Engineering the band gap of $\alpha $-Fe$_{2}$O$_{3}$ by isovalent surfur doping

Hematite $\alpha $-Fe$_{2}$O$_{3}$ is one of the potential materials for solar energy conversion due to its nontoxic, abundant, low-cost and environment-friendly characters. But its indirect band gap of the value of $\sim$2.1 eV causes the low efficiency in the optical absorption and hence the solar energy conversion. We study the isovalent substitutional doping of sulfur on oxygen sites in $\alpha $-Fe$_{2}$O$_{3}$ by means of the first-principles calculations based on DFT. Our results show that the band gap of $\alpha $-Fe$_{2}$O$_{3-x}$S$_{x}$ decreases monotonically with increasing the sulfur concentration $x$, resulting in an obvious increase of the optical absorption edge in the visible range. Most intriguingly, unlike the pure $\alpha $-Fe$_{2}$O$_{3}$ material, the $\alpha $-Fe$_{2}$O$_{3-x}$S$_{x}$ with $x\approx $0.17 exhibits a direct band gap of an ideal value ($\sim$ 1.45 eV), together with high optical absorption ($\sim$ 10$^{5}$ cm$^{-1}$) and lower carriers effective masses. These results indicate that $\alpha $-Fe$_{2}$O$_{3-x}$S$_{x}$, with a proper concentration of sulfur, may serve as a promising candidate for low-cost solar-cell materials.