Modified Hummers Method for Graphene Oxide

Research will be limited to:
- Material: Graphene Oxide
- Method: Modified Hummers Method
- Reactants: KMnO4, H2SO4
- Process: Washing and Exfoliation

Strategy: The objective is to identify the precise experimental parameters for the modified Hummers method, specifically focusing on the temperature control during the addition of KMnO4 to H2SO4, the subsequent washing protocols (HCl vs. DI water) to remove metal ions, and the mechanical/ultrasonic exfoliation conditions required to achieve high-quality graphene oxide. I will search for standardized protocols and comparative studies that optimize these specific steps.
Queries:
- modified Hummers method KMnO4 H2SO4 temperature control protocol
- graphene oxide washing procedure HCl deionized water purification
- exfoliation of graphite oxide to graphene oxide sonication vs stirring
- modified Hummers method safety thermal runaway prevention KMnO4
- characterization of graphene oxide exfoliation efficiency modified Hummers

Executing 5 concurrent searches:
- modified Hummers method KMnO4 H2SO4 temperature control protocol
- graphene oxide washing procedure HCl deionized water purification
- exfoliation of graphite oxide to graphene oxide sonication vs stirring
- modified Hummers method safety thermal runaway prevention KMnO4
- characterization of graphene oxide exfoliation efficiency modified Hummers

Found 9 papers. Key excerpts:

Graphene oxide based coconut shell waste: synthesis by modified Hummers method and characterization — E.H. Sujiono et al., 2020

1. IntroductionGraphene is an exciting material that has an uncommon two-dimensional skeleton with a hexagonal structure of single monomolecular layer of sp2-hybridized carbon atoms [[1]Alam S.N. Sharma N. Kumar L. Synthesis of graphene oxide (GO) by modified Hummers method and its thermal reduction to obtain reduced graphene oxide (rGO).Graphene. 2017; 6: 1-18Crossref
   Google Scholar, [2]Guerrero-Contreras J. Caballero-Briones F. Graphene oxide powders with different oxidation degree, prepared by synthesis variations of the Hummers method.Mater. Chem. Phys. Mar. 2015; 153: 209-220Crossref
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   Google Scholar]. Graphene has attracted intense interest in the many areas of science and technology because of its unique properties [[3]Zhang C. et al.Graphene oxide reduced and modified by environmentally friendly glycylglycine and its excellent catalytic performance.Nanotechnology. Apr. 2014; 25: 135707Crossref
   PubMed
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   Google Scholar] such as excellent electronic [[4]Meyer J.C. Geim A.K. Katsnelson M.I. Novoselov K.S. Booth T.J. Roth S. The structure of suspended graphene sheets.Nature. Mar. 2007; 446: 60-63Crossref
   PubMed
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   Google Scholar, [5]Stankovich S. et al.Graphene-based composite materials.Nature. Jul. 2006; 442: 282-286Crossref
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   Google Scholar, [6]Heersche H.B. Jarillo-Herrero P. Oostinga J.B. Vandersypen L.M.K. Morpurgo A.F. Bipolar supercurrent in graphene.Nature. Mar. 2007; 446: 56-59Crossref
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   Google Scholar], thermodynamic, and mechanical properties [[7]Balandin A.A. et al.Superior thermal conductivity of single-layer graphene.Nano Lett. Mar. 2008; 8: 902-907Crossref
   PubMed
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opus (11445)
Google Scholar, [8]Rattanaet al. Preparation and characterization of graphene oxide nanosheets.Procedia Eng. 2012; 32: 759-764Crossref
Scopus (323)
Google Scholar]. Graphene has a wide range of applications such as transparent conductive films, field effect transistors (FET), water purification, energy storage devices, and sensors due to its outstanding physical and chemical properties [[9]Ding Y.H. Zhang P. Zhuo Q. Ren H.M. Yang Z.M. Jiang Y. A green approach to the synthesis of reduced graphene oxide nanosheets under UV irradiation.Nanotechnology. May 2011; 22: 215601Crossref
PubMed
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Google Scholar, [10]Ramesha G.K. Vijaya Kumara A. Muralidhara H.B. Sampath S. Graphene and graphene oxide as effective adsorbents toward anionic and cationic dyes.J. Colloid Interface Sci. Sep. 2011; 361: 270-277Crossref
PubMed
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Google Scholar, [11]Song J. Wang X. Chang C.-T. Preparation and characterization of graphene oxide.J. Nanomater. 2014; 2014: 1-6Google Scholar, [12]Zheng Q. Li Z. Yang J. Kim J.-K. Graphene oxide-based transparent conductive films.Prog. Mater. Sci. Jul. 2014; 64: 200-247Crossref
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Google Scholar, [13]Huang X. et al.Graphene-based materials: synthesis, characterization, properties, and applications.Small. Jul. 2011; 7: 1876-1902Crossref
PubMed
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Google Scholar]. The first fabrication of single-layer graphene nanosheets was achieved by an exfoliation technique called the Scotch-tape method from bulk graphite [[14]Guo T. Chen X. Su L. Li C. Huang X. Tang X.-Z. “Stretched graphene nanosheets formed the ‘obstacle walls’ in melamine sponge towards effective electromagnetic interference shielding applications.Mater. Des.

Nov. 2019; 182: 108029Crossref
Scopus (45)
Google Scholar] and by epitaxial chemical vapor deposition. However, the drawback of these methods is that they are not applicable for graphene manufacturing in industrial production [[15]Luo J. Wang J. Xia F. Huang X. Direct growth of large area uniform double layer graphene films on MgO(100) substrates by chemical vapor deposition.Mater. Chem. Phys. May 2019; 233: 213-219Crossref
Scopus (6)
Google Scholar]. Synthesis of graphene nanosheet using mechanical exfoliation method is not applicable for large scale production. Therefore, the development of large-scale synthesis methods from materials that are structurally similar to graphene has attracted increasing research attention [[16]Cinar A. Baskut S. Seyhan A.T. Turan S. Tailoring the properties of spark plasma sintered SiAlON containing graphene nanoplatelets by using different exfoliation and size reduction techniques: anisotropic mechanical and thermal properties.J. Eur. Ceram. Soc. Apr. 2018; 38: 1299-1310Crossref
Scopus (10)
Google Scholar]. One of the most prevalent and interesting approaches for graphite exfoliation on a large scale is through the use of active oxidizing agents in a chemical reaction to produce graphene oxide (GO) which is a carbon material with nonconductive hydrophilic properties [[17]Gurunathan S. Woong Han J. Eppakayala V. Kim J. Green synthesis of graphene and its cytotoxic effects in human breast cancer cells.Int. J. Nanomed. Mar. 2013; 1015Google Scholar].GO is a carbon material that shows chemical, optical, and electrical properties similar to those of graphene because it is based on the graphene framework [[18]Shao G. Lu Y. Wu F. Yang C. Zeng F. Wu Q. Graphene oxide: the mechanisms of oxidation and exfoliation.J. Mater. Sci. May 2012; 47: 4400-4409Crossref
Scopus (318)
Google Scholar]. However, GO differs from graphene in that oxygen functional groups such as epoxy and oxygen groups are located on the basal plane of GO, and small amounts of carbonyl and carboxyl groups are present at its sheet edges

