WS2 nanosheets modified via ZnO nanorods (2D/1D) nanocomposite: An efficient photocatalyst for the removal of Cr (VI), humic acid and textile dyes from wastewater

In present study, photocatalytic removal of (i) chromium hexavalent Cr(VI) - heavy metal ion (HMI), (ii) humic acid (HA) - natural organic pollutant and (iii) textile dyes - (cationic: malachite green (MG), methylene blue (MB) and anionic: congo red (CR)) have been carried out using 2D/1D (tungsten disulphide (WS2) sheets/zinc oxide (ZnO) nanorods) nanocomposite. The synthesized 2D/1D heterostructure acts as a barrier for the recombination of photogenerated species and hence, enhanced the photocatalytic activity. The synthesis of high-purity and crystalline nanocomposite was confirmed by the X-ray diffraction studies. The uniform distribution of ZnO nanorods with an average diameter and length of -50 and -280 nm, respectively on the surface of WS2 nanosheets were confirmed by Field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). The photocatalytic study shows that the photocatalytic removal efficiencies significantly, enhanced using synthesized nanocomposite from 4.1% to 83.6% for Cr(VI) and 23.5%- 75.0% for HA as compared to pristine WS2, respectively. The enhanced removal efficiency achieved by the synthesized nanocomposite was explained with the help of (i) higher carrier concentration, (ii) low charge transfer resistance, (iii) appropriate surface charge and (iv) large surface area (85.48 m2/g), as determined from Mott-Schottky (MSK), Electrochemical Impedance Spectroscopy (EIS), Zeta potential, and Brunauer-EmmettTeller (BET) experiments. The significant improvement in the removal efficiencies were also attributed to the pore size of nanocomposite, pore size belongs to the micropores (0.8-2.0 nm) region which facilitates large active sites for the sorption of contaminants. The observed optical bandgap 2.32 eV of the nanocomposite makes it a favourable material for photocatalytic removal and degradation of contaminants. The semblance of active species in photocatalytic degradation was analyzed through a trapping study and shows that superoxide radicals (O.-2 ) are the primary species in the removal process. Further, the confirmation of active species was carried out using EPR analysis and value of g-factor is also calculated. The intermediate products during the photocatalytic degradation are analyzed using high performance liquid chromatography (HPLC) technique. The synthesized WZ nanocomposite is an effective and promising material for the treatment of wastewater in the textile, mining, and agricultural industries for getting out the cleaner production.

Automated summary

In this study, a 2D/1D nanocomposite consisting of tungsten disulphide (WS2) sheets and zinc oxide (ZnO) nanorods was synthesized and used for the photocatalytic removal of various pollutants. The nanocomposite showed enhanced photocatalytic activity due to its ability to prevent recombination of photogenerated species. The removal efficiency of heavy metal ions, organic pollutants, and textile dyes was significantly improved compared to pristine WS2. The enhanced removal efficiency was attributed to factors such as higher carrier concentration, low charge transfer resistance, appropriate surface charge, and large surface area.