Mass Production of Graphene Oxide Beyond the Laboratory: Bridging the Gap Between Academic Research and Industry

Abstract

The mass production of graphene oxide (GO) has garnered significant attention in recent years due to its potential applications in various fields, from materials science to biomedicine. Graphene, known for its unique properties, such as high conductivity and mechanical strength, has been extensively studied. However, traditional production methods such as the exfoliation of graphite with scotch tape are not suitable for large-scale production. This has led to an increased focus on GO as a viable alternative to graphene production. Nonetheless, challenges, including the optimization of oxidation processes, the control of structural uniformity, and the reproducibility of production, have not been solved so far. This review critically examines GO production advancements by analyzing experimental and mechanistic studies to identify significant developments that enable high-yield and reproducible methods suitable for industrial-scale production. Special attention is given to oxidation techniques and postsynthesis purification and storage, with a focus on controlled oxidation to achieve homogeneous and single-layer GO. Through this lens, the review outlines the path forward for the industrialization of GO, aiming to bridge the gap between academic research and industrial production.

1. Introduction

Twenty years have passed since the isolation and fundamental properties of graphene were reported. With the recognition of the Nobel Prize in Physics and the advancement of research projects such as Graphene Flagship, graphene has become widely recognized not only in academic circles but also in industry and society. The first graphene preparation method was reported in 2004,¹ which involved the simple process of exfoliating highly oriented graphite with scotch tape, revealing key properties of graphene such as high electron conductivity, thermal conductivity, electron mobility, and mechanical strength. Following the elucidation of the fundamental properties of graphene, endeavors have been directed toward integrating graphene into composite materials and devices.²⁻⁴

While the scotch tape exfoliation of graphite (Figure 1a-i) is still employed in academic research, the method presents challenges for mass production in practical applications. This is primarily because the crystal size of graphite typically ranges less than millimeters, making it nearly impossible to achieve defect-free graphene over centimeter-scale areas. Thermal decomposition of SiC (Figure 1a-ii) and chemical vapor deposition (CVD) of organic molecules on metal substrates (Figure 1a-iii) can yield high-quality graphene over large areas. However, obtaining substantial quantities (on the order of tons) remains a challenging task. This is mainly attributed to the specific surface area of graphene being 2,630 m²/g (considering both sides of the graphene sheet), requiring an extensive area of about 1.315 km² to produce only 1 kg of single-layer graphene on a substrate. Therefore, in practical applications, it is crucial to manufacture, store, and utilize graphene in a confined three-dimensional space. Recently, dispersing graphene in solvents has emerged as a solution to these challenges in mass production.

Figure 1.

Overview of the production methods for graphene family materials: (a) Solid- or gas-phase methods, which produce high-quality graphene but lack scalability. (b) Liquid-phase methods, which offer high scalability but result in lower-quality graphene.

Achieving a single-layer dispersion of graphene has been challenging due to the strong π–π interactions between graphene sheets. Nevertheless, scientists have developed dispersants that effectively interact with the graphene surface to stabilize it in liquid form (Figure 1b-i). Unfortunately, this process generally yields multilayer graphene. Surface charges are introduced on graphene to consistently obtain atomic-layer materials and induce electrostatic repulsion between the sheets. Therefore, converting graphite to graphene oxide (GO) is recognized as a simple and mass-producible method for producing single-layer materials (Figure 1b-ii).

Graphite oxidation was initially documented almost two centuries ago by Schafhaeutl.⁵ However, only about 25 years have passed since the discovery of single-layer GO.⁶ This would be because prior to the discovery of 2D materials, researchers were not primarily focused on producing single-layer materials. The use of GO in research purposes has increased significantly since 2010 because GO can be produced on a large scale by using inexpensive graphite and an oxidizing agent or electrolytic equipment. In addition, GO has recently become commercially available at reasonable prices, allowing researchers from various fields to utilize it. As a result, GO has been applied in a wide range of fields, and numerous papers have been published.⁷ Consequently, there has been growing interest and demand for the mass production of GO with high reproducibility. Many papers toward GO industrialization have also been published; however, the definition of “mass” production is ambiguous, with some papers referring to a few gram-scale syntheses as large-scale. In addition, there is no consensus on the production yield of GO, as typical GO contains more than 30 at. % oxygen, resulting in a weight increase of more than 1.5 times compared to the original graphite. It is worth noting that almost all of the literature has overlooked this point. Moreover, unoptimized oxidation conditions can lead to a nonuniform mixture of unoxidized graphite and GO, necessitating a separation process that results in a low yield of GO. An ideal synthesis process for producing GO involves uniform preparation of graphite oxide without exfoliation, followed by sequential purification to remove unoxidized oxidants and acids or salts, with exfoliation performed as the final step. Due to its high dispersibility, single-layer GO is difficult to purify by centrifugation or filtration, making controlled and uniform oxidation essential for achieving homogeneous GO with a high yield (Figure 2).

