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Additions of fluorine atoms to the surfaces of graphene Nanoflakes:A density functional theory study
Solid State Sciences 97 (2019) 106007
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Additions of fluorine atoms to the surfaces of graphene Nanoflakes:A density functional theory study
T
Hiroto Tachikawa∗, Hiroshi Kawabata Division of Applied Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluorination PAH Potential energy IR and Raman spectra Absorption spectra
The electronic states and band gaps of carbon materials such as graphene, fullerene, and carbon nanotubes are strongly affected by radical addition. In the present study, the reactions of graphene nanoflakes with fluorine (F) atoms were investigated by density functional theory (DFT) in order to elucidate the nature of the bonding between the F atoms and the graphene nanoflakes. Graphene nanoflakes composed of 4–37 benzene rings were used as graphene models. The present calculations reveal that F atoms react with graphene surfaces and bind directly to carbon atoms to form strong C–F bonds. The binding sites are locally Cδ+-Fδ– polarized and have large dipole moments. The binding energy of F to graphene was calculated to be 30 kcal/mol, which is 1.5 times larger than that for the addition of hydrogen. Fluorine atoms also add to these surfaces without activation barriers, which is very different to hydrogen-addition reactions that have barriers of 5–6 kcal/mol. The electronic states of the fluorinated graphenes are discussed on the basis of the theoretical results.
1. Introduction The interactions between radicals and carbon materials have recently attracted considerable interest because the electronic properties of carbon materials are dramatically altered by the addition of radicals [1–3]. Halogen and hydrogen atoms are the simplest organic radicals and are sometimes used to modify the surfaces of carbon materials such as diamond [4], graphene, and fullerenes [5]. A radical strongly influences the electronic conductivity and band gap of the semi-conductor. In addition, fullerene and graphene materials are potential radical-storage media. Therefore, the elucidation of the electronic states and the formation mechanisms of radical-added carbon materials are important in the development of high-performance materials. Fluorine-modified graphenes have received significant attention because their electronic structures are dramatically changed on fluorination. The fluorination of graphene proceeds readily through a chemical reaction that has been mechanistically studied [6–19]. Robinson et al. [11] reported that the reaction of graphene with XeF2 leads to partial or full fluorination of graphene and showed that the properties of graphene, such as band gap, considerably depend on the degree of fluorination. The reaction of the graphene surface with F was found to significantly depend on the reaction conditions; only 25% coverage was observed when a single graphene surface was exposed to XeF2; however, complete (100%) coverage resulted when both surfaces were
∗
allowed to react with XeF2. Delabarre et al. used fluorinated graphite as the cathode in a primary lithium battery [20], and observed higher capacities for lowtemperature-fluorinated graphite. While the halogenation of carbon materials may also possibly lead to new materials chemistry, the information on the electronic structure of fluorinated graphene is limited. The density functional theory (DFT) calculations of fluorinated graphenes have been carried out by several groups [21–28]. Hutama et al. showed that the F atom can be added to the graphene surface without an activation barrier [25]. The binding energy was calculated to be 26.2 kcal/mol at the B3LYP/cc-pVDZ level. Moreover, Bulat et al. achieved a binding energy of 26.0 kcal/mol through B3LYP/6311G(d,p) calculation [26]. Previous calculations were performed only for small-sized graphene nanoflakes. However, these sizes are far from those of real graphene nanoflakes. In addition, theoretical information on the infrared and absorption spectra of fluorinated graphene nanoflakes are quite limited. In this study, fluorinated graphene nanoflakes with large sizes were systematically investigated by means of the DFT method with the aim of gaining insight into the electronic structures of fluorine–graphene (FGR) complexes. Potential energy curves for the approach of F to these surfaces were also investigated, and UV, IR, and Raman spectra were simulated on the basis of the DFT results. In our previous study [29], we investigated hydrogen-atom-added graphenes by DFT and found that
Corresponding author. E-mail address: [email protected] (H. Tachikawa).
https://doi.org/10.1016/j.solidstatesciences.2019.106007 Received 4 August 2019; Received in revised form 26 August 2019; Accepted 15 September 2019 Available online 18 September 2019 1293-2558/ © 2019 Published by Elsevier Masson SAS.
Solid State Sciences 97 (2019) 106007
H. Tachikawa and H. Kawabata
the addition of a hydrogen atom is associated with an activation barrier, although the magnitude of the barrier is very low (5–7 kcal/mol). The mechanism for F-atom addition is also compared to that involving a hydrogen atom.
2. Computational methods Graphene nanoflakes composed of 4–37 benzene rings (denoted as: GR(n), n = 4, 7, 19, and 37) were used in the present study. The edgesites of the GR(n) species were terminated with hydrogen atoms. A fluorine (F) atom was added to the central region of the graphene nanoflake. Hereafter, F-added graphene nanoflakes are referred to as F-GR (n). The structures of GR(n) and F-GR(n) were fully optimized using the coulomb-attenuating-method functional (CAM-B3LYP) [30] with the 6311G(d,p) basis set [31]. Atomic charges were calculated by natural bond population analysis (NPA) and the natural bond orbital (NBO) method [32]. Excitation energies were calculated using the time-dependent (TD) DFT method [33]; fifty states were solved in these TD-DFT calculations. The binding energy (Ebind) is defined as:
− Ebind = E (F − GR) − [E (F ) − E (GR)]
(1) Fig. 2. Optimized structure of fluorinated graphene, F-GR(37), and geometric parameters around the binding site. The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level.
where E(F-GR), E(F), and E(GR) are the total energies of F-GR, the F atom, and GR, respectively. All calculations were carried out using the Gaussian 09 package [34]. In previous studies [35–38], we investigated the interactions of graphene and several molecules using DFT at the same levels of theory; a similar technique was used in the present study.
