Hummers' method
Hummers' method is a widely used chemical oxidation technique for synthesizing graphitic oxide, also known as graphene oxide, from graphite flakes. Developed by William S. Hummers Jr. and Richard E. Offeman in 1958, the process involves slowly adding potassium permanganate to a mixture of graphite, sodium nitrate, and concentrated sulfuric acid at low temperatures to facilitate intercalation and oxidation, followed by dilution with water, treatment with hydrogen peroxide to reduce excess permanganate, and repeated washing to purify the product into a brownish, dispersible powder.[1] This method revolutionized the preparation of oxidized carbon materials by providing a rapid, efficient alternative to earlier techniques like the Staudenmaier process, achieving higher yields and greater oxidation levels while using relatively accessible reagents.[2] With over 27,000 citations to the original publication as of 2025, it remains the foundational protocol in the field, particularly since the 2004 isolation of graphene sparked renewed interest in graphene oxide as a versatile precursor for reduced graphene oxide and graphene derivatives.[1] The resulting graphene oxide features abundant oxygen-containing functional groups (such as hydroxyl, epoxy, and carboxyl) on its basal planes and edges, enabling facile exfoliation in aqueous media and imparting properties like hydrophilicity and reactivity for further chemical modifications.[3] Despite its advantages in scalability and cost-effectiveness, the original Hummers' method generates hazardous byproducts, including toxic nitrogen dioxide gas from sodium nitrate decomposition, prompting numerous improvements over the decades.[4] Notable modifications, such as the 2010 version by Marcano et al., eliminate sodium nitrate, increase permanganate loading, and use phosphoric acid co-solvent to enhance safety, yield, and oxidation efficiency while minimizing environmental impact.[5] These variants have expanded the method's applicability in large-scale production for diverse fields, including energy storage (e.g., supercapacitors and batteries), environmental remediation (e.g., pollutant adsorption), biomedical applications (e.g., drug delivery and biosensors), and polymer composites for enhanced mechanical and electrical properties.[3] Ongoing research continues to refine the process for greener synthesis and better control over graphene oxide's structural defects and functionality.[4]Background and Context
Graphite Oxide Fundamentals
Graphite oxide is a layered material produced by the chemical oxidation of graphite, incorporating oxygen-containing functional groups such as hydroxyl (-OH), epoxy (C-O-C), carbonyl (C=O), and carboxyl (-COOH) primarily on the basal planes and edges of the carbon sheets.[6] According to the widely accepted Lerf-Klinowski structural model, these oxidized regions feature sp³-hybridized carbon atoms forming aliphatic bridges and rings, alternating with unoxidized sp²-hybridized aromatic domains that preserve some conjugated structure.[7] This heterogeneous composition arises from the oxidation process, which partially disrupts the pristine sp² carbon network of graphite, converting it to sp³ hybridization and enabling easier separation of layers through intercalation.[8] A key structural change in graphite oxide is the expansion of interlayer spacing from 0.335 nm in graphite to approximately 0.7–1.1 nm, influenced by the degree of oxidation, hydration, and intercalated species like water molecules that exert repulsive forces between layers due to the polar functional groups.[9] This increased d-spacing facilitates the material's exfoliation into thinner sheets and contrasts with the tightly packed structure of graphite.[10] Physically, graphite oxide is hydrophilic, attributed to its abundance of polar oxygen groups, which allow it to disperse readily in aqueous media to form stable colloidal suspensions—a property absent in non-polar graphite.[6] Electrically, it behaves as an insulator, as the introduced functional groups interrupt the delocalized π-electron system responsible for graphite's conductivity, though partial reduction can restore some electronic properties. Graphite oxide, often denoted as the bulk oxidized form, must be distinguished from graphene oxide, which refers to its exfoliated single-layer variant; the latter acts as a versatile precursor for graphene production via reduction.[6] This material is typically synthesized through oxidative methods like the Hummers' procedure, providing a foundational intermediate for advanced carbon-based nanomaterials.[11]Historical Development
