www.MaterialsViews.com
Fluorographene
Dichlorocarbene-Functionalized Fluorographene: Synthesis and Reaction Mechanism Petr Lazar, Chun Kiang Chua, Kater˘ina Holá, Radek Zbor˘il, Michal Otyepka,* and Martin Pumera*
Halogen functionalization of graphene is an important branch of graphene research as it provides opportunities to tailor the band gap and catalytic properties of graphene. Monovalent C–X bond obviates pitfalls of functionalization with atoms of groups 13, 15, and 16, which can introduce various poorly defined groups. Here, the preparation of functionalized graphene containing both fluorine and chlorine atoms is shown. The starting material, fluorographite, undergoes a reaction with dichlorocarbene to provide dichlorocarbene-functionalized fluorographene (DCC-FG). The material is characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, and highresolution transmission electron microscopy with X-ray dispersive spectroscopy. It is found that the chlorine atoms in DCC-FG are distributed homogeneously over the entire area of the fluorographene sheet. Further density functional theory calculations show that the mechanism of dichlorocarbene attack on fluorographene sheet is a two-step process. Dichlorocarbene detaches fluorine atoms from fluorographene sheet and subsequently adds to the newly formed sp2 carbons. Halogenated graphene consisting of two (or eventually three) types of halogen atoms is envisioned to find its way as new graphene materials with tailored properties.
1. Introduction Graphene exhibits a wide range of interesting properties, which shall find applications in nanoelectronics, nanomedicine, catalysis, electrochemistry, to name a few.[1–5] However, many applications require the premodification of graphene in Dr. P. Lazar, K. Holá, Prof. R. Zbor˘il, Prof. M. Otyepka Regional Centre of Advanced Technologies and Materials Faculty of Science Department of Physical Chemistry Palack´y University Olomouc 77146, Czech Republic E-mail: [email protected] Dr. C. K. Chua, Prof. M. Pumera Division of Chemistry & Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University 21 Nanyang Link, Singapore 637371, Singapore E-mail: [email protected] DOI: 10.1002/smll.201500364 small 2015, DOI: 10.1002/smll.201500364
order to manipulate its band gap or catalytic activity. This can be achieved by incorporating graphene with heteroatoms, i.e., nitrogen, boron, sulfur, and halogens, or hydrogen.[6–9] While the modification of graphene with polyvalent elements, such as N, P, S, or B is popular, it is highly challenging to control their final chemical states.[6–10] On the other hand, the addition of monovalent atoms, such as halogens (F, Cl, Br, or I), offers an excellent way to control the manipulations of band gap and electrocatalytic properties of graphene.[11–13] It has been shown that with increasing fluorination of the sp2 carbon sheets of graphene, the heterogeneous electron transfer (HET) rates of graphene increases;[14,15] similarly, the HET rate of graphene increases when functionalized with more electronegative elements from group 17 (χCl > χBr > χI).[13] Given the unique properties conferred by the halogens, it is of huge importance to prepare graphene which is concurrently modified with more than one type of halogen to further tune its electronic and electrochemical properties.[16] Herein, we show a facile preparation route for dichlorocarbene-functionalized fluorographene (DCC-FG) (C2FxCl1−x−δ)n. In order to ensure a high coverage of halogen,
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
1
full papers www.MaterialsViews.com
we prepared DCC-FG via the addition of dichlorocarbene onto fluorographene, obtained from the subsequent exfoliation of fluorographite. This ensures a high halogen coverage as the reaction mechanism is based on the cycloaddition of reactive carbene species. The resulting DCC-FG was characterized in detail by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). We have further elucidated the mechanism of dichlorocarbene interaction with pristine graphene, partially fluorinated graphene (C4F), and fluorographene (CF) using density functional theory (DFT) calculations.
2. Results and Discussion 2.1. Synthesis and Properties DCC-FG was prepared by ultrasonicating fluorographite powder in a mixture of CHCl3 and NaOH to yield few-layer fluorographene[17] and consequently refluxing in the same mixture with phase transfer catalyst according to previously reported procedure.[18] The starting material and product of the reaction were characterized by XPS. XPS survey spectra (Figure 1, top panels) showed the fluorographite starting material containing a composition of 66.47 at% of C, 29.41 at% of F, and 4.12 at% of O, which effectively resulted in the formula of (C2F1−δ)n, where δ stands for the contribution of other elements, such as O, present on the surface of fluorographite (δ = 0.12). The reaction of dichlorocarbene with fluorographene, which was obtained upon ultrasonication treatment of fluorographite, resulted in DCC-FG with a composition of 67.09 at% of C, 11.52 at% of F, 7.62 at% of Cl, and
13.77 at% of O, which provided a formula of (C2F1−x−δClx)n, where x = 0.23 and δ = 0.41. The δ represents oxygen functionalities created on the surface of DCC-FG during synthesis. A control study indicated that the ultrasonication treatment of fluorographite did not result in significant oxidation and defluorination effects. High-resolution XPS (HR-XPS) of C1s showed profoundly different spectra for fluorographite and DCC-FG (Figure 1). HR-XPS C1s of fluorographite showed a weak peak at C C/C C bond (≈284.5–285.5 eV) and strong peaks corresponding to covalent C F, C F2, and C F3 bonds (288–293 eV).[14,19] The C1s peak of DCC-FG showed a different signature whereby significantly less intense peaks of C Fx bonds and new CCl2 were observed. Both the C1s peak of fluorographite and DCC-FG were also fitted with oxygen functionalities (C O, C O, and O C O bonds). The presences of these oxygen functional groups were reflected in the HR-XPS O1s (Figure S1, Supporting Information). Overall, the amount of oxygen functionalities was low in fluorographite and increased up to threefold upon functionalization. Consequently, the DCC-FG was further exfoliated by ultrasonication in N,N-dimethylformamide (DMF)[11] (0.5 mg of dry DCC-FG was mixed with 1 mL of DMF and ultrasonicated in a bath for 2 h). The obtained dispersion was further characterized by high-resolution transmission electron microscopy/energy dispersive X-ray spectroscopy (HR-TEM/EDX) and Raman spectroscopy. The Raman spectrum of DCC-FG (Figure 2, black line) clearly showed a characteristic vibration in the region of 730 cm−1 corresponding to C–Cl2 antisymmetric stretching. The presence of D band (1339 cm−1) and G band (1586 cm−1) as well as the shape and intensity of 2D peak (2682 cm−1) indicated the presence of few-layer defective graphene flakes in the
Figure 1. X-ray photoelectron spectra of A,C) fluorographite and B,D) DCC-FG. Top panel: survey spectra; bottom panel: high-resolution scans of C1s orbital.
