The field of nanoscience continues to grow at an impressive rate and, with such a vast landscape of material, careful distillation of the most important discoveries will help researchers find the key information they require. Nanoscience Volume 4 provides a critical and comprehensive assessment of the most recent research and opinion from across the globe. Coverage includes diverse topics such as 2D nanomaterials, quantum dot solar cells and core nanoparticles for drug delivery applications. Anyone practising in any nano-allied field, or wishing to enter the nano-world will benefit from this resource, presenting the current thought and applications of nanoscience.
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Role of ligands in the synthesis of bi- and multi-metallic nanocrystals
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.
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 Au core–Pd shell structures which was explained on the basis of reduction kinetics and reduction potential. In general, in the coreduction approaches the standard reduction potential (SRP) of metal precursors employed can play a pivotal role in determining the segregation or mixing of metals. The metal with the more positive SRP, i.e. more noble, easily reducible metal precursor will be reduced first, leading to the formation of the core. The reduction of the second metal precursor (with a lower SRP) as a shell on such cores will lead to the formation of core–shell architectures. From a practical point of view, relative concentration can be effectively employed to circumvent the core–shell formation dictated by SRPs. For example, Sun and coworkers showed that the reaction of AgNO3 (4 mmol) and HAuCl4 (0.2 mmol) at 120 °C in the presence of oleylamine generated alloy nanoparticles instead of the expected core–shell architectures. Further, they showed that the exact alloy composition depended on the duration of the reaction, with Au0.52Ag0.48 formed after 1 h of heating and Au0.39Ag0.61 at 2 h. On the other hand, metals with comparable SRPs will lead to the formation of alloyed nanostructures. The difference in SRPs between the metal and the reducing agent can also impact the rate of the reduction and the dimensions of the resulting nanoparticles. pH of the reaction medium can also influence the SRP of the metals and thereby influence the formation of nanoparticles.
The growth of anisotropic nanostructures under co-reduction approaches could be classified as (a) continuous growth or (b) crystallite coalescence. In the continuous growth approach, the metal atoms generated upon reduction of metal salts, grow into nuclei, and further into uniform alloy nanoparticles. In this approach, there is a need to control both nucleation and growth of the nanoparticles. The crystallinity of the nuclei generated is controlled by the reduction rate, and their subsequent growth into shaped nanoparticles depends on the select passivation of certain facets by capping agents present in the medium. The growth is restricted on such stabilized facets. Nuclei generated from the metal salts, coalesce into one dimensional or dendritic structures under crystallite coalescence route. When rapid reduction of the metal precursors leads to the formation of a number of nuclei with high surface energy whose subsequent growth is limited by the availability of the metal atoms, they can coalesce in to one large nanoparticle, whose shape is dictated by the growth rate of different facets. Again, the capping agents can control the growth rate of certain facets, and regulate the facet-oriented coalescence process.
In the seeded growth approach, the nucleation and the subsequent growth stages of nanoparticles are temporally separated. The deposition and growth of the second metal on a seed nuclei will depend on the difference in SRPs between the seed metal and the metal to be deposited, lattice match or lack thereof, bond strength between the two metals and finally the facet-specific binding ligands or surfactants. If the SRP of the second metal is more than the SRP of the core metal, then galvanic exchange reaction can occur. For example, Murray et al. have shown that thiolate monolayer protected clusters with a Ag, Cu or Pd nanoparticle cores can react with Au–thiolate yielding Au containing bimetallic nanoclusters. In this approach, the added metal ions adsorbed on the seed nanoparticles get reduced by the oxidation of the nanoparticle seeds, and again capping agents can influence the reduction and oxidation sites. If the SRP of the metal to be deposited is less than the SRP of the core metal, then seed mediated growth could occur. For example, silver acetate can be reduced on Au nanoparticle surface to yield core–shell architectures. If there is a minimum lattice mismatch between the two metals and if they can bond strongly, it will lead to epitaxial growth. The shape controlled synthesis of nanostructures can proceed simultaneously via one or more of the pathways described above. Currently, seed-mediated co-reduction approach is emerging as an important tool in the synthesis of anisotropic nanoparticles. In this approach, two metals are co-reduced in the presence of a pre-formed seed whose symmetry is transferred to the final nanoparticles.
Current synthetic advances have enabled the synthesis of bi- and multi-metallic nanoparticles with distinct morphologies including single-crystalline and twinned polyhedrons, nanodendrites, nanowires, nanorods, multipods, and hollow architectures. The key to achieving such shape control depends on a number of experimentally controllable parameters such as choice of metal precursors, stabilizing surfactants, reductants, reaction temperature and media.