Fig. 1. Optimized structures of the graphene nanoflakes, GR(n), used in the present study, where n indicates the number of benzene rings in GR(n). The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level. 2
Solid State Sciences 97 (2019) 106007
H. Tachikawa and H. Kawabata
Table 1 Optimized geometrical parameters of fluorinated graphene, F-GR(n), calculated at the CAM-B3LYP/6-311G(d,p) level. The bond distance and angles are in Å and degrees, respectively. n
R1
R2
R3
θ
φ
4 7 19 37
1.456 1.456 1.456 1.457
1.498 1.499 1.493 1.491
1.501 1.495 1.491 1.491
104.4 103.5 104.1 104.0
114.7 114.2 114.2 114.4
3. Results 3.1. Structures of fluorinated graphene flakes The optimized structures of GR(n) (n = 7, 19, and 37) calculated at the CAM-B3LYP/6-311G(d,p) level are displayed in Fig. 1, which reveal that the GR(n) moieties are fully planar. The C–C bond distances were calculated to be ~1.418 Å around the center of the graphene. A single fluorine atom was then added to the carbon atom (C0) in the central region of the GR(n), after which the F-GR(n) structures were fully optimized; the optimized structure of F-GR(37) is shown in Fig. 2, while the optimized geometrical parameters of all F-GR(n) molecules are listed in Table 1. The C0–F bond distance (R1) in F-GR(19) was calculated to be 1.456 Å. The local structure around the C0–F bond is bent, with C0 displaced by 0.405 Å from the graphene plane. The C–C bond lengths (R2 and R3) around the binding site were calculated to be 1.491 Å (C0–C1) and 1.491 Å (C0–C2), which are ~0.08 Å longer than that of GR(37) prior to the addition of the F atom. The C2–C0–C2’ angle φ was calculated to be 114.4°, which is smaller than that of normal graphene (φ = 120.0°) in GR(19) due to the bent structure around C0. Similar structures were obtained for F-GR(n) (n = 4, 7, and 37), as summarized in Table 1. 3.2. Atomic charges at the C–F binding site
Fig. 3. (A) Potential energy curves for F atom addition to a graphene surface. The zero energy level corresponds to the total energy of the optimized structure of F-GR(37) (PD). Curve 1 (solid line) was calculated using the optimized structure of F-GR(37), while the C0–F distance (R1) was only varied over the 1.30–5.00 Å range. Curve 2 (dashed line) was calculated using the planar structure of GR(37), while the C0–F distance was varied. (B) NPA atomic charges on the C0 and F atoms in F-GR(37) along curve 1. The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level.
The atomic charges of the central carbon atom, C0, and the adsorbed F atom were calculated by the NPA method, the results of which are listed in Table 2. In the case of F-GR(37), the NPA atomic charges on C0 and F were found to be +0.334 and −0.415, respectively, indicating that the C–F bond is locally Cδ+-Fδ– polarized, with a large dipole moment associated with the C–F bond.
difference between curves 1 and 2 was 25.9 kcal/mol at R1 = 5.00 Å, which corresponds to the sp3-sp2 relaxation energy of C0. The curve-1-to-2 crossing point, at R1 = 1.80 Å, was lower in energy than the dissociation limits, at R1 = 5.0 Å. The three dots indicate the energies of the optimized structures obtained at R1 = 1.457, 1.845, and 2.010 Å; the energies at these points were calculated to be 0.0 (R1 = 1.457), 9.0 (1.845 Å), and 12.5 kcal/mol (2.010 Å), which reveal that the addition of the F atom proceeds without any activation barrier. The NPA-calculated atomic charges on the F and C0 atoms are shown in Fig. 3B as functions of C0–F distance (R1). At R1 = 5.00 Å, the F and C0 charges were −0.01 and + 0.03, respectively, which are close to zero (neutral state). The charge on the F atom became more negative with decreasing C–F distance, and was −0.41 at the equilibrium distance (R1 = 1.457 Å). On the other hand, the charge on the C0 atom increased with decreasing R1, and was +0.33 at R1 = 1.457 Å. These results indicate that charge transfer between the F atom and graphene surface takes place gradually during the approach of F.
3.3. Potential energy curves Potential energy curves (PECs) for the additions of F atoms to the graphene surface of GR(37) are displayed in Fig. 3, as functions of C0–F distance (R1). The zero level corresponds to the total energy of the optimized F-GR(37) structure. Using the optimized structure of F-GR (37), the C0–F distance was only varied over the 1.30–5.00 Å range (curve 1), with the energy calculated at each point. The shape of the PEC was always attractive from the dissociation limit to the equilibrium point. Curve 2 (dashed line) was calculated using the planar structure of GR(37), with the C0–F distance varied over the 1.30–5.00 Å range. The energies of curve 2 were lower than those of curve 1 in the 1.80–5.00 Å range, indicating that structural relaxation around the C0 atom of GR is important during the addition of F to the GR surface. The energy Table 2 NPA atomic charges on the C0 and F atoms of F-GR(n). n
C0
F
4 7 19 37
+0.312 +0.321 +0.333 +0.334
−0.409 −0.412 −0.415 −0.415
3.4. Spin density along the reaction coordinate The spatial distributions of the spin density along the reaction coordinate in F-GR(37) are shown in Fig. 4. Spin was only localized on the F atom at longer separations, namely R1 = 5.00 Å; in particular, the 3
Solid State Sciences 97 (2019) 106007
H. Tachikawa and H. Kawabata
Fig. 5. Simulated (A) IR and (B) Raman spectra of GR(37) and F-GR(37). The calculations were carried out at the CAM-B3LYP/6-31G(d) level. Fig. 4. Spatial distributions of spin density in F-GR(37) along the reaction coordinate (R1 = 1.457–5.00 Å). The calculations were carried out at the CAMB3LYP/6-311G(d,p) level.
Table 3 Vibrational frequencies of the main peaks in GR(37) calculated at the CAMB3LYP/6-31G(d) level, and their assignments.
unpaired electron was localized in the 2pz-orbital of the F atom and the spin density on the F atom was calculated to be 0.988. At R1 = 3.00 Å, the unpaired electron was distributed on the F atom and part of the graphene surface. The spin densities on F and GR were 0.822 and 0.056, respectively, at this distance, indicating that the unpaired electron was mainly distributed on the F atom and only slightly transferred into the GR. In final product (PD state; R1 = 1.457 Å), i.e., the optimized structure of F-GR(37), the spin density was widely distributed onto the graphene surface. The values of spin density on F and GR were calculated to be 0.056 and 0.944, respectively. This result indicates that the unpaired electron (spin density) from the F atom was fully transferred to the graphene surface following the addition of F.
Peak
frequency/cm−1
assignment
a b c d
3225 1212–1709 944 1343–1696
(C–H)str in edge region (C]C)str in edge and bulk region (C–H)bend in edge (C]C)str in bulk
Table 4 Vibrational frequencies of the main peaks in F-GR(37) calculated at the CAMB3LYP/6-31G(d) level, and their assignments.
