Amorphous structural models for graphene oxides
Introduction
Since the synthesis in 2004 [1], graphene has demonstrated many excellent physical/chemical properties and held promise for a variety of applications [2], [3], [4], [5], [6], [7]. In the family of graphene-based materials, graphene oxide (GO), a single layer of graphite oxide (first produced by treating graphite with strong aqueous oxidizing agents [8]), is also a focus of intensive studies partially because it is an important material to massively produce graphene [9], [10], [11], [12], [13]. More importantly, GO itself has manifested many unique properties that may lead to technological applications in many fields, such as electronic devices [14], [15], [16], [17], chemical sensors [18], [19], optical devices [20], [21], [22], [23], [24], energy storage [7], [13], [25], [26], and composite materials [3], [27].
Determining the atomic structure of GO is essential for a better understanding of its fundamental properties and for realization of the future technological applications. Over the past decade, various efforts have been given to solve this issue.
Among numerous experimental techniques, the most relevant one is the nuclear magnetic resonance (NMR) measurement [28], [29], [30], [31], [32]. The solid-state 13C NMR and 1H CPMAS NMR reveal the evidence of epoxy (C–O–C), hydroxyl (OH), carboxylic (COOH), and a small amount of other groups in GOs. However, it is extremely challenging to determine the detailed atomic structure of GO due to the following factors: (1) GO is a nonstoichiometric compound with a variety of compositions depending on its synthesis condition; (2) GO is strongly hydrophilic and hygroscopic; (3) GO is thermally unstable and slowly decomposes above 60–80 °C [31], [33], [34], [35].
In addition to NMR, various microscopic means were employed to characterize the atomic structures of GOs, including transmission electron microscopy (TEM) [34], [36], [37], [38], [39], [40], scanning electron microscopy (SEM) [35], [36], atomic force microscopy (AFM) [37], [38], [41], [42], [43], [44], scanning tunneling microscopy (STM) [45]. Besides, measurements using optical analysis techniques [34], [35], [36], [37], [41], [42], [43], [44], [45], [46], such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy, demonstrated that GO contains carbon atoms with different types of bonding states, for example, graphitic C, C–O–C, C–O, CO, O–CO, and so on.
Now it is generally accepted that GO bears hydroxyl and epoxy groups mostly on its basal plane [37], [46]. The ratio of each bonding type of carbon can be tailored by the synthesis conditions, which enhances the complexity of atomic structure of GO. All these experimental observations suggest that the atomic structure of GO is nearly amorphous in large length scale.
Theoretically, many GO structural models have been proposed. Decades ago, four structural models respectively proposed by Hofmann and Holst [47], Ruess [48], Scholz and Boehm [49], Nakajima et al. [50], [51], and Lerf and co-workers [30], [31], [32], were generally accepted, depending on the characterization technique prevalent at that time. Among them, the model proposed by Lerf and co-workers, in which the hydroxyl and epoxy groups distribute on the basal layer in a nearly disordered manner, explains the aforementioned experimental results very well. Recently, based on Cai’s NMR experiment [46], Yan et al. [52], [53] and our group [25], [54] have identified the energetically favorable atomic configuration of GO, which contains epoxy and hydroxyl groups in close proximity with each other, and found that these functional groups prefer to aggregate. In addition to hydroxyl and epoxy, groups of epoxy pair and epoxy-hydroxyl pair in GO were also proposed by Zhang et al. [55]. Employing genetic algorithm and first-principles approaches, Xiang et al. [56] searched the most stable structure of oxidized graphene by considering various epoxy groups (normal epoxy, unzipped epoxy and epoxy pair), and found that phase separation between bare graphene and fully oxidized graphene is thermodynamically favorable in partially oxidized graphene, which agrees with our results [54]. Later, Lu et al. [57] found the long hydroxyl chains are not expected to be widely present in the real GO samples owing to their high chemical shifts according to the simulated NMR spectra.
Note that although the disordered GO models proposed by Hofmann and Holst [47], Ruess [48], Scholz and Boehm [49], Nakajima et al. [50], [51], and Lerf et al. [30], [31], [32], are not thermodynamically most favorable, they can meet with the experimental observations to some extent; while the thermodynamically more favorable ordered GO models proposed recently are difficult to be synthesized. This leads to a plausible discrepancy between the experiments and theories: the structures of GOs are amorphous from experimental observations; but the ordered GO models are thermodynamically preferred from theoretical point of view.
To address this controversial issue and gain a deeper insight into the structural characteristics of GOs, here we considered the possibility of amorphous structural models for GOs and compared them with the ordered structures. It is interesting that the energetically preferred structures of our constructed amorphous GO models always exhibit some locally ordered structural motifs in the short range. Moreover, the energetically preferred amorphous GOs are nearly as stable as the ordered ones at low oxygen coverage. Our computations reveal an overall trend for the amorphous GO structures, which exhibit a locally ordered configuration but are disordered in the long range. These results not only are in line with the experimental observations of amorphous GOs, but also help understand the previous theoretical ordered models with lower energies.
