We demonstrate trifluoromethylation of graphene by copper-catalyzed free radical reaction. The covalent addition of CF3 to graphene, which changes the carbon atom hybridization from sp2 to sp3, and modifies graphene in a homogeneous and nondestructive manner, was verified with Raman spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy reveals that CF3 groups are grafted to the basal plane of graphene, with about 4 at. % CF3 coverage. After trifluoromethylation, the average resistance increases by nearly one order of magnitude, and an energy gap of about 98 meV appears. The noninvasive and mild reaction to synthesize trifluoromethylated graphene paves the way for graphene's applications in electronics and biomedical areas.
Graphene, a single layer of sp2-bonded carbon atoms arranged in a honeycomb pattern, has attracted enormous attention because of its fascinating properties. Due to its superior electronic properties and structure, graphene is now used in numerous areas, such as high-frequency circuits,1 transparent conductive films,2 energy storage,3 and sensors.4 Despite these prosperous applications, graphene needs modification of properties to enlarge its application field, such as bandgap opening for digital electronics. In order to rationally tailor graphene's properties and readily apply graphene in technologies, researchers have devoted considerable efforts to chemical functionalization of graphene. However, because of the chemical inertness and high kinetic barrier in transition from sp2 to sp3 carbon of graphene, new methods and techniques for covalent functionalization of graphene without side reactions or damages on carbon lattice are highly desired.
The trifluoromethyl group (CF3) is a very important moiety in medicinal chemistry5 and agrochemicals.6,7 Compounds after trifluoromethylation dramatically change their solubility, lipophilicity, and antioxidizability, thus have better membrane permeability, increased bioavailability, and increased metabolic stability.5,7 Therefore, implanting trifluoromethyl group into graphene basal plane may open up a gateway to graphene's applications in biotechnology. Moreover, since covalent modification of graphene such as hydrogenation,8 fluorination,9,10 and chlorination11–13 can tailor graphene's band structure and open up a band gap, it is expected that a band gap can be induced by CF3 groups covalently binding to the graphene lattice.
Here, we demonstrate for the first time that trifluoromethylation of graphene can be realized by using highly reactive trifluoromethyl free radicals to react with graphene. The covalent addition of CF3 to the large conjugated structure of graphene, which changes the carbon atom hybridization from sp2 to sp3, was verified with Raman spectroscopy (Raman), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Furthermore, the properties of trifluoromethylated graphene were studied using electronic measurements. The resistance increased by about one order of magnitude, and exhibited a temperature dependence that is consistent with the possible opening of an energy gap. Overall, we have developed an environmentally friendly and nontoxic reaction to realize the nondestructive and homogenous trifluoromethylation of graphene, which may help graphene fulfill its applications in electronics and biomedical areas.
The schematic diagram of trifluoromethylation of graphene is depicted in Figure 1. Togni electrophilic trifluoromethylating reagent 1-trifluoromethyl-1,2-benziodoxol-3-(1H)-one and CuCl were utilized to generate CF3 free radicals.14 A possible mechanism is that the iodine atom in Togni reagent gets an electron through a single-electron-transfer from CuCl, then the C-I bond breaks homolytically and generates a CF3 free radical. The highly reactive CF3 free radicals can attach to carbon atoms on graphene skeleton presumably through free radical addition reaction. Two kinds of graphene samples were used for study of trifluoromethylation: graphene flakes fabricated by mechanical exfoliation of Kish graphite and large-area graphene films grown on copper foils through chemical vapor deposition (CVD). Before reaction, residues on graphene samples were removed by thermal annealing at 370 °C for 2 h in a forming gas (50% H2/50% Ar). In a typical reaction, graphene samples were immersed in a solution of 30 mM Togni reagent and 6 mM CuCl in anhydrous methanol under the protection of nitrogen or argon gas. The reaction mixture was then kept at 60 °C with stirring or low-power ultrasound for 30 min. After reaction, samples were rinsed with methanol, chloroform, and ethanol to remove possible contamination, and then dried with nitrogen gas.
