Structure, Characterization and Exploration of Synthesis of Conical and Polyhedral Crystals of

A Thesis

Submitted to the Faculty

of

Drexel University

by

Svetlana Dimovski

in partial fulfillment of the

requirements for the degree

of

Doctor of Philosophy

January 2006

© Copyright 2006 Svetlana Dimovski. All Rights Reserved.

iii

DEDICATIONS

To my family and the divine circle of my friends, for shaping and enriching my life with their love and dedication, and

To the memory of my beloved step-father, Antonije Djuric, for a legacy of love, courage, humor, independence, and freedom of choice.

iv

ACKNOWLEDGEMENTS

This work would not be accomplished without the genuine support of a large

number of people. First and foremost, I would like to thank my graduate advisor, Prof.

Yury Gogotsi, for his support, coaching, encouragement and guidance throughout my

PhD studies. His vision, trust, enthusiasm and an extraordinary abundance of energy were a large source of motivation and a major driving force that kept me on this path. As a teacher, Prof. Gogotsi showed me how to look outside the box and how to define, analyze and solve problems. As a leader and manager, he taught me the importance of setting high goals, developing personal mastery, and staying firm and enthusiastic during the process. I am grateful that under his tutelage and his enduring optimism I had plenty of opportunities to practice and develop new skills and apply these learnings towards my professional and personal development.

Thanks are further due to my thesis committee members; Prof. Frank Ko, Prof.

Richard Knight and Prof. Alexander Fridman from Drexel University, and Prof. John E.

Fischer from the University of Pennsylvania. Their constructive criticism, challenging questions and stimulating discussions were of crucial importance towards completion of this work. They helped me address the problem from various directions and establish clear boundaries for my work.

During the course of my research I had an opportunity to collaborate with Prof.

Slava V. Rotkin from Lehigh University, Prof. John Jaszczak, Prof. Stephen Hackney and v

Prof. George W. Robinson from Michigan Technological University, and Prof. Pin-Heng

Tan from the Chinese Academy of Sciences. Prof. Rotkin’s expertise in modeling of the energetics of nanostructures was very instructional in understanding the mechanism of formation of graphite polyhedral crystals. Prof. John Jaszczak and the team from MTU shared their insights, knowledge, expertise and some data about naturally occurring counterparts of carbon cones. Working with Prof. Jaszczak has been a greatly rewarding experience for me and I owe him my sincere special thanks for the past few years of productive collaboration. Prof. Ping-Heng Tan provided his valuable insights in the analysis of Raman spectra of graphite single crystal edge planes and graphite polyhedral crystals. These collaborations contributed to the research in providing various pieces of the puzzle that brought me closer to the solution of problem.

I would like to thank Mr. David von Rohr for introducing me to the world of microscopy and for kindly sharing his knowledge and experience of this analytical technique. I am thankful to Mr. Davide Mattia for his experimental help with hydrothermal experiments and Dr. Haihui Ye and Dr. Joseph Libera for help with transmission electron microscopy. J. Libera started the research on polyhedral crystals and cones in that I continued. Dr. L. Rotkina and Dr. D. Ge helped me with

FIB work. Mr. Jay Bhatt, Hagerty library information services librarian, provided an excellent support in getting access to literature material utilized in the present study.

On a number of occasions Beth Carroll, Maria Pia Rossi, Holly Burnside and

Kristopher Behler proof-read my manuscripts, presentations or posters and helped me say what I really meant to say. I am grateful for their kind support. Members of the vi

Nanomaterials Group at Drexel University have always been supportive colleagues and friends, and for that I owe them greatest respect.

My new colleagues and great management team at Procter & Gamble Miami

Valley Information Center have been very supportive and understanding during the writing of this thesis. Mr. Matthew H. Chestnut kindly helped with proof-reading many of pages that follow. His input and suggestions were of a great value for me.

Thanks are further due to my undergraduate mentor, Prof. Leposava Sidjanin from the University of Novi Sad, Serbia and Montenegro, a person with charismatic energy, integrity and optimism. It was her professional expertise, guidance and coaching craftsmanship that had a vital influence in forming my desire to study materials science and engineering.

The Department of Materials Science and Engineering at Drexel University has been a venue of a tremendous change over the last few years. It was an exciting period of time for both faculty and students, and it was a valuable experience for me to participate in this growth process. I greatly enjoyed the friendliness and supportive atmosphere at the department.

Mr. Bill Jones from Solar Atmospheres, Inc. helped me run my initial experiments on their production site. Solar Atmospheres, Inc. built and donated a customized furnace for our experimental needs. Mr. Jones’s generosity and support are indeed far beyond ordinary, and I owe him my sincere thanks.

I am very grateful for getting to know every person that life had brought to me so far, but I would like to mention an inner circle of special friends whose presence had the vii

greatest impact on me since I came to the USA. They are: Riad, Korhan, Goknur, Mesut,

Alexei and Eda. Some of these people I’ve known for years and some of them I happened to meet recently, nevertheless, they all amaze me over and over with their compassion, strength, wisdom, passion and balance.

As a mentor and a friend, Prof. Riad Gobran truly enlightened my life with his wisdom, an interesting outlook on life, unlimited encouragement and support. I thank him for always going out of his way in helping me and guiding me through my trying times. I am thankful beyond words for his joie-de-vivre and the way he affected my perception of life.

Finally, my dear family, I thank them for their compassion, love, patience and strength. Unfortunately, the time spent away from each other can never be restored.

This research has been sponsored by the Department of Energy grant DE-FJ02-

01ER45932. The purchase of the Raman spectrometer and environmental SEM was supported by NSF grants DMR-0116645 and BES-0216343. Kind thanks go to Zonta

International and Nikolich Trust for financial support through the Amelia Earhart and

Studenica Foundation scholarships. viii

TABLE OF CONTENTS

LIST OF TABLES...... xi

LIST OF FIGURES ...... xii

ABSTRACT...... xxi

CHAPTER 1: INTRODUCTION...... 1

CHAPTER 2: LITERATURE REVIEW...... 5

2.1 Graphite...... 7

2.2 Carbon nanotubes...... 11

2.3 Observation of multi-shell non-planar graphitic materials ...... 18

2.3.1 Carbon whiskers and cones...... 19

2.3.2 Carbon nanotubes with polygonal cross-section...... 28

2.3.3 Properties of carbon cones and polygonal tubes: theoretical

considerations...... 31

2.4 Nanotubes and cones made of inorganic compounds...... 35

2.5 Summary, motivation and study objectives...... 41

CHAPTER 3: MATERIALS AND METHODS ...... 45

3.1 Materials ...... 45

3.1.1 Glassy carbon...... 46

3.1.2 Carbon cones in natural graphite samples ...... 51

3.2 Materials characterization techniques...... 55

3.2.1 Scanning electron microscopy ...... 57 ix

3.2.2 Transmission electron microscopy ...... 65

3.2.3 Micro-Raman spectroscopy ...... 76

CHAPTER 4: RESULTS AND DISCUSSION ...... 88

4.1 Carbon cones...... 88

4.1.1 Occurrence of cones in glassy carbon...... 88

4.1.2 Naturally occurring carbon cones ...... 101

4.1.3 Carbon cones – geometry considerations ...... 121

4.2 Graphite polyhedral crystals (GPCs) ...... 125

4.2.1 Morphology and size distribution ...... 125

4.2.2 Structure...... 128

4.2.3 Raman spectra of GPCs ...... 135

4.2.4 GPCs - geometry considerations...... 139

4.3 Testing the model of structure ...... 144

4.3.1 Scrolls or “Russian Dolls”? ...... 144

4.3.2 Intercalation and exfoliation ...... 146

CHAPTER 5: DISCUSSION OF POSSIBLE SYNTHETIC ROUTES TO CONICAL

AND POLYHEDRAL ...... 150

5.1 Parallel studies ...... 150

5.2 Hydrothermal routes to GPCs and GCCs ...... 155

5.2 Relation of GPCs and GCCs to hydrothermal nanotubes...... 160

CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS...... 166

6.1 Conclusions...... 166

6.2 Future work...... 170 x

LIST OF REFERENCES...... 174

APPENDIX A: MEASUREMENTS AND SUPPORTING MATERIAL ...... 192

APPENDIX B: EULER’S THEOREM ...... 196

APPENDIX C: ELECTRON DIFFRACTION PATTERN OF A MWNT...... 198

APPENDIX D: ENERGETICS OF POLYGONIZATION OF MULTIWALLED

CARBON NANOTUBES ...... 199

APPENDIX E: LIST OF EXPERIMENTS ...... 204

APPENDIX F: LIST OF ABBREVIATIONS (in order of appearance) ...... 216

VITA...... 219 xi

LIST OF TABLES

1. Impurity (ash) level and its breakdown126 ...... 49

A1. Pore size distribution in glassy carbon GL-200 ...... 192

A2. Summary of inorganic nanotubes reported in the literature and the synthetic procedures used for their production117 ...... 194

A3. Calculated values for Raman active vibrational modes of SWCNTs181 ...... 195

E1. Plan of experiments for oxidation of GPCs and GCCs in air...... 208

E2. Experimental parameters for annealing of MWNTs ...... 195

E3. Experimental parameters for growth of GPCs by mass transport in gas...... 195

E4. Experimental parameters for growth of GPCs by pyrolysis of phenolic resin...... 195

xii

LIST OF FIGURES

1. Ternary bonding diagram of carbon (adopted and modified from M. Inagaki23) ...... 6

2. Crystal structure of hexagonal graphite24 ...... 8

3. Schematic of: a) hexagonal graphite crystal; and b) rhombohedral graphite crystal. View is perpendicular to the basal plane.24 ...... 9

4. Imperfections in graphite: a) random rotation between adjacent planes; b) stacking faults and disclinations (deviation from parallel orientation); and c) vacancies.24 ...... 10

5. Basic structural unit of disordered graphite14 ...... 11

6. structure. Carbon nanotube can be considered both as an extended or a rolled sheet of . Arrows show two of many different rolling directions. The two indicated directions give rise to armchair and zigzag type of nanotubes. Adopted and modified from Refs.12, 26 ...... 12

7. Formation of a chiral nanotube: connecting atoms along A-B and A’-B’ lines in the direction indicated by arrow. Adopted and modified from Ref.12, 26 ...... 13

8. Indexing a carbon nanotube: Hamada indices nomenclature. Adopted and modified from Ref.12, 26 ...... 14

9. High resolution transmission electron micrograph of: a) single-walled (courtesy of Dr. H. Ye), and b) a bundle of single-walled carbon nanotubes26 ...... 15

10. Multi-walled carbon nanotube: a) High resolution transmission electron micrograph, b) Russian-doll, and c) Swiss-roll model of structure...... 16

11. Near-linear relationship between the sheet number and helix angle number in helical tubes. The helix angle changes approximately every four sheets.27 ...... 17

12. Electron micrographs of (a) pencil-like carbon columns; (b) columnar carbon specimen with screw-like markings on side46 ...... 20

13. Model illustrating the formation of cigar-like graphite: (a) longitudinal cross section of the cigars showing their texture; (b) cone-helix structure of graphitic filaments51 ...... 22

14. Conical graphite whiskers. (a) SEM micrograph of a cleavage fragment of conically wound graphite whisker. Note the 140° apex angle of the conical cleavage plane. (b) Electron diffraction pattern with the incident electron beam along the normal to the cleavage ‘plane’ of the conically wound whiskers. Note the 126-fold rotation symmetry of the pattern.53 ...... 23

15. Model illustrating the formation of a conical helix. (a) Sector β is removed from a disc. (b) The angular gap is closed and a cone is formed. (c) Twisted nucleus of the conical helix xiii

containing one pentagonal ring in the graphene network. Conical helix is formed through rotation of successive graphene sheets over a constant angle.53 ...... 24

16. Carbon cone showing separation of layers in fullerenic end caps63 ...... 25

17. Fullerene cones. (a) The five possible seamless graphitic cones, with cone angles of 19.2, 38.9, 60, 86.6, and 123.6°.19, 20 Electron micrographs of the corresponding five types of cones (scale bars in b–f, 200 nm). Apex angles: (b) 19.2°, (c) 38.9°, (d) 60.0°, (e) 84.6°, and (f) 112.9°.21 ...... 26

18. Scanning electron microscopy and HRTEM images of TGCs. a) Aligned TGCs grown on an iron needle surface. b) A high-resolution view of one TGC shows the faceted and helical appearance. c) TGC tip. The graphite sheet steps can be clearly seen on the surface of the cone in c).72, 74...... 27

19. (a) HRTEM micrograph of a nine-sheet nonsymmetric tube. A d spacing of 0.34 nm is found on the left side and a d spacing of 0.45 nm is seen on the right side.27 (b) Model illustrating the formation of the nonsymmetric fringes from a tube (a) with polygonal cross section.27 (c) The graphitic interplanar spacing decreases as the tube diameter increases, and approaches 0.344 nm at roughly D=10 nm. The data were measured from three different nanotubes indicated by different symbols. Hollow circles: from a seven-shell tube with innermost diameter Dmin =1.7 nm. For large D, graphitization may occur resulting in a polygonal cross section. The broken line indicates the expected decrease in interplanar spacing owing to local graphitic stacking.87 ...... 29

20. Electronic structure and localized states at carbon nanotube tips: (a) STM image of a Fullerene carbon cone; (b) local densities of states for a cone tip derived from scanning tunneling spectra at four (A–D) points along the tip; and (c–d) tight-binding calculations for two different configurations of cone tips.95 ...... 32

21. Tight-binding densities of states (states/eV/cell) for the (10, 0) cylindrical (a), and the (10, 0)5 pentagonal (b) cross-section nanotubes. The Fermi level is positioned at zero energy. Both nanotubes are also represented in inset on the right of their respective DOS. 101...... 33

22. Tight-binding band structures of the metallic (12, 0) nanotube, illustrating the effect of the degree of polygonization of the cross section on electronic behavior. Given here are examples of (a) the triangle (12, 0)3, (b) square (12, 0)4, and (c) hexagonal (12, 0)6 geometries that are compared to the pure cylinder case tube (d).101 ...... 34

23. Layered inorganic compounds. (a) Schematic representation of the mineral kaolinite, (b) 105 Schematic presentation of MoS2 layer with both bulk and rim atoms delineated...... 36

105 24. TEM image of a WS2 nanotube ...... 37

25. TEM image and schematic representation of H2Ti3O7 nanotubes: (a) axial direction; (b) cross section clearly showing the winding of the sheets into nanoscroll structure; (c) Schematic rendering of the cross section; and (d) Schematic rendering of the rolling up of the nanosheet into a nanoscroll.105 ...... 38

xiv

26. BN nanocones: a) SEM image of a stacked BN cone, b) HREM image of a tip region of a BN cone with an apex angle of about 20°. Tips of monolayer BN indicated by arrows have the closed shapes.114 c) A schematic of a BN cone. The apex is constituted by four pentagons with termination in two three-coordinated atoms.113 ...... 39

115 27. Faceted microtubules of Ga2O3. Tubes are found to have hexagonal cross section...... 40

28. Structure of glassy carbon: a) Two models of the glassy carbon structure.126, 127, and b) High- resolution transmission electron micrograph of GL-200 glassy carbon. The image shows nano-size cavities and short range order of lamellae...... 48

29. Glassy carbon GL-200: a) Condensation (curing) reaction illustrates the process of formation of phenolic resin from phenol-formaldehyde; b) A typical scanning electron micrograph of GL-200 fracture surface shows featureless amorphous material and occasional microporosity (circled); and c) Raman map collected around one such pore. Intensity of graphitic (G) peak relative to disorder-induced (D) band indicated that more of the graphitic carbon existed within the pore than in the surrounding matrix...... 49

30. Graphitic microgeodes - the pores of GL-200 glassy carbon: a) A detail of the GL-200 fracture surface with four crystal-bearing coalesced pores. The needle-like carbon crystals as grown inside the pores; b) Plate-like morphology of the inner surface of pore walls; and c). Detail of a pore showing a graphite conical crystal (GCC) and two graphite polyhedral crystals (GPCs) ...... 51

31. Occurrence of graphite in nature: a) Tabular; b) Spherical, and c) Fibrous graphite (Prof. J. Jaszczak’s collection of photographs134) ...... 52

32. Natural carbon samples: a) Reflected cross-polarized light image of a portion of a 2-mm graphite sphere cross-section. b) Optical image of a 1.5-mm diameter graphite sphere with pronounced ridges and tiny cones on the surface. c) FESEM image of a graphite triskelion covered with cones.119 ...... 54

33. Signals resulting from interaction of the primary electron beam with the specimen in an SEM.147 ...... 57

34. Variations of secondary electron signal intensity due to changes in specimen topography.14758

35. Schematic of an SEM (Copyright: The McGraw-Hill Companies, Inc.) ...... 60

36. Schematics of backscattered and secondary electron detectors. Inset shows BSE and SE signals emitted with a finely focused high-resolution beam.147...... 61

37. Carbon sample sensitivity to beam energy: a) Image obtained with 25 kV acceleration voltage. b) Image obtained with 5 kV. One can see charging and the lack of surface detail in a)...... 63

38. Schematic of a transmission .151 ...... 67

39. Schematic of electron diffraction in TEM...... 70 xv

40. The diffraction patterns of graphite: a) Dotted pattern of a hexagonal pyrolytic graphite monocrystal; and b) Beaded ring pattern of a polycrystalline graphite sample.24 ...... 71

41. HRTEM image of a multi-walled carbon nanotube (courtesy of Dr. Haihui Ye). Arrows show defects in tube’s walls...... 72

42. FIB sample preparation: a) A spot of interest is located on the glassy carbon sample; b) A protective thin platinum layer is deposited onto the surface; c) One trench is etched out from the sample, d) and the second one. The sample is thinned further with a soft ion beam. e) An FIB manipulator is used to pluck and lift the sample from the specimen. f.) Sample is transferred to the TEM grid. Scale bar is 10 µm for a)-e) and 5 µm for f)...... 75

43. a) Generic model of Raman spectra. b) First- and second-order Stokes (SR) and anti-Stokes Raman (ASR) spectra of multi-walled carbon nanotubes obtained with 2.54-eV excitation. For convenience, the negative portion of the x-axis is inverted and overlapped with the positive portion.157 ...... 78

44. Characteristic Raman spectra of various carbon materials (courtesy of Prof. Dresselhaus) ... 79

45. First- and second-order Raman spectra of graphite crystal basal and edge planes as a function of incident beam energy and edge orientation with respect to polarized beam.172 ...... 80

46. Raman spectra of turbostratically stacked (TS) carbon and an individual graphite whisker excited with 632.8 nm laser excitation. The inset gives the energy 172 dependence of the frequencies of the L1 and L2 modes...... 82

47. a) Raman spectrum (514.5 nm excitation wavelength) of single-walled carbon nanotubes produced by an electric arc, b) Enlarged detail of the spectrum shows the peaks in 100-250 cm-1 region.14 ...... 83

48. Micro-Raman spectra (low-frequency region) of A, B and C HADE MWNTs specimens, SWNTs and HOPG.186...... 84

49. Schematic of a Renishaw Raman microspectrometer. The monochromatic incident beam is redirected through a set of optical components into the microscope objective. Objective is used for illuminating the sample and for collecting light scattered on the sample. Inelastically scattered light is then dispersed into a spectrum inside the main spectrometer unit. The computer collects Raman signal from the CCD detector attached to the spectrometer and optical images from the video camera attached to the microscope.187 ...... 86

50. SEM micrograph of GL-200 fracture surface showing the interior microstructure of two pores. Conical crystals are marked with arrows...... 90

51. SEM micrograph of hydrothermally etched GL-200 graphitic pore shells (egg-like structures) and microstructure of several pores...... 91

52. SEM micrographs of two GCCs. a) A 17º-GCC attached to the pore wall, b) A small apex angle GCC (~3º) with an almost flat tip...... 92 xvi

53. The structure of the dome-capped cones: a) A TEM micrograph of a 14.5º-GCC. Low resolution TEM shows the hollow structure of the cone. The inner and outer surfaces are marked by dashed lines. HRTEM inset shows lattice fringes of the 002 graphitic planes of the cone tip. Central nano-size channel extended throughout the cone tip. The features such as dislocations (D) and the edge-terminating loops (L) were observed. b) Schematic model of the hollow cones structure...... 94

54. (a) TEM image showing the hollow morphology of an 8º-GCC. Apex angle of the inner surface reduced by 2º due to wall thickening close to the tip. (b) HRTEM of cone’s walls shows a zone A of graphitic (0.34 nm) fringes and zone B that showed a change in stacking sequence and non-uniform lattice spacing...... 95

55. Schematic representation of a scroll-type structure with a dislocation that potentially can explain the unusual spacing observed in Figure 54b.203 ...... 97

56. TEM analysis of a small apex angle cone. Shown here is a 2.7º-GCC (low resolution TEM). The (002) lattice fringes from the side of the cone are shown in the inset. The walls of the GCC were well-ordered, the interplanar spacing being 0.34-nm...... 98

57. SAED pattern of the 2.7º-GCC taken from the circled area (Figure 56). Conicity of the structure contributed to elongation of diffraction spots in the direction parallel to the cone axis, as shown schematically for 0004 spot...... 99

58. a) Scanning electron micrograph of graphite sphere partially etched out from calcite. The surface of the sphere was covered with carbon cones. b) Model of spherical graphite originally developed by Double and Hellawell52 to explain the growth of graphite spheres in nodular cast iron...... 103

59. FESEM images of a cone-covered graphite aggregate. (a) Low- magnification image showing complete coverage of the aggregate surface with conical structures. A ~39º cone is marked by an arrow. (b) Higher magnification image of the sample showing a variety of large cones with different apex angles and sharp and blunt tips. Arrows show changes in the apex angle. (c) Close up view of two surfaces which are almost perpendicular and show different cone morphologies - large cones on one surface and globular (artichoke-like) structure on the other. The latter are clusters of large-angle cones...... 104

60. Kola graphite: a) Optical photograph of a 7-mm spherical cluster of radiating graphite fibers with albite, apatite and aegirine (courtesy of Prof. J. Jaszczak) b) SEM image of graphite hollow polyhedral fibers and associated cones. Strontium-bearing apatite crystals inside graphite channels. A graphite cone is circled at the left. c) Kola graphite cones...... 105

61. Scanning electron micrograph of graphite whiskers of varying morphologies, coating the surfaces of aegirine and associated minerals...... 106

62. Histogram of the frequency of occurrence of apex angles on a single sample of graphite. Outlined green bars indicate the positions of expected apex angles from the disclination model for cone-helix structures (see Section 4.1.3). Arrows show expected apex angles for the pentagon defect model; these angles also preserve the ideal graphite stacking and are xvii

therefore expected to have relatively low energies and be more predominant. Inset: the inclination of cones relative to the focal plane of microscope has been taken into account…107

63. Typical Gooderham cone morphologies. (a) SEM image of a cone with a 60º apex angle, the most common apex angle. The slightly uneven surface of the cone suggests their layer- growth mechanism. (b) FESEM and (c) SEM images of large cones with numerous smaller cones growing on their surface. Smaller cones covering surfaces of large cones have a broad distribution of shapes, but large apex angles prevail (c). (d) FESEM image of four cones having sharp and broad tips (multiple tips are marked by arrows). The cones are oriented to reveal their circular cross sections around the tips and layered growth (ripples)138...... 109

64. Growth irregularities in Gooderham cones: a) Inter-growth of two cones. b) A cone with a wedge defect...... 110

65. Surface of a carbon aggregate covered with curved graphite shells and hollow or incomplete carbon cones...... 111

66. FESEM micrograph of carbon cones with a) open, and b) faceted tips...... 112

67. Graphite scrolls and tubes from Hackman valley, Mt .Yukspor, Kola Peninsula, Russia. a-d) FESEM images of graphite scrolls of varying morphologies, coating the surfaces of aegirine and associated minerals in fractures in the pegmatite. a) Cigar-like carbon scrolls. b) Broken scroll revealing a hollow center. c) A carbon whisker with scroll morphology. d) Cone with a smooth dome-shaped tip. e) The hole revealed at the surface where the crystal have been broken from the graphite substrate119 ...... 113

68. Kola scrolls and cones showing two different tip morphologies...... 114

69. Impurities in Kola samples: a) FESEM of a cone covered with a grainy layer of a mineral. b) Tubular nanostructures observed on the surfaces of some cones. c) EDS spectrum of another cone showing a high content of boron and presence of nitrogen...... 115

70. HRTEM micrograph of a graphite nanocone with ~39° apex angle. The (002) lattice fringes were well-ordered and parallel to the cone surface...... 116

71. a) HRTEM image of a graphite nanocone with ~126° apex angle showing partially ordered lattice fringes parallel to the cone surface. b) HRTEM micrograph from the same cone. The image shows a discontinuity of the apex angle in the vicinity of the tip139 ...... 118

72. a) TEM micrograph of a 60º cone. b) Electron diffraction pattern from the center of the cone shows clear 0002 spots from the two sides of the wall ...... 119

73. Comparison of Raman spectra from single crystal graphite, naturally occurring graphite cones from Gooderham, Ontario, and scroll-type whiskers of graphite from the Kola Peninsula. Cross-hairs in the optical images of the insets indicate the positions of the Raman probe for the respective spectra...... 120

74. “Goodness of fit” according to cone-helix model: Four out of several energetically favorable (001) twist grain-boundary angles: a) 13.2º, b) 21.8º, c) 27.8º and d) 30º...... 124 xviii

75. High resolution FESEM micrographs of three typical GPCs with 9 facets: a) An axially true GPC with a tip in the form of a step-like pyramid, b) Helical GPC with a flat pyramid tip and most probably the growth spiral on the surface. One face appears slightly larger and two appear noticeably smaller than the other six faces. c) A perfect helical GPC with flat tip appears to have nine-fold rotational symmetry...... 126

76. FESEM images of various needle-like crystal’ cores and extensions: a) A straight 12 nm in diameter carbon nanotube core extends out of a straight and thin body. b) A long and thin carbon nanotube is shared by a large (1) and a small (2) GPC. Helix angles of the two GPCs appear to be different. c) Irregular shapes occur in some of the thin cores. d) A broken GPC reveals a very flexible and strong core, suggested to be a carbon nanotube...... 127

77. FESEM images of: a) A hollow cylindrical carbon nano-rod (most probably a thick and straight MWNT). b) A GPC with multi-faceted closed tip. It was expected that such tips contain sp3 carbon...... 128

78. TEM micrographs of: a) GPC with pentagonal cross-section. The multi-walled carbon nanotube core was nested in the polygonized body. b) HRTEM of two GPCs sharing a single multi-walled nano-size core of a conical shape...... 129

79. Low (a) and high (b, c) resolution TEM images of a GPC with a MWNT extending from the tip. HRTEM image near the core of the GPC shows the MWNT core of 23 nm in diameter. GPC body terminated with the dome-like end, the surface of which was covered with loop structures...... 130

80. HRTEM of a surface of the dome-like GPC end. Loops (1) are formed through zipping of 4 or more adjacent layers. The loops have configurations of half-tori (2). Some of the layers (3) do not zip...... 131

81. Structure of a graphite polyhedral crystal: a) Inverted SAED pattern of a GPC with multi- walled core nanotube. b) Corresponding TEM image of selected area of the GPC. Dark rectangle is the beam stopper...... 133

82. Structure of a long cylindrical GPC: a) Inverted SAED pattern of the GPC. b) Corresponding TEM image of the GPC supported by a lacey carbon film of the TEM grid. The circle shows the area that gives the pattern in a)...... 135

83. Fundamental, combination modes and overtones in Raman spectra taken from the side face and the tip of an individual graphite polyhedral crystal (514.5 nm excitation)...... 136

84. Raman scattering from graphite polyhedral crystals. a) 1400–1500 cm−1 lines, and b) 3151 and 3174 cm−1 features (514.5 nm excitation). Different colors correspond to 5 different crystals...... 138

85. Polygonization of multi-walled carbon nanotubes: a) Schematic of dislocation model,85 b) Planar and near-planar regions of graphitic stacking are separated with the regions of high curvature, c) 2000°C annealed tubes showing overgrowth of nanotube over the original in a sheath like structure, and d) Polygonal structure of a MWCNT annealed at 2000°C...... 140 xix

86. Interrupted growth, imprints on crystal’ faces and ring-like structures found in GPCs...... 145

87. FESEM micrograph of GL-200 GC after exfoliation. Etching, fracture and exfoliation of GPCs, and pullout and unwrapping of GCCs are observed...... 147

88. FESEM micrographs of exfoliated GPCs. a) Two GPCs with etched surfaces. Etching occurs in the planes perpendicular to axis. b) GPCs retained their polygonal shape after exfoliation. Delamination of shells occurs occasionally...... 148

89. a) Fractured GPC reveals strong nanotube core. Fracture occurred along a preferred plane. Unwinding of b) a conical and c) a cylindrical graphite crystal (this can also possibly be a small apex angle cone). The mechanism of the degradation of conical crystals showed that they grew through a scroll mechanism...... 149

90. Conical and polyhedral carbon nanostructures reported after 2002: a) SEM image of a small apex angle carbon cone grown on VGCF surface by thermal decomposition of propane,65 b) TEM image of a typical hollow carbon cone produced by thermal decomposition of butyl alcohol in presence of metallic Mg,222 c) Pin-like polyhedral rod crystal produced by a combustion flame method,90 d) Cross-sectional HTTEM view of faceted MWNTs (a facet marked with arrow)223...... 153

91. a) SEM micrograph of cones.227 b) TEM micrograph of a polygonized nanotube similar to GPC produced from supercritical C-O-H fluids.195 ...... 157

92. Proposed process of formation of GPCs and GCCs in glassy carbon: a) Formation of closed pores filled with gas in C-O-H system, b) Precipitation of carbon from supersaturated C-H-O fluid and nanotube growth on the walls toward the center of pores. c) Thickening of nanotubes and formation of GPCs. Growth is limited by pore dimensions...... 161

93. TEM images of (a) glassy carbon matrix, (b) nanocones, (c) nanotubes and , (d) nanotubes and multi-shell carbon nanoparticles...... 164

C1. Normal incidence electron diffraction pattern and its analysis of a multishell tube containing several isochiral clusters of chiral and achiral tubes. Note the presence of graphite-like reflections as reinforcements in streaks.62 ...... 197

D1. Model of a coaxial cylindrical MWNT vs. polygonized coaxial MWNT...... 199

D2. The energy difference ∆E between a round and a polygonized MWNT as a function of number of shells (n) for various numbers of faces (r) and: a) N1=120 atoms, and b) N1=36 atoms in the inner shell. Going from red to blue curves, the number of facets (r) was varied from 5 to 20...... 201

D3. Energy difference between a round and a polygonized MWNT as a function of number of shells (n) and number of atoms in the first shell (N1). The plot shows domains of round and polygonized multiwall nanotubes and defines the region of relative stability of polygonized MWNT. A and B are energy contributions of incompatibility of layers in cylindrical MWNT and energy of edges curvature in polygonized MWNT ...... 202 xx

E1. Roadmap of experiments for characterization of conical and polyhedral crystals of graphite206

E2. Roadmap of experiments for degradation of GPCs and GCCs ...... 206

E3. Roadmap of synthesis experiments...... 207

E4. SEM image of GC-200 fracture surface after oxidation at 400ºC for 10h ...... 208

E5. (a) Custom made and (b) commercial crucibles. (c) Vacuum furnace at Drexel University, donation to the Department of Materials Science and Engineering by Solar Atmospheres, Inc. A large (7.5” diameter x 14.5” deep) hot zone constructed with energy efficient graphite insulation allows a maximum temperature of ~ 2200ºC with extremely high thermal uniformity within the zone (± 5ºF)...... 210

E6. Pyrolytically stripped carbon nanotubes annealed in presence of carbon source: (a-b) argon flow at 1750°C and (c-d) in vacuum at 2000°C. Observed was thickening around core nanotubes coming from mass transport of carbon from carbon source to nanotubes...... 212

E7. Carbonaceous material synthesized in pores of glassy carbon through carbonization and pyrolysis of phenolic resin at 2000ºC: (a) Carbon spherical and spheroidal particles. (b) Filamentous carbon structures...... 214

E8. Products of hydrothermal synthesis of short and straight carbon filaments from SWNTs treated in deionized water at 760ºC and 14,000 psi for 24 hours...... 215

xxi

ABSTRACT Conical and Polyhedral Crystals of Graphite Svetlana Dimovski Advisor: Yury Gogotsi, Ph.D.

