Lecture 10 Flashcards
Give a recap of previous knowledge you should remember.
This is depicted in the notes
What are carbon nanotubes?
They are 1D materials which are viewed as rolled-up sheets of graphite (graphene sheets), as you can see in Figure 7.1. They can be rolled up along several directions of the graphene sheet (which determines the electrical properties as either semiconducting or metallic, and their chirality), have different sizes, and be single-walled (SWCNTs) or multiwalled (MWCNTs).
Why is the synthesis of CNTs still a subject of debate?
The chemistry of CNTs is complicated by a series of facts, the first and foremost being their extremely small solubility in any solvent. This has driven the exploration of surface functionalization pathways with the aim of improving their solubility.
What are the issues faced by CNT’s surface functionalization?
Carbon nanotube (CNT) functionalization presents several challenges. Modifying the surface often disrupts the material’s electronic properties, especially through covalent bonding, which compromises conductivity. However, this can be an advantage for sensor development if conductivity changes are controlled by adsorbing molecules. Another issue is the relative inertness of the CNT’s sp²-bonded graphene sheets, though the curvature increases reactivity. Functionalization also varies based on nanotube diameter and producing monodisperse CNTs requires complex methods. Additionally, sparse functionalization leads to difficulties in dispersing CNTs, as nanotubes tend to form bundles. Impurities from the fabrication catalyst further complicate the process.
What are the possible ways to functionalize the surface of CNT?
- Halogenation
- Nitrene addition
- Bingel’s reaction
- Acid oxidative cutting
- Pi interactions
- Van der Waals interactions
Note that the reactions we are going to see in this chapter are largely applicable to fullerenes as well, and in some cases, they might also apply to graphene sheets.
How do we functionalize CNTs using halogenation?
A common route for the functionalization of polyolefines- of which CNTs can be seen as a relative - is halogenation. Chlorine, bromine, fluorine, or iodide can be reacted with the double bond of polyolefins. Fluorine is generally the candidate of choice for CNTs. The degree of fluorination can be controlled with the temperature, as shown in Figure 7.1
The advantage of this methodology is that fluorine is a good “Leaving group”, as it can be replaced easily by other functional groups by using standard Grignard regents (RMgBr), alkyl lithium regents (RLi), or alkyl peroxides. In this way, it is possible to attach, in principle, any organic compound R to the walls of the CNTs. Another approach, maybe simpler, is to react primary amines with the fluorinated CNTs to yield secondary amines bound to the CNTs. This fluorination route is very nice and is generally performed in the gas phase also because of the temperatures required.
How do we functionalize CNTs by nitrene addition?
Another addition to the CNT walls can be performed by reacting them with nitrenes, compounds of formula R-OOC-N3, Where R can be virtually anything. The driving force is the elimination of N2 as a gas, which drives the reaction to completion. Additional stimulus can be given by irradiation or heat. The advantage of this reaction is that it can be safer than fluorination reactions, even though the evolution of a gas as a product has to be considered.
How do we functionalize CNTs by Bingel’s reaction?
Bingel’s reaction involves the reaction of a bromomalonate (Figure 7.1) of formula BrC(H)(COOEt)2 with the walls of the CNT. The reaction results in a nucleophilic cyclopropanation, leading to the formation of two covalent bonds with the CNT. The ester groups in the product can then be replaced by arbitrary R groups by exposing them to ROH alcohol. To give an example, the R groups were chosen to be R = -Ch2CH2SCH3. Given the affinity of sulfur for gold, teh resulting CNTs were tagged with gold nanocrystals, which would selectively bind to the functionalized nanotubes. Since more than one nanotube could bind to the same gold colloid, this methodology could be used to form crosslinked networks of nanotubes, the crosslinks being gold colloids
How do we functionalize CNTs by Acid oxidative cutting?
Usually, the CNTs are capped by a hemisphere which is essentially a half fullerene. What was discovered was that by exposing the nanotubes to strong oxidizing acids they would leave open and that the cut would be terminated by carboxylic acid groups produced by the oxidation. This methodology can functionalize the walls of the CNT in case defects like pentagons and heptagons are present in the hexagonal graphitic wall structure.
The extent of the cutting as well as of the wall functionalization are dependent on the temperature of the reaction, the acid composition, and the curvature of the CNT
What is the importance of being able to functionalize the ends of CNTs?
