Synergetic donor-donor codoping strategy for enhanced photoelectrochemical activity of hematite

Hematite is a promising semiconductor for photoelectrochemical water splitting owing to its ideal band gap, nontoxicity, abundance, and chemical stability. However, the conversion efficiency remains less than its theoretical limit, which is in part due to a low carrier density. Although efforts have been made to increase the carrier density by incorporating appropriate donor dopants, structural instability caused by high donor doping has restricted the enhancement achieved and the resulting photocatalytic performance. Herein, we used density functional theory calculations to show that an enhanced carrier density and photocatalytic performance can be achieved without causing structural instability by using a donor???donor codoping strategy to introduce a 3d transition metal (Ti/V) and Si into hematite. Despite the Coulombic repulsion among the electrons from donors, the Coulombic attraction between donors with an oxidation number of +4 (Ti4+/V4+) and negatively charged small polarons contribute to a strong binding energy. The compensatory binding energy stabilizes the crystal structure and thus increases the density of carriers, most of which are small polarons. We also suggest that the carrier density can be further enhanced by increasing the ratio of Si interstitial doping, which produces four times more polarons than Si substitutional doping under experimental conditions of high temperature and low oxygen partial pressure. Our findings pave a way for an environment-friendly and efficient photocatalysis toward improvement of hydrogen fuel generation