Nanomaterials: Graphene and graphene oxide

Graphene and its applications

1. Hydrazine-reduction of graphene oxide Colloidal suspension of graphene oxide platelets in purified water (3g/L) is prepared by sonication of GO in ultrasonic bath for 3h Hydrazine monohydrate added to the GO suspension Stirring in an oil bath at 80  C for 12 h

2. Synthesis of graphene Chemical Vapour Deposition (CVD) Among various approaches, CVD using transition metal substrates is considered most promising, inexpensive and feasible method to produce single layer or multi-layers graphene A tube furnace for high temperature heating, a quartz vacuum chamber, a vacuum and pressure control system for the growth condition adjustment, and several mass flow controllers to provide carbon source and reactant gases with required flow rate

3. Graphene deposition is divided into two parts: 1. Precursor pyrolysis to carbon 2. Formation of graphitic structure from dissociated carbon atoms The precursor dissociation should be done only on the substrate surface (i.e. heterogeneous reaction) to avoid the precipitation of carbon clusters in the gas phase which is typically in the form of carbon soot [ Sits on synthesized graphene ( graphene /soot mixture)]. Soot sits also on reactor walls For heterogeneous decomposition of precursors on surface, various catalysts of elemental metals are typically used.

4. Requirement of catalysts In addition to the undesirable need for very high temperature, high energy barriers result in reaction rates to be dependent on temperature making it difficult to control the reaction rate Because the film quality is determined by reaction kinetics, high energy barrier leads directly to difficulty in controlling the film quality In the graphitic structure formation step, large area graphitic structure forms only when reaction temp. is raised beyond 2500  C without catalysts , which is too high & requires special setup Introduction of catalysts lowers energy barriers not only for the pyrolysis of precursors, but also for the graphitic structure formation

5. Using Cu as catalyst Annealing at high temp. in H 2 environment is to remove the native oxide layer on the Cu surface, while Cu grains will also develop With the exposure of Cu foil in CH 4 /H 2 environment, nucleation of graphene islands take place randomly but mostly at the grain boundary of Cu surface Exposure to CH 4 continues-- graphene domains grow in size & eventually aggregate into a continuous graphene film

6. Graphene growth mechanism in CVD CVD method has shown the capability for growing high-quality graphene film & controlled graphene nucleation over a large area

7. Mechanism I : Carbon atoms surface segregation Decomposed carbon atoms diffuse into the catalyst bulk at high temperature. Then, carbon atoms precipitate on the catalyst surface during cooling period Mechanism II : Carbon atoms surface deposition Carbon atoms are directly deposited on catalyst surface without segregation resulting in a graphene layer

8. Method I: Impact of Cooling rate

9. Transfer mechanism for device fabrication ** Spin-coating a thin polymeric layer, such as PMMA or PDMS on top of the as-grown graphene for support to graphene framework ** Underneath Cu substrate is etched away by FeCl 3 soln , HCl , HNO 3 etc ** After the Cu dissolution, the floating membrane is placed on a desired substrate. After drying, the polymeric film is dissolved with acetone or chloroform In another approach a thermal release tape is coated instead of PMMA. After Cu removal, the tape is placed on target and passed through roller using mild heat— Graphene on PET can be made by this method to function as touch screen

10. Electrolytic exfoliation DC bias Graphite rod Graphite rod Electrolyte Materials: 1.High purity graphite rods 2.Poly(sodium-4-styrenesulfonate) 3.De-ionized (DI) water Processes: 1.Apply constant potential of 5V 2.Dispersion obtained is centrifuged 3.Washed with DI water and ethanol 4.Dried to make powder 5.Vacuum filtration using Anodic Aluminium Oxide to obtain graphene paper When PSS dissolved in water, it will dissociate into Na + & PSS - ions During the electrolysis PSS ion is forced to move to the + ve graphite electrode and interact with graphite, leading to the electrolysis exfoliation of the graphite rod. Mechanism

11. Synthesis of graphene on glass by plasma-enhanced CVDs Low temperature PECVD using pure CH 4 as the precursor Without aid of metallic catalyst , uniform, transfer-free vertically oriented graphene flakes can be directly formed on various glass substrates Temperature range is 400-600 °C RF Plasma used

12. Mono- and multilayer graphene sheets are produced from commercial graphite powder in a wet grinding process under mild ball milling conditions The shear forces in the milling chamber lead to a continuous delamination of ultrathin graphene flakes which are dispersed in a liquid medium To avoid agglomeration of exfoliated flakes anionic surfactant sodium dodecyl sulfate is used Process parameters adjusted to overcome the weak interlayer forces without breaking them leads to fabrication of thin flakes with high aspect ratios A high-yield and low-cost production Synthesis from commercial graphite by direct mechanical grinding

