Alloy Nanopowders: In materials science, the range of properties of metallic systems can be greatly extended by taking mixtures of elements to generate intermetallic compounds and alloys. In many cases, there is an enhancement in specific properties upon alloying due to synergistic effects, and the rich diversity of compositions, structures, and properties of metallic alloys has led to widespread applications in electronics, engineering, and catalysis. The desire to fabricate materials with well-defined, controllable properties and structures on the nanometer scale coupled with the flexibility afforded by intermetallic materials has generated interest in bimetallic and trimetallic nanoclusters, which will be referred to as alloy nanoclusters or nanoalloys.
As for bulk alloys, a very wide range of combinations and compositions are possible for nanoalloys. Bimetallic nanoalloys can be generated with, more or less, controlled size and composition. The cluster structures and degree of A-B segregation or mixing may depend on the method and conditions of cluster generation (type of cluster source, temperature, pressure, etc.).
Nanoalloys can be generated in a variety of media, such as cluster beams, colloidal solutions, immobilized on surfaces, or inside pores. One of the major reasons for interest in alloy nanoparticles is the fact that their chemical and physical properties may be tuned by varying the composition and atomic ordering as well as the size of the clusters. In fact, nanoalloys may display not only magic sizes but also magic compositions, i.e., compositions at which the alloy nanoclusters present a special stability. Surface structures, compositions, and segregation properties of nanoalloys are of interest as they are important in determining chemical reactivity and especially catalytic activity.
Nanoalloys are also of interest as they may display structures and properties which are distinct from those of the pure elemental clusters: the structures of binary clusters may be quite different from the structures of the corresponding pure clusters of the same size; synergism is sometimes observed in catalysis by bimetallic nanoalloys. They may also display properties which are distinct from the corresponding bulk alloys due to finite size effects, e.g., there are examples of pairs of elements (such as iron and silver) which are immiscible in the bulk but readily mix in finite clusters.
Alloy Nanopowders: Mixing Patterns
Four main types of mixing patterns can be identified for nanoalloys.
Core-shell segregated nanoalloys consist of a shell of one type of atom (B) surrounding a core of another (A), though there may be some mixing between the shells. This mixing pattern is common to a large variety of systems, as we will see below.
Sub-cluster segregated nanoalloys consist of A and B sub clusters, which may share a mixed interface (left) or may only have a small number of A-B bonds (right).
Mixed nanoalloys may be either ordered (left) or random (i.e., a solid solution, right). Random mixed nanoalloys are often termed “alloyed” nanoparticles in the literature, but we shall not use this term in the following, preferring the terms “mixed” or “intermixed” and specifying whether mixing is ordered or random. The intermixed pattern is common to many systems.
Multishell nanoalloys may present layered or onion-like alternating shells. Metastable structures of this type were observed in simulations of the growth of Cu-Ag, Ni-Ag and Pd-Ag clusters; there has also been evidence in favor of stable A-B-A and A-B-A-B arrangements for Co-Rh and Pd-Pt clusters, respectively. Very recently, three-shell Pd-Au nanoparticles have been experimentally produced. These nanoparticles present an intermixed core, an Au-rich intermediate shell, and a Pd-rich outer shell.
Nanoalloys have already been utilized in a number of technologically important areas, ranging from catalysis (e.g., catalytic converters in automobiles and electrochemical fuel cells) to optoelectronic, magnetic, and even medical applications:
The properties, including the catalytic activity, of metals may be modified and fine tuned by alloying, i.e., forming bimetallic solids. The same is true for small metal particles and clusters, and the field of alloy nanocatalysis is currently attracting a lot of attention in the field of catalysis, the mutual influence of different neighboring atoms can lead to catalytic behavior which is different (and often better) than that of the monometallic clusters, i.e., “synergistic effects” are observed. Layered (core-shell) bimetallic clusters offer fascinating prospects for the design of new catalysts.
Another important driving force for research into catalysis by NAs is the cost/rarity of the metals typically used in catalysis. It is clearly desirable to use common (cheaper) metals, such as Fe, Co and Ni, to replace expensive metals, such as Pt and Ir, while achieving a NP surface whose chemistry mimics (or betters) that of the monometallic catalyst. Bimetallic NA catalysts containing Pt and Ir or Re have found extensive use in the reforming of petrochemicals, while NAs containing Pt, Pd and other metals are of importance in automobile catalytic converters.
In the past decade, there has been tremendous growth in the use of nanoparticles and other nanostructures in Biodiagnostics-molecular diagnostics for biomedical applications, e.g., for bioconjugation, as cellular labels, and in assays for gases, metal ions, and DNA/protein markers for disease. In this respect, nanoparticles offer the possibility of enhanced robustness, sensitivity, and selectivity.
An increasingly important area of application of Alloy NPs is in biomedical applications, Including: Biodiagnostics (e.g. taking advantage of the sensitivity of Plasmon resonances and other physical properties to the coordination of biomolecules), imaging (e.g. in fluorescence microscopy and Magnetic Resonance Imaging), drug delivery (typically employing relatively inert hollow gold nanospheres and nanorods) and other therapeutic applications (e.g. using the MNP as an agent for localized heating, radiation, etc.). The combination of the size of MNPs and the possibility of modifying their surfaces by coordinating surfactants to increase their lipophilicity or hydrophiliciy – or to target specific cells—makes them particularly attractive in medical applications.
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