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  • Publication
    Electric field Phenomena at Water/Metal-Oxide Interfaces
    (University College Dublin. School of Chemical and Bioprocess Engineering, 2022) ;
    Understanding effective energy-conversion systems and dealing with the problem of intermittency through scalable energy-storage systems are the two major difficulties in renewable energy. At the Grid size, relatively little progress has been done, and two considerable issues remain: (i) minimizing environmental harm, and (ii) the issue of ecologically friendly energy conversion. Light-driven photoelectrochemical (PEC) water-splitting can create hydrogen, but it is inefficient; instead, we focus on how electric fields can be applied to metal-oxide/water systems to adjust the interplay with their intrinsic electric fields, and how this can change and increase PEC activity, drawing both on experiment and non-equilibrium molecular simulation. Non-equilibrium molecular-dynamics simulations of liquid water were carried out in the canonical ensemble in the presence of both external static and oscillating electric fields of(r.m.s.) intensities 0.05 V/Å and 0.10 V/Å, with oscillating-field frequencies 50, 100 and 200 GHz. The rigid potential model TIP4P/2005 was used, and NEMD simulations were performed, including in the supercooled region, at temperatures ranging from 200 to 310 K. Significant changes in the percentage dipole alignment and self-diffusion constant were found vis-à-vis zero-field conditions, as well as shifting of the probability distribution of individual molecular self- diffusivities. The application of static fields was typically found to reduce the self-diffusion of liquid water, effectively due to some extent of "dipole-locking", or suppression of rotational motion, whereas diffusivity was found to be enhanced in oscillating fields, especially at high frequencies and outside the supercooled region. Classical molecular-dynamics techniques were used to evaluate the distribution of individual water molecules’ self-diffusivities in adsorbed layers at TiO2 surfaces anatase (101) and rutile (110) at 300 K for inner and outer adsorbed layers. Using local order parameters, the layered-water structure was identified and classed in layers, which proved to be an equally viable way of "self-ordering" molecules in layers. Anatase and rutile differed significantly in disrupting these molecular distributions, particularly in the adsorbed outer layer. Anatase (101) had much greater self-diffusivity values, owing to its "corrugated" structure, which allows for increased hydrogen bonding interaction with adsorbed molecules beyond the initial hydration layer. On the contrary, rutile (110) has more securely "trapped" water molecules in the region between Ob atoms, resulting in less mobile adsorbed layers. Finally, the dynamical properties of physically and chemically adsorbed water molecules on pristine hematite-(001) surfaces were investigated using non-equilibrium ab-initio molecular dynamics (NE -AIMD) in the NV T ensemble at room temperature, in the presence of externally applied, uniform static electric fields of increasing intensity. Significant changes in the dipole moment and self-diffusion constant were observed in comparison to zero-field circumstances, as well as a shift in the probability distribution of individual molecule self-diffusivities. For example, static fields were shown to promote the self-diffusion of water molecules at the a-Fe2O3 surface, owing to some degree of ’dipole-locking’ in the applied direction of the field.