Synchrotron X-ray Diffraction and Infrared Spectroscopy Studies of

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Synchrotron X-ray Diffraction and Infrared Spectroscopy Studies of C60H18 under High Pressure Honglei Ma, Xuemei Zhang, Bingbing Liu,* Quanjun Li, Qifeng Zeng, Shidan Yu, Bo Zou, Tian Cui, and Guangtian Zou State Key Lab of Super hard Materials, Jilin University, Changchun 130012, China

Zhenxian Liu Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Northwest, Washington, DC 20015

T. Wågberg and B. Sundqvist Department of Physics, Umeå University, SE-90187 Umeå, Sweden

Dag Noreus Department of Structural Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden

ABSTRACT In situ high-pressure angle-dispersive synchrotron X-ray diffraction and high-pressure mid-infrared (IR) spectrum measurements of C60H18 were carried out up to 32 and 10.2 GPa, respectively. Our diffraction data indicated that the fcc structure of C60H18 was stable up to 32 GPa. The bulk modulus B0 was determined to be 21 ( 1.16 GPa, about 40% higher than that of C60. The C-H vibrations still existed up to 10.2 GPa, and the vibrational frequencies decreased with increasing pressure. IR-active vibrational frequencies and their corresponding eigenvectors of C60H18 were simulated by DMOL3. The effects of the hydrogen atoms attached to the fullerene molecular cage on the stability of the structure under high pressure are discussed. SECTION Kinetics, Spectroscopy

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ydrogenated fullerenes have various potential applications, including their use as hydrogen storage materials, as solid lubricants, and in lithium ion cells.1 At the present time, a number of hydrogenated fullerene molecules from C60H2 to C60H36 have been successfully synthesized and characterized,2 and compounds heavier than C60H40 have been reported.3 C60H36 and C60H18 are the most prominent examples in this class of compounds, which are the most stable hydrogenated derivatives of the fullerene. It has been predicted that there are more than 1014 different isomeric forms of C60H36, but only four isomers have been proposed to be stable. The fractions of the various isomers in C60H36 samples depend mostly on the preparation method. Up to now, it is hard to get a pure single isomer of C60H36. In contrast, C60H18 has only a single stable isomer with C3v symmetry, and recently, pure C60H18 has been synthesized in high yield by hydrogenation of C60 at a high pressure of H2 and high temperature.4 This provides an ideal material to study the structure and properties of hydrofullerenes. A few other methods to produce C60H18 with high yield have also been reported.5,6 It is well known that high pressure is one of the most effective methods to change the structure of materials.7 For the hydrogenated fullerenes, the high-pressure method can

r 2010 American Chemical Society

be applied to study the structural stability of the materials and the effects of the hydrogen atoms attached to the fullerene molecular cage on the crystalline structure under high pressure. Furthermore, due to their potential applications in energy storage and as lubricants, it is also important to study the properties of hydrofullerenes under conditions relevant to this. Meletov8 studied the pressure behavior of C60H36 up to 12 GPa using Raman spectroscopy and reported an anomaly at 0.6 GPa in the pressure dependence of the Raman spectra that could be attributed to a phase transformation. All of the observed features were reversible with pressure. Kawasaki9 performed in situ XRD measurements in C60H36 up to 2.7 GPa, but no structural phase transformation was observed in this range. Recently, Molodets et al. have measured the XRD and IR spectrum of C60H36 after shock-wave compression treatment.10 They found that the samples retained the bcc structure and that hydrogen atoms were still attached to the carbon cages after shock compression up to 40 GPa and 750 K. Obviously, the

Received Date: November 29, 2009 Accepted Date: January 19, 2010 Published on Web Date: January 26, 2010

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DOI: 10.1021/jz9003262 |J. Phys. Chem. Lett. 2010, 1, 714–719

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Figure 1. XRD patterns of C60H18 under atmospheric pressure.

