. (E) HRTEM image of one of the Pd-Au core-shell nanocrystals. (F) HRTEM image of a Pd-Au dimer, showing the partial decahedral structure of an Au particle attached to a Pd nanocube [Reprinted with permission from Lim et al. (2010), Copyright 2010, American Chemical Society].

The shell thickness of Au on Pd is dependent on different concentrations of HAuCl₄, forming a core-shell. Upon increasing the concentration of HAuCl₄, the thickness of Au shells increased 3–5 nm accordingly on Pd cubes without altering the shape of the Pd core as shown in Figure 2A. A bimetallic core-shell nanostructure was clearly seen in a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image as shown in Figure 2D. This can be seen due to the contrast between the Pd and Au metals. At a low concentration of HAuCl₄, the thinner shell of Au coated over all facets of the Pd nanocube. The high-resolution transmission electron microscopy (HRTEM) image in Figure 2E shows the inter planar distance of Pd and Au indicated an epitaxial growth of Au on Pd. Figure 2F shows the high resolution TEM image of Pd-Au dimer, the Au develops with partial decahedral structure over Pd nanocube.

The seed mediated co-reduction (SMCR) technique is used to synthesize Pd-Cu nanostructures of larger lattice mismatch (~7%) between precursor metals (Kunz et al., 2017). In this technique, a variety of alloy structures are synthesized due to the co-deposition of Pd and Cu on top of cubic Pd seeds. Pd nanocube seed was prepared by mixing a Pd precursor with cetyltrimethylammonium bromide (CTAB) as a capping agent and AA as a reducing agent. A calculated amount of seed particles were added to the solution containing metals precursors, H₂PdCl₄ and CuCl₂, with AA and CTAB for the growth of the alloy. CTAB acts not only as a capping agent but also slows the reduction process of the Pd precursor through its coordination with Pd (II) and forms a [CTA]₂PdBr_4−xCl_x complex. Diverse bimetallic Pd-Cu nanostructures are synthesized by altering the Pd and Cu precursor ratio as well as the pH of the reaction. The concentration of copper was kept constant whereas the concentration of the Pd precursor was increased and the ratio of Pd: Cu maintained from 1:10 to 3:2. There was a gradual increase in the size due to enough availability of the Pd precursor for deposition.

Reaction pH was monitored by varying HCl concentrations in the reaction. When the reaction pH was less (<2), polyhedral structure was favored, while a gradual increase in the reaction pH (2–3) saw the formation of multi-branched and octopodal structures. This happened because at a lower pH, the slow reduction rate allows the metal atoms to diffuse to a lower energy site throughout the growth, forming a polyhedral structure. The cubic Pd remained at the center and acted as seed for the growth of Cu. The metal with a higher concentration remained in the core part of the structure and the metals with less concentration shifted to the tips of the nanostructures as a shell.

Au-Pd bimetallic nanorods were prepared by the co-reduction of Au and Pd using the seed-mediated method where the growth was restricted kinetically (Sun et al., 2017). First, single crystalline seeds of Au nanorods were synthesized by mixing 0.5 mM HAuCl₄ with 0.2 M CTAB in (1:1) volume ratio. Then, freshly prepared 0.6 mL (0.01 M) NaBH₄ in water was injected into the prepared Au (III)-CTAB mixture with continuous stirring at 12,000 rpm. The color of solution altered from yellow to brownish yellow. Then, the seed solution was allowed to grow for 30 min (Ye et al., 2013). After the synthesis of single crystalline seeds of Au nanorods, multifaceted Au-Pd bimetallic alloy shells were overgrown via seed mediated electroless plating. The electroless plating was conducted by mixing metal precursors, HAuCl₄ and H₂PdCl₄, with CTAB and AA at 30°C under ambient air. Since Au nanorods hold diverse facets, including local high-index and low-index facets, these can be tuned with different parameters to engineer into diverse architect with the overgrowth of the second metal in the growth step. Metal precursors ratio, reducing agent concentration, and the capping surfactants are responsible for the thickness dependent growth and surface co-deposition. These parameters also influenced atomic coordination and the stoichiometry maintained in the compositional form, resulting in different polyhedral nanostructures. For example, varying CTAB concentrations alters packing densities on different facets which favors growth in different geometries in BNS. CTAB forms bilayers on the surface of Au nanorods and regulates the diffusion rate and co-deposition ensuing alloy of Au-Pd. Surface growth of metals on Au nanorods leads to the overgrowth of nanocrystals and transforms into structurally distinct multifaceted geometries of Au and Pd alloy and core-shell nanorods. The electroless deposition of Au-Pd alloy over the Au nanorod seed was manipulated kinetically, which possibly resulted in huge families of polyhedral Au-Pd bimetallic nanorods with fine geometries including elongated hexoctahedral, truncated concave cuboidal, and truncated cuboidal nanorods.