13. Wet-spun graphene fibers (GFs) exhibit great potential as a straightforward method for high-performance fibers in electronic devices, sensors, and supercapacitors , owing to their ultralight weight, good mechanical strength, electric conductivity and possibility of large-scale production. Graphene oxide (GO) with abundant oxygen functional groups on its basal planes and edges can form colloidal dispersions of single sheets in water and other polar organic solvents. Aqueous GO solution is continuously spun into metres of macroscopic GO fibers . Subsequent chemical reduction gives the macroscopic neat GFs with high conductivity ( ∼ 2.5 × 10 4 S m −1 ) and good mechanical performance ( ∼ 140 MPa at ultimate elongation of 5.8%). The flexible, strong GFs can be knitted into designed patterns and into directionally conductive textiles . Dry spinning of GO fiber On a homemade dry spinning apparatus, GO organic dopes are extruded through a spinneret with an inner diameter of 130 µm at a rate ranging from 1 to 10 mm s−1 . Infrared light is used to heat gel fibers to form dry fibers before collection. The reel together with the fibers is transferred into an oven and dried thoroughly at 60 °C for 12 h.

14. Biosensing by Graphene Graphene does not oxidize in air or in biological fluids, making it an attractive material for use as a biosensor Consists of a recognition layer, a transducer as well as electronic components R ecognition layer determines biological response which is further converted into an electrical signal with the help of the transducer Electrical signal is then amplified and processed by the external electronic system Recognition Layer Targets

15. Scaffolds for bioengineered organs Surface area of a monolayer graphene is approx. one order magnitude higher than other biomaterials making it a very interesting biomaterial Graphene flat surface nature along with its mechanical properties can be exploited for applications such as structural reinforcement of biocompatible films & scaffolds for bioengineered organs

16. Processing Using lasers to tailor the properties of graphene The laser beam can be focused precisely on graphene film to produce distinct behaviour useful for producing devices The key is the use of short, highly controlled laser pulses, which will induce chemical changes in the carbon lattice Graphene lattice can be patterned by cutting, adding external molecules or binding compounds (functional groups like oxygen or hydroxyl etc.)

17. Graphene with outstanding anti-irradiation capacity   Graphene with high mechanical strength, chemical inertness and outstanding anti–irradiation capacity can be used as lubricant additive for space application MECHANICAL PROPERTIES OF COMPOSITES WITH GRAPHENE INCLUSIONS Due to ultrahigh strength of graphene, its inclusions are effectively used in enhancement of both strength and fracture toughness in composite materials Ex. Fabricated silicon nitride + 1 wt.% graphene platelets (GPL; thickness: 1-10 nm) nanocomposites with relatively homogeneously dispersed GPLs exhibit 50% enhancement in the fracture toughness

18. Ultrafiltration Membranes with Graphene-Oxide Coatings for Oil/Water Separation In oil/water separation, water preferentially permeates through GO coated membranes over oil, forming numerous water pockets inside the structure which serve as a repulsive support for oil droplets GO coating which has been proved to be hydrophilic help trap the water, reduces the effective permeation pore size, and introduces additional nano-scaled roughness for smaller oil droplets repulsion Oil droplets are thus repelled to the membrane surface and accumulate to form bigger droplets in size Then, the oil is released from the underwater superoleophobic GO membrane surface and floats on top of oil/water mixture

19. Graphene for desalination Perforated graphene filters can handle the water pressures of desalination plants while offering much better permeability The energy required for pumping is also much lower The challenge is to find the sweet spot of about 0.8 nanometers . If pores are at 1.5 nm, then both the water and salt will pass through. If they’re 0.5 nm, nothing gets through Nanometre-sized pores are created in a graphene monolayer using an oxygen plasma etching process , which allows the size of the pores to be tuned Resulting membranes exhibit a salt rejection rate of nearly 100% and rapid water transport Water fluxes of up to 10 6  g m −2  s −1  at 40 °C are measured using pressure difference as a driving force.

