Creating a toolbox for nanoparticle synthesis

Hybrid nanoparticles made from several different materials that can be built up in a controlled and directed manner have been created by chemists in the US. The work is a step towards a predictable framework for making complex particles with applications in catalysis, solar energy conversion, medicine and electronics.

Organic chemists have a multitude of tools and reactions available to them for constructing molecules. But as team leader Ray Schaak from Pennsylvania State University explains, when it comes to making nanoparticles ’we are way behind’.

Schaak’s goal is to build up a ’total synthesis framework’ for hybrid nanoparticles, and to that end his team has come up with ways to join four different materials together in a controlled, linear order. ’There are many competing reactions and many possible products, much like in many molecular syntheses,’ he says. ’We need to understand how to form one product relative to another in a very precise way, which becomes more and more difficult as the number of material components increases.’

Starting with a platinum seed crystal, the team grew an iron oxide (Fe₃O₄) particle. Next, they added a salt source of a third metal - gold, nickel, palladium or silver - which grew exclusively on the platinum crystal of the hybrid particles rather than the iron oxide, even though these metals would normally grow on either material in isolation. Growing a fourth material, a metal sulfide, on the Au-Pt-Fe₃O₄ hybrids gave hybrids where the sulfide was exclusively attached to the gold crystal.

This control, explains Schaak, comes from the way the materials interact within the hybrid particles. ’The electronic interactions across the hybrid Fe₃O₄-Pt interface make Fe₃O₄-Pt behave differently to either Fe₃O₄ or Pt that are isolated from one another,’ he says, ’and this causes the next step of the reaction to occur only on the Pt surface.’ Schaak draws parallels between these interactions and, for example, the electronic interactions between metal nanoparticles and oxide supports in heterogeneous catalysis, and hopes that drawing on this knowledge will help the design of larger hybrid nanoparticles in a more predictable way.

’I’m particularly impressed by the ability to make a tetramer,’ says Uri Banin from the Hebrew University of Jerusalem in Israel, a pioneer in the hybrid nanoparticle field. ’It’s something we couldn’t have imagined even a few years ago.’ He adds that, unlike in organic synthesis, adding even one more material to a nanostructure is challenging as the materials are often not fully compatible. ’I look forward to having this high level of control over these systems and I think we’re getting there.’

Phillip Broadwith