Chemical Approaches to Layered Materials: Special Issue of Advanced Materials

This special issue was edited by three Graphene Flagship scientists, Professor Xinliang Feng (Technische Universität Dresden, Germany), Professor Paolo Samorì (University of Strasbourg, France) and Professor Vincenzo Palermo (CNR – National Research Council of Italy, Italy). This issue showcases enlightening research into chemical approaches to layered materials from both within the Graphene Flagship and the wider community. By highlighting chemical approaches, it is possible to see how the structure of layered materials could be controlled at the atomic or molecular level to create multifunctional materials with exceptional properties.

The reviews collected in this special issue provide a timely overview of the different chemical approaches being explored in graphene and other layered materials. Explaining the motivation, Professor Palermo says “The idea behind creating this special issue was to give an update and a broad view of the state of the art of the chemical processing of graphene. At first, graphene was a game for physicists, with the initial focus on the physical properties; but now chemistry has come into play because the properties need to tuned for application in devices. The aim for this special issue is to show to all the people working on chemistry what others are doing at a world-wide level, including a good overview of what is going on in the Flagship.”

Talking more about the importance of good understanding of the chemistry of graphene and other layered materials, Prof Palermo adds “Graphene oxide is a good example of the difference between the chemical and physical approaches. Originally, it was the underdog of the graphene family because it is disordered and not conductive. However, if you look at practical applications, more and more people are now studying graphene oxide, as one can tune its properties more easily.  Now, big scale production of graphene oxide is a reality.”

The issue focuses not only on the synthesis of novel layered materials, but also on their covalent and non-covalent functionalisation, enabling their properties to be tailored for particular uses.  For example, these could be used in applications ranging from energy storage, harvesting and conversion, opto-electronics, and electro- and photocatalysis, all the way through to coating, foams, composites, membranes and nanomedicine.

Within the Special Issue, researchers from the Graphene Flagship provide detailed review of the recent work within their fields:

• New-Generation Graphene from Electrochemical Approaches: Production and Applications - Sheng Yang et al. discuss the production of graphene via electrochemical exfoliation.
• Supramolecular Approaches to Graphene: From Self-Assembly to Molecule-Assisted Liquid-Phase Exfoliation - Dr Artur Ciesielski and Professor Paolo Samori review the supramolecular functionalisation of graphene for exfoliation and processing.
• Nanoscale Mechanics of Graphene and Graphene Oxide in Composites: A Scientific and Technological Perspective - Dr Vincenzo Palermo et al. discuss in depth the challenges for graphene in composites whilst also clarifying the current position of graphene in industrial applications, taking through the graphene hype cycle.
• Biomedical Uses for 2d Materials Beyond Graphene: Current Advances and Challenges Ahead – Dr Rajendra Kurapati et al. give a thorough overview of the various biomedical applications for layered materials.
• 2d-Crystal-Based Functional Inks - Dr Francesco Bonaccorso et al. review of the different strategies and requirements for developing inks for printing layered materials.
• Carbon Nanomembranes - Professor Andrey Turchanin and Professor Armin Gölzhäuser provide a detailed review of the synthesis and functionalisation of carbon nanomembranes, including their use as a precursor for graphene.

Special Issue: Chemical Approaches to 2D Materials, Advanced Materials Volume 28, Issue 29, August 3, 2016, Page 6019

New-Generation Graphene from Electrochemical Approaches: Production and Applications

Sheng Yang, Martin R. Lohe, Klaus Müllen, and Xinliang Feng

Electrochemical exfoliation is a relatively new method of producing exfoliated graphene flakes. In an electrochemical cell containing a specially chosen electrolyte, graphene flakes are exfoliated from a graphite electrode. Ions from the electrolyte intercalate into the graphite, and the current through the cell causes the graphite to expand, breaking up into layers. The exfoliation process can be very fast with high yields at low cost, which, as the authors show, makes electrochemical exfoliation an excellent candidate for industrially scaled production of graphene flakes. As well as the potential for upscale production, the method is also versatile; the properties of the exfoliated graphene can be tailored by choice of electrolyte, additives, electrode and electrochemical potential to control the number of layers, lateral size, oxidation ratio and defect density of the flakes. This customisability means that the graphene properties can be specifically tailored for a range of applications including energy generation and storage, sensors, composites, as well as graphene papers and transparent conductive films.

Since the electrochemical reaction occurs so fast, the dynamics of the exfoliation process are not fully understood. The authors examine the open questions regarding the optimum conditions for consistent exfoliation of high quality monolayers without too much oxidation. These research questions are important targets for refinement of the exfoliation process for industrial scale-production. Nevertheless, electrochemically exfoliated graphene has been successfully demonstrated as composite electrodes for lithium batteries, as efficient catalyst carriers for a range of catalysis reactions, and in composites for anti-corrosion coatings and quantum dots. Exfoliated graphene has also been demonstrated in flexible and transparent electrodes and applied in organic photodetectors that match state-of-the-art silicon-based photodetectors. These wide-ranging achievements show excellent prospects for electrochemically exfoliated graphene, and coupled with the potential for upscaling production, point to a bright future for this versatile technique.