Nanocrystals with hollow interiors have attracted great interest as they contain a larger surface-to-volume ratio in comparison to the solid counterpart which crucially affects the performance in different applications (Banaee and Crozier, 2010). Au nanorings are active materials and show significant tunability in localized surface plasmon resonance (LSPR) properties connected to variable transverse and longitudinal axes (Ozel et al., 2015). Wang et al. studied a seed-mediated route for the synthesis of Pd nanosheets supported by Au nanorings (Wang W. et al., 2016). The island growth of Au NPs due to the selective deposition on the sidelines of Pd nanosheets formed Pd ultrathin nanosheet supported by Au nanorings. Pure Au rings were further synthesized by selectively removing Pd cores by etching the Pd with excess nitric acid (HNO₃) at room temperature. The expansion of Au rings over the Pd nanosheets was monitored in the ultraviolet-visible (UV-Vis) spectroscopy. The nanosheets formed from Pd showed a large LSPR band around ~940 nm. As the Au NPs slowly grown on the Pd nanosheets surface were covered completely, Pd peak disappeared due to the plasmon interaction between the Au and Pd nanosheets and two distinct peaks appeared at 520 and 770–915 nm. The peak appeared at 520 nm was due to the out of plane dipole mode of the Au rings and peak in the near infrared region was due to the combination of the in-plane dipole and face resonance modes in the nanorings (Ozel et al., 2015).

Anisotropic Au/SiO₂/Pd nanobipyramids (NBPs) were successfully synthesized through the seed-mediated growth mechanism with a silica coating, which acts as a hard template to provide a preferential deposition of Pd over exposed surfaces of Au NBPs. The first step was the formation of Au NBPs seed by the reduction of a gold (HAuCl₄) precursor with ice cold NaBH₄ in the presence of trisodium citrate. Then, the seed solution was injected into the growth solution containing CTAB, HAuCl₄, AgNO₃, HCl, and AA (Zhu et al., 2017). Synthesis of Au NBP/mesoporous silica (mSiO₂) involved coating of the silica [through tetraethyl orthosilicate (TEOS)] in sideline of Au NBPs surface with the help of effective blocking of methoxy poly(ethylene glycol) (mPEG-SH) to the ends. mPEG-SH has a tendency to bind preferentially to the ends of Au NBPs, which is very important in further side growth of silica on the surface of Au NBPs. The synthesized anisotropic Au NBP/mSiO₂ further used as a hard template for position selective deposition of Pd on Au NBPs. The schematic representation for the synthesis of Au NBP/side-Pd nanostructures is shown in Figures 3A,B shows TEM image of the Au NBP, Au NBP/end-mSiO₂, Au NBP/end-dSiO₂, and Au NBP/side-Pd nanostructure. In addition to the small amount of TEOS on mSiO₂ components, there was sequential deposition of dense SiO₂ at the end. Finally, succeeding Pd deposition was directed by the dense silica components that occurred on the side surface of the Au NBPs.

Side Pd deposition on the Au NBP/end-SiO₂ nanostructures. (A) Schematic illustration of the process for side Pd deposition. (B) TEM images of the Au NBP, Au NBP/end-mSiO₂, Au NBP/end-dSiO₂, and Au NBP/side-Pd nanostructure samples [Reprinted with permission from Zhu et al. (2017), Copyright 2010, Advanced Functional Materials].

Janus nanostructures that possess two or more different surface areas containing different optical and electronic properties are interesting candidates in diverse technological and biomedical applications. Janus nanoparticles with an iron oxide nanosphere and a branched gold nanostar (referred as Janus magnetic nanostar) was successfully synthesized by the two step seed-mediated-growth method (Reguera et al., 2017). Gold nanospheres were synthesized first and used as seeds for the growth of gold-iron oxide nanodumbbells. Subsequently, it acted as seeds for the directional growth of asymmetric gold nanostars: (1) The synthesis of asymmetric nano dumbbells involves a reaction between 1-octadecane, oleylamine, 1,2-hexadecanediol, oleic acid, and the two metal precursor Fe(CO)₅, HAuCl₄ at 300°C under N₂ atmosphere. (2) The Janus magnetic nanostars synthesized by injecting nano dumbbells solution redispersed in chloroform with carboxyl terminated polyethylene glycol (PEG) into N, N-dimethylformamide (DMF) gold solution containing PVP after 1h of stirring. The addition of carboxyl-terminated-PEG to the nanodumbbells provides a good dispersion in DMF that evades the obscurity in phase-transformation of nanodumbbells, as these are stabilized by oleic acid and oleylamine in water.