(158b) Layer-Dependent Band Gap and Solar Energy Conversion with 2D Black Phosphorus (Invited)

Two-dimensional (2D) black phosphorus (BP), called phosphorene, is a new semiconductor with high mobility and a band gap that increases with decreasing flake thickness. 2D BP may be suited for solar energy harvesting but its performance for converting solar to chemical energy is unknown. Here we use hole- and electron-scavenging redox couples to probe whether bulk BP or 2D BP can act as a photocatalyst. We find that catalytic activity depends strongly on flake thickness, with thinner flakes exhibiting higher activity. To understand the underlying mechanism, we performed transient absorption spectroscopy and found that thinner flakes also have shorter excited state lifetimes. This apparent discrepancy highlights two competing effects: thinner flakes have larger band gaps, providing a greater driving force for electron transfer but thinner flakes also have a small lateral size, which makes edge defects more accessible and facilitates charge recombination. This work highlights a design goal for 2D materials: solar energy conversion requires flakes with large lateral sizes or edge passivation in order to reduce pathways for electron-hole recombination.