Polymer Functionalized Graphene

Arun Kumar Nandi

• Publisher: Royal Society of Chemistry
• Year: 2021
• ISBN:
  9781788019682
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

There is an immense variety of research on polymer functionalized graphene (PFG). Functionalization of graphene is necessary for improvement of the compatibility with polymers. Applications of these graphene polymer hybrids include in chemical and biological sensing, photovoltaic devices, supercapacitors and batteries, dielectric materials and drug/gene delivery vehicles. This book will shed light on the synthesis, properties and applications of these new materials, covering two methods (covalent and noncovalent) for producing polymer functionalized graphene. Chapters cover physical, optical, mechanical and electronic properties, applications of polymer functionalized graphene in energy harvesting and storage, and uses in biomedicine and bioengineering. Written by an expert in the field, Polymer Functionalized Graphene will be of interest to graduate students and researchers in polymer chemistry and nanoscience.

CHAPTER 1

Introduction

A short history of graphene is discussed, including the importance of graphene. The synthesis of graphene, graphene oxide, reduced graphene oxide and graphene quantum dot is introduced. The characterization of the above materials using different spectroscopic techniques is also introduced. A brief discussion of the optoelectronic properties of the materials and the necessity of functionalization, specifically polymer functionalization is discussed in detail.

1.1 Introduction

After the invention of X-ray crystallography, the layered structure of graphite was unveiled and was found to be a deck of weakly bonded graphene planes. Graphene is a two-dimensional, sp² hybridized flat crystalline form of carbon atoms in a hexagonal lattice structure (Figure 1.1). ^1,3

Figure 1.1The crystal structure of graphene – carbon atoms arranged in a honeycomb lattice. Figure By AlexanderAlUS, Reproduced from https://en.wikipedia.org/wiki/Graphene#/media/File:Graphen.webp under the terms of the CC BY-SA 3.0 license, https://creativecommons.org/licenses/by-sa/3.0/.

This highly optically transparent and mechanically strong material has exciting electrical properties and heat conductivity, leading to many applications in technology. It has a C–C bond length of 0.142 nm and an atomic thickness of 0.345 nm. Its tensile strength of ∼0.4 GPa is much higher than steel, its Young's modulus is 500 GPa, and it is more elastic than rubber. ⁴ Due to these incredible properties it is much studied in chemistry, physics, materials science, engineering and also in biology.

1.2 Short History of Graphene

Sporadic attempts at graphene synthesis started in 1859,⁵ but pre-2004 results were experimental, where they obtained ultra-thin graphitic films, but did not report any of graphene's characteristic properties. Earlier attempts at producing graphene were concentrated on a chemical method of exfoliation. Bulk graphite was first intercalated⁶ to separate layers by intervening atoms or molecules. The insertion of large molecules between atomic planes causes greater separation so that it results in isolated graphene layers into a 3D matrix. However, due its uncontrollable nature, the graphitic muck found very low interest. In another method, few-layer graphene was grown epitaxially by chemical vapour deposition (CVD) of hydrocarbons on metal substrates.^7,8 Also in other techniques, SiC on thermal decomposition yielded graphene films that were characterized using surface science techniques. ^9,10 In 2004, Novoselov et al. first isolated graphene in large quantity using a simple ‘Scotch tape method’. ^1,2 This micromechanical cleavage (Scotch tape) method was very simple and effective for growing graphene science very quickly. This technique does not require sophisticated equipment, hence it helped to grow graphene science enormously. For their new Scotch tape method of synthesizing graphene, correct characterization and delineating the beautiful physics of graphene, Geim and Novoselov of University of Manchester, UK obtained the Nobel Prize in Physics in the year 2010. Another physical method to prepare graphene is the thermal expansion of graphite followed by ultra-sonication in an aqueous medium yielding dispersion of graphene sheets ^11,12 and this has also gained popularity to produce graphene. Then chemists become interested in synthesizing graphene by oxidizing graphite followed by exfoliation on sonication and the produced graphene oxide is then reduced chemically to get reduced graphene oxide in plentiful amounts. The preparation methods of graphene, graphene oxide, reduced graphene oxide and graphene quantum dots are described in detail below.

