A reflection on “Formation and processability of liquid crystalline dispersions of graphene oxide”

Abstract

Over the last decade, significant progress has been made in researching liquid crystal (LC) dispersions derived from graphene oxide and, subsequently, from other two-dimensional (2D) materials. Our communication in Materials Horizons in 2014 (R. Jalili, S. H. Aboutalebi, D. Esrafilzadeh, K. Konstantinov, J. M. Razal, S. E. Moulton and G. G. Wallace, Mater. Horiz., 2014, 1, 87–91, https://doi.org/10.1039/C3MH00050H) provided a foundation for further research. It outlined the critical role of interactions between graphene oxide (GO) sheets and water and the importance of sheet size in determining the ability to exhibit LC behaviour, enabling control of rheological properties and hence rendering dispersions amenable to subsequent fabrication processes such as fibre spinning and 3D printing. The primary objective of our research was to clarify the complexities involved in determining the production and processability of these dispersions, thereby establishing a forward-thinking foundation that has had a lasting impact on the practical applications of graphene, other 2D materials and structures containing them.

1. Introduction

Our first article in Materials Horizons a decade ago contributed towards understanding critical factors influencing the processability of graphene and its derivatives (https://doi.org/10.1039/C3MH00050H).¹ This paved the way for a significant period of research that advanced bottom-up device fabrication. Our study sought to unravel the complexities of controlling the formation and processability of graphene oxide-based dispersions, providing a contribution that has since resonated within the scientific community.

The emergence of graphene sparked a surge in intensive research, yielding a wealth of knowledge on novel 2D materials.² Despite the remarkable properties of graphene measured at the nanoscale, widespread incorporation into real-world commercial applications faced obstacles due to the limited options available for processing and fabrication.³ Achieving solution processability and straightforward incorporation into three-dimensional structures proved critical for graphene and many other nanoscale materials.

Our journey began with recognising that the promise of graphene and other 2D materials could only be fully realised by developing reliable methods to manipulate them in a highly dispersed solution. Our contribution guided the scientific community, showing how to produce processable graphene suitable for standard large-scale fabrication methods like fibre spinning, 3D printing, and coating methodologies. The approaches developed ensured the preservation of graphene’s nanoscale properties and the integration into the production of new devices.

Unlike any other, the liquid crystalline phase in graphene oxide (LCGO) provides an unprecedented handle for aligning, manipulating, and processing graphene on a large scale, offering a customisable approach to material design. However, the production of LCGO was uniquely challenging due to the considerable shape anisotropy of the mesogenic units and the polydispersity in size and shape. LCGO has demonstrated numerous applications in various fields, including structural materials, electrical and thermal materials, separation media, energy storage and conversion, catalysis, and model display technologies. Readers interested in learning more about the characteristics and applications of LCGO can access reviews that we, as well as other researchers, have conducted on the subject.^4–8

As we reflect on our initial publication, we recognise the lasting impact of LCGO on various application fields. Our work laid a foundational understanding of the fundamental principles governing liquid crystalline dispersions and their practical implications in many other 2D materials. The journey from that seminal publication has set the stage for ongoing advancements, and we remain poised at the forefront of a dynamic and evolving field that continues to make possible the myriad applications of graphene derivatives and 2D materials.

2. The originally published findings

The study tackled the persistent challenge of controlling the properties of graphene derivatives in bottom-up device fabrication, with a critical focus on fine-tuning dispersion properties as a crucial step for subsequent fabrication. The journey began with optimising GO sheet dimensions, ranging from small to extremely large, across a broad spectrum of aspect ratios while preserving their liquid crystalline properties.¹ This undertaking required an understanding of the core forces and complexities associated with the soft self-assembly of GO sheets. The interaction between steric hindrance, excluded-volume effects, and the amphiphilic properties of solvated GO sheets has been identified as crucial factors that affect the soft self-assembly of LCGO colloidal suspensions. Our theoretical study provided a guiding structure, emphasising the role of entropy in the formation of nematic liquid crystals for such anisotropic-shaped particles with high polydispersity. Thus, it examined the soft-self-assembly aspects of LCGO dispersions, providing insights into the conditions necessary for controlling GO sheet sizes across a broad range while preserving liquid crystallinity.

Our system, crucially, deviated from theoretical considerations of completely rigid rods by incorporating the flexibility of GO sheets into the equation. This flexibility, associated with configurational entropy, significantly impacted entropy loss in the nematic phase. We investigated the effects of electrostatic interactions, steric hindrance, and the role of solvent molecules in the structural formation of LCGO dispersions. Then, the relationship between GO sheet size, theoretical concentrations for isotropic–nematic phase transition, and LC formation paved the way for a comprehensive understanding of LCGO dispersions’ rheological behaviour. Non-Newtonian properties such as yield values and viscosity differences demonstrated the transition between isotropic and nematic phases, essential for industrial processes such as wet spinning. To this end, we investigated modifying LCGO dispersions for graphene fibre production. We systematically induced liquid crystallinity in small sheet GO dispersions by introducing ultra-large GO sheets into isotropic dispersions, providing a route to spinnable LCGO dispersions. This novel method demonstrated the interaction of excluded volume effects, entropic rearrangement, and the competitive nature of various-sized GO sheets.