2 www.small-journal.com
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201500364
www.MaterialsViews.com
Figure 2. The Raman spectrum of DCC-FG (top line) and fluorographite precursor (bottom line).
sample. In comparison, the fluorographite precursor was not Raman active (Figure 2, red line). This phenomenon is closely related to the optical transparency of highly fluorinated material to the laser applied (780 nm in our case).[20] The chemical mapping of element composition by HR-TEM/
EDX was performed to visualize the distribution of halogen substituents on the graphene sheet. An overlay of fluorine and chlorine maps showed the presence of small islands containing either fluorine or chlorine atoms, which do not overlap (Figure 3). This suggested that the dichlorocarbene replaced fluorine atoms on fluorographene and that the functionalization was rather homogeneous. The functionalization also altered the hydrophilicity and solvent dispersibility of the sample dramatically. Fluorinated graphene/graphite usually disperse well in DMF but not in water.[21,22] However, the functionalization of the fluorographene resulted in an opposite trend (Figure S2, Supporting Information). The DCC-FG was well dispersed in water but not in DMF. The hydrophilicity of DCC-FG was largely due to the presence of oxygen-containing groups and possibly rendered biocompatible. Due to the fact that dichlorocarbene prefers to add to sp2-hybridized carbon network, which is naturally lacking in fluorographene, we turned our attention to the mechanism of dichlorocarbene reaction with fluorographene.
2.2. Mechanism of Dichlorocarbene Functionalization of Graphene The mechanism of the dichlorocarbene addition on sp2 network was investigated theoretically using DFT calculations.[23] We
Figure 3. The microscopic mapping of elemental composition of the prepared DCC-FG obtained by HR-TEM/EDX: a) overlaid elemental maps of fluorine (green) and chlorine (red) on the sample; b) elemental map of carbon (blue); c) overlaid elemental maps of fluorine (green), chlorine (red), and carbon (blue) on the sample; d) HR-TEM image of DCC-FG. small 2015, DOI: 10.1002/smll.201500364
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
3
full papers www.MaterialsViews.com −1.
of 7.6 kcal mol It should be noted that the chemisorbed state is separated by a rather high barrier (31.8 kcal mol−1) indicating that the chemisorbed :CCl2 groups are kinetically trapped to graphene. Although the geometry of :CCl2 bonded in the global minimum at 1.3 Å (Figure 4) is very similar to that of BH2 adsorbed on graphene,[6] the local deformation of graphene is much stronger, indicating relative strength of CCl2–graphene interaction. On the other hand, :CCl2 does not alter the electronic properties of graphene. We calculated the density of states for :CCl2 bonded on graphene, and no shift of the Fermi level occurs. We also calculated respective workfunction, and its value of 4.44 eV is close to that of pristine graphene (4.2 eV) calculated with the same setup.
Figure 4. The potential energy surface of the :CCl2 molecule for the adsorption on the bond position on graphene. The distance between the carbon atom of the molecule and its two neighboring atoms carbon atoms of graphene was fixed, while all other atoms were allowed to relax (solid curve with diamonds). The solid curve with crosses shows the energies without the relaxation, and dashed line the energy gained by the relaxation. Inset figures show the geometry of a) physisorbed :CCl2 and b) bonded :CCl2.
first inspected the thermodynamic stability of :CCl2 on pristine graphene. The molecule adsorbs preferentially onto the bond position with adsorption energy of −13.8 kcal mol−1. The adsorption energy in the hollow position is −5.0 kcal mol−1 and the top site is the least probable with just −3.0 kcal mol−1. In the bond position, :CCl2 pulls two neighboring carbon atoms by 0.52 Å above the graphene sheet and the length of the C C bond is 1.55 Å, indicating the formation of a covalent bond. In order to elucidate the adsorption properties of :CCl2 in detail, we calculated the potential energy surface as a function of the distance of the molecule from graphene. The distance between the carbon atom of the molecule and its two neighboring carbon atoms of graphene was constrained, while all the other atoms were allowed to relax. The potential energy surface (Figure 4) shows rather peculiar energetics of the :CCl2 adsorption. The molecule adsorbs first at the distance of 3.1 Å with the adsorption energy of −6.2 kcal mol−1. This minimum corresponds to a physisorption, and is caused by nonlocal van der Waals interaction, because test calculation with the Perdew–Burke–Ernzerhof (PBE)[24] functional (which does not cover van der Waals interaction) yields no attraction in this region. The geometrical changes of the graphene upon physisorption of :CCl2 are negligible, as expected. Nevertheless, the global minimum on the potential energy surface lies at the distance of 1.3 Å and corresponds to :CCl2 chemisorption, although the adsorption energy in this constrained calculation is only −6.5 kcal mol−1. Releasing the constrain and letting the system to fully relax, the adsorption energy becomes −13.8 kcal mol−1, as mentioned in the preceding paragraph. This energy consists of favorable binding of :CCl2 to graphene, which is counterbalanced by deformation energy of graphene of 45.9 kcal mol−1 (Figure 4). The high deformation penalty causes the adsorption energies for physisorption and chemisorption to differ just by the factor
4 www.small-journal.com
2.3. Mechanism of Dichlorocarbene Functionalization of Fluorographene (C1F1)n The reaction of the :CCl2 molecule with fluorographene (C1F1) is illustrated in Figure 5. The interaction of :CCl2 with C1F1 is weak due to the strong repulsion between the Cl atoms of the carbene and the F atoms of the C1F1. The adsorption can be characterized as a physisorption with the interaction energy of −8.2 kcal mol−1. The molecule remains at 2.8 Å above fluorographene (measured as the distance between carbon atom of :CCl2 and the nearest fluorine atom of CF). In contrast to the barrier-less defluorination observed for C4F (see below), the detachment of one of fluorine atoms is hampered by an energy barrier of 33.1 kcal mol−1 on fluorographene. The detachment may be facilitated in the vicinity of defect site (holes, missing fluorine atoms), which might be present in CF. The resulting CCl2F molecule stays physisorbed to the surface of CF and the energy of this configuration is −14.8 kcal mol−1. The CCl2F molecule can easily desorb from the CF surface, as the desorption energy is only 4.2 kcal mol−1. On the other hand, when it manages to cut additional fluorine atom from CF, the complex of the CCl2F2 molecule physisorbed to CF can be formed via a barrier of 28 kcal mol−1. Such complex has the energy of −17.2 kcal mol−1 with respect to initial reactants, and represents therefore the global minimum of this process.