Figure 2. Synchrotron X-ray diffraction pattern of C60H18 under different pressures.

existence of structural phase transformations in hydrogenated fullerenes is still controversial, and the properties of hydrogenated fullerenes under high pressure should be further studied. To the best of our knowledge, there is no investigation so far about the in situ high-pressure properties of C60H18, and it is thus important to carry out such studies to gain insight into the stability of C60H18. In this Letter, we report the first studies of the stability of the structure and of changes in the behavior of the hydrogen atoms of C60H18 under high pressure by in situ angle-dispersive synchrotron X-ray diffraction experiments and IR spectroscopy up to 32 and 10.2 GPa, respectively. The IR spectra of C60H18 and the eigenvectors of hydrogen atoms of C60H18 were simulated by DMOL3. We found that the fcc structure of C60H18 is stable up to 32 GPa, higher than that for pure C60. The C-H vibrational frequencies decrease with increasing pressure. These results indicate that hydrogen plays an important role in the stability of the material and protects the structure from distortion under high pressure. Figure 1 shows the observed XRD patterns of C60H18 under atmospheric pressure. We analyzed the diffraction data and obtained the lattice constant a = 14.56 ( 0.035 Å, which is consistent with the report of Talyzin11 but larger than the value for C60 (a = 14.17 Å).12 It was also found that the diffraction lines of C60H18 could be indexed by a fcc lattice, similar to that of C60, indicating that the attached hydrogen atoms induce an expansion of the fullerene lattice. In contrast to C60, where molecules rotate freely above 260 K at atmospheric pressure,13 it is believed that the C60H18 molecules are static in the solid phase. The presence of spinning side bands in the highresolution NMR spectrum4 strongly indicates that the C60H18 molecules are indeed static in the solid phase. Figure 2 gives the pressure dependence of the X-ray diffraction pattern of C60H18 with increasing pressure. It is evident that all of the diffraction peaks (111), (220), (311), and (222) of C60H18 shift to higher angles with increasing pressure, indicating a decrease in unit cell volume. As pressure increases, the peaks become broader and less intense. Though the intensities of all of these diffraction lines are very weak, most original diffraction lines of C60H18 could be observed up to 32 GPa, with no new diffraction lines appearing. This indicates that the crystalline structure of C60H18 is stable and that there is no structural phase transformation under the present experimental conditions. This behavior is different from that of C60, in which the fcc structure becomes almost amorphous


at 28 GPa,13 indicating that the structure of C60H18 is more stable against pressure deformation than C60. In contrast to C60H36, no structural phase transformation occurs up to 32 GPa. Figure 2 shows the X-ray diffraction pattern after releasing to ambient pressure. The high-pressure behavior of C60H18 is reversible, and the sample keeps its fcc structure with lattice parameters in good agreement with those of the pristine sample. Regarding the relative intensity of diffraction peaks, the intensity of the (220) peak decreases at a slower rate compared to that of the others, indicating that the corresponding d spacing remains relatively well-defined. Considering that the [110] directions correspond to the direction to the nearest molecular neighbor in the fcc structure,13 it is reasonable to assume that this d spacing should remain well-defined even if the lattice is locally distorted by the applied high pressure since both C60 and C60H18 molecules are quite incompressible. Figure 3a shows the pressure dependence of the d values for C60H18. The d values decrease homogeneously with pressure, which indicates an isotropic volume decrease with increasing pressure. Figure 3b shows the pressure dependence of the relative unit cell volume of C60H18. In order to obtain the bulk modulus B0 of C60H18 and its pressure derivative B00, the third order Birch-Murnaghan equation of state14 " 5=3 # V 0 7=3 V0 PðV Þ ¼ 1:5B0 V V 8 " #9 2=3 < = 3 V0 1 þ ðB00 - 4Þ -1 : ; 4 V was fitted to the data for pressure versus relative volume V/V0, where V is the volume at pressure P and V0 is the volume at ambient pressure. The bulk modulus for C60H18 was determined to be 21 (1.16 GPa, with a pressure derivative of B00 = 6.68 ( 0.25. Compared with the bulk modulus of C60, we found that the C60H18 is 40% less compressible than C60.13 These different behaviors could be explained by the effect of added hydrogen. IR spectroscopy is a powerful tool to study the lattice dynamics of C60H18. It should be pointed out that IR spectroscopy is more sensitive to changes in the C-H vibration

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Figure 3. (a) Pressure dependence of d values for C60H18. (b) Pressure dependence of the relative unit cell volume of C60H18; the line represents a fit of Murnaghan's equation of state to the data for C60H18.

