Heteroatom doped graphene in photocatalysis: A review
Graphical abstract
Introduction
Photocatalytic processes, which make use of the energy of photons to propel useful chemical reactions, have attracted tremendous scientific and technological interest over the last 40 years. The origin of this interest is founded on the notion that they can serve to potentially tackle contemporary global challenges such as environmental pollutant remediation [1] and the solar production of fuels for an alternative green energy source [2], [3]. This energy conversion to chemical form has an upper hand over direct solar-electric energy conversion as the latter process relies on intermittent light source from the sun rendering it weather- and time-dependent, whereas the photocatalytic production of fuel stores solar energy in the form of chemical bonds, which can be more flexibly utilized for various future end uses. While these photocatalytic processes are potentially useful, their efficiency are still far too low for their practical mass application. Photocatalysis is a semiconductor-mediated process. Therefore, several strategies have emerged over the years in the hope to improve the photocatalytic performance of semiconductor photocatalyst, which include appropriate structural design [4], [5], [6], doping [7], [8], [9], noble metal loading [10], [11], [12] and the formation of semiconductor composite [13], [14], [15]. Recently, increasing attention has turned to the use of carbonaceous nanomaterials such as carbon nanotubes (CNT) and fullerenes (C60) to couple with semiconductor photocatalyst to produce high-performance composite. This is because the delocalized conjugated π-system present in these carbon allotropes can function as electron reservoirs to accept and shuttle photogenerated electrons for rapid charge transfer and separation. In addition to that, they also help to facilitate the photocatalyst adsorptive properties and contribute to catalytically active sites.
More recently, the groundbreaking isolation of graphene by Novoselov and Geim [16], which is a 2D carbon allotrope has received significant attention over its forebears (CNT and C60) for hosting many novel properties, many of which is ascribed to its unique electronic band structure. The band anomaly seen for an ideal graphene sheet permits ballistic conduction in the micrometre range, meaning that electrons can travel at remarkably high mobility (15,000 cm2 V−1 s−1) with little scattering at room temperature [17], [18], [19]. This makes graphene the ideal high performance candidate as an electron sink or electron transfer media. Furthermore, its unique single-atomic planar structure allows high transparency [20], [21] and offers a theoretical surface area of ∼2600 m2 g−1 [22], [23], making them highly fitting as a large surface-area support for easy uniform dispersion of the semiconductor particles. Due to its highly tailorable nature, its surface properties can also be favourably adjusted through chemical modification, allowing its use as part of a composite material. For these reasons, it is no surprise that the interest in graphene has also extended to photocatalysis, in the hope that harnessing these superlative qualities will potentially open up new opportunities for next generation photocatalyst systems [24], [25], [26], [27], [28], [29], [30], [31], [32], [33].
Very recently, the heteroatom doping of graphene has been a rising research category to enhance the performance of graphene-based devices for a wide range of applications [34]. This presents an avenue to further extend the role of graphene in the photocatalysis scene. Chemical doping helps to tailor the electronic properties of graphene to unlock many potential features that may benefit photocatalysis. In a pristine undoped graphene, its antibonding π* orbitals (which makes up its conduction band) and bonding π orbitals (which makes up its valence band) degenerate and touch at Brillouin zone corners, making graphene a zero bandgap material as shown in Fig. 1. This non-existent bandgap is attributed to the identical environment of the two sub-lattices of carbon atoms in the graphene unit cell. It is worth noting that for an unperturbed graphene sheet, the Fermi level coincides with the Dirac point, where the conductivity of the system is at its minimum [35]. Since the conductivity of graphene stems from its delocalized π electrons, it should follow that the presence of foreign atoms and defect sites on the lattice would influence its electrical properties. Understanding these consequences of doping on the electrical performance of graphene is thus a key-point to the discovery of its possible functionalities for photocatalysis.
It is noteworthy to mention that chemical doping of graphene has two types. (1) Surface-transfer doping: this type of doping occurs due to additional functionalized groups on the graphene sheet, and depending on their natural tendency to donate or withdraw electrons from graphene, would lead to an n- or p-type conductivity respectively. (2) Substitutional doping: this type of doping is achieved through the substitution of the carbon atoms in the graphene lattice by atoms with a different number of valence electrons. Atoms with more valence electrons then carbon result in an n-type conductivity while, on the other hand, atoms with lesser valence electrons lead to a p-type conductivity. This type of doping will in general introduce additional states in the density of states due to the additional free charge carriers introduced by the dopant [36]. In addition to that, the Fermi level of graphene shifts from the minimum Dirac point to a finite density of state, which subsequently serve in promoting its electrical conductivity [37], [38]. The magnitude of this shift increases with dopant concentration. This improved conductivity and tailorable n- and p-type transport properties would subsequently cater to enhancing the photoactivity of graphene-based photocatalyst systems, as described in the following sections.
