Metallic nanoparticles
The term metal nanoparticle is used to describe nano sized metals with dimensions (length, width or thickness) within the size range 1‐100 nm. Metallic nanoparticles display properties that are quite different from those of individual atoms, surfaces or bulk materials. The main characteristics of MNPs are large surface‐area‐to‐volume ratio as compared to the bulk equivalents, large surface energies, existence as a transition between molecular and metallic states providing specific electronic structure (local density of states LDOS), have plasmon excitation, quantum confinement, short range ordering, increased number of kinks, contain a large number of low‐coordination sites such as corners and edges, having a large number of ˝dangling bonds˝ and consequently specific and chemical properties and the ability to store excess electrons.
Their potential applications include, for example, use in biochemistry, in catalysis and as chemical and biological sensors, as systems for nanoelectronics and nanostructured magnetism.
Synthesis
Chemical methods Include chemical reduction of metal salts, alcohol reduction process, polyol process, microemulsions, thermal decomposition of metal salts and electrochemical synthesis. Physical methods include exploding wire technique, Plasma, chemical vapour deposition, microwave irradiation, pulsed laser ablation, supercritical fluids, sonochemical reduction and gamma radiation.
Reduction of metal complexes in dilute solutions is the general method of synthesis of metal colloidal dispersions, and a variety of methods have been developed to initiate and control the reduction reactions. In most cases the formation of monosized metallic nanoparticles is achieved by a combination of a low concentration of solute and polymeric monolayer adhering onto the growth surfaces. Both a low concentration and a polymeric monolayer can hinder the diffusion of growth species from the surrounding solution to the growth surfaces and the diffusion process is likely to be the rate limiting step of subsequent growth of initial nuclei, resulting in the formation of uniformly sized nanoparticles.
Precursors and reagents
In the synthesis of metallic nanoparticles, or more specifically, metallic colloidal dispersion, various types of precursors, reduction reagents, other chemicals, and methods are used to promote or control the reduction reactions, the initial nucleation and the subsequent growth of initial nuclei. The precursors include: elemental metals, inorganic salts and metal complexes, such as, Ni, Co, HAuC14, H,PtCl,, RhC1, and PdCI2. Reduction reagents includes: sodium citrate, hydrogen peroxide, hydroxylamine hydrochloride, citric acid, carbon monoxide, phosphorus, hydrogen, formaldehyde, aqueous methanol, sodium carbonate and sodium hydroxide.
Other synthesis methods
Metallic nanoparticles can also be prepared by an electrochemical deposition method employing a simple electrochemical cell containing only a metal anode and a metal or glassy carbon cathode. The electrolyte consists of organic solutions of tetra alkyl ammonium halogenides, which also serve as stabilizers for the produced metal nanoparticles. Upon application of an electric field, the anode undergoes oxidative dissolution forming metal ions, which would migrate toward the cathode. The reduction of metal ions by ammonium ions leads to the nucleation and subsequent growth of metallic nanoparticles in the solution. With this method, nanoparticles of Pd, Ni and Co with diameters ranging from 1.4 to 4.8 nm can be produced.
Gold nanoparticles
Colloidal gold has been studied extensively for a long time. In 1857 Faraday published a comprehensive study on the preparation and properties of colloidal gold. A variety of methods have been developed for the synthesis of gold nanoparticles, and among them, sodium citrate reduction of chlorauric acid at 100°C was developed more than 50 years ago and remains the most commonly used method.
Silver nanoparticles
Various methods have been developed for the formation of silver nanoparticles. Synthesis of Ag nanoparticles can be achieved by the UV illumination of aqueous solutions containing AgC104, acetone, 2-propanol and various polymer stabilizers. UV illumination generates ketyl radicals via excitation of acetone and subsequent hydrogen atom abstraction from 2-propanol and the ketyl radical may further undergo protolytic dissociation reaction. Both the ketyl radical and radical anions react with and reduce silver ions to silver atoms.
The reactions have a low reaction rate and favor the production of monosized silver nanoparticles. With the presence of polyethyleneimine as polymer stabilizer, silver nanoparticles formed using the above photochemical reduction process have a mean size of 7nm with a narrow size distribution.
Although polymer stabilizers play a very important role in the synthesis of metal nanoparticles, they can be prepared without using any polymer stabilizer. Silver nanoparticles can be prepared using commercially available set of solutions. Without adding any stabilizing reagent, it can be synthesized using aqueous dispersion of silver nanoparticles of 20-30 nm in size. The dispersion is likely to be stabilized by electrostatic stabilization mechanism. However, the particle size is sensitively dependent on the synthesis temperature. A small variation of temperature would result in a significant change of diameters of metal nanoparticles.