Nonequilibrium Ab Initio Molecular Dynamics Simulation of Water Splitting at Fe2O3–Hematite/Water Interfaces in an External Electric Field

In the exploration of the optimal material for achieving the photoelectrochemical dissociation of water into hydrogen, hematite (α-Fe2O3) emerges as a highly promising candidate for proof-of-concept demonstrations. Recent studies suggest that the concurrent application of external electric fields could enhance the photoelectrochemical (PEC) process. To delve into this, we conducted nonequilibrium ab initio molecular dynamics (NE-AIMD) simulations in this study, focusing on hematite–water interfaces at room temperature under progressively stronger electric fields. Our findings reveal intriguing evidence of water molecule adsorption and dissociation, as evidenced by an analysis of the structural properties of the hydrated layered surface of the hematite–water interface. Additionally, we scrutinized intermolecular structures using radial distribution functions (RDFs) to explore the interaction between the hematite slab and water. Notably, the presence of a Grotthuss hopping mechanism became apparent as the electric field strength increased. A comprehensive discussion based on intramolecular geometry highlighted aspects such as hydrogen-bond lengths, H-bond angles, average H-bond numbers, and the observed correlation existing among the hydrogen-bond strength, bond-dissociation energy, and H-bond lifetime. Furthermore, we assessed the impact of electric fields on the librational, bending, and stretching modes of hydrogen atoms in water by calculating the vibrational density of states (VDOS). This analysis revealed distinct field effects for the three characteristic band modes, both in the bulk region and at the hematite–water interface. We also evaluated the charge density of active elements at the aqueous hematite surface, delving into field-induced electronic charge-density variations through the Hirshfeld charge density analysis of atomic elements. Throughout this work, we drew clear distinctions between parallel and antiparallel field alignments at the hematite–water interface, aiming to elucidate crucial differences in local behavior for each surface direction of the hematite–water interface.

The structural and hydrogen-bond analysis implemented for the bulk and adsorbed water layer were analysed by calculating the Hw−Ow−Hw valence angles, water Ow−Hw bond lengths, and individual-water dipoles as mean value integration of 〈〉 = ∫ () of the corresponding normalised histograms () obtained from the simulation.Moreover, the respective histograms depicting hydrogen bond analysis; hydrogen-bond angles, lengths, hydrogen bonds number are presented therein.The hydrogen bond were selected on the geometry criteria for arrangement of donor (D), acceptor (A), and the hydrogen (H).Classifying the hydrogen bond arrangement was predicated on maximum A−D distance set at 3.5 Å and while A−D−H angle was required not to exceed 35°.Furthermore, dynamics of the hydrogen bonds was described using Luzar-Chandler model 1 for predicting average hydrogen-bond lifetime.
The vibrational density of state (VDOS) for zero-field and external electric-fields implemented was obtained by Fourier-transforming the velocity autocorrelation function of hydrogen atoms.

Figure S2.
Radial distribution functions (RDF) detecting interaction of hematite surface iron (Fesurf) and oxygen (Osurf) atoms with hydrogen (HW) and water oxygen (OW) under zero-field condition for both surface direction alignment as depicted in Figure 1 denoting the left-hand side (LHS) as the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal and the right-hand side (RHS) denoting the upper surface with parallel field-vector alignment.

Figure S1 .
Figure S1.Representative model of the aqueous hematite interface implemented for the calculation.The snapshot of the supercell was performed under periodic boundary condition (PBC) after zero-field conditions equilibration.As clearly shown, the direction of the applied static electric field  is indicated by the darkish orange arrows, pointing along the + z-axis of both supercell.I) The left-hand side (LHS) denotes the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal.II) The hematite slab located at the central part of the supercell where iron (Fe) atoms are shown in pink colour, and oxygen are shown in red.III) The right-hand side (RHS) denotes the upper surface with parallel field-vector alignment.The water molecules where oxygen are depicted in red and hydrogen as white, fills the supercell on both sides, under which the period boundary conditions are imposed.

Figure S3 .
Figure S3.Radial distribution functions (RDF) detecting interaction of hematite surface iron (Fesurf) and oxygen (Osurf) atoms with hydrogen (HW) and water oxygen (OW) under external static electric field of 0.05  Å ⁄ for both surface direction alignment as depicted in Figure 1 denoting the left-hand side (LHS) as the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal and the right-hand side (RHS) denoting the upper surface with parallel field-vector alignment.

Figure S4 .
Figure S4.Radial distribution functions (RDF) detecting interaction of hematite surface iron (Fesurf) and oxygen (Osurf) atoms with hydrogen (HW) and water oxygen (OW) under external static electric field of 0.075  Å ⁄ for both surface direction alignment as depicted in Figure 1 denoting the left-hand side (LHS) as the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal and the right-hand side (RHS) denoting the upper surface with parallel field-vector alignment.

