Influence of Carboxylation on Structural and Mechanical Properties of Carbon Nanotubes: Composite Reinforcement and Toxicity Reduction Perspectives

Abstract

Carboxylation of carbon nanotubes (CNTs) is an important process that is applied routinely for various applications, in particular for biomedical usage and the manufacturing of next-generation composite materials. This study investigates the influence of carboxylation on the structural and mechanical properties of CNTs. Ab initio calculations were performed within the density functional theory framework for metallic and semiconducting single- and multiwalled CNTs, imperfect and carboxylated at various concentrations, including disorder. The morphologies were analyzed, the stabilities of the carboxylated CNTs were determined, relevant electronic properties were evaluated, and elastic moduli were calculated (Young’s, shear, and bulk moduli, as well as Poisson’s ratio). The properties of grafted (COOH, OH, O) and imperfect (vacancy defects) CNTs were compared with those of carboxylated CNTs. In particular, both the structural and elastic properties were found to exhibit significant differences between COOH-grafted and carboxylated systems, with the dependence of the Young’s modulus on the concentration of functional groups or vacancies being the most striking. Experimental observations showing that small concentrations of COOH groups improve the strength of CNTs in comparison to that of defected CNTs are explained. The critical concentration above which the carboxylation of CNTs leads to significant structural changes of the nanotubes was determined. These changes were found to result in a significant decrease of the Young’s modulus, making the CNTs unsuitable as composite reinforcements. Moreover, mechanisms governing the toxicity reduction of carboxylated CNTs are explained.

In vitroreprotoxicity of carboxyl-functionalised single- and multi-walled carbon nanotubes on human spermatozoa

“…Chemical functionalisation of CNTs has attracted considerable interest and is widely used to improve solubility of CNTs in different aqueous media (Yang et al., ). Among numerous chemical functionalisation methods, carboxylation is the most commonly used approach for preparation of CNTs that are well applicable in biomedical technology and also manufacturing of next‐generation composite materials (Milowska, ). Additionally, carboxyl groups are generated on CNTs as a by‐product of acidic purification of raw CNTs to remove metallic and carbonaceous impurities.…”

Breaking the electrical barrier between copper and carbon nanotubes

“…Interestingly, the greatest deformation of the CNT structure is observed in the Al-doped embedded system. The deformation even exceeds that observed in the Ni embedded case and is almost the same degree of distortion that can be induced by covalent functionalization 16 . Al atoms, which tend to create longer bonds with C atoms than do Cu atoms 21 , are trapped by surrounding Cu layers inside the composite and exhibit behaviour which is different from metal-CNT systems containing only one type of metal 21,28 .…”

“…Good wettability reduces the metal-CNT electrical resistance by enhancing the contact area 34 . Wetting properties can be theoretically described by combined analysis of the matrix internal surface energy 28 γ, the interface strength 32 τ, and changes in CNT structure 26 described by the coefficient of CNT radius variation, CV 16 (see ESI † for details). Formation of a continuous or quasi-continuous metallic layer adsorbed on the lateral surface of a nanotube is preferred when τ is high, whereas the separation tendency leading to the formation of a metal cluster is observed when γ is low.…”

“…Covalent functionalization of the CNTs enables improved bonding with the composite matrix and their debundling 13 , but it may lead to certain unwanted changes in the CNT's mechanical and electronic properties 14 . Adsorbates form covalent bonds to the lateral surface of the CNTs, causing local deformations, sp 2 to sp 3 rehybridization, possible formation of heptagon/pentagon defects, vacancies, and substitutional doping depending on the type and concentration of the particular functionals, and as a consequence hamper the conducting sub-bands [14][15][16] . Non-covalent functionalization, mainly based on supramolecular complexation using various adsorption forces, maintains CNT structure 17,18 but its feasibility in multiwalled CNTs is limited to the outer wall, while inner walls remain electrically inactive.…”

Improved Functionalization of Multiwalled Carbon Nanotubes in Ultra-Low Acid Volume: Effect of Solid/Liquid Interface

“…Chemical modifications of CNT surface comprise the main strategy to enhance their interfacial interaction with other materials. [4][5][6] Acid treatments are the most commonly used method for insertion of covalently bonded functional groups, [7][8][9] among other less usual approaches involving ozone, [10][11][12] oxygen plasma [12][13][14] and ultraviolet radiation. 15 Acid treatments generally use nitric (HNO 3 ) and sulfuric (H 2 SO 4 ) acids in ultrasound, [16][17][18][19][20][21][22][23][24] reflux 8,9,[21][22][23][25][26][27][28][29] or ultrasound followed by reflux, 22,30 besides the less common use of microwave.…”