Synthesis, characterization and aggregation behavior of carbon nanotube-metal oxide nanohybrids

Date

2017-05

Authors

Das, Dipesh, 1986-

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Abstract

Extracting multifunctional benefits by combining multiple nano-scale materials has driven materials science to develop nano-heterostructures, which are known as nanohybrids (NHs). Many such composite materials have been researched for applications in the energy sector and in biomedical devices and processes. Among these NHs, carbon nanotubes combined with metal oxides (MOs) are one of the most studied materials that provide unique advantages as electrocatalyst supports, and are currently being commercialized as embedded electrodes for fuels cells. NHs are not only a new class of complex materials but also brings in novel physicochemical properties that most likely cannot be captured by the sum of the properties of their components materials. Thus, understanding the environmental health and safety (EHS) of this new class of composite NHs is imperative. The first challenge that the nano-EHS community faces is to synthesize these materials with a range of MO loadings or composition under a controlled and comparable set of experimental conditions. In this dissertation, a set of carbonaceous-metal oxide NHs have been synthesized and characterized under comparable synthesis conditions. After synthesis, the underlying mechanisms of metal oxide formation on multiwalled carbon nanotubes (MWNT) surfaces has been enumerated, and finally, aggregation behavior of a select NH and its components has been assessed as a function of the metal oxide loading. A modified sol-gel technique has been developed to grow TiO₂, ZnO, Er₂O₃, and Pr₆O₁₁ nanocrystals on MWNT surfaces. The novelty of this technique is that, by varying reagent composition, metal oxide content on the MWNT surfaces can be controlled, keeping all other parameters unchanged. The modified synthesis protocol has been successfully developed to produce a relatively large amount of NHs (100s of mg per batch of synthesis), adequate for systematic nano EHS studies. Following detailed characterization of the materials, underlying hybridization and MO crystal formation mechanism(s) have been enumerated. Furthermore, standard electron potential of the metal species (while considering electron transfer between their oxidized state to zero valent form) has been found to be the controlling factor for the formation of metal or metal oxide crystals from the precursors on MWNT surfaces, using the sol-gel synthesis technique. Self-aggregation, one of the dominant environmental processes that particles undergo upon release into aquatic environment, has been assessed for one of the most used and commercialized NHs MWNT-TiO₂ and its components. This study investigated the role of TiO₂ loading on the aggregation behavior, MWNT-TiO₂ NH with three different TiO₂ loadings. Results suggested that TiO₂ loading on MWNT surfaces control aggregation behavior of the composite NHs. NHs with all TiO₂ loading demonstrated strong dependence on electrokinetics. Deoxygenation of the NHs with decreased TiO₂ loading due to the NH synthesis process appeared to be a key contributor on the electrokinetics of the NHs. The van der Waals interaction forces of the NHs decreased with decrease in TiO₂ loading. This study also concluded that classical DLVO theory may be inadequate to capture the aggregation behavior of the NHs. The controlled synthesis technique developed during this research, as well as the mechanisms of metal vs. metal oxide formation identified will be valuable to prepare a large set of NHs for nano-EHS studies. Aggregation behavior of the composites can be very complex in nature and cannot be predicted form the sum of the behavior of their component materials. The deviation of DLVO prediction from the experimental aggregation data calls for further investigation on direct measurement of other complex surface properties of the NHs upon hybridization such as surface roughness and surface charge heterogeneity

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