ABSTRACT
The exponential growth of graphene research in both the scientific and engineering communities has taken place after the Geim group isolated “free” and “perfect” graphene sheets and demonstrated the unprecedented electronic properties of graphene in 2004 (Graphene, 2010 Nobel Prize for Physics) [1]. Graphene, a single layer of two-dimensional carbon lattice, has shown many unique properties, such as the quantum hall effect (QHE), high carrier mobility at room tem-
perature (~10,000 cm2 V-1 s-1) [2], large theoretical specific surface area (~2630 m2 g-1) [3], good optical transparency (~97.7% per layer) [4], high Young’s modulus (~1 TPa) [5], and excellent thermal conductivity (~3000-5000 W m-1 K-1) [6]. To exploit these proper-ties in various kinds of applications, several synthetic routes have been developed for the preparation of graphene and its derivatives, ranging from the bottom-up epitaxial growth to the top-down ex-foliation of graphite. In particular, chemical exfoliation and reduction starting from the oxidation of graphite is an efficient process to produce graphene sheets in a low-cost, scalable, controllable, and reproducible manner [7]. Owing to the highly versatile and tunable properties, graphene has attracted a great deal of attention in many important applications, such as optoelectronic devices [8], energy storage materials [9], photo-catalysis [10], chemical and biological sensors [11], and polymer composites [12]. In the 21st century, aggravating energy and environmental prob-lems such as fossil fuel depletion, global warming, and pollution are ringing the alarm bell to the human society. Thus, green energy and environment technologies have been the urgent and important ar-eas. Among several possible alternatives for fossil energy, eventually solar energy is probably the only one that can meet the multifold-demand for the long-term human needs. The utilization of solar energy consists of two steps: First, solar energy can be effectively converted to applicable forms (electricity or fuel) from solar power to suppress energy crisis and global warming. Aiming at this goal, solar cells and photo-catalysts for production of H2 and reduction of CO2 are mostly concerned [13-17]. Second, high performance energy storage devices are also required. This is mainly due to the intermittent characteristics of solar energy and other renewable energy sources. Supercapacitor is one of the promising devices for this purpose [18-20]. Due to highly remarkable properties, graph-ene has been useful in various energy and environment applications, such as transparent conductive electrodes or active materials in thin film solar cells, counter electrodes in dye-sensitized solar cells, high-performance electrodes in supercapacitors, and photo-catalysts for reduction of CO2 and degradation of organic pollutants. Therefore, this review mainly focuses on recent advances in the synthesis of
graphene and graphene-based materials and their applications in the energy and environment related systems described above. 6.2 Preparations of Graphene-Based Materials
The first piece of single-layer graphene sheet was prepared by mechanical exfoliation, in which highly oriented pyrolitic graphite (HOPG) was peeled using scotch tape and the subsequent release of the graphene flake on Si/SiO2 after the tape was removed [1]. The exfoliated graphene manifests a unique structure and superior properties, although this production method is not applicable on a large scale. Inspired by the pioneering work, several alternative techniques have been developed for fabricating graphene materials. These methods for fabricating graphene materials used in energy and environment applications can be generally classified into the bottom-up and top-down approaches. The bottom-up approach involves the direct synthesis of graph-ene materials from the carbon sources, such as the chemical vapor deposition (CVD), which is a typical method used to grow large-area, single, and few-layer graphene sheets on metal substrates. When the metal surfaces are heated, hydrocarbon (or carbon oxide) de-composes into carbon atoms and hydrogen gas (or oxygen gas), and the carbon atoms then form a graphene monolayer (Fig. 6.1a). Furthermore, as seen from Fig. 6.1b, the obtained graphene films on metal surface can be transferred to other target substrates via metal etching, which is very important for device applications [21]. Recently, this approach was employed to prepare large-area, contin-uous, few-layered graphene as photo-anodes for organic photovolta-ic devices [14]. Moreover, the epitaxial growth process has also been exploited to prepare single-layer graphene via the sublimation of SiC. This process can provide a higher yield with much less defects, but cannot easily fabricate a large-area graphene [22]. In addition to the above methods based on the solid-phase deposition, graphene is also obtainable via the wet chemical reaction of ethanol and sodium followed by pyrolysis [23], or through the organic synthesis to give graphene-like polyaromatic hydrocarbons (Fig. 6.1c) [24].
