Chemical reduction of graphene oxide (GO) to reduced graphene oxide (rGO) is a critical process for restoring the electrical and structural properties of graphene while retaining some oxygen-containing functional groups for further functionalization. The choice of reductant significantly influences the degree of reduction, residual functional groups, and the resulting material properties. Key reduction methods include hydrazine-based reduction, sodium borohydride reduction, and environmentally friendly alternatives such as ascorbic acid and plant extracts. Each method operates through distinct mechanisms, offering varying efficiencies and impacts on the final rGO characteristics.

Hydrazine-based reduction is one of the most widely studied methods due to its high efficiency in removing oxygen functionalities. Hydrazine hydrate (N2H4·H2O) reacts with epoxy and hydroxyl groups on the basal plane of GO, as well as carbonyl and carboxyl groups at the edges, through nucleophilic substitution and elimination reactions. The mechanism involves the formation of hydrazone and diazine derivatives, which subsequently decompose to release nitrogen and water, leaving behind a reduced graphene structure. This method achieves a carbon-to-oxygen (C/O) ratio of approximately 10:1, significantly higher than that of pristine GO (typically 2:1). However, hydrazine reduction introduces nitrogen dopants into the graphene lattice, which can alter electronic properties. The electrical conductivity of hydrazine-reduced rGO reaches around 1000 S/m, depending on the reduction conditions. A drawback of this method is the toxicity of hydrazine, necessitating careful handling and posing environmental concerns.

Sodium borohydride (NaBH4) is another strong reductant, though less aggressive than hydrazine. It primarily targets epoxy and carbonyl groups via hydride transfer reactions, converting them to alcohols and secondary alcohols, respectively. Unlike hydrazine, NaBH4 does not readily reduce carboxyl groups, leaving some residual oxygen content. The C/O ratio after NaBH4 reduction typically ranges between 6:1 and 8:1, lower than that achieved with hydrazine. The electrical conductivity of NaBH4-reduced rGO is consequently lower, often in the range of 200–500 S/m. However, NaBH4 is less toxic than hydrazine and operates effectively at room temperature, making it a safer alternative for certain applications. The structural integrity of rGO produced via NaBH4 reduction remains high, with fewer defects compared to hydrazine-reduced samples, as the reaction does not introduce nitrogen or other heteroatoms.

In recent years, green reductants have gained attention due to their low toxicity and environmental compatibility. Ascorbic acid (vitamin C) is a prominent example, functioning as a mild reducing agent through its enediol structure, which donates electrons to oxygen-containing groups on GO. The reduction mechanism involves the oxidation of ascorbic acid to dehydroascorbic acid, while epoxy and hydroxyl groups on GO are converted to sp2 carbon domains. The C/O ratio achieved with ascorbic acid is comparable to that of NaBH4 (6:1–8:1), but the process is slower, often requiring heating to 60–95°C for several hours. The electrical conductivity of ascorbic acid-reduced rGO ranges between 300–700 S/m, influenced by the reduction duration and temperature. A key advantage is the absence of hazardous byproducts, making this method suitable for biomedical and environmentally sensitive applications.

Plant extracts have also emerged as viable reductants, leveraging the presence of polyphenols, flavonoids, and other reducing agents naturally occurring in biomass. Extracts from plants such as green tea, aloe vera, and eucalyptus leaves contain multiple reducing and stabilizing compounds that act synergistically to remove oxygen groups from GO. The reduction mechanism is complex, often involving electron transfer from phenolic hydroxyl groups to GO, resulting in the formation of quinones and other oxidized derivatives. The C/O ratio varies widely depending on the plant source and extraction conditions, typically falling between 4:1 and 7:1. Electrical conductivity is generally lower than with chemical reductants, ranging from 50–400 S/m, due to incomplete reduction and residual organic moieties from the extracts. However, plant-based reduction offers additional benefits such as simultaneous functionalization and stabilization of rGO, eliminating the need for additional surfactants or dispersants.

The choice of reductant also affects the residual functional groups on rGO, which in turn influence its properties and potential applications. Hydrazine reduction leaves minimal oxygen content but introduces nitrogen, which can be beneficial for catalytic applications. Sodium borohydride retains more hydroxyl groups, enhancing hydrophilicity and dispersion in aqueous media. Ascorbic acid and plant extracts preserve some carboxyl and hydroxyl groups, improving biocompatibility but potentially compromising electrical performance. Structural integrity varies as well; harsh reductants like hydrazine may create defects due to aggressive oxygen removal, while milder methods like ascorbic acid reduction maintain a more intact sp2 carbon network.

In summary, chemical reduction methods for converting GO to rGO each present distinct advantages and limitations. Hydrazine offers high reduction efficiency and conductivity but poses toxicity risks. Sodium borohydride provides a safer alternative with moderate reduction levels. Green reductants like ascorbic acid and plant extracts are environmentally friendly but may yield rGO with higher residual oxygen content and lower conductivity. The selection of a reduction method ultimately depends on the intended application, balancing factors such as electrical performance, structural quality, and environmental impact.