Graphene based materials and their composites as coatings

Review Article

Austin J Nanomed Nanotechnol. 2014;1(1): 1003.

Graphene based materials and their composites as coatings

Yao Tong1, Siva Bohm2 and Mo Song1*

1Department of Materials, Loughborough University, Loughborough LE11 3TU, United Kingdom

2TATA Steel RD&T, Swinden Technology Centre, Moorgate, Rotherham S60 3AR, United Kingdom

*Corresponding author: : Mo Song, Department of Materials, Loughborough University, Loughborough, LE11 3TU, United Kingdom

Received: December 05, 2013; Accepted: December 27, 2013; Published: December 31, 2013


Coating plays a vital role in improving surface quality and providing protection for a substrate. Consistent efforts have been made to produce excellent properties coating. Graphene, a new discovered carbon allotrope, has received worldwide attention and it was studied in almost every area due to its extraordinary properties arisen from its unique structure. It is a very promising new generation material for developing advanced coating. In this article, the recent activities about utilizing pristine graphene based materials and graphene based composites as coatings were reviewed. The synthesis and functionalization of graphene were also described briefly. The future perspectives of graphene based coatings were suggested.

Keywords: Graphene; Graphene based composites coating applications.


Coating is usually used to improve the surface properties of a substrate, wettability, corrosion resistance and adhesion, for example. The coating industry has been driven to seek new technologies and materials to improve the efficiency of coatings by economic benefits and growing environmental concerns. There are several factors affect the effectiveness of a coating against all the possible damaging sources: they are the quality of the coating, the substrate characteristics, the properties of the coating⁄substrate interface, and the corrosiveness of the environment [1]. In order to satisfy the industrial requirements nowadays, polymer nanocomposites has been more and more investigated and applied in coating because nanocomposites provide superior properties with a relative low cost. Additionally, the processing procedure can be much less complicated than multi–layer coatings [1,2].

Graphene, a new generation material, is an allotrope of carbon element which was first isolated by simple mechanical exfoliation in 2004 [3]. It is a two dimension honey comb single layer crystal lattice formed by the tightly packed sp2 bonded carbon atoms. Due to the unique structure of graphene, these carbon atoms form an excellent electrons carrier space. Therefore, graphene has extraordinary electrical properties such as high electron mobility at room temperature (250,000 cm2⁄V) [4,5] and ballistic transport and quantum hall effect at room temperature [6]. In addition, graphene also has excellent optical properties [5]. Excellent mechanical properties of graphene (i.e. 1TPa Young’s Modulus and 130GPa tensile strength) were also reported and the mechanical properties relate to the number of graphene layers and the internal defects of the graphene layers [5,7,8]. It is possible that the energy band gap of graphene can be changed by the uniaxial strain on graphene and this indicates that the uniaxial strain applied on graphene is able to affect the electronic properties of graphene [9,10]. In terms of thermal properties, the highest thermal conductivity at room temperature was reported as 5000 Wm−1K−1 [11]. Some potential applications of graphene have been suggested by researchers such as gas detection [12], transistors [13], nanocomposites [14], energy storage devices [6], barrier applications [15] and so on. However, graphene is still agiant gold mine that can be dug deeper.

Due to the excellent properties of graphene, it is believed that it could be used to enhance the performance of coating significantly. Graphene is very ideal to be efficient filler for high quality polymer matrix nanocomposite coating. In addition, it can be used with nanoparticles to form graphene–nanoparticles composites coating and as a high quality coating materials solely. Graphene is identified as a high water and oil repellent material while graphene oxide (GO) is hydrophilic [16]. This property can make graphene suitable for the coating that provides water and oil resistance. A test about frictional properties and wear resistance of multi–layers graphene film has been performed using an AFM by Lin and his co–workers [17]. Superb frictional properties and high wear resistance were reported. These results mean that graphene is able to become a protective coating against scratch or other physical damage toward a substrate. Graphene was also proved to be an effective corrosion barrier material because it was considered inert under the conditions where chemical reactions of other substrates will take place [18]. As a result, it is also promising in improving anti–corrosion property of a coating system.