3. How we have built upon this

Building upon the similarities between polymers (1D carbon chains) and LCGO, a 2D network of carbon molecules, we have further expanded our understanding.⁹ From this perspective, we demonstrate how the polymeric resemblance can be leveraged for subsequent additive manufacturing processes by utilising established, scalable polymer processing methods. Unlike polymers, where flow behaviour is determined by factors such as chain entanglement, functionality, and side chains, LCGO provides a distinct alternative pathway. Its sheet size, solvent, functionality, and liquid crystalline phase allow for the tuning of rheological properties to best fit a specific application. This comparison implies that graphene-based devices can be produced with current processing and fabrication tools if they are comprehended and managed. This would close the gap between industrial adoption and graphene-based device applications.

Using the liquid crystalline properties of GO, we measured the bending rigidity of graphene directly for the first time and provided some insight into its extreme mechanical flexibility.¹⁰ This quality allows for flexibility during processing without compromising the strength of the final fabricated structures. We used synchrotron radiation to study LCGO sheets undergoing shear-induced alignment, which is critical for creating flow-aligned structures and devices. We also learned about the material’s thermal fluctuations and bending rigidity. The alignment of LCGO particles under flow challenges conventional beliefs, with fractions aligning in different directions based on flow velocity and viscous forces shrinking the spacing between flakes. This behaviour impacts the material structure and properties. It facilitates the measurement of graphene bending rigidity, which we found to be within a similar range as that of liquid self-assembled bilayers. LCGO exhibits super flexibility by disrupting the aromatic system and altering dihedral angles in the graphene structure, providing a significant advantage in developing highly bendable coatings, films, composites, and fibres. This unique combination of in-plane stiffness and flexibility allows for versatile processing and fabrication, ensuring the final structure’s robustness.

After determining the realisation and processability of LCGO, it became clear that further research into their rheological behaviour, particularly as a function of liquid crystal phase and flakes’ physical properties, was required. This quest aimed to identify the optimum rheological properties needed to expand the application of LCGO to a new level. Our subsequent investigation served as a crucial facilitator, ready to enhance the effectiveness of LCGO in a range of processing methods, including but not limited to 3D printing, coating methodologies, and fibre spinning.⁹ The motivation for this effort was to fully realise the potential of LCGO and, subsequently, other 2D materials, allowing for easy integration into a wide range of industrial processes and applications. Using this knowledge as a foundation, the production of graphene fibres became a focus, a novel discovery with vast potential for two-dimensional materials science and technology. Our graphene fibre produced from LCGO dispersions represented a significant development in fabrication, paving the way for new opportunities in the carbonaceous and 2D material domains.¹¹

The development of graphene fibre, initially achieved in our lab through the wet spinning of LCGO dispersions, is a notable example demonstrating high electrical and mechanical performance and scalability.¹² Using cost-effective graphite as the starting material, graphene fibres can be directly synthesised from LCGO, followed by chemical reduction to restore their electrical properties. Unlike earlier efforts that produced high-performance synthetic fibres using carbon nanotube (CNT) solutions in superacid, our novel approach utilises aqueous LCGO dispersions. This enables large-scale fabrication of robust conducting fibres in a single step without harsh temperatures, super acids, or environmentally damaging chemicals. Subsequently, LCGO and graphene fibres gained significant attention from the scientific community, as evidenced by a notable increase in publications and projects dedicated to their exploration.¹³ We have also successfully scaled up our lab processes via the Australian National Fabrication Facility’s (ANFF)-Materials Node. It is worth noting that the scalability of LCGO has been successfully demonstrated, with batches of 20 litres containing five wt% LCGO, being readily obtained.

