Transition metal impurities in carbon-based materials: Pitfalls, artifacts and deleterious effects

Abstract

From carbides in steel to cobalt–carbon covalent bonds in vitamin B12, transition metal (TM)–carbon interactions play a critical role in so many aspects of human life that we rarely realize it. While insoluble in organic and inorganic solvents, elemental carbon does dissolve in transition metals of specific valence electron configurations. This property, accompanied by carbon’s low vapor pressure, high melting/sublimation point, resistance to corrosion and reducing properties create both challenges and opportunities for the handling and separation of carbon–TM systems. Natural and synthetic carbon-based materials are contaminated by transition metals to various extents, which hinders or totally precludes their advanced applications. Whether it is high-rank coal to be burned in power plants, graphite for electric car batteries or nuclear reactors, or graphene and single-wall carbon nanotubes (SWCNTs) for nanoelectronics or medical purposes – TM impurities can cause a range of unexpected deleterious effects even at parts per billion levels. Despite impressive progress in the field of carbon-based material chemistry and physics, a commercial breakthrough in advanced carbon-based technology has not yet been achieved. One of a few major reasons for this failure is that the industrial production of homogeneous and truly metal-free carbon (nano)structures with particular properties remains a great challenge yet to be overcome and hence impedes high-end applications. A part of the scientific community erroneously considers the purification of carbon (nano)structures a resolved issue, however there is still much to be done in this area. Herein we discuss the problem of TM impurities in carbon-based (nano)materials in a broad context concerning the vast spectrum of materials from natural high-rank coals and graphite to synthetic SWCNTs and graphene. Fundamental considerations on TM–carbon interactions, artifacts and deleterious effects of TM impurities (e.g. in catalysis or medical applications), analytical techniques for the assessment of their concentration, purification and demetalation of carbonaceous materials and catalyst-free synthesis of carbon nanostructures are discussed in detail. Finally, we also show how seemingly detrimental TM impurities may present unexpected benefits.