Abstract
Small metal clusters exhibit physical and chemical properties that differ substantially from those of corresponding bulk metals. Furthermore, the properties of clusters vary greatly depending on the number of constituent atoms. Metal clusters with these characteristics currently attract great attention in a wide range of fields as new nanoscale functional materials. In recent years, the techniques to precisely synthesize metal clusters protected with organic ligands and polymers with atomic precision have advanced dramatically. In addition, substantial knowledge of the size-specific physical/chemical properties exhibited by these metal clusters has been accumulated. In this chapter, we describe the precise synthesis methods of the most studied thiolate (SR)-protected gold clusters Aun(SR)m and their heteroatom-substituted clusters (alloy clusters).
Keywords
- gold clusters
- alloy clusters
- precise synthesis
- fractionation
- size focusing
- metal exchange
1. Introduction
Substances in our surroundings are composed of assemblies of atoms. For example, a metal is a conglomerate of a nearly infinite number of metal atoms. By contrast, certain substances are made up of a countable number of metal atoms. These substances are called “metal clusters” because their shape resembles grape clusters. Although no clear definition of metal clusters has been established, the term generally refers to an aggregate of two to several hundred metal atoms (Figure 1); most such aggregates have a superfine size of 2 nm or less.
The proportion of surface atoms in metal clusters differs substantially from that in bulk metals. Taking a metal cluster with an icosahedral structure as an example, a metal cluster with 55 atoms (Figure 1) has 42 surface atoms, corresponding to 76.3% of the total atom number. In the case of a 13-atom metal cluster (Figure 1), 12 atoms are on the surface, corresponding to 92.3% of the total atoms. In bulk metals (Figure 1), the proportion of surface atoms is only approximately 0.00001% in a cube of 1 cm3. Thus, compared with bulk metals, metal clusters have a much higher proportion of surface atoms available to react with other substances. Moreover, in addition to these geometric features, metal clusters also exhibit particular characteristics related to their electronic structures. Bulk metals have an electronic structure in which the valence and conduction bands are connected. Conversely, discretization of the electronic structure occurs in metal clusters because of the small number of constituent atoms.
Because of these geometric and electronic features, metal clusters exhibit physical and chemical properties that differ from those of the corresponding bulk metals. For example, although bulk gold (Au) is an inactive metal, as its size decreases to the cluster level, Au exhibits high catalytic activity in various oxidation and reduction reactions [1, 2]. Furthermore, the size-specific properties of clusters greatly vary depending on the number of constituent atoms. Figure 2 shows a photograph of aqueous solutions of thiolate (SR)-protected Au clusters (approximately 1 nm in size) with 10–39 gold atoms [3]. The color of the cluster solutions differs substantially depending on the number of constituent atoms in the clusters. This diversity of colors can be attributed to the aforementioned discrete electronic structure of clusters.
As illustrated above, metal clusters exhibit physical and chemical properties that differ substantially from those of bulk metals despite being composed of the same elements. Furthermore, the properties of clusters vary greatly depending on the number of constituent atoms. Because of their very small size, clusters contribute to the miniaturization of materials and conservation of resources. Thus, metal clusters currently attract great attention in a wide range of fields as new nanoscale functional materials.
In recent years, the atomically precise synthesis of metal clusters protected with organic ligands [4–19] and polymers [20, 21] has advanced dramatically. In addition, substantial knowledge about the size-specific physical/chemical properties exhibited by these metal clusters has been gathered. In this chapter, we describe the precise synthesis methods of the most-studied SR-protected Au clusters, Au
2. Gold clusters
As described in Section 1, the properties of metal clusters vary greatly depending on the number of constituent atoms (Figure 2). Therefore, it is important to synthesize clusters with atomic precision to produce clusters with controlled functions. Typically, Au
High-resolution separation of a mixture of clusters of various sizes according to the number of constituent atoms (Figure 3(a)).
Exposure of a mixture of clusters of various sizes to extreme conditions followed by the collection of only those clusters stable under such conditions (Figure 3(b)).
