Article pubs.acs.org/JPCC
Chemical Bonding of Partially Fluorinated Graphene Si Zhou,†,‡ Sonam D. Sherpa,§ Dennis W. Hess,§ and Angelo Bongiorno*,†,‡ †
School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430, United States School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States § School of Chemical and Biochemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, United States ‡
S Supporting Information *
ABSTRACT: Understanding the interactions of graphene and chemical additives is crucial to achieve control over chemically modified graphene materials. Here, density functional theory and experiments are used to elucidate the chemical bonding of partially fluorinated graphene. This material includes F−C bonds whose ionicity varies depending upon local concentration and structure. Three bonding states dominate. Single fluorine on graphene forms a semi-ionic bond with a C atom in an sp2 configuration. Fluorine in highly stable domains of poly(carbon fluoride) consists of covalent F−C bonds alternating in ortho position one to another on both sides of graphene. Fluorine in regions of poly(tetracarbon fluoride) has F species on one side of graphene in para position one to another, and F−C bonds with a character intermediate between the semi-ionic and covalent. Such variability in chemical bonding could be exploited to tailor fluorinated graphene materials to achieve a variety of properties.
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INTRODUCTION 1,2
The origin of this concentration effect remains open to debate.34,37 In recent years, fluorinated graphene has been produced by exposure to XeF2,4,21 exfoliation of graphite fluorite,8 and plasma treatments,10,38 and both high and partial levels of fluorination of graphene have been achieved, leading to 2D analogues of graphite fluorite and F-GICs, respectively. While fully fluorinated graphene is quite stable, and experimental and computational studies agree well on its structure and electronic properties,4,7,39−42 partially fluorinated graphene exhibits a complex nature and its properties remain little understood.10,21,38 Recent experimental studies show, in particular, that partially fluorinated graphene exhibits (analogously to FGICs) fluorination levels and structural and electronic properties that vary and are very sensitive to synthesis parameters, the substrate, and postsynthesis treatments.10,21,38 Quantum chemical methods have been regularly employed to investigate both graphite fluoride43,44 and, most recently, fluorinated graphene compounds.40,45,46 These studies have focused so far on crystalline phases and on structural43,44 and electronic and vibration properties of these carbon materials.40,45,46 Computational efforts to explain the concentrationdependent properties of fluorinated graphite/graphene compounds10,21,38,47,48 remain limited in number and inconclusive.37,44 As a result, the current understanding of the chemical bonding of these materials remains based on two opposed scenarios. First, the nature of the F−C bond changes depending upon fluorine concentration, from purely covalent to semi-ionic
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Fluorine adatoms confer remarkable chemical, tribological, electronic,4 and magnetic5 properties to graphene, and as a result, fluorinated graphene (FG)4−8 has potential applications as electrode material for batteries2 and as lubricant,3 as well as in microelectronics,7,9 organic electronics,10 and photovoltaics.10 FG has indeed been successfully integrated into fieldeffect transistors,7 and most recently, it has also been used to produce high-quality graphene nanoribbons,9 opening the way to novel all-carbon nanocircuitry. Similar to the case of graphene oxide11−18 and other graphene derivatives,19,20 the key challenge faced by a FG-based technology is to achieve control of both the synthesis and postsynthesis treatments of FG, such that by varying the concentration of fluorine10,21,22 or the structure of FG, the chemical and physical properties of this 2D material can be tailored to achieve a variety of properties. To this end, it is of paramount importance to attain a basic understanding of the chemical interaction of fluorine and sp2 carbon in partially fluorinated graphene. Fluorination of sp2 carbon dates back decades, first with graphite23−27 and more recently with carbon nanotubes,28−32 and numerous studies exist on the synthesis and properties of this class of materials.23−27,33−36 Two essential facts can be drawn from this extensive literature. First, various fluorination levels of sp2 carbon can be achieved: from the very dilute up to F/C ratios of 0.5 in the fluorine−graphite intercalation compounds (F-GICs),24,27,33,34 and up to F/C ratios of 0.5 and 1 in poly(dicarbon monofluoride) ((C2F)n) and poly(carbon monofluoride) ((CF)n) compounds,24 respectively. Second, the physical chemical properties of these materials vary considerably depending upon fluorine concentration.24,27,33,34 © 2014 American Chemical Society
Received: September 4, 2014 Revised: October 23, 2014 Published: October 24, 2014 26402
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or semicovalent and to ionic at very low fluorine concentrations;24,27,33,34 such variability is at the origin of and correlates well with the observed properties of fluorinated graphite/graphene compounds.10,21,38,47,48 Second, F−C bonds are covalent and do not exhibit a variable chemical character, and the concentration-dependent properties of these materials stem from the structure-dependent hyperconjugation of C−C bonds of the carbon sheets and F−C bonds.34,49 In this study, we exploit density functional theory (DFT) calculations and Xray photoelectron spectroscopy (XPS) to elucidate the nature of the chemical bonding of fluorine and C(sp2) in partially fluorinated graphene. This study provides a demonstration of the occurrence of a variable covalent-to-ionic chemical character of the F−C bond in fluorinated graphene (and most likely also in fluorinated graphite).