2.4. Mechanism of Dichlorocarbene Functionalization of Fluorographene (C4F1)n The adsorption on partially fluorinated graphene C4F is different from that on CF. The repulsion between the Cl and F atoms is reduced (compared to the case of CF), allowing stronger interaction between the molecule and C4F support. The molecule detaches one of fluorine atoms, creating the CCl2F molecule physisorbed to the C4F support (Figure 6). This process happens as a direct result of the force relaxation, thus there is no significant energy barrier blocking the detachment of the fluorine atom. The energy of this configuration with respect to the initial reactants is −43.9 kcal mol−1, much lower than that of the analogical configuration on C1F1
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201500364
www.MaterialsViews.com
Figure 5. The two-step mechanism of the reaction of :CCl2 with fluorographene. :CCl2 detaches the fluorine atoms from the surface of CF, resulting in physisorbed CCl2F2 molecule. Carbon atoms are colored in gray, fluorine atoms in yellow, and chlorine in green.
(−14.8 kcal mol−1). The binding energy of the CCl2F molecule to the C4F support is 14.2 kcal mol−1. The reaction can continue, i.e., the CCl2F molecule may cut an additional fluorine atom from the surface of fluorinated graphene, which results in the CCl2F2 molecule physisorbed to defluorinated support. The energy of the final state (C32F6 support + CCl2F2) is very favorable with respect to the initial state (C32F8 support + :CCl2) and amounts to −109.8 kcal mol−1. We also inspected the adsorption into other positions on C4F, in which CCl2 is bound directly to one of carbon atoms. The adsorption into the hollow position is exothermic, and the adsorption energy is −8.5 kcal mol−1. The adsorption energy is positive for both nonequivalent on bond positions (carbon atoms are not equivalent in C4F, because one quarter of them bears the fluorine atom; this creates two nonequivalent on bond positions), thus CCl2 does not bind above the carbon–carbon bond in contrast to the adsorption on pristine graphene. The reason for this behavior lies in the repulsion between the Cl atoms of CCl2 and surrounding fluorine atoms on C4F. If one fluorine atom is removed (by the reaction with other CCl2 molecule, for instance), the adsorption into defluorinated region of C4F (or strictly speaking, of C32F7) becomes much more favorable—the adsorption energy on
the C C bond is −21.2 kcal mol−1, making the adsorption onto the C C bond of partially fluorinated graphene more favorable than onto the pure graphene (−13.8 kcal mol−1). The reaction with fluorinated graphene depends on the concentration of fluorine atoms. The fluorographene reacts only weakly with the :CCl2 molecule (the adsorption energy is −8.2 kcal mol−1) and the removal of the fluorine atom by the CCl2 molecule is blocked by the energy barrier of 33.1 kcal mol−1. This is caused by the high energy needed to remove the F atom from the surface of CF.[25] The situation changes when the graphene is only partially fluorinated. On C4F, the CCl2 molecule can detach the F atom without the need to overcome any energy barrier. This process creates energetically favorable (−43.9 kcal mol−1) configuration in which the CCl2F molecule is weakly bound to the C4F support, and can be followed by attachment of further fluorine atom to the CCl2F molecule, creating very stable complex of fluorinated graphene and the CCl2F2 molecule (−109.8 kcal mol−1). The adsorption of another :CCl2 into the region of C4F, from which the F atom was removed, becomes more favorable—the adsorption energy on the C C bond is −21.2 kcal mol−1, lower than the analogical adsorption energy on pure graphene (−13.8 kcal mol−1). In summary, the reaction of :CCl2 with nonstoichiometric CxFy is a two-step process; in the first step the CCl2 detaches the fluorine atoms from the support and in the next step CCl2 chemisorbs to the sp2 carbon atoms.
3. Conclusions
Figure 6. The CCl2F molecule after detaching one of fluorine atoms from the partially fluorinated graphene (C4F). The energy of this configuration is −43.9 kcal mol−1 with respect to noninteracting C4F and :CCl2. small 2015, DOI: 10.1002/smll.201500364
We prepared dichlorocarbene-functionalized fluorographene by the reaction of few-layer fluorographene with dichlorocarbene in chloroform. The material was homogenously functionalized across the flake as observed from high-resolution TEM/EDX mapping. The DCC-FG shows completely different dispersibility in DMF and water than the pristine
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
5
full papers www.MaterialsViews.com
fluorographite. DFT calculations showed that reaction of dichlorocarbene with fluorographene is a two-step process. In the first step, fluorographene is defluorinated and in the latter step, the dichlorocarbene adds to the formed sp2 carbons. The DCC-FG represents a new member of the graphene family, which was synthetized from fluorographite. This indicates that fluorographite is a viable pristine material for the synthesis of other graphene derivatives. We envision that the modification of graphene with two or more halogen atoms may lead to well-defined tuning of the properties of graphene derivatives.
