By Liam Critchley (chemistry and nanotechnology writer)

First isolated in 2004 by Nobel Laureates Andre Geim and Konstantin Novoselov, graphene has excited global interest because of its fantastic properties resulting from being two dimensional (2D). These University of Manchester researchers famously used scotch tape to obtain a single atomic layer of graphite – which is thousands (or millions) of atomic layers of graphene held together by van der Waals forces. While graphite is, well, boring, 2D graphene may be the most exciting practical material ever discovered.

The term “graphene” is currently used to describe a range of materials, from monolayer graphene produced through chemical vapor deposition (CVD) to few-layered graphene in the form of nanoparticle powders up to large, thin and nearly defect free (LTDF) flakes. Even graphene oxide (GO), reduced graphene oxide (rGO) and nanoplatelets are called “graphene”.

A new material has emerged called “3D graphene”. You could be forgiven for asking if that is not an oxymoron, since graphene’s fantastic properties are made possible because of its 2D structure. After all, isn’t graphite 3D graphene?

What Is 2D Graphene?

The descriptive ‘2D’ doesn’t originate from the spatial dimension graphene occupies. Instead, it is derived from how graphene’s one free electron per carbon atom is made available. 2D graphene’s fantastic properties occur along the surface of each atom because, in each graphene layer of atoms, the electrons are confined in 1 dimension (between the layers) but the electrons are free to move in 2 dimensions (across the two dimensions of the graphene layer). Thus, graphene is a 2D material.

This ordering of the free electrons makes high quality graphene capable of transmitting electricity at nearly the speed of light. For the same reason, it enables highly efficient transmission of heat and cold. When LTDF graphene flakes are arranged in a planar configuration, they become 200 times stronger than steel.

What is 3D Graphene?

There is no single definition of 3D graphene because various researchers and companies have each defined it to suit their need. While 3D graphene materials do vary, the commonality is that they contain interconnected graphene layers that form 3D networks. So, in this case, the term ‘3D’ refers to spatial arrangement.

Different graphene materials in different form factors can be used to create 3D graphene structures―including ‘crumpled’ and ‘twisted’ graphene, CVD graphene sheets or graphene flakes. Regardless of the graphene type, the different graphene layers are connected to form 3D networks which can be used to create 3D macrostructures―such as graphene foams, graphene aerogels and graphene hydrogels.

The ability to form 3D networks opens graphene up to new application areas. These graphene macrostructures contain interconnected 3D porous networks―and the lightweight, mechanical and conductive properties of graphene layers can be exploited in different ways compared to using planar graphene layers. For example, Scottish company Integrated Graphene grows 3D scaffolds (named GiiTM) directly onto different surfaces.

The pores in 3D graphene tend to be well defined in the nanometre range, with the walls of the pores being composed of thin layers of interconnected graphene layers. There are several methods to make 3D graphene, with template-assisted and solution-based methods among the most common. Over the last several years, graphene networks have been created with a whole range of graphene materials―including CVD graphene, graphene powder and graphene oxide materials. It almost goes without saying that the properties of the formed 3D graphene will differ based on the type of graphene used.

3D graphene’s interconnected network does possess useful properties due to its highly resilient and interconnected pore network and ultra-high surface area. For many applications, it is the pore network that is being exploited. 3D graphene materials are certainly lighter than other nanoscale and microscale porous materials. In fact, graphene aerogels are one of the lightest solid materials in existence.

When it comes to making 3D structures using ‘crumpled’ CVD graphene sheets, Lyten appears to be the leader with a material it calls “Lyten 3D Graphene”. From the available literature, Lyten appears to crumple graphene sheets to increase the number of active sites, making the exposed surfaces reactive with other materials.

