image: Several theoretically predicted 3D sp3 carbon structures.
Credit: ©Science China Press
1. The Infinite Possibilities of Carbon
While graphite in pencil and diamonds in the Earth share the same element - carbon, they exhibit vastly different properties due to the different hybridization and arrangement of atoms. Scientists are no longer satisfied with these two classical structures and are trying to construct novel three-dimensional (3D) carbon crystals with superhard, conducting, porous and other properties through “Lego” -style assembly of carbon units. Connecting carbon atoms with different hybridizations leads to an essentially countless number of carbon allotropes: some are harder than diamond yet light as a foam, some have honeycomb channels for efficient hydrogen storage, and some conduct electricity like metals.
2. Theoretical Predictions: Building Crystal Genome Library of Carbons
With the development of computational materials science, over 1,600 3D carbon structures have been predicted using tools like particle swarm optimization and genetic algorithms. For instance, T-carbon is a lightweight superhard material with less than half the density of diamond constructed by replacing each carbon atom in diamond with a carbon tetrahedron (Phys Rev Lett 2011, 106, 155703). The “carbon schwarzite” is a honeycomb structure with negative curvature constructed according to the triply periodic minimal surface proposed by mathematician Hermann Schwarz, exhibiting Dirac cone band structure and exceptional adsorption capacity (Phys Rev B 2014, 90, 125434).
The article points out that although countless 3D structures can be constructed theoretically, experimental synthesis remains limited. This situation mainly stems from the fact that carbon tends to form graphite or diamond in thermodynamics, while other structures are mostly metastable and require much finer control conditions.
3. Experimental Breakthroughs: Multiple Synthesis Techniques
1）Template-Assisted Carbonization: Using zeolites with regular channels as templates, microporous 3D carbon networks can be prepared by chemical vapor deposition. For example, lanthanide-catalyzed-synthesized zeolite-templated carbon (ZTC) exhibits a periodic 3D graphene-like structure with a specific surface area of 4100 m2/g (Nature 2016, 535, 131–135), deemed as the experimental preparation of carbon schwarzites.
2）Organic Synthesis: Bottom-up preparation of macroscopic 3D carbons through the assembly of curved molecular carbons (e.g., [8]circulene and [7]circulene derivatives), which can be seen as building blocks of negatively curved 3D carbon crystals.
3）High-Pressure Processing: At tens of thousands of atmospheres of pressure, fullerene C60 molecules can connect with each other to form 3D polymers. Compressing the C70-carbon nanotube “pea-pod” structure at a pressure of more than 60 GPa can obtain V-carbon with a hardness of 89 GPa (Phys Rev Lett 2017, 118, 245701), which opens up a new path for the preparation of superhard materials.
4）Charge Injection: Several syntheses rely on the charge-injection-induced connection of carbon nanostructures, as highlighted by Zhu’s team. By heating the mixture of C60 molecules and lithium nitride, covalent bonds formed between C60 cages, thus achieving the gram-scale preparation of 3D long-range ordered porous carbon (LOPC) for the first time (Nature 2023, 614, 95–101). This method can be used to precisely control the structure at atmospheric pressure, for the preparation of more 3D carbon crystals.
4. Dawn of Future Materials
3D carbon crystals promise revolutionary applications: periodic porous structures for hydrogen storage, superhard crystals for cutting tools, and carbon-based semiconductors for next-gen electronics. Prof. Zhu remarks, “We’re unlocking the species library of carbon, like building future materials with different functions using atomic Lego blocks.” With the advancement of computational simulation and experimental techniques, 3D carbon crystals are moving from theory to reality, which not only expands the boundaries of carbon materials, but also indicates that the design of carbon materials would enter a new era of “atomic-level customization”.
Yanbo Zhang, a PhD candidate at University of Science and Technology of China, is the first author of the paper, and Prof. Yanwu Zhu is the corresponding author. This work was supported by the National Key R&D Program of China and the National Natural Science Foundation of China.