Fulerenos: the chemistry of today, the bet of tomorrow

2000/08/01 Imaz Oiartzabal, Alaitz | Matxain Beraza, Jon Mattin - EHUko Kimika Fakultateko eta DIPCko ikertzailea Iturria: Elhuyar aldizkaria

• Kimika

If the abundance of surface elements is analyzed, it is observed that carbon is not one of the most abundant elements (the average is 320 g of carbon per ton of surface). Combined with other elements in the lithosphere, it is found in limestones and oil. However, in the atmosphere and in marine waters the CO is in the form of 2. In general, carbon is the basic element of organic molecules. Carbon is the basis of most of the components needed for life. Many of the fuels we use in our daily lives (gasoline and butane extracted from oil) and/or plastics, largely made up of carbon. In short, it can be said that in nature carbon, without associating or mixing with other elements, is found in three main forms: diamond, graphite and active carbon. Diamond and graphite have a crystalline structure and carbon an amorphous structure. Carbon is therefore the element that generates the most compounds.

Each carbon atom has four electrons in the last layer, through which it can be connected with a maximum of four atoms. Both graphite and diamond are formed by the orderly bonding of carbon atoms. In the diamond each atom joins four other carbons to form tetrahedral structures. Each of these tetrahedrons joins another tetrahedron through the vertices, forming a symmetrical three-dimensional network. In the case of graphite, however, each carbon atom binds to three other carbons forming hexagon sheets that accumulate to form three-dimensional structures. The joints between these plates are weaker than diamond joints. So graphite is much softer than diamond.

Diamond and graphite have different structures and properties, and their use is different. The importance of diamond in jewelry is known, but due to the intrinsic hardness of diamond, large impurities are used in industry. Graphite is used in the manufacture of electrodes, such as solid lubricant, pencil mines, etc.

New carbon structures

As has already been indicated, science has advanced greatly and, in many cases, when unforeseen discoveries have occurred, research lines have been diverted, previous work has been discarded and new avenues have been initiated. This is what happened to them in 1985 Robert F of the USA. Curl and Richard E. Www.euskaltel.com and British Harold W Krotori. This team discovered and identified a new carbon structure: fulerene. These compounds were already seen by other groups, but were not identified as "unknown carbon structures". Astrochemist Kroto found fulerenes while researching the composition of carbon-rich stars. In order to observe the carbon structures of the outer atmosphere of the giant red stars, the graphite was evaporated by laser rays; after analyzing the carbon plasma thus obtained, it is observed time and again that the tendency to carbon collection by sexennia was very high (even seventy in volume). This discovery led him to the Nobel Prize in Chemistry in 1996.

Structure of fulerenes

It was no easy task to imagine the structure of this substance they just found. After several headaches, R. The aforementioned scientists discovered that the structure of the geodesic domes of the American architect and philosopher Buckminster Fuller (1895-1983) corresponded to them. Therefore, although initially called "Buckminsterfullereno", later became popular the term "fulereno", but also called "buckyballs" and "buckys". This structure resembles the football, that is, it is a circular and hollow structure of 60 carbon composed of 12 pentagons and 20 hexagons, C 60 . Around each of the pentagons there are five hexagons that allow to give a resounding structure. This structure is obtained by associating each carbon atom with three other carbons, as in the case of graphite. This structure, almost spherically shaped, has an average diameter of 7Á (7x10 -10 m) and a molecular mass of 720.64 (12.01 of the carbon atom). Fulerene is the most spherical and symmetrical molecule known. 94% of esfericity.

As already mentioned, in the beginning we found structures C 60 and C 70 that, despite being the most numerous, gradually were discovering others: C 20 (the smallest possible fulerene, regular dodecahedron, consisting of 12 pentagons), C 32, C 50, C 76, C 78, C 84, C 240, C 540, etc.

Intermediate structures have also been found between fulerenes and graphite: full nanotubes and fuleroids. Nanotubes are very small cylindrical tubes that are formed by folding graphite sheets and whose ends are closed by fulerene structures. Conversely, full fuleroids are formed by accumulation of concentric layers of graphite-fulerene of different size (onion style). However, they still do not know if macroscopic amounts of these full fuleroids can be produced.

How are they done?

