Reduction of graphene oxide (GO) to graphene is a critical step in producing processable graphene materials with restored electrical and structural properties. The process aims to remove oxygen-containing functional groups while minimizing structural defects. Three primary reduction methods—chemical, thermal, and photocatalytic—have been extensively studied, each with distinct mechanisms, efficiencies, and trade-offs in C/O ratio restoration and defect introduction. Hydrazine, hydroiodic acid (HI), and laser reduction serve as representative techniques for these categories, each offering different advantages and limitations in conductivity recovery. Additionally, eco-friendly approaches are gaining traction as sustainable alternatives to traditional reductants.

Chemical reduction is one of the most widely used methods due to its scalability and relatively mild processing conditions. Hydrazine monohydrate has been a standard reductant, effectively removing epoxy and hydroxyl groups through nucleophilic attack, restoring sp² carbon networks. Studies report C/O ratios increasing from ~2.0 in GO to ~10.0 after hydrazine treatment. However, hydrazine introduces nitrogen doping, which can alter electronic properties, and residual functional groups often remain, limiting conductivity to ~1000 S/cm, significantly lower than pristine graphene (~10,000 S/cm). Additionally, hydrazine is toxic and environmentally hazardous, prompting the search for safer alternatives. HI reduction offers a less toxic route, achieving C/O ratios of ~12.0 with higher conductivity (~3000 S/cm) due to more complete oxygen removal. HI selectively targets epoxides and carbonyls, leaving fewer defects than hydrazine, but iodine residues may require post-treatment cleaning.

Thermal reduction employs high temperatures to decompose oxygen functionalities, often exceeding 1000°C in inert or reducing atmospheres. This method efficiently removes most functional groups, yielding C/O ratios above 15.0, but the violent release of CO₂ and CO gases creates lattice vacancies and tears the graphene sheets. While conductivity can reach ~2000 S/cm, the defect density remains high, compromising mechanical strength. Rapid thermal annealing and microwave-assisted heating reduce energy consumption and processing time but still struggle with defect control. Laser reduction is a localized thermal approach that selectively irradiates GO films, achieving rapid deoxygenation with minimal substrate damage. The C/O ratio can exceed 20.0 in optimized conditions, and conductivity approaches ~2500 S/cm. However, laser parameters (wavelength, power, scan rate) must be carefully tuned to avoid excessive ablation or incomplete reduction.

Photocatalytic reduction leverages light-activated catalysts, such as TiO₂ or UV-sensitive organic agents, to strip oxygen groups under mild conditions. This method avoids high temperatures and harsh chemicals, making it environmentally favorable. Titanium dioxide under UV irradiation generates electron-hole pairs that react with GO, reducing epoxides and carbonyls. The C/O ratio typically reaches ~8.0, lower than chemical or thermal methods, and conductivity remains modest (~500 S/cm) due to residual defects. However, photocatalytic reduction can be combined with chemical or thermal treatments to enhance efficiency while reducing environmental impact.

Defect introduction varies significantly across methods. Hydrazine and HI produce point defects and small holes due to incomplete oxygen removal and chemical etching. Thermal reduction generates larger vacancies from gas evolution, while laser ablation can cause edge defects if power settings are too high. Photocatalytic methods introduce fewer defects but often leave behind unreacted functional groups. The trade-off between C/O ratio restoration and defect density directly impacts conductivity recovery. Even with high C/O ratios, defects disrupt charge carrier mobility, preventing graphene from reaching its intrinsic conductivity.

Eco-friendly reduction approaches focus on replacing toxic reagents with benign alternatives. Ascorbic acid, green tea extracts, and bacterial metabolites have demonstrated effective reducing capabilities, though with lower C/O ratios (~6.0–8.0) and conductivities (~100–500 S/cm). These methods rely on mild redox reactions, avoiding harsh conditions but sacrificing efficiency. Solvothermal reduction in water or ethanol at moderate temperatures (150–200°C) offers a balance, achieving C/O ratios of ~10.0 with fewer defects than conventional thermal treatment. Electrochemical reduction is another promising avenue, applying a voltage to selectively remove oxygen groups without chemical reagents, yielding conductivities up to ~1500 S/cm.

Conductivity recovery is inherently limited by the irreversible damage caused during GO synthesis and reduction. The oxidation process creates topological defects that persist even after reduction, disrupting the π-conjugated network. While some methods restore sp² domains effectively, others leave behind residual sp³ carbons or holes that act as scattering centers. Doping effects from reductants like hydrazine or HI further complicate electronic properties, sometimes enhancing conductivity at the cost of altered charge carrier concentrations. Achieving near-pristine graphene conductivity remains challenging, but hybrid approaches combining chemical, thermal, and photocatalytic steps show promise in balancing performance and practicality.

In summary, the choice of reduction method depends on the application requirements and environmental considerations. Hydrazine offers high C/O ratios but introduces toxicity, HI provides cleaner reduction with residual challenges, and laser ablation delivers precision at the cost of scalability. Photocatalytic and eco-friendly methods prioritize sustainability over performance, highlighting the need for continued innovation in defect-minimized, green reduction strategies. Conductivity recovery is ultimately constrained by the intrinsic trade-offs between deoxygenation efficiency and structural preservation, underscoring the importance of tailored reduction protocols for specific use cases.