Role of chloride ions in the formation of Au@Ag core–shell nanocrystal structures by using a microwave–polyol method

Roles of chloride ion in the formation of Au@Ag core–shell nanocrystal structures through a separated two-step process have been studied using a microwave (MW)-polyol method. The first step is to reduce HAuCl4 precursor to form Au cores and the second is to grow Ag shells epitaxially around the pre-existing Au nuclei by reducing AgNO3 at various Cl− concentrations. Without Cl− ions in the second step, dominant product is spherical Ag particles and few Au@Ag particles can be produced. While, higher Cl− ion concentrations lead to the formation of a significant amount of AgCl due to the reaction between Cl− from HAuCl4 or NaCl and Ag+ from AgNO3, which will remarkably interfere with the formation of Au@Ag nanocrystals. A small amount of Cl− anions (∼0.3 mM) has been found to be a key factor for the preferential formation of Au@Ag nanocrystals with well-defined shapes. The role of Cl− anions has been discussed in terms of shape selective oxidative etching of spherical Ag nanoparticles and the formation of AgCl leading to slow face-selective crystal growth by decreasing the concentration of free Ag+ ions. Under our MW heating condition, spherical Ag particles or shells are preferentially etched, while single crystal cubic, twinned bi-pyramidal, and five-twinned nano-rod/-wire structures can survive and further grow into larger particles or Au@Ag core–shell nanostructures with clear facets and shape edges. This finding provides new information in stability of twinned Au@Ag core–shell and Ag nanoparticles against oxidative etching by Cl−/O2 (dissolved in solvent).