3.5. Simulated IR and Raman spectra of F-GR The IR and Raman spectra of the optimized structures of GR(37) and F-GR(37) were calculated at the CAM-B3LYP/6-31G(d) level and are displayed in Fig. 5A and B, respectively. The assignments of vibrationalmodes were made on the basis of normal mode analyses, and the main peaks and their assignments are summarized in Table 3 (GR) and 4 (FGR). Peak a in the spectrum of GR, at 3225 cm−1, is assigned to the C–H stretching mode in the edge region of GR. Peak b, which appears in the 1212–1709 cm−1 range, is composed of two components, namely the C]C stretching modes in the edge and bulk-surface regions of GR. Peak c, at 944 cm−1, is assigned to the C–H bending mode in the edge region of GR. In the Raman spectrum of GR, peak d is assigned to the C]C stretching modes of the bulk surface region of graphene (1343–1696 cm−1). The IR and Raman spectra of GR following the addition of the F atom were different as shown in Fig. 5B. In the IR spectrum of F-GR, peaks a′ and c′ are located at 3225 and 939 cm−1, respectively; these peaks are essentially at the same positions as those prior to the addition of F. A new low-intensity peak appeared at 944 cm−1 following F
Peak
frequency/cm−1
assignment
a' b' c' d' e f g
3225 1144–1694 939 1312–1665 944 821–1226 1672
(C–H)str in edge region (C]C)str in edge and bulk region (C–H)bend in edge (C]C)str in bulk (C–F)str (C]C)str near the C–F site (C]C)str around C–F site
addition (peak e in the IR spectrum and peak e′ in the Raman spectrum, indicated by the arrow); the peaks are assigned to the C–F stretching mode of F-GR. In the Raman spectrum, new peaks were observed between at 662–1226 cm−1 (peaks f), which are assigned to the C]C stretching modes perturbed by the C–F bond. The new strong peak appeared at 1672 cm−1 in Raman spectrum (peak g). This peak is assigned to be a stretching mode of C]C double bond around C–F binding site. 3.6. Simulated absorption spectra of GR(19) and F-GR(19) The simulated absorption spectra of GR and F-GR are shown in Fig. 6. A strong peak corresponding to the π-π* transition in GR was observed at 3.49 eV in the spectrum of GR (the main peak in Fig. 6A). Following the addition of the F atom to GR, a low-energy tail was 4
Solid State Sciences 97 (2019) 106007
H. Tachikawa and H. Kawabata
Fig. 8. Binding energies (kcal/mol) for the additions of F atom to several graphenes of different size, GR(n) (n = 4, 7, 19, and 37). Binding and activation energies for the additions of hydrogen atom are also shown. The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level.
Fig. 6. Simulated absorption spectra of (A) GR(19) and (B) F-GR(19) calculated at the CAM-B3LYP/6-311G(d,p) level of theory.
corresponds to the electronic excitation from the HOMO to the SOMO, namely the electronic transition from the π-orbital of F-GR to the C–F binding site. Peak 2 corresponds to a transition within the C–F binding site, while peak 3 is due to a π-π* transition on the GR surface. In other words, the lowest-energy transition (the new band, peak 1) corresponds to the electronic excitation from the π-orbital to the C–F binding site of F-GR, and is a GR defect band.
observed in the 1.30–2.80 eV range (Fig. 6B). Peaks that contribute to the long tail are indicated by arrows in Fig. 6B; peaks 1, 2, and 3 are located at 1.36, 1.72, and 2.05 eV, respectively. Hence, the addition of F atom affects the absorption spectrum of GR. In particular, a long tail appears in the low-energy region of the absorption spectrum. In order to assign the absorption peaks in the low-energy region of the spectrum of F-GR, the configuration state functions (CSFs) and molecular orbitals (MOs) of F-GR were analyzed in detail. Fig. 7 displays an energy diagram along with the spatial distributions of key MOs. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are widely distributed over the surface, although the C–F site has no orbital component in LUMO. In the singly occupied molecular orbital (SOMO) and HOMO-1, the spatial distributions are fully distributed and include the C–F site. Peak 1
3.7. Binding energies The binding energies for the additions of F to graphenes of different size, GR(n) (n = 4, 7, 19, and 37) are summarized in Fig. 8, together with those for H-GR(n). The binding energies for the F-GR species were calculated to be 23.6 (n = 4), 26.1 (n = 7), 29.4 (n = 19), and 30.1 kcal/mol (n = 37) at the CAM-B3LYP/6-311G(d,p) level. The binding energy was calculated to increase with increasing n, and reached a limiting value at around n = 19. A similar trend was observed for the addition of H to GR, although the magnitude of the binding energies obtained for the addition of H were significantly different to those obtained for the addition of F (about two-thirds less). In addition, large-activation barrier differences were found. The addition of hydrogen is associated with an activation energy, with barrier heights of 6–7 kcal/mol; in contrast, the addition of F is barrierless. 3.8. Interaction of chlorine atom (Cl) with graphene surface PEC for the additions of Cl atom to the graphene surface of GR(37) is plotted in Fig. 9A, as functions of C0–Cl distance (R1). The zero level corresponds to the total energy of dissociation limit (GR(37) + Cl). For comparison, PEC for F addition is also plotted by dashed line. The shape of the PEC for Cl was always attractive from the dissociation limit to the equilibrium point as well as that of F atom. The Cl atom is located in the on-top site (above C0 atom) at the equilibrium structure. However, the binding energy of Cl atom is significantly smaller than that of F atom: 6.5 kcal/mol (Cl) vs. 29.9 kcal/mol (F). Also, the equilibrium point of Cl atom is longer than that of F atom: 2.785 Å (Cl) vs. 1.457 Å (F). The Cl atom is shallow trapped on graphene surface. The spatial distributions of the spin density along the reaction coordinate in Cl-GR(37) are shown in Fig. 9B. Spin density of Cl at R1 = 5.00 Å was calculated to be 0.814, indicating that about 20% of
Fig. 7. Energy diagram depicting the molecular orbitals (MOs) of F-GR, and absorption-band assignments in the low-energy region. 5
Solid State Sciences 97 (2019) 106007
H. Tachikawa and H. Kawabata
Fig. 9. (A) Potential energy curve for Cl atom addition to a graphene surface. Dashed line indicates PEC for F atom addition. Spatial distributions of spin density in ClGR(37) along the reaction coordinate. The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level.
the spin density was transferred from Cl to graphene. At R1 = 3.50 Å, the value of spin density decreased to 0.674. In product state (R1 = 2.785 Å), the spin density was distributed onto the graphene surface. The values of spin density on Cl and GR were calculated to be 0.624 and 0.386, respectively, suggesting that about 40% of the spin density is transferred from Cl to graphene. The optimized geometrical parameters of Cl-GR(n) (n = 19 and 37) are given in Table 5. The bond lengths of C0–Cl were 2.732 Å (n = 19) and 2.785 Å (n = 37), which are significantly longer than those of F-GR (n) (1.456–1.457 Å). The NPA charges of C0 and Cl atoms were +0.048 and −0.308, respectively, in Cl-GR(37) (Table 6). About 70% of unpaired electron is transferred from Cl to graphene. However, the positive charge on C0 atom is small and is close to zero, indicating that the unpaired electron is delocalized over the graphene surface in Cl-GR (37). The similar features were obtained in Cl-GR(19). The magnitude of charge transfer from Cl to graphene is smaller compared with the F atom (see Table 6).
