Scalable Syntheses of Graphene Oxide and Reduced Graphene Oxide using Cascade Design Oxidation and Highly Basic Reduction Reactions

Summary

The article presents the protocols of a cascade design oxidation process and a highly basic reduction reaction for innovative, scalable conversion of graphite into multilayer graphite oxide powder, graphene oxide nanosheets, supramolecular reduced graphene oxide hydrogel, and reduced graphene oxide nanomaterials.

Abstract

This present work describes the scalable synthetic methods of oxidation and reduction reactions for producing graphene oxide and reduced graphene oxide nanomaterials. Cascade design of Mn(VII)-based oxidation reaction is applied to convert graphite into multilayer graphite oxide powder, the precursor of single-layer graphene oxide nanosheets. Since exothermic heats, relevant chemical ratios, and combinative strategy are utilized, the cascade design process saves considerable amounts of heating energy, chemical reagents, and synthetic time. In the next stage, the hydrated multilayer structure of graphite oxide is exfoliated in water with the support of sonication. Graphene oxide nanosheets are electrostatically stabilized and chemically reduced using highly basic ammonia solution at pH >11 and a temperature of 90 °C. Alkaline ammonium hydroxide solution is considered an environmentally friendly and inexpensive chemical agent for synthesizing reduced graphene oxide nanosheets assembled in a hydrogel structure. Hydrated reduced graphene oxide nanosheets in the supramolecular hydrogel can be ultrasonically exfoliated to produce homogeneous aqueous dispersions. The general protocol of oxidation-reduction reactions sequentially synthesizes graphite oxide powder, graphene oxide nanosheet, reduced graphene oxide hydrogel, and reduced graphene oxide nanosheet for various scientific research and multidisciplinary applications. Prospective development of the synthetic approach from laboratory scale to industrial production is envisioned to elaborate on the potential.

Introduction

Graphene-based materials have great potential to revolutionize many applications and industries, including water purification, polymer nanocomposites, paints, coatings, inks, supercapacitors, lithium-ion batteries, solar cells, fuel cells, and artificial photosynthesis¹. Today, graphene-based materials are synthesized from various raw materials, such as natural/artificial graphite, biocarbons, and hydrocarbon/hydrogen gases. Methods for large-scale production of graphene-based materials include liquid-phase exfoliation of graphite, oxidation-reduction processes, and chemical vapor deposition (CVD)¹. Liquid-phase exfoliation method has been scaled up for industrial manufacturing of pristine graphene nanosheets. Efficiency, standard, and processability of pristine graphene are crucial issues for development in this approach². About the CVD method, single-layer graphene layers are synthesized from the deposition of hydrocarbon/hydrogen gases (typically methane and hydrogen) on metal substrates. CVD graphene is atomically thin, conductive, and transparent for biosensing, optoelectronic, and electronic applications. In the approach of oxidation-reduction processes, natural/artificial graphite structures are oxidized to synthesize graphite oxide (GrO) and graphene oxide nanosheets (GO) that are then reduced to obtain reduced graphene oxide nanosheets (RGO)³. The oxidation-reduction pathway produces both GO and RGO nanomaterials that are solution-processable and versatile building blocks for a variety of formulations and applications.

In the aspect of the primary process in the oxidation-reduction approach, graphite oxidation can be conducted using an electrochemical method or a chemical reaction. At present, graphite oxidation reaction using Hummers reagents (sulfuric acid, potassium permanganate, water, and hydrogen peroxide) is considered the most popular chemical method for GrO/GO production in scientific laboratories and industrial factories^4,5,6. Manganyl (VII) or Mn(VII) compound, deriving from the mixture of potassium permanganate and concentrated sulfuric acid, is an inexpensive and potent agent for oxidation reactions, but it is also dangerous and explosive at temperatures above 55 °C⁷. Conventional designs of graphite oxidation reaction use a high amount of Mn(VII) compound (graphite: KMnO₄ = 1:3 w/w) and the exothermic addition of water to graphite/Mn(VII) reaction mixture, leading to serious risks of thermal runaway explosion and additional energy-consuming cooling/heating processes⁸. Therefore, our recent research papers presented the cascade design of Mn(VII)-based oxidation reaction (using graphite: KMnO₄ ratio of 1:2 w/w and two exothermic cascade steps), providing the advantages of process safety, energy saving, and chemical efficiency for scalable production technology^3,8. As a result, graphite is chemically converted into GrO, a multilayer structure intercalated with water molecules in intersheet galleries. When sonicated in water, ultrasonic vibrations and cavitations in aqueous channels exfoliate multilayer GrO into single-layer GO nanosheets.

