Based on the density function theory (DFT) method, the interaction between the graphene and oxygen atom is simulated by the B3LYP functional with the 6-31G basis set. Due to the symmetry of graphene (C54H18, D6h), a representative patch is put forward to represent the whole graphene to simplify the description. The representative patch on the surface is considered to gain the potential energy surface (PES). By the calculation of the PES, four possible stable isomers of the C54H18-O radical can be obtained. Meanwhile, the structures and energies of the four possible stable isomers, are further investigated thermodynamically, kinetically, and chemically. According to the transition states, the possible reaction mechanism between the graphene and oxygen atom is given.
Since being discovered in 2004 , graphene, as the first truly two-dimensional crystal, with large surface area, high chemical stability, flexibility, superior electric and thermal conductivity [2–5], has drawn dramatic attention for use in such areas as sensors, energy conversion, photodetectors, and aerospace [6–10]. Graphene oxide (GO) as a significant raw material of mass-production graphene via reduction, is an important carbon nanomaterial in the graphene family [11–14]. Because GO is rich for oxygen-containing groups in the structure, and the ratio of sp2 to sp3 hybridized carbon atoms is easy to control, GO possesses brilliant performance in electronics, optics, energy conversion, environmental science, and biotechnology. It is reported that GO nanoplatelets were self-assembled to form carbon films with tunable porous morphologies and mechanical flexibility, which incorporated good electrical properties . Yeh and his co-workers used GO in the photocatalytic of H2 evolution activity. According to the Mott-Schottky equation, the results of electrochemical analysis showed that H2 could be generated stably from the pure water or methanol solution through the GO films under mercury vapor light irradiation, even under the condition of lack of cocatalyst Pt . Meanwhile, compared with other benchmark material, i.e. C60, GO exhibits a good performance on the optical power limiting. In order to investigate optical power limiting behaviors of GO, Jiang et al.  prepared GO films on the plastic substrates and glass, which shows excellent broadband optical limiting. Besides, compounded GO, with other materials, can enhance the thermodynamic properties of the material [18–20]. In addition, graphene quantum dots (GQDs) as an emerging fluorescent nanomaterial, with excellent optical performance , easy to surface modify , high chemical stability [23– 26], low toxicity, and good bio-compatibility [27, 28], have drawn dramatic attention in the scientific community. All in all, GO as well as GQDs have dramatically grabbed the attention of the scientific community due to their brilliant performances. Due to most of them being prepared by oxidation, it is of vital importance to understand the interaction between the graphene and oxygen atom.
In this paper, a systematic study of the interaction between the graphene (C54H18, D6h) and oxygen atom is performed by the density function theory. Based on the symmetry, a representative patch is put forward to represent the whole graphene (C54H18, D6h) to simplify the description. By the single point energy scan on the entire surface of the graphene, the PES between the graphene (C54H18, D6h) and oxygen atom was obtained. Meanwhile, all the possible isomers, as well as all the TSs, were obtained to explain the possible reaction mechanism.
2 Computational details
In this study, C54H18 (D6h) is chosen as the whole system. Based on the symmetry of graphene, a representative patch is put forward to represent this whole graphene (C54H18, D6h) to simplify the description. In calculation, we adopted the B3LYP hybrid functional with the Gaussian09 program [29– 31]. The STO-3G , as well as the standard 6-31G(d), were set as the basis sets. Firstly, graphene (C54H18, D6h) and its corresponding symmetry group, as the initial geometries are optimized with the basis set of B3LYP/6-31G (d). Secondly, in terms of the calculation of the single point energy, the potential energy curve and the PES were obtained at the basic level of STO-3G. When calculating PES, we just need to consider the energy distribution in the whole graphene (C54H18, D6h). So, the basis set of STO-3G can satisfy our requirements according to our experience. Thirdly, the isomers of C54H18-O on the representative patch were optimized respectively at the standard 6-31G (d). In addition, the harmonic vibrational frequency was analyzed to make sure that the stationary points at the same basis set, were real.
3 Results and discussion
3.1 The representative patch
Based on the symmetry, we chose a triangular section of the graphene (C54H18, D6h) as a representative patch to make it more simple(as shown in Fig. 1). All the key points have their own symmetry. For instance, the key point of R1 is C6v symmetry; the key points of Rn (n = 2, 3, 4), bn (n = 3, 4, 5, 6), C3, C4 are C2v symmetry; the key points of bn (n = 1, 2, 7, 8) C1, C2 are Cs symmetry. The area of the total surface is equal to 24 and the representative patch is the 1/24.When calculated to the representative patch, it can contain the whole surface of the graphene (C54H18, D6h). Hence, we just need to calculate the smallest structure unit (the patch DEF) to replace the entire graphene. On the representative patch, there are eighteen key points including four six-membered rings’ centres (key points of R1, R2, R3, R4), eight bond centres (key points of b1, b2, b3, b4, b5, b6, b7, b8), six carbon atoms. Because the graphene (C54H18, D6h) edge is saturated with hydrogenation, we just need to take the four carbon atom (key points of C1, C2, C3, C4) into account.