Table 6 NPA atomic charges on the C0 and Cl atoms of Cl-GR(n), calculated at the CAM-B3LYP/6-311G(d,p) level. n
C0
Cl
19 37
+0.048 +0.050
−0.308 −0.337
4. Discussion 4.1. Comparison with hydrogen addition reaction The interactions of fluorine (F) atoms with the surfaces of graphene nanoflakes were investigated by the DFT method. We found that the addition of F and Cl atoms to graphene surfaces was barrierless; in contrast, the addition of a hydrogen atom to a graphene surface is associated with an activation barrier of around 7 kcal/mol. Potential energy curves (PECs) for the addition of F and H atoms to a graphene surface are shown in Fig. 10. The present calculations reveal that the addition of F proceeds without an activation barrier, leading to high
Fig. 10. Illustrating the PECs for the interaction of F atom (Cl atom) with a graphene surface. The dotted line is the PEC for the addition of H atom to a graphene surface.
Table 5 Optimized geometrical parameters of fluorinated graphene, Cl-GR(n), calculated at the CAM-B3LYP/6-311G(d,p) level. The bond distance and angles are in Å and degrees, respectively. n
R1
R2
R3
θ
φ
19 37
2.732 2.785
1.430 1.422
1.422 1.422
101.2 85.1
119.7 120.0
binding energies (~30 kcal/mol). On the other hand, the addition of a hydrogen atom proceeds through a transition state with a C–H distance of ~1.70 Å, and the barrier is due to the avoided crossing of the ground and excited states. For the addition of F, the barrier disappears due to the high binding energy of the F atom, which is a typical example of the Evans–Polanyi principle, i.e., activation barriers diminish with
6
Solid State Sciences 97 (2019) 106007
H. Tachikawa and H. Kawabata
Table 7 Binding Energies of the Fluorine Atom to Several Polyaromatic Hydrocarbons (PAHs) (in kcal/mol). system
C/F ratio
method
binding energy (kcal/mol)
ref
pyrene–F (n = 4)
16
coronene–F (n = 7)
24
circumcoronene–F (n = 19)
54
GR(37)-F (n = 37)
96
M052X/6-31 + G(d,p) B3LYP/cc-pVDZ CAM-B3LYP/6-311G(d,p) M052X/6-31 + G(d,p) B3LYP/6-311G(d,p) B3LYP/cc-pVDZ B3LYP/6-31G CAM-B3LYP/6-311G(d,p) B3LYP/6-311G(d,p) B3LYP/cc-pVDZ CAM-B3LYP/6-311G(d,p) CAM-B3LYP/6-311G(d,p)
21.6 20.9 23.6 24.3 22.9 22.9 20 26.1 26.2 26.2 29.4 30.1
Nijamudheen et al. [27] Hutama et al. [25] this work Nijamudheen et al. [27] Bulat et al. [26] Hutama et al. [25] Papuitz et al. [28] this work Bulat et al. [26] Hutama et al. [25] this work this work
increasing exothermicity (binding energy) [39]. It was also found that the interaction of Cl with the graphene is very weak and Cl is shallow trapped on the surface.
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4.2. Comparison with previous studies Previously, several groups reported the binding energies of a F atom to graphene nanoflakes. The polyatomic hydrocarbons, such as capyrene, coronene, and circumcoronene, were employed as nanoflakes. The binding energies are listed in Table 7 together with present calculated values. The binding energies of the F atom to the nanoflakes were calculated to be 20–26 kcal/mol, which are in good agreement with the present values (23–30 kcal/mol). The present study indicates that the F atom can bind to the graphene surface without an activation barrier [25]. This result is essentially in accordance with previous calculations with small-sized nanoflakes. Therefore, it can be concluded that size dependency on binding energy and electronic states is small in the case of fluorinated graphene. The present simulated IR and Raman spectra indicate that new peaks have appeared to be 1672 cm−1 (peak g), 944 cm−1 (peak e’), and 8211226 cm−1 (peak f) following the F addition to graphene surface. These results are essentially previous experiments and theoretical calculations [23,40]. The chlorinated graphene Cl-GR was also investigated by the DFT calculations. However, the information on the detailed electronic states and structures were limited. Garcia-Fernandez et al. calculated the binding energies of Cl atom to small sized graphene nano-flakes [41,42]. They obtained the binding energy of 7.1 kcal/mol in Cl-GR(7). This value is in reasonably agreement with the binding energies obtained for larger sized graphene, 6.7 kcal/mol in Cl-GR(19) and 7.0 kcal/mol in Cl-GR(37), calculated in the present study. In this study, we used several approximations to calculate the interaction of the F atom with graphene nanoflakes. First, polycyclic aromatic hydrocarbons (PAHs) were used as a model of the graphene nanoflake, because they have been widely used as models of graphene and nanoflakes. Second, the F atom was placed in the central region of GR. Third, GR defects were completely neglected. In the future, we will investigate these issues. Despite the assumptions used in this study, valuable information regarding the electronic states of F-GR was obtained, which will be useful in the development of fluorinated carbon materials. Acknowledgments The authors acknowledge partial support from JSPS KAKENHI Grant Numbers 18K05021 and 17H03292. References [1] B. Narayanan, Y. Zhao, C.V. Ciobanu, Appl. Phys. Lett. 100 (2012) 203901.
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[37] [38] [39] [40]
E.S. Snow, Nano Lett. 10 (2010) 3001. [41] C. Garcia-Fernandez, B. Radola, L. Martin-Gondre, S. Picaud, M.T. Rayez, J.C. Rayez, F.X. Ouf, J. Rubayo-Soneira, Chem. Phys. 483–484 (2017) 46–55. [42] Bastien Radola, Ludovic Martin-Gondre, Sylvain Picaud, Marie-Therese Rayez, Jean-Claude Rayez, Chem. Phys. 523 (2019) 18–27.
