First-principles study of doped hematite surfaces for photoelectrochemical water splitting

Abstract

Photoelectrochemical (PEC) water splitting, using sunlight and appropriate semiconductors to produce hydrogen (H2) fuel, is a promising route to solve both the production of clean H2 fuel and storage for solar energy. Owing to its various advantages, hematite (α-Fe2O3) has emerged as a promising photoanode material for PEC water splitting. However, its poor electrical conductivity, low carrier mobility, short-hole diffusion length, and fast recombination rates of the electron-hole pairs have greatly limited its full potential for PEC performance. One way to improve the PEC activity of α-Fe2O3 is by doping with other elements. In particular, surface doping is proved to be more beneficial than bulk doping because it reduces the distance moved by the charge carriers from inside the bulk to the surface where they are required for interfacial transfer.

In this study first-principles calculations based on density functional theory (DFT) were carried out to investigate the influence of Cu, Zn, Ti and Zr on the {0001} and {01 2} hematite surfaces for enhanced PEC water splitting. Various surfaces of hematite were constructed and their thermodynamic stabilities were determined by calculating surface and formation energies. The {0001} and {01 2} surfaces were found to be the most stable. Besides, all the doped systems were found thermodynamically stable. Furthermore, it was found that Cu doped surface systems does not only decrease the bandgap but also leads to the correct conduction band alignment for spontaneous water splitting. In all calculations, the charge density difference plots and the Bader charge analysis showed accumulation of charge at the top outmost surface, implying the photogenerated charge carriers can efficiently diffuse to the surface for enhanced interfacial charge transfer to the adsorbates.

Morever, it was found that even with mono doping of Zn on the topmost layer of the {0001} α-Fe2O3 surface, the bandgap can be decreased without impurity states in the band structure which normally acts as recombination centres. Furthermore, the energetic stability and electronic properties of bimetallic doped {0001} α-Fe2O3 surface with (Zn, Ti) and (Zn, Zr) pairs for enhanced PEC water splitting was also studied. Bimetallic doping is viewed as an important and executable way of not only increasing the conductivity of a semiconductor material but also reducing the quick recombination of the electron-hole pairs. The doped systems showed negative formation energies under both O-rich and Fe-rich conditions implying that they are thermodynamically stable and could be prepared experimentally. Additionally, bimetallic doping of (Zn, Ti) and (Zn, Zr) on the {0001} surface is expected to enhance the PEC performance of α-Fe2O3 because Ti or Zr is capable of increasing the conductivity of α-Fe2O3 due to the substitution of Fe3+ with Ti4+ or Zr4+, while Zn can foster the surface reaction and reduce quick recombination of the electron-hole pairs. We hope that our results provided here will be of great interest to both experimental and theoretical researchers.