Bulk-scale synthesis of randomly stacked graphene with high crystallinity

Abstract

Since the strong interlayer interaction of AB-stacked graphene in bulk form degrades the superior property of single-layer graphene, formation of randomly stacked graphene is required to apply the high performances of graphene to macroscopic devices. However, conventional methods to obtain bulk-scale graphene suffer from a low crystallinity and/or the formation of a thermodynamically stable AB-stacked structure. This study develops a novel approach to produce bulk-scale graphene with a high crystallinity and high fractions of random stacking by utilizing the porous morphology of a graphene oxide sponge and an ultrahigh temperature treatment of 1500–1800 °C with ethanol vapor. Raman spectroscopy indicates that the obtained bulk-scale graphene sponge possesses a high crystallinity and a high fraction of random stacking of 80%. The large difference in the random-stacking ratio between the sponge and the aggregate samples confirms the importance of accessibility of ethanol-derived species into the internal area. By investigating the effect of treatment temperature, a higher random-stacking ratio is obtained at 1500 °C. Moreover, the AB-stacking fraction was reduced to less than 10% by introducing cellulose nanofiber as a spacer to prevent direct stacking of graphene. The proposed method is effective for large-scale production of high-performance bulk-scale graphene.

Introduction

Graphene possesses numerous excellent properties such as a high carrier mobility, electrochemical performance [1], optical transparency, thermal conductivity [2], and mechanical strength [3]. These properties are attributed to the unique electronic structure derived from a one-atom-thick honeycomb lattice of single-layer graphene. Due to these excellent properties, graphene and graphene-containing materials have been studied extensively toward applications in electronics [[4], [5], [6]], electrode material [[7], [8], [9], [10], [11]], etc. One problem is that the thinness and small volume of a single-layer graphene flake limits the electrical, mechanical, and other performances. Thus, bulk-scale graphene, which is an aggregate composed of plenty of graphene flakes, is required for various daily applications [12] such as pressure sensors [13,14] and battery electrodes [[7], [8], [9], [10], [11]]. Preparation of high-quality bulk-scale graphene is a critical issue for practical applications.

The production of bulk-scale graphene starting from graphene oxide (GO) is a promising approach due to mass-production compatibility and structure controllability. In this production process, GO flakes are dispersed in solution through the functionalization of bulk graphite with oxygen-containing groups. GO then reduced into reduced graphene oxide (rGO). The most common approach for reduction is a hydro-thermal method or a chemical method [15,16], but these do not address the defect issue, such as vacancy. Alternatively, a high-temperature treatment method can produce rGO with the highest crystallinity [17]. However, the stacking structure of multilayer graphene is problematic. The thermodynamically favorable AB-stacked structure of multilayer graphene is formed during reduction of GO at high-temperature. A strong interlayer interaction in AB-stacked multilayer graphene causes its electronic structure to deviate from that of single-layer graphene, degrading the superior properties [18]. On the other hand, theoretical calculations have predicted that randomly stacked graphene, where adjacent graphene layers are randomly rotated or translated, can preserve the properties of single-layer graphene because it has an electronic structure similar to that of single-layer graphene [19]. Experimental studies have confirmed that randomly stacked graphene keeps a single-layer-like electronic structure [20] and has superior properties compared to AB-stacked graphene. Richter et al. [21] realized a high mobility of 7 × 10⁴ cm²/V·s for individual flakes of multilayer graphene with a rotationally stacked structure, while Liu et al. [18] found that AB-stacked bilayer graphene films exhibited a mobility of 4.4 × 10³ cm²/V·s. The development of fabrication methods for bulk-scale graphene with controlled interlayer stacking is crucial to realize graphene-based applications in numerous fields.

We previously produced graphene with a high fraction of the randomly stacked structure from GO aggregates by ultrahigh temperature reduction under an ethanol vapor supply [22]. At ultrahigh temperature, ethanol is decomposed into different species and further chemical reactions in gas phase occurs [23]. Reaction products containing carbon atoms and OH groups (hereafter called as “ethanol-derived species”) mainly act as carbon source and etchant, respectively. This reduction method has also been utilized for few-layer rGO on substrates and achieved a high carrier mobility [24]. However, the analysis of the bulk-scale graphene was limited to the outer surface of a GO aggregate [22], and the stacking structure of the internal area was not clarified. The repairing process and the formation of a randomly stacked structure should be limited to the surface area of the GO aggregates because the ethanol-derived species cannot enter the internal area of dense samples.

Accessibility of ethanol-derived species to GO flakes should be a critical factor for the successful formation of a randomly stacked structure induced by ethanol-mediated reduction of GO. Preparation of a GO sponge with a highly porous structure by freeze-drying is a potential solution for the inaccessibility problem of GO flakes [25]. Freeze-drying is a method by which liquid-containing materials are frozen below the freezing point and the solvent is sublimated into a vapor and removed under vacuum. The original structure and shape of the material are maintained because the solvent in the solid phase is sublimated directly into gas phase. Instead of aggregation, the GO dispersion can be dried into a GO sponge where GO flakes maintain separation similar to the dispersion. It should be mentioned that GO flakes randomly rotated in three dimensions, which was less impacted by nearby GO flakes compares to aggregate or film samples. The as-prepared GO sponge has a porous structure with a large surface area, which is suitable for further reduction reactions.

Besides inhibiting AB-stacked structure formation during the reduction process, the addition of other materials as spacers may effectively prevent graphene layers from stacking physically. Spacers must be chemically inert or transformed into inert materials at high temperature to prevent reactions between GO and spacers. Additionally, the spacers must be water soluble to promote molecular level mixing of GO and the spacers. Cellulose nanofiber (CNF) fulfills these requirements for spacers. CNF is a natural cylindrical polymer with plenty of resources, renewability [26], high strength, high stiffness [[27], [28], [29], [30]], and a low weight [14,31], which makes it widely used in many areas [[32], [33], [34]]. CNF can be prepared by (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl and NaClO, which is known as the TEMPO method. CNF possesses a high aspect ratio of 4–10 nm in diameter and 1 μm in length [26], which is suitable for intercalation into the GO interlayers in a dispersion before reduction. The water-solubility was improved by carboxylate groups on CNF [35,36]. These effects make CNF a promising candidate as a spacer for graphene.

Herein we propose a method that combines freeze-drying and an ultrahigh temperature process to produce bulk-scale graphene to tackle property degradation due to the strong interlayer interaction in AB-stacked structure. This method can repair and reduce GO on the bulk scale and realize a high random-stacking fraction, which should preserve the properties of single-layer graphene. A GO sponge prepared by freeze-drying of a GO dispersion presents a larger surface area than a GO aggregate. An increase in the accessible area of the GO sponge by ethanol vapor contributes to a high crystallinity and high fractions of random stacking. Additionally, this study provides an approach to further decrease the AB-stacked structure ratio of a graphene sponge utilizing CNF. CNF serves as a spacer that intercalates between the graphene layers to prevent the stacking where graphene layer contact directly (direct stacking). This rGO/CNF sponge features a low defect density, large surface area, reduced interlayer stacking, and bulk-scale production compatibility, increasing its potential applicability.