H. Tachikawa, T. Iyama, J. Phys. Chem. C 123 (2019) 8709. H. Tachikawa, Appl. Surf. Sci. 396 (2017) 1335. G.G. Wubbels, Tetrahedron Lett. 56 (2015) 1716. J.T. Robinson, J.S. Burgess, C.E. Junkermeier, S.C. Badescu, T.L. Reinecke, F.K. Perkins, M.K. Zalalutdniov, J.W. Baldwin, J.C. Culbertson, P.E. Sheehan,
8
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Additions of fluorine atoms to the surfaces of graphene Nanoflakes:A density functional theory study
T
Hiroto Tachikawa∗, Hiroshi Kawabata Division of Applied Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluorination PAH Potential energy IR and Raman spectra Absorption spectra
The electronic states and band gaps of carbon materials such as graphene, fullerene, and carbon nanotubes are strongly affected by radical addition. In the present study, the reactions of graphene nanoflakes with fluorine (F) atoms were investigated by density functional theory (DFT) in order to elucidate the nature of the bonding between the F atoms and the graphene nanoflakes. Graphene nanoflakes composed of 4–37 benzene rings were used as graphene models. The present calculations reveal that F atoms react with graphene surfaces and bind directly to carbon atoms to form strong C–F bonds. The binding sites are locally Cδ+-Fδ– polarized and have large dipole moments. The binding energy of F to graphene was calculated to be 30 kcal/mol, which is 1.5 times larger than that for the addition of hydrogen. Fluorine atoms also add to these surfaces without activation barriers, which is very different to hydrogen-addition reactions that have barriers of 5–6 kcal/mol. The electronic states of the fluorinated graphenes are discussed on the basis of the theoretical results.
1. Introduction The interactions between radicals and carbon materials have recently attracted considerable interest because the electronic properties of carbon materials are dramatically altered by the addition of radicals [1–3]. Halogen and hydrogen atoms are the simplest organic radicals and are sometimes used to modify the surfaces of carbon materials such as diamond [4], graphene, and fullerenes [5]. A radical strongly influences the electronic conductivity and band gap of the semi-conductor. In addition, fullerene and graphene materials are potential radical-storage media. Therefore, the elucidation of the electronic states and the formation mechanisms of radical-added carbon materials are important in the development of high-performance materials. Fluorine-modified graphenes have received significant attention because their electronic structures are dramatically changed on fluorination. The fluorination of graphene proceeds readily through a chemical reaction that has been mechanistically studied [6–19]. Robinson et al. [11] reported that the reaction of graphene with XeF2 leads to partial or full fluorination of graphene and showed that the properties of graphene, such as band gap, considerably depend on the degree of fluorination. The reaction of the graphene surface with F was found to significantly depend on the reaction conditions; only 25% coverage was observed when a single graphene surface was exposed to XeF2; however, complete (100%) coverage resulted when both surfaces were
∗
allowed to react with XeF2. Delabarre et al. used fluorinated graphite as the cathode in a primary lithium battery [20], and observed higher capacities for lowtemperature-fluorinated graphite. While the halogenation of carbon materials may also possibly lead to new materials chemistry, the information on the electronic structure of fluorinated graphene is limited. The density functional theory (DFT) calculations of fluorinated graphenes have been carried out by several groups [21–28]. Hutama et al. showed that the F atom can be added to the graphene surface without an activation barrier [25]. The binding energy was calculated to be 26.2 kcal/mol at the B3LYP/cc-pVDZ level. Moreover, Bulat et al. achieved a binding energy of 26.0 kcal/mol through B3LYP/6311G(d,p) calculation [26]. Previous calculations were performed only for small-sized graphene nanoflakes. However, these sizes are far from those of real graphene nanoflakes. In addition, theoretical information on the infrared and absorption spectra of fluorinated graphene nanoflakes are quite limited. In this study, fluorinated graphene nanoflakes with large sizes were systematically investigated by means of the DFT method with the aim of gaining insight into the electronic structures of fluorine–graphene (FGR) complexes. Potential energy curves for the approach of F to these surfaces were also investigated, and UV, IR, and Raman spectra were simulated on the basis of the DFT results. In our previous study [29], we investigated hydrogen-atom-added graphenes by DFT and found that
Corresponding author. E-mail address: [email protected] (H. Tachikawa).
https://doi.org/10.1016/j.solidstatesciences.2019.106007 Received 4 August 2019; Received in revised form 26 August 2019; Accepted 15 September 2019 Available online 18 September 2019 1293-2558/ © 2019 Published by Elsevier Masson SAS.
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the addition of a hydrogen atom is associated with an activation barrier, although the magnitude of the barrier is very low (5–7 kcal/mol). The mechanism for F-atom addition is also compared to that involving a hydrogen atom.
2. Computational methods Graphene nanoflakes composed of 4–37 benzene rings (denoted as: GR(n), n = 4, 7, 19, and 37) were used in the present study. The edgesites of the GR(n) species were terminated with hydrogen atoms. A fluorine (F) atom was added to the central region of the graphene nanoflake. Hereafter, F-added graphene nanoflakes are referred to as F-GR (n). The structures of GR(n) and F-GR(n) were fully optimized using the coulomb-attenuating-method functional (CAM-B3LYP) [30] with the 6311G(d,p) basis set [31]. Atomic charges were calculated by natural bond population analysis (NPA) and the natural bond orbital (NBO) method [32]. Excitation energies were calculated using the time-dependent (TD) DFT method [33]; fifty states were solved in these TD-DFT calculations. The binding energy (Ebind) is defined as:
− Ebind = E (F − GR) − [E (F ) − E (GR)]
(1) Fig. 2. Optimized structure of fluorinated graphene, F-GR(37), and geometric parameters around the binding site. The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level.
where E(F-GR), E(F), and E(GR) are the total energies of F-GR, the F atom, and GR, respectively. All calculations were carried out using the Gaussian 09 package [34]. In previous studies [35–38], we investigated the interactions of graphene and several molecules using DFT at the same levels of theory; a similar technique was used in the present study.
