Reports: DNI355497-DNI3: Design of Catalytic Phosphorus-Metal Monolayers for Hydrocarbon Transformation

Scott C. Warren, PhD, University of North Carolina, Chapel Hill

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.

During the course of our work, we discovered that the phosphorene has a tendency to oxidize, similar to molecular phosphines. In order to extend the analogy of metal-phosphine binding to phosphine oxidation chemistry, we began to explore the chemical structure of the phosphorene oxides. This led to two studies that explore the mechanism and chemistry that occurs on the surface of the phosphorene, which may be of particular interest in, for example, catalysis.

In our study on phosphorene oxidation, we found that oxygen can bind to the surface of phosphorene. This is supported by experimental data (X-ray photoemission spectroscopy) as well as computational analysis. The analysis consisted of density functional theory-based predictions of the x-ray photoemission spectroscopy binding energy, thereby allowing us to identify all possible chemical species on the phosphorene’s surface. The XPS was performed with several oxidants and examined as a function of exposure time. The data was cross-correlated with other analytical methods, such as TEM, which allowed us to identify the spatial distribution of the chemical species that were produced.

The key findings of our study is that the use of oxygen as an oxidant leads to the formation of a relatively uniform phosphorene oxide coating on the basal surface of phosphorene, with phosphorus is oxidation states primarily from 1 to 3. The use of water as an oxidant leads to the oxidation at defect and edge sites, with oxidation states primarily from 3 to 5. The use of both water and oxygen leads to the rapid degradation of the phosphorene flakes, with pitting, etching, and oxidation occurring throughout the material.

This new insight opens up pathways for chemical modification of phosphorene and will allow catalytic studies into the performance of phosphorene oxide.