Intermetallic Au3LixM1-x (M = Fe, Ni or Co) nanoalloys: Effect of synthetic conditions on the composition and order-disorder transition

Nanoalloys combining magnetic and plasmonic properties are interesting systems for catalysis and photo-catalysis, magneto-optics, nanomagnetism and fundamental studies. Nevertheless, their synthesis is challenging due to the immiscibility of Au and 3d magnetic metals at mild temperatures in equilibrium conditions. In this work we prepared through colloidal chemistry synthesis Au3LixM1-x (M = Fe, Ni or Co) nanoalloys and studied the synthetic conditions that affect the transition between a disordered and an ordered L12 intermetallic alloy crystal phase. We found that Au seeds act as templates for the formation of the intermetallic nanoalloys, and that lithium (coming from the Butyllithium reducing agent injected after the seeds formation) plays a fundamental role in the stabilization of the intermetallic phase. By tuning the synthetic parameters, we were able to tune the Fe content in Au3LixFe1-x intermetallic nanoalloys from 0.6 to 2.6 %, also tuning the magnetic moment of the nanoalloys. All the synthesized nanoalloys were able to sustain a plasmonic resonance in the visible range, which is blue shifted and broadened with respect to Au NPs. Our results can open colloidal synthesis to novel crystal phases and nanomaterials combining plasmonic and magnetic functionalities at the nanoscale.