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CMG provides a route to produce large scale, solution processable graphene and opens possibilities for modification and further functionalization of graphene.84, 104_{GO is a}

member of the CMG family and is the first step towards fabrication of a wide variety of CMGs.

2.2.1 Synthesis of graphene oxide

Graphene oxide can be synthesized by using the Brodie, Staudenmeir, or Hummers method.50_{In this work a modified Hummers method as reported by Eda et al.,}44_{is used.}

Modified Hummers method: 5 g of graphite powder was mixed with a solution of 4.5 g of KNO3 and 169 ml of concentrated H2SO4 and placed in an ice bath. This mixture

was continuously stirred and 22.5 g of KMnO4was slowly added over a period of 1 hour.

The mixture was left to stir for another 2 hours in the ice bath and then removed from the ice bath and left to stir continuously for 5 days. This resulted in a black viscous liquid. To this, 500 ml aqueous solution of 5 wt% H2SO4 was added over 1 hour while stirring

% H2SO4, and 0.5 wt

supernatant. T

slurry was dispersed in de colloidal su

Figure 2 right.

Vacuum filtration

water soluble contaminants in the GO suspension obtained using the modified Hummers method. This step was carried out for all the GO based samples produced in this study.

A

was used to vacuum filter filtration membrane

filtration membranes were dried in a vacuum peeled off resulting in a free

storage. When required

concentrations in water by mild sonication concentrations 1, 0.5, 0.25 and 0.13 mg

, and 0.5 wt. %

supernatant. The procedure of washing was repeated 10 times. The resulting dark brown slurry was dispersed in de

colloidal suspension of GO.

2.2 Aqueous dispersions of GO of concentrations 1, 1/2, 1/4, and 1/8, mg ml

Vacuum filtration

water soluble contaminants in the GO suspension obtained using the modified Hummers method. This step was carried out for all the GO based samples produced in this study.

mixed cellulose used to vacuum filter filtration membrane w

filtration membranes were dried in a vacuum peeled off resulting in a free

storage. When required

concentrations in water by mild sonication ncentrations 1, 0.5, 0.25 and 0.13 mg

. % H2O2, leaving to precipitate for 2 days and then discarding the clear

he procedure of washing was repeated 10 times. The resulting dark brown slurry was dispersed in de-ionised (DI) water by mild sonication to give an aqueous

spension of GO.

Aqueous dispersions of GO of concentrations 1, 1/2, 1/4, and 1/8, mg ml

Vacuum filtration: Vacuum filtration was an important step to get rid of all the water soluble contaminants in the GO suspension obtained using the modified Hummers method. This step was carried out for all the GO based samples produced in this study.

mixed cellulose membrane (Millipore) used to vacuum filter the GO dispersion

whilst the water

filtration membranes were dried in a vacuum

peeled off resulting in a free-standing graphene oxide paper storage. When required, the graphene oxide

concentrations in water by mild sonication ncentrations 1, 0.5, 0.25 and 0.13 mg

, leaving to precipitate for 2 days and then discarding the clear he procedure of washing was repeated 10 times. The resulting dark brown ionised (DI) water by mild sonication to give an aqueous

Aqueous dispersions of GO of concentrations 1, 1/2, 1/4, and 1/8, mg ml

Vacuum filtration was an important step to get rid of all the water soluble contaminants in the GO suspension obtained using the modified Hummers method. This step was carried out for all the GO based samples produced in this study.

membrane (Millipore) GO dispersion. A

the water soluble contaminants were washed through. The filtration membranes were dried in a vacuum

standing graphene oxide paper the graphene oxide

concentrations in water by mild sonication or stirring ncentrations 1, 0.5, 0.25 and 0.13 mg ml-1_.

, leaving to precipitate for 2 days and then discarding the clear he procedure of washing was repeated 10 times. The resulting dark brown ionised (DI) water by mild sonication to give an aqueous

Aqueous dispersions of GO of concentrations 1, 1/2, 1/4, and 1/8, mg ml

Vacuum filtration was an important step to get rid of all the water soluble contaminants in the GO suspension obtained using the modified Hummers method. This step was carried out for all the GO based samples produced in this study.

membrane (Millipore), having an average pore size of 0.22 A thin, uniform

soluble contaminants were washed through. The filtration membranes were dried in a vacuum desiccator

standing graphene oxide paper the graphene oxide paper

or stirringFigure 2.