To make the potential energy surface at a reasonable interaction distance between the graphene and oxygen atom, the potential energy curves of the representative patch at the different key points were discussed by the scanning of the single point energy. Fig. 2 is the potential energy curves of the representative patch, and it can be seen that two different trends are shown from the curves. The top curves show a relatively sharp trend, which belongs to the oxygen atom moving away from the bond centre of b1 and b2. Meanwhile, the bottom curves have potential wells, the height of which is about 1.4 Å. It expressed that there is a stable chemical adsorption site at the bond centre of the b3 and b4 when the oxygen atom moved away from the graphene at the normal directions.
According to the potential energy curves of the C54H18–O radical, the potential energy surfaces were scanned at the height of 1.4 Å, as shown in Fig. 3. From the representative patch, it can be clearly observed that the key points of b1, b2, b3, b4 are the minimum value of the potential energy in the wells, and the key points of R1, R2, R3, and R4 are the maximum value of the potential energy. Therefore, four possible isomers are proposed theoretically, which are named corresponding to the bond centre of bn (n = 1, 2, 3, 4).
3.2 The thermodynamic stability
Table 1 is the geometric parameters, HOMO-LUMO gap energies, and relative energies for the four possible isomers of C54H18-O. However, the balanced distance of the C-O bond (including the bonds of C-O and C-O) is 1.52Å and the mean angel of C-O-C is 60.64°.
|Structure||C-C (Å)||C-O (Å)||C-O (Å)||C-O-C(°)||Gap Energy(eV)||Rel Energ(eV)|
Among the different isomers of C54H18-O, it can be noted that C54H18-O2 is the lowest energy in total, so we set it as the zero point of the relative energy. Then C54H18-O4 is only higher by 0.003 eV than C54H18-O2, which is the second most stable. And C54H18-O3 possesses the highest energy, which is the least stable. Based on the above analysis, the ordering of thermodynamic stability is received for C54H18-O2>C54H18-O4> C54H18-O1> C54H18-O3.
3.3 The chemical reaction activity
The HOMO–LUMO energy gap sequence enable us to describe the chemical reaction activity of the four C54H18-O isomers, which can also be shown in Table 1. Comparing with the HOMO–LUMO energy gaps, we can get that C54H18-O2 is the highest gap energy at 2.728 eV, and the C54H18-O3 is the lowest gap energy at 1.932 eV. According to the gap energy of HOMO–LUMO, the sequence of the chemical reaction activity is C54H18-O2 > C54H18-O4 > C54H18-O1 > C54H18-O3, which is consistent with the sequence of the thermodynamic stability.
3.4 The kinetics activity
TSs between different C54H18-O radicals are calculated at the 6-31G (d) level. The results show that each TS only has one single imaginary frequency. TS1 represents the transition state of C54H18-O1 and C54H18-O2, and the value of single imaginary frequency is 522.24 i cm−1. TS2 represents the transition state of C54H18-O2 and C54H18-O3 with the only imaginary frequency valu of 517.55 i cm−1. TS3 represents the transition state of C54H18-O3 and C54H18-O4 and the value of single imaginary frequency is 439.86 i cm−1.
Table 2 is the activation energies (Ea) as well as the relative energy of transition states (TS1, TS2, TS3). It shows that the Ea value of the oxygen atom to be transferred from the isomer of C54H18-O1 to the isomer of C54H18-O2 is 0.747 eV. On the reverse reaction path, the Ea value of the oxygen atom to be transferred from the isomer of C54H18-O2 to the isomer of C54H18-O1 is 0.902 eV. Another reaction path for the isomer of C54H18-O2 is the oxygen atom to be transferred from the isomer of C54H18-O2 to the isomer of C54H18-O3, the Ea value of which is 1.048 eV. So the minimum Ea value of the isomer C54H18-O2 is 0.902 eV. In addition, the Ea value of C54H18-O3 (including the isomer of C54H18-O3 to the isomer of C54H18-O2 and the isomer of C54H18-O3 to the isomer of C54H18-O4) is 0.703 eV and 0.604 eV. Thus, the minimum Ea value of the isomer C54H18-O3 is 0.604 eV. Meanwhile, the Ea value of the oxygen atom to be transferred from the isomer of C54H18-O4 to the isomer of C54H18-O3 is 0.946 eV. Therefore, the stability order of the kinetics activity is C54H18-O4 > C54H18-O2 > C54H18-O1 > C54H18-O3 for the four isomers of C54H18-O. This is a little different to the sequence of the thermodynamic stability and the chemical reaction activity.
According to the PES gained by the scanning of the single point energy, four possible isomers of C54H18-O on the representative patch are optimized by the density functional theory method. In short, the oxygen atom is easy to absorb above the bond to form the epoxy. C54H18-O2 is the most stable isomer, calculated by analyzing the the thermodynamics and chemical reaction activity. C54H18-O4 is the most stable isomer as calculated by analyzing the kinetics activity of radicals. Furthermore, investigation of the interaction between the oxygen atom and graphene is also crucial to further understanding the details of the GO and GQDs.
The financial support of the National Natural Science Foundation of China (Grant Nos. 21373042), the Fundamental Research Funds for the Central Universities of China (Grant No. DUT15RC(4)11) and Liaoning Provincial Doctor Startup Foundation of China (Grant No. 20141019). And the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry is greatly appreciated.
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