stretching vibrations, as shown in Figure 5d. The third (V3) band can be attributed to the C(H(Bn))-C(H(Cn))-C(H(An)) (n = 1, 2, 3) stretching vibrations, as shown in Figure 5e. We have measured in situ IR spectra of C60H18 up to 10.2 GPa. Figure 6a shows IR spectra of C60H18 from 1150 to 3050 cm-1 at different pressures, including one spectrum measured at 1.6 GPa under decompression. Figure 6b shows peak positions of the seven main IR bands for C60H18 as functions of pressure. In order to determine the peak positions of those C-H vibrational bands more precisely, a peak fit to the experimental data with pseudo-Voigt functions was made at each pressure. It should be pointed out that the IR absorption bands for C-H stretching vibrations are saturated at 0.3 and 1.7 GPa. Therefore, it is impossible to accurately distinguish the peak positions at these pressures. In Figure 6b, we show error bars for the uncertainty in peak positions; however, the indicated error is similar in size to the symbols. This figure also clearly shows that the bands related to C-C vibrations below 1800 cm-1 shift to higher frequencies while bands related to C-H vibrations shift to lower frequencies with increasing pressures. These two families of bands thus have significantly different behaviors. The red shift of the C-H vibrations is interesting because it is different from that observed in other aromatic hydrocarbons, such as benzene.17 The red shift indicates that the C-H stretching vibration weakens with increasing pressure, which might relate to a pressure-driven enhancement of the C-H interaction between the hydrogen and neighboring molecular cages due the change of the van der Waals interactions in the lattice, which acts as a spring between the molecular cages. When the molecular cages approach each other with increasing pressure, this force might be strengthened. As a result, it will effectively hinder the lattice compression. Compared with C60, we found that the C60H18 cage became more stable due to the addition of hydrogen atoms such that no intermolecular carbon-carbon bands were formed. This also confirms that molecules tend to orient in such a way as to maximize interactions between hydrogen atoms and the carbons on neighboring molecules and to minimize interactions between the neighboring carbon atoms. From our data in the high-frequency range, we also found that the pressure dependence of the band V3 (-2 cm-1/GPa) is similar to that of the V2 band (-3 cm-1/GPa) and higher than the pressure dependence of the V1 band (approximately

Figure 4. Experimental (a) and calculated mid-IR absorption spectra (b) of C60H18 at ambient pressure.

than Raman spectroscopy, and it also avoids the problem of interfering luminescence from the hydrogenated fullerenes. As shown in Figure 4, the measured mid-infrared (MIR) spectrum of C60H18 at ambient pressure is in good agreement with previous reports15,16 as well as with the spectrum calculated by DMOL3. The IR spectrum can be roughly divided into two parts. (1) Spectral bands below 1800 cm-1 are mainly from the vibration of the carbon cage. For example, two representative bands at 1474 and 1611 cm-1 are attributed to breathing vibrational modes of the carbon cage and are only linked to carbon atoms, while two other low-frequency bands at 1201 and 1272 cm-1 are related with vibrations of carbon atoms with bonds to hydrogen atoms. (2) The spectral bands from 2800 cm-1 to 3000 cm-1 are attributed to C-H stretching vibrations. The corresponding eigenvectors of this high-frequency region were also calculated by DMOL3 methods and are shown in Figure 5. Figure 5a,b shows that the 18 hydrogen atoms can be divided into three equivalent AnBnB0nCnDnD0n (n = 1, 2, 3) spin groups, and the proton positions are given in gray. Each band at high frequency in Figure 4 is caused by similar equivalent vibrations from three groups. According to our simulation of the motion model, the first band (V1) in the IR spectra at high frequency is mainly caused by the groups of C-H(Dn) (n = 1, 6 n) 2, 3) and C(H(Bn))-C(H(Bm)) (n = m = 1, 2, 3; m ¼ antisymmetric stretching vibrations, and the directions of these vibrations are shown in Figure 5c. The second band (V2) mainly 6 n) comes from the C(H(Bn))-C(H(Bm)) (n = m = 1, 2, 3; m ¼


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Figure 5. (a) The molecular model of C60H18; panels (b-e) indicate the eigenvectors of C-H vibration modes for the three bands in the high-frequency region.

