Synthesis of Copper Nanoparticles

Copper nanoparticles have found a wide variety of applications everywhere, owing to their electrical, mechanical, catalytic and optical properties. The metal is a pretty good alternative to the noble metals like gold and silver, as it's highly conductive and far more economical than the latter. Copper is also consistently used in electronic circuits as it's an excellent conductor of electricity. Furthermore, not only are the copper nanoparticles inexpensive, their properties can be manipulated by altering the synthesis methods.

Copper nanoparticles get easily oxidised when placed in open air, resulting in creation of copper oxides. Therefore, such synthesis methods are normally implemented in non-aqueous media, in an inert atmosphere (using nitrogen or argon) and at a low precursor concentration, so as to prevent any oxidation.

Two main methods for synthesis of copper nanoparticles

The methods used for synthesising copper nanoparticles can be broadly classified into two different categories: the ones involving the physical or top-down approach and the ones involving chemical or bottom-up approach.

While, the bottom-up approach involves the construction of nanoparticles' structures from clusters, molecules or atoms, the physical or top-down approach makes use of a bulk piece of the required material, which is then reduced to nanosize, with the help of etching, grinding and cutting methods. Hence, in the latter, nanomaterials are obtained from larger entities, and there's no atomic-level control involved.

Chemical methods - Colloidal or micro-emulsion techniques, chemical reduction, sauna chemical reduction, hydrothermal, microwave-assisted and electrochemical synthesis are some of the main methods used for synthesis of copper nanoparticles through the bottom-up or chemical approach. Apart from these, biosynthesis or biological methods are also considered a part of the chemical or bottom-up approach.

Physical methods - Mechanical milling, vacuum vapour deposition, lase pulse ablation and pulsed wire discharge are some of the commonly known physical methods for synthesis of copper nanoparticles.

A bit more about these two methods of synthesis of copper nanoparticles, and how they differ

Although the physical methods of synthesis of copper nanoparticles can be used for creation of a wide variety of these particles, the main drawback of these methods is the quality of the product that is produced, which is comparatively lower than the quality of copper nanparticles obtained from the chemical methods. Furthermore, the physical methods involve usage of costly equipment or vacuum systems.

On the other hand, the morphology and growth of the nanoparticles can be altered during the chemical synthesis, by optimizing the reaction's conditions, for instance, the type of solvent used, the surface agent's concentration, the temperature of the surface agent, the stabilizing or capping agent used and the precursor. A narrow size distribution of the copper nanoparticles can be obtained by using such optimum reaction conditions during the chemical synthesis process. However, such methods of synthesis of copper nanoparticles are ideal for lab scale synthesis, and not feasible when it comes to production in a commercial facility or on a large scale.

About the synthesis of copper nanoparticles through the chemical reduction method

Chemical reduction is easily the most important and frequently used technique for synthesis of copper nanoparticles. In this method, a reducing agent like ascorbic acid, polyol, hypophosphite,

hydrazine, glucose or sodium borohydride is used for reducing a copper salt. It was Michael Faraday, who in the year 1857 came up with a systematic study of synthesis of gold nanoparticles using the chemical reduction method.

Using this method, nano-sized metal copper nanoparticles can be produced with good control over their sizes and morphologies.