4. Experimental Section Materials: Fluorographite was obtained from Alfa Aesar. Chloroform and sodium hydroxide were obtained from Sigma Aldrich, Singapore. Triethylbenzylammonium chloride was obtained from Alfa Aesar, Singapore. Milli-Q water (resistivity: 18.2 MΩ cm) was used throughout the experiments. Methods: Synthesis of Dichlorocarbene-Functionalized Fluorographene: Fluorographite (10 mg) and triethylbenzylammonium chloride (phase transfer catalyst, PTC, 10 mg) were mixed in water (7.5 mL) and chloroform (10 mL). The mixture was ultrasonicated (180 W) for 1.5 h before the addition of NaOH (10 g) and chloroform (10 mL). After vigorous stirring for 24 h, PTC (10 mg), NaOH (10 g in 7.5 mL water), and chloroform (10 mL) were added into the mixture and left to stir for 24 h at 80 °C. The mixture was brought to room temperature and further diluted with water (250 mL). The filtered solid was washed with a copious amount of acetone and ethanol. This was followed by subsequent rinsing with hexane, chloroform, ethanol, water, ethanol, water, and diethyl ether. The obtained powder was left to dry at 70 °C in the oven for 5 d to provide dichlorocarbene-functionalized fluorographene. Characterization: XPS data were acquired using Phoibos 100 spectrometer and an Mg X-ray radiation source (SPECS, Germany). Relative sensitivity factors were used for the evaluation of atomic percentages from the XPS survey spectra. XPS samples were prepared by coating a carbon tape with a uniform layer of the material under study. Raman spectroscopy was performed using DXR Raman microscope with 780 nm excitation line of a diode laser. The HR-TEM image and the STEM-HAADF (high-angle annular darkfield imaging) images were obtained using a FEI Titan electron microscope operating at 80 kV. DFT calculations of functionalized graphene were performed using the projector-augmented wave method in the Vienna ab initio simulation package (VASP) suite.[26,27] The energy cutoff for the plane-wave expansion was set to 400 eV. The graphene sheet was modeled using a 4 × 4 supercell (32 carbon atoms) with a calculated C C bond length of 1.44 Å. Additionally, we used two models of fluorinated graphene: (1) fluorographene (CF) modeled by the 4 × 4 supercell (32 carbon and 32 fluorine atoms) and (2) C4F (32 carbon and 8 fluorine atoms), which was identified as thermodynamically possible and dynamically stable structure of partially fluorinated graphene. The periodically repeated sheets were separated by 18 Å of vacuum in all cases. Adsorption of :CCl2 was modeled by placing the molecule into one high symmetry positions, (a) above one of carbon (fluorine) atoms, (b) above the bond connecting two neighboring carbon
6 www.small-journal.com
atoms, or (c) into the hollow positions in the middle of benzene ring. The atomic forces acting on the molecule were then fully relaxed using conjugate gradient algorithm. Thermodynamic stability of various reaction products was calculated by comparing the total energy of respective product with respect to the sum of the total energies of initial reactants. We used the optB86b-vdW functional[28] in following calculations of reaction and adsorption properties. This functional yielded the adsorption energies of organic molecules on graphene in an excellent agreement with experiment.[29]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors gratefully acknowledge the support by the project LO1305 of the Ministry of Education, Youth and Sports of the Czech Republic. M.O. acknowledges support from the Grant Agency of the Czech Republic (P208/12/G016). K.H. would like to gratefully acknowledge the Operational Program Education for Competitiveness – European Social Fund (CZ.1.07/2.3.00/20.0155 of the Ministry of Education, Youth and Sports of the Czech Republic). M.P. acknowledges Nanyang Technological University and Singapore Ministry of Education Academic Research Fund AcRFTier 1 (2013T1-001-014, RGT1/13) for the funding support. The authors also acknowledge Dr. Cˇépe and Ing. Tomanec (RCPTM) for HR-TEM measurements.
[1] M. J. Allen, V. C. Tung, R. B. Kaner, Chem. Rev. 2010, 110, 132. [2] K. S. Novoselov, V. I. Fal’ko, L. Colombo, P. R. Gellert, M. G. Schwab, K. Kim, Nature 2012, 490, 192. [3] V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza, R. Zboril, K. S. Kim, Chem. Rev. 2012, 112, 6156. [4] A. Ambrosi, C. K. Chua, A. Bonanni, M. Pumera, Chem. Rev. 2014, 114, 7150. [5] Q. Tang, Z. Zhou, Z. Chen, Nanoscale 2013, 5, 4541. [6] X. Wang, G. Sun, P. Routh, D. H. Kim, W. Huang, P. Chen, Chem. Soc. Rev. 2014, 43, 7067. [7] M. Pumera, J. Mater. Chem. C 2014, 2, 6454. [8] Y. Li, Z. Zhou, P. Shen, Z. Chen, ACS Nano 2009, 3, 1952. [9] Y. Li, Z. Zhou, P. Shen, Z. Chen, J. Phys. Chem. C 2009, 113, 15043. [10] P. Lazar, R. Zbor˘il, M. Pumera, M. Otyepka, Phys. Chem. Chem. Phys. 2014, 16, 14231. [11] R. Zbor˘il, F. Karlický, A. B. Bourlinos, T. A. Steriotis, A. K. Stubos, V. Georgakilas, V. Šafár˘ová, D. Janc˘ík, C. Trapalis, M. Otyepka, Small 2010, 6, 2885. [12] F. Karlický, K. K. R. Datta, M. Otyepka, R. Zbor˘il, ACS Nano 2013, 7, 6434. [13] H. L. Poh, P. Šimek, Z. Šofer, M. Pumera, Chem. Eur. J. 2013, 19, 2655.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201500364
www.MaterialsViews.com [14] X. Chia, A. Ambrosi, M. Otyepka, R. Zbor˘il, M. Pumera, Chem. Eur. J. 2014, 20, 6665. [15] S. Boopathi, T. N. Narayanan, S. S. Kumar, Nanoscale 2014, 6, 10140. [16] F. Karlický, R. Zbor˘il, M. Otyepka, J. Chem. Phys. 2012, 137, 034709. [17] M. Zhu, X. Xie, Y. Guo, P. Chen, X. Ou, G. Yu, M. Liu, Phys. Chem. Chem. Phys. 2013, 15, 20992. [18] C. K. Chua, A. Ambrosi, M. Pumera, Chem. Commun. 2012, 48, 5376. [19] H. Touhara, Y. Goto, N. Watanabe, K. Imaeda, T. Enoki, H. Inokuchi, Y. Mizutani, Synth. Met. 1988, 23, 461. [20] R. R. Nair, W. Ren, R. Jalil, I. Riaz, V. G. Kravets, L. Britnell, P. Blake, F. Schedin, A. S. Mayorov, S. Yuan, M. I. Katsnelson, H.-M. Cheng, W. Strupinski, L. G. Bulusheva, A. V Okotrub, I. V Grigorieva, A. N. Grigorenko, K. S. Novoselov, A. K. Geim, Small 2010, 6, 2877. [21] Y. Hernandez, M. Lotya, D. Rickard, S. D. Bergin, J. N. Coleman, Langmuir 2009, 26, 3208.