Fig. 1. Optimized structures of the graphene nanoflakes, GR(n), used in the present study, where n indicates the number of benzene rings in GR(n). The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level. 2
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Table 1 Optimized geometrical parameters of fluorinated graphene, F-GR(n), calculated at the CAM-B3LYP/6-311G(d,p) level. The bond distance and angles are in Å and degrees, respectively. n
R1
R2
R3
θ
φ
4 7 19 37
1.456 1.456 1.456 1.457
1.498 1.499 1.493 1.491
1.501 1.495 1.491 1.491
104.4 103.5 104.1 104.0
114.7 114.2 114.2 114.4
3. Results 3.1. Structures of fluorinated graphene flakes The optimized structures of GR(n) (n = 7, 19, and 37) calculated at the CAM-B3LYP/6-311G(d,p) level are displayed in Fig. 1, which reveal that the GR(n) moieties are fully planar. The C–C bond distances were calculated to be ~1.418 Å around the center of the graphene. A single fluorine atom was then added to the carbon atom (C0) in the central region of the GR(n), after which the F-GR(n) structures were fully optimized; the optimized structure of F-GR(37) is shown in Fig. 2, while the optimized geometrical parameters of all F-GR(n) molecules are listed in Table 1. The C0–F bond distance (R1) in F-GR(19) was calculated to be 1.456 Å. The local structure around the C0–F bond is bent, with C0 displaced by 0.405 Å from the graphene plane. The C–C bond lengths (R2 and R3) around the binding site were calculated to be 1.491 Å (C0–C1) and 1.491 Å (C0–C2), which are ~0.08 Å longer than that of GR(37) prior to the addition of the F atom. The C2–C0–C2’ angle φ was calculated to be 114.4°, which is smaller than that of normal graphene (φ = 120.0°) in GR(19) due to the bent structure around C0. Similar structures were obtained for F-GR(n) (n = 4, 7, and 37), as summarized in Table 1. 3.2. Atomic charges at the C–F binding site
Fig. 3. (A) Potential energy curves for F atom addition to a graphene surface. The zero energy level corresponds to the total energy of the optimized structure of F-GR(37) (PD). Curve 1 (solid line) was calculated using the optimized structure of F-GR(37), while the C0–F distance (R1) was only varied over the 1.30–5.00 Å range. Curve 2 (dashed line) was calculated using the planar structure of GR(37), while the C0–F distance was varied. (B) NPA atomic charges on the C0 and F atoms in F-GR(37) along curve 1. The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level.
The atomic charges of the central carbon atom, C0, and the adsorbed F atom were calculated by the NPA method, the results of which are listed in Table 2. In the case of F-GR(37), the NPA atomic charges on C0 and F were found to be +0.334 and −0.415, respectively, indicating that the C–F bond is locally Cδ+-Fδ– polarized, with a large dipole moment associated with the C–F bond.
difference between curves 1 and 2 was 25.9 kcal/mol at R1 = 5.00 Å, which corresponds to the sp3-sp2 relaxation energy of C0. The curve-1-to-2 crossing point, at R1 = 1.80 Å, was lower in energy than the dissociation limits, at R1 = 5.0 Å. The three dots indicate the energies of the optimized structures obtained at R1 = 1.457, 1.845, and 2.010 Å; the energies at these points were calculated to be 0.0 (R1 = 1.457), 9.0 (1.845 Å), and 12.5 kcal/mol (2.010 Å), which reveal that the addition of the F atom proceeds without any activation barrier. The NPA-calculated atomic charges on the F and C0 atoms are shown in Fig. 3B as functions of C0–F distance (R1). At R1 = 5.00 Å, the F and C0 charges were −0.01 and + 0.03, respectively, which are close to zero (neutral state). The charge on the F atom became more negative with decreasing C–F distance, and was −0.41 at the equilibrium distance (R1 = 1.457 Å). On the other hand, the charge on the C0 atom increased with decreasing R1, and was +0.33 at R1 = 1.457 Å. These results indicate that charge transfer between the F atom and graphene surface takes place gradually during the approach of F.
3.3. Potential energy curves Potential energy curves (PECs) for the additions of F atoms to the graphene surface of GR(37) are displayed in Fig. 3, as functions of C0–F distance (R1). The zero level corresponds to the total energy of the optimized F-GR(37) structure. Using the optimized structure of F-GR (37), the C0–F distance was only varied over the 1.30–5.00 Å range (curve 1), with the energy calculated at each point. The shape of the PEC was always attractive from the dissociation limit to the equilibrium point. Curve 2 (dashed line) was calculated using the planar structure of GR(37), with the C0–F distance varied over the 1.30–5.00 Å range. The energies of curve 2 were lower than those of curve 1 in the 1.80–5.00 Å range, indicating that structural relaxation around the C0 atom of GR is important during the addition of F to the GR surface. The energy Table 2 NPA atomic charges on the C0 and F atoms of F-GR(n). n
C0
F
4 7 19 37
+0.312 +0.321 +0.333 +0.334
−0.409 −0.412 −0.415 −0.415
3.4. Spin density along the reaction coordinate The spatial distributions of the spin density along the reaction coordinate in F-GR(37) are shown in Fig. 4. Spin was only localized on the F atom at longer separations, namely R1 = 5.00 Å; in particular, the 3
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Fig. 5. Simulated (A) IR and (B) Raman spectra of GR(37) and F-GR(37). The calculations were carried out at the CAM-B3LYP/6-31G(d) level. Fig. 4. Spatial distributions of spin density in F-GR(37) along the reaction coordinate (R1 = 1.457–5.00 Å). The calculations were carried out at the CAMB3LYP/6-311G(d,p) level.
Table 3 Vibrational frequencies of the main peaks in GR(37) calculated at the CAMB3LYP/6-31G(d) level, and their assignments.
unpaired electron was localized in the 2pz-orbital of the F atom and the spin density on the F atom was calculated to be 0.988. At R1 = 3.00 Å, the unpaired electron was distributed on the F atom and part of the graphene surface. The spin densities on F and GR were 0.822 and 0.056, respectively, at this distance, indicating that the unpaired electron was mainly distributed on the F atom and only slightly transferred into the GR. In final product (PD state; R1 = 1.457 Å), i.e., the optimized structure of F-GR(37), the spin density was widely distributed onto the graphene surface. The values of spin density on F and GR were calculated to be 0.056 and 0.944, respectively. This result indicates that the unpaired electron (spin density) from the F atom was fully transferred to the graphene surface following the addition of F.
Peak
frequency/cm−1
assignment
a b c d
3225 1212–1709 944 1343–1696
(C–H)str in edge region (C]C)str in edge and bulk region (C–H)bend in edge (C]C)str in bulk
Table 4 Vibrational frequencies of the main peaks in F-GR(37) calculated at the CAMB3LYP/6-31G(d) level, and their assignments.