Figure S5 .
Figure S5.Radial distribution functions (RDF) detecting interaction of hematite surface iron (Fesurf) and oxygen (Osurf) atoms with hydrogen (HW) and water oxygen (OW) under external static electric field of 0.0875  Å ⁄ for both surface direction alignment as depicted in Figure 1 denoting the left-hand side (LHS) as the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal and the right-hand side (RHS) denoting the upper surface with parallel field-vector alignment.

Figure S6 .
Figure S6.Distribution of hydrogen-bond A-H lengths of the bulk water region and aqueous hematite interfaces under the effect of applied static electric field in either surface direction alignment as depicted in Figure1denoting the left-hand side (LHS) as the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal and the right-hand side (RHS) denoting the upper surface with parallel field-vector alignment

Figure S8 .
Figure S8.Distribution of average number of hydrogen bonds per water molecule in the bulk water region and aqueous hematite interfaces under the effect of applied static electric field in either surface direction alignment as depicted in Figure1denoting the left-hand side (LHS) as the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal and the right-hand side (RHS) denoting the upper surface with parallel fieldvector alignment.

Figure S9 .
Figure S9.Hydrogen-bond lifetime correlation function of the bulk water region and aqueous hematite interfaces under the effect of applied static electric field in either surface direction alignment as depicted in Figure1denoting the left-hand side (LHS) as the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal and the right-hand side (RHS) denoting the upper surface with parallel field-vector alignment.

Figure S10 .
Figure S10.Atomic charge density () distribution in the hematite surface iron (Fesurf), oxygen (Osurf) atoms, hydrogen (HW) and water oxygen (OW) of the supercell collected during (NE-)AIMD under externally applied static field.The measured distances was calculated from the average position of the outermost Fe-atom layer on the corresponding hematite surface.As depicted, the dotted blue lines indicates the de facto boundary of the interfacial region and the direction of applied electric field follows accordingly as described in Figure1.

Figure S11 .
Figure S11.Time distribution of Hirshfeld charge density of the surface iron (Fesurf) hematite slab collected during (NE-) AIMD simulation under static field conditions of magnitudes 0.05  Å ⁄ , 0.075  Å ⁄ , 0.0875  Å ⁄ and 0.10  Å ⁄ in comparison with zero-field condition of magnitude 0.00  Å ⁄ .The effect of applied static electric field in either surface direction alignment as depicted in Figure 1 denotes (i) the left-hand side (LHS) as the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal and the (ii) right-hand side (RHS) represents the upper surface with parallel field-vector alignment.

Figure S12 .
Figure S12.Time distribution of Hirshfeld charge density of the hydrogen water (Hw) in the hematite-water interface collected during (NE-) AIMD simulation under static field conditions of magnitudes 0.05  Å ⁄ , 0.075  Å ⁄ , 0.0875  Å ⁄ and 0.10  Å ⁄ in comparison with zerofield condition of magnitude 0.00  Å ⁄ .The effect of applied static electric field in either surface direction alignment as depicted in Figure 1 denotes (i) the left-hand side (LHS) as the lower surface wherein the external-field vector is in antiparallel alignment vis-à-vis its local surface normal and the (ii) right-hand side (RHS) represents the upper surface with parallel field-vector alignment.

Figure S13 .
Figure S13.Time distribution of Hirshfeld charge density of the (i) hydrogen water (Hw) and (ii) oxygen atoms in the bulk water region collected during (NE-) AIMD simulation under static field conditions of magnitudes 0.05  Å ⁄ , 0.075  Å ⁄ , 0.0875  Å ⁄ and 0.10  Å ⁄ in comparison with zero-field spectrum of magnitude 0.00  Å ⁄ .

Table S1 :
Standard Deviation and Mean of the Hirshfeld charges of the individual atom type located in the Hematite/Water Interface at Zero Electric Field Intensity  Å ⁄ of Given Time [ps] after equilibration

Table S2 :
Standard Deviation and Mean of the Hirshfeld charges of the individual atom type located in the Hematite/Water Interface at the specified Static Electric Field Intensity [ Å] ⁄ of Given Time [ps] after equilibration

Table S3 :
Standard Deviation and Mean of the Hirshfeld charges of the individual atom type located in the Hematite/Water Interface at the specified Static Electric Field Intensity [ Å] ⁄ of Given Time [ps] after equilibration

Table S4 :
Standard Deviation and Mean of the Hirshfeld charges of the individual atom type located in the Hematite/Water Interface at the specified Static Electric Field Intensity [ Å] ⁄ of Given Time [ps] after equilibration