Nonradiative Charge Recombination Dynamics in Fully Hydroxylated Hematite Surface: A Time-Domain Ab Initio Study

Hematite (alpha-Fe2O3) is a typical semiconducting transition metal oxide that exhibits attractive properties for photoelectrochemical (PEC) water splitting and other applications. However, it has been widely reported that charge recombination in alpha-Fe2O3 photoanodes is a serious problem, hindering further improvement in the efficiency of PEC water splitting. We used ab initio nonadiabatic molecular dynamics (NAMD) to investigate the charge recombination in bulk and surface phases of alpha-Fe2O3. The NAMD simulations employ the decoherence-induced surface hopping (DISH) method implemented within time-dependent density functional theory. We test and demonstrate the need to incorporate both the phase-consistency correction and the all-electron calculation of the NA coupling, the latter needed for transition metals with d-shell electrons. The NAMD simulations show that the time scale of intrinsic charge recombination in bulk alpha-Fe2O3 can reach microseconds, in accordance with the existence of long-lived photogenerated charge carriers observed in transient absorption measurements on alpha-Fe2O3 electrodes. However, the hydroxylated iron-termination of the alpha-Fe2O3(0001) surface, one of the most stable alpha-Fe2O3 surfaces in aqueous solution, exhibits a much faster charge recombination, about 1 order of magnitude faster compared to bulk alpha-Fe2O3. The key factor for the fast charge recombination in the surface can mainly be assigned to the strong fluctuations in the NAC and the energy gap between the conduction band minimum (CBM) and valence band maximum (VBM) and the quantum anti-Zeno effect. This work extends the charge recombination dynamics from bulk phase to aqueous interfacial phase of alpha-Fe2O3, approaching the PEC reaction conditions, and thus assists in understanding of the charge dynamics underlying the oxygen evolution reaction (OER) at the Fe2O3-water interface and design of new efficient Fe2O3-based photoanode materials.