61st Annual Report on Research 2016

The work conducted in this project has focused on the development of a new class of materials in which metal atoms are bound to the surface of 2D black phosphorus, also known as phosphorene. 2D phosphorus is similar to graphene in that the atoms are covalently bonded in two dimensions, and the material therefore has a high surface area. In our proposal, we suggested an analogy between the lone pairs of a typical trialkyl phosphine, which can bind to a metal atom, and the lone pairs of phosphorene, which might be able to bind to metals.

We expected that late transition metals would bind favorably to 2D phosphorene, according to standard “soft-soft” pairings. We focused on Au(I) species owing to their solubility in the same solvents that phosphorene can be suspended and we sought to synthesize Au(I) bound to phosphorene via a ligand exchange process. In particular, we used Au(I)PPh₃Cl, gold (I) triphenylphosphine chloride and examined whether the triphenylphosphine ligand could be displaced by 2D phosphorus.

We conducted these ligand exchange processes in the dark and in the light, with various additives to promote the reaction. Through our survey, we identified conditions in which Au(I) would be reduced to Au(0) and remain on phosphorene’s surface as Au nanoparticles. This outcome was quite surprising, because the reduction potential of Au(I)PPh₃Cl was more negative than the conduction band of phosphorene. Through control experiments, we determined that the electron doing the reducing was coming from the conduction band of phosphorene. This implied that the species being reduced was not Au(I)PPh₃Cl—an interesting mystery.

To understand this result, we proposed a model in which Au(I)PPh₃Cl was being converted to some other Au(I) species that had a more positive reduction potential. It is known, in fact, that the reduction potentials of Au(I) phosphines vary widely depending on the binding strength between the Au(I) and the phosphine: weaker ligands have more positive potentials. From our experiments, we knew that the only available ligand to bind Au(I) was 2D phosphorus. If there could be an equilibria that promoted some proportion of Au(I)PPh₃Cl to form Au(I)-phosphorene, then the phosphorene-bound gold could be reduced to form Au(0).

We performed a series of experiments and suitable controls to test this idea. We found that the reduction of Au(I) to Au(0) only occurred in the presence of 2D phosphorus and light; lacking either light or 2D phosphorus, the reduction did not occur. This allowed us to ascertain that the electron transfer that reduced Au(I) was driven by electrons in the conduction band.

Ultimately, these experiments have provided the way to anchor high concentrations of Au(0) and other metals on phosphorene’s surface. This new chemistry opens up a possible route for achieving unique catalytic properties of phosphorene-supported nobel metals.