Design of novel SnIX (X = Br/Cl) Janus layers: electronic, optical, and photocatalytic properties, as well as defect and strain engineering

Abstract

We design SnIX (X = Br/Cl) Janus layers (JLs) using first principles-based calculations. The overall stability of the designed structures is validated using the formation energy, phonon spectra, elastic constants and ab initio molecular dynamics (AIMD) simulations. The calculated phase diagrams suggest suitable chemical conditions for the experimental realization of the JL. The as-designed JLs show electron mobility in the zig–zag direction which is about one order higher than that previously reported for 2D materials. The exciton binding energy calculated using the Bethe Salpeter equation (BSE) method is 0.60 and 0.97 eV for SnIBr and SnCl respectively. The band alignment (BA), calculated using the generalised gradient approximation (GGA), the Heyd–Scuseria–Ernzerhof (HSE) functional and Green's function-based GW approximations, straddles the water redox potentials and favours overall water splitting (OWS). The reaction rate determining steps (RDS) for the hydrogen evolution reaction and oxygen evolution reaction (HER and OER) are calculated from Gibbs free energy (GFE) changes. For light-on conditions it is found that the RDS of the HER is smallest at pH = 0 and can be further reduced below zero by defect (I monovacancy) and compressive (biaxial or uniaxial) strain engineering. On the other hand, the RDS data for the OER are smallest at pH = 14 and can be further diminished by tensile strain. Under compressive strain and at medium pH values there are optimum conditions for OWS (i.e. for both the HER and OER) with the RDS close to zero eV. Also, we could achieve a solar to hydrogen efficiency of 15.71% in SnIBr and 12.62% in SnICl by applying biaxial tensile strain.