Oxidative Molecular Layer Deposition of Amine-Containing Conjugated Polymer Thin Films

Abstract

Conjugated polymers such as polyethylenedioxythiophene (pEDOT), polypyrrole (pPy), and polyaniline (pAni) exhibit high electrochemical capacities, making them appealing as electrode materials for energy storage, electrochemical desalination, and chemical sensing. Recent work has established the growth of thin films of pEDOT using alternating gas-phase exposures of the EDOT monomer and a metal chloride (e.g., MoCl 5 ) oxidant in a process termed oxidative molecular layer deposition (oMLD). Here, we describe the first demonstration of oMLD of amine-containing conjugated polymers. We find that pyrrole (Py) and MoCl 5 undergo self-limiting surface reactions during oMLD exposures to form conformal pPy thin films, but oMLD using aniline (Ani) and p-phenylenediamine (PDA) monomers yields unexpected azo functionality. The formation of azo groups is attributed to an MoCl 5 -amine surface adduct that spatially constrains polymerization reactions near the amine group and produces azo groups when coupling two primary amines. pPy grown by oMLD exhibits a record-breaking 282 mAh/g capacity in an aqueous electrolyte, and PDA/MoCl 5 oMLD yields azo polymers of interest as anode materials for alkali-ion batteries. Alternating between Py and PDA monomers during oMLD produces molecularly assembled 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.

Mechanistic Insights into Oxidative Molecular Layer Deposition of Conjugated Polymers

“…Likewise, cyclic voltammetry (CV) electrochemical measurements in 0.1 M NaCl electrolyte yielded characteristic electrochemical response consistent with pPy, pPDA, and pEDOT (Figure f–h). As reported in recent work, PDA/MoCl 5 produces a blend of phenazine and azo functionality with corresponding mixed redox activity . However, we observed no electrochemical response above the bare substrate signal for Fu and Thi monomers.…”

“…Deposition of polymer thin-films by oMLD was carried following previously established conditions . Briefly, the oMLD reactor chamber was held at 150 °C using PID temperature controllers.…”

“…Polymers deposited onto PGS substrates were characterized by cyclic voltammetry (CV) with a Biologic SP-150 potentiostat using a 3-electrode custom glass electrochemical cell, as described previously. , Aqueous electrochemical measurements were performed in 0.1 M NaCl aqueous electrolyte degassed using argon purge at circumneutral pH using an Ag/AgCl reference electrode (BASi) and graphite rod counter electrode (99.999%, Fischer Scientific). CV experiments were performed at a sweep rate of 50 mV/s over a potential range of −1.00 to +1.00 V vs Ag/AgCl, unless otherwise noted.…”

“…oMLD is distinct from gas phase polymerization studies in which an oxidant and monomer are codosed in the gas phase to produce polymer films, − because it separates the gas phase chemical precursors into sequential exposure steps. To date, oMLD processes have been demonstrated using 3,4-ethylenedioxythiophene (EDOT, Figure b), ,, pyrrole (Py, Figure c), p-phenylenediamine (PDA, Figure d), and 3-hexylthiophene (3HT, Figure e) monomers and MoCl 5 , ReCl 5 , and SbCl 5 chemical oxidants. These species undergo self-limiting surface reactions to grow thin-film polymers monomer-by-monomer.…”

“…Additionally, some reactions do not yield the same products as observed from homogeneous mixtures. For example, primary amines in Ani react to form azo species (e.g., azobenzene) during sequential oMLD doses instead of forming polyaniline (pAni) observed by homogeneous oxidation. Here, we (1) establish key insights into the oMLD growth mechanism that help to explain these unexpected phenomena, (2) identify design rules to guide future development of new oMLD chemistries and processes, and (3) use this understanding to control the molecular assembly of copolymers with record electrochemical capacity.…”