Structure and Synthesis of graphene oxide — Sun et al., 2019

Graphene oxide (GO) is regarded as one typical two-dimension structured oxygenated planar molecular material. Researchers across multiple disciplines have paid enormous attention to it due to unique physiochemical properties. However, models used to describe the structure of GO are still in argument and ongoing to update. Currently, synthesis methods for massive production are seemingly abundant but in fact, dominated by few thought systems. We herein aim to give a mini but critical review over the synthesis of graphene oxide as well as its structure, involving relative peer work.
Keywords
Graphene oxide; structure; synthesis; preparation
* Corresponding author. E-mail: sunling@bjut.edu.cn (Ling Sun)
1. Introduction
Graphene oxide (GO) is the oxidized analogy of graphene, recognized as the only intermediate or precursor for obtaining the latter in large scale, [1] since the English chemist, sir Brodie first reported about the oxidation of graphite centuries ago [2]. About thirty years ago, the term graphene was officially claimed to define the single atom-thin carbon layer of graphite [3], which structurally comprises sp2 hybridized carbon atoms arranged in a honeycomb lattice, rendering itself large surface area and some promising properties in terms of mechanical, electrical, and others. [4, 5] Despite these extraordinary properties, purely single-layer graphene remains very limited success in practical applications due to the difficulties in the large-scale formation of specifically organized structures. [6] But the precursor GO has advanced much in both academics and industries in the last decades because of its readiness by exfoliating bulk graphite oxide facilely prepared from the oxidation of graphite. [7, 8] This bottom-down chemical strategy features the upmost flexibility and effectiveness thereby arousing the greatest interest in practical applications.
Now it is seemingly clear that GO is a non‐stoichiometric chemical compound of carbon, oxygen, and hydrogen in variable ratios which largely yet partly depend on the processing methodologies. [2, 9–11] GO possesses abundant oxygen functional groups that are introduced to the flat carbon grid during chemical exfoliation, evidenced as oxygen epoxide groups (bridging oxygen atoms), carbonyl (C=O), hydroxyl (-OH), phenol, and even organosulfate groups (impurity of Sulphur). [12, 13]In other word, these defects of various kinds are brought into

the naturally intact graphene structure, further categorized into on‐plane functionalization defects and in‐plane lattice defects (vacancy defects and hole defects) which are semi-randomly distributed in GO’s σ‐ framework of the hexagonal lattice. [1, 7] Such a defect-rich structure leads to a set of unique properties of GO and render it availability and scalability of consequent applications via post treatments, e.g. chemically-derived graphene-like materials, functionalized graphene-based polymer composites, sensors, photovoltaics, membranes[14] and purification materials. On the detailed structure of GO, however, it remains some ambiguous and literature reports are still in argument (Fig.1). [11, 13, 15–23] In addition, methods in regard to synthesis of GO has been massively researched in the past few years. The effectiveness and environmental benignity was core driven force for the continuous evolvement. Herein, we update the progress and make a short yet critical review on model structures of GO as well as the synthesis.
2. Graphene oxide structure
In 1936, Hofmann and Rudolf [22] proposed the first GO structure (Figure 1, the top-left 1st schematic structure) in which a deal of epoxy groups are randomly distributed on the graphite layer, and then in 1946, Ruess [21] updated the Hofmann model by incorporation with hydroxyl entities and alternation of the basal plane structure (sp2 hybridized model) with a sp3 hybridized carbon system. By contrast, in 1969, Scholz and Boehm [19] proposed a less ordered structure with C=C double bonds and periodically cleaved C-C bonds within the corrugated carbon layers and hydroxyl, carbonyl groups in different surroundings, free from ether oxygen. More further in 1994, Nakajima and Matsuo [18] proposed a stage 2 graphite intercalation compound (GIC)resembled lattice framework based on the fact that fluorination of graphite oxide gives the same X-ray diffraction pattern as that of stage 2-type graphite fluoride, (C2F)n. In 1998, Lerf and Klinowski et al. [17] characterized their GO by the 13C and 1H nuclear magnetic resonance (NMR), and subsequently found the 60 ppm line better related to epoxide groups

(1,2-ethers) other than 1,3 ethers, and the 130 ppm line to aromatic entities and conjugated double bonds. The carbon atoms attached to OH groups slightly distorted their tetrahedral structure, resulting in partial wrinkling of the layers. Accordingly, they proposed a model featuring a nearly-flat carbon grid structure with randomly distributed aromatic regions with unoxidized benzene rings and regions with aliphatic six-membered rings. However, all these earlier models could not well explain the origin of the planar acidity of GO, which is now a well-understood chemical property for GO. Thereafter, Szabó and coworkers in 2006 [16] revived but a little modified the Scholz-Boehm model, etc. by again examining the results from elemental analysis, transmission electron microscopy, X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, X-ray photoelectron spectroscopy, and electron spin resonance besides NMR. They then proposed a carboxylic acid-free model comprising two distinct domains: trans-linked cyclohexyl species interspersed with tertiary alcohols and 1,3-ethers, and a keto/quinoidal species corrugated network. Interestingly, as to the phenomenon of GO in basic solution Roukre et al. [23] found that GO decomposed into
slightly oxygenated graphene part and strongly graphene-bound oxidative debris (OD) upon suffering a base washing, and then suggested a simple OD-base washed GO two-component model, which was much different from those previously proposed, upgrading the way we used to understand about GO. Besides, they also mentioned about the metastability of unwashed GO, which reminded us of previous room-temperature metastable GO film [24], while the internal mechanism of external stimuli-responded structural instability was lack of sufficient investigation. In 2013, Dimiev et al. [11] revisited the structure via acid titration and ion exchange experiment in term of acidity of GO and proposed a novel dynamical structural mode (DSM), which describes the evolution of several carbon structures with attached water beyond the static Lerf’s model. More recently, Liu et al. [25] experimentally observed oxygen bonding and evidenced the C=O bonds on the edge and plane of GO, confirming parts of earlier proposed models, especially the L-K model.