Regarding the secondary process in the oxidation-reduction approach, GO nanosheets dispersed in aqueous solution are chemically reduced to obtain RGO nanosheets. Our recent paper demonstrated that highly basic ammonia solution at pH >11 is an effective environment for the reduction reaction³. In comparison with the reducing agents of hydrazine, hydroiodic acid, ascorbic acid, and sodium borohydride, ammonia solution is more inexpensive and alkaline for synthesizing supramolecular RGO hydrogel³. As-synthesized RGO nanosheets still contain a number of oxygen-containing functional groups (C/O ratio of RGO nanosheets is about 4.17) for retaining surface hydration. With the water-intercalated hydrogel structure, hydrated RGO nanosheets can be ultrasonically redispersed in water at appropriate concentrations.

In this manuscript, protocols of cascade design oxidation reaction and highly basic reduction reaction are described for safe, efficient, and scalable production of GO and RGO in laboratory and pilot scales. Considerable improvements in the synthetic procedures are reported for scale-up development. In the aspect of GrO/GO synthesis, the combinative cascade strategy of adding several chemical units into water is implemented and analyzed to elaborate on the scale-up potential. In the aspect of RGO synthesis, GrO precursor structures are electrostatically stabilized, ultrasonically exfoliated, and fully converted into RGO hydrogel. The improved procedure of alkaline reduction reaction significantly enhances production efficiency because of the complete conversion of GrO material into RGO hydrogel. The RGO hydrogel obtained has good properties of three-dimensional morphology, supramolecular self-assembly, aqueous dispersibility, and graphene-based nanostructures. In general, the chemical oxidation-reduction processes produce high-quality products of GrO powder, GO nanosheets, RGO hydrogel, and RGO nanosheets.

