Voorbereiding van de Carbon Nanosheets bij kamertemperatuur
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
Tweedimensionale koolstofnanostructuren trekken veel aandacht vanwege de gerapporteerde uitstekende elektrische, thermische en mechanische eigenschappen 1-5. Deze materialen zouden moeten de technische vooruitgang op het gebied van polymeersamenstellingen 6, energiereservoirs 7 en moleculaire elektronica 8-10 bevorderen. Ondanks intensief onderzoek inspanningen in de afgelopen jaren echter, de toegang tot grotere hoeveelheden welbepaalde koolstof nanomaterialen is nog steeds beperkt, waardoor hun grootschalige implementatie in technologische toepassingen 11,12 belemmert.
Carbon nanomaterialen zijn bereikbaar door een top-down of bottom-up aanpak. Typische benaderingen zoals afschilfering technieken 13 of hoogenergetische processen op oppervlakken 14-16 bieden de mogelijkheid om materialen met een hoge mate van structurele perfectie en zeer goede prestatie te verkrijgen. De isolatie en zuivering van ee producten blijft een uitdaging, en de grootschalige productie van gedefinieerde nanostructuur materialen is moeilijk 12. Anderzijds kunnen bottom-up benaderingen worden gebruikt die afhankelijk van het gebruik van moleculaire precursoren, hun opstelling in gedefinieerde structuren, en een daaropvolgende carbonisatie dat de koolstofnanostructuren 17-23 oplevert. In dit geval, de voorlopers zelf zijn complexer en hun bereiding vereist vaak meerdere synthesestappen. Deze benaderingen kunnen een hoge mate van controle over de chemische en fysische eigenschappen van de resulterende materialen bieden en kunnen een directe toegang tot speciaal materiaal leveren. Echter de omzetting van de voorlopers tot koolstof nanomaterialen typisch uitgevoerd bij temperaturen boven 800 ° C, wat leidt tot een verlies van de ingebedde chemische functionalisering 24-27.
De hierboven genoemde beperkingen zijn in onze groep aangepakt door het gebruik van zeer reactieve oligoynes dat can worden omgezet in koolstofvezels nanomaterialen bij kamertemperatuur 28,29. Vooral amfifielen die een hydrofiele kopgroep en een hexayne segment toegankelijk via een opeenvolging van bromering en palladium gemedieerde Negishi kruis-koppelingsreacties 30,31. De omzetting van deze precursor moleculen in de beoogde structuur optreedt bij of beneden kamertemperatuur bij bestraling met UV-licht. De hoge reactiviteit van de oligoyne amfifielen maakt het gebruik van zachte templates, zoals de lucht-water grensvlak of vloeistof-vloeistof interface mogelijk. In eerdere onderzoeken, hebben we met succes bereid blaasjes uit oplossingen van hexayne glycoside amfifielen 28. Verknoping van deze vesicles werd onder milde omstandigheden bereikt door UV bestraling van de monsters. Bovendien hebben we onlangs bereid zelf-geassembleerde monolagen van hexaynes met een methylgroep carboxylaat kopgroep en een hydrofobe alkyl staart in de lucht-water-interface in een Langmuir trog. De dichtbevolkte Packed moleculaire voorlopers werden daarna rechtlijnig omgezet in zelfdragend carbon nanosheets bij kamertemperatuur door UV-bestraling. In verwante benaderingen gedefinieerde moleculaire precursoren zijn onlangs gebruikt voor de bereiding van tweedimensionaal uitgebreide nanosheets de lucht-water grensvlak 32-38.
Het doel van dit werk is om een beknopt, praktisch overzicht van de totale synthese en fabricage stappen die het mogelijk maken voor de bereiding van koolstof nanosheets van hexayne amfifielen geven. De focus ligt op de experimentele aanpak en voorbereidende vragen.
Protocol
Representative Results
Discussion
De gewenste hexayne amfifiel (3) is rechtlijnig bereid door sequentiële bromering 52,53 en Pd-gekatalyseerde verlenging van het alkyn 30,31 segment, gevolgd door een laatste ontschermingsreactie van de tritylphenyl ester (2) (figuur 1a) 29. De succesvolle synthese wordt bevestigd door de 13 C NMR-spectrum (figuur 1b) en de UV-Vis-absorptiespectrum (figuur 1c) 31,54. Dit toont de gemakkelijke aard waardoor hogere ol…
Disclosures
The authors have nothing to disclose.
Acknowledgements
Funding from the European Research Council (ERC Grant 239831) and a Humboldt Fellowship (BS) is gratefully acknowledged.
Materials
| Methyllithium lithium bromide complex (2.2M solution in diethylether) | Acros | 18129-1000 | air-sensitive, flammable |
| Zinc chloride (0.7M 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 (2.5M 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, 40-60µm) | 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 |
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