Further Reading

S. Yang, M. R. Lohe, K. Müllen, X. Feng, Adv. Mat. 28, 6213 (2016)

Supramolecular Approaches to Graphene: From Self-Assembly to Molecule-Assisted Liquid-Phase Exfoliation

Artur Ciesielski and Paolo Samorì

Functionalisation of graphene by covalent bonding with functional molecules tends to disrupt the conduction properties of graphene and degrade its electrical, thermal, and optical properties. As the authors show, supramolecular approaches to functionalisation – in which molecules are bound to the graphene sheet via non-covalent forces – provides a promising route to decorating graphene with functional molecules while tuning its properties in a more controlled manner. For example, non-covalently bonded molecules could be used induce carrier doping in the graphene sheet, or allow for easier deposition of insulators and semiconductors onto the graphene surface. In order to exploit this potential, the arrangements of the target molecules on the graphene surface must be well understood and controllable. Molecules that self-assemble into ordered patterns are therefore excellent candidates for supramolecular functionalisation of graphene.  As well, precise knowledge of the effects of the functionalisation on the underlying graphene is also needed.

Supramolecular chemistry also plays a role in improving the efficiency of liquid phase exfoliation techniques, as the authors describe. Liquid-phase exfoliation requires breaking the van der Waals forces holding graphene sheets together as graphite. In ultrasound-induced liquid-phase exfoliation, ultrasound is used to exfoliate graphene flakes and disperse them in a solvent. Using carefully chosen additives, the graphene dispersion can be stabilised against re-aggregation of the graphene flakes, by the formation of non-covalently bonded supramolecular complexes between the graphene flakes and the additive molecules. Additionally, the additive molecules can also lead to functionalisation of the graphene sheets, further tuning the properties of the graphene ink. Here too, understanding the chemistry of the supramolecular interactions is necessary for full control over the final properties of the graphene dispersion. While the supramolecular chemistry of graphene is relatively unexplored, this versatile and modular approach suggests an excellent route to tune physical and chemical properties.

Further Reading

A. Ciesielski, P. Samorì, Adv. Mat. 28, 6030 (2016)

Nanoscale Mechanics of Graphene and Graphene Oxide in Composites: A Scientific and Technological Perspective

Vincenzo Palermo, Ian A. Kinloch, Simone Ligi, and Nicola M. Pugno

Graphene’s excellent tensile strength, Young’s modulus, and flexibility make it attractive for structural reinforcement in composites. Structural composites are among the most mature graphene technologies, as several graphene-reinforced products are available on the market. However, as shown by the authors, there is still much room for improvement in this area, as the structural properties of these composite materials are considerably less than those of the graphene within them. Typically, the composites are constructed of a host polymer matrix with graphene flakes dispersed throughout the material, to absorb mechanical stress applied to the composite. There are several factors that affect the structural properties of the resulting composite, including the size of the graphene flakes, the quality of the graphene, and the bonding between the graphene and the polymer.

Chemical functionalisation of the graphene sheets is a promising method to improve the bonding between the graphene and the polymer. In particular, modifying the surface of graphene so that it is able to trigger polymerisation would lead to excellent interaction between the graphene and the polymer, with polymer chains growing directly from the graphene sheets. While modification and functionalisation of graphene tend to decrease its tensile strength and Young’s modulus, the increased contact between the polymer and the graphene might prove worth the trade-off, as the higher interaction will allow better transfer of stress to the flake, providing better reinforcement. In order to improve graphene structural composites, clear understanding of the different factors influencing the performance of graphene is necessary, and is a growing research area. The authors suggest that progress could be pursued through exploration of new chemistries of graphene, and new models of mechanical stress in graphene composites.

Further Reading

V. Palermo, I. A. Kinloch, S. Ligi, N. M. Pugno, Adv. Mat. 28, 6232 (2016)

Biomedical Uses for 2d Materials Beyond Graphene: Current Advances and Challenges Ahead

Rajendra Kurapati, Kostas Kostarelos, Maurizio Prato, and Alberto Bianco

Aside from graphene, other layered materials such as transition metal dichalcogenides, hexagonal boron nitride, transition metal dioxides and black phosphorous monolayers are promising for biomedical applications due to their unique physical and chemical properties. Stable dispersions of these and other layered materials can be obtained in water or other biologically relevant fluids through functionalisation or treatment with surfactants. As illustrated by the authors, these layered materials are being explored for use in biomedical applications such as tissue engineering, bioimgaing, and anti-cancer therapies and theranostics. In cancer therapies, combined chemo-, photothermal and photodynamic therapies have been shown alongside image-guiding, with functionalised transition metal dichalcogenides and oxides proving promising candidates. Inorganic layered materials such as laponite and layered double hydroxides have potential in tissue engineering, showing high biocompatibility and promotion of bone cell growth.  

