Electrolyte engineering for the mass exfoliation of graphene oxide across wide oxidation degrees

Abstract

Oxygen-containing functional groups play crucial roles in graphene oxide due to their enhanced processability, stability, and functionalization. However, achieving precise control over the oxidation degrees of graphene oxide through a straightforward and effective method remains a significant challenge. Herein, we report a two-step electrochemical approach encompassing pre-intercalation and post-exfoliation/oxidation, enabling the mass exfoliation of graphene oxide with customizable oxidation levels. Initially, the pre-intercalation of concentrated sulfuric acid into graphite foil promotes uniform expansion, transforming it into a quasi-monolayer graphene structure. Subsequently, post-exfoliation in reductive/oxidative electrolytes triggers the simultaneous detachment and oxidation process, resulting in well-dispersed graphene nanosheets with quantified oxidation levels on a timescale of minutes. Comprehensive characterization studies confirm the varied oxidation levels of the exfoliated graphene oxide, spanning conventional oxidation degrees obtained via Staudenmaier's, Hofmann's, and Hummers' methods. Furthermore, we evaluate the scalability of this method and the solution processability of exfoliated graphene nanosheets, demonstrating the continuous production of graphene oxide at the kilogram scale and the fabrication of meter-length nanocomposite films with exceptional mechanical properties.

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Jing Li

Jing Li, a professor in the School of Chemistry at Beihang University obtained his B.S. degree from Wuhan University in 2011. He earned his Ph.D. at the Institute of Chemistry, Chinese Academy of Sciences in 2016, under the guidance of Prof. Li-Jun Wan and Prof. Dong Wang, focusing on the chemical synthesis and functionalization of graphene. Subsequently, he served as a research fellow with Prof. Jiong Lu at National University of Singapore from 2016 to 2022, working on the solution synthesis and surface study of two-dimensional materials. Currently, his research interests encompass the surface study and functional assembly of low-dimensional materials.

Introduction

Oxygen-containing graphene, including graphene oxide (GO), reduced graphene oxide (rGO), and their derivatives, has diverse applications across electronics, catalysis, nanocomposites, and energy-related devices.¹ Over the past decade, GO-based research has made tremendous advances in material synthesis and property tailoring, which has paved the way for technological breakthroughs with exceptional performance.² Among these, GO stands out as one of the most extensively studied materials, owing to its versatility and adaptability for further chemical derivatization.^3,4 Generally, graphite raw materials undergo deep oxidation in harsh oxidative solutions to produce thoroughly exfoliated GO monolayers, resulting in nanosheets with excellent processability, stability, and functionalization induced by the surface oxygen-containing functional groups.⁵ Subsequently, the obtained GO is further reduced to rGO with varying oxidation degrees tailored to specific applications.⁶

However, the extensive oxidation of graphite typically introduces two additional challenges. Firstly, it leads to the formation of permanent defects in GO, which significantly degrade the electronic, mechanical, and thermal properties of graphene and derivatives. For instance, the exceptionally high carrier mobility in graphene⁷ is markedly reduced in rGO (generally less than 10 cm² V⁻¹ s⁻¹)⁸ due to the irreparable defects formed during the oxidation process of GO.⁹ Secondly, the post-reduction of GO into rGO with a desirable oxidation level generally involves a trade-off between dispersibility in solutions and restoration of the conjugated electronic structure in rGO.¹⁰ As a result, low-oxidation degree graphene generally suffers from severe processability issues such as restacking, precipitation, or phase separation, drastically limiting its further applications.¹¹

