Nitric acid CNTs oxidation

The acid reflux procedure was first described by Rinzler el al. [28], in which raw nanotube materials are refluxed in nitric acid to oxidize the metals and carbon impurities. Acid-treated CNTs are considered to have carboxylic acid groups at the tube ends and, possibly, at defects on the side walls. The functionalized SWNTs have considerably different properties from those of the pristine tubes. 

Tsang et al [20] have used nitric acid to open CNTs, but the simplest opening technique is thermal oxidation [29,30] where CNTs are heated in air or oxygen atmosphere to temperatures of the order of 600-700°C. As mentioned above, tips are eroded first. 

Jiang and Gao22 reported the preparation of MWCNT-TiN and MWCNT-Fe2N composites using CCVD CNTs oxidized in nitric acid (140°C, 24 h). A mixed solution of Ti(OC4H9)4 and ethanol was added to an aqueous suspension of CNTs. The dried precursor was calcinated in N2 (450°C, 2 h)... 

A more drastic scheme proposed the aeidie oxidation using a mixture of concentrated sulfuric (98 %) and nitric acid (65 %) in a ratio 3 1 for 8 hours at 40 °C [39]. Under these conditions the tube eaps are opened and the tubules are shortened in fragments of different length. After that, the CNTs were washed with pure water and dried at 100 °C overnight. 

Luo et al. [43] proposed the dispersion of CNTs in 4.0 M HCl for 4 hours imder ultrasonie agitation to eliminate metal oxide catalysts. After rinsing earefully with water until neutral pH and drying, the CNTs were dispersed in 60 mL mixture of eoneentrated nitric acid plus sulfuric acid (1 3) with ultrasonic agitation for 4 hours in a water bath followed by washing until neutral pH and finally drying. 

Functionalization of CNTs by covalent chemistry. Covalent functionalization of CNTs has attracted a great interest for biosensors development. This t5T)e of functionalization is expected to play a crucial role in tailoring the properties of materials and the engineering of CNT biosensing devices. The first step in a covalent functionalization process involves a chemical treatment of CNTs under oxidizing conditions, such as sonication in a mixture of sulfuric and nitric acids or treatment with piranha... 

Besides the classical nitric acid treatments, other oxidative agents have been used to prepare functionalized CNTs. Rao et al. [170] have compared the effects of concentrated nitric acid, aqua regia, HF-BF3, aqueous OSO4, and KMn04 (acid/alkali) solutions on MWCNT structures [170]. Indeed, all these oxidants... 

In another work," Kevlar-functionalised nanotubes were prepared by heating Kevlar with MWNTs in the presence of a sulphuric/nitric acid mixture under reflux. This resulted in the partial oxidation and functionalisation of CNTs with carboxylic acid groups, which formed hydrogen bonds with amido groups, as well as terminal carboxylic acid and amino groups in Kevlar. This process produced Kevlar-coated CNTs, which have been utilised for the fabrication of MWNT-poly(vinyl chloride) (PVQ composites." ... 

Purification techniques developed so far can be classified as physical, chemical and mechanical processes. Oxidation in the presence of various oxidizing agents like KMnO, and is one of the first attempts made to purify CNTs by oxidizing less stable amorphous carbon [61]. The different oxidization abilities of CNTs and other carbon species has been utilized in this process [62]. Based on the fact that the etching rate of amorphous carbons is faster than that of nanotubes, acid treatment with nitric acid has been used to eliminate impurities from CNTs [63]. Amorphous carbon and the residual catalyst particles are removed by this process. Both oxidation and acid treatment destroy the CNTs significantly and modify the structure [64,65]. Ultrasonication and centrifugation methods have also been tried to separate SWCNTs physically from low yield soot. Bandow et al. achieved 70% pure SWCNTs using ultrasonic... 

Firstly it was found that, contrary to treatments based on nitric acid, this mild HCl-based purification did not seem to significantly oxidize the surface of the CNT walls, as shown by the comparison of the IR spectra of purified and non-purified CNTs. Secondly, the results of TGA measurements clearly pointed out that the efficiency of the purification was batch-dependent in terms of relative percentage of iron particle removal. The HIPCO SWCNTs of a batch containing a... 

Oxidation of CNTs by 8 M nitric acid with sonication... 

Oxidation of CNTs using 2.6 M nitric acid by reflux... 

A purification of the CNTs has been performed by oxidation using a mixture of sulfuric and nitric acid at 100 °C for 3 h. The separation of the oxidized nanotubes from the reaction solution was performed via centrifugation and membrane microfiltration. 

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