Figure 2.

Comparison of the controlled and uncontrolled oxidation of graphite and its effect on the GO yield.

The author meticulously examines previously published papers, specifically focusing on experimental sections and mechanistic studies. This work aims to highlight the important contributions enabling the mass production of GO and supporting the industrialization of graphene materials.

2. Classification of GO Synthesis

In 1840, Schafhaeutl documented the oxidation of graphite, with small molecules such as sulfuric and nitric acid intercalated between graphite layers.⁵ However, the technique developed by Brodie,⁸ which was published later and utilized a similar oxidizing system, is often regarded as the pioneering method in this field, although other graphite oxidations have been investigated by Staudenmaier,⁹ Charpy,¹⁰ Kohlschütter,¹¹ Thiele,¹² Hofmann,^13,14 and Boehm.¹⁵ Currently, Hummers’ method, introduced in 1958, employing KMnO₄ as an oxidant in H₂SO₄,¹⁶ is widely used for the preparation of GO due to its rapid reaction time and high reproducibility. Recently, electrochemical methods have emerged as more cost-effective and environmentally friendly alternatives. This section examines potential GO mass production methods for industrial applications, focusing on Brodie’s, Hummers’, and electrochemical methods. The characteristics of each method are summarized in Table 1.

Table 1. Comparison of the GO Preparation Methods.

Method	Oxidant	Solvent	Wastes/byproduct	Safety	Cost
Brodie’s	NaClO3, KClO3	conc. HNO3	unreacted oxidant, acidic water, Cl2 and NOx gases	Explosive	High due to dangerous reagents
Hummers’	NaNO3, KMnO4	conc. H2SO4	Mn salt, acidic water, NOx gas	Explosive	Moderate
Optimized Hummers’	KMnO4	conc. H2SO4	Mn salt, acidic water	Less explosive	Lower than Hummers’
Electrochemical	None (Electric current)	H2O	Electrolyte (only adsorbed by GO)	Electrical shock	Minimal (graphite sheet dependent)

2-1. Chemical Oxidation: Brodie’s Method

The methods developed by Schafhaeutl and Brodie have contributed to research on the oxidation of graphite. Currently, Brodie’s method, which was reported 19 years after Schafhaeutl’s work, is more renowned. Brodie’s method involves oxidizing graphite in concentrated nitric acid using potassium chlorate as the oxidizing agent. The method reported by B. C. Brodie in 1859 is shown below.⁸

Step 1. Graphite powder (10 g) and potassium chlorate (30 g) are mixed in a flask, and an excess amount of fuming nitric acid is added.

Step 2. Leave the mixture at 60 °C for 3 to 4 days (until no yellow vapor is generated).

Step 3. Dilute with a large amount of water, remove the supernatant, and then wash with distilled water to remove acids and salts.

Step 4. Repeat steps 1 to 3 four times.

Repeating the oxidation treatment increases the oxygen content, but after four repetitions, there is almost no change in the elemental composition. Brodie’s method produces GO with higher crystallinity and thermal stability, indicating less damage to the graphene layer compared to other chemical methods. Brodie’s GO is easier to restore the sp² honeycomb carbon framework through reduction treatments. This means that properties similar to graphene can be obtained using Brodie’s oxidation and following reduction treatments. However, Brodie’s method poses a safety risk due to the prolonged presence of the explosive oxidizing agent in the reaction solution. This risk becomes particularly relevant when the process is scaled up for industrial applications.

To address this issue, researchers have developed optimized synthesis conditions of Brodie’s method. By investigating the amount of oxidizing agent, reaction time, and number of oxidation cycles, it became possible to introduce a comparable amount of oxygen in one oxidation treatment as in the original Brodie’s method with four oxidation treatments.¹⁷ It has been reported that adequate oxidation can be achieved by adding graphite, potassium chlorate, and fuming nitric acid, allowing it to sit at room temperature for 24 h, and then heating it at 60 °C for 3 h.¹⁸ Staudenmaier developed an efficient oxidation method,⁹ which involved introducing concentrated sulfuric acid into Brodie’s synthesis conditions. This modification has proven effective in achieving optimal oxidation in a single treatment, while significantly reducing the amount of fuming nitric acid. The efficacy of this technique has been attributed to the generation of nitroium ions (NO₂⁺) resulting from the reaction between sulfuric acid and nitric acid. However, the precise mechanism underlying this enhancement remains a subject of ongoing investigation. For example, a new oxidation system using KClO₃, H₂SO₄, and H₃PO₄ was reported recently, resulting in avoiding the formation of toxic NO_x.¹⁹ However, the ton-scale production of GO through Brodie’s method has not yet been reported. The establishment of optimized synthesis conditions for GO holds the potential to substantially mitigate the safety risks associated with the traditional Brodie’s method while simultaneously enhancing process efficiency.

