FIELD OF THE INVENTION

The present invention generally relates to a novel, rapid, eco-friendly method to fabricate holey graphene nanoplatelets using dry microwave irradiation. In particular, the method can be used to treat graphite intercalation compounds either with or without partial oxidation to obtain holey graphene nanoplatelets with predetermined hole size, geometrical structure of hole edges, thickness and lateral dimension.

BACKGROUND OF THE INVENTION

Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice and is a basic building block for graphitic materials of all other dimensionalities. It can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite. Due to its excellent electronic, thermal and mechanical properties, and its large surface area and low mass, graphene holds great potential for a range of applications. Examples include energy and hydrogen storage devices, inexpensive, flexible macroelectronic devices, and mechanically reinforced conductive coatings including films for electromagnetic interference (EMI) shielding in aerospace applications.

Holey graphene, referred to graphene sheets with nanoholes in their basal plane, recently attracts increasing interest from both fundamental and practical application point of views. Its unique properties lead to a wide range of applications, which cannot be achieved by its non-holey counterpart. It is important to note that holey graphene is different from porous graphene. In porous graphene, the porous structures are formed by creating physical spaces between intact graphene sheets, while holey graphene is synthesized by etching through the graphene sheets to form holes in their basal planes. In bulk 3D materials assembled from holey graphene materials for practical applications, both nanoholes and pores existed. The existence of the nanoholes provides desired “short-cuts” for efficient mass transport across the graphene plane and ultimate access to the inner surfaces. The existence of holes in graphene increases the accessible surface area (ASA) of graphene assemblies. Most importantly, generating nanoholes also naturally transforms a large number of in-plane atoms into edge atoms. It has been theoretically predicted and experimentally demonstrated that the edges of graphene sheets have different electronic states and chemical functionalities from their basal planes, which render them unique molecule and ionic absorption capability, dramatically increased quantum capacitance and electrochemical double layer capacitances. Recent years have witnessed the wide applications of holey graphene in gas storage/separation, oil absorption, photonic devices, catalysis, sensing, electrochemical energy generation, and storage. In particular, the recent development in compressible, dense, ultrathick electrode architectures with high mass loading can deliver high areal capacity and high-rate capability, which represents one of the critical steps forward in using holey graphene-based materials toward practical applications.

Due to the wide range of applications, various approaches have been developed for the production of holey graphene sheets. Bottom-up approaches based on chemical vapor deposition (CVD) methods, and top-down photo, electron or plasma etching of graphene utilize various templates, which provide good control over the sizes, shapes, and the positions of holes. These strategies start with high-quality graphene and generate holes on the basal planes of graphene while leaving other parts of the basal planes intact. This type of holey graphene is referred as pristine holey graphene to differentiate from those holey graphene materials with a large number of defects (i.e., oxygen-containing groups, 5-8 member rings, sp3 carbons) on the basal planes. However all these methods are designed for electronic and spintronic device applications, some of them even need a solid substrate to support the graphene sheets during the hole drilling process, so suffering from high cost and difficulties in scaling up to produce a large quantity of materials for bulk applications.

On the other hand, the bulk chemical etching approaches, such as KOH etching, H3PO4activation, HNO3oxidation, hot steam etching, enzymatic oxidation, and oxidative etching with or without catalytic nanoparticles, have advantages for large-scale and cost-effective synthesis. Most of these chemical etching based-approaches require graphene oxide (GO) or reduced graphene oxide (rGO) as a starting material, which contains various defects on their basal planes. During the hole generating process, some of these defects are partially gasified or etched away. While some of them may evolve to other defective forms such as 5-8 member rings, carbonyl, ether, and lactose groups, which are difficult to be removed even after high-temperature annealing. Further, fabrication of the GO and/or rGO starting materials usually takes hours to days, depending on the oxidation methods applied. Recently, we developed a one-step-one-pot microwave-assisted approach to rapidly, directly and controllably fabricate holey GO from graphite powder. However, this one-step-one pot microwave-assisted approach, similar to the widely used graphite oxidation methods, such as Staudenmaier, Hofmann, Hummers, or Tour's methods, also utilize metal-containing oxidants, such as KMnO4and/or KClO3. Trace residues of these oxidants and metal ions used or generated in these approaches can further participate in undesired reactions and can be detrimental to a wide range of applications. However, purification of GO remains a challenge mainly due to its tendency of gelation. Even though these metal ions are water soluble, they were trapped due to the gelation tendency of GO, causing the GO product to be highly flammable. Therefore, extensive cleaning and purification steps are required, making industrial-scale production expensive and time-consuming. Therefore the fundamental molecular basis in fabricating holey graphene materials determined that the existence of various defects on their basal planes and possible metal residues, which not only decrease their electrical and thermal conductivity, also negatively influence the chemical and thermal stability of holey graphene materials for their long-term practical applications. There are no scalable approaches reported to date which is capable of mass production of highly conductive and chemically stable pristine holey graphene materials without involving metallic containing compounds and at low cost for practical bulk applications. Furthermore, none of the approaches could generate pristine holey graphene materials with controlled geometric structures, such as zigzag or armchair, of their hole edges.