Protocol

1. Scalable production of graphite oxide using cascade design oxidation reaction

1. Preparation of three beakers of graphite/H₂SO₄ suspension
   1. Add 5.0 g graphite powder to a 50 mL glass beaker. Add 50 mL of 97% sulfuric acid (H₂SO₄) to the beaker. Prepare an additional two beakers of graphite/H₂SO₄ suspension in the same way.
      CAUTION: Concentrated sulfuric acid 97 % is a strong corrosive chemical that may cause severe burns upon contact with organic substances.
   2. Stir (magnetically) the three graphite/H₂SO₄ suspensions in ambient conditions for at least 1 h.
2. Preparation of three erlens of Mn(VII)/H₂SO₄ solution
   1. Add 100 mL of 97% sulfuric acid to a 250 mL glass erlen. Slowly add 10 g potassium permanganate (KMnO₄) to dissolve in the H₂SO₄ solution under magnetic stirring, resulting in about 100 mL of Mn(VII)/H₂SO₄ solution. Similarly, prepare the second and third erlens of Mn(VII)/H₂SO₄ solution, respectively.
      ​CAUTION: The Mn(VII)/H₂SO₄ solution is a dangerous oxidizing compound that may become explosive at temperatures above 55 °C or upon contact with organic substances (tissue paper, lubricant grease, ethanol, and acetone).
   2. Use an infrared thermometer for measuring reactor temperatures. Stir (magnetically) the three Mn(VII)/H₂SO₄ solutions for at least 15 min before the next reaction step.
3. First cascade of graphite/H₂SO₄ suspension into Mn(VII)/H₂SO₄ solution
   1. Put one as-prepared erlen of Mn(VII)/H₂SO₄ solution in a water bath with ambient water temperature of around 27 °C. Use an infrared thermometer for measuring reactor temperatures during reaction processes.
   2. Conduct the first cascade step by pouring one above-prepared graphite/H₂SO₄ suspension (50 mL) very slowly into the Mn(VII)/H₂SO₄ solution (100 mL) under magnetic stirring in the 250 mL glass Erlen.
      CAUTION: This reaction stage may be explosive if the exothermic reaction temperature increases to above 55 °C or the reaction mixture is contaminated with organic impurities and/or water.
   3. After 9 min of putting the Erlen in the water bath, take the Erlen with the reaction mixture (graphite/Mn(VII)/H2SO4) out of the water bath and keep stirring the reaction mixture at room temperature. The reaction temperature gradually increases to a peak in the range of 48 - 52 °C and then slowly decreases to near-room temperature.
   4. Prepare the three erlens of graphite/Mn(VII)/H₂SO₄ mixtures as described above by the slow addition of graphite/H₂SO₄ suspension to Mn(VII)/H₂SO₄ solution.
   5. Stir (magnetically) the three mixtures of graphite/Mn(VII)/H₂SO₄ reaction for at least 4 h, resulting in three erlens of graphite/Mn(VII)/H₂SO₄ mixtures.
4. Second cascade of graphite/Mn(VII)/H₂SO₄ mixture into water
   1. Conduct the second cascade step using the combinative strategy of adding three as-prepared graphite/Mn(VII)/H₂SO₄ mixtures to an appropriate amount of water. At first, add 1080 mL of pure water to a 2 L glass beaker, followed by magnetic agitation.
   2. Carefully pour the first Erlen of graphite/Mn(VII)/H₂SO₄ mixture into water under agitation in the 2 L beaker. Exothermic heat from the reaction of water and concentrated sulfuric acid raises the reaction temperature to about 59 °C. To measure reactor temperatures, use an infrared thermometer.
   3. Add slowly the second Erlen of graphite/Mn(VII)/H₂SO₄ mixture to the reaction. As a result, the mixture temperature increases to about 80 °C.
   4. Add the third Erlen of graphite/Mn(VII)/H₂SO₄ mixture to the reaction, making the mixture temperature rise to approximately 94 °C.
   5. Agitate the reaction suspension for 2 h. The reaction temperature spontaneously declines to near-room temperature.
5. Addition of hydrogen peroxide solution
   1. Prepare 450 mL of 5% hydrogen peroxide solution by mixing approximately 75 mL of 30% hydrogen peroxide solution and 375 mL of pure water.
   2. Gradually pour the obtained 450 mL of 5% H₂O₂ solution into the reaction mixture in the 2 L beaker. A small increase in mixture temperature occurs due to the exothermic reaction.
   3. Keep stirring the reaction mixture in ambient conditions for about 1 day.
6. Washing graphite oxide with 5% hydrochloric acid solution and water
   1. Pour the reaction mixture in the 2 L beaker into 50 mL centrifuge tubes for centrifugation at 1500 x g for 5 min at ambient temperature conditions.
   2. After centrifugation, collect the sedimented solids for mixing with 1 L of 5% hydrochloric acid solution. Agitate the acidic suspension for at least 1 h using a magnetic stirrer.
   3. Centrifuge the acidic suspension at 1500 x g for 10 min, at an ambient solution temperature of 28 °C to remove the supernatant acidic solution. Wash the sedimented solids with pure water until the supernatant solution reaches pH 4. Measure the solution pH using pH papers.
   4. Use filtration and pure water to wash the sedimented solid of graphite oxide in a vacuum filtration system until the aqueous filtrate shows a pH of nearly 7. Use a porcelain Buchner funnel (diameter of 125 mm) and filter papers (diameter of 110 mm) for filtration.
7. Drying, grinding, and packaging
   1. After the washing stage, dry the graphite oxide slurry at 80 °C in a drying oven for about 2 h. Use a stainless steel or ceramic mortar with a corresponding pestle to grind the obtained material to produce a powder of graphite oxide product (GrO).
   2. Weigh and analyze the moisture of GrO powder (the moisture of GrO should be about 20% wt). To measure moisture content, use an electronic moisture analyzer. Store the GrO product in a plastic bag.
8. Characterization of the powder
   1. Measure the weight and moisture of GrO powder using an analytical balance and an electronic moisture analyzer. Calculate solid contents (by subtracting moisture weight from total GrO weight), product yields (the ratio of GrO solid weight to initial graphite weight), and chemical efficiencies (equivalent to the mass ratio of GrO solid product to the KMnO4 reactant used).
   2. Characterize GrO product using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) as described in previously published papers^3,8. Raman spectroscopy is analyzed with the laser excitation wavelength of 532 nm.