The direct synthesis of graphene with quantified oxidation degrees is pivotal for achieving a delicate balance between solution processability and the preservation of intrinsic properties stemming from the conjugated lattice in graphene.¹² This endeavour holds immense potential for fully harnessing the remarkable properties of graphene.¹³ Notably, extensive oxidation of graphite typically employed in conventional synthetic methods is primarily dictated by the intercalation energy barrier of oxidants.¹⁴ Consequently, prolonged reaction times, elevated temperatures, and stronger oxidants are often necessary to achieve dispersible graphene oxide down to the monolayer thickness.^15–17 And the concentration of the oxidizing agent is proven to be a key parameter for manipulating the oxidation degrees of graphene oxide.¹⁸ The electrochemical exfoliation method typically involves the intercalation of foreign species (ions or small molecules) into the interlayer spaces of layered materials, followed by a mild exfoliation process, ultimately yielding monolayer two-dimensional materials.^19–21 Among these steps, the application of electrochemical voltage serves as an effective means to drive the intercalation of various chemicals into the van der Waals gap of layered materials,^22–24 leading to the formation of quasi-monolayer structures that readily disperse in the solution phase under mild external stimuli.^25,26 The electrochemical exfoliation method is not only applicable to the exfoliation of van der Waals materials but also to the preparation of monolayer non-van der Waals materials,^27,28 and is highly versatile and universally applicable. In this regard, decoupling the oxidation process from the intercalation procedure in graphite represents a rational approach for finely tuning the oxidation degree of graphene.

Herein, we present a two-step electrochemical method involving the pre-intercalation of concentrated sulfuric acid into graphite foil (priced at 4.15 US dollars per kilogram) via a minimal anodic voltage, achieving an integrated intercalation structure. Notably, this intercalation process efficiently expands the interlayer distance of graphite to approximately 1 nm, with negligible structural damage to the graphene lattice, as confirmed by Raman spectroscopy and electronic transport measurement. Subsequently, the weakly stacked graphite is simultaneously exfoliated and oxidized into monolayers with various oxidation degrees via electrolyte engineering. Specifically, reductive hydrazine sulfate yields the GO monolayer with a minor amount of surface functional groups, revealing record high carrier mobility up to 892 cm² V⁻¹ s⁻¹ for electrons and 562 cm² V⁻¹ s⁻¹ for holes, surpassing those of rGO. Additionally, the oxidation level of graphene can be continuously tailored with high controllability, yielding a high monolayer yield (∼80%), a rapid exfoliation rate (down to a few seconds), and a wide range of oxidation degrees across those in conventional Staudenmaier's, Hofmann's, and Hummers' methods. We also assess the mass production and solution processability of the exfoliated GOs. As a proof of concept, GO sheets are exfoliated at a rate of hundreds of grams per hour, demonstrating excellent solution processability and enabling continuous assembly into meter-length GO nanocomposite films with outstanding mechanical properties surpassing those of commercial GO.

Results and discussion

A synthetic schematic of GO nanosheets, featuring a broad range of oxidation degrees via the two-step electrochemical approach, is depicted in Fig. 1a. Initially, pre-intercalation was conducted in a two-electrode electrochemical cell using concentrated sulfuric acid (>95%). The procedure involved a cell voltage of 2.2 V for approximately 10 minutes. During this step, the graphite anode uniformly expanded into a weakly stacked structure, forming a quasi-monolayer configuration.²⁷ Subsequently, the expanded graphite foil was immersed into a secondary electrolyte for post-exfoliation and simultaneous oxidation. Notably, due to the sufficient intercalation of sulfuric acid into each interlayer space of graphite, the van der Waals interaction was drastically weakened, allowing for easy oxidation degree manipulation of graphite through adjustable oxidizing properties of the electrolyte. Notably, the commercial graphite foil, which costs as low as 4.15 US dollars per kilogram (Fig. 1b), could be readily transformed into the second electrochemical cell for the continuous production of GOs with various oxidation degrees in different electrolytes (Fig. 1c).

Fig. 1 The mass exfoliation of GO over a wide range of oxidation levels via a two-step exfoliation approach. (a) Schematic illustration of the electrochemical exfoliation process for preparing GOs. Specifically, graphite foil was pre-intercalated in concentrated H₂SO₄ electrolyte, followed by simultaneous exfoliation and oxidation of intercalated graphite foil in a secondary electrolyte with varying oxidizability. This process yielded GO nanosheets with controllable oxidation levels. (b) Photograph of commercial graphite foil used in this work. (c) Representative photographs of exfoliated GOs with controllable oxidation levels, ranging from less oxidized GOs (left, EcE-G1) to highly oxidized GOs (right, EcE-G6).