Eco-Friendly Approach for Graphene Oxide Synthesis by Modified Hummers Method. — Néstor Méndez-Lozano et al., 2022

Graphene has excellent mechanical, electronic, optical and thermal properties. It has a unique two-dimensional structure one atom thick [ 1 ]. Many researchers have been interested in investigating this two-dimensional (2D) form of carbon because it has become a relevant topic for the development of materials with many applications [ 2 ]. As reported in the literature, graphene has a large specific surface area [ 3 ], an efficient electron mobility (200,000 cm 2 v − 1 s − 1 ) [ 4 , 5 ], a high Young’s modulus (1 TPa) [ 6 ], and good thermal conductivity (4.84 × 10 3 to 5.30 × 10 3 W/mK) [ 7 ]. Graphene oxide (GO) can be manufactured or self-assembled into materials with controlled compositions and microstructures for different applications [ 8 ]. Previous work has reported the use of graphene oxide combined with fullerene in thin-film form to produce lightweight three-dimensional hybrid structures with high surface area [ 9 ]. The arrangement of other molecules within graphene oxide layers has shown that multilayer structures exhibit high biocatalytic activity [ 10 ]. The Langmuir–Blodgett process has recently been used for the production of graphene oxide by which a uniform dispersion and controllable development of graphene oxide flakes has been achieved [ 11 ].
The most important and widely applied method for GO synthesis is that developed by Hummers and Offeman [ 12 ]. This method has three important advantages over other techniques. First, the reaction is complete in a few hours, second, potassium chlorate can be replaced by potassium permanganate for a safer reaction, and third, the use of sodium nitrate eliminates acid mist formation. However, the method also has some defects, since in the oxidation process, some toxic gases such as nitrogen dioxide and dinitrogen tetroxide are released. In addition, sodium and nitrate ions are difficult to remove from the wastewater formed during the process of synthesis and purification of graphene oxide.
In previous works, the Hummers method has been improved by excluding sodium nitrate and increasing the amount of potassium permanganate, carrying out the reaction in a single mixture [ 13 ]. With this modification, it is possible to increase the performance of the reaction and reduce the release of toxic gases; also, phosphoric acid is introduced in the reaction system. Previous research has reported that the mixture of sulfuric acid and nitric acid used

in the Hummers method acts as a “chemical scissors” for graphene planes that facilitates the penetration of the oxidation solution [ 14 ].
On the other hand, potassium permanganate can achieve the complete intercalation of graphite, forming graphite bisulfate [ 15 , 16 ]. This interaction ensures the effective penetration of potassium permanganate into the graphene layers for graphite oxidation. Due to this, potassium permanganate replaces the function of sodium nitrate, so it is not necessary for the reaction. In this investigation, we show an easy synthesis route to produce GO using a low-cost and environmentally friendly modified Hummers method. In addition, the synthesis route is highly reproducible in obtaining graphene oxide for its subsequent reduction (rOG) for possible biocatalytic applications as reported in previous works.
Materials included: natural graphite (99%) supplied by Aldrich chemistry, potassium permanganate (KMnO 4 ) supplied by Sigma Aldrich, sulfuric acid (H 2 SO 4 ) supplied by Jalmex, hydrochloric acid (HCl) supplied by Sigma Aldrich, and hydrogen peroxide (H 2 O 2 ) supplied by J.T Baker. All the reagents were obtained in the city of Queretaro, Mexico.
The synthesis process for obtaining graphene oxide is described below. Two glycerin baths are preheated to 45 °C and 98 °C, respectively. Then, 1 g of graphite was added in a ball flask in a cold bath for 5 min with 23 mL of sulfuric acid (H 2 SO 4 ), it was stirred for 5 min. Subsequently, potassium permanganate (KMnO 4 ) was added and placed in the glycerin bath at 45 °C for 2 h. After 2 h, the mixture was transferred to a glycerin bath at 98 °C, adding 46 mL of distilled water at room temperature; then, it was kept for 15 min. After 15 min, 140 mL of hot water was added along with 10 mL of hydrogen peroxide (H 2 O 2 ). The mixture obtained was emptied into the strainer to filter by vacuum. The sample was removed and placed in 6 jars with 1 g of sample. Finally, 2 mL of hydrochloric acid (HCl) and distilled water were added to wash the samples by centrifugation. The final sample was placed in a petri dish to be dried in oven

at 60 °C and 90 °C for 24 h. In Table 1 , the number of washes, the centrifugation revolutions and the time for each sample is shown. In addition, in Figure 1 , the results after washing and after drying, respectively, are observed. Finally, in Figure 2 , the synthesis process is shown in a flow chart. Table 2 shows a comparison between the traditional synthesis method and the modified Hummers method used in this work.
In this study, X-Ray Diffraction (XRD) was used to determine the crystal structure and verify the spacing between the GO layers.
The XRD pattern for the sample dried at 60 °C, GO60-1 and GO60-2 is presented in Figure 3 . This sample exhibits a diffraction peak at 9.28° due to the (002) plane of GO [ 17 ]. In addition, a small peak at ≈26° is observed; according to the literature, this peak corresponds to graphite. When the graphite is oxidized, the diffraction peak should change from ≈26° to ≈11°; this agrees with the results observed in Figure 3 .
On the other hand, the XRD pattern for the sample dried at 90 °C is presented in Figure 4 GO90-1 and GO90-2. This sample presents a diffraction peak at 9.6°, which is slightly different from the GO60 sample. In GO90-1, a small peak at ≈26 corresponding to graphite can also be observed; however, in GO90-2, this peak is no longer present, which suggests that in said sample, all the graphite was oxidized to become GO. In addition, the intensity of it is three times greater compared to GO90-1, which suggests a greater number of planes (002) in that direction. This difference could be due to the increase in the drying temperature compared to the GO60 sample.
In both samples, the observed peaks are sharp, which indicates that the graphite was completely oxidized by this method. The spacing between the GO layers was calculated using Bragg’s law [ 18 ]. λ = 2 d s i n θ where n is the diffraction series and λ is the X-ray wavelength 0.154 nm . The spacing between the GO60 and GO90 layers was 0.95 nm and 0.92 nm, respectively, according to Br

Testing the influence of the temperature, RH and filler type and content on the Universal Power Law for new reduced graphene oxide TPU composites — J Gómez et al., 2017

In this paper, 6 different reduced graphene oxide (rGO) were prepared by a modified Hummers’ method and reduced by thermochemical methods. rGO materials were intentionally prepared to obtain different BET and thickness and oxygen content maintaining constant the lateral size to compare its performance on thermoplastic polyurethane (TPU) matrix. Microstructure and the effect of the incorporation of rGO on the hardness and electrical properties of TPU were investigated. It has been studied the temperature and humidity dependence of the electrical conductivity and the sensitivity and the response time to humidity changes have been determined. Influence of the filler content, temperature and humidity on the Jonscher’s universal power law (UPL) for ac conductivity vs frequency and its fitting parameters A and n were determined. It has been observed an anomalous behaviour (according to UPL) and a linear correlation between log A and n independently of the filler content, humidity and temperature, however there is an influence of the rGO used for the preparation of the composite. To study the transport mechanisms the experimental results were adjusted to the equation = 0 exp[-(TMott/T)] and the maximum adjustment for = 1/4 like other carbon nanocomposites however there is not an unequivocal behaviour.
1. Introduction
Graphene based nanocomposites have attracted much attention in the last decade to improve physicochemical properties of polymer matrices: electrical and thermal conductivity, mechanical performance [1-6].
Thanks to the particular chemical structure of thermoplastic polyurethanes (TPUs), characterized by hard and soft segment, these materials are very versatile offering a wide range of service temperatures and hardness options and excellent chemical and mechanical performance [7]. It is possible to increase and adapt mechanical or electrical properties of TPU resins using nanofillers, such as graphene in the preparation of composites. These properties make TPUs suitable for their use in several products and different industries such as footwear, engineering, building & construction, automotive, hose & tubing,
wires & cables, and medical, and growth is expected in automotive, engineering and medical applications [8].
Currently, a variety of techniques have been developed to prepare graphene nanosheets (GNS). Reduced graphene oxide (rGO) has attracted much attention to obtain large lateral-size graphene materials, due to of their excellent electrical conductivity properties, and scalability, that makes it in

