Preparation of Carbon Nanosheets at Room Temperature

Summary

We present the synthesis of an amphiphilic hexayne and its use in the preparation of carbon nanosheets at the air-water interface from a self-assembled monolayer of these reactive, carbon-rich molecular precursors.

Abstract

Amphiphilic molecules equipped with a reactive, carbon-rich "oligoyne" segment consisting of conjugated carbon-carbon triple bonds self-assemble into defined aggregates in aqueous media and at the air-water interface. In the aggregated state, the oligoynes can then be carbonized under mild conditions while preserving the morphology and the embedded chemical functionalization. This novel approach provides direct access to functionalized carbon nanomaterials. In this article, we present a synthetic approach that allows us to prepare hexayne carboxylate amphiphiles as carbon-rich siblings of typical fatty acid esters through a series of repeated bromination and Negishi-type cross-coupling reactions. The obtained compounds are designed to self-assemble into monolayers at the air-water interface, and we show how this can be achieved in a Langmuir trough. Thus, compression of the molecules at the air-water interface triggers the film formation and leads to a densely packed layer of the molecules. The complete carbonization of the films at the air-water interface is then accomplished by cross-linking of the hexayne layer at room temperature, using UV irradiation as a mild external stimulus. The changes in the layer during this process can be monitored with the help of infrared reflection-absorption spectroscopy and Brewster angle microscopy. Moreover, a transfer of the carbonized films onto solid substrates by the Langmuir-Blodgett technique has enabled us to prove that they were carbon nanosheets with lateral dimensions on the order of centimeters.

Introduction

Two-dimensional carbon nanostructures attract significant attention due to the reported outstanding electrical, thermal, as well as mechanical properties^1-5. These materials are expected to further the technical progress in the fields of polymer composites⁶, energy storage devices⁷, and molecular electronics^8-10. Despite intensive research efforts in recent years, however, access to larger amounts of well-defined carbon nanomaterials is still limited, which impedes their large-scale implementation in technological applications^11,12.

Carbon nanomaterials are accessible by either top-down....

Protocol

Caution: Please make sure to consult the relevant materials safety data sheets (MSDS) before the use of any chemical compounds. Some of the chemicals used in these syntheses are acutely toxic and carcinogenic. Prepared nanomaterials may have additional hazards compared to their bulk counterpart. It is imperative to use all appropriate safety practices when performing reactions (fume hood) and personal protective equipment (safety glasses, gloves, lab coat, full length pants, closed-toe shoes). If not otherwise stated the following procedures involve standard Schlenk techniques³⁹.

1. Preparation of the Molecular Precursor<....

Results

The ¹³C nuclear magnetic resonance (NMR) spectrum of the prepared precursor molecule 3 displays the 12 sp-hybridized carbon atoms of the hexayne segment with the corresponding chemical shifts of δ = 82-60 ppm (Figure 1b). Moreover, the signals at δ = 173 ppm and at δ = 52 ppm are assigned to the carbonyl and methyl carbon of the ester, respectively. The signals between δ = 33-14 ppm are ascribed to the aliphatic carbons of the dodecyl .......

Discussion

The desired hexayne amphiphile (3) is straightforwardly prepared by the sequential bromination^52,53 and Pd-catalyzed elongation^30,31 of the alkyne segment, followed by a final deprotection reaction of the tritylphenyl ester (2) (Figure 1a)²⁹. The successful synthesis is confirmed by the ¹³C NMR spectrum (Figure 1b) as well as the UV-Vis absorption spectrum (Figure 1c)^31,54. This demonstrates the facile.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Methyllithium lithium bromide complex (2.2 M solution in diethylether)	Acros	18129-1000	air-sensitive, flammable
Zinc chloride (0.7 M solution in THF)	Acros	38945-1000	air-sensitive, flammable
1,1'-Bis(diphenylphosphino)ferrocene] dichloropalladium(II), DCM adduct	Boron Molecular	BM187
N-Bromosuccinimide	Acros	10745	light-sensitive
Silver fluoride	Fluorochem	002862-10g	light-sensitive
n-Butyllithium (2.5 M solution in hexanes)	Acros	21335-1000	air-sensitive, flammable
Sodium methanolate	Acros	17312-0050
Tetrahydrofuran (unstabilized, for HPLC)	Fisher Chemicals	T/0706/PB17	This solvent was dried as well as degassed using a solvent purification system (Innovative Technology, Inc, Amesbury, MA, USA)
Toluene (for HPLC)	Fisher Chemicals	T/2306/17	This solvent was dried as well as degassed using a solvent purification system (Innovative Technology, Inc, Amesbury, MA, USA)
Acetonitrile (for HPLC)	Fisher Chemicals	A/0627/17	This solvent was dried as well as degassed using a solvent purification system (Innovative Technology, Inc, Amesbury, MA, USA)
Dichloromethane (Extra Dry over Molecular Sieve)	Acros	34846-0010
Chloroforme (p.a.)	VWR International	1.02445.1000
Pentane	Reactolab	99050	Purchased as reagent grade and distilled once prior to use
Heptane	Reactolab	99733	Purchased as reagent grade and distilled once prior to use
Dichloromethane	Reactolab	99375	Purchased as reagent grade and distilled once prior to use
Diethylether	Reactolab	99362	Purchased as reagent grade and distilled once prior to use
Geduran silica gel (Si 60, 40-60 µm)	Merck	1115671000
Langmuir trough	R&K, Potsdam
Thermostat	E1 Medingen
Hamilton syringe	Model 1810 RN SYR
Vertex 70 FT-IR spectrometer	Bruker
External air/water reflection unit (XA-511)	Bruker
UV lamp (250 W, Ga-doped metal halide bulb)	UV-Light Technology
Brewster angle microscope (BAM1+)	NFT Göttingen
Sapphire substrates	Stecher Ceramics
Quantifoil holey carbon TEM grids	Electron Microscopy Sciences
Nuclear magnetic resonance spectrometer (Bruker Avance III 400)	Bruker
JASCO V-670 UV/Vis spectrometer	JASCO
Scanning Electron Microscope (Zeiss Merlin FE-SEM)	Zeiss