Janus is the Roman god of beginnings and endings. He is usually depicted as having two asymmetric faces, since he looks to the future and to the past. In 1991, Pierre Gilles de Gennes raised the concept of “Janus particles” in his Nobel Prize address.

The term “Janus particles” or “Janus-like particles” was used to describe particles with two chemically or physically different hemispheres. These kinds of Janus-like particles usually exhibit altered properties in contrast to the single component particles. In contrast to core-shell particles where only the shell surface is exposed, Janus-like particles have two different surfaces and their interface, which provide more options for functionalization.

Janus-like nanoparticles are not only attractive nanomotors, nanorobots or highly efficient catalysts, but are also interesting for the fields of information storage and smart nanomedicine. However, differences in the material’s physical properties as crystalline structure, density, atomic radius, etc. render the synthesis of Janus-like nanoparticles difficult. It is still challenging to fabricate Janus-like nanoparticles whose constituents have a large lattice mismatch.

Prof. Yanglong Hou and his team from Peking University have recently developed a method to synthesize monodisperse 12 nm-14 nm Ag-Fe@Fe₃O₄ Janus-like nanoparticles (AFHs) at room temperature. To eliminate the restriction of lattice mismatch between Fe and Ag, amorphous Fe@Fe₃O₄ nanoparticles (NPs) are selected as seeds, meanwhile, reductive Fe can reduce Ag precursor directly even at as low as 20 °C without additional reductant.

The thickness of the Fe₃O₄ shell can influence the amorphous properties of the particles, and a series of Janus‐ and satellite‐like heterostructures are synthesized. A “cut‐off thickness” effect is proposed based on the abnormal phenomenon that with the increase of reaction temperature, the diameter of Ag in AFHs decreases. Because of the interphase interaction and the coupling effect of Ag and Fe@Fe₃O₄, the AFHs exhibit unique optical and magnetic properties. This strategy for synthesis of monodisperse heterostructures can be extended for other metals, such as Au and Cu.

This article is part of the Advanced Science 5th anniversary interdisciplinary article series, in which the journal’s executive advisory board members highlight top research in their fields. For further information on this and other hot topics, check out this virtual issue.