How 2D and 3D Graphene Differ

Because 2D and 3D graphene have different macrostructure arrangements, there are some differences in properties―especially structural properties. 3D graphene’s mechanical properties vastly differ from 2D graphene. While no definitive study has been found, reports suggest that 3D graphene has substantially less tensile strength than LTDF graphene flakes, for example. Likewise, while 3D graphene networks are better than graphite for electrical conductivity and charge carrier mobility, they are substantially less conductive than 2D LTDF graphene flakes. While estimates vary, 3D graphene’s conductivity has been reported at 0.1 – 1 MS/M while LTDF graphene has conductivity up to 100 MS/m – meaning that LTDF 2D graphene is between 100 and 1,000 times more conductive than 3D graphene.

Is 3D Graphene Impacted by Van Der Waals Forces?

Like 2D graphene, van der Waals forces play a key role in 3D graphene materials―and 3D graphene uses graphene flakes/sheets in a very similar manner to 2D graphene. But there are some differences in the resulting effect on the overall material structure. In 2D graphene, the van der Waals forces are used to hold the layers on top of each other, but when it comes to 3D graphene, the individual graphene layers are interlinked into a 3D network using van der Waals forces. π-π stacking and hydrogen bonds (if GO) also play a role in forming the networks but van der Waals forces are a key driver as to why 3D graphene networks hold the rigid, porous structure.

Application Areas of 3D Graphene

LTDF graphene flakes can be used as an additive material in thousands of products.While 3D graphene retains some limited properties and characteristics of 2D LTDF graphene, it is its porous nature that enables a range of applications cantered around ions/molecules absorbing and desorbing.

Many (perhaps most) of the applications cited in the literature have used 3D graphene in one form or another as binder materials in supercapacitors and batteries. Much of the interest has been in metal-air batteries (Li-air, Zn-air, Al-air, and Na-air), presumably because the 3D graphene networks have more active catalytic sites. This improves the catalytic activity of the cathode and improves the overall energy density of the battery. Lyten has also been focusing its application development in less commercialized sectors and has been using 3D graphene to create Li-S (lithium-sulfur) battery packs for electric vehicles. There’s also a growing interest in using 3D graphene materials as electrode substrates in biological fuel cells because there is a higher surface area for bacterial colonization or biocatalytic immobilization.

Another application area is in thermoelectric devices, i.e., devices that convert heat to electricity. An ideal thermoelectric material should have the electrical conductivity of metal, with the Seebeck coefficient (the thermoelectric voltage in response to a temperature difference) of an insulator, and a low thermal conductivity (like that of a semiconductor). 3D graphene has a high Seebeck coefficient, a decent electrical conductivity and low thermal conductivity (two orders of magnitude lower than 2D graphene). These properties allow for a good thermoelectric performance because the pores interrupt phonon transport but do not interfere with electron transport, allowing heat to be harvested and converted into electricity because the interruption of phonons creates temperature differences across the material.

Biochemical sensing is another key application area for 3D graphene and is an area that has been commercially targeted by Integrated Graphene and their 3D graphene sensor product Gii-SensTM. 3D graphene can be functionalized to detect peptides, cellulose molecules, living cells, tumor cells, and cancer cells. The main reason is that the high surface area of 3D graphene allows for more enzymatic/catalytic activity, and a lot of receptors can be functionalized across the material. For cancer cells specifically, the low thermal conductivity and electrochemically active surface is advantageous because a high thermal conductivity makes it problematic to use temperature to distinguish between cancerous and healthy cells. In the biotech space, 3D graphene has also gathered interest as biocompatible scaffolding for tissue and bone regeneration applications because the porous network provides a good platform for cells to proliferate from and integrate into the surrounding tissue.

Overall, 3D graphene is an adjacent extension to your conventional graphene materials―with many 3D graphene materials being composed of interconnecting networks of conventional 2D graphene layers. They offer potential in a number of application areas―particularly due to their porous nature―but for most materials, the core macro properties that they impart into other materials are generally dramatically less than LTDF 2D graphene. It will be interesting to see which niche uses continue to utilize 3D graphene once industrial volumes of LTDF graphene flakes become widely available.

SEM view of 3D graphene.


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