In coating applications, graphene is believed to be promising but the articles designated for coating applications are limited. In this review, the synthesis methods and functionalization of graphene were described briefly. The articles about utilization of graphene in coating published in these years were reviewed. The conclusions summarized the published researches and suggested the future research perspectives.

Graphene based materials: Synthesis methods and functionalization

Synthesis methods: Different methods have been developed to synthesize graphene. However, not all of them can be used to synthesize good quality graphene efficiently. Three major routes, mechanical exfoliation, reduction from graphene oxide and chemical vapour deposition (CVD), are regarded as the promising routes to synthesize graphene and they are possible to be used for large scale graphene production in the future. Among them, the chemical route, graphene reduced from graphene oxide were frequently used to study the utilization of graphene based materials in different applications.

Mechanical exfoliation: The first graphene sheet was produced by simple mechanical exfoliation [3]. Therefore, mechanical exfoliation became a very attractive method for the researchers to produce graphene initially. The very first mechanical exfoliation method was a simple peeling process where pre–treated graphite was fixed onto a photoresist and graphene layers were peeled off by a scotch tape [3]. Although this simple method can produce graphene with extraordinary properties, it is limited by its low efficiency. Many efforts have been made to improve mechanical exfoliation method. Ultrasonic devices, solvent and surfactants were used to modify this process to produce high quality graphene in larger scale. The solvents and the surfactants can be intercalated into the atomic layers of graphite to form graphite intercalation compounds to prevent agglomeration and assist further separation of graphene single layer [5,19,20]. The influence of ultrasonic power, time and solvent used on the volume of graphite intercalation compounds was also investigated [5]. Although the use of solvent and surfactants can help to produce good quality graphene in larger scale, their major drawbacks are high solvent cost and the difficulties in following graphene deposition caused by high solvent boiling point [5,19]. Graphite oxides produced by chemical methods were also used in mechanical exfoliation. However, the subsequent produced graphene has inevitable structure defects which could disrupt the electronic structure of graphene. These structure defects could not be restored by chemical reduction or thermal annealing [21-23]. Hence, physical exfoliated graphene is preferred when graphene structure is required in an application. However, it is still extremely challenging to scale up mechanical exfoliation process to produce large amount of graphene in a cost effective way with commercial available technologies and devices.

Chemical vapour deposition (CVD): Chemical vapour deposition was first reported in 2006. Ni foil was used as a substrate and camphor was used as precursor [24]. Since then, chemical vapour deposition method was received more and more attention for being regarded as a new promising route to produce graphene in large scale [5]. In addition, this method was able to control the number of graphene layers and minimize the folding of graphene, and this meant controlled thickness graphene film could be synthesized [5,23,25,26]. Medium–high carbon solubility (>0.1 atomic%) substrate like Ni and low carbon solubility (<0.1 atomic%) substrate like Cu have different graphene growth mechanisms [5]. For high carbon solubility substrate, graphene layer is grown from the precipitation of carbon on the substrate, which is dissolved into the substrate earlier, after cooling. A typical CVD process generally has three steps [26,27].

1. The substrate is put into a chemical vapour deposition chamber at a setting vacuum and temperature with a diluted hydrocarbon gas.

