Role of ligands in the synthesis of bi- and multi-metallic nanocrystals

Ramjee Balasubramanian

DOI: 10.1039/9781782620358-00001

Wet-chemical methods have enabled the size-, shape- and composition-controlled synthesis of bi- and multi-metallic nanoparticles with varying degrees of success. Among several variables involved in colloidal synthesis, coordination ligands surrounding the metal prior to the generation of nanoparticles and ligand surfactants eventually stabilizing the nanoparticles have been known to play a major role in dictating the reduction kinetics and the size- and shape- of the resulting nanoparticles. This review will discuss some of the recent examples of such ligand effects in the synthesis of bi- and multi-metallic nanoparticles.

1 Introduction

Metals such as Au, Cu, Ag, Pb, Sn, Fe and Hg, collectively referred to as "metals of antiquity" were discovered long back and used by a number of ancient civilizations. Alloys are solid solutions of various metals or metals and non-metals. They have been known since the Bronze and Iron ages and continue to play a major role in our day to day life. Bronze, primarily made of Cu and Sn, in reality is a multimetallic system also containing smaller amounts of Ni, Fe, Pb, As, Co, Sb, S etc., whose exact composition was dependent on the geographical region and the time period. The alloying of iron with carbon enabling the production of oxidation resistant steel and cast iron has also been known for a very long time. An investigation of various steel blades and other weaponry from the Middle ages revealed that Turkish (Ottoman empire) and Italian (Middle ages) steel contained the lowest amount of carbon, Japanese steel contained an intermediate amount while Indian steel (from the Mughal period) had the highest amount of carbon. Such alloys with varying compositions show significant variation in their physical properties and even today research efforts are directed towards optimizing the composition of different alloys for various applications.

There is currently a lot of interest in the synthesis and study of nanomaterials, i.e., those with at least one dimension in the 1–100 nm size range, due to their unique electronic, optical, magnetic, catalytic and chemical properties. A variety of nanomaterials comprising non-metals, metals and their alloys have been widely investigated. The study of alloy based metallic systems is particularly fascinating as their physical and chemical properties and ensuing applications can be modulated by varying their size, shape, elemental composition, and surface elemental distribution. When compared to the investigations on bimetallic nanoparticles, tri- and multi-metallic nanoparticles are relatively unexplored and are currently attracting a lot of interest. Bi- and multi-metallic nanoparticles hold promise in wide ranging areas from biology to material science in various applications including catalysis, electrocatalysis sensing, and multimodal imaging.

In principle, when two metals are mixed they could form a variety of distinct architectures based on their mixing pattern. They could be core–shell segregated alloys, entirely segregated alloys, multishell nanoalloys and randomly or orderly mixed alloys (Fig. 1). Certain inherent parameters of the constituent metals such as bond strength, surface energies, size and electronegativity dictate the formation of various architectures. For example, if the bond between the two metals is stronger than the homonuclear bonds, mixing will be favored. Metals with lower surface energy or larger sizes will tend to occupy the surface of these nanoparticles. In addition to magic sizes, alloy nanoparticles may also offer magic compositions. It is worth noting that the mixing pattern strongly depends on the preparation conditions, composition and dimension of the bimetallic nanoparticles.

There is currently a lot of interest in the shape controlled synthesis of bimetallic nanoparticles. In general, alloy nanoparticles can adopt either single crystalline geometric structures such as octahedra or truncated octahedra or non-single crystalline compact structures such as icosahedra, decahedra, polytetrahedra and polyicosahedra. The efficient packing in non-single crystalline structures leading to nonoptimal interatomic distances causes some internal strain, which will not favor the formation of such geometrical structures with larger dimensions.

Similar to the synthesis of monometallic nanoparticles, the preparation methods for multimetallic nanoparticles can be grouped as either "top-down" physical methods or "bottom-up" wet-chemical methods. This review will primarily focus on select wet-chemical methods, which allow precise control of the size and shape of the nanoparticles without involving specialized equipment. Preparation methods of bi- and multi- metallic nanoparticles can be generally grouped as (a) coreduction and (b) successive reduction approaches. While core–shell architectures can be prepared by seed-mediated approaches, random alloy preparation typically involves co-reduction approaches. In addition, alloy nanoparticles can also be generated by thermal decomposition of organometallic precursors and a variety of methods involving electrochemical, photochemical, sonochemical, biosynthesis, and radiolysis approaches.

In the co-reduction approach, appropriate mixtures of metal precursors are reduced in the presence of a reducing agent. When compared to seed mediated approaches, this approach offers the simplicity of a one-pot reaction. For example, when palladium and gold salts are co-reduced, given the bond energies of 218.6 [+ or -] 6 kJ mol-1, 143_21 kJ mol-1, 4136 kJ mol-1 for Au–Au, Au–Pd and Pd–Pd bonds respectively, segregation can be predicted. Given the surface energies and sizes of 1.506 Jm-2 and 144 pm for Au and 2.003 Jm-2 and 137 pm for Pd, a Pd core–Au shell could be predicted. Indeed molecular dynamics simulations have supported the formation of Pd core–Au shell nanostructures. However, several co-reduction approaches exclusively yield...