Figure 2. From left to right, horizontal segments show the energy of an F anion intercalated in bilayer graphene, and of F species chemisorbed on single-layer graphene in the isolated form and in small clusters of two up to four units. Right, open squares show the energy of F species in two stable fully fluorinated states of a graphene layer. Formation energies are given per F unit and are calculated with respect to pristine graphene and half an F2 molecule. Ball and stick illustrations above or below the symbols show the various bonding states of F (colored discs) and graphene (gray sticks). F atoms chemisorbed on opposites sides of the carbon plane are shown in blue and yellow colors, respectively.
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METHODS We carried out DFT calculations by using the software provided with the QUANTUM-Espresso toolkit.50 We used an energy cutoff of 100 Ry, norm-conserving pseudopotentials,51 and a generalized gradient approximation of the exchange and correlation energy functional.52 Within our semilocal DFT approach, London dispersion interactions were accounted for within the semiempirical corrective scheme proposed by Grimme.53 To compute core-level XPS binding energies, we used the core-excited pseudopotential technique.14,50,54 This method allows an estimate of core-level XPS binding energies that account for the vertical photoexcitation transition and core−hole final-state relaxation effects. We used the fhi98PP package55 to generate the pseudopotentials of the photoexcited C and F species. Additional technical details of our calculations and the full list of results are reported in the Supporting Information (SI).
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Figure 3. F−C bond length (ordinates) and F 1s core-level energy shift (abscissa) of fluorine species chemisorbed on graphene in various bonding environments. From left to right, F bonded to edge C atoms of an armchair-type graphene edge, energetically most stable F−C tetramers, trimers, and dimers, and an individual fluorine species chemisorbed on a pristine region of graphene (bottom) and nearby a tightly packed agglomerate of F−C bonds on graphene (top). For clarity, the various F species within dashed boxes are sketched (see Figure 2 for stylistic conventions) as belonging to the same partially fluorinated graphene layer. The color of the vertical segments and open circles is used to establish the correspondence between core-level energy shift and the fluorine species drawn in the ball-and-stick illustration. The height of the vertical segments is used to show the length of the F−C bonds in the various bonding environments. The F 1s core-level binding energies calculated from DFT are referred to that of F in single F−C species chemisorbed on a pristine region of graphene. The core-level energy shift of an F anion in bilayer graphene is also reported on the right (vertical dashed segment).
RESULTS AND DISCUSSION In a dilute regime, fluorine chemisorbs on graphene, forming a species whose formation energy is ∼0.7 eV lower than that of
Figure 1. Formation energy of an F anion (black squares) intercalated in bilayer graphene, and of an F species chemisorbed on graphene on either the interior (red squares) or exterior (green squares) of the bilayer structure. The energy of these three fluorine species is plotted vs the distance between the graphene layers. A schematic illustration of bilayer graphene and the three aforementioned fluorine species is shown in the inset. For clarity, a colored filled square is placed beside each F species to establish a correspondence with the energy values.
