Reactivity and energy level of a localized hole in liquid water

We study the reactivity and the energy level associated with the reduction of the H2O+ radical cation in liquid water by combining ab initio molecular dynamics simulations at the hybrid functional level, a grand-canonical formulation of solutes in aqueous solution, and nudge-elastic-band calculations. We demonstrate that this extremely oxidative solute promptly dissociates and calculate an energy barrier for the reaction of 0.06 eV, consistent with the short measured lifetimes. We calculate the energy level related to the H2O+/H2O redox couple with respect to the vacuum level and to the computational standard hydrogen electrode (SHE), using the thermodynamic integration method. This energy level is found to lie at 3.8 +/- 0.1 eV above the SHE level, in remarkable agreement with a previous estimate based on thermodynamical data. The implications of the present results for the mechanism of water splitting at the heterogenous semiconductor-water interface are discussed.

Electronic Levels of Excess Electrons in Liquid Water

Francesco Ambrosio, Giacomo Francesco Leonardo Miceli, Alfredo Pasquarello

We provide a consistent description of the electronic levels associated with localized and delocalized excess electrons in liquid water by combining hybrid-functional molecular dynamics simulations with a grand canonical formulation of solutes in aqueous solution. The excess electron localizes in a cavity with an average radius of 1.8 angstrom and a majority coordination of five water molecules. The vertical binding energy, the optical s p transitions, and the adiabatic redox level are found to agree closely with their experimental counterparts. The energy level associated with electron delocalization V-0 is inferred to lie at -0.97 eV with respect to the vacuum level.

Amer Chemical Soc2017

Spectroscopy and dynamics at liquid water interfaces

Bruno Credidio

This thesis is a detailed description of three experimental investigations on aqueous interfaces. All projects made use of the microjet technology or the more recently developed flat-jet technique which enables the implementation of liquid water in vacuum chambers. In the first study presented here we show that a flat-jet created from the impingement of two liquid microjets generates a laminar turbulence-free water-water interface. By colliding a Luminol solution microjet with another microjet of hydrogen peroxide solution, the chemiluminescence of Luminol reveals where the liquids mix in the flat-jet. The liquids readily mix in the rims of the flat-jet, while in the middle part the different liquids only meet via diffusion. Flat-jets are stationary systems such that the longitudinal flow direction translates into a time dimension, providing access to the interface in different total interaction times ranging from 10 to 200 µs. This timescale is difficult to access by current techniques. The water-vacuum interface is the next topic, in which we have studied aqueous solutions using microjets and photoelectron spectroscopy (MJ-PES). For the first time, we have experimentally demonstrated the implementation of an absolute photoelectron energy reference in MJ-PES. We could show that there is a concentration-dependent shift of valence bands which were previously (without this absolute energy reference) undetectable. Even though this study was done using a tabletop He plasma light source, it creates the foundation of absolute energy referencing for MJ-PES works done in, e.g., synchrotron laboratories. Lastly, we have built an experimental apparatus to investigate the water-gas interface. Such interfaces are ubiquitous in nature but lack direct experimental works on the molecular scale, namely scattering studies. This is largely due to the technical difficulty imposed by the high vapour pressure of water. The dense layer of vapour shields the liquid surface in such a way that conventional molecular beams cannot travel to and from the liquid surface without interacting with the vapour phase, destroying the initial and final states required to understand the surface scattering event at the quantum level (transmission probability of ~e^(-100)). We propose an alternative method to this: by approaching the molecular beam source and the water flat-jet to a distance of 200 µm or less, the probability of scattering solely at the liquid surface becomes realistic. We have performed preliminary experiments using the developed apparatus.

EPFL2022

Spectroscopy and dynamics at liquid water interfaces

Bruno Credidio

EPFL2022