small 2015, DOI: 10.1002/smll.201500364
[22] M. Zhu, X. Xie, Y. Guo, P. Chen, X. Ou, G. Yu, M. Liu, Phys. Chem. Chem. Phys. 2013, 15, 20992. [23] E. R. Margine, M. L. Bocquet, X. Blase, Nano Lett. 2008, 8, 3315. [24] P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865. [25] M. Dubecký, E. Otyepková, P. Lazar, F. Karlický, M. Petr, K. C˘épe, P. Banáš, R. Zbor˘il, M. Otyepka, J. Phys. Chem. Lett. 2015, 6, 1413. [26] P. E. Blöchl, Phys. Rev. B 1994, 50, 17953. [27] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758. [28] J. Klimeš, D. R. Bowler, A. Michaelides, Phys. Rev. B 2011, 83, 195131. [29] P. Lazar, F. Karlický, P. Jurec˘ka, M. Kocman, E. Otyepková, K. Šafár˘ová, M. Otyepka, J. Am. Chem. Soc. 2013, 135, 6372.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: February 5, 2015 Revised: March 15, 2015 Published online:
www.small-journal.com
7
Fluorographene
Dichlorocarbene-Functionalized Fluorographene: Synthesis and Reaction Mechanism Petr Lazar, Chun Kiang Chua, Kater˘ina Holá, Radek Zbor˘il, Michal Otyepka,* and Martin Pumera*
Halogen functionalization of graphene is an important branch of graphene research as it provides opportunities to tailor the band gap and catalytic properties of graphene. Monovalent C–X bond obviates pitfalls of functionalization with atoms of groups 13, 15, and 16, which can introduce various poorly defined groups. Here, the preparation of functionalized graphene containing both fluorine and chlorine atoms is shown. The starting material, fluorographite, undergoes a reaction with dichlorocarbene to provide dichlorocarbene-functionalized fluorographene (DCC-FG). The material is characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, and highresolution transmission electron microscopy with X-ray dispersive spectroscopy. It is found that the chlorine atoms in DCC-FG are distributed homogeneously over the entire area of the fluorographene sheet. Further density functional theory calculations show that the mechanism of dichlorocarbene attack on fluorographene sheet is a two-step process. Dichlorocarbene detaches fluorine atoms from fluorographene sheet and subsequently adds to the newly formed sp2 carbons. Halogenated graphene consisting of two (or eventually three) types of halogen atoms is envisioned to find its way as new graphene materials with tailored properties.
1. Introduction Graphene exhibits a wide range of interesting properties, which shall find applications in nanoelectronics, nanomedicine, catalysis, electrochemistry, to name a few.[1–5] However, many applications require the premodification of graphene in Dr. P. Lazar, K. Holá, Prof. R. Zbor˘il, Prof. M. Otyepka Regional Centre of Advanced Technologies and Materials Faculty of Science Department of Physical Chemistry Palack´y University Olomouc 77146, Czech Republic E-mail: [email protected] Dr. C. K. Chua, Prof. M. Pumera Division of Chemistry & Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University 21 Nanyang Link, Singapore 637371, Singapore E-mail: [email protected] DOI: 10.1002/smll.201500364 small 2015, DOI: 10.1002/smll.201500364
order to manipulate its band gap or catalytic activity. This can be achieved by incorporating graphene with heteroatoms, i.e., nitrogen, boron, sulfur, and halogens, or hydrogen.[6–9] While the modification of graphene with polyvalent elements, such as N, P, S, or B is popular, it is highly challenging to control their final chemical states.[6–10] On the other hand, the addition of monovalent atoms, such as halogens (F, Cl, Br, or I), offers an excellent way to control the manipulations of band gap and electrocatalytic properties of graphene.[11–13] It has been shown that with increasing fluorination of the sp2 carbon sheets of graphene, the heterogeneous electron transfer (HET) rates of graphene increases;[14,15] similarly, the HET rate of graphene increases when functionalized with more electronegative elements from group 17 (χCl > χBr > χI).[13] Given the unique properties conferred by the halogens, it is of huge importance to prepare graphene which is concurrently modified with more than one type of halogen to further tune its electronic and electrochemical properties.[16] Herein, we show a facile preparation route for dichlorocarbene-functionalized fluorographene (DCC-FG) (C2FxCl1−x−δ)n. In order to ensure a high coverage of halogen,
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
1
full papers www.MaterialsViews.com
we prepared DCC-FG via the addition of dichlorocarbene onto fluorographene, obtained from the subsequent exfoliation of fluorographite. This ensures a high halogen coverage as the reaction mechanism is based on the cycloaddition of reactive carbene species. The resulting DCC-FG was characterized in detail by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). We have further elucidated the mechanism of dichlorocarbene interaction with pristine graphene, partially fluorinated graphene (C4F), and fluorographene (CF) using density functional theory (DFT) calculations.
2. Results and Discussion 2.1. Synthesis and Properties DCC-FG was prepared by ultrasonicating fluorographite powder in a mixture of CHCl3 and NaOH to yield few-layer fluorographene[17] and consequently refluxing in the same mixture with phase transfer catalyst according to previously reported procedure.[18] The starting material and product of the reaction were characterized by XPS. XPS survey spectra (Figure 1, top panels) showed the fluorographite starting material containing a composition of 66.47 at% of C, 29.41 at% of F, and 4.12 at% of O, which effectively resulted in the formula of (C2F1−δ)n, where δ stands for the contribution of other elements, such as O, present on the surface of fluorographite (δ = 0.12). The reaction of dichlorocarbene with fluorographene, which was obtained upon ultrasonication treatment of fluorographite, resulted in DCC-FG with a composition of 67.09 at% of C, 11.52 at% of F, 7.62 at% of Cl, and
13.77 at% of O, which provided a formula of (C2F1−x−δClx)n, where x = 0.23 and δ = 0.41. The δ represents oxygen functionalities created on the surface of DCC-FG during synthesis. A control study indicated that the ultrasonication treatment of fluorographite did not result in significant oxidation and defluorination effects. High-resolution XPS (HR-XPS) of C1s showed profoundly different spectra for fluorographite and DCC-FG (Figure 1). HR-XPS C1s of fluorographite showed a weak peak at C C/C C bond (≈284.5–285.5 eV) and strong peaks corresponding to covalent C F, C F2, and C F3 bonds (288–293 eV).[14,19] The C1s peak of DCC-FG showed a different signature whereby significantly less intense peaks of C Fx bonds and new CCl2 were observed. Both the C1s peak of fluorographite and DCC-FG were also fitted with oxygen functionalities (C O, C O, and O C O bonds). The presences of these oxygen functional groups were reflected in the HR-XPS O1s (Figure S1, Supporting Information). Overall, the amount of oxygen functionalities was low in fluorographite and increased up to threefold upon functionalization. Consequently, the DCC-FG was further exfoliated by ultrasonication in N,N-dimethylformamide (DMF)[11] (0.5 mg of dry DCC-FG was mixed with 1 mL of DMF and ultrasonicated in a bath for 2 h). The obtained dispersion was further characterized by high-resolution transmission electron microscopy/energy dispersive X-ray spectroscopy (HR-TEM/EDX) and Raman spectroscopy. The Raman spectrum of DCC-FG (Figure 2, black line) clearly showed a characteristic vibration in the region of 730 cm−1 corresponding to C–Cl2 antisymmetric stretching. The presence of D band (1339 cm−1) and G band (1586 cm−1) as well as the shape and intensity of 2D peak (2682 cm−1) indicated the presence of few-layer defective graphene flakes in the
Figure 1. X-ray photoelectron spectra of A,C) fluorographite and B,D) DCC-FG. Top panel: survey spectra; bottom panel: high-resolution scans of C1s orbital.
