ISIS Report 07/08/13
Graphene from Greenhouse Gases to Save the Climate
Graphene can be synthesized in large qualities and with
minimum environmental impacts from methane and also carbon dioxide, thereby
also saving the climate Dr Mae-Wan
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Industrial production of graphene has already begun in China
(see  Graphene
and the New Age of Carbon, SiS 59), though the precise methods used
in production are not known. My choice is for making it from greenhouse gases
in an environmentally sustainable way.
Production methods vary in environmental and health
impacts and energy efficiency
Current methods that can mass produce graphene are chemical
exfoliation of graphite, epitaxial growth on silicon carbide (SiC) and chemical
vapour deposition on transition metal foil. Chemical exfoliation requires harsh
chemicals such as potassium permanganate (KMnO4)
in concentrated sulphuric acid (H2SO4), and hydrogen
peroxide (H2O2) to oxidize graphite and separate
the layers to make a colloidal dispersion of graphene oxide, which is then
reduced back to graphene with highly toxic chemicals such as hydrazine .
Epitaxial growth on SiC requires heating SiC to high temperatures in excess of
1 100 ºC in an atmosphere of argon and the quality and size of the product
depend on the SiC wafer, which makes the process quite expensive as well as
energy intensive . Chemical vapour deposition (CVD) on transition metal foil
looks the most promising, producing
single layer graphene in large sheets using methane CH4
as carbon source, but the process also requires high temperatures. Plasma (electrically
ionized gases) CVD reduces the temperature required, but involves expensive
equipment and producing multilayer graphene with low transparency and many
defects resulting from ion bombardment by the plasma.
Plasma assisted chemical vapour deposition from methane
Researchers at the National Central University of Taiwan led
by Chien-Cheng Kuo have devised a low-cost plasma-assisted CVD to grow
monolayer graphene at low temperature and without the defects from the ion
bombardment . The plasma is generated from the gases in a separate chamber
between two electrodes, which then flows into the chamber where deposition
occurs onto copper foil (see Figure 1). The flow rate of H2 was
varied from 5 to 20 standard cubic centimetres (sccm) per second at a temperature
as low as 600 ºC. A direct current pulse power supply of 200 W with pulsing
frequency of 20 kHz maintains the plasma. A spectrum analyser was used to
obtain the plasma emission spectra through an optical fibre.
Figure 1 Plasma
assisted CVD at low temperature for graphene synthesis
Graphene films were grown on 25 mm thick copper foil electro-polished and
washed, then mounted in the CVD chamber and the furnace heated to 1 035 ºC to
anneal the sample for 30 min. Graphene was grown at a lower temperature of 600 ºC.
Methane gas flowing at 1 sccm was the carbon source. It was mixed with various
flows of H2 and fed into the tube for 5 min to form a monolayer of
graphene (Figure 2).
Figure 2 Plasma
decomposed fragments for CVD graphene synthesis
Graphene growth involves the decomposition of
CH4 and H2 in the mixed plasma into fragments including CHx
radicals (chemical species with unpaired electrons). The gaseous CHx
radicals recombine with each other after they had floated for a certain
distance, and the metastable carbon atoms and molecules form graphene structure
on the copper surface. Crucially, the most effective length for growing
graphene between the plasma and the centre of the hot zone was approximately 30
Subsequently, the sample was
rapidly cooled by removing it from the hot zone of the furnace. The synthesized
graphene films were transferred onto SiO2/Si substrates by etching away the
copper foil in an ion chloride solution. Prior to wet etching, a 200-nm thick
film of PMMA (poly-methyl methacrylate) was spin-coated onto the top of
graphene/copperfoil and baking at 13o ºC for 1 min. The PMMA/graphene thin
films were washed with dilute hydrochloric acid to remove the metal ions and
then rinsed deionized water. PMMA/graphene films were placed on the SiO2/Si
substrate and the PMMA was dissolved away in an acetone bath over 24h.
Spectroscopic analysis showed
that higher quality graphene was obtained with increased H2 flow
Electroplating Cu for better and bigger graphene
Another group at Chinese Academy of Sciences in Shanghai,
China, led by Li Tie and Wang Yue-Lin found that graphene domains can be better
synthesized on electroplated Cu with a smooth and uniform surface from dilute
methane gas . The copper electroplating was performed on a SiO2/Si substrate
after 50 nm ni was sputtered on for the adhesion layer, and then 200 nm Cu
sputtered on as the seed layer. Electrodeposition was finished in a plating
tank with copper sulphate to about 4mm.
