Structural characterization of water-metal interfaces

We analyze and compare the structural, dynamical, and electronic properties of liquid water next to prototypical metals including Pt, graphite, and graphene. Our results are built on Born-Oppenheimer molecular dynamics (BOMD) generated using density functional theory (DFT) which explicitly include van der Waals (vdW) interactions within a first principles approach. All calculations reported use large simulation cells, allowing for an accurate treatment of the water-electrode interfaces. We have included vdW interactions through the use of the optB86b-vdW exchange correlation functional. Comparisons with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional are also shown. We find an initial peak, due to chemisorption, in the density profile of the liquid water-Pt interface not seen in the liquid water-graphite interface, liquid water-graphene interface, nor interfaces studied previously. To further investigate this chemisorption peak, we also report differences in the electronic structure of single water molecules on both Pt and graphite surfaces. We find that a covalent bond forms between the single water molecule and the Pt surface but not between the single water molecule and the graphite surface. We also discuss the effects that defects and dopants in the graphite and graphene surfaces have on the structure and dynamics of liquid water.

Figure 1

Visualizations of the liquid water-Pt interface (a), liquid water-graphite interface (b), and liquid water-graphene interface (c) used in the DFT calculations. Here, the transparent surfaces around the atoms are the computed self-consistent charge densities for the geometries seen.

Figure 2

Left: Mean squared displacement of 100 water molecules in a cubic supercell $(14.{41}^3$ Å${}^3)$ with a thermostat of 400 K using the vdW-optB86b exchange correlation functional. The fit of the asymptotic behavior gives a diffusion coefficient of $2.0\times {10}^{-9}\mathrm{m}{}^2\mathrm{s}{}^{-1}$, which is slightly lower than the experimental value of $2.3\times {10}^{-9}\mathrm{m}{}^2\mathrm{s}{}^{-1}$. Middle: Molecular density profiles of a 10 ps long simulation (with a thermostat of 400 K) of a liquid water-Pt interface using the vdW-optB86b exchange correlation functional. 10 ps of MD was generated and then divided in half to give two trajectories, each 5 ps long labeled as ‘first 5 ps’ or ‘last 5 ps’. The error bars are the standard error found from analyzing the molecular density profile at each time step. Right: O-H radial distribution functions for liquid water-Pt interfaces at 330 K using the PBE functional. Three independent MD trajectories, each 5 ps long, were generated and are labeled as simulation A, B, and C. Here, it is clear that the O-H radial distribution functions have converged when comparing the different simulations.

Figure 3

Molecular (top 3 figures) and atomic (bottom 3 figures) density profiles for the liquid water-Pt interfaces at 330 K, 400 K, and 1000 K. The top two curves in the molecular density profiles are from two independent MD trajectories, one with vdW interactions (labelled as vdW) and one without (labelled as PBE). The bottom four curves are from the same MD trajectories but have been split up by either O or H atoms to give atomic density profiles.

Figure 4

Normalized probability distributions of a water molecule having a certain orientation. In Fig. 1, we divided the supercell into regions (I, II, and III) separating bulk from surface water molecules. Here, we only show region I; regions II and III show little structure and a more random orientation. Here, $\theta$ is the angle between the geometric dipole to the $z$ normal. In the left figure, we compare two independent water-Pt interfaces; one with vdW interactions (labelled as vdW) and one without (labelled as PBE). In the right figure, we compare a water-Pt (labelled as Pt) and water-graphite interface (labelled as graphite), both with vdW interactions.

Figure 5

Network reorganization probability functions $\left[\mathrm{\Pi }\right(t\left)\right]$ giving the likelihood of finding the same neighboring water molecules as a function of time and the mean squared displacements (subfigures) for selected simulations. In the left plot, we compare the liquid water-Pt interfaces with vdW interactions (labelled as vdW) and without (labelled as PBE) as well as temperature effects (330 and 400 K). In the right plot we compare the liquid water-Pt (labelled as Pt), liquid water-graphite (labelled as graphite), and liquid water-graphene (labelled as graphene) interfaces, all with a thermostat of 400 K using the optB86b-vdW exchange correlation functional.

Figure 6

Molecular density profiles for the liquid water-Pt (labelled at Pt), liquid water-graphite (labelled at graphite), and liquid water-graphene (labelled as graphene) interfaces. For all three interfaces, the simulations were run with a thermostat of 400 K, and the optB86b-vdW exchange-correlation functional was used.

Figure 7

Visualizations of the HOMO orbital (top) for the graphite (left) and Pt (right) surface as well as the PDOS (bottom) for the graphite $p_z$ orbitals, Pt $d$ orbitals, and O $p$ orbitals. The opaque purple surface (1.1e-5) and the transparent black surface (8.2e-5) are at different isovalues.