2 www.small-journal.com
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201500364
www.MaterialsViews.com
Figure 2. The Raman spectrum of DCC-FG (top line) and fluorographite precursor (bottom line).
sample. In comparison, the fluorographite precursor was not Raman active (Figure 2, red line). This phenomenon is closely related to the optical transparency of highly fluorinated material to the laser applied (780 nm in our case).[20] The chemical mapping of element composition by HR-TEM/
EDX was performed to visualize the distribution of halogen substituents on the graphene sheet. An overlay of fluorine and chlorine maps showed the presence of small islands containing either fluorine or chlorine atoms, which do not overlap (Figure 3). This suggested that the dichlorocarbene replaced fluorine atoms on fluorographene and that the functionalization was rather homogeneous. The functionalization also altered the hydrophilicity and solvent dispersibility of the sample dramatically. Fluorinated graphene/graphite usually disperse well in DMF but not in water.[21,22] However, the functionalization of the fluorographene resulted in an opposite trend (Figure S2, Supporting Information). The DCC-FG was well dispersed in water but not in DMF. The hydrophilicity of DCC-FG was largely due to the presence of oxygen-containing groups and possibly rendered biocompatible. Due to the fact that dichlorocarbene prefers to add to sp2-hybridized carbon network, which is naturally lacking in fluorographene, we turned our attention to the mechanism of dichlorocarbene reaction with fluorographene.
2.2. Mechanism of Dichlorocarbene Functionalization of Graphene The mechanism of the dichlorocarbene addition on sp2 network was investigated theoretically using DFT calculations.[23] We
Figure 3. The microscopic mapping of elemental composition of the prepared DCC-FG obtained by HR-TEM/EDX: a) overlaid elemental maps of fluorine (green) and chlorine (red) on the sample; b) elemental map of carbon (blue); c) overlaid elemental maps of fluorine (green), chlorine (red), and carbon (blue) on the sample; d) HR-TEM image of DCC-FG. small 2015, DOI: 10.1002/smll.201500364
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
3
full papers www.MaterialsViews.com −1.
of 7.6 kcal mol It should be noted that the chemisorbed state is separated by a rather high barrier (31.8 kcal mol−1) indicating that the chemisorbed :CCl2 groups are kinetically trapped to graphene. Although the geometry of :CCl2 bonded in the global minimum at 1.3 Å (Figure 4) is very similar to that of BH2 adsorbed on graphene,[6] the local deformation of graphene is much stronger, indicating relative strength of CCl2–graphene interaction. On the other hand, :CCl2 does not alter the electronic properties of graphene. We calculated the density of states for :CCl2 bonded on graphene, and no shift of the Fermi level occurs. We also calculated respective workfunction, and its value of 4.44 eV is close to that of pristine graphene (4.2 eV) calculated with the same setup.
Figure 4. The potential energy surface of the :CCl2 molecule for the adsorption on the bond position on graphene. The distance between the carbon atom of the molecule and its two neighboring atoms carbon atoms of graphene was fixed, while all other atoms were allowed to relax (solid curve with diamonds). The solid curve with crosses shows the energies without the relaxation, and dashed line the energy gained by the relaxation. Inset figures show the geometry of a) physisorbed :CCl2 and b) bonded :CCl2.
first inspected the thermodynamic stability of :CCl2 on pristine graphene. The molecule adsorbs preferentially onto the bond position with adsorption energy of −13.8 kcal mol−1. The adsorption energy in the hollow position is −5.0 kcal mol−1 and the top site is the least probable with just −3.0 kcal mol−1. In the bond position, :CCl2 pulls two neighboring carbon atoms by 0.52 Å above the graphene sheet and the length of the C C bond is 1.55 Å, indicating the formation of a covalent bond. In order to elucidate the adsorption properties of :CCl2 in detail, we calculated the potential energy surface as a function of the distance of the molecule from graphene. The distance between the carbon atom of the molecule and its two neighboring carbon atoms of graphene was constrained, while all the other atoms were allowed to relax. The potential energy surface (Figure 4) shows rather peculiar energetics of the :CCl2 adsorption. The molecule adsorbs first at the distance of 3.1 Å with the adsorption energy of −6.2 kcal mol−1. This minimum corresponds to a physisorption, and is caused by nonlocal van der Waals interaction, because test calculation with the Perdew–Burke–Ernzerhof (PBE)[24] functional (which does not cover van der Waals interaction) yields no attraction in this region. The geometrical changes of the graphene upon physisorption of :CCl2 are negligible, as expected. Nevertheless, the global minimum on the potential energy surface lies at the distance of 1.3 Å and corresponds to :CCl2 chemisorption, although the adsorption energy in this constrained calculation is only −6.5 kcal mol−1. Releasing the constrain and letting the system to fully relax, the adsorption energy becomes −13.8 kcal mol−1, as mentioned in the preceding paragraph. This energy consists of favorable binding of :CCl2 to graphene, which is counterbalanced by deformation energy of graphene of 45.9 kcal mol−1 (Figure 4). The high deformation penalty causes the adsorption energies for physisorption and chemisorption to differ just by the factor
4 www.small-journal.com
2.3. Mechanism of Dichlorocarbene Functionalization of Fluorographene (C1F1)n The reaction of the :CCl2 molecule with fluorographene (C1F1) is illustrated in Figure 5. The interaction of :CCl2 with C1F1 is weak due to the strong repulsion between the Cl atoms of the carbene and the F atoms of the C1F1. The adsorption can be characterized as a physisorption with the interaction energy of −8.2 kcal mol−1. The molecule remains at 2.8 Å above fluorographene (measured as the distance between carbon atom of :CCl2 and the nearest fluorine atom of CF). In contrast to the barrier-less defluorination observed for C4F (see below), the detachment of one of fluorine atoms is hampered by an energy barrier of 33.1 kcal mol−1 on fluorographene. The detachment may be facilitated in the vicinity of defect site (holes, missing fluorine atoms), which might be present in CF. The resulting CCl2F molecule stays physisorbed to the surface of CF and the energy of this configuration is −14.8 kcal mol−1. The CCl2F molecule can easily desorb from the CF surface, as the desorption energy is only 4.2 kcal mol−1. On the other hand, when it manages to cut additional fluorine atom from CF, the complex of the CCl2F2 molecule physisorbed to CF can be formed via a barrier of 28 kcal mol−1. Such complex has the energy of −17.2 kcal mol−1 with respect to initial reactants, and represents therefore the global minimum of this process.
