Seedless synthesis of nanocomposites, optical properties, and effects of additives on their surface resonance plasmon bands.

The work describes an easy seedless competitive chemical reduction method for the synthesis of Ag@Au/Ag bimetallic nanoparticles by mixing AgNO, HAuCl and cysteine. Transmission electron microscope (TEM) images show that the large number of irregular, cross-linking, and aggregated Ag@Au/Ag are formed in a reaction mixture (HAuCl+AgNO+cysteine), whereas flower-like nanocomposites are obtained in presence of cetyltrimethylammonium bromide (CTAB), which acted as a shape-directing agent. Optical images reveal that the initially reaction proceeds through formation of purple color, which changes into dark brown color with the reaction time, indicating the formation of Ag@Au/Ag nanocomposites. The Ag has strong tendency to form complex with cysteine. Firstly, the reduction of Ag ions to Ag occurred by the HS group of the cysteine-Ag complex. Secondly, AuCl ions adsorbed on the positive surface of Ag, which undergoes reduction by potential deposition, and leads to the formation of Ag@Au/Ag bimetallic nanoparticles. Inorganic electrolytes (NaCl, NaBr, NaNO and NaSO) have significant impact on the stability and aggregation of Ag@Au/Ag nanocomposites.