The present study describes the structure, Raman spectra and growth model of

synthetic and naturally occurring conical and polyhedral nano- and micro- crystals of

graphite. While planar graphite and carbon nanotubes have been extensively studied, and

their structure and properties are well documented in the literature, the world of non-

planar multishell graphitic materials was somewhat neglected. With a growing tendency

to design and utilize materials that will bridge the gap between traditional micro- and

“emerging” nano- technologies, the importance of understanding such structures is

continuously increasing.

In the present work, Raman spectroscopy, scanning and transmission electron

microscopy with electron diffraction were used as primary techniques to characterize the

morphology and structure of conical and polyhedral nano- and micro- crystals of

graphite. Such information was used to understand the environment suitable for their

growth.

Graphitic carbon cones (GCCs) and graphite polyhedral crystals (GPCs) have an

elongated needle-like and, typically, axially symmetric morphology. The length of these low-symmetry crystals ranges from several tens of nanometers up to several microns.

Graphite has been known to form seamless graphite nano- and micro-cones that can have xxii

up to five different apex angles determined by disclinations in graphite. Scrolled conical structures, which grow with a different mechanism, may have virtually any apex angle between 2-3 degrees up to 150 degrees. However, the “magic” angles of 19.2, 38.9, 60.0,

83.6 and 112.9 degrees are still most frequently observed. These apex angles are found to be energetically preferred as they allow the registry between graphene sheets in the cone.

Polygonization of nanotubes accompanied by growth in the radial direction leads to the formation of graphite polyhedral crystals (GPC). They have nanotube cores and graphite crystal faces and remarkably unusual axial symmetries.

The structure of carbon cones and polyhedral crystals has been studied, and the mechanism of their formation has been proposed in the present work. The study reports formation of small apex angle cones that have not been predicted by theory, and it also provides detailed information about the occurrence of carbon cones in nature. The morphology, structure and diffraction patterns of graphite polyhedral crystals were explained. 1

CHAPTER 1: INTRODUCTION

Layered solid materials have been used for many millennia when ancient man

drew his first cave painting, shaped his first habitat, and formed clay pots. Layered solids

have also been studied for centuries by geologists, chemists, physicists and materials

scientists. Natural layered materials such as rocks, graphite, mica and clay, as well as

human-developed layered superconductors,1,2 ceramics3 and composites4 have shaped the

face of the Earth and society to an amazing extent. Nowadays we find these materials

practically everywhere: from houses, roads and bridges, to nuclear power plants, aircraft

and space shuttles.

With very few exceptions, all studies on layered solids deal with materials

composed of planar sheets, meaning that atoms or species in every single layer have

planar conformation; layers, on the other hand, do not necessarily have to be identical.

Certainly one of the most ubiquitous of this kind of solid is graphite.5 Unlike all other

layered solids, graphite is comprised of a single element – carbon. Carbon atoms, when

sp2 hybridized, form a hexagonal arrangement within planes that are called graphene

sheets. Graphene sheets are stacked in a hexagonal, rhombohedral or a random

(turbostratic) pattern and are weakly bonded together by van der Waals forces. Among many other attractive properties, the weak bonds between graphene layers make graphite one of the softest engineering materials, and the best solid lubricant known. 2

Until recently, graphite and were the only two known carbon

polymorphs. The discovery of carbines,6 fullerenes,7-9 and carbon nanotubes8 in the late

80s and early 90s, however, extended the family of solid carbon materials to linear

(chain) and non-planar (curved) . Curved carbon surfaces have the same

coordination number CN = 3 as graphite, yet their bonding nature and properties render

them as a separate class of carbon materials. Furthermore, the discovery of closed shell carbon materials launched a whole new era of nanoscience and that in a very short period of less than fifteen years has become one of the top research initiatives across the globe. Moreover, the global market for is expected to reach

$20.5 billion by the year 2010.10

Graphite and graphitizable carbon materials have been studied in depth for more

then a century. For a decade, fullerenes, single-wall, and multi-wall carbon nanotubes

have been the subject of extensive studies, and their structure and properties have also

been well documented in the literature.11-15 The present study focuses on structural

characterization of nano- and micro-size non-planar multi-layered sp2 carbon materials,

such as graphite whiskers, cones and graphite polyhedral crystals that according to their

structural features may be considered as intermediate materials between graphite and

carbon nanotubes.

Although non-planar graphitic microstructures in the shape of cones had been

reported as early as 1957,16,17 it was not until several years ago18-21 that more attention

was paid to these exotic classes of graphitic materials. Until now, extensive studies have

not been conducted to determine the parameters and define the morphology of complex 3

non-planar axial multi-shell carbon nanomaterials. To understand the morphology,

structure and mechanisms of their nucleation and growth, a study addressing these questions is necessary. In the present work, the morphology and structure of several novel types of axial carbon nano- and microcrystals, including natural and man-made

carbon cones and graphite polyhedral crystals have been probed using different techniques. High resolution scanning (SEM) and transmission electron microscopy

(TEM) have been successfully employed to reveal the morphology and to gather

statistical information about crystal size and geometry distribution. Structural information, in addition to high resolution TEM (HRTEM), has been also assessed with

Raman spectroscopy, a powerful tool for the analysis of carbon materials.15,22 The

approach and the model proposed here are applicable for the analysis of many different

layered materials.

Although considered exotic at present, cones, whiskers and graphite polyhedral crystals may have numerous applications where sizes between nanotubes and are required, and fill the gap between the nano- and micro-worlds. It is easy to envision that one day these materials will find applications in various areas of

engineering because of their structure, morphology and unique properties; many of which

are theoretical predictions at present. Examples of such applications include:

• materials engineering: graphitic cones and polyhedral crystals as reinforcements

for nanocomposites and new functional nanomaterials;

• chemistry and biomedicine: chemical sensors, cellular probes, and micro/nano

electrodes; 4

• analytical tools and instrumentation development: conical and polyhedral probes

for atomic force and scanning tunneling microscopes and ultra-small indenters;

• energy, transportation and electronic devices: materials for energy storage, field

emitters and components for nano-electromechanical systems (NEMS).

On the path to these industrial applications, properties of these exotic new forms of elementary carbon must be determined and low-cost methods for large-scale manufacturing must be developed.

5

CHAPTER 2: LITERATURE REVIEW

Carbon is a unique element in terms of its unprecedented aptness to form a myriad

of solid carbon forms. These forms exist naturally in the Earth’s crust, in extraterrestrial

space, and can be synthesized artificially. Two possible ways to classify them are

according to their origin and their application. Another approach, which is particularly

important for this study, takes into account two important parameters: type (nature) of

carbon bond, and characteristic size of carbon entity (Figure 1).

Carbon atoms form bonds with their nearest neighbors by various combinations of

s and p orbitals. This phenomenon is known as orbital hybridization. The compact

electronic core of the innermost shell of the carbon atom allows valence electrons from

the outer shell to adjust when forming bonds with their nearest neighbors, which results

in the formation of one-dimensional (1D) chains, two-dimensional (2D) planes, and

three-dimensional (3D) tetrahedral bond networks. In addition to sp, sp2 and sp3 types of hybridization, carbon also yields systems with bonds that are intermediate between the sp2 and sp3, resulting in curved atomic carbon surfaces (Figure 1). This diagram emphasizes the topological similarity between organic molecular carbon species and bulk inorganic all-carbon materials according to the nature of bonding and characteristic size of the carbon/ or pure carbon entity.

This work focuses on new nano- and micro-size solid forms of elementary carbon with sp2 and sp2-sp3 bonds. An attempt has been made to fill the gap in knowledge 6

Graphite (sp2)

Amorphous carbon, , DLC, glassy carbon, rs e carbon blacks, etc. k is d Nano-size h n l a a morphology of w r n s d ? graphitic o e e s b n h l r o y ta materials c l a o s C p ry c sp3+sp2+sp

e n e Ovalene r y e l ll i u m Nanotubes n F fa sp sp2 + π

Corannulene C Fullerenes sp3 sp + 2π

=C=C= Adamantane Cumulene Hydrocarbons

Nanodiamond Diamond (sp3) spn, (1

Figure 1: Ternary bonding diagram of carbon (adopted and modified from M. Inagaki23).

existing between fullerene and graphite families of carbon materials, as indicated by the red text and question mark in Figure 1.

This chapter gives an overview of the literature on various types of non-planar carbon nano- and micro- size materials concluding with the year 2000, when this research was launched. Summarized here are a brief history of the discovery of cones and polyhedral crystals along with synthesis methods, structural and geometrical considerations, and theoretical predictions of their properties. Progress and discoveries made by other teams in parallel to this study are used as references in data discussion

(Chapters 4 and 5). 7

2.1 Graphite

Although graphite has been used for drawing and writing since “the dawn of

history”, not until the 18th century was it realized that graphite and diamond were actually

two allotropes of the same element – carbon. The disparity of their properties, which is

simply amazing, is the result of different arrangements of atoms in the unit cell, and it can

be traced to the nature, strength and spatial orientation of bonds between the atoms.

Graphite is easily distinguished by its superlubricious properties as one of the best and

most cost effective solid lubricants. The most common crystal structure of graphite is

hexagonal (alpha), with unit cell dimensions a = b = 2.456 Å, c = 6.708 Å, and a c/a ratio

= 2.73 (Figure 2).

In graphite, every carbon atom is covalently bonded to three other surrounding atoms with short (1.418 Å) and strong (524 kJ/mole) sigma bonds in a form of hexagonal pattern. All of the sigma bonds belong to a single plane, in crystallography known as a basal plane. When isolated, this plane of sp2 bonded carbon atoms is called a graphene layer. Weak van der Waals bonds (pi bonds) of only 7 kJ/mole are formed between atoms of adjacent layers through the formation of electron pairs between fourth delocalized valence electrons of the hybridized p orbitals. The spacing between layer planes is therefore relatively large: c/2 = 3.347 Å. Due to pi orbital electrons that are delocalized across hexagonal atomic layers, carbon also conducts electricity in planes parallel to the basal planes. Conductivity in the direction perpendicular to the basal planes (c direction) 8

Figure 2: Crystal structure of hexagonal graphite.24

is negligible. Similarly, thermal and acoustic properties are highly anisotropic, since

phonons propagate quickly along the tightly-bound planes, but are significantly slower to

travel from one plane to another.

Sequential stacking of graphene layers yields both hexagonal and rhombohedral

forms (Figure 3). In the more common hexagonal graphite, graphene layers are stacked in

an –ABABAB – pattern. In other words, the carbon atoms in every other layer are superimposed over each other, as shown in Figure 3a. Here one can distinguish between alpha and beta carbon atoms (Figure 2), the former having neighbor atoms in the adjacent plane directly above and below, and the latter having no neighbors in the corresponding

planes. 9

a) b)

Figure 3: Schematic of: a) hexagonal graphite crystal; and b) rhombohedral graphite crystal. View is perpendicular to the basal plane.24

Rhombohedral phases (Figure 3b) have been observed when graphite is subjected

to deformation processes such as grinding.25 Here, the stacking sequence of layers

follows an –ABCABCABC– pattern. The carbon atoms in every third layer are

superimposed. The unit cell has dimensions of a = b = 2.256 Å, and c = 10.06 Å.

Rhombohedral graphite is thermodynamically unstable, and it easily converts to the hexagonal form during heat treatment above 1300°C. It is never found in its pure form but always as an associated phase to hexagonal graphite. From that perspective, rhombohedral graphite can be considered as an extended stacking fault of the hexagonal form.

Stacking order between adjacent graphene layers can be completely disturbed when the value of planar translation departs from the values that correspond to hexagonal or rhombohedral stacking, or adjacent basal planes are randomly rotated around the direction of the c-axis (Figure 4). 10

a) b) c)

Stack of two layers with random rotation

Figure 4: Imperfections in graphite: a) random rotation between adjacent planes; b) stacking faults and disclinations (deviation from parallel orientation); and c) vacancies.24

Graphite that lacks the three-dimensional order is called turbostratic graphite and

is distinguished by increased interlayer distance. In addition, disclinations (planes that are

no longer parallel) and vacancies (missing atoms within the planes) (Figure 4) add to the

overall disorder of the structure. Real carbon materials are therefore polycrystalline and

can be considered as solid aggregates of graphite crystallites, as schematically

represented in Figure 5. Properties of polycrystalline graphite depend greatly on the

apparent size (L1) of the basic structural unit (crystallite).

Graphite is chemically inert to and alkalis under most conditions and it is

heat resistant up to 3200°C under vacuum or inert atmospheres. In or air,

however, it oxidizes around 400°C. The heat resistance and chemical stability of graphite

are exploited in crucibles and refractories, rocket nozzles, space shuttle heat shields as well as hot pressing and hot extrusion dies. High purity graphite also finds use as a matrix and neutron moderator in nuclear reactors. Unlike diamond, graphite is an electrical conductor and as such is widely used in electrodes for metal refining and in electric arc lamps. 11

Figure 5: Basic structural unit of disordered graphite.14

Graphite also finds applications as a structural material. Its strength to weight

ratio is remarkable. Graphite in the form of fibers is widely used for reinforcement in

lightweight composite materials. Its strength, preserved even at very high temperatures

(40MPa at 1600°C), has made graphite the material of choice for hot pressing and

extrusion dies.

2.2 Carbon nanotubes

As discussed in previous sections and shown in Figure 1, graphene sheets are

formed from pure sp2 carbon, whereas the three-dimensional bonding network of sp3 carbon is found in diamond. When a curvature is introduced in a planar graphene sheet, as in fullerenes and carbon nanotubes, a partial loss of the sp2 hybridization occurs, and

the sp3 bond character increases due to change in the bond angles. This event is called

rehybridization of sp2/sp3 bonds. 12

carbon nanotubes as elongated fullerenes armchair nanotube

zigzag nanotube

formation of larger fullerenes

two of many possible C60 fullerene rolling directions

Figure 6: Carbon nanotube structure. Carbon nanotube can be considered both as an extended fullerene or a rolled sheet of graphene. Arrows show two of many different rolling directions. The two indicated directions give rise to armchair and zigzag type of nanotubes. Adopted and modified from Refs.12,26

A carbon nanotube, in essence, is a cylindrically shaped shell structure of three- coordinated carbon atoms connected together with rehybridised sp2/sp3 bonds. Ideally,

both ends of the cylinder are closed with a hemi-fullerene. Conceptually, carbon

nanotubes can be considered both as extended fullerenes (extension coming through the

insertion of carbon belts), or as seamless carbon cylinders formed by rolling up a

graphene sheet (shown schematically in Figure 6). The structural similarity of carbon

nanotubes and fullerenes justifies placing these two types of materials in the same family of carbon materials.

There are many possible ways to roll up a graphene sheet into a seamless cylinder

(Figure 6). Two extreme cases result in an armchair and a zig-zag type of tubes, while any other arbitrary direction gives rise to chiral nanotubes. 13

B’

ChiralChiral angleangleis is defineddefined asas angleangle betweenbetween wrappingwrapping directiondirection andand aa unitunit cellcell vectorvector θ A’ directiondirection B

a1

A a2 chiral carbon nanotube

Figure 7: Formation of a chiral nanotube: connecting atoms along A-B and A’-B’ lines in the direction indicated by arrow. Adopted and modified from Ref.12,26

A model of a chiral nanotube is shown in Figure 7. If a rectangle indicated by A-

B-B’-A’ is cut out of a graphene sheet, rolled up in direction indicated by the arrow, and atoms along A’-B’ superimposed to their matching A-B counterparts, a chiral tube with a chiral angle θ is formed. The chiral angle θ is defined as the angle between the direction

r of rolling (A-A’) and the direction of unit cell vector ( a1 ). Since the physical properties of carbon nanotubes strongly depend on the chiral angle θ and the tube diameter d, a set of these two parameters is necessary to unambiguously define each tube. Since the length of the tube affects mechanical properties of a bulk carbon nanotube material, but does not have any impact on the tubes’ physical properties per se, it is not usually specified in tube’s nomenclature.

Often, for the purpose of simplicity, a pair of integers (m,n) called Hamada indices is used to indicate a particular tube geometry (Figure 8). In an (m,n) carbon nanotube, the numbers m and n correspond to a particular position of a carbon atom in an 14

(0,0) (3,0) (6,0) ZigzagZigzag

(5,2)

(2,2)

(4,4) ArmchairArmchair a1

a2 (n,m)(n,m)--HamadaHamada indicesindices

Figure 8: Indexing a carbon nanotube: Hamada indices nomenclature. Adopted and modified from Ref.12,26

arbitrary coordinate system of a two-dimensional graphene network which forms the

(m,n) tube when superimposed to the (0,0) atom in the same coordinate system.

There is a simple correlation between the tube diameter, the chiral angle and

Hamada indices:

a d = ⋅ ()n2 + m2 + n ⋅ m (1) π

and:

⎡ 3 n + m ⎤ θ = arccos⎢ ⋅ ⎥ (2) ⎣ 2 n2 + m2 + n ⋅ m ⎦

15

a b

Figure 9: High resolution transmission electron micrograph of: a) single-walled (courtesy of Dr. H. Ye), and b) a bundle of single-walled carbon nanotubes.26

One can easily see that all (m,n), where m = n, indicate an armchair tubular structure, and tubes (m,n) where n=0 are zig-zag tubes. For every other combination of m and n, the tube structure is called chiral. In chiral tubes, chains of atoms spiral around the tube axis instead of closing a circumference. If a nanotube consists of a single carbon cylinder, it is called single-walled carbon nanotube (SWCNT) (Figure 9a). SWCNTs tend to align themselves into bundles or ropes (Figure 9b) held together by weak van der

Waals forces. When the tube is comprised of the two or more layers, it is called multi- walled carbon nanotube (MWCNT) (Figure 10).

If a MWCNT consists of several nested tubes, it gives rise to a Russian-doll type of MWCNT. When the walls are comprised through a manifold scrolling of a single graphene layer, the so-called Swiss-roll structure is produced. Nomenclature for indexing of multiwalled carbon nanotubes is somewhat different than for SWCNTs in the sense that it takes into account the number of graphene layers within the tube walls. Interlayer spacing in MWCNTs is somewhat larger than that in graphite because of very high strain 16

c) a) b)

Figure 10: Multi-walled carbon nanotube: a) High resolution transmission electron micrograph, b) Russian-doll, and c) Swiss-roll model of structure.

. energy in smaller diameter tubes. It becomes closer to the value of turbostratic graphite of

0.34 nm when a sufficiently large number of layers are present in the tube walls.

Characteristic for MWCNTs is that the chiral angle is almost never constant along the tube radius. Very often, θ changes with the tube diameter (Figure 11), which suggests that the mechanism of competing energies plays a role in the formation of multi-layered tubular carbon structures. For inner shells, very small tube diameters give rise to a very high strain energy, which prevents atoms of adjacent layers from satisfying both epitaxy and geometry conditions (graphite interplanar spacing of 3.347 Å). 17 Number ofthe tube sheets

Number of the helix angles

Figure 11: Near-linear relationship between the sheet number and helix angle number in helical tubes. The helix angle changes approximately every four sheets.27

Single-wall and multi-wall carbon nanotubes come in various sizes. The smallest stable freestanding single-walled carbon nanotube is found to have the diameter of 0.4 nm.28 Freestanding SWCNTs with diameters larger than 6 nm tend to collapse.29 The smallest stable armchair (2,2) SWNT grown inside a MWNT was measured30,31 to be 0.3 nm in diameter. MWCNTs can be significantly larger, depending on the number of shells in the wall, which varies from 232-34 to a few tens, hundreds, and sometimes even thousands35 of layers. Carbon nanotubes are high aspect ratio structures, and their length in some cases can reach more than 10 micrometers.

Formation of rehybridised sp2/sp3 bonds resulting from curvature completely alters the physical, mechanical and electronic properties of a graphene sheet. For example, a planar graphene sheet is a zero band gap semiconductor,22 armchair carbon nanotubes are all metallic,22 while zig-zag and chiral nanotubes, depending on their size 18

and chiral angle, can be either semiconducting or metallic.11,22 Wrapping of graphene in a certain way introduces new periodic boundary conditions that alter permitted electron wave functions of a planar infinite graphene sheet according to new symmetry rules.

Capped ends of carbon nanotubes are locations of a high strain concentration which allows for an easier functionalization of tube tips compared to their cylindrical walls.

2.3 Observation of multi-shell non-planar graphitic materials

Graphite whiskers, carbon cones and polyhedral crystals are all multi-shell needle-like all-carbon structures, meaning that their length is considerably larger than their width or diameter. The major difference between the graphitic cones and polyhedral crystals, besides their shape, is in their texture, i.e. in the orientation of the atomic planes within the structure.

The common feature of carbon whiskers, cones, scrolls and graphite polyhedral crystals, besides their chemistry and the graphitic nature of their bonds, is their morphology and the high length-to-diameter aspect ratio that places them between graphite and carbon nanotube materials. Various kinds of graphitic materials are described in the following sections. More details about their structure will be provided, and their properties and potential applications will also be mentioned.

19

2.3.1 Carbon whiskers and cones

Graphite whiskers are the first known non-planar graphitic structures that were obtained through a controlled preparation. Bacon36 succeeded in growing high strength graphite whiskers on carbon electrodes using a DC arc under an argon pressure of 92 atm.

The temperatures developed in the arc were sufficiently close to the sublimation point of graphite (above 3600°C), to enable the vaporization of carbon from the tip of the positively charged electrode and formation of cylindrical deposits embedded with whiskers of up to 3 cm in length and a few microns in diameter.37 Carbon deposition under extreme conditions such as a “flash CVD” process also resulted in growth of very peculiar micron-sized tree-like carbon structures.38 Whiskers and filaments of graphite have also been observed to form during pyrolytic deposition of various hydrocarbon materials. Hillert and Lange39 studied the thermal decomposition of n-heptane and reported the formation of filamentous graphite on iron surfaces at elevated temperatures.

Pyrolysis of methane,40 and carbon monoxide41 on iron surfaces or heated carbon filaments42 also resulted in the formation of similar structures, as well as the thermal decomposition of on Nichrome wires below 700°C.43 Whisker growth during the pyrolytic deposition process is generally considered as being catalyzed by metals.44,45

Haanstra et al.,46 however, observed non-catalytic columnar growth of carbon on β-SiC crystals by pyrolysis of carbon monoxide at 1 atm pressure above 1800°C (Figure 12). 20

Figure 12: Electron micrographs of (a) pencil-like carbon columns; (b) columnar carbon specimen with screw-like markings on side.46

The experiments47 demonstrated that the growth of carbon whiskers in this case was defined by rotation twinning and stacking faults on habit plane of the β-SiC substrate. The cylindrical carbon columns formed by this mechanism were observed to consist of parallel conical graphitic layers stacked along the column axis. Most specimens from one experiment had diameters between 3 and 6 µm, and were several tens of microns long. The apex angle of the conical mantle was measured to be about 141°. The conical nuclei of these columns were produced on defects in twinned β-SiC, such as the dislocation with a screw component perpendicular to the surface.

Very similar “needle”-like or “spine”-like graphitic materials, were also reported by Knox et al.48 In an attempt to synthesize a porous graphitic carbon material that would be capable of withstanding considerable shear forces, such as those met in high performance liquid chromatography, they produced porous glassy carbon spheres that in most cases contained graphitic needles. Knox et al. impregnated high porosity silica gel spheres with a melt of phenol and hexamethylenetetramine (HEXA) in a 6:1 weight ratio. 21

The impregnated material was first heated gradually to 150°C to form phenol- formaldehyde resin within the pores of the silica gel, and then carbonized slowly at

900°C in a stream of oxygen-free nitrogen. The silica was then dissolved with hot aqueous potassium (at least 99% complete) and the remaining porous glassy carbon was subsequently heated to 2500°C in oxygen-free argon. In addition to the expected glassy carbon structure, the resulting product often contained a considerable amount of needle-like material that was determined to have a three-dimensional graphitic structure. The graphitic whiskers resulting from their experiments were usually a few microns long and about 1 µm thick. Electron diffraction and transmission electron microscopy revealed the twinned structure of the whiskers. The angle between graphene planes near the tip was measured to be about 135°. Since this material was a side product of their experiment, Knox et al. did not provide any further details of its structure nor did they propose the nucleation mechanism. It is, however, highly likely that their graphitic needles were nucleated and grown in a similar fashion to Haanstra’s whiskers shown in

Figure 12. Incomplete dissolution of the silica matrix could have caused formation of twinned β-SiC phase during the glassy carbon pyrolysis between 1000°C and 2500°C, which further induced growth of columnar graphite from disproportionated CO within the porous glassy carbon spheres. The 135° whiskers were also synthesized recently at

2100°C from gaseous carbon monoxide and ball-milled natural graphite49 contaminated with zirconia particles during milling. When heated above 1900°C, zirconia particles react with the carbon to form ZrC.50 22

Figure13: Model illustrating the formation of cigar-like graphite: (a) longitudinal cross section of the cigars showing their texture; (b) cone-helix structure of graphitic filaments.51

The growth of the graphitic whiskers under these considerations was probably initiated by screw dislocations on the surfaces of the ZrC particles. Similarly, Gillot et al.51 studied the heat-treatment of products of the martensite electrolytic dissolution and observed formation of “cigar”-shaped crystals of graphite at 2800°C. The model of their texture is shown in Figure 13a. The length of the crystals ranged from few microns to 250

µm with an aspect ratio of about 10. The growth mechanism of the “cigars” was suggested to involve mass transfer through the gas phase. In the whiskers, the graphitic layers had the shape of an obtuse cone, the axis of which was coincident with the axis of the whisker, and is basically the same structure noted by, but not fully described by,

Knox et al. Such whiskers were assumed to form when a single graphene sheet coiled around the axis in a helix, each turn of the helix having the shape of a cone (Figure 13b).

The angular shift θ of the (hk0) crystallographic directions from one gyre to the next one in the helix was measured to be θ ≈ 60°. All graphitic layers were found to have the stacking arrangement of a perfect graphite crystal. Later, Double and Hellawell proposed 23

Figure 14: Conical graphite whiskers. (a) SEM micrograph of a cleavage fragment of conically wound graphite whisker. Note the 140° apex angle of the conical cleavage plane. (b) Electron diffraction pattern with the incident electron beam along the normal to the cleavage ‘plane’ of the conically wound whiskers. Note the 126-fold rotation symmetry of the pattern.53

the cone-helix growth mechanism for such structures,52 which relies on the formation of a negative wedge disclination within a graphene sheet.

Ge,19 Sattler19,20 and Krishnan21 were among the first to observe and study fullerene nanocones, i.e. seamless conical structures formed when one or more pentagonal rings are incorporated into a graphene network. Incorporation of pentagonal and heptagonal defects into graphene sheets and nanotubes was at that time already discussed by Iijima,54 Ajayan,55,56 Ebbesen57,58 and others59-61 with the goal of explaining the conical morphologies of carbon nanotube tips observed by HRTEM. The importance of pentagonal defects in the formation of three-dimensional conical graphitic structures was not fully recognized until thorough investigation of their electron diffraction patterns by Amelinckx et al.,18,53,62 who studied helically wound conical graphite whiskers (Figure

14a) by electron microscopy and electron diffraction. Whiskers gave rise to unusual 24

a c

b

Figure 15: Model illustrating the formation of a conical helix. (a) Sector β is removed from a disc. (b) The angular gap is closed and a cone is formed. (c) Twisted nucleus of the conical helix containing one pentagonal ring in the graphene network. Conical helix is formed through rotation of successive graphene sheets over a constant angle.53

diffraction effects consisting of periodically interrupted circular ring patterns (Figure

14b).

Very similar diffraction patterns had been previously obtained from whiskers described in the literature.46 Amelinckx et al. proposed a growth mechanism whereby the initial graphite layer adopts a slitted dome shaped configuration (Figure 15a,b) by removing a sector β and introducing a fivefold carbon ring in the six-fold carbon network

(Figure 15c). Successive graphene sheets were then rotated with respect to the previous one over a constant angle, thus realizing a helical cone around a 'disclination' with a fivefold carbon ring core. The model explains the morphological features and the particular diffraction effects observed on these reproducibly prepared columnar graphite crystals and it also builds on the other cone models.46,52 25

Figure 16: Carbon cone showing separation of layers in fullerenic end caps.63

The first true multishell fullerene graphitic cones consisting of seamless axially stacked conical surfaces (Figure 16) were observed in the products of chlorination of silicon carbide at temperatures above 1000°C sometime in 1972,63 and then reported by

Millward and Jefferson in 1978.64 Since these structures were rather singular observations in the products of the reaction, they were not recognized as a new material until much later.63

Large quantities of similar structures were successfully produced for the first time by Ge and Sattler.19,20 Up to 24 nm in length and 8 nm in base diameter, these nanometer- sized structures were generated by vapor condensation of carbon atoms on a highly oriented pyrolytic graphite substrate. All of the cones had the same apex angle ~19°, which is the smallest among five possible opening angles for perfect graphitic cones

(Figure 17a). The growth of these nanostructures is considered to be initiated exclusively by fullerene-type nucleation seeds with different numbers of pentagons. Fullerene cones of other apex angles corresponding to 1, 2, 3 and 4 pentagons were produced and reported three years later by Krishnan et al.21 (Figure 17c-f), who also reproduced the

~19° cone (Figure 17b). 26

Figure 17: Fullerene cones. (a) The five possible seamless graphitic cones, with cone angles of 19.2, 38.9, 60, 86.6, and 123.6°.19,20 Electron micrographs of the corresponding five types of cones (scale bars in b–f, 200 nm). Apex angles: (b) 19.2°, (c) 38.9°, (d) 60.0°, (e) 84.6°, and (f) 112.9°.21

Similarly, a few other conical structures of graphite were produced by thermal decomposition of hydrocarbons65 with or without the aid of a catalyst, or by employing various thermochemical routes.66 Orientation of the layers in catalytically produced cones is closely related to the shape of the catalyst particle, which they resemble.67-69

Catalytically produced cones can adopt open,69-71 helical,69,71 or a close-shell69 structure.

There have been several new types of cone-shaped carbon filaments composed of cylindrical graphite sheets reported recently.72-74 These so-called tubular graphite cones

(TGC) (Figure 18) have been synthesized on an iron needle using a microwave plasma

72 assisted chemical vapor deposition (MWCVD) method in a CH4/N2 gaseous environment. Corn-shape carbon nanofibers (CCNFs) with metal-free tips have also been

73 synthesized by a MWCVD method using CH4 and H2 gases. Graphitic coils wound 27

Figure 18: Scanning electron microscopy and HRTEM images of TGCs. a) Aligned TGCs grown on an iron needle surface. b) A high-resolution view of one TGC shows the faceted and helical appearance. c) TGC tip. The graphite sheet steps can be clearly seen on the surface of the cone in c).72,74

around a tapered carbon nanotube core were also produced by the same technique using a different substrate material.75

What makes these and similar structures cone-shaped is not an inclination of graphitic layers with respect to the cone axis, but rather the continuous shortening of graphene layers from the interior to the exterior of the walls,66 or a combination of both mechanisms, as in the case of carbon nanopipettes.75 Although their morphology resembles the shape of a cone, intrinsically their microstructure is the one of the multiwall carbon nanotubes, which implies different mechanical and electronic properties. Tailoring carbon nanotubes to the cone shapes can now be done routinely.76-79 28

2.3.2 Carbon nanotubes with polygonal cross-section

The structure of single and multiwall carbon nanotubes and single-wall carbon nanotube ropes has been studied in depth over the last fourteen years8,12,57,80-83 and is described briefly in Section 2.2.

While the ability of carbon to form multiwall tubular nanostructures is well known and has been studied extensively, very little information is available about carbon nanotube structures having polygonal cross-sections. Although an occurrence of polygonal vapor-grown carbon fibers with a core carbon nanotube protrusion was noted by Speck et al.84 as early as 1989, no details were given about core fiber structure and its polygonization.

Zhang et al.85 studied the structure of an arc-discharge produced carbon soot by high resolution transmission electron microscopy (HRTEM), and they were the first to indicate the possibility of polygonal multiwall carbon nanotubes, assuming that the tubes consisted of closed coaxial concentric layers. The first evidence for the occurrence of polygonized carbon nanotubes came from Liu and Cowley,80 who used nanodiffraction in conjunction with HRTEM and selected area electron diffraction (SAED) to investigate the structures of carbon nanotubes having diameters of a few nanometers.