Synthesis of C 60 W. Krätschmer and D. R. Huffman is due to physicists (1990). At first they got very small amounts, which was the biggest problem. Although the properties of fulerenes are very important, quantities were obtained so small (and therefore so expensive) that it was difficult to advance research. Therefore, the biggest initial challenge was to synthesize macroscopic amounts of fulerene. Today, specialized laboratories generate their own fulerenes. Although the procedure discovered by these scientists has slightly varied since then, it is based on the voltaic arc between the graphite electrodes established in the controlled atmosphere of helium. This generates soot with small amounts of fulerene, the proportion C 60 /C 70 of soot also depends on the current intensity used in the procedure. This procedure is very simple compared to the complex tool initially used, but other sources of fulerene have been investigated and continued to investigate.

On the other hand, many substances are currently analyzed through computer programs. These programs carry out very complex calculations and produce good results. From the theoretical analysis of the connections between atoms, virtual images of chemicals are obtained and their geometry and energy are calculated. Given the difficulty of obtaining fulerenes and derivatives in macroscopic, cheap and pure quantities, the realization of these simulation studies is very useful, since the realization of stability studies of these molecules allows to predict their properties.

For what?

Before analyzing the use of any material, it is necessary to investigate and understand its properties, since depending on its properties the material will have different uses. In fact, if we look at the properties of fulerenes, we will see that they are very interesting: they have conductive, photochemical, structural properties, etc. Thanks to these multiple properties, the possibility of several uses for fulerenes was initially speculated.

As for the structural properties, the small spherical structure of fulerenes makes them able to store atoms and small molecules inside them: heavy metals, drugs, etc. The latter can have a great influence on medicine, since the drugs thus gathered could reach the diseased organ without destroying it on the way or damaging it in other organs. On the other hand, fulerene has also been used in the most precise microscopes based on the tunnel effect of electrons. These microscopes have a small tip in which by placing a fulerene molecule, its small size has allowed the graphite atoms to be "seen" individually. The aforementioned nanotubes, combined with metals, conduct electrical current. Its nanometric structure allows the construction of very small threads, which allows, among other things, to reduce the size of computer chips. In addition, it has been speculated that they have greater mechanical strength (i.e., they are more resistant) than carbon fibres with graphite structure.

Fulerenes can collide at a speed of 27,000 km/h without breaking into a steel plate, indicating the hardness of these molecules. In addition, they are able to withstand very high pressures, 220,000 times higher than human beings suffer from the atmosphere. Consequently, its packaging capacity is very high, more than double that of diamond. This hardness and packaging capacity has also referred to its industrial potential as a diamond substitute.

However, the conductive and photochemical properties are some of the most important. In addition, taking into account the ability of fulerenes to combine with other molecules, materials with enormous potential are obtained. Fulerenes can be used as electrical insulators or as semiconductors, depending on the compound to which it is associated. Insulating materials do not let the electric current pass, while semiconductors, despite acting as insulators in normal conditions, are able to absorb a small amount of energy from the medium, thus transporting the electric current. For this it is necessary to combine fulerenes with other molecules. The photochemical properties of fulerenes allow obtaining from sunlight the energy necessary to conduct electric current. All these properties have allowed the Fulerenes to start their journey in the field of renewable energies. Several laboratories are investigating the usefulness and possible advantages of fulerenes in this field.

In 1999 they presented the revolutionary solar photovoltaic cell formed by molecules derived from fulerene. In them, the same molecule supplies and captures electrons, which allows to increase cellular efficiency. However, it is still too early to draw conclusions. The authors have still recognized that the effectiveness of this cell is very low, but with the use of derivatives that absorb more solar energy, improvements can be expected.

In general, the materials driving the electric current lose energy due to the phenomenon called electrical resistance. This is because electrons moving from the conductor have obstacles in their movement. This, of course, has its influence on solar cells, as a quantity of solar energy is lost. During this century, however, materials have been found without resistance! This means that electrons do not lose energy in their movement. These materials are called superconductors. Many superconductive materials have been found but all have the same problem: very low temperatures are needed for superconductivity to appear, as thus the obstacles suffered by electrons disappear.

Fulerenes combined with potassium (K) or barium (Ba) at 33K or -240°C are superconductors.

We have seen that fulerenes can have many uses. Every day there are numerous applications to new materials and, as a consequence, those that are currently only speculations, are becoming reality. When a high synthesis of fulerenes is achieved, these will become reality in our society.

The photovoltaic solar cell converts the energy coming from the sun into electric current. To do this, the solar cell needs two different materials. One absorbs light, releasing electrons and the other captures them. The electric current arises as a result of the movement of these electrons.