Heteroatom carbon spheres incorporated with Titania for photocatalytic degradation of organic pollutants

Abstract

Continuous efforts to produce catalysts that are environmentally friendly, low-cost, and less toxic have been investigated for the treatment of organic pollutants in wastewater. In this research, an onion raw material that is environmentally friendly, abundant and is a green carbon precursor was used to synthesize carbon material. The onion material consists of heteroatoms such as nitrogen, sulfur, oxygen, hydrogen, and carbon elements. Here, we demonstrated an onion material synthesized by the hydrothermal process followed by the CVD method to produce nitrogen, sulfur co-doped carbon spheres (NSCS). The resultant product was subjected to various synthesis temperatures (850, 900, and 950 °C), in order to understand the effect of synthesis temperature on the NSCS material. Natural-based carbon precursors require activating agents, in this study potassium hydroxide (KOH) and zinc chloride (ZnCl2) were used as activating agents to produce spherical carbon products. The NSCS material was later denoted as NSCS without activation, NSCS2 (NSCS2 KOH; NSCS2 ZnCl2), and NSCS3 (NSCS3 KOH;NSCS3 ZnCl2) depending on the mass ratio of activating agent used. The presence of heteroatoms in the synthesized NSCS material was confirmed by the CHNS elemental analysis. FTIR and XPS analysis confirmed the presence of covalent bonding between carbon and nitrogen and sulfur. Formation of covalent bonding such as C=C, C-S, and C=N was observed. The morphology, size, and structural features of the carbon spheres were studied using TEM, SEM, and laser Raman spectroscopy. The PXRD of the produced NSCS was found to be graphitic with a low graphitization degree. TiO2 nanoparticles were produced by the sol-gel method followed by the CVD method annealed at 450 °C with a crystallite size of 12 nm observed from PXRD. The resultant TiO2 showed a high surface area of 51.2 m2/g. The catalyst (TiO2_NSCS) was obtained following the same procedure as that of the formation of TiO2 nanoparticles. Successful incorporation of the NSCS to TiO2 was confirmed by XPS with different configurations of the material being observed. The results obtained from BET showed a high surface area of NSCS material and in turn when they were used as support material for TiO2,a high surface area of the composite was observed. The catalyst was tested by photodegradation of Rhodamine B dye, to evaluate its activity and stability. High-pressure liquid chromatography coupled with mass spectroscopy (HPLC-MS) technique was used to confirm the degradation of the RhB dye compound. Additionally, the breakdown of the complex RhB dye structure was reported. It was observed that a lower molecular mass was plausible when the composite was used as an alternative to using TiO2 alone