Nanodiffraction is a form of convergent beam electron diffraction (CBED) in which a diffraction pattern is obtained from regions of the specimen about 1 nm or less in diameter. The tubes used in this study were produced by a variant of Kratschmer- 29

a b

c

Figure 19: (a) HRTEM micrograph of a nine-sheet nonsymmetric tube. A d spacing of 0.34 nm is found on the left side and a d spacing of 0.45 nm is seen on the right side.27 (b) Model illustrating the formation of the nonsymmetric fringes from a tube (a) with polygonal cross section.27 (c) The graphitic interplanar spacing decreases as the tube diameter increases, and approaches 0.344 nm at roughly D=10 nm. The data were measured from three different nanotubes indicated by different symbols. Hollow circles: from a seven-shell tube with innermost diameter Dmin =1.7 nm. For large D, graphitization may occur resulting in a polygonal cross section. The broken line indicates the expected decrease in interplanar spacing owing to local graphitic stacking.87

Huffman arc-discharge method86 in helium gas at a pressure of 550 Torr. The DC voltage applied to the electrodes was 26-28 V and the corresponding current was 70 A.

The carbon nanotubes obtained at the given experimental conditions consisted of

3 to 30 carbon sheets and had a length of up to 1 µm. The inner diameters of these tubes 30

ranged from 2.2 to 6 nm and the outer diameters ranged from 5 to 26 nm. In addition to nanotubes of circular cylindrical cross-section, with zero, one or several helix angles, there were many tubes having polygonal cross-sections, made up of flat regions joined by regions of high and uniform curvature.80 A HRTEM image of one such structure is given in Figure 19a. Polygonization of the cross-section is observed indirectly through formation of uneven patterns of lattice fringes on the two sides of the tube, with spacings varying from the 0.34 nm given by circular cylinder tubes, up to 0.45 nm from the regions of high curvature (Figure 19b).

In their study of the intershell spacing of multiwall carbon nanotubes prepared by the same Kratschmer-Huffman arc-discharge method, Kiang et al.,87 similarly, found that the intershell spacing in carbon nanotubes ranged from 0.34 to 0.39 nm among different nanotubes, decreasing with an increase of the tube diameter (Figure 19c). Some other reports have also shown variation of the values from 0.344 nm (obtained by electron and powder x-ray diffraction measurements)88 to 0.375 nm (based on HRTEM images).89

Faceted multiwall carbon nanotubes with larger diameters, called graphite polyhedral crystals (GPCs), were reported to grow at high temperatures in the pores of a glassy carbon.35 The present study in particular deals with understanding their structure and vibrational properties. GPCs of somewhat less perfect structures have been recently successfully produced by using the flame combustion method.90 Since flame combustion synthesis of GPCs has been conducted in parallel to this work, it will be discussed further in the text.

31

2.3.3 Properties of carbon cones and polygonal tubes: theoretical considerations

Carbon nanotubes are known to be either metallic or semiconducting, depending on their diameter and chirality.61,91-95 The role of pentagon, heptagon, or pentagon-heptagon pair topological defects in the structural and electronic properties of nanotubes have also been studied theoretically,96-98 and experimentally by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS).96 Special attention has been paid to the curved surfaces of capped carbon nanotube tips, since these can be considered as regions with a high density of defects. As the density of defect states increases at the tube ends, it can be expected that the electronic band structure of the ends will differ significantly from that elsewhere on the tube.96,98 This has been successfully demonstrated via spatially resolved STM/STS of a conically shaped tube end (Figure

20a,b).96

An STM image of such one conical tip is shown in Figure 20a. The apex of the cone has a diameter of 2.0 nm. The tunneling spectra had been acquired at four different positions along the tube (marked with white letters in Figure 20a). Local densities of states (LDOS), derived from the scanning tunneling spectra, are represented in Figure

20b. In addition, the tight binding calculations performed on two different tip morphologies are given in Figures 20c,d. As one moves along the tube from the position

A to the position D the density of topological defects increases, since the topological defects are concentrated in a smaller volume. As a result, the effect of confinement on the electronic structure becomes more and more pronounced. This does not seem to be very 32

Figure 20: Electronic structure and localized states at carbon nanotube tips: (a) STM image of a fullerene carbon cone; (b) local densities of states for a cone tip derived from scanning tunneling spectra at four (A–D) points along the tip; and (c–d) tight- binding calculations for two different configurations of cone tips.96

striking in the case of the conduction band, where only a slight and broad enhancement has been noted in the LDOS at the cone apex. The valence band, however, is found to alter considerably, exhibiting sharp resonant states at the cone tip, Figure 20b, curve (D).

The strength and position of these resonant states with respect to the Fermi level is very sensitive to the distribution and position of defects within the cone. This is illustrated with two models of the cones having different morphologies obtained by altering the position of pentagons within the tip structure (Figure 20c,d). In the two examples, the (A), (B), and (C) LDOS calculated by the tight-binding method are very similar. Strong and sharp peaks in (D) LDOS have a different shape and position in the case of model I and model II, the latter consistent with the experimental observations in

Figure 20b. The distribution of the defects and their effect on the electronic properties of 33

Figure 21: Tight-binding densities of states (states/eV/cell) for the (10, 0) cylindrical (a), and the (10, 0)5 pentagonal (b) cross-section nanotubes. The Fermi level is positioned at zero energy. Both nanotubes are also represented in inset on the right of their respective DOS.101

the cones has been studied in detail elsewhere.99 LDOS calculations of helix-type carbon cones are obtained through establishing a tight-binding model of a screw dislocation in the graphite.98 Localized resonant states are very important in predicting the electronic behavior of carbon cones. They can also strongly influence the field emission properties100 of cones.

The electronic properties of the cylindrical singlewall and multiwall carbon nanotubes have been widely studied both theoretically and experimentally over the past fifteen years, and the findings are summarized in several carbon nanotube books.11,12

Electronic structure of polygonal singlewall carbon nanotubes has been investigated theoretically within a tight-binding and ab initio frameworks93,102 and it has been found that polygonization changes the electronic band structure qualitatively and quantitatively.

An example is given for the case a (10, 0) nanotube with: a circular (a), and pentagonal

(b) cross-section (Figure 21). The (10, 0) zigzag carbon nanotube with a circular cross 34

Figure 22: Tight-binding band structures of the metallic (12, 0) nanotube, illustrating the effect of the degree of polygonization of the cross section on electronic behavior. Given here are examples of (a) the triangle (12, 0)3, (b) square (12, 0)4, and (c) hexagonal (12, 0)6 geometries that are compared to the pure cylinder case tube (d).101

section is a semiconductor, with a band gap of 0.82 eV. In calculating the band structure of a polygonal tube, it is reasonable to assume that the zones of strong curvature near the edges of the polygonal tube will introduce a σ*-π* hybridization of carbon bonds. This local variation of the bonding with strong sp3 character in the folds creates a sort of defect line in the sp2 carbon network.103 In addition to the effect of bond hybridization, polygonization of the cross section lowers the symmetry from a ten- to five-fold axis.

Furthermore, out-of-plane bending of the hexagonal carbon rings along the polygonal edges brings a new pair of atoms closer than the second-neighbor distance in graphite.

This results in a modification of the electronic band structure, and as a consequence, the semiconducting band gap of the (10, 0) polygonal tube is almost completely closed (Figure 21b). The ab initio calculations93 confirm these tight-binding results and predict a gap of 0.08 eV for the pentagonal cross section. The electronic 35

behavior of metallic armchair nanotubes is not so strongly altered with the polygonization because the σ*-π* hybridization is not possible in the case of armchair configuration.

Theoretical studies also suggest that the perturbation of electronic properties of carbon nanotubes will be different for various degrees of polygonization (i.e. various numbers of facets).93 An example is given for a (12, 0) nanotube. The (12, 0) zigzag nanotube of a circular cross section is metallic. When different polygonal cross sections

(triangle, square, and hexagon) are considered, the results indicate that all kinds of electronic properties arise (Figure 22). The first two cases are metallic, while the third one is a 0.5 eV band gap semiconductor. It is important to bear in mind that these calculations are given for a carbon nanotube comprised of a single shell. For polygonized multi-walled carbon nanotubes, it is expected that with an increasing number of shells and diameter their electronic properties become similar to those of graphite.

2.4 Nanotubes and cones made of inorganic compounds

The explosion of research on fullerenes and carbon nanotubes stimulated the scientific community to explore the possibility of synthesis of fullerene-like structures and nanotubes from other layered compounds.104-108 Perhaps the most known inorganic tubular structures of submicrometer size are asbestos fibers, which are made of kaolinite and similar minerals (Figure 23a). The folding of a kaolinite sheet to form scrolls was attributed to the lattice mismatch between fused silica tetrahedra and alumina octahedra.105 36

a) b)

Figure 23: Layered inorganic compounds. (a) Schematic representation of the mineral kaolinite, (b) Schematic presentation of MoS2 layer with both bulk and rim atoms delineated.105

Many other inorganic compounds, such as metal dichaclogenides (sulfides, selenides and tellurides), metal dihalides (chlorides, bromides and iodides), metal oxides, and numerous ternary and quaternary compounds have layered structures. The example of MoS2, well known for its lubricious properties, is shown in Figure 23b. Molybdenum layers and the two chalcogen layers with the metal in a trigonal pyramidal or octahedral coordination mode are kept together by van der Waals forces. A distinction has been made between the metal and chalcogen atoms in the bulk of the sheet and at the rim, the former being fully bonded with its nearest neighbors and therefore inactive, and the later having free dangling bonds that are considered as the source of instability, and well as the reason for the formation of tubular structures. A high resolution transmission electron micrograph of yet another dichalcogenide tube (WS2) is shown in Figure 24.

Both nested nanotubes and inorganic nanoscrolls are often synthesized.

Conceptually, nanoscrolls can be obtained by rolling up different molecular layers around an imaginary axis. Evidence of an inorganic nanoscroll is shown in Figure 25. H2Ti3O7 37

105 Figure 24: TEM image of a WS2 nanotube.

nanoscrolls are obtained by treating TiO2 of the anatase or rutile form with NaOH at

120°C, and subsequently washing with HCl solution.109,110 A longitudinal TEM view of one such scroll is shown in Figure 25a. A cross-sectional TEM image of another H2Ti3O7 tube (Figure 25b) shows the curling of sheets into the scroll, as illustrated schematically in Figures 25c and d.

The diameter of the resulting tubes was measured to be around 10 nm. The tubes were around 50 to 200 nm long. Interlayer distance was measured to be 7.8 Å. The number of layers for the tube shown in Figure 25a was different at the two sides of tube walls. This is also an indication that the tube is not a seamless cylinder. A concise 38

Figure 25: TEM image and schematic representation of H2Ti3O7 nanotubes: (a) Axial direction; (b) Cross section showing clearly the winding of the sheets into nanoscroll structure; (c) Schematic rendering of the cross section; and (d) Schematic rendering of the rolling up of the nanosheet into a nanoscroll.105

summary of dichalcogenide compounds and methods that are known to result in the formation of tubular nanostructures is given in Appendix A (Table A2).

Hexagonal boron nitride (BN) is another layered graphite-like structure that has been found to easily form tubes, fullerenes and cones.111,112 In hexagonal BN, the iso- electronic B-N pair replaces a C-C pair in the graphene sheet. The replacement of pairs can be partial or full, which gives rise to a myriad of possible BC2N structures.

Unlike inorganic nanotubes, which have open tips, or carbon nanotubes that have conical or half-spherical caps at their ends, most BN nanocones are found to have flat tips.111,112 This is consistent with the theoretical demonstration that incorporation of four squares (that gives rise to flat tip morphology) instead of five pentagons, gives BN tubes

113 energetically more stable structures. Nevertheless, BxCyNz nanocones (Figure 26) had 39

a b

c

boron nitrogen

Figure 26: BN nanocones: a) Electron microscope image of a stacked BN cone, b) HREM image of a tip region of a BN cone with an apex angle of about 20°. Tips of monolayer BN indicated by arrows have the closed shapes,114 c) A schematic of a BN cone. The apex is constituted by four pentagons with termination in two three- coordinated atoms.113

been successfully produced by thermal annealing of a mixed powder of β-rhombohedral boron and hexagonal boron nitride at 1200°C under lithium vapor.114 Figure 26a shows a stack of several BN nanocones. BN chemical composition of the material was confirmed by EDS analysis. A HREM image of a tip region of another BN cone with an apex angle of about 20° is shown in Figure 26b. Arrows indicate that the tips of monolayer BN cones have closed shapes. BN cone with a 20° apex angle BN cones have also been independently reported in the literature.111 40

Figure 27: Faceted microtubules of Ga2O3. Tubes are found to have a hexagonal cross section.115

We have seen in the previous section that graphitic cones with an apex angle of

19.2° were previously observed,20 and that this apex angle arises by introducing a disclination of 5π/3 in the form of a pentagonal defect in the monolayer graphitic sheet.

The BN cone with an apex angle of about 20° can be interpreted in a similar manner.

Thus, it is reasonable to consider that the BN cones shown in Figure 26a are actually formed by stacking a large number of BN single conical shells (Figure 26c). These can be considered as BN counterparts of true “fullerene” type of cones.

Development of new synthetic methods has resulted in the production of nanotubes from isotropic compounds.115,116 Figure 27 shows a scanning electron

115 micrograph of faceted microtubules of Ga2O3.

Formation of nanotubes from 3D compounds through folding is energetically unfavorable. One way to overcome the energy barrier is to introduce grain boundaries

117 and dislocations, which gives rise to polycrystalline tubes. In the case of Ga2O3, however, highly crystalline faceted microtubules (Figure 27) were obtained by fast 41

growth along the 0001 c-axis. Similarly, growth of crystalline faceted nanotubes of

AlN with hexagonal cross-sections was demonstrated recently.116

Although the actual atomic structure and chemistry of these tubes and graphite polyhedral crystals are completely different, the examples presented here indicate an increasing ability to create materials of similar morphology and size that may be useful in nanotechnology.

2.5 Summary, motivation and study objectives

Carbon is one of the best studied and yet the most intriguing elements in the

Periodic Table. The centuries-old study of elemental carbon solids is now experiencing a renaissance initiated by the discovery of fullerenes and carbon nanotubes. Since 1960 many sporadic studies have shown that carbon can form a number of highly organized solids in addition to the amorphous forms and four allotropes.

These carbon cones and polyhedral crystals have much in common with carbon nanotubes and nanofibers, yet form a separate class of carbon solids due to the ability of their layers to order 3-dimensionally. This, in turn, allows complex shapes and unusual architectures that differ from both nanotubes and planar graphite. It is also clear that they cannot be comfortably classified in the same group with other graphite materials because they are built of non-planar layers. Hence, they should be considered separately as a family of structures positioned between nanotubes and graphite. 42

Two types of carbon cones have been observed: helically wound cones and seamless fullerene cones. This classification refers to the way the adjacent layers are stacked within a cone. Helically wound cones were discovered as a side product of carbonization of silicon carbide at high temperatures about three decades ago. Except for a few studies, these materials were all but forgotten for a long period of time after their discovery. With growing interest in nanocarbon and the discovery of fullerene carbon cones, interest in these earlier materials is now increasing.

Seamless fullerene cones differ from the helical cones in the way that the conically shaped carbon crystallites are formed by stacking (not scrolling) of a large number of graphene layers. Each of the layers therefore is a graphene cone, formed by introducing one or more pentagonal defects in a planar graphene sheet. Fullerene carbon cones therefore have a limited number of apex angles, depending on how many pentagonal rings have been incorporated in their tips. These angles are: 19.2°, 38.9°,

60.0°, 83.6° and 112.9°, and they correspond to 1, 2, 3, 4 and 5 pentagonal rings.

Introduction of an additional pentagon would result in a hemi-fullerene and a cylindrical

(as seen in closed carbon nanotubes) rather than conical morphology.

One of the most peculiar ways that carbon atoms can form a solid structure is found in graphite polyhedral crystals. Polygonization of nanotubes accompanied by growth in the radial direction leads to the formation of nano- and micro-size graphitic crystallites that have nanotube cores and graphite-like crystal faces. Unusual rotational symmetry (including, but not limited to 5-, 6- 7- and 9-fold rotation) and helicity has been observed in these crystals. 43

Several routes for the synthesis of carbon cones have been demonstrated over the past three decades. Additionally, a very brief communication118 from 1970 reports the occurrence of needle-like graphite in natural sources of carbon, but it does not provide any detailed information on their structure and morphology. Graphite polyhedral tubes have been discovered to have coincidentally formed in the pores of a glassy carbon and the exact method and parameters involved in their nucleation and growth are unknown.

To the author’s best knowledge, no published information exists on the occurrence of this material in nature. The fine details of their structure are also unavailable to the scientific community, because no systematic study has been published.

A certain number of theoretical studies have been conducted with the aim of predicting the electronic properties of conical and polygonized tubular graphene. All calculations show unique features of the electronic band structure of both types of materials. Real measurements of any kind of physical properties on carbon cones and polygonal tubular crystals have not been performed.

Carbon cones, whiskers and polyhedral tubular crystals are a class of solid carbon materials that has been studied much less then other families of carbon materials. On the other hand, their unique structure, morphology and theoretically predicted properties make these materials an excellent bridge between the nano- and micro-worlds. If understood better, these materials may find numerous applications where sizes between nanotubes and carbon fibers are required. Moreover, the insights of the structure and the underlying principles of their nucleation and growth will aid understanding of their inorganic tubular and conical cousins. 44

The present study was motivated by a desire to fill in current knowledge gaps and systematically analyze available information, improving understanding of carbon cones and tubular graphite polyhedral crystals. Understanding their structure is a key first step toward this goal. Hence the goals of these studies have been to:

• Determine the morphology and structure of synthetic carbon nano- and micro-size

materials observed in pores of glassy carbon using SEM, TEM and Raman

spectroscopy;

• Explore the stability of carbon cones and polyhedral crystals;

• Explore the presence of carbon cones and polygonal tubular structures in natural

carbons;

• Investigate the morphology and structure of natural cones and polyhedral tubular

crystals and compare them with similar structures produced under laboratory

conditions;

• Analyze the mechanisms of nucleation and growth and outline possible

manufacturing routes.

This study contributes to understanding the structure, vibrational properties, and the mechanisms involved in nucleation and growth of conical and polyhedral crystals of graphite. 45

CHAPTER 3: MATERIALS AND METHODS

This Chapter describes materials and methods used to access information about

the morphology and structure of axial graphite nano-size crystals. Carbon samples from

pores of a glassy carbon and natural graphite samples from two different localities were

studied using a variety of techniques, including optical microscopy, field emission

scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and

micro-Raman spectroscopy.

3.1 Materials

High purity glassy carbon GL-200 produced by Toyo Tanso Co. Ltd. (Japan) was

reported to contain a novel type of nano-size graphitic crystals, named graphite

polyhedral crystals (GPCs).35 The present study reports a detailed investigation of carbon

materials discovered in the pores of GL-200.

Here, the occurrence of carbon cones and polyhedral tubular nanocrystals in

nature is explored in parallel with studies performed on the original crystal-bearing

synthetic glassy carbon materials. The Earth’s diverse geological environments form a wide range of kinetic and thermodynamic conditions that produce a wealth of natural minerals. From this "natural laboratory" come not only minerals of complex chemistry 46

and structure, but also simple and common minerals, such as graphite.119 Graphite occurs

in a wide range of geological environments worldwide and is widely used in many

industries. Most of the studies on natural graphite samples reported their planar

morphology. However, in a very few locations graphite forms compact spherical

aggregates with radial internal textures120-124 similar to those observed in graphite spheres in cast iron.52 These samples are of particular interest in the present study because they

are expected to possess curved graphite layers. If natural cones and polyhedral tubular

graphite crystals exist, knowing their geological origins would help elucidate

mechanisms of the formation of this kind of graphitic material.

3.1.1 Glassy carbon

Glassy carbon belongs to the family of non-graphitizable, so-called hard carbons.

This strategically very important material was developed in the beginning of the 1960s.

The first patent on glassy carbon was dated 1961 and was issued in Great Britain.125

Because of its rapid commercialization, the details of the manufacturing process were secret for more than a decade. Remarkably high mechanical strength, thermal shock resistance, chemical and thermal stability, and gas/liquid impermeability are properties that are found highly desirable in a large number of industries. Some applications of glassy carbon include semiconductor devices (dummy wafers, crucibles, target substrates of CVD), mechanical seals, biomaterials (heart valves), recording devices (magnetic 47

recording substrates), metallurgical applications (crucibles for molten metals), electrodes for analytical chemistry instrumentation etc.126

Glassy carbon is usually produced from various thermoset resins through a long and complex process of molding, carbonization and final firing at high temperatures and pressures. The structure of glassy carbon can be schematically illustrated as a three- dimensional network of carbon ribbons or lamellae (Figure 27a), which are fused together and locked into positions that do not allow for alignment into planar sheets without breaking the chemical bonds between atoms. A transmission electron micrograph of the GL-200 glassy carbon used in this study is shown in Figure 27b. Short range graphitic order exists within each lamella or ribbon. The average size of the basic structural unit (BSU) in glassy carbon is about 5 – 10 Å. The bulk material is isotropic due to the nature of the three-dimensional ribbon network. The combination of low specific area (0.1 – 1 m2/g) and low density (1.4 – 1.5 g/cm3) indicates large proportion of closed cavities in glassy carbon materials.

Glassy carbon GL-200 produced by Toyo Tanso, Co. Ltd. is one of the high purity glassy carbons with a very high oxidation resistance (5 times higher than that of graphite). The GL-200 is made from phenolic resin (Figure 28a) by carbonization at

2000°C in N2 atmosphere at ~10 Torr. The density of GL-200 was measured to be 1.48 g/cm3. Open porosity was determined to be less than 1 vol%. Total content of impurities, which include Si, Al, Ca, Ti, V and Fe was determined to be less than 100 ppm. Results of ultimate combustion analysis of ash and its breakdown are given in Table 1. 48

a) Shiraishi model

Jenkins model

b)

10 nm

Figure 28: Structure of glassy carbon: a) Two models of the glassy carbon structure126,127, and b) High-resolution transmission electron micrograph of GL-200 glassy carbon. The image shows nano-size cavities and short range order of lamellae.

A scanning electron micrograph of a typical GL-200 fracture surface is shown in

Figure 29b. Other than occasional micro-size pores (marked with circles in Figure 29b), the surface of the sample is featureless. The pores usually have the shape of a hollow ellipsoid, with an average size of 4.7 µm along the shorter and about 7.8 µm along the longer diagonal measured on 50 pores (Appendix A, Table A1). Even small pores can be easily identified on fracture surfaces by the long “tails” formed as a result of crack propagation through the material. 49

Table 1. Impurity (ash) level and its breakdown.126

Present ratio of impurities (wt. %) Material Ash (ppm) Al Si Ca Ti V Fe Other GL-200 <100 10 38 37 7 4 3 1

Careful investigation of the micropores, however, showed that they contain a very

unusual type of crystalline carbon material. This is further illustrated by a Raman map generated from a flat 25x25 µm sample surface that includes two pores (Figure 29c). The

map showed variations of the peak intensity ratio (IG/ID) of graphitic (G) vs. disorder-

a)

Curing, T Phenol + nH O Formaldehyde 2 HEXA

b) c) 2.0 intensity ratio intensity D /I G 15 µm I 5 µm 0.5 Figure 29: Glassy carbon GL-200: a) Condensation (curing) reaction illustrates the process of formation of phenolic resin from phenol-formaldehyde; b) A typical scanning electron micrograph of GL-200 fracture surface shows featureless amorphous material and occasional microporosity (circled); and c) Raman map collected around one such pore. Intensity of graphitic (G) peak relative to disorder-induced (D) band indicated that more of the graphitic carbon existed within the pore than in the surrounding matrix. 50

induced (D) carbon band. The peak intensity ratio is an indicator of the level of graphitic ordering and is an estimate of the average crystallite size.128 The graph in Figure 29c confirmed a greater presence of graphitic carbon (red, orange and yellow) inside the pores than in the surrounding glassy matrix (blue and black).Figure 30a shows a detail of the GL-200 fracture surface with four interconnected crystal-bearing pores. The fracture plane, which coincides with the microscope view plane, split the pores to reveal their inner morphology. The most striking feature of the inner pore microstructure was an abundance of needle-like faceted carbon structures. All the needle-like crystals in the pores of GL-200 had one of their ends attached to the pore wall, the pore with the crystals thus resembling the shape of a geode.129 The wall itself was built of short and bent

graphitic plates (Figure 30b).

One can distinguish between the two types of needle-like carbon crystals (Figure

30c): the rods with polygonal cross-section, so-called graphite polyhedral crystals35

(GPCs), and the conically shaped carbon nano-whiskers that are further referred in this study as graphite conical crystals130 (GCCs).

Several methods have been considered and attempted to separate the crystalline phases in the pores from the glassy carbon matrix for TEM and SEM studies. These include oxygen plasma etching, separation by oxidation in air, chemical etching and

hydrothermal treatment.131

Hydrothermal treatment was demonstrated to be the most efficient and highly

selective way to separate the stable crystalline phase from the amorphous glassy matrix.

Glassy carbon was crushed in a mortar into smaller particles. Capsules were prepared 51

2 µm 500 nm a b c MWNT Pores

GPCs

GCC 1 µm GC fracture surface

Figure 30: Graphitic microgeodes - the pores of GL-200 glassy carbon: a) A detail of the GL-200 fracture surface with four crystal-bearing coalesced pores. The needle-like carbon crystals as grown inside the pores; b) Plate-like morphology of the inner surface of pore walls; and c). Detail of a pore showing a graphite conical crystal (GCC) and two graphite polyhedral crystals (GPCs).

from 24K pure gold tubes with diameters of 5 mm. The tubes were first cut into

approximately 30 mm long pieces and then were annealed at 800 °C for 30 minutes to

remove any stresses remaining in the material from mechanical deformation. Glassy

carbon particles and deionized water were then sealed in the gold capsules using an in-

house built arc welding set up. Complete separation of crystallite-bearing pores is

achieved by etching GL-200 in a Tuttle-type autoclave for 24h at pressures around 100

MPa and temperature of 750°C.

3.1.2 Carbon cones in natural graphite samples

Along with hydrogen and helium, carbon is one of the most abundant132 elements in the Universe. Carbon polymorphs, such as , near-perfect crystals of graphite and microscopic diamond have been discovered in some meteorites.133 In the Earth’s 52

a b c

Figure 31: Occurrence of graphite in nature: a) Tabular; b) Spherical, and c) Fibrous graphite (Prof. J. Jaszczak’s collection of photographs134).

crust, carbon is found at a concentration of 3100 ppm132 mainly in the form of its

compounds. Besides diamond, elemental carbon in nature also exists in the form of

graphite, i.e. coal.

Natural graphite is an abundant mineral and is found in many parts of the world,

but because of its softness good quality crystals are extremely rare and therefore graphite

is very seldom represented in mineral collections. Graphite in nature occurs in the form

of flakes (plates), crystallites (plates, needles) or an amorphous (granular) sp2 phase.24

Flaky graphite is found in metamorphosed silica-rich quartzites, gneisses and marbles.

Crystalline (vein) graphite occurs in transverse, igneous, or metamorphic rocks, where it was formed by transformation of oil precursors. Granular graphite is the result of the metamorphosis of coal exposed to high pressures.24

The best quality samples of natural graphite are those that have a structure similar

to that of a perfect graphite single crystal (Chapter 2.1). Such samples can be found in

California (Crestmore quarry), New Jersey (Lime Crest, Sterling Hill, Trotter Mine

Dump and Valentine Mine #4 quarries), South Africa (Umzimkulu quarry), Namibia

(Namib Grube) and Finland (Pargas)134, although the best quality samples are claimed to 53

exist in meteorites. Other than tabular plates (Figure 31a), natural graphite also adopts

fibrous (Figure 31b) and spherical (Figure 31c) morphologies. Spherical graphite is

reported to occur in both metamorphic and igneous rocks, while fibrous texture is seen

only in igneous rocks.134

In the present study, the occurrence of natural carbon cones is investigated in graphite samples that are expected to have an abundance of non-planar graphene

surfaces. Because of their morphology, the spherical and fibrous graphites are good

candidates for in-depth analysis. Natural graphite samples were obtained from two

different geological locations. They are subsequently referred to as Gooderham and Kola

graphite. Gooderham graphite is typically spherical, while Kola graphite has a radiating

fibrous morphology.

3.1.2.1 Gooderham graphite

One of the prominent natural occurrences of spherical graphite is in the Central

Metasedimentary Belt of the Grenville province, Canada, at the south-western end of the

Bancroft shear zone.135,136 Spherical graphite is found there in calcite boudins that were

exposed by a roadcut south of Gooderham, Ontario.124,135,136

In addition to spheres, graphite is also present there in the form of tabular flakes,

spheroidal, and triskelial124 polycrystalline aggregates (Figure 32). Sphere sizes range

from 0.1 to 10 mm in diameter. Pyrite is the only mineral in addition to the host calcite 54

a b c

Figure 32: Natural carbon samples: a) Reflected cross-polarized light image of a portion of a 2-mm graphite sphere cross-section. b) Optical image of a 1.5-mm diameter graphite sphere with pronounced ridges and tiny cones on the surface. c) FESEM image of a graphite triskelion covered with cones.119

that commonly occurs with the carbon aggregates. Peak metamorphic temperatures in the area are believed to be below 700°C.135 After mechanical trimming, the graphite spheres were partially or fully separated from calcite by a careful dissolution of the enclosing mineral with dilute (5%) HCl. Etched samples were rinsed thoroughly in distilled water and dried in air at room temperature. Several Gooderham graphite spheres with diameters ranging from 1 to 3 mm and a triskelion (Figure 31c) have been analyzed in the present study.

3.1.2.2 Kola graphite

Samples of fibrous graphite were obtained from a graphite-bearing alkaline syenitic pegmatite near the bed of Hackman Creek in the Hackman Valley on the eastern slope of Mt. Yukspor, Khibiny Massif, Kola Peninsula, Russia.137,138 55

The graphite at Hackman Valley occurs macroscopically in two forms.139 The first

are spherical aggregates ranging in diameter from 3 mm to 2 cm. The second

macroscopic form of graphite’s occurrence appears optically as fine-grained surface

coatings in aegirine-rich cavities and is associated with apatite and feldspar. However,

under SEM magnification these graphite coatings are seen to be almost solely comprised

of micro- and nano-scale graphite whiskers. The whiskers range in size from tens of

nanometers in length and diameter to over 10 µm in length and several µm in diameter.

3.2 Materials characterization techniques

All techniques for materials characterization have one thing in common: a probe

is used to interact with a material and create perturbations; these perturbations are then analyzed to extract qualitative or quantitative information about the material. One needs to be aware that every perturbation can result in some level of damage to the material.

Therefore, the main objective and challenge of materials characterization is to obtain maximum information about the material without altering the original structure noticeably, i.e. to keep the damage created during the process and the artefacts of imaging and interpretation at the lowest possible level.

Depending on the nature of the probe, various materials characterization techniques had been developed over time. A whole range of tools is now available to probe materials structure, chemistry, surface topology, magnetic properties, etc. Each of 56

the techniques and tools can reveal a unique piece of information about material, and are

therefore complementary to one another.

In studying the morphology and structural features of nano- and micro-size

crystals, high spatial resolution microscopy techniques are needed. While transmission

electron microscopy with electron diffraction capabilities had proved unparalleled in

providing information about materials structure and atomic arrangement variations, high

resolution scanning electron microscopy complements this understanding with

information about the topological and morphological features of the material.

Raman spectroscopy has proved helpful in understanding the vibrational

properties and the microstructure of various carbon materials22,140-146 because of its

sensitivity to differences in bonding and the atomic structure of carbon. The type of

bonds present, their structure, order-disorder ratio, average crystallite size, and surface

chemistry can all be obtained from Raman spectroscopy. The relationship between

spectra and structure have been discussed extensively and well documented in the

literature. When a Raman spectrometer is coupled with a light microscope, micro-Raman

spectroscopy enables one to determine variations of Raman spectra across the sample.

The drawback of micro-Raman technique is low spatial resolution, limited by the optics

of the microscope.

In the present study, scanning and transmission electron microscopy and micro-

Raman spectroscopy have been used extensively to study the carbon materials described

in the previous section. The principles of these techniques are described in the pages that

follow. 57

3.2.1 Scanning electron microscopy

Scanning electron microscopy (SEM) is an analytical method that uses

accelerated electrons as a radiation source for the visualization and characterization of a specimen’s topography, composition and microstructure on a nanometer (nm) to micrometer (µm) scale. In the scanning electron microscope, a finely focused electron beam is swept in a raster across a specimen’s surface to form an image or a static beam is used to obtain an analysis of a micro-size volume at one position. When the surface of a sample is probed by an electron beam, it interacts with the high energy electrons from the beam and various signals are produced in this process. These include secondary electrons

(SE), backscattered electrons (BSE), characteristic x-rays, and other photons of various

energies (Figure 33).