Therefore, a solution that overcomes the above-described inadequacies and shortcomings in the process of producing holey graphene or holey graphene nanoplatelets containing more than one layer of graphene nano sheets while keeping the rest of the basal plane nearly intact, at low cost and in an eco-friendly manner, is desired. In particular, it would be desirable to produce holey graphene nanoplatelets with a large number of zigzag edges associated with the holes.

SUMMARY OF THE INVENTION

This disclosure provides a method for fabricating holey graphene nanoplatelets. The method includes subjecting partially oxidized graphite intercalation compounds (PO-GICs), oxygen intercalated graphite intercalation compounds (OI-GICs) or commercial expandable graphite (CEG) to one or more microwave irradiation treatments to generated holey graphene nanoplatelets. The generated holey graphene nanoplatelets include one or more expanded graphene sheets having a plurality of holes on the basal planes thereof.

In preparing the partially oxidized graphite intercalation compounds (PO-GICs), the method may include adding graphite flakes to a mixture of ammonium persulfate and sulfuric acid to form a reversible graphite intercalation compound and purging the mixture containing the reversible graphite intercalation compound with oxygen gas. The method may further include heating the mixture to obtain the partially oxidized graphite intercalation compounds (PO-GICs).

In preparing the partially oxidized graphite intercalation compounds (PO-GICs), the method may include exposing graphite flakes to a mixture of hydrogen peroxide and sulfuric acid.

In preparing the oxygen intercalated graphite intercalation compounds (OI-GICs), the method may include adding graphite flakes to a mixture of ammonium persulfate and sulfuric acid to form a reversible graphite intercalation compound and purging the mixture containing the reversible graphite intercalation compound with oxygen gas.

In some embodiments, in preparing the partially oxidized graphite intercalation compounds (PO-GICs) or the oxygen intercalated graphite intercalation compounds (OI-GICs), the method may include purging the mixture containing the reversible graphite intercalation compound with oxygen gas at a rate of between about 90 ml/min and about 120 ml/min for a period of between about 30 minutes and about 120 minutes.

In some embodiments, one or more parameters of the microwave irradiation treatments are selected to induce a Joule heating mechanism or a combination of a Joule heating mechanism and a micro-plasma etching mechanism to obtain a predetermined number of holes of predetermined size and geometry on the basal planes of the expanded graphene sheets, one or more parameters including duration, power, and intervals of microwave irradiation pulses.

In some embodiments, the microwave irradiation treatments include one or more microwave irradiation pulses at the power of between about 10 Watt and about 300 Watt for a period of about 3 seconds and about 150 seconds.

In some embodiments, the microwave irradiation treatments further include (1) a first microwave irradiation to remedy existing defects on the basal planes of the expanded graphene sheets; (2) a second microwave irradiation to generate baby holes on the basal planes of the expanded graphene sheets; and (3) a third microwave irradiation to obtain said plurality of holes by fine-tuning the generated baby holes, such that the expanded graphene sheets of the holey graphene nanoplatelets have a predetermined number of holes of predetermined size and geometry on the basal planes of the expanded graphene sheets.

In some embodiments, said plurality of holes have a geometrical structure of hole edges predominantly in a zigzag configuration. In some embodiments, said plurality of holes are uniformly distributed across the basal planes of the expanded graphene sheets. In some embodiments, the mean hole size of said plurality of holes is about 5 to about 500 nm. In some embodiments, the mean thickness of the holey graphene nanoplatelets is from about 2 nm to about 14 nm. In some embodiments, the mean lateral dimension of the holey graphene nanoplatelets is from about 0.5 μm to about 20 μm. In some embodiments, the holey graphene nanoplatelets have a conductivity between about 7000 S/m and about 100,000 S/m. In some embodiments, the holey graphene nanoplatelets have a C:O molar ratio between 15:1 and 55:1.

This disclosure also provides holey graphene nanoplatelets prepared by the above-disclosed methods. In some embodiments, said plurality of holes having a geometrical structure of hole edges predominantly in a zigzag configuration. In some embodiments, the holey graphene nanoplatelets have a conductivity between about 7000 S/m and about 100,000 S/m. In some embodiments, the holey graphene nanoplatelets have a C:O ratio between 15:1 and 55:1.

This disclosure additionally provides a product including the holey graphene nanoplatelets. The product may be electrodes of batteries or electrodes of capacitors.

