One Component Systems Ex

The nanoparticles (NPs) and nanostructured films of different metallic elements (Fe, Co, Ni, Cu, Ge, Ag, Au, Pd, Pt, Ru, etc.) have been the focus of intense research in recent years (Table 1) [12, 13, 386, 388]. A large number of methods have been devised for a controlled and size-tunable synthesis of colloidal nanoparticles. A variety of solution chemical methods such as laser pyrolysis and thermal

Precursor flow

Transport to surface cm

Adsorption of precursor

Surface reactions

Desorption w of precursor

_Oft

Surface diffusion

Precursor flow

Transport to surface cm

Adsorption of precursor

Surface reactions

Desorption w of precursor

_Oft

Figure 11. Fundamental processes active in the CVD process.

decomposition of organometallic compounds have recently been used to generate nanostructured materials [389, 390]. Fendler and Meldrum have reviewed the colloidal methods applied to generate nanostructured materials [391]. In terms of size selectivity and standardized procedures, this area has reached a certain level of maturity, due in particular to the large number of methods applied and efforts dedicated to the synthesis of monodisperse metal nanoparticles. However, the following section contains only selected examples that are related or show a close relevance to the scope of this article.

One general scheme for preparing nearly monodisperse (standard deviation, <5%) 3d transition metal nanoparticles relies on producing a single short nucleation event followed by slower growth on the nuclei formed. This is best achieved by introducing thermally unstable, zero-valent metal precursors, usually carbonyl compounds (e.g., Fe(CO)5, Co2(CO)8 [392, 393], Ni(CO)4 for Fe, Co, and Ni NPs), into a hot solution containing colloidal stabilizers (Scheme 13) [394-396]. Alternatively, the metal precursors (e.g., acetates, halides) can be reduced by appropriate reagent (e.g., hydrides, polyols) to the elemental form [397, 398]. Metal carbonyls are interesting SSPs for the preparation of metallic nano-particles and thin films (Table 1) because the metals within these compounds are formally in the zero-valent state and the departing CO ligand is by itself a stable noncondens-able gas. For this reasons, thermolysis, even in the absence of a reducing environment, can, in principle, produce high-quality metallic films or nanoparticles. The as-produced metal nanoparticles can be collected and consolidated to obtain nanopowders of metals. Alternatively, they can be capped with appropriate passivating agents to obtain stable colloids or can be incorporated within the pores of meso-porous organic or inorganic matrices to obtain composite materials (Scheme 13). Furthermore, a number of metal car-bonyls are reasonably stable under ambient conditions and possess sufficient volatility to be transported in the vapor phase at relatively low temperatures. The volatility and stability of carbonyl compounds depend upon their nuclearity and can be altered through the stabilization of the metal center via donor ligands. Both simple and modified metal carbonyl precursors (Table 5) have been used in the CVD process to deposit metallic films of a huge number of metals (CVD BOOK).

Scheme 13.

Although the choice of metal precursor is commonly based on the ease of handling and ready availability of the starting material, a judicious choice may nevertheless affect the process parameters. For instance, the size of NPs in the case of thermal or sonochemical reduction of metal sources can be controlled by adjusting the temperature and precursor-to-surfactant ratio, whereas the chemical reduction of metal ions (minimum two sources) is additionally subject to the influence of the concentration of the reducing agent and the kinetics of the reduction (e.g., a fast reduction step will produce a large number of nuclei). The presence of inert atmosphere and surfactant is necessary to avoid the oxidation of NPs, which otherwise results in the isolation of metal oxide particles [399]. The adsorbed surfactant molecules provide a dynamic organic shell that mediates growth, stabilizes the particles in solution, and limits oxidation after synthesis. The chemical strength of NP-surfactant interaction can also be used to monitor the size; for example, a combination of tight (e.g., long-chain carboxylic acid that binds strongly because of the strong affinity of metal for oxygen) and weak (e.g., tri-octyl or butyl phosphines) binding surfactants will favor slow and rapid growth, respectively. Although weak binding surfactants are important in mediating the growth of NPs, the surfactants that remain coordinated to the metal surface upon the isolation of NPs are the ones that bind tightly. Similarly, the steric profile of the surfactant molecules can be changed to tune the NP size. Furthermore, the surfactant can be exchanged for surfactants of different length or chemical functionality. In the case of transition metal NPs, the carbonyl decomposition methodology is scalable to large quantities for the production of magnetic fluids [400, 401]. With the use of molecular precursors, metal nanoparticles can be synthesized with a very narrow size dispersion that allows investigation of the scaling effect on the fundamental properties of the material. For instance, the magnetization data on cobalt nanoparticles of different sizes clearly show that the saturation magnetization decreases with the decreasing particle size (Fig. 12) [402, 403]. The coercivity Hc, the field required to reverse the magnetization, decreases with decreasing nanocrystal size, as seen in M versus H loops measured at 5 K (Fig. 12) for a series of hcp Co nanoparticles [401].

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