Other than controlling the type and concentration of charge injected into graphene, chemical doping can also result in the opening of a bandgap, which converts graphene into a semiconductor. By the addition of foreign atoms, its former lattice symmetry is broken, which results in the formation of gap between π and π* bands as seen in Fig. 1. Irrelevant to which dopant, this gap linearly scales with the dopant concentration [41]. This adjustability offers the ability to engineer the bandgap of graphene to suit a certain absorption specification. Thus, graphene can be tailored to exhibit organic semiconductor properties and, with adequate research, can be made to efficiently replace classic semiconductor photocatalyst.
Moreover, it should be known that for an ideal, undoped graphene, it is chemically inert since its unpaired electrons are strongly bound and passivated in its delocalized π system, which impede its adsorptivity and reactivity [42]. Although this π system can facilitate favourable adsorption through a π–π, π–CH or π–cation interaction, the interaction is however weak and minimal. Fortunately, the introduction of heteroatoms can in addition bestow graphene with highly abundant active sites. According to a theoretical study done by Yang et al., adding foreign heteroatom with a different electronegativity than carbon will break the electroneutrality in graphene and create unbalanced charged areas, which serve as active sites [43]. Moreover, heteroatoms with unpaired electrons also cause a localized distribution of molecular orbitals which makes it susceptible to chemical reactivity. A study by Zhang [44] also revealed the heteroatom doping leads to the asymmetry of spin density, which also function as important catalytic active sites [45]. Not only that, active sites also exist in the form of structural defects which can arise from the strain in the lattice imparted from the atomic size differences with the dopants [46]. All these origins of active sites made graphene rich in chemical properties for stronger interaction to help assist the photocatalytic performance. In all, since doping can simultaneously introduce a bandgap and made graphene catalytically capable, it can thus be utilized as an intrinsic photocatalyst on its own. It should be recognized that metallic resources are progressively depleting from its excessive exploitation, and the chemical doping of graphene can be a highly desirable practical strategy for an alternative metal-free and low-cost semiconductor photocatalyst. This way, part of precious metals is spared and environmental contamination from toxic metals will be reduced.
However, this research area is still in its infancy and much research requires to be done for its effective implementation. Up until now, the rapid invasion of graphene in the photocatalysis scene has prompted many review articles [47], [48], [49], [50], [51], however with none focusing on the implication of heteroatom doping. Herein, the recent increase on the number of investigations on heteroatom doped graphene materials for their usage in photocatalysis have motivated demand for reviewing this topic. It is desired that this short review will promote awareness and encourage further investigations for the development in this budding research area. In this article, the different doped graphene and their reported photocatalytic applications are introduced according to the dopant element. The progress of both theoretical and experimental investigations will be discussed. Finally, this review will close with a summary and future perspective of heteroatom doped graphene in the photocatalysis field.
Section snippets
Oxygen-doped graphene
The addition of atomic oxygen has to be the most famous derivate of graphene yet. The top-down wet chemical oxidation and consecutive reduction of graphite as the starting material is the most widely employed route to produce graphene sheets for many research activities, including photocatalysis [52], [53]. This approach has the advantage of generating high graphene yield and vitally endows graphene with its excellent dispersibility in a broad range of solvents [42], [54]. This not only
Conclusion and perspectives
Table 1 summarizes the different roles heteroatoms play in a photocatalytic system. Heteroatom addition to graphene can instil favourable properties propitious for photocatalysis and further extend the ability of graphene for photocatalysis. As of today, heteroatom doped graphene have clearly emerged as a relevant research arena that is still undergoing rapid advances, as evident by the explosive number of recent publications. The appearance of these newly doped carbon materials have attracted
Acknowledgement
This work was funded by the Ministry of Science, Technology and Innovation (MOSTI) Malaysia under the e-Science Fund (Project no.: 03-02-10-SF0244).
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