2 Ligand effects
Ligands coordinating to the metal precursors can play several important roles during the synthesis of nanoparticles. Metal halides are very common precursors employed in both aqueous and two-phase syntheses of nanoparticles. The SRPs of silver salts containing chloride, bromide and iodide are 0.2223, 0.00713, and -0.1522 respectively, which can be used to modulate the reduction rate. The kinetics of nanoparticle growth also depends on the availability of the metal ion, i.e., halides can effectively control the amount of metal ions in the solution. The solubility and the rate of reduction of various gold halides follows the same order: (AuI2)-< (AuBr2)-< (AuCl2)-. The stability of the metal precursors which varies with the nature of the coordinating ligand also determines the temperature at which the onset of the nucleation occurs as demonstrated in the case of rhodium nanoparticle synthesis. While RhCl3 is slightly less stable than RhBr3, Rh2(TFA)4 bridged by a bidentate trifluoroacetate ligand is endowed with exceptional stability requiring higher temperatures.
Such ligands can also determine the morphology of the resulting nanocrystals. In the case of Au nanoparticles the binding strength of various halides increases in the following order: Cl-< Br-< I-. The binding of ions such as halides to growing metal nanoparticle surfaces can lead to the formation of a physical barrier and thereby retard subsequent metal deposition. Noble metal nanoparticles adopt a face centered cubic lattice and their different crystal planes have different energies. While the surface energies (γ) of fcc metals follows the trend: γ(111)< γ(100)< γ(110), they can be modified by capping ligands or surfactants leading to the formation of nanostructures that would not be thermodynamically favored. In the synthesis of rhodium nanoparticles, the selective adsorption of Br- to (100) planes of rhodium leads to the formation of nanocubes. On the other hand, both TFA- and Cl- adsorb to (111) surfaces and lead to the formation of icosahedra and triangular planar plates respectively. In general, Br- and I- are known to selectively stabilize the (100) facets of Pt, Pd, Ag, Au and their alloys.
Halides can also undertake additional tasks during the synthesis of nanoparticles in addition to the above described roles, i.e. their role in (a) modulating the metal ion's SRP, stability, solubility etc. and (b) stabilization of select facets of nuclei/nanoparticles. Halides can also modulate silver under potential deposition, interact with ancillary reagents etc. and this has been reviewed elsewhere. In addition, they can promote oxidative etching in the presence of oxygen and etch away twinned structures, and initiate, facilitate and direct galvanic exchange reaction. On the other hand, ligands such as citrate ions or citric acid can block the oxidative etching of multiply twinned particles and allow preparation of icosahedra shaped nanoparticles.
To prevent their agglomeration to thermodynamically stable bulk metal, during the synthesis and thereafter nanoparticles are often stabilized with various surface passivating agents, which may be categorized as electrostatic, steric or electrosteric stabilizers. Among steric stabilizers, ligand surfactants, i.e., those that can coordinate to the metal nanoparticle surfaces, bestow metal nanoparticles with enhanced stability against irreversible flocculation or precipitation. Thiol based surfactants have been extensively used for the passivation of various metal nanoparticles, though other surface active functional groups such as phosphine, amine, carboxylic acid, isocyanide, pyridone etc. have also been employed. Additionally, a variety of polymeric and dendritic surfactants have also been used for stabilizing nanoparticles.
Ligand surfactants can exert substantial influence in determining key physical and chemical properties. Stronger binding ligands which can effectively stabilize the nanoparticles often lead to undesirable side effects such as reduced catalytic activity and dampening of optical properties. Hence nanoparticles are often passivated with weakly-binding surfactants like anions, polymers or amine surfactants. On the other hand, the influence of the strongly interacting organic ligands on nanoparticles are not always detrimental. Despite the somewhat reduced conversion (40% of the unmodified catalyst) the modification of Pd with alkanethiol led to improved selectivity during the hydrogenation of 1-epoxy-3-butene.
Ligands and surfactants have been known to show varying affinity for and stabilization of different nanoparticles. While tris(hydroxymethyl) phosphine oxide (THPO) stabilized gold nanoparticles have been known to be unstable, THPO stabilized platinum nanoparticles remain stable. Recently, Rodionov and coworkers exploited the differential binding affinity of the surface passivating ligand towards the constituent metals in influencing the catalysis of FePt bimetallic nanoparticles (Fig. 2). Unlike literature reports which often employ both oleic acid and oleylamine as stabilizers, they synthesized FePt nanoparticles exclusively in the presence of oleic acid. Not surprisingly, they noted that such nanoparticles were more prone to aggregation than those prepared in the presence of both oleic acid and oleylamine. Also, oleic acid could be readily exchanged with other fluorous carboxylic acids, which bind strongly to the surface Fe atoms exclusively while weakly binding and preserving the catalytic activity of surface Pt atoms (Fig. 2). Specifically they showed that longer fluorous carboxylic acid ligands led to the selective and rapid hydrogenation of the C=O group of cinnamaldehyde.
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Table of Contents
Supramolecular Chemistry of AIE-active Luminophores; 2D Nanomaterials; Quantum Dot Solar Cells; Core-shell Nanoparticles for Drug Delivery Applications; Ligands for Enhancing Charge Transport in Films of Nanocrystals; TEM and Advances in Microscopy; Role of Ligands in the Synthesis of Bi- and Multi-metallic Nanocrystals