an alternative to other carbon-based materials such as CNTs, Carbon Blacks and conductive graphite materials.
Among chemical oxidation methods, Hummers’ method [9] has become the most popular approach to obtain graphene oxide as starting material in large scale production of graphene sheets by reduction methods: chemical or thermal. However, graphene material produced by this method contains a high amount of sp3 defects. Chemical reduction and thermal annealing can be used for reducing sp3 defects. Beside thermal, chemical and thermochemical reduction of graphene and graphite oxides are easy customizable and versatile methods for preparing different types of reduced graphene oxide grades. [10- 12]
We have prepared and characterized 6 different rGO materials, controlling the exfoliation step. We have achieved low oxygen content (2,6%) for RGO1 and RGO2, similar lateral size, while the specific surface area (SSA), measured by BET isotherm, has shown significant differences in average thickness. We have also modified the reduction step to obtain rGO materials with different oxygen content and defects concentration. We produced rGO-TPU composites by solution blending with these rGO.
We published the preparation and measurement of the electrical conductivity of TPU-rGO
nanocomposites using 20x20 m graphene sheets, which shows low percolation limit [13].
Recently, it has been reported high electrical conductivity over 10 S/m at 10% of loading in graphene nanoplatelets/CNTs/PU composites and similar electrical conductivity (10-4 S/m) at 1% of loading than the composites prepared for this study that show similar conductivity than previously reported ones [14-17]. Easy and quick preparation of these composites is one of their main advantages; however, we also want to highlight their high sensibility to humidity.
Empirical Jonscher´s UPL allows studying conduction mechanism into disordered matrices. The
measured ac conductivity () of conducting and semi-conducting materials is characterized by the
transition above a critical (angular) frequency 0 from a low-frequency dc plateau to a dispersive highfrequency region [18-25]. Anomalous power law dispersion has been observed in all kind of materials: single crystals, polymers, glasses, technical ceramics, conductive polymer composites, etc. [26-28]. Mauritz

has recently reported a linear evolution of log A vs n and has opened the question of the link between A and n and the influence of the different materials [19]. In this paper, we test UPL behaviour of the prepared rGO/TPU composites analysing the relationship between log A vs n. We have observed linear evolution log A vs n at different filler contents, temperatures and humidity levels and there is a different evolution depending on the rGO used in the production of the nanocomposite.
In this paper, we also explore the effects of the average thickness of rGO in electrical properties of TPU composites. Temperature and humidity effect in the electrical conductivity are also analysed in addition to their potential application in low cost humidity self-sensing material.
2. Experimental
2.1. Materials
Flake Graphite powder with particle size of 600 m was obtained from Grafitos BARCO, Spain. For
preparation of rGO samples; KMnO4, concentrated H2SO4, sodium hydroxide, hydrochloric acid and ascorbic acid were bought to COFARCAS (Spain). Thermoplastic polyurethane (TPU) GOLDENPLAST049 A85 NP30 with a density of 1,16 g/cm3 (at 20ºC) was used as received. N,Ndimethylformamide 99,9% (DMF; Labkem), methanol, and isopropanol (Cofarcas Spain), were used as received for the study.
2.2 Synthesis of graphene sheets
The 2 rGO were prepared by a modified Hummers’ method using flake graphite powders as the starting
material.
rGO Preparation:
Graphene oxide was prepared using a modified Hummers’ method [9] in H2SO4. Starting from graphite flakes (20 g) and using a proportion of graphite/KMnO4 1:3,75. The reaction temperature inside the reactor was kept between 0 and 4 ºC during the oxidants addition (48 h). After that time, resulting solution was slowly warmed up to 20ºC and maintained for 72 hours of reaction. To remove the excess of MnO4-, H2O2 solution was added to the reaction mixture and stirred overnight. After sedimentation, the solution was washed with a mechanical stirred H

Effect of the degree of oxidation of graphene oxide on As(III) adsorption — A. C. Reynosa-Martínez et al., 2019

*Corresponding author. Tel: 844 438-9637. E-mail: eddie.lopez@cinvestav.edu.mx (Eddie López)
The study of the interaction between graphene oxide and arsenic is of great relevance towards the development of adsorbent materials and as a way to understand how these two materials interact in the environment. In this work we show that As(III) adsorption, primarily H3AsO3, by graphene oxide is dependent on its degree of oxidation. Variations in the concentration of potassium permanganate resulted in an increase on the C/O ratio from 1.98 to 1.35 with C-OH and C-O-C concentrations of 18 and 32%, respectively. Three oxidation degrees were studied, the less oxidized material reached a maximum As(III) adsorption capacity of 124 mg/g, whereas the graphene with the highest degree of oxidation reached a value of 288 mg/g at pH 7, to the authors knowledge, the highest reported in the literature. The interaction between graphene oxide and As(III) was also studied by Density Functional Theory (DFT) computer models showing that graphene oxide interacts with As(III) primarily through hydrogen bonds, having interaction energies with the hydroxyl and epoxide groups of 378 and 361 kcal/mol, respectively. Finally, cytotoxicity tests showed that the graphene oxide had a cellular viability of 57% with 50 μg/ml, regardless of its degree of oxidation.
1. Introduction
Arsenic water pollution is a global challenge that affects countries in Europe, Asia and America [1]. For example, in Latin America 14 out of 20 countries suffer of this problem [2]. In Mexico, half of the country has shown water wells contaminated with arsenic with concentrations as high as 0.624 mg/L, particularly in arid and semiarid areas where other water sources are limited [3]. These concentrations are considerably high taking into account the limit of 0.01 mg/L established by the World Health Organization.
Currently, there are different techniques designed to removed arsenic, including adsorption, which is widely used due to its simplicity of operation and scalability [4]. A wide range of adsorbent material have been used such as iron oxides, activated alumina [5], activated carbon [6] and reduced graphene oxide (rGO) with nanoparticles such as magnetite,