2. Scalable synthesis of reduced graphene oxide hydrogel using highly basic reduction reaction

1. Preparation of graphene oxide dispersion
   1. Weigh 1.25 g of GrO powder and add 1000 mL of pure water. Drop 25%-28% ammonia solution drop by drop into the aqueous dispersion to make the solution pH 10.
   2. Agitate the aqueous alkaline dispersion for 1 h. Sonicate the GrO dispersion using an ultrasonic probe with power of 100 W, continuous cycle, and amplitude of 80%. Sonicate for 15 min, followed by a cooling period of 15 min in ambient conditions. Perform a total of four sonication cycles for a total of 1 h of sonication.
2. Reduction of graphene oxide dispersion using highly basic ammonia solution
   1. Pour the obtained 1 L dispersion of graphene oxide (GO) into a 1-L round glass reactor. Add 111 mL of 25%-28 % ammonia solution to the reactor, making the reaction mixture highly basic (pH > 11). 
      NOTE: The measurement of solution pH using pH papers shows violet colour (pH 14), and the measurement using pH meters presents basic pH > 11.
   2. Seal the glass reactor with polyethylene films and put in a water bath. Heat the reaction mixture (1111 mL) to 90 °C, without agitation. Maintain at 90 °C for 3 h. Then, turn off the heating and leave the reactor to cool down gradually.
3. Filtration, washing, gelation, and storage
   1. Filter the highly basic mixture of reduced graphene oxide (RGO) using filter fabrics or cellulose filter papers in a vacuum filtration system. Wash RGO solid with pure water until the filtrate solution reaches pH 8.
   2. Dry the obtained RGO slurry at 80 °C for gelation. Use an electronic moisture analyzer to measure the moisture content of the RGO hydrogel (it should be about 97%). Collect and store the RGO hydrogel product in a closed plastic box.
4. Characterization of the hydrogel
   1. Measure weight and moisture of RGO hydrogel to calculate solid weights and product yields. RGO solid weight is the difference between RGO hydrogel weight and the moisture weight. The product yield or production efficiency is the mass ratio of RGO solid product to GrO solid precursor.
   2. Characterize RGO hydrogel product using XRD, FTIR, Raman spectroscopy, SEM and EDS techniques as presented in our previous publications^3,8. The wavelength of incident laser in Raman spectroscopy is 532 nm.
   3. Disperse 0.1 g RGO hydrogel in 300 mL of water by agitation and sonication. Ensure homogeneous dispersion of RGO nanosheets to indicate the aqueous dispersibility of the supramolecular RGO hydrogel.

Results

The experimental schemes of cascade design, oxidation reaction, and highly basic reduction reaction are presented in Figure 1. As described earlier, the graphite oxidation reaction includes two main cascade steps^3,8. In the protocol, after steps 1.1 and 1.2, step 1.3 was implemented (Figure 1A). The increase in mixture temperature from room temperature to about 48-52 °C indicates that an exothermic oxida...

Discussion

In the aspect of GrO synthesis, oxidation methods using Hummers reagents (KMnO₄, H₂SO₄, H₂O, and H₂O₂) have been applied in GrO productions in laboratory and industrial scales worldwide, owing to basic chemical availability and simple oxidation processes^1,3,5,7. However, serious explosive risks of Mn(VII) compounds derived from K...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ammonia solution 25% - 28%	Xilong Scientific Company	1336-21-6
Graphite powder	Shanghai Zhanyun Chemical Company	7782-42-5
Hydrochloric acid 36%	Xilong Scientific Company	7647-01-0
Hydrogen peroxide solution 30%	Xilong Scientific Company	7722-84-1
Potassium permanganate 99.5%	Duc Giang Chemicals Group	7722-64-7
Sulfuric acid 97%	Xilong Chemical Company	7664-93-9