Figure 33: Signals resulting from interaction of the primary electron beam with the specimen in an SEM.147 58

Figure 34: Variations of secondary electron signal intensity due to changes in specimen topography.147

For imaging, secondary and backscattered electrons are of the greatest interest.

The number of backscattered electrons is proportional to Z2, where Z is the atomic number. The number of secondary electrons produced and therefore the signal intensity vary primarily as a result of differences in surface topography (Figure 34). The analysis of characteristic x-rays emitted from a sample as a result of electron bombardment enables both qualitative identification and quantitative elemental information to be obtained from microvolume regions of a specimen. SEM, in addition, can be used to obtain information about crystal orientation, magnetic fields or currents induced in specimen by the beam.147

Development of scanning electron microscopy instrumentation and imaging techniques currently allows routine characterization of a wide range of specimens. With slightly more sophisticated sample preparation techniques, it is also possible to analyze fine particles, films on a substrate, fixed biological specimens, wet samples or frozen 59

liquids, and non-conductive or uncoated materials. The magnification that can be

achieved with an SEM is limited by the beam spot size. The lateral resolution for a field

emission type of electron source usually falls in the range of 1-5 nm. The current state of

the art in high resolution scanning electron microscopy (HRSEM) is a spatial resolution

of 0.4 nm at 30kV, which enables imaging at magnifications above 1,000 000x.148

Figure 35 shows a schematic of a typical scanning electron microscope. The

system operates in a vacuum. The electron column, consisting of an electron gun and two

or more electron lenses, produces a focused electron beam with energies in the range of

100V - 40kV. The electron beam is scanned over the surface of a specimen by two pairs

of deflection coils. The signal intensity produced during the beam-specimen interaction is

measured at every point of the scanned surface and converted to the specimen image on

the viewing screen (computer monitor or cathode ray tube). The ratio of the linear size of

the specimen image on the viewing screen to the linear size of the raster on the specimen

is known as the magnification (M) of the SEM. Since the linear size of the screen is

constant, the magnification is therefore controlled by changing the size of the scanned

area on the sample. Higher magnification images are obtained by scanning a smaller area

of the sample, which in turn requires an electron beam of a sufficiently small diameter

(spot size). In addition, the beam deflection system has to be sensitive enough to provide

the small angles of deflection needed. Reduction of the beam spot size, however, reduces

the intensity of the emitted signal because the primary beam does not provide sufficient

probe current to excite emission of a sufficient number of secondary and backscattered

electrons. This can be partially corrected through a reduction of the working distance (the distance of the specimen from the final lens of the SEM). Optimization of the working 60

Figure 35: Schematic of an SEM (Copyright: The McGraw-Hill Companies, Inc.)

distance and beam parameters (spot size, current, acceleration voltage) along with alignment and correction of astigmatism is a crucial part of SEM imaging.

All SEMs used in this study were equipped with at least one secondary and one backscattered electron detector (Figure 36). These two detectors collect specific information about the sample, which depends on the nature of the electron scattering events in the material.

When electrons enter the specimen, they interact with the electron clouds of the specimen atoms. These interactions may result in elastic or inelastic scattering events. In 61

Figure 36: Schematics of backscattered and secondary electron detectors. Inset shows BSE and SE signals emitted with a finely focused high-resolution beam.147

the elastic scattering process, electrons are deflected from their original path but they do not suffer kinetic energy loss. During elastic scattering events some of the electrons may actually leave the specimen, which gives rise to the backscattering signal. The probability of this event increases with the atomic number (Z) of the specimen atoms because of the stronger positively charged nuclei, and decreases as the electron energy increases:

−20 2 2 2 Q()> φ0 = 1.62 ×10 (Z E )cot (φ0 2), (3)

62

where Q is the probability of elastic scattering at angles greater than a specified angle

φ0 .

In addition, electrons can lose energy gradually through inelastic scattering when primary beam electrons transfer some of their energy to the specimen atoms. Inelastic scattering results in secondary electrons and X-rays. The rate of electron energy loss

( dE ) with distance traveled ( ds ) is again a function of atomic number Z, the atomic weight and the specimen’ density:

dE Zρ ⎛ 1.166E ⎞ = −2πe4 N ln⎜ i ⎟ (4) 0 ⎜ −0.19 −3 ⎟ ds AEi ⎝ ()9.76Z + 58.5Z ×10 ⎠

where e is the electron charge, N0 Avogadro’s number, Z the atomic number, ρ the

density, A the atomic weight, and Ei the electron energy at any point of the specimen.

In imaging carbon materials ( ρ = 2.1 g/cm3), the rate of energy loss due to inelastic scattering is calculated to be 2.24 eV/nm.147 This number is significantly higher for heavier elements and higher density materials. This means that an electron beam penetrates relatively easily through carbon. In order to minimize the imaging artifacts

(Figure 37a) and obtain information from the very top surface layer (Figure 37b), it is therefore highly recommended for characterization of carbon materials to use a relatively low voltage beam that will not penetrate too deeply into the sample. 63

a b

2 µm 2 µm

Figure 37: Carbon sample sensitivity to beam energy: a) Image obtained with 25 kV acceleration voltage. b) Image obtained with 5 kV. One can see charging and the lack of surface detail in a).

While beam electrons are losing their energy, this energy is transfered to specimen electrons during inelastic collisions. Secondary electrons are, therefore, specimen electrons that are emitted during the inelastic scattering process and their energy typically ranges between 0 and 50 eV, with a most probable energy of 3-5 eV.147

In addition to the most commonly used Everhart-Thornley electron detector (able to collect both secondary and backscattered signals) and solid-state backscattered detectors, which are mounted in the specimen chamber or on the pole piece of the final lens (Figure 36), the latest high performance field-emission SEM instruments offer a

“through-the-lens” (TTL) or “in-lens” (IL) electron detection capability. With such design, the secondary electrons (type SE1 and SE2) are trapped by the strong objective lens magnetic field and spiraled up to the detector. Secondary electrons created by the

BSE colliding with the bottom of the lens and chamber walls (SE3 type) are practically completely eliminated by TTL or IL detectors. A TTL detector, in addition, has a capability of discriminating between BSE and SE electrons, because the magnetic field 64

does not collect off-axis BSE and on-axis BSEs are too strong to be pulled to the detector from their straight line path.

This study was conducted using an in-house environmental scanning electron microscope (XL-30 ESEM, made by FEI Company), and several externally available microscopy sources (JEOL JSM 6500, JSM 6700, and JSM 7000F and FEI Sirion 200).

The in-house Philips/FEI XL-30 model is a field-emission ESEM with a nominal lateral resolution of 2 nm at 15kV. It operated at accelerating voltages from 1eV to 30 kV and it can efficiently reach magnifications of 150,000x. Although the instrument has three operation modes (high vacuum, gaseous and wet mode), it was used here only for high- vacuum imaging. The FEI XL-30 ESEM is equipped with a digital EDAX EDS prism X- ray detector for composition analysis.

SEM principles have been briefly outlined here. For a more comprehensive coverage of SEM fundamentals, sample preparation and imaging techniques, readers are referred to general SEM literature.147,149

65

3.2.2 Transmission electron microscopy

Historically, the development of the transmission electron microscopy came as an answer to overcoming the limitations of light optical microscopy. The low imaging resolution of a light microscope is imposed by the wavelength of visible light (400-700 nm). The electron diffraction experiments from 1927 (Davison and Germer, and

Thompson and Reid) practically demonstrated the wave nature of the electron beam predicted theoretically by de Broglie in 1925, and therefore opened the door for the development of electron microscopy.

The concept of resolution in an electron microscope is the same as that in conventional light microscopy, which means that it is directly proportional to the wavelength of the probe (an electron beam in the case of TEM). The wavelength of the beam ( λ ) depends on the energy of electrons that comprise the beam, and the energy is dictated by the accelerating voltage ( E ). Therefore:

1.22 λ = (5) E

Theoretically, a resolution of 0.2 Å is achievable by using a 100-keV electron beam with an electron wavelength of 0.04 Å. Unfortunately, the actual resolution of a

TEM is limited not by the accelerating voltage (E), but by the lens aberrations, mainly spherical and chromatic aberrations. 66

Spherical aberration is caused by the more widely scattered electrons coming to a focal point closer to the lens than those that are scattered at shallower angles. Chromatic aberration, the other major distortion, is caused by the faster (shorter wavelength) electrons focusing at a different position than the slower ones, a problem that can be resolved only with a monochromatic electron beam.

Currently, ultra-high resolution microscopy is performed at resolutions between 1 and 2 Å. This corresponds approximately to the separation of atoms in solids. Moreover, the FEI Company just revealed their new state-of-the-art Titan line of high resolution

S/TEM (scanning and transmission electron microscope) with new corrector and monochromator technologies that allow a sub-Ångstrom resolution.150 This opens up further opportunities for direct observation of individual nanostructures at an unprecedented resolution of 0.5 Å, which is approximately one-third the size of a carbon atom.

Still, conventional high resolution TEM can be considered as the method of choice for the characterization of nanostructures. Using various imaging modes, TEM is able to provide information about diameters, lengths, stacking arrangements and defects in carbon nanotubes, nanocones and graphite polyhedral crystals.

In a conventional transmission electron microscope, similarly to SEM, a sample is irradiated by an accelerated electron beam (Figure 38) produced by an electron gun and then accelerated through the anode plate and focused by a set of magnetic lenses. Unlike in an SEM, where the beam is limited to an interaction volume, the beam in a TEM travels all the way through the specimen and the images or diffraction patterns are formed 67

Figure 38: Schematic of a transmission electron microscope.151

on phosphor screen or other imaging detectors that are placed behind the specimen. In order to obtain an image with a TEM, the specimen needs to be transparent to the electron beam, which requires sample thicknesses of about 100 nm or less.

Images are recorded either by direct exposure of a photographic emulsion, or digitally by a charged-coupled device (CCD) camera.

68

3.2.2.1 Electron diffraction and TEM image formation

Electron diffraction is the oldest, and for materials scientists, the most useful aspect of transmission electron microscopy that provides information on the crystal structure of the specimen, lattice repeat distance, atom arrangement and crystal defects etc.

Similarly to SEM, scattering of electrons around atoms is the essential phenomenon in gathering information from the specimen. While an SEM relies on electrons that are scattered at a large angle, TEM is primarily interested in electrons that do not deviate far from their original path. On its way through the sample, not all the electrons of an electron beam will be scattered. Some of them will remain unaffected by the specimen. The end result is that a non-uniform beam leaves the rear surface of the specimen and TEM uses this nonuniformity to infer information about the materials of the specimen. The nonuniformity can be angular or spatial, where the former is viewed in the form of diffraction patterns and the latter is observed as contrast in the images of the specimen.

The concept of low-angle scattering and electron diffraction is easy to understand if the electrons are considered in terms of electron waves and not in terms of particle interactions that characterize high-angle scattering. According to von Laue, the diffracted waves are in-phase if the path difference between waves scattered by adjacent scattering centers is an integer number of wavelengths. In addition, Bragg argued that during 69

diffraction waves behave as if they are reflected off atomic planes. The diffraction condition then obeys the following equation:

2d sinθ B = nλ , (6)

where d is the interplanar spacing in the crystal, λ the wavelength of the electron beam,

θB the Bragg’s angle and n is an integer.

According to Bragg’s equation, atomic planes that are closer together give rise to larger angles of diffraction. For the first order diffraction, n = 1. In addition, sinθ can be approximated by θ (at small angles). Hence the Bragg equation can be simplified to:

λ = 2dθ (7)

For a small diffraction angle:

r λ = 2θ = (8) L d

Formation of a diffraction pattern in TEM is illustrated in Figure 39. The parameter Lλ is known as the “camera constant” because it does not depend on the material of the specimen. If the camera length is known, then the interplanar d spacing can be determined by simply measuring the distance r in the pattern: 70

Figure 39. Schematic of electron diffraction in TEM.

λL d = . (9) r

The most common electron diffraction in TEM is selected area electron diffraction (SAED). In SAED, a specific area of the specimen is selected by inserting a selected area aperture into the image plane of the objective lens. In this way, only the selected area of the specimen contributes to the diffraction pattern (DP). In addition, other electron diffraction techniques are available, such as nanobeam electron micro- diffraction, convergent beam electron diffraction (CBED) etc. Details of different diffraction techniques can be found in general TEM literature.149,152,153

By using electron diffraction, a number of questions can be addressed. TEM with

DP helps to determine whether the material is crystalline or amorphous. For a crystalline material, it is possible to determine whether the material is a monocrystalline or 71

a b

Figure 40: Diffraction patterns of graphite: a) Dotted pattern of a hexagonal pyrolytic graphite monocrystal; and b) Beaded ring pattern of a polycrystalline graphite sample.24

polycrystalline, and to determine the crystallographic characteristics such as the lattice parameter and crystal symmetry. If the material is polycrystalline, DPs can determine the grain size, and how individual grains are oriented within the specimen. Figures 40 show examples of diffraction patterns of hexagonal pyrolytic (a) and polycrystalline (b) graphite. The indexing of diffraction patterns by measuring the distance and the interplanar angle between spots, allows us to identify the structure of the materials.

Phase contrast arises due to phase differences of the electrons that pass through the specimen. Since most electron scattering mechanisms involve certain phase changes and it is impossible to prevent deflected electrons from contributing to the image by using any size of aperture, some level of phase contrast is always present in every image.

The most common and useful phase contrast images are formed by many strongly diffracted electron beams when the incident beam is exactly parallel to a zone axis of the crystal. Such phase contrast images are often called high resolution transmission electron microscopy (HRTEM) images or lattice fringes. An HRTEM micrograph obtained from a 72

Figure 41: HRTEM image of a multi-walled carbon nanotube (courtesy of Dr. Haihui Ye). Arrows show defects in tube’s walls.

9 nm outer diameter multi-walled carbon nanotube is shown in Figure 41. The arrows show the spots where the tube’s structure is disrupted by defects.

In addition to the lattice fringe image (HRTEM), other phase contrast images like

Moiré fringe images and Fresnel fringe images can be used to analyze dislocations, grain boundaries and other crystal defects.

3.2.2.2 TEM Instrumentation and sample preparation

In this study, the TEM investigations were carried out using three different microscopes: a JEOL 2010F TEM/Scanning TEM, JEOL 3010, and a JEOL 4000 EX

TEM. The JEOL 2010F field-emission TEM operated at 200 kV. It delivers a point-to- point resolution of 0.23 nm and a lattice resolution of 0.10 nm. The fully loaded PGT

EDS detector and Gatan electron energy loss spectrometer (EELS) make this microscope 73

an ideal tool for high-resolution analytical and energy-filtering imaging. A variety of sample holders, including single tilt, double tilt, in situ hot stage (25 ~ 1000 °C) and cryo-stage (-175 ~ 50 °C), make the 2010F an instrument with full capabilities of various electron diffraction methods, bright/dark field and high resolution phase contrast imaging. The JEOL 3010 and the 4000EX are fully optimized for high resolution phase contrast imaging with a 0.19 nm point-to-point spatial resolution and a lattice resolution of 0.14 nm. These microscopes operated at 300 kV and 200 kV respectively.

Although TEM has extraordinary advantages over many other techniques, the actual TEM investigation of materials can be very challenging because of the difficulties arising from the preparation of electron-transparent specimens. Over the years, various techniques and methods have been developed to allow characterization of the samples using TEM.

The focused ion beam (FIB) renders itself as a pin-point accurate technique for

TEM specimen preparation from a pre-selected area. An FIB system operates similarly to an SEM, but in addition to an electron source, it is also equipped with a gallium liquid metal ion source. While the electron beam enables imaging of the selected area and monitoring of the work progress, the focused ion beam is used to section the material by etching the sample into any shape needed. The present study utilized a dual beam focused ion beam microscope (FEI Strata DB-235) located at the University of Illinois at Urbana-

Champaign. More than 10 attempts were made to prepare samples from glassy carbon, however, due to the difficulty of sample preparation, only 3 specimens of GL-200 were obtained and finally investigated by TEM. Some of the naturally occurring carbon cones 74

were also sectioned by this technique. The FEI Strata DB-235 is a combination of a high- resolution field emission scanning electron microscope (FE-SEM) and a scanning metal ion beam microscope. The SEM column is equipped with a Schottky thermal field emission gun that provided a resolution of about 2.5 nm for accelerating voltages between 0.2 to 30 keV. The ion column with a gallium metal ion source (up to 30 keV) provided a current range from 1 pA to 40 nA.

FIB is used to prepare samples by the so-called ‘lift-out’ method. In this method, no sample preparation prior to FIB milling is necessary, which reduces the overall time of preparation (no prior mechanical thinning, dimpling or ion milling is needed).

A typical “lift-out” procedure is demonstrated in Figure 42 on a glassy carbon sample. An attempt is made to isolate a pore with graphite polyhedral crystals. The point of interest is located on a fracture surface of the sample (Figure 42a). A thin layer of platinum is then deposited to protect the surface (Figure 42b). The material around the pore is then removed by etching using a high current ion beam from one (Figure 42c) and then the other side of the pore. This creates a thin membrane between the two etched trenches. The membrane is further etched using a low ion beam current to a final thickness of 100~300 nm (Figure 42d). Additional cutting on the bottom and both side edges of the membrane (Figure 42e) helps its release with a manipulator in the FIB.

Finally, the sample is detached and delivered onto a TEM grid (Figure 42f).

During FIB milling, material can often undergo some level of structural changes and/or damage caused by the ion beam. Other TEM preparation methods were also considered, and several very good samples were successfully prepared from the powder. 75

a b

Pt protection layer Spot of interest

c d Etching of the other side

Etching of one side

e f Isolated pore FIB with crystals manipulator

Figure 42: FIB sample preparation: a) A spot of interest is located on the glassy carbon sample; b) A protective thin platinum layer is deposited onto the surface; c) One trench is etched out from the sample, d) and the second one. The sample is thinned further using a soft ion beam. e) An FIB manipulator is used to pluck and lift the sample from the specimen. f.) Sample is transferred to the TEM grid. Scale bar is 10 µm for a)-e) and 5 µm for f).

Bulk and hydrothermally treated glassy carbon materials were crushed and gently ground in a mortar. The powders were then mixed with isopropyl alcohol and the powder-alcohol suspension was carefully transferred to TEM copper grids coated with a film of lacey carbon. Finally, the grids were dried, first over a warm light bulb (60 Watts) for about one minute. The yield of graphite polyhedral crystals in such prepared samples was very low; however, crushing was still found to be advantageous in providing more of the isolated crystals (crystals detached from the shell of the pore). 76

3.2.3 Micro-Raman spectroscopy

Raman spectroscopy is a non-destructive analytical method used for

“fingerprinting” materials structure and chemistry based on the detection of fundamental vibrations in materials. Raman analysis requires minimal sample preparation and is therefore considered as a very efficient technique that is often widely used in industrial settings for monitoring of processes. The method utilizes changes in a monochromatic light beam energy that occurs due to the inelastic collision of light with molecular regions of the specimen, to gather information about the bonding nature and the structural organization of atoms in the specimen.

The term “Raman scattering” refers to the inelastic scattering of photons primarily by elementary excitations associated with the degrees of freedom of ions and electrons in crystalline and amorphous solids 154. The inelastic scattering process is a two-photon event that involves the simultaneous annihilation of an incident photon and the creation of a scattered photon. During the inelastic scattering process, the energy (and therefore the frequency) of the incident photon is altered (increased or decreased) for a certain value by a multiple of quanta of vibrational energy of the material being examined. The dominant form of Raman scattering, first-order scattering, involves a single quantum of excitation.

If a wave vector of a photon in free space is denoted as q before and q' after the scattering event, then the corresponding wave vectors of a photon in a crystal are nq and 77

nq' , where n is the index of refraction of the crystal. If photons frequencies are denoted by ν and ν ', the principles of conservation of energy and momentum yield:

hhhννν'()=± k S (10) hnqqk' =± hn h

where h is Planck’s constant.

The frequency of a phonon νS ()k is then determined from the shift in the photon frequency (')ν −ν .

The Raman spectra are usually measured in wavenumbers (ω ) that easily translates into frequencies according to the expression:

1 ν ω = ν~ = = (11) λ c

where λ is the wavelength of the scattered photon and c is the velocity of light.

Depending on whether the energy of the photon is decreased (through emission) or increased (through absorption, rarer) during the scattering event, one can differentiate between Stokes and anti-Stokes spectral regions (Figure 43).

The first-order spectra display a discrete set of peak values associated with elementary excitations in materials that in solid-state physics are determined with the center of the Brillouin zone (BZ) ( k = 0). Because the magnitudes of the photon wave vectors q and q' are comparably small to the size of the Brillouin zone of typical 78

a b

Rayleigh scattering

Stokes anti-Stokes

300 200 100 0 -100 -200 -300 Raman shift (cm-1)

Figure 43: a) Generic model of Raman spectra. b) First- and second-order Stokes (SR) and anti-Stokes Raman (ASR) spectra of multi-walled carbon nanotubes obtained with 2.54-eV excitation. For convenience, the negative portion of the x-axis is inverted and overlapped with the positive portion.157

crystals155, the first-order processes that conserve the wave vector can access only elementary excitations at or near ( k = 0). For higher-order processes, it is necessary to take into account all elementary excitations that take part in the scattering. In contrast to the first-order spectra, higher-order spectra may exhibit continuous features that correspond to the wave vectors spanning the entire Brillouin zone of the crystal. In the material has no translation symmetry, such as in amorphous materials, the wave vector conservation rule (Eq. 10) breaks down, and the Raman spectrum again is expected to display features reflecting the density of states of the particular excitation. In such cases,

Raman spectra generally consist of several broad bands with maxima corresponding to peaks in the broadened phonon DOS for the crystalline phase.156

79

Figure 44: Characteristic Raman spectra of various carbon materials (Courtesy of Prof. Dresselhaus).

3.2.3.1 Raman spectroscopy of carbon materials

Raman spectroscopy is a very useful technique for studying carbon materials due to its high sensitivity to changes in the carbon atom bonding state and atomic structure, as illustrated in Figure 44. Raman spectroscopy studies cover a whole range of carbon materials, from amorphous to its crystalline forms,22,140-146,158-161 including highly oriented pyrolytic graphite (HOPG),146,162,163 microcrystalline and pyrolytic graphite

(PG),144 amorphous164,165 and glassy carbon, fullerenes, carbon onions,166 carbon nanotubes,157,161,167-169 etc. Continued breakthroughs in the discovery of new peaks and theories that explain their formation make this field or science very dynamic. For detailed information about Raman spectroscopy of carbon nanomaterials several excellent books are available to the reader.22,170,171 80

1582 514 nm 1581 633 nm 1580 780 nm

2727 2685 2646 2440 2454 3244 Edge 3248 863 plane ⊥ 864

1344 1620 1624 1322 1366 1649 1615 Edge 2328 plane ll

Basal plane

1000 1500 2000 2500 3000 1000 1500 2000 2500 3000 1000 1500 2000 2500 3000 Raman Shift (cm-1)

Figure 45: First- and second-order Raman spectra of graphite crystal basal and edge planes as a function of incident beam energy and edge orientation with respect to polarized beam.172

Raman spectra of a single crystal of graphite are shown in Figure 45. A single

4 crystal of graphite belongs to the D6h symmetry group; the irreducible representation for the Brillouin zone center optical modes can be decomposed into:

Γ = A2u + 2B2g + E1u + 2E2g (12)

For a graphite single crystal, only the 2E2g modes are Raman active, and they

−1 162 have been observed at 42 and 1582 (G) cm , respectively. Infrared-active A2u and E1u modes have been observed at 867.8 and 1588 cm−1,173 and one of the two optically

−1 174 inactive B2g modes has been found at 127 cm by neutron scattering. The other B2g 81

mode that corresponds to ‘out-of-plane’ atomic displacements was reported in a Raman spectrum of HOPG159 and natural graphite crystal172 edge planes. The presence of this peak is possible when the Raman inactive B2g mode becomes Raman active by modification of the crystalline point group symmetry due to a slight rearrangement of the lattice structure in the vicinity of the edge.159

In addition to the k = 0 zone-center modes, several other bands are observed that can be attributed to k ≠ 0 Raman forbidden fundamental modes. The two first-order lines at ca. 1360 (D) cm−1 and ca. 1620 (D’) cm−1 (excited using 514.5 nm laser light) have been reported for microcrystalline graphite,162 pristine HOPG163 and ion-implanted

HOPG.175 Furthermore, the D and D’ bands have also been observed in the Raman spectrum of graphite edge planes144,172 and in the inner/outer surfaces of shortened cup- stacked-type carbon nanotubes.176

Such Raman forbidden bands (k ≠ 0) become Raman active due to the double- resonance (DR) Raman mechanism. For the DR process in graphite,177,178 an electron with momentum k (and electron energy Ei(k)) is excited by the incident laser photon in an electron–hole creation process. The electron is then scattered by emitting or absorbing a phonon with momentum q to the state with momentum k +q (and energy E(k +q)), then scattered again back to the state with momentum k (and electron energy Ef(k)) to recombine with a hole. In general, there exist four possible DR scattering processes for the Raman Stokes and anti-Stokes scatterings.178 82

Figure 46. Raman spectra of turbostratically stacked (TS) carbon nanoparticles and an individual graphite whisker excited with 632.8 nm laser excitation. The inset gives the 172 energy dependence of the frequencies of the L1 and L2 modes.

The D mode is explained by an inter-valley DR mechanism that occurs around two nonequivalent K points at neighboring corners of the first BZ, while the D’, and L1

−1 −1 (ca. 228 cm ) and L2 (ca. 355 cm ) modes that had been observed in single carbon whiskers, (Figure 46) are explained by an intra-valley DR mechanism that occurs around the K point in the BZ of graphite.157,160,178,179

Raman spectra of carbon nanotubes have been extensively studied both theoretically and experimentally and great progress in understanding and assigning their characteristic peaks has been made over the last decade. Theoretical simulations of individual graphene layers performed even before actual Raman spectra of carbon nanotubes were collected,168,180,181 indicated that SWNTs would have a unique Raman active vibration modes. A typical Raman spectrum of SWNTs obtained experimentally is 83

Figure 47: a) Raman spectrum (514.5 nm excitation wavelength) of single-walled carbon nanotubes produced by an electric arc, b) Enlarged detail of the spectrum shows the peaks in 100-250 cm-1 region.14

shown in Figure 47. The first-order spectrum is dominated by the 1562 - 1589 cm-1 doublet, and a multi-component band at 183 cm-1.

The splitting of the 1582 cm-1 graphitic peak into the 1562 - 1589 cm-1 doublet

(sometimes also seen as a triplet) is explained as a degeneracy of the 1582 cm-1 brought in by the curvature of the graphene layer. Introduction of the curvature results in transition from 2D to 3D symmetry, changes in the Brillouin zone of graphene, and the optical phonon dispersion around 1580 cm-1. The intensity of these peaks decreases as the tube diameter d increases and becomes 1582 cm-1 for d = ∞ (planar graphene).

The low-frequency band at ~ 183 cm-1 represents the radial breathing mode

(RBM) of a carbon nanotube. RBMs are diameter dependant, meaning that each distinctive component of the 183 cm-1 band (Figure 47b) corresponds to a specific tube diameter.22,168,182,183 Calculated values of Raman active modes for single-walled carbon 84

Figure 48: Micro-Raman spectra (low-frequency region) of A, B and C HADE MWNTs specimens, SWNTs and HOPG.186

nanotubes are listed in Table A3 of Appendix A. For a detailed explanation of Raman active vibrational modes of single-walled carbon nanotubes reader is referred to the recent literature in this field.184,185

Raman spectra of multi-walled carbon nanotubes have also been investigated thoroughly over the last decade. The majority of these studies have been carried out on nanotubes synthesized by helium arc discharge evaporation and chemical vapor deposition methods. Such tubes are found to have a large number of defects and wide inner diameters, so the Raman spectra collected from them showed typically no significant difference from the spectra of microcrystalline graphite. However, recent

Micro-Raman, polarized Raman and surface enhanced Raman spectroscopy (SERS) studies186 conducted on highly ordered MWNTs, produced by the hydrogen arc discharge evaporation (HADE) method, reported multiple new features in the low frequency region

(Figure 48) and peak splitting around 1580 cm-1. Many of the low frequency Raman 85

active modes can be contributed to A1g radial breathing modes of the extremely thin inner-most nanotube of HADE MWNTs (~ 1 nm). Similarly, the splitting at 1580 cm-1 is assigned to the one-dimensional confinement effects of electrons and optical phonons in the core nanotube.

3.2.3.2 The instrument and the experimental procedure

A schematic of the Renishaw micro-Raman spectrometer used in this study is shown in Figure 49. A micro-Raman spectrometer uses a monochromatic light source for sample illumination. In this case, the light is directed at the surface of the sample through the objective lens of the microscope. The scattered light is collected through the same objective and redirected into the wavelength analyzer and detector by a collection optics system. The computer further collects the Raman signal from the charged-coupled device

(CCD) detector attached to the spectrometer and optical images from a video camera attached to the microscope. Raman signals in the form of a spectrum are projected on the screen of the monitor. Integration of a Raman spectrometer with a microscope enables analysis of very small regions of the sample. A motorized XY or XYZ stage enables continuous scanning of the sample surface that is a very valuable solution for Raman mapping mode. 86

Reflecting mirror Laser source Computer Laser beam

CCD Camera Raman spectrometer main unit

Microscope

Figure 49: Schematic of a Renishaw Raman microspectrometer. The monochromatic incident beam is redirected through a set of optical components into the microscope objective. Objective is used for illuminating the sample and for collecting light scattered on the sample. Inelastically scattered light is then dispersed into a spectrum inside the main spectrometer unit. The computer collects Raman signal from the CCD detector attached to the spectrometer and optical images from the video camera attached to the microscope.187

Depending on the wavelength analyzer design, Raman spectrometers are classified as dispersive and non-dispersive. Non-dispersive spectrometers separate the

Raman scattered light either using tunable bandpass filters or by modulation of the scattered light into characteristic frequencies to be monitored by a single detector and demodulated by a Fourier transform (FT). The dispersive wavelength analyzer separates the Raman scattered light spatially, to be scanned across a single detector or monitored by multiple parallel detectors. This is achieved by using a diffraction grating as the dispersing element. The dispersed scattered light is detected using a multichannel charge coupled device (CCD) detector. Integration of a Raman spectrometer with a microscope provides higher spatial resolution suitable for performing analysis on very small regions 87

of a sample. The spatial resolution that Raman instrumentation can achieve depends on the optical systems used for light delivery and collection. The spatial resolution is fundamentally limited by diffraction effects to approximately half the wavelength (λ/2) of the light used to illuminate the sample. However, few Raman microspectrometers can achieve this theoretical limit and a spatial resolution of ~1λ–3λ (1–2 µm) is more typical.

In the present study, a Ramascope 1000 Raman microspectrometer made by

Renishaw, UK, was used to analyze carbon samples. Three monochromatic sources of light (a 514.5 nm Ar ion laser, a 633 nm He-Ne laser, and a 785 nm diode laser), were used with this system. Scattered light was dispersed into spectra with one of the three available diffraction gratings (1200 grooves/mm for the 780 nm laser, 1800 grooves/mm for the 633 nm laser, and 2400 grooves/mm for the 514.5 nm laser). The gratings provided spectral resolution of approximately 1 cm-1 per pixel of the charge coupled device (CCD) camera. The spatial resolution depends on the microscope objectives and can reach up to 1 µm for the 100x objective. The spectra from various carbon samples were typically collected using reduced laser power intensity (1 and 10% of a nominal 25 mW) to avoid burning and damaging the samples. Collection time was typically set to 10 seconds per single scan and at least 25 accumulations per point were used for a better signal-to-noise ratio. Both extended and static spectra were collected in at least 5 points within the zone of interest. Galactic Grams and Renishaw Wire software packages were used to analyze the single and multiple spectra collected.

88

CHAPTER 4: RESULTS AND DISCUSSION

4.1 Carbon cones

4.1.1 Synthetic cones from the pores of glassy carbon

Carbon cones from different sources have different structures. Therefore, conical

crystals of different origins will be considered separately.

