Bias-dependent local structure of water molecules at a metallic interface

Abstract

DOE Early Career Awards ; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) ; ICTP-Simons Associate Scheme ; FP7 FET-ICT Planar Atomic and Molecular Scale devices (PAMS) project (European Commission) ; Spanish Agencia Estatal de Investigacion ; Dep. de Educacion of the Basque Government ; UPV/EHU ; US Department of Energy ; DOE Early Career Awards: DE-SC0003871 ; DOE Early Career Awards: DE-FG02-09ER16052 ; Processo FAPESP: 2011/11973-4 ; FP7 FET-ICT Planar Atomic and Molecular Scale devices (PAMS) project (European Commission): 610446 ; Spanish Agencia Estatal de Investigacion: MAT2016-78293-C6-4-R ; UPV/EHU: IT-756-13 ; US Department of Energy: DE-AC02-98CH10886 ; Understanding the local structure of water at the interfaces of metallic electrodes is a key issue in aqueousbased electrochemistry. Nevertheless a realistic simulation of such a setup is challenging, particularly when the electrodes are maintained at different potentials. To correctly compute the effect of an external bias potential applied to truly semi-infinite surfaces, we combine Density Functional Theory (DFT) and NonEquilibrium Green's Function (NEGF) methods. This framework allows for the out-of-equilibrium calculation of forces and dynamics, and directly correlates to the chemical potential of the electrodes, which is introduced experimentally. In this work, we apply this methodology to study the electronic properties and atomic forces of a water molecule at the interface of a gold surface. We find that the water molecule tends to align its dipole moment with the electric field, and it is either repelled or attracted to the metal depending on the sign and magnitude of the applied bias, in an asymmetric fashion.