the F2 binding energy. We expect that the computed F−C bond energies reported in this work have a similar accuracy. It is interesting to compare the energetic stability of a single fluorine species chemisorbed on graphene to that of a fluorine anion intercalated in bilayer graphene (Figure 1). The limitations of semilocal DFT to describe charge-transfer states in nonbonded systems are well-known58 (our calculations show that graphene transfers 0.5e to F). Nevertheless, these calculations lead to the following qualitatively correct and interesting results. At the optimal interlayer separation of 5 Å, the energy of a fluorine anion in bilayer graphene is only 0.2 eV larger than that of a fluorine species chemisorbed on either side
pristine graphene and (half) an F2 molecule (Figure 1). Our DFT scheme yields a binding energy for F2 of 1.81 eV, an electron affinity for atomic F of 3.57 eV, and a work function for a graphene layer of 4.22 eV. These energy values compare well with the experimental data of 1.65, 3.41,56 and 4.55 eV,57 respectively. The largest percent error of 10% is obtained for 26403
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Figure 4. C 1s core-level energy shifts computed from DFT of carbon species in pristine and fluorinated regions of graphene. These species include a C atom in a sp2 orbital configuration and not bonded or near F species chemisorbed on graphene (open black circle and top table row), a C species in a sp2 orbital configuration and bonded to a fluorinated C atom (open purple circle and second entry in the table), a C atom bonded to an F atom and surrounded by C atoms in a sp2 orbital configuration (green open circle; (FC)1), a C atom bonded to F and belonging to a dimeric complex of F−C bonds (magenta open circles; 1,2(FC)2 in the table), C species embedded in fluorinated regions of graphene and thus bonded to other fluorinated C species (cyan open circle; 1,2(FC)n in the table), and C atom at a graphene edge forming a covalent bond with F (orange circle; last table row). See captions of Figures 3 and 4 for the stylistic conventions used in the ball and stick illustration.
3 and 4, and for the full list of results, see SI). In particular, we considered F−C bonds at graphene edges; isolated F species chemisorbed on graphene and the C atoms in its immediate vicinity; F and C atoms at the periphery and in the interior of fluorine agglomerates on graphene of various sizes and geometry; and a fluorine anion intercalated in bilayer graphene. In these calculations, all periodic models used to mimic regions of partially fluorinated graphene included an area of pristine graphene encompassing C atoms in sp2 configuration and a single F−C species. The core-level binding energies associated with these latter species were used as reference core-level zeroenergy shifts (see SI for additional details). Core-level XPS binding energies are, in first approximation, sensitive to the amount (charge state) and distribution (bonding type) of the electronic charge at an atomic site. Our calculations show in particular that the 1s core-level binding energy of an F anion intercalated in bilayer graphene is separated by about 8 eV from that of an F atom forming a covalent bond at a graphene edge. This result shows that these computed spectroscopic quantities can be used as indicators of the chemical nature of the F−C bond in fluorinated graphene (Figure 3). In particular, our calculations show that, depending on local environment, fluorine chemisorbed on graphene exhibits core-level binding energies varying over about 3.5 eV (Figure 3). F in an isolated F−C species yields a core-level binding energy about midway between those of an F anion in graphite and a covalent F−C bond at a graphene edge. Fluorine in this bonding state forms a chemical bond with graphene exhibiting a nature halfway between the ionic and covalent bonds. In the literature, this type of bond is referred to as semiionic or semicovalent F−C bond,34 and hereafter this fluorine species will be indicated as (FC)1. Figure 4 reports the values of the C 1s core-level energy shift of several different C sites in partially fluorinated graphene. These energy values vary considerably depending on whether the C atom is bonded or not to fluorine. However, it is interesting to note that these C 1s core-level XPS energy shifts are not good indicators of either the type of bond existing between F and C or the specific chemical environment surrounding a C atom. In fact, C bonded to F at graphene edges or forming (FC)1 species give rise to C 1s core-level energy shifts differing by only 0.2 eV, and fluorinated C species in dimer, trimer, or larger agglomerates of F−C bonds yield approximately the same energy shift regardless of the type and symmetry of the F−C bonding structure, i.e., 1(FC)n versus 2 (FC)n. The ionicity of the F−C bond depends strongly on the local chemical environment (Figure 3 and SI). A (FC)1 bond loses
