Carbon nanotubes purification constrains due to large Fe–Ni/Al ₂ O ₃ catalyst particles encapsulation

Abstract

Purification efficiency of carbon nanotubes (CNTs) by the method of chemical oxidation was considered as a function of position and size of catalyst remains and consequently of the tubes morphology. Oxidation of CNTs by means of both HNO₃ and NaOH treatment efficiently removes small catalyst particles embedded in the tubes top, following “tip-mode” CNTs growth mechanism. Destructive character of the purification can be assumed due to the resulting tiniest tube population increase as a consequence of their body tearing. However, limited purification efficiency was observed in the case of bigger metal particles with variable size and position in CNTs. Bigger particles occur on account of catalyst instability portrayed as small metal particles of active phase migration and merging. The formed agglomerates are not stable in the tubes hollow, but disintegrate leading to different sizes and position of metal particles in the tubes body. Consequently, CNT may be obtained with non-uniform thickness and morphology. The phenomenon is due to liquid-like behaviour of the active phase at reaction temperature (700 °C) which is higher than both Huttig and Tamman temperatures of applied metals. A mechanism is proposed assuming that an isolated bigger part of the mother particle stayed encapsulated inside the tube body inactive for further tube growth, while a smaller fragment of the collapsed particle resided at the tube top acting as a new-born active site. Owing to “replica effect” the tube further grows thinner following the size of the new active site. Consequently CNTs of irregular morphology occur as they resemble metal particles of various sizes following their disintegration.