High-viscosity constraints in the solution-processing of carbon nanomaterials using cyrene

The transition toward sustainable solution-processing of carbon nanomaterials has positioned the bio-derived solvent Cyrene as a green alternative to toxic amides like N-methyl-2-pyrrolidone. While Cyrene possesses ideal surface energy for stabilising carbon allotropes, its high viscosity (14.5 cP) can complicate the dynamics of liquid-phase processing, a problem that highlights the relative lack of investigation into nanomaterial processing in high-viscosity environments. In this work, we provide a comprehensive investigation into how this constraint dictates the morphological and electronic properties of carbon nanomaterial dispersions. We first demonstrate that Cyrene is susceptible to degradation under high-power sonication (45 W), creating to non-volatile residues that can hinder thin-film performance. In the case of CNTs, high viscosity establishes a fundamental debundling limit, where the average bundle diameter is constrained to approximately 4.7 nm and the yield of individual nanotubes is capped at ∼16%. For graphene, the high viscosity necessitates centrifugal forces approximately 50 times greater than those used in aqueous systems to achieve desired nanosheet size fractions, resulting in nanosheets with vastly higher average thicknesses. While high-temperature annealing is required to remove the high-boiling solvent from thin films, residual degradation products limit the maximum conductivity to ∼1000 S m⁻¹. Nevertheless, transparent conducting thin films of nanotubes were successfully fabricated, achieving a conductivity of ∼500 S m⁻¹ at > 80% optical transmittance. These results emphasise that while Cyrene is a viable green solvent, its high viscosity represents a critical bottleneck that must be managed to drive the performance of sustainable carbon electronics.