, leaving to precipitate for 2 days and then discarding the clear he procedure of washing was repeated 10 times. The resulting dark brown ionised (DI) water by mild sonication to give an aqueous

Aqueous dispersions of GO of concentrations 1, 1/2, 1/4, and 1/8, mg ml

Vacuum filtration was an important step to get rid of all the water soluble contaminants in the GO suspension obtained using the modified Hummers method. This step was carried out for all the GO based samples produced in this study.

having an average pore size of 0.22 thin, uniform film of GO was

soluble contaminants were washed through. The desiccator, and then the GO films were standing graphene oxide paper which is convenient for can be re-dispersed at known

Figure 2.2shows GO dispersions of , leaving to precipitate for 2 days and then discarding the clear he procedure of washing was repeated 10 times. The resulting dark brown ionised (DI) water by mild sonication to give an aqueous

Aqueous dispersions of GO of concentrations 1, 1/2, 1/4, and 1/8, mg ml-1from left to

Vacuum filtration was an important step to get rid of all the water soluble contaminants in the GO suspension obtained using the modified Hummers method. This step was carried out for all the GO based samples produced in this study.

having an average pore size of 0.22 of GO was formed on the soluble contaminants were washed through. The , and then the GO films were which is convenient for dispersed at known shows GO dispersions of , leaving to precipitate for 2 days and then discarding the clear he procedure of washing was repeated 10 times. The resulting dark brown ionised (DI) water by mild sonication to give an aqueous

from left to

Vacuum filtration was an important step to get rid of all the water soluble contaminants in the GO suspension obtained using the modified Hummers method. This step was carried out for all the GO based samples produced in this study.

having an average pore size of 0.22 µm, formed on the soluble contaminants were washed through. The , and then the GO films were which is convenient for dispersed at known shows GO dispersions of

Deposition on a substrate: GO can be deposited from an aqueous suspension directly onto substrates such as silicon oxide (300 nm SiO2on degenerately doped Si) for

further study. SiO2 is hydrophobic and has been previously treated with 3-amino-

propyltriethoxysilane prior to deposition of GO.68 _{Here, O}

2-plasma treatment (Quorum

Technologies Plasma Etcher-Asher-Cleaner) was used (100 W for 2 minutes), to make the SiO2hydrophilic. After this, two different approaches were used.

Spin Coating: graphene oxide suspension was placed on the substrate and the substrate was rotated at 3000 rpm for 45 s. The acceleration and deceleration times were 0.1 s. This technique is efficient as it results in a controllable and uniform distribution of GO sheets. As discussed in Section 2.2.2, a near monolayer coverage was obtained with GO concentration of 1 mg ml-1_{. For higher concentrations of GO (5 – 10 mg ml}-1_{) longer spin}

coating time (60 s) was required for complete drying of the film.

TEM Grids: samples for TEM were prepared by drop casting GO dispersion on lacey carbon grids. Prior to drop casting the lacey carbon grids were air-plasma cleaned for 20 s to make them hydrophilic. Some samples were also prepared by drop casting GO dispersion on lacey carbon grids that were sputter coated with Au at rate of 5 nm/min. These can be used to calibrate GO diffraction patterns (DP) against gold DP as will be shown in Chapter 3.

Reduction of GO: in this study two techniques were used for reducing GO.

Reduction in suspension: 112 l of ammonia solution (35 wt. % Fisher Scientific) and 17.6 l of hydrazine monohydrate solution (62 wt.%, Fisher Scientific) were added to 10 ml of 1 mg ml-1_{GO suspension and the mixture was left to stir for 1 h at 90 .}51_The

Figure 2

Reduction on the substrate

followed.46

then place

Aldrich). The whole system was sealed using a glass cover and parafilm and bath at 40

heated at 300

2.2.2 Characterisation of

AFM topography was obtained using an Asylum Research MFP3D AFM in tapping mode. Figure 2.4a shows the AFM height image of a GO film spin coated on Si/SiO2 substrate from a suspension of concentration 1 mg ml

of fully exfoliated GO sheets. This layer shows uniform thickness and almost full coverage on the substrate. The lateral dimensions of these sheets range from 100 nm to 2 μm. Occasionally, a second layer is also visible on top of the first GO layer.

2.3 Aqueous dispersion

Reduction on the substrate

46_{GO spin coated on Si/SiO}

then placed in a larger petri dish containing 1 ml of hydrazine anhydrous (98 Aldrich). The whole system was sealed using a glass cover and parafilm and

40 for 18 h. Th at 300 in air for 1 h.