The role of graphene as an overlayer on nanostructured hematite photoanodes for improved solar water oxidation

Photocatalysis is a clean, sustainable method of producing hydrogen fuel. Hematite (alpha-Fe2O3) is a promising photocatalytic material due to its favorable band gap, earth abundance and low cost. However, overcoming hematite's poor surface and electrical properties is crucial for reaching its theoretical potential. To this end, we show that single layer graphene transferred to the surface of nanostructured, titanium doped hematite results in a photocurrent density 1.6 times greater than bare hematite. Understanding the mechanism behind this enhancement is important for optimizing graphene coatings on photocatalysts. Electrical Impedance Spectroscopy reveals that graphene affects the surface-electrolyte interface without modifying the bulk of the hematite. Kinetic studies were performed using Intensity Modulated Photocurrent Spectroscopy, revealing that hematite with the graphene overlayer is able to transfer charge more efficiently due to a decrease in the surface recombination rate. Therefore, the main contribution of graphene on hematite is reduced surface recombination, resulting in a higher yield of charge carriers on the catalytic surface to participate in the oxygen evolution reaction. Single layer graphene overlayers have the potential to act as a general surface-modification technique to improve the photocurrent density of other metal oxide photocatalysts. (C) 2018 Elsevier Ltd. All rights reserved.