2-2. Chemical Oxidation: Hummers’ Method

Hummers’ method, which was developed in 1958,¹⁶ is different from Brodie’s methods because it uses potassium permanganate (KMnO₄) as an oxidizing agent. In sulfuric acid, KMnO₄ is converted into Mn₂O₇ and reacts quickly and exothermically with graphite. Therefore, certain precautions, such as adding it in portions while it is cooled, have been taken to suppress the explosive reactions. The synthesis method of GO, as reported by Hummers, is shown below.

Step 1. Mix 100 g of graphite, 50 g of sodium nitrate, and 2.3 L of sulfuric acid. Stir the mixture while it was cooled with ice.

Step 2. Gradually add 300 g of KMnO₄.

Step 3. Leave the mixture for 30 min in a 35 °C water bath.

Step 4. After 30 min, 4.6 L of water was added and the mixture allowed to stand for 15 min in a 98 °C water bath.

Step 5. Dilute the reaction solution with 14 L of water.

Step 6. Add 3 wt % hydrogen peroxide solution to the obtained solution until foaming stops.

Step 7. Filtrate with filter and wash with water to remove impurities.

In general, uniformly oxidized and exfoliated GO clog filter membranes, rendering filter-based purification quite challenging. Therefore, the filterability of the final product obtained through this original method indicates the production of nonuniformly oxidized product, known as graphite oxide. This underscores the importance of optimizing oxidation conditions to produce a uniform and single-layered GO.

In 1999, an efficient synthesis method of single-layer GO was developed to enhance oxidation efficiency by using preoxidized graphite.⁶ This technique reduces the reaction time and uniformly oxidizes graphite, resulting in a high yield of GO. Preoxidizing graphite eliminates the need for sodium nitrate during oxidation, reducing harmful NO_x gas emissions. Although the precise effects of preoxidation of graphite are not clear, it is considered that factors such as improved affinity between graphite and sulfuric acid or Mn oxidizing agent, and increased formation rate of graphite intercalation compounds by removing impurities contribute to the effective synthesis of GO. In 2010, an improved method for the synthesis of GO was introduced,²¹ which utilized phosphoric acid instead of sodium nitrate. This method not only resulted in deeper and uniform oxidation of graphite but also reduced the production of harmful gases like NO_x. However, a direct comparison of cost and efficiency between this method and the traditional Hummers’ method is not feasible due to the large amounts of sulfuric acid and KMnO₄ used. Apart from these methods, several other techniques have been explored for the oxidation of graphite, including various pretreatment methods such as expanding graphite, irradiating the microwave, and employing additives such as manganese oxide, water, and nitric acid. However, the efficacy of these methods has not been scientifically proven, and many aspects of them remain empirical. After comprehensive investigations into different GO production methods were conducted, unnecessary steps and reagents were eliminated to determine the minimum necessary conditions for GO synthesis. According to the findings, the process of creating GO from graphite requires only KMnO₄ and sulfuric acid as reagents. A detailed investigation of the oxidation conditions indicated that a reaction time of 2 h at 35 °C was adequate, and that a minimum KMnO₄/graphite weight ratio of 3:1 (w/w) was required; however, 4:1 (w/w) or larger amount of KMnO₄ is preferred if complete oxidation and exfoliation are required.²⁰ In contrast, 6:1 is too much from our experience unless otherwise defective and highly dispersible GO is required, although many researchers employ this condition. The effect of the reaction temperature on the defects in GO was examined. High temperatures in GO synthesis led to excessive CO₂ generation, which caused numerous defects. Conversely, lower temperatures resulted in a milder reaction and less defective GO, but required a longer reaction time.²²

The implementation of optimized conditions led to the successful formation of GO in quantities over 500 g in a laboratory batch (Figure 3a). Moreover, continuous flow production of GO was achieved by precisely controlling the ratio of raw materials, their concentrations, reaction temperature, and reaction time. Currently, the scaling-up process is being carried out using modified Hummers’ method, resulting in batches producing over 10 kg (Figure 3b).

Figure 3.