Controlling the growth rate of the clusters to obtain a uniform chemical composition (Figure 3(c)).
Replacing the ligands of the cluster with ligands having different bulkiness to render clusters with a different chemical composition stable (Figure 3(d)).
Hereafter, each of these methods is explained in detail.
2.1. Fractionation
Au
Polyacrylamide gel electrophoresis is a highly effective technique for separating hydrophilic SR-protected Au
2.2. Size focusing
The fractionation methods noted above are suitable for the systematic isolation of a series of Au
2.3. Slow reduction
Typically, NaBH4 is employed as the reducing agent to generate Au atoms. However, CO can also be used as the reducing agent. Au atoms are generated more slowly using CO than using NaBH4 and so the Au
2.4. Transformation from one stable size to another
The chemical composition of stable clusters varies depending on the bulk of the SR functional group [45]. Therefore, when the ligand of a stable cluster is replaced with a bulkier SR, distortion of the metal core is induced, resulting in the formation of clusters with a different composition (Figure 3(d)) [46]. An example is the reaction of phenylethanethiolate (SC2H4Ph)-protected Au38(SC2H4Ph)24 with 4-
3. Alloy clusters
The physical and chemical properties of metal clusters also strongly depend on the chemical composition as well as on the size of the metal core. For example, the catalytic activity of polymer-stabilized Pd147 clusters is remarkably improved when the Pd at the surface is partially substituted by Au [47]. In addition, alloy nanoclusters composed of Pd and Ru exhibit markedly different catalytic activities compared with those of their monometallic nanocluster counterparts. The catalytic activity obtained by mixing these two metals is higher than that of monometallic nanoclusters of Rh, which is located between these two elements in the periodic table [48]. As illustrated by these examples, synergistic effects caused by mixing different elements generate physical and chemical properties that differ from those of monometallic clusters. Thus, the composition control of metal clusters is very interesting from the viewpoint of modification of the physical and chemical properties of clusters, and results in new applications for clusters.
It is well known that SR forms strong bonds with Au (Section 2). Furthermore, stable Au
3.1. Co-reduction of multiple kinds of metal ions
The most common method for mixing other metallic elements with Au is the simultaneous reduction of the other metal ions with Au ions using a reducing agent (Figure 8(a)). This approach is called the co-reduction method. For example, to synthesize SR-protected alloy clusters, Au and other metal ions are mixed in solution, followed by the addition of thiol. A strong reducing agent (NaBH4) is then added, resulting in the simultaneous reduction of all of the metal ions present. Examples of alloy clusters synthesized using this method include Au25−
3.2. Metal exchange with metal complexes
Metal clusters can exchange metal atoms with metal complexes (Figure 8(b)). This reaction enables heteroelements to be introduced into metal clusters to synthesize alloy clusters [61]. Although there are some exceptions [62], the number of constituent atoms of the metal core generally does not change during this exchange [63–71]. Therefore, this reaction enables some of the atoms in a cluster to be replaced with other elements while maintaining the original number of constituent atoms and geometry. In addition, this reaction allows heteroelements to be mixed more easily than the co-reduction method. The metal exchange reaction enables the synthesis of alloy clusters composed of metal elements with very different redox potentials, and a larger number of heteroatoms can be replaced than that achieved by the co-reduction method. Using this type of exchange reaction, alloy clusters such as Au25−
3.3. Deposition of metal atoms onto metal clusters
When Au
4. Conclusions and prospects
In this chapter, we focused on Au
Acknowledgments
The authors thank all the coauthors whose names appear in the cited references. This work was supported by JSPS KAKENHI (grant numbers JP15H00763, JP15H00883, JP16H04099, 16K17480, and 16K21402). Funding from the Nippon Sheet Foundation for Materials Science and Engineering, the Sumitomo Foundation, the Takahashi Industrial and Economic Research Foundation, the Tanaka Kikinzoku Memorial Foundation, and the Futaba Electronics Memorial Foundation is also gratefully acknowledged.
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