-0.5 cm-1/GPa). Compared with the model in Figure 5, we found that the hydrogen atoms involved in modes V3 and V2 are located at the outer margin of the bowl-shaped part of the C60H18 molecule, and it is reasonable to believe that these hydrogen atoms are most sensitive to the pressure. The blue shift for the bands related to C-C vibrations may result from the shrinkage of the C60H18 cage with increasing pressure or could indicate that the C60H18 cage deforms in an anisotropic way and thus changes its shape under high pressure. However, the resolution of our diffraction experiment is much too low to verify any such deformation, especially since the molecules are probably orientationally disordered. In summary, in situ angle-dispersive synchrotron X-ray diffraction and IR spectra of C60H18 under high pressure were measured up to 32 and 10.2 GPa, respectively. The IR spectra of C60H18 and the eigenvectors of hydrogen atoms of C60H18 were simulated by DMOL3. In the high-pressure XRD experiments, no structural phase transformation was observed up to 32 GPa, and C60H18 retained the initial fcc structure when the pressure was released. We have assigned the eigenvectors of the three bands in the high-frequency region to the C-H stretching vibrations and four other representative bands in the low-frequency region to vibrations in the carbon cage by analyzing the vibrations of the IR-active bands. The vibrations linked to only carbon atoms in the cage exhibit a blue shift, while the C-H stretching vibrations show red shift with increasing pressure. These results indicate a change in the interaction between hydrogen and neighboring cages and


could explain the low compressibility of the structure under high pressure. The presence of hydrogen in the structure could also explain why no phase transitions occur for this material under pressure.

EXPERIMENTAL METHODS The C60H18 was synthesized by hydrogenation of C60 at 100 bar of H2 and 673 K for 10 h.4 For synchrotron X-ray diffraction high-pressure measurements, a symmetrical diamond anvil cell18 was used with liquid argon as the pressure medium. To eliminate orientation effects, single crystals of C60H18 were first crushed into fine powder. Samples were loaded into a 120 μm diameter hole drilled in the T301 stainless steel gasket. The synchrotron XRD experiments were performed at the wiggler beamline X17C of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), using the angle-dispersive X-ray diffraction (ADXRD) technique and a MAR345 image plate detector to record the diffracted X-rays.19 The wavelength was 0.4066 Å. Two-dimensional patterns were radially integrated using the software FIT2D. For mid-IR high-pressure measurements, the IR beam was focused onto the sample under study by a custom-made IR microscope purged with dry air. The IR spectra were collected in transmission mode by a Bruker Vertex80 V FTIR spectrometer. The spectra were collected from 400 to 3000 cm-1 with a nitrogen-cooled broad-band MCT detector, with a resolution of 4 cm-1, and with 2048 scans used for all spectra collection. The pressure was calibrated

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(J0730311), the Project for Scientific and Technical Development of Jilin Province, and also an exchange grant from the Swedish Research Council through the SIDA-Swedish Research Links exchange program. The use of the National Synchrotron Light Source beamlines X17C and U2A is supported by NSF COMPRES EAR01-35554 and by U.S. DOE Contract DE-AC02-10886.

REFERENCES (1) (2)

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Figure 6. (a) Mid-IR absorption spectra of C60H18 under high pressures. The solid lines in the C-H stretching vibrational region indicate a least-squares fit of pseudo-Voigt functions to the measured data (solid circles). (b) Positions of representative IR bands of C60H18 as a function of pressure. The solid lines result from the linear least-squares fit to the data under compression. Open circles represent the peak positions of the IR bands from decompression.

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with the ruby fluorescence technique20 for both X-ray and IR measurements. The IR experiment has also been repeated at the U2A beamline of the NSLS, BNL. In this work, all ab initio calculations (including geometry optimizations, vibrational frequencies, and their corresponding eigenvectors) were carried out by the local density first-principles DMOL3 methods within the local density approximation (LDA).

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AUTHOR INFORMATION (12)

Corresponding Author: *To whom correspondence should be addressed. E-mail: liubb@ jlu.edu.cn. Tel/Fax: 86-431-85168256.