Figure 2(a) shows the evolution of Raman spectra for a single-layer graphene sample as a function of trifluoromethylation time. The intensity of the D peak at 1350 cm−1 for pristine graphene was negligible, suggesting the defect-free nature of graphene before reaction. As the reaction time prolonged, the disorder induced D peak prominently emerged. Meanwhile, the 2D peak around 2700 cm−1 gradually decreased. These observations indicate that a significant number of defects were created which were presumably due to the covalent grafting of CF3 onto graphene after reaction. In contrast, controlled experiments in which CuCl or Togni reagent was not present did not show similar trifluoromethylation phenomena (see Figure S1 of the supplementary material.15) Micro-Raman mapping was utilized to evaluate the uniformity of trifluoromethylation. For large-area graphene films grown by CVD (Figure 2(b), inset), the intensity map of I(D)/I(G) ratio for trifluoromethylated graphene (Figure 2(b)) shows uniformly distributed green color, indicating high homogeneity of trifluoromethylation within the spatial resolution of the Raman instrument (∼1 μm). The surface morphology of graphene before and after trifluoromethylation was studied by atomic force microscopy (AFM). The AFM image of trifluoromethylated graphene (see Figure S2 of the supplementary material15) reveals that graphene still maintained its basal plane after reaction, suggesting the noninvasive nature of trifluoromethylation (Figure 2(c)). We performed a statistical analysis of 105 positions on single-layer graphene samples before and after reaction and summarized their height distribution in Figure 2(d). The height of the trifluoromethylated single-layer graphene falls into a range of 1.3–2.0 nm, with a mean value of 1.6 nm. For pristine graphene, the height is in the range 0.7–1.1 nm, with a mean value of 0.87 nm. The increased height and broader distribution after trifluoromethylation presumably originate from the height of CF3 group and lattice distortion of graphene after bonding transformation from sp2 to sp3 hybridization.
To quantify the elemental composition and bonding type of the trifluoromethylated graphene, XPS was performed on graphene films grown by CVD before and after trifluoromethylation due to the large area required for data acquisition. High-resolution spectra for F 1s and C 1s are shown in Figures 3(a) and 3(b), respectively. The prominent F 1s peak at 689.0 eV, which significantly emerged after reaction, is assigned to the CF3 species on the trifluoromethylated graphene16 (Figure 3(a)). The C 1s peak of trifluoromethylated graphene can be fitted into three different peaks at 284.5, 286.0, and 291.9 eV. The peak at 291.9 eV is attributed to CF3 species. The physical adsorption of Togni reagent is excluded because of the absence of iodine element (see Figure S3 of the supplementary material15). The surface coverage of CF3 groups on graphene can be estimated by taking the ratio of the CF3 peak area in F 1s to the triple area of graphene's peak in C 1s, after taking the relevant atomic sensitivity factors for F 1s and C 1s into accounts. The coverage of CF3 group is around 4% for this sample.
Transport measurements were used to characterize the electrical properties of trifluoromethylated graphene. The graphene films grown on Cu foils by CVD were transferred onto silicon dioxide substrates using the “dry transfer” method17 to avoid unintentionally oxygen and water doping. After photolithography, graphene films were tailored into bars using reactive-ion etching process. Graphene field-effect transistors were then fabricated on Si/SiO2 using electron-beam lithography followed by metal deposition (5 nm Cr/ 45 nm Au). The doped silicon substrate (300 nm thick SiO2) was used as a gate electrode to change the density of charge carriers in the graphene layer. After trifluoromethylation, the graphene sheet became more resistive. Indeed, as the degree of trifluoromethylation increased, the room temperature resistance of graphenes at zero gate voltage increased from 0.9 kΩ (pristine graphene) to 7 kΩ (moderately trifluoromethylated graphene), and finally reached a value of 26.3 kΩ (highly trifluoromethylated graphene) (Figure 4(a), the corresponding Raman spectra which indicate the extent of modification are shown in Figure S4 of the supplementary material15). The carrier mobility decreased from ∼ 2100 cm2 V−1 s−1 before to ∼50 cm2 V−1 s−1 after trifluoromethylation (Figure 4(b)). The ambipolar behavior was preserved after reaction, and the Dirac point shifted positively from 10 V to 70 V, indicating that the reaction was accompanied by a p-type doping. Density functional theory (DFT) calculations indicate that the CF3 group attracts electrons from graphene with a charge transfer of 0.22 e, thus inducing a p-type doping of graphene (see the supplementary material for more details15). In addition, we statistically analyzed the sheet resistance of graphene devices before and after reaction at zero gate voltage. The average sheet resistance increased by nearly one order of magnitude after trifluoromethylation (see Figures 4(c) and 4(d)). The reduction of conductivity of the trifluoromethylated graphene presumably originates from the covalent bonding of CF3 which disrupts the highly delocalized electronic structure of graphene, reduces the charges in the conducting π orbitals, and introduces scattering centers.