As well as direct medical applications, layered materials are also interesting as biosensors and antimicrobial agents. Biosensors are important tools in diagnostics, forensics, and food safety, so there is high demand for new sensitive, selective and cost-effective sensing platforms. The semiconducting properties of layered materials such as transition metal dichalcogenides provide tunability in sensing, and their other properties such as fluorescence and high conductivity provide extra functionality to the biosensing platform. Biomedical and biological applications of layered materials are still in the early stages, and there is much research needed into the biocompatibility and toxicity of these materials in different formats, as well as their persistence in biological and environmental systems. The authors show that the wide range of layered materials with distinct electrical and chemical properties suggests that careful selection and optimisation will provide tailored solutions to different clinical demands.

Further Reading

R. Kurapati, K. Kostarelos, M. Prato, A. Bianco, Adv. Mat. 28, 6025 (2016)

2d-Crystal-Based Functional Inks

Francesco Bonaccorso, Antonino Bartolotta, Jonathan N. Coleman, and Claudia Backes

Graphene and other layered materials such as boron nitride and transition metal dichalcogenides can be exfoliated and processed into inks with different properties for a wide range of applications. Liquid-phase exfoliation (LPE), in which shear forces are applied to separate the layers into flakes in a suitable solvent, is a versatile technique that can be used to produce large quantities of inks, with high-quality, pristine flakes. While LPE has been successfully demonstrated for several different applications including composites, solar cells, flexible electronics and energy storage, there are several challenges that still remain, as discussed by the authors. One main concern is that the LPE produces a broad range of flake sizes in each dispersion, so a method is needed to sort the flakes into different lateral sizes and numbers of layers for use in different applications. Not only will this require development in LPE methods, but improvements in measuring techniques are also required to ensure that the inks can be fully characterised on the same scale as production.

In addition to the properties of the flakes, the authors illustrate several challenges regarding the formulation of the inks themselves. The liquid characteristics such as viscosity and density must be carefully tuned for different printing methods such as spray-coating, gravure and screen printing, and so on. Flake re-aggregation must also be controlled to prevent nozzles clogging in ink-jet printing. This is achieved by careful selection of solvent and additives; however, the demands are severe. For each different layered material, it must be possible to produce different inks with defined boiling point and viscosity in a non-toxic solvent. The development of high-quality, tailored inks of layered materials is of immense interest for applications in composites, sensors, and flexible electronics, among others, and excellent progress is being made in meeting the demanding requirements to fully exploit this versatile technique.

Further Reading

F. Bonaccorso, A. Bartolotta, J. N. Coleman, C. Backes, Adv. Mat. 28, 6232 (2016)

Carbon Nanomembranes

Andrey Turchanin and Armin Gölzhäuser

Carbon nanomembranes are synthetic materials that can have highly tailored physical and chemical properties, and at the nanoscale behave much like every-day plastic films. The authors outline the straightforward and flexible synthesis route, in which membranes are formed from self-assembled carbon-based molecules that can contain various functional groups for specific surface chemistry. The molecules are self-assembled onto a surface, and then subjected to radiation, either in the form of low-energy electrons or light. This energy breaks the bonds within the molecules, and new bonds are formed in a network across the surface – joining the molecules into a new nanomembranes. The nanomembranes can then be transferred to different surfaces, or used as freestanding layers. The carbon nanomembranes are around 1 nm thick, and – due to the controllability of the electron-beam or laser irradiation – can be formed over specifically targeted areas or patterns. Not only this, but the nanomembranes can also be annealed at high temperatures to form graphene at nano- to microcrystalline scales and controllable patterns.

As shown by the authors, the carbon nanomembranes have exciting applications in the formation of electronic and optoelectronic devices, as well as in biosensors, where their thinness and controllable functionalisation will lead to high sensitivity and selectivity. The membranes can also be functionalised differently on each side, which provides excellent opportunities for multi-functional sensors, or as energy funnelling devices in artificial photosynthesis with photoactive functionalisation.  However, the ultimate application envisioned for these nanomembranes is in nano-scale separation and filtration of gases and liquids. The nanomembranes are incredibly thin compared to the current state-of-the-art in filtration, and their thinness leads to lower pressure differences for good filtration. While carbon nanomembranes are well-established, there is still huge potential for many applications in this straightforward and versatile synthesis method.

Further Reading

A. Turchanin, A. Gölzhäuser, Adv. Mat. 28, 6075 (2016)
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