(a) 500 g-scale production of GO in the laboratory: (i) 50 L reactor, (ii) crude mixture after the reaction is completed, (iii) 10 L continuous-flow centrifuge for purification, (iv) > 700 g of GO after freeze-drying. (b) Bench plant facility at NSC Co., Ltd. for 10 kg-scale GO production.

2-3. Electrochemical Oxidation of Graphite Electrode

Exfoliation of graphite through electrochemical oxidation is studied as an alternative to chemical oxidation methods that use oxidants, such as KMnO₄ or KClO₃. The electrochemical treatment of graphite has been studied since the 1970s, during which the production of graphite intercalation compounds (GICs) was actively investigated. When a voltage higher than that required for GIC formation is applied, oxidation and exfoliation of graphite occur, producing GO. The progression of electrochemical oxidation is illustrated in Figure 4. Before intercalation, polarization occurs with no noticeable change. At 1.5 V, intercalation begins, leading to a color change to blue. Applying higher potentials induces water decomposition, generating reactive oxygen species, which, in turn, leads to the production of O₂ and the oxidation of graphite. At voltages exceeding 4 V, the graphite sheets are fully oxidized and expanded, forming GO.

Figure 4.

Electrochemical oxidation process of the graphite anode at different potentials.

Compared to chemical oxidation, the electrochemical process offers several advantages: 1) it does not require chemical oxidants; 2) the degree of oxidation can be controlled by electrochemical potential and current; 3) simultaneous functionalization can be achieved by adding appropriate chemical reagents to the electrolyte solution; and 4) the equipment and procedure are straightforward and pose no risk of oxidative explosion. The electrochemical method requires only an aqueous electrolyte, a power supply, and electrodes. The reaction can be easily stopped by turning off the power. In addition, unlike the chemical synthesis of GO which requires strong oxidizing agents and acids, leading to defective GO, the electrochemical approaches can produce GO with minimal defects using a low environmental impact process.²³

Applying higher potential leads to water electrolysis, which generates active oxygen species (OH or O radicals). A typical electrochemical oxidation of graphite involves using graphite as the anode and platinum as the cathode in an aqueous electrolyte solution with an applied voltage of around 10 V. It is believed that the reactive oxygen species contribute to opening up the edges of graphite, facilitating the intercalation of ions and water. The ions, such as sulfate or perchlorate, and water then decompose between the graphite layers, producing gas species. The generated gas pressure causes the exfoliation of the layers, resulting in the exfoliation (Figure 5).

Figure 5.

Proposed pathway for the electrochemical oxidation of graphite: (a) intercalation occurs when an electric potential is applied, (b) water decomposes, generating reactive oxygen species that attack the graphite surface, (c) oxy-functional groups and defects form, leading to electrostatic repulsion among the oxy-functional groups and an increase in interlayer distance, and (d) successive intercalation and oxidation progress between the layers, resulting in the formation of GO. Adapted with permission from ref (23). Copyright 2020 Oxford University Press.

Highly oriented pyrolytic graphite (HOPG) is a type of synthetic graphite with high crystallinity. When HOPG is electrochemically oxidized, the interlayer space expands, causing the material to visibly swell uniformly.²⁴⁻²⁹ However, HOPG is expensive and not suitable for mass production. On the other hand, graphite rods obtained from kneading, forming, and sintering needle coke and pitch are available at low cost. Commercially available graphite rods usually have a diameter of 0.5–2 cm and a height of 10 cm for research purposes. Consequently, numerous studies have been conducted on the oxidation of graphite rods using various electrolytes, such as sulfuric acid, sulfates, mixed acids, alkaline solutions, and organic surfactants. In addition, electrodes from used batteries³⁰ and pencils³¹ can also be used as graphite sources. However, one important problem with graphite rods is that they tend to collapse as oxidation takes place.^32,33 The resulting product was then deposited at the bottom of the electrolyte solution (Figure 6). Once the product is isolated from the electrode, no further reaction occurs, leading to the formation of products with low-level and nonuniform oxidation.

Figure 6.

Example of nonuniform and destructive electrochemical oxidation of graphite rod.