of the graphene layers. At shorter interlayer spacings, however, while a single F species chemisorbed on an outer face of bilayer graphene preserves an energy of about −0.7 eV (Figures 1 and 2), both anion and F species chemisorbed on an inner carbon face lose energetic stability. At the interlayer distance of 3.5 Å, in particular, the energy of the anion increases up to 0.3 eV (i.e., 1 eV larger than that of F chemisorbed on an external face of bilayer graphene), while the energy of an F species chemisorbed on an inner face reaches a value of 0.1 eV. These results indicate that the incorporation of fluorine in a multilayer graphene film is controlled by factors such as interlayer separation and thus fluorine concentration, and that the intercalated species comprise F atoms chemisorbed on the carbon layers and, to some extent, F anions as well. In the chemisorbed state, individual fluorine species on graphene attract each other and are prone to the formation of orderly, packed aggregates (Figure 2). Our calculations show that the energy gain per F unit resulting from the formation of a dimer with F−C bonds in ortho and para positions are 0.18 and 0.23 eV, respectively, when the two F are chemisorbed on the same side of graphene, and 0.51 and 0.22 eV, respectively, when they are chemisorbed on opposites sides of the carbon plane (Figure 2). Similar results are found also in the case of small clusters and large agglomerates of F−C bonds. In particular, our calculations show that the energetically most stable configurations of orderly, packed aggregates with fluorine species chemisorbed on the same side of the graphene plane are obtained when the F−C bonds are in para positions to one another (Figure 2). Large ordered aggregates of this type have the composition of poly(tetracarbon fluoride) and exhibit a cohesive energy (relative to an isolated F−C species on graphene) of 0.37 eV. In the case of large aggregates with fluorine species on both sides of graphene, the energetically most favorable configuration is obtained when the F species are chemisorbed alternatively on the two sides of graphene in ortho position to one another (Figure 2). These aggregates of F−C bonds have the composition of poly(carbon fluoride) and a cohesive energy of 0.87 eV, and thus they are denser and more stable than the aggregates having the F species only on one side of the graphene layer (Figure 2). As the concentration of fluorine increases from dilute to concentrated, the bonding state of fluorine on graphene evolves from that of individual F−C species dispersed on the carbon plane to that of F−C bonds in orderly, packed agglomerates with F/C ratios up to 1, respectively. To gain insight on the nature of F−C chemical bond, we used our DFT scheme to calculate 1s core-level XPS binding energies of F and C atoms in different bonding states and chemical environments (Figures 26404
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of F−C bonds alternating on opposites sides of graphene in ortho position one to another gives rise to a core-level binding energy differing by only 0.4 eV from that of F species at graphene edges (Figure 3). This result shows that in this bonding state, the F−C bond exhibits a strong covalent character and a chemical nature markedly different from that of (FC)1 (in fact, F in these two bonding states gives rise to corelevel binding energies differing by about 3.5 eV). This type of covalent F−C bonds will be indicated as 2(FC)n, where the superscript “2” underlines that F chemisorption occurs on both sides of the carbon plane and the subscript “n” implies that the F−C bond belongs to large domains of fluorinated graphene with an F/C ratio close to 1. Another bonding state of fluorine and graphene is achieved when the F−C bonds form orderly, packed domains of fluorinated graphene with F species chemisorbed in para position one to another on one side of the carbon plane (Figure 3). In this bonding state, fluorine yields a core-level binding energy that lies midway between those associated with (FC)1 and F species at graphene edges and is separated from both of them by about 2 eV. This result shows that these types of F−C bonds exhibit a chemical nature that is intermediate between the semi-ionic and covalent characters of (FC)1 and 2(FC)n bonds, respectively. This latter bonding state for F on graphene will be referred to as 1(FC4)n, where the subscript “4” specifies that the local F/C ratio in the interior of these fluoro-graphene domains is about 0.25. (FC)1, 1(FC4)n, and 2(FC)n species form F−C