Photoinduced small electron polarons generation and recombination in hematite

Abstract

Polarons generally affect adversely the photochemical and photophysical properties of transition metal oxides. However, the excited-state dynamics of polarons are not fully established to date and thus require an atomistic understanding. We focus on α-Fe2O3 with photoexcitation, electron injection, and heterovalent doping as the small polaron models, and conduct simulations of ab initio adiabatic molecular dynamics (AIMD) and nonadiabatic molecular dynamics (NA-MD). The elaborately designed AIMD simulations show that localization of electron at a single Fe site is an adiabatic and ultrafast process within sub-15 fs. Fe2O3 doping with an electron or a Si and Ti dopant forms a localized electron polaron while photoexcitation forms localized electron and hole polarons simultaneously, leading to diverse electron–hole recombination dynamics. NA-MD simulations demonstrate that recombination of an electron polaron created by doping with a delocalized hole at the valence band maximum of α-Fe2O3 takes place around 5 ps, while recombination between a pair of small electron and hole polarons in photoexcited Fe2O3 delays to about 110 ps owing to weak NA coupling and fast decoherence process. The ultrafast formation of small electron polarons in α-Fe2O3 impedes the accumulation of delocalized holes in the valence band that directly participate in water oxidation at photoanodes. The detrimental effect can be partially circumvented in photoexcited Fe2O3 for slowing electron–hole recombination despite polarons may retain low charge mobility. These findings provide a fundamental understanding of the excited-state dynamics of small electron polaron in α-Fe2O3 and may help design efficient transition metal oxides photoanodes.