3D Printed Structures of Carbon Allotropes

Co-Authored by Dean J. Campbell* and Matthew E. Hill

*Bradley University, Peoria, Illinois

Models representing the arrangement of atoms within structures have been used for many years.^1-4 With the development of 3D printing, complex models can be produced that are sufficiently robust to be handled repeatedly.⁵ For a recent student project in the Materials Chemistry course that was taught at Bradley University, 3D printed models were made that represented several of the allotropes of carbon. Allotropes are differing structural forms of the same element.⁶ One goal was to print all of these models to have approximately the same carbon atom size and the same bond lengths, enabling optimum comparison between the models.

The files used to print these models were obtained from various online sources.

For example, one good source of information for 3D models is the Cambridge Crystallographic Data Center (CCDC). To search through the files, one can go to https://www.ccdc.cam.ac.uk/structures/ to Search - Access Structures. If a reference code or number is known, the identifier box can be used to search for the molecule.

If the reference code or number is not known, one can use the compound name box to search for the one of the common names of the structure. It might take multiple search iterations to find the desired file.

Two alternatives to the CCDC website are https://www.crystallography.net/cod/search.html and http://rruff.geo.arizona.edu/AMS/amcsd.php

These sites are not preferable since they do not provide a preview of the structure. The cif file must be downloaded to be viewed. Readers who are interested in using the files that were used to print these 3D models can find them in the Supporting Information. Readers must log into their account to access. Not a member? Register for free!

Once the structures were printed, the support material needed to be removed. A needle-nosed pliers was helpful in some cases, and it was also found that a rotary grinding tool or a drill press was useful to produce holes in the support material to help give a pliers a place to start. The diamond structure was somewhat fragile, and some carbon atoms were glued back on to the structure after accidentally being broken off while the support was being removed.

Figure 1 shows 3D printed models of (LEFT) diamond and (RIGHT) lonsdaleite. Both have carbon atoms with sp³ hybridization that has a tetrahedral bonding geometry to four other carbon atoms. Normal diamond has a cubic symmetry, as shown by the square pattern in Figure 1 (LEFT), and the rare lonsdaleite has a hexagonal symmetry, as shown by hexagon pattern in Figure 1 (RIGHT).

fig_1.jpg

Figure 1. 3D printed models of (LEFT) diamond and (RIGHT) lonsdaleite.

Figure 2 shows two 3D printed models of graphene stacked above each other to represent two layers of the graphite structure.⁷ The structures have carbon atoms with sp² hybridization that each has trigonal planar bonding geometry to three other carbon atoms. Note that the top and bottom layers are offset with respect to each other.

fig_2.jpg

Figure 2. Two 3D printed models of graphene stacked above each other to represent two layers of the graphite structure.

Figure 3 shows 3D printed models of (LEFT) carbon nanotubes and (RIGHT) C₆₀ fullerene. Both structures have carbon atoms with sp² hybridization that each has a trigonal planar bonding geometry to three other carbon atoms. Unlike the other models, the model of the carbon nanotube was acquired from a ThingiVerse site online.⁸ The nanotube model has eight hexagon structures and eight additional bond lengths around its circumference. This alternation of bonds and hexagons is referred to as an armchair geometry for the nanotube (other geometries include zigzag and chiral).^9,10 This particular armchair nanotube is also called a (8,8) nanotube, referring to how a single graphene sheet can be rolled to produce this nanotube.^9.10 Since each hexagon length is 0.284 nm, and each bond length is 0.142 nm, a real nanotube with this structure has a circumference of 3.408 and a diameter of 1.085 nm.

fig_3.jpg

Figure 3. 3D printed models of (LEFT) carbon nanotubes and (RIGHT) C₆₀ fullerene.

The goal to print all of these models to have approximately the same carbon atom size and the same bond lengths was achieved. Since their production, the models have been passed around Materials Chemistry and General Chemistry II classes at Bradley University to illustrate carbon allotrope structures. It was noted in a classroom that the fullerene model fits inside the carbon nanotube model, resembling a basketball in a net. This might be useful to making a sports connection in the classroom, maybe by pointing out that a real basketball is 200-300 million times larger than a C₆₀ fullerene.^11,12

Printing 3D models can be connected to environmental issues. The polylactic acid used to print the models can be sourced from biomass, rather than fossil fuels. Waste production and E-factors can also be part of classroom discussion. In a Materials Chemistry course at Bradley University, a model was shown that was produced on a 3D printer, as well as the scraps of the supporting material that were also produced by the printer. The mass of the waste was divided by the mass of the product to obtain the E-factor, Figure 4, and the advantages and limitations of this approach were discussed. Of course, E-factor not the only measure of the greenness of a process.¹³

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Figure 4. E-factor calculated from model and waste supporting material produced by a 3D printer.

It is anticipated that these 3D printed models will see additional use at STEM outreach events. Readers who are interested in using the files that were used to print these 3D models can find them in the Supporting Information. Readers must log into their account to access. Not a member? Register for free!

Safety

3D printers involve moving parts and heating elements. Please use caution to avoid burns and pinches. Cutting the extra supporting filament away from the models may require the use of sharp objects. Consider the hand-eye coordination of individuals being asked to cut the extra material away from the model.

Acknowledgements

We thank Rhiannon Davids, Karl Jung, and Edward Flint for assistance with the 3D printing. We also thank the Bradley University Education Department and Edward Flint for access to their printers. This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society. The material contained in this document is based upon work supported by a National Aeronautics and Space Administration (NASA) grant or cooperative agreement. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NASA. This work was supported through a NASA grant awarded to the Illinois/NASA Space Grant Consortium.

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