Graphene should have revolutionized electronics by now. When physicists first isolated the single-atom-thick carbon material in 2004, its properties were remarkable: electrical conductivity surpassing copper, thermal transfer outperforming diamond, mechanical strength exceeding steel despite near-total transparency. The material promised flexible displays, wearable sensors, and printed circuits that could be manufactured like newspapers.
Yet graphene remains largely confined to laboratories. Production methods introduce defects, contamination, and inconsistent layer counts that compromise the material’s theoretical properties. And even when researchers obtain usable material, processing it into functional inks creates additional obstacles.
To print graphene onto a surface, you first need to disperse it in liquid. But graphene consists of flexible sheets with exceptionally high aspect ratios, meaning they are far wider than they are thick. They behave like microscopic pieces of cling wrap suspended in water. At low concentrations, they float freely. Push beyond about 2 mg·mL⁻¹, and the sheets begin sticking to each other, restacking uncontrollably into aggregates that settle out of suspension and clog printing equipment. The same massive surface area that gives graphene its remarkable properties also amplifies the attractive forces driving aggregation.
Researchers have tried stabilizing high-concentration dispersions by adding polymers, surfactants, and other binding agents. These additives work, but they dilute the functional material. An ink that is half graphene and half inactive polymer will never match the performance of pure graphene. Every additive represents a compromise between processability and performance.
A team led by researchers at Monash University in Australia, collaborating with Ruhr University Bochum in Germany and Swansea University in Wales, has now taken a different approach. Their work, published in Advanced Materials Technologies (« Solvation-Modulated Dispersions of Reduced Graphite Oxide Toward Binder-Free Conductive Inks »), sidesteps the aggregation problem by changing graphene’s physical shape entirely.
The strategy begins with graphite oxide heated to 900 °C through thermal expansion and annealing. This produces expanded reduced graphite oxide, a fluffy, accordion-like material with recognizable two-dimensional layers. The researchers then subject this expanded material to ball milling, a mechanical process involving repeated high-speed collisions with zirconia balls inside a rotating jar.
The collisions compact the fluffy structure into dense, three-dimensional particles the team calls dense-block reduced graphite oxide, or DB-rGtO. « Rather than behaving as individual 2D sheets, DB-rGtO forms microparticles consisting of loosely-stacked thin sheets and pore networks that still interact strongly with solvents, »
Packing density increases more than threefold, from 0.53 g·cm⁻³ to 1.69 g·cm⁻³. Lateral particle sizes range from 1 to 10 µm, while thicknesses span 0.1 to 1 µm. The resulting particles approximate squashed spheres with an aspect ratio around 0.1, a geometry distinct from graphene’s nanoscale thickness.
This shape change carries important consequences. Although DB-rGtO particles are themselves compacted multi-layer structures, they differ fundamentally from uncontrolled graphene aggregates. When flat graphene sheets restack in solution, they form irregular clumps through extensive surface-to-surface contact, a process that continues until the dispersion destabilizes.
Dense-block particles, by contrast, interact through limited point contacts. Their three-dimensional shape prevents the runaway restacking that plagues conventional dispersions, allowing them to remain uniformly suspended at high concentrations.
Unlike graphene nanoplatelets, which share a similar block-like appearance but consist of tightly bound layers that remain inert in solution, the compacted layers in DB-rGtO stay accessible to surrounding liquid. That accessibility enables solvent-dependent solvation. Different liquids penetrate and swell the compacted structures to varying degrees, depending on their physical properties.
Using polarized light microscopy, the team directly quantified this solvation behavior. They observed that water produces negligible solvation because its high surface tension prevents infiltration. Dibasic ester, a low-viscosity industrial solvent, penetrates readily and causes significant particle swelling. Terpineol and Cyrene, a bio-derived alternative, produce intermediate effects.
Two factors govern this behavior. Surface tension determines whether solvent molecules can initially wet the particle surface. Viscosity controls how easily liquid propagates through the internal structure. Separately, the researchers found that Hansen Solubility Parameters, a set of values describing molecular interaction preferences, govern whether particles aggregate or remain dispersed. Solvents with parameters closely matching graphene’s optimal values, such as terpineol and Cyrene, produce stable dispersions with minimal clumping. Water, whose parameters diverge significantly from graphene’s, causes severe aggregation regardless of solvation level.
« This structural transition allows us to formulate stable graphene-based inks at extremely high concentrations, up to 200 mg·mL⁻¹, while still keeping the ink printable, » Nguyen explains. « By carefully controlling how these particles interact with solvents, we can tune ink viscosity even at high solid content without relying on large amounts of additives. »
Variable swelling directly affects how dispersions flow. By selecting appropriate solvents, the team tuned viscosity across orders of magnitude at identical mass concentrations. At 150 mg·mL⁻¹, dispersions in dibasic ester showed viscosity values four orders of magnitude higher than those in terpineol at low flow rates.
To validate that solvation explains these differences, the researchers estimated effective particle volumes from microscopy images and compared them against predictions from the Krieger-Dougherty equation, a standard model relating suspension viscosity to particle volume fraction. The experimental data matched the model well, confirming that solvation-induced swelling accounts for the observed variations.
Screen printing demands inks thick enough at rest to hold their shape but thin enough under pressure to pass through a fine mesh. The team formulated an ink using terpineol at 200 mg·mL⁻¹, a concentration that substantially exceeds what conventional graphene dispersions can achieve while remaining processable.
This ink produced well-defined interdigitated patterns, a comb-like electrode arrangement common in sensors and energy storage devices, with line widths and gaps in the hundreds of micrometers on flexible plastic substrates. A comparable ink using dibasic ester failed despite meeting some viscosity targets. Excessive thickness at rest prevented proper leveling after printing, leaving visible marks where the mesh had pressed against the substrate.
Ball milling inevitably damages electrical conductivity by fragmenting the continuous carbon network. To compensate, the researchers prepared inks from nitrogen-doped starting material. Nitrogen atoms substituting for carbon in the lattice donate extra electrons, enhancing current flow. This doped precursor exhibits roughly fivefold higher conductivity than undoped versions before processing.
Films printed from nitrogen-doped dense-block material achieved sheet resistances between 5 and 10 ohms per square (a standard measure of thin-film conductivity) after a single printing pass, roughly a hundredfold improvement over undoped counterparts. When configured as electrothermal heaters, printed strips measuring 1 × 3 cm reached temperatures above 200 °C within 20 seconds under 10 V.
The team points out that this performance compares favorably with other reported graphene-based heating elements while requiring minimal additives and only one printing pass.
The approach proves versatile across multiple manufacturing methods. « We achieved single-pass printed films and patterns with conductivities ranging from 5 to 10 ohms per square, adaptable across various deposition techniques such as blade coating, screen printing, and flexographic printing for producing high-resolution, high-fidelity features, » Nguyen notes.
What distinguishes this work is reshaping problematic thin particles into manageable three-dimensional ones. Achieving 200 mg·mL⁻¹ concentrations with minimal additives while maintaining printable viscosity addresses a persistent bottleneck in graphene commercialization. Rather than accepting the material’s limitations and engineering around them chemically, the researchers eliminated those limitations at their geometric source.
For flexible printed electronics, from wearable temperature sensors to rapid-heating elements, the approach offers a manufacturing pathway that preserves graphene’s functional properties instead of diluting them.