2.4. Mechanism of Dichlorocarbene Functionalization of Fluorographene (C4F1)n The adsorption on partially fluorinated graphene C4F is different from that on CF. The repulsion between the Cl and F atoms is reduced (compared to the case of CF), allowing stronger interaction between the molecule and C4F support. The molecule detaches one of fluorine atoms, creating the CCl2F molecule physisorbed to the C4F support (Figure 6). This process happens as a direct result of the force relaxation, thus there is no significant energy barrier blocking the detachment of the fluorine atom. The energy of this configuration with respect to the initial reactants is −43.9 kcal mol−1, much lower than that of the analogical configuration on C1F1
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201500364
www.MaterialsViews.com
Figure 5. The two-step mechanism of the reaction of :CCl2 with fluorographene. :CCl2 detaches the fluorine atoms from the surface of CF, resulting in physisorbed CCl2F2 molecule. Carbon atoms are colored in gray, fluorine atoms in yellow, and chlorine in green.
(−14.8 kcal mol−1). The binding energy of the CCl2F molecule to the C4F support is 14.2 kcal mol−1. The reaction can continue, i.e., the CCl2F molecule may cut an additional fluorine atom from the surface of fluorinated graphene, which results in the CCl2F2 molecule physisorbed to defluorinated support. The energy of the final state (C32F6 support + CCl2F2) is very favorable with respect to the initial state (C32F8 support + :CCl2) and amounts to −109.8 kcal mol−1. We also inspected the adsorption into other positions on C4F, in which CCl2 is bound directly to one of carbon atoms. The adsorption into the hollow position is exothermic, and the adsorption energy is −8.5 kcal mol−1. The adsorption energy is positive for both nonequivalent on bond positions (carbon atoms are not equivalent in C4F, because one quarter of them bears the fluorine atom; this creates two nonequivalent on bond positions), thus CCl2 does not bind above the carbon–carbon bond in contrast to the adsorption on pristine graphene. The reason for this behavior lies in the repulsion between the Cl atoms of CCl2 and surrounding fluorine atoms on C4F. If one fluorine atom is removed (by the reaction with other CCl2 molecule, for instance), the adsorption into defluorinated region of C4F (or strictly speaking, of C32F7) becomes much more favorable—the adsorption energy on
the C C bond is −21.2 kcal mol−1, making the adsorption onto the C C bond of partially fluorinated graphene more favorable than onto the pure graphene (−13.8 kcal mol−1). The reaction with fluorinated graphene depends on the concentration of fluorine atoms. The fluorographene reacts only weakly with the :CCl2 molecule (the adsorption energy is −8.2 kcal mol−1) and the removal of the fluorine atom by the CCl2 molecule is blocked by the energy barrier of 33.1 kcal mol−1. This is caused by the high energy needed to remove the F atom from the surface of CF.[25] The situation changes when the graphene is only partially fluorinated. On C4F, the CCl2 molecule can detach the F atom without the need to overcome any energy barrier. This process creates energetically favorable (−43.9 kcal mol−1) configuration in which the CCl2F molecule is weakly bound to the C4F support, and can be followed by attachment of further fluorine atom to the CCl2F molecule, creating very stable complex of fluorinated graphene and the CCl2F2 molecule (−109.8 kcal mol−1). The adsorption of another :CCl2 into the region of C4F, from which the F atom was removed, becomes more favorable—the adsorption energy on the C C bond is −21.2 kcal mol−1, lower than the analogical adsorption energy on pure graphene (−13.8 kcal mol−1). In summary, the reaction of :CCl2 with nonstoichiometric CxFy is a two-step process; in the first step the CCl2 detaches the fluorine atoms from the support and in the next step CCl2 chemisorbs to the sp2 carbon atoms.
3. Conclusions
Figure 6. The CCl2F molecule after detaching one of fluorine atoms from the partially fluorinated graphene (C4F). The energy of this configuration is −43.9 kcal mol−1 with respect to noninteracting C4F and :CCl2. small 2015, DOI: 10.1002/smll.201500364
We prepared dichlorocarbene-functionalized fluorographene by the reaction of few-layer fluorographene with dichlorocarbene in chloroform. The material was homogenously functionalized across the flake as observed from high-resolution TEM/EDX mapping. The DCC-FG shows completely different dispersibility in DMF and water than the pristine
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
5
full papers www.MaterialsViews.com
fluorographite. DFT calculations showed that reaction of dichlorocarbene with fluorographene is a two-step process. In the first step, fluorographene is defluorinated and in the latter step, the dichlorocarbene adds to the formed sp2 carbons. The DCC-FG represents a new member of the graphene family, which was synthetized from fluorographite. This indicates that fluorographite is a viable pristine material for the synthesis of other graphene derivatives. We envision that the modification of graphene with two or more halogen atoms may lead to well-defined tuning of the properties of graphene derivatives.
