Watching Polarons Dance: Coherent Carrier-Phonon Coupling in Hematite Revealed by Transient Absorption Spectroscopy

Abstract

Hematite remains a prominent photoanode candidate for the oxygen evolution reaction in solar water splitting, despite efficiency limitations from rapid trapping of photoexcited electrons and holes. While the formation of polarons, quasiparticles formed by electron-hole interactions with lattice vibrations, is a proposed trapping mechanism, direct evidence of such states has been elusive. Here, we use potential-dependent transient absorption spectroscopy to identify the coherent phonon mode and strong exciton-phonon coupling responsible for exciton-polaron formation after band gap excitation in α-hematite and identify the three underlying d-d transitions that are strongly modulated by this phonon. The equilibrium geometry of exciton-polarons in α-hematite is displaced from the ground state geometry along the vibrational coordinate of an A_1g symmetric Fe-O stretching motion at 225 ± 7 cm^-1, resulting in vibrational coherence with a lifetime of 1.9 ± 0.1 ps. Our comparative ex situ and in situ experiments reveal that the energy and dephasing time of the A_1g mode are remarkably resilient to applied potential and the addition of an Al₂O₃ overlayer; however, the dephasing time is sensitive to substrate identity. This potential-dependent transient absorption approach establishes a powerful platform for directly probing polaron dynamics in photoelectrochemical systems, opening new pathways to rationally design modified hematite and other transition metal oxide electrodes with enhanced charge transport properties for more efficient solar water splitting.