4.1.1.1 Occurrence of cones in glassy carbon

While the possibility that the surface of conventional non-graphitized

microporous carbons prepared by pyrolysis of organic compounds (such as GC) can be converted into structures containing graphitic nano-size carbon filaments has been indicated before,188-193 and while some particles in the pores of GC produced by

decomposition of phenolic resins have been previously reported, the quality of the

published images194 has not allowed the analysis of their morphology or structure. The fractured surfaces of glassy carbon GL-200 made by Toyo Tanso Co. Ltd., Japan, however, revealed unique needle-like crystalline carbon shapes35 inside the pores that had

been formed during processing. Scanning electron micrographs of pore interiors (Figure

50) showed at least three different types of axial structure: 89

• Graphite conical crystals (GCCs),

• Graphite polyhedral crystals (GPCs), and

• Straight multiwall carbon nanotubes (MWNTs).

In all of the pores examined (more than 500), GCCs (marked with arrows in

Figure 50), GPCs, MWNTs and plate-like graphitic nanocrystals (Figure 31b) occurred together. Not taking into account the small plate-like nanocrystals covering the pore walls, typically about 8 % of pore crystals had a conical morphology, about 83 % were polyhedral crystals and 9 % were straight MWNTs (calculated on a population of 50 pores). 90

Figure 50: SEM micrograph of GL-200 fracture surface showing the interior microstructure of two pores. Conical crystals are marked with arrows.

Hydrothermal treatment was used to separate some of the pores with crystals from

the glassy carbon matrix. Preferential etching of the glassy phase relative to the

crystalline phase resulted in isolation of the crystal-containing “eggs”, as shown in Figure

51. More resistant to etching, the crystalline pore walls formed a closed-shell barrier

around the needle-like crystalline material, thus protecting the original structure from exposure to high temperature supercritical water. 91

Isolated crystal-containing pores

Open pores

Gold substrate 10 µm

Figure 51: SEM micrograph of hydrothermally etched GL-200 graphitic pore shells (egg-like structures) and microstructure of several pores.

4.1.1.2 Morphology, size and apex angle distribution

Further SEM examination of the pores and the GCCs morphology shows that the size of the graphite conical crystals ranged from a few tens of nanometers to 300 nm in cone base diameter, and their lengths ranged from about 500 nm up to a couple of micrometers. The measured values of apex angles varied within a very narrow range

(from ~ 3° to ~ 20°). Two types of GCCs were observed in the SEM. The first type, shown in Figure 52a, had a somewhat larger apex angle and a dome-shaped tip, and the 92

a b 17º-GCC

GPC

3º-GCC 500 nm 500 nm Figure 52: SEM micrographs of two GCCs. a) A 17º-GCC attached to the pore wall, b) A small apex angle GCC (~3º) with an almost flat tip.

second (Figure 52b) had a smaller apex angle, usually between 3º and 5º, and almost flat

tip.

The surfaces of both types of cones appeared to be perfectly smooth. No crystal

growth steps were observed on the cone walls. Some of the glassy carbon material with

cones and other crystals had been subjected to mechanical grinding in a mortar. Small

particles were examined in SEM after grinding, and no fractures were found on the

GCCs. GCCs proved mechanically very strong.

Several types of synthetic carbon cones have been produced with or without the

aid of a catalyst, or by various thermochemical routes.66 However, the morphology of the dome-like tip cones in terms of size and apex angle values does not resemble any other previously reported cones. The morphology of the second type of GCCs was similar to some of the carbon whiskers reported by Haanstra et al.46 However, Haanstra’s whiskers

were significantly larger in size (a few microns and above), and they easily cleave

(Figure 14a) when a shear stress is applied, which is not a typical behavior for GCCs. 93

4.1.1.3 Structure

Additional details of the GCC structure were revealed by transmission electron

microscopy (TEM) and high resolution (HR) TEM studies. Figure 53a shows a TEM

image of a conical crystal with a 14.5º apex angle. The crystal was hollow, and consisted

of straight and well-defined ~110-nm-thick walls. Small amounts of amorphous carbon

were observed on the cone surfaces and around the tip. The amorphous carbon was

probably deposited from solution onto the cone during sample preparation. The dome-

like morphology of the cone tip that has been noted in high resolution SEM images

(Figure 52a), was also observed in low resolution TEM (Figure 53a).

High resolution TEM image of the cone tip, shown in the inset, revealed few

important details of the cone structure. The HRTEM image showed the (002) lattice

fringes and the orientation of the graphene layers within the cone. In GCCs, graphene

layers are parallel to the cone walls, and the cone walls are, therefore, atomically smooth.

The core of the particular 14.5º-cone was not completely graphitized (lattice fringes were

not perfectly straight lines). The central nano-size channel was observed in the cone tip.

The channel extended throughout the tip and was connected with the central cone cavity.

Unlike the smooth surfaces of the cavity, the graphene planes terminated on the walls of

the channel, which gave an appearance of a “herring-bone” nanostructure. Herring-bone

structures are seen in hydrothermally produced carbon nanotubes195 and graphitic carbon

nanofibers.196 94

a a-carbon central cone channel b

nm 0 11 D L

o .5 14 L L L HRTEM of the cone tip a-carbon

Herring-bone tube 200 nm

Figure 53: The structure of the dome-capped cones: a) A TEM micrograph of a 14.5º- GCC. Low resolution TEM shows the hollow structure of the cone. The inner and outer surfaces are marked by dashed lines. HRTEM inset shows lattice fringes of the 002 graphitic planes of the cone tip. Central nano-size channel extended throughout the cone tip. The features such as dislocations (D) and edge-terminating loops (L) were observed. b) Schematic model of the hollow cones structure.

Other unique features of the GCCs cone structure were the edge-terminating loops, marked with arrows and letters (L) in the inset (Figure 53a). Edge-terminating loops have been reported in high-temperature annealed planar graphite particles,197-199

GPCs,35 and annealed cup-like176,200 carbon nanofibers. Such loops are formed as a result

of lip–lip interactions201 between the dangling bonds at the edges of two adjacent

graphene layers. Noted with (D) is the dislocation in the cone wall (Figure 53a, inset).

The dislocation was created by an extra layer of graphene in the bottom part of the cone.

A model of the cone structure generated from the TEM image analysis is given in

Figure 53b. In further text, this model will be referred as GCC type A model. 95

a outer surface

m n

7 7 inner surface

8o

m m n o n 6 4 8 71 FP

200 nm

b A - 002 graphitic inner surface lattice fringes of the cone

B – change in stacking sequence

10 nm

Figure 54: (a) TEM image showing the hollow morphology of an 8º-GCC. Apex angle of the inner surface was reduced by 2º due to wall thickening close to the tip. (b) HRTEM of cone’s walls shows a zone A of graphitic (0.34 nm) fringes and zone B that showed a change in stacking sequence, and non-uniform lattice spacing.

A model of the cone structure generated from the TEM image analysis is given in

Figure 53b. In further text, this model will be referred as GCC type A model. A TEM image of another GCC is shown in Figure 54a. Shown here is an 8º-cone with a smooth outer surface. The cone was sufficiently transparent to the electron beam to show the inner contours of the walls and their finite thickness. The outer surface of the GCC was 96

smooth. The cone wall thickness, however, appeareded to be different at the tip and the

base of the cone. Marked in Figure 54a is the flexion point (FP) with the change of the

wall thickness at one side of the wall. The difference in apex angle between the inner and

outer cone surfaces was about 2º (the inner apex angle measured to be smaller than the

outer). The observed deviation might be the result of a prolonged growth of the innermost

graphene layers of the cone.

A high resolution TEM micrograph of a cone wall is shown in Figure 54b. The

(002) lattice fringes of the cone walls demonstrated graphitic order with a corresponding

interlayer spacing of 0.34 nm. Within the wall, a zone (B) that had different fringes than

the rest of the wall was observed. In zone B, some of the (002) lattice fringes appeared

darker or separated by a different interplanar spacing value. This indicated the disturbed

stacking sequence between adjacent graphene planes (stacking fault). Within zone (B) the stacking of graphene planes changed several times from hexagonal to rhombohedral and back.

As discussed in Chapter 2, an increase of interplanar spacing was observed earlier in polygonal multiwall carbon nanotubes27,83,202 and was atributed to the phenomenon of

the tubes’ polygonization (Figure 19a,b). In the case of cones, high resolution SEM and

TEM, however, were unable to confirm and give any support or further evidence of

GCCs polygonization. An alternative explanation for the irregular fringing that involves visibly larger interplanar spacing was given by Van Tendeloo203 and Amelinckx.62,203

Their explanation was related to the presence of dislocations in cone walls (Figure 55) due to their scroll structure. In the case of the small apex angle GCC in Figure 54, the 97

Figure 55: Schematic representation of a scroll-type structure with a dislocation that potentially can explain the unusual spacing observed in Figure 54b.203

most probable explanation for pattern in region (B) was the stacking fault in the form of hexagonal-rhombohedral transitions, although other possibilities such as the scroll

structure resulting in a poor match between conical layers and dislocations were not fully

excluded. Ultra high resolution TEM would be an appropriate technique to resolve this

issue.

The structure of the small apex angle GCCs (type B) has also been investigated by

TEM, and an example of one such cone is given in Figure 56. The cone shown in Figure

56 had a 2.7°-GCC. The length of the cone was 2 µm and the diameter in the tip was dT =

168 nm. One can distinguish a long, (small-apex-angle) conical body of the crystal, and a 98

192 nm

c 2.7o on e ax is 168 nm

m n 4 .3 0

002 lattice fringes 500 nm

Figure 56: TEM analysis of a small apex angle cone. Shown here is a 2.7º-GCC (low resolution TEM). The (002) lattice fringes from the side of the cone are shown in the inset. The walls of the GCC were well-ordered, the interplanar spacing being 0.34 nm.

short large-angle conical tip of the cone. The cone was built up from more than 250 graphene layers.

The inset in Figure 56 shows the (002) lattice fringes from the GCC body with an interplanar spacing of 0.34 nm, which confirmed that the GCCs were graphitic. The selected area electron diffraction (SAED) pattern (Figure 57) obtained from the circled zone in Figure 56, suggested the axial symmetry of GCCs. It is known that hk.l reflections of axially symmetric structures are streaked away from the fiber axis.62 The hk.l reflections from the GCC form continuous rings (Figure 57b), which streak away from the indicated cone axis creating observed bands. 99

abSAED Pattern

0004`

2.7o

f ibe r a xis 0002 0004 0004 • 0004 from graphene layers of the left wall 0006 • 0004` from graphene layers of the right wall • same reasoning applies to all 00.l spots

Figure 57: SAED pattern of the 2.7º-GCC taken from the circled area (Figure 56). Conicity of the structure contributed to elongation of diffraction spots in the direction parallel to the cone axis, as shown schematically for 0004 spot.

The sets of graphene layers on the left and the right walls parallel to the beam give two rows of 00.l reflections that are almost overlapped, thus giving rise to the elliptical shaped dots, as shown in Figure 64b. A similar shape of 00.l spots was observed for carbon nanotubes having a ‘herring-bone’ structure. The size of the apex angle (2.7°) was measured directly from the diffraction pattern.

The structure of GCCs from glassy carbon is interesting for several reasons. First, the cones were highly graphitic and had fairly thick and rigid walls. The structure of the cones was more-or-less very regular. Dislocations and stacking faults were present in the material to a very small extent and they did not significantly affect the morphology of the cones. Secondly, the apex angles of GCCs were relatively small compared with other previously reported cones18-21,48,51,204 and they varied over a narrow range from about 3º to 20º. Finally, all of the cones examined by the TEM were catalyst-free. 100

As detailed in Chapter 2, in principle all of the cones that have been previously

reported in the literature can be classified into three groups. These are: catalytically

grown fibrous carbon structures with conical orientation of graphene planes, cones

nucleated by a dislocation on a substrate and grown by the cone-helix mechanism, and

fullerene cones, which follow the pentagon-defect route. The orientation of graphene

planes relative to the main axis of the filament or cone varies over a very wide range of

values for all 3 cases.

Conical carbon layers with variable apex angles were observed in some nanotubes

filled with metal or metal alloys,205 but also in herring-bone195 and bamboo-like206,207 nanotubes. These tubes were found to nucleate on catalyst particles and grow axially due to significantly faster longitudinal growth rate in comparison to radial thickening.

For catalytically produced fibers and cones, the orientation of the graphene planes depends strongly on the shape and type of the catalyst particle and in theory, any value of the apex angle is possible. From EDS analysis of GC ultimate combustion products it was found that Si, Al, Ca, Ti, V and Fe impurities were present in the glassy carbon samples in concentrations of less than 100 ppm. Both the cone tips and the cone roots (when allowed) were carefully examined for the presence of these elements, but the cones appeared to be pure carbon. The likelihood that GCCs were catalytically produced is very small; however it was not completely excluded. Since the GC was produced by a long- term treatment at ~2000 ºC, metals could have evaporated from the cone tips. Still, no cavities from catalyst particles were found in cone tips, suggesting that noncatalytic growth occurred. 101

The morphology of the GCCs was most similar to the cones grown by the cone-

helix mechanism,52 except in one key respect the orientation of the graphene layers in

GCCs was parallel to the cone walls, while in the cone-helix type of cones, the layers

form very large apex angles, so they terminate on the cone’s outer surfaces at a nearly

perpendicular angle of entrance.

The morphology and structure of the last type of carbon cones are determined by a number of pentagonal rings incorporated in the cone tip. The orientation of graphene

planes in the cones was such as in the case of GCCs (parallel to the cone’s surface), however, the tips of these cones were completely closed and the values of their apex angles can have only five very discrete values (Figure 17). The apex angle values measured for GCCs did not correspond to any of these five values. In addition, the

HRTEM of GCCs confirmed their open tip structure.

The GCCs cone structure model will be discussed in detail in Section 4.1.3 and

the possible mechanisms of nucleation and growth of GCCs will be discussed in detail in

Chapter 5.

4.1.2 Naturally occurring carbon cones

4.1.2.1 Occurrence of carbon cones in nature

To the author’s best knowledge, there has been only one publication that briefly

noted the occurrence of graphite whiskers and cones in nature.118 The cones have been 102

observed growing on natural Ticonderoga graphite crystals. In their brief communication,

Patel and Deshapande118 reported 65 to 125 µm thick graphite whiskers which grew in

the <0001> direction, the graphene planes of graphite being perpendicular to the whisker axis. The growth of the whiskers was presumably assigned to a screw dislocation mechanism during the growth of graphite, but no further details of their structure or the

structure -geological origin relationship were given.

A large number of natural graphite samples of various geological origins was

examined, and the presence of carbon cones was observed in spherical graphite samples

from Gooderham, Canada, and in syenitic igneous rock from the Kola Peninsula, Russia.

Spherical, spheroidal and triskelial polycrystalline carbon aggregates (Figure 32)

from Gooderham, Canada were examined by X-ray diffraction, energy dispersive

spectroscopy (EDS), polarized light microscopy and scanning electron microscopy. EDS

studies showed that the cones were indeed made of carbon. X-ray diffraction studies of

whole spheres indicated that the graphite was well crystallized and hexagonal.

Reflected light microscopy of aggregates in polished cross-section showed that

they typically had a radially elongated texture. The cones were highly lustrous in

reflected light. The surfaces of the aggregates can be smooth, scalloped, velvety, or well

crystallized. On a small fraction of the velvety surfaces, graphite indeed occurred as

cones up to 40 µm high. They were circular in cross section, and could exhibit nearly

continuous coverage of the graphite substrate surfaces. Graphite spheres embedded in

partially dissolved calcite are shown in Figures 32b and 58. An SEM micrograph in

Figure 58a shows a detail of a cone-covered surface. 103

a b

Calcite matrix

Cone-covered surface of the carbon sphere 5 µm

Figure 58: a) Scanning electron micrograph of graphite sphere partially etched out from calcite. The surface of the sphere was covered with carbon cones. b) Model of spherical graphite originally developed by Double and Hellawell52 to explain the growth of graphite spheres in nodular cast iron.

The surface and texture of natural graphitic spheres were similar to those seen in nodular cast iron (Figure 58b). The graphite spheres are commonly circumscribed with one or more thin protruding graphite ridges, which in thin section can often clearly be correlated with calcite grain boundaries, thus suggesting fluid deposition of the graphite.

Deposition of graphite by metamorphic fluids in other geological environments has been well documented.208

Of over 1000 Gooderham spheres and spheroids that have been examined, approximately 20 have been observed to have clearly identifiable cones on their surfaces.

The cones appeared highly reflective with a silver-white metallic luster.

A small pyrite crystal is commonly associated with cone-bearing graphite aggregates. Surfaces of few samples were found to be completely covered with large arrays of graphitic cones (Figure 59). 104

a 25 µm c 20 µm

b 10 µm

Figure 59: FESEM images of a cone-covered graphite aggregate. (a) Low- magnification image showing complete coverage of the aggregate surface with conical structures. A ~39º cone is marked by an arrow. (b) Higher magnification image of the sample showing a variety of large cones with different apex angles and sharp and blunt tips. Arrows show changes in the apex angle. (c) Close up view of two surfaces which are almost perpendicular and show different cone morphologies - large cones on one surface and globular (artichoke-like) structure on the other. The latter are clusters of large-angle cones.

It is striking that cone morphologies, which are extremely rare in the mineral and material kingdom, can dominate the graphite surfaces (Figure 59a). Moreover, all surface features, including large artichoke-like shapes (upper part of Figure 59c) and nanoscale cones or overgrowths on large cones observed on the samples, have conical shapes. This fact suggested that the growth environment rather than the substrate was responsible for the conical growth. 105

Cones and carbon whiskers have also been observed in another sample found in

Kola Peninsula, Russia (Figure 60 and 61). Kola spherical aggregates are composed of

friable, radially-aligned fibers (Figure 60a). Scanning electron micrograph of the sample’s surface showed that these macroscopic fibers were actually hollow channels composed of tabular graphite crystals greatly elongated in the direction of the fiber axis

(Figure 60b). In cross section, the fibers ranged in size from 10 to 60 mm across. The

graphite fibers tended to be brittle, and some appeared to be broken. Although the

channels were typically empty, some were filled with aegirine or strontium-apatite

(Fig. 60b).

This graphite type may be a result of an overgrowth or replacement of earlier-

grown crystals, such as aegirine, apatite or astrophyllite, which occur as radially-aligned

aggregates of prismatic crystals at several locations in the Kola Peninsula.137 Individual

aegirine crystals partly coated with graphite are also quite common in the graphitic host

rock.

a b c

Carbon cone Carbon cones

Sr-apatite

Planar graphite

20 µm 12 µm Figure 60: Kola graphite: a) Optical photograph of a 7-mm spherical cluster of radiating graphite fibers with albite, apatite and aegirine (courtesy of Prof. J. Jaszczak). b) SEM image of graphite hollow polyhedral fibers and associated cones. Strontium-bearing apatite crystals inside graphite channels. A graphite cone is circled at the left. c) Kola graphite cones. 106

5 µm

Figure 61: Scanning electron micrograph of graphite whiskers of varying morphologies, coating the surfaces of aegirine and associated minerals.

Inside and among the hollow channels found were numerous graphite whiskers up

to 4 µm in length and up to 1.5 µm in diameter (Figure 60c) in clusters (Figure 61) or in isolation (Figure 60c), and grown out of the secondary-graphite overgrowth. The

whiskers morphologically ranged from conical to tubular, and ranged in size from tens of

nanometers in length and diameter to over 10 µm in length and several µm in diameter.

107

4.1.2.2 Morphology, size and apex angle distribution

Microscopy studies of Gooderham cones showed that they ranged in size from sub-micrometer to 40 microns long. Gooderham cones also have a variety of apex angles, and can have sharp or rounded tips. Cone heights ranged from less then a micron to 40

µm, and unlike most laboratory produced cones, showed a wide distribution of apex angles. The apex angles were found to vary from 38° to ~140° with 60° being most common.

α

60º cone inclination 3 pentagons axis angle 8 φ 7 6 projection plane 5 4

Frequency [%] 83.6º 38.9º 112.9º 3 2 pentagons 4 pentagons 1 pentagon 2 19.2º 5 pentagons 1 0 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Apex angle [degrees]

Figure 62: Histogram of the frequency of occurrence of apex angles on a single sample of graphite. Outlined green bars indicate the positions of expected apex angles from the disclination model for cone-helix structures (see Section 4.1.3). Arrows show expected apex angles for the pentagon defect model; these angles also preserve the ideal graphite stacking and are therefore expected to have relatively low energies and be more predominant. Inset: the inclination of cones relative to the focal plane of microscope has been taken into account.138 108

The distribution of apex angles, measured from digitalized images for 554 cones,

is shown in Figure 62. An uncertainty of ~2% was estimated for most of the

measurements. Uncertainties were larger, however, for many of the smaller cones, which

frequently had the largest apex angles (≥90º). Some cones were noted to have slightly

curved surfaces in cross section (marked with an arrow in Figure 59b), making

determination of the apex angle difficult. Also, the very tip of the cone may have a

slightly different apex angle compared to the main part of the cone and smaller cones can

grow on the tip or side walls of larger ones (Figures 59b and 63b,c), ultimately

transforming them to artichoke-shaped cones (upper portion of Figure 59c).

Additional uncertainty of the measured angles arose from the random inclination

of the cones with respect to the plane of the image. For an actual apex angle α (Figure 62,

inset), the measured (projected) apex angle α′ as a function of the inclination angle φ of

the cone with respect to its axis lying in the plane of the image is given by

tan()α′ 2 = tan()α 2 cos ()φ . For both positive and negative φ values, α′ is always

greater than α. For φ ≤ 15° , α′ -α is on the order of 2º or less. Thus, only cones whose central axes were judged to lie nearly parallel to the plane of the image were measured for inclusion in the graph shown in Figure 62. Approximately 3% of the observed cones had apex angles near 39º. These cones occur up to 45 microns tall and were up to twice as tall as the next-largest cones with other apex angles. The most common apex angle was near 60º (Figure 63a), with cones having 60±2º apex angles accounting for nearly 23% of

those measured (Figure 62). 109

a 50 µm c 60º

5 µm b 20 µm

d

2 µm

Figure 63: Typical Gooderham cone morphologies. (a) SEM image of a cone with a 60º apex angle, the most common apex angle. The slightly uneven surface of the cone suggests their layer-growth mechanism. (b) FESEM and (c) SEM images of large cones with numerous smaller cones growing on their surface. Smaller cones covering surfaces of large cones have a broad distribution of shapes, but large apex angles prevail (c). (d) FESEM image of four cones having sharp and broad tips (multiple tips are marked by arrows). The cones are oriented to reveal their circular cross sections around the tips and layered growth (ripples).138

Figure 63 shows typical morphologies of Gooderham cones. A cone with the most common 60º apex angle is shown in Figure 63a. This cone had a round tip and relatively smooth surfaces. Secondary cone growth was observed at the base of the cone. The slightly uneven surface of the cone suggested the layer-growth mechanism. Figures 63b and 63c show large cones with numerous smaller cones growing on their surface. These 110

a b

A

B

5 µm 5 µm

Figure 64: Growth irregularities in Gooderham cones: a) Inter-growth of two cones. b) A cone with a wedge defect.

secondary cones typically had a broad distribution of shapes, but large apex angles prevailed (Figure 63b). Some of the cones, like those shown in Figure 63d, had multiple conical tips. The tip of the large cone in Figure 63d (circled) had four sharp and broad tips (marked by arrows). The cones were oriented to reveal their circular cross sections around the tips and layered growth (ripples).

Figure 64 shows SEM micrographs of other two cones with large steps on their surface. These steps suggested a layered growth of cones through the deposition of carbon from hydrothermal fluid. Figure 64a shows growth of cone B through cone A.

This, in addition, supported an assumption that the cones grew from a hydrothermal fluid.

It is important to note that a similar inter-growth of crystals has been also observed in the glassy carbon samples.

Other than full solid cones, some Gooderham samples also revealed partly conical hollow surface structures composed of curved graphite shells. Figure 65 shows the 111

surface of one such sample. The samples with curved carbon surfaces may possibly be

ancestors of the large cones and are therefore referred to here as "protocones".

Unlike large solid cones, many of these graphitic structures had partly faceted

surfaces (Figure 66). The tips of the faceted cones typically had six facets, and these

facets only extended part way down the surfaces of the cones, which maintained a circular base. The faceted cones can reach up to 1 µm in diameter. To the author’s best knowledge, this is the first record of such an unusual graphitic morphology.

The faceted cones were reminiscent of the polyhedral graphite crystals from glassy carbon pores. The morphology and the surface topography of the cones and petrologic relations of the samples suggested that the cones formed from metamorphic fluids.

10 µm

Figure 65: Surface of a carbon aggregate covered with curved graphite shells and hollow or incomplete carbon cones. 112

a b

2 µm 1 µm

Figure 66: FESEM micrograph of carbon cones with a) open, and b) faceted tips.

Another occurrence of natural carbon cones was discovered in samples of alkaline syenitic pegmatite of Kola Peninsula, Russia (Figures 60 and 61). Numerous graphite cones and whiskers up to 15 µm in length and up to 1 µm in diameter cover the inner and the outer surfaces of channels comprised of tabular graphite crystals.

For several Kola samples, the surfaces of cavities in the host rock were found to

be coated with a fine-grained graphite layer comprised solely of carbon whiskers. The

morphology of some of the Kola whiskers was nearly cylindrical (Figure 67a) while other crystals exhibited true conical (Figure 67b-d) morphologies with dome-shaped tips

(Figure 67c,d). The conical whiskers appeared to be significantly larger and more abundant than the cylindrical whiskers.

Many Kola cones showed distinct spiral growth steps (Figure 67c) at the surfaces of their tips, suggesting they have a scroll-type structure, as seen previously in carbon synthetic whiskers.36,42,46,51 SEM images of the broken cones and the surface where a

crystal has been broken off from the graphite substrate (Figure 67e) revealed their hollow

structure. 113

a b

5 µm 1 µm

c d e

1 µm 2 µm 500 nm

Figure 67: Graphite scrolls and tubes from Hackman valley, Mt .Yukspor, Kola Peninsula, Russia. a-d) FESEM images of graphite scrolls of varying morphologies, coating the surfaces of aegirine and associated minerals in fractures in the pegmatite. a) Cigar-like carbon scrolls. b) Broken scroll revealing a hollow center. c) A carbon whisker with scroll morphology. d) Cone with a smooth dome-shaped tip. e) The hole revealed at the surface where the crystal have been broken from the graphite substrate.119

Three different tip morphologies are shown in Figure 68. Shown are sharp conical tip (Figure 68a) and a tip with two secondary cones (Figure 68b) versus the more common rounded tip (Figure 67c-d). The tips of many cones also showed a change in morphology relative to the rest of the whisker.

The surfaces of many Kola cones were decorated with round nanoparticles

(Figure 67b and 68a) observed easily in the SEM as a difference in image contrast. In 114

a b

500 nm 2 µm

Figure 68: Kola scrolls and cones showing two different tip morphologies.

some cases, the cone surfaces were completely wrapped in a thin layer of grainy material.

Moreover, associated rigid tubular nanostructures (Figures 69a,b) have been occasionally observed with these layers. A thorough EDS analysis had been conducted on Kola

samples, and it was found that the films and the nanotubular structures (most likely

asbestos) comprised of Si, Ca, Fe and O in varying amounts. In addition, strong carbon

peaks from the graphite tabular and conical substrate and some traces of titanium were

recorded by EDS.

More important, however, was the observation of mineral-free carbon cones with

a relatively strong boron peak (Figure 69c). The content of boron in one such cone was

determined to be about 12.5 atomic % (the content of other impurities in the same cone

was less than 0.9 atomic %, including ~ 0.5% of nitrogen). EDS K-line maps are shown

in the inset of Figure 69c. Both boron and nitrogen were spatially associated with carbon.

Carbon is very well known for its ability to form layered C-B-N compounds due to the

substitution of some of the carbon atoms with B and/or N. This is observed in graphite,

carbon nanotubes and carbon cones. The relatively high amount of boron detected in the 115

a b

1 µm 300 nm

c

SEM NK

CK BK

Figure 69: Impurities in Kola samples: a) FESEM of a cone covered with a grainy layer of mineral. b) Tubular nanostructures observed on the surfaces of some cones. c) EDS spectrum of another cone showing a high content of boron and presence of nitrogen.

cone in Figure 69c suggested that some of its carbon atoms may have been substituted

with boron. Nitrogen may be bonded or just physisorbed on the cone surface.

EDS, however, does not distinguish between bonded or free atoms. Also, the accuracy of EDS in measuring both, B and N is low. Thus, further investigation, such as 116

TEM with EELS, for example, is needed to determine the actual structure and content of impurities in these cones.

4.1.2.3 Structure

Utilizing the FIB sectioning method, three TEM samples were prepared from

Gooderham cones. Unfortunately, none of the 3 samples were sufficiently good to provide unambiguous information about the cones’ structure.

Preparation of a TEM sample of Kola cones was far more successful. Graphite scraped from the cavities in the host rock resulted in a cone-containing powder that was directly placed on lacey carbon-coated copper grids for examination by TEM.

A HRTEM micrograph of a Kola cone is shown in Figure 70. The TEM suggested that the graphite in the nano-size cone was well-ordered and the surface of the cone

Loop plane terminations in the cone tip and the surfaces of central channel

Smooth graphitic surfaces

Dislocation Central “channel” 5 nm

Figure 70: HRTEM micrograph of a graphite nanocone with ~39° apex angle. The (002) lattice fringes were well-ordered and parallel to the cone surface.139 117

appeared atomically smooth. The average (002) interplanar spacing measured from lattice

fringes was 0.356 nm. The (002) lattice fringes were arranged parallel to the cone

surfaces, and formed a 39º apex angle. An amorphous region along the axis might have

been the central “channel” of the cone. The “channel” was not as clearly distinguishable

as that observed in GCCs (Figure 60a, inset) partly because of the loop edge terminations

on the inner cone surfaces. Noted in Figure 70 was also a dislocation observed in one side

of the cone wall.

According to Euler’s theory (Appendix B), incorporation of 4 pentagonal rings in

a planar graphene sheet yields its transformation to a cone with a closed tip and an apex

angle of ~39º. Although the apex size of the cone in Figure 70 corresponded to the value

calculated for the 4-pentagon fullerene cone, the structure of the tip (compare to the cone

in Figure 17), as observed in HRTEM, suggested the morphology schematically shown in

Figure 53b.

Figure 71 shows an HRTEM micrograph of another Kola cone. The apex angle of

the cone was ~126º (Figure 71a). The average spacing between graphene planes

measured from lattice fringes of the cone walls was 0.38 nm. The surfaces of the 126º

cone appeared to be coated with few nanometers thick amorphous carbon film.

Significant disorder was evident in the central part of the cone, as marked with the dashed line in Figure 71a. 118

a 126º -Kola cone Graphitic surfaces Loop plane terminations in the cone tip

Disordered core 5 nm

b Graphitic surfaces

A three-layer loop within the wall of the cone tip

1 nm

Figure 71: a) HRTEM image of a graphite nanocone with ~126° apex angle showing partially ordered lattice fringes parallel to the cone surface. b) HRTEM micrograph from the same cone. The image shows a discontinuity of the apex angle in the vicinity of the tip.139

The HRTEM image in Figure 71b showed a three-layer loop structure in the cone wall near the tip. The feature was sufficiently large to create the structural discontinuity and change of the apex angle in the very top several surface layers of the cone. Loop terminations of graphene planes created the effect of the round cone tip. 119

a b

60°

0002

C o 0002 n C e o n a x e i a s x is 50 nm

Figure 72: a) TEM micrograph of a 60º cone. b) Electron diffraction pattern from the center of the cone shows clear 0002 spots from the two sides of the wall.

The selected area electron diffraction pattern from the center of another Kola cone

(Figure 72a) was shown in Figure 72b. Strong 0002 spots indicated that the cone was

graphitic and its walls highly ordered. Similar to the cone in Figure 57, the two sets of

00.l diffraction spots come from graphene planes of the cone walls locally parallel to the

electron beam. The angle between the lines connecting these two sets of spots

corresponded to the relative orientation of graphene sheets in the cone walls, which, in

this case is the same at the apex angle of the cone. This implied that the graphene sheets have to be parallel to the cone surfaces.

Samples of Kola carbon from the cavity surface coatings were very rich with cones. In addition, TEM micrographs of this material have also recently revealed the

potential existence of naturally occurring carbon nanotubes.

120

4.1.2.4 Raman spectra

The first- and second-order Raman spectra from a Gooderham graphite cone and a

Kola graphite scroll obtained using a 514.5 nm excitation wavelength are shown in

Figure 73. Solid lines denote the spectra after baseline correction. For reference, also given is the Raman spectrum from the basal plane of a single crystal of graphite.