20. Super-stretchable Graphene Oxide Macroscopic Fibers Using bar coating and drying of water/GO dispersions large-area GO thin films can be prepared with an outstanding mechanical behaviour and excellent tear resistance Dried films are scrolled to prepare GO fibers with extremely large elongation to fracture (up to 76%), high toughness (up to 17 J/m 3 ), and attractive macroscopic properties, such as uniform circular cross section, smooth surface, and great knotability Electrical conductivity increases to 416 S/cm at room temperature

21. Composite Materials Graphene is strong, stiff and very light-- Currently, aerospace engineers are incorporating carbon fibre into the production of aircraft as it is also very strong and light Graphene is much stronger and much lighter --it can be used as polymer composite to create a material that can replace steel in the structure of aircraft, improving fuel efficiency, range and reducing weight   Electrical conductivity —Can be used to coat composite aircraft surface to prevent electrical damage resulting from lightning strikes Structural health monitoring - Graphene coating can also be used to measure strain rate, notifying the pilot of any changes in the stress levels on the aircraft wings   Body Armour : High strength requirement applications such as body armour for military personnel and vehicles

22. Photovoltaic Cells Low production cost, high flexibility and high electron mobility means that graphene can be used as an alternative to silicon or ITO in the manufacture of photovoltaic cells Silicon cells are very expensive to produce, graphene based cells are not expensive Silicon turn light into electricity it uses a photon for every electron produced-- graphene absorbs a photon and generates multiple electrons Silicon is able to generate electricity from certain wavelength bands of light -- graphene is able to work on all wavelengths Flexible and thin means that graphene based photovoltaic cells could be used in clothing

23. Energy Storage Present problem : Battery can potentially hold a lot of energy, but it can take a long time to charge Capacitor can be charged very quickly, but can’t hold that much energy The capabilities of lithium ion batteries can be enhanced by incorporating graphene as an anode to offer much higher storage capacities with much better longevity and charge rate Supercapacitors which are able to be charged very quickly, yet also be able to store a large amount of electricity --  Graphene based micro- supercapacitors are being developed for use in low energy applications such as smart phones and portable computing devices

24. Spintronics Large-area graphene created by CVD on a SiO 2 substrate can preserve electron spin  over an extended period & communicate. Spintronics  varies electron spin rather than current flow. The spin signal is preserved in graphene channels over a nanosecond with a spin lifetime of 1.2 ns. Spintronics is used in disk drives for data storage and in magnetic random-access memory . Electronic spin is generally short-lived and fragile. As the spin-based information in current devices needs to travel only a few nanometers , it performs satisfactorily. However, in processors, the information must cross several tens of micrometers with aligned spins. Graphene is the only known candidate for such behaviour.

25. Conductive ink Fully integrated conductive circuit consists of graphene - which shows excellent conductivity and can also be flexed and wrinkled without damage to the circuit. Packaging uses flexographic roll-to-roll printing to process graphene -based Ink. The circuit is completed with reusable electronic module that gives the alarm sound via an integrated speaker if removed from the package or other damage to the circuit. ***This was the world's first commercially available product based on graphene Infrared light detection Graphene reacts to the infrared spectrum at room temperature, with sensitivity 100 to 1000 times-- too low for practical applications . However, two graphene layers separated by an insulator allow an electric field produced by holes left by photo-freed electrons in one layer to affect a current running through the other layer. The process produces little heat, making it suitable for use in night-vision optics. The sandwich is thin enough to be integrated in handheld devices, eyeglass-mounted computers and even contact lenses.

26. Ultrafiltration Outstanding property of graphene oxide : Only material that allows water to pass through and being almost completely impervious to gases and liquids- - Very small molecules of gases such as Helium molecules cannot pass through this wonder material ***** This phenomenon has been used for further distilling of vodka to higher alcohol concentrations, in a room-temperature laboratory, without the application of heat or vacuum as used in traditional distillation methods Pressure sensors The in-plane pressure sensor consisting of graphene sandwiched between hexagonal boron nitride (h-BN) The electronic properties of the sandwiched structure can be modulated by changing the interlayer distances via applying external pressure Because h-BN and graphene are inert to high temperature, the device can support ultra-thin pressure sensors for application under extreme conditions. *** A biocompatible pressure sensor is made from mixing graphene flakes with cross-linked polysiloxane .

27. Radio wave absorption Stacked graphene layers on a quartz substrate increases the absorption of millimeter (radio) waves by 90 per cent over 125 – 165 GHz bandwidth, extensible to microwave and low-terahertz frequencies, while remaining transparent to visible light. For example , graphene could be used as a coating for buildings or windows to block radio waves. Coolant additive Graphene's high thermal conductivity suggests that it can be used as an additive in coolants. Preliminary research work showed that 5% graphene by volume can enhance the thermal conductivity of a base fluid by 86%. Oil spillage management Polystyrene/Fe3O4/ Graphene aerogel removes about 40 times its weight even after 10 repetitive cycles