Green synthesis of p-NiO/n-ZnO nanocomposites: Excellent adsorbent for removal of congo red and efficient catalyst for reduction of 4-nitrophenol present in wastewater

Globally water pollution caused by mainly the discharge of industrial effluents, human activities and natural processes is the major issue and concern. Adsorption and heterogeneous catalysis among various developed technologies for the remediation of wastewater treatment have attracted much attention due to the involvement of versatile and cheap materials such as mixed metal oxides. NiO/ZnO nanocomposites with different molar ratios (Ni:Zn 1:4, 2:3, 1:1, 3:2, 4:1 abbreviated as N1Z4, N2Z3, N1Z1, N3Z2, N4Z1, respectively) were synthesized via sol-gel method using zinc 2,4-pentanedionate and nickel acetate as precursors. Nanocomposites, NiO and ZnO were characterized by powder X-ray diffraction (PXRD), field emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-Ray photoelectron spectroscopy (XPS), Fourier transform infra-red (FT-IR) spectroscopy, Brunauer-Emmett-Teller surface analysis (BET) and ultraviolet diffuse reflectance spectroscopy (UV-DRS). XRD demonstrated the presence of cubic NiO and hexagonal ZnO phases. FE-SEM and TEM images demonstrated mixed morphology including spheres, hexagons and rods, and their particle size varies from 15 to 45 nm. Among all the composites, N3Z2 has probed as excellent adsorbent towards congo red (CR) (% removal efficiency: N3Z2, N2Z3, N1Z1, N4Z1, N1Z4, NiO, ZnO: 94.9, 88.8, 86.5, 67.9, 57.4, 52.3, 22.8, respectively, for 60 ppm CR in 1 h). N3Z2 can also be reused upto 5 cycles. On the other hand, N1Z1 has catalysed reduction of 4-nitrophenol (4-NP, 0.2 mM) to 4-aminophenol (4-AP) within 9 min with 100% conversion efficiency upto 3 cycles. The molar ratio of NiO and ZnO in the synthesized nanocomposites plays an important role in controlling their morphology, surface area and their selective application for wastewater treatment.