Abstract
We have used ab initio molecular dynamics and density-functional theory (DFT) calculations at the B3LYP/6-31G** level of theory to evaluate the energy and localisation of excess electrons at a number of representative interfaces of polymer nanocomposites. These modelled interfaces are made by combining liquid water and amorphous slabs of polyethylene and silica. The walls of the amorphous silica slabs are built with two surface chemistries: Q4 or fully-dehydroxylated and Q3/Q4 or partially-hydroxylated with a silanol content between 1.62 and 6.86 groups per nm2. Our results indicate that in silica/polyethylene systems an excess electron would sit at the interface with energies between -1.75 eV with no hydroxylation and -0.99 eV with the highest silanol content. However, in the presence of a free water film, the chemistry of the silica surface has a negligible influence on the behaviour of the excess electron. The electron sits preferentially at the water/vapour interface with an energy of minus a few tenths of an eV. We conclude that the moisture content in a wet polymer nanocomposite has a profound influence on the electron trapping behaviour as it produces much lower trapping energies and a higher excess-electron mobility compared to the dry material.
Highlights
Electron transfer underpins technologies such as photovoltaics,[1,2] organic thin film transistors, light-emitting diodes,[3] photocatalysis,[4] DNA based molecular electronics,[5,6] as well as energy transfer in nature[7] including radiation damage and repair.[8,9] The injection of excess electrons plays a major role in the performance of electrical insulation with significant economic consequences.[10]
We have calculated the energy and degree of localisation of excess electrons at a number of interfaces made by combining amorphous polyethylene, silica, and liquid water to be representative of interfaces to be found in wet and dry polymer nanocomposites
For pure silica we find an excess electron would be strongly localised with an energy of around À1.5 eV
Summary
Electron transfer underpins technologies such as photovoltaics,[1,2] organic thin film transistors, light-emitting diodes,[3] photocatalysis,[4] DNA based molecular electronics,[5,6] as well as energy transfer in nature[7] including radiation damage and repair.[8,9] The injection of excess electrons plays a major role in the performance of electrical insulation with significant economic consequences.[10]. Polymer nanocomposites made by blending a polymer with oxide nanoparticles have been reported to have higher effective permittivities,[22,23] and enhanced electrical breakdown strength[24] than those of the base polymer
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