To confirm the successful intercalation of H₂SO₄-based electrolytes in the pre-intercalation stage, both the graphite precursor and intercalated graphite were characterized by electron microscopy. The commercial graphite foil shows compact stacking configuration (Fig. 2a–c). The intercalation of HSO₄⁻ anions and H₂SO₄ molecules²⁸ efficiently expanded the graphite into an accordion-like film with overall thickness augmented by approximately 250% (Fig. 2d and e), as measured by large-scale scanning electron microscopy (SEM). This expansion is further attested by the high-resolution electron microscopy (HRTEM) in Fig. 2f. The cross-sectional interlayer distance of intercalated graphite was measured to be 1.10 nm on average, indicating the uniform intercalation of the electrolyte in the van der Waals space. Notably, despite the intercalated graphite consisting of weakly stacked graphene and electrolyte alternating units, the integrated film remains conformally processable for transfer into the secondary electrolyte for further exfoliation and oxidation.

Fig. 2 Cross-sectional morphology characterization of graphite before and after pre-intercalation. (a) Large-area and (b) zoom-in (dashed-box area) cross-sectional SEM images of intrinsic graphite foil before pre-intercalation. (c) HRTEM images (top panel) and the corresponding intensity profile (bottom panel) along the dashed line of the graphite edge. (d) Large-area and (e) zoom-in (dashed-box area) cross-sectional SEM images after intercalation. (f) HRTEM images (top panel) and the corresponding intensity profile (bottom panel) along the dashed line of the edge of intercalated graphite.

The intercalated graphite was initially exfoliated in a less oxidative electrolyte to evaluate the pre-intercalation damage toward the exfoliated graphene nanosheets. Notably, the Raman spectrum has been proven to be sensitive to a variety of defective configurations in graphene, including atomic vacancies, flake edges, in-plane strain, and surface functional groups,²⁸ indicated by the activation of the D peak around 1350 cm⁻¹.²⁴ Intriguingly, the absence of the D peak in intercalated graphite in Fig. 3a indicates a defect-free nature in the pre-intercalation step. This observation aligns with phenomena previously reported in the reversible intercalation of the ammonium persulfate and sulfuric acid mixture,²⁹ revealing the feasibility of isolating the oxidation step from the conventional oxidation procedure to further fine-tune the oxidation level in GO. Following this hypothesis, we first conducted exfoliation of pre-intercalated graphite in an electrolyte mixture containing 50 wt% H₂SO₄ and 0.01 M hydrazine sulfate. Due to the weak van der Waals interaction in pre-intercalated graphite, it rapidly disentangles into few-layer nanosheets in the electrolyte. The fluffy morphology in SEM images (Fig. 3b and c) demonstrates complete intercalation and exfoliation of graphite foil by the two-step electrochemical approach. Electrochemically exfoliated graphene (denoted as EcE-G1) was further characterized using HRTEM. The low contrast of EcE-G1 under the electron beam shows the effectiveness of the exfoliation method towards ultrathin nanosheets (Fig. 3d).

Fig. 3 Morphological, structural, and electronic characterization of exfoliated EcE-G1. (a) Raman spectra of graphite foil before and after pre-intercalation. (b) Large-area and (c) close-up SEM images of exfoliated EcE-G1. (d) Representative TEM image and (e) corresponding SAED pattern of exfoliated EcE-G1 monolayer. (f) Intensity profiles along the dashed line marked in panel (e). (g) HRTEM image of the intact graphene lattice domain in EcE-G1. (h) A plot of the source-drain current (ISD) versus gate voltage (V_g) of the EcE-G1-based FET device, with the inset showing the atomic force microscopy image of a FET device. (i) Comparison of measured carrier mobilities in GO, rGO, liquid-phase exfoliated graphene (LPE-G), and electrochemically exfoliated graphene (EcE-G), with detailed comparison available in Table S1†.

