Its ability to transport electrons, i.e., the property of conductivity: The conductivity is based on the relation between occupied and unoccupied electronic bands, as electrons can become mobile only if the energy band of which they are part is not fully occupied. Most of the d-type transition metals are characterized by only partially filled d-orbitals so that incompletely filled bands result in any case. d10 elements, such as palladium, platinum or gold, have nearby s-bands that can be used for electron transport.
Magnetism: The existence of unpaired electrons is a condition for magnetism; however, only the uniform orientation of free spins over a large area results in ferromagnetism (for example, the well-known ferromagnetism of iron, cobalt and nickel); while non-oriented free spins produce paramagnetic materials (for example, copper and gold).
Metallic NPs, also called nanoclusters, are pieces of metal at the nanometer scale. They can be nanocrystalline, aggregates of crystallites or single crystallites (nanocrystals). Due to the number of bound metal atoms they contain, metallic nanoparticles display intermediate electronic energy levels in comparison with molecules and metal bulks. (As a result, particular physical and chemical properties are expected for metallic nanoparticles that can lead to applications in various areas such as in catalysis [4]. Metallic nanoparticles are generally considered as intermediate species between metal complexes and metal surfaces, and the term “nanocatalyst” is now commonly used to describe them.
For synthesizing carbon nanomaterials (CNMs), typically nanometer-sized metal particles are required to enable hydrocarbon decomposition at a lower temperature than the spontaneous decomposition temperature of the hydrocarbon [4, 5]. The most commonly used metals are iron, cobalt and nickel for two main reasons: i) the high solubility of carbon in these metals at high temperatures, and ii) the high carbon diffusion rate in these metals. Besides that, the high melting point and low equilibrium-vapor pressure of these metals offer a wide temperature window of CVD for a wide range of carbon precursors [6].
Nanoparticles consisting of small metal or metal oxide crystallites in the range of a few nanometers (1–100 nm) are important for catalysis and adsorption. Nanoparticles have higher effective surface area and improved physicochemical properties giving better performance. The uniform shape and size of catalysts are particularly important for structure-sensitive reactions where different types of surface metal atoms, such as corners, edges or terrace atoms, possess quite different properties. Menezes et al. (2013) have studied the effect of particle size on catalytic activity of titania-supported Au-Ag (1:1) nanoparticles for CO oxidation. They observed that reactivity of the catalysts increased as size of the nanometals decreased [7].
3.2.3 Types of Nanometals as Catalyst
The types of nanometals as catalyst discussed here are based on their physical characteristics.
3.2.3.1 Nanometal Colloids as Catalysts
In the precursor concept, the nanometal colloids can be tailored for support by modifying them with lipophilic or hydrophilic protective coatings. Adsorption onto the support is achieved by dipping the material into a solution of the particles. Surfactant-stabilized nanometal colloid catalysts have been found to surpass conventional catalysts for hydrogenation and oxidation reactions. The first intramolecular Pauson-Khand reaction in water was successfully carried out by using aqueous colloidal cobalt nanoparticles as the catalyst.
3.2.3.2 Nanoclusters as Catalysts
Metal nanoclusters have also been found to be good catalysts. Metal clusters retain their activity for extended periods of time and over a range of substrates. Gold nanoclusters have also exhibited catalytic activity for the low temperature oxidation of carbon monoxide, even though bulk gold is inactive. Nanoparticles supported on polymers have been found to catalyze hydrogenations and carbon-carbon coupling reactions. Colloids of bi- and tri-metallic nanoclusters have been shown to be active and selective catalysts in the Suzuki cross-coupling, Pauson-Khand, and hydrogenation reactions.
3.2.3.3 Nanoparticles as Catalysts
Metal nanoparticles on a variety of supports have also been investigated as catalysts. Zinc and platinum nanoparticles supported on a zeolite matrix exhibited high aromatizing activity in the conversion of lower alkanes. Nanoscale cobalt particles dispersed in charcoal were used as catalyst for the Pauson- Khand, reductive Pauson-Khand, and hydrogenation reactions.
3.2.3.4 Nanopowder as Catalysts
Usually powder of silica and platinum nanoparticles are used as nanocatalyst. It has exhibited very strong catalytic activity for hydrolyzation reactions. Intradendrimer hydrogenation and carbon-carbon coupling reactions took place in a variety of solvents (water, organics, biphasics, supercritical CO2) using dendrimer-encapsulated metal nanoparticles.
3.3 Methods for the Preparation of Nanoparticles
It is a challenge to prepare nanoparticles of uniform shape and size by conventional methods. Various new preparation methods are reported that give nanoparticles with narrow size distribution. Some of the reported methods are:
Hydrothermal method of metal nanoparticles
Microwave-irradiated method of metal nanoparticles
Dendrimer-assisted method of metal nanoparticles
Reverse micelle method of metal nanoparticles
Co-precipitation method of metal nanoparticles
Biogenic synthesis of metal nanoparticles
3.3.1 Hydrothermal Method of Metal Nanoparticles
In a hydrothermal process, precursor solution in the presence of an alkali is autoclaved at a certain temperature for a specific time. In a typical process, aqueous solution of precursor and KOH is mixed and placed in a Teflon-lined stainless-steel autoclave. The autoclave is maintained at 70–200 °C for 10–24 h depending on the process and then air-cooled to room temperature. Resulting precipitate is collected by filtration, washed and finally dried. For example, nickel ferrite particles prepared by hydrothermal process resulted in particles of size distribution from 40–90 nm [8].
3.3.2 Microwave-Irradiated Synthesis of Metal Nanoparticles
Microwave-irradiated synthesis is a new promising technique for the preparation of size-controlled metallic nanostructures. Xu et al. (2010) reported the preparation of Pt nanoparticles supported on CNTs [9]. In this method, solutions of Pt precursor (H2PtCl6. 6H2O), ethylene glycol and KOH are mixed in a vial and CNTs are uniformly dispersed in mixed solution. The closed vial is then placed in a microwave oven (2450 MHz, 800 W) and heated for the required time. The resulting suspension was filtered and dried at 120 °C. This preparation method resulted in Pt nanoparticles on the surface of CNTs having uniform spherical shape with diameter of 15 ± 3 nm.
3.3.3 Dendrimer-Assisted Synthesis of Metal Nanoparticles
Dendrimer is highly ordered branched poly-amidoamine macromolecule with tree-like topology and is used in the preparation of nanomaterial. Dendrimers have multiple coordination sites that are utilized to coordinate metal ions and synthesize metal clusters. In this method, metal ions in solution are complexed to dendrimer, mostly with amine groups in the outer shell of OH-terminated poly-amidoamine. Complexed metal ions are subsequently reduced