Effects of Molecular Geometry on the Properties of Compressed

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Effects of Molecular Geometry on the Properties of Compressed Diamondoid Crystals Fan Yang, Yu Lin, Maria Baldini, Jeremy E. P. Dahl, Robert M. K. Carlson, and Wendy L Mao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02161 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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The Journal of Physical Chemistry Letters

Effects of Molecular Geometry on the Properties of Compressed Diamondoid Crystals Fan Yang1, Yu Lin*2,3, Maria Baldini4, Jeremy E. P. Dahl2, Robert M. K. Carlson2, Wendy L. Mao1,2 1

Department of Geological Sciences, Stanford University, Stanford, CA, 94305

2

Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory,

Menlo Park, CA, 94025 3

Department of Chemistry, Stanford University, Stanford, CA, 94305

4

Geophysical Laboratory, Carnegie Institution of Washington, Advanced Photon Source,

Argonne National Laboratory, Argonne, IL, 60439 AUTHOR INFORMATION Corresponding Author *[email protected]

ACS Paragon Plus Environment

1


ABSTRACT: Diamondoids are an intriguing group of carbon-based nanomaterials, which combine desired properties of inorganic nanomaterials and small hydrocarbon molecules with atomic-level uniformity. In this paper, we report the first comparative study on the effect of pressure on a series of diamondoid crystals with systematically varying molecular geometries and shapes, including zero-dimensional (0D) adamantane, one-dimensional (1D) diamantane, [121]tetramantane,

[123]tetramantane,

and

[1212]pentamantane,

two-dimensional

(2D)

[12312]hexamantane, and three-dimensional (3D) triamantane and [1(2,3)4]pentamantane. We find the bulk moduli of these diamondoid crystals are strongly dependent on the diamondoids’ molecular geometry with 3D [1(2,3)4]pentamantane being the least compressible and 0D adamantane being the most compressible. These diamondoid crystals possess excellent structural rigidity and are able to sustain large volume deformation without structural failure even after repetitive pressure loading cycles. These properties are desirable for constructing cushioning devices. We also demonstrate that lower diamondoids outperform the conventional cushioning materials

in

both

the

working

pressure

range

and

energy

absorption

density.

TOC GRAPHICS

Synchrotron XRD

Raman spectroscopy 1.00 0.95

0D

1D

2D

3D

0.90

V/V0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.85 0.80

2&3D

0.75 0.70

0D

1D

0.65 0

2

4

6

8

10

12

14

16

Pressure (GPa)

KEYWORDS diamondoid, high pressure, geometry, compressibility


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Diamondoids are a class of saturated hydrocarbons consisting of fused cyclohexane rings in the form of “cage-like” structures superimposable on the diamond lattice1,2. These molecules occur naturally in petroleum and can be distilled from crude oil3,4. Lower diamondoids, namely adamantane, diamantane, and triamantane, contain one, two, and three fused carbon cages, respectively5. Higher diamondoids, beginning with the four-cage tetramantane, have structural isomers and are very difficult to synthesize6-12. These molecules exhibit increasing structural complexity and their molecular geometries vary. With their unique, diverse, and stable molecular structures, diamondoids, as well as their functionalized derivatives, have potential for enabling a new generation of novel and durable materials and devices for nanotechnology applications13-18. At ambient temperature and pressure, all these diamondoids form soft and transparent molecular crystals. Although they visually resemble macroscopic diamonds, these crystals demonstrate very different properties and are held together by weak intermolecular van der Waals forces. The diamond-like properties only emerge at the intramolecular sp3 carbon framework, while their intermolecular interactions and associated properties are not well explored. Pressure, which can dramatically alter a material’s physical and chemical behavior, can provide insight into structure-property relationships and offer an opportunity for optimizing desirable properties19,20. Pressure-induced bond breaking and bond formation have been documented in many carbon-based molecular systems21-24. So far, however, pressure-driven structural changes in diamondoids are not well understood25-29. In this article, we report for the first time the systematic investigation of the effect of pressure on eight selected diamondoid crystals with various molecular geometries as previously categorized by Lasse30 and Filik31, including one 0D molecule: adamantane, four 1D rod shaped molecules: diamantane, [121]tetramantane, [123]tetramantane, [1212]pentamantane, one 2D


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planar shaped molecule: cyclohexamantane, and two 3D pyramidal shaped molecules: triamantane and [1(2,3)4]pentamantane (Figure 1). We observed that the bulk moduli of these diamondoid crystals are strongly dependent on their molecular geometries. Elucidating the connection between shape and properties is a challenging but essential task for a rational design of nanomaterials at the atomic level. These crystalline materials with systematic molecular geometrical variations allow us the rare opportunity of understanding how their bulk mechanical properties and microscopic molecular geometries are correlated. Furthermore, we also explored and discussed possible applications of diamondoid crystals based on their unique elastic properties, for instance as cushioning materials.