The spontaneously observed highly ordered liquid crystalline behaviours of spinning solutions enable the creation of intricate three-dimensional graphene fibre architectures. Furthermore, wet-spun graphene fibres possess intrinsic self-orientation characteristics, enhancing their physical properties. The combination of these technological advances highlights the great potential of graphene fibres and positions them as leaders in the quest for new materials to incorporate graphene into innovative applications. To this end, their superior attributes, including high porosity and conductivity, make them more appealing than conventional carbon fibres, particularly in fields such as electrochemistry and biomedicine. In our laboratory, we have already demonstrated the versatility of graphene fibres in various applications, including supercapacitors, batteries, strain sensors, composites, plasma catalysis, ammonia synthesis, implantable microelectrodes, and a wide range of new cutting-edge applications based on graphene integration.^14–24

These fibres facilitated the development of implantable electrodes, critical for recording human neural activity in biomedical research. For neural systems and machines to communicate with each other effectively, it is essential to develop precise, low-cost microelectrodes. These characteristics include microscale size (<50 μm), similar to individual neurons, low impedance, large surface area, significant charge injection capacity, flexibility, strength, and biocompatibility. Our graphene fibres emerged as a promising candidate to meet these high standards.²⁵

By adjusting the wet-spinning apparatus, we controlled the diameter of graphene fibres to below 50 μm, approaching the size of individual neurons. The inherent liquid crystalline properties facilitated the self-ordering of graphene sheets, resulting in a large surface area and flexibility resembling the structure of human tissue. Using these advantageous properties, we created high-performance microelectrodes for recording and detecting neuronal activity. Surface modifications such as polymers and Pt coatings were used to reduce impedance and improve biocompatibility, ensuring electrochemical and mechanical compatibility with surrounding tissues. Our modified graphene microelectrodes had a higher charge injection capacity, lower specific impedance, and larger specific surface area than previously reported microfibers and commercial electrodes. In vivo cortical neural recording experiments with liquid crystalline-based graphene fibres as microelectrodes, showed intriguing findings. These graphene fibre microelectrodes had a high signal-to-noise ratio of 9.2 dB and effectively transmitted signals from rat neurons.²⁵ This exercise inspired numerous additional research efforts within our team and among other researchers. Some of these efforts are referenced here for readers who want to learn more about graphene fibre-based implantable electrodes, which we first introduced.^26–30

Further advances from our research included extending LCGO dispersion formation in organic solvents and composites.³¹ This advanced our understanding of the solvophobic effect and the factors influencing the self-assembly process in composites. This expansion improved the self-assembly process and lyotropic liquid crystallinity, enabling the development of processable, self-assembled, self-oriented hybrid composites with superior mechanical and electrical properties.^19,22,23 These composites retain both the functionality of the added nanomaterials and the inherent orientation of LCGO.

Scaling up complex structures to innovate processes and systems is possible by integrating an emerging technology with a well-established application. This methodology holds promise for a variety of applications, including electrocatalysis, supercapacitors, and batteries, as it requires scaffolding with 3D conductive substrates, precise tuning of micro and macropore channels, configuration of hierarchical structures, and alignment of internal constructions via self-assembly processes, to optimise the electrochemical properties of the finished device. Nanocatalysts, such as bismuth, have demonstrated efficacy in improving electrochemical properties and serving as the primary building block for electrochemical ammonia production. This advancement provides unprecedented design flexibility in traditional fibre spinning, coating, and 3D printing processes (Fig. 1). To this end, by investigating the anisotropic properties of LCGO, we were able to sandwich functional materials like nanocellulose and bismuth nanocatalysts between graphene sheets.^32,33 In these works, we created a fibre-spinning and 3D printable ink to demonstrate this process at scale. This method combines the design flexibility of additive manufacturing with nano-level ordering made possible by the liquid crystalline phase. Our directed self-assembly approach proved valuable when leveraging nanoassembly in fibre spinning and 3D printing, to create mechanically robust architectures with added new functionality.

Fig. 1 (A) A schematic representation of our scalable approach combines the design flexibility of additive manufacturing with precise nano-level engineering of functional materials, such as nanocatalysts, enabled by the liquid crystalline phase and subsequent additive manufacturing processes. (B) SEM micrographs of a layer-by-layer nanocomposite created by integrating Bi nanocatalysts with LCGO to improve electrocatalytic performance. A typical sample is depicted from both the top and cross-sectional perspectives. Insets show the layer-by-layer arrangement of catalysts and graphene sheets.

The ability of GO dispersions to support equivalent LC phases in various organic solvents enabled us to develop composites with layer-by-layer 3D structures containing ordered nanomaterials unsuitable for water processing.^31,32,34 The simplicity of synthesis, short processing time, and high scalability make it easier to create 3D frameworks with new properties. We used these architectures for electrochemical ammonia synthesis and CO₂ reduction, where the addition of the liquid crystalline phase resulted in a one-order-of-magnitude increase in performance.^32,35 As an illustration, self-assembling layered electrocatalytic systems were produced by employing self-ordering Bi nano-electrocatalysts in the LCGO dispersion. Using 3D printing technology, we could precisely control the geometry, microporosity, and number of deposited layers of the electrocatalytic scaffold electrode. This led to improved mass transport properties, increased durability, and the prevention of catalyst detachment. The 3D-printed electrodes achieved industrially acceptable current densities of more than 350 mA cm⁻² for ammonia synthesis and significantly higher durability.³²