Section snippets
Amorphous GO models
Our amorphous GO models start from a rectangular supercell of graphene consisting of 80 carbon atoms (17.21 Å × 12.45 Å in dimension). Based on the structural characteristics of the stable GO structures proposed by previous computations [25], [52], [53], [54], [55], [56], we summarize the following rules to construct the amorphous structural models: (1) two functional groups cannot locate at the same carbon atom; (2) paired hydroxyl groups are added to two adjacent carbon atoms, one above and
OH:O = 2.00 with R = 10–70%
In this part, we have considered the amorphous GOs with OH:O = 2.00 but different coverage rates. As an example, Fig. 2a displays the further optimized amorphous GO structure with R = 70%. Clearly the whole system is amorphous, but some ordered motifs exist in the short range, which are highlighted and shown individually on the top of the graph. After relaxation, the epoxy and hydroxyl groups in the highlighted parts are arranged in an ordered manner, i.e., aggregating along either armchair or
Conclusions
A series of amorphous GO structures with different coverage rates and OH:O ratios were constructed by randomly adding epoxy and hydroxyl groups onto a perfect graphene supercell with 80 carbon atoms following some structural rules. Ordered GO structures with epoxy and hydroxyl chains along the armchair direction were considered for comparison. The main conclusions from our first-principles computations are itemized as follows.
First, the thermodynamically stable amorphous GO is usually
Acknowledgements
This work is supported in China by the Fundamental Research Funds for the Central Universities of China (No. DUT10ZD211) and the National Natural Science Foundation of China (No. 11134005), and in USA by NSF Grant EPS-1010094 and the Environmental Protection Agency (EPA Grant No. RD-83385601).
References (74)
- et al.
Study of oxygen-containing groups in a series of graphite oxides: physical and chemical characterization
Carbon
(1995) - et al.
A new structural model for graphite oxide
Chem Phys Lett
(1998) - et al.
Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide
Carbon
(2007) - et al.
Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy
Carbon
(2009) - et al.
A new structure model of graphite oxide
Carbon
(1988) - et al.
Formation process and structure of graphite oxide
Carbon
(1994) - et al.
Reduction of graphene oxide by electron beam generated plasmas produced in methane/argon mixtures
Carbon
(2010) - et al.
Determination of the density of localized states in semiconductors from the pre-recombination transient photoconductivity
Solid-State Electron
(2008) - et al.
Structure and mechanical properties of cubic BC2N crystals within a random solid solution model
Diamond Relat Mater
(2010) - et al.
Electric field effect in atomically thin carbon films
Science
(2004)
Two-dimensional gas of massless Dirac fermions in graphene
Nature (London)
Graphene-based composite materials
Nature (London)
The rise of graphene
Nat Mater
Tunable quantum dots in bilayer graphene
Nano Lett
Graphenes as potential material for electronics
Chem Rev
Graphene-based ultracapacitors
Nano Lett
On the atomic weight of graphite
Philos Trans R Soc Lond
Single sheet functionalized graphene by oxidation and thermal expansion of graphite
Chem Mater
Honeycomb carbon: a review of graphene
Chem Rev
Chemical methods for the production of graphenes
Nat Nanotechnol
High-throughput solution processing of large-scale graphene
Nat Nanotechnol
The chemistry of graphene oxide
Chem Soc Rev
Electronic transport properties of individual chemically reduced graphene oxide sheets
Nano Lett
Evaluation of solution-processed reduced graphene oxide films as transparent conductors
ACS Nano
Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material
Nat Nanotechnol
Epitaxial-graphene/graphene-oxide junction: an essential step towards epitaxial graphene electronics
Phys Rev Lett
Reduced graphene oxide molecular sensors
Nano Lett
Practical chemical sensors from chemically derived graphene
ACS Nano
Nonlinear optical properties of graphene oxide in nanosecond and picosecond regimes
Appl Phys Lett
Photoluminescence and band gap modulation in graphene oxide
Appl Phys Lett
Hydrothermal dehydration for the “Green” reduction of exfoliated graphene oxide to graphene and demonstration of tunable optical limiting properties
Chem Mater
Position dependent photodetector from large area reduced graphene oxide thin films
Appl Phys Lett
Nonlinear optical transmission of nanographene and its composites
J Phys Chem C
Graphene oxide as an ideal substrate for hydrogen storage
ACS Nano
Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage
ACS Nano
Functionalized graphene sheets for polymer nanocomposites
Nat Nanotechnol
Solid-state NMR studies of the structure of graphite oxide
J Phys Chem
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