Ullmann Reactions of Carbon Nanotubes-Advantageous and Unexplored Functionalization toward Tunable Surface Chemistry

Abstract

We demonstrate Ullmann-type reactions as novel and advantageous functionalization of carbon nanotubes (CNTs) toward tunable surface chemistry. The functionalization routes comprise O-, N-, and C-arylation of chlorinated CNTs. We confirm the versatility and efficiency of the reaction allowing functionalization degrees up to 3.5 mmol g^-1 by applying both various nanotube substrates, i.e., single-wall (SWCNTs) and multi-wall CNTs (MWCNTs) of various chirality, geometry, and morphology as well as diverse Ullmann-type reagents: phenol, aniline, and iodobenzene. The reactivity of nanotubes was correlatable with the nanotube diameter and morphology revealing SWCNTs as the most reactive representatives. We have determined the optimized conditions of this two-step synthetic protocol as: (1) chlorination using iodine trichloride (ICl₃), and (2) Ullmann-type reaction in the presence of: copper(I) iodide (CuI), 1,10-phenanthroline as chelating agent and caesium carbonate (Cs₂CO₃) as base. We have analyzed functionalized CNTs using a variety of techniques, i.e., scanning and transmission electron microscopy, energy dispersive spectroscopy, thermogravimetry, comprehensive Raman spectroscopy, and X-ray photoelectron spectroscopy. The analyses confirmed the purely covalent nature of those modifications at all stages. Eventually, we have proved the elaborated protocol as exceptionally tunable since it enabled us: (a) to synthesize superhydrophilic films from-the intrinsically hydrophobic-vertically aligned MWCNT arrays and (b) to produce printable highly electroconductive pastes of enhanced characteristics-as compared for non-modified and otherwise modified MWCNTs-for textronics.

Keywords: Ullmann reaction; carbon nanotubes; chlorination; electroconductive coatings; functionalization; hydrophilization.

Figure 1

General scheme of the preferential sites of chlorination of carbon nanotubes (CNTs) by their treatment with ICl₃ (A); optimized conditions for Ullmann-type reactions of CNTs (B); for clarity, only a fragment of the (outer) nanotube wall is shown; ∅ stands for an empty set, i.e., no atoms in the case of C-arylation. The figure was prepared on the basis of a hypothesis made by Abdelkader et al. [33].

Figure 2

High-Resolution Transmission Microscope images (HRTEM) of chlorinated TUBALL™ single-walled carbon nanotubes (SWCNTs) by their treatment with: ICl₃ (A), subjected to an Ullmann-type reaction with phenol (B), subjected to Ullmann-type reaction with iodobenzene (C), subjected to Ullmann-type reaction with aniline (D).

Figure 3

The Radial Breathing Mode (RBM) region of pristine and functionalized SWCNTs.

Figure 4

The G-band (A) and D-band (B) region of SWCNT samples; (C) I_D/I_G and I_D/I_G’ ratios presented as blue and red bars, respectively.

Figure 5

The G-band range after deconvolution for pristine and functionalized single-walled carbon nanotubes (SWCNTs).

Figure 6

Thermogravimetric (TG) curve for 1,3,5-trihydroxybenzene (THB) modified in-house vs. a pristine MWCNT array sample.

Figure 7

O-1s region X-ray photoelectron spectroscopy (XPS) signals and deconvoluted peaks for O-arylated Nanocyl NC7000^TM MWCNTs modified via reaction of chlorinated nanotubes with phenol (A) and 1,3,5-trihydroxybenzene (B).

Figure 8

High resolution X-ray photoelectron spectroscopy (XPS) of Nanocyl NC7000^TM MWCNTs obtained in the C 1s bonding energy region for: halogenated CNTs with ICl₃ (A), halogenated CNTs using ICl (B), O-arylated CNTs (C), N-arylated CNTs (D), C-arylated CNTs (E), modified with 1,3,5-trihydroxybenzene (THB) (F).

Figure 9

High resolution X-ray photoelectron spectroscopy (XPS) of Nanocyl NC7000^TM MWCNTs obtained in the N 1s bonding energy region for N-arylated MWCNTs.

Figure 10

Aqueous dispersions of in-house MWCNT array tiny flakes: pristine (A) and covalently modified by Ullmann-type reaction of chlorinated arrays with 1,3,5-trihydroxybenzene—identical weight and no ultrasonication applied (B); wetting dynamics of 1,3,5-trihydroxybenzene (THB) covalently modified in-house ‘MWCNT carpet’—snapshots of photographs in the timescale of seconds for a drop released from a goniometer (C); the correlation between wetting contact angle (WCA) and the enthalpy of immersion for three samples: pristine, blank experiment (no catalyst in the Ullmann reaction) and non-covalently treated with 1,3,5-trihydroxybenzene in-house vertically aligned MWCNT array (D).