0D

2D

adamantane

1D

cyclohexamantane

3D

diamantane

[123]tetramantane

[121]tetramantane [1212]pentamantane

triamantane

[1(2,3)4]pentamantane

Figure 1. Molecular structures of eight diamondoids studied showing their different geometries and dimensionalities. (Red dashed arrows indicate the molecular size expanding directions.) Raman spectroscopy was used to monitor the vibrational dynamics of diamondoids in-situ under pressure. The spectra can be divided into several distinct vibrational regions31,32: CCC bending and CC stretching (200-900 cm-1), CH wagging/CH2 twisting/CH2 scissoring (900-1600


4

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cm-1), and CH stretching (2800-3200 cm-1). Upon compression, only continuous changes were observed in the CC stretching region while peak merging and splitting arose in higher wavenumber regions as shown in Figure 2a. A dominant CC stretching mode is observed in all of these molecules31,32 which is located at 755 cm-1 in adamantane (Figure 2a), 703 cm-1 in diamantane (Figure S1a)29, and around 680 cm-1 in triamantane (Figure S1c), [121]tetramantane (Figure S1e)28, and [1212]pentamantane (Figure S1g). Calculations have assigned this mode to the breathing vibration across a single adamantane cage32. The evolution of this peak position as a function of pressure for various diamondoid crystals is shown in Figure 2b. Raman shifts as a function of pressure could be fairly explained by a parabolic relationship which is presumably due to the changes of C-C bond force constant under compression. There are obvious discontinuities in the developing trend of this breathing mode in adamantane, diamantane and [121]tetramantane which are marked by the dashed lines. The pressures at which discontinuities occur are generally in good agreement with the phase transition pressures observed from our xray diffraction (XRD) experiments or reported in the literature1,28,29. This observation indicates that the changes in intermolecular packing under pressure do affect the intramolecular vibrations. Furthermore, the persistence of this mode indicates that diamond-like cages remain intact up to the highest pressure studied. Upon releasing pressure, the changes seen in the Raman spectra were completely reversible. This observation provides evidence against the breaking and formation of new bonds at high pressure. For carbon-based molecular solids, decomposition and amorphization have been widely observed under pressure33-35. The pressure range in our experiments is comparable to those where formation of new carbon structures or dissociation of the original structures have been reported, however, in the case of the diamondoid crystals we


5


studied, only modifications of molecular packing were observed. This is likely due to their rigid diamond-like molecular cages.

(a)

34.5

(b)

860 840

)

800

-1

820

Raman Shift (cm

14.5

5.3

1.0

P↑

900 880

25.0

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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780 760 740 720

adamantane diamantane triamantane [121]tetramantane [1212]pentamantane

700

0

700

800

900

1000

1100 -1

Raman Shift (cm )

1200

680 0

5

10

15

20

25

30

35

Pressure (GPa)

Figure 2. (a) Raman spectra of the CC stretching, CCC bending, CH wagging/CH2 twisting and CH2 scissoring region in adamantane as a function of pressure. The star sign indicates the carbon cage breathing vibration. Numbers on the right axis are pressures in the GPa unit. (b) The evolution of the carbon cage breathing mode with pressure for several studied diamondoids. Dashed lines indicate the obvious discontinuities in the developing trend of this breathing mode. We used powder XRD to track the structural evolution of the diamondoid crystals as a function of pressure. Pressure-induced molecular packing rearrangements were observed in all the investigated diamondoids except triamantane within the pressure range studied (Figure 3 and Figure S2). All structural transitions are reversible upon releasing pressure. Due to the uncertainties in structural determinations at high pressure, we focused on the structural evolution for the low pressure phases of all diamondoid crystals studied here, except adamantane which transforms from an orientationally disordered cubic phase to an ordered tetragonal phase at 0.5 GPa25,26,27. The relative volume changes as a function of pressure for the low pressure phases of


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these diamondoid crystals are plotted in Figure 4a and were fit to a 3rd order Birch-Murnaghan equation of state (EoS)36,37 (see Supporting Information for the description of the EoS and the detailed fitting curves of each diamondoid crystal). Fitting statistics are summarized in Table 1. For all of the diamondoid crystals, the small ambient bulk moduli (K0) are consistent with the fact that intermolecular van der Waals interactions are weak and compression is essentially achieved by reduction of the intermolecular spacing at low pressures. As molecules are pushed closer together under further compression, volume reduction becomes increasingly difficult due to the strong Coulomb repulsion and the stiffness of the intramolecular covalent bonds which result in rapid increases in bulk moduli as evidenced by the large pressure derivative (K0’).


