Enthalpy of formation of carboxylated carbon nanotubes depending on the degree of functionalization

Carbon nanotubes (CNTs) with different content of carboxylated groups on their surface (depending on the duration of their treatment with nitric acid) were synthesized. All samples were analyzed by thermal analyses, X-ray photoelectron spectroscopy, Raman and energy-dispersive X-ray spectroscopy, transmission electron microscopy and S-BET. The adiabatic bomb calorimetry technique was used for the determination of enthalpy of formation. With the increase in time of treatment from 3 to 9 h, the content of oxygen increased from 7.49 to 8.22 at%. After 15-h treatment in nitric acid, CNTs contained 7.86 at%. The enthalpies of formation of all samples were negative and had nonlinear character. The changes of surface and bulk physicochemical characteristics of oxidized CNTs were analyzed. It was shown that despite decrease in surface enthalpy of formation a dagger(f) H (0)(298(surf.)) with the increase in oxygen content, the bulk enthalpy of formation a dagger(f) H (0)(298(bulk)) was very sensitive to defectiveness and structure of carbon layers. It resulted in the difficult correlation between oxygen content, morphology, defectiveness and a dagger(f) H (0)(298).