2-4. Electrochemical Oxidation of Graphite Sheet

Graphite sheet is a flexible film of less than 500 μm in thickness. It is commercially available in sheet form, ranging from several square centimeters to continuous rolls spanning meters in length. Graphite foil is produced using two main methods: pressing expanded graphite or high-temperature carbonization of a polyimide sheet. The latter method yields a material with superior crystallinity, resulting in enhanced electrical and thermal conductivities, albeit at a higher production cost. Using graphite sheets allows for easy surface area adjustment, facilitating precise calculation of electric current density (A/m²) and enabling discussions on reaction efficiency and oxidation rate. However, a simple aqueous sulfuric acid cannot produce single-layer GO by electrochemical oxidation due to the nonuniform and destructive reactions, similar to the case of the graphite rod (Figure 6). Consequently, attempts have been made to optimize the reaction conditions to improve the yield of GO and the uniformity of the reaction. By carefully selecting electrolysis conditions, graphite sheets can undergo oxidation and expansion while maintaining their original sheet shape, allowing for easier recovery and purification.^34,35 The electrolyte selection and the pretreatment of graphite are crucial factors in increasing the yield of GO. A two-step electrochemical exfoliation process through stage 1 GIC was developed to synthesize GO with a yield of over 70% and more than 90% single-layer.³⁶ The preparation of GIC was accomplished via electrochemical treatment of the graphite anode in concentrated sulfuric acid, and electrochemical oxidation was performed in 0.1 M (NH₄)₂SO₄. GIC facilitates the smooth and uniform insertion of water molecules between layers, resulting in a uniform oxidation reaction. When graphite undergoes electrochemical oxidation without an intercalation process, achieving a higher yield and uniform oxidation is impossible. A similar two-step electrochemical method is also reported, with a change in the second-step electrolytes by combining different ratios of hydrazine sulfate, diluted H₂SO₄, and KMnO₄.³⁷ This method enables the production of GO > 200 g/h with controllable oxidation levels (Figure 7).

Figure 7.

Mass production of GO over a wide range of oxidation levels via a two-step electrochemical exfoliation approach. (a) Schematic illustration of the process; graphite foil was preintercalated in concentrated H₂SO₄ electrolyte, followed by simultaneous exfoliation and oxidation of intercalated graphite foil in various secondary electrolytes. (b) A photograph of graphite foil. (c) Representative photographs of exfoliated GOs with controllable oxidation levels, ranging from less oxidized GO (left) to highly oxidized GO (right). Adapted with permission from ref (37). Copyright 2024 Royal Society of Chemistry.

From our experience, the oxidizing methods mentioned earlier are not ideal for uniformly oxidizing and obtaining single-layer GO. As a result of optimization, a graphite sheet with high thermal conductivity has been identified as a superior graphite source, with HBF₄ being used as the electrolyte to achieve uniform oxidation (Figure 8a).³⁸ After the electrochemical treatment, the graphite sheet swells about 200 times but retains its original sheet form. By washing and dispersing the product with ultrasonication, a uniform GO is obtained. The nondestructive nature of this method allows for continuous-flow production in a laboratory setting using a roll of graphite sheet connected to a specially designed anode. In the flow reaction, the graphite sheet is immersed in an electrolyte, with the cathode positioned above it. This prototype reactor enabled the production of 100 g/day of GO (Figure 8b), and further scaling up to 1 kg/day was successfully achieved (Figure 8c).

Figure 8.

(a) HBF₄ electrolyte results in a uniformly oxidized and expanded graphite sheet. (b) Continuous-flow electrochemical oxidation equipment for 100 g/day production. (c) Advanced continuous-flow electrochemical oxidation equipment for 1 kg/day production. Adapted with permission from ref (38). Copyright 2020 Elsevier.

3. Differences in GO Structure and Physical Properties

Functional groups and physical properties of GO prepared by different synthesis methods have been confirmed.³⁹ Although the chemical composition of GO prepared by Brodie’s and Hummers’ methods is almost the same, there is a difference in the distribution of functional groups. Hummers’ GO has more C=O bonds and C–C bonds as well as a random structure compared to Brodie’s GO. These differences in functional groups influence the degree of hydration and solvation: when water is soaked, the interlayer distance increases more in Hummers’ GO due to the strong interaction.

The thermal behavior of GO and the properties of thermally reduced GO prepared by different methods have been studied. The temperature at which rapid heat generation occurs was compared for Hummers’ and Brodie’s methods. DSC analysis confirmed the rapid heat generation at 195 °C for Hummers’ GO and 270 °C for Brodie’s GO. XRD analysis revealed that Brodie’s GO has high crystallinity, which affects its thermal stability. The specific surface areas of thermally reduced GOs are significantly different: 550 m²/g for Brodie’s GO and 130 m²/g for Hummers’ GO. The specific surface area increases as each graphene sheet is isolated by physically expanding due to the gas (carbon dioxide, carbon monoxide, and water) generation during the heating. It is considered that the gas pressure is higher for Brodie’s GO, which undergoes thermal reduction at a higher temperature. In other words, the higher the crystallinity and thermal stability of GO, the higher the specific surface area, because reduction (gas generation) occurs at a high temperature and high pressure. Hummers’ GO, which has a strong interaction with a polar solvent, is preferable when dispersing in the solvent, whereas Brodie’s GO is preferable when obtaining graphene-like materials with a large specific surface area after thermal treatment. Comparative investigations of chemical and electrochemical methods are limited; GO prepared by both methods has shown similar performance in Li-ion storage capacity, membrane separation, and catalytic reactions.³⁸

4. Transition from Laboratory to Industry in Chemical Oxidation of Graphite

With the increasing commercial demand for GO, the scale-up of the optimized laboratory method to an industrial level has been investigated. Laboratory-level investigations have identified several critical challenges and solutions.