3.5. Simulated IR and Raman spectra of F-GR The IR and Raman spectra of the optimized structures of GR(37) and F-GR(37) were calculated at the CAM-B3LYP/6-31G(d) level and are displayed in Fig. 5A and B, respectively. The assignments of vibrationalmodes were made on the basis of normal mode analyses, and the main peaks and their assignments are summarized in Table 3 (GR) and 4 (FGR). Peak a in the spectrum of GR, at 3225 cm−1, is assigned to the C–H stretching mode in the edge region of GR. Peak b, which appears in the 1212–1709 cm−1 range, is composed of two components, namely the C]C stretching modes in the edge and bulk-surface regions of GR. Peak c, at 944 cm−1, is assigned to the C–H bending mode in the edge region of GR. In the Raman spectrum of GR, peak d is assigned to the C]C stretching modes of the bulk surface region of graphene (1343–1696 cm−1). The IR and Raman spectra of GR following the addition of the F atom were different as shown in Fig. 5B. In the IR spectrum of F-GR, peaks a′ and c′ are located at 3225 and 939 cm−1, respectively; these peaks are essentially at the same positions as those prior to the addition of F. A new low-intensity peak appeared at 944 cm−1 following F
Peak
frequency/cm−1
assignment
a' b' c' d' e f g
3225 1144–1694 939 1312–1665 944 821–1226 1672
(C–H)str in edge region (C]C)str in edge and bulk region (C–H)bend in edge (C]C)str in bulk (C–F)str (C]C)str near the C–F site (C]C)str around C–F site
addition (peak e in the IR spectrum and peak e′ in the Raman spectrum, indicated by the arrow); the peaks are assigned to the C–F stretching mode of F-GR. In the Raman spectrum, new peaks were observed between at 662–1226 cm−1 (peaks f), which are assigned to the C]C stretching modes perturbed by the C–F bond. The new strong peak appeared at 1672 cm−1 in Raman spectrum (peak g). This peak is assigned to be a stretching mode of C]C double bond around C–F binding site. 3.6. Simulated absorption spectra of GR(19) and F-GR(19) The simulated absorption spectra of GR and F-GR are shown in Fig. 6. A strong peak corresponding to the π-π* transition in GR was observed at 3.49 eV in the spectrum of GR (the main peak in Fig. 6A). Following the addition of the F atom to GR, a low-energy tail was 4
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Fig. 8. Binding energies (kcal/mol) for the additions of F atom to several graphenes of different size, GR(n) (n = 4, 7, 19, and 37). Binding and activation energies for the additions of hydrogen atom are also shown. The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level.
Fig. 6. Simulated absorption spectra of (A) GR(19) and (B) F-GR(19) calculated at the CAM-B3LYP/6-311G(d,p) level of theory.
corresponds to the electronic excitation from the HOMO to the SOMO, namely the electronic transition from the π-orbital of F-GR to the C–F binding site. Peak 2 corresponds to a transition within the C–F binding site, while peak 3 is due to a π-π* transition on the GR surface. In other words, the lowest-energy transition (the new band, peak 1) corresponds to the electronic excitation from the π-orbital to the C–F binding site of F-GR, and is a GR defect band.
observed in the 1.30–2.80 eV range (Fig. 6B). Peaks that contribute to the long tail are indicated by arrows in Fig. 6B; peaks 1, 2, and 3 are located at 1.36, 1.72, and 2.05 eV, respectively. Hence, the addition of F atom affects the absorption spectrum of GR. In particular, a long tail appears in the low-energy region of the absorption spectrum. In order to assign the absorption peaks in the low-energy region of the spectrum of F-GR, the configuration state functions (CSFs) and molecular orbitals (MOs) of F-GR were analyzed in detail. Fig. 7 displays an energy diagram along with the spatial distributions of key MOs. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are widely distributed over the surface, although the C–F site has no orbital component in LUMO. In the singly occupied molecular orbital (SOMO) and HOMO-1, the spatial distributions are fully distributed and include the C–F site. Peak 1
3.7. Binding energies The binding energies for the additions of F to graphenes of different size, GR(n) (n = 4, 7, 19, and 37) are summarized in Fig. 8, together with those for H-GR(n). The binding energies for the F-GR species were calculated to be 23.6 (n = 4), 26.1 (n = 7), 29.4 (n = 19), and 30.1 kcal/mol (n = 37) at the CAM-B3LYP/6-311G(d,p) level. The binding energy was calculated to increase with increasing n, and reached a limiting value at around n = 19. A similar trend was observed for the addition of H to GR, although the magnitude of the binding energies obtained for the addition of H were significantly different to those obtained for the addition of F (about two-thirds less). In addition, large-activation barrier differences were found. The addition of hydrogen is associated with an activation energy, with barrier heights of 6–7 kcal/mol; in contrast, the addition of F is barrierless. 3.8. Interaction of chlorine atom (Cl) with graphene surface PEC for the additions of Cl atom to the graphene surface of GR(37) is plotted in Fig. 9A, as functions of C0–Cl distance (R1). The zero level corresponds to the total energy of dissociation limit (GR(37) + Cl). For comparison, PEC for F addition is also plotted by dashed line. The shape of the PEC for Cl was always attractive from the dissociation limit to the equilibrium point as well as that of F atom. The Cl atom is located in the on-top site (above C0 atom) at the equilibrium structure. However, the binding energy of Cl atom is significantly smaller than that of F atom: 6.5 kcal/mol (Cl) vs. 29.9 kcal/mol (F). Also, the equilibrium point of Cl atom is longer than that of F atom: 2.785 Å (Cl) vs. 1.457 Å (F). The Cl atom is shallow trapped on graphene surface. The spatial distributions of the spin density along the reaction coordinate in Cl-GR(37) are shown in Fig. 9B. Spin density of Cl at R1 = 5.00 Å was calculated to be 0.814, indicating that about 20% of
Fig. 7. Energy diagram depicting the molecular orbitals (MOs) of F-GR, and absorption-band assignments in the low-energy region. 5
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H. Tachikawa and H. Kawabata
Fig. 9. (A) Potential energy curve for Cl atom addition to a graphene surface. Dashed line indicates PEC for F atom addition. Spatial distributions of spin density in ClGR(37) along the reaction coordinate. The calculations were carried out at the CAM-B3LYP/6-311G(d,p) level.
the spin density was transferred from Cl to graphene. At R1 = 3.50 Å, the value of spin density decreased to 0.674. In product state (R1 = 2.785 Å), the spin density was distributed onto the graphene surface. The values of spin density on Cl and GR were calculated to be 0.624 and 0.386, respectively, suggesting that about 40% of the spin density is transferred from Cl to graphene. The optimized geometrical parameters of Cl-GR(n) (n = 19 and 37) are given in Table 5. The bond lengths of C0–Cl were 2.732 Å (n = 19) and 2.785 Å (n = 37), which are significantly longer than those of F-GR (n) (1.456–1.457 Å). The NPA charges of C0 and Cl atoms were +0.048 and −0.308, respectively, in Cl-GR(37) (Table 6). About 70% of unpaired electron is transferred from Cl to graphene. However, the positive charge on C0 atom is small and is close to zero, indicating that the unpaired electron is delocalized over the graphene surface in Cl-GR (37). The similar features were obtained in Cl-GR(19). The magnitude of charge transfer from Cl to graphene is smaller compared with the F atom (see Table 6).