2. The dissolve of carbon atoms into the substrate starts at a relatively low temperature.

3. Graphene layers are formed from the out–diffused dissolved carbon atoms in the followed rapid quenching.

The type and concentration of the hydrocarbon gas and the thickness of the substrate determine the concentration of dissolved carbon atoms. Both cooling rate and the concentration of dissolved carbon atoms control the thickness and the crystal structure of the graphene layers [26,27]. For the low carbon solubility substrate, the growth of graphene does not company with a diffusion process. The graphene layers are grown on the surface of the substrate and this process is a four–steps process [28]:

1. Methane is deposited on the substrate to form CxHy with exposing the substrate to hydrogen.

2. Nuclei stars to form from the local super–saturation of CxHy on the substrate.

3. Graphene islands are grown from the nuclei on the substrate surface.

4. Graphene covers the substrate surface.

Whether graphene can cover the whole substrate surface depends on the amount of CxHy on the substrate. Some modifications of chemical vapour deposition have been carried out in recent years. For example, plasma can be used to enhance chemical vapour deposition process that provides a route to synthesize graphene with lower temperature and shorter deposition time [29]. Although CVD is believed to be an ideal route to synthesize large area graphene sheet, the graphene produced from this technique still has intrinsic defects which allow the transportation of some molecules. Therefore, detrimental effect on the barrier properties of graphene sheet is resulted [30]. In addition, the costs of equipment and the time required for synthesis of large amount of graphene are the key limitations.

Reduction and synthesis of graphene oxide: Although mechanical exfoliation and CVD can produce high quality graphene, it is still extremely challenging to enlarge the synthesis scale cost effectively with commercial available technologies and devices. Researchers more focus on the synthesis and reduction of graphene oxide (GO) because graphene oxide and reduced graphene oxide can be synthesized easily. Graphene oxide is usually synthesized from the oxidation of graphite by strong oxidants based on Brodie [31], Staudenmaier [32], Hummers’ method [33] or some other modification of these methods. Hummers’ method was more widely used and many modifications had been made to synthesize graphene oxide for designated applications [5,13,34]. GO can be easily dispersed in many solvents and especially well in water which facilitate any subsequent processing [35]. The reduction of GO to is basically a chemical route to produce graphene with compromised properties induced by the chemical process. GO can be reduced in either chemical routes or thermal routes. Various chemicals had been reported to reduce GO such as hydrazine [23], hydroquinone [36], sodium borohydride (NaBH4) [37] and ascorbic acid [38]. Hydrazine hydrate reductant was found to be the best one to produce thin and fine reduced graphene oxide (RGO). However, NaBH4 exhibited the best efficient to reduce graphene oxide although it can slowly react with water [5]. Thermal reduction of GO utilizes heat treatment to remove oxide functional group on GO to produce RGO [5,13]. A simple and low temperature one–step solvothermal method was reported by Dubin et al.[39]. The reduction of GO was resulted from thermal reduction at 200 and the reaction with N–methyl–2– pyrrolidinone (NMP) molecules. The detailed chemistry of reduction and the study toward synthesizing better properties RGO will grow rapidly as the research in graphene moving forward.

Functionalization of graphene

The functionalization of graphene is the major route to stabilise graphene suspension in a complex environment without agglomeration takes place. For composites based coating, functionalization of graphene plays a very important role to achieve good interfacial bonding between matrixes and graphene sheets. Graphene functionalization can be achieved via physical or chemical approaches and there are three major categories of functionalization: functionalization via organic species, functionalization via macromolecules and functionalization via nanoparticles [40].

Many organic substances can react with the p bonds of graphene and, therefore, different functional groups can be introduced for different purposes. The oxidation of graphite can generate oxidised functional group on graphene layers which give the opportunity to produce stable graphene suspension in water or some organic solvents. GO is easy to disperse in water because of its hydrophilic nature. However, it is not soluble in every organic solvent and, therefore, functionalization of graphene is necessary to enable formation of stable graphene suspension with different organic solvent [40,41]. Further treatments of the oxidised groups of GO by organic species can also introduce functional groups such as carboxylic groups, enabling graphene to be available for more applications [42,43]. According to literature, radical reaction can be used as the second route to synthesize graphene oxide and further functionalization. One example is the utilization of benzoyl peroxide to synthesize graphene oxide [44]. The preparation procedure is shown in Figure 1.