Structural analysis of x-ray irradiated carbon nanostructures

Abstract

Emerging nanotechnologies in which nuclear applications and radiations play key roles are: nano-electronics in environmental monitoring and remediation, electrode materials in hydrogen economy, polymer based nanocomposites in biotechnology, diagnostics and therapy. Radiation based technology using x-rays, e-beams and ion-beams is the key to avariety of different approaches. Due to the various ionizing irradiations, physical, chemical and biological properties of the materials can be significantly modified. Compared with conventional chemical reduction, the irradiation techniques are environmentally friendly, easily controlled, highly pure and less destructive. The most common defects induced by irradiation are vacancies and interstitials. Carbon based nanostructures with sp2-like hybridization, are exclusive due to the fact that its valence permitted researchers to engineer a large collection of molecular architectures. What makes all these structures truly phenomenal is that they are indeed built from the same component and they still can differ in shape and dimensionality. The most prolific irradiation-induced defects in graphenic carbon nanostructures are vacancies (single or multi vacancies). These carbon sp2nanostrucutres develop an extended reconstruction of the atomic network near the vacancy by saturating two dangling bonds and forming a pentagon. In graphene, single vacancies reconstruct, but in CNT the reconstruction is much stronger owing to the curvature and inherent nanoscale size of the system. It was found that for a CNTs to contract locally to “heal” the hole and thus saturate energetically unfavorable danging bonds. Thus, curved graphitic structures such as CNTs can be referred to as self-healing materials under irradiation. Some of the last experimental studies on the irradiation of MWCNTs reported a broad range of interesting phenomena such as surface reconstructions, modified mechanical properties, ion-irradiation induced changes in electrical coupling between nanotubes.1-3 Kis et al., have shown a strong stiffening of bundles of CNTs after electron irradiation.4 Last years, irradiation with γ-rays was studied as one of the clean and easy method for modification of carbon nanostructures. Namely, the effects of γ-irradiation strongly depend on the irradiation conditions, the materials type and the irradiation medium. Guo et al. observed a dramatic increase in the ID/IG of the Raman spectrum of γ-ray irradiated multi-walled CNTs (MWCNTs), which was attributed to the large presence of sp3-hybridized carbon atoms.5 This is opposite to the trend reported by Xu et al.,6 who noted an 8% decrease in ID/ IG for MWCNTs irradiated to 20Mrad in air, signaling improved graphitic order.6 Also, it was found that γ-irradiation decreased the diameter of MWCNTs, increased their specific surface area and modified their oxygen functional groups.7 The graphitization of MWCNTs was improved with doses of 100kGy, while a higher dose of 150kGy induced structural damage.7 Regarding the graphene, γ-irradiation was used for the reduction of graphene oxide in different liquid media.8 Bardi et al.,9 studied x-ray irradiation induced structural changes on single wall carbon nanotubes.9 Based on the Raman and XPS measurements, they confirmed the modifications in the structure of the nanotube surfaces, and found that the degree of disorder in the CNTs structure correlates with the x-ray irradiation dose.9 Although a huge amount of theoretical works were done to understand the origin of various kinds of irradiated induced structural changes and defects in carbon nanostructures, very little is known experimentally. Thus, the present work is aimed to focus on the influence of X-ray irradiation on the structural identification of changes and defects formed in carbon based nanostructures (G, MWCNTs, hybrid G/MWCNTs).