Theoretical understanding of graphene supported hematite photoanode for solar-driven water splitting applications

Graphene, an allotrope of carbon, is an intriguing material because it has potentially influential properties including high electrical conductivity and zero band gap on water splitting. Nanostructured hematite (Fe2O3) has been reported as a benchmark catalyst for investigating solar water splitting. In this study, we aim to understand the role of graphene as an underlayer material of hematite using density functional theory+U methodology. To understand the effect of graphene substrate on hematite's catalytic efficiency, we consider pristine graphene as well as graphene with defects (carbon vacancies in different positions and concentrations). Overpotential is found to be reduced for graphene supported hematite as compared to stand-alone hematite. Charge density difference analysis confirms more charge delocalization when carbon vacancies is present in the system. This observation is supported by smaller magnetization values and net Bader charge of the active site. Furthermore, graphene plays an important role in reducing the band gap significantly which is beneficial for catalytic efficiency. In particular, hematite supported by graphene having vacancies had nearly zero band gap which is expected to help charge carrier to be transported to the surface. We have calculated the cumulative probability of a charge to reach hematite's surface using a wave propagation simulator. Graphene supported hematite has higher cumulative probability of charge transfer than bare hematite. Graphene supported hematite having carbon vacancies in graphene shows higher cumulative probability than its pristine counterpart. These indicatives for improved carrier transport and catalysis are beneficial for water splitting. These observations also support the previously reported experimental electron impedance spectroscopy (EIS) results where graphene overlayered hematite has reported to have lower band gap, higher photocurrent density and promotable solar-driven water oxidation reaction. [Display omitted] •Graphene underlayer helps to reduce the overpotential of nanostructured hematite.•Graphene plays a key role to reduce band gap significantly benefitting catalysis.•Vacancies in graphene lead to more charge delocalization.•Carbon vacancies increase the probability of a charge to reach hematite's surface.