An efficient and stable narrow bandgap carbon dot-brookite composite over other CD-TiO2 polymorphs in rhodamine B degradation under LED light

Abstract

Carbon dot (CD) obtained from pine bark was loaded on three TiO2 polymorphs (anatase, rutile, and brookite abbreviated in this study as Ana, Rut, and Brk respectively) to evaluate their photocatalytic performance in rhodamine B (RhB) degradation under visible-LED light. The materials were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), x-ray powder diffraction (XRD), N2 sorption isotherm, and Fourier transform infrared (FT-IR), UV-diffused reflectance spectroscopy (UV-DRS), and x-ray photoelectron spectroscopy (XPS). The x-ray result shows that the different pure pristine polymorphs and their composites were successfully prepared with Rut and CD-Rut composite having the highest crystallite size of 23.06 and 21.64 nm respectively. The N2 sorption isotherm of the materials shows Ana, Brk and their composite exhibited type IV isotherm with H series hysteresis loop, which implies that they are mesoporous materials. The isotherm exhibited by Rut and CD-Rut suggests that they are non-porous. However, their pore size distribution (PSD) shows that the materials possess mesopores that may have resulted from interparticle voids due to particle aggregation. Results from the photocatalytic degradation experiments showed that the materials performed in the following order CD-Brk>Brk>CD-Ana>CD-Rut>Rut>Ana with a corresponding percentage RhB removal of 99.91, 76.78, 36.28, 27.50, 22.65, and 12% respectively. The excellent performance of CD-Brk and Brk over the others is attributed to their narrow bandgap energy of 2.00 eV making them more active under the visible-LED light. CD-Brk performed better than Brk because it possesses more surface-active sites resulting from its relatively higher surface area. In terms of degradation mechanism, CD-Brk which was stable after 4 cycles of degradation, showed that superoxide anion radical (O2•−) played major role in the degradation process.