What is happening at the atomic level during the seed-mediated growth of copper on gold?

Bimetallic nanocrystals obtained via seed-mediated growth are typically analyzed using conventional electron microscopy and many interesting features at the atomic level are often missed. Here we report the seed-mediated growth of copper (Cu) on gold (Au) nanospheres for the synthesis of Au-Cu bimetallic nanocrystals enclosed by a Janus surface comprised of an ordered intermetallic phase and a disordered alloy. Initially, the abundant Cu(II) precursor enables uniform Cu deposition, followed by inter-diffusion to generate an ultrathin shell made of intermetallic Au₁Cu₁. The growth subsequently evolves into an asymmetric mode, where Cu deposition preferentially proceeds at one activated site to generate Au_xCu_y alloy with a graded composition. When tested for nitrate reduction, the Janus nanocrystals show 100% Faradaic efficiency across a wide potential range and a high ammonia yield rate.