Selected area electron diffraction (SAED) reveals that the diffraction intensity ratio between (11̄00) and (12̄00) spots is 0.62, indicating the representative monolayer thickness of exfoliated EcE-G1.^30,31 Additionally, intact graphene lattice domains up to tens of nanometres could also be frequently observed in HRTEM. Finally, the electronic properties of exfoliated EcE-G1 flakes were evaluated via transport measurement in bottom-gated field-effect transistor devices on a standard Si substrate with a 300 nm thickness SiO₂ layer.^32,33 The electron and hole mobilities were measured to be 892 cm² V⁻¹ s⁻¹ and 562 cm² V⁻¹ s⁻¹, respectively, surpassing those reported in rGO.^34,35

The pre-intercalated graphite foil pieces were subsequently exfoliated in 50 wt% H₂SO₄ aqueous solution with varied oxidants to assess the controllability over the oxidation levels of exfoliated GO. Detailed experimental protocol and the influence of electrolyte components on GO can be found in ESI, Fig. S1–S3†. Based on the enhanced oxidizability of the electrolyte, the exfoliated GO was further denoted as EcE-G2 to EcE-G6 to show the increasing oxidation levels of GO flakes.

The controllability of oxidation degrees in GO via electrolyte engineering was comprehensively assessed through various characterization studies. The X-ray diffraction (XRD) spectra (Fig. 4a) indicate the absence of discernible peaks for the EcE-G1 sample and a distinct peak at 12.65° for the EcE-G2 sample, corresponding to a calculated interlayer distance of approximately 6.997 Å according to the Bragg equation. The gradually decreasing diffraction angles from EcE-G3 (12.35°), EcE-G4 (12.37°), EcE-G5 (12.45°) to EcE-G6 (12.15°) reveal an increasing coverage of oxidative functional groups in each graphene lattice, corresponding to an overall augmentation in the interlayer distance from 7.167 Å (EcE-G3), 7.063 Å (EcE-G4), 7.109 Å (EcE-G5), and 7.284 Å (EcE-G6).

Fig. 4 Oxidation degree modulation in exfoliated GO in a wide range. (a) XRD, (b) Raman spectra, (c) UV-vis spectra. (d) Derived-TGA, and (e) XPS characterization of the exfoliated GO series. (f) A comparison of carbon-to-oxygen elemental ratios in exfoliated GO with those reported for conventional synthesis methods.^36,37

The Raman spectra (Fig. 4b) display three characteristic peaks (D, G, and 2D peaks) in exfoliated GO. As a result of the growing oxidation degree, the intensity of the 2D peak at ∼2700 cm⁻¹ is suppressed due to the “Pauli blocking” effect.^14,38 The prominent D peak at ∼1350 cm⁻¹ signifies structural defects introduced by oxygen-containing groups to the carbon basal plane.³ The steadily increased peak intensity and widened full width at half-maximum of D peaks from EcE-G1 to EcE-G6 further prove the elevated oxidation level in GO. Moreover, the detailed influence of electrolyte concentration on the Raman spectra of GO is depicted in Fig. S2.† The UV-vis absorption spectra (Fig. 4c) exhibit a blue shift of the absorption peak from 262 nm for EcE-G1 to 233 nm for EcE-G6, corroborating successful oxygen functionalization via the two-step process and a progressive increase in oxidation.^39,40

Thermogravimetric analysis (TGA) was conducted to investigate the thermal stability of the prepared GO. Fig. 4d illustrates the TGA derivative curves of exfoliated samples, covering a temperature range from room temperature to 800 °C in a N₂ atmosphere. Overall, the weight loss percentages were observed to increase in the following order: EcE-G1 < EcE-G2 < EcE-G3 < EcE-G4 < EcE-G5 < EcE-G6, consistent with the incremental oxidation levels in the exfoliated GO.⁴¹ The exfoliated GO predominantly reveals four temperature ranges for the total weight loss: the slight weight loss below 100 °C is assigned to the desorption of water trapped between GO nanosheets,^42,43 while the weight loss within the temperature ranges of 100–160 °C, 160–250 °C, and 250–360 °C is assigned to the surface hydroxyl, epoxy, and carboxyl functional groups, respectively.^44,45 The oxidation structure of graphene evolves in the following order: from epoxy in less oxidized GO, to hydroxyl in medium oxidized GO, and finally transforming into deeply oxidized carboxyl functional groups. This chemical oxidation tendency is also consistent with those documented for conventional Hummers' method, indicating the effectiveness of electrolyte engineering in modulating the oxidation levels of exfoliated GO.^36,46

