| | Process |
Description | References |
| | Gas phase oxidation | (i) In general, chemical oxidation includes gas phase oxidation (using air, O2, Cl2, H2O, etc.), liquid phase oxidation (acid treatment and refluxing, etc.), and electrochemical oxidation.
(ii) Tubes are heated at a controlled rate either in wet air or vacuum or other oxidizing agents for an extended time at about 330°C.
(ii) This is a good way to remove carbonaceous impurities.
(iii) The disadvantages of this method are that it often opens the end of CNTs, cuts CNTs, damages surface structure and introduces oxygenated functional groups (–OH, –C=O, and –COOH) on CNTs. Metal particles cannot be directly removed, and further acid treatment is needed. | [134–145] |
| | Liquid phase oxidation (acid treatment) | (i) Usually, the acid treatment will remove the metal catalyst and some fullerenes.
(ii) Reflux in HNO3, HCl, or other acid for period of time ranging from 4 to 48 hours.
(iii) HNO3 is the only acid that does not cause degradation to tubes (unless left in HNO3 for extended time frames, usually >16 hrs).
(iv) The metal catalyst is solvated while the CNTs remain in suspended form. | [135, 140, 144–147] |
| | Electrochemical oxidation | (i) CNTs with fewer defects show higher electrochemical oxidationresistance than CNTs with more defects. Suitable for purifying CNT arrays without destroying their alignment.
(ii) The CNTs electrode is immersed in 0.2 M HNO3 solution or 0.2 M HCl (purging with N2 for 20 min prior to use) and the potential was cycled between +1.00 and +2.00 V at a scan rate of 50 mV s−1.
(iii) Increases the specific area of nanotubes by cutting off the nanotube tips and by converting the surface property of nanotubes from the hydrophobic state to the hydrophilic state. | [148, 149] |
| Centrifugation
| (i) Centrifuge at 7,000 g or more for 30 min to 3 hours. Often supernatant is removed after one run and centrifuged again.
(ii) This removes nanospheres, metal nanoparticles, other carbon particles, but some methods produce a low yield of nanotubes, especially when centrifugation is repeated many times. | [150–152] |
| | Filtration | (i) This technique is often used in conjunction with oxidation. The acid decomposition products are highly soluble in basic solution and CNTs are not and are separated using a basic solution of pH 11 and filtered using 3–5 μm filter, often under vacuum.
(ii) This removes nanospheres, metal nanoparticles, polyatomic carbons, and fullerenes, without any noted negative effects. | [153] |
| | Ultrasonication | (i) CNTs are suspended in distilled water, toluene, or acid solution and sonicated for 5–30 minutes.
(ii) This separates tubes from attached particles, creating dispersion of nanotubes and other particles for better centrifugation.
(iii) The separation of the particles is highly dependable on the surfactant, solvent, and reagent used. The solvent influences the stability of the dispersed tubes in the system. | [143, 145, 150, 154–156] |
| | Magnetic | (i) CNTs is suspended in soap or tolulene and nanoparticle powder (ZrO2, NHCl4, CaCo3, diamond) is added. This particles will attach to the CNTs and make them magnetic.
(ii) The slurry is sonicated for 2 hr and subsequently the magnetic particles are trapped using permanent magnetic poles.
(iii) This is then followed by chemical treatment.
(iv) This removes catalyst materials and small inorganic particles. Few to no known negative effects. | [157–159] |
| | Microwave purification | (i) CNTs are sonicated, then diluted in HNO3 (or other acid).
(ii) Microwaved at 100–200 W, and microwave is ramped up to ~200°C over 30 min.
(iii) Microwave is then held at temperature 200°C for 30–90 minutes. This removes amorphous carbon, metals, and other nanoparticles, with no known negative effects. | [160–162] |
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