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Production of carbon nanotubes

Lecture 2. Production of carbon nanotubes. Methods:. Pyrolysis of hydrocarbons in the presence of metal catalysts (1950 - present). Main products: multi-walled carbon nanotubes . Catalysts: Fe, Co, Ni Temperature: 800 -1200 C

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Production of carbon nanotubes

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  1. Lecture 2 Production of carbon nanotubes

  2. Methods: • Pyrolysis of hydrocarbons in the presence of • metal catalysts (1950 - present) Main products: multi-walled carbon nanotubes. Catalysts: Fe, Co, Ni Temperature: 800 -1200 C Precursors: C2H2, CH4 (gases), benzene (solution), camphour (solid)

  3. Experimental set up a. One-step process (gas precursors) catalysts Furnace 800-1200 C Quartz tube or ceramic tube CNTs grown in substrate H2/Ar C2H2, or CH4

  4. One-step process (solution precursors) catalysts Furnace 800-1200 C benzene H2/Ar

  5. b. Two step-process (solid precursors) camphour catalysts 150-300 C 800-1200 C Ar/H2

  6. Two step-process Metallic-organics as precursors: Ferrocene (五環鐵) Fe ferrocene 150-300 C 800-1200 C CNTs deposited on tube walls Ar/H2

  7. Random arrangement of multi-walled CNTs

  8. 2. DC Arc discharge of graphite (1990-present) 20- 35 V, 70-120 A Anode (+) Cathode (-) Arc temperature 2500-3500 C He or (Ar) atmosphere Products: Fullerenes and multi-walled CNTs

  9. See website at http://tw.youtube.com/watch?v=8N79nlhwcgM&feature=related

  10. 負極長出銀灰色碳積物稱為(carbon deposit) 正極消耗 真空腔壁層積碳灰, 稱為carbon soot: fullerenes藏于soot中

  11. 分離 Multi-walled CNTs Side view Front view Cut open

  12. Carbon deposit at cathode

  13. cathode Hollow anode Filling with Fe, Co, Ni or others to produce single-walled CNTs

  14. 5cm 原始煤灰 陰極 陽極 腔體

  15. 負極長出銀灰色碳積物稱為(carbon deposit) 正極消耗 真空腔壁層積碳灰, 稱為carbon soot: fullerenes及SWNTs藏于soot中

  16. anode Filling with B, B2O3 or boron compounds to produce B-doped multi-walled CNTs B

  17. anode Filling with Ta, Ta2O5 or Tatanlum compounds to produce encapsulated TaC multi-walled CNTs

  18. 3. Laser ablation of Co-Ni-graphite (1995-present) Products: single-walled carbon nanotubes SWNTs found on cooler furnace He Laser beam cooler 1200 C Co-Ni-graphite

  19. 4. Electrolysis of graphite in molten ionic salts (1995-present) Product: multi-walled CNTs, low melting points metals (Sn, Pb, Bi) encapsulated CNTs Graphite rod LiCl, KCl, NaCl furnace Graphite crucible

  20. 5. others, derivatives of pyrolysis, e.g. plasma CVD, MOCVD,… etc Products: variety of CNTs, including SWNTs, MWNTs

  21. Improved techniques on nanotube growth Substrate and catalyst modification

  22. Thin film of Fe 改變金屬催化劑形態(簡單矩陣) Science, 283,512,1999

  23. 比較複雜碳管矩陣 APL. 79, 3155, 2001 Chem.Phys.Lett, 371, 433, 2003 APL, 79, 1534, 2001

  24. Nature, 416, 495, 2002.

  25. 碳管整齊排列之意義為何? • 提昇場發射性能 (field emission) ? (not true, because • threshold field and turn-on-voltage is even higher than • random arranged CNTs) 2. 陣列成長時, 碳管之結晶性, 結構 或 長度, 直徑比雜亂成長 較為均勻一致 (possibly true, at least tube length and diameter are rather uniform than random growth of CNTs on substrate).

  26. Growth mechanisms of CNTs Characteristics of CNT growth in arc discharge process 1. No catalyst --- multi-walled CNTs (fact) E 5-10V/m 2. Electric field driven growth (not certain ?) 3. High temperature (fact) + - 4. Less defects on CNTs and high degree of graphitization (high crystallinity) (Fact). Pyrolysis-CNTs Arc-CNTs

  27. 5. CNT length is limited to 5-15 m, tube diameter is similar to pyrolysis grown CNTs (fact) 6. open-ended growth (reasonable) Both-end growth Open-end wrong Possibly right 7. Carbon ions exchange in arc zone (C+ > C-) C+ C+ - C- C- + 8. CNT growth in arc discharge requires buffer gas (He, Ar, 300-500 torr). In a vacuum tube cannot grow (Fact).

  28. 9. Probability of hexagon formation is higher than that of pentagons in arc process, so CNT growth continues, otherwise pentagons lead to tube closure. When tube closure occurs tube growth stops. (fact) 3 2 1 Incoming ions

  29. Growth mechanisms of CNTs in pyrolysis process 1. Metal catalysts are needed, metal particles are always at tube tips (fact). 2. H2 is always needed (not true), because pyrolysis of C60 can also produce CNTs in Ar or N2 flow. 3. Lower production temperature compared with arc discharge, so CNTs are defective structures (fact). 4. Two types of growths have been argued since 1970 by M. Endo and RTK Baker 5. Lift-off growth and rooting growth

  30. Modification of Baker’s model C2H2 C2H2 C2H2 C2H2 droplet H Cold Fe Fe Hot Fe 800-1200C Room temp C Atoms strongly vibrate in lattice Migration of C through Fe droplet

  31. When C diffuses through Fe, carbidic species form Carbidic species: Intermediate phase of carbide

  32. Baker’s model cannot explain why CNTs are hollow structure Migration of C through Fe droplet solid structure

  33. Lift-off growth

  34. Rooting growth

  35. None-sense mechanisms on CNT growth 極其無聊之碳管成長機制 1. Scooter mechanism by rice group Co C growth

  36. 2. Co-C60 catalyzed growth in arc process C Co

  37. Why they proposed these none sense mechanisms? Because at a time they did not see metal articles at tube tips!

  38. Particularly, the Co-C60 catalyzed growth model was proposed based on two wrong images. Image 1 1.1-1.2Å 7Å 1.1-1.2Å Image 2

  39. True is finally found by Rice group itself Chem.Phys.Lett, 260, 471, 1996 Endo and Baker’s models

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