By functionalizing the ends of the nanotubes you can also promote the solubility of the tubes in polar solvents (the carboxylic acids can also be functionalized with a long alkyl chain to improve the solubility in apolar solvents) and you open the door to possible end-to-end coupling in designed patterns
The idea of using end-functionalized CNTs as the tip of an atomic force microscope (AFM) has also enabled a neat atomic-scale imaging technique called “chemical force microscopy”
How is the surface of CNTs functionalized by using Pi interactions and VdW forces?
CNTs could be transferred to the aqueous phase by using surfactants. Investigation of these functionalized CNTs under the transmission electron microscope (TEM) showed that the surfactant molecules self-assembled on the surface of the CNTs in ways that depended on the radius of the tube. The interaction between the alkane tails of the surfactant molecules and the walls of the CNTs can be defined as van der Waals, as shown in Figure 7.1. In the case of aromatic molecules, such as pyrene-containing derivatives, the interaction is a pi-interaction, where the two aromatic systems interact through the pi-orbitals (Figure 7.1). These kinds of weak bonding interactions have a minimal perturbation effect on the electronic properties of CNTs
What are fullerenes?
Fullerenes are a family of closed-cage stable carbon clusters, of which C60 is the most stable member (See Figure 7.3). Other members include C76, C84, and so forth.
How were fullerenes discovered?
They were first detected while exploring the carbon chemistry of red giant stars. Interstellar clouds contain long carbon chains such as HC5N, HC7N, and HC9N, and scientists were trying to reproduce their formation by laser vaporization of carbon. Such techniques would produce carbon plasma which would then be sampled by mass spectrometry. It was then seen that the peak corresponding to 60 carbon atoms was strikingly more common than the others!
This was the discovery of C60
This is supported by info in the slides shown in the digital notes
What is the chemistry of fullerenes?
The chemistry of fullerenes is similar to the chemistry of CNTs. Their finite structures, however, makes fullerenes easier to understand and model. In fullerenes, the number of electrons does not provide for aromaticity, so the carbons’ pi electrons are not fully delocalized around the molecule.
This is supported by info in the slides shown in the digital notes
What is C60 generally used for?
C60 is generally used for its ability to “store” a large number of electrons (up to 7); these can be extracted easily by reduction-oxidation (redox) processes and observed electrochemically. You will thus see fullerenes being investigated as components of catalysts for redox reactions, of plastic solar cells and light-emitting diodes, or plastic superconductors.
This is supported by info in the slides shown in the digital notes
How are CNTs produced?
CNTs were initially produced by an arc-discharge method in the presence of a metal catalyst, such as iron or cobalt. This method is very similar to the one used for the mass production of fullerenes. The more modern method employs an acetylene plasma or methane plasma in the presence of a suitable substrate where the catalyst particles are deposited. This allows the production of straight and thick CNT “mats” on a variety of surfaces.
This is supported from info in the slides shown in digital notes
What is the advantage of graphene?
The advantage of graphene is that it is planar and thus largely compatible with most of our patterning and lithographic techniques, which were designed for planar silicon wafers.
How can we synthesise graphene?
Graphene can now be produced in a variety of ways, one from graphite, one from SiC single crystals which, when exposed to a certain temperature, desorb the outmost layer of silicon atoms exposing a single underlying graphene sheet.
It can also be produced by organic synthesis or as a colloid in water solution.
This last method uses graphite oxide as an intermediate with increased layer spacing. Graphite is first oxidized, which produces graphite oxide, which still has a layered structure, but a larger interplay distance, and hence a weaker interlayer bonding. This solid is then exfoliated by ultrasound and dispersed in water, where oxygen-based functional groups like epoxides, alcohols, and carboxylic acids are removed by hydrazine, a strong reducing agent. The graphite oxide sheets are thus reverted to graphene sheets which though remain water soluble in the presence of ammonia allowing spin- and dip-coating of graphene.
This is probably one of the most important differences between graphene and CNTs for a chemist. The possibility to process in solution large amounts of graphene will allow nanochemists to dream up applications of graphene in self-assembly of nanocomposites.
This is supported by info in the slides shown in the digital notes
How can we detemine the thickness of graphene layers and how does a single layer look like?
This is depicted in the notes
Look at the notes for leftovers
I AM SO SORRY IT WASNT ME IT WAS THE BOOK