To elucidate the transport mechanism of the trifluoromethylated graphene, variable-temperature electrical measurements in vacuum were carried out. Figure 5(a) displays the current Isd flowing through the trifluoromethylated graphene as a function of the applied source-drain voltage Vsd for various temperatures. The Isd − Vsd characteristics are linear within the range −0.1 V < Vsd < 0.1 V, allowing us to extract the resistance of the device as R = dVsd/dIsd. The resistance of trifluoromethylated graphene increases by two orders of magnitude upon cooling from 300 K down to 78 K. In addition, the natural logarithm of the resistance as a function of temperature behaves as ln(R) ∼ T−1/3 between 78 K and 200 K (Figure 5(b)). This suggests that the relevant transport mechanism in this temperature range is variable range hopping (VRH) of charge carriers between localized states which are separated in space but close in energy. Indeed, in the VRH model, the temperature dependence of the resistance follows a power law, and reads:
where T0 is constant, and d is the dimensionality of the material. This transport mechanism is expected in trifluoromethylated graphene, since the trifluoromethylation may be incomplete and graphene nano-islands are preserved. Charge carriers can hop between such isolated nano-islands, or percolate through weakly interconnected nano-islands, a behavior well captured by the VRH model.
Interestingly, above 200 K the resistance follows a thermally activated behavior described by R(T) = Aexp(−Δ/2kBT), where A is a constant, and Δ is an energy gap (Figure 5(c)). Extracted from the fit, this gap is about 98 meV. A similar behavior has been observed in fluorinated graphene18 and chlorinated graphene;11 there, the Δ was ascribed to a band gap originating from the chemical modification of graphene. Altogether, our transport data show that trifluoromethylation graphene behaves like a granular metal19 in which charge carriers hop between localized sites; the thermally activated behavior we observe above 200 K indicates that a gap ∼98 meV is present.
In summary, we have developed a nondestructive and homogenous trifluoromethylation method, for the first time to our knowledge, to covalently attach CF3 groups to graphene. The successful covalent attachment of CF3 was unequivocally demonstrated by Raman, AFM, and XPS. We estimate the coverage of CF3 to be nearly 4%. After trifluoromethylation, the resistance increases by one order of magnitude, and charge transport is dominated by charge hopping in combination with a thermally activated behavior across an energy gap. Further work will be dedicated to improving the regularity of CF3 grafting in atomic scale on the whole sample. Indeed, theoretical calculations show that CF3 groups prefer to uniformly adsorb on graphene, and that the most stable configuration is a superlattice that gives rise to a band gap as large as 1.2 eV. We expect that the method described here will bring graphene closer to applications in the fields of electronics and the biomedical areas, such as drug delivery.
We thank Joel Moser and Yufeng Nie for helpful discussion. This study was financially supported by the Ministry of Science and Technology of China (Grant Nos. 2013CB932603, 2012CB933404, and 2011CB933003), the National Natural Science Foundation of China (Grant Nos. 51290272 and 51121091), the Ministry of Education (20120001130010), and the International Postdoctoral Exchange Fellowship Program (Grant No. 20130002).