ceria, zirconium and zirconium oxide and Fe3O4/MnO2 [7–13]. These materials have shown maximum adsorption capacities of 14.04 up to 212.33 mg/g for As(III) species (H3AsO3, H2AsO3 −, HAsO32− and AsO3 3−), which are the most dangerous species of As. Among these materials, graphene oxide has been used primarily to work as supporting material after reduction. Nevertheless, since graphene oxide contain oxygenated functional groups such as hydroxyl and epoxide, it has shown the capability to remover other contaminants such as uranium, plutonium and toxic dyes [14–16]. Since these functional groups are responsible for the adsorption capacity of graphene oxide, its degree of oxidation has a strong effect on the adsorption capacity of contaminants, therefore graphene oxide could be developed to adsorb high concentrations of arsenic without the use of other nanoparticles [17, 18]. This could simply the production of adsorbent materials and reduce its cost towards a practical use. Furthermore, since GO is widely used in the industry it is expected to be realized into the environment, therefore, studies on the interaction between GO and contaminants such as As, have important environmental implications as they also could help to elucidate contaminant transport processes as they have already suggested[19].
Different methods have been used to synthetize GO from graphite, one of the most known is the improved Hummers method in which KMnO4 is used as oxidant agent in a mixture of H2SO4:H3PO4 in a ratio 9:1[19, 20]. The oxidation process starts on carbon atoms at the edges and on defects of the graphene layer, where hydroxyl groups are formed. As oxidation progress the
basal plane undergoes oxidation and more hydroxyl groups are formed. Later on, the hydroxyl group on the edges are further oxidized to ketone and quinone groups, whereas the hydroxyl groups on basal plane are condensed to epoxide groups by dehydration. Finally, the ketone group is oxidized to carboxylic acid through the breakage of the carbon ring [22]. The complete oxidation of hydroxyl group to epoxide and carboxylic acid is achieved, for example, by increasing the amount of KMnO4 [22, 23].

In this work we produced three different graphene oxides with a different degree of oxidation. We characterized these GOn and tested their As(III) capacity based on pH and As(III) concentration, showing maximum adsorption capacities up to 288 mg/g, to our knowledge the highest reported in the literature for single graphene oxide and even reduced graphene oxide with nanoparticles of ferrite, 147 mg/g [25]. This work also shows the effect of secondary salts on its adsorption capacity and provided computer models to explain the adsorption process and cytotoxicity to understand their impact as suitable adsorbent material. Our results show that graphene oxide can achieve high As(III) adsorption capacities, thus simplifying the possible use of graphene oxide as adsorbent material for As(III).
2. Experimental

2.1. Synthesis of graphene oxide (GO1, GO2 and GO3)
The synthesis of GO was performed using the improved Hummers method proposed by Marcano [21], as follows: 400 ml of sulfuric acid (95-98%, Jalmek) and phosphoric acid (85.8%, J.T. Beaker) were mixed in a 9:1 volume ratio with 3 g of graphite flakes (Sigma-Aldrich 95%) and 9 g of potassium permanganate (99%, Sigma-Aldrich), 1 to 3 ratio, with a weight ratio of 1:3 at 50 °C for 24 hours. After that the mixture was cooled to 2 °C and 3 ml of hydrogen peroxide (30%, Jalmek) was added. The mixture was then diluted with deionized water until pH 1. The material was washed twice with 3 different solutions, the first one with hydrochloric acid (36.5-38%, Jalmek) at 30% v/v, then with deionized water and finally with ethanol (99.5%, Jalmek). The solid material was coagulated with ethyl ether (99%, Jalmek) and centrifuged at 3500 rmp during 30 minutes (in a centrifuge XC-2450 PREMIERE). Subsequently, the material was dispersed in ethanol (99.5%, Jalmek) and exfoliated in an ultrasonic bath (Branson 3800, Frequency 40
kHz/Sonics Power 110) during 1 hour. Finally, the solid was dried overnight at

FABRICATION OF ZINC OXIDE/GRAPHENE-CARBON NANOTUBES NANOCOMPOSITE WITH ENHANCED DYE DEGRADATION ABILITY — Intekhab Alam et al., 2021

and ZnO nanoparticles was carried out to investigate their abilities in the degradation of Rhodamine B (RhB) dye. We utilized the Modified Hummer’s method to prepare the graphene oxide (GO) nanosheet. Moreover, Gr-CNTs had been synthesized from GO and multi-walled carbon nanotubes with hydroxyl group (MWCNTs-OH). The hydrothermal method was employed to fabricate the ZnO/Gr-CNTs nanocomposite from ZnO nanoparticles and Gr-CNTs. During the characterization by X-ray Diffraction (XRD), all the significant peaks of ZnO and ZnO/Gr-CNTs were found in the same phase angle. In addition, the final nanocomposite was also characterized by Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-ray Spectroscopy (EDS). Finally, from the dye degradation test, it was apparent that ZnO/Gr-CNTs nanocomposite showed superior dye removal ability compared to the ZnO nanoparticles.
INTRODUCTION
Synthetic dyes are widely used by the textile industry as they are inexpensive, brighter, and easier to apply. However, these synthetic dyes have good solubility in water that makes them common water pollutants [1]. Their existence in the industrial wastewaters causes serious environmental hazards because of their mutagenicity to humans and toxicity to aquatic life [2]. Therefore, it is mandatory to remove dyes from water prior to their discharge into water bodies. Adsorption, photocatalytic degradation, biodegradation, advanced oxidation, electrochemical, and chemical oxidation are generally utilized processes for dye removal [3]. Among them, the photocatalysis process has attracted researchers because of its cost-effectiveness and biodegradable and non-toxic end products. ZnO, TiO2, SrO2, WO3, Fe2O3, ZrO2, CdS, SrTiO3, and ZnS have been employed as heterogeneous photocatalysts for
ZnO has a wide band-gap (~3.4 eV), good stability, low cost, and excellent optoelectronic properties [5]. All of these properties make ZnO a potential candidate for the photoc

atalysis and degradation of various dyes. However, ZnO is not an efficient dye degradant on its own because of its rapid recombination of electron-hole pairs, insufficient absorption efficiency in the visible sunlight region, and other chemical and physical obstacles [3].
Meanwhile, adsorption is one of the most efficient and cost-effective processes for the degradation of textile dyes. For the adsorption of different dyes, carbon-based inorganic supports have been widely employed [2]. Among the carbonaceous materials utilized as adsorbents, the relatively new graphene oxide (GO) and carbon nanotubes (CNTs) have attained much attention as they have superior adsorption capabilities due to their large specific surface area and the presence of a wide spectrum of surface functional groups [6]. Moreover, the adsorption of textile dye on GO and multiwalled carbon nanotubes (MWCNTs) have been ascribed to 𝜋−𝜋 electron donor-acceptor interactions and electrostatic interaction [7]. It has been reported that GO can absorb the CNTs by the strong 𝜋−𝜋 interaction to construct graphene-carbon nanotubes (Gr-CNTs) hybrid structure [8]. The hybridized structure has shown superior performance than its constituents CNTs and GO and can be applied as transparent conductive electrodes for various applications [9]. Previously, Chen et al. have reported the preparation of Gr/MWCNTs/ZnO nanocrystalline aggregate by the spray-drying method. The Gr/MWCNTs hybrid drastically enhanced the adsorption capacity and photocatalytic activity of ZnO nanoparticles and made it an excellent degradant of methyl orange dye [8]. However, in this study, we fabricated ZnO/Gr-CNTs nanocomposite by simple hydrothermal method for the first time, considering the potentials of ZnO nanoparticles and Gr-CNTs hybrid [10]. We performed different characterization tests on the final nanocomposite and explored its efficiency to degrade Rhodamine B (RhB) dye to find its suitability in wastewater treatment.
EXPERIMENTAL PROCEDURE
Synthesis of GO using modified Hummer’s method
We synthesized GO by modified Hummer’s method that was previously reported by Paulchamy et al

