Solvent-based and surfactant-assisted exfoliation are two prominent methods for producing graphene from graphite. These techniques rely on breaking the weak van der Waals forces between graphite layers while minimizing damage to the graphene structure. Both approaches aim to achieve high yields of few-layer or monolayer graphene with minimal defects, suitable for applications such as conductive inks, composites, and flexible electronics.

In solvent-based exfoliation, graphite is dispersed in a liquid medium with a surface energy close to that of graphene, reducing the energy required to separate the layers. The process typically involves sonication to provide the necessary energy for exfoliation. N-Methyl-2-pyrrolidone (NMP) is one of the most effective solvents due to its surface tension of approximately 40 mJ/m², which closely matches graphene's surface energy (~54 mJ/m²). Other solvents, such as dimethylformamide (DMF), gamma-butyrolactone (GBL), and cyclopentanone, have also been used successfully. The choice of solvent affects the concentration of exfoliated graphene, with NMP typically yielding dispersions of 0.01–0.1 mg/mL after processing.

Sonication is a critical step in solvent-based exfoliation. Parameters such as power, duration, and temperature must be optimized to balance exfoliation efficiency against defect formation. Bath sonication is gentler than probe sonication, reducing the likelihood of tearing graphene sheets but requiring longer processing times (often 10–100 hours). Probe sonication, operating at 100–500 W for 1–10 hours, achieves higher yields but increases the risk of defects. To minimize damage, pulsed sonication cycles (e.g., 30 seconds on, 30 seconds off) are often employed. Post-sonication centrifugation (1000–5000 rpm for 10–60 minutes) removes unexfoliated graphite and thick flakes, leaving a supernatant enriched with few-layer graphene.

Surfactant-assisted exfoliation uses amphiphilic molecules to stabilize graphene in aqueous solutions, reducing reaggregation. Common surfactants include sodium cholate, sodium dodecylbenzene sulfonate (SDBS), and Triton X-100. These molecules adsorb onto graphene surfaces, creating electrostatic or steric repulsion between sheets. Sodium cholate, for example, achieves concentrations up to 0.3 mg/mL in water, with sonication times of 5–20 hours at 50–150 W. The surfactant-to-graphite ratio is crucial; excess surfactant can hinder electrical conductivity in final applications, while insufficient amounts lead to poor dispersion stability.

Yield optimization depends on multiple factors. In solvent-based exfoliation, higher initial graphite concentrations (5–20 mg/mL) can improve yields but may require longer sonication or multiple centrifugation steps. For surfactant-assisted methods, the critical micelle concentration (CMC) of the surfactant must be considered. Operating near the CMC ensures efficient exfoliation without excessive residue. Post-processing techniques like vacuum filtration or freeze-drying can concentrate graphene dispersions, though care must be taken to avoid restacking.

Defect introduction remains a challenge in both methods. Prolonged sonication or excessive power generates lattice vacancies, edge defects, and oxidative damage, which degrade electrical and mechanical properties. Raman spectroscopy is commonly used to assess defect density, with the D/G peak intensity ratio (ID/IG) serving as a key metric. Pristine graphene exhibits an ID/IG ratio below 0.1, while heavily damaged material may exceed 1.0. To mitigate defects, some protocols incorporate mild chemical treatments or thermal annealing post-exfoliation.

Scalability is another hurdle. While lab-scale sonication can produce gram quantities, industrial-scale processes require alternatives like shear mixing or high-pressure homogenization. These methods offer higher throughput but may necessitate adjustments to solvent or surfactant formulations. For conductive inks and composites, achieving uniform dispersions without aggregates is essential. Functionalization with polymers or small molecules can enhance compatibility with matrices like plastics or ceramics, though this often trades off against intrinsic graphene properties.

Ink formulation demands careful control of viscosity and surface tension. Graphene concentrations of 1–10 mg/mL are typical, with additives like ethyl cellulose or polyvinylpyrrolidone (PVP) improving film-forming ability. Surfactant residues can impair conductivity, necessitating washing steps or alternative stabilizers like cellulose nanocrystals. For composites, direct mixing of graphene dispersions with polymer melts or solutions is common, followed by casting or extrusion. The percolation threshold—the minimum graphene loading required for electrical conductivity—varies with dispersion quality but often falls between 0.5–5 vol%.

Environmental and safety considerations also influence method selection. Many organic solvents (e.g., NMP, DMF) are toxic or hazardous, driving interest in water-based surfactant systems. However, removing surfactants without damaging graphene remains difficult. Emerging approaches use bio-derived solvents like terpenes or ionic liquids, though costs and scalability are still under evaluation.

In summary, solvent-based and surfactant-assisted exfoliation offer viable routes to graphene production, each with trade-offs between yield, quality, and scalability. Optimizing sonication parameters, solvent or surfactant selection, and post-processing steps is critical for applications requiring high-performance graphene. Defect minimization and large-scale processing remain active research areas, with advances in alternative exfoliation techniques and green chemistry holding promise for future developments.