Density Functional Theory study of Cu doped (0001) and (012) surfaces of hematite for photoelectrochemical water splitting.
J. Simfukwe1,2*, R. E. Mapasha1, A. Braun3 and M. Diale1
1Physics Department, University of Pretoria, Pretoria 0002, South Africa.
2Physics Department, Copperbelt University. Riverside, Kitwe 10101, Zambia
3Empa, Zurich, Switzerland
The production of energy from fossil fuels and nuclear materials has a number of environmental draw backs. These draw backs include the creation of nuclear waste and the pollution associated with fossil fuels which lead to global warming and climate change. Increasing demand for sustainable, carbon free energy is the motivation behind the development of solar energy conversion and storage technologies. In Photoelectrochemical (PEC) water splitting, we need suitable semiconductors to directly dissociate water molecules into hydrogen and oxygen. Hematite (α-Fe2O3) possesses several advantages over other semiconductor materials for water splitting technique. Its band gap of ~ 2.1 eV makes it possible to absorb about 40 % of the incident solar spectrum. In addition, it is a very stable material in a broad pH range, non-toxic and abundant in the Earth’s crust. These advantages have attracted a lot of research on hematite as a photoanode material for PEC. However, hematite also presents a number of challenges for its full applications in PEC water splitting. Hematite as an n-type oxide, its conduction band which lies below that of H+/H2 redox potential, does not favour the self reduction of hydrogen. This means an external bias is needed to drive this reaction. Poor carrier mobilities and short hole diffusion length are other challenges for hematite. In this study, we have carried out Density Functional Theory calculations to study the Cu doped (0001) and (012) surfaces of hematite for enhanced water splitting. It is envisaged that surface doping is more beneficial than bulk doping because it reduces the distance moved by the charge carriers and further reduce quick recombination resulting in efficient use of the charges. Our results show that the (0001) and (012) surfaces are more stable. Furthermore, our preliminary results indicate that the band gap of hematite can be reduced by surface doping with Cu which leads to increased absorption of incident solar light.
Keywords: hematite (α-Fe2O3); surfaces; copper (Cu); Photoelectrochemical and density functional theory