Carbon electrode interfaces for synthesis, sensing and electrocatalysis: general discussion.

Abstract

Robert Hamers replied: One can think of water as being like an amorphous semiconductor, in that there are strong interactions between adjacent atoms but there is no long-range order. J.V. Coe published several important papers in which he describes the water conduction band. Terminology is important, as one can refer to an optical conduction band and an adiabatic conduction band. The optical conduction band is determined by nding the position of the valence band relative to vacuum, and then using the optical bandgap (onset of optical absorption) to dene the position of the conduction band. For our purposes, the more important denition is that of the adiabatic conduction band, which is essentially determined by the electron affinity level of the water. That is, it refers to a state in which there is an excess electron, but the water molecules have not yet polarized around it to form a solvated electron. In this limit, the electron is essentially a plane wave and water behaves like a continuum dielectric, with the electron stabilized with respect to vacuum by ~0.1 V due to the long-range polarization forces that the electron exerts. A key aspect of the ‘adiabatic conduction band’ is that it has been used to explain how electrons created in water (usually created by high-energy ionizing radiation) are able to travel signicant distances in water before the water nally polarizes around them to form solvated electrons, which then have much lower mobility. In our studies, the conduction band may be important in establishing how far the solvated electrons are located away from the electrode surface. If they enter the water with ~1 eV of kinetic energy (roughly the energy difference between the conduction band of diamond and the conduction band of water), then they may be able to travel some