The effect of oxidation on physicochemical properties and aqueous stabilization of multiwalled carbon nanotubes：comparison of multiple analysis methods

Surface oxidation can alter physicochemical properties of multiwalled carbon nanotubes(MWCNTs) and influence their aqueous stabilization.Many techniques have been used to characterize the physicochemical properties and aqueous stabilization of MWCNTs.However,the relationship between the change in physicochemical property and the aqueous stabilization of MWCNTs merits more studies,and the multiple characterization techniques have not been well compared.This study systematically and comparatively investigated the effect of oxidation on the physicochemical properties and aqueous stabilization of MWCNTs using multiple analysis methods.Increased surface area,disclosed tube ends,defects on the sidewalls,disruption of the electronic structure,and removal of metal catalysts and amorphous carbon were observed for the oxidized MWCNTs(o-MWCNTs) using the multipoint Brunauer-Emmett-Teller(BET) method,transmission electron microscope observation,Raman spectroscopy,UV-Vis spectroscopy,and thermogravimetric analysis.An oxidation-time-dependent increase in oxygen content of the MWCNTs was verified by the methods of elemental analysis,mass difference calculation,and X-ray photoelectron spectroscopy(XPS).Fourier transform infrared spectroscopy,XPS,and the Boehm titration were employed to study the functionalities on the MWCNT surfaces.Despite the limitations of these techniques,the results indicated that the dramatic increase in carboxyl groups was mainly responsible for the significant increase in oxygen content after the oxidation.The dissociation of the grafted functional groups increased electronegativity of the o-MWCNTs and facilitated the aqueous stabilization of o-MWCNTs through electrostatic repulsions.The oxidation affected the UV-Vis absorbance of MWCNT suspensions.The absorbances at 800 nm of the stabilized MWCNT suspensions had a good correlation with the MWCNT concentrations and could be used to quantify the MWCNT suspensions.The findings of this work are expected to boost the research on carbon nanotubes and their environmental behaviors.

Effect of precursor thermal history on the formation of amorphous and crystalline calcium carbonate

The role of the thermal history of the precursor was studied for amorphous and crystalline calcium carbon- ate phases synthesized from calcium nitrate. The X-ray diffraction patterns of these phases are influenced by their annealing temperature of 0, 300, 400, and 500 ℃. However, the effect of the precursor thermal history on the X-ray diffraction pattern of the resulting calcium carbonate phase is negligible. Transmis- sion electron microscopy indicates that materials annealed at 400 ℃ consist of amorphous aggregates, irrespective of the precursor thermal history. The crystallite size of crystalline calcium carbonate is influ- enced by the precursor thermal history, and ranges from 23 to 26 rim. Near-edge X-ray absorption fine structure measurements indicate that the annealing temperature plays an important role in determining the local electronic structure. The role of the thermal history of the precursor is also important for the resultinu electronic structure.