CARBON MATERIALS AND TUBES & RODS OF NON-CARBON MATERIALS Flashcards
How large is the C60 molecule? How many 5-rings and 6-rings of carbon atoms does it contain? What is the approximate C-C bond distance?
7 Å diameter, 12 five rings, 20 six rings, 1.41 Å average bond length.
Describe the chemical bonding in graphene (type of bonds etc), and in graphite.
sp2-sigma bonds, pi-bonds. The latter are responsible for the metallic conductivity
Graphite: van der Waals interactions between the 2D-layers.
Consider lecture slides and compare the electronic band structure of C (diamond), Si and Ge. N-doping of diamond gives yellow crystals. How will you explain that based on the band structure?
Nitrogen impurity creates a impurity level within the band gap which can absorb energy, and in this case is responsible for the absorbance of yellow light.
Give chemical formulas for some graphene analogue materials. How do the electronic properties of BN differ from those of graphene?
h-BN, BCN2, BC2N, BN is a semiconductor.
Describe how fullerenes and carbon nanotubes are formed in an arc reactor. Why is cooling of the reactor walls required?
Heat up to create plasma, which will then combine to C2 and further bond formation. Cooling required owing to large release of bonding energy when individual C-atoms combine and make bonds.
Describe synthesis routes of graphene.
Scotch tape, sonication in a liquid medium followed by centrifugation, bottom-up synthesis based on sugars as reactants, reduction of ethanol with sodium followed by pyrolysis, catalytic CVD processes, reduction of graphene oxide.
What is graphene oxide? Describe synthesis route, and its electronic properties. Which types of C-C bonds are oxidized in the synthesis process? How can the various functional groups of the GO be investigated/determined?
GO has graphite layer structure but with irregular structure with oxygen and hydrogen species. pi bonds are oxidized. Spectroscopy IR/RAMAN.
Synthesis:
- potassium chlorate with nitric acid or sulfuric acid are used to oxidize graphite.
VERY VIOLENT REACTION, NEEDS TO BE BELOW 150 K.
How can RGO be obtained? Which properties makes RGO attractive for use as e.g. as an additive to electrode materials for LI-ion batteries?
Reduced graphite oxide is made by heating graphite to 180 degrees, reducing with hydrazine, and then use Cu catalyst + hydrazine again.
What is understood with pillaring of RGO (GOpr)? Why is RGO considered of potential interest in catalysis?
GOpr is electrochemically activated and the orderly pillated. Interesting because it is a good electric conductor and can function as a electron reservoir.
Comment on the mechanical strength of carbon nanotubes. How will defects affect these properties? What is understood by topological defects in graphene?
Extremely strong mechanically. Defects can make the tubes more plastic like.
Describe the difference between multiwall tubes and a scroll. Do both of these types of structures occur both for carbon materials and e.g. V-based oxide materials? How can you experimentally verify which of these situations do occur for a given sample –scroll versus tube?
Difference is that scroll has sheet ends on inside and outside. Both types can occur. It is required to collect an image that is perpendicular to the tube/scroll axis to distinguish between them.
Describe the building principle of different variants (ways of rolling up) of carbon nanotubes. Are there systematic correlations between building principle and electronic properties of the CNT?
It is possible to roll the CNT different, so that the pattern on the tube will be different. Zigzag, armchair and chiral. This difference leads to different electronic structure of the tube.
What is the difference between armchair and zig-zag CNT structures?
Difference in the pattern of the honeycomb pattern. Armchair resembles a chair, and zig-zag is alternating.
Describe possible low-temperature chemical synthesis routes to CNTs. What is the Boudoir reaction? In which organic/inorganic compounds can we find reactive C-atoms or C-C dimers?
Can use carbonyls, also ferrocene.
Boudouard reaction is a redox reaction of carbon monoxide and carbon dioxide, 2CO = CO2 + C.
Can use ionic carbides.
Describe carbon cones. Why do they form with specific opening angles for the cone?
The specific opening angles: caused by how one can role up a sheet. The open carbon cone can be modeled as a wrapped graphene sheet. In order to have strain-free, seamless wrapping, a sector has to be cut out of the sheet. That sector should have an angle of n×60°, where n=1,…,5.
Therefore, the resulting cone angle should have only certain, discrete values α = 2arcsin(1−n/6) = 112.9°, 83.6°, 60.0°, 38.9°, and 19.2° for n = 1,…,5, respectively. The graphene sheet is composed solely of carbon hexagons which can not form a continuous cone cap. As in the fullerenes, pentagons have to be added to form a curved cone tip, and their number is correspondingly n = 1,…,5.