1.3 Synthesis of Graphene

The different methods of producing graphene from graphite are presented in Scheme 1.1.

Scheme 1.1 Schematic illustration of different methods to obtain graphene from graphite Reproduced from ref. 24 with permission from Elsevier, Copyright 2015.

It can be produced in a ‘top–down’ method to obtain a single-layer graphene from graphite crystals, or using a ‘bottom–up’ method to form graphene on self-assembling small molecules used to create graphene, respectively. Among the top–down methods the mechanical peeling by the ‘Scotch tape technique’, forms a single-layer graphene material. This is made by repeated peeling of small mesas of greatly oriented pyrolytic graphite. It is the first method to produce large quantities of good quality of graphene, and is the original method of its invention. ^1,2 The films are two-dimensional semimetal having small overlap between conductance and valence bands, exhibiting a ambipolar electric field effect generating holes and electrons in concentrations up to 10¹³ cm⁻² with mobilities of about 10 000 cm² V⁻¹ S⁻¹ on application of gate voltage.

Another top–down technique is the liquid-phase exfoliation where delamination is done using exfoliating agents (usually solvents) to prevent the overlap between adjacent layers by making colloidal dispersions. For example, graphene powder mixed with N-methyl pyrrolidone (NMP) is sonicated vigorously producing a colloidal dispersion of graphene and removing the colloidal aggregates by centrifugation. ¹³ This is possible because the required energy to exfoliate graphene is coming from the graphene–solvent interaction, particularly for solvents whose surface energies match with that of graphene. N,N′ dimethyl formamide (DMF) has also the similar property to exfoliate graphene and the yield of graphene could be up to 12% of the graphite used for exfoliation. Then a stabilizer is used to stop restacking of the graphene layers and thus produce stable graphene sheets.

In a bottom-up technique of producing graphene, chemical synthesis is used to produce small-sized graphene or graphene nanoribbon (GNRs). In a microwave plasma reactor substrate-free gas-phase synthesis of graphene is well reported. ¹⁴ Also arc discharge synthesis ¹⁵ of multi-layered graphene is reported. GNRs are produced from the reaction between high aromatic rings and polycyclic aromatic hydrocarbons. However, the most current and trending method to create graphene from the bottom–up approach is via chemical vapour deposition (CVD), which is well established, providing the material in sufficient quantity and requires an easy laboratory set-up. In the CVD process, reactive gases (methane, etc.) at a regulated rate are passed through a gas-mixing unit mixing the gases uniformly before feeding into the reactor. In the reactor a chemical reaction occurs and the solid products become deposited on the metal substrates. The reactor has a heating arrangement to maintain the high temperatures required for the reaction. The source of C is mostly CH₄, which is catalytically dehydrogenated on the Cu surface, then a solid solution of C is formed near the metal surface, resulting in graphene. This route facilitates surface migration and monolayer graphene growth. A schematic model of the working process is shown in Figure 1.2. ¹⁶

Figure 1.2 Schematic illustration of the chemical vapour deposition process: (1) Diffusion of reactant precursors through the boundary layer. (2) Adsorption of reactants onto the substrate surface. (3) Onset of a chemical reaction at the substrate surface. (4) Desorption of the adsorbed product from the surface. (5) diffusion of by-products through the boundary layer. Reproduced from https://commons.wikimedia.org/wiki/File:Sequence_during_CVD_%28en%29.svg under the terms of the CC BY-SA 3.0 license, https://creativecommons.org/licenses/by-sa/3.0/deed.en.

The by-products of reaction with the unused gases get removed by the gas outlet system. Graphene is grown on the surface of metals by decomposition of hydrocarbons or by segregation of carbon. ¹⁷ To improve graphene formation and to obtain a smooth graphene surface, treatment of the metal substrate before beginning the CVD process is generally done. Wet chemical treatments are made for metal films to avoid oxide reduction by soaking in acetic acid, etc. Then, a metal substrate, is heated under low vacuum in the CVD process. Good quality gra...