Chapter 1 Introduction
1.7. Synthesis of graphene oxide and its derivative materials
The properties of GO are sensitive to the synthesis process, and also GO is the source material for its derivative materials (RGO and GQDs). A comprehensive understanding of the synthesis of GO is required and essential for use in its potential applications. The evolution of the GO synthesis process has been discussed in section 1.1. The production of GO and graphene (RGO) on an industrial scale can be done using the top-down approach. Its mass production can be carried out at a low cost.
1.7.1. Synthesis of graphene oxide
Graphite oxide can be synthesized using the modified or simplified Hummers method.
Graphite/ expanded graphite flakes as source material, concentrated H2SO4, H3SO4, or mixture are used as a protonated solvent, and KMnO4 as an oxidizing agent is used for synthesis of graphite oxide. Adding H2O2 to end the oxidation reaction, filtration and centrifugation processes are used to obtain purified graphite oxide after oxidation. Various proposed techniques exist to exfoliate graphite oxide to obtain GO, such as ultrasonication, electrochemical, microwave-assisted, shear-mechanical, hydrothermal, and freezing-thawing- based exfoliation.
Table 1.1 collates various exfoliation techniques for graphene-based sheets, the nature of the solvent used, the resulting lateral size, and their applications. Although several approaches have been reported to exfoliate graphene derivatives into a few layers of large Table 1.1: Various techniques for graphene-based sheet exfoliation, nature of solvent used, their resulting lateral dimension and applications
lateral size, the hydrothermal method has shown to be a promising technique exhibiting its relative cost-effectiveness, producing excellent yield with controllable and large lateral size
Exfoliation method
Nature of solvent Lateral size Applications Reference
Electrochemical Aq. Ammonium sulfate
>30 µm - 50
Ultrasonic Imidazole 10 µm Supercapacitor 51
Ultrasonic Pyridine <1 µm Mechanical 52
Ultrasonic Water >30 µm Highly conductive
thin film
53
Ultrasonic Aq. salt Up to 30 µm Thin film devices 54
Ultrasonic Aq. PTCA 10 – 12 µm Conductive wire 55
Ultrasonic Aq. salt 5 – 10 µm Electrochemical
sensors
56
Ultrasonic Aq.salt 10 µm Li-battery
electrode
57
Ultrasonic Water/SL/DDAC /DDBAC
100 nm – 1 µm
- 58
Ultrasonic Aq. salt 100 nm– 6
µm
Ultracapacitor 59
Microwave Water 30 µm - 60
Microwave CHP/DMF/NMP/ionic liquid
2 – 7 µm 3D Printing 61
Electrochemical Aq. Ammonium sulfate
>30 µm - 50
Shear force Water 50 µm Energy storage 62
Freeze-thaw Anhy. THF/DMSO 4 µm Electronics and energy storage
63
Hydrothermal Aq. Glucose 20 – 100 µm Photodetector 28
with outstanding electrical properties. However, this method requires additional autoclave equipment and the exfoliation processes at high temperatures up to 220 °C. Therefore, there is a further demand for a simpler and more cost-effective approach to exfoliate huge lateral dimension graphene-based sheets, exhibiting high electrical and sensing performances.
1.7.2. Synthesis of reduced graphene oxide
RGO is one of the derivative materials of GO where the presence of oxygen functional groups is minimal, and its atomic structures have resorted to sp2 hybridized graphitic structure from sp3. The applications of GO and RGO are vastly different due to the difference in their properties, even though both are the similar derivative materials. The conversion process from GO to RGO can be achieved using various chemical reactions, thermal treatment, and electrochemical methods. The above techniques are used to remove the oxygen functional groups and restore the graphitic structure of GO. However, the RGO exhibits difference in surface morphology, and structural, electronic, and physical properties.
The conversion from GO to RGO can be observed by change in the colour to black from bright yellow, its hydrophobic nature, increase in the C/O ratio, high electrical conductivity, etc.
Table 1.2: The list of chemically reducing agents and its details to synthesise RGO
GO can be reduced to RGO chemically using hydrazine, borohydrides, aluminum hydride, hydrohalic acid, thiourea dioxide, methanol, etc. The list of chemically reducing agents and their details for synthesized RGO are mentioned in Table 1.2. Reduction of GO by using a chemical method has its limitations, such as a) it cannot retain the same lateral size
Reducing agent C/O ratio Conductivity (S/m) Expt.
parameters
Reference
Hydrazine 10.3 2420 100 °C, 24 hrs 64
NH3BH3 14.2 19300 80 °C, 12 hrs 65
LiAlH4 12 ---- 70 °C, 24 hrs 66
HI 12 29800 100 °C, 1 hr 67
Thiourea dioxide 14.5 ---- 90 °C, 1 hr 68
Methanol 4 3.2 ×10-5 100 °C, 5 days 69
Thiophene 10.9 ---- 80 °C, 24 hrs 70
of RGO as similar to GO, b) it is challenging to exfoliation into mono-layer, c) it becomes hydrophobic nature leads to agglomerate in a water solvent, d) this process produces harmful bi-products causes environmental hazard and e) unwanted doping on RGO, etc.
The thermal treatment process is the simplest and has overcome most of the limitations caused by chemical methods such as retaining the lateral size and monolayer RGO, which are significant for various applications, especially to fabricate 2-D devices with two-electrode terminals or transistors. In thermally reduced RGO, oxygen functional groups from the GO are removed in the form of CO2 and CO gas during heat treatment. Zhao et al.44 studied the thermal treatment of GO in a range of temperatures from 200 to 900 °C. They analysed the transformation of GO to RGO by using Raman and FTIR analysis and studied its capacitance properties.
1.7.3. Synthesis of graphene quantum dots
GQDs are another derivative material of GO. They have significant edge defects due to the breaking down from a large size to a small size (down to 1.53 nm), exhibit quantum confinement effect, offer large surface area, high photoluminescence, and minimum oxygen functional groups.
Table 1.3: List of GQDs synthesis methods and obtained sizes.
They are potentially used in various applications such as a photodetector, bio-medical sensors, solar cell, super capacitor, memory device, catalysis, etc.45 GQDs can be synthesized using both top-down and bottom-up approaches. GQDs can be synthesized from GO by using various methods such as hydrothermal, high-power sonication, chemical reactions, electrochemical processes, pulsed laser ablation, etc. The list of some of the protocols adopted for synthesis of GQDs is summarized in Table 1.3. The hydrothermal method is one of the most well-known and standardized methods to synthesize GQDs from the critic acid,
Techniques Source material Size (nm) Reference
Electrochemical Graphite rods 14 72
Pulsed laser ablation GO 3.4 73
Chemical reaction GO 5.6 74
Microwave Triethanolamine 5.6 75
Hydrothermal Citric acid 15 76
1,5-dinitronaphalene, 1,3,6-trinitropyrene, GO, etc. This process is time-consuming (12-24 hours), costly, requires high temperature and pressure. Moreover, it yields QDs with a non- uniform size distribution, that agglomerate after removal of oxygen functional groups from GO.