Graphite oxide

Graphite, graphite formerly acid is a nonstoichiometric compound of the chemical elements carbon, oxygen and hydrogen; they can be obtained from graphite under the action of strong oxidants. In its maximally oxidized form of graphite oxide forms a yellow solid; the ratio of carbon to oxygen ranging from 2.1:1 to 2.9:1. Graphite having a layer structure similar to graphite, even if the distances between the molecular planes are larger and more irregular.

In basic solutions decomposes into graphite flakes with monomolecular layer thickness, which are referred to as graphene oxide (based on graphs, the single-layer form of graphite ). For graphene, a high-strength paper-like material, Graphenoxidpapier be established, the has become interesting in recent years as a possible intermediate in the production of graphs. However (as of 2010 ) has produced this way the graphene to numerous chemical and structural irregularities.

History and presentation

The first representation of graphite succeeded in 1859 the British chemist Benjamin Collins Brodie Jr., treated the graphite with a mixture of potassium chlorate and fuming nitric acid. Faster and less dangerous in higher yield, the method described by William S. Hummers and Richard E. Offeman 1957; they used a mixture of sulfuric acid H2SO4, sodium nitrate, NaNO3, and potassium permanganate, KMnO4. This method is still (as of 2013) in use.

Recently, a mixture of H2SO4 and KMnO4 was used to cut open carbon nanotubes in the longitudinal direction, thereby arise microscopically small, flat graphene ribbons with a width of a few atoms which carry a cap made of oxygen atoms ( = O) or hydroxyl groups at the ends ( -OH ).

Structure

Structure and properties of the graphite oxide are determined by the synthesis method used and the oxidation rate was achieved. The layer structure of the graphite used is typically obtained, but the layers are irregular in terms of their planarity, and have a greater distance up to two times ( about 0.7 nm ) than graphite. Unlike the historically established name graphite suggests, it is in the strict sense is not an oxide. Besides epoxy experimentally been found in the following functional groups: the carbonyl ( = CO), hydroxy and phenyl groups.

The exact structure is not well understood because of the rather variable spacing of the layers as well as the low overall order in detail. The distance of 1.1 ± 0.2 nm Graphenoxidlagen is recording with the scanning tunneling microscope can detect regions in which oxygen atoms are arranged in a rectangular pattern having a lattice constant of 0.27 nm x 0.41 nm. The edges of each layer bounded by carboxyl and carbonyl groups. By means of X-ray photoelectron spectroscopy can be shown that carbon atoms are also present in rings without oxygen ( 284.8 eV ) (see 286.2 eV for C -O, 287.8 eV for C = O and 289.0 eV for O- C = O).

Graphite takes very light on water, whereby the distance between the levels increases significantly ( up to 1.2 nm in the saturated state ). At higher pressures, water is additionally installed in the space between the individual layers. The Graphitoxidmasse retains moisture from the ambient air in proportion to the humidity. A complete drying appears difficult, since heating leads to 60-80 ° C for the partial decomposition of the material.

By rapidly heating to 280-300 ° C, graphite oxide decomposes (English exfoliation ), a finely divided, amorphous carbon powder, similar to activated carbon arises.

Applications

Graph production

Graphite recently become of interest as a possible precursor for the production of a large graph to scale. Graphite oxide is an insulator; with a differential electrical conductivity of 5.10 -3 S · cm -1; at a bias voltage of 10 V, it is almost a semiconductor. Due to its hydrophilic properties, graphite oxide dissolves in water easily, and it breaks down into small microscopic Graphenoxidflocken, which are usually one layer thick. Theoretically, it should be obtained by chemical reduction of a suspension of graphene flakes.

A partial reduction of graphene oxide is achieved by a 24- hour treatment with hydrazine at 100 ° C, short-term, lasting a few seconds treatment with plasma of hydrogen gas, or a strong light pulse, such as a xenon flash.

The conductivity of graphene produced in this way is less than 10 S · cm -1, the electron mobility between 2-200 cm2 · V-1 · s- 1 for flaws and 0.5 and -30 cm2 · V-1 · s- 1 for electrons. These values ​​are significantly larger than that of the oxide, but still several orders of magnitude smaller than that of pure graphene. Recordings with the atomic force microscope show that the carbon layers are deformed by oxygen bonds, as well as a significant roughness of the oxide layers, which remains after the reduction. These defects appear in the Raman spectrum of the graphene oxide.

Graphenoxidpapier

As in papermaking Graphenoxidflocken can be drawn from an aqueous dispersion, an extremely tear-resistant Graphenoxidpapier arises.