Strain-induced water dissociation on supported ultrathin oxide films

Controlling the dissociation of single water molecule on an insulating surface plays a crucial role in many catalytic reactions. In this work, we have identified the enhanced chemical reactivity of ultrathin MgO(100) films deposited on Mo(100) substrate that causes water dissociation. We reveal that the ability to split water on insulating surface closely depends on the lattice mismatch between ultrathin films and the underlying substrate, and substrate-induced in-plane tensile strain dramatically results in water dissociation on MgO(100). Three dissociative adsorption configurations of water with lower energy are predicted, and the structural transition going from molecular form to dissociative form is almost barrierless. Our results provide an effective avenue to achieve water dissociation at the single-molecule level and shed light on how to tune the chemical reactions of insulating surfaces by choosing the suitable substrates.

MgO(100) and Mo(100) surfaces, it is usually to use Mo(100) as the substrate to study ultrathin MgO(100) films. The lattice mismatch between MgO(100) and Mo(100) is 5.1%, therefore MgO ultrathin films supported on Mo(100) will slightly expand compared with their bulk position. The interlayer distance between Mo substrate and 1 ML MgO(100) is 2.10 Å, while this distance increases to 2.15 Å for 2-5 ML MgO(100). Oxygen atoms at the interface prefer to bond to surface Mo atoms, which is in line with prior results 17 .
It is well known that water molecule prefers to adsorb on the stoichiometric MgO(100) surface in molecular form at low coverage 7 . Then it will form two nearly degenerate adsorption structures with one or two hydrogen bonds between water and surface oxygen, and the corresponding adsorption energies per water are around − 0.45 eV. We then study water behaviors on MgO(1-5 ML)/Mo(001) surfaces. Water will initially lands on MgO(001)/Mo(001) surfaces in the molecular form. Similarly, it is also found that water molecules have two possible adsorption configurations in molecular form with nearly degenerate adsorption energy. One molecular configuration M 1 is that there is one strong hydrogen bond between water and surface oxygen with the distance of 1.38 Å (see Fig. 1(a)), while another molecular adsorption (M 2 ) has two identical weak hydrogen bonds with the distance of around 1.68 Å (see Fig. 1(b)). The adsorption energies per water for both M 1 and M 2 on MgO(1-5 ML)/Mo(100) are from − 0.67 eV to − 0.75 eV, while the adsorption energies per water on ultrathin MgO(100) films deposited on Ag(100) are around − 0.5 eV. The results indicate that molecular adsorption of water can be significantly strengthened by the Mo(100) substrate. In addition, the adsorption energy per water are almost insensitive to film thickness.
The questions is where the adsorption energy differences for water adsorption on ultrathin MgO(100) films deposited on different metal substrates come from. It is clearly that MgO lattice is slightly contracted by 1.8% on Ag while expanded by 5.1% on Mo substrate. Is MgO lattice expansion induced by Mo substrate responsible for the enhancement of water adsorption? To verify our assumption, we have calculated the adsorption energy per water as a function of MgO lattice on MgO(2 ML)/Ag(100) and MgO(2 ML)/Mo(100) surfaces shown in Fig. 2. When ultrathin MgO(100) films deposited on Ag(100) substrate, the lattice of MgO will be shortened by 1.8%, and the corresponding adsorption energy per water is − 0.41 eV. While the adsorption energy per water is − 0.74 eV on Mo-supported MgO(100). If we keep the lattice parameters of MgO(100)/Mo(100) unchanged, and just replace Mo by Ag, then in this case the adsorption energy per water is − 0.73 eV. Our results indicate that the adsorption energy for molecular adsorption almost linearly increases with the increasing of MgO lattice constant. In other words, the adsorption energy closely depends on the lattice parameter of MgO, while charge effect does not play an important role in water dissociation. The results definitely indicate that the expansion of MgO lattice will remarkably strengthen the interaction of water with MgO(100) surface. This is because the increment of the bond length of MgO will reduce bond strength significantly, resulting in the enhancement of their reactivity for water splitting.
Now that the interaction of water with ultrathin MgO(100) films has been greatly improved by Mo(100) substrate, it is interesting to study whether MgO(100)/Mo(100) is reactive for water dissociation. In contrast to adsorption behaviors of water on MgO(100)/Ag(100), water will easily dissociate on MgO(100)/Mo(100) surface, which implies that the ability to split water on ultrathin MgO(100) films is notably improved by Mo(100) substrate. Three possible dissociative configurations D 1 , D 2 , and D 3 are shown in Fig. 1(c-e), respectively. The adsorption energies for molecular and dissociative adsorption for MgO(1-5 ML)/Mo(100) are listed in Table 1. From Table 1, we can find that the dissociative configurations are favored over molecular adsorption.
To uncover the dissociative mechanism of water, we systematically study the structural configurations of M 1 and D 1 using MgO(2 ML)/Mo(100) surface. The corresponding structural parameters and adsorption energy per water as a function of MgO lattice are listed in Table 2. The MgO lattice increases gradually from + 0.0% to 5.1%, where MgO lattice with 5.1% expansion is equal to that of Mo lattice. The results clearly show that the bond length of O w -H 1 in water steadily increases from 1.02 Å to 1.12 Å along with MgO lattice expansion range from 0.0% to 5.1%, where the bond length of O w -H 2 in water is unaffected by the change of MgO lattice. Accordingly, the hydrogen bond between water and surface oxygen (O 1 -H 1 ) gets shorter by 0.29 Å. The bond length elongation of O w -H 1 and the shortening of O 1 -H 1 indicate that water molecule tends to dissociate. In addition, from Table 2 we can clearly note that the bond length of O w -Mg 1 decreases significantly with the increase of unit cell size, which implies the stronger interaction between water and surface. Furthermore, the bond length of O 1 -Mg 1 increases by around 0.5 Å with the induced strain by Mo substrate. The angle of O 1 -Mg 1 -O w (θ) also decreases by 10°.
As shown in Fig. 2, the slopes of adsorption energy for molecular and dissociative water behave differently. The dissociated water has a steeper slope than that of molecular one, as a result water prefers to dissociate on the MgO(100) surface when 4% interfacial strain is applied. As we know that the interfacial strain will change the lattice of ultrathin MgO films as the lattice constants of metal substrates vary. When ultrathin MgO films deposited on Mo(100) substrate, the MgO lattice is enlarged by 5.1%, so water prefers to dissociate on MgO(100)/Mo(100) surface. While MgO lattice shrinks 1.8% constrained on the Ag(100) substrate, thus water does not prefer to dissociate on this system. In fact, if we assume that Ag has the same lattice as Mo, water will also dissociate on MgO(100)/Ag(100) surface (see Fig. 2). In addition to metal substrate, the thickness of MgO(100) films also have     Fig. 3), M 1 will spontaneously transfer to D 1 passing through a barriless pathway with the energy gain of 0.08 eV. Then D 1 can easily transfer to D 3 by climbing over a small barrier of 0.02 eV. D 3 is the most energetically favorable adsorption configuration with the lowest dissociative adsorption energy of − 0.88 eV. For D 3 , the O w H group binds to two surface Mg atoms forming two strong bonds. In addition, there exists one strong hydrogen bond between O w of the dissociated water and hydrogen binding to surface oxygen. Another dissociation channel (see green line in Fig. 3) is from M 2 to D 3 via D 2 . It needs to overcome a very small barrier of 0.02 eV for water to dissociate initially, then it will form the meta-stable dissociative configuration of D 2 . There are two hydrogen bonds for D 2 . One hydrogen bond is that the dissociated H points to dissociated O w H and another one forms between H from dissociated O w H and surface oxygen. Afterwards, D 3 also forms by striding over the energy barrier of 0.04 eV. Furthermore, M 2 may transfer to M 1 due to the small reaction barrier of 0.02 eV, then D 3 forms going across D 1 , which is the third dissociation channel (see red line in Fig. 3). As energy barriers during water dissociation are relatively low for all the dissociation channels, there may exist multiple dissociation pathways for water on MgO(100)/Mo(100) surface. Among these, the channel one should be the most likely channel for water dissociation.

