Distinct interface behaviors of Ni(ii) on graphene oxide and oxidized carbon nanotubes triggered by different topological aggregations

Abstract

Although carbon nanotubes can be described as a seamlessly curled graphene nanosheet, two-dimensional graphene oxide (GO) and one-dimensional oxidized carbon nanotubes (OCNTs) have different fates and environmental risks, such as deposition, degradation and cytotoxicity. In particular, coexisting heavy metal ions (HMs) trigger distinct morphological transformations of both of these carbon derivatives. In addition, these morphological transformations can change the interface behaviors of HMs on both of these materials. In this study, the differences in the morphological changes of GO and OCNTs and the interface behaviors of Ni(II) were explored via the intrinsically microscopic structural changes of both of these typical carbon materials. Batch experiments revealed that Ni(II) sorption on GO drastically decreased with increasing ionic strength, while it was almost independent of ionic strength on the OCNTs. This phenomenon is attributed to the aggregation and wrinkling of GO sheets at higher Na⁺ concentrations, resulting in a decrease in the GO surface area and number of sorption sites. Meanwhile, the intertwining aggregations of OCNTs still ensured that the sorption sites were naked. For the first time, Ni²⁺ ions were observed to persist as inner-sphere complexes on GO even under alkaline conditions, where the Ni(OH)₂(s) phase was determined on the OCNTs. This could be attributed to the fact that the fast aggregation of GO, which fixed Ni²⁺ ions into the interlayers, inhibited the nucleation of Ni(OH)₂. Stable layered structures of GO aggregations were difficult to exfoliate, leading to a decreased release of Ni(II) from GO with increasing Ni(II) loading. For the OCNTs, naked Ni²⁺ ions could be easily and effectively released. These findings are critical to assess the mobility, transformation and cytotoxicity of nanomaterials and HMs in aquatic environments.