Oxidative Molecular Layer Deposition of Conjugated Amine Polymer Thin Films

Abstract

Conjugated amine polymers such as polypyrrole (pPy), and polyaniline (pAni) exhibit high electrochemical capacities, making them appealing as electrode materials for energy storage, electrochemical desalination, and chemical sensing. Nanoscale thin films of these polymers are attractive to provide lower weight, faster charging, and higher sensitivity in these applications. This report describes the first demonstration of molecular layer-by-layer growth of conjugated amine polymers using alternating gas-phase exposures of amine monomers and MoCl5 oxidant in a process termed oxidative molecular layer deposition (oMLD). Pyrrole (Py) and MoCl5 undergo self-limiting surface reactions by oMLD to form conformal pPy thin films, but oMLD using aniline (Ani) and p-phenylenediamine (PDA) monomers yield unexpected azo functionality. This is attributed to a MoCl5-amine surface adduct that directs polymerization near amine groups, producing azo groups for primary amines. oMLD pPy exhibits record 282 mAh/g capacity in aqueous electrolyte, and PDA/MoCl5 oMLD yields azo-polymers of interest as anode materials for alkali-ion batteries. Alternating between Py and PDA monomers during oMLD produces molecularly mixed copolymers with qualitatively different electrochemical responses from the isolated monomer structures. This work lays the foundation for the growth of conformal thin films of conjugated amine polymers with molecular-level control of composition and thickness.