bonds exhibiting distinct chemical characters, from semi-ionic to covalent. In particular, our calculations show that a (FC)1 species introduces minimal distortions in the graphene layer: the F−C bond is perpendicular to the carbon plane, its length is about 1.6 Å, and the C atom bonded to F is uplifted by about 0.35 Å with respect to the carbon plane. They also show that, in this chemical state, fluorine deprives the carbon layer of electronic charge, introducing a p-doping character in the modified graphene layer (Figure 5). These results corroborate the fact that (FC)1 bonds have an ionic nature and that these isolated species do not disrupt the sp2 bonding properties of the graphene layer. When (FC)1 species agglomerate, leading to domains of 1(FC)n, and 2(FC)n species, or more simply to the dimer shown in Figure 5, the degree of local distortions introduced by the nearest-neighbor F−C bonds increases considerably. These distortions allow the C atoms bonded to F to switch from an sp2 to an sp3 orbital configuration, thereby enhancing the covalent character of the F−C bonds (Figure 5). In this bonding state and as covalency increases, the F−C bonds shorten (Figures 3 and 5), and the band structure of the partially fluorinated graphene layer shows the opening of a band gap (Figure 5). To corroborate the computational study, we used our results to interpret recent experimental XPS spectra and gain insight into the structure of a partially fluorinated graphene system. In particular, we considered F 1s XPS spectra of fluorinated graphene obtained by a SF6 plasma treatment of epitaxial graphene films on SiC. The carbon films were grown on both the C-face and Si-face surface termination of 4H-SiC wafers,59 and they consisted of multilayer (C-face) and bilayer (Si-face) graphene films on SiC, respectively.59 The plasma treatment led to fluorination of the top one or two graphene layers of the films, and to F/C ratios of 0.05 and 0.1 in the case of films obtained from C-face and Si-face terminations of 4H-SiC, respectively10,38 (SI). As discussed elsewhere10,38 and in agreement with the behavior of fluorographene films obtained
Figure 5. (a) Electronic band structure of a periodic arrangement of (FC)1 species on graphene (red solid lines), superimposed to the band structure of the pristine carbon layer obtained by using the same periodicity in the direct and reciprocal spaces; the Fermi level is set at 0 eV. Top (left) and side (right) views of the periodic supercell used in these DFT calculations are shown above the band structure plot. (b) Same as (a) in the case of two fluorine species chemisorbed on graphene and forming a dimeric species. (c) Kohn−Sham energies (horizontal segments) of highest occupied moleculer orbitals (sketched with a light-blue contour line) of, from left to right, methyl groups in a planar (top) and distorted (bottom) geometry, fluoromethane molecules with the methyl group in the same planar (top) and distorted (bottom) configurations, and a fluorine atom.
some ionicity when, instead of being isolated, it is in proximity of a highly fluorinated region of graphene. In fact, a (FC)1 species in this latter chemical environment gives rise to a F 1s core-level binding energy 0.7 eV larger than that of an isolated (FC)1. Most remarkable, however, is the case of F−C bonds forming orderly, packed domains of fluorinated graphene. In particular, our calculations show that fluorine in agglomerates 26405
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Figure 6. F 1s XPS spectra of partially fluorinated graphene obtained from epitaxial graphene films grown on the C-face (left) and Si-face (right) of 4H-SiC wafers and presenting fluorination levels of 0.05 and 0.1, respectively. Open circles show raw experimental data to which a Shirley background has been subtracted. The spectra are fitted with three Gaussian peaks of equal width, attributed to (FC)1 (green solid line), 1(FC4)n (red solid line), and 2(FC)n (blue solid lines) species, respectively. The energy separations of the first and second peak and of the second and third peak are held at the constant values of 1.3 and 1.5 eV, respectively. In each panel, the inset shows a sketch of the fluorinated films comprising the three different bonding states for F and graphene; in these illustrations the SiC substrate is shown in light blue, the graphene layers are represented by black segments, and the (FC)1, 1(FC4)n, and 2(FC)n species are represented by using green vertical sticks on either side of graphene, thin red rectangles on either the top or down side of the top carbon layer, and blue thick rectangles across the carbon plane, respectively. The fractions of these fluorine species resulting from our fit are reported in the legend.