4. Other research activities and future directions

Following our first publication in 2013, significant progress has been made because of the ongoing effort to develop new carbon materials such as graphite, carbon nanotubes, carbon nanofibers, graphene, and graphene oxide. While the electrochemical properties of graphene fibres have been extensively studied, there is still plenty of room for improvement in their mechanical properties. Tailoring the rheological behaviour of graphene precursors to reduce disorder and wrinkles and incorporating polymer additives, offers potential solutions, but these additives may impair electrochemical and electronic performance. To overcome these obstacles, precise manipulation of critical properties and post-processing techniques, including stretching and heating, are necessary.

Furthermore, achieving scalable fabrication of graphene fibre yarns will be essential for realising their full potential in advanced industrial and textile applications. Wet spinning of LCGO provides scalability, as demonstrated by our 2013 study using multifilament spinning nozzles to produce graphene yarns.¹² However, challenges remain in ensuring consistent continuity and outstanding mechanical performance of each filament, preventing self-fusion behaviour, and reducing adhesion to substrates during filament collection.

Graphene fibres have become essential functional materials due to their exceptional mechanical and electrochemical characteristics. Further investigation into compressive strength is required to expand the range of applications beyond electronics, electrocatalysis, and electrochemistry. This critical parameter signifies the ability to withstand fatigue and maintain flexibility under compression. This property is particularly significant in the case of composite production. In carbon materials, compression strength is influenced by shear slippage between graphite crystals, and graphene fibres made from LCGO may outperform carbon fibres due to larger crystalline sizes and optimised inherent orders in their microstructures. Graphene fibres are used in various engineering applications, including general engineering, textiles, healthcare, and wearable energy storage devices.³⁵ Their inherent flexibility, excellent electron transport properties, biocompatibility, and ability to be further functionalised with surface treatment make them ideal for various industries, including biomedical applications, wearable energy storage, and textile-based electronics.¹⁹ This demonstration validates the versatility of LCGO but also highlights its role in promoting innovation in integrating graphene and other similar 2D materials, such as MoS₂ and MXene, into practical devices.^11,36

Furthermore, significant progress has been made in exploring other LC dispersions from 2D materials besides graphene and producing high-performance 2D materials based on wet-spinning techniques. Our original study served as a road map for these research activities by describing the critical role of interactions between 2D sheets and water and their sheet sizes in the spinning process. Other fibres, such as 2D MoS₂ and MXene, demonstrated exceptional electrical and mechanical properties, rivalling other carbon-based and graphene hybrid fibres, indicating a high potential for wearable electronics, including conductors, Joule heaters, light heaters, and electrochemical electrodes.

However, research into graphene and other 2D materials-based fibres is still in the early stages, with many areas that need further exploration and challenges that must be overcome. Optimised strategies for improving the scalability and quality of their dispersions are critical, especially for the continuous spinning of other 2D fibres. Understanding LC formation mechanisms and factors influencing LC properties is essential for tailoring rheological behaviours and producing formulations suitable for fibre processing.¹⁰ Further research into the coagulation process of 2D material-based LC in various solutions may reveal gelation mechanisms during wet-spinning of 2D material-based fibres.³⁷

Developing real-time monitoring technology for the transition from fluid to gel state in fibres could help control structure and properties. Additionally, while neat graphene fibres have higher conductivities than composite or hybrid fibres, the latter typically have better mechanical properties.^22,23 Effective fibre fabrication methods are thus required to create neat 2D material-based fibres with high conductivity and mechanical properties. Uncontrolled volumetric shrinkage during the coagulation and drying processes of layered fibres can negatively impact mechanical and electrical properties. Understanding the effects of spinning solvent, LC dispersion rheology, spinning parameters, and coagulation mechanisms is critical for producing wrinkle-free fibres with circular cross-sections and high structural packing density. Post-stretching of graphene dispersions after solidification improves the alignment of LC domains and 2D sheets in fibres while increasing fibre density. Analysing post-spinning fibre drawing at different temperatures can help to optimise 2D flake alignment in new fibres and improve performance. It is critical for textile processing and wearable applications to produce 2D material-based fibres with mechanical properties comparable to conventional clothing fibres. Washability, breathability, and handling are all vital considerations for practical wearable applications. Despite their compelling properties, graphene and other 2D material-based fibres have only been used in limited applications. Highly tailored fibres with reliable properties and geometries are needed to realise their full potential in sensing, energy harvesting, electromagnetic interference shielding, electrochemistry, electrocatalysis, and biomedical applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The Australian Research Council financially supported this work under the Future Fellowship award to A.R.J. (FT230100396).

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