4-1. Temperature Control

Achieving maximum oxidation requires heating; however, excessively high temperatures can pose significant safety risks. Effective temperature management and installation of appropriate safety interlocks are essential in this situation. It is crucial to control the temperature when adding KMnO₄ because unstable dimanganese heptaoxide (Mn₂O₇) can explode at temperatures above 55 °C. Adding KMnO₄ at temperatures lower than 20 °C results in defect-less GO, while high temperature (>35 °C) oxidation proceeds smoothly but leads to the formation of defects in GO.²²

Thermal runaway is a major cause of industrial chemical accidents. It begins with a highly exothermic reaction that raises the temperature, which, in turn, accelerates the reaction rate. Once the temperature exceeds a critical limit, an explosion can occur. In the synthesis of GO, the exothermic heat generated by adding KMnO₄ to the H₂SO₄/graphite mixture and by adding water to the KMnO₄/H₂SO₄/graphite mixture can cause local overheating, raising the temperature above the safety limit of 55 °C. The thermal analysis revealed that just after the addition of KMnO₄, exothermic peaks were observed by calorimetric analysis.⁴⁰ Additionally, heating GO can lead to a self-propagating reaction that poses a combustion hazard (Figure 9). Therefore, thermal runaway is a significant explosive risk in the scaled-up production of GO.

Figure 9.

(a) Schematic of reaction calorimeter. (b) Calorimetric data: The orange line represents the reactor temperature (Tr), the green line represents the jacket temperature (Tj), and the blue line represents the heat released by the reaction, the heat flux (q). Adapted with permission from ref (40). Copyright 2020 American Chemical Society.

4-2. Addition Rate, Sequence, and Reaction Time

As in many chemical processes, controlling the addition rate of reagents is crucial for managing the product properties and mitigating any exothermic reactions. Dimiev et al. concluded that the oxidation reaction is diffusion-controlled and dependent on the graphite grain size;²⁰ for the same source of graphite, small-size flakes are oxidized significantly faster than large flakes.

In a general chemical oxidation of graphite, an oxidant such as KMnO₄ is added stepwise to the graphite/acid media mixture. It is reported that preformed acidic oxidizing media accelerates the oxidation of graphite;⁴¹ however, storing large amount of explosive and unstable Mn₂O₇ would not be recommended for industrial production of GO.

Perera et al. studied the impact of reaction time on the oxidation of graphite at intervals of 1, 2, 3, and 4 h at 50 °C.⁴² The results showed that all GO products had consistent percentages of oxygen. However, the distribution of functional groups varied; longer reaction times led to an increase in the number of C=O bonds, indicating the formation of more defects. Morimoto reported that 35 °C for 2 h is the optimum reaction condition,⁴³ which was confirmed by in situ XRD and in situ XAFS analyses (Figure 10). The progress of the oxidation can be monitored by measuring the remaining oxidant with an iodometric titration. When synthesizing GO using a method that requires a large amount of an oxidizing agent, it is crucial not to halt the reaction while the oxidizing agent is still present.

Figure 10.

(a) In situ XRD and (b) XAFS analysis of standard Mn samples (left) and in situ XAFS analysis using the synchrotron facility to understand the progress of graphite disassembling and oxidation. Adapted with permission from ref (43). Copyright 2020 American Chemical Society.

