Carbon Allotropes: Metal-Complex Chemistry, Properties and Applications

Carbon, the 6th element in the periodic table denoted by the letter “C” and true element of life, provides the chemical basis for life on Earth due to its ability to form stable bonds with other carbon atoms, oxygen, nitrogen, sulfur, and many other elements in Mendeleev’s Periodic Table. Carbon is found almost everywhere, and it is one of the most abundant materials on earth. It is the 4th most common element in the universe and 15th most common on earth’s crust. All life on Earth contains various forms of carbonic structures, from proteins to the tallest trees [1]. Existence of a host of carbon inorganic forms is the also responsibility of stable single and multiple carbon-carbon covalent bonds. This process is called catenation, in which an element can bond with itself to form long chains. During much time, only two conventional carbon allotropes, graphite (black, soft, and conductive) and diamond (shiny, transparent, and extremely hard), have been known. Only in the last few decades have new synthetic carbon allotropes such as carbon nanotubes, fullerenes (buckminsterfullerene C₆₀, smaller and higher fullerenes), and graphene been discovered. Their outstanding properties, current and potential applications, testify their unique scientific and technological importance [2]. In addition, a host of other carbon structures, both obtained and still predicted, have been reported up to date.

According to much available information, graphite and diamond belong to well-known classic carbon allotropes. In several classifications, natural coal, amorphous carbon, and commercially produced carbon black are added to this non-strict list of conventional carbon forms.

The era of carbon-based nanotechnology, as it is well-known, started from 1985 when the fullerene C₆₀ was discovered. The rediscovery of carbon nanotubes and unexpected discovery of graphene gave a powerful impulse to the further development of carbon nanostructures. At present, these nanocarbons, as well as nanodiamonds or nanofibers, can already be considered as “conventional” carbon nanostructures.

When the area of nanotechnology began to develop intensively as an independent field in the frontiers of physics, chemistry, materials chemistry and physics, medicine, biology, and other disciplines two decades ago, terms such as “nanoparticle,” “nanopowder,” “nanotube,” and “nanoplate,” and other terms related to shape rapidly became very common. At the same time, during the last years, efforts of researchers have led to reports of a large number of the nanostructure types mentioned earlier and the discovery of rarer species, such as “nanodumbbells,” “nanoflowers,” “nanorices,” “nanolines,” “nanotowers,” “nanoshuttles,” “nanobowlings,” “nanowheels,” “nanofans,” “nanopencils,” “nanotrees,” “nanoarrows,” “nanonails,” “nanobottles,” and “nanovolcanoes,” among many others.

In this section, we show several other carbon allotropes, from those rare as, for example, lonsdaleite to common glassy carbon and “carbon black,” xerogels, or hydrogels. In case of carbide- and MOF-derived carbons (relatively new research areas, especially the last one), the production methods vary and structures of formed carbons can be distinct (carbon nanotubes, fullerene- or onion-like nanostructures, nanocrystalline graphitic carbon, amorphous carbon, nanodiamonds, etc.); this is not a special structural type of carbon.

As it has been shown above, a grand variety of carbon allotropes and forms is currently known. They can be very common (graphite, coal) or rare (nanoplates or nanocups) and can be well-developed industrially (carbon black) or intensively studied on nano-level (carbon nanotubes or graphene), doped with metals and functionalized with organic and organometallic moieties. At the same time, applying modern computational methods, a host of new carbon nanoforms (e.g., novamene [1] or protomene [2]) are possible, which have not yet been observed experimentally. An efficient and reliable methodology for crystal structure prediction was developed [3], merging ab initio total energy calculations and a specifically devised evolutionary algorithm. This method allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input. While in many cases it is possible to solve crystal structure from experimental data, theoretical structure prediction is crucially important for several reasons.

Metal complexes have a lot of useful applications in organic and organometallic chemistry, catalysis [1], medicine as anticancer pharmaceutics and for drug delivery [2], various biological systems [3], polymers [4] and dyes, separation of isotopes [5], and heavy metals [6], among many other uses. Sometimes they are applied for increasing solubility [7, 8] of classic objects, carbon nanotubes (CNTs), which form bundle-like structures with very complex morphologies with a high number of Van der Waals interactions, causing extremely poor solubility in water or organic solvents. Metal complexes are also able to serve as precursors to fill CNTs with metals [9] or oxides [10], to decorate CNTs with metal nanoparticles [11], as well as to be encapsulated by CNTs [12].

