Purified SWNTs Become Diamagnetic

"High-Purity Diamagnetic Single-Walled Carbon Nanotube Buckypaper"
Kim, Y.; Torrens, O. N.; Kikkawa, J. M.; Abou-Hamad, E.; Goze-Bac, C.; Luzzi, D. E. Chem. Mater. ASAP published on 05/16/2007.
DOI: 10.1021/cm063006h

Comment:
The magnetic impurities to SWNTs have been like the shadow of things. These leftover from catalysts, such as Fe (for HiPco SWNTs) or Co/Ni (for arc SWNTs) compounds, are encapsulated in stiff, multilayer carbon shells. They are the really bad guys: silencing ESR signals, broadening NMR peaks, misleading phonon/electron mobilities, etc.

Getting rid of them is not nearly as trivial. Chemical methods should be carefully experimented since distroying the carbon layers - to access and dissolve the metals - is shooting the nanotubes in the foot as well. Physical methods are apparently more plausible. In the literature, Kitaygorodskiy et al. (JACS, 2005, 127, 7517) used magnetic separator - often used in biological separation - to separte the magnetic species from a soluble SWNT sample.

Procedure:
Purified SWNTs (PII-SWNTs from Carbon Solutions, Inc.) was heated to 560C in air for 10-30 min (40-60% yield), followed by sonication in HCl (37%) at 60C for 40min and filtering and brief drying. The solids (~200 mg) were suspended in DMF, and filtered through the 1.1T magnetic gradient apparatus containing a "column" of iron granules (for 3 times with fresh iron granules). The suspensions were then filtered through nylon filter and dried (vacuum annealed at 650C for 30min) to obtain the purified buckypaper (yield 10-20%). Speed: 5 mg/h.

Properties:
Magnetically purer:

• Using Quantum Design phyiscal property measurement system (PPMS)
• Magnetic moment reduced from 1.04 to 0.012 emu/g.

• Beautiful 13C-NMR Spectrum: FWHM only 25 ppm (maximum: 116ppm, double T1)
• Estimated SWNT susceptibility: -5x10(-6) emu/g (theoretical -0.64 x 10(-6) emu/g) (Tag: better or worse?)

Generally purer:

• UV/vis/NIR: From absorption peaks (The Haddon Method)
• XRD: SWNT rope peak @5.6 vs MWNT peak @25.6
• C60-Filling (Signature of Luzzi Group): 95% filled
• Ni:Y ratio change: from 4:1 (catalyst ratio) to 20:1 (obtained) to 2:1 (purified).