Elsevier

Applied Surface Science

Volume 358, Part A, 15 December 2015, Pages 2-14
Applied Surface Science

Heteroatom doped graphene in photocatalysis: A review

https://doi.org/10.1016/j.apsusc.2015.08.177Get rights and content

Highlights

  • Doping graphene with foreign atoms extends its function in the photocatalyst system.

  • Chemically doped graphene improved the electrical conductivity.

  • Chemically doped graphene outperform conventional rGO as a semiconductor support.

  • Chemically doped graphene cause bandgap opening and formation of catalytic sites.

  • Chemically doped graphene can behave as functional standalone photocatalyst.

Abstract

Photocatalysis has been a focus of great attention due to its useful environmental applications such as eliminating hazardous pollutants and generating sustainable energy. Coincidentally, graphene, a 2D allotrope of carbon, has also infiltrated many research fields due to its outstanding properties – photocatalysis being no exception. As of recent, there has been growing research focus on heteroatom (O, N, B, P and S) doping of graphene and its emergent application opportunities. In this study, rather than the familiar graphene as the electron transfer medium that is normally integrated in a photocatalyst system, we contrarily explore the implication of heteroatom doped graphene and the underlying mechanism behind their advantageous uses in photocatalysis. This review surveys the literature and highlights recent progress and challenges in the development of chemically doped graphene in the photocatalysis scene. It is desired that this review will promote awareness and encourage further investigations for the development in this budding research area.

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).

References (123)

  • Z. Zhang et al.

    Enhanced photo-electrochemical performances of graphene-based composite functionalized by Zn2+ tetraphenylporphyrin

    Appl. Surf. Sci.

    (2014)
  • M. Aleksandrzak et al.

    Effect of graphene thickness on photocatalytic activity of TiO2-graphene nanocomposites

    Appl. Surf. Sci.

    (2015)
  • Y. Gu et al.

    Synthesis and photocatalytic activity of graphene based doped TiO2 nanocomposites

    Appl. Surf. Sci.

    (2014)
  • H. Mousavi et al.

    Nitrogen and boron doping effects on the electrical conductivity of graphene and nanotube

    Solid State Sci.

    (2011)
  • S. Stankovich et al.

    Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide

    Carbon

    (2007)
  • P. Lin et al.

    Simultaneously functionalization and reduction of graphene oxide containing isocyanate groups

    Appl. Surf. Sci.

    (2015)
  • H. Hu et al.

    Metal-free graphene-based catalyst – insight into the catalytic activity: a short review

    Appl. Catal. A: Gen.

    (2015)
  • T.-F. Yeh et al.

    Roles of graphene oxide in photocatalytic water splitting

    Mater. Today

    (2013)
  • T.-D. Nguyen-Phan et al.

    The role of graphene oxide content on the adsorption-enhanced photocatalysis of titanium dioxide/graphene oxide composites

    Chem. Eng. J.

    (2011)
  • P. Shi et al.

    Co3O4 nanocrystals on graphene oxide as a synergistic catalyst for degradation of Orange II in water by advanced oxidation technology based on sulfate radicals

    Appl. Catal. B: Environ.

    (2012)
  • Y. Min et al.

    Ionic liquid assisting synthesis of ZnO/graphene heterostructure photocatalysts with tunable photoresponse properties

    Diam. Relat. Mater.

    (2012)
  • B. Han et al.

    Electrostatic self-assembly of CdS nanowires-nitrogen doped graphene nanocomposites for enhanced visible light photocatalysis

    J. Energy Chem.

    (2015)
  • A. Fujishima et al.

    Electrochemical photolysis of water at a semiconductor electrode

    Nature

    (1972)
  • T. Inoue et al.

    Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders

    Nature

    (1979)
  • G. Liu et al.

    Nanosized anatase TiO2 single crystals for enhanced photocatalytic activity

    Chem. Commun.

    (2010)
  • Z. Wu et al.

    The fabrication and characterization of novel carbon doped TiO2 nanotubes, nanowires and nanorods with high visible light photocatalytic activity

    Nanotechnology

    (2009)
  • J. Wang et al.

    Origin of photocatalytic activity of nitrogen-doped TiO2 nanobelts

    J. Am. Chem. Soc.

    (2009)
  • T. Ohno et al.

    Photocatalytic activity of S-doped TiO2 photocatalyst under visible light

    Chem. Lett.

    (2003)
  • H. Li et al.

    Mesoporous Au/TiO2 nanocomposites with enhanced photocatalytic activity

    J. Am. Chem. Soc.

    (2007)
  • W.W. Wang et al.

    ZnO–SnO2 hollow spheres and hierarchical nanosheets: hydrothermal preparation, formation mechanism, and photocatalytic properties

    Adv. Funct. Mater.

    (2007)
  • K. Vinodgopal et al.

    Nanostructured semiconductor films for photocatalysis. Photoelectrochemical behavior of SnO2/TiO2 composite systems and its role in photocatalytic degradation of a textile azo dye

    Chem. Mater.

    (1996)
  • T. Arai et al.

    Efficient complete oxidation of acetaldehyde into CO2 over CuBi2O4/WO3 composite photocatalyst under visible and UV light irradiation

    J. Phys. Chem. C

    (2007)
  • K.S. Novoselov et al.

    Electric field effect in atomically thin carbon films

    Science

    (2004)
  • K.S. Novoselov et al.

    Two-dimensional gas of massless Dirac fermions in graphene

    Nature

    (2005)
  • X. Du et al.

    Approaching ballistic transport in suspended graphene

    Nat. Nanotechnol.

    (2008)
  • R. Nair et al.

    Fine structure constant defines visual transparency of graphene

    Science

    (2008)
  • K.S. Kim et al.

    Large-scale pattern growth of graphene films for stretchable transparent electrodes

    Nature

    (2009)
  • M.J. McAllister et al.

    Single sheet functionalized graphene by oxidation and thermal expansion of graphite

    Chem. Mater.

    (2007)
  • H. Zhang et al.

    P25-graphene composite as a high performance photocatalyst

    ACS Nano

    (2009)
  • Q. Li et al.

    Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-cluster-decorated graphene nanosheets

    J. Am. Chem. Soc.

    (2011)
  • Q. Xiang et al.

    Preparation and enhanced visible-light photocatalytic H2-production activity of graphene/C3N4 composites

    J. Phys. Chem. C

    (2011)
  • A. Iwase et al.

    Reduced graphene oxide as a solid-state electron mediator in Z-scheme photocatalytic water splitting under visible light

    J. Am. Chem. Soc.

    (2011)
  • X. Wang et al.

    Heteroatom-doped graphene materials: syntheses, properties and applications

    Chem. Soc. Rev.

    (2014)
  • A.C. Neto et al.

    The electronic properties of graphene

    Rev. Mod. Phys.

    (2009)
  • F. Joucken et al.

    Localized state and charge transfer in nitrogen-doped graphene

    Phys. Rev. B

    (2012)
  • Y. Zhang et al.

    Experimental observation of the quantum Hall effect and Berry's phase in graphene

    Nature

    (2005)
  • P. Avouris

    Graphene: electronic and photonic properties and devices

    Nano Lett.

    (2010)
  • G. Xie et al.

    Graphene-based materials for hydrogen generation from light-driven water splitting

    Adv. Mater.

    (2013)
  • P. Rani et al.

    Designing band gap of graphene by B and N dopant atoms

    RSC Adv.

    (2013)
  • L. Yan et al.

    Chemistry and physics of a single atomic layer: strategies and challenges for functionalization of graphene and graphene-based materials

    Chem. Soc. Rev.

    (2012)
  • Cited by (0)

    View full text