Flame spray pyrolysis of onion-like carbon nanostructures and composites containing reduced titania: A study on their morphological and optical features.

Abstract

Carbon nano onions (CNOs) exhibit unique structural and morphological features owing to graphitic layered structures. However, these nanostructures present limited optical activity in visible region due to their higher degree of sp2 hybridization which results in fast recombination of charge carrier species. This necessitates the structural modification of CNOs to impart redshift absorption. Previously, doping of metals and non-metals have been reported for the structural modifications, however, incorporation of metal oxides and their contribution to optical features have not been yet studied. This study specifically demonstrates one step synthesis of visible light driven TiO2 embedded CNOs via modified flame spray pyrolysis (FSP) and provides insights about their structural and optical features. Transmission electron microscopy (TEM) results show that CNO composites present core to shell morphology and TiO2 particles are embedded in the layered structures. Inner core of CNOs is associated with organic carbon (OC) while elemental carbon (EC) is responsible for the outer shell which originates due to high temperature residence time and thereby formation of higher EC4-6 fractions in the closed FSP. Thermal optical carbon analysis shows that core to shell ratio is proportionally affected by titania concentration leading to enhanced defects induced structures. This is further supported by Raman spectroscopic analysis which exhibit rise of ID/IG from 0.76 to 0.82 for 0.5% to 5% titania respectively. These structural defects appear due to sp2 disrupted domains and serve as anchoring sites for the functional groups such as C=O, C-O, C-H and C=C as evidenced by FTIR findings. Furthermore, titania induces synergistic effect and promotes redshift absorption of CNOs, leading to widening its band gap from 1.55 eV to 2.04 eV. This visible light driven CNO composites can act as photocatalyst for different photocatalytic and photochemical applications.