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

Onion-like carbon nanostructures (CNO-like structures) exhibit unique structural and morphological features owing to their graphitic layered structure. However, these nanostructures present limited optical activity in the visible region due to their higher degree of sp²hybridization, which results in fast recombination of charge carrier species. This necessitates structural modification of CNO-like structures to impart redshift absorption. Previously, doping of metals and non-metals has been reported for these structural modifications; however, the incorporation of metal oxides and their contribution to optical features have not been yet studied. This study specifically demonstrates the simultaneous synthesis of visible-light-driven TiO₂/CNO-like nanostructures via modified flame spray pyrolysis (FSP) and provides insights about their structural and optical features. Transmission electron microscopy results show that CNO-like composites present core-to-shell morphology and TiO₂particles are embedded in the carbon layered structures. The inner core of CNO-like structures is associated with organic carbon, while elemental carbon (EC) is responsible for the outer shell, which originates due to high temperature residence time and consequent formation of higher EC_4-6fractions in the closed FSP. Thermal optical carbon analysis shows that the core-to-shell ratio is proportionally affected by titania concentration, leading to enhanced defect-induced structures. This is further supported by Raman spectroscopic analysis, which exhibits rise of<i>I</i>_D/<i>I</i>_Gfrom 0.76 to 0.82 for 0.5 wt% to 5 wt% titania, respectively. In the context of Raman spectroscopic analysis,<i>I</i>_Dstands for the for the intensity of the D-band (disorded band) while<i>I</i>_Grepresents the intensity of the G-band (graphitic band). Higher<i>I</i>_Dshows that carbon nanostrucrure is more disorded and amorphous in nature while higher value of<i>I</i>_Gexhibits the ordered and graphitic nature of the carbon material. These structural defects appear due to sp²disrupted domains and serve as anchoring sites for functional groups such as C=O, C-O, C-H and C=C, as evidenced by Fourier tranform infrared findings. Furthermore, titania induces a synergistic effect and promotes redshift absorption of CNO-like structures, leading to widening of the band gap from 1.55 eV to 2.04 eV. These visible-light-driven CNO-like composites can act as photocatalysts for different photocatalytic and photochemical applications.