Conclusion
In summary, we have performed a systematic study to investigate the interaction of water with Mo-supported ultrathin MgO(100) films. The understanding of how water interacts with metal oxide surfaces is important in uncovering the interfacial phenomena. The single water molecule has been successfully split on insulating surface by choosing the suitable metal substrate. The mechanism of water dissociation on MgO(100)/Mo(100) surface has been revealed. The interfacial tensile strain due to lattice mismatch will cause the expansion of MgO lattice, and 4% expansion of MgO lattice will result in the dissociation of water on supported MgO(100) surface. Our results provide an effective method to enhance the surface reactivity towards water by choosing the suitable substrate.

Methods
Density-functional theory (DFT) calculations have been performed using Vienna ab initio simulation package (VASP) 18,19 to study the water adsorption behaviors. Perdew-Burke-Ernzerhof (PBE) functional 20 within generalized gradient approximation (GGA) is chose to describe exchange and correlation effects, as PBE functional gives the excellent description of hydrogen bonds 21 . Projector augmented wave (PAW) method 22 is used to describe the interactions between valence and core electrons. The energy cutoff is 500 eV, and the convergence criterion on each atom during structural relaxations is less than 0.02 eV/Å. In order to avoid the inter-molecular interaction we present results using a p(4 × 4) Mo(100) surface, where the distance between the adjacent water molecules is 12.60 Å. Four atomic Mo layers with the bottom two layers fixed at their bulk positions are used to mimic the substrate, which give the converged results. One to five monolayers (ML) of MgO(100) are adopted as the ultrathin MgO films. A vacuum region of 15 Å is introduced to separate the neighbouring slabs. The (2 × 2 × 1) and (4 × 4 × 1) k-point Monkhorst-Pack samplings 23 are used for structural relaxations and total energy calculations, respectively. The energy barriers and transition states are estimated by using the climbing image nudged elastic band (CI-NEB) method 24 .