All of the observed Raman modes can be assigned according to the selection rules and the double resonance Raman mechanism.177 Of particular interest here, the first-order region of the spectra of both scrolls and cones showed a strong and narrow slightly upwardly shifted graphitic (G) ~1582 cm-1 line, as well as two additional features at

G 1584

ID/IG = 0.30 - cones 10 µm 5 µm FWHM 33.5 ID/IG = 0.25 - scrolls

D’ D 1622 2D 2726 1363 D+G 2D’ 2948 Gooderham cones 2456 3243 1583 2703 2722 1359

Intensity [a.u.] 1621 2946 3242 Kola scrolls 2466 2725 2686

1582 3246 Single crystal (basal plane) 2440

1500 2000 2500 3000 Raman Shift [cm-1] Figure 73: Comparison of Raman spectra from single crystal graphite, naturally occurring graphite cones from Gooderham, Ontario, and scroll-type whiskers of graphite from the Kola Peninsula. Cross-hairs in the optical images of the insets indicate the positions of the Raman probe for the respective spectra. 121

∼1360 (D) and 1620 cm-1 (D’). The second-order Raman spectra exhibited four distinct

energy dispersive features at ~ 2460, ~ 2726 (2D), ~2948 (D+G) and ~ 3242 cm-1 (2D’).

The full width at half maximum (FWHM) of the G line was measured to be ~ 33.5 cm-1 for Gooderham cones and ~ 24 cm-1 for Kola scrolls (FWHM = 14 cm-1 in the case of

graphite single crystal).

The calculated value of D to G line intensity ratio (ID/IG) was slightly higher for

the cones (0.30) than for scrolls (0.25). This and the FWHM values indicated a somewhat

higher degree of graphitization in the case of the Kola scrolls. Raman spectra of the

graphite cones were similar to those from microcrystalline graphite, and showed little

variation with respect to apex angle.

4.1.3 Carbon cones – geometry considerations

In principle, two possible models19,52 are currently used to explain the formation

of graphitic cones from graphene sheets (Figures 13 and 17). Both models are here

considered in an attempt to explain variations of morphology and observations of

structure of GPCs and natural graphite cones.

According to the first model, also known as the “cone-helix” structure, a cone is

made by winding a single sheet of graphite around an axis into a scroll. The formation of

this kind of structure generally requires a line defect, a wedge disclination, and a screw dislocation. On the other hand, an ideal “fullerene” cone is comprised of a number of 122

seamless conical graphene layers stacked along their axis. This model is known as the

“fullerene cone” model, because the tip of such a cone theoretically requires only

pentagonal defects and as such resembles the structure of fullerenes. Fullerene cones can

also be obtained by incorporation of pentagonal/heptagonal defect pairs or lower/higher

order carbon rings and their various combinations, as long as their number satisfies

Euler’s condition209 (Appendix B). In the world of graphite cones, this classification is an equivalent to the debate between “scroll” and “Russian-doll” model of multiwall carbon nanotubes.12

According to Euler’s theorem, exactly 12 pentagons are needed to form a

fullerene from graphene sheet, and exactly 6 pentagons yield a half-dome structure, as seen in carbon nanotube caps. Similarly, five different cones can be generated by having respectively 1 to 5 pentagonal rings in their structure, as experimentally demonstrated19,20 and shown in Figure 17. The apex angles for these cones can take only five discrete values (112.9º, 83.6º, 60º, 38.9º and 19.2º) which can be calculated according to the following relation:210

sin()θ 2 = 1 − ()N5 6 (12)

where N5 is the number of pentagons in the cone structure.

A departure from ideal is seen when a screw dislocation is added to the wedge

disclination, resulting in conical structures with a spectrum of apex angles.17,18,46,48,51,53

As a graphene sheet wraps around the disclination, adjacent overlapping layers are 123

rotated with respect to each other by an angle equal to the disclination angle. Among

practically unlimited number of disclination angles, some of them should be energetically more favorable (Figure 74). Their value can be calculated from the following equation:

α = n×60° , or α = n×60° ±ω (13)

where n = 0, 1, 2, … 6, and ω = 13.2°, 21.8°, 27.8° … are expected low energy (001)

twist grain-boundary angles based on lattice coincides, which are a measure of “goodness

of fit”, but do not account for atomistic interactions and the curvature of the sheets.

Disclinations with overlap angles equal to integer multiples of 60° (the same

values given by Euler’s theory) should be energetically the most favorable because they

preserve the graphite crystal structure without stacking faults, provided the screw

component of the disclination has a Burgers vector corresponding to an even multiple of

graphite’s c-axis interplanar spacing. Values of corresponding apex angles are calculated

from the following relation:

θ = 2⋅sin −1 ()1−α 360° , (14)

and range from 6° to 149°. Graphitic cones having such apex angles should predominate

over others.

The apex-angle distribution in the sample of natural graphite cones is shown in

Figure 62. This spread of apex angle values can only be explained by the cone-helix 124

13.2º 21.8º

apex α angle

27.8º 30.0º θ disclination angle

Figure 74: “Goodness of fit” according to cone-helix model: Four out of several energetically favorable (001) twist grain-boundary angles: a) 13.2º, b) 21.8º, c) 27.8º and d) 30º.

model. The smaller apex angles were not observed in the sample. Such cones may be energetically disfavored because of the higher elastic energy due to bending required to form the corresponding disclinations.

In contrast to natural cones, GCCs from glassy carbon yield only small apex angle values that range from ~3º to ~20º. None of these values corresponded to apex angle values of fullerene cones. In addition, most of them can be predicted by the cone-helix model. The structure of GCCs tip suggested their further stabilization through lip-lip interactions that resulted in semi-toroidal edge terminations.

125

4.2 Graphite polyhedral crystals (GPCs)

4.2.1 Morphology and size distribution

Graphite polyhedral crystals (GPCs) are another form of needle-like graphitic crystals found in the pores of GL-200 glassy carbon. The size of the graphite polyhedral crystals ranged from 100 to 1000 nm in diameter and can reach up to few micrometers in length. The size of GPCs in this case was limited by the size of the pores. Similar to

GCCs, polygonal crystals were attached with one of their sides to the walls of the pore.

The morphology of GPCs mostly resembled faceted rods. The number of facets can vary from 5 to 14 and these facets are distributed uniformly around the rod axis.

Depending on the number of facets, GPCs can therefore have various rotational symmetries. The most frequently observed number of facets was 9. It can be seen in

Figure 50 that GPCs grow in a geode-like fashion and have a random orientation within a pore. This was found to be a major difficulty in determining the actual number of facets.

Another difficulty is the ability to resolve faceted morphology on crystals of submicrometer diameter.

Figure 75 shows FESEM micrographs of three typical morphologies of GPCs.

The body of a GPC can be axially true (Figure 75a) or it can possess a helical habit

(Figures 75b,c). Various chiral angles have been observed. Equal probability of left- handed and right-handed helix is noted. In addition, the outer surface of helical crystals can sometimes contain a growth spiral (Figure 75b). The central tubular or polygonal channel (typically 25 nm or less) is clearly visible in the core of the crystals. 126

a b c

195 nm 200 nm 100 nm

Figure 75: High resolution FESEM micrographs of three typical GPCs with 9 facets: a) An axially true GPC with a tip in a form of a step-like pyramid, b) Helical GPC with a flat pyramid tip and most probably the growth spiral on the surface. One face appears slightly larger and two appear noticeably smaller than the other six faces. c) A perfect helical GPC with flat tip appears to have the nine-fold rotational symmetry.

About half of the crystals in the analyzed samples had pyramidal (Figure 75a,b) or

flat (Figure 75c) tip faces. Pyramidal faces may be smooth, as in the case of the crystal in

Figure 75b, or step-like, as in the case of GPC in Figure 75a.

The other half of the crystals terminated with a thin central “needle” or “tube”

(Figure 76) typically with a diameter of about 5 to 20 nm, as shown in Figure 76a. The

“needle” or “rope” goes throughout the whole GPC body (Figure 76d) and sometimes

extended well beyond the tip of the polyhedral body (Figures 76a-c). Not rarely, 2 or

more GPCs (Figure 76b,c) shared a common core. In extreme cases the core could extend

2-3 times longer than the whole body.

Although it appeared as stiff and rigid, the core was actually much more flexible

(as expected for a smaller diameter) and stronger than the body of the GPC, as noticed in

Figure 76d. The broken GPC revealed the central core which is bent 180º. This suggested 127

ab

2

1

12 nm 200 nm

cd

100 nm 100 nm

Figure 76: FESEM images of various needle-like crystal’ cores and extensions: a) A straight 12 nm in diameter carbon nanotube core extends out of a straight and thin body. b) A long and thin carbon nanotube is shared by a large (1) and a small (2) GPC. Helix angles of the two GPCs appear to be different. c) Irregular shapes occur in some of the thin cores. d) A broken GPC reveals a very flexible and strong core, suggested to be a carbon nanotube.

that the central core had the structure of a carbon nanotube. A large majority of GPCs had smooth and planar crystal faces; the presence of imperfect shapes, however, was also noted in some cases (Figure 76c).

In very small amounts, the pores of GC also contained larger cylindrical carbon rods (Figure 77a) and polygonal GPC-like structures with multi-faceted tips and no axial symmetry (Figure 77b). These structures occurred together in less than 3% of all non- conical needle-like crystals. The cylindrical rod shown in Figure 77a has a hollow and 128

a b

90 nm 500 nm

Figure 77: FESEM images of: a) A hollow cylindrical carbon nano-rod (most probably a thick and straight MWNT). b) A GPC with multi-faceted closed tip. It was expected that such tips contain sp3 carbon.

open tip structure. Its size was ~ 50 nm and above and the structure is most probably that of a MWNT. Transmission electron microscopy of a multifaceted crystal similar to that in

Figure 77 has shown that the main body of the crystal is hollow, the diameter of the cavity being about half of the diameter of the crystal. Multifaceted tips were expected to contain sp3 carbon.

4.2.2 Structure

Figure 78a shows a GPC with a polygonal body and a straight nested multi-walled carbon nanotube in the core of the GPC. Inside the MWNT core, there was a smaller

MWNT. A gap between the outer walls of the inner tube and the inner walls of the outer tube was observed. Similarly, the graphene planes of the GPC’s body closer to the nanotube core terminated earlier than the rest of the planes further away from the axis, 129

leaving, thus, a vacant space in the shape of a cup at the end of the GPC. The same features were observed in TEM analysis of another GPC shown in Figure 78b. The body of the GPC in Figure 78a, as marked schematically, had a cross-section in the shape of a regular pentagon.

Figure 78b shows two GPCs grown on the same core. The core appeared to be conical multi-walled nano-size hollow structure. Both GPCs had barrel-like shapes, with outermost graphene layers shorter than the inner ones. The terminations of layers were

seen as parts of the elliptical lines around the axis of the GPCs. From Figure 78b, it was

not possible to clearly resolve whether the layers formed a scroll around the core or

whether the shells were coaxial, but it was clear that they did not have facets as in the

GPC shown in Figure 78a.

a Multiwall carbon b Multi-walled nano- nanotube core size core

Pentagonal Central channel GPC body

The body of the secondary GPC

The body of the 50 nm primary GPC

Figure 78: TEM micrographs of: a) GPC with pentagonal cross-section. The multi- walled carbon nanotube core was nested in the polygonized body. b) HRTEM of two GPCs sharing a single multi-walled nano-size core of a conical shape. 130

200 nm 002 lattice fringe image

m n 23

Core m nanotube n 34 0.

Dome-like end of the GPC

Looped edge terminations

Polygonized body 5 nm

Figure 79: Low (a) and high (b, c) resolution TEM images of a GPC with a MWNT extending from the tip. HRTEM image near the core of the GPC shows the MWNT core 23 nm in diameter. GPC body terminated with a dome-like end, the surface of which was covered with loop structures.

The GPC shown in Figure 79 had a single MWNT core 23 nm in diameter, which was built from well graphitized walls with 0.34 nm interplanar spacing. The end of the

GPC had a dome-like shape, the surface of which was covered with loop structures.

Almost all of the dangling bonds were eliminated through the zipping of the adjacent graphitic shells into these loops.

A detail from the dome-like area of yet another GPC with loop structures is shown in Figure 80. Typically atoms at the ends of 4 or more adjacent layers closed 131

together in a more stable end loop configuration (1). In the case of the planar graphite, zipping of adjacent graphene layers would result in semi-cylindrical sleeves on the surfaces perpendicular to the basal plane of the crystal, while in the case of multiwall nanotubes, the resulting loop sleeve structure resembled the shape of a half torus (2).

Formation of loops between 2 or more layers may cause formation of the loops in few other adjacent layers because the curvature introduced in one part of the structure causes other layers in the vicinity to bend, as indicated by (1) in Figure 80. Some of the layers (3) in GPCs were found free of loops.

Zipping of graphene layers is also found in graphite conical crystals and natural cones, and it also has been observed in edge planes of some high temperature planar graphites,29,199 and on the surfaces of cup-like multi-walled carbon nanotubes annealed in argon atmosphere above 900 °C. 29,35,71,176,211,212

3

1 1

2

10 nm

Figure 80: HRTEM of a surface of the dome-like GPC end. Loops (1) are formed through zipping of 4 or more adjacent layers. The loops have configurations of half-tori (2). Some of the layers (3) do not zip. 132

Elimination of dangling bonds through the zipping mechanism is closely related

to the temperature to which the sample has been heated. For zipping to occur, it is

necessary for the edge atoms to overcome an energy barrier and form a bond with atoms

from an adjacent layer. The single loop structures are noted to form at temperatures

between 900°C and 1200°C, while 1500°C is considered as the onset for the formation of

multi-layer loops.176 However, in the case of well-aligned planes and appropriate environment, loop formation may occur at temperatures of 600ºC – 800ºC, as observed in the carbonization of liquid-crystalline nanofibers.213

The structure of GPCs was further investigated using selected-area electron

diffraction (SAED). Figure 81a shows a diffraction pattern obtained from the area of the

GPC shown in Figure 81b, which is the tip of the GPC and part of the core MWNT. It

was assumed that the GPC had flat outer surfaces and the axis of the GPC was therefore

located within the plane perpendicular or near-perpendicular to the electron beam.

The diffraction pattern of a GPC consisted of several groups of spots. Noted first

was the group of spots along a row perpendicular to the tube axis. These were 00.l spots

and corresponded to the c-spacing of graphite. They were produced by diffraction of the

electron beam by a set of graphene sheets in the tube walls oriented locally parallel to the

electron beam. The graphene planes locally perpendicular to the incident beam produce

hk.0 reflections. For graphite, zig-zag and armchair MWNTs, these spots form a hexagon,

as marked in Figure 81a. 133

a b

Tube axis

1013H

1010 1100 1210

0110 0 0110

0002 1210 0004 1010 1100

Figure 81: Structure of a graphite polyhedral crystal: a) Inverted SAED pattern of a GPC with multi-walled core nanotube. b) Corresponding TEM image of selected area of the GPC. Dark rectangle is the beam stopper.

The difference between the three structures can be observed by the shape and the

position of these spots. For MWNTs, the hk.0 reflections were streaked towards higher spatial frequencies and had the sharp cut-off at some well defined maximum interplanar distance of graphite. The streaking was related to the curvature of the graphene sheets which caused “projected” spacings along the incident beam direction to decrease with increasing local inclination of the lattice planes belonging to a given spot. The positions of the 0110 , 0110 , 1100 and 1100 spots in Figure 81a corresponded to the ziz-zag

arrangement of carbon atoms. For armchair tubes, the position of these four spots would

be on the same circle but placed closer to the projection of axis (compared to the

diffraction pattern of a polychiral MWNT in Appendix C). Furthermore, hk.l reflections

marked with red circles in Figure 81a suggested the hexagonal …ABABAB… stacking

of the tubes. 134

SAED pattern of a long cylindrical GPC is shown in Figure 82a. The pattern was obtained from the area of the GPC marked with circle in Figure 82b. Similar to the previous SAED pattern, an observed set of 00.l spots in the row perpendicular to the axis, and the set of streaked hk.0 reflections that forms a hexagon. Unlike in Figure 81a, the hk.0 reflections consisted of two spots streaked outward, which corresponded to chiral

MWNTs. In chiral nanotubes, the patterns produced by the sets of planes along the “top” and “bottom” of the tube will not coincide because of chirality, so the tube gives rise to the angular “doubling” 2θ of the hexagons of hko spots. The angular “doubling” is a direct measure for the helical angle in the case of normal incidence only. For the GPC shown in Figure 82b, the 4.5° value of the chiral angle was measured from its diffraction pattern in Figure 82a (assuming that the axis of the GPC was parallel to the electron beam). According to the diffraction pattern, there was only one extra set of hko spots, meaning that all tubes (layers) from the particular GPC had one single chiral angle. Since the pattern gave strong and sharp spots, the adjacent shells (tubes) should satisfy graphitic epitaxy. Marked with red circles in Figure 82a are reflections that point out to both hexagonal and rhombohedral stacking of the adjacent layers (note the difference relative to the previous SAED). The intensity of the spots that correspond to the rhombohedral stacking was stronger. Presence of both hexagonal and rhombohedral stacking patterns in a monochiral structure may support the “Russian doll” model of structure. If the structure consisted of coaxial nested graphene cylinders, it is hardly possible to simultaneously satisfy the chirality, stacking and geometry conditions, which may lead to eventual stacking faults. 135

a b

Tube axis

1010 1210 0110 0004 0002 1100 4.5° 1100 0

0110 1210

1010

500 nm

Figure 82: Structure of a long cylindrical GPC: a) Inverted SAED pattern of the GPC. b) Corresponding TEM image of the GPC supported by a lacey carbon film of the TEM grid. The circle shows the area that gives the pattern in a).

In summary, GPCs are highly ordered nanotubular graphite materials that consist of a surprisingly large number of graphene layers. These layers grow on a core nanotube, which results in complex axial morphologies of GPCs.

4.2.3 Raman spectra of GPCs

Vibrational properties of graphite polyhedral crystals have been studied by Raman spectroscopy. The size and aspect ratio of some of the GPCs enabled acquisition of

Raman spectra from the individual crystals. Submicron-sized GPCs were highly graphitized structures with the extinct D band and the G band of about the same width

(FWHM = 14 cm−1) as in crystals of natural graphite.214 This can be explained by a 136

smaller number of terminated graphene planes in GPC compared with a graphite crystal of the same size. The largest crystals were sufficient in size to enable selective micro-

Raman analysis from the side face and the tip, as shown in Figure 83.

Spectra from the crystal faces corresponded to perfect graphite with a narrow G band and no D band, as expected from the TEM analysis of the GPC structure (Figures

78 and 79). Spectra from the tips featured an unusually strong 2D (2706 cm−1) overtone that exceeded the intensity of the G band in graphite. It was similar to the spectra of graphite whiskers (as seen in Figure 46) but slightly weaker. The spectrum featured in

Figure 83 showed the strongest 2D band recorded for GPCs. Again, similar to whiskers, two additional bands were observed in the second-order frequency range at around 1895 and 2045 cm−1 (Figure 83).

1120 cm-1 184 cm-1 1580 cm-1 2706 cm-1 192 cm-1

1100 1140 100 nm along the crystal axis 1609 cm-1 3246 cm-1 -1 1618 cm-1 1352 cm-1 1442 cm 1495 cm-1

3174 cm-1 1300 1400 1500 1600 3151 cm-1

1895 cm-1 2045 cm-1 Side face The tip 3100 3200 3300 2446 cm-1

1500 2000 2500 3000 Raman Shift (cm-1) Figure 83: Fundamental, combination modes and overtones in Raman spectra taken from the side face and the tip of an individual graphite polyhedral crystal (514.5 nm excitation). 137

A number of weak low-frequency bands were observed in the initial study of

GPCs, including a doublet at 184/192 cm−1.35 The peak position of the doublet corresponded to the position of radial breathing modes (RBMs) of single-walled carbon nanotubes. In several studies, RBMs were observed in Raman spectra obtained from small diameter MWNTs. It was suggested that actually the innermost tubes of the

MWNTs contributed to these peaks. In the case of graphite polyhedral crystals, it was highly likely that the doublet at 184/192 cm−1 came from the inner shells of the multi- walled nanotube core often seen in the crystals.

The mode dependency of the excitation energy was not measured, since finding the same GPCs after switching lasers was not a trivial issue. Similar low-frequency bands were observed in spectra from the graphite crystal edges.172 Several bands were also observed in the range 1440–1500 cm−1 (Figures 83 and 84a). The positions of those bands varied from crystal to crystal and may reflect variations in the GPC structure.

Two new bands were also observed in the second-order spectra at ~3151 and

~3174 cm−1 (Figure 84b). Unlike the 1440–1500 cm−1 lines, the positions of these modes had fixed values; the assignment of these modes requires further investigation.

For acquisition of Raman spectra from a single GPC, it is necessary to take into account the fact that even the largest crystals are poorly resolved under a light microscope. Therefore, it was not possible to give a detailed correlation between the

Raman spectra and the structural features of GPCs (helical or straight crystals, symmetric or not). Still, based on an analysis of the Raman spectra (narrow G band, no D band and almost perfect graphite second-order spectrum) and TEM, GPCs were closer to perfect 138

graphite than multi-walled nanotubes or cylindrical graphite whiskers, which always showed a distinct D band.

a 1353

1452 1468 1464 1478 1381

1442 1495 1477

1300 1350 1400 1450 1500 Raman Shift (cm-1)

b

3246

3151 3174

3100 3150 3200 3250 Raman Shift (cm-1)

Figure 84: Raman scattering from graphite polyhedral crystals. a) 1400–1500 cm−1 lines, and b) 3151 and 3174 cm−1 features (514.5 nm excitation). Different colors correspond to 5 different crystals.

139

4.2.4 GPCs: - geometry considerations

An explanation for polygonization of multi-walled carbon nanotubes was

proposed by Zhang et al. 85 and the explanation was based on the assumption that

MWNTs have a structure of nested coaxial cylinders (“Russian doll” model of structure).

In such tubes, the circumference increase between two successive graphene cylinders implies the insertion of extra carbon atoms in the outer cylinder. The structure of the tube can take two possible further configurations. According to one model, the successive tube can adopt a different helical angle to accommodate the change in circumference, which will result in a tube with orientationally disordered (turbostratic) stacking. According to the second model (Figure 85a), the successive tube will have the same chiral angle and the rows of hexagons in successive tubes will tend to align themselves as close as possible to an energetically preferred graphitic stacking. Since such stacking cannot be satisfied along the whole circumference, edge dislocations (marked with bold lines in

Figure 85a) and regions with stacking faults are generated within the tube.

The optimum configuration is achieved when the dislocations are distributed as uniformly as possible around the tube’s periphery, this being induced by a mutual repulsion of dislocations of the same sign in a single glade plane perpendicular to the axis of the tube. Moreover, the interaction between dislocations of the same sign in parallel glade planes tends to align the dislocation lines in vertical walls. The number of such dislocation walls is of the order of 18 (resulting in 9 facets) and can vary slightly with the chiral angle.203 140

a b Regions of high curvature

Planar and near-planar regions with graphitic stacking

c d

2

2

1 3

Figure 85: Polygonization of multi-walled carbon nanotubes: a) Schematic of dislocation model,85 b) Planar and near-planar regions of graphitic stacking are separated with the regions of high curvature, c) 2000°C annealed tubes showing overgrowth of nanotube over the original in a sheath like structure, and d) polygonal structure of a MWCNT annealed at 2000°C.

FESEM and TEM studies of GPCs were in a very good agreement with the proposed model. The most common number of facets observed in FESEM micrographs of GPCs was nine (Figure 75). Clear and sharp axial edges and bright lines like those observed in the conical portion of the GPC shown in Figure 75a suggest a single chiral angle throughout the structure. SAED patterns from GPCs body, in addition confirmed graphitic stacking in GPCs’ walls. 141

It is easy to envision a trade-off between the energy associated with a turbostratic

stacking versus the dislocation and strain energy associated with shape changes and

stacking faults. Which mechanism will prevail depends strongly on the tube size. The relative energy required to maintain graphitic order is smaller with increasing tube size,

therefore thicker tubes are expected to have graphitic ordering and polygonal cross- sections with discrete regions of high curvature and stacking faults (Figure 85b), while thin tubes are more likely to maintain cylindrical cross-sections and turbostratic stacking with varying helicity between individual shells. It has been also observed that not all

GPCs are faceted, and some of the tubes, such that shown in Figure 77a are cylindrical.

The dislocation model is also very suitable to explain the effect of temperature on

the tube shape. Polygonization of MWNTs due to thermal treatment was further explored

experimentally. Solid state vacuum annealing experiments were conducted at

temperatures of 1600°C and above. Several types of MWNTs, including large nanotubes

(nanofibers with diameters of 50-200 nm) produced by Hyperion and Applied Sciences,

nanopipes produced at low temperatures in alumina membranes (Chemistry Department,

Drexel University) and cylindrical CCVD MWNTs (5-20 nm in diameter) produced by

Arkema, France215 were annealed at high temperatures (1600°C – 2300°C) for 3 hours in

a vacuum furnace and their structure before and after annealing was examined by Raman

spectroscopy, FESEM and HRTEM. The structure and morphology of synthesized

products revealed the role of surface diffusion in the growth of multiwall carbon

nanotubes. Discussion of possible synthetic routes for conical and polyhedral graphite

crystals will be given in detail in Chapter 5. Figures 85b and c show HRTEM

micrographs of Arkema nanotubes after annealing at 2000°C. Compared to as-received 142

tubes, graphitization, polygonization, mass transfer and over growth was observed in the annealed samples.

The increase of graphitic order in MWCNTs structure was observed by HRTEM and Raman spectroscopy. Raman spectra of the annealed tubes showed the separation of the G and D’ peaks, a lower R-value (ID/IG ratio), and an increase in intensity of the

second order peaks of MWCNTs. This thickening of the tubes indicated that mass

transfer was occurring during the annealing process. The original tube, observed in the

center of the tube in Figure 85c, was enclosed by new sheath-like graphene layers. There

was also an elimination of dangling bonds at the tips of the nanotubes as seen in Figure

85c. Due to annealing, the edge atoms of the successive graphene sheets formed loop

structures (shown in the inset of Figure 85c) like those seen in GPCs (Figure 80). Most

importantly, the morphology of the tubes after annealing changed to polygonal.

Polygonal cross-sections were readily observed in HRTEM micrographs of the tips of the tubes in a direction perpendicular or near-perpendicular to the tube’ axis, as shown in Figure 85d. Polygonization, in this case, as well as for all other heat-treated nanotubes, was irregular, and various other defects were seen in the tube. Examination of the regions of high curvature revealed that some of the graphene layers (1) were continuous throughout the whole curvature and underwent sharp bending, while the others (2) appeared broken and showed atomic plane mismatch. For continuous layers, the positions of bends in two successive graphene layers did not perfectly coincide along a straight radial line; however, the deviation was still on the order of 0.5 nm. Marked with (3) in Figure 85d are incomplete graphene sheets that suggested the scroll model of 143

MWNT structure in at least a part of the nanotube (compare to the schematic in Figure

55).

The real MWNTs differed significantly from ideal models used to explain the

morphology. The dislocation model of MWNT structure proposed by Zhang et al.,

(Figure 85a) was, nevertheless, very helpful in explaining the effect of temperature on the

morphology of the tubes. High temperatures will increase the dislocation mobility, which

will allow walls to graphitize, and in addition, the morphology of the nanotube will adjust to its energetically preferred polygonal configuration.

Some empirical studies suggest that polygonization of the tube cross-section will occur for inner tube diameters larger than ~ 12 nm (see Figure 19c), but the micrograph

in Figure 85d suggested that this value can be smaller than 8 nm.

It has further been proposed216 that multiwall nanotubes most likely consist of

circular core shells and polygonal outer layers. This assumption can only be confirmed by

a cross-sectional TEM of suitable MWNTs (small inner diameter and polygonized outer

walls), which is a challenging task, keeping in mind the difficulty in preparing cross-

sectional samples of nanotubes. While the limits of the FESEM resolution may not enable

one to observe polygonization of very small tubes, HRTEM along the tube axis showed

that the core nanotubes of GPCs (Figure 79) had uniform fringes on both side of walls

(no abnormal increase of interplanar spacing that can be attributed to polygonization).

This can be considered as indirect evidence that the core nanotube was cylindrical. New

layers grown over the core nanotube in the radial direction polygonized at a certain diameter, which further results in a polyhedral morphology of GPCs. 144

4.3 Testing the model of structure

4.3.1 Scrolls or “Russian-Dolls”?

Graphite polyhedral crystals (GPCs) have nanotube cores and graphite faces, and they exhibit very unusual and complex polygonal morphology. Graphite conical crystals

(GCCs) have very small apex angles and smooth graphitic walls. As-produced GPCs and

GCCs grow together within single pores of glassy carbon. The environment for growth of both types of structures can, therefore, be considered the same. The most natural question to be asked is why then are their morphologies so different? If the growth conditions are the same, the differences in GPCs’ and GCCs’ morphologies may be due to different nucleation, structure or growth mechanisms. If the conditions were not the same (if one were formed earlier than the other ones and the environment changed meanwhile), is it possible to correlate it with the morphologies observed? Since no evidence was found in favor of catalytic growth of cones or polyhedral crystals, the problem was simplified by an assumption that perhaps the shape of catalytic particles (that eventually sublimed at high temperatures) determined the morphology of the crystals in pores. Moreover, all of the results point out the role of the stacking sequence and its evolution in time as a key mechanism that affects the morphology. In going from 2D to 3D analysis, the first question to answer is: are these materials made of a scrolled or coaxial carbon layers?

Several interesting features observed in the samples of GPCs were analyzed.

These features are shown in Figure 86. The growth of a horizontal GPC (1) was 145

Interrupted growth and secondary GPCs Imprints on crystal faces and ring-like structures

1 4

3 5 2

200 nm 500 nm

Scrolls or Russian dolls?

Figure 86: Interrupted growth, imprints on crystal’ faces and ring-like structures found in GPCs.

interrupted with another crystal (2); however, the left and the right portion of the crystal

(1) had approximately the same diameter and helicity. Some core nanotubes were

covered with secondary GPCs. This seems to be possible only if the growth continued in

a spiral fashion. Some other crystals contained pyramidal imprints (4) that would be hard

to explain in the case of scrolled growth. Finally, a ring (5) formed by the pulling-out of

inner shells also suggests the Russian doll structure.

In order to obtain direct evidence of the tube microstructure, continued attempts have been made by several research groups in performing cross-sectional transmission electron microscopic studies217-220 on MWNTs. These studies had been conducted on

various types of carbon nanotubes and they still did not result in a complete solution.

Although cross-sectional HRTEM of GPCs and GCCs was considered as the most

informative method to verify the model, a challenge has been faced in the feasibility of

preparing good representative samples. The aspect ratio of the crystals and the way they 146

grow in the pores of glassy carbon with one end attached to the pore walls and the other one free-standing, do not make these materials the most suitable for this kind of study.

The present study employed an indirect method to examine the structure of GPCs

and GCCs by analyzing their behavior in the process of intercalation and exfoliation.

4.3.2 Intercalation and exfoliation

Intercalation and exfoliation experiments were performed to reveal the fine

structure of carbon nanocrystals from the pores of GL-200. Intercalation of graphite by

sulfuric and nitric acids is widely used in graphite industry to produce exfoliated graphite

flakes. Intercalation of SWNT bundles by nitric results in partial exfoliation of

bundles into individual single wall nanotubes.221 In the present study, glassy carbon GL-

200 was gently crushed in a mortar into smaller particles. The particles were examined

under FESEM to confirm that the GPCs and GCCs were not damaged during grinding.

The material was then soaked in concentrated H2SO4/HNO3 (1:1) for 1 hour to intercalate

the crystals with sulfuric acid (glassy carbon belongs to the group of hard carbons and is

therefore not expected to intercalate). The acid mixture was decanted and the sample

washed with deionized water and dried on a filter paper for about 24 hours. The acid-

treated material was rapidly heated at 980±10°C for 15 seconds to reach the maximum

volume expansion.

Formation of volatile species during annealing expands and exfoliates the graphitic phase into fine lamellae and ideally into single graphene layers. The abrupt release of volatile species between the graphene layers during exfoliation on a microscale 147

creates various types of deformations in the crystals. An SEM micrograph of exfoliated pore crystals is shown in Figure 87. Two different ways of deformation were observed in conical graphite crystals: lateral pullout of outer shells and unwrapping (unfolding). The first phenomenon favors a hypothesis of a cup-stacked coaxial layer structure, while the second one suggested the scroll-model of structure.