7


(a)

14.0 9.5

Intensity (a.u.)

5.4

P↑ 2.4

Pnma 2

3

4

5

6

7

8

0.6 0

9

Å

2θ (degree)

H---H: 2.30

(b)

21.0

Intensity (a.u.)

14.1

8.6 4.5

R3

4

P↑

6

8

2θ (degree)

10

0.5

Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12

H---H: 2.63

Figure 3. High pressure powder XRD patterns of [1(2,3)4]pentamantane (a) and cyclohexamantane (b) along with their ambient crystal structures. Different phases are shown by different colors (black: the low pressure phase, red: a mixture of low pressure and high pressure phases). The stars mark the region where notable changes were observed indicating the onset of phase transitions. The numbers on the right axis are pressures in the GPa unit. Right figures show ambient crystal structures and nearest hydrogen-hydrogen distances (red dotted lines). The unit cells are reoriented in a fashion that H-H distances are recognizable.


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The pressure-volume (P-V) diagram (Figure 4b) clearly illustrates that the bulk moduli display

systematic

correlations

with

diamondoids’

molecular

geometry,

with

3D

[1(2,3)4]pentamantane being the least compressible compared with 0D adamantane being the most compressible and all the 1D and 2D molecules lying in between. In addition, the EoSs of linear

molecules

(i.e.

diamantane,

[121]tetramantane,

[123]tetramantane

and

[1212]pentamantane) are clustered in the diagram, further indicating the molecular geometry governs the elastic compressional behavior of these systems. The EoSs of diamond38, graphite39, crystalline forms of C6040, benzene41, and methane42-44 are plotted for comparison. It is interesting to note that the EoS of C60 falls within a similar region in the P-V space as diamondoid crystals, which could be a result of similar molecular size, geometry and intermolecular interactions. It should be noted that although molecular geometry plays the dominant role in affecting the bulk crystal’s compressibility, there are other factors that contribute to the compressibility which may explain why 2D cyclohexamantane does not rigorously fall between 1D and 3D molecules as shown in Figure 4a. In the pressure range studied, the compressibility of these crystals is largely affected by the surface hydrogen interactions between adjacent molecules which have direct correlations with diamondoid molecular geometry and their packing orientations.


9


(a) 1.00

adamantane LP phase adamantane HP phase diamantane triamantane [121]tetramantane [123]tetramantane [1212]pentamantane [1(2,3)4]pentamantane cyclohexamantane

0.95

0.90

V/V0

0.85

0.80

0.75

0.70

0

2

4

6

8

10

12

14

16

Pressure (GPa)

(b) 1.00 diamond

0.95

0.90

0.85

V/V0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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graphite

0.80

2&3D

0.75

C60

0.70

methane

benzene

1D

0D

0.65 0

2

4

6

8

10

12

14

16

Pressure (GPa)

Figure 4. (a) Normalized unit cell volume changes versus pressure for various diamondoid crystals. The dashed lines are the 3rd order Birch-Murnaghan EoS fits to the P-V data. (b) The EoS distributions for various diamondoids with different dimensionalities. EoSs of several hydrocarbons and carbon allotropes are also plotted for comparison. A common feature among all these diamondoid crystals is the easily triggered pressureinduced structural transitions. Only the crystal structure of triamantane persists up to the highest pressure studied, while all other diamondoids undergo at least one structural phase transition,


10

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with some of the transitions occurring at fairly low threshold pressures. The hydrogen separation distance has been found to be positively correlated with the transition pressures. It is defined as the distance between the nearest hydrogen atoms in adjacent molecules. The values we measured at ambient conditions for these diamondoids are given in Table 1. The lower values for adamantane (2.37Å) and [1(2,3)4]pentamantane (2.30Å) mean the adjacent molecules are relatively close and repulsive forces will build up quickly, therefore less compression is needed to trigger phase transitions, which is consistent with their very low transition pressures. The significantly larger H-H distance in triamantane (2.81Å) implies greater compression is needed to reach the critical transition point. This may explain why we did not observe a phase transition in triamantane even up to 18.1 GPa. Table 1. Structural information for the investigated diamondoid crystals Name

Ambient Phase

V0/Z (Å3)

K0 (GPa)

K0’

Adamantane (0D)

Fm3m

209.7

3.2(0.2)*

19(1.0)

Diamantane (1D)

