Synergistic effect of polypyrrole modified WS2 nanosheets on visible light assisted catalysis for the removal of chromium (VI) and humic acid

Abstract

In this work, a novel nanocomposite was synthesized by the surface modification of tungsten disulphide (WS2) with polypyrrole (PPy) for the photocatalytic removal of heavy metal ions (chromium hexavalent (Cr (VI)) and natural organic contaminant (humic acid (HA)). The morphological analysis of synthesized samples was carried out using Field Emission Scanning Electron Microscopy (FESEM) which shows that PPy nanoparticles (NPs) are randomly distributed on the surface of WS2 nanosheets (NSs). The interlayer spacing of fringe width of WS2 NSs calculated from High Resolution Transmission Electron Microscopy (HRTEM) studies comes out to be 0.65 nm also in coherence with XRD analysis. Elemental analysis of synthesized nanocomposite was confirmed by using X-Ray Photoelectron Spectroscopy (XPS). The synthesized WP nanocomposite shown enhanced efficiencies (ɳCr ∼98.75% and ɳHA ∼77.78%) as compared to negligible efficiency of pristine WS2 (∼15% and 16%) and PPy (∼49% and 40%) for the reduction of Cr (VI) and degradation of HA, respectively. The enhanced removal efficiency of nanocomposite was attributed to decrease in the bandgap (2.03 to 1.80 eV), enhanced carrier concentrations (4.0 × 1030 cm−3 to 3.2 × 1031 cm−3) and low recombination rate (1.85 to 7.40 ns) of WP nanocomposite on the incorporation of PPy in WS2 as evident from UV–Vis, Mott-Schottky technique (MSK) and Time-correlated single-photon counting (TCSPC) decay measurement, respectively. The enhancement in removal efficiency is also attributed to the high surface area (160.36 m2/g) of the nanocomposite as analyzed using Brunauer-Emmett-Teller (BET) technique. The photocatalytic reduction efficiency was also studied as a function of pH (4,7 and 9). The reduction pathways follow second-order rate kinetics with high value of correlation coefficient (R2 ∼ 0.99). The proposed photocatalytic reduction mechanism was supported by a trapping study which shows that the superoxide radicals (⋅O2−) were the primary species taking part in photocatalytic reaction. Reusability test and stability of materials demonstrates promising photocatalytic performance of the synthesized nanocomposite.