Remarkable enhancement of the catalytic properties of gold nanostars with silver coating

Gold nanostars (AuNSs) have attracted great attention owing to their interesting morphology, which makes them excellent for numerous applications, especially for catalysis and surface-enhanced Raman scattering (SERS). The SERS properties enhance as the size of AuNSs increases. On the contrary, there is a drastic decrease in their catalytic activity. In this work, we show that the catalytic activity of AuNSs of larger size (roughly 500 nm) can be markedly improved by coating them with silver (Ag). The AuNSs and their Ag coating were performed using the seed-mediated method. We optimized the reaction conditions during the coating process to prevent secondary nucleation within the growth solution. The Ag-shell thickness can be increased by simply using a higher concentration of AgNO3 and ascorbic acid in the growth medium. Furthermore, the Ag-coated AuNSs (Ag@AuNSs) were used for the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) and the catalytic degradation of anionic azo dye methyl orange (MO). The catalytic activity of AuNSs increases with an increase in Ag-shell thickness. For optimum silver shell thickness, the catalytic activity increased by 45-fold and 18-fold, compared to AuNSs, for the reduction of 4-NP and MO, respectively. This remarkable enhancement in catalytic properties implies that Ag@AuNSs have a great potential for multifunctional applications e.g., catalytic mechanistic studies using SERS.