Dr. Nicolas Schaeffer (Researcher at Universite Pierre et Marie Curie, Paris, France)
Materials with sizes at the nanometer scale with various sizes, shapes and natures are nowadays produced, sometimes at industrial scales. Amongst those nanomaterials, noble metal (Au, Ag) spherical nanoparticles are particularly interesting because of their unique physical and chemical properties.
Bottom-up synthetic routes consist in engineering nanoparticles through chemical reactions by inducing the release of the metallic part of stable precursors. Uncharged metal atoms are unstable and attract each others, forming small seeds that can grow and form colloidal suspensions. Generally, molecules able to adsorb onto those particles surfaces are used to stop their growth. A careful control of the seeding and growth kinetics, and the use of suitable ligands give some control over the size of the particles.
The main synthetic methods can be classified as follows:
- Single phase methods; the release of metallic atoms, their aggregation and stabilization occurs in one medium. The production of metal atoms is achieved by (a) thermal decomposition, in which case heat is used to break the bond between the metal and the rest of the precursor, or (b) by addition of a reducing chemical that will affect the electronic properties of the precursor in order to release metal atoms.
- Biphasic methods implies the transfer of chemicals from one solvent to another (in general from water into organic solvent) using surfactants. These can occur in “open” media like in the Brust method (c), in which gold is transferred from water to an organic phase and reduced to produce particles in this solvent. Reactions can also be confined within micelles (assemblies of surfactant molecules that can trap solvent and reactants). Mixing micelles containing either metallic precursors or a reducer enables controlled reduction step reactions through dynamic interactions of the micelles (d).
All these methods are specific to the metal that is used and the size and size distribution one wants to achieve. They are effective for predefined sets of conditions, but can rarely be applied to different systems, regardless of the sometimes-similar chemicals and physical properties of the materials. Hence, from a fundamental point of view, several aspects of the formation of nanomaterials remain unexplored, hence limiting the development of new strategies for their productions.
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