. [11]. This method of synthesis consists of both oxidation and exfoliation of graphite sheets because of the thermal treatment of the solution. For fabricating GO by this method, 2 g graphite nanoparticles
concentration) in a 1000 mL volumetric flask initially. Then the volumetric flask with the mixture was kept in an ice bath under 5 °C temperature with continuous stirring with a magnetic stirrer for 4 hours. Subsequently, 12 g KMnO4 was mixed with the suspension at a slow rate, which was regulated cautiously for maintaining the reaction temperature under 15 °C. After that, 184 mL deionized (DI) water was added to the mixture very slowly for diluting the mixture that was maintained under stirring for 2 hours. Next, the mixture was stirred at 35 °C temperature for 2 hours after removing the ice bath, and it was kept in a reflux system at 98 °C for 10 minutes afterward. The temperature was altered to 30 °C after 10 minutes to get a brown-colored solution. Again, the temperature was modulated to 25 °C after 10 minutes, which was then kept for 2 hours. Afterward, the solution was treated with 40 mL H2O2 that altered the color to bright yellow. An equal amount of prepared solution was added to 200 mL of DI water taken in two separate beakers, which were stirred for 1 hour. Then the solution mixed with water was kept unstirred for 3-4 hours for settling the particles at the bottom. Subsequently, the separated water was poured into another beaker for filtering purposes. Next, for washing the mixture, centrifugation with HCl (10% concentration) was performed repeatedly. After that, the mixture was also washed by centrifugation with DI water multiple times. Finally, a pH-neutral gel-like substance was formed, which was then placed in a Teflon-lined autoclave and vacuum-dried at 60 °C temperature using a programmable oven for 6 hours to obtain the GO powder.
Fabrication of Gr-CNTs and ZnO/Gr-CNTs
For the fabrication of Gr-CNTs, primarily, 0.1 g of functionalized MWCNTs with 5.58 wt% hydroxyl groups (MWCNTs-OH) (95% purity, US Research Nanomaterials) was exfoliated in 40 g of

Characterization of reduced graphene oxide produced through a modified Hoffman method — Qaiser Ali Khan et al., 2017

Public Interest StatementLarge scale production yet maintaining quality has been an engrossing area of research in the production of graphene derivatives globally since 2004. Due to their simplicity and lower cost, wet chemical methods result in lower-cost alternative to produce reduced graphene oxide (rGO) with higher yield. The present research is an effort to synthesize rGO through an improved chemical route and quality analysis of the product by appropriate techniques. Rapid thermal treatment combined with chemical reduction step improved the exfoliation and isolation of rGO nanoplatelets.
1. IntroductionGraphene is a monoatomic thick (~0.35 nm) sheet consisting of carbon atoms arranged in a honeycomb-like (hexagonal) structure. High electrical conductivity, large specific surface area and excellent charge mobility are the main characteristics that make graphene-based materials excellent candidates for a wide variety of electrical applications (Pinto, Jones, Goss, & Briddon, 2010Pinto, H., Jones, R., Goss, J. P., & Briddon, P. R. (2010). Mechanisms of doping graphene. physica status solidi (a), 207, 2131–2136. https://doi.org/10.1002/pssa.201000009 [Crossref], [Web of Science ®], [Google Scholar]). A charge carrier mobility up to 3,700 cm2/Vs (Liao & Duan, 2010Liao, L., & Duan, X. (2010, November). Graphene–dielectric integration for graphene transistors. Materials Science and Engineering: R: Reports, 70, 354–370.10.1016/j.mser.2010.07.003 [Crossref], [PubMed], [Web of Science ®], [Google Scholar]), ability to withstand current density of 108 A/cm² (Lin, et al., 2013Lin, J., Peng, Z., Xiang, C., Ruan, G., Yan, Z., Natelson, D., & Tour, J. M. (2013, July). Graphene nanoribbon and nanostructured SnO 2 composite anodes for lithium ion batteries. ACS Nano, 7, 6001–6006.10.1021/nn4016899 [Crossref], [PubMed], [Web of Science ®], [Google Scholar]), transmittance up to

97% to the visible light (Mkhoyan et al., 2009Mkhoyan, K., Contryman, A. W., Silcox, J., Stewart, D. A., Eda, G., Mattevi, C., … Chhowalla, M. (2009, March). Atomic and electronic structure of graphene-oxide. Nano Letters, 9, 1058–1063.10.1021/nl8034256 [Crossref], [PubMed], [Web of Science ®], [Google Scholar]) and high thermal conductivity of the order of 5 × 10³ W/mK (Balandin et al., 2008Balandin, A. A., Ghosh, S., Bao, W., Calizo, I., Teweldebrhan, D., Miao, F., & Lau, C. N. (2008, March). Superior thermal conductivity of single-layer graphene. Nano Letters, 8, 902–907.10.1021/nl0731872 [Crossref], [PubMed], [Web of Science ®], [Google Scholar]) has made graphene and its derivatives among the hot topics of research today.GO and rGO are intriguing materials for photonics and electronic devices both for intrinsic characteristics and as precursors for the synthesis of graphene. Overall, a fine control of the chemical and physical properties is required since the performances of graphene-based systems depend on the reduction state of graphene oxide that is strongly affected by processing. Since its first isolation (Park & Ruoff, 2009Park, S., & Ruoff, R. S. (2009, April). Chemical methods for the production of graphenes. Nature Nanotechnology, 4, 217–224. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]), graphene has witnessed a surge of interest to develop methods for a larger-scale and economical production. Chemical solution methods have been popular because of the inexpensive starting materials and the requirement of simple processing equipment. Methods proposed by Staudenmaier, Hofmann and Hummers remained more prominent (Poh et al., 2012Poh, H. L., Šaněk, F., Ambrosi, A., Zhao, G., Sofer, Z., & Pumera, M. (2012). Graphenes prepared by Staudenmaier