Currently, the area of carbon allotropes, in particular nanocarbons, is one of the most developing fields in chemistry and nanotechnology, where carbon nanotubes and graphene are leaders in the number of publications. Taking into account their existing and potential technical, biological, medical applications (in particular for drug delivery purposes), and many others, we note that the main difficulty to integrate such materials into devices and biological systems derives from their lack of solubility in organic and physiological solutions. Functionalization of carbon allotropes with the assistance of biological molecules remarkably improves their solubility in aqueous or organic environment and, thus, facilitates the development of novel biotechnology, biomedicine, and bioengineering. For example, the nanodiamonds (NDs) have got a series of distinct applications in various areas, in particular medicine, electrochemistry and creation of novel materials. Biomedical applications of NDs are well-developed and related with the recently established fact that carbon NDs are much more biocompatible than most other carbon nanomaterials, including carbon blacks, fullerenes, and carbon nanotubes [1]. Their tiny size, large surface area, and ease functionalization with biomolecules make NDs attractive for various biomedical applications both in vitro and in vivo, for instance, for single particle imaging in cells, drug delivery, protein separation, and biosensing [2, 3]. Similarly, water-soluble carbon nanoonions (CNOs) are used for biological imaging [4] and as promising theranostic agents [5].

As well as other contaminants (particular matter, heavy metal ions, toxic gases, etc.), carbon allotropes are severe contaminants in air, water, and soil. For example, for diesel vehicles, the black carbon (BC), organic carbon (OC), and other inorganic components of fine particulate matter (PM), as well as carbon monoxide (CO), nitrogen oxides (NO_x), sulfur dioxide (SO₂), ethane, acetylene, benzene, toluene, and other compounds, are typical contaminants under real-world driving conditions [1]. Among carbon allotropes in the environment, the most important carbons in the elemental form are black carbon (mainly), carbon nanotubes, graphene, and fullerenes in lesser quantities. Engineered carbon nanoparticles range from the well-established multi-ton production of carbon black (CB) and other carbon allotropes for applications in plastics and car tires to microgram quantities of fluorescent quantum dots used as markers in biological imaging. All of them possess distinct toxicity, depending on many factors (type of allotrope, particle size, form, structural defects, coating molecules, grade of functionalization, etc.). So, the nanotoxicology, as a scientific discipline, shall be quite different from occupational hygiene in approach and context. Understanding the toxicity of carbon nanomaterials and nano-enabled products is important for human and environmental health and safety as well as public acceptance.

According to the statistic reports, graphite prices were up 30–40% in the second half of 2017 due to an improving steel industry, environmental related production problems in China, and continued strong demand growth from the lithium-ion battery industry. Prices for large flake graphite are currently up to $1200/t from US$750 in 2017. This is still well below the 2012 peak of US$2800/t which was entirely due to the commodity super cycle and strong steel demand. With steel demand also recovering and production issues in China, the supply/demand picture for graphite is very favorable [1]. Graphite prices depend on two factors – flake size and purity. Large flake (+80 mesh) and high-carbon (+94%) varieties command the premium pricing segment [2]. Graphite is applied in the following products and processes, among others:

In this section, we present several selected experimental procedures (including characterization of samples in some cases), directly borrowed from original articles. These experiments are of distinct grades of difficulty (for professors/researchers or students), requiring or not a special equipment. Not all of them can be easily reproduced; students and their advisors need to make a selection of appropriate practices. We hope this part of the book will be very useful as educational material for M.Sc. and Ph.D. students, working in the areas of nanochemistry and nanotechnology, biochemistry and drug delivery, fabrication of carbon thin films from dispersion, application of composites of various carbon allotropes with polymers, and so on.

As it has been shown above, a grand variety of carbon allotropes and forms is currently known. They can be very common (graphite, coal) or rare (nanobuds, nanoplates, or nanocups) and can be well-developed industrially (carbon black) or intensively studied on nano-level (carbon nanotubes or graphene), doped with metals and functionalized with organic and organometallic moieties. The hexagonal network of nanotube cylinders, graphene sheets, or fullerene molecules consisting of hexagons and pentagons of carbon atoms are highly aesthetically pleasing, providing inspiration both to artists and scientists [1]. The shape of carbon nanotubes also influences their outstanding physical properties, such as electrical and heat conductance, which are already exceeding many traditional materials, as well as mechanical strength. Fully carbon electronic devices are now a main dream and great expectative of modern technologies, where a lot of researchers are working.