Student Projects/Monometallic nanoparticles

MONOMETALLIC NANOPARTICLES

Particles having dimensions up to 100 nanometers are defined as nanoparticles. They have very unique physical and chemical properties associated with them which are not seen in their bulk counterparts. This is called the ‘Quantum Size Effect’ (QSE). The main reason for the unique properties is the fact that the ratio of surface atoms to that of bulk atom is very high. For nanoparticles, the number of atoms present on their grain boundary is similar, or sometimes higher than the number of atoms in the bulk. These properties are shape and size-dependent and therefore can be designed according to the demand of the application. One of the most important applications of nanoparticles is as catalysts as catalytic properties are directly related to their surface area.

There are two distinct techniques to synthesize monometallic nanoparticles; a physical method and a chemical method. In the physical method, a bulk or chunk of the metal is taken, and its size is continuously reduced by physical vapour deposition, ball milling, lithography or micromachining. In the chemical method, the metal is brought to its atomic state and allowed to self-assemble. The source of the metal atoms is either molecular or ionic precursors. The chemical method is a better option while synthesizing uniform nanoparticles. By manipulating and controlling the conditions like pH, temperature, reduction potentials of the precursors and binding agents, desired nanoparticles can be custom designed which are suitable for different applications.

Certain natural products or synthetic polymers are often used as stabilizing agents to prevent the agglomeration of nanoparticles, which might lead to their becoming particles beyond the nanometer range. These polymers are solvent-soluble and have an affinity towards metals. The polymer-stabilized metal nanoparticle synthesis consists of two steps:

1. the conversion of metal ions in the ionic or molecular precursors to their zerovalent states

2. the binding of the polymer on to the surface of the nanoparticle.

The agglomeration of the particles in the nano-size range can be controlled by the polymers. The binding of the polymer on to the nanoparticle surface may take place before or after the reduction of the metal to its zerovalent state. If the binding occurs before the reduction, the size and structure might be affected by the interaction between the metal ions and the polymer. If the binding is followed by the reduction then only the reduction conditions influence the structural properties.

The stabilizing polymers have both a hydrophobic ‘train’ and a hydrophilic ‘tail’ or ‘loop’. The hydrophobic part gets adsorbed on to the surface of the metal nanoparticles while the hydrophilic part is dissolved in the polar solvent. The adsorption of the polymer on to the surface can be explained using of adsorption isotherm and this mode of stabilization is explained by steric stabilization mechanism.