Pa3

257.7

5.0(0.6)

[121]Tetramantane (1D)

P21/n

391.4

[123]Tetramantane (1D)

P1

[1212]Pentamantane (1D)

Transition Pressure (GPa)

H-H (Å)

0.5

2.37

19.5(2.8)

>13**

2.65

5.5(0.6)

18.8(2.3)

12

2.53

386.5

5.0(1.3)

20.0(1.1)

5.1

2.52

P212121

476.6

8.1(0.8)

8.5(1.5)

1.2

2.48

Cyclohexamantane (2D)

R3

413.0

13.5(1.5)

11.9(1.8)

4.5

2.63

Triamantane (3D)

Fddd

319.8

13.9(1.6)

9.6(1.5)

>18

2.81

[1(2,3)4]Pentamantane (3D) Pnma

453.9

16.0(1.2)

11.8(1.0)

* K0 and K0’ are for the high pressure phase25,26.

0.6

2.30

** Helium was used as a pressure medium29.


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The unique elastic properties of diamondoid crystals suggest a number of possible applications. Controlling and tailoring the property of a system by altering its building blocks have long been one of the main goals of crystal engineering. Diamondoid networks have been recognized as potential structural motifs for constructing metal-organic frameworks, supramolecular systems45-47, etc. Therefore, the molecular-geometry-dependent mechanical properties presented here might provide guidance in designing nanoscale structures with tunable mechanical strength based on diamondoids. The small K0, structural stability, and compressional reversibility indicate that the diamondoids can be compacted to achieve large volume reductions (>30%) and expanded to original volumes without structural failure over a wide pressure range. Large compression energies can therefore be absorbed. This property would be useful in cushioning devices. We conducted repetitive pressure loading experiments on three lower diamondoids up to 5 GPa in order to test the cushioning performance, particularly the structure recovery capability. We found the complete reversibility of the Raman spectra for the quenched sample is maintained after 20 pressure cycles (Figure 5a and Figure S3). In fact, diamondoids exhibit good cushioning performance over a much higher pressure range (in the GPa scale) while conventional materials such as foamy polystyrene and newly proposed carbon nanotube cushion materials mainly work over much smaller pressure ranges (in the MPa scale). We estimate the energy absorption density (calculated at 2 GPa, see SI for detailed calculations) to be on the order of 102 KJ/Kg for lower diamondoids (e.g. adamantane: 157 KJ/Kg, diamantane: 115 KJ/Kg, triamantane: 80 KJ/Kg) that is at least one order of magnitude higher than conventional cushion materials48 (Figure 5b).


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(a)

(b)

160

Energy absorption density (kJ/kg)

Intensity (a.u.)

after

before

140 120 100 80 60 40 20

T1

C N TTr 2 ia m an ta ne D ia m an ta ne Ad am an ta ne

Raman Shift (cm )

CN

400 600 800 1000 12002800 2900 3000 -1

M gO

0

Ve rm icu lit Po e ly st yr en e

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a) Raman spectra of adamantane before (black) and after (red) 20 pressure cycles. (b) Energy absorption density (calculated at 2 GPa, see SI for detailed discussions) for the three investigated lower diamondoids (red) compared with some conventional cushioning materials (black). CNT-1 and CNT-2 refer to the microagglomerate and superagglomerate carbon nanotubes48 respectively. In summary, we reveal for the first time the correlation between molecular geometry and bulk crystals’ compressibility of various diamondoids. This discovery can provide guidance in tailoring the mechanical strength of the target systems based on diamondoids which might find applications in crystal engineering, for example, in constructing metal-organic frameworks and supramolecular systems. Besides, all the diamondoids possess excellent molecular unit rigidity under pressure which could find applications as a desirable cushioning material. We demonstrate that the diamondoids, in particular the lower ones, outperform the conventional cushioning material in both the working pressure range and energy absorption density.