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ACKNOWLEDGMENT We thank Dr. Jingzhu Hu for her technical (14)

support at X17C, NSLS. This work was supported financially by NSFC (10674053, 10574053, 10979001, 20773043), the National Basic Research Program of China (2005 CB724400), the Cultivation Fund of the Key Scientific and Technical Innovation Project (2004295), the Program for Changjiang Scholar and Innovative Research Team in University (IRT0625), the Cheung Kong Scholars Program, the National Foundation for Fostering Talents of Basic Science


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Withers, J. C.; Loutfy, R. O.; Lowe, T. P. Fullerene Commercial Vision. Fullerenes, Nanotubes, Carbon Nanostruct. 1997, 5, 1–31. Drelinkiewicz, A.; Byszewski, P.; Bielanski, A. Catalytic Hydrogenation of C60 Fullerene. React. Kinet. Catal. Lett. 1996, 59, 1588–2837. Talyzin, A. V.; Tsybin, Y. O.; Schaub, T. M.; Mauron, P.; Shulga, Y. M.; Z€ uttel, A.; Sundqvist, B.; Marshall, A. G. Composition of Hydrofullerene Mixtures Produced by C60 Reaction with Hydrogen Gas Revealed by High-Resolution Mass Spectrometry. J. Phys. Chem. B 2005, 109, 12742–12747. Wågberg, T; Johnels, D; Peera, A; Hedenstr€ om, M; Schulga, Y. M; Tsybin, Y. O; Purcell, J. M; Marshall, A. G; Noreus, D; Sato, T; Talyzin, A. V. Selective Synthesis of the C3v-Isomer of C60H18. Org. Lett. 2005, 7, 5557–5560. Vieira, S. M. C.; Ahmed, W.; Birkett, P. R.; Rego, C. A. Hydrogenation of [60]Fullerene Using a Novel Chemical Vapour Modification (CVM) Method. Chem. Phys. Lett. 2001, 347, 355–360. Briggs, J. B.; Montgomery, M.; Silva, L.; Miller, G. P. Facile, Scalable, Regioselective Synthesis of C3v C60H18 Using Organic Polyamines. Org. Lett. 2005, 7, 5553–5555. Mujica, A.; Rubio, A.; Munoz, A.; Needs, R. J. High-Pressure Phases of Group-IV, III-V, and II-VI Compounds. Rev. Mod. Phys. 2003, 75, 863–912. Meletov, K. P.; Tsilika, I.; Assimopoulos, S.; Kourouklis, G. A.; Vel, S.; Bashkin, I. O.; Kulakov, V. I.; Khasanov, S. S. HighPressure Effects in Hydrofullerene C60H36 Studied by Raman Spectroscopy. Phys. Status Solidi B 2001, 223, 459–467. Kawasaki, S.; Yao, A.; Okino, F.; Touhara, H.; Suito, K. High Pressure Phases of Fullerenes, Hydrofullerenes, and Fluorofullerenes. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2002, 386, 109–114. Molodets, A. M.; Lobach, A. S.; Zhukov, A. N.; Shulga, Yu. M.; Fortov, V. E. Stability of Crystalline Structure and Molecules of Hydrofullerene C60H36 under High Shock Pressures. Dokl. Akad. Nauk 2008, 423, 181–184. Talyzin, A. V.; Tsybin, Y. O.; Purcell, J. M.; Schaub, T. M.; Shulga, Y. M.; Nor eus, D.; Sato, T.; Dzwilewski, A.; Sundqvist, B.; Marshall, A. G. Reaction of Hydrogen Gas with C60 at Elevated Pressure and Temperature: Hydrogenation and Cage Fragmentation. J. Phys. Chem. A 2006, 110, 8528– 8534. Fleming, R. M.; Pamirez, A. P.; Rosseinsky, M. J.; Murphy, D. W.; Haddon, R. C.; Zahurak, S. M.; Makhija, A. V. Relation of Structure and Superconducting Transition Temperatures in A3C60. Nature 1991, 352, 787–788. Sundqvist, B. Fullerenes under High Pressures. Adv. Phys. 1999, 48, 1–134. Birch, F. Finite Strain Isotherm and Velocities for Single-Crystal and Polycrystalline NaCl at High Pressures and 300 K. J. Geophys. Res. 1978, 83, 1257–1268. Popov, A. A.; Senyavin, V. V.; Granovsky, A. A.; Lobach, A. S. In Hydrogen Materials Science and Chemistry of Carbon Nanomaterials; Veziroglu, T. N., Svetlana, Y., Eds.; Springer: The Netherlands, 2005; pp 347-356.


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