The synthesis of graphite oxide originated in the 19th century with early oxidation techniques that laid the groundwork for later advancements. In 1859, British chemist Benjamin C. Brodie introduced the first method, treating graphite flakes with fuming nitric acid and potassium chlorate at elevated temperatures over several days, yielding a product with a carbon-to-oxygen ratio of approximately 2.2.[12] This Brodie method, while groundbreaking, was inefficient, requiring multiple repetitions and posing risks from explosive byproducts like chlorine dioxide gas.[12] Nearly four decades later, in 1898, German chemist Ludwig Staudenmaier refined Brodie's approach by adding concentrated sulfuric acid to the mixture of nitric acid and potassium chlorate, enabling a more gradual and consistent oxidation.[12] Staudenmaier's modification shortened reaction times and improved yield but retained the hazards of chlorate-based oxidants, limiting practical scalability for larger-scale production.[13] To overcome these challenges, William S. Hummers Jr. and Richard E. Offeman, researchers at the University of Minnesota, invented a novel procedure in 1957 that employed potassium permanganate as the primary oxidant in concentrated sulfuric acid with sodium nitrate.[1] Published in 1958 in the Journal of the American Chemical Society (volume 80, issue 6, page 1339), this method—now known as Hummers' method—dramatically enhanced safety by eliminating chlorates, accelerated reaction times to 8–12 hours, and achieved a comparable carbon-to-oxygen ratio of about 2.25, making it more reproducible and suitable for broader use.[1][12] The innovation was driven by the demand for an efficient, low-risk oxidation route amid growing interest in modified carbon materials during mid-20th-century research.[13] Hummers' method experienced rapid uptake in the 1960s, becoming the preferred technique for graphite oxide preparation in studies of layered materials, including clay-polymer composites and pioneering nanocomposite investigations that explored intercalation and exfoliation behaviors.[14] This early adoption underscored its role in advancing understanding of nanostructured hybrids, setting the foundation for subsequent materials science applications.[12]Original Procedure
Step-by-Step Process
The original Hummers' method begins with the preparation phase, where 100 g of graphite flakes, 50 g of sodium nitrate, and 2,300 mL of concentrated sulfuric acid (98%) are mixed in a 4 L beaker and cooled to 0-5°C using an ice bath to control the exothermic process.[1] The mixture is stirred to ensure even dispersion.[1] Next, oxidation is initiated by the careful addition of 300 g of potassium permanganate gradually over about 2 hours, with continuous mechanical stirring and temperature monitoring to keep it below 20°C, as exceeding this threshold risks a runaway reaction or explosion due to the strong oxidizing agent.[1] This step forms a thick, pasty suspension as the intercalation and initial oxidation occur. The mixture is then allowed to stand for 24 hours at room temperature.[1] The reaction then progresses with dilution. The mixture is diluted slowly with 4,600 mL of deionized water, which causes the temperature to rise to 98°C; this is maintained for 15 minutes until the solution turns a brownish color, indicating substantial exfoliation.[1] The mixture is then further diluted with 14,000 mL of deionized water. Termination follows to quench excess oxidant. Approximately 100 mL of 30% H₂O₂ is added until effervescence ceases and the purple color from permanganate disappears, yielding a bright yellow suspension of graphite oxide.[1] Purification completes the process. The suspension is filtered, and the solid is washed with dilute HCl to remove manganese residues, followed by repeated washes with deionized water until the filtrate is free of sulfate and chloride ions.[1] The purified material is then dried in air to obtain graphite oxide as a brownish powder, with yields corresponding to approximately 188 g of product from 100 g of graphite (or 80-90% based on carbon content).[1][15] The method was originally developed for batch processing at a lab scale of 100 g of graphite, allowing for safe handling in standard laboratory equipment while completing the core reaction steps over about 24 hours plus additional processing time.[1] Smaller scales (e.g., 1-5 g) are commonly used today by proportionally scaling reagents (e.g., 23 mL H₂SO₄, 0.5 g NaNO₃, 3 g KMnO₄ per 1 g graphite) to minimize risks.Key Reagents and Conditions
