Chapter 2
Controlled Synthesis
Size Control
Shinjiro Takano
∗ and Tatsuya Tsukuda
∗,
§,
1 ∗Department of Chemistry, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, Japan §Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto, Japan 1 Corresponding author: E-mail:
tsukuda@chem.s.u-tokyo.ac.jp Abstract
This chapter focuses on the state-of-the-art methods of atomically precise synthesis of Au and Ag clusters whose surfaces are protected by monolayers of organic ligands such as thiols, phosphines, and terminal alkynes. Chemical compositions of ligand-protected Au and Ag clusters that had been isolated by the end of 2014 are surveyed. High stability of the isolated Au and Ag clusters is explained in terms of geometrical and electronic structures. Manifestation of molecular-like optical properties and non-face-centered cubic atomic packing below a critical size is demonstrated by taking alkanethiolate-protected Au clusters as an example.
Keywords
Ligand-protected metal clusters; Size-dependent properties; Size-selected synthesis
2.1. Atomically Precise Size Control: Why?
Metal nanoparticles and
metal clusters are states located between the bulk and atomic states of the corresponding metal. Both systems exhibit novel physicochemical properties that are very different from those of the bulk state and strongly dependent on their “size” because of their size-dependent structures. The essential requirement for the study of their basic properties and for practical applications is to synthesize nanoparticles and clusters as stable compounds while controlling their size. Both systems can be stabilized against aggregation by using similar methods including complete passivation by organic ligands, partial passivation by polymers, and immobilization on solids. However, the precision required for size control and size evaluation of nanoparticles and clusters is significantly different. Evolution of physicochemical properties of nanoparticles is not noticeable by just removing or adding a single atom but is recognizable when one layer of an atomic shell is removed or added (
Figure 1). Thus, the “diameter” of a nanoparticle should be controlled with an accuracy of ∼0.5
nm, which can be determined by conventional transmission electron microscopy (TEM) or scanning electron microscopy (SEM). A large variety of “monodisperse” metal nanoparticles with a standard deviation in diameter of less than 10% have been synthesized by controlling the kinetics of the particle growth. Under optimized conditions, nucleation of atoms occurs abruptly in a supersaturated solution without the formation of new nuclei, and the nuclei grow uniformly thereafter to form monodisperse nanoparticles.
1 In contrast, it has been elucidated by experimental and theoretical studies of bare metal clusters that their intrinsic properties dramatically change even by the addition or removal of a single atom (
Figure 1).
2–
4 A typical example of such striking size dependence manifests as the “magic numbers” in the mass spectra of metal clusters. Observation of the magic numbers indicates that clusters composed of these specific numbers of atoms are extraordinarily stable compared with neighboring clusters. Therefore, in order to synthesize metal clusters as chemical compounds in the real world, “the number of constituent atoms” must be controlled with atomic precision, which is a nontrivial task. In addition, the cluster size must be determined precisely by mass spectrometry (MS) and not conventional microscopic analysis techniques such as TEM and SEM.
This chapter summarizes the state-of-the-art methods of size-controlled synthesis of metal clusters with a focus on Au and Ag clusters whose surfaces are passivated by monolayers of organic ligands such as thiols (RSH), phosphines (R3P), and terminal alkynes (RCCH).
5–
7 In this chapter, these systems are referred to as Au(Ag):SR, Au:PR3, and Au:CCR, respectively. The chemical compositions of the protected clusters that had been isolated by the end of 2014 are extensively surveyed. The origin of the high stabilities of these isolated clusters will be discussed in terms of the closure of electronic and geometric shells. Size-dependent evolution of electronic and geometric structures will be demonstrated by taking Au:SR as an example.
Figure 1 Metal nanoparticles and clusters.
2.2. Atomically Precise Size Control: How?
This section describes experimental techniques for chemical synthesis of metal clusters with atomically defined sizes. The first step in the synthesis of metal clusters is to reduce the precursor metal ions in the presence of protective ligands as in the case of that of protected metal nanoparticles. The yield of small (<2
nm) metal clusters can be enhanced by suppressing the growth of the nuclei. To retard the growth, the surfaces of the nuclei should be rapidly passivated with ligands and all the precursor metal ions should be transformed immediately into the nuclei. Although careful optimization of the synthetic conditions allows us to prepare monodisperse metal clusters, there is a distribution in size in terms of the number of constituent atoms, and the reproducibility of the cluster sizes obtained is not sufficiently high. In what follows, three representative approaches are introduced to overcome these difficulties: template-mediated synthesis, fractionation, and size-focusing synthesis via postsynthetic core etching or slow core growth (
Figure 2).
Precise determination of the cluster size is a key for the characterization of small clusters regardless of the synthetic method employed. MS is an indispensable tool for determining the number of not only the metal atoms but also the protecting ligands (
Figure 2). Information on molecular formulas is essential for theoretical studies of the structures of ligand-protected metal clusters whose structures cannot always be determined by single crystal X-ray diffraction (XRD). For such a determination, it is crucial to ionize the ligand-protected metal clusters intact (without fragmentation) by using a soft ionization method such as electrospray ionization (ESI)
8–
11 or matrix-assisted laser desorption ionization (MALDI).
12 Water-dispersible clusters protected by hydrophilic ligands containing carboxylic, amino, or hydroxyl groups are ionized into multiply charged ions via protonation or deprotonation of the functional groups under given pH conditions.
8,
9 To facilitate the dehydration of the clusters, a volatile solvent such as acetonitrile or methanol is added to the aqueous media.
8,
9 In contrast, organic dispersible clusters protected by hydrophobic ligands are charged by electrochemical ionization of the metal core
10 or by forming noncovalent adducts with ions such as Cs+.
11 MALDI-MS is applicable for the analysis of both hydrophobic and hydrophilic Au:SRs by using specific matrices such as
trans-2-[3-(4-
tert-butylphenyl)-2-propenyldidene]malononitrile
12 or α,β-diphenylfumaronitrile.
13 The clusters can be ionized intact by careful optimization of the matrix-to-cluster molar ratio (typically >100) and the fluence of the laser (usually a N2 laser). However, one has to bear in mind that ion intensities in the mass spectra of individual clusters do not directly reflect their relative abundance or populations because the ionization efficiency may depend on the cluster size.
Figure 2 Schematic illustration of strategies for size-controlled synthesis of Au clusters.
2.2.1. Template-Mediated Synthesis
The first approach for the cluster size control utilizes molecular templates such as dendrimers (DEN) (an arborized polymer with a regularly branched structure from a core),
14,
15 ferritin (a self-aggregate constructed by 24-protein subunits with a 7-nm inner diameter),
16 and porphyrins.
17 The basic idea is to regulate the number of precursors ions coordinated to the binding sites located within the cavity or to limit the space for cluster growth by template cages. For example, mass spectrometric studies have demonstrated that a series of Au clusters (Au5, Au8, Au13, Au23, and Au31) within poly(amidoamine)-DEN can be synthesized
18 and that highly monodisperse Au clusters (Au∼66) are confined within a cavity made of six porphyrin molecules.
172.2.2. Fractionation
The second approach is to fractionate size-defined clusters from crude mixtures.
19 The success of the fractionation method lies in the fact that the size distributions of the clusters formed in crude mixtures are not atomically continuous but discrete because of the significant differences in their stabilities. In other words, the crude samples contain only a limited number of stable clusters. In practice, one has to choose a suitable fractionation method depending on the hydrophobic or hydrophilic nature of the clusters, which is determined by the protective ligands. In the following, representative experimental techniques of fractionation or isolation of ligand-protected clusters are presented.
2.2.2.1. Fractional Precipitation or...
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