, Hofmann and Hummers methods with consequent thermal exfoliation exhibit very different electrochemical properties. Nanoscale, 4, 3515–3522. 10.1039/c2nr30490b [Crossref], [PubMed], [Web of Science ®], [Google Scholar]) to synthesize graphene by chemical oxidation of graphite and subsequent reduction.In order to facilitate separation of the individual graphene layers, the van der Waals attraction between graphite basal planes has to be overcome, for which thermal or chemical exfoliation has been proposed (Stankovich et al., 2007Stankovich, S., Dikin, D. A., Piner, R. D., Kohlhaas, K. A., Kleinhammes, A., Jia, Y., … Ruoff, R. S. (2007). Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon, 45, 1558–1565.10.1016/j.carbon.2007.02.034 [Crossref], [Web of Science ®], [Google Scholar]). One approach is to expand the interlayer distance by oxidation of graphite and intercalation of compounds (Ang, et al., 2009Ang, P. K., Wang, S., Bao, Q., Bao, Q., Thong, J. T., & Loh, K. P. (2009). High-throughput synthesis of graphene by intercalation−exfoliation of graphite oxide and study of ionic screening in graphene transistor. ACS Nano, 3, 3587–3594.10.1021/nn901111s [Crossref], [PubMed], [Web of Science ®], [Google Scholar]). The resulting oxidized graphite revealed a disordered graphitic stacking with d-spacing increased from ~0.34 to 0.7 nm (Ang et al., 2009Ang, P. K., Wang, S., Bao, Q., Bao, Q., Thong, J. T., & Loh, K. P. (2009). High-throughput synthesis of graphene by intercalation−exfoliation of graphite oxide and study of ionic screening in graphene transistor. ACS Nano, 3, 3587–3594.10.1021/nn901111s [Crossref], [Pub

Study of Different Properties of Graphene Oxide (GO) and Reduced Graphene Oxide (rGO) — Prateek Viprya et al., 2023

1. IntroductionGraphene oxide (GO) is a material that attracts considerable attention in the scientific community due to its unique physical and chemical characteristics. Its properties can be tuned by varying the degree of oxidation, the size and shape of the flakes, and the chemical functionalization, which makes it a versatile material with great potential for various applications. Graphene-based material has excellent mechanical [1], thermal [2,3], and electrical [1,4] properties, making it a potential candidate for use in energy storage [5], biosensors [5], biomedical engineering [6], hydrogen storage, displays, and solar cells [7,8]. GO has potential application in environmental remediation due to its ability to absorb various pollutants, such as heavy metals and organic contaminants, making it a promising material for water purification and desalination [9].The synthesis of graphene oxide (GO) was first carried out by Brodie [10] and modified by Hummers [11]. The molecular structure of GO, according to these authors, is that of a carbon compound with oxygen functional groups bonded to carbon atoms in the hexagonal plane [12]. Graphene has a flexible structural shape, remarkable mechanical strength, and zero bandgaps [13,14]. 2. Method for Synthesis of GO and rGO Synthesis of GO and rGOReduced GO is obtained by the removal of oxygen from the GO structure. GO is produced via methods such as Hummers’ (oxidation with sulfuric acid, potassium permanganate) and Brodie’s (nitric acid oxidation) [14]. By using sulfuric acid to raise the mixture’s acidity and multiple aliquots of solid KClO3 throughout the reaction in 1898, Staudenmaier improved Brodie’s process. This method, known as Staudenmaier’s method, produces hazardous ClO2 gas, which breaks down in the air and causes explosions [15].Alghyamah et al. (2022) created rGO by reducing GO in situ inside the PEO matrix while employing L (+) Ascorbic acid as a green reductant [16]. Chaiyakun et al. (2012) synthesized the GO nanosheet using Hummers’ method by mixing natural graphite powder and sodium nitrate with sulfuric acid and stirring with potassium permanganate [17]. Ban

et al. (2012) used strong oxidizing agents, stirred with a magnetic stirrer, added potassium permanganate, and stirred for three days to obtain GO [18]. Zhou et al. (2013) employed Hummers’ method, involving graphite, H2SO4, NaNO3, KMnO4, heat, water, and centrifugation, to produce graphene oxide (GO) powder [19]. Lavin-Lopez et al. (2017) created rGO by combining thermal and chemical processes, employing hydrazine and ascorbic acid as reducing agents. The products’ names were Hydrazine Multiphase Reduced Graphene Oxide (HMP-rGO) and Ascorbic acid Multiphase Reduced Graphene Oxide (AMP-rGO) [20]. Graphite modified via Hummers’ method; mixed with KMnO4, H2SO4; water, H2O2 added for brown GO solution. Filtered, washed, and thermally reduced to rGO at 750 °C with argon [21]. 3. Characterization 3.1. Scanning Electron Microscope (SEM)Scanning electron microscopy (SEM) determines surface morphology and the number of graphene layers. This is achieved by focusing a beam of energetic electrons onto the sample. The morphological characteristics of GO and rGO can be explained using SEM [22]. Handayani et al. (2019) analyzed GO samples using SEM at 10k, 25k, and 40k magnifications, revealing thicker edges due to oxygen functional groups. EDS (Energy Dispersive X-ray Spectroscopy) confirmed carbon and oxygen composition (shown in Figure 1) [1]. Analysis of graphite, GO, and rGO reveals platelet-like crystals, wrinkled flakes, and disordered crumpled sheets, respectively, through SEM (Figure 2) [23,24]. Cheng-an et al. (2017) used SEM to observe the lamella structures of GO composite films. Increased GO concentration led to organized and stratified deposition, with well-organized GO stacks in 50% GO composite film [25]. During a comparison between graphite powder and rGO using SEM at 10k magnification, lemon juice was found to be effective in reducing rGO thickness from 26.4–29.3 nm [26]. Long-term ball milling results in shrinking

ZnO particle size and damaged hexagonal crystals. In the hybrid nanocomposite, ZnO nanoparticles adhere to the rGO surface [27]. A study of the surface morphology of pristine GO foils revealed smooth surface images at low and high magnification, with an average roughness profile of 0.6 μm at a sub-micrometric scale [28]. Analyzing fractured surfaces in composites. Plain cement showed cracks passing through hydration products, while GO-cement displayed crack deflection, impeding crack propagation on increasing load [29]. Salinization makes GO shorter and smoother [30]. An SEM image of rGO that had been dried at 80 °C for a day shows a folded shape typical of a few-layer rGO (thickness 10 nm), huge dimensions (>100 nm), and re-stacked layers [31]. While analyzing GO papers through SEM, thicker papers showed increased roughness due to flake stacking and blockage of the flow path, resulting in wrinkles and surface roughness [32]. SEM reveals unevenly scattered rGO on fibers and displays cycle-dependent accumulation, color change, and dispersed rGO [33]. While GO nanosheets with characteristic folds and wrinkles show that graphite oxide exfoliation successfully produced 2D nanosheets [34].