Graphite polyhedral crystals in most cases retain their polygonal shape after exfoliation (Figure 88). The most damage in the form of progressive etching was seen around the GPCs’ conical terminations and along the folding edge lines (Figure 88a).

Etching of a GPC

Unwrapping of a cone

Fractured GPC

MWCNTs

Pullout in cones

Exfoliated GPC

1 µm

Figure 87. FESEM micrograph of GL-200 GC after exfoliation. Etching, fracture and exfoliation of GPCs, and pullout and unwrapping of GCCs are observed. 148

The spacing between graphene layers in the vicinity of the folding edge was larger than in the near-planar regions, which allowed their better intercalation with sulfuric acid. Since these were the regions of high curvature, they were also more deformed and more frustrated with respect to near-planar regions. Similarly, looped terminations in GPCs tips are highly susceptible for intercalation. Graphite polyhedral crystals (GPCs) tended to retain their polygonal shape after exfoliation (Figure 88) or they fractured along preferred planes (Figure 89a). Fracture along preferred planes suggested the presence of 3D order. Radial steps on crystal' surfaces perpendicular to the axis (Figure 88a) supported this observation. Epitaxy and helicity of adjacent shells, however, cannot be satisfied throughout the whole structure, as suggested by the morphology of the steps. A major difference in the mechanism of exfoliation of GCCs and GPCs is illustrated in Figure 90. While polyhedral crystals typically fractured (Figure

89a), cones rather tended to unwrap (Figure 89b,c).

a 200 nm b Steps on surfaces

Delaminating of shells

Faster etching occurs along the folding edge lines 500 nm

Figure 88. FESEM micrographs of exfoliated GPCs. a) Two GPCs with etched surfaces. Etching occurs in the planes perpendicular to axis. b) GPCs retained their polygonal shape after exfoliation. Delamination of shells occurs occasionally. 149

a 500 nm b c 200 nm

Fracture Unfolding along plane of a cone

Nanotube core of a broken GPC Unfolding of broken cylindrical or a small apex angle conical 200 nm crystal

Figure 89. a) Fractured GPC reveals strong nanotube core. Fracture occurred along a preferred plane. Unwinding of b) a conical and c) a cylindrical graphite crystal (this can also possibly be a small apex angle cone). The mechanism of the degradation of conical crystals showed that they grew through a scroll mechanism.

The compactness of GPCs cannot be explained by a scroll-type of structure.

Moreover, such a peculiar and regular polygonization can be envisioned only for coaxial

(“Russian-doll”) structures. The strain energy of a single shell decreases with an increase of the tube diameter and the stacking fault energy becomes a dominant part in the total energy of crystals. GPCs are, therefore, most likely coaxial type structures. Unwrapping of the conical crystals, on the other hand, confirmed that they had a scroll type of structure and they grew by a cone-helix mechanism. 150

CHAPTER 5: DISCUSSION OF POSSIBLE SYNTHETIC ROUTES TO CONICAL AND POLYHEDRAL GRAPHITES

5.1 Parallel studies

Growing interest in carbon nanostructures during the past five years has resulted

in the discovery of several new methods for the synthesis of carbon nanocones65,72,222 and

GPC-like morphologies.90

Muradov and Schwitter65 investigated the non-catalyzed thermal decomposition

of light hydrocarbons over vapor-grown carbon filaments (VGCF), and found that carbon

cones (Figure 90a) were produced on the surfaces of filaments at ambient pressures and

temperatures ranging from 900°C to 1500°C. In this process, the filaments were

resistively heated in a quartz tube for 5-10 minutes under the vapor of an undiluted hydrocarbon gas (5 ml/min) and then cooled to room temperature in the flow of the same gas. Disproportionation of hydrocarbon molecules in contact with hot filaments resulted in the formation of carbon deposits and randomly scattered cones. The base diameter and height of these cones varied in the range of 70-2000 nm and 300-6500 nm, respectively.

The values of the most commonly observed apex angles were: 6°, 10°, 18° and 37°.

Slicing of cones with FIB showed that they were hollow. It is interesting to note that the morphologies of these cones did not depend significantly on the gas concentration,

suggesting that perhaps the surface diffusion mechanism may be responsible for cone growth. Thorough structural studies of these cones were not conducted; therefore, no further information is available about orientation and stacking arrangement of graphene 151

planes within cone walls. Relatively small apex angles of these cones showed that they may be related to cones observed in pores of glassy carbon.

In another study,222 hollow carbon cones with maximum outer diameters of 100–

600 nm (Figure 90b) were synthesized by thermal reduction of butanol over metallic Mg

at 500°C. In these experiments, butyl alcohol and metallic Mg powder were mixed and

sealed in a stainless steel autoclave and maintained at 500°C for 12 hours. Metallic Mg

reacted with butyl alcohol to produce carbon, magnesia and hydrogen. Carbon upon

cooling formed hollow graphitic cones. According to HRTEM and Raman spectroscopy

data, these cones were not fully graphitized, although alignment of graphene planes

parallel to cone surfaces was evident in their HRTEM images.

A microwave (750 W) plasma assisted chemical vapor deposition using a mixture of N2 and CH4 as the reaction gas (N2:CH4=200:3 vol.) resulted in growth of tubular

graphite cones (TGCs)72 on electrochemically eroded iron needles as substrates. The graphite sample holder with iron needles pointing upward into the plasma was heated to

600°C before introducing the gas mixture. TGCs of different sizes were formed with their roots varying from nanometers to micrometers in size. TGCs grew directly from the iron needle surface. The tips of the cones were catalyst-free, suggesting growth from the

catalyst particle in the base. TGCs had very high aspect ratio and were very sharp. The average length was about 12 µm, and the average tip apex angle was 6° to 7°, similar to the cones from pores of glassy carbon. Moreover, the TGCs possessed faceted and helical morphology. HRTEM of TGCs, however, shows that the (002) graphene planes were parallel to the cone axis and the conical shape of TGC was a result of uniform shortening 152

of graphene layers from the center towards the periphery. In other words, the TGCs were

multi-walled polygonal carbon nanotubes with conically tailored walls.

Polyhedral crystals of morphologies very similar to GPCs (Figure 90c) were

produced by using a flame combustion method.90 Flame combustion is actually a high

temperature chemical deposition process that involves complicated chemical and physical

processes far from thermodynamic equilibrium. In their study, Okuno et al. used an oxyacetylene torch (O2/C2H2 volume gas ratio typically 0.9) with a C2H2 gas flow of 2.0

l/min to deposit carbon over molybdenum plate substrates. The molybdenum plates were

heated at 1200°C to 1300°C prior to deposition. The deposition time was fixed to 3 min.

Experiments were performed with an electro-deposited Ni–Co catalyst layer and without

catalyst, and in both cases GPC-like morphologies were formed. When no catalyst was

used, crystals with rod-like and pin-like helical polyhedral morphologies grew up to 3 µm

in diameter and 15 µm in length. The number of facets was frequently eight. The

diameter of crystals was smaller towards the tips, leading to a sharp apex. HRTEM

confirmed that highly graphitic carbon layers in rod-like crystals were oriented parallel to

the surface sides of the crystal. In pin-like crystals, graphitic layers were also parallel to

the surface side but not to the crystal axis. Thus, the polygonized shapes made these

structures similar to GPCs. However, the different faceting suggested a different

polygonization mechanism.

When a catalytic layer was introduced on the substrate, multiwall carbon

nanotubes (MWNTs) were formed in the early stages of the deposition (5–10 s). After a

deposition time of 1 min, polyhedral crystals were synthesized around these MWNTs. In 153

that case, nucleation appeared simultaneously in several places and gave rise to several

crystal segments along the core MWNT, similar to GPCs from glassy carbon pores

(marked with (3) in Figure 86). The morphologies of rod-like and pin-like crystals

reported by Okuno et al. did not have the perfection of shape of those formed in the pores of glassy carbon. This might be the result of their fast growth far from thermodynamic equilibrium.

a b

c d

Figure 90: Conical and polyhedral carbon nanostructures reported after 2002: a) SEM image of a small apex angle carbon cone grown on VGCF surface by thermal decomposition of propane,65 b) TEM image of a typical hollow carbon cone produced by thermal decomposition of butyl alcohol in presence of metallic Mg,222 c) Pin-like polyhedral rod crystal produced by a combustion flame method,90 d) Cross- sectional HRTEM view of faceted MWNTs (a facet marked with arrow).223 154

Very interesting partially faceted tubular carbon nanostructures223 were formed by

a high temperature thermal treatment of MWNTs produced by a catalytic CVD method.

The main characteristics of these nanotubes were highly straight and crystalline layers,

very low interlayer spacing (0.3385 nm), low R value (ID/IG = 0.0717) in their Raman

spectra, and unevenly spaced lattice fringes on one or both sides of the hollow core.

Similar to the results obtained in the present study (Figure 85b), catalytic CVD MWNTs with highly undulated graphene layers were transformed into straight crystalline layers and their round tip morphology was transformed into faceted morphology. Direct

HRTEM observation of cross-sectional shape of annealed carbon nanotubes (shown in

Figure 90d) confirmed that the hollow core of the nanotube was not circular, but rather partially faceted. Partially faceted cross-sectional shapes (marked with an arrow) were produced during thermal treatment up to 2600°C. Faceting was ascribed to abrupt density changes within a confined nano-size space, with accompanying phase separation. SAED on these structures was not conducted to analyze the stacking sequence of carbon layers

within the tube wall segments.

In brief, the studies conducted by others in parallel to the present study

demonstrated that it is possible to synthetically produce nano- and micro-size carbon

materials of various conical and polyhedral morphologies in a predictable manner. None

of the morphologies achieved, however, exactly duplicated those of GPCs and GCCs.

This showed that conical and polyhedral graphitic structures can be produced by different

methods and various precursors in a broad temperature range with growth parameters and

methods scattered over a very broad range. Finally, the results of Okuno et al.,90 Kim et al.,223 and the present study all agree that high temperatures help graphitization and 155

polygonization of carbon layers around core carbon nanotube, but it did not result in

perfectly uniform facets as observed in GPCs.

5.2 Hydrothermal routes to GPCs and GCCs

Hydrothermal techniques have been used for the synthesis and crystallization of

different materials such as quartz, zeolites, ultrafine ferrites, hydroxyapatite and

diamond.224 The term “hydrothermal” refers to any heterogeneous reaction which occurs

in the presence of aqueous solvents under high pressure and temperature conditions to

dissolve and recrystallize (recover) materials that are relatively insoluble under ordinary conditions.224

Recent developments in the area of hydrothermal treatment of carbon materials

showed that this method can also be used for the production of various carbon

nanostructures. The studies showed that nano-size carbon cells,225 carbon nanotubes,226 planar and bamboo-like carbon filaments227,228 can be grown from supercritical C-H-O

fluids.

When solid carbon and water are sealed in a capsule and exposed to high pressures and temperatures, water dissolves carbon to form a supercritical C-H-O fluid.

Various forms of solid carbons are formed from this fluid upon cooling. In addition, solid carbon can be replaced by virtually any (ethylene glycol, polyethylene, paraformaldehyde and others have all been used successfully) to form the 156

supercritical C-H-O fluid. Thermodynamic calculations done using the ChemSage 5.1

226 Gibbs energy minimization program show that H2O, H2, CO and CH4 are expected to

be present in the growth environment. CH4 and CO are in equilibrium with solid carbon

under hydrothermal synthesis conditions 229 and growth of solid carbon from these species is assumed.

The behavior of fullerenes under hydrothermal conditions was investigated by

Suchanek et al.230 Their study demonstrated that fullerenes can transform to amorphous

carbon at temperatures above 400°C over a long time (48 hours). In addition, when nickel particles were used, open-ended MWNTs were formed from fullerenes under hydrothermal conditions in the vicinity of nickel particles. This process typically occurred at temperatures around 700°C. Such nanotubes typically had an outer diameter of 30-40 nm and a wall thickness of 5 nm. Similarly, Gogotsi et al.226 reported hydrothermal synthesis of MWNTs by treating organic precursors in the C-H-O system at temperatures up to 800 °C and pressures up to 100 MPa in the presence of a catalyst. The diameters of these hydrothermal MWNTs typically ranged from 50 to 200 nm.195 TEM and electron diffraction analysis showed that these nanotubes had nearly perfect graphene layers in the walls if the synthesis temperature was high and a catalyst was used. Most of the large tubes did not have cylindrical cross-sections, but they did not develop sharp edges either.

Calderon-Moreno et al. reported the behavior of SWNTs under hydrothermal treatment231 and formation of catalyst-free MWNTs. In their study, SWNT were

hydrothermally treated at temperatures between 200 and 800°C and pressures of 100 157

MPa. After treatment, SWNT transformed completely to shorter MWNT and graphitic

particles.

It has also been demonstrated that amorphous conical carbon microstructures can

grow from a supercritical C-O-H fluid formed by the decomposition of paraformaldehyde

(Fig. 91a).227 Well ordered hollow carbon nanotubes with multiwall structures (Figure

91b) composed of graphitic layers have been produced by treating amorphous carbon in pure water at 800 °C and 100 MPa.232 High resolution TEM micrographs of some of

these tubes showed faceted tips and variations in contrast and lattice spacing along the

tube diameter, suggesting that their cross-sections may be polygonal. Similarly, some

GPCs with faceted tips were shown in the present study (Figure 77b).

Hydrothermal carbon nanotube growth from supercritical water is supposed to

occur under equilibrium conditions, the surface transport rates under equilibrium

conditions are very high, and therefore tubes with well-ordered graphite walls are

a b

4 µm

Figure 91. a) SEM micrograph of amorphous carbon cones.227 b) TEM micrograph of a polygonized nanotube similar to GPC produced from supercritical C-O-H fluids.195 158

typically produced.195,226 Similarly, the walls of GPCs in the present study were shown to

be highly graphitic. Finally, the morphologies, petrologic associations, and the values of

the stable carbon isotope δ13C analysis of cone-containing natural graphite samples

described in the present study all point in the direction of a hydrothermal route of

formation. Natural cones, therefore, may also be formed by a C-O-H fluid deposition

during cooling or by hydration reactions with dissolved minerals.

A very recent study by Wang et al.233 reported the synthesis of highly crystallized

MWNTs under hydrothermal conditions at the surprisingly low reaction temperatures of

180°C. In their study, Wang et al. synthesized MWNTs through the reduction of ethyl

alcohol by NaBH4 without conventional catalysts such as Fe/Ni/Co, following a modified

Wolff-Kishner reduction process. The Wolff-Kishner reduction normally refers to a base catalyzed process of production of alkanes from the corresponding and . The growth of MWNTs takes place under a strong basic solvent with high concentration of NaOH. In a typical reaction, ethyl alcohol, NaBH4 and 10 M NaOH

solution were thoroughly mixed and sealed in a Parr reactor which was then kept at

180°C for 20 hours. Examination of the reaction products after cooling, washing and

drying, confirmed the synthesis of high quality MWNTs. The size of such nanotubes

typically ranged from 10 to 40 nm in diameter and up to several micrometers in length.

The inner diameters of these tubes were 3 to 6 nm. HTREM and SAED studies showed

that the tubes had bamboo-like structure and well-crystallized walls. The tips of the tubes

were mostly closed, the closure typically having a conical shape (formed by

incorporation of pentagonal defects in graphene sheets). These experiments have also

revealed an essential role of concentrated NaOH in aqueous solution for the formation of 159

the carbon nanotubes. The NaOH in this process was assumed to chemically catalyze the

reduction of ethyl alcohol by NaBH4 under hydrothermal conditions.

Similarly, recent studies of Liu et al.222 (explained in the previous section) demonstrated that hollow carbon cones can be formed at 500°C by thermal reduction of butyl alcohol over metallic Mg. Metallic calcium and magnesium are not known to catalyze MWNT growth; however, they can promote thermal decomposition of certain organic precursors. Furthermore, calcite has been shown to enhance graphitization in hydrothermal treatment of amorphous carbon,234 and few earlier studies235,236 also demonstrated the crucial role of alkali and earth-alkali compounds (typically and carbonates) in carbonization of 3,5-dimethyl-phenol-formaldehyde resins, where they reduced the temperature of liberation of hydrogen from the resin. The present study has also shown that natural carbon cones form in calcite boudins, and that trace amounts (less than 25 ppm) of calcium and magnesium were detected in GL-200 glassy carbon that contained GPCs and GCCs.

While the exact role of alkali and earth-alkali elements and their compounds in nucleation and growth of curved graphene surfaces is still not completely understood, it is clear that they can lower the temperature of alcohol reduction under hydrothermal conditions and thus alter the composition of C-H-O fluids. In return, this may create hydrothermal system conditions (concentration, pressure and temperature) that favor the deposition of curved form of carbon versus planar graphite.

160

5.3 Relation of GPCs and GCCs to hydrothermal nanotubes

A major question arising in the present work is the mechanism by which GPCs

and GCCs grow in glassy carbon. Here it is hypothesized their formation by a

hydrothermal route, although the precise thermal and chemical history of GL-200 glassy

carbon samples is speculative. The sequence of the events proposed here seems logical in

explaining the formation of GPCs and GCCs in glassy carbon pores and relies heavily on

experimental observations of the present and other parallel studies.

Glassy carbon GL-200 is produced in a process of carbonization of cured phenolic resin at very low heating rates (to minimize thermal stresses and avoid crack formation) and final firing temperatures of ~2000°C. Curing of phenolic resin occurs through a condensation reaction (Figure 29a) where phenol reacts with formaldehyde to form a three-dimensional polymer network during which the water is given up. Most likely, the pockets of fluid (water vapor and some other small molecules such as oxygen, hydrogen, carbon monoxide, etc.) create bubbles in the structure while the resin was still curing and before the structure was completely cured. As curing continues and the viscosity of the system increases, the bubbles remain trapped inside the structure as shown schematically in Figure 92a.

161

abc

C-O-H

GC matrix GC matrix GC matrix Figure 92: Proposed process of formation of GPCs and GCCs in glassy carbon: a) Formation of closed pores filled with gas in C-O-H system, b) Precipitation of carbon from supersaturated C-H-O fluid and nanotube growth on the walls toward the center of pores. c) Thickening of nanotubes and formation of GPCs. Growth is limited by pore dimensions.

Carbonization of glassy carbon is typically done by heating the material very slowly (0.1°C/min) from 120-240°C, when reaction occurs, to 900-1000°C, when most of the non-carbon species are removed. During this process, any remaining water molecules, as well as hydrogen, oxygen, and potentially CO and CH4 continuously evolve from the carbonizing network into the space available around. This may be very important in the early stages of structure development, as the gas evolving from the process can balance the pressure inside the pores to prevent leakage of species entrapped in them. As the temperature increases, the structure of glassy carbon and the gas dynamics across pore surfaces change, as does the concentration of species inside pores. But, since the process is very slow and changes are incremental during most of the process it is reasonable to assume that the whole system is close to thermodynamic equilibrium.

The development of the three-dimensional glassy carbon network (shown in

Figure 28) reduces the gas permeability of the pore walls. Pores with entrapped H, O and 162

potentially some CO species at one point may act as tightly closed systems and enable the formation of hydrothermal conditions. Once such conditions are established, the hydrothermal fluid may act to partially dissolve and recrystallize the inner surface of the pore walls. Deposition of carbon from C-H-O fluid may then yield reorganization of carbon atoms from the pore inner surface into cylindrical, polygonal or herring-bone carbon nanotubes and conical surfaces (Figure 92b), planar graphene and various other carbon filaments, in a manner dependent on the surface topology and experimental conditions. If the process occurs close to thermal equilibrium, these structures can be expected to have highly graphitized walls. These carbon nanotubes and conical carbon surfaces serve as precursors for the growth of GPCs and GCCs, as illustrated in Figure

92c. Nucleation of tubular structures will promote fast growth of core nanotubes in the longitudinal direction and very slow growth in the radial direction. Rapid consumption of available carbon atoms after nucleation of nanotubes can alter the thermodynamic equilibrium of the closed pore system and change the ratio of radial vs. longitudinal growth rate. Preformed MWNTs then continue to play the role of the deposition substrate, and the tube continues to grow radially, giving rise to GPCs and GCCs (Figure

92c). The growth of graphite polyhedral structures around MWNTs in a combustion flame method has also been confirmed experimentally (see Chapter 5.1).

As shown in the previous section, nucleation and growth of carbon nanotubes and nanocones is possible by treatment of various organic precursors and solid carbons under hydrothermal conditions. Amorphous carbon, fullerenes and SWNTs can all yield

MWNTs if processed appropriately. The hypothesis that carbon nanotubes and nano- cones can nucleate hydrothermally in the pores of the glassy carbon has been tested 163

experimentally in the present study. A small amount of ground glassy carbon powder was

sealed in a gold capsule (3 mm in diameter and 1.5 cm in length) filled with deionized

water. The capsule with glassy carbon was then placed in a Tuttle type autoclave, which

was pressurized to ~ 80 MPa and slowly heated to 760° where it was kept for about 4 h.

Reaction products were cooled, dried and then analyzed by TEM. Figure 93 shows the

changes of the glassy carbon structure caused by a short hydrothermal treatment. A

typical structure of the original glassy carbon is shown in Figure 93a. HRTEM images of

the hydrothermally treated material are shown in Figures 93b-d. These show multi-walled

carbon nanotubes, nanocones and polyhedral nanoparticles with 2-5 graphene layers in

the walls. Figure 93b shows a nano-size carbon cone and Figure 93c shows a double-

walled carbon nanotube with a large fullerene inside and possibly a smaller one on the outer surface of the tube tip. Figure 93d shows another double-walled nanotube, closed conical structures and onion nanoparticles. None of these MWNTs has catalyst particles in their cores or tips. Since some of the short tubes show closed tips on both sides, it can be concluded that MWNTs can nucleate non-catalytically from glassy carbon under hydrothermal conditions.

It has also been shown that growth of MWNTs can occur by both catalytic and non-catalytic routes. TEM examination of GPCs and GCCs tips and bases, when possible, did not provide any evidence that can support the notion of their catalytic growth, although some trace amounts of iron were found in glassy carbon by the final analysis of GL-200 ash. The final analysis had also shown the presence of trace amounts

of Mg and Ca. The chemical reactions of carbonization and reduction of phenol- 164

formaldehyde may be affected by these species, but it is very hard to define their actual role in formation of morphologies such as GPCs and GCCs.

Growth of GPCs and GCCs from a hydrothermal fluid is further supported by the appearance and surface topology of the pore walls. As shown in Figure 30b, the inner surfaces of the pores were completely covered with tiny graphitic crystals (confirmed experimentally by Raman spectroscopy). Moreover, hydrothermally produced MWNTs

a b

c d

Figure 93: TEM images of (a) glassy carbon matrix, (b) nanocones, (c) nanotubes and fullerenes, (d) nanotubes and multi-shell carbon nanoparticles. 165

reported in several other studies produced straight, needle-like morphologies, similar to

GPCs. This suggests that the growth of ordered carbon structures in the pores of an

amorphous material is not a random event related to the presence of catalyst particles, but

is rather governed by the thermodynamic conditions suitable for growth.

Non-equilibrium structures are difficult to make in high temperature processes

when all the reactants are mobile.237 The experiments of Okuno et al.90 and the present

study show that large polygonal nanotubes can be synthesized both close to and far from

thermodynamic equilibrium. Although polygonal tubes may be considered more stable

than cylindrical MWNTs, large polygonal MWNTs with regular size and morphology

similar to GPCs were not reproduced under combustion flame conditions. At the moment,

there is no experimental data that confirms the formation of regular polygonal GPCs far

from thermodynamic equilibrium. The present study suggests that GPCs with regular

morphologies, such as those observed in the pores of GL-200 glassy carbon, are

equilibrium structures and that they were formed by an epitaxial growth of carbon over

the core carbon nanotube. Their perfect shape (uniform facets and constant helical angle)

was intrinsically determined by the structure of the core nanotube and preserved during

the epitaxial growth around it. 166

CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS

Carbon is a unique element due to its unprecedented tendency to form a myriad of solid carbon forms. These forms exist naturally in the Earth’s crust, extraterrestrially, or can be produced via different synthetic routes.

Although the properties of bulk graphite, single-walled and multi-walled carbon nanotubes have been studied fairly well, very little is still known about various non- planar graphitic materials of intermediate (nano and micro) size. The present work examined the morphology, structure and vibrational properties of two classes of meso-

scale carbon materials: carbon cones and polyhedral crystals. Conclusions and suggested guidelines for future work are outlined.

Synthesis of carbon cones and polygonal crystals still remains in its infancy, and

it is a challenge to produce sufficient amounts of these materials for actual measurements of their physical properties that will take them a step further towards actual applications.

6.1 Conclusions

In the present work, high resolution scanning and transmission electron

microscopy, selected area electron diffraction and Raman spectroscopy studies have been

conducted on graphite polyhedral crystals and carbon cones from the pores of glassy

carbon and two types of naturally occurring carbon cones. The materials studied are a

new class of three-dimensionally ordered layered non-planar nano- and micro-size 167

graphitic carbons. The structure of GPCs and GCCs has been further tested by

intercalation and exfoliation experiments. Potential nucleation and growth mechanisms of

GPCs, GCCs and naturally occurring carbon cones were analyzed and hydrothermal

synthetic routes have been suggested.

SEM analysis of GL-200 glassy carbon fracture surfaces revealed pores that

contain nano-size graphitic cones in addition to graphite polyhedral crystals and

multiwalled carbon nanotubes. These cones were found to have very small apex angles

that varied over a narrow range from ~3° to ~20°. The size of the cones ranged from a

few tens of nanometers to ~300 nm in cone base diameter, and the length ranged from a

few hundred nanometers up to few micrometers. Two types of cones were observed in

the pores of glassy carbon: cones with dome-shaped tips and cones with almost flat tips.

The latter had smaller apex angles (typically from 3° to 5°) than the former type. Cones

from glassy carbon typically were hollow, with the wall thickness sometimes exceeding

100 nm. GCCs were highly graphitic (interplanar spacing of ~0.34 nm as in turbostratic

graphite), with basal planes parallel to the cone walls.

Carbon cones and graphite whiskers have been discovered in the samples of

natural graphite. Graphite in nature occurs as cones in several localities. Examined in the

present study were spherical, spheroidal and triskelial polycrystalline carbon aggregates

from Gooderham, Canada, and cones have been found to form nearly continuous coverage of the aggregates’ surfaces. Graphite whiskers and scrolls were discovered in fibrous carbon samples from Kola Peninsula, Russia. 168

The carbon cones from Canada were relatively large in size, some of them

reaching up to 40 µm in height. Unlike GCCs from the pores of glassy carbon, these cones had a broad range of apex angles that varied from 38° to ~140° with 60° being

most common. Surfaces of the Canadian cones were smooth and their tips were dome-

shaped. Often, the surfaces of larger cones were covered with smaller secondary cones.

Apex angles of secondary cones were typically in the upper range. As shown by TEM

and SEM analysis of sliced cones, the majority of them were fully solid, however some

of the samples from the same locality contained curved and hollow graphite shells. In

addition, submicrometer size carbon cones with faceted tips were observed on several

occasions. TEM and Raman analysis of natural carbon cones showed that they were

partly graphitic (interplanar spacing of 0.35 nm and ID/IG ratio of 0.3). Graphite basal

planes were parallel to the walls, similar to GCCs with dome-shaped tips. The size of

graphite whiskers from Kola Peninsula ranged from several hundred nanometers to over

15 µm in length and several µm in diameter (for large whiskers). They grew in clusters or

in isolation, but never covered large surfaces as seen in the Canadian samples. A variety

of morphologies ranging from cones with dome-shaped tips to scrolls and intermediate

shapes were present in Kola samples. Kola whiskers and cones were on average

somewhat more graphitic (ID/IG ~ 0.25) compared to the Canadian samples.

The structure of natural carbon cones with wide distribution of apex angles, many

of which did not satisfy Euler’s condition for fullerene cones, was explained through the

cone-helix model. Exfoliation of GCCs also confirmed that at least some of them were 169

made by scrolling of graphene layers at a small angle, although a “cup-stacked”

(analogous to Russian-doll) type of structure was not completely excluded.

GPCs were another class of non-planar crystalline nano- and micro-size graphitic material that grew in the pores of GL-200. The present study showed that they have the structure of large polygonal multiwalled carbon nanotubes with three-dimensional ordering of the carbon layers. The size of the GPCs ranged from few tens of nanometers to almost 1 µm in diameter, and up to few micrometers in length. The morphology of

GPCs resembled faceted rods, the number of facets varying from 5 to 14, with 9 being the most frequent. The cross-section of GPCs typically had a shape of a regular polygon.

Axially straight and helical GPCs were observed. Chiral angle of GPCs varied from crystal to crystal. SAED patterns showed that hexagonal epitaxy of adjacent graphene layers was at least partly satisfied within a single GPC. A multiwalled carbon core nanotube was observed in many GPCs to extend at their tips. Occasionally, two or more

GPCs were observed on the same core nanotube. Nanotube cores were stronger than the outer layers. The mechanism of failure along with the unique regularities and rotational axial symmetry of GPCs suggested that they had “Russian-doll” structures.

Besides suitable composition, pressure and temperature of the growth environment, formation of highly ordered structures within pores of glassy carbon was promoted by slow reaction kinetics, and the presence of active species such as hydrogen and oxygen atoms that balanced the crystal growth rate with the surface etching rate. This could explain the surprisingly large number of ordered carbon layers (over 1500) growing on the core nanotube that resulted in complex but regular axially-symmetric structures. 170

While exposure to high temperatures was shown to facilitate polygonization of cylindrical multiwalled carbon nanotubes, regular polygonal shape of GPCs could not be achieved by annealing only. Their perfect shape (uniform facets and constant helical angle) was intrinsically determined by the structure of the core nanotube and preserved during the epitaxial growth around it.

Hypothesized in the present study was formation of GPCs and GCCs from supercritical fluids oversaturated with carbon, although the precise thermal and chemical history of GL-200 glassy carbon samples was not known. The sequence of the events proposed seemed logical in explaining their formation in glassy carbon pores and relied heavily on experimental observations of the present and other parallel studies. Geological origins of naturally occurring carbon cones also suggested their growth under hydrothermal conditions from a metamorphic fluid.

The present study on several new types of solid carbon materials provides information on their morphology, structure and the mechanisms of their formation.

Certain parts of the study could be further elaborated and developed, as suggested in the following section.

6.2 Future work

Nano- and micro-size GPCs and GCCs combine geometry and attractive properties of carbon nanotubes with properties of a finite size 3D material, and, as such, may play the key role in practical realization of meso-scale devices, field emitters, chemical 171

sensors, reinforcement in composite materials etc. Due to their peculiar morphology and

rigid structure, GPCs are the material of choice for nano-size probes. The bottom-up

approach in manufacturing of probes by self-assembly does not require sophisticated

tools or complex procedures typical of top-down methods. Moreover, making the axial

multi-layered shell or scroll type nano-size structures by top-down methods is impossible

for the time being.

In order to make these materials available for use, further effort is required in

synthesis of GPCs and GCCs. A better understanding of their properties is also needed.

Recommended here are a few areas that future research should focus on:

• Development of an optimized large scale and high-yield synthesis method;

• Modeling the morphology and the electronic structure of GPCs and GCCs;

• Measurement of the mechanical and electrical properties;

• Testing the chemical stability, aptness for functionalization and oxidation

kinetics.

Growth of GPCs and GCCs from supercritical C-H-O fluids needs to be explored in depth. Understanding the mechanism of GPCs and GCCs formation from C-H-O fluids will (1) expand current fundamental understanding of carbon nucleation and growth kinetics in hydrothermal systems, and (2) will contribute to better understanding of the process of non-planar graphite formation in the Earth’s crust. Effects of chemical composition, pressure, temperature, cooling rate and reaction catalyst also need further attention. The exact mechanism of nucleation and growth of carbon from the C-O-H system and the precise role of the precursor are not known. Once the growth conditions 172

for GCCs and GPCs are established, growing the arrays of these materials on a substrate

can be attempted. Moreover, hydrothermal synthesis using supercritical water in a high- pressure, high-temperature autoclave offers many advantages over other preparation methods: it is relatively inexpensive, environmentally benign, and allows the reduction of free energies for various equilibria and synthesis of phases that may not be stable under other conditions. Enhanced transport in supercritical fluids allows growth of ordered crystal structures at temperatures well below those of solid or gas phase synthesis. Taking into account the fact that the formation of hydrothermal carbon occurs under relatively low temperatures (200-800°C) and pressures (<100 MPa), this method is attractive for growing new carbon structures.

