Nonradiative Charge\nRecombination Dynamics in Fully\nHydroxylated Hematite Surface: A Time-Domain Ab Initio Study

Hematite (α-Fe<sub>2</sub>O<sub>3</sub>) is a typical\nsemiconducting\ntransition metal oxide that exhibits attractive properties for photoelectrochemical\n(PEC) water splitting and other applications. However, it has been\nwidely reported that charge recombination in α-Fe<sub>2</sub>O<sub>3</sub> photoanodes is a serious problem, hindering further\nimprovement in the efficiency of PEC water splitting. We used ab initio\nnonadiabatic molecular dynamics (NAMD) to investigate the charge recombination\nin bulk and surface phases of α-Fe<sub>2</sub>O<sub>3</sub>.\nThe NAMD simulations employ the decoherence-induced surface hopping\n(DISH) method implemented within time-dependent density functional\ntheory. We test and demonstrate the need to incorporate both the phase-consistency\ncorrection and the all-electron calculation of the NA coupling, the\nlatter needed for transition metals with d-shell electrons. The NAMD\nsimulations show that the time scale of intrinsic charge recombination\nin bulk α-Fe<sub>2</sub>O<sub>3</sub> can reach microseconds,\nin accordance with the existence of long-lived photogenerated charge\ncarriers observed in transient absorption measurements on α-Fe<sub>2</sub>O<sub>3</sub> electrodes. However, the hydroxylated iron-termination\nof the α-Fe<sub>2</sub>O<sub>3</sub>(0001) surface, one of the\nmost stable α-Fe<sub>2</sub>O<sub>3</sub> surfaces in aqueous\nsolution, exhibits a much faster charge recombination, about 1 order\nof magnitude faster compared to bulk α-Fe<sub>2</sub>O<sub>3</sub>. The key factor for the fast charge recombination in the surface\ncan mainly be assigned to the strong fluctuations in the NAC and the\nenergy gap between the conduction band minimum (CBM) and valence band\nmaximum (VBM) and the quantum anti-Zeno effect. This work extends\nthe charge recombination dynamics from bulk phase to aqueous interfacial\nphase of α-Fe<sub>2</sub>O<sub>3</sub>, approaching the PEC\nreaction conditions, and thus assists in understanding of the charge\ndynamics underlying the oxygen evolution reaction (OER) at the Fe<sub>2</sub>O<sub>3</sub>–water interface and design of new efficient\nFe<sub>2</sub>O<sub>3</sub>-based photoanode materials.