4. Experimental Section Materials: Fluorographite was obtained from Alfa Aesar. Chloroform and sodium hydroxide were obtained from Sigma Aldrich, Singapore. Triethylbenzylammonium chloride was obtained from Alfa Aesar, Singapore. Milli-Q water (resistivity: 18.2 MΩ cm) was used throughout the experiments. Methods: Synthesis of Dichlorocarbene-Functionalized Fluorographene: Fluorographite (10 mg) and triethylbenzylammonium chloride (phase transfer catalyst, PTC, 10 mg) were mixed in water (7.5 mL) and chloroform (10 mL). The mixture was ultrasonicated (180 W) for 1.5 h before the addition of NaOH (10 g) and chloroform (10 mL). After vigorous stirring for 24 h, PTC (10 mg), NaOH (10 g in 7.5 mL water), and chloroform (10 mL) were added into the mixture and left to stir for 24 h at 80 °C. The mixture was brought to room temperature and further diluted with water (250 mL). The filtered solid was washed with a copious amount of acetone and ethanol. This was followed by subsequent rinsing with hexane, chloroform, ethanol, water, ethanol, water, and diethyl ether. The obtained powder was left to dry at 70 °C in the oven for 5 d to provide dichlorocarbene-functionalized fluorographene. Characterization: XPS data were acquired using Phoibos 100 spectrometer and an Mg X-ray radiation source (SPECS, Germany). Relative sensitivity factors were used for the evaluation of atomic percentages from the XPS survey spectra. XPS samples were prepared by coating a carbon tape with a uniform layer of the material under study. Raman spectroscopy was performed using DXR Raman microscope with 780 nm excitation line of a diode laser. The HR-TEM image and the STEM-HAADF (high-angle annular darkfield imaging) images were obtained using a FEI Titan electron microscope operating at 80 kV. DFT calculations of functionalized graphene were performed using the projector-augmented wave method in the Vienna ab initio simulation package (VASP) suite.[26,27] The energy cutoff for the plane-wave expansion was set to 400 eV. The graphene sheet was modeled using a 4 × 4 supercell (32 carbon atoms) with a calculated C C bond length of 1.44 Å. Additionally, we used two models of fluorinated graphene: (1) fluorographene (CF) modeled by the 4 × 4 supercell (32 carbon and 32 fluorine atoms) and (2) C4F (32 carbon and 8 fluorine atoms), which was identified as thermodynamically possible and dynamically stable structure of partially fluorinated graphene. The periodically repeated sheets were separated by 18 Å of vacuum in all cases. Adsorption of :CCl2 was modeled by placing the molecule into one high symmetry positions, (a) above one of carbon (fluorine) atoms, (b) above the bond connecting two neighboring carbon
6 www.small-journal.com
atoms, or (c) into the hollow positions in the middle of benzene ring. The atomic forces acting on the molecule were then fully relaxed using conjugate gradient algorithm. Thermodynamic stability of various reaction products was calculated by comparing the total energy of respective product with respect to the sum of the total energies of initial reactants. We used the optB86b-vdW functional[28] in following calculations of reaction and adsorption properties. This functional yielded the adsorption energies of organic molecules on graphene in an excellent agreement with experiment.[29]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors gratefully acknowledge the support by the project LO1305 of the Ministry of Education, Youth and Sports of the Czech Republic. M.O. acknowledges support from the Grant Agency of the Czech Republic (P208/12/G016). K.H. would like to gratefully acknowledge the Operational Program Education for Competitiveness – European Social Fund (CZ.1.07/2.3.00/20.0155 of the Ministry of Education, Youth and Sports of the Czech Republic). M.P. acknowledges Nanyang Technological University and Singapore Ministry of Education Academic Research Fund AcRFTier 1 (2013T1-001-014, RGT1/13) for the funding support. The authors also acknowledge Dr. Cˇépe and Ing. Tomanec (RCPTM) for HR-TEM measurements.
[1] M. J. Allen, V. C. Tung, R. B. Kaner, Chem. Rev. 2010, 110, 132. [2] K. S. Novoselov, V. I. Fal’ko, L. Colombo, P. R. Gellert, M. G. Schwab, K. Kim, Nature 2012, 490, 192. [3] V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza, R. Zboril, K. S. Kim, Chem. Rev. 2012, 112, 6156. [4] A. Ambrosi, C. K. Chua, A. Bonanni, M. Pumera, Chem. Rev. 2014, 114, 7150. [5] Q. Tang, Z. Zhou, Z. Chen, Nanoscale 2013, 5, 4541. [6] X. Wang, G. Sun, P. Routh, D. H. Kim, W. Huang, P. Chen, Chem. Soc. Rev. 2014, 43, 7067. [7] M. Pumera, J. Mater. Chem. C 2014, 2, 6454. [8] Y. Li, Z. Zhou, P. Shen, Z. Chen, ACS Nano 2009, 3, 1952. [9] Y. Li, Z. Zhou, P. Shen, Z. Chen, J. Phys. Chem. C 2009, 113, 15043. [10] P. Lazar, R. Zbor˘il, M. Pumera, M. Otyepka, Phys. Chem. Chem. Phys. 2014, 16, 14231. [11] R. Zbor˘il, F. Karlický, A. B. Bourlinos, T. A. Steriotis, A. K. Stubos, V. Georgakilas, V. Šafár˘ová, D. Janc˘ík, C. Trapalis, M. Otyepka, Small 2010, 6, 2885. [12] F. Karlický, K. K. R. Datta, M. Otyepka, R. Zbor˘il, ACS Nano 2013, 7, 6434. [13] H. L. Poh, P. Šimek, Z. Šofer, M. Pumera, Chem. Eur. J. 2013, 19, 2655.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201500364
www.MaterialsViews.com [14] X. Chia, A. Ambrosi, M. Otyepka, R. Zbor˘il, M. Pumera, Chem. Eur. J. 2014, 20, 6665. [15] S. Boopathi, T. N. Narayanan, S. S. Kumar, Nanoscale 2014, 6, 10140. [16] F. Karlický, R. Zbor˘il, M. Otyepka, J. Chem. Phys. 2012, 137, 034709. [17] M. Zhu, X. Xie, Y. Guo, P. Chen, X. Ou, G. Yu, M. Liu, Phys. Chem. Chem. Phys. 2013, 15, 20992. [18] C. K. Chua, A. Ambrosi, M. Pumera, Chem. Commun. 2012, 48, 5376. [19] H. Touhara, Y. Goto, N. Watanabe, K. Imaeda, T. Enoki, H. Inokuchi, Y. Mizutani, Synth. Met. 1988, 23, 461. [20] R. R. Nair, W. Ren, R. Jalil, I. Riaz, V. G. Kravets, L. Britnell, P. Blake, F. Schedin, A. S. Mayorov, S. Yuan, M. I. Katsnelson, H.-M. Cheng, W. Strupinski, L. G. Bulusheva, A. V Okotrub, I. V Grigorieva, A. N. Grigorenko, K. S. Novoselov, A. K. Geim, Small 2010, 6, 2877. [21] Y. Hernandez, M. Lotya, D. Rickard, S. D. Bergin, J. N. Coleman, Langmuir 2009, 26, 3208.
small 2015, DOI: 10.1002/smll.201500364
[22] M. Zhu, X. Xie, Y. Guo, P. Chen, X. Ou, G. Yu, M. Liu, Phys. Chem. Chem. Phys. 2013, 15, 20992. [23] E. R. Margine, M. L. Bocquet, X. Blase, Nano Lett. 2008, 8, 3315. [24] P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865. [25] M. Dubecký, E. Otyepková, P. Lazar, F. Karlický, M. Petr, K. C˘épe, P. Banáš, R. Zbor˘il, M. Otyepka, J. Phys. Chem. Lett. 2015, 6, 1413. [26] P. E. Blöchl, Phys. Rev. B 1994, 50, 17953. [27] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758. [28] J. Klimeš, D. R. Bowler, A. Michaelides, Phys. Rev. B 2011, 83, 195131. [29] P. Lazar, F. Karlický, P. Jurec˘ka, M. Kocman, E. Otyepková, K. Šafár˘ová, M. Otyepka, J. Am. Chem. Soc. 2013, 135, 6372.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: February 5, 2015 Revised: March 15, 2015 Published online:
www.small-journal.com
7
