Miracle material GRAPHENE: synthetic allotrope of carbon

Graphene is usually called as miracle material owing to incredible properties and numerous applications in almost all fields of science and research. It is the fourth synthetic allotrope of carbon with a fused ring of hydrocarbons, which was first chemically synthesized in 2004 for the first time. It differs from graphite in arrangement of atomic layers as graphene is a singled layered structure of sp² hybridized carbon atoms while graphite contain a multilayered atomic arrangement of carbon atoms having sp² hybridization that is why it is called synthetic allotropic form of carbon.

1. Properties of graphene:

Graphene is a two-dimensional (2D) compound in which carbon atoms are inter-linked with each other in a hexagonal geometry, which results in 2D arrangement.

The basic raw material used in the synthesis of graphene is simple graphite powder, which is very affordable to use. Graphene shows excellent mechanical, electrical, catalytic, additive, absorption and adsorption properties. Mechanical strength of this compound is far more than steel; nevertheless, it is considerably thinner than it, which makes it a spellbinder material among others. It can tolerate high pressure because it is a lightweight material with high tensile strength. Graphene gives magnificent results in photocatalytic degradation reaction whenever used as nanocatalyst due to large surface to volume ratio and absorption capability. As a consequence, active sites of a catalyst increases, which allows more reactants or contaminants to adsorb and boost up the rate of degradation with more efficient results. The structure of graphene possesses good porosity, which enhances its adsorption capacity and ultimately the absorption power. Another contributing factor that makes it better adsorbent among other bulk materials is the large specific surface area, which allows more adsorption space. Electron conductivity is also very high that also speeds up the photocatalytic reactions by absorbing electromagnetic radiations. These extraordinary properties increase the demand of this miracle product in scientific research and various industries.

2. Precursors of graphene:

Graphene is commercially available in two basic precursors to use directly of in the production of graphene based materials like nanocomposites/hybrids. These two precursors are

Ø  Graphene oxide (GO)

Ø  Pristine graphene (reduced form)

Figure 1 shows a schematic diagram of two precursor materials of graphene commonly used in synthesis of nanocomposites and hybrid products.

Figure. 1. Schematic diagram of precursors of graphene

2.1. Graphene oxide (GO)

Graphene oxide possess a porous structure which shows arrangement of carbon atoms which resembles to honeycomb with highly oxidized negatively charged oxygenated functionalities. This is an oxidized form of graphene having frequent oxygenated species present on the active sites of the surface as well as in internal structure. These species exist in three types of oxygen containing functional groups, which contain

Ø  Hydroxyl group (OH^-)

Ø  Carbonyl group (C=O)

Ø  Carboxylic group (COOH^-)

These species are fundamentally responsible to bind the metallic nanoparticles on the surface of graphene oxide strongly and firmly via electrostatic interactions and inter molecular forces, in the formation of graphene-metal nano hybrids. The abundance of these oxygen containing species is more useful in strong interracial bonds among GO and metallic particles, whether present in single metal oxide or bimetallic compounds.

The main contributing reason of hydrophilic nature of graphene oxide is the presence of excessive amount of these highly reactive species on the surface of graphene oxide. Another promising feature of oxygenated graphene is that these species remarkably contribute in enhancing the adsorption capacity of GO as compared to reduced form of graphene. In catalytic applications these species absorb electromagnetic radiations rapidly and high electron conductivity of these species make the degradation of organic as well as inorganic pollutants easier consuming less time and giving high efficiency. Figure 2 shows structure of graphene oxide in which oxygenated groups are attached at different positions.

Figure. 2. Structure of graphene oxide (GO)

2.1.1. Defects in GO structure:

Downside of using graphene oxide is production of defects while using it as precursor material in the synthesis of graphene-metal nanocomposites/hybrids, in which it reduces to rGO when wet chemical or hydrothermal techniques are used. Figure 3 shows defects in the structure of graphene oxide sheet that probably result due to incomplete oxidation or uneven conditions of temperature and pressure provided to the reaction system.

 Figure. 3. Defects produced in graphene oxide structure

2.2 Pristine graphene:

Reduced form of graphene is called as pristine graphene containing minor amount or totally absence of oxygenated species present on the surface. Paradoxically to graphene oxide, this form of graphene is hydrophobic in nature due to deficiency of oxygen. This property of reduced graphene is helpful in its removal from water and easy to handle if dissolved in waste water. It is normally difficult to use as precursor in the synthesis of graphene-metal nano products because it inhibits the interaction of metallic particles with the surface of graphhen. Consequently, GO is mostly used as precursor material in the synthesis of nanohybrids that is converted to reduced form during product formation.