Table 6 NPA atomic charges on the C0 and Cl atoms of Cl-GR(n), calculated at the CAM-B3LYP/6-311G(d,p) level. n
C0
Cl
19 37
+0.048 +0.050
−0.308 −0.337
4. Discussion 4.1. Comparison with hydrogen addition reaction The interactions of fluorine (F) atoms with the surfaces of graphene nanoflakes were investigated by the DFT method. We found that the addition of F and Cl atoms to graphene surfaces was barrierless; in contrast, the addition of a hydrogen atom to a graphene surface is associated with an activation barrier of around 7 kcal/mol. Potential energy curves (PECs) for the addition of F and H atoms to a graphene surface are shown in Fig. 10. The present calculations reveal that the addition of F proceeds without an activation barrier, leading to high
Fig. 10. Illustrating the PECs for the interaction of F atom (Cl atom) with a graphene surface. The dotted line is the PEC for the addition of H atom to a graphene surface.
Table 5 Optimized geometrical parameters of fluorinated graphene, Cl-GR(n), calculated at the CAM-B3LYP/6-311G(d,p) level. The bond distance and angles are in Å and degrees, respectively. n
R1
R2
R3
θ
φ
19 37
2.732 2.785
1.430 1.422
1.422 1.422
101.2 85.1
119.7 120.0
binding energies (~30 kcal/mol). On the other hand, the addition of a hydrogen atom proceeds through a transition state with a C–H distance of ~1.70 Å, and the barrier is due to the avoided crossing of the ground and excited states. For the addition of F, the barrier disappears due to the high binding energy of the F atom, which is a typical example of the Evans–Polanyi principle, i.e., activation barriers diminish with
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Table 7 Binding Energies of the Fluorine Atom to Several Polyaromatic Hydrocarbons (PAHs) (in kcal/mol). system
C/F ratio
method
binding energy (kcal/mol)
ref
pyrene–F (n = 4)
16
coronene–F (n = 7)
24
circumcoronene–F (n = 19)
54
GR(37)-F (n = 37)
96
M052X/6-31 + G(d,p) B3LYP/cc-pVDZ CAM-B3LYP/6-311G(d,p) M052X/6-31 + G(d,p) B3LYP/6-311G(d,p) B3LYP/cc-pVDZ B3LYP/6-31G CAM-B3LYP/6-311G(d,p) B3LYP/6-311G(d,p) B3LYP/cc-pVDZ CAM-B3LYP/6-311G(d,p) CAM-B3LYP/6-311G(d,p)
21.6 20.9 23.6 24.3 22.9 22.9 20 26.1 26.2 26.2 29.4 30.1
Nijamudheen et al. [27] Hutama et al. [25] this work Nijamudheen et al. [27] Bulat et al. [26] Hutama et al. [25] Papuitz et al. [28] this work Bulat et al. [26] Hutama et al. [25] this work this work
increasing exothermicity (binding energy) [39]. It was also found that the interaction of Cl with the graphene is very weak and Cl is shallow trapped on the surface.
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4.2. Comparison with previous studies Previously, several groups reported the binding energies of a F atom to graphene nanoflakes. The polyatomic hydrocarbons, such as capyrene, coronene, and circumcoronene, were employed as nanoflakes. The binding energies are listed in Table 7 together with present calculated values. The binding energies of the F atom to the nanoflakes were calculated to be 20–26 kcal/mol, which are in good agreement with the present values (23–30 kcal/mol). The present study indicates that the F atom can bind to the graphene surface without an activation barrier [25]. This result is essentially in accordance with previous calculations with small-sized nanoflakes. Therefore, it can be concluded that size dependency on binding energy and electronic states is small in the case of fluorinated graphene. The present simulated IR and Raman spectra indicate that new peaks have appeared to be 1672 cm−1 (peak g), 944 cm−1 (peak e’), and 8211226 cm−1 (peak f) following the F addition to graphene surface. These results are essentially previous experiments and theoretical calculations [23,40]. The chlorinated graphene Cl-GR was also investigated by the DFT calculations. However, the information on the detailed electronic states and structures were limited. Garcia-Fernandez et al. calculated the binding energies of Cl atom to small sized graphene nano-flakes [41,42]. They obtained the binding energy of 7.1 kcal/mol in Cl-GR(7). This value is in reasonably agreement with the binding energies obtained for larger sized graphene, 6.7 kcal/mol in Cl-GR(19) and 7.0 kcal/mol in Cl-GR(37), calculated in the present study. In this study, we used several approximations to calculate the interaction of the F atom with graphene nanoflakes. First, polycyclic aromatic hydrocarbons (PAHs) were used as a model of the graphene nanoflake, because they have been widely used as models of graphene and nanoflakes. Second, the F atom was placed in the central region of GR. Third, GR defects were completely neglected. In the future, we will investigate these issues. Despite the assumptions used in this study, valuable information regarding the electronic states of F-GR was obtained, which will be useful in the development of fluorinated carbon materials. Acknowledgments The authors acknowledge partial support from JSPS KAKENHI Grant Numbers 18K05021 and 17H03292. References [1] B. Narayanan, Y. Zhao, C.V. Ciobanu, Appl. Phys. Lett. 100 (2012) 203901.
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E.S. Snow, Nano Lett. 10 (2010) 3001. [41] C. Garcia-Fernandez, B. Radola, L. Martin-Gondre, S. Picaud, M.T. Rayez, J.C. Rayez, F.X. Ouf, J. Rubayo-Soneira, Chem. Phys. 483–484 (2017) 46–55. [42] Bastien Radola, Ludovic Martin-Gondre, Sylvain Picaud, Marie-Therese Rayez, Jean-Claude Rayez, Chem. Phys. 523 (2019) 18–27.
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