Currently, there are no available theoretical models that describe the geometry of polygonized tubes by taking into account the size of the inner tube diameter, the chiral angle, strain energies of individual shells and free energy due to graphitic stacking. An advanced model is therefore required to demonstrate the correlation between the shape of the GPCs and the intrinsic nature of their core nanotubes.

One can expect the need for developing a mesoscopic theory of electronic structure of GPCs and GCCs. It essentially differs from the theory developed for cylindrical multiwall nanotubes because of: (1) the stronger interactions between neighboring walls in the facet as a result of perfect registry, and (2) additional quantization resulting from discontinuity in the electron wave-function over the edge of the facet. In the case of GCCs, the curvature and the strain are not constant over the surface, and they increase at the tip because of the topology of the conical manifold. 173

Mechanical, electrical and other physical properties of GPCs and GCCs need to be

measured. Limited amounts of material and the small size of the individual particles was

a major obstacle in doing it earlier. The development of new methods and tools for

manipulation of nano-size objects such as Zyvex nano-manipulator, however, is now

opening the door to new measurement capabilities. Studying how the properties depend

on the symmetry and/or morphology of the GPC/GCC is another exciting challenge.

The interaction of given chemical moieties (C-H-O) with the graphitic surfaces

(planar and non-planar) at various temperatures and pressures should be thoroughly investigated both experimentally and theoretically. Kinetics of interaction of GPCs and

GCCs with hydrogen, oxygen and oxidizing acids, can be very helpful in maximizing the yield and optimizing the growth parameters. Chemical functionalization of GPCs and

GCCs can add to the versatility of properties, but can also test the realm of using GPCs and GCCs for chemical sensors.

174

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230. Suchanek, W. L., Libera, J. A., Gogotsi, Y. & Yoshimura, M. Behavior of C60 under Hydrothermal Conditions: Transformation to Amorphous Carbon and Formation of Carbon Nanotubes. Journal of Solid State Chemistry 160, 184-188 (2001).

191

231. Calderon-Moreno, J., Swamy, S. S. & Yoshimura, M. Evolution of Single-Wall Carbon Nanotubes During Hydrothermal Treatment. Solid State Ionics 151, 205- 211 (2002).

232. Calderon Moreno, J. M. & Yoshimura, M. Hydrothermal Processing of High- Quality Multiwall Nanotubes from Amorphous Carbon. J. Am. Chem. Soc. 123, 741-742 (2001).

233. Wang, W., Poudel, B., Wang, D. Z. & Ren, Z. F. Synthesis of Carbon Nanotubes Through a Modified Wolff-Kishner Reduction Process. (in press) (2006).

234. Somiya, S. (ed.) Hydrothermal Reactions for Engineering ( Elsevier, Applied Sci., 1989).

235. Yamashita, Y. & Ouchi, K. Influence of Alkali on the Carbonization Process - I: Carbonization of 3,5-Dimethylphenol-Formaldehyde Resin with NaOH. Carbon 20, 41-45 (1982).

236. Yamashita, Y. & Ouchi, K. Influence of Alkali on the Carbonization Process - III: Dependence of Type of Alkali and of Alkali Earth Compounds. Carbon 20, 55-58 (1982).

237. Van der Put, P. J. The Inorganic Chemistry of Materials: How to Make Things out of Elements (Plenum Press, New York and London, 1998).

192

APPENDIX A: MEASUREMENTS AND SUPPORTING MATERIAL

Table A1: Pore size distribution in glassy carbon GL-200.

Pore number long side (µm) short side (µm) mean size (µm) 1 9.023 4.681 6.852 2 16.646 11.394 14.020 3 9.616 4.267 6.941 4 9.859 8.333 9.096 5 16.580 11.931 14.255 6 9.561 4.193 6.877 7 9.934 8.147 9.041 8 7.422 6.250 6.836 9 9.440 6.569 8.004 10 12.397 5.628 9.012 11 9.296 4.237 6.766 12 8.093 2.971 5.532 13 9.300 6.680 7.990 14 5.230 2.487 3.858 15 4.580 3.490 4.035 16 9.223 5.017 7.120 17 5.696 3.188 4.442 18 10.268 3.958 7.113 19 5.969 5.187 5.578 20 9.819 4.845 7.332 21 9.012 5.297 7.155 22 10.006 5.121 7.564 23 11.273 3.920 7.596 24 9.188 4.082 6.635 25 5.596 5.793 5.695 26 2.943 2.982 2.963 27 3.845 3.175 3.510 28 2.353 1.646 2.000 29 8.383 5.562 6.973 30 10.482 6.006 8.2445 31 11.534 8.895 10.214 32 6.148 5.498 5.823

193

Table A1: Pore size distribution in glassy carbon GL-200 (Continued) Pore number long side (µm) short side (µm) mean size (µm) 33 5.243 3.340 4.302 34 4.819 3.504 4.162 35 4.057 2.344 3.200 36 5.097 3.603 4.350 37 5.837 2.581 4.209 38 4.476 4.169 4.322 39 6.034 4.320 5.177 40 7.811 3.801 5.805 41 9.005 4.026 6.515 42 7.096 3.838 5.467 43 5.393 3.979 4.686 44 9.112 4.601 6.857 45 2.968 2.346 2.657 46 5.326 2.677 4.002 47 4.209 2.656 3.432 48 8.498 3.869 6.183 49 9.376 5.099 7.237 50 7.743 3.597 5.670 mean value 7.816 4.716 6.266 standard deviation 3.082 2.106 2.458

194

Table A2: Summary of inorganic nanotubes reported in the literature and the synthetic procedures used for their production.117

Compound Synthetic Approach Comments

ReS is a 2-D Reaction with the respective oxide at elevated 2 MS (M=W, Mo) compound with Re- 2 temperature Re bonds

MS2 (M=W, Mo) Chemical vapor transport (CVT) Single wall MoS 2 CVT(I )+C catalyst nanotubes 2 60 Ammonium thiometallate (ATM) solution in Not perfectly MoS2 porous alumina template, reaction with H2S crystalline MS (M=Nb, Ta, Ti, Decomposition of MS at elevated 2 3 ZrHt) temperature

MoS2 Heating MoS2 powder in a closed Mo crucible

Depositing NbCl2 (ReCl3) from solution onto NbS2, ReS2 carbon nanotubes; firing in H2S at elevated temperature

NbSe2 Direct reaction of elements at 800ºC Hydrothermal reaction of ATM; crystallization Non-perfect MoS2 from acetone; firing H2S at 600ºC crystallinity Firing of ATM in thiophene/hydrogen at 360- WS 2 450ºC

TiS2 CVT(I2)

MoS2 Firing of ATM in H2 Firing of MoO nanobelts in the presence of MoS 3 2 sulfur Hydrothermal synthesis with organic amines WS 2 and cationic surfactant and firing at 850ºC

Growth of WO3-x nanowhiskers; annealing in WS2 H2S/H2 atmosphere at elevated temperature

Deposition of NbCl4 on carbon nanotubes NbS2/C template/firing in H2S MX (M=Mo, Nb; X=S, 2 Electron beam irradiation of MX powder Se, Te) 2

MS2 Arc discharge of submerged electrodes

MoS2, WS2 Microwave plasma

SnS2/SnS (misfit) Laser ablation of SnS2 Reacting t-Bu In with H S in aprotic solvent at lnS 3 2 Metastable phase 203ºC in the presence of benzenethiol present

195

Table A3: Calculated values for Raman active vibrational modes of SWCNTs181 Mode Symmetry Peak Positions 165 A (3) 1g 1587 120 E (6) 1g 1585 18

E2g (6) 368 1591

196

APPENDIX B: EULER’S THEOREM

Suppose that a polyhedral object is formed by enclosing the space with polygons.

The number of polygons is therefore equal to the number of faces (F) of such object. If

(V) is the number of vertices, (E) is the number of edges, and (g) is a genus of the structure, then the four parameters correlate as follows:

V − E + F = 2⋅()1− g . (B1)

For bulk three-dimensional solids, g is equivalent to the number of cuts required

to transform a solid structure into a structure topologically equivalent to a sphere (for

instance, g = 0 for a polygonal sphere such as C60 or C70, and g = 1 for a torus). Suppose,

further, that the object is formed of polygons having different (i) number of sides. The

total number of faces (F) is then:

F = ∑ Ni (B2)

where Ni is the number of polygons with (i) sides. Each edge, by definition, is shared between the two, and each vertex between the three adjacent faces, which denotes as:

E = ()1 2 ∑ i ⋅ Ni , (B3) 197

and:

V = ()2 3 ⋅ E (B4)

By substituting equations (2), (3) and (4) in (1), Euler’s postulate for i ≥ 3 denotes:

3N3 + 2N 4 + N5 − N7 − 2N8 − 3N9 −... =12(1− g) (B5)

∑()6 − i Ni =12(1− g) (B6)

It yields from the equation (B5) that the number of hexagons does not play a role,

and the balance between the number of pentagons and higher order polygons (i ≥ 7 ) is required in order to form an enclosed structure. If each vertex is considered an atomic site containing an sp2-hybridized C atom, and each edge is assigned to one C-C bond, then

according to equations (B5) and (B6), only 12 pentagons are needed to form a fullerene

or a nanotube. If one heptagon is present, then 13 pentagons will close the structure.

198

APPENDIX C: ELECTRON DIFFRACTION PATTERN OF A MWNT

Figure C1: Normal incidence electron diffraction pattern and its analysis of a multishell tube containing several isochiral clusters of chiral and achiral tubes. Note the presence of graphite-like reflections as reinforcements in streaks.62

199

APPENDIX D: ENERGETICS OF POLYGONIZATION OF MULTIWALLED CARBON NANOTUBES1

Energy minimization approach can be used to predict shape of a tube. Energy per unit length of a MWNT shell consists of four components:

E L = in plane stain + interlayer strain + curvature + incompatibility of neighbor layers

Considered here is an achiral coaxial multiwalled carbon nanotube (Figure D1). The

following assumptions have been made:

• Strain is uniform within a shell in a round NT geometry, and

• Curvature is uniform within a shell in a round NT geometry.

n a0 1 Rn

O r

R1

Cylindrical vs. Polygonized MWNT Graphene layers (nanotube shells) local graphitic c-axesc Figure D1: Model of a coaxial cylindrical MWNT vs. polygonized coaxial MWNT.

Energy per unit length of i-th shell of a cylindrical MWNT is then given by the following equation:

1 Model proposed by Prof. S.V. Rotkin, Physics Department, Lehigh University 200

2 2 ⎡ Ni+1 ⎤ 1 Ei = 2πλRiε i + 2πµRi ⎢ε i + ()ε i+1 − ε i ⎥ + π 3 ⋅ Ec + incomp. (D1) ⎣ Ni+1 − Ni ⎦ Ri

where Ri is radius of the shell, Ni and Ni+1 number of atoms in i and i+1 shell

respectively, ε i and ε i+1 corresponding strains,λ and µ Lame elastic constants, and Ec energy of curvature.

Similarly, energy per unit length of i-th shell of a polygonized MWNT is:

2 ()r ()r 2 ⎡ ()r Ni+1 ()r ()r ⎤ 1 ()r Ei = 2πλRi ()ε i + 2πµRi ⎢ε i + ()ε i+1 − ε i ⎥ + π 3 ⋅ Ec + incomp. (D2) ⎣ Ni+1 − Ni ⎦ Re

where superscript (r) denotes relaxation in polygonal tube and Re is effective radius of curvature at edges.

Ni+1 ()r ()r For polygonized tubes, ε i ~ ε i+1 and the term ()ε i+1 − ε i = 0 . Ni+1 − Ni

Similarly, contribution of incompatibility of the layers in the polygonal MWNT can be neglected assuming graphitic stacking between layers in near-planar regions of the tube:

incomp.()r = 0 .

The diagrams in Figure D2 plot the energy difference ∆E between a round and a polygonized MWNT as a function of number of shells (n) and numbers of facets (r).

They also illustrate the effect of size on the MWNT shape. The MWNT was assumed to have large number of shells and is therefore treated as continuum. The total energies were 201

calculated for values of λ = 52.82 kg/(sec2) = 330 eV/(nm2) and µ = 57.38 kg/(sec2) =

358 eV/(nm2), which were literature values for carbon nanotubes.2

a r = 5 r = 17 50 r = 18

40 r = 19

30

20 Energy difference Energy

10 r = 20

20 40 60 80 100 120 140 n

-10

b

20 r = 5 r = 6

10

20 40 60 80 100 120 140 n r = 7

Energy difference -10

-20

r = 8 Polygonization -30

Figure D2: The energy difference ∆E between a round and a polygonized MWNT as a function of number of shells (n) for various numbers of facets (r) and: a) N1=120 atoms and b) N1=36 atoms in the inner shell. Going from red to blue curves, the number of facets (r) was varied from 5 to 20.

2 H. Suzuura and T. Ando, Phys. Rev. B, 65, 235412 (2002) 202

Figure D2a shows the ∆E as a function of n for a MWNT that has 120 atoms along the circumference of the inner shell of the tube. The plot in Figure D2b shows the

∆E = f ()n when the inner shell consists of 36 atoms (R1~1.5 nm). According to the energy balance, when the energy difference becomes negative, preferred shape of

MWNT is polygonal. This model also predicts that for a certain diameter of the inner shell, the number of polygonal facets depends on the number of shells and can change along the tube diameter. Moreover, there are critical numbers of facets rc that stabilize the polygonal shape of the tube in its c-th shell and further growth of layers (n>c) does not affect the tube shape, i.e. tube continues to grow epitaxially. This, however, has to be compensated with rearrangements of carbon atoms at the edges.

100 Round A increasing MWNT 80

Polygonized 60 MWNT

40 B increasing

20 n -n shells of number

10 20 30 40 50 N 1 Figure D3: Energy difference between a round and a polygonized MWNT as a function of number of shells (n) and number of atoms in the first shell (N1). The plot shows domains of round and polygonized multiwall nanotubes and defines the region of relative stability of polygonized MWNT. A and B are energy contributions of incompatibility of layers in cylindrical MWNT and energy of edges curvature in polygonized MWNT. 203

Contour plot of energy difference between a round and a polygonized MWNT as a function of number of shells (n) and number of atoms in the first shell (N1) is shown in

Figure D3. The plot shows domains of round and polygonized multiwall nanotubes and defines the region of relative stability of polygonized multiwall nanotubes. Depending on the value of energy contribution (A) that corresponds to incompatibility of layers in cylindrical MWNT and contribution (B) that takes into account the energy of curvature at edges of polygonized MWNT, several possible phase diagrams can be realized, as noted by red arrows. 204

APPENDIX E: LIST OF EXPERIMENTS

Experiments performed in the present work can be classified into three groups:

1. Characterization of conical and polyhedral crystals of graphite (Figure E1);

2. Degradation of GPCs and GCCs (Figure E2);

3. Synthesis experiments (Figure E3).

The experiments on characterization of GPCs, GCCs and naturally occurring carbon cones include:

• FIB sample preparation and SEM/TEM thin section study of GL-200 glassy

carbon.

• SEM, TEM and SAED characterization of GPC and GPCs isolated from GL-

200 glassy carbon sample (Toyo Tanso, Co.).

• AFM characterization of GPCs’ surface morphology.

• Raman spectroscopy of planar graphite edge planes, graphite tubular crystals

and graphite polyhedral crystals.

• SEM, FIB, TEM and Raman spectroscopy study of naturally occurring

graphitic cones.

205

The experiments on degradation of GPCs, GCCs include:

• Intercalation and exfoliation of GPCs and GCCs from GL-200 glassy carbon

with sulfuric acid.

• Oxidation of GPCs and GCCs from GL-200 glassy carbon in air.

The experiments on synthesis of GPCs and GCCs include:

• Polygonization of MWNTs in vacuum: effect of temperature on MWCNTs

structure (1500ºC-2000ºC).

• Growth of GPCs and GCCs by mass transport under constant flow of gas at

1500ºC-1650ºC.

• Synthesis of GPCs and GCCs from Novolac phenolic resins.

• SEM, TEM and Raman spectroscopy characterization of samples obtained in

the above experiments.

Roadmaps of experiments are given in Figures E1-E3. Experimental techniques and results of characterization of GPCs, GCCs and naturally occurring carbon cones (Figure

E1) were explained in detail in the present work. Additional work performed towards degradation and synthesis of GPCs and GCCs is briefly summarized in the pages that follow.

206

Characterization of conical and polyhedral crystals of graphite Natural graphite Synthetic carbon

Carbon cones and whiskers Glassy carbon GL-200

Gooderham carbon Graphite conical crystals (GCCs) Graphite polyhedral crystals (GCCs) Kola carbon

SEM, EDS SEM, EDS SEM, EDS Raman spectroscopy TEM, SAED TEM, SAED Raman spectroscopy TEM, SAED

Figure E1: Roadmap of experiments for characterization of conical and polyhedral crystals of graphite.

Degradation of GPCs and GCCs

Intercalation and exfoliation Oxidation in air

Objective: - to distinguish between: (a) soft and hard Objective: - (a) to separate soft and hard carbon phase carbon phase of GL-200 glassy carbon; (b) scroll and of GL-200 glassy carbon; and (b) to separate the effect “Russian doll” type of structure; and (c) understand of temperature vs. effect of intercalation in previous failure mechanism of GCCs and CPCs. experiment.

Glassy carbon GL-200 was crushed in mortar, particles GIC are formed by insertion of atomic were placed into alumina crucibles and heated in air at or molecular layers of guest into graphite various temperatures and different lengths of times.

Intercalation of GL-200 with H2SO4/HNO3 Time was varied from 15 sec to 10 hours. Temperature was (1:1) at room temperature for 1 hour varied from 375°C to 980°C. Full list is given in Table E1. Graphene sheets Guest molecules

(H2SO4)

Formation of volatile compounds during rapid heating at high temperature breaks (exfoliates) the structure of host graphite

Exfoliation by annealing at 980°C for 15 s

Figure E2: Roadmap of experiments for degradation of GPCs and GCCs. 207

Growth of polyhedral crystals hydrothermally: To Annealing of MWNTs: - (a) to understand the effect of confirm hypothesis of hydrothermal growth of GPCs and temperature on carbon nanotubes as potential precursors for GCCs in the pores of glassy carbon. Additionally, naturally Annealing GPCc/GCCs growth; (b) to compare results with mass transport occurring carbon cones also suggest that they were grown of growth at high temperature. from hydrothermal fluid. MWNTs Controlling parameters: type of the tubes, temperature, time. Controlling parameters: Type (source and morphology) of carbon precursor material, temperature, pressure, time, Samples were placed in custom-made graphite crucibles and heating and cooling rate. annealed in vacuum furnace (Solar Atmospheres, Inc.).

Hydro- Synthetic Mass thermally routes transport

Growth of crystals in pores of glassy carbon: -to Growth of crystals from CNTs by mass transport at high reproduce growth in GL-200 temperatures: - (a) to avoid problems of nucleation of carbon nanotubes in the pores of glassy carbon; (b) to compare results Controlling parameters: type of seed carbon nanotubes, with annealing at high temperature. type of resin, temperature, curing history of resin, porosity, Vapor in time. pores Controlling parameters: type (size, morphology, structure, synthesis methods) of seed carbon nanotubes, source of carbon, Novolac type of phenolic resin (powder) with or without CNT type of environment, temperature, time. seeds was step-cured in two different ways (a) in graphite crucibles at low temperature (150°C - 250°C for over 36 h Carbon nanotube samples were dispersed over carbon fibers time for the structure to set and create closed porosity, and (Hercules Inc., type AS4C-GP-6K) and placed next to the source of (b) using mounting machine (pressure and temperature). amorphous carbon in graphite crucibles for vacuum annealing, or in an alumina boat for annealing in flow of Ar. Samples were carbonized slowly (0.5°C/min from RT to 1000°C) to prevent cracking, then graphitized up to 2000°C.

Figure E3: Roadmap of synthesis experiments.

Degradation of GPCs and GCCs

Intercalation of GPCs and GCCs was explained in detail in the body of the present

work (Chapter 4). Additionally, oxidation experiments were conducted on GC-200 glassy

carbon to separate soft and hard carbon phase (GPCs and GCCs from amorphous matrix)

and to better understand the effect of temperature alone in exfoliation experiments.

Oxidation was performed in air. Time and temperature were varied according to the

Table E1. Selective etching of crystals from amorphous glassy carbon matrix can be

achieved at mild temperatures (400ºC) for a long period of time (~10 hours), as shown in

Figure E4. 208

Table E1: Plan of experiments for oxidation of GPCs and GCCs in air. Temperature Long time (h) Medium time (h) Short time* (min) (°C) 10 5 31020 40 60 80 375 x

400 x

425 x

450 x

475 x

500 x x x

550 x x

600 x x x x x x x

650 x x x x x x x

700 x x x x x x x

750 x x x x x x x

800 x x x x x x x

* In addition, 15 seconds oxidation at 980°C is conducted on one sample to compare results with exfoliation experiments

Figure E4: SEM image of GC-200 fracture surface after oxidation at 400ºC for 10h.

209

Synthesis of GPCs and GCCs

Four different synthetic routes to synthesis of GPCs and GCCs were explored experimentally. They are:

1. Annealing of MWNTs;

2. Growth by mass transport in a gas environment;

3. Growth of crystals by pyrolysis of phenolic resins; and

4. Hydrothermal route.

1. Annealing of MWNTs

Annealing of MWNTs was performed using a vacuum furnace donated to Drexel

University by Solar Atmospheres Inc., Lansdowne, PA. Maximum temperature explored in the experiments was 2000ºC. Three types of MWNTs were used in experiments and parameters were varied according to Table E2. Small amounts of MWNT samples were placed in custom-made or commercially available graphite crucibles (Figure E5a,b) and annealed in the vacuum furnace (Figure E5c). Results of the annealing were explained in detail in the body of the present work (Chapters 4 and 5).

210

Table E2: Experimental parameters for annealing of MWNTs Type of MWNT Temperature (°C) Time (h) Large diameter CVD 1500, 1600, 1750, 1800, 3 (alumina membrane) 2000 Atofina CVD 1600, 1800, 2000 3 Pyrolytically stripped 1600, 1800, 2000 3 (Pyrograph, Inc.)

(a)

(b) (c)

Figure E5. (a) Custom made and (b) commercial crucibles. (c) Vacuum furnace at Drexel University, donation to the Department of Materials Science and Engineering by Solar Atmospheres, Inc. A large (7.5” diameter x 14.5” deep) hot zone constructed with energy efficient graphite insulation allows a maximum temperature of ~ 2200ºC with extremely high thermal uniformity within the zone (± 5ºF).

211

2. Growth by mass transport in a gas environment

Explored was ability to grow GPCs and GCCs by mass transport using single- and multi-walled carbon nanotubes as seeds, and various types of solid carbon materials as a source of carbon. The experiments were performed in high temperature tube furnace

Carbolite® CTF 17/300 at constant flow of argon. Maximum temperature explored in the experiments (limited by the furnace design) was 1750ºC. Acetylene by Alfa

Aeser, carbon powder by Superior Graphite, Inc., and on-site produced carbide-derived nanocarbon) were used as carbon source. Experimental parameters were varied according to Table E3. As control, the same materials were annealed in vacuum (10-7 Torr) in presence of acetylene carbon black. Analysis of product materials shows minor thickening and irregular polygonization of MWNTs (Figure E6). This suggests insufficient mobility of carbon atoms in gaseous environment and inability to uniformly control growth rate along the surface of seed nanotubes.

Table E3: Experimental parameters for growth of GPCs by mass transport in gas. Type of MWNT Carbon source Temperature (°C) Time (h) SWNT (Bucky, USA) Carbon black, CDC, carbon 1600, 1750 3 powder (Superior Graphite) Large diameter CVD Carbon black, CDC, carbon 1600, 1750 3 (alumina membrane) powder (Superior Graphite) Atofina® CVD Carbon black, CDC, carbon 1600, 1750 3 powder (Superior Graphite) Pyrolytically stripped Carbon black, CDC, carbon 1600, 1750 3 MWNT powder (Superior Graphite)

212

a b

200 nm 200 nm

c d

Core nanotube

Carbon coating

Edge loop structure

Figure E6: Pyrolytically stripped carbon nanotubes annealed in presence of carbon source: (a-b) argon flow at 1750°C and (c-d) in vacuum at 2000°C. Observed was thickening around core nanotubes coming from mass transport of carbon from carbon source to nanotubes.

3. Synthesis of GPCs from phenol-formaldehyde resins

Synthesis of large quantities of GPCs through carbonization and pyrolysis of phenolic resin (phenol-formaldehyde) as a source of carbon was attempted. Phenolic resin by Plenco, Inc. in a form of solid powder (Novolac type) was used in preparation of the green samples. Phenolic resin powders were thoroughly mixed with hexamethylenetetramine in various proportions. Heating of the powder mixtures was 213

performed in closed graphite crucibles using a high temperature vacuum furnace (Figure

E4c). The decomposition of hexamethylenetetramine upon slow heating at 170°C –

250°C introduces additional formaldehyde into the system, which enables cross-linking and hardening of the molten resin with simultaneous formation of porosity. Carbonization of the precursors upon heating at 800°C -1000°C provides formation of gas-impermeable glassy carbon matrix, and a C-H-O (N) gaseous environment in the pores of the matrix, which is a suitable system for growth of various nanocarbon species. In addition to seedless synthesis, sets of experiments were conducted with carbon nanotube seeds.

Parameters were varied according to Table E4. Figure E7 shows products of reaction of carbonization and annealing of phenolic resins without carbon nanotube seeds. Spherical, spheroidal and filamentous carbon was found in pores of vitrified phenolic matrix.

Problem with this process is complicated and time consuming procedure for samples preparation (carbonization at 1000ºC for 24 hours, slow heating and cooling to avoid fracture) and their further characterization, since the products of synthesis are always entrapped in a glassy carbon matrix.

Table E4: Experimental parameters for growth of GPCs by pyrolysis of phenolic resin. Nanotube seed Carbon source Temperature (°C) Time (h) N/A Plenco® phenolic resin 2000 1, 2, 3, 5 Large diameter CVD Plenco® phenolic resin 2000 1, 2, 3 (alumina membrane) Atofina CVD Plenco® phenolic resin 1800, 2000 1, 2, 3 Pyrolytically stripped Plenco® phenolic resin 1800, 2000 1, 2, 3 MWNT

214

a b

10 µm 5 µm

Figure E7. Carbonaceous material synthesized in pores of glassy carbon through carbonization and pyrolysis of phenolic resin at 2000ºC: (a) Carbon spherical and spheroidal particles. (b) Filamentous carbon structures.

4. Synthesis of GPCs from a hydrothermal fluid

A custom designed autoclave setup at Drexel University is utilized to explore synthesis of

GPCs and GCCs from a supercritical C-H-O fluid. Some of the results (nucleation of multi-walled cones and cylindrical nanotubes were summarized in the body of the present work (Chapter 5). Figure E8 shows products of synthesis using SWNTs as precursor carbon material treated in deionized water at 760ºC and 14,000 psi for 24 hours. SWNTs transformed to short and straight multiwall carbon nanotubes under hydrothermal conditions.

215

Short and straight carbon filaments

Inner surface of gold capsule

Carbon particles

Figure E8. Products of hydrothermal synthesis of short and straight carbon filaments from SWNTs treated in deionized water at 760ºC and 14,000 psi for 24 hours.

216

APPENDIX F: LIST OF ABBREVIATIONS (in order of appearance)

CN coordination number

SEM scanning electron microscope/microscopy

TEM transmission electron microscope/microscopy

HRTEM high resolution transmission electron microscopy

HRSEM high resolution scanning electron microscopy

NEMS nano-electromechanical systems

1D one-dimensional

2D two-dimensional

3D three-dimensional

SWCNT single-walled carbon nanotube

MWCNT multi-walled carbon nanotube

DC direct current

CVD chemical vapor deposition

HEXA hexamethylenetetramine

TGC tubular graphite cones

CCNF corn-shape

SAED selected area electron diffraction

CBED convergent beam electron diffraction

GPC graphite polyhedral crystal

STM scanning tunneling microscopy

STS scanning tunneling spectroscopy 217

LDOS local densities of states

EDS energy dispersive spectroscopy

FESEM field-emission scanning electron microscopy

GCC graphite conical crystal

SE secondary electrons

BSE backscattered electrons

TTL through-the-lens

IL in-lens

ESEM environmental scanning electron microscope/microscopy

S/TEM scanning and transmission electron microscope

CCD charged-coupled device

DP diffraction pattern

CBED convergent beam electron diffraction

EELS electron energy loss spectrometer

FIB focused ion beam

BZ Brillouin zone

SR Stokes Raman

ASR anti-Stokes Raman

DOS densities of states

PG pyrolytic graphite

HOPG highly oriented pyrolytic graphite

DR double-resonance

218

TS turbostratically stacked

RBM radial breathing modes

HADE hydrogen arc discharge evaporation

SERS surface enhanced Raman spectroscopy

FT Fourier transform

SAED selected area electron diffraction

FWHM full width at half maximum

VGCF vapor-grown carbon filaments

219

VITA NAME Svetlana Dimovski

EDUCATION • PhD in Materials Science and Engineering, Drexel University, Philadelphia, USA, 2006 • BS/MS in Mechanical Engineering, University of Novi Sad, Novi Sad, Serbia and Montenegro, 1996

EXPERIENCE Scientist - Corporate Analytical Department, Procter & Gamble, Cincinnati OH, (08/05 – present); Research Assistant - Department of Materials Science and Engineering, Drexel University, Philadelphia, PA (09/00 – 08/05)

PUBLICATIONS S. Dimovski, Y. Gogotsi. Graphite Whiskers, Cones, and Polyhedral Crystals, in Handbook of Nanomaterials. Ed. Y. Gogotsi, CRC Press, December 2005 P.-H. Tan, S. Dimovski, Y. Gogotsi. Raman Scattering of Non-Planar Graphite: Arched Edges, Polyhedral Crystals, Whiskers and Cones. Phil. Trans. Royal Soc. Lond. A, 362 (1824), 2004, 2289-2310 – invited paper J. A. Jaszczak, G. W. Robinson, S. Dimovski, Y. Gogotsi. Naturally Occurring Graphite Cones. Carbon, 41(11), 2003, 2085–2092 S. Dimovski, A. Nikitin, H. Ye, Y. Gogotsi. Synthesis of Graphite by Chlorination of Iron Carbide at Moderate Temperatures. J. Materials Chem., 14 (2), 2004, 238-243 Y. Gogotsi, S. Dimovski, J. A. Libera. Conical Crystals of Graphite. Carbon, 40 (12), 2002, 2263–2284 V. Kayastha, Y. K. Yap, S. Dimovski, Y. Gogotsi. In-situ Control of Dissociative Adsorption for Effective Growth of Multiwall Carbon Nanotubes. Appl. Phys. Lett., 85 (15), 2004, 3265-3267 S. Dimovski, A. Nikitin, H. Ye, Y. Gogotsi. Low Temperature Synthesis of Graphite from Iron Carbide in “New Carbon Based Materials for Electrochemical Energy Storage Systems”, Eds. Barsukov I., Barsukov V., Doninger J. and Johnson C., NATO ARW, Kluwer Academic Press, Netherlands, February 2005 S. Dimovski, J. A. Jaszczak, G. W. Robinson, Y. Gogotsi, S. A. Hackney. Naturally Occurring Cones and Tubes of Graphite. Carbon 2004 Conference Proceedings, Providence, RI, July 2004 S. Dimovski, J. A. Libera, Y. Gogotsi. A Novel Class of Carbon Nanocones. Mat. Res. Soc. Symp. Proc., 706, 2002, Z6.27.1-Z6.27.6 (peer-reviewed) Carbon, cover page image (entire 2004 and 2005); Journal of the American Ceramic Society, back-cover, 86 (10), 2003; Journal of the American Ceramic Society, back-cover, 87 (4), 2004

PRESENTATIONS Three faculty candidate seminars, two corporate seminars, 15 international conference presentations, and 10 presentations at local and internal meetings

HONORS AND AWARDS Outstanding Poster Award by the MRS (2001); Graduate Student Research Award, Drexel University, (2002); Best Poster Award by The Electrochemical Society (2002); Dragomir Nikolitch Trust Scholarship from Studenica Foundation (2002); Amelia Earhart Fellowship from Zonta International (2002); The Am.Cer.Soc. Ceramographic Competition: 2nd place (SEM) and 3rd place (Combined Techniques) (2004); Excellence Award, University of Novi Sad (1995). 220