4-3. Oxidizer Quenching and Post Heating

The use of strong oxidizing agents necessitates reaction quenching to ensure safe handling of the reaction mixture. It is important to manage this process carefully to keep temperature fluctuations to a minimum, ensure safe operation, and maintain quality, especially when excess KMnO₄ is used (KMnO₄/graphite weight ratio >3). After oxidation, water is added to the reaction mixture, followed by the addition of H₂O₂. Before adding water, most Mn species are located between GO layers and not in the solution.⁴³ The addition of water releases Mn species, predominantly Mn³⁺. If excess oxidative Mn⁷⁺ remains in the mixture, then a homogenization reaction occurs, such as Mn³⁺ + Mn⁷⁺ → Mn⁵⁺. This suggests that adding water can stop the reaction. Additionally, the addition of H₂O₂ leads to further reduction of Mn species to Mn²⁺, which are easily soluble in water and can be removed from GO by water washing. In this step, H₂O₂ acts as a reducing agent; it can also be replaced by other mild reducing agents, such as citric acid. After adding H₂O₂, the original Hummers’ method involved heating the mixture to 95 °C. This step is not essential for enhancing oxidation, but it does improve the thermal stability of GO.⁴⁴

4-4. Purification

The purification of GO is crucial in its production, especially by chemical oxidation methods, as impurities can affect its properties for specific applications. Traditional purification methods like filtration and centrifugation are not effective, requiring a lot of water, energy, and time. An efficient method using a fluidic diffusion cell system with a porous membrane has been developed.⁴⁵ This method outperforms traditional approaches by removing major contaminants more effectively, using about 95% less water. Gas press filtration using ceramic paper, glass wool, or woven glass fiber beds also accelerates the purification process.⁴⁶ The loss of GO occurs during purification by centrifugation; thus, centrifuge-free purification is desirable to obtain GO in high yield. It is reported that GO remains unexfoliated by adding aqueous HCl, enabling filtration and washing.⁴⁷ This washing effectively removes metal ions and acid media (H₂SO₄) and achieves a 170% yield of GO, calculated from the weight of starting graphite.

Cross-flow filtration has been investigated as an advanced purification for the mass production of GO.⁴⁸ In a larger-scale process, filtration is time-consuming due to severe clogging of conventional filters by water-swollen GO, as mentioned above. While dialysis membranes avoid clogging, the increasing back permeation significantly prolongs the dialysis time and drastically increases water consumption. As a result, neither filtration nor dialysis is suitable for scaling up the GO process. In contrast, a rough prefiltration of the as-synthesized GO, followed by cross-flow filtration, proves far more effective. This method significantly reduces the processing time and minimizes the level of wastewater. Unlike traditional filtration and dialysis, cross-flow filtration can be easily automated for continuous operation. Furthermore, the aqueous GO dispersion can be concentrated in the final stage when online electrical conductivity monitoring confirms the desired removal of ionic impurities (Figure 11).

Figure 11.

Scheme of the pilot plant, enabling GO purification by automated cross-flow filtration. 1) water reservoir, 2) pump for water feed, 3) control of filling level and controller for water feed pump, 4) blade stirrer with mixing holes, 5) 30 L vessel containing GO dispersion, 6) hollow fiber cross-flow filtration membrane cartridge, 7) peristaltic pump for GO circulation, 8) online ion conductivity measurement for monitoring the ion content of the wastewater, 9) wastewater stream, 10) containment. Adapted with permission from ref (48). Copyright 2014 Elsevier.

The treatment of wastewater generated during the synthesis and purification of GO is essential for its commercial production, although it has received limited attention in academic research. In the case of Hummers’ type chemical oxidation, the wastewater contains K⁺, Mn²⁺, and SO₄^2– ions. Mn²⁺ ions can be effectively removed by adding KOH and adjusting the pH to around 10, resulting in a precipitate identified as Mn₃O₄ with some Mn(OH)₂. It was confirmed that the Mn²⁺ concentration in the KOH-treated wastewater was as low as 120 μg/L, which is below the World Health Organization’s guideline of 400 μg/L for drinking water.⁴⁹

4-5. Recycling Acidic Solvent

H₂SO₄ is commonly used as a solvent in the chemical oxidation of graphite. Typically, more than 30 mL of H₂SO₄ is required to oxidize 1 g of graphite. To recycle H₂SO₄, filtration or centrifugation should be performed before quenching with water, as the water content in recycled H₂SO₄ must be below 10% for efficient graphite oxidation. By controlling the viscosity of the reaction mixture, membrane filtration has enabled H₂SO₄ recovery without altering the properties of the resulting GO product (Figure 12).⁵⁰ Recycling H₂SO₄ can be achieved because most Mn species are present between GO layers and are not present in solution before water addition. The filtration before adding water is not limited to recycling H₂SO₄; it also reduces the overall amount of wastewater, as less water is needed for purification. A similar approach has been documented where the solid product and acid are separated directly, allowing for acid recycling and quick centrifugal purification of GO.⁵¹

Figure 12.

Separation of the GO product and H₂SO₄ by filtration for recycling the acidic solvent.