Figures 1 and 2, the majority of the fluorine species in the partially fluorinated graphene films form agglomerates of either the 1(FC4)n or 2(FC)n type (Figure 6). The latter types of domains exhibit high stability and F/C ratios of 1. The domains formed by 1(FC4)n species include F/C ratios of about 0.25 and F−C bonds exhibiting an intermediate semi-ionic and covalent character. These types of agglomerates can form, a priori, on either side of the top graphene layer. Figure 6 shows that the distribution of (FC)1, 1(FC4)n, and 2(FC)n species in fluorinated epitaxial graphene film varies considerably from film to film. We conjecture that these large structural differences might be related to either the difficulty for fluorine to penetrate the top graphene layer and intercalate in the multilayer structure or structural differences between the various graphene samples due to random defects generated during the epitaxial graphene growth.
by exposing graphene to XeF2,21 the partially fluorinated epitaxial graphene films exhibit a complex and variable structure, highly sensitive to factors such as the nature of the substrate, synthesis parameters, and postsynthesis treatments and ambient conditions.10,21,38 Nevertheless, all the F 1s XPS spectra of partially fluorinated epitaxial graphene share an intriguing property: they show a multicomponent structure extending over an energy interval as wide as 5−6 eV. In accord with our results, this suggests that the partially fluorinated epitaxial graphene films are composed of all three possible bonding states of fluorine and graphene, i.e., the (FC)1, 1 (FC4)n, and 2(FC)n species. Hence, we used a three-Gaussian fit and the core-level energy shifts in Figure 3 to estimate the fractions of (FC)1, 1(FC4)n, and 2(FC)n species in the films (Figure 6 and SI). The analysis of the XPS spectra led to the model structures and compositions of fluorinated epitaxial graphene films shown in Figure 6 (see also SI). These results show that, at fluorination levels of 5% or larger, fluorine is chemisorbed on graphene in the form of (FC)1, 1(FC4)n, or 2(FC)n species (fluorine species at graphene edges are assumed to be equivalent to 2(FC)n species, i.e., energetically very stable, and not contributing to variation of the electronic doping level of graphene or the dipole moment perpendicular to films). (FC)1 species can be chemisorbed on either side of the top graphene layer. These semi-ionic species deprive the carbon layer of electronic charge, introducing a certain degree of pdoping in the top graphene layer of the films. Furthermore, the (FC)1 and to same extent also the 1(FC4)n species are polar, and their number on either side of the top graphene layer determines the net dipole moment perpendicular to the film surface. In agreement with experiments, our analyses show that the films showing larger variation of the work function with respect to the pristine graphene films are those comprising larger fractions of (FC)1 and 1(FC4)n species. Furthermore, Figure 6 shows that, in agreement with the results reported in
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CONCLUSIONS In this study, we demonstrated that the ionicity (or covalency) of the fluorine−graphene bond varies considerably depending on local concentration and arrangement of the fluorine species. Three bonding states of fluorine on graphene are dominant. First, an isolated fluorine species on graphene forming a semiionic bond with a C atom in an sp2 orbital configuration; in this bonding state F acts as a p-dopant for graphene. Second, a fluorine species belonging to highly stable domains of poly(carbon fluoride), comprising covalent F−C bonds alternating in ortho position one to another on both sides of graphene. Third, fluorine species forming regions of poly(tetracarbon fluoride), with F−C bonds in para position one to another and exposed on only one side of graphene; these latter bonds exhibit a chemical character intermediate between the semi-ionic and the covalent. Furthermore, we showed that partially fluorinated epitaxial graphene on silicon carbide includes, in variable distributions, all three aforementioned bonding states of fluorine and graphene. These findings 26406
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combined with a future ability to control the bonding state or structure of fluorinated graphene could lead to a class of graphene-based materials with tunable properties for a variety of applications.
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ASSOCIATED CONTENT
S Supporting Information *
Technical details of calculations, and additional and complementary results. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
A.B. and D.W.H. conceived the main ideas and goal of the work. S.Z. carried out the DFT calculations. S.Z. and S.D.S. performed the analysis of the experimental spectra. A.B. wrote the article. All the authors contributed to the scientific development of the project. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support of the National Science Foundation (NSF) under MRSEC grant DMR-0820382. A.B. acknowledges also the support of NSF grants CMMI-1436375 and CHE-0946869. The authors sincerely thank Drs. Walt de Heer and Claire Berger for supplying the epitaxial graphene samples used in this work.
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