Optical properties and photocatalytic activity of 2D materials

Abstract

Two-dimensional semiconductors, owing to their large active surface area and extraordinary electronic properties, are promising candidates for catalysis in clean energy applications. In this context, we investigate the optical properties of various two-dimensional materials such as phosphorene, graphene nitride and transition- meal dichalcogenides within different first-principles hierarchies. First, we will discuss the two-dimensional phosphorene and its derivatives. Since the pristine phosphorene is unstable in air and easily oxidized, we exploit this chemical instability to our advantage. We argue that the stable derivatives with N, O and S coverages have the correct band alignment necessary for photo- catalysis along with high photo-absorption capability. Next, we will discuss the possibilities in designer van der Waals heterostructures with graphitic carbon ni- tride g-C 3 N 4 and Janus transition-metal dichalcogenides MoXY. The results are discussed in the context of photocatalytic water splitting and solar-to-hydrogen conversion efficiency. In the end, we will discuss excitonic properties in the two-dimensional materials, which is fundamentally very different from their bulk counterparts due to much weaker electron screening in the reduced dimension. We describe a tractable hydrogenic model, where the parameters are calculated from inexpensive and conventional density functional theory. The results, at first, are benchmarked against relatively more accurate GW-BSE calculations and available experimental results. Within this methodology, the effects of strain in the lattice and impurities are investigated in phosphorene. Exciton renormalization due to better electron screening is investigated in thicker samples and heterostructures. Excellent agreement between the results and the available experimental results indicates the general applicability of the hydrogenic model to other two-dimensional materials.