The original Hummers' method employs potassium permanganate (KMnO₄) as the primary oxidant, which facilitates electrophilic attack on the carbon edges of graphite flakes through the generation of permanganate species in the acidic environment. Hydrogen peroxide (H₂O₂, 30% solution) serves as a quenching agent to reduce excess permanganate and halt the oxidation process, preventing over-oxidation of the graphite structure.[1] The reaction occurs in a strongly acidic medium provided by concentrated sulfuric acid (H₂SO₄, 95-98% purity), which protonates the graphite layers and enables intercalation of oxidizing species. Sodium nitrate (NaNO₃) is included as a supplementary reagent, supplying nitrate ions that assist in the initial oxidation and help maintain the anhydrous conditions during the early stages. Deionized water is used exclusively for the dilution and washing stages to avoid introducing impurities that could contaminate the product.[16] Standardized proportions in the original procedure include a 3:1 weight ratio of KMnO₄ to graphite (e.g., 300 g KMnO₄ for 100 g graphite), with 23 mL of concentrated H₂SO₄ and 0.5 g NaNO₃ per gram of graphite, allowing for scalability while maintaining reaction control. Temperature is rigorously managed using an ice bath (0-5°C) during initial mixing and KMnO₄ addition to control the exothermic reaction and prevent runaway oxidation; the mixture stands at room temperature for 24 hours, followed by dilution causing heating to 98°C for 15 minutes.[1] Essential equipment includes a suitable reaction vessel such as a beaker for initial mixing under mechanical stirring, a thermometer for precise monitoring, and an ice bath setup; post-reaction filtration employs a funnel to separate the graphite oxide solid efficiently.[1]Chemical Mechanisms
Oxidation Reactions
The oxidation reactions in Hummers' method begin with the initial intercalation of sulfuric acid and sodium nitrate into the graphite structure, forming graphite intercalation compounds (GICs) such as graphite bisulfate and nitrate complexes that expand the interlayer spacing and facilitate subsequent oxidation. This step involves the reaction of NaNO₃ with H₂SO₄ to generate HNO₃ in situ, which further decomposes to provide nascent oxygen for initial edge-site oxidation:NaNO₃ + H₂SO₄ → HNO₃ + NaHSO₄,
followed by 2HNO₃ → 2NO₂ + H₂O + [O], and C (graphite edge) + [O] → CO.[2] These complexes weaken van der Waals forces between graphene layers, enabling oxidant penetration.[2] The core permanganate oxidation follows, where KMnO₄ in concentrated H₂SO₄ generates reactive species like Mn₂O₇ and MnO₃⁺ that intercalate into the expanded graphite layers and add oxygen functionalities. At low temperatures (0–4°C), the primary reaction forms the key oxidant:
2KMnO₄ + H₂SO₄ → Mn₂O₇ + K₂SO₄ + H₂O.
Upon heating to 35–45°C, Mn₂O₇ decomposes, releasing atomic oxygen that attacks carbon atoms, primarily at defects and edges:
Mn₂O₇ → 2MnO₃⁺ + O (nascent oxygen species).
The permanganate oxidation leads to the formation of C-O bonds through electrophilic addition and oxygen insertion at carbon sites.[2][17] This electrophilic addition targets π-electrons in the graphene lattice, leading to ring-opening and oxygen group attachment. While various mechanisms have been proposed, including the formation of MnO₃⁺ or direct decomposition to MnO₂ and atomic oxygen, the core process involves nascent oxygen attacking carbon sites.[2][17] Hydroxylation and epoxidation occur as the nascent oxygen reacts with the basal plane of graphene sheets, forming hydroxyl (-OH) and epoxy (-O-) groups, while edges undergo further carboxylation to yield -COOH functionalities. These transformations disrupt the sp² hybridization, converting it to sp³, and are driven by the high reactivity of MnO₃⁺ and related radicals. For instance, epoxy formation proceeds via direct oxygen bridging across carbon atoms, while hydroxyl groups arise from protonation of initial adducts. Carboxyl groups predominate at sheet edges due to preferential cleavage of C-C bonds there.[2][17] In the hydrolysis stage, controlled addition of water further functionalizes the partially oxidized graphite, hydrolyzing epoxides to additional -OH groups and promoting hydration of carbonyl intermediates. This is accompanied by H₂O₂ treatment to reduce residual manganese oxides:
2MnO₂ + H₂O₂ + 2H⁺ → 2Mn²⁺ + O₂ + 2H₂O,
which completes the removal of Mn residues and stabilizes the oxygen groups.[2] A secondary oxidation may occur here under aqueous acidic conditions, enhancing defect formation via MnO₄⁻-mediated cleavage of C=C bonds to carbonyls:
R-CH=CH-R + MnO₄⁻ → R-C(=O)-C(=O)-R.[17] The overall stoichiometry of the oxidized product, graphite oxide, approximates C₂(OH)₂O or similar empirical formulas, reflecting a C/O atomic ratio of approximately 2–3, depending on reaction conditions. Byproducts include MnO₂ residues, which are reduced during purification, as well as evolved gases like CO, CO₂, NO₂, and O₂ from decarboxylation and decomposition.[2][17] These transformations, first detailed in the seminal procedure, underpin the method's efficacy in achieving high degrees of oxidation.[1]