EXPERIMENTAL METHODS


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Sample preparations. Adamantane was purchased from Sigma-Aldrich, while other high purity diamondoid samples were isolated from petroleum. Higher diamondoids were obtained by vacuum distillation above 345 °C and are pyrolyzed at 400 to 450 °C to remove nondiamondoid hydrocarbons. Size and shape selectivity were determined via high-performance liquid chromatography. All samples have purities exceeding 99%, and their structures are confirmed by single crystal x-ray diffraction. More detailed descriptions on isolation and characterization approaches can be found in the recently published articles6,7,8. [123]tetramantane is chiral with a helical twist in its carbon framework. The [123]tetramantane drawn in the manuscript Figure 1 is the “P” form (i.e., positive (clockwise twist). Our measurements were performed on the racemic mixture (equal amount of P and M-[123]tetramantane). High pressure Raman spectroscopy and powder X-ray diffraction. Raman measurements were carried out at high pressure and room temperature using a symmetric diamond anvil cell with 500 µm diamond culets. Polycrystalline diamondoid samples were loaded into the sample chamber created by drilling a 150 µm diameter hole in a preindented stainless steel gasket. A 5 µm diameter ruby ball was loaded for pressure calibration49. No pressure transmitting medium was used for the high pressure Raman measurements. Raman measurements were conducted by exciting a 514 nm Ar+ laser using the Renishaw Invia system in the Extreme Environments Laboratory at Stanford University. For [123]tetramantane. [1(2,3)4]pentamantane and cyclohexamantane, strong fluorescence was observed during Raman measurements which obscured the Raman signal and prevented us from tracking the carbon cage breathing mode. The fluorescence comes from the sample isolation approach. In-situ high pressure XRD experiments were conducted at beamline 16BMD of the Advanced Photon Source (APS), Argonne National Laboratory (ANL). The wavelength of the incident


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monochromatic x-rays was 0.3066 Å for [123]tetramantane and [1(2,3)4]pentamantane, and 0.4246 Å for all of the other diamondoids. Except diamantane, all XRD experiments were carried out without using any pressure medium. Phase transitions in diamantane are easily affected by the deviatoric stress29, therefore helium gas was applied as pressure medium in its XRD measurement. In order to obtain satisfactory patterns for diamondoids composed of low-Z elements, the exposure time for every pressure point were usually more than half an hour. The collected XRD patterns were integrated using FIT2D software50. These diamondoid crystals maintain good crystallinity even with extensive grinding, which often yield spotty diffraction patterns instead of smooth Debye-Scherrer rings. Therefore, we only use the peak positions to index the space group and determine the unit cell volume. The JADE5 software package was used to index diffraction patterns and refine unit cell parameters. All these diamondoid crystals except triamantane underwent at least one pressure-induced phase transition. Sluggish transition kinetics is also commonly observed among these diamondoids where the low pressure and high pressure phase coexists. Due to the uncertainty in determining high pressure phases, we only tracked the unit cell information for the low pressure phases to the highest pressure possible in the phase mixture region. EoS fitting details of each diamondoid crystals are discussed in the supporting information.

ASSOCIATED CONTENT Supporting Information: Some of the high pressure Raman spectra and powder XRD patterns, EoS fitting details of each diamondoid crystals, energy absorption density calculations are available in the supporting information (SI). AUTHOR INFORMATION


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Department of Energy (DOE) under Contract DE-AC0276SF00515. HPCAT are supported by DOE-NNSA under Award Number DE-NA0001974 and DOE-BES under Award Number DE-FG02-99ER45775, with partial instrumentation funding by NSF MRI-1126249. APS is supported by DOE-BES, under Contract Number DE-AC0206CH11357. M. B. is supported by EFree, under Award Number DE-SC0001057. REFERENCES (1) Bostedt, C.; Landt, L.; Moller, T.; Dahl, J. E. P.; Carlson, R. M. K. Diamondoids in Nature's Nanostructures (ed. Barnard, A. S. and Guo, H.), Pan Stanford Publishing Pte., Ltd. 2012, pp. 169-194. (2) Marchand, A. P. Diamondoid Hydrocarbons-Delving into Nature’s Bounty. Science 2003, 299, 52. (3) Dahl, J. E. P.; Moldowan, J. M.; Peters, K. E.; Claypool, G. E.; Rooney, M. A.; Michael, G. E.; Mello, M. R.; Kohnen, M. L. Diamondoid Hydrocarbons as Indicators of Natural Oil Cracking. Nature 1999, 399, 54-57. (4) Wei, Z. B.; Moldowan, J. M.; Zhang, S. C.; Hill, R.; Jarvie, D. M.; Wang, H.; Song, F. Q.; Fago, F. Diamondoid Hydrocarbons as Molecular Proxy for Thermal Maturity and Oil Cracking: Geochemical Models from Hydrous Pyrolysis. Org. Geochem. 2006, 38, 227-249. (5) Gunawan, M. A.; Poinsot, D.; Domenichini, B.; Dirand, C.; Chevalier, S.; Fokin, A. A.; Schreiner, P. R.; Hierso, J. C. The Functionalization of Nanodiamonds (Diamondoids) as a Key Parameter of Their Easily Controlled Self-Assembly in Micro-and Nanocrystals from the Vapor Phase. Nanoscale 2015, 7, 1956.


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