Photoinduced small electron polarons generation and recombination in hematite

• Published in: npj computational materials 2022-07, Vol.8 (1), p.1-8, Article 148
• Main Authors: Cheng, Cheng, Zhu, Yonghao, Zhou, Zhaohui, Long, Run, Fang, Wei-Hai
• Format: Article
• Language: English
• Subjects: 639/301/1034/1038; 639/301/299/890; Adiabatic; Adiabatic flow; Characterization and Evaluation of Materials; Chemistry and Materials Science; Computational Intelligence; Doping; Electrons; Excitation; Ferric oxide; Hematite; Localization; Materials Science; Mathematical and Computational Engineering; Mathematical and Computational Physics; Mathematical Modeling and Industrial Mathematics; Metal oxides; Molecular dynamics; Oxidation; Photoexcitation; Polarons; Recombination; Simulation; Theoretical; Titanium; Transition metal oxides; Valence band
• ISSN: 2057-3960; 2057-3960
• DOI: 10.1038/s41524-022-00814-7

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-Fe 2 O 3 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. Fe 2 O 3 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-Fe 2 O 3 takes place around 5 ps, while recombination between a pair of small electron and hole polarons in photoexcited Fe 2 O 3 delays to about 110 ps owing to weak NA coupling and fast decoherence process. The ultrafast formation of small electron polarons in α-Fe 2 O 3 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 Fe 2 O 3 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-Fe 2 O 3 and may help design efficient transition metal oxides photoanodes.