4-6. Storage of Graphene Oxide

GO can be reduced through heating or light irradiation.⁵² It has been reported that the structure of GO changes over time, transitioning through intrinsic, metastable, and transient states.⁵³ These structural changes are caused by the rearrangement of functional groups on the surface, leading to the formation of large sp² domains and causing aggregation of GO.⁵⁴ Radical species are considered to contribute to these processes. In addition, different pH values in the GO dispersion cause structural changes in GO. Under alkaline conditions, deoxygenation and C–C bond cleavage occur.⁵⁵ The metastability of GO is also observed in the dried state. Even at room temperature, GO undergoes spontaneous chemical transformation and reduction, with a relaxation time of 35 days, and continues to change its structure for up to 100 days.⁵⁶ This is why GO powder is not easily dispersed again in water (6.6 mg/L), although pristine GO dispersion can reach over 10 g/L.⁵⁷

For optimal preservation of GO, it is recommended to store it in the form of a water dispersion under weakly acidic pH and low temperature, preferably in a dark environment or covered with aluminum foil. It is important to note that the stability of GO depends on the preparation method as the quantity of reactive radical species is subject to variation based on the degree of oxidation and defects.

5. Conclusion and Future Perspective

In conclusion, GO exhibits a range of properties that make it suitable for diverse applications across industries. The synthesis methods, including notable approaches such as Brodie’s method and the widely adopted Hummers’ method as well as electrochemical approaches, provide essential insights into tailoring structural and physical properties of GO for specific applications. These methods offer control over key characteristics such as the degree of oxidation, crystallinity, and defect density, making GO adaptable to both high-performance materials and scalable production processes. Additionally, efforts to scale up GO production, including optimizing the temperature, oxidant amount, reaction time, and purification processes, underscore the increasing significance of GO in the industry.

While challenges in mass production remain, including safety risks, exothermic reactions, and the need for efficient purification methods, advancements are gradually making industrial-scale synthesis feasible. For instance, the careful management of thermal runaway risks during synthesis and the refinement of quenching processes are critical for ensuring safe, large-scale production with chemical oxidation methods. Electrochemical oxidation is an emerging technology for GO production; however, the selection of the starting graphite sheet significantly impacts the quality of the resulting GO, as cheap expanded graphite sheets generally yield low-quality GO.

Advanced purification techniques such as cross-flow filtration and solvent recycling are creating opportunities for sustainable large-scale production. Moreover, purification methods that minimize water usage and energy consumption, such as the development of new filtration membranes, represent promising directions for industrial application.

Looking forward, the future of GO relies not only on overcoming technical obstacles but also on fully leveraging its potential to revolutionize industries, ranging from energy storage and water purification to biomedical applications. However, concerns about the biocompatibility of GO cannot be overlooked. Some studies suggest that GO has the potential to induce adverse biological responses, including carcinogenic effects. Furthermore, if released into the environment, GO may cause harm toward aquatic organisms and may also limit its ability to accumulate in ecosystems. Addressing these risks through rigorous toxicity studies, environmental assessments, and the development of safer GO derivatives will be critical for the responsible advancement of GO technologies. The next frontier of research should focus on enhancing the reproducibility and scalability of GO production, while addressing environmental concerns.

Continued research aimed at selecting optimum graphite sources, refining oxidation methods of graphite, and postprocessing techniques may soon position GO from a mere academic curiosity to a key material of the future. GO’s exceptional surface area, versatile functional groups, and modifiability make it an ideal candidate for diverse applications that could significantly influence electronic technologies and environmental sustainability. In this regard, interdisciplinary collaboration among material scientists, chemical engineers, and industrial stakeholders will be essential for maximizing GO’s practical applications and contributions to technological advancement.

Glossary

Vocabulary

Graphene oxide (GO)

A material derived from graphite that contains oxygen functional groups, enhancing its dispersibility and chemical reactivity for various applications, including energy storage, water purification, and biomedicine.

Graphite powder

A fine, particulate form of graphite that is commonly used as a precursor in the production of GO by chemical oxidation methods.

Graphite sheet

A flat, thin form of graphite, typically used in large-scale production of GO. It serves as the starting material for GO by electrochemical oxidation processes.

Electrochemical oxidation

A method for producing GO by applying an electric current to a graphite anode in an electrolyte solution, which leads to the intercalation and oxidation of graphite.

Purification

The process of removing impurities from GO after synthesis, typically through filtration, centrifugation, or other methods, to ensure the desired material properties for specific applications.

Biocompatibility

The ability of a material, such as GO, to interact safely with biological systems, avoiding harmful effects when used in medical or environmental applications.

This work was supported by JST, CREST Grant Number JPMJCR18R3 and JPMJCR24S6, Japan.

The author declares no competing financial interest.