Nanoparticles synthesis
Its because the gold nanoparticles are the star of the show. The researchers were just looking at how surface modification using the silver would alter the morphologies of the gold nanoparticles. Probably the researchers wondered to themselves one day, "How could we possibly control the shape of these gold nanoparticles?" and decided to try with silver.
The bimetallic part would suggest that the researchers were aiming for gold-silver nanoparticles (agglomerated together, not one on the surface of another like in the research)