X-ray photoelectron spectroscopy (XPS) was utilized to quantitatively assess the chemical composition and functional groups grafted on exfoliated GO.⁴⁷ The corresponding XPS spectra are shown in Fig. 4e and S3.† The C 1s spectrum can be fitted into three distinct deconvoluted peaks. The peak at 284.6 eV corresponds to the carbon bonds, including C–C (sp³) and C=C (sp²).⁴⁸ The binding energies at 286.6 eV and 288.7 eV are ascribed to hydroxyl and epoxy groups, as well as carbonyl and carboxyl/ester groups, respectively.¹⁷ These results unequivocally reveal the effective incorporation of controllable oxygenated functional groups onto the graphene surface through the two-step electrochemical method. Importantly, the intensities of peaks corresponding to oxygen-containing functional groups in GO are observed to progressively increase with the oxidizability of the electrolytes. These results align well with the data obtained from Raman spectroscopy, UV-vis spectroscopy, and TGA, reinforcing the viability of manipulating graphene's oxidation levels through the two-step electrochemical process. In addition, Fig. 4e also provides insights into various percentages of carbon and oxygen at different oxidation stages of GO, facilitating quantitative analysis (Table S2†). Notably, the range of C/O elemental ratios in exfoliated GO could be readily tuned across a wide oxidation range, matching or even surpassing those obtained via Staudenmaier's, Hofmann's, and Hummers' methods (Fig. 4f).³⁶

The exfoliation efficiency of the two-step electrochemical exfoliation methods was evaluated through thickness statistics conducted by atomic force microscopy (AFM). Aqueous dispersions of GO were diluted with water and drop-cast onto freshly exfoliated mica surfaces for AFM imaging. Representative GO flakes exfoliated in various electrolytes are shown in Fig. 5a–f, revealing an average thickness of approximately 1.0 nm, indicative of a monolayer.⁴⁹ This is drastically different from those reported for rGO, which generally suffer from severe aggregation when their surface functional groups are reduced;^6,50 we ascribe this to the efficient pre-intercalation treatment, which significantly weakens the van der Waals interactions, thereby facilitating the repaid detachment into thin flakes. Quantitatively, thickness statistics (Fig. 5g) conducted among 200 individual flakes through AFM measurements in each GO sample manifest a monolayer yield approaching 80%, with negligible influence from the post-exfoliation electrolyte.

Fig. 5 Thickness statistics of exfoliated GOs with varying oxidation degrees. Representative AFM image of exfoliated GO flakes sourced from (a) EcE-G1, (b) EcE-G2, (c) EcE-G3, (d) EcE-G4, (e) EcE-G5, and (f) EcE-G6. Insets show the vertical height profiles along the dashed line in each panel. (g) Thickness distribution of exfoliated GO flakes with varying oxidation degrees. Notably, the statistics were performed on more than 200 individual GO flakes in each sample through AFM imaging.