Graphene synthesis was done in a tube furnace with a mixture of CH4
(0.15 sccm), H2 (50 sccm) and Ar (500 sccm) at ambient pressure and
1 040 ºC for 3 to 10 minutes.
Hexagonal domains of graphene were deposited
increasing in size with increasing length of deposition time ranging from 2 up
to 7 mm in diameter. These were far
superior to those grown on copper foil: denser coverage and larger domains. Further
work indicates that the graphene nucleation density was proportional to the CH4
concentration kand the graphene coverage rate on electroplated Cu was
proportional to the CH4 concentration and growth time.
There is clearly much room for improving the
efficiency and cost-effectiveness of the process, as well as the quality of the
Graphene from CO2
Carbon dioxide, the major greenhouse gas responsible for
global warming could be reduced converting it into liquid fuels, useful chemicals
or carbon materials. Currently attractive options for transformation of carbon
include reducing CO2 to diamonds and nanotubes, either through
direct CO2 splitting or by reacting with metals at pressures > 70
MPa (~ 700 atm).
Scientists at City College of New
York in the United States have succeeded in converting CO2 to
graphene oxide using relatively non-toxic materials and mild conditions .
The conversion takes place in two steps: CO2 fixation (at CO2
pressure < 3 MPa and temperature < 100 ºC) and graphenization (600 -75o C
under 0.1 MPa of N2). The first step generates a solid that contains
methoxy (OCH3), formate (HCOO) and aliphatic (C-C) groups while the
second step pyrolysis the solid compound to graphene oxide-boron-oxide
nanocomposites. The formation of aliphatic groups without using
metal-containing compounds at mild conditions is of great interest to the
synthesis of various organic products starting from CO2.
An extremely simple method of arc discharge
shows promise in producing graphene sheets with a few layers. An arc discharge is simply a bright electric current that
forms across a gap in a circuit or between two electrodes. Researchers
led by Yongsheng Chen at Nankai University, Tianjin, China, used a direct
current arc-discharge in a home-made water-cooled stainless steel chamber
filled with a mixture of CO2 and helium . Different gas compositions
ranging from 5 vol% to 40 vol% CO2, and direct currents of 100 to
200 A were used. The discharge voltage was kept around 30 V by controlling the
distance between the two electrodes. Both electrodes were normal graphite rods
obtained commercially with an anode diameter of 13 mm and a cathode diameter of
40 mm. After discharge, the cotton-like deposits that forms on the inner wall
of the chamber was collected and examined. The optimum conditions for producing
a few layered graphene were a low voltage < 35 V, high gas pressure (>1
270 Torr ~1,7 atm), high current (~150 A) and 25-40 vol% CO2. In a
typical run, tens of grams of high-quality graphene sheets with four to five
layers can be generated in minutes.
Researchers led by Narayan
Hosmane at North Illinois University DeKalb in the United States discovered a
quick method for converting carbon dioxide directly into graphene simply by
burning the metal magnesium in dry ice according to the reaction :
2Mg + CO2 → 2MgO + C
In a typical experiment, 3 g of
Mg ribbon was ignited inside a dry ice bowl and covered by another dry ice
slab. After combustion, the black products were transferred to a beaker
containing 100 mL of 1M HCl and stirred overnight at room temperature to remove
Mg and MgO as MgCl2, which is soluble. After washing, the isolated solid
carbon product was dried under high vacuum overnight at 100 ºC, yielding 680 mg
(92 % of theoretical). The product was characterized by TEM, Raman
spectroscopy, wnergy-dispersive X-ray analysis and X-ray powder diffraction,
and found to be 3-7 layers graphene, along with trace amounts of Mg and O.
A company Graphene Technologies has patented
a technology to produce graphene from CO2, and was named Company of
the year in 2013 by the Futures in Review conference.
There are 2 comments on this article so far. Add your comment
|Todd Millions Comment left 25th August 2013 10:10:18|
Since you mentioned diamonds,back in the pre interweb days(bless the sainted Al Gore.),Diamond films where reported grown on substrats,covered with diamond dust 'seed'.In methane filled chambers,by using a laser to catalysis the film growth.I don't recall the temps,pressures or wavelenghts.The source was popular science-I think.I never got the scratch proof lexan windsheild,glasses or unbreakable solar panel glazing I expected from this-fart to diamond trick-but graphene 'seed',and CO2 for same approach may be worth persuing.Cheers.
|Philip Ward Comment left 20th October 2013 14:02:59|
Before you claim such a tehcno-fix can "save the climate", isn't an audit of embodied energy in the product and a discussion of issues of scale is necessary?