Oppositely charged twin-head electrospray: a general strategy for building Janus particles with controlled structures

Because of their unique heterostructure characteristics and anisotropic surface properties, Janus particles have gained growing interest in a number of novel applications. For the first time we demonstrate a facile, but versatile and general strategy for large-scale building of Janus particles with controlled structures and chemical composition pairs by an oppositely charged twin-head electrospray. In this protocol, two different droplets electrosprayed respectively from two tip-to-tip nozzles at high voltages of opposite polarities, after solvent evaporation and precursor gelation, collide with each other and coagulate into one Janus particle because of the Coulombic attractive forces. The as-electrosprayed droplets show different transient phase states at collision depending on the kinetic parameters such as the chemical compositions of precursors, humidity, concentration of solvent vapour, etc. Thus the resultant Janus particles have various morphologies and structures controlled by the transient phase state of the eletrosprayed droplets as well as the post-heat-treatment parameters. As examples, we demonstrate here the controlled fabrication of metal oxide-metal oxide and metal oxide-metal sulphide Janus particles with solid snowman-like, hollow-bowl snowman-like, and pot-like structures. Because of their unique heterostructure and novel morphology characteristics, the as-prepared Janus particles, despite a polydispersity in size and inhomogeneity in morphology, have some important potential applications including photocatalytic hydrogen production, environment remediation, and nanomotors.