The scalability of exfoliated GO production and its solution processability were assessed through a proof-of-concept application involving the fabrication of mechanical composite thin films. Leveraging the favourable balance between microscopic weak-stacking and macroscopic conformability inherent in pre-intercalated graphite foil, a continuous immersion process was developed wherein graphite rolls are sequentially treated with pre-intercalation and post-exfoliation electrolytes, allowing for mass production of GO with tailored oxidation degrees by adjusting the post-exfoliation electrolyte composition. The large-scale production process of aqueous dispersions containing GO nanosheets is illustrated in Fig. 6a and b. Fig. 6a depicts the setup before pre-intercalation, while Fig. 6b showcases the rapid and thorough detachment of GO flakes from immersed graphite with a production rate approaching 240 grams per hour, which shall be further scalable with a larger electrochemical cell. Fig. 6c presents a photograph of the mass-produced GO solutions exhibiting excellent stability over 100 days of storage without significant precipitation. Subsequently, GO with varying oxidation levels was incorporated into polymer matrices to fabricate uniform composite films using a superspreading approach.^51,52 A photograph of meter-length and homogeneous composite films is presented in Fig. 6d, and the preparation process of GO/polymer nanocomposite films is shown in Fig. S4.† Moreover, small-angle X-ray scattering (SAXS) was employed to analyse and quantitatively assess the orientation of GO nanosheets within the composite polymer films.⁵³ As shown in Fig. 6e, the two-dimensional (2D) SAXS pattern of the nanocomposite films exhibits two pronounced diffuse spots along the equatorial direction, with an azimuthal angle (φ) plot characterized by peaks at φ = 90° and 270°. The orientation order parameter (f), calculated to be 0.852 ± 0.016 (n = 3), indicates well-assembled GO nanosheets. This demonstrates the successful synthesis of GO with various oxidation degrees, which resembles the excellent solution processability of commercial GO samples, facilitating the continuous fabrication of highly aligned nanocomposite films.⁵⁰ Further, the mechanical properties of these composite films, with a consistent nanofiller-to-polymer ratio of 10 wt%, were examined (Fig. 6f). By adjusting the oxidation degrees of GO, the mechanical performance, including tensile strength and modulus, of the nanocomposite films reaches a maximum at a C/O elemental ratio of approximately 2.3 (EcE-G5), exhibiting a high tensile strength (910.81 ± 48.17 MPa) and modulus (77.15 ± 2.43 GPa). The corresponding stress–strain curves are illustrated in Fig. S6.† This optimized performance may be attributed to a rational balance between the intrinsic mechanical properties of individual GO filler flakes and the interfacial interaction between the oxygen-containing group and the polymer matrix. The intact sp² hexagonal lattice in graphene contributes to opposing the deformation forces,⁵⁴ while hydrogen-bonding and the reversible crack arresting mechanism originating from epoxy-to-ether transformation enhance fatigue resistance between grafted oxygen-containing groups and polymer matrices.⁵⁵ Overall, the reinforcing effect of GO as a nanofiller in the composite involves complex chemo-mechanical energy dissipation mechanisms, necessitating further insightful understanding in future research endeavours.

Fig. 6 The scalable production and solution processability demonstration of exfoliated GO. Photographs of graphite foil (a) prior to and (b) after a 60-second post-exfoliation process. (c) Produced GO solutions with good stability. (d) Photographs of fabricated meter-length GO-polymer composite films. (e) 2D SAXS images and azimuthal angle plots for the composite films. (f) Mechanical properties (strength and modulus) of composite films with various oxidation degrees in GO.

Conclusions

We have developed a scalable electrochemical strategy that involves sequential pre-intercalation and post-exfoliation/oxidation processes. This approach facilitates precise control over the oxidation degree of graphene oxide (GO) through simple adjustments in the oxidizability of the electrolyte. By employing this two-step exfoliation approach, we achieve rapid and efficient transformation of inexpensive graphite foil into nanosheets with high monolayer yield (averaging ∼80%). The broad range of oxidation degrees achievable in GO allows for the synthesis of both lightly oxidized graphene flakes with high carrier mobility, as well as deeply oxidized nanosheets possessing a high oxygen-to-carbon elemental ratio comparable to those obtained via Staudenmaier's, Hofmann's, and Hummers' methods. Furthermore, this method is compatible with continuous exfoliation processes, enabling the mass production of scalable GO with outstanding stability, precise controllability, and excellent solution processability.

Data availability

All data are available in the main text or the ESI material.†

Author contributions

J. Li and M. Liu conceived the project. H. Ren, X. Xia, Y. Sun, Y. Zhai, Z. Zhang, J. Wu and J. Li conducted the experiments. H. Ren and X. Xia contributed equally. All the authors contributed to the discussion and writing of the paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

J. Li. acknowledges the support from the National Natural Science Foundation of China (No. 22272004), and Fundamental Research Funds for the Central Universities (YWF-22-L-1256).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta02654c

‡ These authors contributed equally to this work.

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