Figure 1.
Examination of GO sample using SEM: (a) 10k magnification; (b) 25k magnification; (c) 40k magnification.

Figure 2.
10,000× magnification SEM image of (a) graphite; (b) GO; (c) rGO.

3.2. FTIR SpectroscopyFTIR (Fourier transform infrared) is the most common form of infrared spectroscopy. FTIR spectra analysis was performed to investigate the structure and functional groups of the materials [35]. FTIR analysis of rGO showed O-H (~3464 cm⁻1), C=O (~1639 cm⁻1), C-OH (~1288 cm⁻1), C-O (~1003 cm⁻1) peaks, with weak C=C (~1493 cm⁻1), indicating GO oxidation. Reduction weakens/eliminates peaks over time [1,36]. Peaks at 1081 cm−1 (C-O bond), 1630–1650 cm−1 (

Toward Green Synthesis of Graphene Oxide Using Recycled Sulfuric Acid via Couette-Taylor Flow. — Won Kyu Park et al., 2017

Introduction Graphene is a free
standing, two-dimensional monolayer carbon-based
nanomaterial with remarkable physical properties, which has been studied
in various applications such as transistor, transparent electrode,
supercapacitor, sensor, and polymer composite. 1 − 11 Typical synthesis routes of graphene include mechanical exfoliation
from bulk graphite (the “Scotch-tape” method), 4 chemical vapor deposition (CVD) through the reaction
of metal catalysts and precursors, 12 − 16 or chemical exfoliation of graphite using strong
oxidants. 9 , 17 − 29 For practical industrial applications of graphene, the synthesis
routes should guarantee high-quality, low-cost, and high-yield eco-friendly
processes. The mechanical exfoliation method can yield the highest
quality of graphene, but the associated process is not suitable for
the mass production. 4 Although large-area
as well as single-layered graphene sheets can be produced by the CVD
method, the fabrication process is rather complex and requires metal
catalysts, which can potentially raise the overall production costs. 12 − 16 On the other hand, the solution-processed chemical exfoliation technique
is desirable for the large-scale production process with relatively
lower costs, 9 , 17 − 29 and various functional composite structures can also be readily
constructed utilizing the oxygen-containing functional groups created
on the basal plane and edge sites of graphene. 17 , 21 In chemical exfoliation processes, strong acids are typically
used,
and graphene oxide (GO) produced from Hummers’ method is one
of the most widely studied graphene derivatives synthesized by such
an approach. Sulfuric acid (H 2 SO 4 ) is frequently
used in the oxidation process of GO, 30 − 32 which raises serious
environmental concerns and also increases the overall production cost
due to the large amount of water required to handle the discharged
acid waste and to purify the resulting GO from the acid. A few studies
suggested adjusting the concentration and volume of the applied acid
to mitigate the aforementioned issues. 31 − 33 Herein, we report
a facile filter system to recycle the H 2 SO 4 and
reduce the amount of water required for the GO
production process, which can facilitate the reduction of

overall
production cost and commercialization of GO-based materials with alleviated
environmental concerns. H 2 SO 4 and graphite oxide
were separated through the filter system after the oxidation of graphite
using the Couette–Taylor reactor and before the washing process.
The Couette–Taylor flow reactor is equipped with rotating inner
and fixed outer coaxial cylinders. Toroidal vortices are generated
and evenly spaced along the axis at a critical rotating speed of the
inner cylinder. 34 − 36 In our previous study, the toroidal flow of solutions
led to excellent blending of graphite with oxidants (KMnO 4 and H 2 SO 4 ), thus enhancing the oxidation efficiency
with high yields of singleand few-layered GO production. 37 The Couette–Taylor flow reactor comprises
two coaxial cylinders. Whereas the outer cylinder remains standstill,
the inner one rotates at controlled speed. When the rotational speed
of the inner cylinder reaches a threshold value, doughnut-shaped vortexes
are generated, which rotate in opposite directions with constant arrays
along the cylinder axis. This Couette–Taylor vortex induces
highly effective radial mixing and uniform fluidic motion within each
vortex cell, enabling enhanced mass transfer of the reactants. The
toroidal motion also generates high wall shear stress, which can facilitate
GO fabrication. 34 − 37 The key parameter that renders the acid filtration process possible
is the viscosity of the reactant mixtures, which shows distinctive
characteristics between the Hummers and Couette–Taylor methods
as discussed later. The filtered acid can also be recycled in subsequent
GO production processes, and such consecutive oxidation reactions
from the Couette–Taylor flow reactor were successfully demonstrated
utilizing the recycled H 2 SO 4 . Results and Discussion The modified Hummers method has been widely adopted as the standardized
synthesis technique for GO production because of its relatively simple
approach. 30 − 32 However, the long reaction time and use of large
volume of water in Hummers’ method have been the major bottleneck
to its widespread industrial applications. In our previous work, we
demonstrated that the Couette–Taylor flow reactor can dramatically
reduce the process time with high yield of singleand few-layered
GOs ( Figure 1 a). 37 In a typical GO synthesis process, the

dissolved
oxidizing agent (KMnO 4 ) in acid (H 2 SO 4 ) is diffused into the graphite interlayer during the oxidation reaction
of graphite, which leads to increase in the viscosity of the mixture
(graphite, KMnO 4 , and H 2 SO 4 ). 32 As shown in Figure 1 b, the viscosity of the mixture before the
oxidation reaction is ∼50 cP at 25 °C, which increases
to ∼85 and ∼200 cP for the Hummers and Couette–Taylor
flow methods, respectively, after the oxidation reaction of 60 min.
For this reaction time, the recovery rate of GO prepared by Couette–Taylor
flow is ∼98%, indicating that the graphite oxides are mostly
well-oxidized and exfoliated into singleand few-layered GOs, whereas
the recovery rate is only ∼34% for Hummers’ method,
implying inefficient oxidation and exfoliation. Although the recovery
rate of Hummers’ method (∼93%) becomes comparable to
that of Couette–Taylor flow after prolonged treatment time
(1440 min), the viscosity of the mixture is extremely increasing (10 000
cP), which impedes the acid filtration. Therefore, the Couette–Taylor
flow method enables the acid filtration process by producing the reactant
mixtures with relatively low viscosity (<200 cP) and can achieve
the high recovery rate at the same time, whereas in Hummers’
method, these two important conditions cannot be realized simultaneously.
We note that the graphite oxide mixture prepared from Hummers’
method for 60 min of reaction time can be used in the filtering process
with its low viscosity (∼85 cP). However, the low recovery
rate of the resulting GO (∼34%) is limiting the efficacy of
Hummers’ method in the filtration process. On the basis of
these results, for the filtering process of H 2 SO 4 , the oxidation reaction of 60 min by the Couette–Taylor flow
method was used for the following sets of analysis. Figure 1 Comparison of the Hummers
and Couette–Taylor flow methods.
(a) Schematic of the GO synthesis process. (b) Visc