Due to the van der Waals forces between graphene layers, graphene sheets tend to restack and agglomerate, creating significant challenges in applications such as nanomaterials and biomaterials-based technologies.
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This article aims to provide insight into graphene functionalization approaches, one of the most prominent methods for improving the dispersibility of graphene sheets.
The Functionalization of Graphene and its Advantages
Graphene oxide is composed of oxygenated functional groups, the most prevalent of which are carboxyl, carbonyl, epoxy, and hydroxyl. These functional groups present on graphe are polar, making it very hydrophilic. However, when the oxygen functionalities in graphene oxide nanosheets are removed during the rGO preparation process, they lose their water solubility and consequently undergo irreversible aggregation.
Furthermore, graphene derivatives are easily aggregated in solvents due to strong π–π stacking or van der Waals forces, posing significant challenges during preparation for further application. T o overcome this issue, various functionalization processes are adopted to improve graphene dispersibility in water and other polar solvents.
The ultimate focus of functionalization is to improve graphene dispersibility in various solvents, which is a critical step in fabricating graphene-based nanocomposites for coating applications.
The functional groups in functionalized graphene (FGs) facilitate catalyst adsorption and intercalation, making FGs promising materials for catalytic applications. The functionalization of graphene improves not only its solubility but also the interaction between the nano reinforcing material and the polymer matrix.
The use of oxygen-containing groups in functionalization can improve charge transfer, which has immense potential for sensing and detecting organic and gas molecules. Thus, the study of graphene functionalization has greatly expanded graphene's application area. For the functionalization of graphene, two approaches have been taken. They are covalent and non-covalent graphene modifications.
Covalent Functionalization Approaches
Covalent functionalization occurs when a suitable functional group forms a covalent bond with graphene's sp2 carbon structure, specifically at the edges or basal plane. The introduction of functional groups via a covalent approach can be used to tune the aromatic character of graphene, which can result in a change in its electronic properties, improving solubility and stability and opening of bandgap.
Different covalent functionalization methods have been developed in general, including the formation of covalent bonds between the oxygen groups of GO and functional groups such as organic molecules, biomolecules, metal nanoparticles, polymers, and so on.
Graphene can be covalently functionalized directly with organic molecules such as epoxide, carboxylic acid groups, amino groups, hydroxyl groups. As an example, the carboxylic acid groups in the GO edge can be replaced with amine groups such as ethylenediamine and ethanolamine to improve dispersibility.
Furthermore, highly reactive intermediates like nitriles, carbenes, and aryl diazonium salts can be functionalized directly on the aromatic basal plane of GO. Upon functionalization, the sp2 hybridization of the basal plane could be altered to sp3 via this reaction. GO functional groups significantly improved dispersion stability in water, dimethyl sulfoxide and N, N-dimethylformamide (DMF), homogeneously, compared to non-functionalized GO.
Non-Covalent Functionalization Approaches
Non-covalent functionalization of graphene through π -interactions allows for the attachment of functional groups to graphene without interfering with the electronic structure of the material.
The functionalities are not covalently bound to graphene in this case; instead, they are typically adsorbed on the surface of graphene by relatively weak interactions., e.g., through cation−π interaction, anion−π interaction, H−π interaction, π–π interaction, and surfactant−π interaction.
For example, the π–π stacking interaction occurs between negatively charged π electron cloud of graphene and the foreign molecules having π orbitals. Similarly, cation−π interaction occurs in between π electron cloud of graphene and with positively charged metal cation such as Na+, Cu+, and Ag+.
Non-covalent functionalization of graphene has been shown to improve its water solubility since non-covalent functionalization will not change the intrinsic electronic properties. Hence, the organic molecules with electrochemically active groups have recently been introduced into graphene surfaces via non-covalent interactions, allowing them to be used as supercapacitor electrode materials.
Some aromatic dyes, such as methylene blue (MB), methylene green (MG) and congo red (CR), have recently been used to make non-covalently functionalized graphene sheets, which are primarily used in biosensors, solar cells, and electrocatalysis.
Applications of Graphene Functionalization
The interactions between the solvents and the functional groups reduce intersheet van der Waals forces and hydrogen bonding between GO nanosheets, increasing colloidal stability in solvents. For example, a sulfanilic acid group was chemically incorporated into GO nanosheets, resulting in improved water dispersibility due to ionic repulsion.
The covalent functionalization of reduced GO aerogel (rGOA) with polyaniline (PANI) improves electrical conductivity and prevents graphene nanosheet agglomeration while increasing the surface area of the rGOA. As a result, the capacitive performance of the PANI-grafted rGOA was higher (396 F/g at 10 A /g) than that of the rGOA (183 F/g at 10 A/g). These characteristics demonstrated that PANI-grafted rGOA is suitable for the high supercapacitor electrodes.
Many studies say that adding N and B-containing functional groups like amino acids and boric acids to the surface of GO can enhance its electrocatalytic activity.
Researchers from Spain's Instituto de Carboquimica demonstrated the possibility of covalently functionalizing GO with poly (vinyl alcohol) (PVA) to improve PVA's mechanical properties. As a result, the team demonstrated a 60% increase in Young's modulus and a 400% increase in tensile strength when compared to non-modified PVA.
Comparison Between Two Approaches
Covalent functionalization is typically based on the chemical bond formed between organic molecules and graphene, and it has the potential to significantly improve solubility and processability. Covalent functionalization is more stable and powerful than non-covalent functionalization. Furthermore, the electronic and chemical properties of graphene and its derivatives can be easily and effectively tailored using the covalent functionalization approach. However, introducing reactive molecules into graphene would increase the number of sp3-defects or even cause the carbon framework to rupture. As a result, the intrinsic electronic properties will be significantly altered.
On the contrary, non-covalent modification is usually preferable to take advantage of graphene's intrinsic properties. The non-covalent modification of graphene does not introduce defects into the graphitic lattice or disrupt its structure, preserving graphene's intrinsic electronic and mechanical properties.
Future Remark
The functionalization of graphene can improve its dispersion and impart new properties, which is important for broadening its applications. The functionalization approaches provide various ways to expand on current applications of graphene, such as, supercapacitors, bioimaging, fuel cells, coating, and drug delivery, as well as band gap opening, which can be used in electronics.
References and Further Reading
Cano, M., et al. (2013). Improving the mechanical properties of graphene oxide-based materials by covalent attachment of polymer chains. Carbon, 52, 363-371.
Punetha, V. D., et al. (2017). Functionalization of carbon nanomaterials for advanced polymer nanocomposites: A comparison study between CNT and graphene. Progress in Polymer Science, 67, 1-47.
Georgakilas, V., et al. (2012). Functionalization of graphene: covalent and non-covalent approaches, derivatives and applications. Chemical reviews, 112(11), 6156-6214.
Maio, A., et al. (2021). An overview of functionalized graphene nanomaterials for advanced applications. Nanomaterials, 11(7), 1717.
Abdelhalim, A. O., et al. (2021). Functionalization of graphene as a tool for developing nanomaterials with predefined properties. Journal of Molecular Liquids, 118368.
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