Novel Z-scheme Co3O4/WO3 nanocomposite performance in adsorption and photocatalytic degradation of ethylparaben and methylene blue in water
Eric M Ngigi, Philiswa N Nomngongo and J Catherine Ngila
Abstract
Effective removal of organic pollutants from water bodies using both adsorption and photocatalysis provides an effective solution for both ecological and environmental importance. Degradation of organics such as parabens using photocatalytic process with visible light irradiation is challenging. However, semiconductor photocatalysis based on solar radiation utilisation whose mechanism involves absorption of photons when irradiated by sunlight or illuminated light, is possible. WO 3 has been exploited for its visible light utilisation whereas a novel Z–scheme photocatalyst formed when WO 3 is doped with Co 3O 4 has shown enhanced photocatalysis and adsorption performance. Z-scheme WO 3/Co 3O 4 semiconductor nanocomposite exhibits high photoactivity due to the suitable band gap matching between the two semiconductors as they share photoexcited electrons in the conduction band (CB) that can easily migrate to each other’s valence band (VB) to recombine with holes under solar light irradiation. In the present work, novel Co 3O 4/WO 3 (ratio 3:7) nanocomposites were successfully synthesised via two-step route including the solvothermal process and subsequent wet impregnation method. The Co 3O 4/WO 3 nanocomposites were applied for efficient removal of methylene blue and ethylparaben from water. Powder x-ray diffractometer (pXRD), scanning electron microscope (SEM), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), x–ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET) were used to characterise the prepared nanocomposites. The adsorption and photocatalytic activities of WO 3 and Z-scheme Co 3O 4/WO 3 nanocomposites (30 wt% of Co 3O 4) were evaluated by monitoring adsorption and photocatalytic activities of methylene blue and ethylparaben from water. The results confirmed that adsorption and photoactivity efficiencies of Co 3O 4/WO 3 nanocomposites were higher than either WO 3 or Co 3O 4 individually. The percentage removal efficiencies using Z-scheme Co 3O 4/WO 3 in the degradation of methylene blue and ethylparaben were 100% and 88% respectively, after 15 min. The improvement of adsorption capability could be attributed to the increase of specific surface areas by addition of Co 3O 4 whereas photocatalytic performance may be attributed to lower recombination rates. Furthermore, the effects of water parameters and radical scavengers on the adsorption and degradation processes were also investigated. It was noted that 
were major active species in the removal of methylene blue and ethylparaben. A probable mechanism on the enhanced photocatalytic performance was proposed based on the band structure and radical trapping experiment. The novel Z-scheme Co 3O 4/WO 3 nanocomposite developed in this study has great potential for degradation of organic pollutants in wastewater matrix. To the best of our knowledge, there is no literature that has reported the synthesis of Co 3O 4/WO 3 nanocomposite for the application of removal of organic pollutants.
